Investigations On Flow And Combustion Characteristics Of The Centrally Fuel Rich Swirl Burner In Industrial Pulverized Coal Boiler

As the critical equipment for heat and steam supply,industrial boilers are widely used in many fields for the production and our life.Most of the industrial boilers currently used in China are coal-fired industrial boilers,and their coal consumption accounts for about 30%of the total coal consumption in China.Therefore,emission of pollutants produced by coal-fired industrial boilers cannot be ignored.In actual operation,most of those operational industrial pulverized coal boilers in service in China have the following problems:high NO_x emission,high carbon content in fly ash and severe slagging.Therefore,it is necessary to develop pulverized coal combustion technology with low pollutant emission,anti-slagging,good combustion stability,and high combustion efficiency for industrial boilers.In this thesis,the centrally fuel rich（CFR）swirl pulverized coal combustion technology of industrial pulverized coal boilers is developed,based on the CFR coal combustion technology in power station boilers,and a comprehensive study is carried out.Taking the CFR swirl burner in industrial pulverized coal boilers as the research object,a single-phase cold test system is established.Through the constant temperature hot wire anemometer,the three-dimensional velocitices of the air flow at the burner outlet and in the pre-combustion chamber under different operating parameters are measured.Operating parameters’effects on the flow characteristics of the outlet of the swirl burner and the pre-combustion chamber are analyzed.When the ratio of inner and outer secondary air increases from 0:100 to 60:40,at the burner outlet,a stable central reflux zone can be formed.At this time,the diameter of the central reflux zone is 1.23～1.55D_e and the length is 3.15～3.82D_e.Compared with the CFR swirl burner in power station boilers,the central reflux zone of the burner in this thesis has a significantly longer length.When the ratio of inner and outer secondary air increases to 60:40,the maximum reflux flue gas is 8 times of the primary air volume.As the excess air coefficient is 0.2,no stable central reflux zone can be shaped inside or outside the pre-combustion chamber.When the excess air coefficient chamber increases from 0.4 to 1.2,the maximum reflux flue gas increases by 5.5 times,and the combustion stability of the burner is significantly enhanced.Taking the CFR swirl burner in a 29 MW industrial pulverized coal boiler as the research object,a gas-particle two-phase test facility is established.By particle dynamic analyzer（PDA）,the three-dimensional gas-particle velocity,particle size and particle volume flux are measured under different structures and operating parameters.When the outer secondary air vane angle is greater than 60°,the peak value of the particle-phase flow is close to the water wall,and more large particles are carried to the separated secondary air area.When the separated secondary air is arranged as double layers,the size of the central reflux zone and the reflux velocity are larger,and the large particles are far away from the separated secondary air.This is conducive to the burnout of pulverized coal and is not easy to lead to high-temperature corrosion and slagging on the waterwall.With the increase of outer secondary air volume,the mixing of air flow between pre-combustion chamber and separated secondary air is accelerated.When the ratio of outer and separated secondary air is greater than 65:35,the peak value of particle axial velocity is close to the water wall.This is not conducive to preventing slagging and high-temperature corrosion of the waterwall.When the excess air coefficient of the pre-combustion chamber is 0.2,the position of the maximum particle volume flux is always close to the furnace centerline,and the swirl burner can not establish a good combustion process.When the excess air coefficient of pre-combustion chamber is greater than0.4,with the increase of excess air coefficient of pre-combustion chamber,the area of central reflux zone,the reflux velocity and the particle volume reflux flow all increase.But the tangential and radial average velocity and particle volume flux near the water wall are large at this time.Thus,it is suggested that the angle of secondary air,the structure of separated secondary air,the ratio of outer and separated secondary air and the excess air coefficient of pre-combustion chamber be set at 60°and below,as double layers,below 45:55,and at 0.4 to 0.8,respectively.The CFR swirl pulverized coal combustion technology of industrial pulverized coal boilers is applied to the 29 MW hot water boiler.Through industry test,the combustion characteristics of pulverized coal are studied under unde different operating parameters.Low-NO_x combustion for industrial pulverized coal boiler is achieved.At 9～12 MW load,when adjusting the damper opening,the maximum and minimum concentration of NO_x emission at furnace outlet is 364.9 mg/m³@O₂=9%,and 64.1 mg/m³@O₂=9%,respectively.The influence of the opening of the inner secondary air damper on the burner performance is greater.The flue gas temperatures in the burner area are all below 1100℃,which is lower than the coal’s ash fusion temperature.The operation of multiple heating seasons shows that the burner has good anti-slagging performance.At 11～12 MW load,the NO_x emission concentration is reduced by 22%through introducting flue gas recirculation.Low load combustion stability is achieved when the CFR swirl pulverized coal combustion technology in industrial pulverized coal boiler is adopted.At 3.6 MW,the temperature in the burner area is less than 1000℃,and the NO_x emission concentration is 385.7 mg/m³@O₂=9%,during which the boiler operates stably.The minimum load for stable combustion without natural gas support can reach 3 MW.The load regulation rate is close to 10:1.Finally,through the numerical calculation method,the combustion characteristics of 29 MW hot water boiler are numerically predicted.Modeling according to 1:1 size,after grid division,thermal simulation is carried out through Fluent software,and the flow,combustion and NO_x generation characteristics in the furnace of full load boilers with different operating parameters are compared.When the rate of separated secondary air is 85.7%,the furnace central reflux zone can not be formed.When the rate of separated secondary air is 71.4%,the central reflux zone and the combustion are asymmetric,which threatenes the boiler’s safe operation.As the rate of separated secondary air is set at 14.3%,the central reflux zone and flame shape changes obviously,and the carbon content in the fly ash raises.As the rate of separated secondary air is 55.4%,the area of high temperature zone is large and the heating distribution in the furnace is more uniform.The better rate of separated secondary air is 55.4%.As the rate of recirculation flue gas increases,the furnace’s high temperature area gradually reduced,the burnout degree of pulverized coal become worse,and the NO_x emission decreases gradually.When the rate of the recirculation flue gas is 30%,the carbon content of the fly ash increases by 12.03points and the NO_x emission concentration decreases by 19%.In this thesis,a new swirl combustion organization mode of industrial pulverized coal boiler is proposed.Combined with laboratory test,numerical simulation and industry test,the main design parameters of the new type of CFR swirl burner for industrial pulverized coal boilers are optimized,and a reasonable coupling among velocity distribution,temperature distribution and concentration distribution is formed in the furnace.Finally,low-NO_x emission and stable combustion are realized in the industrial pulverized coal boiler.