Study On Off-Design Performance And Configuration Optimization Of STIG With Throttle Valve

In order to relieve the energy crisis and environmental pollution caused by unbalanced structure of energy consumption in our country,the government vigorously promotes the reform of energy policy and encourages the use of natural gas.As the major application mode of natural gas to generate electricity,the gas turbine is widely used in cogeneration system.To improve the system thermal efficiency under off-design conditions,the steam injected gas turbine(STIG)cycle could be used.For the in-depth study on the performances of STIG,the results show that the cogeneration system with STIG has the ability of adjust the ratio of heat to power elastically and independently.The small-scale STIG reduces the output power by decreasing the turbine inlet temperature.However,the quantity of heat produced by heat recovery steam generator(HRSG)will reduce,so that the ability of adjust the ratio of heat to power weakened after that.By learning from the predecessors’ research achievements and referring to the literature,this study presents a mode of operation in which the turbine inlet temperature keep the design values all the time.In order to achieve that mode,an ideal throttle valve is installed before the compressor.When the STIG work under the off-design condition,both compressor inlet pressure and air mass flow decrease with the power output reduction.But the turbine outlet temperature increase.As a result,the exergy of the exhaust increase and efficiency of HRSG is improved.Therefore the STIG with throttle valve has the ability of adjust the ratio of heat to power elastically under the lower load.Cause the energy demand usually varies from time to time,the part-load operation of cogeneration system account for long periods of the total plant operation time.Thus,the investigation of the off-design performance is significant.Therefore,in this study,the author compares this two operation mode under off-design condition and obtains the overall off-design characteristics,also gets the result of prime energy saving ratio(PESR)by comparing with the separation production system.Under the lower load condition,the performance of the STIG with throttle valve is better.By reduce the power output,the PESR of STIG without throttle valve lower than the PESR of STIG with throttle valve.The gap is more noticeable in high value of steam injected ratio.Under the off-design condition,every operation point of this two operation mode has the thermodynamic parameters determined by 3(,)sX T and1(,)sX p,respectively.For this,this study obtains all the operation points and the range of variation.For the STIG without throttle valve,the maximum quantity of heat is the limitation.For the STIG with throttle valve,the pressure ratio is the limitation.In addition,both inject steam into combustor and reduce the output power will make the pressure ratio increase.The maximum steam injected ratio of STIG with and with throttle valve is 0.10 and 0.12,respectively.At the present,the main operation strategies of cogeneration system are following the electricity load(FEL)and following the heating load(FHL).In order to guide the selection of operation strategy,obtain the operation points which have the equal power output or the equal quantity of heat.Choosing the appropriate load model and based on the above method,optimize the system configuration using the two kind of operation mode STIG,respectively.The optimal MEI of the STIG with throttle valve is 0.29 and the optimal MEI of the normal STIG is 0.28.Comparing the results,get the conclusion that the STIG with throttle valve has a better performance in that load model.