22 and with air directly or through an air box 24 the air box may be mounted on the outside of the walls 20 opposite to the combustion zone 12 inside the vessel the air box is a manifold that distributes air to each of the burners combustion of the fuel injected by the main burners 18 and air from the air box 24 occurs in the combustion zone 12 of the vessel the flue gas 26 produced by the combustion flows in a downstream direction that is upward from the combustion zone 12 to the burnout zone 14 in the vessel 11 the main burners supply the heat energy input into the vessel additional heat may be released into the vessel 11 at the reburning zone 16 where a reburn fuel such as natural gas is combusted the reburn fuel enters the vessel 11 through a reburn fuel injector 28 downstream of reburning zone 16 is the burnout zone 14 where overfire air enters the vessel 11 through an overfire air injector 30 downstream of the burnout zone in the vessel 11 the flue gas 26 optionally passes through a series of heat exchangers 32 solid particles remaining in the flue gas may be removed by a particulate control device 33 such as an electrostatic precipitator u201c esp u201d or baghouse a selective reducing agent n agent is sprayed into the burnout.