One of the most reliable and efficient thermal power stations in the country, Stanwell Power Station has an important role to play in Queensland’s power system – now, and into the future. 

Stanwell Power Station (SPS) is a coal-fired power station located in Stanwell, 22 kilometres west of Rockhampton. 

At the time it became fully operational in 1996, SPS was one of the largest industrial developments ever undertaken in Queensland. And with a capacity to generate 1460 megawatts between its four units, it’s still one of the largest suppliers of electricity to the National Electricity Market (NEM).

The station’s performance has been recognised both nationally and internationally, and it previously held the world record at 1,073 days of continuous operation.

So, how does it do it? 

Fuelling up

First, low sulphur black coal – sourced from the Curragh Mine, near Blackwater in Central Queensland – is transported via rail to the power station. This coal is stockpiled in huge mounds within the power station complex, until it’s required in boiler bunkers capable of holding approximately 350 tonnes of coal each.

When operating at full load, SPS burns about four million tonnes of coal a year. Regulated by the coal feeder, the coal flows from boiler bunkers to the pulverisers (sometimes called mills). Inside these pulverisers – of which there are six for each of the station’s four generating units – steel balls the size of large beach balls roll around, crushing the coal until it’s ground as fine as talcum powder.

Full steam ahead

After being pulverised, the finely ground coal is mixed with warm air and burnt inside a boiler. Each of the station’s four units has one of these boilers, which is essentially a large room with about 200 kilometres of boiler tubing attached to its inside walls.

The burning coal heats the water flowing through the boiler tubes, and when the temperature gets high enough, it converts to steam. The boilers are fitted with low nitrogen oxide burners, one of the key environmental controls at the station.

The steam from the boiler then gets even hotter in the superheater, and passes through high pressure pipework to the turbine. The steam is actually hot enough, at 541°C, to make the steam pipe glow a dull red (yes, just like in the cartoons). These high temperatures help to make the process more efficient.

Within the turbine, the steam pressure drops from about 17,000 kilopascals to about 8kPA absolute, depending on ambient conditions. This causes the turbine to rotate at 3,000 revolutions per minute. The used steam is then condensed to water, which is pumped back to the boiler, and the process of creating steam begins all over again.

Heat from the condensing steam is removed by recycled water, which then flows back to one of two 130-metre-high cooling towers, both designed to withstand cyclonic winds. Here, water is cooled by air, and then falls to the bottom of the cooling tower, where it will be recycled through the condenser. 

The absorbed heat from the water is released into the atmosphere – the plumes you’ll see emerging from the cooling towers are made up of water vapour (or steam) lost through evaporation during the cooling process.

Electrostatic shock 

Meanwhile, the ash and dust emissions from the burnt coal are kept within regulatory limits through the use of electrostatic precipitators. There are two of these precipitators in each generating unit – they use large plates to collect ash, dust and fine particles. The particles are attracted to the plates, then collected in hoppers and carried away by a conveyor to silos, where they are mixed with water and ash.

This process creates a slurry which is pumped to the ash disposal area, where it dries hard like cement. Low amounts of water in the ash keep the risk of groundwater contamination to a minimum.

The gases that pass through the electrostatic precipitators are discharged up a 210-metre-high chimney. There are four flues – one for each unit – within the chimney.

Switching on 

Each of the station’s four units has a generator, the main components of which are the rotor and the stator. The motion of the steam-powered turbine turns the rotor, which creates a magnetic field. This induces a current in the stator, which is a stationary coil of wires around the rotor. This current then flows out to the generator transformer through large metal conductors encased in metal ducts.

The last step in the electricity generation process is the generator transformer. The transformer increases the electricity voltage from the stator from 20,000 volts to 275,000 volts, which allows the power to be transported efficiently through the electricity grid to the customer at the end of the distribution system.

Finally, it’s your turn. You use the electricity generated by the station to power your lights, air conditioner, fridge, appliances and so on – and now, you’ll know the process it went through to get there.

The spark for a bright future 

Under the Queensland Energy and Jobs Plan (QEJP), Queensland will gradually become less reliant on coal-fired power stations for energy over the next decade. But even when renewable generation and storage provides the majority of the generation in the grid, stations like SPS will continue to play an important role. 

That’s because coal-fired power stations don’t just generate electricity. They also provide system services that are essential to the security and reliability of the grid.

Coal-fired power stations are synchronous generators, which means they’re purposely designed to spin at the same frequency as the power system (and resist changes to this frequency). By maintaining this frequency, they help to provide system strength and inertia. 

System strength refers to the power system’s ability to respond to disturbances, like generator outages and transmission line faults. Essentially, the more system strength there is in an energy grid, the more resilient it will be when disturbances occur. 

Similarly, inertia acts as a shock absorber, giving the grid more ability to withstand surges and imbalances in supply and demand. A lack of inertia exposes the grid to instability.

Non-synchronous generators – such as solar panels and wind turbines – are currently unable to provide inertia, because they’re not synchronised to the grid by a rotating mass, but rather by inverters.      

That’s why, like all of Queensland’s publicly owned coal-fired power stations, SPS will gradually be converted into a clean energy hub. As well as installing energy storage technologies and serving as a maintenance hub for nearby government-owned renewable projects, this will involve converting one or more of the generating units to synchronous condensers. 

This will enable the site to continue providing the grid with the essential system services currently provided by SPS, without exporting power.  

The coal-fired units will only be converted when the newly established Queensland Energy System Advisory Board is confident there is enough replacement generation, storage and supporting infrastructure in place for energy reliability to be assured. 

Until then, and as this transition takes place, traditional generation assets like SPS will continue working to ensure the grid remains stable, secure and reliable as we move towards the renewable future.