(280a) Analysis of An Air Separation Unit as Part of An IGCC Power Plant

The air separation unit (ASU) plays a key role in improving the efficiency, availability, and operability of an oxygen fed integrated gasification combined cycle (IGCC) power plant. An improved integration between the ASU and the balance of the plant, especially the gasifier, and the gas turbine (GT) has significant potential for enhancing the overall plant efficiency. Considering the higher operating pressure of the GT, an elevated-pressure air separation unit (EP ASU) is usually favored instead of the conventional low-pressure air separation units (LP ASU). This process is generally chosen when most, or all, of the nitrogen produced is used as a fuel diluent for the gas turbine. In addition, if air is extracted from the GT to be used as an air feed to the ASU, it is generally desired to have the operating pressure of the ASU close to that of the GT in order to maximize the benefit of the high-pressure air. Due to the decrease in separation efficiency with increasing pressure and because of stringent purity requirements, for both oxygen and nitrogen, considerable modifications to the existing LP ASU technology is required. A detailed study on various possible configurations of an EP ASU and their impacts on the overall process efficiency are scarce in the open literature. Some configurations have been patented which are found to be very power intensive. In this study, a number of possible configurations has been considered with an objective of reducing the auxiliary power requirement. Plant operability is also considered with each of the configurations. It has been suggested in the open literature that in order to maximize the benefit of the high-pressure air available by air extraction an EP ASU should be used so as to increase the savings from the decreased load on the main air compressors (MAC). No study has been found within the open literature investigating whether equivalent savings are possible from an LP ASU with a pumped liquid oxygen cycle at varying degrees of integration. Studies carried out here have shown that an LP ASU with a pumped liquid oxygen cycle has nearly equivalent power savings as that of an EP ASU up to approximately 60 - 70% integration. A study was also completed for determining the optimal operating pressure of the ASU considering the degree of integration, and fraction of nitrogen injection. Results of this study will also be discussed.