WO2009055946A1

WO2009055946A1 - Method and device for analysing gaseous atmosphere

Info

Publication number: WO2009055946A1
Application number: PCT/CH2008/000443
Authority: WO
Inventors: Peter Graf; Christoph HÜGLIN; Heinz Burtscher
Original assignee: Omnisens S.A.
Priority date: 2007-10-29
Filing date: 2008-10-23
Publication date: 2009-05-07

Abstract

The invention relates to a method and a device for the automatic and continuous analysis of a polluting component in the form of at least one salt in a gaseous atmosphere, wherein said method comprises, during a sampling step, trapping the salt on a neutral substrate and, during a measuring step, carrying out a dissociation of said salt by thermal dissociation for extracting therefrom the polluting component in a gaseous state, and measuring the polluting component content of said atmosphere. To this end, the device (10) includes a sampling means (11) for circulating the atmosphere to be analysed through said device, and a measuring means (12) for determining the content of polluting substances in the sampled atmosphere. The sampling means (11) is associated with a system (13) for drying the atmosphere before feeding the same through a valve (9) into a chamber (14). The chamber (14) is provided with a cooling device (15) and a heating device (16) for thermal dissociation. A neutral porous substrate (17) provided in the housing is used for collecting the salts contained in the sampled atmosphere, and a filter (18) provided downstream from said porous substrate is used for removing the side products resulting from the thermal dissociation so that a gas analyser (19) can measure the exact content of the gaseous polluting component in the analysed gaseous atmosphere.

Description

METHOD AND DEVICE FOR ANALYZING A GAS ATMOSPHERE

TECHNICAL FIELD The present invention relates to a method for the automatic and continuous analysis of a gaseous atmosphere and, in particular, for obtaining and measuring at least one pollutant component present in particular in the form of at least one salt. in said gaseous atmosphere.

It also relates to a device for the automatic and continuous analysis of a gaseous atmosphere and, in particular, for obtaining and measuring at least one pollutant component present in particular in the form of at least one salt in said atmosphere gas, in order to implement this process.

Prior art

The amount of polluting components, present in a gaseous atmosphere in traces, is often very difficult to measure, in particular because some of them, in particular the ammonia gas Nhh present in the ambient air, tend to to combine rapidly with other particles to form salts such as NHx ammonium salts, for example ammonium nitrates NHUNOa and ammonium sulphates (NH4) ₂ SO4.

Both gaseous ammonia and ammonium salts contribute to soil pollution by their nitrogen content. Knowledge of their respective concentration is therefore important for the monitoring of air quality and the estimation of eutrophication of soils. Indeed, the sources of pollution are the same for both types of components and it is necessary to know the content of these two types of components to perform accurate quantification of the emission by said sources.

In view of setting up an automatic surveillance network, it is necessary to measure in a reliable way, both the content the gaseous component and the content of salts in suspension, these measurements being carried out continuously, in particular uninterrupted by human interventions. This principle prohibits the use of a method of sampling and laboratory analysis. Furthermore, a high temporal resolution is desirable to permit analysis of the sources limited in _t ime as the effects, such as eg fattening soil, normally disappear with time, even if the loss occurs on average long term.

There are measuring devices, including the apparatus marketed under the name "TGA300" by the Applicant, for the continuous measurement of ammonia gas, with a high temporal resolution. On the other hand, the measurement of the ammonium salt content of a gaseous atmosphere almost in real time is currently not possible. The method used consists of sampling the atmosphere to be controlled at the site and performing a chromatographic analysis in the laboratory. The disadvantages of this process are the lack of temporal resolution, the use of a large workforce due to multiple manipulations and the use of complex laboratory equipment. In the end, this method is practically not applicable outside a purely scientific framework, and does not lend itself to industrial applications in the form of a real-time control in the field and in an automated way .

The article "Alternative on-line and sorbent tube-based sampling procedures for 24- hr air pollution profiling" on behalf of Woolfenden E. et al published in the journal

INTERNATIONAL LABORATORY vol 23, No. 8, of September 1, 1993, is concerned with the sampling of gases by capture according to their volatility on cooled surfaces. In this process, the captured volatile organic elements are desorbed by heating the adsorbent and then detected by an appropriate detection system. There is no chemical transformation of the elements. DE 37 41 930 A1 in the names Wurz Dieter and Wachter Wurz Christine also relates to a sampling device for determining the content of NH3 and SO3 flue gas. This invention is similar to that described in the previous document because the desired gases are captured by a cold condenser and then analyzed out of the device. In the process used in this device there is no chemical transformation of the elements.

These methods differ from that of the invention in which the polluting salts contained in the atmosphere are chemically decomposed and the gaseous pollutant component resulting from this decomposition is analyzed. For this purpose, the particles suspended in the air are collected on a porous support and the cooling of this support is carried out only to avoid an uncontrolled thermal decomposition of the most volatile ammonium salts, in particular the nitrate of Ammonium present in the atmosphere at ambient temperatures. There is a chemical decomposition of the ammonium salts which leads to a formation of ammonia which is detected.

Presentation of the invention

The present invention proposes to overcome the drawbacks of known techniques by proposing an automatic and continuous measurement method capable of determining the content of pollutant components, in the form of traces, contained in the form of at least one salt, in a gaseous atmosphere without the assistance of a measuring laboratory.

This object is achieved by the method according to the invention as defined in the preamble and characterized in that during a first phase, said sampling phase, said salt is trapped on a neutral porous support by circulating said gas atmosphere through said porous support and, during a second phase, said measurement phase, dissociation of said salt to extract said pollutant component in the gaseous state, is made to pass through the gases from said dissociation through at least one filter arranged downstream of the porous support in order to trap certain components, these components are trapped by impregnating at least said filter with a substance which reacts chemically with said components, and the content of pollutant component in the gaseous state is measured extracted by means of at least one gas analyzer.

Advantageously, said dissociation of said salt is preferably a thermal dissociation. This thermal dissociation can be carried out in steps of temperature, each of these steps corresponding to a dissociation temperature of a specific salt of said pollutant component.

Advantageously, during said sampling phase, said atmosphere is dried so that its dew point is lower than the sampling temperature.

To avoid premature thermal dissociation during the sampling phase, said atmosphere is preferably cooled upstream of said porous support.

In a particularly advantageous manner, when the polluting substance is ammonium salts, in order to measure the content of ammonium salts contained in the gaseous atmosphere, during the sampling phase, a capture of said salts contained in this atmosphere, and the ammonium salts contained in said atmosphere are collected on the porous support, and during the measurement phase, a thermal dissociation of said ammonium salts is carried out in order to transform them into ammonia gas, the nitric acid generated is neutralized by impregnating a filter disposed downstream of said porous support of a basic solution, and the gaseous ammonium from said salts is measured by means of a gas analyzer to determine the ammonia content contained in the gaseous atmosphere. When said ammonium salts are ammonium nitrates and sulphates, the thermal dissociation is advantageously carried out during said measuring phase, in two temperature stages, one of which is at about 80 ^° C. in order to dissociate the ammonium nitrates and the other is at about 150 ⁰ C to dissociate the ammonium sulfates.

Said thermal dissociation is carried out by heating, during determined time intervals, said ammonium salts collected on said porous support.

This object is also achieved by the device according to the invention, as defined in the preamble and characterized in that it comprises a neutral support disposed in an enclosure and arranged to trap said salt, means arranged to dissociate said salt. for extracting said pollutant component in the gaseous state, at least one filter disposed downstream of the porous support and arranged to trap certain components derived from this dissociation, and at least one gas analyzer arranged to measure the content of gaseous pollutant component extracted said gaseous atmosphere.

In the preferred embodiment of the invention, the device further comprises means arranged to dry said gaseous atmosphere before its introduction into the chamber, means arranged to cool said gaseous atmosphere introduced into the chamber upstream of said porous support and means arranged to circulate said gaseous atmosphere through the porous support and to trap said salt by means of said porous support.

Advantageously, said means arranged to dissociate said salt to extract said pollutant component in the gaseous state comprise a heating device arranged to perform a thermal dissociation of said salt. Said heating device is advantageously arranged to heat said salt in steps of temperature, each of these steps corresponding to a dissociation temperature of a specific salt of said pollutant component.

Preferably, said at least one filter for trapping components derived from the dissociation is impregnated with a substance which chemically reacts with said components.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention and its advantages will be better understood on reading the detailed description of the method of the invention and the device for carrying out this method, with reference to the appended drawings given for information and not limitation. , wherein:

FIG. 1 is a schematic view showing the device for implementing the method according to the invention, and

Figure 2 is a graph showing NH3 emission peaks as a function of time on the abscissa.

Best way to realize the invention

Certain polluting components, and in particular ammonium salts, are contained in the atmosphere in the form of an aerosol. The affected salts are nitrates and sulphates, including NH4NO3 ammonium nitrate and ammonium sulphate (NH 4) ₂ SO 4, the thermal stability of atomic bonds is considerably different. Thus ammonium nitrate NH4NO3 heated to about 60 ^° C. decomposes into NHb ammonia plus residues while ammonium sulphate (NH 4) ₂ SO 4 decomposes only from a temperature of the order of 100 ⁰ C.

The dissociation, and in particular the thermal dissociation of salts of a polluting substance present in an atmosphere to be analyzed, makes it possible to decompose said salts in gas. The thermal dissociation, possibly differentiated according to the salts treated, decomposes the polluting salt into a gaseous component and various residues. The analysis of the components in the gaseous state being controlled, measurement of the salt content of said atmosphere is possible. The process provides a fair result, the content of these salts can not be identified in a simple, efficient and fast for real-time analysis.

The device 10 according to the invention as schematically represented in FIG. 1, mainly comprises so-called sampling means, represented diagrammatically by the arrow 11, intended to circulate through the device, for a given time, an atmosphere with analyzing and means, said measurement, shown schematically by the arrow 12, for determining the content of one or more polluting substances of said sampled atmosphere. The sampling means 11 are associated with a drying system 13 for drying the atmosphere at the time of its sampling before its introduction by a valve 9 in a chamber 14 associated, on the one hand, with a cooling device 15 and, on the other hand, to a heating device 16, these two devices being arranged to be controlled independently. Inside the enclosure 14 are mounted, on the one hand, a neutral porous support 17 arranged to collect salts contained in the sampled atmosphere and, on the other hand, a filter 18 disposed downstream of this porous support 17 and arranged to remove parasitic products resulting from the thermal dissociation mentioned above.

The operating mode of the device 10 for carrying out the method of the invention will be described in more detail below in the context of an analysis of the ambient atmosphere. In a first phase, called the sampling phase, the air is sampled by blowing it or by sucking it inside the enclosure 14 and passing it through the drying system 13 to its dew point (temperature below which the condensation appears) is lower than the temperature of the chamber 14. With the cooling device 15, the air temperature is gradually lowered to approximately 5 ° C so that the thermal dissociation can in no case begin during the sampling phase. The flow and duration of sampling depend on the desired time resolution and have no influence on the operation of the device as a whole.

During a second phase, called measuring phase, the valve 9 blocks the flow of air flowing inwardly of the chamber 14. On the other hand, dry synthetic air is injected containing no pollutants, especially no ammonia NH3. This dry synthetic air is heated by means of the heating device 16 and, during its passage through the porous support 17, heats the salts deposited during the sampling phase, causes their thermal dissociation and releases ammonia gas NH3 measurable by an analyzer 19 such as the TGA300.

The adapted management of the temperature of the air flow arriving on the porous support 17, thanks to the heating device 16, makes it possible to separate the species of polluting substances. Thus, stepwise heating makes it possible to emit ammonia volumes at different times, which result in emission peaks A and B on a curve 20 (see FIG. 2) indicating the presence of ammonia in the analyzed atmosphere. . By way of example, the two salts mentioned above, the nitrate and the ammonium sulphate are identified when the atmosphere to be analyzed passes on the porous support 17 for about fifteen minutes at about 80 ^° C. and then for about fifteen minutes at about 150 ⁰ C. Preferably, each sequence corresponding to a peak is measured by an independent analyzer, which makes it possible to measure the respective amounts of the two types of ammonium salts. Curve 20 of FIG. 2 represents the two ammonia peaks Nhb A and B which appear following the first and the second thermal dissociation of the two salts mentioned above.

The dissociation of NH4NO3 ammonium nitrate produces an acid compound, nitric acid HNO3. Nitric acid tends to settle on surfaces which are located after the porous support 17. If this is the case, this deposit acts as a gaseous ammonia trap, which reduces the amount of ammonia actually transmitted to the analyzer and systematically distorts the measurement. It is therefore essential to prevent the deposition of this acid, which is possible if the HNCb is captured or neutralized before it settles on the walls. For this purpose, the filter 18, which is disposed directly after the porous support 17, is impregnated with a substance which reacts chemically with said components. Thus it will be impregnated with a basic solution, for example a solution of caustic soda, when the components from the thermal dissociation are acidic, or an acid solution when said components are basic. Such a filter does not alter the measurement of the ammonia content, but traps the chemical component that is likely to distort the measurement.

Since the dissociation principle involves a sampling phase followed by a measurement phase, this makes a continuous measurement of the concentration of the salts theoretically impossible. Indeed, according to the configuration described, the sampling phase or capture salts on a porous support is carried out for about two hours, and the measurement is substantially in thirty minutes. To overcome this drawback, it is proposed for example to combine two thermal dissociation units with a single analyzer. As a result, one of the units is used to sample while the other is in the analysis phase, allowing continuous work without losing any information. Moreover, the analyzer can be used effectively between two measurements of salts to measure the concentration of elements identical to those of the dissociation and naturally present in the atmosphere. Thus, the combination of an analyzer with the dissociation unit allows the measurement of the total concentration of the pollutant component in its forms of gas and salts.

The method and the device described meet the objectives set forth in the preamble. They can undergo various variations and be presented according to various embodiments. In particular, it is possible to provide in the device means for heating the porous support itself to heat the salts collected on said porous support. It is also conceivable to dissociate by radiation or electrochemical method without departing from its sphere of the present invention defined by the claims.

Claims

1. A method for the automatic and continuous analysis of a gaseous atmosphere and, in particular, for obtaining and measuring at least one pollutant component present in particular in the form of at least one salt in said gaseous atmosphere, characterized in that, during a first phase, said sampling phase, said salt is trapped on a neutral porous support by circulating said gaseous atmosphere through said porous support and, during a second phase , said measuring phase, dissociation of said salt is carried out in order to extract said pollutant component in the gaseous state, the gases coming from said dissociation are passed through at least one filter arranged downstream of the porous support in order to trap certain components, these components are trapped by impregnating at least said filter with a substance which reacts chemically with said components, and the content of pollutant component in the gaseous state extracted by means ofminus a gas analyzer.

2. Method according to claim 1, characterized in that said dissociation of the salt is a thermal dissociation.

3. Method according to claim 2, characterized in that said thermal dissociation is carried out in steps of temperature, each of these steps corresponding to a dissociation temperature of a specific salt of said pollutant component.

4. Method according to claim 1, characterized in that said atmosphere is dried during said sampling phase so that its dew point is lower than the sampling temperature.

5. Method according to claim 1, characterized in that said atmosphere is cooled upstream of said porous support during said sampling phase.

6. Process according to claim 1, in which the polluting substance is ammonium salts, characterized in that, for measuring the content of ammonium salts contained in the gaseous atmosphere, during the sampling phase, the said salts contained in this atmosphere are captured, and the ammonium salts contained in the said atmosphere are collected on the porous support, and during the measurement phase, a thermal dissociation of the said salts is carried out; ammonium hydroxide to turn them into gaseous ammonia, the nitric acid generated is neutralized by impregnating a filter disposed downstream of said porous support of a basic solution and the gaseous ammonium from said salts is measured by means of a gas analyzer to determine the ammonia content in the gaseous atmosphere.

7. The process as claimed in claim 6, wherein said ammonium salts are ammonium nitrates and sulphates, characterized in that, during said measurement phase, the thermal dissociation is carried out in two temperature stages. one of which is approximately at 80 ^° C. to dissociate the ammonium nitrates and the other of which is approximately at 150 ^° C. to dissociate the ammonium sulphates.

8. Process according to claim 7, characterized in that said thermal dissociation is carried out by heating, during determined time intervals, the ammonium salts collected on said porous support.

9. Device for the automatic and continuous analysis of a gaseous atmosphere and in particular for obtaining and measuring at least one polluting component present in particular in the form of at least one salt in said gaseous atmosphere, for the implementation of the method according to claims 1 to 8, characterized in that it comprises a neutral support (17) disposed in an enclosure (14) and arranged to trap said salt, means arranged to dissociate said salt to extract said pollutant component to the gaseous state, at least one filter (18) disposed downstream of the porous support (17) and arranged to trap certain components resulting from this dissociation, and at least one gas analyzer (19) arranged to measure the pollutant component content gas extracted from said gaseous atmosphere.

10. Device according to claim 9, characterized in that it further comprises means (13) arranged to dry said gaseous atmosphere before its introduction into the chamber (14), means arranged to cool said gaseous atmosphere introduced into the chamber. enclosure upstream of said porous support (17), and means arranged to circulate this gas atmosphere through the porous support (17) and to trap said salt by means of said porous support.

11. Device according to claim 9, characterized in that said means arranged to dissociate said salt to extract said pollutant component in the gaseous state comprise a heating device

(16) arranged to effect thermal dissociation of said salt.

12. Device according to claim 11, characterized in that said heating device (16) is arranged to heat said salt in steps of temperature, each of these steps corresponding to a dissociation temperature of a specific salt of said pollutant component.

13. Device according to claim 9, characterized in that said at least one filter (18) for trapping components resulting from the dissociation is impregnated with a substance that chemically reacts with said components.