Coal in its solid form is dirtier than gasoline or diesel as a power source, but if you're converting it to gas, there's no difference. The reason that coal is dirty is because of poor or incomplete combustion and carbon particles that are released. Since it's purified in the transformation process, okay na yan.

Coal won't run out as quickly as gasoline or diesel. The half-life of the coal supply will be reached around ten to twenty years after oil peaks (I think oil is peaking already). But if they find a really cheap way to produce gas from coal, it will definitely run out quicker.