As a renewable alternative to fossil fuels, the demand for and use of wood pellets has grown sharply. However, the climate benefit of wood pellets varies significantly according to a new thesis from the Swedish University of Agricultural Science (SLU).
Biomass is often regarded as carbon neutral in climate calculations meaning that the calculations assume that the amount of carbon dioxide emitted during combustion is simultaneously taken up by new vegetation.
– In conventional life cycle assessment (LCA), all emissions from the system under study are usually summed up into a single pulse, irrespective of when in time they occur, but this approach overlooks temporal carbon dioxide (CO2) fluxes between the soil, biomass, and atmosphere connected to bioenergy systems, said Charlotta Porsö, Doctorate at the Swedish University of Agricultural Science (SLU) in Uppsala, Sweden.
Factoring in time
In her recently published doctoral thesis “Time-dependent climate impact of production and use of wood pellets from short rotation forestry and logging residues”, Porsö has developed a method for evaluating the climate effects over time, taking into account both fossil and biogenic emissions in wood pellet production and use.
– This thesis seeks to increase the knowledge base for planning new wood pellet systems with the development of a new time-dependent approach for conducting LCA in which both the timing and magnitude of greenhouse gas (GHG) fluxes are considered in climate impact assessment, Porsö explained.
The method was applied to calculate the climate impact of three different systems for wood pellet production supplied to the Swedish heat and power sector; fast growing popular (Populus spp.) and willow (Salix spp.) on fallow agricultural land in central Sweden, short-rotation eucalyptus (Eucalyptus grandis) on disused agricultural land in Mozambique and logging residue from a final felling of a boreal coniferous forest stand (Norway spruce, Picea abies) in northern Sweden.
All better than coal
The main finding was that all three of the wood pellet systems evaluated by Porsö were a better alternative than using fossil coal for heat and power production from a climate impact perspective, both in terms of global warming potential (GWP) and global mean surface temperature change (ΔTS).
Establishing short-rotation forest plantations on former agricultural land, both in Sweden and Mozambique, provided carbon sequestration potential in both live biomass and soil organic carbon, which resulted in an initial negative ΔTS. In other words, both had a cooling effect on the temperature.
Porsö notes that over time, the cooling effect due to carbon sequestration in soil and biomass will decline as a new steady state is reached, whereas the warming effect due to GHG emissions from the production system will continue to increase, resulting in a net warming temperature effect of the wood pellet systems over time.
Wood pellets produced from logging residues extracted from final felling of a boreal coniferous forest stand in northern Sweden resulted in a positive ΔTS. Net emissions of biogenic CO2 accounted for by far the largest part of this temperature effect, while GHG emissions from harvesting, upgrading and transport were of less importance.
However, Porsö stresses that even this system has a much smaller climate impact than using fossil carbon.
– From a climate perspective, it is beneficial to replace fossil carbon with pellets from all the systems I’ve investigated. And whilst energy is used to produce the pellets, you get out 7-11 times the primary energy input in these production systems, said Charlotta Porsö.
Electricity used in the upgrading and pelleting process accounted for a large part of the total primary energy input to the wood pellet systems thus the higher the share of renewable electricity, the lower the GHG impact. Porsö also notes that torrefied wood pellets were better from a climate perspective due to assumed higher electrical efficiency and a higher co-firing rate with coal compared with non-torrefied wood pellets.