Air Separation

Providing oxygen to a gasification plant results in significant initial capital and operating costs. The air separation unit (ASU) can account for up to 15% of the total gasification plant cost, and it consumes a major portion of the total in-plant power use. The technology of choice is predominately based on cryogenic distillation, which has been in practice for over 75 years. The market is dominated by a small number of highly competitive companies who are willing to offer lump-sum-turnkey systems for a project or even build and operate a plant near the client’s project site and supply the needed oxygen ‘over the fence’ under a long-term contract.

Other oxygen supply technologies, such as pressure swing adsorption (PSA) and polymeric membrane (for more see ion-transport membranes [ITM’s]) are available, but they are in general either on a much smaller scale or cannot provide oxygen at a high enough purity (>95%) for gasification. Cryogenic distillation is the predominant air separation technology for high-throughput, high-purity oxygen demands, such as gasification.

Cryogenic ASU
Cryogenic distillation separates oxygen from air by liquefying air at very low temperatures (-300°F).  Ambient air is compressed in multiple stages with inter-stage cooling then further cooled with chilled water.  Residual water vapor, carbon dioxide, and atmospheric contaminants are removed in molecular sieve adsorbers. Cooling to cryogenic temperatures is achieved by heat exchanger with product gases as well as after-coolers and expanders.  The air then enters the “cold box,” which contains a distillation column with many stages, and an argon column for additional oxygen purification.

Oxygen and nitrogen products are warmed by heat exchange with the cold box feeds and pressurized by compressors to the final delivery pressure.  Alternatively, products may be pressurized by small boost compressors.  Oxygen storage may be advisable to ensure steady gasifier operation through periods of high oxygen demand.

Nitrogen may be released at low pressure to atmosphere, or compressed to high pressure and used as a byproduct or as a diluent for a syngas/hydrogen fired turbine. An elevated-pressure nitrogen stream may be useful in integrating the ASU with the gasification plant. ASUs can also be used to separate other useful industrial gases like argon, neon, and krypton.

A typical ASU flow diagram

	Aerial view of a typical ASU plant

Applications & Integration
The ASU may be integrated with the power island, within the gasification complex, to improve its overall efficiency and reduce costs.  The NUON IGCC plant, at Buggenum Netherlands, for example, was designed with such integration.  Elements of integration, for example, include:

• Extracting air from the gas turbine for use in the ASU,
• Returning the compressed nitrogen from the ASU to the gas turbine combustor,
• Using power produced from the turbine generator to operate the electric motor-driven compressors in the ASU.

These types of integration, however, do add complexity to the design and operation of the gasification plant.

Major Technology Providers
Providers of large-scale ASUs include, but are not limited to: Liquid Air Engineering, Airco, Air Products and Chemicals, Praxair, BOC Gases, Air Liquide, and Linde.  The following table lists examples of a few gasification plants and their respective ASU providers:

New Technology Development
Air separation is an integrated, but costly, part of a gasification plant.  As such, there is great incentive to improve its efficiency and cost.  DOE/NETL is currently supporting efforts to develop new ion-transport membranes as an alternative to cryogenic distillation technology for air separation.

Return to Supporting Technologies