Primary Formation of PCDD/F in a Circulating Fluidized Bed Boiler Co-Firing Waste with a "Clean" Biomass - Impacts of Fuel Quality

Paper i proceeding, 2012

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low temperatures (<700 C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of the fuel quality was studied in the 12 MWth circulation fluidized bed (CFB) boiler located at Chalmers University of Technology by comparing the concentrations of PCDD/Fs found in the raw gas after the convection pass of the boiler and the PCDD/Fs found in the fly ashes. Sorted, crushed and pre-dried waste pellets from IcoPower in the Netherlands were compared to a clean biomass case consisting of only stem wood in the form of a mixture of wood chips. PCDD/Fs were also found in the fly ashes which may be a problem in the future during ash disposal. The formation of PCDD/Fs was significantly reduced in the raw gas after the convection pass as well as in the fly ashes during co-combustion of waste pellets with municipal sewage sludge compared to the case where the waste pellets were used without the addition of the sludge. A link is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and the formation of PCDD/Fs.