WO2015036725A1

WO2015036725A1 - Gas sensor

Info

Publication number: WO2015036725A1
Application number: PCT/GB2014/000350
Authority: WO
Inventors: Charles David Coe
Original assignee: Codel International Limited
Priority date: 2013-09-11
Filing date: 2014-09-04
Publication date: 2015-03-19
Also published as: GB2518155A; GB2518155B; GB201316159D0

Abstract

A gas sensor (300) for sensing gases in a stream of gas/ particles, said sensor comprising: an outer casing (301, 302) having one end (304) adapted to be exposed to said stream of particles; an electro-chemical sensor (500) for sensing chemicals in a gas; and a gas permeable material (504) positioned between said electro- chemical sensor and said exposed end of said outer casing, for allowing gas to permeate to said electro-chemical sensor from said exposed end.

Description

GAS SENSOR

Field of the Invention

[0001] The present invention relates to gas sensors, and particularly although not exclusively to gas sensors for use in sampling gases in harsh environments such as chimneys, coal mills, or flues.

Background of the Invention

[0002] There are several industrial applications where it is necessary to examine the chemical composition of gases in a harsh environment. A harsh environment can include highly abrasive environments, high temperatures, high pressures, and corrosive environments.

[0003] In known coal fired power stations, crushed or powdered coal is fed into a furnace, to be burnt, producing heat. The heat creates steam in a boiler, which is used to drive a set of turbines, which drive generators to produce electricity.

[0004] Coal fired power stations consume large amounts of coal, which is delivered in large lumps, with particle size up to 30cm or 40cm. The coal must be crushed to create powdered coal to ensure uniform combustion. Bulk coal is pulverized to create a fine granular powder which is fed into the coal station boilers for combustion. Pulverization of the coal is performed in large coal mills, which are off the order of 10m high. Known coal mills comprise large stainless steel chambers containing ball bearings and a set of grinding rollers.

[0005] Referring to Figure 1 herein, there is shown one type of known coal mill 100 for pulverizing coal into a fine powder. [0006] Coal is fed through a coal inlet 101 at the top of the chamber, ground by the grinding rollers 102, 103 and ball bearings, and pulverized coal is fed through an outlet at the top of the coal mill. [0007] The powdered coal is fed into the boiler via a carrier air stream, along one or more coal powder supply pipes 104,105, 106.

[0008] A large amount of energy is involved in pulverizing the coal, and the interior of the coal mill becomes heated, and sometimes this leads to fires within the coal mill. Fires within coal mills can be catastrophic. At minimum, the interior of the mill becomes damaged, but potentially such fires are also life threatening and can lead to mill explosions. Where a mill explosion occurs, this can destroy the whole building in which the mill is situated, and accidents have been known where several persons have been killed by coal mill explosions.

[0009] Therefore, an early indication of the onset of a fire within a coal mill is desirable. However, this provides a problem in sensing a fire within a mill, because the environment within the mill is very harsh, being extremely abrasive. The properties of the coal dust are such that any sensors placed in the mill or in the carrier air stream will become eroded very quickly and would be worn away within a few minutes of installation.

[0010] Combustion within the mill gives rise to carbon monoxide (CO) as a combustion product. Hence, measuring the carbon monoxide content of gas within the mill can give an indication of any fires or potential fires within the mill. Normally, there should only be very low levels of background carbon monoxide within the mill, typically 10 or 20 parts per million (ppm). If a fire occurs, the carbon monoxide content will fairly quickly increase to levels of between 100ppm and 500ppm. At these carbon monoxide levels, operators would consider shutting the mill down and purging it to quench any latent combustion. [0011] Due to the harshly abrasive environment, there is a problem in measuring the carbon monoxide content of gas within the mill.

[0012] There are several companies providing known systems whereby a sample of gas is extracted from the main chamber of the mill, and a gas is fed into an analyzer. These systems are cumbersome and the main problem is extracting a gas sample, because any sampling device placed in the main chamber of the mill becomes eroded and destroyed very quickly. Such prior art analyzers are produced by Godel International Limited.

[0013] However, no one has so far produced a truly satisfactory analysis system. The known systems are complicated and have high maintenance requirements. [0014] There remains an ongoing problem of detecting fires within coal crushing mills.

[0015] There are other applications for measuring the chemical composition of gases, such as in chimneys or flues, where the composition of the gases in the center of the gas stream needs to be measured, typically at a position of a minimum of one metre from the chimney wall. However, because the gases present a harsh environment, such as high temperature in the range typically 60°C to 500°C, this precludes inserting a gas sensor directly into the gas stream.

[0016] There are generally two prior art solutions for measuring gas streams in chimneys or flues:

• Optical measurement, by optical sensors outside the chimney or flue directly viewing the gases to be measured. These are often referred to as ln-situ analysers; and • Remote measurement, by gas extraction, transporting the gas to a position remote from the chimney or flue, where a laboratory type analysis can be carried out. [0017] The optical type of sensor often has a probe which extends into the chimney stack, with an optical system located at the end of the probe near and outside the chimney wall containing a light source, a light modulator, and optical detectors. An optical beam is transmitted from a position outside the chimney wall to a position inside of the chimney along the probe, and is reflected from the end of the probe. The returned optical signal is detected, and an intensity and/ or spectrographic analysis is made to give information about the composition of the gas. There is no active sampling of the gas by drawing gas into the probe. Rather gas diffuses into the probe, and its effect on a light beam is measured. Alternatively the light is transmitted across the entire duct or chimney stack obviating the need for a probe. These are often referred to as cross stack analysers.

[0018] The second type of analysis involves extracting a sample of the gas from the chimney or flue, and passing the gas to an analyzer at a location several meters away from the chimney. The analyzer is typically a laboratory type equipment, which although accurate, is also delicate and bulky. A variety of known analysers are currently used.

[0019] Optical sensors are generally not as accurate as the remote analysers. Therefore, the prior art methods are either non sampling optical sensors, which are placed in situ on the chimney, but which suffer from relatively low accuracy, or remote analysis systems, which extract a sample of gas at a remote station, which give good accuracy, but which cannot be positioned at the chimney or flue and require transfer of gas along pipes to a relatively bulky analysis station, and are subject to various sampling errors which can change the constituency of the sampled gas These sampling systems often require regular service and maintenance because of the need to present a clean gas to the analyser thus requiring the filtering of solid particles from the sampled gas.

[0020] Specific embodiments disclosed herein aim to provide an improved gas sensor which can be positioned near to the gas to be sampled, and which give improved accuracy compared to optical sensors.

[0021] Specific embodiments disclosed herein also aim to provide a relatively low maintenance, easily maintained gas sensor device.

[0022] Specific embodiments disclosed herein also aim to provide a sensor device which provides a measurement accuracy similar to the prior art remote sampling analyzers, but without the need to transport gases to a remote location away from the gas flow.

Summary of the Invention

[0023] According to a first aspect there is provided a gas sensor comprising: an outer casing having one end adapted to be exposed to a stream of gas; at least one electro-chemical sensor for sensing chemicals in a gas; and a gas permeable material positioned between said exposed end of said outer casing and said at least one electro-chemical sensor, for allowing gas to permeate to said electro-chemical sensor from said exposed end.

[0024] According to a second aspect there is provided a gas sensor for sensing gases in an output pipe of a coal mill, said pipe carrying a stream of gases and particles, said sensor comprising: an outer casing adapted for fitment to said output pipe, and having one end adapted to be exposed to an interior of said output pipe; an electrochemical sensor for sensing chemicals in a gas sampled from said stream; an inner housing for containing said sensor; a chamber between said electrochemical sensor and said exposed end of said outer casing; and a gas permeable material between said electrochemical sensor and said exposed end of said outer casing, for allowing gas to permeate to said electrochemical sensor.

[0025] According to a third aspect there is provided a method of sensing gases in a stream of gases in a conduit, said method comprising: placing a gas sensor adjacent said stream of gases passing through said conduit, such that said gas sensor can receive gas from said conduit; determining a level of said measured gas in said gas stream; comparing said measured gas level with a predetermined safe level of said gas; and if said determined measured gas level exceeds said predetermined safe level of gas, generating an alert signal. [0026] According to a fourth aspect there is provided a method of sensing gases in chamber of a coal pulverisation mill, said method comprising: placing a gas sensor adjacent a stream of coal particles passing through an output pipe of said coal mill, such that said gas sensor can receive gas from said coal particle stream; determining a level of carbon monoxide in said coal particle stream; comparing said carbon monoxide level with a predetermined safe level of carbon monoxide; and if said determined carbon monoxide level exceeds said predetermined safe level of carbon monoxide, generating an alarm signal.

[0027] Other aspects are as set out in the claims herein. Brief Description of the Drawings

[0028] For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which:

Figure 1 herein illustrates schematically a known coal crushing mill for providing a supply of pulverized coal to a coal fired power station;

Figure 2 herein illustrates schematically a coal crushing mill having a pulverized coal delivery pipe, and a novel gas sensor according to a specific embodiment of the present invention, and showing a specific measurement method according to the present invention;

Figure 3 illustrates schematically a specific embodiment gas sensor according to the present invention in a first view; Figure 4 illustrates schematically a second view of the gas sensor, from the rear;

Figure 5 illustrates schematically in cut away view, the gas sensor of figures 3 and 4 showing internal components;

Figure 6 illustrates schematically in dissembled view the gas sensor of figures 4 3 to 5 herein; Figure 7 illustrates schematically a rear plate of an outer casing of the gas sensor of figures 3 to 6 herein;

Figure 8 illustrates schematically in dissembled view a rear plate, inner insulation, temperature controlled inner housing and electrochemical sensor of the gas sensor of figures 3 to 7 herein;

Figure 9 illustrates schematically the view of figure 8, with the insulation removed, showing in more detail the temperature controlled inner housing and electrochemical sensor;

Figures 10 and 11 show two views of the prior art electrochemical sensor;

Figure 12 shows the view of figure 8 herein, with the insulation and electrochemical sensor removed, showing a seating and connectors for the electrochemical sensor within the temperature controlled housing;

Figure 13 illustrates schematically a method of monitoring carbon monoxide in an output pipe of a coal pulverization mill; Figure 14 illustrates schematically in cut away view, a second gas sensor according to a second specific embodiment, for sensing gases in a flue, transmission pipe, chamber or other environment containing high temperature and/or corrosive gases;

Figure 15 illustrates schematically the second sensor in cut away view as installed in a chimney or flue;

Figure 16 illustrates schematically a third gas sensor in cut away view from one side installed in a chimney or flue; Figure 17 illustrates schematically the third gas sensor in external perspective view;

Figure 18 illustrates schematically components of the third gas sensor of Figures 14 to 17 in exploded view;

Figure 19 illustrates schematically a head portion of the third gas sensor, in position fitted to a flue or pipe; and

Figure 20 illustrates in perspective view, the third gas sensor with the optional electric cooling fan removed.

Detailed Description of the Embodiments

[0029] There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

[0030] In the specification, the term "gas conduit" is used to include a chimney stack, a flue, a gas pipe line, or any other tubular structure which carries a gas, either with or without other solid or liquid components flowing with the gas stream.

[0031] Referring to Figure 2 herein, there is illustrated schematically a gas sensor 202 fitted to an output pipe 201 of a coal mill 200. The output pipe conveys pulverized coal powder from the coal mill to a furnace of a coal fired boiler.

[0032] The gas sensor 202 provides an electrical signal to an electronic detection and control system 203, for detecting the carbon monoxide level within gases passing through the pulverized coal delivery outlet 201.

[0033] Referring to figures 3 and 4 herein, there is shown in perspective view a gas sensor 300 for sensing gases within the coal powder delivery pipe output of a coal mill.

[0034] Figure 3 herein shows the side of the sensor which is exposed to the flow of powdered/pulverized coal dust, and figure 4 shows the reverse side of the sensor which is presented on the outside of the coal powder delivery pipe.

[0035] The sensor 300 comprises a cylindrical metal plate 301 ; centrally located in the cylindrical metal plate, is provided a protruding cylindrical metal tubular casing 302; an annular ring cover plate 303, covering one end of the upstanding cylindrical casing 302; a circular stainless steel mesh filter 304 held in place by the annular ring 303; a cylindrical back plate 305, which is attached to the underside of the main body plate 301 by a plurality of bolts or screws 306- 311 ; a first electrical connector 312 for connecting a cable to read a signal from a sensor device within the housing; a second connector/entry point 313 for receiving an electrical cable for providing power to a Peltier heat pump within the casing; and a third electrical connector/electrical entry port 314 for receiving a electrical cable for providing electrical connections to a temperature sensor within the housing.

[0036] Referring to Figure 5 herein, there is shown schematically the sensor in cut away view bisecting the gas sensor device along a plane coincident with a main central axis of the sensor. Inside the cylindrical casing 302, is provided an electro-chemical sensor 500, within an internal substantially cylindrical tubular metal housing 501 having an annular flange 502 at one end, the other end being open; surrounding the metal housing 501 , is provided a cylindrical tubular insulating component 503; a porous ceramic disc 504 is fitted over the electro-chemical sensor 500 and across the opening of the inner housing 501 , the porous ceramic disc being held in place by an annular cylindrical plate 505. [0037] The housing 501 , electro-chemical gas sensor 500; annular plate

505 and porous ceramic disc 504 are positioned within a hollow cavity formed within the cylindrical tube 502.

[0038] The disc shaped stainless steel filter 304 closes off an opening at one end of the cylindrical tube 302, within which the electro-chemical gas sensor 500, metal housing 501 and temperature insulating material 503 is contained, so that gas can permeate through the stainless steel filter 304, and through the gas porous ceramic disc 504, to reach the electro-chemical gas sensor 500 within the housing 501.

[0039] Between the lower flange 502 of the housing 501 and the back plate 305, is provided a circular, disc shaped or square/rectangular Peltier heat pump 506, connected to an electrical power supply by a set of electrical wires, which exit the gas sensor device through the electrical connections on the rear of the back plate 305. Also provided within the cavity, is a temperature sensor positioned in a drilled hole in the housing 501 to measure the temperature of the metal housing. [0040] The Peltier effect heat pump 506 transfers heat from the housing 501 to the back plate 305. Since the back plate 305 is securely fixed to the main body plate 301 , heat can be transferred from the internal housing 501 to the body plate 301 , from where it transfers by conduction to a metal pipe or housing located on a metal pipe which is part of the coal mill, to which the sensor is attached in use, and which dissipates the heat. Heat is also dissipated by radiation and convection from the rear of the device into the atmosphere thereby keeping the housing 501 at a lower temperature than the rest of the casing, in order to keep the electro-chemical gas sensor 500 within its range of operating temperatures which are typically between 0°C and 60°C.

[0041] In use, the sensor is installed within the wall of a coal delivery pipe, by cutting a substantially circular aperture in the pipe, and bolting the main body 301 to the pipe. Alternatively, the gas sensor can be fitted into a cylindrical housing welded to the pipe with a flange at the end, such that the front surface of the sensor, containing the steel filter, is located almost flush and certainly not protruding past the inner wall of the duct and into the carrier air stream. [0042] When in situ, the cover plate 303 and stainless steel filter 304 are exposed to the flow of coal powder passing along the delivery pipe at the output of the coal mill. The sensor device is subjected to a flow of coal powder, which flows across the cover plate in a direction substantially parallel to a main plane of the cover plate, which results in less abrasion than being located in the actual coal laden air stream.

[0043] In use, gas permeates through the stainless steel filter 304, across the gap or void 507 behind the stainless steel filter and in front of the porous ceramic disc, and permeates the gas porous ceramic disc 504. Gas reaches the electro-chemical sensor 500 via the gas porous ceramic disc 504, allowing the electro-chemical sensor to measure the amount of carbon monoxide in the gas flow. [0044] The measurement and monitoring electronics 203 detect high carbon monoxide content, typically of more than 50 parts per million, and this reading can be used to generate an alarm if the carbon monoxide content reaches a dangerously high content indicating combustion within the coal mill.

[0045] Referring to Figure 6 herein, there is illustrated schematically in perspective view, the back plate 305 and internal insulation material 503, dissembled from the main plate 301 and cylindrical housing 302, showing the electrical connections through the insulating material to the electro-chemical sensor and Peltier effect heat pump.

[0046] Referring to Figure 7, there is illustrated schematically the back plate 305 showing the electrical leads for supplying power to the Peltier effect heat pump, and for carrying the electrical signals from the internal temperature sensor which monitors the temperature of the housing within the sensor.

[0047] Referring to Figure 8 herein, there is illustrated schematically in view from above, the back plate 305, removed from the main plate 302 and main casing. There is shown the cylindrical annular insulation material 503; the electro-chemical sensor 500; and the upper open end of the temperature controlled housing 501, with the gas permeable disc removed.

[0048] Referring to Figure 9 herein, there is shown the back plate 305 and housing assembly, with the cylindrical temperature insulating material removed, showing electrical connections to the temperature sensor and to the Peltier heat effect pump.

[0049] Referring to Figures 10 and 11 herein, there are shown in view from the front, and from the rear, the electro-chemical sensor 500 for sensing Carbon monoxide within the gas flow. Electro-chemical sensor is a known off the shelf component comprising a cylindrical plastics housing having two or more electrical contacts at the rear of the housing, and at the front of the housing a porous membrane, allowing gas through to a set of sensors within the device.

[0050] Referring to Figure 12 herein, there is illustrated schematically the back plate 305 with temperature controlled housing 501 and a set of electrical connectors for plugging the rear of the electro-chemical sensor into.

[0051] In use, the gas sensor device is mounted in the exit outlet pipe of the coal mill, where the coal powder is being transferred away from the coal mill. The exit pipe carries coal powder within a compressed air stream. Typically on a mill, there can be anywhere between 1 and 3 such exit pipes. A flange is welded onto the outlet pipe, and the gas sensor, can be bolted to the flange to sample the gas flowing through the outlet delivery pipe, such that the top of the sensor is flush with the inside of the delivery tube, so that the coal dust passes the sensor in a direction 90° to a main central axis of the sensor, and substantially parallel to the exposed upper stainless steel filter.

[0052] The outwardly facing ring 303 is preferably made from a durable metal, such as Inconel, so as to increase reliability and reduce maintenance due to wear.

[0053] Dust or powder particles will pass by the exposed outer ring 303, but gas will defuse through the metal filter 304. Because gases diffuse through this filter by natural permeation there is no tendency for the filter to become blocked with solid particles from the carrier air stream, thus offering a low maintenance operation without the need for frequent filter cleaning or replacement.

[0054] The electro-chemical cell typically may measure Carbon monoxide content in the range Oppm to 1000ppm. A typical lifetime of a known electrochemical cell is typically around 12 months, and at maximum around 24 months. Hence, the sensor will need maintenance approximately once every year with replacement of the electro-chemical cell and the stainless steel filter 304.

[0055] Referring to Figure 13 herein, there is illustrated schematically method steps for a method of measuring carbon monoxide content in a coal mill pulverization chamber, by reading the carbon monoxide content in a stream of output coal particles in an output pipe of the coal mill.

[0056] In process 1301 , the gas sensor as described herein continuously monitors a carbon monoxide content of gas in the pulverized coal stream passing through the output pipe, as herein before described. The output of the gas sensor is an electrical signal which is related to the carbon monoxide content.

[0057] In process 1302, the output of the gas sensor is compared with a pre-determined output signal level corresponding to a safe level of carbon monoxide, for example less than 50ppm.

[0058] In process 1303, if the real time gas sensor reading exceeds the pre-determined safe carbon monoxide level signal (i.e. a signal corresponding to greater than 50ppm) then an alarm signal is generated by the monitoring equipment 203.

[0059] The monitoring equipment can implement monitoring of the output signal of the gas sensor either by an analogue electronics circuit, or a digitalized circuit. Continuous monitoring and alarm setting can be performed by a microprocessor or computer.

[0060] In the present description a gas sensor for sensing carbon monoxide in the output of a coal mill is described. However the sensor disclosed, may sense other gases such as Sulphur Dioxide, Carbon Dioxide, Methane, Oxygen, Oxides of Nitrogen, with appropriate substitution of the electrochemical sensor. [0061] The first specific embodiment disclosed herein may have an advantage of allowing measurement of a condition within a crushing chamber of coal pulverizing mill without the need to position any sensors within the crushing chamber.

[0062] The first specific embodiment disclosed herein may have an advantage of increased reliability and decreased maintenance time of a sensing system for sensing conditions within a coal crushing chamber.

[0063] The first specific embodiment disclosed herein may have an advantage of greater simplicity and reduced cost compared to conventional methods of sensing conditions within a coal crushing chamber. [0064] In the general case the sensor is not restricted to use in the output of a coal mill, but can be used in ducts, chimneys or the like where there is a flow of gas in a tubular conduit, and the sensor can be fitted to the wall of the conduit, without interfering with the flow of gas through the conduit. The sensor and measurement technique is particularly suited to fluid flows consisting of a stream of particles and gas combined.

[0065] Referring to Figure 14 herein, there is illustrated schematically in perspective view, a second gas sensor according to a second specific embodiment. The second gas sensor is suitable for sampling and sensing gas in a flue, pipe or chamber in a wide variety of applications, including but not limited to petro-chemical plants, chemical distillation plants, chimney flues for incineration plants, boilers or the like. In these applications, the composition of the gas flowing in a pipe or conduit needs to be established. The gas may be at high temperature, and/or may contain high levels of suspended solid particles. The gas flow near the wall of the chimney or flue may not be representative of the bulk gas flow in the central region of the chimney or flue and gas in the center of the flow needs to be analysed. [0066] The second gas sensor 1400 comprises an elongate tubular sampling probe 1401 having at a first end at least one gas inlet aperture 1402, and at a second end one or a plurality of gas outlet apertures 1403; the probe being fitted at its second end with a peripheral annular flange 1404 extending in the direction transverse to a main axial direction of the probe; a cylindrical metal plate 1405 having a protruding cylindrical metal tubular part 1406; an annular ring cover plate 1407 covering one end of the cylindrical tube 1406, the cover plate 1407 having a substantially cylindrical central aperture, the ring plate covering one end of the cylindrical tube 1406 and retaining a circular sintered stainless steel mesh filter 1408 across the otherwise open end of the cylindrical tube, the circular stainless steel mesh filter 1408 being held in place by the annular ring 1407; a sensor casing 1409, one end of which fits into the cylindrical metal plate 1405 and partially into an upper end of the cylindrical tube 1406, said sensor casing 1409 being a cast or machined metal component; the sensor casing 1409 comprising a chamber 1410, into which fits a sensor housing 1411 ; said sensor housing 1411 containing at least one electro-chemical sensor 1412, and in the embodiment shown, four electrochemical sensors; the chamber 1410 also containing at least one Peltier heat pump 1413, and a thermally insulating tubular cylindrical sleeve 1414 for fitting around the electro-chemical sensor and Peltier heat pump; the heat pump 1413 transmits heat from the electro-chemical sensors, via the sensor housing 1411 to the sensor casing 1409; connected to the top of the casing 1409 is a cast metal finned heat sink 1415, in mechanical and thermal contact with the metal casing 1409 to dissipate heat in the housing and keep the housing cool; and on top of the finned heat sink 1415, an electric fan 1416 for blowing air through the finned heat sink 1415 for forced air cooling of the sensor casing.

[0067] Whilst an upper end of the chamber 1410 is closed off by the material of the casing 1409, a second, opposite end of the chamber is closed off by an annular metal plate 1418, having an aperture behind which is retained a Porous ceramic diffuser or membrane 1420, one surface of which is opened to a cavity 1421 formed between the ceramic diffuser, the stainless steel diffuser 1408 and the inner walls of the cylindrical tubular part 1406.

[0068] The casing 1409 has one or a plurality of passages, through which electrical cables can be fitted to one or a plurality of plug/socket electrical terminals 1417 mounted at the side of the casing. At least one first electrical connector is fitted for controlling the current to the Peltier heat pump, and at least one second electrical connector is provided for connecting to an electrical signal cable for carrying a signal from the at least one electro-chemical sensor.

[0069] The first end of the tubular probe 1401 has a rounded, curved or angled end 1422 on an opposite side of the tube to the aperture 1402.

[0070] Referring to Figure 15 herein, the wall of the chimney, pipe or chamber has formed in it a substantially cylindrical aperture, into which is fitted a cylindrical tubular liner 1500. The tubular liner 1500 is fitted at one end with a peripheral annular flange 1501 , having a plurality of through apertures. The through apertures may be tapped, and suitable for fitting a set of bolts 1502 to secure the flanged plate 1405 of the sensor to the flange 1501 of the tubular liner. Alternatively, the apertures may be clearance holes, so that a bolt can be passed through the clearance hole, and bolted to the reverse of the flange using a nut and spring washer.

[0071] The cylindrical liner 1500 is of a diameter greater than the outside diameter of the tubular probe 1401 , leaving a cylindrical gap 1503 between the liner and the probe.

[0072] In the installation shown, the tubular probe is inserted into the upward flow of the chimney stack, having the inlet aperture 1402 at a lower height than the upper end of the tubular probe, so as to allow for any condensation of liquid inside the probe to flow down along the probe and out of the aperture 1402. [0073] Operation of the second sensor will now be described.

[0074] The sensor may be installed in a chimney stack as shown in Figure 15, having the open aperture 1402 of the probe projecting into the upward gas flow at a position away from the side wall of the chimney stack. Typically, measurements, of gases need to be taken towards the center of the gas flow, and the purpose of the probe is to deliver a supply of gas to the stainless steel filter, from where it can diffuse to the inlet membranes of the electrochemical sensor(s). Gases enter the aperture 1402, and due to the increased dynamic gas pressure towards the center of the ohimnoy, as compared to tho rolativoly lowor dynamic gas pressure near the chimney wall, gases flow towards the electro-chemical sensors at the second end of the probe as shown by the arrows in Figure 15. Gases flow into the probe, along the probe to the sensor head containing the temperature controlled electrochemical sensors, and out of the exit vents or apertures 1403 near the sensor head. Gas flow is delivered to the stainless steel diffusing filter 1408 and permeates through the filter into the cavity 1421 behind the filter. The gas permeates through the ceramic diffuser 1414, to the electro-chemical sensors 1410. Excess gas leaves the probe tube 1401 through the second apertures 1403 and return to the chimney via the cylindrical gap between the liner 1500 and the probe.

[0075] The dynamic pressure of the gas in the center of the chimney or flue cause the gases to travel along the inside of the probe to the relatively lower pressure region at the second end of the probe, which is near the wall of the chimney or flue. Chimney gases escape from the second end of the probe tube via the second apertures 1403, and travel along the gap between the liner 15 and the outer wall of the probe 1400, to be released back into the gas flow of the chimney or flue.

[0076] Cooling of the electrochemical sensors is effected by transfer of heat from the sensors to the thermally controlled heat sink housing 1411. The heat sink housing or mounting is temperature controlled by transfer of heat from between the heat sink housing 1411 and the sensor casing 1409 via the Peltier heat pump 1413. [0077] The sensor casing 1409 may be cooled by conduction of heat to the external heat sink 1415, from where it dissipates into the atmosphere by convection, assisted by the fan.

[0078] Referring to Figure 16 herein, there is illustrated schematically in perspective cut away view, a third sensor according to a third specific embodiment. The construction, operation and installation of the third sensor is substantially the same as that for the second sensor described herein before, with the exception that a distal first end 1600 of the probe 1601 consists of a circular end, cut across perpendicular to a main axial length of the sensor probe and closed off by a circular plate 1602.

[0079] In the installation shown in Figure 16, the cylindrical liner 1603 is inserted into a concrete chimney wall, with an open aperture 1604 of the probe collecting rising gases which are transported substantially horizontally along the length of the probe, to the sensing head 1605, which is positioned outside the chimney, and bolted to a flange 1606 at one end of a cylindrical wall liner 1607. This arrangement enables the sensor devices to be placed immediately adjacent the flue or chimney, but outside the flue or chimney, where the temperatures are lower.

Maintenance and Servicing

[0080] Since the complete sensor head and probe can be removed by undoing the bolts which attach the flange at the sensor head to the flange on the liner, the whole gas sensor assembly can be easily removed and swapped with a replacement unit, if the gas sensor needs servicing, or needs to be replaced. [0081] For maintenance or servicing, the sensor head can be dissembled by undoing the bolts which attach the sensor housing to the cylindrical tube. From there, the stainless steel filter, ceramic filter, and electrochemical sensors can be replaced. In the best mode embodiment, the sensor housing including the filters, sensors, heat sink mounting and Peltier effect heat pump can be removed as a replaceable cartridge unit. The fan unit bolts or screws to the sensor housing, and may be removed without removing the gas sensor from the chimney or flue. [0082] A cartridge unit as above can be replaced with a replacement cartridge having a set of one or more electrochemical sensors for detecting different elements or components, thereby changing the use of the sensor to detect different components using the same probe, without the need to install a complete new mounting to the chimney or flue.

[0083] The sensor head can be fitted with a replacement probe unit, if the probe unit needs cleaning or is damaged.

[0084] Referring to Figure 17 herein, there is illustrated schematically the third gas sensor of Figure 16, shown in perspective view and removed from the chimney or flue installation.

[0085] Referring to Figure 18 herein, there is illustrated schematically in exploded view main components of the third gas sensor device shown in Figures 16 and 17 herein.

[0086] The main components include probe 1601 having a flange 1801 , distal aperture 1604 at a first end of the probe, and one or a plurality of proximal apertures 1802 at a second end of the probe; cylindrical housing 1804 with attached cylindrical flange plate 1805; sensor housing 1806; heat sink 1809 and electric ian 1810. [0087] Referring to Figure 19 herein, there is illustrated schematically in perspective view the third gas sensor installed in a chimney or flue, showing the sensor head in external view. [0088] Referring to Figure 20 herein, there is shown in perspective view, the third gas sensor of Figures 16 to 19 herein, with the optional electric fan removed from the heat sink. In some applications, the additional cooling provided by the electric fan may be unnecessary, and sufficient heat may be dissipated by the finned heat sink in contact with the sensor casing, which are both open to the atmosphere at a position outside the chimney or flue.

[0089] The gas sensors disclosed herein may have an advantage of providing a more accurate gas measurement taken at a position immediately adjacent to a gas flow, and may provide improved reliability, easy maintenance, and lower cost operation compared to known optical gas sensors.

[0090] The gas sensors disclosed herein may provide a device which, like for like with comparable prior art optical gas sensors, is almost an order of magnitude less expensive due to simpler construction.

[0091] The gas sensors described herein have a feature of performing sampling of a gas by a sensor, at a position immediately adjacent a pipe, flue or chimney, without transporting the sampled gas to a remote location away from the pipe, flue or chimney.

Claims

1. A gas sensor comprising: an outer casing having one end adapted to be exposed to a stream of gas; at least one electro-chemical sensor for sensing chemicals in a gas; and a gas permeable material positioned between an exposed end of said outer casing and said at least one electro-chemical sensor, for allowing gas to permeate to said at least one electro-chemical sensor from said exposed end.

2. A gas sensor according to claim 1 , comprising an inner housing for containing said at least one electro-chemical sensor.

3. A gas sensor according to claim 2, wherein said inner housing is temperature controlled.

4. A gas sensor according to claim 2 or 3, comprising a Peltier effect heat pump having one side in thermal contact with said outer casing, and another side in thermal contact with said housing, for transferring heat from said inner housing to said outer casing.

5. A gas sensor according to any one of claims 2 to 4, wherein said gas permeable material closes off said inner housing, whilst allowing gas to reach said electrochemical sensor.

6. A gas sensor according to any one of the preceding claims, comprising a thermally insulating material positioned between said electrochemical sensor and said outer casing, for thermally insulating said electrochemical sensor from said outer casing.

7. A gas sensor according to any one of the preceding claims, comprising a temperature sensor for sensing a temperature in or around said electrochemical sensor.

8. A gas sensor according to any one of the preceding claims, further comprising a filter located at said exposed end of said outer casing, for filtering out solid particles from a gas flow to said electrochemical sensor.

9. A gas sensor according to claim 8, wherein said filter is located at said exposed end of said outer casing.

10. A gas sensor according to claim 8 or 9, wherein said filter is a removable replaceable part.

11. A gas sensor according to any one of claims 8 to 10, wherein said gas permeable material is positioned between said electro-chemical sensor and said filter.

12. A gas sensor as claimed in any one of claims 8 to 11 , wherein said filter comprises a stainless steel filter.

13. A gas sensor according to any one of the preceding claims, further comprising a cover plate located at said exposed end for covering said exposed end, said cover plate being formed of a hard metal or hard metal alloy material.

14. A gas sensor according to claim 13, wherein said cover plate comprises Inconel.

15. A gas sensor according to claim 13 or 14, as appendant to claim 8, wherein said filter is retained in place by said cover plate.

16. A gas sensor according to any one of the preceding claims, wherein said outer casing comprises: a tubular housing for containing said electro chemical sensor, said tubular housing being adapted to extend through a side wall of said output pipe; an outer flange plate connected to said tubular housing; and a rear cover plate attaching to said tubular housing and / or flange plate.

17. A gas sensor according to claim 16, wherein said tubular housing comprises a gas permeable channel for allowing a gas present at an outer exposed end of said tubular housing to permeate through said housing to reach said electrochemical sensor within said housing.

18. A gas sensor according to claim 16 or 17, wherein said rear cover plate is provided with one or a plurality of electrical outlets for passage of electrical contacts to a cooling apparatus within said outer casing.

19. A gas sensor according to any one of claims 16 to 18, wherein said outer flange plate is adapted for securing to a wall of a pulverised coal delivery pipe of a coal pulverising mill.

20. A gas sensor according to any one of claims 1 to 18, comprising an elongate probe having at a first end, a first aperture for inlet of gas; and at a second end, one or a plurality of second apertures for outlet of gas; said second end of said probe delivering gas to said electro-chemical sensor.

21. A gas sensor for sensing gases in a conduit or chamber, said conduit or chamber carrying a stream of gases and particles, said sensor comprising: an outer casing adapted for fitment to said output pipe, and having one end adapted to be exposed to an interior of said output pipe; an electrochemical sensor for sensing chemicals in a gas sampled from said stream; an inner housing for containing said sensor; a chamber between said electrochemical sensor and said exposed end of said outer casing; and a gas permeable material between said electrochemical sensor and said exposed end of said outer casing, for allowing gas to permeate to said electrochemical sensor.

22. A method of sensing gases in a gas stream in a conduit or chamber, said method comprising: placing a gas sensor adjacent said stream of gases passing through said conduit or chamber, such that said gas sensor can receive gas from said conduit or chamber; determining a level of said measured gas in said gas stream; comparing said measured gas level with a predetermined safe level of said gas; and if said determined measured gas level exceeds said predetermined safe level of gas, generating an alert signal.

23. The method according to claim 22, wherein said gas sensor comprises: an outer casing having one end adapted to be exposed to said gas stream; an electro-chemical sensor for sensing chemicals in a gas; and a gas permeable material positioned between said aperture and said exposed end of said outer casing, for allowing gas to permeate to said electrochemical sensor from said exposed end.

24. The method according to claim 23, comprising regulating a temperature of said electrochemical gas sensor by enclosing said sensor in a temperature controlled housing.

25. The method according to claim 23 or 24, comprising maintaining said electrochemical sensor within a temperature range of 0°C to 60°C.

26. A method of sensing gases in a chamber of a coal pulverisation mill, said method comprising: placing a gas sensor adjacent a stream of coal particles passing through an output pipe of said coal mill, such that said gas sensor can receive gas from said coal particle stream; determining a level of carbon monoxide in said coal particle stream; comparing said carbon monoxide level with a predetermined safe level of carbon monoxide; and if said determined carbon monoxide level exceeds said predetermined safe level of carbon monoxide, generating an alarm signal.