Readily available lignocellulosic biomass as substrate for biogas plants is gaining popularity amongst biogas plant operators. Results of low‐temperature pretreatment (light cooking) of wheat straw to remove waxes and prepare the biomass for microbial action are described. Benefits of light cooking are low thermal energy demand and low investment cost compared to conventional techniques such as steam explosion. The novelty lies in utilizing the low temperature range 25–100 °C for pre‐soaking the biomass. Two different types of wheat straws were pretreated at varying temperatures and sizes. The results were compared with Buswell's equation for theoretical maximum biomethane yield. Compared to untreated straw, pre‐soaking leads to a significantly higher methane yield. Size reduction combined with light‐cooking does not affect the methane yield in the same manner as pre‐soaking of the biomass.
Appropriate evaluation of the process performance of biogas plants needs to consider the anaerobic biodegradability of the used biomass. Anaerobic biodegradability is limited by lignin, which is part in most substrates and, by extension, in digestates of biogas plants. Previous research has shown that the content of acid detergent lignin (ADL) in digestates can be predicted from measured gross calorific values (GCVs). The correlation of GCV of 34 digestate samples to an alternative measure for lignin, the Klason lignin (KL), is evaluated as well as the correlation of KL content and other chemical constituents to residual biomethane potential (BMP). Results indicate a very low correlation of chemical composition to GCV and BMP. A correlation of GCV to BMP was not observable. The results let conclude that evaluation of anaerobic biodegradability of digestates by measuring KL or predicting KL from GCV is not productive.
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