(91f) Novel Integration of Gas Separation Membranes for CO2 Capture from Igcc Power Plants

A core mission of the U.S. Department of Energy's (DOE) Carbon Sequestration Program is to foster the development of commercially-ready technologies for CO2 capture and sequestration. The DOE's Coal Fuels and Hydrogen Program has a similar mission for the production of hydrogen for use with advanced fuel-cells. R&D goals have been established for electric power generation from next-generation Integrated Gasification Combined Cycle (IGCC) plants and for future coal-to-hydrogen fuel plants. The missions of the two programs are intertwined in that both involve the production of hydrogen from coal using advanced coal gasification and synthesis gas clean-up systems.

For carbon capture, the best available coal gasification system currently employs a two-stage Selexol? system for H2S removal and CO2 capture. Coal is fed to a high-efficiency gasifier, and the conversion of CO in the raw syngas to H2 is maximized using water-gas shift (WGS) reactors. For IGCC applications, the Selexol acid gas removal (AGR) system located downstream of the WGS produces a hydrogen-rich fuel gas that is combusted in a gas turbine (GT) topping cycle. H2S removed in the AGR is sent to a Claus sulfur plant; while the CO2 recovered is dehydrated and compressed for transport to an appropriate sequestration site. When hydrogen is the desired product, the optimal design is slightly different; less shifting of the raw syngas is required and the AGR is followed by a pressure swing adsorption (PSA) system to recover high-purity hydrogen for compression and sale. The by-product gas from the PSA, which contains H2, CO and CO2 is combusted with oxygen to generate electric power for plant use. The CO2 from the AGR and the exhaust from the combined-cycle is dehydrated and compressed for transport and sequestration.

R&D supported by the U.S. DOE is investigating alternatives to Selexol for capturing CO2 that may exceed the performance of Selexol and achieve the DOE program goals. Membrane gas separation has been touted as one possible approach. Membranes have a number of advantages, in that they are compact, have no moving parts, have low maintenance, and can be highly reliable. The National Energy Technology Laboratory (NETL) is investigating alternative flowsheets incorporating membranes that may out-perform Selexol for CO2 capture. An initial screening study has identified several novel integrations of the membrane for IGCC and coal to hydrogen. For IGCC applications, the use of a N2 sweep gas along with integration of the membrane within the CO2 compression train is particularly attractive since high-purity hydrogen is not required for power generation. These options coupled with bulk H2 recovery from the syngas after the WGS, can also produce a high-purity side stream for use in other applications. PSA can be used for bulk separation with H2 recoveries in the range of 70% to 80%. This plant configuration allows for maximum flexibility in either the production of hydrogen for sale, or of power generation and sale, or both. Other potential advantages of this approach in addition to the elimination of the two-stage Selexol unit are:

? The H2/CO2 separation membranes operate at high feed pressures with a large partial-pressure differential across the membrane

? The use of a sweep gas improves the driving force across the membrane and also serves as the GT diluent

? Hydrogen re-compression is eliminated and the hydrogen fuel and sales gas are delivered at the required pipeline and turbine pressures, respectively

? The membrane is placed well downstream of the gasifier at a location where most of the contaminants in the raw syngas have been removed

A major challenge for implementing these CO2 capture strategies is the development of membrane materials with high selectivities for H2 relative to CO2. The IGCC process with CO2 capture using gas separation membranes, along with a similar process co-producing hydrogen and power, have been modeled with the Aspen process flowsheet simulator. Cases were developed for both warm and cold gas clean-up, and for no gas clean-up, where both CO2 and H2S are co-sequestered. Based on the results of the modeling, membrane performance and cost targets were developed consistent with the overall goals of the DOE R&D program.