Valorization of Waste Oxytree Biomass for Impregnated Solid Fuel Production—Process Assessment and Fuel Property Evaluation

The increasing generation of organic and liquid wastes calls for sustainable strategies to convert residues into valuable energy resources. This study investigates waste Oxytree biomass (Paulownia Clon In Vitro 112®) as a sorbent for producing impregnated solid fuels from selected liquid wastes, including used cooking oil, spent mineral oil, and pyrolysis condensate, targeting industrial energy applications. Oxytree biomass was selected due to its high and predictable yield, uniform composition, and favorable physical properties compared to conventional lignocellulosic residues such as pine sawdust. Biomass and liquid wastes were characterized in terms of fuel properties and elemental composition. Several empirical combinations of sorbent and liquid fractions were tested to optimize homogeneity and fuel quality, resulting in a final composition of sorbent:used cooking oil:used machine oil:pyrolytic condensate equal to 3:1:1:3. The temporal stability of this selected fuel was verified over 24 h, 3 days, and 1 week. The resulting fuels exhibited an energy value of approximately 15 MJ/kg, low ash content (