Purified coal gas is mainly molecular hydrogen, methane and carbon monoxide, with smaller quantities of other hydrocarbons and diluent gases. It is made by heating coal in retorts with little or no admitted air, not by completely burning the coal.
The hot products include permanent gases, condensable organic vapours and water vapour, leaving coke in the retort. Cooling removes much of the coal tar and condenses water, which captures Ammonia as ammoniacal liquor. Further washing and purification remove much of the remaining Ammonia, hydrogen sulfide and other objectionable constituents before the gas is distributed. A crude retort product and purified household coal gas therefore have different compositions.
Molecular hydrogen is commonly the largest component by volume. Methane is another major combustible constituent, while carbon monoxide contributes fuel value and serious toxicity. Ethylene and other unsaturated hydrocarbons, together with vapours such as benzene, contribute strongly to the luminous flame. Their thermal breakdown can produce incandescent soot, explaining why a small fraction of heavier hydrocarbons can disproportionately affect lighting quality. Smaller or variable quantities of carbon dioxide and molecular nitrogen dilute the combustible mixture. Molecular oxygen is not an intended major constituent; air ingress can change the mixture.
There is no single balanced chemical equation for making coal gas: coal is a complex feed and produces many products simultaneously. Nor is one exact percentage composition universal; it depends on the coal, retort temperature, operating conditions and purification. The main distinction is between the light combustible gases, the smaller illuminating hydrocarbon fraction, and contaminants or diluents. Modern natural gas, usually rich in methane, should not be used as a synonym for historical coal gas.
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