Ammonia catalytic abatement apparatus and method

Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component

Reexamination Certificate

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Details

C423S239100

Reexamination Certificate

active

06299847

ABSTRACT:

BACKGROUND OF THE INVENTION
Ammonia abatement is a particular problem in the treatment of human and animal waste, sludge, manure decomposition and fertilizer manufacturing facilities. At present, ammonia gas is generally abated with a wet scrubbing system which is relatively expensive and requires secondary treatment of the wastewater. Combustion catalytic converters are also known for abatement of ammonia gas. However, the effluent from such catalytic converters include unacceptable levels of oxides of nitrogen (NOx) in many applications.
The ammonia catalytic abatement apparatus and method of this invention solves this problem on a dry basis, such that there is no wastewater associated with abating the ammonia laden gas, and it is less expensive and easier to operate while achieving equal or better results than wet scrubbers.
SUMMARY OF THE INVENTION
The ammonia catalytic abatement apparatus and method of this invention utilizes two types of catalytic converters in line, wherein the first catalytic converter converts the ammonia in an ammonia laden gas stream to nitrogen in approximately equal stoichiometric proportions of oxides of nitrogen and the remaining unconverted ammonia gas. The second catalytic converter then converts substantially all the remaining ammonia gas and oxides of nitrogen into nitrogen and water vapor, thus achieving greater conversion of the ammonia to nitrogen while minimizing oxides of nitrogen in the effluent vented to atmosphere. The ammonia laden gas stream is first heated to the required conversion temperature of the first catalytic converter, which is generally about 300° C. or greater. The first catalytic converter in the disclosed embodiment includes platinum as the catalytic agent, which converts approximately 80% of the ammonia to nitrogen gas and about 10% of the ammonia to oxides of nitrogen. The gas stream leaving the first catalytic converter thus includes approximately equal stoichiometric proportions of oxides of nitrogen and ammonia which, in the disclosed embodiment of the first catalytic converter, is about 10% each. The gas stream can now be treated with a second selective catalytic reduction catalyst, wherein substantially all of the oxides of nitrogen and remaining ammonia gas can be converted to nitrogen and water vapor by the following equation:
NH
3
+NO
x
→N
2
+H
2
O
The disclosed preferred embodiment of the apparatus for abating ammonia gas in an ammonia laden gas stream while minimizing oxides of nitrogen in the effluent entering the atmosphere thus includes a heater which receives the ammonia laden gas and which heats the gas to the conversion temperature of the first catalytic converter, a first catalytic converter which converts the ammonia gas to nitrogen, water vapor and approximately equal stoichiometric proportions of oxides of nitrogen (NOx) and the remaining ammonia (NH
3
) gas, and a second catalytic converter which receives this gas stream and which converts substantially all of the remaining NH
3
and NOx. In the disclosed embodiment, ammonia laden gas, typically air, is conveyed by a fan through a recuperative heat exchanger, where the air is preheated. A heater, such as a natural gas burner, then heats the gas stream to approximately 300° C. or greater. The gas stream is then passed through the first catalytic converter, wherein approximately 80% of the NH
3
is converted to N
2
+H
2
O and about 10% NOx, leaving about 10% unreacted NH
3
. The second selective catalytic reduction catalyst then converts substantially all of the remaining NH
3
and NOx to N
2
and H
2
O, as described above. Finally, the gas stream is returned to the regenerative heat exchanger prior to exiting the system to atmosphere.
The method of abating ammonia gas in an ammonia laden air or gas stream while minimizing oxides of nitrogen in the effluent entering atmosphere of this invention thus includes heating the ammonia laden gas stream to the reaction temperature of the first catalytic converter, then converting the ammonia gas in the heated ammonia laden gas stream to nitrogen, water vapor and approximately equal proportions of the remaining unreacted ammonia gas and oxides of nitrogen, then converting substantially all of the remaining ammonia gas and oxides of nitrogen to nitrogen and water vapor. As set forth above, the ammonia gas is preferably converted in a catalytic converter, wherein the first catalytic converter includes platinum as the catalytic agent and the second catalytic converter is preferably is a selective catalytic reagent catalyst generally including base metal oxides as the catalytic agents such as titanium dioxide, vanadium pentoxide and tungsten trioxide. Typical catalytic converters of these types are comprised of a monolithic ceramic block substrate having numerous holes or cells through which the gas stream passes. The first catalytic converter preferably has about two hundred cells per square inch and the catalytic agent, preferably platinum, is calcined onto the ceramic. The second catalytic converter, which is preferably a selective catalytic reduction (SCR) catalyst, may also have a monolithic ceramic substrate structure having about one hundred cells per square inch, preferably calcined with base metal oxides including titanium dioxide, vanadium pentoxide and tungsten trioxide. An alternative embodiment includes an optional bypass around the first catalytic converter which receives heated ammonia laden gas from the heater and directs the heated ammonia laden gas to the second catalytic converter to compensate for excess oxides of nitrogen received from the first catalytic converter. The preferred embodiment includes a mixer which mixes the diverted heated ammonia laden gas received from the bypass and the effluent from the first catalytic converter to achieve approximately equal proportions of ammonia and oxides of nitrogen.
The ammonia catalytic abatement apparatus and method of this invention thus converts the ammonia gas in an ammonia laden gas or air stream while minimizing oxides of nitrogen in the effluent entering the atmosphere without requiring a wet scrubber and there is no wastewater associated with the process which must be further treated. The dry method and apparatus of this invention is approximately as effective as a wet scrubbing system and is less expensive and easier to operate while achieving equal or better results. Other advantages and meritorious features of the ammonia catalytic abatement apparatus and method of this invention will be more fully understood from the description of the preferred embodiments, the appended claims and the drawing, a brief description of which follows.


REFERENCES:
patent: 4138469 (1979-02-01), Kato et al.
patent: 4179407 (1979-12-01), Iiyama et al.
patent: 4438082 (1984-03-01), Dettling et al.
patent: 5534236 (1996-07-01), Woldhuis
patent: 5679313 (1997-10-01), Nojima et al.
patent: 5728356 (1998-03-01), Iida et al.

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