A computational method for coupled water-gas transport in the overburden of solid waste landfills

Abstract

The present invention provides a calculation method for coupled water-gas migration in a solid waste landfill cover layer, relates to the field of environmental geotechnical engineering technology, and aims to solve the problem that in the prior art, only the migration of water in the cover layer is considered, while the migration of landfill gas and the occasional influence of water migration and gas migration are lacking. The technical scheme adopted is to comprehensively consider the mutual influence between water infiltration, water evaporation, gas convection, gas diffusion, and water and gas migration, so as to simulate the process of coupled water-gas migration in the cover layer under different working conditions and boundary conditions, calculate the influence of complex climate changes such as rainfall and evaporation on gas migration, and evaluate the anti-seepage and air-tightness performance of the cover layer according to the cumulative leakage at the bottom of the cover layer and the gas emission rate at the top; the problem that the calculation result has a large error due to not considering the mutual coupling between water and gas migration in the cover layer is solved, and the method is simple, convenient and easy to implement.

Description

Technical Field

[0001] The invention relates to the technical field of environmental geotechnical engineering, and in particular to a calculation method for water-gas coupled migration of a solid waste landfill cover layer. Background Art

[0002] Landfill is one of the main ways to dispose of solid waste. In recent years, with the continuous increase in the output of solid waste and the enhancement of environmental protection and governance awareness, the pollution problem of solid waste landfills has received more and more attention. Due to the complex composition of solid waste in landfills, after a series of biochemical reactions, polluting leachate and gas will be produced. If these pollutants are discharged into groundwater and the atmosphere, they will cause serious damage to the ecological environment around the solid waste landfill. The cover layer is an important component of the landfill, just like the "artificial skin" of the landfill. The cover layer not only isolates people and animals from direct contact with the waste, but also needs to have anti-seepage and air-tight functions, that is, effectively prevent rainwater from infiltrating into the solid waste pile to reduce the generation of leachate; prevent the harmful gases generated by the solid waste pile from being discharged into the atmosphere to cause air pollution and isolate the oxygen in the atmosphere from invading the solid waste pile. However, since the cover layer is directly exposed to the complex and changeable atmospheric environment, it must withstand long-term dry-wet cycles, cold-warm and freeze-thaw cycles. At the same time, the continuous generation of gas in the landfill leads to an increase in the air pressure inside the landfill, which constantly tests the anti-seepage and air-tightness performance of the cover layer. Therefore, calculating the water vapor migration process in the landfill cover layer is of great significance for evaluating the anti-seepage and air-tightness performance of the cover layer and guiding the design of the cover layer.

[0003] At present, relevant technicians in this field have done a lot of research, but they mainly focus on the anti-seepage performance of the cover layer, that is, only consider the migration of water in the cover layer, but do not consider the migration of landfill gas and the influence of the coupling effect of water migration and gas migration, which will cause large errors in actual engineering applications. Therefore, this field needs to develop a calculation method for the coupled migration of water and gas in the cover layer of landfills. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention discloses a method for calculating the water-gas coupled migration of the cover layer of a solid waste landfill. The technical solution adopted is to include the following steps:

[0005] Step 1, establish a model of water vapor migration in a single-layer cover soil of a solid waste landfill, and set model parameters and boundary conditions;

[0006] Step 2, calculating the distribution of moisture and gas in the cover layer;

[0007] Step 3, evaluating the anti-seepage and air-tightness performance of the covering layer.

[0008] As a preferred technical solution of the present invention, the model parameters in step 1 include soil property parameters and gas property parameters.

[0009] As a preferred technical solution of the present invention, the boundary conditions include a moisture boundary at the bottom of the covering layer, a moisture boundary at the top of the covering layer, a gas boundary at the bottom of the covering layer and a gas boundary at the top of the covering layer.

[0010] Preferably, for water migration, the water boundary condition at the bottom of the cover layer is a constant pore water pressure head boundary; the water boundary condition at the top of the cover layer is during the rainfall period t p The time is the water infiltration boundary, and at t p After a certain time, it is the water evaporation boundary. For gas migration, the gas boundary condition at the bottom of the cover layer can be set to a constant concentration boundary or a constant flux boundary; the gas boundary condition at the top of the cover layer is a constant concentration boundary.

[0011] As a preferred technical solution of the present invention, in step 2, the calculation of moisture distribution of the covering layer includes: B(T)=exp(αh 0 +T / 4), Z=αz,T=αk s t / (θ s -θ r ), L = αl, 2λ k cos(λ k L)+sin(λ k L) = 0; where q(Z, T) is the infiltration or evaporation rate of the top of the cover layer, which is a positive value for infiltration and a negative value for evaporation, and the unit is mm / day; α is the soil desaturation coefficient, and the unit is m -1 ; z is the coordinate height from the bottom of the covering layer, in meters; t is the time, in days; k s is the saturated permeability coefficient of soil, in m / s; θ s is the saturated volumetric moisture content of soil; θ r is the residual volume moisture content of soil; θ w is the volumetric moisture content of the soil; l is the thickness of the cover layer, in m; h 0 is the pore water pressure head at the bottom of the overburden layer, in m; h i is the initial pore water pressure head of the overburden layer, in m; λ k is the characteristic value.

[0012] As a preferred technical solution of the present invention, the calculation of the gas concentration distribution of the cover layer in step 2 includes:

[0013]

[0014]

[0015] D s =τD g , In the formula, C g is the gas concentration in the overburden, in mol / m 3 ;Y is the reason n×1 matrix composed of nr n×r matrix composed of n n×1 matrix composed of G n The n×1 matrix composed of g D is the volumetric air content of the soil; s is the effective diffusion coefficient of gas, in m 2 / s; τ is the apparent bending coefficient of soil pores for gas migration; D g is the gas diffusion coefficient, in m 2 / s; k = k s e αh is the water unsaturated permeability coefficient, in m / s; k rg is the relative gas permeability coefficient; μ g is the gas dynamic viscosity coefficient, in Pa·s; k i is the inherent permeability of the soil, in m 2 ;P g is the gas pressure, in Pa; S w is the soil saturation; φ is the soil porosity; H g is Henry's constant; v w is the water seepage velocity, in m / s; v g is the gas convection velocity, in m / s; C 0 is the gas concentration at the bottom of the overburden layer, in mol / m 3 ; C t is the gas concentration at the top of the overburden layer, in m / s; F 0 is the gas inflow rate at the bottom of the overburden layer, in mol / m 2 / day.

[0016] As a preferred technical solution of the present invention, the calculation of the gas concentration distribution of the cover layer in step 2 also includes

[0017] k is a positive integer.

[0018] As a preferred technical solution of the present invention, the calculation of the gas concentration distribution of the cover layer in step 2 also includes

[0019] k is a positive integer.

[0020] As a preferred technical solution of the present invention, the evaluation parameters in step 3 include the moisture leakage rate at the bottom of the covering layer, the cumulative moisture leakage at the bottom of the covering layer, the gas emission rate at the top of the covering layer and the cumulative gas emission of the covering layer. According to the pollution emission standards or requirements, when the leakage at the bottom of the covering layer and the gas emission at the top exceed the standard requirements, the covering layer is broken down, thereby obtaining the moisture and gas breakdown time of the covering layer; if the leakage at the bottom of the covering layer and the gas emission at the top are still less than the emission requirements when they reach stability, it means that the covering layer meets the design requirements and has good anti-seepage and air-tightness performance.

[0021] As a preferred technical solution of the present invention, the calculation formula for the cumulative water leakage at the bottom of the covering layer is: Where Q is the water leakage at the bottom of the cover layer, in mm; t 0 is the calculation time, the unit is day; S f It is the water storage capacity of the covering soil, in mm.

[0022] As a preferred technical solution of the present invention, the calculation formula for the gas emission rate at the top of the covering layer is: Where F t is the gas emission rate at the top of the overburden layer, in mol / m 2 / day.

[0023] The beneficial effects of the present invention are as follows: the present invention comprehensively considers the mutual influence among water infiltration, water evaporation, gas convection, gas diffusion, and water and gas migration, can simulate the process of water-gas coupled migration in the covering layer under different working conditions and boundary conditions, calculates the influence of complex climate changes such as rainfall and evaporation on gas migration, and evaluates the anti-seepage and air-tightness performance of the covering layer according to the accumulated leakage at the bottom of the covering layer and the gas emission rate at the top; can accurately calculate the process of water-gas coupled migration in the covering layer, and provides valuable basis and data for the actual landfill covering layer design; solves the problem of large errors in calculation results caused by not considering the mutual coupling between water and gas migration in the covering layer, and is simple, convenient and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0025] Figure 1 A schematic diagram of the process of calculating the water-gas coupled migration process of the cover layer of a solid waste landfill according to the present invention;

[0026] Figure 2 This is a calculation model diagram of the solid waste landfill cover layer of the present invention;

[0027] Figure 3 It is a diagram of the accumulated water leakage at the bottom of the covering layer of the present invention;

[0028] Figure 4 This is a diagram of the gas emission rate from the top of the covering layer of the present invention. DETAILED DESCRIPTION

[0029] Example 1

[0030] like Figures 1 to 4 As shown, the present invention discloses a method for calculating the water-gas coupled migration of the cover layer of a solid waste landfill, and the technical solution adopted is, comprising the following steps:

[0031] Step 1 is to establish a coupled migration model of water and gas in the overburden layer, mainly inputting soil property parameters, water migration related parameters, gas migration related parameters, and boundary conditions;

[0032] Step 2 is to calculate the water-gas coupled migration process in the overburden layer and obtain the pore water pressure head distribution and gas concentration distribution;

[0033] Step 3 is to evaluate the anti-seepage and air-tightness performance of the covering layer, mainly by calculating the leakage at the bottom of the covering layer and the gas emission at the top of the covering layer, and combining relevant standards to evaluate the anti-seepage and air-tightness performance of the covering layer.

[0034] Step 1, establish a model of water vapor migration in the landfill cover soil, and set the parameters and boundary conditions of the model;

[0035] The specific parameter values ​​and boundary conditions are set as follows: the thickness of the covering soil layer is l = 1m, the saturated volume moisture content of the soil is θ s =0.43, soil residual volume moisture content θ r =0.0956, soil saturated permeability coefficient k s =5×10 -7 m / s, soil desaturation coefficient α=0.3m -1 ; Soil intrinsic permeability k i =0.2×10-15 m 2 ; Gas dynamic viscosity coefficient μ g =1.1×10 -5 Pa·s. Bottom pore water pressure head h 0 =-2.5m; initial pore water pressure head of overburden layer h i = -2.5m; q(A, T) is the infiltration or evaporation rate of the top of the cover layer (40 mm / day for infiltration and -2 mm / day for evaporation, and the duration of rainfall is t p =5day). Henry constant for methane H g =0.0316; gas diffusion coefficient D g =2.137×10 -5 m 2 / s; Gas concentration at the bottom of the covering layer C 0 =10mol / m 3 ; Gas concentration at the top of the overburden layer C t =0mol / m 3 ; The initial gas concentration is 0 mol / m 3 .

[0036] Step 2, calculating the distribution of moisture and gas in the cover layer;

[0037] After establishing the calculation model of water-gas coupled migration in the landfill cover and inputting relevant parameters, the boundary conditions are set according to the formula B(T)=exp(αh 0 +T / 4), Z=αz,T=αk s t / (θ s -θ r ), L = αl, 2λcos(λL) + sin(λL) = 0, the water migration distribution in the cover layer can be calculated;

[0038] When k is a positive integer, the gas migration distribution in the overburden layer can be calculated.

[0039] Step 3, evaluating the anti-seepage and air-tightness performance of the covering layer;

[0040] The calculated moisture and gas migration distribution based on the coupled water-gas migration in the overburden layer is given by Eq. The accumulated water leakage at the bottom of the cover layer and the gas emission rate at the top can then be calculated.

[0041] According to the requirement of the US ACAP project that the annual seepage of soil cover in humid areas should not exceed 30 mm, and the minimum methane gas emission rate stipulated in the regulatory baseline calculation guideline for the Australian legacy landfill methane project is 0.45 mol / m 2 / day requirements, the anti-seepage and air-tightness performance of the landfill cover can be evaluated.

[0042] According to the diagram of the accumulated water leakage at the bottom of the cover layer and the diagram of the gas emission rate at the top, it can be seen that the gas emission rate at the top of the cover layer and the water leakage at the bottom of the cover layer in this embodiment meet the corresponding standard requirements. At the same time, the calculation results show that the traditional method will underestimate the leakage at the bottom of the cover layer if it does not consider the water-gas coupling effect, which will cause the designed cover layer to have a greater safety risk.

[0043] Components not described in detail herein are prior art.

[0044] Although the specific embodiments of the present invention are described in detail above, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in this field without departing from the purpose of the present invention, and modifications or deformations without creative labor are still within the protection scope of the present invention.

Claims

1. A method for calculating the coupled water-gas migration of the cover layer of a solid waste landfill, characterized in that: The following steps are involved: Step 1, establish a model of water vapor migration in a single-layer cover soil of a solid waste landfill, and set model parameters and boundary conditions; Step 2, calculating the distribution of moisture and gas in the cover layer; Step 3, evaluating the anti-seepage and air-tightness performance of the covering layer; In step 2, the calculation of moisture distribution of the covering layer includes: B(T)=exp(αh0+T / 4), Z=αz,T=αk s t / (θ s -θ r ), L = αl, 2λ k cos(λ k L)+sin(λ k L) = 0; where q(Z,T) is the infiltration or evaporation rate at the top of the cover layer; α is the soil desaturation coefficient; z is the coordinate height from the bottom of the cover layer; t is the calculation time; k s is the saturated permeability coefficient of soil; θ s is the saturated volumetric moisture content of soil; θ r is the residual volume moisture content of soil; θ w is the volumetric moisture content of the soil; l is the thickness of the cover layer; h0 is the pore water pressure head at the bottom of the cover layer; h i is the initial pore water pressure head of the overburden layer; k is the characteristic value; The calculation of the gas concentration distribution of the blanket layer in step 2 includes: D s =τD g , In the formula, C g is the gas concentration in the covering layer; Y is n×1 matrix composed of nr n×r matrix composed of n n×1 matrix composed of G n The n×1 matrix composed of g is the volumetric air content of the soil; D s is the effective diffusion coefficient of gas; τ is the apparent bending coefficient of soil pores to gas migration; D g is the gas diffusion coefficient; k = k s e αh is the water unsaturated permeability coefficient; k rg is the relative gas permeability coefficient; μ g k is the gas dynamic viscosity coefficient; i is the inherent permeability of soil; P g is the gas pressure; S w is the soil saturation; is the soil porosity; H g is Henry's constant; v w is the water seepage velocity; v g is the gas convection velocity; C0 is the gas concentration at the bottom of the covering layer; C t is the gas concentration at the top of the covering layer; F0 is the gas flux at the bottom of the covering layer; The calculation of the gas concentration distribution of the cover layer in step 2 also includes k is a positive integer; The calculation of the gas concentration distribution of the cover layer in step 2 also includes k is a positive integer.

2. The method for calculating coupled water-gas transport in the cover layer of a solid waste landfill according to claim 1, characterized in that: The model parameters in step 1 include soil property parameters and gas property parameters.

3. The method for calculating coupled water-gas transport in the cover layer of a solid waste landfill according to claim 1, characterized in that: The boundary conditions include a blanket bottom moisture boundary, a blanket top moisture boundary, a blanket bottom gas boundary, and a blanket top gas boundary.

4. The method for calculating coupled water-gas transport in the cover layer of a solid waste landfill according to claim 1, characterized in that: The evaluation parameters in step 3 include the moisture leakage rate at the bottom of the covering layer, the cumulative moisture leakage at the bottom of the covering layer, the gas emission rate at the top of the covering layer, and the cumulative gas emission of the covering layer.

5. The method for calculating coupled water-gas migration in the cover layer of a solid waste landfill according to claim 4, characterized in that: The calculation formula for the cumulative water leakage at the bottom of the covering layer is: Where Q is the water seepage rate at the bottom of the cover layer; t0 is the calculation time; S f The water storage capacity of the covering soil.

6. The method for calculating the coupled water-gas migration of the solid waste landfill cover layer according to claim 4, characterized in that: The calculation formula for the gas emission rate at the top of the covering layer is: Where F t is the gas emission rate from the top of the overburden.

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