Hydrogen production processes

Hydrogen is not found free in nature; it is an energy carrier made from water or hydrocarbons, so how clean it is depends on the source of energy used.

1. Steam methane reforming (SMR)

Natural gas reacts with steam over a nickel catalyst at 700–900 °C and 3–25 bar:

$$CH_4 + H_2O \rightarrow CO + 3H_2 \quad (\text{endothermic})$$

This is followed by the water-gas shift reaction at 200–400 °C: $CO + H_2O \rightarrow CO_2 + H_2$. Pressure swing adsorption purifies the H₂. Natural gas reforming is the main industrial route (about two-thirds of world hydrogen, and about 95 % in the USA), with an efficiency of 70–80 % and CO₂ emissions of about 9–10 kg per kg of H₂ ("grey hydrogen"; with carbon capture it is "blue").

2. Water electrolysis

Direct current splits water:

• Cathode: $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$
• Anode: $2OH^- \rightarrow \tfrac{1}{2}O_2 + H_2O + 2e^-$
• Overall: $H_2O \rightarrow H_2 + \tfrac{1}{2}O_2$

Types: alkaline, PEM and solid oxide electrolysers. It needs about 50–55 kWh of electricity per kg of H₂. With electricity from hydro, solar or wind, the product is "green hydrogen" of very high purity (above 99.9 %).

3. Gasification

Coal or biomass is heated with limited oxygen and steam at 800–1500 °C to form syngas (CO + H₂), mainly by $C + H_2O \rightarrow CO + H_2$; the CO is then shifted to more H₂ and the gas is purified. It suits places where coal or biomass is cheap; coal gasification gives high CO₂ emissions.

Comparison

Other methods

Thermochemical cycles (sulphur-iodine) using high-temperature nuclear or solar heat, photo-electrochemical water splitting and biological (algae) production are under development.