**Preface to "Solid Fuels Technology and Applications"**

We are delighted to deliver the "Solid Fuels Technology and Applications" Special Issue of Energies. This Special Issue aims to promote research and technological development on the integration and exploitation of solid fuels and their by-products. Solid fuels are still a key factor in energy production, having been enhanced through recent developments. Under the current trend of climate change and the necessity to mitigate greenhouse gas emissions, high efficiency is more than ever a prerequisite for low financial and environmental cost production.

The seven papers selected in the current Special Issue present novel applications that bring forward the current state-of-the-art on solid fuels utilization. Multi-faceted solutions to a challenging environment are delivered to the readers of this unique collection of articles that combine innovative applications to emerging problems and needs.

In this Special Issue, O'Brien et al. [1] developed a research model that addresses both the sustainability and the profitability of bioethanol production due to synergy with the torrefaction of DDGS and using produced biochar as a marketable fuel. Their results clearly demonstrated the relationship between reduction in environmental footprint ( 24% reduction in CO2 emissions) and the introduction of comprehensive on-site valorization of dried distillers' grains with solubles (DDGS). Wu et al. [2] found that the increased OH/H ratio in staged O2/CO2 combustion offsets part of their reproducibility, resulting in the final NOx emissions being higher than those in air combustion under similar conditions.

Where are the potential sites for underground energy and CO2 storage in Greece? Arvanitis et al. [3] provided a geological and petrological research approach that can serve as a basis for future research and deployment of promising areas. The use of mechanically activated serpentinite was considered by Petrounias et al. [4] to remove Cu(II) from industrial wastewaters. It was proposed that the higher mechanical activation of the studied serpentinites (being subjected to a 1500 revolutions LA test) is related to their higher capability of performing Cu removal. Checko et al. [5] presented ´ an assessment of the resources of methane, considered as the main phase in the prospective areas of the Upper Silesian Coal Basin, Poland. The results of the numerical simulations confirm that the application of multilateral well systems combined with hydraulic fracturing considerably improves the efficiency of CBM extraction from seams characterized by low coal permeability. Nazos et al. [6] dealt with the treatment of barley straw by acid-catalyzed wet torrefaction (ACWT) in a Parr 4553 3.75 L batch reactor (autoclave). The findings indicated that the composition changes of the straw due to ACWT had a significant effect on the HHV of the pretreated material. The petrographic features of sandstones in Central Greece were examined by Petrounias et al. [7] to evaluate their behavior in construction and in energy storage applications. Petrographic methodologies were combined with the quantification of modal composition (GIS proposed method) and 3D depictions of their petrographic features (3D Builder software).

The guest editors are confident that readers will enjoy the articles presented in this beneficiary Special Issue of "Solid Fuels Technology and Applications".

#### **Nikolaos Koukouzas, Pavlos Tyrologou, Petros Koutsovitis** *Editors*

#### **References**


**Nikolaos Koukouzas, Pavlos Tyrologou, Petros Koutsovitis** *Editors*
