California Institute for Energy and Environment (CIEE)

Systematic mineralogical and chemical changes occur in the ash or slag of biomass during combustion. These changes are a function of fuelcomposition, firing temperature, and firing duration. Controlled temperature experiments were conducted using clean samples of wood, rice straw, and wheat straw to evaluate trace element mobility as a function of firing temperature. The concentrations of major and trace elements in ash and slag were analyzed using various multi-element instrumental techniques. The recommended analytical methods and element combinations consists of X-ray fluorescence (XRF) (or similar methods) for major and minor elements, the majority of trace elements using inductively coupled plasma mass spectroscopy (ICPMS), and short irradiation duration instrumental neutron activation analysis (INAA) for the alkali metals and chlorine. There are distinct differences between the individual fuel ash types that easily allow fuel sources to be identified based on knowledge of ash compositions. A general increase occurs in residual concentrations of many elements with the removal of volatile constituents and increasing temperature. Depletion with increasing firing temperature was in this study particularly observed for the alkali metals (Na, K, Rb, Cs), but could also be seen for other elements (Cl, Ag, Cd, As, Se, and Pb). Fly ash from fuel-intake controlled experiments was also analyzed. Observations include: 1) strong fractionation occurs during combustion; 2) trace element concentrations are often strongly affected by contamination from plant construction materials; and 3) incorporation of urban fuel types can strongly affect many heavy metal concentrations. Among the fuel samples tested, enrichment of both barium and lead in wood ash resulted in concentrations slightly exceeding US Environmental Protection Agency’s (EPA) total constituent analysis limits for heavy metals. In ashes and fly ashes for which a significant component of urban wood material is involved, As, Cr, and Pb may variably exceed EPA limits. Metals in allstraw ashes tested were below the permitted limits. Leaching of straw ashes also depletes many alkali major and trace elements, although concerns may then arise as to concentrations in water or other solvent employed.