Conclusions
A pilot scale 2.8 kWe SOFC unit (two 1.4 kWe-stack in parallel
with 60 electrolyte-supported cells each) was powered with
cleaned sewage biogas for around 700 h in a Wastewater Treatment
Plant in Spain. Biogas reforming conditions were set at an O/C ratio
of 2 (through steam addition) and a reforming temperature of
550 C to avoid soot formation and guarantee long-term fuel cell
operation. On the other hand, the SOFC stack was operated at
800 C and at a constant voltage of 43 V (0.7 V per cell).
At optimized conditions for electrical power production satisfying heat demand in the WWTP, system electrical and thermal
efficiencies accounted for 34% and 28%; and the heat-to-power
ratio was 0.8. Although stack electrical efficiencies of 52e53%
were obtained at fuel utilisations of 75e77%, biogas use in the afterburner was required to achieve thermal self-sufficiency; which
reduced system electrical efficiency. Moreover, cogeneration effi-
ciency remained constant at around 59e62% for all the heat-topower ratios tested. The obtained efficiency levels are lower
compared to simulation-based performances, which highlights the
necessity for more pilot experimentation at this scale (rather than
simulations) to overcome the barriers for SOFC technology
deployment in WWTP. Future works should focus on the optimization of the system by improving the thermal integration of the
unit and the reforming conditions to allow operation at lower heatto-power ratios and at reduced thermal demand.
Finally, the selected biogas treatment system combining biological desulphurisation and deep cleaning through adsorption
proved to be to be suitable and reliable solution for fuel cell applications. However, as experiments at different oxygen levels
showed, the biotrickling filter caused biogas dilution, increasing the
oxygen and nitrogen contents in the treated gas; which had a
negative effect on fuel cell electrical performance. As a result, bioscrubbers (or other scrubbing technologies not injecting oxygen
in the biogas) followed by adsorption would be recommended for
fuel cell applications.