Kinetics of Devolatilization of Black Liquor Droplets in Chemical Recovery Boilers - Pyrolysis of Dry Black Liquor Solids

Black liquor is a by-product of the pulping process in the manufacture of paper. It is a complex mixture of both organic and inorganic chemicals with both chemical value and energy content. To recover these two quantities, the black liquor is concentrated to around 60 – 85 % solids concentration and fired into a recovery boiler. In the boiler, the black liquor is sprayed in the form of droplets which fall through the atmosphere of the furnace to the bottom. During this process, the droplet undergoes drying, devolatilization and char burning in succession before coalescing with the smelt at the bottom. The modeling of these processes during the movement of the black liquor droplet is quite complex and in this paper, we attempt to simulate the weight loss during the devolatilization stage by studying the pyrolysis of dry black liquor solids in a muffle furnace. Experiments were carried out at temperatures varying from 973 to 1273 K and for residence times varying from 30 to 180 s. The kinetic scheme of dry black liquor solids decomposition by a single reaction giving gaseous volatiles and char is used. The proposed single reaction model is simulated and the best values of the kinetic parameters i.e. activation energy and Arrhenius constant are found by using the two-dimensional surface fitting non-linear regression algorithm. The results indicate good agreement between predicted and experimental data.