A method for determining methane concentration based on ambient gas composition and volume fraction using optical interferometry.

Abstract

This invention relates to an optical interferometric method for determining methane concentration based on ambient gas composition and volume fraction, belonging to the field of underground coal mine gas detection technology. The method includes: simplifying the gas composition in the underground environment according to the gas composition and the degree of influence of the corresponding gases on methane concentration determination; establishing reference values ​​for ambient temperature and atmospheric pressure in the methane concentration calculation process, and obtaining the gas refractive index at the corresponding reference values; obtaining underground air filtered with water vapor and carbon dioxide, and air filtered only with water vapor, and sequentially filling the gas chamber to obtain two sets of mixed gas refractive index and interference fringe shift data; constructing and solving a set of equations regarding methane and carbon dioxide concentrations based on the two sets of data; and correcting the methane and carbon dioxide concentrations using temperature and pressure correction coefficients calculated from the measured temperature and pressure at the test point. This invention comprehensively considers various factors affecting the accuracy of methane concentration determination, thus improving the accuracy of methane concentration measurement.

Description

Technical Field

[0001] This invention belongs to the field of underground gas detection technology in coal mines, and relates to a method for determining methane concentration based on optical interference of ambient gas components and volume fractions. Background Technology

[0002] Effective prevention and control of gas disasters remains a persistent challenge for coal mining enterprises, and accurate detection of gas concentration has significant practical guiding significance. Methane constitutes approximately 90% of gas, therefore, accurate measurement of methane concentration is the cornerstone and guarantee for effective gas disaster prevention and control. Currently, optical interference methane concentration detectors, employing optical principles, are widely used in coal mining enterprises due to their simple structure and high stability and reliability.

[0003] In traditional optical interferometry methane concentration measurement models, only the effects of moisture and CO2 on the measurement results are considered, except for temperature and pressure correction coefficients, and these are filtered out before measurement. However, the influence of the composition and volume fraction of the mixed gas in the underground coal mine environment on the measurement results is not fully considered. In the traditional model, the refractive index of the mixed gas entering the gas chamber (measuring chamber) of the optical interferometry methane concentration meter is expressed as Xn. m +(1-X)n a Where X represents the volume fraction of methane, and n m n represents the refractive index of methane. a This represents the air refractive index. As can be seen from this formula, in traditional models, the refractive indices of all gases in the downhole mixed gas except methane are simplified and treated as constant air refractive indices. That is, regardless of whether the gas composition and volume fraction at the measurement point are the same, the refractive index of the mixed gas except methane is considered constant. However, research results show that, in addition to moisture and CO2 significantly affecting the measurement results, the concentrations of O2 and N2 in the downhole environment also have a significant impact on the methane concentration measurement results, while CO, H2S, SO2, and other gases have very little impact due to their negligible concentrations. This leads to large fluctuations or data distortion in the detection results of optical interferometry methane concentration detectors in low-oxygen environments such as the return air corner of the downhole working face, limiting the measurement range, wasting manpower, and affecting the effective judgment of the true gas concentration value. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method for determining methane concentration based on the composition and volume fraction of ambient gases, thereby improving the accuracy and applicability of the optical interferometric methane concentration meter, solving the problem of unstable measurement results in the low-oxygen environment of coal mines, promoting the development of accurate methane concentration measurement technology, reducing labor input costs, and improving mine safety.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for determining methane concentration based on ambient gas components and volume fraction using optical interferometry, the method comprising:

[0007] S1. Simplify the gas composition in the downhole environment based on the gas composition of the downhole environment and the degree of influence of the corresponding gas on the methane concentration measurement.

[0008] CO, H2S, SO2, and nitrogen oxides, which have very low concentrations in the downhole environment and do not affect the methane concentration measurement results, can all be regarded as N2. Thus, the downhole environment gas can be regarded as being composed of O2, N2, CH4, and CO2, thereby simplifying the gas composition in the downhole environment.

[0009] S2. Establish reference values ​​for ambient temperature and atmospheric pressure in the methane concentration calculation process, and obtain the gas refractive index under the corresponding ambient temperature and atmospheric pressure reference values.

[0010] S3. At the point where the methane concentration is to be measured downhole, collect downhole atmospheric data and filter out water vapor and carbon dioxide. Then, fill the gas chamber of the optical interferometric methane concentration measuring device with the gas. Calculate the refractive index of the first mixed gas in the gas chamber and record the first displacement of the interference fringes in the optical interferometric methane concentration measuring device.

[0011] S4. Clean the gas in the gas chamber of the optical interference methane concentration measuring device, then fill the gas chamber with underground air that has only had water vapor filtered out, calculate the refractive index of the second mixed gas in the gas chamber, and record the second displacement of the interference fringes in the optical interference methane concentration measuring device.

[0012] S5. Based on the refractive index of the first mixed gas, the refractive index of the second mixed gas, the first displacement, and the second displacement, construct and solve a system of equations concerning the methane concentration and carbon dioxide concentration.

[0013] S6. Measure and obtain the ambient temperature and atmospheric pressure values ​​of the downhole methane concentration test point to calculate the temperature and pressure correction coefficient. Correct the obtained methane and carbon dioxide concentration data according to the temperature and pressure correction coefficient to obtain the true methane and carbon dioxide concentrations of the downhole methane concentration test point.

[0014] Furthermore, in step S2, the reference values ​​for ambient temperature and atmospheric pressure are as follows: ambient temperature is 20°C and atmospheric pressure is 1013.25 hPa.

[0015] Furthermore, in step S3, the refractive index of the first mixed gas is calculated using the following formula:

[0016] n t=c1n1 / (1-c2)+c3n3 / (1-c2)+(1-c1-c2-c3)n4 / (1-c2)

[0017] In the formula, n t The refractive index of the first mixed gas is represented by c1, c2, and c3, which represent the concentrations of methane, carbon dioxide, and oxygen in the downhole gas, respectively. n1 and n3 represent the refractive indices of methane and oxygen at the reference values ​​of ambient temperature and atmospheric pressure, respectively. n4 represents the refractive index of the remaining gas other than oxygen, methane, and carbon dioxide in the atmospheric environment at the downhole methane concentration test point at the reference values ​​of ambient temperature and atmospheric pressure. Since the concentrations of gases such as CO, H2S, SO2, and nitrogen oxides in the remaining gas are very small and have no effect on the methane concentration measurement results, the value of the refractive index n4 of the remaining gas is taken as the refractive index value of nitrogen at the reference values ​​of ambient temperature and atmospheric pressure.

[0018] In step S4, the refractive index of the second mixed gas is calculated using the following formula:

[0019] n=c1n1+c2n2+c3n3+(1-c1-c2-c3)n4

[0020] In the formula, n represents the refractive index of the second mixed gas, and n2 represents the refractive index of carbon dioxide at the reference values ​​of ambient temperature and atmospheric pressure.

[0021] Furthermore, in step S5, the constructed system of equations concerning methane and carbon dioxide concentrations is expressed as follows:

[0022]

[0023] n t =c1n1 / (1-c2)+c3n3 / (1-c2)+(1-c1-c2-c3)n4 / (1-c2)

[0024] n=c1n1+c2n2+c3n3+(1-c1-c2-c3)n4

[0025] In the formula, n a The refractive index of the gas in the reference chamber of the methane concentration measuring device at ambient temperature and atmospheric pressure reference values; N t N b Let represent the first displacement and the second displacement, respectively; L represents the length of the gas chamber in the methane concentration measuring device, and λ represents the wavelength of light. The oxygen concentration c3 can be obtained through a sensor, and c1 and c2 are solved based on the above equations.

[0026] Furthermore, step S6 includes first calculating the temperature and pressure correction coefficient Q:

[0027] Q=[(273.15+T) / 293.15]×[760 / (0.75P)]

[0028] Then, the obtained methane and carbon dioxide concentrations are corrected using the temperature and pressure correction coefficients.

[0029]

[0030] In the formula, T represents the ambient temperature at the point where the downhole methane concentration is to be measured, P represents the atmospheric pressure at the point where the downhole methane concentration is to be measured, and c 1ture c 2ture These represent the actual methane and carbon dioxide concentrations at the test point, respectively.

[0031] The beneficial effects of this invention are as follows:

[0032] (1) This invention optimizes the traditional optical interference method for measuring methane concentration by treating the refractive index of gases other than methane in the mine as a constant value. Based on the actual composition and volume fraction of gases in the mine environment, it proposes a method for measuring methane concentration with the refractive index of gases other than methane as a variable, which is more suitable for the variable atmospheric environment in coal mines.

[0033] (2) This invention comprehensively considers various factors that affect the accuracy of methane concentration measurement, including environmental factors such as ambient temperature and atmospheric pressure at the measurement site, and also considers the influence of underground oxygen, nitrogen and other gas concentrations on methane concentration measurement, effectively improving the accuracy of methane concentration measurement and promoting safe production in coal mines.

[0034] (3) The present invention can simultaneously measure the carbon dioxide gas concentration in the underground environment, and the operation process is consistent with the traditional methane concentration measuring instrument. Therefore, for coal mining enterprises, they only need to refer to the calculation method proposed in the present invention to measure the carbon dioxide gas concentration, and no longer need to replace a large number of measuring instruments, thereby effectively saving production input costs while ensuring measurement accuracy.

[0035] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0036] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0037] Figure 1This is a schematic diagram of the process for determining methane concentration using optical interferometry based on ambient gas components and volume fractions, as described in this invention.

[0038] Figure 2 This is a schematic diagram of the optical structure of an optical interference methane concentration meter. Detailed Implementation

[0039] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0040] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0041] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0042] To improve the accuracy and applicability of optical interferometric methane concentration measuring instruments and solve the problem of unstable measurement results in low-oxygen environments in coal mines, this invention proposes an optical interferometric methane concentration measurement method based on environmental gas components and volume fractions, based on the traditional optical interferometric methane concentration measurement model. This aims to improve the accuracy of methane concentration measurement, promote safe production in coal mines, and save production input costs.

[0043] like Figure 1 As shown, an embodiment of the present invention provides a method for determining methane concentration based on ambient gas components and volume fractions using optical interferometry, comprising the following steps:

[0044] S1: Based on the gas composition of the downhole environment and the degree of influence of the corresponding gases on the methane concentration measurement, the gas composition of the downhole environment is simplified. It is set that the gas in the downhole environment in the methane concentration measurement method consists of O2, N2, CH4, and CO2. The volume concentrations of CH4, CO2, and O2 are denoted as c1, c2, and c3, respectively. The remaining gas is treated as N2 and its volume concentration is denoted as c4, where c4 = 1 - (c1 + c2 + c3).

[0045] The remaining gases include N2, CO, H2S, SO2, and nitrogen oxides. The concentrations of CO, H2S, SO2, and nitrogen oxides are very small and have no effect on the methane concentration measurement results. Therefore, a simplified treatment can be made, and the remaining gases can be treated as N2. That is, the concentration and refractive index values ​​of the remaining gases are taken as the concentration and refractive index values ​​of N2.

[0046] The gas volume concentration c3 is obtained through a sensor, while c1 and c2 are unknowns to be calculated.

[0047] S2: Standardize the ambient temperature and atmospheric pressure values ​​in the methane concentration calculation process. Since the refractive index of a gas is affected by ambient temperature and atmospheric pressure, and varies under different ambient temperatures or atmospheric pressures, it is necessary to first establish reference values ​​for ambient temperature and atmospheric pressure, and then obtain the specific values ​​of the gas refractive index under the corresponding ambient temperature and atmospheric pressure conditions by looking up tables.

[0048] Generally, ambient temperature and pressure (temperature T0 = 20℃, pressure P0 = 1013.25hPa) are used as a reference standard to determine the exact values ​​of the refractive indices of four gases: CH4, CO2, O2, and N2, which are denoted as n1, n2, n3, and n4, respectively.

[0049] Furthermore, under the aforementioned standardized ambient temperature and atmospheric pressure conditions, the refractive index of fresh air is n. a The refractive index is a known parameter and can be obtained by looking up a table.

[0050] S3: Before testing the optical interference methane concentration meter, clean the gas chamber, check the performance of the gas absorbent, check the airtightness, and zero the scale in a fresh airflow on the ground. After passing the inspection, fill the reference gas chamber of the meter with fresh air and seal it.

[0051] The gas absorbents are sodium lime and silica gel, which are loaded into the drying tube assembly. Sodium lime is used to absorb carbon dioxide, and silica gel is used to absorb moisture.

[0052] S4: At the location where methane concentration is to be measured downhole, the downhole atmosphere (filtered to remove water vapor and carbon dioxide) is drawn into the gas chamber of the optical interferometry methane concentration meter to determine the refractive index n of the mixed gas inside the gas chamber. t And read the distance N of the interference fringes in the methane concentration meter. t .

[0053] Among them, the refractive index n of the mixed gas in the gas chamber t The calculation formula is expressed as:

[0054] n t =c1n1 / (1-c2)+c3n3 / (1-c2)+(1-c1-c2-c3)n4 / (1-c2)

[0055] S5: Clean the gas in the gas chamber, then draw downhole air (filtered only to remove water vapor) into the gas chamber of the optical interferometer methane concentration meter. Re-establish the refractive index n of the mixed gas in the gas chamber, and read the movement distance N of the interference fringes in the methane concentration meter. b .

[0056] The formula for calculating the refractive index n is as follows:

[0057] n=c1n1+c2n2+c3n3+(1-c1-c2-c3)n4

[0058] S6: Based on the linear relationship between the displacement of the interference fringes and the methane concentration, and combined with the data read after two intakes of downhole air, establish and solve a system of equations containing unknowns c1 and c2.

[0059] The established system of equations is as follows:

[0060]

[0061] Where L is the length of the gas chamber in the measuring device, and λ is the wavelength of light.

[0062] S7: Measure and obtain the ambient temperature and atmospheric pressure values ​​at the downhole methane concentration measurement site. Correct the methane and carbon dioxide concentration data obtained from the equations using the temperature and pressure correction factor Q, and finally determine the true methane concentration c at the measurement point. 1ture With carbon dioxide concentration c 2ture .

[0063] The method for determining the temperature and pressure correction coefficient Q is as follows:

[0064] Q=[(273.15+T) / 293.15]×[760 / (0.75P)]

[0065] In the above formula, T is the ambient temperature at the location to be measured, in °C; P is the atmospheric pressure at the location to be measured, in hPa.

[0066] Then c 1ture and c 2ture The following corrections are made respectively:

[0067]

[0068] This embodiment optimizes the traditional optical interferometry method for methane concentration measurement, which treats the refractive index of gases other than methane as constant values ​​in the mine. Based on the actual composition and volume fraction of gases in the mine environment, it proposes a methane concentration measurement method where the refractive index of gases other than methane is variable, making it more suitable for the variable atmospheric environment in coal mines. The method provided in this embodiment comprehensively considers various factors affecting the accuracy of methane concentration measurement, including environmental factors such as ambient temperature and atmospheric pressure at the measurement site. It also considers the influence of the concentrations of gases such as oxygen and nitrogen in the mine on methane concentration measurement, thus improving the accuracy of methane concentration measurement. Furthermore, the method provided in this embodiment can also measure the concentration of carbon dioxide in the mine environment, and the operation process is consistent with that of traditional methane concentration measuring instruments.

[0069] like Figure 2 The diagram shown is a simplified optical structure of the optical interference type methane concentration meter, which has two reference gas chambers and one gas chamber. The two reference gas chambers are located on both sides of the gas chamber, and the reference gas chambers and the gas chamber are not interconnected.

[0070] based on Figure 2 The working principle of the optical interference methane concentration meter with the described optical structure is as follows: a laser light source generates a laser beam, which is incident on a parallel plane mirror through lens A and prism A and split into two beams. The first beam is reflected by the first surface of the parallel plane mirror and enters a reference gas chamber on one side. The second beam is refracted by the first surface of the parallel plane mirror and then reflected by the second surface and enters a gas chamber. After exiting the reference gas chamber, the first beam is reflected twice by prism C and enters the reference gas chamber on the other side. Then it exits the parallel plane mirror, is refracted by the first surface and reflected by the second surface, and reaches prism B. It then passes through lens B and lens C and enters the eyepiece. The second beam exits the gas chamber, is reflected by prism C and enters the gas chamber again. After exiting the gas chamber, it reaches the parallel plane mirror, is refracted by the first surface and reflected by the second surface, and reaches prism B. It then passes through lens B and lens C and enters the eyepiece. Since the first and second beams come from the same coherent light source, they produce optical interference. The displacement of the scale of the interference fringes relative to the zero mark of the interference fringes is observed and recorded through the eyepiece.

[0071] Based on the above principle, two light absorption experiments were conducted. In the first experiment, downhole gas filtered to remove water vapor and carbon dioxide was filled into the gas chamber. After the laser was absorbed by the gas, interference was generated, and the displacement of the first interference fringe scale was recorded. In the second experiment, downhole gas filtered to remove only carbon dioxide was filled into the gas chamber. After the laser was absorbed by the gas, interference was generated, and the displacement of the second interference fringe scale was recorded.

[0072] In summary, this invention proposes an optical interferometric methane concentration measuring instrument based on the composition and volume fraction of ambient gases. It optimizes the traditional optical interferometric methane concentration measurement principle, which treats the refractive index of gases other than methane as a constant value in underground mines. Based on the actual composition and volume fraction of ambient gases in underground mines, it proposes a methane concentration measurement method where the refractive index of gases other than methane is variable. This improves the accuracy of methane concentration measurement and is more suitable for the variable atmospheric environment in coal mines.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining methane concentration based on optical interference of environmental gas components and volume fractions, characterized in that, The method includes: S1. Based on the gas composition of the downhole environment and the degree of influence of the corresponding gases on the methane concentration measurement, the gas composition of the downhole environment is simplified. S2. Establish reference values ​​for ambient temperature and atmospheric pressure in the methane concentration calculation process, and obtain the gas refractive index under the corresponding ambient temperature and atmospheric pressure reference values; S3. At the point where the methane concentration is to be measured downhole, collect downhole atmospheric data and filter out water vapor and carbon dioxide. Then, fill the gas chamber of the optical interferometric methane concentration measuring device with the gas. Calculate the refractive index of the first mixed gas in the gas chamber and record the first displacement of the interference fringes in the optical interferometric methane concentration measuring device. The refractive index of the first mixed gas is calculated using the following formula: In the formula, This represents the refractive index of the first gas mixture. , , These represent the concentrations of methane, carbon dioxide, and oxygen in the downhole gas, respectively. , These represent the refractive indices of methane and oxygen at ambient temperature and atmospheric pressure reference values, respectively. This represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment at the point where the downhole methane concentration is measured, under reference ambient temperature and atmospheric pressure values. The value is taken as the nitrogen refractive index value under the reference values ​​of ambient temperature and atmospheric pressure; S4. Clean the gas in the gas chamber of the optical interference methane concentration measuring device, then fill the gas chamber with underground air that has only had water vapor filtered out, calculate the refractive index of the second mixed gas in the gas chamber, and record the second displacement of the interference fringes in the optical interference methane concentration measuring device. The refractive index of the second gas mixture is calculated using the following formula: In the formula, Indicates the refractive index of the second gas mixture. , , These represent the concentrations of methane, carbon dioxide, and oxygen in the downhole gas, respectively. , , These represent the refractive indices of methane, carbon dioxide, and oxygen at ambient temperature and atmospheric pressure reference values, respectively. This represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment at the point where the downhole methane concentration is measured, under reference ambient temperature and atmospheric pressure values. The value is taken as the nitrogen refractive index value under the reference values ​​of ambient temperature and atmospheric pressure; S5. Based on the refractive index of the first mixed gas, the refractive index of the second mixed gas, the first displacement, and the second displacement, construct and solve a system of equations concerning the methane concentration and carbon dioxide concentration. The constructed system of equations concerning methane and carbon dioxide concentrations is expressed as follows: In the formula, This represents the refractive index of the first gas mixture. This represents the refractive index of the second gas mixture; , , Let methane, carbon dioxide, and oxygen concentrations be represented respectively in the downhole gas. The solution is obtained using this system of equations. and ; , , These represent the refractive indices of methane, carbon dioxide, and oxygen at ambient temperature and atmospheric pressure reference values, respectively. This represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment at the point where the downhole methane concentration is measured, under reference ambient temperature and atmospheric pressure values. The value is taken as the nitrogen refractive index value under the reference values ​​of ambient temperature and atmospheric pressure; The refractive index of the gas in the reference chamber of the methane concentration measuring device is indicated under ambient temperature and atmospheric pressure reference values. , These represent the first displacement and the second displacement, respectively. This indicates the length of the gas chamber in the methane concentration measuring device. Indicates the wavelength of light; S6. Measure and obtain the ambient temperature and atmospheric pressure values ​​of the downhole methane concentration test point to calculate the temperature and pressure correction coefficient. Correct the obtained methane and carbon dioxide concentration data according to the temperature and pressure correction coefficient to obtain the true methane and carbon dioxide concentrations of the downhole methane concentration test point.

2. The method according to claim 1, characterized in that, The specific reference values ​​for ambient temperature and atmospheric pressure are: ambient temperature of 20 ℃ and atmospheric pressure of 1013.25 hPa.

3. The method according to claim 1, characterized in that, Step S6 includes first calculating the temperature and pressure correction coefficient. Q : Then, the obtained methane and carbon dioxide concentrations are corrected using the temperature and pressure correction coefficients. In the formula, This indicates the ambient temperature at the point where the downhole methane concentration is measured. The atmospheric pressure at the point where the downhole methane concentration is to be measured indicates the concentration of the methane. , These represent the methane and carbon dioxide concentrations in the downhole gas obtained by solving the constructed system of equations, respectively. , These represent the actual methane and carbon dioxide concentrations at the test point, respectively.

4. The method according to claim 1, characterized in that, Step S1 includes treating CO, H2S, SO2, and nitrogen oxides, which have very low concentrations in the downhole environment gas and do not affect the methane concentration measurement results, as N2, thereby considering the downhole environment gas as being composed of O2, N2, CH4, and CO2.

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