The Simulation Research Of Residual Ash Formation Process In Pulverized Coal Combustion And Char Fragmentation

Although the proportion of coal has been descending for several years, as Chinese main energy, coal accounts for 66 percent of primary energy at present. The emitting of SO2、NOx and particle matter caused by burning a lot of coal is still the main source of pollutants of Chinese air pollution. Especially the contribution of particle matter in air pollution is particularly serious in recent years. The formation process of particle matter in pulverized coal combustion is a very complicated process, the mass of residual ash dominates the total mass of particulate matter and the char fragment plays a decisive role in residual ash formation and its size distribution, so the research of char fragmentation and residual ash’s size distribution in pulverized coal combustion is significant. However, due to limited conditions, it’s difficult to take a detailed and intuitive research about char fragment and residual ash formation in pulverized coal combustion by experimental methods, while as an important supplement of experimental study, the numerical simulation plays a very important role in describing char fragment and residual ash formation in pulverized coal combustion. The existing char fragment models have their own shortage, the physical properties of char are ignored in population balance model, the size of pore and inclusion is assumed to be monosized in three-dimensional fragment model, there are significant differences between these models and real char. Two models(the continuum model and the discrete model) are developed to simulate char fragmentation and residual ash formation in the thesis. Firstly, the continuum model of monosized mineral inclusions and monosized pores is simulated and then simulate the model which has pore size distribution and mineral inclusion size distributionFirst of all, the geometry of the two models is analyzed and get some important parameters such as separation length scale Ls、coalescence length scale Lc and ash growth number Λ, then program to simulate each of the two models. In continuum model, the char particle is assumed to be a sphere with known radius, pores and mineral inclusions as discrete phase randomly distributed to char sphere. Firstly, the continuum model of monosized mineral inclusions and monosized pores is simulated and analysis the results of simulation, and then simulate the model which has pore size distribution and mineral inclusion size distribution and analysis the simulation results. The discrete model is assuming the char particle as a diamond-shaped cluster, the cluster consists of many unoccupied site or an occupied site, it means that this position is a pore when a site is unoccupied and carbon matrix when a site is occupied, distributing same quality ash in all occupied seats in a certain probability. Reaction of the particle is represented by random removal of sites from the perimeter of th e cluster, thereby simulating reaction in the external diffusion controlled regime, and analysis the results of simulation. At last, a mineral size distribution is utilized to explore its influence on the ash particle size distribution and analysis the res ults of simulation.From the simulation results of the continuum model, we find that the residual ash size distributions are bimodal; The number of residual ash increases and the size of ash reduces with the initial char porosity increases; Residual ash size could be forecasted by ash growth number Λ to some extent. When using a mineral size distribution and pore size distribution, the residual ash size distributions will be more widely.From the simulation results of the discrete model, we can find that the residual ash size distributions are bimodal to; The larger size of the model, the more widely the ash size distribution is, when using a large matrix, the simulation results will be more accurate; The size of residual ash increases with parameter λ increasing; When the porosity increases, the number of char fragmentation increases and the size of residual ash decreases. When using a mineral size distribution, the ash size distributions are more uneven.