So there's this process in real life where syngas is made by reacting superheated steam with coal to make flammable syngas (a mix of carbon monoxide and hydrogen). Then the Fischer-Tropsch process where the syngas is passed over a metal catalyst (iron-based) at high pressure to form hydrocarbon chains where the higher the pressure, the longer the hydrocarbon produced.

This could be a nice addition to the game. The Fischer-Tropsch can technically be implemented as a chemical plant recipe to turn syngas into either fuel gas or diesel. The steam-reforming of coal (or potentially even biomass) might require a new machine that takes in coal and superheated steam to output syngas and pollution. The syngas itself can also be used as a fuel in boilers or rotary kilns.

While obviously I would leave balancing such an addition to the devs, I think the requirement of superheated steam, the pollution and catalyst expenditure might be a good tradeoff for a mechanism to burn through excess coal reserves.