There are two highly inaccurate statements that are often made about the use of wood pellets as a substitute for coal in power generation. (1) The CO2 released from the combustion of wood pellets is greater than the CO2 released from the combustion of coal; (2) Using wood pellets for heat or power creates a carbon debt that takes decades to repay.
The Manomet study1 released in June, 2010 codified both of those so-called facts about using wood for fuel. Since then both the “pellets are worse than coal” and the “carbon debt” arguments have become ingrained in the anti-biomass literature.
In this white paper we will show why those statements, often presented as facts, are inaccurate.
Do pellets release more CO2 in combustion than coal? Coal started its life a very long time ago as biomass. And, it turns out, on a dry basis, coal and wood yield similar results in terms of the CO2 produced (in kilograms of CO2 per unit of potential energy)2. Of course wood does not have zero moisture content (MC). But neither does coal. The typical moisture contents by weight of coal are anthracite 3% - 16%, bituminous and sub-bituminous 8% - 20%, and lignite 39% or more.
Carbon contents also vary. Hardwood to softwood ranges from 48.5% to 51.5%. Coal varies widely. Lignite is about 40%, sub-bituminous is about 67%, bituminous is about 82%, and anthracite is about 95%.
It is the water in the solid fuel that causes its CO2 emissions to increase over the dry weight basis. The underlying process that drives this is “the enthalpy of vaporization”. In simple terms, it takes energy to evaporate the water in the wood or coal and convert it to vapor (steam). All of that energy is sent into the atmosphere in the form of water vapor and is lost. So to get a million BTUs of useful energy from the solid fuel, a larger mass of wood or coal is necessary to compensate for the losses from vaporizing the water. More wood or coal per unit of energy means more CO2 per unit of energy.
The analysis of carbon emissions from wood and coal will vary depending on the grade and MC of the coal. At 45% MC for wood, which is the level used by the Manomet study (page 103) and which is a common MC for green wood chips, and at 15% MC for sub-bituminous coal, the combustion of wood for power yields about 34% more CO2 per unit of useful energy then power generated from sub-bituminous coal3.