Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2000-06-08
2001-12-04
Lazarus, Ira S. (Department: 3743)
Furnaces
Process
Treating fuel constituent or combustion product
C110S214000, C110S302000, C110S308000, C110S347000, C110S348000, C431S010000, C431S011000
Reexamination Certificate
active
06325003
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method that provides for reduction of nitrogen oxides from the combustion of carbonaceous fuels. More particularly, it refers to a combustion technique that uses two sequential stages of partial oxidation followed by a final stage of complete oxidation that can be easily retrofitted to existing pulverized coal-fired and oil-fired utility boilers.
2. Description of the Prior Art
There are several patents that describe staged combustion techniques to reduce nitrogen oxides emissions from the combustion of fuels containing nitrogen. U.S. Pat. No. 3,727,562 describes a three stage process for reducing nitrogen oxides (NO
x
) emissions wherein the first stage of combustion is operated with a deficiency of air and the unburned fuel from this stage is separated and burned in a second zone with excess air and then the second stage gases are burned in a third excess air stage. U.S. Pat. No. 4,343,606 describes a multi-stage combustion process wherein fuel gas produced in a first stage partial oxidation zone, operated at a stoichiometric air to fuel ratio of 0.50 to 0.625, followed by a second stage of oxidation wherein it is operated at an air to fuel stoichiometric ratio of 1.0 or slightly greater. Following this, additional air is added to insure that the fuel is completely oxidized. While these methods accomplish their intended purposes, they do not provide the NO
x
reduction required under current regulations, which reductions are achievable with the present invention.
The Clean Air Act Amendments of 1990 set NO
x
emission limits for coal-fired utility boilers, to be met in the year 2000, that range from 0.40 to 0.86 lb NO
x
/10
6
Btu depending on boiler type. However, the U.S. EPA has promulgated more stringent regulations for Eastern and Mid-Western States that will limit nitrogen oxides emissions for all types of coal-fired boilers during the ozone season (May through September) to 0.15 lb NO
x
/10
6
Btu. The combustion technologies commercially available today cannot meet this limit.
The only technology available to the carbonaceous fuel fired utility boiler industry that will guarantee this low level of NO
x
emissions is the Selective Catalytic Reduction (SCR) technology. The SCR method uses ammonia addition and a downstream catalyst to destroy the NO
x
produced in the coal combustion process. This approach is expensive both from capital and operating cost perspectives.
Further, arsenic concentrations (>10 ppmw) in the coal can also poison the catalyst, shortening its life. In addition, ammonium sulfites/sulfates and calcium sulfates from the combustion process can blind the catalyst, reducing catalyst effectiveness. Still further any ammonia that passes through the catalyst unconverted (ammonia slip) will enter the atmosphere, react with air born sulfur dioxide and nitrogen dioxide to form fine particulate (PM
2.5
). This is an environmental debit for SCR technology because the U.S. EPA regulates emissions of fine particulate. Very fine particles are not filtered out by nose hairs (particles less than 6 microns) and enter the lungs.
It would therefore be very advantageous to have an improved combustion process that will yield nitrogen oxide emissions, when firing carbonaceous nitrogen containing fuels, of 0.15 lb NO
x
/10
6
Btu or less. In such a system, catalyst is not used; therefore, coal products of combustion have no effect on the process other than that of normal combustion processes, making staged combustion is a more reliable technology than SCR. Further, ammonia is not used and fine particulate emissions will not increase.
Such a system will also provide a much lower cost per ton of NO
x
reduced compared to SCR, providing the electric utility industry with an economical technology to meet the level of nitrogen oxides emissions to be imposed on Eastern and Mid-Western utilities in the year 2003.
SUMMARY OF THE INVENTION
We have discovered a process using staged combustion techniques that will reduce NO
x
emissions to the levels to be imposed on Eastern and Mid-Western utilities by the U.S. EPA in the year 2003 (<0.15 lb NO
x
/10
6
Btu). To accomplish this, any existing pulverized coal-fired or oil-fired burners may be used wherein the burner air-to-fuel stoichiometric ratio (SR) is operated in the range of about 0.55 to 0.75.
Typically, the carbonaceous fuel is fired in a wall-fired or corner-fired boiler furnace in a plug flow fashion under a sub-stoichiometric air condition (SR at about 0.55 to 0.75). The plug flow firing technique creates a high temperature reducing condition that minimizes the NO
x
produced from the oxidation of fuel bound nitrogen. Some of the first stage combustion air can be added below the burners. By adding air underneath the burners, the bottom of the furnace is maintained in an excess air condition to preclude the potential of reducing gas corrosion in this part of the furnace. In the second combustion stage, preheated air is introduced into the furnace above the first stage burners, the air rate being set to yield an overall SR in the middle furnace of 0.80 to 0.99. With this technique, the upper middle of the furnace is slightly reducing (oxygen deficient) and the production of thermal NO
x
is minimized.
Another alternative for furnaces with multiple rows of burners is to operate the lower level of burners in an excess air condition, (SR of 1.05 to 1.15) with air introduction through the upper rows of burners such that an SR in the first stage of about 0.55 to 0.75 is maintained. This technique will also keep the bottom of the furnace in an oxidizing condition, but will form slightly more NO
x
.
The products of partial combustion from the second stage of combustion rise up through the boiler furnace and are cooled by radiant heat transfer to the furnace water-walls. When these gases have been cooled down to a range of about 2300° F. to 2700° F., overfire air (OFA) is added to bring the overall SR at this point to a range of approximately 1.05 to 1.25 to complete the combustion process. Thermal NO
x
production is greatly reduced in this OFA zone because the temperatures are relatively low and NO
x
production reactions are not favored. This method of the present invention can be retro-fitted to any existing boiler, furnace and can also be implemented during the construction of new boiler furnaces.
REFERENCES:
patent: 2925069 (1960-02-01), Terpe
patent: 3727562 (1973-04-01), Bauer
patent: 3955512 (1976-05-01), Martin et al.
patent: 4343606 (1982-08-01), Blair et al.
patent: 4423702 (1984-01-01), Ashworth et al.
patent: 4427362 (1984-01-01), Dykema
patent: 4501204 (1985-02-01), McCartney et al.
patent: 4685404 (1987-08-01), Sheppard et al.
patent: 4765258 (1988-08-01), Zauderer
patent: 4971552 (1990-11-01), Fukuda et al.
patent: 5246364 (1993-09-01), Landreth et al.
patent: 5284438 (1994-02-01), McGill et al.
patent: 5458659 (1995-10-01), Ashworth
patent: 5937772 (1999-08-01), Khinkis et al.
Ashworth Robert
Murrell Frederick J.
Zawadzki Edward A.
Buchanan Ingersoll P.C.
Ciric Ljiljana V.
Clearstack Combustion Corporation
Lazarus Ira S.
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