Numerical investigation of diesel particulate matter dispersion in an underground development face during key mining activities

Highlights

• Numerical modelling of diesel particulate matter distribution were conducted.

• The effectiveness of the auxiliary ventilation system was evaluated.

• High DPM concentration areas were identified in different mining activities.

• CFD simulation results were validated with onsite experimental data.

Abstract

Diesel particulate matter (DPM) is carcinogenic to humans. Underground miners have a high risk of over-exposure to high concentrations of DPM. To control DPM effectively, it is essential to understand the DPM dispersion characteristics. In this study, the DPM distributions of three key and representative mining activities, shotcreting, charging and loading activity, in an underground development face were studied. A computational model for the mining activities was developed using 3D imagery, onsite data and OpenFOAM. Tracer gas experiments were first conducted in the underground mine for the validation of CFD simulation. The simulations were carried out at a steady-state using the standard k-ε turbulence model, and the transport and dispersion of DPM were modelled using a segregated species transport model. DPM distribution characteristics for each mining activity were analysed, and the regions with high concentration (>0.1 mg/m³) were identified, and the reasons for the high concentrations were also discussed. At last, the efficiency of the current auxiliary ventilation system on DPM dilution was evaluated based on the simulation results. The results show that a broader region with high DPM concentration was identified in the downstream of the loader during the loading activity, and this issue could be solved by simply increasing the ventilation rate. The findings in this paper could be used for optimizing the auxiliary ventilation design for future mining activities in this development face.

Introduction

With the rapid development of mechanization in the mining industry, diesel equipment has been widely used for mining activities. Nevertheless, this also generates a severe health issue on the miners who are overexposed to diesel particulate matter (DPM). A previous study [1], showed that more than 90% of the diesel particles by mass are less than 100 nm. Such size of particles is capable of penetrating the human respiratory system and deposit at the deepest part of the human lung [2],. In 2012, the International Agency for Research on Cancer (IARC) classified DPM as a carcinogen to humans (Group 1) [3],. A number of studies [3], [4], [5] indicated that both acute- and long-term exposure to high concentration could result in adverse health effects, such as asthma, light-headedness, lung cancer, etc. Due to the confined working environment, the underground miners have a high risk of over-exposure to high DPM concentration than that of the worker who works in a normal environment. For this reason, it is vital to remain the DPM concentration under an acceptable level. In 2017, the Australian Institute of Occupational Hygienists recommended an 8-hour time-weighted average (TWA) exposure limit of 0.1 mg/m³ and an “action limit” of 0.05 mg/m³ on the basis of elemental carbon (EC) for underground mines [6],. However, it is very challenging for most of the mining industries to meet this standard [7],.

Currently, ventilation is still the primary strategy to dilute DPM concentrations in underground mines. In an underground development face, usually, the auxiliary ventilation set up does not change in a complete mining cycle, including shotcreting, charging, loading activities, etc. Diesel particulate emissions vary in the mining cycle due to the different type of diesel equipment used. Thus, it is essential to understand the DPM dispersion characteristics in the development face for various mining activities, especially for the key mining activities which highly rely on the diesel machines. Computational fluid dynamics (CFD) is an effective way to simulate the DPM distribution based on the onsite experiments to assess the effectiveness and optimize mine ventilation systems.

Numerous studies have used CFD simulation to investigate the transport and accumulation of contaminants in underground mines. One of the commonly used models in these studies is the species transport model, which solves a transport equation for a scalar under a specified air velocity field. This scalar could be the temperature, contaminant concentration, etc. in the simulation. Kurnia et al. [8], used a species transport method to simulate the methane behaviours in an underground mine tunnel. Various methane release conditions were evaluated, and the effects of the ventilation on the methane distributions were also investigated. The results showed that the methane behaviours were highly influenced by the gas release conditions, such as release location, release rate and source numbers. Based on the methane distributions under different scenarios, effective methane management was provided. A numerical study was conducted by Fang et al. [9], to investigate gas dispersion in twin tunnels by using the species transport method. The effects of the ventilation systems parameters, such as duct airflow quantity, auxiliary fan locations on the gas dilution efficiency, were also evaluated. An auxiliary ventilation system with a jet fan placed at 50 m before the cross-aisle was recommended to control the gas concentration effectively. A similar study was conducted to analyse the methane distribution in an underground development face [10],. The species transport model was applied to study gas dispersion characteristics. The performance of different auxiliary ventilation systems on the gas control was considered, and the best auxiliary ventilation system design was suggested based on the simulation results. Torno and his colleagues [11], analysed the blasting gas behaviours after blasting in underground headings by using the CFD methods. The results were validated with the onsite gas concentrations during 40 min and a good agreement was obtained.

Currently, two main modelling methods are generally used for the DPM simulations, include species transport method, which treats DPM as a continuous phase, and Eulerian-Lagrangian method, which treats DPM as a solid discrete phase. The species transport has also been applied in numbers of studies to investigate the diesel particulate transport distribution in different regions of underground mines. Kurnia et al. [12], used this method to investigate the hazardous gases from the diesel emission in an underground development face under various auxiliary ventilation systems. The auxiliary ventilation systems performance on hazard gases dilution were further evaluated, and the hazardous gases control strategies were provided based on the numerical results. Zheng and his colleagues conducted a series of numerical studies to investigate diesel particles’ dispersion and distribution characteristics using the species transport method. They predicted the DPM distributions in an underground isolated zone with a loader at six locations to present the dynamic activity [13],. They evaluated the buoyancy effect on the DPM dispersion in an underground development face [14],. They investigated the efficiency of four auxiliary ventilation systems on DPM dilution [15],. Also, they analysed the DPM patterns with the impact of moving vehicle in a heading face [16],.

Many other studies also used Eulerian-Lagrangian model to simulate diesel particles dispersion particles in underground mines [17],[18]. A good agreement between the simulation data and onsite data was achieved by treating DPM as discrete particles. Although Eulerian-Lagrangian method gives more details about the DPM dispersion characteristics, this method is computationally expensive. Recent studies [19],[20] have compared the effectiveness and efficiency of using different computational modelling approaches for CFD simulation of particulate dispersions. Both of the studies showed that the species transport model could provide similar results as the model which considers DPM as solid particles. In this study, the DPM distributions under three key mining activities would be simulated. According to the computational cost, the species transport method was applied in this study to predict the DPM dispersion characteristics in underground mines.

A mining cycle in a development heading includes activities such as shotcreting, loading, and charging, etc. Various diesel machines with different engine powers are used in these mining activities, which emit a different amount of DPM. However, such activities are performed under the same auxiliary ventilation set up, which may not guarantee the DPM concentrations to be maintained under the safe limit of 0.1 mg/m³ in the working heading. Thus, it is important to investigate the performance of an auxiliary ventilation system in a development face and recommend the optimum ventilation design for effective DPM dilution. The aim of this work is to study the DPM dispersion characteristics under various key mining activities and evaluate the efficiency of the current auxiliary ventilation system on DPM dilution in an underground development face by using CFD simulation. This study is based on an onsite experiment in a development face in a gold mine in Western Australia. A steady-state CFD simulation was carried out to study the particulate matters’ dispersion characteristics by using standard k-ε turbulence, and the transport and dispersion of DPM was modelled by using a segregated species transport model. The high DPM concentration areas were identified where the ventilation could be improved. In addition, Sulfur hexafluoride (SF₆) was used in the onsite experiment as a tracer gas for the CFD validation purpose. The results provided in this study will be useful in designing new and optimizing existing mine ventilation systems to better control the DPM concentrations in the development face.