A Theoretical Study of Reaction and Diffusion in Microstructured Materials

Configurational averaging, volume averaging, effective medium, coal combustion, cenospheres, combustion-generated aerosols

Loewenberg, Michael ; Gavalas, George R.

University:California Institute of Technology

Department:Chemistry and Chemical Engineering

关键词: Configurational averaging, volume averaging, effective medium, coal combustion, cenospheres, combustion-generated aerosols;   

Others  :  https://thesis.library.caltech.edu/2592/3/Loewenberg_m_1988.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

A fundamental study of heterogeneous reaction and diffusion in random, microstructured materials has been conducted. A detailed, ensemble averaging approach has been developed for the analysis of diffusion-controlled reactions. The method was used to determine the reactant flux into a bounded or semi-infinite medium containing a dilute suspension of reactive, spherical particles under steady-state or transient conditions. The influence of the boundary was given explicit, detailed consideration and the results were compared with a mean-field treatment. Physical motivation is provided by the process of ash vaporization during pulverized coal combustion. The analysis was subsequently extended to study the decay of an initially uniform distribution of reactant which allows comparison with other theoretical approaches. The result suggests that the present method reproduces the solution to this well-known problem by a seemingly simpler, more physical approach. The configurational averaging technique was employed in a study of heterogeneous reaction in a porous material under diffusion-controlled conditions. The porous solid is assumed to have a bimodal pore structure with a random, isotropic distribution of cylindrical macro-pores. The results are relevant to the pulverized combustion of char from softening coals. In the diffusion-controlled limit, the results coincide with a simpler, single pore model.

A simplified model of char combustion has been developed which treats pore diffusion and growth coupled to gas-phase heat and mass transport. An efficient model-based algorithm was developed for the determination of oxidation rates from combustion data. These models were applied in two studies involving well-defined laboratory combustion experiments.