CA2036612C - Apparatus and method for reducing nox emissions - Google Patents

Abstract

Apparatus and method for reducing NOX emissions from furnace flue gas are provided in which a stream of natural gas or other fluid fuel which has little or no fixed nitrogen is introduced into the upper portion of the furnace. The fuel stream has sufficient turbulence to form eddies surrounded by the flue gas so that the fuel reacts with the nitrogen oxides in the flue gas to form ammonia-like and cyanide-like compounds and nitrogen gas. The ammonia-like and cyanide-like compounds react with additional amounts of nitrogen oxides in the flue gas to form nitrogen gas, water vapor and carbon dioxide.
In this manner, the amount of nitrogen oxides in the flue gas is reduced. The invention can be easily applied to new furnaces or retrofitted on existing furnaces.

Description

ji, f"~ J ~ ~, 2 TITLE

APPARATUS AND ~h ~ r..~D FOR R~DUCING
NO~ ~.~T.~STONS

BACKGROUND OF THE INVENTION

Field of the Invention The presen~ invention relates to an apparatus and method for in-furnace reduction of nitrogen oxide emissions in flue gas.

Description of the Prior Art In the combustion of fuels with fixed nitrogen such as coal, oxygen from the air ma~ combine with the.
nitrogen to produce nitrogen oxide NO or nitrogen dioxide NO2 together called nitrogen oxides or NOX. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. Production of nitrogen oxides is regarded as undesirableO There are numerous government regulations which limit the amount of nitrogen oxides which may be emitted ~rom a combustion furnace. Furthermore, the presence of nitrogen oxides in a furnace flue gas causes the condensed gases to become more corrosive and acidic. Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions in the furnace flue gas.

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Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions in a furnace flue gas. One such approach is a process known as in-furnace NOX reduction, reburning, or fuel staging. In reburning, coal, oil, or gas is injected above the normal flame zone to form a fuel-rich zone. In this zone, part of the nitrog~n oxides are reduced to a~monia-like and cyanide-like fragments which are then oxidized to form N2 and nitrogen oxide.
Several problems occur when this process is used: first, coal may be an inefficient reburn fuel because o~ its high fixed-nitrogen composition. The fixed nitrogen introduced at this location in the ~urnace will have less chance of being converted to N2, and therefore have a higher chance of ending up as nitrogen oxide and may, depending on the nitrogen oxide concentration of the flue gas, increase the emissions of nitrogen oxides.
Furthermore, the fuel must be injected with a sufficient volume of gas. I~ air is used as this gas, there must be enough fuel to consume the oxygen in the flue gas and air, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as rehurn fuel. Furthermore, the necessity of using carrier air requires extensi~e duct work in ~he upper part of the furnace.

Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made so as to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. The exit temperature of the furnace must be limited in order to preserve the heat exchanger's 10 surface. Therefore, a tall furnace is required to complete this second stage process.
Moreover, the fuel must be injected in such ~uantities as to make the upper furnace zone fuel-rich.
This fuel is supplied in excess of the amount of air in 15 the ~urnace and ultimately requires more air in order to be completely combusted. Thus, air must be injected above the reburn fuel injection. This requires even more duct work and furnace volume.
Finally, most coal furnaces which are now in 20 operation are not designed to accommodate the prior art methods. Major modifications such as the provision of extensive duct work and the addition of a second stage to the process are required to utilize the prior art methods. Such retrofitting is expensive. Conse~uently, 25 there is a need for a combustion apparatus and process fb ~ , r~ ~ J,~

which will reduce NOX emissions in flue gas and which can be readily used in existing furnaces.
In United States Patent 4,779,545, a reburn process wherein natural gas is introduced into the upper furnace through pulse combustors is described. The patent teaches that the natural gas must be injected in pulses to achieve NOX reduction. This process does not require any make-up air to reduce NO~ emissions.
However, it does re~uire the expense of obtaining and 10 operating pulse combustors. Therefore, there is a need for an improved process for in-furnace reduction of nitrogen oxides which can be implemented at low cost.

SUMMA~Y OF TH~. lNV .ll~W
In accordance with the present invention, there is provided an improved apparatus and process for reducing the nitroyen oxide emissions in furnace flue gas. A stream of combustible fluid such as natural gas is introduced into the upper furnace through pipes, nozzles, orifices, or other suitable devices. The stream 20 must have sufficient turbulence to break apart as it enters the furnace. The stream disperses and mixes with combustion products and forms fuel-rich eddies therewith.
In these fuel-rich eddies the nitrogen oxides formed in the coal burner will be reduced to ammonia-like and 25 cyanide-like fragments and N2. As these eddies decay and _5_ ~i3 ,,~

mix with more of the flue gas, they encounter an oxidizing environment and more NOX reduction is caused by the ammonia-like and cyanide-like compounds reacting with more of the nitrogen oxides to form N2 and water. The excess fuel and reduced nitrogen fragments react with oxygen. As a result, the nitrogen oxides in the air-rich flue gas and the ammonia-like and cyanide-like fragments in the fuel-rich eddies are converted to N2 at the same time the combustion of the natural gas is completed.
Because of the simplicity of this system, it is ideal for retrofitting existing coal furnaces. Because the process relies on controlled mixing to provide fuel-rich and then air-rich environments, there is no need for an air addition stage. Because gas burns more rapidly at a lower temperature than coal, the fuel can be introduced at a higher elevation and lower temperature. Lower temperature acts to reduce the e~uilibrium level of nitrogen oxide in the flue gas and, hence, increases the NOX reduction possible. Finally, duct work is not necessary for injection air nor for completion air. As a consequence, the cost of reducing the NOX emissions in the flue gas is greatly reduced. Other objects and advantages of the invention will become apparent as a description of the preferred embodiments proceeds.

BRIEF D~S~~ ON OF TH~ DRAWINGS
Figure 1 is a schematic drawing of a furnace having our apparatus for reducing nitrogen oxide emissions in accordance with the principles of the present invention.
Figure 2 is a perspecl:ive view partially in section of a pipe having an annular opening which can be used in our apparatus.
Figure 3 is a perspective view partially in section of a pipe having a restricted orific0 which can be used in our apparatus.
Figure 4 is a perspective view partially in section of a pipe having a converging and diverging nozzle which can be used in our apparatus.
Figure 5 is a perspective view partially in section of a pipe having a Helmholtz Resonator which can be used in our apparatus.
Figure 6 is a perspective view partially in section of a pipe having a reed type vibrator which can be used in our apparatus.
Figure 7 is a perspective view partially in section of a pipe having an acoustical vibration generator which can be used in our apparatus.
Figure 8 is a perspective view partially in section of a pipe having a fluidic oscillator which can be used in our apparatus.

Figure 9 is a perspective view of a Y-type pipe which can be used in our apparatus.
Figure 10 is a perspective view of a three pipe arrangement which can be usecl in our apparatus.

~ ON OF T~ ~KK~U E~B~D~ENT
As shown in Figure 1 our improved apparatus for reducing nitrogen oxide emissions in ~urnace flue gas 10 can be readily retrofitted to an existing furnace 12.
The furnace lZ is designed to consume coal or any other fuel. The stream of fuel enters the furnace 12 by way of fuel entries 13, 14 and 15 that pass through furnace wall 11 and which are located in the lower portion of the furnace 12. It burns in primary combustion zone 16 which typically has a temperature above 3000~F. Flue 18 provides an exit for the flue gas which is created in primary combustion zone 16 durin~ the combustion of the fuel. The flue gas has a temperature in the range of 2100~F to 2400~F when it passes from the furnace to contact the heat exchangers 20. Those heat exchangers 20 which are in the upper portion of the furnace cause a rapid temperature drop of the flue gas. During the combustion of the fuel, some of the fixed nitrogen reacts with oxygen to form nitrogen oxide, and nitrogen oxide is also formed from atmospheric nitrogen and oxygen.

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We provide fuel injection apparatus 22 and 23 to introduce a stream of natural gas or other suitable fuel in a manner so that the gas will form fuel-rich eddies surrounded by flue gas. These eddies and flue gas react to reduce the nitrogen oxide emissions in the flue gas.
The fuel injection apparatus 22 and 23 introduce streams of natural gas or other fuel having little or no fixed nitrogen content. As the stream enters the furnace it disperses through the flue gas to form fuel-rich eddies 24 an~ 25 in the upper portions of the furnace 12 above the primary combustion zone 16. These eddies are surrounded by flue gas.
The stream of fuel entering through injectors 22 and 23 must have the correct internal turbulence to form eddies. Some sources of fuel such as low ~TU gas may have suf f icient volume and velocity to provide a stream having such internal turbulence so a common plpe 22 and 23 can-be used as injectors. However, we prefer to use injectors of the types shown in Figures 2 through 10.
They are designed to induce eddies in the gas stream.
As shown in Figure 2 we may provide a pipe 26 having a solid core 28 and annular opening 27 therearound pipe 26 passes through furnace wall 11. This type of injector produces a stream which will disperse as it exits pipe 26 and enters the furnace. In Figure 3 we show yet another pipe 29 having a restricted orifice 30 _ 9 _ ~ J

passing through wall 11 of furnace 12. Yet another way to produce a turbulent stream is shown in Figure 4 wherein pipe 31 has a convergent/divergent nozzle 3~. In Figure 5 we show pipe 33 having a Helmholtz Resonator 34 attached thereto. This resonator will induce turbulence in the fuel stream passing through pipe 33. Yet another way to induce turbulence is to use a reed type vibrator 36 shown in Figure 6 on pipe 35. In the embodiment of Figure 7 an acoustical vibxation generator 38 is attached to pipe 37 to induce turbulence into the fuel stream passing through that pipe. Still another way to create a turbulent stream is to use a fluidic oscillator of the type shown in Figure 8. The stream flowing in pipe 39 impinges upon diverter 40 causing the stream to pass out through outlets 42 or 43. A feed back fluid line 41 is also provided in this type of oscillator. The arrangement causes the stream to alternate hetween outlet 42 and outlet 43. In Figure 9 we show a Y-type injector 41 having an upper entry pipe 45 through which air or flue gas is directed and the lower entry pipe 46 through which fuel is directed. As these two streams combine in pipe 44 they have sufficient turbulence so as to break apart upon entry into the furnace. In Figure 10 we show yet another way of inducing sufficient turbulence in a stream as it enters the furnace. There we provide a pipe 47 through which natural gas or other suitable fuel is injected. Above pipe 47 we provide injection pipe 48 and below pipe 47 yet another injector pipe 49 which pipes 48 and 49 are used to inject air or fuel gas~ This arrangement also induces turbulence in the fuel stream entering through pipe 47 so that it will break apart and form eddies in accordance wit:h the present invention.
The injectors introcluce fuel and little if any air so that the eddies are formed primarily from the fuel and induced combustion products~ Other fluid fuels which usually contain li~tle fixed nitrogen include those o~
the general forms CXHy or CXHyOz. The eddies initially equilibrate in a fuel-rich condition and then mix with more flue gas containing some excess ~2' to complete the oxidation of the fuel~
The natural gas eddy, as it begins to burn, reacts with a portion of the nitrogen oxide in the flue gas to form molecular nitrogen, N2, ammonia-like fragments, NHi and cyanide-like fragments HjNC:
(1) CH4 + NOX ~~> N2 ~ NHi As the eddy of fuel and flue gas combusts, the cyanide-like fragments and ammonia-like compounds react with additional nitrogen oxide in the flue gas to form N2 and water vapor:
~2) HjCN ~ NO --> N2 + H2O
(3) 2N~i + NO --> N2 + H2O

The above equations characterize the process, but do not comprise all reactions, pathways and intermediate products which may occur.
We induce the eddies of natural gas and combustion products in the upper portion of the furnace so that the eddies do not interfere with the primary combustion of the coal taking place in the furnace below.
Because natural gas, which burns more readily and rapidly than coal, is used as fuel, it can be introduced at a level in the furnace where the temperature is as low as 2100~F to 2400~F. Since this is the desired exit temperature of the flue gas from the furnace, our gas injectors 22 and 23 can be located near thP furnace exit.
No second stage air addition to the furnace is re~uired.
This lower reaction temperature also reduces the temperature-dependent equilibrium level of nitrogen oxide and allows greater reduction of nitrogen oxide.
Our fuel injectors do not require any air.
Because they generate eddies which educ~ furnace gases, ; 20 there is no need for an air duct to bring pressurized air to these ~uel injectors. Since no duct work is needed to ; carry the air to the upper portions of the ~urnace 12, a major retxofitting problem, which is especially acute for those furnaces which have no space to accommodate any duct work, has been eliminated.

-12- ~J --The eddies of natural gas reduce the amount of nitrogen oxide in the flue gas in four ways. First, the natural gas does not contain any fixed nitrogen.
Consequently, unlike a fuel c:ont~in;ng fixed nitrogen, the combustion of natural gas creates very little additional nitrogen oxide D ';econd, the natural gas reduces the amount of nitrogen oxide in the ~lue gas directly by the chemical reactions set forth in equations ~1), (2) and (3) above. Third, the natural gas also reduces the amount of nitrogen oxide by consuming the excess oxygen in the flue gas. The reduction in the amount of oxygen in the flue gas reduces the equilibrium level of nitrogen oxide in that gas. Finally, since the natural gas is introduced at a higher level in the furnace where the temperature is lower, the equilibrium level of nitrogen oxide is lower, allowing for more complete reduction. In this manner, our injectors 22 and 23 provide effective reduction of nitrogen oxide in the combustion products.
In addition to providing a suitable reduction in the amount of nitrogen oxide in the flue gas, our invention is cost-effective as a retrofit to existing coal furnaces. No additional duct work is necessary for our natural gas injectors 22 and 23. Furthermore, our fuel injectors can be placed near the furnace exit and still operate at a proper operating temperature, -13~ 2 eliminating the need for second stage air addition to the furnace. Finally, our system is so simple that it can be inexpensively applied to retrofit any fossil fuel fired furnace currently in use.
A further advantage of this invention is the use of flue gas rather than air as the oxidizing fluid in the eddies. This improvement reduces the amount of natural gas to be used in order to reach the desired air/fuel ratio, since no air is introduced through the fuel injectors. This absence has the additional advantage of reducing the gas flow per unit energy released, through the convective passes, producing a richer eddy, and directly reducing some of the flue gas nitrogen oxide in the fuel-rich eddy.
While we have shown and described a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same, it is to be distinctly understood that the invention is not limited thereto, but ma~ be otherwise variously embodied and practices within the scope of the following claims.

Claims (17)

1. An improved apparatus for reducing NOX in flue gas in a furnace wherein a fuel is burned in a primary combustion zone to produce a flue gas containing nitrogen oxide, wherein the improvement comprises at least one fuel introducing device attached to said furnace above its primary combustion zone, which device introduces into combustion gases a stream of fluid fuel having sufficient turbulence so that the stream forms fuel-rich eddies surrounded by flue gas, the fluid fuel being selected from the group of fluids consisting of natural gas, CxHy compounds and CxHyOz compounds.

2. The apparatus of claim 1 wherein said fuel introducing device is positioned to introduce said fluid into a region of said furnace wherein the flue gas is at a temperature in the range of 2100°F to 2400°F.

3. The apparatus of claim 1 wherein the injector is sized to produce eddies in sufficient numbers and sizes to promote a reaction of the fluid with a first portion of said nitrogen oxide in said flue gas to form ammonia-like and cyanide-like compounds, N2, water and carbon dioxide and said ammonia-like and cyanide-like compounds further react with a second portion of said nitrogen oxide and said flue gas to form N2, water and carbon dioxide.

4. The apparatus of claim 1 wherein the fuel is introduced into the furnace through a pipe.

5. The apparatus of claim 1 wherein the fuel is introduced into the furnace through a fuel introducing device having an annular opening.

6. The apparatus of claim 1 wherein the fuel is introduced into the furnace through a fuel introducing device having a restricted orifice.

7. The apparatus of claim 1 wherein the fuel is introduced into the furnace through a fuel introducing device having a convergent-divergent nozzle.

8. The apparatus of claim 1 wherein the fuel is introduced through a fuel introducing device having a harmonic resonator.

9. The apparatus of claim 1 wherein the fuel is introduced through a fuel introducing device having a reed type vibrator to produce regular wake shedding.

10. The apparatus of claim 1 wherein the fuel is introduced through a fuel introducing device having an acoustical vibration generator.

11. The apparatus in claim 1 wherein the fuel is introduced through a fuel introducing device which employs a fluidic oscillator to generate discrete eddies.

12. The apparatus of claim 1 wherein the fuel introduced in the furnace above its primary combustion zone is premixed with at least some of the air or flue gas.

13. The apparatus of claim 12 wherein at least some of the air or flue gas is introduced adjacent to the introduction points of the fuel in the furnace above its primary combustion zone.

14. A method of reducing nitrogen oxides in flue gas comprising the step of:
injecting a stream of fluid into said flue gas which stream has sufficient turbulence so as to produce eddies surrounded by flue gas, said fluid being selected from the group of fluids consisting of natural gas, CxHy compounds, CxHyOz compounds and mixtures primarily of these compounds, and being injected into the flue gas in quantities sufficient to promote a reaction between the nitrogen oxides in the flue gas and the fluid to form ammonia-like and cyanide-like compounds and N2 and to promote a secondary reaction of said ammonia-like and cyanide-like compounds and additional nitrogen oxides from the flue gas to form N2, water and carbon dioxide.

15. The method of claim 14 wherein the flue gas has a temperature within the range of 2100° F to 2400°F.

16. A method for reducing nitrogen oxides in flue gas comprising the step of:
injecting into said flue gas a mixture formed by the reaction between flue gas and/or fluid selected from the group of compounds consisting of CxHy compounds, CxHyOz compounds, natural gas and mixtures primarily of those compounds in quantities sufficient to promote a reaction between the nitrogen oxides in the flue gas and the fluid to form ammonia-like and cyanide-like compounds and N2 and a secondary reaction of said ammonia-like and cyanide-like compounds and additional nitrogen oxide from the flue gas to form N2, water and carbon dioxide.

17. The method of claim 16 wherein the flue gas has a temperature within the range of 2100°F to 2400°F.