Gas is an important energy source for Queensland homes and businesses. But the energy mix is evolving – and so is the role that gas will play in it.

What is gas as an energy source? 

Gas is a fossil fuel formed from the layered remains of plants and animals that are gradually buried by sand, sediment and rocks.

As these remains decompose and are exposed to immense heat and pressure under the surface of the Earth over the course of millions of years, their molecules break down, and the energy stored in their cells produces a naturally occurring hydrocarbon gas mixture. This mixture is mostly made up of methane, but can also include carbon dioxide, nitrogen, hydrogen sulphide and helium.

You might have heard of ‘conventional’ and ‘unconventional’ gas, but in reality, it’s all the same gas – it’s only the extraction methods that are different.

Conventional gas is extracted from porous and permeable sandstone rock formations. This gas will flow freely from rock formations that have been tapped by a drilled well.

Since the 1990s, unconventional gas has become more common. This is gas that’s found in complex geological formations, requiring extra steps to extract. Unconventional gas is categorised by the type of rock it’s extracted from.

Coal seam gas is found in coal seams, hundreds of metres below the Earth’s surface. It’s typically held in place by rock and groundwater that has to be removed to access the gas.

Shale gas is extracted from fine-grained sedimentary rocks, usually located between 1000 and 2000 metres underground. These rocks are of low permeability, but advances in drilling technology have made it somewhat more practical and economical to extract gas from them.

Then there’s tight gas, found in sandstone or limestone rocks of low permeability more than 1000 metres below the ground.

While shale gas and tight gas reserves have been identified in Queensland, the vast majority of Queensland’s gas supply is coal seam gas, sourced from the Surat and Bowen basins.

Once gas has been extracted and processed, it’s stored in tanks ready for transmission and distribution. It’s both colourless and odourless – the unmistakable smell of ‘gas’ is actually added artificially, so you don’t forget to turn it off.

Gas that’s intended to be exported is converted into liquefied natural gas (LNG) – a process that involves cooling the gas to −273.15 °C, reducing its volume so that it can be more easily transported around the world.

Gas produced for domestic consumption is sent through high pressure transmission pipelines to ‘city gates’ – points where the transmission pipelines connect with networks of low pressure gas distribution pipelines that take gas to homes and businesses.

Once the gas reaches you, it’s ready for use – most commonly for cooking, whether in the kitchen or on the BBQ; water heating; and to run appliances such as fireplaces and outdoor heaters.

Transmission pipelines deliver gas directly to large industrial users, who use it as a fuel for a variety of mining and manufacturing processes, including smelting and refining metals. Gas is also key to the production of fertiliser, and used to produce a wide range of industrial products, including plastics, polymers and textiles.

Transmission pipelines also deliver gas to electricity generators, who use it to power gas turbines.

There are two main types of gas turbine power stations – open cycle plants and combined cycle plants – that use gas to generate electricity.

In an open cycle gas-fired power plant, gas combusts and expands through a gas turbine. Essentially, the hot pressurised gas spins the blades of the turbine, which is connected to a generator – when the turbine blades rotate, it causes magnets to spin inside the generator, forcing electrons to move through wires, creating an electrical current.

A combined cycle gas-fired power plant does the same thing, but also uses the ‘waste heat’ generated by this process to boil water and produce steam that drives a second turbine. This makes them more efficient than open cycle plants, generating about 50 per cent more electricity.

What role does gas play in Queensland’s electricity market? 

Queensland’s electricity system primarily consists of dispatchable generation. This is electricity generation that can be scheduled on or off, and increased or decreased on command, to ensure supply always meets demand.

Dispatchable generators are classified as either baseload units or peaking plants. In Australia, baseload units are typically coal-fired generators. They run continuously throughout the year, except during maintenance outages, and provide the majority of power in the electricity grid.

But although these coal-fired generators are capable of ramping their production up and down, they tend to move relatively slowly, and they can’t operate below certain minimum output levels.

Peaking plants, on the other hand, produce little or no energy during periods of average demand, but can ramp up to full power within minutes during periods of peak demand.

These peaking plants are usually gas-fired generators, which can start up and respond far more rapidly than coal-fired generators. They’re more expensive to operate than coal-fired generators, which is why they typically only run for a few hours at a time, when demand for energy from the grid is at its highest.

In a nutshell, gas-fired generators are flexible and dynamic plants that are able to fill the gap between supply from coal-fired generators and demand from consumers.

What role will gas play in the future? 

In the years to come, solar and wind generation are expected to make up a larger proportion of the energy in the grid. But because their output is dependent on the weather, they provide neither the consistent, ‘always on’ supply of coal-fired generators, or the dynamic, on-demand capability of gas-fired generators.

For supply to continue to closely match demand, then, there’s a need for firming capacity – a flexible supply of energy that can be quickly switched on to support variable renewable generation as wind and solar output changes, or when there’s a sudden increase in demand.

Gas is expected to be a key source of firming capacity in the future energy system. Although gas-fuelled generators have high running costs, the capital cost of gas-fired generation – i.e. the amount it costs to install the generation in the first place – is relatively low. That’s why, when used strategically, gas-fired generation is considered the lowest capital cost per megawatt way to provide backup and peaking generation for an energy system with large amounts of variable renewable energy, like wind and solar.

Gas also burns the cleanest of all fossil fuels, with gas-fired power generation producing about half the emissions of an equivalent amount of coal-fired power. (It’s often referred to as ‘natural gas’, but don’t let the word ‘natural’ fool you – it’s still a non-renewable energy source).

As Queensland continues its shift to renewable energy, gas is set to remain a critical bridging fuel – supporting energy security and grid stability when and where it’s needed most.