Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2001-02-28
2002-09-24
Lazarus, Ira S. (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S210000, C110S342000, C110S348000
Reexamination Certificate
active
06453830
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention discloses a method for the reduction of the concentration of nitrogen oxides, NO
x
, in the products of combustion of a solid fossil fuel. The method comprises of creating a fuel rich combustion zone downstream of a primary excess air combustion zone in a combustor or furnace by introducing additional fuel into the downstream zone. The preferred embodiment of this invention is to inject into the downstream zone, aqueous liquid droplets containing dispersed solid fuel particles. Dispersion of the solid fuel particles is maintained by continuous mixing of the liquid and, if necessary, by the addition of a surfactant in the liquid-solid mixing vessel. Injection takes place in the furnace at a temperature range of 2000° F. to 2500° F. downstream of the main combustion zone where about 90% of the total fuel is burned under excess air conditions. The additional fuel injected in said zone results in slightly fuel rich temperature conditions, which converts the NO
x
to molecular nitrogen. For economic reasons, it is preferred to limit the additional fuel to 10% of the total fuel heat input to the furnace or boiler. However, the process has been found to be effective even with more than 30% of the total heat input injected in the downstream zone. Further downstream, sufficient additional air is introduced into the furnace to oxidize all unburned combustion gases and remaining fuel particles. Alternatively, the fuel rich NO
x
reduction zone can be limited to a central region of the furnace. The combustion gases containing excess air that surrounds this fuel rich zone will mix further downstream with the fuel rich gas and complete the combustion process. Specially designed atomized water droplet injectors are utilized to disperse and vaporize the droplets that contain the solid fuel particles throughout the gas temperature zone that yields optimum NO
x
reduction.
Alternatively, the fuel rich combustion zone can be produced by dispersion of liquid fuel droplets of varying size throughout the gas temperature zone at which NO
x
reduction is effective.
A third alternative method to produce the fuel rich combustion zone is to utilize pyrolysis gas, derived from coal or biomass produced in a separate vessel.
A fourth alternative method to introduce the additional fuel is to use pulverized coal particles or shredded biomass particles. The selection of the specific fuel and means for introducing this fuel is determined by fuel availability and economics and by combustor and boiler design considerations. The staged combustion method of this invention using these fuels and fuel injection methods can be combined with other NO
x
reduction processes to yield large overall NO
x
emission reductions.
2. Description of Prior Art
The combustion of fossil fuels under excess air conditions leads to the formation of NO
x
, a pollutant that leads to smog and acid rain over wide areas far removed from the combustion source, and it is especially a problem in urban environments. There are two sources of NO
x
, one is primarily formed during the combustion of solid fossil fuels, especially coal. The fuel bound nitrogen whose concentration is generally in the range of 1% by weight in the coal is the primary source of NO
x
in coal combustion. The three primary NO
x
precursors released in the combustion of fuel bound nitrogen are hydrogen cyanide, HCN, ammonia, NH
3
, and nitrogen oxide, NO. In fuel rich combustion, these three species are converted to nitrogen. Many researchers have measured the rate of destruction of these species under fuel rich conditions, (e.g. J. W. Glass and J. O. L. Wendt, “Mechanisms Governing the Destruction of Nitrogenous Species During Fuel Rich Combustion of Pulverized Coal”, in
Proceedings
19
th
Symposium
(
International
)
on Combustion,
[The Combustion Institute, Pittsburgh, Pa. 1982] p.1243).
These rates can be used to estimate the time required to reduce these three species by a specific amount, such as a factor of 10. It was found that as the stoichiometric ratio approaches unity, i.e. as it proceeds from very fuel rich to leaner conditions, the concentration of NO predominates and the other two species are sharply reduced, (see for example, Y. H. Song, et.al., “Conversion of Fixed Nitrogen in Rich Combustion”, in
Proceedings
19
th
Symposium
(
International
)
on Combustion,
[The Combustion Institute, Pittsburgh, Pa. 1982] p.53). Therefore, a conventional fuel lean combustor will produce almost completely NO species.
Calculations were performed for the time needed for a factor of 10 reduction of NO in a Western U.S. coal, using Glass' reaction rates, for two fuel rich stoichiometric ratios of 0.5 and 0.7, i.e. 50% and 30% oxygen deficiency, respectively. Initial concentrations of NO at these stoichiometric ratios were taken from D. P. Rees, et.al., “NO Formation in a Laboratory Pulverized Coal”, in
Proceedings
19
th
Symposium
(
International
)
on Combustion,
[The Combustion Institute, Pittsburgh, Pa. 1982] p. 1305). It was found that temperature was by far the primary rate-governing factor, a factor of ten reduction required several seconds, while at 2500° F., it required about 0.1 seconds, and at 3000° F., about 0.01 seconds, for both 50% and 30% fuel rich stoichiometry. At the highest of these temperatures, however, thermal NO
x
begins to form in significant quantities under excess air conditions. Additionally, combustion with oxygen in excess of the amount required for stoichiometric combustion, which is required for all fossil fuels to minimize other pollutants, such as carbon monoxide, results in the formation of thermal NO
x
. Thermal NO
x
is formed from the reaction of nitrogen with oxygen in the combustion air, and its concentration rises substantially at temperatures above about 3000° F.
The combustion gas velocity in the furnace region upstream of the superheater in large boilers is in the range of 20 to 25 feet per second. Therefore to create a,fuel rich zone in combustion gases containing excess oxygen at 2000° F. would require a distance of 10's of feet in the gas flow direction to achieve the factor of ten NO reduction. It thus appears that temperatures nearer to 2500° F. are preferred to reduce the NO concentration in a thin gas slab perpendicular to the combustion gas flow direction. Based on the NO reduction calculation, it is only necessary to introduce the additional fuel at the base of a slab of gas having the proper temperature in a cross-section perpendicular to the gas flow direction. At 2500° F., only 2 to 2-½ feet in the gas flow direction are required to effect a high NO concentration reduction. While the calculation given here demonstrates the disclosed reburn approach, in actual practice a gas temperature range of 2000° F. to 2500° F. should be evaluated for optimizing the location of adding reburn fuel and the amount of reburn fuel needed for each specific boiler or furnace.
This additional fuel, or reburn fuel, produces a fuel rich zone downstream of the primary combustion zone of the furnace or boiler where the combustion gases contain an excess of oxygen. It is essential to implement the reburn process in as short a distance in the gas flow direction as possible. Furthermore, the reburn fuel should be introduced at the outer gas temperature boundary at which the NO reduction rate is optimum. This location for introducing the reburn fuel should be several feet away from the boiler walls in large boilers because the combustion gas temperature near the wall is lower and the reburn reaction rate is slower. The present invention discloses how this reburn fuel is preferably introduced into the boiler or furnace.
Coal is the primary fuel for utility boilers, and to efficiently burn it requires combustion at 3000° F. or higher. Consequently, both fuel bound and thermal NO
x
form in high concentration, especially in large coal fired boilers used in electric utility power plants.
Several currently practiced tec
Lazarus Ira S.
McGuireWoods LLP
Rinehart K. B.
Zauderer Bert
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