(64e) Methods for Determining Chemical Mechanisms of Biomass Torrefaction

The main objective of torrefaction is to produce a heat-treated solid, char-like material. Torrefaction (or slow to mild pyrolysis) serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation; for example, combustion and co-combustion with coal. The topic has gathered interest in the past two decades but further understanding is required for optimization of the process thus enhancing economic efficiency, which is crucial to the success of the commercial operations. In particular, there is a noticeable gap in current literature regarding kinetic mechanisms and rate data. Limited kinetic data and reaction models have been recently published for woody materials and recently reed canary grass, wheat straw, and willow. A quantitative understanding of torrefaction for many varieties of biomass may be attained by conducting appropriate torrefaction tests; particularly softwoods and herbaceous biomass that contain a significant fraction of hemicelluloses as glucomannan as opposed to hardwoods and wheat straw which contains relatively high amounts of hemicelluloses as xylan. Organic matter in biomass is comprised of lignins, hemicelluloses, and cellulose. While the ratio of these constituents varies depending on the type plant species and growing conditions, it has been postulated that the thermal-chemical behavior of these constituents in the temperature range of torrefaction (i.e., 200-300°C) is primarily a function of heating rate, peak temperature, time at peak temperature, and gas temporal conditions. Torrefaction appears to be a two-stage process where hemicelluloses decomposition and devolatilization precedes cellulose breakdown and dehydration into smaller chains. Molecular cross-linking has also been observed, which may be a detriment in biochemical conversion of torrefied feedstock. A method for elucidating the mechanisms of hydrogen and function group extraction and chemical cross-linking of the three major constituents has been developed. This requires control of thermal energy deposition and real-time measurement of species evolution, followed by post torrefaction sample chemical and physical analysis. A heated fluid-sand bed is used to achieve consistent heating conditions of closely uniform particles sizes. Species devolatilization is monitored using a set of analytical instrument and laboratory methods for volatile and semi-volatile species determination. Mass evolution rates are correlated to obtain devolatilization rates that can then be applied to single particles or particle ensembles. NMR is useful for determining cross linking that occurs within the samples.

An early set of data, rate correlations, and physical/chemical properties of torrefied corn stover are presented in this paper to illustrate application of the testing and characterization methods.