Patent Application
20080210090
1. Field of the Invention
The present invention relates generally to a ceramic substrate used as a fuel cell reformer cleanup substrate, and in one particular implementation to an extruded porous ceramic fuel cell reformer cleanup substrate.
2. Description of the Related Art
Fuel cells are electrochemical energy conversion systems and devices that produce electricity from fuel. Generally, a fuel cell extracts the energy released from the chemical reaction in the formation of water from hydrogen and oxygen. In a fuel cell, the reaction occurs in the presence of a catalyst without combustion in a highly efficient manner, with water vapor as the predominate byproduct of the reaction. Accordingly, fuel cells are considered to be efficient and clean sources of energy.
Hydrogen as a fuel source for fuel cells requires high pressure compression of the fuel in a gaseous form, or in a super-cooled liquid form. In either case, the fuel is extremely expensive to generate, store, and to handle safely. Hydrogen in a pure form is highly combustible, and requires specialized equipment and handling procedures.
An alternative way to provide hydrogen as a fuel for a fuel cell is through the use of a fuel cell reformer. A reformer is a device that removes hydrogen from hydrocarbon fuels, such as methane or gasoline. The output of a fuel cell reformer is ideally hydrogen gas, though due to the complexities in the chemical reaction within the reformer, and due to the impurities that naturally occur within hydrocarbon-based fuels, impurities are often emitted from the reformate stream. These impurities, if left within the reformate stream, can destroy the efficiency of the fuel cell, or even totally destroy the fuel cell itself.
Fuel cell reformer cleanup can be performed through the use of a honeycomb ceramic substrate having a catalyst applied to attract and adsorb the impurities passing through the substrate. A honeycomb substrate provides high surface area as a support for the catalyst, so that the catalyst is accessible to the reformate stream. However, to ensure that the substrate can collect all of the impurities so that the fuel cell efficiency is not destroyed, the substrate must be large enough to ensure sufficient residence time of the reformate stream within the substrate to capture all of the impurities.
Accordingly, the industry has a need for a fuel cell reformer cleanup substrate that has high porosity and an associated high permeability, so that the reformate stream has can be effectively cleaned in a compact unit without high backpressure. Preferably, the substrate would be cost-effective to manufacture, and could be manufactured with flexible physical, chemical, and reaction properties.
Briefly, the present invention provides a fuel cell reformer cleanup substrate that is a highly porous substrate formed in an extrusion process. More particularly, the present invention enables fibers, such as organic, inorganic, glass, ceramic or metal fibers, to be mixed into a mass that when extruded and cured, forms a highly porous substrate. Depending on the particular mixture, the present invention enables substrate porosities of about 60% to about 85%, and enables process advantages at other porosities, as well. The extrudable mixture may use a wide variety of fibers and additives, and is adaptable to a wide variety of operating environments and applications. Fibers are mixed with binders, pore-formers, extrusion aids, and fluid to form a homogeneous extrudable mass. The homogeneous mass is extruded into a green substrate. The more volatile material is preferentially removed from the green substrate, which allows the fibers to interconnect and contact. As the curing process continues, fiber-to-fiber bonds are formed to produce a structure having a substantially open pore network. The resulting porous substrate provides structural support for a washcoat in an adsorber type application, such as in the cleanup system for a fuel cell reformate stream.
In a more specific example, ceramic fibers are the mullite phase of aluminosilicate fibers.
In another specific example, a porous substrate may be used to clean up the reformate stream of a fuel cell reformer. The porous substrate in this example can be formed by extrusion of a mixture of ceramic-material fiber with additives. The porous substrate has a washcoat applied to at least a portion of the porous substrate adapted to reversibly adsorb components in the reformate stream, such as in the use of a revolver assembly. In this specific example, the porous substrate can be used in a stationary system or a mobile vehicle.
In other examples, the washcoat has an affinity for the adsorption of hydrogen sulfide, which is typically a component of a fuel cell reformate stream that can destroy a fuel cell if not removed from the reformate stream. Embodiments of this example includes washcoats comprising zinc oxide, lanthanum oxide, and rare-earth oxides. The penetration of the washcoat into the porous substrate provides significant advantages in the efficiency and performance of the porous substrate as a fuel cell reformate cleanup filter.
Advantageously, the disclosed fiber extrusion system produces a substrate having high porosity, and having an open pore network that enables an associated high permeability, as well as having sufficient strength according to application needs. The fiber extrusion system also produces a substrate with sufficient cost effectiveness to enable widespread use of the resulting filters and catalytic converters. The extrusion system is easily scalable to mass production, and allows for flexible chemistries and constructions to support multitudes of applications. The present invention represents a pioneering use of fiber material in an extrudable mixture. This fibrous extrudable mixture enables extrusion of substrates with very high porosities, at a scalable production, and in a cost-effective manner. By enabling fibers to be used in the repeatable and robust extrusion process, the present invention enables mass production of filters and catalytic substrates for wide use throughout the world.
These and other features of the present invention will become apparent from a reading of the following description, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The drawings constitute a part of this specification and include exemplary embodiments of the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention.
Detailed descriptions of examples of the invention are provided herein. It is to be understood, however, that the present invention may be exemplified in various forms. Therefore, the specific details disclosed herein are not to be interpreted as limiting, but rather as a representative basis for teaching one skilled in the art how to employ the present invention in virtually any detailed system, structure, or manner.
Referring now to
System 10 enables a highly flexible extrusion process, so is able to accommodate a wide range of specific applications. In using system 10, the substrate designer first establishes the requirements for the substrate. These requirements may include, for example, size, fluid permeability, desired porosity, pore size, mechanical strength and shock characteristics, thermal stability, and chemical reactivity limitations. According to these and other requirements, the designer selects materials to use in forming an extrudable mixture. Importantly, system 10 enables the use of fibers 12 in the formation of an extruded substrate. These fibers may be, for example, ceramic fibers, organic fibers, inorganic fibers, polymeric fibers, oxide fibers, vitreous fibers, glass fibers, amorphous fibers, crystalline fibers, monocrystalline fibers, polycrystalline fibers, non-oxide fibers, carbide fibers, metal fibers, other inorganic fiber structures, or a combination of these. However, for ease of explanation, the use of ceramic fibers will be described, although it will be appreciated that other fibers may be used. Also, the substrate will often be described as a filtering substrate or a catalytic substrate, although other uses are contemplated and within the scope of this teaching. The designer selects the particular type of fiber based upon application specific needs. For example, the ceramic fiber may be selected as a mullite fiber, an aluminum silicate fiber, or other commonly available ceramic fiber material. The fibers typically need to be processed 14 to cut the fibers to a usable length, which may include a chopping process prior to mixing the fibers with additives. Also, the various mixing and forming steps in the extrusion process will further cut the fibers.
According to specific requirements, additives 16 are added. These additives 16 may include binders, dispersants, pore formers, plasticizers, processing aids, and strengthening materials. Also, fluid 18, which is typically water, is combined with the additives 16 and the fibers 12. The fibers, additives, and fluid are mixed to an extrudable rheology 21. This mixing may include dry mixing, wet mixing, and shear mixing. The fibers, additives, and fluid are mixed until a homogeneous mass is produced, which evenly distributes and arranges fibers within the mass. The fibrous and homogenous mass is then extruded to form a green substrate 23. The green substrate has sufficient strength to hold together through the remaining processes.
The green substrate is then cured 25. As used in this description, “curing” is defined to include at least two important process steps: 1) binder removal and 2) bond formation. The binder removal process removes free water, removes most of the additives, and enables fiber-to-fiber contact. Often the binder is removed using a heating process that burns off the binder, but it will be understood that other removal processes may be used dependent on the specific binder used. For example, some binder may be removed using an evaporation or sublimation process. Some binders and or other organic components may melt before degrading into a vapor phase. As the curing process continues, fiber-to-fiber bonds are formed. These bonds facilitate overall structural rigidity, as well as create the desirable porosity and permeability for the substrate. Accordingly, the cured substrate 30 is a highly porous substrate of mostly fibers bonded into an open pore network 30. The substrate may then be used as a substrate for many applications, including as a substrate for filtering applications, adsorber applications, and catalytic conversion applications. Advantageously, system 10 has enabled a desirable extrusion process to produce substrates having porosities of up to about 90%.
Referring now to
Generally, a fiber is considered to be a material with a relatively small diameter having an aspect ratio greater than one. The aspect ratio is the ratio of the length of the fiber divided by the diameter of the fiber. As used herein, the ‘diameter’ of the fiber assumes for simplicity that the sectional shape of the fiber is a circle; this simplifying assumption is applied to fibers regardless of their true sectional shape. For example, a fiber with an aspect ratio of 10 has a length that is 10 times the diameter of the fiber. The diameter of the fiber may be 6 micron, although diameters in the range of about 1 micron to about 25 microns are readily available. It will be understood that fibers of many different diameters and aspect ratios may be successfully used in system 10. As will be described in more detail with reference to later figures, several alternatives exist for selecting aspect ratios for the fibers. It will also be appreciated that the shape of fibers is in sharp contrast to the typical ceramic powder, where the aspect ratio of each ceramic particle is approximately 1.
The fibers for the extrudable mixture 52 may be metallic (some times also referred to as thin-diameter metallic wires), although
In order to produce an extrudable mixture, the fibers are typically combined with a plasticizer. In this way, the fibers are combined with other selected organic or inorganic additives. These additives provide three key properties for the extrudate. First, the additives allow the extrudable mixture to have a rheology proper for extruding. Second, the additives provide the extruded substrate, which is typically called a green substrate, sufficient strength to hold its form and position the fibers until these additives are removed during the curing process. And third, the additives are selected so that they burn off in the curing process in a way that facilitates arranging the fibers into an overlapping construction, and in a way that does not weaken the forming rigid structure. Typically, the additives will include a binder, such as binder 61. The binder 61 acts as a medium to hold the fibers into position and provide strength to the green substrate. The fibers and binder(s) may be used to produce a porous substrate having a relatively high porosity. However, to increase porosity even further, additional pore formers, such as pore former 63, may be added. Pore formers are added to increase open space in the final cured substrate. Pore formers may be spherical, elongated, fibrous, or irregular in shape. Pore formers are selected not only for their ability to create open space and based upon their thermal degradation behavior, but also for assisting in orienting the fibers. In this way, the pore formers assist in arranging fibers into an overlapping pattern to facilitate proper bonding between fibers during later stage of the curing. Additionally, pore-formers also play a role in the alignment of the fibers in preferred directions, which affects the thermal expansion of the extruded material and the strength along different axes.
As briefly described above, extrudable mixture 52 may use one or more fibers selected from many types of available fibers. Further, the selected fiber may be combined with one or more binders selected from a wide variety of binders. Also, one or more pore formers may be added selected from a wide variety of pore formers. The extrudable mixture may use water or other fluid as its plasticizing agent, and may have other additives added. This flexibility in formation chemistry enables the extrudable mixture 52 to be advantageously used in many different types of applications. For example, mixture combinations may be selected according to required environmental, temperature, chemical, physical, or other requirement needs. Further, since extrudable mixture 52 is prepared for extrusion, the final extruded product may be flexibly and economically formed. Although not illustrated in
The present invention represents a pioneering use of fiber material in a plastic batch or mixture for extrusion. This fibrous extrudable mixture enables extrusion of substrates with very high porosities, at a scalable production, and in a cost-effective manner. By enabling fibers to be used in the repeatable and robust extrusion process, the present invention enables mass production of filters and catalytic substrates for wide use throughout the world.
Referring to
Advantageously, the formation of bonds, such as bonds 112 facilitates forming a substantially rigid structure with the fibers. The bonds also enable the formation of an open pore network having very high porosity. For example, open-space 116 is created naturally by the space between fibers. Open space 114 is created as pore former 105 degrades or burns off. In this way, the fiber bond formation process creates an open pore network with no or virtually no terminated channels. This open pore network generates high permeability, high filtration efficiency, and allows high surface area for addition of catalyst, for example. It will be appreciated that the formation of bonds can depend upon the type of bond desired, such as solid-state or liquid-assisted/liquid-state sintering, and additives present during the curing process. For example, the additives, particular fiber selection, the time of heat, the level of heat, and the reaction environment may all be adjusted to create a particular type of bond.
Referring now to
Referring now to
The extrudable mixture and process generally described thus far is used to produce a highly advantageous and porous substrate. In one example, the porous substrate may be extruded in to a filter block substrate 175 as illustrated in
When used as a flow-through device, the high porosity of block 176 enables a large surface area for the application of the washcoat or catalytic material. In this way, a highly effective and efficient catalytic converter may be made, with the converter having a low thermal mass. With such a low thermal mass, the resulting catalytic converter has good light off characteristics, and efficiently uses catalytic material. When used in a wall flow or wall filtering example, the high permeability of the substrate walls enable relatively low back pressures, while facilitating depth filtration. This depth filtration enables efficient particulate removal, as well as facilitates more effective regeneration. In some cases, however, mostly cake or surface filtration is observed. In wall-flow design, the fluid flowing through the substrate is forced to move through the walls of the substrate, hence enabling a more direct contact with the fibers making up the wall. Those fibers present a high surface area for potential reactions to take place, such as if a catalyst or adsorber washcoat is present. Since the extrudable mixture may be formed from a wide variety of fibers, additives, and fluids, the chemistry of the extrudable mixture may be adjusted to generate a block having specific characteristics. For example, if the final block is desired to be a diesel particulate filter, the fibers are selected to account for safe operation even at the extreme temperature of an uncontrolled regeneration. In another example, if the block is going to be used to filter a particular type of exhaust gas, the fiber and bonds are selected so as not to react with the exhaust gas across the expected operational temperature range. Although the advantages of the high porosity substrate have been described with reference to filters and catalytic converters, it will be appreciated that many other applications exist for the highly porous substrate.
The fibrous extrudable mixture as described with reference to
Binders and pore formers may then be selected according to the type of fibers selected, as well as other desired characteristics. In one example, the binder is selected to facilitate a particular type of liquid state bonding between the selected fibers. More particularly, the binder has a component, which at a bonding temperature, reacts to facilitate the flow of a liquid bond to the nodes of intersecting fibers. Also, the binder is selected for its ability to plasticize the selected fiber, as well as to maintain its green state strength. In one example, the binder is also selected according to the type of extrusion being used, and the required temperature for the extrusion. For example, some binders form a gelatinous mass when heated too much, and therefore may only be used in lower temperature extrusion processes. In another example, the binder may be selected according to its impact on shear mixing characteristics. In this way, the binder may facilitate chopping fibers to the desired aspect ratio during the mixing process. The binder may also be selected according to its degradation or burnoff characteristics. The binder needs to be able to hold the fibers generally into place, and not disrupt the forming fiber structure during burnoff. For example, if the binder burns off too rapidly or violently, the escaping gases may disrupt the forming structure. Also, the binder may be selected according to the amount of residue the binder leaves behind after burnout. Some applications may be highly sensitive to such residue.
Pore formers may not be needed for the formation of relatively moderate porosities. For example, the natural arrangement and packing of the fibers within the binder may cooperate to enable a porosity of about 40% to about 60%. In this way, a moderate porosity substrate may be generated using an extrusion process without the use of pore formers. In some cases, the elimination of pore formers enables a more economical porous substrate to be manufactured as compared to known processes. However, when a porosity of more than about 60% is required, pore formers may be used to cause additional airspace within the substrate after curing. The pore formers also may be selected according to their degradation or burnoff characteristics, and also may be selected according to their size and shape. Pore size may be important, for example, for trapping particular types of particulate matter, or for enabling particularly high permeability. The shape of the pores may also be adjusted, for example, to assist in proper alignment of the fibers. For example, a relatively elongated pore shape may arrange fibers into a more aligned pattern, while a more irregular or spherical shape may arrange the fibers into a more random pattern.
The fiber may be provided from a manufacturer as a chopped fiber, and used directly in the process, or a fiber may be provided in a bulk format, which is typically processed prior to use. Either way, process considerations should take into account how the fiber is to be processed into its final desirable aspect ratio distribution. Generally, the fiber is initially chopped prior to mixing with other additives, and then is further chopped during the mixing, shearing, and extrusion steps. However, extrusion can also be carried out with unchopped fibers by setting the rheology to make the extrusion mix extrudable at reasonable extrusion pressures and without causing dilatency flows in the extrusion mix when placed under pressure at the extrusion die face. It will be appreciated that the chopping of fibers to the proper aspect ratio distribution may be done at various points in the overall process. Once the fiber has been selected and chopped to a usable length, it is mixed with the binder and pore former. This mixing may first be done in a dry form to initiate the mixing process, or may be done as a wet mix process. Fluid, which is typically water, is added to the mixture. In order to obtain the required level of homogeneous distribution, the mixture is shear mixed through one or more stages. The shear mixing or dispersive mixing provides a highly desirable homogeneous mixing process for evenly distributing the fibers in the mixture, as well as further cutting fibers to the desired aspect ratio.
In general, the mixture may be adjusted to have a rheology appropriate for advantageous extrusion. Typically, proper rheology results from the proper selection and mixing of fibers, binders, dispersants, plasticizers, pore formers, and fluids. A high degree of mixing is needed to adequately provide plasticity to the fibers. Once the proper fiber, binder, and pore former have been selected, the amount of fluid is typically finally adjusted to meet the proper rheology. A proper rheology may be indicated, such as by one of two tests. The first test is a subjective, informal test where a bead of mixture is removed and formed between the fingers of a skilled extrusion operator. The operator is able to identify when the mixture properly slides between the fingers, indicating that the mixture is in a proper condition for extrusion. A second more objective test relies on measuring physical characteristics of the mixture. Generally, the shear strength versus compaction pressure can be measured using a confined (i.e. high pressure) annular rheometer. Measurements are taken and plotted according to a comparison of cohesion strength versus pressure dependence. By measuring the mixture at various mixtures and levels of fluid, a rheology chart identifying rheology points may be created. For example, Table 5
Once the proper rheology has been reached, the mixture is extruded through an extruder. The extruder may be a piston extruder, a single screw extruder, or a twin screw extruder. The extruding process may be highly automated, or may require human intervention. The mixture is extruded through a die having the desired cross sectional shape for the substrate block. The die has been selected to sufficiently form the green substrate. In this way, a stable green substrate is created that may be handled through the curing process, while maintaining its shape and fiber alignment.
The green substrate is then dried and cured. The drying can take place in room conditions, in controlled temperature and humidity conditions (such as in controlled ovens), in microwave ovens, RF ovens, and convection ovens. Curing generally requires the removal of free water to dry the green substrate. It is important to dry the green substrate in a controlled manner so as not to introduce cracks or other structural defects. The temperature may then be raised to burn off additives, such as binders and pore formers. The temperature is controlled to assure the additives are burnt off in a controlled manner. It will be appreciated that additive burn off may require cycling of temperatures through various timed cycles and various levels of heat. Once the additives are burned off, the substrate is heated to the required temperature to form structural bonds at fiber intersection points or nodes. The required temperature is selected according to the type of bond required and the chemistry of the fibers. For example, liquid-assisted sintered bonds are typically formed at a temperature lower than solid state bonds. It will also be appreciated that the amount of time at the bonding temperature may be adjusted according to the specific type of bond being produced. The entire thermal cycle can be performed in the same furnace, in different furnaces, in batch or continuous processes and in air or controlled atmosphere conditions. After the fiber bonds have been formed, the substrate is slowly cooled down to room temperature. It will be appreciated that the curing process may be accomplished in one oven or multiple ovens/furnaces, and may be automated in a production ovens/furnaces, such as tunnel kilns.
Referring now to
Once the substrate requirements have been defined, a fiber is selected from Table 1 of
A binder is then selected from Table 2 of
As shown in block 254, the fibers selected in block 252 should be processed to have a proper aspect ratio distribution. This aspect ratio is preferred to be in the range of about 3 to about 500 and may have one or more modes of distribution. It will be appreciated that other ranges may be selected, for example, to about an aspect ratio of 1000. In one example, the distribution of aspect ratios may be randomly distributed throughout the desired range, and in other examples the aspect ratios may be selected at more discrete mode values. It has been found that the aspect ratio is an important factor in defining the packing characteristics for the fibers. Accordingly, the aspect ratio and distribution of aspect ratios is selected to implement a particular strength and porosity requirement. Also, it will be appreciated that the processing of fibers into their preferred aspect ratio distribution may be performed at various points in the process. For example, fibers may be chopped by a third-party processor and delivered at a predetermined aspect ratio distribution. In another example, the fibers may be provided in a bulk form, and processed into an appropriate aspect ratio as a preliminary step in the extrusion process. It will be appreciated that the mixing, shear mixing or dispersive mixing, and extrusion aspects of process 250 may also contribute to cutting and chopping of the fibers. Accordingly, the aspect ratio of the fibers introduced originally into the mixture will be different than the aspect ratio in the final cured substrate. Accordingly, the chopping and cutting effect of the mixing, shear mixing, and extrusion should be taken into consideration when selecting the proper aspect ratio distribution 254 introduced into the process.
With the fibers processed to the appropriate aspect ratio distribution, the fibers, binders, pore formers, and fluids are mixed to a homogeneous mass as shown in block 262. This mixing process may include a drying mix aspect, a wet mix aspect, and a shear mixing aspect. It has been found that shear or dispersive mixing is desirable to produce a highly homogeneous distribution of fibers within the mass. This distribution is particularly important due to the relatively low concentration of ceramic material in the mixture. As the homogeneous mixture is being mixed, the rheology of the mixture may be adjusted as shown in block 264. As the mixture is mixed, its rheology continues to change. The rheology may be subjectively tested, or may be measured to comply with the desirable area as illustrated in Table 5 of
Referring now to
Referring now to
After the fibers have been chopped to an appropriate aspect distribution, the water is mostly removed using a filter press 316 or by pressing against a filter in another equipment. It will be appreciated that other water removal processes may be used, such as freeze drying. The filter press may use pressure, vacuum or other means to remove water. In one example the chopped fibers are further dried to a complete dry state as shown in block 318. These dried fibers may then be used in a dry mix process 323 where they are mixed with other binders and dry pore formers as shown in block 327. This initial dry mixing assists in generating a homogeneous mass. In another example, the water content of the filtered fibers is adjusted for proper moisture content as shown in block 321. More particularly, enough water is left in the chopped fiber cake to facilitate wet mixing as shown in block 325. It has been found that by leaving some of the slurry water with the fibers, additional separation and distribution of the fibers may be obtained. Binders and pore formers may also be added at the wet mix stage, and water 329 may be added to obtain the correct rheology. The mass is also shear mixed as shown in block 332. The shear mixing may also be done by passing the mixture through spaghetti shaped dies using a screw extruder, a double screw extruder, or a shear mixer (such as sigma blade-type mixer). The shear mixing or kneading can also take place in a sigma mixer, a high shear mixer, and inside the screw extruder. The shear mixing process is desirable for creating a more homogeneous mass 335 that has desirable plasticity and extrudable rheology for extrusion to work. The homogeneous mass 335 has an even distribution of fibers, with the fibers positioned into an overlapping matrix. In this way, as the homogeneous mass is extruded into a substrate block and cured, the fibers are allowed to bond into a rigid structure. Further, this rigid structure forms an open pore network having high porosity, high permeability, and high surface area.
The fiber extrusion system offers great flexibility in implementation. For example, a wide range of fibers and additives, may be selected to form the mixture. Several mixing and extrusion options exist, as well as options related to curing method, time, and temperature. With the disclosed teachings, one skilled in the extrusion arts will understand that many variations may be used. Honeycomb substrate is a common design to be produced using the technique described in the present invention, but other shapes, sizes, contours, designs can be extruded for various applications.
A fuel cell is an electrochemical energy conversion device similar to a battery, differing in that it is designed for continuous replenishment of the reactants consumed; i.e. it produces electricity from an external supply of fuel and oxygen as opposed to the limited internal energy storage capacity of a battery. Additionally, the electrodes within a battery react and change as a battery is charged, or discharged, whereas a fuel cell's electrodes are catalytic and relatively stable. Fuel cells are used for energy generation in a variety of applications, from electricity generation and portable power units to powering mobile phones and automobiles.
Typical reactants used in a fuel cell are hydrogen on the anode side, and oxygen on the cathode side (a hydrogen cell). Usually, reactants flow in and reaction products flow out. Virtually continuous long-term operation is feasible as long as these flows are maintained.
In many fuel cell systems, there is a fuel reformer that is placed in the stream of the main fuel cell stack. The fuel reformer converts input fuels into hydrogen, often using catalytic processes. In some conditions, this reaction can take place at room temperature while in most places, this reaction takes place under elevated conditions. The reformate stream (i.e. the gases exiting from the reformer and flowing down the path to enter the fuel cell) typically contain many components, including H2, N2, CO, CO2, H2S, etc. Several of these gases can be cleaned up easily using oxidation type catalysts using, for example, porous substrates adapted as a flow through or wall flow filter. Most importantly, the H2S gas needs to be discarded or else it will quickly foul and destroy the precious catalysts in the fuel cell reactor. Even miniscule amounts of H2S (i.e. in low ppm levels) can foul the fuel cell catalysts, resulting in poor performance or non-operation of the fuel cell.
The removal of H2S is an important step in cleaning up of the reformate stream since most naturally available (or synthesized fuels) contain some sulfur, H2S is produced in the reformer system. Some interesting methods have been found to remove H2S from gas streams. Some methods rely on molecular membrane type technologies but those are hard to implement and not very durable. Some recent advances utilize washcoat technology to adsorb H2S onto surfaces that have particular affinity for H2S even at ppm levels. In such cases, the washcoat composition is specially designed and configured to attract H2S and hold onto it while there is H2S present in the gas stream. Then, at a later time, when the H2S concentration goes down, or a clean environment is presented, the H2S is released and captured elsewhere. Such a system can be easily configured for use in a fuel cell environment by using two or more such adsorption cells in parallel in a revolver type system. For example, a flow through honeycomb catalyst providing a high surface area for the washcoat to be placed on can be put in the main reformate stream so that the H2S in the stream is captured and a gas stream devoid of any H2S is allowed to pass through to the fuel cell stack. When the washcoat reaches its maximum H2S holding capacity, the revolver system can be triggered so a new honeycomb substrate is exposed into the main gas stream and the ‘loaded’ substrate is exposed to a clean air or N2 environment. In such a H2S-less environment, the H2S can be released and stored elsewhere or converted into other species. In such a revolving mechanism, no H2S is allowed to pass through to the main fuel cell stack and the H2S is essentially captured from the main reformate stream and released outside the system. The bed that adsorbs the gas is fully re-generable and hence lasts a very long time in an application.
Some of the washcoats used include zinc oxide, lanthanum oxide, other rare-earth oxides etc. Often the washcoat loadings can be quite high and washcoats about 20-50 microns thick can be put onto the walls of a honeycomb substrate. Some materials are used to soak completely in sulfur, i.e. to reach a saturation point, while some new studies (e.g. Stephanapoulous et al. Science, Vol. 312, p. 1508 (2006)) have shown using only surface adsorption phenomenon to quickly adsorb and then quickly desorb even trace quantities of sulfur compounds in a gas stream. When applied to the porous substrate of the present invention, the washcoat can penetrate into the porous material, thereby increasing the effective surface area to which the reformate stream can be exposed.
Referring to
In an exemplary embodiment the porous substrate 502 is composed of mullite fibers having a porosity of about 85%. Mullite is the mineralogical name given to the only chemically stable intermediate phase in the Al2O3—SiO2 system. The natural mineral is rare, naturally occurring on the Isle of Mull off the west coast of Scotland. Mullite is commonly denoted as 3Al2O3.2SiO2 (i.e., 60 mol % Al2O3 and 40 mol % SiO2), though mullite fibers, in this exemplary embodiment, can include a metastable phase of 2Al2O3SiO2 or compositions from 60 mol % to 67 mol % alumina. An alumina-silica phase diagram showing the mullite composition is shown as
The porous substrate 514, and particularly the surfaces of the porous substrate forming the walls of the plurality of channels, such as walls 528 and 529 that form channel 524, are coated with a washcoat described above having an affinity for adsorption of H2S. As the fuel 522 passes through the channels the H2S from the fuel is trapped on the washcoat material 519 coated on the surface of the substrate.
Typical sizes of the substrates used in a reformate stream filter in either the flow-through configuration of
The advantages of a porous substrate to trap H2S in a reformate stream filter is that when the washcoat becomes saturated with H2S, the substrate can be taken off-line and regenerated.
In an exemplary embodiment, the H2S-less exhaust gas leaving the first adsorbing substrate in a revolver-type system is moved into the fuel cell assembly for energy conversion, and then the exhaust gas from the fuel cell is passed through the second desorbing substrate in the revolver system to release the H2S into the exhaust stream. This way no extra air-flow system is required for desorbing the H2S from the saturated substrates.
In the exemplary embodiment shown in
While particular preferred and alternative embodiments of the present intention have been disclosed, it will be apparent to one of ordinary skill in the art that many various modifications and extensions of the above described technology may be implemented using the teaching of this invention described herein. All such modifications and extensions are intended to be included within the true spirit and scope of the invention as discussed in the appended claims.
This application claims priority to U.S. provisional patent application No. 60/824,088, filed Aug. 31, 2006, and entitled “Extruded Porous Ceramic Fuel Cell Reformer Cleanup Substrate”, which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60824088 | Aug 2006 | US |
