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 ba ck to the forest and farm lands. During their growth, plants accumulat e significant amounts of heavy metals of anthropogenic origin, normall y enriched in the ashes during the energy conversion processes. If som e kind of heavy metal separation technique could be applied during or after the processes, a more ecologically safe ash fraction may be prod uced for the recirculation of the nutrients. In addition, contaminated soils could be efficiently cleaned by controlled cultivation and comb ustion of biomass fuels. Previous experimental results from full-scale combustion plants have indicated that a significant fraction of a hea vy metal-free ash may be obtained at high temperatures due to the lowe r volatilization temperatures of these metals. In the present work, th e theoretical feasibility of a proposed high-temperature cyclone separ ation technique was evaluated by means of chemical equilibrium model c alculations. The equilibrium behavior of both nutrient elements (Ca, M g, K, Na, P) and heavy metals (Cd, Cu, Cr, Pb, Ni, Zn, As, V) as funct ions of temperature was determined. The results indicate that Cd, Cu, Pb, and possibly As and Ct may be volatilized, and thus separated, thr ough a hot cyclone (800-850 degrees C), still keeping all Ca, Mg, and P and 75% of K and Na in a condensed form in the cyclone ash.