Integrated modelling of the thermal, chemical and geomechanical processes in underground coal gasification

The considerate focus on unconventional fossil fuel resources is a natural consequence of emerging global energy requirements and the ever more limited opportunities to deploy new conventional resources. Underground Coal Gasification (UCG) is an unconventional method for recovering energy from coal resources through in-situ conversion to gas. An oxidising gas agent is injected to initiate and sustain the in-situ coal gasification. The quality of the collected product syngas is characterised by its carbon monoxide (CO), hydrogen (H2) and methane (CH4) content. However, as it is an unconventional method of energy production it evolves through research conducted through modelling studies, laboratory and in-situ trials which support this evolution process. The purpose of this PhD research project is to identify and model the critical parameters which will give increased control on the underground process and ultimately the composition of the final syngas product. In order to achieve this objective, it is necessary to breakdown the UCG process to interrelated stages and design component models that realistically simulate the chemical and physical processes that take place. In the core of the UCG lays the coal gasification process and the simultaneous cavity growth within the coal seam and these will be studied as part of this PhD research. An integrated simulation methodology, which considers the thermal, chemical and geomechanical processes has led to the development of the coupled Thermo-Mechanical-Chemical (TMC) model. Experimental and literature data is used to validate and calibrate the developed models. In addition to increased understanding of the UCG process and its control, this research allows for UCG investors to maximise the financial value sourced from the end-product gas as well as reduce the risk of making unprofitable investments. A number of geologically representative UCG scenarios are simulated through the developed TMC model. The scenarios aim at evaluating the impact of various operational parameters to the UCG operation. The coal panel thickness, the panel depth below the surface, the operating pressure, the type of the injected agent as well as the type of coal where UCG takes place are among the tested parameters. The simulation methodology is based on coupling two industry standard simulators, Advanced System for Process ENgineering (ASPEN) Plus, used for the thermo-chemical simulation, and FLAC3D, which enables the thermo-mechanical simulation of the UCG process. The coupling of the two simulation tools is achieved through sequential interchange of data and through the development of an additional transitional Gasification Support module. The Gasification Support module facilitates the exchange of data between the two simulators and focus on the participating heat and mass transport phenomena within the growing UCG cavity. Principally, the Aspen Plus model simulates the chemical processes taking place in the coal seam and focuses on the thermodynamic, mass and heat transfer modelling components in order to calculate the amount of produced heat, as well as gas under restricted Gibbs minimisation and equilibrium conditions. In addition to the different chemical reactors that constitute the Aspen Plus model constructed, calculator blocks written in Fortran code were introduced to regulate modelling performance in line with experimental data. The Aspen Plus simulation also facilitated the development of different process designs depending on the employed UCG layout (i.e. Linked Vertical Wells, Continuous Retracting Injection Point). The FLAC3D model reflects realistically the 3D spatial features of a gasified coal seam underground. This module produces the resulting thermo-mechanical stress distributions on the coal seam and the surrounding strata, taking account of both mechanical failure and coal spalling effects, heat transfer rates within the cavity and the surrounding strata. The cavity growth modelling results include the extent and the growth rate of the developing UCG cavity given the specified operational parameters such as the coal characteristics (e.g. composition, formation thickness, depth), the composition of reagents injected (i.e. air, oxygen, steam) and the feed rate, the pressure, the gasification and the combustion temperatures. In addition, the UCG product gas characteristics (e.g. composition, heating value) and the participating heat and mass transfer phenomena are also analysed in comparison with the operational parameters of the UCG process.Open Acces