New study dispels assumption that burning forest residues for energy is carbon neutral. Burning wood in power plants will significantly undermine efforts to reduce greenhouse emissions over the next 10 – 50 years even under industry best-case scenarios where only forestry wastes are burned as fuel, according to a study published in Environmental Research Letters.
“Not Carbon Neutral: Assessing the net emissions impact of residues burned for bioenergy,” by Mary S. Booth, Ph.D., assesses net CO2 emissions from burning tree tops and branches left over from forestry operations. Such materials are often considered to have zero net greenhouse gas emissions since they are assumed to emit CO2 from decomposition or incineration even if they are not burned for energy. The paper explodes this fallacy by demonstrating that even when power plants burn true wood residues and exclude whole trees specifically cut for fuel, net emissions are still significant.
“To avoid dangerous climate warming requires us to reduce power sector CO2 emissions immediately,” said Mary Booth, author of the study. “However, this analysis shows that power plants burning residues-derived chips and wood pellets are a net source of carbon pollution in the coming decades just when it is most urgent to reduce emissions.”
The study examines the net CO2 emissions impacts of biomass burned in US power plants and exported wood pellets that are burned to replace coal at the UK’s massive Drax power station and other power plants in the EU. Combined, these facilities consume tens of millions of tonnes of wood per year. The study acknowledges that wood pellets are often sourced from whole trees, not forestry residues, but evaluates carbon emissions from residues-derived pellets because the biomass industry so often claims residues are a main pellet feedstock. It finds that even assuming the materials burned are true residues, up to 95% of the cumulative CO2 emitted represents a net addition to the atmosphere over decades.
“Drax and other wood-burning power plants emit as much or more CO2 per megawatt-hour as when they burn coal, but the policy of treating biomass as having zero emissions means companies avoid paying fees for biomass carbon pollution,” said Mary Booth. “This analysis shows that holding companies accountable for biomass CO2 would reflect the atmospheric carbon pollution impact better than treating biomass as carbon neutral.”
The paper includes a method for weighting CO2 emissions from biomass power plants so they can be counted under carbon trading and renewable energy subsidy programs.
The UK paid £809 m (about $1.2 b) in renewable energy subsidies to bioenergy in 2015, in addition to subsidies for zero-emissions wind and solar energy, and bioenergy is anticipated to grow significantly in the coming years. The analysis shows that increasing bioenergy to levels projected by international climate modeling would emit around 9 billion tonnes of CO2 per year at the smokestack, and that cumulative atmospheric CO2 loading would be a high percentage of those stack emissions.
Like other recent papers on bioenergy, Booth’s study finds that burning wood for energy is not compatible with Paris Agreement goals to reduce carbon pollution in the coming decades, but additionally demonstrates that net CO2 emissions are large even under the biomass industry’s best case scenario where biomass is sourced from residues, rather than whole trees.