Hydrocarbon cracking furnace with steam addition to lower mono-nitrogen oxide emissions

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, a roof and an exhaust stack for exhausting flue gases, the method comprising the steps of:

• providing a set of N floor burners disposed on the floor adjacent to the two side walls;
• providing a set of M wall burners disposed along 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;
• attaching a set of 3N primary burner tips, connected to the source of fuel gas, to the set of N floor burners;
• attaching a set of 3N secondary burner tips, connected to the source of fuel gas, to the set of N floor burners at a height below the set of 3N primary burner tips and at a position between the set of 3N primary burner tips and the vertical central plane;
• attaching a set of 2N steam injection ports, connected to the source of steam, to the set of N floor burners between the set of 3N secondary burner tips and the 3N secondary tips;
• delivering a combined fuel gas flow into the firebox with a combined flow rate F;
• delivering a steam flow into the set of 2N steam injection ports with a flow rate S of between about one-fourth and about equal to the combined flow rate F;
• dividing the combined fuel gas flow into a first flow to the set of M wall burner burners with flow rate F1 and into a second flow to the set of N floor burners with flow rate F2, where F1/F2 is between zero and about 0.25;
• subdividing the second flow into a third flow to the set of 3N primary burner tips with flow rate F3 and a fourth flow to the set of 3N secondary burner tips with flow rate F4, where F3/F4 is between about 0.1 and about 0.2;
• delivering a fresh air flow to the N floor burners by adjusting an air intake baffle connected to ambient air;
• mixing the fresh air flow with the third flow;
• burning fuel gas from the first flow to create a first set of flames and a first flue gas;
• burning fuel gas from the third flow to create a second set of flames and a second flue gas;
• burning fuel gas from the fourth flow to create a third set of flames and a third flue gas;
• recirculating a portion of the second and third flue gases as a recirculating flue gas;
• spraying the steam flow from the 2N steam injection ports into a set of 2N conical steam flows which make contact with the second set of flames thereby reducing the mono-nitrogen oxide emissions into the first, second, third and recirculating flue gases;
• and,
• exhausting the first, second, third and recirculating flue gases through the exhaust stack.

A method for upgrading an existing hydrocarbon cracking furnace to reduce mono-nitrogen oxide emissions where the hydrocarbon cracking furnace has a firebox with a floor, two side walls, two end walls and a roof with an exhaust stack for exhausting flue gases, a set of floor burners attached to the floor adjacent to the two side walls, a source of fuel gas, and a set of hybrid radiant tubes situated near a vertical central plane bisecting each of the two end walls, the set of hybrid radiant tubes attached to a feedstock source at a first end and a heat exchanger on a second end, the method comprising the steps of retrofitting and operating the hydrocarbon cracking furnace according to the sub-steps:

• providing a set of primary burner tips on the set of floor burners above the level of the floor, connected to the source of fuel gas;
• providing a set of secondary burner tips on the set of floor burners at about the level of the floor, fixed between the set of primary burner tips and the vertical central plane, connected to the source of fuel gas;
• providing an adjustable source of fresh air;
• providing a source of steam;
• adding a set of steam injection ports to the set of floor burners, between the set of secondary burner tips and the primary burner tips at about the height of the primary burner tips,
• connecting the set of steam injection ports to the source of steam;
• flowing fuel gas into a first fuel gas flow from the source of fuel gas to the set of primary burner tips at a flow rate F1;
• flowing the fuel gas into a second fuel gas flow to the set of secondary burner tips at a flow rate F2;
• flowing steam from the source of steam at a mass flow rate S to the set of steam injection ports to create a set of conical steam flows, where the mass flow rate S is between about one-fourth and about equal to the combined flow rate F=F1+F2;
• adjusting a fresh air flow to 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 containing a sub-stoichiometric amount of 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 as a recirculating flue gas;
• mixing the second fuel gas flow with the recirculated fraction of first flue gas as a second mixture containing a sub-stoichiometric amount of oxygen;
• burning the second mixture 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;
• and,
• exhausting the first flue gas, second flue gas, third flue gas and recirculating flue gas through the exhaust stack as an exhausted flue gas with reduced mono-nitrogen oxide emissions.

A hydrocarbon cracking furnace producing low mono-nitrogen oxide emissions comprising:

• a firebox with a floor, two side walls, two end walls and a roof;
• an exhaust stack connected to the roof for exhausting flue gases from the firebox;
• a set of floor burners attached to the floor adjacent to the two side walls;
• a set of hybrid radiant tubes positioned near a vertical central plane bisecting each of the two end walls and further attached to a feedstock source at one end and a heat exchanger on the other end;
• a source of fuel gas;
• a source of steam;
• the set of floor burners including a set of burner tiles having a top tile surface adjacent to the side wall, a lower surface positioned between the top tile surface and the vertical central plane, a beveled surface connecting the lower surface to the top tile surface where the top tile surface is between about 4 inches and about 9 inches above the floor and the lower surface is at the level of the floor;
• the set of burner tiles further including a chamber;
• a set of primary burner tips attached to the set of burner tiles inside the chamber at a height of between about 3 inches below the top tile surface and about the level of the top surface, and connected to the source of fuel gas;
• an air intake baffle connected between ambient air and the chamber;
• a set of secondary burner tips attached to the set of burner tiles at the lower surface and extending to a height of between about 3 inches above the lower surface and about the level of the lower surface, the set of secondary burner tips connected to the source of fuel gas;
• a set of steam injection ports, connected to the source of steam, and further attached to the set of burner tiles between the set of primary burner tips and the set of secondary burner tips at a height about the same as the set of primary burner tips;
• a first fuel gas flow from the source of fuel gas to the set of primary burner tips having a flow rate F1;
• an ambient air flow to the set of primary burner tips from the ambient air sufficient to enable a fuel-lean combustion at the primary burner tips;
• a second fuel gas flow from the source of fuel gas to the set of secondary burner tips having a flow rate F2;
• a steam flow at a flow rate S from the source of steam to the set of steam injection ports and emerging from the set of steam injection ports into the firebox as a set of conical steam flows, where the flow rate S is between about one-fourth and about equal to the combined flow rate F=F1+F2;
• a recirculating flue gas flow from the combustion of the first and second fuel gas flows, recirculated to the set of secondary burners;
• a set of pilot burners attached to the set of floor burners in proximity to the primary burner tips and the secondary burner tips and attached to the wall burners, the set of pilot burners connected via a set of pilot valves to the source of fuel gas;
• a feedstock flow from the feedstock source into the hybrid radiant tubes;
• and,
• a hydrocarbon product flow from the hybrid radiant tube into the quenching stack.
• a set of primary flames generated by fuel-lean combustion at each of the set of primary burner tips;
• a set of secondary flames generated by fuel-rich combustion at each of the set of secondary burner tips;
• wherein each conical steam flow of the set of conical steam flows is in contact with at least two primary flames in the set of primary flames.