Preeti Aghalayam

Underground Coal Gasification (UCG) is considered to be a clean coal technology primarily intended to utilize deep underground (>300 m) coal deposits. In this process, a mixture of reactant gases like air/oxygen and steam are injected directly to an ignited portion of underground coal seam. UCG involves complex interactions of different processes like drying, pyrolysis, chemical reactions and spalling. Spalling is detachment of small coal particles from the coal seam due to interconnection of cracks developed in it. It plays an important role by offering higher surface area to give improved performance. The mechanism of spalling and its characterization are not well understood. Furthermore, there are no well established experimental techniques to measure the spalling rates. This paper studies spalling behavior of a lignite coal, which is characterized by high moisture and volatile matter, and suggests a possible underlying mechanism. The rate of spalling was measured using an experimental setup under the UCG-like conditions. In this setup, a reacting coal block was attached to a load cell and suspended in a UCG-like environment. When the experiments were repeated under similar conditions with different blocks of same coal, it was found that there were variations in the rates of spalling. This might be due to the heterogeneity in coal blocks in the form of originally present fissures or weak regions. A UCG process model was used to explain these experimental results and also to investigate the effect of spalling rate on product gas calorific value. We believe that spalling happens due to formation and extension of cracks. Hence a microscopic crack pattern on a heated coal monolith was examined in different stages of heating to understand the mechanism of spalling. It is concluded that cracks are first formed during the initial stage of drying due to the capillary stresses developed due to removal of moisture from the pores and were further extended due to shrinkage of coal during pyrolysis. The detachment of coal particles happens due to horizontal linking of vertical cracks, which might result out of either horizontal cracks, if any, or available fissures and weak regions or relatively weak interlayer bonding at the bedding planes.

Underground gas gasification (UCG) is a clean coal technology which involves in-situ gasification of deep-seated underground coal. The process can be divided in two phases based on state of coal seam and direction of cavity growth. In phase-I, cavity grows mainly in vertical direction while in phase-II it grows in horizontal direction. The in-house simulator developed for both the phases of UCG has been reported earlier Samdani et al. (2016a,b). It incorporates reaction kinetics, flow patterns, spalling, heat and mass transfer effects. In this work, we take further insight and perform parametric studies to examine the effects of different operating conditions, coal properties and design parameters on key performance indicators i.e. exit gas quality, energy generation rates etc. The investigation revealed that the exit gas quality and rate of coal consumption are strong functions of spalling rates and kinetics of reactions; the coal having very low spalling tendency or less reactivity may not be favorable for the UCG process. An important parameter called critical spalling rate has emerged through this analysis. It is the property of given coal above which UCG is sustainable. In addition, model performance is also sensitive to inlet gas temperature, pressure and composition. Optimum performance of UCG is obtained at a steam to oxygen ratio of 2.5 and at the highest possible inlet gas temperature, operating pressure, and oxygen content in the feed. Among the design parameters, the length of outflow channel is very important as it strongly affects both the exit gas calorific value and its fluctuations with time. The predicted effects of different parameters are in accord with the observations during lab-scale UCG experiments and different field trials. This study demonstrates the importance of a process model to determine the best conditions for UCG process and to evaluate feasibility of the process for a coal seam under consideration.

Underground Coal Gasification (UCG) is a process of gasifying coal in-situ to produce syn-gas. The gas thus produced, passes through the outflow channel that leads to the production well. As explained in part-I of this paper (Samdani et al., 2015), UCG can be divided in two distinct phases. The phase-I corresponds to initial vertical growth of the cavity and the output from phase-I model provides input to the phase-II model. This paper presents an unsteady state model for phase-II of UCG, wherein, the growth occurs in the horizontal direction towards the production well through the outflow channel. A compartment model, based on tracer studies performed on actual UCG cavity, is developed for phase-II of UCG. Here, the outflow channel is divided in small sections along the length, each consisting of rubble zone, void zone and roof at the top. This reduces the complexity caused by non-ideal flow patterns and changing sizes of different subzones inside the outflow channel. The subzones and the sections are linked appropriately, for mass and energy flow, to give overall performance of UCG. The proposed approach combines chemical reactions, heat and mass transfer effects, spalling characteristic and complex flow patterns to achieve meaningful results. In all, seven gas species, three solid species and eleven reactions are included. The simulation results such as variation in solid density, dynamics of different zones, exit gas quality are presented. The model is validated by comparing the predicted exit gas quality and that observed during similar laboratory scale experiments. Finally the results are also compared with pilot scale field-trials. This model along with the phase-I model provides a complete modeling solution for UCG process.

Underground Coal Gasification (UCG) process is studied through systematic laboratory scale experiments. Our earlier published work (Daggupati et al., 2011) demonstrated the various features of the Indian lignite coal in context of its applicability for UCG. In the present work, we study a hard Indian coal, with low volatile matter and moisture content. These results are compared with that of lignite type soft Indian coal, which has relatively high volatile matter and moisture content. The syn-gas produced from hard coal has a calorific value of 69 kJ/mol whereas the syn-gas from lignite coal has a higher calorific value of 170 kJ/mol, under similar conditions, in the laboratory experiments that mimic UCG process. Since UCG is a complex process involving different phenomena like spalling, gas-solid reactions of char and diffusion of gas in the char layer, separate studies on these aspects are required to explain the difference in the behaviors of these two coals in UCG. Spalling tendencies of these two coals are studied qualitatively by performing separate sets of experiments and the findings are used to explain the laboratory scale UCG results. The spalling experiments show that the hard coal has no tendency to spall, but lignite coal spalls, especially under high temperature and reactive atmosphere. The reactivity of the respective chars is studied separately using Thermo Gravimetric Analyzer. It is found that the char produced from lignite coal has higher reactivity of around ten times than the char produced from the hard coal. The paper thus presents a simple laboratory method to evaluate the feasibility of a given coal for UCG with theoretical analysis of the results obtained. © 2013 Elsevier Ltd. All rights reserved.

In underground coal gasification (UCG), a cavity is formed in the coal seam due to consumption of coal. The irregular-shaped cavity consists of a spalled-rubble on the cavity floor, a cavity roof and a void zone between the two. Depending on the cavity growth pattern, UCG process can be divided into two distinct phases. In phase-I, coal/char near injection well gets consumed and cavity grows in a vertical direction and hits the overburden. Phase-II starts thereafter, in which the cavity grows in the horizontal direction toward the production well. This paper presents an unsteady-state model for gas production during phase-I for a coal under consideration for UCG. The non-ideal flow patterns in the cavity are determined using computational fluid dynamics (CFD). The CFD results and residence time distribution (RTD) studies show that the complex UCG cavity can be reduced to a computationally less time consuming compartment model consisting of a radial plug flow reactor (PFR) followed by a continuous stirred tank reactor (CSTR). The developed compartment model incorporates reaction kinetics, heat-transfer, mass-transfer, diffusional limitations and thermo-mechanical failure effects for the coal of interest. The model is tested on a lab scale UCG; it can predict the location of reaction and drying fronts, profiles of solid and gas compositions, exit gas calorific value and cavity growth rates. Further, the model predictions show an excellent match with the cavity growth rate and exit gas quality observed during laboratory-scale UCG-like experiments on the coal of interest. Therefore, the model can potentially be used to determine feasibility of UCG for any other coal for the known kinetics and spalling parameters.