CA2004907A1 - Method of combustion for reducing the formation of nitrogen oxides during combustion and an apparatus for applying the method - Google Patents
Method of combustion for reducing the formation of nitrogen oxides during combustion and an apparatus for applying the methodInfo
- Publication number
- CA2004907A1 CA2004907A1 CA002004907A CA2004907A CA2004907A1 CA 2004907 A1 CA2004907 A1 CA 2004907A1 CA 002004907 A CA002004907 A CA 002004907A CA 2004907 A CA2004907 A CA 2004907A CA 2004907 A1 CA2004907 A1 CA 2004907A1
- Authority
- CA
- Canada
- Prior art keywords
- air
- combustion
- gas
- reducing
- mixed
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 238000002485 combustion reaction Methods 0.000 title claims abstract description 82
- MWUXSHHQAYIFBG-UHFFFAOYSA-N nitrogen oxide Inorganic materials O=[N] MWUXSHHQAYIFBG-UHFFFAOYSA-N 0.000 title claims abstract description 72
- 230000001603 reducing effect Effects 0.000 title claims abstract description 39
- 238000000034 method Methods 0.000 title claims abstract description 33
- 230000015572 biosynthetic process Effects 0.000 title claims abstract description 28
- 239000007789 gas Substances 0.000 claims abstract description 59
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 claims abstract description 36
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 32
- 239000001301 oxygen Substances 0.000 claims abstract description 32
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 32
- 239000003546 flue gas Substances 0.000 claims abstract description 30
- 239000003638 chemical reducing agent Substances 0.000 claims abstract description 16
- 238000005755 formation reaction Methods 0.000 claims description 26
- 239000000203 mixture Substances 0.000 claims description 24
- 239000000446 fuel Substances 0.000 claims description 17
- 238000002156 mixing Methods 0.000 claims description 7
- 229960003903 oxygen Drugs 0.000 claims description 3
- 238000001816 cooling Methods 0.000 claims description 2
- 239000000567 combustion gas Substances 0.000 claims 1
- 235000019391 nitrogen oxide Nutrition 0.000 description 20
- 229910002089 NOx Inorganic materials 0.000 description 15
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 8
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 8
- 229910002091 carbon monoxide Inorganic materials 0.000 description 6
- 230000003247 decreasing effect Effects 0.000 description 5
- 229910052757 nitrogen Inorganic materials 0.000 description 5
- 238000006243 chemical reaction Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 229960005419 nitrogen Drugs 0.000 description 4
- 238000004064 recycling Methods 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 238000010521 absorption reaction Methods 0.000 description 3
- 239000003054 catalyst Substances 0.000 description 3
- 239000006096 absorbing agent Substances 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
- 239000001257 hydrogen Substances 0.000 description 2
- 229910052739 hydrogen Inorganic materials 0.000 description 2
- 125000004435 hydrogen atom Chemical class [H]* 0.000 description 2
- JCXJVPUVTGWSNB-UHFFFAOYSA-N nitrogen dioxide Inorganic materials O=[N]=O JCXJVPUVTGWSNB-UHFFFAOYSA-N 0.000 description 2
- MGWGWNFMUOTEHG-UHFFFAOYSA-N 4-(3,5-dimethylphenyl)-1,3-thiazol-2-amine Chemical compound CC1=CC(C)=CC(C=2N=C(N)SC=2)=C1 MGWGWNFMUOTEHG-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 206010037660 Pyrexia Diseases 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 239000005864 Sulphur Substances 0.000 description 1
- 238000007792 addition Methods 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 206010022000 influenza Diseases 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000013021 overheating Methods 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 239000010970 precious metal Substances 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 239000010913 used oil Substances 0.000 description 1
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN A CARRIER GAS OR AIR
- F23C6/00—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion
- F23C6/04—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection
Abstract
Abstract A method of combustion and an apparatus for reducing the formation of nitrogen oxides during reducing com-bustion, particular in the flame. In the method, the oxygen-containing gas required for the combustion contains elementary oxygen less than atmospheric air.
The gas consists of air and a low-oxygen-content or non-oxygen-containing gas containing reducing agents, preferably of separately cooled flue gas obtained from the reducing combustion space. The apparatus comprises at least one gas flue (6) through which flue gas containing reducing agents from the reducing combustion is passed via a cooler (9) into an air mixer (8) in which it is mixed with the primary air to be introduced into the boiler.
The gas consists of air and a low-oxygen-content or non-oxygen-containing gas containing reducing agents, preferably of separately cooled flue gas obtained from the reducing combustion space. The apparatus comprises at least one gas flue (6) through which flue gas containing reducing agents from the reducing combustion is passed via a cooler (9) into an air mixer (8) in which it is mixed with the primary air to be introduced into the boiler.
Description
0~ 7 A method of combus-tion for ~educing the formation of nitrogen oxides during combustion and an apparatus for applying the method The invention relates to a method of combustion for reducing the formation of nitrogen oxides during combustion, wherein air required for the combustion of a fuel is introduced in at least two steps, the air being introduced understoi hiometrically in the first step, preferably with an air coefficient ranging from 0.80 to 0.95, and a gas or gas mixture substantially free from elementary oxygen is mixed with the air to be introduced into the first step.
The invention is also concerned with an ap-paratus for applying the method comprising means for introducing air into a furnace, means for introducing fuel into the furnace, and means for mixing a gas or gas mixture containing less oxygen than air with the air to be introduced into the first understoichio-metric combustion step before the air is introduced into tha furnace.
All combustion processes produce nitrogen oxides when the nitrogen of both air and fuel com-bines with oxygen to form oxides of different kinds.
In the reducing flame, N0x is derived mainly from the nitrogen of the fuel through rapid formation, that is, so called prompt N0x is obtained. At high tem-peratures, mostly nitrogen oxide (N0) is obtained.
When ~the temperature drops, N0 is easily co~verted into the other nitrogen oxides in the presence of oxygen, mainly into nitrogen dioxide (N02). The for-mation of nitrogen oxides occurs at a rapid reaction rate as soon as the required chemical equilibrium conditions are met, i.e., mainly at high temperature and in the presence of oxygen. If the equilibrium .~ . .
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conditions are altered after the formation of nitro-gen oxi~es so as to cause decomposition of the nitro-gen oxides, the reaction cate o the decomposing pro-cess is very slow, the clecomposing requiring mainly time, catalysts or additional chemicals. From the environmental point of view, nitrogen oxides are highly disadvantageous. They are formed abundantly in industrial processes as well as in power plants and other boiler works, and one of the most important ob-jectives of environmental protection is to reduce NOx emissions to the atmosphere.
In an attempt to reduce NOx emissions, nitrogen oxides are converted into another form in various ways. Such techniques include various reduction methods based on the use of catalysts, and the use of absorbing agents for simultaneous absorption of sulphur and nitrogen oxides in various ~ays. These methods involve various problems difficult to solve, such as the high price and difficult availability of precious metals used as catalysts and the poor absorption properties of ~he absorbing agents. More-over, it is often difficult to dimension the ap-paratus when applying absorption methods due to variation in boiler capacities and other such factors.
Technically, it is more advantageous to try to prevent the formation of nitrogen oxides during the combustion step instead of removing them. For this purpose, a variety of low NOx burners have been de-veloped, and attemp-ts have been made to carry out -the combustion in a pressurized space, in addition to which supply of air into the boiler has been carried ~ out in stages before the superheaters. Contrary to i expectations, these methods, however, have not pro-~ vided any particularly good results, because in prac-i ~00~9~7 tice the operation of -the methods has been prevented or substantially deteriorated by such factors as variation in the formation conditions of nitrogen ox-ides, reaction kinetics, operational conditions of boilers and variation occurring therein. Furthermore, removal of nitrogen oxides has been attempted by means of circulation bed furnaces operating at very low temperatures (about 800C), that is, at condi-tions disadvantageous for the formation of NOx. This, however, has deteriorated the efficiency of the fur-naces as well as their ability to burn different kinds of fuel as it has been necessary to drop the temperature as low as near the minimum temperature required for continuously maintaining the combustion process. The methods described above are widely known and therefore will not be described more closely (Finnish Ministry of Trade and Industry/Energy De-partment D:140, Helsinki 1987).
DE Offenlegungsschrift 30 40 830 discloses a method ln which completely combusted cooled flue gas obtained from a gas flue after the boiler is mixed with the air to be introduced into the under-stoichiometric first combustion zone in order to re-duce the amount of nitrogen oxides. Even though this .,: ~ ,..
method helps to prevent the formation of nitrogen oxides to some extent, it does not enable sufficient control of the amount of nitrogen oxides. In addi-tion, the recycling of the flue gas increases the mass flow of gas flowing through the boiler, thus re-~uiring a somewhat larger combustion space and larger conduits everywhere in the boiler.
The NO content is usually low within the re-ducing area, depending on the reducing effect of hydrogen (H2) and carbon monoxide (C02). These sub-stances decompose the possibly formed NO roughly ac-1. '""'.'-.'' ' z~
cording to the following reactions:
NO ~ CO -> 1/2 N2 + C2 NO + H2 -~ 1/2 N2 ~ H2O
In combustion carried ou-t understoichiometric-ally in a manner known per se, the NO concentra-tion can, in principle, be kept on a low level. Problems occur only when the conditions become reducing or when very high temperatures occur, that is, over 1500C. The problems result even ~rom a minor excess of air, which under furnace conditions causes rapid formation of NO, or from very high temperatures (over 1500C) at which H2 and CO cannot any more prevent the formation of NO due to their reduced reducing po-tential. In prior art apparatuses such situations occur particularly in the primary flame but also in connection with the introduction of secondary and tertiary air. One of the most important reasons for the formation of NO in the primary flame of prior art apparatuses is that the heterogeneous flame contains, e.g., oil drops or carbon particles and, as a conse-quence, there occurs high concentration gradients of oxygen and burning gases as well as high temperature gradients. Thereby it is always possible that minor temperature peaks occur locally at phase boundaries, for instance, if the amount of oxygen at such a point is stoichiometric or slightly overstoichiometric. In a typical combustion apparatus, the temperature may rise instantaneously and locally up to about 2000C.
As a result, the local NO concentration rises rapidly up to about 3500 ppm (prompt NO~). The NO so formed will not decompose to any greater degree under boiler conditions. Accordingly, it is obvious that even minor locally and instantaneously occurring tempera-ture peaks increase rapidly the average NO value of exhaust gas, which should remain on a concen-tration ~0~345~
level of about 100 ppm.
The object of the present invention is to pro-vide a method by means of which the formation of ~x during a reducing combustion step, usually in so called primary combustion, particularly in the flame, can be minimized and in which conditions prerequisite for the formation of NOx are prevented without any complicated apparatuses. Removal of NOx after combus-tion is not required. The method is characterized in that a gas or gas mixture containing reducing agents, such as H2 and CO, is mixed with the air to be intro-duced into the first step, that the oxygen content of the gas mixture introduced into the first step is preferably 12-19%, and that the oxygen content and reducing potential of the air mixture to be intro-duced are adjus~ed so that the nitrogen oxide con-centration of flue gas from combustion carried out at the adiabatic combustion temperature of the fuel used, corresponding to the supplied oxygen content and reducing potential, is no more than a prede-termined concentration value.
The basic idea of the invention is that air is introduced into the combustion process in such a man-; ~:
ner that the NOx formation in the reducing part ofthe furnace, particularly in the difficultly con-trollable flame, remains on a sufficiently low level at all temperatures and oxygen/fuel ratios possibly occurring during this combustion step. This is achieved by carrying out the combustion under re-ducing conditions by using a gas or a gas mixture having an oxygen content lower than that of ordinary air and containing reducing agents. By means of the method according to the invention, the concentration of nitrogen oxides can be controlled so that the equilibrium concentration of the nitrogen oxides in ,,"~.
Z0~9637 : :
the flue gas, in practice, also the maximum con-centration, remains all the time at a very low value.
A further object of the invention is to provide an apparatus for applying the method. The apparatus is characterized in -that the mixing means comprise at least one gas flue for passing part of the flue gas from the first combustion step into the air to be introduced into the first step to be mixed with it.
The basic idea of the apparatus of the inven-tion is that the reducing gas or gas mixture, that is, non-oxygen-containing or low-oxygen-content gas containing reducing agents, and air are mixed thoroughly with each other and introduced at least into the boiler zone in which fuel and air are nor-mally poorly mixed with each other so that local tem-perature peaks are likely to occur. Typically, this zone is the reducing combustion zone of the boiler, mainly the flame.
The invention will be described in greater de-tail in the attached drawings, wherein Figure 1 illustrates the interdependence of the ;
temperature and the air coefficient (ratio of oxygen to the amount of theoretical oxygen required for the combustion irrespective of the other components pre-sent in the gas mixture, such as inert components and reducing agents) in a prior art application with re-spect to a predetermined NO concentration level when the combustion is carried out normally with air, and the interdependence of the adiabatic temperature and the air coefficient in a typical oil combustion pro-cess carried out normally with air and with a mixture of air and gas consisting of completely combusted flue gas from a boiler, the mixture having an oxygen content of 17~ (see DE Patent Application 3 040 830);
r igure 2 illustrates by way of example the , ':
;~;(3~14907 ~`
interdependence of the maximal NO amount obtained in the combustion of pure methane (CH4) and the air co-efficient when the gas maintaining the combustion is air, a mixture of completely combusted flue gas and air, as disclosed in Figure 1, or a mixture of cooled gas recycled from the reducing combustion and air;
and Figure 3 is a schematic view of an apparatus for applying the method of the invention.
In Figure 1, the curve A-B shows by way of example the adiabatic combustion temperature of a widely used oil type as a function of the air coeffi~
cient when the combustion is carried out normally with air. The curve C-D illustrates by way of example the adiabatic combustion temperature of the same oil type as a function of the air coefficient when the combustion is carried out with air diluted with com~
pletely combusted flue gas, the oxygen content of the mixture being 17%. The curve E-F shows by way of example the pairs of temperature and air coefficient values corresponding to the N0 concentration 100 ppm when the combustion is carried out normally with air.
Above the curve the N0 concentration is more than 100 ppm. Very high temperatures (over 1500C) in par~
ticular are important for the invention. As the local temperatures in the hottest portions of the flame may rise very close to the adiabatic temperature, it is to be seen from the figure that the N0 concentration of 100 ppm (point G) can be achieved with an air co-efficient as low as 0.82 when the combustion is carried out normally with air. When using air diluted with completely combusted flue gas, the concentration of 100 ppm is achieved with an air coefficient of 0.93 (point H). At worst, the maximum N0 concentra-tion within the reducing area is about 2700 ppm when .
. . ~.
- ;~0~49~37 :-the combustion is carried out normally with air and only ~00 ppm when the co~bustion is carried out with diluted air as disclosed in the example. The first-mentioned value is represented by point I and the last-mentioned by point J in Figure 1.
It has now been found unexpectedly that the NO
formati~n can be prevented within the reducing area of the burner, particularly in the flame, when oxy-gen required by the combustion is introduced under-stoichiometrically and at an uniform oxygen content of less than 21% by mixing a gas or gas mixture con-taining considerable amounts of reducing agents with the combustion air. In this way the combustion tem-perature, particularly that of the flame, can be de-creased, simultaneously increasing reducing potential so that abundant formation of NOx is no more possible, not even locally or instantaneously. This preferably takes place so that the local temperature peak of the flame does not exceed about 1500C and the oxygen concentration is reduced by means of cool-ed flue gas recycled from the reducing combustion step, typically containing hydrogen (H2) and carbon monoxide (CO). The formation of NOx is efficiently prevented both by the temperature drop and the in-crease in the reducing potential.
In Figure 2, the curve K-L illustrates the maximum NO concentration obtained in the combustion of pure methane (CH4) when the combustion is carried out normally with air and the burning takesi place adiabatically. The curve M-N illustrates the maximum NO concentration when completely combusted flue gas has been mixed with the combustion air. The curve 0-N, in turn, illustrates an example where the oxygen content of the combustion air has been decreased by adding to it properly cooled flue gas from the re-,. . .
~0~9~
g ducing step, operated wit:h the same air coefficient,in an amount of 24% on the volume flow of the primary air, whereby reducing agents H2 and C0 will also be recycled to a considerable degree. It clearly appears from the figure that the recycling of reducing gases decreases greatly the N0 concentration while the tem~
perature is decreased and the reducing potential in-creased. For instance, the maximum decrease in N0 concentxation with the air coefficient 0.80 is as great as 97% with the air coefficient 0.80 as com-pared with combustion with air, that is, the con-centration drops from 0.048 mol N0/kg CH4 (point P in Figure 2) to 0.0012 mol N0/kg CH4 (point Q in Figure 2) and about 73~ on the value obtained when complete-ly combusted flue gas is mixed with the combustion air (corresponding to the ratio between Q and R).
When the method of the invention is applied, the oxy-gen concentration as well as the amount and reduction capability of the reducing agents, that is, their reducing potential, can be adjusted in a desired man-ner according to the used fuel and other combustion conditions.
It has been unexpectedly found that the maximum efficacy of the method of the present invention, that is, the greatest decreasa in N0 formation as compared with the prior art, is to be obtained with air co-efficients ranging from 0.80 to 0.95. It is likewise unexpected that the maximum decrease in N0 concentra-tion occurs within the air coefficient range conven-tionally applied in typical prior art primary combus-tion in power plant boilers. Accordingly, the method of the present invention decisively decreases N0x formation in the flame of the burner, that is, the formation of prompt N0x, which has been most diffi-cult if not impossible to prevent in prior art appar-: ", ~
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atuses. When comparing the curves M-N and O-N of Fig-ure 2, it appears that when a gas or gas mixture con-taining reducing agents :is mixed with air according to the invention, the air coefficient 0.95 still gives a NOx concentration (point S) which is about 92~ lower than that obtained by combustion with ordi-nary air (point T) and 40~ lower than the value ob-tained by adding completely combusted flue gas (corresponding to the ratio between points U and S).
Furthermore, the use of completely combusted flue gas increases the amount of gas used, which requires a greater boiler and greater gas flues, whereas in the method of the invention the increased amount of gas and the greater space requirement concern only that part of the boiler in which the reducing combustion takes place. As further appears from Figure 2, the curves M-N and O-N join when the air coeficient is 1, which is due to the fact that flue gas from fuel burned with stoichiometric ratio cannot any more con-tain reducing agents to any greater degree. This, however, is unimportant ~or the end result in combus-tion processes carried out with lower air coeffi-cients. Essential in the invention is that local overheating is prevented in an understoichiometric combustion so that nitrogen oxides will not be form-ed.
Figure 3 shows schematically an apparatus for applying the method of the invention. The apparatus comprises a burner such as a boiler 1 with a furnace 2. Fuel is introduced into the furnace 2 by means of one or more feeding devices 3. Oxygen-containing gas mixture required for the combustion is introduced into the same part of the furnace 2 through a conduit 4 belonging to air supply means. Air is supplied into the conduit 4 through a conduit 5, while reducing Z(~4~7 :
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gas, that is, at least substantially non-oxygen-con-taining gas mixture containing substantial amounts of reducing agents, mainly H2 and C0, is supplied through a conduit 6 belonging to mixing means via a blower 7 and a gas mi~er 8. The gas to be mixed is preferably flue gas deri~ed from the furnace 2, in which raducing combustion takes place. The flue gas, which contains reducing agents, is cooled by means of coolers 9 and 10 and its amount is controlled by means of a valve 11. The reducing flue gas is mixed in the mixer 8 with the air to be introduced into the furnace. A major portion of the flue gas produced during the reducing combustion step i5 passed into subsequent combustion steps, shown schematically with a single combustion step 12. During the subsequent combustion steps, additional air is introduced into the boiler by means of a valve 13 through a conduit 14, whereby the fuel will be combusted as completely as possible. At this stage the 1ue gas can be cooled b~ means of a heat exchanger 15 and thereafter by means o coolers 16, whereafter it is passed through a blower 17 into a gas flue 18. Depending on the re-quired amount of reducing agents, final cooled flue gas can be mixed with the air to be introduced into the furnace 2 during the first step in a manner known per se through a conduit 19 in addition to the flue gas from the reducing step introduced through the conduit 6, the amount of the final flue gas being adjusted by a valve 20. In this way, both the oxygen content of the mixture of air and gas to be intro-duced into the furnace and the concentration of the reducing agents in it can be adjusted according to the combustion conditions and fuel used. If there exists a danger that the flame or a portion of it becomes too hot at the beginning of the reducing com-,' ' :, .' ~'.~.
':~ ` ` ','. '. .- j .' . ~ .,; , . ' :
bustion step 12 with resultant excessive formation of nitrogen oxide, the flame temperature during this step can also be decreased by feeding reducing gas through a valve 21 and a conduit 22 at the beginning of the reducing combustion step 12. Heat losses from the burner can be decreased by insulating the combus--tion chambers by insulations 23 and 24, shown schema-tically in the figure~
I~ is to be understood that some of the devices described above can be combined into a single entity to obtain a structurally more advantageous solution.
For instance, the parts 2, 10, 12, 15 and 16 can be easily combined.
It is essential in the apparatus of the inven-tion that the air and reducing gas are mixed properly before their introduction into the reducing part of the furnace and that the temperature of the flame or a flame portion is decreased only to such an extent as is required for preventing the formation of NO
without any risk of the combustion process being interrupted. In the present method the mix ratio of air and fusl is ~etermined, e.g., by the thermal value of the fuel used, the minimum temperature re-quired for maintaining combustion, the chemical analysis of the gas, the desired NOx level, the di-mensions of the heat surfaces of the boiler, the de-gree of cooling (temperature~ of the recycled gas, and the positions of the gas introduction steps. As a consequence, this ratio may vary widely; typically, the amount of gas is 10 to 70% on the amount of air supplied.
It is obvious that when the same combustion efficiency is to be obtained, the mass volume of the gas used in the apparatus of the invention is greater as compared with prior art apparatuses, though mainly i ,, . . ~ ., , ~ , ., " ,. . .- , . . .
;;~ 9L9~7 ::
only in the reducing part of the boiler. However, the dimensions of the boiler will not change to any greater degree because the recycling preferably takes place only during the understoichiometric combustion step(s) and because increase in the flow volume of gas is for a major part compensated for by change in the gas density caused by the temperature drop. It is also obvious that, theoretically, the recycling of gas does not reduce the efficiency of the boiler;
varying heat losses, however, may resul-t in slightly reduced efficiency. In view of the advantages ob-tained, this drawback is not of any greater im-portance.
The invention has the advantage that the ap-paratus can be constructed by means of well-known inexpensive constructions and no separate expensive means for removing NOx are needed because the forma-tion of NOx has been prevented sufficiently. Further, the method of the invention is easy to realize and very easy to control when its principles are applied to apparatuses and control systems known per se. It is likewise possible to control the high local forma-tion o~ NOx in the hottest, spotlike portions of the flame of the burner because the formation of NOx is so restricted that its concentration cannot exceed a set limit value. The NO concentration of the flue gas emission to the surrounding is, of course, dependent on the operational properties and structure of the oxidizing part of the boiler.
.., ".
The invention is also concerned with an ap-paratus for applying the method comprising means for introducing air into a furnace, means for introducing fuel into the furnace, and means for mixing a gas or gas mixture containing less oxygen than air with the air to be introduced into the first understoichio-metric combustion step before the air is introduced into tha furnace.
All combustion processes produce nitrogen oxides when the nitrogen of both air and fuel com-bines with oxygen to form oxides of different kinds.
In the reducing flame, N0x is derived mainly from the nitrogen of the fuel through rapid formation, that is, so called prompt N0x is obtained. At high tem-peratures, mostly nitrogen oxide (N0) is obtained.
When ~the temperature drops, N0 is easily co~verted into the other nitrogen oxides in the presence of oxygen, mainly into nitrogen dioxide (N02). The for-mation of nitrogen oxides occurs at a rapid reaction rate as soon as the required chemical equilibrium conditions are met, i.e., mainly at high temperature and in the presence of oxygen. If the equilibrium .~ . .
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conditions are altered after the formation of nitro-gen oxi~es so as to cause decomposition of the nitro-gen oxides, the reaction cate o the decomposing pro-cess is very slow, the clecomposing requiring mainly time, catalysts or additional chemicals. From the environmental point of view, nitrogen oxides are highly disadvantageous. They are formed abundantly in industrial processes as well as in power plants and other boiler works, and one of the most important ob-jectives of environmental protection is to reduce NOx emissions to the atmosphere.
In an attempt to reduce NOx emissions, nitrogen oxides are converted into another form in various ways. Such techniques include various reduction methods based on the use of catalysts, and the use of absorbing agents for simultaneous absorption of sulphur and nitrogen oxides in various ~ays. These methods involve various problems difficult to solve, such as the high price and difficult availability of precious metals used as catalysts and the poor absorption properties of ~he absorbing agents. More-over, it is often difficult to dimension the ap-paratus when applying absorption methods due to variation in boiler capacities and other such factors.
Technically, it is more advantageous to try to prevent the formation of nitrogen oxides during the combustion step instead of removing them. For this purpose, a variety of low NOx burners have been de-veloped, and attemp-ts have been made to carry out -the combustion in a pressurized space, in addition to which supply of air into the boiler has been carried ~ out in stages before the superheaters. Contrary to i expectations, these methods, however, have not pro-~ vided any particularly good results, because in prac-i ~00~9~7 tice the operation of -the methods has been prevented or substantially deteriorated by such factors as variation in the formation conditions of nitrogen ox-ides, reaction kinetics, operational conditions of boilers and variation occurring therein. Furthermore, removal of nitrogen oxides has been attempted by means of circulation bed furnaces operating at very low temperatures (about 800C), that is, at condi-tions disadvantageous for the formation of NOx. This, however, has deteriorated the efficiency of the fur-naces as well as their ability to burn different kinds of fuel as it has been necessary to drop the temperature as low as near the minimum temperature required for continuously maintaining the combustion process. The methods described above are widely known and therefore will not be described more closely (Finnish Ministry of Trade and Industry/Energy De-partment D:140, Helsinki 1987).
DE Offenlegungsschrift 30 40 830 discloses a method ln which completely combusted cooled flue gas obtained from a gas flue after the boiler is mixed with the air to be introduced into the under-stoichiometric first combustion zone in order to re-duce the amount of nitrogen oxides. Even though this .,: ~ ,..
method helps to prevent the formation of nitrogen oxides to some extent, it does not enable sufficient control of the amount of nitrogen oxides. In addi-tion, the recycling of the flue gas increases the mass flow of gas flowing through the boiler, thus re-~uiring a somewhat larger combustion space and larger conduits everywhere in the boiler.
The NO content is usually low within the re-ducing area, depending on the reducing effect of hydrogen (H2) and carbon monoxide (C02). These sub-stances decompose the possibly formed NO roughly ac-1. '""'.'-.'' ' z~
cording to the following reactions:
NO ~ CO -> 1/2 N2 + C2 NO + H2 -~ 1/2 N2 ~ H2O
In combustion carried ou-t understoichiometric-ally in a manner known per se, the NO concentra-tion can, in principle, be kept on a low level. Problems occur only when the conditions become reducing or when very high temperatures occur, that is, over 1500C. The problems result even ~rom a minor excess of air, which under furnace conditions causes rapid formation of NO, or from very high temperatures (over 1500C) at which H2 and CO cannot any more prevent the formation of NO due to their reduced reducing po-tential. In prior art apparatuses such situations occur particularly in the primary flame but also in connection with the introduction of secondary and tertiary air. One of the most important reasons for the formation of NO in the primary flame of prior art apparatuses is that the heterogeneous flame contains, e.g., oil drops or carbon particles and, as a conse-quence, there occurs high concentration gradients of oxygen and burning gases as well as high temperature gradients. Thereby it is always possible that minor temperature peaks occur locally at phase boundaries, for instance, if the amount of oxygen at such a point is stoichiometric or slightly overstoichiometric. In a typical combustion apparatus, the temperature may rise instantaneously and locally up to about 2000C.
As a result, the local NO concentration rises rapidly up to about 3500 ppm (prompt NO~). The NO so formed will not decompose to any greater degree under boiler conditions. Accordingly, it is obvious that even minor locally and instantaneously occurring tempera-ture peaks increase rapidly the average NO value of exhaust gas, which should remain on a concen-tration ~0~345~
level of about 100 ppm.
The object of the present invention is to pro-vide a method by means of which the formation of ~x during a reducing combustion step, usually in so called primary combustion, particularly in the flame, can be minimized and in which conditions prerequisite for the formation of NOx are prevented without any complicated apparatuses. Removal of NOx after combus-tion is not required. The method is characterized in that a gas or gas mixture containing reducing agents, such as H2 and CO, is mixed with the air to be intro-duced into the first step, that the oxygen content of the gas mixture introduced into the first step is preferably 12-19%, and that the oxygen content and reducing potential of the air mixture to be intro-duced are adjus~ed so that the nitrogen oxide con-centration of flue gas from combustion carried out at the adiabatic combustion temperature of the fuel used, corresponding to the supplied oxygen content and reducing potential, is no more than a prede-termined concentration value.
The basic idea of the invention is that air is introduced into the combustion process in such a man-; ~:
ner that the NOx formation in the reducing part ofthe furnace, particularly in the difficultly con-trollable flame, remains on a sufficiently low level at all temperatures and oxygen/fuel ratios possibly occurring during this combustion step. This is achieved by carrying out the combustion under re-ducing conditions by using a gas or a gas mixture having an oxygen content lower than that of ordinary air and containing reducing agents. By means of the method according to the invention, the concentration of nitrogen oxides can be controlled so that the equilibrium concentration of the nitrogen oxides in ,,"~.
Z0~9637 : :
the flue gas, in practice, also the maximum con-centration, remains all the time at a very low value.
A further object of the invention is to provide an apparatus for applying the method. The apparatus is characterized in -that the mixing means comprise at least one gas flue for passing part of the flue gas from the first combustion step into the air to be introduced into the first step to be mixed with it.
The basic idea of the apparatus of the inven-tion is that the reducing gas or gas mixture, that is, non-oxygen-containing or low-oxygen-content gas containing reducing agents, and air are mixed thoroughly with each other and introduced at least into the boiler zone in which fuel and air are nor-mally poorly mixed with each other so that local tem-perature peaks are likely to occur. Typically, this zone is the reducing combustion zone of the boiler, mainly the flame.
The invention will be described in greater de-tail in the attached drawings, wherein Figure 1 illustrates the interdependence of the ;
temperature and the air coefficient (ratio of oxygen to the amount of theoretical oxygen required for the combustion irrespective of the other components pre-sent in the gas mixture, such as inert components and reducing agents) in a prior art application with re-spect to a predetermined NO concentration level when the combustion is carried out normally with air, and the interdependence of the adiabatic temperature and the air coefficient in a typical oil combustion pro-cess carried out normally with air and with a mixture of air and gas consisting of completely combusted flue gas from a boiler, the mixture having an oxygen content of 17~ (see DE Patent Application 3 040 830);
r igure 2 illustrates by way of example the , ':
;~;(3~14907 ~`
interdependence of the maximal NO amount obtained in the combustion of pure methane (CH4) and the air co-efficient when the gas maintaining the combustion is air, a mixture of completely combusted flue gas and air, as disclosed in Figure 1, or a mixture of cooled gas recycled from the reducing combustion and air;
and Figure 3 is a schematic view of an apparatus for applying the method of the invention.
In Figure 1, the curve A-B shows by way of example the adiabatic combustion temperature of a widely used oil type as a function of the air coeffi~
cient when the combustion is carried out normally with air. The curve C-D illustrates by way of example the adiabatic combustion temperature of the same oil type as a function of the air coefficient when the combustion is carried out with air diluted with com~
pletely combusted flue gas, the oxygen content of the mixture being 17%. The curve E-F shows by way of example the pairs of temperature and air coefficient values corresponding to the N0 concentration 100 ppm when the combustion is carried out normally with air.
Above the curve the N0 concentration is more than 100 ppm. Very high temperatures (over 1500C) in par~
ticular are important for the invention. As the local temperatures in the hottest portions of the flame may rise very close to the adiabatic temperature, it is to be seen from the figure that the N0 concentration of 100 ppm (point G) can be achieved with an air co-efficient as low as 0.82 when the combustion is carried out normally with air. When using air diluted with completely combusted flue gas, the concentration of 100 ppm is achieved with an air coefficient of 0.93 (point H). At worst, the maximum N0 concentra-tion within the reducing area is about 2700 ppm when .
. . ~.
- ;~0~49~37 :-the combustion is carried out normally with air and only ~00 ppm when the co~bustion is carried out with diluted air as disclosed in the example. The first-mentioned value is represented by point I and the last-mentioned by point J in Figure 1.
It has now been found unexpectedly that the NO
formati~n can be prevented within the reducing area of the burner, particularly in the flame, when oxy-gen required by the combustion is introduced under-stoichiometrically and at an uniform oxygen content of less than 21% by mixing a gas or gas mixture con-taining considerable amounts of reducing agents with the combustion air. In this way the combustion tem-perature, particularly that of the flame, can be de-creased, simultaneously increasing reducing potential so that abundant formation of NOx is no more possible, not even locally or instantaneously. This preferably takes place so that the local temperature peak of the flame does not exceed about 1500C and the oxygen concentration is reduced by means of cool-ed flue gas recycled from the reducing combustion step, typically containing hydrogen (H2) and carbon monoxide (CO). The formation of NOx is efficiently prevented both by the temperature drop and the in-crease in the reducing potential.
In Figure 2, the curve K-L illustrates the maximum NO concentration obtained in the combustion of pure methane (CH4) when the combustion is carried out normally with air and the burning takesi place adiabatically. The curve M-N illustrates the maximum NO concentration when completely combusted flue gas has been mixed with the combustion air. The curve 0-N, in turn, illustrates an example where the oxygen content of the combustion air has been decreased by adding to it properly cooled flue gas from the re-,. . .
~0~9~
g ducing step, operated wit:h the same air coefficient,in an amount of 24% on the volume flow of the primary air, whereby reducing agents H2 and C0 will also be recycled to a considerable degree. It clearly appears from the figure that the recycling of reducing gases decreases greatly the N0 concentration while the tem~
perature is decreased and the reducing potential in-creased. For instance, the maximum decrease in N0 concentxation with the air coefficient 0.80 is as great as 97% with the air coefficient 0.80 as com-pared with combustion with air, that is, the con-centration drops from 0.048 mol N0/kg CH4 (point P in Figure 2) to 0.0012 mol N0/kg CH4 (point Q in Figure 2) and about 73~ on the value obtained when complete-ly combusted flue gas is mixed with the combustion air (corresponding to the ratio between Q and R).
When the method of the invention is applied, the oxy-gen concentration as well as the amount and reduction capability of the reducing agents, that is, their reducing potential, can be adjusted in a desired man-ner according to the used fuel and other combustion conditions.
It has been unexpectedly found that the maximum efficacy of the method of the present invention, that is, the greatest decreasa in N0 formation as compared with the prior art, is to be obtained with air co-efficients ranging from 0.80 to 0.95. It is likewise unexpected that the maximum decrease in N0 concentra-tion occurs within the air coefficient range conven-tionally applied in typical prior art primary combus-tion in power plant boilers. Accordingly, the method of the present invention decisively decreases N0x formation in the flame of the burner, that is, the formation of prompt N0x, which has been most diffi-cult if not impossible to prevent in prior art appar-: ", ~
:-`'''' .:;
~,' ,', ,.'.'~,"
~0~ 7 , ~ .
, . . .~, ~.
atuses. When comparing the curves M-N and O-N of Fig-ure 2, it appears that when a gas or gas mixture con-taining reducing agents :is mixed with air according to the invention, the air coefficient 0.95 still gives a NOx concentration (point S) which is about 92~ lower than that obtained by combustion with ordi-nary air (point T) and 40~ lower than the value ob-tained by adding completely combusted flue gas (corresponding to the ratio between points U and S).
Furthermore, the use of completely combusted flue gas increases the amount of gas used, which requires a greater boiler and greater gas flues, whereas in the method of the invention the increased amount of gas and the greater space requirement concern only that part of the boiler in which the reducing combustion takes place. As further appears from Figure 2, the curves M-N and O-N join when the air coeficient is 1, which is due to the fact that flue gas from fuel burned with stoichiometric ratio cannot any more con-tain reducing agents to any greater degree. This, however, is unimportant ~or the end result in combus-tion processes carried out with lower air coeffi-cients. Essential in the invention is that local overheating is prevented in an understoichiometric combustion so that nitrogen oxides will not be form-ed.
Figure 3 shows schematically an apparatus for applying the method of the invention. The apparatus comprises a burner such as a boiler 1 with a furnace 2. Fuel is introduced into the furnace 2 by means of one or more feeding devices 3. Oxygen-containing gas mixture required for the combustion is introduced into the same part of the furnace 2 through a conduit 4 belonging to air supply means. Air is supplied into the conduit 4 through a conduit 5, while reducing Z(~4~7 :
!
gas, that is, at least substantially non-oxygen-con-taining gas mixture containing substantial amounts of reducing agents, mainly H2 and C0, is supplied through a conduit 6 belonging to mixing means via a blower 7 and a gas mi~er 8. The gas to be mixed is preferably flue gas deri~ed from the furnace 2, in which raducing combustion takes place. The flue gas, which contains reducing agents, is cooled by means of coolers 9 and 10 and its amount is controlled by means of a valve 11. The reducing flue gas is mixed in the mixer 8 with the air to be introduced into the furnace. A major portion of the flue gas produced during the reducing combustion step i5 passed into subsequent combustion steps, shown schematically with a single combustion step 12. During the subsequent combustion steps, additional air is introduced into the boiler by means of a valve 13 through a conduit 14, whereby the fuel will be combusted as completely as possible. At this stage the 1ue gas can be cooled b~ means of a heat exchanger 15 and thereafter by means o coolers 16, whereafter it is passed through a blower 17 into a gas flue 18. Depending on the re-quired amount of reducing agents, final cooled flue gas can be mixed with the air to be introduced into the furnace 2 during the first step in a manner known per se through a conduit 19 in addition to the flue gas from the reducing step introduced through the conduit 6, the amount of the final flue gas being adjusted by a valve 20. In this way, both the oxygen content of the mixture of air and gas to be intro-duced into the furnace and the concentration of the reducing agents in it can be adjusted according to the combustion conditions and fuel used. If there exists a danger that the flame or a portion of it becomes too hot at the beginning of the reducing com-,' ' :, .' ~'.~.
':~ ` ` ','. '. .- j .' . ~ .,; , . ' :
bustion step 12 with resultant excessive formation of nitrogen oxide, the flame temperature during this step can also be decreased by feeding reducing gas through a valve 21 and a conduit 22 at the beginning of the reducing combustion step 12. Heat losses from the burner can be decreased by insulating the combus--tion chambers by insulations 23 and 24, shown schema-tically in the figure~
I~ is to be understood that some of the devices described above can be combined into a single entity to obtain a structurally more advantageous solution.
For instance, the parts 2, 10, 12, 15 and 16 can be easily combined.
It is essential in the apparatus of the inven-tion that the air and reducing gas are mixed properly before their introduction into the reducing part of the furnace and that the temperature of the flame or a flame portion is decreased only to such an extent as is required for preventing the formation of NO
without any risk of the combustion process being interrupted. In the present method the mix ratio of air and fusl is ~etermined, e.g., by the thermal value of the fuel used, the minimum temperature re-quired for maintaining combustion, the chemical analysis of the gas, the desired NOx level, the di-mensions of the heat surfaces of the boiler, the de-gree of cooling (temperature~ of the recycled gas, and the positions of the gas introduction steps. As a consequence, this ratio may vary widely; typically, the amount of gas is 10 to 70% on the amount of air supplied.
It is obvious that when the same combustion efficiency is to be obtained, the mass volume of the gas used in the apparatus of the invention is greater as compared with prior art apparatuses, though mainly i ,, . . ~ ., , ~ , ., " ,. . .- , . . .
;;~ 9L9~7 ::
only in the reducing part of the boiler. However, the dimensions of the boiler will not change to any greater degree because the recycling preferably takes place only during the understoichiometric combustion step(s) and because increase in the flow volume of gas is for a major part compensated for by change in the gas density caused by the temperature drop. It is also obvious that, theoretically, the recycling of gas does not reduce the efficiency of the boiler;
varying heat losses, however, may resul-t in slightly reduced efficiency. In view of the advantages ob-tained, this drawback is not of any greater im-portance.
The invention has the advantage that the ap-paratus can be constructed by means of well-known inexpensive constructions and no separate expensive means for removing NOx are needed because the forma-tion of NOx has been prevented sufficiently. Further, the method of the invention is easy to realize and very easy to control when its principles are applied to apparatuses and control systems known per se. It is likewise possible to control the high local forma-tion o~ NOx in the hottest, spotlike portions of the flame of the burner because the formation of NOx is so restricted that its concentration cannot exceed a set limit value. The NO concentration of the flue gas emission to the surrounding is, of course, dependent on the operational properties and structure of the oxidizing part of the boiler.
.., ".
Claims (4)
1. A method of combustion for reducing the for-mation of nitrogen oxides during combustion, wherein air required for the combustion of a fuel is intro-duced in at least two steps, the air being introduced understoichiometrically in the first step, preferably with an air coefficient ranging from 0.80 to 0,95, a gas or gas mixture substantially free from elementary oxygen is mixed with the air to be introduced into the first step, and a gas or gas mixture containing reducing agents is mixed with the air to be intro-duced into said first step, c h a r a c t e r i z e d in that H2 and CO containing combustion gases from the understoichiometrical combustion step, preferably from the first combustion step, are mixed with the air to be introduced into the first step so that the oxygen content of the gas mixture to be introduced into the formed first step is preferably 12 to 19%, and that the oxygen content and reducing potential of the air mixture to be introduced are adjusted so that the nitrogen oxide concentration of flue gas formed during combustion carried out at the adiabatic com-bustion temperature of the fuel used, corresponding to the supplied oxygen content and reducing potential, is no more than a predetermined concentra-tion value.
2. A method according to claim 2, c h a r a c -t e r i z e d in that the flue gas is cooled before being mixed with air.
3. An apparatus for applying a method according to claim 1, comprising means (4, 5) for introducing air into a furnace (2), means (3) for introducing fuel into the furnace (2), and means (6, 7, 8, 9, 11) for mixing a gas or gas mixture containing less oxy-gen than air with the air to be introduced into the first understoichiometric combustion step before the air is introduced into the furnace (2), c h a r a c -t e r i z e d in that he mixing means (6, 7, 8, 9, 11) comprise at least one gas flue (6) for passing part of the flue gas from the first combustion step into the air to be introduced into the first step to be mixed with it.
4. An apparatus according to claim 3, c h a r -a c t e r i z e d in that the mixing means (6, 7, 8, 9, 11) comprise means (9, 10) for cooling the flue gas before it is mixed with the air.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI885800 | 1988-12-15 | ||
| FI885800A FI88199B (en) | 1988-12-15 | 1988-12-15 | BRAENNFOERFARANDE FOER REDUCERING AV KVAEVEOXIDBILDNINGEN VID FOERBRAENNING SAMT APPARATUR FOER TILLAEMPNING AV FOERFARANDET |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2004907A1 true CA2004907A1 (en) | 1990-06-15 |
Family
ID=8527574
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002004907A Abandoned CA2004907A1 (en) | 1988-12-15 | 1989-12-07 | Method of combustion for reducing the formation of nitrogen oxides during combustion and an apparatus for applying the method |
Country Status (14)
| Country | Link |
|---|---|
| CN (1) | CN1043522A (en) |
| AU (1) | AU4671089A (en) |
| CA (1) | CA2004907A1 (en) |
| CS (1) | CS712789A2 (en) |
| DD (1) | DD290042A5 (en) |
| DE (1) | DE3941307A1 (en) |
| DK (1) | DK639589A (en) |
| ES (1) | ES2020611A6 (en) |
| FI (1) | FI88199B (en) |
| FR (1) | FR2640728A1 (en) |
| GB (1) | GB2226122A (en) |
| HU (1) | HUT55521A (en) |
| IT (1) | IT1237909B (en) |
| SE (1) | SE8904187L (en) |
Families Citing this family (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NO172704C (en) * | 1990-02-23 | 1993-08-25 | Norsk Hydro As | ARC HEATING AND REACTION SYSTEM |
| DE4302847A1 (en) * | 1993-02-02 | 1994-08-04 | Abb Research Ltd | Two-stage non-polluting fuel combustion system |
| GB2281964A (en) * | 1993-09-18 | 1995-03-22 | Enertek International Limited | Reducing emissions from naturally aspirated burners |
| US5823124A (en) * | 1995-11-03 | 1998-10-20 | Gas Research Institute | Method and system to reduced NOx and fuel emissions from a furnace |
| CN1114464C (en) * | 1999-12-22 | 2003-07-16 | 中国科学院山西煤炭化学研究所 | Method for treating high concentration nitrogen dioxide waste gas and its equipment |
| CN1102419C (en) * | 1999-12-22 | 2003-03-05 | 中国科学院山西煤炭化学研究所 | Method of removing high-concentration nitrogen dioxide from fuel oil and its equipment |
| NO312260B1 (en) * | 2000-03-24 | 2002-04-15 | Organic Power As | Method and device for the conversion of energy by the combustion of solid fuel |
| WO2008004281A1 (en) * | 2006-07-04 | 2008-01-10 | Miura Co., Ltd. | Combustion apparatus |
| CN101907296B (en) * | 2010-07-12 | 2012-05-30 | 昆明理工大学 | Effective dimethyl ether/air low NOx high-temperature burning system |
| FR3039251B1 (en) | 2015-07-21 | 2017-07-28 | Ifp Energies Now | PROCESS AND INSTALLATION CLC WITH PRODUCTION OF HIGH PURITY NITROGEN |
| BE1023010B1 (en) * | 2015-10-06 | 2016-11-04 | Lhoist Recherche Et Developpement Sa | Process for calcining mineral rock in a vertical right furnace with regenerative parallel flows and furnace used |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3781162A (en) * | 1972-03-24 | 1973-12-25 | Babcock & Wilcox Co | Reducing nox formation by combustion |
| JPS5232977Y2 (en) * | 1973-07-12 | 1977-07-27 | ||
| US3880570A (en) * | 1973-09-04 | 1975-04-29 | Babcock & Wilcox Co | Method and apparatus for reducing nitric in combustion furnaces |
| US4135874A (en) * | 1976-03-31 | 1979-01-23 | Ishikawajima-Harima Jukogyo Kabushiki Kaisha | Two stage combustion furnace |
-
1988
- 1988-12-15 FI FI885800A patent/FI88199B/en not_active Application Discontinuation
-
1989
- 1989-12-06 GB GB8927547A patent/GB2226122A/en not_active Withdrawn
- 1989-12-07 CA CA002004907A patent/CA2004907A1/en not_active Abandoned
- 1989-12-12 SE SE8904187A patent/SE8904187L/en not_active Application Discontinuation
- 1989-12-13 AU AU46710/89A patent/AU4671089A/en not_active Abandoned
- 1989-12-14 IT IT02270089A patent/IT1237909B/en active IP Right Grant
- 1989-12-14 HU HU896607A patent/HUT55521A/en unknown
- 1989-12-14 DD DD89335681A patent/DD290042A5/en not_active IP Right Cessation
- 1989-12-14 DE DE3941307A patent/DE3941307A1/en not_active Withdrawn
- 1989-12-15 CS CS897127A patent/CS712789A2/en unknown
- 1989-12-15 DK DK639589A patent/DK639589A/en not_active Application Discontinuation
- 1989-12-15 FR FR8916660A patent/FR2640728A1/en not_active Withdrawn
- 1989-12-15 CN CN89109301.XA patent/CN1043522A/en active Pending
- 1989-12-15 ES ES8904234A patent/ES2020611A6/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| AU4671089A (en) | 1990-06-21 |
| ES2020611A6 (en) | 1991-08-16 |
| HUT55521A (en) | 1991-05-28 |
| CS712789A2 (en) | 1991-09-15 |
| DD290042A5 (en) | 1991-05-16 |
| GB8927547D0 (en) | 1990-02-07 |
| CN1043522A (en) | 1990-07-04 |
| SE8904187D0 (en) | 1989-12-12 |
| IT1237909B (en) | 1993-06-18 |
| SE8904187L (en) | 1990-06-16 |
| DK639589D0 (en) | 1989-12-15 |
| GB2226122A (en) | 1990-06-20 |
| DE3941307A1 (en) | 1990-06-21 |
| DK639589A (en) | 1990-06-16 |
| HU896607D0 (en) | 1990-02-28 |
| FI885800A (en) | 1990-06-16 |
| FI885800A0 (en) | 1988-12-15 |
| FI88199B (en) | 1992-12-31 |
| FR2640728A1 (en) | 1990-06-22 |
| IT8922700A0 (en) | 1989-12-14 |
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