Improvement Of The Low Reheat Steam Temperature And Comparison Of Extraction Heating Modes For 660MW Ultra-supercritical Thermal Power Unit

This paper takes the 2×660MW ultra-supercritical power plant as an example.The problem of low reheat steam temperature is discussed to find out the influencing factors and formulate reasonable modification scheme.Meanwhile,focused on the extraction and heating reconstruction project of the power plant,the economy of different extraction heating modes and the impact on the safety of boiler high-temperature reheater are studied,which is of great significance in optimizing the design and operation of ultra-supercritical units.Through analyzing the problem of low reheat steam temperature,boiler thermal calculation and single-input change method are adopted to analyze the effects of coal ash content,moisture content,reheat steam flow,excess air coefficient,and flame-center height on the reheat steam temperature.The engineering calculation formulas for reheat steam temperature are obtained by thermal calculation data and incremental model.Considering field data,the practical operating condition is simulated,it is determined that the low reheat steam temperature of the boiler is due to the low exhaust temperature of the high pressure cylinder,the deviation of the coal quality,the larger cross-sectional area of the furnace than that of the design coal,the higher feed water temperature and installation defects.In order to increase the temperature of reheat steam,the modification scheme of increasing the heating area of vertical low temperature reheater can be adopted while repairing the space of the middle partition wall of the flue at the end of boiler.After the modification,the reheat steam temperature will be increased to the design value,and there will be a certain adjustment margin.Extraction of steam heating will affect the economic and security of the unit,and the two aspects should be taken into account when renovating.In terms of economy,the heat balance method is used to establish the heat balance calculation model for the inlet and outlet extraction of the lowtemperature reheater,and the influence of different extraction heating modes on the thermal economy of the unit is calculated.It is concluded that both of the two methods will reduce the standard coal consumption rate of power generation,and improve the economic efficiency of the unit,and the extraction of steam from the low-temperature reheater will be more economical.In terms of safety,based on the calculation data of the thermal system and the boiler,a thermal deviation calculation software is established to obtain the distribution curve of the fluid temperature and the metal outer wall temperature along the tube length before and after the extraction.By calculating the temperature change of the outer wall of the pipe with different material extraction points under different extraction modes,and combining the outer wall temperature and alarm temperature provided by the boiler plant,it is determined that both extraction modes will cause overheating of the high-temperature reheater tube panel.Among them,the maximum temperature of the inlet extraction of the low-temperature reheater exceeds the alarm value of 3.5°C,and the maximum temperature of the outlet extraction of the low-temperature reheater exceeds the alarm value of 5°C.The analysis and transformation of the low reheat steam temperature and the comparison of the economic safety of the steam heating reforms which can provide the basis for the similar units to solve the similar problems and formulate the transformation technology plan.