AU1242499A

AU1242499A - Method, device and installation for analysing a gas effluent for determining dust rate

Info

Publication number: AU1242499A
Application number: AU12424/99A
Authority: AU
Inventors: Giancarlo Bertole
Original assignee: CGIA
Current assignee: CGIA
Priority date: 1997-11-17
Filing date: 1998-11-16
Publication date: 1999-06-07
Also published as: JP2001523816A; FR2771174B1; WO1999026055A1; FR2771174A1; EP1032815A1

Description

1 METHOD, DEVICE AND INSTALLATION FOR ANALYSING A GAS EFFLUENT FOR DETERMINING DUST RATE The present invention relates to methods and equipment which serve for analysing gaseous effluents. It applies mainly 5 to the examination of the gases discharged into the atmosphere by industrial plants within the context of pollution control. It aims to solve the difficulties encountered in this field, by making it easier to automate simple procedures, but also providing reliable results. From this standpoint, it goes 10 as far as taking into account the particularly demanding requirements of plants using industrial furnaces or boilers, waste-reprocessing plants, incineration plants, etc., by being well adaptable to diversified situations in which the available personnel do not necessarily have a high degree of technical 15 competence. The present invention relates more particularly to the weight measurement of the dust present in an effluent in a wet medium, as is the case in particular in flue-gas discharge stacks with which waste incinerators operating by a wet or 20 semi-wet process are equipped. In these chimneys or smoke stacks, the flue gases are at temperatures and water concentrations such that they are below the dew point and the moisture easily condenses as water droplets, which explains moreover the white plume that may be seen leaving the stack, 25 the temperature prevailing outside the stack being lower than inside to the point of being below the dew point of the flue gases. Currently, no satisfactory technique seems to have been proposed for permanently verifying that the amount of solid 30 dust particles that a stack of a plant carrying out a wet treatment releases into the atmosphere does not exceed the permitted tolerance.

2 In a known manner, the weight measurement of dust in the discharged gas (in other words the weight of dust per unit volume of gas in question under normal temperature and pressure conditions) is calculated from measurements made on samples 5 taken of the effluent which are tapped off to a suitable analyser. On this subject, mention may be made of two methods. The first consists in depositing the dust on a strip of paper during its exposure to the gaseous effluent and in then 10 subjecting it to beta radiation provided by a carbon 14 radioactive source and in examining the attenuation of the beta radiation, which depends on the amount of dust deposited on the strip. A second method operates on the principle of scattering of the light illuminating a sample of the gaseous effluent at a 15 low angle of incidence (in practice, 15 degrees); in this case, it is in fact the turbidity of the sample that is measured. In reality, another method exists, which would be more satisfactory from the point of view of costs and of reliability under continuous monitoring conditions, but which unfortunately 20 has the major drawback of being impossible to implement if the gaseous effluent to be analysed is wet. This is the triboelectric method, which involves detecting the density of impacts of dust particles on a sensitive triboelectric probe head connected to means suitable for translating said density 25 into an electric current. However, this method is completely inoperable on an industrial scale when the effluent to be analysed is likely to contain water vapour capable of condensing as droplets on the sensitive head, since this causes electrical short circuits 30 which make the probe unusable, thereby giving an exaggerated deposition of touching solid particles on its surface. It is therefore not known how to operate other than by applying triboelectric detection to a sample of gaseous effluent extracted from the stack in which the sample is conditioned 35 before it passes close to the sensitive head.

3 The object of the present invention is therefore to propose a better solution which is technologically exploitable on an industrial scale, even in the case of pollution monitoring applied to gaseous effluents (also called by the 5 general term "flue gases" escaping from plant stacks) which are regarded as being wet, which means to say that they are capable of causing water condensation on any unheated surface with which they come into contact. The aim of the invention is thus to obviate the need to 10 have to extract samples of the effluent gas to be analysed, thereby significantly increasing the manufacturing cost and the running and maintenance costs. Its aim is also to make it possible to obtain results which are able to meet, without question, the legal 15 requirements under economically acceptable conditions. In addition, it has the advantage of being easily integrated into a complete system for monitoring the pollution by the gaseous effluents discharged into the atmosphere by industrial plants, and this being so under attractive economic 20 conditions. For this purpose, the subject of the invention is a method of analysing a gaseous effluent flowing through a stack, especially applicable to the case of flue gases discharged from wet-treatment plants, characterized in that it includes a step 25 of determining a weight concentration of dust by detection of the density of impacts of dust particles on a sensitive part of a triboelectric probe introduced directly into said stack, along the path of a portion of the gas stream flowing longitudinally in a measurement duct, and downstream of a muff 30 for heating said measurement duct so as to take said portion of the stream locally to a temperature above the dew point of the water vapour in the effluent. In this way, while still carrying out the weight measurement by in situ detection in the stack, condensation of 4 the water vapour into liquid droplets, which would disturb the density of impacts on the triboelectric probe and create short circuits prejudicial to its operation, is avoided. At the same time, since detection takes place directly 5 in the flowing effluent, the triboelectric probe makes it possible to obtain, in a simple manner, an accurate measurement which does not carry the risk of being influenced by the formation of deposits, as would be the case if the triboelectric measurement were made outside the stack, on a 10 tapped-off sample undergoing velocity variations. The dust concentration may be determined continuously, at the very least as long as the variations in the velocity of the effluent remain within the usual limits found in plant stacks, especially in the case of stacks discharging 15 incinerator flue gases into the atmosphere, in so far as these variations in the steady-state conditions are less than plus or minus 20% of the nominal velocity for a given incinerator. According to another characteristic of the invention, the velocity of the gaseous effluent in the measurement duct is 20 measured periodically in order to check that it has not deviated excessively from a value close (to within + 20%) to a theoretical isokinetic condition with the velocity of the gaseous effluent in the stack and, if necessary, to correct the positioning of the measurement duct in the stack. This makes it 25 possible, in a simple manner, to maintain a constant measurement accuracy. In addition, the positioning correction may be carried out without in any way dismantling the equipment, in so far as means are advantageously provided for adjusting the transverse position and the longitudinal 30 orientation of the measurement duct in the stack. This velocity measurement, taking 10 minutes, may, for example, be carried out once a month. As a complement or a variant, the corrections deemed necessary may be made by acting on electronic means for 35 processing the signals coming from the triboelectric probe, 5 which means have the purpose, among others, of converting these signals into a numerical measurement result. Advantageously, the measurement of the velocity of the gaseous effluent in the measurement duct is made alternately 5 with the measurement of the dust concentration. In this case, for the measurement of the velocity of the gaseous effluent in the measurement duct, provision is advantageously made for the triboelectric probe to be temporarily replaced by a Pitot tube fitted so as to emerge in the measurement zone where the 10 sensitive part of the probe is placed. There are cases in which it would be more advantageous to have means which remain permanently mounted in the stack in order to carry out this measurement as well as, moreover, to carry out other measurements which would be useful for the 15 calculations necessary for converting the detected data into information about the weight of dust contained in a unit volume of gas under standard temperature and pressure conditions'. Thus, it will be advantageous for a temperature sensor, such as a thermocouple, to be permanently located in the measurement 20 zone near the sensitive part of the triboelectric probe. According to yet another characteristic of the invention, the measurement duct is located sufficiently far from the wall of the duct to avoid the influence of the skin effects of the flow of gaseous effluent. For the usual 25 dimensions of plant stacks, this minimum distance is generally one tenth of the diameter of the stack, this being sufficient for it to be in the plateau region of the transverse velocity profile of the flue-gas stream. The measurement duct is thus located in a zone of the stack where the velocity of the 30 gaseous effluent has an average value with respect to the velocity distribution in the stack. The object of the invention is also a device comprising the means necessary for implementing the above method. This device must provide passages through the wall of the stack, 35 particularly for the triboelectric probe and for means for 6 supplying the heating muff from an external energy source. From this standpoint, this will obviously be preferably an electrical supply. The invention therefore consists in its principle in benefiting from these requirements by constructing 5 the device as a rigid structure comprising two guide tubes which are fixed at their ends, on the one hand, to the measurement duct to be introduced into the stack and, on the other hand, to a plate capable of forming a mounting flange on the wall of the stack. 10 According to a preferred embodiment of the invention, the device is characterized in that it includes a measurement cell fastened to a mounting plate capable of forming a flange for closing off an opening made in the stack, said cell comprising a measurement duct whose upstream part is surrounded 15 by a heating muff, and in that an electrical heating device is placed in the heating muff in thermal contact with the measurement duct and supplied via a lower guide tube passing through the flange and emerging in said muff, and in that an upper guide tube for a triboelectric probe emerges in the 20 downstream part of the measurement duct passing through the flange. Preferably, since these two tubes extend from the measurement duct to the mounting flange and provide passages through the wall of the stack, they are fastened to the measurement duct or its heating muff at variable distances so 25 as to allow fine adjustment in the positioning of the measurement duct in the stack, both in terms of longitudinal orientation and in terms of distance from the wall of the stack. The device according to the invention therefore 30 constitutes an assembly which is easily installed in an opening in the stack provided for the introduction of measurement systems. This assembly includes a rigid rectangular structure consisting of the mounting flange, the measurement cell and the two guide tubes reserved for the probe (or the velocity sensor) 35 and for the electrical connections to the heating muff, respectively.

7 When it is mounted, the precise positioning of the measurement duct for obtaining a constant velocity close to the isokinetic condition is easily achieved by varying the fastening distance between the measurement duct and each of the 5 tubes. Advantageously, the two tubes are fastened to the measurement duct by screwing. The measurement tube allows the triboelectric probe or the velocity measurement device to be easily introduced into the measurement duct; in particular, it facilitates the 10 operation of verifying the isokinetic condition. According to yet another characteristic of the invention, a heat-insulating element is placed between the shell of the heating muff and its electrical resistance element helically surrounding the wall of the measurement duct. This 15 makes it possible to reduce the external skin temperature of the heating muff while keeping it at a temperature above 130 0 C (the dew point of acid gases) so as to prevent corrosion of the measurement duct and of the heating muff. Furthermore, the device according to the invention 20 advantageously includes a sleeve intended for the introduction of a temperature probe in the downstream part of the measurement duct, said sleeve emerging in the downstream part of the measurement duct and passing through the flange. Such a temperature sensor is useful for controlling the temperature of 25 the gaseous effluent, in the dust-concentration measurement zone, by regulating the supply to the electrical heating device. In general, it is desirable for the electrical heating device to be one with an oblong electrical conductor. This 30 section variation advantageously replaces the brazed or welded hot-part/cold-part junctions and eliminates the expansion problems encountered at high temperatures and/or under large mechanical loads.

8 As an option, the device may include a device for blowing air onto the triboelectric probe. This makes it possible, without handling the triboelectric probe, for the latter to be cleaned in a regular periodic manner or when it is 5 found to be delivering aberrant measurements. The device according to the invention may include a device for measuring the skin temperature of the electrical heating device and a device for cutting off its supply. This provides safety against any deterioration of the electrical 10 heating device, by preventing this temperature from exceeding 400 0 C, for example. The subject of the invention is also an apparatus for analysing a gaseous effluent which includes an above device, characterized in that it includes an electronic circuit which 15 receives the measurement signals from the triboelectric probe and converts them into dust concentrations. Advantageously, the apparatus according to the invention includes a programmable controller which receives the measurements delivered by the triboelectric probe and the 20 measurements delivered by other sensors and sampling analysers, said controller being connected to a supervision system able to act on gas-effluent monitoring alarms and controls. According to preferred characteristics of the apparatus according to the invention, to be used separately or in their 25 operating combinations, the supervision system permanently receives measurements of the velocity and of the temperature of the gaseous effluent in the stack. It detects the aberrant measurements delivered by the triboelectric probe and causes the triboelectric probe to be cleaned by blowing air in the 30 case in which the corresponding equipment is provided. It causes the supply to the electrical heating device to be cut off when its skin temperature exceeds a threshold value. It operates the controller in its calculations to convert the results of the various measurements, so as to express them as 9 values per unit volume under standard temperature and pressure conditions. The invention will now be more fully described within the context of preferred characteristics and of their 5 advantages, with reference to the figures of the appended drawings which illustrate them and in which: - Figure 1 is a schematic sectional illustration of a dust weight measurement device according to the invention; and - Figure 2 is a schematic illustration of the 10 electronic means for automatically managing an apparatus for monitoring the flue gases in plant stacks, which includes the device of Figure 1 among the equipment for detecting and measuring the contents of the polluting constituents. Figures 1 and 2 illustrate an apparatus intended for 15 analysing a gaseous effluent with determination of a dust concentration, particularly in the case of stacks for discharging flue gases into the atmosphere from incinerators employing a wet or semi-wet treatment. Figure 1 shows the outer wall 1 of a stack of an 20 incinerator. This wall includes an inner lining 2 made of a heat-resistant material such as ebonite. In practice, the latter does not have to withstand temperatures above 80 0 C during operation of the stack. Flowing through this stack is a gaseous effluent whose 25 temperature is between 60 and 80*C and whose velocity is less than 25 m/s; the pressure may be positive or negative and the water content is at saturation. This gaseous effluent may include a certain number of polluting constituents whose contents must be maintained below a maximum value and, in 30 particular, dust particles whose content must not exceed 10 g/m3. The apparatus for analysing the gaseous effluent therefore comprises analysers for samples taken off for each of 10 the gaseous pollutants, plus a sensor for the dust concentration weight measurement which is illustrated in Figure 1. This sensor is based on the known principle of the impact technique (triboelectric probe), but the measurements 5 are made in line, that is to say the triboelectric probe is placed directly in the stack. The entire dust-concentration measurement cell 20 is mounted on a rectangular flange 3 of standardized dimensions, which closes off an opening 4 made in the wall 1 of the stack, 10 for the purpose of introducing measurement apparatuses. This flange is firmly attached to a measurement duct 5 which forms part of the measurement cell 20 and is placed in the stack parallel to the axis of the latter, a portion of the gaseous effluent flowing through said measurement duct. 15 The measurement duct 5 is a cylindrical tube made of Inconel, 385 mm in length and approximately 50 mm in diameter, which may be easily introduced into the openings such as 4 provided in the existing stacks. In the case of a new plant, a larger opening, for example 600 ( 200 mm, may be provided, 20 making it possible to use a measurement duct according to the invention which is 500 mm in length. The positioning of the measurement duct 5 in the stack is done with respect to the flange 3, that is to say the measurement duct 5 is fastened to the flange 3 with the 25 possibility of varying its orientation and its distance from the flange 3. This fastening is carried out by means of two guide tubes 6 and 12 which are perpendicular to the measurement duct 5 and fastened at their ends to the measurement cell and to the 30 flange 3, respectively, so as to form an approximately rectangular rigid structure of good mechanical strength. These two tubes are made of Inconel 600 and their diameter is 38 mm. As a variant, other materials are the alloys Monel 400 and Hastelloy C.

11 The length of these two tubes depends on the diameter of the stack. It is defined in order to position the measurement duct 5 at least at a sufficient distance from the wall 1 to avoid the influence of the skin effects of the gas 5 effluent flow. The distance is, for example, equal to 1/10 of the internal diameter of the stack in the case of its minimum distance and one half of its radius in the case of its maximum distance. Thus, it is ensured that the velocity of the gaseous 10 effluent in the measurement duct 5 is at least close, if not equal, to the average value of the velocity of the gaseous effluent in the stack, given the distribution of velocities over the cross section of the stack, and that it follows the same variations. It has been able to be observed in fact that 15 the isokinetic condition is not an absolute condition, provided that, in practice, the deviation remains constant and within the plus or minus 20% limits. The upper guide tube 6 passes through the flange 3 and is fastened to the actual measurement duct 5 by means of a 20 tapped tee connector 7 into which the measurement tube 6 is screwed. This tube 6 emerges in the measurement duct 5 in the downstream part of the latter, that is to say at the outlet along the path of the gaseous effluent. It serves for introducing and fixably mounting a triboelectric probe 10, the 25 terminal sensitive part 8 of which is placed in a measurement zone 9 inside the measurement duct 5. This tube 6 may also serve for introducing and fastening another sensor in this measurement zone, such as a Pitot tube interchangeable with the removably mounted probe and intended to measure the velocity of 30 the gaseous effluent in the measurement duct 5 by comparing the dynamic pressure with the static pressure. The measurement tube 6 is fastened to the flange 3 by means of two clamping collars, or nut and counternut, 17. The second tube 12 fastening the measurement duct 5 to 35 the flange 3 is a lower guide tube which includes the electrical supply necessary for the operation of the device of 12 the invention. It is fastened to the upstream part of the measurement cell 20. It passes through the flange 3 and is fastened thereto by means of two clamping collars 13. Its other end is fastened to a heating muff 14 which surrounds the 5 upstream part of the measurement duct 5. This fastening, at a variable distance, is achieved by screwing into a tee connector 15 of the heating muff 14. The heating muff 14, made of Inconel sheet, forms part of the measurement cell 20. It is placed around the upstream 10 part of the measurement duct 5. Its length is equal to approximately three quarters of the length of the measurement duct 5, for example 315 m. The guide tube 12 serves mainly as the passage for the electrical supply conductors 16 from the outside of the stack to an electrical resistance heating 15 element helically wound around the duct 5 inside the muff 14. The electrical wire constituting the resistance heating element 17 is flattened from its cylindrical cross section in order for it to bear against the measurement duct better and thus increase the area for heat exchange. Moreover, a 20 sufficient temperature difference allows direct contacting, dispensing with the brazed or welded hot-part/cold-part junctions and eliminating the expansion problems encountered at high temperatures and/or large loads. The power of the electrical heating device is approximately 1500 W. 25 A thermocouple 19 is inserted between the turns of the conductive heating wire 17. It is connected to an indicating thermostat 21 external to the stack. An on/off safety contact cuts off the supply should the skin temperature of the heating wire 17, namely 450 0 C, be exceeded, knowing that the maximum 30 temperature that the wire can withstand is 6000C. The second contact of this thermostat 21 can be used for transmitting a malfunction alarm. A heat-insulating jacket 18 is interposed between the electrical resistance element 17 and the heating muff 14. It 35 includes a sealing bead protected by an aluminium adhesive 13 tape. Two collars 23 are welded to the duct 5 at the ends of the muff 14 in order to completely insulate the heating wire 17 and the heat-insulating jacket 18 from being corroded by the gaseous effluent. By virtue of this heat insulation 18, the 5 temperature drops to 60 0 C at 2 cm from the measurement cell 20, this being suitable for the lining 2 since the latter can withstand up to 80 0 C. A sleeve 24 is mounted in a similar way on the two tubes 6 and 12 downstream of the measurement tube 6; it houses 10 a temperature probe sensor 25, introduced into the measurement duct 5, just downstream of the measurement zone 9. The temperature measurement that it provides at 27 serves for regulating the heating power at 22 in order to obtain a temperature of between 80 and 95 0 C in the measurement zone 9. 15 The device may also include a pipe 26 for blowing in air in order to clean the triboelectric probe 8, should this be required, without dismantling it. It emerges in the measurement tube 6 via a lateral cap near the mounting flange 3. The mechanical mounting is designed to allow the blowing device to 20 be an optional variant to the basic equipment. Figure 2 is a diagram of the complete analytical apparatus. The output of the triboelectric probe 8 is connected to an electronic unit 31 which processes the detection signals 25 coming from its shaping module 28 (see also Figure 1) in order to deliver a dust-concentration weight measurement. The measurements delivered by other sensors and various analysers relating to the other pollutants are sent to a controller 33 which also receives the dust concentration value. 30 The controller 33 is connected to a supervision system 34 (in practice, a computer) which uses the various values, under the control of preprogrammed software, and manages the information useful for monitoring the pollution and for meeting 14 the legal standards with regard to the cleanliness of the gaseous effluent discharged by the stack. The velocity of the gaseous effluent in the measurement duct 5 is periodically measured by removing the triboelectric 5 probe 8 from the measurement duct 5 and replacing it with a Pitot tube. This checking operation, lasting approximately 10 minutes, may be carried out once a month. By means of the functional information-transmission and control connections appearing in Figure 2, it has been 10 attempted to symbolize most of the automatic management procedures. They involve not only the measurement cell in its probe 10 (with its own module 28 for supply and for shaping the associated triboelectric signal) and its heating muff 14, as has already been described with regard to Figure 1, but also 15 sensors 35, for measuring temperature, pressure, flow rate, velocity of the effluent in the stack bounded by the wall 1, and sampling taps 36 conveying the samples to the set of analysers 32 for monitoring the gaseous pollutants. The sensors 35 and the sampling-tap equipment 36 are assumed to be located 20 at other levels on the stack than .the dust-concentration measurement cell. The sensors 35 transmit their information directly to the controller 33, while, as in the case of the information relating to the dust concentration measurement by the probe 10, 25 the temperature signals via the sensor 25 and the velocity signals via the sporadically used Pitot tube, which correspond to measurements carried out at the triboelectric probe, see their results processed by the electrical unit 31 specific to the dust detection device according to the invention. The same 30 applies to the information regarding the skin temperature of the electrical resistance element of the heating muff 14. The latter receives, back from the unit 31, the commands which control its supply with electric current, either in on/off mode in the case of the local threshold temperature being exceeded, 35 or in analogue variation as a function of the temperature recorded in the measurement zone.

15 The electronic unit 31 transmits the results of its calculations to the controller 33, from which it receives information relating to a maximum tolerable threshold for the dust weight concentration that it generates, as well as, where 5 appropriate, comparative calibration information and commands such as the one which determines, if required (detected by aberrant dust concentration results), the switching-on of the blown-air cleaning device for a predetermined or calculated period of time. 10 From Figure 2 it may be seen, moreover, that it is at the controller 33 that the calculations for converting the assays into units under standard temperature and pressure conditions are carried out and that various commands, which in particular control the gas samples to be taken from the 15 effluent for assaying the non-condensable polluting gases, together with the calibrations other than those which relate to the dust concentration, are managed. As is conventional per se, the controller also serves for the acquisition of the various data in a form enabling them to be used by the supervision 20 system on the computer 34, which is equipped with all useful means for display, programming and alarm-signal emission. Further features of the invention that perhaps have not been sufficiently explained in the case of the illustrative embodiment considered above relate to the construction of the 25 triboelectric probe and that of the measurement duct. Thus, Figure 1 in its lower part shows that the tube 5, which constitutes the actual measurement duct, extends slightly from the heating muff 14 beyond the lower collar 23. The reason for this arrangement is to guide the streams of fluid in order 30 to separate those which pass into the duct from those which clearly pass around it, causing no turbulence. Nevertheless, the overall height of the duct 5 remains less than the opening 4 made in the wall of the stack. With regard to the construction of the device at the 35 triboelectric probe, the latter consists of a solid rod coated 16 with Teflon (polytetrafluoroethylene), as is conventional per se. On the other hand, since the sensitive part 8 and even the body of the probe 10 in its guide tube 6 are in the presence of other pollutants in gaseous form and at high 5 temperature, it is desirable to avoid any deterioration likely to occur in the guide tube 6. Provision is therefore made within the context of the invention for the metal surfaces exposed to the effluent inside the stack, although they are heated, to be provided with a coating for protecting them 10 against corrosion by acid flue gases (especially sulphur containing acids and hydrochloric acid) . For this purpose, it is possible, in particular, to use a polytetrafluoroethylene spray, plus a sheath made of a film of heat-shrinkable plastic, such as polyethylene. This precaution is recommended most 15 particularly in the case of the outer surface of the wall of the upper guide tube 6, in its half adjoining the wall 1 of the stack, as shown at 29 in Figure 1. This is because experience has shown that there is a tendency for pitting to occur at this point, something which may be explained by the presence of hot 20 gas (coming from the measurement zone 9) in the annular space between the guide tube 6 and the rod which it contains and which operates as the triboelectric probe. Moreover, it will certainly have been understood from the foregoing description that, in so far as the upper guide 25 tube 6 is made of a conducting metal alloy, only so as to define the electrical earth reference for the triboelectric measurement in the module 28 outside the stack, it is important to ensure that there is no leakage of electrical charge between the solid rod of the probe 10 and its guide tube 6. In order to 30 better provide non-conductive guiding, provision has been made, on the measurement cell side, in the immediate proximity of the mechanically strong support with respect to the connector 7, for the rod of the probe 10 to be equipped, as illustrated in Figure 1, with a centring ring 30 made of 35 polytetrafluoroethylene (an electrically non-conductive material) and held fixed onto the probe between two elastically deformable circular clips.

17 However, the invention is not limited, of course, to the embodiment that has just been described. For example, the dimensional characteristics indicated are especially advantageous in the application of the invention 5 to the analysis of flue gases discharged by plant stacks such as those normally present in an incineration plant. These data have the advantage of emphasizing ratios for transposition to other applications. Thus, it may be seen that the diameter of the measurement duct, including that with its heating muff, 10 corresponds to a fraction of the radius of the stack and that it is placed, in a section of the stack, both away from the outer wall of the stack and from its longitudinal axis. Consequently, it is located in an average-velocity zone in the transverse velocity profile across the stack, even if there is 15 no obvious plateau in the velocity profile. Moreover, the device may be simplified with respect to the example shown in detail above. In particular, rather than monitoring the temperature of the electrical resistance heating element (means 19-21) supplemented by true analogue control of 20 the power which supplies it depending on the temperature detected in the measurement zone (means 25, 27, 22), it will often be possible just to adjust the heating power to a set value calculated to be sufficient under the operating conditions, depending on the predetermined parameters .(or else 25 calculated in the plant) relating to the moisture content and the velocity of the effluent to be analysed. With regard to the response sensitivity of the triboelectric probe, this is generally set beforehand, in the electronic signal shaping and processing unit specific to it, 30 based on calibration measurements carried out by assaying the dust deposits on a strip of paper. From another standpoint, it should be noted that the vocabulary used corresponds to a preferred application of the method forming the subject of the invention, but it should be 35 understood in a broad sense to extend to other applications.

18 The same applies to a notion of a stack. The function of this stack is to convey a gaseous effluent in which it is desired to detect the impacts of solid particles on a probe, allowing a weight assay to be deduced therefrom. It could therefore be, 5 for example, a pipe in which a gas containing suspended particles is flowing from one station to another in an industrial process. It may also be seen here that the term dust extends to the notion of all liquid or solid particles. However, it remains the case that the invention is most 10 particularly beneficial in contexts involving wet treatments.

Claims

1. Method of analysing a gaseous effluent flowing through a gaseous effluent discharge stack, applicable to the case of a water-saturated effluent, characterized in that it 5 includes a step of dust weight measurement by in situ detection of the density of impacts of dust particles on a sensitive part (8) of a triboelectric probe (10) introduced into said stack (1), along the path of a portion of the gas stream flowing longitudinally in a measurement duct (5) , placed in the stack 10 and downstream of a muff (14) for heating said measurement duct (5) so as to take said portion of the stream locally to a temperature above the dew point of the water vapour in the effluent.

2. Method according to Claim 1, characterized in that 15 said measurement duct (5) is positioned in the stack (1) in such a way that the average velocity of the gaseous effluent in the stack (1) and the velocity of said portion passing through the measurement duct (5) are close to the isokinetic condition and in that an external supply for said heating muff (14), 20 preferably an electrical resistance heating muff, is adjusted depending on predetermined parameters relating to the moisture content and the velocity of the gaseous effluent in such a way that the temperature of said portion of the stream remains, in operation, above the dew point in the vicinity of said 25 sensitive part (8) of the probe in order to avoid any condensation of water droplets thereon.

3. Method according to Claim 2, characterized in that the velocity of the gaseous effluent in the measurement duct (5) is measured periodically, in a measurement zone (9) where 30 said sensitive part (8) is placed, in order to check that it is close to the isokinetic condition with the average velocity of the gaseous effluent in the stack (1).

4. Device for implementing the method according to one of the preceding claims, characterized in that it comprises a 20 mechanically strong structure which includes two passage tubes through the wall of the stack (1), namely an upper tube (6) forming a guide tube for the triboelectric probe (10) and a lower tube (12) for means for supplying the heating muff (14), 5 said tubes (6, 12) extending from said measurement duct (5) or from its heating muff (14), respectively, to a plate (3) capable of forming a mounting flange on the wall of the stack (1).

5. Device according to Claim 4, characterized in that 10 at least one of said tubes (6, 12) is adjustable in the way it is fastened to the measurement duct (5), so as to allow the position of the measurement duct (5) in the stack (1) to be modified, said fastening being achieved in particular by screwing. 15

6. Device according to Claim 4 or 5, characterized in that said triboelectric probe (10) is removably mounted in its guide tube (6) and in that it includes means for measuring the velocity of the gaseous effluent in the measurement duct (5), these means being interchangeable with said probe (10). 20

7. Device according to any one of Claims 4 to 6, characterized in that it includes a sleeve (24) intended for the introduction of a temperature sensor (25) in the downstream part of the measurement duct (5), said sleeve (24) emerging in the downstream part of the measurement duct (5) and passing 25 through the flange (3).

8. Device according to any one of Claims 7 to 10, characterized in that the heating muff (14) includes an electrical resistance element (17) applied against the wall of the measurement duct (5) and covered with a heat-insulating 30 jacket (18).

9. Device according to Claim 8, characterized in that it includes means (19, 21) for monitoring the skin temperature of said electrical resistance element (17) and causing its supply to be cut off if the temperature exceeds a predetermined 21 value, and/or means (27, 22) for regulating this temperature depending on the information delivered by the temperature sensor (25) of Claim 7.

10. Device according to any one of Claims 4 to 8, 5 characterized in that it includes a device (26) for blowing air from the outside into said upper guide tube (6), thereby cleaning the triboelectric probe (10).

11. Device according to any one of the preceding claims, characterized in that it includes an anticorrosion 10 protective coating on the metal surfaces which are heated in operation and exposed to the effluent, especially on the upper guide tube (6).

12. Apparatus for analysing a gaseous effluent, which includes a device according to any one of Claims 4 to 11, 15 characterized in that said device is associated therein with means known per se for taking samples of the effluent for analysis of gaseous pollutants by an analyser (32) and in that it includes an electronic unit (31) for processing the detection information from the triboelectric probe (10), 20 transmitting this information to a controller (33) which also receives the results delivered by said analyser (32) and, in particular, ensures their conversion in order for it to be expressed, as well as the dust weight concentration per unit volume of gas under standard temperature and pressure 25 conditions, said controller (33) furthermore being advantageously completed by a computer supervision system (34) able to manage pollution control information with regard to the gaseous effluent.