Open access publications in the SLU publication database

Abstract

With diminishing fossil fuel reserves and concerns about global warming, the agricultural sector needs to reduce its use of fossil fuels. The objective of this thesis was to evaluate different systems for biomass-based production of tractor fuel and mineral nitrogen fertilisers, which at present are the two largest fossil energy carriers in Swedish agriculture. The land use, energy input and environmental load of the systems were calculated using life cycle assessment methodology. Two categories of renewable tractor fuel were studied: first generation fuels and second generation fuels, the latter defined as fuels not yet produced on a commercial scale. An organic farm self-sufficient in tractor fuel was modelled. Raw material from the farm was assumed to be delivered to a large fuel production facility and fuel transported back to the farm, where it was utilised. In general, the second generation renewable fuels had higher energy balance and lower environmental impact than the first generation fuels. However all systems studied reduced the use of fossil fuels to a great extent and lowered the contribution to global warming. The land needed to be set aside for tractor fuel varied between 2% and 5% of the farm's available land. Two major routes for biomass-based production of mineral nitrogen for conventional agriculture were studied, one based on anaerobic digestion and one on thermochemical gasification of biomass. The crops studied were able to produce between 1.6 and 3.9 tonnes N per hectare in the form of ammonium nitrate. The use of fossil fuel for ammonium nitrate production was 35 MJ per kg N in the fossil reference scenario, but only 1-4 MJ per kg N in the biomass systems. The contribution to global warming can be greatly reduced by the biomass systems, but there is an increased risk of eutrophication and acidification. It is clear that the agricultural sector has great potential to reduce the use of fossil fuel and to lower the emissions of greenhouse gases by utilising biomass resources. However, there are many other issues to be addressed when utilising biomass energy, for example the trade-off between different environmental impacts and the use of limited resources such as fresh water, phosphorus and ecosystem services.