Device and method for measuring a CH4 concentration in a gas mixture containing water vapor

Abstract

The invention relates to a device (1) for measuring a CH4 concentration in a gas mixture containing water vapor. To increase the accuracy of an NDIR measurement of a CH4 concentration in a gas mixture containing water vapor, the device (1) comprises at least one NDIR measuring device (2) and at least one gas guide unit (6) arranged upstream of the NDIR measuring device (2) with respect to a flow of the gas mixture through the device (1) and designed to be selectively permeable to water vapor for drying and / or humidifying the gas mixture, wherein the gas guide unit (6) is designed such that the water content of the gas mixture exiting it either corresponds to the water content of a CH4-free ambient air surrounding the gas guide unit (6) or deviates by a maximum of 5% from the water content of the CH4-free ambient air surrounding the gas guide unit (6).

Description

Technical field

[0001] The invention relates to a device and a method for measuring a CH4 concentration in a gas mixture containing water vapor. The invention also relates to the use of such a device for measuring a CH4 concentration in a gas mixture containing water vapor. State of the art

[0002] Non-dispersive infrared absorption (NDIR) is an established method in gas sensing. A suitable NDIR gas detector is known, for example, from EP 0 794 423 A1. In this method, the radiation emitted from an infrared radiation source passes through a measuring cuvette containing a gas mixture and strikes an infrared-sensitive gas sensor at the opposite end of the cuvette. According to the Lambert-Beer law, the attenuation of the electromagnetic radiation by the gas mixture enclosed in the cuvette is a measure of the concentration of a gas component being measured.

[0003] NDIR gas detectors that react to a specific gas component of a gas mixture containing a sample gas can exhibit cross-sensitivities to other gas components (also called interfering components) present in the sample gas. These cross-sensitivities must be compensated for to obtain accurate measurement results. This can be done, for example, according to utility model DE 20 2019 101 137 U1. However, this approach incurs additional costs due to the structurally expanded measurement setup required.

[0004] Furthermore, water vapor contained in a sample gas poses a problem in NDIR measurements, as it also causes inaccurate results. To reduce and maintain a constant water content in a sample gas, the gas can be cooled and its temperature regulated using a sample gas cooler. However, this also involves high costs for setting up the measurement equipment.

[0005] Another problem with NDIR gas detectors is long-term drift, especially zero-point drift, which can be corrected by regular adjustment of the NDIR gas detectors. Disclosure of the invention

[0006] One object of the invention is to increase the accuracy of an NDIR measurement of a CH4 concentration in a gas mixture containing water vapor.

[0007] This problem is solved by the independent claims. Advantageous embodiments are described in the dependent claims, the following description, and the figures, whereby these embodiments, either individually or in combination with at least two of them, can represent an advantageous and / or further developing aspect of the invention. Advantageous embodiments of the device can correspond to advantageous embodiments of the method, and vice versa, even if this is not explicitly stated in detail below.

[0008] An inventive device for measuring a CH4 concentration in a gas mixture containing water vapor comprises at least one NDIR measuring device and at least one gas guidance unit arranged upstream of the NDIR measuring device with respect to a flow of the gas mixture through the device and designed to be selectively permeable to water vapor for drying and / or humidifying the gas mixture, wherein the gas guidance unit is designed such that the water content of the gas mixture exiting it corresponds to the water content of the CH4-free ambient air surrounding the gas guidance unit or deviates by a maximum of 5% from the water content of the CH4-free ambient air surrounding the gas guidance unit.

[0009] With the device according to the invention, it is possible to dry and / or humidify a CH4 (methane)-containing measuring gas supplied to the NDIR measuring device by means of the gas guide unit, such that the water content of the gas mixture exiting the gas guide unit corresponds to the water content of the CH4-free ambient air surrounding the gas guide unit or deviates from the water content of the CH4-free ambient air surrounding the gas guide unit by a maximum of 5%. The latter can be achieved by the water content of the measuring gas supplied to the NDIR measuring device being slightly higher or lower than the water content of the zero gas supplied to the NDIR measuring device.The invention makes it possible, in particular, to easily perform a calibration measurement using CH4-free ambient air by passing the ambient air (zero gas) through the gas flow unit and the NDIR measuring device for a predetermined period instead of the measuring gas, and at the end of this period, setting at least one measuring channel of the NDIR measuring device to zero for measuring the CH4 concentration. The measuring gas supplied to the NDIR measuring device and the zero gas supplied to the NDIR measuring device have the same water content or differ from each other by a maximum of 5%.

[0010] If the sample gas supplied to the NDIR measuring device and the control gas supplied to the NDIR measuring device have the same water content, any disturbance caused by water vapor in the sample gas during the measurement of the CH4 concentration in the water vapor-containing gas mixture corresponds to the disturbance caused by water vapor in the control gas during the calibration measurement. Subtracting the calibration measurement result from the NDIR measurement result completely compensates for the interfering influence of the water vapor in the sample gas, resulting in a higher accuracy for the NDIR measurement of the CH4 concentration in the water vapor-containing gas mixture.

[0011] If the sample gas supplied to the NDIR measuring device and the control gas supplied to the NDIR measuring device have a water content that differs by a maximum of 5%, a disturbance caused by water vapor in the sample gas during the measurement of the CH4 concentration in the water vapor-containing gas mixture is essentially equivalent to the disturbance caused by water vapor in the control gas during the calibration measurement. Subtracting the calibration measurement result from the NDIR measurement result largely compensates for the interfering influence of the water vapor in the sample gas, resulting in a higher accuracy for the NDIR measurement of the CH4 concentration in the water vapor-containing gas mixture.The gas supply unit can be designed such that the difference between the water content of the measuring gas supplied to the NDIR measuring device and the water content of the zero gas supplied to the NDIR measuring device is so small that the resulting deviation of the NDIR measurement of the CH4 concentration in the water vapor-containing gas mixture is less than 1% of the corresponding end-range value of the NDIR measuring device.

[0012] The period during which zero gas is supplied to the NDIR measuring device for calibration can be, for example, in the range of seconds, such as from 2 seconds to 60 seconds. This calibration period can be predefined by the device's electronics and set using software executable by the electronics.

[0013] For NDIR measurement of the CH4 concentration in the water vapor-containing gas mixture, the sample gas can be passed through the NDIR measuring device for a predetermined measurement period, which can be in the range of hours, minutes, or seconds. The measurement period can, for example, be chosen so short that the CH4 concentration drift Δc during this period is less than 1% of the full-scale value of the NDIR measuring device.

[0014] At the end of the calibration period, each measurement channel of the NDIR measuring device, in particular the measurement channel for measuring the CH4 concentration in the water vapor-containing gas mixture, which may have drifted by a concentration amount Δc in a previous measurement period, can be set to zero (Δc=0). This also compensates for the zero-point drift.

[0015] The device according to the invention can, for example, be operated such that, after a measurement phase, i.e., an NDIR measurement of the CH4 concentration in the gas mixture containing water vapor, the zero gas is passed through the gas flow unit and the NDIR measuring device for a predetermined period. Subsequently, another measurement process can be carried out, so that measurement and adjustment processes can be performed alternately, for example, making the device suitable, for example, for continuous monitoring of the CH4 concentration of exhaust air from which the gas mixture containing water vapor can be extracted for each measurement process.

[0016] A change in the water content of the ambient air does not adversely affect the accuracy of the CH4 concentration measurement that can be carried out with the device, since the gas guide unit always ensures that the measuring gas exiting the gas guide unit has the same water content or approximately the same water content as the ambient air or the zero gas.

[0017] The NDIR measuring device can comprise at least one infrared radiation source, at least one measuring cuvette, and at least one infrared-sensitive radiation sensor. The radiation emitted by the infrared radiation source passes through the measuring cuvette, which contains a gas mixture to be analyzed, and strikes the infrared-sensitive radiation sensor at the opposite end of the measuring cuvette. The measuring cuvette can have a window transparent to IR radiation at both its inlet and outlet, relative to the direction of the IR radiation passing through it. This window can be made, for example, of calcium fluoride (CaF₂). It has been observed that water vapor present in the measuring cuvette can accumulate in or on the window material, leading to a change in the window's transmission properties.This change is accompanied by a change in the spectral distribution of the electromagnetic radiation emitted from the measuring cuvette. Furthermore, the electromagnetic radiation passing through the respective cuvette window is delayed by the absorption or deposition of water vapor in or on the window material (so-called hang-up effect), with this delay being caused by adsorption and desorption effects on the cuvette window. These interfering effects can also be reliably and largely to completely compensated for by the device according to the invention. Therefore, the device according to the invention can be used to compensate for disturbances in the measurement of a CH4 concentration in a gas mixture containing water vapor, disturbances caused by the absorption and / or deposition of water vapor in or on the window material of a measuring cuvette window.

[0018] The NDIR measuring device can also have at least one additional measuring channel for measuring the concentration of at least one other gas component of the water vapor-containing gas mixture. The additional gas component can be, for example, nitrous oxide (N₂O) or carbon dioxide (CO₂). The NDIR measuring device can have both a measuring channel for nitrous oxide and a measuring channel for carbon dioxide.

[0019] The gas guidance unit for drying and / or humidifying the gas mixture can, for example, be tubular or hose-like. The gas guidance unit can also be made, for example, of a perfluorinated copolymer with ionic properties, where a sulfonic group can function as the ionic group. For example, the gas guidance unit can be made partially or completely of NAFION. ®The gas supply unit can consist of a single gas supply pipe or hose, or two or more gas supply pipes or hoses connected in parallel. If there is high relative humidity inside the gas supply unit and low relative humidity outside, water vapor passes through the wall of the gas supply unit from the inside to the outside (drying). Conversely, if there is low relative humidity inside the gas supply unit and higher relative humidity outside, water vapor passes through the wall of the gas supply unit from the outside to the inside (humidification). This passage can continue until the water vapor partial pressure inside and outside the gas supply unit is identical. Thus, an equalization of relative humidity (RH) takes place.The length of the gas guide unit is preferably selected such that the moisture exchange between the inside and outside is ≥95%. This means that the initial difference between the humidity of the measuring gas or a subsequently described test gas on the one hand and the ambient air on the other hand is 100% before the measuring gas or test gas enters the gas guide unit, and is only ≤5% at the outlet of the gas guide unit, thus ensuring a moisture exchange between the inside and outside of ≥95%. The length of the gas guide unit is preferably selected depending on the gas flow rate V through the gas guide unit, such that the length of the gas guide unit is chosen to be greater with increasing gas flow rate. For example, with a gas flow rate of <1 liter / minute, the length of the gas guide unit can be approximately 50 cm.

[0020] The gas flow unit can additionally be designed, for example, such that the water content of a test gas exiting it and used for end-point adjustment corresponds to the water content of the CH4-free ambient air surrounding the gas flow unit, or deviates from the water content of the CH4-free ambient air surrounding the gas flow unit by a maximum of 5%. End-point adjustment is generally only performed every few months or once a year. Since the test gas used for end-point adjustment is usually very dry, its humidity or water content should also be adjusted to match the humidity of the ambient air. The water content of the test gas flowing through the gas flow unit would thus be adjusted to the water content of the ambient air by transferring moisture from the ambient air through the gas flow unit to the test gas (humidification).The test gas would then be guided in the same way as the measuring gas. Endpoint adjustment can be performed manually or automatically.

[0021] The device according to the invention for measuring a CH4 concentration in a gas mixture containing water vapor can be used, for example, for continuous methane measurement in sewage treatment plants and cowsheds. For example, air from the surroundings of a sewage treatment plant or a cowshed can be used as CH4-free ambient air.

[0022] According to an advantageous embodiment, the device has at least one electrically controllable valve unit arranged upstream of the gas supply unit. Depending on the switching state of the valve unit, either ambient air for adjusting the NDIR measuring device or the gas mixture for measuring CH4 concentration can be supplied to the gas supply unit via this valve unit. The valve unit can be controlled by the device's electronics, for example, to enable automated and alternating measurement and adjustment processes. The valve unit can have at least one inlet for CH4-free ambient air, at least one inlet for the measuring gas, and at least one outlet through which either the measuring gas or the ambient air exits the valve unit to flow to the gas supply unit.

[0023] According to a further advantageous embodiment, the valve unit comprises at least one 2 / 3-way valve. This allows the device to be built in a compact manner. Alternatively, the valve unit can have at least one valve for the measuring gas and at least one valve for the CH4-free ambient air.

[0024] According to a further advantageous embodiment, the device has at least one conveying unit arranged downstream of the valve unit, the gas guide unit, or the NDIR measuring device, by means of which either the ambient air or the gas mixture can be conveyed through the NDIR measuring device, depending on the switching state of the valve unit. This allows the device to be equipped with only one appropriately arranged conveying unit to effect the respective air or gas flows through the device. This enables a compact design of the device. Furthermore, the volume flows of the air or gas flows through the device can be kept essentially constant in a simple manner using a single conveying unit. Alternatively, at least one conveying unit for the measuring gas and at least one further conveying unit for the CH4-free ambient air can be provided.

[0025] According to a further advantageous embodiment, the device has at least one filter unit upstream of the valve unit for filtering the ambient air, with which at least one gas component of the ambient air that interferes with a CH4 concentration measurement can be filtered out. For example, traces of gases may be present in the CH4-free ambient air, which could interfere with the zero adjustment of at least one measuring channel of the NDIR measuring device described above. These traces of gases can be filtered out by the filter unit. The filter unit can be designed as a chemical absorber (so-called scrubber).

[0026] According to a further advantageous embodiment, the device comprises at least one electronic control unit configured to control the valve unit such that, firstly, ambient air can be supplied to the gas supply unit for the adjustment of the NDIR measuring device, and, after completion of the NDIR measuring device adjustment, the gas mixture can be supplied to the gas supply unit for the CH4 concentration measurement. The aforementioned operation of the device can be automated via the electronic control unit. For this purpose, the electronic control unit can comprise at least one processor on which suitable software can be executed. The electronic control unit can be configured to control the valve unit such that the adjustment and the CH4 concentration measurement can be performed continuously and alternately.

[0027] According to a further advantageous embodiment, the device electronics are configured to set all measurement channels of the NDIR measuring device to zero at the end of the adjustment process. This embodiment offers the advantages mentioned above with reference to the corresponding embodiment of the device.

[0028] The invention also proposes the use of the device according to one of the aforementioned embodiments or a combination of at least two of these embodiments for measuring a CH4 concentration in a gas mixture containing water vapor.

[0029] This use brings with it the advantages mentioned above in relation to the device.

[0030] According to a method according to the invention for measuring a CH4 concentration in a gas mixture containing water vapor using at least one NDIR measuring device, the gas mixture is dried or moistened by means of at least one gas guidance unit arranged upstream of the NDIR measuring device with respect to a flow of the gas mixture and selectively permeable to water vapor, such that a water content of the gas mixture exiting the gas guidance unit corresponds to a water content of a CH4-free ambient air surrounding the gas guidance unit.

[0031] The method offers the same advantages as those mentioned above with regard to the device. In particular, the device can be used to carry out the method according to one of the embodiments mentioned above or a combination of at least two of these embodiments.

[0032] According to an advantageous embodiment, the gas supply unit is supplied with either ambient air for adjusting the NDIR measuring device or the gas mixture for measuring CH4 concentration. This embodiment offers the advantages mentioned above with reference to the corresponding embodiment of the device.

[0033] According to a further advantageous embodiment, at least one gas component of the ambient air that interferes with CH4 concentration measurements is filtered out. This embodiment offers the advantages mentioned above with reference to the corresponding embodiment of the device.

[0034] According to a further advantageous embodiment, ambient air is first supplied to the gas supply unit for the adjustment of the NDIR measuring device, and after the adjustment of the NDIR measuring device is complete, the gas mixture is supplied to the gas supply unit for the CH4 concentration measurement. This embodiment offers the advantages mentioned above with reference to the corresponding embodiment of the device.

[0035] According to a further advantageous embodiment, at the end of the adjustment of the NDIR measuring device, all measuring channels of the NDIR measuring device used for CH4 concentration measurement are set to zero. This embodiment offers the advantages mentioned above with reference to the corresponding embodiment of the device.

[0036] The invention is explained below by way of example with reference to the attached figures using a preferred embodiment, wherein the features explained below can represent an advantageous and / or further developing aspect of the invention both individually and in different combinations with one another. Brief description of the characters

[0037] It shows: Fig. 1 a schematic representation of an embodiment of a device according to the invention; and Fig. 2 a diagram illustrating the function of the gas supply unit of the in Fig. Device shown in 1. Detailed description of the characters

[0038] In the figures, identical or functionally equivalent components are marked with the same reference symbols. To avoid unnecessary repetition, a detailed description of such components may be omitted below.

[0039] Fig. Figure 1 shows a schematic representation of an embodiment of a device 1 according to the invention for measuring a CH4 concentration in a gas mixture containing water vapor.

[0040] The device 1 comprises an NDIR measuring device 2. The NDIR measuring device 2 comprises a measuring cuvette 3, an IR radiation source 4, and an IR radiation sensor 5. The measuring cuvette 3 has an inlet (not shown in detail) through which a gas with a volume flow rate V̇3 can be supplied to the measuring cuvette 3. Furthermore, the measuring cuvette 3 has an outlet (not shown in detail) through which a gas can be discharged from the measuring cuvette 3 in accordance with arrow P. With respect to electromagnetic radiation (not shown) traveling from the IR radiation source 4 to the IR radiation sensor 5, the measuring cuvette 3 has an inlet and outlet window (not shown) made of calcium fluoride, which is transparent to IR radiation.

[0041] The device 1 also includes a gas guide unit 6, arranged upstream of the NDIR measuring device 2 with respect to the flow of the gas mixture through the device 1 and designed to be selectively permeable to water vapor, for drying and / or humidifying the gas mixture. The gas guide unit 6 is designated as NAFION ® -Hose formed with a length L.

[0042] The gas guidance unit 6 is designed such that the water content of the gas mixture exiting it either corresponds to the water content of the CH4-free ambient air surrounding the gas guidance unit 6 or deviates by a maximum of 5% from the water content of the CH4-free ambient air surrounding the gas guidance unit 6.

[0043] Furthermore, the device 1 has an electrically controllable valve unit 7 arranged upstream of the gas supply unit 6, via which, depending on the switching state of the valve unit 7, either ambient air with a volume flow rate V̇2 for adjusting the NDIR measuring device 2 or the gas mixture with a volume flow rate V̇1 for measuring CH4 concentration can be supplied to the gas supply unit 6. For this purpose, the valve unit 7 is designed as a 2 / 3-way valve and as a solenoid valve.

[0044] Furthermore, the device 1 has a pumping unit 8 arranged downstream of the valve unit 7 and upstream of the gas guide unit 6, by means of which either ambient air or the gas mixture can be pumped through the NDIR measuring device 2, depending on the switching state of the valve unit 7. The pumping unit 8 is designed as a pump. Due to the specific arrangement of the pumping unit 8, the following applies: V̇1 = V̇2 = V̇3.

[0045] Furthermore, the device 1 has a filter unit 9 upstream of the valve unit 7 with respect to an ambient airflow for filtering the ambient air, with which at least one gas component of the ambient air that interferes with a CH4 concentration measurement can be filtered out of the ambient air. For this purpose, the filter unit 9 is designed as a chemical absorber.

[0046] Furthermore, the device 1 has electronic components 10 which are configured to control the valve unit 7 when the conveying unit 8 is switched on, such that initially, ambient air can be supplied to the gas supply unit 6 for the adjustment of the NDIR measuring device 2, and after completion of the adjustment of the NDIR measuring device 2, the gas mixture can be supplied to the gas supply unit 6 for the CH4 concentration measurement. For this purpose, the electronic components 10 are connected to the valve unit 7 via a control line S.

[0047] The device electronics 10 are configured to supply ambient air to the gas supply unit 6 for a period t1 for the adjustment of the NDIR measuring device 2, and to supply the gas mixture to the gas supply unit 6 for a period t2 after the adjustment of the NDIR measuring device 2 for the CH4 concentration measurement, whereby period t2 is longer than period t3. The device electronics 10 are configured to alternate these periods.

[0048] Furthermore, the device electronics 10 are configured to set all measurement channels of the NDIR measuring device 2 to zero at the end of the adjustment process. The device electronics 10 can also serve as part of the NDIR measuring device 2.

[0049] Fig. Figure 2 shows a diagram illustrating the function of the gas guidance unit 6 of the [unclear text]. Fig. 1. Device shown. The relative humidity RH is plotted against the length L of the gas guidance unit 6. Fig. 2 is presented in conjunction with Fig. 1 explained.

[0050] If the relative humidity of the CH4-free ambient air is RH2 and a sample gas with a relative humidity of RH1 is passed through the gas guide unit 6, the relative humidity RH1 of the sample gas adjusts to the relative humidity RH1 of the ambient air, meaning that the sample gas is dried by the gas guide unit 6. The greater the length L of the gas guide unit 6, the more closely the relative humidity RH1 of the sample gas adjusts to the relative humidity RH1 of the ambient air.

[0051] In Fig.Figure 2 shows another situation in which the relative humidity of the CH4-free ambient air has decreased to the value RH*2. Then the relative humidity RH*1 of the measuring gas approaches the relative humidity RH*2 of the ambient air. The effect of device 1 is therefore independent of the current relative humidity RH*2 of the ambient air. Reference symbol list 1 Device 2 NDIR measuring device 3 measuring cuvettes 4 IR radiation source 5 IR radiation sensors 6 Gas guide unit 7 Valve unit 8 conveying units 9 filter unit 10 Device Electronics L Length of 6 P Arrow (Exit) RH relative humidity RH1 relative humidity (measuring gas) RH*1 relative humidity (measuring gas) RH2 relative humidity (ambient air) RH*2 relative humidity (ambient air) S control line t1 period (adjustment) t2 period (measurement) V̇1 Volume flow rate (measuring gas) V̇2 Volume flow (ambient air) V̇3 Volume flow QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 0 794 423 A1

[0002] DE 20 2019 101 137 U1

[0003]

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