| China is rich in coal resources,ranking third in the world’s total coal reserves,so China’s annual coal mining volume is huge.In the process of coal mining,a large amount of coalbed methane is produced,but most of the methane is too low to be used directly and can only be discharged into the air.This is not only a waste of resources,but also a huge damage to the environment.Methane’s greenhouse effect is 23 times that of carbon dioxide,and its effect on the ozone layer is seven times that of carbon dioxide.In 2020,China proposed the goal of"carbon peak"in 2030 and"carbon neutral"in 2060,so it is urgent to solve the problem of low-concentration mine gas.In this paper,a 600MW unit direct through boiler is taken as the research object,and the coal burning boiler is connected with coal mine ventilation,and the combustion characteristics of mixed firing are studied and analyzed by numerical simulation method.SOLIDWORKS is used to model the object of study,ICEM in ANSYS is used to mesh the object of study,comprehensive consideration of boiler heat and mass transfer characteristics,the corresponding mathematical model is established,through FLUENT numerical simulation to obtain the velocity,circulation,components and nitrogen oxide distribution under various conditions.Finally,design-Expert software was used to determine the optimal compatibility parameters.Taking the BMCR working condition as the basic working condition,the combustion characteristics under this working condition are analyzed.The numerical simulation of the original working conditions of the boiler shows that the error of the average temperature and the average oxygen volume fraction of the outlet section of the furnace is less than 8%compared with the actual measured results.The model can truly simulate the operation process in the furnace.In this paper,the effects of methane concentration in exhausted wind,four different entry modes of exhausted wind,the rate of exhausted air on combustion characteristics,the limit of depleted combustion and the optimum compatibility parameters of mixed combustion were studied respectively.The results show that with the increase of methane concentration,the average temperature in the main combustion region increases,and the maximum increase is about 102K.In the main combustion region,the volume fraction of oxygen decreases while that of carbon monoxide increases.The NOx emission can be effectively reduced by adding spent air into the inlet of the boiler for combustion.The NO emission at the VAM is reduced from974.8 mg/m~3 to 436.3 mg/m~3,which is reduced by 55.24%.When the methane concentration is0.75%,Into four modes also can make the temperature rise on the combustion chamber of a stove or furnace combustion produces positive effect,reduces oxygen volume fraction,and compared with no access to spent significantly increased consumption of oxygen and carbon monoxide wind volume fraction increases because of incomplete combustion,nitric oxide emissions as a reducing atmosphere increased and the increase of methane will turn nitric oxide reaction.When the proportion of exhaust air increases from 20%to 35%,the overall trend of temperature distribution in the burner area is the same,but the greater the exhaust air rate,the more uniform the temperature distribution in the burner area,the lower the oxygen concentration in the burner area,resulting in incomplete combustion of coal,increasing the concentration of carbon monoxide;At the same time,increasing the burnout air rate can reduce the emission of nitric oxide.When the methane concentration is lower than 0.01%,the contribution of heat to combustion is low and the carbon dioxide emission is almost unchanged.When the concentration is lower than 0.00025%,the effect on the production of nitric oxide is small and the emission of nitric oxide is greatly reduced.Therefore,considering comprehensively,the concentration of ventilation into the boiler should not be lower than0.01%.The quadratic model between boiler combustion and various test factors(methane concentration in exhausted wind,exhaust air rate and boiler load)was established and optimized by Design Expert,and the significance degree of single factor and multi-factor second-order interaction terms on each response value was analyzed.Among the single factors,exhaust air rate had the greatest influence on temperature.The boiler load has the greatest influence on NO generation,and the exhaust air rate and boiler load have the most significant influence on temperature and NO concentration under the interaction.The optimal test conditions are as follows:when the methane concentration is 0.25%,the exhaust air rate is 33.51%,and the boiler load is 564.11MW,the temperature response value is 1691.41,the NO concentration response value is 0.00031589 and the predicted value is in good agreement with the measured value. |
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