Reheat Gas Turbine Research Articles

Impact of acoustic pressure on autoignition and heat release

A feedback mechanism for thermoacoustic pulsations in gas turbine reheat combustors is proposed and investigated, namely the impact of acoustic pressure waves on the reaction kinetics of autoignition. An analytical model framework is developed, which represents the combustor as a plug flow reactor. This analogy allows one to derive modulations of the heat release rate as a result of the history of pressure perturbations during the autoignition process. Numerical studies are conducted on homogeneous reactors with detailed chemistry simulations, in order to assess quantitatively the pressure sensitivity of the reaction kinetics of autoignition. From such sensitivities, flame transfer functions of autoignition flames are derived. The expressions obtained are successfully compared with time-domain simulations in one-dimensional space, and used in acoustic network models for stability predictions. The wider applicability of the model is demonstrated by extending it to nonlinear dynamics, transverse modes and technical premix conditions. The results obtained indicate that in general the feedback mechanism results in a positive contribution to the acoustic source term, in particular at elevated frequencies.

Optimisation Potentials for the Heat Recovery in a Semi-closed Oxyfuel-combustion Combined Cycle with a Reheat Gas Turbine

Abstract Semi-closed oxyfuel-combustion combined cycle plants are one possibility to realise CCS technologies with natural gas-fired power plants. Due to the recirculation of exhaust gas the working fluid in the gas turbine consists mainly of CO2. This leads to the necessity for an increased pressure ratio to achieve the efficiency and the exhaust gas temperatures of conventional gas turbines. In this work a conventional natural gas-fired combined cycle plant with a reheat gas turbine modified for oxyfuel operation will be presented. As it is assumed that the necessary pressure ratio is not realisable with axial compressors two different scenarios are considered. On the one hand the pressure ratio is the same as in the conventional process and on the other hand the pressure ratio is set to an assumed maximum of 60. The exhaust gas temperature lies significantly above the values of the conventional plant. Generally the higher pressure ratio results in a higher net efficiency of the plant. An unchanged heat recovery process results in a loss of efficiency of at least 9.3 to 10.8%-pts. depending on the pressure ratio. Increasing the steam parameters can achieve a reduction of this loss ranging from 0.5%-pts for the high pressure ratio to 1.0%-pts for the low pressure ratio. An alternative approach for the heat recovery is the application of an internal recuperator in the gas turbine. When the achievable pressure ratio is far from the optimum value, as in the low pressure ratio case, this approach can lead to significantly lower losses of 9.1%-pts. With a higher pressure ratio the effects of the optimisation options are similar.

GHGT-12 Performance of the IGCC with distributed feeding of H2 in the gas turbine burner

Abstract A main hurdle for realizing the IGCC with CO 2 capture is the lack of gas turbines capable of burning hydrogen in an environmental-friendly, yet energy-efficient manner. With currently proposed combustion schemes an efficiency penalty is caused by the preparation (separation and compression) of the inert diluent, i.e. nitrogen. Distributed Fuel Injection (DFI) , where hydrogen is provided to the combustion air through a H2-separating membrane or a porous wall, could be a means of avoiding both concentrated fuel point sources and N2 dilution of the fuel. The paper presents two potential schemes for DFI: Scheme 1 with upstream H2 separation and scheme 2 with integrated H2 separation from the shifted syngas. First process simulation results for an IGCC with a reheat gas turbine yielded an IGCC efficiency increase of 0.7%-points with Scheme 1 and of 1.3%-points with scheme 2, compared to a reference case with nitrogen fuel dilution. This is mainly due to savings in N2 compression work but for scheme 2 also because the partial pressure difference of H2 over the Pd membrane can be employed as driving force. The results should however be regarded as indicative only one of the reasons being that fuel injection pressure drop for DFI is yet unknown, and is presumably a trade-off between membrane/porous wall size.

OPTIMUM OPERATING PARAMETERS OF AN IRREVERSIBLE GAS TURBINE CYCLE

The present paper discusses the performance of an irreversible regenerative intercooler–reheat gas turbine cycle under wide ranges of design and operating parameters. The study aims to determine the optimum regions of these parameters that satisfy the most requirements of the gas turbine cycle. The operating parameters are the inlet air temperature to the first stage compressor, inlet temperature and pressure of air entering the high pressure turbine and the thermal size (the heat transfer coefficient–area product) of the whole heat exchangers used in the cycle. A mathematical simulation model is developed to calculate the performance parameters of the cycle under different operating conditions. The developed model achieves most of the requirements of the gas turbine cycle (maximum first and second law efficiencies, minimum back work ratio (BWR), maximum ecological coefficient of performance (ECOP), maximum work output, minimum exergy losses, and finally minimum heat added to the cycle). Achieving some or all of these requirements is based on selection of certain small region in the applicable range of operating parameters. The optimum region resulted from the given operating parameters are: 302 K – 315 K for the minimum temperature, 1340 K – 1360 K for the maximum temperature, 1440 kPa – 2830 kPa for the maximum pressure, and finally 20.7 kW/K – 29.6 kW/K for the heat exchangers’ thermal size.

Hydrogen fuel supply system and re-heat gas turbine combustion

Carbon Capture and Storage (CCS) based on the pre-combustion routine involves a fuel supply and gas turbine combustion system which must handle a fuel with a very high content of Hydrogen. This might cause problems related to ignition and auto-oxidation in the piping in addition to thermo-acoustic instabilities, flash back and increased emissions in the gas turbine. In this work it is found that for typical fuel compositions problems with self ignition and auto-oxidation in the fuel supply system is avoided if the mixture temperature is kept below 400°C and the fraction of Oxygen in the fuel does not exceed 3%. Furthermore it is shown that even a reheat combustor can be fuelled with up to 70% Hydrogen and 30% Nitrogen as diluent at full load without problems with combustion instabilities or NOx emissions when inline fuel injection is used together with appropriate mixing devices.

Assessment of existing H 2/O 2 chemical reaction mechanisms at reheat gas turbine conditions

This paper provides detailed comparisons of chemical reaction mechanisms of H 2 applicable at high preheat temperatures and pressures relevant to gas turbine and particularly Alstom’s reheat gas turbine conditions. It is shown that the available reaction mechanisms exhibit large differences in several important elementary reaction coefficients. The reaction mechanisms are assessed by comparing ignition delay and laminar flame speed results obtained from CHEMKIN with available data, however, the amount of data at these conditions is scarce and a recommended candidate among the mechanisms can presently not be selected. Generally, the results with the GRI-Mech and Leeds mechanisms deviate from the Davis, Li, Ó Conaire, Konnov and San Diego mechanisms, but there are also significant deviations between the latter five mechanisms that altogether are better adapted to hydrogen. The differences in ignition delay times between the dedicated hydrogen mechanisms (Ó Conaire, Li and Konnov) range from approximately a maximum factor of 2 for the H 2-air cases, to more than a factor 5 for the H 2/O 2/AR cases. The application of the computed ignition delay time to reheat burner development is briefly discussed.

Parametric study of a closed cycle reheat gas turbine power plant based on the harmonic mean isentropic exponent

SYNOPSIS A closed cycle gas turbine power plant with reheater is investigated on the basis of harmonic mean isentropic exponent. An analytical expression is derived for the pressure ratio corresponding to maximum power and maximum dimensionless net work output. It is found that the pressure ratio corresponding to maximum power and the maximum non-dimensional net work is high when the ratio of extreme temperatures of cycle (i.e. Tmax and Tmin) and isentropic efficiencies of compressor and turbine (i.e. ηc and ηt) are high. The theoretical results are also compared with the results obtained by considering practical reheat turbine data.

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).

A novel high-performance low-NOx fuel-rich/fuel-lean two-stage combustion gas and steam turbine system for power and heat generation

This paper proposes, describes, and analyses a novel high temperature and performance reheat gas turbine (GT) cogeneration system. This newly proposed system employing two-stage fuel-rich and fuel-lean combustion system is named as the ‘Fuel Rich-Lean Combustion Reheat GT Cogeneration System’ (RLCC). Energy and exergy analyses (the latter mainly to evaluate and demonstrate the reduction in exergy loss of this combustion process compared with conventional single-stage and fuel lean-lean combustion) of the proposed cogeneration system were performed and compared with a conventional cogeneration system. The proposed RLCC system is predicted to have an overall power generation efficiency of up to 53.0 per cent (low heating value (LHV) basis), exergy efficiency up to 61.9 per cent, and specific power up to 524.3 kJ/kg. Compared with a conventional cogeneration (CGC) system, its energy utilization efficiency was as high as 95.0 percent, 4.0 percentage points higher than that of the CGC system, the overall power generation efficiency was higher by 6.2 percentage points, and the exergy efficiency was higher by 7.8 percentage points, and the overall power-based NO x emissions was reduced by up to 34.0 per cent.

Comparative study of steam injection effects on operation of gas turbine cycles

In the present work, gas turbine cycles are modified with steam injection between the combustion chamber exit and the gas turbine inlet. Heat recovery steam generators, utilising the exhaust gases, provide these cycles with the injected steam at saturated vapour. The irreversibility of the different composing units of the cycles and the variation of gas properties owing to steam injection as well as changes in the interrelation of component performance parameters are taken into account. The isentropic temperature ratio and maximum to minimum cycle temperature ratio are varied over some ranges that slightly exceed their practically acceptable bounds in order to comprehensively investigate their effects on cycle characteristics. At the chosen values of the operating parameters, the enhancement achieved in the overall efficiency for the simple, reheat (with steam injection at high and low pressures) and partial oxidation (with steam injection at high and low pressures) gas turbine cycles are of about 20∼30%, 120∼200%, 10∼12%, 120∼260%, 20% respectively. The present modified cycles technique can be considered among the possible ways to improve the performance of gas turbine cycles-based power plants at feasible costs.

Thermodynamic optimization of a reheat chemically recuperated gas turbine

Abstract The feasibility of integrating a commercially available reheat gas turbine with a methane steam reformer is analyzed. A slight modification to the original reheat design is proposed to improve the methane conversion rate in the reforming process and, consequently, the efficiency in recovering waste exhaust heat from the gas turbine. Two solutions are proposed for the heat recovery scheme: a first reformer has a single pressure level while the second has two in order to match the different pressures of the combustors. While the single pressure scheme gives good performance with respect to the stand alone gas turbine, the dual pressure reformer can give a further benefit, as far as an accurate optimization of the steam management is performed.

Performance evaluation of reheat gas turbine cycles

The role of gas turbine power plants in electrical energy production has been considerably increased in the last two to three decades. Various methods have been proposed to improve the performance of gas turbine cycles. In this research, two methods, a reheat cycle (RC) and a cycle with a reheat and a heat exchanger (RHC), were investigated and compared with a simple cycle (SC). Today, to achieve a higher efficiency and capacity in gas turbines, higher turbine inlet temperatures (1300 °C and more) are used. Basically, application of such temperatures without turbine blade cooling is impossible. Therefore, analysis of gas turbine cycles without considering blade cooling in modelling will certainly not lead to a valid and correct result. The main objective of this paper is to study the performance of an RC and RHC under actual conditions. In this regard, all processes are treated as actual, and in particular a relatively simple and reliable approach is used to predict the amount of cooling air. It should be noted that there are many attempts being made to produce ceramic turbines, for which a much-reduced cooling requirement will be necessary. The results obtained on the basis of a model developed for this research show that reheating in the context of a realistic study may lead to an improvement both in efficiency and in specific net work.

RP of Si3N4 burner arrays via assembly mould SDM

A rapid prototyping process applying ceramic gelcasting and assembly mould shape deposition manufacturing has been developed to fabricate mesoscale silicon nitride components with complex 3D geometry. An overview of the fabrication of Si3N4 burner arrays for gas turbine reheat applications is demonstrated. The design of burner arrays features internal fuel passages and integrated fuel injection elements, which cannot be manufactured by conventional methods. Geometrical decomposition of design models and recent developments in ceramic processing provide a novel approach in prototyping the burner arrays. In this paper, issues of materials compatibility, mould fabrication strategies, ceramic processing, and design iterations will be discussed along with the functional test of the 4×4 burner array.

Cycle Optimization and High Performance Analysis on Gas Engine-Steam/Gas Turbine Combined Cycles

The reciprocating engine has the maximum pressure and temperature in the cylinder, higher than that of the conventional gas turbines. When the gas engine is integrated with gas turbine instead of turbocharger, ETCS (the engine-turbo compound system) is composed. This paper defines the compound system with ERGT (the Engine Reheat Gas Turbine) and optimizes the engine operating condition. Natural gas firing steam/gas turbine combined cycle is selected as the standard system of thermal power plant. The higher TIT (Turbine Inlet Temperature) of gas turbine usually enables the higher power generation efficiency. This paper shows the effect of engine exhaust temperature on the cycle performance with no change of TIT and that the exhaust of 900°C increases the system power generation efficiency from 46 to 50% LHV (TIT 1 150°C) and 54 to 57% LHV (TIT 1 350°C), respectively.

A Unique Approach for Thermoeconomic Optimization of an Intercooled, Reheat, and Recuperated Gas Turbine for Cogeneration Applications

In the present paper, a comprehensive methodology for the thermoeconomic performance optimization of an intercooled reheat (ICRH) gas turbine with recuperation for cogenerative applications has been presented covering a wide range of power-to-heat ratio values achievable. To show relative changes in the thermoeconomic performance for the recuperated ICRH gas turbine cycle, results for ICRH, recuperated Brayton and simple Brayton cycles are also included in the paper. For the three load cases investigated, the recuperated ICRH gas turbine cycle provides the highest values of electric efficiency and Energy Saving Index for the cogenerative systems requiring low thermal loads (high power-to-heat ratio) compared to the other cycles. Also, this study showed, in general, that the recuperated ICRH cycle permits wider power-to-heat ratio range compared to the other cycles and for different load cases examined, a beneficial thermodynamic characteristic for the cogeneration applications. Furthermore, this study clearly shows that implementation of the recuperated ICRH cycle in a cogeneration system will permit to design a gas turbine which has the high specific work capacity and high electric efficiency at low value of the overall cycle pressure ratio compared to the other cycles studied. Economic performance of the investigated gas turbine cycles have been found dependent on the power-to-heat ratio value and the selected cost structure (fuel cost, electric sale price, steam sale price, etc.), the results for a selected cost structure in the study are discussed in this paper.

On some perspectives for increasing the efficiency of combined cycle power plants

The paper proposes an analysis of some possibilities to increase the combined cycle plant efficiency to values higher than the 60% without resorting to a new gas turbine technology. Optimization of heat recovery steam generator (HRSG) with the use of parallel sections and of limit subcritical conditions (up to 220 bar) is the key elements to obtain this result. The HRSG optimization is sufficient to obtain combined cycle plant efficiencies of the order of 60% while, joining HRSG optimization with the use of gas turbine reheat (postcombustion) and gas to gas recuperation can lead the efficiency of the whole plant to the limit value of 65%. Results are proposed with reference to a turbine inlet temperature of 1500 K, corresponding to those of usual commercial D–F series gas turbine.

An insight on intercooling and reheat gas turbine cycles

The current drive for high efficiencies and low emissions has resulted in the examination and development of advanced power systems based on complex gas turbine cycles. Reheat and intercooling are two such schemes. The basic objective of introducing intercooling and reheat is to sources of loss in a gas turbine engine are those arising from the turbomachinery and the need to cool the turbine blades. In this paper the concepts of intercooling and reheat for gas turbines are assessed, in a systematic way, using a model that includes the above losses in order to evaluate their effects on the engine performance. Also examined is the choice of the position where intercooling or reheat is implemented which can have a large effect on the engine output. A comparison is made with the simple cycle and it is shown that these schemes show much promise. Some of the development difficulties are also outlined. Intercooling promises large improvements in efficiency over the simple cycle, especially at high pressure ratios. Reheat on the other hand is much more suited to combined cycles.

Extension of Partial Regenerative Dual Fluid Gas Turbine.

For improvement in thermal efficiency of steam injected gas turbines for electric power generation, Partial Regenerative Dual Fluid Gas Turbine System (PRDF) was proposed in the previous paper. In this paper, heat recovery processes of PRDF and steam injected gas turbine are evaluated in view of exergy loss. It has been shown that decrease in exergy loss in the heat recovery process can contribute to improvement in thermal efficiency. Extended systems of PRDF for further improvement in thermal efficiency were also proposed and evaluated : one is a Partial Regenerative Dual Fluid Reheat Gas Turbine, and another is a combined system of PRDF with NH3 bottoming cycle.

Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation

The Department of Heat and Power Engineering at Lund University in Sweden has been conducting theoretical studies of combined SOFC and gas turbine (SOFC/GT) cycles. The overall goal is an unbiased evaluation of performance prospects and operational behaviour of such systems. The project is part of a Swedish national program on high-temperature fuel cells. Results of continuous studies started earlier by authors are presented. Recent developments in modelling techniques has resulted in a more accurate fuel cell model giving an advantage over previous system studies based on simplified SOFC models. The fuel cell model has been improved by detailed representation of resistive cell losses, reaction kinetics for the reforming reaction and heat conduction through the solid part of the cell. This SOFC model has further been confirmed against the literature and integrated into simulation software, Aspen Plus™. Recent calculations have focused on a system with external pre-reforming and anode gas recirculation for the internal supply of steam. A reference system, sized at 500 kW, has also been analyzed in variants with gas turbine reheat and air compression intercooling. In addition, knowledge of stack and system behaviour has been gained from sensitivity studies. It is shown that the pressure ratio has a large impact on performance and that electrical efficiencies of more than 65% are possible at low pressure ratios.

Natural Gas Decarbonization to Reduce CO2 Emission From Combined Cycles—Part II: Steam-Methane Reforming

This paper discusses novel schemes of combined cycle, where natural gas is chemically treated to remove carbon, rather than being directly used as fuel. Carbon conversion to CO2 is achieved before gas turbine combustion. The first part of the paper discussed plant configurations based on natural gas partial oxidation to produce carbon monoxide, converted to carbon dioxide by shift reaction and therefore separated from the fuel gas. The second part will address methane reforming as a starting reaction to achieve the same goal. Plant configuration and performance differs from the previous case because reforming is endothermic and requires high temperature heat and low operating pressure to obtain an elevated carbon conversion. The performance estimation shows that the reformer configuration has a lower efficiency and power output than the systems addressed in Part I. To improve the results, a reheat gas turbine can be used, with different characteristics from commercial machines. The thermodynamic efficiency of the systems of the two papers is compared by an exergetic analysis. The economic performance of natural gas fired power plants including CO2 sequestration is therefore addressed, finding a superiority of the partial oxidation system with chemical absorption. The additional cost of the kWh, due to the ability of CO2 capturing, can be estimated at about 13–14 mill$/kWh.