Coal as one of fuels for combustion process contains fix carbon, ash, volatile matter and moisture. Coal can be classified based on them. In the combustion system of boiler, coal needs oxygen and heat (certain temperature) to perform firing. Coal can be fired by two methods; underfeed stoker firing and overfeed stoker firing, for more details see here.
When we discuss overfeed stoker firing in which approximately 8 inch coal bed is arranged above stoker grate, air should be distributed from bottom the grate and a layer of ash are passed first. Oxidizing zone occur when oxygen content in air pass through the first layer of burning coal which contain fix carbon. When burning process, heat temperature will increase and drying raw coal from moisture and volatile matter.
Over fire air as excess air is needed to perform complete combustion because distributed air from bottom grate can not burn volatile matter and carbon monoxide perfectly. Composition of air combustion should be controlled, does not be over and less regarding to efficiency of combustion. At firing which has low rate, level of over fire air should be minimized to eliminate possibility of cooling on volatile matter, so the combustion will be incomplete. At firing which has high rate, amount excess air is determined by the ability to totally mix over fire air, carbon monoxide and volatile matter to perform complete combustion.