A Review on Combustion Instability of Hydrogen-Enriched Marine Gas Turbines

A comparative study on NH3/H2 and NH3/CH3OH combustion and emission in an optical SI engine

Hydrogen-enriched flames are a viable solution to reduce greenhouse gas emissions and improve combustion efficiency in gas turbines and industrial applications. Hydrogen has advantages over conventional fuels, such as higher flame speed and a broader flammability range. These benefits lead to more efficient and stable combustion, resulting in improved efficiency, reduced emissions, and increased power output. However, a drawback of hydrogen-enriched flames is the elevated temperature, leading to higher NOx emissions. To tackle this issue, lean swirl-stabilized combustion and stratification techniques are used. The first one enhances combustion efficiency, stability, and reduces emissions, while stratification introduces a non-uniform distribution of fuel, improving the static stability of flames. The DACRS burner is an example of stratified combustors used in gas turbines. By combining hydrogen-enrichment, stratification, swirl-stabilization, and lean operation, combustion systems in gas turbines and industrial combustors can be significantly improved. The present study examined the stability and combustion properties of a hydrogen-enriched, lean, stratified CH4/air flame in a gas-turbine burner. The study varied the hydrogen fraction and equivalence ratio and found that the combination of stratification and hydrogen enrichment had conflicting effects on flame stability. The flame blowout limit remained unchanged, even with up to 20% hydrogen enrichment. NOx emissions were not affected by hydrogen addition up to 20%, but increasing the equivalence ratio promotes NOx formation, as anticipated.

SummaryThis paper reviewed theoretical, numerical, and experimental studies on combustion instabilities in combustors for energy‐producing devices such as gas turbines, boilers, and furnaces utilizing oxyfuel combustion technology. Oxyfuel combustion as a CO2 capture technology gives encouraging and promising solutions for controlling greenhouse gases emission from combustion devices. However, the major concerns with oxyfuel systems are the combustion instabilities mainly arising from the CO2 dilution required for controlling excessive temperatures. Higher dilution levels or significantly nonuniform distribution of CO2 in the mixture can potentially lead to static and/or dynamic instabilities associated with local flame extinction and unsteady heat release rate, respectively. As in the case of lean premixed air/fuel flames, combustion instabilities are among the most critical operational challenges encountered in CO2‐diluted oxyfuel systems as well. Existing literature on flame instabilities has predominately focused on air‐fuel flames, with very few research studies dedicated to the same phenomena in oxyfuel combustion. With an ever‐increasing push toward carbon capture and reduction in NOx emissions, it has become important for the combustion community to accelerate research efforts in the direction of oxyfuel combustion systems. This review study on flame instabilities in oxyfuel combustors is one small step in that very direction. This paper starts with discussions on the theoretical mechanisms of combustion instabilities. Then a range of numerical and experimental studies, from premixed to non‐premixed flames, are presented and discussed. The main objective is to summarize recent progress in understanding instabilities in oxyfuel combustion systems, focusing primarily on the effect of CO2 dilution on flame stability. A range of experimental and numerical studies by various groups have strongly associated both static and dynamic instabilities in oxyfuel systems with diluent concentration, suggesting higher concentrations potentially result in a stratified mixture field which leads to higher heat release fluctuations and pressure oscillations. As such, many combustion systems use hydrogen enrichment to enhance static stability while employing flow and/or fuel modulation to dampen dynamic instabilities in oxyfuel combustors. Finally, we highlight and summarize some of the key issues and unsolved questions that still challenge researchers and, therefore, need to be the focus of future research work as regards to oxyfuel combustion.

Modern, low emission combustion systems with improved fuel-air mixing are more prone to combustion instabilities and therefore use advanced control methods to balance minimum NOx emissions and and the presence of thermoacoustic combustion instabilities. The exact operating conditions at which the system encounters an instability is uncertain because of sources of stochasticity, such as turbulent combustion, and the influence of hidden variables, such as un-measured wall temperatures or differences in machine geometry within manufacturing tolerances. Practical systems tend to be more elaborate than laboratory systems and tend to have less instrumentation, meaning that they suffer more from uncertainty induced by hidden variables. In many commercial systems, the only direct measurement of the combustor comes from a dynamic pressure sensor. In this study we train a Bayesain Neural Network (BNN) to predict the probability of onset of thermoacoustic instability at various times in the future, using only dynamic pressure measurements and the current operating condition. We show that, on a practical system, the error in the onset time predicted by the BNNs is 45% lower than the error when using the operating condition alone and more informative than the warning provided by commonly used precursor detection methods. This is demonstrated on two systems: (i) a premixed hydrogen/methane annular combustor, where the hidden variables are wall temperatures that depend on the rate of change of operating condition, and (ii) full scale prototype combustion system, where the hidden variables arise from differences between the systems.

Effects of H2 and CO2 addition on the heat transfer characteristics of laminar premixed biogas–hydrogen Bunsen flame

A challenging issue in the gas turbine industry is to develop a practical dual fuel (DF), dry low emission (DLE) combustion system. Especially for the onshore-based power generation systems, and liquid DLE for aeroderivative engines used for marine propulsion. A novel mid-size (3MW) gas turbine is being developed mainly targeted for marine propulsion, where a dual fuel DLE combustion system aiming at single digit NOx emission figures has been explored. As a part of this development, the present technology available from different gas turbine manufacturers has been surveyed. Status of the different techniques applied in dual fuel DLE combustors today and their achievements are presented, including the available information on fuel injectors, cooling schemes, combustion air distribution, noise control and combustor performance. The techniques utilized and explained are such as flame temperature control (water/steam injection), staged combustion, lean premixing and lean prevaporized premixing, rich-quench-lean-burning (RQLB) and catalytic combustion. These are also documented for the different concepts commercially available, describing both advantages and drawbacks. Conclusions are made towards the dominating trends for the different parameters mentioned above, and how they affect the final combustor design. A survey of the dominating parameters for low emission combustion systems is presented.

In engineering, the combustion chamber with a backward step is very popular, and it is a kind of flame stabilizer. In this type of combustion chamber, there will be shedding vortices at the step due to the instability of the flow field. The shedding vortices will carry reactants to move downstream and burn, resulting in unstable heat release and then pressure and velocity fluctuations of the sound field, thereby, finally, forming a combustion-vortex-acoustic interaction process. If a positive feedback loop is formed between the unstable heat release and the pressure fluctuation of sound field, combustion instability will occur, and it is also referred to as thermoacoustic oscillation due to vortex shedding. Combustion instability frequently occurs in many practical systems or equipment, and its induced significant pressure oscillations have a serious influence on the normal operation of the equipment. Recently, the combustion instability has been extensively studied experimentally, but the theoretical investigation on its nature is still rare. Since combustion instability is a complicated nonlinear phenomenon, it is necessary to study its nature from the viewpoint of nonlinear dynamics. Based on the one-dimensional simplified model of thermoacoustic instability involving vortex shedding proposed by Matveev and Culick, the typical nonlinear phenomenon in thermoacoustic oscillation induced by vortex shedding is studied. The study focuses on the initial value sensitivity of the system, the influence of key parameters on thermoacoustic oscillation, and the phenomenon of vortex-acoustic lock-on. Firstly, the Galerkin method is used to approximate the governing equation, and the partial differential equations are reduced to a set of ordinary differential equations. Then, the first ten modes are selected, and the pressure and velocity fluctuations of sound field under different system parameters are obtained by MATLAB program. Finally, the thermoacoustic instability of the system under different initial disturbances, the influences of different steady flow velocity on the thermoacoustic oscillation of the system, and the phenomenon of vortex-acoustic lock-on in thermoacoustic oscillation are studied in detail. The results show that the system of thermoacoustic oscillation involving vortex shedding is extremely sensitive to initial values, and there are a rich variety of nonlinear phenomena. With steady flow velocity increasing, the amplitude of pressure fluctuation augments generally. However, the similar structures are found in several intervals of steady flow velocity, and the amplitude first decreases and then increases. In particular, it is verified that the system oscillates periodically by integer (fp/fs) multiple of the vortex impinging frequency (fs), that is, the vortex-acoustic frequency locking with the number of revolutions fp/fs, which is found in experiment and can be regarded as an important characteristic of periodic thermoacoustic oscillation.

Hydrogen addition effects on high intensity distributed combustion

For the development of high efficiency and low emission combustion systems, high temperature air combustion technology has been tested by utilizing preheated air over 1100 K and exhaust gas recirculation. In this system, combustion air is diluted with large amount of recirculated exhaust gases, such that the oxygen concentration is relatively low in the reaction zone, leading to low flame temperature. Since, the temperature fluctuations and sound emissions from the flame are small and flame luminosity is low, the combustion mode is expected to be flameless or mild combustion. Experiment was performed to investigate the turbulent flame structure and NO emission characteristics in the high temperature air combustion focused on coflowing jet diffusion flames which has a fundamental structure of many practical combustion systems. The effect of turbulence has also been evaluated by installing perforated plate in the oxidizer inlet nozzle. LPG was used as a fuel. Results showed that even though NO emission is sensitive to the combustion air temperature, the present high temperature air combustion system produce low NO emission because it is operated in low oxygen concentration condition by the high exhaust gas recirculation.

This work concerns the prediction of potentially damaging thermoacoustic oscillations in gas turbine combustion systems by computational means. A framework is laid out to predict numerically the frequency and stability of thermoacoustic oscillations, with focus on the high frequency screech instability of afterburners. A hybrid numerical method is used that includes separate calculations of the mean flow and of the perturbed field due to the acoustic oscillations. This modularity supports the choice of models that are the most appropriate for combustion and for acoustic wave propagation, which are the processes that make up the feedback mechanism that can lead to the establishment of an instability. This gives flexibility, improved accuracy and more insight into the physics of the thermoacoustic system at a potentially reduced computational cost. The mechanism leading to screech involves the formation of vortices induced by acoustic transverse modes at the afterburner flameholder. These vortices trap fresh reactants that burn after a certain time delay, therefore feeding energy into the oscillation. Within a linear approximation, the effect of small amplitude acoustic fluctuations on the flame is studied by perturbing harmonically the transverse velocity at the flameholder lip over a range of frequencies using forced combustion CFD calculations. The response in heat release rate, which is a thermoacoustic source of sound, is represented by a flame transfer function (FTF). It is argued that for the investigation of screech oscillations, this FTF must be multidimensional because of the transverse nature of the acoustic oscillation. For fully premixed flames, the main contributor to heat release rate fluctuations is the variation in flame surface area. This information is used to develop a novel flame model that represents the multidimensional, frequency dependent response of the flame to velocity perturbations. Compared to FTFs, which require computationally expensive forced calculations, this model has the advantage of providing the frequency dependent flame response as part of the acoustic calculation. After verification and validation of each of the tools used for the acoustic and combustion simulations, this flame model is used in the analysis of a simplified afterburner, where a high frequency, radial and longitudinal resonant mode was computed. Convective modes, which are important in the prediction of the frequency of thermoacoustic oscillations are predicted as a result of the interaction between the acoustic wave and the flame.

There is a rapid increase in the demand for low NOx hydrogen/methane combustion systems in recent years, and low NOx and low emission burners and furnaces are promising platforms for such applications. Combustion instability is the most important drawback in low NOx burners and uniform thermal field. This paper investigates the influence of CO2 in methane and hydrogen on combustion instability. Numerical simulations were conducted using a small swirl burner and furnace combustion system. The effects of fuel type and CO2 dilution rate, which is a major contributor of hydrogen and methane combustion, on the combustion characteristics and instability are investigated. Combustion instability decreases for higher CO2 dilution rates.

Comparison of the effects of EGR and lean burn on an SI engine fueled by hydrogen-enriched low calorific gas

An experimental study was carried out to evaluate the potential of hydrogen enrichment to increase the tolerance of a stoichiometrically fuelled natural gas engine to high levels of dilution by exhaust gas recirculation (EGR). This provides significant gains in terms of exhaust emissions without the rapid reduction in combustion stability typically seen when applying EGR to a methane-fuelled engine. Presented results give the envelope of benefits from hydrogen enrichment. In parallel, the performance of a catalytic exhaust gas reforming reactor was investigated in order that it could be used as an onboard source of hydrogen-rich EGR. It was shown that sufficient hydrogen was generated with currently available prototype catalysts to allow the engine, at the operating points considered, to tolerate up to 25 per cent EGR, while maintaining a coefficient of variability of indicated mean effective pressure below 5 per cent. This level of EGR gives a reduction in NO emissions greater than 80 per cent in all test cases.

Development of efficient and low-emission colorless distributed combustion (CDC) systems for gas turbine applications require careful examination of the effects of various parameters on the fuel-air mixing and combustion in these systems. These effects are systematically studied by experimental and numerical simulations of turbulent flow, mixing and combustion in a laboratory-scale CDC system. The high air temperature CDC has shown to significantly reduce the NOx and hydrocarbon emissions while improving the reaction pattern factor and stability with low pressure drop and noise without using any flame stabilizer. Numerical simulations conducted in this study are based on the large eddy simulation (LES) and high order numerical methods. The main objective is to investigate the flow field and fuel-air mixing within the combustor by LES for the same conditions as experiments. The numerical results establish the reliability of the computational model for the CDC simulations. They also indicate the importance of fuel injector configuration and show the effects it has on the velocity, scalar and temperature fields within the combustor. HE Colorless Distributed Combustion (CDC) has been demonstrated to provide ultra-low NOx and CO emissions, improved pattern factor and reduced combustion noise in stationary gas turbine combustors. 1-3 The flame in the CDC is homogeneous and is formed in the entire combustion volume instead of a limited reaction zone. 1-4 The key factor to achieve CDC is the controlled flow distribution, reduce ignition delay, and rapid injection and mixing of fuel and air jets to promote the distributed reaction in the entire combustion volume without any flame stabilizer. 1-3 Large gas recirculation and high turbulent mixing rates are desirable and helpful for achieving the distributed reaction and for avoiding hot spot zones in the flame. 9 Air and fuel jets are normally injected separately to prevent the formation of thin reaction zones between the heated air stream and the fuel jet. Both air and fuel jets entrain hot and diluted burned gases to a desirable degree and with controlled shear layer mixing, to establish “complete” mixing of fuel and oxygen and suitable fuel-air mixture temperature in the entire combustor priori to autoignition 1 . The CDC may be established by various fuel and air injection configurations. The performance of non-premixed CDC for various geometries and fuel-air injection configurations has been studied experimentally at the University of Maryland (UMD). The focus of this study was to develop CDC for stationary gas turbine combustors, operating in the thermal intensity range of 5–85 MW/m 3 -atm. The thermal load was scaled with volume as well as the operating pressure. 7,8 The present work uses the LES model for detailed and systematic study of the turbulent flow pattern and fuel-air mixing in the UMD CDC system. The numerical predictions are first compared with the available experimental data for the case without fuel jet injection. The CDC system is then simulated by LES for various fuel-air jet configurations, and inflow air conditions.

This paper describes the conceptual design and aerothermal approach of the ABB ALSTOM POWER UK Limited (ABB ALSTOM POWER) dry low emission (DLE) combustion system directed towards all current and future power ranges. A set of parameters was employed as a common criterion to design the company's combustion system across the product range. The parameters made it possible to scale the combustion system to operate at any required firing temperature without the penalty of achieving the low emission targets. The logic and sensible air mass mapping and testing management based on a basic knowledge of the aerothermodynamics of the combustion system and its integral parts produced reliable operation and engine results. The combustion system aerothermodynamic design considers the constraints of maintaining combustor specifications while introducing a reliable ultra-low NO x concept with minimal structural change to engine design. Furthermore, the ignition loop, piloting and stability margins were issues continuously assessed through a defined application programme to examine their impact on engine operability at various load conditions. A main radial swirler injector is utilized for both gas and liquid to produce a fuel lean mixture for combustion at full-load operating conditions, while a pilot fuel injector guarantees a local fuel-rich zone to sustain stability at low power conditions. The carefully planned switch-over between the pilot and main swirler provides smooth operation and low emission across the load range conditions. The company combustion system relies upon the principle of aerodynamic variable mixing with a single vortex generator burner. The partial premixing process takes place at the radial inflow swirler slots before the fuel—air mixture is introduced into a prechamber and thereafter into the combustion chamber. For vane passage injection the liquid liquid fuel atomization should be at an optimum and the vane passage and prechamber length set to provide the vaporization period. The impingement cooling scheme aided by a thin layer of thermal barrier coating (TBC) achieved an excellent combustor wall temperature at all operating conditions and across the DLE engines product range. The scheme made it possible to direct a large amount of air towards the swirler to achieve the lean premix combustion. Substantial CO emission reduction at part load was achieved by variable guide vanes (VGV) modulation for the single-shaft engines. This action inhibits NO formation without slowing the CO oxidation process to a frozen condition. The variable mixing regime approach and accurate air management has demonstrated an ultra-low NO x emission of 12–15 and 32–35 ppmv (corrected to 15% O2) at the full-load condition while operating on gas and distillate-2 fuel respectively. Moreover, the system exhibited low CO and dynamics oscillation capability for both gas and liquid fuel operation. At the present time, the one-dimensional analysis has been proved to be a reliable tool for combustion system designers to produce a preliminary design configuration. The particular choice for a commercial computational fluid dynamics (CFD) code to satisfy various criteria of the swirling combusting flow is proving to be difficult due to constraints regarding slover and/or methods limitation. The results obtained from the high-pressure air facilities, engine beds and field trials represent a true technological achievement through collective engineering design efforts towards emission and pollution control for small- and medium-range gas turbine engines.