• Energy Research

Cotton ginning trash (CGT) collected from Australian cotton gins was evaluated for bioethanol production. CGT composition varied between ginning operations and contained high levels of extractives (26-28%), acid-insoluble material (17-22%) and holocellulose (42-50%). Pretreatment conditions of time (4-20 min), temperature (160-220 °C) and sulfuric acid concentration (0-2%) were optimised using a central composite design. Response surface modelling revealed that CGT fibre pretreated at 180 °C in 0.8% H2SO4 for 12 min was optimal for maximising enzymatic glucose recoveries and achieved yields of 89% theoretical, whilst the total accumulated levels of furans and acetic acid remained relatively low at <1 and 2 g/L respectively. Response surface modelling also estimated maximum xylose recovery in pretreated liquors (87% theoretical) under the set conditions of 150 °C in 1.9% H2SO4 for 23.8 min. Yeast fermentations yielded high ethanol titres of 85%, 88% and 70% theoretical from glucose generated from: (a) enzymatic hydrolysis of washed pretreated fibres, (b) enzymatic hydrolysis of whole pretreated slurries and (c) simultaneous saccharification fermentations, respectively.

Abstract Lignocellulosic materials are potentially a relatively inexpensive and abundant feedstock for biofuel production. The key to unlocking lignocelluloses recalcitrance is in part, the development of an effective pretreatment process. A promising new pretreatment method for lignocellulosic materials is the use of ionic liquids (ILs). ILs are non-volatile solvents that exhibit unique solvating properties. In this review, the dissolution of cellulose and lignocellulose in various ionic liquids is described including key properties such as high hydrogen-bonding basicity, which increases the ability of the IL to dissolve cellulose. As a pretreatment in biofuel production, the review details aspects such as the regeneration of cellulose from ionic liquids, structural changes that arise in the regenerated cellulose and its effect on enzymatic hydrolysis, the potential for IL recycling and finally, exploiting ILs in an integrated bioprocess.

The impacts of varying pretreatment parameters (temperature, time, and alkalinity) on enzymatic hydrolysis of sorghum straw were investigated. Following pretreatment, both solids and lignin content was found to be inversely proportional to the severity of the treatments. Higher temperatures and alkali strength were quintessential for maximising sugar recoveries from enzyme saccharifications. Total sugar release peaked when sorghum straw was pretreated in 2% NaOH at 121 degrees C for 60 min; representing a 5.6-fold higher yield compared to samples pretreated at 60 degrees C in the absence of alkali. Similarly, 4.3-fold increases in total sugars from samples treated with 2% NaOH at 60 degrees C for 90 min, confirmed the importance of alkali inclusion. Addition of beta-glucosidase and xylanase to saccharification mixtures enhanced reaction rates and final sugar yields, whilst reducing cellulase dosage 4-fold. Saccharification efficiency of pretreated solids approached 90% and 95% (w/w) with as little as 2.5 and 5.0 FPU cellulase/g, respectively.

Conditions for optimal pretreatment of eucalypt (Eucalyptus dunnii) and spotted gum (Corymbia citriodora) forestry thinning residues for bioethanol production were empirically determined using a 3(3) factorial design. Up to 161mg/g xylose (93% theoretical) was achieved at moderate combined severity factors (CSF) of 1.0-1.6. At CSF>2.0, xylose levels declined, owing to degradation. Moreover at high CSF, depolymerisation of cellulose was evident and corresponded to glucose (155mg/g, ∼33% cellulose) recovery in prehydrolysate. Likewise, efficient saccharification with Cellic® CTec 2 cellulase correlated well with increasing process severity. The best condition yielded 74% of the theoretical conversion and was attained at the height of severity (CSF of 2.48). Saccharomyces cerevisiae efficiently fermented crude E. dunnii hydrolysate within 30h, yielding 18g/L ethanol, representing a glucose to ethanol conversion rate of 0.475g/g (92%). Based on our findings, eucalyptus forest thinnings represent a potential feedstock option for the emerging Australian biofuel industry.

Abstract Dilute sulphuric acid pretreatment followed by enzyme saccharification of Sorghum bicolor straw was undertaken to examine its potential as a feedstock in bioethanol production in Australia. Factorial design experiments evaluated the impact of pretreatment parameters on hemicellulose solubilisation and cellulose enzymatic hydrolysis. Sugar yields in prehydrolysate and saccharified liquors were found to increase with treatment severity; temperature was found to have the greatest impact. Degradation products were minimal; acetate and total phenolics peaked at 33 and 1.5 mg/g respectively. Conditions for maximum hemicellulose solubilisation (2% H 2 SO 4 for 60 min at 121 °C) differed to those associated with maximum glucose release from solid residue saccharifications (1% H 2 SO 4 /90 min /121 °C). Water extractive sugars accounted for over 20% total sugars recovered. Addition of β-glucosidase and xylanase to enzyme saccharification enhanced reaction rates and final sugar yields three-fold, whilst reducing cellulase dosage. Considering its abundance, high sugar potential and apparent ease of conversion, sorghum straw is an appropriate feedstock for the production of second generation fuels.

Physico-chemical pretreatment of lignocellulosic biomass is critical in removing substrate-specific barriers to cellulolytic enzyme attack. Alkaline pretreatment successfully delignifies biomass by disrupting the ester bonds cross-linking lignin and xylan, resulting in cellulose and hemicellulose enriched fractions. Here we report the use of dilute alkaline (NaOH) pretreatment followed by enzyme saccharifications of wheat straw to produce fermentable sugars. Specifically, we have assessed the impacts of varying pretreatment parameters (temperature, time and alkalinity) on enzymatic digestion of residual solid materials. Following pretreatment, recoverable solids and lignin contents were found to be inversely proportional to the severity of the pretreatment process. Elevating temperature and alkaline strengths maximised hemicellulose and lignin solubilisation and enhanced enzymatic saccharifications. Pretreating wheat straw with 2% NaOH for 30 min at 121 °C improved enzyme saccharification 6.3-fold when compared to control samples. Similarly, a 4.9-fold increase in total sugar yields from samples treated with 2% NaOH at 60 °C for 90min, confirmed the importance of alkali inclusion. A combination of three commercial enzyme preparations (cellulase, β-glucosidase and xylanase) was found to maximise monomeric sugar release, particularly for substrates with higher xylan contents. In essence, the combined enzyme activities increased total sugar release 1.65-fold and effectively reduced cellulase enzyme loadings 3-fold. Prehydrolysate liquors contained 4-fold more total phenolics compared to enzyme saccharification mixtures. Harsher pretreatment conditions provide saccharified hydrolysates with reduced phenolic content and greater fermentation potential.

This paper reports on processing options for the conversion of feedlot cattle manures into composite sugars for ethanol fermentation. Small-scale anaerobic digestion trials revealed that the process significantly reduces the content of glucan and xylan (ca. 70%) without effecting lignin. Moreover, anaerobic digestate (AD) fibres were poor substrates for cellulase (Cellic® CTec 2) saccharification, generating a maximum combined sugar yield of ca. 12% per original dry weight. Dilute acid pretreatment and enzyme saccharification of raw manures significantly improved total sugar recoveries, totalling 264 mg/g (79% theoretical). This was attained when manures were pretreated with 2.5% H2SO4 for 90 min at 121°C and saccharified with 50 FPU CTec 2/g glucan. Saccharomyces cerevisiae efficiently fermented crude hydrolysates within 6 h, yielding 7.3 g/L ethanol, representing glucose to ethanol conversion rate of 70%. With further developments (i.e., fermentation of xylose), this process could deliver greater yields, reinforcing its potential as a biofuel feedstock.