Kinetics of the Thermal Decomposition of Biomass

Abstract

This paper is concerned with the kinetics of the thermal decomposition of a woody biomass, willow. It addresses two questions. First, what method of data analysis is appropriate for extracting reliable kinetic data from thermogravimetric analysis (TGA) experiments? Second, what kinetics are most suitable for high heating rate situations such as those present in pulverized fuel power stations? It contains kinetic analysis of willow TGA data using a variety of approaches. A review of previously published work on biomass and its polymeric components helps ascertain the variation in kinetics, reasons for differences, and extrapolation to flame temperatures. The data falls into two main categories: (1) very high E and A values (>100 and up to 270 kJ/mol, and up to 1017 s−1) derived when model biomass components are studied, for example, cellulose; or the data is interpreted as the sum of a number of individual first-order reactions, for example, FG-BioMass; (2) intermediate and low E and A values (50−100 kJ/mol and <∼105 s−1) derived using a number of global approaches. For low heating rate experiments (101 K/min), the global first-order reaction kinetic models that tend to yield low E values, such as the reaction rate constant method, work well. High E kinetics can also work well at low heating rate, but only if the reaction is assumed to be due to the sum of a number of individual steps, such as the biochemical components degrading independently, or the functional group approach. For higher heating rates (>103 K/s) high E kinetics predict conversion well, and this can be rationalized since primary cracking reactions will dominate under these conditions. However, at heating rates of 105 K/s and temperatures of 1500 °C (i.e., flame conditions), a compensation on the rates is seen and the choice of rate parameters is less critical. Two sets of kinetic data, E = 178.7 kJ/mol, A = 2.2 × 1013 s−1 and E = 48.7 kJ/mol, A = 6.84 × 103 s−1, both predict conversions in keeping with the available experimental data.