How much CO2 does wood produce versus coal?
The results of our analysis shows that wood is generally about the same or slightly lower in CO2 emissions on a dry basis, but both wood and coal do not naturally have zero moisture content (MC). The typical moisture content of coal is:
Anthracite Coal : 2.8% - 16.3% by weight
Bituminous Coal : 2.2% - 15.9% by weight
Lignite Coal : 39% or more by weight
It is the water that causes CO2 emissions to increase over the dry weight. The underlying cause that drives this is “the enthalpy of vaporization.” In simple terms, it takes energy to evaporate the water in wood or coal and convert it to vapor, and all of that energy is sent out the chimney and into the atmosphere in the form of water vapor, unless a condensing boiler is used which may claim part of the escaping energy. To get a million BTUs of useful energy from the fuel, a larger mass of wood or coal is necessary to compensate for the losses from vaporizing all that water. And more wood/coal burned means more CO2 produced. With coal, the higher water content grades also have lower carbon content and higher volatiles. The net effect of this is that, on average, CO2 outputs are relatively consistent across grades (see Table 2). At 45 percent, the combustion of wood yields about 9.0 percent more CO2 per unit of useful energy than an average of the coal grades’ outputs.