Developmental Technology for Oxygen Separation
In spite of its established role in reliably providing high-throughput, high-purity oxygen for gasification, cryogenic distillation-based air separation is costly and energy-intensive to operate, accounting for up to 15% of the total gasification plant capital cost, and consuming a major portion of in-plant power use. Moreover, any outages of the air separation unit (ASU) can disrupt the entire gasification plant process. Accordingly there is great incentive to develop a new approach or technology for air separation.
Other oxygen supply technologies, such as pressure swing adsorption (PSA) and polymeric membranes are available, but at present commercially available offerings are limited in applicability to much smaller scales or cannot provide oxygen at a high enough purity (>95%) for gasification.
Accordingly, one of DOE/NETL's major gasification R&D program objectives is the development of cost-effective, ion-transport membranes as an alternative to cryogenic distillation technology for air separation/oxygen production. Ultimately, these are expected to provide substantial cost reduction for oxygen separation in comparison with the current cryogenic technology and other available options.
Ion Transport Membranes (ITM)
Ion Transport Membrane (ITM) technology uses a radically different approach to produce high-purity oxygen (O2). It offers tremendous opportunities to improve the efficiency and cost for air separation, and thus, on the overall gasification economics.
ITMs are nonporous ceramic membranes that are permeable only to oxygen ions and are therefore 100% selective. At elevated temperatures (800-900°C), oxygen from the air feed adsorbs and dissociates on the membrane to form oxygen ions by electron transfer. The oxygen anions migrate through the ceramic lattice counter-currently with electrons, and are driven toward the permeate side by the oxygen partial pressure differential. Flux through the membrane can be increased by introducing a sweep gas on the permeate side to maintain a low oxygen partial pressure. To minimize the mechanical load on the membrane, the feed stream is typically pressurized to 100-300 psia, while the permeate side is kept sub-atmospheric.
The energy of the hot, pressurized, non-permeate stream is recovered by a gas turbine for power generation and an optional steam turbine for low-level heat recovery.
Air Products and Chemicals, Inc., (APCI), with key partner Ceramatec and others, are pursuing a project with an objective to develop, scale-up, and demonstrate the ITM concept for large-scale gasification applications. The project is co-sponsored by the Department of Energy (DOE). It is a phased program with Phase I focusing on the materials and processing R&D, and the design, construction and operation of an approximately 0.1 ton/day (TPD) Technology Development Unit (TDU). During Phase I, APCI successfully built a 0.5 TPD commercial-scale ITM TDU unit and demonstrated its ability to produce high-purity (>99%) O2 at a high flux rate.
The success of Phase I led to Phases II and III, which continued the development of high-flux materials, and the design and construction of a 5 TPD Subscale Engineering Prototype (SEP) pilot plant to verify the performance of commercial-scale modules. Currently, APCI is operating a 5 TPD SEP unit at its Sparrows Point gas plant, and has demonstrated that the unit can be operated under full driving-force conditions, meet/exceed wafer performance for flux and purity, and that it can be cycled from idle to operating conditions without loss of performance. Design and construction of a 100-tpd SEP unit has begun.
Subscale Engineering Prototype (SEP) ITM Test unit at APCI's Sparrows Point gas plant.
Since ITMs are thermally activated, the basic process cycle must include heating the pressurized air feed to high operating temperatures, either by indirect heat exchange or direct firing. APCI has performed studies to integrate the ITM module into a gas turbine power cycle to produce oxygen, power and/or steam, and concluded that integration is practically feasible to achieve acceptable cycle efficiency. The energy associated with the hot, pressurized, non-permeate stream can be recovered.
Partial or minimum integration is also feasible to design a "stand-alone" ITM system with reduced power co-production. It potentially offers good early entry prospects for ITM technology.
Air Products and Chemicals, Inc. compared the economics of ITM against a state-of-the-art cryogenic ASU and projected that ITM would decrease the installed capital cost of air separation equipment by 35%. This translates to a 7% decrease in the installed capital cost of an IGCC plant and a 1% increase in efficiency.
APCI and DOE are planning to extend work in ITM development, with the following activities occurring from present to September 2015:
- Conduct preliminary testing of 1 TPD modules in the ISTU to establish module performance.
- Design, build and test a 100 TPD test system with at least thirty 1 TPD ITM modules (construction currently continuing).
- Measure flux and purity performance of ceramic ITM modules designed for use in advanced energy systems and industrial systems with low carbon emissions.
- Develop project-quality cost estimations for a 2000 TPD Test Unit that will meet requirements for a test facility that addresses technical risk to enable a demonstration of the technology at large scale.
For further information see the NETL Gasification Projects page about the Air Products ITM Oxygen project.