WO1989003720A1

WO1989003720A1 - Methods and devices for gas cleaning

Info

Publication number: WO1989003720A1
Application number: PCT/US1988/003726
Authority: WO
Inventors: Maria Flytzani-Stephanopoulos
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1987-10-23
Filing date: 1988-10-21
Publication date: 1989-05-05
Anticipated expiration: 1990-04-23

Abstract

Methods and devices for simultaneously removing major pollutants (e.g., fly ash, nitrogen oxides and sulfur oxides) from flue gases and other gas streams requiring purification are disclosed. The untreated gas is passed through a cross-flow structure (10) which includes a porous wall (20) treated with a chemically-active substance. As the gas flows through the porous wall (20), particulate pollutants, such as fly ash, are trapped. At the same time, non-particulate pollutants, such as nitrogen oxides, sulfur oxides and hydrogen sulfide, react with the active substance and are converted to species which can be absorbed to the walls of the cross-flow structure. The non-particulate pollutants can also be converted to harmless reaction products and released into the environment.

Description

METHODS AND DEVICES FOR GAS CLEANING

Background of the Invention

The present invention is directed to the cleaning of gases which result from combustion and other chemical processes. More particularly, the invention is directed to the removal of pollutants from gases exhausted as a result of power generation reactions, from gases produced during coal gasification and from similar pollutant-laden gases.

Electric power is often generated by the combustion of coal and other fossil fuels or other organic fuels. Such combustion processes emit what is known as "flue gas", a reaction by-product which comprises of noxious pollutants, such as nitrogen oxides (NO ) , sulfur oxides (SO ) and particulate pollutants, such as fly ash. Raw, gaseous products from coal gasification and other similar processes also contain particulate and gaseous pollutants, particularly hydrogen sulfide, organic sulfur compounds such as carbonyl sulfide (COS) , carbon disulfide (CS₂), mercaptans and occasionally ammonia (NH_) .

Normally, these pollutants can require up to three different technologies to effect their removal. Particulate pollutants typically are removed by cyclones, bag houses and other filtration technology. Sulfur compounds are often removed by scrubbing technology, while nitrogen oxides generally are removed by catalytic conversion technology. At present, there does not appear to be any single technique or apparatus which can effectively remove all of the above pollutants.

It is an object of the present invention to provide a method for simultaneously removing major pollutants from exhaust gases which result from power generation processes. Another object of the invention is to provide an apparatus for simultaneously removing pollutants from gases which result from other processes, such as coal gasification. Finally, an object of the invention is to provide a regenerable, cross-flow structure which simultaneously can. filter particulate pollutants from a gas and act as a substrate for the absorption and catalytic conversion of non-particulate pollutants to innocuous materials. Other objects will be apparent to those of ordinary skill in the art upon reading this disclosure.

Summarv of the Invention

According to the invention, methods and devices are provided for cleaning a gas to remove pollutants. The methods and devices of the present invention perform a variety of gas cleaning functions which normally require separate methods. In one aspect of the invention, a cross-flow structure is disclosed for cleaning gases. The cross-flow structure features a plurality of gas inlet passages which are displaced by approximately 90° from gas outlet passages. Each gas inlet passage is also vertically displaced from the gas outlet with which it communicates and is separated therefrom by a thin porous wall.

Particulate impurities of a size larger than the pores of the porous layer are prevented from traversing the pores of the porous walls as gas flows from the inlet passages to the outlet passages. In addition, the porous walls carry at least one active composition for converting non-particulate pollutants into innocuous materials. The non-particulate pollutants can be converted into materials which are retained on the porous walls, or non-toxic substances which can be discharged into the environment.

In another aspect of the invention, the porous walls of the cleaning apparatus can be constructed as multi-layer, ceramic structures. For example, in a bi-layer configuration, the first layer to which the untreated gas is exposed has relatively small pores of a size sufficient to trap particulate pollutants entrained with the gas. The second porous layer, which may have a greater thickness, can have pores which are relatively larger in size. The second layer also carries at least one chemically-active composition (e.g., a metal oxide or a noble metal catalyst) which can convert the pollutant into a harmless species or one which can react with and readily absorb the pollutant. Composite porous materials, which include multiple layers carrying different active agents, are also disclosed. For example, in a three-layer configuration, a tight-pore, particulate-trapping layer can be followed first by an SO absorbing layer and then an NO converting layer.

Preferably, the pollutants that are trapped and absorbed by the cross-flow structure can also be periodically removed by a backflushing procedure. The active composition may be renewed by a regeneration procedure.

Through this invention, a pollutant- containing gas can be cleaned using a single method and a single apparatus. This invention avoids the application of several different cleaning methods to remove the various pollutants found in flue and process gases.

The invention will next be described in connection with certain illustrated embodiments; however, it should be clear that various additions, subtractions and modifications can be made by those skilled in the art without departing from the spirit or scope of the invention. For example, the term "cross-flow structure" as used herein is intended to encompass a wide variety of devices having porous walls which permit a gas to pass therethrough. Although the illustrated embodiments are "double-side" cross-flow monoliths (i.e., the gas enters from both the left and right sides and exits in an orthogonal direction) , it should be clear that single-side monoliths, as well as structures that do not require an orthogonal exit, can likewise employ the teachings of the present invention.

Generally, cross-flow filter structures of the type which can be used in conjunction with the present invention are disclosed in U.S. Patent No. ,343,631, which is hereby incorporated by reference.

Moreover, the gases treated by the methods ^•and devices of the present invention can also be pre-treated by the introduction of pre-filtering techniques to remove gross particulate matter and/or by the addition of reactants (such as ammonia in the case of NO catalysis) . Similarly, the present invention can be practiced in conjunction with existing technologies, such as bag houses, scrubbers, fludized bed processes, and the like, whenever tandem or multi-step cleaning operations are warranted.

Brief Description of the Drawings

For a fuller understanding of the nature and objects of" the invention, reference should be made to the following figures, in which:

FIG. 1 is a perspective view of a cross-flow filter cleaning apparatus of the present invention.

FIG. 1A is a detailed, perspective view showing the construction of the apparatus of FIG. 1.

FIG. 2 is a schematic sectional view of one embodiment of a porous wall of a cross-flow filter according to FIG. 1.

FIG. 3 is a schematic, sectional view of another embodiment of a porous wall of a cross-flow filter according to FIG. 1.

FIG. 4 is a perspective view of an alternative cross-flow filter cleaning apparatus of the present invention.

FIG. 4A is a detailed, perspective view showing the construction of the apparatus of FIG. 4.

Detailed Description of the Drawings

Referring to FIG. 1, a gas cleaning apparatus is shown comprising a cross-flow filter structure 10 disposed within a flow path for pollutant-containing gases exhausted from a power-generating combustion process or the like. The untreated gas 12 may enter apparatus 10 through inlets 14 disposed in the lateral walls 16A, 16B of filter 10. Cross-flow filter 10 has a multitude of inlets 14 disposed in lateral walls 16A and 16B. Each inlet 14 communicates with an inlet flow channel 18 which, in turn, communicates with one or more discharge channels 22 and discharge ports 24 disposed in an end wall 26 of filter 10. Channels 22 are open to discharge ports 24 at one end, but are closed at their opposite end by sealed wall 28.

The inlet flow channels 18 can be of virtually any desired shape, including rectangular, square, triangular or hexagonal. The flow channels 18 each lead into the interior of cleaning apparatus 10 and communicate with inlets disposed in lateral wall 16B. The bottom of the flow channel 18 is defined by a porous wall 20 which enables flow channel 18 to communicate with discharge channel 22.

Discharge channel 22 is of essentially the same construction as inlet flow channel 18. Discharge channel 22 leads from the sealed wall 28, through the interior of cleaning apparatus 10 to discharge port 24. Channels 22 are defined by side walls 25 which may or may not be permeable to gases flowing through channels 22. In a preferred embodiment, as illustrated in FIG. 1, lateral walls 16 each have several rows 30, each of which has several inlet ports 14 and corresponding inlet channels 18. Each row of inlet ports 14 is disposed between rows of discharge channels 22 and discharge ports 24. All of the inlet ports in a given row communicate with one or more discharge channels 22 and discharge ports 24 which are disposed immediately above and below channel 18. Generally, the pressure differential which results from the flow of gas through channels 18, prevents or minimizes communication between inlet channels 18 in adjacent rows.

The elements of the cleaning- apparatus 10 can be constructed from a wide variety of materials. The choice of a suitable material will depend upon the operating conditions of a particular application. However, preferred materials are those which are able to withstand high temperatures and long-term exposure to pollutants, such as nitrogen oxides, sulfur oxides and hydrogen sulfides. Examples of such materials include ceramics, metal alloys, cermet composites and even certain polymers. The porous walls 20 are preferably constructed of a porous ceramic material. Examples of such materials include ceramics such as alumina, silica, magnesium oxide and cordierite. The apparatus 10 can be constructed, for example, from ceramic components to form a cross-flow monolith. In such a construction, layers of ceramic wall elements are built up and then fused together by techniques known in the art to form the overall structure. In one preferred embodiment, thin, flat sheets of alpha-alumina having regularly spaced vertical strips of the same material are extruded. The flat sheets are treated with an active agent as described below, and then disposed to form the composite porous walls 20 of the structure of FIG. 1. Consecutive layers of these structures are then positioned orthogonally to each other to form the rows of inlet and outlet channels until a complete cross-flow structure of the desired size is obtained. End walls and other housing elements can then be added, as desired. The layers are then bonded together with a bonding agent or fused together (with or without a flux material) at an elevated temperature and/or pressure. Once the components are bonded or fused together to provide gas-tight joints, the cross-flow monolith is essentially completed and needs only to be disposed within the flow path of the untreated gas to commence operations.

FIGURES 4 and 4A illustrate an alternative design of a cross-flow cleaning apparatus of the present invention, in which inlet channels 19 are substantially sinusoidally or triangularly shaped and communicate with similarly shaped, orthogonally disposed outlet channels 23. In this embodiment, the inlet and outlet channels have side walls 25 which are defined by a corrugated sheet 21 disposed between porous walls 20. Corrugated sheets 21 can be extruded as part of the flat sheets which form porous walls 20, or, alternatively, can be extruded separately and disposed between walls 20. In operation, multiple cross-flow structure elements can be connected to an exhaust manifold (not shown) with the gas exit end of each element attached to the manifold. Several such modules can then be disposed in a single vessel thereby providing a relatively large surface-to-volume ratio, typically an order of magnitude greater than that of an ordinary baghouse.

As noted above, the pores of walls 20 are preferably of such size that they are able to prevent the passage of particulate pollutants, such as fly ash, entrained in the exhaust gas. The overall thickness of the walls 20 can range from about 100 microns to about 4.0 millimeters. The pores may range in size from 0.1 to 10.0 microns in diameter. Preferably, the pores are less than 1.0 micron in diameter.

The porous wall 20 can comprise a bi-layered ceramic structure 31, as illustrated in FIG. 2. In this embodiment, bi-layered wall 31 comprises a first layer 32 and a second layer 34. The first layer 32 can range in thickness from about 1 micron to about 10 microns and has pores ranging in average diameter up to about 0.5 micron and preferably less than 0.2 micron. The second layer can range in thickness from about 50 microns to about 4.0 millimeters, having pores ranging in size from about 0.5 micron to about 20 microns in diameter.

In FIG. 3, another embodiment of the porous walls 20 is shown in which the porous wall consists of a three-layered material 40. The first layer 42 again serves to block the passage of particulates and can range from about 1 micron to about 10 microns in diameter and have pores ranging in diameter up to about 0.5 micron. The second layer 44 and the third layer 46 can be similar in thickness and pore size. However, layer 46 preferably is slightly thinner than layer 44. Each layer 44 and 46 can range in thickness from about 50 microns to about 2.0 millimeters and have pores ranging in size from about 0.5 micron to about 20 microns in diameter. Layer 44 is preferably coated with or otherwise carries an SO or H S absorbing composition, such as a metal oxide, while the layer 46 includes a NO converting composition, such as a noble metal. In this embodiment the noble metal is protected against "poisoning" by SO compounds as the SO pollutants in the gas stream are reacted with and absorbed by the active composition before the noble metal, disposed in layer 46, is reached.

Preferably, porous wall 20, and alternatively layer 34 of wall 31 and layers 44 and 46 of wall 40, are treated with an active composition which serves to convert non-particulate pollutants into harmless species. For example, most flue gases contain common pollutants, such as nitrogen oxides (NO ) and sulfur oxides (SO ) . The active composition typically converts sulfur oxides into solid metal sulfites and metal sulfates which are retained within the porous wall itself. Also, the active composition can simultaneously catalytically convert the nitrogen oxides (in the presence of ammonia) into harmless molecular nitrogen which may be discharged into the environment. In another embodiment, the active composition may be used for high-temperature cleanup of coal-gasifier exit type gases, which contain H₂S, COS, CS₂ and other organic sulfur compounds as well as, occasionally, ammonia (NH₃). In this embodiment the active composition converts sulfur compounds into metal sulfides, which are retained within the porous wall itself. Simultaneously, ammonia can be catalytically decomposed into molecular nitrogen and hydrogen which may be discharged into the environment. The active composition may also serve to catalyze the water gas shift reaction (CO + H2O → CO2 + H2 under these conditions.

The active compositions of the present invention include various metal oxides, such as copper oxide (CuO), iron oxide (Fe₂0₃), titanium oxide (Ti0₂), aluminum oxide (A1₂0_), zinc oxide (Zn0₂), cerium oxide (Ce0₂), as well as oxides of other elements such as Manganese and Nickel. Mixtures of two or more of the above compounds can also serve as effective active compositions. These metal oxides react with sulfur oxide compounds to yield metal sulfates which are absorbed within the porous wall and thus removed from the gas. Moreover, ammonia can be preferably added to the gas before it enters the cross-flow filter 10. Nitrogen oxide compounds react with ammonia, in the presence of metal oxides and metal sulfates, to yield molecular nitrogen (N_) and water which are released into the environment. Hydrogen sulfide and organic sulfur compounds present in coal and fuel gases will react with the metal oxides (and certain metals such as copper) to yield metal sulfides, which are retained within the walls of the structure. Examples of useful combinations of metal oxides include CuO/Fe₂0₃/Al₂0_; CuO/CeO,/Al₂0₃; ZnO/Ti0₂; and ZnO/Fe₂0₃. A preferred combination metal oxide is formed by combining three parts copper oxide (CuO) with one part each of iron oxide (Fe₂0₃) and aluminum oxide (A1₂0₃).

The active composition can also include a noble metal catalyst, such as platinum, palladium, iridium, rhodium or ruthenium, supported on alumina or another oxide carrier. These are also effective for NO reduction in the presence of NH,. In another embodiment, the active composition can be a liquid phase absorbent-catalyst coating the porous ceramic walls. Examples of such absorbent-catalysts include molten salts of alkali sulfates with vanadium pentoxide or a molten salt of ammonium and alkali bisulfates with various dissolved catalysts.

The active compositions of the present invention can be incorporated into the porous walls by a variety of techniques. The active compositions can be mixed into the ceramic in its molten state or prior to sintering. Alternatively, the active composition can be sprayed onto the porous material by conventional liquid carrier spray or vapor deposition techniques. In another alternative technique, the porous material can be immersed in a liquid solution containing the active composition to imbibe or absorb the active substance. In the case of metal oxides, the porous material can be formed from aqueous or organic solutions carrying the metal ions after dehydration and/or pyrolysis, followed by an oxidation (calcination) . The desired thickness and pure structure of the active phase layers can be controlled by multiple applications prior to final sintering, stack assembly or the like. Preferably, the metal oxide active compositions can be applied to the porous walls in quantities ranging from about 10 wt. percent to about 50 wt. percent. Noble metals may be used in much smaller quantities, typically less than 1 wt. percent.

In operation, the filter 10 is disposed in the flow path of pollutant-containing flue gas. The gas enters the filter through inlets in lateral walls 16A, 16B of the filter. Inlets may be disposed on both lateral walls 16A and 16B. The gas flows laterally through the inlet port into a laterally-aligned flow channel. The wall defining the bottom portion of the flow channel is constructed of a porous material, thereby allowing the gas to flow through the porous material into a discharge channel disposed below and offset by 90 degrees from the flow pathway. In an alternative embodiment, the inlets on internal walls 16A, 16B can be prevented from communicating with each other by a separator wall (not shown) .

While passing through the porous wall, most particulate pollutants, such as fly ash, are filtered by the thin surface layer of the porous wall. In addition, non-particulate pollutants, such as nitrogen oxides, sulfur oxides and the like, are reacted with an active component within the porous wall and are absorbed or converted into relatively harmless species, as described above. Upon traversing the porous wall and entering the discharge flow path, the clean gas exits the filter through outlet ports disposed in the end wall of the cleaning apparatus 10.

The cleaning apparatus, constructed according to the embodiment illustrated in FIGS. 2 and 3, operates essentially in the same way as described above. However, in the case of the embodiments of FIGS. 2 and 3, the porous wall is multi-layered. The first layer filters the particulate matter, while the subsequent layers are treated with active components which react with the non-particulate gaseous pollutants.

The cross-flow structure can be periodically cleaned by backflushing, and the active compositions can be thermally or chemically regenerated by well known techniques. Trapped particulate matter can be blown out and collected for disposal. When the active composition is a metal oxide or other sulfate-forming compound (i.e., to remove SO ), regeneration can be accomplished with a reducing gas, such as hydrogen, carbon monoxide, methane or the like, preferably at an elevated temperature (e.g., 300°C to about 500°C). When the active composition is a metal sulfide-forming composition (i.e., to remove H₂S) , it can be regenerated by an oxidizing gas, such as air or air-steam mixtures at an elevated temperature (e.g., about 500°C to about 800°C).

What is claimed is:

Claims

1. A method of cleaning a gas to remove pollutants, comprising the steps of: disposing a cross-flow structure having porous walls within the flow path of an untreated gas, such that the gas is forced to pass through the porous walls, the passage through said walls serving to filter particulates entrained in the gas; and disposing at least one chemically- active composition within said porous walls, such that non-particulate pollutants are chemically- converted to innocuous compounds as a result of passage through the cross-flow structure.

2. The method of claim 1 wherein the gas is a flue gas.

3. The method of claim 2 wherein the active composition is a nitrogen oxide-reducing compound, and the method further includes adding ammonia to said gas to promote the conversion of nitrogen oxides to molecular nitrogen.

4. The method of claim 3 wherein the active composition is a noble metal catalyst.

5. The method of claim 3 wherein the active composition is selected from the group consisting of copper oxides, iron oxides, aluminum oxides, cerium oxides and mixtures thereof.

6. The method of claim 1 wherein the method further includes chemically-absorbing the pollutants into the porous ceramic walls of the structure.

7. The method of claim 1 wherein the gas is a flue gas derived from coal combustion.

8. The method of claim 1 wherein the method further includes periodically regenerating the absorbent capacity of the structure.

9. The method of claim 1 wherein the method further includes periodically backflushing the structure to remove caked particulate matter.

10. The method of claim 1 wherein said porous walls are ceramic.

11. An apparatus for cleaning a gas to remove pollutants, comprising at least one cross-flow structure, including at least one porous wall material through which the gas is forced to pass, the porous wall material having a pore size suitable to block passage of particulate pollutants entrained in the gas, and further including at least one active composition for converting non-particulate pollutants into an innocuous material.

12. The apparatus of claim 11 wherein the active composition converts said non-particulate pollutants by reactive absorption into a material retained within the porous wall.^"

13. The apparatus of claim 11 wherein the active composition converts said non-particulate pollutants into a non-toxic emission.

14. The apparatus of claim 11 wherein the porous wall material is a ceramic material.

15. A porous composite for treating gases to remove pollutants, comprising: a first ceramic layer adapted to serve as an input face through which a pollutant-containing gas is forced to pass, the first ceramic layer having a structure of pores suitable to block particulates entrained in the gas; and at least one further ceramic layer having a structure of relatively larger pores and impregnated with at least one chemically-active composition which acts to chemically convert pollutants in the gas to innocuous compounds after passage through the first layer.

16. The porous composite of claim 15 wherein the thickness of said first layer ranges from about 1.0 micron to about 10.0 microns.

17. The porous composite of claim 15 wherein the thickness of said further layer ranges from about 50.0 microns to about 4.0 millimeters.

18. The porous composite of claim 16 wherein the pores in said first layer range in size up to about 0.5 microns.

19. The porous composite of claim 17 wherein the pores in said further layer range in size from about 0.5 micron to about 20.0 microns.

20. The porous composite of claim 15 wherein the active composition in said further layer is a noble metal catalyst.

21. The porous composite of claim 15 wherein the active composition is an absorbent chosen from the group consisting of copper oxides, iron oxides, titanium oxides, aluminum oxides, zinc oxides, and cerium oxides and mixtures thereof.

22. The porous composite of claim 15 wherein the active composition is an absorbant formed by combining copper oxide, iron oxide and aluminum oxide.

23. The porous composite of claim 15 wherein the first ceramic layer is a porous alumina layer.

24. The porous composite of claim 15 wherein the further ceramic layer is a porous alumina layer.

25. A porous composite for treating gases to remove pollutants, comprising: a first ceramic layer adapted to serve as an input face through which a pollutant-containing gas is forced to pass, the first ceramic layer having a structure of pores suitable to block particulates entrained in the gas; . a second ceramic layer having a structure of relatively larger pores and impregnated with at least one chemically-active composition which reacts with and chemically absorbs said pollutants into the second ceramic layer; and a third ceramic layer having pores substantially similar in size to those of said second layer, and impregnated with an active composition which converts said pollutants into innoculous compounds after passage through said first layer.

26. The porous composite of claim 25 wherein said second layer is of a greater thickness than said first layer.

27. The porous composite of claim 25 wherein said third layer is of a lesser thickness than said second layer.

28. The porous composite of claim 25 wherein said second layer carries a metal oxide composition.

29. The porous composite of claim 25 wherein said third layer carries a noble metal.