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  • Writer's pictureMitchell Wilkie

Wood Biomass Fuel is Carbon-Neutral…Oh, Really?

Image shows a pile of wood biomass that includes tree branches and construction material.
Wood biomass pile for fuel

Wood biomass is suitable as fuel for generating power to heating greenhouses and more. It’s often claimed that biomass is a ‘low carbon’ or ‘carbon neutral’ fuel, meaning that carbon emitted by biomass burning won’t contribute to climate change.

As a good thing, biomass is technically a 'renewable' energy source, in that trees can be replanted after they’re harvested and burnt. Canada has among the best sustainable forestry practices globally and these resources are well suited for biomass fuel use. Also, Canadian companies have innovative technologies for using biomass as fuel derived directly from woodchips or becoming refined bio-oils. For example, Ensyn Technologies (Ottawa) produces biocrude from forest and agricultural residues using its proprietary fast-pyrolysis thermal technology. They are the world’s leading biocrude producer.

Okay, biomass is renewable, but if burning biomass emits carbon dioxide (CO2), how can it be “carbon neutral”?

Some argue that because trees store carbon as they grow, replacement forests will gradually resequester or recapture the carbon dioxide emitted when the previous trees were burned for energy, making the whole process carbon neutral. Essentially this means that no net emissions are put into the atmosphere. Since these wood materials are expected to eventually decay while also emitting carbon dioxide in the process, it is argued that burning them to generate energy will emit the same amount of carbon as if they were left to decompose on a forest floor or in a landfill.

Biomass fuel is typically derived from waste materials from commercial timber harvesting, sawmill residue and construction demolition among other sources. Though relative to fuel demand, these wood residues are extremely limited, and many facilities already harvest and chip whole trees for fuel.

It takes decades for trees and branches to decompose on the forest floor and release carbon, and during that process, a portion of that decomposing carbon is also incorporated into new soil carbon. However, burning biomass propels the carbon stored in this wood into the atmosphere instantaneously. There is a significant difference between the immediate emissions from burning residues and the slow evolution of carbon from natural decomposition.

So, its the matter of timing when arguing carbon neutrality. Burning biomass for energy releases large amounts of carbon into the atmosphere all at once. But depending on the type of tree, forests may take decades or even a century to resequester the same amount of carbon back out of the air. The main point is that this biomass form of energy can’t be considered carbon-neutral unless critical factors like time are ignored.

Proponents of biomass power further argue it is better from a greenhouse gas perspective to burn this material, and emit the carbon as CO2, rather than let it decompose in the forest, where some of it may be emitted as methane (CH4) which has a greater global warming potential. But methane is not produced in upland areas where well-aerated logging residues are decomposing. Rather, it is mostly produced in wet, low-oxygen environments like wetland soils. Forest soils contain bacteria that produce methane, but also bacteria that consume methane, so the net emissions are small. Studies show that only about 3% of wood deposited in landfills is emitted as methane or carbon dioxide because they are resistant to anaerobic decomposition in that environment. Where biomass is very likely to be a source of methane emissions are the typically large 95,000 cubic metre wet, baking, and poorly aerated piles of chipped wood fuel at many biomass plants. Temperatures in such piles can reach 82°C-110°C (180°F-230°F) within two months after mounding and foster a high probability of spontaneous combustion. In addition to carbon dioxide, off-gassing from relatively dry wood fuels can produce carbon monoxide, methane, butane, ethylene, and other toxic gases.

Since it takes a long time for the carbon to accumulate in the forest, it is referred to as ‘slow in’ and ‘fast out’ when you’re burning it because it goes into the atmosphere rapidly. Thus, it has the potential to be carbon neutral, but only over a very long time. It certainly isn’t in the short term if we are strategically working to reduce global carbon emissions now. To be considered carbon neutral on long time scales, there would have to be replacement trees planted that are given enough time to store the same amount of carbon that their predecessors contained when they were harvested. Older, carbon-rich forests cleared to make room for faster-growing, easier-to-harvest trees would require even more carbon to be stored away to make up the difference.

Essentially, scientific evidence says that biomass fuels will be bad for global climate change mitigation efforts. What do you think?

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