B.S.

Zainal et Renewable and Sustainable Energy Reviews 189 (2024)

al. 113941
Table 3 (continued )

Ref. Title Summarised Review Scopes Categories Findings

decarbonisation strategy, technological

mandates, and certification systems.
[40] “Global hydrogen development - A The paper covers the various technical and The technical advancements include
technological and geopolitical overview” technological developments related to purification, compression, transportation,
hydrogen production, refinement, and usage. Green hydrogen’s
compression, storage, transport, and competitiveness depends on extensive
utilisation. development of renewable energies, such
as
creating large-scale facilities for
manufacturing electrolysers and fuel cells
and establishing hydrogen refuelling
stations, particularly for transportation
needs.
[41] “Global warming consequences of It aims to measure the impact of hydrogen Potential benefits and Replacing natural gas with green
natural gas with hydrogen in the domestic
replacing an
as indirect GHG on the distribution of challenges of using green in domestic sectors could substantially
hydrogen
energy sectors of future low-carbon methane in the troposphere and hydrogen reduce the consequences of global
economies in the United Kingdom (UK) tropospheric
ozone. warming, provided hydrogen leakage is
and
the United States” minimised.
[42] “Cost-benefit analysis, levelised cost of The article discusses the concept of cost- Feasibility and Biohydrogen’s economic viability is less
energy (LCOE), and evaluation of financial benefit analysis and uses the LCOE to sustainability of sensitive to the cost of biomass feedstock
feasibility of full commercialisation of evaluate the economic viability of biohydrogen but more sensitive to capital and
biohydrogen” biohydrogen technology. The authors commercialisation operational expenditures.
examine the costs of biohydrogen
production,
storage, and transportation and compare
them to the potential benefits.

Fig. 4. Fossil fuel-based production technologies.

material is crucial in determining the efficiency of the SR process, with a Despite significant progress, industrial SMR remains inefficient
higher ratio resulting in lower carbon dioxide emissions. A membrane regarding energy integration, intensity, and GHG emissions due to the
reactor can be used instead of traditional reactors to complete the re- complex multistep process. Therefore, there is a high demand for
action [66]. In industrial applications, the thermal efficiency of developing technologies to produce blue hydrogen instead of grey
hydrogen synthesis through the SMR is typically around 70–85 % [67]. hydrogen using CCSU technology, making the process economically
SMR is a vital process for producing hydrogen, which has many uses, viable. Several new technologies have been proposed to improve the
including fuel cell power generation, fuel for transportation, and as a efficiency of SMR while reducing energy input and GHG emissions [70].
chemical feedstock for producing ammonia and other chemicals. It is These technologies include:
also used to produce carbon monoxide, which can be used as a feedstock
for many chemicals, including methanol and acetic acid. SR has the • Membrane technology: This technology uses membranes to separate
potential to achieve both the highest feasible hydrogen efficiency and hydrogen from the other SMR reaction products, allowing for a more
the lowest possible carbon monoxide content. Meanwhile, steam refor- efficient process [71].
mation is an endothermic process that requires much energy. The per- • Sorption-enhanced steam methane reforming (SESMR): This tech-
formance of traditional steam reformers is hindered by the low nology uses sorbents to capture carbon dioxide from the SMR reac-
effectiveness of pelletised catalysts, which typically have an efficacy tion, which reduces GHG emissions [72].
factor of less than 5 % due to limitations in mass and heat transport [68]. • Chemical looping steam methane reforming (CL-SMR): This tech-
As a result, kinetics is a limiting factor in traditional steam reformer nology uses a cyclic process of oxidation and reduction to produce
reactors [69], and less expensive nickel catalysts are utilised in industry.

9
B.S. Zainal et Renewable and Sustainable Energy Reviews 189 (2024)
al. 113941
Table 4
Different studies of fossil fuel-based production technologies for hydrogen production.

Process Parameters Findings References

Supercritical water gasification Temperature: autoclaved at 750 ◦ C, H2 increased by ≈ 20 % by K2CO3 [45]

30 min
Pressure: 24–26 MPa
Additives: K2CO3/γ-Al2O3
Supercritical water gasification Temperature: 400–450 ◦ C H2 production (mol/kg OM) increased by ≈ 930 % (0.026–0.37 mol/kg [46]
Time: 1–30 min OM)
Pressure: 24–26 MPa when added KOH
Additives: H2O2 and KOH
Supercritical water gasification Temperature: 350–750 ◦ C H2 yield: increased by ≈ 137 % (from 33.18 to 78.62 mol/kg) [47]
Time: 0–90 min
Pressure: 16.87–24.87 MPa
Additives: K2CO3
Electrochemical catalytic coal Enhancement: Ni–Cr wire in ECG CO yield increased 185 % at 800 ◦ C from 0 to 400 W. [48]
gasification
(ECG) Temperature: 800 ◦ C CO is the main combustible product with a small amount of CH4 and H2.
Power: 400 W
Autothermal reforming (ATR) Catalyst: Rh10, Rh20, Rh60 Highest H2 concentration (dry basis): ATR (≈40 vol%, Rh20) [49]
Temperature: 750 ◦ C
Autothermal reforming (ATR) Catalyst: Rh/CeO2 and Rh/Al2O3 Highest H2 concentration: 28 vol% [50]
Temperature: 850 ◦ C
Gas hourly space velocity (GHSV):
5000/h
Autothermal reforming (ATR) Catalyst: RhPt Highest H2 concentration: 37 to 39 vol% [51]
Temperature: 700–900 ◦ C
Gas hourly space velocity (GHSV):
30,000/h
◦ C.
Hydrocarbon Pyrolysis Catalyst: H-ZSM-11 zeolite Gas yield: increased up to 80.8 wt% at 700 [52]
Temperature: 500–900 ◦ C
Heating rate: 10 ◦ C/min
Hydrocarbon Pyrolysis Catalyst: chromium wires Conversion efficiency of alkanes to H2: increased to 100 % at 1475 ◦ C [53]
Temperature: 1200–1475 ◦ C (2.42 %)
Hydrocarbon Pyrolysis Catalyst: HZSM-5 Highest H2 yield (%): 68.6 % when co-pyrolysis with polyethene [54]
Temperature: 500–700 ◦ C
Hydrocarbon partial oxidation Oxidant type: Air, enriched air, Highest H2 concentration: 50.2 vol% [55]
oxygen
Pressure: 0.2–8.0 MPa
Temperature: 700–3300 ◦ C
Methane partial oxidation Oxidant type: with and without Highest H2 concentration (without oxygen): 2.5–3.0 mol/mol CH4 [56]
oxygen
Pressure: 0.101–5.05 MPa
Temperature: 500–1300 ◦ C
Oxygen/alkane ratio: 1.0–4.0
Post-flame zone: pyrolysis, steam
reforming

hydrogen from methane, which is more efficient and produces fewer ranging from 3 to 8 MPa. More carbon monoxide is generated (H2: CO =
GHG emissions than conventional SMR [73]. 1: 1 or 2: 1) when compared to SR (H2: CO = 3: 1). As a result, the
• Chemical looping sorption-enhanced steam methane reforming (CL- process is completed by converting carbon monoxide with steam into
SESMR): This technology combines the benefits of CL-SMR and hydrogen and carbon dioxide.
SESMR to improve the efficiency further and reduce GHG emissions
CH + O → CO + (7)
2H 4 2 2
of SMR [74].
• Solar-assisted steam methane reforming (SASMR): This technology CH4 + 2O2 → CO2 + 2H2 O
uses solar energy to provide the heat required for the SMR reaction, (8)
which reduces the need for fossil fuels and GHG emissions [75].
• Electrified steam methane reforming (ESMR): This technology uses CH4 + H2 O (g) → CO + 3H2
electricity to drive the SMR reaction, which is more efficient and
(9) In order to bring down the working temperature, which is
produces fewer GHG emissions than conventional SMR [76].
typically
These technologies are still under development, but they have the in the range of 700–1000 ◦ C, catalysts can be introduced to the CPOX.
potential to improve the efficiency and environmental performance of The employment of a catalyst in a reforming process typically allows for
SMR significantly. lower operating temperatures and higher hydrogen yields, resulting in
higher efficiency and reduced production costs. Maintaining tempera-
5.1.1.2. Partial oxidation (POX) and catalytic partial oxidation (CPOX). ture control has been proving difficult due to the exothermic nature of
Hydrocarbon POX and CPOX have been proposed to produce hydrogen the reactions, which results in the development of coke and hot spots
for transport fuel cells and other commercial applications [77,78]. The [77,80].
raw materials used in these methods can be methane or biogas, but they The exact products of POX depend on the type of hydrocarbon being
are primarily heavy oil fractions that are challenging to process and oxidised and the reaction conditions. For example, the POX of methane
utilise [79]. POX is a non-catalytic process that gasifies raw material in produces a mixture of carbon dioxide and water, while the POX of
the presence of oxygen (reactions (7) and (8)) and possibly steam ((9), propane (C3H8) produces a mixture of carbon dioxide, water, and
ATR) at temperatures ranging from 1300 to 1500 ◦ C and pressures hydrogen. Temperatures between 800 and 1000 ◦ C, an oxygen-to-
ethanol molar ratio of 0.6–0.8, and atmospheric pressure are the best
operating conditions for POX of ethanol (POE) for hydrogen production.

1
B.S. Zainal et Renewable and Sustainable Energy Reviews 189 (2024)
al. 113941
Under these conditions, complete ethanol conversion and a hydrogen 5.1.1.4. Dry reforming (DR) (also known as carbon dioxide reforming).
yield of 86–95 % are possible without coke formation [81]. DR is a process used to produce hydrogen and carbon monoxide from
POX is a widely used method for producing hydrogen, and it has natural gas and carbon dioxide. The process involves reacting methane
several benefits over other methods, such as SR or electrolysis. It has (the main component of natural gas) with carbon dioxide to produce
high efficiency, can use a wide range of feedstocks, and does not produce syngas [84]. The general reaction is as follows:
carbon dioxide as a by-product. However, it also has some drawbacks,
o kJ
such as expensive equipment, a specialised catalyst, and water produc- CH + CO 2→ 2CO + 2H 2, H = + 247.4
4 298
tion as a by-product, which can be challenging to separate from the
hydrogen gas. Compared to ATR, the POX process in fossil fuel (11) The reaction is typically carried out at high
reforming occurs at high pressure. Overall, POX of hydrocarbons is a
complex and multifaceted chemical reaction that plays a significant temperatures
role in many industrial processes. (700–1100 ◦ C) and pressures (1–10 bar) in the presence of a catalyst,
such as nickel or cobalt. The reaction is endothermic, meaning that it
5.1.1.3. Autothermal reforming (ATR). ATR was developed to solve the requires an input of heat to proceed. This reaction not only helps in
problem of reduced hydrogen yield in POX and the endothermic heat of waste management through methanation but also has the benefit of
reaction in SR. ATR combines these two processes, making it an combusting methane and carbon dioxide, both greenhouse gases [24].
attractive option for the onboard reforming of complex hydrocarbons Additionally, the syngas obtained in this reaction, with a hydro-
such as kerosene and diesel to provide hydrogen to fuel cells. As pre- gen/carbon monoxide ratio close to one, is valuable in the chemical
viously mentioned, steam is added to the catalytic POX process in ATR. industry as it is used in various types of highly specific syntheses to
ATR is a combination of both SR (endothermic) and POX (exothermic) produce a wide range of chemical compounds such as formaldehyde,
reactions [64]. acetic acid, and liquid hydrocarbons [85].
The ATR process is generally defined by the following (Eq. (10)): Ethanol can also be converted into hydrogen through dry reforming.
However, it is much less known [86]. The general reaction is as follows:
Cn Hm Op + x(O2 + 3.76 N2 ) + (2n 2x p)H2 O ↔
nCO2 C2 H5 OH + CO2 ↔ 2CO + 2 H2 + H2 O + C
(12) The main disadvantage of this process is that the carbon

( dioxide
) m (10) reformation produces inert solid carbon, which can deactivate the cat-
+ 2n 2x p + H2 + 3.76xN2 alysts utilised [87]. The DR of ethanol for hydrogen synthesis is a
2
The oxygen-to-fuel molar ratio (x) impacts the hydrogen production, complex multiple-reaction system with several unwanted side reactions
reaction heat, and amount of water required to convert fuel carbon to that affect product distribution [88]. As a result, hydrogen yield is
carbon dioxide [82]. Even after the water/gas shift reaction, the con- complexly dependent on process variables such as pressure, tempera-
centration of carbon monoxide in the gas produced by POX of hydro- ture, and carbon dioxide-to-ethanol molar ratios, to name a few. One of
carbon fuels frequently exceeds 1 % by volume. Other processes are the benefits of DR is that it allows for hydrogen production using a
necessary to lower the carbon monoxide level further. The POX must feedstock that is abundant and relatively inexpensive (natural gas). The
generate enough heat to produce hydrogen endothermically; thus, the process also captures and utilises carbon dioxide, which can help miti-
(x) must be such that the reaction is exothermic. gate GHG emissions. However, DR can be expensive, energy-intensive,
ATR is advantageous because it does not call for any additional heat and unsuitable for all applications. The thermodynamics for the DR re-
source and is easier and more cost-effective than SR of methane. Fig. 5 is action is not as favourable as the ATR or SMR reactions. However, the
a diagram that illustrates the several modes of operation that a fuel fact that 1 mol of carbon dioxide is consumed per mole of methane can
processor can go through to produce hydrogen. lower the carbon footprint, making the consumption of natural gas (and
From Fig. 5, a key benefit of the ATR process compared to the SR methane) more environmentally friendly.
process is that it may be turned off and restarted relatively quickly while
simultaneously creating more hydrogen than POX alone [64]. Even 5.1.2. Hydrocarbon thermal decomposition (TD)
though ATR and POX procedures do not require external heat supplies, TD is a method for producing carbon dioxide-free hydrogen from
they are more expensive than other alternatives [83]. Pure oxygen input natural gas. The reaction can be written as follows:
and separation equipment, such as an oxygen separation system, raise
kJ
the cost of these procedures. o
CH 4 → C + 2H2 , H298 = + 75.6 (13)
mol

1
B.S. Zainal et Renewable and Sustainable Energy Reviews 189 (2024)
al. 113941

Fig. 5. Operating conditions for POX, ATR, and SR [43].

1
B.S. Zainal et Renewable and Sustainable Energy Reviews 189 (2024)
al. 113941
The temperature of the pyrolysis process can be significantly from water using electricity (electrolysis), photonic energy (photolysis),
decreased by utilising a transition metal catalyst such as (Ni, Fe, or Co) or thermal energy (thermolysis) [106,107].
[89]. The energy needed to produce 1 mol of hydrogen (75.6/2 = 37.8
kJ/mol H2) is slightly lower than the SR process. The process is only 5.2.1.1. Electrolysis. Fig. 7 represents the conceptual setup for four
slightly endothermic, requiring less than 10 % of the heat generated by electrolyser technologies: alkaline-based electrolysis, PEM, SOEC, and
burning methane to be driven forward. The process also produces a AEM. Electrolysis is one of the most straightforward processes for
valuable by-product, pure carbon, in addition to the main product, creating hydrogen from water. The reaction, however, is very endo-
hydrogen. Because the process creates only trace amounts of carbon thermic; as a result, the necessary energy input is accomplished through
dioxide (about 0.05 m3 of carbon dioxide for every m3 of hydrogen electricity obtained from PV panels or steam turbines [108–110]. It is a
manufactured if methane is used as a fuel), it does not harm the envi- process in which electrical energy is converted into chemical energy in
ronment [62]. It is worth noting that the process can produce no carbon the form of H2 and O2 as a byproduct. Two reactions occur in each
dioxide if a small portion (about 14 %) of the generated hydrogen is electrode (anode and cathode) [111]. A separator between the anode
used to fuel the process. However, despite the benefits of pyrolysis, and cathode electrodes guarantees that the products remain segregated.
there is a significant potential for fouling from the carbon produced, Water splits when electricity is applied, producing H2 at the cathode and
which can be mitigated using an appropriate reactor design [90]. O2 at the anode via the following reaction:
2H2 O → 2H2 + O2 (14)
5.2. Renewable sources-based production
The cathode produces hydrogen, while the anode produces oxygen.
technologies
In an alkaline environment, the following reaction mechanism can be
considered [43,108]:
The categorisation of hydrogen production methodologies derived
from renewable sources, segregating them into biomass-related and Cathode : 2H2 O (l) + 2e → H2 (g) + 2 OH (aq) (15)
water-splitting processes as presented in Fig. 6. The water-splitting
category encompasses techniques such as electrolysis, thermolysis, and Anode : 4 OH (aq) → O2 (g) + 2H2 O(l) + 4e
photolysis. Conversely, biomass-centric methodologies integrate both
(16) Although alkaline-based electrolysis is the most prevalent
biological and thermochemical procedures. Table 5 provides a synthesis
of the cutting-edge technologies rooted in renewable sources for electrol-
hydrogen generation. These techniques encompass dark fermentation, ysis technique, more proton exchange membrane (PEM) electrolysers,
photo fermentation, pyrolysis, photocatalytic water splitting (photol- anion exchange membrane (AEM) electrolysers and solid oxide elec-
ysis), thermolysis, and electrolysis. For each technique, the table enu- trolysis cells (SOEC) units are being developed [112,113]. Water is
merates the parameters utilised within the investigation, the employed injected into the PEM electrolyser at the anode. It is split into protons
substrate and inoculum, additives incorporated, and the zenith of (H+ ) that move through the membrane to the cathode to generate
hydrogen yield observed. The optimal methodology for green hydrogen hydrogen and oxygen that remain with the water [40]. In alkaline and
generation may exhibit variability contingent upon the bespoke appli- SOEC, water is supplied at the cathode and split into hydrogen, sepa-
cation demands, inclusive of parameters, additive components, and rated from water in an external separation unit, and hydroxide ions
prevailing conditions. (OH ), which flow through the aqueous electrolyte to the anode to
generate oxygen. AEM electrolysers operate in an alkaline environment,
5.2.1. Water-splitting technology but the typical diaphragms (asbestos) in the alkaline-based method are
Hydrogen, composed of hydrogen and oxygen, may be produced substituted with anion exchange membranes (immobilised positively
from water, the most readily available resource [105]. Therefore, if charged functional groups on the polymer backbone or pendant poly-
sufficient energy is applied, the water molecule can be broken down into meric side chains) [114].
hydrogen and oxygen parts. The process of water splitting can be carried Catalysts and materials also play a critical role in the development
out using a variety of different technologies. Hydrogen can be extracted and improvement of electrolysers. Innovations in catalysts and materials

Fig. 6. Renewable sources-based production technologies.