AN APPARATUS FOR MEASURING THE UNBURNT RESIDUALS IN COAL ASHES AND A METHOD FOR EMPLOYING THE APPARATUS

Abstract: A variable-frequency microwave generator (12) generates an electromagnetic field in a microwave resonance chamber (1) in which there are situated a sample-carrier chamber (3) not absorbing microwaves and an open capacitive antenna (2), the resonance chamber (1) and the sample-carrier chamber (3) being set in motion relative to each other, so as to provide a linear signal that can be correlated with the content of unburnt matter in the coal ash.

Description: An apparatus for measuring the unburnt residuals in coal ashes and a method for employing the apparatus. The present invention relates to an apparatus for measuring the unburnt residuals in coal ashes and a method for employing the apparatus, more particularly, a microwave apparatus and a method for measuring in real time the quantity of such unburnt residuals present in the ashes produced by coal-fired power stations. Several measurement methods that employ microwaves for measuring the content of carbon (unburnt coal residuals) present in ash are known from the state of the art, cases in point being those described in U.S. Patent No. 4,705,409, U.S. Patent No. 4,754,214 and U.S. Patent No. 5,109,201. These methods are generally based on measurements of the conductivity of the ash sample and, depending on the particular method, the percentage of unburnt matter may then be obtained from the dielectric constant of the ashes (U.S. Patent No. 4,754,214), the energy absorbed by the sample (U.S. Patent No. 5,109,201) or indirectly from the temperature increase of some substance, water for example, placed along a wave guide (U.S. Patent No. 4,705,409). Also known are such documents as U.S. Patent No. 5,173,662 and the subsequent U.S. Patent No. 5,369,369, in which the percentage of unburnt matter is obtained by measuring the attenuation or phase shift of a microwave signal that crosses the ash sample as compared with the reference signal. The percentages of unburnt residuals that can be measured by this method do not exceed 5% and the experimental tests do not yield measurements of a high reproducibility and accuracy. More generally, the known methods are associated with measurement limits that derive from the saturation of the signal absorbed by the sample and the range of validity of the measurements is therefore rather narrow and comprised between 0 and 10%. In the known methods, furthermore, the measurements depend not only on the percentage of the coal that has not been burnt, but are also strongly dependent on the mineral matrix of the coal and the chemical elements contained in the coal. None of the existing microwave instrumentation is therefore capable of measuring high percentages of unburnt residuals and this represents a serious limitation when these instruments are applied in recently constructed thermoelectric power stations that employ low-NO x combustion systems. These systems produce ashes with high contents of unburnt matter (often between 10 and 30%) as price to be paid for the low nitrogen oxide emissions. The impossibility of measuring high percentages of unburnt matter with the known instrumentation also represents a serious limitation whenever it is desired to measure the unburnt matter in the heavy boiler- bottom ashes, which always have high contents of unburnt residuals, often in excess of 15%, and this no matter whether the plant in question is old or new. Furthermore, all the known techniques require the apparatus to be initially calibrated with ashes having a matrix of the same type as that of the sample to be analyzed, and this makes it very difficult to use the instruments in power stations that are not situated in the immediate vicinity of a coal mine and therefore stock coal lots of different origins that, more often than not, become mixed in unknown proportions in the stock yards. The purpose of the present invention is therefore to provide an apparatus and a method for measuring the unburnt residuals that can readily be used in coal-fired power stations that produce ashes with highly variable contents of such unburnt matter and burn coals from different sources. The technical problem that the invention sets out to solve is that of making it possible to determine the contents of unburnt matter in heavy ashes over a wide range of concentrations of these residuals and this quite independently of the mineral matrix of the coal employed. The apparatus of the invention comprises a chamber resonant to microwaves (resonance chamber) in which there is situated a sarnple- carrier chamber that is transparent to the microwaves, a variable- frequency microwave generator and a data acquisition and processing system. The resonance chamber contains a capacitive antenna that is connected to the microwave generator by means of a connector and a microwave line. In a new manner, the apparatus is provided with an open capacitive antenna consisting of two sections that are inclined with respect to each other and arranged inside the resonance chamber and is also such that - during the measurements - the resonance chamber and the sample- carrier chamber can be set in motion with respect to each other by means of a conventional device that will not be further considered in the present description. Preferably, the resonance chamber is cylindrical, while the sample-carrier chamber is likewise cylindrical and coaxial with the resonance chamber. It has been found that if the desired effect is to be maximized, the capacitive antenna must be positioned at roughly half the height of the cylindrical resonance chamber. The said antenna consists of two rectilinear sections. A first section rests on a radius of the circular section of the resonance chamber identified by a plane orthogonal to the longitudinal axis of the resonance chamber and passing through the connector of the antenna. The said first section has a length of the order of half the aforesaid radius. This first section of the antenna is followed by a second section which has a length equal to the first one and forms an angle with respect to the plane of the circular section comprised in the range between 50° and 90°. For the sake of simplicity of description we shall hereinafter use the term "resonant termination" to designate the assembly consisting of the resonance chamber, the sample-carrier chamber and the capacitive antenna. The invented method employs the microwaves that resonate inside the resonance chamber and consists of the following phases: - using the microwave generator to excite two electromagnetic field distributions, conventionally denoted by TMOIO mode and TE111 mode , of which the first attains its maximum along the axis on which the ash sample is situated, while the second attains its maximum in a plane that is orthogonal to the axis and is situated at roughly half the height of the resonance chamber; - acquiring the resonance spectrum by recording the behaviour of the reflecting resonance signal as the excitation frequency varies; the said resonance spectrum provides two minima (resonance peaks) corresponding to the two distinct central resonance frequencies of TM010 and TEl l l modes; the said method is further characterized by the fact that comprises the following phases: - correlating the variation of one or more characteristics of the resonance peaks, such as the frequency, the amplitude and the half-height width of one or both the resonance peaks, with the quantity of ash present in the sample; - defining a linear correlation between the slope parameters obtained from one or more of the said correlations and the content of unburnt matter; - determining the quantity of unburnt matter present in the sample on the basis of the said linear correlation of the slopes. Still for the sake of simplicity of description we shall hereinafter use the term "representative characteristics" to designate the set of such resonance peak characteristics as the frequency, the amplitude and the half-height width of one or both the resonance peaks. Furthermore, we shall use the term "linear combination" to designate the correlation between the slopes obtained on the basis of the representative characteristics of the resonance peaks and the percentage of unburnt matter in the sample, thus distinguishing it from the aforesaid other correlations. A first key aspect of the invention is constituted by the fact that the experimental analysis of both the aforesaid electromagnetic field distributions (TM010 and TE111 modes) yielded the unexpected effect that it made it possible to arrive at measurements that are independent of the mineral matrix of the sample. A second key aspect of the invention is constituted by the fact that, analyzing the resonance signal characteristics of different parts of the sample, it is possible to obtain measurements covering a wide range of unburnt matter percentages. A third key aspect of the invention is constituted by the fact that the open capacitive antenna positioned at roughly half the height of the internal side wall of the resonance chamber consists of two sections that are inclined with respect to each other at a predetermined angle. In fact, a linear relationship between the quantity of ash and the representative characteristics of the two resonance peaks can be obtained only by using an open capacitive antenna with the aforesaid inclination. In a preferred application of the method the measurements of the percentage of unburnt matter are carried out by making use of a linear combination - obtained by means of statistical methods - of obtained six slope parameters (three for each of the two resonance peaks). In a reduced application, moreover, it is possible to carry out measurements of the percentage of unburnt matter, by using optionally even only one of the obtained slope parameters. Such an application calls for the prior calibration of the instrument with at least two samples of a known content of unbumt matter, knowledge of this content having previously been obtained, for example, by means of chemical analysis. It will then be possible to correlate any one of the said slopes with at least two of the known percentages of unburnt matter and obtain the so-called calibration straight for the parameter under consideration. The content of unburnt matter of unknown samples having the similar mineral matrix can then be obtained from this calibration straight by means of interpolation measurements. The advantages offered by the invention as compared with the known techniques consist mainly of the fact that it overcomes the problem of signal saturation associated with percentages of unburnt matter in excess of 10% and therefore makes it possible to determine the percentage of unbumt matter over a wide range of concentrations. It also provides sufficiently accurate measurements independently of the mineral matrix of the coal from which the sample was obtained even without the prior calibration of the equipment. The present invention is described in detail by way of examples and with reference to the drawings wherein Figure 1 is a schematic view, Figure 2 is a variant of Figure 1 , Figure 3 is a block diagram, Figure 4 is a resonance spectrum, and Figures 5 to 14 are graphs of linear correlations. Referring to Figure 1, the resonance chamber (1) can be seen to be cylindrical, since this is the most obvious and preferred solution, though not a necessary one. The resonant termination consists of the resonance chamber (1), an open capacitive antenna (2), which consists of two rectilinear sections (2a, 2b) arranged at 90° with respect to each other and is situated at half the height of the resonance chamber(l), and a sample-carrier chamber (3) in the form of a quartz tube to accommodate the sample that is to be examined, the quartz being appropriately transparent to the microwaves. The aforesaid parts are linked by means of a connector (4) and a microwave line (5) to a microwave generator (12) and a data acquisition and processing system (17), the function of which will subsequently be clarified with the help of Figure 3. The said quartz tube has a closed lower end, while its upper end is connected to a cyclone (6) for capturing the ashes originating from the smoke stack of the boiler. In this particular implementation the sample is gradually loaded into the quartz tube time by time and, while the quantity of ash dropped into the tube varies, the signal reflected by the resonance chamber is detected, possibly compared with a reference signal. In a possible configuration an analysis of various portions of one and the same sample can be obtained by partially filling the quartz tube (3) with the entire sample and gradually inserting the tube into the resonance chamber (1) or, alternately, by moving the resonance chamber with respect to the quartz tube (3) a little at a time and in the direction of the arrow f, thus exposing successive portions of the ash column to the microwaves. This makes it possible to avoid the rapid saturation of the resonance signal that occurs in the presence of high percentages of unbumt matter. In a preferred configuration the partially filled quartz tube (3) is fixed, while the resonance chamber (1), to which the antenna (2) is rigidly attached, is displaced longitudinally in such a manner that the microwaves will successively come to strike different portions of the ash column. Figure 2 illustrates an alternative implementation example in which the sample-carrier chamber (3) has its upper end connected to a dosing device (7) for the ashes (8) consisting of a hopper (9) and a conveyor belt (10) for moving the said ashes (8), while the lower end of the chamber is provided with an on-off valve (11). As compared with the implementation of Figure 1, this implementation makes it possible for the quartz tube (3), which is fixed with respect to the resonance chamber (3), to be loaded with different quantities of ash by means of an appropriate dosing device (7), thus assuring that there will be a continuous passage of the sample across the sample-carrier chamber. Figure 3 shows the block diagram of the microwave circuit, which basically consists of a variable-frequency microwave generator (12), a directional coupler (13) and two detectors (14, 15) for, respectively, the reflected signal and the reference signal. After having been attenuated, the microwave power is subdivided by means of the directional coupler (13) in such a manner that a part is sent to the reference signal detector (15), while the remaining part reaches the resonance cavity (16). The reflected microwave power is then recorded by the second detector (14) arranged at the free output side of the directional coupler (13). The data acquisition and processing system (17) collects the experimental results obtained from the aforesaid analysis. The applied frequencies may be varied according to the geometry of the chamber, preferably from 1,500 MHz to 3,500 MHz. As previously described, the coupling between the microwave line and the resonant termination makes it possible to excite the two aforementioned electromagnetic field distributions TM010 and TE111. Referring to Figure 4, the graph there reproduced shows the resonance spectrum (18) as a function of the excitation frequency, the said spectrum having two resonance peaks (18a, 18b) experimentally obtained in a cylindrical brass cavity that becomes resonant at about 1900 MHz and contains a quartz tube having an internal diameter of 6 mm, this tube being partially filled with ash. It has been found experimentally that, using an open capacitive antenna situated at about half the height of the cylindrical resonance chamber, the said antenna having a first rectilinear section aligned along a radius of the circular section of the cavity that is defined by a plane orthogonal to the axis of symmetry and passing through the antenna connector, the said first rectilinear section having a length of about half the said radius, and a second rectilinear section, which has the same length as the first section and is positioned in such a manner as to make an angle of between 50° and 90° with respect to the plane of the said circular section, one assures that all the correlations between the representative characteristics associated up with the resonance signal and the quantity of ash present in the said termination will be linear in a convenient range of cell deplacement. When working with ash samples derived from different coals and containing different percentages of unbumt matter, the linear variation of the said representative characteristics of the resonance signal (frequency, amplitude and half-height width of the resonance peaks) makes it possible to obtain the slopes of the respective straights for both the distribution modes TM010 and TE111. Figures 5 to 14 illustrate the linear correlations of the representative characteristics of the resonance signal obtained in a brass cavity as the percentage of carbon was varied. Figures 5 and 8 show the variations of the resonance frequency of respectively TMOIO and TE111 modes (ordinate) as a function of the gradually diminishing ash quantity (abscissa) in the absence of unbumt matter. Figures 6 and 9 show the variations of the amplitude of the resonance signal of respectively TM010 and TE111 modes (ordinate) as a function of the gradually diminishing ash quantity (abscissa) in the absence of unbumt matter. Figures 7 and 10 show the variations of the half-height width of the resonance peaks of respectively TMOIO and TE111 modes (ordinate) as a function of the gradually diminishing ash quantity (abscissa) in the absence of unbumt matter. In a further preferred embodiment, ashes fall from the cyclone 6 into the quartz tube 3 until the upper part of the tube is filled in (as upper part is intended that comprised between the cyclone at the top and a porous filter at the bottom) and the lower part of the tube is empty. During the tube loading step the resonance chamber is positioned at the bottom, under the filter, and consequently it does not strike any ash portion, but only the empty lower part of the quartz tube. In this position a "blank" measurement of the resonance spectrum is determined. The real measurement step starts when the chamber is lifted until it strikes a convenient ash portion. By further moving up the chamber step by step, the microwaves strike ash portions little by little so that for each increasing ash portion inside the chamber a resonance spectrum is obtained which is normalized with respect to the blank measurement and from which the concerned parameters are then calculated. Finally, each parameter is recorded according to the ash quantity (viz. according to the chamber lifting due to the fact that the tube section is constant). As the relationship between the said recorded parameters and the said chamber lifting is linear, provided the chamber lifting steps are properly selected, the slope of said linear relationship can be calculated. It has been found that this slope shows a good correlation with unbumt residuals of the sample. Once the measurement are completed, cleaning air is blown from the chamber bottom and, after passing through the empty portion of the tube and the porous filter, the air purges the ashes. Following appropriate processing with statistical methods, the slopes obtained from the said linear correlations furnish a linear combination that makes it possible to arrive at the unknown percentage of unbumt matter quite independently of the type of coal involved. Figure 11 shows the variation of the percentages of unbumt matter obtained with the method just described (ordinate) as a function of the percentages of unbumt matter obtained by means of chemical analysis (abscissa) when making use of the linear combination of all the six slope parameters for the two modes TM010 and TM111. The graph shows that there is good agreement between the two types of measurement and that the described method yields the percentage of unbumt matter with an error of the order of 1% (one percent) over the entire range between 0 and 36% (thirty-six percent). Figure 12 is an example of the reduced application of the method and shows the calibration straight referred to the resonance peak shift TM010 per unit of mass (ordinate) as a function of the different percentages of carbon (abscissa). Figure 14 shows that this relationship remains linear for percentages of unbumt carbon up to 36%. Figure 13 provides another example of the reduced application of the method and shows the calibration straight referred to the half-height width of the resonance peak TM010 per unit of mass (ordinate) as a function of the different percentages of carbon (abscissa). The aforesaid linear combinations can be further optimized by considering, for example, a combination of the representative characteristics of the resonance peaks that comprises not only the linear terms, but also the mixed and quadratic terms.

Claims: Claims 1. An apparatus for measuring the unbumt residuals in coal ashes (8) consisting of a microwave resonance chamber (1) in which there is situated a sample-carrier chamber (3) not absorbing microwaves, a variable-frequency microwave generator (12) and a data acquisition system (17), where the resonance chamber (1) contains a capacitive antenna (2) connected by means of a microwave line (5) to the microwave generator (12), characterized in that the capacitive antenna is an open antenna consisting of two sections (2a, 2b) that are inclined with respect to each other and is situated inside the resonance chamber (1) in a position such that all the correlations of the characteristics of the resonance signal as a function of the quantity of ash present in the sample-carrier chamber (3) will be linear, and in that the resonance chamber (1) and the sample- carrier chamber (3) can be set in a motion relative to each other. 2. An apparatus in accordance with claim 1, characterized in that the open capacitive antenna (2) is positioned at half the height of the resonance chamber (1), the said capacitive antenna (2) consisting of two rectilinear sections (2a, 2b), the first of which rests in a plane orthogonal to the longitudinal axis of the resonance chamber (1) that passes through the connector (4) of the antenna (2) and has a length approximately equal to one quarter of the chamber width, while the second, which has the same length as the first, is positioned in such a manner as to form an angle between 50° and 90° with respect to the said orthogonal plane. 3. An apparatus in accordance with claims 1 and 2,. characterized in that the sample-carrier chamber (3) is fixed with respect to the resonance chamber (1) and is gradually filled with the sample. 4. An apparatus in accordance with claims 1 and 2, characterized in that the sample-carrier chamber (3) and the resonance chamber (1) are in longitudinal motion with respect to each other. 5. A method for determining the quantity of unbumt matter in coal ash by means of the use of the apparatus in accordance with claims 1 to 4 and consisting of the following phases: - using the microwave generator (12) to excite two electromagnetic field distributions (TM010 mode and TEl l l mode), of which the first attains its maximum along the axis on which the ash sample is situated, while the second attains its maximum in a plane orthogonal to the axis and situated at roughly half the height of the resonance chamber; - acquiring the resonance spectrum (18) by recording the behaviour of the resonance signal as the excitation frequency varies, the said resonance spectrum (18) providing two resonance peaks (18a, 18b) corresponding to the two distinct excitation frequencies of modes TM010, TEl l l, said method being further characterized by the fact that it also comprises the following phases: - correlating the variation of one or more characteristics of the resonance peaks (18a, 18b), such as the frequency, the amplitude and the half-height width of one or both the resonance peaks, with the quantity of ash present in the sample; - defining a linear correlation between the slope parameters obtained from one or more of the said correlations and the content of unbumt matter; - determining the quantity of unbumt matter present in the sample on the basis of the said linear correlation of the slopes. 6. A method in accordance with claim 5, characterized in that the linear correlation of the slopes comprises either all or a part of the slopes obtained from the said correlations. 7. A method in accordance with claim 5, characterized in that the linear correlation of the slopes may also comprise only one of the slopes obtained from the said correlations. 8. A method in accordance with claim 5, characterized in that the characteristics of the resonance signal are analyzed for different portions of one and the same sample.

Inventors: BOTTI LAURA, CELLI RIZIERI MARCO, GRAZIADIO MARIO, BOTTI, LAURA, CELLI RIZIERI, MARCO, GRAZIADIO, MARIO
Applicants: BOTTI LAURA, CELLI RIZIERI MARCO, ENEL PRODUZIONE S P A RICERCA, GRAZIADIO MARIO, BOTTI, LAURA, CELLI RIZIERI, MARCO, ENEL PRODUZIONE S.P.A.-RICERCA, GRAZIADIO, MARIO
Application year: 2000
Main classification: G01N22
IPC codes: G01N22