Chemical Looping Combustion
One of the proposed ways of dealing with global warming is the capture of CO2 at stationary powerplants, with the gas being injected into wells, deep aquifers, or even (after liquifaction) into the deep ocean.
The showstopper here is the cost of separating CO2 from the flue gases. Existing techniques led to estimates of this taking perhaps 1/3 of the energy roduced by the powerplant, raising the cost significantly.
Several groups are now looking at a new technology for this problem, called chemical looping combustion, or CLC. Here, combustion is separated into two phases. In the first, particles containing a transition metal oxide (typically an oxide of Fe, Mn, Cu, Co, or Ni) is reduced by a fuel gas, either to another oxide or to the base metal. In the second phase, the metal is reoxidized with air. The oxide particles are recirculated between the two fluidized bed reactors in which these reactions occur.
Because the air and fuel never mix, no nitrogen is mixed into the combustion products of the fuel. For a hydrocarbon fuel, the exhaust of the combustion reactor is CO2, water, and unburned fuel. This gas is cooled, compressed, and the CO2 liquified; any unburned fuel gas is recirculated to be burned again (or bled off to the other reactor for disposal to the atmosphere).
How efficient is this? Calculations show the system can achieve overall efficiency on natural gas of better than 54%; this is reduced by about 2% if the cost of compressing/liquifying the CO2 is included. The high efficiency is in part due to a reduced irreversibility of combustion, and also due to the lack of a separation stage to remove nitrogen from either air or the flue gases. The cost estimates appear reasonable; with lab measurements of fluid bed erosion rates the cost of separating a ton of CO2 is less than 1 euro.