(143a) Optimal Integrated Design of Air Separation Unit and Gas Turbine Block for IGCC Systems

The Integrated Gasification Combined Cycle (IGCC) systems are considered as a promising technology for power generation. However, they are not yet in widespread commercial use and opportunities remain to improve system feasibility and profitability via improved process integration. This work focuses on the integrated design of gasification system, air separation unit (ASU) and the gas turbine (GT) block. The ASU supplies oxygen to the gasification system and it can also supply nitrogen (if required as a diluent) to the gas turbine block with minimal incremental cost. Since both GT and the ASU require a source of compressed air, integrating the air requirement of these units is a logical starting point for facility optimization (Smith et al., 1997). Air extraction from the GT can reduce or avoid the compression cost in the ASU and the nitrogen injection can reduce NOx emissions and promote trouble-free operation of the GT block (Wimer et al., 2006).

There are several possible degrees of integration between the ASU and the GT (Smith and Klosek, 2001). In the case of ?total' integration, where all the air required for the ASU is supplied by the GT compressor and the ASU is expected to be an elevated-pressure (EP) type. Alternatively, the ASU can be ?stand alone' without any integration with the GT. In this case, the ASU operates at low pressure (LP), with its own air compressor delivering air to the cryogenic process at the minimum energy cost. Here, nitrogen may or may not be injected because of the energy penalty issue and instead, syngas humidification may be preferred. A design, which is intermediate between these two cases, involves partial supply of air by the gas turbine and the remainder by a separate air compressor. These integration schemes have been utilized in some IGCC projects. Examples include Nuon Power Plant at Buggenum, Netherlands (both air and nitrogen integration), Polk Power Station at Tampa, US (nitrogen-only integration) and LGTI at Plaquemine, US (stand-alone). However, there is very little information on systematic assessment of air extraction, nitrogen injection and configuration and operating conditions of the ASU and it is not clear which scheme is optimal for a given IGCC application.

In this work, we address the above mentioned problem systematically using mixed-integer optimization. This approach allows the use of various objectives such as minimizing the investment and operating cost or SOx and NOx emissions, maximizing power output or overall efficiency or a weighted combination of these factors. A superstructure is proposed which incorporates all the integration schemes described above. Simplified models for ASU, gas turbine system and steam cycle are used which provide reasonable estimates for performance and cost (Frey and Zhu, 2006). The optimal structural configuration and operating conditions are presented for several case studies and it is observed that the optimal solution changes significantly depending on the specified objective.

References

Frey, H. C. and Zhu, Y. (2006) Improved System Integration for Integrated Gasification Combined Cycle (IGCC) Systems. Environ. Sci. Technol., 40, 1693-1699.

Smith, A. R., Klosek, J. and Woodward, D. W. (1997) Next-generation integration concepts for air separation units and gas turbines. J. Eng. Gas Turbines Power, 119, 298-304.

Smith, A. R. and Klosek, J. (2001). A review of air separation technologies and their integration with energy conversion processes. Fuel Processing Technology, 70, 115-134.

Wimer, J. G., Keairns, D., Parsons, E. L., and Ruether, J. A. (2006). Integration of Gas Turbines Adapted for Syngas Fuel With Cryogenic and Membrane-Based Air Separation Units: Issues to Consider for System Studies. J. Eng. Gas Turbines Power, 128, 271-280.