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.