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Lean Combustion in Gas Turbines

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Abstract

This chapter addresses the use of lean combustion in gas turbines. Lean combustion is now a standard practice for reaching ultralow pollutant emissions through control of combustion temperature. Innovation allows operation at lean conditions while avoiding operability issues such as premixer flashback and autoignition. Ground-based systems adopt innovations sooner than aviation systems because of the latter's safety requirements. The ability of current lean gas turbine technology to provide rapid response, good turndown, good stability, and low-emissions performance is remarkable. Recent adoption of advanced manufacturing methods is allowing further innovation such as multipoint injector arrays to be developed which have shown good performance with highly reactive fuels such as hydrogen.

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... Because the combustion of hydrogen generates more water in the exhaust than natural gas on a per mole of fuel basis and consumes less air, drying the sample for the analyzer and correcting the values to 15% O2 introduces another shift in the result. Essentially, the correction for hydrogen to 15% O2 on a dry basis increases the NO x concentration for operation on hydrogen when compared to natural gas [27]. In the extreme case of 100% hydrogen vs 100% natural gas at a 2000 K flame temperature, a 40% increase in the stated NO x value results [28]. ...
Conference Paper
Achieving carbon neutrality and sustainability has prompted research efforts into reducing carbon dioxide and carbon monoxide emissions in combustion systems without significantly increasing nitrogen oxides (NOx). Because of its high specific energy density, hydrogen is a prime candidate to eventually replace natural gas. Presently, using renewable hydrogen mixtures in natural gas turbines can reduce carbon dioxide and carbon monoxide emission as a step towards carbon neutrality. In the present effort, a commercial natural gas 200kW microturbine generator produced by Capstone Green Energy Corporation is used to study the impact of hydrogen addition to natural gas. Exhaust emissions, injector temperatures, and power output are evaluated for increasing levels of hydrogen in the fuel mixture. Relative to emissions on 100% natural gas, the experimental results show that, for full load, carbon monoxide and carbon dioxide emissions decrease as the amount of hydrogen in the fuel mixture increases while the NOx emissions increase. This is attributed to the increased reactivity of hydrogen resulting in a jet flame stabilized closer to the injector exit as the amount of hydrogen is increased. While the injector tip temperatures increased modestly with additional hydrogen, visual inspection of the injectors following hydrogen addition tests indicate that flashback or combustion inside the injectors was not occurring. This is consistent with the modest changes in NOx with hydrogen addition. If flashback were occurring, the emissions levels would be expected to be significantly higher. The results also demonstrate that added hydrogen to the fuel does not have a statistically significant impact on overall system efficiency. In general, the results show that the injectors can withstand the temperature increase from hydrogen addition while maintaining low emissions at full load, indicating that addition of hydrogen is possible without any modification to this gas turbine engine which has been optimized for performance on natural gas. Parametric studies were carried out to demonstrate how modest changes in engine operating points and fuel injectors can be incorporated to attain single digit NOx levels with up to 30% hydrogen.
... It has the advantage that only the water and oxygen content of the exhaust gas sample have to be measured sufficiently accurate while the complete composition does not have to be known. However, as pointed out in [13], a concentration based metric does not inherently "reward" more efficient processes, in contrast to emission indices which can also be decreased by increasing fuel efficiency. ...
... With the increasingly stringent NO x emission regulations and standards, various measures to mitigate the NO x emissions by controlling their formation during combustion have been studied and applied [12]. Among them, the low-temperature combustion technologies, such as ultra-lean combustion [13] and oxygendeficient combustion [14], can effectively limit the formation of thermal NO x . For some low-NO x combustors, the flue gas recirculation (FGR), either external (EFGR) [15] or internal flue gas recirculation (IFGR) [16], is the main strategy to create local oxygendeficient condition and achieve low-temperature combustion. ...
Article
The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13 ~ 0.21) on flame configuration, combustion stability, combustion efficiency, and NOx emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NOx generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NOx emission of below 10 ppm (@ 15 vol.% O2) due to low oxygen concentration and flame temperature. Furthermore, part of NOx entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H2) to further lowering NOx emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.
... In order to cope with increasingly stringent emission requirements, the low-temperature combustion has been widely used to achieve low NO x (nitric oxides) emissions by reducing thermal NO x formation [1], including ultra-lean combustion [2], moderate or intense low-oxygen dilution (MILD) combustion [3], and flue gas recirculation (FGR) [4,5]. However, these methods often lead to combustion instability and narrow the stability limit, making the simultaneous implementation of low NO x emissions a challenge. ...
Article
A novel vortex-tube combustor with axial fuel injection for NOx reduction was proposed to overcome the instability of ultra-lean combustion. The stability limit, flame configuration, NOx and CO emissions, and flame structure were investigated experimentally under various global equivalence ratios and fuel flow rates. The mechanisms of NOx emission reduction were analyzed by numerical simulation. Results show that the limit of global equivalence ratio can be as low as 0.01 and the amplitude of pressure fluctuation is always less than 1300 Pa, indicating fairly good performance in combustion stabilization of the combustor. In the operation range, there is a trade-off region with low NOx and CO simultaneously. The diffusion-like flame structure in this combustor can enhance the local equivalence ratio, whilst the flow field structure can also promote the transport of chemical enthalpy to the flame front, thus facilitating the stabilization. The enhanced stabilization enables the ultra-lean combustion and the ensued small area of the high-temperature zone to conduct, as well as the low NOx formation through reducing thermal NO. The local fuel-rich region and the flow field structure can promote the NOx to be reduced further via the Fenimore mechanism.
... In order to cope with increasingly stringent emission requirements, the low-temperature combustion has been widely used to achieve low NO x emissions by reducing thermal NO x formation (Baukal, 2003), including ultra-lean combustion (McDonell et al., 2016), moderate or intense low-oxygen dilution (MILD) combustion (Saha et al., 2020), oxygen-deficient combustion (Yang and Blasiak, 2005), and flue gas recirculation (FGR) . However, these methods often lead to combustion instability and narrow the stability limit (Yan et al., 2017). ...
Article
In order to overcome the instability of ultra-lean combustion, a novel vortex-tube combustor with axial fuel injection was proposed. The stability limit, flame configuration, NOx emissions, and burn-off rate of the combustor were investigated experimentally under various global equivalence ratios (φg) and fuel flow rates (qf). Results show that the combustor exhibits a large stability limit with φg and qf as low as 0.01 and 6×10⁻⁵ m³/s respectively. Complete combustion and ultra-low NOx emissions of less than 3 ppm can be achieved at φg of 0.3 and qf of more than 30 ×10⁻⁵ m³/s, indicating that the combustor has a good potential for ultra-lean combustion and low NOx emission. The pressure fluctuation amplitude is always less than 1300 Pa during the entire experiments. The vortex-induced flame has a diffusion-like flame structure, which provides a suitable equivalence ratio zone under ultra-lean conditions, whilst the high peak combustion temperature indicates an intensified combustion, which is responsible for the large stability limit and low-pressure fluctuation amplitude. Subsequently, the enhanced stabilization can enable the ultra-lean combustion and the ensued low temperature to conduct, whilst the vortex-flow can decrease the local flow velocity and enable the turbulent diffusion velocity of NO to be dominant, which can make the NO emission reduced further. The decreased area of high-temperature region and the appearance of the negative reaction rate region of NO is the essential reason for the decrease of the NO emission at lean cases.
... combustors can reach up to 16 and 26 kPa, respectively, exceeding 5% of the average value of operation pressure and leading to instability. 17 In some demanding industrial combustors, 18 the required pressure fluctuation will be much lower. Thereby, the current pressure fluctuation in the typical vortex-tube combustors is still large to some extent. ...
Article
Full-text available
Experimental and numerical (via ANSYS FLUENT) studies have been conducted on the combustion stability and stabilization mechanisms in a localized stratified vortex-tube combustor (LSVC) under lean conditions. The stability limit and flame configuration were obtained under different combustion conditions. Combined with the flow field distribution, the formation mechanisms of the local stratification of species and the resultant flame configuration were analyzed. Results show that the local stratification peculiarity is responsible for the dual flame appearance. On the basis of the local stratification of species, the local equivalence ratio is close to stoichiometry in the vicinity of the flame front, while it is above 1.0 in the interior, enabling the achievement of stable combustion at a global equivalence ratio as low as 0.12 in the LSVC. The flow field can help the transport of the reactive species and yields an intensified combustion and a large density gradient. The peak heat release rate of 0.5 W/mm3 in the LSVC is much higher than that of 0.1 W/mm3 in the rapidly mixed vortex-tube combustor (RMVC) at the global equivalence ratio of 0.6 and the maximum tangential velocity of 26.44 m/s. The flame-vortex interaction theory provides a new perspective to interpret the rapid flame propagation in vortex-tube combustors. Based on the pressure jump theory, the flame speed was obtained via a specific formula closely related to the density gradient and the injection velocity. It turns out that the flame speed in the LSVC is remarkably higher than that in the RMVC at a certain same combustion condition. Moreover, the decrease of local flow velocity resulted from the strong swirl provides a favorable guarantee for the balance with the local flame speed.
... Lean premixed combustors facilitate efficient combustion of natural gas with lower NO x emission levels [1]. However, such combustors are susceptible to combustion instability, a thermoacoustic feedback process [2]. ...
Article
The characterization and mitigation of thermoacoustic combustion instabilities in gas turbine engines are necessary to reduce pollutant emissions, premature wear, and component failure associated with unstable flames. Fuel staging, a technique in which the fuel flow to a multinozzle combustor is unevenly distributed between the nozzles, has been shown to mitigate the intensity of self-excited combustion instabilities in multiple nozzle combustors. In our previous work, we hypothesized that staging suppresses instability through a phase-cancelation effect in which the heat release rate from the staged nozzle oscillates out of phase with that of the other nozzles, leading to destructive interference that suppresses the instability. This previous theory, however, was based on chemiluminescence imaging, which is a line-of-sight integrated technique. In this work, we use high-speed laser-induced fluorescence to further investigate instability suppression in two staging configurations: center-nozzle and outer-nozzle staging. An edge-tracking algorithm is used to compute local flame edge displacement as a function of time, allowing instability-driven edge oscillation phase coherence and other instantaneous flame dynamics to be spectrally and spatially resolved. Analysis of flame edge oscillations shows the presence of convecting coherent fluctuations of the flame edge caused by periodic vortex shedding. When the system is unstable, these two flame edges oscillate together as a result of high-intensity longitudinal-mode acoustic oscillations in the combustor that drive periodic vortex shedding at each of the nozzle exits. In the stable cases, however, the phase between the oscillations of the center and outer flame edges is greater than 90 deg (∼114 deg), suggesting that the phase-cancelation hypothesis may be valid. This analysis allows a better understanding of the instantaneous flame dynamics behind flame edge oscillation phase offset and fuel staging-based instability suppression.
... Multipoint injection strategies are among the most interesting liquid fuel injection technologies aimed to improve efficiency and reduce emissions by means of a more efficient control of combustion processes in gas turbine ( McDonell, 2008). A successful design of premixing devices based on these injection strategies calls for the definition of the best operating conditions of liquid jets in cross flow in dependence of geometries, fuel and airflow rates, and pressure and temperature of the inlet airstream. ...
Article
Full-text available
Determination of velocities and locations of liquid droplets formed during the atomization process of a liquid jet injected in a high-pressure air crossflow represents an essential step in the definition of appropriate boundary conditions for the elaboration and validation ofreliable numerical models. In this paper, three test cases relative to different air-to-liquid velocity ratios and air temperatures are analyzed by means ofelastic scattering imaging and particle image velocimetry (PIV). The significance of the PIV measurements, evaluated as a function of spatial position and operating conditions, was adequately high with the exception of the nearest field to the injection point, where the spray is too dense. Velocity data, measured in the axial and transversal planes, are presented along with corresponding velocity components' profiles at selected positions. Results show the rapid alignment of droplet trajectories to the airflow and the dominant role played by the airflow in determining final droplet velocities and, hence, their placement in the spray plume. The droplets close to the windward profile move parallel to the jet leading edge, whereas in the leeward region they cross the scattering intensities' isocontours. On the grounds of these results, a synthetic description of liquid droplet velocities' behavior in the different regions of the spray plume is given that represents a useful conceptual tool in the attempt to correlate physical mechanisms, underpinning atomization processes, to the observed phenomenologies.
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In this paper, an experimental study of the non-reacting turbulent flow field characteristics of a piloted premixed Bunsen burner designed for operational at elevated pressure conditions is presented. The generated turbulent flow fields were experimentally investigated at atmospheric and elevated pressure by means of high-speed particle image velocimetry (PIV). The in-nozzle flow through the burner was computed using large-eddy simulation (LES), and the turbulent flow field predicted at the burner exit was compared against the experimental results. The findings show that the burner yields a reasonably homogeneous, nearly isotropic turbulence at the nozzle exit with highly reproducible boundary conditions that can be well predicted by numerical simulations. Similar levels of turbulence intensities and turbulent length scales were obtained at varied pressures and bulk velocities with turbulent Reynolds numbers up to 5300. This work demonstrates the burner’s potential for the study of premixed flames subject to intermediate and extreme turbulence at the elevated pressure conditions found in gas turbine combustors. Graphical abstract
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