Method and apparatus for radiant tube combustion

1. An apparatus for use with a radiant tube in which combustion proceeds downstream through a combustion zone extending along the length of the tube, the apparatus comprising:

a staging structure configured to direct reactants into the combustion zone in stages;

the staging structure including a primary reactant tube having an open end configured as a first stage reactant inlet to the combustion zone, with the primary reactant tube configured to extend within the radiant tube to define an annular mixing chamber radially between the primary reactant tube and the radiant tube;

the staging structure further including a baffle configured to be interposed between the mixing chamber and the combustion zone, with the baffle defining a second stage reactant inlet communicating the mixing chamber with the combustion zone, and defining a third stage reactant inlet communicating the mixing chamber with the combustion zone at a location downstream of the second stage reactant inlet.

2. An apparatus as defined in claim 1 wherein the baffle comprises a can-shaped structure having an annular end wall mounted on the primary reactant tube and a cylindrical body wall extending downstream from the annular end wall.

3. An apparatus as defined in claim 2 wherein the second stage reactant inlet is a gas flow opening through the annular end wall, and the third stage reactant inlet is a gas flow opening radially between the cylindrical body wall and the radiant tube.

4. An apparatus as defined in claim 1 wherein the staging structure further includes a jet pump configured to communicate a flue end portion of the radiant tube with the mixing chamber.

5. A method of providing combustion in a radiant tube in which combustion proceeds downstream through a combustion zone extending toward a flue end of the radiant tube, the method comprising:

forming a first reactant stream which contains fuel and combustion air, and which is free of gas recirculated from the flue end of the radiant tube;

directing the first reactant stream into the combustion zone to provide a first combustion stage;

forming a second reactant stream which contains combustion air and gas recirculated from the flue end of the radiant tube;

separately directing the second reactant stream into the combustion zone to provide a second combustion stage;

forming a third reactant stream which contains combustion air and flue gas recirculated from the flue end of the radiant tube; and

separately directing the third reactant stream into the combustion zone at a location downstream of the location at which the second reactant stream is directed into the combustion zone to provide a third combustion stage downstream of the second combustion stage;

wherein the combustion air is mixed with gas recirculated from the flue end of the radiant tube in a mixing chamber that is located within the radiant tube but outside the combustion zone, and wherein the second and third reactant streams are both drawn from the mixing chamber.

6. An apparatus for use with a stream of fuel, a stream of combustion air, a stream of recirculated flue gas, and a radiant tube in which combustion proceeds downstream through a combustion zone extending along the length of the tube, the apparatus comprising:

a staging structure configured to extend within the radiant tube to define first, second and third stage reactant inlets at upstream ends of respective first, second and third stage regions of the combustion zone within the radiant tube;

a staging structure configured to extend within the radiant tube to define first, second and third stage reactant inlets at upstream ends of respective first, second and third stage regions of the combustion zone within the radiant tube;

said staging structure being further configured to communicate the first reactant inlet with the streams of fuel and combustion air to the exclusion of the stream of recirculated flue gas, and to communicate the second and third reactant inlets with the streams of combustion air and recirculated flue gas to the exclusion of the stream of fuel;

wherein the third reactant inlet is located downstream of the second reactant inlet; and

wherein the staging structure is configured to define a mixing chamber inside the radiant tube but outside the combustion zone, and to communicate the mixing chamber with the streams of combustion air and recirculated flue gas to the exclusion of the stream of fuel, and wherein the second and third reactant inlets communicate the mixing chamber with the second and third stage regions of the combustion zone, respectively.