Air classification of herbaceous feedstocks for ash removal and quality improvement

Ash in biomass originates from the plant (physiological) and from the harvest and collection process (introduced).  Ash has been shown to be at best inert in conversion processes, but can also decrease yields, degrade products, and poison catalysts.  Corn stover (CS), switchgrass (SG), and grass clippings (GC) were air classified into 5-7 fractions using air velocities from 1.3 m/s to 5.9 m/s.  A fine fraction (< 80 mesh, 0.177mm) was removed from the lightest air classified fraction.  Ash content and composition were measured for all fractions.  Elevated ash levels, as high as 46.3%, were observed in the lightest and fine fractions.  Removal of the highest ash fractions reduced initial ash content by 9.4% in CS, and by 8.5% in both SG and GC. Typical soil elements (silicon, iron, aluminum) were highest in the lightest fractions. Elements associated with plant physiological processes (calcium, potassium, magnesium) were evenly distributed throughout all fractions, and attempts at decreasing concentrations of these elements were ineffective.  Air classification isolated several anatomical fractions such as cobs, leaves, stems, and/or husks. The isolation of these fractions enables the possibility of fraction specific processes and markets, including reduced severity pretreatment of cobs, leaves, or husks, C5 specific conversions for cobs, biopower applications for high ash fractions, and fiber based coproducts from stems. Air classification is economically feasible ($2.23-$4.93 per ton), and this figure is likely to decrease with the identification of additional markets and reductions in required processing.