As mentioned earlier, many cogeneration or combined cycle gas turbine power plants use a duct burner to increase the steam capacity of the HRSG and power output. Duct burners usually burn natural gas, but they may also burn oil. Duct burners can almost double the temperatures of exhaust gases in the HRSG to 200°F, but most duct burners are run to increase temperatures in the neighborhood of 1400°F.

Different designs of duct burners exist, but Figure 1  illustrates a cross-sectional view of a low NO x duct burner that burns natural gas. Exhaust gases from the gas turbine flow around the fuel manifold and mix with the natural gas flowing from orifices in the manifold. The zone around the fuel manifold (zone A) is a fuel rich zone since only a small amount of the exhaust gases can pass through the small slot between the manifold and the stabilizer casing of the duct burner. A “diffusion flame” burns in zone “A” forming a recirculation pattern. When exhaust gases flow into zone “B” they mix with combustion products from the turbine, more eddies are formed behind the stabilizer casing and combustion is completed.

The effect of duct burners on the total NO x produced from a power plant varies with different systems, but duct burners produce relatively small amounts of NO x emissions. In some systems, a duct burner can even reduce NO x emissions.