Comparison of Different Mechanical Pretreatment Methods for the Anaerobic Digestion of Landscape Management Grass

Abstract

The aim of this study was to use landscape grass from species-rich orchards for biogas production, thus preserving these very valuable areas for future generations. Since these grass clippings have high lignocellulose content, the substrate has to be pretreated before being fed into the biogas digester. In this study, three different mechanical treatment processes (cross-flow grinder, ball mill and a mounted mower) were investigated and compared with untreated grass clippings. Chemical composition, specific methane yield, degradation kinetics and microscopic images were analyzed. In order to derive recommendations, the harvesting and pretreatment processes were examined in terms of energy demand, additional methane yield, and suitability of the substrate for use in biogas plants, taking into account conservation aspects. Within the pretreatment process, ball milling leads to the highest significant increase in specific methane yield of up to 5.8% and the fastest gas formation kinetics (lag time λBM: 0.01 ± 0.0 d; duration to reach half of total gas production ½M(x)BM: 5.4 ± 0.2 d) compared to the untreated variant (λUT: 1.02 ± 0.2 d; ½M(x)UT: 6.5 ± 0.2 d). A comparison of the energy required for the mechanical disintegration of the substrates with the increased yield of methane during the digestion process shows that the mechanical processing of these substrates appears to be useful. A positive energy balance was achieved for the cross-flow grinder (12.3 kWh tVS−1) and the ball mill (21.4 kWh tVS−1), while the Amazone Grasshopper left a negative balance (−18.3 kWh tVS−1), requiring more energy for substrate pretreatment than was generated as methane surplus. In summary, the pretreatment of landscape management grass is a suitable approach for utilizing agricultural residues efficiently in a biogas plant and thus contributing to sustainable energy production.

Anaerobic Digestion of Cuttings from Grassland in Protected Landscape Areas

“…However, mowing orchard meadows is a labour-intensive task that cannot be compensated for by the expected biogas yields alone [68]. Nevertheless, the maintenance of these meadows represents a cultural landscape and nature conservation measure that has a high ecological value, which must be taken into account in monetary terms.…”

“…In contrast, the substrate can be used in biogas plants after mechanical pre-treatment at both the first and second cutting time without any further restrictions [68][69][70][71].…”

“…Zheng et al (2014) provide a good overview of the various methods of pre-treatment, the effect of the pretreatment and the resulting increase in methane yield from lignocellulosic material [76]. For the substrate delivered directly from the field to the biogas plant, the actual biogas yields to be obtained depend on the harvesting technique and also on the mechanical pre-treatment method to increase the conversion of the cell wall structural substances to biogas [68,77,78]. Specific methane yields were significantly correlated with the parameters describing the chemical composition of the harvested biomass (Table 1).…”

“…Pretreatment of the material has a positive influence on the kinetics of the biogas process [68,80]. However, the parameters for R m and ½ M are in ranges of values that would generally be too inert for demand-oriented biogas production, as currently promoted by policy in Germany [81].…”