Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-10-20
2001-08-28
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
Reexamination Certificate
active
06280694
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a single stage process for removing NOx compounds from waste products, compounds and wastewaters. More specifically, the invention relates to a single stage process utilizing a fluidized bed container to remove NOx compounds from explosive, hazardous and/or radioactive materials.
BACKGROUND OF THE INVENTION
Nitrogen oxides can be commonly found in many waste products and compounds. Nitrogen oxides (referred to herein as “NOx”) include such compounds as nitric acid, aluminum nitrate, sodium nitrate, ammonium nitrate, potassium nitrate and the like.
Traditional approaches to removing NOx include dry contact reduction processes for solid and gaseous nitrate compounds and wet absorption processes for gaseous NOx. Dry contact reduction processes may be either catalytic or non-catalytic and may be either selective or non-selective. Selective reduction processes are characterized by the selective reduction of gaseous nitrogen oxides and their consequent removal in the presence of oxygen. A common selective reduction agent for gaseous NOx is ammonia. Ammonia, however, oxidizes to form unwanted nitrogen oxide at high temperatures. Moreover, excess ammonia is itself a pollutant.
In the drawings, Other selective reduction methods employ catalysts such as iridium. The problem with catalyst reduction is that the presence of particulates, sulfurous acid gases and other poisons reduce catalyst effectiveness and life thereby increasing costs.
Non-selective reduction processes generally involve the addition of a reducing agent to the gaseous NOx containing material, consuming all free oxygen through combustion and reducing the NOx to nitrogen by the remaining reducing agent. Catalysts are typically utilized in these processes. Reducing agents useful in these processes are both scarce and expensive.
Wet absorption processes typically require large and expensive equipment such as absorption towers. An example of a wet absorption process is the absorption of nitrogen oxides by water or alkali solution. Another shortcoming of the wet absorption process is that these methods are not economically effective where the NOx concentration in the gaseous waste stream is above 5,000 ppm.
In the nuclear industry, there is an annual production of significant amounts of wastes which are classified as radioactively contaminated salt cakes, ion exchange media, sludges and solvents. These radioactive wastes either contain nitrogen oxides or nitrogen oxides are produced as part of the treatment of these wastes. In particular, nuclear fuel reprocessing with nitric acid produces highly radioactive nitric acid and sodium nitrate waste by-products.
For solid or slurry NOx wastes and compounds a variety of processes have been tried for NOx destruction. Rotary calciner and fluid bed processors have been utilized with typical results yielding less than 90% conversion of solid nitrates to gaseous NOx and nitrogen. The gaseous NOx generally exceeded 10,000 ppm which requires addition of extensive gaseous NOx removal methods as described above. In addition, severe agglomerations occur in processors as well as the presence of flammable or explosive mixtures of nitrates and reducing agents in the processors.
A process which does not have the limitations and shortcomings of the above described prior art methods for nitrogen oxide removal from waste streams and compounds would be highly desirable.
SUMMARY OF THE INVENTION
According to its major aspects and briefly recited, the present invention is a method and apparatus for converting nitrogen oxides directly to nitrogen using a single steam-reformer vessel. Nitrate compounds or wastes are fed into the single vessel along with a fluidized gas composed of steam and oxygen. The single vessel contains an inert media bed made of high-density media, such as amorphous alumina beads up to 3000 microns in diameter. The fluidizing gases are injected at relatively high speeds, ranging up to 800 feet per second.
In a first embodiment of the present invention carbonous materials are used as the heat source to evaporate water in the waste feed and as the principal reducing agent. The vessel is divided into three zones with the lowest most zone operated under oxidizing conditions via the addition of superheated steam with oxygen that reacts with the carbon to form CO/CO
2
and generate heat to evaporate water content and heat nitrate compounds to reduction temperature. The middle zone is operated under strongly reducing conditions in which NO
3
, NO, N
2
O and NO
2
are reduced to N
2
. Steam reforming of carboneceous materials in this zone forms CO, H
2
and CH
4
that serve as strong gaseous reducing agents. The upper zone is operated under oxidizing conditions via the addition of more oxygen that oxidizes the remaining C, CO, CH
4
and H
2
form in the second or middle zone to form CO
2
and water. This process result in only trace NOx, CO and H
2
in off-gas from the single vessel and requires little auxiliary energy to be added.
In a second embodiment of the present invention, the lowest zone is operated under oxidizing conditions and the middle and upper segments operated under strongly reducing conditions. This process results in less NOx, more CO and H
2
output and also requires low auxiliary energy. Auxiliary energy can be provided by electrical heaters.
In the third embodiment of the present invention, all three segments are operated under strongly reducing conditions. This process results in less NOx, increased CO and H
2
and requires additional auxiliary energy.
In a fourth embodiment of the present invention, the lower and middle segments are operated under strongly reducing conditions and the upper segment is operated under oxidizing conditions. This process results in low NOx, no CO and H
2
output but requires auxiliary energy to be added.
BRIEF DESCRIPTION OF THE DRAWINGS
The FIGURE is a schematic illustration of a system for removing NOx from a waste stream or compound according to a preferred embodiment of the present invention.
REFERENCES:
patent: 4218427 (1980-08-01), Yan
patent: 4226830 (1980-10-01), Davis
patent: 4483692 (1984-11-01), Patel
patent: 4609537 (1986-09-01), Tolpin
patent: 4656147 (1987-04-01), Iida et al.
patent: 4662081 (1987-05-01), Greenwood
patent: 4664678 (1987-05-01), Rehmat et al.
patent: 4665632 (1987-05-01), Greenwood
patent: 4966101 (1990-10-01), Maeda et al.
Griffin Steven P.
Mann Michael A
Medina Maribel
Nexsen Pruet Jacobs & Pollard LLC
Studsvik, Inc.
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