Produção de hidrogênio e metano a partir de vinhaça e hemicelulose da palha de cana-de-açúcar: processos individuais e integrados para o aproveitamento biotecnológico de subprodutos da indústria sucroalcooleira

Abstract

Sugarcane plays a strategic role in the Brazilian economy, serving as the primary raw material for sugar and ethanol production. In the 2025/26 harvest, Brazil consolidated its position as the world's largest producer, producing 669 million tons of sugarcane and approximately 27 billion liters of ethanol. Ethanol production generates significant volumes of effluents, such as vinasse, with about 10 to 15 liters produced per liter of distilled ethanol. Additionally, sugarcane harvesting and processing generate around 140 kg of straw per ton of harvested cane. Currently, the main uses of these byproducts are fertigation for vinasse, while sugarcane straw is used for soil management after harvest. Vinasse, a material rich in organic load (COD) and minerals, can lead to soil and groundwater pollution when applied continuously and without proper management. Given the large volumes produced, sustainable management strategies are essential. Sugarcane straw, a lignocellulosic residue, also presents challenges: its accumulation in the field and its energy potential remain underutilized. Although it contains ne-third of the structural carbohydrates of sugarcane, part of these sugars, particularly hemicellulosic pentoses from the hemicellulosic fraction, can only be converted into second-generation ethanol by genetically modified microorganisms, making the process economically challenging and still less competitive than first-generation ethanol. In this context, the high COD of vinasse and the carbohydrates from straw hemicellulose emerge as strategic substrates for hydrogen (H₂) and methane (CH₄) production through fermentation and anaerobic digestion, providing both waste treatment and bioenergy generation. This study aimed to produce H₂ from vinasse and hemicellulose hydrolysate (HH) of sugarcane straw, both individually and through co-fermentation, and to sequentially generate CH₄ from the fermentation effluents. Anaerobic sludge from a wastewater treatment plant was used as inoculum. Both sealed systems and gas-releasing systems were evaluated to assess the effects of partial pressure. Hydrogen production from vinasse (25 gCOD/L) reached 245.86 NmL/L within 48 hours under thermophilic conditions (50 °C, pH 5.5) using a manometric reactor equipped with a gas release valve to prevent pressure from exceeding 0.6 bar. Partial pressure was not a dominant inhibitory factor for H₂ production, as comparison with sealed reactors revealed no significant differences, indicating that the complexity of the process is more closely associated with the characteristics of the effluent. Fermentation with hemicellulose hydrolysate was evaluated for carbohydrate concentrations of 1, 5, and 10 g/L in sealed reactors under the same pH and temperature conditions. Pathway shifts, primarily towards the homoacetogenic pathway, were observed due to gas accumulation, which favored its activation. At higher concentrations, such as 10 g/L, fermentation using a manometric reactor with gas release achieved 1107.97 NmL/L in 24 hours, an 82% increase compared to the sealed system. Co-fermentation of vinasse and hydrolysate improved microbial adaptation to the medium, resulting in a 58% increase in H₂ production compared to vinasse mono-fermentation, reaching 386.60 NmL/L of H₂ with a C/N ratio of 24. This study demonstrated that carbohydrates from the hydrolysate enhanced the digestibility of vinasse by microorganisms. Sequential CH₄ production from the co-fermentation effluent reached 4529 NmL/L (343 NmL/gCOD), while the effluent from single-stage vinasse fermentation achieved 3618 NmL/L (386 NmL/gCOD), compared to 2628 NmL/L (185 NmL/gCOD) obtained from the direct digestion of raw vinasse. Two-stage digestion of the hydrolysate effluent increased CH₄ production approximately five-fold (1259.37 NmL/L; 190 NmL/gCOD) compared to single-stage digestion (253.40 NmL/L; 20 NmL/gCOD). The separation of anaerobic digestion into two stages allowed for substrate pretreatment for the methanogenesis, resulting in greater CH4 production and concentrations in the biogas. Overall, this study highlights the benefits of co-fermentation and two-stage digestion for enhancing bioenergy integration within sugarcane biorefineries.