subject: Flue Gas Recirculation [print this page] Flue gas recirculation is a great way to optimize the efficiency of an industrial plant with respect to the reduction of harmful NOx gases produced by high temperature boiler flames. Flue gases are notorious for having many components that are harmful to both life and the environment if they escape into the atmosphere.
For boilers that run extremely high temperatures, NOx gases are formed from nitrogen in the air used to burn the fuel. Some industrial facilities have added the step of flue gas recirculation to this process as a means of not only incinerating a portion of the gas twice, but also recirculating gas that is relatively cool when compared to the boiler temperatures back through the boiler burner. This accomplishes NOx reduction in two ways. First, depending on where it is injected, it can lower the temperature of the combustion chamber which results in lower NOx formation and it also dilutes the amount of oxygen in the boiler which also lowers the amount of NOx formation.
In addition to lowering NOx emissions, running flue gas back through the burner is beneficial for the atmosphere because flue gas also contains other harmful gases. Common compounds in flue gases include, but are not limited to: 1,3-butadiene, 1-butene, n-butane, carbon dioxide, carbon monoxide, cis-2-butene, ethane, ethylene, n-hexane, isobutane, isobutylene, isopentane, methane, nitrogen, n-pentane, propadiene, propane, propylene, trans-2-butene, hydrogen and hydrogen sulfide.
Most of these are harmless in themselves, but some of them are very harmful to life and the environment. Harmful ones include: 1,3-butadiene, carbon monoxide and sulfur dioxide.
Incineration is the best way to reduce or eliminate the toxicity of flue gases. Incineration through a flare converts the harmful hydrocarbons such as 1,3-butadiene to carbon dioxide and water. Other harmful gases such as carbon monoxide are also converted to carbon dioxide. Using flue gas recirculation not only reduces NOx emissions, but also assists in the incinertion of toxic flue gases at the same time.
