Estudo do pré-tratamento hidrotérmico e do tratamento com água quente da palha e do bagaço de cana-de-açúcar visando a produção de etanol

Abstract

Renewable alternatives are increasingly being discussed with the aim of expanding the diversity of sustainable raw materials for the biofuel sector, promoting the transition to a cleaner and more renewable energy matrix. In this scenario, second-generation ethanol production emerges as an efficient alternative, since the carbohydrates present in lignocellulosic residues, such as sugarcane straw and bagasse, can be solubilized through pretreatments that release fermentable sugar monomers. Therefore, this work aimed to evaluate two distinct processes: hot water treatment and hydrothermal pretreatment of sugarcane straw and bagasse. In this context, from the immediate analysis, high levels of volatile matter and fixed carbon were observed for both sugarcane straw and bagasse, confirming a composition rich in residual organic compounds such as cellulose, hemicellulose, and lignin. In the hot water treatment, a full 2³ factorial experimental design was carried out, with three central points and nine unique combinations, analyzed using Minitab 17 software. The independent factors were: time (20 and 80 min, with a central point at 50 min), temperature (60 and 100 °C, with a central point at 80 °C), and straw/sugarcane bagasse ratio (25 and 75%, with a central point at 50%). Trial 6 (80 min, 60°C, and 75% straw) showed the highest release of reducing sugars (5.50 g/L). In the hydrothermal pretreatment, carried out in a closed system (autoclave) under 1 bar pressure, a 2² experimental design was adopted, considering as independent variables the time (20 and 80 min, with a central point at 50 min) and the straw/bagasse ratio (50 and 90%, with a central point at 70%) in the solubilization of lignocellulosic components. Trial 4 (80 min, 90% straw) showed the highest release of reducing sugars (6.64 g/L), highlighting the positive influence of pressure on the solubilization process of lignocellulosic components. The statistical analysis of the process showed that both processes were significantly influenced by the operational variables evaluated. The factorial design allowed the identification of statistically robust models for reducing sugars and yield, with adequate coefficients of determination (R²) validated by analysis of variance (ANOVA). In the hot water treatment, the straw/bagasse ratio had the greatest effect on the response, while the application of the Steepest Ascent method did not result in additional gains, confirming the proximity to the initial optimum region. In the hydrothermal pretreatment, the greater severity of the process intensified the solubilization of lignocellulosic components, observed by the reduction in the mass yield of the solid fraction and the increase in the release of reducing sugars. These results confirm the viability of hydrothermal pretreatment as an efficient alternative for reusing sugarcane straw and bagasse in the production of second- generation ethanol. From the perspective of Environmental, Social and Governance (ESG) criteria, this study demonstrates high relevance in encouraging sustainable practices in the biofuel industry, contributing to the mitigation of environmental impacts and the promotion of the circular economy, in addition to aligning with the Sustainable Development Goals (SDGs) 7, 9, 12 and 13 of the United Nations (UN).