Study On Submicron Particle Formation And Emission During Coal Combustion

Most of fuel consumed in power station in China, now and in the future, is coal. Coal combustion supplies a large amount of thermal energy, and also releases a great deal of particulate matter to the atmosphere. Some researchers indicate that coal combustion is one main source of inhalable particles in the atmosphere, which are greatly harmful for human health and ecological environment. However, little information about the formation mechanism of submicron particulate matter (SPM) and its controlling in the combustion process was reported because of the complexity of the SPM formation. Therefore, it has great scientific and practical significances to investigate the formation mechanisms, evolvement process and emission characteristic of the SPM during coal combustion.Firstly, this thesis discussed the serious harm of the SPM emission on human and inhabitation environment, which means that it was important and necessary to study the SPM formation and controlling method. Simultaneously, this thesis provided an overview of the present researches on the formation, emission and numerical simulation of the SPM, and analyzed the deficiency of the existing research. Then the relevant research work was carried out as follows.In first section, the mass size distribution, physicochemical characteristics and formation mechanism of the SPM from pulverized coal combustion in a drop tube furnace were discussed. Factors that influencing the emission characteristics and elemental partition of the SPM were also investigated, which included the distribution of furnace temperature, coal particle size and oxygen concentration. A similar bimodal distribution of PM₁₀ with a small and a large mode at 0.0944μm and 3.95μm, respectively, was obtained in all runs. Based on the comparison of concentrations of ash-forming elements in the size-segregated ash with their filter concentrations, it was concluded that the ash smaller than 0.377μm were formed by the nucleation of vaporized ash components and growth via coagulation and heterogeneous condensation. The microstructure of submicron PM was measured by a scanning electron microscopy, showing that these particles were approximately spherical in shape. Coal particle size, oxygen concentration, and especially furnace temperature affected the SPM emission significantly in the combustion process. Increasing furnace temperature and oxygen concentration, and decreasing coal particle size would lead to more the SPM formation.To better understand the emission characteristics and chemical composition of the SPM, fly ash was sampled in the entrance and exit of the dust cleaning equipments, such as ESP and venturi scrubber, in several large-scale utility boilers. A similar bimodal distribution of PM₁₀ was obtained in the studied boilers. The small and large modes are located at 0.1μm and 4.0μm, respectively. The chemical components were the oxides and sulfates of alkali metal in the SPM, while the refractory oxides contributed to the major composition of coarse particle. The collection efficiency of the dust cleaning equipments had a minimum in particle size range of 0.01-1μm. The minimum is 65% and 50% for ESP and venturi scrubber, respectively. The study provided the bases for researching and controlling the emission of particulate matter from the thermal power plant.In this thesis, a model of ash vaporization was established and integrated into a self-developed CFD code to predict ash vaporization in coal combustion process. Experimental data from a single particle combustion was used to validate the above model. The calibrated model was then applied to simulate the ash vaporization in a 92.9 kW laboratory-scale single-burner furnace. The effects of different combustion conditions, including air staging, on the ash vaporization were investigated. The results showed that the fraction of ash vaporization is mostly sensitive to coal particle temperature. Ash vaporization primarily occurred after a short interval along the coal particle trajectories when the particle temperatures increased to 1800K. Air staging influenced the ash vaporization by changing the gas temperature distribution in the furnace. The simulation results showed that the more extreme the staging condition, the lower the overall peak temperature, and hence the lower the amount of ash vaporization.Finally, the dynamic characteristics of the SPM in coal combustion process were theoretically analyzed, and a mathematic model was developed to predict the formation of the SPM. The results showed that the number concentration of the SPM was higher in the high temperature zone, but the mass concentration of the SPM increased with the increase of the furnace height. This indicated that controlling the furnace temperature could decrease the formation of the SPM. Therefore, the simulation of deflected second air system was also conducted to study the influence on the formation of the SPM. The results illustrated that the simulated method was effective on depressing the formation of the SPM, and for the simulated boiler, the deflected angle of the two middle second air should be 10°. This study could provide the theoretical direction to depress the formation of the SPM, and also give reference to design and operation the boiler with low pollutant emission.