Mutual effects of porosity and reactivity in char oxidation

The motivation for this review is the need to understand the interdependenc e of porous structure and reactivity of highly porous carbonaceous material s during oxidation. These materials can be oxidized in three regimes: regim e I, kinetically controlled conditions; regime II, partial diffusion-contro lled conditions; regime III, diffusion controlled conditions. Since the emp hasis here is on the porous structure and its influence on reactivity, cond itions where transport processes are dominant were not included for they ma sk the view of interest. Therefore, the review discusses only physicochemic al processes occurring during oxidation of highly porous chars in regime I. Furthermore, reactivity is influenced by many factors, such as catalysis, volatile matter, and water content. To avoid the effect of these factors, h ighly porous synthetic chars with nothing but elemental carbon and residual hydrogen and oxygen was chosen. Mainly, we discuss a commercial product kn own as Spherocarb which consists of spherical particles with specific surfa ce area of about 1000 m(2) g(-1) and porosity of about 0.6. These particles are well defined and reproducible in their properties. They serve well as model materials for various synthetic chars, coal chars, and other carbonac eous materials. The review presents in a systematic manner macroscopic prop erties and processes that shed light on different aspects of porosity and r eactivity. These are presented both from experimental observation as well a s modeling view. An attempt was made to present a porous structure model th at can reconstruct all available experimental data on these particles durin g oxidation. In the review the following processes and properties are discu ssed: shrinkage, fragmentation, and porosity. All are directly connected to porous structure and reactivity. (C) 2001 Elsevier Science Ltd. All rights reserved.