Determination of Reaction Kinetics of Rice Husks in Air Using Thermogravimetric Analysis

Abstract

Rice husk is produced in large quantities as a by-product of rice milling in rice-producing countries and has posed disposal problems in these countries. Disposal of or energy recovery from rice husk can be accomplished by thermochemical conversion processes (pyrolysis, combustion, and gasification). However, it appears that the kinetics of rice husk, which can contribute to the accurate modeling and design of thermochemical conversion processes, have not been studied extensively. In this paper the technique of thermogravimetric analysis (TGA) was used to study the thermochemical behavior of four varieties of rice husk (Lemont LG, ROK 14, CP 4, and Pa Potho). The thermal degradation of rice husk was studied in an air atmosphere (21 % oxygen and 79 % nitrogen) from ambient temperature to 700 degrees C at the heating rate of 20 degrees C min-1. The thermograms showed two distinct reaction zones. The kinetic parameters (activation energy, preexponential factor, and order of reaction) were determined for the two reaction zones by applying thermoanalytical techniques to the reaction kinetics. Higher thermal degradation rates were observed in the first reaction zone due to rapid release of volatiles as compared to those in the second reaction zone. In the first reaction zone the activation energies ranged from 37.0 to 54.7 kJ mol-1. Relatively lower activation energies (18.0 - 21.0 kJ mol-1) were obtained in the second reaction zone. The preexponential factors were in the range of 4.3 x 104 to 6.4 x 10 6 min- 1 in the first reaction zone and 4.5 x 102 to 1.5 x 103 min- 1 in the second reaction zone. The orders of reaction were in the range of 1.2 - 1.6 and 0.4 - 0.5 for the first and second reaction zones, respectively. The predicted thermal degradations were in good agreement with the experimental data in both the first and second reaction zones.