An Integrated Model for Underground Coal Gasification

Abstract

Underground coal gasification receives renewed interest in both Western and Eastern Europe because of the vast amounts of otherwise unminable coal deposits that occur on the European continent. A field test is currently being carried out in Spain. Other countries in Europe show interest in this method of utilizing coal resources. In this study we present a model that describes the development of an underground coal gasifier. Both permeable bed and channel surface gasification are combined with thermo-mechanical failure of both coal and overlying rock. By this combination the model predicts the channel development and thus the sweep efficiency of the gasification process. The model consists of two modules: the first module solves the flow equations in the entire flow domain. The second module selects a block of coal for gasification and a block of coal and/or rock for thermo-mechanically induced spalling. Other features such as ash content and heterogeneity are included in the model as well. Model calculations show that the ash content of the coal and the coal and rock spall rate determine largely the sweep efFiciency of the gasification process. The model is used for field test evaluation purposes and can be used for extrapolation of the test results to other situations, e.g. the deep lying thin coal layers that occur in North-Western Europe. Introduction Underground coal gasification is a method of utilizing the vast otherwise unminable coal resources. Deep lying, thin coal seams such as occur in North-Western Europe can possibly be economically recovered by the application of standard directional drilling techniques. A typical configuration consists of a horizontal injection well with a vertical producer. Prior to the gasification step a final linkage path is created between injector and producer using, for example, reverse combustion techniques to char the coal. The gasification step starts by ignition of the coal and the injection of air or air enriched with oxygen. Subsequently, a cavity is formed in the coal seam. When the gas quality deteriorates the injection well is burnt to inject further upstream. This technique is called CRIP i.e. controlled retraction injection point. The situation during the first CRIP step is schematically shown in Figure 1. The successful underground coal gasification field tests were designed for two or more of these CRIP steps. The design of a field test and its interpretation would greatly benefit from a simple (i.e., low computational cost) dedicated simulator. The model must be able to describe the development from the early stages of gasification to a fully developed gasifier. The field test in Pricetown shows a possible mature configuration of a gasifier; gases percolate through ash and rock debris to a teardrop shaped channel where surface gasification takes place. A model for the initial stages has been developed by Britten and Thorsness. Their model was successful to describe field tests in thick coal layers. For economical gasification of thin coal layers, however, the cavity must develop to cover a radius much larger than the layer thickness. P. 191^

Innovative thermodynamic underground coal gasification model for coupled synthesis gas quality and tar production analyses

“…water ingress, heat losses), in consequence partially leading to lower synthesis gas qualities and undesired hazardous byproducts [1,3,[8][9][10][11][12][13][14]. Pollutant charged gas leakage from the UCG reactor due to roof rock failure as a thermo-mechanical response of in-situ pressure and temperature conditions recently marks one of the most severe potential environmental impacts [1,6,[15][16][17][18][19]. With focus on the growing public debate on gas leakage affiliated with organic groundwater pollution risks as well as general tar plugging problems, a key future UCG by-product challenge thus lies in better tar production control and aligned organic pollutant minimization in UCG gases [20][21][22][23].…”

Modelling Underground Coal Gasification—A Review

“…From the observable entity, by changing the injection point, they concluded that oxygen should be injected at the bottom of the channel, otherwise valuable gasification products would be oxidized, leading to a low heating value of the production gas. The model was verified with Biezen's experiments [71] for double-diffusive natural convection in a trapezoidal channel.…”

“…This model is very limited, as the temperature of the gases and composition are constant in each region. From the same research group, Biezen et al [93] extended the concept of the probabilistic simulation by including the movement of several interfaces including ash-void, ash-coal, void-coal, and void-rock. The model consists of two modules: one module solves flow in the entire domain and the other selects a block of coal/rock for gasification or roof spalling.…”

“…Recently, case studies of the feasibility for UCG-CCS storage have been carried out in the Powder River Basin of Wyoming, US [7,93,113,119], at a selected site in Bulgaria [91,93], and at the Williston Basin, North Dakota, US [108]. Roddy and Younger [111] studied the potential of carbon storage and addressed surface subsidence and aquifer contamination as the main challenges for UCG-CCS application; however, in a later paper, Younger [120] studied hydrogeological and geomechanical aspects of UCG and proposed an action plan in order to avoid subsidence and aquifer contamination in the life cycle of UCG-CCS operations.…”

A process model for underground coal gasification – Part-I: Cavity growth

“…The model should provide useful inputs at the design stage for determining capacity of a pair of wells and should be able to predict the effects of unforeseen events such as water intrusion or sudden spalling. Several modeling efforts are reported in the literature and reviews are found elsewhere 2,7 .Available models can be classified into packed bed models [8][9][10] , channel models [11][12][13][14] , coal block models [15][16][17][18] and process models [19][20][21] . However, first three classifications have idealized flow patterns, and hence cannot adequately include effect of actual flow patterns on product gas composition.…”