Research On Flue Gas Thermal Deviation At Furnace Exit In A1000MW Lignite Boiler

Developing1000MW ultra-supercritical lignite boiler is of great importance toclean and high-efficient utilizing large abundant lignite resource in China. Lignite isa kind of low ash fusion point coal and easy to slag. So the thermal load ofcross-section in the furnace can’t be too high and it’s necessary to using largefurnace size. Then the actual tangential circle diameter in furnace increased, leadingto aggravation of flow and heat transfer non-uniformity. The single chamber dualcircle tangentially fired method is widely used during1000MW Ultra-supercriticallignite boilers. This paper researched the non-uniformity at the furnace exit in a1000MW Ultra-supercritical lignite boiler under the support of the National HighTechnology Research and Development of China (863Program). This work canprovide theoretical basis for reduction of thermal non-uniformity at the furnace exit.Using empirical formula method simulated the combustion process in a600MW lignite boiler to consider the influence of moisture on the ComputationalFluid Dynamics software Fluent. The calculating results agreed well withexperimental ones. Then simulated the combustion process in a1000MW ligniteboiler with three different number of grids,1.2million,1.5million and3millions.The results shows that1.5million of grids can satisfy request of calculation accuracy.Then we got the numerical simulation method of the1000MW lignite boiler.The thermal non-uniformity at the furnace exit in a tangentially fired boiler iscaused by the uneven distributions of velocity, temperature and thermal radiation ofhigh-temperature flame center. This paper analyzed the similarities and differencesbetween single and dual circle tangentially fired boiler on radiation heating surface,radiation-convection heating surface and convection heating surface. This paperthen studied the relationship between furnace cross-section and the thermaldeviation of radiant heating surface. The results showd that it’s the airflow residualrotation at furnace exit caused the thermal deviation.This paper then studied the influence of OFA reverse tangent angle on thedeviation of velocity and temperature at furnace exit. The non-uniformity of velocityand temperature would be decreased when reverse tangent angle is proper. If reversetangent angle is large, the airflow in furnace would rotate inversely and thenon-uniformity would increase conversely. The optimal reverse tangent angle is15°.This paper also studied the vertical swing angle of the4st and7st OFA nozzles onthe non-uniformity at furnace exit. Got that both the two nozzles down swing5°canreduce the non-uniformity greatly.