Patent Application
20070237692
Reference is hereby made to the following copending application, which was filed on even date with the present application: “Catalytic Reactor with Swirl”, Steven W. Burd and Meredith B. Colket, application Ser. No. ______.
The present invention is directed to a system and method for combusting hydrocarbon fuels in an efficient manner which minimizes pollutant emissions, particularly NOX emissions.
Exhaust gases produced in the combustion of hydrocarbon fuels by engines contribute to atmospheric pollution. Exhaust gases typically contain nitric oxide (NO), nitrogen dioxide (NO2), carbon monoxide, and unburned hydrocarbons. Nitrogen oxides are a cause of smog, acid rain, and depletion of stratospheric ozone. With high combustion temperatures in an engine, oxygen and nitrogen combine to form the pollutants NO and NO2 (collectively known as “NOX”). Typical fuels reacting with air exceed the threshold temperature which results in NOX formation.
A reduction in the formation of NOX is desirable. One method to control NOX is to employ a catalyst that allows low-temperature reaction of fuel and air. Most, if not all, of the fuel can be reacted at a moderate temperature, thus inhibiting formation of NOX. The use of a catalyst results in a pre-reaction of a portion of a fuel to stabilize the main combustion process. The catalytic process is referred to as catalytic combustion.
When reacting fuel with a catalyst, heat is generated. This heat must be controlled to result in a lower combustion temperature. Typically only a portion of the total fuel to be burned is reacted in the catalyst chamber. One solution to the problem of heat production is to provide a stream of cooling air about a stream of fluid that is in contact with the catalyst or a substrate to which the catalyst is attached or resides. Such a process uses heat exchange in which certain channels contain the catalyzed fluid, while other channels contain air for cooling and absorbing heat from the catalytic reaction. These two fluid streams can then be mixed upon exiting the heat exchanger and combusted with a reduction of NOX.
In catalytic combustors (or catalytic reactors), hydrocarbon fuel is mixed with a first air stream to form a fuel and air mixture having an equivalent ratio greater than unity, or stoichiometry greater than 1.0, and partially oxidized by contacting the fuel/air fluid mixture with an oxidation catalyst stage, thereby generating the heat of reaction in a partial oxidation product stream comprising hydrogen and carbon dioxides. The reaction is intended to be pure catalytic, thus minimizing the formation of oxides of nitrogen (NOX).
A portion of the heat of reaction is conducted through the wall of a substrate on which the catalyst resides and is removed via the back side convection and conduction heat transfer to the second air stream and/or compatible cooling fluid. The partial oxidation product stream is mixed with a second air stream, which is raised in temperature from its initial state via the heat of reaction, to combust the fuel in a down stream combustor. The down stream combustor can include additional fuel or air mixtures that contribute to a combustion in single or multiple zones.
The fuel/air mixture flows into a catalytic oxidation stage and contacts an oxidation catalyst which partially oxidizes the mixture to generate heat and a partial oxidation product stream comprising hydrogen, carbon oxides (primarily CO), and unreacted hydrocarbon fuel. Catalytic oxidation in this context is intended to drive a rapid oxidation or oxidative pyrolysis reaction carried out at a temperature below that required to support thermal combustion or combustion without a flame at a temperatures below which thermal NOX forms in appreciable amounts. Partial oxidation means that there is insufficient oxygen available to completely convert fuel to carbon dioxide and water, and thus fully liberate the chemical energy stored in the fuel.
The current invention seeks to improve current catalytic reactor designs. A catalytic tube insert promotes turbulence in fluid flow passing through the catalytic tube. The insert is used in a catalytic reactor to allow for a method of improved combusting of a catalyzed fuel by either adding to the catalyst surface area, or increasing the residence time of a fluid with the catalyst, or both.
Catalyst tubes 14 are made from a heat conducting material and adapted for conducting fluid flow internally and within the housing 12. Catalyst tubes 14 have a tube entrance 26, a tube exit 32, an interior surface 34, and an exterior surface 36. The interior surface 34 creates an interior flow path 38 within the catalyst tube 14. In one embodiment, a portion 40 of catalyst tube 14 adjacent the tube exit 32 contains a differing cross section to change the velocity or direction of the fluid exiting the catalyst tube 14.
Although shown as cylindrical, the catalyst tubes can be of varying geometries, for example, round, lobed, polygonal, elliptical, or other cross-sectional shapes. Similarly, although illustrated as generally linear or straight, catalyst tubes 14 may be twisted, or of varying cross-sectional shapes. In all embodiments, interior fluid flow paths 38 exist within the interior surface 34 of catalyst tubes 14.
Flow paths 28 within housing 12 are defined by the conduit exterior surfaces 36 and wall 24. Flow paths 28 are in communication with aperture 22 and exit 20. Flow paths 28 can have varying configurations dependent upon the shape and spacing of catalyst tubes 14 and wall 24 of housing 12. Flow paths 28 permit the flow and diffusion of a fluid in a manner to allow heat transfer with the fluids flowing within catalyst tubes 14.
An insert 16 is located within each catalyst tube 14. Insert 16 is constructed from a heat conducting material, and may be composed of the same material as the catalyst tubes 14. Tabs 42 secure insert 16 within catalyst tube 14. Tabs 42 are attached to the ends 45 and 46 of insert 16, and extend to the interior surface 34 of catalyst tube 14, where tabs 42 are secured such as by welding or other means. Alternately, tabs 42 or insert 16 itself may extend to the end of tube 14 for attachment to the end of tube 14 or grate 30. Other attachment or contact points may be provided to assist in heat removal from insert 16 to substrate comprising catalytic tube 14. In the embodiment illustrated, insert 16 is a twisted metal strip that extends the length of the catalyst tube 14 from entrance 30 to beginning of portion 40. Insert 16 creates a tortuous flow path internal to the catalyst tube 14. The result is a production of swirl to increase residence time within the catalyst tube, which results in more oxidation of a fluid comprised of fuel and air. Thus, the reaction per unit of length of catalyst tube 14 is increased.
A catalyst 44 is deposited on a portion of the conduit interior surface 34 and on a portion of insert 16. Catalyst 44 can be deposited anywhere in the flow path 38 through tube 14. A fuel-rich fuel/air mixture is mixed prior to delivery to interior flow path 38 created by catalytic tube 14. Alternately, mixing of fuel and air within flow path 38 is possible due to the turbulence of fluid flow created by insert 16.
When the fuel is a hydrocarbon and oxygen is the oxidizer, catalyst 44 may include group VIII noble metals, base metals, metal oxides, or a combination thereof. Elements including zirconium, vanadium, chromium, manganese, copper, platinum, palladium, ruthenium, osmium, iridium, rhodium, cerium, lanthanum, other elements of the lanthanide series, cobalt, nickel, and iron are all suitable, as well as chromium oxides, cobalt oxides, and alumina, or mixtures thereof. Catalyst 44 is applied directly to the substrate, or in an alternate embodiment, is applied to a bonding coat or washcoat composed of such materials as alumina, silica, zirconia, titania, magnesia, other refractory metal oxides, or a combination thereof.
The substrate comprising the catalytic tubes 14 and inserts 16 is fabricated from high-temperature materials. In one embodiment, high temperature nickel alloy is used. In alternate embodiments, high-temperature metal alloys are used, including alloys composed of iron, nickel, and/or cobalt, in combination with aluminum, chromium, and/or other alloying materials. Other substrate materials include ceramics, metal oxides, intermetallic materials, carbides, and nitrides. Metallic substrates and refractories are most preferred due to their excellent thermal conductivity, allowing effective backside cooling of the catalyst.
In the embodiment of
In this embodiment, grate 30 only allows for the introduction of fluid into catalyst tubes 14, while aperture 22 introduces fluid into flow path 28. Grate 30 may be constructed from a metal plate that contains apertures to allow the introduction of a fluid only within the catalyst tubes 14 at entrance 18. A plate 48 generally parallel to grate 30 is present at exit 20 and contains one or more apertures that act as an exit port 49 for fluids. Plate 48 also contains openings corresponding to tube exits 32 to permit the flow of fluid out of the catalytic tubes 14. In this embodiment, tabs 42 may be secured along the length of insert 16, and/or at the ends 45 and 46 of insert 16 or grate 30 to provide additional support compliance of insert 16 within tube 14.
Inserts 16a-16d show a variety of differing profiles possible as inserts into catalytic tubes 14. Though four tubes are shown, applications may dictate that the tube array be more numerous or less in number that shown. Moreover, relative size of tubes and flow paths to chamber size may differ. Insert 16a is a flat metal strip. Insert 16b is a star shaped polygon. Insert 16c is a cross, while insert 16d is another conduit smaller in size than the internal diameter of conduit comprising catalytic tube 14. All inserts 16a-16d may be comprised of metals or conductive material, and may be covered entirely or on a portion thereof with a catalyst 44 (as illustrated by inserts 16a and 16b) as described above. Similarly, all inserts 16a-16d may be twisted about a longitudinal axis of catalytic tubes 14. The various shapes of inserts 16a-16d will create different flow paths 38 within similarly shaped catalytic tubes 14.
Inserts 16a-16c may be held in place in catalytic tubes 14 by securing the outer perimeter of the insert to the interior surface 34 of conduits 14. For example, inserts 16a-16c may be welded or adhered in place with adhesives or other means (friction or diffusion bond). In an alternate embodiment, inserts 16a-16c are held in place through an interference or friction fit of the insert within the catalytic tube. In yet another alternate embodiment, inserts 16a-16d are secured through the use of tabs 42 that are connected to the interior surface 34 of catalytic tubes 14 and the surface of inserts 16a-16d. Inserts 16a-16d are meant to be illustrative examples, and not limit the profile of inserts 16.
Other variations of inserts are envisioned, including metallic tape, static mixers, linear metal strips or rods which are sinusoidally waved, or corrugated, in the longitudinal direction, or any combination thereof. Similarly, the aforementioned inserts can be twisted, braided, coiled, or similarly manipulated prior to or after insertion into the catalytic tubes 14. Those skilled in the art will appreciate the numerous structures that can be designed based upon the specific application, thus the invention should not be considered limited to the insert geometries depicted in the embodiments presented.
A first fluid of fuel/air mixture enters the catalytic tubes 14 at or near entrance 18 as represented by arrow 50. Simultaneously, a second fluid comprising ambient air is introduced into the flow paths between adjacent catalytic tubes 14 as represented by arrow 52. In an alternate embodiment, the second fluid is similar in composition to the first fluid and contains a fuel/air mixture. The interior surface of catalytic tubes 14 contain a catalyst which reacts with the first fluid and generates heat. The second fluid acts a cooling fluid and absorbs the heat generated by the reaction. The first fluid leaves the catalytic reactor 10 at exit 20. Due to the presence of the inserts 16 within the catalytic tubes 14, the fluid exits in an agitated flow pattern as represented by arrow 54. The second fluid is leaving exit 20 as a laminar flow as represented by arrow 56. The fluids 50, 52 mix upon leaving exit 20. The agitation of fluid flow created by inserts 16 aids in the mixing of the first and second fluids at exit 20 to create a single heated, and partially reacted, third fluid for combustion.
In the embodiment illustrated
A catalytic reactor of any of the disclosed embodiments is designed for maximum catalytic reaction in normal operating conditions, while minimizing variations in chemical reaction rates and mass transfer rates due to fluid flow fluctuations. Catalysts are applied in coatings on a weight basis in relationship to geometric surface area of catalyst. Insufficient coating area will result in an insufficient catalytic reaction for the desired reactor and combustor design due to insufficient total mass transfer from the fluid to the catalytic surface. The required coating area depends upon operating conditions (e.g. reactant temperature, pressure, velocity, composition) and catalyst activity, and can be determined by methods known in chemical engineering practice.
With the presence of inserts within the catalytic tubes of a catalytic reactor, fluid flow will be slowed, adjusted, or disturbed in a controlled fashion, thus allowing for greater residence time with the coating area containing the catalyst. This produces in a decrease of the required surface area of the catalyst, which results in smaller and less expensive reactors. By additionally coating the inserts themselves, additional catalyst surface area is added to further reduce reactor size and cost. Thus, design of a reactor will balance the surface area for reaction and cooling with the desired volumetric flow rates of the relevant fluids passing through the reactor. Inserts provide the ability to tailor new or existing reactors for a specific application and also provide improved reactor overhaul capability.
The catalytic reactor of the current invention is used for combusting a catalyzed hydrocarbon fuel. A first fluid is passed into catalytic tubes of the catalytic reactor, while a second fluid is passed adjacent the catalytic tubes in the flow paths of the chamber of the catalytic reactor. Both the first fluid and the second fluid can be comprised of air/fuel mixture, or an air stream. At least one of the fluids is comprised of air/fuel mixture and is in contact with a catalyst. The other fluid can be comprised of air/fuel mixture or a different gas or gas mixture such as air. The insert is provided for at least a portion of the tube to modify the flow of the first fluid, and/or add to the catalytic reaction. The flow of the first fluid leaving the catalytic tubes is mixed with the second fluid, and this mixture is combusted. The heat of the catalytic reaction is extracted from fluid in contact with the catalyst and transferred to the other fluid. In one embodiment, the second fluid is controllably introduced into the catalytic reactor. A third fluid, such as additional air or additional air/fuel mixture, can be added to the mixture of the first fluid and the second fluid upon exiting the reactor to achieve a desired stoichiomety for combustion.
Although illustrated in
Placing inserts into the catalytic tubes of catalytic reactor results in increased surface area per unit length for oxidizing the fuel/air mixture internal to the conduits where the catalyst is applied. The result is a tortuous flow path internal to the conduits or tubes which produces swirl within the conduits or tubes to increase residence time for oxidizing the fuel/air mixture. The shape of a conduit can be designed to promote swirl and shear mixing at the exit of the catalytic tube or conduit. With the turbulated flow pattern, the product stream and secondary air streams mix rapidly upon leaving the catalytic reactor and effectively combine to allow combustion of fuel mixture. This reduces emissions from levels currently present in the state of the art, and the potential to remove the size of this secondary region for combustion and the required complexity therein. Overall, adding inserts which increase residence time, i.e. the time the fluid is in the catalytic tube, resulting in greater reaction per unit length. This will allow for the reduction of the size of equipment and the complexity of combustor assemblies. Another advantage of the current invention is its ability to be retrofit into existing catalytic reactor configurations. For example, a twisted metal strip containing a catalyst can easily be added into the internal pathway of the catalytic tube in a current catalytic reactor.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
