Alabama Power Co.’s Barry Electric Generating Plant north of Mobile will be the site of the nation’s first large-scale attempt to capture carbon dioxide emitted from a coal-fired power plant and to inject the gas deep underground.

The four-year, $175 million experiment begins in 2011 and will test the future viability of coal as a source of electrical power, Alabama Power’s parent firm, the Atlanta-based Southern Company, announced Wednesday.

Tens of thousands of tons of the potent greenhouse gas will be piped 9,000 feet underground around the Citronelle oil fields, about 10 miles from the Barry plant.

“These old oil fields, we know they have held oil for geologic time. The reason they could hold the hydrocarbons in was because of a sealing layer deep below the surface,” said Gerald Hill with the Southern States Energy Board.

Partners in the project include the U.S. Department of Energy, Mitsubishi Heavy Industries Ltd., the Electric Power Research Institute and others.

In a statement, Southern Company Chairman and CEO David Ratcliffe said the main challenge facing deployment of carbon capture and sequestration technology is demonstrating its effectiveness on a large scale.

As electric plants are the largest single source of CO2, regulators have focused on cleaning up their emissions in an effort to curb global warming. The 150,000 tons of CO2 to be injected each year represent a tiny fraction of the 12.5 million tons generated at Barry annually.

But, according to project managers, if the experiment succeeds, it may not be too long before all of the carbon emissions from Barry and hundreds of similarly sized coal plants are piped underground.

Mississippi Power’s Plant Daniel was the site for a demonstration in which 3,000 tons of CO2 recently were injected into a saline rock formation 8,500 feet below ground. Monitoring of its movement at that depth and under multiple geological seals is now under way.