Different mixing modes for biogas plants using energy crops

Biogas is produced by anaerobic digestion of organic matter in digesters, which must be stirred. Stirring is responsible for up to 54% of the power consumption of current biogas plants [Dachs; Rehm 2006]. One possibility to operate biogas plants more efficiently is to reduce the energy consumption by avoiding constant stirring. In this study different mixing modes were investigated in three equal laboratory scale digesters with a liquid volume of 22 l. During the experiments the gas volume flow was measured continuously, the gas quality once a day and the dry matter and volatile solids once a week. Two experimental studies were carried out. In the first one corn cob mix and cow manure were used as input materials. The mixing mode for digester 1 was 2 h stirring and 1 h break, for digester 2 7 h stirring and 1 h break. Digester 3 was stirred continuously as reference. The relative deviation of digesters 1 and 2 to the reference digester stayed below 10% (corresponding to the reproducibility, as analyzed in previous studies). After increasing the organic loading rate the relative deviation started to fluctuate. The average of the methane concentration in the produced biogas was about 60 mol-%, and varied for the three digesters only within the uncertainty of the measurement. For the second experimental study maize silage and cow manure were used as input materials. Digester 3 was again stirred continuously as reference. Digester 1 was stirred for 10 mm followed by a 230 min break. Digester 2 was also stirred for 10 min, but a break of only 50 min followed. In the first days digesters 1 and 2 produced up to 20% more gas compared to the reference. After that the gas production decreased and fluctuated in a range of +/- 10% relative to the reference gas production. The deviations in the methane concentration between the three digesters were within the uncertainty of the measurement. The average of the methane concentration was about 57 mol-%. During the study swelling and foaming of the digestate could be observed for the digesters with non-continuous mixing. This should be taken into account for applications at industrial scale biogas plants. During both experimental studies no negative impact of the mixing modes on the gas volume flow and the methane rate could be observed. During the second study the gas volume flow of digesters 1 and 2 was even higher at the beginning than the one of the reference digester. 29% of the power consumption used for mixing could be saved in case of study 1, digester 1 using alternating mixing modes, without loss in gas flow and methane yield. In the second study with longer breaks more power could be saved. Additionally layering of microorganisms for non stirred digesters was investigated. For the first study the digester was not stirred for 24 h and for about 2 months in the second study. The results indicated that methanogene microorganisms prefer the lower layers of a non stirred digester. (C) 2013 Elsevier Ltd. All rights reserved.