Pressurised hot water extraction (PHWE) and alkaline extraction of spruce

Abstract

Lignocellulosic biomass is among the potential renewable feedstock for the production of energy and chemicals. The pathway to biofuel from biomass involves four core steps: pretreatment, hydrolysis, fermentation, and separation process to obtain highly concentrated bioethanol. The study is mainly focused on the pretreatment of the Norway spruce sawdust. Two pretreatments methods, namely Pressurised hot water extraction (PHWE) and alkaline extraction were applied separately and in combination, in order to fractionate lignocelluloses of the Norway spruce sawdust. NaOH (0.55 M) solution was used during alkaline extractions. The consequences of cooling inbetween the combined extractions in the composition of the residual wood were analysed in the analysis part. Cellulose and hemicellulose in the wood residual were analysed by acid hydrolysis and acid metanolysis, respectively. Both the Klason lignin and acid soluble lignin, in the wood residual were determined to find the total amount of lignin content in the sample. The extracts were analysed for brix and pH.

Separate PHWEs (180 °C and 210 °C) and alkaline extractions (150 °C and 170 °C) were effective in removing hemicellulose and lignin, respectively. Cellulose recovery from both the PHWEs and the alkaline extractions was above 90%. Combined extractions (PHWE/alkaline) 180 °C /150 °C and 180 °C /170 °C were found more effective than combined extraction 210 °C /150 °C and 210 °C /170 °C in terms of cellulose yield. However, lignin contents in all of the combined extractions were higher than the expected. More than 60% of the cellulose was lost during combined extractions 210 °C /150 °C and 210 °C /170 °C. Surprisingly, lignin content in the wood residual from the combined extractions 210 °C /150 °C and 210 °C /170 °C was higher than the lignin content in the wood residual from combined extractions 180 °C /150 °C and 180 °C /170 °C. Cooling resulted in condensation of lignin into the wood fibre, resulting in high lignin content in the wood residual. In combined extraction, the temperature of PHWE affected the cellulose recovery and the delignification process. Higher temperature PHWE resulted in low cellulose yield and high lignin content in the extracted spruce sawdust.