Reduction of ammonia slip in nitrogen oxides reduction process

The present invention is an ammonia injection scheme for NO.sub.x reductions that also reduces ammonia slip wherein the NO.sub.x containing gas to be treated is upstream of a cyclone separator. The reduction in ammonia slip is accomplished by injecting the ammonia through the outside wall of the cyclone inlet duct.

TECHNICAL FIELD 
The present invention relates to a process for reducing the concentration 
of nitrogen oxides (NO.sub.x) in a NO.sub.x containing flue gas. 
BACKGROUND OF THE INVENTION 
Processes for reducing the concentration of NO.sub.x (a common industrial 
pollutant) in a NO.sub.x containing gas are well known in the art. These 
processes generally comprise: 
(a) withdrawing the NO.sub.x containing flue gas as portion of the 
combustion effluent from the combustion of a carbonaceous fuel 
(fluidized-bed boilers are among the most efficient devices for burning 
carbonaceous fuels; the NO.sub.x is primarily formed from the oxidation of 
fuel-bound nitrogen); 
(b) injecting ammonia into the NO.sub.x containing flue gas in order to 
reduce the concentration of NO.sub.x in the flue gas (the reduction of 
NO.sub.x by ammonia is feasible within a narrow temperature range of 
approximately 1600.degree. F.-2000.degree. F. with an optimum temperature 
of about 1785.degree. F.; at higher temperatures, the ammonia is converted 
to NO.sub.x while at lower temperatures, reduction of NO.sub.x by ammonia 
is less effective); and 
(c) feeding the ammonia treated flue gas to a cyclone in order to separate 
entrained particulate matter from the ammonia treated flue gas. 
See for example U.S. Pat. No. 4,756,890 by Tang et al. and European 
published patent application No. 176,293 by Cooper et al. U.S. Pat. No. 
4,770,863 by Epperly et al. teaches that further reductions of NO.sub.x 
can be obtained by also injecting various enhancers into the flue gas such 
as ethylene glycol or sugar. Epperly further teaches that the use of his 
enhancer also reduces the concentration of unreacted ammonia in his 
ammonia/enhancer treated gas. Such a reduction in the "ammonia slip" (as 
it is often referred to in the art) is very advantageous since ammonia is 
a pollutant in and of itself. There is a need in the industry, however, to 
reduce the ammonia slip in a NO.sub.x reducing ammonia injection scheme 
without resorting to the use of an enhancer. It is an object of the 
present invention to meet this need. 
SUMMARY OF THE INVENTION 
The present invention is an ammonia injection scheme for NO.sub.x 
reductions that also reduces ammonia slip wherein the NO.sub.x containing 
gas to be treated is upstream of a cyclone separator. The reduction in 
ammonia slip is accomplished by injecting the ammonia through the outside 
wall of the cyclone inlet duct.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is an improved process for reducing the concentration 
of NO.sub.x in a NO.sub.x containing flue gas. In the process to which the 
improvement of the present invention pertains: 
(a) the NO.sub.x containing flue gas is withdrawn as a portion of the 
combustion effluent from the combustion of a carbonaceous fuel; 
(b) ammonia is injected into the NO.sub.x containing flue gas in order to 
reduce the concentration of NO.sub.x in the flue gas; and 
(c) the ammonia treated flue gas is fed to a cyclone in order to separate 
entrained particulate matter from the ammonia treated flue gas. 
To describe the improvement of the present invention, it is helpful to 
refer to FIG. 1's drawing of a standard cyclone. As shown in FIG. 1, the 
standard cyclone includes an inlet duct 10 for introducing the feed in, to 
the cyclone in a direction which is substantially tangential to the 
circular flow within the cyclone. The entrained particulate matter is 
removed from the bottom of the cyclone while the particulate free gas is 
removed from the top of the cyclone. The inlet duct consists of four 
perimeter walls which form a rectangular cross sectional area: a top wall 
20, a bottom wall 22 opposite the top wall, an outside wall 24 and an 
inside wall 26 opposite the outside wall. The outside wall is more 
specifically defined herein as that perimeter wall of the inlet duct which 
is most nearly at a tangential angle to the circular flow within the 
cyclone. 
The improvement of the present invention is for reducing the concentration 
of ammonia in the ammonia treated gas (ie reducing the ammonia slip) and 
comprises injecting the ammonia through the outside wall of the cyclone 
inlet duct. Although not shown in FIG. 1, the ammonia can be injected 
through a nozzle configuration on the outside wall comprising one or more 
nozzles. 
The term ammonia as employed in this description includes the compound 
ammonia itself, and/or ammonia containing compounds (such as ammonia 
carbonate which will yield ammonia upon vaporization), in aqueous 
solutions or otherwise. 
The reason for the present invention's reduction in ammonia slip is 
probably a function of the fact that injecting the ammonia through the 
outside wall of the cyclone inlet duct makes the entry of the ammonia into 
the cyclone far away from the vortex finder which is located in the center 
of the cyclone. This in turn increases the residence time of the ammonia 
in the cyclone which in turn gives the ammonia more time to form ammonia 
radicals before exiting the cyclone via the vortex finder. (In what is not 
fully understood, the ammonia radicals subsequently react with the 
NO.sub.x to form nitrogen and water or subsequently decompose into 
different species.) 
It should be noted that to the extent that the key to the present invention 
is allowing the ammonia to enter the cyclone far away from the vortex 
finder, the present invention can be broadened to other areas of the inlet 
duct's perimeter which are near the outside wall. (For example, the 
ammonia could also be injected through those portions of the top and 
bottom walls that are closest to the outside wall.) It should also be 
noted that the present invention can be extended to cyclone inlet ducts 
having cross sectional shapes areas other than the usual rectangular shape 
such as a duct having a circular cross section. In such a case, the 
ammonia would be injected through that portion of the cyclone inlet duct's 
perimeter which corresponds to the outside wall of a rectangular inlet 
duct. 
It should be further be noted that, as taught in copending U.S. patent 
application Ser. No. 07/068,725, a tradeoff of the present invention is a 
reduction in ammonia utilization. In actual operation, this tradeoff is 
optimized depending on the importance one assigns to ammonia utilization 
vis-a-vis ammonia slip. 
The following example is offered to demonstrate the efficacy of the present 
invention. 
EXAMPLE 
This example is the result of experimentation conducted at a commercial 
coal fired cogeneration facility in Stockton, Calif. The purpose of this 
example is to demonstrate the present invention's reduction in ammonia 
slip. This was accomplished by showing that, in reducing a flue gas 
NO.sub.x concentration from approximately 190 parts per million (ppm) to 
approximately 21 ppm, the amount of ammonia slip is reduced as a higher 
percentage of the ammonia is injected through the outside wall via-a-vis 
the inside wall of the cyclone inlet duct. As can be seen in the following 
Table, as the percentage of the ammonia injected through the outside wall 
vis-a-vis the inside wall is increased from 0% to 100%, the relative 
amount of ammonia slip is reduced by 78%. When 100% of NH.sub.3 is 
injected through outside wall vis-a-vis inside wall, as shown in Table I, 
ammonia is not injected through inside wall. 
TABLE I 
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% of NH.sub.3 Injected 
Through Outside 
Relative Amount of NH.sub.3 Slip in 
Wall vis-a-vis Inside Wall 
Achieving a 169 PPM NO.sub.x Reduction 
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0 1.00 
17 0.91 
66 0.47 
100 0.22 
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