Research On Parameter Design And Process Characteristic Of 700℃ USC Boiler

The development goal of thermal power unit is that the thermal efficiency of 36MPa/700℃ /720℃ ultra supercritical units can reach above 50% and the power supply coal consumption can be reduced to 250g/(kW·h). Because of the great increase of the steam parameters, the heat absorption characteristics of the steam water system of the boiler is greatly changed, so the design technology of the existing unit can not be copied .According to the theory of thermodynamics and heat transfer,Several new problems were studied in the design of 700 ℃ USC boiler. On the basis of studying the parameters of the steam water system and the practical problems of the 600℃ USC boiler. Main contents include:1) The water wall temperature and the intermediate point temperature were redesigned, and the calculation method of heat transfer deterioration was put forward.In this paper, the heat load distribution and heat transfer characteristics of water cooled wall under 4 variable load conditions and 3 variable coal quality conditions are calculated and analyzed. The results show that, at the rated load, the 700℃ ultra supercritical boiler only needs to be arranged a section of water wall, the design temperature of the outlet working fluid temperature of the water wall outlet should be 450℃～460℃ and the material should be T92 steel tube. In this paper design parameters, the working fluid in the water cooling wall tube is in the critical pressure zone around the 57% load, and has a high margin of safety; but the influence of coal quality changes on the change of the enthalpy of the outlet of the water cooling wall increases.2) Put forward 2 kinds of design scheme (no flue gas recirculation and flue gas recirculation)of 700℃ USC boiler superheater and reheater system and carry on the calculation of heat transfer; analysis some important problems in the calculation method of furnace design; according to the thermal properties of refrigerants, the mechanism of superheater and reheater heating surface heat transfer characteristics is analyzed. The results show that, the heat transfer coefficient of high temperature superheater and reheater is improved with the increase of parameter of 700℃ USC boiler, but heat transfer quantity reduces due to the reduction of heat transfer temperature difference; when the flue gas recirculation is used to regulate the steam temperature, it is better to adjust the steam temperature, but when heat transfer temperature difference of heating surface decreases, heated surface of superheater and reheater needs to increase.3) The technical scheme of high efficiency low oxygen combustion furnace type were studied and some problems by theories were analyzed, such as heat load deviation expansion due to uneven combustion heat load of Single burner,combustion instability of a single burner, decrease of combustion efficiency, larger production of NOx.4) According to the new achievements of science and modern combustion theory,to facilitate the analysis of effect of coal quality variation on 700℃ USC boiler operation characteristics, The macroscopic rule of the 176 kinds of power coal ignition characteristics, stable combustion and burn out characteristics inland were studied and the computational model of pulverized coal’s ignition temperature and ignition heat were established.5) Several new methods were proposed: meridional throttle technology of the inlet of vertical tube platen;the solution of the unbalance between evaporation heat absorption and superheated heat absorption under full load variable pressure operation; method for preventing low load superheater over temperature; a new method for the calculation of heat transfer deterioration of water wall under supercritical pressure; a new method of analysis of the function of the ignition temperature of pulverized coal and the combustion model to distinguish between the coal quality and the stability; the new idea of .Hypoxia high efficient combustion and low NOx emission and low heat deviation though methods of optimizing distribution of pulverized coal flow and two secondary air flow through powder conveying pipe.