Forest Carbon Coalition – Science Synthesis

Why is burning trees for electricity worse than coal?

Substituting wood pellets for coal increases the concentration of carbon dioxide in the atmosphere via several mechanisms:

  • For the same volume or weight, wood pellets contain much less energy, and much more water, than coal. Hence, utilities must burn much more wood to generate the same amount of electricity. Overall, switching from coal to wood pellets raises carbon dioxide emissions from the utility’s generator.
  • Additional emissions occur when the larger volume/weight of pellets must be transported greater distances than would occur if the utility continued to burn coal from sources closer to the generator.
  • Trees that are cut down to produce wood pellets and woody biomass no longer pull carbon dioxide out of the atmosphere, causing CO2 concentrations to rise.

Some argue that burning wood pellets results in no net increase in emissions because the pellets are made from limbs and other leftovers from timber production, which would be burned as slash or left to decompose and release carbon dioxide to the air anyway. In reality, though, manufacturers produce pellets from whole trees logged solely for this purpose. If not logged, the trees would have continued to grow, pulling carbon dioxide from the air. Hence, the production and combustion of pellets results in a net increase in atmospheric carbon dioxide.

Some argue that the emissions from transporting and burning wood pellets are offset if pellet manufacturers replace the logged trees with tree seedlings that pull carbon dioxide from the air as the grow. In reality, though, it can take decades, perhaps centuries, before the carbon sequestered by the seedlings equals the carbon that the initial trees would have sequestered if they’d not been logged to provide raw material for making wood pellets. This time lag—emissions now, sequestration later—has devastating consequences insofar as the globe faces catastrophic impacts from climate change without immediate, dramatic reductions in emissions.

Key research on the relative climate impacts of wood pellets and fossil fuels for energy:

Brack, D., R. Birdsey, and W. Walker, 2021. Greenhouse gas emissions from burning US-sourced woody biomass in the EU and UK. Environment and Society Programme, Chatham House.

Link: https://assets-woodwell.s3.us-east-2.amazonaws.com/wp-content/uploads/2021/10/02114836/2021-10-14-woody-biomass-us-eu-uk-research-paper.pdf

Key excerpts:

  • “In 2019, according to our analysis, US-sourced wood pellets burnt for energy in the UK were responsible for 13 million–16 million tonnes of CO2 emissions, when taking into account emissions from their combustion and their supply chain, forgone removals of CO2 from the atmosphere due to the harvest of live trees and emissions from the decay of roots and unused logging residues left in the forest after harvest.”

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. Environ. Res. Lett. 13 015007.

Link: https://iopscience.iop.org/article/10.1088/1748-9326/aaa512/pdf.

Key excerpts:

  • “We simulate substitution of wood for coal in power generation, estimating the parameters governing NPP [net primary production] and other fluxes using data for forests in the eastern US and using published estimates for supply chain emissions.”
  • “Because combustion and processing efficiencies for wood are less than coal, the immediate impact of substituting wood for coal is an increase in atmospheric CO2 relative to coal.”
  • “The payback time for this carbon debt ranges from 44–104 years after clearcut, depending on forest type—assuming the land remains forest. Surprisingly, replanting hardwood forests with fast-growing pine plantations raises the CO2 impact of wood because the equilibrium carbon density of plantations is lower than natural forests.”
  • “Further, projected growth in wood harvest for bioenergy would increase atmospheric CO2 for at least a century because new carbon debt continuously exceeds NPP.”

Brack, D., 2017. Woody Biomass and Heat: Impacts on the Global Climate. Chatham House Research Paper ISBN 978-1-78413-190-6. London, UK: The Royal Institute of International Affairs, Chatham House.

Link: https://www.chathamhouse.org/2017/02/woody-biomass-power-and-heat/executive-summary.

Key excerpts:

  • “…since in general woody biomass is less energy dense than fossil fuels, and contains higher quantities of moisture and less hydrogen, at the point of combustion burning wood for energy usually emits more greenhouse gases per unit of energy produced than fossil fuels.”
  • “ The volume of emissions per unit of energy actually delivered in real-world situations will also depend on the efficiency of the technology in which the fuel is burnt; dedicated biomass plants tend to have lower efficiencies than fossil fuel plants depending on the age and size of the unit.”
  • “ The impact on the climate will also depend on the supply-chain emissions from harvesting, collecting, processing and transport. Estimates of these factors vary widely but they can be very significant, particularly where methane emissions from wood storage are taken into account.”
  • “ Overall, while some instances of biomass energy use may result in lower life-cycle emissions than fossil fuels, in most circumstances, comparing technologies of similar ages, the use of woody biomass for energy will release higher levels of emissions than coal and considerably higher levels than gas.”