CFD study of the thermochemical characteristics of mesoscale bubbles in a BFB gasifier

Using the computational fluid dynamic model based on the multiphase particle-in-cell approach, reactive gas-solid flows in a three-dimensional bubbling fluidised bed gasifier regarding coal gasification is simulated. The predicted results of gas components are firstly validated with experimental measurements. Subsequently, the dynamics (e.g. chord length, aspect ratio, velocity, volume) together with the essential thermal features (e.g. temperature, pressure, density, viscosity, conductivity, specific heat capacity) of mesoscale bubbles are explored. Bubbles in the central part of the gasifier have a large lateral chord length and volume. The restriction effect of the gasifier wall elongates the vertical chord length and enlarges the aspect ratio of bubbles close to the wall. The density and pressure of bubbles decrease along with the bed height, and the thermochemical properties of bubbles are strongly related to bubble temperature. A large content of the CO₂ exists for those bubbles close to the bed surface. The gas velocity, operating pressure and gasifying agent obviously affect the bubble property, while the effects of inlet gas temperature and coal flow rate are relatively weak. The results obtained provide meaningful insight regarding the mesoscale structures involved in fluidised bed coal gasification.