Enzymatic pretreatment of wood to reduce energy consumption in thermo-mechanical pulp refining

Abstract

Mechanical pulping processes continue to play an important role in world pulp and paper production. Of these, thermomechanical pulping (TMP) is the most energy intensive process. Direct application of enzymes has some commercial applications in the pulp and paper industry, especially in bleaching of kraft pulps. However, results have been mixed when using enzymes to reduce energy consumption in mechanical refining. The goal of this study was to develop a more thorough understanding of the fundamental mechanisms by which enzymatic modification can achieve energy savings during thermomechanical refining. First, the actions of individual enzymes on different wood substrates was studied to determine which enzyme or enzyme combination would be most capable of producing results on native wood species. It was found that when using an enzyme cocktail containing a mix of cellulolytic and hemicellulolytic enzymes to pretreat wood chips prior to refining, and depending on mechanical pre-treatment, reductions of 15-36% in energy consumption could be achieved. In fungal treatments, fungi hyphae and mycelia grow into the wood and can deliver the enzymes deeper into the wood. To mimic this process and further improve enzyme penetration into wood chips, light mechanical pre-treatments including downsizing and/or compression/decompression cycles were used prior to enzyme applications to determine if they could help improve enzyme penetration in wood chips. Using a fluorescent molecular probe of similar dimensions as the enzymes used in our experiments to help visualise penetration, it was observed that the treatments did improve accessibility to potential enzyme attack sites by creating cracks and forcing enzyme solution into fibre lumens and walls. Pulps prepared using these enzymatically and mechanically pretreated wood samples showed, using electron microscopic imaging, that fibre separation during refining occurred at locations which allowed for the development of more favourable pulp physical properties (e.g. improved mechanical strength, enhanced printability, improved drainability, etc.). These changes in fibre separation patterns also shifted energy consumption patterns during multi-stage refining which played a role in the overall energy savings obtained. These results were supported by pulp characterization and paper properties analyses. Finally, Fourier Transform-Raman (FT-Raman) and Nuclear Magnetic Resonance (NMR) were used to investigate lignin and lignin-carbohydrate-complexe (LCC) structural changes caused by the enzyme treatment. FT-Raman analysis showed that, with increasing enzyme dosage, native lignin peaks became more pronounced indicating that lignin is being dissociated from the LCC within the wood cell walls. Cross Polarization-Magic Angle Spinning NMR (CP-MAS NMR) results also indicated that certain bonds related to LCC, along with peaks representative of β-glycosidic bonds, were being increasingly cut with increasing enzyme dosage. These changes to the molecular connectivity within the cell walls could help explain how enzyme treatments affect fibre material strength properties and also explain changes to observed fibre bundle fracture patterns which in turn explains the mechanisms behind the reduction in specific energy consumption (SEC). Fibre quality analysis, light microscopy images and paper properties were also used to helped corroborate these observations.