CATALYST MEMBER MOUNTING MEANS, STAGED CATALYTIC FLAME ARRESTOR AND METHOD FOR PREVENTING FLAME INITIATION OF EXHAUST GAS

Abstract: A method and apparatus for preventing flame initiation of the flammable exhaust gas from a small engine involves flowing the exhaust gas through a first flame arrestor stage (10), through a cooling means (11, 32), and then through at least a second flame arrestor stage (34). The flame arrestor stages (10, 34) are capable of catalytically oxidizing the flammable components of the exhaust gas and the cooling means (11, 32) dissipates some of the heat of oxidation between stages so the pollutants in the exhaust gas stream can be oxidized without increasing the temperature of the exhaust gas to its auto-ignition temperature. Each stage (10, 34) has a catalyst member (14, 14') secured in a canister (15, 35) through which the exhaust gas flows. The catalyst members (14, 14') contain catalytic materials effective for the conversion of at least some of the noxious components to innocuous substances. A ceramic fiber fabric (16) formed from fibers having an average diameter of about 10 microns and an average length greater than 10 inches (about 25.4 cm) is wrapped around the catalyst member to facilitate mounting the catalyst member in the canister. Optionally, the ceramic fiber fabric is substantially free of intumescent fibers.

Description: CATALYST MEMBER MOUNTING MEANS, STAGED CATALYTIC FLAME ARRESTOR AND METHOD FOR PREVENTING FLAME INITIATION OF EXHAUST GAS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the mounting of catalyst members in canisters and to a flame arrestor for flammable exhaust gases, and further to a flame arrestor and catalytic emission abator useful to prevent the ignition of exhaust gases from small engines. Flame arrestors are known for use in processing facilities to prevent the propagation of a flame front through a flammable or potentially explosive gas stream. For example, vents on atmospheric-pressure storage tanks for liquids having vapor pressures in the flammable range are often equipped with arrestors to prevent flames outside the tank from traveling back through the tank's atmospheric vent and igniting the flammable vapor-air mixture inside the tank's base. Other types of flame arrestors are disposed in transfer conduits of flammable fluids as safety precautions between processing stations. Generally, flame arrestors are designed to function as a result of their physical configurations, e.g., one conventional configuration for a flame arrestor is to have a cross-sectional area significantly smaller than the cross section of the conduit through which the flammable gas flows. Another design consideration is to make the flame arrestor from a material having high heat conduction ability so that the arrestor acts as a heat sink to cool a flammable gas mixture safely away from its auto-ignition temperature . The exhaust gases of internal combustion engines, including small engines, are known to contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NO χ ) that foul the air. Governmental restrictions on such emissions from larger engines, such as those for automobiles, require that they be equipped with fuel control devices designed to keep the air/fuel mixture used in the engine in stoichiometric balance, so that substantially complete combustion and catalytic treatment of the fuel can yield carbon dioxide without substantial formation of N0 χ . Automobiles are typically also equipped with flow-through underfloor catalyst members, commonly known as catalytic converters, that comprise environmental catalysts that convert unburned hydrocarbons, carbon monoxide and NO χ to innocuous substances, e.g., carbon dioxide, nitrogen and water vapor. The fuel control devices help optimize the conversion performance of catalytic converters by keeping the air/fuel combustion mixture for the engine in stoichiometric balance. Even though the activity of the environmental catalyst is generally exothermic, exhaust gases are not ignited even when heated by the exothermic action of a single environmental catalyst because the exhaust gas produced by an engine running on a stoichiometric air/fuel mixture has an auto-ignition temperature that is higher than that reached by the catalyst. Small internal combustion engines, usually two-stroke and four-stroke spark ignition engines, e.g., gasoline-engine powered lawn mowers, motor scooters, motorcycles, mopeds and the like, provide a severe environment for a catalytic exhaust treatment apparatus . This is because small engines typically power machines that are not large enough to accommodate a length of exhaust pipe between the engine and the catalytic treatment apparatus sufficient to cool the exhaust before it contacts the catalyst. In addition, the exhaust gas contains a high concentration of unburned fuel and unconsumed oxygen which release large quantities of heat when they react over the catalyst. The heat conditions are often severe because a catalyst member is often mounted inside another structure such as a muffler, or it may be insulated to protect the user; in either case, the heat it produces cannot quickly be dissipated. Furthermore, the vibrational force in a two-stroke engine can be three or four times that of a four-stroke engine. For example, vibrational accelerations of 70 G to 90 G (G s gravitational acceleration) at 150 hertz (Hz) have been reported for small engines. The harsh vibration and exhaust gas temperature conditions associated with small engines lead to several modes of failure in the exhaust gas catalytic treatment apparatus, including failure of the mounting structure by which a catalyst member is secured in the apparatus and consequential damage or destruction of the catalyst member due to the mechanical vibration and to flow fluctuation of the exhaust gas under high temperature conditions. The catalyst member usually comprises a ceramic-like carrier member that has a plurality of fine parallel gas flow passages extending therethrough (sometimes referred to as a ■■ honeycomb") and which is typically made of, e.g., cordierite, mullite, etc., on which a catalytic material is coated. A typical carrier member has cells spaced 2.54 millimeters ("mm") apart (100 cpsi) and walls 0.3 mm thick. The ceramic-like material is subject to cracking and pulverization by excessive vibration. The catalyst member is typically mounted in a canister by wrapping the catalyst member with a resilient mat and enclosing the canister about the wrapped catalyst member. The mat resides in a gap between the canister and the catalyst member. The canister is configured so that it is easily mounted in the exhaust line of the vehicle. Related Art In SAE technical paper number 922252 entitled New Developments In Packaging of Ceramic Honeycomb Catalysts, dated October 19-22, 1992, Gulati et al discuss the mounting of ceramic honeycomb catalysts in canisters to be emplaced in the exhaust lines of diesel and gasoline trucks. They teach a preference for gap bulk density (GBD) of a ceramic mat used to mount a honeycomb catalyst in a canister as being from 0.78 to 1.2 grams per cubic centimeter ( "g/cm 3 " ) , referring to the density of the ceramic fiber fabric when it is compressed between the canister shell and the catalyst member therein, and a corresponding mean mounting pressure of from 0.67 to 23 kg/cm 2 . For temperatures above 700°C, a ceramic material sold commercially under the- trademark FIBERMAX-LS™ by Carborundum Company is suggested. A hybrid mat, in which an inner layer of highly insulating FIBERFRAX" 1 material is placed next to an intumescent XPE-FF material, is disclosed for use in the heavy-duty gasoline truck engine catalytic converter, where the inlet gas temperature may approach 1100°C and monolith skin temperature may exceed 900°C. Product literature for a prior art ceramic fiber fabric material used to mount catalyst members states that the fabric comprises fiber having a length of up to approximately 254 millimeters (10 inches). U.S. Patent 3,795,524 to Sowman, dated March 5, 1974, discloses a ceramic fiber fabric whose fibers are continuous and comprise alumina, from 5 percent to 25 percent boria, and from 0 to 65 percent silica, but the material is not taught for use as a mounting mat for a catalyst member. SUMMARY OF THE INVENTION The present invention relates to a method for preventing flame initiation of a flammable exhaust gas from an engine, the exhaust gas containing at least oxygen and unburned hydrocarbons. The method comprises flowing the exhaust gas in contact with a plurality of sequentially arranged flow-through catalytic flame arrestor stages comprising a first stage and at least one subsequent stage. Each stage comprises a catalyst member comprising a catalytic material effective to oxidize a portion of the pollutant species in the exhaust gas, to increase the auto-ignition temperature of the exhaust gas by reducing the quantity of unburned hydrocarbons in the exhaust gas while releasing insufficient heat to raise the temperature of the exhaust gas to its auto-ignition temperature. The partially oxidized exhaust gas effluent of each stage is cooled before flowing the exhaust gas to the subsequent stage. According to one aspect of the invention, the method may comprise oxidizing from about 10 to about 60 percent of the pollutant species in the exhaust gas in the first stage. The method may comprise oxidizing at the at least one subsequent stage substantially all of the pollutants remaining in the exhaust gas effluent from the first stage. According to another aspect of the invention, the exhaust gas may comprise nitrogen oxides and at least one subsequent stage may comprise a three-way catalytic material so that flowing the exhaust gas through the at least one subsequent stage comprises reducing the nitrogen oxides to nitrogen. Optionally, the method may comprise selectively converting hydrocarbons in the first stage. The invention also has an apparatus aspect pertaining to a staged catalytic flame arrestor apparatus for preventing flame initiation of flammable exhaust gas from a small engine. The engine has an exhaust outlet and the apparatus comprises a flow-through first stage in gas flow communication with the exhaust outlet of the small engine. The first stage comprises a catalyst member comprising a first catalytic material effective to oxidize a first portion of the hydrocarbons and carbon monoxide in the exhaust gas so that the heat of oxidation released by the first stage is insufficient to raise the effluent from the first stage to its auto-ignition temperature. There is a cooling means for cooling the effluent of the first stage. There is also at least one subsequent stage in gas flow communication with the cooling means to receive cooled effluent from the first stage. The at least one subsequent stage comprises a catalyst member comprising a subsequent catalytic material effective to oxidize a subsequent portion of the hydrocarbons and carbon monoxide in the exhaust gas, the subsequent portion releasing insufficient heat of oxidation to raise the exhaust gas to its auto-ignition temperature. Optionally, the first catalytic material may comprise a platinum group metal component that principally comprises a rhodium component. The at least one subsequent stage may comprise a three-way catalytic material . The present invention also relates to improvements in an exhaust treatment apparatus for abating noxious components of the exhaust gas of a small internal combustion engine. The exhaust treatment apparatus of the present invention comprises a catalyst member secured in a canister through which the exhaust gas flows, the catalyst member comprising a catalytic material effective for the conversion of at least some of the noxious components to innocuous substances. A ceramic fiber fabric is disposed in a gap between the catalyst member and the canister for mounting the catalyst member in the canister. One improvement comprises that the ceramic fiber fabric has a density of greater than 0.089 grams per cubic centimeter ("grams per cc"). For example, the ceramic fiber fabric may have a density of at least about 0.1 grams per cc, or at least about 0.5 grams per cc, or even at least about 0.74 grams per cc. Another, independent improvement comprises that the fabric principally comprises fibers having an average length greater than 10 inches, e.g., an average length of at least about 1.5 meters, preferably at least about 3 meters. The fabric may be configured to have a width that is not greater than the length of the catalyst member, and preferably may have woven edges. The fabric may be woven from yarn comprising effectively continuous ceramic fibers. Preferably, the ceramic fiber fabric may be substantially free of intumescent fibers. The ceramic fiber fabric may have a gap bulk density, meaning the density of the ceramic fiber fabric when it is compressed in the gap between a canister shell and a catalyst member therein, of at least about 0.8 g/cm 3 . According to another aspect of the invention, the gap may have a width of less than about 0.1 inch. For example, the gap may have a width of from about 0.05 to 0.06 inch. Optionally, the gap may be less than 0.05 inch. According to yet another aspect of the invention, the catalyst member may have an inlet end face, an outlet end face and a circumferential surface, and the ceramic fiber may be confined to the circumferential surface. Optionally, the apparatus may comprise end rings dimensioned and configured to confine the fabric to the circumferential surface without obstructing the end faces of the catalyst member. In a particular embodiment, the ceramic fiber fabric may have a weight of about 1 gram per square inch. The present invention also relates to a method for assembling a gas treatment apparatus. The method comprises wrapping a catalyst member in a ceramic fiber fabric, the catalyst member comprising a catalytic material effective for the conversion of components of a gas stream and being dimensioned and configured to have two end faces, a circumferential surface, and a plurality of gas flow passages extending therethrough from one end face to the other. The wrapped catalyst member is surrounded with at least one shell having edges that can be joined together. An assembly force is applied on the at least one shell and is maintained while securing the shell edges together to form a canister about the catalyst member and to define a gap between the canister and the catalyst member wherein the ceramic fiber fabric is disposed. According to one aspect of the invention, the gas flow passages of the catalyst member may be defined by sets of parallel cell walls that may be angled relative to each other, and the method may comprise applying the assembly forces at a non-zero angle relative to each set of cell walls. For example, the sets of cell walls may be perpendicular to each other, and the assembly forces may be applied at an oblique angle relative to the cell walls, e.g., at an angle of 45 degrees. In a particular embodiment, the method may comprise confining the ceramic fiber fabric to the circumferential surface of the catalyst member. BRIEF DESCRIPTION OF THE DRAWINGS Figure IA shows plots of compression and relaxation thicknesses for aged and non-aged woven fabric used in one aspect of the present invention; Figure IB shows plots like those of Figure IA for a felt-type fabric of the prior art; Figure 2 is a cross-sectional view of a racetrack-shaped catalyst member mounted in a clamshell canister in accordance with one embodiment of the present invention; Figure 3A is a schematic view of an apparatus for assembling a shell-and-catalyst member assembly; Figure 3B is a cross-sectional view of a round catalyst member oriented in canister shells with assembly forces applied at a 45 degree angle relative to the cell walls in the catalyst member; Figure 4 is a schematic cross-sectional view of a staged catalytic flame arrestor in accordance with one embodiment of the present invention, comprising two catalyst members mounted in the engine muffler with ceramic fiber fabrics in accordance with one embodiment of the present invention; Figure 4A is an enlarged view of that portion of Figure 4 indicated by the circle A in Figure 4; and Figure 5 is a cross-sectional view of a fabric-mounted catalyst member with a fabric that does not extend to the ends of the catalyst member and with end rings that confine the fabric but do not obstruct the end faces of the catalyst member. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF Small engines, such as one- and two-cylinder engines used, for example, to power lawn mowers, are generally not equipped with the stoichiometric air/fuel control devices that are common for larger engines, e.g., automobile engines. Accordingly, the applicants have discovered that under certain operating conditions, the air/fuel mixture provided to a small engine can be inappropriately rich, resulting in inefficient fuel combustion and the emission of hydrocarbon-rich exhaust gases. When flowed through a conventional single stage catalytic converter, the exothermic action of the catalyst can heat hydrocarbon-rich exhaust gas to its auto-ignition temperature, and a flame may be initiated in the exhaust pipe of the device, posing a threat of deactivation of the catalyst due to over-temperature, presenting a hazard to the user and creating a risk that the flame may initiate the combustion of nearby flammable materials. In accordance with the present invention, the exhaust gas of a small engine is flowed through a staged flame arrestor which is effective for raising the auto-ignition temperature of the exhaust gas by converting the flammable components of the exhaust gas to less flammable species, e.g., by converting unburned hydrocarbons to carbon dioxide. The staged flame arrestor catalytically oxidizes the flammable species of the exhaust gas in stages and dissipates some of the heat of oxidation between stages so that the flammable exhaust gas is not heated to its auto-ignition temperature. Accordingly, a flame arrestor apparatus in accordance with the present invention comprises at least two catalytic flame arrestor members in the exhaust gas flow path with cooling means between them. A catalytic flame arrestor stage of the present invention generally comprises a catalyst member comprising a flow-through carrier that defines a series of fine gas flow passages through which the exhaust gas can flow. In some embodiments, the carrier is in the form of a bed of inert pellets, but more typically the carrier has a so-called "honeycomb" configuration in which the gas flow passages are mutually parallel. A coating of catalytic material is deposited on the carrier and the exhaust gas is flowed through the coated carrier to contact the exhaust gas with the catalytic material. The upstream or first catalytic flame arrestor stage comprises a catalyst member comprising a catalytic material effective to catalyze the oxidation of a portion, e.g., about 10 to 60 percent, preferably about 30 to 60 percent, of the flammable components in the exhaust gas without raising the exhaust gas to its auto-ignition temperature. A typical catalytic material for use in the present invention may comprise a platinum group metal component dispersed on a refractory inorganic oxide support material. In preparing a suitable catalytic material, a compound and/or complex of any catalytic component, e.g., one or more platinum group metal compounds or complexes, may be utilized in dispersing the catalytic component onto a bulk support material. As used herein, the term "compound", as in "platinum group metal compound" means any compound, complex, or the like of a catalytic component which, upon calcination or use of the catalyst, decomposes or otherwise converts to a catalytically active form, which is often an oxide. The compounds or complexes of one or more catalytic components may be dissolved or suspended in any liquid which will wet or impregnate the support material, which does not adversely react with other components of the catalytic material and which is capable of being removed from the catalyst by volatilization or decomposition upon heating and/or the application of a vacuum. Generally, both from the point of view of economics and environmental aspects, aqueous solutions of soluble compounds or complexes are preferred. For example, suitable water-soluble platinum group metal compounds are chloroplatinic acid, a ine solubilized platinum hydroxide, rhodium chloride, rhodium nitrate. hexamine rhodium chloride, palladium nitrate or palladium chloride, etc. The compound-containing liquid is impregnated into the pores of the bulk support particles of the catalyst, and the impregnated material is dried and preferably calcined to remove the liquid and bind the platinum group metal into the support material. In some cases, the completion of removal of the liquid (which may be present as, e.g., water of crystallization) may not occur until the catalyst is placed into use and subjected to the high temperature exhaust gas. During the calcination step, or at least during the initial phase of use of the catalyst, such compounds are converted into a catalytically active form of the platinum group metal or a compound thereof. An analogous approach can be taken to incorporate the other components into the catalytic material . Suitable support materials include alumina, silica, titania, silica-alumina, aluminosilicates, aluminum-zirconium oxide, aluminum-chromium oxide, etc. Such materials are preferably used in their high surface area forms. For example, gamma-alumina is preferred over alpha-alumina. It is known to stabilize high surface area support materials by impregnating the material with a stabilizer species. For example, gamma-alumina can be stabilized against thermal degradation by impregnating the material with a solution of a cerium compound and then calcining the impregnated material to remove the solvent and convert the cerium compound to a cerium oxide. The stabilizing species may be present in an amount of from about, e.g., 5 percent by weight of the support material. The catalytic materials are typically used in particulate form with particles in the micron-sized range, e.g., 10 to 20 microns in diameter, so that they can be formed into a slurry and applied as a washcoat on a carrier member. Any suitable carrier member may be employed, such as a honeycomb-type carrier of the type having a plurality of fine, parallel gas-flow passages extending therethrough from an inlet or an outlet face of the carrier, so that the passages are open to fluid-flow therethrough. The passages, which are typically essentially straight from their fluid inlet to their fluid outlet, are defined by walls on which the catalytic material is coated as a "washcoat" so that the gases flowing through the passages contact the catalytic material. The flow passages of the carrier member are thin-walled channels which can be of any suitable cross-sectional shape and size such has trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, or circular. Such structures may contain from about 60 to about 700 or more gas inlet openings ("cells") per square inch of cross section ("cpsi"), more typically 200 to 400 cpsi. Such honeycomb-type carrier may be made of any suitable refractory material such as cordierite, cordierite-alpha-alumina, silicon nitride, zirconium mullite, spodumene, alumina-silica magnesia, zirconium silicate, sillimanite, magnesium silicates, zirconium oxide, petallite, alpha-alumina and aluminosilicates. Typically, such materials are extruded into a honeycomb configuration and then calcined. To attain a carrier sized to fit within a canister with a predetermined gap as described below, a properly sized carrier may be "cored" or otherwise sculpted from a larger honeycomb structure. To produce a cored or sculpted carrier requires that the smooth outer surface or "skin" of a larger, extruded honeycomb structure be removed, leaving the outer surface of the carrier jagged and more vulnerable to degradation than a smooth-surfaced carrier. It is preferred, however, that the carrier be extruded in its proper size, since the smooth outer surface or "skin" of an extruded carrier lends structural support to the finished catalyst member. Alternatively, a honeycomb-type carrier may be made of a refractory metal such as a stainless steel or other suitable iron-based, corrosion-resistant alloys. The coated carrier is disposed in a canister suited to protect the catalyst member and to facilitate establishment of a gas flow path through the catalyst member, as is known in the art. When deposited as a washcoat onto a honeycomb-type carrier, the amounts of the various catalytic components of the catalytic material are often presented based on grams per volume basis, e.g., grams per cubic foot (g/ft 3 ) for platinum group metal components and grams per cubic inch (g/in 3 ) for catalytic material as a whole, as these measures accommodate different gas-flow passage cell sizes in different honeycomb-type carrier substances. Catalyst members suitable for use in the present invention may comprise a platinum group metal component loading of 25.5 g/ft 3 with a weight ratio of platinum-to-rhodium of 5:1. Preferably, the catalyst member of the first catalytic flame arrestor stage, i.e., the most upstream catalytic flame arrestor stage, or "first" stage, comprises a catalytic material that selectively or preferentially oxidizes hydrocarbons rather than carbon monoxide. For example, the first stage may comprise a platinum group metal component that comprises rhodium as the principal catalytic component. The effluent from the first stage then flows through a cooling means so that some, of the heat of oxidation released by the catalyst member in the first stage is dissipated. The cooling means may comprise a heat-conductive conduit through which the effluent from the first stage flows to the next and through which at least some of the heat of oxidation generated by the first stage is dissipated. For example, the cooling means may comprise or even consist of a length of pipe, e.g., an exhaust pipe, between catalyst stages. Optionally, the cooling means may comprise cooling fins or a heat exchanger or other means for dissipating heat from the exhaust gas. The cooling means may also comprise means for injecting ambient gases, e.g., air, to the exhaust gas stream, to cool it down. Air or another source of oxygen at ambient temperature will both cool the exhaust gas and facilitate oxidation of the hydrocarbons and carbon monoxide at a downstream catalytic flame arrestor stage. When the engine is equipped with a muffler, the first stage may be positioned upstream from, or even within, the muffler, and the second stage may be positioned downstream from the muffler, so that at least part of the muffler acts as a cooling means for the exhaust gases flowing therethrough. The cooled, partially treated exhaust gas then flows to a second or subsequent catalytic flame arrestor stage where at least a portion, or preferably substantially all, of the remaining flammable components in the exhaust gas may likewise be oxidized, preferably so that the overall conversion rate for combustible constituents of the exhaust gas to non-flammable species, e.g., carbon dioxide, is at least about 10 percent. Since the exhaust gas is cooled before it encounters the subsequent stage, the remaining unburned hydrocarbons and carbon monoxide therein can be further oxidized in the subsequent stage without raising the temperature of the exhaust gas to its auto-ignition temperature. There should be at least one downstream catalytic flame arrestor stage in the exhaust gas flow path, but there may be more downstream stages, separated by cooling means, if desired. The second or subsequent downstream stage(s) may then comprise platinum group metals that are more active than rhodium for the conversion of carbon monoxide, e.g., platinum, to convert the carbon monoxide that flowed past the first stage. Since the flame arrestor of the present invention operates principally by catalytically raising the auto-ignition temperature of the exhaust gas, the flame arrestor need not be designed in accordance with conventional design constraints, e.g., heat conductivity and decreased size relative to the regular flow lines, that are crucial in prior art flame arrestors. In addition, once the flammable gas has been treated by the flame arrestor of the present invention, it is less easily ignited, so that even if the gas is then exposed to outside temperatures or other conditions, e.g., an environmental flame or spark, that might have initiated a flame prior to treatment by the flame arrestor, such conditions may not initiate a flame after treatment by the flame arrestor. Thus, the flame arrestor of the present invention solves a problem not recognized in the prior art, lends greater design flexibility and intrinsically improved safety levels relative to flame arrestors and prior art small engine catalytic converters. As indicated above, each stage of the flame arrestor of the present invention may comprise a catalyst member comprising a ceramic flow-through carrier bearing a thin washcoat of catalytic material. The catalyst members may be mounted in metal canisters through which the exhaust gas is flowed by wrapping the catalyst members in a swatch of ceramic fiber fabric and securing the wrapped catalyst member within a flow-through canister. The canister is then attached, usually by welding, to the exhaust pipe of the engine. Some prior art ceramic fiber fabrics used for this purpose are intumescent, so that as the temperature of the exhaust gas treatment apparatus rises and the canister expands, the mat expands as well to maintain the "grip" exerted on the catalyst member by the canister. However, intumescent fabrics usually lose their intumescence at temperatures above about 700°C, which are routinely reached by exhaust gases entering a treatment apparatus installed inside a muffler for a small engine. Further, to stabilize the catalyst member against the severe vibrations, the canister is usually designed to tightly compress the fabric around the catalyst member, i.e., to impose a high mounting pressure on the fabric and on the catalyst member, which can crush the fabric and thus cause the mounting to fail. Prior art ceramic fiber fabrics have not exhibited sufficient durability to permit the mounting of flow-through catalyst members in the mufflers of small engines such as are found in lawn mowers. As indicated above, conditions in the mufflers of these devices are severe and include high temperatures, e.g., temperatures in excess of 700°C, as well as high mounting pressures which are required to securely mount the catalyst member against slippage caused by vibrations which may impose acceleration forces of 50 G's or more on the catalyst member. One commercially available prior art ceramic fiber fabric partly disintegrated when subjected to a mounting pressure exceeding 2.1 kilograms per square centimeter ("kg/cm 2 "). That is, a significant proportion of the fibers were crushed by the imposed mounting pressure. Other prior art fabrics exhibit unacceptable erosion under conditions of use. In the prior art, the ceramic fiber fabrics used to mount catalyst members are non-woven, felt-type fabric. One aspect of the present invention arises from the discovery that a particular type of non-intumescent ceramic fiber fabric that is sold commercially under the trade designation NEXTEL" as insulation material for furnaces can be used as a mounting mat for a catalyst member disposed in a muffler for a small engine, with superior results relative to other fabrics used for this purpose. The use of such a ceramic fiber fabric in accordance with the present invention yields the advantages of providing a mounting fabric that is stable at temperatures well in excess of 700°C and that can withstand the compression forces required to secure a catalyst member in a canister despite vigorous vibrations and exhaust flow fluctuations. Without wishing to be bound by any particular theory, it is believed that the NEXTEL" fabrics, which are described below, provide superior performance because they are more dense than fabrics previously used as mounting mats. When a low-density fabric is compressed in a canister about a catalyst member, the fabric bulges from the gap between the canister and the catalyst member and fibers tend to shed from the fabric. In addition, as discussed more fully below, compression of such a fabric by a canister crushes the fibers, causing felt-type fabrics to fail as mounting mats. In contrast, fabrics in accordance with the present invention have densities greater than 0.089 grams per cc (1.46 gram per cubic inch ("per in 3 ")). One way that this can be achieved is to impose an order on the arrangement of fibers in the fabric, e.g., by spinning the fibers into yarn and forming the fabric from the yarn by weaving, knitting, etc., instead of creating a felt material as in the prior art. As used herein and in the claims, the term "woven fabric" is meant to broadly encompass fabrics having regularly interlaced fibers or yarn strands, whether knitted, braided, woven by warp and weft, etc. Such fibers may have densities of at least 0.1 grams per cc (1.6 grams per in 3 ), or of at least about 0.5 grams per cc (8.2 grams per in 3 ). For example, the density of the fabric may be about 0.76 grams per cc (12.45 grams per in 3 ). In contrast, a typical 0.5 inch thick felt-type fabric has a density of only 0.024 grams per cc (0.38 grams per in 3 ), and a "high density" felt-type fabric has a density of only 0.089 grams per cc (1.46 grams per in 3 ). Only one prior art fabric has a high density, i.e., a density of about 0.6 grams per cc, but this is an intumescent material and the high density is attributable to the intumescent mineral therein. To the knowledge of the inventors, no non-intumescent ceramic fiber fabrics having densities greater than 0.089 grams per cc have been used as mounting mats as taught herein. A ceramic fiber fabric of the present invention preferably comprises fibers having an average length that is longer than that of fibers in prior art materials used as mounting mats for catalyst members. Fabrics used in accordance with the present invention comprise fibers may have an average length of greater than 10 inches, e.g., at least about 1.5 meters (about 5 feet), preferably at least about 3 meters (about 10 feet). Most preferably, the fabric comprises yarn spun from effectively continuous fibers, i.e., fibers that extend for substantially the entire length of the yarn in the fabric. Prior art materials, in contrast, generally principally comprise fibers whose average length is less than ten inches, in some cases, only one or two inches. The fibers of materials of the present invention typically have a diameter of at least about 5 microns, and the diameter may be greater than 20 microns. Generally, the fibers should have a diameter in the range of about 5 to 15 microns, i.e., about 10 microns, although it is not crucial that the fiber diameter fall in this range. The fibers principally comprise alumina and silica, with a relatively minor proportion of boria. The boria may constitute from about 0.5 to 10 percent of the fiber weight. The fibers are typically spun into yarn strands containing roughly from 500 to 3000 fibers per strand. Typically, the yarn has 700-800 fibers per strand yielding yarn strands that typically have a diameter of 0.01-0.07 inch in diameter. Preferably, the fabric used for the present invention is knitted or woven in a warp-and-weft pattern to yield a material that exhibits a resiliency in response to a compressive force even during exposure to temperatures as high as 1050°C. Such resilience under high compressive force is not seen in prior art felted fabrics. In a preferred embodiment, each swatch of fabric is woven to a width sized to match the length of the catalyst member. The edges of the fabric that are disposed at the ends of the catalyst member will therefore not expose cut, frayed yarn ends that would make the fabric vulnerable to degradation. Whereas prior art fabrics must occupy an annular gap of about 0.1 to 0.25 inch between the catalyst member and the canister to attain a suitable degree of resiliency, fabrics according to the present invention can occupy a gap of less than 0.1 inch, e.g., the gap may be about 0.05 to 0.07 inch or less. Fabrics used in accordance with the present invention thus allow a given catalyst member to be mounted in a smaller canister than conventional fabrics. Conversely, since catalyst members are sometimes mounted inside other pre-existing structures such as mufflers, the fabrics of the present invention will permit the mounting in such structures of larger catalyst members than will conventional fabrics, thereby yielding increased catalytic activity. The Minnesota Mining and Manufacturing Company ("3M") makes two fabrics under the trade designations NEXTEL" 440 and NEXTEL" 312 that are suitable for use in the present invention. These fabrics are woven from continuous fibers having an average diameter of about 10 to 12 microns. NEXTEL" 440 comprises about 70% alumina, 28% silica and about 2% boria by weight. NEXTEL" 312 comprises about 62% alumina, 24% silica and 14% boria. These fabrics range in weight from 7 to 32 ounces per square yard and in weave from 32 x 21 to 40 x 20. Preferred fabrics used in double layers have weaves of 40 x 20 and a weight of 29.5 ounces per square yard (commercial designation AF-62) and 32 ounces per square yard (commercial designation BF-54 ) . A single layer NEXTEL"-type fabric woven to 1 gram per square inch (about 45.7 ounces per square yard) has also been useful in the present invention. The superior suitability of woven fabrics as mounting materials is seen by subjecting a double layer of such material to pressure, noting the thickness of the material under such compression and then observing the thickness recovered when the pressure is released. A graph illustrating these compressions and relaxations for an aged and non-aged material sample of a double layer of NEXTEL" 440 woven ceramic fiber fabric for use in the present invention is shown in Figure IA. In Figure IA, curve #3 plots the compression thicknesses of a non-aged woven sample of a NEXTEL" 440 fabric and curve #4 shows the relaxation thickness of the fabric when the compression force is released. The compression force was raised incrementally between tests so the plot is read from left to right. Curves #1 and #2 likewise show compression and relaxation thicknesses over the same pressure range as for curves #3, #4 for a sample of material that was aged at 1000°C for 48 hours before testing. In such tests performed using pressures that were increased incrementally over a range from about 20 to about 200 pounds per square inch, the non-aged woven fabric compressed under pressure to a thickness that decreased from about 0.084 inch to about 0.055 inch as the compression force was increased, and relaxed to thicknesses ranging from 0.088 inch to about 0.071 inch, respectively. In other words, the compression/relaxation thickness differential of the non-aged woven fabric ranged from about 0.004 inch at about 20 psi to about 0.016 inch at about 190 psi. Woven fabric that was aged by exposure to calcination at 1000°C for 48 hours relaxed from about 0.087 inch thickness at about 20 psi to 0.098 inch, and it relaxed from about 0.060 inch thickness at about 190 psi to a relaxation thickness of about 0.079 inch. In other words, the compression/relaxation thickness differential of the aged woven fabric ranged from about 0.011 at 20 psi to about 0.019 at 190 psi, which generally coincides with the range of differentials of the non-aged sample. Correspondingly, the differential gap width for a racetrack-shaped cordierite-based catalyst member measuring about 1.8 inch X 1.1 inch in a 316 stainless steel canister over a 1832°F (1017°C) temperature change is about 0.016 inch along the major axis and about 0.01 inch along the minor axis. The thickness differential for the double layer of woven fabric, whether aged or not, corresponds roughly over a wide pressure range to the changes in the gap between such a catalyst member and canister that occur as a result of thermal expansion of the canister. Accordingly, in a suitably configured catalyst member-and-canister assembly, the fabrics used for this invention can be placed under high compression at ambient temperature and can maintain a suitable gripping force even when the canister expands thermally because the fabric exhibits resiliency. In Figure IB, curves SI and #2 plot the compression and relaxation thickness data for a single layer of a non-aged felt-type fabric of the prior art, and curves #3 and #4 plot like data of an aged sample. The compression/relaxation thickness differential of the felt non-aged sample ranged from about 0.048 inch at 20 psi to about 0.034 inch at 190 psi, with compressed thicknesses of about 0.170 inch at 20 psi and 0.082 inch at 190 psi. It is clear from Figure IB that the felt-type fabric loses its resiliency after aging, since the compressed and relaxed thicknesses of the aged sample fall within a more narrow range than corresponding thicknesses of the non-aged sample and the compression/relaxation thickness differential ranges from only about 0.032 inch at about 35 psi to about 0.012 inch at about 190 psi. A comparison of Figure IA to Figure IB illustrates that a woven fabric is better able to maintain its resiliency after aging than a felt fabric. Without wishing to be bound by any particular theory, it is believed that the superior resiliency retention of a woven fabric contributes to the superior performance such fabrics exhibit as mounting media for catalyst members in accordance with this invention. Ceramic fiber fabrics used in accordance with the present invention can withstand continuous exposure to temperatures over 1200°C while resisting disintegration &nd maintaining resiliency while under compression about a catalyst member in a canister. The fabrics used in accordance with the present invention serve to mount the catalyst member in canisters in any suitable manner. Typically, the catalyst member is wrapped in one or more layers of the fabric and the wrapped catalyst member is deposited into a metallic clamshell-type canister. Sometimes, a strip of fabric having a width corresponding to the length of the catalyst member is cut from a wider swatch of material. The cut strip is then wrapped around the catalyst member. In such case, however, the cut edge of the fabric has loose fibers that are exposed at the ends of the catalyst member. The fabric can become frayed and can be a site of failure for the mounting. It is therefore preferred that the fabric have woven edges and that it be sized so that its width corresponds to the length of the catalyst member. In this way, the fabric can be wrapped around the catalyst member without any need to cut the fabric and there will be no exposed frayed edges. The fabric may also be folded for wrapping about the catalyst member, and the fold seam may be placed at an end of the catalyst member to provide a woven edge. The canister is preferably made from metal having a low coefficient of thermal expansion, thus 409 or 430 stainless steel is preferred over 300 series stainless steel. The canister is closed around the wrapped catalyst member under suitable pressure and is sealed, usually by welding. The interior surface of the canister, which bears on the ceramic fiber fabric, may either be smooth or roughened, e.g., by sand blasting. In a typical embodiment, a double layer of a 1 gram per square inch NEXTEL" 440 fabric is wrapped around a racetrack-shaped catalyst member (i.e., one shaped to have two parallel straight sides joined by two rounded sides) measuring 4.32 centimeters ("cm") along its major axis and 2.79 cm x 2.54 cm along the minor axis. The wrapped catalyst member is placed in a clamshell canister which is welded closed to impose a pressure on the fabric that may exceed 2.1 kg/cm 2 . Typically, the canister comprises two shells that surround the rounded sides of the catalyst member and that overlap as shown in Figure 2. During assembly, the shells 115a, 115b are placed about the catalyst member 114 and the surrounding ceramic fiber fabric 116 so that the shells overlap along the minor axis M χ of the catalyst member. (M χ is seen as a line in a cross section of the catalyst member, but it comprises a plane that bisects the catalyst member along its length, as does M 2 . ) The shell-catalyst member assembly is placed in a vise assembly that applies pressure on the shells while the shells are being welded together. In the finished assembly, the fabric 116 is compressed in gap g to a thickness corresponding to the width of the gap. In alternative embodiments, the canister shells may overlap along major axis i l , or the canister may be formed from a single shell that encircles the wrapped catalyst member and that has edges that overlap and that can be welded together. One embodiment of a vise assembly for canning a catalyst member is shown in Figure 3A, where the shell-catalyst member assembly is disposed between minor axis vise shoes 142a, 142b and major axis vise shoes 144a, 144b. Minor axis vise shoes 142a, 142b are dimensioned and configured to engage the exterior of the canister shells of shell-catalyst member assembly 146 along the sides of shell-catalyst member assembly 146 while leaving exposed the seam where the shells overlap so that they can be welded together while pressure is being applied. The vise shoes and shell-catalyst member assembly are suspended in a frame 148 by load cells 148a, 148b and 150a, 150b that apply and measure forces imposed on the vise shoes. The forces, which may differ along the major and minor axes of the catalyst member, can be adjusted by turning one or both of screws 152, 154. In alternative embodiments, the forces imposed on shoes 142a, 142b and 144a, 144b can be controlled by hydraulic or pneumatic or other conventional pressure devices. In use, the catalyst member is wrapped with the ceramic fiber fabric and one or more canister shells, the catalyst member and shell assembly is placed in the vise shoes and a predetermined force is applied to impose a predetermined compression force on the ceramic fiber fabric. The shell or shells are then welded or otherwise secured together. This procedure differs from the prior art in which the assembly force was applied as necessary to achieve a precise configuration of the canister shells prior to welding. In the prior art procedure, the applied assembly force varied to accommodate variations in the physical dimensions of the components of the assembly; as a result, the compression force on the mounting mat could vary from one assembly to the next, causing significant variations in the performance of the finished products. By controlling the force applied during assembly in accordance with the present invention, a predetermined compression force to be imposed on the ceramic fiber fabric (and thus on the catalyst member) can be achieved consistently despite variances in the dimensions of the canister shells, catalyst member or fabric thickness. For example, ceramic substrate tolerances for catalyst members for automobiles can range from about ± 0.035 to ± 0.065 inch. The use of canister shells that have overlapping edges (as seen in Figure 2) to be welded together facilitates control of the assembly force because the degree to which shell edges overlap is not critical to the formation of a satisfactory weld. Therefore, variances in the degree of overlap that are required to permit the use of a predetermined compression force despite variations in the dimensions of the shells, catalyst member, etc., are easily accommodated. The predetermined assembly forces may be in the range of about 100 to 1150 pounds, more typically from about 200 to 500 pounds. The assembly force is selected to attain a desired compression force on the ceramic fiber mat. For a canister holding a catalyst member for a two-stroke, high-vibration engine, the compression force is typically in the range of about 80 to about 120 pounds per square inch (psi), e.g., from about 90 to 100 psi. The compression force may be higher but must not exceed the isostatic strength of the catalyst member. In devices for four-stroke engines, the compression force may be less than 80 psi, e.g., it may be as low as about 30 psi. Preferably, frame 148 is pivotably mounted on a base so that it can be rotated to allow the welder to position one canister seam and then the other for convenient welding. As seen in Figures 2 and 3B, the gas flow passages of a catalyst member may be defined by sets of parallel cell walls that are angled relative to one another. Preferably, assembly forces are applied at a non-zero angle relative to each set of parallel cell walls. When, as seen in Figures 2 and 3B, the sets of cell walls are mutually orthogonal to each other, i.e., when one set of walls extends vertically and another set extends horizontally, the assembly forces are imposed at oblique angles relative to the cell walls. As seen in Figure 3B, forces F are applied on shells 15a, 15b at a 45 degree angle relative to both sets of cell walls of catalyst member 14' ' ' . Another assembly technique that can be used is a stuffing technique in which the catalyst member is wrapped with the ceramic fiber fabric and is pushed through a cone from the large end of the cone. The small end of the cone is dimensioned to correspond to the inside diameter of the canister and is situated at the canister opening. As the catalyst member is pushed through the cone, the cone compresses the fabric so that it can fit into the canister so the catalyst member and fabric slide into the canister from the small end of the cone. After the canister is secured about the wrapped catalyst member, it is attached to an exhaust pipe connected to the engine exhaust outlet or to an engine exhaust manifold outlet. The canister is typically metallic whereas the catalyst member and the mounting mat are mainly ceramic in composition. Accordingly, as the exhaust gases heat the apparatus, the canister experiences greater thermal expansion than the catalyst member and the ceramic fiber fabric mat, and the mounting pressure holding the catalyst member in place is reduced. While the ceramic fiber fabrics of the present invention are resilient and maintain their resiliency with aging, they may not be able to maintain a constant pressure on the catalyst member as the canister size changes with temperature for all canisters. Since lower mounting pressure can leave the catalyst member vulnerable to vibrational damage, it may be advantageous to supplement the ceramic mat with a metal wire mesh disposed between the ceramic mat and the canister. A wire mesh about the wrapped catalyst member in the canister serves to ameliorate the difference in thermal expansion between the metallic canister and the ceramic components therein and thus tends to maintain the mounting pressure on the catalyst member as the temperature of the apparatus rises. A wire mesh useful in this aspect of the invention is formed from Inconel" alloy wire having a diameter of 0.1 millimeters (mm) and a weave period (the length from one peak in a sinusoidally configured woven wire to the next peak) of 3.18 mm. The mounting material and method described above can be used to mount single catalyst members for use solely to treat noxious exhaust gases from small engines, as well as for multiple catalyst members for use in a staged flame arrestor in accordance with the present invention. A staged catalytic flame arrestor in accordance with one embodiment of the present invention is shown schematically in Figures 4 and 4A. The first stage 10 of the flame arrestor comprises a canister 15 mounted on the end of an exhaust pipe 12 which collects exhaust gas flowing from the exhaust outlet of a small engine (not shown), as indicated by arrow 13. Canister 15 is a clamshell-type canister which contains a catalyst member 14 mounted therein. Surrounding catalyst member 14 within canister 15 is a double layer of ceramic fiber fabric 16 which, in accordance with the present invention, comprises continuous, large diameter fibers. Surrounding ceramic fiber fabric 16 is an optional wire mesh 18 which is preferably made of Inconel" wire. Fabric 16 and wire mesh 18 are wrapped around the sides of catalyst member 14 and are folded over ends 14a, 14b of catalyst member 14. Optional annular end rings 20 and 22 are welded to canister 15 to apply axial pressure on ends 14a and 14b of catalyst member 14 and help to secure catalyst member 14 within canister 15. In alternative embodiments, canister 15 can be configured to form end rings as an integral part of the canister. Canister 15 is situated within muffler 11. Muffler 11 vents into pipe 32, on which is mounted a second stage 34 of the flame arrestor. The second stage comprises a canister 35 that houses a catalyst member 14' therein in the same manner that canister 15 houses catalyst member 14. In operation, exhaust gases flow through exhaust pipe 12 into canister 15 of first stage 10. The gases flow through catalyst member 14 and enter first chamber 24 of muffler 11. As gases flow through catalyst member 14, the catalytic material therein stimulates the conversion of some of the hydrocarbons and carbon monoxide in the exhaust gas to innocuous substances, e.g., carbon dioxide and water. The gases then flow through conduit 26 to second chamber 28 and then to third chamber 30. Gases are vented from muffler 11 through pipe 32. The flow path of gas through muffler 11 attenuates the pressure pulses created by the engine, thus reducing the noise output associated with engine operation. In addition, muffler 11 and pipe 32 act as cooling means that dissipate some of the heat produced by the oxidation of hydrocarbons and carbon monoxide over catalyst member 14. The cooled effluent from first stage 10 flows from pipe 32 to the subsequent second flame arrestor stage 34 , where substantially all the rest of the hydrocarbons and carbon monoxide in the gas stream are converted to carbon dioxide without raising the temperature of the exhaust gas to its auto-ignition temperature. The apparatus further comprises optional air inlets 36a, 36b through which optional air pump 38 may inject air or another oxygen-containing gas into the exhaust gas stream via air injection lines 40a and 40b. In the configuration of Figure 4, it is seen that small circumferential portions of ends or end faces 14a, 14b, i.e., of the inlet and outlet end faces 14a, 14b of catalyst member 14, are obstructed by a folded-over portion of fabric 16 and by end rings 22. In some situations, however, the configuration of the muffler can only accommodate a catalyst member of small size, so it may be desirable to assure that no part of the inlet or outlet end faces is obstructed. In any case, such obstruction reduces the effectiveness volume of the catalyst member and is to be minimized or avoided. Accordingly, it may be desired to confine the wrapped ceramic fiber fabric to the circumferential surface of the catalyst member, i.e., to confine the fabric to the gap between the catalyst member and the canister. For example, the fabric may be configured to have a width not greater than the length of the catalyst member. Figure 5 shows a ceramic fabric 16' that has been trimmed to have a width that is smaller than the length L of the catalyst member 14''. Configured in this way, the fabric 16' can be centered on catalyst member 14'' and will not fold over and obstruct either end face 14a' or 14b' of the catalyst member. Optional end rings 23a, 23b mounted on canister 15' extend across the gap between canister 15' and catalyst member 14'' to prevent fabric 16' from extending beyond the end faces 14a', 14b' and obstructing end faces 14a' and 14b'. The use of such end rings is preferred if ceramic fiber 16' extends for the entire length L of catalyst member 14''. Avoiding obstruction of the end faces of the catalyst member in these ways is especially important when the catalyst member is short relative to its diameter, i.e., when the length of the catalyst member (measured as the flow path through the catalyst member) is less than its diameter. Example 1 (Comparative Example) A blank, i.e., uncoated, ceramic carrier having 300 cpsi was mounted at the outlet of the muffler for a Briggs and Stratton 18 horsepower twin cylinder engine on a Coleman Powermate 7-kilowatt power generator. Under ordinary operating conditions, the exhaust gas from the engine was not flammable. However, when the choke was pulled and the exhaust gas was exposed to a spark, the exhaust gas produced a substantial flame. This illustrates that the engine is capable of producing hazardous flammable exhaust gas under certain operating conditions. Example 2 (Comparative Example) The generator described in Example 1 was equipped with a conventional small engine catalytic converter to treat the exhaust gases. The catalytic converter was mounted in a canister at the muffler outlet as an "external" catalyst member and was sized to occupy 2.37 cubic inches and had 300 cpsi. The catalytic material therein comprised platinum and rhodium in amounts providing 40 g/ft 3 platinum group metals with a Pt:Rh ratio of 5:1 dispersed on an alumina support material. The exhaust gas temperature at the inlet and outlet of the catalyst was measured while the engine was put through aging modes. During various aging cycles, the generator was operated at approximately 3000 RPM and with power loads of zero kilowatts (kw), 2 kw, 4 kw and 7 kw. At first, the idle valve or calibration valve was set at the normal factory setting at one and one-quarter turns from the closed position. At this setting, there were no exhaust gas flames observed. The test was repeated with the calibration valve opened to two and one-quarter turns. The engine rpm, applied load, exhaust gas oxygen concentration, exhaust gas temperature of the mufflers inlet and outlet and the observation of any auto-ignition flame were observed and recorded, as set forth in TABLE I. TABLE I LOAD EXHAUST O a TEMPERATURE (°C) TEST RPM (K ) CONC.(%) INLET OUTLET FLAME 1 2998 0 2.98 426 755 YES 2 2980 2 2.16 437 680 NO 3 2966 4 1.89 453 636 NO 4 2931 7 1.33 490 602 NO 5 2997 0 3.02 426 753 YES 6 2975 2 2.21 434 680 NO 7 2966 4 1.88 451 638 NO 8 2930 7 1.35 486 599 NO 9 1320 0 4.34 444 860 YES 10 1337 4 2.15 382 668 NO The exhaust gas spontaneously combusted in both no-load tests 1 and 5. During these tests, high oxygen concentrations were measured in the exhaust gas, indicating a highly inefficient combustion of the fuel in the engine. Flame was also observed under a no load "idle" mode of 1320 rpm. The temperature of the exhaust gas at the outlet of the external catalyst member in the cases observed was 753°C (at 2997 rpm) and 860°C (at 1320 rpm) . In contrast, no flame occurred when there was a load on the generator, at which time the oxygen concentration in the exhaust gas was lower than under zero load conditions and the exhaust gas temperature at the catalyst member outlet was lower than 700°C. This suggests that the auto-ignition temperature of the exhaust gas produced under no load conditions was about 700°C and was exceeded in the external catalyst member. Example 3 The generator of Example 1 was equipped with a staged flame arrestor in accordance with the present invention by mounting two rectangular catalyst members inside the muffler in addition to a conventional small engine external catalytic converter as described in Example 2. The internal catalyst members had the same catalytic metal loading as the external catalyst and were sized at 0.5 inch by 2.5 inches by 2 inches. The internal stages were wrapped in ceramic material and tightly secured in canisters that were open-ended with an inlet and an outlet to allow the exhaust gas to flow therethrough. Each canister inlet was attached to the exhaust line of one of the engine cylinders where they entered the muffler so that each treated a separate portion of the engine exhaust gas. The two internal catalyst members comprised the first stage of the flame arrestor and the external catalytic converter comprised the second stage. The test described above in Example 2 was repeated, and no flame was observed when the valve was at 1 1/4 turns. The results for tests at 2 1/4 turns are set forth in TABLE II. TABLE II LOAD EXHAUST O j TEMPERATURE (°C) TEST RPM (Kw) CONC.(%) INLET OUTLET FLAME 11 3029 0 2.0 493 733 YES 12 3022 2 1.67 479 674 NO 13 3016 4 1.34 492 635 NO 14 2997 7 0.89 531 614 NO 15 3082 0 1.62 483 726 YES When the idle valve was set at two and one-quarter turns, the flame was initiated in the exhaust gas under zero load conditions, but the flames were much smaller than those observed in Example 2 , and appeared to vanish intermittently. This demonstrates that a flame arrestor in accordance with the present invention was effective to inhibit the exhaust from auto-ignition, under conditions that would have led to a flame over a conventional single stage catalytic converter. Upon inspection after the test, the internal flame arrestor stages were cracked, a condition which can be detrimental to catalytic conversion performance. Better performance could therefore be expected with fully intact catalyst members. Example 4 The generator of Example 1 was equipped with two rectangular catalytic flame arrestor stages mounted inside the muffler ("internal stages") and with an external catalytic converter, as described in Example 3 except that the internal stages were sized at 1 inch by 2.5 inches by 2 inches. The engine was then operated under substantially the same conditions that produced a flame in Example 2. The results are summarized below in TABLE III. No flame was initiated in the exhaust gas. TABLE III LOAD EXHAUST O. j TEMPERATURE (°C) TEST RPM (Kw) CONC.(%) INLET OUTLET FLAME 16 3019 0 2.13 508 600 NO 17 2787 2 1.69 498 556 NO 18 2770 4 1.41 510 551 NO 19 2920 7 1.08 546 563 NO 20 2995 0 2.10 515 594 NO The data of TABLE III show that the flame arrestor was fully operative to prevent the exhaust gas from auto-ignition, even under no load conditions and high oxygen concentrations in the exhaust gas. The low outlet temperatures confirm that the flame arrestor successfully kept the exhaust gas cooler than its auto-ignition temperature, which was demonstrated in Examples 2 and 3 to be above 700°C. Example 5 A series of catalyst members were mounted in mufflers for small engines. The catalyst members were mounted by wrapping them in ceramic fiber fabrics placing the wrapped catalyst members in clamshell-type canisters. Except for device (B) in TABLE IV below, each canister had an end ring mounted therein. Each ceramic fiber fabric wrap was folded over both ends of the catalyst member, one end of which was pressed against the mounted end ring, the folded-over portion serving as a bushing between the catalyst member and the end ring. A second end ring was then pressed against the opposite end of the catalyst member and was welded in place to secure the catalyst member in the clamshell under an axial pressure. Four prior art small diameter, short fiber, felt-type ceramic fabric materials were employed; these are designated "F", "D" and "D-S", as well as a ceramic fabric in accordance with the present invention identified as NEXTEL" 440. In each case the clamshell-type canister was welded closed and imposed a pressure of 2.1 kg/cm 2 on the ceramic fiber fabric and the enclosed catalyst member, which compressed the fabric to a gap bulk density of 1.92 g/cm 3 . Each clamshell-type canister was configured to define an exhaust gas inlet and an outlet so that exhaust gas could be flowed into the canister, through the catalyst member and out from the canister. Each canister was mounted within a muffler as shown in Figure 4 on a Husqvarna model 242 chain saw engine. The catalyst members in devices (A), (B) and (C) were mounted in the canisters without end rings; the other devices included end rings to mount the catalyst members therein. The engine was run continuously and the durability of each ceramic fiber fabric was noted. The materials tested and the corresponding results are indicated in the following TABLE IV. -34- TABLE IV DEVICE FABRIC ( i) DURABILITY (A) "F" 30 minutes or less (B) 0.5 inch thick "D" 10 minutes or less (C) 0.25 inch thick "D" 0.4 hour (D) 0.5 inch thick "D" 5 hours (E) 0.25 inch thick "D-S" 0.5 hour 8 lbs/ft 3 density (F) 0.5 inch thick "D-S" 1.5 hours 8 lbs/ft 3 density (G) NEXTEL- 440 cloth 19 hours ( 2 ) (two layers, total 0.14 inch thick) (H) NEXTEL" 440 cloth 89 hours ( 2 ) (two layers, total 0.14 inch thick) (1 Where available, the thickness and/or density of the fabric prior to compression is indicated. ( 2 } Failure was due to failure of a weld seam, not due to ceramic fiber fabric failure. For the NEXTEL" 400 fabric-containing devices (G) and (H), the welding on the canister failed before the ceramic fiber fabric failed. For the other materials, the converters failed because of gas erosion on the mat materials or because the catalyst member became loose due to mechanical vibration. The data of TABLE IV clearly show that continuous, large diameter, ceramic fiber fabric under a compression of over 2.1 kg/cm 2 is vastly more durable than other commercially available ceramic fiber fabrics of short fiber length and small diameter. Example 6 An exhaust treatment apparatus prepared as described in Example 5 using two layers of a NEXTEL" 440 ceramic fiber fabric mounting material was tested by running an associated engine on a long-term basis and observing the durability of the mounting material. After 400 hours of service, the mounting material was still effectively retaining the catalyst member in the muffler canister. Example 7 Heat Cycle Vibration Test Operating conditions for a catalytic converter used on a two-stroke engine can be simulated for test purposes by flowing hot gases from a propane burner or engine exhaust through a canister within which a catalyst member is mounted using a ceramic fiber fabric mat. The canister is attached to a vibrator for the duration of the test and is wrapped in insulating material to inhibit heat loss. Canisters comprising prior art intumescent ceramic fabric mounting mats to hold the catalyst members last ten hours or less in such a test when the gas inlet temperature exceeds about 1050°C or when. the skin temperature of the canister exceeds 650°C. One prior art reference reported that a canister survived 100 hours with vibrational acceleration of 40 G's and an inlet temperature of 850°C. One hundred hours is considered to indicate acceptable durability. A catalyst member measuring 55 millimeters ("mm") in diameter and 40 mm in length was mounted in a canister using a ceramic fiber fabric in accordance with the present invention and was subjected to a heat cycle vibration test. The test canister was tested under 70 G acceleration, 150 Hz vibrations and with an inlet gas temperature that cycled from 200°C to 1050°C and back over ten-minute intervals for a period of 100 hours. At the end of 100 hours, the test canister was intact and passed the test. Failure would have been indicated by deterioration of the ceramic fiber fabric, structural damage to the catalyst member that impaired conversion performance and/or dislocation of the catalyst member in the canister. The skin temperature of the canister was about 800°C throughout most of the test. The test results show that catalyst members mounted using ceramic fiber fabrics in accordance with the present invention provide suitable durability under conditions that are more severe than prior art materials. While the invention has been described in detail with reference to a particular embodiment thereof, it will be apparent that upon a reading and understanding of the foregoing, numerous alterations to the described embodiment will occur to those skilled in the art and it is intended to include such alterations within the scope of the appended claims.

Claims: THE CLAIMS What is claimed is: 1. A method for preventing flame initiation of a flammable exhaust gas from an engine, the exhaust gas containing at least oxygen and unburned hydrocarbons, the method comprising: flowing the exhaust gas in contact with a plurality of sequentially arranged flow-through catalytic flame arrestor stages comprising a first stage and at least one subsequent stage, each stage comprising a catalyst member comprising a catalytic material effective to oxidize a portion of the pollutant species in the exhaust gas to increase the auto-ignition temperature of the exhaust gas by reducing the quantity of unburned hydrocarbons in the exhaust gas while releasing insufficient heat to raise the temperature of the exhaust gas to its auto-ignition temperature; and cooling the partially oxidized exhaust gas effluent of each stage before flowing the exhaust gas to the subsequent stage. 2. The method of claim 1 comprising oxidizing from about 10 to about 60 percent of the pollutant species in the exhaust gas in the first stage. 3. The method of claim 1 or claim 2 comprising oxidizing at the at least one subsequent stage substantially all of the pollutants remaining in the exhaust gas effluent from the first stage. 4. The method of claim 3 wherein the exhaust gas comprises nitrogen oxides and wherein at least the at least one subsequent stage comprises a three-way catalytic material, wherein flowing the exhaust gas through the at least one subsequent stage comprises reducing the nitrogen oxides to nitrogen. 5. The method of claim 1 or claim 2 comprising selectively converting hydrocarbons in the first stage. 6. A staged catalytic flame arrestor apparatus for preventing flame initiation of flammable exhaust gas from a small engine, the engine having an exhaust outlet, the apparatus comprising: a flow-through first stage in gas flow communication with the exhaust outlet of the small engine, the first stage comprising a catalyst member comprising a first catalytic material effective to oxidize a first portion of the hydrocarbons and carbon monoxide in the exhaust gas so that the heat of oxidation released in the first stage is insufficient to raise the effluent from the first stage to its auto-ignition temperature; cooling means for cooling the effluent of the first stage; and at least one flow-through subsequent stage in gas flow communication with the cooling means to receive cooled effluent from the first stage, the at least one subsequent stage comprising a catalyst member comprising a subsequent catalytic material effective to oxidize a subsequent portion of the hydrocarbons and carbon monoxide in the exhaust gas, the subsequent portion releasing insufficient heat of oxidation to raise the exhaust gas to its auto-ignition temperature. 7. The apparatus of claim 6 wherein the first catalytic material comprises a platinum group metal component that principally comprises a rhodium component. 8. The apparatus of claim 6 or claim 7 wherein the subsequent stage comprises a three-way catalytic material. 9. In an exhaust treatment apparatus for abating noxious components of the exhaust gas of an internal combustion engine, the apparatus comprising a catalyst member secured in a canister through which the exhaust gas flows, the catalyst member comprising a catalytic material effective for the conversion of at least some of the noxious components to innocuous substances, and wherein the apparatus further comprises a ceramic fiber fabric disposed in a gap between the catalyst member and the canister for mounting the catalyst member in the canister, the improvement comprising that the ceramic fiber fabric comprises a fabric having a density of greater than about 0.089 grams per cubic centimeter. 10. The apparatus of claim 9 wherein the ceramic fiber fabric comprises a woven fabric. 11. The apparatus of claim 10 wherein the ceramic fiber fabric has a density of at least about 0.1 grams per cubic centimeter. 12. The apparatus of claim 11 wherein the ceramic fiber fabric has a density of at least about 0.5 grams per cubic centimeter. 13. The apparatus of claim 12 wherein the ceramic fiber fabric is about 0.76 grams per cubic centimeter. 14. In an exhaust treatment apparatus for abating noxious components of the exhaust gas of an internal combustion engine, the apparatus comprising a catalyst member secured in a canister through which the exhaust gas flows, the catalyst member comprising a catalytic material effective for the conversion of at least some of the noxious components to innocuous substances, and wherein the apparatus further comprises a ceramic fiber fabric disposed in a gap between the catalyst member and the canister for mounting the catalyst member in the canister, the improvement comprising that the ceramic fiber fabric comprises fibers having an average length of greater than 10 inches. 15. The apparatus of claim 10 or claim 14 wherein the ceramic fiber fabric comprises fibers having an average length of greater than 10 inches. 16. The apparatus of claim 15 wherein the ceramic fiber fabric comprises fibers having an average length of at least about 1.5 meters. 17. The apparatus of claim 15 wherein the ceramic fiber fabric comprises fibers having an average length of at least about 3 meters. 18. The apparatus of claim 15 wherein the ceramic fiber fabric comprises effectively continuous fibers. 19. The apparatus of claim 10 or claim 15 wherein the ceramic fiber fabric is configured to have a width not greater than the length of the catalyst member, and wherein the fabric has woven edges. 20. The apparatus of claim 10 or claim 15 wherein the ceramic fiber fabric is substantially free of intumescent fibers. 21. The apparatus of claim 19 further comprising a wire mesh surrounding the catalyst member in the canister. 22. The apparatus of claim 10 or claim 15 wherein the gap is less than about 0.1 inch. 23. The apparatus of claim 22 wherein the gap has a width of about 0.05 to 0.06 inch. 24. The apparatus of claim 22 wherein the gap has a width of less than 0.05 inch. 25. The apparatus of claim 10 or claim 14 wherein the catalyst member has a circumferential surface and wherein the ceramic fiber fabric is confined to the circumferential surface. 26. The apparatus of claim 25 wherein the catalyst member has an inlet end face and an outlet end face and the apparatus further comprises end rings dimensioned and configured to confine the ceramic fiber fabric within the gap without obstructing the end faces of the catalyst member. 27. The apparatus of claim 10 or claim 14 wherein the ceramic fiber fabric has a weight of about 1 gram per square inch. 28. In an exhaust treatment apparatus for abating noxious components of the exhaust gas of an internal combustion engine, the apparatus comprising a catalyst member secured in a canister through which the exhaust gas flows, the catalyst member having end faces and a circumferential surface and comprising a catalytic material effective for the conversion of at least some of the noxious components to innocuous substances, and wherein the apparatus further comprises a ceramic fiber fabric disposed in a gap between the catalyst member and the canister for mounting the catalyst member in the canister, the improvement comprising that the ceramic fiber fabric is confined to the circumferential surface of the catalyst member. 29. The exhaust treatment apparatus of claim 28 comprising an end ring that extends across the gap between the canister to the catalyst member without obstructing an end face of the catalyst member. 30. A method for assembling a gas treatment apparatus, comprising: wrapping a catalyst member in a ceramic fiber fabric, the catalyst member comprising a catalytic material effective for the conversion of components of a gas stream and being dimensioned and configured to have two end faces, a circumferential surface, and a plurality of gas flow passages extending therethrough from one end face to the other; surrounding the wrapped catalyst member with at least one shell having edges that can be joined together; and applying a predetermined assembly force on the at least one shell and maintaining the predetermined assembly force while securing the shell edges together to form a canister about the catalyst member and to define a gap between the canister and the catalyst member wherein the ceramic fiber fabric is disposed. 31. The method of claim 30 wherein the gas flow passages of the catalyst member are defined by sets of parallel cell walls that are angled relative to each other, and comprising applying the assembly force at a non-zero angle relative to each set of cell walls. 32. The method of claim 31 wherein the sets of cell walls are mutually perpendicular to each other, and comprising applying the assembly force at an oblique angle relative to the cell walls. 33. The method of claim 32 comprising applying the assembly force at an angle of 45 degrees relative to the cell walls. 34. The method of claim 30 comprising defining a gap having a width of less than 0.1 inch. 35. The method of claim 34 comprising defining a gap having a width of from about 0.05 to 0.06 inch. 36. The apparatus of claim 34 wherein the gap has a width of less than 0.05 inch. 37. The method of claim 30 comprising confining the ceramic fiber fabric to the circumferential surface of the catalyst member.

Inventors: BROWN RONALD J, HWANG H SHINN, MOONEY JOHN J, BROWN, RONALD, J., HWANG, H., SHINN, MOONEY, JOHN, J.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1997
Main classification: F01N3
IPC codes: F01N3, F01N7