DE102021119834A1 - Device for measuring the methane content of a gas - Google Patents

Abstract

The invention relates to a device for measuring the methane content of a gas, preferably in a biogas plant, with a first gas supply that receives gas from a first source, a second gas supply that receives gas from a second source, a first controllable valve that is connected to the first or the second gas supply, a pump, which is connected to the first valve, a cuvette, one end of which is connected to the pump, the length of the cuvette being between 150mm and 800mm, a second controllable valve, which is connected to the cuvette, and adapted to maintain the gas in the cuvette or to vent the gas out of the cuvette, an IR radiation source arranged to transmit IR radiation through the cuvette, the IR radiation source being an ideal black body is formed, a filter means, which is arranged to filter the radiation passed through the cuvette; an IR detector arranged to detect the radiation filtered by the filter device and to generate a measurement signal; and an evaluation device that is connected to the IR detector and is designed to evaluate the measurement signals generated by the IR detector and to determine the methane content of the supplied gas from the first source relative to the methane content of the gas from the second source the invention a method for measuring the methane content in a gas and a biogas plant with a device according to the invention.

Description

The present invention relates to a device for measuring the methane content of a gas, preferably within a membrane gap between an inner membrane and an outer membrane of a membrane system, preferably in a biogas plant. The invention also relates to a method for measuring the methane content in a gas, preferably in a membrane space between an inner membrane and an outer membrane of a membrane system, preferably a biogas plant, and a plant, preferably a biogas plant, with a membrane system that has an inner membrane and an outer membrane, and a device for measuring the methane content of a gas in the membrane space of the membrane system.

Devices for measuring the methane content of a gas are well known. They are used in a wide variety of areas and usually include a so-called non-dispersive infrared sensor, which is also referred to as an NDIR sensor. The most important components of such a sensor for gas analysis are a source of infrared radiation, a tube or cuvette through which the gas to be analyzed is passed, a wavelength filter and an infrared detector. The gas to be examined is either pumped into the sample chamber or deep into this chamber. The concentration of the gas sought is measured electro-optically via the extent of absorption of a specific wavelength in the infrared spectrum. The light from the infrared source (IR = infrared) shines through the gas in the sample chamber and the filter and then hits the IR sensor. The filter has the task of letting through only a very narrow spectrum from the spectrum of the light source, the frequency range of which is selected in such a way that it is effectively absorbed by the molecules of the gas being examined. Ideally, the other gases contained in the gas mixture of the sample should not absorb light of this wavelength. The IR signals used are often modulated or pulsed in order to be able to compensate for thermal phenomena, for example.

As already mentioned, such sensors are used for gas analysis in different technical areas of application. In so-called biogas plants, or generally in plants with a membrane system consisting of an inner membrane and an outer membrane, sensors are used to give an alarm if the methane produced during anaerobic degradation is unintentionally released into the environment, i.e. in particular in a membrane gap between the inner membrane and outer membrane. In the meantime, the government regulations for measuring methane are so strict that existing systems are hardly able to measure methane values of 100 ppm and below.

The object of the present invention is therefore to provide a device for measuring the methane content of a gas which, particularly in the field of biogas plants, is capable of reliably detecting values of 100 ppm methane or less within a membrane gap of a membrane system.

• eine erste Gaszuführung, die Gas aus einer ersten Quelle erhält,
• eine zweite Gaszuführung, die Gas aus einer zweiten Quelle erhält,
• ein erstes steuerbares Ventil, das mit der ersten oder der zweiten Gaszuführung verbunden ist,
• eine Pumpe, die mit dem ersten Ventil verbunden ist,
• eine längliche Küvette, die mit der Pumpe verbunden ist, wobei die Länge der Küvette zwischen 150mm und 800mm beträgt,
• ein zweites steuerbares Ventil, das mit der Küvette verbunden ist, und ausgelegt ist, das Gas in der Küvette zu halten oder aus der Küvette ausströmen zu lassen,
• eine IR-Strahlungsquelle, die angeordnet ist, um IR-Strahlung durch die Küvette zu senden, wobei die IR-Strahlungsquelle als idealer Schwarzstrahler ausgebildet ist,
• eine Filtereinrichtung, die angeordnet ist, um die durch die Küvette hindurchgelaufene Strahlung zu filtern;
• einen IR-Detektor, der angeordnet ist, um die durch die Filtereinrichtung gefilterte Strahlung zu detektieren und ein Messsignal zu erzeugen; und
• eine Auswerteeinrichtung, die mit dem IR-Detektor verbunden ist und ausgelegt ist, die vom IR-Detektor erzeugten Messsignale auszuwerten und daraus den Methangehalt des zugeführten Gases aus der ersten Quelle relativ zu dem Methangehalt des Gases aus der zweiten Quelle zu ermitteln.

This object is achieved by a device for measuring the methane content of a gas, preferably in a biogas plant, which has the following features:

• a first gas supply receiving gas from a first source,
• a second gas supply receiving gas from a second source,
• a first controllable valve which is connected to the first or the second gas supply,
• a pump connected to the first valve,
• an elongated cuvette connected to the pump, the length of the cuvette being between 150mm and 800mm,
• a second controllable valve connected to the cuvette and adapted to keep the gas in the cuvette or to let the gas out of the cuvette,
• an IR radiation source arranged to emit IR radiation through the cuvette, the IR radiation source being designed as an ideal blackbody body,
• a filter means arranged to filter the radiation passed through the cuvette;
• an IR detector arranged to detect the radiation filtered by the filter device and to generate a measurement signal; and
• an evaluation device that is connected to the IR detector and is designed to evaluate the measurement signals generated by the IR detector and to determine the methane content of the supplied gas from the first source relative to the methane content of the gas from the second source.

Significantly improved measurement results can be achieved by using a cuvette that is relatively long compared to previous solutions and an ideal black radiator as the IR radiation source. In addition, through external Measuring errors caused by influences can be avoided, especially when used in biogas plants, by considering the difference between the methane content from the first and second source.

The object of the invention is thus completely achieved.

In a preferred development, the cuvette has a length of 160 mm. This length has proven to be particularly advantageous.

In a preferred development, the filter device has a number of filters which allow radiation of different wavelengths to pass through, the wavelength ranges being as follows: 60 nm, 90 nm, 120 nm and 180 nm.

By using a filter device with multiple filters, it is possible to detect multiple absorption spectra, which improves the measurement accuracy. For example, a filter can be chosen to pass a band of frequencies that are not absorbed by any gas.

In a preferred development, the IR radiation source is pulsed at a frequency of 2 Hz. More preferably, the power of the IR radiation source is modulated, in particular 5 power levels of 2 seconds each are run through.

This measure has proven to be particularly advantageous. The measurement accuracy can be further increased in this way. The amplitude evaluation can be optimized by modulating the power levels. In particular, the modulation achieves peak values of different heights. This means that it is always possible to carry out an optimized evaluation, since the best result between methane and a reference is used for the evaluation.

In a preferred development, a further valve is provided which is designed to alternately supply gas from the first and the second source to the first valve. The first valve and the second valve are preferably closed during a measurement process.

• - Zuführen des zu messenden Gases aus einer ersten Quelle in eine Messkammer, vorzugsweise in eine Küvette, die eine Länge von zwischen 150mm und 800mm aufweist;
• - nach einer vorgebebenen Zeitdauer, die ausreicht, dass sich das Gas in der Messkammer beruhigt, Bestrahlen der Messkammer mit einer von einem idealen Schwarzstrahler erzeugten IR-Strahlung, wobei die Leistung der Strahlung moduliert ist;
• - Filtern der durch die Messkammer hindurchgelaufenen Strahlung;
• - Erfassen der gefilterten Strahlung mit einem IR-Detektor;
• - Auswerten der vom IR-Detektor erfassten Strahlung und Speichern des Ergebnisses;
• - Durchführen der vorgenannten Schritte mit einem Gas aus einer zweiten Quelle, und
• - Vergleich der beiden Ergebnisse, um den Methangehalt des Gases aus der ersten Quelle relativ zu dem des Gases aus der zweiten Quelle zu ermitteln.

The object on which the invention is based is also achieved by a method for measuring the methane content in a gas, which has the following features:

• - supplying the gas to be measured from a first source into a measuring chamber, preferably into a cuvette, which has a length of between 150mm and 800mm;
• - after a predetermined period of time, which is sufficient for the gas in the measuring chamber to calm down, irradiating the measuring chamber with IR radiation generated by an ideal black radiator, the power of the radiation being modulated;
• - filtering the radiation passed through the measuring chamber;
• - detecting the filtered radiation with an IR detector;
• - Evaluating the radiation detected by the IR detector and storing the result;
• - performing the above steps with a gas from a second source, and
• - Comparison of the two results to determine the methane content of the gas from the first source relative to that of the gas from the second source.

In a preferred development, the measuring chamber has a length of 160 mm. More preferably, the IR radiation is pulsed, to be specified at a frequency of 2 Hz. The IR radiation is preferably modulated over 5 power levels of 2 seconds each.

The object on which the invention is based is also achieved by a plant, in particular a biogas plant, which has the device according to the invention for measuring the methane content. The system has a substrate space, a gas reservoir, which is located above the substrate space and is delimited by an inner membrane, and an intermediate membrane space, which is located above the inner membrane and is closed off to the outside by an outer membrane, in particular a weather protection membrane, with the Membrane cavity has an opening for supply air and an opening for exhaust air. The device according to the invention is supplied with the exhaust air as a gas from the first source and the supply air as a gas from the second source.

The use of the measuring device according to the invention in a biogas plant has proven to be particularly advantageous, since very high measuring accuracies can be reliably achieved through the differential measurement, which are not falsified by external influences. Such external influences can be of various kinds, for example a vehicle driving past a biogas plant can contaminate the supply air with its exhaust gases and thus lead to a false measured value.

It goes without saying that the features mentioned above and those to be explained below not only in the combination specified in each case, but also in other combinations NEN or can be used alone without departing from the scope of the present invention.

• 1 eine schematische Darstellung einer Biogasanlage;
• 2 eine schematische Blockschaltdarstellung einer erfindungsgemäßen Vorrichtung zum Messen des Methangehalts; und
• 3 ein Flussdiagramm zur Erläuterung des Messverfahrens.

Further advantages and refinements of the invention result from the description and the attached drawing. show:

• 1 a schematic representation of a biogas plant;
• 2 a schematic block diagram of a device according to the invention for measuring the methane content; and
• 3 a flow chart to explain the measurement method.

In 1 1 is a biogas plant, generally representative of plants with a membrane system, shown in a highly simplified form and identified by reference number 10 . In such a biogas plant 10, the energy-rich gas methane is generated by an anaerobic microbial degradation of biomass.

The biogas plant 10 includes a reactor 12 which is filled with a substrate 14 (biomass). Furthermore, the biogas plant has a membrane system which has at least one inner membrane 18 and one outer membrane 22 . The reactor 12 is closed at the top by the inner membrane 18, which is also referred to below as the gas membrane 18, the biogas being collected between the substrate and the gas membrane 18 in a gas reservoir 16.

The reactor 12 is protected from environmental influences by the outer membrane 22 , which is also referred to below as the weather protection membrane 22 , a membrane intermediate space 20 being formed between the weather protection membrane 22 and the gas membrane 18 .

The weather protection membrane 22 is held in place by what is known as supporting air, which flows into the intermediate membrane space 20 via an air inlet opening 24 and flows out via an air outlet opening 26 . The supporting air is provided by a supporting air blower or compressor 32 and guided to the supply air opening 24 via a pipeline 34 . The inlet air opening 24 is usually on the opposite side of the outlet air opening 26. The pressure exerted by the supporting air on the weather protection membrane 22 is designed in such a way that it is securely supported against different load conditions, which are caused, for example, by wind loads or snow loads .

The biogas produced, which essentially contains methane and carbon dioxide, can be discharged from the gas storage tank 16 via a biogas outlet opening 30 for further processing.

In order to detect leaks in the gas membrane 18, the supporting air discharged from the exhaust air opening 26 is usually checked for its methane content. In the meantime, the legal requirements (TRAS 120 "Safety requirements for biogas plants") are so strict that 100 ppm methane and less must be recognized.

For a gas analysis, the supporting air in the intermediate membrane space 20 is conducted via a pipeline 36 to a measuring device 40 which checks whether the supporting air contains methane. The functioning and structure of the measuring device 40 are described below with reference to FIG 2 and 3 explained in more detail.

Out of 1 it can also be seen that the measuring device 40 is also supplied with the support air provided by the compressor 32 . The two gases supplied to the measuring device 40 are denoted by V1 and V2, with V1 denoting the inflowing supporting air, supply air, and V2 denoting the outflowing supporting air, exhaust air.

2 shows the measuring device 40 in a schematic block diagram, which is used for the gas analysis of the support air located in the intermediate membrane space 20 . The measuring device 40 has a two-way valve 42 to which the supply air and the exhaust air V1 and V2 are supplied. The two-way valve 42 can be controlled in such a way that it directs one of the two air flows V1 and V2 to a downstream sample gas valve 44 .

The sample gas valve 44 can be opened and closed in a controlled manner in order to selectively lead the gas to be analyzed to a downstream pump 46 .

The pump 46 pumps the gas through a preferably cylindrical interior 56 of a cuvette 50 , the gas flowing into the interior 56 through an inlet opening 54 and flowing out through an outlet opening 58 .

A further valve 52 is provided downstream of the outlet opening 58 and can likewise be opened and closed in a controlled manner.

As can be seen from the schematic 2 results, the inlet openings 54 and the outlet opening 58 of the cuvette 50 are provided at opposite end portions of the cuvette, so that the gas to be analyzed flows through the entire inner space 56 as far as possible.

The cuvette 50 preferably has a cylindrical shape and is preferably 150 mm to 800 mm long, with a length of 160 mm having turned out to be particularly advantageous.

The two longitudinal ends of the cuvette 50 are made of a material that lets infrared radiation through. For the gas analysis of the gas in the interior 56, infrared light is radiated from the outside via an infrared emitter 60, so that it spreads parallel to the longitudinal axis over the entire length of the cuvette and exits at the opposite end and hits a detector 62. Since different gases absorb different frequency spectra, the gases contained in the interior 56 can be inferred from the light spectrum detected by the detector.

The infrared emitter 60 is an ideal black emitter, as is available, for example, from Axetris AG, Switzerland, under the product name EMIRS200_AT01T_BR060_Series or EMIRS200_AT02V_BR060_Series.

The detector 62 is a four-channel detector with a filter device 64, such as is available from Infratec GmbH, Germany, under the product name LRM-284-#. Filters in the range of 60nm, 90nm, 120nm and 180nm are preferably used for the four channels.

The detector 62 is electrically connected to an evaluation device 66 which evaluates the detected signals. In addition, the evaluation device 66 also controls the controllable valves and the pump in order to carry out the gas analysis or the measurement method. The measurement method itself will be discussed later with reference to the 3 explained in more detail.

In 2 a line 70 which is connected to the input of the pump 46 can also be seen. A gas used for flushing the interior 56 can be introduced via this line 70, or alternatively a test gas which is used for calibration.

The measuring device 40 also includes a pulsation damping element 48 which is provided between the pump 46 and the inlet opening 54 and has the task of making the gas flow generated by the pump more uniform.

For a gas analysis, as in 3 is shown, first the valve 44 and the valve 52 are opened and the pump 46 is switched on. As a result, the gas to be measured flows through the interior 56 of the cuvette 50, depending on the switching status of the two-way valve 42 V1 or V2. After a period of, for example, 3 minutes, the pump 46 is switched off and the two valves 44, 52 are closed.

After a certain time, which depends on how long it takes for the gas in the interior 56 to settle, the measurement can begin.

For the measurement, the IR emitter 60 is pulsed at a frequency of 2 Hz, so that pulsed IR radiation is emitted through the interior 56 over the entire length L of the cuvette 50 . The opposite detector 62 detects this pulsed IR radiation, with the filter device 64 having four filters that allow four different frequency spectra to pass through to the detector 62 . The four frequency spectrums are preferably as follows: 60nm, 90nm, 120nm and 180nm.

The detector 62 is operated in such a way that the detector power is modulated over a total of 5 power levels for 2 seconds at a time. This modulation enables optimization of the amplitude evaluation of the recorded signals

As already mentioned above, the different gases in the interior 56 absorb different frequency spectra, so that the evaluation of this absorption allows very reliable conclusions to be drawn about the gases contained.

The measurement signals detected by the detector 62 are evaluated and stored. In the next step, the previous measurement method is carried out again, but then for the other gas V1 or V2.

The measured values for gas V1 and gas V2 can then be compared with one another in order to determine how the methane content in the two gas streams differs. If there is a difference, this could indicate a leak in the gas membrane 18 .

Since the intermediate membrane space 20 can have a large spatial volume, it must be assumed that the supporting air flowing in does not flow out of this intermediate membrane space 20 again immediately. Rather, it will remain in the membrane gap 20 for a certain period of time. However, this also means that a single difference reading alone is not sufficient to detect a leak.

Rather, it is necessary to carry out the measurement method described several times within, for example, an hour, ie for example every 5 minutes, and to analyze a measured value trend of the difference measured values. A leak can only be reliably concluded if the trend over this period is towards larger differences. In such a case, the measuring device 40 then issue an alarm to indicate the need to examine the facility.

After a period of 5 hours, for example, the measurement process described can be repeated.

The measuring device described, as in 2 as shown, allows for a very precise measurement of methane levels, where levels of 100 ppm and less can be detected. This is achieved in particular through the use of an ideal black body and the selected length of the cuvette 50 and through the described modulated detector power over a total of 5 power levels. In order to be able to ignore environmental influences, ie impurities already present in the supply air provided by the compressor, when used in a biogas plant, the differential measurements described are carried out. i.e. in other words, that the calculation or gas analysis carried out only represents the relative proportion of methane between the supplied and discharged supporting air.

Claims (16)

Device for measuring the methane content of a gas, preferably within a membrane gap between an inner membrane and an outer membrane of a membrane system, preferably a biogas plant a first gas supply receiving gas from a first source, a second gas supply receiving gas from a second source, a first controllable valve which is connected to the first or the second gas supply, a pump connected to the first valve, a cuvette having one end connected to the pump, the length of the cuvette being between 150mm and 800mm; a second controllable valve which is connected to the cuvette and is designed to keep the gas in the cuvette or to let the gas flow out of the cuvette, an IR radiation source arranged to emit IR radiation through the cuvette, the IR radiation source being a black body body; filter means arranged to filter the radiation passed through the cuvette; an IR detector arranged to detect the radiation filtered by the filter device and to generate a measurement signal; and an evaluation device which is connected to the IR detector and is designed to evaluate the measurement signals generated by the IR detector and from this to determine the methane content of the supplied gas from the first source relative to the methane content of the gas from the second source. device after claim 1 , characterized in that the cuvette has a length of 160mm. Device according to one of the preceding claims, characterized in that the filter device has a plurality of filters which allow radiation of different wavelengths to pass through, the wavelength ranges being as follows: 60 nm, 90 nm, 120 nm and 180 nm. Device according to one of the preceding claims, characterized in that the IR radiation source is pulsed at a frequency of 2 Hz. Device according to one of the preceding claims, characterized in that the power of the IR radiation source is modulated, in particular running through five power levels within two seconds. Device according to one of the preceding claims, characterized in that a further valve is provided which is designed to lead gas from the first and the second source alternately to the first valve. Device according to one of the preceding claims, characterized in that the first valve and the second valve are closed during a measurement process. Device according to one of the preceding claims, characterized in that a pulsation damping element is provided, which is arranged downstream of the pump. Method for measuring the content of methane in a gas, preferably in a membrane gap between an inner membrane and an outer membrane of a membrane system, preferably a biogas plant, preferably using the device according to one of Claims 1 until 8th , with the steps: - supplying the gas to be measured from a first source into a measuring chamber, preferably into a cuvette, - after a predetermined period of time that is sufficient for the gas in the measuring chamber to calm down, irradiating the measuring chamber with one of an ideal Black radiators generate IR radiation, the power of the radiation being modulated; - filtering the radiation passed through the measurement chamber, the measurement chamber having a length of 150mm to 800mm; - detecting the filtered radiation with an IR detector; - Evaluating the radiation detected by the IR detector and storing the result; - performing the above steps with a gas from a second source, and - comparing the two results to determine the methane content of the gas from the first source relative to that of the gas from the second source. procedure after claim 9 , characterized in that the IR radiation is pulsed, preferably with a frequency of 2 Hz. procedure after claim 9 or 10 , characterized in that the IR radiation is modulated over five power levels within two seconds. Procedure according to one of claims 9 until 11 , characterized in that the gas pumped into the measuring chamber by a pump is damped by a pulsation damping element. Plant, preferably biogas plant, with a membrane system having an inner membrane and an outer membrane; a substrate space, a biogas space that lies above the substrate space and is delimited by the inner membrane, a membrane space that lies between the inner membrane and the outer membrane, with the outer membrane closing off the membrane space to the outside, with the membrane space having an opening for supply air and an opening for exhaust air, and a device for measuring the methane content according to any one of Claims 1 until 8th , wherein the first source is the exhaust air and the second source is the supply air, so that the content of methane in the exhaust air is detected relative to that of the supply air. plant after Claim 13 , characterized in that an alarm device is provided which emits an alarm if the content of methane in the exhaust air relative to the supply air exceeds a predetermined value at least twice within a predetermined period of time. plant after Claim 14 , characterized in that the predetermined value is less than or equal to 100ppm methane. plant after Claim 13 , wherein the plant is a biogas plant, the inner membrane is designed as a gas membrane and the outer membrane as a weather protection membrane, and the supply air serves as supporting air to support the weather protection membrane.

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