Water gas is a combustion fuel containing carbon monoxide (CO) and hydrogen gas (H2). Water gas is made by passing steam over heated hydrocarbons. The reaction between steam and hydrocarbons produces synthesis gas. The water-gas shift reaction can be used to reduce carbon dioxide levels and enrich hydrogen content, making water gas. The water-gas shift reaction is:
CO + H2O → CO2 + H2
The water-gas shift reaction was first described in 1780 by Italian physicist Felice Fontana. In 1828, water gas was produced in England by blowing steam across white-hot coke. In 1873, Thaddeus S.C. Lowe patented a process that used the water-gas shift reaction to enrich the gas with hydrogen. In Lowe's process, pressurized steam was shot over hot coal, with heat maintained using chimneys. The resulting gas was cooled and scrubbed before use. Lowe's process led to the rise of the gas manufacturing industry and the development of similar processes for other gases, such as the Haber-Bosch process to synthesize ammonia. As ammonia became available, the refrigeration industry rose. Lowe held patents for ice machines and devices that ran on hydrogen gas.
The principle of water gas production is straightforward. Steam is forced over red-hot or white-hot carbon-based fuel, producing the following reaction:
H2O + C → H2 + CO (ΔH = +131 kJ/mol)
This reaction is endothermic (absorbs heat), so heat must be added to sustain it. There are two ways this is done. One is to alternate between steam and air to cause combustion of some carbon (an exothermic process):
O2 + C → CO2 (ΔH = −393.5 kJ/mol)
The other method is to use oxygen gas rather than air, which yields carbon monoxide rather than carbon dioxide:
O2 + 2 C → 2 CO (ΔH = −221 kJ/mol)
Different Forms of Water Gas
There are different types of water gas. The composition of the resulting gas depends on the process used to make it:
- Water gas shift reaction gas: This is the name given to water gas made using the water-gas shift reaction to obtain pure hydrogen (or at least enriched hydrogen). The carbon monoxide from the initial reaction is reacted with water to remove carbon dioxide, leaving only the hydrogen gas.
- Semi-water gas: Semi-water gas is a mixture of water gas and producer gas. Producer gas is the name of fuel gas derived from coal or coke, as opposed to natural gas. Semi-water gas is made by collecting the gas produced when steam is alternated with air to burn coke to maintain a high enough temperature to sustain the water gas reaction.
- Carburetted water gas: Carburetted water gas is produced to enhance the energy value of water gas, which is ordinarily lower than that of coal gas. Water gas is carburetted by passing it through a heated retort which has been sprayed with oil.
Uses of Water Gas
Water gas used in the synthesis of some industrial processes:
- To remove carbon dioxide from fuel cells.
- Reacted with producer gas to make fuel gas.
- It is used in the Fischer-Tropsch process.
- It is used to obtain pure hydrogen to synthesize ammonia.