A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects
Introduction
The consumption of energy in the world continues to rise. According to the International Energy Agency (2012), global energy consumption will increase by over one-third by 2035 and fossil fuels are still dominating the global energy mix. However, due to climate change and a finite supply of fossil fuels, the use of alternatives such as geothermal energy, wind power, and solar power are set to increase (Bai et al., 2016). There is evidence that climate change is caused by anthropogenic greenhouse gas emissions, especially from fossil energy combustion (Ishida et al., 2014). Therefore, one of the main concerns of energy obtained from the burning of coal is that of the carbon footprint. According to Hancheng et al. (2016), it is crucial that technologies that are more energy efficient and/or that can provide energy with lower carbon emissions are adopted in order to mitigate climate change effects.
The extraction of energy in a sustainable form from unworked areas of coal deposits in the subsurface is challenging. Traditional coal mining methods emit considerable levels of CO2 to the environment from the machinery working a mining site. Underground coal gasification (UCG) is a promising option for extracting energy from coal in unworked areas of the subsurface (Imran et al., 2014). The process also provides a solution to obtain the coal energy of a thin coal seam that cannot be extracted by conventional methods. According to Bhutto et al. (2013), UCG is a combination of mining, exploitation, and gasification that eliminates the need for mining and can be used in deep or steeply dipping, un-mineable coal seams. Therefore, the UCG process offers a promising option to obtain energy from coal in a more environmentally friendly manner than traditional coal mining.
UCG was first trialled as early as 1868 by the German scientist Sir William Siemens; Anson G. Betts obtained the patent for UCG in 1909 (Bielowicz and Kasiński, 2014). The first UCG field test program in England was carried out by Ramsey in 1912 (Bhutto et al., 2013). Several other UCG trials have been carried out in other countries, e.g. in the former Union of Soviet Socialist Republics (USSR) (Derbin et al., 2015); in the United States (Klimenko, 2009), and in China (Creedy et al., 2004).
One method to reduce greenhouse gas effects is the storage of CO2 in the subsurface. The selection of a suitable site for CO2 storage depends on many factors (Lamas and Cámara, 2014). According to Durucan et al. (2014), factors such as geomechanical properties, geological structures, cap rock behaviour, hydrogeology, depth, and tectonic activities are the main factors that need to be considered for storing CO2. The selection of the UCG site also fulfils some of the factors that are necessary for CO2 storage. The UCG process leaves a cavity in the subsurface which could be used for the storage of CO2. Therefore, another significant potential benefit of coal gasification is that the created cavity could be used for the capture of CO2 and its sequestration (Sarhosisa et al., 2013).
The UCG process can be carried out in the field using several techniques. The fundamental methodology for UCG involves two wells, one serving as the injection well and the other as the production well (Fig. 1). Four distinct phases can be identified in the coal gasification process. During the first phase, drilling of the injection and production wells from the surface to the coal seam is carried out. In the second phase, establishment of a highly permeable path between the two wells is created/maintained. Methods such as hydraulic fracturing, electro-linkage, explosives, and in seam linkages can be incorporated for this purpose. A detailed description of the methods is given in Blinderman et al. (2008) and Friedmann et al. (2009). Air and/or oxygen is injected through the injection well to start the ignition of coal in the third phase. Finally, in the fourth phase, the extraction of syngas by the production well is carried out. A detailed description of the in-situ gasification process is presented by Shirsat (1989).
One of the main concerns of the UCG process is the ground induced subsidence. The effects of coal gasification was studied by Yang et al. (2014) using a numerical model for coupled thermal and mechanical responses to estimate ground subsidence. The finite element model, built in ABAQUS, simulated coal gasification at a depth of 1200 m in a 100 m thick coal seam. Several geological layers were presented in their numerical model to simulate real ground conditions. Laboratory measured geomechanical properties at room temperature were assigned to each geological layer. However, the study carried out by Ranjith et al. (2012) demonstrated that temperature had a significant effect on rock properties. According to the Yang et al. (2014) model, which assumed constant properties, the surface subsidence after three days of gasification was 0.08 mm.
A thermal-mechanical coupled model in FLAC-3D, developed by Najafi et al. (2014), provided an estimate of the protection of the pillar during the UCG process. Geomechanical properties assigned to the model were based on previously published data (Tian, 2013). The heating effect due to coal gasification was simulated by applying a fixed temperature of 1500 °C and 1200 °C to the upper and lower boundaries of the gasification cavity, respectively. However, during the gasification process, it is not realistic to assume fixed temperatures for the boundaries; temperatures will vary around the gasification area depending on the emission of energy due to coal burning.
A fully coupled thermal-hydraulic-mechanical (T-H-M) model was developed by Xia et al. (2014) to evaluate the coal mechanical deformation, gas flow and transport, and heat transport using COMSOL. The complex interactions of different phases were defined through a suite of coal property models. The models included the following properties: (1) coal porosity; (2) coal permeability; (3) gas equation-of-state; and (4) self-heating models. By using their oxygen consumption model, they found that during coal–oxidation heating, the oxygen concentration has an “S-type” downwards trend, whereas the heating temperature of coal and the gas velocity show “S-type” upwards trends. One of the significant drawbacks of their model is that it is comparatively very small compared to a real coal gasification chamber and it may not be possible to simulate a real field scale scenario with available computational power.
The industrial application of UCG is carried out with several parallel burning panels, as illustrated in Fig. 2, which enhances the gas production considerably compared to a single panel (Ekneligoda et al., 2015, 2016). One of the major concerns of parallel burning is induced ground subsidence. The selection of distance between parallel burning panels should be made after a thorough study of the effect of the coal burning process on ground subsidence.
In this study, numerical models were created using FLAC-3D to capture the geotechnical situation during the in-situ coal burning process at the Wieczorek UCG trial site in Poland. The model incorporates the variation of temperature in the cavity, which gradually decays with energy emission, backward movement of the burning head (from extraction to injection well), and temperature dependant material properties (Ekneligoda et al., 2015, 2016). Several laboratory experiments were conducted to measure the geomechanical properties of the relevant rock material at high temperature, which were incorporated in the numerical model. The numerical analyses were conducted in two stages. In Stage 1, the numerical model included a section near the UCG panel which was assigned a coupled thermal-mechanical constitutive relationship. Stage 2 involved a similar but more computationally efficient mechanical-only numerical model that incorporated results of maximum cavity size from Stage 1 in order to evaluate the worst-case scenario of ground movements from single and multiple UCG panels.
Section snippets
Model geometry
The FLAC-3D mesh was developed based on the geological profile at the Wieczorek UCG trial site in Poland. The mesh was fixed horizontally and free vertically along all vertical boundaries (representing planes of symmetry). The bottom boundary was fixed in the vertical direction but free horizontally.
An initial analysis was conducted taking into consideration only the mechanical response of the model due to the sudden removal of a zone of coal with dimensions length Y = 60 m, width X = 12 m, and
Simulation of coal burning
The coal burning process emits energy and it is important to take into account this energy emission in the numerical model simulation of the UCG process. In this work, the energy balance equation in zones representing coal was modified such that when one zone started burning (at 200 °C) an additional source term was added.
One of the key features of the applied source term is that it does not stay constant but decays gradually with time. In this way, the coal burning process was numerically
Temperature distribution
The spread of the temperature in the model was governed by the thermal conductivity of the surrounding layer of sandstone. A rise of temperature in the vertical direction at different geological layers was observed up to approximately 8 m after 20 days of coal burning. This temperature distribution verified that the coupling calculation was only required in the area near to the source. Analysis of the temperature distribution inside the cavity showed that cavity temperature was always <1000 °C
Conclusions
The paper presented details of a numerical model that was developed to analyse several key features that take place during the process of underground coal gasification. The model was developed based on the UCG trial site at the Wieczorek mine in Poland. Samples obtained from the site were obtained and laboratory tests were conducted to evaluate the effect of high temperatures (up to 1000 °C) on the mechanical properties of the rock, which were then included as input parameters for the numerical
Acknowledgements
The article was prepared based on research conducted within the Research Project: Underground Coal Gasification in operating mines and areas of high vulnerability (COGAR) funded by the European Commission Research Fund for Coal and Steel (RFCS) (Project No. RFC-PR-12005).
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