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Torrefaction of Cellulosic Biomass Upgrading – Energy and Cost Model

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  2010 Pittsburgh, Pennsylvania, June 20 - June 23, 2010  1009376.(doi:10.13031/2013.32188)
Authors:   Devanand Maski, Matthew Darr, Robert Anex
Keywords:   Biomass upgrading, torrefaction energy, production cost, cellulosic feedstock, biomass moisture, simulation model, torrefaction temperature

The upgrading technology like torrefaction can convert biomass from a highly variable low density feedstock into a consistent high-energy-density commodity, which substantially reduce the production cost. The goal of this study was to quantify torrefaction as a transformative upgrading technology to break current cellulosic feedstock production cost barriers delivered to biorefinery. A robust and expanded torrefaction process simulation model having the capability to quantify biomass torrefaction energy and cost components was developed in Matlab Simulink. Simulation tests were carried out to analyze sensitivity of torrefied biomass energy and production cost scenarios in response to moisture content of corn stover using model key parameters. Torrefaction temperature at critical level is an important requirement for auto-thermal operation of torrefaction process, which can greatly reduce cost of energy requirement and also volatile waste stream. The process can generally be operated as auto-thermal at the temperature 240 0C and above depending upon moisture content of corn stover. For higher torrefaction temperatures of 240, 260, 280, and 300 0C, the rate of increase in cost is gradually linear up to 10, 30, 40, and 50 % moisture content respectively, where torrefaction energy requirement can be met by flue gas energy until this moisture content and above which the operation requires external energy supply (auto thermal operation). For a typical 30 % moisture content of corn stover the normalized net energy ratio is around 0.86 at 240 0C. Torrefaction upgrading should be considered over a minimum 4-month operational period in order to begin minimize production cost. The modeling approach demonstrated in this study may be extended to cost effective and quality enhancing pretreatment of a broad spectrum of other biomass feedstocks and thereby create opportunities in a variety of biorefineries ranging from cellulosic bio-coal to ethanol.

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