Utilization of Hot Burnt Gas for Better Control of Combustion and Emissions
Recirculated hot burned gas has long been used to stabilize flames in continuous flow combustion systems, such as furnaces, boilers, gas turbines, and afterburners in jet engines . The recirculation is created by employing swirl vanes, bluff bodies, or baffle plates. The heat and radicals in the hot combustion products act as the ignition source for fresh mixtures of fuel and air flowing into the flame-stabilization region.
In the late 20th Century, air pollution by combustion-generated smoke, partially burned hydrocarbons, carbon monoxide, and nitric oxides, NOx, became a public concern. With the increasing stringency of the regulations for NOx emissions, recirculation of combustion products or exhaust-gas came to be used as one of the measures for controlling NOx formation. Exhaust-gas recirculation (EGR), for example, is extensively used in reciprocating engines for automobiles , where the engine exhaust gas is recirculated into the intake manifold in order to mix with fresh air. Another example is recirculation of combustion products in furnaces and boilers, which is sometimes called internal EGR. Most of the reduction of NOx emissions achieved by EGR is attributed to reduced peak flame temperatures that are due to increased specific heat capacity of the diluted mixtures, though some of the reduction is attributed to resulting slower NO formation that are due to reduced oxygen concentrations and increased carbon dioxide and water vapor concentrations.