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Optimal Integrated Design of Air Separation Unit and Gas
Turbine Block for IGCC Systems

Monday, November 9, 2009: 3:15 PM

Lincoln D (Gaylord Opryland Hotel)

Ravindra S. Kamath, Department of Chemical Engineering, Carnegie Mellon
University, Pittsburgh, PA

Ignacio E. Grossmann, Department of Chemical Engineering, Carnegie Mellon
University, Pittsburgh, PA

Lorenz T. Biegler, Department of Chemical Engineering, Carnegie Mellon
University, Pittsburgh, PA

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.

Extended Abstract: File Not Uploaded

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