Affordable and sustainable production of blue hydrogen is key to the successful use of fuel cells in transportation.
KALPANA GUPTA, ISHITA AGGARWAL and Maruthi Ethakota
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Worldwide, the transport sector is one of the significant contributors to greenhouse gas emissions. In 2018, global CO2 emissions increased by 2% worldwide and this has focused attention on decreasing these emissions by the transport sector.
Still, there is a negligible contribution of renewable sources to fuel the transport sector. Among alternative fuels, hydrogen is emerging as a promising solution to decarbonise transport, while in the power sector renewable energy has already taken a good share, but hydrogen’s share is negligible.
Complete cycle of hydrogen
The complete chain of hydrogen production, transport, storage, and use is shown in Figure 1. Hydrogen is produced mainly via three platforms: thermal processes, electrolysis, and biological processes. Traditionally, hydrogen is produced from fossil fuel by coal gasification and steam reforming. Globally, 48% of hydrogen is produced by natural gas steam reforming and 30% by oil reforming. Of the remainder, 18% is generated by coal gasification, and only 1% of hydrogen is produced by green processes.
Produced hydrogen is stored in gaseous/liquid or metal hydride form. Hydrogen need not be stored and transported in the case of on-site generation and consumption, as in a refinery or fertiliser plant. At the same time, storage and transportation is an integral part of the use of hydrogen as fuel. Currently, hydrogen is utilised mostly as a chemical in various industries and refineries. Today, 70 million t/y of produced hydrogen is mainly used as a chemical in the fertiliser (30%), refinery (50%), metal processing and food, electrical and aerospace industries. The next highest is the mobility sector. Predicted demand for hydrogen as a fuel is 4 million t/y by 2030.
Serious attention is given to producing low-carbon hydrogen. Indeed, green hydrogen with a zero carbon footprint is the future. With technologies available for CO2 capture and storage, well-developed methods of hydrogen production such as steam methane reforming (SMR) can be relied on for near-term developments. The reforming process, combined with CO2 capture and storage, has emerged as a sustainable solution and is described in detail in this article.
Hydrogen production by SMR
The most reliable and efficient process for hydrogen production is steam reforming of fossil fuels. SMR can be generally divided into the following steps: feed pretreatment, steam reforming, shift process, synthesis gas cooling, and purification (see Figure 2). The primary reaction of reforming is strongly endothermic. The heat needed to drive the reaction forward is usually supplied by burning natural gas and thus producing CO2. Carbon monoxide (CO) in the output stream from the primary reaction is usually converted to CO2 via the water gas shift reaction to increase hydrogen production:
CH4 + H2O + heat ⇌ CO + 3H2 
CO + H2O ⇌ CO2 + H2 + heat 
The reformed gas is cooled and routed to a shift reactor to maximise the hydrogen content. The produced syngas is further cooled and process condensate is separated out. The reformed gas has an approximate composition of H2 74 mol%, CH4 7 mol%, CO 1 mol%, and CO2 18 mol%, the exact proportions depending on feed composition, operating conditions, and the selected process scheme. The gases are purified in the PSA section to remove CO, CO2, and CH4 impurities and produce grey hydrogen.
To produce low-carbon ‘blue hydrogen’, a carbon capture process is integrated into the base scheme. A solvent based CO2 capture process is illustrated in Figure 2. The captured CO2 can be used in a variety of industries.
SMR is an efficient, widely used, and economical process. The efficiency of SMR and its specific energy consumption are best among current commercially available hydrogen production methods. SMR is the most reliable technology and has the highest availability among all hydrogen production methods. The process can be easily integrated with CO2 capture options, has very low NOx emissions, and there is no liquid discharge from the processing unit. SMR also has a small footprint per tonne of hydrogen produced.
Classification of hydrogen
Depending on the production method and feed source, hydrogen is classified as blue, grey, or green hydrogen. Hydrogen from coal, oil, and natural gas is grey hydrogen. All of these non-renewable sources and production methods, when integrated with a CO2 capture unit, then produce blue hydrogen. Hydrogen produced from biomass, wind, solar, and hydro-powered electrolysis is green hydrogen. Green hydrogen is typically produced via electrolysis of water in which water is split into hydrogen and oxygen. Dedicated ‘green’ hydrogen production electrolysis remains a niche component of global hydrogen production. But with renewable energy sourced electricity costs on a downwards trajectory (from solar photovoltaics, onshore, and offshore wind), focus and interest are growing. There are three distinct types of electrolyser: alkaline electrolysis, proton exchange membrane electrolysis (PEM), and solid oxide electrolysis cells (SOECs).
Blue and green hydrogen
Blue and green hydrogen are the solution to decarbonised hydrogen fuel production. According to DNV-GL1, “We will ultimately move to a green hydrogen economy. But we are convinced that you can make the step towards green hydrogen via blue hydrogen first.” Hence blue hydrogen can be a supply step between grey hydrogen and green hydrogen. Production of blue hydrogen offers several advantages in the near to medium term as it utilises conventional, large-scale commercial hydrogen production pathways and infrastructure, with 75% of global hydrogen production relying on natural gas.
Hydrogen as fuel is used in the transportation sector in two ways: as an internal combustion engine (ICE), or as a fuel cell electric vehicle (FCEV). The ICE has a disadvantage in terms of nitrogen oxides or NOx formation, inefficiency, higher maintenance, and unsafe operation.
A fuel cell is a device that generates electricity through an electrochemical reaction, not combustion. In a fuel cell, hydrogen and oxygen are combined to create electricity, heat, and water. The fuel cell can produce direct current (DC) power to run an electric vehicle. There are many types of fuel cell, including the proton exchange membrane fuel cell (PEM), solid oxide fuel cell (SOFC), alkaline cell, and so on. The PEM device is the only fuel cell found to be suitable for automobiles. It has an overall efficiency of 40-60% and, when running on pure hydrogen, the only emission is water vapour (see Figure 3).
Figure 4 shows applications of fuel cells in stationary plant, transportation, and materials handling. Their application to road transportation is discussed in this article. The fuel cell has several benefits including low to zero emissions, high efficiency, reliability, fuel flammability, energy security, durability, scalability, and quiet operation.
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