Steam pretreatment as an effective front end for a softwood based biorefinery

• Main Author: Naduvile Veettil, Linoj Kumar
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/44072

For any bioconversion/biorefinery process, the nature of the pretreatment process used has a significant influence on all of the subsequent process steps. Although steam pretreatment has proven effective on agricultural residues and hardwoods, softwoods are considerably more recalcitrant, usually requiring an acid catalyst to ensure effective pretreatment. One of the initial objectives of the work was to assess how effective acid catalysed steam pretreatment would be on a range of softwood substrates as past work had utilized wood chips that were obtained from one tree. It was apparent that similar pretreatment conditions could be used for a range of softwood substrates, resulting in comparable hemicellulose recovery while providing a cellulosic component which could be readily hydrolysed, but at the expense of using high enzyme loadings. To try to enhance cellulose hydrolysis we assessed the role of the various substrate components that are thought to limit hydrolysis. Lignin was shown to restrict hydrolysis at low enzyme loadings (5 – 10 FPU/g glucan), primarily by limiting the accessible cellulose surface area, but also by unproductive binding of the enzymes. To achieve effective hydrolysis at low enzyme loadings, a post-treatment step that removed/modified lignin to enhance the cellulose accessibility was assessed. Steam pretreatment and post-treatment were further optimised to result in a >85% cellulose hydrolysis at an enzyme loading of 10 FPU/g glucan. To try to increase the concentration of final sugars obtained we next evaluated the use of high substrate concentrations. Increased biomass loading during steam pretreatment not only minimised steam and SO₂ consumption, it also resulted in good recovery of the sugars at high concentration. However this was done at the expense of high enzyme loadings. Past work has primarily utilised pulp chips as the feedstock. However, they are unlikely to be used as a commercial bioconversion feedstock. A more likely feedstock, wood pellets were presoaked and steam pretreated. Surprisingly, little hemicellulose loss occurred while the cellulosic rich, water insoluble fraction was readily enzymatically hydrolysed. It was also possible to apply a single steam pretreatment to facilitate both pelletisation and subsequent enzymatic hydrolysis without the need for subsequent steam pretreatment.