Comparative assessment between hydrothermal treatment and anaerobic digestion as fuel pretreatment for industrial conversion of oil palm empty fruit bunch to methane and electricity-A preparation study to full scale

Abstract

Presence of mineral elements in oil palm empty fruit bunch (EFB) biomass is the major cause of slagging and fouling that lower the efficiency of power plant boilers. Two EFB pretreatment methods, hydrothermal treatment (HTT) and anaerobic digestion (AD), were assessed under industrially feasible conditions, aiming to concurrently improve fuel properties and recover biomethane. For HTT, multivariate regression was employed to establish the model of the key mineral “potassium (K) remaining in EFB” with liquid-to-solid ratio (L/S), reaction time (RT) and temperature. K-removal highly responded to RT while temperature and L/S appeared to play a secondary role. Highest K-removal was 90.7% at 120 °C, RT 120 min, and L/S 20:1. The organics release (100.7 g/kgEFB) in HTT leachate, able to use in various sugar biorefinery pathways, was also assessed for the biomethane generation. For industrial practicality, the treated wastewater (from palm oil mill treatment system) was subsequently tested for HTT under the lowest L/S of 5:1. K-removal at 90.9% and leachate methane yield at 29.1 m3CH4/tonEFBdry were achieved, together with oilrecovery from EFB at 70.6%. For AD of EFB, it was also found efficient in removing K at 92.6% and posed a remarkable methane yield of 117.5 ± 1.4 m3CH4/tonEFBdry. Potassium was, however, reabsorbed to EFB fiber after 14 days of fermentation, and importantly no oil was recovered. While both methods could improve EFB properties and ensure a lower fouling/slagging tendency in boilers, energy and mass balance suggests that the additional oil recovery by HTT provided a clear advantage over AD as well as its ease of integration to existing plant.