Aspects on bioenergy as a technical measure to reduce energy related greenhouse gas emissions

摘要

Greenhouse gas emission assessments of energy supply systems have traditionally included the CO_2 emissions produced as the fuel is burned. A lot of models and calculations for evaluating greenhouse gas emission savings by using bioenergy have been introduced. The approaches often cover a major part or sometimes even the whole energy system. The biofuel production process itself is one piece that is normally covered very briefly or considered insignificant. Unfortunately, this means that the significance of some affecting factors might not have been estimated. The object of the thesis was to study the greenhouse gas balances in connection with the harvesting and production of biofuels and, based on this, evaluate in what situations there is a need to re-evaluate the potentials of greenhouse gas emission savings when using bioenergy for substituting fossil fuels. Different methodologies were used in the separate evaluations: the philosophy and methodology of industrial ecology was used to analyse the sustainability and material flows of the Finnish forest industry. A simple calculation model was developed for analysing the energy consumption and greenhouse gas emissions for biomass production chains, upgrading of biofuels and production of solid recovered fuels. Emission risks from long-time storage of biofuel and biodegradable material were evaluated as well as changes in forest soil carbon due to harvesting of forest residues. The examination of the biofuel production chains showed that in a favourable situation as much as 97-98% of greenhouse gas emissions for a fossil fuel could be avoided by substituting it with a biofuel. On the other hand the investigation also pointed out that e.g. increasing fuel storage and upgrading activities for biofuels are likely to decrease this percentage remarkably. The main conclusion of the thesis was that the neutrality of greenhouse gas emissions when producing bioenergy should be re-evaluated. The author further suggests that tools and stimulants for keeping the greenhouse gas emission levels in fuel production chains as low as possible should be developed.