By Mark Kinver
BBC News science and nature reporter
Coal is back on the agenda as a serious player in meeting the world's future energy demands.
Steady burn: Coal's stable price has given it a new lease of life
After being pushed to one side in the "dash for gas" in the 1990s, attention is returning to the role of coal in the global energy mix because of its widespread availability and stable price.
The recent volatility in the markets for oil and gas, combined with concern of an "energy gap" between rising demand and suppliers' struggle to provide the electricity, has positioned coal as a realistic option - both economically and politically.
Politicians and industry experts hope the development of "clean coal technology" will also make the fuel environmentally acceptable among climate conscious citizens.
Although coal is often portrayed by environmental groups as a lingering legacy of the age of steam, energy analysts say it would be premature to consign this fossil fuel to the history books.
The latest figures from the International Energy Agency (IEA) project coal use to steadily increase each year until 2030, when annual demand will reach nearly 7.3 billion tonnes - almost one billion tonnes more than present levels.
John Topper, managing director of the agency's Clean Coal Centre, says the growth in demand is being driven by emerging economies.
"If you are in China or India where you have huge resources of coal and you have elements of the population that do not have access to electricity then your driver is to build and operate power stations as quickly and as effectively as possible."
The unprecedented rate that China is building new coal-fired power stations has shifted the debate from should we burn coal, to how?
Clean coal technology (CCT) has entered policy parlance as an umbrella term for all the various strands of research and development to improve the environmental performance of coal-fired plants.
The term is widely used to describe a complete process in which the coal is gasified (i.e. turned into a gas as opposed to the conventional method of burning the coal itself in a combustion boiler), burnt to power a turbine, and the carbon dioxide and other emissions captured for storage away from the atmosphere.
This definition of CCT is too narrow and excludes a lot of existing engineering that is reducing coal's impact on the environment, says Mike Farley, director of policy liaison for engineering company Mitsui Babcock.
He says the efficiency of traditional boilers used in coal stations has been steadily improving and can rival the efficiency of gasification plants.
Improving the efficiency of boilers used in coal-fired power stations reduces carbon dioxide emissions because less coal is needed to generate the heat energy that turns the steam turbines powering the electricity generators.
FOSSIL FUEL RESERVES
(Proven reserves based on current production)
Coal: 164 years
Gas: 67 years
Oil: 41 years
(Source: World Coal Institute)
Current systems, called advanced supercritical boilers, have an efficiency level of around 42%. By replacing an old-style boiler, operating at about 30% efficiency with one of the new systems can cut CO2 emissions by 23%, says Mr Farley.
The latest boilers also allow biomass to be added to the fuel mix. Industry figures show that this can reduce emissions by a further 20%.
"If you increase the efficiency of a plant and combine it with biomass/coal firing then you get the emissions down to similar levels as a gas-fired power plant."
Gasification plants and advanced supercritical boilers both have advantages that their advocates are quick to highlight.
Boilers are available commercially and can be "retrofitted" to existing coal-fired plants. This means the operator can improve efficiency while keeping capital costs down by using the existing infrastructure at the site.
Gasification systems, such as Integrated Gasification Combined Cycle (IGCC), can be used for poly-generation. This allows the gas (hydrogen) to be used either to power a gas turbine or to be piped elsewhere, for example in transport or in the chemical industry.
This approach could become a key component role in a future hydrogen economy.
China, which is extending its electricity generating capacity at a rate never seen before, favours the tried-and-tested supercritical boilers.
"This year I think [China] is going to build around 55GW of power plants and most of them are coal-fired," says Prof Li Zheng, director of the Clean Energy Centre at Beijing's Tsinghua University.
"It is expected that by the year 2020, the total generating capacity will be around 900GW to 1,000GW."
Professor Li says there is an awareness of the environmental impact this will have: "The public's and government officials' knowledge of climate change is increasing.
"But for the current situation in China, I think the action will come down to more efficient power plants. That is a current and practical way."
Locking in this amount of coal-based capacity into the global electricity mix concerns environmentalists, like Chris Flavin, president of the US-based Worldwatch Institute.
Climate scientists are warning that carbon dioxide levels in the Earth's atmosphere are approaching a "tipping point" that will trigger irreversible climate change.
"If we go on building dirty plants between now and 2020, we are basically committing ourselves to many more decades of further emissions from those plants."
Environmental campaigners are not convinced coal is the answer
Mr Flavin is uncertain about the option of capturing CO2 emissions and storing them deep underground. "It is something that is not being done apart from on a very small pilot scale. You are talking about an extended period to ramp it up."
Most of the industrialised nations are investing a significant amount of money into research and development of clean coal technology. Japan, Australia and the EU all have their own CCT programmes.
The biggest is the US FutureGen programme, a $1bn (£580m) project that is attempting to demonstrate the commercial viability of near-zero emission coal-fuelled power. It will look at integrating an IGCC electricity plant with hydrogen production, carbon dioxide capture and geological storage.
This sort of project should be welcomed, says Michael Cupit, director of energy at Ernst & Young.
"When you go back and look at where we have got to with the gas turbines, which was paid for by the US government under a gas turbine research programme, it was remarkably similar to the one they have launched for clean coal," he told the BBC News website.
However, he says, there is uncertainty about how long it would take for projects like this to become large-scale commercial operations.
"It all depends upon the uptake among the big companies. Once somebody takes the bull by the horns, you can usually squeeze the technology and make it commercial fairly quickly."
Mr Cupit says businesses need clear signals from governments if a suitable level of investment is to be forthcoming.
"One of the problems we face is that there are a lot of piecemeal incentives being offered, he explains.
"What we need to do is to boil it down to the nub of the problem - reducing greenhouse gas emissions."