In a recent report, FutureMetrics aims to debunk two common myths regarding the use of wood pellets as a substitution for coal: that they release more carbon dioxide during combustion, and that they create a carbon debt that takes decades to repay.
Author William Strauss discusses the comparative moisture content (MC) of wood and coal, which, on a dry weight basis, yield similar results in terms of CO2 produced when combusted. He points out that while wood does not have zero MC, but neither does coal—in fact, lignite coal can possess up to 39 percent MC.
The water in solid fuel causes CO2 emissions to increase over the dry weight basis, Strauss points out, and though green wood chips do commonly possess 45 percent moisture—and sub-bituminous coal only 15 percent—and hence yields about 34 percent more CO2 per unit of useful energy, green chips are not suitable for use in almost all coal power plants.
This calculation does not apply when comparing wood pellets with coal, its suitable replacement, but it is commonly used by wood pellet myth propagators. Wood pellets at 6 percent MC result in significantly less CO2 emissions from combustion than all grades of coal under the same circumstances, Strauss points out.
Moving onto the discussion of carbon debt, Strauss illustrates how a responsible tree farmer harvests and replants his stands, giving the example of harvesting one forty-year-old stand each year and replanting one of 40 stands each year. “The entire forest sequesters 152,640 tons per year, every year,” he writes. “The accumulated carbon in the mature stand exactly equals the carbon accumulated every year by all of the younger stands. So although 152,640 tons of carbon are released by the fiber when the 40-year-old plot is used as pellets for fuel, 152,640 tons of carbon are sequestered in the same year by each of the other plots…”
Strauss concludes by deeming anti-biomass literature as wrong on wood pellets, as the combustion of wood pellets releases less CO2 than coal combustion, and states that as long as there are sustainability criteria that make sure the aggregate stock of carbon in working forests isn’t lowered, there is no carbon debt.