Techno-economic evaluation of integrated lignocellulosic ethanol production
Sammanfattning: Sweden has claimed that it wants to have a fossil-free vehicle fleet in 2030 as a milestone of its ambitious goal for 2050 to be a country with a sustainable and resource-efficient energy supply without any net GHG emissions. Production of bioethanol from plant materials is one potential method of meeting this goal. A large amount of research has been devoted toward the development of sustainable and cost-efficient processes for biofuel production from second-generation (2G) lignocellulosic material. Ethanol production from first-generation (1G) feedstock, such as sugar- and starch-based material, is a well-established concept that has been implemented in many countries. Techno-economic analysis has a significant function in evaluating the process development and the targeting of technical and economic bottlenecks that can arise. This thesis performed techno-economic evaluations for various process designs, including ethanol production from 2G lignocellulosic materials. The starting point for the process design was based on Swedish conditions using raw materials that can be found in Sweden—ie, straw and forest logging residues. However, these materials are also common in other parts of the world. The models can thus be used as foundations for other scenarios. The technical evaluations can largely be transferred to other situations, whereas the economic evaluations require some changes in the economic assumptions. First- and second-generation (1G+2G) ethanol production was also integrated for grain and straw to facilitate the introduction of 2G ethanol production by a consolidated 1G ethanol process and supply the 1G process with heat. Co-products of the process, such as biogas, dry solids for heat generation, electricity, heat for district heating, and distiller’s dried grains with solubles, were considered in the process design. To maximize the production of co-products and decrease the energy demand in the process, heat integration was implemented for all configurations. In addition, the potential biomass availability and integration with district-heating systems and pulp and paper mills were examined to maintain a sustainable forestry practices and energy-efficient use of raw materials. The process models were based largely on experimental results from lab-scale and process development unit trials that were performed in the Department of Chemical Engineering, Lund University. The results were implemented in the flowsheeting program Aspen Plus, in which overall process material and energy balances could be calculated. Moreover, heat integration by pinch analysis was performed in Aspen Energy Analyzer. The results from the material and energy balances analyses could then be used to size process equipment, and the process cost was estimated using Aspen Process Economic Analyzer with quotations from vendors. The process heating and cooling requirements from primary steam and cooling water, respectively, could be substantially decreased by implementing heat exchanger networks, which were more cost-efficient than utilities use only. The biomass availability of forest logging residues is highest in the northern and central parts of Sweden, where a biorefinery that uses 150 to 250 tonne dry forest logging residues per year can be located. Integration with district-heating systems was more important for small-scale versus large-scale plants, wherein the increased ethanol production outpaced the income from heat generation. Moreover, the feasibility of ethanol production from straw depends significantly on high xylose utilization due to the large amount of hemicelluloses in this type of raw material. A study of process improvement by higher water-insoluble solids content in simultaneous saccharification and fermentation of glucose sugar for ethanol production from straw showed that further improvements are likely necessary to increase the economic feasibility, which will, however, also depend on the assumptions. The economic feasibility also increased with xylose fermentation and upgrade of the biogas that was produced in the process to vehicle fuel quality. Integration of 1G+2G ethanol production from straw and grain was also an economically feasible concept.
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