METHOD AND SYSTEM FOR OPTICAL MEASUREMENT OF A PROPERTY OF PARTICLES IN A GAS-FORMED MEDIUM

Description

Domaine technique

[0001] The present invention relates to the measurement of a characteristic relating to particles present in a gaseous medium.

[0002] The present invention is advantageously applicable, but not limited to, the field of exhaust gas or industrial flue gas pollution control and the field of monitoring pollutant emissions from engine exhaust gases or industrial flue gases. The present invention can also be applied to the field of air quality monitoring and air pollution control. Technique antérieure

[0003] Emission standards for pollutants from combustion in internal combustion engines now require, in most cases, the reduction of the concentrations of these substances, for example using post-combustion catalytic treatments.

[0004] Pollutants in exhaust gases are typically unburned or partially oxygenated hydrocarbons, carbon monoxide (CO), nitrogen monoxide (NO), and nitrogen dioxide (NO2), commonly referred to by the acronym NOx, nitrous oxide (N2O), ammonia (NH3), sulfur compounds such as hydrogen sulfide (H2S) or sulfur oxides like sulfur dioxide (SO2), ozone (O3), etc. Some of these substances are subject to strict regulations, according to which concentration limits must be respected during atmospheric release. NOx, for example, has a direct harmful impact on human health and indirectly through the secondary formation of tropospheric ozone.

[0005] Fuel combustion in internal combustion engines is also a source of particulate emissions, such as fine particles rich in PAHs (polycyclic aromatic hydrocarbons), as well as soot particles, metallic particles, particles resulting from nucleation processes of various types of volatile chemical species, or a combination of these different types of particles. These particles can range in size from nanometers to several hundred nanometers, or even exceed one micrometer. These particles, and in particular those emitted by diesel engines, can cause respiratory illnesses and are also carcinogenic. Therefore, current standards require that diesel and gasoline-powered vehicles not exceed 6 x 10¹¹ particles (larger than 23 nm) per cm³ of aerosol, which is conditioned prior to measurement to remove the volatile fraction.Thus, emission standards for pollutants from internal combustion engines require in most cases the reduction of concentrations by post-treatment through filtration and / or catalysis.

[0006] Furthermore, particles can also be present in the atmosphere or ambient air, and can have numerous origins and / or result from complex evolutionary processes involving large-scale transport phenomena and photochemical transformations, among others. These particles, whose sources are numerous, ranging from various human activities to soil erosion, for example, exhibit a very wide variability in size, from nanometers to several tens of micrometers, and a very wide variability in composition, which can be organic and / or inorganic. In particular, fine particles represent one of the main risk factors associated with urban air pollution for developing cancer. Fine particles can be particularly toxic because they penetrate deep into the lungs and bronchial tree.

[0007] In the field of measuring particle content in gaseous media such as exhaust gases, the PPS and PPS-M devices from PEGASOR (Finland) are well-known. These high-dynamic particle analyzers allow for the estimation of particle mass and number concentration, either on-board or remotely. The measurement principle involves electronically charging particles as they pass through the sensor and then measuring the leakage current per unit time as the charged particles leave the sensor. However, this device cannot measure other pollutants, including concentrations of NOx, BTEX (benzene, toluene, ethylbenzene, xylenes), SO2, H2S, etc. Furthermore, this device cannot measure particle size. Finally, due to its size, it cannot be easily integrated, for example, into a vehicle's exhaust system.

[0008] We are familiar with document DE 102017204 037 A1, which concerns an optical sensor for determining the concentration of a fluid component. This sensor also includes a fouling sensor to assess the fouling of the optics and thus correct the concentration measurement of the fluid component. Furthermore, this document teaches how to determine the concentration and size of particles based on an estimation of the scattering phenomenon (and not both absorption and scattering, which corresponds to the extinction phenomenon) caused, for example, by the presence of soot on the optical window. The method described in this document assumes homogeneous fouling of the optics to determine the correction to be applied to the optical sensor signal.Furthermore, this method requires measurements using a wide range of wavelengths, as well as two measurements from at least two different angles, which can be restrictive, for example, in the case of a measurement in an exhaust line. Moreover, it appears difficult to detect low concentrations of particles and small particles in a gaseous medium using this method, since only diffusion is taken into account.

[0009] We are also familiar with document DE 102009054594 A1, which concerns a device and a method for determining the particle size distribution and / or concentration of a circulating gas containing particles, in particular a gas from a motor vehicle exhaust system. More specifically, the device described in this document may include a UV light source and a spectrometer. The method described in this document exploits the fact that the intensity of light scattered (and not absorbed and scattered, which corresponds to the phenomenon of extinction) at a predetermined angle depends on the ratio between the wavelength and the particle size. Thus, the method described in this document requires measurements to be taken at different angles between the light source and the spectrometer, which can be restrictive, for example, in the case of a measurement in an exhaust system.Furthermore, this method does not allow for the simultaneous measurement of a concentration of a gaseous chemical species.

[0010] We are also familiar with document EP 1640707 A1, which concerns an apparatus for monitoring engine exhaust gases, comprising a UV radiation source and a detector of the amount of radiation emitted by the source and passing through a measuring chamber. This document specifically exploits the attenuation of radiation caused by the presence of particles, an attenuation measured for at least two different wavelengths. Furthermore, this document describes that these at least two different wavelengths are wavelengths at which the attenuation of electromagnetic radiation caused by the presence of nitrogen dioxide (NO2) in the exhaust is substantially the same. Thus, in general, this document describes an opacimeter that can correct for the effect of NO2 absorbance on the measurement of soot concentration.To determine particle size, this document relies on Mie theory, which assumes spherical particles, a far cry from the shape of soot, which is more accurately described as complex aggregates. Furthermore, this method does not describe the measurement of the particle number (which is important as it is required by current standards). In addition, NO2 correction at specific wavelengths can be complicated when other interfering particles are present at those wavelengths (e.g., NO, NH3).

[0011] Document WO2019 / 020326 is also known, which relates to a method for the in-situ optical measurement of the concentration of gaseous chemical species contained in exhaust gases and the temperature of such exhaust gases, using a single optical measurement system. More specifically, the method described in this document comprises the emission of UV radiation by a light source through the exhaust gases flowing in an exhaust line, the detection by a spectrometer of at least a portion of said UV radiation that has passed through the exhaust gases, the generation of a digital signal of the light intensity as a function of the wavelength of said portion of the UV radiation that has passed through the exhaust gases, and the estimation of the concentration of said chemical species and the temperature of the exhaust gases from the digital signal.However, this method does not describe the measurement of a characteristic of particles that would be present in the gaseous medium.

[0012] We also know the document: A. Bescond, J. Yon, F.-X. Ouf, C. Rozé, A. Coppalle, P. Parent, D. Ferry, C. Laffon, Soot optical properties determined by analyzing extinction spectra in the visible near-UV: Toward an optical speciation according to constituents and structure, Journal of Aerosol Science, Volume 101, 2016, Pages 118-132, ISSN 0021-8502, https: / / doi.Org / 10.1016 / j.jaerosci.2016.08.001, which concerns a method for determining the optical properties of soot by analyzing the extinction spectrum in the near-UV.

[0013] The method and device according to the invention aim to overcome these drawbacks. Indeed, the method and device according to the invention allow the measurement of a characteristic of particles present in a gaseous medium, based on the exploitation of absorption and diffusion phenomena generated by the particles. Furthermore, the method and device according to the invention do not require measurements from different angles, which allows for greater compactness of the device, making it easier to integrate, for example, into an exhaust system.

[0014] Furthermore, the method according to the invention allows, by means of a single optical measurement, the determination of both a characteristic of the particles present in a gaseous medium and the concentration of gaseous chemical species contained in this gaseous medium.

[0015] Furthermore, these measurements can advantageously be performed in situ, without requiring a step of over-concentrating the particles and / or chemical species present in small quantities in the gas to be analyzed. Advantageously, when a measurement of the concentration of gaseous species is performed, the method according to the invention can also allow for a measurement of the gas temperature using the same optical method. Résumé de l'invention

[0016] The invention relates to a method for measuring at least one characteristic of particles present in a gaseous medium, using at least one optical measurement system, said at least one particle characteristic being selected from a number concentration of said particles and an average particle size, said optical measurement system comprising at least one light source and one spectrometer. The method comprises at least the following steps: a) the emission by the light source of UV radiation through said gaseous medium in a measurement zone; b) the detection by said spectrometer of at least a portion of said UV radiation having passed through said gaseous medium in said measurement zone and the generation of a digital signal of the light intensity as a function of the wavelength of said portion of the UV radiation having passed through said gaseous medium; c) the estimation of at least said characteristic of said particles present in said gaseous medium in the following manner: i) from at least said digital signal of light intensity as a function of wavelength, a curve is determined representing the variations of an extinction coefficient of Beer-Lambert law as a function of wavelength for said gaseous medium containing said particles;and ii) the value of said characteristic is determined by minimizing a difference between said curve representing the variations of a Beer-Lambert law extinction coefficient and a reference model of variations of the Beer-Lambert law extinction coefficient as a function of wavelength and at least said characteristic.

[0017] According to one embodiment of the invention, the said curve representing the variations of a Beer-Lambert law extinction coefficient as a function of wavelength can be determined according to a formula of the type: K ext exp λ = − 1 L ln I s λ I 0 λ where L is the length of the optical path followed by said radiation between said light source and said spectrometer, and λ is said wavelength, I s ( λ ) is said digital signal of said light intensity as a function of the wavelength of said part of said UV radiation having passed through said gaseous medium, and I 0 ( λ ) is a digital reference signal of light intensity as a function of wavelength.

[0018] According to an embodiment of the invention in which said characteristic corresponds to a number concentration of said particles, said reference variation model of the Beer-Lambert extinction coefficient as a function of wavelength and said number concentration can be constructed as follows: a plurality of curves representing the variations of the Beer-Lambert extinction coefficient as a function of wavelength are determined for a plurality of samples of said gaseous medium comprising particles having distinct particle concentrations and particle size distributions; a curve is determined C ext ref ¯ λ representative of the average variations of an extinction cross section as a function of wavelength λ from at least the said plurality of curves and the said particle concentrations of the said samples; the said reference extinction model is constructed K ext ref λ N depending on said wavelength λ and the said concentration of particles N according to a formula of the type: K ext ref λ N = N . C ext ref ¯ λ

[0019] Alternatively, one can construct a reference variation model of the Beer-Lambert law extinction coefficient as a function of wavelength and said number concentration and / or average size of said particles according to the Rayleigh-Debye-Gans theory for fractal aggregates.

[0020] According to an embodiment of the invention in which said characteristic is said number concentration of said particles, one can construct said reference variation model of the extinction coefficient K ext RDG − FA λ N agg depending on the wavelength λ and of said concentration in number N agg based on a formula of type : K ext RDG − FA λ N agg = N agg . C ext RDG − FA λ , Or C ext RDG − FA λ is an extinction cross section of the Rayleigh-Debye-Gans theory for fractal aggregates.

[0021] According to an embodiment of the invention in which said characteristic is said average size of said particles, one can construct said reference variation model of the extinction coefficient K ext RDG − FA λ N p ¯ depending on the wavelength λ and of said average size N p particles expressed as the number of primary spherules constituting the aggregates, using a formula of the type: K ext RDG − FA λ N p ¯ = 4 πx p 3 k 2 E m N agg N p ¯ , Or N agg is the concentration in numbers, x p = πD p λ , k = 2 π λ , D p is a parameter related to the fractal morphology of said particles, E(m) is a parameter relating to the optical absorption properties of said particles.

[0022] According to one embodiment of the invention, it is possible to construct said reference variation model of the extinction coefficient K ext RDG − FA λ N agg N p ¯ depending on the wavelength λ of said concentration in number N agg and of said average size N p particles expressed as the number of primary spherules constituting the aggregates, using a formula of the type: K ext RDG − FA λ N agg N p ¯ = 4 π x p 3 k 2 E m N agg N p ¯ , Or x p = πD p λ , k = 2 π λ , D p is a parameter related to the fractal morphology of said particles, E(m) is a parameter relating to the optical absorption properties of said particles.

[0023] According to one embodiment of the invention, in step c), a concentration of at least one gaseous chemical species contained in said gaseous medium (10) can be further determined in the following manner: an absorbance (A) of said gaseous medium is determined as a function of wavelength from at least said digital signal of light intensity (50) as a function of the wavelength of said part of UV radiation having passed through said gaseous medium (43) and a reference digital signal of light intensity as a function of wavelength; said concentration of said at least one chemical species is determined from said absorbance (A) of said gaseous medium and predetermined absorbance, temperature and pressure characteristics of said chemical species.

[0024] According to one embodiment of the invention, it is possible to measure the concentration of at least one, and preferably several, gaseous chemical species contained in said gaseous medium, and included in the list consisting of: NO, NO2, N2O, BTEX, SO2, H2S, O3, O2, H2O, aldehydes such as acetaldehyde or formaldehyde, non-aromatic hydrocarbons such as acetylene or buta-1,3-diene.

[0025] According to one embodiment of the invention, in step c), the temperature (T) of said gaseous medium can be further determined from said digital signal (50) by modifying the molar extinction coefficient of the absorbance of said chemical species extracted from the absorbance (A) of said gaseous medium, said modification being a shift in wavelength or a modification of amplitude or a combination of both.

[0026] According to one embodiment of the invention, said UV radiation (42) emitted may have a wavelength between 180 and 400 nm, preferably between 190 and 250 nm.

[0027] According to one embodiment of the invention, said gaseous medium may correspond to exhaust gases and said measurement zone corresponds to a conduit in which said exhaust gases flow.

[0028] According to one embodiment of the invention, the method may include a preliminary calibration step of said optical measurement system to provide a digital reference signal of the light intensity as a function of the wavelength, preferably by emission of said UV radiation through a reference gas, and by detection of at least a part of said UV radiation having passed through said reference gas to provide a digital reference signal of the light intensity as a function of the wavelength of said part of the UV radiation having passed through said reference gas.

[0029] The invention further relates to a system for measuring at least one characteristic of particles present in a gaseous medium, implementing the method for the optical measurement of at least one characteristic of particles contained in a gaseous medium as described above, said system comprising: a light source capable of emitting UV radiation through said gas within a measurement zone; a spectrometer capable of detecting at least a portion of said UV radiation having passed through said gas in said measurement zone and generating a digital signal of the light intensity as a function of the wavelength of said portion of the UV radiation having passed through said gas; and means for processing and analyzing said signal to determine at least said characteristic of said particles present in said gas from at least said digital signal

[0030] Other objects and advantages of the invention will become apparent from the following description of particular embodiments of the invention, given by way of non-limiting examples, the description being made with reference to the attached figures described below. Liste des figures

[0031] There figure 1A is a diagram illustrating the optical measurement of at least one particle characteristic in a gas flowing through a duct, according to a transmissive configuration of the optical measurement system for implementing the method according to the invention. figure 1B is a diagram illustrating the optical measurement of at least one particle characteristic in a gas flowing through a duct, according to a reflective configuration of the optical measurement system for implementing the method according to the invention. figure 1C is a diagram illustrating the optical measurement of at least one particle characteristic in an ambient gaseous medium, according to a reflective configuration of the optical measurement system for implementing the method according to the invention. figure 1D is a diagram illustrating the optical measurement of at least one particle characteristic in an ambient gaseous medium, according to a reflective configuration of the optical measurement system for implementing the method according to the invention. figure 2 This schematically represents the absorbance of the gas containing different gaseous chemical species A, B, C that we wish to measure. figure 3 This schematically represents the influence of temperature on the absorbance of a given chemical species contained in a gaseous medium. figures 4 à 14 These are diagrams illustrating different embodiments of the optical measurement system for implementing the method according to an embodiment of the invention in which the gaseous medium or gas mixture flows through a conduit such as an exhaust line. figures 15 à 18 illustrate the different stages of the first variant of the process according to the invention applied to a gaseous medium resulting from the combustion of a diesel engine containing particles in the form of soot. figure 19 presents a particle number concentration determined by the process according to the first embodiment of the invention, compared to a reference particle number concentration. figure 20 illustrates results of the second variant of the process according to the invention applied to a gaseous medium from the combustion of a Diesel type engine comprising particles in the form of soot. Description des modes de réalisation

[0032] According to a first aspect, the invention relates to a method for measuring at least one characteristic of particles contained in a gaseous medium, by means of an optical measurement system.

[0033] By particles, we mean any solid or liquid substance with dimensions less than 100 µm, possibly with a volatile phase that can be adsorbed onto a solid phase. Without limitation, the particles according to the invention may correspond to soot particles, which are fine particles (micrometers, submicrons, and nanometers) rich in PAHs (polycyclic aromatic hydrocarbons), but also to particles resulting from the abrasion of parts, such as metallic particles from brake pads, particles from tire abrasion, as well as pollen, etc. In particular, the method according to the invention is advantageously applicable to determining a characteristic of nanoparticles contained in a gaseous medium.According to ISO TS 80004-1, a nanomaterial is a material with at least one external dimension on the nanoscale, that is, approximately between 1 and 100 nm, or which has an internal or surface structure on the nanoscale. Nanomaterials include nanoparticles, which are nano-objects whose three external dimensions are on the nanoscale.

[0034] A gaseous medium is understood to mean a gas or a mixture of several gases. The invention can in particular be applied to an aerosol, which corresponds to a collection of fine particles suspended in a gaseous medium.

[0035] The method according to the invention is implemented at least by means of an optical measurement system comprising at least a light source and a spectrometer.

[0036] The method according to the invention comprises at least the following steps 1) to 3): 1) the emission by the light source of UV radiation through the gaseous medium of interest in a measurement zone; 2) the detection by the spectrometer of at least a part of said UV radiation having passed through said gaseous medium in said measurement zone and the generation of a digital signal of the light intensity as a function of the wavelength of said part of the UV radiation having passed through said gaseous medium; 3) the determination of at least one characteristic of the particles present in said gaseous medium from at least the digital signal of the light intensity.

[0037] According to a second aspect, the invention relates to a system for the optical measurement of at least one characteristic of particles present in a gaseous medium. The system according to the invention comprises at least one optical system including at least: a light source capable of emitting UV radiation through the gaseous medium within a measurement zone; a spectrometer capable of detecting at least a portion of the UV radiation that has passed through the gaseous medium in the measurement zone and generating a digital signal of the light intensity as a function of the wavelength of the portion of the UV radiation that has passed through the gaseous medium.

[0038] Furthermore, the system according to the invention further includes means for processing and analyzing the digital signal (for example by computer using a microprocessor) to determine at least one characteristic of the particles present in the gaseous medium from said digital signal, in particular according to any of the variants of step 3 of the process according to the invention described below.

[0039] Advantageously, the system for the optical measurement of at least one characteristic of particles present in a gaseous medium further includes means for the transmission (for example by electrical wire, by optical fiber or by a wireless communication system) of the measurements made by means of said optical system to said means for the processing and analysis of the digital signal.

[0040] In general, the characteristic of the particles to be measured can be the particle concentration (for example measured as the number of particles per unit volume of gas), a characteristic of the size distribution of the particles present in the gaseous medium (for example the mean diameter of a log-normal size distribution, or the standard deviation of such a size distribution), or the optical properties of the particles.

[0041] According to the invention, at least one characteristic of the particles is chosen from a number concentration of the particles and an average particle size.

[0042] According to one embodiment of the invention, the gaseous medium comprising the particles whose characteristic is to be measured corresponds to exhaust gases, flowing, for example, through a duct such as an exhaust line. Exhaust gases may originate from an internal combustion engine, particularly, but not exclusively, from a motor vehicle. More generally, however, the term "exhaust gas" in this first embodiment of the invention encompasses all other types of exhaust gases resulting from combustion, such as those from boilers or furnaces, and flowing through a chimney or exhaust duct that can incorporate the optical measurement system according to the invention. The present invention can thus advantageously be applied to industrial fumes comprising particles whose characteristic is to be measured.

[0043] According to another embodiment of the invention, the gaseous medium comprising the particles whose characteristic we seek to measure corresponds to an ambient gaseous medium, such as ambient air, present outdoors (areas close to road or rail networks, polluting industries, etc.) or in a confined or semi-confined space (underground parking, spaces in which employees are exposed to particles at least temporarily, etc.).

[0044] According to the invention, the measurement area can be closed (and can then, for example, correspond to a portion of a conduit such as a portion of an exhaust line) or open (and can then be defined by the volume of a cylinder formed by the UV radiation between the UV light source and the spectrometer).

[0045] The method according to the invention allows for in-situ measurement, for example directly in the conduit through which the gas or gas mixture flows, or directly in the ambient gaseous environment, without taking gas samples or transforming or pre-conditioning the gas. In particular, the method according to the invention does not require transforming non-measurable particles into measurable particles.

[0046] Furthermore, the method according to the invention is non-intrusive (for example, it does not alter the flow of gas such as exhaust gases) and is instantaneous, for example with a response time of less than 0.1 s.

[0047] The optical measurement system according to the invention, enabling in-situ measurement, can be easily mounted in or on a vehicle, or any other mobile device. According to one embodiment of the invention, the optical measurement system can be mounted in an exhaust line for exhaust gases from an internal combustion engine.

[0048] THE figures 1A et 1B These schematically represent the principle of the measurement process and system in the case of a measurement zone corresponding to a portion of a duct (such as an exhaust line) through which a gas or a mixture of gases (such as exhaust gases) flows. figure 1A differs from figure 1B by the optical measurement system, which is in a transmissive configuration in the figure 1A and according to a reflective configuration in the figure 1B The method according to this implementation of the invention comprises at least the following steps: The emission by the light source 41 of UV radiation 42 through the gas 10 flowing within a measurement zone 21 located in a duct 20. The UV radiation 42 enters the measurement zone 21 located in the duct 20 through an optical access point (not shown), for example, a window or a lens. The UV radiation 42 passes through the gas along an optical path that may be substantially perpendicular to the path P of the gas. In the case of the configuration of the figure 1A , the optical path is of length d such that represented on the figure 1A In the case of the alternative configuration of the figure 1B according to which the optical system comprises a reflector positioned as described in relation to the figures 13 And 14 further on, the optical path has a length d+d as shown on the figure 1B The detection by the spectrometer 44 of at least a portion 43 of the UV radiation that has passed through the gas in the measurement zone 21, and the generation of a digital signal 50 of the light intensity as a function of the wavelength of the portion of the UV radiation that has passed through the gas. The UV radiation passing through the gas containing the particles whose characteristic is to be measured undergoes both absorption and scattering, as will be described later. The portion of the UV radiation that has passed through the gas is detected by the spectrometer 44 through an optical access (not shown), for example, a window or a lens, which may be the same optical access as for the passage of the radiation emitted by the light source 41 in the case of the configuration of the figure 1B . the estimation of at least one characteristic of the particles present in the gas 10 from at least the digital signal of the light intensity 50.

[0049] Similarly, the figures 1C et 1D The diagrams schematically represent the principle of the method and the measurement system within a measurement zone corresponding to the cylinder formed by the UV radiation beam between the light source and the spectrometer, a measurement zone in which a gaseous medium such as ambient air is present. The method according to this embodiment of the invention comprises at least the following steps: The emission by the light source 41 of UV radiation 42 through the gas 10 present in a measurement zone 21 located between the light source 41 and the spectrometer 44. The UV radiation 42 passes through the gas 10 along an optical path. In the case of the configuration of the figure 1C The optical path has length d. In the case of the alternative configuration of the figure 1D according to which the optical system comprises a reflector positioned as described in relation to the figures 13 And 14Further along, the optical path is of length d+d. The detection by the spectrometer 44 of at least a portion 43 of the UV radiation that has passed through the gas in the measurement zone 21, and the generation of a digital signal 50 of the light intensity as a function of the wavelength of the portion of the UV radiation that has passed through the gas. The UV radiation passing through the gas containing the particles whose characteristic is to be measured undergoes both absorption and scattering. The portion of the UV radiation that has passed through the gas is detected by the spectrometer 44 placed in line with the light source. The estimation of at least one characteristic of the particles present in the gas 10 from at least the digital signal of the light intensity 50.

[0050] In general, when an electromagnetic wave encounters a particle, two phenomena can occur: The particle stores energy, this is the phenomenon of absorption; The particle spatially redistributes energy: this is diffusion.

[0051] Furthermore, the sum of absorption and total scattering (scattering throughout space) is defined as extinction. These three phenomena can be quantified using quantities called cross sections, which are conventionally denoted C abs , C sca , And C exc the indices abs, sca And ext being related to absorption, total diffusion (or "scattering" (in English) and extinction. Generally, a cross section is the ratio between the energy transformed during the interaction and the particle's illumination (energy received per unit area). The present invention exploits the measurement of the extinction phenomenon (absorption and scattering) undergone by the radiation emitted by the light source as it passes through a gaseous medium containing particles.

[0052] According to the invention, during step 3) of the process according to the invention, at least one characteristic of said particles present in the gaseous medium is determined in the following manner: (i) Starting from at least the digital signal of light intensity as a function of wavelength, a curve representing the variations of the Beer-Lambert law extinction coefficient as a function of wavelength is determined for the gaseous medium containing the particles whose characteristic is to be determined. According to one embodiment of the invention, the extinction coefficient, denoted below K ext exp , can be obtained using a formula of the type: K ext exp λ = − 1 L ln I s λ I 0 λ where L is the optical path length, λ is the wavelength, I s ( λ ) is the digital signal of light intensity as a function of the wavelength of the portion of UV radiation that has passed through the gaseous medium, as measured by the spectrometer, and I 0 ( λ This is a digital reference signal of light intensity as a function of wavelength. The digital reference signal of light intensity as a function of wavelength can be obtained by emitting UV radiation, by the light source according to the invention, through a reference gas (i.e., one not containing particles or gaseous chemical species other than those that predominantly compose air, such as filtered air, nitrogen, or helium), and by detecting, by the spectrometer according to the invention, at least a portion of said UV radiation that has passed through said reference gas. Advantageously, the method according to the invention may include a preliminary step of calibrating the optical measurement system to determine a digital reference signal of light intensity as a function of wavelength.(ii) Said characteristic is determined by minimizing a difference between said curve representing the variations of a Beer-Lambert extinction coefficient and a model of the reference variations of the Beer-Lambert extinction coefficient as a function of wavelength and the characteristic of interest. In other words, according to the invention, the characteristic C of the particles present in the gaseous medium of interest is sought, allowing a functional to be minimized. F ( λ, C) expressed in the form: F λ C = K ext exp λ − K ext ref λ C Or K ext exp λ is the curve representing the variations of the Beer-Lambert law extinction coefficient as a function of the measured wavelength as described above, and K ext ref λ C is said model of the reference variations of the Beer-Lambert law extinction coefficient as a function of wavelength and the characteristic C of interest. Advantageously, step 3) of the method according to the invention can be implemented using the means for processing and analyzing the optical measurement system according to the invention.

[0053] According to this preferred embodiment of the invention, the model of the reference variations of the extinction coefficient of Beer-Lambert's law as a function of wavelength and of the characteristic C of interest, referred to hereafter as the reference extinction model for the sake of simplification, may be derived from the literature, or obtained experimentally, or theoretically.

[0054] According to a first embodiment of the method according to the invention, a reference extinction model can be experimentally determined as a function of particle concentration as follows: a plurality of curves representing the variations of the Beer-Lambert extinction coefficient as a function of wavelength are determined for a plurality of samples of a gaseous medium comprising particles having distinct concentrations and size distributions. Then, from this plurality of curves, a curve is determined. C ext ref ¯ λ representative of the average variations of the extinction cross-section as a function of wavelength for said samples; then said reference extinction model is constructed as a function of particle concentration N according to a formula of the type: K ext ref λ N = N . C ext ref ¯ λ

[0055] Thus, according to this variant of the process according to the invention, a concentration in number of particles is determined N contained in a gaseous medium by determining the concentration N allowing to minimize the functional F ( λ, N) next: F λ N = K ext exp λ − N . C ext ref ¯ λ Or K ext exp λ is the curve representing the variations of the Beer-Lambert law extinction coefficient as a function of the measured wavelength for the gaseous medium whose particle concentration is to be determined. Advantageously, a least-squares method is used, for example by applying it to the aforementioned processing and analysis equipment, to achieve this minimization.

[0056] Advantageously, for the implementation of the first variant of the method according to the invention, the curve is determined C ext ref ¯ λ representative of average variations in the extinction cross-section as a function of wavelength for said samples by applying at least the following steps: We generate a plurality of samples of the gaseous medium of interest (hereafter referred to as "gas" in general, for the sake of simplicity) containing particles with distinct size and concentration distributions. In what follows, each gas sample is referenced by an index i, with i ranging from 1 to 1, and we denote N i the concentration in number of particles. Advantageously, the plurality of gas samples comprising particles is generated by means of a particle generator such as the CAST 5201D device from Jing Ltd (Switzerland). Advantageously, the concentration is measured N i the number of particles for each sample is determined using a reference measuring instrument, for example, the PPS device from PEGASOR (Finland). Advantageously, the number of samples I is at least 2, preferably 5, and most preferably 10. Steps 1) and 2) of the method according to the invention are applied to each of the gas samples, and a curve is determined using formula (1) above. K ext ref , i λ representative of the variations of the extinction coefficient as a function of wavelength for each of the gas samples i, with i varying from 1 to I; From this plurality of curves representing the variations of the extinction coefficient as a function of wavelength for each of the gas samples and the concentration N i Based on the number of particles in each sample, a curve is determined, denoted C ext ref , i λ , representative of the variations in the extinction cross section as a function of wavelength for each of the gas samples i, with i varying from 1 to I, according to a formula of the type: C ext ref , i λ = K ext ref , i λ / N i from the variation curves of the extinction cross sections C ext ref , i λ based on the wavelength determined for each of the samples, a curve is determined, denoted C ext ref ¯ λ Subsequently, representative of the average variations of the extinction cross-section as a function of wavelength for each of the samples considered. This curve C ext ref ¯ λ can be obtained by calculating an average, possibly weighted, of the curves C ext ref , i λ representative of the variations of the extinction cross section as a function of wavelength for each of the gas samples i, with i varying from 1 to I.

[0057] According to a second variant of the process according to the invention, a reference extinction model can be determined theoretically as a function of particle concentration and / or average particle size, using a formula from the theory known by the acronym RDG-FA (Rayleigh-Debye-Gans for Fractal Aggregates).

[0058] In general, a description of the RDG-FA theory can be found in the document: Farias, TL, Köylü, Ü. Ö., & Carvalho, MDG (1996). Range of validity of the Rayleigh-Debye-Gans theory for optics of fractal aggregates. Applied optics, 35(33), 6560-6567).

[0059] According to this theory, the extinction coefficient K ext RDG − FA λ can be expressed using a formula such as: K ext RDG − FA λ = 4 πx p 3 k 2 E m N agg ∫ D min D max N p D m . n D m . dD m ∫ D min D max n D m . dD m + 8 πx p 6 3 k 2 F m N agg ∫ D min D max N p 2 D m g R g k D f . n D m . dD m ∫ D min D max n D m . dD m

[0060] With x p = πD p λ k = 2 π λ n D m = 1 log σ geo 2 π exp − log D m − log D m , geo 2 2 log σ geo 2 N p D m = k f 1.3 D m D p D f Or : D m,geo And σ geo correspond respectively to the mean diameter and standard deviation of the particle size distribution following a log-normal law between the values ​​D min and D max; D p , k f , D f are parameters relating to the fractal morphology of particles; E(m) And F(m) are parameters relating to the optical properties of particles, E ( m ) being an absorption function and F(m) being a diffusion function that depends directly on the complex refractive index; N agg corresponds to the number concentration of particles

[0061] In general, this formula models the interaction between an electromagnetic wave and particles in the form of clusters (which applies particularly to soot). More precisely, the first term of this formula represents the portion of light energy absorbed by the particles, and the second term represents the portion of light energy scattered by the particles (spatial redistribution of the unabsorbed energy).

[0062] According to one embodiment of this second variant of the invention, at least the concentration in number of particles is determined N agg using a reference extinction model written according to a formula of the type: K ext RDG − FA λ N agg = N agg . C ext RDG − FA λ in which C ext RDG − FA λ corresponds to the extinction cross-section varying with wavelength according to the theory in the RDG-FA theory, and which can be expressed according to a formula of the type: C ext RDG − FA λ = 4 πx p 3 k 2 E m ∫ D min D max N p D m . n D m . dD m ∫ D min D max n D m . dD m + 8 πx p 6 3 k 2 F m ∫ D min D max N p 2 D m g R g k D f . n D m . dD m ∫ D min D max n D m . dD m

[0063] Thus, according to this implementation of the second variant of the process according to the invention, a concentration in number is determined N agg of particles contained in a gaseous medium by determining the concentration N agg allowing to minimize the functional F ( λ, N agg ) next: F λ N agg = K ext exp λ − N agg . C ext RDG − FA λ Or K ext exp λ is the curve representing the variations of the Beer-Lambert law extinction coefficient as a function of the measured wavelength for the gaseous medium whose particle concentration is to be determined. Advantageously, the least squares method is used, for example, by applying it to the aforementioned processing and analysis methods, to achieve this minimization.

[0064] Advantageously, values ​​for the parameters appearing in equation (7) above can be obtained from the literature or experimentally. According to an embodiment of the invention in which the particles are soot, the values ​​of k f And D f respectively to 2.3 and 1.8. According to an embodiment of the invention in which the particles are soot from diesel combustion, the parameter values ​​can be fixed E(m) And F(m) respectively at 0.25 and 0.15.

[0065] According to an embodiment of this second variant of the preferred implementation of the invention, applicable when the concentration in number of particles N agg is known, we determine an average particle size N p using a reference extinction model expressed by an equation of the type: K ext RDG − FA λ N p ¯ = 4 πx p 3 k 2 E m N agg N p ¯ with : N p ¯ = ∫ D min D max N p D m . n D m . dD m ∫ D min D max n D m . dD m , in which N p corresponds to the average size of the particles expressed as the number of primary spherules constituting the aggregates. Equation (9) above can be obtained by assuming, as a first approximation, that the second term of equation (4) above is negligible.

[0066] Thus, according to this implementation of the second variant of the process according to the invention, the average particle size is determined N p contained in a gaseous medium by determining the average particle size N p allowing to minimize the functional F (λ, N p ) next: F λ N p ¯ = K ext exp λ − K ext RDG − FA λ N p ¯ Or K ext exp λ is the curve representing the variations of the Beer-Lambert law extinction coefficient as a function of the measured wavelength for the gaseous medium whose particle concentration is to be determined. Advantageously, the least squares method is used, for example by applying it to the aforementioned processing and analysis equipment, to achieve this minimization.

[0067] According to another embodiment of the second variant of the invention, when the particle concentration is unknown, one (or more) pair(s) of values ​​for the number concentration can be determined. N agg of particles and the average size in number of spherules N p constituting the aggregates by searching for the pair ( N agg ,N p allowing to minimize the functional F ( λ, N agg ,N p ) next: F λ N agg N p ¯ = K ext exp λ − K ext RDG − FA λ N agg N p ¯ Or K ext exp λ is the curve representing the variations of the Beer-Lambert law extinction coefficient as a function of the measured wavelength for the gaseous medium whose particle concentration and reference extinction model we are trying to determine K ext RDG − FA λ N agg N p ¯ is written according to a formula of the type: K ext RDG − FA λ N agg N p ¯ = 4 πx p 3 k 2 E m N agg N p ¯

[0068] Advantageously, the average size expressed in the number of primary spherules can be expressed in the electrical mobility diameter via a fractal law of the type: D m ¯ = D p β N p ¯ k f 1 Df with : β = 1.3.

[0069] Advantageously, in order to reduce the number of couples ( N agg ,N p ) possible, we look for the values ​​of the parameters ( N agg ,N p ) within predefined ranges. According to one embodiment of the invention, in the case of soot particles produced by a diesel internal combustion engine, a value of N p between 20 and 200 nm, and a value of N agg between 0 and 10<20<# / cm 3< , where the # sign represents the number of particles.

[0070] Advantageously, we use a conjugate gradient inversion method or a Gauss-Newton method to perform the minimization of the functional according to equation (11).

[0071] In a preferred embodiment of the invention, the method may further include, in step 3), the in situ measurement of the concentration of at least one gaseous chemical species contained in the gaseous medium of interest, from at least the digital signal of the light intensity measured in steps 1) and 2). In other words, from the same measurement as that carried out to estimate a characteristic of the particles present in a gaseous medium, it is also possible to determine the concentration of at least one gaseous chemical species contained in this gaseous medium.

[0072] As will be described below, this preferred embodiment of the invention does not require pre-conditioning of the gaseous medium (by overconcentration for example) in the case of a chemical species present in small quantity in the gaseous medium to be analyzed.

[0073] The measurement of the concentration of a chemical species according to the preferred embodiment of the invention can, in particular but not limited to, be carried out at least according to the configurations 1A to 1D described above.

[0074] In the method according to this preferred embodiment of the invention, the concentration of each chemical species is determined from optical measurements performed on the gaseous medium and from an optical signature specific to each chemical species. Each gaseous chemical species whose concentration is to be measured absorbs a portion of the UV radiation and exhibits its own absorption spectrum (absorbance as a function of wavelength).

[0075] During the step of estimating the concentration of at least one chemical species of the preferred embodiment of the invention, at least steps a) and b) described below are carried out: a) The absorbance A of the gaseous medium is determined as a function of the wavelength λ from the digital signal of the light intensity generated by the spectrometer and resulting from the detection of the portion of UV radiation that has passed through the gaseous medium, and from a reference digital signal. In particular, the absorbance A of the gaseous medium is calculated using a formula of the type: A = − ln I s λ I 0 λ where λ is the wavelength, I s ( λ ) is a digital signal of the light intensity as a function of the wavelength of the portion of UV radiation that has passed through the gaseous medium and has been measured by the spectrometer, and I 0 ( λ ) is a digital reference signal of light intensity. The digital reference signal of light intensity as a function of wavelength can be obtained by emitting UV radiation, by the light source according to the invention, through a reference gas (i.e., not containing particles or gaseous chemical species other than those that predominantly compose air, such as filtered air, nitrogen, or helium), and by detecting, by the spectrometer according to the invention, at least a portion of said UV radiation that has passed through said reference gas. According to this preferred embodiment of the invention, the terms I s ( λ ) And I 0 ( λ ) correspond to the digital signals resulting from measurements performed for the application of formula (1) above. In other words, for this preferred embodiment, there is no need to perform any additional measurements beyond those performed to determine a particle characteristic as described above. Advantageously, the method according to the invention includes a preliminary step of calibrating the optical measurement system to determine a reference digital signal of the light intensity. b) The concentration of each chemical species to be measured is determined, for example using analytical and processing means, from the absorbance A of the gaseous medium, predetermined absorbance characteristics, and an estimate of the pressure and temperature of each chemical species.These predetermined absorbance characteristics of each chemical species are preferably obtained during preliminary measurement campaigns to create a database. Data from the literature can also be used to populate such a database. The absorbance characteristic of a given chemical species is understood to be its molar extinction coefficient. Advantageously, the pressure and / or temperature can be estimated by measurement during the implementation of the process according to the invention, using a pressure sensor and / or a temperature sensor, respectively.

[0076] There figure 2 This schematically represents the absorbance A of the gaseous medium containing different gaseous chemical species A, B, C that we wish to measure. The diagram on the left represents an example of the absorbance A (unitless) of the gaseous medium, expressed as a function of the wavelength W (in nm), calculated from the digital signal of the light intensity 50 generated by the spectrometer and the digital reference signal.

[0077] The absorbance A of a gaseous medium is a function of the absorbance length, that is, the length of the optical path traveled by light in the measurement region located in the escape line, the number density of molecules of the gaseous chemical species (A, B, C) contained in the gaseous medium, and the molar extinction coefficient. The molar extinction coefficient, also called molar absorptivity, is a measure of the probability that a photon will interact with an atom or molecule.

[0078] The number density of molecules of a chemical species is itself a function of the temperature, pressure and concentration of the chemical species, and the molar extinction coefficient is a function of the wavelength, the chemical species, the temperature and the pressure.

[0079] Thus, by having predetermined characteristics of the molar extinction coefficient, temperature, and pressure of each chemical species, it is possible to determine the concentration of each chemical species X from the absorbance A of the gaseous medium. The absorbance values ​​of each chemical species are added together, and their sum is typically equal to the absorbance A values ​​of the gaseous medium, plus noise and the absorbance of the other unmeasured chemical species. This is what is represented on the right of the figure 2 by the absorbance diagrams AA, AB, and AC of chemical species A, B, and C, which add up to form the absorbance A of the exhaust gas, noise, and other undetected chemical species near AD.

[0080] Various types of algorithms can be used for concentration calculations, such as least-squares fitting algorithms applied to the absorbance signals themselves, to the derivatives of the absorbance signals, or to the frequency component of the absorbance signals (typically derived from a Fourier transform). Similarly, a number of chemometric methods can be used for this process, such as principal component analysis (PCA) or partial least squares (PLS) algorithms.

[0081] Thus, in this preferred embodiment of the method according to the invention, the concentration of each chemical species is determined from the optical measurement performed on the gaseous medium and from the optical signature specific to each chemical species. Each gaseous chemical species whose concentration is to be measured absorbs a portion of the UV radiation and exhibits its own absorption spectrum (absorbance as a function of wavelength).

[0082] This preferred embodiment of the invention is highly advantageous because it allows, by means of a single method and a single measurement system, the determination of characteristics of various types of pollutants, namely particles (solid or liquid) and gaseous chemical species, present in a gaseous medium. Monitoring compliance with standards relating to pollutants is therefore simplified.

[0083] According to a variant of the preferred embodiment of the invention, it is possible to measure at least one gaseous chemical species X, and preferably several gaseous chemical species X, included in the non-limiting list of: NO, NO2, N2O, NH3, BTEX, SO2, H2S, O3, O2, H2O, aldehydes such as acetaldehyde or formaldehyde, non-aromatic hydrocarbons such as acetylene or buta-1,3-diene.

[0084] Preferably, at least one, and more preferably several, gaseous chemical species are measured from the list consisting of: NO, NO2, N2O, NH3, BTEX, SO2, H2S, O3, O2, H2O. The acronym BTEX refers to benzene, toluene, ethylbenzene and xylenes belonging to the family of aromatic hydrocarbons.

[0085] Advantageously, the concentration of several of these gaseous chemical species can be measured separately and simultaneously. Separate measurement means access to the specific concentration of each chemical species, as opposed to a single, aggregate measurement of the concentration of several chemical species indiscriminately. For example, the concentration of at least two gaseous chemical species can be measured simultaneously, preferably at least the concentration of NO and the concentration of NO₂.

[0086] Nitrogen oxides NO and NO2 are regulated air pollutants, which are advantageous to measure.

[0087] Such a measure can thus allow control of NOx at the outlet of a post-combustion pollution control system, preferably at the outlet of an SCR system, or control of NOx upstream and downstream of a pollution control system, preferably an SCR system, to estimate for example the real-time conversion of NOx into N2 by said pollution control system, and while controlling the supply of NH3.

[0088] The SCR system selectively reduces NOx using a reducing agent on a dedicated catalyst. The reduction is considered selective because the reducing agent reduces the NOx and not the oxygen present in the exhaust gas mixture. With such a pollution control system, the NOx is primarily converted into nitrogen (N2). This reducing agent, particularly for use with an internal combustion engine in a motor vehicle, is either ammonia (or a substance that can decompose into ammonia), an oxygenated or non-oxygenated hydrocarbon, a mixture of hydrocarbons that may contain one or more oxygenated hydrocarbons, or hydrogen. In the case of exhaust gas treatment with an SCR system using ammonia, for example, the ammoniacal agent used is stored either as a solid complex or as a liquid precursor, such as urea in aqueous solution.

[0089] The measurement of NO and NO2 thus allows control of NOx upstream and downstream of an SCR system using an ammoniacal agent, to estimate for example the real-time conversion of NOx into N2 by said pollution control system while controlling the supply of NH3.

[0090] Thanks to a measurement of NO dissociated from that of NO2, it is possible to know the molar ratio NO / NO2 composing NOx.

[0091] It is important to know the NO / NO2 ratio because the NOx reduction reaction with ammonia varies depending on this ratio (it is faster when NO and NO2 are present in equal amounts). Furthermore, NO2 is useful for aiding the oxidation of soot present in a diesel particulate filter (DPF), for example.

[0092] Advantageously, when the method according to the invention is applied to exhaust gases circulating in an exhaust line, the optical measurement system can comprise two optical sensors placed respectively upstream and downstream of the NOx pollution control system. It is then possible to know the real-time conversion of NOx to N2. An on-board diagnostic (OBD) system including NO2 diagnostics is then possible, such diagnostics potentially being required in the future by regulations on pollutant emissions from internal combustion engines of vehicles.

[0093] Advantageously, it is also possible to measure the concentration of at least SO2, or H2S, and preferably at least both. Quantifying sulfur compounds allows, for example, the diagnosis of possible poisoning of a post-combustion pollution control system that may be linked to the use of fuels with a high sulfur content, or the monitoring of emissions from ships operating in sulfur emission control areas (SECAs).

[0094] Advantageously, the concentration of at least NH3 is measured. In this case, the evolution of the NH3 concentration upstream and / or downstream of the SCR system can be monitored, in particular to control the NH3 concentration upstream of an SCR system, for example by adjusting the injection of an ammonia agent into the SCR system, or to manage problems of unwanted NH3 emissions during operation with an SCR system. An OBD including NH3 diagnostics is then possible, such diagnostics potentially being required in the future by regulations on pollutant emissions from internal combustion engines in vehicles.

[0095] According to a principal embodiment of the preferred method of the invention, in step 3, the temperature (T) of the gaseous medium is determined, in addition to at least one characteristic of the particles present in the gaseous medium and the concentration of at least one gaseous chemical species in the gaseous medium. Advantageously, the temperature (T) of the gaseous medium is determined by modifying the molar extinction coefficient of the absorbance of the chemical species whose concentration is to be measured, said absorbance of the chemical species being extracted from the absorbance of the gaseous medium. The modification of the molar extinction coefficient may be a wavelength shift, leading to absorption at different wavelengths, or a modification of the absorbance amplitude at a given wavelength, or a combination of both.When the exact behavior of the molar extinction coefficient of absorbance as a function of temperature for a chemical species is known, either through prior measurements or data from the literature, allowing the creation of a library, this chemical species can be used as a temperature indicator. The degree of accuracy in determining the temperature depends on the sensitivity of the molar extinction coefficient of the chemical species within the measured wavelength range. figure 3 This illustrates the influence of temperature on the absorbance of a chemical species, in this case ammonia, used to determine the temperature according to the present invention. The A-Tc curve represents the absorbance of NH3 at a low temperature, for example 20°C, and the A-Th curve represents the absorbance of NH3 at a high temperature, for example 450°C. A change in the molar extinction coefficient, for example, results in a shift in the absorption signal. Although the example given concerns ammonia, any other chemical species such as SO2, H2S, NH3, BTEX, siloxanes, halogens, aldehydes such as acetaldehyde or formaldehyde, and non-aromatic hydrocarbons such as acetylene or buta-1,3-diene, can be used to determine the temperature. The same type of algorithm used to determine the concentration of chemical species can be used to determine temperature.

[0096] Thus, the method according to this main variant of the preferred embodiment of the invention allows access to the temperature of the gaseous medium without additional measuring equipment in the control area. Furthermore, this main variant of the preferred embodiment of the invention allows for the instantaneous measurement of temperature, through specific processing of the UV absorption signal, simultaneously with the measurement of the concentration of gaseous chemical species contained in the gaseous medium and the measurement of a characteristic of the particles present in the gaseous medium.

[0097] According to the invention, the UV radiation emitted by the light source 42 has a wavelength of at least 180 to 400 nm to measure a characteristic of particles, preferably at least between 190 and 250 nm, a range which proves sufficient in particular in the case of soot produced by a Diesel type thermal engine.

[0098] Advantageously, when the method is implemented according to its preferred embodiment of the invention to also measure a concentration of at least one gaseous chemical species, the UV radiation emitted by the light source 42 has a wavelength at least between: 180 and 230 nm in the case of NO, and / or 180 and 220 nm in the case of NH3, and / or 250 and 300 nm in the case of BTEX, and / or 180 and 230 nm in the case of NO2, and / or 180 and 240 nm in the case of SO2, 160 and 260 nm in the case of H2S, 120 and 340 nm in the case of O3.

[0099] These wavelength ranges are part of what is referred to as deep UV.

[0100] As an example, the light source may be an LED diode emitting in the UV and in particular in the deep UV as indicated above, or perhaps a xenon, deuterium, zinc, cadmium lamp, or another gas lamp such as KrBr, KrCL, KrF excimer lamps.

[0101] The spectrometer allows the analysis of the light signal in the wavelength range of 180–400 nm, preferably 180–280 nm, and more preferably 180–240 nm. Alternatively, a simplified system for analyzing a reduced wavelength range can be used. The term "spectrometer" is retained in the present invention to designate such a simplified system.

[0102] The assembly consisting of at least the UV light source and the spectrometer, also called the optical system or optical sensor in the present invention, is known per se. Such optical sensors are commercially available.

[0103] The optical system may include other elements, notably optical components such as lenses that modify the light beam as needed (e.g., convergence or divergence), or protective elements designed to safeguard the light source and the spectrometer, particularly during cold operation of the optical measurement system. Indeed, cold operation can cause deposits to form on the optical elements due to condensation. Such protective elements are described below, in relation to the figure 12 The position of the sensor installed on the pipe through which the gas or gas mixture flows can be chosen in such a way as to limit fouling of the pipe.

[0104] The invention is advantageously, but not limited to, the field of exhaust gas pollution control. In this context, the optical measurement system can be positioned at different locations on the exhaust line to perform in situ measurement of a characteristic of the particles present in the exhaust gases, and / or the concentration of at least one gaseous chemical species and the temperature, these measurements being adapted to pollutant emissions upstream and / or downstream of a post-combustion gas pollution control system.

[0105] According to one embodiment, the measurement can be carried out downstream of at least one exhaust gas aftertreatment system such as a Diesel Oxidation Catalyst (DOC), an SCR system, or a DPF. Such an embodiment is schematically represented in the figure 4 The exhaust gases 10 flow through the exhaust line 20 along path P, on which an emissions control system 60 is placed, for example a DOC, an SCR system, or a DPF. The emissions control system may also be that of a gasoline engine, such as a three-way catalytic converter, or another emissions control system commonly used on diesel engines, such as a PNA (partial NOx adsorber) or a NOx trap. The emissions control system comprises one or more elementary emissions control units, each elementary emissions control unit being a catalytic converter and / or a filter, catalyzed or not, and modifies the composition or content of the exhaust gases. According to the embodiment shown in the figure 4 The optical measurement system 40 is located downstream of the emissions control system 60. The upstream and downstream positions are defined relative to the direction of exhaust gas flow in the exhaust system. Measuring the particle characteristics, and optionally the exhaust gas temperature and the concentration of certain chemical species in the exhaust gases downstream of the emissions control system 60, allows verification of compliance with pollutant emission standards, monitoring of changes in these standards, and, if necessary, adjustment of the emissions control system's operation to modify the quantities of these substances emitted.

[0106] According to another embodiment, represented in the figure 5 The in situ measurement is carried out in the same way as in the embodiment shown in the figure 4 except that the optical measurement system includes a reflector 45 for reflective measurement. The optical measurement system is that described in relation to the figure 1B . The source 41 and the spectrometer 44 are placed on the same side of the escape line 20, that is to say at the same end of the measurement area 21, opposite to that where the reflector 45 is positioned.

[0107] According to another embodiment, the in-situ measurement can be carried out upstream of at least one exhaust gas aftertreatment system such as the four systems mentioned above (DOC, PNA, SCR, FAP). Such an embodiment is schematically represented in the figure 6 identical to the figure 4 with the exception of the optical measurement system 40 positioned upstream of the pollution control system 60. Such an embodiment can be useful for obtaining information on the characteristics of the particles present in the exhaust gases, and possibly also on the temperature and concentration of chemical species of the exhaust gases before they enter the pollution control system, and for using this information to influence, for example, the operation of the pollution control system 60.

[0108] According to one embodiment, the in-situ measurement can be carried out upstream and downstream of at least one exhaust gas aftertreatment system such as the four systems mentioned above (DOC, PNA, SCR, FAP). An example of this method is illustrated in the figure 7 in which two optical measurement systems 40 and 40' are placed upstream and downstream of a pollution control system 60, respectively. The second optical measurement system 40' is identical to the first optical measurement system 40 located upstream, and comprises a light source 40' and a light analyzer 44' enabling the emission of UV radiation and the detection and analysis of UV radiation that has passed through the exhaust gases in the measurement zone 21' located on the exhaust line 20, respectively, in order to provide an estimate of the characteristics of the particles present in the exhaust gases, and possibly also of the temperature and concentration of chemical species in the exhaust gases. Another example of this method is illustrated in the figure 8 in which the exhaust line comprises two pollution control systems 60 and 61, and three optical measurement systems 40, 40', and 40", placed respectively upstream of the first pollution control system 60, between pollution control systems 60 and 61, and downstream of pollution control system 61. Such an embodiment also makes it possible to know the effectiveness of each pollution control system on the exhaust line, and possibly to adjust the pollution control systems according to information on the characteristics of the particles present in the exhaust gases, and possibly also on the temperature and concentration of chemical species of the exhaust gases, obtained upstream, downstream and between the pollution control systems, in order to control the pollutant emissions at the outlet of the exhaust line.

[0109] In one embodiment, the UV light source and / or the spectrometer of the optical measurement system can be connected to the exhaust line by an optical fiber, allowing greater flexibility in integrating the optical measurement system into the exhaust line, particularly in the context of an on-board vehicle system. An example of such an embodiment is shown in Figure 1. figure 9 where optical fibers 75 and 76 respectively connect the light source 71 and the spectrometer 74 of the optical measurement system 70 to the exhaust line 20, at the level of the measurement zone 21 where the exhaust gases are traversed by the UV radiation conducted by the optical fibers 75 and 76.

[0110] For each of the embodiments represented in figures 5 à 9 The optical system can be the one described in relation to the figure 1B including a 45 reflector.

[0111] In one embodiment, the optical measurement system can comprise several measurement zones connected to a single light source and a single spectrometer by means of optical fibers. Such an embodiment, for example, reduces the cost of implementing optical measurement when upstream and downstream measurements of pollution control systems are required. An example of such an embodiment is illustrated in the figure 10 , wherein the optical measurement system 80 comprises three measurement zones 21, 22 and 23, and a single lighting system 81 and detection / analysis system 84 connected to the measurement zones by optical fibers 85, 87 and 88.

[0112] According to another embodiment, the in situ measurement can be carried out in the same way as in the embodiment shown in the figure 10 except that the optical measuring system comprises three reflectors 45, 45' and 45" as described in relation to the figure 1B , respectively arranged at the ends of measurement zones 21, 22 and 23. Such a system is illustrated in figure 11 .

[0113] According to one embodiment, illustrated in the figure 12 The optical measuring system 90 includes protective means 95 for the light source 91 and / or the spectrometer 94. Such protective means are useful, for example, during cold operation of the optical measuring system, to prevent fouling of the optical elements, as explained above. These protective means may include a shutter that can be controlled by measuring the temperature of the exhaust gases, an air barrier between the light source 91 or the spectrometer 94 and the exhaust gases passing through the measuring zone 21, a specific coating, for example for thermal insulation or to prevent the adhesion of liquids or solid particles, on the surface(s) separating the light source 91 from the exhaust gases or the spectrometer 94 from the exhaust gases, or a means for heating said surfaces. It may also consist of a specific geometry of the optical sensor, not shown in the figure 12 .

[0114] According to one embodiment, illustrated in the figure 13 The optical measurement system 90 is of the type shown in the figure 1B and the optical path of UV radiation is substantially tangent to the path P of the exhaust gases, the optical system being positioned for example at the level of a bend in the exhaust line 20.

[0115] According to another embodiment, illustrated in the figure 14 The optical measurement system 90 is of the type shown in the figure 1B and the optical path of the UV radiation is substantially parallel to the path P of the exhaust gases. The light source 91 and spectrometer 94 assembly are located at the outlet of the exhaust line.

[0116] The present invention is advantageously applicable to the monitoring of polluting emissions from exhaust gases from an internal combustion engine in the field of transport, in particular that of rolling vehicles, but also to the monitoring of polluting emissions in the case of stationary applications such as industrial fumes from combustion plants or generator sets.

[0117] In the field of transport, particularly that of rolling vehicles, the present invention can be used to monitor post-combustion pollution control systems, and possibly control, i.e. intervene in the adjustment, of such pollution control systems. Exemples

[0118] The advantages of the method and device according to the invention are presented below in two application examples. Exemple 1

[0119] The first example of application of the process according to the invention is implemented by means of the first variant described above, and aims to determine the concentration of particles present in the form of soot in a gaseous medium corresponding to exhaust gases from a Diesel type thermal engine.

[0120] More specifically, for this application example, five gas samples containing particles are generated using five different settings of the CAST DIESEL device (diesel-fired burner) from Jing Ltd (Switzerland). Each CAST DIESEL setting produces soot with a specific size distribution. Curves D1 to D5 of the figure 15 represent the particle size distributions for each sample, more precisely the distribution of the number of particles per unit volume NB as a function of their diameter Dm.

[0121] Furthermore, the soot formed by the CAST is measured using a reference particle size analyzer, in this case the SMPS device from TSI (France). The particle concentration is measured using a PPS from PEGASOR (Finland). These instruments allow us to determine the concentration and size of the soot particles present in each sample.

[0122] In parallel with these measurements, UV absorption measurements are performed as described in steps 1) and 2) of the process according to the invention for each of the samples. Then, using formula (1) above, a curve representing the variations of the extinction coefficient is determined. K ext depending on the wavelength λ for each of the 5 samples. All of these curves, labeled K1 to K5, are represented in figure 16 .

[0123] Then, from the curves representing the variations of the extinction coefficient as a function of wavelength for each gas sample and the particle concentration of each sample, we determine curves representing the variations of the scattering cross-section as a function of wavelength. λ for each of the samples according to formula (3) above. All of these curves, labeled C1 to C5, are presented for illustrative purposes in figure 17 We can observe in this figure that the curves C1 to C5 are very close to each other, which justifies averaging them in the next step.

[0124] There Figure 18 presents a curve denoted Cmoy which corresponds to the average of curves C1 to C5. It can be observed that this curve exhibits very high-frequency variations, especially at short wavelengths. In order to improve the application of formula (2) described above, a smoothed approximation Cmoy-pol of the high-frequency curve Cmoy is further determined using a polynomial of degree 4.

[0125] For this application example, we use the polynomial curve Cmoy-pol for minimization according to formula (2) described above, in order to determine the concentration by number of particles in the exhaust gases.

[0126] There figure 19 The graph shows the particle number concentration Ninv (gray curve) determined by the method according to this first embodiment of the invention, which is compared to a reference particle number concentration Nref (black curve), determined using the PPS PEGASOR device. It can be observed on this graph that... figure 19 that the process according to the invention makes it possible to determine a particle concentration very close to the reference concentration given by the PPS, which demonstrates that the process according to the invention is relevant for the measurement of particle concentration. Exemple 2

[0127] The second example of application of the method according to the invention is implemented by means of the second variant described above, and aims to determine both the average size and the concentration of particles present in the form of soot in a gaseous medium corresponding to exhaust gases from a Diesel type thermal engine.

[0128] More specifically, the particles were generated by a miniCAST-type generator from Jing Ltd (Switzerland), powered by diesel. The particles have an electrically mobile diameter between 30 and 150 nm (i.e., a number of spherules). N p < 200) depending on the operating point (overall richness ϕ ).

[0129] There figure 20 shows the evolution of the number concentration curve (Ninv curve at the top) and the average size curve (Dinv curve at the bottom) of particles as a function of overall richness ϕdetermined using the second variant of the method according to the invention, in comparison with curves (respectively denoted Nref and Dref) obtained using reference measuring devices (PPS from Pegasor for number concentration, SMPS from TSI for diameter mobility). It can thus be concluded by comparison that the method according to the invention is relevant for measuring both the concentration and size of particles, even in the case of nanoparticles.

Claims

1. Method for measuring at least one characteristic of particles present in a gaseous medium (10) by way of at least one optical measuring system (40, 40', 40", 70, 80, 90), said optical measuring system comprising at least one light source (41, 41', 41", 71, 81, 91) and a spectrometer (44, 44', 44", 74, 84, 94), said at least one particle characteristic being selected from a numerical concentration of said particles and an average size of said particles, said method comprising at least the following steps: a) the light source emitting UV radiation (42) through said gaseous medium (43) in a measuring zone (21, 22, 23); b) said spectrometer detecting at least a portion of said UV radiation that has passed through said gaseous medium (43) in said measuring zone (21, 22, 23), and generating a digital signal (50) signalling light intensity as a function of the wavelength of said portion of the UV radiation that has passed through said gaseous medium; c) estimating at least said characteristic of said particles present in said gaseous medium as follows: i) based at least on said digital signal signalling light intensity as a function of wavelength, determining a curve representative of variations of an extinction coefficient of the Beer-Lambert law as a function of wavelength for said gaseous medium comprising said particles; and ii) determining the value of said characteristic by minimizing a deviation between said curve representative of variations of an extinction coefficient of the Beer-Lambert law and a model of reference variations of the extinction coefficient of the Beer-Lambert law as a function of wavelength and at least said characteristic.

2. Method according to Claim 1, wherein said curve representative of variations of an extinction coefficient of the Beer-Lambert law as a function of wavelength is determined using a formula of the type: K ext exp λ = − 1 L ln I s λ I 0 λ where L is the length of the optical path taken by said radiation between said light source and said spectrometer, λ is said wavelength, Is(λ) is said digital signal signalling said light intensity as a function of the wavelength of said portion of said UV radiation that has passed through said gaseous medium, and I0(λ) is a reference digital signal signalling light intensity as a function of wavelength.

3. Method according to either of the preceding claims, wherein said characteristic corresponds to a numerical concentration of said particles, and wherein said model of reference variations of the extinction coefficient of the Beer-Lambert law as a function of wavelength and said numerical concentration is constructed as follows: a plurality of curves representative of variations of the extinction coefficient of the Beer-Lambert law as a function of wavelength are determined for a plurality of samples of said gaseous medium comprising particles having distinct particle concentrations and particle size distributions; a curve C ext ref ¯ λ representative of average variations of an extinction cross section as a function of wavelength λ is determined based at least on said plurality of curves and said particle concentrations of said samples; said reference extinction model K ext ref λ N as a function of said wavelength λ and said particle concentration N is constructed using a formula of the type: K ext ref λ N = N . C ext ref ¯ λ .

4. Method according to one of the preceding claims, wherein a model of reference variations of the extinction coefficient of the Beer-Lambert law as a function of wavelength and said numerical concentration and / or average size of said particles is constructed using the Rayleigh-Debye-Gans fractal aggregate theory.

5. Method according to Claim 4, wherein said characteristic is said numerical concentration of said particles, and wherein said model of reference variations of the extinction coefficient K ext RDG − F λ N agg as a function of wavelength λ and said numerical concentration Nagg is constructed based on a formula of the type: K ext RDG − FA λ N agg = N agg . C ext RDG − FA λ , where C ext RDG − F λ is an extinction cross section of the Rayleigh-Debye-Gans fractal aggregate theory.

6. Method according to Claim 4, wherein said characteristic is said average size of said particles, and wherein said model of reference variations of the extinction coefficient K ext RDG − FA λ N p ¯ as a function of wavelength λ and said average size Np of the particles expressed as a number of primary spherules making up the aggregates is constructed based on a formula of the type: K ext RDG − FA λ N p ¯ = 4 πx p 3 k 2 E m N agg N p ¯ , where Nagg is the numerical concentration, x p = πD p λ , k = 2 π λ , D p is a parameter relating to the fractal morphology of said particles, and E(m) is a parameter relating to the optical absorption properties of said particles.

7. Method according to Claim 4, wherein said model of reference variations of the extinction coefficient K ext RDG − FA λ N agg N p ¯ as a function of wavelength λ, said numerical concentration Nagg and said average size Np of the particles expressed as a number of primary spherules making up the aggregates is constructed based on a formula of the type: K ext RDG − FA λ N agg N p ¯ = 4 πx p 3 k 2 E m N agg N p ¯ , where x p = πD p λ , k = 2 π λ , D p is a parameter relating to the fractal morphology of said particles, and E(m) is a parameter relating to the optical absorption properties of said particles.

8. Method according to one of the preceding claims, wherein, in step c), a concentration of at least one gaseous chemical species contained in said gaseous medium (10) is furthermore determined as follows: - determining an absorbance (A) of said gaseous medium as a function of wavelength based at least on said digital signal signalling light intensity (50) as a function of the wavelength of said portion of the UV radiation that has passed through said gaseous medium (43) and a reference digital signal signalling light intensity as a function of wavelength; - determining said concentration of said at least one chemical species based on said absorbance (A) of said gaseous medium and predetermined characteristics pertaining to the absorbance, temperature and pressure of said chemical species.

9. Method according to Claim 8, wherein said concentration of at least one, and preferably several, gaseous chemical species contained in said gaseous medium and included in the following list: NO, NO2, N2O, BTEX, SO2, H2S, O3, O2, H2O, aldehydes such as acetaldehyde or formaldehyde, and non-aromatic hydrocarbons such as acetylene or buta-1,3-diene, is measured.

10. Method according to Claim 8, wherein, in step c), the temperature (T) of said gaseous medium is furthermore determined based on said digital signal (50) by changing the molar extinction coefficient of the absorbance of said chemical species extracted from the absorbance (A) of said gaseous medium, said change being a wavelength shift or an amplitude change or a combination of both.

11. Method according to one of the preceding claims, wherein the UV radiation (42) that is emitted has a wavelength between 180 and 400 nm, preferably between 190 and 250 nm.

12. Method according to one of the preceding claims, wherein said gaseous medium corresponds to exhaust gases and said measuring zone corresponds to a duct in which said exhaust gases circulate.

13. Method according to one of the preceding claims, comprising a preliminary step of calibrating said optical measuring system in order to supply a reference digital signal signalling light intensity as a function of wavelength, preferably by emitting said UV radiation through a reference gas, and by detecting at least a portion of said UV radiation that has passed through said reference gas in order to supply a reference digital signal signalling light intensity as a function of the wavelength of said portion of the UV radiation that has passed through said reference gas.

14. System for measuring at least one characteristic of particles present in a gaseous medium, said system comprising: I) a light source (41, 41', 41", 71, 81, 91) capable of emitting UV radiation (42) through said gas within a measuring zone (21, 22, 23); II) a spectrometer (44, 44', 44", 74, 84, 94) capable of detecting at least a portion of said UV radiation that has passed through said gas (43) in said measuring zone (21, 22, 23), and generating a digital signal signalling light intensity (50) as a function of the wavelength of said portion of the UV radiation that has passed through said gas (43); and III) means for processing and analysing said signal in order to determine at least said characteristic of said particles present in said gas based at least on said digital signal (50), and the system being characterized in that said light source (41, 41', 41", 71, 81, 91), said spectrometer (44, 44', 44", 74, 84, 94) and said means for processing and analysing said signal are designed to implement the method for optically measuring at least one characteristic of particles contained in a gaseous medium according to one of the preceding claims.