Development and experimental validation of CPOx reforming dynamic model for fault detection and isolation in SOFC systems

Abstract

In the present work an investigation of the reforming technologies available for Solid Oxide Fuel Cell (SOFC) systems and their basic concepts has been carried out, with the aim to describe, test and simulate the reforming process for fault diagnosis application. The final aim of a fault diagnosis activity for SOFC systems is to reach the required criteria for a commercial application, which, besides long lifetime and performance, include high reliability and safety at reasonable costs. The achievement of these targets is necessary to contribute promoting the SOFC technology and finally starting a mass production phase. In this thesis, the attention has been focused on the reforming reactor, responsible for the conversion of the inlet fuel in hydrogen, suitable source fuel for the SOFC. In particular, the Catalytic Partial Oxidation (CPOx) process has been analyzed. The CPOx reforming mechanism is the most attractive technology for the production of syngas or hydrogen in small-medium scale SOFC applications and Micro Combined Heat and Power (μCHP) systems. This is due to the ability of the CPOx reaction to be carried out in compact reactors with rapid dynamic response and with low heat capacity. The reaction is slightly exothermic and therefore does not require external heat to take place. In addition, CPOx technology does not require steam, as the media required for the reforming reaction is air, which is easily available for residential application. This mainly means that CPOx is independent from an external water source and any heating source. The hydrocarbon is both oxidized to CO2 and H2O, either partially or completely, and also converted to synthesis gas by endothermic steam reforming (according to the indirect CPOx mechanism). Despite these advantages, catalytic partial oxidation is less efficient than steam reforming. This indicates that it is most suitable for applications in which the system simplicity has the priority with respect to the hydrogen yield. The high surface temperatures can cause a local loss of activity of the catalyst, leading to the instable performance of the entire reactor. Nevertheless, in the CPOx process even a small difference in the operating air and fuel flow rates could lead to carbon deposition or oxidation of the catalyst, with serious consequences for the SOFC system and for the stack itself. It is therefore extremely important to develop a diagnosis tool able to investigate these phenomena and to detect and isolate the faults that may verify inside the reactor. The most common fault events likely to occur inside a CPOx reformer for SOFC applications have been analyzed through a Failure Mode and Effect Analysis (FMEA) and a Fault Tree Analysis (FTA). These analyses are aimed at identifying the main events responsible for the catalyst deactivation, together with their causes and effects on the SOFC system performance. The Catalytic Partial Oxidation mechanism has then been explored from both modelling and experimental points of view, with the aim to simulate the reforming process and identifying the thermodynamic optimal operating conditions at which natural gas may be converted to hydrogen. At the same time, the main fault scenarios likely to occur during the reforming phase have been analyzed, both in experiments and during simulations, to evaluate the capability of the developed model in performing effective fault detection and isolation for on-board diagnostic application. The CPOx dynamic model developed is based on the minimization of Gibbs free energy and can be easily reconfigured for describing a steam reforming mechanism. The simulation results give useful indication on how operating parameters such as the input conditions of reactants (inlet compositions and temperature) affect the reaction equilibrium and, in turn, the products composition and reactor outlet temperature. A sensitivity analysis for different operating conditions has been carried out. The transient behavior of the reforming reaction and the information about methane conversion and hydrogen selectivity complete the set of model results. The dynamic CPOx model has been validated through experimental data and its behavior during transients has been carefully analyzed during the variations in the set-points of operating phases. Both test data and reactor design were part of the activities performed within the EFESO project, funded by the Italian Ministry of Economic Development and led by Ariston Thermo Spa. The model results demonstrate that the CPOx dynamic model represents a useful tool for fault diagnosis application and its results provide an interesting benchmark for the design and working parameters of a CPOx reforming system for SOFC application. [edited by author]