Emissions from intermediate size industrial boiler furnaces include oxides of nitrogen or NO.sub.x emissions which are undesirable because they pollute the atmosphere; thus, they are to be eliminated if at all possible. Attempts have been made in the past to reduce such emissions without sacrificing the operating efficiency of such a furnace, such as operating the burners of the furnace with a slightly excess amount of air, two stage firing and off-stoichiometric or bias firing of the burners, and by increasing the air supply in the furnace. Such attempts have been described in U.S. Pat. No. 3,890,084.
Other attempts have been made to solve this problem by firing the lower of a vertically spaced pair of burners with 70% air while the upper burner was fired with 160% air. All of the air to the burners was introduced through the burner throats and thereby mixed with the fuel which kept the flame relatively cold. Cold flames discourage the formation of NO.sub.x emissions. Thus, since in this arrangement there is such an abundance of excess air, substantial amounts of NO.sub.x were nevertheless generated. This was an improvement over earlier attempts because relatively less fuel was introduced into the furnace through the upper burner which had high NO.sub.x emissions as compared with the lower burner which had low or virtually no NO.sub.x emissions so that the overall NO.sub.x emissions were tolerable.
Another attempt to solve this problem has been to use an air delivery port above each pair of vertically spaced burners of a furnace. These air ports required a booster fan, a separate wind box and separate ducts to the wind box. This required considerable expense and a large modification to conventional furnace design.
Notwithstanding the foregoing, the NO.sub.x emissions still remain a problem and require further attempts to reduce them to a more tolerable level. Because of this, a need has arisen for further improvements in the furnace of the type described so that the emissions resulting from the combustion of fuel and air in the furnace has a reduced amount of NO.sub.x emissions.