Emissions performance of staged premixed and diffusion combustor concepts for an NH3/air flame with and without reactant humidification

Renewably generated ammonia offers a form of carbon-free chemical energy storage to meet the differences between uncertain renewable supply and fluctuating demand, and has the potential to support future energy requirements as a power-to-X concept. The storage and transportation characteristics of NH3 are favorable compared with H2, however there are significant combustion research challenges to enhance fuel reactivity whilst reducing harmful emissions production. The purpose of the presented work was to evaluate different fuel delivery concepts for a representative GT combustor. An experimental and numerical comparison was made between swirl-stabilized premixed and diffusion NH3-air flames at elevated inlet temperature (473 K). The exhaust NOx and unburned NH3 emissions generated from each concept were quantified to optimize operational combustor performance. High-speed OH* and NH2* chemiluminescence was employed to characterize the change in flame topology with variation in fuel-air equivalence ratio, and the resultant influence on measured emission concentrations. Chemiluminescence intensities were shown to elucidate changes in sampled exhaust emissions, enabling detailed analysis of intermediate chemistry. A comparison was made between experimental data and chemical kinetic simulations with a reactor network model, demonstrating the sensitivity of NOx emissions to premixed fuel-air equivalence ratio. A comparison was also made between exclusive primary airflow, and the staged introduction of secondary air, to quantify the change in NOx production between each configuration and improve fuel burnout. Secondary air loadings were incrementally increased through the combustor, and the change in exhaust emissions mapped. In addition, reactant humidification was employed as a secondary process for NOx reduction, having shown favorable performance with NH3/H2 mixtures to limit thermochemical NO production. The efficacy of humidification was compared for both premixed and diffusion configurations