ACTUALISING SUSTAINABILITY WITH SIBANYE-STILLWATER | MINING SECTOR
We are proud to announce that Sibanye-Stillwater is predicted to achieve a saving of 374.02 GWh from its Shaft 8, Shaft 2 and the upcoming project, Shaft 5, from August 2021 to August 2031. This equates to 393.7M kg of CO2, SOx, and NOx emissions reduced 519.9M litres of water saved and 256.6M kg of coal and ash.
The site uses multiple medium voltage (MV) ventilation fans to provide airflow to their mine. The two vent fans at the shaft are directly coupled to two 2240 kW induction motors – a standard squirrel cage motor and a slip ring motor. These fans ran in a duty/standby fashion (common of a ventilation fan system), with one fan running continuously to provide airflow to the shaft. The second fan was used as a standby fan to ensure redundancy. The fans were controlled from the mine’s central Wonderware SCADA network using Inlet Guide Vanes (IGVs). A third-party energy partner managed the clipping schedule on behalf of the mine which used the IGVs to reduce airflow during peak times.
When the mine and Energy Drive started collaborating, this shaft was quickly identified as a prime candidate for energy savings. Energy Drive engineers saw the opportunity to run both fans together to provide the same airflow as one fan.
While counterintuitive, this concept would allow Energy Drive to run each fan at a much lower speed. Each fan would provide a lower airflow, and the well-documented affinity law relationships would then show a substantial decrease in power demand. Additionally, running two fans with fully open IGVs allows the fans to return closer to their best operating point (BOP), allowing for greater efficiency. Using the mine’s suggested flow rates, the Energy Drive engineers predicted an energy saving of 60%.
A reduction in speed can be efficiently achieved using a variable speed drive, which has become a well-established, proven technology for process control and, in this instance, energy saving. A proposal detailing the technical solution as well as a capex-free commercial solution was tabled and approved by the mine.
After approval of the technical and commercial principles, Energy Drive initiated the project phase, developing a functional design specification to describe the new system’s behaviour in collaboration with Sibanye’s key staff.
The first challenge to overcome was one of space. Energy Drive’s solutions are typically mobile and self-contained. The VSDs intended for use at the mine were too tall for the typical 40 ft-high cube container, prompting project engineers to consider other solutions. With the benefit of being easily transportable and a familiar footprint for logistics companies – shipping containers were therefore utilised for this purpose. The decision was taken to use the same base dimensions as the 40 ft container, but with a bespoke e-Unit constructed on it. This allowed Energy Drive to accommodate the tall VSDs, while still having an easily mobile and self-contained solution.
Construction of the unit occurred entirely off-site, allowing easy access for local contractors, Energy Drive engineers, and logistics companies. While this was taking place, a contractor on site completed the foundations on which the unit would sit and all the preliminary MV and LV cable reticulation. The electrical solution provided the mine with 2 modes of operation: VSD operation, as well as the ability to run the fans via their original DOL starters, which is achieved using ring main units (RMUs).
When the e-Unit was completed, a cold-commissioning of the VSDs was performed, and the unit was sent to site. It could then be placed down into its final location, and all the final terminations and cable work could be performed.
The communication between Energy Drive and the site PLC is crucial, as Energy Drive needs to give feedback to the mine on the state of the VSD and receive input from the mine on clipping/non-clipping modes of operation. The Energy Drive PLC can then also act to keep IGVs fully open when under VSD control to maximise savings for the site.
A data and analytics platform allows Energy Drive engineers to have a live view of VSD health, as well as the health of system communications. In the event of anything out of the ordinary, this data is at the fingertips of engineers who can inform mining personnel and dispatch technicians when needed.
A critical part of the commissioning process was establishing a flow baseline for the mine. This was done in collaboration with a third party, who performed a comprehensive set of pitot tube tests to determine the pressure when on IGV control. These tests were done during the clipping and non-clipping stages of operation. The VSD was then tested at 3 operating speeds, 30 Hz, 27 Hz and 24 Hz. Once all these tests had been performed, a comprehensive comparison between the flows while under IGV control and the flows while under VSD control could be made. The report showed that while under VSD, an operating speed of 27 Hz on each fan would provide the mine with the same flow they received during their non-clipping operations. Both fans could be run at 24 Hz during clipping times to achieve the same flow rates.
With an original baseline of 1710.3 kW, the fans now run at 605.75 kW. In fact, the results of this project have been so successful that Energy Drive was awarded the Energy Award for ‘Industrial Energy Project of the Year’ by The Southern African Energy Efficiency Confederation (SAEEC) for 2021. “A huge thank you to the SAEEC for this acknowledgement, Sibanye for partnering with us on this journey, and to our incredibly talented Energy Drive team.” – James Hynd, Energy Drive CEO.
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