Year

2022

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining, and Environmental Engineering

Abstract

Underground coal mining is facing increased threats from the hazards of spontaneous combustion and heating of coal, abnormal mine gas emissions, and harmful dust concentrations in underground workings, due to increased production outputs and extraction depth of cover. To control and mitigate these engineering problems, there is a need to gain critical knowledge of spontaneous heating in the longwall (LW) goaf, gas migration patterns onto the LW face, and ventilation dynamics and dust dispersion in complex underground environments. Advanced Computational Fluid Dynamics (CFD) modelling can be used to simulate various scenarios portraying these hazards that may occur in underground LWs and provide much-needed knowledge and fundamental science that can be used to develop robust and effective control and mitigation strategies against these hazards.

A comprehensive literature review has been conducted to understand these principal mining hazards (PMH), with a particular emphasis on the applications of CFD modelling in the prevention management and control of those PMH arising during coal extraction process. The insufficiencies and gaps in research on spontaneous combustion in active LW goaf, gas migration onto the LW face, and dust dispersion and transport in the development heading were identified. In addition, several field studies were carried out in underground coal mines in Australia to gain a better understanding of these mining issues and collate essential data for the CFD modelling studies.

In recent years, goaf heating and spontaneous combustion incidents have been reported in several Australian underground coal mines during normal production cycles. The onset of these heating incidents was dictated by many operational and environmental parameters. Based on the site-specific conditions of an underground coal mine, where the coal seam gas is of approximately 80% carbon dioxide (CO2) and 20% methane (CH4) with a gas emission rate of 2000 l/s, CFD models were developed and validated with field gas monitoring data collected from the Tube Bundle System. The CFD models incorporated a user defined function (UDF) of gas emission and permeability variations in a three-dimensional (3D) space of computational domain representing the LW panels and goaf areas. Simulation results indicated that better goaf inertisation could be achieved when nitrogen (N2) was injected via cut-throughs (C/T) at about 250 m behind the LW face on the maingate (MG) side and surface boreholes at 100 m and 700 m on the tailgate (TG) side, with a total injection rate greater than 1750 l/s. The oxygen concentration on the MG and TG side dropped below 5% at distances of 120 m and 75 m behind the LW face, with a confined oxidation zone area of 35375 m2, which was approximately one-third of the oxidation zone area without inert gas injection. The impact of geological variations (i.e., coal seam orientations and goaf gas composition) on spontaneous combustion prevention and management was further studied using CFD models. The influence of ventilation design and operational parameters (e.g., tightness of the goaf seals) on spontaneous combustion control was also investigated by additional CFD models based on field data.

During LW sealing-off, the ventilation flow dynamics change within the goaf, which considerably increases the risk of spontaneous combustion and gas explosion. To prevent these hazards, CFD models were developed and calibrated with field gas monitoring data to simulate a range of operational scenarios of different ventilation arrangements. The modelling studies indicated that at least six gas sensors should be employed and positioned appropriately to ensure effective goaf atmosphere monitoring for risk management during the LW sealing-off process.

Extensive CFD-DPM (Discrete phase model) coupling modelling studies were conducted to investigate dust-related issues in LW gateroad development panels. Based on site-specific conditions, a CFD model incorporating a Continuous Miner (CM), Shuttle Car (SC) and exhausting ventilation tube was established and validated with onsite dust monitoring data. Three scenarios of CM cutting at the middle, floor and roof positions were considered and simulated. In all cases, the simulation results indicated that high levels of dust exposure would occur to left-hand-side (LHS) operators and consequently they should be equipped with high-quality personal protective equipment and stay behind the ventilation duct inlet during coal cutting process, while miners standing at the right-hands-side (RHS) of the CM for roof and/or rib bolting and machine operation should stay immediately behind the bolting rig where dust concentration was relatively low.

The studies conducted in this thesis provided new insights into the current goaf inertisation practices to effectively manage and control spontaneous heating in LW goaf by considering geological variations and mining design. Furthermore, the CFD modelling study of gas flow dynamics during the panel sealing-off process provides new knowledge of ventilation and goaf gas dynamics, which is critical to the positioning of gas monitoring sensors to reliably measure goaf atmosphere changes, thus minimizing spontaneous heating and gas explosion risks with much-improved mine safety. The research work also shed light on the dust and ventilation behaviour in gateroad development panels, and provided several recommendations for operators’ locations and dust mitigation strategies to improve the health and safety of miners. The research outcomes from this study contribute to the improvement of current practices and guidance for PMH management and control in underground mines and tunnelling projects.

FoR codes (2008)

091405 Mining Engineering, 091501 Computational Fluid Dynamics

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.