Influences Of Internal And External Secondary Air Ratio On The Aerodynamical Field In A Down-fired Boiler Equipped With Eccentric Swirl Burners

Low volatile coal is abundant in our country.Low volatile coal,especially anthracite,is widely used in power plants in China.The down-fired boiler is the main furnace type for burning anthracite because it has the characteristics of keeping high temperature in the furnace and long pulverized coal particles.However,it is difficult to ensure a good ignition and burnout of pulverized coal under the condition of stable combustion of anthracite,at the same time,the problem of high NOx emission exists generally.In China,NOx in the flue gas of coal-fired power plants is the culprit for providing NOx.In China,there are relatively many NOx control schemes for all kinds of down-fired boilers with direct current burners,but there are relatively few low nitrogen transformation for the B&W down-fired boilers with swirl burners.For this reason,the Institute of Combustion Engineering,Harbin University of Technology,has carried out a low-nitrogen combustion technical transformation of a 300 MW a down-fired boiler with a swirl burner.The technology mainly comprises the following steps of introducing a direct current slit type OFA nozzle at the throat opening of the upper furnace chamber and adding an arc baffle block on one side of the swirl burner near the center of the hearth,so as to reduce the flow area of the inner and outer secondary air channels on one side of the burner,and form an eccentric rotational flow combustion device.In this paper,by changing the ratio of the internal secondary air rate and the external secondary air rate,the flow field distribution and mixing in the nozzle area of a single eccentric swirl burner are investigated.At the same time,the characteristics of flow field in the furnace of a boiler equipped with a modified eccentric swirl burner are investigated.In order to investigate the flow characteristics in the nozzle region of a single eccentric swirl burner,the velocity distribution experiment of a single burner was carried out,and the two-dimensional plane conversion method of the constant temperature hot wire anemometer was used.The three-dimensional velocity distribution of the plane measured in the nozzle region of a single b urner is obtained.In order to investigate the flow mixing law in the nozzle region of a single eccentric swirl burner,based on the original velocity distribution experimental system,the three-dimensional concentration distribution of the plane primary a ir measured in the nozzle region of the single eccentric swirl burner was obtained by using the thermocouples set;In order to investigate the characteristics of flow field in the furnace of a boiler equipped with a modified eccentric swirl burner,an expe riment was carried out on the flow field characteristics of a boiler.By using a constant-temperature anemometer,the two dimensional velocity distribution of the axial longitudinal profile of the central axis of the primary air passing through the burner was obtained.The ratio of internal and external secondary air ratio is 3:7,4:6 5:5,6:4.By comparing the flow field characteristics of each working condition,the optimal ratio of internal and external secondary air rate is obtained to improve the economic performance of boiler and environmental protection performance.According to the above experiments,the quantitative law of the flow characteristics in the nozzle region of the eccentric burner and the whole furnace is found in the different ratio of the internal secondary air and the external secondary air.With the increase of the ratio of internal and external secondary air,the rigidity of primary air flow increases,the diffusion of the central side of the burner increases,and the entrainment of the radial distal flow of the burner weakens.The overall degree of rotation of the air flow in the nozzle area of the burner does not change much,and the primary air and the internal secondary air have little change,the intensity of axial and radial mixing increases,but the uniformity of primary air decreases in radial direction.On the other hand,the radial diffusion range of the burner to the fire side increases,but the swirl degree is weakened,the primary air concentration is higher than that of the back fire side,and the mixing strength between the primary air concentration and the inner secondary air is lower than that on the back fire side.However,for the quantitative law of the flow characteristics of the whole furnace,with the increase of the ratio of internal and external secondary air,the flow field in the burning area of the upper furnace is always symmetrical and the space utilization ratio is relatively good,while the flow field in the lower furnace tends to be symmetrical from asymmetry to symmetry.However,the depth of air flow is attenuated seriously,and the utilization ratio of lower furnace space is reduced,when the ratio of inner and outer secondary air is 5:5,the smooth symmetry of the lower furnace and the depth of the main air flow reach the optimal equilibrium point,the space utilization ratio of the lower furnace reaches the best,and the flow field characteristic of the whole furnace reaches the best.In the actual operation of the boiler,the ratio of the inner secondary air and the outer secondary air wind ratio is adjusted to be 5:5 as the optimal proportion to adjust the air distribution of the other boilers,so that the economic performance and the environmental protection performance of the boiler are optimized.