How Electricity is Generated in a Practical Environment
SEAN KINDER, CMPA’s Document Officer reports on his tour of TRUenergy Yallourn.
WHILE researching Thermodynamics at University, it has been noted that the electrical generation from coal uses the Rankine Cycle. However, there is only so much which can be learnt from reading. The vast majority of learning actually comes from experience. As part of my work updating the ‘Work Safely’ Reference Manual and with my interest in Victoria’s power generation, I was given the opportunity to tour TRUenergy Yallourn, in the Latrobe Valley (approximately 500 years coal reserve at current usage is estimated).
Initially in the mid 1970’s, the power station started with two generating units (each unit consisting of a milling process, turbine, boiler and furnace). In the mid 1980’s two more generating units were built and commissioned. Each of the four units (W1-W4) is controlled independently of one another by the control centre. Almost every system within the power station has a backup support, in case of a breakdown or an emergency, so that production will never halt. To detect some plant faults and report problems, rather than using human senses, regular inspections are completed using thermal imaging cameras, locating hot spots. During the summer season, extra thermal imaging surveys are conducted to ensure asset protection against fire risk.
TRUenergy has many safety procedures in place to protect its employees from danger and the materials handled. Standard procedure for when each maintenance job is started, include forms containing a JSEA ( Job Safety Environmental Analysis) and Risk Assessment Matrix. These must be attached to a Job Plan and Lift Plan where required.
A small spot fire was detected within the Station Coal Bunker, while we were there. The emergency service was called, operations communicated with and the shuttle conveyor re-positioned from the fire location, to protect the asset. Finally, an incident form was filled out and filed. A return pressure roller, which had failed and become loose, was suspected to be the cause of the fire.
Starting from the mines, the coal is extracted and conveyed to the raw coal bunker, where the coal is crushed into a fine powder ready for burning. In the process, magnets are used to collect any foreign materials (cleaning products, fallen conveyor rollers and scrap metal etc.). To fill the entirety of the hoppers, a shuttle conveyor is used (a unit with a reversible belt drive, which moves the conveyor forward and backward).
At capacity, the bunker could feed power production for twelve hours with a 30,000 tonne capacity and a 2,400 tonne per hour feed rate. To measure out the amount of coal within the bunker, ultrasonics is used (a pulse is sent out, reflected and received if there are any obstructions).
Similar to the raw coal bunker, the finely crushed coal is transported to the station coal bunker at the top of the power station. From here, it is fed to the basement, where the pulverised fuel mills are located. A twenty five tonne Beater wheel, spinning at 500 rpm, pulverises the coal and shoots it to the third or fifth floor, where it enters the burner and is ignited. This is the most efficient way of burning coal as it produces a swirling effect inside the furnace. The conveyors surrounding the furnace have a suction seal, due to the negative pressure that the furnace is controlled to, with the use of induced draft fans. All water used in the evaporator must be pure, so there is a refining unit in place to achieve this. When energy is collected from the boiler, maximum efficiency is achieved when the steam is at the highest temperature (base – middle) of the boiler. Some energy not collected by the turbine is extracted and used by the feedwater pre-heaters.
TRUenergy has four turbines. Each turbine has a rating of 360 MW or 375 MW and runs at a pressure of approximately 12.4 MPa or 16.4 MPa respectively. Currently, two of the turbines are being upgraded to a 390 MW rating. The turbines are monitored carefully by the control room, which operates all the systems from the top of the raw coal bunker to the turbines.
Each computer station has about 6-8 computers which show: the output power of the turbine, pressures and temperatures and what is offline (green) and what is in maintenance (flashing).
Once the coal has been fired, it is mixed with recycled water to form a slurry, is piped out to the ash ponds. There are two ponds, each taking six months to fill. In the ponds, the water is slowly separated from the ash over time and as the ash is denser, it sinks. While one is being filled, the other is emptied, approximately 300,000 cubic metres per year. Left over ash is dumped into landfill. Some power stations have processes to recover any unburnt coal, within the ash for re-use.
Overall, the tour was a great insight into seeing how Melbourne’s electricity is produced and how some of the Occupational, Health and Safety procedures, which I am researching at the moment, are implemented. I wish to formally thank Adrian Zwagerman, Field Supervisor of the Maintenance Team and his co-workers, who shared their knowledge and gave me the opportunity to experience the electrical generation process.
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