Coal comes from ancient plants that have been transformed over time. The type of coal depends on factors like age, moisture, and ash content. The process of turning biomass into coal involves stages like peat, lignite, subbituminous, bituminous, and anthracite. The rank of coal increases from brown coal to anthracite. The composition of coal is complex, but for gasification, factors like age, caking properties, water content, and ash properties matter more than detailed petrographic composition.