From Problematic Residues to Valuable Streams: Analytical Characterization and Valorization of Secondary Fractions from Agricultural-Waste Thermochemical Conversion

Agricultural residues are increasingly used as feedstocks for renewable heat and power generation, but their thermochemical conversion also produces secondary fractions that are often treated as problematic wastes. Condensates, tars, aqueous phases, bio-oil-derived liquids, chars, and ash-rich residues may cause corrosion, fouling, toxicity, microbial inhibition, or additional treatment costs. However, these streams may also contain recoverable chemicals, catalytic feedstocks, biodegradable carbon sources, or functional carbonaceous materials. This narrative review examines secondary fractions generated during the torrefaction, pyrolysis, gasification, and hydrothermal processing of agricultural waste as streams that can be analytically characterized and potentially valorized, rather than simply treated as unavoidable residues. Particular attention is given to the role of chromatographic, spectroscopic, elemental, thermal, and bulk physicochemical analyses in determining whether a fraction is suitable for separation, purification, catalytic upgrading, biological transformation, energy recovery, or controlled disposal. The review also highlights key research gaps, including inconsistent terminology, feedstock variability, excessive reliance on model compounds, insufficient quantitative chromatographic data, limited integration of chemical analysis with biological testing, and the scarcity of techno-economic and life-cycle assessments. An analytical-to-valorization decision framework is proposed to support more complete circular waste-to-energy systems.