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The formation of PCDDs and PCDFs in the catalysed combustion of carbon: implications for coal combustion
The formation of PCDDs and PCDFs in the catalysed combustion of carbon: implications for coal combustion R. Luijk, C. Dot-land,
F. Kapteijn*t
and Ii. A. J. Govers
Department of Environmental and Toxicological Chemistry/*Department Engineering, University of Amsterdam, Nieuwe Achtergracht 766, 7078 The Netherlands (Received 76 April 7992; revised 72 August 7992)
of Chemical WV Amsterdam.
Experiments on combustion of an activated carbon (Norit RX Extra) catalysed by CuCl, (0.5 wt%) were carried out in an air flow containing 5 vol.% HCI and 20~01.~~ Hz0 at 300°C. After partial combustion (2h), the samples were analysed for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The total yield of these compounds amounted to 34pgg-’ carbon, with a PCDD/PCDF ratio of 10. The major constituents were the hexa- and heptachlorinated dibenzo-p-dioxins
(50 and 20wt% respectively). The isomer distribution was characteristic of that of a typical fly ash from incineration of waste. The copper-catalysed combustion of the polyaromatic structure of activated carbon in the presence of HCl gives rise to partial chlorination of the carbon surface and local stabilization carbon burnoff. The isolated chlorinated aromatic fragments are assumed to be intermediates formation of PCDDs and PCDFs.
of the in the
(Keywords: catalyst; combustion: carbon)
During incineration of municipal waste, chemical waste and hospital waste, and during combustion of coal, oil, wood and gasoline, polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are generated as trace compounds’. Most studies concerning the formation of PCDDs and PCDFs deal with municipal waste incineration. Severai studies have been conducted on the contribution of combustion of coal to PCDD and PCDF emissions into the environment, but without unequivocal results. In the UK the contribution to total PCDD and PCDF emissions into the atmosphere from combustion sources is estimated to be -25% from domestic coal fires. 10% from industrial coal-fired plants and 5% from coal-fired power stations2. The importance of these coal combustion saurces together equals that of municipal waste incineration. Fiyurr I shows the molecular structure of the parent compounds dibenzo-p-dioxin and dibenzofuran. There are 210 polychlorinated dibenzo-p-dioxin and dibenzofuran congeners, of which seventeen 2,3,7,8-substituted ones have an extremely high toxicity3. In a municipal waste incinerator, PCDDs and PCDFs are found predominantly on fly ash particles. During electrostatic precipitation, which is performed in a relatively low-temperature region (2004OO”C), the amount of PCDDs and PCDFs on fly ash shows a strong _~. ~~~. ~~ ~__~~ Presented at ‘Environmental Aspects of Coal Utilization and Carbon Science Conference’, 31 March-2 April 1992, The University of Newcastle upon Tyne, UK t Present address: Department of Chemical Engineering, Del0 University of Technology, Julianalaan 136, 2628 EL Delft. The Netherlands
OO16-2361/93/030343~15 ( 1993 Butterworth-Heinemann
Ltd.
increase4. However, major questions about the mechanism of formation of PCDDs and PCDFs on fly ash remain unanswered. Two formation routes are postulated to occur’: (1) gas-phase coupling reaction of chlorinated precursors such as chlorophenols (or chlaraphenoxy radicals) and chlorobenzenes, followed by adsorption on the organic particulate phase; (2) heterogeneously catalysed assembly of chlorinated dioxin structures from a carbon, an oxygen and a chlorine source, i.e. the postulated ‘de nouo synthesis. Copper and iron appear to have a catalytic influence’-*, copper being 20 times as effective as iron. All the elements mentioned above are present in fly ash. Moreover, the gas phase continuously supplies oxygen and hydrogen chloride from the waste incineration. With respect to the formation of PCDDs and PCDFs, the conditions of coal combustion resemble those of
4
6 4
dibenzo-p-dioxin
6
dibenzofuran
2.3.7.8-tetrachlorodibenzo-p-dioxin Figure 1
Molecular structures ofdibenzo-p-choxin and dibenzofuran
FUEL, 1993, Vol 72, March
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The formation
of carbon: R. Lui/k et al.
of PCDDs and PCDFs in the catalysed combustion
municipal waste incineration. During pulverized coal combustion, 8@95% of the original mineral matter is emitted as fly ash’. Therefore mineral constituents such as iron and copper are strongly concentrated in fly ash. Depending on the efficiency of the combustion process and the mineral matter content of the coal, the fly ash contains residual unburnt carbon, typically up to 5 wt%. Chlorine, present in coal in concentrations varying from 0.05 to 2 g kg ‘, is emitted predominantly as hydrogen chloride”. In summary, all ingredients for the formation of PCDDs and PCDFs by the ‘de nouo’ synthesis are present during coal combustion. This study describes the formation of PCDDs and PCDFs from pure carbon in the presence of a copper catalyst on the carbon surface and hydrogen chloride and oxygen in the gas phase. The formation of PCDDs and PCDFs in a more realistic model fly ash system, i.e. a mixture of a silica-alumina-supported catalyst and activated carbon, was used as a reference to study the PCDD and PCDF formation characteristics in the catalysed combustion of pure carbon.
EXPERIMENTAL
13C-labelled PCDD and PCDF standards (CIL, Woburn, MA, USA) Si02-A1203, LA-300-lOP, 420m2g-‘, d,= 106-150pm (AKZO Chemie Nederland bv, Amsterdam, The Netherlands) Norit RX Extra, an acid-washed, steam-activated peat char, 1100 m2 g- ‘, d, = loo-325 pm HCl(4.0) (UCAR Speciality Gases, Nieuw-Vennep, The Netherlands) N,(4.0) and 02(4.0) (Hoekloos, Schiedam, The Netherlands) CuCl, (Janssen Chimica, Tilburg, The Netherlands).
toluene and the clean-up previously”,12.
method
have been described
G.c.-m.s. analvsis The formation of PCDDs and PCDFs in incineration processes shows a typical isomer distribution pattern including almost 210 possible congeners13. This study only deals with tetra- to octachlorinated PCDDs and PCDFs. Within the isomer groups, typical PCDD/PCDF incineration patterns were found. Therefore the results are given as the sum of different PCDD and PCDF isomer groups: tetrachlorinated (xCl,DD and CCl,DF), pentachlorinated (CCl,DD and CCl,DF), etc. PCDD and PCDF analysis has been described previously’ l-1 2.14. All results are presented as averages of three or four experiments with 95% confidence intervals. RESULTS In the presence of HCl in the combustion gas, no sample weight loss was observed. Figures 2a and 2b show the yields of tetra- to octachlorinated PCDDs and PCDFs formed in the catalysed combustion of activated carbon. Among the PCDDs as well as the PCDFs the major constituents are hexa- and heptachlorinated compounds, accounting for 53 and 23 wt% of the total yield of PCDDs and PCDFs respectively. The total yield of PCDDs and PCDFs is 3.5k2.4 and 0.3+O.lpgg-’ carbon. The PCDD/PCDF ratio of 10.5 is much larger than the values of 0.6-1.7 generally observed in municipal waste incinerator fly ash.
m carbon
blank
6000 5000 i
Conlbustion Combustion experiments on activated carbon, loaded with 0.5 wt% CuCl, by pore volume impregnation, were conducted in triplicate in a flow of air containing 5 vol.% HCl and 2Ovol.% H,O at 300°C. The sample size was 0.2g, the reaction time 2 h, and the gas flow rate lOOmlmin_ l. One additional experiment was conducted under the same conditions without HCl in the gas flow, as a blank test. A second series of combustion experiments was conducted in quadruplicate with a physical mixture of 0.5 wt% CuCl,/SiO,-Al,O, and activated carbon in the ratio 25:l by weight. The experimental conditions were identical to those described above, but the sample size was 1 g. Again one blank experiment was conducted without HCl. The combustion experiments were performed in a horizontal Pyrex tube reactor. .4fter preheating of the reactor, an inlet tube with a cylindrical sample basket was introduced into the centre of the reactor. The gas flow through the fixed bed was controlled by mass flow controllers and mixed in a chamber containing ceramic pellets. Water was introduced by a motor-driven syringe. A cold trap (toluene, OOC) was used to collect possible evaporated species. After the experiment the sample was weighed to determine the combustion loss. Other apparatus, the Soxhlet extraction procedure with
344
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Si
4000
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c E a
3000
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‘5.
2000
-
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Gi
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CI,DD
CI,DD
CI,DD
CI,DD
CI,DD
L
carbon 300
-b
250
-
blank
CI,DF
CI,DF
CI,DF
CI,DF
Figure 2 Yield of PCDDs and PCDFs in the catalysed of an activated
carbon
(0.5 wt% CuClJC)
CI,DF
combustion
The formation
of PCDDs and PCDFs in the catalysed combustion
Figures 3a and 3h show the yields of tetra- to octachlorinated PCDDs and PCDFs formed in the model fly ash system consisting of a mixture of 0.5 wt% CuCl,/silica-alumina and activated carbon (4 wt%). A maximum is observed for the octachlorinated compounds. The total yield of PCDDs and PCDFs is 1.2 f 0.3 and model fly ash. The PCDD/PCDF ratio 1.5f0.5~gg~’ of 0.8 is representative of the formation characteristics during municipal waste incineration. In both types of experiment the contribution of the blank run without HCl in the gas phase is negligible. Figures 4u and 4h compare the relative yields of tetra- to octachlorinated PCDDs and PCDFs in the two types of combustion experiment described above. The uncertainties in the absolute yields of different PCDD and PCDF isomer groups are reduced when their yields relative to the sum of PCDDs or the sum of PCDFs are considered. The fly ash model system clearly shows a tendency towards complete chlorination of dibenzo-pdioxin and dibenzofuran. In the catalysed combustion of activated carbon a maximum is found with the hexa- and heptachlorinated compounds for PCDDs as well as for PCDFs. The selectivities are more pronounced for the PCDDs than for the PCDFs.
of carbon: R. Luok et al. I
carbon
+ SiO,-AI,O,
blank
,000
5i G 5.
800
2
600
a,
‘S
400
‘“:, CI,DD
,500
Fan
1 CI,DD
Cl&D
+ SiO,-AI,O,
CI,OD
Cl,00
6?k
DISCUSSION The results presented show unambiguously that PCDDs and PCDFs are formed from activated carbon in the presence of a copper catalyst, air, steam and HCl. Only a few previous studies contain data that imply this. Striimbergl’ investigated the formation of chlorinated hydrocarbons from gaseous precursors and noted a dramatic increase in their production when solid carbon sources such as graphite and bituminous coal were present. Jay and Stieglitz’ heated a physical mixture of magnesium silicate, CuCl, .2H,O and activated carbon (weight ratio 20:5:0.8) in air and observed considerable formation of chlorinated benzenes together with some PCDDs and PCDFs. The combustion of activated carbon normally occurs at temperatures > 600°C but in the presence of copper(H) salts on the carbon surface the combustion temperature is reduced to 300°C I6 . CuCI,, in the presence of oxygen and hydrogen chloride at 3OO”C,acts as an oxychlorination catalyst”. Besides oxidative degradation of carbon, the surface is partly chlorinated, especially at those sites prone to oxidation. The modification of the carbon surface with thermally stable C-Cl bonds will locally prevent the burnoff of carbon18, as has been observed in this study and is found in oxychlorination”. As a consequence, low-molecular-weight chlorinated ring systems will be isolated when the surrounding polyaromatic carbon structure is burnt off. These fragments could be precursors in the formation of PCDDs and PCDFs. This isolation of aromatic fragments from the carbon structure constitutes the first step in the ‘de nom synthesis of PCDDs and PCDFs from carbon. In the catalysed combustion of activated carbon as well as in the model fly ash system, a typical incineration pattern was observed within the different PCDD and PCDF isomer groups. Two main differences between these two combustion processes lay in the PCDD/PCDF ratio and the position of the maximum in the degree of chlorination. In the catalysed combustion of carbon, the PCDD/PCDF ratio was over 10 times that found in the
CI,DF
CI,DF
Figure 3 Yield of PCDDs (0.5 wt% CuC12/SiOZ-A1203
CI,DF
CI,DF
CI,DF
and PCDFs in a model fly ash system mixed with activated carbon)
El carbon
carbon
+ SiO,-AI,O,
100 90 -
a
60
2 .? x
60
.-F z 5 L
40-
50 -
3020 -
CI,DD
CI,DD
CI,DD
CI,DD
CI,DD
I
carbon
+ SiO,-AI,O,
carbon
g”r k _ 7 0 -0
70 60
x ‘X
50
I? .-
40
5 u L
30 20 10 0 CI,DF
CI,DF
C&OF
CI,DF
CI,DF
Figure 4 Relative yield of PCDDs and PCDFs in the combustion an activated carbon and in a model fly ash system
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The formation
of PCDDs and PCDFs in the catalysed combustion
model fly ash system. The latter more or less represents a realistic municipal waste incinerator fly ash. The large PCDD/PCDF ratio in the catalysed combustion of carbon indicates a more efficient incorporation of oxygen in the polyaromatic carbon structure when CuCl, is present on the carbon surface. It is unclear whether the dibenzo-p-dioxin structure directly originates from the degrading polyaromatic carbon structure or whether precursors such as chlorobenzenes and chlorophenols also play an important role, as is suggested by the results of Jay and Stieglitz’. The formation of volatile chlorinated precursors in the catalysed combustion of activated carbon is under further investigation. In the model fly ash system a maximum was observed for the octachlorinated compounds, whereas the catalysed combustion of carbon displayed a maximum for the hexa- and heptachlorinated compounds. The fly ash from municipal waste incineration generally has a maximum for the hexa- and heptachlorinated PCDDs and PCDFs. This indicates that the catalysed combustion of carbon is representative of waste incineration fly ash with respect to the maximum in the degree of chlorination. The differences in the position of the maximum in the degree of chlorination may be explained by different catalyst activities, depending on the surface characteristics of activated carbon and silicaaalumina. In the model fly ash system the catalyst was loaded on to the porous silicaaalumina, so that little contact with the carbon source could be expected. In order to explain the formation of PCDDs and PCDFs in the model fly ash system, transport processes have to be considered. In a humid atmosphere at 3OO”C, CuCl, probably migrates from the silica-alumina matrix, present in large excess, to the carbon surface by evaporation16, followed by the catalysed combustion of carbon. Because of the differences in PCDD!PCDF formation characteristics with and without silicaaalumina, intermediate products could also be expected to migrate to the silica-alumina matrix, where they would undergo subsequent reactions. In both combustion processes the total yield (gg-’ carbon) of PCDDs and PCDFs is of the same order of magnitude. This indicates that the amount of carbon is not rate-determining. In accordance with the results of Stieglitz et al.19, who showed that the yield of PCDDs and PCDFs in a fly ash model system is linear with the catalyst concentration, it is postulated that the CuCl, concentration is rate-determining in the formation of PCDDs and PCDFs. CuCl, is necessary for the catalytic combustion of carbon under the HCl atmosphere used. The catalysed burnoff supplies chlorinated aromatic fragments which are intermediates in the formation of PCDDs and PCDFs. The presence of silica-alumina is not necessary for the formation of PCDDs and PCDFs, but the acidity of the catalyst support is an important factor. An alkaline alumina is a very strong dechlorination agent”. The role of the acidic silica-alumina lies in further chlorination reactions, influencing the maximum in the degree of chlorination, and probably also lies in conversion reactions, influencing the PCDD/PCDF ratio. The findings from this study have implications for other industrial processes such as oxychlorination, catalytic combustion of chlorinated fluorocarbons and pulverized coal combustion, in which similar operating conditions can be identified. In oxychlorination, carbonand alumina-supported copper chloride are used as
346
FUEL, 1993, Vol 72, March
of carbon: R. Luijk et al.
catalysts under similar conditions’ 7 as used in the present study. Carbon deposition on the catalyst often occurs in this process, necessitating thorough product cleaning. Moreover, without additional clean-up, the catalyst should be considered as highly contaminated chemical waste after use. In evaluating the relevance of these results for coal-fired plants, several aspects have to be taken into account. PCDD and PCDF formation is assumed to take place between 200 and 400°C temperatures that are found in electrostatic precipitators. Furthermore, unburnt carbonaceous material, HCl, oxygen and the presence of the proven catalytic elements Cu and Fe are needed, together with a sufficient residence time in the precipitator. Swaine’i reports that coal contains on average 502000ppm Cl, but values of 9100 ppm (UK), 8000ppm (USA) and 17000 ppm (Australia) can be found. Chlorine is emitted 99% as HCl in the gas phase, so all the chlorine in coal passes through the precipitator, unless lime is injected in the combustion process. Although in minor concentrations, bromine is also present in coal (the range is 0.5590ppm in most cases, although some Canadian coals can contain up to 3550 ppm”) and emitted _ 60% as HBr (personal communication, R. Mey, Netherlands Agency of Energy and the Environment). Bromine may play a role in the formation of PCDDs and PCDFs because of its higher reactivity in the halogenation of aromatic compounds and the fast exchange reaction of brominated aromatic compounds with chlorine under the conditions used23*24. Halogen emission in coal combustion is reviewed by Slossi”. Depending on the chlorine concentration in the coal, the type of combustion and the pollution control technique used, 50 to several thousand ppm HCl are present in the flue gas, being of the same order of magnitude as found for waste incineration (200-3000 ppm). Total amounts of HCl emission are of the same order of magnitude as from waste incineration (e.g. in The Netherlands, Italy, France, Denmark) or much larger (in the UK, Spain, West Germany). This implies that under favourable conditions, with copper and iron present in the coal mineral matter, the contribution of PCDD and PCDF formation can be at least as large as that of waste incineration. For the UK a risk assessment arrived at the same conclusion2. Burning of 1 t of coal of 5 wt% ash with an efficiency of 99% would yield 3&40mg of PCDD/PCDFs under the conditions of this study, corresponding to ppm levels in the fly ash. Since these are extreme conditions, these values must be considered as upper levels. Nevertheless, these values are high enough for further research in this area to be quite important and urgent. CONCLUSIONS The copper-catalysed combustion of activated carbon in the presence of HCl at temperatures between 200-400”C gives rise to the formation of PCDDs and PCDFs. The catalysed burnoff of the polyaromatic structure of carbon is locally stabilized when CC1 bonds are present; hence chlorinated aromatic fragments are isolated. Under the conditions used, copper is the active catalyst in the combustion and the partial chlorination of carbon and therefore also in the formation of PCDDs and PCDFs. The presence of acidic silicaaalumina gives rise to further chlorination reactions and influences the PCDD/PCDF ratio.
The formation
of PCDDs
and PCDFs
REFERENCES
IO II 12
in the catalysed 13
Society for Clean Air in the Netherlands, ‘Expertise on the Measurement and Control of Dioxins’, Delft, 1991 Department of the Environment, ‘Dioxins in the Environment’, Pollution Paper 27, HMSO. London, 1989 Paustenbach. D. J. ‘The Risk Assessment of Environmental and Human Health Hazards: Textbook ofCase Studies’, Wiley, New York. 1989 Vogg, H., Metzger. M. and Stieglitz, L. B’uste Manage. Res.
14 15 16
1987, 5. 285
20
Shaub. W. M. andTsang, W. Enriron. Sci. Technol. 1983,17,721 Stieglitz. L.. Zwick. G.. Beck. J.. Roth, W. and Vogg, H. C%rnzo.s@~er~ 1989. 18. 1219 Stieglitz. L.. Vogg. H.. Zwick. G.. Beck, J. and Bautz, H. Chervosphere 1991, 23, 1255 Jav. K. and Stieghtz. L. Chemosphere 1991. 22, 987 Hialmarxson, AT K. ‘Interactions in Emissions Control for Coal-fired Plants’. IEACR,‘47. IEA Coal Research, London, 1991 Sloss. S. A. ‘Halogens from Coal Combustion’. IEACR/45, IEA Coal Research, London, 1991 Addink. R., Drijver, D. 3. and Ohe. K. Chemosphere 1991. 23, 1205 Van Berkel, 0. M., Olie. K. and van den Berg. M. Irzl. J. Emiron. .4m//. Chrm. 1989, 34. 51
17 18 19
combustion
of carbon: R. LuQk et al.
Yasuhara. A., Hiroyasu. I. and Morita. M. Btriron. Sci. Technol. 1987, 21, 971 Olie. K.. Slot, P. C. and Wever. H. Chemo.sphere 1989,19,103 Stromberg, B. Chemosphere 1991, 23. 1515 Singoredjo. L. Ph.D. Thesis. University of Amsterdam. 1992. Ch. 2 Allen. J. A. and Clark. A. J. Rer. Pure Appl. Chem. 1971.21.145 Hall, C. R. and Holmes, R. J. Carbon 1992, 30. 173 Stieglitz. L. and Vogg. H. In Proceedings of the International Workshoo on Municiual Waste Incineration. Montreal. 1987 Schooneboom, M. H.: Smit. P. N. and Olie. K. ‘Alumina as a model support in the formation and dechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans’ (submitted to Chemosphere)
21 22
23
24
Swaine. D. J. ‘Trace Elements in Coal’, Butterworths, London, 1990 Van Egmond. N. D. ‘Air pollution as a result of the emission from coal-fired power stations’. Project no. 20.70-004.10. PEO Management Office for Energy Research. Utrecht, 1986 Luijk. R.. Jansen, J. and Govers, H. A. J. ‘The exchange of bromine and chlorine in 2.3,7.8-tetrabromodibenzo-p-dioxin’ (in preparation) Wilken, M., Beyer, A. and Jager, J. In Short Papers. 10th Int Meeting, DIOXIN90, 1990. 2, pp. 377-381
FUEL, 1993, Vol 72, March
347
F. Kapteijn*t
and Ii. A. J. Govers
Department of Environmental and Toxicological Chemistry/*Department Engineering, University of Amsterdam, Nieuwe Achtergracht 766, 7078 The Netherlands (Received 76 April 7992; revised 72 August 7992)
of Chemical WV Amsterdam.
Experiments on combustion of an activated carbon (Norit RX Extra) catalysed by CuCl, (0.5 wt%) were carried out in an air flow containing 5 vol.% HCI and 20~01.~~ Hz0 at 300°C. After partial combustion (2h), the samples were analysed for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The total yield of these compounds amounted to 34pgg-’ carbon, with a PCDD/PCDF ratio of 10. The major constituents were the hexa- and heptachlorinated dibenzo-p-dioxins
(50 and 20wt% respectively). The isomer distribution was characteristic of that of a typical fly ash from incineration of waste. The copper-catalysed combustion of the polyaromatic structure of activated carbon in the presence of HCl gives rise to partial chlorination of the carbon surface and local stabilization carbon burnoff. The isolated chlorinated aromatic fragments are assumed to be intermediates formation of PCDDs and PCDFs.
of the in the
(Keywords: catalyst; combustion: carbon)
During incineration of municipal waste, chemical waste and hospital waste, and during combustion of coal, oil, wood and gasoline, polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are generated as trace compounds’. Most studies concerning the formation of PCDDs and PCDFs deal with municipal waste incineration. Severai studies have been conducted on the contribution of combustion of coal to PCDD and PCDF emissions into the environment, but without unequivocal results. In the UK the contribution to total PCDD and PCDF emissions into the atmosphere from combustion sources is estimated to be -25% from domestic coal fires. 10% from industrial coal-fired plants and 5% from coal-fired power stations2. The importance of these coal combustion saurces together equals that of municipal waste incineration. Fiyurr I shows the molecular structure of the parent compounds dibenzo-p-dioxin and dibenzofuran. There are 210 polychlorinated dibenzo-p-dioxin and dibenzofuran congeners, of which seventeen 2,3,7,8-substituted ones have an extremely high toxicity3. In a municipal waste incinerator, PCDDs and PCDFs are found predominantly on fly ash particles. During electrostatic precipitation, which is performed in a relatively low-temperature region (2004OO”C), the amount of PCDDs and PCDFs on fly ash shows a strong _~. ~~~. ~~ ~__~~ Presented at ‘Environmental Aspects of Coal Utilization and Carbon Science Conference’, 31 March-2 April 1992, The University of Newcastle upon Tyne, UK t Present address: Department of Chemical Engineering, Del0 University of Technology, Julianalaan 136, 2628 EL Delft. The Netherlands
OO16-2361/93/030343~15 ( 1993 Butterworth-Heinemann
Ltd.
increase4. However, major questions about the mechanism of formation of PCDDs and PCDFs on fly ash remain unanswered. Two formation routes are postulated to occur’: (1) gas-phase coupling reaction of chlorinated precursors such as chlorophenols (or chlaraphenoxy radicals) and chlorobenzenes, followed by adsorption on the organic particulate phase; (2) heterogeneously catalysed assembly of chlorinated dioxin structures from a carbon, an oxygen and a chlorine source, i.e. the postulated ‘de nouo synthesis. Copper and iron appear to have a catalytic influence’-*, copper being 20 times as effective as iron. All the elements mentioned above are present in fly ash. Moreover, the gas phase continuously supplies oxygen and hydrogen chloride from the waste incineration. With respect to the formation of PCDDs and PCDFs, the conditions of coal combustion resemble those of
4
6 4
dibenzo-p-dioxin
6
dibenzofuran
2.3.7.8-tetrachlorodibenzo-p-dioxin Figure 1
Molecular structures ofdibenzo-p-choxin and dibenzofuran
FUEL, 1993, Vol 72, March
343
The formation
of carbon: R. Lui/k et al.
of PCDDs and PCDFs in the catalysed combustion
municipal waste incineration. During pulverized coal combustion, 8@95% of the original mineral matter is emitted as fly ash’. Therefore mineral constituents such as iron and copper are strongly concentrated in fly ash. Depending on the efficiency of the combustion process and the mineral matter content of the coal, the fly ash contains residual unburnt carbon, typically up to 5 wt%. Chlorine, present in coal in concentrations varying from 0.05 to 2 g kg ‘, is emitted predominantly as hydrogen chloride”. In summary, all ingredients for the formation of PCDDs and PCDFs by the ‘de nouo’ synthesis are present during coal combustion. This study describes the formation of PCDDs and PCDFs from pure carbon in the presence of a copper catalyst on the carbon surface and hydrogen chloride and oxygen in the gas phase. The formation of PCDDs and PCDFs in a more realistic model fly ash system, i.e. a mixture of a silica-alumina-supported catalyst and activated carbon, was used as a reference to study the PCDD and PCDF formation characteristics in the catalysed combustion of pure carbon.
EXPERIMENTAL
13C-labelled PCDD and PCDF standards (CIL, Woburn, MA, USA) Si02-A1203, LA-300-lOP, 420m2g-‘, d,= 106-150pm (AKZO Chemie Nederland bv, Amsterdam, The Netherlands) Norit RX Extra, an acid-washed, steam-activated peat char, 1100 m2 g- ‘, d, = loo-325 pm HCl(4.0) (UCAR Speciality Gases, Nieuw-Vennep, The Netherlands) N,(4.0) and 02(4.0) (Hoekloos, Schiedam, The Netherlands) CuCl, (Janssen Chimica, Tilburg, The Netherlands).
toluene and the clean-up previously”,12.
method
have been described
G.c.-m.s. analvsis The formation of PCDDs and PCDFs in incineration processes shows a typical isomer distribution pattern including almost 210 possible congeners13. This study only deals with tetra- to octachlorinated PCDDs and PCDFs. Within the isomer groups, typical PCDD/PCDF incineration patterns were found. Therefore the results are given as the sum of different PCDD and PCDF isomer groups: tetrachlorinated (xCl,DD and CCl,DF), pentachlorinated (CCl,DD and CCl,DF), etc. PCDD and PCDF analysis has been described previously’ l-1 2.14. All results are presented as averages of three or four experiments with 95% confidence intervals. RESULTS In the presence of HCl in the combustion gas, no sample weight loss was observed. Figures 2a and 2b show the yields of tetra- to octachlorinated PCDDs and PCDFs formed in the catalysed combustion of activated carbon. Among the PCDDs as well as the PCDFs the major constituents are hexa- and heptachlorinated compounds, accounting for 53 and 23 wt% of the total yield of PCDDs and PCDFs respectively. The total yield of PCDDs and PCDFs is 3.5k2.4 and 0.3+O.lpgg-’ carbon. The PCDD/PCDF ratio of 10.5 is much larger than the values of 0.6-1.7 generally observed in municipal waste incinerator fly ash.
m carbon
blank
6000 5000 i
Conlbustion Combustion experiments on activated carbon, loaded with 0.5 wt% CuCl, by pore volume impregnation, were conducted in triplicate in a flow of air containing 5 vol.% HCl and 2Ovol.% H,O at 300°C. The sample size was 0.2g, the reaction time 2 h, and the gas flow rate lOOmlmin_ l. One additional experiment was conducted under the same conditions without HCl in the gas flow, as a blank test. A second series of combustion experiments was conducted in quadruplicate with a physical mixture of 0.5 wt% CuCl,/SiO,-Al,O, and activated carbon in the ratio 25:l by weight. The experimental conditions were identical to those described above, but the sample size was 1 g. Again one blank experiment was conducted without HCl. The combustion experiments were performed in a horizontal Pyrex tube reactor. .4fter preheating of the reactor, an inlet tube with a cylindrical sample basket was introduced into the centre of the reactor. The gas flow through the fixed bed was controlled by mass flow controllers and mixed in a chamber containing ceramic pellets. Water was introduced by a motor-driven syringe. A cold trap (toluene, OOC) was used to collect possible evaporated species. After the experiment the sample was weighed to determine the combustion loss. Other apparatus, the Soxhlet extraction procedure with
344
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1993,
Vol 72, March
Si
4000
-
c E a
3000
-
‘5.
2000
-
1000
-
Gi
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CI,DD
CI,DD
CI,DD
L
carbon 300
-b
250
-
blank
CI,DF
CI,DF
CI,DF
CI,DF
Figure 2 Yield of PCDDs and PCDFs in the catalysed of an activated
carbon
(0.5 wt% CuClJC)
CI,DF
combustion
The formation
of PCDDs and PCDFs in the catalysed combustion
Figures 3a and 3h show the yields of tetra- to octachlorinated PCDDs and PCDFs formed in the model fly ash system consisting of a mixture of 0.5 wt% CuCl,/silica-alumina and activated carbon (4 wt%). A maximum is observed for the octachlorinated compounds. The total yield of PCDDs and PCDFs is 1.2 f 0.3 and model fly ash. The PCDD/PCDF ratio 1.5f0.5~gg~’ of 0.8 is representative of the formation characteristics during municipal waste incineration. In both types of experiment the contribution of the blank run without HCl in the gas phase is negligible. Figures 4u and 4h compare the relative yields of tetra- to octachlorinated PCDDs and PCDFs in the two types of combustion experiment described above. The uncertainties in the absolute yields of different PCDD and PCDF isomer groups are reduced when their yields relative to the sum of PCDDs or the sum of PCDFs are considered. The fly ash model system clearly shows a tendency towards complete chlorination of dibenzo-pdioxin and dibenzofuran. In the catalysed combustion of activated carbon a maximum is found with the hexa- and heptachlorinated compounds for PCDDs as well as for PCDFs. The selectivities are more pronounced for the PCDDs than for the PCDFs.
of carbon: R. Luok et al. I
carbon
+ SiO,-AI,O,
blank
,000
5i G 5.
800
2
600
a,
‘S
400
‘“:, CI,DD
,500
Fan
1 CI,DD
Cl&D
+ SiO,-AI,O,
CI,OD
Cl,00
6?k
DISCUSSION The results presented show unambiguously that PCDDs and PCDFs are formed from activated carbon in the presence of a copper catalyst, air, steam and HCl. Only a few previous studies contain data that imply this. Striimbergl’ investigated the formation of chlorinated hydrocarbons from gaseous precursors and noted a dramatic increase in their production when solid carbon sources such as graphite and bituminous coal were present. Jay and Stieglitz’ heated a physical mixture of magnesium silicate, CuCl, .2H,O and activated carbon (weight ratio 20:5:0.8) in air and observed considerable formation of chlorinated benzenes together with some PCDDs and PCDFs. The combustion of activated carbon normally occurs at temperatures > 600°C but in the presence of copper(H) salts on the carbon surface the combustion temperature is reduced to 300°C I6 . CuCI,, in the presence of oxygen and hydrogen chloride at 3OO”C,acts as an oxychlorination catalyst”. Besides oxidative degradation of carbon, the surface is partly chlorinated, especially at those sites prone to oxidation. The modification of the carbon surface with thermally stable C-Cl bonds will locally prevent the burnoff of carbon18, as has been observed in this study and is found in oxychlorination”. As a consequence, low-molecular-weight chlorinated ring systems will be isolated when the surrounding polyaromatic carbon structure is burnt off. These fragments could be precursors in the formation of PCDDs and PCDFs. This isolation of aromatic fragments from the carbon structure constitutes the first step in the ‘de nom synthesis of PCDDs and PCDFs from carbon. In the catalysed combustion of activated carbon as well as in the model fly ash system, a typical incineration pattern was observed within the different PCDD and PCDF isomer groups. Two main differences between these two combustion processes lay in the PCDD/PCDF ratio and the position of the maximum in the degree of chlorination. In the catalysed combustion of carbon, the PCDD/PCDF ratio was over 10 times that found in the
CI,DF
CI,DF
Figure 3 Yield of PCDDs (0.5 wt% CuC12/SiOZ-A1203
CI,DF
CI,DF
CI,DF
and PCDFs in a model fly ash system mixed with activated carbon)
El carbon
carbon
+ SiO,-AI,O,
100 90 -
a
60
2 .? x
60
.-F z 5 L
40-
50 -
3020 -
CI,DD
CI,DD
CI,DD
CI,DD
CI,DD
I
carbon
+ SiO,-AI,O,
carbon
g”r k _ 7 0 -0
70 60
x ‘X
50
I? .-
40
5 u L
30 20 10 0 CI,DF
CI,DF
C&OF
CI,DF
CI,DF
Figure 4 Relative yield of PCDDs and PCDFs in the combustion an activated carbon and in a model fly ash system
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The formation
of PCDDs and PCDFs in the catalysed combustion
model fly ash system. The latter more or less represents a realistic municipal waste incinerator fly ash. The large PCDD/PCDF ratio in the catalysed combustion of carbon indicates a more efficient incorporation of oxygen in the polyaromatic carbon structure when CuCl, is present on the carbon surface. It is unclear whether the dibenzo-p-dioxin structure directly originates from the degrading polyaromatic carbon structure or whether precursors such as chlorobenzenes and chlorophenols also play an important role, as is suggested by the results of Jay and Stieglitz’. The formation of volatile chlorinated precursors in the catalysed combustion of activated carbon is under further investigation. In the model fly ash system a maximum was observed for the octachlorinated compounds, whereas the catalysed combustion of carbon displayed a maximum for the hexa- and heptachlorinated compounds. The fly ash from municipal waste incineration generally has a maximum for the hexa- and heptachlorinated PCDDs and PCDFs. This indicates that the catalysed combustion of carbon is representative of waste incineration fly ash with respect to the maximum in the degree of chlorination. The differences in the position of the maximum in the degree of chlorination may be explained by different catalyst activities, depending on the surface characteristics of activated carbon and silicaaalumina. In the model fly ash system the catalyst was loaded on to the porous silicaaalumina, so that little contact with the carbon source could be expected. In order to explain the formation of PCDDs and PCDFs in the model fly ash system, transport processes have to be considered. In a humid atmosphere at 3OO”C, CuCl, probably migrates from the silica-alumina matrix, present in large excess, to the carbon surface by evaporation16, followed by the catalysed combustion of carbon. Because of the differences in PCDD!PCDF formation characteristics with and without silicaaalumina, intermediate products could also be expected to migrate to the silica-alumina matrix, where they would undergo subsequent reactions. In both combustion processes the total yield (gg-’ carbon) of PCDDs and PCDFs is of the same order of magnitude. This indicates that the amount of carbon is not rate-determining. In accordance with the results of Stieglitz et al.19, who showed that the yield of PCDDs and PCDFs in a fly ash model system is linear with the catalyst concentration, it is postulated that the CuCl, concentration is rate-determining in the formation of PCDDs and PCDFs. CuCl, is necessary for the catalytic combustion of carbon under the HCl atmosphere used. The catalysed burnoff supplies chlorinated aromatic fragments which are intermediates in the formation of PCDDs and PCDFs. The presence of silica-alumina is not necessary for the formation of PCDDs and PCDFs, but the acidity of the catalyst support is an important factor. An alkaline alumina is a very strong dechlorination agent”. The role of the acidic silica-alumina lies in further chlorination reactions, influencing the maximum in the degree of chlorination, and probably also lies in conversion reactions, influencing the PCDD/PCDF ratio. The findings from this study have implications for other industrial processes such as oxychlorination, catalytic combustion of chlorinated fluorocarbons and pulverized coal combustion, in which similar operating conditions can be identified. In oxychlorination, carbonand alumina-supported copper chloride are used as
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FUEL, 1993, Vol 72, March
of carbon: R. Luijk et al.
catalysts under similar conditions’ 7 as used in the present study. Carbon deposition on the catalyst often occurs in this process, necessitating thorough product cleaning. Moreover, without additional clean-up, the catalyst should be considered as highly contaminated chemical waste after use. In evaluating the relevance of these results for coal-fired plants, several aspects have to be taken into account. PCDD and PCDF formation is assumed to take place between 200 and 400°C temperatures that are found in electrostatic precipitators. Furthermore, unburnt carbonaceous material, HCl, oxygen and the presence of the proven catalytic elements Cu and Fe are needed, together with a sufficient residence time in the precipitator. Swaine’i reports that coal contains on average 502000ppm Cl, but values of 9100 ppm (UK), 8000ppm (USA) and 17000 ppm (Australia) can be found. Chlorine is emitted 99% as HCl in the gas phase, so all the chlorine in coal passes through the precipitator, unless lime is injected in the combustion process. Although in minor concentrations, bromine is also present in coal (the range is 0.5590ppm in most cases, although some Canadian coals can contain up to 3550 ppm”) and emitted _ 60% as HBr (personal communication, R. Mey, Netherlands Agency of Energy and the Environment). Bromine may play a role in the formation of PCDDs and PCDFs because of its higher reactivity in the halogenation of aromatic compounds and the fast exchange reaction of brominated aromatic compounds with chlorine under the conditions used23*24. Halogen emission in coal combustion is reviewed by Slossi”. Depending on the chlorine concentration in the coal, the type of combustion and the pollution control technique used, 50 to several thousand ppm HCl are present in the flue gas, being of the same order of magnitude as found for waste incineration (200-3000 ppm). Total amounts of HCl emission are of the same order of magnitude as from waste incineration (e.g. in The Netherlands, Italy, France, Denmark) or much larger (in the UK, Spain, West Germany). This implies that under favourable conditions, with copper and iron present in the coal mineral matter, the contribution of PCDD and PCDF formation can be at least as large as that of waste incineration. For the UK a risk assessment arrived at the same conclusion2. Burning of 1 t of coal of 5 wt% ash with an efficiency of 99% would yield 3&40mg of PCDD/PCDFs under the conditions of this study, corresponding to ppm levels in the fly ash. Since these are extreme conditions, these values must be considered as upper levels. Nevertheless, these values are high enough for further research in this area to be quite important and urgent. CONCLUSIONS The copper-catalysed combustion of activated carbon in the presence of HCl at temperatures between 200-400”C gives rise to the formation of PCDDs and PCDFs. The catalysed burnoff of the polyaromatic structure of carbon is locally stabilized when CC1 bonds are present; hence chlorinated aromatic fragments are isolated. Under the conditions used, copper is the active catalyst in the combustion and the partial chlorination of carbon and therefore also in the formation of PCDDs and PCDFs. The presence of acidic silicaaalumina gives rise to further chlorination reactions and influences the PCDD/PCDF ratio.
The formation
of PCDDs
and PCDFs
REFERENCES
IO II 12
in the catalysed 13
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24
Swaine. D. J. ‘Trace Elements in Coal’, Butterworths, London, 1990 Van Egmond. N. D. ‘Air pollution as a result of the emission from coal-fired power stations’. Project no. 20.70-004.10. PEO Management Office for Energy Research. Utrecht, 1986 Luijk. R.. Jansen, J. and Govers, H. A. J. ‘The exchange of bromine and chlorine in 2.3,7.8-tetrabromodibenzo-p-dioxin’ (in preparation) Wilken, M., Beyer, A. and Jager, J. In Short Papers. 10th Int Meeting, DIOXIN90, 1990. 2, pp. 377-381
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