A mechanistic study on char structure evolution during coal devolatilization - Experiments and model predictions

Char structures evolved during the devolatilization process have been found to play a significant role in the subsequent processes (e.g., char combustion and gasification) and to influence the ash formation mechanisms. In the present paper, a mathematical model has been developed based on the multibubble mechanism to simulate the char structure evolution process. The model is the first to provide predictions of heterogenous char structures evolved during devolatilization (e.g. cenospheric char, foam structure, or dense char structure) as well as transient particle swelling ratios, based on the ultimate and proximate data of the given coal. The devolatilization process is divided into the preplastic stage, plastic stage, and resolidified stage. Bubble number conservation, mass, and force balance are formulated during the plastic stage to predict the transient swelling ratio and resultant char structures. Experiments have been conducted using a single coal particle reactor (SPR) and a drop tube furnace (DTF) with density-separated coal samples prepared using the sink-float method. The SPR experiments confirm that bubble behavior is responsible for the swelling of the particles that develop plasticity on heating. The analysis of the DTF chars shows that the swelling ratio and porosity decrease with increasing the coal density. Chars from lowdensity samples are mainly Group I chars (porosity >80%), while high-density samples yield mainly Group III chars (porosity <50%), and the medium-density samples contain a mixture. The predicted swelling ratio, porosity, and char type distribution of the chars of the density-separated samples are consistent with the experimental data.