A study of demineralization methods and the thermal degradation of wood to assess the impact of mineral compounds on biomass torrefaction

This study reviews the mechanisms of torrefaction and investigates the influence of native minerals on the thermal degradation of beech wood and its isolated constituent biopolymers, including hemicellulose, cellulose, and lignin. Evaluation of demineralization results showed that water treatment removed over 90% of soluble ions but left insoluble mineral fractions intact. Conversely, hydrochloric acid treatment extracted both water-soluble and non-soluble catalytic cations, including Ca²⁺, Mg²⁺, successfully establishing a consistent baseline. Thermogravimetric analyses of subsequent mineral re-introduction via impregnation revealed distinct results across different temperature stages. In the low-temperature region between 200 and 300 ℃, the salts exhibited a catalytic effect that shifted the thermal degradation onset temperature downward by 10 to 20 ℃, accelerating early-stage hemicellulose decomposition. When the temperature reached up to 400 ℃, these minerals functioned as volatilization inhibitors and suppressed the maximum weight loss rate of the matrix. This high-temperature mechanism altered the thermochemical pathways to favor secondary cross-linking reactions, resulting in a distinct 3 to 8 wt% increase in the final solid char yield compared to demineralized wood reference samples. Furthermore, pyrolysis gas chromatography mass spectrometry revealed that specific ion pairs dictate volatile degradation pathways. Potassium salts exhibited a stronger catalytic activity than sodium salts in promoting the cleavage of glycosidic bonds and intense thermal reactions. Consequently, the depolymerization pathways that form heavier anhydrosugars, such as levoglucosan, significantly higher proportions of low molecular weight oxygenates, including acetic acid, formic acid, and hydroxyacetone. By analyzing the extraction efficiencies and thermal response metrics, this research quantifies how specific mineral compounds govern the overall thermochemical trajectories, providing parameters for predicting solid residue yields in biomass torrefaction.