适宜原料压实度改善玉米秸秆厌氧干发酵特性

干发酵原料中固形物含量高,结构疏松,原料的压实度直接关系到发酵罐的利用率和生产成本。为了探讨原料压实度对厌氧干发酵过程的影响,该文以玉米秸秆粉为试验材料,研究了不同固形物含量(TS)条件下原料压实度对厌氧干发酵产气特性及物料特性变化的影响,结果表明:相同TS下,随着压实度的增加,日产气量(基于可挥发性固形物,VS)峰值降低且出峰时间略有滞后;原料压实度对累积产气量、日产气量变化趋势以及甲烷体积分数的影响不明显。基于发酵罐容积计算产气量时,原料压实度与日产气量峰值、累积产气总量以及累积甲烷产量均呈显著正相关。不同TS条件下的原料压实度对厌氧干发酵过程的影响程度不同,TS含量越高原料压实度对产气量的影响越明显。TS为20%和25%时,逐层压实发酵罐的单位容积累积产气量比自然填料罐分别提高了37.1%和60.2%。当发酵料初始TS为20%时,原料压实度的增加可促使物料中形成连续的液相,改善物料层间的传质,从而使发酵料上下层间的差异减小。当发酵料初始TS提高到25%时,原料压实度增加对传质特性无明显影响。综上所述,在该试验研究范围内,提高原料压实度可以保证玉米秸秆厌氧干发酵产气过程的正常进行,且可以显著提高发酵罐的容积利用率。该研究对厌氧干发酵原料压实度与发酵特性之间的关系进行了初步探讨,以期为有效提高厌氧干发酵罐的产能能力提供参考。

Suitable compactness of corn stover improving characteristics of solid-state anaerobic digestion

Abstract: To study the effects of compactness on the performance of solid-state anaerobic digestion (SS-AD), the SS-AD of corn stove was conducted with different total solid (TS) contents. The digestion material with the TS content of 20% was loaded into the reactors at the compactness of 0.56, 0.71 and 0.79 kg/L, and the compactness of the digestion material with the TS content of 25% was 0.43, 0.56 and 0.70 kg/L, respectively. The results showed that with the same TS content, the compaction degree had no significant effect on the peak time of daily methane yields, but the peak value significantly decreased (P<0.05) with the compaction degree increasing. For the reactors with the initial TS of 25%, the peak value of daily methane yield of the reactors with the compactness of 0.70 kg/L decreased by 12.8% and 21.8% compared to that with the compaction degree of 0.56 and 0.43 kg/L, respectively. The variation trends of methane concentration and daily methane yield and cumulated methane yield (based on volatile solid, L/(kg·d)) of the reactors with different compactness of the digestion material had no significant difference. However, if calculating the biogas yield based on the volume of the reactor, the result would be completely different. Both of the peak values of daily methane yield and accumulated methane yield were positively related to the compactness. The effects of compactness on solid state anaerobic digestion process were enhanced with the increase of the initial TS content. With the initial TS content of 20%, the peak values of daily methane yield and accumulated methane yield of the reactors with the compactness of 0.79 kg/L were 0.83 L/(L·d) and 32.48 L/L, respectively, which were 29.2% and 37.1% higher than that of the reactors with the compactness of 0.56 kg/L. When the initial TS content was increased to 25%, the peak values of daily methane yield and accumulated methane yield of the reactors with the compactness of 0.70 kg/L were 0.84 L/(L·d) and 25.08 L/L, respectively, which were 52.7% and 60.2% higher than that of the reactors with the compactness of 0.43 kg/L. For the reactors with the initial TS content of 20%, the physical and chemical properties, such as TS, volatile solid (VS), pH value and total volatile fatty acid (TVFA) concentration of the substrates at top and bottom layer were obviously different during the digestion, while the difference gradually narrowed with the increase of compaction degree of the digestion material. However, the physical and chemical properties of the substrates at top and bottom layer in the reactors with the initial TS content of 25% were similar. This was probably due to the high solids content, which made it difficult to form continuous free flowing mobile phase in the substrate, and then the penetration and diffusion of the intermediates were blocked. The results above indicated that increasing packing density of the substrate with the initial TS content of 20% could improve the mass transfer process of the intermediates. The increase of the compactness made it possible to form a continuous liquid phase in the digestion material, which was beneficial to the degradation of the substrate and diffusion of the intermediates. While for the substrate with the initial TS content of 25%, increasing packing density had no significant effect on mass transfer. In summary, the increase of the compaction degree of the digestion material can maintain the normal process biogas yield of the solid-state anaerobic digestion of corn stover, and also can significantly enhance the volume efficiency of the reactors.