Mine ventilation can account for 30% to 40% of some deep underground mines’ total electricity consumption. Taking that into account, a variety of energy management options offer significant scope for improving mines’ electricity consumption profiles.
Traditionally these mines have been ventilated and cooled on the basis of all-the-mine all-the-time. Although there have been recent improvements in this approach, significant potential remains for further improvement and mine ventilation engineers have developed an appetite for exploiting safe opportunities.
Mine ventilation infrastructure can be divided into three main categories, says Steven Bluhm, CEO of BBE Projects, a company that specialises in mine ventilation and cooling projects. One is the primary ventilation equipment, the main equipment used to force cold air down mine shafts and this includes booster fans where necessary. Assuming ventilation and cooling accounts for, say, 30% to 40% of a mine’s electricity cost, some 30% of this could, in general, be attributed to primary ventilation. The secondary ventilation equipment, which includes all the secondary fans and ducts that distribute air to the various sections of an underground mine, could account for, say, another 25%, a larger percentage than some may have expected. Then there is refrigeration plant, which can be located on surface and underground, and which could account for the other 45% in some hot mines.
Ventilation has typically been supplied within South African mines in a brute force manner with all-the-mine being ventilated and cooled all-the-time. Bluhm says more sophisticated approaches to secondary ventilation could in some mines reduce secondary ventilation electricity requirements by up to 40%. “The scope for energy saving on secondary ventilation depends on how much methane and other gases may be present. If it is a gassy mine, then the scope for reducing secondary ventilation is limited. However, if the mine does not have gas issues, then it can introduce schemes such as ventilation-ondemand.”
Ventilation-on-demand involves fitting controls on the fan units, of which there are literally hundreds in a large mine, and the use of variable speed drives and other controls. “The idea is to provide ventilation in the right places at the required time as opposed to the current brute force approach.
” Trackless mechanised mining operations using diesel powered equipment illustrates the level of sophistication achievable. Where the diesel engines are operating the requirement is for suitably high levels of ventilation. Real time control systems can adjust the ventilation provided, not only based on the size of the engine, but by tracking the vehicle’s location. The system can taper off ventilation in areas the vehicle has left and ramp up ventilation in the areas it has entered. “When a drill team arrives at a mining area it needs a certain amount of ventilation, but this is less than that required for the diesel engine machines and the appropriate amount of ventilation can be provided,” Bluhm says.
In other words, the control of secondary ventilation systems in South African mines can offer substantial scope for energy management systems to be implemented. Frank von Glehn, BBE Director, who was recently at the 12th USA/North American Mine Ventilation Symposium in Nevada attended by some 250 people, says numerous papers were presented on this particular topic and it is now a reality. “The industry has been talking about ventilation-on-demand for some time, but now it is being implemented” von Glehn says.
Bluhm attributes the change to the increases in the cost of power globally and the fact that communication and control systems have evolved to the point where they make such systems feasible. Mines now have fibre optic and wireless broadband systems in place and it is not hard to piggyback on these. “Now that we are able to do it there is no reason to delay any longer.
“That said, it is the diamond and base metal mines in southern Africa which are the most advanced in adapting such technology and have such infrastructure in place.” Bluhm says that South Africa has shown leadership in this field in terms of the research and development done, although the implementation of these systems appears to be most advanced in countries such as Canada.
While retrofitting such technology in old mines will require a significant outlay and could represent a barrier, modern mines are being designed with such infrastructure in mind. Bluhm says that while BBE is seeing about 30% to 40% of its business come directly from energy efficiency related projects and initiatives; in essence it permeates all of its business. BBE is doing some 10 mine designs at the moment – and they inherently incorporate energy efficiency principles regarding secondary ventilation as described, as well as in primary ventilation and refrigeration.
Reduction of electricity used for primary ventilation can be achieved in two ways, through energy efficiency and through load clipping. Pre-rotational inlet guide-vanes can be fitted to main fan stations and enable the type of energy management where at certain times of the day power can be clipped to save kilowatt-hours. This is now being implemented widely in South Africa. “In other energy conscious countries this practice was well established some time ago, and in Australian mines, for example, the primary fans are shut down when the workers go to lunch. In South Africa chilled air has traditionally been continuously sent down the mine at the same rate all the time,” Bluhm says.
BBE is working on a number of these primary ventilation energy management projects in South Africa including at Impala, Driefontein, Kloof and Moab Khostong. Other mines are looking at similar plans. “Eskom’s Demand Side Management (DSM) programme has in the past funded some of these initiatives, but the programme appears to have fallen away,” Bluhm says. Overall, Eskom’s DSM programme appears to be on hold and it has a moratorium on new proposals.
The benefits for the mines are not just that they save money and work towards achieving the 10% electricity reduction stipulated by Eskom without losing production, but that they reduce their overall energy footprint, and corresponding carbon footprint. The latter is something that is becoming increasingly important to mining companies. “A typical large mine will save R4 million a year through such a primary ventilation energy management initiative and save some 6,000 tonnes per annum (tpa) of carbon emitted,” Bluhm says.
On the energy efficiency side, there are also steps being undertaken to make existing primary fans more efficient. “Many of the original fans were designed some time ago for a life of 30 years, and the profile of the mine will have changed over the decades. It will be a different operation and that gives us scope to change impeller designs and speeds and gain energy efficiency.”
In the best case scenarios it may be possible to save possibly 20% of the power usage accounted for by primary ventilation.
In the refrigeration systems, which are always directly associated with the ventilation, there are three areas in which energy can be managed. One of these is load shifting, which is obviously neutral in terms of the mine’s carbon footprint but of benefit to peak demand loads and costs. The other two are energy efficiency and load clipping.
“Load shifting from peak to off-peak periods can be achieved by thermal storage mechanisms,” Bluhm says. In the smaller applications this is done using large water dams. In larger mines the shifting of the cooling power to off-peak time can be achieved by creating ice during offpeak periods for cooling energy thermal storage and then using this during peak periods. BBE has implemented some of these projects, including an initial project for Impala, a 1,000 tonne/day ice storage system for the Mponeng gold mine, and it is undertaking a similar scale project at the Moab Khotsong gold mine. The Mponeng gold mine’s project includes tube bundles containing 100 km of tubing. These projects can shift up to 75% of the surface plant’s refrigeration energy usage from peak to off-peak periods.
Energy efficiency involves ensuring the refrigeration equipment runs more efficiently, and this broadly comes down to how the coolers reject heat. As the ambient temperatures decrease the efficiency goes up. It means that by running the refrigeration machines at certain times of the day, typically at night when it is cooler, the energy efficiency is higher.
Load clipping, which is linked with primary ventilation, amounts to turning down the fans that force refrigerated air from surface during certain times of the day, such as when no people are underground before a blast and the mine has been evacuated.
If one undertakes all the measures related to refrigeration that are possible it may be possible to reduce the refrigeration energy consumption by some 30%.
In some particular ultra-deep cases, this could be even higher if another type of ice plant is used with a different purpose in mind. Here the energy efficiency relates to the temperature of the chilled water that goes down the mine for cooling. “In particular it relates to the pumping cost, since for every degree you can lower the temperature of the chilled water, the less of it you have to pump down the mine. There is a big difference between the amount of water you have to pump at a temperature of 5O C compared with at 1O C; in the former case you need many more litres/second to achieve the same level of cooling underground.”
A step-change in this logic, incentivised by power costs, is that if one uses ice one can greatly save on the volumes of water required to be sent underground for cooling and pump returned to surface. Until now this has been inhibited by the high capital cost of such ice making plants, about R150 million for a reasonable size plant, and the deep level mining industry in South Africa has to date only seen ERPM and Mponeng, the latter currently being expanded, establish such facilities. “However, with the cost of electricity and the desire to save on pumping costs and save energy in that way, we may start to see people justifying the necessary upfront capital costs,” says Bluhm, who is aware of mines in South Africa, such as Driefontein and Kloof, considering ice plants of this nature.
Bluhm says that the energy saving capital projects relating to mine ventilation and pumping do have potential to be funded as projects under the Clean Development Mechanism (CDM), a facility that South Africa has been slow in general in taking advantage of compared with other developing countries.
While no single shaft’s ventilation energy management project will shift sufficient tonnes of carbon to fully justify a CDM project, if enough of them are packaged together they have definite CDM potential and Bluhm says some large mining groups have initiated the CDM accreditation process.
“Overall we perceive a great concern and commitment by the mining industry in South Africa to engage in energy efficiency process, not only for the electricity benefits, but to ensure they manage their carbon footprints in the best way possible,” Bluhm says.