June 24, 2012
A pair of new technologies could reduce the cost of capturing carbon dioxide from coal plants and help utilities comply with existing and proposed environmental regulations, including requirements to reduce greenhouse-gas emissions. Both involve burning coal in the presence of pure oxygen rather than air, which is mostly nitrogen. Major companies including Toshiba, Shaw, and Itea have announced plans to build demonstration plants for the technologies in coming months.
The basic idea of burning fossil fuels in pure oxygen isn't new. The drawback is that it's more expensive than conventional coal plant technology, because it requires additional equipment to separate oxygen and nitrogen. The new technologies attempt to offset at least some of this cost by improving efficiency and reducing capital costs in other areas of a coal plant. Among other things, they simplify the after-treatment required to meet U.S. Environmental Protection Agency regulations.
One of the new technologies, which involves pressurizing the oxygen, is being developed by a partnership between ThermoEnergy, based in Worcester, Massachusetts, and the major Italian engineering firm Itea. A version of it has been demonstrated at a small plant in Singapore that can generate about 15 megawatts of heat (enough for about five megawatts of electricity).
The technology simplifies the clean-up of flue gases; for example, some pollutants are captured in a glass form that results from high-temperature combustion. It also has the ability to quickly change power output, going from 10 percent to 100 percent of its generating capacity in 30 minutes, says Robert Marrs, ThermoEnergy's VP of business development. Conventional coal plants take several hours to do that. More flexible power production could accommodate changes in supply from variable sources of power like wind turbines and solar panels.
Marrs says that these advantages, along with the technology's higher efficiency at converting the energy in coal into electricity, could make it roughly as cost-effective as retrofitting a coal plant with new technology to meet current EPA regulations, while producing a stream of carbon dioxide that's easy to capture. The technology also reduces net energy consumption at coal plants, because the water produced by combustion is captured and can be recycled. This makes it attractive for use in drought-prone areas, such as some parts of China.
The other technology, being developed by the startup Net Power along with Toshiba, the power producer Exelon, and the engineering firm Shaw, is more radical, and it's designed to make coal plants significantly more efficient than they are today—over 50 percent efficient, versus about 30 percent. The most efficient power plants today use a pair of turbines: a gas turbine and a steam turbine that runs off the gas turbine's exhaust heat. The new technology makes use of the exhaust by directing part of the carbon dioxide in the exhaust stream back into the gas turbine, doing away with the steam turbine altogether. That helps offset the cost of the oxygen separation equipment. The carbon dioxide that isn't redirected to the turbine is relatively pure compared to exhaust from a conventional plant, and it is already highly pressurized, making it suitable for sequestering underground. The technology was originally conceived to work with gasified coal, but the company is planning to demonstrate it first with natural gas, which is simpler because it doesn't require a gasifier. The company says the technology will cost about the same as conventional natural gas plants. Shaw is funding a 25-megawatt demonstration power plant that is scheduled to be completed by mid-2014. Net Power plants to sell the carbon dioxide to oil companies to help improve oil production.
The technologies may be "plausible on paper," says Ahmed Ghoniem, a professor of mechanical engineering at MIT, but questions remain "until things get demonstrated." (Ghoniem has consulted for ThermoEnergy.) The economics are still a matter of speculation. For one thing, it is "an open question" how much money the technologies could save over conventional pollution control techniques, he says. As a rule, "any time you add carbon dioxide capture, you increase costs," he points out. "The question is by how much." Selling the carbon dioxide to enhance oil recovery can help justify the extra costs, he says, and retrofitting old power plants might help create an initial market. But he says the new technologies won't become widespread unless a price on carbon dioxide emissions is widely adopted.
Ghoniem adds that even if the technology for capturing carbon proves economical, it's still necessary to demonstrate that it's feasible and safe to permanently sequester carbon underground. The challenges of doing that were highlighted by a recent study suggesting that earthquakes could cause carbon dioxide to leak out.
Source: Technology Review