The influence of char particle morphology on char burnout behavior by atomistic simulation

Abstract

Bituminous coal char particles will differ in morphology, porosity, reactivity, and behavior within the zone II regime (where most practical system operates) due to the influence of maceral, rank, and time–temperature-histories. Effective boiler design requires simulations that appropriately capture char behavior. Most simulations, however, assume solid spherical particles that have homogeneous physical and chemical properties. Unfortunately, coal and time–temperature-history variations make it impossible to make a direct comparison of morphologies alone using traditional experimental approaches. Thus, here the influences of coal char morphologies on its reaction behavior and physical structure were explored via small-scale atomistic simulations. Three (ash-free) chars of solid, thick-, and thin-walled morphologies were examined. The atomistic representations had ∼ 143000 carbon atoms, comprised of similar polycyclic aromatic hydrocarbons (PAH) sizes and stacking distributions. While these structures are far below the actual particle size, the approach permits a direct comparison with a level of control not possible via physical experimentation. The densities of the thick- and thin-walled char models were similar to literature observations (apparent and helium densities). The specific surface area for the solid, thick-, and thin-walled char models were 110, 368, and 509 m2/g, with the latter two being closer to experimental data. A simplified combustion simulation (diffusion and oxygen proximity calculations) demonstrated that the char morphology had a significant impact upon their relative reactivities. The thin-walled structure had the highest overall reactivity due to greater reactive surface access, while the solid and thick-walled structure was respectively 27 and 23% lower. The reaction rate profiles with char conversion for these three structures are close to the experimental observations for pulverized bituminous coal chars, where the rate peak occurs at ∼ 20% conversion. The changes in char size, density, and specific surface area (with char conversion) indicated the thick-walled structure was more similar to the literature data. These atomistic approaches have the potential to aid in exploring the reaction behavior of char particles with various morphologies and complicated structures. With scale-up additional behaviors such as particle fragmentation and ash behavior could be incorporated.