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, there is great interest in 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. To illustrate, a representative estimate comparing an ITM-based unit against a state-of-the-art cryogenic ASU 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.
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.
Since ITMs are thermally activated (i.e. requiring the relatively high operating temperatures for dissociation and transfer of oxygen ions), the basic process cycle must include heating the pressurized air feed either by indirect heat exchange or direct firing. Suitable integration of the ITM module into a power cycle to produce oxygen, power and/or steam is warranted to achieve acceptable cycle efficiency. The energy associated with the hot, pressurized, non-permeate stream can be recovered by a gas turbine for power generation and an optional steam turbine for low-level heat recovery. These principles of air compression, heating, expansion, and heat recovery associated with integration of an ITM within a power cycle are illustrated below.