EP3921625A1

EP3921625A1 - Method for measuring the concentration of gaseous species in a biogas

Info

Publication number: EP3921625A1
Application number: EP20701268.3A
Authority: EP
Inventors: Matthieu LECOMPTE; Philipp SCHIFFMANN; Olivier Laget; Noemie Caillol
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2019-02-07
Filing date: 2020-01-14
Publication date: 2021-12-15
Also published as: CN113424046A; US20220128459A1; WO2020160880A1; FR3092665A1; JP2022520557A; KR20210121234A

Abstract

The invention relates to a method for in situ measurement of the concentration of gaseous chemical species contained in a biogas (10) circulating in a line (20), for example of a biogas treatment plant or of a system utilizing a biogas. The method of the invention is implemented by means of an optical measurement system (40) comprising a light source (41) and a spectrometer (44). The source (41) emits UV radiation (42) through the biogas (10) within a measurement area (21) situated in the line. The spectrometer (44) detects at least a portion of said UV radiation having passed through the biogas (10) and generates a digital signal of the luminous intensity (50) depending on the wavelength of the portion of the UV radiation having passed through the biogas. A determination is then made of the concentration of the chemical species from the digital signal of the luminous intensity (50).

Description

METHOD FOR MEASURING THE CONCENTRATION OF GASEOUS SPECIES IN A BIOGAS

Technical area

The present invention relates to the measurement of the concentration of chemical species contained in a biogas, by means of an optical system. The present invention applies advantageously and without limitation to the field of the treatment of a biogas, which aims to transform a biogas into biomethane, and to the use of this biomethane.

Biogas is the product of anaerobic decomposition of organic wastes, such as sludge from sewage treatment plants, agricultural wastes, landfills. Biogas is mainly composed of methane (40 to 70%), C0 ₂ and water vapor, but it also contains impurities, such as sulfur compounds (H ₂ S, S0 ₂ , ...) of siloxanes, halogens or even VOCs (Volatile Organic Compounds). Biogas is therefore not directly exploitable.

To be able to use a biogas, it must be purified (or even purified), in particular to eliminate carbon dioxide and hydrogen sulphide, but also other impurities. Biomethane is thus obtained which can be injected into the natural gas distribution network, or used as a bio-fuel.

A particular use of a purified biogas is the fuel cell ("fuel cell" in English), for which the tolerance thresholds to impurities or contaminants are particularly demanding in order not to damage the system (cf. for example the document " Biogas and fuel cells workshop ", Argonne, 2012, Dennis Papadias and Shabbir Ahmed, Argonne National Laboratory, Presented at the Biogas and Fuel Cells Workshop Golden, CO, June 1 1 -13, 2012").

The development of sensors and methods for measuring each of the polluting substances is therefore of major interest, in order to control the biogas treatment process as well as the qualification of the biomethane obtained after purification, with a view to its use.

Prior art

Document DE 202008003790 U1 is known which relates to a device and a method for measuring the concentrations of contaminants contained in a biogas. More precisely, a partial flow of biogas passes continuously through a gas cell by means of a pump, then a particularly ultra-violet spectrum is measured by means of a spectrometer. This spectrum is then analyzed according to one or more chemometric calibration models. Thus, this method comprises a step of sampling the gas to be analyzed. The device described in this document therefore requires additional elements (a pump in particular) to the measurement itself, making the device more bulky, expensive, and requiring more maintenance. In addition, the analysis of contaminants is de facto remote and therefore deferred in time, which can be harmful in the case in particular of a fuel cell. Furthermore, the method described in this document requires a prior increase in the concentration of the chemical species to be measured, with an adsorption device on a filter, in the event that the concentrations of the species to be measured are too low to be detected and measured. by a spectrometer.

The method according to the invention aims to overcome these drawbacks. In particular, the method according to the invention aims to provide an optical measurement of the concentration of gaseous chemical species contained in a biogas carried out in situ, and without requiring a step of over-concentration of the chemical species present in small quantities in the biogas at analyze. In addition, the method according to the invention allows a dissociated and simultaneous measurement of different gaseous chemical species contained in the biogas. Finally, the method according to the invention can allow the diagnosis and / or the control of a process for the purification of a biogas, from measurements of the concentration of gaseous chemical species contained in the biogas carried out before, during and after the purification of biogas. The method according to the invention can also be advantageously implemented upstream of an installation using biomethane, such as for example in a fuel cell, so as to guarantee the integrity of the system using this gas.

Summary of the invention

The present invention relates to a method for in situ measurement of the concentration of at least one gaseous chemical species contained in a biogas flowing in a duct (20) by means of an optical measurement system comprising at least one light source emitting a UV radiation and at least one spectrometer capable of analyzing at least said UV radiation, said conduit comprising at least one first optical access formed in a wall of said conduit.

The method according to the invention comprises at least the following steps: a) by means of said light source, said UV radiation is emitted at at least said optical access through said biogas within a measurement zone located at least in part in said duct; b) by means of said spectrometer, at the level of said first optical port and / or of a second optical port, at least part of said UV radiation having passed through said biogas in said measurement zone is measured, and a digital signal of the light intensity as a function of the wavelength (W) of said part of the UV radiation having passed through said biogas;

c) said concentration of said chemical species contained in said biogas is determined from at least said digital signal.

According to one implementation of the method, step c) can include at least:

- determining the absorbance of said biogas as a function of the wavelength from said digital signal of the light intensity as a function of the wavelength of said part of the UV radiation having passed through said biogas and of a signal reference numerical light intensity as a function of the predetermined wavelength for a reference gas;

- determining said concentration of said at least one chemical species from said absorbance of said biogas, predetermined absorbance characteristics of said chemical species and an estimate of the temperature and pressure of said biogas.

Advantageously, said absorbance of said biogas can be a function of the absorbance length, of the density number of the molecules of said chemical species and of the molar extinction coefficient.

According to one implementation of the method, said digital reference signal can be obtained by emitting said UV radiation through said reference gas and by measuring at least part of said UV radiation having passed through said reference gas, said gas having a concentration of said known or no chemical species.

According to an implementation of the method, in step c), a temperature of said biogas can also be determined from said digital signal.

According to one implementation of the method, said temperature can be determined by modifying the molar extinction coefficient of the absorbance of said chemical species extracted from said absorbance of said biogas, said modification being a shift in the wavelength or a modification amplitude or a combination of both. According to one implementation of the method, said optical measurement system may further include a reflector arranged in said measurement zone of said duct. According to this implementation, at least part of said UV radiation having been emitted by said light source at the level of the first optical access and having at least partly reflected on said reflector can be measured at at least said first optical access.

According to one implementation of the method, said first and / or second optical access can be offset from said wall of said duct in which the biogas circulates.

According to one implementation of the method, said UV radiation can be emitted at a wavelength between 180 and 400 nm, preferably between 180 and 280 nm, and even more preferably between 180 and 240 nm.

According to one implementation of the method, it is possible to measure the concentration of at least one, and preferably several, gaseous chemical species contained in said biogas, and included in the list consisting of: S0 ₂ , H ₂ S, NH ₃ , BTEX, siloxanes and halogens.

According to one implementation of the method, the concentration of at least two gaseous chemical species can be measured simultaneously, and preferably at least the concentration of H ₂ S and the concentration of NH ₃ .

According to one implementation of the method, it is possible to measure the concentration of at least one gaseous chemical species chosen from sulfur-containing chemical species S0 ₂ and H ₂ S, and preferably both.

Depending on an implementation of the method, the concentration can be measured at least

NH ₃ .

According to one implementation of the method, said pipe in which said biogas circulates can be a pipe of an installation for the purification of said biogas, and said method can be implemented upstream and / or downstream of said installation.

According to one implementation of the method, said duct in which said biogas circulates can be a duct of a system using said biogas such as a distribution network of said biogas. biogas, a vehicle, or a fuel cell, and said method can be implemented upstream of said system using said biogas.

Other objects and advantages of the invention will become apparent on reading the following description of examples of particular embodiments of the invention, given by way of non-limiting examples, the description being given with reference to the appended figures described below. -after.

List of Figures

FIG. 1A is a diagram illustrating the optical measurement of the concentration of chemical species contained in a biogas, according to a transmissive configuration of the optical measurement system for the implementation of the method according to the invention.

FIG. 1B is a diagram illustrating the optical measurement of the concentration of chemical species contained in a biogas, according to a reflective configuration of the optical measurement system for the implementation of the method according to the invention.

FIGS. 1 C and 1 D respectively represent variants of the embodiments presented in FIGS. 1A and 1 B, comprising optical accesses offset from the duct in which the biogas circulates.

Figure 2 shows schematically the absorbance of biogas comprising different gaseous chemical species A, B, C that we want to measure.

Figure 3 shows schematically the influence of temperature on the absorbance of a given chemical species contained in biogas.

FIGS. 4 to 14 are diagrams illustrating different embodiments of the optical measurement system for implementing the method according to the invention.

Description of embodiments

The present invention relates to a method for measuring in situ the concentration of at least one gaseous chemical species contained in a biogas by means of an optical measuring system.

By biogas, it is understood any gas resulting from the anaerobic decomposition of waste of organic origin, such as sludge from purification plants, agricultural waste, landfills. It follows that the biomethane is a biogas according to the invention. The present invention allows measurement in situ, that is to say directly in a conduit in which the biogas circulates and without taking any sample (s) of the biogas. This pipe can be a pipe from an installation for treating said biogas (for example a pipe from a biogas purification installation) and / or a pipe located upstream of an installation using biogas (for example a pipeline of a biogas distribution network). In general, this is referred to below as the conduit of an installation to be monitored.

In addition, as will be described below, the present invention does not require preconditioning of the biogas (by overconcentration for example) in the case of a chemical species present in small quantity in the biogas to be analyzed.

The method according to the invention is implemented by means of an optical measurement system comprising at least one light source emitting UV radiation and a spectrometer. According to the invention, the conduit through which the biogas circulates comprises at least one optical access formed in the conduit in which the biogas flows, said optical access being capable of allowing at least UV radiation to pass. This optical access can be formed by an opening made in the duct, to which is fixed for example a lens or a window.

Figures 1 A and 1 B show schematically and without limitation the measurement principle according to the invention. Figure 1A differs from Figure 1B by the optical measurement system, which is in a transmissive configuration in Figure 1A and in a reflective configuration in Figure 1B.

The measurement process consists of the following steps:

- remission by the light source 41 of UV radiation 42 through a biogas 10 within a measurement zone 21 located in a pipe 20 (for example a pipe of a biogas treatment plant) in which s' flows the biogas. The UV radiation 42 passes through the biogas, along an optical path of length d, which can be substantially but not limited to perpendicular to the path P of the biogas, as shown in Figures 1 A and 1 B. The UV radiation 42 enters the zone measurement by an optical access, for example a window or a lens, provided in the conduit in which the biogas flows.

- The detection by the spectrometer 44 of at least part 43 of the UV radiation which has passed through the biogas in the measurement zone 21, and the generation of a digital signal 50 of the light intensity as a function of the wavelength the part of UV radiation that has passed through the biogas. The gaseous chemical species whose concentration is to be measured absorb part of the UV radiation and each gaseous chemical species absorbs the rays at certain given wavelengths. Absorption follows, under ideal conditions, Beer-Lambert's law. The UV radiation which has passed through the biogas is detected by the spectrometer 44 through an optical access (another optical access for the embodiment of figure 1A, the same optical access for the embodiment of figure 1B) , arranged as for emission by the light source.

- the estimation of the concentration [X] of the gaseous chemical species from at least the digital signal 50.

While the configuration is transmissive in the embodiment shown in Figure 1A, the configuration is reflective in the embodiment shown in Figure 1 B. According to this reflective configuration, the optical system 40 further comprises a reflector 45. The UV radiation emitted by the source 41 is reflected by the reflector 45, positioned at the end of the measurement zone 21 opposite the end where the light source 41 and the spectrometer 44 are located. The reflector 45 is preferably positioned in the conduit 20 in which the biogas circulates, as illustrated in FIG. 1 B. It can alternatively be integrated into the wall of this element, or be disposed outside the latter. The UV radiation 42 passes through the biogas 10 in the measurement zone 21 for the first time, is reflected by the reflector 45, passes through the biogas in the measurement zone 21 in the opposite direction a second time, and is then detected by the spectrometer 44, as described above. The location of the reflector 45 can be further adjusted depending on the order of magnitude of the expected chemical species concentration (s). In fact, the greater the length of the optical path traveled by UV radiation through the biogas, the more reliable and precise the measurement of the concentration will be in the case of low concentrations of chemical species. It is thus advantageously possible to place the mirror 45 on the wall of the duct 20 opposite the light source 41, so as to increase (in this case double) the length of the optical path compared to the configuration in FIG. 1 A.

According to another variant embodiment of the invention, the length of the optical path traversed by the UV radiation in the biogas can be adjusted by means of at least one optical access offset with respect to the wall of the duct in which the gas circulates. biogas. This remote optical access may consist of a tube fixed at one of its ends to the opening made in the conduit of the installation to be monitored, and the other end of this tube comprising means suitable for letting UV radiation, like a porthole or a lens. The biogas which circulates in the pipe of the installation to be monitored can therefore also occupy the space defined by said remote optical access fixed to said element, thus enlarging the measurement zone. According to this alternative embodiment of the invention, the cross section (relative to the main direction of UV radiation) of the remote optical access is preferably substantially circular, but may be of any other shape, of preferably in relation to the shape of the opening made in the duct of the installation to be monitored. FIG. 1 C shows an exemplary embodiment of this variant of the invention, in the case of a transmissive configuration of the optical system according to the invention as defined above, and which comprises two remote optical accesses 31 ', 31 "in the form of circular tubes of length respectively d1 and d2, the first remote optical access being intended for the passage of UV radiation emitted by the light source 41, and the second remote optical access being intended for the passage of UV radiation having passed through the biogas traversed in the measurement zone 21, 21, 21 ", with a view to its measurement by the spectrometer 44. In this case, the total optical path of the UV radiation which has passed through the biogas is equal to d + d1 + d2. Figure 1D shows another embodiment of this variant of the invention, in the case of a reflective configuration of the optical system according to the invention as defined above, and which comprises a remote optical access 31, under the shape of a circular tube of length d1 in the longitudinal direction, optical access intended for the passage of UV radiation emitted by the light source 41, then reflected by the reflector 45 after having passed through the biogas for the first time in the measurement zone 21 ', 21, and passing through the optical access again to be detected by the spectrometer 44 after having passed through the biogas a second time in the measurement zone 21', 21. In this case, the total optical path of the UV radiation having passed through the biogas is 2 (d + d1). These different configurations of the optical system according to the invention, which are not limiting, comprising at least one remote optical access, make it possible to vary the length of the optical path traversed by the UV radiation, and thus to improve the precision of the measurement of the concentration. in the case of low concentrations of chemical species.

Thus, the method according to the invention, which can be implemented in situ, has the advantage of not modifying the flow of the biogas and of being instantaneous, for example with a response time which may be less than 0.1. s, unlike the known methods by sampling gases, in which there is an addition of back pressure, a possible evolution of the gases to be analyzed during sampling, which is undesirable (in fact, during a sampling, the gas may condense, which can contribute to modifying the gas finally analyzed, for example by adsorption of certain molecules on the walls of the sampling tubes), and a transit of the gases to the measurement cell causing a delay in the measurement.

In addition, the method according to the invention can allow a reliable and precise measurement of the chemical species (s) present in the biogas in small quantities, without having recourse to a prior step of overconcentration of the chemical species, by means of an adjustment of the length of the optical path which depends on the arrangement of the elements of the optical system according to the invention. Whether in the case of a transmissive or reflective configuration, the light source 41 and the spectrometer 44 are preferably positioned outside the duct 20 in which the biogas circulates, for example on the external face of the walls of the duct, or at a distance from this element if radiation transmission means are provided, such as for example optical fibers as shown in Figures 10 and 11 described below. This makes it possible in particular to prevent the fouling of these optical elements.

The method according to the invention preferably comprises a preliminary step of calibrating the optical measuring system making it possible to obtain a reference digital signal of the light intensity as a function of the wavelength.

Preferably, this step consists in emitting the UV radiation through a reference gas, for example a gas not containing any of the chemical species to be measured (such as helium, dinitrogen or air), or through a reference gas which contains certain chemical species which it is desired to measure and of which the concentration in said gas is known. The radiation passes through the reference gas, and is then detected by the spectrometer to provide a reference digital signal of light intensity as a function of the wavelength of the part of the UV radiation that has passed through the reference gas. The reference signal is used in the concentration and temperature estimation step, in particular to calculate the absorbance of biogas, as described in detail below.

The UV radiation emitted by the light source 42 has a wavelength between 180 and 400 nm, preferably between 180 and 280 nm (in particular in the case where the chemical species is NO), or very preferably between 180 and 240 nm (especially in the case where the chemical species is NH ₃ ). These wavelength ranges are part of what is referred to as deep UV.

By way of example, the light source can be an LED diode emitting in UV and in particular in deep UV as indicated above, or perhaps a xenon, deuterium or zinc lamp, cadmium, or another gas lamp such as KrBr, KrCL, KrF excimer lamps.

The spectrometer makes it possible to analyze the light signal in the wavelength range 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 denote such a simplified system. The assembly formed by at least the UV light source and the spectrometer, also called optical system or optical sensor in the present invention, is known per se. Such optical sensors can be found commercially.

The optical system can include other elements, in particular optical elements such as lenses making it possible to modify the light beam if necessary (for example convergence or divergence), or even protection elements aiming to protect the light source and the spectrometer, in particular during cold operation of the optical measuring system. Indeed, cold operation can cause deposits on the optical elements by a phenomenon of condensation. Such protective elements are described below, in relation to Figure 12. The position of the sensor installed on the pipe in which the biogas circulates can be chosen so as to limit the clogging thereof.

According to the invention, it is possible to measure at least one gaseous chemical species X, and preferably several gaseous chemical species X, included in the list consisting of: S0 ₂ , H ₂ S, NH ₃ , BTEX (this which includes benzene, toluene, ethylbenzene and xylene), siloxanes and halogens. Preferably, the measurement of at least one, and more preferably of several, gaseous chemical species included in the list consisting of: hydrocarbons (such as aromatics, alkenes, terpenes and terpenoids), siloxanes ( such as D2 to D7), organic compounds (such as sulphides, mercaptans, thiols) or inorganic sulfur compounds (such as sulphides), halogens. Advantageously, at least the measurement of the concentration of THT (tetrahydrothiophene) is carried out.

Advantageously, the dissociated and simultaneous measurement of the concentration of a plurality of these gaseous chemical species can be carried out.

By dissociated measurement is meant an access to the specific concentration of each chemical species, as opposed to an overall measurement of the concentration of several chemical species without distinction. For example, according to the invention, the simultaneous measurement of the concentration of at least two gaseous chemical species, preferably at least the concentration of H ₂ S and the concentration of NH _{3, is carried out} .

According to one implementation of the invention, it is also possible to measure the concentration of at least S0 ₂ , or H ₂ S, and preferably at least both. The quantification of the sulfur elements in a biogas is particularly useful when the method according to the invention is implemented to qualify the biogas before its use in a fuel cell, for which corrosion can be very harmful. Advantageously, the concentration of at least NH ₃ is measured. By repeating the steps of the process according to the invention at different times, it is possible for example to monitor the evolution over time of the NH ₃ concentration of a biogas purification installation.

In the method according to the invention, the concentration of each chemical species is determined from the optical measurement carried out on the biogas and from an optical signature specific to each chemical species. Each gaseous chemical species whose concentration is to be measured absorbs part of the UV radiation and has its own absorption spectrum (absorbance as a function of wavelength).

During the step of estimating the concentration [X] of at least one chemical species, at least steps a) and b) described below are carried out: a) the absorbance A of the biogas is determined as a function of of the wavelength W from the digital signal of the light intensity 50 generated by the spectrometer and resulting from the detection of the part of the UV radiation which has passed through the biogas, and from a digital reference signal. The digital reference signal is preferably established during the preliminary calibration step described above. In particular, the absorbance of biogas is calculated according to a formula of the type of formula (I) below:

b) using analysis and signal processing means such as a microprocessor, the concentration [X] of each chemical species to be measured is determined from the absorbance A of the biogas , predetermined absorbance characteristics and an estimate of the pressure and temperature of each of the chemical species. These predetermined characteristics of absorbance of each of the chemical species are preferably obtained during preliminary measurement campaigns making it possible to create a library. Data from the literature can also feed into such a library. By absorbance characteristic of a given chemical species is meant its molar extinction coefficient. Advantageously, the pressure and / or the temperature can be estimated by measurement during the implementation of the method according to the invention, by means respectively of a pressure sensor and / or a temperature sensor. Advantageously, according to one implementation of the invention, the temperature of the biogas is estimated by means of the main variant described below. below, which may constitute an additional step c) to steps a) and b) described above.

According to a main variant of the method according to the invention, the temperature (T) of the biogas circulating in the pipe is also determined, in addition to the concentration. According to one implementation of this main variant, the temperature (T) of the biogas circulating in the duct 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 said biogas. The change in the molar extinction coefficient can be a shift in wavelength, leading to absorption at different wavelengths, or a change in the amplitude of the absorbance 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 of a chemical species is known, by means of preliminary 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 in the measured wavelength range. FIG. 3 illustrates the influence of temperature on the absorbance of a chemical species, here ammonia, used to determine the temperature according to the present invention. The curve A-Tc represents the absorbance of NH ₃ for a low temperature, for example 20 ^t C, and the curve A-Th represents the absorbance of NH ₃ for a high temperature, for example 450 ^t C. A modification of the molar extinction coefficient, for example, results in a shift in the absorption signal. Although the example given relates to ammonia, any other chemical species such as SO ₂ , H ₂ S, NH ₃ , BTEX, siloxanes, halogens, aldehydes such as acetaldehyde or formaldehyde, non-hydrocarbons. aromatics such as acetylene or buta-1,3-diene, can be used to determine the temperature. The same type of algorithms that are used to determine the concentration of chemical species can be used to determine the temperature.

Thus, the method according to this main variant of the invention makes it possible to access the temperature of the biogas, without additional measuring device in the control zone. In addition, this main variant allows instantaneous measurement of the temperature, by specific processing of the UV absorption signal, simultaneously with the measurement of the concentration of gaseous chemical species contained in the gases. FIG. 2 schematically represents the absorbance A of the biogas comprising different gaseous chemical species A, B, C that it is desired to measure. The diagram on the left shows an example of absorbance A (unitless) of biogas, expressed as a function of wavelength W (in nm), calculated from the digital signal of light intensity 50 generated by the spectrometer and of the digital reference signal.

The absorbance A of a gas is a function of the absorbance length, that is to say the length of the optical path crossed by the light in the measurement zone, of the numerical density of the molecules of the gaseous chemical species (A, B, C) contained in gases, and the molar extinction coefficient of chemical species. The molar extinction coefficient, also called molar absorptivity, is a measure of the probability that a photon is interacting with an atom or molecule.

The numerical density of the 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, l chemical species, temperature and pressure.

Thus, by having the predetermined characteristics of the molar extinction coefficient, temperature and pressure of each of the chemical species, it is possible to determine the concentration [X] of each chemical species X from the absorbance A of the biogas. The absorbance values of each chemical species are added and their sum is approximately equal (except for noise) to the absorbance A values of the biogas. This is represented on the right in FIG. 2 by the absorbance diagrams AA, AB, and AC of the chemical species A, B and C which one wishes to detect, which are added to the noise and to the absorbance of undetected chemical species AD to form absorbance A of biogas.

Different types of algorithms can be used to determine concentration values, such as least squares adjustment algorithms applied to the absorbance signals themselves, to the derivatives of the absorbance signals, or to the frequency portion of the signals. absorbance (typically derived from a Fourier transform). Likewise, a number of chemometric methods can be used for this process such as, for example, principal component analysis (PCA) or partial least squares (PLS) algorithms. The invention advantageously applies to the field of the treatment of a biogas, which may comprise an installation for the purification (or else for the purification) of the biogas. In this context, the optical measurement system according to the invention can be positioned at different locations of a biogas purification installation, in particular upstream and / or downstream of such an installation. This can make it possible to control the quality of a biogas treatment process, at different stages of this treatment, and what is more, in real time. The method according to the invention can also be advantageously applied upstream of a system using a biogas, in particular an already purified biogas, so as to ensure that the biogas used in said system complies with the operating requirements and / or regulations of this system.

According to one embodiment, the measurement is carried out downstream of an installation for the purification of a biogas and / or of a system using a biogas. Such an embodiment is shown schematically in FIG. 4. The biogas 10 circulates along the path P in a conduit 20 of an installation 60 for purifying a biogas. According to this embodiment, the optical measurement system 40 is placed downstream of the installation 60 for purifying a biogas. The upstream and downstream positions are defined in relation to the direction of circulation of the biogas in the pipe 20. The measurement of the concentration of certain chemical species of the biogas downstream of a biogas purification installation 60 makes it possible to verify compliance with the emission standards for polluting substances from such installations, monitor their development, and if necessary adjust the operation of these installations so as to comply with the standards in force.

According to another embodiment, shown in FIG. 5, the in situ measurement is carried out identically to that of the embodiment shown in FIG. 4, except that the optical measurement system comprises a reflector 45, for reflective measurement. The optical measurement system is that described in relation to FIG. 2. The source 41 and the spectrometer 44 are placed on the same side of the duct 20, that is to say at the same end of the measurement zone 21, opposite to that where the reflector 45 is positioned.

According to another embodiment, the in situ measurement is carried out upstream of at least one installation to be monitored, such as an installation for the purification of a biogas or else a system using a biogas. Such an embodiment is shown diagrammatically in FIG. 6, identical to FIG. 4 except for the optical measuring system 40 positioned upstream of the installation 60 to be monitored. Such an embodiment can be useful for obtaining information on the concentration of chemical species in the biogas before they enter a biogas purification installation, for example to influence the operation of this installation. Such an embodiment can also be advantageously implemented upstream of a system using a biogas, such as a network. distribution of said biogas, a vehicle, or a fuel cell, so as to control, in real time, the quality of the biogas entering this system.

According to one embodiment, the in situ measurement is carried out both upstream and downstream of at least one installation to be monitored, such as an installation for the purification of biogas. An example according to this mode is illustrated in Figure 7, in which two optical measurement systems 40 and 40 ’are placed respectively upstream and downstream of an installation 60 for purification. The second optical measuring system 40 'is identical to the first optical measuring system 40 arranged upstream, and comprises a light source 41' and a light analyzer 44 'respectively allowing the emission of UV radiation and the detection and the detection. analysis of the UV radiation which has passed through the biogas in the measurement zone 21 ′ situated along the conduit 20, in order to provide an estimate of the concentration of gaseous chemical species.

Another example according to this mode is illustrated in FIG. 8, in which the method according to the invention is implemented by means of three optical measuring systems 40, 40 ', and 40 ”, placed respectively upstream of a first installation 60 for purification of a biogas, between the first installation 60 for purification and a second installation 61 for purification, and downstream of the second installation 61 for purification of a biogas. Advantageously, the first installation 60 is an installation using a first type of treatment of biogas, and the second installation 61 is an installation using a second type of treatment of biogas. Such an embodiment makes it possible to monitor the efficiency of each of these installations with a view to converting the biogas into biomethane, and possibly to adjust the parameters of these installations as a function of the concentration information obtained upstream, downstream and between them. different facilities.

According to one embodiment, the UV light source and / or the spectrometer of the optical measurement system is connected to the conduit in which the biogas circulates by an optical fiber, making it possible for example to install the optical measurement elements in a protective enclosure. , without affecting the instantaneousness of the measurement. An example of such an embodiment is shown in FIG. 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 duct 20, at the level of the zone of. measure 21 where the biogas is crossed by the UV radiation conducted by the optical fibers 75 and 76. It is quite clear that the optical fiber 76 connected to the spectrometer 74 can be (not shown) placed on the same side of the duct 20 as the optical fiber 75 connected to the light source 71. It is quite clear that for each of the embodiments represented in FIGS. 6 to 9, the optical system can be that described in relation to FIG. 1B, comprising at least one reflector placed in the measurement zone. In the case of the embodiments shown in Figures 6 to 8, the spectrometer (s) 44, 44 ', 44 "can then be placed (not shown) on the same side of the duct 20 as the light source (s) 41, 41' and

41 ".

According to one embodiment, the optical measurement system comprises several measurement zones connected to a single light source and a single spectrometer, by means of optical fibers. Such an embodiment makes it possible, for example, to reduce the cost of implementing the optical measurement in the event that measurements upstream and downstream of an installation to be monitored (for example an installation for purification) are desired. An example of such an embodiment is illustrated in FIG. 10, in which the optical measurement system 80 comprises three measurement zones 21, 22 and 23, and a single light source 81 and a single spectrometer 84 connected to the measurement zones. measurements by optical fibers 85, 86, 87 and 88.

According to another embodiment, the in situ measurement is performed in an identical manner to that of the embodiment shown in FIG. 10, except that the optical measurement system comprises three reflectors 45, 45 'and 45 ”as described in relationship with Figure 2, respectively arranged at the ends of the measuring zones 21, 22 and 23. As illustrated schematically in this Figure 11, the length of the measuring zones 21, 22 and 23, increases in the upstream-downstream direction, or in other words the length of the optical path in the upstream-downstream direction. This embodiment is particularly advantageous for improving the accuracy of the measurement of the efficiency of a biogas treatment process because, as the biogas passes through biogas purification plants (here the installations 60 and 61), chemical species are present in the biogas with increasingly low concentrations. Therefore, it may be advantageous to adjust the optical path of UV radiation, in particular by lengthening it in the downstream direction, so as to allow a reliable measurement of the concentration of chemical species, even in the case of a concentration. low chemical species. This avoids using devices to overconcentrate the chemical species, in particular at the end of the biogas treatment process. In order to lengthen the optical path in the upstream-downstream direction, it is possible, alternatively or in combination with the embodiment of FIG. 10, to implement the method according to one of the embodiments of the invention described. above, for which the optical system comprises at least one remote optical access. According to one embodiment, illustrated in FIG. 12, the optical measurement system 90 comprises means 95 for protecting the light source 91 and / or the spectrometer 94. Such protection means are useful for example during operation. cold of the optical measurement system, in order to prevent fouling of the optical elements, as already explained above. These protection means can comprise a shutter, an air barrier between the light source 91 or the spectrometer 94 and the biogas passing through the measurement zone 21, a specific coating, for example to prevent the adhesion of liquid or solid particles. , the surface or surfaces separating the light source 91 from the biogas or the spectrometer 94 from the biogas, or a means for heating said surfaces. It can also be a specific geometry of the optical sensor, not shown in FIG. 12.

According to one embodiment, illustrated in FIG. 13, the configuration of the optical measuring system 90 is of the reflective type, the optical measuring system 90 comprising a light source 91, a spectrometer 94 as well as a reflector 45, these elements of the optical measurement system 90 being positioned, for example, at an elbow of the duct 20, so that the optical path of the UV radiation is substantially tangent to the path P of the biogas in the measurement zone 21.

According to another embodiment, illustrated in FIG. 14, the configuration of the optical measuring system 90 is of the reflective type, the optical measuring system 90 comprising a light source 91, a spectrometer 94 as well as a reflector 45, these elements of the optical measurement system being arranged inside the duct 20, and preferably at the outlet of the duct 20, so that the optical path of the UV radiation is substantially parallel to the path P of the biogas in the measurement zone 21.

The invention further relates to an installation comprising at least one optical measurement system as described above for implementing the method according to the invention. According to one implementation of the invention, the installation can be a biogas purification installation and / or a system using biogas. According to one implementation of the invention according to which the installation is a system using a biogas, the method according to the invention can be implemented upstream of said installation.

The invention also relates to a use of the method according to the invention for measuring the concentration of at least one gaseous chemical species contained in a biogas circulating in a pipe of an installation. According to one implementation of the invention, the installation can be a biogas purification installation and / or an installation using biogas. According to an implementation of the invention according to which the installation is a system using a biogas, the method according to the invention can be implemented upstream of said installation.

Claims

1. Method for in situ measurement of the concentration ([X]) of at least one gaseous chemical species contained in a biogas (10) circulating in a duct (20) by means of an optical measuring system (40, 40 ' , 40 ”, 70, 80, 90) comprising at least one light source (41, 41 ', 41”, 71, 81, 91) emitting UV radiation (42) and at least one spectrometer (44, 44', 44 ”, 74, 84, 94) capable of analyzing at least said UV radiation (42, 43), said duct (20) comprising at least a first optical access formed in a wall of said duct (20), said method comprising at least the following steps :

a) by means of said light source (41, 41 ', 41 ”, 71, 81, 91), at least said optical access, said UV radiation (42) is emitted through said biogas (10) within a measurement zone (21, 21 ', 21 ", 22, 23) located at least partly in said duct (20);

b) b) by means of said spectrometer (44, 44 ', 44 ”, 74, 84, 94), at least part of said UV radiation is measured at said first optical port and / or at a second optical port (43) having passed through said biogas (10) in said measuring zone (21, 21 ', 21 ", 22, 23), and a digital signal of the light intensity (50) is generated as a function of the length of wave (W) of said portion of UV radiation (43) having passed through said biogas (10);

c) c) said concentration ([X]) of said chemical species contained in said biogas (10) is determined from at least said digital signal (50).

2. The method of claim 1, wherein step c) comprises at least:

- determining the absorbance (A) of said biogas as a function of the wavelength from said digital signal of the light intensity (50) as a function of the wavelength of said part of the UV radiation having passed through said said biogas (43) and a digital reference signal of light intensity as a function of the predetermined wavelength for a reference gas;

- determining said concentration ([X]) of said at least one chemical species from said absorbance of said biogas, predetermined absorbance characteristics of said chemical species and an estimate of the temperature and pressure of said biogas .

3. The method of claim 2, wherein said absorbance (A) of said biogas (10) is a function of the absorbance length, the density number of molecules of said chemical species and the molar extinction coefficient.

4. Method according to one of claims 2 to 3, wherein said digital reference signal is obtained by emitting said UV radiation (42) through said reference gas and by measuring at least part of said UV radiation (43). ) having passed through said reference gas, said gas having a known or zero concentration of said chemical species.

5. Method according to one of the preceding claims, wherein, in step c), a temperature (T) of said biogas (10) is further determined from said digital signal (50).

6. The method of claim 5, wherein said temperature (T) is determined by modifying the molar extinction coefficient of the absorbance of said chemical species extracted from said absorbance of said biogas, said modification being a shift in the length of. wave or a change in amplitude or a combination of both.

7. Method according to one of the preceding claims, wherein said optical measuring system further comprises a reflector (45, 45 ', 45 ") disposed in said measuring zone (21, 21', 21", 22, 23 ) of said duct (20) and in which, in step b), at least a part of said UV radiation (42, 43) having been emitted by said light source at the first optical port is measured at the level of said first optical port. optical access and being at least partly reflected on said reflector.

8. Method according to one of the preceding claims, wherein said first and / or second optical access (31, 32) are offset from said wall of said conduit (20) in which the biogas (10) circulates.

9. Method according to one of the preceding claims, wherein said UV radiation (42) emitted has a wavelength between 180 and 400 nm, preferably between 180 and 280 nm, and even more preferably between 180 and 240 nm.

10. Method according to one of the preceding claims, wherein the concentration ([X]) of at least one, and preferably several, gaseous chemical species contained in said biogas (10), and included in the list made up is measured. by: S0 ₂ , H ₂ S, NH ₃ , BTEX, siloxanes and halogens.

1 1. Method according to one of the preceding claims, wherein simultaneously measuring the concentration ([X]) of at least two gaseous chemical species, and preferably at least the concentration of H ₂ S and the concentration of NH ₃ .

12. Method according to one of the preceding claims, in which the concentration of at least one gaseous chemical species chosen from sulfur-containing chemical species S0 ₂ and H ₂ S, and preferably both, is measured.

13. Method according to one of the preceding claims, in which the concentration of at least NH ₃ is measured.

14. Method according to one of the preceding claims, wherein said conduit (20) in which said biogas circulates (10) is a conduit of an installation (60, 61) for the purification of said biogas, and in which said process is implemented upstream and / or downstream of said installation.

15. Method according to one of the preceding claims, wherein said conduit (20) in which said biogas (10) circulates is a conduit of a system using said biogas (10) such as a distribution network for said biogas, a vehicle, or a fuel cell, and wherein said method is implemented upstream of said system using said biogas.