Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-10-05
2002-06-11
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S355000, C422S172000, C422S173000, C422S198000
Reexamination Certificate
active
06403046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the reduction of pollutants from flue gas and, more particularly, to a process for reducing the level of gaseous and particulate pollutants from the flue gas of one or more fossil fuel fired furnace, wherein ammonia and urea are respectively used in separate sections of such a generation facility to assist in the removal of such pollutants in such separate sections.
2. History of the Related Art
The combustion of fossil fuels (e.g. coal) in power plants generates undesirable nitrogen oxides (NOx), usually in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO
2
). It is known that under certain operating conditions the NOx level in a flue gas stream can be lowered by reacting the NOx with ammonia to produce harmless water and nitrogen as products. The NOx reducing reaction with ammonia can occur at relatively high temperatures, in the absence of a catalyst, in a process known as selective non-catalytic reduction (SNCR). The reaction can also occur at significantly lower temperatures, in the presence of certain catalysts, in a process known as selective catalytic reduction (SCR).
Several processes have been disclosed in prior art which attempt to combine an SNCR process with an SCR process. Typically, in the known so-called combined or “Staged” SNCR/SCR processes, a nitrogenous treatment agent, such as urea, is introduced within the boiler, at a location where the flue gas temperature is still high enough to effectively promote the non-catalytic reduction of NOx by ammonia, typically in the range of 1600° F. to 2100° F. To achieve additional NOx reduction, catalysts are typically installed downstream from the location of the SNCR temperature region, at a point where the flue gas is at a temperature effective for the SCR process, typically in the range of 550° F. to 780° F. As the flue gas containing the excess ammonia remaining from the SNCR stage passes the catalyst, the excess ammonia reacts with the NOx. In the most rationale approaches, as well as in the instant invention, reliance on simply the vagaries of the excess ammonia from “slip” passed from SNCR stage, is believed to be inefficient, nonreliable, and potentially harmful (i.e. uncontrolled ammonia slip and/or maldistribution). Further, by not relying solely on ammonia slip, or overloading, from the SNCR stage, the necessity for enhancers may be eliminated or greatly alleviated. Finally, limited tests on known SNCR/SCR systems which have been produced to date have illustrated that because of such factors as mal-distribution of NOx and ammonia at the face of catalyst from SNCR ammonia slip, the SCR performance is better enhanced by use of an independent ammonia injection grid to complement the ammonia supplied by slip from the SNCR stage (i.e. see the paper titled “Selective Catalytic Reduction Performance Project at Public Services Electric and Gas Company's Mercer Generating Station Unit No. 2”, which was believed to have been presented at the Spring 1995 EPRI NOx Conference in Kansas City, Mo.). With the invention herein, and in other instances, the primary or supplemental supply of ammonia for the SCR phase, is from a selectively controllable source, independent of slip.
U.S. Pat. No. 4,302,431 to Atsukawa et al. discloses a process and apparatus for controlling nitrogen oxides in exhaust gases involving introducing ammonia into an exhaust gas at 700° C. (1292°) to 1300° C. (2372° F.), and then passing the exhaust gas over a catalyst at a temperature between 300° C. (572° F.) to 500° C. (932° F.) (preferably with the introduction of additional ammonia) to decompose remaining NOx and ammonia.
U.S. Pat. No. 4,978,514 to Hofmann et al. discloses a combined catalytic
on-catalytic process for nitrogen oxides reduction. The process of Hofmann et al. requires that, after the SNCR stage, sufficient ammonia is present in the effluent to react with the remaining effluent NOx in the catalyst stage. The Hofmann et al. process also utilizes an enhancer such as oxygenated hydrocarbons, heterocyclic hydrocarbons having at least one cyclic oxygen, sugar or molasses.
U.S. Pat. No. 4,981,660 to Leach discloses a selective hybrid NOx reduction process which utilizes an upright housing such as a natural draft heater tower. In the Leach process, a sufficient amount of reagent ammonia or ammonia radical must be added for the non-catalyst stage such that excess unreacted reagent remains for the catalyst stage.
U.S. Pat. No. 5,057,293 to Epperly et al. discloses a multi-stage process for reducing the concentration of pollutants in an effluent. The Epperly et al. multi-stage process reduces the nitrogen oxides concentration in the effluent such that an approximately 1:1 ratio of ammonia to nitrogen oxides remains in the effluent exiting the SNCR stage to provide ammonia for the SCR stage.
A related process is disclosed in U.S. Pat. No. 5,139,754 to Luftglass et al. The disclosed catalytic
on-catalytic combination process for nitrogen oxides reduction requires the introduction of a nitrogenous treatment agent for the non-catalytic stage in such an amount that ammonia remains in the treated non-catalytic effluent to be used for the SCR stage.
U.S. Pat. No. 5,233,934 to Krigmont et al. discloses a control method of reducing NOx in flue gas streams utilizing an SNCR treatment followed by an SCR treatment. The Krigmont et al. method tries to maximize the NOx removal in the SNCR stage, subject to certain ammonia slip restrictions, and injecting additional ammonia for the SCR stage.
Another related patent, U.S. Pat. No. 5,286,467 to Sun et al. discloses a hybrid process for nitrogen oxides reduction in which a nitrogenous treatment agent other than ammonia is introduced in such a quantity that ammonia is present in the treated effluent leaving the non-catalytic stage. If the SNCR stage does not generate sufficient ammonia for the SCR portion, the process of the Sun et al. patent also provides a source of ammonia to make up the difference. U.S. Pat. No. 5,853,683 illustrates a process similar to the '467 patent.
U.S. Pat. No. 5,510,092 to Mansour et al. discloses a combined SNCR/SCR process in which SCR is employed for primary NOx reduction and NH
3
is injected into the SNCR zone only when the NOx content of the effluent exceeds a preselected design maximum value.
The known hybrid SNCR/SCR related processes typically attempt to maximize the efficiency and chemical utilization of the SNCR so as to minimize the level of nitrogen oxides remaining in the flue gas for processing by the SCR stage, while simultaneously producing excess NH
3
A known deficiency of such an SNCR/SCR processes is that the intentional injection of excess SNCR reagent is not the least cost methodology for providing for ammonia reagent within flue gas.
A review of the above described prior art clearly illustrates that, in instances where mandated levels of post combustion NOx reduction do not require full scale stand alone SCR systems (i.e. see U.S. Pat. No. 5,853,683), combined SNCR/SCR systems of the general type discussed above may be a reasonable solution. Furthermore, to better insure reliability, uniformity, flexibility, responsiveness and overall system control, the most appropriate combined SNCR/SCR system, will include a source of ammonia for the SCR stage of the system, which is at least in part, separate from any dependence on ammonia slip (natural or forced) from the SNCR stage.
While combined SNCR/SCR systems hereinbefore have to some extent addressed the need to provide a separate ammonia supply for the SCR stage, this very requirement of providing an independent ammonia supply has greatly inhibited this approach. In this regard, the SNCR stage is typically performed by injecting urea solution into the furnace where relatively high temperatures serve to initiate the breakdown of urea to form the transient species, including ammonia, which lead to effective NOx reduction. As such, in a typical urea based SNCR system, there is no need to transpo
Griffin Steven P.
Hera, LLC
Medina Sanabria Maribel
Sandler Howard E.
LandOfFree
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