Coal Mines produce 0.5 Coal per turn each, accumulating to 6 Coal per day. Coal Mines are cheap to build and operate, and never run out of Coal to mine. Each city can have up to 6 Coal Mines.
There are no National Projects that increase Coal production. To increase Coal Production, a nation must increase the number of Coal Mines it has.
Coal is used to fuel Coal Power Plants which are a staple Power Improvement for new nations. Coal, along with Iron, is also a necessary component in producing Steel. Steel is a very valuable manufactured resource, and only nations in Europe and North America have access to both Coal and Iron. Nations in other Continents that wish to produce Steel must import either Coal or Iron, increasing their value as an export.