Control of staged combustion, low NO.sub.x firing systems with single or multiple levels of overfire air

In a fossil fuel-fired furnace having a plurality of walls embodying therewithin a main burner zone, a windbox mounted in supported relation within the main burner zone of the fossil fuel-fired furnace, close coupled overfire air means mounted in the windbox, low separated overfire air means located within the main burner zone of the fossil fuel-fired furnace in spaced relation to the windbox, high separated overfire air located within the main burner zone of the fossil fuel-fired furnace in spaced relation to both the low separated overfire air means and the windbox, means for supplying overfire air to the close coupled overfire air means, the low separated overfire air means and the high separated overfire air means, damper means operative to effect the apportioning of the flow of overfire air amongst the close coupled overfire air means, the low separated overfire air means and the high separated overfire air means, and a firing system operative to effect therewith the combustion within the fossil fuel-fired furnace of fossil fuels such that flue gases are generated as a consequence of the combustion of the fossil fuels, a control system for effecting control over the firing system comprising:

• a. means for determining the mass flow rate of the overfire air required to be supplied to the close coupled overfire air means, the low separated overfire air means and the high separated overfire air means in order to maintain a predetermined stoichiometric ration within the main burner zone of the fossil fuel-fired furnace;
• b. means for determining the windbox-to-furnace pressure differential within the fossil fuel-fired furnace;
• c. means for effecting control over the windbox-to-furnace pressure differential such that the windbox-to-furnace pressure differential maintains precedence over said means for determining the mass flow rate of overfire air supplied to the close coupled overfire air means, the low separated overfire air means and the high separated overfire air means;
• and
• d. means for effecting control over the apportioning of the mass flow rate of overfire air amongst the close coupled overfire air means, the low separated overfire air means and the high separated overfire air means.