ETD system

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Tesi etd-09152017-163019

Thesis type
Tesi di laurea magistrale
Study on the influence of representative Tar and H2S on SOFC performance
Corso di studi
relatore Desideri, Umberto
correlatore Purushothaman Vellayani, Aravind
correlatore Cavalli, Alessandro
Parole chiave
  • SOFC
  • contaminants
Data inizio appello
Riassunto analitico
The increasing demand of energy and the increasing concern about global warming has more and more drawn the attention on the use of renewable sources of energy.
An interesting solution can be the integration of biomass gasifiers with Solid oxide fuel cells (SOFCs). Indeed, these systems allow to achieve high conversion efficiency of biomass to electrical energy. In addition, high quality heat is available for further applications, because SOFCs operate at high temperatures and the processes that take place in them are overall exothermic.
However, solid oxide fuel cells are generally very sensitive to impurities contained in syngas. For this reason, a gas cleaning unit is present between the gasifier and the SOFC. In particular, employing a hot gas clean-up, the overall thermal efficiency of system is greater, because cooling and re-heating steps typical of low temperature gas cleaning are avoided.
In this work an integrated system with an updraft gasifier is considered. This type of gasifiers has several advantages in comparison to the other types: a greater thermal efficiency, it can handle biomass with high moisture content, syngas entrains a lower quantity of particulate matter and has a quite good LHV. On the other hand, a greater amount of tar is produced in an updraft gasifier. However, if SOFC internal tar reforming is feasible, a higher content of this contaminant will lead to a higher amount of available fuel in the cell and, thus, this will be an advantage. Moreover, the endothermic tar reforming reaction can contribute to reducing the excess air necessary to cool down the cell, thus increasing further the system efficiency.
The aims of this thesis are:
• studying the feasibility of tar internal reforming,
• studying the effects of both hydrogen sulfide and tar on SOFC.
• investigating the tolerance limits of SOFCs for tar and hydrogen sulfide,
Solid oxide fuel cells have a suitable environment for tar reforming: in the anode chamber there are catalyst, high temperature, steam and/or CO2. Moreover, energy is available to carry out reforming, which is an endothermic reaction for most of compounds. If tar reforming can be done in the cell, tar reformer will be not needed or, at least, there will be a smaller one or cheaper materials will be used. Moreover, less energy will be required. In this way, a higher system efficiency can be achieve.
Although, tolerance limit is already known for hydrogen sulfide, this are investigated in this work because they depend widely on the operating conditions, such as fuel composition, temperature, current load, anode materials. Indeed, the found tolerance limit is strictly associated to the GCU operating temperature. The higher the tolerance limit, the higher the operating temperature of the GCU. A higher operating temperature of the GCU might result is a higher efficiency for the whole system.
In order to investigate these points, four sets of experiments were carried out. In the first one the cell was exposed to clean syngas to have a reference for the following tests. In the second set of experiments, the effects of several concentrations of acetic acid (20 – 150 g/Nm3) were studied. During the third one, 0.8 ppm(v) and 1.3 ppm(v) of hydrogen sulfide were added to syngas stream. At the end, synergistic effects of tar and H2S on cell performance and the effect of hydrogen sulfide on acetic acid reforming were investigated.
All these sets of tests were carried out in the same operating conditions: at 800°C and under a current density of 68 mA/cm2. The employed SOFCs were electrolyte-supported with Ni-GDC anodes. The cells were exposed to each concentration of contaminants for 24 h. When cells performance were affected by these contaminants, a recovery step was carried out.
The effects of these contaminants on the cell were evaluated by i-V curves, monitoring the cell voltage, tar sampling and micro-GC.
It was found that acetic acid was very reactive and reformed in the cell. Carbon deposition due to homogeneous decomposition of acetic acid seemed to occur because solid carbon was observe in the anode inlet ceramic pipe. Micro-GC data suggested that this tar species may produce CH4 from its decomposition. Hydrogen sulfide affected widely the cell performance, methane steam reforming and water-gas shift reaction. Sulfur poisoning due to both H2S concentrations seemed to be partially reversible. These behaviours were confirmed also in the last set of experiments.