MOISTURE CONTENT MEASURING SYSTEM

Abstract: In order to make it possible to measure a moisture content in coal on an on-line basis at a high precision, a moisture content measuring system including a sample container (5) for holding a sample, a weighing in­ strument (7) having a turn-table (6) adapted to rotate with the sample container placed thereon, an A/D converter (14) respon­ sive to a signal from the weighing instrument, a calculator (15) responsive to a signal from the A/D converter for calculat­ ing a moisture content in the sample, an indicator (16) respon­ sive to a signal from the calculator and a microwave generating and irradiating apparatus (1) for heating the sample, is improved. The improvements reside in that the system comprises hot wind generator means (3a,3b,4a,4b) for heating the sample at a low temperature, and dehumidified dry air circulating means for removing moisture in the sample.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: The present invention relates to a moisture content measuring system for analyzing on an on-line basis a moisture content in coal at a coal burning boiler, in coke at a coal or iron manufacturing plant and in lime, coal or the like at a cement manufacturing plant. Description of the Prior Art: In most of the coal burning thermal power stations, many kinds of coal are used as mixed together, but in order to efficiently burn coal and to smoothly perform a desmoking treatment, delicate operation control adapted to natures of coal is necessary. Normally, coal is stored as classified depending upon its brand, and the thermal power station is operated by determining a coal mixing proportion on the basis of results of analysis of the respective brands. However, since coal is solid, a broad distribution exists in the quality even among the same brand of coal, and for appropriate use of mixed coal and for combustion control, on-line analysis, from which results can be obtained at a high precision and moreover quickly, is desired. Among other things, with regard to moisture in coal, a moisture content value would vary largely even in a same lot, as influenced by sprinkling for preventing scattering of coal dust during coal storage and by rainfall, and so, it is considered to be especially important as an object of on-line analysis. A moisture content measuring process for coal is prescribed in JIS M8811 (Japanese Industrial Standard), and according to this process, a predetermined amount of sample coal representing a lot of coal is picked up, after the sample has been preliminarily dried at a temperature of 35°C or lower, it is pulverized and subjected to sample reduction, then a pulverized sample having a grain size of 3 mm or less or a grain size of 9.5 mm or less is held within a dryer until it takes a constant weight at 107°C, and a total moisture content is calculated from the total amount of reduced weight due to drying. Regarding moisture content measuring apparatuses, besides an apparatus relying upon the above-mentioned JIS process, an infrared moisture meter making use of infrared rays, an electrostatic capacity type moisture meter making use of the correlation between a moisture proportion and a dielectric constant, a microwave moisture meter making use of absorption of microwaves by moisture, a neutron moisture meter making use of moderation and scattering of neutrons by hydrogen atoms, and the like are known, and besides coal, they are widely used as apparatuses for performing on-line analysis such as measurement of a degree of dryness of cereals, papers, woods, etc. A moisture content measuring system of the hot air type is also disclosed in US-A-4,142,403. In addition, a microwave heat-drying moisture meter, in which an article is dried by irradiating microwaves and a moisture content is calculated from a reduced weight, is sold in the market. In the moisture measurement according to the above-mentioned JIS process in the prior art, apart from the precision of analysis, normally it necessitated a half day for measuring a total moisture content, and so, it took too much time that was far longer than 0.5 - 1 hours which was considered to be desirable for operation and control of a boiler. On the other hand, although the on-line analyzing processes in the prior art had an advantage that results of measurement could be obtained almost on a real-time basis, every one of the processes had a large measurement error and was not practical in that the infrared moisture meter could measure only a moisture content in a surface portion, in the electrostatic capacity type moisture meter and the microwave moisture meter the influence of a bulk density upon the measured value was large, and in the neutron moisture meter there existed absorption by hydrogen atoms in coal besides the absorption by the moisture. In addition, while the microwave heat-drying moisture meter sold in the market was composed of a microwave source and a ventilation fan in combination, a water dammy circulating section, an automatic weighing section, a turn table, and a controller-calculator section for the system and it could dry a sample in 1 - 3 minutes and automatically perform calculation to obtain a moisture content value, it involved the following problems: (1) Since the object of measurement is an article having a relatively uniform distribution of moisture including foods such as milk, butter, cheese or the like and powdered or granular medicines, the moisture meter can be applied only to a sample whose amount is as little as 1 - 10 g, it cannot be applied to moisture measurement for a sample of large amount (for instance, applicable even to a sample of 1 kg or more) to which the present invention is directed. (2) Heating of a sample depends solely upon heating by microwave absorption, in the case of a sample of coal having a range of grain size of the sample is as broad as several ten micrometers to several ten millimeters, and moreover, having different shapes and sizes, uniform heating is difficult and the sample would be locally overheated in the middle of the moisture measurement, hence decomposition of coal and the like would occur, and therefore, an accurate moisture content value cannot be obtained. (3) Accordingly, the above-mentioned moisture meter employs a system in which a drying period is controlled by a preset timer, and so, it is necessary that a drying time should be preliminarily known for the respective samples. A moisture content measuring system of the microwave type is also disclosed in US-A-4,606,650. SUMMARY OF THE INVENTION: It is therefore one object of the present invention to provide a moisture content measuring system, in which a sample can be heated so as not to be decomposed and moisture generated by heating can be quickly removed and measured. Another object of the present invention is to provide a moisture content measuring system, in which a moisture content in coal or the like being conveyed to a boiler, etc. can be easily sampled and can be measured in a short period of time, and so on-line processing becomes possible. According to a principal feature of the present invention, there is provided a moisture content measuring system as defined in claim 1, in which as a heat source for heating a sample such as coal, microwave irradiation and hot wind blowing are employed in combination, thus by reducing a load of microwaves the sample is dried at a temperature lower than a decomposition temperature of coal, and thereby moisture content measurement can be done quickly and at a high precision. According to the present invention, owing to the above-featured provision, a sample such as coal or the like is heated up to a predetermined temperature by microwaves and hot wind generated by the hot wind generator means, and moisture generated from the sample is removed by the dehumidified dry air circulating means. On the other hand, the sample is always weighed by the weighing instrument, thus a moisture content in the sample is calculated by the calculator, and the results of measurement are sent to the indicator. Through the above-mentioned process, the sample can be heated so as not to be decomposed, and moisture generated thereby can be removed quickly and can be measured. In this way, moisture contained in coal or the like being conveyed to a boiler or the like is sampled and measured within a short period of time, and therefore, on-line processing becomes possible. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of one preferred embodiment of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a schematic view showing a general construction of one preferred embodiment of the present invention; Fig. 2 is a plan view of the same; and Fig. 3 is a diagram showing one example of a relation of a drying time vs. a moisture content obtained as a result of measurement for a moisture content in coal according to the illustrated embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT: Now one preferred embodiment of the present invention will be described with reference to Figs. 1 to 3. Referring to Figs. 1 and 2, within a casing 20 covered by heat-insulating material is disposed a turn-table 6 having a horizontal rotary surface, and this turn-table 6 is coupled a weighing instrument 7 provided thereunder by the intermediary of a shaft penetrating through the bottom wall of the casing 20. The output of the weighing instrument 7 is connected to a digital indicator 16 and a display-recorder 17 in a boiler operating room successively through an A/D converter 14 and a calculator 15. The above-mentioned casing 20 has a shutter 12 in its one side wall (Fig. 2), while within other two opposed side walls are disposed a plurality of heaters 4a and 4b, respectively, and fans 3a and 3b at the upper corners, so that hot winds can be led by respective ducts, and one of the winds may be blown out upwards from the beneath of the turn-table 6, while the other winds may be blown out from the obliquely above of the turn-table 6 onto the turn-table 6. Furthermore, thermometers 13a, 13b and 13c are disposed at the circumference of the turn-table 6, at an outlet of a hot wind on the side of a first heat-exchanger 8, and in a sample, and they are respectively connected to a sequence controller (not shown). Furthermore, a first heat-exchanger 8 and a second heat-exchanger 9 covered by a wall made of heat-insulating material are provided outside of one side wall on the side of the fan 3b, the first heat-exchanger 8 is positioned above the second heat-exchanger 9, and thus there is provided a structure for performing the so-called gas-gas heat-exchange, such that a hot wind around the sample may pass through a cell side space (or a tube) of the first heat-exchanger 8 and a cold wind having passed through the second heat-exchanger 9 may pass through a tube (or a cell side space) of the first heat-exchanger 8. Whereas, the second heat-exchanger 9 is disposed under the first heat-exchanger 8, and it consists of such heat-exchanger that a hot wind pre-cooled by the first heat-exchanger 8 may pass through a cell side space, while a circulating coolant cooled by a chiller 10 at the outside may pass through a tube, and thereby the second heat-exchanger 9 is adapted to further cool and dehumidify the hot wind pre-cooled by the first heat-exchanger 8. In addition, a cold wind outlet of the second heat-exchanger 9 and the tube (or the cell side space) of the first heat-exchanger 8 are connected via a duct, and they are communicated with the duct of the fan 3b. The bottom of the second heat-exchanger 9 is formed in a funnel shape, and a hole at the bottom end penetrates through the casing 20 to be connected to a drain trap 11 disposed outside of the casing 20. Furthermore, at the top of the casing 20 is disposed a magnetron 1, whose radiation port penetrates through the casing 20 and is directed towards the upper surface of the turn-table 6. In addition, in the vicinity of the same radiation port is provided a stirrer 2. It is to be noted that in the case where a coal sample weighs about 5 kg at maximum, the weighing instrument 7 is assumed to have a precision of about 1 g. In order to maintain the temperature within the casing 20 at 140 - 150°C, the powers of the heaters 4a and 4b and the magnetron 1 are chosen to be 1 - 3 kW, respectively. In operation of the above-described construction, a sample of, for instance, coal of about 1 kg in weight is picked up in the sample container 5, then the shutter 12 is opened to place the sample container 5 on the turn-table 6, and the shutter 12 is closed. Weighing is commenced by means of the weighing instrument 7, and subsequently the turn-table 6 is rotated at a speed of 5 - 6 r.p.m. The heaters 4a and 4b, the fans 3a and 3b, the first heat-exchanger 8, the second heat-exchanger 9 and the chiller 10 are put in a working condition, then power is fed to the magnetron 1, and irradiation of microwaves is commenced while rotating the stirrer 2 for uniformly irradiating the sample with microwaves. On the other hand, temperatures are measured by the thermometers 13a, 13b and 13c, and adjustment is effected to maintain the temperature within the casing 20 at 140 - 150°C. Furthermore, an output of the magnetron 1 is regulated so that the temperature of the sample may not rise up to about 200°C or higher. The operations during this period are all controlled by a sequence controller not shown, a signal representing a variation of the weight of the sample caused by drying is sent via the A/D converter 14 to the calculator 15, thereby a moisture content value changing moment by moment is indicated by the digital indicator 16, and also the results of measurement are transmitted to the display-recorder 17 in a boiler operating room. As described above, the sample of coal is heated by microwaves so as not to exceed the decomposition temperature of 200°C, and also it is heated by hot winds generated by the heaters 4a and 4b and the fans 3a and 3b, in combination. On the other hand, the hot wind at the circumference of the sample is led to the first heat-exchanger 8, then it passes through a cell side space (or a tube) of the first heat-exchanger 8 and the second heat-exchanger 9, and passes through the tube (or the cell side space) of the first heat-exchanger 8, the fan 3b and the heater 4b, and again it comes to the circumference of the sample. In this way, since the hot wind at the circumference of the sample is continuously circulated and dehumidified, drying of the sample can be quickened. One example of the results of measurement is shown in Fig. 3. As indicated by the diagram in Fig. 3, simultaneously with commencement of the measurement, primary reduction in weight of the sample begins, after about 15 minutes the sample attains a constant reduced weight, and then after a while the sample presents secondary reduction in weight. The reduction in weight before the constant reduced weight is attained, is caused by evaporation of moisture, and a moisture content value obtained at this time point was well consistent with the value obtained through the JIS process. It is to be noted that the secondary reduction in weight is caused by decomposition of coal, but it can be easily discriminated from the reduction in weight caused by drying. In practice, at the time point when the primary reduction in weight of the sample has terminated and the weight of the sample has become constant, irradiation of microwaves, circulation of hot winds, the dehumidifying means and the turn-table are stopped, the shutter is opened, and the sample is taken out. A series of these operations are automatically controlled by the sequence controller, a moisture content value is automatically calculated by the calculator on the basis of the weight of the sample and the reduced weight, and is digitally indicated by the digital indicator, and also the moisture content value is transmitted as an electric signal to the boiler operating room. The measuring time covering these processes is about 15 minutes, which can well satisfy the condition of 30 minutes or less which is necessitated for on-line analysis. In this way, with the moisture content measuring system according to the illustrated embodiment of the present invention, it has been practically proved to be possible to measure a moisture content quickly and at a high precision without influenced by decomposition of coal. As described in detail above, if the measure of employing microwaves and a hot wind in combination according to the present invention is practiced, it is made possible to measure, for instance, a total moisture content in coal at a precision equivalent to that of the JIS process, and moreover, in 15 - 20 minutes, a moisture content in coal can be analyzed on an on-line basis by automating a series of operations from sampling of coal being conveyed up to measurement of a moisture content, and thus a moisture content value changing moment to moment can be transmitted to a boiler operating room or the like. Accordingly, very delicate operation control such as regulation of a coal feed rate and a coal mixing proportion or the like can be greatly improved as compared to the prior art.

Claims: A moisture content measuring system including a sample container (5) for holding a sample, a weighing instrument (7) having a turn-table (6) adapted to rotate with said sample container (5) placed thereon, an A/D converter (14) responsive to a signal from said weighing instrument (7), a calculator (15) responsive to a signal from said A/D converter (14) for calculating a moisture content in the sample, an indicator (16) responsive to a signal from said calculator (15), a microwave generating and irradiating apparatus (1) for heating said sample, hot wind generator means, which comprises air heating means (4a, 4b) being different from said microwave apparatus (1) and fan means (3a, 3b) and which directs hot wind to the sample, for heating the sample at a temperature lower than the decomposition temperature of the sample, and means (3b, 8, 9, 10, 11), which continuously circulate and dehumidify the air at the circumference of the sample, for removing moisture in the sample. A moisture content measuring system as claimed in Claim 1, characterized in that said air dehumidifying and circulating means includes a heat-exchanging device (8, 9, 10, 11) for cooling and dehumidifying a hot wind which has heated the sample. A moisture content measuring system as claimed in Claim 2, characterized in that said heat-exchanging device includes a first heat-exchanger (8) for pre-cooling the hot wind which has heated the sample, and a second heat-exchanger (9) for cooling the hot wind which has been pre-cooled by said first heat-exchanger (8) into a cold wind, and after the cold wind flowing out of the second heat-exchanger (9) has pre-cooled, in said first heat-exchanger (8), the hot wind which has heated said sample, said cold wind is sent to said hot wind generator means (3b, 4b).

Inventors: ICHIKAWA, MASAHIKO, KINOSHITA, KENICHIRO C/O MITSUBISHI JUKOGYO K. K., KUBO, KYOJI, NAKANE, TAKEMI, NOGUCHI, TAKEKI, YOSHIDA, HIROSHI, KUBO, KYOJI, C/O MITSUBISHI JUKOGYO K. KAISHA,, NAKANE, TAKEMI, C/O MITSUBISCHI JUKOGYO K. KAISHA,
Applicants: CHUBU ELECTRIC POWER COMPANY INC., MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: G01N5
IPC codes: G01N33, G01N5