Effect of nitrogen-containing compounds on polychlorinated dibenzo-p-dioxin/dibenzofuran formation through de novo synthesis

Article abstract of DOI:10.1021/es049040j

An experimental study was conducted to clarify the suppression effect of nitrogen-containing compounds, that is, ammonia and urea, on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) through the de novo synthesis reaction. In the experiment, graphite and copper chloride contained in a mixture were used as sources of carbon and chlorine, respectively. The granulated sample mixture was charged as a packed-bed in the glass tube and heated at 300 °C in the flow of Ar-O ₂ gas mixture. In some cases, urea was added as aqueous solution to the sample, while ammonia was added to the gas flowed through the sample bed. The amount of PCDD/Fs formed decreases significantly by the addition of both ammonia and urea. Particularly, the addition of urea reduces the amount of PCDD/Fs discharged in the outlet gas by approximately 90%. The oxidation rate of carbon in the early stage of the experiment, that is, the heating period, is promoted by the addition of nitrogen-containing compounds. However, soon after the temperature reaches 300 °C, the formation rate becomes lower than that of the case without the addition of nitrogen-containing compounds. On the other hand, organic compounds containing amino (-NH₂) or cyanide (-CN) groups and those containing nitrogen within the carbon ring frame were detected in the outlet gas in the case of urea addition. Typically observed aromatic compounds are chlorobenzonitriles, chlorobenzeneamines, and chloropyridines. This suggests a possibility that hydrogen and/or chlorine combined with PCDD/Fs are also substituted by such nitrogen-containing groups, and this decreases the formation rate of their frame of carbon rings. This phenomenon was also consistent with the fact that a significant reduction was observed in the amount of PCDD/Fs released to the outlet gas when urea was added.

Full text of DOI:10.1021/es049040j

Environ. Sci. Technol. 2005, 39, 795-799
(
i) from precursor substances such as chlorobenzenes
Effect of Nitrogen-Containing
Compounds on Polychlorinated
Dibenzo-p-dioxin/Dibenzofuran
Formation through de Novo
Synthesis
(
CBzs) and chlorophenols (CPs) (2),
(
aromatic hydrocarbons (PAHs) (3-5), and
ii) by the decomposition/cleavage reaction of polycyclic
(iii) “directly” from carbonaceous materials such as soot,
which has a macrocarbon structure and was produced
normally by the condensation of unburned carbon and
hydrocarbon components (6).
For reaction route i, condensation reactions of precursors
contribute to the formation of PCDD/Fs. Previous reports
suggest that specific congeners may be formed through both
routes i and ii depending on the structure of the starting
compounds. For example, 1,2,8,9-T CDF and 1,4,6,9-T CDF
S H U N S U K E K U Z U H A R A , * H I R O S H I S A T O ,
N A O T O T S U B O U C H I ,
Y A S U O O H T S U K A , A N D E I K I K A S A I
4
4
Institute of Multidisciplinary Research for Advanced
Materials, Tohoku University, 2-1-1,
Katahira, Aobaku, Sendai, 980-8577, Japan
are mainly formed from chlorinated coronene through route
ii. Route iii is referred to as “de novo synthesis.” In the high-
temperature combustion process, the formation of these
compounds occurs in the cooling stage of waste gas
containing fly ash particles and many researches have been
trying to clarify the mechanism of PCDD/Fs formation (7-
9). In practical combustion processes, “soot” particles are
usually observed as complex structures containing various
metal elements and chlorides. However, their composition
largely depends on their formation process. De novo synthesis
reaction is known to proceed in an oxidative atmosphere at
lower temperatures, that is, between 200 and 500 °C, and the
maximum reaction rate is observed at about 300 °C (10-16).
Although the oxidation reaction proceeds slowly at such low
temperatures, coexisting metals/metallic compounds pro-
mote its rate considerably. The promotion effect of metallic
chlorides is usually larger than that of the oxides (17). Further,
metallic chlorides can also act as a source of chlorine in the
formation reaction of chlorinated organic compounds such
An experimental study was conducted to clarify the
suppression effect of nitrogen-containing compounds, that
is, ammonia and urea, on the formation of polychlorinated
dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans
(
PCDFs) through the de novo synthesis reaction. In the
experiment, graphite and copper chloride contained in a
mixture were used as sources of carbon and chlorine,
respectively. The granulated sample mixture was charged
as a packed-bed in the glass tube and heated at 300 °C
in the flow of Ar-O2 gas mixture. In some cases, urea was
added as aqueous solution to the sample, while ammonia
was added to the gas flowed through the sample bed.
The amount of PCDD/Fs formed decreases significantly by
the addition of both ammonia and urea. Particularly, the
addition of urea reduces the amount of PCDD/Fs discharged
in the outlet gas by approximately 90%. The oxidation
rate of carbon in the early stage of the experiment, that
is, the heating period, is promoted by the addition of nitrogen-
containing compounds. However, soon after the temperature
reaches 300 °C, the formation rate becomes lower than
that of the case without the addition of nitrogen-containing
compounds. On the other hand, organic compounds
containing amino (-NH2) or cyanide (-CN) groups and
those containing nitrogen within the carbon ring frame were
detected in the outlet gas in the case of urea addition.
Typically observed aromatic compounds are chlorobenzoni-
triles, chlorobenzeneamines, and chloropyridines. This
suggests a possibility that hydrogen and/or chlorine combined
with PCDD/Fs are also substituted by such nitrogen-
containing groups, and this decreases the formation rate
of their frame of carbon rings. This phenomenon was also
consistent with the fact that a significant reduction was
observed in the amount of PCDD/Fs released to the outlet
gas when urea was added.
2
as PCDD/Fs. Copper chlorides, CuCl and CuCl , are catalytic
compounds that have high ability to promote PCDD/Fs
formation (18). We found a linear relationship between the
rates of carbon oxidation and organic chlorine formation
when copper chloride coexists with carbon (19).
Currently, the control of PCDD/Fs emissions in practical
processes is generally conducted by “end of pipe” technolo-
gies such as activated carbon injection, rapid cooling
scrubber, and/or catalytic decomposition tower for organic
compounds. However, such technologies sometimes lead to
a substantial increase in the process cost and difficulties of
heat recovery from waste gas. To establish stable emission
control and to comply with future emission regulation, it is
an important subject to develop an effective reagent/inhibitor
that reliably suppresses PCDD/Fs emissions even when used
in small quantities. So far, nitrogen- and sulfur-containing
compounds (20-22), hydroxides/oxides of alkali metals (23),
and so forth, have been examined as inhibitors. Particularly,
with regard to nitrogen-containing compounds, noteworthy
suppression effects have been confirmed for urea (21, 24,
25), ammonia (20), monoethanolamine (26), triethanolamine
(26), and EDTA (ethylenediaminetetraacetic acid) (27). The
formations of Cu-N bond and a complex composed of Cu
and N have been proposed as the mechanisms involved in
the suppression of PCDD/Fs formation by the addition of
monoethanolamine and urea, respectively (28, 29). However,
fundamental information to understand the details of the
mechanism is still insufficient.
In the present study, the effect of the addition of urea to
the carbonaceous materials and ammonia to the gas flowed
in the de novo synthesis reaction was examined, focusing on
the species of organic compounds formed and the surface
state of carbon. Temperature-programmed desorption (TPD)
measurement was conducted to evaluate the substances
Introduction
Generally, the following three routes are pointed out for the
formation of polychlorinated dibenzo-p-dioxins (PCDDs) and
polychlorinated dibenzofurans (PCDFs) in the combustion
processes (1):
*
Corresponding author phone: +81-22-217-5154; fax: +81-22-
2
17-5154; e-mail: shunsuke@mail.tagen.tohoku.ac.jp.
1
0.1021/es049040j CCC: $30.25

2005 American Chemical Society
VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
795
Published on Web 12/18/2004

the inside of the gas tubes and the connectors was carefully
rinsed with dichloromethane, which was included to the
sample used for the PCDD/Fs analysis of the gas traps. PCDD/
Fs remained in the solid sample after the experiments were
also analyzed by the Soxhlet extraction method. In this
process, extraction time was prolonged to 24 h in comparison
with the standard time of 16 h (30). Sampling of outlet gas
from the reactor, and the clean-up and analytical procedures
for mono- to octachlorinated homologue of PCDD/Fs were
conducted in accordance with JIS K 0311 (30). The amounts
of PCDD/Fs in the gas traps and solid samples were
determined by gas chromatography/mass spectrometry (GC/
MS) with higher resolution, more than 10 000, and single ion
monitoring (SIM).
TABLE 1. Elemental Composition of the Graphite Reagent and
Its Ash
component
graphite
component ash of graphite
ash content (%)
volatile (%)
C (%)
Cu (ppm)
K (ppm)
Cl (ppm)
N (ppm)
S (ppm)
0.08
0.39
99.3
<5
Cu (%)
K (%)
0.09
1.46
4.47
0.21
1.27
Fe (%)
Na (%)
Ca (%)
Si (%)
Al (%)
Mg (%)
15
60
37.2
50
4.42
0.58
60
O (ppm)
H (ppm)
1020
50
Sampling and Analysis of Chlorinated Organic Com-
pounds. Chlorinated organic compounds discharged along
with the outlet gas were collected by use of a sampling set
consisting of three ice-cooled impingers shown in Figure 1b.
In the order of their arrangement, the first two impingers
contained hexane (100 mL) and the last one was empty. After
the experiment, the three impingers were carefully rinsed
with hexane. A rotary evaporator was used to concentrate
combined/adsorbed on the surface of the carbon samples
after the de novo experiment. On the basis of the results,
suppression mechanism of PCDD/Fs formation has been
discussed.
Experimental Section
Materials Used and Sample Preparation for de Novo
Experiment. Reagent-grade graphite powder was used as a
carbon source. It was rinsed twice with both distilled water
and acetone and then dried at 100 °C for 12 h. The elemental
composition of the graphite particles after rinsing and of its
ash are shown in Table 1. The specific surface area and
the hexane solution from 300 to 15 mL. Subsequently, N
2
was flowed over the hexane solution to concentrate it to 1
mL. In the case with urea as sample, the main focus was the
detection of nitrogen-containing chlorinated organic com-
pounds.
2
average particle size of graphite powder are 7.9 m /g and
2
Measurement of Organic Chlorine and CO-CO . Sampling
1
2.8 µm, respectively. Copper(II) chloride dihydrate (CuCl
O) reagent was used as the chlorine source. Acetone
‚2H O was added to the graphite particles
and hand-mixed thoroughly. It was then dried at 100 °C for
2 h. Standard chlorine content of the sample was set at 1
2
‚
of organic chlorine was performed by flowing the outlet gas
into ice-cooled columns of activated carbon (see Figure 1c).
The sampling column consisted of three beds and each of
them was packed with 1 g of activated carbon particles. After
the experiment, activated carbon was first rinsed with distilled
water followed by rinsing with an aqueous solution of
2H
2
solution of CuCl
2
2
1
mass %. This sample is referred to as “CuCl
present paper.
2
sample” in the
3
potassium nitrate (0.8 mass % KNO ) to remove the inorganic
The sample with urea, referred to as “urea sample”, was
prepared by adding an aqueous solution of urea to the CuCl
chlorides. Subsequently, Ar was passed over the activated
carbon particles to dry it out. The amount of chlorine
remained on the activated carbon particles was determined
by the “total organic chlorine halogen analyzer” (31) em-
ploying the procedure of combustion/potentiometric redox
titration. The obtained value corresponds to the amount of
chlorine combined with an organic compound adsorbed on
the activated carbon; therefore, it is referred to as “organic
chlorine” in the present study. Additionally, the concentra-
2
sample and stirring it well. Subsequently, this sample was
dried at 100 °C for 12 h. The molar ratio of N in the added
urea to Cu contained in the sample was set at 3.
The prepared powder sample is pressed to form a disklike
tablet and then crushed into granules having size between
1
and 2 mm.
Procedure: De Novo Experiment. The outline of the
apparatus for the de novo experiment is shown in Figure 1.
Ten grams of the granulated sample is packed in the reaction
tube made of heat-resistant glass with an inner diameter of
2
tions of CO and CO in the outlet gas were measured
continuously by infrared gas analyzers. The rates of carbon
oxidation were calculated from the obtained data.
2
7 mm. The thickness of the sample bed was about 25 mm.
The reaction tube is first inserted into an electric furnace
and heated at 300 °C. Ar-O (2.5 mol %) gas mixture is
introduced in the reaction tube simultaneously at a flow rate
of 2.0 L/min. Ar-NH (1 mol %) gas mixture was added to
the introduced gas to achieve NH concentrations of 350
and 1000 ppm. These cases are referred to as “NH (350 ppm)”
and “NH (1000 ppm)” samples, respectively.
Measurement of Temperature-Programmed Desorption.
After the de novo experiment, 2.5 g of each solid sample, that
is, CuCl and urea samples, was subjected to TPD analysis.
2
The sample was heated under helium atmosphere from room
temperature up to 800 °C at the rate of 10 °C/min. During
2 3
the heating, concentrations of CO, CO , NH , and HCN
released from the sample into the outlet gas were measured
with time.
2
3
3
3
3
The bed temperature reaches the target value of 300 °C
in 15 min, and it is maintained at this temperature for 120
min. In the present paper, “0 min” is defined as the time
when the temperature reaches 300 °C. Therefore, the initial
heating stage is 15 min, from -15 to 0 min, and the later
holding stage comprises the next 120 min, from 0 to 120 min.
The composition and the flow rate of gas were kept constant
throughout the experiment.
Sampling and Analysis of PCDD/Fs. For the sampling of
PCDD/Fs, outlet gas from the reaction tube was passed
through ice-cooled gas traps consisting of five impingers and
an XAD-II resinous column (see Figure 1a). In the order of
their arrangement, the first two impingers contained distilled
water (100 mL), the fourth one contained diethylene glycol
Results and Discussion
Figure 2 shows the time-variation profiles of the oxidation
rates of carbon obtained for CuCl
NH (1000 ppm) samples. The initial peaks for all samples
appear around 15 min during the holding stage. The peak
values of urea, NH (350 ppm), and NH (1000 ppm) samples
are larger than that of CuCl sample. Particularly, the peak
of NH (1000 ppm) sample appears significantly large. Once
the peaks are attained, the oxidation rate of carbon dropped
drastically and became smaller than that of CuCl sample.
However, the total amount of oxidized carbon during
experiments was quite similar for CuCl , urea, and NH (350
ppm) samples, that is, 28.3, 25.9, and 28.6 mg of C/g, except
2 3
, urea, NH (350 ppm), and
3
3
3
2
3
2
2
3
(
100 mL) and the last one was empty. After the experiment,
7
96
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 3, 2005

FIGURE 1. Schematic diagram of the experimental apparatus for de novo experiment.
FIGURE 2. Changes in the rates of carbon oxidation with time for
CuCl , NH (350 ppm), NH (1000 ppm), and urea samples.
FIGURE 4. Time dependence of the nitrogen concentrations in the
solid residue for CuCl and urea samples.
2
3
3
2
FIGURE 5. Time dependence of the chlorine concentrations in the
solid residue for CuCl and urea samples.
FIGURE 3. Time dependence of the amount of organic chlorine
detected in the gas traps for CuCl
and urea samples.
2
2 3 3
, NH (350 ppm), NH (1000 ppm),
decomposition products of urea is released during the heating
stage.
for the NH
of C/g.
3
(1000 ppm) sample that had a value of 35.6 mg
On the other hand, the change in chlorine concentration
shows similar trends for CuCl
2
and urea samples as shown
Figure 3 shows the integrated amount of organic chlorine
discharged during the de novo experiments for CuCl , urea,
(1000 ppm) samples. Addition of
gas did not significantly affect the process; however, the
in Figure 5, suggesting that the release of chlorine from CuCl
2
2
occurs in a similar manner. Therefore, the suppression of
organic chlorine formation that has been confirmed for urea
sample appears to relate to the reactions between the released
nitrogen-containing compounds/radicals and chlorine. Fur-
ther, it can be assumed that the formation of NH Cl (g) will
4
prevent the chlorination of carbon [C-Cl bond (32)] and
organic compounds.
NH
NH
3
(350 ppm), and NH
3
3
amount of organic chlorine tends to decrease with an increase
in the NH concentration. Significantly low values were
3
observed in the case of urea sample, and it appeared to have
a large effect in the heating stage. The change in nitrogen
concentration of urea sample with time is shown in Figure
4
8
. It decreases rapidly in the heating stage and approximately
7% of nitrogen originally contained in the sample is
Figure 6 shows the amount of PCDD/Fs detected in the
gas traps and the residual solid samples for CuCl
(350 ppm), and NH (1000 ppm) samples. The addition of
nitrogen-containing compounds reduces the amount of
2 3
, urea, NH
decreased during this period. This decrease in nitrogen
concentration suggests that a remarkable amount of the
3
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FIGURE 6. Amount of PCDD/Fs detected in the gas traps and solid
sample for CuCl , NH (350 ppm), NH (1000 ppm), and urea samples.
2
3
3
FIGURE 8. Image of the formation of pentachloropyridine on the
carbon surface.
FIGURE 7. Time dependence of the amount of PCDD/Fs detected
in the gas traps for CuCl and urea samples.
2
TABLE 2. Amounts of Nitrogen-Containing Organic Chlorine
Compounds Detected in the Gas Traps for Urea Sample during
Each Experimental Perioda
-
15 to 0 0-20 20-60 60-120
min
min
min
min
FIGURE 9. HCN-TPD profiles for the urea samples, original and
holding time at 0, 20, 60, and 120 min.
2
4
4
2
,2,2-trichloroacetamide
-chlorobenzonitrile
-chloro-2-methylbenzeneamine
,6-dichlorobenzonitrile
17.07
0.17
0.82
0.09
0.40
0.90 nd
nd
nd
0.42
nd
0.20
0.07 0.03
0.13 0.16
0.10 nd
each experimental time period, that is, from -15 to 0, 0 to
20, 20 to 60, and 60 to 120 min. In the heating stage, a large
amount of 2,2,2-trichloroacetamide is formed. Considering
pentachloropyridine
1.83 0.33
a
Amounts are given in micrograms per gram of sample; nd, not
that 2,2,2-trichloroacetaldehyde is also formed for the CuCl
2
determined.
sample in the same stage (33), coexistence of nitrogen-
containing compound seems to elicit a substitution reaction
by NHi radical to acetaldehyde. In the holding stage,
formation rate of 2,2,2-trichloroacetamide decreases while
that of 4-chlorobenzonitrile, 4-chloro-2-methylbenzeneam-
ine, and 2,6-dichlorobenzonitrile increases. These observa-
tions suggest that a substitution reaction by the CN radical
also proceeds on the surface of carbon in this stage. Formation
of pentachloropyridine was also confirmed in the holding
stage.
PCDD/Fs remained in the solid samples. However, when
NH was added to the gas atmosphere, reduction in the
amount of PCDD/Fs in the gas traps was found only for the
NH (1000 ppm) sample. This is consistent with the result
obtained for organic chlorine formation (Figure 3). On the
other hand, the addition of urea to the solid sample leads to
drastic reduction in the amount of PCDD/Fs in the gas traps.
Assuming that the desorption ratios of PCDD/Fs formed on
the sample surface are not significantly different across all
cases, it is necessary to consider the reaction that causes the
PCDD/Fs molecules to change their forms before/during the
desorption process.
3
3
On the basis of the above results, the suppression
mechanisms of PCDD/Fs due to the addition of urea may be
explained as follows: As the oxidation rate of carbon is not
substantially different between CuCl and urea samples (see
2
Figure 7 shows the changes in the amount of PCDD/Fs
detected in the gas traps for CuCl and urea samples with
2
time. A linear relationship is observed in both cases. The
value in the case of the urea sample is approximately one-
2
tenth that of the CuCl sample. As mentioned earlier, 87%
of the nitrogen, added as urea, is released from the solid
sample during the heating stage. However, the suppression
effect of urea is observed throughout the experiment. Further,
since urea is unstable under such an experimental condition,
it may change to other thermally stable compounds. Thus,
it can be deduced that even small amounts of certain
nitrogen-containing compounds have a significant suppres-
sion effect on the formation of PCDD/Fs.
Figure 2), it can be deduced that the supplying rates of the
carbon frame necessary for the formation of PCDD/Fs are
also similar in both cases. In the case of the urea addition,
substitution reactions by NHi and CN radicals, which are
formed by the decomposition of nitrogen-containing com-
pounds, occur on the organic compounds. This may largely
inhibit the formation of dibenzo-p-dioxins and dibenzofurans
that consist of only C, H, and Cl. Pentachloropyridine can
be also formed through condensation reaction by the CN
radical on the carbon surface as shown in Figure 8.
In this mechanism, the existence of the compounds that
generate active nitrogen-containing radicals is important.
Figure 9 shows the HCN-TPD profiles for the original sample
and the samples after the de novo experiments for the urea
sample at 300 °C with different holding times of 0, 20, 60, and
Table 2 shows nitrogen-containing organic chlorides and
their amount detected for urea sample in the gas traps during
7
98
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 3, 2005

1
2
20 min. For the original sample, a peak appears at about
80 °C. The peak temperature increased to about 300 °C for
(5) Weber, R.; Iino, F.; Imagawa, T.; Takeuchi, M.; Sakurai, T.;
Sadakata, M. Chemosphere 2001, 44, 1429.
(
(
6) Stieglitz, L.; Altwicker, E. R. Chemosphere 1997, 34, 1083.
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Technol. 1987, 21, 1080.
the sample with holding time of 0 min, although the amount
of HCN detected decreased to less than one-tenth the original
sample. For the samples with holding times of 20 and 60
min, two distinct peaks were observed at lower than 250 and
over 400 °C, respectively. Further, the peak observed at 400
(
(
8) Stieglitz, L.; Vogg, H. Chemosphere 1987, 16, 1917.
9) Stieglitz, L.; Bautz, H.; Roth, W.; Zwick, G. Chemosphere 1997,
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°
C shifted further away to the higher temperature range for
(10) Stieglitz, L.; Zwick, G.; Beck, J.; Roth, W.; Vogg, H. Chemosphere
989, 18, 1219.
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1
the sample with the holding time of 120 min, suggesting that
gradual generation of HCN continues up to 800 °C or higher
temperatures. This suggests that urea changes to a stable
form at higher temperatures and remains on the surface of
the sample.
(
1
(
12) Addink, R.; Drijver, D. J.; Olie, K. Chemosphere 1991, 23, 1205.
13) Stieglitz, L.; Eichberger, M.; Schleihauf, J.; Zwick, G.; Will, R.
Chemosphere 1993, 27, 343.
(
As mentioned earlier, addition of urea to the solid sample
produces a significantly larger effect on the suppression of
PCDD/Fs formation through de novo synthesis than that
(14) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 1373.
(
(
(
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3
due to the addition of NH to the gas atmosphere. This can
be ascribed to the formation of nitrogen-containing radicals,
such as NHi and CN, formed due to the decomposition of
urea. Further, the possible cause of the substitution reaction
has been indicated. Certain types of decomposition products
appear to be thermally stable under the conditions of the de
novo experiment; therefore, they remain on the surface of
carbon and gradually release radicals such as NHi and CN.
A pyridine-like structure is thought to be an example of this
form, which endures such high temperatures.
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(
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(
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obtained on the interaction of nitrogen-containing com-
pounds with copper chloride existing on the carbon surface.
However, it does not indicate that the copper chloride in the
de novo synthesis reaction remains unchanged when ni-
trogen-containing compounds coexist. Further study is
necessary to evaluate the effect of these compounds on the
behavior of copper and its role in the de novo synthesis
reaction.
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Acknowledgments
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The financial support provided by the Grants-in-Aid for
Scientific Research of JSPS, Grants for Strategic Research of
ISIJ, and Steel Industry Foundation for the Advancement of
Environmental Protection Technology to the present study
is greatly acknowledged. We thank Dr. K. Kawamoto and Dr.
K. Suzuki for scientific and technical advice.
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Received for review June 24, 2004. Revised manuscript re-
ceived October 20, 2004. Accepted October 25, 2004.
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