WO1994021351A1

WO1994021351A1 - Active filters for integrated cleanup of gas streams

Info

Publication number: WO1994021351A1
Application number: PCT/US1994/002922
Authority: WO
Inventors: Maria Flytzani-Stephanopoulos
Original assignee: Massachusetts Institute Of Technology
Priority date: 1993-03-17
Filing date: 1994-03-17
Publication date: 1994-09-29

Abstract

A filter for multiphase cleanup of gas streams, in particular flue gas streams, includes filter wall composites comprising a bulk macroporous support (40) having a microporous membrane (42) on one side and a chemically-active deposit (50) on the opposite side. The bulk macroporous support has a surface area of less than 5.0 m2/g, a compressive crush strength of between about 1.0 lb/mm to about 10.0 lb/mm, and a thickness of between about 1.0 mm to about 3.0 mm. The microporous membrane and the chemically active deposit have thicknesses of less than about 100.0 microns and less than about 10.0 mm, respectively. The process of multiphase cleanup of gas streams includes passing a gas stream, comprising particulate pollutants (16) and gaseous pollutants, through a filter having composite walls formed of a bulk macroporous support having a microporous membrane on one side, and a chemically-active deposit on the opposite side.

Description

ACTIVE FILTERS FOR INTEGRATED CLEANUP OF GAS STREAMS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to filters in general, and more specifically to using chemically-active and regenerable filter wall composites for the simultaneous removal of entrained particulate matter and gaseous pollutants from flue gases, fuel gases and other industrial gas streams.

²• Description of Prior Art

Continually changing environmental laws and regulations throughout the world have imposed new, more stringent performance standards for the removal of pollutants from effluent gas streams. Better, more efficient control of air emissions from power plants, waste incinerators and various industrial processes, require creative application of new technologies to achieve the required control levels without excessive cost penalties. For example, in coal-fired power plants flue gases containing pollutants such as nitrogen oxides (NO X) , sulfur oxides (SOX), and particulate pollutants such as fly ash are produced. Costly equipment must be used to remove the particulate matter, and gaseous sulfur and nitrogen oxides. Similarly, in integrated coal gasification combined-cycle power generation, removal of particulate matter, alkali metals, hydrogen sulfide, and ammonia must be performed before the fuel gas is combusted in the gas turbine.

FIG. 1 shows an example of a prior art power-plant, wir ooiler 10, and economizer 11, producing flue gas 12 which is treated in seriatim to achieve removal of NO , particulates, and S0₂ using, respectively, selective catalytic reduction of NO X in vessel 13 with injected ammonia gas 14 to produce nitrogen and water vapor, a filter house 15 to remove particulate matter (fly ash) 16 and wet (or dry) scrubbers 17 for S0₂ removal. The S0„ is typically in the form of a waste 18, e.g. calcium sulfate, and needs further disposal. Typically, scrubber off-gas is reheated and sent to stack 19 from which treated gases 20 are released. Removal efficiencies are typically about 80% of NO X, 98% of particulateε, and 90% of S0₉< . Each of these units is operated at different temperature for peak efficiency, which fixes their respective installation at different locations along the flue gas ductwork.

The prior art also teaches partially integrated emissions control devices that combine particle filtration with at least one gaseous pollutant removal. For example, U.S. Patent 4,220,633 to Pirsh teaches baghouses for fly ash removal with simultaneous catalytic reduction of nitric oxides with injected ammonia gas on a catalyst phase coated on or lodged within the fiber structure of the baghouse. The S0₂ in the flue gas is removed downstream of the baghouse by scrubbing to produce a waste as explained above.

U.S. Patent 4,728,503 to Iida describes a filter medium for treating exhaust gases, especially those emitted by municipal waste incinerators. The filter described consists of several layers of different materials performing several functions in a prescribed direction, and is limited to a filter having a free outer surface. More specifically, this filter consists of a porous ceramic filter medium to capture fly ash. On one side (inner) of this filter medium a denitration catalyst is applied as a coating to remove nitric oxides by selective catalytic reduction with ammonia, while on the other side (outer) of the filter medium a composite layer is applied to remove hydrogen chloride by slaked iir-e (as CaCl₂ waste). Because the outer layer is non-regenerable, it has to be scraped off periodically and a fresh powder layer is reapplied. Thus, the outer side of the filter must be free to permit this mechanical cleaning. Filter configurations such as those disclosed in U.S. Patent 4,632,683 to Fukutani et al . for an exhaust gas purifying filter having a honeycomb structure, or EP 0,057,251 to estinghouse for a filtration system configuration useful for cleaning high temperature raw gas containing fine particulates, therefore, cannot be used in combination with the Iida disclosure. The SO_? in the exhaust gas can be removed by the slaked lime, or easier by alkaline substances included in the outer surface coating, but the solid sulfates produced will also need to be removed by mechanical ' scraping. This method leads to large volumes of waste by-product to be disposed.

Therefore, an integrated emissions control device that also avoids waste by-product formation would be highly desirable to reduce costs associated with waste handling and disposal. If the initially dilute pollutant is recovered in concentrated form and further processed into a salable product, additional credits will result. The regenerative removal of S0₂ by metal oxides that form sulfates which can be easily reduced back to the oxide form during regeneration is an attractive method that has been considered, especially in conjunction with NO X removal to eliminate the need for, and the costs of operating, two separate units. In the latter case the metal oxide chosen has to show both high reactivity with SO- and be active for denitration, e.g. by selective catalytic reduction with ammonia, shown generally by the formulae:

MO + S0₂ + l/20₂ → MS0 (1)

MO NO + 2NH3 + 0₂ →- 3/2 N₂ + 3H₂0 (2)

MSO₄

where M, shown here to be of valence 2, can be of any valence and is a transition metal, rare earth element, alkali or alkaline earth or other element or oxide which is not poisoned by S0₂ for catalyzing reaction (2), or the sulfate of which is also a catalyst for reaction (2), as shown above. Additionally, the sulfate should be regenerable by thermal or chemical treatment. There are several metal oxides that meet these criteria, for example, CuO, Fe₂0_, NiO, CeO . Also, mixed metal oxides each possessing a different activity for reactions (1) and (2) may be used.

However, the development of these processes is hindered by sorbent attrition problems in moving bed, or fluidized-bed reactors which allow operation in the presence of fly-ash. Conventional fixed-bed designs employing pellets, tablets, rings, etc., which provide the least attrition to solid sorbent/catalysts, cannot be used under high-dust loading because of excessive pressure drop due to plugging of the bed. On the other hand, in advanced fixed-bed designs employing honeycomb monoliths with gas flowing in an array of parallel channels, very large reactor volumes would be required to process the flue gas, because the sorbent would exist only as a thin coating on the channel walls along which the gas flows. In either of these fixed-bed designs, undesirable (often detrimental) interaction of fly ash particles with the active catalyst-sorbent surface can take place since impaction of fly ash on the sorbent cannot be avoided.

For the fixed-bed to become viable, a combination of a filter/sorbent-catalyst medium is required, first presenting the filtering function, followed by the sorbent-catalyst function. PCT International Application Serial No. PCT/US88/03726, by the present applicant, now abandoned, described such a filter medium. The application described methods and devices for gas cleaning consisting of walls with a first, fine-pore ceramic filter layer for fly-ash removal, followed by an active layer of coarser pore-structure to perform reactions (1) and (2). Backflushing with air would be applied periodically tc clean the filter surface from collected particulates . Further, the sulfate formed via reaction (1) would be regenerated with a reducing gas, back to the oxide form. Although this filter device overcomes prior art deficiencies, it did not provide preparation and performance characteristics of the proposed device to effectively remove all three pollutants without operating difficulties. Significant among these operating problems are pressure drop increase, and structural problems such as non-uniformity of coating, spalling, fly-ash penetration and lodging within the active wall, and the strength of the large-pore active wall.

U.S. Patent, 4,977,123 to Flytzani-Stephanopoulos et al . teaches a method for preparation of extrusions of bulk mixed oxide compounds with both high macroporosity and mechanical strength. Several compositions are described as regenerable absorbents of hydrogen sulfide for application to desulfurization of gasifier exit fuel gas streams at high temperatures. A similar extrusion process could be used for extruding compositions for various applications, such as for combined SO X/NOX removal from flue gases, waste incinerator exhausts, or other industrial gases. Further, the reference generally discloses the possibility of combining the active phase functions with particulate filtration. It is noted however, that based on the porosity requirements of the active phase such a filter would be, in large-part, a depth-filter allowing fly-ash particles to lodge permanently within the porous active phase, increasing the pressure drop and, potentially, decreasing the efficiency of conversion of the active phase.

Ceramic filters including layered or gradiated porosity have been described. For example, U.S. Patent 4,629,483 to Stanton describes a method for forming a monolithic, layered, tubular ceramic filter structure with clearly defined layers of different porosities and permeabilities. Alternatively, U.S. Patent 4,810,273 to Komoda describes how to make a ceramic filtering layer of gradually increasing porosity on a planar ceramic support of larger porosity. Neither reference discloses "active" filters, i.e. filter bodies comprising chemically-active phases useful for combined pollutant removal in the absence of interaction from fly ash which is effectively retained on the surface of the barrier filter. These references utilize refractory ceramic materials to form the filter body, including: alumina, silica, zirconia, cordierite, mullite, or silicon carbide.

In place of refractory ceramics, one can use an active phase composition, e.g. of single or mixed metal oxides, to form the filter body. This would require firing the filter body at high temperatures to increase the strength of the filter walls. It is noted however, that certain active phase compositions are limited to low-temperature preparations, so that they retain certain phases in uncombined form or high dispersion. With lower firing temperatures the strength of the filters described by the above references would be greatly compromised.

It is therefore an object of the present invention to provide an improved multifunctional active filter and method for combined pollutant removal, unlimited by the choice of the specific active phase.

SUMMARY OF THE INVENTION Accordingly, improved compositions and methods are disclosed for active filter/sorbent/catalyst media aimed at simultaneous fly ash/gaseous pollutant removal applications, not limited by the choice of the specific active phase composition.

A filter for multiphase cleanup of gas streams, in particular flue gas streams, includes filter wall composites comprising a bulk macroporous support having a microporous membrane on one side and a chemically-active deposit on the opposite side. The bul.< πacropcrous support has a surface

₂ area typically less than 15.0 m /g, a compressive crush strength of between about 1.0 lb/mm to about 10.0 lbs/mn, and a thickness of between about 1.0 millimeter to about 3.0 millimeter. The microporous membrane on the bulk macroporous support has a thickness of less than about 100.0 microns. Lastly, the chemically-active deposit has a thickness of between about 0.0 millimeter to about 10.0 millimeter. By "zero" thickness, we imply that the macroporous support itself possesses adequate chemical reactivity and regenerability in pollutant removal to render the application of the chemically-active deposit unnecessary. When a deposit is needed, however, it is applied in a separate step as a coating on or as a macroporous solid foam integrated with the support wall.

The process of multiphase clean-up of gas streams comprises passing a gas stream, comprising particulate and gaseous pollutants (such as nitrogen oxides, sulfur oxides, volatile organic compounds (VOC's), hydrogen sulfide, organosulfur compounds, etc.) through a filter having walls formed of a microporous membrane, a bulk macroporous support, and a chemically-active deposit. The gas is forced to pass through the microporous membrane, which acts as a barrier filter, blocking passage of the particulates entrained in the gas. The non-particulate pollutants in the flue gas are then passed through the bulk macroporous support and the chemically-active deposit, where they react so that a clean effluent gas is discharged from the filter. The chemically-active deposit may be a denitration catalyst, an ammonia decomposition catalyst, a VOC oxidation catalyst, a regenerable sulfur dioxide or hydrogen sulfide absorbent, or possess a compatible combination of such functions. Alternatively, as may be desirable for certain gas clean-up applications, the bulk macroporous support may be coated with a high-activity catalyst, e.g. a denitration catalyst, followed by a chemically-active deposit that functions primarily as a regenerable SO„-sorben . Still, in other applications, a certain catalyst may be poisoned by SO_ , whereupon the SO_-sorbent may be deposited upstream of a layer of the sensitive catalyst. Regeneration of the saturated sorbent would be accomplished thermally or chemically to recover the sulfur value in concentrated form and the solid in the original active form. Backflushing with air, jet pulsing or other similar method can be used periodically to remove the particulates collected on the membrane side of the filter

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages will be more fully appreciated from the following drawings in which:

FIG. 1 illustrates a prior art process flow diagram for removing particulate pollutants, sulfur oxides and nitrogen oxides from a flue gas stream;

FIGS. 2A, 2B, and 2C are schematic, sectional views of various embodiments of the filter wall composite of the present invention;

FIGS. 3A and 3B are schematic views of a candle filter utilizing various embodiments of the filter wall composite of the present invention;

FIG. 4 is a schematic sectional view of an alternately plugged-cell, honeycomb-monolith active filter geometry;

FIGS. 5A and 5B are detailed views of various embodiments of the filter shown in FIG. 4;

FIG. 6 illustrates the process flow diagram of FIG. 1 utilizing active filters incorporating the filter wall composites of the present invention;

FIG. 6A is a special case of FIG. 6 wherein sulfur recovery is performed in a single stage with the regeneration of the active filters;

FIG. 7 is a graph of combined SO„/NO removal by sorbent/catalyst composition C (Table 1) at 500°C, wherein conversion of sorbent is based on CuSO and Ce„(SO. )_ ormation,

FIG. 8 is a graph of cyclic sulfation performance of composition A (Table 1) at 400 ^">C, having an inlet gas composition (mol%) 0.15 S0₂, 3 0₂, balance N , wherein conversion is based on CuSO 4. formation; FIG. 9 is a graph of cyclic sulfation performance of composition F (Table 1) at 500°C and regeneration at 600°C; conversion of sorbent is based on CuSO. and Ce₂(S0.)., formation;

FIG. 10 is a graph of sulfur products distribution in regeneration of sorbent composition C (Table 1) with 10%CO, 10%CO₂, balance N₂;

FIG. 11 is a graph of a comparison of S0₂ removal by sorbent composition E (Table 1) and foamed monolith J (Example II) at 400°C from a gas containing 1500 ppm S0₂, 3%0₂, balance ₂; conversion is based on CuSO. formation, S.V. = 25,000h^~~ for powder composition E; S.V. = 2,500h for monolith J; and

FIG. 12 is a graph of NO conversion over powder composition E and foamed monolith J as a function of temperature; inlet gas composition (moll):0.06 NO, 0.06 NH₃, 30₂, bal. ₂; powder: 0.5g, 0.5L/M (SA= 310m²/g), SV=25,000 h^-1, foamed monolith: l.Og, 1.0L/M (SA 310m²/g), S.V. = 2500h^_1.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides active, multifunctional filter media composition and methods for multiphase clean up of gas streams.

Referring to FIG. 2A, a filter wall composite 30 for multiphase clean up of gas streams includes a bulk macroporous support 40 having a microporous membrane 42 on one side, and a chemically-active deposit 50 on the opposite side.

Bulk macroporous support 40 may be made from a wide variety of materials, depending on the operating conditions of a particular application. Preferred materials are those which are able to withstand high temperatures and long-term exposure to pollutants, such as sulfur and nitrogen oxides, alkali metals, etc. For example, these materials include ceramics, metal alloys, cermet composites, and certain polymers. More preferably, bulk macroporous support 40 is constructed of refractory ceramic materials having good mechanical strength, thermal shock resistance, chemical resistance, etc. required for a long service of the filter. Such materials include oxides of aluminum, silicon, magnesium, zirconium, titanium, nitrides and carbides of silicon, binary and ternary oxide compounds, such as mullite and cordierite, and several others. When the above requirements can be relaxed, the macroporous support 40 may be constructed from a mixture of compounds, at least one of which is chemically-active, while another serves as a dispersant or structural or textural promoter. Active compounds include elements and oxides of the groups of the transition metals, alkali, alkaline earths, copper, zinc, rare earths, either single or combined in mixed oxide form, alloy form, oxide compound or oxide solid solution form, etc. The structural promoter can be alumina, chromia, titania, silica, magnesia and other such oxides added to improve dispersion or structural stability, resistance to chemical attack, mechanical strength, etc. of the active phase.

Similar compositions in extruded form, used as single and mixed oxide absorbents and catalysts, having high macroporosity and mechanical strength are described in U.S. Patent No. 4,977,123 to Flytzani-Stephanopoulos et al . , which is incorporated herein by reference. The reference teaches preparation of the extrusion by a complexation technique which includes pyrolysis-calcination of amorphous citrate precursors between about 550°C to about 850°C, sizing the resultant powder to a broad particle size distribution, extruding a paste formed by adding a binder and water, and drying and heat treating the processed extrusion.

The bulk, macroporous support 40 may be extruded in planar, tubular or other form as a single element or a multi-element filter body. The filter may, thus, be of cross-flow configuration, such as the one described by Westinghouse in EP 0,057,251; or a through-flow honeycomb monolith with alternatively plugged inlet and exit passages, such as that described by Fukutani in U.S. Patent 4,632,683; or it may be in the form of ceramic candles, hollow tubes, etc. prepared by techniques well-known in the art.

The bulk, macroporous support 40 typically has a surface

₂ area less than about 20.0 m /g, and a compressive crush strength of between about 1.0 lb/mm to about 15.0 lbs/mm of extrusion length. Preferably, macroporous support 40 has a surface area between about 1.0 m 2/g to about 15.0 m2/g, and a compressive crush strength between about 2.0 lbs/mm to about 10.0 lbs/mm. Typically, the macroporous support 40 is between about 1.0 millimeter to about 3.0 millimeters thick possessing sufficient mechanical strength to withstand stresses that occur in the filter during operation and regeneration. Macroporous support 40 has at least 60% of its porosity in large pores with diameters of between about 2.0 microns to about 50.0 microns, which allows for low pressure drop as gas streams are passed through the macroporous support 40.

Although macroporous support 40, prepared as above, may possess adequate activity for certain reactions with gaseous pollutants, often its reactivity is limited due to the high-temperature heat-treatment used to strengthen the filter body during preparation. High temperature firing decreases the surface area and enhances solid oxide compound formation which may not be the desired chemically-active phases. Also, often, the support 40 may contain a large fraction of inert phases to improve the structural stability and/or mechanical strength of the active phase. In such situations, it will be necessary to incorporate a higher-activity phase in the filter wall. Several methods may be used for this step, depending on the specific application of the active filter. For example, a thin coating of the active composition may be deposited on the discharge side of the macroporous support by wet or dry application techniques, such as washcoati g from

x des, when used n comb nat on w t macroporous support 40. This is because deposit 50 is prepared at a lower temperature than support 40, preserving desirable active phases and high dispersion (high surface area) . If utilized, the active compositions can be applied to the macroporous support 40 in quantities ranging from about 5.0 to about 50.0 percent by weight.

Chemically active deposit 50 may be in the form of a film or a foam, which is coated onto macroporous support 40 on the gas-discharge side (opposite microporous membrane 42) . The chemically-active film deposit typically has a thickness less than 2.0 millimeters; a foam deposit is typically between about 2.0 millimeters and about 10.0 millimeters thick. It is noted, however, that variously sized deposits may be utilized, depending upon the chemical composition of deposit 50, pressure drop through the filter wall, and system design.

The chemically-active film deposit 50 is prepared as described above and is applied to macroporous support 40 prior to construction of a filter monolith, or slip-coated within an existing monolith's channels. A macroporous solid foam, chemically-active deposit is typically prepared from a slurry, consisting of the metal oxide composition, water, a foaming agent, and a binder. Any suitable binder material can oe used, including inorganic binders, such as aluminum phosphate, or organic binders, such as methyl cellulose, ethyl cellulose, polyvinyl alcohol, or a mixture, thereof. The slurry is charged into a monolith s channels where it is dried and foamed, thereby expanding within the passage. A similar foam preparation is described in U.S. Patent No. 4,363,644 to Sato et al. , which is incorporated herein by reference.

As shown in FIGS. 2B and 2C, deposit 50 may occasionally consist of two zones, 50' and 50", of distinct chemical composition, either of which is disposed next to support 40, depending, on the application. For example, in S0₂/NO removal, zone 50' may be an active SCR catalyst that precedes the sorbent zone 50" (lower activity) to improve the NO removal efficiency. Conversely, zone 50' may be a denitration catalyst sensitive to S0₂ which should, therefore, be disposed downstream of the S0₂-sorbent zone 50" to preserve its activity.

Microporous membrane 42 may be made from the same variety of materials used to form macroporous support 40. Similar to macroporous support 40, microporous membrane 42 may be an inert, ceramic phase or a mixture of inert- and active-phase compositions. Extruded microporous membrane 42 compositions are fused with macroporous supports 40 by techniques known in the art. It is noted that at least one component material of the microporous membrane is desirable to be present in the macroporous support 40 to prevent spalling, or delamination.

Typically, microporous membrane 42 is less than about 100.0 microns thick and has an average pore diameter less than about 2.0 micron. Preferably, the average pore diameter of microporous membrane 42 is less than about 0.5 micron. Microporous membrane 42 serves as a barrier filter of particulate pollutants, such as fly ash, typically present in fuel gas or combustion gas streams, thereby reducing filter contamination.

In a preferred embodiment, microporous membrane 42 is unitary with macroporous support 40. For example, they may be produced together from the "green state' according to a technique described in U.S. Patent 4,629,483 to Stanton, which is incorporated herein by reference. Alternatively, a technique described in U.S. Patent 4,810,273 to Komoda, which teaches how to make a ceramic filtering layer of gradually increasing porosity on a planar ceramic support of larger porosity may be used, and is incorporated herein by reference.

In operation, filter wall 30 is part of a candle filter or a multi-element, cross-flow or through-flow monolith (not shown), which is disposed in the flow path of a multiphase pollutant gas stream 12. Gas stream 12 is typically a flue gas, fuel gas, or other industrial gas stream comprising particulate pollutants 16, and gaseous pollutants, such as sulfur and nitrogen oxides, hydrogen sulfide, organosulfur compounds, ammonia, VOC'S, etc. While passing through filter wall 30, the particulate pollutants 16, such as fly ash, are blocked by and collected on the microporous membrane 42. The non-particulate pollutants, such as sulfur and nitrogen oxides and the like, are reacted with chemically-active compositions) disposed in the macroporous support 40 and/or chemically-active deposit 50. The non-particulate pollutants are absorbed or converted into innocuous species by the sorbent or catalyst phases of the deposit, respectively. For example, for SO_/NO removal, such processes occur according to reactions (1) and (2). Upon traversing chemically-active deposit 50, clean gas stream 25 exits filter wall 30. Filter bodies are constructed from filter wall composites 30 (FIGS. 2A, 2B and 2C) , by techniques known in the art, to form overall structures of various configurations. For example, cross-flow filters with orthogonally displaced inlet and discharge channels can be formed as described in EP 0,057,251 to Westinghouse. Chemically-active deposit 50, in either film or foam composition, can be deposited into discharge channels as described above. Similarly, cylindrical candle filters may be used. Schematic, sectional views of such filters are shown in FIGS. 3A and 3B. These can be extruded in various sizes and wall thicknesses to meet required ranges of mechanical strength and pressure drop. Referring now to FIG. 4, a schematic, sectional view of a through-flow filter 70 is shown. In this embodiment gas flows in and out of filter 70 along the same axis. The through-flow filter 70 can be extruded in a variety of configurations, sizes and cell densities, such as a honeycomb monolith design. The filter 70 is modified by plugging the entrance of every other channel 71 with a ceramic plug 72 on each end of the monolith. The alternate channel plugging limits the entrance of flue gases into every other channel 71, and subsequently blocks the exit of the flue gases though the channel 71 it entered. The flue gas 12 is then forced to cross through the wall composites 30, and exits through a parallel channel 71. Particulate pollutants 16 are blocked by microporous membrane 42 (FIGS. 2A, 2B and 2C), while the non-particulate pollutants are removed in macroporous support 40 and/or chemically-active deposit 50, which is deposited into alternate (clean side) channels 71 where clean exhaust gas 25 is discharged.

FIGS. 5A and 5B are detailed views of the elements of through-flow filter 70 shown in FIG. 4. FIG. 5A shows flue gas 12 entering channel 71 of filter 70. The gas 12 is forced to cross through filter wall composite 30. Particulate pollutants 16 are filtered by the microporous membrane 42, while non-particulate pollutants are removed in the macroporous support 40 and/or chemically-active deposit 50. A film 51 deposit is coated onto clean side channel 71 where clean exhaust gas 25 is discharged.

FIG. 5B is similar to FIG. 5A, except chemically-active deposit 50 is a foam 52 deposited in clean-side channel 71.

Referring now to FIG. 6, active filters of the present invention are included in the process flow diagram shown in FIG. 1. It can be seen that the active filters 21 replace traditional in seriatim units, including a filter house to remove particulate matter, a separate vessel for NO reduction, and wet (or dry) scrubbers for S0₂ removal. In the process, boiler 10 and economizer 11 produce flue gas 12, which is passed through filters 21. The filters 21 remove particulate pollutants, and react with S0₂ to form sulfated metal oxides which, together with the original oxides, catalyze the conversion of NO X to molecular nitrogen by reaction with ammonia, fed by ammonia stream 14. Clean gas 28 is taken to stack 19, from which treated gas stream 20' is discharged into the atmosphere without reheating.

Periodically, one active filter 21 is cleaned by backflushing, while a second filter 21 is utilized as described above. Fly ash, or other particles 16, blocked by the microporous membrane, are removed and collected for disposal. Further, the chemically-active compositions can be regenerated with a reducing gas, such as natural gas, carbon monoxide, hydrogen, or mixtures thereof, produced from a gasifier 22 or another source. The reducing gas is typically diluted with nitrogen, steam, or other suitable diluent gas. During reaction of the sulfated metal oxide compositions with the reducing gases, an off-gas is produced, containing primarily S0₂ in variable concentration, depending on the conditions of regeneration. This stream is then taken to a catalytic sulfur recovery unit 23, where SO_? is reacted with a reducing gas stream 24 over a catalyst to produce elemental sulfur 25, which is separated out by condensation. Any unconverted SO_? and reduced sulfur compound products (e.g. COS, H₂S) are then recycled through line 26 to active filters 21 or to the boiler 10 (not shown in FIG. 6). Alternatively, the concentrated SO -containing off-gas stream may be taken to a catalytic oxidation unit to produce oleum, SO , or to produce sulfuric acid, H_S0 . In all these cases, the sulfur value of the flue gas is recovered in the form of salable products, and no waste by-product is generate .

There are situations where the sulfur recovery catalyst material is compatible with the sorbents and conditions used in active filters 21. In such cases, the catalyst can be mixed with the active composition in filters 21. In a limiting case, the catalyst is also an active composition for S0 /N0 removal. When sulfur recovery is performed in active filters 21, sulfur recovery reactor 23 is not needed. In this case, provision is taken to recycle stream 27 to the active filters 21. Figure 6A shows a schematic of such a single-stage, regeneration-sulfur recovery process, where the sulfur value is recovered in the form of elemental sulfur. Accordingly, the regeneration off-gas is recycled (stream 27) to the regenerator vessel of active filters 21 to increase conversion of SO- to elemental sulfur, which is separated out by condensation 25. Any unconverted SO and reduced sulfur compounds in the tail gas 26 are recycled to boiler 10 or to absorption vessel of active filters 21.

According to the present process therefore, no solid or sludge waste is generated. The sulfur value of the S0₂ pollutant in the flue gases is recovered as elemental sulfur, a saleable product.

Removal efficiencies, utilizing the active filters of the present invention, are about 99.9% particulates, and greater than 98% and 80% for S0 and NOX, respectively.

The active compositions used in this invention for combined removal of SO-/NO were bulk mixed oxides of copper and aluminum, copper and cerium, or copper, cerium, and aluminum. Various compositions of the mixed oxides can be used, depending on the flue gas type and clean-up temperature. Occasionally, it may be advantageous to include small amounts of transition metal oxides, such as iron, nickel, etc.; other rare earth or alkaline earth oxides as catalyst/εorbent promoters; or it may be beneficial to replace aluminum oxide by other oxides typically used as dispersants, or structural or textural promoters of the active composition, as discussed above. Such cases are considered to fall within the scope of the present invention.

It is well known that when molecular expansion takes place during reaction, such as from a solid oxide to a solid sulfate as in the present case, diffusion of the gas through the typically non-porous, solid product layer is hindered, so that only partial utilization of the active phase occurs. In such situations, it is beneficial to keep the active phase dispersed in small particle size. A common approach is to support the active phase on high-surface area carriers, such as γ-alumina, silica, titania, and the like. This, however, limits the amount of sorbent on the support to less than 10 percent by weight. If higher amounts are used, the bulk phase properties dominate and grain growth, sintering, etc. are not avoided. When the active phase is a sorbent to be used in a fixed-bed design in conjunction with a filter, as in the present invention, the low loading of the carrier necessitates large, i.e. uneconomical, reactor volumes.

An alternative is to disperse the active phase, in an inert or low-activity phase, such as oxides of aluminum, chromium, titanium in bulk. In other words, use a bulk, mixed oxide approach to retain the active phase in dispersed form and at higher weight fraction than what is possible when a support (carrier) is used.

The present invention will be further illustrated by the following examples which are intended to be illustrative in nature and are not to be construed as limiting the scope of the invention.

EXAMPLE I

Active metal oxide compositions were prepared according to a slightly modified complexation technique, namely, the amorphous-citrate technique. Marcilly, C, Courty, P., Delmon, B., "Preparation of Highly Dispersed Mixed Oxides and Oxide Solid Solutions by Pyrolysis of Amorphous Organic Precursors^", . Ame . Ceram. Soc. , Vol. 53(1), 56-57 (1970). The technique involves rapid dehydration (under vacuum at 70°C) of an aqueous solution containing metal salts (e.g. nitrates, chlorides, sulfates, etc.) and citric acid in desired proportions, followed by pyrolysis and further calcination of the resulting, amorphous solid foam in air at relatively low temperatures (500-650°C). Citric acid is added in a 1:1 mole ratio to the total metal salts in solution. A higher ratio is used when a higher surface area and porosity are desired in the final oxide composition. Compositions A-E were prepared by the amorphous citrate technique. In some cases, a coprecipitation technique is employed to prepare binary or ternary oxides. Active composition F, comprising copper, cerium, and aluminum oxides, was prepared by coprecipitating the carbonates, or hydroxycarbonates, of the metals from an aqueous solution of the respective nitrate salts by addition of sodium carbonate. The precipitate was filtered, washed of sodium several times, dried at 110°C overnight, and then calcined in air at 550°C for 3 hours. Active metal oxide compositions A-F are presented in Table 1.

Table 1

Designation Composition Preparation¹ Citric Acid BET,SA Conditions Metals

(wt%) (°C,h) (mol.mol) (m²/g

A CU-7A1 18CuO-82Al,0, 500,3 2:1 373

B Ce lOOCeO, 550,3 1: 1 25

C Cu-Ce 32CuO: 68CeO, 500,3 1:1 24

D Ce/Cu-7A1 100CeO,/18CuO-82Al,O, B/A³ B/A³ 25/373

E Cu-7A1(H) 18CuO-82Al,0, 550,3 2:1 310

F Cu-Ce-Al 33CuO-33CeO,-34Al,0, 550,3 _2 110

Bulk oxide preparation by amorphous citrate complexing of nitrate salts; pyrolysis in air at 450°C for l/2h preceded the calcination step.

2. Prepared by coprecipitation.

3. Two-zone system with gas flowing first through composition B, then comDosition A. The bulk oxide compositions (in 20-35 mesh particle size) were tested in a fixed-bed, quartz reactor assembly under isothermal conditions. Simplified, simulated flue cases were used in these tests with composition (mol%) : 30 , 0.15SO₂, 0.06NO, balance N . For the denitration tests, gaseous NH_, in 1 : 1 molar ratio with NO, was added in the inlet gas stream after it was preheated. After some time on stream, different for each sorbent type, S0₂ would break through. The time at which 150 ppm SO_ would be measured into the exit gas streams was recorded as the breakthrough-time-at-90%-SO₂ removal efficiency. Regeneration of the sulfated sorbent would then follow, using, carbon monoxide diluted in nitrogen, or in nitrogen and C0₂ gas mixtures, at temperatures of 400-600°C. Gases were analyzed by on-line NO/NO gas analyzers and gas chromatographs equipped with sulfur compound detectors. Gas flowrates were set to give corresponding gas hourly space velocities (G.H.S.V.) of 2,000-25,000 h^_1 (STP).

Table 2 shows the combined SO-/NO removal performance of various active compositions. Clearly, cerium oxide alone is an excellent SCR catalyst, better than CuO-containing compositions, at temperatures exceeding 400°C. On the other hand, copper oxide has a higher sulfation activity, so that it can be fully sulfated (100%) at 400°C, either when it is combined with aluminum oxide, as in composition A, or when it is combined with cerium oxide, as in composition C. Note that 26% sulfation conversion of composition C at 400°C is equivalent to fully εulfating, the CuO-component of that material. A new finding in this work was the promotion of the cerium oxide sulfation by copper. In fact, at 500°C, where CeO„ alone is only 7% sulfated prior to 150 ppm 50--breakthrougn, the equimolar CuO-CeO- composition C is completely sulfated (100%). Since both materials, B and C, nave the same moderately high surface area of about 25

2 m /g, this is not a mere dispersion effect, but a promotion/mixing effect that, to date, has not been reported in the literature. Table 2⁴ Composition Sorbent Sulfation (%)⁵ N0_y Conversion(%)

400°C 500°C 600°C 400°C 500°C

A: CU-7A1 100 100 — 90 56

B: Ce 4 7 100 100 98

C: Cu-Ce 26 100 — 98 70

4. Gas composition (moll): 302, 0.15SO2, 0.06NO, O.O6NH3, balance N₂ at S.V. = 2000 h^_1 (STP) .

5. At 150 ppm S02~breakthrough (SO2 removal efficiency is 100% prior to breakthrough) .

To improve the SCR catalyst activity of compositions A or C at 500°C, a two-zone approach can be followed. Thus, composition D, which was not an admixture of B and A, but a two-layered B/A bed with B first in the gas stream flow and A following, achieved 98% NO removal at 500°C, identical to the performance of composition B, while its sulfation conversion was similar to that of composition A.

The NO X conversion shown in Table 2 is attained after partial sulfation of the oxide compositions. The initial activity (for fresh or regenerated composition) is typically lower. In other words, at 400 and 500°C, the sulfates are better SCR catalysts than the corresponding oxides. FIG. 7 shows an example of the combined

removal performance of composition C at 500°C.

The regenerability of performance in multiple sulfation/regeneration cycles is shown for composition A in

FIG. 8. There is a small loss of sulfation conversion after the first cycle, which corresponds to loss of surface area.

From then on, however, no further loss of performance is observed. The inclusion of NOx in the simulated flue gas does not affect the sulfation performance of the sorbent. The regeneration off-gas consisted of SO-, CO-, and N , i.e. in regenerating composition A with diluted CO gas, the only sulfur product was SO-. The multi-cycle sulfation performance of composition C was inferior to that of composition A. While the latter could be regenerated at 400°C, the former had to be regenerated at 600°C. Apparently grain growth during this higher regeneration temperature reduces the extent of subsequent sulfation.

Composition F is an example of a ternary oxide mixture of copper, cerium, and aluminum, prepared by coprecipitation, containing 33 percent by weight of each oxide, i.e. 66 percent by weight active phase. FIG. 9 shows the sulfation performance of composition F (sulfation at 500°C, regeneration at 600°C) . After initial loss of conversion, the second and third cycle performances were identical, a definite improvement over composition C. Thus, the use of an inert alumina framework suppressed the structural degradation of the active phases caused by volume expansion, or grain growth, and the like. Large porosity also helps to moderate similar structural problems in mixed metal oxides. Such guidelines will help to optimize the compositional structure of these and similar sorbent/catalyst materials which have to show performance regenerability over multi-cycle operation at high temperatures.

Sulfated cerium oxide, supported on alumina, has been reported to produce a mixture of S0₂ and reduced sulfur compounds (H₂S) during regeneration with hydrogen. In addition, it is known that when CO is used in regeneration of sulfated ceria, a mixture of S0₂, COS, and elemental sulfur is produced. This is also true when mixed oxides of copper and cerium are used. For example, FIG. 10 shows the sulfur products distribution during regeneration of composition C at various temperatures. Elemental sulfur produced was collected in an ice bath and is not indicated in the figure.

The selectivity of cerium oxide for the production of elemental sulfur during regeneration offers the possibility of recycling the off-gas back to the regenerator to continue reduction of SO- with CO over the now-regenerated cerium oxide catalyzing the formation of elemental sulfur. In such a process, the need for downstream processing of regenerator off-gases is eliminated. The only product is elemental sulfur condensed out of the off-gas and a tail gas carrying any unconverted SO- and product COS back to the boiler or the oxidative absorption unit.

EXAMPLE II The sulfation, deNO X, and regeneration properties of the active compositions were compared in powder form and as a solid-foam integrated within the channels of a monolith filter, such as that shown in FIGS. 4 and 5B. In one such comparison, we used composition E (Table 1). To prepare the foam, we ground the material into a fine powder

(<10μm-sized particles). A slurry of the powder was made by mixing, under continuous stirring, 3.77 grams of composition E with 11 milliliters of de-ionized water and

0.26 grams methyl cellulose (as binder). To this, 0.02 gram aluminum and 0.03 gram hydrochloric acid were added to form a foam. The foam was poured into the discharge channels of a

₂ monolith piece (3/4" dia., 100 cells/in ), supplied by

CeraMem Corporation, with a filtering membrane slip-coated on the walls (as in FIG. 4). The loaded monolith (designated J) was dried in an oven at 175°C overnight and then transferred to a muffle furnace, where it was heated in air at 500°C for

3 hours. The weight change of monolith J, after cooling, corresponded to 12 percent by weight loading with the active phase E.

Active filter J exhibited overall performance characteristics similar to powder composition E used to make the foam. Also, the pressure drop through the foamed filter, while higher than that of the bare one, was below 4" H-0 for a gas flowrate of 1 SLM. All of the powder and active filter tests were conducted at the same ratio of sorbent mass to gas flowrate (0.5 gram sorbent per liter/min of gas). Because of the different geometry of the two systems (packed powder in a tube versus a honeycomb filter with foam-containing channels), the two systems had different space velocities: 25,000 h^~ for the powder and 2,500 h for the active filter.

FIG. 11 shows the stabilized sulfur dioxide removal performance of the active filter, compared to the sorbent powder composition E at 400°C. Both systems achieved 100% SO--removal efficiency prior to breakthrough. Similarly high (80-90%) sorbent conversion at breakthrough was measured for the powder and the active filter. Since these similarities were obtained at very different space velocities, design flexibility is indicated for the active filters, namely, that they can be operated at relatively high space velocities (>20,000 h^~ ). FIG. 12 shows the catalytic activity of the two systems in fresh (unsulfated) form for NO reduction by NH_ as a function of reaction temperature. Both tests were conducted with a gas mixture containing (moll): 0.06NO, 0.06NH-, 30_2# and N₂ balance. The results show that the SCR characteristics of active filter J are very similar to those of powder composition E.

Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present invention. Design considerations may alter the configuration of the active filters, the number of elements carrying active phase compositions with or without a microporous membrane, to optimize the efficiency of certain applications and minimize the cost associated with gas cleanup operation. Accordingly, the invention is not to be limited except oy the appended claims .

What is claimed s:

Claims

1. A filter wall composite for multiphase cleanup of gas streams comprising particulate and gaseous pollutants, comprising: a bulk macroporous support having a microporous membrane on one side thereof for blocking passage of particulate pollutants, and a chemically-active deposit on the opposite side thereof for reacting with gaseous pollutants; said bulk macroporous support having a surface area of 2 less than 35.0 /g, a compressive crush strength of between about 1.0 lb/mm and about 10.0 lbs/mm, and a thickness of between about 1.0 millimeter to about 3.0 millimeters; wherein said microporous membrane has a thickness of less than about 100.0 microns; and said chemically-active deposit has a thickness of less than about 10.0 millimeters.

2. The filter wall of claim 1 wherein said gas stream is a flue gas comprising particulate pollutants, and gaseous pollutants from the group consisting of sulfur oxides, nitrogen oxides, volatile organic compounds, and mixtures thereof.

3. The filter wall of claim l wherein said gas stream is a fuel gaεifier exit gas comprising particulate pollutants, and gaseous pollutants from the group consisting of hydrogen sulfide, carbonyl sulfide, organosulfur compounds, ammonia and mixtures thereof.

4. The filter wall of claim 2 wherein said chemically-active deposit reacts with said sulfur oxides to form sulfated metal oxides.

5. The filter wall of claim 4 wherein said sulfated metal oxides and said chemically-active deposit are selective catalytic reduction catalysts for said nitrogen oxideε.

6. The filter wall of claim 5 wherein said nitrogen oxideε are reacted with ammonia in the preεence of εaid selective catalytic reduction catalysts to produce molecular nitrogen.

7. The filter wall of claim 4 wherein said sulfated metal oxides are regenerated with a reducing gas producing a sulfur dioxide-containing gas stream which is converted to elemental sulfur.

8. The filter wall of claim 3 wherein said chemically-active deposit reacts with said hydrogen sulfide, carbonyl sulfide and other organosulfur compounds to form sulfided metal oxideε, and catalyzeε decomposition of εaid ammonia.

9. The filter wall of claim 1 wherein said bulk macroporous support is made of refractory ceramic materials having high mechanical strength, thermal shock resistance, and chemical resistance.

10. The filt. wall of claim 9 wherein said refractory ceramic matei are selected from the group consisting of mullite, core . -rite, oxides of aluminum, silicon, magnesium, zirconium, titanium, and nitrides and carbides of silicon.

11. The filter wall of claim 1 wherein said bulk macroporous εupport comprises: a chemically-active component selected from the group consisting of elements and oxides of transition metals, alkali metals, alkaline earth metals, rare earth elements, and mixtures thereof; and a structural promoter component selected from the group consisting of alumina, chromia, titania, silica, magnesia, zirconia, and mixtures thereof.

12. The filter wall of claim 11 wherein said gas stream is a flue gas comprising particulate pollutants, and gaseous pollutants from the group consisting of sulfur oxides, nitrogen oxides, volatile organic compounds, and mixtures thereof.

13. The filter wall of claim 12 wherein said chemically-active depoεit reactε with εaid sulfur oxides to form sulfated metal oxides.

14. The filter wall of claim 13 wherein said εulfated metal oxideε and εaid chemically-active depoεit are εelective catalytic reduction catalysts for said nitrogen oxideε.

15. The filter wall of claim 14 wherein said nitrogen oxideε are reacted with ammonia in the presence of εaid selective catalytic reduction catalyεtε to produce molecular nitrogen.

16. The filter wall of claim 1 wherein εaid bulk macroporouε εupport has at least 401 of itε porosity in large pores with an average pore diameter of between about 2.0 micron to about 50.0 micronε.

17. The filter wall of claim 1 wherein εaid chemically-active deposit is selected from the group consisting of elements and oxides of transition metals, alkali metals, alkaline earth metalε, rare earth elements, and mixtures thereof.

18. The filter wall of claim 1 wherein εaid icroporouε membrane haε an average pore diameter of leεs than about 2.0 microns.

19. The filter wall of claim 1 wherein said macroporous support and said microporous membrane have a unitary construction.

20. The filter wall of claim 1 wherein said microporous membrane and said bulk macroporous εupport have at leaεt one common component.

21. The filter wall of claim 1 wherein εaid chemically-active deposit is a film, having a thickneεε of less than about 2.0 millimeters, deposited on said bulk macroporous support.

22. The filter wall of claim 1 wherein said chemically-active deposit iε a macroporouε εolid foam, having a thickneεs of between about 2.0 millimeters to about 10.0 millimeters, depoεited on said bulk macroporous support

23. The filter wall of claim 1 wherein εaid chemically-active deposit comprises two zones, and said two-zone chemically-active deposit achieves about 98% nitrogen oxide removal in a first zone, and about 100% sulfur oxide conversion in a second zone.

24. The filter wall of claim 23 wherein said zones are layered, and said first zone comprises CeO- and said second zone comprises CuO and A1₂0_.

25. The filter wall of claim 17 wherein said chemically-active deposit comprises a ternary oxide of copper, cerium, and aluminum.

26. The filter wall of claim 25 wherein said ternary oxide comprises equal parts of CuO, Ce0₂, and A1₂0_ .

27. A filter for integrated clean up of gas streamε compriεing particulate and gaseous pollutants, compriεing: filter walls comprising bulk macroporous supports having microporous membranes on one side thereof for blocking pasεage of particulate pollutantε, and chemically-active depoεitε on the oppoεite side thereof for reacting with gaseous pollutants; wherein εaid bulk macroporous supports have a surface area of lesε than 35.0 m /g, a compreεεive crush strength of between about 1.0 lb/mm to about 10.0 lbε/mm, and a thickneεs of between about 1.0 millimeter to about 3.0 millimeters, said microporous membranes have a thicknesε of leεε than about 100.0 micronε, and said chemically-active deposits have a thickness of less than 10.0 millimeters.

28. The filter of claim 27 wherein said filter is a crosε-flow device.

29. The filter of claim 27 wherein εaid filter iε a through-flow device.

30. The filter of claim 27 wherein said filter is periodically backflushed with a gaε stream to remove particulate pollutants deposited on εaid microporous membrane.

31. The filter of claim 27 wherein εaid gas stream is a flue gas comprising particulate pollutantε, and gaεeouε pollutantε from the group consisting of sulfur oxides, nitrogen oxideε, volatile organic compounds, and mixtures thereof.

32. The filter of claim 27 wherein εaid gaε stream is a fuel gasifier exit gas compriεing particulate pollutantε, and 4/21351

- 31 -

gaεeouε pollutantε from the group consisting of hydrogen sulfide, carbonyl sulfide, organoεulfur compoundε, ammonia and mixtures thereof.

33. The filter of claim 31 wherein εaid chemically-active deposit reacts with said sulfur oxides to form sulfated metal oxides.

34. The filter of claim 33 wherein said εulfated metal oxides and said chemically-active depoεit are εelective catalytic reduction catalyεtε for said nitrogen oxides.

35. The filter of claim 34 wherein said nitrogen oxides are reacted with ammonia in the presence of said selective catalytic reduction catalysts to produce molecular nitrogen.

36. The filter of claim 33 wherein said sulfated metal oxides are regenerated with a reducing gas producing a sulfur dioxide-containing gas stream which is converted to elemental sulfur.