As the world prepares to gather in Paris to discuss how to stop global warming, many are questioning the role of coal for future energy needs.

Prime Minister Malcolm Turnbull told journalists he did not agree with a moratorium on Australian coal.

He said that if Australia stopped exporting coal, the countries that buy it would import it from somewhere else.

"So if Australia were to stop all of its coal exports, it would not affect - it would not reduce global emissions one iota," Mr Turnbull said.

The claim: Prime Minister Malcolm Turnbull says Australia exports "by and large" cleaner coal than other countries.

Prime Minister Malcolm Turnbull says Australia exports "by and large" cleaner coal than other countries. The verdict: Experts say Australian export coal is of a higher quality on average compared with other countries, meaning less is needed to generate the same amount of energy. This leads to lower carbon emissions by weight compared with coal from other countries. Mr Turnbull's claim checks out.

"In fact, arguably it would increase them because our coal, by and large, is cleaner than the coal in many other countries."

The claim

It is not possible to check what would happen to global emissions if Australia stopped exporting coal.

However, Mr Turnbull asserted that Australian coal was "by and large" cleaner than the coal from other countries.

ABC Fact Check investigates what is meant by "cleaner" coal and how Australia's export coal compares with the global export coal market.

The global coal market

Australia is the second largest coal exporter in the world after Indonesia.

Other large coal exporters are Russia, Colombia, the United States and South Africa.

But while Australia is a major export player in the coal market, it is only the fourth largest coal producer, behind China, the United States and India.

China and India, despite being large coal producers themselves, are in the top five buyers of Australian coal (map).

Source: Department of Industry

Types of coal

Coal is formed when plant material accumulates in swamps and peat bogs, is buried by movements of the earth's crust and is then subjected to high pressures and temperatures over millions of years.

It is made up mainly of carbon, hydrogen and oxygen.

The quality of a coal deposit is determined by the temperature, pressure and formation time it has been subjected to.

Peat is first converted to lignite or brown coal, then to increasingly mature black coals - first sub-bituminous, then bituminous, and finally anthracite.

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A panel of experts has agreed to advise ABC Fact Check for its work on climate issues. For this fact check, Tony Wood was consulted.

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Australia, in contrast to Europe, regards sub-bituminous coal as black coal.

Brown coal (lignite) is classed as a low-rank coal because it has a high ash and moisture content, and less carbon, which means that it produces less energy when it is heated.

It is not suitable for the export market and is used to generate power for the Australian domestic market.

Black coals are ranked higher because they are harder, have more carbon, less ash and less moisture and thus a higher energy content.

There are two kinds of black coal: thermal coal, and metallurgical or coking coal.

Thermal coal, also called steaming coal, has a lower energy content and higher moisture and is used to generate electricity.

Metallurgical or coking coal has a higher energy content and lower moisture and is used to make iron, steel and other metals.

Different types of coal and their uses. ( Department of Industry, Innovation and Science )

Source: Department of Industry, Coal in India, 2015

Tim Buckley, an energy financial analyst with independent energy think tank the Institute for Energy Economics and Financial Analysis, told Fact Check that thermal coal was the most relevant to electricity markets and represented around 80 per cent of the world's coal use.

Bill Koppe, an adjunct professor in the school of earth sciences at the University of Queensland told Fact Check the focus on emissions related to thermal coal.

"There is no greenhouse issue around coking coal frankly because there is no other way of making steel," he said.

Fact Check's analysis therefore focuses on the characteristics of thermal coal.

What is 'cleaner' coal?

Black coals, such as bituminous coal, are ranked higher because they are harder, have more carbon, less ash and less moisture and thus a higher energy content ( Geology.com )

Several properties, including energy content, volatile gases, sulphur, moisture, ash and trace elements, all affect the quality of coal.

A spokeswoman for Mr Turnbull's office provided Fact Check with the Department of Industry's 'Coal in India' report to support the PM's claim.

She cited the lower levels of sulphur, ash, mercury, selenium and other trace elements in Australian coal compared with other coal exporting countries.

"These high quality coals produce fewer emissions per unit of electricity produced and command a higher price in export markets," she said.

The energy content is the most important property of coal for generating power effectively, representing the amount of energy produced from burning a given quantity.

The carbon content, and therefore the energy content of coal, increases over time.

Energy content is measured in kilocalories, with brown coal generating the least energy and anthracite the most:

Brown coal (lignite) - 2,200-4,600 kilocalories per kg

Brown coal (lignite) - 2,200-4,600 kilocalories per kg Sub-bituminous coal - 4,700-7,200 kilocalories per kg

Sub-bituminous coal - 4,700-7,200 kilocalories per kg Bituminous coal - 6,100-8,300 kilocalories per kg

Bituminous coal - 6,100-8,300 kilocalories per kg Anthracite - 7,200-8,300 kilocalories per kg

More polluting emissions are generated from burning low energy coals because greater quantities are needed to get the same amount of electricity as higher-energy coals.

Lignite or 'brown coal' is the lowest-quality coal by energy content. ( Geology.com )

Professor Koppe told Fact Check that in terms of carbon emissions, the most important aspect of coal was moisture content, and to a lesser extent, ash.

"Basically the more moisture you have in a coal the less useful energy you recover per unit of energy," he said.

"Ash has something of the same function because when you combust coal you heat that up and that heat generally isn't recovered, but moisture is the main issue."

He said that Australian coal was exported with around 9 to 10 per cent moisture, whereas Indonesian coal would have around 30 per cent moisture.

Ismet Canbulat, the chair of rock mechanics at UNSW's school of mining engineering, told Fact Check that Australia "on average" has cleaner coal than other countries.

"But in fact it's not really clean coal - it's the amount of coal burnt for the required amount of energy is less if you use Australian coal because the ash content is lower, which means you burn less coal, which means you put less CO 2 into the atmosphere," he said.

He said the ash content of Australian coal varied between eight per cent and 20 per cent, so not all Australian coal was cleaner but on average it was.

A report supplied to Fact Check by the Minerals Council of Australia showed that Australia was a significant exporter of high energy-content, thermal coals.

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Removing the ash

Ash is the material in coal that won't burn.

Coal can have between three and 50 per cent ash and Australian coal is usually at the lower end of this spectrum.

Anthracite is considered the world's highest quality coal in terms of energy content. ( Geology.com )

However, Australian coal is usually washed before export, which removes some of the ash.

Daniel Roberts, research group leader at CSIRO Energy's high efficiency thermal and electrochemical technologies told Fact Check washing increased the energy density and thus the price of coal but even if other countries washed their coal, it would not necessarily bring its quality up to the standard of Australian coal.

He said Australian coals were ranked higher than Indonesian coals because they had been in the ground longer and thus exposed to greater temperature and pressure.

"You certainly couldn't wash an Indonesian sub-bituminous coal and have it as good a quality as an Australian bituminous coal, it's much more molecular than that," he said.

Other pollutants

Coal also contains sulphur, which forms sulphur dioxide when burnt, resulting in acid rain and soot particles, which cause smog.

Dr Roberts said Australian coal was low in sulphur and was sought after internationally as a consequence.

Sorry, this video has expired Malcolm Turnbull makes the claim at a press conference.

Coal also contains trace elements like mercury, arsenic, lead and chromium that are left behind in the ash and can affect health.

A 2006 study conducted by the CSIRO for Australian coal producers showed that Australian thermal coals had lower levels of arsenic, selenium, mercury and boron compared with international coal samples.

Colin Ward, an honorary professorial fellow in coal geology at UNSW said Australian coals were low in potentially harmful trace elements compared to many other coals because Australian coal formation wasn't influenced by sea water and the coals therefore contained less iron sulphide minerals.

"Iron sulphide is where most nasty trace elements, such as mercury, usually occur," he said.

Transport emissions

Transport of coal is another contributor to carbon emissions - the longer distance the coal has to be transported, the more carbon emissions associated with it.

Coal carried by sea now accounts for more than three quarters of the thermal coal trade.

However, Professor Koppe said total emissions from transporting coal was "fairly insignificant", accounting for just 1.5 per cent of total emissions, according to a CSIRO lifecycle analysis.

Australia's new coal mines

Sorry, this video has expired Watch John Barron present the facts.

Mr Buckley told Fact Check that while Australia's coal had traditionally been of a higher energy value, and therefore higher quality, new coal mines being developed in the Galilee Basin in Queensland would produce lower energy coal.

"It's a low quality deposit, which is why it's never been taken in the past, it is 400 km from the coast," he said.

He said Adani's Carmichael mine in the north Galilee Basin will produce coal with an energy value of 4,950 kcal and 25 per cent raw ash.

Professor Koppe said the coal in the Galilee Basin was not as high quality as the coal from the Bowen Basin closer to the coast.

"Normally most Australian mines would wash thermal coal to get the ash down to something like 13 to 14 per cent," he said.

"I know that they are only planning to wash part of the coal and they're planning to export it with an ash content around 20 per cent."

The verdict

Mr Turnbull claimed that Australia's coal, "by and large", was cleaner than the coal in many other countries.

His claim was made in the context of what would happen to global emissions if Australia stopped exporting coal.

Experts told Fact Check that Australia's coal is on average high quality as a result of its low moisture and ash content, and high carbon content, compared with other countries, particularly Indonesia, the largest coal exporter in the world.

As a consequence, less Australian coal is needed to generate the same amount of energy, leading to lower carbon emissions by weight compared with coal from other countries.

Mr Turnbull's claim checks out.

Sources