Coal's 19th-century image of puffing steam trains and billowing factory chimneys belies the fact it is firmly cemented in the 21st century, accounting for nearly half of the planet's energy mix.

It is especially popular in China, which is expected to outstrip the United States within the next couple of years as the world's biggest carbon polluter.

"Every week, China installs an additional 1,000 megawatts of coal-fired power, the equivalent to Britain's annual output," says Cedric Philibert, an expert with the International Energy Agency (IEA).

The love affair with coal is not restricted to China -- and for some, it is not something to be shouted from the rooftops.

The United States is a massive burner of the black stuff, as is Europe. But while the US is unapologetic, Europe seems almost painfully discreet.

In the world's most ecologically sensitive region, coal accounts for 48 percent of Germany's electricity supply -- ironically, to help compensate for the country's abandonment of nuclear power -- while in Poland, coal provides more than 90 percent of power needs.

Australia, meanwhile, which pitches itself to foreign tourists as clean and green, is a major exporter of an energy blamed for global environmental damage.

Australian coal exports in 2006 were worth around 24.5 billion Australian dollars (20.09 billion US), an increase of 43 per cent over 2004-05, and were the country's biggest single commodity, the Australian Coal Association says.

Coal's big downside, as far as climate change is concerned, is that it has low energy efficiency.

If the three fossil fuels are compared on a kilowatt basis, coal emits nearly a third more carbon dioxide (CO2) than fuel oil and three-quarters more than natural gas.

But coal scores big in that it can be dug out in many parts of the world -- unlike oil and gas, where only a small number of countries have plentiful and easily accessible reserves.

Extracting national supplies of coal obviates the geopolitical risk of importing oil and gas from volatile parts of the world -- and, at current prices, it is far cheaper.

That makes it a boon for the fiercely-growing economies of China and India, says, Antoine-Tristan Mocilnikar, an expert at France's Interministerial Delegation on Sustainable Development, and author of a report last year on the future of coal.

"To secure security of supply while staying competitive, coal has a major role to play, especially in some developing countries," says Mocilnikar.

With cost and risk weighing so much in its favour, coal appears to be unassailable.

As a result, emphasis is being given to diminishing coal's impact rather than replacing it with a cleaner alternative.

One of the options being examined by experts, meeting this week in Bangkok in the UN's Intergovernmental Panel on Climate Change to assess measures to tackle global-warming emissions, is carbon sequestration and storage.

Under this, CO2 is drawn off the coal as it is burned. The gas is then pumped deep underground and stored in geological chambers, such as disused gas fields.

The technology would be installed at coal power plants or factories that are big coal consumers.

Several test projects are underway in various parts of the world to assess the feasibility of sequestration, with the United States the most enthusiastic proponent.

A sequestration plant in Illinois should be ready in 2012, the US ambassador to the European Union, Boyden Gray, told AFP in an interview last month.

"We now have the technology, but we have to pull out all the stops to make it as cost-effective as possible," Gray said.

"At the moment, the cost is in a range of 30-80 euros (40-108 dollars) per tonne" of stored CO2, said Francois Klaydjian of the French Oil Institute.

The IEA's Philibert believes this will have to fall between 25-40 dollars per tonne by 2020 to ensure a wide uptake of the nascent technology.

Another question is whether storage is safe.

Scientists see the biggest risks in earthquakes or geological collapse that could breach a C02 chamber, causing the stored carbon to be released into the atmosphere, and thus accelerating global warming at a stroke.

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