Thermomechanical biorefining of Pinus radiata biomass to produce biochemicals using reactive extrusion

Abstract

Currently, ca. 30 million m³ of Pinus radiata are harvested annually in New Zealand to produce timber, pulp and paper, with by-products such as bark and sawdust generated during processing. The most common use for sawdust is as a solid fuel for process heat. However, it is a feedstock that can be processed into platform biochemicals. Although conversion processes focusing on biochemical production from wood are scarce, they are becoming more commercially established. Here, reactive extrusion was explored as a continuous, fast method to depolymerise sawdust into soluble biochemicals with residence times of less than two minutes. This is substantially shorter than other biotechnology routes or conventional batch processing and highlights the potential for integration of reactive extrusion into biorefinery operations. While conventional wood extrusion focused on the solid fraction, this work extensively investigated the liquid biochemical profile. The effects of temperature, moisture content, screw speed, and screw design on the biochemical yield from sawdust were studied. The results indicated that kneading elements in the screw design were key to achieving good processing of the sawdust. A high moisture content of 50% (by weight) was instrumental in the isolation of biochemicals. Moreover, the screw speed had little to no effect on the biochemical composition obtained from the reactive extrusion process. Finally, a maximum of 6.5-7.5% of biochemicals were recovered from sawdust in the liquid phase when processed between 325 °C and 375 °C. The biochemical analyses of the liquor showed a high amount of acetic acid (up to 7913 mg/L) and methanol (up to 2277 mg/L). Furthermore, the furanic content increased with an increase in temperature between 275 °C and 375 °C, while an inverse trend was observed for aromatic phenols. The analyses also revealed that lignin and hemicellulose were depolymerised to produce oligomeric and monomeric breakdown products, while cellulose was untouched. This study successfully demonstrated the successful use of a twin-screw reactive extruder to continuously produce a biochemical-rich liquor from sawdust.

Keywords: Biochemicals; Biorefinery; Green manufacturing; Lignocellulosic biomass; Reactive extrusion.

Fig. 1

Setup of the reactive extruder

Fig. 2

Extrusion screw configuration A (extrusion direction left to right). Green solid blocks represent spacer elements

Fig. 3

Extrusion screw configuration B (extrusion direction left to right). Red blocks represent kneading elements

Fig. 4

Freeze-dried biochemical content in liquor after thermomechanical degradation of sawdust as a function of moisture content and screw speed

Fig. 5

Biochemical content in the liquor determined by Karl-Fisher titration (light grey) and freeze-drying depending on the extrusion processing temperature. The error bars represent the standard deviations of the observations (n = 3 replicates)

Fig. 6

CP MAS ¹³C solid state NMR spectra of raw P. radiata sawdust and sawdust extruded at 275 °C, 325 °C, and 375 °C

Fig. 7

Total lignin, hemicellulose and glucan extraction percentage for the three different temperatures (275 °C, 325 °C, and 375 °C) at constant screw speed (50 rpm), and moisture content (50%)

Fig. 8

¹H NMR spectra of the water-soluble fraction of the liquor obtained at 275 °C, 325 °C, and 375 °C. Signal from d6-DMSO appears at 2.50 ppm

Fig. 9

2D HSQC NMR spectrum of the water-soluble fraction of the liquor obtained with an extrusion temperature of 325 °C

Fig. 10

¹H NMR spectra of the CDCl₃ extracted chemicals from liquor produced at 275 °C(blue), 325 °C(red), and 375 °C(green). Identified compounds include coniferyl aldehyde (c), vanillin (v), furfural (f), 5-HMF (h) and guaiacols (g). CDCl₃ residual solvent signal is at 7.26 ppm

Fig. 11

Percentage of compounds whose likely origin was from carbohydrates, lignin or other wood components and side reactions