A new membrane is combined with a novel process design to reduce energy costs in a CO2 recovery system developed by Membrane Technology and Research Inc. (MTR, Menlo Park, Calif.; www.mtrinc.com). The two-step system uses a membrane made of a hydrophilic rubbery polymer, formed into spiral-wound modules. The membrane is 10 times more permeable to CO2 than conventional membranes used for CO2, says Timothy Merkel, director of process research and development.
In the first step, CO2 is extracted from the fluegas in a conventional manner by applying a slight vacuum to the permeate side of the membrane. However, in the second stage the driving force for permeation is a sweep stream of combustion air which carries the CO2 back into the boiler. "This saves energy because no pressure difference is required to move CO2 through the membrane," says Merkel. Another benefit is that the recycle increases the CO2 concentration in the fluegas from 13% to 19%.
Merkel estimates that a commercial system could recover CO2 for $20–30/ton, versus $40–80/ton for amine absorption. The process is expected to use 12% of an electricity plant’s energy, or 18% when compression costs for sequestration are added on. However, when a field test starts up in 2009 at Arizona Public Service’s coal-fired Cholla plant, the CO2 may be shipped to a nearby algae farm to help biofuel production.