A numerical calculation method for simulating thermochemical conversion of solid fuel

Abstract

The application provides a numerical calculation method for simulating solid fuel thermochemical conversion, belonging to the field of thermochemistry, and more specifically, the method comprises the following steps: constructing a geometric model containing solid fuel and a gas environment; establishing control equations for the constructed geometric model; defining a chemical reaction kinetics model of the solid fuel, and determining source terms of the control equations according to the chemical reaction kinetics model, so as to obtain actual control equations; based on the Knudsen diffusion mechanism, the diffusion fluxes of different gases in the actual control equations are corrected to obtain corrected actual control equations; and the thermochemical conversion process is calculated according to the constructed geometric model and the corrected actual control equations. The application can avoid the local over-temperature collapse of the calculation model caused by the rapid concentration of combustible gas and oxidant or gasification agent to a higher temperature area, thereby making up for the defects of the existing numerical calculation method for solid fuel thermochemical conversion.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of thermochemistry, and more particularly, to a numerical calculation method for simulating the thermochemical conversion of solid fuel. BACKGROUND

[0002] In the field of thermochemical conversion, numerical calculation is a research method that can accurately and quickly describe the process of thermochemical conversion. However, due to the complexity of the thermochemical conversion process of solid fuel, the characteristics of solid fuel must be fully considered when using numerical calculation to study the thermochemical conversion process of solid fuel.

[0003] The thermochemical conversion of fuel includes the process of oxidant or gasification agent reaching the surface of the fuel and the process of oxidant or gasification agent reacting with the fuel. The speed of these two processes is the main factor determining the rate of solid thermochemical conversion. The process of oxidant moving to the surface of the fuel refers to the mass transfer process of the oxidant, which relies on the flow and diffusion of the oxidant. For the mass transfer of oxidant in gas, both flow and diffusion are important. However, for the thermochemical conversion of solid, the oxidant not only needs to reach the surface of the solid, but also needs to pass through the inert layer of the solid surface to contact the active components inside the solid to complete the thermochemical conversion. That is, the thermochemical conversion of solid must also consider the mass transfer process of oxidant or gasification agent inside the solid, and the larger the size of the solid, the greater the influence of the internal mass transfer process. In the solid, the flow resistance of gas is usually large, and the oxidant or gasification agent is difficult to flow freely inside the solid. The mass transfer of oxidant or gasification agent can only rely on diffusion. To simulate the thermochemical conversion of solid fuel, not only the diffusion process of gas in gas must be considered, but also the diffusion ability of oxidant or gasification agent inside the solid must be determined. For example, for coal particles, after the active substances on the surface of the particles are consumed, the internal fuel of the particles is wrapped by an incombustible compact ash layer. In the ash layer, the flow resistance of the oxidant or gasification agent is large, and the flow is difficult. The mass transfer of gas is seriously hindered, which leads to the internal fuel being unable to contact the oxidant or gasification agent, and the thermochemical conversion rate is significantly reduced.

[0004] Therefore, in order to efficiently simulate the thermo-chemical conversion process of solid fuel, a concise and accurate mathematical description of the mass transfer physical process is needed. However, the numerical model currently used to simulate the thermo-chemical conversion process of solid fuel generally ignores a large number of physical processes, especially the mathematical description of the mass transfer process is too simple, making the calculation result difficult to be true and reliable. CN110727906A discloses a method for solving the combustion characteristics of solid combustible based on multi-component pyrolysis gas, which gives the combustion process of pyrolysis gas in the solid combustion process, and more comprehensively gives the reaction kinetics process of solid volatile decomposition combustion, but the invention does not consider the influence of the slow mass transfer characteristics of solid fuel on the combustion of solid fuel, which makes the simulation of the combustion process of solid fuel inevitably distorted, in the distorted gas concentration field, the combustible gas and the oxidant or gasification agent are easy to concentrate in the higher temperature area, and the rapid combustion heat release leads to the local over-temperature of the numerical calculation model, and finally the calculation result deviates from the actual situation and loses reference value. In summary, the prior art lacks a simple, accurate and robust numerical simulation method for processing the thermo-chemical conversion of solid fuel. SUMMARY

[0005] In view of the defects of the prior art, the purpose of the present application is to provide a numerical calculation method for simulating the thermo-chemical conversion of solid fuel, which aims to solve the problem of simulation distortion of the existing solid fuel combustion process.

[0006] To achieve the above-mentioned purpose, the present application provides a numerical calculation method for simulating the thermo-chemical conversion of solid fuel, which comprises the following steps:

[0007] S1, constructing a geometric model containing solid fuel and gas environment;

[0008] S2, establishing control equations for the geometric model constructed in step S1;

[0009] S3, defining the chemical reaction kinetics model of the solid fuel, and determining the source term of the control equation in step S2 according to the same, thereby obtaining the actual control equation;

[0010] S4, based on the Knudsen diffusion mechanism, the diffusion flux of different gases in the actual control equation is corrected by using the following formula to obtain the corrected actual control equation,

[0011]

[0012] In the formula, is the porosity, J i is the diffusion flux of gas i, p is the gas density, D fe,i is the Fick diffusion coefficient of gas i, mu t is the eddy viscosity of the fluid, Sc tis the turbulent Schmidt number of the fluid, is the gradient of the mass fraction of gas i, D T,i is the temperature diffusion coefficient of gas i, K is the diffusion correction coefficient, and T is the temperature;

[0013] S5 calculates the thermo-chemical conversion process according to the geometric model constructed in step S1 and the corrected actual control equation obtained in step S4.

[0014] As a further preference, in step S1, the geometric model is discretized and the degree of discretization and accuracy is verified by using flow calculation.

[0015] As a further preference, in step S2, the control equation established includes continuity equation, momentum equation, energy equation and component transport equation, wherein:

[0016] The continuity equation is:

[0017]

[0018] The momentum equation is:

[0019]

[0020] The energy equation is:

[0021]

[0022] The component transport equation is:

[0023]

[0024] In the formula, t is time, is porosity, ρ g is gas density, ρ s is solid density, v is velocity, μ is dynamic viscosity, p is pressure, μv / K is the viscosity term in the momentum equation source term, β is the viscous resistance coefficient of the porous medium, βρ g |v|v is the diffusion term in the momentum equation source term, c pg is the specific heat capacity at constant pressure of the gas, c ps is the specific heat capacity at constant pressure of the solid, T is temperature, k eff is the effective heat transfer coefficient of the porous medium, Y ig is the mass fraction of gas i, J i is the diffusion flux of gas i, S g is the source term of the continuity equation, S T is the source term of the energy equation, S Yig is the source term of the component transport equation.

[0025] As further preferred, in step S3, the reaction rate of drying, pyrolysis, combustion and gasification of the solid fuel is described by Arrhenius formula, and the source term of the control equation is determined according to the reaction rate, wherein:

[0026] The reaction rate of drying is:

[0027]

[0028] The reaction rate of pyrolysis is:

[0029]

[0030] The reaction rate of combustion and gasification is:

[0031]

[0032] The source term of the continuity equation is:

[0033] S g = ρ g R j , j = 0, 1, 2, 3, 4

[0034] The source term of the energy equation is:

[0035] S T = Δhρ g R j , j = 0, 1, 2, 3, 4

[0036] The source term of the component transport equation is:

[0037]

[0038] In the formula, R0 is the reaction rate of drying, R1 is the reaction rate of pyrolysis, R2 is the reaction rate of fixed carbon combustion, R3 is the reaction rate of C-CO2 gasification, R4 is the reaction rate of C-H2O gasification, k j is the reaction rate constant of reaction j, M is the real-time content of moisture of the solid fuel, VM is the real-time content of volatile matter of the solid fuel, FC is the real-time content of fixed carbon of the solid fuel, M * is the initial content of moisture of the solid fuel, VM * is the initial content of volatile matter of the solid fuel, FC * is the initial content of fixed carbon of the solid fuel, t is time, P i is the partial pressure of gas i, P 0 is the environmental pressure, n is the pressure correction factor, S g is the source term of the continuity equation, S T is the source term of the energy equation, S Yig is the source term of the component transport equation, ρ gLet Δh be the gas density. j For the enthalpy change of reaction j, M i Let M be the molar mass of gas i. j denoted as the difference in molar mass between the gaseous products and gaseous reactants in reaction j.

[0039] As a further preferred embodiment, in step S4, porosity The functional relationship between the degree of solid fuel consumption and the following is:

[0040]

[0041] In the formula, It is the initial porosity of solid fuel, determined experimentally, k V0 k V1 k V2 These represent the volume ratios of moisture, volatile matter, and fixed carbon, respectively. The chemical reaction rates of moisture, volatile matter, and fixed carbon are respectively determined and calculated using the following formula.

[0042]

[0043] In the formula, ρ g Let be the gas density, and t be the time. for The generation rate.

[0044] As a further preferred method, in step S4, the diffusion correction coefficient K is determined as follows:

[0045] (a) Read the real-time state of the computational domain, including the average pore diameter of the solid fuel and the average molecular free path of the gas in the gas environment;

[0046] (b) Based on the real-time state of the computational domain read in step (a), calculate the Knudsen number Kn using the Knudsen diffusion mechanism, and determine the diffusion type of the gas in the solid, including Fick diffusion, transition diffusion, and Knudsen diffusion, and calculate the gas diffusion coefficient D corresponding to each diffusion type. fe D te and D ke ;

[0047] (c) Define the computational domain diffusion mechanism and, based on D determined in step (b), fe D te and D ke Obtain the gas diffusion coefficient D and D / D for the diffusion mechanism in this computational domain. fe That is, the value of K.

[0048] In summary, compared with the prior art, the above-described technical solutions conceived by this invention have the following advantages:

[0049] Beneficial effects:

[0050] 1. The present application is based on the existing thermal chemical conversion mathematical model, and the porosity in the control equation is corrected based on the Knudsen diffusion mechanism, so as to accurately describe the mass transfer process of gas in the gas phase and the solid phase in the thermal chemical conversion process of solid fuel, reduce the distortion of the gas concentration field, avoid the local over-temperature collapse of the calculation model caused by the rapid concentration of combustible gas and oxidant or gasification agent to the higher temperature area, and make up for the defects of the existing numerical calculation method of solid fuel thermal chemical conversion.

[0051] 2. In particular, the present application can be applied to the simulation of thermal chemical conversion processes of various solid fuels by optimizing the functional relationship between the porosity and the consumption degree of solid fuel, and has good universality.

[0052] 3. In addition, the present application also optimizes the determination method of the diffusion correction coefficient K, and can adaptively define the gas diffusion mechanism conforming to the actual situation and calculate the appropriate diffusion correction coefficient for different solid fuels and different reaction conditions. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 is a numerical calculation flowchart for simulating the thermal chemical conversion of solid fuel provided by the embodiment of the present application;

[0054] Figure 2 is a calculation flowchart of the diffusion correction coefficient K provided by the embodiment of the present application. DETAILED DESCRIPTION

[0055] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0056] As shown in Figure 1 , the present application provides a numerical calculation method for simulating the thermal chemical conversion of solid fuel, which comprises the following steps:

[0057] S1, a geometric model containing solid fuel and gas environment is constructed, the constructed geometric model is discretized and subjected to simple flowability calculation test, and it is ensured that the discretization degree of the geometric model is fine enough, and the discretized geometric model does not have obvious difference or error in the flow calculation process;

[0058] S2, the control equation is established for the geometric model constructed in step S1, the state of solid fuel in the flow field is described by using the porous medium area, and the solid filling condition of the solid fuel area is described by using the porosity ​It is the ratio of the volume occupied by the gas phase component to the solid phase component in the discretized unit of the geometric model, and its value ranges from [0,1].

[0059] S3 defines a chemical reaction kinetic model for solid fuels and determines the source terms of the governing equations in step S2 based on it, thereby obtaining the actual governing equations;

[0060] Based on the Knudsen diffusion mechanism, the presence of solids significantly alters the gas diffusion characteristics across the entire computational domain. The diffusion mechanism for porous solid fuel regions is modified. In the computational domain, the gas diffusion flux is composed of Fick diffusion and turbulent diffusion terms in the gas, Fick diffusion and turbulent diffusion terms in the gas within the solid, and a temperature diffusion term. The diffusion fluxes of different gases in the actual control equations can be corrected using the following equation to obtain the modified actual control equations.

[0061]

[0062] In the formula, For porosity, J i Let ρ be the diffusion flux of gas i, ρ be the gas density, and D be the diffusion flux of gas i. fe,i μ is the Fick diffusion coefficient of gas i. t It is the eddy viscosity of the fluid, Sc t It is the turbulent Schmidt number of the fluid. It is the gradient of the mass fraction of gas i, D T,i is the temperature diffusion coefficient of gas i, K is the diffusion correction coefficient, and T is the temperature;

[0063] Specifically, in the porous media solid fuel region, the gas, when diffused through a discrete unit, not only has to pass through " The gas, which is "× discrete unit volume", also needs to be processed by " The solid has a volume of "× discrete unit volume", and the diffusion flux is still affected by the temperature gradient. Therefore, the sum of the Fick diffusion term and the turbulent diffusion term in equation (1) is multiplied by As the diffusion term of gas within a solid discrete unit, the sum of the Fick diffusion term and the turbulent diffusion term is multiplied by... As the diffusion term of gas in a solid within a solid discrete unit, the temperature diffusion term remains unchanged;

[0064] S5 calculates the thermochemical conversion process based on the geometric model constructed in step S1 and the modified actual control equations obtained in step S4.

[0065] Furthermore, in step S2, the established governing equations include the continuity equation, momentum equation, energy equation, and component transport equation, wherein:

[0066] The continuity equation is:

[0067]

[0068] The momentum equation is:

[0069]

[0070] The energy equation is:

[0071]

[0072] The component transport equation is:

[0073]

[0074] where t is time, is the porosity, p g is the gas density, p s is the solid density, v is the velocity, μ is the dynamic viscosity, p is the pressure, μv / K is the viscous term in the momentum equation source term, βρ g |v|v is the diffusion term in the momentum equation source term, c pg is the specific heat at constant pressure of the gas, c ps is the specific heat at constant pressure of the solid, T is the temperature, k eff is the effective heat transfer coefficient of the porous medium, Y ig is the mass fraction of gas i, J i is the diffusion flux of gas i, S g is the source term of the continuity equation, S T is the source term of the energy equation, S Yig is the source term of the component transport equation.

[0075] Further, in step S3, the reaction rate of the solid fuel drying, pyrolysis, combustion and gasification is described by the Arrhenius formula, and the source term of the control equation is determined according to the Arrhenius formula, so as to realize the accurate control of the reaction rate, the reaction temperature and the type of reactants, and the combustible gas such as CO, CH4 and H2 generated by the pyrolysis and gasification of the solid fuel will continue to react when it contacts with the oxidant or gasification agent in the gas phase. For this purpose, the secondary reaction of the combustible gas and the oxidant or gasification agent is given by the Arrhenius formula according to the actual reaction condition, which can also be expressed by the following formula, wherein:

[0076] The reaction rate of drying is:

[0077]

[0078] The reaction rate of pyrolysis is:

[0079]

[0080] The reaction rates for combustion and gasification are:

[0081]

[0082] k j = A exp(-E / RT) (9)

[0083] The source term for the continuity equation is:

[0084] S g = p g R j , j = 0, 1, 2, 3, 4 (10)

[0085] The source term for the energy equation is:

[0086] S T = Ah p g R j , j = 0, 1, 2, 3, 4 (11)

[0087] The source term for the species transport equation is:

[0088]

[0089] where R0is the dry reaction rate, R1is the pyrolysis reaction rate, R2is the fixed carbon combustion reaction rate, R3is the C-CO2gasification reaction rate, and R4is the C-H2O gasification reaction ratek j is the reaction rate constant for reaction j, A is the pre-exponential factor, E is the activation energy, R is the ideal gas constant, T is the temperature, M is the real-time content of the solid fuel moisture, VMis the real-time content of the solid fuel volatiles, FCis the real-time content of the solid fuel fixed carbon, M * is the initial content of the solid fuel moisture, VM * is the initial content of the solid fuel volatiles, FC * is the initial content of the solid fuel fixed carbon, t is the time, P i is the partial pressure of gas i, P 0 is the ambient pressure, n is the pressure correction factor, S g is the source term for the continuity equation, S T is the source term for the energy equation, S Yig is the source term for the species transport equation, p g is the gas density, Ah j is the enthalpy change for reaction j, M i is the molar mass of gas i, M j is the difference in molar mass between the gaseous products and gaseous reactants in reaction j.

[0090] Further, in step S4, the porosity The function relationship with the solid fuel consumption degree is:

[0091]

[0092] In the formula, is the initial porosity of the solid fuel, which is measured by experiment, k V0 , k V1 , and k V2 are the volume proportions of water, volatile matter, and fixed carbon respectively, are the chemical reaction degrees of water, volatile matter, and fixed carbon respectively, and are calculated by using the following formula,

[0093]

[0094] In the formula, ρ g is the gas density, t is time, is the generation rate of H2O.

[0095] Further, in step S4, as shown in Figure 2 , the determination method of the diffusion correction coefficient K is as follows:

[0096] (a) reading the real-time state of the calculation domain, including the average pore diameter of the solid fuel and the average molecular free path of the gas in the gas environment;

[0097] (b) according to the real-time state of the calculation domain read in step (a), calculating the Knudsen number Kn by using the Knudsen diffusion mechanism, and judging the diffusion type of the gas in the solid according to the Knudsen number Kn, including Fick diffusion, transition diffusion, and Knudsen diffusion, and calculating the gas diffusion coefficients D fe , D te , and D ke corresponding to each diffusion type;

[0098] (c) defining the diffusion mechanism of the calculation domain, and obtaining the gas diffusion coefficient D of the diffusion mechanism of the calculation domain according to D fe , D te , and D ke determined in step (b), and D / D fe is the value of K.

[0099] The technical solutions provided by the present application will be further described below according to specific embodiments.

[0100] ​Taking lignite as an example, first, a geometric model of the lignite and its nearby flow field is constructed, control equations based on a porous medium model are set, and experimental data of industrial analysis of lignite, chemical reaction kinetics data, and pyrolysis / gasification gas secondary combustion conversion reaction kinetics data are defined to establish a chemical reaction kinetics model. The diffusion form of the thermo-chemical conversion of lignite in air is modified. At the beginning, the value range of Kn of gas diffusion in lignite is [4.65, 7.36], indicating that the diffusion form of gas in lignite is transition diffusion, and the calculation obtains D fe about 2.88×10 -5 m 2 / s. D ke about 6.93×10 -7 m 2 / s. Further, D te / D fe ≈0.03, so the value of K is set to 0.03. At this point, when simulating the thermo-chemical conversion process of lignite in air, the diffusion rate of gas in the lignite block is about 3% of the diffusion rate in the gas. When part of the lignite is reacted, the porosity of the reacted lignite reaches 0.8 or more, and the proportion of the solid phase diffusion part in the diffusion flux is significantly reduced, and the diffusion of gas mainly depends on the Fick diffusion and temperature diffusion of gas in gas. The control of gas diffusion in the reaction process of different fuel types and different reaction degree fuels is realized. Finally, the lignite thermo-chemical conversion model is calculated, and it is found that the fluctuation of the temperature field is significantly reduced, and the maximum temperature mutation is reduced by more than 200℃, indicating that the robustness of the thermo-chemical conversion model has been significantly improved.

[0101] Those skilled in the art will readily understand that the above description is only preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A numerical calculation method for simulating the thermochemical conversion of solid fuels, characterized in that, The method includes the following steps: S1 constructs a geometric model that includes solid fuel and gaseous environment; S2 establishes the governing equations for the geometric model constructed in step S1; S3 defines the chemical reaction kinetic model of the solid fuel and determines the source terms of the governing equation in step S2 based on it, thereby obtaining the actual governing equation; S4, based on the Knudsen diffusion mechanism, uses the following equation to correct the diffusion flux of different gases in the actual control equation, thereby obtaining the corrected actual control equation. In the formula, φ Porosity J i gas i diffusion flux, ρ For gas density, D fe,i It is a gas i Fick diffusion coefficient, μ t It is the eddy viscosity of the fluid. Sc t It is the turbulent Schmidt number of the fluid. Y i It is a gas i Gradient of quality fraction, D T,i It is a gas i The temperature diffusivity, K It is the diffusion correction coefficient. T For temperature; diffusion correction coefficient K The method for determining it is as follows: (a) Read the real-time state of the computational domain, including the average pore diameter of the solid fuel and the average molecular free path of the gas in the gas environment; (b) Based on the real-time state of the computational domain read in step (a), calculate the Knudsen number using the Knudsen diffusion mechanism. Kn, Based on this, the diffusion type of the gas in the solid is determined, including Fick diffusion, transition diffusion, and Knudsen diffusion, and the gas diffusion coefficient corresponding to each diffusion type is calculated. D fe , D te and D ke ; (c) Define the computational domain diffusion mechanism, and based on the determination in step (b). D fe , D te and D ke Obtain the gas diffusion coefficient D for this computational domain diffusion mechanism. D / D fe That is K The value of ; S5 calculates the thermochemical conversion process based on the geometric model constructed in step S1 and the modified actual control equations obtained in step S4.

2. The numerical calculation method for simulating the thermochemical conversion of solid fuels as described in claim 1, characterized in that, In step S1, the geometric model is discretized, and the degree of discretization and accuracy are verified by flow calculation.

3. The numerical calculation method for simulating the thermochemical conversion of solid fuels as described in claim 1, characterized in that, In step S2, the governing equations established include the continuity equation, momentum equation, energy equation, and component transport equation, among which: The continuity equation is: The momentum equation is: The energy equation is as follows: The component transport equation is as follows: In the formula ，t For time, φ Porosity ρ g For gas density , ρ s For solid density, v For speed, μ For dynamic viscosity, p For pressure, μv / K This refers to the viscous term in the source term of the momentum equation. β The viscous resistance coefficient of porous media, βρ g |v|v This is the diffusion term in the source term of the momentum equation. c pg The specific heat capacity at constant pressure of a gas ，c ps The specific heat capacity at constant pressure of a solid. T For temperature, k eff The effective heat transfer coefficient of porous media, Y ig gas i The mass fraction, J i gas i diffusion flux, S g For the source terms of the continuity equation, S T For the source terms of the energy equation, S Y ig This is the source term of the component transport equation.

4. The numerical calculation method for simulating the thermochemical conversion of solid fuels as described in claim 1, characterized in that, In step S3, the reaction rates of solid fuel drying, pyrolysis, combustion, and gasification are described using the Arrhenius equation, and the source terms of the governing equation are determined accordingly, where: The drying reaction rate is: The reaction rate of pyrolysis is: The reaction rates of combustion and gasification are: The source terms of the continuity equation are: The source terms of the energy equation are: The source term of the component transport equation is: In the formula, R 0 represents the drying reaction rate. R 1 represents the pyrolysis reaction rate. R 2 represents the reaction rate of fixed carbon combustion. R 3 represents the reaction rate of C-CO2 gasification. R 4 represents the reaction rate of C-H2O vaporization. k j For the reaction j The reaction rate constant, M This represents the real-time moisture content of solid fuels. VM This represents the real-time content of volatile matter in solid fuels. FC The real-time carbon content fixed for solid fuels. M This represents the initial moisture content of the solid fuel. VM This represents the initial content of the volatility of solid fuels. FC To fix the initial carbon content of solid fuels, t For time, P i gas i The partial voltage, P 0 Due to environmental pressures, n As a pressure correction factor, S g For the source terms of the continuity equation, S T For the source terms of the energy equation, S Y ig This is the source term of the component transport equation. ρ g For gas density, Δ h j For the reaction j enthalpy change, M i gas i molar mass, M j For the reaction j The difference in molar mass between the gas phase products and the gas phase reactants.

5. The numerical calculation method for simulating the thermochemical conversion of solid fuels as described in claim 1, characterized in that, In step S4, porosity The functional relationship between the degree of solid fuel consumption and the following is: In the formula , φ ini It is the initial porosity of solid fuel, determined experimentally. k V0 、k V1 、k V2 respectively The volume ratio of moisture, volatile matter, and fixed carbon. φ 0、 φ 1. φ 2 represents the degree of chemical reaction of moisture, volatile matter, and fixed carbon, respectively, and is calculated using the following formula. In the formula, Let be the gas density, and t be the time. for φk The generation rate.

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