Theoretical Feasibility for Ecological Biomass Ash Recirculation:  Chemical Equilibrium Behavior of Nutrient Elements and Heavy Metals during Combustion

To obtain a sustainable increased use of the CO2-neutral biomass fuels, the nutrient elements in the ashes formed have to be recirculated back to the forest and farm lands. During their growth, plants accumulate significant amounts of heavy metals of anthropogenic origin, normally enriched in the ashes during the energy conversion processes. If some kind of heavy metal separation technique could be applied during or after the processes, a more ecologically safe ash fraction may be produced for the recirculation of the nutrients. In addition, contaminated soils could be efficiently cleaned by controlled cultivation and combustion of biomass fuels. Previous experimental results from full-scale combustion plants have indicated that a significant fraction of a heavy metal-free ash may be obtained at high temperatures due to the lower volatilization temperatures of these metals. In the present work, the theoretical feasibility of a proposed high-temperature cyclone separation technique was evaluated by means of chemical equilibrium model calculations. The equilibrium behavior of both nutrient elements (Ca, Mg, K, Na, P) and heavy metals (Cd, Cu, Cr, Pb, Ni, Zn, As, V) as functions of temperature was determined. The results indicate that Cd, Cu, Pb, and possibly As and Cr may be volatilized, and thus separated, through a hot cyclone (800−850 °C), still keeping all Ca, Mg, and P and 75% of K and Na in a condensed form in the cyclone ash.