(476d) Modeling And Control Of Air Separations Units For Igcc Power Plants

Integrated Gasification Combined Cycle (IGCC) plants represents one of the most promising options for processing fossil fuels, such as coal or heavy refinery residues, to meet future CO₂ trading and more stringent environment regulations for NO_x, SO₂ or mercury emissions. These plants, depending on the degree of integration, extract some or all of the air demanded by the air separation unit from a gas turbine compressor that also supplies air to the gas turbine. Different responses from the interacting units can cause undesired mass flow fluctuations within the system, especially during changing load demands. These complicate the prediction of operational behavior and require the application of dynamic plant simulations.

In this study, a dynamic model (using ASPEN DYNAMICS) for a heat-integrated double column air separation unit has been developed to investigate feasibility and applicability of constrained multivariable control algorithms. The preliminary ?flow-driven? model has been extended to a ?pressure-driven? simulation model to provide a better understanding of equipment level constraints also able to describe the pressure dynamics responsible for mass-flow fluctuations. Initially, model based PID controllers were implemented for desired product purities and column pressures.

The process variables showed large amount of interaction, which is responsible for the difficulties with the commonly used PID-based control scheme. In this paper we present a detailed operability analysis to better understand inherent performance limitations. Further, we develop a model predictive control strategy that handles rate-of-change constraints imposed by the process design of the air separations unit. Finally, we close with a discussion of future work on the simulation and control of an entire IGCC power plant.