In recent years, political and economic circles have discussed a particular way of making coal-fired power stations more climate friendly. This method is known as “carbon capture and storage”. The technique involves capturing the carbon-dioxide emissions from power plants and factories, and storing them in geological formations deep underground. Some scientists and environmentalists hope that this will decelerate the rise of carbon dioxide in the atmosphere, or perhaps even reduce it. Many of the scenarios prepared by the Intergovernmental Panel on Climate Change assume that if carbon capture and storage is used the probable warming level will stay below 2°C. But such assumptions carry a critical flaw. It is already evident that the technologies currently under development cannot achieve what they promise.
It is now possible to capture only 85 to 90 percent of the CO2 from power stations. Doing so takes energy, which has to come from the power plant itself. The plant, therefore, works 11 to 15 percent less efficiently, cutting its operating efficiency from 35 to 30 percent – back to levels common in the 1980s. The plant would have to burn up as much as one-third more coal to produce the same amount of energy. The commercial use of carbon capture and storage would require digging up yet more coal – with all the accompanying negative environmental consequences.
Where could the captured CO2 be stored? One possibility is in depleted oil and gas fields. In the United States and Norway injecting CO2 into oilfields is a common procedure to boost the yield of oil. A much bigger but more controversial potential store is in saline aquifers: porous rock formations filled with saline water that are capped by impermeable layers of rock.
The Norwegian energy firm Statoil launched one such storage-and-capture project in 1996 at the Sleipner gas field under the North Sea. Because the natural gas extracted from this field contains too much CO2, Statoil separates almost a million tonnes of the gas each year, and injects it into rock formations above the gas field to reduce its carbon tax bill.
But it is uncertain whether the storage locations will stay sealed over the long term, whether gas can leak out, or whether the seals on the boreholes will corrode. A sudden release of a lot of CO2 would endanger humans and other living creatures. The saline water displaced by the CO2 might be forced up into shallower rock layers and contaminate groundwater with salt and toxic substances. The risks are just as high if the CO2 is injected into rock formations below the seabed, as planned in countries including Australia and Britain. This type of offshore storage can severely damage the marine environment through leaks of CO2 and contaminated saline water.
No technique yet exists to monitor CO2 storage sites, systematically identify leaks or plug them when they are found. A flagship project at In Salah in Algeria was shut down in 2011 because of concerns about storage safety. At present, as a result of technical difficulties and the high cost, which would amount to several billion euros for a big power plant, no plant anywhere in the world separates significant amounts of CO2 for storage. A small power station in Canada is the only project that gets support from the public purse to boost production from an oilfield. A major project in the United States to demonstrate carbon capture and storage, called FutureGen, would have cost over $1.6 billion. It was suspended in 2015.
Technically, there are several ways of capturing carbon. One is to use chemicals to “wash” CO2 out of the stream of exhaust gases after combustion. A second approach relies on the principle of coal gasification; it extracts the CO2 before combustion takes place. A third method involves burning coal using pure oxygen, making it easier to extract the CO2 from the exhaust. From a technical point of view, carbon capture is better suited to the steel and cement industries because they are less able to avoid producing CO2.
Despite all the failures, the promise of “clean coal” is still used as a justification for building new coal-fired power plants and thus extending the life of the fossil-fuel business model and decelerating the transition to renewable energy. Carbon-capture plants are less flexible than traditional coal-fired plants in responding to fluctuations in demand for power.
Some coal-fired plants, such as the Drax station in Britain, are able to burn wood as well as coal. In theory, such power stations are supposed to achieve negative carbon emissions by combining carbon capture and storage with the use of bioenergy. Trees absorb CO2 as they grow. When they are burned, the resulting CO2 can be pulled out of the cycle if it is captured and stored. A nice idea – but experts say the sums do not add up. Monoculture plantations of fast-growing trees merely displace intact forests, and store a lot less CO2.
In addition, it is questionable whether the trees absorb as much CO2 as is released by fertilizer applications, wood processing, transport and the destruction of intact soils. Using bioenergy would further raise the pressure on arable land as investors acquire large areas to plant biomass. Critics call attention to the connection between this “land grabbing” and the violation of traditional land-use rights of local people who lose their means of subsistence.
At Drax, however, an ambitious carbon-capture project hit an obstacle when the plant owner halted its investment. A cut in subsidies for renewable energy caused a sharp decline in the company’s share price. The other partners in the consortium say the project will continue; a feasibility study will be completed in 2016.