This surprised me, because surely you get one molecule of CO2 per atom of carbon in the coal, regardless of the kind of coal. But it seems there are more things in heaven and earth than are dreamt of in my stoichiometry - lignite has a higher moisture content than other coals, so more of the energy liberated goes into boiling that water, so there is less energy produced per unit of carbon, which means more CO2 per unit of energy:
https://www.sciencedirect.com/science/article/pii/B978008100...