Strategy Optimization And Performance Improvement Of Gas-steam Combined Cycles Under Design/Off-design Conditions

Gas-steam combined cycle has become one of the most suggested thermal power generation systems around the world with several advantages over coal-fired stations including high efficiency,low pollution and rapid startup.Both natural gas and biomass gas could be used as applicable fuel in the gas turbine cycle which widen its utilization.In many countries,gas turbine power plants usually have to operate under part load conditions to regulate the load and peak for power grid which will cause the decrement on cycle performance.So that it is necessary to investigate improvement methods for gas turbine combined cycle thermal efficiency both under design condition and off-design conditions.In this paper,the PG9351FA gas turbine cycle from GE Company with matching bottoming cycle has selected as the basic combined cycle arrangement.The performance improvement investigation has been carried out through the modification of combined cycle under off-design condition and the design parameter of combustion chamber with detailed contents as following:Firstly,the model of a gas-steam combined cycle with/without an unbalanced flow rate recuperator is established and the influence on gas-steam combined cycle performance by a recuperation retrofitting is simulated.The mainly equations and methods in this section lay the foundation of strategy optimization and cycle innovation.Moreover,the measurement of biomass laminar burning speed is briefly introduced and the derivation process of the multi-shell flame combustion thermodynamic model is given for experimental data processing.Secondly,the combined cycle off-design performances with different strategies using IGV control are compared based on the reliable combined cycle model to analysis the influence of the main parameters on the cycle efficiency.Then,the strategy related to the highest combined cycle performance within the safe region is found,and higher gas turbine inlet/outlet temperature is believed to improve the performance of combined cycle under off-design conditions.After that,a combined cycle with inner recuperator is proposed based on the principle of cascade utilization among the energy of the whole combined cycle.Better part load efficiency could be obtained for the retrofitted combined cycle by adding a recuperator in the optimized HRSG to produce the advantage of the recuperation modification meanwhile reduce the negative impact on the bottoming cycle performance.The off-design model of the retrofitted combined cycle is established,and the operation strategy is innovated based on the characteristics of the cycle.The performance coupling with two novel strategies are compared with the basic combined cycle coupling,with two IGV strategies through energy analysis and exergy analysis under part load conditions to demonstrate the advantage of the inner recuperation modification and interpretate the mechanism of part load performance improvement.And then,a combined cycle with compressor inlet heating process is proposed based on the principle of cascade utilization to utilize the waste heat in the HRSG exhaust flue gas.Novel low grade recuperation cycle could be built for the new combined cycle through the compressor inlet air heat exchanger to obtain better combined cycle part load efficiency by recovering the waste heat in flue gas and maintaining the steam turbine cycle performance at the same time.The off-design model of the modified combined cycle is established and a new operation method is first created based on the characteristics of the retrofitted cycle.The performance coupling with two novel strategies are compared with the basic combined cycle coupling two IGV strategies by energy analysis and exergy analysis under part load conditions to demonstrate the advantage of the compressor inlet air heat exchanger and interpretate the mechanism of part load performance improvement.Furthermore,the affection of the pressure loss of the heat exchanger on the cycle performance is investigated,and the possibility of the combination of IGV control and the new method is presented with expected good results.Finally,the combustion characteristics of the biomass laminar burning speed is studies with the experimental bench and the multi-shell thermodynamic model.Both the spherical bomb method and the one-dimensional steady-state combustion simulation have been utilized to analysis the influence of several factors(including temperature,pressure,and fuel component)on the laminar buring speed.The experimental result is compared with simulation result and data from other articles for reliable verification,and two power law correlations for difference range of carbon dioxide have been developed based on it.Moreover,the affection of factors are discussed,and the modification mechanism is explained for the gas turbine combustion chamber designation with applicable fuel changing from natural gas to biomass.