| Coal-fired power generation is the main power production way in China.About70%of China's electricity needs are met by coal-fired power plants.Among those coal-fired furnaces,the opposite-wall-firing furnace(OWFF)is one of the most widely used types.In this type of furnace,each burner organizes the airflow structure separately and the flames of them is independent,so the combustion process and component concentration distribution should be more uniform theoretically.However,the distribution of carbon monoxide(CO)concentration along the width of the furnace is generally U-shaped in OWFFs in the factual operation.This means that the CO concentration is low in the middle and high near the side walls of the furnace.The enrichment of CO in the side walls aggravates the risks of slagging and high temperature corrosion of the water wall.In view of this phenomenon,numerical simulation and experiment were performed to explore the mechanism and technical improvement of CO enrichment on the side walls of OWFFs in current work.First,a full-scale numerical model of a 600MW opposite-wall-firing furnace(OWFF)was established and validated.On the basis of this model,the distribution coefficient and the mixing coefficient of pulverized coal and air along the width of the furnace were defined in order to explain the relationship between the mixing distribution characteristics of pulverized coal and air and the distribution characteristics of CO.It was found that the airflow distribution in the furnace is relatively uniform,and coal particles are enriched in the vicinity of the side wall.It is the diffusion difference of the coal particles that causes uneven combustion in the furnace,which leads to deviation of CO concentration.Using argon(Ar)as tracer gas,the diffusion process of primary air,secondary air and over fire air was studied,and the deviation degree of different distribution air and coal particles was obtained.The deviation of primary air and pulverized coal mainly occurred in the center of side wall,while for the secondary air,the deviation is advanced to the furnace center.The mixing degree of over fire air and coal particles is the worst.Based on the characteristics of the airflow structure in the furnace,the forming process of the characteristics of CO distribution was expounded.And it was concluded that the corner vortexes formed by the opposite flows is the main cause for the accumulation of coal particles and the high CO concentration in the side wall areas.Second,according to the gas-solid flow characteristics of HT-NR3 swirl burner,the effects of swirl intensity,inner secondary air ratio,primary air ratio and vane angle of outer secondary air on the flow and mixing process of pulverized coal and air in the furnace were studied.Decreasing the swirl intensity will lower the proportion of coal particles in the periphery of the burner flow,which will reduce coal particles thrown to the sidewall.When the swirl intensity decreases from 0.8 to 0,the coal-air mixing coefficient of the sidewalls in the burner region declines from 1.43 to 1.21.The inner secondary air ratio hardly affects the distribution of coal in furnace.Increasing the primary air ratio will increase the upward momentum of flue gas in the center and reduce the diffusion degree of coal particles to the sidewall.When the primary air ratio increases from 23%to 27%,the air-coal mixing coefficient of the sidewalls in the burner region decreases from 1.37 to 1.18.Reducing the angle of vane angle of outer secondary air can also reduce the diffusion of coal particles to the sidewalls to some extent.But it is the same as the adjustment results of other parameters,the fuel-rich condition of the sidewall area cannot be eliminated.Third,aiming the problem of CO concentration deviation in the opposite-wall-firing furnace(OWFF),the effect of bowl-shaped secondary air distribution(BSAD)on the coal-air mixing and combustion process in the furnace was studied by combining field test and numerical simulation.The bowl-shaped secondary air distribution(BSAD)can effectively reduce the enrichment of coal particles of the sidewalls in the burner region,improve the coal-air mixing process along the width direction of the furnace,as a result relieve the deviation of CO concentration,decrease the CO emission and the carbon content of fly ash at the outlet of the furnace,so that it can improve the combustion efficiency of the boiler.With the deviation of bowl-shaped air distribution increasing,the NO_x emission at the outlet of the furnace increases.But when the deviation of air volume is within the range of10%,the NO_x emission concentration does not vary more than 4.4%.Considering the effect of bowl-shaped air distribution of burner on the mixing characteristics of coal and air in the furnace and the NO_x emission concentration in the flue gas at the furnace outlet,the deviation of air volume of bowl-shaped air distribution should be controlled within 10%when utilizing the common coal.The simulated CO and NO_xconcentration curves at furnace outlet and the measured ones at economizer outlet show great consistence.In the actual operation,the improvement effect of bowl-shaped air distribution on the holistic concentration and distribution of CO is more significant.Finally,according to the influence mechanism of the corner vortexes on CO enrichment in the sidewalls,a method using the side air to eliminate vortexes is proposed.Based on the structural characteristics of the corner vortexes,two schemes of arranging side air on the front and rear walls and sidewalls are formed.The effect of nozzle position on the vortex elimination was compared and analyzed,and the optimization design of the two schemes was carried out respectively.The results show that using side air on the sidewalls to eliminate the corner vortexes is more effective.When the nozzle spacing is set at 2.4 m,the side air nozzles of the middle and lower layers are arranged in the same layer with the burner,and the upper nozzle is interlaced arranged with the burner,compared with the original working condition,the high concentration area of CO near the sidewalls is reduced by 67%and the CO emission concentration and carbon content of fly ash at the furnace outlet are slightly reduced,while the NO_x mass concentration increases by less than 6%.and the general effect is favorable. |
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