Pressure and Heat Loss Effects On Flame Structure and Emissions In an NH3 RRQL Combustor

Abstract Ammonia (NH3) is being explored as a carbon-free energy carrier, supported by an extensive global production, distribution, and storage infrastructure. Rich-staged combustor designs with sufficient residence time, so-called Rich-Relaxation-Quick-mix-Lean (RRQL), are particularly promising for direct NH3 combustion without significant nitric oxide (NOx) production, but nitrous oxide (N2O) and unburned NH3 must also be managed. At atmospheric conditions, RRQL configurations have been shown to reduce NOx to near-equilibrium levels while promoting NH3 cracking to hydrogen (H2), provided that the primary stage maintains sufficiently long residence times (tres,primary) and that thermal losses through the combustor walls remain low. However, the influence of elevated pressure on RRQL systems operating with long residence times has not yet been characterized. The present work examines the sensitivity of emissions to pressures over the 1-5 bar range. Measurements of natural luminosity and OH*/NH2* chemiluminescence are used to assess pressure-dependent changes in flame structure and heat release across the rich primary zone and lean secondary zone. Corresponding exhaust data for NOx, NH3, and N2O quantify the degree of NOx relaxation at elevated pressures. Results show that NH3 slip increases with pressure in both the rich and relaxation zones, a behavior attributed to higher heat losses that inhibit effective NH3 cracking. Overall, the study highlights that staged NH3 combustion systems employing long τres,primary, particularly those operating at small laboratory scales and thermal loads, require careful management of relaxation stage heat losses to maintain low-emission performance.