Effects of External Injection of Deionized Water and Water with Impurities on Water-Assisted Flares

Water-assisted flares are used on offshore platforms and occasionally at downstream oil and gas facilities to reduce soot and oxides of nitrogen (NOx) emissions and flame luminosity. Typically, water-assisted flares inject water into the flame from outside the flare stack (external injection), and the water often contains high con-centrations of impurities. Different configurations of water injection were explored using a range of water flow rates, atomizer angles, and radial displacements from the burner center to understand the effects of water flow rate and location of water entrainment into the flame on emissions. A range of sodium chloride (NaCl) con-centrations was also tested as a surrogate species for the impurities contained in the water used at offshore and downstream oil and gas facilities. Flow visualization analysis was carried out, and the water droplet velocity field was used to discuss their motion transitioning from being dominated by their momentum to that of the buoyancy of the combustion products. This study used atmospheric turbulent non-premixed flames on a two-inch circular burner at a constant propane flow rate of 20 standard L/min. It was discovered that the atomizer's angular displacement plays a significant role in black carbon (BC) and NOx emissions suppression, but the atomizer's horizontal displacement does not affect NOx emission. Water directed very close to the base of the flame ensured water was present early in the combustion process relative to other external injected configurations and reduced emissions by a greater extent compared to when the water was directed higher in the flame. The buoyancy of the flame affects the velocity of the injected water, helps to redirect its flow in the flame, and increases its mixing with fuel and air. Large amounts of water droplets pass through the flame front and evaporate within the fuel side of the flame envelope. The flame color and luminosity were affected by an increased NaCl concentration and the different configurations of the injected water. Higher NaCl concentration in the water causes an increase in BC emissions and a slight decrease in NOx emissions relative to deionized water. The smaller NaCl concentration in the water does not affect NOx and BC emissions compared to deionized water.