AN ARRANGEMENT AND METHOD FOR MONITORING THE CONDITION OF A BED OF A FLUIDIZED BED REACTOR

The present invention relates to a measuring arrangement based on acoustic emission for monitoring the condition of a bed of a fluidized bed reactor, especially a fluidized bed boiler, which arrangement allows for real-time detection of changes taking place in the fluidized bed condition, especially in the coarse material content thereof.

Fluidized bed reactors are widely used for different purposes, such as combustion, gasifying, chemical and metallurgic processes. Fluidizing refers to a condition, where a fine solid material is made to fluidize by means of gas or liquid. Depending on the process, various solid bed materials are made to fluidize and/or circulate in systems. In combustion processes of the fluidized bed boilers, the bed material is typically sand or corresponding ma- terial. In a fluidized bed boiler, the fluidizing is effected by blowing air through a solid bed material resting on an air distribution grid. The fluidizing layer may include particulate fuel such as charcoal, coke, wood, waste or peat, and other particulate materials such as sand, ash, sulfur absorbent, catalyst or metal oxides. The coarse fraction of fuel ash, stones etc. accumulate in the bed, which requires their removal. Fuel alkalis also accumu- late in the bed material, which leads to formation of aggregates in the bed. Finally, the presence of excessively coarse material may lead to the collapse of the whole fluidizing bed and thus prevent the operation of the whole process. For ensuring the operation of the fluidized bed process, the particle size of the fluidized bed material has to stay within certain limits. Therefore, it is important to continuously monitor the bed's condition, to change the conditions and thus prevent excess formation and accumulation of coarse material in the bed.

WO publication 2005/038420 discloses a method, in which the coarse material content of the fluidized bed is monitored by determining the temperatures in the upper and lower part of the bed and by controlling the change of temperature difference between these. An increase in the temperature difference is an indication of increasing portion of coarse material. Simultaneously it is advantageous to take samples of the bed for determining the coarse material, especially when the temperature difference reveals that the coarse material content exceeds a certain value.

In EP publication 1106984, agglomerates are detected in a reaction vessel, such as an olefin polymerization fluidized bed reactor, by providing a detecting rod, the strain/bend of which is measured when the agglomerates collide with it. The rod is inserted at an angle of 20-70 degrees in relation to the flow of gas and particles.

Publication WO 00/43118 discloses a method, in which the condition of a fluidized bed is monitored by determining pressure over the fluidized bed segment at various levels in the vertical direction.

A drawback of the above-mentioned methods may be inaccuracy, due to e.g. material attaching to the detecting sensor, which hampers the measuring of the proper variable.

US 5022266 presents a method, which allows monitoring e.g. the uniformity/non- uniformity of the flow distribution in the fluidized bed. By means of detectors (typically an acceleration transducer or a pressure sensor) attached onto the outer wall of the process vessel, the wall vibration is measured and the power spectrum is determined as a function of frequency. The measurements are carried out at multiple points at the circumference of the vessel and additionally, at all these points, a power spectrum area containing a resonance peak for each measuring point is determined. Later a new measurement is carried out and the results are compared to the earlier one. The basic measurement is carried out right after the start-up of the process, when it is known that the flow distribution in the bed is uniform. This measurement is indirect and periodic, and does not thus provide continuous information.

An object of the present invention is to provide an arrangement for continuous monitoring of the condition of a fluidized bed reactor, such as a fluidized bed boiler, with increased reliability. A specific object is to determine and monitor the coarse material content in the fluidized bed.

For achieving these objects, a characteristic feature of the arrangement according to the present invention is that it comprises at least

- a sensor for measuring the acoustic emission caused by the particles in the fluidized bed, said sensor comprising a rod-like wave guide, one end of which is located at a distance inside the reactor and the other end, external to the reactor, is provided with a piezoelectric sensor part for converting the received emission into

an electric analogic signal, whereby the wave guide part inside the reactor comprises an uninsulated portion for receiving emission and an insulated portion; and means for processing the received signal in order to determine a frequency spectrum and/or envelope within a certain frequency range for monitoring the changes taking place in the fluidized bed.

The method is characterized in that

- acoustic emission caused by the collision of the particles in the bed is continuously received by means of a sensor extending into the interior of the reactor, - the received emission is converted into a digital signal,

- a frequency spectrum and/or envelope is determined from the digital signal in real time within a certain frequency range appropriate for the application in question and the frequency spectrum/envelope is compared to the corresponding frequency spectrum/envelope of the normal condition of the bed for detecting changes in the bed's condition.

The invention also relates to a fluidized bed reactor utilizing and applying the arrangement and method according to the invention.

The invention is based on the idea that the condition of the fluidized bed is monitored by inserting through the wall of the reaction vessel a detector rod (wave guide) protruding to the bed, which rod receives acoustic emission, i.e. high frequency vibration wave caused by the collision of particles onto the head of the detector rod located inside the reactor. The rod conducts the vibration waves into a piezoelectric sensor part connected to the rod and located outside the reactor vessel, which sensor part measures a selected frequency and converts the high frequency energy applied to the rod directly to an electric signal. The electric signal is conducted via an analogic filter and an amplifier to an A/D converter, which gives a digital signal that is filtered and conducted into data processing.

The filtered signal is used for calculating a real-time continuous Fourier transformation (FFT) i.e. frequency spectrum, which shows instantaneous, rapid changes and which can be used as a criterion in determining an instantaneous condition. An averaged envelope is calculated from a longer period, which allows monitoring the natural change of the bed, its pulverization or its agglomeration.

The frequency spectra of three fluidized beds with different coarsenesses were determined in preliminary tests. The spectra showed clear differences especially within a certain frequency band. The spectrum of the coarsest bed had the most powerful peaks, while the spectrum of the finest bed was more even. By determining the averaged enve- lopes of these spectra it can be seen that there are essential differences between the envelopes of beds with different coarsenesses. Thus, when the envelope is determined during an appropriately long period of time it gives reliable information on the changes in the coarseness in the fluidized bed.

The bed monitoring method according to the invention reacts consistently to parameters revealing the state of the bed, which are fluidization velocity (the energy level of the collision of the particles), the density of the suspension (collision/unit of time, a second) the coarseness of the fluidized matter (the energy of the collisions). The mentioned parameters are also indirectly influenced by e.g. the location, length and shape of the detecting rod as well as the hardness, grain form and surface structure of the fluidized material.

The arrangement according to the invention preferably comprises the following elements:

A wave guide (detector rod) - a piezoelectric sensor - a filter - an amplifier - an A/D converter - digital filtration - FFT transformation - data processing - a user interface.

The invention is described in more detail with reference to the accompanying figures, of which

Fig. 1 illustrates a preferred construction of the wave guide rod, Fig. 2 illustrates a typical frequency spectrum (intensity/frequency) of the fluidized bed and the respective envelopes, and Fig. 3 illustrates the envelopes of a coarse and a fine fluidized bed.

The construction of the wave guide is illustrated in Fig. 1. An elongated rod-like wave guide 2 having a length L1 is mounted through a wall 4 of a reactor, such as a fluidized bed boiler, to protrude into the bed in the boiler. The rod material has to be heat and wear resistant. Part of the wave guide rod in the reactor is provided with insulation 3 for preventing the generation of acoustic emission at that point of the rod. The length L2 of the uninsulated part, which affects the "sampling distance", i.e. the length of the rod onto which particles may collide, is optimized for each object of measurement (each bed). On the other hand, the width and length of the uninsulated part determine the sampling sur-

face area, i.e. the number of collisions within a time interval per m ² . Thus, the dimensions of the rod determine the specificity of the measurement.

The shape of the rod (form of cross section; solid/hollow) affects the flow resistance and the collision angle of particles. The shape of the wave guide rod must not be such that it would locally resist the flow to an adverse extent and thus disturb the operation of the bed. Excess resistance also increases the sticking of the bed material onto the rod. On the whole, the sticking of any excess material onto the wave guide rod should be prevented as effectively as possible, because excess material will passivate the wave guide, the sur- face area of the measurement zone thereon will change and finally the wave guide will loose its measuring capability, if it becomes totally coated. The shape of the wave guide also affects the collision angle between the guide and a particle. The shape is such that the collision angle of a particle in respect of the surface of the wave guide rod should be over 0 degrees, preferably 45-90 degrees, most preferably close to 90 degrees, i.e. about 90 degrees, because an upright collision to the wave guide generates the best energy level for the signal. The location of the wave guide in relation to the flow of the bed viewed in 3-dimensional perspective should also be such that the collision angle of a particle is preferably 45-90 degrees, most preferably about 90 degrees. An angle of 90 degrees in respect of the flow provides a "pure collision".

The length L3 of the insulated part is determined on the basis of e.g. the location of the measuring point in the bed, i.e. how deep into the reactor the rod must protrude in order to reach the desired measuring point (the location and length of the uninsulated part).

Further, the wave guide rod must be insulated from the reactor body so that the measurement will not be subjected to disturbances therefrom. At the lead-in point in the reactor wall the rod is protected by means of a bushing 5 as well as vibration isolation 6, which eliminates the disturbance caused by the reactor body's vibrations to the monitoring of the bed. Also, sufficient support and resistance against the pressure difference between the interior of the reactor and the surroundings have been taken into account in the lead-in. A rod thus inserted is also easy to check and replace if needed.

The piezoelectric sensor 7 is not mounted inside the reactor and preferably not attached directly onto the reactor wall, but at a distance therefrom. This allows mounting this sen- sor outside the thermal insulation, facilitates maintenance and calibration operations and prevents the sensor from overheating.

In an analog filter, high-pass, low-pass and band-pass are usually made when necessary. In such a filter, a frequency range with the best correlation is selected according to the application in question, and any interfering frequencies are filtered out. After the filtering, the signal is led via an analog amplifier and it is amplified with the intensity required by the application. After that, the signal is converted to digital in an A/D converter, whereby the treatment thereof is facilitated.

The converted signal is filtered digitally utilizing an optimal method selected for the appli- cation in such a way that one or several selected frequency bands can be emphasized and the result is a system for optimal monitoring of the coarseness of the bed. The frequency range is selected such that it best highlights the changes taking place in the fluid- ized bed of the process in question. An optimal frequency range may be determined by means of preliminary tests for each process, i.e. a range in which the changes in the proc- ess, especially the changes in the coarseness of the bed, are most clearly seen.

Typically, several detector-measurement wave guide rods are inserted in a fluidized bed reactor at locations, which are optimal in view of the process. This way, the condition of the bed can be monitored in elevational and latitudinal directions at various points of the bed. The location of the wave guide is chosen such that it is in an optimal range in view of the flows as described above, whereby the risk of sticking is minimized.

Data received from the sensors of each measuring point is led as a filtered digital signal. The treated signal is printed to an interface according to the selected manner, e.g. as a continuous frequency spectrum display, a long-time envelope, or only by printing the ex- ceedings of predetermined action limits.

Figure 2 illustrates a typical frequency spectrum and a corresponding envelope for three fluidized beds having a known coarseness. The coarsest bed has the strongest spectrum and envelope.

Figure 3 illustrates the envelopes of a coarse and a fine bed, when the measurements have been made at two levels in the bed. The intensity of a coarse bed's frequency is higher than that of a fine bed. The intensities show a significant, substantial difference. The intensity of the finer bed is on average half of that of the coarse bed. Thus, during continuous monitoring of the bed's condition, an increase in the intensity of the finer bed is

an indication of an increase in the particle size, i.e. an increase in coarseness. When the intensity exceeds a certain threshold value determined for each application, the conditions in the fluidized bed are changed. For each application, for continuous monitoring of the bed, a frequency range with the highest intensity and greatest difference is most suitably selected.

In the method of the invention, the frequency spectrum and envelopes are the main tools for analyzing the bed's coarseness. An envelope determined within a suitably long period of time shows reliably and clearly the change in the coarseness of the bed. By measuring the acoustic emission caused by the collision of particles, the tests have revealed that it has a direct, clear correlation to the conditions of the bed:

- correlation to the fluidization velocity of the bed

- correlation to the size of the bed

- correlation to the coarseness of the bed and any change therein - correlation to the location of the wave guide / different flow zones of the bed.

E.g. the following advantages are reached by means of the present invention:

- Utilization of acoustic emission (AE) measurement enables monitoring the condition of the fluidized bed and changes therein; - The system may be taught to recognize the normal situation and give alarm on abnormal changes in the bed;

- The combination of AE data with other normal process data enables the avoidance of unnecessary alarms in connection with normal changes in the running situation.

The invention is not limited to the embodiments presented as examples, but it can be modified and applied within the inventive idea defined in the claims.