= Solution
\b[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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