Is Wood Energy Climate Neutral?

 I am sitting in my wood heated cabin in the north woods reading about how wood energy is worse for the climate than coal.

John Upton at Climate Central,  has a piece (cross posted at Grist) highly critical of the EU’S treatment of wood pellet burning power plants as essentially greenhouse gas neutral (the EU counts the GHG emissions of production and transportation, but not the GHG emissions of burning pellets). According to Upton, wood pellet energy produces “more heat trapping gases than coal.” If true, then biomass (wood) energy joins natural gas and hydro power as climate mitigating energy possibilities panned by enviros as “worse than coal.” Upton rejects any offsetting carbon sequestration of biomass growth in reaching this “worse than coal” conclusion.

Wood burning is not worse than coal for the climate, or the environment.  Every phase of coal energy — from destruction of ecosystems and landscapes to remove it from the ground, to the release of mercury, acid rain, sulfur oxides, and radiation when burning it, to the toxic piles of coal ash around the country — is an environmental disaster — in addition to the highest GHG emissions per megawatt of power resulting from combustion of any fossil fuel. Like all fossil fuels, coal is solar energy that has been stored over eons and cannot renegerate in human time frames.

The EU emissions accounting policy treats the wood pellets as a form of renewable energy — since the combustion emissions are potentially offset in a human time frame by the regrowth of the trees and sequestration of carbon dioxide through photosynthesis at the same rate that it is being consumed. Upton glosses over this assumption, asserting (incorrectly) that the cutting of forests for wood pellets itself releases the carbon stored in the forest, and that burning the pellets for electricity constitutes a second set of ghg emissions.He says “double counting would occur if one country reported carbon pollution from deforestation when pellets were produced, reducing forest carbon . . ..”

Of course, carbon neutrality for wood pellets would depend on what happens to the forests that are cut down — whether they are allowed to regenerate (sequestering an equivalent amount of carbon), or converted to agricultural or development land. But young forests have a larger carbon absorption capacity than old forests. Managing land for wood production could theoretically be carbon neutral.

This is not to deny the huge environmental impact of converting an natural forest ecosystem into a monoculture timber production zone. But biomass fuels mitigate climate change more than natural gas or (apparently) hydro power, and have potential to ease the intermittency problems associated with more direct forms of solar enegy (photovoltaics and wind). And climate change will wreak a more severe disruption to sensitive ecosystems than even monoculture forestry will.

Wood burning also has local air quality impacts that can be severe.  But modern EPA approved stoves are better than the old ones.  Wood heat is never going to be a workable solution for widespread use in densely populated areas or areas subject to air pollution trapping inversions. Wood burning power plants can incorporate emissions controls.

Its 20° F out and I am comfortable next to my wood stove in my cabin.  Both here and in my house downstate I heat primarily with deadwood from my property – I never cut a live tree for firewood, so there is no danger of destroying any ecosystems.

I think there is even an argument that burning dead wood is better for the climate than letting it rot — I haven’t been able to find the definitive study on this, but rotting wood releases methane, a much more potent greenhouse gas than carbon dioxide.Here’s my attempt to compare the climate warming potential of burning wood versus letting it rot. The chemical formula for the cellulose in wood is C6-H10-O5. So one molecule of wood cellulose could theoretically produce six molecules of CO2 if burned completely, or about 2.5 molecules of CH4 (methane) + 2.5 molecules of CO2 if allowed to completely biodegrade. The molar mass of CO2 is about 44g.  The molar mass of CH4 is about 16g. According to EPA, methane has a global warming potential of from 28-36 times that of CO2.  So for a given amount of wood cellulose (say, one mole), the global warming impact of burning it is 6 molecules of CO2 times 44 g/ mole = 264 grams CO2. The global warming impact of letting it rot completely would be 2.5 molecules c02 times 44g/mole PLUS 2.5 molecules CH4 times 16 g/mole times 28 times potency of methane = 1,230 grams CO2 equivalent.

So it looks like burning dead wood is about four times better for the climate than letting it rot completely into methane and CO2.  Of course wood burns much faster than it rots, and I haven’t studied chemistry since high school, so if someone has a better analysis, let me know.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s