CA1145657A - Method of reducing no.sub.x-emission - Google Patents
Method of reducing no.sub.x-emissionInfo
- Publication number
- CA1145657A CA1145657A CA000359730A CA359730A CA1145657A CA 1145657 A CA1145657 A CA 1145657A CA 000359730 A CA000359730 A CA 000359730A CA 359730 A CA359730 A CA 359730A CA 1145657 A CA1145657 A CA 1145657A
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- CA
- Canada
- Prior art keywords
- air
- combustion
- zone
- burner
- balance
- 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.)
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of reducing NOx emission during combustion of nitrogen-containing fuels with burners in a closed com-bustion chamber, according to which the combustion occurs in a below-stoichiometric primary combustion zone with direct air supply through the associated burner, and in an above-stoichiometric secondary combustion zone by ad-dition of the remaining or balance of the air. The re-sidue air is supplied to the combustion chamber, regulated as a function of the load, around the particular burner, in such a way that after development of the primary com-bustion zone, the remaining air flows into the outer region of the flame of the burner while supplying the secondary combustion zone.
A method of reducing NOx emission during combustion of nitrogen-containing fuels with burners in a closed com-bustion chamber, according to which the combustion occurs in a below-stoichiometric primary combustion zone with direct air supply through the associated burner, and in an above-stoichiometric secondary combustion zone by ad-dition of the remaining or balance of the air. The re-sidue air is supplied to the combustion chamber, regulated as a function of the load, around the particular burner, in such a way that after development of the primary com-bustion zone, the remaining air flows into the outer region of the flame of the burner while supplying the secondary combustion zone.
Description
~ ~ 5 ~7 ..
The present illvention rela,e lo a method of reducing NOx emission during combustion of nitrogen-containing fuels with burners in a closed combustion chamber, according to which the combustion occurs in a below-stoichiometric primary combustion zone with direct air supply through the associated burner, and in an above-stoichiometric secondary combustion zone by additon of the remaining or balance of the air.
The reaction mechanisms which cause the formation of nitrogen oxides in technical firing or combustion are mostly known. At the present, there are essentially two different formation reactions as follows:
(1) The thermic NOx formation, which is based upon oxi-dation of molecular nitrogen, which occurs for instance abundantly in the combustion air. Since the oxidation of molecular nitrogen requires atomic oxygen or aggressive radicals (for instance OH, 03, etc.), such oxidation is strongly temperature dependent, thus thermic NOx; and
The present illvention rela,e lo a method of reducing NOx emission during combustion of nitrogen-containing fuels with burners in a closed combustion chamber, according to which the combustion occurs in a below-stoichiometric primary combustion zone with direct air supply through the associated burner, and in an above-stoichiometric secondary combustion zone by additon of the remaining or balance of the air.
The reaction mechanisms which cause the formation of nitrogen oxides in technical firing or combustion are mostly known. At the present, there are essentially two different formation reactions as follows:
(1) The thermic NOx formation, which is based upon oxi-dation of molecular nitrogen, which occurs for instance abundantly in the combustion air. Since the oxidation of molecular nitrogen requires atomic oxygen or aggressive radicals (for instance OH, 03, etc.), such oxidation is strongly temperature dependent, thus thermic NOx; and
(2) the formation of Euel NOx, which occurs by oxi-dation of nitrogen compounds bound in the fuel. During the pyrolysis, nltrogen-carbon and nitrogen-hydrogen radicals (CH, HCN, CH, etc.) form from these nitrogen compounds.
These radicals oxidize into NOx in the presence of oxygen already at relatively low temperatures because of their reactivity with molecular oxygen.
A reduction of the thermic NOx-formation is accordingly primarily obtained by lowering the combustion temperature and ~k 5'~
~hat retention times at high temperatures. Since with the combustion of fuels with bound nitrogen, however, a large portion of the total N ~-formation results from the fuel-N0~-reaction, the aforementioned measures with such fuels are not sufficient for complying with the emission standards existing in certain countries. For this purpose, it îs necessary to reduce the nitrogen compounds into molecular nitrogen (N2) still during the pyrolysis in the absence of oxygen. Tests have shown that these reduction reactions to molecular nitro-gen occur, for example, when the fuels are burned or combusted at below-stoichiometric conditions, that is, with less oxygen or air addition than needed for complete combustion. To achieve optimum results, an air ratio between 0.9 and O.S is selected for the primary combustion zone as a function of the limiting or edge conditions (for lnstance wall temperat-lre of the combustion chamber). Ilowever, to achleve a complete com-bustion of the hydrocarbon compounds of the fuel, the reaction products resulting in the below-stoichiometric primary region must then be afterburned.
Tests have shown that with such a two-stage combustion, both the fuel-N0x-formation (with simultaneous heat removal from the below-stoichiometric region) as well as the thermic Nox-formation can be considerably reduced. In tests, with the utilization of the two-stage combustion, the N0x-emission values were reduced approximately up to 70% compared with unstepped combustion.
~ ~ ~ 5 6S~
Through tests it was proven that the formation of fuel NOx could be clearly reduced by operating the burner in the near-stoichiometric or below-stoichiometric range. To avoid losses by incomplete combustion, and to avoid increase of other noxious material emissions (CO, hydrocarbons, and particles), it is necessary for additonal air to be blown in above the burners in the combustion chamber during below-stoichiometric operation of the burners. The disadvantage of this manner of operation is that in the below-stoichio-metrically operated lower part of the combustion chamber,scoring and corrosion of the tube walls can occur. Accord-ingly, the operational certainty of the system is in dangerO
It has furthermore been determined that by slowing the mixture between air flow and fuel flow, likewise consider-able reduction of NOx-emission can be achieved. For this purpose, flow or spray burners, for lnstance, are suitable, with which both the air stream and the fuel stream are blown in parallel into the combustion chamber. To achieve a satis-factory ignition, the burner streams, however, must support 2~ each other, for instance in a corner firing or combustion.
With the arrangement of the burners in a front or counter-firing or combustion, the mixture of air and fuel can, for example, be slowed thereby that the secondary air surround-ing the dust stream is blown-in at substantially the same speed.
In a known burner, the secondary air stream is added ~ ~ ~ S 6S'7 separately in two tubes, which are arranged annularly rela-tive to each other, to permit discharge of, for example, the outer secondary air flow with higher speed, and of the inner secondary air flow, with low speed, directly adjoining the dust stream. DisadvanLageous with this arrangement is that an extension or lengthening of the flame occurs, which has as a consequence larger combustion chambers, and that with the load-conditioned reduction of the secondary air, the secondary air speed is reduced below the dust-air speed, whereby the character and shape of the flame change. The ignition could also be disadvantageously influenced hereby.
Furthermore, it is known to undertake a primary combus-tion at below-stoichiometric conditions in an antechamber of the combustion chamber, and to admix the air necessary for complete combustion with the combustion gases which leave the antechamber. The disadvantage of this arrangement exists in the danger of tube wall corrosion of the antechamber, which is operated at below-stoichiometric conditions.
The object of the present invention accordingly is to provide a method of reducing N0x-emission during combustion of nitrogen-containing fuels with burners in closed combustion chambers, with the method assuring that a low formation of N0x is obtained by influencing the secondary air flow, and simultaneously maintaining the certainty of operation against scoring and corrosion of the pipe walls while simultaneously maintaining an intensive ignition and a good combustion or ~4S~S7 burning out.
This object, and other objects and advantages of the present invention, will appear rnore clearly from the follow-ing specification in connection with the accompanying drawing, which illustrates the principle of a burner for the present inventive method, along with the flame generated therewith.
By one aspect of this invention, a method is provided of reducing N0x emission during combustion of nitrogen-containing fuels with burners in a closed combustion chamber, said method including the steps of: effecting combustion in a below-stoichi-ometric primary combustion zone with direct air supply through the associated burner; effecting combustion in an above-stoi-chiometric secondary combustion zone with the addition of the balance of the air; regulating said balance of the air as a unction of load; and adding said balance of the air to said combustion chamber around the associated burner in such a way that after Eormfltion of said primary combustion zone, said bfllance of the air flows into the ou~er region o the flame while feeding said secondary combustion zone.
There has been discovered as especially advantageous that the secondary air flow supplied directly to the burner be such that in the primary zone an air number between n c 0.9 and n = 0.5 is obtained. Additionally, provision is made according to th~ present invention that the second partial flow of the secondary air (stepped air), itself be divided into at least two partial flows, which are introduced 5~i57 into the combustion chamber from a divided circle which con-centrically surrounds the burner, and that smoke or flue gases be drawn out of the combustion chamber into the primary zone by the impulse of the flame by means of the free space located between the stepped air flows~
By dividing the secondary air flow, the following ad-vantages are obtained with the inventive method: low N0x-emission, no sintering and corrosion on the tube walls of the combustion chamber, as well as a certain ignition and satis-factory combustion over a wide operating range.
While that part of the secondary air which is supplied directly to the burner can be so influenced in relation to twist and speed that an intensive ignition is assured over the entire load range, the second part of the secondary air flow, which is added to the flame as a stepped-air flow ex-ternally o the burner, is such that flfter successful igni-tion and primary combustion there is realized the combustion in the secondary zone and the object o the present inven-tion by means of the mixing energy of the stepped-air strearn.
Referring now to the drawing in detail, the burner in-cludes a core-air pipe or tube 2, a fuel and carrier-air portion 1, and a mantle-air part 3. With this burner, there is obtained a partial combustion zone (primary zone) 6, the air number of which is between 0.9 and 0.5 times the stoi-chiometric relationship~
The burner is embodied in such a way that by predeter-,,5~j~37 mined measures (twist of the mantle-air, conically widened burner outlet, closed core-air~ there is generated in the interior of the flame a zone of intensive return flow or back-flow 5 from a region of already advanced combustion. In so doing, the fuel-air mixture is quickly heated up and ignited. The heating-up and ignition can be influenced by adding core-air. Accordingly, the best ignition is assured wllen the core-air is closed.
The air necessary for the burning-out of the remainder or residue is blown in along the periphery as stepped- or staged-air 4 by means of several jets or nozzles in such a way that not until after formation of the primary flame is the secondary flame or also the afterburner zone 7 supplied with oxygen. For this purpose, the stepped-air flow 4 is arranged in a divided circle which corresponds to twice the diameter of the mantle-air tube. This assures that the stepped-air 4 only reaches the actual flame downstream from the burner outlet after a distance of approximately one to two times the dlameter of the mantle-air tube.
Smoke or flue gases are drawn in from the combustion chamber by impulse exchange at those segments o~ the peripheral surface of the flame not adjoining the stepped-air flow 4. In this manner, the flame temperature is re-ducedO
These radicals oxidize into NOx in the presence of oxygen already at relatively low temperatures because of their reactivity with molecular oxygen.
A reduction of the thermic NOx-formation is accordingly primarily obtained by lowering the combustion temperature and ~k 5'~
~hat retention times at high temperatures. Since with the combustion of fuels with bound nitrogen, however, a large portion of the total N ~-formation results from the fuel-N0~-reaction, the aforementioned measures with such fuels are not sufficient for complying with the emission standards existing in certain countries. For this purpose, it îs necessary to reduce the nitrogen compounds into molecular nitrogen (N2) still during the pyrolysis in the absence of oxygen. Tests have shown that these reduction reactions to molecular nitro-gen occur, for example, when the fuels are burned or combusted at below-stoichiometric conditions, that is, with less oxygen or air addition than needed for complete combustion. To achieve optimum results, an air ratio between 0.9 and O.S is selected for the primary combustion zone as a function of the limiting or edge conditions (for lnstance wall temperat-lre of the combustion chamber). Ilowever, to achleve a complete com-bustion of the hydrocarbon compounds of the fuel, the reaction products resulting in the below-stoichiometric primary region must then be afterburned.
Tests have shown that with such a two-stage combustion, both the fuel-N0x-formation (with simultaneous heat removal from the below-stoichiometric region) as well as the thermic Nox-formation can be considerably reduced. In tests, with the utilization of the two-stage combustion, the N0x-emission values were reduced approximately up to 70% compared with unstepped combustion.
~ ~ ~ 5 6S~
Through tests it was proven that the formation of fuel NOx could be clearly reduced by operating the burner in the near-stoichiometric or below-stoichiometric range. To avoid losses by incomplete combustion, and to avoid increase of other noxious material emissions (CO, hydrocarbons, and particles), it is necessary for additonal air to be blown in above the burners in the combustion chamber during below-stoichiometric operation of the burners. The disadvantage of this manner of operation is that in the below-stoichio-metrically operated lower part of the combustion chamber,scoring and corrosion of the tube walls can occur. Accord-ingly, the operational certainty of the system is in dangerO
It has furthermore been determined that by slowing the mixture between air flow and fuel flow, likewise consider-able reduction of NOx-emission can be achieved. For this purpose, flow or spray burners, for lnstance, are suitable, with which both the air stream and the fuel stream are blown in parallel into the combustion chamber. To achieve a satis-factory ignition, the burner streams, however, must support 2~ each other, for instance in a corner firing or combustion.
With the arrangement of the burners in a front or counter-firing or combustion, the mixture of air and fuel can, for example, be slowed thereby that the secondary air surround-ing the dust stream is blown-in at substantially the same speed.
In a known burner, the secondary air stream is added ~ ~ ~ S 6S'7 separately in two tubes, which are arranged annularly rela-tive to each other, to permit discharge of, for example, the outer secondary air flow with higher speed, and of the inner secondary air flow, with low speed, directly adjoining the dust stream. DisadvanLageous with this arrangement is that an extension or lengthening of the flame occurs, which has as a consequence larger combustion chambers, and that with the load-conditioned reduction of the secondary air, the secondary air speed is reduced below the dust-air speed, whereby the character and shape of the flame change. The ignition could also be disadvantageously influenced hereby.
Furthermore, it is known to undertake a primary combus-tion at below-stoichiometric conditions in an antechamber of the combustion chamber, and to admix the air necessary for complete combustion with the combustion gases which leave the antechamber. The disadvantage of this arrangement exists in the danger of tube wall corrosion of the antechamber, which is operated at below-stoichiometric conditions.
The object of the present invention accordingly is to provide a method of reducing N0x-emission during combustion of nitrogen-containing fuels with burners in closed combustion chambers, with the method assuring that a low formation of N0x is obtained by influencing the secondary air flow, and simultaneously maintaining the certainty of operation against scoring and corrosion of the pipe walls while simultaneously maintaining an intensive ignition and a good combustion or ~4S~S7 burning out.
This object, and other objects and advantages of the present invention, will appear rnore clearly from the follow-ing specification in connection with the accompanying drawing, which illustrates the principle of a burner for the present inventive method, along with the flame generated therewith.
By one aspect of this invention, a method is provided of reducing N0x emission during combustion of nitrogen-containing fuels with burners in a closed combustion chamber, said method including the steps of: effecting combustion in a below-stoichi-ometric primary combustion zone with direct air supply through the associated burner; effecting combustion in an above-stoi-chiometric secondary combustion zone with the addition of the balance of the air; regulating said balance of the air as a unction of load; and adding said balance of the air to said combustion chamber around the associated burner in such a way that after Eormfltion of said primary combustion zone, said bfllance of the air flows into the ou~er region o the flame while feeding said secondary combustion zone.
There has been discovered as especially advantageous that the secondary air flow supplied directly to the burner be such that in the primary zone an air number between n c 0.9 and n = 0.5 is obtained. Additionally, provision is made according to th~ present invention that the second partial flow of the secondary air (stepped air), itself be divided into at least two partial flows, which are introduced 5~i57 into the combustion chamber from a divided circle which con-centrically surrounds the burner, and that smoke or flue gases be drawn out of the combustion chamber into the primary zone by the impulse of the flame by means of the free space located between the stepped air flows~
By dividing the secondary air flow, the following ad-vantages are obtained with the inventive method: low N0x-emission, no sintering and corrosion on the tube walls of the combustion chamber, as well as a certain ignition and satis-factory combustion over a wide operating range.
While that part of the secondary air which is supplied directly to the burner can be so influenced in relation to twist and speed that an intensive ignition is assured over the entire load range, the second part of the secondary air flow, which is added to the flame as a stepped-air flow ex-ternally o the burner, is such that flfter successful igni-tion and primary combustion there is realized the combustion in the secondary zone and the object o the present inven-tion by means of the mixing energy of the stepped-air strearn.
Referring now to the drawing in detail, the burner in-cludes a core-air pipe or tube 2, a fuel and carrier-air portion 1, and a mantle-air part 3. With this burner, there is obtained a partial combustion zone (primary zone) 6, the air number of which is between 0.9 and 0.5 times the stoi-chiometric relationship~
The burner is embodied in such a way that by predeter-,,5~j~37 mined measures (twist of the mantle-air, conically widened burner outlet, closed core-air~ there is generated in the interior of the flame a zone of intensive return flow or back-flow 5 from a region of already advanced combustion. In so doing, the fuel-air mixture is quickly heated up and ignited. The heating-up and ignition can be influenced by adding core-air. Accordingly, the best ignition is assured wllen the core-air is closed.
The air necessary for the burning-out of the remainder or residue is blown in along the periphery as stepped- or staged-air 4 by means of several jets or nozzles in such a way that not until after formation of the primary flame is the secondary flame or also the afterburner zone 7 supplied with oxygen. For this purpose, the stepped-air flow 4 is arranged in a divided circle which corresponds to twice the diameter of the mantle-air tube. This assures that the stepped-air 4 only reaches the actual flame downstream from the burner outlet after a distance of approximately one to two times the dlameter of the mantle-air tube.
Smoke or flue gases are drawn in from the combustion chamber by impulse exchange at those segments o~ the peripheral surface of the flame not adjoining the stepped-air flow 4. In this manner, the flame temperature is re-ducedO
Claims (3)
1. A method of reducing NO emission during com-bustion of nitrogen-containing fuels with burners in a closed combustion chamber, said method including the steps of:
effecting combustion in a below-stoichiometric primary combustion zone with direct air supply through the asso-ciated burner;
effecting combustion in an above-stoichiometric se-condary combustion zone with the addition of the balance of the air;
regulating said balance of the air as a function of load; and adding said balance of the air to said combustion chamber around the associated burner in such a way that after formation of said primary combustion zone, said balance of the air flows into the outer region of the flame while feeding said secondary combustion zone.
effecting combustion in a below-stoichiometric primary combustion zone with direct air supply through the asso-ciated burner;
effecting combustion in an above-stoichiometric se-condary combustion zone with the addition of the balance of the air;
regulating said balance of the air as a function of load; and adding said balance of the air to said combustion chamber around the associated burner in such a way that after formation of said primary combustion zone, said balance of the air flows into the outer region of the flame while feeding said secondary combustion zone.
2. A method according to claim 1, which includes the step of adding air directly through the associated burner in such a quantity that an air number between n = 0.9 and n = 0.5 is obtained in said primary zone.
3. A method according to claim 2, which includes the steps of dividing said balance-of-the-air flow into at least two separated partial streams, introducing said partial streams to said combustion chamber from a divided circle which concentrically surrounds the associated burner, and withdrawing flue gases from said combustion chamber into said primary zone by the impulse of the flame by means of the free space between said separated partial air streams.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000359730A CA1145657A (en) | 1980-09-04 | 1980-09-04 | Method of reducing no.sub.x-emission |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000359730A CA1145657A (en) | 1980-09-04 | 1980-09-04 | Method of reducing no.sub.x-emission |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1145657A true CA1145657A (en) | 1983-05-03 |
Family
ID=4117819
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000359730A Expired CA1145657A (en) | 1980-09-04 | 1980-09-04 | Method of reducing no.sub.x-emission |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1145657A (en) |
-
1980
- 1980-09-04 CA CA000359730A patent/CA1145657A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |