The Three-Dimensional Percolation Simulation Of The Porosity Effect On Coal Char Fragmentation And Particulate Formation

The particulate emissions during the coal combustion account for about 25% of the total particulate emissions, becoming the major pollutant of atmospheric pollution. At present, technology and fundamental research of the particulate emission reduction basing on the combustion process are still devoid. Therefore, the paper carries out the research about main formation mechanism of ultra-micrometer particle, including the char fragmentation and the mineral coalescence during the combustion process.The complex char pores have direct impact on combustion, fragmentation and particulate formulation. The research built three-dimensional percolation models in order to study above process on the microscopic view, including simulating the combustion reaction in diffusion-controlled-regime by removing the occupied-seats layer by layer, and simulating the ash formation simultaneously by recording mineral redistribution and coalescence conditons in the burning process. The findings are as follow: the increase of porosity can accelerate the reaction rate and intensify the char fragmentation; the generated particles are moving to the direction of smaller size; the trend of the ash mass-distribution is similar to that in relevant experiment reaearches; the initial mineral content does not affect the fragmentation circumstance. The external-diffusion-controlled model is improved by changing the select method of burning seats to make the combustion reaction proceed both inside and outside the char surface simultaneously to simulate the combustion reaction in chemical-reaction-controlled regime. The simulation results show that the increase of the initial porosity will also accelerate the reaction rate and intensify the char fragmentation, and what makes the severe fragmentation different from the diffusion-controlled-regime is the reaction on the inner surface. So that at different porosity the numbers of separated ash are all greater than the numbers in external-diffusion-controlled regime, and more smallsized particles produce and distribute evenly and the particle sizes concentrate. To simulate the pulverized coal combustion reaction in transiton temperature controlled zone in boiler, this paper comes up with the concept of diffusion depth based on the kinetics controlled regime, in order to control the depth that the reaction enter into the coal-char’s interior. The results show that: the reaction transit from kinetics controlled regime to diffusion controlled regime with the diffusion depth reducing; the fragmentation degree slow down, resulting in increasing of the number of separated ash and discreasing of the size of large diameter particles. The result illustrates that the diffusion depth can realize the inner surface reaction in transition controlled regime well, and it can also simulate the actual situation that the reaction gases cannot diffuse into the inner core of coal-char.The research’s results can tell us that improving the reaction temperature in the process of combustion will make the reaction carry out near the diffusion controlled regime, which will weaken the coal-char fragmentation phenomenon, reduce the amount of particulate and increase the particle size. It is good for particulate capture after burning, and providing theoretical basis for reducing the environmental impact of coal particles.