Laboratory studies on combustion cavity growth in lignite coal blocks in the context of underground coal gasification

Cavity formation is an important phenomenon in the underground coal gasification (UCG) process. In the early stages of cavity formation, only the combustion reaction is performed in order to stabilize the temperature field. In the present work, we study the formation of the combustion cavity and the effect of various design and operating parameters such as the distance between the wells, feed flow rate and operation time, on its evolution. This paper presents details of laboratory-scale experiments that demonstrate the shape and size of the combustion cavity, and their dependence on various parameters. Empirical correlations for the cavity volume and dimensions in various directions are developed, which indicate the strong effects of mass transport. Results from computational fluid dynamics (CFD), which map the velocity profiles in the cavity, support the experimental observations. (C) 2010 Elsevier Ltd. All rights reserved

Compartment Modeling and Flow Characterization in Nonisothermal Underground Coal Gasification Cavities

Characterization of reactant gas flow patterns in the underground coal gasification (UCG) cavity is important, because the flow is highly nonideal and likely to influence the quality of the product gas. In our earlier work [Daggupati et al., Energy 2010, 35, 2374-2386], we have demonstrated a computational fluid dynamics (CFD)-based modeling approach to analyze the flow patterns in the cavity. A compartment model (network of ideal reactors) for the UCG cavity was developed based on the CFD simulation results. These studies were performed assuming that the UCG cavity is isothermal. In reality, large temperature gradients may prevail under certain conditions and, in turn, may influence the flow patterns. In this work, we consider different possible nonisothermal scenarios in the UCG cavity and propose a simplified compartment modeling strategy to reduce the computational burden. We also examine the effect of various operating and design parameters such as coal spalling, feed flow rate, feed temperature, and orientation of the inlet nozzle. All these effects are quantified by determining the corresponding compartment model parameters. The sensitivity of the compartment model parameters, with respect to the changes in various conditions, is studied. Furthermore, we validate the compartment modeling approach by comparing predicted conversions for a water-gas shift reaction with that of reaction-enabled CFD simulations under nonisothermal conditions. The results presented here provide adequate insight into the UCG process and can be conveniently used in the development of a computationally inexpensive phenomenological process model for the complex UCG process

Laboratory studies on cavity growth and product gas composition in the context of underground coal gasification

Systematic laboratory scale experiments on coal blocks can provide significant insight into the underground coal gasification (UCG) process. Our earlier work has demonstrated the various features of the early UCG cavity shape and rate of growth through lab-scale experiments on coal combustion, wherein the feed gas is oxygen. In this paper, we study the feasibility of in situ gasification of coal in a similar laboratory scale reactor set-up, under conditions relevant for field practice of UCG, using an oxygen-steam mixture as the feed gas. By performing the gasification reaction in a cyclic manner, we have been able to obtain a product gas with hydrogen concentrations as high as 39% and a calorific value of 178 kJ/mol. The effect of various operating parameters such as feed temperature, feed steam to oxygen ratio, initial combustion time and so on, on the product gas composition is studied and the optimum operating conditions in order to achieve desired conversion to syngas, are determined. We also study the effect of various design and operating parameters on the evolution of the gasification cavity. Empirical correlations are proposed for the change in cavity volume and its dimensions in various directions. The results of the previous study on the combustion cavity evolution are compared with this gasification study. (C) 2011 Elsevier Ltd. All rights reserved

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