WO2005038420A1

WO2005038420A1 - A method for monitoring a fluidized bed

Info

Publication number: WO2005038420A1
Application number: PCT/FI2004/050149
Authority: WO
Inventors: Jaani Silvennoinen; Eino Niemi; Jouni Kinni
Original assignee: Kvaerner Power Oy
Priority date: 2003-10-14
Filing date: 2004-10-13
Publication date: 2005-04-28
Also published as: FI20035182A0; FI117451B; FI20035182A

Abstract

A method for monitoring the condition of the fluidized bed in a fluidized bed boiler, for monitoring the content of coarse material in the fluidized bed (6). In the method, e.g. the temperature (T2) of the upper part of the fluidized bed (6) as well as the temperature (T1) of the lower part of the fluidized bed (6) are determined, and the change in the difference between said temperatures (T1, T2) is monitored. Furthermore, the invention relates to a system for monitoring the condition of a fluidized bed (6) in a fluidized bed boiler, as well as a fluidized bed boiler comprising a system for monitoring the condition of a fluidized bed.

Description

A ETHOD FOR MONITORING A FLUIDIZED BED

The invention relates to a method for monitoring the condition of a fluidized bed in a fluidized bed boiler according to the preamble of the appended claim 1. The invention also relates to a system for monitoring the condition of the fluidized bed in fluidized bed boiler according to the preamble of the appended claim 6, as well as to a fluidized bed boiler using the system for monitoring the condition according to the preamble of the appended claim 11.

The techniques of fluidized bed combustion and gasification are based on the use of sand or a corresponding material as a fluidizing material. The function of the fluidizing material is to dry, pyrolyze and ignite a solid fuel as well as to contribute to the "evacuation" of coarse material from the fluidized bed. The fluidized bed material is an essential part of the fluidized bed boiler, and its condition has a significant effect on the whole function of the boiler.

In pure and/or unused fluidized bed material, the granule size is typically in the order of 0.5 to 1.2 mm and 0.1 to 0.6 mm in bubbling fluidized bed applications and in circulating fluidized bed applications, respectively. So-called coarse material, or large-size solid particles, such as e.g. stones, metals and sintered pieces, are accumulated or formed in the fluidized bed material during the combustion. Typically, the coarse material is classified as material with a granule size greater than 1.5 mm in bubbling fluidized bed applications and as material with a granule size greater than 0.8 mm in circulating fluidized bed applications. The presence of these particles is partly due to the uneven com- bustion process in the fluidized bed as well as impurities in the fuel. Thus, for example at hot points in the fluidized bed, sand may accumulate in clots. A basic requirement for the trouble-fee operation of the fluidized bed boiler is that the particle size of the fluidized bed material remains within given limits. If the particle size of the fluidized bed mate- rial increases too much, i.e. the proportion of coarse material increases, the fluidizing properties of the fluidized bed material are reduced, which will result in failures in the combustion process and, in the worst case, the stopping of the boiler.

For the trouble-free operation of the fluidized bed boiler, the coarse material must be removed from the fluidized bed material, and typically this is performed gradually during the combustion process. The removal of the coarse material can be performed in advance at regular intervals, wherein a given part of the fluidized bed material is removed, irrespective of the real content of the coarse material in the fluidized bed material. Thus, as a general rule, a wastage is formed by the excessive consumption of the fluidized bed material, because it is safer to exchange too much than too little fluidized bed material.

To reduce the "unnecessary" wastage of the fluidized bed material, various methods have been developed to determine the content of the coarse material. The basic idea is to take a sample of the fluidized bed material and to analyze it. On the basis of the result, the content of the coarse material is known, and if the content has exceeded a given limit value, some of the fluidized bed material can be exchanged. Typically, the sampling and the material exchange are manually controlled. One problem in said method based on sampling is that it is slow and laborious, particularly when the aim is to monitor the condition of the fluidized bed material as continuously as possible.

The primary aim of the present invention is to provide a method for monitoring the condition of the fluidized bed in a fluidized bed boiler to produce condition monitoring data about the state of the fluidized bed on a continuous basis.

To attain this purpose, the method for monitoring the condition according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1. The system for monitoring the condition according to the invention is primarily characterized in what will be presented in the characterizing part of the inde- pendent claim 6. The fluidized bed boiler according to the invention, in turn, is primarily characterized in what will be presented in the charac- terizing part of the independent claim 11. The other, dependent claims will present some advantageous embodiments of the invention.

One of the main ideas of the invention is to monitor the temperature of the fluidized bed on at least two levels and to produce data to indicate the condition of the fluidized bed from the variation in the temperatures on these levels.

The invention is based on the simultaneous monitoring of temperatures of the fluidized bed in different locations of the fluidized bed, because clear measurable temperature variations are found in different areas of the fluidized bed when the content of coarse material in the fluidized bed is changed. The temperature of the fluidized bed has been found to decrease when the content of the coarse material increases. It has also been found that the temperature first begins to drop in the lower part of the fluidized bed, and the temperatures of the upper parts begin to drop later. In typical boilers, the time difference between the starting points of the temperature drops in the lower and upper parts ranges from several hours to days. Consequently, an increase in the coarse material content of the fluidized bed material can be detected by measuring the temperature of the fluidized bed, and in one embodiment of the invention, the temperature is measured from the lower and upper parts of the fluidized bed, and by comparing these temperatures with each other, the increase in the coarse material content is detected from the increase in the temperature differences.

In one embodiment, the upper temperature of the fluidized bed is measured in the upper part of the fluidized bed, i.e., for example at the height of 300 to 350 mm from the fluidizing nozzles. The lower tem- perature, in turn, is measured, in one embodiment, at the height of 30 to 50 mm from the fluidizing nozzles. The uppermost and lowest temperature measurement levels should be placed in the upper part of the fluidized bed and as low as possible in the fluidized bed, respectively. In this way, a sufficient difference is achieved in the measuring results between the lower level and the upper level (levels). However, as a general rule, the lowest measuring level should be placed above the nozzles. The placement of the lowest temperature measurement is affected e.g. by incoming air from the nozzles, which is cool and will thereby distort the result from a measuring means placed too close. In this case, the measurement should easily include the temperature of the fluidizing air and not of the fluidized bed material. Because temperature measurements inside the boiler are taken for several purposes, such as the determination of the conditions for stabilizing the combustion, the calculation of the thickness of the fluidized bed, and the control of the bed temperature, it is advantageous in some embodiments to select the temperature measurement level in such a way that the same level can be used for both the condition monitoring and other purposes.

However, since it is possible that the difference between the lower and upper temperatures increases for another reason than an increase in the coarse material content, some embodiments of the invention present methods for reducing the susceptibility to malfunctions. One way is to measure temperatures at as many locations of the fluidized bed as possible and to compare these values with each other. In practice, temperatures in the boiler are measured for several purposes including, for example, the above-mentioned determination of the conditions for stabilizing the combustion, the calculation of the thickness of the fluidized bed, and the control of the bed temperature. Some of these measurements are taken at such a height that they do not necessarily give the best value for the measurement at two heights only. When this temperature data, which is originally intended primarily for another purpose, is used as auxiliary data for monitoring the condition, the reliability of the condition monitoring according to the invention can be improved in a very simple way.

Another embodiment of the invention comprises the above-mentioned detection of coarse material based on the temperature measurement and, in addition, the detection of coarse material based on sampling. It is thus possible, for example, to take a sample of the fluidized bed material when the temperature measurement indicates that the content of coarse material has exceeded a given limit value. The sample can be taken of the fluidized bed material in a number of different ways, and the analysis of the sample can also be implemented in a variety of ways. One way is to take the sample through a door in the bottom part of the boiler, after which the sample is cooled down. The sample can then be processed through a suitable sieve, wherein the coarse material is left in the sieve. On the basis of the temperature measurement result and/or the sieving result, an exchange of the fluidized bed material can be turned on, if necessary.

Typically, the exchange of the fluidized bed material is turned on manually by operating persons, but it is also possible to implement the material exchange automatically. In the latter case, the material exchange is carried out when the measurement result obtained on the basis of the temperature measurement result and/or the sieving result exceeds a limit value.

By means of condition monitoring according to the invention, information is continuously obtained about the condition of the fluidized bed. Furthermore, by measuring different temperatures of the fluidized bed, a prognosis can be made on the increase rate of the coarse material content in the fluidized bed, wherein the necessary operations can be foreseen and planned better than by arrangements of prior art.

In one embodiment of the invention, a data documentation is produced from the measurement results onto a graphic user interface to facilitate the decision-making by the operating persons. In this way, it is easier for the operating persons to detect changes in the boiler and to foresee possible factors of malfunction.

In the following, the invention will be described in more detail with reference to the appended principle drawings, in which

Fig. 1 shows one embodiment of the system for monitoring the condition of a fluidized bed boiler, Fig. 2 shows one embodiment of the system for monitoring the condition, and

Fig. 3 shows graphs of temperatures measured by a system for monitoring the condition of a fluidized bed boiler.

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for anyone skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention.

Fig. 1 shows, in principle, the lower part of a fluidized bed boiler, in which a fluidized bed 6, whose condition is to be monitored according to the invention, is located. For example, structures related to the supply of fuel and combustion air as well as the channels and structures related to the discharge of flue gases are excluded from the figure. Figure 1 shows fluidizing air nozzles 4 as well as lower and upper tem- perature measurement levels L1 and L2, respectively, placed according to the invention. The number of the fluidizing air nozzles 4 as well as of the measurement points 1 , 2 forming the temperature measurement level L1 , L2 may be greater than that shown in the figure, but for the sake of clarity, only one measurement point is shown on each measurement level in the figure.

The function of the fluidizing nozzles 4 is to fluidize the fluidized bed material to constitute the so-called fluidized bed 6. In the example, the temperature measurement levels L1 , L2 are placed in such a way that the upper level L2 is placed in the upper part of the fluidized bed 6, wherein the temperature of the upper part of the fluidized bed is obtained as the measurement result. In a conventional boiler, in which the upper temperature is measured at the height of about 300 mm, the temperature at this point is typically from 750 to 930 °C during optimal combustion. The lower temperature measurement level L1 should be placed as low as possible in the fluidized bed 6. Thus, a sufficient difference is achieved between the measurement results from the lower temperature measurement level L1 and the upper temperature measurement level L2 (and possibly other levels to be presented hereinbelow). However, as a general rule, the lowest temperature measurement level L1 must be placed slightly above the nozzles 4. The placement of the lowest temperature measurement is affected by, for example, incoming air from the nozzles 4, which is cool and will thereby distort the result of a measuring means 1 placed too close. A measuring means 1 placed too close to the nozzle 4 would measure primarily the temperature of the fluidizing air and not the temperature in the lower part of the fluidized bed 6. Typically, the lower temperature measurement level L1 is advantageously placed at the height of 15 to 50 mm (preferably 20 to 50 mm) from the upper surface of the nozzle 4. Thus, in a conventional boiler, the temperature at this point is typically 700 to 890 °C during optimal combustion, and the fluidized bed material does not contain so much coarse material that it would disturb the combustion process.

Figure 2 shows an embodiment which differs from the embodiment of Fig. 1 e.g. in that it comprises a third temperature measurement level L3 placed between the lower L1 and the upper L2 levels. Such a third temperature measurement level L3 can often be combined with another temperature measurement in the boiler, because in practice, temperatures are measured inside the boiler for several different purposes, such as, for example, the determination of the conditions for stabilizing the combustion, the calculation of the thickness of the fluidized bed 6, and the control of the bed temperature. Some of these measurements are taken at a height established in the field or at such a height that the result obtained from that height does not necessarily give the best value for the measurement taken from two heights only. When this temperature data, which was originally intended primarily for another purpose, is used as auxiliary data for monitoring the condition, the reliability of the condition monitoring according to the invention can be improved in a very simple way. For example, in a conventional boiler in which the temperature is measured at the height of about 100 mm, the temperature at this point 3 is typically 740 to 930 °C during optimal combustion (in other words, the temperature value in this case is closer to the upper temperature than the lower temperature). There may also be more of these auxiliary temperature meas- urement levels, and they may also be placed in ways different from those shown in the example.

For the sake of clarity, Figs. 1 and 2 only show one measurement point 1 , 2, 3 for each temperature measurement level L1, L2, L3. Particularly in larger boilers, it is sometimes useful to use several measurement points 1 , 2, 3 on one temperature measurement level L1 , L2, L3. In this way, it is possible to measure the temperatures at different points of the fluidized bed 6 more accurately, and simultaneously to detect better the local reductions in the heat transfer, caused by coarse material. The type and placement of the measuring means 1 , 2, 3 may vary, and Figs. 1 and 2 only show, in principle, the placement of the measuring means by some kinds of arms. In some embodiments, one arm or corresponding suspending means can also be used for the placement of one or more measuring means 1 , 2, 3. It is also possible to use the measuring means 1 , 2, 3 for another purpose. The measurement data from several measurement points 1 , 2, 3 on the same temperature measurement level L1, L2, L3 can be used in different ways according to the application. For example, the value of a measurement point can be weighted for the formation of control data or information documen- tation.

In addition to the detection of the content of the coarse material, the examples shown in Figs. 1 and 2 also include the possibility of determining the content of the coarse material on the basis of sampling. It is thus possible, for example, to take a sample of the fluidized bed material always when needed, for example when it is found, on the basis of the temperature measurement, that the content of coarse material has exceeded a given limit value. The sample can be taken of the fluidized bed material in a number of different ways, and the analysis of the sample can also be implemented in a variety of ways. One way is to take the sample through a door 5 or another suitable structure in the bottom part of the boiler. Because the fluidized bed material is very hot, the sample is typically cooled down before the analysis. In the analysis, the sample can, for example, be driven through a suitable sieve, wherein the coarse material is left in the sieve. By estimating or meas- uring the content of the coarse material in the sample taken, for example by weighing, it is possible to make the estimate of the coarse material content, obtained by temperature measurement, more accurate. On the basis of the estimate, it is possible to prepare a prognosis which is sufficiently reliable for the planning of measures to be taken in practice, particularly when a register of measurement results is available, wherein the prognosis can be made by comparing the result with the measured values in the register.

Figure 3 shows, in relation to time t, the values of temperatures T obtained from the measurement levels in an arrangement for condition monitoring according to one embodiment. The system for condition monitoring according to the example comprises three measurement levels L1 , L2, L3, from which each a separate value curve T1 , T2, T3 is obtained. The value curve T2 of the uppermost temperature measure- ment level L2 is also uppermost in the figure, and the value curve T1 of the lowest temperature measurement level L1 is the lowest. Between them, there is the value curve T3 of a third temperature measurement level l_3, and in this example, the third temperature measurement level is placed at the height of 100 mm. In addition, the figure shows a curve K which illustrates the relative content of coarse material in the fluidized bed material.

In Fig. 3, all the three value curves T1, T2, T3 are first substantially aligned; that is, in normal use, the temperature T varies little in different parts of the boiler, and the magnitude of the variation is approximately equal in different parts of the fluidized bed 6. An increase in the coarse material content of the fluidized bed 6 gives rise to considerable changes in the temperature (in the figure, from the moment of time t1 onward), because the accumulation of coarse material reduces heat transfer locally. The changes can first be found in the lower part of the fluidized bed 6, because the coarse material tends to be placed in the lower part of the fluidized bed. As seen from Fig. 3, the temperature curve T1 of the lowest temperature measurement level L1 starts to descend at the moment t1 , and the temperature curves T2, T3 of the upper temperature measurement levels L2, L3 remain substantially on the previous levels.

The increase in the coarse material gradually expands the range of descending temperature upwards in the fluidized bed 6. In Fig. 3, at the moment of time t2, the coarse material has caused a drop in the tem- perature of the upper part of the fluidized bed 6, which results in a relatively strong drop in the temperature curves T2, T3 of the upper temperature measurement levels L2, L3. Thus, measures must be taken to exchange some of the fluidized bed material almost immediately, to keep the boiler in operation. If the fluidized bed material is not exchanged, its fluidizing properties are reduced so strongly that the process cannot be continued.

As seen from Fig. 3, between the moment t1 of indication of the lower temperature measurement level L1 and the moment t2 of indication of the upper temperature measurement level L2 there is a clear time difference which is several hours, even several days, for conventional boilers. This time difference depends e.g. on the size of the boiler and on the fuel used, and the time difference can be determined for each use at a sufficient precision.

According to the invention, the change in the temperature T1 of the lowest temperature measurement level L1 can be monitored to control the increase in the coarse material content before it causes problems to the combustion process; that is, in practice, it is not necessarily appropriate to wait for the moment t2 when the exchange of the fluidized bed material should be performed almost immediately. Because the temperature also varies slightly in the normal use, in practice it is good to compare the temperature T1 of the lowest temperature measurement level L1 with at least one, preferably several temperatures T2, T3 of upper temperature measurement levels L2, L3. Thus, a change in the difference between the temperature T1 of the lowest temperature measurement level L1 and the temperatures T2, T3 of the other temperature measurement levels L2, L3 gives information about an increase in the coarse material.

Because the temperature T1 of the lower part of the fuidized bed 6 drops significantly earlier than the combustion process is substantially disturbed, the necessary maintenance measures can be planned and started sufficiently early. By forming a register of the measurement results and, if necessary, by supplementing the data with data obtained from sampling, it is possible to set up a very reliable prediction method for monitoring the condition of the boiler. The register data does not necessarily need to be data obtained from the very power plant in question, but the register data may also be data produced in another way for the power plant in question.

The system for condition monitoring can also be used for the automatic control of the boiler. Thus, for example when, according to the measurement results, the content of the coarse material exceeds a limit value, the exchange of the fluidized bed material is automatically turned on. The exchange of the fluidized bed material can be controlled to be turned on on the basis of a temperature measurement result and/or a sieving result, depending on the application and the different functions therein.

In one embodiment of the invention, a data documentation is produced from the measurement results of the temperature measurement levels L1 , L2, L3 onto a graphic user interface to facilitate the decision-making by the operating persons. The temperatures T1 , T2, T3 of the different temperature measurement levels L1 , L2, L3 are preferably dis- played as curves in the same coordinate system. According to the application, it is also possible to display various alarm limits, alarms, as well as other data relating to the use of the boiler. By utilizing the data in the register, it is also possible to display an estimate of the coarse material content K as well as a prognosis of the moment for exchang- ing the fluidized bed material. With the help of the data documentation, it is easier for the operating persons to detect changes in the boiler and to foresee possible factors of malfunction.

By combining, in various ways, the modes and structures disclosed in connection with the different embodiments of the invention presented above, it is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention may be freely varied within the scope of the inventive features presented in the claims hereinbelow.

Claims

Claims:

1. A method for monitoring the condition of the fluidized bed in a fluidized bed boiler, for monitoring the coarse material content of the fluid- ized bed (6), characterized by at least determining the temperature (T2) of the upper part of the fluidized bed (6), determining the temperature (T1 ) of the lower part of the fluidized bed (6), as well as - monitoring the change in the difference between said temperatures (T1, T2).

2. The method according to claim 1 , characterized by determining also a third temperature (T3), wherein the change is monitored in the difference between the third temperature (T3) and the temperature (T1 ) of the lower part and/or the temperature (T2) of the upper part.

3. The method according to claim 1 or 2, characterized in that the change in the difference between the temperatures (T1 , T2, T3) con- trols sampling.

4. The method according to any of the preceding claims 1 to 3, characterized by using the change in the difference between the temperatures (T1 , T2, T3) and/or the sampling for controlling the exchange of the material in the fluidized bed (6).

5. The method according to claim 1 or 2, characterized in that the temperatures (T1 , T2, T3) are displayed as curves in the same coordinate system.

6. A system for monitoring the condition of the fluidized bed in a fluidized bed boiler, for monitoring the coarse material content of the fluidized bed (6), characterized in that the system comprises at least a first means (1 ) for determining the temperature (T1 ) of the lower part of the fluidized bed (6), and a second means (2) for determining the temperature (T2) of the upper part of the fluidized bed (6).

7. A system for monitoring the condition according to claim 6, charac- terized in that the system also comprises a third means (3) for determining the temperature (T3) in some part of the fluidized bed boiler.

8. The system for monitoring the condition according to claim 6 or 7, characterized in that the system also comprises a means for deter- mining differences between the temperatures (T1 , T2, T3).

9. The system for monitoring the condition according to claim 6, characterized in that the first means (1 ) is arranged to monitor the temperature (T1) of the lower part of the fluidized bed (6) at the height of 20 to 50 mm from nozzles (4) intended for fluidizing the fluidized bed.

10. The system for condition monitoring according to any of the claims 6 to 9, characterized in that the system also comprises a sieve.

11. A fluidized bed boiler which comprises at least a condition monitoring system for monitoring the coarse material content in the fluidized bed, characterized in that the condition monitoring system also comprises at least a first means (1 ) for determining the temperature (T1 ) of the lower part of the fluidized bed (6), and a second means (2) for determining the temperature (T2) of the upper part of the fluidized bed (6).

12. The fluidized bed boiler according to claim 11 , characterized in that the first means (1) is arranged to monitor the temperature (T1 ) of the lower part of the fluidized bed (6) at the height of 20 to 50 mm from nozzles (4) intended for fluidizing the fluidized bed.

13. The fluidized bed boiler according to any of the claims 11 to 12, characterized in that the condition monitoring system also comprises a sieve.