Life cycle assessment of rapeseed oil, rape methyl ester and ethanol as fuels

摘要

Production of rapeseed oil, rape methyl ester (RME) and ethanol fuel for heavy diesel engines can be carried out with different systems solutions, in which the choice of system is usually related to the scale of the production. The main purpose of this study was to analyse whether the use of a small-scale rapeseed oil, RME and ethanol fuel production system reduced the environmental load in comparison to a medium- and a large-scale system. To fulfil this purpose, a limited LCA, including air-emissions and energy requirements, was carried out for the three fuels and the three plant sizes. Four different methods to allocate the environmental burden between different products were compared: physical allocation according to the lower heat value in the products [MJ/kg], economic allocation according to the product prices [SEK/kg], no allocation and allocation with a system expansion so that rapemeal and distiller’s waste could replace soymeal mixed with soyoil and glycerine could replace glycerine produced from fossil raw material. The functional unit, to which the total environmental load was related, was 1.0 MJ of energy delivered on the engine shaft to the final consumer. Production of raw materials, cultivation, transport, fuel production and use of the fuels produced were included in the systems studied. The results for small-scale plants (physical allocation) are shown in Table I. It was also shown in the study that the differences in environmental impact and energy requirement between small-, medium- and large-scale systems were small or even negligible in most cases for all three fuels, except for the photochemical ozone creation potential (POCP) during ethanol fuel production. The longer transport distances to a certain degree outweighed the higher oil extraction efficiency, the higher energy efficiency and the more efficient use of machinery and buildings in the large-scale system. The dominating production step was the cultivation, in which production of fertilisers, followed by soil emissions and tractive power, made major contributions to the environmental load. [Table 1] The results were, however, largely dependent on the method used for allocation of the environmental burden between the products, i.e.: rapeseed oil and meal, RME; meal and glycerine; and ethanol fuel and distiller’s waste. The results were also dependent on uncertainty in input data, e.g. yield of rapeseed and wheat and use of fertilisers, and on alternative production strategies such as use of catalysts when the fuels produced are consumed, use of an ignition improver of biomass origin during production of ethanol fuel, or use of methanol with biomass origin during production of methanol for transesterification of rapeseed. The costs for production of the fuels in a small-scale plant from raw products grown on a small farm excl. EU area compensation were: rapeseed oil 0.85 SEK/MJengine; RME 1.07 SEK/MJengine; and ethanol fuel 1.29 SEK/MJengine. The corresponding costs for production of the fuels in a large-scale plant from raw products grown on a large farm incl. EU area compensation were: rapeseed oil 0.33 SEK/MJengine; RME 0.35 SEK/MJengine; and ethanol fuel 0.57 SEK/MJengine.