Inorganic Composition and Environmental Impact of Biomass Feedstock

Knowledge of the inorganic components of biomass feedstock is important for process control and for handling coproducts and wastes resulting from energy and fuel utilization of biomass. Analytical survey of forestry thinnings (wood chips), agricultural residues (rice straw, wheat straw, corn stover), and dedicated perennial grass crops (switchgrass, wheatgrass, and miscanthus) shows that, potentially, the whole periodic table may be present in biomass. The main effect of ashing is bonding of oxygen in the ash mainly as silicate, oxides, hydroxides, phosphates, and carbonate residual minerals. Carbon is partially retained as carbonates and graphite (char). Nitrogen is dominantly released to the flue gas, while sulfur is mostly retained in the ash as sulfates. Small losses (∼19%) for both sulfur and chlorine were detected during ashing at 575 °C. The majority of the alkali metals (Li, Na, K, and Rb) will substantially modify soil if applied as a fertilizer. Only Mg, Ca, and Sr of the alkali earth metals, Mn, Cu, and Zn of the period 4 transition metals, and Mo and Cd of the period 5 transition metals may exceed regulatory limits if used as a fertilizer. The heavy elements occur in concentrations too low to cause concern with the exception of Se. The high alkali content of some biomass ash thus makes them good candidates for use as fertilizers provided that they are applied in low proportions (<50%) to soil. Ash of wood material is a carrier of many of the alkali elements (Li, K, Rb, Mg, Ca, Sr, Ba) and some transition elements (Mn, Cu, Zn, Mo, Ag, Cd). In contrast, ash of herbaceous plant material is in addition to K only variably enriched: Li, Na, Se, and Mo in wheatgrass, Mg, and Ca in switchgrass, Mg, Ca, and Cd in corn stover, Mn in rice straw, Mo in wheat straw, and Mn and Cd in miscanthus. Water leaching results in significant losses for anionic chlorine and sulfur as well as for most of the alkali metals, thus making resulting ash from such treated feedstock less attractive as K fertilizers although fuel properties are enhanced for thermal conversion.