Environ. Sci. Technol. 2004, 38, 1201-1207
For example, the carbon injection process has the advantage
Removal of PCDD/Fs from Flue Gas
by a Fixed-Bed Activated Carbon
Filter in a Hazardous Waste
Incinerator
of causing less pressure drop than a fixed-bed process, but
the use of the adsorbent is less efficient (3, 5, 6). Both methods
are widely used on municipal waste combustors and
hazardous waste incinerators. The main objective of em-
ploying activated carbon (AC) units in waste incinerators is
the removal of PCDD/ Fs. Since AC adsorbs gaseous dioxins,
the gas/ particle partitioning of PCDD/ Fs through the air
pollution control equipments (APCE) should also be taken
into consideration. Partitioning of PCDD/ Fs between gas
and particle phases depends mainly on the temperature and
saturation vapor pressure of the compound besides other
factors such as solid-surface area in the gas volume, mass
amount of the compound at monomolecular surface cover-
ing, etc. (7). A highly chlorinated congener has a lower vapor
pressure as compared to a lower chlorinated congener and
has a higher tendency to condense on particles (8). As the
vapor pressure of dioxin congeners decreases with increasing
chlorination, the percentage of particle-bound dioxins and
the removal efficiency of each dioxin congener in an AC unit
will be different. Generally lower chlorinated dioxins with
higher gaseous fractions are removed by carbon adsorption
more efficiently than the highly chlorinated ones (9).
IZAYDAS, the first hazardous waste incinerator of Turkey,
started to operate in 1997 in the Izmit region, the most
polluted area in Turkey. The capacity of the plant is 35 000
t/ yr. The facility has a two-stage combustion system consist-
ing of a rotary kiln that is 12 m in length and 4.2 m in diameter;
with combustion temperatures in the range of 950-1050 °C
and a retention time of 2-2.5 h for solids. The vertical post-
combustion chamber is 12 m in height and 4.1 m in diameter.
This chamber incinerates the gases produced from the rotary
kiln, vapors from the ash quench chamber, and some liquid
wastes. It has a retention time of about 2 s at 1150-1250 °C,
thus ensuring the complete destruction of hazardous organic
compounds. The original air pollution control system of the
plant consisted of an electrostatic precipitator (ESP) and dual
wet scrubbers (a venturi scrubber and a lime scrubber) in
series. However, operation of the incinerator created a large
public debate on the atmospheric emissions of PCDD/ Fs.
Therefore, an additional fixed-bed AC unit was installed
downstream of wet scrubbers in 1999, although PCDD/ F
emissions were measured below the European limit in several
trial burns (10). Figure 1 shows the flow diagram of post-
combustion units with associated temperature profiles and
retention times. This paper presents the results and assess-
ment of dioxin tests conducted at the AC unit between 2000
and 2002 to observe its dioxin removal efficiency.
A YK A N K A R A D E M I R , * , †
M I T H A T B A K O G L U , †
F A T I H T A S P I N A R , ‡ A N D S A V A S A YB E R K †
Department of Environmental Engineering and ARV College,
University of Kocaeli, 41100 Kocaeli, Turkey
The adsorption of polychlorinated dibenzodioxins and
dibenzofurans (PCDD/Fs) by activated carbon (AC) was
examined in a fixed-bed AC unit in a hazardous waste
incinerator (IZAYDAS) in Turkey. Results showed that the
removal efficiencies of PCDD/Fs decrease as the
chlorination level increases, which was explained by the
difference in gas/particle partitioning of the compounds. Since
dioxins are tightly adsorbed by activated carbon, other
flue gas constituents showed no clear effect on the dioxin
removal. Adsorption kinetics indicated that the adsorption
of volatile congeners and homologues fits well with
Henry’s law, possibly due to the higher gaseous fractions,
while the correlation was lower for lowly volatile ones.
PCDD/F congeners and homologues had a concentration
value up to which no adsorption occurred, which could be
attributed to the insufficient contact times at the low
concentrations.
Introduction
Carbon has been used for many years for the removal of
vapor-phase organics in low concentrations from industrial
waste gas streams. Data from waste incinerators in Europe
have shown that dioxins, furans, and other semi-volatile
products of incomplete combustion (PICs) at parts per billion
(ppb) or lower concentrations can be effectively removed
from the combustion gases by carbon. Adsorption of organic
compounds on carbon is a function of many parameters
including properties of the sorbent and the gases to be
removed, concentration of other adsorbable gases and vapors,
operating temperature, and residence time (1-4).
Experimental Section
Two different methods of contacting gas with carbon
adsorbent have been used. In fixed-bed (or passive) mode
of operation, the gas flows through a bed (or series of beds)
of solid adsorbent. Sufficient adsorbent is provided so that
the operation can continue for a long time, from several
hours to 1 yr or more, before the bed becomes saturated.
The second mode of operation, which is commonly used for
cleanup of flue gas, involves injecting a small amount of
solid adsorbent into the flowing gas, providing sufficient
contact time for the efficient use of the carbon, and then
separating the adsorbent before discharging the gas up the
stack. Each method has some advantages and drawbacks.
Description of the AC Unit. Flue gases leaving the wet
scrubbers at 60-65 °C are generally supersaturated with water
vapor (about 22-24% (v/ v) at normal conditions, corre-
sponding to relative humidities (RH) between 110 and 130%);
therefore, a condensation process is required before the AC.
High humidity can negatively affect the adsorption of a carbon
bed by filling up the pores in the carbon particles with
condensed water. The extent to which the RH of the gas
must be reduced in order to prevent this depends on the gas
temperature (e.g., 80% RH would be satisfactory at 77 °C and
about 50% RH at 57 °C) (3). Condensation occurs in the
condensation chamber, which is combined with the AC unit,
by a sudden decrease in the velocity of the flue gas and contact
with the cold metallic surfaces. A volume of 72 m 3 is provided
for condensation, corresponding to a retention time of 6.5-9
s for the actual gas flow rates of 30 000-40 000 m3/ h. A total
* Corresponding author phone: +90-262-3355559; fax: +90-262-
3355559; e-mail: aykan@kou.edu.tr.
† Department of Environmental Engineering.
‡ ARV College.
9
10.1021/es034681k CCC: $27.50
Published on Web 01/10/2004
2004 Am erican Chem ical Society
VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 2 0 1