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See detailDigestion of grass silage in a semi-continuously fed reactor at increasing loading rates - Process stability and kinetics evaluatio
Benito Martin, Patricia UL; Greger, Manfred UL; Schlienz, Markus UL

in Venice 2014 5th International Symposium on Energy from Biomass and Waste Proceedings (2014, December 01)

Grass is a popular substrate in agricultural biogas plants in Central and Northern Europe. There is currently limited information about the impact of the organic loading rates (OLR) on methane yields ... [more ▼]

Grass is a popular substrate in agricultural biogas plants in Central and Northern Europe. There is currently limited information about the impact of the organic loading rates (OLR) on methane yields, process stability and kinetics for continuous systems digesting grass silage. Anaerobic mono-digestion of grass silage was evaluated in semi-continuously fed laboratory continuously stirred tank reactors (CSTRs) at increasing loading rates between 1.9 and 4.7gVS/l/d. Results show that digestion of grass silage in one-stage CSTR is feasible and do not present any loading-induced inhibition for the tested OLR range. While the volumetric methane production experienced an increase of 61%, the methane yield only decreased by 13% for the highest OLR. On the other hand, the estimated first-order hydrolysis constant decreased by 24% when increasing the loading from 1.9 to 4.7gVS/l/d, which related with an accumulation of the organic material in the reactor. It was also found that the composition of the grass silage affected the digester performance significantly. [less ▲]

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See detailBiodegradability of Cellulose in a Continuous Reactor with Increasing Feeding Rates
Benito Martin, Patricia UL; Greger, Manfred UL; Beierlein, Nomathamsanqa Nonjabulo UL

in Hoffmann, C.; Maniatis, K.; Grassi, A. (Eds.) et al 22nd European Biomass Conference Proceedings - Setting the course for a biobased economy (2014, July)

Cellulose is significantly present during the anaerobic digestion of energy crops and agricultural waste for biogas production. Nevertheless, its degradation poses some challenges due to its structure and ... [more ▼]

Cellulose is significantly present during the anaerobic digestion of energy crops and agricultural waste for biogas production. Nevertheless, its degradation poses some challenges due to its structure and resulting slow hydrolysis. The purpose of the current research was to analyse the response of a continuous anaerobic system digesting cellulose to increasing loading rates, in terms of the impact on the specific methane yield and biogas composition as well as on the process dynamics. To this end, a continuously stirred tank reactor with a working volume of 10 litres was operated under mesophilic conditions for over 60 days. Cellulose was added in the reactor daily as the sole carbon source at different organic loading rates (1gVS/l/d and 1.5 gVS/l/d). An increase in the OLR up to 1.5gVS/l/d resulted in an important decrease of the degree of solubilisation, thus creating an accumulation of the cellulose in the reactor and fluctuating methane production, below potential values. In particular, the results suggest that an acetate concentration of 1.3g/l in the medium, which was reached after increasing the loading, could have an effect on the enzymatic cellulose hydrolysis. This work contributes to the limited knowledge of cellulose solubilisation in continuous systems under anaerobic conditions. [less ▲]

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See detailOptimisation of the Co-Digestion of Maize and Grass Silage Influence of the Substrate Mixture on the Biogas Yield
Benito Martin, Patricia UL; Sibisi-Beierlein, Nonjabulo N.; Greger, Manfred UL

in Bilitewski, B.; Clarke, W. P.; Cossu, R. (Eds.) et al Venice 2012 Fourth International Symposium on Energy from Biomass and Waste Proceedings (2012, November)

In recent years, energy crops have been increasingly introduced as co-substrates in biogas plants due to their high biogas yield potential. The purpose of the current research was to investigate the ... [more ▼]

In recent years, energy crops have been increasingly introduced as co-substrates in biogas plants due to their high biogas yield potential. The purpose of the current research was to investigate the influence of the mixture of maize and grass silages, added as feedstock, on the specific methane production (SMP) and on the process dynamics. To this end, biological methane potential (BMP) tests were carried out in 500ml reactors with grass and maize silage in mono-digestion and 3 different co-digestion mixtures. For selected mixtures, process dynamics were further investigated in batch anaerobic experiments in 1 litre reactors by measuring different control parameters. A positive correlation was observed between the proportion of grass in the mixture and the specific methane yield due to the higher protein content (a yield up to 6% higher in comparison with mono-digestion of maize). Nevertheless, increasing amounts of grass were also related with slower degradation (lower hydrolysis rate constants) and higher instability during the first days of digestion. [less ▲]

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See detailMonitoring biodegradability of lignocellulosic biomass during anaerobic digestion
Benito Martin, Patricia UL; Sibisi-Beierlein, Nonjabulo N.; Greger, Manfred UL

in Book of abstracts of the 8th International Conference ORBIT2012 (2012, June)

Energy crops and crop residues can be digested anaerobically either alone or as co-substrate to produce biogas, a versatile renewable energy source. In the agricultural sector, this biomass is often co ... [more ▼]

Energy crops and crop residues can be digested anaerobically either alone or as co-substrate to produce biogas, a versatile renewable energy source. In the agricultural sector, this biomass is often co-digested with animal manure and slurry, one of the most significant agricultural waste streams. In spite of the growing number of agricultural biogas plants using agricultural waste and energy crops, the use of process simulation models for predicting and defining plant behaviour is still very limited, mainly due to the complexity of the process and the heterogeneity of the substrates. Also, the lack of standardized substrate characterisation methodology contributes to this issue. The chemical oxygen demand (COD), widely used as a mass balance term in modelling of wastewater treatment plants, could be applied in this field. However, COD is difficult to measure in the case of heterogeneous and solid substrates, including samples with high solids content. This, in turn, has limited its use to date to evaluate substrate biodegradability over time and methane production potential in the field of co-digestion of agricultural by-products. This research paper reports and analyses experimental results on the applicability of COD as a substrate characterisation and process monitoring parameter in the context of lignocellulosic biomass digestion, as collected from the research work currently being undertaken at the University of Luxembourg. The reliability of this parameter was investigated using different silages. Four sets of batch experiments were carried out using maize silage (12.17gVS/l), grass silage (14.58gVS/l), and two co-digestion mixtures of 40% maize and 60% grass (18.31 gVS/l) and 70% maize and 30% grass (12 gVS/l) in 1 litre reactors running under mesophilic conditions. The reactors were inoculated with pre-acclimatized inoculum from Beckerich agricultural biogas plant in Luxembourg. Soluble and particulate COD, volatile solids (VS), total solids (TS), volatile fatty acids (VFAs), and gas production, including composition, were monitored over a period of 24 days. A few adapted analytical procedures for measuring COD in solid and heterogeneous substrates and samples with high suspended solids concentration, in comparison to standard methods widely used for wastewater, have been proposed in recent years but these have rarely been applied and validated in the field of digestion of agricultural by-products. The COD methodology adopted in this study was developed by the process research group at the University of Luxembourg as a modification of the DIN 38414-S9 considering some suggestions from Raposo et al. (2008). The modified COD method was applied to different substrates (i.e. grass, maize silages, and cellulose) and also to monitor particulate COD (pCOD) evolution during anaerobic digestion of the different silages. The soluble COD (sCOD) method as established in the ISO 15705 standard method was modified for these experiments as the digestate has relatively high suspended solids concentration. First, the measured substrate COD concentration was compared with the theoretical oxygen demand for each substrate. COD recovery obtained was 96% for cellulose, 92.5% on average for the different maize silages tested and 91.5% on average for the grass silages tested. For all different batch series, the pCOD decreased overtime as a result of the hydrolysis of the particulate fraction of the substrate. The pCOD evolution along with the methane accumulative yield suggested that the solubilisation of the substrate was slower in the case of the co-digestion experiments. sCOD, on the other hand, initially increased sharply as a result of the solubilisation of the substrate and subsequent acidification and then decreased as digestion progressed. After 9 days of digestion, 77%, 71%, 69% and 48% substrate to methane conversion rates had been achieved for the experiments fed with grass silage, maize silage, the 70% maize/30% grass mixture and the 40% maize/60% grass mixture, respectively. Measuring the pCOD and sCOD together with biogas and VFAs concentration during digestion allows, not only to estimate the degradation advancement through COD mass balance, but also to obtain relevant information about the solubilisation of the particulate fraction (hydrolysis step). [less ▲]

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