Air Separation Technology focuses on identification of new concepts and technologies for production of oxygen for use in gasification systems. Many gasification-based energy plants run more efficiently if the oxidant is oxygen rather air, but they rely on conventional cryogenic air separation which is expensive both in terms of capital expenditure and cost to operate. Accordingly, the technologies under development target both low cost and high levels of operational efficiency. Fields of investigation under Air Separation Technology currently include:
The historical context for development in this area was oxygen production for large integrated gasification combined cycle power plants (e.g. by scaling up high-temperature ceramic membrane-based ion transport membrane technology). This has shifted to innovative technologies that can deploy in concert with modular gasification-based systems. NETL continues to be interested in potential of development of innovative air separation technology, including hollow fiber membranes and modules, improved sorbents, various cryogenic system improvements, use of solid oxygen carriers in chemical looping to provide in situ oxygen production in gasifiers, and innovative concepts including magnetic field-based air separation.
The Gasification Systems Program plans to continue to develop air separation technologies to be utilized in advanced modular energy systems that will make substantial progress toward enabling cost-competitive, coal-based power generation with near-zero emissions. Consistent with the overall Program strategy, technological development of air separation systems will be suitably sized for integration in modular gasification systems. The Program continues a non-restrictive strategy of fostering technology advancement in any of the air separation technology areas, from cryogenic to innovative concepts, if potential for cost reduction and performance advancement exist.
Ceramic Membranes (Ion transport)
At suitably elevated temperatures, certain formulations of ceramic membranes are permeable only to oxygen ions and are therefore 100% selective. Examples have included OTM (Oxygen Transport Membranes) and ITM (Ion Transport Membranes) which have undergone significant development (notably by Praxair and Air Products). At temperatures of 1450-1650°F, oxygen from feed air 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 that can be established variously by pressurizing the feed air, establishing vacuum on the permeate side, or gas sweeping the permeate side. Ceramic membrane systems afford the opportunity of integrated operation with turbines, and their operation at elevated temperatures increases efficiency/reduces parasitic energy penalty compared to conventional cryogenic oxygen production systems. Systems analyses on a variety of gasification-based processes have shown significant cost and efficiency advantages compared to conventional cryogenic technology; however, technical issues with membrane life, durability of system components, and manufacturing yield continue to be problematic. Approaches such as identifying different materials including dual or mixed matrix ceramic materials which can operate at more moderate temperatures (1100-1300°F) with suitable flux, and membrane configurations and engineering to improve durability and operational integrity of air separation modules, and application to smaller, modular systems have developmental promise.
Cryogenic-based Air Separation Systems Improvements and Modularization
The cryogenic air separation unit (ASU) in a conventional IGCC plant typically accounts for 12 to 15 percent of the overall capital cost of the plant, requiring a large parasitic power load primarily to operate gas compressors. Improvement in operational efficiency of these systems can significantly decrease their high operating costs.
The portfolio includes an interesting project which is applying the magnetocaloric effect for refrigeration/liquefaction of air. This approach uses solid state refrigerants, involving minimal gas compression and no expansion refrigeration (saving the capital and operating costs of turbomachinery).
Onsite NETL Research—Oxygen Carrier Development:
NETL has been developing tailored oxygen carrier materials for possible use in both stand-alone oxygen production modules, and in production of in-situ oxygen within reactors. These carrier materials have tunable oxygen delivery properties to respond to a variety of opportunities and fuels. Investigations are providing information about the link between composition and carrier performance with a focus on use of these materials in modular reactor systems.
As part of the support for Air Separation key technology, 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 key technologies within Gasification Systems include the following: