**4. Conclusions**

Hydrochar from lignocellulosic residual biomass furnishes porous carbon materials via usual activation. The best reaction conditions were 200 ◦C and 0 min. The best results were BET area of 881 m<sup>2</sup>/g, average pore diameter of 11 Å, and sorbent capacity of 6.569 mmol/g at 5 bar. The hydrochar exhibits higher CO2 adsorption than that of some traditional sorbents. The material possesses adequate capacity to adsorb CO2 from mixtures with CH4 selectively. This residual material could be exploited in a sustainable biogas upgrading process, contextually reducing CO2 emissions and the related environmental impact.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3417/10/5/1879/s1.

**Author Contributions:** Funding acquisition, K.G.; Investigation, L.T., A.A.P., F.D.L. and Z.A.; Methodology, K.G.; Project administration, K.G.; Supervision, A.G.; Writing—original draft and revision K.G., L.T. and A.A.P. All authors have read and approved the manuscript for publication.

**Funding:** This research was funded by European Regional Development Fund 2014–2020 gran<sup>t</sup> number PON—R&I E12H18000230001 for Industrial PhD scholarship (http://www.ponricerca.gov.it/) and the APC was funded by the dedicated budget for PhD research activities.

**Conflicts of Interest:** The authors declare no conflict of interest.
