Biomass in a Sustainable Energy System

Abstract: The increased use of biomass for energy is as a key strategy in reducing carbon dioxide (CO2) emission, which represents the largest anthropogenic contribution to the greenhouse effect. In this thesis, aspects of an increase in the utilisation of biomass in the Swedish energy system are treated. Modern bioenergy systems should be based on high energy and land-use efficiency since biomass resources and productive land are limited. The energy input, including transportation, per unit biomass produced is about 4-5% for logging residues, straw and short-rotation forest (Salix). Salix has the highest net energy yield per hectare among the various energy crops cultivated in Sweden (Article I). The CO2 emissions from the production and transportation of logging residues, straw and Salix, are equivalent to 2-3% of those from a complete fuel-cycle for coal (Article II). Substituting biomass for fossil fuels in electricity and heat production is, in general, less costly and leads to a greater CO2 reduction per unit biomass than substituting biomass-derived transportation fuels for petrol or diesel. Transportation fuels produced from cellulosic biomass provide larger and less expensive CO2-emission reductions than transportation fuels from annual crops (Article III). Biomass has the potential to become the dominating energy source in Sweden. The current use of about 80 TWh/yr could increase to about 200 TWh/yr, taking into account estimated production conditions around 2015. Swedish CO2 emissions could be reduced by about 50% from the present level if fossil fuels are replaced and the energy demand is unchanged (Articles III and IV). There is a good balance between potential regional production and utilisation of biomass in Sweden. Future biomass transportation distances need not be longer than, on average, about 40 km. About 22 TWh electricity could be produced annually from biomass in large district heating systems by cogeneration (Article IV). Cultivation of Salix and energy grass could be utilised to reduce the negative environmental impact of current agricultural practices, such as the emission of greenhouse gases, nutrient leaching, decreased soil fertility and erosion, and for the treatment of municipal waste water and sludge, leading to increased recirculation of nutrients (Article V). About 20 TWh biomass could theoretically be produced per year at an average cost of less than 50% of current production cost, if the economic value of these local environmental benefits is included (Article VI).