Sustainable bioethanol production from Zambian corn stover

Abstract

Commercialization of second-generation bioethanol production is hindered by the lack of sustainable, cost-effective, and environmentally friendly pretreatment technology. The use of Deep Eutectic Solvents (DES) is a promising alternative. This study aimed to optimize DES pretreatment of Zambian corn stover to maximize bioethanol production. The specific objectives were to determine engine performance and emissions of bioethanol/gasoline blends; ascertain the ideal conditions for cellulose yield, enzymatic hydrolysis, and bioethanol generation; and conduct a techno-economic feasibility study of major scale DES-based bioethanol production. The factors studied during pretreatment included time (6–15 hours), temperature (60°C–150°C), choline chloride to lactic acid ratio (1:2, 1:6, and 1:10), and substrate-to-solvent ratio (SLR) (1:08–1:32). Hydrolysis was conducted at temperatures between 45°C and 50°C for 60– 72 hours. Optimization of pretreatment and hydrolysis was performed using Central Composite Design (CCD), Response Surface Methodology (RSM), Artificial Neural Networks (ANN), and Gradient Boosted Regression Trees (GBRT). Mathematical models were developed to estimate cellulose and fermentable sugar yields. The optimal pretreatment conditions:105°C, 10.5-hour reaction time, and a 1:6 ChCl:LA ratio yielded a 46.1% cellulose recovery, with model predictions achieving 43% (quadratic) and 46.1% (GBRT) at R2 values of 91% and 80%, respectively. Optimal enzymatic hydrolysis conditions enzyme loading of 10 mg per gram of biomass, 50°C, and 72- hour reaction time resulted in a fermentable sugar yield of 78%, validated through High- Performance Liquid Chromatography (HPLC). Fermentation using Saccharomyces cerevisiae produced bioethanol with an 80% yield, confirmed via Gas Chromatography- Mass Spectrometry (GC-MS). Distillation was conducted at 78.5°C using a computer- controlled bioethanol process unit. Through laboratory-level distillation, 2.82 g of bioethanol was obtained, leading to a final production volume of 3.57 L. Bioethanol/gasoline blends (G100, E10, E20, E30, and E40) were tested on an Atico computer-controlled hybrid test bench engine. Brake power and brake specific fuel consumption (BSFC) results were 31.42, 32.72, 34.03, 30.11, and 28.8 kW and 0.2706, 0.2516, 0.2333, 0.2765, and 0.3194 kg/kWh for G100, E10, E20, E30, and E40 blends, respectively. E20 provided the best balance between performance and emissions, increasing brake thermal efficiency (BTE) by 7.4% while reducing carbon monoxide (CO) and hydrocarbon (HC) emissions by 21% and 26%, respectively. Higher ethanol blends (E30 and E40) further reduced emissions but required modifications in ignition timing and fuel injection for optimal engine performance. A techno-economic analysis (TEA) assessed the feasibility of scaling up DES-based bioethanol production for a 50,000-liter capacity plant. The DES process was found to be 27% more cost-effective than conventional methods due to the recyclability and biodegradability of lactic acid and choline chloride, reducing overall fuel costs. A life cycle assessment (LCA) showed a 32% reduction in greenhouse gas emissions compared to fossil fuel-based gasoline. The results confirm the potential of DES-based pretreatment to enhance bioethanol production and improve economic viability.