Numerical Analysis Of Fine Particles Formation During Coal Combustion

Inhaled particulate matter from combustion does serious harm to human health and environment. To know how the particulate matter generates and grows in the combustion process is the prerequisite to control and prevent them. Nucleation, condensation, coagulation of fine particles and the diffusion to the coarse particles are main courses which influence the size distribution of fine particles. To simulate and predict these processes is the hotspot of combustion process and aerosol studies nowadays. Here in this article we firstly summarize the study evolution and models about particle nucleation, condensation, Brownian coagulation and the diffusion of fine particles to the surface of the coarse particles, then give a reasonable initial log-normal size distribution to the particle system, using moment method finally compute the change of size distribution with time and discuss the change of size distribution during the process of transportation by combining the moment method and the modeling of the fluid field.First three basic processes: nucleation, condensation and coagulation for poly-disperse particles have been modeled. The results show that, the number concentration of particles first increases rapidly and then reduce gradually, the average particle size first reduce and then increase gradually, and the processes spread the size distribution first, then make it narrower, finally the particles can achieve a self-persevere state. It has been found that coagulation has a most important effect on the final particle size distribution while comparing the single process. Considering particles are not spherical actually, the coagulation of non-spherical particles ahs been discussed. It has been found that the fractal dimension can accelerate the coagulation process and as the spherical particles, those fractal particles can obtain the self-persevere size distribution gradually.According to the actual condition in the furnace, the scattering between fine particles and coarse particles and coagulation between fine particles are discussed. The results show that, for fine particles, coagulation between fine particles still control the final size distribution. If the initial size distribution is set to be broader, the scattering has a more obvious effect on the change of size distribution, but coagulation is still the main process. After coagulation and scattering, fine particles have a diameter between 0.1μm and 0.9μm, and particles with a diameter about 0.5μm have the largest number concentration.Finally transportation and coagulation of particles are considered, which realizes the combination of moment method and the fluid field simulation. Coagulation of particles in the reflux and upper part of the back step flow occurs much faster than other part of fluid field. Particles in the main flow region can obtain the self-persevere distribution. For the particles with a broad distribution, the strong flow in the reflux region makes the distribution much broader. When particles transport to the part out of the reflux region, the distribution changes to be narrower, and finally it gets to be as broad as its original distribution.