Compartment Modeling for Flow Characterization of Underground Coal Gasification Cavity

During underground coal gasification (UCG), a cavity is formed in the coal seam when coal is converted to gaseous products. This cavity grows three dimensionally in a nonlinear fashion as gasification proceeds. The cavity shape is determined by the flow field, which is a strong function of various parameters such as the position and orientation of the inlet nozzle and the temperature distribution and coal properties such as thermal conductivity. In addition to the complex flow patterns in the UCG cavity, several phenomena occur simultaneously. They include chemical reactions (both homogeneous and heterogeneous), water influx, thermomechanical failure of the coal, heat and mass transfer, and so on. Thus, enormous computational efforts are required to simulate the performance of UCG through a mathematical model. It is therefore necessary to simplify the modeling approach for relatively quick but reliable predictions for application in process design and optimization. The primary objective of this work is to understand the velocity distribution and quantify the nonideal flow patterns in a UCG cavity by performing residence time distribution (RTD) studies using computational fluid dynamics (CFD). The methodology of obtaining RTD by CFD is validated by means of of representative laboratory-scale tracer experiments. Based on the RTD studies, the actual UCG cavity at different times is modeled as a simplified network of ideal reactors, called compartments. The compartment model thus obtained could offer a computationally less expensive and easier option for determining UCG process performance at any given time, when used in a reactor-scale model including reactions. The network of ideal reactors can be easily simulated using a flowsheet simulator (e.g., Aspen Plus). We illustrate the proposed modeling approach by presenting selected simulation results for a single gas-phase second-order water gas shift reaction.