Ventilation Requirement for Electric Underground Hard Rock Mines A Conceptual Study

Transcription

1 Ventilation Requirement for Electric Underground Hard Rock Mines A Conceptual Study A Halim 1 and M Kerai 2 ABSTRACT The electric power price in mining countries such as Australia and South Africa has increased significantly in the past five years and is likely to continue to increase in the foreseeable future. This can make a mine uneconomic to operate. Replacing diesel vehicles with electric ones can reduce ventilation power consumption, which can comprise up to 40 per cent of total mine power consumption. However, no such airflow requirement for electric vehicles is stated in any mining regulations in the world. In this paper, the authors investigate the ventilation requirement of an electric vehicle operating in an underground hard rock mine. Quantification of atmospheric contaminant emitted by an electric vehicle was done at Rio Tinto s Northparkes mine, followed by thermodynamic and ventilation network simulations using Ventsim Visual software. INTRODUCTION The electric power required for the ventilation system for a mine is one of the major components of the total electric power consumption, which can be up to 40 per cent (Mining Magazine, 2010). To reduce ventilation power consumption, ventilation requirement must be reduced. One option to achieve this is to replace diesel vehicles with electric ones. An electric motor produces zero emissions (ie no gases and diesel particulate matter DPM) and only emits one-third the heat of an equivalent diesel engine. Airflow specification can therefore be less (Marks, 2012; McPherson, 2009). Currently ventilation requirement in underground hard rock mines is determined by multiplying the rated diesel engine power of all vehicles with the regulatory airflow requirement. Some examples are: 0.05 m 3 /s per kw rated engine power is the requirement in Western Australia Mines Safety and Inspection Regulation (WAMSIR) 1995, regulation (6) (WA Government, 1995) 0.06 m 3 /s per kw is the requirement in New South Wales (NSW) Mining Design Guideline (MDG) 29 Guidelines for the management of diesel engine pollutants in underground environments (NSW Government, 2008) m 3 /s per kw is the requirement in Ontario, Canada (Kocsis, 2003) m 3 /s per kw is the requirement in Indonesian mines (Brake, 2010). Currently, there is no such requirement for electric vehicles in any mine regulations in the world. To the authors knowledge, the only regulation concerning ventilation for electric vehicles is WAMSIR 1995 regulation 9.34 which states that a minimum air velocity of 0.25 m/s is maintained in all underground areas in the mine where vehicles or locomotives powered by electricity is used. However, this means that the airflow quantity will be different depending on the dimension of the area where they work. For example, a 14 tonne loadhaul-dump (LHD) unit with motor power of 178 kw will have airflow quantity of 4 m 3 /s in a 4 4 m heading and 6.25 m 3 /s in a 5 5 m heading. This means that the smaller heading will have higher temperature than the larger one. This is not the right approach as the amount of heat produced by an electric vehicle depends on the of all electric motors in that vehicle, not on the dimension of the heading. The airflow quantity should therefore be governed by the of the motors. METHODOLOGY The first step of this study was to quantify the contaminant produced by an electric heavy vehicle, which is heat. The measurement to quantify heat produced by an electric heavy vehicle was done at Rio Tinto s Northparkes coppergold mine in New South Wales. This mine was selected as it exclusively uses electric vehicles, which are electric LHD units, as its main production equipment. The LHD unit is Sandvik LH514E. Data collected were dry bulb (DB) temperature, wet bulb (WB) temperature, Barometric pressure, airflow quantity (air velocity and airway dimension). By using psychometric equations, the amount of heat produced by an electric LHD can be calculated. In addition to this, power consumed by the unit was also determined by measuring current, voltage and power factor. This is to validate a popular assumption 1. MAusIMM, Lecturer, Department of Mining Engineering, Western Australian School of Mines, Curtin University, Locked Bag 30 Kalgoorlie WA A.Halim@curtin.edu.au 2. Graduate Mining Engineer, Barrick Australia Pacific. Formerly undergraduate student, Department of Mining Engineering, Western Australian School of Mines, Curtin University, Locked Bag 30 Kalgoorlie WA

2 A HALIM AND M KERAI that heat produced by an electric vehicle is equal to the power consumed by it if the vehicle does not do any work against gravity. The literature review found that this assumption had never been validated prior to this research. Thermodynamic and ventilation network simulations were done as the next step. The software Ventsim Visual was used. The aim of these simulations was to determine the total airflow quantity that is required in order to have WB temperature at the deepest part of the mine less than or equal to 30 C (design reject temperature). Although the common design limit used is 28 C, with the extensive utilisation of air conditioned cabin vehicles in modern mines the design limit can be increased to 30 C. These vehicles assist heat stress management by providing microclimate cooling, ie the vehicles operator spend most of his/her time in cool environment, personnel working outside air conditioned cabin can take regular breaks inside the vehicle. A similar limit is used in Callie underground gold mine in Northern Territory, which is 30.5 C (Howes and Clarke, 2007). Model of two currently operating hard rock underground mines in Western Australia were used with their known fleet of machinery. The first is a deep mine which extends to over a kilometre deep and the second is a shallow mine which extends to 600 m deep. The models were generated using two distinct thermal conditions found in Australian mining regions, namely Kalgoorlie (cool strata) and Mount Isa (hot strata). Their thermal parameters can be altered in Ventsim Visual. One input of a thermodynamic simulation using Ventsim Visual is the heat emitted by an electric vehicle. For this purpose, it was essential to determine the ratio between the heat emitted and total electric motor of the LH514E. The heat emitted was measured as 145 kw and the total motor of this LHD is kw. This results in a ratio of 81 per cent. However, since this ratio varies in every electrical equipment, a conservative approach was taken in the simulation. Therefore, it was assumed that the amount of heat emitted from every conceptual electric vehicle is equal to its motor. This approach also takes into account potential energy and braking heat dissipated by the vehicle when it travels downhill in decline. It was then necessary to determine the motor of every conceptual electric vehicle. The only way to do this is to compare the power of an electric vehicle and a diesel vehicle that have same workload. The only large vehicle that is manufactured as a diesel and as an electric is Sandvik s LH514 LHD unit. The motor of the LH514 electric is 70 per cent of its diesel equivalent (Sandvik Mining, 2012). Therefore, it was assumed that every conceptual electric vehicle in the fleet has motor power of 70 per cent of its diesel equivalent. With this assumption, the existing mining diesel fleet was converted into a conceptual electric fleet by multiplying their engine power with 70 per cent. The final step was running Ventsim Visual simulations. After mine thermal parameters and heat emitted from conceptual electric vehicles were set, the primary fan was adjusted with various values of fixed quantity that is sufficient enough to have the WB temperature less than or equal to 30 C at the deepest part of the mine. This fixed quantity was then subtracted with leakage and fixed facilities (fuel bay/ workshop and magazine) quantities, and then divided by the total fleet power to obtain unit requirement as m 3 /s per kw. Refrigeration plant was included in the simulation if required. It was assumed that these vehicles are battery powered, which do not produce moisture. Therefore, all the machinery will be inputted as a point source of sensible heat. Although an electric vehicle can have fuel cell as its power source, fuel cell produces moisture and therefore emits sensible and latent heat. The main reason for using this assumption is that the quantification of heat emitted by a mining electric heavy vehicle was done on a cable trailed LHD, which does not produces moisture and hence the heat emitted by this vehicle should be equal to that emitted by a battery powered vehicle. QUANTIFYING HEAT EMITTED BY A MINING ELECTRIC HEAVY VEHICLE As described before, the quantification of heat emitted by a mining electric heavy vehicle was done at Rio Tinto s Northparkes Copper-Gold mine in New South Wales, which exclusively uses Sandvik LH514E LHD units as its main production equipment. The unit operates on a delivered voltage of about 1000 V. It is equipped with three electric motors, one drive motor (132 kw), one pump motor (45 kw), and one fan motor (1.5 kw). The LH514E carries an arrow head design bucket with a capacity of 7 m 3. To determine heat emitted by an LH514E, the following parameters were measured upstream and downstream of the LHD operating in a production drive (refer to Figure 1): DB temperature WB temperature Barometric pressure. In addition to these, airflow quantity that flows in the production drive was measured by measuring air velocity and drive cross sectional area. Using these measurement results and psychometric equations, the amount of heat that was emitted by the unit could be calculated. The measurements were taken while the LHD unit had come to a halt. Unlike diesel vehicles, there is no difference between idling and full throttle as the electric motor always runs at constant speed. This is because when accelerator pedal is pressed, the motor is engaged to the drive wheel and when the pedal is released, the motor is disengaged from the drive wheel. FIG 1 - Locations of LH514E heat quantification measurements. 216

3 VENTILATION REQUIREMENT FOR ELECTRIC UNDERGROUND HARD ROCK MINES A CONCEPTUAL STUDY Various instruments were required in order to complete the measurements. They were supplied by the Western Australian School of Mines (WASM). The Kestrel 4000 pocket weather meter was used to measure the DB and WB temperatures, the Druck DPI 740 digital barometer was used to measure the barometric pressures, Alnor RVA501 digital anemometer was used to measure air velocity, and the Leica Disto was used to measure dimension of the airway. The main aim of the measurements is to have sufficient and reliable data regarding the heat emitted by the LHD unit. Therefore, the measurements were undertaken as close as possible to the LHD unit in order to prevent the data being interfered from other heat sources present in an underground hard rock mine such as ground, the concrete pavement, the caved ore in drawpoints, and water in the production drive. However, it was also understood that there is a high safety risk for personnel to get close to a running electric LHD unit. Therefore, a risk assessment (RA) and a job safety analysis (JSA) were conducted before the measurements were taken. There is a popular assumption that heat emitted by an electric vehicle is equal to the power consumed by the vehicle. In order to validate this assumption, the current, voltage and power factor were also measured. Power consumption by the unit was calculated using the following equations: E = ( 3 V I cosθ)/1000 (1) where: E = power consumption V = voltage (V) I = current (A) cosθ = power factor Once these calculations were done, comparison between power consumed by the LHD unit and heat emitted by it could be done and the assumption could be validated. Measurements were done across five points at the front and rear of the LHD unit to minimise the errors. Results of these measurements are shown in Table 1. As shown in Table 1, the variations between the values of each point do not deviate by a huge amount. The air quantity in the drive was measured as 17.2 m 3 /s (21.6 kg/s). Using these results, heat emitted by the electric LHD was determined using psychometric equations included in an excel spreadsheet. Table 2 shows the heat emitted by the LHD for different trials. The average heat generated from Table 2 was then compared with the power consumed by the LHD. Power consumption was calculated as 147 kw. The voltage, current, and power factor for the LHD were measured as 1000 V, 100 A, and 0.85 respectively. The heat generated (145 kw) and power consumption (147 kw) is very similar and the variation between these values is due to the measurement errors. This finding validates assumption that heat generated by an electric vehicle equals to its power consumption. VENTSIM VISUAL SIMULATIONS Simulation of the deep mine The mine fleet (existing diesel and conceptual electric) of the deep mine is listed in Table 3. The mine employs parallel circuit except at the deepest block which employs series circuit. Trial Heat Average TABLE 1 Data collected at Northparkes Mine. Front of load-haul-dump End of load-haul-dump Barometric Barometric Trial DB WB pressure (Pa) DB WB pressure (Pa) Average TABLE 2 Heat emitted by LH514E. TABLE 3 Deep mine fleet. Diesel engine power Electric motor Fleet size Total electric Light vehicle Light truck (stores) long Light truck (stores) short Charge up rig IT Jumbo Production drill Medium LHD Large LHD Small truck Large truck Grader Water cart Shotcrete rig Concrete agitator TOTAL

4 A HALIM AND M KERAI Not all vehicles listed in Table 3 are present inside the mine at the same time. The simulation was run with the vehicles listed in Table 4 operating in the mine at the same time. This is the typical operational situation in this mine. Jumbos and longhole drills were not included as they run with their small electric motor while drilling. Their motor is so small and therefore can be negligible. TABLE 4 Vehicles operating in the deep mine at the same time. Equipment IT 1 Large truck 4 Grader 1 Water cart 1 Large LHD 3 Small truck 3 Light vehicle 7 As described before, two distinct thermal conditions were included in this simulation, namely Kalgoorlie (cool strata) and Mount Isa (hot strata). Tables 5 and 6 list the thermal parameters for cool and hot strata respectively. These values were obtained from measurements in mines around Kalgoorlie and Mount Isa (Derrington, 2009; Nixon, Gillies and Howes, 1992; H W Wu, pers. comm., 16 August 2012). The average airway age of this mine is nine years. TABLE 5 Kalgoorlie (cool strata) thermal parameters. Parameters Values Rock thermal conductivity 1.75 W/m C Rock thermal diffusivity m 2 /s Rock temperature at surface/portal 23 C Geothermal gradient 8.5 C/km vertical metres Airway wetness factor 10% Average surface barometric pressure 98 kpa Average surface summer temperatures 23 C WB, 35 C DB TABLE 6 Mount Isa (hot strata) thermal parameters. Parameters Values Rock thermal conductivity 3.67 W/m C Rock thermal diffusivity 2.07 x 10-6 m 2 /s Rock temperature at surface/portal 28 C Geothermal gradient C/km vertical metres Airway wetness factor 10% Average surface barometric pressure 98.5 kpa Average surface summer temperatures 25 C WB, 35 C DB In order to address the shortage of airflow in its deepest block, this mine utilises a booster fan. Table 7 shows the conceptual electric vehicle fleet that is used in the deepest block of the mine. This fleet was based on the actual diesel fleet used in this mining block. The electric motor was estimated based on the 70 per cent diesel power assumption described before. The fixed quantity of the booster fan was adjusted along with that of the primary fan in order to find total quantity that will cause WB temperature less than or equal to 30 C in the deepest part of the mine. TABLE 7 Electric vehicle fleet in the deepest block of the deep mine. Motor Total Light vehicle Charge up rig IT Jumbo Production drill Large LHD Small truck Large truck Grader Water cart Shorcrete rig Concrete agitator TOTAL As with the fixed quantity in the primary fan, this fixed quantity was then adjusted to exclude leakage quantity and then divided by the total motor shown in Table 7 to obtain unit ventilation requirement in m 3 /s per kw. No fixed facilities quantity was subtracted from this fixed quantity as the fixed facilities are located above this block. This value was then compared with that obtained from the fixed quantity in the primary fan. Both fixed quantities were then varied in the model, until a WB temperature of 30 C was reached in the deepest part of the mine and both produced similar unit ventilation requirement. It was found that if a refrigeration plant is not employed, the mine located in cool strata requires a total quantity of 900 m 3 /s in its primary fans and 415 m 3 /s in its booster fan. The fixed quantity in the primary fans takes into account the 20 m 3 /s requirement for fixed facilities and the leakage factor of 25 per cent. Therefore, the quantity required for the fleet had to be adjusted accordingly as shown: Total quantity = 900 m 3 /s Quantity without leakage = 900/1.25 = 720 m 3 /s Quantity for vehicle fleet (less quantity for fixed facilities) = = 700 m 3 /s. The total power of the conceptual electric vehicles is noted to be kw. By simply dividing the required quantity of 700 m 3 /s with the total power of kw, the unit requirement was calculated as 0.09 m 3 /s per kw electric motor. For the fleet used in the deepest block, the quantity that was allocated for the fleet was calculated by excluding 20 per cent leakage factor from the booster fan fixed quantity. It was found that the fleet quantity is 415/1.2 = 350 m 3 /s. The unit requirement was calculated by dividing this quantity by the 218

5 VENTILATION REQUIREMENT FOR ELECTRIC UNDERGROUND HARD ROCK MINES A CONCEPTUAL STUDY total power of the conceptual electric vehicles used in this block ( kw). The requirement is 0.09 m 3 /s per kw, which is same with that calculated from the fixed quantity in the primary fan. However, this requirement causes air velocity greater than 5 m/s in many parts of decline, which causes visibility issues due to dust pick up. Therefore, another simulation was done in which a refrigeration plant is employed in order to reduce airflow requirement and air velocity in decline. In order to reduce air velocity to be less than 5 m/s, the total quantity in the primary fans and booster fan have to be reduced to 420 m 3 /s and 185 m 3 /s. A 1.5 MW of refrigeration capacity (R) surface refrigeration plant has to be employed. These correspond to a unit requirement of 0.04 m 3 /s per kw electric motor, which is less than the current world regulatory requirements for the diesel fleet which are 0.05 to m 3 /s per rated kw diesel engine power. For the mine that is located in hot strata, it was found that to achieve same requirement, a 5 MW(R) surface refrigeration plant has to be employed. Simulation of the shallow mine The mine fleet at this mine is listed below in Table 8 with the vehicles operating in a mine at the same time listed in Table 9. The shallow mine is a series ventilation circuit in which each level (ore drive) is ventilated by auxiliary fan and lay flat duct. In addition to heat from the vehicles listed on Table 9, ten TABLE 8 Shallow mine fleet. Diesel engine power Electric motor Total electric Light vehicle Charge up rig Small LHD type Small LHD type Large LHD Large truck Medium truck IT Shotcrete rig Concrete agitator TOTAL TABLE 9 Vehicles operating in the shallow mine at the same time. Large truck 1 Medium truck 1 IT 1 Light vehicle 3 Small LHD type 1 1 Small LHD type 2 1 Large LHD 1 90 kw ore drive auxiliary fans were turned on in the model to simulate heat emitted by them. This reflects the typical condition in this mine where ten ore drives are active at a time. The average airway age in this mine is three years. The mine has a magazine and a fuel bay. The shallow mine located in cool strata requires a total quantity of 90 m 3 /s at its primary fan without any refrigeration plant. This quantity includes 20 per cent leakage factor. Therefore, the ventilation requirement was obtained by adjusted this quantity to exclude leakage and fixed facilities, and then dividing the adjusted quantity by the total power of kw. This resulted in required quantity of m 3 /s per kw electric motor, which is less than the current world regulatory requirements of 0.05 to m 3 /s per rated kw diesel engine power. The shallow mine located in hot strata requires a total quantity of 200 m 3 /s at its primary fan without any refrigeration plant. The higher requirement reflects the additional heat load from warmer strata. The ventilation requirement was calculated as m 3 /s per kw. This requirement causes air velocity greater than 5 m/s in many parts of decline. Therefore, another simulation was done in which a refrigeration plant is employed in order to reduce airflow requirement and air velocity in decline. With a 1 MW(R) surface plant installed, the primary fan quantity is reduced to 120 m 3 /s, which corresponds to unit requirement of m 3 /s per kw. This is less than the current world regulatory requirements for the diesel fleet which are 0.05 to m 3 /s per rated kw diesel engine power. CONCLUSIONS AND RECOMMENDATIONS Table 10 summarises the estimated unit ventilation requirement for electric vehicles with the size of refrigeration plant installed if required. TABLE 10 Summary of estimated unit ventilation requirement for electric vehicles. Mine Thermal condition Airflow requirement (m 3 /s /kw) Note Deep mine Cool 0.04 A 1.5 MW(R) surface refrigeration plant is required Deep mine Hot 0.04 A 5 MW(R) surface refrigeration plant is required Shallow mine Cool No refrigeration plant is required Shallow mine Hot A 1 MW(R) surface refrigeration plant is required It can be seen from Table 10 that for shallow mine located in cool strata, there is an indication that utilising electric vehicles will require less ventilation than utilising diesel vehicles, and therefore will save primary fan power cost. In deep mines located in cool strata and mines located in hot strata, a refrigeration plant is required to achieve this condition. Increasing refrigeration plant size will reduce ventilation requirement. However, the caveat is that the mine will incur additional cost to install, operate and maintain the plant. An optimisation study to gain an understanding of the balance between ventilation requirement and refrigeration plant size should be carried out in the future. 219

6 A HALIM AND M KERAI The popular assumption that heat emitted by electric vehicle equals to its power consumption was validated in this study. The simulations done in this study were only based on two mines. As each mine is unique and has its specific ventilation circuit, vehicle fleet, and geothermal parameters, a similar simulation of other mines might produce different results. It is recommended that similar simulation is done in different mines in order to support the indication found in this study. This study was done with an assumption that all electric vehicles are powered by battery due to the inability to quantifying heat emitted by a fuel cell powered mining vehicle since such a vehicle is yet to be commercially available. It is recommended that when fuel cell powered mining vehicles are available, the heat emitted by these vehicles is quantified and therefore a similar study could be carried out on fuel cell powered vehicles. ACKNOWLEDGEMENTS This study required visiting Rio Tinto s Northparkes Mines, which was made possible through the funding from Mining Education Australia (MEA). The authors would like to thank Professor Peter Knight, Executive Director of MEA, and Paulette Schmidt, Finance Officer and Executive Assistant of MEA for arranging the visit to Northparkes Mines, Eddy Samosir, Claudia Vejrazka, and Mat Allan from Northparkes Mines for their assistance during the visit. REFERENCES Brake, D J, The design of push-pull primary and secondary ventilation systems and a vertically-split intake-exhaust ventilation shaft, in Proceedings 13th US/North American Mine Ventilation Symposium, (eds: S Hardcastle and D McKinnon) pp (MIRARCO, Laurentian University: Sudbury). Derrington, A, Confidential company report, unpublished. Howes, M J and Clarke, B, The Granites Callie Mine Justification and design of a mine cooling plant suitable for wet or dry condensing, in Proceedings Fourth International Seminar on Deep and High Stress Mining (Deep Mining 07) (ed: Y Potvin) pp (Australian Centre for Geomechanics, University of Western Australia: Perth). Kocsis, C, Ventilation benefit analysis for Canadian mines Fuelcell loader project, Report , CANMET-MMSL. Marks, J R, Airflow specification for metal and non-metal mines, in Proceedings 14th US/North American Mine Ventilation Symposium, (eds: F Calizaya and M Nelson) pp (University of Utah: Salt Lake City). McPherson, M J, Subsurface Ventilation Engineering, p 505 (Mine Ventilation Services, Inc: Fresno). Mining Magazine, Plugging The Gap Underground [online], Available from: < equipment/plugging-the-gap-underground> [Accessed: 10 December 2012]. New South Wales (NSW) Government, Mining Design Guideline (MDG) 29 Guidelines for the management of diesel engine pollutants in underground environments [online], Available from: < publications/mdg/1-999> [Accessed: 10 December 2012]. Nixon, C A, Gillies, A D S and Howes, M J, Analysis of heat sources in a large mechanized development end at Mount Isa Mine, in Proceedings 5th International Mine Ventilation Congress, Johannesburg (ed: R. Hemp) pp (Mine Ventilation Society of South Africa: Johannesburg). Sandvik Mining, Specifications of LH514 and LH514E [online], Available from: < [Accessed: 15 August 2012]. Western Australian (WA) Government, Mines Safety and Inspection Regulation 1995 [online], Available from: < html> [Accessed: 10 December 2012]. Wu, H W, Gillies Wu Mining Technology, Brisbane, Qld, Australia (Personal communication, 16 August). 220