Selection of Control Structure of Elevated Pressure Air Separation Unit in an IGCC Power Plant for an Economical Operation

IGCC (Integrated Gasification Combined Cycle) is an alternative power generation system that can utilize fossil fuels in a more eco-friendly way than the conventional pulverized coal-fired plant. An IGCC plant requires an (Elevated Pressure) Air Separation Unit (EP ASU) that separates the air into pure oxygen and nitrogen, to be sent to the gasifier and the gas turbine, respectively. The ASU consumes about 10% of the gross power output generated in IGCC, so an economical operation of the ASU is important for lowering the overall power generation cost. In this research, controlled variable selection for an EP ASU is studied from the viewpoint of economics, i.e., with the objective of maintaining an economically (near-)optimal operation in the presence of load changes. Instead of the full-scale real-time optimization (RTO), we adopt a simpler approach known as self-optimizing control (SOC), which attempts to achieve the objective through a systematic selection of controlled variables. For the purpose of designing and testing a self-optimizing control structure, equation-based modeling of EP ASU is carried out using the software platform of gPROMS. Then, the SOC approach is applied based on the model to select the best set of controlled variables, which will lead to the most economical operation in the presence of load changes. Finally, PI control loops are designed and their dynamic control performances are tested. In addition, the economic loss in the presence of load changes is analyzed and compared with that achievable from the use of RTO.