Industrial Flare Performance at Low Flow Conditions. 2. Steam- and Air-Assisted Flares

Abstract

Full-scale tests of steam- and air-assisted industrial flares were conducted using low BTU content (lower heating value) vent gases at low flow rates. A 36″ diameter steam-assisted flare with a flow capacity of 937,000 lb/h and a 24″ diameter air-assisted flare with a flow capacity of 144,000 pounds per hour were operated with mixtures of natural gas, propylene, and nitrogen or natural gas, propane, and nitrogen at flow rates less than 1% of maximum flow. Combustion efficiency (percentage of the flared gases converted to carbon dioxide and water) ranged from less than 50% to more than 99%. For the steam-assisted flare, combustion efficiency (CE) at low steam-to-vent gas flow ratios (0.5–1.0) was typically in excess of 95%. CE would gradually decrease as steam-to-vent gas ratio increased, to a point, after which CE would decrease dramatically. The steam-to-vent gas ratio at which CE would decrease dramatically depended on the heating value of the vent gas and the position of the steam injection. Higher heating values of the vent gas (600 vs 350 BTU/scf) and the minimization of steam coinjected with the vent gas, rather than injected at the flare tip, promoted higher CE. For the air-assisted flare, CE at low air assist rates (<30–50 times the vent gas flow rate) was typically above 90%. The CE decreased linearly with increasing air assist to vent gas ratio and did not exhibit the rapid decrease exhibited by the steam-assisted flare. Combustion efficiencies for vent gases with higher heating values (∼600 BTU/scf versus ∼350 BTU/scf) decreased more slowly in the air assisted flare. For both the steam- and air-assisted flares, the composition of the vent gas (propane vs propylene) had a much smaller effect on combustion characteristics than steam or air injection and vent gas heating value. Products of incomplete combustion were dominated by CO; other significant species included acetylene, ethylene, formaldehyde, acetaldehyde, and acrolein. The effect of wind speed on CE was estimated to be less than 2.5% of the CE, over the range of wind speeds 0–16 mph.