realitybase - Journal

[Corrected 7/15/10 at 0952.]

One of the things we all "know" about global warming is that if we burn "renewable" biomass instead of coal, we can have clean energy that does not contribute to global warming. I've been suspicious about that, and then last week this caught my eye: Massachusetts is revising its rules to disallow to certain biomass combustion plants "green credits" toward the Commonwealth's goal of reducing its carbon footprint. I decided to do some calculations, and it turns out that burning dead trees and some other biomass sources does not necessarily reduce CO2 emissions and may actually increases them, at least in the short term. It's all about inventories of sequestered CO2, as I'll explain.

Assume we have a self-renewing "forest" of 40 trees, each of which sprouted in a different year and each of which will die on its 40^th birthday (sproutday?). Assume further that each tree grows at a linear rate and incorporates into its structure 5 tons of CO2 by the day it dies, i.e., 250 pounds per year. Finally, assume that Uupon its death, each tree decays at a linear rate of 500 pounds per year, with the assistance of termites, fungi, and other critters, and 20 years after its death has released all of its CO2 back to the atmosphere. Thus, while our forest is in this steady state, we have 40 live trees containing an average of 2.5 tons of CO2 each, for a total of 100 tons, and 20 dead trees each containing an average of 2.5 tons of CO2, another 100 50 tons. Our little forest keeps sequestered, year after year, 200 150 tons of CO2.

Now, let's build a biomass combustion facility at the edge of our forest. Each year we will harvest the tree that died that year and burn it to produce electricity, releasing 5 tons of CO2 to the atmosphere. The trees that died in prior years go on naturally decomposing, but after 20 years they are all gone, and 100 50 tons of CO2 formerly sequestered in our stock of dead trees is all back in the atmosphere. After that, burning each tree as it dies will not further increase the amount of CO2 in the atmosphere, because the rest of the forest assimilates 5 tons per year of CO2. However, to get to that new steady state we will have destroyed the 100 50-ton CO2 dead-tree sink, the same size as the one in our live trees. The planet will be worse off by 100 50 tons of CO2.

Would the planet nevertheless be better off if the wood burning backed out some coal combustion? That would clearly be true in the 21^st and later years after loss of the dead-tree CO2 sink, but what about during those first 20 years? When wood is burned, it releases less energy than coal because the carbon in wood is already "partially oxidized" from pure C to cellulose, a polymer of glucose (C6 H10 O5) units. Wood pulp has a higher heating value of 7,510 Btu/lb and will release 1.630 lbs of CO2 (1 x 264/162). Pure carbon, which is responsible for nearly all the energy content in coal, yields 14,500 Btu/lb and releases 3.667 lbs of CO2 (1 x 44/12). Thus, burning wood releases 86% as much CO2 as burning coal for the same energy output, not a very big advantage. (By similar calculations, one can show that burning natural gas is far better for the planet in the short term than burning dead trees, because natural gas releases only about 56% as much CO2 per unit of energy as does coal.)

One general lesson to take away from this model is that, in steady-state conditions, there may be as much CO2 sequestered in dead biomass as in live biomass—as in any plant the CO2 content goes from zero to maximum and then back to zero. (If the growth and/or decay processes are non-linear, the two sectors may not have exactly equal CO2 content.) The dead tree sink could be either larger or smaller than the live-tree sink, depending on rates of growth and decay which, of course, are probably not linear. I assume tree decay is relatively rapid in the tropics and relatively slow in high latitudes. My impression (only an impression—I haven't done my homework on this) is that some folks consider only the CO2 content of live biomass in their efforts to show project benefits. Oh, and here's another counter-intuitive fact: Turning trees into lumber and incorporating them into long-lived structures may be better for the planet than letting the trees die and decay more quickly in their natural habitats.

This analysis does not necessarily indict every "biomass" project. For example, if barren desert were covered with tanks of cultivated algae to produce biofuels, there would be no destruction of existing sinks of CO2, and the process could be very "climate friendly" if it displaced fossil fuels. However, one should perhaps compare that project with providing water to the area to develop a self-sustaining forest as a CO2 sink, which might be an even better way to slow down global warming. I mean only to show that one cannot assume biomass projects are climate friendly—you have to do your homework and get the accounting right. Heads up to my friends in northern Michigan: Traverse City Power & Light is moving ahead with two projects based on biomass from trees.