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Chemical Reactions and explanation

Today, synthesis gas is most commonly produced from the methane component in natural gas rather than from coal. Three processes are commercially practiced. At moderate pressures of 10–20 atmospheres and high temperatures of around 850 °C, methane reacts with steam on a nickel catalyst to produce syngas according to the chemical equation:

CH4 + H2O → CO + 3 H2

This reaction (also called steam-methane reforming) is endothermic. When methane is partially oxidised with oxygen, it produce syngas, in the following equation:

2 CH4 + O2 → 2 CO + 4 H2

This reaction is exothermic and the heat given off can be used to drive the steam-methane reforming reaction. When the two processes are combined, it is referred to as autothermal reforming. The ratio of CO and H2 can be adjusted by using the water-gas shift reaction,

CO + H2O → CO2 + H2,

to provide the appropriate stoichiometry for methanol synthesis.

The carbon monoxide and hydrogen then react on a second catalyst to produce methanol. Today, the most widely used catalyst is a mixture of copper, zinc oxide, and alumina. At 550–100 atmospheres and 250 °C, it can catalyze the production of methanol from carbon monoxide and hydrogen with high selectivity.

CO + 2 H2 → CH3OH

The production of synthesis gas from methane produces 3 moles of hydrogen for every mole of carbon monoxide, while the methanol synthesis consumes only 2 moles of hydrogen for every mole of carbon monoxide. The excess hydrogen is dealt with by injecting carbon dioxide into the methanol synthesis reactor, where it, too, reacts to form methanol according to the chemical equation

CO2 + 3 H2 → CH3OH + H2O