Simulation Of Fine Particles Coagulation During Pulverized Coal Combustion By Moment Method

The aerosol emissions from coal combustion and waste incineration generally have seriously impact on the human health and environment, which are too small to be captured and enriched with heavy metals. To know how the particulate matters (PM) generates and grows in the combustion process is the prerequisite to control and prevent them. After vaporization, the fine particles are formed as nucleation,condensation and coagulation processes during coal combustion. Among all these processes, coagulation plays a dominant role for fine particles growth and removal.Here in this article we firstly summarize the evolution and mathematical models about fine particles nucleation, condensation and Brownian coagulation, then give a reasonable initial log-normal size distribution to the particle system, using moment method simulate the particle size distribution (PSD) changed during Brownian coagulation processes in the poly-disperse system. Further more, we will discuss the changed of PSD during the process of transportation by combining the moment method and the modeling of the fluid field.Generally speaking, the particles generated from combustion have a typical bimodal mass distribution, so the coagulation can be divided as intra-coagulation which happened within the same mode, and inter-coagulation happened between different modes. The results show that when the ratio of number concentration between fine and coarse mode larger than a certain value, the intra-coagulation will be dominant for particles growth, while the number of fine particles reduced, the inter-coagulation become more effective comparatively, but the inter-coagulation process leads to a smaller and narrower size distribution, and finally could not reach the self-preserving state. From the results, it is also found that the initial deviation of particles and the properties of flue gas can make significant difference on the PSD.Finally, combined the moment method and the modeling of the combustion fluid field, the dynamic characteristics of these fine particles in coal combustion process are theoretically analyzed. The results show that the number concentration of the PM is higher in the high temperature zone, and less in the upper part of the furnace where temperature is lower, but its diameter tends to be larger at the outlet of furnace. Another hand, the number concentration and the mass concentration of particle are higher in the reflux region. These result indicated that the structure of flow field has significant influence on particle coagulation behavior and its PSD.