**4. Conclusions**

Steam gasification tests with lignite were carried out in a bench-scale fluidized-bed gasifier, in order to study the quality of the gas produced at di fferent operating conditions. The correct operation of the gasification process with small lignite particles as feedstock (~44 μm) was possible thanks to the ceramic filter candle integrated in the freeboard of the gasifier, which prevented the entrainment of particles outside the reactor. The bed material used was olivine, and the Steam/ Fuel ratio (S/F) was kept at approximately 0.65. The effect of the temperature and air injections in the freeboard was investigated in terms of the gas composition and tar produced.

The results obtained showed that the increase of the operating temperature caused an improvement of the gas quality, in particular, higher conversion rates and gas yields, and a lower amount of tar produced. Tests carried out with air injections in the freeboard did not show the desired effect of tar reduction, probably because the combustion of part of the syngas did not cause the increase of temperature expected in the externally heated bench-scale gasifier used in this work. However, the amount of tar produced was smaller compared to other tests carried out with biomass at similar operating conditions in the same bench-scale gasifier. Lignite could be less prone to tar production because of its lower volatile content compared to lignocellulosic biomass.

The ashes produced during the gasification test were analysed with XRD and SEM/EDS analysis, and an affinity between Ca and S was noticed, probably indicating the capacity of Ca to retain S in the ashes.

Pressure fluctuations were acquired in the freeboard of the fluidized bed during each test, in order to diagnose possible alterations in the fluidization quality, related to sintering phenomena involving bed particles. The results from the signal analyses, in terms of dominant frequencies in the power spectral density functions and standard deviations, did not show any worsening of the fluidization quality for all investigated gasification conditions. This was confirmed by the SEM analysis, which did not exhibit clusters of particles.

**Author Contributions:** Conceptualization, S.R., K.G. and A.D.C.; methodology, S.R., K.G. and A.D.C.; software, A.D.G.; validation, K.G., A.D.C. and S.R.; formal analysis, E.S., J.M. and A.D.G.; investigation, E.S., J.M. and S.R.; resources, K.G., S.R. and A.D.C.; data curation, E.S., J.M. and A.D.G.; writing—original draft preparation, E.S.; writing—review and editing, E.S., K.G., A.D.C., A.D.G., J.M.; visualization, E.S. and A.D.G.; supervision, K.G. and A.D.C.; project administration, K.G.; funding acquisition, K.G. All authors have read and agree to the published version of the manuscript.

**Funding:** This research was funded by the European Commission managed Research Fund for Coal and Steel (RFCS), gran<sup>t</sup> number 796585. The APC was funded by the University of L'Aquila with the budget of the EU Project LIG2LIQ for dissemination activity.

**Acknowledgments:** The authors kindly acknowledge the financial support of the European Project LIG2LIQ (RFCS-01-2017 n◦796585) co-funded by the European Commission managed Research Fund for Coal and Steel (RFCS). In particular, the authors would like to thank the ICHPW and TUDA, partners of the LIG2LIQ Project, for their characterizations, as well as Fabiola Ferrante (University of L'Aquila), for the XRF analysis. Moreover, the authors would like to thank RWE Power AG for the lignite feedstock supplied. We also give thanks for the financial support provided by the DFG in the framework of the Excellence Initiative, Darmstadt Graduate School of Excellence Energy Science and Engineering (GSC 1070).

**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
