WO1996007899A1

WO1996007899A1 - Process and device for determining gas concentrations in a gas mixture

Info

Publication number: WO1996007899A1
Application number: PCT/CH1995/000197
Authority: WO
Inventors: Andreas Rüetschi
Original assignee: Scr Crevoiserat S.A.
Priority date: 1994-09-07
Filing date: 1995-09-07
Publication date: 1996-03-14
Also published as: AU3251395A; CH685889A5

Abstract

The proposal is for a process for determining gas concentrations in gas mixtures in the monitoring of flue gases of heating installations. According to the invention, one determines in a measuring system (100) comprising a sensor system (110), a control and evaluation unit (120) and data lines (130), the concentration in a gas mixture in the sensor system (110) of a first gas component, especially CO2, in an acoustic resonator (10) and of a second gas component, especially CO, in a photo-acoustic cell (20). The sensor system is controlled by a computer unit (80) to which the measurement signals (52, 62, 72) are also transmitted for evaluation after suitable preparation in the signal processing units (50, 60, 70). The computer unit provides the measured values (81) of the gas concentrations, especially those of CO2 and CO, for display or further use. To implement said process there are measuring devices, one of which integrates the acoustic resonator (10) and the photoacousitc cell (20) in a single measuring cell. This embodiment particularly compact and easy to construct. The process and device of the kind described may be used in flue gas monitoring and especially for the control of heating installations.

Description

Method and device for determining gas concentrations in a gas mixture

The present invention relates to a method and a device for determining gas concentrations in a gas mixture in the exhaust gas monitoring of heating systems according to claims 1-8 and 9-14, as well as uses according to claims 15-17.

Previously known systems for exhaust gas monitoring in heating systems record and process, in addition to the temperature, the pressure of the supply air and, if necessary, the oxygen content of the exhaust gas. Further control requires not only determining the content of carbon dioxide in the exhaust gas, but also that of carbon monoxide.

PS-EP-277622 describes a detection device for the optical-spectroscopic detection of gases by means of the photo-acoustic effect. This device essentially consists of a light source, a detector designed as a gas collection cell and signal processing means. After passing through a narrowband interference filter, the light from the source is radiated into a photoacoustic gas cuvette containing the measurement gas, and the measurement is carried out with the cuvette closed or nearly closed. The disadvantage here is the fact that this arrangement has no means of avoiding, or correcting or compensating, for contaminants which are deposited on the entrance window and thus significantly impair the service life.

According to PS-JP 2-116737, a CO analysis device is known which essentially has a main detector and a compensation detector, as a result of which interference components are effectively eliminated. This transmission process is characterized by a complicated design of the arrangement, which includes requires two sealed gas cells, which has an adverse effect on inexpensive production.

Furthermore, TF Deaton et al. a differential photoacoustic method described, which is particularly suitable for the determination of very small concentrations of methane, carbon dioxide and nitrogen dioxide (Appl. Physics Lett., Vol.26, 300-303 (1975)). A disadvantage is the high experimental effort, which is primarily due to the use of a deuterium fluoride (DF) laser.

The object of the present invention is to provide methods and devices with which gas concentrations in a gas mixture (carbon dioxide and carbon monoxide) are determined in combination in a measuring system and which are inexpensive to manufacture and can be used in the exhaust gas monitoring of heating systems. In addition, such a method is intended to ensure that the measuring system based thereon can work maintenance-free over a long period of time and has a long service life. The measuring accuracy that can be achieved must be sufficient to control a heating burner so that the legal requirements, e.g. those of an Air Pollution Control Ordinance (LRV) are met.

According to the invention, this object is achieved by means of methods according to the wording of claims 1-8 and by means of devices according to the wording of claims 9-14. Show it:

1 shows a schematic representation of the measuring principle for the combined determination of gas concentrations in a gas mixture

2 shows a flow diagram of a measurement method for the combined determination of gas concentrations in a gas mixture

3 shows a schematic structure of a measuring system with a combined sensor system for photoacoustic and sound measurements carried out in the same measuring cell

The invention is described in more detail below with reference to FIGS. 1-3. 1 shows a schematic representation of the measuring principle for the combined determination of gas concentrations in a gas mixture. The measuring system 100 consists of a sensor system 110, a control and evaluation unit 120 and the data lines 130 which connect the sensor system and the control and evaluation unit. The sensor system 110 is connected via a thermally shielded line 102 to a sample chamber, for example a chimney 103, in which the exhaust gases are present as a hot gas mixture mixed with particles, in particular with soot. For the sampling of a gas sample 104, a pump 1 is provided, which via a line 5, a filter 6, a line 7, an acoustic resonator 10, a line 9, a valve 8, a line 11 and a photoacoustic cell 20 leads the sensor system 110. The gas sample is conducted out of the sensor system 110 again via a line 2, a valve 3, a line 2 ′ and a line 4. The line 5, which is surrounded by a thermal shield 102, tightly connects the interior of the chimney 103 to the filter 6. At the outlet of the filter 6 there is a gas mixture free of particles of any kind, in particular of soot particles, ie a cleaned gas sample 105 is available, which is fed via line 7 into resonator 10. In the resonator 10 there is a transmitter 41 and a receiver 42, which are located at a fixed distance from one another and thus meet the requirements for measuring the speed of sound in the cleaned gas sample 105. In the resonator

10, the concentration of the first gas component, for example a CO ₂ concentration, is determined via the speed of sound of the gas mixture or the cleaned gas sample 105. At the resonator 10 is a further valve 8 with the lines 9 and

11 is provided, which leads the cleaned gas sample 105 via line 11 into a compartment 21 of the photoacoustic cell 20. The compartment 21 is provided with a partition 23, which has an optical window 24. Via a valve 12 and the lines 13, 14, the cleaned gas sample 105 reaches a compartment 22, which in turn is connected to the valve 3 via a line 2. The compartment 21 has a further partition 25 with an interference filter 26, which is opposite the partition 23 and which on the compartment 21 opposite side in the compartment 27 has an IR source 28. The compartment 22 also has a detector 29 and a microphone 31 which is attached to the side of the photoacoustic cell 20. The IR source 28, the microphone 31 and the detector 29 are connected via lines 51, 51 'and 51 "to a signal processing unit 50, which in turn is connected via a line 52 to a computer unit 80. The transmitter 41 and the receiver 42 are connected via lines 61 and 61 'to a signal processing unit 60, which in turn is connected via a line 62 to the computer unit 80. A temperature probe 73 is connected via a line 71 to a signal processing unit 70, which in turn is connected via a line 72 is connected to the computer unit 80. At the output 81 of the computer unit 80, the measured values of the concentrations of the first and second gas concentrations (CO ₂ and CO concentrations) are available for further use, be it for display or for documentation of any kind cher kind, but especially for the control and regulation of combustion plants.

The method according to the invention is explained in more detail with reference to the flow chart (FIG. 2), the general steps being shown in FIG. 2. For the description of the different method steps, those reference numbers of the flow chart are used which are given for the structure of the individual steps.

2 shows the flowchart for a method for the combined determination of gas concentrations in gas mixtures, which is described by steps 1-5 below:

1. Sampling

The starting point of the process is a gas sample that is to be examined or analyzed. This is generally a mixture of several components and is usually in a contaminated form, the contaminants being able to be both liquid and solid. For example, the exhaust gas sample from an incineration plant contains soot components. The sampling has to be carried out at a location which enables a representative sample to be taken. It is made possible, for example, by means of a controlled vacuum system, in that a pump generates the necessary vacuum, and a circuit of valves which are arranged in the feed lines ensures that a sufficient amount of the gas mixture in the valve is opened during the valve opening times Analysis room is led.

2. Cleaning

The gas mixture is fed to a filter, or a soot filter, which ensures that the cleaned gas sample leaving the filter no longer has any disturbing amounts of solids due to the selected pore size. This cleaning of the gas sample is of crucial importance, since the lifespan of a measuring system essentially depends on the measures taken to avoid any kind of contamination. The pump performance, the cross-sections of the supply lines, the valve cross-sections, the filter area and the pore size of the filter, with their interdependencies, determine the valve opening times that are necessary to ensure the presence of a homogeneous sample. The gas line that is intended for sampling and the filter are designed so that the gas sample in the subsequent units cannot fall below the dew point until the measurement system is left. This is ensured by ^■ appropriate thermostatting of the measuring system.

3.1. Determination of the first gas component

After the cleaned gas sample has been fed to an acoustic resonator, it is measured there after a dwell time of a few seconds. This is done in a manner known per se by measuring the speed of sound of the cleaned gas sample. In the acoustic resonator there is a transmitter and a receiver at a fixed distance. The transmitter frequency is now determined in such a way that the electrical receiver signal is in phase with the electrical transmitter signal. This frequency is a measure of the speed of sound, from which the concentration of the first gas component, for example the CO ₂ concentration tration, is determined. This measurement technique is not discussed in more detail here (BA Younglove, NV Frederik, Int.Journal of Thermophysics, Vol. 11/5, 897-905 (1990)). It is essential that a CO concentration that may be present in the cleaned gas sample can practically not influence the measurement accuracy, because on the one hand the CO concentrations are in the range of 0-1000 ppm, ie 0-0.1%, and on the other hand because the carbon monoxide and the nitrogen, which is present in large excess, has practically identical molecular weights. The concentration of the first gas component, for example the CO ₂ concentration, is thus determined undisturbed in addition to the carbon monoxide. At the end of this process step, a frequency proportional to the concentration of the first gas component, for example the C0 ₂ concentration, is used, which is further processed in process step 4.

3.2. Determination of the second gas component

After the cleaned gas sample has been fed to a photoacoustic cell, it is measured there after a residence time of a few seconds. This is done in a manner known per se by using the photoacoustic effect. An IR light source is modulated and the modulation of the energy absorbed in the cleaned gas sample is measured via a microphone. An interference filter, which is arranged between the light source and the gas cell, is selected such that the transmission wavelength of the interference filter corresponds exactly to that absorption band of the second gas component, for example the CO absorption band. This largely suppresses disruptive influences. Further details of this measurement technology are not discussed in more detail here. At the end of this process step there is a measurement signal which has a linear relationship to the concentration of the second gas component, for example the CO concentration, and which process step 4 is processed further.

4. Signal processing

The frequency proportional to the concentration of the first gas component, for example the C0 ₂ concentration, and that to the con- Concentration of the second gas component, for example the CO concentration, a measurement signal having a linear relationship are each fed to a signal processing unit and electronically processed such that signals are present at the output of these signal processing units which can be supplied to the computer unit.

5. Evaluate

The measurement signal of the photoacoustic cell is recorded once with the valve closed and once with the valve open. The two amplitude values of the measurement signal change in different ways with the concentrations of the gas components, for example with the concentrations of the CO or the CO ₂ . This characteristic and the value of the first gas component, for example the CO ₂ concentration, determined by means of the speed of sound, allow the superimposed portion and thus the concentrations of the second gas component, for example the CO concentration, to be extracted.

The measurement signals are processed in the computer unit in such a way that digital signals are present at the output, which can be supplied, for example, to a display, or which are kept available for any other use. Operations such as linearizations, offset corrections, smoothing and averaging of any kind are possible.

The possibilities for determining the concentrations of the first and second gas components, for example the CO ₂ and CO concentrations, set out in process steps 3.1 and 3.2 represent only one type of problem solution. Furthermore, this method can be simplified, for example that the two gas concentrations are supplied to a single measuring cell, which on the one hand has the properties of the acoustic resonator, and on the other hand that of a photoacoustic cell. This naturally simplifies the necessary supply and discharge lines for the cleaned gas sample and the requirements for the control, both of which have an advantageous effect on the production costs. It is also advantageous to omit one measuring system, but for that remaining something becomes more complex.

3 shows a schematic representation of the construction of a measuring system with a combined sensor system for photoacoustic and sound measurements carried out in the same measuring cell. The measuring system 100 consists of a sensor system 110, a control and evaluation unit 120 and the data lines 130, which connects the sensor system and the control and evaluation unit. The sensor system 110 is connected via a thermal shield 102 to a sample space, for example a chimney 103, in which the exhaust gases are present as a hot gas mixture mixed with particles, in particular with soot. For sampling a gas sample 104, a pump 1 is provided, which leads the gas sample into the sensor system 110 via a line 5, a filter 6, a line 7 and a measuring cell 30. The gas sample is conducted out of the sensor system 110 again via a line 2, a valve 3, a line 2 ′ and a line 4. The line 5, which is surrounded by a thermal shield 102, connects the interior of the chimney 103 tightly to the filter 6. At the outlet of the filter 6 there is a gas mixture free of particles of any kind, in particular of soot particles, ie a cleaned gas sample 105 Available, which is led via the line 7 and an opening 153 into a compartment 121 of the measuring cell 30. The compartment 121 is provided with a partition 123 which has an optical window 124. The compartment 121 has a further partition 125 with an interference filter 126 or an IR bandpass filter, which is opposite the partition 123. Furthermore, the compartment 121 is equipped with a transmitter 152 and a receiver 151, which are at a fixed distance from one another, and thus meet the requirements for measuring the speed of sound in the cleaned gas sample 105. The line 7 ends with the opening 153, which is embedded in the receiver 152; it is chosen so small that it cannot interfere with the sound measurement. Of course, it can also be attached to another location in the compartment 121. In the compartment 121 of the measuring cell 30, the speed of sound of the cleaned gas sample 105, for example the C0 ₂ concentration, determined. A valve 112 and the lines 113, 114 are provided on the compartment 121 of the measuring cell 30, via which the cleaned gas sample 105 reaches a compartment 122, which in turn is connected to the valve 3 via a line 2 and adjoins the compartment 121 . Adjacent to the partition wall 125, on the side opposite the compartment 121, is a compartment 127 in which an IR source 128 is located. The compartment 122 also has a detector 129 and a microphone 131, which is attached to the side of the measuring cell 30. The IR source 128, the microphone 131 and the detector 129 are connected via lines 51, 51 'and 51 "to a signal processing unit 50, which in turn is connected via a line 52 to a computer unit 80. The transmitter 151 and the receiver 152 are connected via lines 61 and 61 'to a signal processing unit 60, which in turn is connected to the computer unit 80 via a line 62. A temperature probe 73 is connected via a line 71 to a signal processing unit 70, which in turn is connected via a line 72 to is connected to the computer unit 80. At the output 81 of the computer unit 80, the measured values of the concentrations of the first and second gas components, for example the CO ₂ and CO concentrations, are available for further use, be it for a display or for Documentation of any kind, but especially for the control and regulation of heating systems.

Applications for such a method and the corresponding devices are the determination of concentrations in gas mixtures, such as, for example, of carbon dioxide and carbon monoxide, in all types of exhaust gases, but especially those of heating systems and the like. With the measured parameters determined, the control and regulation of combustion processes can be significantly improved and optimized.

Of course, this method is not limited to the determination of carbon dioxide and carbon monoxide. Rather, there are a number of further gas combinations which can be determined using the described method or the corresponding devices. For example, in addition to C0 ₂ also determine NO and other nitrogen oxides; or methane (CH ₄ ), ethane (C ₂ H _β ), ethene (C ₂ H ₄ ), ethine (C ₂ H ₂ ), water and ammonia (NH ₃ ).

It is essential to the invention that the method described provides for the combination of two measuring methods in one sensor system. The gas mixture in the sensor system (110) is converted into the concentration of the first gas component, for example the CO ₂ concentration, in an acoustic resonator (10) and the concentration of the second gas component, for example the CO concentration, in a photoacoustic cell (20). The integration of the acoustic resonator (10) and the photoacoustic cell (20) in a single measuring cell (30) is a particularly advantageous exemplary embodiment.

Claims

Claims:

1. A method for determining concentrations in gas mixtures of a gas sample (104) by means of a measuring system (100), comprising a sensor system (110), a control and evaluation unit (120) and data lines (130), characterized in that the gas sample (104 ) after sampling in the measuring system (100) is passed through a filter (6) that in the cleaned gas sample (105) the concentration of a first gas component is determined by means of a sound measurement and the concentration of a second gas component is determined by means of the photoacoustic effect, and that Measurement signals (61, 61 ') of the sound measurement and the measurement signals (51, 51', 51 ") of the photoacoustic measurement are processed in a computer unit (80).

2. The method according to claim 1, characterized in that the cleaned gas sample (105) is fed to an acoustic resonator (10) in which the speed of sound of the cleaned gas sample is determined, from which the concentration of the first gas component is determined, that the acoustic Resonator (10) is connected to a photoacoustic cell (20) for transferring the cleaned gas sample via first lines (9, 11), via which the cleaned gas sample is fed to the photoacoustic cell (20), and that in the photoacoustic cell (20 ) the concentration of the second gas component of the cleaned gas sample is determined via the photoacoustic effect.

3. The method according to claim 1, characterized in that the cleaned gas sample (105) in the measuring system (100) of a measuring cell

(30) is supplied, in which the speed of sound of the cleaned gas sample is determined, from which the concentration of the first gas component is determined, and in which the concentration of a second gas component of the cleaned gas sample is determined via the photoacoustic effect.

4. The method according to any one of claims 2 or 3, characterized in that the first gas component is carbon dioxide and the second gas component is carbon monoxide.

5. The method according to any one of claims 2 or 3, characterized in that the first gas component is carbon dioxide and the second gas component is nitrogen monoxide or a higher nitrogen oxide (N0 _X ).

6. The method according to any one of claims 2 or 3, characterized in that the first gas component is water and the second gas component is nitrogen monoxide or a higher nitrogen oxide (N0 _X ).

7. The method according to any one of claims 2 or 3, characterized in that the first gas component water and the second gas component methane (CH _< ), ethane (C ₂ H ₆ ), ethene (C ₂ H _< ) or ethyne (C ₂ H ₂ ) is.

8. The method according to any one of claims 2 or 3, characterized in that the first gas component is water and the second gas component is ammonia (NH ₃ ).

9. Device for carrying out the method according to one of claims 4-8, characterized in that an acoustic resonator (10) is provided for determining the concentration of the first gas component, that a photoacoustic cell (20) is provided for determining the concentration of the second gas component and that the photoacoustic cell (20) has two compartments (21, 22) lying one behind the other, which can be optionally connected acoustically via second lines (13, 14) and a valve (12).

10. The device according to claim 9, characterized in that a connection via the first lines (9, 11) is provided between the acoustic resonator (10) and the photoacoustic cell (20).

11. The device according to claim 10, characterized in that the acoustic resonator (10) has a transmitter (41) and a receiver (42), the receiver being arranged at a fixed distance from the transmitter, and in that means for operating the acoustic resonator (10) are provided.

12. The device according to claims 9-11, characterized in that a computer unit (80) is provided for the control of the acoustic resonator (10) and the photoacoustic cell (20), in particular for sampling.

13. Device for carrying out the method according to claims 1-8, characterized in that the acoustic resonator (10) and the photoacoustic cell (20) are integrated in a single measuring cell (30).

14. The apparatus according to claim 13, characterized in that a computer unit (80) is provided for controlling the measuring cell (30), in particular for sampling.

15. Use of a device according to claims 13 or 14 for the determination of carbon dioxide and carbon monoxide in the exhaust gas monitoring of heating systems.

16. Use of a device according to claims 13 or 14 for the determination of carbon dioxide and nitrogen monoxide in combustion technology.

17. Use of a device according to claims 13 or 14 for the determination of carbon dioxide and higher nitrogen oxides (NO _x ) in combustion technology.