Energy Conversion and Management 272 (2022) 116326

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Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

The future of hydrogen: Challenges on production, storage and applications

M.G. Rasul a, *, M.A Hazrat a, M.A. Sattar a, b, M.I. Jahirul a, M.J. Shearer c
a
Fuel and Energy Research Group, School of Engineering and Technology, Central Queensland University, Rockhampton, Queensland 4702, Australia
b
Engineering School, Chisholm Institute, 121 Stud Road, Dandenong, Victoria, Australia
c
Hydrogen Renewable Energies Centre, Central Queensland Energy, Gladstone, Queensland 4680, Australia

A R T I C L E I N F O A B S T R A C T

Keywords: With the demand for hydrogen being expected to increase by about 8-folds in 2050 over 2020, there are several
Hydrogen production challenges factors that can turn into challenges for effective roll out of hydrogen applications in energy sector. Hydrogen has
Hydrogen applications the second highest calorific value, 120-142 MJ/kg, which is the best energy-weight ratio among all conventional
Hydrogen storage
fuels. Among all hydrogen production processes, the green hydrogen production through mature water elec­
Hydrogen infrastructure development
Hydrogen as key driver of energy for future
trolysis process, with technology readiness level of ~7-8 (demonstration/system development) and commercial
readiness index of ~ 4-5 (deployment) contributed about 30% to market share with efficiency of 55-80% and
production cost of ~ $4-7/kg H2. This study found that the current hydrogen production costs may reduce to
desired 1-2 $/kg H2 within a couple of decades, but there is still a lack of plans for combining various hydrogen
production processes where necessary, rather than only focusing on producing hydrogen in mass scale. Among all
the hydrogen storage systems, the specific volumetric storage cost of metal hydride is less, about $125/m3, than
other systems. Ammonia has lowest specific energy cost, about $13/GJ amongst other storage systems. There is a
requirement of rapid progression in relevant infrastructure development for efficient supply chain management
for storage, transportation, and delivery of hydrogen to the stakeholders. This paper reviewed 400+ articles and
summarised hydrogen production processes, storage options, production costs and applications. The synthesis of
key information and deep analysis of limitations of existing studies has been provided followed by deep dis­
cussion on the challenges of hydrogen as energy carrier for future. To achieve sustainable development goals,
integrated plans, infrastructure development, reduction of production costs, achieving net zero emissions and
novel storage development need to be achieved within 2050. This reviews thus could be used as a guideline by
policymakers, researchers, and scientists for shaping future of hydrogen.

Integrating carbon capture, utilisation and storage (CCUS) in the

hydrogen production system from the wastes can solve multiple prob­
1. Introduction
lems, i.e., sustainable waste management, emission reduction, and
cleaner fuel production. With about 13.5% recycling rate of wastes
Hydrogen is now considered to be the future form of leading energy
around the globe there may be about 3.4 billion tonnes of solid waste
system and multipurpose industrial raw material due to its significant
generation by 2050 which is about 70% more than that of year 2016
potential to shape building a cleaner and sustainable earth for the
[10]. Also, the overall quantity of solid wastes generated by 2050 may
human being. There are various production pathways (i.e., electrical,
lead to emission of more than 2.6 giga tonnes of CO2-e [10]. The techno-
thermal, hybrid, and biological) for hydrogen generation from various
economic progress of coal gasification process [11,12] can help to
feedstocks [1,2]. That is why, optimal integration of all of these path­
accelerate the establishment of large-scale commercial gasification of
ways based on regional variation of feedstocks and technical resources
wastes [13] for decarbonised hydrogen production. Begum et al [14]
can efficiently make hydrogen a potential energy transformer and lower
developed a numerical model and investigated the pyrolysis of munic­
the emissions significantly to achieve the sustainable development goals
ipal solid waste. Also, the pyrolysis of wastes for energy production can
(SDG) [3-5]. Apart from the conventional fossil feedstocks biodegrad­
be integrated with the reforming process to produce hydrogen [15,16].
able and non-biodegradable wastes can be used to produce hydrogen by
Efficient use of catalysts [17,18] along with optimisation of operating
improving the waste-to-energy producing pathways [6-9]. Using wastes
parameters [19] are also undergoing extensive research works to
can be considered as net-zero emission producing feedstocks.

* Corresponding author.
E-mail address: m.rasul@cqu.edu.au (M.G. Rasul).

https://doi.org/10.1016/j.enconman.2022.116326
Received 6 July 2022; Received in revised form 23 September 2022; Accepted 4 October 2022
Available online 21 October 2022
0196-8904/© 2022 Elsevier Ltd. All rights reserved.
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Nomenclature IPCC Intergovernmental Panel on Climate Change

LTS Low temperature shift
AHW Atomic hydrogen welding LOHC Liquid organic hydrogen carrier
APR Aqueous phase reforming LH2 Liquid hydrogen
ATR Autothermal steam reforming MCFC Molten carbonate fuel cell
BTH Biomass-to-hydrogen MeAFC Metal-air fuel cell
BWGS Bio-water-gas shift NERA National Energy Resources Australia
CAPEX Capital expenditure OSR Oxidative Steam Reforming
CCU Carbon capture and utilization OPEX Operating expenditure
CCS Carbon capture and sequestration PEFC Polymer electrolyte fuel cell
CODH Carbon monoxide dehydrogenase PEM Polymer Electrolyte Membrane
COS Carbonyl sulphide POX Partial Oxidation
CRI Commercial readiness index PSA Pressure swing adsorption
CSIRO Commonwealth Scientific and Industrial Research P2G or PtG Power-to-Gas
Organisation PEMFC Proton exchange membrane fuel cell
CSP Concentrated Solar Power PAFC Phosphoric acid fuel cell
CSR Catalytic steam reforming SCWR Supercritical water reforming
CTH Coal-to-hydrogen SI HEV Spark ignition hybrid electrical vehicle
DRM Dry reforming methane SI ICEV Spark ignition internal combustion engine vehicle
EBS Environmentally “benign” sequestration SO Solid Oxide
EOR Enhanced oil recovery SR Steam reforming
EWGS Electrochemical water–gas shift SOFC Solid oxide fuel cell
FCEV Fuel cell electric vehicle TRL Technology readiness level
FCV Fuel cell vehicle UNIDO United Nations Industrial Development Organization
GHG Greenhouse gas USDOE United States Department of Energy
HFCV Hydrogen fuel cell vehicle WGS Water-gas shift
IEA International Energy Agency WFPP Waterloo flash pyrolysis process

produce high percentage of hydrogen from thermochemical and bio­ integrate the hydrogen in the energy-economy system, total hydrogen
logical processes [20]. Rapid growth in technologies have been observed supply chain needs an effective set of optimal and low-cost technological
in recent years along with advanced economic analyses to produce evolution for massive scaling-up of hydrogen production, storage,
hydrogen commercially at an affordable level of total cost of ownership transmission and distribution purposes [34-38].
(TCO) [4,5,21]. The physical and chemical properties of hydrogen have made this
The extensive deployment of hydrogen production facilities via beneficial energy vector highly challenging to store and transfer
currently available mature electrolysis processes can be coupled with economically for mass application in comparison to other existing en­
various energy utilising sectors and efficiently achieve decarbonisation ergy sources in recent times. The lower volumetric and energy densities
[22,23]. In addition, countries which produce extra renewable energy of hydrogen at room temperature, compressed (700 bar) and liquid
can use that to produce hydrogen and export or transport it to other conditions are 0.0107 MJ/L, 5.6 MJ/L, and 10.1 MJ/L respectively
regions of the world as green energy in the form of compressed hydrogen considering higher heating value of 143 MJ/kg [39] have made the
gas or liquefied hydrogen or in other carriers like ammonia or methane storage system highly challenging. Besides, the highly active electron of
[22]. This process is currently referred to as “Power-to-Gas (P2G or hydrogen has made it highly susceptible to the metal storage systems
PtG)” technology [24,25]. There are two key challenges to make the which is known as “hydrogen embrittlement” [39,40]. Hydrogen can be
vision successful: firstly, establishing a market for hydrogen and sec­ stored physically through phase changing or through surface adsorption
ondly, decarbonising the hydrogen production processes to restrict the or absorption processes [39,41]. Ziver et al. [42] have recently reviewed
emissions from across sectors [22]. There are several factors for underground storage options of hydrogen gas and indicated that the salt
hydrogen production rather than any mutual exclusiveness between caverns, and saline aquifers offer better technical efficacy than the
these two key challenges for hydrogen deployment for cleaner climate depleted oil reservoirs to prevent loss of hydrogen due to chemical
condition and energy supply. These factors are production capacity reactiveness with residual oil. Besides, the depth, types of rocks, and
improvement and market size development, support from governments permeability are other issues to decide scale of hydrogen storage into
and investors, feedstock selection, and economically feasible technology underground facilities. Based on the applications of the hydrogen in the
selection for production, decarbonisation, storage, transmission and energy transformation systems (i.e., scaling-up [34], mobile, or sta­
applications, which are influenced by the hydrogen industry establish­ tionary [35,36]), technology implementation and various in­
ment. In reality, without economically beneficial and efficient decar­ frastructures planning in regions worldwide [43-46], there are
bonisation processes, the hydrogen production processes from various opportunity to utilise cost-efficient hydrogen storage processes [47] as a
feedstocks other than wind, solar and nuclear energies will not be form of energy storage in renewable way. Though currently almost all
considered as clean fuel [26-29]. hydrogen production from the fossil fuels do not use CCUS processes (i.
In spite of several unsuccessful efforts at campaigning for hydrogen e., grey hydrogen), there is a high demand of efficient and large scale
to be considered as a clean energy producing fuel in the various sectors CCUS facilities (about 50 giga tonnes by 2050 [5]) to be integrated with
previously, there are positive expectations about the successful imple­ the emission producing processes to produce blue and green hydrogen
mentation of hydrogen infrastructure due to demonstrated successes of for decarbonised energy scenario and reduce overall levelised cost of
alternatives like fossil fuel conversion to compressed natural gas, re­ hydrogen production and deliveries (LCOH).
newables like solar photovoltaic (solar PV), wind, energy storage sys­ IRENA [48] has recently summarised the factors those have been
tems, hybrid and electric transportation systems [30-33]. In order to impeding the scaling up of the hydrogen production in recent years to

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

achieve greener transformation of energy sector. These are the costs of Table 1
establishment and ownership, technical matureness, and lower effi­ Recent review articles on hydrogen production, storage, and applications
ciency in each of the value adding stages, insufficient progress for Reference Reviewed Areas Why this review? Key Information
renewable electricity generation, uncertain policies and regulations, Article
lack of common institutional standardisation and certification processes, Chau et al. Hydrogen In order to Maturity of
and chicken-and-egg crisis in establishing the reliable infrastructure for [53] Production through determine the technology and
hydrogen production and supply chain management. Indeed, the key reforming (steam comparative decision-making
concern of transforming the hydrogen as fuel for energy production to methane, efficacies of the through parametric
autothermal, partial reviewed analysis like design
replace the emission intensive energy sources for the energy consuming oxidation), technologies from improvement,
sectors has been found to be the costs due dependence of all other factors electrolysis the available data by operating
on it [48-51]. Time is another key factor as there is potential of complete (alkaline, polymer using modified conditions, yields,
transformation of costs as well as other factors from medium-term electrode multi-criteria infrastructure
membrane, solid decision-making capabilities, and
projection (until 2030) to long-term projection (2050 and beyond) [52].
oxide electrolyser) (MCDM) analysis. resource
In order to develop the novelty of this review article the Clarivate management
Web of Science database has been used to find out the current reviews on analysis.
hydrogen by using few key words (e.g., title including - “Hydrogen”, Faye et al. Hydrogen To identify the most Partial oxidation
document type - “review”, web of science category - “Energy Fuels” or, [38] production, storage, prospective and reforming process is
and transportation effective process efficient along with
“Electrochemistry” or, “Engineering Chemical”, publication years – processes. technologies which CCUS technology
“2020, 2021, 2022”), which yielded 569 review articles. Though are will bring good when fossil fuels are
numerous review articles on various production technologies, there are impact to future used as feedstock.
only a few articles which have reviewed the production, storage, hydrogen goals. Carbon deposition
can be removed
transportations technologies altogether along with the critical goals,
from catalysts with
policies and projection which are indeed driving the key development CeO2/ZrO2 support.
requirements of hydrogen supply chain [53-56]. Table 1 presents Tube trailer
reviewed information on combined sectoral effect for hydrogen supply transporting can be
chain. However, Faye et al. [38] reviewed various production technol­ more efficient than
the liquefied
ogies, storage, and transport options to identify the overall cost/kg and
hydrogen.If
found that hydrogen can be supplied at a cost of $2.86/kg when tube Hydrogen is to be
trailer are used for transportation after hydrogen production through blended with
efficient production process and implementing CCUS. With ever natural gas, it is
effective with
increasing risk of financial crisis all around the world, increasing living
concentrated
cost people may not be ready to pay higher cost for cleaner fuel unless hydrogen.
there is clear indication of lowering cost of renewable energy through Dawood Renewable Due to absence of a It is necessary to link
technology development transition period. Lebrouhi et al. [57] stated in et al. hydrogen – single review article these reviewed areas
their review that about 60% of the global GHG emissions reduction will [60] production, storage which reviewed along with the
options, safety interlinks among purification process,
be driven by the renewables in the later phase of the transition period.
concerns, and uses. these areas. which indeed
But this will only occur when there is efficient coordination and decision defines the
making through respective parties within the energy transition chain. cleanliness level of
Hence, it not only the technology but also the maturity of policies and the hydrogen.
Yue et al. Electrolysis, fuel cell To integrate the The power to
planning which should happen within a very short period to allow
[54] power conversion, energy system with hydrogen, and
reliable progress of the technology development processes. During these storage of energy in the hydrogen hydrogen to power
periods, the fossil fuel fed thermochemical processes should be eco­ the form of production system, mainly linked to
nomic enough to compete well in the energy mix. Gonzalez-Garay et al. hydrogen, energy-hydrogen- cost, efficiency,
[58] reviewed on overall hydrogen supply chain for mobility and fore­ transportation. energy conversion investment,
for application production scaling
casted that the currently valued at $6.0/kgH2 from renewables will drop
requirement and up, and policy
to about $2.6/kgH2 by 2030 if there are optimal prospect in the chal­ transport. support.
lenging areas like electrolyser’s efficiency, higher investment with lower Sazali et al. Hydrogen To investigate the More demand of
interest rate, and rapid technical maturity. With future goals, it is highly [61] production and existing hydrogen
applications. technological production will
essential to go through overall technical review on production, storage,
capabilities and influence the
and applications to propel the growth all around. The more the explo­ necessary currently high-cost
rations there is a chance of obtaining effective solution within antici­ improvements as energy values to be
pated timeline. Investment, public awareness and infrastructure defined by economic dropped down,
maturity can control how the hydrogen will enter into domestic and development. which will also
influence the cost
international market for a prospective producer of renewable hydrogen,
reduction of
for which Australia may enter into international market before it enter renewable hydrogen
into the domestic market with renewable hydrogen [59]. production along
Though hydrogen production, storage, and applications have been with storage
reviewed earlier, the rapidly evolving projections and energy market­ facilities .
Kannah Hydrogen To investigate the Rate of return,
isation due to technological progress, economic analyses, and relevant et al. production, cost- global energy capital cost, and
policies have attracted necessity of reviewing again. Thus, this paper [55] effectiveness of the market share feedstock cost will
analysed and reported different aspects of hydrogen usage such as technologies. potential of influence the
production processes, storage, distribution, applications, sustainability, hydrogen by mixing hydrogen cost. Since
various production the renewable is still
and cleanliness based on the energy sources and feedstocks that may
and purification expensive the highly
help progressing towards future goals. Besides, global scenarios of processes along with efficient reforming
hydrogen and the related research and development are discussed here. (continued on next page)
Finally, the challenges of utilising hydrogen and the technology

3
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Table 1 (continued ) the currently established and prospective hydrogen production

Reference Reviewed Areas Why this review? Key Information processes.
Article Hydrogen is lighter than air and readily disperses (liquid density:
cost of production process can be
70.8 kg/m3 at standard boiling point and 1 atm, vapour density: 0.08376
through detailed integrated with kg/m3 at 20◦ C and 1 atm) [77]. It is also termed as an efficient energy
cost analyses. purification process carrier instead of considering it as a source of energy due to its versatility
and effort can be of production processes and sources (i.e., natural gas, heavy and light
made to reduce the
hydrocarbon oils, coal, solar, wind, geothermal, biomass, biofuels, nu­
overall cost of
production and clear, and electricity) [22,74,78]. When used as fuel, the calorific value
transportation due of hydrogen (120-142 MJ/kg) indicates the best possible energy-to-
to lower feedstock weight ratio in comparison to any other fuels [77,79]. Fig. 1 shows
cost. that only the radioactive material Uranium has a higher calorific value
than hydrogen, and the former is a fuel for nuclear plants rather than for
readiness level and the commercial readiness index of hydrogen are transportation [79,80]. As per the current continual increase in energy-
discussed. Reviews along with discussions thus could be used as a related CO2-e (carbon dioxide equivalent) emissions to meet rapid
guideline by policymakers, researchers, and scientists for shaping future projected economic advances by 2050, global temperature rise has been
of hydrogen. Hence, the novelty for this study is to identify and report projected to increase by 3-6◦ C above that of the pre-industrial era
the efficient thermochemical and electrochemical hydrogen production [65,66]. The Intergovernmental Panel on Climate Change (IPCC) has
processes, their storage, CCUS, and applications which would define the recognized the requirements for combined and efficient global efforts to
future fuel/energy mix for cleaner energy production. tackle such temperature rises and emission reduction from all possible
sectors [65,67-69]. Based on techno-economic development and
2. Hydrogen as key driver for global GHG emission reduction implementation of various activities to curb the GHG emissions and their
global warming effects, the current goal is to reduce the overall emission
The global economic status by 2050 has been predicted to observe level as low as possible to maintain the global warming temperature rise
rapid growth of development [62] that will demand extensive use of at no more than 1.5◦ C by 2050 in comparison to that of the pre-
energy [63] in each of the respective emerging and established sectors. industrial level [65]. Though ambitious and challenging [33,70,71],
In the Paris agreement , 196 signatory countries pledged to decrease the shifting technological dependencies on using direct combustion of fossil
greenhouse gas emissions to bring down the global warming to the pre- fuels for energy production, implementing regulations to strictly check
industrial level [64]. In addition to that, the commitment by the G20 the overall CO2-e emissions to limit the global warming potential (GWP)
nations to reduce the CO2 emissions has accelerated the research and [72] of the emitted pollutants, and an overall concern for cleaner air for
development in the field of hydrogen production, storage, transmission, the comfort of living on earth can inspire all of humanity to focus on
and distribution. As per the current continual increase in energy-related producing cleaner alternatives like hydrogen fuel, and wind or solar
CO2-e (carbon dioxide equivalent) greenhouse gas (GHG) emission, electricity. Recently, the world has become aware of the quality of the
global temperature rise has been projected to increase by 3-6 ◦ C above air for breathing and the extent of emissions related pollution. As a
that of pre-industrial era by 2050 [65,66]. Necessity of an efficient result, strict air pollution regulations have been introducing gradually
global effort has been identified by the intergovernmental panel on and attention has been shifting to look for alternative cleaner fuels like
climate change (IPCC) to keep the global warming related risks under hydrogen for engines [74]. Ugurlu and Oztuna [81] have reported that,
control [65,67-69]. Based on techno-economic analyses and imple­ when liquid hydrogen (LH2) was used as fuel in the fuel cell vehicles
mentation of various activities to curb the GHG emission, thus checking (FCVs), spark ignition hybrid electrical vehicles (SI HEVs) and spark
the global warming effect, the current goal is to reduce overall GHG ignition internal combustion engine vehicles (SI ICEVs), the FCVs effi­
emission level as low as possible to maintain the global warming tem­ ciently reduced the emissions by 35.1% and 49.6% in comparison to that
perature rise up to 1.5 ◦ C by 2050 in comparison to that of pre-industrial of SI HEVs and SI ICEVs respectively. Production of hydrogen gas and its
level [65]. Though ambitious and challenging [33,70,71], an overall application as fuel for the FCEVs has the possibility to decrease overall
shift of technological dependencies from direct combustion of fossil fuels worldwide greenhouse gas emissions (GHGs) by 50%, which will
for energy production and implementation of strict regulations to check effectively reduce the demand for petroleum for internal combustion
the overall CO2-e emission are required to limit the global warming engines by 90% approximately [82].
potential (GWP) [72] of the emitted pollutants. The real concern for With supporting instances of reducing significant amounts of global
better environment that can offer sustainable living on earth for human emissions due to the increasing application of natural gas as an alter­
being has motivated the global attention towards producing cleaner native to use of fossil fuels like coal and crude oil alongside the emer­
energy alternatives like decarbonised hydrogen fuel, renewables (e.g., gence since 2010 of renewable as well as nuclear energy resources [31],
wind, hydro, and solar electricity) for energy production. Recently, the hydrogen has more opportunity to reduce emissions from hard to abate
world has become aware of the quality of the air for breathing and extent sectors [30]. Hydrogen has the potential to be utilised as fuel for internal
of emission related pollution [73]. As a result, strict air pollution regu­ combustion engines in both the gaseous [83] and liquid [84] forms that
lations have been introducing gradually and attention has been shifting can effectively help in reducing the emissions of greenhouse gases
to look for alternative cleaner fuels like hydrogen (H2) for engines [73- (GHGs) [85]. It has potential to be used as a dual fuel with methane (i.e.,
75]. Hydrogen is deemed as a prospective future paradigm shift for compressed natural gas, CNG) [86] or with petroleum [83,87] as well.
renewable energy supply, which can play a most important role for low- The earlier instances of using liquid hydrogen (LH2) in the space
carbon economics alongside electricity [22]. Since it has been identified transportation field were the Apollo , the Voyager, the Skylab space
that there requires a rapid progress and implementation of action plans station, and the Viking projects [85]. Another environmentally friendly
over the next 3 decades to decarbonise the global energy supply system option is to use the hydrogen to produce electricity through fuel cells for
to achieve net-zero emissions (NZE) goals [63,76] by improving efficient vehicle operation, known as hydrogen fuel cell vehicles (HFCVs), which
energy intensity, changing behaviour of energy uses, producing highly can reduce the overall fossil fuel consumption along with ceasing the
efficient renewable energies, carbon neutral hydrogen and hydrogen- vehicular emissions [80,88,89]. The hydrogen-air mixture can be
based fuels, expansion of sustainable bioenergy, and efficient carbon ignited and may resemble a torch that is forced in one direction by the
capture and sequestration processes, this article focuses on reviewing pressure [78]. Due to significant expenditure on technology adoption,
fire hazard, storage, transportation and delivery challenges along with

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 1. Calorific values of conventional fuels [79].

lower engine efficiency (20-25%), hydrogen needs to overcome these focusing on increased renewable and net-zero energy production, and
challenges with economic technical advancement to become the leading cleaner hydrogen production. At this moment hydrogen is counted as
cleaner combustion fuel for vehicles in the next three decades [80]. omnicompetent substance that can be used as source of energy pro­
Hydrogen is not a new commodity for some industries, rather it has duction and raw materials for emission reduction in the industrial pro­
been effectively in use for more than 50 years in industries for which duction processes. Thus, with a production forecast of more than 500 M
relevant practices for design of experiments, codes, legislations, and tonnes of hydrogen by 2050 has a vast opportunity to meet the gap
specifications have been established to ensure safe use and applications between the overall energy demand and that of supplied from cleaner
[78]. Since the fuels are containing energy in its inherent structures, electrical and sustainable net-zero bioenergy cumulatively [63,100].
these can be lethal if handled improperly. As of 2020, more than 70 The total energy mix for net-zero-emissions (NZE) scenario predicts that
million metric tonnes (Mt) of hydrogen were being produced globally there will be a huge reduction of fossil fuel (i.e., Coal, Oil, and Natural
every year for industrial applications [90]. The demand for hydrogen by Gas) consumption in 2050 than that of the year 2020 [63]. As per NZE,
various industries has increased by 3 times over the years since 1975 coal, oil, and natural gas demand will reduce from 5250 Mtce (million
which has resulted in the production from fossil fuels of a global share of tonnes of coal equivalent), 88 Mb/d (million barrels per day), and 4300
about 6% of natural gas as well as 2% of coal for hydrogen production bcm (billion cubic metres) in 2020 to 600 Mtce, 24 Mb/d, and 1750 bcm
[30]. Decarbonisation of energy sectors and hard-to-abate sectors like in 2050, respectively [63]. In order to make the best use of these fossil
heating, process manufacturing as well as the transport sector (e.g., resources, the relevant techno-economic challenges should be tackled
passenger vehicles, trucks, aviation and shipping) are the key promising within the transition period of energy mixes [101] where the renewable
activities [28,29,91,92] to achieve the Kyoto protocol objective which energies will be easily available to consumers at an affordable cost.
has been set as a global strategy for stabilising climate instability by
2050 [93,94]. Also, adoption decarbonisation pathways through both 3. Global hydrogen production scenario
green electricity and green hydrogen (i.e., about 85% of total demand)
will require rapid growth of decarbonised electricity production capa­ As shown in Fig. 2a, around 88 Mt of hydrogen was produced
bility should be about 120,000 TWh by 2050, which is about 4.5 time globally in 2020 which is 23% more than that in 2015. The global
more, within next three decades at an affordable price to lower the total production of hydrogen, refineries, ammonia, and others are shown in
cost of hydrogen lower enough for mass adoption [95]. Deploying the Fig. 2a. The production of hydrogen has been increasing every year since
mix of different efficient processes developed over the years will find 1975 as presented in Fig. 2a. The PwC has projected hydrogen demand
better acceptability to meet the ever-increasing global demand of and analysed the demand until year 2050. They found that the demand
hydrogen production as predicted for checking the global warming of will grow with moderate and steady pace until 2030 and then demand
crisis. will grow stronger from 2035 and onward (Fig. 2b). They also surmised
The goal of hydrogen production for energy sector focuses on that the cost of production will also decrease by 50% during 2030 and
reducing GHG emission to net-zero level to avoid global warming crisis further reduction is predicted in 2050 [62]. Hydrogen is produced using
[65] by creating highly efficient technologies and value chain in­ different energy sources and technologies and subsequent sections
frastructures for stable and rapid globalisation [96-98], and bringing elaborate on the details of the different source. Data from different
down the levelised cost of hydrogen (LCOH) production [21,99] to an sources revealed that energy from fossil fuel dominate the production of
affordable level for the consumers for prompt adaptation. The net zero hydrogen. The availability and the reduction of electricity production
roadmap by International Energy Agency (IEA) [63] indicates that cost from renewable energy source pave the way to produce green
decarbonisation of the global economy within 2050 will need more than hydrogen.
400 milestones to accomplish a complete transformation of global en­ Future prospects of hydrogen fuel for various energy applications are
ergy system. Among these the massive reduction of fossil fuel con­ very promising in the long-term [102]. Currently about 10 countries
sumption without CCUS integration in every energy consuming sectors, (United States of America, United Kingdom , Australia, Canada, France,

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 2a. Global hydrogen production data between 1975 and 2020 [30].

Fig. 2b. Yearly Hydrogen demand to reduce global warming temperature rise [62]

Germany, China, Japan, Norway, and South Korea) around the globe are 2020 and 2050 into four categories, namely (a) Intermediate next steps
moving ahead with establishment of their green hydrogen economy (2020-2022), (b) Early scale-up (2023-2025), (c) Diversification (2026-
[103]. A recent hydrogen roadmap report in the USA [104] has pro­ 2030), and (d) Broad rollout (2031-2050). The hydrogen strategy pub­
jected that the demand for hydrogen fuel across sectors can rise from 1% lished by the Office of Fossil Energy (FE) of the United States Depart­
to 14% of the total final energy share between 2030 and 2050. The ment of Energy (USDOE) has stated that the transition to the lower or
roadmap [104] has divided the technology enabling period between near zero carbon emissions based energy economy will be possible with

Fig. 3. Integration of hydrogen production streams and applications across sectors for US economic growth, redrawn from ref [105].

6
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

the integration of efficient strategies, research and development (R&D) water, and biomass. The hydrogen is used mainly in transport and power
projects, and technology deployment [105]. Fig. 3 shows how the generation, the building and manufacturing industries, and as feedstock
hydrogen strategy can bring benefits by integrating widespread actions in creating chemicals and other products. Hydrogen can be stored as gas,
over existing fossil fuel-based energy applications. Based on the sources, liquid, ammonia, and a liquid organic hydrogen carrier (LOHC). The
production processes, emissions capture or reduction and the way the transmission and distribution methods are pipelines, trucks, and ships
hydrogen economy can offer benefits to the clean energy and environ­ based on the state of storage as presented in Fig. 5.
mental value chains, various colours are used to name the hydrogen Fig. 6 depicts various types of hydrogen production processes. The
[102,106-108]. major conversion processes are thermochemical, photoconversion,
Fig. 4 briefly features the most frequently categorised and termed electrolysis, and biological processes which are already used technolo­
hydrogen colours collected from various publications, which shows that gies for hydrogen production purposes. Though not new, these tech­
the processes which do not have an integrated carbon emissions capture nologies need further investigation to optimise their production
facility need to be integrated to establish the clean hydrogen economy scalability to use hydrogen as a fuel. So, while applying these existing
value chain. technologies, the key challenges are to overcome the cost burden,
emission capture issues, and to enhance the process efficiency to make
4. Feedstocks and production processes of hydrogen the adopted technologies more competitive in terms of the current fuel/
energy supply systems. Different hydrogen production processes are
All the fuels and energy sources are used directly or indirectly as elaborated in the subsequent sections and sub-sections.
feedstock for producing hydrogen in their respective efficient method­
ologies [111-114]. For instance, the direct use of feedstocks occurs in
4.1. Reforming processes
various thermochemical processes where hydrocarbons (i.e., coal, con­
ventional and unconventional [115] natural gases, and oils), biogas,
In chemical process industries, the reforming process has long been
biomass, biofuels, and waste polymers are used as feedstock to produce
used to crack the low-quality hydrocarbons and convert them into high-
hydrogen gas. In contrast, the indirect process is the usage of renewable
quality hydrocarbons by rearranging the hydrocarbon chain structures.
energies (e.g., hydro energy, solar energy, and wind energy) and non-
Usually, the low-octane hydrocarbons are converted into the higher-
renewable electricity (e.g., coal, gas, oil or nuclear powered power­
octane number gasolines in the refineries so as to improve the com­
plants) to crack or electrolyse water into hydrogen gas. The electrolysis
bustion quality of the fuel [122]. Thermal reforming requires high
process does not produce any CO2 while cracking the H2O molecule, but
temperatures and pressures to reform the chemical structures at the
the other thermochemical processes need additional processes with
desired level, whereas the catalytic reforming process uses metallic
which to be integrated to remove the CO2 so as to validly claim to be a
catalysts like platinum (Pt) at a reduced energy level than that of the
completely clean application of hydrogen as an energy carrier or in­
thermal reforming process [122,123]. Due to attention towards climate
dustrial raw material [116,117]. Based on the types of production pro­
protection and greenhouse gas reduction targets with the cleaner energy
cesses and feedstocks used to produce hydrogen, there are varying
resources, the reforming process has received much attention to produce
amounts of CO2 emissions, investment and infrastructure requirements
hydrogen from cracking the lighter hydrocarbons like natural gas,
[118]. Fig. 5 shows the feedstocks, production processes, distribution
gaseous hydrocarbons, methanol, ethanol, naphtha, coal, and liquid
methods, and applications of hydrogen. From the figure it can be stated
hydrocarbons which can be used as feedstocks [112,123-125]. The
that the key feedstocks for hydrogen generation are natural gas, coal,
following sections explain the different types of reforming processes for

Fig. 4. Different types of hydrogen based on energy source [102,106-110].

7
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 5. Feedstocks, production processes, distribution, and applications of hydrogen, redrawn from ref [119].

Fig. 6. Types of hydrogen production processes [114,120,121].

production of hydrogen that efficiently utilise different types of 4.1.1. Steam reforming (SR)
feedstocks. In the steam reforming (SR) process, steam is used at high temper­
atures and pressures to crack the organic compounds, which are used as

8
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

feedstocks, in presence or absence of catalysts to yield hydrogen (H2)

Cx Hy Oz + (x − z)H2 O(+heat)→xCO + (x + y/2 − z)H2 (1)
enriched syngas (i.e., mixture of carbon monoxide and hydrogen [126])
[127]. The SR reactors can be operated at a steady-state condition for SR of non-oxygenated hydrocarbon compounds:
longer periods with highly efficient (>70% dry basis) amounts of
hydrogen production [128]. Multi-stage steam reforming in the pres­ Cx Hy + (x − z)H2 O(+heat)→xCO + (x + y/2)H2 (2)
ence of optimal temperatures, pressures and a suitable catalyst can in­ Step 2: Converting carbon monoxide (CO) into carbon dioxide (CO2)
crease the yield efficiency [129,130]. Nahar et al. [130] reported 94% by the WGS process.
yield efficiency of biodiesel steam reforming at 650◦ C, 10 wt.% Ni/Ce-Zr
catalyst, feedstock preheating up to 190◦ C, molar ratio of steam-and CO + H2 O(− heat)→CO2 + H2 (3)
carbon (S/C) of 3 and an operating period of 100 hours for achieving In the case of converting methane (CH4) gas by the SR process [139],
steady state conditions. On the other hand, the SR process of methanol the reaction steps are presented in Eqns. 4-5.
(CH3OH) can occur between 100-300◦ C [131,132]. A typical steam
reforming reaction process of natural gas (i.e., methane) can occur be­ kJ
SteamReforming(SR) : CH4 + H2 O + 206.1( )→CO + 3H2 (4)
tween 3-25 bar pressure and 700-1000◦ C in the presence of a suitable mol
catalyst (predominantly Ni-based) [82]. While using natural gas as kJ
feedstock, it is crucial to eliminate sulphur (S) found in the natural gas Water − gasshift(WGS) : CO + H2 O − 41.2( )→CO2 + H2 (5)
mol
stream to avoid loss of catalytic activities during the SR process [133]. In
the non-catalytic SR process, the higher energy input requirement leads Steam reforming (SR) of ethanol (C2H2OH) is presented in Eqns. 6-7
to high temperature reactors. For instance, CH4 cracks into various [140].
radicals (e.g., C2H4, C2H2, and C) at 1000◦ C, which undergo further kJ
cracking at over 1500◦ C to produce H2 gas [134]. To determine the SteamReforming(SR) : C2 H5 OH + H2 O + 255.54( )→2CO + 4H2 (6)
mol
optimal design pathway with a feedstock and various variables, multi-
criteria decision analysis (MCDA) with analytic hierarchy process Water − gasshift(WGS) : CO + H2 O − 41.2(
kJ
)→CO2 + H2 (7)
(AHP) can make good help. For instance, Janošovský et al. [135] con­ mol
ducted analysis with natural gas and biogas as feedstock options for The reforming process of ethanol can happen in two stages, i.e.,
steam reforming on the basis of criteria like economics, material and ethanol decomposed into methane that is reformed further. While the
energy utilisation, inherent safety and environmental effect. The reforming reaction of ethanol occurs, higher rate of hydrogen produc­
decision-making hierarchical analysis (about 9027 scenarios) shows that tion can be disturbed by a large number of auxiliary reactions if the
natural gas turns into a good selection when economics, material and reactions are not guided with the help of efficient catalysts as per the
energy utilisation are dominant criteria, and the biogas turns into a objective of the reactions [141]. Liu et at. [141] demonstrated that 15Ni-
favourable feedstock when inherent safety is dominant. Hence, the HCa (15 wt.% Ni catalyst with hydrocalumite) catalyst can convert
further sub-criteria need to be carefully selected to avoid any biased about 99 wt.% of total ethanol into H2 (86 wt.%) and CO (7 wt.%) at 650
error in the analyses. ◦
C. On the other hand, 6% NiO/NaF catalyst can demonstrate about 94%
The reactions in the water-gas shift (WGS) method are performed methanol conversion into 100% H2 under 450 ◦ C [142]. NiO/NaF has
following the steam reforming to use the CO portion of the syngas to very high-level recyclability and low-cost consumption is observed to
dissociate water (H2O) molecules and maximise the overall hydrogen contain the catalyst. Also, the bio-oil can be converted into 1.49 Nm3/kg
gas production [136]. CO is reduced into CO2 while producing H2 in the of hydrogen with CaO catalyst (at 650 ◦ C, S/C 2, and Ca/C = 1) [143].
WGS process. For efficient WGS process output, the syngas (H2/CO) The SR reaction accompanied with WGS contributes about 48% and
content should have a high hydrogen to carbon monoxide (CO) ratio 30% of the total global quantity of hydrogen production by using natural
(>3), whereas the carbon dioxide (CO2) from the syngas can be removed gas and petroleum oils, respectively [81,129,144]. Also, the coal gasi­
along with the CO2 produced in the WGS process [81,136]. Indeed, the fication process and electrolysis processes are used to produce 18% and
famous Haber-Bosch process uses this WGS process to produce hydrogen 3.9% of the total hydrogen in addition to 0.1% contribution from other
since 1913 [137]. Such a combination of two different reaction pro­ production processes [81,129]. Natural gas is utilised to generate more
cesses (i.e., SR and WGS) has increased the interest on hydrogen pro­ than 95% of the total hydrogen in the USA, which makes the production
duction from various hydrocarbon compounds for the purpose of cost greatly reliant upon the gas price [82]. Similarly, countries which
producing carbon free energy. One of the key challenges of these pro­ are reliant upon coal or hydrocarbons to generate hydrogen for indus­
cesses is the emission of CO2, which is a harmful greenhouse gas trial requirements need to consider the price of the feedstock while
component but a major adopted process of producing hydrogen [81]. producing hydrogen. Typical temperature and pressure requirements for
Steam reforming is an extremely endothermic reaction, however the the SR process range between 450◦ C and 925◦ C, and 290 psig and 500
WGS process is exothermic [127]. Hence, the SR process is conducted in psig approximately (i.e., 20 to 35 bar) in the presence of suitable cata­
an adiabatic reactor to avoid high temperature effects on catalysts of the lysts [26,145,146]. SR in the presence of catalysts, known as catalytic
WGS reactors [137]. WGS reactions are conducted in two stages in the steam reforming (CSR), can increase the process efficiency [147]. It is
typical industrial process, namely low temperature (LT)–CO and high essential to remove the unconverted substances from the reactor
temperature (HT)–CO shift reactions. These multistage reaction pro­ because the presence or accumulation of carbon or ashes in the reactor
cesses, in the presence of Fe-based or Cu-based catalysts, can convert can reduce the activity capacity of catalysts [147]. Part of the reformate
more than 99.5% CO into CO2, thus increasing the concentration of H2 in is combusted (without forming oxides of nitrogen (NOx) [77,116]) to
the final product line [137]. To maximise the hydrogen production, provide energy in the first step of reaction, and the heat produced in the
most industries adopt both the HT-CO and LT-CO shift reactions [138]. second step is diverted partly to the process. As a result, thermal effi­
Due to the exothermic nature the HT-CO shift reaction, which generally ciency of steam reformers can reach up to 85%-90%. The case of a non-
occurs above 350◦ C, it is executed very rapidly but is unable to convert catalytic SR process requires more than 1000◦ C and it can be uneco­
all the CO. The rest of the CO (usually <10%) is then converted into CO2 nomical because methane breaks down at above 1500◦ C [134]. Bio-oils
by the LT-CO synthesis at about 180-250◦ C [137,138]. The generalised derived from thermochemical processes contain an aqueous portion for
steam reforming reaction of oxygenated hydrocarbons and non- which the CSR process is effective to increase production of H2 rich gas
oxygenated hydrocarbon compounds [130] are presented in Eqns. 1-3. from the renewable resources like biomass [147,148]. Bio-oil has mainly
Step 1: SR of oxygenated hydrocarbon compounds: two fractions, water-soluble (e.g., light weight organics) and insoluble

9
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

(e.g., lignin-based organics) [148]. Both of these fractions can be sepa­ cm2, catalyst stability period 475 hours) helps optimising the CO
rated through water addition [149] or molecular distillation [150] adsorption in the anode end and about 99.99% pure H2 is obtained
processes. Natural gas, methane, and biogas have been widely used in [152]. That tiny amount of impurity (i.e., 0.01% or 27ppm) is caused by
the CSR process economically, where natural gas has offered better the CO released in the cathode end due to microleakage of the anion
economic outcomes in terms of capital expenditure (CAPEX) and oper­ exchange membrane (AEM).
ating expenditure (OPEX) issues [147]. The biological way of WGS reaction occurs in the presence of en­
In general, production of 1kg H2 from natural gas produces about zymes like carbon monoxide dehydrogenase (CODH) and hydrogenase
8.33 kg of CO2 in the SR-WGS process [144]. Removal of CO and CO2 are [138]. These enzymes can contain metallic ions or cofactors like NiFe,
essential for purified hydrogen production from the reforming process. which forms complex substances such as NiFe-CODH and NiFe-
Following the WGS reaction, the methanation, CO2 absorption in amine hydrogenase [138,155,156]. Microorganisms like hydrogenogenic bac­
solutions, pressure swing adsorption (PSA), membrane separation (MS), teria, methanogenic archaea, acetogenic bacteria, and sulphate-reducer
and cryogenic distillation (CD) are the most common chemical and bacteria can ferment the syngas to produce alcohol, methane, hydrogen,
scrubbing techniques which are adopted to clean up H2 from the mixture or biofuels [138,157-159]. That is why these are also recognised as
with CO and CO2 [128,137]. Among the wide varieties of membranes for biocatalysts. In the case of the focus to produce hydrogen from the
separating H2, the palladium (Pd) and Pd-alloy based metallic mem­ syngas, the carboxydotrophic hydrogenogenic bacteria can conduct the
branes have been found as extremely selective to H2 gas [151]. Though WGS reaction of CO with H2O to generate H2 and CO2 gases [138,158].
H2 itself does not emit any GHG pollutants due to combustion for energy In some cases, biocatalysts offer better effectiveness (i.e., lesser energy
production, its production process needs efficient removal of CO2 during consumption, cessation of catalyst poisoning) and advantages (i.e., no
the production processes. Without efficient capture and sequestration of influence of H2:CO in the syngas, reactions can occur at lower temper­
CO2 from the WGS reactors, the production process will not be defined atures) than that of organic and non-organic chemical catalysts [158]. In
as a clean fuel production technology. a conventional WGS reaction of natural gas, the feedstock is first
As a key step to the H2 production, the energy consumption and reformed into CO, H2 and CO2 gases. Then the catalytic WGS reactions
pressure input (1-6 MPa) in the multiple stages (HT-CO and LT-CO shift occur in low temperature shift (LTS) and high temperature shift (HTS)
reactions) of the WGS process still faces a risk of incomplete conversion reactors to get higher conversion percentages of H2 gas from the refor­
of CO into CO2. A trace quantity of CO in the H2 stream in the fuel cell mate gas (more than 90%) mixtures [153]. The mixture of H2 and other
can damage the efficiency very badly [152]. Besides, the economic gains gases are passed through a PSA process to collect almost 99.9% pure H2
due to the transition from carbonaceous feedstocks to H2 feedstock for gas [153]. On the other hand, the BWGS reaction process is conducted
multipurpose applications require that the relevant production pro­ by transferring electrons from the CO to H2O, which dissociates the H+
cesses be economic and efficient. Therefore, an alternative, or a modi­ from H2O and converts into H2 gas as shown in Eqns. 11-13 [153].
fied WGS process to produce highly purified H2 at reduced expenses has
Electron transfer from CO : CO + H2 O→CO2 + 2e− + 2H + ; (CODH) (11)
been a necessity. Electrochemical water–gas shift (EWGS) process (i.e.,
25◦ C, atmospheric pressure) [152] and bio-water-gas shift (BWGS)
Hydrogen gas production : 2H + + 2e− →H2 ; (hydrogenase) (12)
process (i.e., in the presence of enzymes, anaerobic, 25◦ C, atmospheric
pressure) [153] have been under investigation, which are potentially Complete BWGS reaction : CO + H2 O→CO2 + H2 (13)
offering very high purity of H2 production along with economic gain
from the hydrogen value chain. In the EWGS process [152], the oxida­ About 4.46 kcal/mol of energy is produced when the conversion
tion of CO occurs at the anode to produce CO2.This CO2 is further oxi­ process of CO into CO2 occurs in an anaerobic environment, whereas the
dised with hydroxide (OH-) of KOH to produce CO2- 3 . At the cathode end, aerobic transformation of CO into CO2 occurs in the presence of oxygen
H2O is reduced to H2 and OH-. The K+ ion from the cathode thus reacts and the reaction produces about 61.1 kcal/mol of CO conversion [153].
with CO2- 3 to produce K2CO3. The EWGS reactions may occur like the Economic sensitivity analysis is highly recommended to study the
high temperature polymer electrolyte fuel cell (HT-PEMFC) [154] and benefit of employing the right water-gas-shift reaction process [160-
an electrochemical pumping is created that helps producing pure 162]. The schematic comparison of WS, EWGS and BWGS is presented in
hydrogen [154]. The Total EWGS reaction can be presented as shown in Fig. 7.
Eqns. 8-10. The specification and different types of catalysts used for the SR
processes are presented in Table 2.
Anode : CO + 4OH − →CO2−3 + 2H2 O + 2e− (8) The temperature and a sufficient supply of steam in comparison to
feedstock (e.g., H2O/C2H2OH ratio) in the SR process play a significant
Cathode : 2H2 O + 2e− →H2 + 2OH − (9) role to optimise the hydrogen production. Though the temperature of
the SR process can vary from 400-2000 K, the H2O/C2H2OH ratio (be­
TotalEWGSreaction : CO + 2OH − →H2 + CO2−3 (10)
tween 3 and 6) yields optimum production of hydrogen at 900 K and 1
Advantages [152] of the EWGS process have been reported as: (i) atm pressure [164]. Change of reaction pressure condition does not have
removal of CO2 in the form of K2CO3, thus producing cleaner H2 without significant effect when the target is to produce hydrogen from complete
necessity of CO2 purification stage; and (ii) application of K2CO3 as in­ breakdown of the feedstock [164]. If the reaction conditions are not set
dustrial feedstock in the soap and glass industries as a circular economic well, ethanol can be thermally decomposed into various other organic
contribution. On the other hand [152], oxidation of CO from the SR compounds like ethylene (C2H4), acetone (CH3COCH3), methane (CH4),
process can be affected due to its lower solubility to water phase com­ and black carbon (C) at various stages while yielding H2 in an uneco­
pounds in the anode end. This can be eliminated by increasing con­ nomic way. Fig. 8 shows pathways for thermal decomposition options of
centration of CO in the anode end so that the anode catalysts can help C2H2OH in the presence of platinum (Pt) catalyst at lower temperatures
with improving the oxidation process. The increased oxidation of CO in (around 300◦ C and 1 atm), which has been explained by Sutton et al.
the electrochemical process increases the yield of highly purified H2 [165]. Inefficient catalyst selection and dosage rate lead mainly to for­
[154]. However, the catalyst quantity needs to be optimised to enhance mation of carbon in both SR and WGS reactions [164]. Fig. 9(a) shows
the process efficiency. To enhance the CO oxidation method, the anode the influence of the H2O/C2H2OH ratio and reaction temperature on
end’s catalyst should have weaker interaction with it. Among various hydrogen production efficiency from C2H2OH, whereas the loading ef­
metallic compounds, Pt3Cu has demonstrated efficient activities for fect of catalyst in the case of the formation of coke in the SR process is
increasing the CO oxidation and reduce overpotential of the oxidation presented in Fig. 9(b) [164].
process by 0.1V. The Pt2.7Cu@CNT catalyst (current density 70 mA/ Use of effective catalysts is required to conduct efficient SR reaction

10
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 7. Schematic comparison between (a) WGS, EWGS and (b) BWGS [138,152].

of the feedstocks used for reforming. The key objectives of using cata­ react to generate synthesis gas (CO and H2) between 700◦ C and 900 ◦ C
lysts in the SR reactions are assuring optimal H2/CO ratio and the ratio [127], but the reverse water gas shift (RWGS) reactions are favoured
of steam and carbon (S/C) [139,140,166], reducing process energy beyond 800 ◦ C [178]. This is one of the sustainably promising thermos-
consumption [129,130,134,167], and increasing process efficiency catalytical processes as it is very favourable for both industrial and
[167,168]. While the catalyst selectiveness, thermal stability, impact of environmental effects. Key challenges have been reported as the deac­
pressure in the reaction chamber, reusability, conversion efficiency, tivation of catalytic performances, and lower H2/CO ratio (<1) [178].
turnover frequency, and avoidance of reduction of catalytic effective­ The DR process can be considered as stoichiometrically similar to that of
ness because of formation of coke inside the reactor are the key chal­ the biogas production process. The biogas reaction process comprises a
lenges to optimise the reaction process for feedstocks like natural gas, ratio of 1:1–1.5 of CO2 and CH4, which is near to the stoichiometry of
fossil fuel, and alcohol [166,169-172]. The coke formation and catalytic this chemical reaction [127]. Dry reforming of methane (DRM) has been
efficiency loss can be eliminated significantly with an optimal S/C ratio always followed by another four side reactions including the Boudouard
[166]. While qualities (i.e., chemical, thermal, and physical) are reaction, reverse water–gas shift (RWGS) reaction, methane cracking, or
essential, the cost involvement also influences selecting and continuous CO reduction.
development of various types of catalysts [168]. Noble transition ma­ The dry reforming (DR), Boudouard, and RWGS reactions are shown
terials and non-precious materials are actively used as catalysts, where in the Eqns. 14-a, 14-b and 14-c, respectively.
the precious materials are more expensive than the non-precious ones
DRreaction : CH 4 + co2 ↔ 2H2 + 2CO; (ΔH = 247kJ/mol) (14-a)
[166,173-175]. Nickel (Ni), iron (Fe), cobalt (Co), copper (Cu), man­
ganese (Mn), chromium (Cr), silver (Ag), titanium (Ti), gold (Au), mo­
Boudouard reaction : 2CO ↔ CO2 + C(s) ; (ΔH = − 172 kJ/mol) (14-b)
lybdenum (Mo), palladium (Pd), platinum (Pt), tungsten (W), rhenium
(Re) , osmium (Os),iridium (Ir), rhodium (Rh), and ruthenium (Ru) are RWGS reaction : CO2 + H2 ↔ H2 O + CO; (ΔH = 41 kJ/mol) (14-c)
the renowned active noble and non-noble active materials which are
used as the catalyst or component for composite or multi-metallic cat­ The total efficiency of the method is affected by the reduction of
alysts to conduct reaction processes efficiently [166,173,176,177]. Use efficiency of the catalysts. Therefore, highly sustainable and active
of an appropriate catalyst support or promotor (directly or indirectly) catalyst is required to perform the desired reaction activities of the
demonstrates significant improvement in the reactions, i.e., reduction of process [127,179]. Table 3 presents the detailed operating specifications
coke and by-products formation, increased selectivity of the reactants to of various catalysts for DR of Methane and CO2 to produce CH4.
produce higher yield of hydrogen and stability of the catalysts [127,179]. Higher catalytic activities have been reported for noble
[166,177]. So, preparing and activating the catalysts with appropriate metallic catalysts (i.e., Ru, Pt, Pd)) for the DR process [178]. Ballarini
catalyst supporter for reforming of certain feedstocks is one of the key et al. [180] used several materials like K-L Zeolite, K-Al2O3, K-Mg/Al
steps before the reforming process begins. oxides, and MgO along with Pt-based catalyst as support for dry
reforming of CH4, for which there were higher stability in the reaction
4.1.2. Drying reforming (DR) activities to produce hydrogen from methane. MgO/Pt demonstrated
In the DR process, there are two reactants (i.e., CO2 and CH4) which higher activity with maximum yield of H2/CO as 0.73. On the other

11
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Table 2
Catalysts used in SR reaction processes conducted in membrane reactors [163].
Process Catalyst and catalytic properties Reaction conditions Type of membrane reactor

MSR Cu/ZnO/Al2O3 (Commercial), MDC-3, G66B (CuO 31~38%, T = 350 C, P = 6~15 atm, H2O/CH3OH = 1.2,
◦
Double-jacketed supported Pd
ZnO 41~60%, Al2O3 9~21%) WHSV = 1 hr− 1
Cu/ZnO/Al2O3 (Commercial) modified with ZrO2 (4.5~25.5 g, T = 250~300 ◦ C, P = 1~5 bar, H2O/CH3OH = 1:1 Packed-bed reactor (single-fibre)
and 0.5 mm particle diameter) with Pd–Ag/Al2O3
Cu-based - Cu/Zn/Mg (commercial) T = 300◦ C, P = 1.5~3.5 bar, H2O/ CH3OH = 3:1 , Dense Pd-Ag*
WHSV = 0.36~1.82 hr− 1
CuO (64%) + Al2O3 (10%) + ZnO (24%) + MgO (2%) T = 350 ◦ C, P = 1.3 bar, H2O/CH3OH = 6:1, l/ Dense Pd-Ag*
l/cat = 3g
LaNi0.95Co0.05O3/Al2O3 (co-precipitation method, Alumina as T = 400 ◦ C, P = 0.05 bar Pd-Ag CMR
binder)
CuO/ZnO/Al2O3 (Commercial) T = 310 ◦ C Pd
Ru (5 wt.%)-Al2O3 (Commercial, 3g)Wcat = 3g, 5 wt% Ru T = 400 ◦ C, P = 1.3 bar Pd-Ag supported and dense
CuO/ZnO/Al2O3 (Commercial) (G66MR, 250–355 μm particle T = 200 ◦ C, P= 1 bar Carbon molecular sieve
size)
ZnO/CuO/ Al2O3 (Commercial) T = 200~260 ◦ C, P = 1.2 bar, H2O /CH3OH = 1.3 Dense Pd-Ag20%*
ESR Ni (25 wt.%)/ZrO2 and Co (15 wt.%)/Al2O3 (Commercial) T = 400 ◦ C, P = 8–12 bar, GHSV = 800–3200 h− 1 Pd/PSS composite
Ru (5 wt.%)/Al2O3 (Commercial, 4~7g, size 1~2 mm) T = 400~450 ◦ C, P= 1.5~2.0 bar, H2O: Dense Pd–Ag*
C2H5OH = 8.4~13.0
Pt (0.5 wt.%)/Al2O3 (Commercial, 6.3g, 2 mm size).NiO (25 wt. T = 400~450 ◦ C, P = 1.5~2.0 bar, H2O: Dense Pd–Ag*
%)/SiO2, (Commercial, 6.0 g, 2 mm size) C2H5OH = 8.4–13.0
Rh/SiO2 and Pt/TiO2, Degussa P25 for WGS T = 300–600 ◦ C Composite Pt/PSS
MDC-3: Zn–Cu/Al2O3 (Commercial) T = 320~450 ◦ C, P = 3~10 atm Pd–Ag/PSS composite
1
Ru (0.5 wt.%)/Al2O3 (Commercial, 3 g) T = 400 ◦ C, H2O:C2H5OH = 11:1, GHSV = 2000 h− Dense Pd–Ag*
Pt/Al2O3 (7.2 g, 2~3 mm)Wcat = T = 400 ◦ C and 450◦ C, P = 150~200 kPa, H2O: Dense Pd–Ag*
C2H5OH = 4, 10 and 13
Ru (5 wt.%)/Al2O3, (Commercial. 3g) T = 400 ◦ C, P = 1.3 bar Dense Pd–Ag*
CO (15 wt.%)/Al2O3 (Commercial. 3g) T = 400 ◦ C, P = 3~8 bar Pd/PSS composite
Co (15 wt.%) + La (3.1 wt.%), Rh (4 wt.%), (Commercial. 3g, T = 400 ◦ C, P= 3–8 bar Dense Pd–Ag
143m2 g− 1, 150~250 mm)
Na (0.2~2 wt.%)–Co (12.5 wt.%,)/ZnO, (0.45 g, 0.6~0.85 mm) T = 600~750 ◦ C, P= 7~70 atm, S/C = 3~12, Silica–alumina composite MR
GHSV = 8500~83000 h− 1
Acetic acid SR Ni (4g)and Ru (2g of 5 wt.%), (Commercial) T = 400~450 ◦ C, P = 1.5~2.5 bar Dense Pd–Ag*
Commercial Ni T = 400 ◦ C, P= 1.5~4.0 bar, H2O/CH3COOH = 10:1 Pd–Ag dense*
Glycerol SR Ru (0.5 wt.%)/Al2O3 (Commercial, 3g) T = 400 ◦ C, P= 1.5~4.0 bar, H2O/C3H8O3 = 6:1 Pd–Ag dense*
WHSV = 0.1~1 h− 1
Naphtha Pt (0.3 wt.%) + Re (0.3 wt.%) T = 505 ◦ C, P = 34~37 bar Fluidised bed Pd–Ag23%
reforming

* SSP = self-supported palladium membrane; T = temperature; SBET = BET specific surface area; P= pressure; WHSV = weight-hourly-space-velocity; GHSV = gas-
hourly-space-velocity.

Fig. 8. C2H2OH decomposition pathways (dehydrogenation/hydrogenation reactions) in presence of Pt catalyst [165].

hand, Xie et al. [181] reported that the bimetallic catalyst combination Hence, the DR process has greater prospect to use either natural gas or
of PtCo/CeO2 demonstrates better effectiveness to produce hydrogen biogas which have more methane content to crack into hydrogen. For
and the catalyst itself shows higher resistance to the formation of coke, instance, Hajizadeh et al. [182] reported that at a rate of 48.07 kg/h
thus eliminates the risk of deactivation of catalysts. Also, higher tem­ biogas input to the DR reactor may produce about 8.11 kgH2/h in
perature and pressure (i.e., 850 ◦ C, 1 MPa) in the DR process with presence of Co-Ni-Al2O3 catalyst. This process was reported to be less
hydrotalcite-derived 100Ni20Ir catalyst has higher methane conversion expensive counting about US$1.39/ kgH2.
rate (68.5%) and the catalyst exhibited strong carbon tolerance against
the usual catalyst deactivation due to carbon deposition on the catalyst.

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

total hydrogen production globally. The generic POX reaction of hy­

drocarbon fuels is presented in Eqn.16-a.
(m)
Generic hydrocarbon fuel : Cn Hm + (n/2)O2 →nCO + H2 + heat
2
(16-a)
In comparison to that of the catalytic reforming processes, the POX
process does not require any catalyst to conduct the reforming process.
As a result, this reaction process does not experience the loss of catalytic
effectiveness due to coke formation and deposition on catalysts Though
catalysts are absent, the key challenging issue of the POX reaction pro­
cess is the requirement of high temperatures which is more costly than
other reactors used. Also, the higher temperature leads to generation of
soot and the lower H2/CO ratio (1:1 to 2:1) for lesser H2 production from
the same amount of feedstock used for the other reforming processes like
SR and AR processes, yet the DR process still requires the WGS process to
increase the quantity of H2 production. Besides, the WGS reaction is
conducted to convert the hazardous CO gas into CO2. Generally, the POX
process is effective for reforming the heavier hydrocarbons like petrol,
diesel, and heavy oils at temperature ranging between 1150 ◦ C and 1315
◦
C in presence of 600 kPa reactor pressure for efficient reforming to
hydrogen [185]. Table 4 shows a comparative study of reforming of
methane, methanol, diesel/gasoline, and coal via SR and POX processes
and how efficient the chemical processing is. Certainly, the produced
hydrogen quantity in the SR method is considerably more than that of
the POX method for all these feedstocks [77].
POX reaction for methanol and ethanol can be expresses as in the
Eqns. 16-b and 16-c, respectively [186].
kJ
POXofMethanol : CH3 OH +1/2O2 →2H2 +CO2 +(ΔH ≈ 192.2 mol )
(16-b)
kJ
POXofEthanol : C2 H5 OH +3/2O2 →3H2 +2CO2 +(ΔH ≈ 620.3 mol )
(16-c)
[186] Use of catalysts to conduct the POX reactions of methanol and
ethanol can improve the temperature management ability and improved
hydrogen production rate. Agrell et al. [187] conducted POX reaction of
methanol in presence of Cu40Zn60 catalyst at around 185-215 ◦ C for
O2/CH3OH =0.1, for which the hydrogen production improved signifi­
cantly at lower temperature [188]. Yang et al. [189] conducted meth­
anol partial oxidation reforming in presence of Au-CuO-ZnO (w/w: 3%
Au, 37%Cu, and 60%Zn) catalyst at 523K that produced 97.7% selec­
Fig. 9. Effect of reaction temperature (T) and H2O/C2H2OH ratio on: (a) tivity type hydrogen from the process. Whereas, in case of catalytic
hydrogen production; and (b) coke (Carbon) formation SR reaction process of partial oxidation of both methanol and ethanol, Hohn and Lin [186]
C2H2OH at equilibrium condition [164]. have discussed all the possible and complex reactions pathways that
may take place. In presence of h-BN-Pt.Al2O3 catalyst, the partial
4.1.3. Partial oxidation (POX) oxidation of methanol can be started at room temperature, which has
Steam reforming (SR) of natural gas, biofuels or various hydrocarbon been termed as cold start by Chen et al. [190]. The authors reported that
fuels requires heat energy input to conduct the reaction and produces hydrogen was generated at a rate of 1.3mol/mol of methanol for O2/
CO2. On the contrary, a partial oxidation (POX) reaction produces heat C=0.7, and S/C=1.5. Still there is no highly recognised catalyst and
and lesser amounts of CO2. While comparing with the steam reforming optimised process parameters reported for the POX reactions to produce
process, 1 mole of methane gas (as natural gas) may produce about 25% hydrogen though addition of catalysts in the POX process has been
less hydrogen by the partial oxidation process. Partial oxidation re­ capable of controlling the temperature changes during reactions to
formers react the fuel with a sub-stoichiometric quantity of oxygen produce hydrogen.
[128] as shown in Eqns. 15-a and 15-b.
4.1.4. Autothermal reforming (ATR)/ Oxidative steam reforming (OSR)
Step 1 : CH4 + 1/2O2 →CO + 2H2 (+ΔH) (15-a) Autothermal steam reforming (ATR) is a combination of both SR
(endothermic) and POX (exothermic) reactions through combined input
Step2 : WGSreaction : CO + H2 O→CO2 + H2 (+smallamountofheat) (15-b)
of fuel, steam, and air together into the reactor [128], which can be used
The reaction process differs from the steam reforming process with to efficiently reform the bio-oil to produce H2 [191]. Due to generation
the thermodynamic behaviour, i.e., the POX process is exothermic with of heat in the POX method, reforming methods do not require external
H2/CO=2, whereas the steam reforming process is highly endothermic heating sources in this reaction process, but it is considered as one of the
(e.g., 206 kJ/mol for steam reforming of 1 mol methane gas) [183]. most efficient processes of its kind. Nevertheless, both POX and ATR
Following this, pure H2 is separated usually using a Pressure Swing processes are expensive, and these processes require an intricate oxygen
Adsorption (PSA) technology [184]. About 6% of the total worldwide gas separation process from the atmosphere for further reactions. A WGS
production of natural gas is accounted for as dedicated raw material for reaction process is conducted to convert the CO and boost the hydrogen
production of hydrogen gas. That accounts about three-fourths of the generation policies [184]. Oni et al. [192] have reported that the ATR
process along with CCUS process can offer production of blue hydrogen

13
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Table 3
Latest studies on dry reforming of methane [127,179].
Input CH4/CO2 Metals Temperature (◦ C) Output H2/CO CH4Conversion (%) CO2Conversion (%) Carbon Formation mg/g-Catalyst

0.5 Ce–Gd–O 800 1.07 50 88 Nr

0.8 Rh–Al 700 1 42 nr Nr
1 NiCo/CeZrO2 800 0.84 79 84 0.24%–8.2%
1 WC 900 0.96 95 95 None
1 Ni–Al 700 0.67 19 31 Nr
1 Ni–Pb–Al 700 0.88 60 78 Nr
1 Ni–Pb–1P–Al 700 0.77 55 71 Nr
1 Ni 700 1 54 66 41
1 Co 700 1 75 67–80 20–268
1 Ni–Co 700 1 56–71 83 290
1 Pt–Ru 700 <0.5 90 48 Nr
1 La–NiMgAlO 700 0.8 80 85 Nr
1 Ni 750 nr 32 36 3.6% of inlet C
1 Ce–Gd–O 800 0.96 68 72 Nr
1 Pt–Al 900 nr nr nr 22% of inlet C
1 Ni–La–Al 950 nr 99 90 Nr
1 NiO–MgO 700 0.87 67 77 Nr
1.5 Ni–Al 750 0.9 49 81 Nr
1.5 Ni–Mg–Al 750 0.86 59 70 Nr
1.5 Ni–La–Mg–Al 750 0.95 61 70 Nr
1.5 Rh–Ni–Mg–Al 750 1 58 85 Nr
1.5 Rh–Ni–La–Mg–Al 750 1.06 50 94 Nr
1.5 Rh–Ni 800 1 65 100 Nr
1.5 Ni–Al 850 0.55 72 96 180
1.5 Ni–Ce–Al 850 0.65 73 97 170
2 Ce–Gd–O 800 0.84 66 46 Nr
2.1 Ni 750 nr 21 29 3.6% of inlet C

from PET (49.4%). Addition of HDPE increased the overall hydrogen

Table 4
production from the plastic/biomass mixed streams. As a result, if these
Steam reforming and POX processes with methane, methanol, gasoline, and coal
processes are integrated with efficient CCUS process, the levelised cost
[77].
of hydrogen production can be effectively controlled while resolving the
Products Methane Methanol Diesel/ Coal
global waste accumulation problems due to poor recycling of waste
Gasoline
plastics [192,193].
SR POX SR POX SR POX SR POX
(%) (%) (%) (%) (%) (%) (%) (%)
4.1.5. Aqueous phase reforming (APR)
H2 75.7 47.3 71.1 37.8 71.1 37.8 63.1 23.6 In the aqueous phase reforming (APR), the feedstock (e.g.,
N2 1.9 33.5 1.9 39.8 1.9 39.8 1.9 49.2
oxygenated/non-oxygenated hydrocarbons [196]) is treated at lower
CO2 19.9 16.7 24.5 19.9 24.5 19.9 32.5 24.7
Others 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 temperatures in an aqueous solution in a single step without the ne­
cessity of vaporising the reactants as required in the steam reforming
process to produce hydrogen gas [197]. Usually, this reaction occurs at
at a cost range of $1.69-$2.55 per kg. The CO2 emission from the ATR about 200-250◦ C temperature and up to 60 bar pressure in the presence
process (i.e., 3.91 kgCO2-eq/ kg H2) is very low in comparison to other of platinum (Pt), tin (Sn), cobalt (Co) or nickel (Ni)-based metallic cat­
processes. Cost for emission capture is added to the production expenses, alysts in addition to alumina as a catalyst support [196,198-200]. This
which also affects the feedstock price. Catalyst selection has been process is less energy consuming and “greener” than the other reforming
effective to the ATR process. For instance, Zhang et al. [193] reported processes [196]. This process has a great potential to be considered as
that the addition of Pd-Zn/γ-Al2O3 at 400 ◦ C can produce about 45% (v/ one of the economic methods to generate hydrogen gas from organic
v) of hydrogen efficiently. It has been observed that the precise ther­ compounds [198]. When biomass (usually long chain hydrocarbons or
modynamic analysis of the ATR process of the components of the bio-oils polymers) is directly used as feedstock in the hydrogen production
are absent [194]. The ATR reaction process is also termed as the process, there are additional complexities in the process implementation
oxidative steam reforming (OSR) process due to its nature of reacting due to their variation of composition, sizes, low-energy density, and
components [195]. The typical reaction process of the ATR or OSR transportation. To ease these complexities, biomass could be converted
process is shown in Eqn.17. into liquid (e.g., ethylene glycol, bio-ethanol, polyols, bio-methanol,
( ) bio-oil, cellulose) or gas (e.g., syngas, biogas, methane) as an interme­
Fuel Cx Hy Oz + air + steam → CO2 + H2 + N2 (− ΔH) (17)
diate raw material to be transported, treated and processed efficiently in
While producing hydrogen from various hydrocarbons, the SR, DR, the reforming process to produce hydrogen [199,201]. Cortright et al.
and POX processes generally release CO2 which needs to be integrated [197] first demonstrated that, at about 500K temperature, the hydro­
with the carbon capture, utilization, and storage (CCUS) techniques to carbons having at least 1:1 stoichiometric ratio between C and H mol­
decarbonise these processes and save the world from the impact of CO2 ecules can be reformed to generate H2 and CO2 gases. The stoichiometric
emissions. Also, pyrolysis and ATR process combination can solve waste APR reaction of sugar–alcohol sorbitol (C6O6H14) in the presence of Pt
plastics to hydrogen production issues. Cortazar et al. [16] have re­ catalyst can be shown as in Eqn. (18) [197]:
ported optimised production of hydrogen from several waste plastics
C6 O6 H14 (l) + 6H2 O(l) ↔ 13H2 (g) + 6CO2 (g) (18)
(HDPE, PP, PS, PET), mixed plastics, biomass, and HDPE:Biomass
(25:75, 50:50, 75:25) waste streams by using pyrolysis and ATR pro­ To make the APR process efficient for dedicated hydrogen gas pro­
cesses in line. Among the liquid streams, the highest hydrogen was re­ duction, it is essential to continuously capture CO2 from the product
ported from PP (64.1%), and HDPE (64%). Also, the lowest yield was side. Otherwise, both the H2 and CO2 gases react readily in the presence

14
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

of the same catalysts at low temperature to produce short chain hy­ acetogenic bacteria, and sulphate-reducer bacteria can ferment the
drocarbons like methane (CH4) as per Eqn. (19). syngas to produce alcohol, methane, hydrogen, or biofuels [138,157-
159]. That is why these are also recognised as biocatalysts. In the case of
4H2 (g) + CO2 (g) ↔ CH4 (g) + 2H2 O(g) (19)
the focus being to generate hydrogen from the syngas, the carboxydo­
Though higher hydrogen yield is possible with the APR process, such trophic hydrogenogenic bacteria can conduct the water gas shift reac­
consumption of hydrogen is undesirable (i.e., series-selectivity chal­ tion of CO with H2O to generate H2 and CO2 gases [138,158]. In some
lenge) in the hydrogen industry. On the other hand, this hydrogen cases, biocatalysts offer better effectiveness (i.e., lesser energy con­
consumption process can be efficiently employed in the hydrogenation sumption, cessation of catalyst poisoning) and advantages (i.e., no in­
processes in the biorefineries [197,198]. If the catalysts in the APR fluence of H2:CO in the syngas, reactions can occur at lower
process possess lower rates of cleaving the C-O bonds, the resulting temperatures) than that of organic and non-organic chemical catalysts
alkane production rate could be reduced as well [202]. Though Ni-based [158]. In a conventional WGS reaction of natural gas, the feedstock is
monometallic catalysts are very common, the Pt-based monometallic first reformed into CO, H2 and CO2 gases. Then the catalytic WGS re­
catalysts demonstrate better activity as well as selectivity [202] in terms actions occur in low temperature shift (LTS) and high temperature shift
of abating conversion of H2 into alkanes with further reactions. More­ (HTS) reactors to attain higher percentages of H2 conversion from the
over, the bimetallic catalysts like PtNi, PdFe, PtFe in addition to PtCo reformate gas (more than 90%) mixtures [153]. The mixture of H2 and
demonstrated better activities in APR processes than that of the mono­ other gases are purified through a pressure swing adsorption (PSA)
metallic catalysts [202]. Besides, the catalyst supports like TiO2,CeO2, C process to collect almost 99.9% pure H2 gas [153]. The BWGS reaction
(black and activated carbon), SiO2, ZrO2, ZnO, SiO2-Al2O3 and Al2O3 can process is carried out by transferring electrons from the CO to H2O,
improve the activity of the catalyst [203]. which dissociates the H+ from H2O and converts into H2 gas as shown in
Due to variation of activities and selectivity characteristics, the Eqns. 20-22 [153].
appropriate catalysts can be determined by following the methodologies
Electron transfer from CO : CO + H2 O→CO2 + 2e− + 2H + ; (CODH) (20)
depicted in Fig. 10 as presented by Huber and Dumesic [202]. When 10
wt.% aqueous ethylene glycol is reformed at 498K and at 29.4 bar in the
Hydrogen gas production : 2H + + 2e− →H2 ; (hydrogenase) (21)
APR process in the presence of Pt catalyst and various catalyst supports,
then the effectiveness of producing hydrogen gas with the Pt/catalyst Complete BWGS reaction : CO + H2 O→CO2 + H2 (22)
supports were obtained as, Pt/TiO2>Pt-Black carbon>Pt/activated
C>Pt/Al2O3>Pt/ZrO2 [203]. Higher amounts of alkane formation can If the conversion of CO into CO2 occurs in an anaerobic environment,
be observed when black carbon is used as the catalyst support. By then the reaction yields 4.46 kcal/mol of CO conversion. However, the
comparing the hydrogen production capacity with the alkane formation aerobic transformation of CO into CO2 occurs in the presence of oxygen
route in the presence of a catalyst system, Pt/Al2O3 has shown efficient and the reaction produces about 61.1 kcal/mol of CO conversion [153].
selectivity for conversion of ethylene glycol to hydrogen via the APR Economic sensitivity analysis is recommended to examine the benefit of
process [203]. employing the right water-gas-shift reaction process [160-162].

4.2. Bio-water-gas shift (BWGS) reaction 4.3. Biomass pyrolysis

The BWGS reaction occurs in the presence of enzymes like hydrog­ Biomass can easily be converted into secondary energy without
enase and carbon monoxide dehydrogenase (CODH) [138]. Microor­ investing huge capital and the biomass is mostly available locally [14].
ganisms like hydrogenogenic bacteria, methanogenic archaea, Biomass pyrolysis process is considered as cleaner process to convert

Fig. 10. Methodologies to develop catalysts [202].

15
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

biomass and organic waste into secondary energy sources. The biomass the softness of the woods increases with the increased amount of lignin
pyrolysis process can be integrated in the process of hydrogen produc­ and extractives, whereas hardwood is comprised of more of the cellulose
tion. Aziz et al. [204] reviewed different process to produce hydrogen and hemicellulose biopolymers [220]. The overall activation energy
using the pyrolysis process. The thermal breakdown of lignocellulosic requirement of the thermo-chemical breakdown of the biomass can be
derivatives in an inert condition without oxygen is called pyrolysis varied due to variation of quantity of these biopolymers. Kinetic
[205]. The generation of H2 from biomass starts with fast pyrolysis of the modelling [224-226] of the individual thermal decomposition of cellu­
biomass [206]. In this thermochemical process, biomass feedstock is lose [227-230], hemicellulose [231-233] and lignin [233-236] can
cracked into charcoal, gases, and liquid (bio-oil or bio-crude), methanol, explain the necessity of primary investigation of the feedstock compo­
acetone, and acetic acid by heating the biomass to around 1025K sitions in order to optimise the desired product stream [237-239] among
without oxygen [207]. The key products of the biomass pyrolysis pro­ liquid, gaseous and char. Also, there could be combinations of a few
cess can be presented as in Eqn. (23). In the pyrolysis process, the parallel pathways to complete the pyrolysis of complex biopolymers like
organic material is decomposed by a thermal process in the absence of hemicellulose and lignin [205,224]. A higher heating rate of cellulose
oxygen to produce volatile and inorganic elements containing carbo­ reduces char yield, whereas lowering the heating rate reduces the char
naceous char [208]. Pyrolysis reactions can occur in the presence or yield from the thermos-degradation of lignin [224]. Lowering the
absence of catalysts to produce the desired products [209,210]. Çağlar heating rate also increases the activation energy requirement for
and Demirbaş reported that an increased amount of ZnCl2 catalyst thermos-degradation of both celluloses. Being multicomponent poly­
(>17%) in the pyrolysis reaction of biomass like Olive husk produces mers, both hemicellulose and lignin respond to the heating rate based on
more hydrogen gas (mixture of H2 and paraffins > 70%) from the py­ their variations of components in the respective biomass [224]. A higher
rolysis process [207]. Akubo et al. [17] conducted integrated pyrolysis- proportion of liquid from the biomass thermos-degradation is possible
catalytic steam reforming of biomass feedstocks (hemicellulose (xylan), with higher amounts of cellulose and hemicellulose in the biomass
lignin, and cellulose) and biomass wastes (sugarcane bagasse, wheat feedstock [211]. Also, char yield reduces remarkably with the biomass
straw, palm kernel shell, rice husk, coconut shell, and cotton stalk) that feedstocks containing lesser amounts of lignin [240]. The pyrolysis
increased the productivity of hydrogen gas in the presence of 10 wt% process of the biomass produces water as a by-product [240]. Correla­
Ni/Al2O3 catalyst. The syngas (mixture of H2, CO and CO2) is fed into a tions for measuring higher heating values of the biomass feedstocks can
pressure swing adsorbent at room temperature to separate the H2 gas be obtained from published articles [214,241] which have extensively
from the gas mixtures. The pyrolysis reaction is presented in Eqn. (23). analysed various correlations. Percentages of biopolymers in different
biomass feedstocks and their heating values are presented in Table 5.
Pyrolysis reaction : Biomass→H2 + CO2 + CO + Hydrocarbon gases (23)
Although various research works have been performed on individual
There are four types of biomass feedstocks available in the world: (1) biopolymers (lignin, hemicellulose, and cellulose) of the biomass, it is
woody plants; (2) herbaceous plants or grasses; (3) aquatic plants; and found that the process is not economical in the industrial scale to
(4) manures [211]. Biomass feedstocks can be acquired abundantly from separate these chemical compounds from the biomass feedstocks prior
diverse sources like the wastes from animals, municipal solid waste further reaction processes [239]. Besides, there are technical challenges
streams, food processing industries, rivers or oceans, residual wastes to recover and distinctly separate the pure form of the hemicellulose and
from agriculture, forests and wooden materials [212]. Biomass, being a lignin. Shorter residence timing results in incomplete depolymerization
mixture of various low molecular weight and macromolecular based of lignin. As a result, the lignin macromolecules are randomly broken to
substances, can be considered one of the key renewable energy resources form liquid substances. But the longer residence periods can lead to
for fuel production [213-215]. Besides, the available conversion pro­ secondary thermal cracking of the lignin macromolecules, which results
cesses (e.g., thermo-catalytic or bio-chemical processes) can efficiently in reduced liquid produced and adversely affects the bio-oil’s quality
convert these biomass raw materials into gas, liquid or solid commod­ [239]. Usually, the biomass pyrolysis process is a combination of ther­
ities (i.e., biomass to chemical/hydrocarbon) which can be further mal cracking of the key biopolymers in the biomass at various temper­
treated to produce a desired fuel stream [216]. Converting biomass into atures. Initially the moisture is removed from the biomass (<200◦ C),
liquified hydrocarbon mixtures offers better volumetric energy density then the decomposition of lignin (280-500◦ C), cellulose (<240-350◦ C),
and efficient responses on catalytic processes due to reduction of cata­ and hemicellulose (200-280◦ C) take place in the fast pyrolysis process
lytic poisoning [217]. The bio-oil is typically an intricate mixture of [205]. Most of the thermo-degradation process occurs within two-thirds
various hydrocarbons resulting from depolymerization of biopolymers of the maximum operating temperature of the pyrolysis process. Not
like cellulose linear chain polysaccharides (C6H10O5)n, hemicellulose only the rate of heating, reaction temperature, vapour residence time
comprising a diverse mixture of various polysaccharides categorised as inside the reactor, and types of biomass feedstocks, but also the particle
xylans, mannans, mixed linkage β-glucans and xyloglucans, lignin which size of the feedstocks, gas sweeping rate, amount of moisture in the
is a heterogenous amorphous biopolymer with an approximate formula biomass, presence and catalyst types, reactor type and the variation of
of (C31H34O11)n and various extractives, which are the key constituents reactor types of the pyrolysis reaction process govern the yield charac­
of biomass [211,215,217-219]. Biomass also contains ashes to some teristics [209,228,242]. Given that all other operating conditions are the
extent [220]. About 30-50% of the biomass contents by weight is cel­ same, the variation of particle size can significantly impact the yield
lulose, 10-40% by weight is hemicellulose, and 5-30% by weight could quantification of char and liquid from the pyrolysis method. If the par­
be lignin content based on the variation of the biomass types (i.e., ticle size is smaller, there is better opportunity to overcome the depo­
softwood, hardwood, wheat straw, switchgrasses) (Table 5) [211]. lymerization obstacles of the complex hydrocarbons from biopolymers,
Tannin (a naturally occurring water soluble phenolic compound) is thus increasing the rate of reaction to produce more of the desired
another biopolymer that prevent the plant and algae from microbial, product. On the other hand, the larger size of particles require more
viral and microbial activities [221-223]. It has also been affirmed that activation energy input to overcome the chain disintegration barriers,

Table 5
Energy value variation of biopolymers of various biomass feedstocks [211].
Biopolymers Softwood(%) Hardwood(%) Wheat straw (%) Switchgrasses(%) Higher heating value(MJ/kg) Activation Energy (KJ/mol)

Cellulose 35-40 45-50 33-40 30-50 16.16 227.27-287.56

Hemi-cellulose 25-30 20-25 20-25 10-40 16.10 30.09-194.35
Lignin 27-30 20-25 15-20 5-20 19.40 20-672.97

16
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

which can also lead to formation of more char due to secondary re­ Argon, and flue gases); (b) mechanical (i.e., the way the materials are
actions [243]. If the particle sizes are small enough (i.e., fraction of mm mixed or stirred to accomplish the desired thermal cracking); and (c)
scale), there will be better efficiency for rapid and consistent heat gravity fed [245,269]. A brief classification of the reactors is presented
transfer to the particles to disintegrate and form desired yields [205]. In in Fig. 11 based on the discussions in publications [245,267].
such cases, an efficient reactor bed is also necessary to help with con­ Based on the technique for conducting the pyrolysis reactions and
ducting the reactions uniformly. Though the most frequently used the desired yield types, the pyrolysis methods can be categorised as
reactor bed for pyrolysis process is the fluidised bed reactor, it struggles conventional (slow pyrolysis), flash, and fast [9,205]. Based on the yield
to meet that quick heat transfer requirement to complete the reactions of requirements, flash pyrolysis can be divided into flash-liquid (for
all the fed particles [205]. This matter was resolved with the use of liquid), flash-gas (for syngas and various mixtures of chemicals) and
ablative pyrolysis as well as auger pyrolysis methods to conduct the fast ultra-fast pyrolysis [246,268]. Some more purpose driven pyrolysis
or flash pyrolysis [205,244,245]. processes are also in practice, namely, vacuum pyrolysis, carbonization,
Apart from the variation of sources of biomass feedstocks, studies on hydro-pyrolysis, and methano-pyrolysis [260,270-272]. Any of these
various pyrolysis processes and reactors are mainly available based on types of pyrolysis reactions may occur in the absence or presence of
the yield [246-248], heating and temperature effect [249-252], particle catalysts [206,273-275]. Catalysts help with increasing the reaction rate
size [253-256], vapour residence timing [257-259], and kinetic by reducing the activation energies, optimise the liquid yield and reduce
modelling [260-262]. Other key parameters considered for estimating both gases as well as char yield from the feedstocks processed by the
the efficiency of the pyrolysis process are the moisture content and pre- respective pyrolysis process [209,210]. Then the liquid substances can
heating for biomass drying, composition of biomass, feeding into the be reprocessed easily than the non-condensed gases [212,276].
reactor, reaction period, reactor type, gas flow rate, types and quantity Fast pyrolysis of biomass yields higher amounts of liquid (>75%)
of catalysts, yield and char removal frequency, quenching or condensing based on the dry feed quantity of biomass at optimal temperature,
efficiency of syngas, treating the non-condensing gases, and retreating strictly maintaining a short vapour residence timing and effective
or upgrading the bio-oil [263-265]. Typically, the bio-oil is a combi­ cooling-condensation capacity of the biorefinery plant [217,276,277].
nation of water, insoluble lignin, aldehydes, ketones, organic acids, Initially, the fast pyrolysis process yields vapour of various hydrocar­
carbohydrates, phenol, furfurals and alcohols [257]. Optimal manage­ bons, non-condensable gases and char within few seconds [276]. The
ment of the pyrolysis process parameters can lead to an increased Waterloo flash pyrolysis process (WFPP) decomposes biomass materials
amount of desired chemical in the bio-oil. Selection of the appropriate into organic liquids without the presence of oxygen at continuous at­
type of pyrolysis reactor is imperative to obtain the desired yields from mospheric pressure. Usually, it is a precisely regulated process that
biomass feedstocks. The quantity of bio-oil yield differs with the type of yields high amounts of liquid [205]. The process has been broadly
pyrolysis reactor, e.g., ablative, bubbling fluidised bed and circulating exhibited using hardwood solid waste to yield as high as 70% organic
fluidised bed reactors can yield about 75 wt.% bio-oil, compared with 70 liquid of the feed material [278].
wt.% by the spouted fluidised bed, 65 wt.% by the entrained flow Apart from fast pyrolysis, the methano-pyrolysis [279-281] and
reactor and rotating cone reactor, 60 wt.% by the vacuum pyrolysis hydro-pyrolysis [271,272,282] processes have shown promising fea­
reactor, about 45 wt.% by the rotating screw (auger) reactor and fixed tures on producing sustainable fuels more efficiently. The methano-
bed reactor, as reported and reviewed in various publications pyrolysis process to crack the Methane gas or other hydrocarbons has
[225,239,257,266-268]. Most commercial scale pyrolysis plants run on the potential to establish technological linkage with the sustainable
fluidised bed reactors. Typically, the pyrolysis plants can be categorised green fuel economy. The products of methano-pyrolysis are black car­
into 3 types based on the feeding methodology of the feedstock: (a) bon and hydrogen, carbon being a raw material for carbon-based in­
pneumatic (i.e., flow of atmospherically inert gases like Nitrogen, dustries and hydrogen being a fuel that would not produce any harmful

Fig. 11. Different types of reactors used for pyrolysis processes with or without catalysts [245,267].

17
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

emission products through combustion [283,284]. The Gibbs free en­ to generate hydrogen with natural gas separation. Chen et al [290] in
ergy values to conduct these chemical reactions are dependent on the their research produced hydrogen from the pyrolysis of biomass using
reaction temperature [285]. The following process reactions [284] sol–gel-prepared Fe2O3/MgAl2O4 as an oxygen carrier. They found that
shown in Eqns. 24-28 will clarify the sustainability of methano-pyrolysis the ideal gasification temperature is 950◦ C to attain a high conversion of
process. optimum gasification temperature and gas yield through chemical
looping. But the generated hydrogen was contaminated by CO2 and
Natural gas combustion : CH4 + O2 (air)→2H2 O + CO2 + 890kJ (24)
CO severely due to the formation of coke during reduction. The
hydrogen purity was increased by adjusting the atmosphere in the
Hydrogen combustion : 2H2 + O2 (air)→2H2 O + 572kJ (25)
reduction area with the addition of steam and it was an efficient tech­
Carbon combustion : C + O2 (air)→CO2 + 394kJ (26) nique. The purity of hydrogen was achieved at 96% when the steam and
oil ratio was 1.5 as compared to 84% when there was no steam. Addition
Steam methane reforming (SMR) : CH4 + 2H2 O + 206kJ ↔ CO2 + 4H2 of steam also enhances performance of the cycle by making the porosity
(27) more stable [291]. Table 6 presents different types of catalysts used for
different types of reactors.
Methano − pyrolysis of methane (natural gas)
4.3.2. Syngas to hydrogen production
(28)
819K
: CH4 + 74.52kJ → C(solid) + 2H2 Hydrogen can be produced from syngas which is the by-product of
Though combustion of 1 mole of natural gas produces 890kJ of heat some pyrolysis processes. Syngas can be produced from the pyrolysis of
energy, about 44.27% of the total heat is generated due to combustion of biomass which is available worldwide. This waste can be collected from
carbon. It can be observed that 1 mole of H2 generates about 286kJ of municipal waste, industry waste and other sources. Biomass pyrolysis as
heat due to its combustion. The steam methane reforming process needs explained in previous section can be utilised to produce hydrogen as
206 kJ of heat to produce 4 moles of H2 along with 1 mole of CO2. But well as syngas. The produced syngas can be used as feedstock to produce
the methano-pyrolysis process requires about 74.52kJ of heat energy (at hydrogen. In addition to that, hydrogen can be produced directly using
819k) to produce 2 moles of H2 along with another industrial raw ma­ fast pyrolysis with a high temperature and a certain volatile phase
terial, namely black carbon. Though there is more hydrogen production residence time [204]. The process that is used to produce hydrogen is
in the SMR process, production of CO2 does not make the process the reformer steam-iron process (combination of steam-iron process and
completely sustainable. Conversely, methano-pyrolysis provides a sus­ conventional steam reforming process) which can be potentially
tainable way to convert hydrocarbon fuels into carbon free fuel for clean employed to generate hydrogen in a decentralised facility for on demand
energy production, thus contributing to reduction of environmental hydrogen supply using syngas as the feedstock [292]. The beneficial
pollution. Biomethane or other hydrocarbon gases obtained from the features of the reformer-steam iron process are the use of two different
pyrolysis of biomass feedstocks can be processed in such a way so as to reaction conditions, i.e., reduction process at atmospheric condition and
produce cleaner hydrogen fuel. But carbon removal and reactivity of the oxidation at higher pressure (~5 MPa) condition. In the case of
carbon with the catalysts are key challenging factors for hydrocarbon syngas production from pyrolysis of biomass, the syngas can be directly
cracking processes [286,287]. For a sustainable and economic way of processed by the iron-oxide (hematite, Fe2O3). Therefore, the pyrolysis
hydrogen fuel production [269], it is necessary to compare these py­ of biomass and solid waste will help reduce the burden solid waste
rolysis processes and reactors at optimal operating conditions on an landfilling and the prosecution of hydrogen will add energy security.
industrial scale. The main challenge of biomass pyrolysis is the pro­
duction cost and environmental concerns as identified by Jahirul et al 4.3.3. Biomass derived liquid bio-oil to hydrogen
[288]. The production of bio-oil using the pyrolysis process must over­ The highest product of the pyrolysis and thermochemical process of
come the economic, technical, and social barriers [288,289]. biomass is bio-oil. Different technique can be employed to produced
hydrogen from the bio-oil. Bio-oil, an important biofuel, can be gener­
4.3.1. Use of catalysts in the pyrolysis process ated from thermochemical processes of biomass, such as hydrothermal
Chemical looping of iron oxides has appeared as a favourable method liquefaction and pyrolysis [293-295]. Bio-oil is considered as a

Table 6
Various thermochemical processes of biomass [127].
Technology Feeding Reactor Process Condition Product yield

Fast pyrolysis Lignocellulosic, wood Circulating fluidised Atm. pressure, 400–550◦ C, residence times of 1 s, particle sizes 72% bio-oil, 12% gases and
waste bed Reactor <0.5 mm 16% char
Flash pyrolysis Wood sawdust Conical spouted bed 425–525◦ C bio-oil and gases
reactor
Slow pyrolysis Algal biomass TGA and Py-GC/MS Low heating rate, 350–750◦ C, atm. pressure, long residence 45–55% liquids, 25–35% gases,
time and 15–25 char
Air gasification Pine sawdust Fluidised bed reactor 700–900◦ C, ambient, Air with steam mixture High temperature favoured H2
and gas yields
Steam gasification Olivine particles Fluidised bed gasifier 770◦ C and a steam/biomass ratio of 1 with calcined dolomite
Steam-oxygen woody biomass and Circulating fluidised 800–820◦ C, steam to biomass ratios of 1.1–1.51; oxygen to 20–25% H2, 20–25% CO,
gasification agriculture residue bed gasifier biomass stoichiometric ratios (ER) of 0.36–0.4 35–40% CO2
Steam-air gasification Wood waste Fixed bed downdraft Max. bed temp.: 900–1050◦ C and exit temp.: 700◦ C; 25–30% H2, 40–45% CO,
equivalence ratios of 0.2–0.3 20–25% CO2
Steam-air gasification Wood pellets gasifier Max. bed temp. of 950–1150◦ C and exit temp. of 150–400◦ C, 25–35% H2, 30–40% CO,
and equivalence ratios of 0.24–0.35 20–30% CO2, 5–10% CH4
Supercritical water lignin, cellulose and waste tubular batch reactor Fixed heating rate of 30 ◦ C/min to 650◦ C and residence time of 30–40% H2
gasification biomass 50s with K2CO3 and 20NiA 0.36 Ce/Al2O3 catalyst
Chemical looping biomass char Fix-bed reactor 850◦ C with natural iron ore H2, CO
gasification
Solar gasification pine and spruce wood Tubular reactor with 1000–1400◦ C 35–45% H2, High carbon
solar energy conversion rate of 93.5%

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

favourable feedstock to substitute petroleum fuel for power generation the biomass is the gasification process. In this process, biomass is con­
because it has high energy density and bio-oil transport and storage is verted into combustible gas mixture which has been used for decades for
convenient as compared to gaseous fuel [296,297]. Bio-oil can be coal gasification. To produce hydrogen by the gasification process from
derived from various biomass feedstocks, green wastes (landfill wastes), biomass (e.g., Renewable organic materials, Agro-based crops or crop
animal wastes, and municipal wastes [184]. Reducing the capital and residues, forest residues, specific crops used only for energy production,
operational costs related to biomass feedstock production, logistics and organic municipal wastes, and animal wastes) requires a very high
process efficiency improvement can increase this industry’s contribution temperature (>700◦ C) non-combustion reaction in the presence of a
effectively. The aqueous phase reforming (APR) can also be utilised to controlled amount of oxygen. Hydrogen gas, CO and CO2 are the typical
transform bio-derived liquid (bio-oil) or biomass into hydrogen directly products of the reaction. An adsorption or molecular membrane sepa­
like the autothermal reforming processes, partial oxidation, and steam ration process can be used to remove hydrogen gas from this mixture. On
reforming. Hydrogen is produced in the aqueous phase reforming pro­ the other hand, CO can react with waste to generate more hydrogen and
cess by decomposing the water-soluble organics elements at high pres­ CO2 gases by a waste-gas shift reaction. Production of CO2 is still a
sure (~2 MPa) with relatively low temperatures (<300◦ C). The high challenging task to handle in this process. A simplified example of the
pressure holds the elements in the liquid state and the low temperatures gasification and water gas shift reactions are presented in Eqns. 31-32.
favours CO2 production over CO, hence increase the hydrogen produc­ Step 1: Gasification:
tion without the water gas shift (WGS) reactor.
C6 H12 O6 (Glucose) + O2 + H2 O→CO + CO2 + H2 + otherspecies (31)
The process of reforming biomass-derived liquids and natural gas to
produce hydrogen is same which includes the steps: The reaction of Step 2: Water-gas shift reaction:
liquid fuel with steam at high temperatures in the presence of a catalyst
CO + H2 O→CO2 + H2 (+smallamountofheat) (32)
which produces H2 , CO , and some CO2 . Additional H2 , and CO2 are
formed by reacting the CO with steam in the “water-gas shift” reaction. Integration of the pyrolysis process in the absence of oxygen with a
Lastly, the hydrogen is removed and then purified. The SRR and WGS water-gas shift reaction and catalytic hydrocarbon reforming can be an
reaction is presented in Eqns. 29-30. alternative to the usual high temperature gasification process. Various
hydrocarbon gas mixtures are produced in the process of biomass py­
Steam reforming reaction (ethanol) : C2 H5 OH + H2 O(+heat)→2CO + 4H2
rolysis. Then the mixture of hydrocarbons goes through catalytic hy­
(29)
drocarbon reforming to produce syngas mixture (i.e., hydrogen, CO, and
Water − gas shift reaction : CO + H2 O→CO2 + H2 (+smallamountofheat) CO2). The water-gas shift reaction produces more hydrogen by the re­
action between CO and steam, in which CO converts into CO2 thus a
(30)
significant amount of CO2 is produced. Fig. 12 presents different kinds of
gasifiers. Characteristics of different types of gasifiers are shown in
4.3.4. Supercritical water reforming (SCWR)
Table 7. The table presents the operation parameter and the percentage
The waste aqueous portion of the bio-oil produced from the thermal
of char conversion.
or thermo-catalytic processes may be converted into hydrogen gas or
Syngas contains sulphur contaminants in the form of carbonyl sul­
other hydrocarbons by the supercritical water reforming (SCWR) pro­
phide (COS) and hydrogen sulphide (H2S) which can be separated
cess, thus valorising the wastes. The aqueous portion may contain about
individually or collectively with other gases such as CO2. Numerous
80% water as a result it may not be economical to recover the rest of the
processes which utilise physical or chemical adsorption or a combina­
value-added products (e.g., acetic acids, hydroxy-acetones, 1-butanols,
tion are available to remove acid gas [300].
glucose [298]) from this waste stream. But the SCWR process can
There are some technical and economic challenges with biomass in a
potentially convert the mixture into fuel or other usable chemicals at
decentralised H2 production plant [302]. In a thermo-catalytic process
high pressure (up to 24 MPa), high temperatures (up to 800◦ C), and
like gasification, the plant will need a gasification reactor and portable
higher residence time (about 3-6 hours) to conduct the reaction process
purification unit (PPU) consisting of catalytic filter candles, pressure
[298]. When the water portion of bio-oil is reformed by the SCWR
swing adsorbent (PSA) and a water gas shift reactor (WGS) in order to
process in a tubular fixed-bed reactor in the presence of Ni/Al2O3-SiO2
produce and separate H2 gas from other gases [302]. Optimal adsorption
catalyst, the method’s yield of H2 is 2.3 mol/mol at 800◦ C and 24 MPa
time and linear velocity of the pressurised continuous feed of syngas
[299].
through the adsorbent can lead to highly purified H2 gas (99.99%) with
Biomass pyrolysis can be used to produce hydrogen which will help
more than two-thirds of the H2 gas recovery from the gas mixtures
alleviate the global waste management. The biomass can be collected
[303,304]. In the presence of activated carbon and zeolite catalysts, the
from diverse sources which includes municipal solid waste, industrial
polybed PSA process integrated with the Skarstrom PSA cycle [305] or
waste in food processing, and agricultural waste in industry. These
selective surface flow (SSF) membrane (thin nano-porous layer of car­
sectors are grappling to manage the huge waste they produced each day.
bon membrane supported on a macro-porous tube of alumina) [306] can
This solid waste can be converted into hydrogen, solid char and syngas
improve the H2 gas recovery from the waste gases, resulting in cumu­
which can be synthesised later to convert into usable product. Different
lative H2 gas recovery of more than 90% [307,308]. The polybed PSA
types of pyrolysis are available, selection of these process depends on the
process can be comprised of 9 cyclic steps [309] (i.e., adsorption, con­
types of wastage, yield requirements and other factors. Appropriate
current depressurization I, concurrent depressurization II, concurrent
pyrolysis process can be selected based on the method of feeding, types,
depressurization III, counter-current depressurization, counter-current
and the process suitable for the pyrolysis of waste, economical consid­
purge, counter-current pressurisation I, counter-current pressurisation
eration, and yield requirement. Purpose driven pyrolysis processes are
II, counter-current pressurization III); or the Lofin process consisting of
also in practice, such as vacuum pyrolysis, carbonization, hydro-
9-cyclic steps [310] (i.e., adsorption, concurrent depressurization I,
pyrolysis, and methano-pyrolysis, flash-liquid (for liquid), flash-gas
concurrent depressurization II, concurrent depressurization III, counter-
(for syngas and various mixtures of chemicals). The pyrolysis process
current depressurization, counter-current purge, counter-current pres­
can crack not only the biomass but also the hydrocarbon, therefore this
surization I, counter-current pressurization II, counter-current pressur­
process can be used to crack the hydrocarbon for hydrogen production.
ization III) with the help of activated carbon (in the feed side) and zeolite
(in the product separation side) catalysts [308].
4.4. Biomass gasification Hydrogen can be produced from biomass using the biomass gasifi­
cation process. Different types of gasification process are available, and
Another technique that can be used to produce the hydrogen from each process has different operating conditions and efficiency. This

19
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 12. Different types of gasifiers [300].

Table 7
Different types of gasifiers and their characteristic [301].
Gasifier types Flows Gasification temperature (◦ C) Cold gas efficiency (%) Char conversion (%) Tar content (g/Nm3)
Biomass GA

Updraft Downward Upward 950–1150 exit temperature :150–400 20–60 40–85 1–150
Downdraft Downward Downward 900–1050 exit temperature :700 30–60 <85 0.015–1.5
Fluidised bed Upward Upward 800–900 <70 <70 10–40
Circulating fluidised bed Upward Upward 750–850 50–70 70–95 5–12
Entrained flow Downward Downward 1300–1500 30–90 60–90 = 0–0.2
͂

process generates CO2 which needs to be captured otherwise the process feedstocks because of their low cost of removal and processing, but only
will have impact on the atmosphere and increase the carbon footprint. provide average Decarbonisation quantities due to earlier un­
Synthetic e-fuel can be produced by capturing the CO2 which will derestimations of supply chain emissions contributions [99]. Solid car­
overcome the issue of producing CO2. The process will benefit society by bon is safer, cheaper, and easier to store than that of gaseous CO2.
converting waste into useful fuel and alleviate global waste problem. Furthermore, the pure carbon can be used in printing ink, car tyres, as
well as in many new applications such as soil improvement and carbon
electrodes [284]. Methane (CH4, natural gas) contains the highest H/C
4.5. Decarbonisation
ratio among all hydrocarbons. As a result, it is obvious to favour natural
gas feedstock to produce hydrogen over other hydrocarbons. The
All the hydrogen production processes which use other than water as
cracking’ or ‘Decarbonisation’ of 1 m3 of methane will produce 2 m3 of
a feedstock produce carbon di-oxide. If the carbon di-oxide is released
hydrogen. The decarburization process decomposes the hydrocarbons
into the atmosphere, then the purposed of producing hydrogen will not
into carbon black and hydrogen gas with the help of either the “Hypro
serve. Therefore, to fulfill the objective of implementing new viable
process” or “Hydrocarb process” [313,314]. Carbon can be used as raw
technologies for reduction of GHG emissions to the atmosphere due to
material in various industries as activated carbon, carbon black and
combustion of fuels, the release of carbon compounds should be pre­
graphitic carbon [314]. The general reaction is presented in Eqn. (33)
vented. The possible options are direct decarbonisation, carbon capture
[313]:
and utilisation (CCU), and carbon capture and sequestration (CCS)
[311]. The CCS method is encountering various economic, technolog­ ◦
ΔH =32.43
kJ kJ/mol
H2

ical, and social challenges. Some specialists and researcher have CH4 →C + 2H2 (33)
concluded that “alternative emission mitigation technologies are The decomposition process is greatly governed by the input power,
potentially the only solution that could rescue us from the dire situation temperature, residence time, type and quantity of catalysts, feed rate
that we are heading toward by the end of this century” [312]. Therefore, and molar concentration of raw materials [313,315]. Though at 500◦ C
the fossil-fuel Decarbonisation methods may create an crucial alterna­ only 10% of the feedstock is decomposed, about 95% feedstock can be
tive in the technological portfolio that could alleviate the negative decomposed at 1327◦ C [313,315,316]. Methane Decarbonisation can be
impact of climate change, facilitating the application of natural gas a vital technology employed near the energy end-user on power-to-gas
during a speedy energy shift [313]. There is a trade-off between the options [313]. The development of a sizable and modular method to
mitigation cost and the amount of Decarbonisation reached [99]. The separate methane into pure hydrogen and carbon may be relevant for
most cost-effective techniques of Decarbonisation still use fossil

20
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

domestic application of a few kW of energy, and for up to several MW for

industrial uses. The schematic of methane Decarbonisation is presented
in Fig. 13.
The incorporation of natural gas breakdown in the hydrogen econ­
omy and mobility is feasible based on the assessment with an IRR of
14%. Considering its incorporation in a combined cycle plant, even
though the hydrogen generated is not expensive due to the higher scale,
the efficiency of the plant declines to a 34% and is only offset by carbon Fig. 14. Carbon production due to cracking of hydrocarbons in various
taxes between 51 and 105 €/ton CO2 [313]. Carbon and hydrogen methods [317].
production from cracking of hydrocarbon is shown in Fig. 14.
Hydrocarbons decomposition using single-step cracking (splitting, or 4.6. Electrolysis
dissociation, decomposition, pyrolysis, Decarbonisation, and dehydro­
genation) facilitates the lowering of greenhouse gases by simultaneously Electrolysis process can be used to produce hydrogen from water
producing important carbon products such as graphite or carbon black without any carbon di-oxide generation as a by-product. As this process
which can be used in carbon filaments or carbon nanotubes [317]. The does not release carbon di-oxide, this process is the greener hydrogen
catalytic method includes carbon and metal-based catalysts while production process among all the process if the electricity used is
plasma-based breakdown relies on thermal or non-thermal methods. generated from renewable sources. Electrolysis is a method of breaking
Nearly all the recommended processes are appropriate to a range of water into oxygen and hydrogen by using a direct current, transforming
liquid and gaseous hydrocarbon fuels, and a few of these methods can electricity into chemical energy. At present, about 8 GW of electrolysis
possibly generate a stream of high-purity hydrogen. There have been capacity are in operation globally [318]. Different kinds of electrolysers
positive efforts to utilise catalysts to decrease the highest temperature of are characterised by the charge carrier and their electrolyte and can be
the thermal disintegration of hydrocarbons. Conventional catalysts categorised into: Solid Oxide (SO) electrolysers; Polymer Electrolyte
employed are transition and noble metals such as Ni, Pd, Fe, Mo, Co, Membrane (PEM) electrolysers; and Alkaline electrolysers. Similar to
etc., which are deposited on the high surface area ceramic substrates fuel cells, electrolysers comprise of an anode and a cathode which are
such as SiO2 and A12O3, etc. Various publications reveal the application separated by an electrolyte. Different electrolysers work in somewhat
of carbon-based substances as catalysts for disintegration of hydrocar­ different ways, primarily due to the difference of electrolyte material.
bons into H2 and C. The non-catalytic disintegration methods are plasma The electrolyte in a polymer electrolyte membrane (PEM) electro­
jets, direct current generators, non-thermal low-temperature plasmas lyser is a special plastic material. At the anode, water reacts to produce
such as radio frequency, and microwave plasmatrons. positively charged hydrogen ions (protons) and oxygen. The electrons
Burning fossil fuel or gaseous fuel produces cardon dioxide which travel through an outside circuit and the hydrogen ions particularly
has negative impact on the environment but burning hydrogen produces travel through the PEM to the cathode. Hydrogen ions join with elec­
only water. Therefore, instead of burning fossil fuel or gaseous fuel, the trons at the cathode from the exterior circuit to produce hydrogen gas as
fuels can be decomposed into carbon and hydrogen. Hydrogen can be shown in Eqns. 34-35.
used for energy and other industrial application and carbon can be used
Anode Reaction : 2H2 O → O2 + 4H + + 4e− (34)
for printing ink, tyre manufacturing, carbon electrode and many other
applications. The decomposition of hydrocarbon can be carried out with
Cathode Reaction : 4H + + 4e− →2H2 (35)
catalyst and without catalyst using plasma jet and plasmatron. The
-
production of carbon di-oxide through this process can be captured and Alkaline electrolysers work by carrying hydroxide ions (OH ) from
synthesised for synthetic fuel which can be used for engine fuel. the cathode to the anode through the electrolyte with hydrogen being
produced on the cathode side. Electrolytes of liquid alkaline solution of
sodium or potassium hydroxide have been commercially available for

Fig. 13. Methane Decarbonisation near the end–user in power-to-gas options, redrawn from ref [313].

21
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Cracking the water into hydrogen and oxygen is greener hydrogen

production process than any other process provided that the electricity
used for electrolysis is generated renewably. This process does not
produce any harmful gasses as by product nor does the combustion of
hydrogen. The main electrolysis process used for the decomposition of
water into hydrogen and oxygen are alkaline, PEM and SO. Both PEM
and SO are in initial stage and need further research and development
for their performance improvement. J corridor et al [322] presented the
comparative performance of different hydrogen production method
such as heterogeneous photocatalytic (HETPHP), homogeneous photo­
catalytic (HOMPHP), and hybrid photocatalytic (HYBPHP) using pho­
tocatalytic hydrogen generation technique. They found that HETPHPs
can be used for large-scale application and this method offers longer
Fig. 15. Performance of different electrolysers [319].
operation times due to the high semiconductor photocatalyst stability
and ability to be recovered from the treated solution. They concluded
that highest H2 production rates been achieved with HOMPHPs or
many years for electrolysers. Newer attempts of utilising solid alkaline
HYBPHPs using visible light irradiation.
exchange membranes as the electrolyte are showing potential at the
laboratory size.
5. Hydrogen storage, transmission, and distribution
SO electrolysers, which utilise a solid ceramic electrolyte that
selectively carries negatively charged oxygen ions (O2-) at high tem­
Many methods are available to produce hydrogen efficiently
peratures, produce hydrogen in a somewhat unique way. Water at the
although there are pros and cons of each of these techniques. The main
cathode unites with electrons from the exterior circuit to produce
draws back of the hydrogen economy is the storage, transmission, and
hydrogen gas and negatively charged oxygen ions. The oxygen ions go
through the solid ceramic membrane and react at the anode to produce Table 9
oxygen gas and produce electrons for the external circuit. Solid oxide Typical specifications of production processes [321].
electrolysers must work at high temperatures sufficient for the solid Specification Alkaline PEM SOE
oxide membranes to operate properly (about 700◦ –800◦ C, PEM elec­
Technology maturity State of the Demonstration R &D
trolysers, 70◦ –90◦ C, and commercial alkaline electrolysers,
art
100◦ –150◦ C). The SO electrolysers can efficiently utilise the heat Cell temperature C
◦
60–80 50–80 900–100
available at these high temperatures (from different sources, like nuclear Cell pressure, kPa <3000 <3000 <3000
energy) to reduce the total of electrical energy required to generate Current density, A/cm2 0.2–0.4 0.6–2.0 0.3–1.0
hydrogen from water. The performance of various types of electrolysers Cell voltage, V 1.8–2.4 1.8–2.2 0.95–1.3
Power density, W/cm2 Up to 1.0 Up to 4.4 -
are presented in Fig. 15. The figure shows that the solid oxide (SO) has Voltage efficiency, % 62–82 67–82 81–86
the highest efficiency, but this electrolysis is in the R&D state. Specific system energy consumption, 4.5–7.0 4.5–7.5 2.5–3.5
Though alkaline electrolysers at present have higher efficiencies than kWh/Nm2
electrolysers using PEM and solid electrolytes but SO electrolysers have Partial load range, % 20–40 0–10 -
Cell area, m2 -
considerably higher possibility for potential cost reduction and effi­ <4 <300
Hydrogen production, Nm2/hr <760 <30 -
ciency improvements (Fig. 15). Different hydrogen production processes Stack lifetime, hrs. <90000 <20000 <40000
and their key data are presented in Table 8. System lifetime, yrs. 20–30 10–20 -
The specifications of alkaline, PEM and SOE production processes are Hydrogen purity, % >99.8 99.999 -
presented in Table 9. Cold start-up time, min 15 <15 >60

Table 8
Different types of hydrogen production processes and their key data [320].
Technology SMR POX Electrolysis Gasification

Energy input NG+ electricity HC+ electricity Electricity HC+ electricity

Efficiency % 70–85 60–78 62–82 50–70
Size (Nm3/h) 10000–20000 10000–20000 0.5–10 10000–20000
Lifetime 2–5 years 2–5 years 40000 hrs. 2–5 years
Market share (%) (2012) 48% 30% 4% 18%
Production (2011) cost (€/Nm3H2) 0.05–0.1 0.07–0.15 0.16–0.30 0.05–0.1
Cost breakdown (%) 30% materials 30% materials 50% materials 30% materials
40% process 40% process 30% process 40% process
30% labour 30% labour 20% labour 30% labour
Projection
Efficiency % 70–85 (2015) 60–78 (2015) 50–80 (2015) 50–70 (2015)
70–85 (2020) 60–80 (2020) 55–80 (2020) 55–70 (2020)
75–85 (2030) 65–80(2030) 80–85(2030) 60–75(2030)
Production cost (€/Nm3H2) 0.090 (2015) 0.110 (2015) 0.15–0.27 (2015) 0.095 (2015)
0.085 (2020) 0.105 (2020) 0.13–0.20 (2020) 0.090 (2020)
0.080 (2030) 0.100 (2030) 0.10–0.15 (2030) 0.085 (2030)
Lifetime 2–5 years (2015) 2–5 years (2015) 40000 h (2015) 2–5 years (2015)
2–5 years (2020) 2–5 years (2020) 45000 h (2020) 2–5 years (2020)
2–5 years (2030) 2–5 years (2030) 65000 h (2030) 2–5 years (2030)
Market share 48% (2015) 30% (2015) 5% (2015) 17% (2015)
45% (2020) 25% (2020) 15% (2020) 15% (2020)
40% (2030) 20% (2030) 30% (2030) 10% (2030)

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

distribution. More research should focus in this area to overcome these period. Present automotive hydrogen storage tank technology gives a
drawbacks. A major hurdle in the hydrogen economy lies in its transport venting (or “boil-off”) rate of around 1 to 2% per day due to embrit­
and storage. Being an ultralight gaseous molecule, the density of tlement of the tank material to this gas. A schematic of hydrogen
hydrogen at the normal atmospheric condition (20◦ C and 100 kPa liquefaction is presented in Fig. 17.
pressure) is 0.082 kg/m3 and its lower heating value (LHV) is 119.22 Capital costs for processing the liquid hydrogen (LH2) has been a
MJ/kg [323]. Hydrogen has a very low liquid state temperature range critical concern due to higher real energy demand (kWh energy input
profile. It turns into liquid below -253◦ C and is solid below -259 ◦ C, thus per kg of LH2). But if the energy demand drops below 6 kWh per kg of
the boiling point of hydrogen is just greater than >-253◦ C [77]. Unless LH2, then the liquefaction process could be an economic one [329].
cryogenic compression is adopted [324], its boiling point can be Numerous alternatives have been examined to solve the low-density
increased up to -240◦ C by applying a maximum of 1.4 MPa pressure problem of hydrogen like transporting of hydrogen in the liquid state
[77]. The cost of distribution from the hydrogen generating facility to rather than the gaseous state. Liquid hydrogen has a higher energy
the consumers may require about 40-75% of the total hydrogen supply density than that of gaseous hydrogen and is lower than that of the
chain expenses [325]. On-board tanks are a major limitation of using conventional fossil fuels when being transported long distances
hydrogen as an automobile fuel because of low energy density [85]. (Fig. 18). The boiling point of hydrogen is -253◦ C which is very low; as a
Characteristics of different technologies of hydrogen conversion and result, 30-40% of the energy of the hydrogen is needed to liquefy it and
storage are shown in Table 10. additional energy is needed to keep the hydrogen in liquid state during
The carbon footprint of different pathways to hydrogen is presented storing and transporting to reduce the losses. The energy densities of
in Fig. 16. The figure shows that the lowest carbon footprint is from different fuels are presented in Fig. 18.
centralised electrolysis and the highest is decentralised electrolysis. The
figure also shows that the highest carbon footprint comes from trans­
5.2. Hydrogen carriers
formation near market.
Apart from liquefaction of hydrogen, hydrogen can be combined
5.1. Liquefaction of hydrogen with other chemical or element which is called hydrogen carrier and can
be transported. Then the hydrogen can be released from the carrier using
Hydrogen can be compressed into liquid hydrogen at high pressure different technique and use to serve the purposes. The primary candi­
and low temperature. This process requires high energy compared to dates for the role of hydrogen carriers are liquid ammonia, organic
other storage technique. For a variety of reasons, the most favourable molecules (e.g., methanol), liquid organic hydrogen carriers (LOHCs),
option to store hydrogen is as liquid hydrogen (LH2)[81]. Different and metal alloy hydrides. All the candidates contain carbon except
methods are used to produce liquid hydrogen [81], but these are mainly ammonia and metal alloy hydrides. Dibenzyl toluene, which is safe to
categorised as central production and distributed production facilities. use and currently used as heat transfer fluid, can be used as alternatives
The hydrogen liquefaction process consumes a huge amount of energy; to LOHC. However, the production and handling in large quantities is
as a result, the reduction of energy consumption in this process as well as limited except in specified chemical facilities, although it does not pose a
increasing the efficiency will reduce the cost of the entire process [327]. safety issue with strict controls in place. Ammonia can be used as a
The precooled liquefaction method and cascade liquefaction method are hydrogen carrier and can also be cracked into hydrogen as shown in
the two parts of hydrogen liquefaction methods based on the refriger­ Fig. 19. The advantage of using ammonia as a hydrogen carrier are:
ation cycles.
The boiling point of hydrogen is –253◦ C so it is estimated that the • Ammonia is 6 times more compact than hydrogen stored at 20 MPa.
liquefaction of hydrogen requires 40% of the LHV of hydrogen [77]. It • Ammonia requires low pressure storage and transportation.
could be economical to convert the hydrogen into a liquid substance if • Ammonia is free from carbon and high-density storage of H2.
the liquified hydrogen is directly used in the respective applications. At a • Ammonia can reduce the cost of storage and transportation.
temperature of –30◦ C , carbon steel turns brittle and is susceptible to
fracture [77]. At the boiling temperature of hydrogen, air in contact The hydrogen carrier must be generated using a renewable source
with the hydrogen line may liquify which can create a fire hazard. which should not have any carbon footprint for the carrier then it can be
Moreover, hydrogen cannot be kept in liquid form for an indefinite called zero-carbon or very low-carbon, otherwise the overall process

Table 10
Characteristics of different technologies of hydrogen conversion and storage [326].
Application Capacity Efficiency Initial investment cost Lifetime Maturity

Alkaline FC Up to 250 kW ~50%(HHV) USD 200-700/kW 5000-8000 hr. Early market

PEMFC stationary 0.5-400kW 32%-49% (HHV) USD 3000-4000/kW ~60000 hr. Early market
PEMFC mobile 80-100 kW Up to 60% (HHV) USD -500/kW <5000 hr. Early market
SOFC Up to 200 kW 50%-70% (HHV) USD 3000-4000/kW Up to 90000 Demonstration
hr.
PAFC Up to 11 MW 30%-40% (HHV) USD 4000-5000/kW 30000-60000 Mature
hr.
MCFC kW to several More than 60% (HHV) USD 4000-6000/kW 20000-30000 Early market
MW hr.
Compressor 18MPa - 88%-95% USD -70/kWH2 20 years Mature
Compressor 70MPa - 80%-91% USD 200-400/kWH2 20 years Early market
Liquefier 15-80 MW ~70% USD 900-2000/kW 30 years Mature
FCEV on-board storge tank 5 to 6 kg H2 Almost 100% without USD 33-17/kWh (10000 and 500000 units produced/year 15 years Early market
70 MPa compression
Pressurised tank 0.1-10 MWh Almost 100% without USD 6000-10000/MWh 20 years Mature
compression
Liquid storage 0.1-100 GWh Boil-off steam: 0.3% loss per USD 8000-10000/MWh 20 years Mature
day
Pipeline - 95% inc. compression Rural: USD 0.3-1.2 M /km, Urban: USD 0.7-1.5 M/km 40 years Mature
(Depend on diameter)

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 16. Carbon footprint of different hydrogen pathways, redrawn from ref [336 .

Fig. 17. Hydrogen liquefaction process schematic [328].

cannot be called net-zero. The cost of transporting hydrogen in the electrochemical processes. They pointed out that the thermocyclic pro­
liquid form is considerably lower than that of the compressed gas form cess of metal oxide by splitting the NH3 synthesis reaction into two re­
as obtained from the feasibility study [332]. The cost of transporting actions is the promising method for NH3. They also found that the
gaseous hydrogen is £51.85/MWh per 100 miles (£1.73/kg H2) and the electrochemical can integrate renewable energy. This process can
cost of transporting the equivalent liquid ammonia is £6.55/MWh operate at high and low temperature, but high temperature yields more
(£0.22/kg H2) as of 2018; these are operator costs published by the NH3 than low temperature process.
Freight Transport Association [333]. Lamb et al. [334] reviewed the The remnant of ammonia in the fuel supply during the cracking of
ammonia as hydrogen carrier and discuss the decomposition, separa­ ammonia into hydrogen can poison the PEM fuel cell. The palladium
tion, and purification of hydrogen. They found that dense-metal mem­ catalyst based purifying technology of hydrogen is very expensive which
branes are uninhibited by ammonia and can achieve the required needs processing at 600◦ C and 1 MPa, thus the process has not been
product purity but recommended further research for efficiency economically feasible to date.
improvement and cost reduction. Firman et al. [335] reviewed different The metal oxide method can operate at below 300◦ C and 100 kPa,
ammonia production process such as thermochemical and thus reducing the system cost. RenCat technology utilises a cheap mixed

24
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 18. Comparison of energy densities of conventional fuels, redrawn from ref [330,331].

Fig. 19. Schematic process of cracking ammonia into hydrogen.

Table 11
Comparison among hydrogen storage systems [337].
Fuel / Storage Pressure Energy Specific Specific
System (kPa) Density Volumetric cost Energy Cost
(GJ/m3) (US$/m3) (US$/GJ)

Ammonia gas / 1000 13.6 181 13.3

pressurised
tank
Hydrogen / 1400 3.6 125 35.2
metal hydride
Gasoline 100 34.4 1000 29.1
(C8H18) /
liquid tank
LPG (C3H8) / 1400 19.0 542 28.5
pressurised
tank
CNG (CH4) / 25000 10.4 400 38.3
integrated
storage
system
Methanol 100 11.4 693 60.9
(CH3OH) / Fig. 20. Volumetric capacity of different carriers [337].
liquid tank

The volumetric capacities of different hydrogen carriers are pre­

metal oxide-based catalyst to oxidise the remnant of ammonia after sented in Fig. 20. The figure shows that the Mg2FeH6 has the highest
cracking to produce a PEMFC quality H2. The catalyst, which is pending volumetric capacity and hydrogen at 35 MPa has the lowset volumetric
patent, oxidises only ammonia in the H2 stream and thus makes capacity. From the figure it can be concluded that the hydrogen com­
ammonia concentration zero which is called selective ammonia oxida­ bined with carrier has better volumetric capacity than that of hydrogen
tion (SAO)[336]. The schematic of cracking ammonia into hydrogen is alone.
shown in Fig. 19. The differences of various hydrogen storage systems Ammonia Borane (NH3BH3) can be used as a hydrogen carrier. Its
are presented in Table 11. thermal decomposition process and the chemical equations are

25
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Table 12 Table 13
Ammonia borane decomposition of hydrogen [175]. Volumetric hydrogen density of liquid NH3 and H2.
Thermal Chemical Equation Processes Pressure, temperature H2 density of NH3/ H2 density of H2
Decomposition
Liquid H2 Liquid NH3
First step (110 ◦ C) NH3 BH3 →NH2 BH2 + H2 The first yield of
0.1 MPa, -253◦ C 1 MPa, 25◦ C 1.5 times
hydrogen
0.1 MPa, -253◦ C 0.1 MPa, -33◦ C 1.7 times
Second step (125 ◦ C) nNH2 BH2 →(NH2 BH2 )n Intramolecular
1 MPa, -242◦ C 1 MPa, -25◦ C 2.2 times
polymerisation
Third step (150 ◦ C) (NH2 BH2 )n →(NHBH)n + The second yield of
nH hydrogen
Remaining step (500 (NHBH)n →nBN + nH2 generation of excess
Table 14
◦
C) hydrogen
Brief contrast of various CCS processes [345].
Methods of CCS Advantages Limitations
presented in Table 12. For all the CCS Potential reduction of GHG. Storage longevity and
Ammonia has a lower storage cost over 182 days of 0.54 $/kg-H2 in methods sustainability issue.

contrast to hydrogen at 14.95 $/kg-H2. In addition to that, the volu­ 1. Nonbiological

metric energy density of ammonia is high at 7.1–2.9 MJ/L, and ammonia i. Oceanic Less leakage. Larger sinks for Limited efficiency due to
(Direct CO2. injection depth
is easy to produce, handle and distribute. Ammonia has better com­
Injection) requirement. Adversity to
mercial viability than that of pure hydrogen. If ammonia is produced marine biota.
using renewable sources, it will add more advantages [338-340]. ii. Geological Greater sink depth favours Not fully durable to leak
The properties and advantages of ammonia are: CCS stability. free storage. Expensive.
Enhanced oil and natural gas Unpredicted storage
recovery. Tertiary volume.
• Free from carbon and greenhouse gas.
sequestration.
• The energy density of ammonia is 22.5 MJ/kg whereas that of coal is iii. Chemical Longer CO2 storage period and Unsuitable for industrial
20 MJ/kg. lower leakage. applications.
• At normal temperature and 0.8 MPa, ammonia can be compressed to Thermodynamically more Higher temperature
favourable. required to overcome
liquid.
slow reaction rates.
• Existing infrastructure can be used for ammonia storage and 2. Biological
distribution. i. Oceanic Fixing CO2 issues of It can deteriorate the
(ocean Phytoplankton. ecology of the oceanic
The ammonia decomposition process is schematically shown in fertilization) system.
ii. Terrestrial
Fig. 21.
a) Soil carbon The naturally occurring CO2 Various factors relating to
The comparison of hydrogen density is presented in Table 13. As sequestration sequestration can be further the environment limit the
ammonia has more energy than hydrogen, an ammonia tank (1 MPa) modified with efficient effectiveness of the
with a similar volume of hydrogen holds 2.5 times the energy of a ecosystem establishment. process.
b) Phyto- Larger sinks for CO2. Deforestation risk is
hydrogen tank (70 MPa), so a hydrogen tank of 770 L (350 kg) can be
sequestration Economic. emerged. Both the global
substituted by 315 L of ammonia (172 kg) [342]. The natural photosynthesis scale plantation and
can help in CCS. photosynthesis efficiency
5.3. CO2 capture and storage (CCS), utilization (CCU) need to be increased.
3. Engineering With proper management to Expensive and still more
improve the naturally R&D works need to be
Carbon di-oxide produced from the pyrolysis and other processes occurring photosynthetic done to make the process
must be stopped from releasing into the atmosphere. This can be done by process, it can lead to efficient successful. For instance,
capturing the carbon di-oxide and storing carbon di-oxide or capturing CCS. For instance, engineering the engineering microbial
the carbon di-oxide and using caron di-oxide to produce usable chem­ microbial CCS technology has CCS process still require
potential to become great lot of R&D to achieve
icals. Therefore, to claim the processes to produce renewable or cleaner carbon sink [355]. target efficiency [355] .
fuels, the reaction by-products such as CO and CO2 must be captured and
sequestrated from atmosphere. The concentration of CO2 in the exhaust
gas is high, so steam methane reforming (SMR) methods are potential has a different decarbonisation process and the methods have several
candidates for the usage of carbon capture and sequestration (CCS) obstacles of production, cost and the scale of production [344]. The GHG
technology which can decrease 80% of carbon emissions [343]. Various emissions in terms of CO2 equivalent have been found to be varying due
options are available to generate low-carbon hydrogen and each method to variations of processes and feedstocks [216]. CCS technologies can be
divided into biological, non-biological and engineering approaches
[345]. The non-biological process can be categorised as oceanic [346-
348], geological [349,350] and chemical sequestration [351]. The bio­
logical process [352] can be oceanic fertilization [353] and terrestrial
sequestration [354], whereas the latter can be categorised as soil-carbon
sequestration and Phyto-sequestration processes. Table 14 presents a
brief comparison of various CCS processes.
Moreover, Nanda et al. [356] and Al Mamoori et al. [357] reviewed
various CCS technologies and categorised them under three key routes, i.
e., biological (algae and bacteria, dedicated energy crops, and coalbed
methanogenesis), physiochemical (absorption, adsorption, methane
separation, membrane separation and cryogenic distillation), and
geological routes (oceanic storage and biochar). In such classifications,
the physiochemical processes are mainly considered as fast-track tech­
Fig. 21. Schematic presentation of NH3 decomposition [341].
nologies. Biological processes have been recognised as sustainable

26
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

approaches for carbon fixation, but the geological routes may require Apart from the CCS technologies for carbon free energy supply, the
dealing with carbon trading to establish larger industrial applications captured CO2 can also be utilised as raw materials in several industries.
[356]. Regardless of the CCS technologies, the main stages of CO2 cap­ The technology that assesses the quality of produced CO2 from various
ture are at pre-combustion (about 15-40% v/v CO2 can be captured), sources and reprocessing [312,365,366] the CO2 into usable feedstock
post-combustion (about 4-14% v/v CO2 can be captured) and oxy-fuel for respective industries is known as the carbon capture and utilization
combustion (about 80% v/v CO2 can be captured) [356]. All of these (CCU) process. Ho et al. [366] observed that the established CCU tech­
options and processes have their individual challenges and advantages, nologies require a highly purified and uniformly pressurised CO2 supply
but mostly include the increasing expenditure due to higher capital cost, in order to use that as feedstock in the respective industries. These au­
scalability, energy penalty, integrating and coordinating the auxiliary thors have reviewed various publications and listed various sources of
process plants, air capture technology implementation and analysis of CO2 gas along with its percentile quantity of purity (varying between
the combustion stream yields [351,356,358]. 7.4% and 90%) in the emitter gas mixtures. The carbonated beverage,
Typically, CO2 sequestration requires various steps involving the enhanced coalbed methane recovery, methanol, methane, refrigerant,
capture, pressurization, carrying, and pumping of liquid CO2 (L-CO2) and decaffeination agent require 99% pure CO2, while the required
below the ocean or into geologic developments. At normal temperature pressure can vary from 0.04-300 bar atmospheric pressure condition as
and pressures in the ocean, CO2 will remain as a gaseous form at less per the respective process requirement. On the other hand, enhanced gas
than 500 m depth and in the liquid form at more than 500 m depth. The recovery, urea production and mineral carbonation require 99.9% pure
L-CO2 will float in ocean water at a depth down to 3000 m and will sink CO2 with pressures ranging between 1 and 150 bar atmospheric pres­
below that depth because L-CO2 has relatively low density (Fig. 22). sure. Moreover, CO2 can also be used at atmospheric pressure in various
The CO2 Lake idea shown in Fig. 22 may require further innovative industries. Pure CO2 obtained through the CCU technologies integrated
technology and higher expense, but the lake can possibly reduce the with the plastics (e.g., polycarbonate production) industry [367],
seepage to the atmosphere and exposure to biology. The economics of petrochemical (e.g., methanol, methane) synthesis industry [368,369],
CO2 sequestration related to hydrogen generation from fossil fuels are biofixation of CO2 though microalgae cultivation [370,371] with the
reported in the literature [348,359,360]. Some challenges [344] are still potential of developing biorefineries, building materials and carbon
there for CO2 sequestration: (i) reduction of cost; and (ii) Technical fibre industries [372,373] all use atmospheric pressurised CO2. Purifi­
knowledge of the artificial lake (e.g., volume, longevity, and, most cation and pressurization of CO2 feedstock to a reprocessing industry
importantly, ecological impact). The crucial risk issues are vague pro­ may cost about 70-75% of the overall process where this raw material is
longed environmental effects, catastrophic release of CO2 from the un­ used, but it is highly essential to valorise this compound to establish
derwater sink due to oversaturation, and the substantially reduced pH circular economy [374,375]. With proper CCU policies, the circular
value of sea water surrounding CO2 disposal under the sea. More R&D economy market may grow to 700-800 billion USD by 2030 which may
advancement is required to resolve these issues. Environmentally consume about 15% of the present worldwide annual CO2 emissions
“benign” sequestration (EBS) of CO2 [344,361], mineral carbonation [373].
[362], enhanced oil recovery (EOR) [361] and mixing liquid-carbon Carbon dioxide from atmosphere can also be used to produce e-fuel
dioxide [363,364] with pulverised limestone and water to produce with hydrogen and the e-fuel can be used in transport sector such as
aqueous Ca2+ and HCO-3 (Eqn. (36)) will all lessen the environmental vehicle in road, ship in sea, and airplane in air. The produced e-fuel is
effects (e.g., pH change) of dumping CO2 into the ocean as well as carbon neutral because it will release the carbon during the combustion
enhance the average specific gravity of the system to 1.4g/ml, thus which is captured to produce e-fuel. So, the overall carbon di-oxide in
enabling CO2 disposal at ocean depths higher than 1000 m. the atmosphere will not be increased due to combustion of e-fuel and
hence the e-fuel can be termed as carbon neutral which can solve the
CO2 (l) + CaCO3 (s) + H2 O(l) ↔ Ca2+ (aq) + 2HCO−3 (aq) (36)
issue of hydrogen storage partly [376] .
It has been projected that there may be about 850 CCS projects
around the world by 2030, and about 3,400 by 2050 (Fig. 23) in order to 5.4. Energy conversion and storage development
capture the targeted amount (about 10 Gt) of CO2 from various energy
sectors [358]. As a result, it has been projected that it may require about Public and private sector should focus on the development of
5070 billion USD of cumulative investment between the period of 2010 hydrogen storage because hydrogen economy cannot progress without
and 2050 [358]. proper hydrogen storage. The International Energy Agency (IEA)
Hydrogen TCP consists of 31 members including 23 countries, the
United Nations Industrial Development Organization (UNIDO), the Eu­
ropean Commission, and 6 sponsors. The ongoing task of the IEA
Hydrogen TCP is to develop reversible or regenerative hydrogen storage
materials. An IEA Hydrogen TCP task force identifies that storage ca­
pacities and operating temperatures depend on the application types
such as mobile systems, transport and delivery stations, or storage sys­
tems. The development of improvement of materials is linked with the
understanding of hydrogen storage mechanism. Fig. 24 shows the
mechanism of hydrogen storage development [377].
The main challenge of hydrogen after production is storage and
transport. Hydrogen has very low boiling temperature which makes it
difficult to store. After storing hydrogen for long time at low tempera­
ture, the container material degrades and make it difficult for further
storage. The research and development of storing hydrogen as liquid
organic hydrogen carriers (LOHCs), metal alloy hydrides, and liquid
ammonia is on. If a suitable solution of storing hydrogen is found, then
the hydrogen will jump a big leap in its way to be the alternative of fossil
fuel.

Fig. 22. H2 production via SMR process and CO2 sequestration options [344].

27
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 23. Projection of global CCS deployment projects by 2050 in various energy sectors [358].

(H2O2), analytical chemistry (e.g., atomic absorption spectroscopy, gas

chromatography), aerospace, electronics (semiconductors), weather
balloons, and fuel for rockets as well as the transport industry (e.g., fuel
cells, and liquid hydrogen combustion) [332,378-380]. In transport
sector, hydrogen will be used in hydrogen fuel cell and in internal
combustion engine but in energy sector, hydrogen will be used as energy
carrier. Capital costs, operating costs, infrastructure requirements, range
to refuelling, refuelling facilities, refuelling time, safe life expectancy of
alternative options, emissions, space restrictions, and overall safety
concern are the key factors to facilitate the implementation of hydrogen
fuel as an alternative fuel for the transport sector [318,381,382]. The
performance of the hydrogen systems in the transport sector is presented
in Table 15.
Source: Technology Roadmap Hydrogen and Fuel Cells [382].
The cost of fuel cell is higher as compared with the vehicle with ICE.
The lifetime of fuel cells is only 150000 km with a cost of $60-100k.
People will not buy the FCEV unless the price is reduced, and the lifetime
is prolonged. Therefore, there is little chance of replacing ordinary
vehicle with the fuel cell vehicle very soon.

6.1. Hydrogen as fuel in internal combustion engines

Hydrogen can be used directly or as a mixture with fossil fuels into

Fig. 24. Development strategy of hydrogen energy storage and conversion the internal combustion engine. The application of hydrogen into the
using reversible or regenerative hydrogen storage [377]. internal combustion engine requires the hydrogen storage and trans­
mission. Moe research of hydrogen in internal combustion engine is
6. Applications of hydrogen in different sectors needed to investigate the characteristic of hydrogen combustion in ICEs.
Peeters [383] has investigated the performance of hydrogen in ICE using
There are extensive use of hydrogen as raw material for fertiliser CFD modelling. He found that the combustion is very fast and severe
production (i.e., ammonia (NH3) production), petroleum refineries, because of high flame speed when hydrogen was used and hence the
methanol (CH3OH) production, reducing agent for metal (steel, pressure peaks is high with low thermal efficiency. He suggested and
aluminium) ore processing and manufacturing of glass, hydrochloric found that by optimising the hydrogen injection, combustion can be
acid (HCl) production, food industries (e.g., hydrogenation of oils or slowed down, resulting in lower pressure peaks, lower NOx emissions
fats), atomic hydrogen welding (AHW), coolant, hydrogen peroxide and higher power outputs can be achieved. Shi et al. [384] conducted
CFD simulation with hydrogen-enriched fuelled into proposed

Table 15
Performance of hydrogen systems in the transport sector [382].
Application Capacity Energy efficiency* Investment cost** lifetime Maturity

Fuel cell vehicle 80-120 kW Tank to wheel 43-60% (HHV) $ 60 K-100 K 150,000 km Early market
Hydrogen retail station 200 kg/day -80% inc. compression to 70 MPa $1.5-2.5 M - Early market
Tube trailer (gaseous) Up to 1000 kg -100% (without compression) $1M ($ 1000 per kg payload) - Maturity
Liquid tankers Up to 4000 kg Boil-off stream: 0.3% loss per day $ 75 K - Maturity
• Lower heating value*, $=USD
• All power specific investment costs refer to the energy output
**
• HHV-higher heating value

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M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

modification of rotary engine by configuring the turbulence-induced- MPa bar) hydrogen in comparison to the standard percentile energy
blade (TIB) and observed that NOx emissions can be reduced with production of gasoline combustion. Volumetrically at normal gaseous
trailing TIB while the combustion is better in leading TIB. Hence the condition, about 29.6% hydrogen occupies the combustion chamber in
authors have proposed for more parametric investigations to find out the comparison to that of 1-2% for gasoline [185] as calculated in Eq. (35).
optimised operating conditions for hydrogen enriched combustion along The density (kg/m3) of hydrogen varies with the applied pressure, e.g.,
with minimal NOx emissions. 0.0838 kg/m3 (1 atm), 23.65 kg/m3 (35 MPa), 39.69 kg/m3 (70 MPa)
Traditional internal combustion engines can be customised to oper­ and 72.41 kg/m3 (liquified) [393]. On the other hand, the densities of
ate on pure hydrogen (‘HICEs’) and may possibly see early imple­ gasoline, diesel, LPG and natural gas are about 745 kg/m3, 845 kg/m3,
mentation because they are significantly less expensive than fuel cells. 541 kg/m3, and 0.77 kg/m3 respectively at normal temperature and
Hydrogen can be used all types of internal combustion engines. Besides, pressure (STP) [393]. While comparing the density of the fuels,
the HICEs can adopt all the available advanced automotive engine hydrogen demonstrates better efficacy in terms of energy production per
combustion technologies like direct injection (DI), turbocharging, pre- unit weight equivalent to the gasoline fuel. It also shows that, to gain
chamber ignition, Miller cycle, lean combustion, cylinder deactivation, more energy efficiency from combustion of hydrogen fuel in the internal
start-stop of the engine, and modern transmission system advancements combustion engines, highly compressed hydrogen offers higher energy
which have proved significant improvement in reducing emissions while efficiency as shown in Table 16.
reducing fuel consumption [385]. The release of NOx and lower fuel
%H2 = [volumeH2 ]/[(volumeair + volumeH2 ) = [2/(4.762 + 2)] × 100%
efficiency (~30%) for combustion of hydrogen in the ICEs may impede
the acceptance of HICEs for long term usage in transport [118]. But, = 29.6%
various exhaust treatments like exhaust gas recirculation (EGR) (37)
[386,387], leaner NOx trapping with 3-way catalytic converter (TWC)
E-fuel can be an alternative fuel to internal combustion engine which
[388,389], selective catalytic reduction (SCR) [390,391], and pre-intake
can be used in transport sector such as road vehicle, aviation, rail, and
as well as in-cylinder control strategies [386] can effectively help to
sea-transport. E-fuel can be produced by trapping the CO2 from atmo­
reduce NOx emissions from a HICEs. Hydrogen can be mixed with diesel
sphere or emission from industry to produce oligomeric oxy-methylene
in dual-fuel vehicles or natural gas (‘hythane’) or in bi-fuel to switch
dimethyl ether (OME). Sai et al [376] suggested that E-fuel produced
between them. The retrofitting of hydrogen into the existing engine will
using the mechanism Fischer Tropsch reaction can alleviate the issue of
permit the use of the present infrastructure available. Although the
storing hydrogen. The authors also presented a comparative analysis of
blending of hydrogen is not a net-zero emission outcome, it is an option
combustion in ICE of OME1 fuel blended with diesel with that of the
to lower carbon emissions [381]. The benefits of the addition of H2 can
diesel and other fuels. They found that the OME blended with diesel has
be summarised as follows [317]:
favourable characteristic such as less soot production. Although there is
less maximum rate of energy conversion due to low energy density of e-
• Enhance fuel octane number.
fuel, this can be overcome by mixing with high energy density fuel.
• Engine performance increases with the increase of octane number.
• Engines size can be reduced with more efficient.
6.2. Hydrogen fuel cells in vehicles
• Facilitates ultra-lean burn.
• Up to 30% increase of engine efficiency.
Hydrogen can be used in hydrogen fuel cell to produce electricity and
this electricity can used in different applications such as electric vehicle
The stoichiometrically calculated air-fuel ratio (A/F ratio) for the
and industry and domestic uses. The fuel cell can be installed in vehicle
hydrogen combustion engine is almost 34.3:1 (w/w) (2.4:1 v/v),
and produce electricity to run the vehicle. Fuel cells are considered for
whereas, the gasoline engine is 14.7:1 (w/w) and a diesel engine is
use in fuel cell vehicles and fuel cell systems are for stationary appli­
14.5:1 (w/w) [77]. Due to diverse flammability, hydrogen can burn at a
cation in large central power stations and distributed power generators.
higher A/F ratios like 180:1 as well [77]. This indicates that more air is
This system can be used for residences and buildings in urban and
required to burn the amount of hydrogen drawn into the combustion
remote areas [128]. Power plant companies are exploring the usage of
chamber than for other vehicular fuels. At stoichiometric combustion
solid oxide fuel cells (SOFCs), phosphoric acid fuel cells (PAFCs), and
conditions, 2 moles H2 and 1 mole O2 are required to generate 2 moles
molten carbonate fuel cells (MCFCs) [128]. Central power plants use the
H2O to produce about 0.572 mega joules (MJ) of energy [392]. Table 16
higher temperature fuel cells (MCFCs and SOFCs) and the smaller power
briefly presents the amount of hydrogen being drawn into the combus­
generators mainly use polymer electrolyte fuel cells (PEFCs) for resi­
tion chamber at various metering conditions, i.e., gaseous hydrogen at
dential units and in automobiles for propulsion power. A comparison of
normal pressure, liquid hydrogen and very highly compressed (~70
emissions from BEV (Battery electric vehicle), FCEV (Fuel cell electric

Table 16
Comparison of volumetric efficiency and energy output between gasoline and hydrogen fuelled engines [77].
Types of fuel

Fuel 17 cc 300 cc 405 cc 420 cc

Air 983 cc 700 cc 965 cc 1000 cc
Energy (%) 3.5 kJ (100) 3.0 kJ (85) 4.0 kJ (115) 4.2 kJ (120)

29
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 25. Comparison of emissions for different vehicle [377,382].

vehicle), plug in hybrid vehicles (PHEVs), and ICE vehicles is presented energy content of hydrogen (EH) is estimated using the volume or mole
in Fig. 25. fraction of methane (VM) and hydrogen (VH) as in Eqn. (38).
Fuel cell can be a good option to use hydrogen to generate electricity
11.88VH
which can be used to run the vehicle as electric vehicle. Unlike hydrogen EH = (38)
11.88VH + 39.05VM
combustion in ICE, hydrogen can be used to generate electricity using
different types of cells. Using hydrogen in fuel cell will help reduce the where 11.88 is the volumetric HHV (MJm-3) energy content of hydrogen
emission into atmosphere and fulfill the goal of carbon neutral in 2050. and 39.05 is the volumetric HHV (MJm-3) energy content of methane
To get traction in the market, the price, durability and availability of fuel [393], and VM = 1 -VH. From Fig. 26 it can seen that mixing of 20%
cell must be arranged. Not many car makers are manufacturing the fuel hydrogen by volume will save 13 gCO2 per kWh [118]. Various issues
cell vehicle and the vehicles are not easily available worldwide. must be considered to evaluate the safety matters linked with mixing
Currently, BEV and hybrid car is available in the market but no car with hydrogen into the present natural gas pipeline network to be used at the
fuel cell is readily available in the market. consumer end [118,395].
Hydrogen can used with the existing fuel such as natural gas and
6.3. Other progressive industries transmitted through natural gas network by mixing with them with
certain proration. Due to policy and regulation for safety, there are
Hybrid systems which consist of hydrogen technologies (fuel cells, limitation of hydrogen amount to mix with natural gas. Mixing hydrogen
hydrogen stores, and electrolysers), integrated with battery or wind/ with natural gas will save carbon emission into the atmosphere.
solar power backup, are recommended to fulfill the growing demand for Netherlands and Germany are using high volume of hydrogen into the
power in telecom base stations in rural and remote areas [394]. The grid and other country should follow their footsteps to increase the
hybrid systems can meet the demand of growing power in the telecom percentage hydrogen in the grid.
system by shipping hydrogen to remote areas.
Hydrogen can be mixed or injected into the gas network safely with 7. Economic analysis of H2
small quantities because the safe amount of hydrogen is limited by
administrative and technical constraints [118,395]. Hydrogen has a The cost of producing hydrogen gas through a specific process is the
lower energy density than methane, as a result, a 20% blend of hydrogen combination of investment expenditures, operation expenses, and
with natural gas is equivalent to 7% hydrogen by energy content. The maintenance costs. The factors involved in the cost of hydrogen are

Fig. 26. (a) Percentage of Hydrogen in the grid around the world; and (b) Hydrogen fraction, energy content and carbon saving [118].

30
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Table 17
Comparison of cost of production of hydrogen with different methods [399].
Process Energy Feedstock Capital cost Hydrogen
source (M$) cost ($/kg)

SMR with CCS Fossil fuels NG 226.4 2.27

SMR without Fossil fuels NG 180.7 2.08
CCS
CC with CCS Fossil fuels Coal 545.6 1.63
CG without Fossil fuels Coal 435.9 1.34
CCS
ATR of Fossil fuels NG 183.8 1.48
methane
with CCS
Methane Internally NG – 1.59–1.70
Fig. 27. Factors involved in overall cost of hydrogen for the consumers, pyrolysis generated
redrawn from ref [396]. steam
Biomass Internally Woody 53.4–3.1 1.25–2.20
pyrolysis generated biomass
presented in Fig. 27.
steam
With Biogas as feedstock for hydrogen production, if the payback Biomass Internally Woody 149.3–6.4 1.77–2.05
period is considered as equivalent to 8 years of production period, it may gasification generated biomass
cost about 0.27 US$/kWh [397]. Wang et al. [398] have carried out steam
techno-economic (i.e., energy efficiency, capital investment expendi­ Direct bio- Solar Water+ 50 $/m2 2.13
photolysis algae
tures, consumption of raw materials, costs related to regular production Indirect bio- Solar Water + 135 $/m2 1.42
and cost incurred due to carbon emission or management) performance photolysis algae
analysis between coal-to-hydrogen (CTH) processes and biomass-to- Dark – Organic – 2.57
hydrogen (BTH). Both the CTH (37.82%) and BTH (37.88%) have fermentation biomass
Photo- Solar Organic 2.83
comparable energy efficiency. The BTH process has about 70.92% more
–
fermentation biomass
material consumption than that of CTH (6.43 tonnes/tonnes of H2) for Solar PV Solar Water 12–54.5 5.78–23.27
production of per tonne of hydrogen gas which indicates the possibility electrolysis
of higher CO2 emission from the BTH process. In fact, the BTH process Solar thermal Solar Water 421–22.1 5.10–10.49
emits about 6.7% less CO2 than that of CTH (16.39 tonnes CO2-e/tonne electrolysis
Wind Wind Water 504.8–499.6 5.89–6.03
of H2) due to the variation of composition of the raw materials. How­ electrolysis
ever, the higher raw material costs of the CTH process (51.4% of total Nuclear Nuclear Water – 4.15–7.00
production expenses) can make the BTH process (44.7% of overall electrolysis
production cost) competitive enough to be chosen for industrial devel­ Nuclear Nuclear Water 39.6–2107.6 2.17–2.63
thermolysis
opment. From another point of view, biomass resources could be mainly
Solar Solar Water 5.7–16 7.98–8.40
consisting of green wastes, but coal mining is related to fugitive emis­ thermolysis
sions as well in addition to its own carbon content emission during the Photo- Solar Water – 10.36
hydrogen production process. The total capital investment cost of the electrolysis
BTH process was 19.24% higher than that of CTH (141.8 M US$). Both
the processes have very high amounts of energy losses which need to be
improved. In the case of hydrogen gas recovery and purification from Table 18
the mixture of gases, the cost of the PPU is linked to the efficiency of the Techno-economic analysis of ALK and PEM electrolysers [326].
hydrogen production process [302]. When biomass gasification is per­ Technology ALK PEM
formed, the steam to biomass ratio (S/B) variation between 1 and 1.5 in
Year 2017 2025 2017 2025
addition to the remarkable reduction of PPU cost can lead to fluctuation
of H2 gas in a decentralised station of about 12.75-9.5 €/kg [302]. The Efficiency, kW /kg of H2 51 49 58 52
Efficiency (LHV) % 65 68 57 64
cost of production of hydrogen using different methods are presented in
Lifetime stack Operating hours 80,000 hr. 90,000 40,000 50,000
Table 17. The lowest production cost per kg of hydrogen is CG without hr. hr. hr.
CCS and the highest is photo-electrolysis. The highest capital cost is CAPEX-total system cost (inc. 750 480 1200 700
nuclear thermolysis and the lowest is biomass pyrolysis. power supply and system
The techno-economic analysis of ALK and PEM electrolysers is pre­ costs) €/kW
OPEX % of initial CAPEX/Year 2% 2% 2% 2%
sented in Table 18. The efficiency of ALK and PEM are almost similar. CAPEX-Stack replacement 340 215 420 210
The total system cost of ALK is lower than that of PEM, but the system €/kW
lifetime is same. Typical output pressure kPa Atmospheric 1500 3000 6000
Many factors contribute the cost of hydrogen which includes the System lifetime Years 20 20
feedstock, production, storage, distribution, transmission, and other
factors as can be seen in Fig. 27. The crucial challenge for hydrogen is to
development stage but has high possibility of improvement with lower
reduce the price of the hydrogen. The highest cost of hydrogen is the
capital cost.
hydrogen from SMR with CCS. The low-cost hydrogen can be produced
from CC with CCS, CG without CCS and indirect bio-photocatalysis.
8. Technology and commercial readiness of H2 production
Nuclear thermolysis is the most expensive to install to produce
hydrogen and the cost of hydrogen is $2.17-2.63 per kg of hydrogen. The
Technology readiness levels (TRLs) and Commercial readiness in­
cost of hydrogen from electrolysis process is costlier than other process
dexes (CRIs) are two globally accepted benchmarks that express the
but the positive side of electrolysis process is that they use water as
maturity of technologies. TRLs elaborate the growth and continuing
feedstock. ALK and PEM electrolyser are also used for hydrogen pro­
development of a particular technology through the initial phases of the
duction, and they are in commercial sate. SO is still in research and
innovation chain and CRIs provide a ranking to evaluate industrial

31
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

Fig. 28. TRL and CRI levels, redrawn from ref [343].

Table 19
Recent status of key technologies for hydrogen generation [343].
Applications Capacity Efficiency Initial investment cost Lifetime Maturity

Large scale SMR 150-300 MW 70-85% (LHV) 400-600 USD/kW 30 yrs. Mature
Small scale SMR 0.15-15 MW ~51% (LHV) 3000-5000 USD/kW 15 yrs. Demonstration
AE electrolyser Up to 150 MW 65-82% (HHV) 850-1500 USD/kW 60,000-90,00 hrs. Mature
PEM electrolyser Up to 150 MW (stacks), and up to 1 MW 65-78% (HHV) 1500-3800 USD/kW 20,000-60,000 hrs. Early market
SO electrolyser Lab scale 85-90% (HHV) - ~1000 hrs. R&D

9. Discussion and challenges for hydrogen

Table 20
TRL of different hydrogen production process [400].
Although hydrogen has huge potential to replace conventional fossil
Hydrogen production technologies TRL Feedstock fuels, there are many challenges that need to be addressed so that pro­
Alkaline electrolysis 9 H2O + electricity cess evolves smoothly. This section has presented the current challenges
PEM electrolysis 7–8 H2O + electricity for hydrogen as a replacement to existing fuels.
Solid oxide electrolysis 3–5 H2O + electricity + heat
Current capability of hydrogen production, storage and supply
Biomass gasification 4 Biomass + heat
Biological 1–3 Biomass + microbes (+ light)
worldwide is far less than what will be required in future [401].
Photoelectrochemical 1–3 H2O + light Hydrogen related infrastructure including production, storage, and de­
Thermochemical 1–3 H2O + heat livery to end users must be developed based on the demand. The
infrastructure for hydrogen transportation, storage and delivery are not
fully developed anywhere in the world to cope with the demand that will
barriers and risk which facilitates funding decisions to decrease risks and
arise when vehicles are progressively converted to be powered by
impediments at the different phases moving towards commercialization.
hydrogen fuel or fuel cells. There are very limited numbers of hydrogen
When a technology attains the demonstration and implementation level,
refuelling stations worldwide as compared to conventional fuel stations.
then a set of separate elements are introduced to help in determining the
Current hydrogen storage systems are not adequate to meet the de­
commercial readiness of a technology or project. Details of the TRLs and
mand for the weight and volume of storage of hydrogen which are high
CRIs are presented in Fig. 28 [343].
as compared to conventional vehicle fuel storage, so novel hydrogen
Applications and the status of different technologies are presented in
storage development is imminently required. Ammonia can be a good
Table 19. The technology readiness levels of different production pro­
carrier for hydrogen, but the ammonia-based energy system has many
cesses are presented in Table 20.
challenges to face. Ammonia is produced worldwide using fossil fuels
SMR process has the highest lifetime than that of the other tech­
and, as a result, production of carbon-free ammonia by synthesis is a
nology. This technology requires less initial investment than other
difficult area which needs extensive research and renewable energy
technologies. SMR technology is not completely sustainable process
utilization in a suitable way can assist to solve this issue [402]. The
because it produces CO2. AE and PEM electrolysis technology is
sustainability of ammonia should be measured to identify the applica­
expensive as compared to SMR. AE is matured technology but PEM and
bility of ammonia as developed by Liu et al [402]. However, current
SO is in nascent state in the technology readiness level. Electrolysis
research and development in this area have mainly been aimed towards
processes use water as their feedstock which is renewable, abundant,
the improvement of small and medium size devices such as reciprocating
and does not produce GHG therefore this process can be termed as
engines. In addition to that, toxicity of ammonia is another major barrier
sustainable if the energy is taken from renewable sources such as wind
which is holding back the application of these technologies [342].
and solar. Among all technology presented in this study, Alkaline and
Cost of hydrogen is another factor that needs to be addressed to make
PEM is at the top of the TRL chart as shown in the Table 20 but more
hydrogen a viable alternative to fossil fuels. The price of hydrogen is
research is needed to reduce the initial cost and the durability of the
higher than that of conventional fuel, so it is critical that it be reduced.
process.
Hence factors that contribute to the overall price must be improved such

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as the development of efficient and low-cost solar energy production. solar energy production for hydrogen, nanoscale catalyst development,
Public acceptance and awareness are other issues that can impede development of novel hydrogen storage materials, low-cost and durable
the use of hydrogen. For any technology to flourish, it first needs to be cathodes in fuel cells at low temperature, and a thorough study on
accepted by the general community. This will require a rigorous public hydrogen safety.
awareness program, and this must include details of the relevant health The following research should be carried out in the near future;
and safety analysis as well as a review of legislation so that all these
issues are able to be addressed. • Maturity of hydrogen conversion technology.
Hydrogen related techno-economic feasibility is a big challenge for
hydrogen. The affordability of the technology of green hydrogen pro­ Many technologies available for hydrogen production such as PEM
duction is another factor. The cost of the integrated system that will and SO are in an initial stage or in the pilot scale. More research is
produce the green hydrogen should be within the reach of the potential needed to scale up production into industrial scale which can bring
users within the community. down the cost of hydrogen.
Agyekum et al [403] reviews the hydrogen production and identified
the factor affecting their scale up. They identified, storage and trans­ • Transition of ICEs from fossil fuel to hydrogen fuel in vehicles.
portation of hydrogen, high cost of production as factor affecting their
scale up and role in future energy which has been mentioned above. In ICEs are mainly designed to run with fossil fuels as a result hydrogen
addition to that, they also identified that the absence of a value chain for in ICEs poses many challenges which opens opportunity to carry out
clean hydrogen, lack of international standards, risks in investment, and research in this field. Among the challenges, the crucial points to note
flammability are factor that affecting their scale up and role in future are back-fire, low output power, high NOx production, pumps capable of
energy. delivering liquid hydrogen, spark plug to deal with hydrogen, and
electrode compatible with hydrogen. Most of the research is focused on
10. Conclusions and recommendations the hydrogen production and virtually no research and development
have been carried out to design the ICE suitable for hydrogen fuel.
This paper presents and discusses hydrogen as energy carrier by Therefore, research should be focused on the compatibility of ICE or new
considering various aspect of hydrogen such as production, storage, design of ICEs to suit the hydrogen fuel from the storage tank in vehicle
transmission, and distribution. Literature reviewed indicates that to the combustion chamber in engine.
hydrogen has been used in different sectors for a century and recently
the application of hydrogen as a fuel or energy carrier has gained mo­ • Reduction of production and delivery cost of hydrogen.
mentum due to the concern of global warming and greenhouse gas
emissions which produce adverse effects on the climate. To produce The overall cost of hydrogen is higher than that of the fossil fuels.
hydrogen, different energy sources can be used including renewable More search is needed to innovate the low-cost production of hydrogen
energy, natural gas, oil, nuclear energy, and coal but to claim green and delivery. The cost of production of hydrogen is currently in the
hydrogen the whole process of production should be carbon neutral. range of $4-7/kg H2 excluding the cost of network and delivery system.
Hydrogen gas can be transported by pipelines and liquid hydrogen can The delivery system requires high pressure compressor, storage tank and
be transported by ships, but a complete network of hydrogen trans­ colling system. To bring the price down, more research is needed around
mission, storage, distribution, and delivery is very important to meet the low-cost and efficient separation and purification, development of
global demand which is long way to achieve. Hydrogen can be used to catalyst for low temperature, durable and less expensive membrane,
produce electricity and can be transformed into methane. Methane can develop low cost photocatalyst and electron transfer catalyst.
be used for domestic power and as feedstock for industry and used
directly as fuel in vehicles and planes. Many countries have invested • Reduction of cost of fuel cells and enhance their performance.
billions of dollars in this field to produce green hydrogen but still need
more funding and support from government to achieve 2050 carbon Different types of fuel cell have different shortcomings which need to
neutral goal. be overcome. For automotive application, the power density of fuel cell
The cost of hydrogen is expected to reduce to the desired 1-2 $/kg of needs to improve. The cost of fuel cells is higher compared to ICE as well
H2 from current 4-7 $/kg H2. Hydrogen has the best energy-weight ratio as the durability is less than that of ICE. The cost of fuel cells come from
120-142 MJ/kg among all conventional fuels. Among all hydrogen the expensive material used as well as the production of fuel cell stack.
production process, SMR process has about 48% share of global The chemical stability and conductivity should be improved to enhance
hydrogen production with highest efficiency of 70-85% with lowest the durability and performance of the fuel cell.
production cost. Solid oxide fuel cell (SOFC) technology of hydrogen
conversion and storage has highest efficiency of around 70% but this • Overcoming the shortcomings of hydrogen storage.
technology is in demonstration stage. Metal hydride storage system cost
less (125 $/m3) as per specific volumetric cost and ammonia has lowest The main challenge of hydrogen as a substitute fuel is storage.
specific energy cost (13.3 $/GJ) among all storage system. Hydrogen needs extremely low temperature to store which make it
A widespread application of hydrogen as fuel for the transport sector difficult to store, carry and deliver. Novel storage technology is immi­
and as feedstock for the chemical industry is imminent and to cope with nent to develop. Research should be carried out to develop novel storage
this expected future demand, a complete infrastructure of hydrogen devices to cope with the challenging of hydrogen storage.
from production to delivery is needed. Different countries are formu­
lating policies and enacting laws to include hydrogen as fuel in the • Developing infrastructure to cope with hydrogen transport, distri­
transport and energy sectors to achieve net-zero emission within a bution, and storage.
decade and hence more research and development funds are needed in
this field. Technological advancement of hydrogen production, storage, The current infrastructure around the globe is not sufficient to cope
distribution, and transmission is essential which needs support from with the overall energy demand in the world. Hence, worldwide
government. network of hydrogen is important to meet the demand around the world.
More research is needed to make hydrogen versatile and available There are no details outline as well as the infrastructure for hydrogen
worldwide to be used in different sectors as an alternative to conven­ transport, distribution, and storage. More research should be carried out
tional fuels. These include, but are not limited to, low-cost and efficient for the development of efficient transport, distribution, and storage.

33
M.G. Rasul et al. Energy Conversion and Management 272 (2022) 116326

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