GB2071513A

GB2071513A - Fluidised bed system

Info

Publication number: GB2071513A
Application number: GB8103706A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1980-03-11
Filing date: 1981-02-06
Publication date: 1981-09-23
Also published as: GB2071513B

Abstract

This invention relates to a fluidised bed system in which the condition of the bed may be continuously monitored by vibration detectors. The system preferably comprises a container (1) having located in its walls a series of rigid rods (5). Each rod (5) has associated with it a vibration detector (9) which detects vibrations of the rods (5) caused by collisions with the bed particles and which transmits a signal to a comparator (13). By comparing signals from any selected combination of detectors (9) it is possible to gain an indication of the condition of the fluidised bed (4). <IMAGE>

Description

SPECIFICATION Fluidised bed system This invention relates to a fluidised bed system in which the condition of the bed may be continuously monitored. In particuiar, but not exclusively, the invention relates to a fluidised bed system to be used in the combustion or gasification of coal.

In a fluidised bed, a fluidising gas is passed through a distributor at the bottom of a container into a bed of particles to be fluidised. The action of the gas on the particles can cause the bed to behave like a fluid, the particles of the bed executing random motion. However, to achievefluidisation of the bed it is necessary carefully to control the conditions of its operation. If not enough gas is introduced into the bed, the particles will not be fluidised, and if too much gas is introduced, the particles will be elutriated from the bed with the gas.

In some applications material is added to the bed for treatment, for instance coal for burning or gasification. It is necessary to monitorthe behaviour of the introduced material so that proper control of the process is maintained. Bubbles of gas are formed in the fluidised bed, and it is desirable to be able to characterise their behaviour, since they affect the properties of the bed. It is also desirable to be able to monitor other parameters of the bed, such as its depth and degree of fluidisation.

At present it is only possible to observe bed condition by noting gross defects as they occur. It is therefore an object of the present invention to provide a fluidised bed system in which the condition of the bed is easily monitored.

According to the present invention there is provided a fluidised bed system comprising a container for enclosing a bed of fluidised particles, at least two detectors mounted on the outside of the container for detecting vibrations caused, in use, by collisions of the bed particles and for producing a signal proportional to the vibration, and a comparator for comparing the signal from one detector with the signal from another detector, and which, in use, produces an indication of the condition of the bed.

Preferably, each detector is associated with a rigid rod located in a wall of the container with one end inside and the other end outside the container, such that each detector detects the vibration of the outside end of its respective rod caused, in use, by collisions of the bed particles with the inside end of the rod.

In a perfectly operating fluidised bed, particles will collide at random with the container or with the inside ends of the rods, causing the container or the rods to vibrate at random, giving an apparent noise signal from each detector. If the bed contains particles of a variety of sizes, the signals will still be random although the magnitude of the signals will vary to a greater extent. If the larger particles in a bed begin to sink towards the bottom of the container, the bed will become improperly fluidised and the signals from the detectors will differ from one another. If a bubble formed in the bed rises past a series of vertically aligned detectors, the vibration detected by each detector will change in sequence as the bubble and its associated wake pass the detector.The speed of movement of the bubble may be determined by comparing the times at which the signal produced by the detectors undergoes a characteristic change.

The rods may be solid but preferably are hollow tubes which may be sealed at their inside ends. This enables other detection devices, such as thermocouples to be placed in them, and also enables them to be cooled by, for instance, blowing air through them. It is also possible to use rods or tubes already fitted in the container, for instance tubes used for measuring the in-bed pressure. Since the rods are to be placed in an abrasive and possibly hot and corrosive environment they should be made of a robust and corrosion resistant material such as stainless steel. If the rods are only to be used in a cold fluidised bed, for instance in a model fluidised bed as part of a diagnostic experiment, they can be made of any suitable rigid material.If the rods are not substantially rigid the vibrations will not be transmitted along them, or will be considerably dampened, thus making detection of the vibrations difficult.

The rods may be spaced apart along a vertical line, thus enabling the monitoring of bed depth and bubble movement as well as vertical movement of large size material.

The rods may also be spaced apart horizontally in order to monitor the degree of fluidisation of the bed over the whole area of the container. If an area of defluidisation is detected it may indicate the build up of large particles orthe incorrect working of a fluidising nozzle. The early detection of such problems can increase the efficiency of the operation of the system.

Preferably, an array of rods spaced from one another both vertically and horizontally is proved so that all the above mentioned bed parameters may be monitored by suitable selection of signals for comparison.

The detectors may be any suitable vibration detector, such as optical, capacitance, resistive, magnetic or piezo-electric devices. Preferably the detectors are similar two a gramophone pick-up, since in general the vibration experienced by the outside ends of the rods will be of the same order of magnitude as those caused by a gramophone disc. It would be possible by using two detectors or a stereo pick-up to obtain information in two channels, for instance a horizontal and a vertical component of the vibration of each rod. This will enable a better diagnosis of the beds condition.

The detectors may be clamped to the container adjacent its respective rod and held rigidly against the outside end of the rod. Alternative the detector may be clamped onto the rod itself.

The comparator may be an oscilloscope with a facility for displaying two signals on the same axis. It would be necessary to include a switching device which would enable an operator to select the signals which are to be compared. Such a comparator is useful where model beds are being tested as it can give an immediate response to a change in bed condition following a change in one of the operating parameters of the bed.

Alternatively or additionally, the comparator may be a micro-processor or computer system. The sign als will all be fed to the system and selected for comparison according to the programming instruc tions or instructions inserted by an operator or both.

For instance the system may be programmed to monitor continuously bed depth and degree of fluid isation over the area of the bed, and may be instructed to give indications from time to time of the speed of bubble travel or flow of large particles in the bed.

A further alternative is to feed the selected signal to a loudspeaker. The signal from the detectors may then be used as a process control by comparison of the loudspeaker output with recordings of outputs from a properly operating bed.

The present system is envisaged as being of especial but not exclusive use in the continuous monitoring of a coal combuster or gasifier and in the design of fluidised bed systems using model beds. In the former case the computer system may also be programmed to control coal feed, gas mixture, bed temperature etc.

The present invention will now be described, by way of example only, with reference to the accompanying drawing, which shows diagrammatically a system according to the invention.

Referring to the drawing, there is shown a fluidised bed system comprising a container 1 having a gas inlet 2 which leads a gas, such as compressed air, to a distributor 3. The distributor 3 causes the gas to be evenly distributed through and fluidise a bed of particles 4. A series of hollow stainless steel rods 5 (only three are shown for the sake of clarity) is located in the wall of the container by means of glands 6. One end 7 of the rods is closed and is located inside the container 1. The other end 8 of the rods is open and is located outside the container 1. A piezo-electric transducer 9 (in this case a gramophone pick-up) is clamped to the side of the container 1 adjacent each rod 5 by clamps 10 which hold the transducers 9 against the outside ends of their respective rods 5.The transducers 9 are electrically connected to amplifiers 11, which amplify the signals generated by the transducers 9. The amplified signals are fed to a computer system 13 programmed to compare the signals generated by the transducers 9. The signals may also be fed to an oscilloscope 12 by manual operation of a switching system 14. The oscilloscope 12 will display simultaneously signals from any two of the transducers for visual comparison.

By comparing signals from vertically spaced rods, the system 13 can continuously monitor the depth of the bed. By a similar comparison of signals from horizontally spaced rods, the system 13 can continuously monitor the degree of fluidisation of the bed.

Other conditions of the bed may be monitored by manual selection of suitable signals for comparison.

In use, when the bed is fluidised, the bed particles collide with the inside end 7 of the rods, causing the rods to vibrate. Since the rods are rigid, the vibrations are transmitted to the outside end 8 of the rods, which vibrations are detected by the transducers 9.

Signals are generated by the transducers 9 in reponse to the vibrations and are transmitted either to the system 13 for processing according to the programmed procedures or to the oscilloscope for display.

Since the delicate parts of the system are located outside the container and only one end of a robust stainless steel rod is located inside the bed, it will be possible to get useful information from the probe over a long period, limited mainly by the life of the steel rod. Thus the present invention provides a useful tool for investigation of fluidised beds.

Claims

CLAIMS:

1. A fluidised bed system comprising a container for enclosing a bed of fluidised particles, at least two detectors mounted on the outside of the container for detecting vibrations caused, in use, by collisions of the bed particles and for producing a signal proportional to the vibrations, and a comparator for comparing the signal from one detector with the signal from another detector and which, in use, produces an indication of the condition of the bed.

2. A system according to claim 1, wherein each detector is associated with a rigid rod located in a wall of the container with one end inside and the other end outside the container, such that each detector detects the vibration of the outside end of its respective rod caused, in use, by collisions of the bed particles with the inside end of the rod.

3. A system according to claim 2, wherein the rigid rods are hollow tubes.

4. A system according to claim 3, wherein the hollow tubes are sealed at their inside ends.

5. A system according to either one of claims 3 and 4, wherein the tubes contain other detectors.

6. A system according to any one of claims 3 to 5, wherein the tubes are cooled.

7. A system according to any one of the preceding claims, wherein each detector is an optical, capacitance, resistance, magnetic or piezo-electric device.

8. A system according to claim 7, wherein the detector is a gramophone pick-up.

9. A system according to any one of the preceding claims, wherein the detectors are vertically aligned.

10. A system according to any one of the preceding claims, wherein the detectors form an array including horizontally and vertically aligned sets.

11. A system according to any one of the preced-, ing claims, wherein the comparator is a microprocessor or computer system.

12. A system according to any one of the preceding claims, wherein the comparator includes an oscilloscope for displaying two signals for comparison

13. A system according to any one of the preceding claims, and including a loudspeaker for receiving a signal from the comparator for producing an output for process control.

14. Afluidised bed system, substantially as hereinbefore described with reference to the accompanying drawings.