* CO2 capture from biomass could be big part of climate fight
* But costs high, at $60-$250 a tonne of carbon dioxide-IPCC
* Would be big shift from focus on cutting fossil fuel emissions
OSLO, March 26 (Reuters) - A little-known technology that may be able to take the equivalent of China's greenhouse gas emissions out of the carbon cycle could be the radical policy shift needed to slow climate change this century, a draft U.N. report shows.
Using the technology, power plants would burn biomass - wood, wood pellets, or plant waste like from sugar cane - to generate electricity while the carbon dioxide in the biomass is extracted, piped away and buried deep underground.
Among techniques, a chemical process can strip carbon dioxide from the flue gases from combustion.
The process - called bio-energy with carbon capture and storage (BECCS) - would make the power plants not only carbon-neutral but actively a part of extracting carbon dioxide from a natural cycle of plant growth and decay.
The technology could be twinned in coming decades with planting forests that absorb carbon as they grow, according to the study obtained by Reuters.
It would be a big shift from efforts to fight global warming mainly by cutting emissions of greenhouse gases from mankind's use of fossil fuels in factories, power plants and cars, but may be necessary given the failure so far to cut rising emissions.
"BECCS forms an essential component of the response strategy for climate change in the majority of scenarios in the literature" to keep temperatures low, according to a report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
The IPCC, grouping leading scientists, is the main guide for almost 200 governments that have promised to work out a deal by the end of 2015 to slow warming to avert more floods, heatwaves, more powerful storms, droughts and rising seas.
The leaked report is Chapter 6 in a mammoth study due in mid-April in Berlin about solving climate change. It has details of BECCS not included in a draft summary.
In theory, BECCS could extract between 3.0 and above 10 billion tonnes of carbon dioxide from the air a year, it says, and seems more promising than technologies such as blocking sunlight or building machines to extract carbon from the air.
China, the top carbon producer ahead of the United States, emitted 9.86 billion tonnes in 2012, according to the PBL Netherlands Environmental Assessment Agency.
BECCS, also known as bio-CCS, would cost from $60 to $250 a tonne of carbon dioxide eliminated, the IPCC says.
"BECCS faces large challenges in financing and currently no such plants have been built and tested at scale," it says.
Archer Daniels Midland Co has a facility in Illinois, partly funded by the U.S. Department of Energy, to inject 333,000 tonnes of carbon dioxide a year into the ground from a plant producing ethanol from corn.
"Bio-CCS technology is becoming increasingly recognised as a credible option," said Brad Page, head of the Australia-based Global CCS Institute, but added it was only a partial fix. Like all other experts interviewed, he had not seen the draft.
The IPCC is meeting this week in Japan to approve another report about the risks of climate change, from food and water shortages to a slowdown in economic growth.
Most carbon capture and storage focuses on fossil fuels. Saskatchewan Power's Boundary Dam coal-fired power plant in Canada, due to start this year as the first commercial project of its type, will capture a million tonnes of carbon dioxide a year.
Apart from the high costs of BECCS, "the area you need is vast," said Joris Koornneef, an expert at sustainable energy consultancy Ecofys in the Netherlands.
He estimated that it would require 350 million hectares (864 million acres) - bigger than India - to be producing biomass for BECCS to make enough to suck 10 billion tonnes of carbon dioxide from the air, which would risk taking land from food crops.
Erwin Jackson, deputy head of The Climate Institute, an independent research group in Australia, said governments and companies should do more to research BECCS technologies. "At the moment we're ignoring them and that's risky," he said.
The IPCC says it is at least 95 percent probable that climate change is mainly man-made, rather than caused by natural swings, but opinion polls show voters in many nations are unconvinced. ($1 = 0.7258 Euros)
(Editing by Sonya Hepinstall)