BEIJING China is pushing to complete its first commercial-scale power plant that can capture and store emissions, but must do more research on how and where to lock away carbon dioxide if the technology is to get wide roll-out.
Pressure is building on the world's top emitter of greenhouse gas to curb the growth of its carbon dioxide (CO2) output. China itself is also worried about the impact of rising world temperatures on its climate and food output.
But coal is China's most plentiful domestic source of energy, and Beijing hopes for several more decades of rapid economic growth to lift millions from poverty.
That means capturing and storing carbon dioxide -- the main greenhouse gas blamed for global warming -- in underground reservoirs is likely to be crucial to containing emissions.
But officials worry about the expense and the environmental impact of the process.
"There are still a number of outstanding issues in relation to this technology," said Ma Yanhe, Director-General of the Chinese Ministry of Science and Technology. "Apart from reducing greenhouse gas emissions, it is not making very significant contributions to sustainable development.
"The technology itself is also energy intensive and the significant energy consumption is quite worrisome. Finally, there is no reliable assessment methodology for the long-term environmental impact of this technology."
Among the considerable obstacles facing scientists is uncertainty about how best to store CO2.
If there were problems with storage and large amounts of gas were released at once, perhaps in an earthquake, it could kill people at the surface, while leaks would void the expected contribution to fighting climate change.
Work has already begun on the first stage of the power project, a high-tech plant near the port city of Tianjin that will eventually strip CO2 out of gasified coal before combustion, but will run first for several years as a cleaner power station.
"We plan to start construction in 2014 and complete the works and start operations in 2016," Su Wenbin, head of China Huaneng Group's Greengen zero-emissions project, told a recent CCS conference.
Greengen also has a demonstration plant in Beijing where some of the gas stripped out is used to carbonate soft drinks.
China's storage needs will be vast if it decides to push sequestration as a key part of efforts to curb emissions.
A 1 gigawatt (GW) power plant with a 40-year life span will generate about 200 million tonnes of carbon dioxide, according to a recent study by the China-Britain Near-Zero Emissions Coal initiative (NZEC), which is exploring China's CCS options.
China's installed generating capacity was already 793 GW in 2008, and is forecast to hit 1,600 GW by 2020.
Some geological forecasts are optimistic. There is potential to store 3,066 gigatonnes of gas underground or under the seabed, equivalent to more than 400 years of current emissions, the U.S. National Resource Defense Council said in a report.
More than 90 percent of the country's major CO2 producing centers are no more than 160 kilometers (100 miles) from a potential large underground storage site, the report added.
But 99 percent of this potential capacity is in saline aquifers, a storage option that has not been fully researched.
Although PetroChina this year started a trial project in the northeast to pump CO2 into depleted oil and gas fields to extract more fuel, using a proven technology, this is an option for only a small portion of the country's CO2 output.
There are worries that leakage could be a major problem at the major, older fields, which have been punctured by numerous wells during their exploitation.
Offshore storage has been suggested as an alternative, but this increases cost and technological challenges.
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The cost of most carbon transport and storage in China should be the equivalent of $2 to $8 per tonne, the Pacific Northwest National Laboratory said in a report.
But the separation of carbon emissions can be hugely expensive, whether retrofitting plants or building new ones.
There is also a question mark over who will pay for seismic and other studies rarely included in costings for CCS plants -- but if companies themselves are hunting for storage sites or paying others to do so, it could push up costs.
"Data access will be an important issue because much data held by oil and gas companies is commercially sensitive," said Graham White, at the British Department of Energy and Climate Change.
Further down the line, carbon capture projects might be covered by the Clean Development Mechanism, an international scheme to tackle global warming that allows rich nation polluters to pay for output cuts in poor nations.
For now, however, it is not eligible as opponents say the system is meant to fund verifiable reductions, not technological innovation, and CCS is as yet unproven. They also worry that it could divert investment from renewables and efficiency. (Editing by David Fogarty)