M.B. Chang, T.F. Huang / Chemosphere 40 (2000) 159±164
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3. Results and discussion
temperature range for PCDD/Fs formation is 250±
300°C for cyclone and ESP ashes. The particle size in the
downstream of incinerators is generally smaller than
that in the upstream. It is concluded that the particle size
distribution from large to small sizes is boiler ash > cy-
clone ash > ESP ash. These results indicate that the op-
timum temperature decreases with the decreasing
particle size. The concentration of dioxin for ¯y ash is
ESP ash > cyclone ash. The results indicate that the di-
oxin concentration will increase as the particle size de-
creases and are in agreement with the previous ®ndings
by Chang and Chung (1998).
In this work, the extracted MWC ¯y ash sample is
used as a reactant for dioxin formation via a laboratory-
scale reactor. The blank tests have been carried out in
triplicate in order to check the dioxin content in ex-
tracted ¯y ash. The purpose of this work is to ensure
that the dioxin detected is generated from ¯y ash during
the dioxin formation experiment, rather from the dioxin
remaining in the extracted ¯y ash. The results indicate
that trace amounts of dioxins are still found on the ex-
tracted ¯y ash, but the residual dioxins in the blank
experiment are only 1±3% of the lowest concentration
found in formation experiments. Therefore, the dioxin
content in the extracted ¯y ash can be neglected and it
can be assumed there is no dioxin existing in the ex-
tracted ¯y ash which is used as the reactant for all dioxin
formation experiments. In this study, the eects of the
temperature, oxygen content in the gas stream and
carbon content in the ash on PCDD/Fs formation on the
¯y ash have been investigated. The eects of these fac-
tors on dioxin formation are as follows:
(2) Oxygen content. Since the waste materials can be
incinerated with enriched oxygen or even pure oxygen
the O2 content of the gas stream introduced is varied
from 0% to 100% by volume, and the carrier gas is ni-
trogen. The reaction time (RT) is 1 h and the tempera-
ture is controlled at 300°C. The eect of the oxygen
content on the PCDD/Fs formation is shown in Fig. 3.
The results indicate that the optimum oxygen contents
for dioxin formation for cyclone ash are at 7.5% and
40%, and the maximum dioxin formation is found at
7.5%. Since the optimum oxygen content was found at
two reactive regions, it is concluded that the dioxin
formation may take place via two dierent routes re-
sulting from the dierent oxygen-needed concentrations
for dioxin formation. As the temperature (300°C) is not
the optimal temperature (250°C) for ESP ash, the opti-
mum oxygen content has not been found for ESP ash.
The PCDD/PCDFs ratio for all ¯y ashes experimented
is < 1. In other words, the homologue patterns are more
(1) Temperature. Many publications have indicated
that the optimum temperature of dioxin formation is
between 250°C and 400°C (Addink and Olie, 1995b;
Boscak, 1990). A series of experiments have been carried
out at the temperatures of 200°C, 250°C, 300°C, 350°C
and 400°C. The reaction time (RT) is 1 h and oxygen
content is controlled at 10% by volume. The eects of
the temperature on the PCDD/Fs formation for ashes
from cyclone and ESP are shown in Fig. 2. The optimum
Fig. 2. The eect of the temperature on the PCDD/Fs forma-
tion (Q 10 l/min, RT 1 h, [O2] 10%).
Fig. 3. The eect of the O2 content on the PCDD/Fs formation
(Q 10 l/min, RT 1 h, T 300°C).