Patent ID: 8703064

Claim:
A method for reducing mono-nitrogen oxide emissions from a hydrocarbon cracking furnace having a firebox with a floor, two side walls, two end walls and a roof with an exhaust stack for exhausting flue gases, the method including the steps of: providing a set of floor burners disposed on the floor adjacent to the two side walls; providing a set of hybrid radiant tubes near a vertical central plane bisecting each of the two end walls; providing a source of fuel gas; providing a source of steam; providing an adjustable source of fresh air; providing the set of floor burners with a set of primary burner tips, connected to the source of fuel gas, and a set of secondary burner tips, connected to the source of fuel gas, where the secondary burner tips are positioned at a height below the set of primary burner tips and fixed between the set of primary burner tips and the vertical central plane; providing a set of steam injection ports, connected to the source of steam, placing the set of steam injection ports between the set of secondary burner tips and the primary burner tips; flowing a first fuel gas flow from the source of fuel gas to the set of primary burner tips at a flow rate F 1 ; flowing a second fuel gas flow from the source of fuel gas to the set of secondary burner tips at a flow rate F 2 resulting in a combined flow rate F=F 1 +F 2 ; flowing steam from the source of steam at a flow rate S to the set of steam injection ports to create a set of conical steam flows; adjusting the flow rate S to between about one-fourth of the combined flow rate F to about equal to the combined flow rate F; adjusting a fresh air flow from the source of fresh air to the set of primary burner tips; mixing the first fuel gas flow with the fresh air flow as a first mixture which is sub-stoichiometric with respect to fuel gas; burning the first mixture to create a first set of flames and a first flue gas; contacting the first set of flames with the set of conical steam flows; recirculating a portion of the first flue gas to mix with the second fuel gas flow as a second mixture containing a sub-stoichiometric amount of oxygen; burning the second mixture at the secondary burner tips to create a second set of flames and a second flue gas; burning the second flue gas along with the first mixture at the primary burner tips to contribute to the first flue gas; adjusting the first and second sets of flames to deliver a total heat flux through a horizontal cross-section of the firebox at between about 250,000 and about 400,000 BTU/ ft 2 ; and, exhausting the first flue gas, the second flue gas, and the portion of the first flue gas through the exhaust stack as a combined flue gas whereby the combined flue gas has a reduced mono-nitrogen oxide concentration.