Upgrading and multistage separation of rice straw by degradative solvent extraction

Upgrading and multi-stage separation (UMSS) of rice straw was conducted at different temperatures using 1-methylnaphthalene (1-MN) as solvent. Three main solid products were obtained: low molecular weight extract (soluble), high molecular weight extract (deposit) and extraction residue (residue). The elemental composition, chemical structure and physicochemical characteristic of each component were analyzed in detail. Alkali and alkaline earth metal (AAEM) contents of solid products and rice straw were also measured by ICP-MS. The results showed the yield of soluble increased with temperature, and the carbon-based yield of soluble reached 33.48% at 350°C. The carbon content and oxygen content of three solid products (soluble, deposit, and residue) increased and decreased with temperature, respectively. The carbon content of soluble and deposit reached up to 82.36% and 80.59% respectively. Meanwhile the oxygen contents of them were as low as 9.50% and 12.03% respectively. More than 86.99% oxygen of rice straw was removed as H2O and CO2. Soluble was almost free from ash, and the ash content of deposit was also less than 1.50%. The higher heating values (HHV) of three solid products were significantly higher than that of rice straw. The FT-IR results indicated that not only dehydration reaction and decarboxylation reaction occurred, but also including obvious aromatization reaction. The contents of Na, Mg and K of soluble and deposit were extremely low, and they decreased with temperature gradually. In conclusion, the degradative solvent extraction method realized dehydration, deoxygenation and multistage separation of biomass under mild condition, and obtained a variety of products of low ash and oxygen content as well as high carbon content and HHV.