Analysis and ways for advancing of mathematical model of pulverized coal ignition and combustion

Abstract

An analysis of mathematical models of ignition and burning of a single particle and a coal cloud is given. Models which take into account the presence of ash in particles, the influence of the ratio of the amount of coal and primary air (excess coefficient) and the size of coal particles on the ignition process are presented and analyzed. It is shown that simplifications in mathematical models in most cases lead to a loss of accuracy and therefore the results cannot be used for practical purposes. Simulation of complex air supply processes by approximation of uniformity also leads to questionable results. A significant influence on the ignition and combustion of coal particles affects the content and intensity of the release of volatile substances and the chemical reactions that occur in this case. The rate of volatiles yield is proposed to be calculated according to the Arrhenius law, and the activation energy and frequency factor are considered to be those that do not depend on the type of coal, but are determined only by the temperature of the particle. Taking into account heat transfer by radiation and a decrease in the particle diameter during combustion has a positive effect on the results obtained. According to the results, the reactivity and losses with underburning significantly depend on the initial diameter of coal particles. Most models do not take into account the change in temperature inside the particles during heating, ignition and combustion in the apparatus, but there are works that are specifically devoted to the study of temperature fields inside the particles and the influence of the particle shape on the combustion rate. Modeling showed that motion relative to the gas leads to an intensification of heat transfer between the particles and the environment, while the volatile matter yield time decreases at a high ambient gas temperature. A decrease in the rate of combustion chemical reactions is noted with an increase in the concentration of water vapor in the gaseous medium around the particle, i.e. oxygen diffusion is the limiting factor in particle combustion. The most complete and physically correct gas dynamics during combustion is calculated in models where known turbulence models are used, such as the standard k-e model, RNG k-e model, BSL model and SST model. At the end of the article, the concepts for improving the model of ignition and combustion of coal particles are outlined.