| The energy consumption mix of China gives a factor that the coal-fired power generation will be the main part, over50%, of electricity power supply, during a long period of time in the future, which arises challenges for sustainable development. In this dissertation, the energy efficiency of coal-fired generation unit systems is studied by means of establishing a Cycle-Deck model for the performance analysis of off-design operating cases, to contribute for the challenges mentioned. The study and research for modeling mechanism, operating performance, optimized technology application, etc. are presented in this dissertation.The researches in this dissertation are unfold around the four aspects:mechanism, model construction, operating performance and application.1) MechanismA quasi two-dimensional modeling method of the boiler heat transfer surface has been put forward to solve the problem that the traditional lumped parameter model called zero dimension is unable to calculate the parameters distribution inside. The quasi two-dimensional model is developed by dividing the heat transfer surface into several coupled layers. The simulation results of the model agree well with actual operation data, the maximum relative error is0.77%. The parameters distribution within the boiler heat transfer surface along the flue gas flow is calculated.The steam turbine variable condition calculation method based on characteristic function of flow area is raised to solve the problem that the Flugel formula is unable to complete the calculation from flow rate to pressure. A model is developed to analyzing the internal physical relationship among the steam turbine stages. A monotonic function between compression ratios and G (?) is found. The function is determined by the flow area and the adiabatic coefficient of the stage, so called characteristic function of flow area. The operating data from a real steam turbine is applied to test the model. The results show that the method is well applied to the calculation form flow to pressures, maximum relative error is2.78%, which is much more accuracy than the Flugel formula.2) Model ConstructionA thermal system Cycle-Deck model of a coal-fired power generating unit is built. The word Cycle-Deck is referenced by the ideas of an aviation engine Cycle-Deck model, which is organized in the accordance with the order of the equipment, subsystems and the whole system. In the dissertation a Cycle-Deck model of coal-fired power generating units is developed based on the object mechanism to simulate the detailed and accurate operating performance. The model enables the simulations for complex off-design conditions, the strengthen modelling of the key device focused on. The model serves as a platform for the research of operating characteristic and the explorer of the varying thermal system. Real data obtained from a600MW supercritical generation unit is applied to test the accuracy of the Cycle-Deck model. The results show that this Cycle-Deck model is with simulation accuracy, a total of177data points is used to verify the calculation, the relative error of97%data points is less than3%, and the relative error of79%data points is under3%.3) Operating PerformanceThe coal-fired power generating unit operating performance was obtained by the simulation of the Cycle-Deck model. The operating performance of varying parameters is studied. The Parameters such as excess air coefficient, burner angle, environment temperature, desuperheating water temperature, feedwater temperature, reheat steam flow, wind temperature, main steam pressure, main steam temperature, reheat steam pressure, reheat pipe pressure loss, reheat steam temperature, exhaust steam pressure, terminal temperature difference are analytical investigated to describe their connections between energy consumption, under different operating conditions.The operating performance of varying structure system is studied. It is found that the heateres removal effect the heat rate are mutually reinforced, three high pressure feedwater heateres removal at the same time the increases of heat rate by heaters removed at the same time is36%above of heaters removal separately.The operation performance varying devices performance is also studied. It is found that the impact of the performance decrease of heater in full load is about3-6times when under the50%load. The reason is that the heaters are designed under full load. It is found that the feedwater heater will decrease with the load decreasing. Take a600MW generation unit for example, the thermal temperature difference decreases more than5℃from110%load to50%load. The difference of the heat rate calculated between thermal temperature difference varying condition and the thermal temperature difference staying condition is up to74kJ/kWh and the relative error is0.92%. The actual operating data verified this discovery. It is found that the operating parameters of feedwater heater is coupled, the change of one heater may affect the upstream or downstream heaters on heat transfer performance. The maximum difference of the heat rate calculated between considering coupling and without considering the coupling, when high pressure heater thermal temperature difference change1℃, is17.1%.4) Applications The soft measurement technology based on the generator power to monitor the heat rate is developed and applied. For the inaccuracy of the steam flow measurement in coal-fired power generating units, which leads to the unsteady measurement of the main steam flow rate. The method verifies the measurement of condensate steam flow and main steam flow and to improve on-line monitoring of the heat rate. The method has been applied in a generation unit. The standard deviation of the monitoring data is12.2kJ/kWh, which is one-third of the result using condensate flow to calculate heat rate during the same period. The results of thermal performance test and design data prove that the monitoring results by this method is accurate. The soft measurement technology can be used for real-time online monitoring and condition assessment of thermal system performance.The active control technology of exhaust gas bypass cascade economizer design is developed. For the boiler exhaust gas temperature is usually higher than the design data and uncontrollable, a cascade bypass economizer can be located in parallel circuits. The part of exhaust gas is used to heat feedwater and condensate water. The technology reduces the exhaust gas temperature, at the same time, decreases the heat rate of steam turbine regenerative system. It can be seen from the results that the use of bypass cascade economizer will eventually cool the exhaust gas temperature to95℃and reduce the coal consumption for2.8g/kWh. Compared with the similar technology using low temperature economizer, the technology can reduce coal consumption for an extra1.59g/kWh. In addition, the technology also realize the active control of exhaust gas temperature by changing the share of bypass gas. |
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