Optimization of the operation of a complex combined-cycle cogeneration plant using a professional process simulator

Abstract

Thermoeconomic optimization is a relatively modern technique to analyze and design more efficient energy systems, leading to a better compromise between energetic efficiency and cost. Thermoeconomic optimization can be parametric (plant configuration is fixed), applicable both at the design phase or the operation phase of an energy system, or structural (plant configuration may vary). In practice, mathematical thermoeconomic optimization may be accomplished in two ways: (i) the conventional way, which manipulates all pertinent equations simultaneously, or (ii) integrated with a professional process simulator, such that the equations are manipulated separately. In the latter case, the simulator deals with the thermodynamic property and balance equations, while an external optimization routine, linked to the simulator, deals with the economic equations and objective function. In this work a previous implementation of an integrated approach for parametric mathematical thermoeconomic optimization of complex thermal systems is applied to an actual combined-cycle cogeneration plant located in the outskirts of the city of Rio de Janeiro, in Brazil. The simulator is the Thermoflex software, which interfaces with the MS-Excel program. Thus, the Powell's method for optimization integrated with Thermoflex is written in the VBA language. The plant contains more than 60 thermal components, including two gas turbines, one steam turbine, and two heat recovery steam generators. Simulation of one operational condition of the plant requires several hundred variables. The plant produces nominally 380 MW of power, and exports a mass flow rate between 200 and 400 ton/h of superheated process steam, at 45 bar and 404°C, to a neighboring refinery. The cogeneration plant operates subjected to an economic scenario, which changes with time, because of varying fuel, electricity, and steam prices. Therefore, to manage the plant, it is important to know the minimum operational cost, when a fixed contracted hourly-rate oi process steam has to he exported, while a variable amount of electrical power is produced. An optimization problem can thus he formulated, for which the objective is to minimize the cost of consumed resources per unit electrical power generated. Results of optimization exercises to determine the optimal operational conditions of the plant for various exported mass flow rates of process steam are presented and discussed. Copyright © 2008 by ASME.