Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-05-03
2002-02-19
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S237000, C423S239100, C423S245300, C423S246000
Reexamination Certificate
active
06348178
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the removal of nitrogen oxides or “NOx” from exhaust gases and the like, and more particularly to processes and apparatus for reducing NOx selectively using autocatalytic, autothermal reactions in a manner to also remove other exhaust contaminants from the combustion of carbonaceous fuels, and also to industrial processes using the same.
BACKGROUND OF THE INVENTION
Without being bound by any particular theory, the background of the present invention will be described by way of a description of particular problems discussed in the art and various proposed solutions to such problems. For brevity, various references will be briefly and generally summarized herein. A more complete understanding of such background art may be obtained by a complete review of the documents cited herein, etc. What should be understood from the following discussion is that, despite such extensive prior efforts to provide various methods of NOx removal and the like, a continuing need exists for practical and low-cost methods of NOx removal in a variety of industrial processes, which may utilize a variety of commercially-available reductants.
Carbonaceous fuels are burned in internal combustion engines and other equipment such as boilers, furnaces, heaters and incinerators, and the like (i.e., in a wide variety of industrial process). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons and soot. High temperature combustion using excess air, however, tends to generate oxides of nitrogen (often referred to as NOx).
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO
2
). Free radicals of nitrogen (N
2
) and oxygen (O
2
) combine chemically primarily to form NO at high combustion temperatures. This thermal NOx tends to form even when nitrogen is removed from the fuel. Combustion modifications which decrease the formation of thermal NOx generally are limited by the generation of objectionable byproducts.
Mobile and stationary combustion equipment are concentrated sources of NOx emissions. When discharged to the air, emissions of NO oxidize to form NO
2
, which tends to accumulate excessively in many urban atmospheres. In sunlight, the NO
2
reacts with volatile organic compounds to form groundlevel ozone, eye irritants and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions to low levels. Despite advancements in fuel and combustion technology, groundlevel ozone concentrations still exceed federal guidelines in many urban regions. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement stringent NOx emissions regulations. Such regulations will require low NOx emissions levels that are attained only by exhaust aftertreatment.
Exhaust aftertreatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods are known in the art and involve nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO
2
for removal by wet scrubbers. Such aftertreatment methods typically require some type of reactant for removal of NOx emissions.
Wet scrubbing of NO
2
produces waste solutions that represent potential sources of water pollution. Wet scrubbers primarily are used for NOx emissions from nitric acid plants or for concurrent removal of NO
2
with sulfur dioxide (SO
2
). High costs and complexity generally limit scrubber technology to such special applications. Wet scrubbers are applied to combustion exhaust by converting NO to NO
2
, such as is described in U.S. Pat. No. 5,047,219.
The NSCR method typically uses unburned hydrocarbons and CO to reduce NOx emissions in the absence of O
2
. Fuel/air ratios must be controlled carefully to ensure low excess O
2
. Both reduction and oxidation catalysts are needed to remove emissions of CO and hydrocarbons while also reducing NOx. The cost of removing excess O
2
precludes practical applications of NSCR methods to many O
2
-containing exhaust gases.
Combustion exhaust containing excess O
2
generally requires chemical reductant(s) for NOx removal. Commercial SCR systems primarily use ammonia (NH
3
) as the reductant. Chemical reactions on a solid catalyst surface convert NOx to N
2
. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Excess NH
3
needed to achieve low NOx levels tends to result in NH
3
breakthrough as a byproduct emission.
Large catalyst volumes are normally needed to maintain low levels of NOx and NH
3
breakthrough. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants. Even under normal operating conditions, the SCR method requires a uniform distribution of NH
3
relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed nonuniformly, so low levels of both NOx and NH
3
breakthrough may be achieved only by controlling the distribution of injected NH
3
or mixing the exhaust to a uniform NOx level.
NH
3
breakthrough is alternatively limited by decomposing excess NH
3
on the surface of a catalyst as described in U.S. Pat. No. 4,302,431. In this case, the excess NH
3
is decomposed catalytically following an initially equivalent decomposition of NOx and NH
3
together. The decomposition of excess NH
3
, however, reduces the selectivity of the SCR method, increasing the molar ratio of NH
3
with respect to NOx as much as 1.5 or higher.
In a combination of catalytic and noncatalytic reduction methods, both NOx and NH
3
removal may be controlled by SCR following an initial stage of NOx reduction by SNCR. In the SNCR method, NOx emissions may be reduced partially without controlling NH
3
breakthrough to a low level. The SCR method may decrease NOx further while also lowering NH
3
breakthrough to an acceptable level.
The use of excess NH
3
to enhance NOx removal by the SNCR method is described in detail in U.S. Pat. Nos. 4,978,514 and 5,139,754. With such methods, the NH
3
injection to SNCR is controlled so that the unreacted NH
3
remains sufficient for the subsequent catalytic reduction of NOx to a low level. This injection strategy is based on the use of excess NH
3
for reducing NOx to lower levels, as with the SCR method described above.
Another method for combining SNCR and SCR methods is described in U.S. Pat. No. 5,510,092. In this method, the catalytic NOx reduction is always maximized using a separate NH
3
injection grid, and the NOx emissions are reduced noncatalytically only as needed to maintain a final low NOx level. This method decreases the consumption of NH3 by minimizing the use of SNCR which removes NOx less selectively than the catalytic method.
The low selectivity of the SNCR method and the use of excess NH
3
for decreasing NOx levels is reported by Lyon, who is believed to have first suggested the noncatalytic reduction of NOx (U.S. Pat. No. 3,900,554). In commercial coal-fired boiler tests, 73% NO reduction has been reported with 2.2 ppm NH
3
breakthrough using a 0.9 molar ratio of NH
3
with respect to NO, while 86% NO reduction required 11 ppm NH
3
breakthrough and a 2.2 molar ratio of NH
3
with respect to NOx. These results are reported in Environ. Sci. Technol., Vol. 21, No. 3, 1987.
In another article (Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986), Lyon also reports the inhibiting effect of NH
3
on CO oxidation. This observation in experiments and commercial tests is confirmed by modeling studies. The inhibition has been explained in terms of competition between NH
3
and CO for reaction with the OH free radical. It is believed that, while NH
3
inhibits the oxidation of CO, the CO a
Palekar Vishwesh
Ramavajjala Madhu
Slone Ralph J.
Sudduth Bruce C.
Griffin Steven P.
Loudermilk & Associates
Medina Marebel
Noxtech, Inc.
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