Pretreatment and/or fractionation process of lignocellulosic materials using gamma-valerolactone to produce biofuels and green chemicals

The largest waste stream in the world is Lignocellulosic materials.  Traditionally, these materials are burned, landfilled, or left to degrade causing substantial greenhouse gas emissions- thus making it a significant global issue. Global forestry and agricultural practices generate large quantities of lignocellulosic waste making this a significant global issue. This material is composed of three main compounds: Lignin, Cellulose, and Hemicellulose.

Biorefining is a new approach for the processing of lignocellulosic-based waste materials. It offers many advantages as an advanced waste processing system as it holds potential for higher levels of valorisation of organic feedstocks. Biorefining of lignocellulosic material is a process in which the biomass is valorised by isolating lignocellulosic fractions and generating new product(s). The most investigated process is pre-treatment of lignocellulosic materials to produce residual cellulose that can be utilised for more accessible fermentation of enzymes with an aim to enhance bioethanol production.

This study investigates the use of Australian timber and forestry wastes as a resource to produce platform compounds that can be used for biofuel and green-chemical production. The organic solvent `γ-Valerolactone’ (GVL) has demonstrated potential use for organosolv fractionation of lignocellulosic biomass to enhance cellulose hydrolysis and fermentation yields.

The effectiveness of GVL pre-treatment of Australian eucalyptus sawdust for high cellulose biomass and bioethanol production was assessed for GVL concentrations of 35-50% w/w, temperatures of 120-180 °C, and reaction durations of 0.5-2.0 hours. Optimum conditions were determined using the response surface method (RSM) according to the central composite face-centred design. Cellulose content increased from 39.9% to a maximum of 89.3% w/w using treatments with 50% GVL at 156°C for 0.5 hours. Temperature had the most significant effect (RSM p < .05) on cellulose content of residual biomass; however, reducing operational duration of < 0.5 hours may also be viable according to RSM. Pre-Hydrolysis Simulations Saccharification and Fermentation PSSF fermentations of optimised pre-treated eucalyptus sawdust produced up to 94% of theoretical ethanol yield, which corresponded to approximately 181 kg of ethanol per dry ton of eucalyptus sawdust.

Following the GVL fractionation, changes to the nanostructure of the wood fibres were investigated using Carbon 13 Magic Angle Spinning Solid State Nuclear Magnetic Resonance (C13 SS-NMR), (Fourier-transform infrared spectroscopy and (FT-IR) and Scanning electron microscopy (SEM) . These analytical instruments were used to quantify changes to the cellulose crystallinity. FT-IR was also used to identify surface crystallinity and lignin grouping. SEM and compositional data indicated that the solubilised lignin reprecipitated as consolidated lignin spheres on the fibril surface following cooling of the fractionation liquor. It was found that the GVL fractionation process caused an 82-126% increase in the crystallinity of the cellulose in the residual biomass. This significant increase was due to the hydrolysis of the hemicellulose and surface cellulose during the fractionation process, whereas paracrystalline and pure crystalline cellulose did not undergo significant hydrolysis under the conditions tested. The microfibrils showed significant aggregation and disruption following fractionation and it was noted that complete disruption of the microfibrils occurred at the high temperature treatments.

The outcomes from the first phase of pre-treatment effectiveness evaluation showed that reaction time of 30 minutes was optimal for the range of reactions times investigated. Due to this result, it was hypothesised that an opportunity to reduce the reaction duration exists and would make the process more economically viable due to reduction in energy demands. Scoping trials found that reaction duration as short as 5 minutes could be undertaken with significant fractionation occurring. To investigate this, a subsequent RSM was undertaken at these shorter reaction durations, this RSM focused on optimisation to maximise cellulose content in the recovered pulp (RP) and minimal cellulose hydrolysis. It was found that an RP of 88.3% w/w cellulose could be produced using 56% w/w GVL at 167 °C for 10 minutes.

The next phase of this study examined the effectiveness of GVL fractionation for the fractionation of several other common Australian lignocellulosic biomass at the optimum conditions. A variety of different lignocellulosic biomass were assessed, including two industrial types of sawdust, forestry bark waste, mulched garden waste, and two grain crop straws. It was found that an RP of 85.8% w/w and 81.9% w/w cellulose can be produced from the two sawdust's supplied by industry. The fractionation process was more effective at isolating cellulose from the sawdust compared to the straws and garden wastes (a fraction of these produced RP with a cellulose content of only 55.3-71.9% w/w). GVL fractionation increased cellulose crystallinity for all biomass tested. Ultimately it was determined that the GVL process has high potential for rapid fractionation of woody biomass and production of high crystallinity cellulose enriched pulp.

The final stage of this study was a techno-economic analysis of the GVL fractionation of timber and forestry sawdust. This was undertaken using Aspen Plus V10. The model produced was based on an operational capacity of processing 8,179 tonne of mixed Australian timber processing sawdust per year. The modelled site had a production capability of 2,266 kL per year of ethanol or 2,669 tonnes per year of RP with high cellulose of 85% w/w. It was shown that the site would have a total installed equipment cost of $98,400,000 AUD and a total investment cost required for all direct and indirect costs of $167,900,000 AUD. This techno-economic analysis showed the importance of improving the solvent recovery phase and ensuring onsite GVL production. It was found that cost for replacement of the non-recovered GVL would account for a large component of the operational expenditure (OPEX) equating to 82% of chemical costs for the sites.

This study demonstrated that GVL fractionation has a high potential for use in an Australian forestry biorefining system. The feasibility of GVL Organosolv fractionation for the production of renewable fuels and green bioproducts has been assessed in this study. The implementation of a lignocellulosic biorefinery system allows for diversification of products for the Australian forestry and agricultural sectors, providing further valorisation of their operation as well as reducing the Green House Gas (GHG) emissions of the sectors.