by Bill Schlesinger
Wood-pellet producer, Enviva, recently announced that it would double its harvest of forest biomass in the Southeast as a contribution to reducing the impact of climate change. After all, when used to generate electricity, forests grow back, removing carbon dioxide from the atmosphere.
Some simple calculations, here taken from the EPA, suggest otherwise. A 50-megawatt electric power plant burning wood pellets would emit 43,730 tons of carbon as carbon dioxide each year, whereas the same plant burning coal would emit 39,200 tons/year. The difference stems from the lower energy content of wood, so you need to burn more of it.
Wood containing 43,730 tons of carbon could be obtained from the harvest of 875 acres of land with 40-year-old trees. Granted, a young forest replanted on this land would take up more carbon each year than the old forest it replaced, but it would take 40 years of regrowth to recapture the carbon from the harvest. Thus, until the year 2062, a harvest in 2022 would leave a legacy of carbon dioxide in the atmosphere. And, of course, the harvest would be repeated on another 875 acres in 2023. This is not a particularly helpful contribution to net zero emissions by 2050 and to reducing carbon dioxide emissions during the period of greatest impact on future climate.
The forest products industry argues that much of the Southeast shows an increase in the overall area of forest. This is good, but those forests were growing (storing carbon) before a decision was made to cut trees for wood pellet production. Existing trees do not offer an offset to balance the carbon released by burning woody-biomass pellets. To do so would be “double-counting.”
The industry is giving us a smoke-and-mirrors analysis of the benefits of woody biomass as a renewable energy—best story that Washington lobbyists can muster. But, if we are serious about mitigating climate change, we must shift our attention to sources of energy that emit no greenhouse gases to the atmosphere—like solar and wind. Forests should not be cut for fuel, and they must remain green to hold and accumulate carbon dioxide from the atmosphere as the coming decades unfold.
References:
Jonker, J.GG., M. Junginger, and A. Faaij. 2014. Carbon payback period and carbon offset parity point of wood pellet production in the southeastern United States. Global Change Biology Bioenergy 6: 371-389.
Schiffman, P.M. and W. C. Johnson. 1989. Phytomass and detritial carbon storage during forest regrowth in the southeastern United States Piedmont. Canadian Journal of Forest Research 19: 69-78.
Schlesinger, W.H. 2018. Are wood pellets a green fuel? Science 359: 1328-1329.
Sterman, J.D., L. Siegel, and J.N. Rooney-Varga. 2018. Does replacing coal with wood lower CO2 emissions? Dynamic lifecycle analysis of wood bioenergy. Environmental Research Letters 13: doi: 10.1088/1748-9326/aaa512
US Environmental Protection Agency AP-42 Emission Factors, page 1.1-42, Table 1.1-20
