In-situ catalytic pyrolysis of biomass using low-dose alkali metal salts: The effect of salt type and temperature

To elucidate the chemical activation mechanism in the in-situ catalytic pyrolysis of biomass using low-dose alkali salts, this study examined the effect of alkali salt type and temperature on char properties by co-pyrolysis of low-dose potash alkali salt (1:10 mass ratio of molten salt to biomass) and biomass. The results revealed distinct differences in pyrolysis behavior, char yields, and porosities among various alkali salts. Notably, the experimental group with the addition of KOH yielded the highest carbon production of 25.3 wt% with a microporous area of 342.6 m²/g compared to 19.2 wt% char yield and 39.5 m²/g by direct pyrolysis. KCl and KNO₃ facilitated the breakdown of glycosidic bonds, enabling biomass pyrolysis at lower temperatures (a reduction of 79.5 °C and 74 °C, respectively, compared to the raw material). Furthermore, a mechanism for in-situ catalytic pyrolysis at different temperatures using low-dose alkali salts was proposed. Potassium salts promote the cleavage of amino acid hydrogen bonds in biomass at low temperatures, accelerating the removal of phenolic hydroxyl groups and leading to the precipitation of large amounts of volatile matter to increase the mesopore area at low temperature (<400 °C). At 400–500 °C, KOH interacted with the char skeleton to generate active sites, where OH⁻ ions combined with free carbonyl and hydroxyls. Potassium ions simultaneously acted as template agents within etched pores, enhancing specific surface area to 448.64 m²/g. At 800 °C, potassium salts promoted the polycondensation of carbonyl, hydroxyl, and ether-bonding groups into polybenzene ring char structures, significantly increasing char yield. The pyrolytic char prepared with a low dose of KOH was rich in oxygenated groups, demonstrating its high suitability for catalytic applications.