| With the increasingly prominent contradiction between economy development and energy and environment in China, energy conservation and emission reduction of thermal power units becomes an issue of great concern. The current wide use of large-capacity super criticality units provides an effective means of improving thermal efficiency. Meanwhile, to utilize the waste heat is an important measure to save energy. The huge amount of boiler flue gas waste heat can be used but haven’t been fully used. The flue gas heat has a low temperature and can be conducted into either regenerative system to heat the condensate water or the boiler through heating the inlet air of air preheater. However, the economy of the two ways of boiler flue gas waste heat utilization (BFGWHU) is disputed due to the disagreement over the calculation methods of cold source loss. In this context, it’s urgent to figure out the theoretical questions and find out exact evaluation methodology and best ways to utilize it, thus we can improve the energy conversion efficiency of flue gas waste heat and lower the energy consumption rate accordingly.Based on the original steam regenerative cycle, the concepts of flue gas regenerative cycles which include turbine side regenerative cycle (TRC), boiler side regenerative cycle (BRC) and united regenerative cycle (URC) are presented according to the inputting location of flue gas waste heat. Following the entropy equilibrium principle, working ability models of all the flue gas regenerative cycles which can evaluate thermal economy are built and verified by field tests. So the energy transfer mechanism of flue gas heat during utilization is systematically studied and the effective analytical method is put forward, thus the doubt in the evaluation of BFGWHU is clarified.In this thesis, various means of BFGWHU in power station are summarized, the drawbacks and limitations of which are discussed. Flue gas waste heat being regeneratively utilized inside the unit is determined as the basic mode. The main saving energy analysis theories and methods of BFGWHU in thermal plants are discussed, and the advantages and problems are analyzed. According to the characteristics of BFGWHU in power station, the entropy generation analysis and equivalent enthalpy drop method are used as the research basis of theoretical analysis and calculation. By combining the advantages of the two methods, the thermal economy of BFGWHU is deeply studied.On the basis of steam regenerative cycle, the recovered flue gas waste heat is used in steam regeneration and air regeneration, and the concepts of TRC and BRC are presented. TRC is quantitatively analyzed using the equivalent enthalpy drop method. The concepts of extraced steam actual working efficiency and equivalent increment of equivalent enthalpy drop are put forward. The original thermal economy computing model is modified for better understand and application. Based on trisector air preheater system model, the heat transfer of BRC is analyzed. Through the concept of flue gas regenerative heat recovery conversion numbers, the actual amount of utilized flue gas heat is determined, which lays the fundation of exact thermal economy analysis.Based on the entropy equilibrium principle, entropy generation model of external heat entering temprature difference heat-transfer system is built, which supplies a simplified and feasible method of thermal economy analysis. Working ability models of TRC&BRC are built using this method. Subject to the thermal parameter regulation of partial quantitative analysis, the application in a300MW unit shows that the thermal economy of TRC is confined by the original temperature of boiler flue gas while BRC is free of it, but the reduction in actual amount of utilized flue heat leads to lower thermal economy than TRC. Calculation shows that equivalent enthalpy drop method has good uniformity with entropy generation method. However, the latter can reveal the distribution characteristics of the irreversible loss in thermal system and can more easily find the key factor to affect thermal economy, thus can indicate the way to decrease the irreversible loss and optimize the system.Using the thermal economy evaluation models of TRC, the thermal economy of separated type heat pipe and low pressure economizer are respectively analyzed, which shows that the latter is better at energy saving. According to the time-interval feature of BRC, two flue gas regenerative cycles are combined in the manner of running separately in respective time-interval, the BFGWHU system is preliminarily optimized.The heat rate field tests with TRC on and off is conducted in a300MW unit. Except for the errer actor of the exhaust steam pressure change by TRC, the test result is corrected to calculation standard through variable working condition computing model of condenser. Comparison between the test and the calculated result shows that the deviation is1.83%, thus validating the accuracy of the research method and the thermal economy computing models.Currently the application research about low temperature flue gas heat transfer elements has made great progress. According to the analysis conclusion has been obtained, the concept of united regenerative cycle (URC) on boiler and turbine is presented to deeply utilize low temperature flue gas waste heat, which is a combination of TRC&BRC. URC can be extended to series system and parallel system. The heat transfer model of URC is built. The coupling mechanism of TRC&BRC is analyzed and thermal economy evaluation models are built through the concept of heat recovery conversion numbers. Considering the calculated results of a600MW unit, the mechanism of the flue gas heat energy level increasing is analyzed. It shows that the thermal economy of URC is higher than that of TRC, the root cause of which is the entropy generation reducing and the working ability increasing of the air preheater.To further learn the influence of BFGWHU on unit thermal efficiency, the BFGWHU process is discussed independently using the partial quantitative analysis thought, and the concept of equivalent unit efficiency of BFGWHU is presented. According to the original thermodynamic meaning of efficiency, the generalized unit efficiency is defined on the basis of traditional unit efficiency and analyzed with the use of BFGWHU system. This efficiency analysis provides a new simple and effective way for researches into more complex BFGWHU systems.The influencing factors of URC are analyzed from the perspective of actual running. According to the adjusting principle of air inlet temprature of coal mill, the conditions of ensuring boiler combustion stability are obtained. The thermal economy variation of URC parallel system operation adjustment under rated conditions and variable conditions is analyzed, which reveals that the adjustment of flue gas bypass portion is able to meet the requirement of seasonal characteristics. The sensitivities of various BFGWHU systems to original boiler exhaust gas temperature are analyzed, which shows that the sensitivity of URC series system and TRC is higher than that of URC parallel system, and the suitable original exhaust temperature range of each flue gas regenerative system is determined. The influences of flue gas regenerative systems on whole plant net efficiency are analyzed through the concept of equivalent auxiliary power rate, which provides foudation for evaluating the power supply thermal efficiency indices of BFGWHU systems. |
PDF Full Text Request