Time-dependent climate impact of production and use of wood pellets from short rotation forestry and logging residues

Anthropogenic greenhouse gas (GHG) emissions are the main cause of climate change, with combustion of fossil fuels and land use changes being the main source of emissions. Wood pellets are considered a viable replacement for fossil fuels. Therefore this thesis sought to increase the knowledge base for planning new wood pellet systems. This was done by investigating the energy efficiency and time-dependent climate impact of production and use of wood pellets supplied to the Swedish heat and power sector from short rotation forestry grown in central Sweden (willow and poplar) and in central Mozambique (eucalyptus), and from residual forest biomass extracted from final felling in Sweden. 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 CO2 fluxes between the soil, biomass and atmosphere connected to bioenergy systems. This motivated the development of a new time-dependent approach for conducting LCA in which both the timing and magnitude of GHG fluxes are considered in climate impact assessment. The main findings were that all three wood pellet systems investigated were a better alternative than 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 (∆TS). Establishing short rotation forest plantations on former agricultural land provided carbon sequestration potential in both live biomass and soil, which resulted in an initial negative ∆TS, i.e. a cooling effect on the temperature. However, wood pellets produced from logging residues extracted from final felling of a boreal coniferous forest stand (Norway spruce) in northern Sweden resulted in a positive ∆TS, i.e. warming temperature effect. 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. The energy output of the wood pellet systems studied was 7 to 11 times the primary energy input