热化学气固反应涉及的物理过程，跨越从微观到宏观的多个尺度，包括原子—表面—晶粒—颗粒—反应器。对单一尺度的物理过程，已有成熟的模型计算方法：原子尺度的反应机理以量子化学计算，表面尺度的表面反应以微观动力学计算，晶粒尺度的离子扩散和颗粒尺度的气体扩散以菲克定律计算，反应器尺度的流态化以CFD–DEM（计算流体力学—离散元方法）计算。以上模型在跨尺度过程中存在耦合，通过模型的输入、输出参数在不同尺度间的传递实现，其中微观尺度的结构参数影响宏观尺度的性能，宏观尺度的设计参数传递至微观尺度作为外部条件；但受计算规模的限制，无法简单将每个尺度的模型联立求解，而需在相邻尺度间引入耦合简化。现有的跨尺度模型研究均未实现由微观原子到宏观反应器的完整耦合，无法以严格理论方法描述反应过程。本文以降低计算量为导向，从微观到宏观尺度逐级建立模型，并在各自的参数传递机制下，相应给出耦合简化策略：在原子—表面尺度，复用单次量子化学计算结果，以其描述周期性表面上所有粒子的行为，并以统计力学预测系统宏观热力学量；在表面—晶粒尺度，将微观动力学作为固态扩散的边界条件，重点描述了非对等异种位点参与的表面和固态扩散，通过控制步骤和准稳态假设减少独立组分数，以构造解析速率方程；在晶粒—颗粒尺度，以表面气体浓度预测内部晶粒处浓度，将晶粒—孔隙的离散分布近似为连续分布，获得形式较简单的两相流控制方程；在颗粒—反应器尺度，数值计算流态化过程，以将反应器设计参数施加至每个颗粒，对颗粒内浓度场使用准稳态假设，以构造解析速率方程。逐级耦合得到完整跨尺度模型，适用于多种气固反应体系，编写为可扩展软件包并嵌入开源CFD–DEM软件进行计算，计算结果可通过微型流化床热重分析实验验证。该模型可揭示各尺度过程对总反应速率的影响，尤其是无法直接调整的微观参数。在原子尺度，温度对反应概率的影响主要取决于能垒。在表面尺度，总反应速率主要由控制步骤速率决定，对反应机理的判断可通过准稳态影响反应阶数。在晶粒和颗粒尺度，扩散与本征反应速率的相对快慢决定了反应物的空间分布。在反应器尺度，设计温度和浓度具有决定性影响；单颗粒反应速率受其轨迹影响较大，但在稳定流态化下系统总体表现稳定。
The physical processes during thermochemical heterogeneous reactions span across micro- and macroscopic scales, including the atoms, surface, grain, particle, and reactor. Single-scale models have been developed and used in the literature: quantum chemistry calculation for atom-scale mechanisms, microkinetics for surface-scale reactions, Fick’s Law for grain-scale and particle-scale diffusion, and CFD–DEM (computational fluid dynamics – discrete element method) for reactor-scale fluidization. The above models shall be coupled in multiscale processes by input/output parameter transmission between scales; structural parameters from microscopic scales affect the macroscopic performance, while design parameters from macroscopic scales are transmitted as external conditions to microscopic scales. However, due to the limit of computational complexity, a multiscale model cannot be developed by simply coupling every above model; thus, coupling simplifications are required between adjacent scales. No multiscale model in the literature has received complete coupling from the atoms to the reactor, which cannot provide a rigorous theoretical prediction of the entire reaction process.This study aims to reduce computational cost by hierarchically construct the model for every scale, and meanwhile apply corresponding coupling simplifications under specific parameter transmission. For the atom and surface scales, results from quantum chemistry are reused for all atoms on a periodical surface, and thermodynamic properties are calculated with statistical mechanics. For the surface and grain scales, the microkinetics act as boundary conditions for ion diffusion, emphasizing the surface and bulk diffusion on asymmetrical dual sites; an analytic rate equation is derived by eliminating variables based on the rate-determining step and quasi-steady assumptions. For the grain and particle scales, gas concentrations at inner grains are predicted from the particle surface condition; the discrete distribution of grains and pores is treated continuous, simplifying the governing equation to a two-phase mode. For the particle and reactor scales, numerical computation is conducted for fluidization, applying the reactor’s design parameters to every particle; the intraparticle gas concentration is assumed quasi-steady, which results in an analytic rate equation for the particle. A complete multiscale model is established as above, which is applicable to various heterogeneous reaction systems; an extensible software package is programmed and embedded into an open source CFD–DEM software, whose computation results can be experimentally validated through micro-fluidized-bed thermogravimetric analysis.The impact of parameters from every scale on the overall performance, especially the microscopic ones which are not adjustable, are revealed by the model. At the atom scale, temperature affects the reaction probability based on the energy barrier. At the surface scale, the rate-determining step controls the overall rate, while the mechanisms can determine the reaction order. At the grain and particle scales, the relative rate of diffusion versus intrinsic reaction determines the spatial distribution of reactants. At the reactor scale, the designed temperature and concentration have decisive impacts; single-particle reaction rate is deviated by its trajectory, while the average rate remains steady.
