A numerical simulation method for pyrolysis of large-diameter biomass particles

Abstract

The present application relates to a kind of numerical simulation method of large particle biomass particle pyrolysis, the method includes the following steps: step S1, create meshed large particle biomass particle pyrolysis geometric model in OpenFOAM, model includes biomass particle internal region and surrounding fluid region;Wherein, the boundary of biomass particle and fluid is set as dynamic mesh, to characterize the migration process of biomass pyrolysis reaction boundary;Step S2, based on the heat conduction process in particle interior, the heat transfer process between particle and heating fluid, and the shape change in particle reaction process, establish biomass particle pyrolysis model;Step S3, by pyrolysis parameter and the migration setting of pyrolysis reaction boundary, solve biomass particle pyrolysis model in step S2, numerical simulation is carried out to large particle biomass particle pyrolysis process.Compared with prior art, the present application can obtain the temperature distribution and composition change in particle interior in pyrolysis process, more accurately predict the heat transfer between particle and fluid.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization, and in particular to a numerical simulation method for the pyrolysis of large-diameter biomass particles. Background Technology

[0002] Pyrolysis is one of the important methods for utilizing biomass resources. During pyrolysis, biomass particles absorb heat from the outside while undergoing phase changes and deformations due to chemical reactions. It is a complex coupled process of flow, heat transfer, mass transfer, and deformation. Pyrolysis requires the biomass to be crushed, with particle sizes typically ranging from 5 to 50 mm. Biomass has a thermal conductivity of approximately 0.21–0.71 W / (m·K), making it a poor conductor of heat.

[0003] Traditional biomass pellet pyrolysis models often treat them as point masses and use the lumped parameter method. However, in the actual reaction process, there is a significant temperature gradient inside large-diameter particles, causing different parts of the particles to be in different reaction processes at the same time, which cannot be predicted by a uniform model.

[0004] To address the shortcomings of the existing technologies, it is necessary to design a numerical simulation method for the pyrolysis of large-diameter biomass particles, which can obtain the temperature distribution and composition changes inside the particles during the pyrolysis process and more accurately predict the heat transfer between the particles and the fluid. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a numerical simulation method for the pyrolysis of large-diameter biomass particles. This method is based on gridded modeling and dynamic grid migration settings of reaction boundaries to establish and solve a biomass particle pyrolysis model, thereby realistically simulating the pyrolysis process of large-diameter biomass particles, obtaining the temperature distribution and composition changes inside the particles during pyrolysis, and more accurately predicting the heat transfer between particles and fluid.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] This invention provides a numerical simulation method for the pyrolysis of large-diameter biomass particles, the method comprising the following steps:

[0008] Step S1: Create a gridded geometric model of large-diameter biomass pyrolysis using modeling software in OpenFOAM. The model includes the internal region of the biomass particles and the surrounding fluid region. The boundary between the biomass particles and the fluid is set as a moving grid to characterize the migration process of the biomass pyrolysis reaction boundary.

[0009] Step S2: Based on the heat conduction process inside the particles, the convective heat transfer process between the particles and the heating fluid, and the shape changes during the particle reaction process, a biomass particle pyrolysis model is established.

[0010] Step S3: By setting the pyrolysis parameters and the migration of the pyrolysis reaction boundary, solve the biomass particle pyrolysis model in step S2 and perform numerical simulation of the pyrolysis process of large-diameter biomass particles.

[0011] Preferably, the modeling software in step S1 is Block Mesh.

[0012] Preferably, the modeling software in step S1 is Gambit or ICEM.

[0013] Preferably, step S2 specifically comprises:

[0014] Based on the internal heat conduction process of the particles, the convective heat transfer process between the particles and the heating fluid, and the shape changes during the particle reaction process, a biomass particle pyrolysis model is established to determine the chemical elemental composition of the biomass particles, the reaction equation, the gas-solid two-phase heat transfer equation, the reaction kinetic equation, the physical property parameters of the biomass particles, and the physical property parameters of the heating gas.

[0015] Preferably, the physical properties of the biomass pellets include the outer layer composition, porosity, and thermal conductivity of the biomass pellets.

[0016] Preferably, in the biomass particle pyrolysis model in step S2, the particle pyrolysis rate is controlled by a reaction kinetic model.

[0017] Preferably, the parameters of the reaction kinetic model are obtained by thermogravimetric analysis or DSC analysis.

[0018] Preferably, step S3 specifically includes the following sub-steps:

[0019] Step S31: Based on the grid settings in step S1, divide the computational domain into two computational domains: biomass particles and fluid.

[0020] Step S32: Set the physical property parameters, initial conditions, and boundary conditions for the two computational domains respectively;

[0021] Step S33: By controlling the moving speed of the moving mesh that characterizes the boundary between biomass particles and fluid, the reaction rate at different locations during the biomass particle reaction is simulated.

[0022] Step S34: Solve the biomass pellet pyrolysis model to obtain pyrolysis data of large-diameter biomass pellets under set operating conditions;

[0023] Step S35: Extract data and analyze results.

[0024] Preferably, step S32 further includes: setting the particulate phase as a porous medium and setting the fluid phase as a high-temperature gas.

[0025] Preferably, the data extraction and result analysis in step S35 specifically includes:

[0026] Step S351: Obtain the temperature distribution, composition changes, moisture content changes, and shape changes inside the particles during the pyrolysis process;

[0027] Step S352: Analyze the effects of different moisture contents and particle sizes on the heating rate, the temperature difference between the particle surface and the center, and the surface heat transfer coefficient.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1) By creating a gridded geometric model of large-diameter biomass pyrolysis on the open-source software OpenFOAM, the model is divided into a solid region inside the particle and a fluid region around it. At the same time, the boundary between the biomass particle and the fluid is set as a moving grid. By controlling the moving rate of the moving grid, the reaction rate at different locations of the particle during the reaction process is simulated, resulting in different shape changes. Through the heat conduction process inside the particle and the migration of the reaction interface during the pyrolysis process, the uneven temperature distribution on the particle surface and inside can be realistically simulated.

[0030] 2) The modeling software Block Mesh, which comes with the open-source software OpenFOAM, is used. Mesh generation can be achieved at the same time as modeling. The structured mesh generation and the use of more blocks can better adapt to dynamic meshes.

[0031] 3) Compared with the existing biomass particle pyrolysis model that treats particles as particles and uses the lumped parameter method for numerical simulation, the biomass particle pyrolysis model constructed in this invention comprehensively considers the heat conduction process inside the particle, the convective heat transfer process between the particle and the heating fluid, and the shape change of the particle during the reaction process. This facilitates the existence of a significant temperature gradient inside large-diameter particles, so that different parts of the particle are in different reaction processes at the same time.

[0032] 4) By solving the reaction kinetic parameters through thermogravimetric analysis or DSC analysis, the pyrolysis rate of biomass pellets in the pyrolysis model can be precisely controlled.

[0033] 5) This invention considers the changes in porosity inside the particles during pyrolysis, tracks the changes in particle shape during pyrolysis, and considers the coupling between the fluid region and the particle boundary. It can be used to study the temperature distribution, composition changes and shape changes inside the particles during the pyrolysis of a single large-diameter particle, so as to analyze the heat transfer and reaction characteristics of a single large-diameter particle of different sizes and types during pyrolysis. It is of great significance for a detailed understanding of the biomass particle pyrolysis process and for guiding engineering practice. Attached Figure Description

[0034] Figure 1 This is a three-dimensional geometric model diagram;

[0035] Figure 2 This is a temperature distribution diagram;

[0036] Figure 3 This is a map showing the distribution of hemicellulose.

[0037] Figure 4 A map showing the distribution of cellulose;

[0038] Figure 5 This is a lignin distribution diagram;

[0039] Figure 6 A schematic diagram showing temperature changes at different locations inside a biomass pellet;

[0040] Figure 7 This is a flowchart of the method of the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0042] Example

[0043] like Figure 7 As shown in the figure, this embodiment presents a method for simulating the pyrolysis of large-diameter biomass particles, including the following steps:

[0044] Step S1: Create a 3D geometric model using the block mesh software included with OpenFOAM, including the particle part and the surrounding heated gas part, such as... Figure 1 As shown;

[0045] Block Mesh performs mesh generation simultaneously with modeling; a structured mesh generation is used, with a total of 13 blocks across the two computational domains of particles and fluid in this embodiment; among them, particles are divided into 7 blocks, and the use of a larger number of blocks can better adapt to dynamic meshes. Its 3D model is as follows: Figure 1 As shown. Alternatively, mesh generation can be performed on OpenFOAM using Gambit or ICEM software.

[0046] Step S2: Establish a biomass pellet pyrolysis model, including biomass reaction and flue gas flow and heat transfer models:

[0047] 1) Conservation equations for the particle phase in porous media, including the conservation equations for particle phase composition and particle phase energy.

[0048] During pyrolysis, as pyrolysis volatiles are released, a porous medium semi-coke is formed on the outside of the particles; the particle phase refers to the unreacted area inside the particles and the porous medium area formed on the particle surface after the reaction.

[0049] Particulate phase composition conservation equation:

[0050]

[0051] In the formula, α si It is the mass fraction of component i; m s This represents the mass of the solid phase, expressed in kg. This is the heterogeneous reaction rate of component i, expressed in kg·m³. -3 ·s -1 ;χ i It is the ratio of the mass of component i reacting to its initial mass; P i The total number of components;

[0052] Energy conservation equation for the particulate phase (considering heat conduction in the unreacted internal region and heat transfer between the external porous medium region and the gas phase):

[0053]

[0054] In this equation, the left side represents the internal energy of the particles, and the right side represents the following terms: the first term is the thermally conductive phase, the second term is the heat of pyrolysis reaction, the third term is the total energy of the pyrolysis gas, the fourth term is the heat convection, the fifth term is the effective diffusion heat flux, and the sixth term is the dissipative phase. For Hamiltonian operators; N p The total number of components; α si It is the mass fraction of component i; c p,i The specific heat of component i is expressed in J·kg⁻¹. -1 ·K -1 m s denoted as ρ, where ρ is the mass of the solid phase; T is the temperature of the solid phase. Thermal conductivity, unit: J·m -2 ·s -1 ·K -1 h represents specific enthalpy, measured in J·kg⁻¹. -1 ;∈ g Porosity; ρ g The density of the pyrolysis gas is expressed in kg·m³. -3 ;e g The specific internal energy of the pyrolysis gas is expressed in J·kg⁻¹. -1 ;v g The velocity of the pyrolysis gas is expressed in m / s. -1 Qg To effectively diffuse heat flux, the unit is J·m -2 ·s -1 μ represents dynamic viscosity. This is the permeability tensor, in meters. 2 ;

[0055] 2) Gas phase conservation equation in porous media:

[0056] Gas phase mass conservation in porous media regions:

[0057]

[0058] The first term on the left side of the equation represents the change in gas phase density, ∈ g Porosity; ρ g The first term is the density of the pyrolysis gas; the second term is the convection term, v g The velocity of the pyrolysis gas is expressed in m / s. -1 The first term S on the right side of the equation g It is the rate of pyrolysis gas generation, measured in kg·m³. -3 ·s -1 The second term Ω h It is the heterogeneous reaction rate of the solid surface due to vaporization, and the unit is kg·m. -3 ·s -1 ,satisfy:

[0059]

[0060] In the formula, The heterogeneous reaction rate of component i is expressed in kg·m³. -3 ·s -1 N p The total number of components;

[0061] Gas phase momentum conservation equation in porous media:

[0062] In porous media regions, the gas phase obeys Darcy's law with the addition of the Klingberg correction factor:

[0063]

[0064] Among them, v g It is the gas flow rate, m·s -1 ;∈ g It is porosity; It is the permeability tensor, m 2 ; This is the Klinkenberg correction factor, in Pa; p is the gas pressure; μ is the dynamic viscosity.

[0065] 3) Gas phase conservation equation:

[0066] Gas phase mass conservation equation (biomass pellets decompose continuously upon heating, causing changes in the mass of the gas phase):

[0067]

[0068] Where, ρ g The density of the pyrolysis gas is expressed in kg·m³. -3 v g It is the gas flow rate, measured in m / s. -1 S g It is the rate of pyrolysis gas generation, with units of kg·m³. -3 ·s -1 ;

[0069] Gas phase momentum conservation equation:

[0070]

[0071] In the formula, ρ g The density of the pyrolysis gas is expressed in kg·m³. -3 ;v g It is the flow rate of the pyrolysis gas, measured in m / s. -1 μ is the dynamic viscosity; I is the identity matrix;

[0072] Gas phase energy conservation equation:

[0073]

[0074] In the formula, K is kinetic energy, and the unit is J·kg. -1 ·s -1 ;α eff It is the effective thermal diffusivity, W·m -1 ·K -1 ;R heat It is the heat of pyrolysis reaction, W / m -3 ;

[0075] Gas phase component transport equations:

[0076]

[0077] Where, μ eff It is dynamic viscosity, measured in Pa·s. -1 ;Y i R is the mass fraction of gas phase i. i It is the formation rate of substance i, kg·m -3 ·s -1 ;

[0078] 4) Pyrolysis equation and reaction rate:

[0079]

[0080] In the formula, χ i The mass ratio of component i to its initial mass in the reaction; A i E is the pre-exponential factor; i The activation energy of component i during pyrolysis is expressed in J·kg⁻¹. -1 R is the molar gas constant, T is the pyrolysis temperature; N p This represents the total number of components.

[0081] Step S3: Use OpenFOAM to read the computational mesh file, and use splitMeshRegions-cellZones-overwrite to divide the 3D geometric model into two computational domains: granular and fluid. Then, establish the initial and boundary conditions, and specify the solution control parameters, etc., specifically:

[0082] The controlDict setting is used to control the start / end time of the solution, the time step, and the output data parameters, etc.

[0083] vSchemes sets discrete formats;

[0084] fvSolution sets the matrix solver, specifically as follows:

[0085] 1) The 0 folder is set as the initial and boundary conditions, where the gas composition is set to 20% CO2 and 80% N2 by volume; the particle size is 10 mm; the initial flue gas temperature is 973 K; the flue gas velocity is 0.2 m / s; and the gas-solid coupling boundary condition is coupledMixed.

[0086] 2) Dynamic mesh settings: In the constant folder, specify the dynamic mesh settings in the dynamicMeshDict file under the porousMat folder which specifies the physical properties of the porous medium. Since the particle surface undergoes large deformation, diffusivity is specified as quadratic. The deformed surface is selected as the gas-solid intersection plane porousMat_to_flow, and corresponding to it is in the flow folder.

[0087] 3) fvSolution is set to PBiCGStab (Stabilized Preconditioned Biconjugate Solver) for the asymmetric matrix solver to adapt to the dynamic mesh and to set other algorithm controls;

[0088] 4) Set the physical property parameters for two regions in constant:

[0089] The particulate phase is set as a porous medium, and the fluid phase is set as high-temperature flue gas;

[0090] Particle property parameter settings: The pyrolysis model uses LinearArrhenius, the mass model uses DarcyLaw_Heterogeneous, the material property is selected as porous medium, and the specific properties and pyrolysis parameter files are specified in constantProperties under the data folder;

[0091] Hemicellulose pyrolysis: Hemicellulose pyrolysis is simulated by two steps. First, hemicellulose decomposes into volatiles and intermediate solids. In the subsequent reaction, the intermediate solids decompose into volatiles and char.

[0092] Cellulose pyrolysis: Simulated using two parallel reactions. Lignin decomposes into volatiles and charcoal via an Arrhenius reaction. Adsorbed water can be simulated as a fourth-phase solid, and its evaporation process is considered as pyrolysis.

[0093] Gas phase settings: Define the properties of the product gas in thermo.compressibleGas. The moving mesh is coupled with the particle phase and deforms together.

[0094] Step S4: After setting up, perform the calculation:

[0095] By setting different parameters such as moisture content and particle size, the pyrolysis of large-particle-size biomass under different structural parameters is calculated, and the temperature and composition changes under different working conditions are comprehensively analyzed.

[0096] Figures 2-5 These are pyrolysis temperature distribution diagrams, hemicellulose distribution diagrams, cellulose distribution diagrams, and lignin distribution diagrams, respectively. Figure 6 This diagram illustrates the temperature change over time at different locations inside the particle; the gas direction is from left to right, a is the leftmost end of the particle, b is the point near the half-radius of the incoming flow direction, c is the center of the particle, d is the point far from the half-radius of the incoming flow direction, and e is the rightmost end of the particle.

[0097] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A numerical simulation method for the pyrolysis of large-diameter biomass particles, characterized in that, The method includes the following steps: Step S1: Create a gridded geometric model of large-diameter biomass pyrolysis using modeling software in OpenFOAM. The model includes the internal region of the biomass particles and the surrounding fluid region. The boundary between the biomass particles and the fluid is set as a moving grid to characterize the migration process of the biomass pyrolysis reaction boundary. Step S2: Based on the heat conduction process inside the particles, the convective heat transfer process between the particles and the heating fluid, and the shape changes during the particle reaction process, a biomass particle pyrolysis model is established. Step S3: By setting the pyrolysis parameters and the migration of the pyrolysis reaction boundary, solve the biomass particle pyrolysis model in step S2 to perform numerical simulation of the pyrolysis process of large-diameter biomass particles. This specifically includes the following sub-steps: Step S31: Based on the grid settings in step S1, divide the computational domain into two computational domains: biomass particles and fluid. Step S32: Set the physical property parameters, initial conditions, and boundary conditions for the two computational domains respectively; Step S33: By controlling the moving speed of the moving mesh that characterizes the boundary between biomass particles and fluid, the reaction rate at different locations during the biomass particle reaction is simulated. Step S34: Solve the biomass pellet pyrolysis model to obtain pyrolysis data of large-diameter biomass pellets under set operating conditions; Step S35: Extract data and analyze results, specifically as follows: Step S351: Obtain the temperature distribution, composition changes, moisture content changes, and shape changes inside the particles during the pyrolysis process; Step S352: Analyze the effects of different moisture contents and particle sizes on the heating rate, the temperature difference between the particle surface and the center, and the surface heat transfer coefficient.

2. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 1, characterized in that, The modeling software used in step S1 is Block Mesh.

3. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 1, characterized in that, The modeling software used in step S1 is Gambit or ICEM.

4. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 1, characterized in that, Step S2 specifically involves: Based on the internal heat conduction process of the particles, the convective heat transfer process between the particles and the heating fluid, and the shape changes during the particle reaction process, a biomass particle pyrolysis model is established to determine the chemical elemental composition of the biomass particles, the reaction equation, the gas-solid two-phase heat transfer equation, the reaction kinetic equation, the physical property parameters of the biomass particles, and the physical property parameters of the heating gas.

5. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 4, characterized in that, The physical properties of the biomass pellets include the outer layer composition, porosity, and thermal conductivity.

6. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 4, characterized in that, In the biomass pellet pyrolysis model in step S2, the pellet pyrolysis rate is controlled by a reaction kinetic model.

7. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 6, characterized in that, The parameters of the reaction kinetic model were obtained through thermogravimetric analysis or DSC analysis.

8. The numerical simulation method for the pyrolysis of large-diameter biomass particles according to claim 1, characterized in that, Step S32 further includes: setting the particulate phase as a porous medium and setting the fluid phase as a high-temperature gas.

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