Mass Balance, Energy, and Exergy Analysis of Bio-Oil Production by Fast Pyrolysis

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture (USDA). USDA is an equal opportunity provider and employer. Mass, energy, and exergy balances are analyzed for bio-oil production in a bench-scale fast pyrolysis system developed by the USDA’s Agricultural Research Service (ARS) for the processing of commodity crops to fuel intermediates. Because mass balance closure is difficult to achieve due, in part, to the system’s small size and complexity a linear programming optimization model is developed to improve closure of elemental balances without losing the overall representation of the pyrolysis products. The model results provide an opportunity to analyze true energy and exergy balances for the system. While energy comparisons are based on heating values, exergy flows are computed using statistical relationships and other standard techniques. Comparisons were made for a variety of biomass feedstocks including energy crops and various byproducts of agriculture and bioenergy industry. The mass model allows for proper accounting of sources of mass loss and suggestions for improved system performance. Energy recovery and exergetic efficiency are compared for a variety of pyrolysis product utilization scenarios including use of biochar and noncondensable gases as heat sources. Exergetic efficiencies show high potential for energy utilization when all the pyrolysis product streams can be recycled to recuperate their internal energy. The exergy analysis can be beneficial to developing exergetic life cycle assessments (ELCA) for the fast pyrolysis process as sustainable technology for advanced biofuels production.