ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-02042020-164202


Tipo di tesi
Tesi di laurea magistrale
Autore
BOSI, ALESSIO
URN
etd-02042020-164202
Titolo
An Experimental Study to Detect Carbon Deposition in Solid Oxide Fuel Cells
Dipartimento
INGEGNERIA DELL'ENERGIA, DEI SISTEMI, DEL TERRITORIO E DELLE COSTRUZIONI
Corso di studi
INGEGNERIA ENERGETICA
Relatori
relatore Prof. Desideri, Umberto
supervisore Prof. Purushothaman Vellayani, Aravind
tutor Ing. Stam, Jelle
Parole chiave
  • electrochemical impedance spectroscopy
  • distribution of relaxation times
  • direct internal reforming
  • carbon deposition
  • SOFCs
Data inizio appello
27/02/2020
Consultabilità
Completa
Riassunto
Solid oxide fuel cells (SOFCs) are electrochemical devices that convert chemical energy stored in fuels in electrical power, with high efficiencies and without a significant environmental impact. Since their elevated operating temperature, combined with the ability to process carbon monoxide and the resistance to poisoning by impurities present in the fuels, SOFCs can directly process hydrocarbon fuels. The use of nickel as catalyst at the anode leads the possibility to convert fuels within the cell, generally to carbon monoxide and hydrogen, with a process called direct internal reforming (DIR), in which steam is mainly used as reforming agent. This aspect leads to several advantages of SOFCs over low temperature fuel cells: internal reforming significantly increases the overall efficiency for hydrocarbon fuelled system, by reducing the need for cooling and the need for external reformers, which significantly influence complexity and cost of the system. Although the clear benefits, DIR also has some drawbacks and operating problems: one of the great challenges is carbon deposition on the anode, which primarily occurs due to the propensity of nickel to promote the hydrocarbons cracking. This process is commonly avoided by adding an abundance of steam, but this leads to a decrease in the power density due to fuel dilution. In order to detect carbon deposition, which can block the anode pores and eventually leads to the cell cracking, several experiments have been performed in this study: electrolyte-supported button SOFCs have been fuelled with different simulated reformed hydrocarbon fuels, monitoring the cells behaviour with electrochemical impedance spectroscopy (EIS). Furthermore, an impedance deconvolution technique called distribution of relaxation times (DRT) has been applied as a useful method for interpreting EIS data. Due to the observation made on different SOFCs, it is possible to state that carbon deposition likely occurred during the experiments performed on cells operating in open-circuit voltage (OCV) at T=800 [°C], and fuelled with a H_2-〖CH〗_4 dry mixture with a methane concentration equals to 25 [vol%]. Post-mortem surface analysis of the cells is required to confirm the observations and the possibility to extract the influence of carbon deposition from EIS measurements, which could be used for diagnostic and to validate carbon deposition models, that can lead to a better prediction.
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