Submitted to: Journal of Energy Resources Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2012
Publication Date: 10/19/2012
Citation: Boateng, A.A., Mullen, C.A., Osgood-Jacobs, L., Carlson, P., Macken, N. 2012. Mass balance, energy and exergy analysis of bio-oil production by fast pyrolysis. Journal of Energy Resources Technology. 134/042001-1-9. Interpretive Summary: The U.S. Departments of Agriculture and Energy are committed to achieving the country’s energy security through the development of domestic renewable energy and advanced biofuels which will at the same time create opportunities for the farm and rural communities. Of the various biomass conversion technologies being studied, fast pyrolysis has received the farmers’ attention due to its small footprint and potential ease of deployment on-farm. At ARS, a bench scale unit has been developed to test this process on many sources of biomass, including dedicated energy crops (e.g. switchgrass), crop residues (e.g. corn stover, straw) and other waste materials. For the pilot scale system, small size, the high viscosity of bio-oil and the variability of feedstock make it difficult to achieve a balance of the material flows in and out. Therefore, a mathematical mass balance model was developed which can take data generated from test runs and determine how much of the carbon, hydrogen, oxygen and nitrogen in the biomass is ultimately found in each of the products from this process. It can also help researchers determine the amount of each material that is not collected because it is stuck in the system or escaped the system prior to collection. These losses will be minimized in scale up to a production system, and this model allows for prediction of product yields on a larger scale. Another model was then developed which takes the outcome of the mass balance model and data from testing on the products and determines the exergy efficiency of the system. Exergy is an advanced theory which is related to energy, but measures the “real world” work which can be done by a material. The exergy results for the system can be incorporated into a larger context where the process is part of a system where biomass is grown, converted to fuel and then consumed as fuel and be used to determine the sustainability of the entire system. This information will be useful to those developing and commercializing biofuel products via pyrolysis and those studying sustainable biofuels development.
Technical Abstract: 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 non-condensable 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.