Devolatilization characteristics of a bio-residue (potato stems) with high contents of alkali metals and non-structural components – Influences of the washing pretreatment severity

A bio-residue (potato stems), rich in alkali metals and non-structural components, is subjected to hot (353 K) or acidic (0.1 mol/L hydrochloric acid) water washing. The acidic pretreatment is more effective for the demineralization, with alkalis reduced by 99 (powders) or 90 (particles) %, corresponding to organic matter dissolution around 35 or 17 wt%, respectively. The latter figures remain approximately the same for hot water washing (38 and 19 wt%) but alkalis are reduced only by 78 or 64 %. The leachate properties are significantly dependent on the solvent type and the residue granulometry, as indicated by thermogravimetric and SEM-EDX analysis. Washed bio-residues decompose at temperatures and with rates progressively increasing as the pretreatment becomes more severe (fine granulometry and acidic water washing). Multi-step global kinetics, previously developed for the untreated material and properly revisited to incorporate washing effects, reveals trends with important compositional changes. Despite starch dissolution, volatile product yields for the absolute peak rate zone increase by about 40–65 %, testifying augmented cellulose contents. The pectin compounds are almost eliminated, to the advantage of the shoulder rate zone, representing hemicellulose decomposition, with an increase in the volatile product yields around 22–78 %.