Modelado, análisis y control dinámico de un reformador de etanol en tres etapas para aplicaciones en pilas de combustible

Abstract

This thesis presents results related to the design, modeling and control of a low temperature ethanol reformer as a source of hydrogen designed to feed a fuel cell (PEMFC). As fuel, ethanol has been selected as one of the renewable energy sources currently under study due to its interest for scientific and economic fields. The reformer has been separated into three stages in order to guarantee the maximum production of H2 and the minimum production of CO. SnO2 and Co(Fe)Na+/ZnO catalytic monoliths have been used, where in the first stage ethanol is dehydrogenated over SnO2 and, in the second stage, the reforming of acetaldehyde over Co(Fe)Na+/ZnO takes place under the presence of hydrogen, generated in stage 1. Then we add a third stage with a Fe2O3-Cr2O3 commercial catalyst to complete the WGS reaction. In addition to ensure a reducing environment in the reforming step, this configuration also allows optimizing the conditions of each stage of the reaction. Constant pressure has been imposed as a condition of the reformer design, but the variation of temperatures in each stage has been considered. In this thesis dynamic models have been developed for the reformer of ethanol with vapour at low temperatures described previously, the stationary and dynamic response has been analysed and controllers have been designed to meet the requirements of the system.

Up until now there have only been a few works that address the design and implementation of controllers for fuel reformers, and none of these have used ethanol fuel. Some are in the area of automatic control and have been applied to tubular reformers but consider other type of fuel, not ethanol, for its alimentation In this thesis the mathematical model used to represent the state of the monolithic reformer has been presented, considered and implemented. Through a study of the stationary state one can validate the different hypothesis of assumed design and through an analysis of the parametric sensitivity the operational conditions for the reform process are discussed.

This way the mathematic model of the reactor can be used to explore other alternatives for the operation of the reformer putting an emphasis on the influence of temperature in the second stage as this is a variable. Afterwards a study is made of the control of the models based on the balance of mass and linearised energy. In order to realise the control study the mathematic tools RGA, MRI and CN have been used. Through this analysis the control structures have been selected which will be most suitable for design controllers which can guarantee the requirements for the fuel cell.