CN114781285B - Biomass large particle pyrolysis simulation method based on sphere cluster hypothesis and Laguerre-Voronoi structure - Google Patents
Biomass large particle pyrolysis simulation method based on sphere cluster hypothesis and Laguerre-Voronoi structure Download PDFInfo
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Abstract
The invention discloses a biomass large particle pyrolysis simulation method based on a sphere cluster hypothesis and a Laguerre-Voronoi structure. The method belongs to the field of biomass pyrolysis, and can be used for exploring pyrolysis behaviors and phenomena of biomass, and comprises three main steps of initial sphere cluster generation, time step dispersion, pyrolysis iteration and result output. The invention utilizes sphere cluster hypothesis to characterize irregularly-shaped biomass particles; the shrinkage of the mother particles is realized by the volume reduction and movement of the constituent son spheres; the internal region of the particle is divided by using the Laguerre-Voronoi structure, so that the internal heat transmission process of the particle is realized, and the internal pore structure of the particle is analyzed; the anisotropic thermal conduction and shrinkage process of the particles is achieved with a direction dependent thermal conductivity and shrinkage factor. The method is beneficial to obtaining internal details which are difficult to observe in the experiment in the biomass pyrolysis process, and can be expanded to be applied to a reactor scale computational fluid dynamics model containing multiple biomass particles, so that the prediction precision and accuracy of large-scale simulation are greatly improved.
Description
Technical Field
The invention belongs to the field of biomass pyrolysis, and particularly relates to a biomass large-particle pyrolysis simulation method based on a sphere cluster hypothesis and a Laguerre-Voronoi structure, which can be used for exploring biomass pyrolysis behaviors and phenomena in single-particle and multi-particle systems.
Background
Biomass pyrolysis has important application prospects in new energy conversion and utilization due to the reasons of raw material reproducibility, product diversity and the like. At present, besides experimental measurement, the research in the field is performed by utilizing a numerical simulation method, so that details which cannot be obtained in a pyrolysis experiment can be provided, the design and operation of the experiment can be further guided, and the research, development and application costs are reduced.
In the actual process, biomass pyrolysis mostly occurs in a fluidized bed, a fixed bed and other reactors. Multi-particle simulation work at this scale often takes too much simplifying assumptions, such as assuming spherical particles, ignoring thermal gradients inside particles, assuming equal volume or equal density transformations of particles, etc. These assumptions do not apply to anisotropic biomass particles, and therefore there is a need to develop finer and efficient particle-scale pyrolysis models and use them in multiparticulate systems to improve the accuracy and performance of reactor-scale simulation.
Existing particle scale models that take into account fluid transport within the particles are computationally expensive. The single-particle pyrolysis model neglecting the flow field in the particle is mostly only suitable for particles with regular shapes such as spheres, cubes and the like, and the anisotropic heat transmission and shrinkage process in the particle cannot be analyzed.
Disclosure of Invention
Aiming at the technical difficulties, the invention provides a biomass large particle pyrolysis simulation method based on a sphere cluster hypothesis and a Laguerre-Voronoi structure. The method belongs to the field of biomass pyrolysis, can be used for exploring the behavior and phenomenon of biomass pyrolysis in single-particle and multi-particle systems, and mainly comprises three steps of initial sphere cluster generation, time step dispersion, pyrolysis iteration and result output. The invention realizes pyrolysis characterization of any irregularly-shaped particles under any thermal boundary condition by utilizing sphere cluster hypothesis; the shrinkage of the mother particles is realized by the combination of volume reduction and movement of the constituent son spheres; the Laguerre-Voronoi structure is utilized to divide the internal region of the particle, so that not only is the internal (inter-sphere) heat transmission process of the particle realized, but also the internal pore structure of the particle is analyzed, and meanwhile, support is provided for sphere movement calculation; the anisotropic thermal conduction and shrinkage processes of the particles are taken into account by the use of direction dependent thermal conductivity and shrinkage factors. The method provides a rapid and effective biomass large-particle pyrolysis simulation method based on the sphere cluster assumption and the Laguerre-Voronoi structure, is beneficial to obtaining internal details which are difficult to observe in the biomass pyrolysis process, can be expanded to be used in a reactor scale comprehensive model containing multiple biomass particles, and further improves the prediction precision and accuracy of large-scale simulation.
Specifically, the invention provides a biomass large particle pyrolysis simulation method based on a sphere cluster hypothesis and a Laguerre-Voronoi structure, which comprises the following steps:
S1, initial ball cluster generation
Simulating a real biomass material by adopting a sub-sphere with the size, density, temperature and material composition properties, forming a sphere cluster by a plurality of sub-spheres, and equivalently representing original biomass large particles by the sphere cluster;
S2, dividing the pyrolysis process into a plurality of time intervals on a time scale, and carrying out particle pyrolysis iteration on each time interval; the particle pyrolysis iterative process comprises four sub-steps of particle structure updating, heat transfer calculation, pyrolysis reaction calculation and particle shrinkage calculation;
S3, finishing iteration and outputting a calculation result.
As a preferred embodiment of the present invention, the initial ball cluster generation method includes: grid division is carried out on the original real biomass by utilizing a grid generation technology; according to the porosity of the original real biomass and the grid position and volume, generating sub-spheres with specific positions and radiuses in sequence, wherein the sub-spheres have intrinsic density consistent with that of the original real biomass; the space between the sub-spheres corresponds to the pores inside the real biomass material; the temperature and the material composition properties of the subspheres are consistent with those of the original real biomass.
As a preferred scheme of the invention, the particle structure update comprises an outer envelope surface update and an inner Laguerre-Voronoi structure update;
the method for updating the outer envelope surface specifically comprises the following steps: enveloping the whole sphere cluster in a convex polyhedron by adopting a plurality of planes tangential to the sphere of the sphere cluster surface, and forming a boundary of the represented biomass particles by the formed enveloping surface set;
The specific method for updating the internal Laguerre-Voronoi structure comprises the following steps: dividing a space region to which any sub-sphere belongs by using a Laguerre-Voronoi structure: the space region to which a sphere i with a center position of x i and a radius of r i belongs is defined as a Laguerre-Voronoi unit, wherein any point x meets the following conditions
The constant holds for any sphere j (j+.i), where x j,rj is the center position and radius of sphere j, respectively, and d (x, x i) and d (x, x j) represent the distance of point x from the center of spheres i and j.
As a preferred embodiment of the present invention, the heat transfer calculation includes an external convection radiation calculation and an internal heat conduction calculation;
The external convection radiation calculation only affects the temperature change of the outer surface subsphere, and the heat flux is calculated according to the area of the envelope surface and the local environment convection and radiation related parameters;
the internal heat conduction calculation influences the temperature change of any sub-sphere, and the heat flux is calculated by the heat conduction coefficient, the temperature difference and the relative position of the adjacent sub-spheres and the interface area of the Laguerre-Voronoi unit; if the thermal conductivity is correlated to the relative positions of adjacent pellets, the anisotropic thermal conduction process within the biomass particles can be simulated.
According to the invention, pyrolysis calculation is carried out on the nanospheres sequentially by adopting a pyrolysis mechanism and dynamics model, and the quality and density change of the nanospheres are obtained according to the reacted product.
Further, the pyrolysis mechanism and dynamics model can be an Arrhenius model.
As a preferred embodiment of the present invention, the particle shrinkage calculation includes a subspheric volume update and a subspheric translation calculation;
Wherein the regeneration of the subsphere volume is realized by mass change and density change in the pyrolysis process, and the formula is that
Wherein V i,mi,ρi represents the volume, mass and density of the subsphere i, respectively;
The ball translation calculation includes an active translation ball calculation and a passive translation ball calculation.
The active translation sub-sphere calculation method comprises the following steps: firstly, defining a shrinkage threshold gamma threshold, wherein the value of the shrinkage threshold gamma threshold is selected in the range of [0,1 ]; then the sub-spheres are exhausted, which satisfies
Ωi＞γthreshold·max(Ω)
The related subspheres all actively move, but do not have active moving speed, wherein omega i represents the solid consumption rate of the subsphere i in unit volume, and max (omega) represents the maximum solid consumption rate of the subsphere i in unit volume at the current moment;
The active speed shifting of the nanospheres is based on the assumption that the porosity of the corresponding Laguerre-Voronoi units is unchanged in time steps, and the calculation formula is as follows:
Wherein u i is the active moving speed of the pellet i, S celli and V celli represent the surface area and volume of the Laguerre-Voronoi unit to which the pellet i belongs, (x c,yc,zc) and (x i,yi,zi) are the center coordinates of the biomass pellet and the ith constituent pellet thereof, and χ x,χy and χ z are shrinkage factors in the x, y and z directions, respectively, for achieving anisotropic shrinkage of the biomass pellet.
The passive translation sub-sphere calculation method comprises the following steps: aiming at any actively moving sub-ball i, searching to obtain a sub-ball j on a boundary, so that a connecting line vector of the sub-ball j and the sub-ball i is closest to the active speed moving direction of the sub-ball i; searching between the sub-balls i and j to obtain the shortest path; and finally, all the subspheres except the subsphere i on the path are subjected to passive translation, and the corresponding passive translation speed is 2u i.
Further, the output calculation result may include biomass large particle pyrolysis details such as temperature distribution and evolution, density and porosity distribution and evolution, particle morphology evolution, particle quality evolution and the like in the particle, wherein relevant information and evolution process in the particle are difficult to observe and obtain by an experimental method.
Compared with the prior art, the invention has the following beneficial effects:
(1) The adopted sphere cluster hypothesis can realize the representation of biomass particles with any irregular shape and analyze the morphological change of the biomass particles in the pyrolysis process;
(2) The internal region of the particle is divided by the Laguerre-Voronoi structure, so that not only can the internal (inter-subsphere) heat transmission process of the particle be realized, but also the internal pore structure of the particle can be analyzed, and further support is provided for the calculation of the shrinkage of the particle;
(3) The anisotropic heat conduction and shrinkage process of the particles can be solved by utilizing the heat conductivity and shrinkage factors related to the directions;
(4) Not only can the detailed information of the interior of the particles which are difficult to observe in the experiment in the single biomass particle pyrolysis process be obtained, but also the detailed information can be integrated with a multi-particle reactor scale computational fluid dynamics model, so that the prediction precision and accuracy of large-scale simulation are remarkably improved.
Drawings
FIG. 1 is a flow chart of the method of the present invention.
Fig. 2 is a schematic representation of a typical particle representation using the sphere cluster hypothesis.
Fig. 3 shows a certain sphere particle structure.
Fig. 4 is a schematic diagram of heat transfer calculation.
Fig. 5 is a schematic diagram of a passive contraction calculation of a sphere.
FIG. 6 is a graph showing the evolution of the center temperature and the residual mass fraction of a certain spherical biomass particle under a certain working condition.
Fig. 7 shows the morphological evolution of a certain cubic biomass particle in a bottom radiant heating state in a pyrolysis process.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in further detail with reference to the accompanying drawings.
As shown in fig. 1, a flow chart of the method for acquiring internal detail parameters in the biomass large particle pyrolysis process based on the sphere cluster hypothesis and the Laguerre-Voronoi structure is shown. The method mainly comprises the following steps:
Step one, initial ball cluster generation. As shown in fig. 2, several sub-spheres with the properties of size, density, temperature, material composition and the like are adopted to jointly form an initial sphere cluster so as to equivalently represent the original biomass large particles; the subspheres have an intrinsic density of locally true biomass material; the space between the subspheres is abstracted into a pore structure inside the material; fig. 2 shows a sphere and irregular shape of a sub-sphere representation of two primary biomass large particles.
Specifically, the method for generating the initial ball cluster comprises the following steps: firstly, carrying out grid division on original biomass by utilizing a grid generation technology, such as hexahedral structured grids; and then generating the subspheres with specific positions and radiuses according to the porosity of the original biomass, the grid positions and the volumes in sequence, wherein the temperature, the material composition and other properties of the subspheres are consistent with those of the original biomass.
Dividing the pyrolysis process into a plurality of intervals on a time scale, wherein the intervals can be divided in a mode of equally dividing the time intervals, and performing particle pyrolysis iteration on each interval, wherein the steps comprise particle structure updating, heat transfer calculation, pyrolysis reaction calculation and particle shrinkage calculation;
The particle structure update sub-step includes an outer envelope surface update and an inner Laguerre-Voronoi structure update. The outer envelope surface updating method is to use a plurality of planes tangent to the surface sub-sphere to envelope the whole sphere cluster in a convex polyhedron, and the formed envelope surface set forms the boundary of the represented biomass particles. The internal Laguerre-Voronoi structure updating method is to divide a space region to which any sub-sphere belongs by utilizing the Laguerre-Voronoi structure; the center position is x i, and any point x in a space region (called Laguerre-Voronoi unit below) to which a sphere i with radius r i belongs meets
The constant holds for any sphere j (j+.i), where x j,rj is the center position and radius of sphere j, respectively, and d (x, x i) and d (x, x j) represent the distance of point x from the center of spheres i and j. A typical spherical particle structure is shown in fig. 3.
The heat transfer calculation sub-step aims at updating the temperature of the sub-sphere, including the external convective radiation calculation and the internal thermal conduction calculation, as shown in fig. 4. The external convective radiation calculation is only valid for the outer surface sub-spheres, the heat flux of which is calculated from the envelope surface area and the local environment convection and radiation related parameters. The internal heat conduction and heat flux is obtained by calculation of the heat conduction coefficient, the temperature difference and the relative position of adjacent sub-spheres and the interface area of the Laguerre-Voronoi unit; the thermal conductivity is related to the relative positions of adjacent sub-spheres.
The pyrolysis reaction substep is calculated by adopting a specific pyrolysis mechanism and a dynamic model (such as an Arrhenius model) to sequentially carry out pyrolysis calculation on the sub-spheres, and the quality and density change of the sub-spheres are obtained according to the reacted product.
The particle shrinkage calculation substep includes a subspheric volume update and a subspheric translation calculation. The sphere volume update is calculated by
Wherein V i,mi,ρi represents the volume, mass and density of the sub-sphere i, respectively. The ball translation calculation includes an active translation ball calculation and a passive translation ball calculation.
The method for judging the active translation sub-ball comprises the following steps: firstly, defining a shrinkage threshold gamma threshold, wherein the value of the shrinkage threshold gamma threshold is selected in the range of [0,1 ]; then the sub-spheres are exhausted, which satisfies
Ωi＞γthreshold·max(Ω)
The related subspheres all actively move, but do not have active moving speed, wherein omega i represents the solid consumption rate of the subsphere i in unit volume, and max (omega) represents the maximum solid consumption rate of the subsphere i in unit volume at the current moment; the calculation formula of the active speed of the ball is as follows:
Wherein u i is the active moving speed of the sub-sphere i, S celli and V celli represent the surface area and volume of the Laguerre-Voronoi unit to which the sub-sphere i belongs, (x c,yc,zc) and (x i,yi,zi) are the center coordinates of the biomass particles and the i-th constituent sub-sphere thereof, and χ x,χy and χ z are shrinkage factors in the x, y and z directions, respectively, for realizing anisotropic shrinkage of the biomass particles;
As shown in fig. 5, the passive translational balloon determination and solving method is as follows: aiming at any actively moving sub-ball i, searching to obtain a sub-ball j on a boundary, so that a connecting line vector of the sub-ball j and the sub-ball i is closest to the active speed moving direction of the sub-ball i; searching between the sub-balls i and j to obtain the shortest path; finally, all the subspheres (except subsphere i) on the path are subjected to passive translation, and the corresponding passive translation speed is 2u i.
And step three, finishing iteration and outputting a calculation result. The results can include biomass large particle pyrolysis details such as temperature distribution and evolution, density and porosity distribution and evolution, particle morphology evolution, particle quality evolution and the like in the particles.
The method of the invention can be implanted, operated and simulated in any programmable platform, and the convection and radiation states of the particle surface are required to be specified by oneself during simulation. For example, the simulation method is realized by adopting C++ language, and the pyrolysis process of a certain sphere and a certain cube particle is solved to obtain fig. 6 and 7, wherein fig. 6 is a particle center temperature and residual mass fraction evolution diagram of the certain sphere particle under a certain working condition, and fig. 7 is a pyrolysis process morphological evolution of the certain cube particle under a bottom radiation heating state; from the results, the center temperature and the mass loss of the spherical particles are well compared with those of experiments, and the morphological change of the cubic particles is also in accordance with the actual process. The method can obtain the detail information of the interior of the particles which is difficult to observe in the experiment in the single biomass particle pyrolysis process, and can also be integrated with a multi-particle reactor scale computational fluid dynamics model, so that the prediction precision and accuracy of large-scale simulation are remarkably improved.
Claims (6)
1. The biomass large particle pyrolysis simulation method based on the sphere cluster hypothesis and the Laguerre-Voronoi structure is characterized by comprising the following steps of:
S1, initial ball cluster generation
Simulating a real biomass material by adopting a sub-sphere with the size, density, temperature and material composition properties, forming a sphere cluster by a plurality of sub-spheres, and equivalently representing original biomass large particles by the sphere cluster;
S2, dividing the pyrolysis process into a plurality of time intervals on a time scale, and carrying out particle pyrolysis iteration on each time interval; the particle pyrolysis iterative process comprises four sub-steps of particle structure updating, heat transfer calculation, pyrolysis reaction calculation and particle shrinkage calculation;
The particle structure update comprises an outer envelope surface update and an inner Laguerre-Voronoi structure update;
the method for updating the outer envelope surface specifically comprises the following steps: enveloping the whole sphere cluster in a convex polyhedron by adopting a plurality of planes tangential to the sphere of the sphere cluster surface, and forming a boundary of the represented biomass particles by the formed enveloping surface set;
The specific method for updating the internal Laguerre-Voronoi structure comprises the following steps: dividing a space region to which any sub-sphere belongs by using a Laguerre-Voronoi structure: the space region to which a sphere i with a center position of x i and a radius of r i belongs is defined as a Laguerre-Voronoi unit, wherein any point x meets the following conditions
The constant holds for any sub-sphere j (j not equal to i), where x j,rj is the center position and radius of sub-sphere j, d (x, x i) and d (x, x j) represent the distance of point x from the centers of sub-spheres i and j, respectively;
The particle shrinkage calculation comprises the steps of sphere volume updating and sphere translation calculation;
Wherein the regeneration of the subsphere volume is realized by mass change and density change in the pyrolysis process, and the formula is that
Wherein V i,mi,ρi represents the volume, mass and density of the subsphere i, respectively;
the ball translation calculation comprises active ball translation calculation and passive ball translation calculation;
The active translation sub-sphere calculation method comprises the following steps: firstly, defining a shrinkage threshold gamma threshold, wherein the value of the shrinkage threshold gamma threshold is selected in the range of [0,1 ]; then the sub-spheres are exhausted, which satisfies
Ωi＞γthreshold·max(Ω)
The related subspheres all actively move, but do not have active moving speed, wherein omega i represents the solid consumption rate of the subsphere i in unit volume, and max (omega) represents the maximum solid consumption rate of the subsphere i in unit volume at the current moment;
The active speed shifting of the nanospheres is based on the assumption that the porosity of the corresponding Laguerre-Voronoi units is unchanged in time steps, and the calculation formula is as follows:
Wherein u i is the active moving speed of the sub-sphere i, S celli and V celli represent the surface area and volume of the Laguerre-Voronoi unit to which the sub-sphere i belongs, (x c,yc,zc) and (x i,yi,zi) are the center coordinates of the biomass particles and the i-th constituent sub-sphere thereof, and χ x,χy and χ z are shrinkage factors in the x, y and z directions, respectively, for realizing anisotropic shrinkage of the biomass particles;
s3, finishing iteration and outputting a calculation result, wherein the calculation result comprises internal temperature distribution and evolution, density and porosity distribution and evolution, particle morphology evolution and particle quality evolution.
2. The method of claim 1, wherein the initial cluster generation method comprises: grid division is carried out on the original real biomass by utilizing a grid generation technology; according to the porosity of the original real biomass and the grid position and volume, generating sub-spheres with specific positions and radiuses in sequence, wherein the sub-spheres have intrinsic density consistent with that of the original real biomass; the space between the sub-spheres corresponds to the pores inside the real biomass material; the temperature and the material composition properties of the subspheres are consistent with those of the original real biomass.
3. The method of claim 1, wherein the heat transfer calculations include external convective radiation calculations and internal thermal conductivity calculations;
The external convection radiation calculation only affects the temperature change of the outer surface subsphere, and the heat flux is calculated according to the area of the envelope surface and the local environment convection and radiation related parameters;
The internal heat conduction calculation influences the temperature change of any sub-sphere, and the heat flux is calculated by the heat conduction coefficient, the temperature difference and the relative position of the adjacent sub-spheres and the interface area of the Laguerre-Voronoi unit; if the thermal conductivity coefficient is related to the relative position of adjacent sub-spheres, the anisotropic thermal conduction process in the biomass particles is simulated.
4. The method of claim 1, wherein the pyrolysis calculation is performed on the pellets sequentially using a kinetic model, and the pellet mass and density change is obtained from the product change before and after the reaction.
5. The method of claim 4, wherein the kinetic model is an Arrhenius model.
6. The method according to claim 1, wherein the passive translational subs calculating method comprises: aiming at any actively moving sub-ball i, searching to obtain a sub-ball j on a boundary, so that a connecting line vector of the sub-ball j and the sub-ball i is closest to the active speed moving direction of the sub-ball i; searching between the sub-balls i and j to obtain the shortest path; and finally, all the subspheres except the subsphere i on the path are subjected to passive translation, and the corresponding passive translation speed is 2u i.
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