Numerical analysis of kinetics of char combustion process and selection of mechanism functions

Abstract

Low and medium temperature pyrolysis technology is an important way of clean and utilization of low metamorphic coal. In this paper, three kinds of char samples were prepared in tube furnace according to different pyrolysis temperatures, and their combustion characteristics were compared by means of thermal analysis. Three modeless kinetic methods, KAS, FWO and Fredman, were used to study the combustion reaction activity under nonisothermal conditions from TG curve and DSC curve. In the end, Malek method is used to explore the mechanism function of the reaction process, and the results are analyzed numerically. The objective programming model and MATLAB are used to calculate the combined mechanism function of the reaction process. The results show that with the increase in the pyrolysis temperature, the combustion of char gradually shifts to the high temperature zone and shows a secondary effect between the volatile matter and the combustion characteristic temperature. The results show that the flammability index and burnout index increase first and then decrease, and the char prepared at 550 °C has the most developed pores and the best combustion performance with the increase in pyrolysis temperature, the average reaction activation energy decreases first and then increases, and the char prepared at 550 °C has the lowest activation energy. When the combustion heating rate is 5 °C min−1, the experimental curve and theoretical curve have a higher coincidence, and the combined mechanism function is adopted. The number is more suitable than the single most probable mechanism function to describe the combustion process of char. At this heating rate, the fuel reaction conforms to Z–L–T equation and reaction order model, which is limited by three-dimensional diffusion and chemical reaction. The distribution coefficients of the combined mechanism function are 0.4207 and 0.5886.