Selective removal of ash and specific elements of ash in air classified fractions of loblolly pine forest residues

Ash contained in biomass can decrease the efficiency of thermochemical conversion processes.  Whole ash represents unconvertible material, while certain elements of ash are destructive to catalysts or conversion products.  Pine forest residues are an underutilized source of biomass for thermochemical conversions, yet the ash content and composition makes them less desirable.  Air classification represents an effective and economical tool to isolate high ash biomass fractions.  Loblolly pine forest residues were air classified into 10 fractions, and ash content and composition were measured.  Ash concentrations were highest in the lightest fractions (5.8-8.5%), and a heavy fraction of the fines (8.9-15.1%).  Removal of high ash fractions resulted in a substantial reduction in ash content of the remaining biomass in forest thinnings (1.69%-1.07%) and logging residues (1.09%-0.68%).  These high ash fractions represented only 6.7% of the biomass, yet they contained over 40% of the ash.  Elemental analysis of the air classified fractions revealed that most of the ash in the lightest fractions was introduced ash, as it was easily removed and contained high concentrations of soil elements (silicon, aluminum, iron, sodium, and titanium).  Calcium, potassium, magnesium, sulfur, manganese, and phosphate, known to be of biological origin, were more evenly distributed throughout the air classified fractions.  A preliminary economic analysis showed that air classification could be used for ash reduction for as little as $1.93 per ton of product biomass, making it a potentially attractive technology for the removal of ash and selective removal of elements originating from introduced ash from pine forest residues.