Browsing by Author "Mihiretu, Gezahegn Teklu"

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    Advancing Lignocellulosic Biorefineries through Co-Production of Hemicellulosic Biopolymers and Bioenergy
    (Stellenbosch : Stellenbosch University, 2022-04) Mihiretu, Gezahegn Teklu; Görgens, Johann Ferdinand; Chimphango, Annie Fabian Abel; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: This research project was conceived in the context of advancing a lignocellulosic biorefinery for co-production of xylan biopolymers, bioethanol and electricity from two agro-industrial materials, namely sugarcane residues (SCT) and aspen wood (AW). The research was primarily designed to include two full-fledged experimental studies and one techno-economic case study. Accordingly, two biomass pretreatment approaches, namely: microwave-assisted pressurised hot water (MWA-PHW) and alkalinised steam explosion pretreatment (ASEPT) methods were experimentally investigated for their effect on the extraction of xylan from SCT and AW. Extraction experiments (via MWAPHW and ASEPT) were conducted by varying temperatures between 165 – 205 ℃ and retention times 3 - 22 min at test points identified using Central Composite Design (CCD) as response surface methodology (RSM). Pretreatment conditions were intended for a dual purpose: maximizing xylan extraction yield while simultaneously enhancing cellulose digestibility. Experimental results on xylan yield and cellulose digestibility were analysed using ANOVA method to establish optimal conditions for significantly enhanced values. Accordingly, under the MWA-PHW method, maximum xylan yields of 66 and 50%, and highest cellulose digestibility of 78 and 74%, were respectively attained for AW at (195℃, 20 min) and SCT at (195 ℃, 15 min). Whereas maximum xylan yields of 51 and 24%, and highest cellulose digestibility of 92 and 81%, were attained for SCT and AW respectively, following their pretreatment under ASEPT at (204 ℃, 10 min). Under both methods, the xylan extracts were predominantly non-monomeric with insignificant formation of degradation products. This strongly suggested both MWA-PHW and ASEPT were viable approaches for xylan extraction purposes. ANOVA results also revealed that temperature was the dominant factor influencing the xylan yield and cellulose digestibility. The techno-economic case study was aimed at evaluating the economic viability of the biorefinery for co-production of xylan biopolymers, bioethanol and electricity (i.e. main-case scenario, MCS) against two benchmark processes, i.e. Base-case (BCS) and Intermediate-case (ICS) scenarios, where only bioethanol and electricity are produced from sugarcane residues (Basis: daily capacity of 1000 tons of dry biomass subjected to ASEPT condition of 204 ℃ and 10 min). The study results showed that co-production of xylan biopolymers substantially improved the economic performance of the main biorefinery case (i.e. MCS) by lowering the selling price of ethanol against higher values under the benchmark processes. A minimum hemicellulose selling price (MHSP) of 809 USD/ton of xylan co-product was determined by fixing ethanol selling price at 0.70 USD/L (market price of ethanol in 2019); higher MHSP values certainly lead to further lower prices. Minimum ethanol selling prices (MESP) under the MCS, BCS and ICS were respectively estimated at 0.61, 0.95 and 0.81 USD/L, where the xylan price was assumed at 1000 USD/ton (=> MCS). Even though the economic viability of the main biorefinery case was significantly enhanced with co-production of xylan than without, this multiproduct biorefinery complex was rendered rather energy-intensive as a result of such coproduction scheme where the recovery of xylan biopolymers necessitated substantial thermal and electrical energy demands. From environmental point of view, the coproduction of xylan biopolymers along with bioethanol and electricity was shown to have a positive contribution towards mitigating GHG emissions from fossil sources. The GHG emissions savings under the MCS, BCS and ICS were estimated around 69, 64 and 65% against gasoline as fossil baseline of 90 gCO2eq/MJ (RSB-Global), but there was only marginal difference between the savings under the main biorefinery case and that under the benchmark processes.