Estudo da produção de etanol lignocelulósico a partir de resíduos do cultivo de coco

Abstract

The use of lignocellulosic biomass for the production of second-generation ethanol, such as green coconut husk, implies the development and improvement of biomass conversion steps into sugars and these into ethanol via fermentation. Thus, the execution of the pretreatment step, responsible for breaking the lignocellulosic matrix leaving it accessible to enzymatic attack in the hydrolysis step, is necessary. With the available monomers (mainly xylose and glucose), the yeast converts them into ethanol, in the fermentation step. Thus, this work aimed to study the conditions of saccharification and fermentation, using residues from coconut cultivation as biomass, with the objective of evaluating the best conditions of saccharification and fermentation for the production of E2G. Initially, the hydrothermal pretreatment (HPT) was carried out in a PARR type reactor using 15% (w/v) of solids, at 195°C, 10 min and 200 rpm. In parallel, an experimental design was carried out for the acid pretreatment (APT) (factors: concentration 1 and 2% of H2SO4 and temperature of 100 and 120°C) aiming at optimizing the pretreatment operating conditions. Depending on the glucose concentration, the conditions of 1% (v/v) H2SO4 at 100°C for green coconut shell (GCS), 2% (v/v) H2SO4 at 120°C for coconut-tree leaflet (CLL) and 2% (v/v) H2SO4 at 100°C for the coconut-tree leaf stalk (CLS) followed by enzymatic hydrolysis, using 7.5% (w/v) of solids and 20 FPU/gbiomass dry Cellic Ctec2 (58 FPU/mL), supplemented with 10% Cellic Htec (7326 IU/mL), relative to the volume of Cellic Ctec2. After 72h of reaction, the glucose concentrations reached 31.85 and 19.07 g/L for the hydrolysis of GCS subjected to HPT and APT, respectively. Regarding the behavior of CLL in hydrolysis, 21.31 and 13.65 g/L of glucose concentration were reached when this biomass was submitted to HPT and APT, respectively. For CLS, 45.39 and 21.01 g/L of glucose were obtained after HPT and APT, respectively. These results showed that enzymatic hydrolysis using hydrothermal pretreatment was more efficient than the results obtained for pretreated acidic biomasses. Then, all samples were sent to fermentation with Kluyveromyces marxianus yeast at 37°C/24 h. At this stage it was possible to verify that practically all glucose and xylose were consumed by the yeast. However, the ethanol yield reached values between 47 and 80%, indicating possible problems of a toxic nature, that is, the production of secondary compounds, such as acetic acid, furfural and 5-hydroxymethylfurfural (HMF). A preliminary study was carried out to increase the fermentation efficiency in sugarcane bagasse hydrolyzate, biomass well studied for this purpose, and submitted to hydrothermal (195ºC, using 200 rpm for 10 min) and acid (0.5% (v/v) of sulfuric acid at 121°C for 15 min) and in both, the solid load of (10% m/v). The enzymatic hydrolysis of the pretreated material was carried out using the CellicCtec® enzymatic complex (60 FPU/gdry biomass, 50mM citrate buffer and pH 4.8) at 50ºC using 150 rpm for 72h. Before the detoxification process, an experimental design was carried out with the yeast Saccharomyces cerevisiae to verify if the compounds furfural (1 and 4g/L) and acetic acid (1 and 5% v/v) and yeast mass (0.2 and 1% m/v) exerted significant inhibition on the tested species. The presence of furfural and acetic acid had a strong influence on the species considered, affecting the consumption of sugars in the environment by more than 90%. The detoxification process evaluated the concentration through an experimental design with activated charcoal (1, 3 and 5% m/v) and the process time (30, 45 and 60 min) at 30 ºC, 150 rpm. Detoxification increased the fermentation efficiency by 13% for the hydrolyzate obtained hydrothermally, while for the acid there was no significant difference. Thus, it was possible to conclude that the study of process conditions enabled the increase in ethanol production compared to the values already reported in previous studies with green coconut husk. The potentiality of the coconut leaf for the production of second-generation ethanol was identified, and it was verified that the detoxification conditions can be adapted to the biomass of coconut cultivation to increase efficiency and make the results feasible for industrial production.