METHOD AND IMPROVED FURNANCE FOR REDUCING EMISSIONS OF NITROGEN OXIDES

Abstract

A method and improved furnace for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors and a plurality of spaced apart over-fire air injectors positioned above the primary fuel injectors are disclosed. Injection of over-fire air produces zones of cooler combustion gasses containing over-fire air that separate zones of hot combustion gasses containing nitrogen oxides. Reburn fuel injectors inject a reburn fuel into the zones of hot combustion making the effluent combustion gases containing nitrogen oxides partially or totally fuel-rich in order to further reduce nitric oxide.

Claims

1. An improved furnace of the type having a plurality of primary fuel injectors on a wall of the furnace and a plurality of spaced apart over-fire air injectors on the wall of the furnace and positioned above the primary fuel injectors such that centerlines through each a pair of adjacent over-fire air injectors define a region on the wall of the furnace that extends upward and downward on the wall of the furnace between that pair of adjacent over-fire air injectors, wherein the improvement comprises a plurality of reburn fuel injectors, each reburn fuel injector located within one of the regions.

2. The improved furnace of claim 1 wherein the reburn fuel injectors are slot injectors.

3. The improved furnace of claim 1 wherein the reburn fuel injectors are capable of being tilted to direct the flow of reburn fuel in a selected upward direction or a selected downward direction.

4. The improved furnace of claim 1 wherein the reburn fuel injectors are capable of being turned left or turned right to direct the reburn fuel in one of several selected directions.

5. The improved furnace of claim 1 wherein the furnace is comprised of a series of substantially parallel boiler tubes and webbing extending between each pair of adjacent boiler tubes and the reburn fuel injectors are in the webbing.

6. The improved furnace of claim 1 wherein the reburn fuel injectors are positioned above the over-fire air injectors.

7. The improved furnace of claim 6 wherein the reburn fuel injectors are positioned about fourteen feet above the over-fire air injectors.

8. The improved furnace of claim 1 wherein the furnace is a face-fired furnace, an opposed-fired furnace, a tangentially-fired furnace, a turbo-fired furnace or grate-fired furnace.

9. The improved furnace of claim 1 wherein the reburn fuel injectors have an orifice that is round, square or oblong.

10. The improved furnace of claim 1 wherein the injectors penetrate the boiler through existing openings.

11. The improved furnace of claim 10 wherein the furnace has a primary burner air duct and at least one of the existing openings are the primary burner air duct.

12. A method for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors on a wall of the furnace and a plurality of spaced apart over-fire air injectors on the wall of the furnace and positioned above the primary fuel injectors wherein during operation of the furnace combustion gasses will flow from the ignition zone adjacent the fuel injectors toward and past the over-fire air injectors and the injection of over-fire air creates zones of hot combustion gases containing nitrogen oxides, the zones separated by zones of cooler combustion gases containing over-fire air, the method comprising injecting a reburn fuel into at least one of the zones of hot combustion gases.

13. The method of claim 12 wherein the reburn fuel is injected at a downward or upward angle into the at least one of the zones of hot combustion gasses.

14. The method of claim 12 wherein the reburn fuel is a high hydrogen content gas.

15. The method of claim 14 wherein the high hydrogen content gas is natural gas or coal syn-gas.

16. The method of claim 12 also comprising identifying the zones of hot combustion gases.

17. The method of claim 16 wherein the zones of hot combustion gases are identified by observation or by computer modeling.

18. The method of claim 17 wherein the computer modeling includes flow modeling of high velocity injectors designed to form nitric oxide reburn zones.

19. The method of claim 12 also comprising injecting reburn fuel at velocities sufficient to form fuel-rich reburn zones within at least one of the zones of hot combustion gases.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0015] FIG. 1a is a diagram of a wall in a conventional furnace which has no over-fire air ports.

[0016] FIG. 1b is a diagram of a wall in a conventional furnace which has over-fire air ports.

[0017] FIG. 2a is a side-view diagram showing the flow of combustion gases in a conventional furnace which has no over-fire air ports.

[0018] FIG. 2b is a side-view diagram showing the flow of combustion gases in a conventional furnace which has over-fire air ports.

[0019] FIG. 3 is a diagram showing zones in the flow of combustion gases that we have observed in a furnace which has over-fire air ports.

[0020] FIG. 4 is a diagram similar to FIG. 3 showing zones in the flow of combustion gases in a present preferred embodiment of our furnace.

[0021] FIG. 5a is a diagram of a present preferred reburn fuel injector that can be used in the furnace shown in FIG. 4.

[0022] FIG. 5b is a diagram of a portion of furnace wall having a slot in the webbing between boiler tubes, into which slot the reburn fuel injector may be placed.

[0023] FIG. 6 is a diagram showing plume penetration of the reburn fuel into the combustion gas flowing through the furnace shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] It inherently takes the total volume of the upper furnace to fully mix the over-fire air with the combustion effluent from the lower furnace; other-wise, the lower -primary flame combustion zone might be made even more fuel-rich with increased over-fire air and improved NOx reductions.

[0025] Considering the effect of high temperature upon increasing nitric oxide kinetics, over-fire air injection and mixing is generally accomplished within the colder, upper two-thirds of the furnace. This is accomplished by diverting a part of the normally used wind box combustion air. We have discovered that as the over-fire air mixing takes place, there develop upward flowing parallel zones of buoyant effluent combustion gases which are in the process of mixing but are not yet mixed. The wind box combustion air is preheated to between 500 F. and 900 F., but it is relatively cold in comparison to the combustion effluent which may be hotter than 3000 F. Thus, the injected over-fire air is two or three times more dense than the uprising combustion effluent flue gas. Because of this large density difference the hot and buoyant combustion effluent chimneys around the colder over-fire air and the over-fire air mixes very poorly. This parallel flow of over-fire air is depicted in FIG. 3, in an actual 460MW Utility Boiler application.

[0026] These upward flow, buoyant columns of hot gases have been visually observed, empirically measured with a matrix of CO, O.sub.2 and NO measurements and replicated through Computer Furnace Models. These columns of hot gases or spirals, in the case of tangential-fired furnaces, contain the nitric oxide formed in the lower primary combustion region of the furnace.

[0027] We identify and target these parallel columns or spirals of combustion effluent, containing nitric oxide for injection of a reburn fuel. We call this injection Zonal Gas Reburn injection. Zonal Gas Reburn may also be called ZGR or Zonal Off-Stoichiometric Gas Reburn or ZOGR.

[0028] In a present preferred embodiment shown in FIGS. 4 and 5b, multiple slot injectors 21 are inserted between the boiler tubes 22 of an actual 460 MW Utility Boiler 20. These zonal gas reburn injectors are targeted to inject the reburn gas into the combustion effluent containing nitric oxide formed in the primary flame region 12 of the furnace.

[0029] In the case of this 460MW Boiler shown in FIG. 3, the reburn fuel injectors 21 are placed in a region between the centerlines 14 of adjacent over-fire air injectors 8 and above those injectors. The reburn fuel injectors in the furnace shown in FIG. 4 are 14 feet above the over-fire air injectors and over-fire air injectors are 13 feet above the top row of burners. In general the reburn fuel injectors may be placed above or below the over-fire air injectors but in position to mix with the trajectory of the effluent combustion gases, containing the nitric oxide to be reduced. This applies to all type of furnaces whether face-fired, opposed-fired as here, tangentially-fired, turbo-fired or even grate-fired; in every case with over-fire air, the targeted zone will exist before and as the over-fire air mixes. The resulting zonal reburn zone, regardless of furnace type, must extend to the coldest possible region of the upper furnace in order to take full advantage of lowered temperature nitric oxide equilibration, as this reburn zone mixes to complete combustion with the over-fire air.

[0030] The gas reburn fuel injectors 21 may be any type orifice, round, square, oblong, etc., so long as it may fit through a penetration into the furnace or even through the air duct around the primary burners and they inject the gas reburn fuel into the targeted nitric oxide containing effluent.

[0031] We prefer to provide multiple reburn fuel slot injectors to allow for a-priori targeting of the high NOx zones with multiple injectors. We prefer to provide reburn fuel injectors which fit between tubes 22 without requiring expensive tube modifications as shown in FIGS. 5a and 5b. The reburn fuel injectors 21 may be horizontally adjustable as they fit through the slots 24 in the webbing 26 between the boiler tubes, as is the one installed as shown in FIG. 5b.

[0032] Vertical tilt or up/down adjustment of the slot injector mechanisms shown in FIG. 5a allows for empirical experimentation and adjustments for maximizing nitric oxide reduction and maintaining combustion completion with minimum CO emissions.

[0033] Alternatively, the injectors could be positioned to penetrate the boiler through existing openings. Such openings could be openings for the primary burner air duct.

[0034] The use of high hydrogen containing natural gas or other high hydrogen gas, such as coal or syn-gas improves the burn-out of lower hydrogen primary fuels such as coal or heavy oils. With natural gas reburn injection the moisture content of the effluent combustion gas goes from 6% in the primary furnace to nominally 12% in the Zonal Gas Reburn zone of our process. This increased hydrogen content greatly enhances carbon burn-out; and our adjustable and parallel flow reburn injection method allows the process to take place in the most favorable temperature region of the furnace which will allow for both combustion completion and lowered temperature nitric oxide equilibration.

[0035] In the case of the 460MW boiler shown in FIG. 3, two rows of seven Zonal Gas Reburn injectors were installed on the front wall and two rows of seven Zonal Gas Reburn injectors were also installed on the rear wall to allow flexibility when operated at reduced load and with different combinations of coal mills in service. Also one row of four injectors were installed on the boiler sides to allow reduced load following and emission minimization. The measured penetration produced by the gas injected through these ZGR injectors was twenty-eight feet. This is shown proportionately in FIG. 6 for this seventy-eight foot high furnace volume boiler.

[0036] In the demonstration case shown in FIG. 6, the ZGR injector is pointed downward at 20 degrees and the ZGR injection plume mixes for 28 feet before losing its identity within the uprising combustion effluent. Each gas plume spread to 6 feet in diameter at 28 feet penetration and moved upward with the combustion gases at a velocity of 10 feet to 30 feet per half second. At 28 feet the plume eddies could still be seen and are fuel rich. Six feet of the plume had mixed with approximately 60% of the combustion gases such that the plume contained 9 percent fuel (0.615% excess air=9% fuel). Then the plume mixed out to form final CO.

[0037] In our method a gaseous fuel can be injected into a furnace which already is using over-fire air for nitric oxide reduction and through our process of targeted Zonal Gas Reburn further nitric oxide reductions can be achieved.

[0038] The gaseous fuel injection can be selectively targeted to zones of buoyant parallel or spiral flow combustion effluent, which chimney around the colder over-fire air flow, to form multiple reburn zones. Our targeted reburn zones rise with the buoyant effluent, which contains nitric oxide formed in the lower furnace, and our fuel-rich Zonal Gas Reburn reduces this nitric oxide. Also, the hydrogen content of the reburn fuel improves the carbon burn-out of solid or liquid primary fuel; while the targeted reburn zone dissipates in the upper furnace, with the over-fire air serving as reburn completion air. Our targeted Zonal Reburn Injection reduces both nitric oxide, CO and carbon emissions.

[0039] Buoyant, vertical or spiral (encountered with tangential firing) zones of effluent, containing nitric oxide from the lower primary fuel combustion region of the furnace can be spatially identified and zonal gas reburn injection can be used to form upward flowing fuel-rich reburn zones which reduce the nitric oxide and then mix with the over-fire air to complete combustion. These zones can be identified by observation of a furnace in which temperatures of the combustion gases are measured at selected locations or by computer modeling. The computer modeling may include flow modeling of high velocity injectors designed to form nitric oxide reburn zones.

[0040] Our process spatially targets multiple zones of a furnace to reduce nitric oxide emissions and at the same time improve combustion and burn-out.

[0041] Our process is compatible with the implementation of over-fire air. Our Zonal Gas Reburn in fact uses the already implemented over-fire air as completion air to complete the combustion of the Zonal Reburn Fuel.

[0042] Reburn gas or higher hydrogen reburn fuel can be injected through boiler wall penetrations or even through the primary burner air supply to target the primary combustion effluent and reduce nitric oxide. Such reburn fuel injectors are purposely targeted to cause the combustion effluent to become fuel-rich thereby reducing its nitric oxide concentration. These injectors can be designed to maximize the reduction of nitric oxide within the targeted effluent. Their design may involve any orifice suitable to provide the design penetration into the targeted nitric oxide containing effluent. The orifice can be round, square or oblong orientation and aspect to provide the design penetration into the targeted nitric oxide containing effluent.

[0043] Slots can be installed in the webbing between boiler tubes, without modification of these tubes for using our method. Through use of these slots significant gas reburn fuel can be injected into targeted furnace effluent flow zones. And further these targeted zones of low excess air combustion effluent are thereby made fuel-rich, so that resulting off-stoichiometric combustion of the reburn gas will reduce nitric oxide in this effluent.

[0044] Slots can be cut between tubes allow the insertion of practical injection nozzles for the purpose of controlled injection and mixing of reburn fuel into targeted effluent arising from the lower furnace.

[0045] Practical injection nozzle assemblies can be fastened to the boiler wall so as to fit through the slots in the webbing. These injection nozzle assemblies allow vertical directional adjustment of injection flow, and the tolerance space around the inserted nozzles allows for their air cooling.

[0046] Our reburn fuel injection assemblies may be installed to provide multiple Zonal Gas Reburn injection points without expensive high pressure boiler tube modifications.

[0047] Multiple reburn fuel injection nozzles can have a high vertical aspect ratio of vertical height to horizontal width from 1.0 to 100 in the direction of the upward flowing effluent, thus providing more targeted gas flow in the upward direction of the effluent and less horizontal spreading into the over-fire air (where there is no nitric oxide).

[0048] Slotted reburn fuel injectors can be positioned to target observed zones of primary combustion effluent, before the effluent mixes with over-fire air, for any type furnace such as, but not limited to: face-fired, opposed-fired, tangential-fired or turbo-fired furnace.

[0049] Although we have shown and described certain present preferred embodiments of our furnace and method for reducing nitrogen oxide emissions it should be distinctly understood that our invention is not so limited and may be variously embodied within the scope of the following claims.