Temperature dependence of biomass combustion kinetics

Biomass residues, such as rice straw, are available abundantly in most Asian countries, where rice is replaceable. In order to better utilize the wastes as fuels, they are pyrolyzed prior to its briquetting in order to improve the Heating Value and carbon content. The pyrolysis process is carried out at 400°C for 30 minutes. The heat required for pyrolysis is supplied by electric resistance nickel wires coiled around the cylindrical reactor. The obtained char is crushed, mixed with natural binding material and then briquetted in form of a bar with rectangular cross sections 16 × 16 mm and height between 28 - 35 mm. The briquettes weigh around 5 gram, consists of 4 gram char and 1 gram binder. Subsequently the briquettes undergo combustion process in the same reactor but the wall temperature is kept constant at 350°C, while the initial temperature of the combustion process is varied 125°C, 150°C, 175°C, 200°C, and 225°C. The observed combustion processes (represented by reaction rates, combustion periods and maximum temperatures) reveal their strong dependence on briquette temperatures, as shown in the Arrhenius formula. The acquired thermogravimetry data (temperatures and masses) are stored and analyzed in order to determine the Activation Energy and Pre-exponential Coefficient of the combustion kinetics. The activation energy varies between 87 and 370 kJ/mol, while the Pre-exponential Coefficient changes even more terribly between 195,000 and 8.54 × 10³⁵ m/s. Theoretically, these values should be closely similar, because they should be specific to carbon, regardless of the source material, the process, and the initial temperature. Nevertheless, since combustion of solids is highly unstable, the acquired fluctuated data lead to these varying results.