Techno-economic analysis of Xylitol, Citric Acid and Glutamic Acid biorefinery scenarios utilizing sugarcane lignocellulose
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH SUMMARY: A decrease in global sugar prices, lower crop yields (caused by drought) and the continued use of aging and inefficient infrastructure have been listed as external factors contributing to decreasing profitability of the South African (SA) sugar milling industry. In an effort to diversify product revenues and ensure future economic competitiveness within the sugar industry, it was proposed that second-generation (2G), lignocellulosic biomass (sugarcane bagasse and trash) be valorised for the co-production of chemical products and electricity in biorefineries annexed to existing mills. Detailed techno-economic analyses were conducted on three separate bioproduct scenarios with electricity co-generation, for a total feed capacity of 113.5 tons/h of sugarcane bagasse and trash. Products for biorefinery scenarios were selected based on potential applications, global demands and technological maturities, and included catalytic xylitol production, fermentative citric acid (CA) production (solvent extraction (SE) route) and fermentative glutamic acid (GA) production (hybrid membrane-integrated route). Detailed process simulations for each product scenario were developed using reported technology performances and Aspen Plus® software. Simulation results were used to define equipment, processes and utility requirements, and determine the total operating cost (TOC) and capital investment (TCI) for discounted cash flow (DCF) analyses at a 9.7% hurdle rate (real term basis). Profitability was characterised over a 25-year project lifespan in terms of financial parameters such as the internal rate of return (IRR), net-present (NPV), pay-back period (PBP). The economic profitability (IRR’s) of product scenarios were compared to a baseline scenario with electricity-only production from combined heat and power (CHP) plant, to assess whether it was more profitable to continue burning sugarcane biomass for electricity or to convert biomass to bioproducts with electricity co-production in integrated biorefineries. The co-production of xylitol (utilising a Raney-nickel catalyst) and electricity was achieved through a 46% bypass of the lignocellulose feed to the CHP plant. This scenario was deemed to be profitable, based on an average xylitol market price of $4538/ton, achieving an IRR of 30% (> 9.7% minimum real term rate) and an NPV of $405 million for a TCI of $220 million. CA production (SE purification route) requiring a 45% biomass bypass to the CHP plant for electricity co-production, was deemed to be the least profitable, based on an average CA market price of $1102/ton, achieving an IRR of 13%, above the minimum acceptable rate of 9.7%. A NPV of $77 million was achieved within a 13 year pay-back period, for a TCI of $291 million. The GA-electricity co-production biorefinery (utilizing a hybrid membrane integrated fermenter) required a 35% biomass bypass to the CHP plant, was deemed to be the most profitable product scenario, based on an average GA price of $3625/ton, resulting an NPV of $866 million and an IRR of 32%. The TCI ($422 million) was paid back over a period of 5 years. All product scenarios (xylitol, CA and GA) were able to achieve IRR’s in excess of the 8.3% achieved by the CHP baseline scenario, with electricity-only production (i.e. 100% biomass bypass).The profitability of all biorefinery scenarios were highly dependent on 3 main variables, namely the product selling price, FCI and the TOC. In addition to these variables, the xylitol biorefinery profitability was highly affected by catalyst price, whereas GA and CA profitability were sensitive to enzyme and solvent costs respectively. In terms of environmental impact, the GA scenario consumed the most process water and produced the highest quantity of solid waste and non-biogenic carbon emissions, whereas the CHP baseline scenario had the lowest environmental impact. Although the investigated xylitol and GA scenarios were deemed profitable, the proposed production scale of xylitol (38 789 tons/annum), based on lignocellulose availability, represented 20% of the world’s total xylitol demand; a reduction in the production scale to a more realistic market penetration will negatively affect profitability. The production scales for GA and CA represented much smaller and more realistic market penetration values of 3% (83 005 tons/annum) and 5% (97 893 tons/annum) of global product demand, respectively. Thus, whereas the GA scenario offered the most compelling investment case, its negative environmental impacts warrant further consideration.
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