Techno-economic analysis of a production-scale torrefaction system for cellulosic biomass upgrading



Torrefaction of biomass early in the supply chain is one method of enhancing or preservin biomass quality during storage; reducing storage and transportation costs; attaining uniformity among different feedstock types; and meeting biorefineries' quality and quantity demands for feedstock in a cost effective and sustainable manner. The main objectives of this study were to analyze the mass and energy balance, to assess the techno-economic feasibility of a production-scale torrefaction system, and to quantify the sensitivity of energy and cost results to changes in initial feedstock moisture content, torrefaction process temperature, plant operating window, and initial capital investment. The output from this analysis provides clear guidance on the opportunities and costs associated with production-scale torrefaction of biomass as a feedstock intermediate. Analysis of energy results indicated that the net external energy required for the torrefaction process increased and energy efficiency of the torrefaction system decreased with increasing moisture contents. However, both of these energy metrics exhibited a decreasing trend with increasing process temperatures. In addition to the energy results, unit torrefaction process cost exhibited a decreasing trend with decreasing initial moisture contents of feedstock and decreasing torrefaction process temperatures. For the typical moisture content of 30%wb, process temperature of 240°C, plant operating window of 6 mo.yr-1 and initial capital investment of $7.5 million for the system with rated capacity of 25, unit torrefaction process cost was estimated to be 17.5 $-Tton-1. Additional system improvements through capital cost reduction and wider operating windows can yield a torrefaction product cost of ∼12 $-Tton-1. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd