Influence of SO_2 on the formation of PCDD/F in MSWI

摘要

Experimental investigations on the influence of SO_2 on PCDD/F formation at a full scale municipal solid waste incinerator and at a pilot plant document that PCDD/F formation during effective combustion is mainly caused by the fly ash deposits in the boiler and not by the particles currently flying through the boiler. The findings indicate that PCDD/F formation takes place dominantly inside the bulk. Mechanisms of boiler corrosion forming high amounts of Cl_2 were identified to cause enhanced PCDD/F formation inside the fly ash bulk on the boiler surface. The porous structure of the deposits is connected with the overflowing flue gas by fine pores. Reactions inside the ash layer, the transfer of SO_2 and other gaseous flue gas constituents into and PCDD/F out of the deposits are controlled by diffusion. Since the diffusion velocity inside the deposits is very slow short term fluctuations of the SO_2 concentration cannot influence PCDD/F formation immediately. Only permanent high SO_2 concentrations lead to the formation of mainly sulfate rich fly ashes containing only trace amounts of chlorine and will lead after the removal of chloride containing ash deposits from the boiler surface to a minimized PCDD/F formation as well as low boiler corrosion.