Process Development for Ethanol Production from Cellulose-Rich Bagasse Residues after Furfural Production
Date
2025-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
Cellulosic ethanol production is often costly, where the pretreatment process and enzymatic
hydrolysis contribute to high capital and operational costs. In this study, cellulose-rich furfural
residues (FRs) from a South African sugar mill were used as feedstock for cellulosic ethanol
production. The FRs used are the by-product of the furfural production process, which uses
sugarcane bagasse as raw material. Furfural is produced industrially by treating bagasse at 184 °C
and 10 bar for 80-90 minutes, using the by-products, acetic and formic acids to catalyse the
process. After removal of furfural by steam-stripping, the resulting cellulose-rich FRs have the
potential to be converted into ethanol, and provide process energy. The FRs can eliminate the need
for a pretreatment process, provided that the residues exhibit excellent enzymatic
digestibility. Conversion of FRs can add value to the sugar industry, which is under increasing
pressure from economic forces in the sugar markets. This study investigated critical aspects of FRs
to 2G ethanol process, aiming to achieve acceptable process performance, while minimising the
operational costs of enzymes and yeasts for the process. The latter was essential since the
hydrolytic enzymes that digest the cellulose into fermentable sugars are by far the greatest
operational cost, which, if left unchecked, would render this process economically infeasible. To
achieve this target, four commercial enzyme cocktails, namely, Cellic® CTec3, SacchariSEB C6L Plus,
Viscamyl™ Flow, and Cellic® CTec3 HS, were screened to identify the most efficient and least costly
options, together with seven Saccharomyces cerevisiae strains, of which two strains, namely
Cellusec® 2.0 and Cellusec® 3.3 secreted cellulase enzymes. The latter two strains were essential
to minimise commercial enzyme dosages, while achieving attractive ethanol yields, concentration and
productivity to minimise operational costs. To maximise performance and productivity, fermentations
were done in fed-batch simultaneous saccharification and fermentation (SSF) to maximise the solids
loading and maintain an ethanol titre of > 30g/L. Whereas ethanol concentrations of 36 g/L at 89%
yield could be achieved at a high enzyme dosage of 5 FPU/g DS and a solid loading of 15% (w/w),
which served as a benchmark, the enzyme-producing yeast S. cerevisiae strain Cellusec® 3.3 could
reduce the exogenous enzyme dosage to 2 FPU/g DS, albeit at a
decreased ethanol concentration of 24.9 g/L (60% yield) at a solid loading of 15% (w/w).
Further decrease of exogenous enzyme dosage to 1.5 FPU/g DS was achieved at a high solid loading of
25% (w/w), but this was associated with accumulation of fermentation inhibitors, thus limiting the
final ethanol concentration to 19 g/L at 21% yield, with residual glucose of 45 g/L remaining at
the end of fermentation. Whereas research is ongoing on further process development to circumvent
the issue of residual glucose accumulation, an ethanol titre of 41 g/L was theoretically possible
should all the residual glucose at the end of fermentation be converted into ethanol. The study
successfully lowered commercial enzyme dosage, which could lead to reduced operational costs
associated with exogenous enzymes. Consolidated bioprocessing (CBP) yeasts that secrete cellulase
enzymes should be engineered to withstand inhibitors, to maintain
viable yeast cells and increase ethanol production.
Description
Thesis (MEng)--Stellenbosch University, 2025.
Bunga, G. E. 2025. Process Development for Ethanol Production from Cellulose-Rich Bagasse Residues after Furfural Production. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/6fbfddf3-9bf1-4612-bec0-20b4e4edb985
Bunga, G. E. 2025. Process Development for Ethanol Production from Cellulose-Rich Bagasse Residues after Furfural Production. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/6fbfddf3-9bf1-4612-bec0-20b4e4edb985