Systems and Processes for Removal and Reduction of NOx and CO Gases from Flue/Exhaust Gas Streams

Abstract

A system and method for the reduction of NOx and CO contaminants using an ion-exchange resin media having lower-valency ions of the transitional metal elements, such as ferrous ions, cuprous ions and/or manganese ions, such that gases containing NOx and/or CO contaminants may be passed over the media so that the contaminants are absorbed by the lower-valency ions of the transitional metal elements, the media configured so that it can be regenerated to remove the NOx and/or CO contaminants. Regeneration includes exposing the media to a heated stream of hydrogen gas or exposing the media to hydrogen ions in an electrochemical cell.

Claims

1-4. (canceled)

5. A method of reducing NOx and CO contaminants, the method comprising: providing an ion-exchange resin media comprising lower-valency ions of the transitional metal elements, and directing gases containing NOx and/or CO contaminants over the media such that the contaminants are absorbed by the lower-valency ions of the transitional metal elements.

6. The method of claim 5, where the lower-valency ions of the transitional metal elements comprise ferrous ions, cuprous ions and/or manganese ions.

7. The method of claim 5, further comprising regenerating the media to remove the NOx and/or CO contaminants.

8. The method of claim 7, wherein regenerating comprises exposing the media to a heated stream of hydrogen gases.

9. The method of claim 7, wherein regenerating comprises exposing the media to hydrogen ions in an electrochemical cell.

Description

DESCRIPTION OF CERTAIN EMBODIMENTS

[0026] The object of the present invention is a more effective and less expensive process for removing NO.sub.x from combustion flue gases. In one application, an embodiment of the present invention is preferably positioned downstream of the particulate removal systems and flue gas desulfurization systems.

[0027] The ferrous ion, as well as the cuprous ion and ions of some transition metal elements, show a propensity for absorbing gas molecules like NO and CO, both of which are present in flue gas to various levels. Thus, aqueous solutions of ferrous sulfate, ferrous perchlorate, ferrous chloride and ferrous sulfate-ethylenediamine tetraacetic acid (FeSO.sub.4-EDTA) have shown significant absorption of nitric oxide (NO), ranging from 15-25 mg NO absorbed per milliliter (ml) of a unimolar (1M) solution, due to the activity of the Fe.sup.++ ions for NO absorption. However, the use of aqueous solutions of these slats is not always favorable for various emission-control applications.

[0028] One embodiment of the present invention comprises a solid-state media, wherein lower-valency ions of transitional metal elements (such as ferrous, cuprous or manganese ions, for example) have been absorbed in an ionic form. The solid-state media can be comprised of a cation-exchange resin (CEX), wherein the ferrous ion (Fe.sup.++) (for example) has been absorbed onto the resin by exchange with the existing Na.sup.+ or H.sup.+ ions, generally available as the commercial form of the cation exchange resin (CEX-Na.sup.+ or CEX-H.sup.+). Thus, exposure of the Na.sup.+ or H.sup.+ form of the cation exchange resin to a ferrous sulfate or ferrous chloride solution (for example) results in a cation exchange resin with the Fe.sup.++ ion (for example) (CEX-Fe.sup.++).

[0029] Ion exchange resins are available widely and cheaply in industry, and are commonly used for water treatment applications, as well many other applications where ions need to be sequestered or exchanged. Common cation exchange resins are made up of insoluble organic polymers having either sulfonic acid groups (RSO.sub.3.sup.) or carbocyclic acid groups (RCO.sub.2.sup.), resulting in a negative charge on the resin. Cation exchange resins can be strongly acidic or weakly acidic (RSO.sub.3.sup. or RCO.sub.2.sup.), respectively. The Fe.sup.++ form of the strongly acidic cation exchange resin would be Fe.sup.++(RSO.sub.3.sup.).sub.2. Other cation exchange resins include chelating resins (made from iminodiacetic acid, thiourea, etc.). In one example, an embodiment includes an Fe.sup.++ form of a cation exchange resin as a solid-state media through which the flue gas is passed as a filter system, and the NO component of the flue gas is preferentially absorbed by the solid-state media.

[0030] Advantages of the proposed process include the complete absorption of the nitrogen oxides in the Fe.sup.++ form of the cation exchange resin, (CEX-Fe.sup.++), their substantially complete, if not totally complete, passage through the filtration/separation system, and the substantially complete, if not totally complete, reduction of the nitrogen oxides in the flue gas, without use of expensive catalysts and catalyst support structures commonly used in SCR systems. Embodiments of the invention need not use ammonia injection or urea injection as the NO.sub.x reductant; hence there are no ammonia slip problems or production of other ammonium salts.

[0031] Some embodiments of the present invention include a process for removal of various contaminants from a gaseous mixture, like CO and NO, either alone or together, by a Fe.sup.++ form of a cation exchange resin (herein after referred to as CEX-Fe.sup.++ ) filtration device, either at low or high pressures. After absorption of NO, the resultant molecule becomes CEX-Fe.sup.++NO or CEX-Fe.sup.++CO. Once the resin is saturated with NO or CO, a new replacement filter can be used for continued absorption of these exhaust gas components.

[0032] Recognizing the regeneration of the solid-state media is desirably from an energy efficiency standpoint, the spent media can be obtained in its original CEX-Fe.sup.++ by exposing the saturated CEX-Fe.sup.++NO to a stream of heated hydrogen gas in a separate regeneration step. The absorbed NO gets converted to nitrogen and water vapor. A suitable temperature of the incoming hydrogen stream for resin regeneration is 85-95 C., close enough to the vaporization temperature of water to enable a stream of nitrogen and water vapor. Likewise, regeneration of the media containing CO can also be obtained by exposing the media to a heated hydrogen stream, where the CO gets converted to carbon and water vapor, where the carbon can be washed away.

[0033] Some embodiments of the present invention comprise a process for the removal of various contaminants like nitric oxide or carbon monoxide from a gaseous mixture, either alone or together, by a cation exchange resin containing the ferrous ion, either at low or high pressures, followed by regeneration of the resin to its original state, CEX-Fe.sup.++, by exposure of the NO/CO saturated solid-state media to a heated stream of hydrogen.

[0034] An additional process for regeneration of the CEC-Fe++ solid-state media from its CEX-Fe.sup.++NO or CEX-Fe.sup.++CO form after absorption of CO or NO would be an electrochemical reduction of the absorbed gas species by hydrogen ions in an electrochemical cell, wherein the anode and cathode compartments are separated by an ion exchange membrane, preferably a cationic membrane like Nafion. The cathode compartment of the cell contains the saturated from of the CEX-Fe.sup.++NO, and water is passed through the anode compartment. The water is electrolyzed to oxygen at the anode, and hydrogen ions pass through the membrane to the cathode compartment, wherein they reduce the absorbed NO to nitrogen and water. The solid-state media gets regenerated to its original form of CEX-Fe.sup.++, and can be used again for removal of nitric oxide or carbon monoxide from exhaust or flue gas streams.

[0035] The table below shows various experiments for NO absorption and media regeneration done by a stream of heated hydrogen.

TABLE-US-00001 TABLE 1 NO Absorption and Regeneration Resin used: Seimens water strongly acidic CEX, Na.sup.+ form. Exchanged with 1M FeSO4 solution to convert to Fe.sup.++ form Initial NO NO capacity capacity, Time for Hydrogen after regen, mgNO/gm absorption regeneration mgNO/gm Test # resin (minutes) temperature resin 1 21.70 60 85 C. 20.69 2 29.39 60 85 C. 30.22 3 28.22 60 85 C. 27.45 4 29.42 60 85 C. 30.28 NO stream: 80% purity at room temperature.