Produção de hidrogênio a partir da fermentação anaeróbia de hidrolisados de resíduos agroindustriais

Abstract

Considering the increasing global demand for energy, the reduction of nonrenewable energy resources and concern for the environmental quality, hydrogen (H2) emerges as a clean, renewable and promising alternative, since its combustion results only in water and energy. Among the biological methods for H2 production, anaerobic fermentation is considered the most viable technology, since it does not require an external source of energy and several types of biomass can be used as carbon source by the microorganisms. Therefore, the aim of this work was to select an agroindustrial residue and to evaluate the production of H2 from anaerobic fermentation of its hydrolysate under different experimental conditions. The raw materials used were rice husk (CA), rice bran (FA), brewers’ spent grain (BC), soybean waste (RS) and wheat waste (RT). All materials were characterized, prepared and submitted to the acid hydrolysis process for the carbohydrate’s conversion into fermentable sugars under different conditions of temperature (104.4-127.0°C), time (20-60 min), acid concentration (5-15% H3PO4), and moisture (60-90%). The best experimental condition and the raw material to be used for H2 production were selected by total reducing sugars (ART) of obtained hydrolysates. Selected hydrolysate was then used as a source of substrate in the anaerobic fermentation process for H2 production, using anaerobic sludge as inoculum. Fermentation tests were conducted according to the factorial experimental design, evaluating the effect of temperature (35- 45°C), initial pH (5.5-7.5) and inoculum ratio (10-30%) in H2 production and maximum H2 production rate. All experiments were monitored for 20 h. The composition of the medium was analyzed before and after each assay in terms of sugars, inhibitors, and acetic acid. The final pH of each experiment was also determined. The experimental condition of acid hydrolysis that resulted in the highest concentration of ART in the hydrolysates of rice husk and rice bran was assay 3 (127°C, 60 min, 15% (w/w) H3PO4 and 60% moisture). In this condition, the concentration of ART was 118.16 g kg CA -1 and 170.38 g kg FA -1. On the other hand, the assay 1 (127°C, 20 min, 5% (w/w) H3PO4 and 90% moisture) resulted in higher ART concentration in brewers’ spent grain, soybean waste and wheat waste hydrolysates. The concentration of ART was 600.97 g kg BC -1, 80.37 g kg RS -1 and 228.04 g kg RT -1. From the results of hydrolysis, brewers’ spent grain was the raw material chosen for the H2 production process, as it resulted in the highest ART concentration. In addition, the BC will be available in large quantity during all over the year. The highest H2 yield (4160 mL L-1) and maximum H2 production rate (760 mL L-1 h-1) were found in assays 7 (35°C, pH 7.5 and 30% inoculum) and 8 (45°C, pH 7.5 and 30% inoculum), respectively. In some assays, glucose consumption of the BC hydrolysate reached 100%, while xylose was consumed up to 91.56%. The concentration of inhibitors (furfural and 5-HMF) before and after the fermentation process did not have significant alteration and did not show negative interference in H2 production. On the other hand, the concentration of acetic acid had an expressive increase from a range of 0.1392-0.3001 g L-1 to 1.1490- 5.8392 g L-1, which explains the drop in pH in the fermentation medium at the end of the tests. Finally, it was concluded that brewers’ spent grain hydrolysate presented appropriate characteristics for H2 production by anaerobic fermentation, leading to promising results when compared to the literature.