Main Area: Gas Separation
Gas separation unit operations represent major cost elements in gasification plants. The gas separation technology being supported in the DOE program promises significant reduction in cost of electricity, improved thermal efficiency, and superior environmental performance.
Ion-Transport Membrane Oxygen Separation Modules
Gasification-based energy conversion systems rely on two gas separation processes: (1) separation of oxygen from air for feed to oxygen-blown gasifiers; and (2) post-gasification separation of hydrogen from carbon dioxide following (or along with) the shifting of gas composition when carbon dioxide capture is required or hydrogen is the desired product.
Research efforts include development of advanced gas separation alternatives such as oxygen separation membranes that will provide substantial cost reduction compared to energy-intensive commercial technologies for oxygen production such as cryogenic air separation. In addition, the Gasification Systems program, in concert with other DOE programs, is developing membranes and other novel methods for hydrogen recovery from gas streams that will minimize the cost and efficiency losses for hydrogen and carbon dioxide separation.
A major program objective is the development of advanced air separation technologies that can produce commercial-scale quantities of oxygen at lower cost than conventional cryogenic systems. Many existing and future gasification plants are or will be oxygen-blown. The cryogenic air separation unit used in these plants typically accounts for 12-15% of the overall capital cost of the plant, so there is a substantial room for improvement with advanced air separation technologies.
Efforts are primarily focused on Ion Transport Membranes (ITM), which currently represent the most promising technological pathway. The installed capital cost of an ITM system versus a state-of-the-art cryogenic oxygen air separation unit (ASU) is projected to be 25% lower. In addition, ITMs allow for the production of high purity oxygen at elevated temperatures as needed in gasification, thereby reducing the parasitic energy penalty associated with using cryogenic oxygen.
ITMs are nonporous ceramic membranes that are permeable only to oxygen ions and are therefore 100% selective. At temperatures of 1450-1650°F, oxygen from the air feed adsorbs on the membrane and dissociates to form oxygen ions by electron transfer. The oxygen anions enter and migrate through the ceramic lattice counter-currently with electrons, and are driven toward the permeate side by the oxygen partial pressure differential.
Air Products and Chemicals, Inc., is developing ITM oxygen separation technology in a multi-phase DOE program. Commercial scale ITM oxygen production modules, which can produce 1 TPD (ton per day) of oxygen, have been successfully fabricated and tested. Efforts are underway to construct and test a 100 TPD system that will use these 1 TPD modules. It is anticipated that this technology will be ready to be scaled up to a 2,000 TPD in 2015.
and Carbon Dioxide Separation
Hydrogen separation technologies focus on development of advanced membranes to separate of hydrogen and carbon dioxide from syngas, after the syngas has undergone a water gas shift reaction to increase hydrogen and carbon dioxide concentration. For effective integration with advanced IGCC technologies, and to be able to realize the full advantages of high-temperature gas cleaning technologies, hydrogen/carbon dioxide separation must be accomplished at temperatures higher than conventional separation processes. Operation at higher process temperatures also offers the possibility of enhancing the water gas shift through integration with advanced membranes, since both processes operate at similar temperatures. Technologies that are capable of producing both hydrogen and carbon dioxide at high pressure can avoid significant recompression costs that would further enhance the economics of these plants.
The primary technical challenges for membrane-based technologies include optimization of the composition and microstructure of membrane materials, development of thin defect-free membrane films for enhancing flux, development of robust seals, ability to accommodate contaminants in the syngas, and operation at high-permeate pressures.
The Eltron Hydrogen Transport Membrane technology under development in the DOE program uses metal or metal alloy materials for separating hydrogen from carbon dioxide. The carbon dioxide remains on the feed side of the membrane, and therefore also remains at high pressure so compression costs for sequestration are reduced. The operating temperature of 535-825°F for these novel membranes is compatible with emerging warm gas-cleaning technology, enabling even better thermal efficiency and process economics for future coal-to-hydrogen plants using both technologies.
Praxair and Worcester Polytechnic Institute are developing an integrated, cost-effective hydrogen production and separation process that employs palladium and palladium-alloy membranes.
Completed Gas Separation Projects include the following:
Systems Analyses – As part of the support for the Gas Separation program element, systems studies are being conducted to provide unbiased comparisons of competing technologies, determine the best way to integrate process technology steps, and predict the economic and environmental impacts of successful development.
Other main areas within Gasification Systems include the following: