I thought I'd posted this earlier, but perhaps it didn't take...

According to Engineering Toolbox, the typical components of coal gas as produced in the Liverpool Road Gasworks of DOT fame (and every other gasworks in the country) were roughly:

CO2 3.8% CO 28.4% CH4 0.2% H2 17.0% N 50.6%

Note that useless (as a fuel) carbon dioxide and nitrogen make up over half the composition. That's an unwelcome, if unavoidable, result of roasting coal in air with its preponderance of nitrogen.

If my memory plays right, there was also a touch of hydrogen sulphide in the air around Liverpool Street, Cross Lane and Regent Road.