The concentration of a flammable vapor or gas in the circulating air must be kept below the flash point. To avoid risk, the exhaust air from some reactors, furnaces, and dryers cannot be recirculated directly. This air must be depleted of most of the molecules that can burn or explode in the presence of oxygen when their concentrations reach the flash point.  

Several proposed processes can decrease emissions from such a lean fuel. These emissions are often hazardous and polluting, and sometimes they emit a foul odor. Aside from condensation and absorption, processes may involve the combustion of concentrated vapors in incinerators or the assisted combustion of dilute vapors in incinerators.  

Conventional incinerators fall into two categories: catalytic and thermal. Catalytic incinerators can easily become poisoned. Thermal incinerators, on the other hand, are expensive. The exhaust from thermal incinerators has to be heated to 850°C by adding fuel, leading to more CO₂ emissions.