In this study, the mechanical size fractionation technique and the water leaching technique were applied in sequence to reduce the ash content of crop residues to target value of 6% db (dry basis) following the applicable standard. Results showed that the finest fraction of ground crop
residues (< 0.25 mm) had much higher ash content than the coarser fractions, thus demonstrating that size fractionation can reduce the need for leaching as ash removal technique, and it was most effective for corn stover. For a water-to-biomass ratio (w/w) of 30, the final ash contents
of the biomass materials involved in all experimental treatments were effectively reduced to below the target value. Canola straw had the highest ash removal efficiency and element removal efficiency as compared to wheat straw and corn stover. Even though the leaching condition (12 h, 25 °C)
was adequate for effective ash reduction, SiO2 removal efficiency could be significantly improved when the leaching temperature was raised from 25 °C to 45 °C. A preliminary cost analysis indicated that biomass pretreatment by mechanical size fraction followed by water leaching
as needed would lead to a 30% increase in the total production cost (TPC) of agro-pellets, and this is significantly lower than the increase in TPC by 66% if the pretreatment is done by water leaching only of the ground crop residues.