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Assessment of PAH emissions as a function of coal combustion variables in fluidised bed. 2. Air excess percentage
ELSEVIER
Fuel Vol. 77, No. 13, pp. 1513-1516, 1998 © 1998ElsevierScienceLtd. All rightsreserved Printedin GreatBritain 0016-2361/98$19.00+0.00
PII: S0016-2361(98)00069-6
Short Communication Assessment of PAH emissions as a function of coal combustion variables in fluidised bed. 2. Air excess percentage Ana M. Mastral*, Marisol Callen, Ram6n Murillo and Tomas Garcia Instituto de Carboquimica, CSIC, PO Box 589, 50080 Zaragoza, Spain (Received 26 January 7998; revised 3 April 1998) A low rank coal combustion was carried out in a fluidised bed combustion pilot plant at laboratory scale, with the aim of studying the influence of the air excess percentage on the PAH formation and emission. The experiments were performed at 850°C with a constant air total flow of 860 L h-l, varying the coal feeding rate and therefore the air excess percentages. In each experiment, five samples have been collected from two cyclones, bubbling system, nylon filter, and adsorption system. The PAH contained in these five samples have been analysed by FS (fuorescence spectroscopy in mode synchronous) after extraction by sonication with dimethyl-formamide (D/VlF) The analyses show that the total PAH amount keeps a close relationship with the air excess percentage: the higher the air excess, the lower the total PAH amount emitted. © 1998 Elsevier Science Ltd. All rights reserved. (Keywords: coal; fluidised bed combustion; air excess percentages; PAH emissions)
INTRODUCTION Coal constitutes the main source in power stations to produce electrical energy and for many years, the only limitation in these stations has been to control the NOx, SOxa, COx and particulate matter emissions. Atmospheric fluidised bed combustion (AFBC) is generally considered to be an environmentally favourable combustion technology where control of emissions can be integrated into the combustion system. AFBC systems burning coal usually operate with a combustion zone temperature of around 800-900°C. These low temperatures not only prevent thermal NO formation but promote NO-reducing reactions during the combustion process, The temperature range in AFBC is also suitable for addition of sorbent into the bed for sulphur removal by reducing SOz emissions. In any case, optimum process conditions have to be identified whenever opposing effects are observed. For instance, there is a strong effect of temperature on sorbent demand if the combustion temperature in the FBC is outside the optimum range of about 800850°C. However, NO emissions increase as temperature increases, whereas NzO emissions show exactly the opposite trend. As a result of the range of temperatures used in FBC, the emissions of CO are in general higher than from conventional boilers, * Correspondingauthor,
Another important factor is the excess air, which mainly affects the emissions of NO, N20 and CO, together with the economical
regulation on emissions of these cornpounds will be very restrictive. Concerning the PAH nature, one of the ways to abate
aspect of the process. The NOx decreases with decreasing amount of excess air. A decrease in excess air will, however, increase the CO emission and result in decreased combustion efficiency. Both operational factors, bed temperature and
their emissions would be to control the previously different variables that affect combustion. Therefore, it would be necessary to study the PAH formation in relation to combustion process conditions12-18 with the aim of minimising their formation.
excess air, and others as primary air ratio or sorbent feed rate, have been extensively studied and reported 2, but only for the pollutants cited earlier, In current research, the efforts are aimed toward technical and economic improve-
Some of these variables regarding the nature of coal 15 are difficult to modify but others, as temperature combustion and air excess percentages, can be controlled and studied avoiding higher emissions16-18. In this paper, the coal FBC is carried out
ments seeking new ways to get cheaper and cleaner coal-conversion processes. In this way, the work conditions and the possible modifications carried out have been focused on the use of different mechanisms as washed and desulphurization of coal, addition of CaSO4 3 to retain and to avoid
in different air excess percentages and the corresponding PAH emissions are studied as a function of this combustion variable.
high emissions of sulphur compounds, all of them to abate atmospheric pollution, In reported work, the nature of the Volatile Organic Compounds (VOC) 4 has not been taken into account. From these VOC, the PAC, and within these, the
A low rank coal from NE Spain (SAMCA) has been burned (0.5-1 mm particle size) in a fluidised bed combustion (6.7 cm id) plant t6 with sand (1 mm) as the fluidifying agent. The coal ultimate and proximate analysis are: C (%daf): 73.8; N (%daf): 0.9;
PAH5-s, constitute some of the most dangerous compounds because of the possibility of their interacting with biological nucleophiles inhibiting their regular functions. Nowadays, PAH emissions are reaching a growing interest in the concern about the emissions of VOC 9-11 because it is expected that new legislation about the
S (%db): 6.3; H (%daf): 6.4; ash (%ar): 23.9; volatiles (%ar): 15.0; and moisture (%ar): 15.7. The combustion experiments were performed at fixed temperature (850°C), at fixed flow (860 L h - ' ) and different percentages of air excess: 5, 10, 20 and 40%. In all cases, the sampling time was for
EXPERIMENTAL
Fuel 1998 Volume 77 Number 13
1513
PAH emissions as a function of coal combustion: A. M. Mastral et al.
2 h, when the combustion was in optimal operation conditions, in regime, with the aim of avoiding the starting times, which increase PAH emissions. From each experiment, five samples from the two cyclones (first and second cyclone), bubbling system, nylon filter (20/~m) and XAD-2 resin, were extracted by sonication for 30 rain. Through the bubbling system, composed of a DMF solution, the air flow was forced to pass with the object of retaining possible PAH and to condense the water generated during this process. The samples were analysed by spectroscopy of fluorescence in synchronous mode (FS) following the analytical conditions reported in previous works 16-18 To avoid the possibility of quenching, the solutions were diluted. With regard to the analytical technique used, it is worth remarking that FS is an easy, rapid and non-destructive method which allows, once the work conditions for each compound have been determined, determination of the quali and quantitative analyses of every PAH in complex samples. Preparation work, such as fractionation and clean-up procedures are not necessary with this technique.
Influence of efficiency combustion on PAH emissions Two aspects must be considered with regard to the coal combustion and PAH emissions relationship. On the one hand, the incomplete coal combustion caused by bad processing, with the emissions of unburned fragments, mainly aromatics from coal structure; and on the other hand, the pyrolytic process joined to any combustion process. The PAH emissions resulting from incomplete combustion could be controlled by optimising the combustion. To study this factor, the combustion efficiency has to be calculated in each experiment. Table 1 shows those that reached the discussed conditions, Table 1 shows the close relationship between efficiency values and the air excess percentages. As it could be expected, when the percentages of air excess increase, the efficiencies also increase, giving a more complete combustion. The material burned will have a lower contribution on PAH total amount emitted when the experiments are carded out in optimal conditions. As efficiencies reached are very close to 100%, an incomplete combustion is not a factor to be taken into account on PAH emissions. Therefore, the results obtained with such high efficiencies are going to provide information about the Table 1 Combustion efficiencies reached as a function of the air excess (SAMCA coal, FBC, 850°C, 860 1 h -1)
1514
5 98.7
10 99.3
20 99.5
% Air excess 5% 10% l'tcyclone 121.5 30.8 2 ndcyclone 39.7 12.6 Bubbling system 33.8 20.3 Nylon filter 18.0 25.8 XAD-2 32.8 14.9 Total (/xgkg-1) a 245.9 104.4 aConcentrations for the 11 PAIl listed and analysed by FS
40 99.6
20% 4.0 1.9 1.3 6.2 5.7 19.1
40% 3.1 0.9 54.1 0.0 3.7 61.8
Table 3 Individual PAIl (#g kg -1) total amount emitted from coal combustion in a FBC at 850°C, 8601 h -1 and different percentages of air excess % Air excess 5% 10% 20% 40% Fluorene 43.2 20.9 11.8 2.5 Benzo[a]pyrene 3.4 0.5 n.d. 1.3 Pyrene 23.2 4.7 1.3 9.7 Chrysene 38.1 8.2 n.d. 0.6 Anthracene 1.9 0.7 n.d. 0.1 Acenaphthene 36.1 14.7 n.d. 25.5 Benzo[a]anthracene 16.2 6.7 n.d. 2.7 Dibenzo[a,h]anthracene 1.5 2.6 n.d. n.d. Coronene 81.8 45.2 5.1 19.3 Perylene 0.5 0.1 0.9 n.d. Benzo[k]fluoranthene n.d. n.d. n.d. n.d. Total (/zg kg -1) 245.9 104.4 19.1 61.8
importance of the radical mechanisms involved in the pyrolytic process,
RESULTS AND DISCUSSION
% Air excess % Efficiency
Table 2 PAH (/~g kg -1) trapped, as a function of each trap, in coal FBC at 850°C, 860 1 h -1 and different percentages of air excess
Influence of the combustion variables on PAH emissions The PAH analysed in this work are acenaphthene, fluorene, anthracene, pyrene, benz[a]anthracene, coronene, chrysene, benzo[a]pyrene, dibenz[a,h]anthracene, benzo[k]fluoranthene and perylene. The rest of the PAH from the USEPA list, indenopyrene, fluoranthene, benzo[b]fluoranthene, naphthalene, acenaphthylene, phenanthrene and benzoperylene were not suitable for quantification by FS because of their low fluorescence, in some cases, and the lack of identification in others. Coronene and perylene were also quantified, in spite of not being listed, because they were considered interesting for mechanistic studies because of their high chemical stability, although they do not show carcinogenic activity. Each of these PAH was quantified and the total PAH amount collected in each trap is shown in Table 2. The distributionofthetotalPAHamount emitted in each experiment (Table 2), is observed to be higher with the lowest percentage of air excess, showing a minimum emission when coal is burnt with 20% of air excess. Great air excess generates lower PAH emissions, From the PAH amounts collected in each trap, it was not possible to guess a determined tendency on PAH distribution as a function of each trap. This random distribution has already been observed in previous works 17 when other combustion variables were studied. However, at the lowest percentages of air excess, most of the total amount of PAH emitted is supported on the particulate matter trapped in first cyclone. When the percentage of air
Fuel 1998 Volume 77 Number 13
excess increases, in general, the first cyclone amount decreases. The contribution of each individual PAH to the total amount collected in each trap has been studied. Again, no correlation was found between the molecular volume of each PAH and the correlative disposition of the traps. That is to say, for instance, coronene, has been detected simultaneously in the first (first cyclone) and in the last trap (resin). The individual PAH amounts as a function of the percentages of air excess are shown in Table 3. One of the most interesting compounds to study is coronene. In most of the samples, coronene is the majority component, especially for those obtained at low air excess. This fact could indicate that the aromatic radicals formed in the first stages of combustion tend to stabilise by forming species as coronene by pyrosynthesis, and this mechanism seems to be favoured by low velocities in the post-combustion area. In a first approach, the formation of coronene is an advantage in terms of pollution control. Coronene stability is related to its inertness, so these results seem to indicate that the way to reduce hazardous PAH emissions could be to increase residence time of radicals, but in the presence of a bulky oxygen excess, to promote the formation of more stable PAH. One of the main reasons previously mentioned to study PAH emissions is the carcinogenic power of some of these compounds. Inside this group, benzo[a]pyrene and dibenzo[a,h]anthracene, are of the greatest interest because they are the most carcinogenic compounds. The results obtained show low values and a minimum contribution of these two compounds to the total PAH emissions from coal combustion.
PAH emissions as a function o f coal combustion: A. M. Mastral et al. 170 160
150 140
130
Solid phase
120 II0 I00 90 80
PAH7o
6o so 4o 3o 2o 1o
~
o 5%
~
l
G
a
phase s
i
i
i
i
~
i
10%
15%
20%
25%
30%
35%
40%
% air excess Figure I PAH (/zg kg -I) distributions between solid and gas phase as a function of the air excess percentages in coal FBC 850°C, 860 1h -1, SAMCA coal)
m4 rings [ D5 rings E]6-7 ring~
0,0-
" .......
5%
'''''"
......
10%
"
' ' " ' "
-
20%
":':"
":+:':':':':"
40%
% air excess Figure 2
PAH (/xg kg-1) distribution by number of rings as a function of the air excess percentage from coal combustion in a FBC at 850°C,
8601 h -1 In contrast, because of carcinogenic power, it should be interesting to know the PAH distribution once emitted to the atmosphere. While those emitted on particulate matter and trapped on cyclones are not released to the atmosphere, the PAH emitted in the gas phase would be released to the atmosphere affecting large areas, These last compounds could be transported
long distances undergoing photochemical reactions which would turn them into even more hazardous pollutants. Therefore, the distribution between solid and gas phases has been studied assuming that those PAH in solid state or supported on the particulate matter were trapped by the first and second cyclone and those collected in the bubbling system, nylon filter and adsorbent were
included in the gas phase. Moreover, those PAH trapped on the cyclones, in commercial plants are recycled to the bed in order to improve combustion efficiency. In this work, the results on PAH distribution into solid/gas phase is shown in Figure 1. From Figure 1, it is deduced that in the experiments carried out with a 5% air excess, the PAH amount detected is higher
Fuel 1998 V o l u m e 77 N u m b e r
13
1515
PAH emissions as a function of coal combustion: A. M. Mastral et al.
in the solid phase than in gas phase. When this excess is increased to 10, 20 or 40%, the results change, increasing the gas phase and the ratio between PAH gas phase-PAH solid phase. The highest the air excess percentage, the higher the amount of PAH formed in the gas phase. That is to say, the lowest percentages of air excess could favour the highest PAH deposition on particulate matter, while the highest percentages of air excess could favour the sweep of PAH to the gas phase, Under the true conditions in which a power plant works, according to this data distribution, it could be extrapolated that, at the lowest percentages of air excess, the PAH would mostly affect the surroundings of the power plant, by deposition of the particulate matter on the field. At the highest percentages of air excess, the contamination would affect non-specific larger areas. In addition, as a result of the different PAH nature with respect to stability and volatility, the distribution behaviour of PAH by rings number, as a function of the air excess percentages, has been also plotted and is shown in Figure 2. Figure 2 shows that, in all the cases, independently of the oxygen excess, the PAH with 3 and 6-7 rings in their molecules are the most abundant. PAH with 5 rings are always in the minority and their relative variation with the air excess is minimum. In contrast, when the percentages of air excess increase, the more the volatile compounds (3 rings) contribute in higher proportions to the total PAH. So, the highest percentages of air excess seem to minimise possible interactions between PAH, making their inter-conversion difficult in association with other condensation reactions, resulting in the lowest hydrocarbons emissions. Perhaps this is a result of the formation of substituted PAH, PAC and other oxygenated compounds, as a consequence of the interaction between oxygen atoms and radicals. In this way, at
1516
the lowest percentages of air excess, this interaction between oxygen and radicals should be less favoured, and as result, the PAH amount emitted would be higher. Summarising, from the two mechanisms implied in PAH formation and emission from coal FBC (an incomplete combustion and the release of radicals from the previous pyrolysis), once the coal combustion efficiency is optimised, the pyrosynthetic process has a very important influence. From this second possibility, the reactions between the released radicals can be retrogressive reactions, by interaction between themselves, or radical elimination reactions by oxidation. The radicals disappearance by oxidation breakdown, according to the data shown, is favoured when the coal combustion is carded out at high percentages of air excess keeping the total air flow constant. With regard to the PAH distribution by ring number, the low percentages of air excess (5, 10%) produce higher amounts of more stable compounds which decrease with the increment of percentage excess air. It is these cases when the more volatile PAH seem to have a major relative contribution to the total PAH emission. ACKNOWLEDGEMENTS Authors would like to thank the partiai financial support of this work to ECSC, DG XVII, Contract 7220/ED/089.
5 6
7
tion Systems and its Toxic Properties, ed. J. Lahaye and G. Prado. 8
9
10 11 12 13
14
emissions control for coal-fired plants. IEACR/47, IEA Coal 3 4
Fuel 1998 Volume 77 Number 13
Research, London, March 1992. Khan, W. Z. and Gibbs, B. M., Fuel, 1995, 74(6), 800. Chagger, H. K., Jones, J. M., Pourkashanian, M. and Williams, A., Fuel, 1997, 76(9), 861.
Dennis. Battelle Press, Columbus, Ohio, 1980. Williams, P. T. and Nazzal, J. M.,
Journal of Analytical and Applied Pyrolysis, 1995, 35, 181. Cunningham, D., Journal of Cleaner Production, 1995, 3(4), 225. Barrefors, G. and Peterson, G., Chemosphere, 1995, 30, 1551. Jay, K. and Stieglitz, G., Chemosphere, 1995, 30, 1249. Clarke, L. B. and Sloss, L. L., Trace Elements--Emission from Coal Combustion and Gasification. IEACR/49, IEA Coal Research, London, 1992. Bonfanti, L., de Michele, G., Riccardi, J. and L6pez-Doriga, E., Corn-
bustion Science and Technology,
REFERENCES Nordin, A., Fuel, 1995, 74(4), 615. Hjalmarsson, K. A., Interactions in
Plenum Press, New York, 1983. Barfnedit, T. R., Andon, B. M., Thilly, W. G. and Hites, R. A., in
Chemical Analysis and Biological Fate: Polycyclic Aromatic Hydrocarbons, ed. M. Cooke and A. J.
15
1 2
Williams, P. T., Journal of Institute of Energy, 1990, 63, 22. Lee, M., Novotny, M. and Bartle, K.D., Analytical Chemistry of Polycyclic Aromatic Compounds. Academic Compounds, Academic Press, New York, 1981. Longwell, J. P., in Soot in Combus-
16
17
18
1994, 101, 505. Akers, D. J. and Dospoy, R. L., Fuel Processing Technology, 1994, 39, 73. Mastral, A. M., Call6n, M. S., Mayoral, M. C. and Galb~in, J., Fuel, 1995, 74(12), 1762. Mastral, A. M., Call6n, M. S. and Murillo, R., Fuel, 1996, 75(13), 1533. Mastral, A. M., Galb~in, J., Pardos, C. and Rubio, B., Microanalytical Letters, 1995, 28(10), 1883.
Fuel Vol. 77, No. 13, pp. 1513-1516, 1998 © 1998ElsevierScienceLtd. All rightsreserved Printedin GreatBritain 0016-2361/98$19.00+0.00
PII: S0016-2361(98)00069-6
Short Communication Assessment of PAH emissions as a function of coal combustion variables in fluidised bed. 2. Air excess percentage Ana M. Mastral*, Marisol Callen, Ram6n Murillo and Tomas Garcia Instituto de Carboquimica, CSIC, PO Box 589, 50080 Zaragoza, Spain (Received 26 January 7998; revised 3 April 1998) A low rank coal combustion was carried out in a fluidised bed combustion pilot plant at laboratory scale, with the aim of studying the influence of the air excess percentage on the PAH formation and emission. The experiments were performed at 850°C with a constant air total flow of 860 L h-l, varying the coal feeding rate and therefore the air excess percentages. In each experiment, five samples have been collected from two cyclones, bubbling system, nylon filter, and adsorption system. The PAH contained in these five samples have been analysed by FS (fuorescence spectroscopy in mode synchronous) after extraction by sonication with dimethyl-formamide (D/VlF) The analyses show that the total PAH amount keeps a close relationship with the air excess percentage: the higher the air excess, the lower the total PAH amount emitted. © 1998 Elsevier Science Ltd. All rights reserved. (Keywords: coal; fluidised bed combustion; air excess percentages; PAH emissions)
INTRODUCTION Coal constitutes the main source in power stations to produce electrical energy and for many years, the only limitation in these stations has been to control the NOx, SOxa, COx and particulate matter emissions. Atmospheric fluidised bed combustion (AFBC) is generally considered to be an environmentally favourable combustion technology where control of emissions can be integrated into the combustion system. AFBC systems burning coal usually operate with a combustion zone temperature of around 800-900°C. These low temperatures not only prevent thermal NO formation but promote NO-reducing reactions during the combustion process, The temperature range in AFBC is also suitable for addition of sorbent into the bed for sulphur removal by reducing SOz emissions. In any case, optimum process conditions have to be identified whenever opposing effects are observed. For instance, there is a strong effect of temperature on sorbent demand if the combustion temperature in the FBC is outside the optimum range of about 800850°C. However, NO emissions increase as temperature increases, whereas NzO emissions show exactly the opposite trend. As a result of the range of temperatures used in FBC, the emissions of CO are in general higher than from conventional boilers, * Correspondingauthor,
Another important factor is the excess air, which mainly affects the emissions of NO, N20 and CO, together with the economical
regulation on emissions of these cornpounds will be very restrictive. Concerning the PAH nature, one of the ways to abate
aspect of the process. The NOx decreases with decreasing amount of excess air. A decrease in excess air will, however, increase the CO emission and result in decreased combustion efficiency. Both operational factors, bed temperature and
their emissions would be to control the previously different variables that affect combustion. Therefore, it would be necessary to study the PAH formation in relation to combustion process conditions12-18 with the aim of minimising their formation.
excess air, and others as primary air ratio or sorbent feed rate, have been extensively studied and reported 2, but only for the pollutants cited earlier, In current research, the efforts are aimed toward technical and economic improve-
Some of these variables regarding the nature of coal 15 are difficult to modify but others, as temperature combustion and air excess percentages, can be controlled and studied avoiding higher emissions16-18. In this paper, the coal FBC is carried out
ments seeking new ways to get cheaper and cleaner coal-conversion processes. In this way, the work conditions and the possible modifications carried out have been focused on the use of different mechanisms as washed and desulphurization of coal, addition of CaSO4 3 to retain and to avoid
in different air excess percentages and the corresponding PAH emissions are studied as a function of this combustion variable.
high emissions of sulphur compounds, all of them to abate atmospheric pollution, In reported work, the nature of the Volatile Organic Compounds (VOC) 4 has not been taken into account. From these VOC, the PAC, and within these, the
A low rank coal from NE Spain (SAMCA) has been burned (0.5-1 mm particle size) in a fluidised bed combustion (6.7 cm id) plant t6 with sand (1 mm) as the fluidifying agent. The coal ultimate and proximate analysis are: C (%daf): 73.8; N (%daf): 0.9;
PAH5-s, constitute some of the most dangerous compounds because of the possibility of their interacting with biological nucleophiles inhibiting their regular functions. Nowadays, PAH emissions are reaching a growing interest in the concern about the emissions of VOC 9-11 because it is expected that new legislation about the
S (%db): 6.3; H (%daf): 6.4; ash (%ar): 23.9; volatiles (%ar): 15.0; and moisture (%ar): 15.7. The combustion experiments were performed at fixed temperature (850°C), at fixed flow (860 L h - ' ) and different percentages of air excess: 5, 10, 20 and 40%. In all cases, the sampling time was for
EXPERIMENTAL
Fuel 1998 Volume 77 Number 13
1513
PAH emissions as a function of coal combustion: A. M. Mastral et al.
2 h, when the combustion was in optimal operation conditions, in regime, with the aim of avoiding the starting times, which increase PAH emissions. From each experiment, five samples from the two cyclones (first and second cyclone), bubbling system, nylon filter (20/~m) and XAD-2 resin, were extracted by sonication for 30 rain. Through the bubbling system, composed of a DMF solution, the air flow was forced to pass with the object of retaining possible PAH and to condense the water generated during this process. The samples were analysed by spectroscopy of fluorescence in synchronous mode (FS) following the analytical conditions reported in previous works 16-18 To avoid the possibility of quenching, the solutions were diluted. With regard to the analytical technique used, it is worth remarking that FS is an easy, rapid and non-destructive method which allows, once the work conditions for each compound have been determined, determination of the quali and quantitative analyses of every PAH in complex samples. Preparation work, such as fractionation and clean-up procedures are not necessary with this technique.
Influence of efficiency combustion on PAH emissions Two aspects must be considered with regard to the coal combustion and PAH emissions relationship. On the one hand, the incomplete coal combustion caused by bad processing, with the emissions of unburned fragments, mainly aromatics from coal structure; and on the other hand, the pyrolytic process joined to any combustion process. The PAH emissions resulting from incomplete combustion could be controlled by optimising the combustion. To study this factor, the combustion efficiency has to be calculated in each experiment. Table 1 shows those that reached the discussed conditions, Table 1 shows the close relationship between efficiency values and the air excess percentages. As it could be expected, when the percentages of air excess increase, the efficiencies also increase, giving a more complete combustion. The material burned will have a lower contribution on PAH total amount emitted when the experiments are carded out in optimal conditions. As efficiencies reached are very close to 100%, an incomplete combustion is not a factor to be taken into account on PAH emissions. Therefore, the results obtained with such high efficiencies are going to provide information about the Table 1 Combustion efficiencies reached as a function of the air excess (SAMCA coal, FBC, 850°C, 860 1 h -1)
1514
5 98.7
10 99.3
20 99.5
% Air excess 5% 10% l'tcyclone 121.5 30.8 2 ndcyclone 39.7 12.6 Bubbling system 33.8 20.3 Nylon filter 18.0 25.8 XAD-2 32.8 14.9 Total (/xgkg-1) a 245.9 104.4 aConcentrations for the 11 PAIl listed and analysed by FS
40 99.6
20% 4.0 1.9 1.3 6.2 5.7 19.1
40% 3.1 0.9 54.1 0.0 3.7 61.8
Table 3 Individual PAIl (#g kg -1) total amount emitted from coal combustion in a FBC at 850°C, 8601 h -1 and different percentages of air excess % Air excess 5% 10% 20% 40% Fluorene 43.2 20.9 11.8 2.5 Benzo[a]pyrene 3.4 0.5 n.d. 1.3 Pyrene 23.2 4.7 1.3 9.7 Chrysene 38.1 8.2 n.d. 0.6 Anthracene 1.9 0.7 n.d. 0.1 Acenaphthene 36.1 14.7 n.d. 25.5 Benzo[a]anthracene 16.2 6.7 n.d. 2.7 Dibenzo[a,h]anthracene 1.5 2.6 n.d. n.d. Coronene 81.8 45.2 5.1 19.3 Perylene 0.5 0.1 0.9 n.d. Benzo[k]fluoranthene n.d. n.d. n.d. n.d. Total (/zg kg -1) 245.9 104.4 19.1 61.8
importance of the radical mechanisms involved in the pyrolytic process,
RESULTS AND DISCUSSION
% Air excess % Efficiency
Table 2 PAH (/~g kg -1) trapped, as a function of each trap, in coal FBC at 850°C, 860 1 h -1 and different percentages of air excess
Influence of the combustion variables on PAH emissions The PAH analysed in this work are acenaphthene, fluorene, anthracene, pyrene, benz[a]anthracene, coronene, chrysene, benzo[a]pyrene, dibenz[a,h]anthracene, benzo[k]fluoranthene and perylene. The rest of the PAH from the USEPA list, indenopyrene, fluoranthene, benzo[b]fluoranthene, naphthalene, acenaphthylene, phenanthrene and benzoperylene were not suitable for quantification by FS because of their low fluorescence, in some cases, and the lack of identification in others. Coronene and perylene were also quantified, in spite of not being listed, because they were considered interesting for mechanistic studies because of their high chemical stability, although they do not show carcinogenic activity. Each of these PAH was quantified and the total PAH amount collected in each trap is shown in Table 2. The distributionofthetotalPAHamount emitted in each experiment (Table 2), is observed to be higher with the lowest percentage of air excess, showing a minimum emission when coal is burnt with 20% of air excess. Great air excess generates lower PAH emissions, From the PAH amounts collected in each trap, it was not possible to guess a determined tendency on PAH distribution as a function of each trap. This random distribution has already been observed in previous works 17 when other combustion variables were studied. However, at the lowest percentages of air excess, most of the total amount of PAH emitted is supported on the particulate matter trapped in first cyclone. When the percentage of air
Fuel 1998 Volume 77 Number 13
excess increases, in general, the first cyclone amount decreases. The contribution of each individual PAH to the total amount collected in each trap has been studied. Again, no correlation was found between the molecular volume of each PAH and the correlative disposition of the traps. That is to say, for instance, coronene, has been detected simultaneously in the first (first cyclone) and in the last trap (resin). The individual PAH amounts as a function of the percentages of air excess are shown in Table 3. One of the most interesting compounds to study is coronene. In most of the samples, coronene is the majority component, especially for those obtained at low air excess. This fact could indicate that the aromatic radicals formed in the first stages of combustion tend to stabilise by forming species as coronene by pyrosynthesis, and this mechanism seems to be favoured by low velocities in the post-combustion area. In a first approach, the formation of coronene is an advantage in terms of pollution control. Coronene stability is related to its inertness, so these results seem to indicate that the way to reduce hazardous PAH emissions could be to increase residence time of radicals, but in the presence of a bulky oxygen excess, to promote the formation of more stable PAH. One of the main reasons previously mentioned to study PAH emissions is the carcinogenic power of some of these compounds. Inside this group, benzo[a]pyrene and dibenzo[a,h]anthracene, are of the greatest interest because they are the most carcinogenic compounds. The results obtained show low values and a minimum contribution of these two compounds to the total PAH emissions from coal combustion.
PAH emissions as a function o f coal combustion: A. M. Mastral et al. 170 160
150 140
130
Solid phase
120 II0 I00 90 80
PAH7o
6o so 4o 3o 2o 1o
~
o 5%
~
l
G
a
phase s
i
i
i
i
~
i
10%
15%
20%
25%
30%
35%
40%
% air excess Figure I PAH (/zg kg -I) distributions between solid and gas phase as a function of the air excess percentages in coal FBC 850°C, 860 1h -1, SAMCA coal)
m4 rings [ D5 rings E]6-7 ring~
0,0-
" .......
5%
'''''"
......
10%
"
' ' " ' "
-
20%
":':"
":+:':':':':"
40%
% air excess Figure 2
PAH (/xg kg-1) distribution by number of rings as a function of the air excess percentage from coal combustion in a FBC at 850°C,
8601 h -1 In contrast, because of carcinogenic power, it should be interesting to know the PAH distribution once emitted to the atmosphere. While those emitted on particulate matter and trapped on cyclones are not released to the atmosphere, the PAH emitted in the gas phase would be released to the atmosphere affecting large areas, These last compounds could be transported
long distances undergoing photochemical reactions which would turn them into even more hazardous pollutants. Therefore, the distribution between solid and gas phases has been studied assuming that those PAH in solid state or supported on the particulate matter were trapped by the first and second cyclone and those collected in the bubbling system, nylon filter and adsorbent were
included in the gas phase. Moreover, those PAH trapped on the cyclones, in commercial plants are recycled to the bed in order to improve combustion efficiency. In this work, the results on PAH distribution into solid/gas phase is shown in Figure 1. From Figure 1, it is deduced that in the experiments carried out with a 5% air excess, the PAH amount detected is higher
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PAH emissions as a function of coal combustion: A. M. Mastral et al.
in the solid phase than in gas phase. When this excess is increased to 10, 20 or 40%, the results change, increasing the gas phase and the ratio between PAH gas phase-PAH solid phase. The highest the air excess percentage, the higher the amount of PAH formed in the gas phase. That is to say, the lowest percentages of air excess could favour the highest PAH deposition on particulate matter, while the highest percentages of air excess could favour the sweep of PAH to the gas phase, Under the true conditions in which a power plant works, according to this data distribution, it could be extrapolated that, at the lowest percentages of air excess, the PAH would mostly affect the surroundings of the power plant, by deposition of the particulate matter on the field. At the highest percentages of air excess, the contamination would affect non-specific larger areas. In addition, as a result of the different PAH nature with respect to stability and volatility, the distribution behaviour of PAH by rings number, as a function of the air excess percentages, has been also plotted and is shown in Figure 2. Figure 2 shows that, in all the cases, independently of the oxygen excess, the PAH with 3 and 6-7 rings in their molecules are the most abundant. PAH with 5 rings are always in the minority and their relative variation with the air excess is minimum. In contrast, when the percentages of air excess increase, the more the volatile compounds (3 rings) contribute in higher proportions to the total PAH. So, the highest percentages of air excess seem to minimise possible interactions between PAH, making their inter-conversion difficult in association with other condensation reactions, resulting in the lowest hydrocarbons emissions. Perhaps this is a result of the formation of substituted PAH, PAC and other oxygenated compounds, as a consequence of the interaction between oxygen atoms and radicals. In this way, at
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the lowest percentages of air excess, this interaction between oxygen and radicals should be less favoured, and as result, the PAH amount emitted would be higher. Summarising, from the two mechanisms implied in PAH formation and emission from coal FBC (an incomplete combustion and the release of radicals from the previous pyrolysis), once the coal combustion efficiency is optimised, the pyrosynthetic process has a very important influence. From this second possibility, the reactions between the released radicals can be retrogressive reactions, by interaction between themselves, or radical elimination reactions by oxidation. The radicals disappearance by oxidation breakdown, according to the data shown, is favoured when the coal combustion is carded out at high percentages of air excess keeping the total air flow constant. With regard to the PAH distribution by ring number, the low percentages of air excess (5, 10%) produce higher amounts of more stable compounds which decrease with the increment of percentage excess air. It is these cases when the more volatile PAH seem to have a major relative contribution to the total PAH emission. ACKNOWLEDGEMENTS Authors would like to thank the partiai financial support of this work to ECSC, DG XVII, Contract 7220/ED/089.
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