3.1. Production

To use hydrogen as an energy carrier, it must be separated from the other elements in the molecules that contain it. It can be produced from fossil fuels or from renewable energy sources. The main production processes to produce hydrogen from fossil fuels include steam methane reforming (SMR), catalytic decomposition of natural gas, partial oxidation of heavy oils, and coal gasification. The predominant production processes to produce hydrogen from renewable energy sources are water electrolysis, thermochemical water decomposition, photochemical, photoelectrochemical, and photobiological.


3.2. Steam Methane Reforming

SMR is currently the least expensive way to produce hydrogen and uses light hydrocarbons (e.g. methane and naphtha) as the source. First step is synthesis gas generation, in which a desulfurized hydrocarbon is mixed with process steam over a nickel-based catalyst in the reformer. The second step is supplemental hydrogen generation, in which the synthesis gas enters the shift converter. Finally, the third step is gas purification, where the primary diluent, CO2, is removed in a scrubbing unit. The hydrogen produced typically has a purity of 97–98%. This method produces CO2 emissions.

3.3. Partial Oxidation

Partial oxidation is similar to the catalytic steam reforming used in SMR, but with the synthesis gas generation step slightly modified. This process includes both heavy oils and coal as sources.

A hydrocarbon feedstock is mixed with process steam and oxygen in the partial-oxidation unit. The hydrogen produced typically has a purity of 97–98%. Three principal types of partial-oxidation processes exist: light hydrocarbons (catalyst at about 600 ºC, e.g. methane and naphtha), heavy hydrocarbons (without catalyst at about 1400 ºC, e.g. heavy oils), and solid hydrocarbons (without a catalyst, e.g. coal gasification). This method produces CO2 emissions.


3.4. Electrolysis

Water electrolysis involves the catalytic decomposition of water into hydrogen and oxygen using electricity. This process does not produce any emissions other than hydrogen and oxygen, and the electricity used can come from renewable sources such as wind or solar energy.


3.5. Thermochemical decomposition

Thermochemical water decomposition generally uses multi-step cycles because very high temperatures are required before an appreciable amount of water decomposes in single-step cycles.


3.6. Sources

Around 48% of the global demand for hydrogen is currently generated from natural gas, about 30% from oil/naphtha from refinery/chemical industrial off-gases, 18% from coal, 3.9% from water electrolysis, and the remaining 0.1% from other resources such as nuclear, biomass, wind, solar, geothermal, or hydroelectric energy.

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