• SOFC
• solid oxide fuel cell
• updraft gasifier
• HCl
• tar
• acetic acid
• cell degradation
• synergistic effect
• biomass gasification

Biomass – Integrated gasification fuel cell (B – IGFC) systems represent an interesting field of research due to their high overall efficiency and low emissions. The system studied is composed by a biomass updraft gasifier, a high temperature gas-cleaning unit (GCU) and a solid oxide fuel cell (SOFC). The aim of this thesis is the improvement of a B – IGFC system, through higher efficiency and lower costs, by acting mainly on the GCU operating temperature and components. This study is focussed on the possibility of increase the HCl reactor temperature and reduce the costs of the tar reformer; in order to do so, the downstream SOFC have to tolerate larger concentration of HCl and tar into the syngas stream.
The experimental campaign concerned three set of tests in which a Ni-GDC cell was fed with syngas and different concentrations of the contaminants, HCl and tar, separately or together. The model tar chosen was acetic acid, because it is the most abundant species in the updraft gasifier produced gasses. The concentrations (dry basis) tested were 17 – 41 – 82 – 128 g/Nm3 for tar and 3,4 – 20 – 50 ppm for hydrogen chloride. Furthermore, the synergistic effects of both contaminants together were examined, testing a constant tar content of 41 g/Nm3 and an increase concentration of HCl equal to 3,4 – 20 – 50 ppm.
Further objectives of this study were to determine the feasibility of internal acetic acid reforming, evaluate the effect of hydrogen chloride on cell performance and on tar reforming.
All the experiments were carried out under a constant current density of 68 mA/cm2, at about 800°C and feeding the cell with simulated wet syngas (9,5% H2; 12,6% CO; 9,5% CO2; 1,3% CH4; 30,3% N2; 36,8% H2O).
The results revealed that tar injection led to higher immediate cell performance due to internal tar reforming, lower degradation rates and it has been demonstrated that the cell exposed to 128 g/Nm3 of acetic acid had stable operation, for overall two days of test. Thus, since the cell can internally reform the whole load of tar produced by the updraft gasifier, the tar reformer can be removed from the system.
The HCl set of test shown that the tolerance limit in the tested conditions was 3,4 ppm, indeed, the higher concentrations resulted in voltage drop and higher cell degradation rates. This outcome lead to two possibilities: a) increase the HCl reactor temperature up to 600°C, which implies higher thermal efficiency and a lower water content in the syngas needed to avoid carbon deposition; or b) remove the HCl clean-up step, but then limiting the choice of the feedstock to biomass with low chlorine content, such as paper residue sludge or cacao shells. In addition, lower H2O content into syngas stream means higher cell performance due to the higher fuel concentrations and lower cost, derived for the evaporation of water and the system for the injection of steam.
Once the two contaminants were fed together into the SOFC, it was observed positive synergistic effect, which led to lower cell degradation rates in comparison with the set of test with HCl only.
In conclusion, the aim of this thesis has been achieved through a redefinition of the tolerance limits for tar and HCl, inside a SOFC, allowing the removal of the external tar reformer and an increase in HCl reactor temperature or the possibility of remove it, when feedstock with low chlorine content are used.