In this section taken from the Courrier de la Planète, our journalists answer questions from our readers. Click here to subscribe.
During its growth, each tree siphons CO2 which floats in the air. It uses this element to form its roots, its trunk, its branches. When a log is burned — or a forest fire is raging, although that is not the subject of this article — the carbon in the wood releases intense heat and reverts to CO2. A nice campfire therefore generates greenhouse gases (GHG).
So let’s come to the question of Serge Blondin, one of the readers of Le Courrier de la Planète: why do we consider that burning biomass — logs or wood pellets, for example — is less harmful to the climate than burning natural gas, gasoline or coal?
Short answer: unlike fossil fuels, which take millions of years to form, trees grow in decades. The carbon released when the wood is burned will therefore be swallowed up by the new maple, new birch or new spruce that will replace the cut tree. In the case of deforestation, however, the combustion of biomass results in a net addition of CO2 in the air.
“If you are in a sustainably managed forest, where you let the trees grow back, the neutrality effect [carbone] is relatively there. According to certain scenarios, in the boreal forest, we sometimes even tend to capture more carbon, because it accumulates in the soil,” explains Annie Levasseur, professor at the École de technologie supérieure in Montreal specializing in the analysis of bioenergy life cycle.
Now that we’ve given that short answer, let’s go over some other important details. First, note that, for the same amount of energy, burning wood releases more CO2 than fossil fuels. There are 96 kg of CO equivalent2 per gigajoule of energy for dry wood, 73 kg for diesel, 68 kg for automotive gasoline and 50 kg for natural gas.
Thus, initially, burning biomass sends more CO2 in the atmosphere than that of a fossil fuel. This “carbon debt” will last a few years after converting an oil furnace to a device that burns wood pellets. It will gradually repay itself as a new tree grows in place of the unfortunate one that went up in smoke.
“Biogenic” carbon
To judge the environmental performance of a biomass project, it must always be compared to a reference scenario. Let’s take the example of the branches of trees cut in the boreal forest — one of the most advantageous sources of biomass, according to Ms. Levasseur. Conventionally, the forestry industry leaves these branches on the ground. They decompose there, releasing CO2 in the air.
Using these branches for energy does not reduce carbon sequestration. Studies indicate that in Quebec’s boreal forest, half of it can be harvested without harming ecosystems. In addition, the release of CO2 by burning does not occur much earlier than in the reference scenario — decomposition — which limits the warming in the interval. The duration of the “carbon debt” is minimized.
End-of-life wood (old pallets, planks from deconstructed buildings) is another “super profitable” source of biomass, emphasizes Professor Levasseur. Conversely, it is undesirable to use wood residues that are already recycled, such as residues from sawmills, from which the industry often makes pressed wood panels (presswood).
As for whole trees and trunks, it is the source of forest biomass whose carbon debt takes the longest to repay. We are talking about several decades to achieve zero effect on the climate. Moreover, it is better to use this wood to make long-lasting products—such as furniture or buildings—that sequester carbon and mitigate climate change.
In Quebec, as elsewhere in the world, CO emissions2 “Biogenic”, destined to be absorbed by new plants, are not included in the GHG balance. However, according to a study by Ms. Levasseur which will be published soon, the lag between carbon emission and reabsorption should be taken into account. “It affects the trajectories to get to carbon neutrality in 2050,” she says.
Forest bioenergy represents about 7% of the energy consumed in Quebec. Residues can be burned to heat, to make electricity or both (cogeneration). They can also be transformed into pyrolytic oil, a biofuel used by certain industries. Half of the wood pellets produced in Quebec are exported to the United States and Europe