Microwave- Versus Conventional Acid-Assisted Thermohydrolysis of Willow and Maize Silage: Methane Yield and the Energy Cost of Pretreatment

Abstract

Hydrothermal pretreatment is widely proposed to improve the anaerobic digestibility of lignocellulosic biomass, yet whether it repays its own energy input is rarely quantified. We compare microwave- and conventional (conductive) acid-assisted thermohydrolysis of willow (Salix viminalis) and maize silage at 110–130 °C, coupling biochemical methane potential (BMP, n = 3) with an incremental energy balance against an untreated control. Substrate, heating mode and temperature were all significant (p < 0.001), with heating mode dominating. Maize silage responded monotonically, reaching 306.0 ± 4.0 NmL CH4 g−1 VS at 130 °C under microwave heating (+31.4%), whereas willow behaved erratically: its best variant reached 310.6 ± 2.5 NmL CH4 g−1 VS (+28.9%), yet two conductively heated variants fell below the control. Furanic by-products stayed at trace levels, well below inhibitory thresholds, excluding toxicity. Critically, no variant recovered its own energy input: every treatment was net-negative, the least unfavourable being microwave heating at 110 °C (−3.57 and −3.48 kJ g−1 DM). Heating mode changed the penalty three- to five-fold, microwave being superior in all six paired comparisons. Under the mild conditions tested, acid-assisted thermohydrolysis was not self-financing in energy terms; where applied for other reasons, microwave heating is the less unfavourable of the two modes tested, although neither returns a net energy gain.