Device and method for cleaning selective catalytic reduction protective devices

Abstract

One embodiment described herein relates to a system for removing pollutants from a flue gas. The system includes a selective catalytic reduction (SCR) system having a SCR reactor containing a NOx reducing catalyst and one or more SCR protective devices. At least one of the SCR protective devices is connected to a rapping hammer system that actively remove fly ash collected on the SCR protective devices.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For the purposes of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 shows SCR protective devices placed at various points in ductwork upstream of the SCR reactor.

FIG. 2 shows a rapping hammer assembly within a flue gas stream.

FIG. 3 shows a rapping hammer assembly outside of a flue gas stream.

FIG. 4 shows a side view of the rapping hammer assembly.

FIG. 5 shows a side view of a SCR protective device.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION OF THE INVENTION

The term “SCR protective device” as used in the present specification and claims refers to any device that prevents appreciable quantities of large fly ash particles (LPA) and other large particulate material in flue gases from entering the NO_x reducing catalyst channels or accumulating on other SCR catalyst surfaces. One example of an SCR protective device is a wire mesh screen that has openings that are slightly smaller than the diameters of the NO_x reducing catalyst channels. Typically, the SCR protective device is a screen that is surrounded by a supporting frame.

It is noted that while the SCR protective device prevents appreciable quantities of fly ash from entering the NO_x reducing catalyst channels, it does not hinder the flow of the gas from entering the NO_x reducing catalyst.

Now referring to the figures in which like numerals correspond to like parts, and in particular to FIG. 1, an SCR protective device 20 may be placed in various locations upstream of the SCR reactor 22. One embodiment described herein relates to the active removal of accumulated fly ash on any SCR protective device 20 placed upstream of SCR reactor 22 as well as any SCR protective device that is placed directly over the catalyst material. In one embodiment, SCR protective device 20 can be placed in a sloped or angled orientation in relation to a flue duct wall (“ductwork”) 35. In another embodiment, SCR protective device 20 can be placed in a perpendicular orientation in relation to flue duct wall 35.

As shown in FIG. 2, one embodiment of the invention has a mechanical rapping system 24 operatively connected to SCR protective device 20. Mechanical rapping system 24 generally includes a rapping hammer assembly 26 and a control unit 28. Rapping hammer assembly 26 includes hammers 30 that are attached to a rotating shaft 32. Hammers 30 can be made of any material suitable to contact SCR protective device 20. Examples of such materials include, but are not limited to: metal, plastic, rubber, concrete, and any other suitable synthetic or naturally occurring material. The weight and size of hammers 30 will vary depending on the system, the amount of fly ash, and the size of SCR protective device 20. Hammers 30 can be replaced from time to time, or as necessary with hammers that weigh more or less than the typical hammers used in the system. Additionally, hammers 30 can be replaced with hammers that are larger or smaller than the typical hammers used in the system.

Hammers 30 contact SCR protective device 20 with a hitting, rapping, or striking motion of sufficient force to cause at least a portion of fly ash that has accumulated on the SCR protective device to slough off and be removed therefrom. It is contemplated that hammers 30 can contact any portion of SCR protective device 20, including any surrounding supporting frame.

Rotating shaft 32 is attached to hammers 30. Preferably, rotating shaft 32 is made of steel; however one skilled in the art will recognize that other materials, such as plastic, or other synthetic or naturally occurring material may be used for the rotating shaft.

Rotating shaft 32 is typically rotated by control unit 28 thereby causing hammers 30 to contact SCR protective device 20. Rapping hammer assembly 26 may be operated by an electric or battery operated motor located in control unit 28. Alternatively, rapping hammer assembly 26 could be operated by pneumatic cylinders or magnetic impulse devices, or by any other power source that would allow hammers 30 to contact SCR protective device 20 in a forceful motion to remove accumulated fly ash.

Typically, control unit 28 is connected to rapping hammer assembly 26 via rotating shaft 32. The motor, or other power means, actuates the movement of hammers 30.

In one embodiment of the present invention, control unit 28 includes a user interface 33 such as a desktop computer, a laptop computer, a monitor, or other display device that allows a user to vary the settings of rapper hammer assembly 26. User interface 33 would allow the user to control several variables, including but not limited to, the pressure of hammers 30 striking SCR protective device 20, the amount of times the hammers strike the SCR protective device in a specific time period, and/or the continuity of the hammer strikes on the SCR protective device. These variables would vary and are specific to each plant. Control of these variables will facilitate the removal of at least a portion of any fly ash accumulated on SCR protective device 20.

In one embodiment of the invention, hammers 30 continuously strike SCR protective device 20. In another embodiment, hammers 30 strike SCR protective device 20 at predetermined times. In yet another embodiment, a sensor or measuring device 34, such as a differential pressure transmitter, may be employed to determine when a certain amount of fly ash accumulates on SCR protective device 20. Once a certain amount of fly ash accumulates on SCR protective device 20, hammers 30 will be activated and will strike the SCR protective device.

As shown in FIG. 2, at least a portion of rapping hammer assembly 26 is within ductwork that has a flue gas stream flowing through it. Typically, in this embodiment control unit 28 is located outside flue duct wall 35. A wall seal 36 prevents the flue gas from escaping from flue duct wall 35.

In another embodiment, as shown in FIG. 3, SCR protective device 20 includes a plurality of contact elements 38 that protrude from the SCR protective device. Contact elements 38 also protrude at least partially outside flue duct wall 35. Contact elements 38 may be made of any material that is suitable to be contacted with hammers 30. Examples of appropriate materials include, but are not limited to, metal, plastic, rubber, concrete, and other synthetic or naturally occurring materials. Contact elements 38 provide a surface which hammers 30 can impact instead of hitting SCR protective device 20 directly.

Typically, rapping hammer assembly 26 is not directly connected to SCR protective device 20. As shown in FIG. 3, hammers 30 strike contact elements 38 which protrude outside flue duct wall 35. In this embodiment, rapping hammer assembly 26 is outside flue duct wall 35 and is not exposed to the flue gas.

FIG. 4 shows a side view of FIG. 3. As seen in this figure, the flow of the flue gas 40 travels towards and goes through SCR protective device 20. Fly ash and other particulates present in the flue gas are captured by SCR protective device 20. Hammers 30 move in a semi-circular direction 42 toward contact elements 38, connected to SCR protective device 20. Rotating shaft 32 rotates hammers 30 toward contact elements 38.

As one skilled in the art will recognize, there may be one or more mechanical rapping systems 24 attached to one SCR protective device 20. The number of hammers 30 per rapping hammer assembly 26 may vary to optimize the point(s) at which SCR protective device 20 is impacted by the hammers. Additionally, one of ordinary skill in the art will recognize that one or more contact elements 38 may be connected to SCR protective device 20.

Once hammers 30 have struck SCR protective device 20 in an effective manner, very little fly ash will remain on the SCR protective device. However, it may be necessary to repeat the contact of hammers 30 to the SCR protective device 20 more than once. Therefore, rapping hammer assembly 26 may be programmed or monitored so hammers 30 strike contact elements 38 numerous times within a certain time period. Alternatively, rapping hammer assembly 26 may repeatedly contact SCR protective device 20 for continuous fly ash removal. In another alternative embodiment, sensor 34 may be used to measure or detect an amount of fly ash present on SCR protective device 20. Once the amount of fly ash reaches a certain level, rapping hammer assembly 26 can be activated, thereby causing hammers 30 to strike contact elements 38.

The manner in which hammers 30 contact SCR protective device 20 will vary from system to system. The action of hammers 30 contacting SCR protective device 20 will allow fly ash particles to slough off and continue through the system. Rapping hammer systems applied to SCR protective devices installed upstream of the catalyst bed dislodge fly ash particles back into the flue gas stream or move the fly ash along SCR protective device 20 to a discharge point. Alternatively, dislodging fly ash can be transported along SCR protective device 20 to an ash collection hopper (not shown).

While the invention is directed to the use of a mechanical rapping system on SCR protective devices, one skilled in the art will recognize that this mechanical rapping system can alternatively be employed on any item or device, including SCR protective devices that are designed to improve fly ash knockout in hoppers upstream of the SCR reactor. These items or devices include, but are not limited to economizer outlet “bull noses,” kicker plates, splitters, and other similar items.

When a mechanical rapping system is used in connection with SCR protective devices upstream of the SCR reactor, the dislodged fly ash particles may be removed back into the flue gas stream or may be removed to a discharge point or fly ash collection hopper. FIG. 5 shows a side view of SCR protective device 20 and a path which a fly ash particle may take once it contacts the SCR protective device. After the fly ash particles are dislodged from SCR protective device 20, a portion of the particles may fall by gravity to an ash collection hopper installed below the screen. Some of the dislodged particles may be carried by the flue gas stream back to SCR protective device 20. When an SCR protective device 20 is installed on a slope, as shown in FIG. 5, the ash particles will eventually work their way to the edge of the SCR protective device where they can be dislodged into a discharge pipe 44, vacuumed out from time to time, or removed by a device that provides a gas seal 46, e.g. a cyclone or loop seal.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.

Claims

1. A system for removing pollutants from a flue gas, the system comprising a selective catalytic reduction (SCR) system comprising: a SCR reactor containing a NOx reducing catalyst;one or more SCR protective devices located upstream of the SCR reactor wherein the one or more SCR protective devices substantially prevent large particles in the flue gas from entering the SCR reactor or otherwise impeding the flow of flue gas therethrough; anda mechanical rapping system for impacting the SCR protective device to dislodge therefrom accumulated large particles.

2. A system according to claim 1, wherein said rapping hammer system comprises: at least one hammer;at least one rotating shaft; anda control unit for controlling the at least one rotating hammer.

3. A system according to claim 2, wherein the control unit comprises an electric motor.

4. A system according to claim 2, wherein the control unit comprises pneumatic pump cylinders.

5. A system according to claim 2, wherein the control unit comprises a magnetic impulse device.

6. A system according to claim 2, wherein the control unit further comprises a user interface.

7. A system according to claim 6, wherein said user interface comprises devices to control at least one variable selected from speed, pressure, time and continuity.

8. A system according to claim 1, wherein the rapping hammer system is operatively connected to the at least one SCR protective device within a ductwork.

9. A system according to claim 1, wherein the rapping hammer system is connected to the SCR protective device outside a ductwork.

10. A system according to claim 9, wherein the hammers strike contact elements extending from the SCR protective device.

11. A method of removing accumulated fly ash from an SCR protective device comprising the steps of: connecting a rapping hammer system comprising at least one hammer and at least one rotating shaft to an SCR protective device;rotating the rotating shaft to turn the at least one hammers; andcontacting the at least one hammer to the SCR protective device, whereby accumulated fly ash present on the SCR protective device is removed.

12. A method according to claim 11, wherein the rapping hammer system further comprises a control unit.

13. A method according to claim 12, wherein the control unit comprises at least one of an electric motor, pneumatic pump cylinders, or a magnetic pulse device.

14. A method according to claim 11, further comprising the step of: altering at least one variable, the variable selected from speed, time, pressure and continuity of the rapping hammer system.