Effect of maize straw storage practice on biogas production performance during anaerobic co-digestion with cattle manure

Abstract:The use of energy crops as feedstock for biogas production is increasing. Since energy crops accumulates seasonally, but biogas plants have to be fed continuously, the feedstock has to be conserved. Therefore 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 the emerging stage. The aim of this paper is to explore the effect of storage methods (air drying or ensiling) of maize straw on biogas production performance, evaluation of anaerobic co-fermentation of maize straw and cattle manure was conducted by batch fermentation in laboratory scale. Biogas production was measured for 45 days in 1.5 L batch digesters at mesophilic conditions 37℃. First of all, the chemical composition and structural properties of air-dried maize straw (ADMS) and ensiled maize straw (EMS) was comparatively analyzed. And on this basis, 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 were 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 the cracks and ruleless pores were taken on the surfaces of EMS, the crystalline degree of cellulose was lower than that of ADMS. The 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, and 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 were higher than that of ADMS except the TMS of 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 methane yield of maize straw, and the increase in biogas potential after ensiling is assumed to be caused by degradation of structural polysaccharides into more easily degradable intermediates. In a word, ensiling are an appropriate feasibly storage methods 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.