Failure Probability And Numerical Simulation On Heating Surface Tube Temperature Field Of Ultra-supercritical Power Boiler

Ultra supercritical units improve power generation efficiency of coal-fired units and achieve low pollution emissions by increasing the steam parameters to reduce coal consumption efficiently. However, the unit installed capacity increased year by year, the pipeline operating conditions deteriorate again and pipe incidents occur sometimes, which brings great economic losses to the normal operation of the power plant. Current wall temperature and thermal deviation calculation methods are not suitable for ultra supercritical units. Therefore, temperature field of combustion in lower furnace and heating surface pipe of upper furnace were carried out with CFX software in this paper.In this paper, ultra supercritical boiler failure modes and various factors that cause the failure were analyzed by using analytic hierarchy process. We found the right weight for each failure factor, obtained the relative importance of different failures types of heating surface after establishing the failure model and judgment matrix of water wall, superheater and reheater. The results contribute to targeted research on the reasons for the failure of the heating surface.For the simulation of combustion chamber, through the calculation and analysis of 1:1 furnace solid model, a detailed discussion of the influence of simulation parameters was carried out under the conditions of different amounts of coal, excess air ratio, coal fineness, coal type and load. Results showed that when the secondary air in certain circumstances, the overall temperature of the furnace along the height under increasing amount of coal conditions showed a decreasing trend after the first increase.When the amount of coal changed, the temperature of each part became different while it had little effect on the shape of tangential flame. Pulverized coal burned completely at its mass flow rate of 11kg/s. When the amount of coal in certain circumstances, the overall temperature of the furnace first increased and then decreased under increasing amount of excess air conditions. The perfect combustion of pulverized coal occurred under the excess air ratio of 1.15 condition. The smaller the coal fineness was, the larger combustion surface area and smaller thermal resistance itself, the shorter time needed to heat it to the ignition temperature. Coal with high calorific value and volatile content was easier to form a high temperature of furnace. More heat released from burnout zone which made the temperature and radiation intensity of furnace exit greater under high heat load conditions.According to the method of lower furnace simulation investigated in this paper,water wall analytical calculation and numerical simulation were carried out to obtainthe danger zone and effects of temperature and stress distribution at different height of the furnace. Results showed that the maximum temperature of water wall appeared at the tip of fin on its side of facing the flame, the temperature of the fin along the height showed a decreasing trend after the first increase and its value reached the maximum at the height of 38 m. The maximum stress occurred in the position between the pipe and fin on the back side of the flame, the hazardous areas appeared in the middle of the lower and upper burners.For the heating surface pipe simulation of upper furnace, the temperature and velocity of flue gas obtained from the lower furnace was passed up through a transport equation to achieve the fluid-structure coupling heat transfer analysis. The temperature distribution and fluctuations of pipe under different load conditions was carried out and verified by plant measured data. Results showed that primary superheater and third stage superheater were similar, the high temperature zone occurred in the center of the furnace where the largest temperature fluctuations happened, the temperature fluctuations of the pipe decreased from the center to the two sides of the furnace under varying conditions. From the height above secondary reheater, the high temperature zone extended from the center to the sides of the furnace, at the same time, the lower load conditions, the longer extension distance.What’s more, the temperature fluctuations of the pipe increased from the center to the sides of the furnace, the hazardous area extended from the center to the edge of the furnace with the increased height.By failure analysis of the superheater pipe and water wall weld crack, to find out the corresponding failure reason, to verify the correctness of the failure model and the simulation results, to put forward the improvement measures.