JPH041255B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for monitoring and controlling ash deposits in the convection section of a steam generator.

[Conventional technology] In operating a boiler that burns pulverized coal,
Most of the ash contained in the coal is deposited on the water walls of the combustion chamber and on the heat exchange tubes of the convection section of the boiler. This ash deposit has low thermal conductivity and changes the radiant properties of the surface, insulating the pipe from fire and combustion gases. As a result of these, effective gas-to-tube heat transfer is inhibited both to the furnace walls as well as to the tubes of the convection section.

Our U.S. Patent No. 4,408,568 involves simultaneously measuring the actual heat flux present in the boiler and the heat flux reaching the walls of the boiler, and calculating the difference between these heat flux values by calculating the amount of ash deposited on the inner walls. A method is described for monitoring the deposition of ash on the inner walls of a boiler due to pulverized coal by determining it as a quantity measure. A signal indicating the degree of fouling of the furnace may be used by the furnace operator as a decision to initiate soot blower operation and/or as a decision to initiate other furnace control actions; used for automatic start of boiler control.

In the convection section of the steam generator, heat is transferred by convection and conduction from the combustion gas stream into the tube wall through which the gas stream contacts the outside and through which the steam flows. Typically, an array of heat exchange tubes is provided which are contacted in sequence by a flowing gas stream. The function of the convection section is to superheat the pressure steam before sending it to the steam-driven turbine, which typically generates power, and to heat the low-pressure steam returned from the high-pressure side of the turbine to the low-pressure The purpose is to reheat it before recycling it to the side.

[Problem to be Solved by the Invention] As mentioned above, ash deposits can also occur on the tubes of the convection section of the boiler. Currently, there is no direct means available to assess the amount of ash deposited on a convection section or the extent to which that deposit reduces the ability of a heat exchange surface to transfer heat from the gas phase to vapor. do not have.

Furthermore, the ash adheres unevenly. Across a particular horizontal plane of the tube array, more fouling may occur on one side or corner than on the other side or corner, resulting in an imbalance in the gas flow commonly referred to as channeling. A distribution occurs. With current methods of operating steam generators, there is no way to determine the extent of the fouling imbalance.

To determine when to operate the soot blower to remove ash deposits from the convection tubes, operators rely on several indirect signals and occasional visual inspection. I have no choice but to. This lack of more direct information can lead to inefficiency, reverse control, and unstable operation, sometimes resulting in severe contamination that forces the operation to stop. There is. Additionally, soot blowers may be used unnecessarily or excessively when the convection pipes are actually clean. Unnecessary soot blowing can be harmful and expensive since the action of soot blowing corrodes the heat exchange tubes.

There are diagnostic systems available on the market, but these are based on measuring conditions at the boiler outlet. Because these systems can only measure dirt indirectly and have long response times, their signals are generally inadequate for satisfactory control.

Therefore, there is a need to provide a method for directly measuring ash deposits in the convection section of a steam generator so that boiler operation is improved.

[Means for Solving the Problems] Therefore, according to the present invention, a heat exchanger in a convection section of a steam generator where a flowing hot gas stream comes into contact with a heat exchanger surface to heat the steam flowing within the heat exchanger surface. Provided is a method for controlling ash deposition on a surface, the method comprising: a surface in a flowing gas stream having a selected number of heat exchange surfaces spaced apart in the flow direction of the gas stream; directly measure the temperature of the gas stream at two locations, calculate the temperature difference between these two locations as a measure of the amount of ash deposit on a selected number of heat exchange surfaces, and calculate the predetermined amount of ash deposit. activating cleaning of a selected number of heat exchange surfaces in response to established criteria.

Therefore, in the present invention, the temperature of a hot gas stream flowing over a heat exchange surface that recovers heat from the gas stream is directly measured at two longitudinally spaced locations in the flowing gas stream. . The temperature difference between these two locations is determined by these direct measurements, and this temperature difference is used as a measure of the amount of ash deposited on the heat exchange surface between the two locations.

The amount of ash deposit, i.e. the degree of fouling, is used to determine the fouling factor, which in turn is
It is used to clean the heat exchange surfaces in response to a predetermined value of the fouling factor, thereby controlling the buildup of ash on the heat exchange surfaces.

Direct measurements of temperature can be carried out in any conventional manner, preferably by means of a radiant heat pyrometer, although clean heat flux meters observing through an aperture in a wall confining the hot gas flow may also be used.

In one embodiment of the invention, the amount of ash deposited in the convection section of the steam generator is determined by using a combination of a radiant heat pyrometer or suitable alternative and a heat flux meter or suitable alternative. A method for determining the distribution status is provided. A radiant heat pyrometer or suitable alternative measures the gas temperature difference across the row of heat exchange tubes in the convection section, while a heat flux meter or suitable alternative measures the difference in gas temperature caused by uneven ash deposition. Monitor channeling.

A radiant heat pyrometer is focused on the gas space between the rows of tubes. The pyrometer readings are corrected for changes in the radioactivity of the gas stream caused by changes in the concentration of water vapor, carbon dioxide, and coal ash particles in the gas stream. This correction is conveniently calculated continuously and on-line using a dedicated minicomputer or microcomputer that monitors the pyrometer readings as well as the coal and air throughput rates and ash content. 2 placed across one tube row
By using two pyrometers, the row measures the decrease in gas temperature. The steam flow rate and its temperature drop across the same tube bank, which are commonly measured in modern steam generator operation, are also sent to the computer.
Using the above information along with the gas temperature drop determined by the pyrometer, the computer can:
As will be described in detail later, the contamination coefficient R _F is continuously calculated. This fouling factor is uniquely proportional to the degree of fouling of the tube row and its value is displayed numerically or visually, for example in the form of a color-coded diagram, on a monitor screen. This value can also be recorded on any convenient medium.

As previously discussed, uneven fouling causes the flow of combustion gases to form channels within the row of tubes. In a preferred embodiment of the invention, heat flux meters are placed at strategic locations on the water tube wall surrounding the convection section. In some cases, contamination of this area can be a problem, and a clean heat flux meter viewed through an aperture is a suitable alternative to a heat flux meter placed on the water tube wall. be able to. In other cases, it is advantageous to use thermocouples that protrude into the gas flow.

When the heat flux monitored by one of these meters is significantly lower and/or higher than the average for that row's particular level, this indicates that the tube row is unevenly fouled, and this information is also displayed numerically or visually on a monitor screen and, if desired, recorded on any convenient medium.

Steam generator operators can use the fouling coefficient and gas channeling information to soot to remove ash deposits from selected convection section rows for optimal operating results and minimal erosion of the tubes. Determine periodic and selective operation of blowers. Alternatively, to automatically activate the soot blower when a specific value R _F is recorded for a specific row of heat exchange tubes,
Signals can be used. The channeling signal can be used to suppress commands or selectively activate the soot blower, thereby eliminating unnecessary blowing action and resulting tube erosion.
Steam loss can be prevented.

[Examples] Hereinafter, the present invention will be described by way of examples with reference to the accompanying drawings.

Referring to the drawings, FIG. 1 schematically depicts a coal-fired boiler 10. As shown in FIG. Pulverized coal and air are fed through burner 12 to combustion chamber 14 of boiler 10 . As is well known, the furnace wall 16 consists of a plurality of parallel tubes in which steam is generated and sent to a steam collection system (not shown).

The combustion gases rise to the convection section 18 of the boiler 10. This convection section 18 consists of rows 20, 2 of heat exchange tubes.
2, through which the steam is sent for superheating and reheating in a known manner. The combustion gases then pass through an economizer 24 and an air heater (not shown) and are exhausted to the atmosphere via line 26.

While the boiler 10 is operating, ash and slag are deposited on the furnace wall 1.
6 as well as the heat exchanger rows 20, 22 and stick to the tubes, reducing heat absorption through the surfaces of these tubes and creating other operational difficulties. Soot blowers (not shown in Figure 1) are located throughout the boiler 10 to remove deposits from the heat exchanger tube surfaces by directing jets of steam at the deposits. It operates.

As previously mentioned, in U.S. Pat. No. 4,408,568, a plurality of heat flux meters are placed within the furnace wall 16 directly toward the flame to monitor the buildup of ash and other deposits on the furnace wall 16. A method is shown.

The present invention relates to the monitoring of deposits on heat exchanger tubes forming rows 20, 22, 24, and for reference of specific parts thereof, a schematic elevational and plan view of row 20 of heat exchanger tubes is provided. Figures 2 and 3 shown respectively are used.

The configuration of the individual rows 28 of heat exchanger tubes is quite conventional and consists of a series of tubes that carry steam and transfer heat from the flowing gas stream 30 through the tube walls to heat the flowing steam. . A horizontally arranged retractable soot blower 32 is associated with each row 28 to remove deposits of deposits from the tube surface.

In accordance with the present invention, radiant heat pyrometers 34 are positioned at both the upper and lower ends of the rows 20 of heat exchange tubes and between vertically adjacent sets. It is also possible to provide a pair of pyrometers 34 for every row of tubes 20 or 22 (or indeed for an economizer 24) in the convection section, or, depending on the requirements of local conditions, a pair of pyrometers 34. A pair of pyrometers 34 may be provided for one unit row 28 or selected plurality of unit rows of tubes. Each pyrometer 34
measures the temperature of the gas stream 30 at that location. The pyrometer 34 is typically focused on the gas flow at the longitudinal centerline of rows 20 or 22.

These pyrometers 34 are of the type that are sensitive to the wavelength range in which carbon dioxide and water absorb and emit radiation. In order to convert the pyrometer signal into a true temperature measurement, a correction for the inherent radiation of the gas space is required. Radioactivity is influenced by percent moisture, percent carbon dioxide, percent ash in the coal, total air flow rate, and gas temperature. The correction is performed by calculation from the gas phase components.
Alternatively, pyrometer 34 may be of a type sensitive to wavelength ranges in which carbon dioxide and water do not emit and/or absorb radiation. In this case, the signal typically follows the solid particulates in the gas stream, and the temperature measurements are typically corrected using data for total air flow, percent ash in the coal, and coal feed rate. Calculations for correction are usually performed online by a dedicated computer. If desired, either type of pyrometer can be used depending on the particular case, such as a pyrometer that is not sensitive to particular wavelengths but measures total radiation. Additionally, in certain other cases, U.S. Pat.
A clean heat flux meter, such as one of the types shown in No. 4408568, can also be used. In this case as well, appropriate corrections are made to the signal.

The pyrometer 34 measures the longitudinal temperature drop across the row 28 of heat exchange tubes along the approximate centerline of the row 28. As the individual tubes in row 28 become dirty, there is less heat transfer across the tube surface to heat the steam, resulting in a smaller temperature drop between each pair of pyrometers 34. The temperature difference measured there is sent to an on-line computer, which is also supplied with data for steam temperature and flow measurements and correction of pyrometer readings, as described above.

The relationship that exists in a row of heat exchangers is given by the following equation (1):

Q=UA△T _1n (1) In the above equation, Q is the amount of heat absorbed by the steam, determined from the measured values of temperature and flow rate on the steam side of the pipe, and △T _1n is the amount of heat absorbed by the steam in the steam line. is the logarithmic mean temperature drop across the column as measured by a radiant heat pyrometer and thermocouple at , where A is the surface area of the tube;
U is the effective heat transfer coefficient of the tube, part of which is affected by contamination of the tube.

The fouling factor R _F can be determined from equation (2).

1/U=1/(hc) _f +1/(hc) _s +L/K+R _F (2) In the above equation, U is the effective heat transfer coefficient determined from equation (1), R _F is the fouling coefficient, (hc) _f is the convective heat transfer coefficient on the gas flow side of the tube, (hc) _s is the convective heat transfer coefficient on the steam side of the tube, and L and K are the thickness and thermal conductivity of the tube in the convection section, respectively.

The calculation to be done using equations (1) and (2) is
This is most effectively done by a computer programmed to accept the measured temperature and flow rate and calculate U, which in turn calculates R _F. The fouling factor R _F can be given to the operator as a numerical value or displayed on the monitor screen as part of a graphical representation of the convection section fouling, to assist the operator in controlling the combustion process. Colors are used to indicate the degree of contamination.

When the contamination reaches a predetermined standard for any particular row 28, either as a result of operator intervention or by automatic action by a computer, deposits are removed from the surface of the heat exchanger tubes in that row. The soot blower 32 for that row is activated to remove the deposits.

In a preferred embodiment of the invention, a soot blower 32 is provided in response to channel ring measurements.
Measures are taken to suppress the operation of the As previously mentioned, gas channeling occurs in the rows 28 of heat exchange tubes as a result of the different degrees of contamination in the horizontal planes. A heat flux meter 36 is arranged on the water tube wall 38 of the convection section 18 in the horizontal plane. Four or more horizontally arranged sets of such heat flux meters 36 can be arranged at vertically spaced locations within the rows 20, 22.

Each of these heat flux meters 36 measures the heat flux reaching that meter. Heat flux meter 36 may be of any convenient construction.
These heat flux meters are of the type that are fixed to the tube wall and get dirty. Or, if the walls are dirty,
It is also possible to use a clean heat flux meter of the type described above, which is viewed through an opening in the wall. In yet other specific cases, it may also be advantageous to use thermocouples whose protective walls protrude into the gas stream.

The heat flux reaching each meter 36 is measured by gas flow 30 through a particular row of heat exchange tubes 28. When there is no channeling, the heat flux reaching each heat flux meter 36 is virtually the same. However, if fouling is likely to occur in a portion of the horizontal end of the heat exchange tube bank 28, the gas flow will flow to the remainder of the tube bank 28 and will be greater than the portion prone to fouling. Under these circumstances, the heat flux reaching the heat flux meter 36 is different. Radiant heat pyrometer 34 does not necessarily detect these changes. This is because the temperature measurements thereby made relate to the gas flow through approximately the central portion of the tube array 28. Therefore, the heat flux meter 36
is used to monitor the degree of channeling, and the heat flux measurements made thereby can be used to manually or manually clean selected soot blowers to selectively clean heat exchange tubes in areas prone to fouling. It is conveniently used to operate in an automatic computer controlled manner. Such selected soot blower operation prevents all soot blowers from operating in response to fouling conditions detected by radiant heat pyrometer 34. Only those areas that require cleaning will be actually exposed to the soot blast. In this way, pipe corrosion and significant cost and operational problems are minimized and steam savings are maximized.

Boiler 1 due to solid deposits from gas stream 30
The contamination of the convection section 18 of 0 is thus monitored by the radiant heat pyrometer 34 and the heat flux meter 36. The measurements made by these instruments are processed to generate information to the operator regarding the condition of the convective section, or are responsive to a predetermined combination of conditions indicated by those measurements. It is used to automatically clean pipes using a soot blower under computer control.

Direct measurement of gas stream temperature is made using a radiant heat pyrometer, and this measurement is used to accurately and instantaneously measure the buildup of ash and other solids in the convection section. By compensating for the radiation properties of the gas flow and by taking into account the effects of channeling, the boiler operator first
Provided with information that allows accurate operation of the boiler. Alternatively, precise automatic cleaning of the convective section can be performed using computer control based on the collected data. In this way, the problems of the prior art regarding fouling of the convection section and corrosion of the steam pipes of the steam generator are overcome.

In summary, the present invention provides a method for monitoring and controlling ash in a convection section such that fouling of heat exchange tubes can be accurately monitored and controlled. Variations are possible within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a typical coal-fired steam generator to which the present invention is applied, and FIG. 2 is an array of heat exchanger tubes in the convection section designed to apply the preferred embodiment of the present invention. FIG. 3 is a plan view of the row of heat exchange tubes in the convection section of FIG. [Code in the figure] 10...Pulverized coal combustion boiler,
12... Burner, 14... Combustion chamber, 16... Furnace wall, 18... Convection section, 20, 22... Row of heat exchange tubes, 24... Economizer, 26... Line, 28... Row, 30... Gas flow, 32... Soot blower, 34... Radiant heat pyrometer, 36... Heat flux meter.

Claims (1)

[Scope of Claims] 1. Controlling the accumulation of ash on a heat exchange surface of a convection section of a steam generator in which a flowing hot gas stream contacts a heat exchange surface to heat steam flowing within the heat exchange surface. a method in which the temperature of a gas stream is continuously and directly measured at two locations in the gas stream spaced apart in the direction of gas flow between which a selected number of heat exchange surfaces are arranged. adjusting the measured temperature to compensate for the radiation of the flowing gas stream; calculating the ash deposit as a fouling factor R _F from said calculated temperature difference; and steam flowing through the selected number of heat exchange surfaces. successively determining the flow rate of the selected number of heat exchange surfaces and the temperature change of the steam across the heat exchange surfaces; and activating cleaning, and the contamination coefficient R _F is determined by the following equation R _F =1/U-1/(hc) _f -1/(hc) _s -L/K where (hc) _f is the convective heat transfer coefficient on the gas flow side of the heat exchange surface, (hc) _s is the convective heat transfer coefficient on the steam side of the heat exchange surface, L and K are the thickness and thermal conductivity of the heat exchange surface, respectively, and U is The heat transfer coefficient is calculated from the following equation: U=Q/A△T _1n , where Q is the heat absorbed by the steam,
determined from measurements of temperature and flow rate on the steam side of the heat exchange surfaces, ΔT _1n is the logarithmic average temperature drop across said selected number of heat exchange surfaces, A determined from temperature measurements of the gas stream; is determined by substituting the measured value for the area of a heat exchange surface. 2. The fouling factor is displayed on a monitor screen for use by a steam generator operator in controlling the steam generation process and/or for cleaning dirty heat exchange surfaces. Methods of controlling ash accumulation as described in Section 1. 3. A method for controlling ash accumulation as claimed in claim 2, wherein said indication represents a convective section illustrating a dirty area. 4. The method of claim 1, wherein the fouling factor is used to automatically clean dirty heat exchange surfaces. 5 determining the heat flux reaching a plurality of circumferential locations of the heat exchange surface, comparing these individual determinations with an average of the determinations, and automatically determining the Claims 1 to 4, wherein the heat exchange surface is actuated to selectively clean only a portion of the heat exchange surface in response to a detected imbalance in the gas flow. A method for controlling ash accumulation according to any one of the preceding items.