Capturing CO2 : Membrane Systems Move Forward

Over the last three decades, operators across the chemical process industries (CPI) have employed membrane-based systems to carry out various types of separations. Because of their fundamental engineering and economic advantages over competing separation technologies, membranes are now being explored for CO2 capture from power plant emissions and other fossilfuel-based fl uegas streams.* The promise of system simplicity is a key driver of membrane R&D. While conventional solvent-based separation systems are both capitaland maintenance-intensive, “membrane systems have no moving parts and just let the gas streams simply pass through — so they are expected to be more reliable and less costly,” says Jeff Phillips, senior program manager of advanced generation, Electric Power Research Institute (EPRI; Charlotte, NC; www.epri.com). The unavoidable pressure and/or temperature cycling required for solvent regeneration adds complexity and imposes cost penalties, in terms of both the capital costs and parasitic energy losses. “All of these attributes make membrane systems a potentially cheaper alternative to absorption-based separation for capturing CO2,” says Jared Ciferno, technology manager, existing plants, emissions and capture, at the U.S. Dept. of Energy’s (DOE) National Energy Technology Laboratory (NETL; Pittsburgh, PA; www.netl.doe.gov). However, the ability to parlay industry’s knowledge and experience into a standard membrane solution for capturing CO2 has turned out to be a much more complex engineering challenge than many had anticipated. Rather, several critical distinctions will shape the ultimate solution, such as: • The nature of the separation — separating CO2 from nitrogen in the fl uegas produced by conventional coalfi red power plants (Figure 1a), versus separating CO2 from hydrogen in the syngas produced by integrated gasifi cation combined cycle (IGCC) power plants (Figure 1b) • The nature of the gas stream — the lower-pressure, larger-volume, relatively dilute post-combustion fl uegas streams produced by coal-fi red plants, versus the higherpressure, smaller-volume, CO2-enriched pre-combustion syngas streams produced by IGCC plants • The anticipated location in the process — at the end of the process in a coal-fi red plant for post-combustion CO2 capture, versus further upstream, between other unit operations within an IGCC process, to separate CO2 from the syngas before it is combusted in the gas turbines.