Coal-fired power stations have been the workhorses of Australia’s electricity supply, running 24/7 to ensure the security and reliability of the grid. And while the energy mix is evolving, there’s still life in these workhorses yet. 

Under the Queensland Energy and Jobs Plan (QEJP), it’s expected that 80 per cent of the state’s energy will come from renewable sources by 2035 – and, by then, the state will no longer be reliant on coal-fired generation. 

That’s a far cry from the traditional energy mix, which has traditionally relied heavily on coal to keep the lights on. At the time the QEJP was published in 2022, for instance, coal was responsible for supplying 70 per cent of Queensland’s annual electricity demand. 

While renewables are rising rapidly, the energy transformation won’t happen overnight. Based on demand forecasts and energy market modelling, the Queensland Supergrid Infrastructure Blueprint estimates that Queensland will need 12,200 megawatts (MW) of new wind generation capacity, and 10,000 MW of new large-scale solar capacity, to reach that target. 

And to ensure demand can still be met when the sun isn’t shining and the wind isn’t blowing, Queensland is also expected to need at least 6,000 MW of long duration storage (i.e. pumped hydro), up to 3,000 MW of grid-scale storage, and up to 3,000 MW of new low-to-zero emission gas-fuelled generation. 

Building that much new generation and storage will take time, which is why Queensland’s publicly owned coal-fired power stations will continue to operate until they’re no longer required.

At Stanwell, our coal-fired power stations – the 1,460 MW Stanwell Power Station at Rockhampton, and the 1,400 MW Tarong Power Station and 443 MW Tarong North Power Station in the South Burnett – continue to be among the most reliable and efficient in the country. 

While we also have Queensland’s largest pipeline of renewable generation and storage in development, our continued maintenance and investments in these existing assets will ensure a reliable, affordable energy supply for Queenslanders as the industry evolves. 

How does coal-fired generation work? 

A thermal power station generates energy by converting heat into electricity. At a coal-fired power station, this is done by burning coal. 

(Every coal-fired power station is a thermal power station, but not every thermal power station is a coal-fired power station. Gas-fired power stations and nuclear power stations, for instance, are also classified as thermal.) 

To begin with, coal is stockpiled on site. This is why, traditionally, there’s been limited need for energy storage in Queensland – because energy can be stored in coal stockpiles until it’s required. 

This coal is then pulverised into a powder, which is then burnted in a boiler known as a furnace. 

The heat generated by this process is used to turn water, which is fed into the power station, into steam. 

This steam then passes through high pressure pipes to a turbine. The steam is used to helps move and spin the turbine, which is connected to an electrical generator. 

The main components of the generator 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 generator transformer increases the electricity voltage from the stator from 20,000 volts to 275,000 volts, which allows power to be transported efficiently through the grid to homes and businesses. 

Those are the broad strokes, but the specifics of the process differ from one power station to the next. Learn more about how coal is used to generate electricity and the environmental controls that are in place at Stanwell Power Station and the Tarong power stations. 

Where does the coal come from? 

Coal, a sedimentary rock, is a non-renewable fossil fuel that forms from dead plant matter over millions of years. It’s a combustible rock composed mainly of carbon, along with other elements such as hydrogen, sulphur, oxygen and nitrogen. 

Australia produces two types of coal – brown coal and black coal. 

The high moisture content of brown coal reduces the amount of energy that can be produced by burning it. It’s not considered viable for the export market, and even within Australia, only three remaining coal-fired power stations – all located in Victoria – are fueled by it. 

Black coal, on the other hand, contains more carbon and less ash and moisture than brown coal, and therefore produces more energy when it’s burnted, making it a more efficient fuel source. Black coal is mainly used for two purposes – to generate electricity (thermal coal) and make steel (metallurgical coal).

Australia has the fourth-largest share of coal reserves in the world. The principal black coal producing locationsbasins are the Bowen Basin (Queensland) and Sydney Basin (New South Wales). 

Stanwell Power Station uses coal sourced from the Curragh Mine, near Blackwater in Central Queensland, which is transported via rail to the power station. 

The Tarong Power Stations receive coal directly from Meandu Mine, which is just 1.5 kilometres away, via a conveyor belt. Both the Meandu Mine and the power stations are owned by Stanwell. 

What’s the difference between a subcritical and supercritical power station? 

You may have heard of supercritical power stations, also known as HELE (High Efficiency, Low Emissions) plants. These are still coal-fired power stations – but to generate the same amount of electricity as a traditional plant, they burn less coal, emit less carbon dioxide and release less pollutants, giving them a smaller environmental footprint. 

They still burn coal to heat water into steam, and use that steam to spin a turbine that generates electricity. The main difference is that the steam goes into the turbine at a higher temperature and pressure. 

Traditional plants use subcritical boilers, which operate below the thermodynamic critical point of water (22.1 megapascals). HELE plants, however, use boilers that operate above 22.1 megapascals – these are referred to as supercritical boilers. 

In a subcritical boiler, there’s a distinct change in phase between feedwater and steam. But in a supercritical boiler, the feedwater doesn’t need to pass through a distinct boiling phase. In fact, the process is so fast that the term ‘supercritical boiler’ is a bit of a misnomer, because there’s no actual boiling happening inside the device. 

There are also ultra-supercritical boilers in operation – these function in much the same way as supercritical boilers, but at even hotter temperatures, above 600°C. 

Because these supercritical and ultra-supercritical plants operate at higher temperatures and pressures than subcritical plants, they need to burn less coal to produce the same amount of energy as subcritical plants – and less coal means less carbon emissions. 

Aside from releasing carbon dioxide, burning coal also produces other atmospheric pollutants, including sulphur dioxide (SO2), nitrogen oxides (NOx) and particulate matter. Most HELE plants utilise a range of state-of-the-art combustion optimisation and flue gas treatment technologies to virtually eliminate these pollutants. 

There are four HELE power stations in Australia, and they’re all in Queensland. (All four are supercritical plants; there are no ultra-supercritical plants in operation in Australia.) 

Tarong North Power Station, for instance, is a supercritical plant. The single 443 MW advanced cycle coal-fired unit’s supercritical boiler technology reduces CO2 emissions by about 10 per cent compared to conventional subcritical boilers. 

Tarong North Power Station also uses an advanced bag filtration system to capture fine particles that are produced by burning coal. This industrial-scale bag filter captures 99.99 per cent of all emitted dust particles, reducing the station’s particulate emissions.

How are thermal power stations maintained? 

Every generating unit in every power station in the National Electricity Market (NEM) is required by law to undergo a regular, scheduled maintenance overhaul. These overhauls are critical for ensuring the efficiency and safety of these units, delivering performance improvements, and providing a reliable supply of electricity to support system security.

The approval for a planned overhaul is given more than a year before the commencement of the project, to allow for accurate scoping, planning and procurement of the parts and resources that are required.

The overhauls are planned to ensure continued safe operation that downtime is kept to a minimum, and the unit is quickly returned to its full generating capacity. They’re often planned to take place during ‘shoulder’ periods of the year, between times of peak demand for energy in summer and winter, to ensure that as many units as possible are operational during these peak times.

The exact length of the overhaul will depend on the scope of the operation. Power stations usually run on either a four or five-year overhaul cycle, which means the overhaul is intended to make the plant safe to return to service for the next four or five years, respectively.

Once the required inspection, maintenance, refurbishment and replacement of plant and equipment has been completed, the unit will be recommissioned and returned to service.

What role do coal-fired power stations play in the energy system? 

Queensland’s electricity system has historically consisted of mainly ‘dispatchable generation’. This is generation that can be scheduled on or off and increased or decreased on command to ensure supply always meets demand, as opposed to variable generation – i.e. solar and wind – that’s dependent on the weather. 

Dispatchable generators are often classified as baseload units and peaking plants, respectively. Baseload units usually run continuously throughout the year, except during maintenance outages, while peaking plants produce little or no energy during periods of average demand, but can ramp up to full power within minutes during periods of peak demand. 

Because of their efficient and low-cost generation, baseload units have traditionally provided the majority of the power in the energy grid. In Australia, baseload units are typically coal-fired power stations, and peaking plants are typically gas-fired generators. 

Importantly, 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.      

So, at least for now, the Australian Energy Market Operator (AEMO), which is responsible for managing the day-to-day operation of the grid, has to ensure that a minimum number of synchronous generators – such as coal-fired and gas-fired generators – are online, at or above minimum generation levels, to supply essential system services at any given time. 

What role will coal-fired power stations play in the future? 

In Queensland, it’s expected that there’ll be enough wind and solar generation, supported (or ‘firmed’) by a mix of pumped hydro energy storage, batteries, and low emissions gas-fuelled generation, to collectively provide the capacity currently provided by coal by 2035. 

But even though the energy mix will no longer be reliant on coal by then, the state’s publicly owned coal-fired generators won’t simply close. Instead, the Queensland Government will invest in repurposing these stations into clean energy hubs, capitalising on their skilled workforces, strong network connections and existing infrastructure. 

This will include installing batteries and low-to-zero gas-fuelled generation at these sites, and using them as maintenance hubs for nearby government-owned wind and solar farms. It’ll also mean repurposing coal-fired generating units into synchronous condensers. This way, these units can continue to provide system strength and inertia, without exporting power to the grid, so that the grid can continue to meet its operational requirements.

The conversions of generator units to synchronous condensers will be designed to be reversible, so these units can return to service in the event of a renewable drought, or the forced outage of other generators and forms of storage. 

Under the Queensland SuperGrid Infrastructure Blueprint, Stanwell Power Station and the Tarong power stations will continue to operate with no change, providing baseload power and keeping downward pressure on electricity prices, until 2026-27, when one or more units at each station will shift to seasonal operation or synchronous condenser conversion.

This will only take place when energy reliability is assured, and there’s enough replacement generation, storage and supporting infrastructure in place. 

The state government will establish a Queensland Energy System Advisory Board to provide expert technical advice and compare the grid’s progress against ‘Blueprint checkpoints’ to determine if it’s possible to move coal-fired power stations to the next phase of their modernisation. 

This ensures these coal-fired power stations will continue to play a critical and ongoing role in ensuring the grid remains stable, secure and reliable as we move towards our renewable future.