Measuring devices for determining the gross or net calorific value of fuel gases containing hydrocarbons

Abstract

The invention relates to a measuring device for determining the gross or net calorific value of a fuel gas containing hydrocarbons, wherein the gross or net calorific value of a fuel gas (3) containing hydrocarbons is determined by means of a Raman photometer (1), wherein the Raman radiation (15) obtained after the interaction of a laser (6) with the fuel gas (3) is limited by means of a bandpass filter (20) to the wavelength of 2900 cm-1 of the C-H-stretching vibrations of the hydrocarbons contained in the fuel gas (3). ‑1 The wavelength of hydrogen is about 4155 cm and is transmitted to a photomultiplier tube (23) and integrated by the photomultiplier tube, wherein the gross calorific value or net calorific value (26) is determined based on the output signal (24) of the photomultiplier tube (23). ‑1 By extending the Raman photometer (1) with an additional measuring channel in order to determine the hydrogen concentration, it is also possible to determine the gross calorific value of a hydrocarbon mixture containing hydrogen.

Description

Technical Field

[0001] The invention relates to a method for determining the gross or net calorific value of a hydrocarbon-containing fuel gas.

[0002] The invention also relates to a measuring device for determining the gross or net calorific value of a hydrocarbon-containing fuel gas. Background Art

[0003] DIN EN ISO 6976:2016 specifies methods for calculating variables relevant to combustion technology, such as the gross calorific value, net calorific value, density, relative density, and gross and net Wobbe index of natural gas, natural gas exchange gas, and other fuel gases, when the components of the gas mixture are known in terms of mole fractions [1]. The specified methods enable the properties of the gas mixture to be calculated under commonly used reference conditions.

[0004] The composition of natural gas is usually measured with the aid of a gas chromatograph (GC), in which the components of a gas mixture are separated by chromatography and successively detected. The combustion technology-related variables of interest can then be calculated from the quantitatively determined components. The entire process usually takes several minutes. Although the accuracy of the combustion value determined with a gas chromatograph is less than 0.1%, a faster, less precise determination of the gross calorific value can be desirable for mixing applications and gas turbines.

[0005] The composition of natural gas or similar gases can also be determined by means of infrared spectroscopy or Raman spectroscopy [2], [3], [4]. In this case, the calculation of the variables of interest relevant to combustion technology is carried out based on the previously quantitatively determined gas composition.

[0006] Raman spectroscopy is based on the inelastic scattering of laser light at a sample. The scattered radiation (Raman radiation) is shifted in frequency (Raman shift) compared to the incident light and is detected with the aid of a spectrometer with a downstream CCD array. The frequency shift between the incident and scattered light is characteristic of the vibrational and rotational structure of the chemical substance of the sample. The Raman spectrum of a substance represents a unique molecular fingerprint and can therefore be used for analytical purposes.

[0007] In practice, however, the spectral resolution and sensitivity of a CCD array are not sufficient to determine the composition of the fuel gas, since high spectral resolution requires small pixels, which in turn produce only weak signals. Summary of the invention

[0008] The object of the invention is to be able to determine the gross calorific value or the net calorific value quickly and with relatively little effort.

[0009] According to the invention, this object is achieved by the method defined in the invention and by the measuring device defined in the invention, advantageous developments of the method and of the measuring device being specified in the invention.

[0010] The invention therefore relates to a method for determining the gross or net calorific value of a hydrocarbon-containing fuel gas by means of a Raman photometer, wherein the Raman radiation obtained after the interaction of the laser with the fuel gas is restricted by means of a bandpass filter to the wavelength of 2900 cm-1 of the CH-stretching vibrations of the hydrocarbons contained in the fuel gas. -1 The Raman radiation is transmitted to a photomultiplier tube and comprehensively detected by the photomultiplier tube, and the gross calorific value or the net calorific value is determined based on the output signal of the photomultiplier tube.

[0011] The invention also relates to a measuring device for determining the gross or net calorific value of a hydrocarbon-containing fuel gas, the measuring device having a Raman photometer with a bandpass filter and a photomultiplier tube, the Raman photometer being designed to limit the Raman radiation obtained after the interaction of laser light with the fuel gas to the 2900 cm-1 region of the CH-stretching vibration of the hydrocarbons contained in the fuel gas by means of the bandpass filter. -1 The Raman radiation is transmitted to a photomultiplier tube in a wave number range of about 1000 Å and the measuring device has an evaluation device which determines the gross calorific value or the net calorific value from the output signal of the photomultiplier tube.

[0012] According to the invention, a Raman photometer is used instead of a Raman spectrometer. Unlike the method of determining all the main components in the fuel gas by means of a Raman spectrometer, the determination of the gross or net calorific value is achieved via spectral integration. The invention is based on the discovery that at 2900 cm -1 The area integral of the Raman features of the hydrocarbons contained in the fuel gas (hereinafter referred to as the Raman signal) within the narrow range of the Raman shift of the fuel gas is related to the gross calorific value and (in practice less relevant) the net calorific value of the fuel gas.

[0013] The hydrocarbons included in the determination of the gross or net calorific value according to the invention are in particular methane and ethylene and also higher alkanes and isomers of alkanes up to pentane.

[0014] In order to obtain a sufficiently strong Raman signal, the determination of the gross or net calorific value can be carried out when the pressure of the fuel gas is increased from 1 bar absolute to 10 bar absolute, preferably at least 5 bar absolute.

[0015] The determination of the gross or net calorific value can also be preferably carried out at a laser wavelength of 405 nm ± 10 nm, wherein the wavelength at 2900 cm is selectively detected by means of a bandpass filter with a central wavelength of 459 nm ± 13 nm and a half-value width of 5 to 10 nm. -1 The short laser wavelength of 405 nm does not cause any fluorescence in the gas and results in a good yield of Raman photons. In addition, the corresponding lasers are available at reasonable prices as diode lasers and have sufficient optical power. The scattering of the laser light is typically ± 5 nm.

[0016] Alternatively, Raman excitation can take place at a laser wavelength of 450 nm ± 10 nm, wherein the central wavelength of the bandpass filter is then 518 nm. The advantage of a longer wavelength is that the bandpass filter is more selective in the wavelength band.

[0017] In order to improve the linear regression of the correlation between the Raman signal and the gross or net calorific value in the case of fuel gases with a relatively high or low methane content, in addition to the area integration of the Raman features of the CH stretching vibrations, reference can also be made to the Raman features around the CC main vibrations with a wavelength of 990 cm -1 Area integration of the broad Raman-shifted Raman features of hydrocarbons with more than one carbon atom (ie, all hydrocarbons except methane) on the left and right.

[0018] The Raman spectrometer is extended by using an additional measurement channel for hydrogen at about 4155 cm -1 The Raman shift of the sensor can be used to determine the concentration of hydrogen and the gross calorific value of hydrocarbon mixtures containing hydrogen. This enables applications in the field of adding hydrogen to natural gas or biomethane and injecting biogas into natural gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is explained below with the aid of exemplary embodiments and with reference to the drawings, in which:

[0020] Figure 1 An example of a measuring device according to the invention with a Raman photometer is shown,

[0021] Figure 2 shows a stereogram of a Raman photometer,

[0022] Figure 3 shows the hydrocarbons at 2900 cm -1 Raman features around the Raman shift,

[0023] Figure 4 An example of the correlation of the Raman signal obtained using a Raman photometer with the gross calorific value of different natural gas mixtures is shown.

[0024] The same reference numerals in the various figures have the same meaning. These representations are purely schematic and do not represent any relative scale. DETAILED DESCRIPTION

[0025] Figure 1 A measuring device with a Raman photometer 1 is shown in a simplified schematic diagram.

[0026] Figure 2 The Raman photometer 1 is shown in a perspective view.

[0027] The Raman photometer 1 has a pressure-tight (10 bar) measuring cell 2 through which a fuel gas 3, for example natural gas, flows at an increasing, controlled pressure of up to 10 bar absolute. The fuel gas 1 enters the measuring cell 2 via a gas inlet 4 and leaves the measuring cell at two gas outlets 5, wherein the gas outlets 5 are connected to the measuring cell 2. Figure 1 In contrast to the schematic diagram in the figure, it is preferred to lead upward and downward from the drawing plane (see Figure 2 ). The laser beam 6 of the laser 7 passes through the aperture 8, falls into the measuring cell 2 via the focusing lens 9 and the interference filter 10 and penetrates the measuring cell in the direction of the gas inlet 4, where it is captured in the backscatter-free light capturer 11. The linear section (beam waist) of the laser beam (Gaussian beam) 6 focused by the lens 9 is located in the center of the measuring cell 2.

[0028] For the Raman photons scattered at the molecules of the fuel gas 3, a first receiving optical system 12 is arranged at right angles to the laser beam 6 at the measuring cell 2. The receiving optical system 12 consists of two stages 13, 14, wherein in the first stage 13, the scattered light 15 is focused onto a rectangular aperture 18 of approximately 1×4 mm by means of one or more lenses 16, 17. This achieves a spatial filtering of the scattered light 15, so that only those photons successfully reach the second stage 14, which are scattered from the confined volume around the focused linear section of the laser beam 6. The first lens 16 can serve as a pressure-tight connection of the measuring cell 2.

[0029] In the second phase 14, the light 15 scattered in parallel by means of a lens 19 passes through a bandpass filter 20 in the form of a narrowband interference filter or, if necessary, in the form of two interference filters arranged one behind the other. The photons selected by the bandpass filter 20 in a gas component-specific manner are imaged by means of a further lens 21 onto a rectangular photocathode 22 (approximately 1×4 mm) of a photomultiplier tube 23, by means of which the photons are detected individually. The photomultiplier tube 23 generates an output signal (Raman signal) 24 which is proportional to the number of photons absorbed by the photocathode 22 per unit time and supplies it to an evaluation device (processor) 25 for evaluation and determination and output of the gross calorific value 26 of the fuel gas 3.

[0030] Preferably, a laser 7 with a wavelength of 405 nm is used. This short wavelength does not cause any fluorescence in the gas and leads to a good yield of Raman photons. In addition, the corresponding laser 7 is a diode laser at a reasonable price. The laser power can be limited to below 35 mW to meet explosion protection, wherein a detection limit of methane <100 ppm can be achieved at a measurement duration of 1 second. The beam quality and line width of the laser beam 6 are improved by the aperture 8 and the narrowband interference filter 10, so that the background signal in the measuring cell 2 is at a low level. In addition, the background signal is greatly reduced by the arrangement and design of the gas outlet 5 and the light trap 11. The photomultiplier 23 is used to convert the Raman photons that are only generated in very small quantities, especially due to the low laser power, into a sufficiently strong Raman signal 24. In addition, when the pressure increases, the measurement is realized in the measuring cell 2 of, for example, 5 bar absolute, because the number of Raman photons generated and thus the Raman signal 24 increases proportionally with the measured pressure. For this purpose, a pressure regulator can be provided at the combined gas outlet 5 if necessary.

[0031] In order to be able to operate the measuring device in an industrial environment with higher laser powers (> 35 mW), the measuring unit 2 can also be embodied in a pressure-tight enclosure (Ex-d).

[0032] Figure 3 shows a cross-section of the Raman spectrum of the main components of natural gas (here methane, ethane, propane and butane) and the minor components nitrogen and hydrogen sulfide. Natural gas also contains low concentrations of higher hydrocarbons and various isomers at <0.1%. As shown in the figure, the Raman shift caused by the CH-stretching vibration of hydrocarbons is about 2900 cm -1 The Raman shift is roughly the same for all hydrocarbons and there are no other chemical bonds in the natural gas mixture 3. For the central wavelength λ of the bandpass filter 20, the laser wavelength λ ex The relationship between Raman Shift and RS applies to:

[0033] 1 / λ[nm]=1 / λ ex [nm]-RS[cm -1 ] / 10 7 .

[0034] In the exemplary embodiment of the Raman photometer 1 described here, a bandpass filter 20 with a central wavelength of approximately 459 nm and a full width at half maximum (FWHM) of 5 to 10 nm is selected so that the mentioned CH stretching vibrations are detected.

[0035] The determination of the gross calorific value with the Raman photometer 1 is not carried out via the determination of the individual combustible components of the fuel gas, but rather via the integral measurement of the concentration of the "combustible chemical bonds" in the relevant hydrocarbons. These are mainly CH-bonds, according to the CH-bond's wavelength of about 2900 cm -1 The Raman shift of the natural gas 3 is detected by means of a bandpass filter 20. It is determined that the Raman signal 24 is correlated with the gross calorific value of the natural gas 3. The correlation occurs because ethane, propane and higher hydrocarbons have a higher Raman effective cross section due to the increased number of CH bonds per unit volume and therefore have a higher Raman effective cross section at 2900 cm -1 The area integration of the Raman feature near the Raman shift of about 1.3° produces a higher scattered light signal. Therefore, in order to have the best correlation with the gross heating value, the position of the central wavelength and the half-value width of the bandpass filter 20 must be correctly selected as described above so that the entire feature is detected, especially in the case of higher alkanes.

[0036] exist Figure 4 In Figure 2 , a correlation is graphically shown based on the Raman signals 24 obtained for 12 natural gas mixtures with known gross calorific values. Measurements #1 to #12 were carried out at an absolute pressure of 5000 hPa, at a flow rate of 1000 ml / min of the measuring gas 3 through the measuring chamber 2 and at a temperature adjusted to a constant 25°C. The gross calorific value statement was determined by the supplier of the natural gas mixture with the aid of GC analysis and stated on the certificate of analysis. The background signal of nitrogen (only about 0.01% of the CH-Raman signal) was subtracted from the Raman signal 24. The regression line for the 12 measurement points (filled in white) is drawn with a solid line. There are 4 clear outliers among the 12 measurement points, of which the outliers #4, #8, #10 below the regression line are obtained from the natural gas mixture with the lowest methane content and the outlier #11 above the regression line is obtained from the natural gas mixture with the highest methane content. It can be seen from these results that higher alkanes are still underestimated in the Raman measurement method. An expansion of the bandpass filter 20 can bring about an improvement in the correlation coefficient. From Figure 4 Based on the measurements, it was possible to deduce that the uncertainty in the determination of the gross calorific value with the aid of the Raman measurement method is 1.5%.

[0037] If the outliers #4, #8, #10, #11 are ignored, the dotted regression line is obtained. Based on the maximum deviation, the uncertainty in determining the gross calorific value is only 0.5% of the gross calorific value according to the certificate.

[0038] In addition to disregarding the outliers #4, #8, #10, #11, these values ​​can also be corrected as a function of the methane concentration in the fuel gas 3. For this purpose, the methane concentration can be measured separately, for example, by means of a selective non-dispersive infrared (NDIR) gas analyzer.

[0039] Alternatively, the Raman radiation 15 is limited in the second receiving optical system 27 by means of a further bandpass filter (interference filter) 28 to the 990 cm-1 band of the CC vibration of hydrocarbons contained in the fuel gas 3 and having more than one carbon atom. -1 The measured values ​​#1 to #12 can be corrected by a higher weighting of all hydrocarbons except methane over a wave number range of about 1000 nm and integrated detection is performed in the second photomultiplier tube 29. The second receiving optical system 27 is installed at the measuring unit 2 relative to the first receiving optical system 12 and has the same structure as the first receiving optical system except for the bandpass filter 28. The correction can be performed in a simple manner, that is, the output signal (Raman signal) 30 of the second photomultiplier tube 29 is added to the output signal 24 of the photomultiplier tube 23 with the calibration factor in the evaluation device 25. The corrected measurement point is Figure 4 The center is filled in black and the regression line for this measurement point is shown as a dotted line.

[0040] If a mixture of hydrogen and natural gas is used in the future (e.g. to store regenerative energy in the natural gas grid), the hydrogen concentration in the gas mixture 3 must also be taken into account when determining the gross calorific value. The Raman shift of hydrogen is at 4155 cm -1 The second receiving optical system 27 is very suitable for this purpose. Therefore, by simply replacing the bandpass filter 28 in the second receiving optical system 27 with another suitable bandpass filter 28 ', or by means of a filter slider or filter wheel containing the corresponding required bandpass filters, other components besides hydrogen, such as carbon dioxide, nitrogen and hydrogen sulfide, can be measured. By including nitrogen measurement or carbon dioxide measurement, more accurate results for the determination of the gross calorific value or net calorific value can be obtained in a larger range of methane concentrations if necessary. The table below lists the required center wavelengths of the corresponding bandpass filters.

[0041]

[0042] For the simultaneous measurement of different components, the Raman photometer 1 can also be equipped with more than the two receiving optical systems 12 , 27 shown here, wherein further receiving optical systems can be installed on the measuring unit 2 , in particular in a direction perpendicular to the recording plane.

[0043] Just for the sake of illustration, Figure 1 Not shown is a window through which the laser beam 6 enters the measuring cell 2 and through which the Raman radiation 15 exits the measuring cell 2. The window can be formed by the interference filter 10 and a lens, for example 16, of the receiving optical system 12, 27.

[0044] The Raman measurement method according to the invention can be easily calibrated using a series of calibration gases. The measurement is performed in a continuous gas flow, wherein a result is obtained every second.

[0045] References:

[0046] [1] DIN EN ISO 6976:2016: Natural gas – Calculation of gross and net calorific values, density, relative density and Wobbe index from constituent components

[0047] [2] EP 1 174 705 A1

[0048] [3] Johannes Kiefer: Recent advances in characterization of gaseous and liquid fuels by vibrational spectroscopy. Energies 2015, 8, 3165-3197

[0049] [4] Christiaan Mul, Master thesis: Raman spectroscopy for natural gas process applications - an instrumental and operational investigation in theory and practice, 12.12.2017, Vrije Universiteit Amsterdam, Analytical Solutions and Products BV

Claims

1. A method for determining the gross or net calorific value of a hydrocarbon-containing fuel gas (3) by means of a Raman photometer (1), wherein: The Raman radiation (15) obtained after the laser (6) interacts with the fuel gas (3) is limited to the CH-stretching vibration of the hydrocarbon contained in the fuel gas (3) at 2900 cm by means of a bandpass filter (20). -1 The Raman radiation (15) is transmitted to a photomultiplier tube (23) and detected by the photomultiplier tube for integration, and the gross calorific value or net calorific value (26) is determined based on an output signal (24) of the photomultiplier tube (23), wherein the Raman radiation (15) is limited to the 990 cm-1 band of the CC vibration of hydrocarbons with two or more carbon atoms contained in the fuel gas (3) by means of a further bandpass filter (28). -1 The Raman radiation is transmitted to another photomultiplier tube (29) and integrated and detected by the photomultiplier tube, and the output signal (30) of the other photomultiplier tube (29) is used to correct the gross calorific value or net calorific value (26) determined based on the output signal (24) of the photomultiplier tube (23).

2. The method according to claim 1, characterized in that The hydrocarbons include C1- to C5-alkanes and isomers of alkanes.

3. The method according to claim 1 or 2, characterized in that: The determination of the gross or net calorific value is carried out when the pressure of the fuel gas (3) is increased from 1 bar absolute to 10 bar absolute.

4. The method according to claim 1 or 2, characterized in that: The determination of the gross calorific value or the net calorific value is achieved when the pressure of the fuel gas (3) is at least 5 bar.

5. The method according to claim 1 or 2, characterized in that: When the wavelength of the laser (6) is 405 nm ± 10 nm or 450 nm ± 10 nm, the determination of the gross calorific value or the net calorific value is achieved.

6. The method according to claim 5, characterized in that A bandpass filter (20) having a central wavelength of 459 nm±13 nm or 518 nm±13 nm and a half-value width of 5 to 10 nm is used.

7. The method according to claim 1 or 2, characterized in that: In order to determine the gross or net calorific value of the fuel gas (3) which contains hydrogen in addition to the hydrocarbons, the Raman radiation (15) is limited to 4155 cm -1 The output signal (24) of the photomultiplier tube (23) and the output signal (30) of the other photomultiplier tube or the additional photomultiplier tube (29) are transmitted to the other photomultiplier tube (29) or the additional photomultiplier tube for integrated detection, and the gross calorific value or the net calorific value (26) is determined based on the output signal (24) of the photomultiplier tube (23) and the output signal (30) of the other photomultiplier tube or the additional photomultiplier tube (29).

8. A measuring device for determining the gross or net calorific value of a hydrocarbon-containing fuel gas (3), the measuring device comprising a Raman photometer (1) comprising a bandpass filter (20) and a photomultiplier tube (23) as well as a further bandpass filter (28) and a further photomultiplier tube (29), the Raman photometer being designed to limit the Raman radiation (15) obtained after the interaction of laser light (6) with the fuel gas (3) to the 2900 cm-wavelength band of the CH-stretching vibrations of the hydrocarbons contained in the fuel gas (3) by means of the bandpass filter (20). -1 The Raman radiation (15) is transmitted to the photomultiplier tube (23) in a wave number range of about 2000 cm / s and supplied to the photomultiplier tube (23), and the measuring device has an evaluation device, which determines the gross calorific value or the net calorific value (26) based on the output signal (24) of the photomultiplier tube (23), and the Raman photometer (1) is designed to limit the Raman radiation (15) to the 990 cm / s band of the CC vibration of hydrocarbons with two or more carbon atoms contained in the fuel gas (3) by means of the further bandpass filter (28). -1 The Raman radiation is transmitted to the other photomultiplier tube (29) and is integrated and detected by the other photomultiplier tube, and the evaluation device uses the output signal (30) of the other photomultiplier tube (29) to correct the gross calorific value or net calorific value (26) determined based on the output signal (24) of the photomultiplier tube (23).

9. The measuring device according to claim 8, for carrying out the method according to any one of claims 1 to 7.

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