Effects of spacing between fuel and oxidizer nozzles on NOx emission from spray combustion furnace operating under various oxidizer temperatures

The aim of the present study was to investigate the effects of burner configuration and oxidizer temperature on NOx emission from a laboratory-scale furnace fueled with liquid kerosene, operating under high-temperature air combustion conditions. The burner employed here consisted of a spray nozzle and an oxidizer nozzle, between which the spacing was varied from 30 to 120 mm. The oxidizer was air, and the preheating temperature was from 1023 to 423 K. The appearance of the flame under each condition was visualized by thermocouple measurements, and the corresponding emission of nitrogen oxide (NOx) was measured by a gas analyzer. The droplets in the spray stream in the operating furnace were visualized using shadowgraph photography. When the oxidizer temperature was high (>= 823 K), NOx emission decreased with increasing spacing between the spray and oxidizer nozzles. This was due to a decrease in the flame temperature resulting from dilution by recirculating burned gas. In contrast, for lower oxidizer temperatures (<= 623 K), NOx emission initially increased and subsequently decreased with increasing inter-nozzle spacing. In these cases, the lifted flame formed in the furnace moved upstream as the inter-nozzle spacing increased. This is attributed to faster ignition induced by increased evaporation of fuel droplets and the higher temperature of the burned gas recirculating in the furnace. The increase in flame temperature associated with this movement increased the amount of NOx emission. A further increase in inter-nozzle spacing led to flame extinction, resulting in a decrease in NOx emission.