Techno-economic assessment of biofuels production from sugarcane bagasse

Abstract

In tropical countries such as South Africa, sugarcane bagasse is one of the most abundant lignocellulosic renewable feedstocks for biofuel production. The adoption of integrated processing routes for utilising by-products is a promising approach for achieving near-complete conversion of organic biomass while reducing waste generation. This study investigates a second-generation sugarcane bagasse biorefinery annexed to a sugar mill. A techno-economic assessment approach was adopted to determine the feasibility of each investigated process route. Five sugarcane bagasse-to-biofuel conversion routes were evaluated, namely the bioethanol fermentation process, the anaerobic digestion process, the dark fermentation process, the gasification process, and the fast pyrolysis process. Each process route was simulated individually in Aspen Plus V11 using input data obtained from literature sources. The simulations generated mass and energy balances, which were subsequently utilised for techno-economic analyses (TEA) and life cycle assessments (LCA). The TEA was performed using Aspen Process Economic Analyzer (APEA) and Microsoft Excel spreadsheets, while the LCA was conducted using SimaPro software.The study considered a typical sugar mill located at Sezela in KwaZulu-Natal, South Africa, with a cane crushing capacity of 300 tonnes of sugarcane per hour, producing approximately 81 tonnes of sugarcane bagasse per hour. It was assumed that 36% of the bagasse is utilised in boilers for steam generation. The remaining portion (51.84 tonnes per hour) was allocated to the biorefinery, which was assumed to be annexed to the sugar mill. This configuration eliminates transportation costs and minimises feedstock costs. Following the completion of process simulations in Aspen Plus, Aspen Process Economic Analyzer was used to estimate the installed equipment costs for each investigated process route. The installed equipment costs were escalated from a 2019 base year, consistent with Aspen V11 cost correlations, to 2024 values using the Chemical Engineering Plant Cost Index (CEPCI). Capital expenditure (CAPEX) for each process route was estimated using the installed equipment costs. Variable operating costs (VOC) and fixed operating costs (FOC) were combined to determine the operating expenditure (OPEX) of each biorefinery. It was assumed that the plant operates for 5000 hours per year. Total sales revenue was calculated based on the sales of valuable final products, and selling prices were obtained from literature sources and online market prices. Discounted cash flow analysis was used to evaluate theeconomic performance of each process route, assuming a tax rate of 28%, a discount rate of 12%, and straight-line depreciation over 5 years at 20% per year. The plant lifetime was assumed to be 25 years. Four economic indicators were calculated, namely net present value (NPV), payback period (PBP), return on investment (ROI), and profitability index (PI). Life cycle assessment (LCA) was conducted to evaluate the environmental impacts of the investigated process routes. SimaPro software was used for the LCA, following ISO 14040 and ISO 14044 methodology. The ISO framework includes four main stages: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation of results. The goal and scope of the LCA were to determine the least environmentally impactful processing route using a cradle-to-gate system boundary. The LCI was based on mass and energy balances obtained from Aspen Plus simulations, and the Ecoinvent database in SimaPro was utilised. The LCIA was performed using two approaches. The first was damage assessment, which applied the ReCiPe 2016 Endpoint (H) method and evaluated three damage categories, namely human health, ecosystems, and resources. The second approach was characterisation, which applied the ReCiPe 2016 Midpoint (H) method. Under the midpoint approach, eight impact categories were evaluated, including global warming, fine particulatematter formation, terrestrial acidification, freshwater eutrophication, marine eutrophication, mineral resource scarcity, fossil resource scarcity, and water consumption. The fast pyrolysis process was identified as the most profitable process route, achieving an NPV of 199.44 million USD, a payback period of 1 year, an ROI of 10.27, and a PI of 46.64. The dark fermentation process ranked second, with an NPV of 67.41 million USD, a payback period of 3.3 years, an ROI of 1.51, and a PI of 7.95. The gasification process ranked third, with an NPV of 23.21 million USD, a payback period of 3.6 years, an ROI of 1.40, and a PI of 7.31. The anaerobic digestion process ranked fourth, with an NPV of 37.57 million USD, a payback period of 4.4 years, an ROI of 1.16, and a PI of 5.85. Under the conditions assumed in this study, the bioethanol fermentation process was economically unfeasible, as it produced a negative NPV, indicating that the project would not recover its initial investment by the end of the plant lifetime.The life cycle assessment results showed that the dark fermentation process was the most environmentally favourable process route, followed by the fast pyrolysis process, anaerobic digestion process, gasification process, and bioethanol fermentation process, respectively. The viiibioethanol fermentation process showed the highest environmental impacts across all evaluated impact categories, making it the least environmentally favourable processing route. Electricity was used as the energy source in the LCA, and energy consumption significantly influenced environmental impacts. Since South African electricity is largely generated from coal, process routes with high energy demand exhibited higher environmental impacts and were therefore less environmentally sustainable.