Demineralization Pretreatments for Reducing Biomass Variability in Pyrolysis

Thermogravimetric and calorimetric analyses are applied to study how washing modifies the pyrolysis rates and heats of five samples of potato plant stems. Hot (553 K) water or dilute (hydrochloric) acid washing of powdered samples causes a reduction in the alkali content by about 62–78 or 97–99%. The feedstock variability is highly reduced, especially for dilute acid treatment. The char yields drastically decrease up to 42–50%, with increases in the peak rates and corresponding temperatures of up to 20–60% and 50–60 K, respectively. Overall, these characteristic parameters closely approach the beech wood values used for comparison. The shape of the rate curves also testifies the dissolution of nonstructural organic components (pectin, starch, and protein) essentially to the advantage of holocellulose. The ratios between activation energy and order of the global devolatilization reaction increase from about 62–98 kJ/mol (no treatment) to 78–104 kJ/mol (hot water) and 113–124 kJ/mol (dilute acid) (versus 141 kJ/mol for wood). Following washing, the strong exothermic character of the crop residues (global reaction heats from −560 to −180 J/g) is lost. The pyrolysis becomes nearly thermally neutral after hot water washing (heats from −106 to −25 J/g). Furthermore, dilute acid washing makes the process shift from exothermic to endothermic with heats around 70–270 J/g (versus 238 J/g of wood).