The formation of polychlorinated dibenzo-p-dioxins and
dibenzofurans (PCDD/Fs) has been examined to investigate
the influence of organic and inorganic chlorine sources
and their contents in municipal solid waste incineration. A
laboratory-scale fluidized-bed reactor with electric
heating was used to control combustion condition identically.
Combustion temperature was set to 900 °C, and the
amount of air supplied was twice as much as the amount
of theoretical air. Artificial wastes containing organic
(polyvinyl chloride, PVC) or inorganic (NaCl) sources of
chlorine at several levels and copper chloride (CuCl2·2H2O)
as a catalyst were prepared to define the waste
composition and make it constant. The experimental
setup had been carefully planned to suppress the effects
of experimental conditions except the waste composition.
Results of combustion experiments revealed that no PCDD/Fs were detected in the absence of Cl sources and
copper chloride, but PCDD/Fs formation was recognized
in the cases with Cl and a catalyst. In our experimental
conditions, both organic and inorganic chlorines affect
PCDD/Fs formation obviously. As Cl content in the waste
was increased, CO concentration in flue gas became higher,
and more PCDD/Fs were formed in both series of
experiments with PVC or NaCl. It seems that combustion
conditions indicated by CO concentration are strongly related
to PCDD/Fs formation during incineration. It cannot be
said that there is a significant difference between the effects
of PVC and NaCl on PCDD/Fs formation in the artificial
solid waste incineration.
Combustion experiments performed in the presence of hydrogen chloride (HCl) in a laboratory-scale fluidized-bed reactor were carried out to elucidate the role of chlorine in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs; together: PCDD/Fs) in various sections of a municipal waste incinerator. We first demonstrated that the homologue profile and the pattern of the congener proportions of PCDD/Fs for a model waste containing poly(vinyl chloride) (PVC) combusted in the absence of HCl were similar to those for a PVC-free waste combusted in the presence of HCl. This showed no difference between PVC in the waste and injected HCl in the role as a chlorine source in PCDD/F formation during incineration. Next, to investigate PCDD/F formation in each section of the incinerator, we carried out combustion experiments with the PVC-free waste, injecting HCl at different locations of the incinerator. The amounts of PCDDs and PCDFs formed were significantly reduced when HCI was not supplied to the main combustion section. The presence of HCI in the main combustion section was essential for the formation of PCDD/Fs, even in the downstream sections. This finding indicates that compounds that were able to form PCDD/Fs in the downstream sections were mainly formed in the main combustion section in the presence of HCl.
Combustion experiments in a laboratory-scale fluidized-bed reactor were performed to elucidate the effects of combustion temperature on PCDD/Fs formation during incineration of model wastes with poly(vinyl chloride) or sodium chloride as a chlorine source and copper chloride as a catalyst. Each temperature of primary and secondary combustion zones in the reactor was set independently to 700, 800, and 900 degrees C using external electric heaters. The PCDD/Fs concentration is reduced as the temperature of the secondary combustion zone increases. It is effective to keep the temperature of the secondary combustion zone high enough to reduce their release during the waste incineration. On the other hand, as the temperature of the primary combustion zone rises, the PCDD/Fs concentration also increases. Lower temperature of the primary combustion zone results in less PCDD/Fs concentration in these experimental conditions. This result is probably related to the devolatilization rate of the solid waste in the primary combustion zone. The temperature decrease slows the devolatilization rate and promotes mixing of oxygen and volatile matters from the solid waste. This contributes to completing combustion reactions, resulting in reducing the PCDD/Fs concentration.
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