The Investigation On Char Characteristics And Particulate Matter Formation During Coal Combustion

Coal, as the most important conventional energy in China, while supplying a large amount of thermal energy during combustion, also releases a great deal of pollutants. Among these pollutant, besides SO_x,NO_x,CO₂, the emission of fine particulate matters gradually attracts the attention of the researchers around the world. Some researchers indicate that coal combustion is one main source of inhalable particles in the atmosphere, which are greatly harmful for human health and ecological environment. Moreover, among the cities which have exceed the standard of air in China, 68% of them has serious problem of inhalable particles pollution. Therefore, it has great scientific and practical significances to investigate the formation mechanisms and emission control of particulate matter during coal combustion. Firstly, the mechanisms of fine particle formation during coal combustion have been analyzed. Main particulate matter formation mechanisms are vaporization and subsequent condensation of inorganic matter, included mineral coalescence, char fragmentation and excluded mineral fragmentation. This paper focuses on the research about char characteristic and its effect on particulate matter formation by model calculation and experiments. Then, the mechanisms of fragmentation by devolatilization are analyzed theoretically. A mathematical model is developed for the prediction of the distribution of pressure and applied force in a coal sphere during devolatilization. Some factors affecting the fragmentation caused by devolatilization such as particle size and furnace temperature are considered. The results indicate that the possibility of fragmentation would increase with volatile matter content and furnace temperature. This model calculation provides support for the subsequent experiments. Finally, the char characteristic and particulate matter emission of Pingdingshan bituminite are investigated experimentally by Drop Tube Furnace (DTF). Char particles are collected by glass fiber filter, and particulate mattes are collected by Dekati Low Pressure Impacter (DLPI) under various conditions. Effects of particle size and temperature on char characteristic are analyzed, including pore structure, morphology, swelling characteristics of char particles. Also, effects of different factors on particulate emission are studied. Decreasing coal particle size and increasing the oxygen concentration lead to more supermicron-sized particulate matter formation. To a great extent, the char characteristics can explain the emission of supermicron-sized particulate matter.