Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen. Green hydrogen is usually understood to be produced from renewable electricity via electrolysis of water. Less frequently, definitions of green hydrogen include hydrogen produced from other low-emission sources such as biomass. Producing green hydrogen is currently more expensive than producing gray hydrogen, and the efficiency of energy conversion is inherently low. Other methods of hydrogen production include biomass gasification, methane pyrolysis, extraction of underground natural hydrogen, and in situ hydrogen synthesis. As of 2023, less than 1% of dedicated hydrogen production is low-carbon, i.e. blue hydrogen, green hydrogen, and hydrogen produced from biomass. In 2020, roughly 87 million tons of hydrogen was produced worldwide for various uses, such as oil refining, in the production of ammonia through the Haber process, and in the production of methanol through reduction of carbon monoxide. The global hydrogen generation market was fairly valued at US$155 billion in 2022, and expected to grow at a compound annual growth rate of 9.3% from 2023 to 2030.
Overview Molecular hydrogen was discovered in the Kola Superdeep Borehole. It is unclear how much molecular hydrogen is available in natural reservoirs, but at least one company specializes in drilling wells to extract hydrogen. Most hydrogen in the lithosphere is bonded to oxygen in water. Manufacturing elemental hydrogen requires the consumption of a hydrogen carrier such as a fossil fuel or water. The former carrier consumes the fossil resource and in the steam methane reforming (SMR) process produces greenhouse gas carbon dioxide. However, in the newer methane pyrolysis process no carbon dioxide is produced. These processes typically require no further energy input beyond the fossil fuel.
Decomposing water, the latter carrier, requires electrical or heat input, generated from some primary energy source (fossil fuel, nuclear power or a renewable energy). Hydrogen produced by electrolysis of water using renewable energy sources such as wind and solar power, referred to as green hydrogen. When derived from natural gas by zero greenhouse emission methane pyrolysis, it is referred to as turquoise hydrogen. When fossil fuel derived with greenhouse gas emissions, is generally referred to as grey hydrogen. If most of the carbon dioxide emission is captured, it is referred to as blue hydrogen. Hydrogen produced from coal may be referred to as brown or black hydrogen.
Classification based on production method Hydrogen is often referred to by various colors to indicate its origin (perhaps because gray symbolizes "dirty hydrogen").
Current production methods
Steam reforming – gray or blue
Hydrogen is industrially produced from steam reforming (SMR), which uses natural gas. The energy content of the produced hydrogen is around 74% of the energy content of the original fuel, as some energy is lost as excess heat during production. In general, steam reforming emits carbon dioxide, a greenhouse gas, and is known as gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen produced is known as blue hydrogen. Steam methane reforming (SMR) produces hydrogen from natural gas, mostly methane (CH4), and water. It is the cheapest source of industrial hydrogen, being the source of nearly 50% of the world's hydrogen. The process consists of heating the gas to 700–1,100 °C (1,300–2,000 °F) in the presence of steam over a nickel catalyst. The resulting endothermic reaction forms carbon monoxide and molecular hydrogen (H2). In the water-gas shift reaction, the carbon monoxide reacts with steam to obtain further quantities of H2. The WGSR also requires a catalyst, typically over iron oxide or other oxides. The byproduct is CO2. Depending on the quality of the feedstock (natural gas, naphtha, etc.), one ton of hydrogen produced will also produce 9 to 12 tons of CO2, a greenhouse gas that may be captured. For this process, high temperature steam (H2O) reacts with methane (CH4) in an endothermic reaction to yield syngas.
CH4 + H2O → CO + 3 H2 In a second stage, additional hydrogen is generated through the lower-temperature, exothermic, water-gas shift reaction, performed at about 360 °C (680 °F):
CO + H2O → CO2 + H2 Essentially, the oxygen (O) atom is stripped from the additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.
Electrified Steam Methane Reforming In May 2019, Science published the results of a Danish study in which the tin catalyst is heated electrically. This reduces natural gas consumption (and CO2-emission) by a third, while the improved heating increases the overall efficiency. SMR needs 4,2 kWh/Nm3 H2, eSMR 3,6 kWh/Nm3 H2 (2,6 kWh natural gas and 1.0 kWh electricity).
From water
Methods to produce hydrogen without the use of fossil fuels involve the process of water splitting, or splitting the water molecule (H2O) into its components oxygen and hydrogen. When the source of energy for water splitting is renewable or low-carbon, the hydrogen produced is sometimes referred to as green hydrogen. The conversion can be accomplished in several ways, but all methods are currently considered more expensive than fossil-fuel based production methods.
Electrolysis of water – green, pink or yellow
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