The main constituents of producer gas are

Producer gas is an industrial fuel gas made by blowing a limited supply of air through a thick bed of red-hot coke or coal in a gas producer. Because atmospheric air is roughly four-fifths nitrogen, the gas leaving the producer is essentially a mixture of carbon monoxide, formed by the partial combustion of carbon (2C + O2 -> 2CO), together with the nitrogen from the air, which passes through largely unreacted.

In the reaction that defines producer gas, a restricted supply of air (roughly four-fifths nitrogen) is blown through the hot coke bed: 2C + O2 -> 2CO. The nitrogen in that air does not take part in the reaction and passes through largely unchanged, so the two majority gases leaving the producer are carbon monoxide and nitrogen, i.e. CO + N2.

The other three composition pairs correspond to different reactions or conditions:

• CO + H2 is the composition of water gas, obtained by passing steam (not air) over red-hot coke: C + H2O -> CO + H2.

• CO2 + H2 would require carbon to be fully oxidized to CO2 in the same step that releases free hydrogen, a combination that does not arise from either the air-blown or steam-blown coke reaction.

• CO2 + N2 would require the carbon in the coke bed to burn all the way to CO2 (C + O2 -> CO2) while the nitrogen passed through unreacted, a complete-combustion condition rather than the partial-combustion condition that defines producer gas.