Controlling cooling flow in a sootblower based on lance tube temperature

A cleaning system and method for cleaning heat transfer surfaces in a boiler using a temperature measuring system for measuring and monitoring wall temperature of an annular wall of the tube of a lance of one or more sootblowers. Controlling a flow of steam or other fluid through the tube during the cooling portions of the strokes based on wall temperature measurements from the temperature measuring system. Infrared or thermocouple temperature measuring systems may be used. The steam or other fluid may be flowed at a default flowrate that may be substantially zero until the temperature measuring system indicates the wall temperature of the annular wall begins to exceed a predetermined temperature limit which may be the softening point of the annular wall. Then the steam or other fluid is flowed at a rate greater than the default flowrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to boilers and sootblowers and, in particular, to methods and apparatus for removing ash deposits on heat exchangers of the boilers and for minimizing a flowrate of steam or other cleaning fluid through the sootblowers when not actively cleaning the ash deposit.

2. Description of Related Art

In the paper-making process, chemical pulping yields, as a by-product, black liquor which contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a boiler. The two main functions of the boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. As used herein, the term boiler includes a top supported boiler that, as described below, burns a fuel which fouls heat transfer surfaces.

A Kraft boiler includes superheaters in an upper furnace that extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section and superheated steam exits at a controlled temperature. The superheaters are constructed of an array of platens that are constructed of tubes for conducting and transferring heat. Superheater heat transfer surfaces are continually being fouled by ash that is being carried out of the furnace chamber. The amount of black liquor that can be burned in a Kraft boiler is often limited by the rate and extent of fouling on the surfaces of the superheater. The fouling, including ash deposited on the superheater surfaces, reduces the heat absorbed from the liquor combustion, resulting in reduced exit steam temperatures from the superheaters and high gas temperatures entering the boiler bank.

Boiler shutdown for cleaning is required when either the exit steam temperature is too low for use in downstream equipment or the temperature entering the boiler bank exceeds the melting temperature of the deposits, resulting in gas side pluggage of the boiler bank. In addition, eventually fouling causes plugging and, in order to remove the plugging, the burning process in the boiler has to be stopped. Kraft boilers are particularly prone to the problem of superheater fouling. Three conventional methods of removing ash deposits from the superheaters in Kraft boilers include:

1) sootblowing, 2) chill-and-blow, and 3) waterwashing. This application addresses only the first of these methods, sootblowing.

Sootblowing is a process that includes blowing deposited ashes off the superheater (or other heat transfer surface that is plagued with ash deposits, with a blast of steam from nozzles of a lance of a sootblower. A sootblower lance has a lance tube for conducting the steam to a nozzle at a distal end of the lance. Sootblowing is performed essentially continuously during normal boiler operation, with different sootblowers turned on at different times. Sootblowing is usually carried out using steam. The steam consumption of an individual sootblower is typically 4-5 kg/s; as many as 4 sootblowers are used simultaneously. Typical sootblower usage is about 3-7% of the steam production of the entire boiler. The sootblowing procedure thus consumes a large amount of thermal energy produced by the boiler.

The sootblowing process may be part of a procedure known as sequence sootblowing, wherein sootblowers operate at determined intervals in an order determined by a certain predetermined list. The sootblowing procedure runs at its own pace according to the list, irrespective of whether sootblowing is needed or not. Often, this leads to plugging that cannot necessarily be prevented even if the sootblowing procedure consumes a high amount of steam. Each sootblowing operation reduces a portion of the nearby ash deposit but the ash deposit nevertheless continues to build up over time. As the deposit grows, sootblowing becomes gradually less effective and results in impairment of the heat transfer. When the ash deposit reaches a certain threshold where boiler efficiency is significantly reduced and sootblowing is insufficiently effective, deposits may need to be removed by another cleaning process.

A steam sootblower, typically, includes a lance having an elongated tube with a nozzle at a distal end of the tube and the nozzle has one or more radial openings. The tube is coupled to a source of pressurized steam. The sootblowers are further structured to be inserted and extracted into the furnace or moved between a first position located outside of the furnace, to a second location within the furnace. As the sootblowers move between the first and second positions, the sootblower rotates and adjacent to the heat transfer surfaces. Sootblowers are arranged to move generally perpendicular to the heat transfer surfaces.

Some of the platens having heat transfer surfaces have passages therethrough to allow movement perpendicular to the heat transfer surfaces. The movement into the furnace, which is typically the movement between the first and second positions, may be identified as a “first stroke” or insertion, and the movement out of the furnace, which is typically the movement between the second position and the first position, may be identified as the “second stroke” or extraction. Generally, sootblowing methods use the full motion of the sootblower between the first position and the second position; however, a partial motion may also be considered a first or second stroke.

As the sootblower moves adjacent to the heat transfer surfaces, the steam is expelled through the openings in the nozzle. The steam contacts the ash deposits on the heat transfer surfaces and dislodges a quantity of ash, some ash, however, remains. As used herein, the term “removed ash” shall refer to the ash deposit that is removed by the sootblowing procedure and “residual ash” shall refer to the ash that remains on a heat transfer surface after the sootblowing procedure. The steam is usually applied during both the first and second strokes.

Rather than simply running the sootblowers on a schedule, it may be desirable to actuate the sootblowers when the ash buildup reaches a predetermined level. One method of determining the amount of buildup of ash on the heat transfer surfaces within the furnace is to measure the weight of the heat transfer surfaces and associated superheater components. One method of determining the weight of the deposits is disclosed in U.S. Pat. No. 6,323,442 and another method is disclosed in U.S. patent application Ser. No. 10/950,707, filed Sep. 27, 2004, both of which are incorporated herein by reference. It is further desirable to conserve energy by having the sootblowers use a minimum amount of steam when cleaning the heat transfer surfaces.

BRIEF SUMMARY OF THE INVENTION

A cleaning system for cleaning heat transfer surfaces of one or more heat exchangers in a boiler includes one or more sootblowers, each of which includes a lance with an elongated hollow tube and two nozzles at a distal end of the tube. A temperature measuring system is used for measuring and monitoring wall temperature of an annular wall of the tube during operation of the one or more sootblowers.

An exemplary embodiment of the cleaning system includes that each of the sootblowers is operable for moving the lance in and out of the boiler in insertion and extraction strokes and a control system is used for controlling a flow of steam or other cleaning fluid through the tube and nozzle during cleaning portions and cooling portions of the strokes. The control means is further operable for controlling the flow of steam during the cooling portions of the strokes based on wall temperature measurements from the temperature measuring system. The control means is further operable for controlling the flow of steam during the cooling portions of the strokes to prevent the wall temperature measurements from exceeding a predetermined temperature limit which may be a softening point or slightly less than the softening point of the tube.

The temperature measuring system may be an infrared temperature measuring system for measuring the wall temperature of the annular wall outside the boiler. The temperature measuring system may be a thermocouple temperature measuring system having thermocouples attached to the annular wall for measuring the wall temperature of the annular wall inside the boiler. The thermocouples may be partially disposed from an inside surface of the annular wall in holes through and along a length of the annular wall.

The method of operating the cleaning system may include flowing the steam or the other hot cleaning fluid through the tube and nozzle during the cooling portions of the strokes at a flowrate equal to a default value unless the wall temperature exceeds or is about to exceed the predetermined temperature limit based on temperature measurements from the temperature measuring system and, then, increasing the flowrate above the default value. The default value may be substantially zero.

DETAILED DESCRIPTION OF THE INVENTION

Diagrammatically illustrated inFIG. 1is an exemplary embodiment of a Kraft black liquor boiler system10having a sootblower system3with one or more sootblowers84. A Kraft black liquor boiler system10having a plurality of sootblowers84is disclosed and described in U.S. patent application Ser. No. 10/950,707, filed Sep. 27, 2004, entitled “Method of Determining Individual Sootblower Effectiveness” which is incorporated herein by reference. A control system300which operates the sootblower84in part based on a measured temperature of an annular wall93of a tube86of a lance91of the sootblower. The sootblower84typically rotates the lance91during operation. The annular wall's93temperature is measured and/or monitored with a temperature measuring system9illustrated inFIG. 1as an infrared temperature measuring system11as illustrated in more detail inFIGS. 3 and 4. Other types of temperature measuring systems may be used such as a thermocouple temperature measuring system13as illustrated inFIGS. 5 and 6.

Black liquor is a by-product of chemical pulping in the paper-making process and which is burned in the boiler system10. The black liquor is concentrated to firing conditions in an evaporator12and then burned in a boiler14. The black liquor is burned in a furnace16of the boiler14. A bullnose20is disposed between a convective heat transfer section18in the boiler14and the furnace16. Combustion converts the black liquor's organic material into gaseous products in a series of processes involving drying, devolatilizing (pyrolyzing, molecular cracking), and char burning/gasification. Some of the liquid organics are burned to a solid carbon particulate called char. Burning of the char occurs largely on a char bed22which covers the floor of the furnace16, though some char burns in flight. As carbon in the char is gasified or burned, the inorganic compounds in the char are released and form a molten salt mixture called smelt, which flows to the bottom of the char bed22, and is continuously tapped from the furnace16through smelt spouts24. Exhaust gases are filtered through an electrostatic precipitator26, and exit through a stack28.

Vertical walls30of the furnace16are lined with vertically aligned wall tubes32, through which water is evaporated from the heat of the furnace16. The furnace16has primary level air ports34, secondary level air ports36, and tertiary level air ports38for introducing air for combustion at three different height levels. Black liquor is sprayed into the furnace16out of black liquor guns40. The heat transfer section18contains three sets of tube banks (heat traps) which successively, in stages, heat the feedwater to superheated steam. The tube banks include an economizer50, in which the feedwater is heated to just below its boiling point; a boiler bank52, or “steam generating bank” in which, along with the wall tubes32, the water is evaporated to steam; and a superheater system60, which increases the steam temperature from saturation to the final superheat temperature.

Referring toFIG. 2, the superheater system60illustrated herein has first, second, and third superheaters61,62, and63for a total of three superheaters, however, more or less superheaters may be incorporated as needed. The construction of the three superheaters is the same. Each superheater is an assembly having at least one but typically more, such as 20-50, heat exchangers64. Steam enters the heat exchangers64through a manifold tube called an inlet header65. Steam is superheated within the heat exchangers64and exits the heat exchangers as superheated steam through another manifold tube called an outlet header66. The heat exchangers64are suspended from the headers65,66which are themselves suspended from the overhead beams by hanger rods not illustrated herein.

Platens67of the heat exchanger64have outer surfaces referred to herein as a heat transfer surfaces69which are exposed to the hot interior of the furnace16. Thus, virtually all parts of the heat transfer surfaces are likely to be coated with ash during normal operation of the furnace16. A substantial portion of the heat transfer surfaces are cleaned, that is, have a portion of ash removed, by a cleaning system80. The cleaning system80includes at least one, and preferably a plurality of steam sootblowers84, which are known in the art. The cleaning system80illustrated herein includes steam sootblowers84; however the cleaning system80may also be used with sootblowers using other cleaning fluids. The sootblowers84are arranged to clean the heat exchangers and, more specifically, the heat transfer surfaces. Sootblowers84include elongated hollow tubes86having two nozzles87at distal ends89of the tubes86. The two nozzles87spaced about 180 degrees apart.

The tubes86are in fluid communication with a steam source90. In one embodiment of the cleaning system80, the steam is supplied at a pressure of between about 200 to 400 psi. The steam is expelled through the nozzles87and onto the heat transfer surfaces. The sootblowers84are structured to move the nozzles87at the end of the tubes86inwardly between a first position, typically outside the furnace16, and a second position, adjacent to the heat exchangers64. The inward motion, between the first and second positions, is called an insertion stroke and an outwardly motion, between the second position and the first position, is called an extraction stroke.

A first set81of the sootblowers84are operable to move the nozzles87at the end of the tubes86generally perpendicular to and in between the heat exchangers64. A second set82of the sootblowers84are operable to move the nozzles87at the end of the tubes86generally parallel to and in between the heat exchangers64. A plurality of tubular openings92through the heat exchangers64are provided for allowing the tubes86of the first set81of the sootblowers84to move generally perpendicular through the heat exchangers64. The heat exchangers64are sealed and the tubes86may pass freely through the tubular openings92.

Steam is expelled from the nozzles87as the nozzles87move between the first and second positions. As the steam contacts the ash coated on the heat transfer surfaces, a portion of the ash is removed. Over time, the buildup of residual ash may become too resilient to be removed by the sootblowers84and an alternate ash cleaning method may be used. The sootblowers84described above utilize steam, it is noted however, that the invention is not so limited and the sootblowers may also use other cleaning fluids that for example may include air and water-steam mixtures.

Operation of the cleaning system80is controlled by a control system300which controls the cleaning system80based on the weight of the ash deposits on one or more of the heat exchangers64. The control system300also controls the amount of steam supplied or the steam's flowrate to the tubes86during cleaning portions of the insertion and extraction strokes and during cooling portions of the insertion and extraction strokes. The control system300is programmed to activate the insertion and extraction of the lances91of the sootblowers84, that is, movement between the lance's91first and second position, speed of travel, and the application and/or quantity of steam.

Cleaning steam is typically applied on the insertion stroke of the lances91but may also be applied on the extraction or both strokes. The steam is applied at a cleaning rate to remove the ash and at a cooling rate to prevent the lance91from getting too hot. In conventional Kraft boilers, steam has been applied at a cleaning rate or cleaning flow of between 15,000-20,000 lbs/hr and at a cooling rate or cooling flow of between 5,000-6,000 lbs/hr to ensure that the sootblower lance is operating well below the temperature limit of the material. The steam may be supplied anywhere from substantially zero to one hundred percent of the maximum quantity that the cleaning system is programmed to deliver. The control system300using the measured temperature of the annular wall93, illustrated inFIGS. 3 and 6of the tube86of the lance91from the temperature measuring system9to control and minimize the cooling flow. For a boiler using cleaning flow of between 15,000-20,000 lbs/hr, a cooling flow of between 0 and 2,000 lbs/hr may be achieved using the temperature measuring system9to control and minimize the cooling flow.

The use of steam to clean heat exchangers64is expensive. Therefore, it is desirable to use only the amount of steam needed to remove the ash. Substantially less steam is used during the cooling portions than the cleaning portions of the strokes. Cleaning or cooling amounts of steam may be used during either the insertion or extraction strokes. In one embodiment of the sootblowing method one-way cleaning is used to reduce the sootblowing steam used. One-way cleaning uses full cleaning flow during the insertion stroke into the boiler and only cooling flow during the extraction stroke or on the way out of the boiler. During the cooling portions of the stroke, steam is used only to keep the lances91of the sootblowers84cool. The temperature measuring system9is used to measure or monitor the temperature of the lance's tube86and minimize the amount of steam used during the cooling portions of the stokes.

The cleaning system80uses the temperature measuring system9to continuously measure or monitor the temperature of a sootblower lance tube86while it is operating in the boiler14. The control system varies the cooling flow within the lance91(using a variable flow control valve not shown) to prevent the wall temperature of the annular wall93of the tube86of the lance91from exceeding a predetermined temperature limit. In one exemplary method of cleaning system80, the amount of steam supplied or the steam's flowrate to the tubes86during the cooling portions of the strokes is set to a default value which may be substantially zero and is increased if the control system300determines that the wall temperature exceeds or is about to exceed the predetermined temperature limit based on temperature measurements from the temperature measuring system9.

In one exemplary method of using the temperature measuring system9, steam is supplied at a flowrate that is as low as possible without the temperature of the tube86rising above its softening point or temperature. Thus, the maximum allowable temperature of the tube86is its softening temperature. The flowrate of steam is minimized without allowing the lance's tube temperature to exceed its softening point based on direct temperature measurements of the tube86.

Two types of temperature measuring systems9are illustrated herein. An infrared temperature measuring system11is illustrated inFIGS. 1 and 3. In the embodiment of the infrared temperature measuring system11illustrated herein an infrared sensor110is located outside and adjacent to the boiler14and, is thus, operable for measuring the wall temperature of the annular wall93of the lance tube86as it is extracted and inserted into the boiler14. Though the infrared sensor110is located outside the boiler14, it gives an accurate reading of the wall temperature because of the large thermal mass of the annular wall93and the rapid extraction of the lance from the furnace. These two factors result in the temperature being measured at this location to be essentially the same temperature of the lance immediately before it exits the boiler14.

Other types of temperature measuring systems may be used. One such system is a thermocouple temperature measuring system13as illustrated inFIGS. 5 and 6. One or more thermocouples114are attached to the annular wall93of the lance tube86to measure the wall temperature of the annular wall93inside the boiler14. As illustrated herein, a number of the thermocouples114are partially disposed from an inside surface130of the annular wall93in tight fitting holes116through and along a length L of the annular wall93. Plugs124are disposed in the holes116between an outer surface128of the annular wall93and the thermocouples114disposed in the holes116. The thermocouples114are welded, indicated by weld126to an inside surface130of the annular wall93. The thermocouples114are connected to a transmitter (not shown) mounted on an outside of the lance91on an outside portion of the lance91that does not enter the boiler14. The transmitter transmits temperature readings of the thermocouples to the control system300which operates the sootblower84.

While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.