玉米秸秆储存方式对其与牛粪混合厌氧消化特性的影响

摘要

农作物秸秆是沼气生产的主要原料，但因其季节性收获特点，必须妥善储存才能为沼气全年生产提供高质量原料。该研究以玉米秸秆为原料，探索了储存方式（风干或青贮）对其厌氧消化产沼气性能的影响。首先研究比较风干秸秆和青贮秸秆的化学成分和结构特性，然后分别考查两种秸秆与牛粪在不同总固体混合比（total solids mixing ratios，TMR）条件下的产气特性。结果表明，玉米秸秆青贮后的纤维素等可消化纤维组分保存良好，风干秸秆的可溶性糖和水分损失殆尽。扫描电镜（scanning electron microscope，SEM）和X-衍射（X-ray differaction， XRD）结果显示，风干秸秆的木质纤维结构紧密，青贮秸秆表面则有很多裂缝和孔洞，木质纤维暴露，纤维素结晶度（0.42）低于风干秸秆（0.46）。厌氧消化结果表明，无论风干或青贮秸秆，当牛粪与秸秆TMR值为7∶3时，累积产气量、TS/VS产气量和TS/VS产甲烷量均高于其他4种比例，且青贮秸秆的产气性能优于风干秸秆。除1∶1比例外，其余TMR条件下青贮秸秆的累积甲烷产量均高于风干秸秆，当TMR值为7∶3时青贮秸秆的累积甲烷产量最高（1816 mL）。秸秆青贮的储存方式不仅能良好保存化学组分和水分，还有利于提高沼气产量和品质，是适用于秸秆跨季节储存和预处理的一种可行方式。%The use of energy crops as feedstock for biogas production is increasing. Since energy crops accumulate seasonally but biogas plants have to be fed continuously, the feedstock has to be conserved. Therefore, the storage of energy crops is an important issue to handle, as crops need to be available throughout the year. Research on preservation and storage of energy crops as feedstock for anaerobic digestion is still at an emerging stage. The aim of this paper is to explore the effects of storage methods (air drying or ensiling) of maize straw on biogas production performance. Evaluations of anaerobic co-fermentation of maize straw and cattle manure were conducted by batch fermentation in a laboratory scale. Biogas production was measured for 45 days in 1.5L batch digesters at mesophilic conditions of 37℃. First, the chemical composition and structural properties of air-dried maize straw (ADMS) and ensiled maize straw (EMS) were comparatively analyzed. Then, the effects of total solids (TS) mixing ratios (cattle manure: maize straw) on biogas conversion were discussed. TS mixing ratios (TMR) were 1:1, 3:7, 7:3, 4:6, and 6:4, respectively. The dynamic analysis of daily biogas production, accumulative biogas production, accumulative methane production, and pH value was implemented. The results show that ensiling had little effect on the cellulosic composition of EMS, however water and WSC of ADMS nearly vanished. In other words, the properties of the maize straw were influenced by the method of storage. The results of SEM and XRD indicate that the lignocellulosic matrix of ADMS is compact, but cracks and ruleless pores were taken on the surfaces of EMS, signaling that the crystalline degree of cellulose was lower than that of ADMS. Storage by ensiling can be considered as a pretreatment process, because the structural polysaccharides of maize straw, which is quite resistant to anaerobic digestion, are partly degraded during storage. Results also show that the anaerobic efficiency of biogas production, the cumulative biogas yield, TS/VS biogas yield, and TS/VS methane yield were highest when the TMS was 7:3, whether ADMS or EMS. The accumulative methane production of EMS was higher than that of ADMS when the TMS was 1:1, and the highest accumulative methane production was 1 816 mL when the TMS was 7:3. The present results demonstrate that the biogas yield of maize straw can be maintained by appropriate ensiling conditions for even after 8 months. Ensiling, which acts as a pretreatment step, showed positive effects on the methane yield of maize straw, and the degradation of structural polysaccharides into more easily degradable intermediates caused the increase in biogas potential after ensiling. In conclusion, ensiling is an appropriate, feasible storage method to preserve fresh crops throughout the year for an anaerobic digestion facility in order to ensure a predictable quality of crops and a constant supply of feedstock.