GB2026276A - Particle monitor - Google Patents

Abstract

Apparatus and method are provided for monitoring the occurrence of agglomerates of flue dust in the gas stream emitted by an industrial chimney, by means of continuous microwave Doppler radar sampling a cross-section, obliquely angled to the chimney axis, of the total gas stream adjacent the chimney mouth. <IMAGE>

Description

SPECIFICATION Particle monitor The present invention relates to the monitoring of the emission, from industrial chimneys, of agglomerates of flue dust. The invention is particularly applicable to cement works' chimneys, the agglomerates being commonly referred to as blobs, which term will be used herein.

The emission of blobs from the chimneys of cement works is undesirable in itself and a potential annoyance particularly if blob material falls in a built-up area. In order to prevent the deposition of blobs it is necessary to identify the sources or mechanisms responsible for the emission. Information available from fall-out reports or from physical and chemical analysis of the blobs themselves proves insufficient for such identification, and there is a need for a means of monitoring the emission of blobs from the tops of the chimneys, so that ideas as to the causes and control of blob emission can be tested and judged with a view to reduction of such emission.

We postulate that the threshold level at which deposition of blobs might cause annoyance to local inhabitants may be about one blob of 1 mm diameter material deposited in 1 M2 in an area of ground in the neighbourhood of the chimney over a three-hour period. If the blobs were emitted continuously from a single stack and deposited into a total area of, say, 104m2 this threshold deposition would correspond to an emission rate from a chimney of about 1 sent1. If the blobs were emitted in a single puff then this puff would contain 104 blobs.

It is accordingly an object of the invention to provide for monitoring instrumentation with the ability to make continuous observation on the emission of low density blobs from the top of a chimney with blob diameters down to about 1 mm at rates down to a sec-', with equipment which would operate without maintenance for several months. It is a further object to make such observation in the presence of the usual background dust load in the chimney gas stream.

It has previously been proposed to employ a pulsed Doppler radar system for monitoring blob emission by scanning the gas stream as a whole in the chimney, but this system required an excessive amount of power and was found insufficiently sensitive to detect blobs at the low rate of emission likely to constitute a nuisance, and the equipment required was susceptible to adverse extraneous influences.

The invention is based on the use of low power microwave Doppler radar to sample the gas stream at the mouth of the chimney. At an effective blob size of 1.5 mm diameter the sensitivity is sufficient to sample about 0.1 per cent of the chimney outflow area. Experience suggests that this sensitivity is quite adequate for practical purposes. The monitoring technique is not sensitive to the normal dust burden in the flue gas stream because of the rapid reduction in radar scattering cross-section, and hence received signal, with decrease in particle size.

Components and sub-systems for continuous wave Doppler radar are readily available.

The use of lower power continuous wave radiation permits lower inherent noise to be achieved, and opens the way to observing objects of small scatter cross-section at close range, leading to the sampling of the rate of occurrence of individual events.

According to the invention a method of monitor ing the occurrence of agglomerates of flue dust in the gas stream emitted by an industrial chimney comprises selecting a non-axial cross-section of the total gas stream adjacent the chimney mouth, detecting the passing of agglomerates through at least one predetermined fractional sample of said stream cross-section by means of low power continuous microwave Doppler radar directed along said stream cross-section and responsive to the scatter cross-sections of individual agglomerates pas sing through said sample to produce signals indicative of said passing agglomerates, amplifying said signals and displaying a representation of the amp lified signals.

In preferred embodiments of the invention rep resentations of the signals are recorded, for instance in chart form, and the signal to be recorded is filtered to select those Dopler frequencies appropriate to the prevailing chimney gas flow velocity and processed to provide an output relating to the rate of occurr ence of Doppler signals above a predetermined threshold signal amplitude.

The invention also provides apparatus for record ing the occurrence of agglomerates of flue dust in the gas stream emitted by an industrial chimney, comprising: (a) a gastight monitor module which comprises a transmission antenna coupled to a source of continuous microwave radiation, a reception antenna coupled to a microwave radiation detector, and means for deriving Doppler radar signals from said radiation, the module being constructed and dimensioned to respond to the scatter cross-sections of indi vidual agglomerates passing within a pre determined range in front of the antennae, and the antennae being adapted for mounting at the mouth of the chimney facing down the interior of the chimney at an angle to the chimney axis with said source and detector outside the chimney;; (b) electronic signal processing circuitry for coupl ing to the monitor module to amplify said signals; and (c) means for representing and recording said signals.

The antennae may suitably be horns with associ I ated waveguides, fitted with gastight windows and mounted as a pair on one or more brackets for fixing to the chimney top so as to face down the chimney at an angle of about 60 to the chimney axis. The signal processing circuitry preferably includes a band pass filter to select those Doppler frequencies appropriate to the chimney gas flow velocities to be encoun tered, and a discriminator circuit and rate meter to provide an output indicating the rate of occurrence of Doppler signals above a predetermined signal threshold amplitude.

The signals produced from the monitor by the associated signal processing equipment and repres enting blob emission may suitably be displayed on an ultra-violet recorder, while the indication of rate i of emission may be presented on a chart recorder, advantageously a wall chart recorder in the plant control room of the works, for correlation of blob emission with plant operation.

A preferred embodiment of the invention is based on theoretical calculations which indicate that it should be feasible to observe the motion of indi vidual low density blobs, with up to 70 percent voids, down to 1 mm diameter at ranges up to about 500 mm using 1 OmW radiated microwave power at 10GHz. Normal dust burdens would have no effect, since the total dust signal will be only of the order of thousandths of the threshold signal level.

The monitor unit has separate small horn antennae for radiating microwave power and for receiving the scattered radiation signal, and these are mounted in the mouth of the chimney looking obliquely down into the gas stream. These antennae are connected by separate waveguides to the microwave source and to the radiation detector and signal preamplifier mounted a short distance down the outside surface of the chimney. With this arrangement the monitors observe the flow of blobs in the chimney gas stream near the antennae but are not susceptible to the motion of rain or birds nearthetop of the chimney. With the aerials mounted at 30 to the horizontal the combination of aerial radiation and reception patterns provide sufficient sensitivity to observe the emission of 1 mm diameter blobs over about 500 cm2 cross-sectional area of gas flow.If blobs were uniformly distributed over the gas stream this detection area would provide a sampling probabiiity of 0.16 percent. The electronics are mounted a short distance down the outside surface of the chimney to keep the temperature sufficiently low for satisfactory and low noise operation over long periods. The power for the electronics is provided by a stabilised power supply mounted near the top oratthe bottom of the chimney. Doppler signals are sent by coaxial cable to recording equipment near the base of the chimney.

The design of the monitors and of the signal processing electronics makes the output signals insensi tiveto mechanical vibration of the monitor units. A design of electrical supply, cabling and earthing arrangements is used which makes the output fairly immune to electrical interference.

It has been found that the monitoring equipment which has been developed is able to withstand the chimney top environment of high temperatures (around 200"C), high humidity, dust and corrosive gases for several months without maintenance or attention.

Positive correlation has been established between the deposition of blobs in the neighbourhood and area of a works, and the rate of occurrence of blob signals from chimney top monitors. Correlation has also been obtained between the observations of two monitors operating together for about an hour on top of a single chimney. Some correlation has been observed between high signal rates and some fea tures of plant operations. The invention therefore provides effective blob monitoring equipment useful in identifying the sources and mechanisms respons ible for blob release and enabling the consideration of modifications to plant or operating procedures for inhibiting significant emission of blobs.

The invention is further described by way of illust ration with reference to the accompanying drawings, in which: FIGURE 1 shows a perspective view of a monitor unit, unmounted; FIGURE 2 shows a monitor module mounted at the top of a chimney, the latter being shown in sectional side elevation; FIGURE 3 shows in semi-schematic form a circuit diagram of suitable electronics directly associ ated with the blob monitor for mounting on the chimney; this circuit includes voltage stabilis ing circuits, the microwave oscillator, the mic rowave detector and the Doppler signal preamplifier, as will be readily apparent to those skilled in the art; FIGURE 4 shows in semi-schematic form a circuit diagram of a unit suitable for processing sign alsfrom the module containing the circuit of Figure 3 for filtering, rate measuring and recording, as will be apparent to those skilled in the art.

The horn antennae 1,2 are made from 18SWG sheet copper and have a front aperture of 85 x 85 mlm and a half cone angle of about 11". The two horns are rigidly mounted together so that their axes converge at an angle of about 15". As shown in Fig ure 2 the horns are mounted at the top 3 of the chim ney with their plane of viewing depressed about 30 below horizontal. This arrangement is designed to maximise the cross-sectional area of the chimney gas flow to which the monitors will be sensitive.

The front surface of the horn antennae is covered by a 10 m/m thick sheet 4 of polytetrafluoroethylene ("Teflon" (Registered Trade Mark)). This thickness is chosen as a minimum for attenuation of radiation at 10 GHz and to provide adequate mechanical stiffness for making gas tight seals.

The whole monitor unit, from the Teflon windows on the horn antennae to the socket connections 5 for the electronics, is built as a single leak-tight enclos ure. Experience has shown that even through the electronics may be adequately protected against corrosion by encapsulation in epoxy resin and by mounting thin Teflon windows nearby in the waveguides, any leakage of high dew-point flue gases into the waveguides results in condensation in the cool region of the guides and eventual obstruc tion to the transmission of microwave power. A pressure rise rate, with the monitor under vacuum, of less than 10-7 torr sec-1 shou Id be suitable for a 3-year monitor lifetime.

A Gunn oscillator (CXY11B) is used to provide about 1 OmW microwave power at 10 GHz. This oscil lator is mounted in an X-band Doppler radar module 6 (Mullard CL 8960). This mounts the Gunn oscillator and a detector diode (BAV 46) in adjacent sections of waveguide. The output power and operating fre quency can be adjusted with a trimmer screw pro jecting into the oscillator section waveguide. The local oscillator signal is leaked from the oscillator waveguide to the detection waveguide through an adjustable slot (not shown) where the module is attached to the main waveguide structure.

The noise level of the detector diode, and thus the threshold Doppler signals to be observed, is only a few microvolts. It is not wise to try and send such small signals through long lengths of cable down the chimney (about 200 m). A low noise preamplifier with a voltage gain of about 2000 and a line driver output stage are therefore mounted directly on the microwave module. The printed circuit card construction and direct help avoid problems with microphonics.

7 volt and 13 volt power supplies may be provided by Farnell regulated power supplies mounted about 10 m from the top of the chimney and conducted to the monitor by three core screened cable.

A more preferred arrangement, as shown in Figure 3, is to provide supply regulation directly on the preamplifier board from a single 20 volt power supply unit at the bottom of the chimney. This arrangement gives better regulation, less equipment at the top of the chimney and easier inspection and maintenance of power supplies. It also reduces the chance of mains frequency interference and the chance of plug connections being damaged by surface tracking in humid or wet conditions.

To minimise electrical interference the monitor unit is electrically isolated from the lightning con ductorsystem the top of the chimney and the monitor, output signal cable sheaths and the negative side and shielding of the low voltage power supply cableto the monitor are all earthed art a single point at the bottom of the chimney. When mains energised units are used to supply the low voltage power near the top of the chimney there is no connection between the earth on the upgoing mains cable and either the power supply output leads or the monitor earth.

The monitor is mounted by Teflon links 7 at three points to a stainless steel Frame 8 over the lip 9 of the chimney and to an extension bar 10 from this frame to the top hoop 11 around the chimney brickwork. This provides the electrical isolation required as above. Although the electronics end of the monitor structure is fairly stiffly supported from the outer surface of the chimney a fair amount of flexibility has been left between the top point of support and the horn antennae. This flexibility and the mass of the antennae horn assembly ensure a natural fre quencyfor movement in airflowturbulencewell below the minimum Doppler frequencies experi- enced in a cement works' chimney with only one kiln running - about 200 Hz.

The preamplifier circuit, Figure 3, incorporates a degree of band-pass filtering to provide optimum gain only over the Doppler frequency range likely to be of practical significance. Reduced gain is pro vided at high frequencies, to minimise possible interference, and at low frequencies to minimise the 1 effect of - noise from the detector system.

f Before being presented to signal selection or recording equipment, signals sent down the chim ney from individual monitors need to be passed through special filter units with a sharp low fre quency cut-off characteristic, such as a filter unit designed to have a -3 dB point at 200 Hz. The cut-off frequency is chosen to be close to the minimum Doppler frequency likely to arise with a chimney gas flow velocity due to operation of just a single kiln and to minimise-- noise level from the microwave f mixer and preamplifier.

The above sharp cut-off filter is incorporated into a signal processing unit (Figure 4) which provides outputs relating to the amplitude of individual Dop pler signals and the rate of occurrence of Doppler signals above a certain threshold signal amplitude.

After the three stage filter unit the signals are rectified with a full wave linear integrated circuit detector to provide output signals relating to the peak amplitude of the Doppler signals. These uni directional signals can be displayed directly on an ultra-violet recorder. These signals are also passed to a discriminator and signals above the chosen threshold level are presented to a rate meter circuit.

The output from the rate meter has been designed to match directly the 1 to 5 volt input requirement of wall chart recorders located in the plant control room. For blob signals above the threshold dis criminator level outputs of 2.3,3.1 and 5 volts cor respond to blobs being emitted every 10, every 5 and every 1 seconds.

Performance features of a typical blob monitor I may be summarised thus: Microwave operating frequency 10 GHz Microwave power output about 10 mW Preamplifier gain about 2800 Pass band of preamp (- 3db points) 200 Hz to 2500 Hz Equivalent input noise signal (P-p) about 5 iLV Power supply requirements ( 15-20V 220 mA Angle of view to direction of gas flow 60 Calculated size of smallest blobs monitored effectively (on the basis of dry low density 1.5 m/m diameter blob material with an effective permittivity of 1.3) Cross-sectional area of gas flow sampled about 300 cm2 Fraction of chimney mouth area sampled 0.1 per cent Range of airflow velocity corresponds to pass band 3 to 38 msec-1 of preamplifier After performance testing the monitor is evacuated with a rotary vacuum pump, back filled to a slight over pressure with dry nitrogen or argon and then sealed off on the soft copper pump-out tube.

When blob observation signals are displayed on a long term basis directly on the ultra-violet recorder, it is necessary to provide some precise time marking to enable observations to be correlated with other i events. Time indication may be achieved using a quartz crystal clock unit to provide different size pulses every minute, every five minutes and every hour on a single subsidiary trace of the ultra-violet recorder.

The monitors developed observe the flow of blobs in a small sample of the outflow gas stream at the top of a chimney. Observations with two monitors spaced about 2 metres apart on a chimney showed essentially the same pattern of output signals. This result suggests that observations with a single monitor may form a fairly representative sample of the whole chimney outflow pattern. This expectation is also supported by correlations which have been found between high signal occurrence rates and blobbing deposits on the ground.

The cross-sectional area of the chimney gas stream sampled for blobs of 1.5 mm diameter is about 300 cm2, or 0.1 per cent of the chimney mouth area. From blobs of this size, a rate of occurrence of signals of 1 sec-1 would correspond to 104 blobs sec-' from the chimney top. Larger blob particles, denser particles, and particles with a higher water content have larger radar scattering cross-sections and would be seen over a larger fraction of the chimney cross-section. There is at present no simple relation between the rate of occurrence of signals, or their size, and the actual rate at which blobs leave the top of a chimney, although in principle the microwave system and the signal processing arrangements could be enhanced to provide direct information on the effective size and range of individual blobs, so that total emission rates could be estimated.However, the present qualitative sampling information should be sufficient to enable the sources and mechanisms responsible for the release of blobs to be indentifed.

Blob monitor signals have been observed with continuous rates ranging from only a few per hour to every 100 per hour~' . At times the monitor signals occur in bursts and these bursts involve signal rates up to over 1 sec-1.

Typical components for the circuit shown in Figure 3 are: REG 1 Type LM 340 REG 2 Type LM 340 R1 270 ohm C1 0.01,aF R2 1,000 ohm C2 330 jaF R3 270 ohm C3 2.2,aF R4 1,000 ohm C4 0.1,aF R5 10,000 ohm C4 820 pF R6 180,000 ohm C6 2.6,mF R7 200 ohm C7 0.1,aF R8 10,000 ohm C8 2.5,aF R9 1,500 ohm C9 920pF R10 200 ohm C10 100,aF R11 10,000 ohm C11 0.01,uF R12 1,500 ohm R13 180 ohm R14 39 ohm D1 Gun diode D2 Mixer diode l.C.1 LM 381(2 halves) I.C.2 LH 0002 Typical components for the circuit shown in Figure 4 are:: R1 10,000 ohms C1 0.1,aF R2 10,000 ohms C2 0.1 CLF R3 6,800 ohms C3 0.01,aF R4 47,000 ohms C4 0.01 ELF R5 8,200 ohms C5 2.2yF R6 47,000 ohms C6 l000F110V) ) or 1 x2000 R7 47,000 ohms C7 1000,uF/10V) F R8 10,000 ohms C8 0.01,us R9 1,000 ohms C9 0.01,uF R10 10,000 ohms C10 47,uF/20V R11 5,600 ohms R12 47,000 ohms Cl Type 741 R13 47,000 ohms l.C.2 Type 741 R14 12,000 ohms I.C.3 Type 711 R15 1,000 ohms I.C.4 Type 741 I.C.5 Type 741 R17 47,000 ohms I.C.6 CD4047 R18 10,000 ohms R19 20,000 ohms I.C.8 Type 741 R20 2,200 ohms l.C.9 Type 741 LH0042CH R21 47 ohms R22 47,000 ohms D1 Type 741 IN4006 R23 50,000 ohms D2 Type 741 IN4006 R24 33,000 ohms D3 Type 741 IN4006 Typical components for the circuit shown in Figure 4 continued: R25 10,000 ohn R26 1,000ohm R27 1,000ohn R28 1,000ohn R29 1,2000hun R30 10,000ohn Meter: 100A

Claims (10)

1. A method of monitoring the occurrence of agglomerates of flue dust in the gas stream emitted by an industrial chimney, comprising: (a) selecting a non-axial cross-section of the total gas stream adjacent the chimney mouth; (b) detecting the passing of agglomerates through at least one predetermined fractional sample of said stream cross-section, by means of continuous microwave Doppler radar directed along said stream cross-section and responsive to the scatter cross-sections of indicidual agglomerates passing through said sample, to produce signals indicative of said passing agglomerates; and (c) amplifying said signals and displaying a rep resentation of the amplified signals.

2. A method according to Claim 1 wherein said representations are recorded.

3. A method according to Claim 1 or 2 wherein said signals are filtered to select those Doppler frequencies appropriate to the prevailing chimney gas flow velocity.

4. A method according to Claim 3 wherein the signals are processed to provide an output related to the rate of occurrence of Doppler signals which exceed a predetermined threshold signal amplitude.

5. A method of monitoring the occurrence of agglomerates of flue dust substantially as described with reference to the accompanying drawings.

6. Apparatus for recording the occurrence of agglomerates of flue dust in the gas stream emitted by an industrial chimney comprising: (a) a gas-tight monitor module which comprises a transmission antenna coupled to a source of continuous microwave radiation, a reception antenna coupled to a microwave radiation detector, and means for deriving Doppler radar signals from said radiation, the module being constructed and dimensioned to respond to the scatter cross-sections of indi vidual agglomerates passing within a pre determined range in front of the antennae, and the antennae being adapted for mounting at the mouth of a chimney facing down the interior of the chimney at an angle to the chim ney axis with said source and detector outside the chimney; (b) electronic circuitry for coupling to the monitor moduleto process said signals; and (c) means for representing and recording said signals.

7. Apparatus according to Claim 6, wherein the signal processing circuitry includes a band pass filter to select those Doppler frequencies appropriate to the chimney gas flow velocities to be encountered.

8. Apparatus according to Claim 7, wherein the signal processing circuitry includes a discriminator circuit and rate meterto indicate the rate of occurrence of Doppler signals which exceed a predetermined signal threshold amplitude.

9. Apparatus according to Claim 6,7 or 8, wherein the antennae are horns with associated waveguides, fitted with gastight windows and mounted as a pair on one or more brackets for fixing to the chimney top so as to face obliquely down into the chimney.

10. Apparatus for recording the occurrence of agglomerates of flue dust substantially as described and shown in the accompanying drawings.