Feasibility of pulse combustion in micro gas turbines

In gas turbines, a fast decrease of efficiency appears when the output decreases; the efficiency of a large gas turbine (20…30 MW) is in the order of 40 %, the efficiency of a 30 kW gas turbine with a recuperator is in the order of 25 %, but the efficiency of a very small gas turbine (2…6 kW) in the order of 4…6 % (or 8…12 % with an optimal recuperator). This is mainly a result of the efficiency decrease in kinetic compressors, due to the Reynolds number effect. Losses in decelerating flow in a flow passage are sensitive to the Reynolds number effects. In contrary to the compression, the efficiency of expansion in turbines is not so sensitive to the Reynolds number; very small turbines are made with rather good efficiency because the flow acceleration stabilizes the boundary layer. This study presents a system where the kinetic compressor of a gas turbine is replaced with a pulse combustor. The combustor is filled with a combustible gas mixture, ignited, and the generated high pressure gas is expanded in the turbine. The process is repeated frequently, thus producing a pulsating flow to the turbine; or almost a uniform flow, if several parallel combustors are used and triggered alternately in a proper way. Almost all the compression work is made by the temperature increase from the combustion. This gas turbine type is investigated theoretically and its combustor also experimentally with the conclusion that in a 2 kW power size, the pulse flow gas turbine is not as attractive as expected due to the big size and weight of parallel combustors and due to the efficiency being in the order of 8 % to 10 %. However, in special applications having a very low power demand, below 1000 W, this solution has better properties when compared to the conventional gas turbine and it could be worth of a more detailed investigation.     

Premixed ammonia/hydrogen swirl combustion under rich fuel conditions for gas turbines operation

Energy storage is one of the highest priority challenges in transitioning to a low-carbon economy. Fluctuating, intermittent primary renewable sources such as wind and solar require low-carbon storage options to enable effective load matching, ensuring security of supply. Chemical storage is one such option, with low or zero carbon fuels such as hydrogen, alcohols and ammonia having been proposed. Ammonia provides zero-carbon hydrogen storage whilst offering liquefaction at relatively low pressures and atmospheric temperatures, enabling ease of transportation in a pre-existing infrastructure. Ammonia can also be used directly as a fuel in power plants such as gas turbines to avoid complete conversion back to hydrogen. It is a relatively unreactive fuel, and so it is of interest to explore the potential utilisation of ammonia/hydrogen mixtures. Hence, the goal of this paper is to provide a first assessment of the suitability of a chosen 70%NH₃30%H₂ (%vol) blend for utilisation within a gas turbine environment, based on primary combustion diagnostics including combustion stability – via OH chemiluminescence - and emissions (NO_x and NH₃). An established optical generic swirl-burner enabled studies of the influence of equivalence ratio (φ > 1), ambient temperature (<484 ± 10 K) and bypass air, with a focus on NO_x reduction, one of the main challenges for ammonia combustion. A numerical GT cycle model is developed alongside the experimental investigation. The results demonstrate that the blend has considerable potential as a fuel substitute with reasonable combustion stability and significant reduction of emissions for the cases without bypass air, due to increased chemical reactivity of unburned ammonia. However, emissions are still above those recommended for gas turbine cycles, with a theoretical cycle that still produces low efficiencies compared to DLN methane, highlighting the requirement for new injection techniques to reduce NO_x/unburned NH₃ in the flue gases whilst ensuring increased power outputs.     

Thermodynamic characteristics of a low concentration methane catalytic combustion gas turbine

Low concentration methane, emitted from coal mines, landfill, animal waste, etc. into the atmosphere, is not only a greenhouse gas, but also a waste energy source if not utilised. Methane is 23 times more potent than CO₂ in terms of trapping heat in the atmosphere over a timeframe of 100 years. This paper studies a novel lean burn catalytic combustion gas turbine, which can be powered with about 1% methane (volume) in air. When this technology is successfully developed, it can be used not only to mitigate the methane for greenhouse gas reduction, but also to utilise such methane as a clean energy source. This paper presents our study results on the thermodynamic characteristics of this new lean burn catalytic combustion gas turbine system by conducting thermal performance analysis of the turbine cycle. The thermodynamic data including thermal efficiencies and exergy loss of main components of the turbine system are presented under different pressure ratios, turbine inlet temperatures and methane concentrations.     

Characteristics of oxy-fuel combustion in gas turbines

This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH₄, O₂, CO₂ and H₂O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas.     

Research and development for steam and gas turbines in low-carbon future

Abstract

Future energy requirements for the UK will need to be met with reduced CO₂ emissions. There is a requirement for 2050 CO₂ emissions to be at 20% of 1990 levels. To achieve this there will need to be considerable investment into the research and development, and construction of renewables technologies. Despite this emphasis on renewable power, fossil fuelled power generation technologies, and in particular turbines, will still have a major role to play in the future. However, the way in which the turbines will be used may change significantly. There will therefore need to be research and development investment for turbines at all levels of technology readiness. The arguments for future R&D investment for steam and gas turbines are reviewed and some directions the R&D may need to take examined.     

Catalytic combustion for industrial gas turbines

This brief review provides a general account of work directed at the use of catalytic combustion in gas turbine engines. A major potential advantage of using catalytic combustion is that the fuel can be burnt efficiently at temperatures low enough (< 1500°C) to avoid significant oxidation of atmospheric nitrogen. This advantage was less important when catalytic combustion was demonstrated in the 1970s than it is today and received relatively little attention until the following decade. After discussion of the principles involved in the design of a combustor that must meet the mixing, size, performance and durability goals of a based gas turbine application, the review turns to accounts of experiments conducted on a laboratory scale with simple configurations. These established basic operating parameters for satisfactory combustion performance and led to larger scale work and to prototype design concepts for industrial gas turbines in the late 70s and early 80s. Test results were encouraging but were not pursued definitively in the U.S.A. Activity continued at several centres in Japan, with exploration of a number of different catalyst arrangements, geometries, and control systems, again with encouraging results. At the same time, there has been renewed interests in the U.S.A. and in Europe, spurred largely by the emphasis on reducing emissions of nitrogen oxides (NOx). The paper concludes with suggestions for further development of catalytically stabilized combustion systems for gas turbines. These systems must ensure adequate pre-catalyst temperature, with evenly premixed fuel and air, and sufficient temperature rise across the catalyst to ensure effective completion of reaction in a homogeneous reaction mode. The outstanding problems are largely concerned with questions of catalyst integrity and longevity in practical configurations and realistic engine operating conditions.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

Distributed swirl combustion for gas turbine application

Colorless distributed combustion (CDC) has been shown to provide significant improvement in gas turbine combustor performance. Colorless distributed combustion with swirl is investigated here to develop ultra-low emissions of NO and CO, and significantly improved pattern factor. Experimental investigations have been performed using a cylindrical geometry combustor with swirling air injection and axial hot gas exit stream from the combustor. Air was injected tangentially to impart swirl to the flow inside the combustor. The results obtained from the combustor have demonstrated very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions at an equivalence ratio of 0.7 and a high heat release intensity of 36 MW/m³-atm under non-premixed combustion. To further simulate gas turbine operating conditions, inlet air to the combustor was preheated to 600 K temperature and the combustor operated at 2 atm pressure. Results showed very low levels of CO (∼10 PPM) but the NO increased somewhat to ∼10 PPM at an equivalence ratio of 0.5 and heat release intensity of 22.5 MW/m³-atm under non-premixed combustion conditions. For premixed combustion, the combustor demonstrated low levels of both NO (5 PPM) and CO (8 PPM) at an equivalence ratio of 0.6 and a heat release intensity of 27 MW/m³-atm. Results are reported at different equivalence ratios on the emission of NO and CO, lean stability limit and OH^* chemiluminescence. These results suggest that further performance improvement can be achieved with improved fuel mixture preparation prior to the ignition of fuel at higher operational pressures using swirling combustor design for our quest to develop ultra low emission high intensity combustor for gas turbine application.     

Exergy-based analysis of gas transmission system with application to Yamal-Europe pipeline

This paper presents a thermodynamic analysis of a gas transmission system consisting compressor stations and pipeline sections. It has been assumed that the compressor station comprises a gas turbine-driven compressor and a gas cooler, and the irreversibility of the processes associated with the gas transmission was investigated. The exergy method was used to determine the amount of work supplied to the components of the pipeline system and the amount of work that is lost during the gas transmission. For the case study, the Yamal-Europe pipeline is chosen. In this study, a nonisothermal, steady-state gas flow model was used for comparing the performance of the gas transmission system under different cooler operating set points. The pipeline flow and the compressor station processes were governed by the equations which include real-gas model based on virial equation of state.     

First and second law analysis of pulse tube refrigerator

The first and second law of thermodynamics was used to analyze the orifice type and the double-inlet type of pulse tube refrigerator (PTR). Detailed dynamic characteristics of the thermodynamics, flow and heat transfer processes in the PTR were revealed, including the dynamic pressure variations, transient gas temperature, mass flow rate in the PTR. The exergy loss method was used to analyze each component in the PTR for the first time, and the performance coefficients of all components of PTR have been obtained. It was found that the performance coefficient of the double-inlet PTR was 0.108, 9% higher than that of the orifice PTR. The analysis also showed that the exergy efficiency of the double-inlet PTR was 29.95%, significantly higher than that of the orifice PTR (25.04%). In addition, it was found that the exergy losses in the regenerator and orifice were substantially larger than in other components of the PTR system. The optimal design of these two key components is, therefore, essential for the further improvement of the PTR performance.     

Investigation of forward flow distributed combustion for gas turbine application

New innovative advanced combustion design methodology for gas turbine applications is presented that is focused on the quest towards zero emissions. The new design methodology is called colorless distributed combustion (CDC) and is significantly different from the currently used methodology. In this paper forward flow modes of CDC have been investigated for application to gas turbine combustors. The CDC provides significant improvement in pattern factor, reduced NO_x emission and uniform thermal field in the entire combustion zone for it to be called as an isothermal reactor. Basic requirement for CDC is carefully tailored mixture preparation through good mixing between the combustion air and product gases prior to rapid mixing with fuel so that the reactants are at much higher temperature to result in hot and diluted oxidant stream at temperatures that are high enough to autoignite the fuel and oxidant mixture. With desirable conditions one can achieve spontaneous ignition of the fuel with distributed combustion reactions. Distributed reactions can also be achieved in premixed mode of operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation forward flow modes consisting of two non-premixed combustion modes and one premixed combustion mode have been examined that provide potential for CDC. In all the configurations the air injection port is positioned at the opposite side of the combustor exit, whereas the location of fuel injection ports is changed to give different configurations. Two combustion geometries resulting in thermal intensity of 5 MW/m³-atm and 28 MW/m³-atm are investigated. Increase in thermal intensity (lower combustion volume) presents many challenges, such as, lower residence time, lower recirculation of gases and effect of confinement on jet characteristics. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra-low NO_x emissions are found for both the premixed and non-premixed combustion modes at the two thermal intensities investigated here. Almost colorless flames (no visible flame signatures) have been observed for the premixed combustion mode. The reaction zone is observed to be significantly different in the two non-premixed modes. Higher thermal intensity case resulted in lower recirculation of gases within the combustion chamber and higher CO levels, possibly due to lower associated residence time. The characteristics at the two thermal intensity combustors investigated here were found to be similar.     

Cycle improvements to steam injected gas turbines

The efficiency and specific power of the steam injected gas turbine is analysed by modelling the thermodynamic cycle. In this model special attention is paid to the blade cooling. The basic cycle as well as cycles with improvements such as intercooling, heat recovery by regenerator and blade cooling using steam are studied. The different cycles are compared with the combined cycle and the intercooled regenerative cycle. The conclusion is that the steam injected cycles have high efficiency and specific power. Adding heat exchangers to the cycle is not beneficial. Using steam as coolant for the blades offers interesting perspectives. Copyright © 2000 John Wiley & Sons, Ltd.     

Reduction of a detailed reaction mechanism for hydrogen combustion under gas turbine conditions

The aim of this study is to find a reduced mechanism that accurately represents chemical kinetics for lean hydrogen combustion at elevated pressures, as present in a typical gas turbine combustor. Calculations of autoignition, extinction, and laminar premixed flames are used to identify the most relevant species and reactions and to compare the results of several reduced mechanisms with those of a detailed reaction mechanism. The investigations show that the species OH and H are generally the radicals with the highest concentrations, followed by the O radical. However, the accumulation of the radical pool in autoignition is dominated by HO₂ for temperatures above, and by H₂O₂ below the crossover temperature. The influence of H₂O₂ reactions is negligible for laminar flames and extinction, but becomes significant for autoignition. At least 11 elementary reactions are necessary for a satisfactory prediction of the processes of ignition, extinction, and laminar flame propagation under gas turbine conditions. A 4-step reduced mechanism using steady-state approximations for HO₂ and H₂O₂ yields good results for laminar flame speed and extinction limits, but fails to predict ignition delay at low temperatures. A further reduction to three steps using a steady-state approximation for O leads to significant errors in the prediction of the laminar flame speed and extinction limit.     

Development of a gas turbine performance analysis program and its application

A general-purpose performance prediction program, which can simulate various types of gas turbine such as simple, recuperative, and reheat cycle engines, has been developed. A stage-stacking method has been adopted for the compressor, and a stage-by-stage model including blade cooling has been used for the turbine. The combustor model has the capability of dealing with various types of gaseous fuels. The program has been validated through simulation of various commercial gas turbines. The simulated design performance has been in good agreement with reference data for all of the gas turbines. The average deviations of the predicted performance parameters (power output, thermal efficiency, and turbine exhaust temperature) were less than 0.5% in the design simulations. The accuracy of the simulation of off-design operation was also good. The maximum root mean square deviations of the predicted off-design performance parameters from the reference data were 0.22% and 0.44% for the two simple cycle engines, 0.22% for the recuperative cycle engine, and 0.21% for the reheat cycle engine. Both the design and off-design simulations confirmed that the component models and the program structure are quite reliable for the performance prediction of various types of gas turbine cycle over a wide range of operations.     

Investigation of reverse flow distributed combustion for gas turbine application

In this paper reverse flow modes of colorless distributed combustion (CDC) have been investigated for application to gas turbine combustors. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation reverse flow modes consisting of three configurations at thermal intensity of 28 MW/m³-atm and five configurations at thermal intensity of 57 MW/m³-atm have been investigated and these high thermal loadings represent characteristic gas turbine combustion conditions. In all the configurations the air injection port is positioned at the combustor exit end, whereas the location of fuel injection ports is changed to give different configurations. The results are presented on the exhaust emissions and radical emissions using experiments, and evaluation of flowfield using numerical simulations. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes investigated here. Cross-flow configuration, wherein the fuel is injected at high velocity cross stream to the air jet resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from closer to diffusion mode to distributed regime. This feature is beneficial for part load operation where higher stability limit is desirable.     

Applications of porous media combustion technology – A review

The rapid advances in technology and improved living standard of the society necessitate abundant use of fossil fuels which poses two major challenges to any nation. One is fast depletion of fossil fuel resources; the other is environmental pollution. The porous medium combustion (PMC) has proved to be one of the feasible options to tackle the aforesaid problems to a remarkable extent. PMC has interesting advantages compared with free flame combustion due to the higher burning rates, the increased power dynamic range, the extension of the lean flammability limits, and the low emissions of pollutants. This article provides a comprehensive picture of the global scenario of applications of PMC so as to enable the researchers to decide the direction of further investigation. The works published so far in this area are reviewed, classified according to their objectives and presented in an organized manner with general conclusions.     

Evaluation of thermal NOx emission characteristics of high efficiency gas turbines using refuse-recovered low BTU gases

The paper evaluates thermal NO_x formation characteristics of high efficiency gas turbines using refuse-recovered low Btu gases. First, an estimation model of nitric oxide (NO) formation density for the gas turbine is constructed by revising a model proposed by W. S. Y. Hung. Next, taking a gas turbine burning high Btu gas with a turbine inlet temperature of 1273 K as a reference case, its NO emission density is estimated. Parametric analyses are done on the parameters that greatly affect the NO formation characteristics for the reference gas turbine, and it is shown that there is a limit to the possible NO density reduction by only controlling these parameters. Thirdly, the effects on the NO density of increasing gas turbine efficiency, by incorporating a regenerator or by raising turbine inlet temperature, are investigated. It is shown that the NO density of a gas turbine with a high efficiency is estimated to be higher than the assumed environmental quality standard (EQS) value. Finally, by utilizing these results, the cases are investigated where refuse-recovered low Btu fuels such as pyrolysis gas from municipal refuse are used. It is shown that the NO densities in these cases are estimated to be notably lower than the EQS value. Further, it is demonstrated that the use of low Btu fuel does not necessarily assure a low NO density, since the NO density is not determined only by the heating value of the fuel.     

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and fundamental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers…. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabilities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs.     

An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions

Chemical kinetics in hydrogen combustion for elevated pressures have recently become more relevant because of the implementation of hydrogen as a fuel in future gas turbine combustion applications, such as IGCC or IRCC systems. The aim of this study is to identify a reaction mechanism that accurately represents H₂/O₂ kinetics over a large range of conditions, particularly at elevated pressures as present in a gas turbine combustor. Based on a literature review, six mechanisms of different research groups have been selected for further comparisons within this study. Reactor calculations of ignition delay times show that the mechanisms of Li et al. and Ó Conaire et al. yield the best agreement with data from shock tube experiments at pressures up to 33 bar. The investigation of one-dimensional laminar hydrogen flames indicate that these two mechanisms also yield the best agreement with experimental data of laminar flame speed, particularly for elevated pressures. The present study suggests that the Li mechanism is best suited for the prediction of H₂/O₂ chemistry since it includes more up-to date data for the range of interest.     

Analysis of the benefits of carbon credits to hydrogen addition to midsize gas turbine feedstocks

The addition of hydrogen to the natural gas feedstocks of midsize (30–150 MW) gas turbines was analyzed as a method of reducing nitrogen oxides (_NOx)$({\mathrm{NO}}_x)$ and _CO2${\mathrm{CO}}_2$ emissions. In particular, the costs of hydrogen addition were evaluated against the combined costs for other current _NOx${\mathrm{NO}}_x$ and _CO2${\mathrm{CO}}_2$ emissions control technologies for both existing and new systems to determine its benefits and market feasibility. Markets for _NOx${\mathrm{NO}}_x$ emissions credits currently exist in California and the Northeast States and are expected to grow. Although regulations are not currently in place in the United States, several other countries have implemented carbon tax and carbon credit programs. The analysis thus assumes that the United States adopts future legislation similar to these programs. Therefore, potential sale of emissions credits for volunteer retrofits was also included in the study. It was found that hydrogen addition is a competitive alternative to traditional emissions abatement techniques under certain conditions. The existence of carbon credits shifts the system economics in favor of hydrogen addition.