Chemistry of Hydrogen Production Processes

Steam methane reforming.

SMR is a widely utilized thermochemical procedure for producing gray hydrogen [30,31]. It consists of converting natural gas, which is the leading feedstock of methane, and steam (i.e., H2O and heat) into synthesis gas. The latter is a combination of hydrogen and carbon monoxide (CO) that can serve as a vital feedstock for several chemical and petrochemical operations

CH₄ + H₂O ⇌ CO + 3H₂

Then, the syngas is passed via an exothermic water-gas shift reaction to transform CO into hydrogen and CO₂.

CO + H₂O ⇌ CO₂ + H₂

Biomass gasification

Biomass gasification is a thermochemical reaction that converts biomass materials into a versatile gas mixture known as syngas. The process involves exposing the biomass to high temperatures within a controlled environment where oxygen or air access is restricted.

Biomass + H₂O + O₂ ⇌ CO + CO₂ + CH₄ + H₂ +  Other gases

Coal gasification

Coal gasification is a chemical procedure that transforms coal into syngas. It involves heating pulverized coal in the presence of an oxidizing agent, typically steam or oxygen. This process initiates various chemical reactions, converting coal into CO and hydrogen, among other gases (Eq. (4)). Typically, the gasification processes of coal and biomass are similar (Fig. 4), with the key difference being that the former uses coal as feedstock, whereas the latter uses biomass.

C+H₂O⇌CO+H₂

Water electrolysis

Water electrolysis is a procedure utilized to transform water into hydrogen and oxygen, using electricity, as depicted in Eq. (5). This transformative reaction is considered among the most common electrochemical processes for generating hydrogen [43]. It holds immense promise for a sustainable future, as it enables the generation of clean, high-purity and versatile hydrogen fuel while releasing oxygen as a valuable byproduct, utterly free from any carbon emissions

2H₂O + Electricity → O₂ + 2H₂