Abstract
The effect of reducing particle size on physical properties, the methane yield and energy flow were investigated through the biochemical methane potential (BMP) experiment of aerobic-anaerobic digestion (AAD) of rice straw (RS). The whole straw was crushed through four sieves of different aperture sizes (1, 3, 5, and 7 mm) to obtain the actual and non-uniform particle size distribution (PSD). The results indicated that the actual particle sizes were normally or logarithmic normally distributed. Reducing particle size could significantly promote the aerobic hydrolysis and acidification process, increase the content of volatile fatty acids (VFAs) from 4408.78 to 6225.15 mg/L and the degradation of volatile solids (VS) from 40.56% to 50.49%. The results of path analysis suggested that particle size reduction played an important role in improving lignocellulosic degradability, which was the main factor affecting methane production with the comprehensive decision of 0.4616. The maximum methane production obtained at 1 mm sieve size was 176.47 mLCH4g−1 VS. The phyla of Firmicutes (61.5%), Proteobacteria (9.3%), Chloroflexi (8.3%), Bacteroidetes (4.1%), Cyanobacteria/Chloroplast (4.6%) were mainly responsible for VFAs production and lignocellulose degradation. However, the net negative energy balance was observed at the 1 mm sieve size due to the increased energy input. Therefore, the optimum sieve size for AAD was 3 mm.
Highlights
Rice straw is one of the most abundant renewable energy sources in China, with an annual output of about 200 million t [1], which is mainly composed of 19–27% hemicellulose, 32–47% cellulose, and 5–24% lignin [2,3]
Energies 2021, 14, x FOR PEER REVIETWhe 0.25–0.425 mm particles accounted for 47.82% at the 1 mm sieve size sho8wofin16g that the particle sizes were in a narrower range than other groups
(1) The particle size distribution was in a narrower range under smaller sieve sizes
Summary
Rice straw is one of the most abundant renewable energy sources in China, with an annual output of about 200 million t [1], which is mainly composed of 19–27% hemicellulose, 32–47% cellulose, and 5–24% lignin [2,3]. Anaerobic digestion (AD) is a highly desirable technology for converting organic wastes into value-added products such as biogas and organic fertilizer [5,6]. It can be divided into two stages: hydrolysis-acidogenesis and methanogenesis, which are regulated by different kinds of microorganisms. Due to the recalcitrant lignocellulosic structure, it is difficult to hydrolyze rice straw, resulting in the low AD efficiency [7]. Various pretreatment methods have been applied to increase the accessibility of microorganisms to lignocellulose [8]. No effective method has been found to strike a balance between economy and environmental friendliness
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