Effect Of Physical Properties And Chemical Composition Changes In Autohydrolysis Pretreatment On The Subsequent Alkali Impregnation Of Poplar Sapwood Chips

Autohydrolysis pretreatment is a promising pretreatment method for pre-extracting hemicelluloses from raw materials in pulp and paper industry owing to its economical and environmentally-friendly advantages.Before pulping,hemicelluloses from lignocellulosie raw materials are partially or completely pre-extracted by using autohydrolysis pretreatment technology to produce high value-added products,and then the rest is used for chemi-mechanical pulping or chemical pulping.In this way,not only traditional pulp products can be produced,but also high value utilization of chemical components in the lignocellulosic raw materials can be realized.In the process of chemi-mechanical pulping,a good impregnation of wood chips can raise the yield of accepted pulp and decrease both the reject content and pulping time.After autohydrolysis pretreatment of lignocellulosic raw materials,how will the changes of physical properties and chemical composition affect the subsequent alkali impregnation in the process of chemi-mechanical pulping?At present,the related research on the above field is not yet fully investigated,and the effect of the changes of surface lignin on the subsequent alkali impregnation has also not been reported.Aiming at this issue,in this study,the autohydrolyzed poplar sapwood chips with different levels of autohydrolysis intensity were selected as raw materials,and the changes of physical properties(total pore volume,volume porosity,and specific surface area)and chemical composition(hemicelluloses,acetyl,lignin,surface lignin,and extractive)of poplar sapwood chips during autohydrolysis pretreatment were studied.Stepwise regression analysis was used to quantitatively study the effects of physical properties and chemical composition of the autohydrolyzed poplar sapwood chips on the subsequent alkali impregnation(the amount of NaOH solution absorbed and the amount of NaOH solution consumed),and the effect of autohydrolysis on the subsequent alkali impregnation process was comprehensively and systematically explored.Firstly,the changes of physical properties and chemical composition of autohydrolyzed poplar sapwood chips were studied.The results showed that,after autohydrolysis pretreatment,the specific surface area,total pore volume and volume porosity of poplar sapwood chips increased significantly.With the increase of the intensity of autohydrolysis pretreatment from 2.78 to 130.59,the specific surface area increased from 1.320 m2/g to 2.731 m2/g,the total pore volume increased from 3.07×10-3 cm3/g to 9.68×10-3 cm3/g,and the volume porosity increased by 5.4%.After autohydrolysis pretreatment,the extractive content of poplar sapwood chips with a combined hydrolysis factor of 130.59 were increased by about 5 times than that of poplar sapwood chips with a combined hydrolysis factor of 2.78.Meanwhile,the contents of xylan and acetyl groups were decreased by 61.3%and 70.6%,respectively,and the contents of acid-insoluble lignin and acid-soluble lignin were decreased by 20.1%and 53.8%,respectively.By means of X-ray photoelectron spectroscopy and scanning electron microscopy,it was found that lignin droplets appeared on the surface of poplar sapwood chips after autohydrolysis pretreatment,and gradually increased with the increase of intensity of autohydrolysis.Then,the autohydrolyzed poplar sapwood chips were used in the subsequent alkali impregnation process.The absorption and consumption of NaOH solution during the alkali impregnation process were used as characterization methods to study the effect of surface lignin produced during autohydrolysis on the subsequent alkali impregnation.The results showed that with the increase of surface lignin concentration from 80.12%to 97.66%,the amounts of NaOH solution absorbed and consumed increased by 51.1%and 25.1%,respectively.Moreover,the effects of surface lignin concentration on the subsequent alkali impregnation were investigated by stepwise regression analysis.The physical properties and chemical composition of autohydrolyzed poplar sapwood chips acted as independent variables,and the amounts of the absorption and consumption for NaOH solution acted as dependent variables.The results showed that the existence of the surface lignin had nearly no impact on the subsequent alkali impregnation in making chemi-mechanical pulps as compared to the volume porosity,and that the effect of physical properties of autohydrolyzed poplar sapwood chips on alkali impregnation in chemi-mechanical pulping process was much greater than that of chemical composition.Finally,it can be further concluded that autohydrolysis can facilitate the subsequent alkali impregnation of the autohydrolyzed poplar sapwood chips in making chemi-mechanical pulp and then soften the lignin in the middle lamella of the cells of wood chips.As a result,the pulp fibers can be separated easily when mechanical refining.The combination of autohydrolysis pretreatment and chemi-mechanical pulping can not only make hemicelluloses get high value-added and efficient utilization but also make the pulping process clean,thereby reducing the negative impact of anionic waste residue in the paper-making process.This study would be of significance for the combination of biomass refinery and pulp and paper industry.