Thermoacoustic instabilities and exhaust emissions from radially-staged H2/NH3/CH4 flames

Abstract

As the severity of the climate crisis resulting from global warming becomes increasingly evident, carbon neutrality has become a top global priority, significantly impacting not only international environmental regulations but also energy industry policies. Achieving carbon neutrality requires the decarbonization of energy sources in the power sector, as well as ensuring grid stability while accommodating a growing demand for electricity. In this context, low-carbon and carbon-free gas turbine power generation is expected to play a key role

however, the transition to next-generation carbon-free fuels (such as hydrogen and ammonia) must be preceded by innovations in combustion technology. This dissertation conducts a detailed analysis of gas turbine combustion phenomena using hydrogen, ammonia, and methane as fuels and also addresses the technical challenges encountered during the development of low-carbon and carbon-free gas turbine combustion systems. To mitigate flashback risk during hydrogen combustion, a combustor nozzle featuring sixty small-scale injectors with an inner diameter of 6.5 mm was employed and this setup was uniformly utilized for all experimental research to exclude the impact of nozzle geometry variations. To gain a comprehensive understanding of the combustion characteristics of these alternative fuels, experimental measurements and analyses were conducted utilizing dynamic pressure signals, OH*/CH*/NH2*/NH* chemiluminescence, OH planar laser-induced fluorescence (OH PLIF), and concentrations of NOx, CO, NH3, H2, O2 in the exhaust gases. The fuel flexibility of gas turbine combustion systems plays a crucial role in enhancing the grid stability, particularly when integrated with renewable energy sources. However, variations in fuel composition give rise to several technical issues

thus, it is imperative to closely examine the combustion characteristics associated with the alternative fuels and to incorporate this understanding into the design of the combustion system. Based on this research background, the first study aims to investigate the combustion dynamics and emissions characteristics of lean-premixed clustered flames under different fuel composition conditions. Methane, propane, and hydrogen were used as fuels to encompass a broad spectrum of thermodynamic, transport, and combustion properties. The experimental results revealed that while the nitrogen oxide emissions are almost unaffected by the fuel composition due to the constant adiabatic flame temperature, the variation of fuel composition has a significant impact on self-excited instabilities, which tend toward higher frequency oscillations with increasing hydrogen concentration. From phase-resolved OH PLIF measurements for a 50/50 mixture of CH4 and H2, we identify the creation, evolution, and annihilation of an array of coherent vortical structures as the mechanism responsible for sound generation and flame surface modulations without strong interactions between adjacent flames. In contrast, the behavior of 50% C3H8 + 50% H2 case is primarily influenced by the periodic merging and separation of neighboring reactant jets, resulting in large-scale asymmetric oscillations in the transverse direction. Given the substantial change in transport properties, effective Lewis number-related interpretation provides a reasonable explanation for the different behaviors of a cluster of small-scale flames. In order to achieve a reduction of over 50% in carbon emissions compared to pure methane, the fuel-flexible combustion system must operate with a hydrogen mole fraction of at least 80%. However, such operating conditions can induce technical issues related to high-frequency and high-amplitude combustion instabilities, which can negatively affect the overall engine system. Therefore, additional research and technical advancements are necessary to address these challenges. In line with these efforts, the second study experimentally investigated the effects of radial fuel staging on the combustion dynamics of clustered lean-premixed hydrogen/methane flames. Also, this study carefully examined the effects of flame asymmetry induced by staging on the emissions of nitrogen oxides and carbon monoxide. In conjunction with phase-averaged OH*/CH* chemiluminescence and OH PLIF flame imaging, we carried out extensive measurements over the full range of 0 to 100% H2/CH4 fuel staging conditions, including even/uneven blends of H2/CH4 fuels between inner and outer nozzle groups, partial/complete fuel split cases, and pure H2 or CH4 fuel for all constituent flames, under a constant thermal power condition of 78 kW. Our measurements demonstrated that whereas carbon monoxide concentrations are largely unaffected by the radial fuel staging conditions except under high and pure hydrogen percentage conditions, there exists a strong correlation between total nitrogen oxides emissions and overall adiabatic flame temperature. Integrated analyses of iso-contour instability maps revealed that a discontinuous mode transition takes place, where the intermediate-amplitude lower frequency oscillations associated with relatively low hydrogen concentration conditions give way to stronger, higher frequency instabilities under high hydrogen content conditions. While even-blend (non-staging) conditions are characterized by coherent oscillations of the constituent flames, the responses of clustered flames to radial fuel staging conditions are eccentric and complex, manifested as large-scale asynchronous modulations between inner- and outer-stage heterogeneous reaction zones. This observation suggests that in a multi-element injector environment spatiotemporal incoherence driven by inhomogeneous heat release can be used to neutralize self-excited pressure oscillations, by disrupting pressure-heat release coupling processes. The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen-doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen-air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetry-induced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and self-excited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observed that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects – such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96% reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.