Chementator: Two solid ways to remove CO2 from fluegas, one based on Na2CO3 . . .

While monoethanolamine (MEA) is the traditional solvent for scrubbing CO₂ from fluegas, more effective methods, including solid absorbents, are being developed (see CE, December 2008, pp. 16–20). Sodium carbonate, for example, is the sorbent in a process being developed, with DOE support, by Research Triangle Institute (RTI, Research Triangle Park, N.C.; www.rti.org). Desulfurized fluegas from a wet scrubber is passed through a fluidized bed of Na₂CO₃ particles, which adsorb CO₂ to form bicarbonate at 50–80°C. When the particles are loaded they are heated with steam or CO₂ to about 120°C to drive off a 99%-pure CO₂ stream and regenerate Na₂CO₃ (diagram).

The process "uses only about half the energy of amine processes and you don’t have to handle any corrosive liquid," says Raghubir Gupta, a senior scientist.
RTI has tested the process at a scale of about 1/3-m.t./d of CO₂ and is now building a 10-m.t./d pilot unit.

… and another with adsorbing beads

In a concept being developed by Adsorption Research, Inc. (ARI; Dublin, Ohio; www.adsorption.com), adsorbent beads are delivered to the top of an adsorber column by a bucket elevator and fall down through an adsorption section, counter to upflowing, cooled fluegas. The beads’ passage is slowed by a series of perforated trays to provide a residence time of about 1-1/2 min for carbon dioxide adsorption.

Next, the beads drop through a heat-exchanger section, where they are indirectly heated to 400 – 600°F by incoming fluegas. The heat releases the CO₂, which is withdrawn through a perforated pipe. Simultaneously, the gas is cooled to about 125°F for the adsorption step.

In laboratory tests, using a zeolite or a proprietary adsorbent, the process has achieved 89% CO₂ recovery, with a purity of 99%, says Kent Knaebel, president of ARI. He adds that, unlike MEA adsorption, the process requires little parasitic heat or cooling energy and the adsorbent is not degraded by SO₂, NOx or O₂. He estimates that a commercial unit could process 15,000 tons/d of CO₂ from a 500-MW power plant for under $20/ton, compared with about $40/ton for MEA.