The Effects of Secondary Air Staging Location, Pressure and Humidification on NOx Emissions for a High Ammonia Blend in Premixed Swirl Combustion

Abstract Ammonia (NH3) is a zero-carbon fuel which can be produced renewably, so has been gaining significant research interest in recent years. It is also found in the waste streams of all industries handling material of organic origin (e.g. agriculture, sewage, oil refining, coal coking, etc.). As ammonia is toxic, it is often destroyed rather than recovered. The Phosam process enables recovery of high purity anhydrous ammonia from industrial waste streams and has been used commercially in the steel industry. Ammonia combustion challenges include low reactivity and a high propensity for NOx emissions. Low reactivity can be overcome by blending with more reactive gases (e.g. hydrogen) and NOx emissions can be reduced via staged combustion. Fortunately, hydrogen-rich process gases are often a byproduct of the same industries. This work progresses previous work that successfully combusted a 15%vol hydrogen-rich industrial process gas (coke oven gas) with both anhydrous ammonia and humidified (30 %vol H2O) ammonia using a premixed swirl burner in a model GT combustor under fuel-rich conditions and ambient pressure. This campaign introduces secondary air at two different locations (15 and 25 cm from the burner exit) and at two pressures (1.1 and 1.3 bara), to demonstrate relative chemical kinetic effects on emissions formation with complete combustion of these relatively complex, multicomponent fuel blends. The equivalence ratio (Φ) was varied, both in the primary zone and globally to evaluate the relative parametric effects on exhaust emissions. The global Φ was manipulated by substitution of a portion of the air with nitrogen of equivalent thermal capacity. When secondary air-staging, the anhydrous blend achieved consistently lower minimized NO than the humidified blend, despite the blends having equivalent minimized emissions in the absence of secondary air. Modest pressure elevation of ∼17%, reduced NO entering the second stage by ∼25%. Staging further downstream produced ∼24% lower NO, when operating at near atmospheric pressure. Observations of OH* and NH2* flame chemiluminescence indicated that flame structure was modestly influenced as the secondary staging moved upstream. Both staging locations enabled adequate secondary air mixing prior to gas sampling (CO reduced from ∼6000 to < 5 ppm). Increases in global Φ (above 0.7) had no measured effect on NO production.