ROCK DOWN TO ELECTRIC AVENUE
With the fuel price soaring and government tightening health and safety screws, now is the time for underground mines to go electric, writes Leon Louw.
It’s not a new conversation: diesel versus electric, or electric versus hybrid. Debates about the pros and cons of using electric or diesel driven equipment (or a hybrid version) in underground mines have been raging in the stopes and boardrooms for more than two decades.
But recently, a renewed emphasis on modernisation has swung the pendulum somewhat in favour of the electric option. But in an industry that has been a little slow to adapt to the demands of business in the wake of a fourth industrial revolution, going the electric route might be a bridge too far.
Diesel driven equipment has many proponents, albeit less than a few years ago. After all, replacing an age-old diesel guzzling fleet with a hip new look electric version requires a significant upfront capital investment. And that’s the glitch. The commodity downturn has put the squeeze on procurement managers. Regardless of the long-term benefits of investing in a clean, green and modern range of underground equipment, cash (for the moment) is king, and most mines have zipped up their once bulging wallets. However, it might be at their own peril, as the benefits of a modern, electrically driven underground fleet slightly outweigh those of a diesel engine.
If anything, the safety benefits of using electric equipment should be enough to convince any hard-core diesel admirer to switch allegiance. Since the first fleet of diesel driven equipment was sent down underground mines in the early 1900s, thousands (if not millions) of miners have inhaled huge quantities of harmful diesel particulate matter (DPM), gasses and odours.
That was when diesel ruled. It was cheap and dirty, and besides, there were no viable alternatives. Mine workers continued toiling in gas filled stopes, and some paid the ultimate price. Numerous studies have shown that long-term exposure to combustion-related fine particulate pollution, including diesel particulate matter (DPM), is an important risk factor for cardiopulmonary and lung cancer mortality (Pope, 2002). Regardless, diesel-powered equipment by the underground mining community has continuously increased over the past several decades.
In the US, for example, about 150 pieces of diesel equipment were operated in underground coal mines in 1974 and by 1995 that number reached 3 000 units (MSHA 2001a). A similar trend was seen in underground metal/non-metal mines (N/NM), also in the US, where the use of diesel equipment first started in 1936 and by 2001 there were more than 4 000 units operating in underground M/NM mines (MSHA 2001b).
Although diesel emissions and its health impacts have been the subject of many studies globally, and in South Africa, diesel driven equipment still dominates the underground mining environment. However, fuel and energy costs are rising, and mining companies are looking for more environmentally friendly solutions, so it might just be the right time to go electric. More than one study has found that a mining operation could save as much as 70% of its operational expenditure when using electric equipment instead of a diesel driven fleet. If you consider the total cost of ownership, including operator, tyres and so on, it is possible that electric could give you a total cost reduction of about 10%.
Advantages of battery versus diesel
- Instant power
- No diesel particulate matter problem
- Lower noise
- Lower operating temperatures
- Less mine ventilation required
- Less machine maintenance
- No diesel fuel handling or storage
- Fully proportional traction speed and hydraulics
- Whisper pump with less noise and uses less power
- Regenerative braking
- Advanced proximity ready
Although South African original equipment manufacturer (OEM) Bell Equipment does not build any electric or hybrid drive machines yet, Tristan du Pisanie, ADT product marketing manager, says that it is an area Bell is interested in. The company manufactures both surface and underground articulated dump trucks (ADTs) and sell a Rockscaler for clearing underground areas of loose overhanging rocks after a blast.
Du Pisanie says there are two big potential benefits in using electric technology underground. “Firstly it does not produce problematic exhaust gases so the ventilation requirements underground are less. Secondly, the efficiency of conversion from electric energy to wheel drives is much higher than the conversion efficiency from diesel to wheel drive so the amount of heat that an electric vehicle produces is much less. This benefits cooling requirements,” says Du Pisanie.
The challenge for electric is the energy density of batteries, it is nowhere near as high as diesel. This is an area that improves every year though and it will be interesting to see how things change in the future. Another possibility is running the machines off electric lines. This can work for well-established haulage tunnels but it will not work in new expansion areas.
“Hybrids offer benefits in efficiency by assisting a diesel engine with an electric motor when peak power is required,” says Du Pisanie. There are also hydraulic solutions that work in a similar way, which allows a smaller and more efficient engine to be used. In addition the hybrid system can harness energy that is normally lost for feeding back. An example of this is vehicle braking, where the energy is usually dissipated as heat.
“In a hybrid the electric motor becomes a generator and charges battery packs, the generator serves to slow the machine down. It also works harnessing energy when excavator booms are lowered or when stopping an excavator during swinging,” says Du Pisanie. The extent of a hybrid system’s benefits depends on the type of application in which the machine is running.
But despite the benefits of an electrically driven engine, diesel engines are the traditional method of earthmoving and they work very well. Du Pisanie says that a diesel engine’s major strength compared to electric is that the energy density of diesel fuel is very high. “Storing a shift's worth of energy is a lot more compact and cost effective for a diesel machine than an electric machine,” says Du Pisanie. He adds that the negatives of diesel, particularly when underground, are the exhaust gases and exhaust gas heat.
Challenges in development
So what has been hampering the development of electric and hybrid driven underground equipment, if the benefits are so great? Du Pisanie says that cost has been the major constraint. “Both electric and hybrid systems tend to be expensive but the mine operator's main priority is to ensure that they have the most commercially profitable solution. Both electric and hybrid have benefits not available from more traditional solutions but these benefits need to outweigh the costs.”
The second factor that has delayed the development of electric and hybrid solutions is energy storage. Although modern batteries are much better than 20 years ago they still have some limitations. For example they charge slower than ideal, they are heavy and more expensive and the energy density is not as high as most people would like it to be.
Hybrid machines are interesting and operate in different ways. Most hybrids have a fully mechanical drivetrain – engine, gearbox, driveshafts and wheels. Inserted into that arrangement is an electric motor. This motor runs off a battery pack and provides additional power when the operator calls for maximum power, and then also provides braking force when the operator wants to slow down. When braking the motor turns into a generator and charges the battery pack.
Besides electric and hybrid technology, hydrogen fuelled equipment is another exciting new development. South African platinum giant Implats recently announced its intention to convert some of its underground diesel driven machines to hydrogen. Du Pisanie is a big fan of hydrogen fuelled technology, but warns that the challenges are even greater for hydrogen than electric and hybrid.
“Production of hydrogen is very energy intensive. The most common method is electrolysis and if the electricity used is from a fossil fuel powered power station then the carbon footprint of the process is very big indeed. Secondly the distribution network for hydrogen needs to be set up for it to become a viable option. In addition, the energy density of hydrogen as a fuel is very low so a very big tank is needed to allow a hydrogen powered vehicle to be driven a practical distance,” says Du Pisanie.
Hydrogen is used as a fuel in internal combustion engines but the more exciting option is fuel cells, first developed for travel in space. They take hydrogen and use it to generate electricity for running an electric motor. The by-product is water. Fuel cells are becoming more cost effective every year but are still quite expensive.
The price of diesel burn
According to Marc de Chalain, business development manager, Africa at OEM Joy Global, diesel equipment offers a high level of mobility and flexibility when compared to electrical equipment.
“However, this comes at the price of burning diesel, which causes heat and creates emissions underground. As a result mines have to invest in extensive cooling and ventilation systems. Joy Global manufactures a full range of underground, electrically powered and flameproof coal equipment that includes continuous miners, shuttle cars, battery haulers, flexible conveyor trains and roof bolters. In addition, the company also builds a full range of diesel non-flameproof equipment including Load Haul Dumpers (LHDs), ADTs and jumbo and production drills rigs. De Chalain says that the company’s hybrid 18 and 22 ton LHDs are increasingly popular.
“Hybrids offer a good compromise between diesel and electrically driven equipment by providing diesel like mobility and flexibility. At Joy, we have found that about 30% less fuel is burned when using a hybrid system, which of course results in lower heat and less emissions,” says De Chalain. He adds that electrical powered equipment is typically easier to flameproof, making it easier to use in coal applications.
Although there are many benefits to using electric and hybrid technology De Chalain says it remains challenging to develop these solutions. Hybrid units need to generate and store electrical energy, which is difficult and can be achieved in various ways. Joy Global, for example, has successfully developed kinetic energy storage systems for both surface and underground applications. Underground applications have the added challenge of scaling energy storage systems to fit into a low-profile package suitably sized for underground mining. According to De Chalain Joy Global pioneered this hybrid technology almost 15 years ago, on its Le Tourneau surface wheel loaders.
The limitations on mobility created by the tethering of electrical machines make haulage units a challenge. Electrically powered equipment is normally best suited for use in continuous miners, drills and roof bolters, as they move extremely slowly. “However to employ an electrical drive on an LHD travelling hundreds of metres is impractical for cable handling,” says De Chalain.
The hybrid LHD delivers its best benefits when employed in a production application over short haul distances. This allows their high torque delivery to create an acceleration advantage over mechanical units, while still delivering a significant fuel saving. The shorter cycle times mean higher production and therefore lower costs per ton.
The best of both worlds [END SUBHEAD]
According to Kevin Reynders, managing director at Rham Equipment, the biggest benefit of underground electrical technology is that there are no gases, smoke or excessive heat emitted from electric motors. Rham has a full range of underground roofbolters, electrical/hydrostatic belt drives, load haul dumpers, utility vehicles, dump trucks and face drills that are completely compatible with diesel engines, electric motors and hybrid options.
“Diesel engines in turn,” says Reynders, “offer underground miners ample flexibility in their stoping or development ends.”
However, for Reynders, a hybrid combination of diesel and electric offers the best of both worlds. “An operator can tram with his diesel engine and work in the section with a clean electric motor and trailing cable,” says Reynders. The downside for Reynders though is that a massive underground power supply configuration is required to accommodate electrical or hybrid powered machines, and the additional power source will have come in the form of gully boxes, power points and sub stations, as diesel is not required for these vehicles.