Combined leaching and steam explosion pretreatment of lignocellulosic biomass for high quality feedstock for thermochemical applications

A novel pretreatment approach was developed in the present study to simultaneously reduce the content of inorganic troubling elements (TEs) and improve biomass energy density, i.e., combined leaching and steam explosion (SE) pretreatment with mechanical pressing. Three different high-ash-containing biomass feedstocks were studied – wheat straw (WS), spruce bark (SB), and empty fruit bunches of oil palm (EFB). Before studying the combined pretreatment, the impact of individual pretreatment was determined in-depth on biomass composition and ash-transformation behaviour. In leaching pretreatment, the effect of leaching with steam explosion condensate (SEC), dilute acetic acid and water was studied comprehensively. The solid (raw and treated) and liquid samples (leachate, SEC, hydrolysate) were characterized for a detailed comparative analysis. The ash-fusion and −transformation behaviour of raw and pretreated biomass was studied in-depth using advanced multicomponent thermodynamic equilibrium modelling (TEM) and push-rod dilatometry. After combined pretreatment, a considerably high reduction in TEs content was noted: 82–98 % K, 93–99 % Na, 80–99 % Cl, 34–90 % S, 3–77 % Mg, ash 43–63 %, 26–67 % P, and 19–63 % Ca. Simultaneously, heating values of all three feedstocks increased by 1.3–2.3 MJ/kg, reaching up to 20–22 MJ/kg. TEM results reveal that much lesser amounts of molten slag and volatile inorganic compounds were formed in pretreated samples compared to raw and SE biomass, such as K₂Si₅MgO₁₂, KCl, KPMgO₄, K₂Si₂O₅, CaSiO₃, and MgSiO₃ in WS; Na₂CO₃, K₂CO₃, K₂CaC₂O₆, KPMgO₄, and CaCO₃ in SB. Due to higher energy density and lower propensity towards ash-related issues, combined pretreated WS and SB have a high potential to be used in large-scale thermochemical applications.