Intermittent ventilation in underground mine

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SIMULATION OF METHANE DISTRIBUTION AND VENTILATION IN UNDERGROUNDMINES: POTENTIAL FOR A NOVEL INTERMITTENT AIRFLOW SYSTEM

J.C. Kurnia 1,2 and A.S. Mujumdar 1,2,3*

1 Department of Mechanical Engineering National University of Singapore,

9 Engineering Drive 1, Singapore 117576(First author: [email protected])

2 Minerals Metals and Material Technology Centre National University of Singapore

9 Engineering Drive 1, Singapore 117576(*Corresponding author: [email protected])

3 Department of Bioresource Engineering McGill University

111 Lakeshore Road Sainte-Anne-de-Bellevue, Québec, Canada H9X 3V9



SIMULATION OF METHANE DISTRIBUTION AND VENTILATION IN UNDERGROUNDMINES: POTENTIAL FOR A NOVEL INTERMITTENT AIRFLOW SYSTEM

ABSTRACT

Methane is recognized as one of the prime causes of underground mine disasters around the globe.A cost-effective mine ventilation system and methane control is required to ensure a safe and productiveenvironment in an underground mine whilst keeping energy usage and operating cost at minimum.Typically methane enters a mine environment or tunnel from localized distributed sources at high initialconcentration. Hence, an understanding of its dispersion from discrete sources is important to design amethane management system via effective ventilation. This study employs computational fluid dynamicsto model methane dispersion in an underground tunnel which has a number of discrete sources of methaneand effective ways to dilute and or disperse it. Air flow distribution within the tunnel as well as methanedispersion is simulated by solving the governing transport equations subject to appropriate boundaryconditions. First, the effectiveness of ventilation duct placement is examined. Several possibilities areexplored and investigated. It is found that the common configuration utilized in underground mineperforms best as compared to other configurations in maintaining the concentration of methane belowpermissible levels. Next, a novel intermittent airflow ventilation system is proposed and evaluated viasimulation with the goal of reducing the energy cost whilst maintaining methane level in the mining facebelow the allowable level. Parametric studies are conducted to investigate effects of various factorsinfluencing the effectiveness and performance of this novel intermittent ventilation system. This study willprovide some new ideas for designing an efficient and effective underground mine ventilation system bylowering the net average amount of air that needs to be circulated in the tunnel for a safe operation.

KEYWORDS

Computational fluid dynamics, Intermittent flow, Mine ventilation, Underground mining, Ventilationmodelling

INTRODUCTION

For decades, the presence of methane has been a major safety hazard in underground mining. It isconsistently found in most underground mines (especially underground coal mines). Methane is the primecause of many mining disasters around the globe. Several major incidents and accidents in undergroundcoal mines with fatalities have been reported (Torano et al., 2009); for example, a devastating mineexplosion in Courrieres, France which killed over 1000 mine workers in 1906, an explosion in the 8 th Coalbed at San Nicolas, Spain in 1995 which claimed 14 lives, the disaster in Upper Big Branch mine,West Virginia USA in 2010 with 29 fatalities (McAtter et al., 2011) and recently a mine blast in southwestChina killed 18 people (Channel NewsAsia, 2012). To prevent such major accidents from occurring in thefuture as well as to ensure a safe and productive environment in an underground mine, a proper ventilationsystem is mandatory. Most ventilation systems installed in underground coal mines nowadays need tosupply excess fresh air to ensure safe methane concentrations as required by local codes. While thisapproach could maintain low methane concentrations, it requires tremendous energy to drive excessive airto various locations in underground mines and in turn impose huge operational costs on the miningindustry. This situation is worsening as more countries start taxing carbon emissions resulted from energyusage which could in many instances in future, cost more than the energy cost itself. As such, effective



mine ventilation strategies is required to ensure a safe and productive environment in an underground minewhilst keeping energy usage and operating cost at minimum.

Figure 1 – Various duct placements in a typical underground mine tunnel with discrete methane sources.Tunnel geometry is taken from previous work by Parra et al. , 2006

In response to this requirement, a large number of experimental and numerical studies focusing onflow behaviour and methane dispersion in underground mines have been carried out and reported in thepublic domain. Nakayama et al. (1999) developed a CFD model for methane gas distribution in a miningface. They validated their model using the experimental results of Ichinose et al. (1998) who conducted

in-situ measurement and CFD simulation to examine methane gas distribution in the mining face zone. Animportant finding of Nakayama’s study was that the methane gas concentration was found to be higher atlocations transverse in and along the corner space where the face end meets the ceiling and the floor as wellas area underneath the ventilation duct. Torano et al. (2009) studied methane dispersion in a Spanishunderground coal mine with an auxiliary ventilation system. Consistent with the finding of Kissel andWallhagen (1976), Schultz et al. (1993), Haney et al. (1982) and Kissel (2006), they found that a forcedventilation system is more effective in delivering fresh air to the mining face as compared to ventilationsystem which utilizes only an exhaust fan.

Wala et al. (2003, 2007, 2008) conducted experimental and numerical studies on methanedistribution in the vicinity of the mining face. They investigated various scenarios which could occur in anunderground coal mine including the box cut and slab cut scenarios. They also examined the effect ofscrubber operation on face ventilation and methane distribution. Even though no definite conclusion hasbeen made, they highlighted the importance of CFD modelling in developing an effective mine ventilation

system. Recently, Sasmito et al. (2012) reported a numerical study examining various auxiliary ventilationsystems which could provide sufficient oxygen for the miner and maintain methane concentrations at a safelevel while keeping energy usage low. They also examined various cutting scenarios in underground mineoperations. They found that proper flow stopping design could enhance gas control throughout the mineswhilst maintaining low pressure drops. In addition, they concluded that a combination of brattice andexhausting system offers the best option for methane handling. Following up on Sasmito’s work, Kurnia et



al, carried out studies to develop mine ventilation systems which could efficiently control dust, methaneand other hazardous gases in an underground mine (Kurnia et al., 2012; Kurnia and Mujumdar, 2012a,b).

Generally, methane enters a mine tunnel as localized sources at high concentration (Kissel, 2006).As methane emerges from these discrete sources, it progressively mixes with the ventilation air as it isdispersed and diluted. Hence, understanding its dispersion from discrete sources is important to design asafe methane management system via effective ventilation. In this study, we investigate the effectivenessof a ventilation system installed in an underground mine tunnel which has a number of discrete sources ofmethane via a computational fluid dynamics (CFD) approach. First, the effectiveness of ventilation ductplacement is examined. Several possibilities are explored and investigated. Next, a novel intermittentairflow ventilation system is proposed and evaluated via simulation with the goal of reducing the energycost whilst maintaining methane level in the mining face below the allowable level. The authors believethis to be an innovative concept being proposed and evaluated for the first time.

MATHEMATICAL MODEL

A three-dimensional model was developed for a typical mine tunnel which is the simplest and themost used in underground coal mining studies (please refer to Figure 1a). This tunnel geometry is takenfrom the previous work by Parra et al. (2006). The mine tunnel is 36 m long, 3.6 m wide and 2.9 m high.Methane is released from ten sources (10 x 10 cm 2) into the mining face with total flow rate of 0.05 m 3 /s.A ventilation duct with a diameter of 0.6 m is hung at the various positions in the access road (details onthe duct placement can be found in Table 1). Its setback distance from the mining face is 6 m.

Table 1– Geometric parametersDuct placements Base case Bottom Side TopHeight (m) 1.9 0.7 1.1 2.5Space from wall (m) 0.6 0.4 0.4 0.8

Governing equations

We solve the mass, momentum, energy and species transport subject to appropriate boundaryconditions. Methane is released from discrete sources in the mining face and dispersed by the ventilationairflow. Conservation equations for mass, momentum, energy and species in vector form are:

,t

ρ ρ ∂

=∂

u (1)

( ) ( )( ) ( ) ( )[ ]2

,3

T p t t k g

t ρ ρ µ µ µ µ ρ ρ = − + + + + + − +

∂ + ∂u u u I I

uu u (2)

( ) ,Pr

p t

p p eff

t

cT c c T k T

t

µ ρ ρ

∂+ = +

∂

u (3)

( ) ( ) ,.t

i i i eff i

t

Dt Sc

µ ρω ρω ρ ω

∂+ = +

∂

u (4)

where ρ is the fluid density, u is the fluid velocity, p is the pressure, µ is the dynamic viscosity of the fluid,I is the identity or second order unit tensor, g is gravity acceleration, c p is the specific heat of the fluid, T isthe temperature, ω i is the mass fraction of species i (O 2, CH 4 and N 2), D i is diffusivity of species i, µ t is



turbulent viscosity and Sc is Schmidt number. Unlike most of the earlier studies, this treatment includedthermal effects which are known to affect the flow and species concentration distributions.

Constitutive relations

A ternary species mixture comprising of oxygen, water vapour and methane exists in theventilation air in the tunnel. The interaction between the species is captured in the mixture density whichfollows incompressible ideal gas law given by (Sasmito et al., 2012):

, pM

RT ρ = (5)

where R is the universal gas constant and M refers to the mixture molecular weight given by

4 2 2 2

4 2 2 2

1

.CH O H O N

CH O H O N

M M M M M

ω ω ω ω −

= + + +

(6)

Here, M i is the molecular mass of species i. Mass fraction of nitrogen is calculated as:

( )2 2 2 4

1 . N O H O CH

ω ω ω ω = − + + (7)

The fluid mixture viscosity is calculated using

4 2 2 2

,

with and = CH , O , H O and Ni i

i i i j

j

xi j

x

µ µ =

Φ∑ ∑ (8)

where xi,j are the mole fraction of species i and j and

21 1 1

2 2 4

,

11 1 .8

i i i

i j

j j j

M M

M M

µ

µ Φ = + +

(9)

The mole fractions are related to the mass fractions by xi= ω i M/M i. In-line with the concentration unitcommonly used in applicable regulations, methane concentration in this paper is presented in % by volume.In addition, fan power is calculated as:

. fan fan fanP P Q= ∆ & (10)

where ∆P fan is the pressure rise across the inlet and outlet of fan and fanQ& is the volumetric flow rate of the

fan. It should be noted that the actual fan power would be different depending on fan efficiency.

Turbulence model

The most commonly used turbulence model in engineering viz. k- ε was selected for this work.This model comprises two-equations which solve for turbulent kinetic energy, k, and its rate of dissipation,



ε , which is coupled to the turbulent viscosity (Wilcox, 2006). This model is also known to becomputationally efficient and reasonably reliable for the configurations under investigation.

Boundary conditions

The applicable boundary conditions are as follows: (i) At walls: the standard wall function is usedin all simulations; (ii) At the duct outlet: air velocity of 12 m/s is prescribed at the duct outlet (Parra et al.,2006); (iii) At the mining face: methane is released at total flow rate of 0.05 m 3 /s (Torrano et al ., 2009);(iv) At the outlet: stream-wise gradient of the temperature and species is set to zero and the pressure is setto standard atmospheric pressure (1 bar).

NUMERICAL METHODOLOGY

The computational domains were created, meshed and labelled in the commercial code Gambit2.3.16. Three different sizes of mesh 6 × 10 5, 1 × 10 6 and 1.8 × 10 6 were implemented and compared interms of local pressure, velocities, and methane concentration to ensure a mesh independent solution. Wefound that the mesh size of around 1 × 10 6 gives about 2% deviation compared to the mesh size of 1.8 ×106; whereas, the results from a mesh size of 6 × 10 5 deviate up to 12% as compared to those from thefinest one. Therefore, a mesh of around 1 million elements was deemed sufficient for the numericalinvestigation purposes, comprising a fine structure near the wall and increasingly coarser mesh in themiddle of the tunnel to reduce the computational cost to manageable level.

The conservation equations together with the constitutive relations, turbulence model andboundary conditions were solved using a finite volume solver, Fluent 6.3.26. The equations were solvedwith the Semi-Implicit Pressure-Linked Equation (SIMPLE) algorithm, first order upwind discretizationand Algebraic Multigrid Method (AGM). On average, each simulation required around 1000-3000iterations to meet convergence criteria of 10 -6 for all variables. Each run required around 5-6 hours onworkstations with six core processor, requiring 4–6 GB RAM.

RESULTS AND DISCUSSION

Flow behaviour and methane dispersion in a mine tunnel model with discrete methane sourceswere investigated. In the following section we will present and discuss the effects of ventilation ductplacement and explore various possible configurations of our newly proposed innovative intermittentventilation system.

Ventilation duct placement

Before proceeding with the study of the innovative intermittent ventilation system, it is of interestto first investigate the effectiveness of ventilation duct placement commonly adopted in undergroundmines. Four possible duct placements are examined: base, bottom, side and top position. One of our goalsin this CFD study was to see if simple changes in geometry and placement of the duct work would affectthe ventilation characteristics in a favourable way. The objective was not to increase capital costs, but toallow retrofits and at the same time reduce the electrical power demand without jeopardizing ventilationperformance.



Figure 2 – Velocity contours (m/s) at plane 1 m from the mine floor for the tunnel with various ventilationduct placement

Figure 2 presents velocity profiles at 1 m height from the mine floor for various duct locations.Variation in duct placement significantly affects the airflow profiles in the mine tunnel. It is observed thatthe base case, where ventilation duct located on the top right of the tunnel, provides more uniform velocityin the mine tunnel as compared to other duct placements, for which a higher velocity is only observed inthe front section of the mine tunnel.

Figure 3 – Methane concentration (% by volume) at plane 1, 8, 16, 24, 32 m from the mining face for thetunnel with various ventilation duct placement



The effect of duct placement on methane concentration distribution is prominent only in the frontentry section of the mine tunnel, as can be seen from Figure 3. As can be seen, base case and top positionsoffer better methane control as they tend to confine higher concentrations to the floor region - presumablythese would be diluted if they tried to migrate upward. At a location far from the mining face, we see thatmethane concentration is relatively uniform and similar for all configurations. Looking further into themethane concentration along the mining tunnel as shown in Figure 4, it is found that all strategies canmaintain methane levels below the explosion limit (5% by volume methane concentration). On average,the base case configuration, where ventilation duct located on the top right of the tunnel, performs best inmanaging methane emission. However, base case position has higher maximum concentration, indicatingmethane concentration build up at certain point along the tunnel.

Figure 4 – a) Maximum and b) cross-section average methane concentration (% by volume) along thetunnel with various ventilation duct placement

Effect of intermittency

In the previous section, we noted that the base case duct placement, which is commonlyimplemented in underground mine, offers the best methane handling strategy as compared to otherplacements. Here, we evaluate several possible intermittency designs and compare their performance interms of methane concentration and possible energy savings as compared to one with a traditional constantventilation flow. A 12 m/s constant air velocity from the main ventilation duct blowing directly towards

the mining face (case i) is compared with three intermittency scenarios: (case ii) 5 min high velocity (12m/s) and 5 min low velocity (6 m/s); (case iii) 5 min high velocity (12 m/s) and 10 min low velocity (6m/s); and (case iv) 5 min high velocity (12 m/s) and 15 min low velocity (6 m/s). These configurationsrepresent an underground mine which has 2, 3 and 4 active mining area and the desire to cycle themaximum ventilation through them.



Figure 5 – Velocity contour (m/s) at height 1 m from the mine floor for case iii (intermittent flow 5 min

high velocity of 12 m/s and 10 min low velocity of 6 m/s)

Figure 6 – Predicted methane concentration contours (% by volume) at 1, 8, 16, 24, 32 m from the mineface for case iii (intermittent flow 5 min high velocity of 12 m/s and 10 min low velocity of 6 m/s)

The intermittency leads to dynamic behaviour in the flow (Figure 5) and methane concentrations(Figure 6). The step changes in ventilation velocity changes the overall velocity behaviour inside thetunnel: when high velocity applies (Figures 5a and d), a relatively high flow velocity develops throughoutthe tunnel dispersing methane emission and forcing it to leave the tunnel (Figures 6a and d); conversely,when low air velocity applies, air flow velocity throughout the tunnel reduces significantly (up to 70% atthe outlet region, see Figures 5b and c); this is further mirrored by the rise in the methane concentration upto twice the full flow scenario (Figures 6b and c) and then reduces back to low methane concentration asthe intermittent flow is periodically applied. It is also noteworthy to mention that during one period ofintermittency, methane concentration at high velocity becomes somewhat higher throughout the tunnel (~10%, please refer Figures 6a and d).





Figure 7 – a) Cross-section average methane concentration at 1 m from the mining face and b) volumeaverage methane concentration (% v/v) throughout the mine tunnel for various intermittent modes

Despite its somewhat inferior performance on handling methane removal, intermittency offerspotential for energy saving as given in Table 3. It is noted that significant amount of energy saving can be

achieved up to 43.5, 58 and 65% for case ii, iii and iv as compared to case i, respectively. The saving willeven be higher when it is translated to the annual operating cost saving and, to some extent, company canclaim for carbon emission trading as well. Clearly, it can be deduced that intermittency has potential to beimplemented for energy saving; on the other hand, an improved intermittency design should be developedand more studies is required to enhance methane removal and optimize the operating condition. If the airflow volume is increased further, intermittent ventilation could be a cost effective method.

CONCLUSIONS

In this study, methane dispersion in an underground tunnel which has a number of discretesources of methane is investigated by utilizing a computational fluid dynamics (CFD) approach. Air flowdistribution within the tunnel as well as methane dispersion is simulated by solving the governing transportequations subject to appropriate boundary conditions. Several different possible ventilation ductplacements in an underground mine tunnel have been studied and examined. The results indicate that base

case position, where the ventilation duct hung at the corner of the tunnel, which traditionally used inunderground mine performs best in dispersing the methane emission and reducing methane concentrationin mine tunnel. In addition, it is found that all configurations could maintain methane concentration belowits explosive level.

Subsequently, we introduce and evaluate intermittent flow ventilation system to reduce energyusage. The results are not so promising, based on the CFD results obtained to date for a modelunderground mine, whereby intermittency by reducing air velocity into half could not maintain a lowmethane concentration. For mines where methane is not a primary issue (non-coal underground mines),however, intermittent flow with 5 min high velocity and 15 min low velocity could offer up to 65% ofenergy savings. Such energy savings will not only reduce expenses in electricity bill but also will result invery significant savings from carbon tax credits. More parametric studies are now being carried out toobtain an optimum ventilation design which could save energy usage and in turn carbon credit taxassociated with it even more. In addition this study will be extended to investigate application of

intermittent flow ventilation system in non-coal underground mine where diesel emission is the primeissues rather than methane as in underground coal mine.



ACKNOWLEDGMENTS

This work was financially supported by Singapore Economic Development Board (EDB) throughMinerals Metals and Materials Technology Centre (M3TC) Research Grant R-261-501-013-414.

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Intermittent ventilation in underground mine