Combined Butyric Acid and Methane Production from Grass Silage in a Novel Green Biorefinery Concept

In a Green Biorefinery, grass silage can be a source for lactic acid, proteins, amino acids and fibres. Processing residues can be used for anaerobic digestion and methane production. But by changing the ensiling conditions, butyric acid fermentation can be achieved. That makes grass silage also a potential substrate for a combined butyric acid and methane production. The objective of this study was to determine the potential of butyric acid production at different ensiling conditions applied to grass and measuring the methane yield potential of solid residues after a separation step. The highest butyric acid concentration in the produced press juice was 20.1 ± 4.5 g kg−1 and was achieved by carbonated lime addition and a reduced dry matter content after 90 days at mesophilic storage conditions. This resulted in a theoretical butyric acid yield of 332 kg ha−1 a−1. For the fibrous leftover press cake, a theoretical methane production potential of 2778 m3CH4 ha−1 a−1 was reached. The results show that theoretically a combined production of butyric acid and methane can be realised in a Green Biorefinery concept.