Improving Micro-Mixing Combustion Stability Using Fuel Preheating and Product Recirculation

Abstract Micro-mixing is a promising combustion approach due to its inherent safety against flashback and autoignition, with multiple studies having demonstrated low NOx levels in the order of premixed combustion. Previous work has extended the operating limits of micro-mixing combustion to the design requirements of highly recuperated gas turbines, demonstrating low NOx performance at inlet temperatures of up to ∼1000 K with hydrogen and hydrocarbon-based fuels such as propane, natural gas, and jet fuel. Due to its highly reactive nature, micro-mixing combustion of hydrogen is very stable, but instabilities were observed with hydrocarbon-based fuels when operating near premixed levels of NOx due to their slower reaction rates, indicating that a trade-off exists between NOx emissions and combustion stability. This paper presents the experimental results of a new micro-mixing injector designed to improve combustion stability by 1) increasing the recirculation of combustion products to the flame zone, and 2) preheating the fuel by heat transfer from the combustion products to the injector. Although this configuration is still susceptible to instabilities, their amplitudes are much lower, with levels of 10 dB compared to ∼20 dB for the standard micro-mixing injector. NOx emissions are similar for both injector configurations, indicating a better trade-off between combustion stability and emissions when operating near the levels of perfectly premixed combustion for the new injector configuration.