Method for low NOx fire tube boiler
A fire tube boiler includes a perforated flame holder configured to hold a combustion reaction that produces very low oxides of nitrogen (NOx).
1. A method for operating a low oxides of nitrogen (NOx) fire tube boiler, comprising:
providing a boiler shell including at least one combustion pipe disposed at least partially inside the shell and a plurality of fire tubes disposed inside the shell, the plurality of fire tubes being configured to receive combustion products from the combustion pipe, the combustion pipe being characterized by a length and an inside diameter, the boiler shell being structured to hold boiler water, the combustion pipe surrounding a combustion volume and forming a continuous volume with the plurality of fire tubes, and the combustion pipe and fire tubes being configured to collectively hold the boiler water out of the combustion volume;
providing a perforated flame holder including a body that defines a plurality of void volumes operable to convey the fuel and air and to hold a combustion reaction;
supporting the perforated flame holder in the combustion pipe;
outputting fuel and combustion air into the combustion volume in a direction aligned to deliver mixed fuel and combustion air to the perforated flame holder;
holding a combustion reaction supported by the fuel and combustion air with the perforated flame holder;
delivering hot combustion products to the fire tubes;
transferring heat from the fire tubes to the boiler water;
outputting hot water or steam from the boiler; and
outputting combustion products including less than 10 parts per million NOx at 3% excess oxygen.
2. The method for operating a low NOx fire tube boiler of claim 1 , wherein outputting combustion products including less than 10 parts per million NOx at 3% excess oxygen includes outputting less than 1 part per million NOx.
3. The method for operating a low NOx fire tube boiler of claim 1 , further comprising:
receiving heat from the combustion reaction held in the void volumes of the perforated flame holder into the body of the perforated flame holder; and
outputting the heat from the body of the perforated flame holder to the mixed fuel and combustion air in the void volumes to maintain combustion.
4. The method for operating a low NOx fire tube boiler of claim 3 , wherein void volumes each comprises an elongated aperture.
5. The method for operating a low NOx fire tube boiler of claim 3 , wherein outputting heat from the body of the perforated flame holder to the mixed fuel and combustion air includes outputting heat into elongated apertures each having a length L sufficient for thermal boundary layers formed along walls defining the elongated apertures to substantially merge to cause the entirety of the fuel and combustion air to be heated to an autoignition temperature of the fuel in the fuel and combustion air mixture.
6. The method for operating a low NOx fire tube boiler of claim 3 , wherein receiving heat from the combustion reaction into the body of the perforated flame holder and outputting the heat from the body of the perforated flame holder to the mixed fuel and combustion air further comprises:
holding the received heat in the body of the perforated flame holder; and
transferring the held heat in an upstream direction toward an unburned portion of the fuel and combustion air mixture.
7. The method for operating a low NOx fire tube boiler of claim 6 , wherein transferring the held heat in an upstream direction toward an unburned portion of the fuel and combustion air mixture includes transferring heat with thermal radiation within the plurality of void volumes defined by the body of the perforated flame holder.
8. The method for operating a low NOx fire tube boiler of claim 6 , wherein transferring the held heat in an upstream direction toward an unburned portion of the fuel and combustion air mixture includes transferring heat with thermal conduction within the body of the perforated flame holder.
9. The method for operating a low NOx fire tube boiler of claim 1 , wherein outputting fuel and combustion air into the combustion volume in a direction aligned to deliver mixed fuel and combustion air to the perforated flame holder includes delivering a leaner fuel mixture than what would stably burn in a stream-stabilized flame at a location corresponding to the perforated flame holder.
10. The method for operating a low NOx fire tube boiler of claim 1 , wherein void volumes are characterized by a void fraction, expressed as (total perforated flame holder volume−body volume)/total perforated flame holder volume, of about 70% (0.70).
11. The method for operating a low NOx fire tube boiler of claim 1 , wherein providing a perforated flame holder includes providing a perforated flame holder wherein the body defines an input surface configured to receive the fuel and air, an output surface opposite to the input surface, and a peripheral surface defining a lateral extent of the perforated flame holder;
wherein the void volumes comprise a plurality of elongated apertures extending from the input surface to the output surface of the perforated flame holder; and
wherein holding the combustion reaction with the perforated flame holder includes holding a majority of the combustion reaction to occur between the input surface and the output surface of the perforated flame holder.
12. The method for operating a low NOx fire tube boiler of claim 11 , wherein holding the combustion reaction with the perforated flame holder includes holding the combustion reaction at least partially within the elongated apertures; and
wherein each elongated aperture has a lateral dimension D equal to or greater than a quenching distance of the fuel in the mixed fuel and combustion air.
13. The method for operating a low NOx fire tube boiler of claim 1 , wherein supporting the perforated flame holder in the combustion pipe includes supporting the perforated flame holder with a flame holder support structure away from the fuel nozzle at a distance sufficient to cause delivery of substantially completely mixed fuel and combustion air to the perforated flame holder.
14. The method for operating a low NOx fire tube boiler of claim 1 , wherein supporting the perforated flame holder in the combustion pipe further comprises:
supporting thermal insulation adjacent to the wall of the combustion pipe along at least a portion of the distance between a fuel nozzle and the perforated flame holder.
15. The method for operating a low NOx fire tube boiler of claim 1 , wherein outputting fuel and combustion air into the combustion volume in a direction aligned to deliver mixed fuel and combustion air to the perforated flame holder further comprises:
outputting a jet of fuel from a fuel nozzle; and
outputting combustion air from an air source disposed adjacent to the fuel nozzle.
16. The method for operating a low NOx fire tube boiler of claim 15 , wherein outputting a jet of fuel from a fuel nozzle includes outputting a jet of a gaseous hydrocarbon fuel.
17. The method for operating a low NOx fire tube boiler of claim 1 , wherein outputting fuel and combustion air into the combustion volume in a direction aligned to deliver mixed fuel and combustion air to the perforated flame holder includes outputting fuel through a fuel nozzle characterized by a nozzle diameter through which the fuel is emitted;
outputting combustion air adjacent to the emitted fuel; and
allowing the fuel and combustion air to flow through a mixing distance before reaching the perforated flame holder;
wherein supporting the perforated flame holder in the combustion pipe includes supporting the perforated flame holder with a flame holder support structure at the mixing distance from the fuel nozzle.
18. The method for operating a low NOx fire tube boiler of claim 17 , wherein the mixing distance is at least 20 nozzle diameters.
19. The method for operating a low NOx fire tube boiler of claim 17 , wherein the mixing distance is 100 nozzle diameters or more.
20. The method for operating a low NOx fire tube boiler of claim 19 , wherein the mixing distance is about 265 nozzle diameters.