Smoke is unburned carbon from incomplete hydrocarbon combustion. The oxygen in the air first combines with hydrogen atoms in the hydrocarbon molecules, freeing carbon atoms for secondary reactions. If the hydrocarbon molecules crack at the high combustion temperature, then more free carbon is produced. Unsaturated hydrocarbon molecules produce more free carbon. This unburned carbon appears as smoke.
The free carbon will be converted to carbon dioxide (and not produce smoke) as long as an excess of oxygen is present within the combustion zone. This requires good air/gas mixing. Thick smoke will result if a hydrocarbon gas is flared without prior mixing with air.
Injection of steam helps control the carbon-hydrogen ratio. Steam catalyzes the combustion reactions and lowers the temperature in the combustion zone, thus suppressing smoke. However the main reason for injecting a high velocity steam-air mixture into the combustion zone is to increase turbulence, which improves combustion and reduces the size of the flame. Unfortunately, steam injection increases the combustion roar.
The usual injection rate is about 0.5 pounds of steam per pound of hydrocarbon. Actual steam requirements for smokeless combustion are hard to predict because they depend on the molecular weight of the relief gas, the percentage of unsaturates, and the air/gas mixing ratio. Butadiene requires 0.8 to 1 pound of steam per pound of gas; methane requires 0.1 to 0.2 pounds per pound, while hydrogen burns smokelessly without steam.