Process for firing a furnace

Claim 1

In a process for firing a furnace by combusting fuel and oxidant gas, thereby producing furnace gases, wherein at least one jet of oxidant gas, selected from the group consisting of oxygen-enriched air and oxygen, and at least one jet of fuel are injected into the furnace aspirating the furnace gases into the oxidant jet prior to combustion, and wherein the furnace, in order to maintain a given temperature range which incorporates a specified temperature, is fired using alternating high and low firing rate periods, said low firing rate period being characterized by injection into the furnace of fuel and oxidant at flow rates such that substantial leakage of air into the furnace is caused to occur followed by a high firing rate period characterized by injection into the furnace of fuel and oxidant at flow rates, each greater than their respective flow rates during the low firing rate period, such that little or no air leakage into the furnace is caused to occur, the improvement comprising reducing NOx emissions to an acceptable level by delaying the injection of oxidant at the high firing rate at the start of the high firing rate period for a finite time no greater than S, wherein S is given by the formula: S=(4.12×10.sup.5)V/FT wherein: S is the time delay in seconds, V is the furnace volume in cubic meters, F is the firing rate during the high firing period in kilocalories per second, and T is the specified temperature in °K.