Methods and systems for reducing emissions of nitrogen oxides from ribbon burners

Abstract

Methods and systems for improving burner, particularly ribbon burner performance, involving admixing a portion of the combustion products from the flue gas resulting from the burner to at least one of a primary and a secondary oxidizer supply to the burner. Admixing a portion of a carbon dioxide-containing flue gas combustion product formed upon combustion of a fuel gas by a ribbon burner significantly reduces NOx emissions resulting from operation of the burner.

Claims

1. A method for reducing emissions of nitrogen oxides from a ribbon burner that upon combustion operation forms a flue gas including combustion products, said method comprising: admixing a portion of the flue gas combustion products with a primary oxidizer to form a flue gas and oxidizer combination having a reduced concentration of oxygen as compared to the primary oxidizer, wherein the oxygen content of the flue gas and oxidizer combination is reduced to no more than 20 volume %; admixing a fuel with the flue gas and oxidizer combination to form a combustible mixture; feeding the combustible mixture to the ribbon burner to combust the combustible mixture, wherein the ribbon burner is elongated and has a long thin slot filled with corrugated metal strips to create a narrow array of short interconnected flames and wherein the reduced concentration of oxygen in the flue gas and oxidizer combination reduces emissions of nitrogen oxides from the ribbon burner; and providing a secondary oxidizer to pass to the flames of the ribbon burner for more complete combustion of the fuel.

2. The method of claim 1 wherein the ribbon burner combusts a fuel gas selected from the group consisting of natural gas, propane, butane, syngas and landfill gas.

3. The method of claim 1 wherein the ribbon burner combusts natural gas.

4. The method of claim 1 wherein the primary oxidizer comprises air.

5. The method of claim 4 wherein the oxygen content of the flue gas and air oxidizer combination is reduced to no more than 20 volume %.

6. The method of claim 4 wherein the oxygen content of the flue gas and air oxidizer combination is reduced to no more than 19 volume %.

7. The method of claim 1 wherein the flue gas combustion products and the primary oxidizer are drawn from respective sources by a single blower.

8. A method for reducing emissions of nitrogen oxides from a partially aerated ribbon burner that combusts a fuel gas of natural gas or propane, said method comprising: admixing a portion of carbon dioxide-containing flue gas combustion product formed upon combustion of the fuel gas by the partially aerated ribbon burner with a primary oxidizer to form a flue gas and oxidizer combination having a reduced concentration of oxygen as compared to the primary oxidizer, wherein the oxygen content of the flue gas and oxidizer combination is reduced to no more than 20 volume %; admixing the fuel gas of natural gas or propane with the flue gas and oxidizer combination to form a combustible mixture; feeding the combustible mixture to the ribbon burner to combust the combustible mixture, wherein the partially aerated ribbon burner is elongated and has a long thin slot filled with corrugated metal strips to create a narrow array of short interconnected flames and wherein the reduced concentration of oxygen in the flue gas and oxidizer combination reduces emissions of nitrogen oxides from the partially aerated ribbon burner; and providing a secondary oxidizer to pass to the flames of the partially aerated ribbon burner for more complete combustion of the fuel gas.

9. The method of claim 8 wherein the fuel gas comprises natural gas.

10. The method of claim 8 wherein the primary oxidizer to which the carbon dioxide-containing flue gas combustion product formed upon combustion of the fuel gas is admixed comprises air.

11. The method of claim 10 wherein the oxygen content of the flue gas and oxidizer combination is reduced to no more than 19 volume %.

12. The method of claim 8 wherein the carbon dioxide-containing flue gas combustion product and the primary oxidizer are drawn from respective sources by a single blower.

13. A method for reducing emissions of nitrogen oxides from a partially aerated ribbon burner that upon combustion operation reacts a fuel gas with air oxidizer to form combustion products, the air oxidizer having an oxygen content, said method comprising: admixing a portion of the combustion products with the air oxidizer to form a oxidant-containing combination having a reduced oxygen content as compared to the air oxidizer alone, wherein the oxygen content of the oxidant-containing combination is reduced to no more than 20 volume %; subsequently admixing a fuel with the oxidant-containing combination to form a combustible mixture; feeding the combustible mixture to the partially aerated ribbon burner to combust the combustible mixture, wherein the partially aerated ribbon burner is elongated and has a long thin slot filled with corrugated metal strips to create a narrow array of short interconnected flames and wherein the reduced oxygen content of the oxidant-containing combination reduces emissions of nitrogen oxides from the partially aerated ribbon burner; and providing a secondary oxidizer to pass to the flames of the partially aerated ribbon burner for more complete combustion of the fuel.

14. The method of claim 13 wherein the fuel gas is selected from the group consisting of natural gas, propane, butane, syngas and landfill gas.

15. The method of claim 13 wherein the fuel gas comprises natural gas.

16. The method of claim 13 wherein the oxygen content of the oxidant-containing combination is reduced to no more than 19 volume %.

17. The method of claim 13 wherein the combustion products and the air oxidizer are drawn from respective sources by a single blower.

18. The method of claim 1 wherein the secondary oxidizer is passed to the flames of the ribbon burner without being premixed with fuel.

19. The method of claim 1 wherein the admixing of the fuel with the flue gas and oxidizer combination to form a combustible mixture comprises all the fuel fed to the ribbon burner.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified schematic of a system for reducing nitrogen oxide emissions in accordance with one embodiment of the invention.

(2) FIG. 2 is a simplified schematic of an experimental set-up used in some examples herein described.

(3) FIG. 3 is a graphical representation of NOx emissions versus oxygen content in the primary oxidant realized at selected burn rates of a ribbon burner in accordance with some examples herein described.

(4) FIG. 4 is a graphical representation of NOx reduction versus oxygen content in the primary oxidant realized at selected burn rates of a ribbon burner in accordance with some examples herein described.

DETAILED DESCRIPTION OF THE INVENTION

(5) As detailed further below, the invention generally provides improved burner performance such as by reducing emissions of nitrogen oxides.

(6) In accordance with one aspect of the invention, significant reductions of NOx emissions from a burner, particularly a ribbon burner, is based at least in part and, in accordance with certain preferred embodiments primarily on modifying the primary and/or secondary oxidizer/air composition by admixing at least a portion of carbon dioxide (CO.sub.2) and/or other combustion products from the flue gas produced or formed by or upon operation of the burn into or with the primary and/or secondary oxidizer/air supply to the burner. That is in a subject invented system in accordance with one aspect of the invention, a portion of the flue gases is recirculated by admixing a selected percentage of the flue gas into or with the primary and/or the secondary air streams. As a result the combustion of the gaseous fuel (e.g., natural gas, propane, butane, syngas, etc.) occurs with an oxidizer/air stream or streams containing a reduced or lower concentration of oxygen and at lower flame temperature, thus producing or resulting in reduced or lower emissions of thermal nitrogen oxides.

(7) While the broader practice of the invention is not to be unnecessarily limited to or by a specific or particular theory of underlying operation, it is believed that dilution of an oxidizer stream with the products of combustion, as realized hereby, leads to a reduction of the oxygen concentration in the combustion zone and as a result reduces flame temperature and consequently reduces NOx formation via the thermal mechanism, one of three or four mechanisms typically associated with NOx formation in combustion processes. In the thermal mechanism, the amount of NOx formation increases with temperature and increases with oxygen concentration. The subject approach can enable significantly reduced NOx emissions without unnecessarily sacrificing the simplicity, low cost, reliability, and safety of processes associated with the utilization of selected designs of burners, such as ribbon burners, for example.

(8) Turning to FIG. 1, there is shown a simplified schematic of a system, generally designated by the reference numeral 10, for reducing nitrogen oxide emissions from a burner in accordance with one embodiment of the invention. The system 10 includes one of more ribbon burners 12 equipped or provided with an appropriate oxidizer from an oxidizer supply 14 and an appropriate fuel from a fuel supply 16. Oxidizer from the oxidizer supply 14 is fed to the burner 12 via a line 20. Fuel from the fuel supply 16 is fed to the burner 12 by the line 24. As shown, the oxidizer and fuel can be premixed prior to introduction into or to the burner 12 such as via a mixing venturi 26, with the oxidizer-fuel mixture being fed via a line 30 to the burner 12.

(9) In one preferred embodiment, the oxidizer can appropriately be air and the fuel can appropriately be natural gas. Those skilled in the art and guided by the teaching herein provide will, however, understand and appreciate that the broader practice of the invention is not necessarily so limited as for example, other fuel materials, particularly gaseous fuel materials such as propane, and other oxidizer media such as oxygen enriched air and oxygen, for example, can be used.

(10) Typically, a combustion air fan is used to supply primary air to the mixing venturi 26 where it is mixed with the fuel (usually natural gas and sometimes propane).

(11) The system 10 is also provided with a flue gas recovery apparatus 40 such as including or having an exhaust hood 42 and an associated exhaust duct 44 whereby flue gas formed upon operation of the burner 12 can be appropriately handled.

(12) As schematically shown in FIG. 1, a portion of the flue gas is passed via a line 50 to a mixing venturi 52 where such flue gas is appropriately mixed with oxidizer (e.g., air) from the line 20, with the flue gas and oxidizer combination being fed via a line 54 to the mixing venture 26.

(13) It has been discovered that supplying higher fractions of combustion oxidizer as primary oxidizer results in reduced NOx emissions from ribbon burners.

(14) It has been found that depending on specific or particular industrial process requirements, ribbon burners can be operated with a PER as high as 2.0, when only 50% of the needed combustion air is supplied as the primary air, or can be operated with a PER as low as 1.0 (i.e., fully aerated). Consequently, because of such process requirements, maximization of the PER cannot always be used as a NOx minimizing technique in industrial settings.

(15) As shown in FIG. 1, where the burner 12 is a partially aerated burner, secondary air can be provided to the burner 12 from a secondary air supply 60. If desired, and as shown, a portion of the flue gas is passed via a line 62 and added to secondary air fed to the burner 12.

(16) A preferred implementation of the described invention requires the inclusion of an appropriate technique or system for admixing a portion of the flue gases from the ribbon burner into the primary and/or secondary air stream(s). For industrial/commercial baking and drying applications, the flue gases are typically already gathered into or via a common flue duct or ducts, aka flue stack(s). Thus, desired implementation of the invention can be realized through the simple inclusion and connection of a branch line to this or these stack(s). For example, a blower can be used to draw a portion of the flue gases out of the stack through the branch line and push the flue gases into the primary and/or secondary air stream(s). Alternatively, an eductor can be used instead of a blower and compressed air (or a pressurized inert gas) can be used to drive the eductor. Another alternative, such as may be appropriate for use in connection the primary air stream and where the fuel is not used to inspirate the primary air, would be to use an eductor with the fuel as the driving force for the eductor. Yet another possible alternative would be to use one blower for both the primary air and a portion of the flue gas, where a tee is installed upstream of the blower and one or more dampers are used to meter the proper ratio of primary air to flue gas. FIG. 1 shows in outline an optional single blower 70 for both primary air and flue gas combustion products (CO.sub.2).

(17) In some applications, such as at least in some direct baking and direct drying operations, water vapor may also be a constituent in the flue gas. The admixing of such water vapor with the oxidizer can and may serve to further reduce NOx emissions such as by cooling or reducing the flame temperature and/or altering process kinetics, for example.

(18) The present invention is described in further detail in connection with the following examples which illustrate or simulate various aspects involved in the practice of the invention. It is to be understood that all changes that come within the spirit of the invention are desired to be protected and thus the invention is not to be construed as limited by these examples.

EXAMPLES

(19) The concept of the invented system has been proven using laboratory facilities. The flue gas recirculation was simulated with CO.sub.2 admixing into the primary air stream using the laboratory experimental arrangement, generally designated by the reference numeral 110, shown in FIG. 2.

(20) As schematically shown in FIG. 2, the system 110 included a ribbon burner 112 with an exhaust hood 114 and exhaust duct 116 in association with the burner 112. The burner 112 was connected to or with a source of fuel, e.g., natural gas 120, via a flow controller 122 and a solenoid valve 124. The burner 112 was also connected to or with a source of primary air 130 and a source of CO.sub.2 131 via respective mass flow controllers 132 and 134, respectively, and a mixer 136. The fuel and the primary air/CO.sub.2 were combined or mixed prior to introduction to the burner 112, as shown. A supply of secondary air, depicted by the arrows 140, was also connected to the burner 112.

(21) The system 110 further included a flame safeguard 170 appropriately interconnected to the fuel supply solenoid 124.

(22) The exhaust from the burner 112 was appropriately interconnected with gas analyzers 180 whereby the exhaust composition was appropriately analyzed and determined.

(23) FIG. 3 depicts reductions of the NOx formation in the ribbon burner's flame with dilution of the primary oxidant supplied to the burner with CO.sub.2, at the burner firing rates of 15,000 BTU/h; 20,000 BTU/h; and 25,000 BTU/h, respectively. The overall oxygen to the fuel ratio was not changed to maintain the complete combustion, but as a result of the dilution the effective concentration of the oxygen in the primary oxidant stream (the mixture of primary air and CO.sub.2) was decreased down to as low as 18.4%.

(24) FIG. 4 shows the effect of the primary oxygen content on NOx reduction at a primary equivalence ratio of 1.053. The reduction of NOx is close to 50% for 25,000 BTU/h (7.3 kW) firing rate and exceeds 50% for the firing rates of 15,000 BTU/h (4.4 kW) and 20,000 BTU/h (5.9 kW).

(25) Thus, the invention provides methods and systems such that desired performance of partially premixed ribbon type burners can be realized without jeopardizing the simplicity, cost, and reliability for which ribbon burners are well known. Thus, the invention allows industrial companies to meet new stricter regulations such as those further limiting NOx emissions without requiring expensive modification or replacement of classic or customary ribbon burners.

(26) The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.

(27) While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.