Fossil fuels can be solid liquid or gas. Solid fossil fuel is coal, liquid fossil fuel is oil while Natural Gas or Coal Seam Gas are a gas fossil fuel. All fossil fuels are formed over millions of years from organic material being buried in the earth under extreme heat and pressure.
Fossil fuel gas is mostly methane (the same gas that occurs naturally from the process of decay) and is used in gas turbines to produce electricity. Gas is used extensively in Australian industry in processing, producing steam for industrial purposes and for generating electricity. Its use in generating electricity is increasing in Australia.
Ergon Energy owns and operates Barcaldine Power Station in Central Queensland, a 55MW combined cycle gas power station. When the power station was first built it used the gas supply to directly produce electricity in a gas turbine. This process creates a large amount of heat.
Later a steam turbine was added to the power station so that the heat could be used to create steam to produce extra electricity. This is why it is referred to as combined cycle.
Combined cycle gas power stations are much more efficient in converting the energy in the fossil fuel to electricity than coal fired power station. The gas comes to the power station via a long gas pipeline.
What is a combined cycle gas turbine?
A combined cycle gas turbine (CCGT) produces electricity using both gas and steam turbines in a single plant. The 'waste' heat from the gas turbine unit is used to produce steam which drives the steam turbine unit to produce 'extra' electricity.
CCGT's have three main components:
- a gas turbine-generator;
- a heat recovery steam generator to recover waste heat from the gas turbine exhaust; and
- a steam turbine-generator to generate additional electricity.
How combined cycle gas turbines work
The gas turbine compresses air, which is mixed with the gas and fired in a combustion chamber producing hot exhaust gases. The hot gas is expanded in a turbine that drives the compressor and an electricity generator producing two thirds of the total electricity output.
The hot gases leaving the gas turbine still contain a lot of energy, which would be wasted if the gas turbine were used alone or in 'open cycle' mode. The heat recovery steam generator extracts most of this remaining energy to produce steam. The steam drives a turbine connected to a generator which produces a third of the total electricity output.
Advantages of using CCGT to produce electricity
Compared to coal-fired power stations of the same size, CCGT's:
- produces less than half the amount of greenhouse gases;
- use less than half the amount of water;
- produce much less sulphurous and nitrous pollutants as well as no ash;
- gas-fired power stations require much less water.
- gas is transported in pipes so there are no 'transport' costs to the environment.
Queensland has a lot of natural and coal seam gas in Queensland.
Coal seam gas is usually 'wasted' by burning or release into the air to make mining coal safe. Using it for electricity makes the waste gas less harmful to the air (methane is more than 20 times more damaging to the atmosphere than the carbon dioxide produced by burning it). Using a waste product for producing electricity means we don't need to burn as much coal for electricity.
Disadvantages of using combined cycle gas turbinesWith fossil fuels taking millions of years to form, the worldwide supply of gas may eventually run out. It is usually more expensive using gas to produce electricity (although gas does produce less greenhouse gases and pollution).