Mass Balance and Exergy Analysis of a Fast Pyrolysis System

Abstract

Mass balance closure and exergetic efficiency is evaluated for a bench scale fast pyrolysis system. The USDA Agricultural Research Service (ARS) has developed this system for processing energy crops and agricultural residues for bio-oil (pyrolysis oil or pyrolysis liquids) production. Mass balance closure cannot be achieved due to the system size and complexity of inputs and outputs. A linear programming optimization model is developed to use the experimental data to achieve improved closure of elemental balances without losing the overall representation of the pyrolysis products. Having improved the mass balance, it is then possible to evaluate the exergy of the system. Exergy flows are computed using statistical relationships and other standard techniques. Computational details and results are discussed for switchgrass, a typical candidate biomass. Solutions for the minimum and maximum bio-oil outputs were generated. These particular results indicated that bio-oil accounted for approximately 10% of the loss mass. Considering all products as useful, the exergy destruction is approximately 20%. If the bio-oil alone is considered useful, the exergy destruction is about 40%. Further exercise of the model can be useful in evaluating mass losses and exergy for other feedstock and experimental runs.