Numerical Calculation And Study Of Water-wall Tube Temperature Of Supercritical Boiler

Many supercritical coal-fired boilers have been put into running in China in recent years. The safety operation of water-wall has become an important subject attracting concern. A 600MW supercritical boiler is studied in present paper. Heating load of spirally-wound water wall tube at different elevation and the thermal states of working medium at supercritical and near-critical conditions were determined. The temperature profiles at different running conditions were calculated, the changing characteristic of the temperature profile was analyzed, the positions of the highest temperature were determined. The research contents and results are as follows:(1) Numerical calculations of water-wall temperature profile of a 600MW supercritical boiler were conducted by the moment method. Two-dimensional heat conduction differential equations were established for the cross sections at different heights. The basic principle and calculation formula of moment method were introduced. The equivalency of matrix method and variational method has been proved. The mesh with bilinear quadrilateral grid has been generated, and numerical calculation formula of finite element based on moment method was deduced.(2)The spirally-wound water wall tube in furnace was partitioned into 7 zones along the height. The heat conservation equations for the 7 zones were established, and thermal calculations in the 7 zones were conducted and the heat loads were determined. The derived heat loads were modified based on the theory of energy balance.(3)Temperature profiles of 49 cross sections under 7 supercritical and near-critical working conditions were calculated. The characteristics of the spirally-wound water wall tube inclination arrangement and circumferentially non-uniform heating were considered. The heat convection coefficient curve along the circumferential direction was determined.(4)The heat absorption difference was analyzed based on the different wound modes of the spirally-wound water wall tube in every furnace zone. The heat absorped difference of the water-wall tube caused by the dimension difference from the entrance header to the outlet of the spirally-wound water wall tube was analyzed in present paper. The deviation tubes which have the highest heat absorption were confirmed. The outlet temperatures of deviation tubes were calculated. The calculated results and the measured results were compared. And the model of the calculation of the water-wall temperature profile was proved reasonable, and the calculation result was credible. (5)The temperature rising rates of the water-wall under different start-up modes were studied on the basis of steady temperature profile calculation under supercritical pressure. The results indicate that the inner side temperature of the water-wall changed quickly. The change rate can exceed the operation requirement obviously.(6)Under near critical condition, the vaporization process was calculated and analyzed along the height. The position of heat transfer deterioration was determined. The overheating caused by transfer deterioration was analyzed. It is indicated that vaporization happened in the burner zone of the furnace. And the heat transfer deterioration in two-phase region can lead to overheating.