Compatibility of sustainability and economic efficiency of bioethanol production with a special focus on small ethanol plants

The EU Biofuels Directive stipulates the substitution of fossil fuels by environmentally compatible alternatives. To that effect EU Member States should ensure a minimum percentage of biofuels and other renewable fuels to be placed on their markets. In Austria the ambitious national target for 2008 is to substitute 5.75 % of the energy content of all fuels for transport purpose. Within the current political framework the production of bioethanol is steadily gaining importance. However, the sustainable development of bioethanol as a renewable source of energy has to allow for ecological considerations in the production process itself, too. The level of sustainability is challenged by the fact that common large-scale bioethanol industry is powered by fossil fuels. To meet the requirements of feedstock, also considerable transportation emissions are involved. The main objective of this work is to evaluate different scenarios of small-scale bioethanol production (1000, 5000, 10000 tons bioethanol/year) from wheat and maize with innovative energy supplying facilities. Since the demands of small plants are much lower, they can be supplied with feedstock at local level. Transportation efforts are abated, while the position of farmers as energy producers is strengthened. Several other environmental aspects are considered, such as sustainable crop rotation concepts for regional feedstock production in accordance with local food production, and – most importantly – the direct as well as indirect thermal utilisation of biogenic residual materials in order to substitute fossil fuels within the bioethanol production process. The three most valuable process options for renewable energy supply are identified by process simulation: (1) biogas production from stillage and co-substrates utilised in a combined heat and power (CHP) plant, (2) biogas production from stillage only utilised in a gas-fired boiler, and (3) process steam production by straw incineration. In order to assess the methane yield of different biogas feedstock, stillage co-fermentation with agricultural residues is examined in batch and continuous experiments. The experimental data is implemented in the process simulation environment. With regard to the ecological footprint, the developed small-scale bioethanol plants are considerably more beneficial than common large-scale industry, which is still powered by fossil fuels. In terms of the Sustainable Process Index (SPI) the ecological impact can be reduced by 61 to 92 %. The main result of the economic feasibility study is that in 12 out of 21 analysed plant scenarios the ethanol price is lower than 0.7 €/l. Thus, it is feasible to produce bioethanol in a sustainable as well as economic way.