Energy pellets from whole-wheat straw processed with a deep eutectic solvent: A comprehensive thermal, molecular and environmental evaluation

Abstract

Storage and transportation are main considerations for seasonal crop residues to become competitive alternatives to non-sustainable fossil fuels and chemicals. Hence, pelletization and briquetting are highly relevant to mass and energy densification and associated supply chains. Here, we propose a green treatment based on recyclable, deep eutectic solvent (DES) to achieve partial swelling and dissolution of residual agricultural biomass (wheat straw), leading to improved structuring and cohesion of energy pellets. The mild DES-based process enhances access to lignin, which migrates and regenerates at interphases for effective binding, enabling improved pellet durability (increased by 90–97% compared to pellets produced from the untreated biomass). The role of lignin was studied by molecular dynamics simulation, which revealed non-bonding interactions with cellulose, impacting the strength of the pellet structure. The DES treatment enhanced the heating value of the pellets (from 16.1 to 19.5 MJ/kg), partially a result of a higher acidy ratio, and simultaneously led to better devolatilization, lignin accessibility and release of volatile matter during pyrolysis. The potential environmental benefit of DES-treated pellets over wood-based counterparts is revealed by a life cycle assessment. Our study further indicates the reduced environmental impact of DES-treated pellets, which can be furthered by recycling of the solvent.