**4. Conclusions**

Torrefaction of biomass is a promising process for improving the characteristics of biomass, as an alternative renewable energy source over the use of fossil fuels. This process provokes the interest of investors in this sector. From this perspective, the potential development of biomass heat conversion technologies, such as combustion, is promising as far as the use of new forms of biomass is concerned, i.e., more eco-friendly, more abundant and more economical, as is the case of barley straw. The fact that publications on this topic have significantly increased indicates the strong academic relevance and industrial interest in this subject in recent years. In the current study, torrefaction conditions were investigated for increasing the Higher Heating Value of combustion for barley straw. An integrated methodology was applied to this end, with the main focus given on the impact of the temperature and time parameters and with the results presented herein eventually indicating that severe treatment conditions are the optimum ones in order to maximize the heating value of barley straw combustion. According to these experimental results, the optimal time that gives the maximum output equal to 21.3 MJ/kg was 47.5 min where *Hg* increases by 21.7%., for *R*0 = 1.99·10<sup>7</sup> and consequently log *R*0 = 7.3. On the other hand, according to the developed models, the maximum Higher Heating Value of the barley straw combustion is expected to be at the maximum material's mass loss percentage, i.e., at the most severe torrefaction conditions. More or less, moderate torrefaction conditions could be chosen to reduce barley straw's mass loss but with enhanced Higher Heating Value of the material combustion compared to the untreated material but lower value compared to the optimal one.

**Author Contributions:** Investigation, A.G.N.; Methodology, G.E.G.; Software, D.V.P.; Supervision, D.K.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work has been partly supported by the University of Piraeus Research Center.

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