Thermal–mechanical modelling around the cavities of underground coal gasification

Abstract

Underground coal gasification (UCG) is an efficient method for the conversion of the deep coal resources into energy. This paper is concerned with a feasibility study of the potential of deeply lying coal seams (> 1200 m) for the application of UCG combined with subsequent storage of CO2 for a site located in Bulgaria. A thermal-mechanical coupled model was developed using the ABAQUS software package to predict the heat transfer, the stress distributions around the UCG and the consequent surface subsidence. Material properties of rocks and coal were obtained from existing literature and geomechanical tests which were carried out on samples derived from the demonstration site in Bulgaria. Three days of gasification has been simulated by assigning a moving heat flux on a cell of 2 m 2 m 2 m at a velocity of 2 m/day. Results of temperature and stress distribution showed that the developed numerical model was able to simulate the heat propagation and the stress distribution around cavities under a thermal-mechanical coupled loading during the UCG process. Also, the surface subsidence was found to be 0.08 mm after three days of gasification for the case studied. It is anticipated that the results of this paper can be used for the prediction and optimization of the UCG process in deep coal seams.

Thermo-Mechanical Simulations of Rock Behavior in Underground Coal Gasification Show Negligible Impact of Temperature-Dependent Parameters on Permeability Changes

“…Initial parameterization of coal (Table 1) considered data from the review of experimental testing data on temperature-dependent thermo-mechanical properties for bituminous coals carried out by Min [38]. From the thermal trends of the coal's elastic modulus (E) and tensile strength (σt), its cohesion (c) and friction angle (ϕ) were assumed and applied for model parameterization [42].…”

“…Their simulation results show increasing ground subsidence with the increase of the reactor size as well as the stress increased at the front of the face and pillar edge due the induced thermal stresses. Thermo-mechanical simulation results of rocks surrounding an UCG reactor elaborated by Yang et al [38] show the temperature distribution and stress increase (around 10%) as well as vertical displacements (0.073 m) above the coal seam after three days of gasification. Sarraf et al [39] present a 2D model for the growth of a UCG reactor based on the CRIP technology, which incorporates the combined effects of fluid flow through porous media, mass transfer of species, heat transfer and reaction kinetics.…”

“…Furthermore, most existing mechanical modeling studies focused on ground subsidence or stress changes by applying only coarsely refined structured numerical grids, resulting in a limited validity considering thermo-mechanical processes in the close reactor vicinity [37,38,41]. Most of the present experimental studies on thermo-mechanical properties of rocks at high temperatures mainly considered crystalline rocks such as granites and marbles.…”

Substantiating parameters of stratification cavities formation in the roof rocks during underground coal gasification

“…При скважинной подземной газификации буро-угольного пласта марки Б3 породы непосредственной кровли у огневого забоя подземного газогенератора испытывают влияния высоких температур (Yang, Sarhosis & Sheng, 2014;Otto & Kempka, 2015). Вели-чина температуры газификации буроугольного пла-ста изменяется по длине реакционного канала (Perkins & Sahajwalla, 2007).…”

Modelling Underground Coal Gasification—A Review

“…In order to predict these risks, geomechanical and hydrogeological models are required. The models that dealt with surface subsidence and water hydrology in the UCG process can be found in recent references: Yang et al [97], Sheng et al [98]. From the studies, it appears that careful planning, appropriate site selection, adequate engineering and modeling, and professional management are the keys in order to implement UCG-CCS schemes successfully.…”