Effects Of Offset Angles Of Mid-secondary Air Nozzles On Gas-solid Flows In A Furnace

The aerodynamics field in the furnace has great influence on the pulverizedcoal (PC) combustion. For tangential fired furnace of PC boiler, the research ofthe effects of offset angles of mid-secondary air nozzles on gas-particle flowcharacteristics in the furnaces will benefit the design and operation of the boiler.Firstly, a cold model was built on the similarity theory by 1:25 according to600MW PC boiler with single furnace andПtype layout of the whole boiler.The adjustment work was performed and experimental parameters were chosencarefully.Tracer test has been done to research the flow properties of the primary airjet with powders in the furnace by the laser light sheet technology and high speedphotography technology. The pictures information was quantized by a Matalabprogram. By the qualitative and quantized analysis, the results show that with theincreasing of offset angles of mid-secondary air nozzles, the offset angles ofprimary air increase and the turning of jet become earlier consequently. Theincreasing of offset angles of mid-secondary also leads to a bigger tangentialrotational circle, which makes a better combustion space utility in the furnace.The ignition condition was improved when the neighboring flame move to thehead of examined flame. But when the offset angle of mid-secondary air nozzleswas shifted too much, the primary air would strike the water wall, which meansthat the slagging will enhance on the water wall. At meantime, the neighboringflame move to the head of examined flame too much would also lead to earlierignition, which would increase the nozzles temperature consequently.On the basis of tracer test results, the gas-solid flow characteristic in themodel furnace were accurately measured with a 3D-PDA system, gained theprimary air's jet track, gas-solid velocity, turbulent kinetic energy andconcentration distribution in the model furnace at different swing angles of midsecondaryairs. The results show that: gas-solid velocity slippages of tangent,radial and axial directions are unconspicuously. Gas-solid turbulent pulsing havethe anisotropic's characteristic, but differences are unconspicuously among gas,fine particles and coarse particles. fire-facing side of jet flow exists shearvortexs, the back side exists wake vortices, so shear stress and turbulent kinetic energy are large at the two sides of the jet flow, exchange of material and energyis strong. In the burner's spout, turbulent kinetic energy and turbulence intensityare large. When the jet flow reaches the hearth center, turbulent kinetic energyand turbulence intensity become small. With the increasing of offset angles ofmid-secondary air nozzles, the circle diameter in the furnace increases and themaximal concentration line is at the inner side of the maximal velocity line,forming"air surrounding fuel"gas-partical aerodynamics field. However, in someregions near the jet's free developing zone, the location of the peak concentrationdoes not change with the mid-secondary offset angles. Properly enlarge the offsetangles of mid-secondary nozzles will delay the mixing of the secondary air intothe primary-air-and-coal mixture and make coal combustion as multistage one,forming more oxidative environment near the water walls.