Experimental Study On Combustion And No_x Emissions Of Pulverized Anthracite Preheated By Circulating Fludized Bed

With the rapid development of electric power industry in China, generator sets are required to be developed of bigger capacity and lower emission level. Thus some higher requirements of coal combustion were put forward, such as burning stability, low pollution and flexibility of boiler load variation. Electric power industry development also leads to the lack of high quality power coal. Since the anthracite coal reserves in China are large, it can be seen that the percentage of anthracite in thermal power plant fuel will be increasing continuously. However, anthracite has some intrinsic problems for combustion. It is difficult to ignite, to combust stably and to burn out. Such features will make the boiler, which burning anthracite, experiences low combustion efficiency, high NOx emissions and unstable combustion in low boiler load.A new technology based on pulverized coal preheated by circulating fluidized bed (CFB) was proposed in this thesis. In the CFB, the properties of pulverized anthracite coal changed through some physical and chemical reactions, such as friction, devolatile, pyrolysis and partial combustion. These changes of property include diameter of pulverized anthracite coal decreasing, specific surface area increasing, pore volume increasing, temperature increasing up to800℃, which are advantaged to stable combustion. The preheated pulverized anthracite entered the combustion chamber to combust, with low NOx emissions.A30kW bench-scale rig of pulverized anthracite combustion in a down-fired combustion chamber (DFCC) was developed. The CFB riser has a diameter of90mm and a height of1,500mm. The down-fired combustion chamber has a diameter of260mm and a height of3,000mm.A series of experiments were carried out by the parameters variation of combustion and air-staged. The results showed that Yangquan anthracite with volatile as air dry basis6.74%can be preheated up to800℃in CFB. The combustion of preheated pulverized anthracite was stable and uniform with the maximum temperature difference less than200℃; the size of preheated pulverized anthracite significantly decreased, the50%cut size decreased from82μm to19μm after preheating. The specific surface area increased from4.9m2/g to111.0m2/g, and the pore volume increased from0.014cm3/g to0.096cm3/g. The gas generated in preheating process included combustible gas, whose net calorific value is1.53MJ/Nm3. Combustion efficiency increased when the size of pulverized anthracite feed into the CFB decreased. As long as the variations of excess air ratio and temperature of preheated pulverized anthracite were in a reasonable scope, these two parameters have little impact on combustion efficiency. Combustion efficiency of Yangquan pulverized anthracite in this30kW bench-scale rig was94.17%.As a result of air-staged combustion, the conversion ratio of fuel-N to NOx was less than32%, and the NOx concentration was less than400mg/m3. The NOx concentration increased with the size of pulverized anthracite decreased, air ratio in the reducing zone increased and the residence time shortened. The excess air ratio has little effect on NOχ concentration.