Browsing by Author "Mokomele, Thapelo"

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    The development and modeling of an ethanol production biocatalytic system with cell retention
    (Stellenbosch : Stellenbosch University, 2014-12) Mokomele, Thapelo; Clarke, Kim G.; Callanan, Linda H.; Stellenbosch University. Faculty of Engineering. Department of Process Engineering.
    ENGLISH ABSTRACT: See PDF for abstract.
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    Ethanol production potential from AFEX™ and steam-exploded sugarcane residues for sugarcane biorefineries
    (BioMed Central, 2018-05-04) Mokomele, Thapelo; Da Costa Sousa, Leonardo; Balan, Venkatesh; Van Rensburg, Eugene; Dale, Bruce E.; Görgens, Johann F.
    Background: Expanding biofuel markets are challenged by the need to meet future biofuel demands and mitigate greenhouse gas emissions, while using domestically available feedstock sustainably. In the context of the sugar industry, exploiting under-utilized cane leaf matter (CLM) in addition to surplus sugarcane bagasse as supplementary feedstock for second-generation ethanol production has the potential to improve bioenergy yields per unit land. In this study, the ethanol yields and processing bottlenecks of ammonia fibre expansion (AFEX™) and steam explosion (StEx) as adopted technologies for pretreating sugarcane bagasse and CLM were experimentally measured and compared for the first time. Results: Ethanol yields between 249 and 256 kg Mg−1 raw dry biomass (RDM) were obtained with AFEX™-pretreated sugarcane bagasse and CLM after high solids loading enzymatic hydrolysis and fermentation. In contrast, StEx-pretreated sugarcane bagasse and CLM resulted in substantially lower ethanol yields that ranged between 162 and 203 kg Mg−1 RDM. The ethanol yields from StEx-treated sugarcane residues were limited by the aggregated effect of sugar degradation during pretreatment, enzyme inhibition during enzymatic hydrolysis and microbial inhibition of S. cerevisiae 424A (LNH-ST) during fermentation. However, relatively high enzyme dosages (> 20 mg g−1 glucan) were required irrespective of pretreatment method to reach 75% carbohydrate conversion, even when optimal combinations of Cellic ® CTec3, Cellic ® HTec3 and Pectinex Ultra-SP were used. Ethanol yields per hectare sugarcane cultivation area were estimated at 4496 and 3416 L ha−1 for biorefineries using AFEX™- or StEx-treated sugarcane residues, respectively. Conclusions: AFEX™ proved to be a more effective pretreatment method for sugarcane residues relative to StEx due to the higher fermentable sugar recovery and enzymatic hydrolysate fermentability after high solids loading enzymatic hydrolysis and fermentation by S. cerevisiae 424A (LNH-ST). The identification of auxiliary enzyme activities, adequate process integration and the use of robust xylose-fermenting ethanologens were identified as opportunities to further improve ethanol yields from AFEX™- and StEx-treated sugarcane residues.
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    Integrated bioenergy and animal feed production from AFEX TM and steam exploded sugarcane residues
    (Stellenbosch : Stellenbosch University., 2019-03) Mokomele, Thapelo; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Current and future trends demonstrate that the increasing world population, dwindling arable land, changing human diets and increased demand for (bio)energy present an opportunity to redesign the way land is used to meet the future food, feed and bioenergy demands. The sustainable integration of bioenergy and highly digestible livestock feed production systems has been touted as a potential avenue to increase the economic returns to agriculture and simultaneously promote energy security, particularly in developing countries. To this end, post-harvest residues from sugarcane processing (i.e. sugarcane bagasse (SCB) and cane leaf matter (CLM)) have emerged as candidate feedstock for integrated bioenergy (e.g. bio-ethanol and biogas) and animal feeds production in South Africa and Brazil. The principal aim of this dissertation was to perform a systematic comparison of the potential use of steam explosion (StEx) and ammonia fiber expansion (AFEXTM) as pretreatment technologies to overcome biomass recalcitrance, thereby generating highly digestible animal feeds, and cellulosic ethanol and biogas production feedstocks from sugarcane residues for future integrated biofuel-animal feed systems. A side-by-side comparison of the effect of StEx and AFEXTM pretreatment of sugarcane residues revealed AFEXTM to be the better pretreatment for maximising ethanol yields per Mg raw dry material (RDM) from both SCB and CLM. Under industrially relevant solids loadings of 16% and dosages of 9.8 mg protein/g RDM, AFEXTM pretreated sugarcane residues generated ethanol yields up to 324 litres/Mg RDM, the highest ethanol yields reported in literature from sugarcane residues. In contrast, ethanol yields from steam exploded sugarcane residues were limited to the range 205 to 257 litres/Mg RDM primarily due to the compounded effect of carbohydrate degradation during pretreatment, enzyme inhibition and microbial inhibition of recomninant Saccharomyces cerevisiae 424A (LNH-ST) during fermentation. To debottleneck microbial inhibition during the fermentation of non-detoxified StEx whole slurry’s, the potential use of industrial xylose-fermenting S. cerevisiae strains as efficient and inhibitor tolerant ethanologens was evaluated. S. cerevisiae strains CelluXTM 4 and TP-1 demonstrated near complete glucose and xylose consumption, with high acetate resistance, furan detoxification and phenolic aldehyde detoxification phenotypes. Ultimately, both strains facilitated the generation of 224 litres/Mg RDM from non-detoxified StEx SCB whole slurry under a pre-hydrolysis simultaneous saccharification and co-fermentation (PSSCF) configuration. In comparison, the same yeast strains generated moderately higher ethanol yields (254 litres/Mg RDM) during the PSSCF of highly fermentable AFEXTM-treated SCB, demonstrating that the difference in the potential ethanol yields that can be recovered from the two pretreatment technologies can be significantly reduced by using inhibitor-tolerant ethanologens. With both AFEXTM and StEx-treated sugarcane residues requiring enzyme dosages of 9.8mg protein/g RDM to achieve high ethanol yields, the potential use a room-temperature Cellulose IIIIactivation (CIIII-activation) process to enhance the digestibility of StEx- or AFEXTM-treated sugarcane residue pellets was investigated as a potential strategy to minimise the enzyme cost contribution per unit volume ethanol produced. Coupling AFEXTM sugarcane lignocelluloses with CIIII-activation reduced of the enzyme dosage requirements by more than 60% (to ~3 mg protein/g RDM), whilst achieving ethanol yields greater than 280 litres/Mg RDM. These results represented the lowest enzyme dosage to achieve ethanol yields of 280 L/Mg RDM reported in literature. In contrast, upgrading StEx-treated sugarcane residue pellets could only facilitate ethanol yields up to 201 litres/Mg RDM at an enzyme dosage of ~3 mg protein/g RDM. Besides ethanol production, both AFEXTM and StEx also demonstrated significant improvements in the animal feed value of SCB and CLM. AFEXTM-treated sugarcane residues were characterized by 230% increase in the non-protein nitrogen content of the biomass, and up to 69% and 26% improvement in the in-vitro true digestibility (IVTD) and metabolizable energy (ME), respectively, relative to untreated controls (P < 0.05). Although StEx did not increase the nitrogen content of the pretreated sugarcane residues, the IVTD and ME of StEx-treated SCB and CLM were improved by 54% and 7%, respectively (P < 0.05). These results demonstrated that both AFEX and StEx pretreatment can simultaneously generate highly digestible animal feeds and enhanced cellulosic ethanol feedstocks from sugarcane residues. The combination of the near optimal C/N ratios and structural modifications of AFEXTM-treated sugarcane residues also facilitated biogas production with methane yields up to 299 L CH4/kg VS, with or without co-digestion with dairy cow manure (DCM). To obtain comparable methane yields, untreated and steam exploded (StEx) sugarcane residues had to be co-digested with DCM, at mass ratios providing initial C/N ratios in the range of 18 to 35. Furthermore, the solid digestates recovered from the co-digestion of the sugarcane lignocelluloses with DCM were enriched in nitrogenphosphate-potassium (NPK), suggesting that they could be used as biofertilizers or partial replacements for the CLM that is typically left on the field during green cane harvesting. The results from this dissertation showed that both AFEXTM and StEx successfully enhanced the ethanol production potential, methane production potential, and animal feed value of sugarcane residues, providing alternative models for the sugarcane industry to create bioenergy and food value from sugarcane residues. Ultimately, these results provide essential information and insights for future techno-economic and life-cycle analyses that are required to establish the preferred pretreatment technology and processing strategies to enable economically viable and environmentally sustainable integrated bioenergy and animal feed production from South African sugarcane residues.