Compartment Modeling for Flow Characterization of Underground Coal Gasification Cavity

Abstract

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.

Modelling Underground Coal Gasification—A Review

“…The higher coal permeability led to faster growth of the cavity and resulted in a much wider cavity ( Figure 50). Unlike some model predictions (i.e., Daggupati et al [90]), a very negligent effect of the temperature of the inlet oxygen was observed on cavity growth. Their model was validated with the lab-scale experiments of Daggupati et al [34] in terms of cavity growth (Table 22).…”

“…Recently, Daggupati et al [90] used computational fluid dynamics (CFD) to describe the cavity as series of ideal reactors ( Figure 46). The residence time distribution (RTD) of gases was calculated based on CFD results.…”

“…Their results indicated the STEP model as the best fitted model, and finally they concluded that the cavity behaves almost like a small number of CSTRs in a series with a high level of back-mixing. Recently, Daggupati et al [90] used computational fluid dynamics (CFD) to describe the cavity as series of ideal reactors ( Figure 46). The residence time distribution (RTD) of gases was calculated based on CFD results.…”

“…In order to account for tar, Shirazi et al [42] followed the kinetic model proposed by Boroson and Howard [90], who considered tar cracking with the following overall reaction: …”

Deployment of Underground Coal Gasification in India

“…Process models for evaluating endpoint use of syn-gas in a SOFC system in a combined cycle system were developed 45 . Development of steady state and dynamic models for computational analysis of the UCG reactor and detailed characterization of flow within the cavity have been reported 46 . This has happened alongside experimental analysis of the same 43 .…”