Combined Cycle Efficiency Research Articles

EFFECT OF OPERATING VARIABLES ON THE PERFORMANCE OF A COMBINED CYCLE COGENERATION SYSTEM WITH MULTIPLE PROCESS HEATERS

Combined cycle power plants with a gas turbine topping cycle and a steam turbine bottoming cycle are widely used due to their high efficiencies. Combined cycle cogeneration has the possibility to produce power and process heat more efficiently, leading to higher performance and reduced green house gas emissions. The objective of the present work is to analyze and simulate a natural gas fired combined cycle cogeneration unit with multiple process heaters and to investigate the effect of operating variables on the performance. The operating conditions investigated include, gas turbine pressure ratio, process heat loads and process steam extraction pressure. The gas turbine pressure ratio significantly influences the performance of the combined cycle cogeneration system. It is also identified that extracting process steam at lower pressures improves the power generation and cogeneration efficiencies. The process heat load influences combined cycle efficiency and combined cycle cogeneration efficiency in opposite ways. It is also observed that using multiple process heaters with different process steam pressures, rather than a single process heater, improves the combined cycle cogeneration plant efficiency.

Simulation of an oxygen membrane-based combined cycle power plant: part-load operation with operational and material constraints

This paper presents the design and part-load performance of a natural gas-fired oxy-combustion combined cycle power plant for CO2 capture. The combustion chamber of a conventional gas turbine was replaced by a membrane reactor, making it possible to obtain a highly concentrated CO2 stream for long-term storage. The focus was on power plant operation with a view to operational and material constraints of individual process components to ensure their proper performance and required lifetime. In this respect, the mixed-conducting membrane modules were of particular interest. Temperature as well as concentration limitations for CO2 and other chemical species narrowed the operating window. Other critical reactor components added further constraints. For part-load operation of the power plant, two load control strategies were analysed for the gas turbine operating at constant rotational speed. In the first load control strategy, variable guide vanes were used to manipulate the mass flow of air to the gas turbine compressor. This degree of freedom was used to control the turbine exit temperature. In the second control strategy, variable guide vanes were not used and the turbine exit temperature was allowed to vary. For both load control strategies, the mean solid-wall temperature of the membrane modules was maintained close to its design value, which led to improved stability. The load-control strategy using variable guide vanes was superior to the strategy without variable guide vanes due to higher combined cycle efficiencies and increased load-reduction capability. Moreover, the performance of the catalytic combustors in the membrane reactor, operating at near stoichiometric conditions, also improved as a result of increased oxygen concentrations at part-load operation. Relevant process components were based on spatially distributed conservation balances for energy, species, mass, and momentum. A stability diagram was incorporated into the membrane module model to investigate the risk of degradation. Performance maps were used for turbomachinery components.

Cycle analysis of low and high H2 utilization SOFCs/gas turbine combined cycle for CO2 recovery

AbstractGlobal warming is mainly caused by CO2 emission from thermal power plants, which burn fossil fuel with air. One of the countermeasure technologies to prevent global warming is CO2 recovery from combustion flue gas and the sequestration of CO2 underground or in the ocean. SOFC and other fuel cells can produce high‐concentration CO2, because the reformed fuel gas reacts with oxygen electrochemically without being mixed with air, or diluted by N2. Thus, we propose to operate the multistage SOFCs under high utilization of reformed fuel for obtaining high‐concentration CO2. In this report, we have estimated the multistage SOFCs' performance considering H2 diffusion and the combined cycle efficiency of multistage SOFC/gas turbine/CO2 recovery power plant. The power generation efficiency of our CO2 recovery combined cycle is 68.5% and the efficiency of conventional SOFC/GT cycle is 57.8% including the CO2 recovery amine process. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(10): 38–45, 2008; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecj.10165

Enhancing the efficiency and power of the triple-pressure reheat combined cycle by means of gas reheat, gas recuperation, and reduction of the irreversibility in the heat recovery steam generator

The main methods for improving the efficiency or power of the combined cycle are: increasing the inlet temperature of the gas turbine (TIT), inlet air-cooling, applying gas reheat, steam or water injection into the gas turbine (GT), and reducing the irreversibility of the heat recovery steam generator (HRSG). In this paper, gas reheat with recuperation was applied to the regular triple-pressure steam-reheat combined cycle (the Regular cycle) by replacing the GT unit with a recuperated gas-reheat GT unit (requires two gas turbines, gas recuperator, and two combustion chambers). The Regular cycle with gas-reheat and gas-recuperation (the Regular Gas Reheat cycle) was modeled including detailed modeling of the combustion and GT cooling processes and a feasible technique to reduce the irreversibility of its HRSG was introduced. The Regular Gas Reheat cycle and the Regular Gas Reheat cycle with reduced-irreversibility HRSG (the Reduced Irreversibility cycle) were compared with the Regular cycle, which is the typical design for a commercial combined cycle. The effects of varying the TIT on the performances of all cycles were presented and discussed. The results indicate that the Reduced Irreversibility cycle is 1.9–2.15 percentage points higher in efficiency and 3.5% higher in the total specific work than the Regular Gas Reheat cycle, which is 3.3–3.6 percentage points higher in efficiency and 22–26% higher in the total specific work than the Regular cycle. The Regular Gas Reheat and Reduced Irreversibility cycles are 1.18 and 3.16 percentage points; respectively, higher in efficiency than the most efficient commercially-available combined cycle at the same value of TIT. Economic analysis was performed and showed that applying gas reheat with recuperation to the Regular cycle could result in an annual saving of 10.2 to 11.2 million US dollars for a 339 MW to 348 MW generating unit using the Regular cycle and that reducing the irreversibility of the HRSG of the Regular Gas Reheat cycle could result in an additional annual saving of 11.8 million US dollars for a 439 MW generating unit using the Regular Gas Reheat cycle.

Binary conversion cycles for concentrating solar power technology

It is recognized that the temperature potential of concentrated solar energy is much higher than needed by standard conversion cycles. High temperature solar receivers are in the development stage hopefully leading to the use of solarized gas turbines or of solar combined cycles. These systems are analyzed and taken as a reference standard. Binary alkali-metal steam cycles are shown to be intrinsically more efficient than combined cycles owing to their fully condensing nature. Even at top temperatures of about 600 °C typical for steam cycles the binary cycle allows, in principle, a significant efficiency gain (49.5% against 43% of a steam cycle). However, the binary high temperature systems are investigated featuring either a direct vaporization of the metal within the receiver or a liquid receiver cooling loop with the working fluid vaporized in a proper heat exchanger. With reference to the second option, the computed efficiency is 56% at a top cooling loop temperature of 1000 °C (the same efficiency is attained in a direct vaporization loop at 720 °C). A 60% thermal efficiency is within the potential of the technology. The above figures can be compared with a combined cycle efficiency of 50% at 1200 °C turbine inlet temperature. Available alkali metals are reviewed for the use of working fluid: potassium being the best known fluid but rubidium (or cesium) offering, in perspective, a better overall performance. Material problems connected with the containment of alkali metals at high temperature are reviewed. Experimental evidence suggests that up to 800–850 °C stainless steel is an adequate material, while for higher temperatures, up to 1200 °C, refractory metals should be used. With reference to heat storage the availability of appropriate high temperature substances either as liquids or as melting solids, storing energy as sensible or as latent heat respectively, is discussed. Finally the critical issue of metal vapour turbine design is considered. The results of a number of computations are presented giving the basic geometrical data of some potassium, rubidium and cesium expanders. Rotor diameters tend to be comparatively large. With reference to a 50 MW overall plant output the maximum tip diameter is 3.9 m for a potassium and 2.8 m for a rubidium turbine.

Performance Simulation of Combined Cycle with Kalina Bottoming Cycle

The Kalina cycle has potential for improved performance re-garding electrical ef ficiency, specific power output and cost of electricitycompared with conventional technology because the mixture of workingfluids enables ef ficient energy recovery. Thermodynamic analysis hasbeen carried out for combined cycle with the Kalina bottoming cycle.In this work, the identi fied key parameters for the Kalina cycle are tur-bine inlet condition (pressure, temperature and concentration), separatortemperature and ambient temperature. The effect of these parameters onexergy ef ficiency of combined cycle is examined. The combined cycleefficiency increases with the increase in the turbine inlet pressure, andthe same decreases with increases in ambient temperature, turbine inlettemperature and its concentration. Heat recovery from exhaust decreaseswith increases in the separator temperature, and it does not alter theoutput of the combined cycle. The ef ficiency of the cycle is very sensi-tive to the turbine inlet concentration and ambient temperature.

Performance Simulation of Combined Cycle with Kalina Bottoming Cycle

ABSTRACT The Kalina cycle has potential for improved performance regarding electrical efficiency, specific power output and cost of electricity compared with conventional technology because the mixture of working fluids enables efficient energy recovery. Thermodynamic analysis has been carried out for combined cycle with the Kalina bottoming cycle. In this work, the identified key parameters for the Kalina cycle are turbine inlet condition (pressure, temperature and concentration), separator temperature and ambient temperature. The effect of these parameters on exergy efficiency of combined cycle is examined. The combined cycle efficiency increases with the increase in the turbine inlet pressure, and the same decreases with increases in ambient temperature, turbine inlet temperature and its concentration. Heat recovery from exhaust decreases with increases in the separator temperature, and it does not alter the output of the combined cycle. The efficiency of the cycle is very sensitive to the turbine inle...

Sensitivity analysis of STIG based combined cycle with dual pressure HRSG

Thermodynamic evaluation has been carried out for a combined cycle with STeam Injected Gas Turbine (STIG) having dual pressure heat recovery steam generator (HRSG). Steam from high-pressure steam turbine is injected into the combustion chamber at a pressure higher than the combustion pressure to improve the exergy efficiency of combined cycle. The effect of steam injection mass ratio, deaerator pressure (or temperature ratio), steam reheat pressure ratio, HP steam turbine pressure, compressor pressure ratio and combustion temperature on combined cycle exergy efficiency has been investigated. It has been found that advantage from steam injection to combined cycle is obtained at high steam reheat pressure and high steam turbine inlet pressure. At this condition, the increasing effect of gas cycle output exceeds the decreasing effect of steam cycle output. The major exergetic loss that occurs in combustion chamber decreases with introduction of steam injection in to the combustion.

Optimum gas turbine cycle for combined cycle power plant

The gas turbine based power plant is characterized by its relatively low capital cost compared with the steam power plant. It has environmental advantages and short construction lead time. However, conventional industrial engines have lower efficiencies, especially at part load. One of the technologies adopted nowadays for efficiency improvement is the “combined cycle”. The combined cycle technology is now well established and offers superior efficiency to any of the competing gas turbine based systems that are likely to be available in the medium term for large scale power generation applications. This paper has as objective the optimization of a combined cycle power plant describing and comparing four different gas turbine cycles: simple cycle, intercooled cycle, reheated cycle and intercooled and reheated cycle. The proposed combined cycle plant would produce 300 MW of power (200 MW from the gas turbine and 100 MW from the steam turbine). The results showed that the reheated gas turbine is the most desirable overall, mainly because of its high turbine exhaust gas temperature and resulting high thermal efficiency of the bottoming steam cycle. The optimal gas turbine (GT) cycle will lead to a more efficient combined cycle power plant (CCPP), and this will result in great savings. The initial approach adopted is to investigate independently the four theoretically possible configurations of the gas plant. On the basis of combining these with a single pressure Rankine cycle, the optimum gas scheme is found. Once the gas turbine is selected, the next step is to investigate the impact of the steam cycle design and parameters on the overall performance of the plant, in order to choose the combined cycle offering the best fit with the objectives of the work as depicted above. Each alterative cycle was studied, aiming to find the best option from the standpoint of overall efficiency, installation and operational costs, maintainability and reliability for a combined power plant working in base load. Several schemes are proposed for investigation. In particular, four configurations were studied: simple cycle (SC), intercooled cycle (IC), reheated cycle (RH) and intercooled and reheated cycle (IC/RH).

Cycle analysis of low and high H 2 utilization SOFC/gas turbine combined cycle for CO 2 recovery

A major factor in global warming is CO 2 emission from thermal power plants, which burn fossil fuels. One technology proposed to prevent global warming is CO 2 recovery from combustion flue gas and the sequestration of CO 2 underground or near the ocean bed. Solid oxide fuel cell (SOFC) can produce highly concentrated CO 2, because the reformed fuel gas reacts with oxygen electrochemically without being mixed with air in the SOFC. We therefore propose to operate multi-staged SOFCs with high utilization of reformed fuel to obtain highly concentrated CO 2. In this study, we estimated the performance of multi-staged SOFCs considering H 2 diffusion and the combined cycle efficiency of a multi-staged SOFC/gas turbine/CO 2 recovery power plant. The power generation efficiency of our CO 2 recovery combined cycle is 68.5%, whereas the efficiency of a conventional SOFC/GT cycle with the CO 2 recovery amine process is 57.8%.

Thermodynamic analysis of heat recovery steam generator in combined cycle power plant

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.

Performance Simulation of Dual Pressure HRSG in Combined Cycle Power Plant

ABSTRACT Combined cycle power plants play an important role in the present energy sector. The main challenge in designing a combined cycle power plant is how to transfer gas turbine exhaust heat to the steam cycle to achieve optimum steam turbine output. Most of the combined cycle developers focus on the gas turbine and steam turbine, but the performance of the heat recovery steam generator (HRSG) strongly affects the overall performance of the combined cycle power plant. Therefore, the present article undertakes the problem of optimal utilization of the waste heat from gas turbine exhaust with dual pressure HRSG configuration. The combined cycle efficiency has been analyzed parametrically from both the first and second law of thermodynamics. It is observed that in the dual cycle the high-pressure steam pressure must be relatively high, and low-pressure steam pressure must be low for better heat recovery from HRSG.

Modelling, analysis, and irreversibility reduction of the triple-pressure reheat H-system combined cycle

The triple-pressure reheat H-system combined cycle is considered the most efficient combined cycle that is commercially available. The gas turbine of the H-system combined cycle is cooled using a closed loop of steam and an open loop of air. Compared with the regular triple-pressure reheat combined cycle, the heat recovery steam generator (HRSG) of the H-system has a different configuration and a reduced load since superheating of steam is partially performed by the gas turbine. In this paper, the triple-pressure reheat H-system combined cycle was modelled, including detailed modelling of the HRSG and the expansion and cooling processes of the gas turbine. The performance of the H-system combined cycle was analysed and a feasible technique to reduce the irreversibility of the HRSG was introduced. The reduced-irreversibility triple-pressure reheat steam-air gas turbine-cooled (H-system) combined cycle was compared with the regularly designed triple-pressure reheat H-system combined cycle, which is the typical design for a commercial combined cycle. The effects of varying the turbine inlet temperature (TIT) on the performance of all cycles were presented and discussed. The results indicate that the reduced-irreversibility H-system combined cycle is 1.45–1.65 per cent higher in efficiency and 2.5 per cent higher in the total specific work than the regularly designed H-system combined cycle when all compared at the same values of TIT and minimum temperature difference for pinch points (PPm)- The reduced-irreversibility H-system combined cycle was 1.35 per cent higher in efficiency than the most efficient commercially available H-system combined cycle when compared at the same value of TIT. Economical analysis of the reduced-irreversibility cycle was performed and showed that applying the introduced technique could result in an annual saving of ten million US dollars for a 440 MW power plant.

Co-utilization of biomass and natural gas in combined cycles through primary steam reforming of the natural gas

Power production from biomass can occur through external combustion (e.g. steam cycles, organic Rankine cycles, Stirling engines), or internal combustion after gasification or pyrolysis (e.g. gas engines, IGCC). External combustion has the disadvantage of delivering limited conversion efficiencies (max 30–35%). Internal combustion has the potential of high efficiencies, but it always needs a severe and mostly problematic gas cleaning. The present article proposes an alternative route where advantages of external firing are combined with the potential high efficiency of combined cycles through co-utilization of natural gas and biomass. Biomass is burned to provide heat for partial reforming of the natural gas feed. In this way, biomass energy is converted into chemical energy contained in the produced syngas. Waste heats from the reformer and from the biomass combustor are recovered through a waste heat recovery system. It is shown that in this way biomass can replace up to 5% of the energy in the natural gas feed. It is also shown that in the case of combined cycles, this alternative route allows for external firing of biomass without important drop in cycle efficiency.

Analysis of design and part load performance of micro gas turbine/organic Rankine cycle combined systems

This study analyzes the design and part load performance of a power generation system combining a micro gas turbine (MGT) and an organic Rankine cycle (ORC). Design performances of cycles adopting several different organic fluids are analyzed and compared with performance of the steam based cycle. All of the organic fluids recover greater MGT exhaust heat than the steam cycle (much lower stack temperature), but their bottoming cycle efficiencies are lower. R123 provides higher combined cycle efficiency than steam does. The efficiencies of the combined cycle with organic fluids are maximized when the turbine exhaust heat of the MGT is fully recovered at the MGT recuperator, whereas the efficiency of the combined cycle with steam shows an almost reverse trend. Since organic fluids have much higher density than steam, they allow more compact systems. The efficiency of the combined cycle, based on a MGT with 30 percent efficiency, can reach almost 40 percent. Also, the part load operation of the combined system is analyzed. Two representative power control methods are considered and their performances are compared. The variable speed control of the MGT exhibits far better combined cycle part load efficiency than the fuel only control despite slightly lower bottoming cycle performance.

Thermo-Economic Optimization of a Solid Oxide Fuel Cell, Gas Turbine Hybrid System

Large scale power production benefits from the high efficiency of gas-steam combined cycles. In the lower power range, fuel cells are a good candidate to combine with gas turbines. Such systems can achieve efficiencies exceeding 60%. High-temperature solid oxide fuel cells (SOFC) offer good opportunities for this coupling. In this paper, a systematic method to select a design according to user specifications is presented. The most attractive configurations of this technology coupling are identified using a thermo-economic multi-objective optimization approach. The SOFC model includes detailed computation of losses of the electrodes and thermal management. The system is integrated using pinch based methods. A thermo-economic approach is then used to compute the integrated system performances, size, and cost. This allows to perform the optimization of the system with regard to two objectives: minimize the specific cost and maximize the efficiency. Optimization results prove the existence of designs with costs from 2400$∕kW for a 44% efficiency to 6700$∕kW for a 70% efficiency. Several design options are analyzed regarding, among others, fuel processing, pressure ratio, or turbine inlet temperature. The model of a pressurized SOFC–μGT hybrid cycle combines a state-of-the-art planar SOFC with a high-speed micro-gas turbine sustained by air bearings.

4340 レーザートルクセンサーを用いた改良型コンバインドサイクルの性能診断(S60-2 ガスタービン(2),S60 ガスタービン)

A new optical torque measurement method was applied to diagnosis of thermal efficiency of advanced combined cycle, i.e. ACC, plants. Since the ACC power plant comprises a steam turbine and a gas turbine and both of them are connected to the same generator, it is difficult to identify which turbine in the plant deteriorates the performance when the plant efficiency is reduced. The sensor measures axial distortion caused by power transmission by use of laser beams, small stainless steel reflectors having bar-code patterns, and a technique of signal processing featuring high frequency. The sensor was applied to the ACC plants of TOKYO ELECTRIC POWER COMPANY in order to analyze each steam turbine performance for selection of the plant that steam was extracted, following the success of inspection of the sensor reliability in ACC.

Diagnosis of Thermal Efficiency of Advanced Combined Cycle Power Plants Using Optical Torque Sensors

A new optical torque measurement method was applied to diagnosis of thermal efficiency of advanced combined cycle, i.e. ACC, plants. Since the ACC power plant comprises a steam turbine and a gas turbine and both of them are connected to the same generator, it is difficult to identify which turbine in the plant deteriorates the performance when the plant efficiency is reduced. The sensor measures axial distortion caused by power transmission by use of He-Ne laser beams, small stainless steel reflectors having bar-code patterns, and a technique of signal processing featuring high frequency. The sensor was applied to the ACC plants of TOKYO ELECTRIC POWER COMPANY, TEPCO, following the success in the application to the early combined cycle plants of TEPCO. The sensor performance was inspected over a year. After an improvement related to the signal process, it is considered that the sensor performance has reached a practical use level.

Cycle Analysis of Low and High H2 Utilization SOFCs/Gas Turbine Combined Cycle for CO2 Recovery

Global warming is mainly caused by CO2 emission from thermal power plants, which burn fossil fuel with air. One of the countermeasure technologies to prevent the global warming is CO2 recovery from combustion flue gas and the sequestration of CO2 at the underground or in the ocean. SOFC and other fuel cells can produce high concentration CO2, because the reformed fuel gas reacts with oxygen electrochemically without being mixed by air, or diluted by N2. So we propose to operate the multi-staged SOFCs under high utilization of reformed fuel for getting high concentration CO2. In this report, we have estimated the multi-staged SOFCs performance considering H2 diffusion and the combined cycle efficiency of multistage SOFC/gas turbine/CO2 recovery power plant. The power generation efficiency of our CO2 recovery combined cycle is 68.5% and the efficiency of conventional SOFC/GT cycle is 57.8% including the CO2 recovery amine process.

Modelling and dynamics of an air separation rectification column as part of an IGCC power plant

An Integrated Gasification Combined Cycle plant (IGCC) opens the well-proven and highly efficient combined cycle process to fossil fuels, like coal or heavy refinery residues. Such a plant thereby possesses a novel linkage of typical energy engineering related units, e.g. a gas turbine and typical process engineering parts, which in the present case is an air separation plant. Different responses from the connected components can cause undesired mass flow fluctuations within the system especially during changing load demands. The cryogenic rectification column, as the core of the air separation plant, strongly affects the system's transient behaviour. The upper part of such a heat-integrated double column, a packed column with structured packing, has therefore been more closely investigated in the present paper. For this purpose, a dynamic model of such a column has been developed which is also able to describe the pressure dynamics supposedly responsible for these mass flow fluctuations. The transient behaviour of the uncontrolled column is analysed and discussed with special regard to pressure dynamics. The column pressure responds to disturbances on two different time scales. The short-term response, which is in the range of 100–200 s, is governed by the transient behaviour of the fluid dynamics and is discussed in detail. The long-term response is dominated by the nonlinear dynamics of the concentration profiles. The time constant of this response depends strongly on the direction and intensity of the disturbance and takes from 10,000 up to several 100,000 s.