Contents lists available at ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/coal A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects T.C. Ekneligoda a, ⁎ , A.M. Marshall b a School of Civil and Environmental Engineering, University of Witwatersrand, Johannesburg, South Africa b Nottingham Centre for Geomechanics, University of Nottingham, Nottingham, UK ARTICLE INFO Keywords: Coal Underground coal gasification Numerical modelling Calorific value Parallel burning ABSTRACT Underground coal gasification (UCG) is a promising option for extracting energy from coal in unworked or hard to access areas of the subsurface. From a geotechnical perspective, UCG involves various complex phenomena resulting from the elevated temperatures induced within the rock surrounding the UCG burn. This paper presents a coupled thermal-mechanical numerical model developed to represent a UCG trial in Wieczorek, Poland. Temperature dependent mechanical properties were assigned according to results obtained from laboratory experiments and data available in the literature. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. This source term was described using a time decay function to reflect the fact that the energy release from coal gradually decreases with time. The mechanical degradation of coal due to burning was simulated by removing the burned zone from the calculation after a specific time, which depended on zone size and type of coal. In this study, it was found that the maximum temperature at the burning zone was always < 1000 °C, which agrees with previous research carried out for other UCG trials. The size of the burning zone was predicted to spread about 15 m laterally after 20 days of burning. Ground subsidence was evaluated for single and multiple (parallel) panel simulations; subsidence at the top of the numerical mesh, corresponding to a depth of 395 m below the surface, ranged from 23 mm for a single panel to 85 mm for seven panels. The degradation of mechanical properties of the rock surrounding the burned zone due to heating was found to have a marginal effect on the ground sub- sidence when parallel burning was carried out. The numerical modelling results obtained from this study may provide guidance for the design and operation of UCG processes. 1. 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 dom- inating 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 anthro- pogenic greenhouse gas emissions, especially from fossil energy com- bustion (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 ef- fects. 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 CO 2 to the environment from the machinery working a mining site. Underground coal gasifi- cation (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 gasi- fication 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 en- vironmentally 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 https://doi.org/10.1016/j.coal.2018.09.015 Received 1 May 2018; Received in revised form 18 September 2018; Accepted 24 September 2018 ⁎ Corresponding author. E-mail address: Thushan.ekneligoda@wits.ac.za (T.C. Ekneligoda). International Journal of Coal Geology 199 (2018) 31–38 Available online 25 September 2018 0166-5162/ © 2018 Elsevier B.V. All rights reserved. T