| China is in a major transformation process of energy structure and utilization mode.At present,coal-fired power plants occupy a dominant position in power generation capacity and installed capacity.The proportion of renewable energy is gradually increasing.Due to the limitation of peak shaving capacity of coal-fired power plants and the intermittent and fluctuation of renewable energy,the grid urgently needs to add more flexible power generation capacity to enhance peak shaving capacity.Gas turbine combined cycle is gaining more and more attention due to its excellent efficiency,peak shaving performance and low emission.Gas turbine technology is making further breakthroughs in the direction of greater capacity and higher parameters,but its wide efficiency range and rapid decline in efficiency with load limit its economic operation under part-load condition.With the further increase of the proportion of renewable energy,the gas turbine combined cycle system will more and more undertake the task of peak shaving.It will operate under part-load conditions for a long time to back-up renewables.It is of great significance to optimize the performance of the combined cycle system under part-load conditions.This article aims to improve the part-load operating performance of the combined cycle system,based on the part-load characteristic prediction model of the gas turbine combined cycle,studies the part-load operation characteristics of key components,and explores methods to improve part-load performance from the perspective of operating strategies and system configuration.Load performance method.The main work and results are as follows:Firstly,on the basis of the general characteristic curve of the compressor stage,considering the influence of the rotation speed and the compressor inlet guide vane(IGV)installation angle on the air flow,an improved stage-stacking method was used to build the compressor characteristic prediction model,which can be used to calculate the operating characteristics of the compressor when the rotation speed,IGV angle and environmental parameters change;using continuity equations constructed by aerodynamic functions,considering the influence of the changes of cooling air,Mach number,angle of attack and gas parameters on turbine stage characteristics,a model for predicting turbine part-load characteristics was constructed;at the same time,a part-load performance prediction model of the bottoming cycle was constructed based on the variable working condition models of each heating surface,and gives the evaluation index of the unit;using the modular modeling method,considering the matching of the operating points of the compressor,combustion chamber and turbine,considering operating strategies,a gas turbine combined cycle part-load characteristic prediction model was constructed for system characteristic estimation under different operating strategies,and it is convenient to estimate the performance of an improved system.The model was verified using a typical F-class gas turbine combined cycle unit with operating points at different loads and different ambient conditions,and the calculation accuracy of the model meets the requirements of unit characteristics analysis.Using the established prediction model,the operating characteristics of key components under part-load are obtained,and the load regulation mechanism of the unit is explored from a theoretical perspective,and the principles and methods of part-load performance optimization are proposed.This content is the basis for the follow-up gas turbine combined cycle system part-load operation performance optimization.It can be used to calculate the operating characteristics of the unit under part-load conditions after the system configuration and operating strategy are changed,and to evaluate the system performance.Secondly,in order to improve the adverse effect on the combined cycle efficiency caused by the reduction of the turbine inlet temperature under part-load conditions,a turbine backpressure adjustment system was proposed and the corresponding operating mode was formulated.The exhaust pressure of the gas turbine is controlled by the induced draft fan set at the outlet of the heat recovery steam generator(HRSG),thereby the turbine inlet temperature can be improved under part-load conditions.The influence of the turbine backpressure on the part-load characteristics,key parameters of the main equipment and the energy conversion were studied.The research results show that reducing the turbine backpressure will not have a significant impact on the operating characteristics of the final turbine stage,and the reduction of the turbine backpressure solves the problem of the turbine expansion ratio and isentropic efficiency decreasing with load.Compared with the benchmark unit,the load application range of the constant T3 strategy can be expanded from 100%?82.4%to 100%?63.7%,and the turbine exhaust will not be overheated,and the efficiency of the gas turbine is significantly improved.Reducing turbine backpressure has a negative impact on the performance of the bottoming cycle,but it has a significant improvement in combined cycle efficiency.Compared with modes of the constant T3+fuel only control(T3-F)and contant T3+constant T4+fuel only control(T3-T4-F),under the same combined cycle load,the method of reducing turbine backpressure can improve the combined cycle efficiency by 1.76 and 0.52 percentage points at most,respectively.The proposed T3-PD-PU-F operating mode is more suitable for turbine backpressure adjustment system.The variation of turbine backpressure has a large influence on the feasible operational region of the combined heat and power(CHP)unit,and the backpressure can be used as a method to expand the heat-to-electric ratio.Thirdly,as the load decreases,the turbine exhaust temperature rises rapidly,which is an important reason for restricting the application of the constant T3 operating mode in a larger load range.To solve this problem,a gas turbine combined cycle configuration with intermediate recuperated gas turbine is proposed.Preheating air at the inlet of the combustion chamber with the gas coming from turbine stage can reduce fuel consumption while ensuring the turbine inlet and exhaust temperatures are at ideal values,thus the operating performance of the gas turbine combined cycle under part-load can be improved.Based on the proposed operating mode,the part-load characteristics of the gas turbine combined cycle under the coordinated control of IGV and flow rate of the gas flowing into the recuperator are studied,and the effects of the pressure drop in the recuperator and minimum recuperative flow on the performance of the gas turbine combined cycle under variable operating conditions are analyzed.The results show that the proposed system can maintain the turbine inlet and exhaust temperature constant at the design value in a wider gas turbine load range,but the load range of IGV action is reduced.From the perspective of gas turbine power,gas turbine efficiency and combined cycle efficiency can be increased by 2.15 and 1.04 percentage points respectively at the same load.The contribution of efficiency improvement mainly comes from the gas turbine cycle.The recuperation between turbine stages has little effect on the turbine exhaust temperature(only exists in the high load region and low load region),and the performance of the bottoming cycle is basically not affected.The design pressure loss of the recuperator has a great influence on the performance of the system.When the design pressure loss increases by 1 percentage point,the efficiency of the gas turbine and the combined cycle efficiency decrease by 0.20 and 0.14 percentage points,respectively.The minimum recuperation flow has little effect on the performance of the combined cycle.Finally,by breaking the conventional operation strategies that taking gas turbine as the critical role of load regulation and taking IGV as the single air flow adjustment method,a system configuration and operating mode that combines recuperation and intake air temperature control were proposed.The effects of recuperation and intake air temperature control on part-load performance of gas turbine and combined cycle were analyzed,and the effect of recuperation on system performance under different conditions was discussed.The calculation and analysis of system characteristics show that in the high load region(recuperation ratio control),the bottoming cycle assumes the load regulation task of the unit,and the gas turbine cycle operating conditions are basically constant.At this region,the combined cycle efficiency shows an upward trend as the load decreases,and the highest efficiency is higher than the design operating efficiency of the reference unit.In the low-medium load region(intake air temperature control),affected by recuperation,the gas turbine operating point is higher than the reference unit.At the same combined cycle load,the turbine inlet temperature and expansion ratio are higher,and the combined cycle efficiency is significantly improved.As the load decreases,the performance of the system increases gradually.When the combined cycle power is reduced to 44%of design power output,the combined cycle efficiency and exergy efficiency can be increased by 2.46 and 2.34 percentage points,respectively.The disadvantage of this configuration is that the full-load power is reduced by 17.92 MW compared to the reference unit.If this operating mode is combined with IGV control,a deeper load reduction and more efficient part-load operating performance can be achieved. |
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