JP2011017524A - Method for cleaning air preheater in operating state - Google Patents

Abstract

PROBLEM TO BE SOLVED: To clean a viscous adhered object from rotating type air preheaters without stopping a steam generating system.SOLUTION: A method of cleaning the air preheaters of the type having a rotor which passes through a stream of flue gas and a stream of intake combustion air is disclosed. A soot blower is operated in a step method to blow a soot blowing medium through passageways in the rotor. The passageways are arranged in concentric rings and move at a tangential velocity depending on the speed of the rotor and the location of the passageway relative to the center of the rotor. In this method, the speed of the rotor is adjusted in accordance with the position of the soot blower so that every passageway moves over the soot blower at the same or substantially the same tangential velocity.

Description

  The present invention relates to a method for cleaning a rotary regenerative air preheater used in a coal-fired power plant.

  Techniques for preheating air are well known and have been used for many years with boilers to improve combustion and boiler efficiency. One such preheating technique uses an air preheater manufactured by Ljungstrom. This air preheater has two parallel ducts, combustion exhaust gas flows through one duct, and combustion air flows through the other duct. The two gas flows are in opposite directions. A rotor is arranged to rotate through both ducts and about an axis between the two ducts to transfer heat from the combustion exhaust gas to the combustion air. Air preheaters are typically operated at sufficiently high temperatures to prevent contaminants such as sulfuric acid vapor present in the flue gas from condensing in the heat exchanger. For example, in a typical published temperature guide line of a Ljungstrom air preheater, the flue gas outlet temperature exceeds at least 149 ° C. (300 ° F.) and is as high as 177 ° C. (350 ° F.). Held at height. At these temperatures, aerosol condensation of gaseous sulfuric acid and the associated preheater corrosion effects are minimized.

  At lower temperatures, condensates, ash, or other related materials from flue gases tend to deposit over a period of time on heat transfer surfaces known in the art as “baskets”. When these adherends are stacked, the flow paths for air and flue gas are blocked, and the heat transfer capability is reduced. Therefore, these air preheaters typically have a device that blows air or steam into the rotor at high speed to remove the deposit. In the industry, this device is referred to as a soot blowing device.

  The soot blower that cleans the air preheater is generally of the “retractable” or “swing arm” method. Both schemes have advantages and disadvantages.

  A swing arm type soot blower for cleaning a regenerative air preheater uses a swing arm mounted to rotate at a set angle or through an arc, as the rotor turns and As the swing arm rotates through the arc, the end is provided with one or more nozzles that blow a soot blowing medium (steam, air or water) onto the rotor. The soot blower is usually attached to the cold end of the rotor, which is the exit end of the flue gas.

  These soot blowers typically use a drive mechanism that includes a worm gear and a worm wheel or chain drive that rotates a lever throw arm. The connecting link attaches the lever throw arm to the lever attached to the arm mounting plate of the soot blower. This link configuration reciprocates the lever and soot blower mounting plate back and forth over the arc. This type of mechanism is disclosed in US Pat. No. 6,065,528 to Fierle et al. In these systems, a swing arm soot blower constantly changes speed or angular velocity as it sweeps across the rotor. The speed is zero at the beginning and end of the sweep, and the speed is maximum at the center of the sweep position. Because of this link configuration, the speed is constantly accelerating or decelerating between the center of the sweep position and the beginning and end. Accordingly, the energy of the soot blowing medium is concentrated towards the two ends of the nozzle stroke. This usually causes more rapid degradation of the heat exchange elements of the rotor towards the center and outer periphery of the rotor.

  Retractable soot blowers have become more widely used in recent years because they resemble soot blowers used in other parts of the furnace and because of their improved ability to control their position during the cleaning process. Has been applied. The retractable soot blower consists of two concentric tubes. One is fixed in place and attached to a medium source (air, steam, water, etc.), and the other is capable of controlled movement in and out of the boiler duct laterally along the same axis as the fixed tube. At the end of the movable tube closest to the media supply line, the filler material is placed between the inner surface of the outer tube and the outer surface of the inner tube. The end of the inner tube that is not connected to the cleaning medium is open. The end of the outer tube furthest from the cleaning media line is surrounded by a rounded cap containing one or more outlet holes (or nozzles).

  The cleaning medium is directed from a line of the medium source to a stationary tube via a device called a poppet valve. When the valve is opened, the cleaning medium is guided inside the fixed tube under pressure, allowing it to move out of the open outer end and into the volume of the outer tube. The filler material prevents the media from exiting the assembly at the junction, causing all media to travel the entire length of the outer tube and out of the cleaning nozzle at the distal end of the movable tube. By controlling the insertion depth of the movable tube, direct control of the cleaning point at the surface of the air preheater is achieved.

  Techniques for cleaning rotary heat exchangers are known in the art. For example, Shoenherr et al. (U.S. Pat. No. 2,812,923) apply a cleaning solution through a fan-shaped plate port above a heat exchanger and apply a fan-shaped plate disposed below the heat exchanger. An apparatus for draining liquid through a slot is described. Such devices have until recently been considered suitable for cleaning air preheaters.

In recent years, coal-fired power plants have been forced to install end systems to further reduce nitrogen oxide emissions. In this step, ammonia or urea is injected as a reducing agent. In one example of this emission control system, ammonia introduced into the flue gas upstream of the catalytic reactor reacts with nitrogen oxides in the catalytic reactor to form nitrogen (N ₂ ) and water (H ₂ O). To do. A small percentage of the injected or introduced ammonia often remains in the flue gas downstream of the catalytic reactor. This action is called ammonia slip. Ammonia slip is essentially a function of the need for nitrogen oxide removal, the activity of the catalyst, and the degree of mixing of the injected ammonia with the flue gas. It is also important that the flow through the reactor is uniform at all locations on the cross section of the reactor with uniform flue gas velocities, and that all catalytic conversion materials are reachable without obstruction. .

  On an industrial scale, these requirements can only be achieved to a limited extent with acceptable capital expenditure. As a result, ammonia slip is inevitable distributed unevenly across the reactor cross-section. On average, ammonia slip reaches only a few ppm. However, at some locations, this average may be several times this average. For this reason, ammonia and sulfuric acid vapor will be included in the flue gas entering the air preheater.

The temperature of the flue gas entering the air preheater is typically 315-371 ° C (600-750 ° F). In this temperature range, the sulfur oxide reacts with ammonia from the ammonia slip according to the following equation to form NH ₃ + H ₂ O + SO ₃ → NH ₄ HSO ₄ to form ammonium bisulfate. Alternatively, 2NH ₃ + SO ₃ + H ₂ O → (NH ₄ ) ₂ SO ₄ to form ammonium sulfate.

  In the air preheater, the gas cools to 110-177 ° C (230-350 ° F). In this temperature range, in addition to sulfuric acid, ammonium bisulfate condenses on the air preheater basket as a sticky liquid.

  Ammonium bisulfate is so sticky that it captures ash particles and quickly blocks the gas passages of the device. These deposits can also cause corrosion. To date, this problem has been mitigated by limiting ammonia slip to less than 5 ppm, and in some installations by limiting to less than 2 ppm. This increases the expenditure corresponding to the required capacity of the catalytic reactor. Nevertheless, it is impossible to rule out the possibility of higher than average ammonia slips occurring at several locations in the cross section of the reactor. Thus, in some cases, relatively high and even extremely high ammonia concentrations may occur, thereby damaging the air preheater in this area by the process described above.

  The soot blowing device is not very effective in removing ammonium bisulfate deposits. Deposits usually occur in the central part of the rotor, where the soot blower energy is dissipated before the deposits arrive. The presence of the deposit is seen by an increase in the pressure drop across the air preheater on both the air and gas sides. When this happens, the power generation unit must be stopped and the air preheater must be washed with high pressure water.

US Pat. No. 6,065,528 US Pat. No. 2,812,923

  The present invention relates to a method for cleaning adhesive deposits containing ammonium bisulfate, sulfuric acid, and other materials from a rotary air preheater without shutting down the steam generation system. Furthermore, this method can be used to reduce the accumulation of deposits at the embolization point by changing the operating strategy of conventional soot blowers. This method is particularly effective when used with a rotary air preheater of the type shown in FIG. This type of preheater includes a heat exchange rotor that includes adjacent passages for conducting air and flue gas and is rotated about an axis. The soot blower is at any selected depth from the outer periphery of the rotor so that as the rotor is rotated, the soot blowing medium is blown from the soot blower to the selected depth in the passage. Can be placed adjacent to each other.

  In a conventional preheater, the rotor is rotated at a constant angular velocity. In the method of the present invention, the angular velocity of the rotor is changed during soot blowing depending on the depth of the nozzle of the soot blowing device from the outer periphery of the rotor. Specifically, the angular velocity is adjusted so that all passages pass through the soot blower at the same or substantially the same tangential velocity.

The perspective view which shows schematic structure of one type of air preheater which can use this invention. FIG. 2 is a diagram of the air preheater and soot blower configuration of FIG. 1 showing the motor and control device required to implement the present invention. The graph which shows the pressure drop ((DELTA) P) of the whole air preheater in the air side and gas side before cleaning during the cleaning which uses a medium of high pressure water, and after cleaning. The perspective view which shows schematic structure of the 2nd type air preheater which can use this invention.

  FIG. 1 is a perspective view of a conventional air preheater in which the flue gas moves in the longitudinal direction, ie upward or downward, and is intended to illustrate one type of air preheater in which the present invention is used. Has been. The present invention is applicable to horizontal, vertical (with the cold side end at the top) and reverse vertical (with the cold side end at the bottom) air preheater. FIG. 1 shows a vertical air preheater with a cold end at the bottom. The air preheater includes a rotor housing 12 to which a heat exchange rotor 14 is attached. The rotor is mounted for rotation on a shaft 16 that extends between an upper central portion 18 and a lower central portion 20. The rotor is divided into small portions or passages 22 by a diaphragm plate 24, and a heat exchange basket 26 is stacked in these small portions 22. Located at the top and bottom of the air preheater and attached to the rotor housing 12 and the upper and lower central portions 18 and 20 are transition duct assemblies, shown as 28, 30, 32 and 34. These transition duct assemblies attach an air preheater to the air supply to the steam generator or other combustion device and to the duct for the flue gas from this device. For example, flue gas can enter the air preheater via the transition duct 28, transfer heat to the rotating rotor 14, and exit via the transition duct 30. Combustion air enters through the transition duct 32, picks up heat from the rotor 14, and exits through the transition duct 34. These transition ducts are constructed to create a transition between the generally circular air preheater and the rectangular generator duct.

  One problem with air preheaters is that the flue gas flowing through the rotor often contains particulate and / or condensable material that can be deposited on the heat transfer surface of the basket 26. This tends to clog the air preheater, reducing heat transfer efficiency and increasing the demand for suction blowers. This problem is usually addressed by providing soot blowers 13,15. The soot blowers 13 and 15 traverse the surface of the rotor while the rotor is rotating, and spray steam, air, or water onto the flow path through the rotor and the heat transfer surface to remove the adherend. There are usually two soot blowers, one at the top (or hot end) of the air preheater and the other at the bottom (or cold end) as shown in FIG. The Since most of the deposits are generated at the low temperature side end (flue gas outlet), when only one soot blowing device is used, the soot blowing device is usually the low temperature side end (the lower end of FIG. 1). Part).

  Typically, the air preheater rotates at 3 / 4-4 revolutions per minute (RPM). When the boiler operator wants to clean the air preheater, a soot blower is inserted from the outer edge and slowly moves toward the center of the rotor. This procedure works on soot deposits, but often does not remove all of the sticky liquid deposits.

  The present invention allows complete cleaning regardless of the nature of the adherend. In a preferred application, the soot blower is inserted completely in the center of the rotor with the air preheater rotating at normal speed. Thereafter, the soot blower is retracted in stages. Each step may be in any convenient manner, but the greatest success has been achieved with steps of 15-60 millimeters. In each step, the RPM of the rotor of the air preheater is adjusted so that the tangential speed of a portion of the rotor passing through the end of the soot blower is constant. When a complete rotation of the rotor at a particular soot blower position is complete, the soot blower advances (or indexes) to another position and stops, and the rotor speed matches the new insertion depth. And a complete rotor cycle is performed. When high pressure water is used as the cleaning medium, complete cleaning in any step can be demonstrated by the high pressure spray from the bottom penetrating all the way to the top of the air preheater. The size of each step should be selected by visual observation or other means.

  In a further improvement of the invention, the rotor RPM is controlled by a continuously variable drive motor 40 (shown in FIG. 2). The controller 48 is programmed to automatically set the RPM of the rotor in proportion to the insertion distance of the soot blower. The control also specifies the minimum RPM (typically 0.2 to 0.5 RPM) that prevents stalling of the motor driving the RPM. The minimum RPM can vary from air preheater to air preheater depending on whether the air preheater is rotating longitudinally or laterally, the type and effectiveness of the support bearings, and the sealing performance of the sealing mechanism. . The control device can also control the motors 43 and 44 that move the soot blower forward and backward, and can also control the motor 46 of the soot blower.

  In addition, we generally show that two soot blowers (top and bottom) are required for complete cleaning, but applying this method requires only the bottom soot blower. It was. Using only the bottom soot blower saves considerable cleaning time and reduces wear on the basket material at the hot end. In this way, it has been shown that the number of daily cleanings is reduced from 4 times per day to once or twice per day.

  The effect of this method is shown in the following example. North Carolina power plant air preheaters were fouled with ammonium bisulfate deposits to the point that the steam medium blower was unable to maintain an acceptable pressure drop (ΔP) through the rotor. The boiler had to be taken out of service every few months to clean the air preheater. The air preheater had a hot end (top portion) having a thickness of 73.66 cm (29 inches), while a cold end (bottom portion) having a thickness of 104.14 cm (41 inches). There is a 25.4 cm (10 inch) spacing above and below these parts where the soot blower works.

  The power plant has two air preheaters of the same size and structure that operate under substantially the same conditions. One preheater was cleaned using the method of the present invention and the other preheater was not cleaned. The cleaned air preheater rotor has been improved so that the rotational speed of the rotor can be changed as directed by the controller. The inventors have programmed the controller to adjust the rotor speed according to the method of the present invention. The soot blower was positioned toward the passage or a small portion of the innermost ring of the rotor and was operated to blow steam through the passage as the passage moved over the soot blower. At this time, each rotor was rotating at a normal operation speed. The soot blower was then moved stepwise toward a small portion or passage of the outer ring. As the soot blower reaches each next depth, the rotor speed is adjusted so that the angular velocity of the small portion is the same as the angular velocity of the small portion at the innermost depth at the start of the process. It was dropped. After all depth was cleaned by the soot blower, the rotor speed was returned to the normal operating speed at which the process started. Throughout the process, the power generator continued to operate normally. The rotor speed was 1.5 RPM when the process started and 0.33 RPM when the final ring was cleaned. The cleaning process took 3-4 hours.

  During the test period, the pressure drop through the rotor of the preheater being cleaned (shown as 2B) was measured by the flue gas side sensor and by the combustion air side sensor of the rotor. The sensor was similarly placed on the uncleaned rotor shown as 2A. FIG. 3 represents the pressure drop (ΔP) across the air and gas side air preheater before, during, and after cleaning using a medium of high pressure water. Stable operation with a partially dirty air preheater is shown on February 22, 2008 on the left side of the figure. The air preheater was cleaned twice using this method in operation during the period starting on February 22, 2008 and ending on February 24, 2008. During cleaning, the boiler power was reduced, which reduced the air flow through the preheater.

  FIG. 3 is a graph showing the pressure drop across the combustion air and flue gas sides of the air preheater and the total air flow through the preheater for the period from February 22 to 24, 2008. The legend at the bottom left shows the position of each sensor. 2A shows an air preheater that has not been cleaned. 2B shows the cleaned preheater. The pressure drop on the input air side of the preheater is shown as SH PHTR AIR DP. The pressure drop on the flue gas side of the preheater is shown as SAH GAS SIDE DP. During the cleaning period, the boiler load was reduced, thereby reducing the total air flow through the preheater. Thus, the total air flow is also shown in FIG. The actual pressure drop at 5:54 am on February 2, 2008 at each of the four sensor locations is shown in the upper left box of FIG. The upper right box in FIG. 3 shows the pressure drop values at these same locations at 10:46 pm on February 24, 2008. The important information in the graph is not the peak and valley of the curve corresponding to boiler load and air flow, but the difference in pressure drop between the cleaned preheater and the uncleaned preheater. Cleaning reduced the pressure drop from 10.41 cm (4.1 water inches) to 8.38 cm (3.3 inches), but the pressure drop of the uncleaned air preheater was 9. It remained between 14 cm (3.6 inches of water) to 9.65 cm (3.8 inches of water). Similarly, effective cleaning using the present invention reduced the ΔP on the gas side from 25.15 cm (9.9 inches of water) to 20.32 cm (8.0 inches) of water. The uncleaned air preheater maintained a ΔP between 22.86 cm (9.0 inches of water) to 24.38 cm (9.6 inches of water).

The benefits generated as a result of practicing the present invention over the two years that the method of the present invention was tested on the referenced generators were as follows: 36 scheduled 36 hours twice a year for cleaning the air preheater. Including loss of suspension. According to the inspection, no damage was found on the end face on the high temperature side. Since withstand more ammonia slip, ammonia reagent is increased to reduce NO _X emissions. The air preheater outlet gas temperature was lowered by minimizing the need for bypass, thereby improving boiler efficiency and reducing CO ₂ emissions.

  Although the method of the present invention was tested using a preheater of the type shown in FIG. 1 in which the gas flows in the longitudinal direction, the method can also be used for a lateral flow preheater. Such a preheater is shown in FIG. In this type of preheater 50, the rotor 54 rotates on a shaft 52 in a longitudinal plane, while flue gas and intake air flows laterally through transition ducts 55, 56, 57 and 58. The rotor is divided into small portions 62 by a diaphragm plate 64, and a heat exchange basket 66 is stacked in these small portions 62. Soot blowers 61 and 63 are provided on the flue gas side of the preheater. The rotor 54 and the soot blowers 61 and / or 63 are operated so that all passages pass through the soot blower at the same or substantially the same angular velocity.

  During cleaning, it is preferred that all the passages move at the same tangential speed. However, in some systems this can be difficult to achieve. The operator will find it easier to clean two or more adjacent depths without changing the rotor speed. It is possible to do this and still achieve the benefits of our cleaning method. Therefore, tangential speed variations of up to 15 percent are acceptable. If there is such variation, it can be considered that all the passages are moving at substantially the same angular velocity.

  Although it is preferred to clean the entire depth of the rotor passage, the method of the present invention does not require the cleaning of all small portions. In some installations, some small parts are cleaned in one pass of the soot blower, but it may be sufficient not to clean all the small parts. Thereafter, in another pass or at another time, other small portions or passages can be cleaned. In fact, there may be several preheaters where a particular passage is little or not cleaned.

  While the preferred embodiment of the present invention of our method for cleaning the air preheater in operation has been described and illustrated, our invention is not limited thereto and is described below. Various modifications can be made within the scope of the claims.

Claims (7)

A method of cleaning an air preheater, wherein the air preheater includes air and flue gas and includes adjacent passages at selected depths from the outer periphery of the rotor that rotate about an axis; and Soot that can be placed adjacent to any selected depth so that as the rotor rotates, soot blowing media can be blown from the soot blowing device into the passage of the selected ring. In the air preheater cleaning method, which is a type of air preheater comprising a blowing device,
Placing the soot blower adjacent to the passage at the first depth;
Rotating the rotor at a first selected speed such that each passage at the first depth passes through the soot blower;
Spraying soot blowing media through the passage at the first depth while rotating the rotor at the first selected speed;
Placing the soot blower adjacent to the passage at the second depth;
Rotating the rotor at a second selected speed such that each passage at the second depth passes through the soot blower;
Spraying soot blowing media through the passage at the second depth while rotating the rotor at the second selected speed;
Selecting the first speed and the second speed such that all passages pass through the soot blower at substantially the same tangential speed. The rotor includes at least one additional set of passages at another selected depth;
Sequentially positioning the soot blower adjacent to the passage at another selected depth;
Rotating the rotor at a speed selected for each additional passage set such that each passage of the additional passage set passes through the soot blower;
Blowing soot blowing media through the passages of the additional passage set while rotating the rotor at the selected speed for the set of additional passages;
The method for cleaning an air preheater according to claim 1, further comprising: selecting all speeds such that all of the passages pass through the soot blower at substantially the same tangential speed.   The method for cleaning an air preheater according to claim 1, wherein the soot blowing medium is air, water, or steam.   The method of cleaning an air preheater according to claim 1, wherein some, but not all, passages in the rotor are blown with soot blowing media.   The method for cleaning an air preheater according to claim 1, wherein the soot blowing device first blows the soot blowing medium through the innermost depth of the passage.   The method for cleaning an air preheater according to claim 1, wherein the soot blowing device first blows the soot blowing body through the outermost depth of the passage. The passages at two adjacent depths are sprayed with soot blowing media,
The method for cleaning an air preheater according to claim 1, wherein the rotor is rotated at the same number of revolutions per minute while spraying all the passages.

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