PCDD/F emissions from co-combustion of RDF with peat, wood waste and coal in FBC boilers

The use of refuse derived fuel (RDF) as additional fuel in multifuel fluidized bed boilers (FBC) is a promising way for rational waste management. Several combustion experiments using RDF as additional fuel have been carried out in the research program of combustion and gasification technologies in Finland under the title Liekki 2.The size of fluidized bed combustors used in these experiments ranged from 15 kWth up to 117 MWth. The chlorine content of fuel rose up to 0.35% as a measured dry base (db). The emissions of the toxic PCDD/F compounds were found to follow fuel-Cl content trendwise, as expected. However, quantitatively the relative emission levels varied with the factor of 103 at the same Cl- content in fuel. Using statistical analysis correlations between PCDD/F emissions and combustion conditions (Tg, CO etc.) as well as fuel and ash properties were found.Most of the earlier measurements in commercial plants have been conducted usually after an electrostatic precipitator (ESP), which has made it difficult to relate emission results to corresponding combustion conditions. According to the laboratory scale fluidized bed reactor (15 kWth) using normal (primary and secondary) and staged (primary + secondary + tertiary) combustion air feeding, it was found that in the staged condition (higher vertical temperature profile) the total amount of PCDD/F compounds (gas and particles) decreased, but the amount of PCDD/F in solid phase increased.Although the required European emission level of PCDD/F (0.1 ng/m3n I-TEQ, in 11% O2) can be achieved, the detailed formation and destruction mechanisms of PCDD/F in combustion conditions are not known well enough for quantitative determination of the emissions