Radiant tube with recirculation

1. An apparatus comprising:

a radiant tube;

a burner configured to fire into the radiant tube;

a recirculation conduit having an inlet within the radiant tube and an outlet within the radiant tube at location between the inlet and the burner;

an injector configured to inject staged fuel that inspirates gaseous contents of the radiant tube into the inlet of the recirculation conduit;

valves configured to control flows of fuel from a source to the burner and the injector; and

a controller configured to operate the valves in a mode in which the valves provide fuel to the burner and the injector simultaneously.

2. An apparatus as defined in claim 1 wherein the outlet of the recirculation conduit is configured for adjacent gaseous contents of the radiant tube to aspirate the staged fuel from the outlet.

3. An apparatus as defined in claim 2 wherein the recirculation conduit has only a single outlet configured for adjacent gaseous contents of the radiant tube to aspirate the staged fuel from the outlet.

4. An apparatus as defined in claim 2 wherein the outlet is one of multiple outlets of the recirculation conduit, each of which is configured for adjacent gaseous contents of the radiant tube to aspirate the staged fuel from the outlet.

5. An apparatus as defined in claim 1 wherein the recirculation conduit has a length between the inlet and the outlet, and that length is located entirely within the radiant tube.

6. An apparatus as defined in claim 5 wherein the recirculation conduit is mounted within the radiant tube to define an elongated annular gas flow passage radially between the recirculation conduit and the radiant tube.

7. An apparatus as defined in claim 1 wherein the radiant tube has a turn between the burner and the outlet of the recirculation conduit.

8. An apparatus as defined in claim 1 wherein the burner, the recirculation conduit and the injector together comprise one of two oppositely oriented assemblies of a burner, a recirculation conduit and an injector that are configured relative to the radiant tube as recited in claim 1 .

9. An apparatus comprising:

a radiant tube;

a burner configured to fire into the radiant tube in a direction downstream from the burner and thereby to generate a downstream flow of combustion products within the radiant tube;

an injector configured to inject staged fuel into the radiant tube in a direction upstream relative to the downstream flow of combustion products;

a recirculation conduit contained within the radiant tube, the recirculation conduit having an inlet aligned with the injector to receive the staged fuel along with combustion products inspirated into the recirculation conduit by the staged fuel, and having an outlet configured to discharge a mixture of the inspirated combustion products and staged fuel into the downstream flow of combustion products at a location between the inlet and the burner;

valves configured to control flows of fuel from a source to the burner and the injector; and

a controller configured to operate the valves in a mode in which the valves provide fuel to the burner and the injector simultaneously.

10. An apparatus as defined in claim 9 wherein the outlet of the recirculation conduit is configured for the downstream flow of combustion products to aspirate the mixture from the outlet.

11. An apparatus as defined in claim 9 wherein the injector is part of a second burner configured to fire into the radiant tube oppositely relative to the burner of claim 9 , and further comprising a second recirculation conduit with an inlet and an outlet arranged and configured within the radiant tube oppositely relative to the inlet and outlet of the recirculation conduit of claim 9 .

12. An apparatus as defined in claim 11 wherein the radiant tube has a turn located between the recirculation conduits.

13. A method of combustion in a radiant tube having a turn between its opposite ends, comprising:

providing a downstream flow of combustion products in the radiant tube by directing air and first stage fuel into the radiant tube at a first location upstream of the turn;

injecting second stage fuel into the radiant tube at a second location downstream of the turn;

withdrawing combustion products from the downstream flow;

forming a mixture of the second stage fuel and withdrawn combustion products;

transporting the mixture upstream relative to the downstream flow along a recirculation flow path where the mixture does not mix with the downstream flow; and

discharging the transported mixture from the recirculation flow path into the downstream flow at a third location between the first and second locations.

14. A method as defined in claim 13 wherein the second stage fuel is injected into the radiant tube in a manner that withdraws combustion products from the downstream flow by inspirating combustion products from the downstream flow into the recirculation flow path.

15. A method as defined in claim 13 wherein the third location where the transported mixture is discharged into the downstream flow also is downstream of the turn.

16. A method as defined in claim 13 wherein the transported mixture is discharged into the downstream flow at only a single location.

17. A method as defined in claim 13 wherein the transported mixture is discharged into the downstream flow at multiple locations.

18. A method as defined in claim 13 wherein the second stage fuel is injected into the radiant tube in a direction upstream relative to the downstream flow.

19. A method as defined in claim 13 wherein the downstream flow is generated by directing a pressurized flow of fuel and a pressurized flow of combustion air into the radiant tube through a first burner at one end portion of the radiant tube, and the second stage fuel is injected into the radiant tube by directing a pressurized flow of staged fuel without a pressurized flow of combustion air into the radiant tube through a second burner at an opposite end portion of the radiant tube.

20. A method as defined in claim 13 wherein the mixture is formed to be fuel rich sufficiently to cause reburning of NOx in the downstream flow.