Mathematical Problems in Engineering

Volume 2015, Article ID 723764, 9 pages

http://dx.doi.org/10.1155/2015/723764

## Computational Fluid Dynamics Simulation of Oxygen Seepage in Coal Mine Goaf with Gas Drainage

^{1}School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China^{2}Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA

Received 7 August 2014; Revised 17 October 2014; Accepted 27 October 2014

Academic Editor: Shaofan Li

Copyright © 2015 Guo-Qing Shi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Mine fires mainly arise from spontaneous combustion of coal seams and are a global issue that has attracted increasing public attention. Particularly in china, the closure of coal workfaces because of spontaneous combustion has contributed to substantial economic loss. To reduce the occurrence of mine fires, the spontaneous coal combustion underground needs to be studied. In this paper, a computational fluid dynamics (CFD) model was developed for coal spontaneous combustion under goaf gas drainage conditions. The CFD model was used to simulate the distribution of oxygen in the goaf at the workface in a fully mechanized cave mine. The goaf was treated as an anisotropic medium, and the effects of methane drainage and oxygen consumption on spontaneous combustion were considered. The simulation results matched observational data from a field study, which indicates CFD simulation is suitable for research on the distribution of oxygen in coalmines. The results also indicated that near the workface spontaneous combustion was more likely to take place in the upper part of the goaf than near the bottom, while further from workface the risk of spontaneous combustion was greater in the lower part of the goaf. These results can be used to develop firefighting approaches for coalmines.

#### 1. Introduction

Spontaneous combustion of coal is an issue that threatens the development of the coal industry worldwide. Among China’s state-owned collieries, 56% of the mines have been jeopardized by spontaneous combustion, and the combustion incidents in these mines account for 90–94% of all coalmine fires [1]. Since the 1990s, the coalmines operating in China have mainly been fully mechanized cave mines. This type of mining leaves a large amount coal in the goaf. The production efficiency is increased compared to nonmechanized mining, but fully mechanized mining results in high air leakage, high rock fall, and more loose coal. These factors increase the probability of coal spontaneous combustion. Furthermore to reduce the risks of gas explosion and improve utilization of methane, especially in mines with high gas content, at some coalmines gas is drained from the mine goaf and coal with high negative pressure technology. These practices increase the air leakage volume and disturb mine ventilation, which elevates the risk of coal spontaneous combustion.

Spontaneous combustion of coal underground takes place mainly in the goaf and occurs through a complex system of thermal, hydraulic, chemical, and mechanical processes [2–6]. The combustion of coal underground is closely related to the concentration and distribution of oxygen in the goaf [7]. Consequently, study of the oxygen concentration and distribution is important to understand coal spontaneous combustion. To date, the oxygen distribution in coal mine goaf has typically been approximated from either a minimal number of actual gas measurements in the goaf or model test results obtained in the laboratory. These two methods have many disadvantages, one of which is the heavy workload required. Although laboratory results are valuable, their extrapolation to the mining environment is not entirely successful because scaling is complicated, and small-scale experiments do not accurately replicate the large-scale environment. Scaling issues typically arise when the coal temperature is high enough that radiative heat transfer cannot be neglected. In these cases, there are problems with scaling of the radiative heat transfer from the small-scale spontaneous combustion results to large-scale mining. For small-scale tests when the coal temperature is low, radiative heat transfer can be neglected but the test results have not been validated [7, 8]. Consequently, it is necessary to establish a new method to study coal spontaneous combustion.

In this paper, to study coal spontaneous combustion, we developed a three-dimensional CFD model of the oxygen concentration under conditions of gas drainage from the goaf. The distribution of oxygen in the goaf was simulated, and the results used to evaluate the coal spontaneous combustion hazard in specific areas of the goaf. The influence of goaf gas drainage on oxygen distribution was also studied using numerical methods. The results could be used for prevention of coal spontaneous combustion and to establish fire-fighting protocols.

#### 2. Theory

##### 2.1. Oxygen Diffusion in the Goaf

In order to simulate the oxygen distribution in goaf under gas drainage conditions, numerical modeling was performed with CFD theories. The finite volume method with the second-order upwind scheme was used to solve the coupled flow, mass transfer, and energy equations using the CFD solver. CFD simulations require solving the Navier-Stokes (N-S) equations, which are formed from a series of partial differential equations governing mass, momentum, and energy conservation. If mass transfer and mixing are part of the process under investigation, then a conservation equation for the components must also be included [9]. The mathematical model for flow of mixed gas in the mine goaf is developed using these equations, along with specific boundary conditions and initial conditions. The following equations apply to gas flow in goaf [10]. The mass conservation equation can be expressed aswhere is the density the mixed gas, represents the , , coordinates in three-dimensional space, is the distance, is the time, is the velocity, and is the source of mass loss of gas in goaf.

The momentum conservation equation iswhere is the gas pressure of cube , represents the , , coordinates in three-dimensional space, is the distance, is the viscous stress tensor which is caused by the viscous effect, is the gravity component in direction , and is source of momentum loss. The momentum loss is caused by fluid flow in porous media and can be expressed aswhere is the gas viscosity in the goaf; is the matrix of the viscous loss coefficient; is the matrix of the inertia loss coefficient; and is the velocity component in direction , where represents the , , coordinates in three-dimensional space. This equation indicates that when the velocity is low in comparison to the viscous loss coefficient, the inertia loss coefficient will be infinitely small.

Equation (3) is equivalent to Darcy’s equation. Convection and diffusion of the multicomponent gas is mainly considered when air transfer occurs in the goaf. From the component mass conservation law, the following conservation equation is obtained:where is the fraction of component and is its density, is the diffusion coefficient of component , is the source term of , and is a scalar quantity. The source term includes events such as methane and CO release and oxygen consumption.

The energy transport equation is formulated under the assumption of thermal equilibrium between the solid matrix and gas. Coal oxidation is an exothermic process, and to provide an accurate description of oxygen concentration, the link between heat production and oxygen consumption must be considered. Therefore, the mathematical model should contain energy conservation equations such aswhere is the specific heat capacity, is the thermodynamic temperature, is the thermal conductivity of gas in the goaf, and is energy source term. For CFD simulation, the geometry, material properties, and boundary conditions need to be specified.

##### 2.2. CFD Model of Oxygen for the Fully Mechanized Cave Mine Workface

The distribution of oxygen was modeled using the widely used CFD software FLUENT. CFD analysis generally involves the following key steps: field studies to obtain basic information on goaf geometry and other parameters; meshing of the established geometric model to a finite element grid by automatic mesh generation software such as Gambit; establishment of flow models and boundary conditions through user-defined functions (UDFs) as described in [11]; model simulations with basic conditions; model calibration and validation with field measured data; and study of the influence of various parameters on the oxygen distribution using the CFD model.

The main factors influencing the distribution of oxygen in mine goaf are viscous flow, which is caused by a pressure gradient, and diffusion, which is caused by a concentration gradient. The longwall goaf permeability and oxygen consumption and diffusion coefficients are the main parameters in a mathematical model of oxygen distribution. Goaf permeability is largely affected by the distribution of pressure in the goaf. Creedy and Clarke highlighted that the permeability at the edge of the goaf is significantly different from that in the middle, and the permeability in these areas can range from 10^{−2} m^{2} to 10^{−7} m^{2} [12]. In the simulation in the present study, goaf permeability was varied from 10^{−2 }m^{2} to 10^{−9 }m^{2}, and the permeability was expressed by a hyperbolic tangent function [13, 14], and the characteristics of permeability distribution can be seen as in Figure 1.