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
C110S213000, C110S214000, C110S302000, C110S308000, C110S347000, C110S348000, C431S004000, C431S009000, C431S010000, C431S011000
Reexamination Certificate
active
06325002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to a method that uses combustion techniques to reduce nitrogen oxides from the combustion of carbonaceous fuels to very low levels. More particularly, it refers to a combustion technique that uses two sequential stages of partial oxidation followed by a final stage of complete oxidation. In-situ furnace flue gas recirculation is incorporated in the second stage of partial oxidation to effect a cooler oxidation zone with lower localized oxygen concentrations.
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 first and 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 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. Other patents describe external flue gas recirculation such as U.S. Pat. 5,002,484 for reducing NOx emissions. Still others use flue gas recirculation within the burners proper, e.g. U.S. Pat. Nos. 5,316,469 and 5,562,438 to reduce NOx emissions. While these methods accomplish their intended purposes, they do not provide the NOx reduction required under current U.S. Environmental Protection Agency (EPA) regulations.
The Clean Air Act Amendments of 1990 set NOx emission limits for coal-fired utility boilers to be met in the year 2000, that range from 0.40 to 0.86 lb NOx/10
6
Btu depending on boiler type and many of the patented techniques mentioned above could have been used to meet these limits. However, in response to the Ozone Transport Assessment Group (OTAG) State Implementation Plan (SIP) call to Eastern and Mid-Western States in 1998, the U.S. EPA has promulgated new rules for nitrogen oxides emissions for all types of coal-fired boilers that will require emissions of 0.15 lb NO
x
/10
6
Btu or less during the ozone season (May through September) in the year 2003.
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 NOx emissions is the Selective Catalytic Reduction (SCR) technology. The SCR method uses ammonia addition and a downstream catalyst to destroy the NOx produced in the coal combustion process. This approach is expensive both from capital and operating cost perspectives. Further, arsenic in the coal can poison the catalyst, shortening its life. Also, ammonium sulfites/sulfates and calcium sulfates from the combustion process can blind the catalyst, thereby reducing its effectiveness.
Therefore, it would 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. Such a system will also provide a lower cost per ton of NO
x
reduced compared to SCR to provide the electric utility industry an economical technology to meet the newly promulgated level of nitrogen oxides emissions.
The staged combustion with in-situ furnace flue gas recirculation method of the present invention is less costly than SCR technology in achieving these reductions and since catalyst, which can be poisoned from the products of carbonaceous fuel combustion, is not required, staged combustion with in-situ furnace flue gas recirculation represents a more reliable technology.
SUMMARY OF THE INVENTION
I have discovered a process employing staged combustor and in-situ furnace flue gas recirculation techniques into one system that will reduce NO
x
emissions to the Year 2003 regulated limit of ≦0.15 lb. NOx/10
6
Btu. To accomplish staged combustion, any of the first stages of existing staged combustors can be used wherein the air to fuel stoichiometric ratio (SR) can be operated in the 0.50 to 0.70 range. These types of staged combustors are described for example in U.S. Pat. Nos. 4,423,702; 4,427,362; 4,685,404; 4,765,258 and 5,458,659, each of which is hereby incorporated by reference herein. Such staged combustion methodologies may also add alkali compounds to reduce the coal ash slag viscosity and/or to capture sulfur in the molten slag. Although any staged combustor type could be used, the preferred types are those that remove molten slag from the combustor proper to minimize ash carryover and reduce slag fouling in the boiler furnace. Further, cyclone fired furnaces that operate under excess air conditions may be retrofitted to implement the three stage combustion technique.
Typically, coal is fired in the first stage of the combustors under a stoichiometric ratio (SR) of air to coal of about 0.50 to 0.70 to minimize the NO
x
produced from the oxidation of fuel bound nitrogen. A fuel gas is exiting and molten slag separated out. In the second stage of combustion, preheated second stage combustion air is introduced into the fuel gas produced in the first stage, an air rate being added to yield an overall SR at this point of about 0.85 to 0.99. The method of air entry to provide for rapid mixing and high localized flame temperatures under a reducing atmosphere to yield the high temperature condition that provides for high combustion efficiency (defined as low unburned carbon content in ash). Since the flame zone is reducing (oxygen deficient) in the second stage partial oxidation zone, minimal thermal NO
x
is produced. The second stage combustion fuel gas and air introduction methods are designed to effect in-furnace flue gas recirculation (FGR) that allows part of the oxygen for the second stage partial oxidation to be provided by the FGR. This technique provides for lower localized oxygen concentrations in the flame zone that helps to minimize NOx formation.
The second stage products of partial combustion rise up through the boiler furnace and are cooled by radiant heat transfer to the furnace water-walls. When the flue 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. NO
x
production is greatly reduced in this OFA zone because the temperatures are relatively low and thermal NOx production reactions are not favored.
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: 4685404 (1987-08-01), Sheppard et al.
patent: 4765258 (1988-08-01), Zauderer
patent: 5181475 (1993-01-01), Breen et al.
patent: 5291841 (1994-03-01), Dykema
patent: 5458659 (1995-10-01), Ashworth
patent: 5462430 (1995-10-01), Khinkis
patent: 5878700 (1999-03-01), Farzan et al.
Buchanan Ingersoll P.C.
Ciric Ljiljana V.
Clearstack Combustion Corporation
Lazarus Ira S.
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