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Abstract The main problem and also challenge was to find a satisfactory effective tool for real-time control the torrefaction/pyrolysis process of biomass and to evaluate (also in real-time conditions) biochar quality. This paper presents results from investigation on a new method for both evaluating biochar quality and control biomass thermal processing at temperatures of 300, 350 and 400 ◦ C. Thermal processing of biomass in this temperature range is torrefaction and pyrolysis. Investigation was focused on processing Virginia Mallow (Sida Hermaphrodita) energetic crops. Several strong correlations were observed as follows: carbon and hydrogen content in biochar (charcoal), oxygen content, ash and volatile matter from biochar, calorific value and temperature of the process and finally, acetic acid in condensate and carbon content in the biochar. The results show that the content of acetic acid, being one of the major compounds present in the condensate (black liquor, biooil), is in satisfactory linear correlation with the process temperature and furthermore, with carbon content in biochar. The mathematical description for this polynomial function was proposed as follows: Carbon (%) = 0.6e-3 x Acetic Acid (ppm)+49.5. The correlation between acetic acid in black liquor and carbon in biochar was used to introduce a new method which can be considered as a diagnostic tool for determining biochar quality in real-time conditions during biomass thermal processing. Hence, carbon content in biochar as well as its calorific value can be determined by measuring percentage of acetic acid in the condensate. This correlation was considered as the main novelty presented here. This method provides relatively fast evaluation of quality of biochar produced in industrial scale torrefaction systems, what makes it possible to optimize process parameters (temperature, retention time) in real time conditions and improve biochar production process in this way. Highlights • C H tradeoff for biochar is in strong linear negative correlation. • Acetic acid in biooil increased from 5 to 25% with process temperature increase. • Acetic acid in biooil is verifiable correlated with carbon content in biochar. • Acetic acid vs. carbon correlation can be used to control biochar quality. [ABSTRACT FROM AUTHOR]