LCA of transport fuels from short rotation forestry in a long term perspective
Examensarbete för masterexamen
A life cycle assessment (LCA) has been carried out on biofuels for the transportation sector in Sweden originating from a feedstock of domestically farmed wood. When talking about biofuels today, there is a distinction between 1st generation's biofuels and 2nd generation's biofuels. Wood is often referred to as 2nd generation's biofuel feedstock. The benefits of 2nd generation's biofuels, compared to that of the 1st generation’s, are for example a higher yield per hectare and a lesser need for fertilisers during the cultivation. The hard wood species salix was chosen as feedstock. The time frame was 30 years and the studied fuel alternatives were expected to have been introduced on a large scale, thus the studied fuel is considered being used in the background system. Prospective attributional LCA has been used throughout the study. The functional unit was 1 hectare*year and the chosen indicators for the environment were green house gases, energy efficiency and land use. The life cycle included 3 major steps: 1. cultivation of wood; including soil preparation, harvest and termination of the cultivation, 2. conversion of energy into a specific fuel and 3. end use, which in this case meant power to the power train from the engine/motor. The conversion of the harvested salix into transport energy was done in 2 major ways, but with 3 different outcomes: 1. Gasification with either; a. fuel synthesis resulting in DME/methanol, or b. electricity generation by burning the synthetic gas instead of synthesis, or 2. fermentation, where ethanol was the main outcome. In other words: DME/methanol, electricity and ethanol were the main outcomes. Even though DME and methanol are two different fuels, the production was similar up to the very last step, thus the reason for putting them together as one outcome. In the fermentation process, a large amount of lignin-fuel was by-produced. In fact the production of lignin was even larger than the produced amount of ethanol. A system expansion solution was therefore carried out for the lignin, which resulted in theoretically higher conversion efficiency. Since the system was looked upon as a closed loop, meaning that it was self sufficient and that the exact amount of wood harvested was replanted, the only significant GHG's emitted could be traced to the manufacturing of fertilisers. The difference between the outcome alternatives was small and of a very low importance for the overall environmental performance. The most energy efficient conversion was gasification with synthesis to DME or methanol followed by gasification to electricity and, as the most inefficient alternative; fermentation to ethanol. However, when the end use was counted in, the tables turned. Since the efficiency of an electric motor is higher than that of the combustion engine, regardless fuel, the overall efficiency of the gasification to electric motor path was more efficient than the other two alternatives.
Miljöteknik , Environmental engineering