http://en.m.wikipedia.org/wiki/Coal_to_liquid
As coal liquefaction generally is a high-temperature/high-pressure process, it requires a significant energy consumption and, at industrial scales (thousands of barrels/day), multi-billion dollar capital investments. Thus, coal liquefaction is only economically viable at historically high oil prices, and therefore presents a high investment risk.
Most[which?] coal liquefaction processes are associated with significant CO2 emissions from the gasification process or from heat and electricity inputs to the reactors.[citation needed], thus contributing to global warming, especially if coal liquefaction is conducted without carbon capture and storage technologies.[13] High water consumption in water-gas shift or methane steam reforming reactions is another adverse environmental effect.[citation needed] On the other hand, synthetic fuels produced by indirect coal liquefaction processes tend to be 'cleaner' than naturally occurring crudes, as heteroatom (e.g. sulfur) compounds are not synthesized or are excluded from the final product.[citation needed]
Pyrolysis of coal produces polycyclic aromatic hydrocarbons, which are known carcinogens.[14]