CHOICE OF OXIDANT
All gasification processes require an oxidant for the partial oxidation reaction. There are essentially two alternatives: air, which is available in unlimited quantities at the location of the gasifier; and oxygen, which has to be separated from the nitrogen in the air at considerable cost. A third alternative, oxygen-enriched air, is essentially a mixture of the two.
Historically, the first continuous partial oxidation systems, producer gas generators, operated with air. The idea of operating with pure oxygen was already developed in the 1890s, but it was only realized in the 1930s after the introduction of large-scale commercial cryogenic oxygen plants. Since then most gasification plants have operated with high purity (>90mol% 02) oxygen. To a large extent this has been dictated by the fact that in the period between 1935 and 1985, most gasifiers were built for chemical applications where the presence of large quantities of nitrogen originating from the air was detrimental to the downstream synthesis process. (Note that this also applies to ammonia, where only about 25-30% of the nitrogen associated with the oxygen used in the gasifier is required for the synthesis.)
These considerations of downstream chemistry do not apply to power applications, which have developed along with the increasing efficiencies of gas turbines, so that it was necessary to review the pros and cons of air versus oxygen for these applications. The result of such reviews in individual cases has been a decision in favor of oxygen in practically all large-scale projects. For small-scale projects (<50MWe), mostly operating with biomass or waste, the decisions have tended to favor air. It is therefore useful to understand the basic issues behind the choice of oxidant.