Exergetic analysis of a fast pyrolysis process for bio-oil production

This paper presents an exergetic analysis of a fast pyrolysis plant simulated in Aspen Plus, producing crude bio-oil from lignocellulosic feedstock (hybrid poplar woodchips). The simulation includes the drying and pretreatment of the biomass, the pyrolysis reactor, product recovery, and a combustion reactor that provides the process heat. Chemical and physical exergies are determined for all process streams and the exergy destruction is calculated at the component level of the plant. The overall exergetic efficiency of the plant is found to be 71.2%, with the gas-and-char combustor of the plant causing the highest exergy destruction. Relatively high irreversibilities are also calculated in the pyrolysis reactor and the bio-oil recovery section (quench and water cooler), as well as in the dryer and the mill. Further investigation shows considerable potential for improvement when introducing the hot exhaust gases of the combustor directly in the dryer without using part of their thermal energy for preheating the combustion air. This measure increases the overall plant efficiency to 73.2% by reducing the inefficiencies in the dryer and the heat exchangers. Lastly, the contribution of the compressors and pumps to the overall exergy destruction is found to be rather small.