Kinetics of the Thermal Degradation of Rice Husks in Nitrogen Atmosphere

Abstract

The use of agricultural residues for power production has generated increasing interest in the conversion of rice husks to higher quality fuels by thermochemical conversion processes such as pyrolysis, combustion, and gasification. However, the reliable design and operation of thermochemical conversion systems requires consid erable knowledge of rice husk thermal degradation characteristics and kinetics. The thrust of this paper therefore was to study the reaction kinetics of four varieties of rice husk at a heating rate of 20 degrees C min-1 in an inert atmosphere of pure nitrogen (99.5% nitrogen, 0.5% oxygen) using thermogravimetric analysis and derivative thermogravimetric techniques. The thermal degradation characteristics and the kinetic parameters (activation energy, preexponential factor, and order of reaction) were determined for the two prominent reaction zones from the thermogravimetric curves. Thermal degradation rates in the first reaction zone were relatively higher than those in the second reaction zone. The initial degradation temperatures were 220, 232, 223, and 240 degrees C for the rice husk varieties Lemont LG, ROK 14, CP 4, and Pa Potho, respectively. Residual weights recorded at 700 degrees C were in the range of 33.9 - 38.7%. Higher activation energies (29.0 - 35.4 kJ mol-1) and preexponential factors (4.7 x 103 to 6.4 x 104 min-1) were obtained for all rice husk varieties in the first reaction zone as compared to those of the second reaction zone (15.1 - 17.7 kJ mol-1 and 1.0 x 102 to 5.1 x 102 min-1 for the activation energy and preexponential factor, respectively). The order of reactions was in the range of 0.90 - 1.08 for the first reaction zone and 0.30 - 0.36 for the second reaction zone.