Combustion method for hydrocarbonic fuels with low emission of nitrogen oxides
United States Patent 3914091
A combustion method for hydrocarbonic fuels with low emission of nitrogen oxides comprising a step of primary combustion where a hydrocarbonic fuel mixed with air in an amount less than 70 percent of the theoretically required amount for completion of combustion of the fuel is subjected to a primary combustion and then allowed to pass through a process of accelerating the partial combustion to decompose into simple inorganic gases consisting mainly of H2 and CO as combustible components, and a step of secondary combustion where the simple inorganic gases are led to a secondary combustion chamber independent from the foregoing steps to have the combustion completed with feed of air for combustion. A combustion method for hydrocarbonic fuels with low emission of nitrogen oxides comprising a step of primary combustion where a hydrocarbonic fuel mixed with air in an amount less than 70 percent of the theoretically required amount for completion of combustion of the fuel is subjected to a primary combustion and then allowed to pass through a process of accelerating the partial combustion to decompose into simple inorganic gases consisting mainly of H2 and CO as combustible components, and a step of final combustion where the simple inorganic gases are led across heat absorbing processes to a combustion chamber independent from the foregoing steps for completion of the combustion with feed of air for combustion.
US Patent References:
Method of burning fuels
Fraser et al. - January 1966 - 3228451
COMBUSTION METHOD OF GAS BURNERS FOR SUPPRESSING THE FORMATION OF NITROGEN OXIDES AND BURNER APPARATUS FOR PRACTICING SAID METHOD
Iida et al. - May 1973 - 3730668
REDUCING NO EMISSIONS BY ADDITIVE INJECTION
Stengel - July 1973 - 3746498
METHOD OF BURNING LIQUID FUEL IN FLUID BED APPARATUS
Mikami et al. - November 1973 - 3770369
REDUCTION OF NITROGEN OXIDES FROM PRODUCTS OF HYDROCARBON COMBUSTION WITH AIR
LaHaye et al. - August 1974 - 3832122
Method of burning fuels
Fraser et al. - January 1966 - 3228451
COMBUSTION METHOD OF GAS BURNERS FOR SUPPRESSING THE FORMATION OF NITROGEN OXIDES AND BURNER APPARATUS FOR PRACTICING SAID METHOD
Iida et al. - May 1973 - 3730668
REDUCING NO EMISSIONS BY ADDITIVE INJECTION
Stengel - July 1973 - 3746498
METHOD OF BURNING LIQUID FUEL IN FLUID BED APPARATUS
Mikami et al. - November 1973 - 3770369
REDUCTION OF NITROGEN OXIDES FROM PRODUCTS OF HYDROCARBON COMBUSTION WITH AIR
LaHaye et al. - August 1974 - 3832122
Inventors:
Yamagishi, Kazuo (Urawa, JA)
Nozawa, Masaaki (Yokohama, JA)
Tokumoto, Tsunenori (Yokosuka, JA)
Nozawa, Masaaki (Yokohama, JA)
Tokumoto, Tsunenori (Yokosuka, JA)
Application Number:
05/447035
Publication Date:
10/21/1975
Filing Date:
02/28/1974
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Export Citation:
Assignee:
Tokyo Gas Company Limited (Tokyo, JA)
Primary Class:
Other Classes:
431/12
International Classes:
F23C6/04; F23C13/00; F23R3/40; F23C6/00; F23R3/00; F23M3/04
Field of Search:
431/10,351,12
Primary Examiner:
Favors, Edward G.
Attorney, Agent or Firm:
Wenderoth, Lind & Ponack
Claims:
1. A combustion method for hydrocarbonic fuel which results in low emission of nitrogen oxides comprising a primary combustion step of mixing a hydrocarbonic fuel with air in the amount of less than 70 per cent of the theoretically required amount for complete combustion of the fuel and then accelerating the partial combustion in the presence of a nickel catalyst to decompose the fuel into simple inorganic gases consisting mainly of H2 and CO as combustible components, and a secondary combustion step of transferring the simply inorganic gases to a secondary combustion chamber for completing combustion, further air being provided in said secondary combustion chamber.
2. A combustion method for hydrocarbonic fuel which results in low emission of nitrogen oxides comprising a primary combustion step of mixing a hydrocarbonic fuel with air in the amount of less than 70 per cent of the theoretically required amount for complete combustion of the fuel and then accelerating the partial combustion in the presence of a nickel catalyst to decompose the fuel into simple inorganic gases consisting mainly of H2 and CO as combustible components, and a secondary combustion step of transferring the simple inorganic gases through a heat absorbing means and thence to a combustion chamber for completing combustion, further air being provided in said combustion chamber.
2. A combustion method for hydrocarbonic fuel which results in low emission of nitrogen oxides comprising a primary combustion step of mixing a hydrocarbonic fuel with air in the amount of less than 70 per cent of the theoretically required amount for complete combustion of the fuel and then accelerating the partial combustion in the presence of a nickel catalyst to decompose the fuel into simple inorganic gases consisting mainly of H2 and CO as combustible components, and a secondary combustion step of transferring the simple inorganic gases through a heat absorbing means and thence to a combustion chamber for completing combustion, further air being provided in said combustion chamber.
Description:
DESCRIPTION OF THE INVENTION
This invention is concerned with a method of combustion of hydrocarbonic fuels with low emission of nitrogen oxides. It is known theoretically as well as experimentally that nitrogen oxides (NO x ) are less produced in combustion at an air ratio less than the theoretical requirement. Further, in the secondary combustion and thereafter, the combustion temperature generally tends to decline so that the amount of nitrogen oxides to be produced tends to decrease. But, when the conventional staged combustion method is used in which diffusion flames are preferred, the following difficulties are involved, viz. (1) although the primary combustion may be of combustion under the air ratio below the theoretical requirement as a whole, there may be locally created an oxygen rich region where NO x are likely to be produced; (2) the partial combustion may not be completed in the primary combustion resulting to the production of the hydrocarbonic gas components which tend to produce soot, leading to troubles of soot formation in the heat absorbing processes after the primary combustion or in the stage of secondary combustion; and (3) the stage of secondary combustion has a large amount of hydrocarbon left so that there is much formation of the so-called prompt NO probably due to the C--H radicals. With such difficulties, there is a limit in the final reduction of No x , and the reduction rate is 50 to 60 percent at best.
The combustion method of the present invention eliminates the difficulties of the conventional staged combustion method entirely. According to the combustion method of the invention, a hydrocarbonic fuel is mixed with air in an amount less than 70 percent of the theoretically required amount, and is subjected to a primary combustion and then allowed to pass through a process of accelerating the partial combustion to decompose into simple inorganic gases consisting mainly of H 2 and CO as combustible components, then the simple inorganic gases are led to an independent secondary combustion chamber from the foregoing stages for completion of the combustion with feed of air for combustion. Any hydrocarbonic fuels are applicable, and as a partial combustion accelerating process, a combustion chamber having a sufficient geometric volume or some catalytic layer kept at an appropriate temperature may be used. As a catalyst, any of the nickel catalysts usually used as a catalyst for partial combustion of hydrocarbons can be used.
Now the low NO x emission characteristics of a staged combustion method having such a partial combustion accelerating process incorporated will be described. In the primary combustion, a fuel is burned at a ratio of air sufficiently lower than 70 percent of the theoretical requirement. For such combustion, a premix flame is preferable in that it produces only a small amount of NO x in the primary combustion, but from the aspect of stability of combustion, the diffusion flame is advantageous. However, when the diffusion flame is used, the amount of NO x produced in the primary combustion tends to increase, as mentioned in the foregoing. In this stage, the partial combustion is not complete, and a large amount of undecomposed hydrocarbon or C--H compounds are present. The gas is then led to a promising partial combustion accelerating process which consists of some catalytic layer or a combustion chamber having sufficient geometric volume and the following reactions proceed:
CO + H 2 O CO 2 + H 2 CH 4 + H 2 O 3H 2 + CO
and the gas is decomposed into simple inorganic gases comprising mainly CO and H 2 as combustible components. It must be noted here that the NO x produced in the primary combustion are reduced in the partial combustion accelerating process by H 2 and CO produced by the partial combustion. Thus, there is obtained a high temperature partial combustion gas containing mainly CO and H 2 as combustible components with little NO x . This gas is then subjected to a secondary combustion together with air for secondary combustion. But, in order to reduce the generation of NO x in the secondary combustion to a low level, it is preferable to pass the gas through a heat absorbing process as is generally practiced in the conventional stage combustion. In this case, the gas contains no unstable intermediate products of combustion as a substance producing soot but consists mainly of relatively stable CO and H 2 so that formation of soot can be suppressed to a minimum. Since the combustible gases in the secondary combustion are for the most part H 2 and CO, a low temperature combustion enabling to suppress the generation of NO x to a sufficiently low level is practicable, and if required, an oxidation catalyst for low temperature combustion can be used. Where the cooling process after the partial combustion has to be omitted from the restriction in the configuration of combustion equipment and thus the partial combustion gas of high temperature is immediately brought to the secondary combustion, the unburned portion according to the present method consists of H 2 and CO so that the final emission of NO x can be reduced to a far lower level than that in the conventional combustion of hydrocarbons.
As an example, experimental data are shown in the following. With the primary and secondary air ratios taken as 70 percent and 50 percent of the theoretical amount respectively (viz. 120 percent in total) and a Ni catalyst is used as the partial combustion accelerating process, the combustion is carried out at a temperature of about 1,100°C. The values of measurement of NO x are 1 to 2 ppm at the exit of the catalytic layer and less than 10 ppm at the point of termination of the secondary combustion, indicating that the NO x content is suppressed to a low level not attainable by the conventional staged combustion method. In the foregoing example, a Ni catalyst is used for the process of accelerating the partial combustion, but the method of the invention is not limited thereto. In the combustion method of the present invention, anything can be used for such purpose if it will decompose the unburned hydrocarbon component in the primary combustion gas burned in air less than 70 percent of that theoretically required to simple inorganic gases comprising CO and H 2 and preferably to accelerate the reduction of NO by hydrogen or CO. For example, the acceleration of partial combustion may be accomplished not by a catalyst but by a combustion chamber having a sufficiently large geometric volume as described in the foregoing. As an example, there are obtained, under the conditions of combustion of the primary and secondary air ratios at 55 percent and 65 percent respectively (viz. 120 percent in total), SV at 1,000 hrs - 1 and temperature at 1,050°C., the values of measurement are 1 to 2 ppm at the exit of the primary combustion chamber and 15 ppm at the point of termination of the secondary combustion chamber. Now referring to the secondary combustion chamber, the combustion method of the present invention has the secondary combustion and subsequent processes isolated from the processes of the prior stages. Thus, the method of the invention has an advantage in that for various configurations of the secondary combustion chamber, any advantageous pattern of secondary combustion for NO x combustion can be determined freely without any adverse effect upon the processes of the prior stages. Further, in the combustion method of the invention having the partial combustion accelerating process incorporated, the secondary combustion is mainly of H 2 and CO which are readily combustible gases so that the combustion is completed with ease and that it is possible to reduce the emission of CO to a minimum even under a low excess air ratio. As described in the foregoing, the combustion method of the present invention is a unique method which reduces the emission of hazardous substances such as NO x and CO and that of unburned gases to a minimum.
DRAWING
The drawing represents a block diagram of the method of the present invention wherein numeral 1 indicates a fuel system and numeral 2 indicates an air system.
This invention is concerned with a method of combustion of hydrocarbonic fuels with low emission of nitrogen oxides. It is known theoretically as well as experimentally that nitrogen oxides (NO x ) are less produced in combustion at an air ratio less than the theoretical requirement. Further, in the secondary combustion and thereafter, the combustion temperature generally tends to decline so that the amount of nitrogen oxides to be produced tends to decrease. But, when the conventional staged combustion method is used in which diffusion flames are preferred, the following difficulties are involved, viz. (1) although the primary combustion may be of combustion under the air ratio below the theoretical requirement as a whole, there may be locally created an oxygen rich region where NO x are likely to be produced; (2) the partial combustion may not be completed in the primary combustion resulting to the production of the hydrocarbonic gas components which tend to produce soot, leading to troubles of soot formation in the heat absorbing processes after the primary combustion or in the stage of secondary combustion; and (3) the stage of secondary combustion has a large amount of hydrocarbon left so that there is much formation of the so-called prompt NO probably due to the C--H radicals. With such difficulties, there is a limit in the final reduction of No x , and the reduction rate is 50 to 60 percent at best.
The combustion method of the present invention eliminates the difficulties of the conventional staged combustion method entirely. According to the combustion method of the invention, a hydrocarbonic fuel is mixed with air in an amount less than 70 percent of the theoretically required amount, and is subjected to a primary combustion and then allowed to pass through a process of accelerating the partial combustion to decompose into simple inorganic gases consisting mainly of H 2 and CO as combustible components, then the simple inorganic gases are led to an independent secondary combustion chamber from the foregoing stages for completion of the combustion with feed of air for combustion. Any hydrocarbonic fuels are applicable, and as a partial combustion accelerating process, a combustion chamber having a sufficient geometric volume or some catalytic layer kept at an appropriate temperature may be used. As a catalyst, any of the nickel catalysts usually used as a catalyst for partial combustion of hydrocarbons can be used.
Now the low NO x emission characteristics of a staged combustion method having such a partial combustion accelerating process incorporated will be described. In the primary combustion, a fuel is burned at a ratio of air sufficiently lower than 70 percent of the theoretical requirement. For such combustion, a premix flame is preferable in that it produces only a small amount of NO x in the primary combustion, but from the aspect of stability of combustion, the diffusion flame is advantageous. However, when the diffusion flame is used, the amount of NO x produced in the primary combustion tends to increase, as mentioned in the foregoing. In this stage, the partial combustion is not complete, and a large amount of undecomposed hydrocarbon or C--H compounds are present. The gas is then led to a promising partial combustion accelerating process which consists of some catalytic layer or a combustion chamber having sufficient geometric volume and the following reactions proceed:
CO + H 2 O CO 2 + H 2 CH 4 + H 2 O 3H 2 + CO
and the gas is decomposed into simple inorganic gases comprising mainly CO and H 2 as combustible components. It must be noted here that the NO x produced in the primary combustion are reduced in the partial combustion accelerating process by H 2 and CO produced by the partial combustion. Thus, there is obtained a high temperature partial combustion gas containing mainly CO and H 2 as combustible components with little NO x . This gas is then subjected to a secondary combustion together with air for secondary combustion. But, in order to reduce the generation of NO x in the secondary combustion to a low level, it is preferable to pass the gas through a heat absorbing process as is generally practiced in the conventional stage combustion. In this case, the gas contains no unstable intermediate products of combustion as a substance producing soot but consists mainly of relatively stable CO and H 2 so that formation of soot can be suppressed to a minimum. Since the combustible gases in the secondary combustion are for the most part H 2 and CO, a low temperature combustion enabling to suppress the generation of NO x to a sufficiently low level is practicable, and if required, an oxidation catalyst for low temperature combustion can be used. Where the cooling process after the partial combustion has to be omitted from the restriction in the configuration of combustion equipment and thus the partial combustion gas of high temperature is immediately brought to the secondary combustion, the unburned portion according to the present method consists of H 2 and CO so that the final emission of NO x can be reduced to a far lower level than that in the conventional combustion of hydrocarbons.
As an example, experimental data are shown in the following. With the primary and secondary air ratios taken as 70 percent and 50 percent of the theoretical amount respectively (viz. 120 percent in total) and a Ni catalyst is used as the partial combustion accelerating process, the combustion is carried out at a temperature of about 1,100°C. The values of measurement of NO x are 1 to 2 ppm at the exit of the catalytic layer and less than 10 ppm at the point of termination of the secondary combustion, indicating that the NO x content is suppressed to a low level not attainable by the conventional staged combustion method. In the foregoing example, a Ni catalyst is used for the process of accelerating the partial combustion, but the method of the invention is not limited thereto. In the combustion method of the present invention, anything can be used for such purpose if it will decompose the unburned hydrocarbon component in the primary combustion gas burned in air less than 70 percent of that theoretically required to simple inorganic gases comprising CO and H 2 and preferably to accelerate the reduction of NO by hydrogen or CO. For example, the acceleration of partial combustion may be accomplished not by a catalyst but by a combustion chamber having a sufficiently large geometric volume as described in the foregoing. As an example, there are obtained, under the conditions of combustion of the primary and secondary air ratios at 55 percent and 65 percent respectively (viz. 120 percent in total), SV at 1,000 hrs - 1 and temperature at 1,050°C., the values of measurement are 1 to 2 ppm at the exit of the primary combustion chamber and 15 ppm at the point of termination of the secondary combustion chamber. Now referring to the secondary combustion chamber, the combustion method of the present invention has the secondary combustion and subsequent processes isolated from the processes of the prior stages. Thus, the method of the invention has an advantage in that for various configurations of the secondary combustion chamber, any advantageous pattern of secondary combustion for NO x combustion can be determined freely without any adverse effect upon the processes of the prior stages. Further, in the combustion method of the invention having the partial combustion accelerating process incorporated, the secondary combustion is mainly of H 2 and CO which are readily combustible gases so that the combustion is completed with ease and that it is possible to reduce the emission of CO to a minimum even under a low excess air ratio. As described in the foregoing, the combustion method of the present invention is a unique method which reduces the emission of hazardous substances such as NO x and CO and that of unburned gases to a minimum.
DRAWING
The drawing represents a block diagram of the method of the present invention wherein numeral 1 indicates a fuel system and numeral 2 indicates an air system.