In coal-fired processes, part of the fuel is first burnt to heat the charge. The product of this combustion is CO. When the temperature reaches 1,000 °C, the CO reacts with the unburned carbon to create CO:
The production of H cannot be achieved by the thermal decomposition of water, as the temperatures involved are too low. Hydrogen is in fact produced along with carbon monoxide by the reaction:
These two reducing gas production reactions, which consume 172.45 and 131.4 kJ/mol respectively, are highly endothermic and operate by limiting charge heating.