Microwave Autohydrolysis with Explosive Decompression of Willow and Maize Silage: Residence Time, Methane Yield and Energy Balance

Biomethane recovery from lignocellulose is limited by the recalcitrance of the fibre matrix, which pretreatment can relieve. Thermal pretreatment is usually optimised through temperature, whereas the residence time of hydrothermal treatment is seldom isolated as an independent variable, and rarely without added acid. Willow (Salix viminalis) and maize silage (Zea mays) were treated by microwave autohydrolysis with explosive decompression at 130 °C, using residence times of 5, 15 or 25 min against an untreated control, and assessed for solubilisation, by-products, methane potential, kinetics and net energy balance. Relative to the control, the 25 min treatment raised methane potential by 189% in maize silage (to 384.8 ± 21.4 NmL CH4 g−1 VS) and by 129% in willow (to 333.5 ± 14.2 NmL CH4 g−1 VS), reaching 72–74% of the theoretical maximum. Silage gained 78% at the first treated point (5 min dwell), whereas willow showed no significant gain before 15 min; in willow, the methane gain tracked continuously released xylose (r = 0.996) rather than the early COD or glucose burst. Furanic by-products remained very low; indicative concentrations and total phenolics stayed below reported methanogenesis inhibition thresholds for phenol (direct equivalence between total phenolics and pure phenol not assumed), so yield rose monotonically without turnover. The incremental energy balance stayed negative; the estimated incremental energy recovery reached 74% of the calculated input for willow and 97% for silage at 25 min.