Toxic elements in biomass and waste fuels and their removal from the flue gases of fluidised bed boilers

Hvb coal and fir bark, were combusted with Cl-rich waste fuels (plastic waste and paper sludge) under bubbling bed (BFB) combustion conditions. Vaporisation and distribution to ash streams were studied for those elements included in the EU's co-incineration directive (Hg, Cd, Pb, Mn, Cu, Cr, Ni, Co, Sb, As, V). Increase of Cl-concentration in the feedstock was observed to enhance the transfer of many toxic elements to finest fly ash, probably due to the formation of volatile chlorides in the FB furnace. The surface area to weight ratio of fly ash increases with decreasing particle size, causing increased condensation of volatilised trace elements onto the finest size fraction. Elements bound to ultra-fine fly as can pass through conventional gas cleaning devices along with genuinely gaseous species. As a result, the measured loss of toxic elements from the combustors studied was higher than predicted by the thermodynamic calculations.