Epsilon Open Archive

Abstract

Using bioenergy to replace fossil fuels has been adopted as a climate mitigation measure, since less greenhouse gases are expected to be released into the atmosphere. In Sweden, the share of bioenergy is relative high (about 23% of total consumption including peat), with a relatively large proportion originating from domestically produced forest biomass.

This thesis examined the climate impact of using two types of woody biomass (willow, logging residues) for district heating, using time-dependent life cycle assessment methodology. The climate impact of the wood-based energy systems was determined and compared with that of the fossil fuels coal and natural gas. The focus was on the temporal dynamics of carbon fluxes between soil, biomass and atmosphere.

Establishing willow on agricultural land provided the potential to sequester carbon from atmosphere to soil, giving a net cooling effect on global mean temperature. However, this effect was shown to be highly dependent on willow yield (i.e. productivity), with low yield potentially decreasing soil carbon content. Moreover, district heating from willow chips gave a lower warming effect than coal and natural gas, irrespective of yield.

Combustion of forest biomass in the form of logging residues also gave a lower warming effect than coal and natural gas. However, the climate benefits compared with natural gas were delayed by 15-20 years (depending on geographical location) due to the chemical composition of natural gas, which generates less greenhouse gas emissions than coal and logging residues during extraction and combustion. Nevertheless, when decomposition of unharvested forest biomass was included in the reference systems, bioenergy from logging residues had climate benefits compared with coal and natural gas.