Epsilon Open Archive

Abstract

Global warming is a result of human-induced greenhouse gas emissions, primarily from fossil fuel use, but also from land use changes. To mitigate climate change, fossil fuel-based energy systems need to be replaced with alternative energy sources. Here bioenergy can play an important role, since this renewable fuel is considered to be carbon-neutral, meaning that no extra carbon dioxide (CO2) is emitted to the atmosphere.

However, carbon-neutral is not the same as climate-neutral and, while the CO2 from biomass use was once, and will again, be captured during plant growth, the temporary imbalance in the atmosphere can have consequences for the climate. Furthermore, bioenergy supply chains generally consume fossil fuels and producing biomass for energy requires land, which can lead to carbon stock changes.

This thesis examined the climate impact and energy performance of bioenergy from short-rotation coppice willow and long-rotation forest residues. Willow is a dedicated energy crop grown on agricultural land for energy, while forest residues (tops, branches and stumps) are a by-product harvested after final felling in conventional forests. A time-dependent life cycle assessment (LCA) method was used to capture the timing of greenhouse gas fluxes, including biogenic carbon (carbon stored in biomass and soil). In addition, a new method that combines time-dependent LCA with GIS mapping, and thus assesses the climate impact over a landscape, was developed.

The results showed that growing willow on former fallow land can give a negative climate impact (cooling effect) by sequestering carbon from the atmosphere in biomass and soil and by achieving high productivity, which is important for the final outcome. Initial soil organic carbon content was shown to have a large influence on future carbon stocks. Harvesting forest residues for energy gave a higher climate impact than harvesting willow, with forest stumps giving a slightly higher climate impact than tops and branches. Moreover, forest residues harvested in northern Sweden gave a slightly higher climate impact than forest residues harvested in the south. All bioenergy feedstocks studied gave a lower climate impact than hard coal and natural gas over time and the climate benefit of replacing these fossil fuels increased over time when studying continuous energy outtake (landscape perspective).