Potassium demineralization of coconut fiber via combined hydrothermal treatment and washing: Effect on pyrolysis kinetics, mechanisms, and bio-oil composition

Producing clean energy from waste biomass via pyrolysis is critical in reducing dependence on fossil fuels and alleviating their environmental impacts. Inherent potassium in low energy density coconut fibers reduces bio-oil yield and compromises bio-oil quality and the integrity of reactors within which pyrolysis occurs. The effect of consecutive hydrothermal treatment (180 degrees C-220 degrees C) and water washing on coconut fiber demineralization, pyrolysis behavior, and bio-oil composition was investigated. Chemical fractionation method classified the most occurring inorganic (potassium) as similar to 76% water-soluble, similar to 19% ion-exchangeable, and similar to 5% acid-soluble species. Demineralization results show that combined pretreatment at <= 200 degrees C leached out all water-soluble and exchangeable potassium. However, at 220 degrees C, electrostatic attraction and increased tortuosity imposed by the high density of oxygenated functional groups and a well-developed porous structure hamper demineralization. Nevertheless, a maximum potassium removal efficiency of 95% could be achieved. Although pretreatment marginally affects apparent activation energy, the reaction mechanism of hydrochars became insensitive to the heating rate between 10 and 50 degrees C min(-1). Due to pretreatment, the selectivity of thermally stable phenolic and anhydrosugar derivatives increased at the expense of the light oxygenates of low thermal stability because of suppressing fragmentation reactions and enriching polymers with a high degree of polymerization. Light oxygenates decreased from 27.45 area % to 24.02, 16.33, and 13.32 area % for hydrochars produced at 180 degrees C, 200 degrees C, and 220 degrees C, respectively. Therefore, this pretreatment approach can produce inorganic-free, thermally stable, and chemically stable bio-oils.

Automated summary

The study found that consecutive hydrothermal treatment and water washing of coconut fibers can effectively remove water-soluble and exchangeable potassium. This pretreatment approach can produce bio-oils that are inorganic-free, thermally stable, and chemically stable.