APPARATUS AND METHOD FOR ANALYSING THE CONSTITUENTS OF A GAS OR A GAS LIKE CLOUD OR PLUME

Abstract: Apparatus for analysing the constituents of a plume comprising optical absorbance determining apparatus for determining the optical absorbance of a first constituent of the plume along a path through the plume, and spatial variation determining apparatus for determining the spatial variation in mass of a second constituent over at least one dimension within said plume, whereby the total mass of said first constituent of said plume may be calculated by relating the optical absorbance of the path through the plume utilised by the optical absorbance determining apparatus to the spatial extent of the whole plume as determined by the spatial variation determining apparatus.

Description: Apparatus and Method For Analysing the Constituents of a Gas or a Gas Like Cloud or Plume BACKGROUND OF THE INVENTION The present invention relates to apparatus and method for analysing the constituents of a gas or a gas like cloud or plume (hereafter referred to as a plume). Whilst the described embodiment will be described with reference to apparatus and method for remotely determining the gaseous constituents of a motor vehicle exhaust plume, the principles of the invention are not restricted thereto and may also be utilized to measure remotely the constituents of a gas plume such as an exhaust gas plume from a chimney such as a power station chimney, or a gas-like plume such as smoke. Measurement of the gaseous constituents of a motor vehicle exhaust plume, or other similar exhaust plumes is desirable to determine the environmental impact of the exhaust gas. However, some constituents of the exhaust gas plume are of greater interest than others, typically carbon monoxide, oxides of nitrogen, unburned or partially burned hydrocarbons. However these constituents are usually present in very low concentrations and so cannot be easily determined. The concept of making optical measurements to determine the emissions constituent in the exhaust plumes of moving vehicles is known. Typically such devices pass a beam of electromagnetic radiation through the plume and measure changes in optical intensity due to the absorptions of specific wavelengths of the electromagnetic radiation caused by the constituents of the plume. Thus, for example, CO 2 absorbs electromagnetic light of particular known wavelengths and NO 2 absorbs light of different wavelengths. After various correlations and assumptions, these optical absorptions are commonly presented as relative concentrations, for example having units of ppmv (parts per million, by volume). Relating an optical absorbance to a relative concentration of a sample of gas is straight forward in theory - a direct application of Beer's law. / = Jo e ~ε ° where I = intensity after passing through plume I 0 = intensity before passing through plume ε = a constant for the relevant constituent, called the absorption coefficient c = concentration of the relevant constituent / = length of beam path For example; Optical absorbance, ^ 4[co2] can be defined such that ^ 4[co2] = - In (7/7 0 ) at a CO 2 absorption wavelength Hence A[co2 = ε[co2] £[C02] /' where ε[co2] is the absorption coefficient for CO 2 and C[co2] is the concentration of CO 2 in the beam path In a remote sensing application however, (e.g. where the device is alongside a road, and receives the beam of electromagnetic radiation which has passed through a vehicle exhaust plume), the device integrates optical absorbance over a path which includes not only the plume but also a large distance through ambient air. The net result of applying Beer's law is a calculated concentration, which appears much lower than, and is not wholly representative of that which would be found in the core of the plume itself. This is a consequence of the length of the beam path not being the same as the length of the path within the plume. To attempt to get around this problem, existing devices use the ratio of optical absorbance for a constituent of interest (e.g. NO 2 ) to optical absorbance for a reference constituent (e.g. CO 2 ). In this way the ratio reflects gas composition in the plume and is unaffected by the part of the beam path passing through ambient air so long as the concentrations of both reference and constituent of interest in the ambient air are zero or low enough to be acceptable sources of error. The determined ratio of optical absorbances is multiplied by the ratio of the absorption coefficients, determined as a calibration step, such that the ratio becomes a ratio of concentrations, independent of path length. E.g. for the ratio of NO 2 to CO 2 : •<4[N02] / = 6[N02] C[N02] / S[C02] C[C02] = 6[N02] C[N02] / ε[C02] C[C02] Hence C[NO2] / qco2] = 04[NO2] / ^ 4[C02] ) x (ε[co2] / S[N02]) where A [ HΌ2] is the optical absorbance of NO 2 , ε[N02] is the absorption coefficient for NO2, and C[N02] is the concentration of NO2 in the beam path. In order to determine the concentration of the constituent of interest (in this case NO2, it is then necessary to multiply the ratio of concentrations by the actual concentration of the reference constituent (CO2), either as determined experimentally, or by some other means of estimation. This is non-trivial and the currently commercialised approach for motor vehicle emissions is to estimate this concentration based on stochiometric combustion equations (determined under laboratory conditions and hence not necessarily reflecting an actual vehicle in the field) and assumptions about the fuel composition (which will vary considerably depending on the fuel used by a particular vehicle), intake air composition, and air/fuel ratios (which will vary depending on the road conditions, the acceleration of the vehicle and the state of tune of the vehicle). [ See US patent 5,498,872] The most useful measures for assessing the environmental impact of emissions are not concentrations of emitted gases, however, but measures of the mass of the pollutant emitted. Environmental impact assessments relating to transport require data in terms of mass of pollutant emitted per second or per kilometre travelled by the vehicle. The effects of fully dispersed emissions in terms of ambient background concentrations of pollutants are usually measured in terms of mass of polluting constituent per cubic metre of ambient air. This is a significant differentiation - while concentrations of gases can provide general information regarding the efficiency of a process, e.g. the state of performance of an engine and catalyst system, only mass emission data can indicate what the effect of the process may be on air quality and related issues such as atmospheric damage or public health. It is for this reason that the Type Approval measures (a statutory regulation with which new vehicles must comply) applied to newly developed vehicles are expressed in terms of mass emissions, while the periodic in-service inspection is measured with reference to concentrations of emitted gases. The measurement of mass emissions requires a different approach if remote optical instrumentation is to be employed. The present invention is arranged to address the above problems. Reference may also be made to patent specifications which disclose various arrangements for analysing an exhaust gas plume: US4241403 US4924095 US5252828 US5319199 WO9212411 US5371367 US5343043 EP564566A1 US5210702 US5418366 US5401967 US5583765 US5591975 US5489777 US5498872 US5644133 US5621166 US5726450 US5831267 US5877862 EP564566 US5719396 US5797682 WO0016068 WO0016070 WO0026641 WO0034755 WO0039556 WO0042415 SUMMARY OF THE INVENTION According to one aspect, the present invention provides apparatus for analysing the constituents of a plume comprising: optical absorbance determining apparatus for determining the optical absorbance of a first constituent of the plume along a path through the plume and, spatial variation deteirnining apparatus for determining the spatial variation in mass of a second constituent over at least one dimension within said plume, whereby the total mass of said first constituent of said plume may be calculated by relating the extent of said plume sampled by the optical absorbance determining apparatus to the spatial extent of the plume as determined by the spatial variation determining apparatus. According to a second aspect, the present invention provides apparatus for measuring the constituents of a plume comprising: concentration deteπnining apparatus for determining the ratio of concentrations of a first and a second constituent of the plume along a path through the plume and, spatial variation determining apparatus for determining the spatial variation in concentration of the second constituent over at least one dimension within the plume, and calculation means to determining the concentration of the first constituent in the plume. Preferably, said spatial variation determining apparatus determines the total amount of the said second constituent in the said plume. An advantage of these arrangements is that the constituents of the plume which are of interest, typically carbon monoxide, oxides of nitrogen, unburaed or partially burned hydrocarbons (i.e. the second constituent in the above paragraphs), are usually present in very low concentrations and so cannot be easily quantified in terms of detecting them across the whole transverse cross-section. It is normally necessary to detect them only along a defined path though the core of the plume. It is usually easier, however, to quantify the spatial distribution of carbon dioxide in the plume (i.e. the first constituent in the above paragraphs) which is present in the exhaust in relatively high concentration, and this may be used to determine the spatial extent of the plume by said spatial variation determining apparatus. Said apparatus for determining the optical absorbances of a first constituent along a path or ratios of a first and second constituent may comprise beam passing apparatus for passing a beam of electromagnetic radiation through the plume along said path and means to measure changes in the optical intensity at predetermined wavelengths (the predetermined wavelength relating to the chosen second constituent). Said beam of electromagnetic radiation is preferably of a limited wavelength range (e.g. monochromatic) and said apparatus for determining the spatial distribution of the second constituent may therefore comprise an appropriate monochromatic source for illumination of the plume or alternatively one or an array of sensors tuned using wavelength selective means, e.g. optical filters, to determine the spatial location and extent of the plume in at least one dimension. The array of sensors may be provided in a line to determine the spatial distribution of the second constituent in a place (which may extend across the path of a vehicle being remotely monitored) In a preferred arrangement, said spatial variation determining apparatus for determining the total amount of the second constituent across plume may operate in three dimensions. Furthermore this measurement may be carried out over time so as to determine the change of the plume with time. The second constituent may comprise CO 2 or H 2 O, and the first constituent may comprise the constituent of interest, for example the oxides of nitrogen, NO 2 or NO . Because the constituent of NO 2 is low in most exhaust plumes, it is not generally possible to directly measure NO 2 across the whole cross section of the plume, particularly as the plume disperses. The apparatus of the invention may be provided in a controlled environment, for example in a laboratory to test the exhaust emissions of a motor car under test, or in a semi-controlled environment such as attached to the stack of a power station, or funnel of a ship or a less controlled environment provided at the road side for determining the exhaust gas emissions of passing vehicles remotely. The present invention also provides a method for analysing the constituents of a plume comprising: determining the optical absorbance of a first constituent of the plume along a path through the plume and, deteπnining the spatial variation in mass of a second constituent over at least one dimension within said plume, and determine the total mass of said first constituent of said plume by relating the extent of said plume sample as determined by the optical absorbance determining step to the total spatial extent of the plume as determined by the spatial variation determining step. The present invention also provides a method for measuring the constituents of a plume comprising: determining the ratio of concentrations of a first and a second constituent of the plume along a path through the plume and, determining the spatial variation in concentration of the second constituent over at least one dimension within the plume, and thereby to deteπnining the concentration of the first constituent in the plume. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described by way of example and with reference to the accompany drawings in which Figure 1 is a perspective view of a first apparatus for analysing an exhaust plume of a moving motor vehicle, and Figure 2 is a perspective view of a second apparatus for analysing an exhaust plume of a moving motor vehicle. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1, there is shown a motor vehicle 10 (which may be a car, van truck or bus) travelling in the direction 11 along a road 12. Motor vehicle 10 includes an exhaust pipe 13 emitting a plume 14 of exhaust gas. The apparatus of the invention comprises a source 15 of electromagnetic radiation at one side of the road 12, and a detector 16 at the opposite side of the road 12. The detector 16 includes two detection means, an optical absorbance determining apparatus comprising a first detection means 17 for detecting electromagnetic radiation along a defined path 20 in the form of a collimated beam 25 of electromagnetic radiation which passes through the exhaust plume, and a spatial variation determining apparatus comprising a second detecting means 18 which detects electromagnetic radiation from a two dimensional plane indicated at 21, the two dimensional plane including the path 20. The outputs of the first and second detector means 17, 18 are passed as digital signals to a memory store 22 at the roadside where the signals may be recorded onto a memory medium 23 such as a magnetic disc, or a CD, or a zip disc or any equivalent means; the memory medium 23 may be removed periodically for analysis in a computer 24 remote from the detectors. Alternatively, the digital signals may be passed by a cable 26 or transmitted by a radio microwave or infra-red link 27 direct to computer 24. The detection means 17 and 18 may be in the form of PbSe (Lead Selenide) temperature stabilised infrared detectors from CalSensors (part BXT2-17T) and the source 15 may be a TomaTech CS-IR21-V small area infrared source. A chopper may be used so that the beam is modulated. The second detector mean 18 comprises a "camera" comprising a two dimensional array of pyroelectric sensors, which is tuned using optical filter techniques in such a way as to visualise the exhaust plume as it is generated, i.e. as a vehicle passes the camera. Specifically the detector means 18 comprises an infrared camera operating in the 2 - 15μm region which can be used to acquire an image, which defines the spatial location of the exhaust plume. The image intensities depend on how the camera 18 and illumination source 15 are deployed, and may result from (a) the specific absorption of electromagnetic radiation by constituents in the exhaust gas, (b) the re-emission of radiation absorbed by the constituents of the gas, or (c) the higher temperature of the exhaust gases compared to the surrounding air. In each case, the image obtained using the device is processed to give direct information about the spatial characteristics of the plume. In the above described arrangement (a) (which is shown in Figure 1) the intensity of the image results from absorbance of the radiation from the source 15 by the exhaust gas plume which is located between the source 15 and the detector/camera 18. In the above arrangement (b) where the source 15 and the camera 18 are co- located (i.e. are arranged alongside one another on one side of the road), the exhaust gas absorbs some light from the source 15 and the detector 18 only detects re-emitted absorbed light. In the above arrangement (c) where the exhaust gases are of a higher temperature such as are produced by jet (gas turbine) engines for example in aircraft or helicopters, one may determine the extent of the exhaust plume using thermal cameras (with no separate radiation source 15). In Figure 1 the apparatus thus far described is set out on each side of the road side where it is intended to measure the exhaust plume 14 of passing motor vehicles. The detection means 17, 18, may operate continuously or may be pulsed or chopped electronically or by providing a physical chopper to interrupt the beam as is already known so as to provide a series of readings over a course of time. Operation of the detection means 17, 18 may be initiated by detection of the passage of the motor vehicle 10. A succession of output signals from the two detection means the 17, 18 is taken and passed to the computer 24 either directly by the cable 26 or radio, microwave or infra-red link 27 or by means of the intermediary of the memory medium 23. Thus by taking a series of images as the plume disperses and decays, the temporal characteristics of its spatial extent can also be determined. A more precise profile of plume characteristics is determined by using a combination of spatial or spatio-temporal characteristics with mathematical models (based on computational fluid dynamics techniques - CFD) in the computer 24. Selecting an arrangement in which the intensity variations in the image are due to differing amounts of optical absorption by a constituent such as carbon dioxide means that the image generated is actually a map of concentration of this constituent in various parts of the plume and surrounding space. If the actual concentration of the imaged constituent is known for just one point in the image, the concentration at all other points in the image can be calculated using the relative image intensities. This allows calculation of the mass of the imaged constituent emitted (e.g.: CO 2 ) since information about all points in the image can be integrated to get a measure for the whole plume. We may then use the concentration ratios established previously to calculate the mass emissions of lower concentration constituents of interest such as NO 2 . If, as with prior arrangements, we were only to have the first detector means 17, then there would be the following main limitations: (a) Necessity to reference optical absorbance of a constituent of interest to the optical absorbance of a reference constituent such as carbon dioxide, and the subsequent use of a estimation technique based on combustion equations because of the lack of information about the amount of exhaust gas being sampled. This introduces errors due to the non-ideal nature of the estimation technique and limits the system to measuring relative concentrations, and, (b) Inability to sample more than a section through the plume, which introduces errors due to non-representative sampling if the plume composition varies spatially, and temporally as it does under the influence of turbulent air disturbance caused by the vehicle's passage and the prevailing wind flow vectors. The addition of the second detector means 18 overcomes the first limitation and associated sources of error, and substantially overcomes the second limitation by relating the dynamics of the gas in the sampling region of the first means to the dynamics of the plume as a whole. Using a beam 25 of electromagnetic radiation with much smaller cross-sectional area than the plume dimensions, for example a collimated beam, passing through the plume 14 and recording the optical absorbance at specific wavelengths due to the presence of each constituent of interest, the detector 16 provides a signal which is proportional to the number of molecules of the constituent of interest located in the beam 25. If such a measurement is made either before the vehicle 10 passes the detector 16 or a sufficiently long time afterwards, a measure of background optical absorbance can be made and by subtraction, optical absorbance proportional to the number of molecules emitted into the beam path by the vehicle 10 can be measured. In practical applications, attenuation of the beam 25 by scattering or masking effects due to dust, smoke, suspended particulates etc. affects the measured optical absorption and it is necessary to compensate for this by making further measurements in a region of the electromagnetic spectrum which is not absorbed by the constituent of interest. Such measurements then provide a baseline for offsetting these effects. Measurement of mass emissions, as exemplified by the measurement of emissions factors (mass of emitted species per kilometre travelled) from moving vehicles is based on the application of Beer's Law. Concentration is expressed in units of number of molecules per unit volume, such that Beer's law can be expressed: A = ε ( n /v) I where n = number of absorbing molecules in beam / = path length of sample beam v = volume of sample beam A term φ = v / / can be defined - this is the same as the effective cross sectional area of the beam. A term γ = n x m can be defined - this is the total mass of the constituent of interest in the sampled beam, where m is the molecular mass of the absorbing constituent. Hence: A = ε γ / m φ γ is a significant value since it represents mass emitted into the beam path in the distance the vehicle travelled while crossing the beam path, which is linearly related to mass emitted into the beam path per kilometre travelled. Since A is measured, ε is determined by calibration of the instrumentation, m is a constant for the absorbing gas species and φ is a known design parameter, γ can be directly measured by the application of the above equation to optical absorbance measurements made by the first detection means 16. Thus we have a direct measure of mass emission, but only in the region of space sampled by the first detection means 16 (ie the beam 25). In order to measure the total mass emission, it is necessary to relate the mass measured in this region to the mass emitted into the regions of the plume 14 which are outside the beam path 25. This is the purpose of the second detection means 18. A short time (a few tens of milliseconds) after the exhaust gas is emitted from a moving vehicle 10 its net movement in the direction 11 of travel of the vehicle can be assumed to be zero. Net transport in the direction 11 of travel can be neglected such that the plume is assumed to disperse only in directions orthogonal to the direction of travel. So long as the sampling beam path 20 is so arranged such that the boundary of the dispersing plume 14 remains within the sampling region of the first and second detection means 17, 18the second detection means 18 provides a measure of the spatial distribution of emitted gas in a single plane 21 orthogonal to the beam path 20 and to the direction 11 of travel. This spatial distribution information is used to form a ratio ct2/ai which relates the amount of exhaust emitted gas in a transverse section 21 through the whole of the plume 14 to the amount of exhaust gas in a region of this transverse section corresponding to the beam path 20. The total mass emission, F, for the vehicle is thus given by:- F = γ ctι / ct2 = n? φ αι / ε α2 In use of the apparatus of Figure !, we provide a method for analysing the constituents of an exhaust plume 14 by determining the optical absorbance of a first constituent (e.g. oxides of nitrogen) of the plume 14 along the path 20 through the plume 14 by means of the first detector means 17 and, determining the spatial variation in mass of a second constituent (e.g.carbon dioxide or water vapour) over at least one dimension within said plume 14 by means of the second detector means 18, and in the computer 24, deteπnining the total mass of said first constituent of said plume 14 by relating the extent of said plume sample as determined by the optical absorbance determining step to the total spatial extent of the plume as determined by the spatial variation determining step utilising the mathematical relationships referred to above. Furthermore in use of the apparatus of Figure 1 we measure the constituents of the plume 14 by: deteπnining the ratio of concentrations of a first (e.g. oxides of nitrogen)and a second (e.g.carbon dioxide or water vapour) constituent of the plume 14 along a path 20 through the plume by means of the first detector means 17 and, determining the spatial variation in concentration of the second constituent over at least one dimension within the plume by means of the second detector means 18, and in the computer 24, determining the concentration of the first constituent in the plume utilising the mathematical relationships referred to above. We now refer to the second embodiment of the invention illustrated in Figure 2. In essence, the second detection means 18 of Figure 1 which provides information regarding the spatial extent of the plume 14, is replaced by a plurality of pairs of illuminated sources and detectors arranged on each side of the path of the vehicle 10. Thus on each side of the road 12 there is provided an upright support 28, 29. Support 28 mounts six illumination sources 31-36 and support 30 mounts six detectors 37-42. Each illumination source/sensor pair (illumination source 31-36 sensor 37-42) are arranged opposite one another so that light from the relevant illumination source 31- 36 passes horizontally along a respective path 43-48 to the sensor 37-42. In this way, the plane 21 provided in the embodiment of Figure 1 by the camera 18 is provided by the spaced paths 43-48. We have found that when the set of values detected by the sensors 37-42 are plotted for an exhaust plume 14, the shape of the graph of the values is regular and thus relatively few sensors are required, the shape of the graph between the plotted values being readily extrapolated to provide the overall shape and hence the spatial extent of the plume 14. Calibration of the instrument - converting the single point optical absorbance measurements into concentrations - requires some form of calibration in which the optical absorbance for known concentrations of the constituent of interest is measured using known path lengths. This may be built into the architecture of the device in such a way that caUbration is performed automatically and periodically - even performed as an integral part of each exhaust plume analysis. The invention is not restricted to the details of the foregoing examples.

Claims: CLAIMS 1. Apparatus for analysing the constituents of a plume comprising: optical absorbance determining apparatus for determining the optical absorbance of a first constituent of the plume along a path through the plume and, spatial variation deteπnining apparatus for determining the spatial variation in mass of a second constituent over at least one dimension within said plume, whereby the total mass of said first constituent of said plume may be calculated by relating the extent of said plume sampled by the optical absorbance determining apparatus to the total determined spatial extent of the plume as determined by the spatial variation determining apparatus. 2. Apparatus as claimed in claim 1 in which said spatial variation determining apparatus determines the total amount of the said second constituent in said plume. 3. Apparatus as claimed in claim 1 or 2 in which said apparatus for determining the optical absorbances of a first constituent along a path comprises beam passing apparatus for passing a beam of electromagnetic radiation through the plume along said path and means to measure changes in the optical intensity at predetermined wavelengths. 4. Apparatus as claimed in claim 1, 2 or 3 in which said apparatus for deteπnining the spatial distribution of the second constituent comprises a source of radiation to illuminate the plume and one or an aπay of sensors sensitive to that radiation , to determine the spatial location and extent of the plume in at least one dimension. 5. Apparatus as claimed in claim 4 in which said one or an array of sensors include(s) wavelength selective means comprising optical filters. 6. Apparatus as claimed in claim 4 or 5 in which said the array of sensors is provided in a line to determine the spatial distribution of the second constituent in a plane. 7. Apparatus as claimed in claim 6 in which the plane extends across the path of a vehicle being remotely monitored. 8. Apparatus as claimed in any of claims 1 to 7 in which said spatial variation determining apparatus for determining the total amount of the second constituent across the plume operates in three dimensions. 9. Apparatus as claimed in any of claims 1 to 8 in which said spatial variation determining means is configured to be operated over time so as to determine the change of the spatial extent of the plume with time. 10. Apparatus as claimed in any of claims 1 to 9 in which the optical absorbance determining apparatus is adapted to determine the optical absorbance of at least one of the oxides of nitrogen. 11. Apparatus as claimed in any of claims 1 to 10 in which the spatial variation determining apparatus comprises apparatus for determining the spatial extent of C0 2 or H 2 0. 12. Apparatus as claimed in any of claims 1 to 11 adapted to be provided at the road side for determining the exhaust gas of passing vehicles remotely. 13 A method for analysing the constituents of a plume comprising: determining the optical absorbance of a first constituent of the plume along a path through the plume and, determining the spatial variation in mass of a second constituent over at least one dimension within said plume, and determine the total mass of said first constituent of said plume by relating the extent of said plume sample as determined by the optical absorbance determining step to the total spatial extent of the plume as determined by the spatial variation determining step.

Inventors: CROOKELL ANDREW, CROOKELL, ANDREW
Applicants: CROOKELL ANDREW, SIRA LTD, CROOKELL, ANDREW, SIRA LIMITED
Application year: 2001
Main classification: G01N21
IPC codes: G01N21