Optimization of liquid hot water pretreatment on Hybrid Pennisetum anaerobic digestion and its effect on energy efficiency

To overcome the inherent complexity of lignocellulosic biomass and improve its biological conversion efficiency, the central composite surface response methodology was used to optimize the process variables (temperature, reaction time and liquid-to-solid ratio) in liquid hot water pretreatment for biomethane production from the anaerobic digestion of Hybrid Pennisetum (a typical energy crop in southern China). Temperature, reaction time and water content were varied between 160–200 °C, 20–60 min and 60–90 mL respectively. The biomethane yields from pretreated material were considered as response variables to different processes conditions. Results showed untreated Hybrid Pennisetum had a biomethane potential of 218.6 L/kg volatile solid (VS) (corresponding to a biodegradability of 62.6%). The effect of liquid hot water pretreatment on the biomethane yield was distinct. Pretreatment severities less than 4.55 resulted in increased biomethane yields. However, further increasing the severity led to lower biomethane yields. The optimum biomethane yield (290.6 L/kg VS) was obtained from pretreated Hybrid Pennisetum at 175 °C for 35 min, with the highest biodegradability of 83.2%. Energy balance showed that, under the optimal pretreatment condition, the highest energy conversion efficiency of 76.1% was attained in theory. If the process energy inputs are included, a process energy efficiency of 51.7% could be realized with proper heat recovery.