Hydrogen

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Abstract: Hydrogen is an energy carrier, currently produced from natural gas and used in large quantities in petroleum refining and other industrial applications. Hydrogen is also a potential low-carbon fuel that could be utilized via hydrogen fuel cells, hydrogen combustion engines, and other pathways. The low-carbon potential of hydrogen depends on the life cycle emissions during its production and utilization. To achieve low life cycle GHG emissions, hydrogen could be produced through electrolysis using low-carbon electricity, through water thermolysis, or through biological processes. Several electrolyzer technologies have been developed that show different efficiency and specific energy demand. A number of challenges also remain about hydrogen storage.

Introduction

Hydrogen is potentially interesting for power production because it contains no carbon. If carbon emissions during its production can be eliminated, hydrogen could be a potential zero-carbon “fuel” option. Hydrogen is classified as a secondary fuel or energy carrier because unlike primary energy sources, it does not occur naturally in sufficient quantities to be extracted and then used for energy production on a large scale. As such, it requires industrial production and requires higher energy input than is contained in the final product. Its production is most attractive in locations with abundant and relatively low-cost energy. Different “types” of hydrogen are commonly referred to using a color scheme that reflects upstream GHG emissions.

According to the IEA, about 90 Mt H₂ was used globally in 2020. Refineries consumed about 40 Mt as feedstock and reagents or as a source of energy. More than 50 Mt was used in the industrial sector, with 45 Mt used mainly as a feedstock to produce chemicals (split approximately as ¾ for ammonia production and ¼ for methanol production) and about 5 Mt in the direct reduced iron (DRI) process for steelmaking. In 2020, hydrogen use in transportation was almost negligible at about 20 kt which represented less than 0.01% of transportation energy consumed. However, hydrogen and hydrogen-based fuels are being widely promoted as offering emissions reduction opportunities, especially in hard-to-electrify transport segments such as long-haul, heavy-duty trucking, shipping, and aviation [5478]. Figure 1 shows the demand for hydrogen from 1950 to 2020. While there are some differences compared to the IEA data, the trends are similar [5560].

Many climate policies project that the use of hydrogen will increase significantly in the coming decades. In one scenario, the IEA’s net-zero scenario, hydrogen demand could increase to 530 Mt by 2050 with about 140 Mt of this demand coming from industry and 100 Mt from transport. About one-third of the hydrogen demand is assumed to be used to produce hydrogen‐based fuels such as ammonia, synthetic kerosene and synthetic methane. Use in the power sector is also expected to increase with hydrogen being used for energy storage to balance increased penetration of photovoltaic and wind generation—the hydrogen could then be used in gas-fired power plants and stationary fuel cells for example. Hydrogen use in buildings is also expected to increase, but its penetration is expected to be limited to situations where other technologies cannot be adopted and/or to increase electricity grid flexibility [5478]. In this scenario, hydrogen and hydrogen-based fuels would be expected to avoid up to 60 Gt CO₂ emissions in 2021-2050, 6.5% of total cumulative emissions reductions assumed for the scenario.

In petroleum refining, over 90% of hydrogen is used for hydrotreating and hydrocracking to reduce sulfur from finished products and increase the yield of transport fuels, respectively. More than 65% of this hydrogen demand is met by hydrogen supplied as a by-product from catalytic reformers and ethylene crackers while the balance is produced from natural gas and coal. The potential low-carbon hydrogen demand by replacing the latter is estimated to be up to 10 Mt/y by 2050 for a 10% or 100 Mt/y reduction in global refinery carbon emissions. However, much larger reductions in carbon emissions would be possible by replacing fossil fuels combusted to generate heat and steam. The potential low-carbon hydrogen requirement for heat and steam generation is estimated to be up to 40 Mt/y by 2050, and up to 300 Mt/y or about 25% reduction in carbon emissions. Therefore, the total potential demand for low-carbon hydrogen in refining could be up to 50 Mt/y by 2050 [5561].

For fueling applications, two methods of hydrogen utilization are considered as a potential low-carbon replacement for combustion engines powered by liquid hydrocarbon fuels such as diesel or gasoline:

Low life-cycle greenhouse gas emissions are critical if hydrogen is to live up to its potential as a low-carbon fuel. Figure 2 summarizes one view of this for marine applications. There are many potential hydrogen production technologies of varying commercial potential—Figure 2 shows three production pathways that might realistically be used for large scale hydrogen production. While using renewable electricity for water electrolysis is one option that is commonly touted as a path to a carbon-free future, given the demand for renewable electricity, it is doubtful that this could also meet all future demands for fuel production as well. Continuing to rely on fossil fuels for hydrogen production is therefore very likely. However, coupling fossil fuel hydrogen production with carbon capture and storage (CCS) would be required to achieve GHG reductions. While direct use of hydrogen might be suitable for many applications, challenges such as low temperature, high pressure and total volume requirements will make storage and transport difficult for many other applications. Thus, further conversion of hydrogen into ammonia, methanol, methane or a liquid hydrocarbon fuel might be required. Any carbon required for these products would need to be captured directly from the air or indirectly via sustainable biomass sources [5562][5563].

Hydrogen Properties. Characteristics of hydrogen include a very high gravimetric and very low volumetric energy contents that can require storage volumes much higher than for other fuels. The very low atmospheric boiling temperature of -253°C makes liquid storage challenging. It’s very low ignition energy and wide flammability limits in air make it very susceptible to ignition within closed spaces, and leaks can be more problematic than for other gaseous fuels.

Another characteristic of hydrogen is its long ignition delay time at high pressure which gives it a lower tendency to auto-ignition (knocking) during the combustion period. On the other hand, at low pressure and high temperature it has a short ignition delay time that makes it prone to pre-ignition [5796].