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Natural gas and coal account for most hydrogen production, which is 95 industries such as cement and steelmaking production, power genera-
% of total production. Meanwhile, as a byproduct of chlorine produc- tion, and fuel cells for EV.
tion, electrolysis produces roughly 5 % of the world’s hydrogen [12]. It may not be easy to be optimistic about the future of FCEVs that run
With the recent growth of the battery electric vehicle (BEV) sector, it on hydrogen and emit only water due to the recent success of BEVs.
may be tempting to be sceptical about the future of fuel cell electric However, with their hydrogen fuel cell vehicle, the Mirai, companies
vehicles (FCEVs) that run on hydrogen and only emit water as a like Toyota demonstrate a commitment to this technology. Subse-
byproduct. However, the demand for hydrogen is projected to increase quently, in an inaugural press conference, Honda president and Chief
to 87 million Mt in 2020 and between 500 and 680 million Mt by 2050. Executive Officer (CEO) Toshihiro Mibe stated that Honda has planned
The hydrogen production market was valued at $130 billion from 2020 to increase its FCEV and EV model line-up in major electrification
to 2021 and is expected to grow at an annual rate of 9.2 % through 2030 markets such as the United States and China to 40 % by 2030. This plan
[19]. According to a report by Gupta [20], the market for hydrogen will then be expanded to cover 80 % of the world by 2035 and 100 % by
generation surpassed $140 billion in 2019 and is expected to increase at 2040 [23].
a compound annual growth rate (CAGR) of more than 6.25 % between
2020 and 2026. Rising crude oil consumption and increased investments 5. Hydrogen production technologies
in expanding existing refining facilities in developing economies will
generate considerable market growth prospects for hydrogen makers Fig. 3 illustrates two different methods for producing hydrogen: from
worldwide. fossil fuels and renewable sources. Hydrogen produced from fossil fuels
is classified as blue hydrogen, and the production methods can be cat-
egorised into hydrocarbon reforming and pyrolysis. Hydrocarbon
4.2. Hydrogen applications reforming methods include steam reforming (SR), partial oxidation
(POX), and autothermal reforming (ATR). Hydrogen produced from
Fig. 2 shows that hydrogen can be used in many applications, such as renewable sources is called green hydrogen, and the production
for exploring outer space, industrial processes, electricity, and vehicles. methods include biomass processes (biological or thermochemical) and
Hydrogen blending also plays a crucial role by allowing for the water splitting (electrolysis, thermolysis, and photolysis). The biological
controlled mixing of hydrogen with other gases, optimising energy ef- biomass pathway includes bio-photolysis, dark fermentation (DF), and
ficiency and reducing GHG emissions across various sectors, from photo fermentation (PF). The thermochemical biomass pathway in-
transportation to industrial processes. According to the Energy Infor- cludes pyrolysis, gasification, combustion, and liquefaction. Details of
mation Administration (EIA) [21], industry uses nearly all the hydrogen these technologies are discussed in subsections 5.1 and 5.2.
to refine petroleum, treat metals, produce fertiliser, and prepare foods in Currently, the principal source of hydrogen production is fossil fuels.
the United States. Hydrogen is also used in petroleum refineries to It has the potential to be marketed as a commercially mature technology
reduce the sulphur level of fuels. The National Aeronautics and Space that can be employed at low cost while reaching high efficiency [25].
Administration (NASA) used liquid hydrogen as rocket fuel in the 1950s. The hydrogen generation efficiency ranges from (65 %–75 %) when
It was among the first to deploy hydrogen fuel cells to power spacecraft employing the SR of methane. Meanwhile, the efficiency of the methane
electrical systems. POX process is estimated to be around 50 % [26]. Water electrolysis may
The oil refining industry is responsible for most hydrogen generation also be used to synthesise hydrogen gas from water, accounting for
and consumption in the United States. As market demand in this in- approximately 95 % of total hydrogen production [27].
dustry has expanded, industrial gas firms have increasingly established Table 3 summarises hydrogen production, technologies, economics,
hydrogen generation plants on-site or near refineries. Merchant roles, and status. The table shows that the demand for hydrogen is
hydrogen, provided by industrial gas firms, now accounts for the vast increasing as hydrogen has a high heating value (HHV) of 142 MJ/kg,
bulk of hydrogen use at refineries, with 2.4 million Mt in 2014 [22]. In and it is suitable for fuel in aviation, steel, chemical, and electricity
recent years, there has been an uptick in interest in green hydrogen storage in the fuel cell. Some car manufacturers, such as Toyota, Honda,
production technologies. The potential applications are expanding and BMW, are willing to encounter issues related to hydrogen produc-
across a wide range of industries, including electricity grid stabilisation, tion costs and not give up on it just yet.
refrigeration, cleaning products, green ammonia production for fertil-
isers, heavy transportation such as shipping, manufacturing processes in

Fig. 2. Hydrogen applications in different sectors.

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Fig. 3. Hydrogen Production Methods [24].

5.1. Fossil fuel-based production technologies operational and manufacturing expenses. SMR is a chemical process that
produces hydrogen, carbon monoxide, and other valuable chemicals
The synthesis of hydrogen from hydrocarbons necessitates precise from natural gas or other hydrocarbon feedstocks. It is a primary
energy and temperature requirements. Endothermic reactions are used hydrogen production method widely used in the chemical and petro-
in industrial operations to produce hydrogen, which requires heat from chemical industries.
an external or internal source [43]. Even though hydrogen generation The conventional process of SMR involves using methane and steam
at atmospheric pressure is thermodynamically advantageous at to produce a combination of hydrogen and carbon monoxide, which is
tempera- tures above 800 ◦ C, temperatures higher than 1000 ◦ C are typically done at high temperatures (700–1000 ◦ C) and pressures (3–25
required to obtain significant conversion rates in systems that do bar) with the aid of a catalyst (reaction (1)). The catalyst can be either
not utilise a catalyst [44]. Fig. 4 illustrates the different non-precious metals, such as nickel, or precious metals from the Group
hydrogen production methods from fossil fuel-based technologies, VIII elements, like platinum or rhodium [58,59]. The CO further reacts
including hydrocarbon reforming, steam reforming, partial oxidation, with steam in the so-called WGS reaction to form carbon dioxide and
autothermal oxidation, and pyrolysis. Table 4 summarises the current additional hydrogen (reaction (2)) [60,61].
state of these technologies and concludes that adding a catalyst can
kJ
enhance hydrogen production and control crucial parameters such as CH 4 + H2 O → CO + 23 H , H
o
= 206 (Reforming reaction)
298
temperature and pressure. It is important to note that Table 4 only (1)
includes studies published between
2020 and 2022. The following conclusion can be made: o kJ
CO + H2 O → CO2 + H2 , H 298 = 41 (Water–Gas Shift reaction) (2)
mol
• the addition of catalysts as additives can increase the production of
The reaction releases heat as a byproduct (reaction (1)), commonly
hydrogen
used to create steam for power generation or other industrial applica-
• different catalysts and temperatures can influence the production of
tions. Additionally, the process includes a step to remove carbon dioxide
hydrogen
[62], and other feedstocks such as lighter hydrocarbons, methanol and
• the use of oxidants and high temperatures can increase the concen-
oxygenated hydrocarbons can also be used [63]. The network of
tration of hydrogen
reforming reactions for feedstocks such as hydrocarbons and methanol
• hydrogen concentration can increase when the process is performed
can be seen in Eqs. (3)–(6) [64]:
without oxygen during pyrolysis and steam reforming in methane
partial oxidation Cm Hn + m H2 O (g) → mCO + (m + 0.5n)H2

(3) Cm Hn + 2m H2 O (g) → mCO2 + (2m + 0.5n)H2

5.1.1. Hydrocarbon reforming
The use of reforming technology to produce hydrogen from hydro- (4) CO + H2 O (g) ↔ CO2 + H2
carbon fuels is explained here. SR of hydrocarbons is considered the
most widely used method for hydrogen production, especially in re- (5)
fineries. The hydrogen production process using hydrocarbon fuels is
CH3 OH + H2 O (g) ↔ CO2 + 3H2
divided into four methods: SR, POX, ATR, and dry reforming (DR) [43].
(6)
These methods produce significant amounts of carbon monoxide.
Therefore, one or more chemical reactors are used in a subsequent stage The SMR process is divided into two stages. In the first stage, hy-
to convert carbon monoxide into carbon dioxide through the water-gas drocarbon raw material is mixed with steam and fed into a reactor [65],
shift (WGS) and methanation processes. where it produces syngas (an equimolar synthesis gas containing
hydrogen and carbon dioxide) with a reduced carbon dioxide content
5.1.1.1. Steam methane reforming (SMR). SMR is now one of the most (reaction (3) and (4)). In the second stage, the cooled product gas is sent
common and least expensive methods of producing hydrogen [57]. Its to a carbon monoxide catalytic converter, where steam helps convert
benefit stems from its excellent operating efficiency and affordable carbon monoxide into carbon dioxide and hydrogen (reaction 5). The
catalytic process of steam reforming, which is used to avoid catalyst

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al. necessitates a feedstock material free of 113941
sulphur-containing compounds. The H: C atom ratio in the feedstock