Kinetic Parameters for Biomass under Self-Ignition Conditions: Low-Temperature Oxidation and Pyrolysis

Pulverized biomass may self-heat and spontaneously ignite when stored or processed at intermediate or even low temperatures. In this work, reaction kinetic parameters for biomass oxidation and pyrolysis were determined for the temperature range 423–523 K. Thermogravimetric analysis was used to determine mass loss kinetics in a stepwise-isothermal heating program. Two wood species (pine and beech), two agricultural residues (wheat straw and sunflower husks), and two commercial wood pellet samples were investigated. Atmospheres with 0, 20, and 80% oxygen were used in the experiments. A pyrolysis model of four parallel reactions for extractives, hemicellulose, cellulose, and lignin fit the experimental data for 0% O2 well. Oxidation kinetics could be modeled by additional reactions in parallel to the pyrolysis mechanism. Two mechanisms were tested: (1) considering oxidation of a lumped “volatilizable” component plus oxidation of char; and (2) separate oxidation reactions for volatilizable extractives, hemicellulose, cellulose, and lignin, plus char. The more complex mechanism did not give a clear advantage over the simpler mechanism. It was further found that pyrolysis and oxidation reactions for the components could be modeled with the same activation energy, regardless of which biomass they appear in. For the lumped component oxidation model, an apparent activation energy of 130 kJ/mol was found. The observed reaction order in oxygen was in the range 0.4–0.5. The models also compared favorably to additional experimental data between 373 and 773 K for a heating rate of 5 K/min. The kinetic models presented here are intended mainly to describe low-temperature reactions, such as self-heating of biomass and the onset of smoldering combustion.