Oxygen

Commercial Technologies for Oxygen Production

Gasification processes require an oxidant, most commonly oxygen; less frequently air or just steam may suffice as the gasification agent depending on the process. Oxygen-blown systems have the advantage of minimizing the size of the gasification reactor and its auxiliary process systems. However, the oxygen for the process must be separated from the atmosphere. Commercial large-scale air separation plants are based on cryogenic distillation technology, capable of supplying oxygen at high purity1 and pressure. This technology is well understood, having been in practice for over 75 years. Cryogenic air separation is recognized for its reliability, and it can be designed for high capacity (up to 5,000 tons per day).

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 exchange 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
A typical ASU flow diagram
   
  Aerial view of a typical ASU plant
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 
The air separation 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. 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:

Company

Plant

Location

Product

ASU Provider

Tampa Electric

Polk Station

Tampa, FL

252 MW electricity

Air Products

SG Solutions/
PSI Energy

Wabash River

Terre Haute, IN

262 MW electricity

Air Liquide

Nuon

Demkolec

Buggenum, Netherlands

250 MW electricity

Air Products

Eastman Chemical

Chemicals from Coal

Kingsport, TN

Chemicals

Air Products

Farmland

Fertilizer Plant

Coffeyville, KS

Fertilizer

BOC Gases


1. While a 95% oxygen purity is believed to be the economic optimum for integrated gasification combined cycle (IGCC) applications, a higher purity oxygen is usually required for the production of clean fuels and chemicals.


Oxygen

 

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