Integrated leaching-steam explosion pretreatment to upgrade low-grade biomass into high-quality black pellets

The primary objective of the present study is to produce durable energy dense biomass pellets with low inorganic content for thermochemical applications. Three different types of biomass residues were studied: Wheat straw, Spruce bark and Empty fruit bunches of oil palm. A novel pretreatment method was applied to upgrade pellet qualities of the low-grade feedstocks, i.e. combined leaching and steam explosion (WSE) pretreatment. The impact of water leaching, steam explosion and WSE was investigated thoroughly on biomass pellet properties. The pellet properties studied were mass and energy density, heating value, mechanical durability, hydrophobicity, compressive strength, tensile strength, length expansion and surface morphology. The results indicated that raw biomass pellets do not meet the requirements of existing international standards for biomass pellets due to poor mechanical durability (<95%) or elevated levels of troubling elements (Cl, N, K; Ash content 2.9-8.1 wt%). However, after combined pretreatment, substantial improvement was noted in all physicochemical and mechanical properties of the treated pellets, especially high-heating values (reaching 18.6-20.8 MJ/kg), mechanical durability (reaching 98.4-99.5%), tensile strength (reaching 12.7-13.1 MPa) and ash content (44-62% removal, 2.3-4.6 wt%). These treated black pellets exhibit superior quality, fulfilling requirements outlined in the international biomass pellets standards (ISO, ENplus®, DINplus, IWPB).