WO2001010539A1

WO2001010539A1 - Method for controlling the catalytic treatment of flue gas

Info

Publication number: WO2001010539A1
Application number: PCT/US2000/017788
Authority: WO
Inventors: Arthur S. Adams; Mitchell B. Cohen; Benjamin T. Greene
Original assignee: Alstom Power Inc.
Priority date: 1999-08-10
Filing date: 2000-06-28
Publication date: 2001-02-15
Also published as: TW442636B; AU5897100A

Abstract

A method for controlling the catalytic treatment of a gas is provided. The method includes providing a catalytic treatment enclosure (30) having an inlet for introducing therein a gas to be treated, an outlet for exhausting gas from the catalytic treatment enclosure (30), and a catalyst contact assembly (100) disposable intermediate the inlet and the outlet for contracting gas flowing thereagainst so as to effect catalytic treatment of the gas. The method also includes the step of flowing gas to be treated through the catalytic treatment enclosure (30) wherein the gas is catalytically treated via contact with the catalyst contact assembly (100). The method further includes retracting the catalyst contact assembly (100) from the catalytic treatment enclosure (30) through a removal port (114) located downstream of the inlet relative to the gas flow. After retraction of the catalyst contact assembly (100), an additional step of the method includes blocking the escape of gas through the removal port (114) to the exterior of the catalytic treatment enclosure (30) contemporaneous with the blocking of the removal port (114). The method additionally includes re-positioning the catalyst contact assembly (100) at its gas contacting location within the catalytic treatment enclosure (30). After re-positioning the catalyst contact assembly (100), at its gas contacting location within the catalytic treatment enclosure (30), another step of the method includes flowing gas to be treated through the catalytic treatment enclosure (30) wherein the gas is catalytically treated via contact with the catalyst contact assembly (100) therein.

Description

METHOD FOR CONTROLLING THE CATALYTIC TREATMENT OF FLUE GAS

BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for customizing the catalytic treatment of flue gas and, in particular, to a method and apparatus for customizing the catalytic treatment of flue gas produced by fossil fuel combustion.

In recent years oxides of nitrogen, also known as NOχ, have been implicated as one of the elements contributing to the generation of acid rain and smog. One post-combustion process for the lowering of NOχ emissions is that of selective catalytic reduction (SCR). Selective catalytic reduction systems use a catalyst and a reactant such as ammonia gas, NH₃, to dissociate NOχ to molecular nitrogen, N₂, and water vapor. A utility steam generating power plant having, for example, a fossil fuel-fired furnace may utilize selective catalytic seduction (SCR) as a NOχ reduction technique. The furnace typically comprises a furnace volume in fluid communication with a backpass volume. Combustion of hydrocarbon fuels occurs within the furnace volume creating hot flue gases that rise within the furnace volume giving up a portion of their energy to the working fluid of a thermodynamic steam cycle. The flue gases are then directed to and through the backpass volume wherein they give up additional energy to the working fluid. Upon exiting the backpass volume the flue gases are directed via a gas duct through a selective catalytic reduction chamber and thence to an air preheater and flue gas cleaning systems thence to the atmosphere via a stack.

In a typical SCR system, at some point in the gas duct after the flue gas stream exits the backpass volume and upstream of the SCR chamber, a reactant, possibly ammonia, in a gaseous form, or a urea water solution is introduced into, and encouraged to mix with, the flue gas stream. The reactant flue gas mixture then enters the SCR chamber wherein the catalytic reductions take place between the reactant/flue gas mixture and the catalytic material. The introduction of the ammonia or urea into the flue gas stream is typically achieved by the use of injector nozzles located at either the periphery of the gas duct, or immersed within the flue gas stream.

The installation of a conventional SCR system demands a not insignificant cost and the operation of such a conventional SCR system also entails a substantial expenditure due to large ammonia consumption rates and the need to periodically replace a portion of the catalytic material. It would be beneficial if there were an option other than the installation and operation of a conventional full scale SCR system. The availability of such an option would be especially beneficial in connection with some of the more recent low NOx burner configurations which are capable of nearly or actually achieving legislatively mandated maximum NOx levels. Such units could benefit from a NOx trimming capability capable of successfully "trimming" or controlling the relatively low NOx output without incurring the capital and operational costs of a conventional full scale SCR.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new and improved selective catalytic reduction system for operation with a flue gas producing unit such as, for example, a fossil fuel-fired furnace, which advantageously permits savings of space, material and operating costs. It is also an object of the present invention to provide such a new and improved selective catalytic reduction system for operation with a fossil fiiel-fired furnace which advantageously affords increased flexibility in adjusting the NOx conversion capability of a fossil fuel-fired furnace.

According to one aspect of the present invention, there is provided a method for controlling the catalytic treatment of a gas. The method includes providing a catalytic treatment enclosure having an inlet for introducing therein a gas to be treated, an outlet for exhausting gas from the catalytic treatment enclosure, and a catalyst contact assembly disposable intermediate the inlet and the outlet for contacting gas flowing thereagainst so as to effect catalytic treatment of the gas. The method also includes the step of flowing gas to be treated through the catalytic treatment enclosure wherein the gas is catalytically treated via contact with the catalyst contact assembly. The method of the one aspect of the present invention further includes retracting the catalyst contact assembly from the catalytic treatment enclosure through a removal port located downstream of the inlet relative to the gas flow. After retraction of the catalyst contact assembly, an additional step of the method includes blocking the escape of gas through the removal port to the exterior of the catalytic treatment enclosure and effecting the flow of gas through the catalytic treatment enclosure contemporaneous with the blocking of the removal port. The method of the one aspect of the present invention further additionally includes repositioning the catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure. After re-positioning the catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure, another step of the method includes flowing gas to be treated through the catalytic treatment enclosure wherein the gas is catalytically treated via contact with the catalyst contact assembly therein.

According to additional features of the one aspect of the method of the present invention, the method preferably includes the additional steps of disposing the retracted catalyst contact assembly at a service location, reducing the level of gas retained in the retracted catalyst contact assembly, and servicing the catalyst contact assembly by at least one of repairing, removing, and replacing catalyst material following the step of reducing the level of gas retained in the catalyst contact assembly. Moreover, it is preferable that the step of disposing the retracted catalyst contact assembly at a service location includes disposing the catalyst contact assembly in an enclosed housing and reducing the level of gas retained in the retracted catalyst contact assembly includes positively pressurizing the enclosed housing to withdraw the retained gas from the catalyst contact assembly.

According to yet further additional features of the method of the one aspect of the present invention, the step of positively pressurizing the enclosed housing includes applying purge air to the enclosed housing to displace the retained gas out of the enclosed housing and conducting the displaced gas to the catalytic treatment enclosure.

In accordance with an alternate feature of the method of the one aspect of the present invention, the method also includes providing a second catalyst contact assembly disposable in the catalytic treatment enclosure and controlling the step of retracting the first catalyst contact assembly in coordination with the disposition of the second catalyst contact assembly in the catalytic treatment enclosure such that the second catalyst contact assembly is disposed in a gas contacting location in the catalytic treatment enclosure during the step of flowing gas through the catalytic treatment enclosure contemporaneous with the sealing isolation of the removal port from the exterior of the catalytic treatment enclosure. This alternate feature of the method preferably also includes the step of retracting the second catalyst contact assembly from the catalyst treatment enclosure after the step of re-positioning the first catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure. According to another aspect of the present invention, there is provided an apparatus for controlling the catalytic treatment of a gas. The apparatus includes a catalytic treatment enclosure having an inlet for introducing therein a gas to be treated, an outlet for exhausting gas from the catalytic treatment enclosure, and a catalyst contact assembly disposable intermediate the inlet and the outlet for contacting gas flowing thereagainst so as to effect catalytic treatment of the gas. Gas to be treated is flowed through the catalytic treatment enclosure wherein the gas is catalytically treated via contact with the catalyst contact assembly. The apparatus further includes means for retracting the catalyst contact assembly from the catalytic treatment enclosure through a removal port located downstream of the inlet relative to the gas flow and means for blocking the escape of gas through the removal port to the exterior of the catalytic treatment enclosure contemporaneous with the flow of gas through the catalytic treatment enclosure. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic front plan view of a fuel-fired steam generating power plant including a fuel/air firing system, a furnace volume, a horizontal pass, a backpass volume, and the preferred embodiment of the selective catalytic reduction chamber of the present invention;

Figure 2 is an enlarged perspective view of one version of the selective catalytic reduction chamber shown in Figure 1 and showing the catalytic contact assembly thereof in its online disposition;

Figure 3 is an enlarged perspective view of one version of the selective catalytic reduction chamber shown in Figure 1 and showing the catalytic contact assembly thereof in its offline disposition; and

Figure 4 is an enlarged perspective view of another version of the selective catalytic reduction chamber shown in Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to Figure 1 there is depicted a generalized schematic diagram in the nature of a side elevation view of a fossil-fuel fired furnace of a steam generating power plant, generally designated by reference numeral 10, and associated structures including the preferred embodiment of the selective catalytic reduction system of the present invention. For a more detailed description of the nature of the construction and the mode of operation of a fossil-fuel fired furnace such as the fossil-fiiel fired furnace 10, one may reference U.S. Patent No. 4,719,587, which issued on Jan. 12, 1987 to F. J. Berte and which is assigned to the same assignee as the present patent application.

Referring further to Figure 1, the fossil-fuel fired furnace 10 includes a furnace volume, generally designated by reference numeral 12. It is within the furnace volume 12 of the fossil-fuel fired furnace 10 that, in a manner well known to those skilled in the art, combustion of fuel and air is initiated. The hot gases that are produced from this combustion, commonly referred to as flue gases 14 and which may act as a heat exchange medium, rise upwardly within the furnace volume 12 and give up heat to the working fluid of a thermodynamic steam cycle. The working fluid passes through the furnace waterwall tubes 16 which in a conventional manner line all four walls of the furnace volume 12. The flue gases 14 then exit the furnace volume 12 through a horizontal pass, generally designated by reference numeral 18. The horizontal pass 18 in turn leads to a backpass volume, generally designated by reference numeral 20. The upper segment of the furnace volume 12 as well as the horizontal pass 18 and the backpass volume 20 commonly contain other heat exchange surfaces 22,24,26 for superheating and reheating steam or heating feedwater in a manner well known to those skilled in the art. Thereafter, the steam generated in the thermodynamic steam cycle commonly is made to flow to a turbine (not shown), which forms one component of a turbine/generator set (not shown). The steam provides the motive power to drive the turbine which thence drives the generator. The generator is, in known fashion, cooperatively associated with the turbine such that electricity is produced thereby. With further reference to Figure 1, the aforesaid flue gases 14, after passing through the backpass volume 20 and giving up heat to the heat exchange surfaces 24,26 therein, are directed via flue gas ductwork 28 through a selective catalytic reduction chamber 30 and thence to an air preheater (not shown), flue gas cleaning systems (not shown), a stack (not shown) and are then vented to the atmosphere. Referring further to Figure 1 there is also depicted a schematic representation of a means, generally designated by the numeral 32, for supplying fuel and air to the furnace volume 12. The fuel and air supply means 32 consists of various ducts 34 so designed and constructed as to transport fuel and air, separately or if need be in combination, from a fuel source 36 and an air source 38 to a main windbox 40 thence therethrough to the furnace volume 12. The air may also be directed to a set of separated overfire air (SOFA) windboxes 42, and thence therethrough to the furnace volume 12 so as to complete the aforesaid combustion. For a more detailed description of the nature of construction and the mode of operation of the fuel and air supply means, one may reference U.S. Patent No. 5,315,939, which issued on May 31, 1994 to M. Rini et al. and which is assigned to the same assignee as the present patent application.

Continuing further in Figure 1 there is also depicted a schematic representation of a means, generally designated by the reference numeral 44, for supplying a reactant to the flue gas 14 flowing through the flue gas ductwork 28. The reactant supply means 44 includes a reactant source 46 and a reactant grid 48 so designed and constructed, in combination, as to transport the reactant from the reactant source 46 to the flue gases 14 for mixture therewith. The flue gas reactant mixture then flows to the selective catalytic reduction chamber 30 wherein it undergoes selective catalytic reduction. The method of the present invention for controlling the catalytic treatment of a gas will now be described with respect to the gas treatment performed in the selective catalytic reactor chamber 30 as an exemplary gas treatment environment suitable for implementation of the method of the present invention. As noted, the selective catalytic reactor chamber 30 is operable to treat the flue gases created by during the combusting of a fossil fuel such as, for example, coal. The method includes the step of providing a catalytic treatment enclosure having an inlet for introducing therein a gas to be treated, an outlet for exhausting gas from the catalytic treatment enclosure, and a catalyst contact assembly disposable intermediate the inlet and the outlet for contacting gas flowing thereagainst so as to effect catalytic treatment of the gas. Thus, it can be seen that the selective catalytic reduction chamber 30 is configured as a catalytic treatment enclosure having an inlet defined by the imaginary pre-treatment threshold PRE past which the flue gas to be treated flows into the selective catalytic reduction chamber 30 and an outlet defined by the imaginary post-treatment threshold POS past which the flue gas flows as it exits the selective catalytic reduction chamber 30. The selective catalytic reduction chamber 30 includes a plurality of catalyst contact assemblies 100 disposable intermediate the imaginary pre-treatment threshold PRE and the imaginary post-treatment threshold POST for contacting the flue gas flowing thereagainst so as to effect catalytic treatment of the flue gas. Before further describing the implementation of the method of the present invention, details of the catalytic contact assemblies 100 will first be described with reference to Figure 2, which is an enlarged perspective view of the selective catalytic reduction chamber 30 having one individual catalyst contact assembly 100 therein in an online disposition, and Figure 3, which is a perspective view of the selective catalytic reduction chamber 30 shown in Figure 2 with the one individual catalyst contact assembly 100 therein disposed in an offline disposition. Referring first to Figure 2, it can be seen that the interior portion of the selective catalytic reduction chamber 30 in which the catalyst contact assembly 100 is disposed has a parallelepiped shape having a height extent HEX and a width extent WID peφendicular to the height extent HEX. The flue gas flows through the selective catalytic reduction chamber 30 in an overall direction parallel to the height extent HEX. The one catalyst contact assembly 100 includes a module retaining carriage 102 for supporting a plurality of treatment modules 104 A - D, a transfer assembly 106, and an associated service bay 108. Each treatment module 104A - D comprises a respective plurality of catalyst surfaces 110 having catalytic material for catalytic interaction with flue gas and supported in the respective treatment module in spaced parallel relation to one another. Each treatment module 104 A - D is preferably independently serviceable relative to the other treatment modules such that each treatment module can be independently repaired or replaced when the module retaining carriage 102 is disposed in an offline disposition, as will be described in more detail below.

Each treatment module 104A - D has an overall parallelepiped shape. The module retaining carriage 102 supports the plurality of treatment modules 104A - D in spaced parallel relation to one another for movement of the treatment modules as a single unit by the transfer assembly 106. The module retaining carriage 102 is comprised of a steel framework having an overall rectangular shape of a longitudinal extent sized in coordination with the width extent WTD of the interior of the selective catalytic reduction chamber 30 such that the module retaining carriage 102 supports the treatment modules 104 A - D across the entire width extent WTD with the catalytic surfaces 110 of the treatment modules 104 A - D oriented parallel to the flow direction of the flue gas.

The transfer assembly 106 is operable to transfer the module retaining carriage 102, with the treatment modules 104A - D supported thereon, between an online disposition in which the treatment modules 104 A - D are supported at a gas contacting location within the selective catalytic reduction chamber 30 at which the catalyst surfaces 110 are contacted by flue gas, and an offline disposition in which treatment modules 104A - D have been retracted from the selective catalytic reduction chamber 30. The transfer movement by the transfer assembly 106 of the module retaining carriage 102 between the online and offline dispositions is preferably accomplished by moving the module retaining carriage 102 in a direction peφendicular to the height extent HEX of the selective catalytic reduction chamber 30 such as, for example, in a direction parallel to the width extent WID, in preference to lifting the module retaining carriage 102 out of the selective catalytic reduction chamber 30 through the top thereof. Figure 2 illustrates one suitable transfer arrangement in which the module retaining carriage 102 is supported in its offline disposition at a location immediately adjacent a side wall 112 of the selective catalytic reduction chamber 30 and is transferred through a removal port 114 in the side wall 112 into its online disposition at a location within the selective catalytic reduction chamber 30. The transfer movement of the module retaining carriage 102 through the removal port 114 is preferably accomplished by a configuration of the transfer assembly 106 which ensures reliable and efficient movement of the module retaining carriage. To this end, Figure 2 illustrates that the transfer assembly 106 preferably comprises a pair of rails 116 extending parallel to one another and spaced apart from one another in the direction of the depth extent DEP of the selective catalytic reduction chamber 30. One half of each rail 116 extends fully across the width extent WID of the selective catalytic reduction chamber 30 and the other half of each rail extends to the outside of the side wall 112 of the selective catalytic reduction chamber 30. The outside extending half of each rail 116 is supported in the service bay 108, which is itself supported by appropriate structural support (not shown) contiguous to the side wall 112 of the selective catalytic reduction chamber 30. The module retaining carriage 102 is provided with a plurality of track following rollers 118 each rotatably mounted to the module retaining carriage and having a cylindrical surface for rolling movement along one of the rails 116. The transfer assembly 106 includes a transfer drive sub-assembly preferably in the form of a rack and pinion arrangement comprising a rack 120 mounted to the module retaining carriage 102 and a pinion gear 122 rotatably driven by a drive motor 124 mounted in the service bay 108. The pinion gear 122 meshingly engages the rack 120 mounted to the module retaining carriage 102 to controllably reversibly move the module retaining carriage 102 between its online and offline dispositions.

Figure 3 shows the module retaining carriage 102 in its offline disposition in which the module retaining carriage is completely retracted from the interior of the selective catalytic reduction chamber 30 and is fully enclosed within the service bay 108. The service bay 108 includes a seal door sub-assembly 126, a purge air sub-assembly 128, and an exhaust air sub-assembly 130. The seal door sub-assembly 126 includes a door movably mounted on the side wall 112 of the selective catalytic reduction chamber 30 for movement between a closed position in which it sealingly closes the removal port 114 and an open position in which it is raised to permit transfer movement of the module retaining carriage 102 through the removal port 114. The purge air sub-assembly 128 includes a pressurized air source communicated with the interior of the service bay 108 to supply air under pressure into the service bay 108. The pressurized air introduced into the service bay 108 acts to drive or purge flue gas from the service bay such as, for example, flue gas resident in the treatment modules 104 supported on the module retaining carriage 102 during its offline disposition in the service bay. The exhaust air sub-assembly 130 includes a duct 132 communicating the service bay 108 and the interior of the selective catalytic reduction chamber 30 with one another and a valve 134 mounted on the duct 132 for controllably permitting or preventing the passage of exhaust gas from the service bay 108 through the duct 132 to the selective catalytic reduction chamber 30.

One aspect of the method of the present invention for controlling the catalytic treatment of a gas can thus be implemented with the arrangement as described with respect to Figures 2 and 3. The basic steps of this one aspect of the method of the present invention include providing a catalytic treatment enclosure such as, for example, the selective catalytic reduction chamber 30, having an inlet for introducing therein a gas to be treated and an outlet for exhausting gas from the catalytic treatment enclosure. The method also includes providing a catalyst contact assembly such as, for example, the catalyst contact assembly 100, which comprises the module retaining carriage 102 disposable intermediate the inlet and the outlet of the selective catalytic reduction chamber 30 for contacting gas flowing thereagainst so as to effect catalytic treatment of the gas.

For the performance of the next step of this one aspect of the method, the module retaining carriage 102 is disposed in its online disposition in the interior of the selective catalytic reduction chamber 30 at which the treatment modules 104 A - D are supported for treating the flue gas flowing through the selective catalytic reduction chamber 30. The method thus also includes the step of flowing gas to be treated through the catalytic treatment enclosure (the selective catalytic reduction chamber 30) wherein the gas is catalytically treated via contact with the catalyst contact assembly (the catalyst contact assembly 100).

The method of the one aspect of the present invention then includes retracting the catalyst contact assembly 100 from the catalytic treatment enclosure through a removal port such as, for example, the removal port 114, located downstream of the inlet relative to the gas flow. This step of the method is performed by the arrangement illustrated in Figures 2 and 3 by controlling the motor 124 to rotate the pinion gear 122 so as to thereby drive the module retaining carriage 102 via its rack 120 in a retracting movement which transfers the module retaining carriage 102 from its online disposition within the selective catalytic reduction chamber 30 into its offline disposition within the service bay 108. After retraction of the catalyst contact assembly 100, an additional step of the method includes blocking the escape of flue gas through the removal port (the removal port 114) to the exterior of the catalytic treatment enclosure (the selective catalytic reduction chamber 30) and effecting the flow of gas through the catalytic treatment enclosure contemporaneous with the blocking of the removal port. This is accomplished by lowering the door of the seal door assembly 126 from its raised position into its lowered sealing position in which it sealingly blocks the removal port 114 while continuing the flow of flue gas through the selective catalytic reduction chamber 30. If desired, servicing of the treatment modules 104 A - D can now be accomplished by first introducing pressurized air via the purge air sub-assembly 128 into the service bay 108 to thereby drive out any flue gas still resident in and between the treatment modules 104 A - D. The flue gas thus driven out from the service bay 108 is conducted via the duct 132 back to the interior of the selective catalytic reduction chamber 30. The valve 134 is disposed in its open position to permit this return flow of flue gas.

Once purged of flue gas, the service bay 108 can be entered to thereby service the treatment modules 104A - D supported therein. For example, individual treatment modules 104 A - D can be serviced by repair or replacement. Additional treatment modules may be added or current treatment modules can be deleted to thereby customize or tailor the catalytic capability of the catalyst contact assembly 100 to the particular NOx trimming requirements. Alternatively, the treatment modules 104 A - D can simply be parked in the service bay 108 without servicing of the modules.

Since the module retaining carriage 102 is disposed in its offline disposition, flue gas flowing through the selective catalytic reduction chamber 30 is not contacted by the treatment modules 104 A - D and thus passes through the selective catalytic reduction chamber 30 without undergoing catalytic treatment. In the event that more than one catalyst contact assembly 100 is provided, a variety of catalytic treatment options are possible. To illustrate the versatility with which the method of the present invention can be implemented, attention is drawn to Figure 4 which is an enlarged perspective view of another version of the selective catalytic reduction chamber 30 shown in Figure 1. The selective catalytic reduction chamber 30 shown in Figure 4 is provided with a lower catalytic contact assembly 100 and an upper catalytic contact assembly 200, each configured and operable identical to the catalytic contact assembly described with respect to Figures 2 and 3. The lower catalytic contact assembly 100 and the upper catalytic contact assembly 200 can be operated independent of one another to customize or tailor the catalytic treatment capability of the selective catalytic reduction chamber 30. For example, as shown in the exemplary operational configuration shown in Figure 4, one of the pair of catalytic contact assemblies (the upper catalytic contact assembly 200) can be disposed offline in its service bay 108 while the other catalytic contact assembly (the lower catalytic contact assembly 100) is disposed online in the selective catalytic reduction chamber 30 to intercept flue gas for catalytic treatment thereof. Alternatively, both of the catalytic contact assemblies 100,200 can be retracted into their respective associated service bays 108 such that none of the flue gas continuing to flow through the selective catalytic reduction chamber 30 is catalytically treated.

Referring again to the basic steps of the method of the one aspect of the present invention, the method also includes re-positioning the catalyst contact assembly 100 at its gas contacting location within the catalytic treatment enclosure (the selective catalytic reduction chamber 30) after the step of retracting the catalytic contact assembly. This step is accomplished, for example, by transferring the module retaining carriage 102 from its offline disposition in the service bay 108 into its online disposition in the interior of the selective catalytic reduction chamber 30 with the transfer movement performed by the arrangement illustrated in Figures 2 and 3 which by controls the motor 124 to rotate the pinion gear 122 so as to thereby effect, via the rack 120, transfer movement of the module retaining carriage 102 from the service bay 108 into the selective catalytic reduction chamber 30. The rollers 118 rollingly support the module retaining carriage 102 on the rails 116 during this transfer movement. After re-positioning of the catalyst contact assembly 100 at its gas contacting location within the catalytic treatment enclosure (the selective catalytic reduction chamber 30), the last basic step of the method is performed which includes flowing gas to be treated through the catalytic treatment enclosure (the selective catalytic reduction chamber 30) wherein the gas is catalytically treated via contact with the catalyst contact assembly 100 therein.

According to yet further additional features of the method of the one aspect of the present invention, the step of positively pressurizing the enclosed housing includes applying purge air to the enclosed housing to displace the retained gas out of the enclosed housing and conducting the displaced gas to the catalytic treatment enclosure. In accordance with an alternate feature of the method of the one aspect of the present invention, the method also includes providing a second catalyst contact assembly disposable in the catalytic treatment enclosure and controlling the step of retracting the first catalyst contact assembly in coordination with the disposition of the second catalyst contact assembly in the catalytic treatment enclosure such that the second catalyst contact assembly is disposed in a gas contacting location in the catalytic treatment enclosure during the step of flowing gas through the catalytic treatment enclosure contemporaneous with the sealing isolation of the removal port from the exterior of the catalytic treatment enclosure. This alternate feature of the method preferably also includes the step of retracting the second catalyst contact assembly from the catalyst treatment enclosure after the step of re-positioning the first catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure.

While a preferred embodiment of the invention has been shown, it will be appreciated by those skilled in the art that modifications may readily be made thereto. It is, therefore, intended that the appended claims shall cover any modifications alluded to herein as well as to all modifications that fall within the true spirit and scope of the invention.

Claims

We claim:

1. A method for controlling the catalytic treatment of a gas, comprising: providing a catalytic treatment enclosure having an inlet for introducing therein a gas to be treated, an outlet for exhausting gas from the catalytic treatment enclosure, and a catalyst contact assembly disposable intermediate the inlet and the outlet for contacting gas flowing thereagainst so as to effect catalytic treatment of the gas; flowing gas to be treated through the catalytic treatment enclosure wherein the gas is catalytically treated via contact with the catalyst contact assembly; retracting the catalyst contact assembly from the catalytic treatment enclosure through a removal port located downstream of the inlet relative to the gas flow; after retraction of the catalyst contact assembly, blocking the escape of gas through the removal port to the exterior of the catalytic treatment enclosure and effecting the flow of gas through the catalytic treatment enclosure contemporaneous with the blocking of the removal port; re-positioning the catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure; and after re-positioning the catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure, flowing gas to be treated through the catalytic treatment enclosure wherein the gas is catalytically treated via contact with the catalyst contact assembly therein.

2. A method for controlling the catalytic treatment of a gas according to claim 1 and further comprising disposing the retracted catalyst contact assembly at a service location, reducing the level of gas retained in the retracted catalyst contact assembly, and servicing the catalyst contact assembly by at least one of repairing, removing, and replacing catalyst material following the step of reducing the level of gas retained in the catalyst contact assembly.

3. A method for controlling the catalytic treatment of a gas according to claim 2 wherein disposing the retracted catalyst contact assembly at a service location includes disposing the catalyst contact assembly in an enclosed housing and reducing the level of gas retained in the retracted catalyst contact assembly includes positively pressurizing the enclosed housing to withdraw the retained gas from the catalyst contact assembly.

4. A method for controlling the catalytic treatment of a gas according to claim 3 wherein positively pressurizing the enclosed housing includes applying purge air to the enclosed housing to displace the retained gas out of the enclosed housing and conducting the displaced gas to the catalytic treatment enclosure.

5. A method for controlling the catalytic treatment of a gas according to claim 1 and further comprising providing a second catalyst contact assembly disposable in the catalytic treatment enclosure and controlling the step of retracting the first catalyst contact assembly in coordination with the disposition of the second catalyst contact assembly in the catalytic treatment enclosure such that the second catalyst contact assembly is disposed in a gas contacting location in the catalytic treatment enclosure during the step of blocking the escape of gas through the removal port to the exterior of the catalytic treatment enclosure and effecting the flow of gas through the catalytic treatment enclosure contemporaneous with the blocking of the removal port.

6. A method for controlling the catalytic treatment of a gas according to claim 5 and further comprising retracting the second catalyst contact assembly from the catalyst treatment enclosure after the step of re-positioning the first catalyst contact assembly at its gas contacting location within the catalytic treatment enclosure.