GB2560870A

GB2560870A - Gas concentration measurement apparatus

Info

Publication number: GB2560870A
Application number: GB1620408.3A
Authority: GB
Inventors: Harley Phil; Shepherd Bill
Original assignee: PHOTON FIRE Ltd
Current assignee: PHOTON FIRE Ltd
Priority date: 2016-12-01
Filing date: 2016-12-01
Publication date: 2018-10-03
Also published as: GB201620408D0

Abstract

A gas concentration measurement apparatus 1 comprises a gas cell with a body having gas inlet 3, a gas outlet 4 and a closed void 5 therebetween, a primary light source 20 emitting monochromatic light of a first wavelength, a secondary light source 22 emitting monochromatic light of a second, different wavelength and a primary light sensor 21. The sources 20, 22 may emit square waves in antiphase. The wavelength of the first source 20 may correspond to an absorption peak of the gas to be detected, for example carbon dioxide. The light sources 20, 22 and sensor 21 are connected to the void 5 via optical fibres 12, 13. A reference detector 23 may receive light from the sources 20, 22 that has not passed through the gas in the void 5. Values of measured and reference light from each of the sources 20, 22 may be used to calculate a gas concentration.

Description

(54) Title of the Invention: Gas concentration measurement apparatus

Abstract Title: Measuring gas concentration with two light sources of different wavelength passing through a gas cell (57) A gas concentration measurement apparatus 1 comprises a gas cell with a body having gas inlet 3, a gas outlet 4 and a closed void 5 therebetween, a primary light source 20 emitting monochromatic light of a first wavelength, a secondary light source 22 emitting monochromatic light of a second, different wavelength and a primary light sensor 21. The sources 20, 22 may emit square waves in antiphase. The wavelength of the first source 20 may correspond to an absorption peak of the gas to be detected, for example carbon dioxide. The light sources 20, 22 and sensor 21 are connected to the void 5 via optical fibres 12, 13. A reference detector 23 may receive light from the sources 20, 22 that has not passed through the gas in the void 5. Values of measured and reference light from each of the sources 20, 22 may be used to calculate a gas concentration.

FIGURE 1

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Gas Concentration Measurement Apparatus

Field of the Invention

The present invention relates to an apparatus for measuring the concentration of specified gases in a gaseous mixture and specifically to measuring small deviations in gas concentrations in harsh environments.

Background of the Invention

In some situations there is a requirement to measure changes in the concentration of a particular gas in a gaseous mixture, where the measured concentration of the particular gas is liable to fluctuate, but forms a major constituent of all the gases in the gaseous mixture. One such situation is in the field of carbon capture and storage. In particular, where carbon dioxide is to be stored, for example in disused oil wells appropriate geological strata or mine workings, and where there is a financial consequence related to the amount of carbon dioxide stored, it necessary to measure not only the volume of gas stored, but also the purity of that gas. Exhaust gas resulting from the combustion of carbon based fuels contains a very large amount of carbon dioxide, but also other compounds in gaseous form. The actual proportion of carbon dioxide in such an exhaust gas stream will depend on the method of carbon capture employed, etc. Hence, in order to verify how much carbon dioxide is present in a stream destined for storage it is necessary to ascertain not just the volume of gas but the concentration of carbon dioxide therein.

Concentrations of carbon dioxide in an exhaust gas from burning a carbon based fuel may be close to 100% and may fluctuate by only very small amounts, typically from fractions of one percent to up to 5%. The task of measuring changes in concentration of a particular gas in a gaseous mixture where the concentration of the measured gas is high is not straightforward.

Exhaust gases from the combustion of carbon based fuels are typically hot, in excess of 125C and may be at high pressure, with operating pressures often exceeding 30 bar.

It would be desirable to develop a gas concentration measurement apparatus that is capable of measuring small fluctuations in the concentration of a gas in a mixture of gases, in particular where the measured gas is the major constituent of the mixture. Further, it would be desirable to develop such a gas sensor that is capable of operating at high temperatures and pressures.

One typical method of measuring the concentration of carbon dioxide in a gaseous mixture involves passing the gaseous mixture through a measurement channel with an infra-red source positioned to shine infra-red light through the measurement channel towards an infra-red sensor.

However such a system, sometimes described as a single beam measurement system, has the shortcoming that fluctuations in the intensity of the infra-red light source, together with undesirable absorption of the light by other gasses, vapours, and suspended particiulate, present in the channel, may erroneously be ascribed to fluctuations in the concentration of carbon dioxide.

Summary of the Invention

According to the invention there is provided a gas concentration measurement apparatus comprising a gas cell that includes a body having gas inlet and a gas outlet and a closed void therebetween, a primary light source, a secondary light source and a primary light sensor, wherein the primary and secondary light sources and the primary light sensor are situated remotely with respect to the body and are optically connected thereto, to shine light into the void, by a plurality of optical fibres, wherein at least one optical fibre extends between the primary light source and the body to a first side of the void, at least one optical fibre extends between the secondary light source and the body to the first side of the void and at least one optical fibre extends between the body to a second side of the void and the primary light sensor, wherein the optical transmission properties of the plurality of optical fibre cables are substantially the same, and wherein the primary light source and the secondary light source are each configured to emit substantially monochromatic light sources of different wave lengths.

Preferably, the optical fibres are comprised in at least two optical fibre cables.

The optical fibres extending between the primary and secondary light sources and the body may be comprised in an optical fibre cable that includes a primary bifurcation providing said cable with at least three ends, the ends of primary bifurcation being connected to the primary and secondary light sources respectively and the third end of the cable being connected to the void.

The optical fibres in the optical fibre cable may be divided into two substantially equal numbers of optical fibres at the primary bifurcation.

The gas concentration measurement apparatus may further comprise a reference light sensor.

The ends of the fibre optic cables may be encapsulated, preferably in a ferrule.

The primary and secondary light sources may be substantially monochromatic light emitting diode’s of differing wavelengths. In another embodiment the primary and secondary light sources comprise a polychromatic source and a filter. In yet another embodiment the monochromatic primary and secondary light sources comprise lasers.

Advantageously, the wavelength of the primary light source is selected to correspond to an absorption peak of the gas, the fluctuation in concentration of which is being measured. Typically, gases display a number of absorption peaks at different wavelengths of incident light. Some of those absorption peaks are more significant than others.

More advantageously, the wavelength of the primary light source is selected to correspond to an absorption peak of the gas, the fluctuation in concentration of which is being measured, which absorption peak is not the most prominent displayed by the gas in question. This provides the advantage that the light sensor is less likely to become saturated and thereby facilitates the measurement of small changes in gas concentration.

Preferably, the primary light source emits substantially monochromatic light having a wavelength of 2.0 micron and the secondary light source emits substantially monochromatic light having a wavelength of 1.8 micron. Whilst any wavelength could be chosen for the secondary light source provided that it is non-coincident with an absorption line of the gas to be detected, it is preferable to use a wavelength that is similar to the primary light source wavelength in order that

Mie-scatter from any particulate present in the gas stream provides similar apparent absorption at both measurement and reference wavelengths, and also that it is similarly affected by broad spectrum absorbers, for instance water vapour, that could potentially be present in the gas stream.

The apparatus may include a temperature sensor and/or a pressure sensor arranged to sense the temperature and/or pressure of the gas the concentration of which is being measured.

The apparatus may include a controller, the controller configured to receive as inputs the electronic output signals of the primary and reference light sensors and the controller programmed with an algorithm that converts the output of the light sensors to an output signal representative of the concentration of the gas passing through the closed void. It is preferred that the algorithm modifies the output signal of the primary light sensor in accordance with the electronic output signal of the secondary light sensor.

Preferably, the outputs of the temperature and/or pressure sensor form inputs to the controller, and more preferably, the algorithm that converts the output of the light sensor to an output signal representative of the concentration of the gas passing through the closed void is modified in accordance with the electronic output signal of the temperature and/or pressure sensor.

This is advantageous because the absorption spectrum changes with both temperature and pressure, and in particular in relation to pressure.

Preferably, the distance between the respective ends of the first and second optical fibre cables across the void of the gas cell is between 20mm and 100 mm, and more preferable, the distance between the respective ends of the first and second optical fibre cables across the void of the gas cell is 23.5mm.

The gas concentration measurement apparatus of the invention allows the concentration of a particular gas within a gaseous mixture to be monitored continuously and in an environment of high pressure (30 bar or greater) and temperature (typically in excess of 125 C). The use of fibre optic cable allows sensitive electronics to be located at a distance from the harsh environment of the gas cell.

The primary and secondary light sources are alternately enabled in time such that only one light source emits light at any one time.

The use of a doubly bifurcated optical fibre cable allows the light from both the primary and secondary light sources to be combined in approximately equal measure for presentation to the gas cell void, whilst also allowing a portion of the light from the primary and secondary light sources to be presented to a secondary light sensor in order to provide compensation for fluctuations in the emission intensity of the primary and secondary light sources.

Brief Description of the Drawings

In the Drawings, which illustrate preferred embodiments of a gas concentration measurement apparatus according to the invention:

Figure 1 is a schematic diagram of the apparatus;

Figure 2 is cross-sectional schematic diagram of a component of the apparatus shown in

Figure 1;

Figure 3 is a schematic representation of a bifurcated fibre optic umbilical;

Figure 4 is a graph representing light emission from a light emitting diode and CO2 absorption; and

Figure 5 is a schematic representation of operation of the apparatus illustrated in Figures 1 to 3.

Detailed Description of the Drawings

Referring first to Figures 1 and 3, gas concentration measurement apparatus 1 comprises a body in the form of a four-way union tube fitting 2. The union tube fitting provides a gas inlet 3 and a gas outlet 4 situated to either side of a void 5. Gas carrying conduits 8, 9 are connected to the gas inlet 3 and gas outlet 4 respectively by connectors 10, 11.

Optical fibre cables 12, 13 are connected to ports 6, 7 in opposing sides of the body 2 by means of securing nuts 14, 15 respectively. The ends of the optical fibre cables 12, 13 that attach to the ports 6, 7 are each provided with a ferrule, as described in greater detail below with reference to Figure 3.

The fitting 2 is an industry standard component for use in high temperature and pressure environments. The ferrule is configured and provided with a suitable seal so that it may be held in place in the fitting by the securing nut and olive 15 with the end of the optical fibre cable situated in the port 6, 7 of the union.

Referring now to Figure 2, gas in the conduit 8 enters the void 5 through inlet 3 and exits through outlet 4 into the conduit 9. The void 5 is filled with the gas. Light of a prescribed wavelength is shone through the void 5 from the light source 20, via the optical fibre cable 12 to the port 6 which is aligned with port 7 where an end of the optical fibre cable 13 is situated. The other end of the optical fibre cable 13 is connected to a primary light sensor 21.

Where a beam of monochromatic light of a certain wavelength traverses a gas cell it is attenuated according to the Beer Lambert Law, which states that the log of relative intensities of the illuminating, and emerging beam will vary according to the distance traversed in the gas, the concentration of the gas, and the absorption coefficient of the gas at the wavelength of the light that traverses the gas. Hence, where the distance traversed by the light and its wavelength are constant the variation in the log of relative intensity is dependent on the concentration of the gas in the void 5 at any particular moment.

The apparatus also includes a secondary light source 22, the output of which is used in the calculation of gas concentration and a power illumination reference light sensor 23. The wavelength of the secondary light source 22 is different to that of the primary light source 20. The light sources 20 and 22 are both near infra-red light emitting diodes in the illustrated embodiment and include mirrored parabolic reflectors, or lenses. The purpose the power illumination reference light sensor is to provide for the correction of errors in the signal received by light sensor 21. It is known that the intensity of light emitted by a near infra-red light emitting diode fluctuates over time.

It is also known that particles and other gases and vapours can cause undesirable obscuration of the ends of the optical fibre cables 12, 13 mounted in the ports 6, 7. It is desirable to be able to correct for such errors. Doubly bifurcated optical fibre cable 12 has one pair of ends to collect light from the infra-red light emitting diodes 20, and 22, and a second pair of ends to conduct light from both sources to the four-way union tube fitting 2, and also to the power illumination reference detector 23, the latter bypassing the four-way union tube fitting 2. Hence, the power illumination reference sensor permits fluctuations in the intensity of light emitted by the light sources 20, and

22, to be corrected. Of the individual optical fibres in the doubly bifurcated cable 12, 95% of the optical fibres connected to each of the light emitting diodes 20, 22 terminate at the fitting 2 and 5% terminate at the power illumination reference light sensor 23.

In relation to obscuration of the ends of the optical fibre cables 12, 13 the apparatus may be calibrated in the absence of any gas of interest in the void 5. The difference between the output signals of the light sensors 21,23 can be measured. After a period of use, again with the void empty of gas, a measurement of the difference between the output signal of the light sensors 21, in comparison with the same measurement made when the ends of the optical fibre cables were clean will reflect the extent to which the said ends of the optical fibre cables have been obscured.

Referring to Figure 3 in particular, the optical fibre cable 12, which comprises many discrete optical fibres, is bifurcated to each side of a central portion 12a. The optical fibres of the cable formed into the left hand bifurcations 12b, 12c are terminated with respective ferrules 12b’, 12c’.

The optical fibres of the cable formed into the right hand bifurcations 12d, 12e are also terminated with respective ferrules 12d’, 12e’. The ferrules 12b’ to 12e’ provide for easy attachment of the optical fibre cable to the port 6, the light sources 20, 22 and the power illumination reference sensor 23. The ends of the fibre optic cable 13 are similarly provided with ferrule terminations for attachment to the port 7 and primary light sensor (which may also be considered as an absorption detector) 21. The individual fibres within the fibre optic cable, typically comprising several hundred in number, are divided as they emerge from the central section such that half the individual optical fibres are constrained in 12b, and 12c respectively, whilst an more than half of the individual optical fibres are constrained in 12d and less than half of the individual optical fibres are constrained in12e. Typically 95% of the fibres are present at 12d, whilst 5% of them are presented at 12e. This arrangement is in order that the maximum amount of light from the two sources is available for measurement of the gas concentration, whilst the smaller amount of light available to the secondary sensor ultimately equates to the light received at the primary sensor as a result of losses in the optical coupling efficiency of the gas cell 5.

Figure 4 illustrates: the spectrum of light emitted by the light emitting diode 20, which has a nominal output wavelength of 2.0 micron, the spectrum being the outermost bell curve (a); the wavelength spectrum passing through an interference filter which is represented by the bell curve (b). As can be seen from the bell curve (b), the interference filter removes light having a wavelength less than 1.94 micron and greater than 2.06 micron, the emission spectrum; and the

CO2 absorption spectrum represented by the twin peaks (c) which start at a wavelength of approximately 1.99 micron and end at a wavelength of approximately 2.025 micron.

The performance of the apparatus is improved by the interference filter. An interference filter is placed in front of the primary light source 20. Without the interference filter present the light emitted from the light emitting diode 20 that is not absorbed by the CO2 in the chamber dilutes the effective absorption by the CO2. Whilst the interference filter removes some light that would be available for absorption by CO2, which is a negative effect, the improvement in performance by removing the dilution effect of having light in the chamber that is not absorbed by the CO2 outweighs this. The interference filter also limits the the intensity variability as the wavelength of light emitted by the light emitting diode varies with time.

The apparatus 1 includes a photonics controller 25 and a micro-controller 30. The function of the photonics controller 25 is to control the output of the light emitting diodes 20, 22 so that they are transmitted in a square wave modulated format, in anti-phase, to receive the signals from the primary light sensor 21 and power illumination reference light sensor 23. The photonics controller outputs signals representative of the signals received by the primary light sensor 21 and the illumination reference light sensor 23 to a micro controller 30, which also receives inputs from temperature and pressure sensors 31,32, which are present in the illustrated example. The microcontroller 30 operates an algorithm which at its heart subtracts the output of the primary light sensor 23 that is associated with the secondary light source 22 from the output of the primary light sensor 23 associated with the primary light source 20.

The apparatus 1 further includes a display 33 and an output port 34 which allows other devices to be connected to the apparatus. In the illustrated example, the display 33 is an OLED display and the output port 34 is a USB port.

The micro-controller 30 performs the following operations:

From the four signal channels available from the photonics controller:

1. A value of measurement light (λ=2.0μ) is received through the detector cell (A_mes)

2. A value of measurement light (λ=2.0μ) is received bypassing the detector cell (A_ref)

3. A value of reference light (λ=1,8μ) is received through the detector cell (B_mes)

A value of reference light (λ=1,8μ) is received bypassing the detector cell (B_ref)

In practice these values will differ at the outset, and so will need to be normalised from time to time, during a period when it is known there is no CO2 in the Gas Cell. This is achieved by a “reset” function during system commissioning that normalises against B_ref, so that:

Ames = Aref = Bmes = Bref

Both the measured and reference illuminating light will vary by a small amount all the time, and so each measurement value is corrected by its corresponding reference value.

A = Ames/Aref and B — Bmes/Bref

Absorption in the cell is thus represented by B - A.

This absorption value needs to be corrected for the Beer Lambert Law to yield a CO2 concentration value.

Concentration CO2 = 1 - (1/βχρ(σδ(Β/Α)))

Where σ is a constant representing the absorption coefficient for CO2 based on the spectral data for the absorption peak under investigation and its relationship to the illuminating light source at standard conditions of temperature and pressure, and δ is the spatial distance travelled by the light across the cell.

The resulting value requires further adjustment using a look-up table to compensate for the temperature and pressure of the gas in the cell.

The final CO2 concentration result is communicated to a host computer by USB, Ethernet, or similar data protocol.

Figure 5 illustrates the operation of the apparatus illustrated in Figures 1 to 3. The photonics controller 25 generates a phase synchronisation signal which forms an input to a twophase oscillator 25a. The two phase oscillator 25a generates the anti-phase light source signal for light sources 20 and 22.

Claims

1. A gas concentration measurement apparatus comprising a gas cell that includes a body having gas inlet and a gas outlet and a closed void therebetween, a primary light source, a secondary light source and a primary light sensor, wherein the primary and secondary light sources and the primary light sensor are situated remotely with respect to the body and are optically connected thereto, to shine light into the void, by a plurality of optical fibres, wherein at least one optical fibre extends between the primary light source and the body to a first side of the void, at least one optical fibre extends between the secondary light source and the body to the first side of the void and at least one optical fibre extends between the body to a second side of the void and the primary light sensor, wherein the optical transmission properties of the plurality of optical fibre cables are substantially the same, and wherein the primary light source and the secondary light source are each configured to emit substantially monochromatic light sources of different wave lengths.

2. A gas concentration measurement apparatus according to Claim 1, wherein the optical fibres are comprised in at least two optical fibre cables.

3. A gas concentration measurement apparatus according to Claim 2, wherein the ends of the fibre optic cables are encapsulated.

4. A gas concentration measurement apparatus according to Claim 3, wherein the ends of the fibre optic cables are encapsulated in respective ferrules.

5. A gas concentration measurement apparatus according to any preceding claim, wherein the optical fibres extending between the primary and secondary light sources and the body are comprised in an optical fibre cable that includes a primary bifurcation providing said cable with at least three ends, the ends of primary bifurcation being connected to the primary and secondary light sources respectively and the third end of the cable being connected to the void.

6. A gas concentration measurement apparatus according to Claim 5, wherein optical fibres in the optical fibre cable are divided into two substantially equal numbers of optical fibres at the primary bifurcation.

7. A gas concentration measurement apparatus according to any preceding claim, further comprising a reference light sensor.

8. A gas concentration measurement apparatus according to Claim 7 when dependent on Claim 5, wherein the optical fibre cable is configured in a doubly bifurcated arrangement, the primary bifurcation and a secondary bifurcation extending from opposing sides of a common central section.

9. A gas concentration measurement apparatus according to Claim 8, wherein a first portion of the optical fibres connected to the primary and secondary light sources are comprised in first part of the secondary bifurcation and are connected to the body and a second portion of the optical fibres connected to the primary and secondary light sources comprised in a second part of the secondary bifurcation and are connected to the reference sensor.

10. A gas concentration measurement apparatus according to Claim 8 or 9, wherein the number of optical fibres in each part of the secondary bifurcation is unequal.

11. A gas concentration measurement apparatus according to Claim 10, wherein 90 percent or more of the optical fibres are comprised in the first part of the bifurcation and 10 percent or fewer of the optical fibres are comprised in the second part of the bifurcation.

12. A gas concentration measurement apparatus according to any preceding claim, wherein the substantially monochromatic light sources are selected from the group comprising: substantially monochromatic light emitting diode’s; substantially monochromatic near infra-red light emitting diode’s; a polychromatic source and an interference filter; lasers; distributed feedback lasers;

quantum well lasers; and optical fibre lasers.

13. A gas concentration measurement apparatus according to any preceding claim, wherein the light sources and/or the light sensors include or have associated therewith reflectors or lenses.

14. A gas concentration measurement apparatus according to Claim 13, wherein the reflectors are parabolic reflectors.

15. A gas concentration measurement apparatus according to any of Claims 12 to 14, wherein at least one of the substantially monochromatic light sources has a filter associated therewith, the filter allowing only light within a certain bandwidth to pass therethrough, the bandwidth of each filter narrower than the bandwidths of the monochromatic light sources.

16. A gas concentration measurement apparatus according to any preceding claim, wherein the wavelength of the primary light source is selected to correspond to an absorption peak of the gas, the fluctuation in concentration of which is being measured.

17. A gas concentration measurement apparatus according to Claim 16, wherein the wavelength of the primary light source is selected to correspond to an absorption peak of the gas, the fluctuation in concentration of which is being measured, which absorption peak is not the most prominent displayed by the gas the concentration of which is being measured

18. A gas concentration measurement apparatus according to Claim 17, wherein the primary light source emits substantially monochromatic light having a nominal wavelength of 2.0 micron and the secondary light source emits substantially monochromatic light having a nominal wavelength of 1.8 micron.

19. A gas concentration measurement apparatus according to any preceding claim, including a temperature sensor and/or a pressure sensor arranged to sense the temperature and/or pressure of the gas the concentration of which is being measured.

20. A gas concentration measurement apparatus according to any preceding claim, including a signal modulator, which signal modulator causes the each of the primary and secondary light sources each to output square wave in anti-phase and a controller, the controller configured to receive as an input the output of the primary light sensor and the controller programmed with an algorithm that causes the controller to output a signal representative of the concentration of the gas passing through the closed void by subtracting the output of the primary light sensor associated with the secondary light source from the output of the primary light sensor associated with the primary light source.

21. A gas concentration measurement apparatus according to Claim 20, when dependent on Claim 7, wherein the controller is configured to receive as inputs the electronic output signals of the primary and reference light sensors and wherein the algorithm modifies the output signal of the primary light sensor in accordance with the electronic output signal of the reference light sensor.

22. A gas concentration measurement apparatus according to Claim 20 or 21, wherein the outputs of the temperature and/or pressure sensor form inputs to the controller.

23. A gas concentration measurement apparatus according to any of Claims 20 to 22, wherein the algorithm that converts the output of the light sensor to an output signal representative of the concentration of the gas passing through the closed void is modified in accordance with the electronic output signal of the temperature and/or pressure sensor.

24. A gas concentration measurement apparatus according to any preceding claim, wherein the distance between the respective ends of the optical fibres connected to the body across the void of the gas cell is between 20mm and 100 mm.

25. A gas concentration measurement apparatus according to Claim 24, wherein the distance between the respective ends of the first and second optical fibre cables across the void of the gas cell is 23.5 mm.

26. A gas concentration measurement apparatus substantially as shown in, and as described with reference to, the drawings.

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GB 1620408.3

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