Pretreatment of agricultural biomass for anaerobic digestion: Current state and challenges

[Display omitted] •Pretreatment is guided for anaerobic digestion of agricultural biomass.•Characteristics of feedstock must be considered for optimizing pretreatment.•Alkali and steam explosion leads to an enhanced anaerobic digestion performance.•PEF, ionic liquids and rumen system can be future challenges for pretreatment.•Pretreatment is significant in a combined biorefinery with agricultural residue. The anaerobic digestion (AD) of agricultural biomass is an attractive second generation biofuel with potential environmental and economic benefits. Most agricultural biomass contains lignocellulose which requires pretreatment prior to AD. For optimization, the pretreatment methods need to be specific to the characteristics of the biomass feedstock. In this review, cereal residue, fruit and vegetable wastes, grasses and animal manure were selected as the agricultural biomass candidates, and the fundamentals and current state of various pretreatment methods used for AD of these feedstocks were investigated. Several nonconventional methods (electrical, ionic liquid-based chemicals, ruminant biological pretreatment) offer potential as targeted pretreatments of lignocellulosic biomass, but each comes with its own challenges. Pursuing an energy-intensive route, a combined bioethanol-biogas production could be a promising a second biofuel refinery option, further emphasizing the importance of pretreatment when lignocellulosic feedstock is used.