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Clean energy special: Going underground
03 September 2005
From New Scientist Print Edition.
Emma Young

Enlarge Carbon sequestration pilot projects

Enlarge Burying the problem

The decaying industrial landscape of Silesia in southern Poland doesn't look the sort of place to give birth to a planet-saving technology. Yet here among the flyblown coalfields and mouldering, Communist-era factories is a project that many climate researchers believe represents humanity's best bet for averting a global disaster.

Deep below the surface, in a coal seam about a kilometre down, 1000 tonnes of carbon dioxide are being held under lock and key. Australian researchers pumped it down there last year in the hope that it would remain held inside the coal instead of wafting around in the atmosphere. The verdict: so far so good.

Of course, 1000 tonnes is a mere drop in the ocean of CO₂ spewed into the atmosphere every day by factories, aeroplanes, cars and power stations. Global emissions of this greenhouse gas are about 23 billion tonnes a year and rising. But many researchers believe that projects like the one in Silesia can be replicated all over the world, with huge positive implications for the global environment.

John Topper, director of the International Energy Agency's Clean Coal Centre in London, says that if we are serious about stabilising CO₂ levels in the atmosphere we have just two options. The first is to build lots of new nuclear power plants. The second is to develop technologies similar the one being trialled in Silesia, collectively known as carbon capture and storage (CCS). And he reckons he knows which one is in pole position. "Nuclear energy is an emotive issue in many countries," he says. "So the carbon capture and storage option is receiving serious attention."

The basic idea of CCS - also known as carbon sequestration - is simple. Instead of dumping CO₂ into the atmosphere, where it contributes to global warming, you collect it and store it out of harm's way. That's not particularly hard to do: researchers know how to snag the gas, and also know how to lock it away securely. But economically and socially, carbon sequestration still has a long way to go.

One challenge is to radically cut the cost of the capture technology. And though storage poses few technical problems, there are serious questions over its public acceptability, says Peter Cook, head of Australia's Cooperative Research Centre for Greenhouse Gas Technologies (CO₂CRC), based in Canberra. "You have to convince people you can do it safely and securely," he says (see "What if it escapes?").

Despite these obstacles, sequestration has begun to get off the ground. The world's first commercial venture to capture and store CO₂ deep underground started up back in 1996, at the Norwegian company Statoil's Sleipner gas field in the North Sea. Every year, 1 million tonnes of CO₂ from the purification of natural gas is injected more than 800 metres down into an underground sandstone formation. The idea is to secure the CO₂ indefinitely, saving the company carbon taxes and cutting Norway's annual output of greenhouse gas by 3 per cent. Numerous other projects have since sprung up around the world (see Map).

As yet, however, carbon sequestration has not made the breakthrough in the sector where it is needed the most. The biggest producers of CO₂ now and into the foreseeable future are not gas platforms like Sleipner but coal-fired power stations, which burn coal in air and vent the waste gases straight into the atmosphere. But alongside clean coal technology (see "A greener shade of black"), most researchers agree that carbon sequestration has enormous potential to mitigate the problem.

In the US, about one-third of carbon emissions come from coal-fired plants, while globally coal-burning power plants produce about 6 billion tonnes of CO₂ every year, around 25 per cent of the total. And with the US, India and China all committed to building more coal-fired plants, the problem can only get worse. Over the next few decades, emissions from coal-fired power stations could double according to some estimates.

To become economically viable in such plants, the cost of carbon sequestration needs to fall to about $10 per tonne of CO₂. At the moment it is far higher, largely because of the expense of capturing the gas. In place of the smokestacks that now vent combustion gases into the air, power stations will have to be equipped with expensive absorption towers designed to trap CO₂, perhaps by using solvents that separate it from other flue gases. CO₂ makes up only about 15 per cent of the emissions from regular coal-burning plants. The rest is nitrogen, plus various impurities. Solvents developed to filter out CO₂ from waste gas streams don't cope very well with this mixture. "Impurities in flue gas can oxidise conventional solvents, which reduces their effectiveness," says John Wright, director of the Energy Transformed programme at CSIRO, Australia's national research organisation.

It's tricky to extrapolate from research models to full-size plants, Cook warns, but the best estimates are that it would cost in the region of US$40 for each tonne of CO₂ removed through retrofitting a traditional coal-burning plant with gas scrubbers.

Improved designs for power plants can help to bring down the costs. But the huge expansion plans in China and India - China has plans for about 560 new coal-fired plants, and India 213 - are going ahead without any consideration for carbon capture, Toppert says. Finding a way to cut the costs of retrofitting traditional power stations will be vital for tackling global emissions over the next 60 years - the lifetime of a power plant.

Various groups are working on improving the capture technologies. Researchers from CO₂CRC and CSIRO have developed new, more efficient solvents. Neither group has filed patents on the technology yet, so details are sketchy. "We are looking for about a 20 per cent improvement in absorption capacity - and we hope we could do much better," Wright says.

Other research is focusing on creating CO₂-filtering membranes to replace solvents. At the US Department of Energy, teams are working on carbon-fibre molecular sieves. In April, the DOE also announced new grants for research into capture technologies for existing coal-burning power plants.

Once the CO₂ has been captured, the next problem is finding underground sequestration sites where it can be locked away. A recent study in Australia suggests that the country's deep saline aquifers could swallow 125 million tonnes of CO₂ a year, equivalent to half Australia's output from power stations. Similar studies in the US, coordinated by the DOE, have tentatively identified underground sites with the combined capacity to store 600 billion tonnes of CO₂, equivalent to 100 years of the national carbon dioxide output.

Saline aquifers at least 800 metres underground are a popular option for sequestration, mainly because it is thought they might provide most of the capacity for the world's unwanted CO₂. These structures are widespread around the globe and could absorb huge volumes of the gas, which at high enough pressures is transformed into a fluid. "As long as you inject it down to that sort of depth, into suitable geological formations, the CO₂ remains dense - and it will stay trapped," Cook says. Once injection holes are sealed, the cap rock above keeps the CO₂ in place.

But while there are plenty of these basins around, they are not necessarily in the same place as existing or planned power plants. New South Wales, home to some of Australia's major coal-producing regions, has few suitable sites, for instance.

But there are alternatives. The Weyburn oilfield in Saskatchewan, Canada, was first drilled in 1954. In 2000 the operator, EnCana, began piping in waste CO₂ from a coal gasification plant in North Dakota, 330 kilometres away. EnCana uses the gas to flush out remaining oil, and about three-quarters of the CO₂ pumped into the oil field stays down there as a supercritical fluid. The plan is that after the last dregs of oil are removed, 20 million tonnes of CO₂ will be sealed in.

And as the Silesia project has shown, coal mines are another option. The approach is based on the fact that coal readily adsorbs CO₂, and once it has been adsorbed it stays in there. Luke Connell, head of CSIRO Petroleum's reservoir engineering group, who is working on the project, stresses that coal beds won't always be ideal CO₂ repositories. For one thing it takes a long time to get the gas in. But they do have plus points. "The main advantage is that coal is often close to carbon dioxide emissions, so in some cases it will have geographical advantages over other options," he says.

Another proposal is to inject CO₂ deep into the ocean to form a stable liquid layer on the seabed. This is perhaps the most controversial option. One major concern is that CO₂ would react with seawater, increasing the acidity of the ocean and affecting marine life, Cook says. Deep ocean storage is also problematic when it comes to another key element of sequestration technology: keeping track of the CO₂. Demonstrating that any stored stocks can be accurately monitored will be essential, not only for securing widespread public acceptance but also for the economics of carbon taxation, stresses Ernie Perkins of CO₂CRC, a former head of geochemical monitoring on a four-year study of the Weyburn project.

Monitoring technologies such as seismic and geochemical surveying are improving, but Perkins says more pilot projects in a range of different settings are needed to fully understand what happens to CO₂ at sites with varying geological characteristics. "We can predict a lot of things, but you never really know how accurate your models are until you've done it in the field," he says.

Unknowns notwithstanding, there are a handful of projects in the pipeline that will attempt to bring everything together. One is Future Gen, a $1 billion prototype coal-fired power plant on the drawing board at the US Department of Energy. Future Gen will employ extensive carbon sequestration - the idea is to reduce emissions to zero - and so could provide answers about the technology's viability in a large power station. If all goes to plan, Future Gen aims to start operation in 2011, and continue testing until 2015. Plans for a similar power plant in Queensland, Australia, were announced in June.

These trials are pointing the way to a cleaner future. But for some researchers the future cannot arrive soon enough. "Unless we crack the retrofitting of existing plants," says Wright, "the world is going to be in a lot of trouble."

From issue 2515 of New Scientist magazine, 03 September 2005, page 34

What if it escapes? On the evening of 21 August 1986, death came silently to the villages around Lake Nyos, Cameroon. A belch of carbon dioxide bubbled up from the lake and down the valley, asphyxiating about 2000 people. What if stores of captured CO2 escaped suddenly in a similar way? Could sequestration put lives at risk? There are two possibilities for waste CO2 release: sudden, large burps, as happened at Lake Nyos, and slow seepages, which would be less likely to threaten life but would mean the subterranean chambers are not the impenetrable vaults the environment modellers might assume them to be. Ernie Perkins of CO2CRC in Canberra, Australia, thinks explosive leaks are unlikely, especially from the deep saline aquifers most favoured for sequestration. The liquid CO2 would be held in place by the cap rock above. It would be buried so deep that there would probably be many such cap-rock layers between it and the surface, Perkins says. If CO2 managed to escape from one, it would be caught by the next. But if CO2 trickled out, it is not clear whether anyone would be able to spot it against the background of natural CO2 production from the soil, Perkins adds. "Could you see, say, 1 per cent of a deposit coming to the surface, if it's a diffuse flow? The answer at the moment is that we don't think so, or we don't know." There is a plan for a project in the US to create an artificial leak to find out. The ideal, of course, is that the underground stores would hold CO2 indefinitely. It's not easy to model what could happen over millennia, but there are large natural subterranean reservoirs of almost pure CO2, including three in the US, which have held onto their stocks over vast periods of time. Studies of these stores may help reveal what is likely to happen to waste stores long into the future.

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