A method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil

Abstract

This invention discloses a method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil, relating to the field of soil gas transport parameter testing technology. It addresses the problems of existing methods, which suffer from parameter testing deviations due to neglecting the interactions of multiple gas components or considering only a single transport mechanism, and the inability to simultaneously obtain the Knudsen diffusion coefficient. By applying two different bottom flux conditions to the same soil column and using the total pressure difference between the two conditions to eliminate coupled unknowns, the method achieves simultaneous analytical solution of the permeability coefficient and Knudsen diffusion coefficient, which can be used for accurate determination of gas transport parameters in landfill soil cover layers.

Description

Technical Field

[0001] This invention belongs to the field of soil gas transport parameter testing technology, specifically involving a method for simultaneously measuring the soil gas permeability coefficient and Knudsen diffusion coefficient. Background Technology

[0002] The primary purpose of laying a top cover layer at landfills is to reduce rainwater infiltration and prevent the uncontrolled release of landfill gas, thereby lowering the pollution load on the surrounding atmosphere. The scientific design of the soil cover layer and the efficient operation of the landfill gas collection and treatment system both depend on the accurate determination of gas transport parameters within the cover layer. These parameters mainly include the gas phase permeability coefficient and the Knudsen diffusion coefficient, both of which provide crucial information for optimizing the design of the soil cover layer and for the efficient collection of landfill gas.

[0003] Currently, the main method for testing gas transport parameters in soil overburden is indoor soil column experiments. Existing assessment methods have the following shortcomings: some methods only consider gas convection, and the calculation of permeability coefficient is based solely on the relationship between gas flux and pressure difference, neglecting the influence of diffusion processes; other methods use the convective-diffusion model (ADM) to describe the gas transport process in the soil, which, while considering convection and diffusion, does not account for the influence of interactions between multiple gas components on gas transport. All of these methods suffer from biased gas transport parameter testing due to neglecting certain transport mechanisms, and none can simultaneously obtain the Knudsen diffusion coefficient. Summary of the Invention

[0004] To address the aforementioned problems, the main objective of this invention is to design a method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil, thereby overcoming the shortcomings of traditional methods, such as their simplistic mechanism, neglect of the influence of multiple components, and inability to simultaneously obtain key parameters.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil includes the following steps: Step 1: Prepare soil sample: After sieving the soil sample, dry it, adjust the moisture content of the soil sample to the target value, and then let it stand. Step 2, Filling the soil sample: Fill the soil sample into the test device containing the bottom air chamber, the middle soil sample chamber and the top air chamber, compact it to the specified density, connect the ventilation device and the air pressure and gas flow test device, and confirm the airtightness of each part; introduce a mixed experimental gas containing at least two gas components through the bottom air chamber, and introduce air through the top and maintain atmospheric pressure boundary conditions. Step 3, Data Acquisition under Condition 1: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 1, wait for the gas to reach a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 4, Data Acquisition under Condition 2: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 2, wait for the gas to migrate to a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 5, Parameter Calculation: Based on the control equations of coupled multi-component gas convection, bulk diffusion and Knudsen diffusion, using the total gas pressure and flux data of each component in operating conditions 1 and 2, the Knudsen diffusion coefficient of air and the Knudsen diffusion coefficient of each component are obtained analytically. Then, combined with the Knudsen diffusion coefficient and gas concentration and pressure data, the gas phase permeability coefficient is analytically solved.

[0006] As a further description of the present invention, step 1 includes the following steps: Step 1.1: Grind the soil sample and sieve it, with the sieve opening size not exceeding 2mm; Step 2.2: The sieved soil sample is dried in an oven at 100-110℃ for 20-24 hours until constant weight is achieved; Step 1.3: Add water and stir until the target moisture content is reached. Slowly spray the water while mixing the soil sample. After adding water, let the soil sample stand in a sealed bag for no less than 24 hours to allow the moisture content to be evenly distributed.

[0007] As a further description of the present invention, in step 2, the testing device is a multi-section hollow column, with sampling holes and air pressure testing holes reserved on the side walls of each section of the column. The bottom air chamber is connected to the air source through a flow control device, and the top air chamber is provided with an air inlet and an air outlet, with the air outlet connected to a flow meter.

[0008] As a further description of the present invention, in step 2, the mixed experimental gas introduced into the bottom of the testing device contains methane and carbon dioxide, and air is introduced into the top.

[0009] In step 2, the testing device is a multi-section hollow column, with sampling holes and air pressure testing holes pre-drilled on the side walls of each section. The bottom air chamber is connected to a gas source via a flow control device, and the top air chamber has an inlet and an outlet. The inlet is connected to an air compressor, and the outlet is connected to a flow meter. Specifically, the mixed experimental gas introduced at the bottom of the testing device contains methane and carbon dioxide, while air is introduced at the top via the air compressor. The gas is discharged through the outlet via an electronic soap film flow meter (accuracy 0.01 mL / min). Methane and carbon dioxide are the main components of landfill gas, and this gas combination can realistically simulate the multi-component gas transport scenario in the soil cover layer of a landfill. The air introduced at the top introduces nitrogen and oxygen as tracer components, facilitating the simultaneous detection of the concentrations of the four components using a gas chromatograph.

[0010] As a further description of the present invention, in steps 3 and 4, the total gas pressure is tested by a U-tube manometer connected to the gas pressure test hole on the side wall of the test device, and the accuracy of the manometer is not less than 0.01 kPa.

[0011] As a further description of the present invention, in steps 3 and 4, gas samples are collected through the sampling holes of the testing device, and the concentrations of each component are analyzed using a gas chromatograph GC9800. The detected components include methane, carbon dioxide, oxygen, and nitrogen. The detection limits for methane, carbon dioxide, oxygen, and nitrogen are 0.5%, 0.5%, 0.1%, and 0.1% (volume fraction), respectively.

[0012] As a further description of the present invention, in step 5, the air Knudsen diffusion coefficient The calculation formula is: ; Knudsen diffusion coefficient of components The calculation formula is: ; in, For the first The total number of gas components is as follows: gaseous components; Let be the ideal gas constant. Absolute temperature For testing depth; and These correspond to working conditions 1 and 2 respectively. Total gas pressure at depth Here, represents the total gas pressure at the top boundary of the soil column, and represents atmospheric pressure. and These are the molecular masses of component i gas and air, respectively; and These correspond to the first and second working conditions, respectively. Gas flux of the component.

[0013] As a further description of the present invention, in step 5, the gas phase permeability coefficient The calculation formula is: ; in, The Knudsen diffusion coefficient for air is... for Knudsen diffusion coefficient of components p is the gas viscosity coefficient. t for Total gas pressure at depth.

[0014] As a further description of the present invention, the method is used to measure gas transport parameters in the soil cover layer of a landfill, wherein the soil sample is loess.

[0015] Compared with the prior art, the technical advantages of the present invention are as follows: This invention provides a method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil. This method constructs governing equations coupling multi-component gas convection, bulk diffusion, and Knudsen diffusion, incorporating the mutual diffusion between multi-component gases and the Knudsen diffusion effect into the gas transport description framework. This overcomes the shortcomings of traditional methods that only consider convection or use an agglomerative-diffusion model (ADM) while neglecting multi-component interactions, eliminating parameter measurement biases caused by incomplete transport mechanisms and significantly improving the accuracy of gas permeability coefficient testing. Simultaneously, this method can simultaneously acquire the Knudsen diffusion coefficient, solving the problem that existing technologies cannot simultaneously measure this key parameter, providing more comprehensive data support for the accurate description of gas transport behavior in soil.

[0016] At the experimental level, this invention employs a two-condition steady-state differential experimental design. By applying two sets of boundary conditions with different bottom gas fluxes to the same soil column sequentially, and utilizing differential processing of the gas pressure data from the two conditions, coupled unknowns are effectively eliminated, enabling the direct derivation of the analytical expressions for the Knudsen diffusion coefficient and the gas phase permeability coefficient. This analytical solution method eliminates the need for numerical iteration, resulting in a concise and clear calculation process that avoids errors and convergence issues that may be introduced by numerical methods. The testing device uses a multi-sectioned hollow column, which is simple in structure and easy to operate. The bottom gas chamber allows for precise adjustment of the flux of each component through a flow control device. Sampling holes and gas pressure testing holes are reserved on the side walls, enabling the acquisition of multi-point data at different depths. The system exhibits good operability and repeatability, making it easy to promote and apply. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall structure of the multi-component gas transport parameter testing device of the present invention; Figure 2 This is a schematic diagram showing the test results of Knudsen diffusion coefficient and permeability coefficient in the soil column experiment of this invention; Figure 3 This is a schematic diagram comparing the experimental and analytical model results of gas pressure distribution in this invention; Figure 4 Table 1 shows the various property parameters of the experimental soil samples.

[0018] In the diagram, 1. Gas cylinder, 2. Mass flow controller, 3. Soil column device, 31. Bottom air chamber, 32. Middle soil sample chamber, 33. Top air chamber, 331. Air inlet, 332. Air outlet, 34. Sampling hole, 35. Air pressure test hole, 4. U-tube manometer, 5. Air compressor, 6. Electronic soap film flow meter. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings: In one embodiment of the present invention, a method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil is disclosed, with reference to... Figure 1-3 As shown, the method described herein is used to determine gas transport parameters in the soil cover layer of a landfill, where the soil sample is loess. Loess is widely distributed in Northwest my country and is a common material for landfill soil cover layers. The method of this invention can be directly applied to the accurate determination of gas transport parameters in loess cover layers, providing parameter basis for the scientific design of soil cover layers and efficient landfill gas collection. The method includes the following steps: Step 1: Prepare soil sample: After sieving the soil sample, dry it, adjust the moisture content of the soil sample to the target value, and then let it stand. Step 2, Filling the soil sample: Fill the soil sample into the test device containing the bottom air chamber, the middle soil sample chamber and the top air chamber, compact it to the specified density, connect the ventilation device and the air pressure and gas flow test device, and confirm the airtightness of each part; introduce a mixed experimental gas containing at least two gas components through the bottom air chamber, and introduce air through the top and maintain atmospheric pressure boundary conditions. Step 3, Data Acquisition under Condition 1: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 1, wait for the gas to reach a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 4, Data Acquisition under Condition 2: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 2, wait for the gas to migrate to a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 5, Parameter Calculation: Based on the control equations of coupled multi-component gas convection, bulk diffusion and Knudsen diffusion, using the total gas pressure and flux data of each component in operating conditions 1 and 2, the Knudsen diffusion coefficient of air and the Knudsen diffusion coefficient of each component are obtained analytically. Then, combined with the Knudsen diffusion coefficient and gas concentration and pressure data, the gas phase permeability coefficient is analytically solved.

[0020] In this embodiment, by applying two different bottom gas flux conditions to the same soil column, and collecting data on total gas pressure and concentration of each component under steady-state conditions of each condition, the coupling unknowns are eliminated by using the difference between the two conditions, thus realizing the synchronous analytical solution of Knudsen diffusion coefficient and gas phase permeability coefficient, overcoming the problem that the two parameters cannot be decoupled under single conditions.

[0021] Specifically, in step 1, during soil sample preparation, a loess sample is taken, ground, and sieved. The sieve aperture size is no larger than 2 mm, preferably 2 mm. The sieved soil sample is then dried in an oven at 100–110°C for 20–24 hours until constant weight, preferably at 105°C for 24 hours. Water is then slowly sprayed onto the sample using a spray bottle and stirred until the target moisture content of 13% is reached. The soil sample is stirred while slowly adding water, and then left to stand in a sealed bag for at least 24 hours to allow for uniform moisture distribution. By controlling the sieve aperture size and drying conditions, a uniform particle size distribution and accurate controllable initial moisture content can be ensured, providing a consistent soil sample for subsequent gas transport parameter testing.

[0022] In this embodiment, the testing system mainly consists of a gas cylinder 1 (including a methane cylinder and a carbon dioxide cylinder), a mass flow controller 2, a soil column device 3, a U-tube manometer 4, a gas chromatograph, an air compressor 5, and an electronic soap film flow meter 6.

[0023] The soil column device 3 is made of plexiglass, and the column body is assembled from four hollow column sections, each with an inner diameter of 200 mm and a height of 200 mm, and the sections are sealed together. The soil column as a whole is divided into three parts: a bottom air chamber 31, a middle soil sampling chamber 32, and a top air chamber 33. Each column section has sampling holes 34 and gas pressure testing holes 35 pre-reserved on its sidewalls. This multi-section splicing structure facilitates the acquisition of gas pressure and concentration distribution data at different depths, providing multi-point verification support for parameter calculations.

[0024] Regarding gas path connections, the bottom gas chamber 31 is connected to the methane and carbon dioxide cylinders via a mass flow controller 2. A regulating valve and a humidifier are installed along the gas path. The mass flow controller 2 is used to precisely regulate the flow rate of each component gas. The top gas chamber 33 has an inlet 331 and an outlet 332. The inlet 331 is connected to the air compressor 5 for air intake. A humidifier, a glass rotor flow meter, a pressure gauge, and a regulating valve are installed along the gas path. The outlet 332 is connected to an electronic soap film flow meter 6 (accuracy 0.01 mL / min) to discharge exhaust gas. For pressure and concentration testing, the pressure testing holes 35 on the side walls of each column section are connected to a U-tube manometer 4 via conduits. The sampling hole 34 is used by a syringe to collect gas samples, which are then sent to a gas chromatograph for concentration analysis. After all connections are completed, a leak test is performed on each part to confirm there are no leaks before the experiment can begin.

[0025] During data acquisition in steps 3 and 4, the flux of each component of the bottom mixed experimental gas was adjusted to the specified values ​​for conditions 1 and 2 using a mass flow controller. Air was introduced at the top at a flow rate of 120 mL / min to maintain atmospheric pressure boundary conditions. After the gas migration reached a steady state, the total gas pressure at each depth was read using a U-tube manometer (accuracy not less than 0.01 kPa). Simultaneously, gas samples were collected at each sampling hole using a syringe and sent to a gas chromatograph for analysis. The detected components included methane, carbon dioxide, oxygen, and nitrogen. The U-tube manometer has a simple structure and intuitive readings. Its accuracy can meet the requirements for accurate measurement of small gas pressure differences in the soil, providing reliable gas pressure data input for differential calculations between the two conditions. The gas chromatograph can simultaneously and quantitatively analyze the concentrations of multiple gas components, completely characterizing the concentration field distribution of multi-component gases within the soil column, and providing complete concentration data for solving the governing equations.

[0026] In some preferred embodiments, the mixed experimental gas introduced at the bottom contains methane and carbon dioxide, while air is introduced at the top via an air compressor. Methane and carbon dioxide are the main components of landfill gas, and this gas combination can realistically simulate the multi-component gas transport scenario in the landfill soil cover layer; the air introduced at the top introduces nitrogen and oxygen as tracer components, facilitating the simultaneous detection of the four components by gas chromatography. Unless otherwise specified, all gases used are commercially available, the gas chromatograph is a GC9800 model, and the manual injection volume is 1 mL.

[0027] In this embodiment, as Figure 1 As shown, the multi-component gas transport model takes the top of the soil column as the reference point. Reference plane, downward is positive The direction is determined by establishing a coordinate system with the axis of symmetry of the soil column. The boundary condition at the top of the soil column is constant atmospheric pressure. The partial pressures of the components satisfy: (1) in, In the atmosphere Partial pressures of the components. Boundary condition at the bottom of the soil column with a specified flux. Landfill gas representing the degradation of waste at the bottom of the cover layer The rate of generation of component gases, i.e.: (2) A gas transport model is established, considering the transport of multi-component gases in soil pores under the combined influence of convection, bulk diffusion, and Knudsen diffusion. The governing equations are: (3) in, and They are respectively Components and Partial pressures of the component gases, total gaseous components; and They are respectively Components and Gas flux of the components The tortuosity coefficient of gas diffusion in porous media. For free phase diffusion Components and The binary diffusion coefficient of the components, for Knudsen diffusion coefficient of the component The gas phase permeability coefficient, The viscosity coefficient of the gas. for Total gas pressure at depth For depth.

[0028] In this embodiment, by solving the gas transport model, a one-dimensional steady-state analytical solution of the total gas pressure distribution considering the convection and diffusion mechanism of multi-component gases is obtained, and an explicit expression of the Knudsen diffusion coefficient and permeability coefficient is further given.

[0029] First, solve for the total air pressure. And summing equation (3), we get: , (4) in, for molar concentration of the component gases equal Partial pressure of component gases Divide by total gas pressure ,Right now: Assuming the average Knudsen diffusion coefficient It is a constant, equal to the Knudsen diffusion coefficient of air: (5) Solving equation (4) and combining it with boundary conditions, we obtain the gas total pressure distribution equation; (6) in, denoted as the Knudsen diffusion coefficient for air.

[0030] The aforementioned total pressure distribution equation contains two unknown parameters, and the equation set is not closed under a single working condition, making it impossible to solve independently. Therefore, this embodiment applies two different bottom gas flux conditions to the same soil column sequentially, achieving simultaneous solution using the two conditions.

[0031] Based on equation (4) and the test data from operating conditions 1 and 2, the Knudsen diffusion coefficient can be obtained. The calculation formula is: (7) Knudsen diffusion coefficient of components The calculation formula is: (8) in, For the first The total number of gas components is as follows: gaseous components; Let be the ideal gas constant. Absolute temperature For testing depth; and These correspond to working conditions 1 and 2 respectively. Total gas pressure at depth Here, represents the total gas pressure at the top boundary of the soil column, and represents atmospheric pressure. and These are the molecular masses of component i gas and air, respectively; and These correspond to the first and second working conditions, respectively. Gas flux of the component.

[0032] The above analytical formula is derived from the governing equations of coupled multi-component gas convection-bulk diffusion-Knusen diffusion under two working boundary conditions. The common unknown is eliminated by using the difference in total pressure between the two working conditions as the difference component, thus obtaining an explicit analytical expression. The Knusen diffusion coefficient of each component is then obtained from the square root relationship with the ratio of the molecular mass of each component.

[0033] Gas phase permeability The calculation formula is: (9) in, The Knudsen diffusion coefficient for air is... for Knudsen diffusion coefficient of components p is the gas viscosity coefficient. t for The total gas pressure at the depth. By substituting the already obtained Knudsen diffusion coefficient into the above formula, and combining the total gas pressure and flux data of each component under any operating condition, the gas phase permeability coefficient can be analytically obtained, realizing the simultaneous determination of the two parameters in a single experiment.

[0034] Furthermore, to verify the accuracy and reliability of the method described above, experimental data reported in existing literature were used for verification calculations. The verification data were taken from the gas transport indoor soil column experiment conducted by Yang Yibiao (2014) on loess samples from the Jiangcungou landfill.

[0035] The loess samples used in this experiment were taken from uncontaminated natural loess near the Jiangcungou landfill. To simulate the double-layer soil cover structure used in actual landfill engineering, the soil samples filled in the soil column were divided into two layers: the upper layer (0–0.3 m) had a dry density of 1.30 g / cm³, and the lower layer (0.3–0.8 m) had a dry density of 1.45 g / cm³. The basic physical properties of the soil samples are listed in Table 1. Based on the results of previous unit cell experiments, fresh loess has almost no methane oxidation capacity after 22 days of incubation; therefore, the methane biodegradation reaction within the soil column was not considered in the verification calculations.

[0036] The soil column apparatus used in the verification experiment was made of plexiglass. The column body was assembled from four hollow columns with an inner diameter of 200 mm and a height of 200 mm, and its structure was consistent with the test apparatus described in this invention. A gas injection device was connected to the bottom cavity of the soil column, through which a mixture of methane and carbon dioxide gas was introduced. An air compressor was connected to the top cavity, introducing air at a flow rate of 120 mL / min. The air was discharged into the atmosphere after passing through an electronic soap film flow meter (accuracy 0.01 mL / min) to maintain atmospheric pressure boundary conditions at the top. Pressure measurement holes were pre-drilled every 200 mm along the height direction on one side wall of the soil column, connected to a U-tube manometer (accuracy 0.01 kPa) to test the total gas pressure at different depths.

[0037] The verification experiment consisted of four different bottom flux conditions, with the following bottom methane and carbon dioxide fluxes for each condition: (1) Operating condition 1: Methane flow rate 4 mL / min; carbon dioxide flow rate 4 mL / min.

[0038] (2) Operating condition 2: Methane flow rate 7.5 mL / min; carbon dioxide flow rate 7.5 mL / min.

[0039] (3) Operating condition 3: Methane flow rate 10.5 mL / min; carbon dioxide flow rate 4.5 mL / min.

[0040] (4) Operating condition 4: Methane flow rate 12.5 mL / min; carbon dioxide flow rate 12.5 mL / min.

[0041] Using the method of steps 1 to 5 of this embodiment, the total gas pressure distribution data of operating conditions 1 and 2 are selected, and the Knudsen diffusion coefficient of air is solved according to equations (7) and (9) respectively. ) and gas phase permeability ( ).

[0042] Depend on Figure 2 (a) The Knudsen diffusion coefficient of air calculated using the method of the present invention is given. Distribution results along the depth of the soil column; Figure 2 (b) The permeability coefficients calculated using the method of this invention are given. The distribution results along the depth of the soil column indicate that the Knudsen diffusion coefficient of air in the soil ranges from 1.1 × 10⁻⁶. -6 ~4.76×10 -4 m 2 / s, the gas phase permeability coefficient ranges from 4.36×10 -14 ~2.7×10 -13 m 2 The upper soil layer (0-30cm) has a lower dry density and higher porosity, resulting in higher Knudsen diffusion coefficient and permeability coefficient than the lower soil layer (30-80cm). This pattern is consistent with the physical properties of the soil, verifying the rationality of the calculation results in terms of physical trends.

[0043] In contrast. Figure 2 (b) simultaneously presents the gas-phase permeability coefficient calculated using the conventional convection-diffusion model (ADM) and the permeability coefficient calculated using the multi-component gas convection-diffusion model of the present invention. The comparison results show that the conventional convection-diffusion model neglects the diffusion interactions between the multi-component gases, resulting in a calculated gas-phase permeability coefficient that is approximately 40% lower than that calculated by the present invention. This demonstrates that the present invention, by introducing a multi-component coupling mechanism, can effectively eliminate parameter measurement bias caused by incomplete transport mechanisms, thereby improving the accuracy of permeability coefficient determination.

[0044] To further verify the accuracy of the parameters measured in this invention, the calculated Knudsen diffusion coefficient and permeability coefficient were substituted into the one-dimensional steady-state multi-component gas transport analytical model (Equation 3) to calculate the frictional distribution of the total gas pressure within the soil column under working conditions 1 to 4. The top boundary condition of the model was set to atmospheric pressure, and the bottom boundary condition was taken as the methane and carbon dioxide fluxes corresponding to each working condition, with the bottom air flux set to zero. Comparison of the calculated results with experimentally measured values ​​is shown below. Figure 3 As shown. By Figure 3 It can be seen that the total gas pressure distribution calculated by the method of the present invention is in good agreement with the experimental results, indicating that the Knudsen diffusion coefficient and gas phase permeability coefficient measured simultaneously by the present invention can accurately describe the convection-diffusion transport process of multi-component gas in soil column, verifying the reliability and engineering applicability of the method of the present invention.

[0045] The technical solution of the present invention has been disclosed through the above content. Compared with the prior art, the present invention has the following beneficial effects: 1. By constructing governing equations that couple multi-component gas convection, bulk diffusion, and Knudsen diffusion, the mutual diffusion between multi-component gases (bulk diffusion) and the Knudsen diffusion effect are incorporated into the bulk transport description framework. This overcomes the shortcomings of existing methods that only consider convection or use convection-diffusion models (ADM) while ignoring the interaction of multi-component gases, eliminates parameter test bias caused by incomplete transport mechanisms, and improves the test accuracy of gas phase permeability coefficient. 2. By designing a steady-state differential experiment with two working conditions, two sets of boundary conditions with different bottom gas fluxes were applied to the same soil column. The coupling unknowns were eliminated by using the difference of the total gas pressure data of the two working conditions. The analytical expressions of the Knudsen diffusion coefficient and the gas phase permeability coefficient were derived. This enabled the simultaneous determination of two gas transport parameters in a single soil column experiment, solving the technical problem that the existing methods could not obtain the Knudsen diffusion coefficient simultaneously. 3. This invention uses an analytical solution method to directly calculate parameters from measured air pressure and concentration data, without the need for numerical iteration, and the calculation process is simple and clear; the test device adopts a multi-section spliced ​​hollow column, which is simple in structure and convenient to operate. The flow rate of each component in the bottom air chamber is precisely adjusted by a flow control device, and sampling holes and air pressure test holes are reserved on the side wall, which can obtain multi-point test data at different depths. The experimental system has good operability and repeatability.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil, characterized in that, Includes the following steps: Step 1: Prepare soil sample: After sieving the soil sample, dry it, adjust the moisture content of the soil sample to the target value, and then let it stand. Step 2, Filling the soil sample: Fill the soil sample into the test device containing the bottom air chamber, the middle soil sample chamber and the top air chamber, compact it to the specified density, connect the ventilation device and the air pressure and gas flow test device, and confirm the airtightness of each part; introduce a mixed experimental gas containing at least two gas components through the bottom air chamber, and introduce air through the top and maintain atmospheric pressure boundary conditions. Step 3, Data Acquisition under Condition 1: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 1, wait for the gas to reach a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 4, Data Acquisition under Condition 2: Adjust the flux of each component of the bottom mixed experimental gas to the specified value under Condition 2, wait for the gas to migrate to a steady state, and then collect the total gas pressure and gas concentration data of each component at the test depth. Step 5, Parameter Calculation: Based on the control equations of coupled multi-component gas convection, bulk diffusion and Knudsen diffusion, using the total gas pressure and flux data of each component in operating conditions 1 and 2, the Knudsen diffusion coefficient of air and the Knudsen diffusion coefficient of each component are obtained analytically. Then, combined with the Knudsen diffusion coefficient and gas concentration and pressure data, the gas phase permeability coefficient is analytically solved.

2. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: Step 1 includes the following steps: Step 1.1: Grind the soil sample and sieve it, with the sieve opening size not exceeding 2mm; Step 2.2: The sieved soil sample is dried in an oven at 100-110℃ for 20-24 hours until constant weight is achieved; Step 1.3: Add water and stir until the target moisture content is reached. Slowly spray the water while mixing the soil sample. After adding water, let the soil sample stand in a sealed bag for no less than 24 hours to allow the moisture content to be evenly distributed.

3. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: In step 2, the testing device is a multi-section hollow column. Each section of the column has a sampling hole and a pressure test hole reserved on its side wall. The bottom air chamber is connected to the air source through a flow control device, and the top air chamber is provided with an air inlet and an air outlet. The air outlet is connected to a flow meter.

4. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: In step 2, the mixed experimental gas introduced into the bottom of the testing device contains methane and carbon dioxide, while air is introduced into the top.

5. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: In steps 3 and 4, the total gas pressure is tested by a U-tube manometer connected to the gas pressure test hole on the side wall of the testing device, and the accuracy of the manometer is not less than 0.01 kPa.

6. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: In steps 3 and 4, gas samples are collected through the sampling holes of the testing device, and the concentrations of each component are analyzed using a gas chromatograph. The detected components include methane, carbon dioxide, oxygen, and nitrogen.

7. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: In step 5, the air Knudsen diffusion coefficient The calculation formula is: ； Knudsen diffusion coefficient of components The calculation formula is: ； in, For the first The total number of gas components is as follows: gaseous components; Let be the ideal gas constant. Absolute temperature For testing depth; and These correspond to working conditions 1 and 2 respectively. Total gas pressure at depth Here, represents the total gas pressure at the top boundary of the soil column, and represents atmospheric pressure. and These are the molecular masses of component i gas and air, respectively; and These correspond to the first and second working conditions, respectively. Gas flux of the component.

8. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 7, characterized in that: In step 5, the gas phase permeability coefficient The calculation formula is: ； in, The Knudsen diffusion coefficient for air is... for Knudsen diffusion coefficient of components p is the gas viscosity coefficient. t for Total gas pressure at depth.

9. The method for simultaneously determining the gas permeability coefficient and Knudsen diffusion coefficient in soil according to claim 1, characterized in that: The method is used to measure gas migration parameters in the soil cover layer of a landfill, and the soil sample is loess.

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