Experimental Study On Preheating And Combustion Characteristics And Pollutants Emission Of Pulverized Anthracite

Coal is the main energy source in china with anthracite accounting for17%of total production. Due to the shortage of coal resource, anthracite is generally used for power generation directly. Meanwhile, large substantial semi-coke with almost no volatile produced by low rank coal utilization technologies also needs to be burned to generate electricity. However, due to the low volatile and high fixed carbon contents, there are some problems in the power plants burning anthracite and semi-coke, such as difficult ignition, unstable combustion at low load, high carbon content in fly ash, and high nitrogen oxide emission. Based on the theory of preheating strengthening combustion, a system for preheating pulverized anthracite in a circulating fluidized bed (CFB) is established. In this thesis, the preheating mechanism of pulverized anthracite is revealed, and the combustion characteristic is explored to achieve stable and efficient combustion and low pollutant emission of preheated pulverized anthracite.In order to explore the preheating and combustion characteristics of pulverized anthracite, the transformation process of coal-nitrogen to NOx, and the desulfurization characteristics, experiments are carried out on a bench scale rig of pulverized anthracite combustion preheated in a circulating fluidized bed. The effects of operating conditions in CFB and the down-fired combustor on the preheating and combustion characteristics of pulverized anthracite and NOx emission are also investigated.The experimental results show that, the preheated pulverized anthracite with a temperature higher than800℃can be obtained steadily and continuously by partial pyrolysis, gasification, and combustion of anthracite coal at a low air equivalence ratio in CFB. After being preheated, the mean particle size of pulverized anthracite significantly decreases, the specific surface area and pore volume increase, and the surface becomes rough with a well-developed pore structure, all of which leads to the improvement of physical structure of the pulverized anthracite. In the preheating process, coal nitrogen mainly converts into NH3and N2, and coal nitrogen reduction in CFB is favorable to reduce NOx emission. The conversion rate of the components in pulverized anthracite increases with the increase in the preheating temperature and the air equivalence ratio in CFB. After the preheated pulverized anthracite and high temperature coal gas with the temperature higher than800℃enter the down-fired combustor encountering with air, fast and efficient combustion occurs. The combustion of the preheated anthracite is steady, and the temperature profile in the down-fired combustor is uniform. There is no any difficulty in ignition in the down-fired combustor. The combustion rate of preheated pulverized anthracite is controlled by both the chemical reaction rate and the diffusion rate. The highest combustion efficiency in the experiments can reach97.5%, which indicates that the combustion and burning-out performance of pulverized anthracite are greatly improved after being preheated.In addition, combing the technology of pulverized coal preheating and air-staging can significantly reduce NOx emission. Especially for pulverized anthracite, the minimum NOx emission is103mg/m3(6%O2) without equipping SCR, which basically reaches the limits of national emission regulations. The NOx emission and fuel-N conversion ratio decrease with the increase in the pulverized coal size and air equivalence ratio in CFB; initially decrease and then increase with the increase in the preheating temperature, reaching a minimum at the preheating temperature of900℃. The NOx emission increases with the increase in the air equivalence ratio in the reducing zone and the excess air ratio, and the decrease in the residence time of preheated anthracite in the reducing zone.SO2emission decreases significantly after the addition of limestone to CFB, and the desulphurization efficiency can reach50%. The desulfurization reaction occurs in CFB, mainly between H2S and CaO. The effect of limestone addition on NOx formation is closely related with the coal type. It is observed that adding limestone to CFB has little effect on NOx emission for Yangquan anthracite coal and Shenmu semi-coke, while it is obviously effective for Dongsheng lignite coal and Datong bituminous coal.The results show that the technique of preheating pulverized anthracite in a CFB can achieve stable, efficient, and clean combustion for the fuels with low volatile content mainly of anthracite coal. The research results in this work provide theoretical basis and technical solution for the utilization of anthracite and smei-coke in the field of combustion.