Deep mine cooling, a case for Northern Ontario: Part I

International Journal of Mining Science and Technology 26 (2016) 721–727

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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Deep mine cooling, a case for Northern Ontario: Part I D. Millar a,b, K. Trapani a,⇑, A. Romero a,b a b

MIRARCO, 935 Ramsey Lake Rd, Sudbury P3E 2C6, Canada Laurentian University, 935 Ramsey Lake Rd, Sudbury P3E 2C6, Canada

a r t i c l e

i n f o

Article history: Received 16 July 2015 Received in revised form 9 January 2016 Accepted 28 April 2016 Available online 31 May 2016 Keywords: Thermal loads Cooling Underground mining Deep mining HVAC mining

a b s t r a c t Cooling energy needs, for mines in Northern Ontario, are mainly driven by the mining depth and its operation. Part I of this research focusses on the thermal energy loads in deep mines as a result of the virgin rock temperature, mining operations and climatic conditions. A breakdown of the various heat sources is outlined, for an underground mine producing 3500 tonnes per day of broken rock, taking into consideration the latent and sensible portions of that heat to properly assess the wet bulb global temperature. The resulting thermal loads indicate that cooling efforts would be needed both at surface and underground to maintain the temperature underground within the legal threshold. In winter the air might also have to be heated at surface and cooled underground, to ensure that icing does not occur in the inlet ventilation shaft-the main reason why cooling cannot be focussed solely at surface. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction Mining at depth creates new challenges not only in extracting the ore at such a depth but also in maintaining the working environment within safe working temperatures. Intake ventilation air increases in temperature as it auto-compresses on its way down the shaft [1], at a rate of approximately 10 °C/km. So for a mine 3 km below the mean sea-level, the increase in temperature due to the compressibility of the air would be in the range of 30 °C. Depending on the geothermal gradient at the mine’s location, there might also be some heat uptake from the surrounding rock mass, especially at deeper levels where the virgin rock temperature is higher [2]. The American Conference of Governmental and Industrial Hygienists (ACGIH) guidance on wet bulb global temperature (WBGT) for work levels and work regimen (Table 1) indicate that working temperatures exceeding 26.7 °C, at a moderate working level, should be subjected to a work-rest regimen. A work-rest regimen requires a proportion of the time working and resting every hour of work, with the proportions determined by how much the temperature exceeds 26.7 °C. Longer periods of time spent at rest lowers production rates and so efficacy of the mine’s operation. Cooling is thus required to ensure that the working temperature is below the 26.7 °C threshold for which a work-rest regimen is necessary. Which form of cooling, surface or sub-surface cooling,

⇑ Corresponding author. Tel.: +1 705 6751151. E-mail address: [email protected] (K. Trapani).

would be better suited to provide the coolth necessary for deep underground mining? This research aims to (i) analyse the sources of heat contributing to the temperature underground and (ii) determine the cooling delivery methods that would be effective in maintaining the WBGT (temperature) below the work-rest temperature regimen threshold. This includes investigation of individual and combined surface and sub-surface cooling. Since this research is focused on mining operations in Northern Ontario, Canada, climatic conditions typical of the region were considered, with sub-zero temperatures in winter and moderate temperatures in summer.

2. Heating sources 2.1. Auto-compression and geothermal gradient As a fluid flows down a shaft its potential energy is converted into enthalpy, resulting in an increase in temperature (by autocompression). At the same time, heat present due to geothermal flux and radiometric decay in the surrounding rock leads to heat transfer from the rock to the fluid. The virgin rock temperature underground is indicative of the geothermal heat transfer. In Northern Ontario, at 3000 m depth, the virgin rock temperature (VRT) is approximately 55 °C [3]. The geothermal step can be taken to be 57.1 m/°C, based on the geothermal resource assessment of Bristow et al. and Grasby et al. [4,5]. The heat uptake depends not only on the geothermal step but also the age of the shaft, its dimensions and its wetness fraction.

http://dx.doi.org/10.1016/j.ijmst.2016.05.026 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

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Table 1 ACGIH guidance on wet bulb globe temperature (WBGT) values for work levels and work regimens. Whole body working level Descriptor

Light

Moderate

Heavy

Rate of work (W) Exemplified by

244 Sitting/standing, light hand or arm work

349 Walking with moderate lifting or pushing

488 Pick and shovel

Work regimen

TLV WBGT temperatures

Continuous work 75% work, 25% rest, each hour 50% work, 50% rest, each hour 25% work, 75% rest, each hour

30.0 °C 30.6 °C 31.4 °C 32.2 °C

26.7 °C 28.0 °C 29.4 °C 31.1 °C

25.0 °C 25.9 °C 27.9 °C 30.0 °C

(86 °F) (87 °F) (89 °F) (90 °F)

(80 °F) (82 °F) (85 °F) (88 °F)

(77 °F) (78 °F) (82 °F) (86 °F)

Table 2 Sensible and latent heat loads from underground tunnel. Depth (m) Virgin rock temperature (°C) Dry surface temperature (°C) Wet surface temperature (°C)

1500 29.3 34.7 27.9

2000 38.0 35.2 28.0

2500 46.8 35.6 28.1

3000 55.5 36.1 28.2

3500 64.3 36.5 28.3

Wetness fraction 0.00-dry Sensible heating power (kW) Latent heating power (kW) Water added to air (kg/s)

1120 0 0

591 0 0

2303 0 0

4014 0 0

5726 0 0

Wetness fraction 0.25-moderately dry Sensible heating power (kW) Latent heating power (kW) Water added to air (kg/s)

7538 6771 2.779

6152 7114 2.921

4766 7458 3.062

3381 7802 3.204

1997 8148 3.346

Wetness fraction 0.50-wet Sensible heating power (kW) Latent heating power (kW) Water added to air (kg/s)

13,955 13,542 5.559

12,895 14,229 5.842

11,836 14,917 6.125

1077 15,606 6.408

9719 16,295 6.692

Tunnel dimension 5 m
5 m of total length 12 km at given depth, for varying wetness fraction and maintained air condition DB = 35 °C, WB = 28 °C and rock with thermal conductivity 4.5 W/ (m °C), rock density 2700 kg/m3 and heat capacity 950 J/(kg °C). Air velocity assumed to be 2 m/s in all openings. Geothermal step 57.1 m/°C and rock temperature at surface = 3 °C. The temperature change due to the conversion from potential energy to enthalpy is dependent upon the following derivation:

ðh2  h1 Þ ¼

1 2 ðv  v 21 Þ þ gðz2  z1 Þ 2 2

But ðh2  h1 Þ ¼ cp ðt 2  t 1 Þ and

1 2 ðv  v 21 Þ ¼ 0 2 2

Therefore ðt2  t1 Þ ¼ gðz2  z1 Þ=cp where h is the coolant enthalpy, v is the coolant velocity, z is the depth, t is the coolant temperature and cp is the specific heat capacity of the coolant. The change in temperature for air, water and ice cooling medium, according to their respective specific heat capacities (1005, 4187 and 2111 J/(kg °C)), is 9.76 °C/km, 2.34 °C/km and 4.65 °C/ km respectively. The coolth stored in each of the cooling mediums, taking the outlet temperature (after heating through the mine tunnel) to be 28 °C, is 25, 105 and 450 kJ/kg for air (at 3 °C), water (at 3 °C) and ice (at 1 °C). Considering the change in potential energy, the energy stored in the media for the same conditions would be 4, 75 and 420 kJ/kg for air, water and ice respectively. So, at 3 km depth, air which is surface cooled to 3 °C (no lower, to prevent icing damage to the hoisting system) is not able to provide any cooling, rather it is a source of heat to the underground workings. Table 2 illustrates the resulting heat loads at varying depths underground, taking into consideration the potential

energy conversion and the geothermal heat gradient. Results show that with a higher wetness fraction the air enthalpy is also higher, mainly due to the latent heat from evaporation of the water. 2.2. Diesel mobile plant Mobile plant, essential in underground mining operations, include LHDs (load–haul–dump), haulage trucks, and service/utility trucks (including drills, bolters and scalers). Some mines are slowly transitioning to an electric fleet primarily to reduce ventilation requirements resulting from exhaust emissions of the diesel engines; yet, most mines in Northern Ontario still operate with a majority diesel mobile plant. Table 3 illustrates the installed capacity of the diesel mobile plant in some mines in Northern Ontario. These machines not only generate combustion product gases and diesel particulate matter but also produce substantial heat [6]. The heat generated from the diesel mobile plant depends on the duty cycle of the equipment as well as the engine’s efficiency. Thus not all of the heat entering the air as a result of diesel combustion does so as sensible heat. The heat output from the diesel mobile plant is a combination of latent and sensible heat. The latent heat component is defined as a product of the water added to the air (kg/s) and the latent heat of the water at the wet bulb temperature (MJ/kg). The sensible heat is the difference between the total heat generated from the diesel engine (delivered useful engine power, MW/engine efficiency, %) and the latent heat. Taking diesel to comprise C12H23 on average, its complete combustion reaction is:

C12 H23 þ 84:5ð0:79N2 þ 0:21O2 Þ ! 12CO2 þ 11:5H2 O þ 66:8N2 Which corresponds to a mass-based air-to-fuel ratio (AFR) of 14.6 and means that for every kilogram of fuel combusted, 1.239 kg of water is released into the air (the water-to-fuel ratio).

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D. Millar et al. / International Journal of Mining Science and Technology 26 (2016) 721–727 Table 3 Fleet installed diesel engine capacity in Northern Ontario mines (kW). Mine

Kidd creek

Lockerby

Lindsley

Onaping/Craig

Vehicle type LHDs Haulage trucks Service/utility/drills

4796 1193 10,615

1910 915 1497

1166 326 719

4241 2265 6083

Totals

16,604

4322

2210

12,589

Table 5 Heat generation comparison of a diesel LHD with an electric LHD of the same performance.

Engine/motor output (kW) Thermal efficiency (%) Heat dissipated from the prime mover (kW) Drive train losses (kW) Total heat dissipation (kW)

Using an average engine efficiency of 37%, in accordance with Grenier et al., the heat loads for the installed capacity of a diesel mobile plant were calculated and are presented in Table 4. 2.3. Electric mobile plant As previously mentioned, electric mobile plant is becoming increasingly popular. This is mainly due to the associated energy savings from reduced ventilation requirement since electric mobile plant does not have combustion emissions. The efficiency of electric mobile plant is also higher, resulting in a lower amount of heat generated. The higher efficiency is mainly due to: (i) the electric motor requiring less power than a diesel motor for the same performance due to the former’s torque characteristics, and (ii) the energy supplied to the mobile plant is converted into useful work at the wheels more efficiently with electrically operated equipment and drive trains than is the case of diesel powered equipment. A comparison between the two is given in Table 5, emphasising the heat generated. In the case of the diesel mobile plant the heat generated from the equipment consists of both sensible and latent heat, as a result of the added water from combustion. In contrast there is no combustion in the electric mobile plant and so all the heat generated is sensible heat. 2.4. Broken rock in sub-surface Freshly blasted rock initially has a temperature equal to the virgin rock temperature (VRT) of the surrounding rock, which in this case is taken to be 55 °C. It is then cooled down by the ventilation air, so contributing to the heat underground [7,8]. The heat load (kW) can be approximated as follows:

_ p ðVRT  t d Þ q ¼ mC _ is the average mass rate of broken rock (kg/s), C p is the where m specific heat capacity of the broken rock (kJ/(kg °C)) and td is the temperature at which the rock exits the underground working. Assuming a broken rock production rate of 3500 tonnes per day underground, the average mass rate of broken rock is 40.5 kg/s. The specific heat capacity of the rock was taken to be 0.95 kJ/(kg °C), with a rock density of 2700 kg/m3 and an existing rock temperature of 29 °C. Hence the approximated heat load from the broken

Diesel LHD

Electric LHD

243 37 516 97.2 614

177 95 9 97.2 107

rock is 960 kW (for dry broken rock mass). These results vary with the wetness fractions of the broken rock, which are given in Table 6. As the mining depth increases, so does the heating from the broken rock since the rock fragments have a surface temperature approximating the virgin rock temperature. The latent heat generated from the broken rock will depend on the fraction of surface area which is wetted. The surface area of the broken rock from blasting was taken to be 150 m2/m3. The wetted surface area so could be estimated for the broken rock, for which the resulting latent heat was calculated based on the various relative humidity underground at the different depths. 2.5. Auxiliary electrical equipment Most of the auxiliary equipment in an underground mine operates on electricity. The heat produced by some of the auxiliary electrical equipment can be substantial. For example, in the Enterprise mine in Australia, the contribution of electrical equipment to the overall heat load is approximately 7% [9]. The most important electrical contributors of heat in an underground mine are:    

Ventilation system fans-10% end heat 100 kW; Pumps-10% end heat 400 kW; Bolters and drill jumbos-7 kW heat load in average mine; Lighting-100% end heat 350 kW.

So the total estimated heat from the auxiliary electrical equipment, based on a production rate of 3500 tonnes per day, is approximately 860 kW. 2.6. Other sources of heat Other sources of heat loading of mine ventilation air that are associated with water include evaporation of water from: (i) still liquid sources such as pools of water and wet fill material, and (ii) flowing streams of liquid such as springs, drainage channels and sprays. While the dominant water source for these deep mines is service water, with flow rates 20–30 kg/s, these are considered in the treatment of broken ore. Smaller heat loads may be expected from resistance or I2R losses in electricity distribution cabling (currently of order 5% of rating of 20 MWe, i.e. 1 MWth).

Table 4 Installed underground diesel engine rated capacity at Kidd Creek Mine in 2000, accounting for water-to-fuel ratio of diesel combustion. Vehicle description

LHDs

Haulage trucks

Pick-up trucks

Drills/bolters/scalers

Utility trucks/tractors

Misc

Total

Peak production conditions Number of operating vehicles Delivered useful power (MW) Peak air heating power (MW) Fuel burn rate (@45.575 MJ/kg) (kg/s)

16 2.1 5.8 0.127

5 0.5 1.3 0.029

10 0.2 0.4 0.010

20 1.0 2.8 0.061

25 0.7 1.8 0.040

20 0.4 1.1 0.025

96 4.9 13.4 0.293

Water-to-fuel ratio 1.239 chemical water only Water added (@1.239 kg/kg fuel) (kg/s) 0.158 Latent heat load (@L = 2.44 MJ/kg) (MW) 0.4 Sensible heat load (MW) 5.4

0.037 0.1 1.3

0.012 0.0 0.4

0.076 0.2 2.6

0.050 0.1 1.7

0.031 0.1 1.1

0.363 0.9 12.5

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Stope filling is also a common practice, which leads to the cement heat of hydration to be considered. The cement heat of hydration is taken to be approximately 250 kJ/kg [10], considering 10% cement backfill the heat released from the backfill is 25 kJ/kg. Assuming the same amount of backfill as rock extracted is re-introduced in the mine, that is 3500 tonnes per day or 40.5 kg/s, and then the heat of hydration from the backfill can be estimated to 1013 kW. Oxidation of sulphide ores also results in exothermic heat release; this heat could be estimated by considering the oxidation reaction for the different sulphides within the ore. The different sulphides considered were pentlandite, chalcopyrite and pyrrhotite taken to consist 1.4%, 1.2% and 19.4% of the ore respectively

[11]. Assuming a 1 mm oxidation depth of the ore as it is being transported underground (broken rock with surface area 150 m2/ m3, density of 2700 kg/m3 and production rate of 40.5 kg/s), the released exothermic heat for the pentlandite, chalcopyrite and pyrrhotite respectively is 75.5, 52.0 and 1469.8 kW based on the reaction’s heat of formation [12–14]. The resulting heat from the oxidation of the sulphide ores for the specified mineral percentages is 1597 kW. 2.7. Summary The heat sources reviewed in this section are compiled in Table 7, in which sensible and latent loads were summed to

Table 6 Average heat loads from freshly broken (1 day old) rock with porosity 30% for air conditions DB = WB = 28 °C and air velocities through muck piles of 5 mm/s with thermal conductivity 4.5 W/(m °C), rock density 2700 kg/m3, heat capacity 950 J/(kg °C) for varying wetness fractions. Depth (m) Virgin rock temperature (°C)

1500 29.3

2000 38.0

2500 46.8

3000 55.5

3500 64.3

Wetness fraction 0.00-dry Specific sensible heating power (kW/m3 broken rock) Specific latent heating power (kW/m3 broken rock) Water added to air from wet surface (kW/m3 broken rock) Sensible heating power for 1.25 mtpa (kW) Latent heating power for 1.25 mtpa (kW) Water added to air for 1.25 mtpa (kg/s)

0.01 0 0 12 0 0

0.21 0 0 346 0 0

0.41 0 0 685 0 0

0.61 0 0 1020 0 0

0.81 0 0 1358 0 0

Wetness fraction 0.25-moderately dry Specific sensible heating power (kW/m3 broken rock) Specific latent heating power (kW/m3 broken rock) Water added to air from wet surface (kW/m3 broken rock) Sensible heating power for 1.25 mtpa (kW) Latent heating power for 1.25 mtpa (kW) Water added to air for 1.25 mtpa (kg/s)

0 0 0 7 5 0.002

0.11 0.10 0 187 159 0.065

0.22 0.19 0 370 315 0.130

0.33 0.28 0 548 472 0.194

0.43 0.38 0 727 631 0.259

Wetness fraction 0.50-wet Specific sensible heating power (kW/m3 broken rock) Specific latent heating power (kW/m3 broken rock) Water added to air from wet surface (kW/m3 broken rock) Sensible heating power for 1.25 mtpa (kW) Latent heating power for 1.25 mtpa (kW) Water added to air for 1.25 mtpa (kg/s)

0 0 0 5 7 0.003

0.08 0.13 0 129 217 0.089

0.15 0.26 0 254 431 0.177

0.22 0.39 0 376 644 0.264

0.30 0.51 0 499 859 0.353

Table 7 Heat loads from underground tunnel, for varying wetness fraction and maintained air condition DB = 35 °C, WB = 28 °C and rock with thermal conductivity 4.5 W/(m °C), rock density 2700 kg/m3 and heat capacity 950 J/(kg °C). Air velocity assumed to be 2 m/s in all openings. Geothermal step 57.1 m/°C and rock temperature at surface = 3 °C. Summary

Depth (m)

1500

2000

2500

3000

3500

Diesel mobile plant (kW) (WFR = 3.6) (kW) Water (kg/s) Electrical mobile plant (kW) Heat transferred on tunnel surfaces (kW) (Wetness fraction 0.25) (kW) Water (kg/s) Broken rock (kW) (Wetness fraction 0.25) (kW) Water (kg/s)

Sensible Latent

10,800 2600 1.055 6700 7538 6771 2.779 5 7 0.003

10,800 2600 1.055 6700 6152 7114 2.921 129 217 0.089

10,800 2600 1.055 6700 4766 7458 3.062 254 431 0.177

10,800 2600 1.055 6700 3381 7802 3.204 376 644 0.264

10,800 2600 1.055 6700 1977 8148 3.346 499 859 0.353

Sensible Sensible Sensible Sensible Sensible

1600 100 24 7 369

1600 100 24 7 369

1600 100 24 7 369

1600 100 24 7 369

1600 100 24 7 369

Sensible Sensible

1013 1597 7977 9378 3.837 3877 6778 2.782

1013 1597 9487 9931 4.065 5387 7331 3.010

1013 1597 10,998 10,489 5.291 6898 7889 3.239

1013 1597 12,505 11,046 4.523 8405 8446 3.468

1013 1597 14,012 11,607 4.754 9912 9007 3.699

Auxiliary electrical equipment Auxiliary fans (kW) Pumps (kW) Bolters (kW) Drills (kW) Lighting (kW) Other heat sources Hydration of cement back fill (kW) Oxidation of sulphide ores (kW) Total sensible (diesel) (kW) Total latent (diesel) (kW) Total water (diesel) (kg/s) Total sensible (electric) (kW) Total latent (electric) (kW) Total water (electric) (kg/s)

Sensible Sensible Latent Sensible Latent

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D. Millar et al. / International Journal of Mining Science and Technology 26 (2016) 721–727 1 R1

8 2

60

7

Down shaft

Working tunnel

Up shaft

50 Temperature (ć)

WBGT

R2

WBGT Max

40

DB 30

WB VRT

20 10

3 4

5

6

0

Fig. 1. Schematic of the underground ventilation and refrigeration system investigated (R1 and R2 are mine air refrigeration systems installed on the surface and in the sub-surface (as a bulk air cooler) respectively).

60

Down shaft

2000

4000

6000

8000

10000

12000

Tunnel length (m)

Fig. 3. Temperature profile of a 3000 m deep mine with a 4000 m long working tunnel for Scenario 2: No cooling, summer (surface temperature, DB = 24 °C and WB = 18 °C).

Working tunnel Up shaft

60

Down shaft

Working tunnel

Up shaft

50

WBGT Max DB

30

WB 20

VRT

WBGT

Temperature (ć)

Temperature (ć)

50 WBGT

40

10

2000

4000 6000 8000 Tunnel length (m)

10000

highlight the total heat load within the mine and therefore the amount of cooling required. The sub-surface climate simulations were performed with the ClimSim code for Windows v1.2. The results given in Table 7 assume a constant airflow of 2 m/s. 3. Coolth deliverance Mine cooling by ventilation air alone can be achieved at depths where the air delivered underground still has some cooling potential, i.e. lower depth, as indicated by the negative values given in Table 7 at 1500 m depths (dependent on surface temperatures). Increased ventilation airflow could be implemented to attain the cooling required, although this would be at the expense of increased ventilation costs. A simple mine network (Fig. 1) was designed, to assess various cooling scenarios for a 3000 m deep mine with diesel mobile plant and with the heat loads specified in Table 7 taken to be linearly distributed along the working tunnel (Points 5 to 6 in Fig. 1). The cooling scenarios investigated, some of which are discussed by Guo et al. [15], are as follows:

Scenario 4 Scenario 5

DB WB

20

0

12000

Fig. 2. Temperature profile of a 3000 m deep mine with a 4000 m long working tunnel for Scenario 1: No cooling, winter (surface temperature, DB = WB = 3 °C).

Scenario 1 Scenario 2 Scenario 3

WBGT Max

30

VRT

10

No cooling-winter (DB = WB = 3 °C) No cooling-summer (DB = 24 °C and WB = 18 °C) Surface cooling (30 MW)-summer (DB = 24 °C and WB = 18 °C) Sub-surface cooling (20 MW)-summer (DB = 24 °C and WB = 18 °C) Surface and sub-surface cooling (30 + 20 MW)summer (DB = 24 °C and WB = 18 °C)

The sub-surface cooling was assumed to be generated at the surface and then delivered underground through bulk air coolers. This was to reduce the cooling costs associated with an underground

2000

4000 6000 8000 Tunnel length (m)

10000

12000

Fig. 4. Temperature profile of a 3000 m deep mine with a 4000 m long working tunnel for Scenario 3: 30 MW surface cooling, summer (surface temperature, DB = 24 °C and WB = 18 °C). 60 Down shaft

Working tunnel

Up shaft

50

Temperature (ć)

0

40

WBGT 40

WBGT Max DB

30

WB 20

VRT

10

0

2000

4000 6000 8000 Tunnel length (m)

10000

12000

Fig. 5. Temperature profile of a 3000 m deep mine with a 4000 m long working tunnel for Scenario 4: 34 MW sub-surface cooling, summer (surface temperature, DB = 24 °C and WB = 18 °C).

refrigeration plant, which has a coefficient of performance (COP) close to 2 rather than 4 for a refrigeration plant at surface. This bigger COP value is due to the higher temperatures and pressures the heat pump and cooling tower would be operating in. The minimum temperature at which the ventilation is pumped underground is 3 °C; this to avoid icing damage in the hoisting shaft. The results for this Scenario 1, which simulates winter conditions with pre-heating as required, are shown in Fig. 2. Wetness fraction was taken to be 0.05 in the downcast shaft and working area, the up shaft was taken to be 0.25 wet due to condensation. Fig. 2 shows that at the lowest surface temperature the end temperature in the working tunnel is about 1 °C higher than the permitted WBGT temperature (red line) for a continuous working regime underground. This indicates that sub-surface cooling will still be required to ensure that working temperatures do not exceed the 27 °C WBGT. An even greater level of cooling will be

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D. Millar et al. / International Journal of Mining Science and Technology 26 (2016) 721–727 60

Down shaft

Working tunnel

Up shaft

Temperature (ć)

50 WBGT

40

WBGT Max 30

DB WB

20

VRT 10

0

2000

4000 6000 8000 Tunnel length (m)

10000

12000

Fig. 6. Temperature profile of a 3000 m deep mine with a 4000 m long working tunnel for Scenario 5: combined surface (20 + 10 MW) and sub-surface (20 MW) cooling, summer (surface temperature, DB = 24 °C and WB = 18 °C).

25 J

F

M

A

M

J

J

A

S

O

N

D Steam

20

Energy demand (MW)

Air heating Chilled water (surface precooling)

15

electric operated plant would result in a significantly lower heat load underground and therefore surface cooling would be sufficient in cooling the underground working area. The diesel consumption, as well as the electricity load (for which some of the energy is ultimately converted into heat underground), are shown in Fig. 7. The baseline underground cooling (air bulk cooler) was set to 20 MW of cooling. This baseline value took into account the almost constant heat sources underground (arising from the mobile diesel plant, auto-compression of air, geothermal heating, broken rock, auxiliary equipment and other heat sources) and the 3 °C cooling threshold at surface (to avoid icing damage in the hoisting shaft). The fluctuation in temperature (in summer) leads to a varying cooling load, which is estimated to peak at 20 MW, taking into consideration the coolth recovery from the return air of the mine. With higher coolth recovery from the mine’s return air, which is approximated at 10 °C cooler than the fresh air supply, the surface cooling plant’s rating could be lowered to 15 MW. Converting to an electric mobile fleet could result in a 20–25 MW reduction in the supply of underground cooling, by increasing the automotive efficiency of the fleet (having lower thermal losses) as well as eliminating the exhaust gases which require additional ventilation volume underground [16]; and so higher auto-compression and geothermal heat uptake.

Electricity

10

Diesel

5. Conclusions

UG cool air 5

0

24 48 72 96 120 144 168 192 216 240 264 288 Hours of 12 representative days of the year (one for each month)

Fig. 7. Energy demand profile for the heat loads given in Table 7, simulating a ‘typical’ mine in Northern Ontario.

required in summer, which is characterised by higher surface temperatures, as depicted in Scenario 2 (Fig. 3). Figs. 4 and 5 show the Scenario 3 and Scenario 4, respectively. The final scenario (Scenario 5) combined surface and subsurface cooling, 30 MW and 10 MW respectively. As shown in Fig. 6, this cooling scenario would achieve underground working temperatures that abide by the ACGIH thresholds for continuous working regimes. For the surface cooling loads, it is assumed that 10 MW of cooling can be recovered from the exhaust air from the return shaft outlet, meaning the effective surface cooling required would be approximately 20 MW. 4. Discussion At deep mining depths (>2500 m), the heat uptake from autocompression and the geothermal gradient of the rock in the downcast shaft of the mine results in higher working temperatures is safely permissible for continuous working regimes. Even in winter in Northern Ontario, where the mine is able to operate at the lowest input surface temperatures, working temperatures remain too high (Fig. 2). This indicates that surface cooling alone would not be sufficient in providing the necessary cooling for deep underground workings, thus a supplement of base underground/subsurface cooling load would be required. An energy demand profile for a typical Northern Ontario mine averaged throughout 12 representative days of the year is shown in Fig. 7. This profile models the climatic conditions and heat loads throughout the year and considers 50% wet broken rocks being produced from the mine at a rate of 3500 tonnes per day. It also assumes that the mine’s mobile plant is diesel operated, since an

This research analysed the major sources of heat underground, in particular the effect of auto-compression and geothermal heat uptake at varying mining depths and with different wetness parameters. The results showed that at depths >2500 m cooling will be necessary to ensure safe working temperatures underground; this is especially the case in the summer months which are characterised by high temperatures. The capacity of surface cooling is limited to a minimum of 3 °C due to safety concerns relating to icing in the hoisting shaft; thus a base load of cooling underground is required. For a ‘typical’ mine in Northern Ontario, designed with the heat loads specified in Table 7 and being operated underground with a diesel mobile plant, the baseline sub-surface cooling was estimated at 20 MW, while the surface cooling, varying with atmospheric temperatures >3 °C, was estimated at 15–20 MW peak (with 10 MW additional cooling from coolth recovery of the return air). Conventional cooling uses vapour compression refrigeration to satisfy such cooling loads. Other natural and mechanical cooling technologies could be implemented to provide the necessary cooling. These various cooling technologies and their technical and economic performance are discussed in Part II of the series ‘Deep Mine Cooling, a Case for Northern Ontario’. Acknowledgments The authors would like to thank CEMI (Centre for Excellence in Mining Innovation) for their funding to support this research. References [1] Donoghue AM. Occupational health hazards in mining: an overview. Occup Med 2004;54(5):283–9. [2] Beck AE, Balling N. Determination of virgin rock temperatures. Berlin: Springer, Netherlands; 1988. [3] O’Connor D. Justifying ventilation-on-demand in a Canadian mine and the need for process based simulations. In: 11th US/North American mine ventilation symposium, Pennsylvania; 2006. [4] Bristow Q, Mwenifumbo C. A new temperature, capacitive-resistivity, and magnetic-susceptibility borehole probe for mineral exploration, groundwater, and environmental applications. J Histochem Cytochem 1968;16:487–9.

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