Ethanol from Softwood - Techno-Economic Evaluation for Development of the Enzymatic Process

Sammanfattning: Ethanol produced from renewable resources can be used to replace fossil fuels such as gasoline and diesel. This thesis deals with the development of the enzymatic process used for ethanol production from softwood. A techno-economic tool, based on Aspen Plus and Icarus Process Evaluator, was developed in which the whole process, from raw material to ethanol, was studied and critical process parameters identified. The base case model consisted of steam pretreatment, enzymatic hydrolysis, fermentation, distillation, washing of fibrous residues, evaporation and drying, as well as steam and solid fuel production. In the subsequent economic evaluation the ethanol production cost was estimated and compared with costs obtained using alternative process configurations and other process conditions. Based on experimental data it was concluded that a process based on simultaneous saccharification and fermentation (SSF) results in a lower ethanol production cost than separate hydrolysis and fermentation (SHF). The evaluation showed that significant savings in ethanol production cost could be made by reducing the yeast concentration in SSF and by reducing the process energy demand. Simulations showed that the latter could be accomplished by recirculation of process streams or by running the SSF process at a higher substrate load. Mechanical vapour recompression applied to the evaporation step and the replacement of the evaporation unit with an anaerobic digester were both identified as promising alternatives to multiple-effect evaporation of the stillage stream. Two-step steam pretreatment resulted in a lower ethanol production cost than pretreatment in one step. This process concept has yet to be proved on pilot scale. The model was also used for a comparison between the investigated lignocellulosic process and a commercial starch-based process. The ethanol production cost was slightly higher for the lignocellulosic process. However, compared to the starch-based process significant improvements can still be made to the lignocellulosic-based process by reduction of the enzyme loading, energy demand and capital cost. The potential for a higher ethanol yield is also much greater for the lignocellulosic process. The tool presented here is of importance for further development of the ethanol-from-wood process.