WCA Case Study | November 2008
BMA is centred on long-term sustainability to provide a platform for growth; meaning that while the company grows in economic importance, it simultaneously progresses its sustainability performance – socially and environmentally.
To achieve a sustainable platform for growth, BMA has introduced an Energy Excellence Programme (EEx), based on the Federal Energy Efficiency Opportunity legislation for large energy-using corporations with an annual energy usage above 0.5 petajoules a year (8 PJ/y). Six BMA sites (except Hay Point Terminal and South Walker Creek and Poitrel Mines because of their lower energy consumption) trigger these legislative requirements with a total energy usage of 15.8 PJ/y. BMA is required to assess 80% of its total energy use within the five year assessment cycle.
Through steering committees and site ownership of the EEx Programme, BMA has identified a number of projects to reduce energy consumption. One of these initiatives - dragline bucket efficiency - is already showing significant results, with energy reductions of 20% when filling the bucket.
Estimated electricity savings of around 630 megawatt hours (MWh) have been achieved, equal to turning off power to approximately 500 homes for one year.
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What is a Dragline Bucket?
Dragline excavation systems are used in surface mining, where they remove overburden above coal. A dragline bucket system consists of a large bucket that is suspended from a boom (a large truss-like structure) with wire ropes. The bucket is manoeuvred by a number of ropes and chains. The hoist rope, powered by large diesel or electric motors, supports the bucket and hoist-coupler assembly from the boom. The dragrope is used to draw the bucket assembly horizontally.
In a typical cycle of excavation, the bucket is positioned above the material to be excavated. The bucket is lowered and the dragrope is drawn so that the bucket is dragged along the surface of the material. The bucket is then lifted by using the hoist rope. A swing operation is performed to move the bucket to the place where the material is to be dumped. The dragrope is then released causing the bucket to tilt and empty.
Dragline Fleet Upgrade Study
The project stemmed from BMA’s dragline fleet upgrade study which aimed to increase the payload (or weight) capacity of each bucket by 25%. Studies were conducted by South Walker Creek Mine, in partnership with Central Queensland Mining Supplies, and included a series of tests and full design specifications to calculate the total weight the boom can carry.
To increase the new bucket’s payload capacity by 25% to the desired 125% of the dragline’s ‘rated suspended load’, the shape of the bucket was altered to obtain the optimal relationship between bucket mass, height and width for filling efficiency. A bigger and wider bucket would usually mean the dragline suspends a heavier payload and uses more energy. However, it was a key goal of this project that although the bucket’s load would increase, the energy required to fill the bucket would stay the same or actually be reduced.
Following the studies, designs were approved for the development of a lightweight bucket, with a more streamlined design to improve the removal of spoil when in operation. The redesigned bucket and rigging weighs 62 tonnes and can carry up to 109 tonnes per load. Now in full operation at South Walker Creek Mine, the energy required to fill the bucket has reduced by 20%. Similar bucket efficiency designs are being replicated at BMA Blackwater, Peak Downs and Goonyella Riverside Mines.
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