Optimization of Combined Cycle Power Plant Using Sequential Quadratic Programming

Thermodynamic assessment of integrated biogas-based micro-power generation system

This article provides the energy and exergy based performance analysis of a solid oxide fuel cell (SOFC) – gas turbine (GT) – steam turbine (ST) combined cycle power plant. The system utilizes the GT exhaust heat for fuel and air preheating subsequently in a fuel recuperator (FR) and an air recuperator (AR) before finally producing steam in a heat recovery steam generator (HRSG) coupled with the ST cycle. It considers 30% external reforming in a pre-reformer (PR) by steam extracted from the bottoming ST plant. The study considers the effect of additional fuel burning in the combustion chamber (CC) as a means for increasing the net GT and ST power output. A detailed parametric analysis based on variation of compressor pressure ratio (CPR), fuel flow rate (FFR), air flow rate (AFR), current density, single level boiler pressure and ST inlet temperature (STIT) is also provided. Results indicate improved system performance at higher CPR. The optimum single level boiler pressure is found to be 40bar with 50% additional fuel burning. Burning of additional fuel improves the GT and ST power output, however with reduction in the plant’s overall efficiency. Further comparison of performance with a similar other system where the AR is placed head of the FR indicates slightly better performance of the proposed system with FR ahead of AR (FRAOAR).

This concept shows that an efficient combined cycle, comprising topping & bottoming cycle, does not have to be privilege of gas turbine plants only, but could also be achieved with steam turbine plants. An efficient power-producing concept of a combined steam-turbine cycle with addition of a recirculating steam compressor is disclosed. Topping part of such a combined steam-turbine cycle operates at elevated steam turbine inlet temperature and pressure, while its “waste heat” is recovered by the bottoming part of the combined cycle in a heat-recovery boiler (steam heat exchanger). The recirculating steam compressor pumps the cooled majority of the entire steam flow to the maximum cycle pressure, while smaller steam flow fraction continues its full expansion to some low pressure in a condenser. The cycle waste heat could be transferred to the bottoming part of the combined cycle in a variety of modalities, depending on the chosen main high-temperature steam-turbine inlet temperature and inlet pressure (supercritical/subcritical). At an assumed constant steam-turbine inlet temperature of 900°C (∼300 bar), a very high gross cycle thermal efficiency could potentially be achieved, ranging from 56 to 62% with the high-temperature steam-turbine pressure ranging from subcritical (30 bar) to supercritical (300 bar). Such a combined steam-turbine cycle seems to be a suitable energy conversion concept that could be applied in classic thermal power plants powered by coal, but also seems as an ideal option for application in the new generation of gas-cooled nuclear rectors, where the gaseous reactor coolant, heated up to 1000°C, would indirectly transfer its heat content to working fluid (superheated steam) of the topping part of the combined steam-turbine cycle. Alternatively, the proposed concept may be combined with renewable energy sources of a sufficient temperature level.

This paper presents the thermodynamic study of a heat and power system which combines an organic Rankine cycle and a gas turbine (GT) cycle. The required power of the compressor in the GT cycle and pump in the Rankine cycle is provided by the WT which according to the amount of cycle required power. For analysis of the cycle, a simulation has been performed using R123 as the working fluid in the Rankine cycle and air and combustion products in the GT cycle. In this end, effect of various parameters such as wind speed, the angular speed of WT, and the gas turbine inlet temperature as well as the compressor pressure ratio, gas turbine isentropic efficiency, condenser temperature and compressor isentropic efficiency on the total thermal efficiency and total exergy efficiency is calculated and analyzed. Thermal efficiency and exergy efficiency of 21.31% and 23.54% is obtained. Also, it is observed that the greatest exergy destruction occurs in the combustion chamber.

ABSTRACTA heat recovery steam generator (HRSG) is the vital equipment in a gas/steam combined cycle power plant (CCPP) which plays an important role in enhancing the overall performance of the plant. An HRSG is designed on the basis of the size of the gas and steam turbines used in the power plant. Therefore, the HRSG directly affects the performance of the CCPP and its optimisation based on physical design can improve the plant performance. In the present study, the plant data of the CCPP of NTPC Ltd. at Auraiya (U.P.), India, has been considered for study. A segmented finned model has been chosen as it is the preferred arrangement [Ganapathy, V. 2003. Industrial Boiler and Heat Recovery Steam Generator Design, Application and Calculation. New York: Marcel Dekker Inc.] and thermodynamic investigations have been carried out upon a dual pressure HRSG in a gas/steam CCPP from the perspective of maximum heat recovery and minimum pressure loss. The performance investigations of varying physical parameters of a dual pressure HRSG with segmented fins are useful for CCPP designers and for all those who are engaged in the area of heat recovery.

A multi-objective optimization method of cogeneration of power and heat in a combined gas turbine and organic Rankine cycle (ORC) is conducted to achieve the best system design parameters from both thermodynamic and economic aspects by utilizing nondominated sorting genetic algorithm-II (NSGA-II). Exergy efficiency and total cost rate of the system have been considered as objective functions. The cogeneration system consists of a gas turbine (GT) and an organic Rankine cycle (ORC) in which the two cycles are connected through a single-pressure heat recovery steam generator (HRSG). In order to optimize the system, air compressor pressure ratio, air compressor isentropic efficiency, air preheater outlet temperature, turbine inlet temperature, isentropic efficiency of the gas turbine, pinch point temperature of HRSG, pinch point temperature of evaporator, evaporator temperature, and condenser temperature have been selected as decision variables. Optimization results indicate that exergy efficiency of the cycle increases from 51.41% at base case to 55.6% while more than 9.15% reduction is achieved in the total cost rate of the cycle. Also by applying multi-objective optimization, the exergo-economic factor has reached from 10.68 to 27.40.

Thermodynamic performance simulation and concise formulas for triple-pressure reheat HRSG of gas–steam combined cycle under off-design condition

Combined cycle power plant (CCPP) is a closed-cycle power plant, where the heat from the gas turbine’s (GT) exhaust gas will be streamed to the heat recovery steam generator (HRSG) to be utilized by steam turbine (ST). CCPP Block 4 (Jawa-2) PT Indonesia Power Priok POMU has an installed capacity of 880 MW, consists of 2 GT units (301.5 MW each) and 1 ST unit (307.5 MW). The performance of a power plant depends on its load, as the efficiency of the turbine generator is low when operated at low loads. The data as of July 2019 showed that 2.2.1 (2 GT, 2 HRSG, 1 ST) configuration has been used in three conditions where the CC net load was around 30 - 45 %, which in fact could be compensated by the 1.1.1 (1 GT, 1 HRSG, 1 ST) configuration. This resulted in a decrease of the CC net efficiency up to 21.34 %. The optimization that can be done is to change the load configuration from 2.2.1 to 1.1.1 at 0 - 50 % of CC net load through simulations, by including the influence of the GT and HRSG start-up processes. The result of this optimization is that the CCPP performance increases due to higher performance of each turbine generator. Thus, the optimization results during July 2019 provided energy saving of 1,146.09 MMBTU or equivalent to cost saving of IDR 152,249,551.76.

Optimization of the combined-cycle efficiency by optimizing the gas or steam cycle efficiencies does not guarantee an optimum combined cycle. In this paper, triple-pressure and dual-pressure non-reheat combined cycles are modelled and optimized for 50 and 300 cases respectively. The layout for the triple-pressure cycle was chosen to reduce NOX emission. Constraints are set on the pressure of the feed water heater, the minimum temperature difference for pinch points, the superheat approach temperature difference, the steam turbine inlet temperature and the dryness fraction at the steam turbine outlet. The combined-cycle efficiency is optimized using two different methods: the direct search method and the variable metric method. The effects of the inlet temperature of both the gas turbine and the steam turbine and of the minimum pinch point temperature difference on the performance of the optimum cycles are presented and discussed. The optimization results indicate that the layout for the triple-pressure cycle redefines the temperature differences for pinch points and adds a constraint on the intermediate pressure of the steam generator, which reduces the optimum efficiency by 2–3 percent when compared with the dual-pressure cycle under the same conditions. For the dual-pressure cycle, the optima of the most efficient cases were found to be 0.5-2 percent higher in efficiency than the optima of the regular design case, which is the usual design case for a commercial combined cycle. The optimum dual-pressure combined cycle was found to be almost as efficient as the most efficient commercially available triple-pressure reheat combined cycle, when compared at the same firing temperature.

Combined cycle power plants play an important role in the present energy sector. The main challenge in designing a combined cycle power plant is proper utilization of gas turbine exhaust heat in the steam cycle in order to achieve optimum steam turbine output. Most of the combined cycle developers focused on the gas turbine output and neglected the role of the heat recovery steam generator which strongly affects the overall performance of the combined cycle power plant. The present paper is aimed at optimal utilization of the flue gas recovery heat with different heat recovery steam generator configurations of single pressure and dual pressure. The combined cycle efficiency with different heat recovery steam generator configurations have been analyzed parametrically by using first law and second law of thermodynamics. It is observed that in the dual cycle high pressure steam turbine pressure must be high and low pressure steam turbine pressure must be low for better heat recovery from heat recovery steam generator.

Thermodynamic sensitivity analysis of hybrid system based on solid oxide fuel cell

Heat recovery steam generator (HRSG) is an equipment part of the combined cycle power plant (CCPP), which consists of the gas turbine, HRSG, and steam turbine. HRSG performs a significant role in defining the efficiency of the CCPP. The fault condition at a subsystem or component of HRSG could cause failure in entire CCPP systems. Therefore, the fault potential should be identified, and the failure should be mitigated once it occurs. The objective of this study is to determine the alternative control structure which can be applied in the HRSG system in case of major fault and to determine which the most optimum alternative control structure based on reliability and cost analysis. First, the fault is determined and ranked based on the severity of the effect. Restructuration will be done for any major fault with severity rank 8 and above. The alternative control structure will be determined, then reliability and cost analysis will be applied to those structures to choose the most optimum alternative structure which will be applied in the system. Modeling and simulation will be done using MATLAB Simulink. Closing the blowdown valve, reducing the opening of the steam outlet control valve, and the combination of the previous two structures are the alternative structures that can be applied in HRSG systems in case of complete loss in the economizer outlet control valve. The third structure is the most optimum alternative structure based on a reliability and a cost analysis. It has reliability rate of 3.99 and cost of $12,078.87. The third structure also has the longest retention time of 436 s.

This article illustrates aspects of heat recovery steam generator (HRSG) design when employing process integration in an integrated reforming combined cycle (IRCC) with pre-combustion CO2 capture. Specifically, the contribution of the paper is to show how heat integration in a pre-combustion CO2 capture plant impacts the selection of HRSG design. The purpose of such a plant is to generate power with very low CO2 emissions, typically below 100 g CO2/net kWh electricity. This should be compared to a state-of-the-art natural gas combined cycle (NGCC) plant with CO2 emissions around 380 g CO2/net kWh electricity. The design of the HRSG for the IRCC process was far from standard because of the significant amount of steam production from the heat generated by the auto-thermal reforming process. This externally generated steam was transferred to the HRSG superheaters and used in a steam turbine. For an NGCC plant, a triple-pressure reheat steam cycle would yield the highest net plant efficiency. However, when generating a significant amount of high-pressure steam external to the HRSG, the picture changed. The complexity of selecting a HRSG design increased when also considering that steam can be superheated, and low-pressure and intermediate-pressure steam can be generated in the process heat exchangers. For the concepts studied it was also of importance to maintain a high net plant efficiency when operating on natural gas. Therefore the selection of HRSG design had to be a compromise between NGCC and IRCC operating modes.

Retrofitting existing power generation plants by repowering is becoming an attractive option to improve plant performance with less cost. “Hot Windbox Repowering” involves utilizing the hot exhaust gas from a combustion gas turbine and using it as combustion air for an existing fossil-fuel boiler. “Combined Cycle Repowering” or “Full Repowering” involves completely replacing the existing boiler with a combined cycle consisting of a gas turbine(s) and a heat recovery steam generator (HRSG). The existing steam turbine will be used in both repowering scenarios. This paper discusses an engineering study and summarizes the results obtained from repowering an existing heavy-oil / natural gas fired steam power plant in the north east of the United States. The plant consists of a 600 MW boiler and steam turbine. Several engineering studies were considered and evaluated thermodynamically and economically to retrofit such plant. Several options were considered involving different gas turbines, gas turbine combinations, and different repowering methods. The best option is based on retrofitting the unit by a combination of both, hot windbox repowering and combined cycle repowering. The proposed design consists of one gas turbine repowering the windbox of the existing boiler, and a second gas turbine operating in a separate combined cycle configuration with the generated superheated steam tying into the main steam line and expanding in the existing steam turbine. Several heat balances were developed to assist in obtaining meaningful results for this feasibility study. Actual costs were obtained for the gas turbines and heat recovery steam generators (HRSG), as well as installation costs for a more accurate evaluation. The results indicate that the combined output of the repowered unit will generate an additional 295 MW and reduce the heat rate by more than 11 percent at full load and annual average ambient conditions. The estimated capital cost of the project is expected to range from $235 to $245 millions.

The system-level simulation model for Natural Gas Combined-Cycle (NGCC) plant combines advanced G class turbine with various fuels was performed in the study. The commercial chemical process simulator, Pro/II® V8.1.1, is used in the study to build the analysis model. There are three major sections which are gas turbine (GT), heat recovery steam generator (HRSG) and steam turbine (ST).The study envisages two analyses as the basic and feasibility cases. The former is the benchmark case which is verified with the reference data of exist NGCC power plant located in Dah-Tarn, Taiwan. The Dah-Tarn NGCC power plant introduces the advanced M501G gas turbine from Mitsubishi Heavy Industries, Ltd. (MHI) and the system efficiency of the combined-cycle is 58.75 % (LHV). The latter introduces a feasibility study with advanced gas turbine and actual parameters in Taiwan. The SNG from carbonate fuel feed to M501G based combined-cycle is evaluated in the feasibility study cases first, and the mixture gas of SNG and syngas is also evaluated to compare the different of overall performance with the pure SNG case.The results show that the system efficiency of pure SNG case is around 57 % (LHV). For the mixture gas case, the maximum ratio of syngas to SNG is 1:19, due to keep the same composition of methane is 85.9 % with the Dah-Tarn NGCC power plant. The efficiency of mixture gas case is 55.75 % (LHV).The study shows the possibility way to use coal and reduce the CO2 emission. The another advantage is the using of SNG could increases the usage of combined-cycle in Taiwan, due to the prices of natural gas is higher than coal in Taiwan and results in the higher idle capacity.