The objective of the present study was to investigate the effect of the distance between fuel and oxidizer nozzles on NOx emissions from a laboratory-scale spray combustion furnace simulating an industrial high-temperature air combustion (HiTAC) furnace. The furnace employed in these trials was fueled with commercially obtainable kerosene in combination with highly preheated oxidizer gases. The oxidizer was pre-diluted by the addition of nitrogen in order to produce dilution levels equivalent to those in industrial HiTAC furnaces. The resulting NOx measurements indicate a trend opposite to that reported by previous studies, such that increasing the distance between the nozzles increases the NOx emissions, and a theory is advanced to explain this unusual observation. In a furnace within which the nozzle distance in the burner is small, the properties of the oxidizer supplied from the nozzle located near the spray nozzle greatly affect NOx emissions. The use of an oxidizer with a much lower preheat temperature and a lower O 2 concentration induces flame lifting further downstream, which significantly reduces NOx emissions through the formation of an invisible flame exhibiting a uniform temperature distribution throughout the furnace. In contrast, in a furnace in which the burner has a large nozzle distance, a highly luminous flame is anchored near the spray nozzle exit due to ignition of fuel vapor by high-temperature burned gases recirculated to the lower section of the furnace. This flame represents a zone with high combustion intensity and generates significant NOx emissions. In such cases, the influence of the oxidizer properties is moderated due to the widely separated nozzle locations.

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