Exergy Calculation Modelling Tool for Mixtures in Power Generation: Application to WGS and ASU units of an IGCC Power Plant with Pre-combustion CO2 Capture

Abstract

Innovative power plant concepts that utilize gaseous fossil fuels is the subject of interest for many researchers in the last decade. Literature studies focus on simulation and thermal analysis of such plants, regarding the net electrical efficiency, expressed as % of the low heating value of the coal input, and the GHG emission levels, measured in kg of CO2/MWh as the key performance indicators of the plant from an energy and environmental perspective. In order to make a sound analysis of thermal processesthe concept ofphysical exergy is introduced, in such a way that it is possible to evaluate the losses that take place in each section of the plant from an energy quality perspective, acknowledging that not all the thermal input from the fuel can be transformed into useful work. Additionally, the process streams undergo composition changes that make the determination of chemical exergy mandatory, accountingfor composition imbalances with the reference environment. The calculation of chemical exergy as presented in Thermodynamics books requires the definition of hypothetical states for different substances as well as the assumption of ideal mixing laws. Moreover, tabulated values of the change in standard Gibbs free energy for devaluation reactions of the components in the mixture are necessary. In order to simplify such calculations, a model in Unisim Design R431 was created allowing the determination of the total exergy of a stream by accessing its specific enthalpy and entropy given by a predefined thermodynamic property package. Thus, the chemical exergy of a stream at environmental conditions is identified with the change of the Gibbs free energy of a system that chemically transforms the components in the stream withdrawing the stoichiometric co-reactants from the environment and yielding the products present in the environmentat the same conditions. If the components in the mixture do not undergo a chemical transformation, the chemical exergy change that takes place in the system only accounts for mixture separation (gM) and the reversible expansion or compression of such components to reach the same chemical potential (i.e. partial pressure) as in the environment. The system that achieves such transformations is composed of ideal internal devices that are not technologically feasible (isothermal compression, isentropic expanders, ideal membranes etc.) However, with basic unit operation models (heaters, conversion reactors, splitters, expanders etc.) this ideal system can be effectively represented.This simple modelling tool allows researchers to effectively incorporate exergy in their power plant evaluations and familiarizes students with the concept making it more accessible and comprehensive. In this work, the exergy model is used to evaluate the performance of an Air Separation Unit (ASU) and a Water Gas Shift (WGS) section of an IGCC plant.