A Distributed Fuel Injection Approach to Suppress Lean Blow-Out and NO_x Emissions in a Methane-Ammonia-Fueled Premixed Swirl Combustor

In an effort to reduce the harmful effects of greenhouse gas emissions, ammonia is being pursued as a fuel for power generation as it is a carbon-free energy source. However, the use of ammonia-air mixtures in premixed swirl combustors poses challenges due to low flame speed, reactivity, and high Nitrous oxide emissions. This study attempts to overcome lean blowout limits of methane-ammonia-air mixtures by a novel, multipoint (⁠$O(103)$⁠) injection strategy, whereby micron-sized holes on the swirler vanes generate a coflowing stream of fuel and air, which is then injected into a swirling air cross-flow. The resulting improvement in mixing facilitated by increases in momentum flux ratio and fine-scale turbulence is found to reduce lean blowout (LBO) limits to equivalence ratios between 0.65 and 0.7 for mixtures containing ammonia as high as 80–90% by volume. The measurements carried out using a model-swirl combustor setup are analyzed further using zero-dimensional chemical kinetic models as well as CH* and OH* chemiluminescence. Chemiluminescence imaging shows the heat release zone to move downstream and broaden with an increase in ammonia content, as a result of decreasing flame speed. This forms a precursor to lean blow out through the action of instabilities at the flame front, which is potentially alleviated by the improved mixing achieved through the multipoint injection strategy. The resulting ultrashort mixing length can lead to a compact combustor design with the ability to lower LBO limits and improve Nitrous oxide emissions while utilizing carbon-free ammonia.