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Monitoring the efficiency of biogas plants – Correlation between gross calorific value and anaerobically non-degradable organic matter of digestates
Highlights.?Substrate and digestate samples were analysed for lignin and calorific value.?Good correlation was found between lignin and calorific value: adj. R2?=?0.88.?The lignin content in digestates can be estimated by calorimetric measurements.?The correlation is not significantly influenced by hydraulic retention time.Abstract.When evaluating the efficiency of biogas plants based on the residual energy content in the digestate, it is necessary to distinguish the anaerobically available and non-available portion of energy. Digestates from agricultural biogas plants typically contain lignin as anaerobically non-degradable matter. The aim of the present study was to examine, if the content of anaerobically non-degradable matter in digestates, as analysed by acid detergent lignin (ADL), can be predicted from gross calorific values (GCV). For that, 31 samples at different degradation states from 9 biogas plants were analysed. The results show that the VS-specific lignin fraction can be predicted from calorimetric measurements in the studied range from 0.03 to 0.36 with an accuracy of ±0.086 (±2RMSECV). The accuracy of prediction of lignin in digestates was not significantly influenced by the passed hydraulic retention time of the material in the digester. The model equation allows distinguishing anaerobically degradable and non-degradable portions of organic matter and inherent energies in digestates, assuming lignin to be the only non-degradable component. Future efforts have to be made to extend the data set for a greater calibration range of ADL and generalise the model equation for a wider spectrum of biogas plants fed with a variety of substrates.
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