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The Emissions of VOCs during Co-Combustion of Coal with Different Waste Materials in a Fluidized Bed I. Gulyurtlu,*,† P. Abelha,† A. Grego´rio,† A. Garcı´a-Garcı´a,‡ D. Boavida,† A. Crujeira,† and I. Cabrita† DEECA-INETI, Edificio J, Estrada do Pac¸ o do Lumiar, 1649-038 Lisbon, Portugal, and Department of Inorganic Chemistry, University of Alicante, Ap 99-E-03080 Alicante, Spain Received June 3, 2003. Revised Manuscript Received November 21, 2003
The combustion of different fuels gives rise to the formation of small but appreciable amounts of volatile organic compounds (VOCs). They basically result from incomplete combustion and their emissions have negative repercussions on health and on the environment in general. As their measurement is difficult, costly, and very time-consuming, very little is reported on the emissions of VOCs from combustion installations. In this study, various blends of two different coals with several wastes were burned in a pilot-scale fluidized bed combustor and measurements of VOCs at several locations along the combustor height as well as just before the stack were carried out. The results demonstrate that the parameters important for the formation of VOCs are temperature, excess air levels, and the effectiveness of the mixing of air with fuel. Furthermore, it was observed that coal was the principal source of VOCs, but the combustion of volatiles from fuels such as biomass, occurring in the freeboard, was important in reducing the emissions of VOCs to almost zero.
Introduction The combustion of solid fuels generally gives rise to the formation of small but significant amounts of volatile organic compounds (VOCs) in combustion equipment. Until recently, it has been difficult and expensive to carry out the quantitative determination of VOCs in chimneys. The new developments in analytical methods and the growing concern to protect the environment, particularly from those compounds emitted to the atmosphere that are toxic and hazardous to human health as is the case with VOCs, have made possible the determination of the concentrations of VOCs released faster and relatively cheaper. Consequently, more attention is currently given to monitoring the emissions of VOCs and recent EU legislations already limit their emissions from combustion systems. The principal reason for the formation of VOCs is the incomplete combustion, and consequently the amount of VOCs formed is dependent on the nature of the fuel used, the design of the combustion equipment, and its operating conditions.1-3 The reasons for incomplete combustion are related to both reaction chemistry and the efficiency in mixing fuel with air. Temperature is the principal parameter in controlling reaction chemistry, and higher temperatures generally result in faster reaction rates and lower emissions of VOCs. Turbulence is what controls the rapid mixing of air with fuel, thus * Corresponding author: Tel.: +351 21 7165141. Fax: +351 21 7166965. E-mail:
[email protected]. † DEECA-INETI. ‡ University of Alicante. (1) Masclet, P.; Bresson, M.; Mouvier, G. Fuel 1987, 66, 556. (2) Schmidt, C.; Brown, T. Combustion modeling, scaling and air toxins. FACT- Vol. 18. New York, ASME, 1994; p 137. (3) Wild, S.; Jones, K. Environ. Pollut. 1995, 88, 91.
leading to optimized combustion, particularly for the gas-phase reactions. In recent years, in an attempt to reduce dependence on fossil fuels, biomass has attracted attention as a potential substitute. However, experience has shown that the availability of biomass could be a serious obstacle for its extensive use for energy. Biomass has by far been utilized either as a base fuel in fairly small boilers or as a co-fuel in larger, mostly coal-based units. There is a possibility of rendering the use of biomass more viable by blending it with nontoxic waste materials which are economically unattractive for recycling and cost high because of their deposition in landfills. The use of these wastes for energy is promising provided that they combine well with other fuels during the combustion process for energy and have no negative effect on the environment. It is, therefore, imperative that there is a satisfactory synergy between coal, biomass, and wastes so that the impact of multi-fuel cofiring could bring about the least negative of each fuel when used separately. This is particularly true for VOCs because the nature of fuel, as referred to above, is an important factor in emissions of VOCs, particularly with respect to their type. The data previously reported are limited and are mostly from coal combustion systems.4 These results clearly suggest that the emissions of VOCs decrease substantially as the equipment size increases and the type of firing takes place at higher temperatures. The size of the combustor is important because it provides greater volume for mixing, thus leading to the reduction in the emissions of VOCs. In general, pulverized fuel firing occurs at temperatures above 1273 (4) Chaggar, H.; Jones, J.; Pourkashanian, M.; Williams; A., Owen, A.; Fynes, G. Fuel 1999, 78, 1527.
10.1021/ef0340155 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/30/2004
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K and this gives rise to significant reduction. However, small-scale grate firing generally results in higher amounts of VOCs formed because the combustor volume is small and the temperature is relatively lower, being in the range of 773-973 K. A very comprehensive review of ongoing work is given in an IEA publication;5 however, very little information is available on VOCs from fluidized bed systems burning coal or other fuels or mixes of fuels. Biomass and waste are characterized as having high volatile matter, considerable moisture, very variable ash content, and relatively low in heating value. The presence of moisture could lead to a decrease in the combustion temperature, which is not so desirable for controlling the formation of VOCs. The combustion of these materials is also characterized by the presence of significant burning in the gas phase because of the high volatile content. Hence, it is important that the mixing of volatiles with air has to be well optimized to minimize incomplete combustion. There is almost no information available of emissions of VOCs during the combustion of waste materials. The authors have recently submitted a paper6 to present some new data on the amounts of VOCs determined during mono-combustion of coal and waste materials in a fluidized bed. In this work, it was observed that the nature of fuel did not have too much influence on the emissions of VOCs. However, the combustion behavior of waste materials is quite different from that of coal because most of the combustion of waste occurs in the freeboard. Since there is a growing interest in co-combustion of coal with different residues, the presence of significant combustion both in the bed and in the freeboard, the present work reports the results during co-combustion. It appears that, provided the combustion in the freeboard is well controlled, the levels of VOCs are almost nonexistent, suggesting that the combustion of volatiles could thermally destroy VOCs originating from coal, thus leading to almost complete destruction of VOCs. It became clear during this work that considerable amounts of VOCs were released near the entry point of fuel where the fuel went through a fast devolatilization and the fuel/air ratio was below that of stoichiometric. Experimental Section The characterization of the fuels used is given in Tables 1 and 2. In comparison with both coals, the waste fuels have higher volatile matter and relatively low fixed carbon content. Furthermore, with the exception of pine sawdust, the other two contain high ash which led to the fast accumulation of ash particles in the bed. Of the waste fuels used, pine sawdust had very small amounts of N and S with no Cl. Sewage sludge had a very large content of N and quite significant amounts of S and Cl. N and Cl contents of the urban waste were relatively large, but the fuel was low in S. The fluidized bed facility of INETI is square in cross section with each side being 300 mm long and its height is 5000 mm. It is previously described in detail.7 Figure 1 gives a schematic representation of the installation at INETI. (5) Sloss, L.; Smith, I. Organic Compounds from Coal Utilisation, IEACR/63. London, UK, IEA Coal Research Center, 1993; pp 1-69. (6) Gulyurtlu, I.; Abelha, P.; Grego´rio, A.; Garcia-Garcia, A.; Boavida, D.; Cabrita, I. Energy Fuels, submitted. (7) Gulyurtlu, I.; Boavida, D.; Abelha, P.; Miranda M.; Cabrita, I. Proceedings of the Sixth International Conference on Circulating Fluidised Beds, 1999, DECHEMA.
Gulyurtlu et al. Table 1. Proximate and Ultimate Analyses of Samples sewage sludge (SS)
pine sawdust (PS)
Polish coal (PC)
urban waste (UW)
carbocol coal (CC)
moisture ash volatile matter fixed carbona
Proximate (d.b., % wt) 5.5 5.0 10.8 43.1 1.6 14.5 45.2 76.6 26.7 6.2 16.8 48.0
4.4 27.2 60.9 7.5
2.8 25.5 31.7 40.0
C H N Cl S Oa
Ultimate (daf, % wt) 52.0 53.0 81.5 8.0 6.0 5.7 6.0 0.2 1.6 0.5 0.0 0.3 1.2 0.1 0.9 32.3 40.7 10.1
43.8 7.2 0.9 0.7 0.2 18.2
58.8 4.6 1.2 0.04 1.3 8.01
a
By difference; d.b. - dry basis; daf - dry ash free basis. Table 2. Physical Characteristics of Samples density (kg/m3) fuel
particle size (µm)
real
bulk
Polish coal carbocoal sewage sludge pine sawdust urban waste
500-4000 500-4000 500-4500 200-4000 briquettes (W × D) 13.5 mm × 8.4 mm
1361 1268 1178 836 970
820 761 678 235 590
The fuel feed rate is related to the air requirement for several excess air levels and the fuel/air ratio is controlled by software developed by INETI. The total air required is divided between fluidizing air and that of secondary air supplied to the freeboard and the software regulates their amounts, which are pre-set. There are ports in the freeboard for the introduction of secondary air in stages. The temperatures in the bed and along the freeboard and that of the flue gases leaving the reactor are continuously monitored. The experimental conditions used to carry out the combustion tests are given in Table 3. There are gas sampling probes situated at various heights of the reactor to carry out instant measurements of O2, CO, CO2, NOx, N2O, SO2, and VOCs. The gases leaving the combustor go through two cyclones in series. The VOCs were sampled through a cooled probe and directly analyzed with a specific AUTOFIM II analyzer fitted with a FID detector, which returns the total VOCs concentration value as CH4 equivalent in the wet gases after the correction accounting for the dilution factor. The AUTOFIM II analyzer has a hand probe which is inserted directly in the sampling point situated at various heights of the reactor. Since the analyzer needs at least 19% oxygen in the samples gas, a dilution ratio of 10:1 was required. This dilution was accomplished by a Range Extender accessories and relies on an adjustable restricts that sets the pressure drop across the ambient air filter assembly to induce a lower level of sample flow through its own filter assembly. The high degree of dilution dispenses the gas sample drying. The AUTOFIM II incorporates a flame ionization detector and is very sensitive to almost all organic gases and vapors. Measurement at concentrations as low as 0.1 ppm by volume in air is possible. The FID employs a hydrogen flame, burning in an excess of air and surrounded by an electrostatic field, with provision for the introduction of a sample into the flame. The conductivity of the flame is low in the absence of organic substances due to low concentrations of the electrons and ions produced by the burning hydrogen. A small current is observed to flow in the electrostatic field. However, the conductivity of the flame greatly increases in the presence of organic substances due to increased production of electrons and carboncontaining ions. Then an increase of current proportional to
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Figure 1. Schematic representation of the experimental facility. Table 3. Experimental Conditions during Combustion Tests
fuel 60% Polish coal + 40% pine sawdust 65% Polish coal + 35% sewage sludge 50% carbocoal + 50% sewage sludge 70% carbocoal + 30% urban waste Polish coal carbocoal
bed temperature (K)
feed rate (kg/h)
excess air (%)
sec. air/ prim. air
1113-1123
8.9
37
0.42
1103-1113
8.3
55
0.25
1113-1118
11.2
70
0.28
1073-1173
10.8
55
0.28
1113-1123 1113-1123
7.3 9.1
36 42
0.18 0.24
the rate of sample introduction, and the concentration of the sample is observed. The FID is most sensitive to oxidizable carbon-containing substances. Examples are alkanes, alkenes, alkynes, and aromatic hydrocarbons. There is less sensitivity to oxidized carbon-containing substances. Examples are aldehydes, ketones, and carboxylic acids. There is also less sensitivity to organic substances containing heteroatoms. Examples are halogenated hydrocarbons which contain fluorine, chlorine, and/or bromine atoms; alcohols, ethers,and esters which contain oxygen, sulfur, and/or phosphorus atoms; and amines, amides, and nitriles which contain nitrogen atoms. The FID responds differently to different substances. Every FID is calibrated using methane. Thus, when responding to an unknown substance, the measurement observed is the concentration of the methane equivalent to the concentration of the unknown substance.
Figure 2. Variation in temperature along the combustor height.
Results and Discussion Figure 2 gives the temperatures measured at different heights along the combustor for different fuels used. It is clear from this figure and in accordance with expectations that most of the combustion of waste fuels occurred in the region of the freeboard close to the top of the bed. It was found that from the top of the bed corresponding to a height of 300 mm from the distributor plate to about 650 mm, the temperature increased when mixtures of waste materials were burned with coal. When carbocol (CC), which has a high volatile content, was used alone, a rise in temperature over the same height was also
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Table 4. Results of Devolatilization Tests fuel
devolatilization rate (g/min)
devolatilization temperature range (K)
Carbocol coal Polish coal
0.67 0.46
558-923 598-993
observed. Some segregation in the combustion was observed during co-combustion as most of the volatiles from waste fuels burned in the freeboard and this is very clear from the temperature profiles given in Figure 2. It is clear that the temperature in the first 2000 mm of the combustor was higher than 1073 K to thermally destroy VOCs in almost all cases except for monocombustion of Polish coal and co-firing of the same coal with waste materials. This rapid reduction in VOCs along the height is in fact verified in all tests. The bed temperature was observed to be higher when coal was co-combusted with waste materials in comparison with waste combustion alone because coal was mostly burned in the bed thus maintaining a stable and higher bed temperature. It is important that the bed temperature is kept at as close to 1103 K as possible, as this temperature is preferred because of the need to optimize SO2 reduction with limestone. Higher bed temperatures generally encourage faster devolatilization rates which then give rise to greater amounts of volatiles released. Maintaining the temperature at about 1103 K, the devolatilization was found to occur at a fast rate and to release most of the volatiles. However, a complete release of volatiles is not possible and there is a slower but gradual liberation of volatiles due to the secondary pyrolysis.8 It is possible that this secondary pyrolysis might have released precursors leading to the formation of VOCs along the freeboard height, particularly in those zones where the O2 level was low. A previous study8 showed that, in fluidized beds, up to 20% of volatiles were released from secondary pyrolysis because particle size was relatively big and slowed the escape of volatiles from the interior of particles despite high heating rates. It appears that this is particularly the case with Polish coal, as the use of this coalsalone or mixed with waste fuelssresulted in VOC levels greater than those of carbocol, considering that the same particle size was used for both coals. The rate of devolatilization of the two coals was determined and the values are presented in Table 4. It is clear that the release of volatiles occurred at a slower pace with Polish coal, which could suggest that there was a significant extent of secondary pyrolysis which could explain higher VOC values along the freeboard height. The temperature range of devolatilization for both coals was also determined and the results are given in Table 4. The carbocol was found to start releasing volatiles at a lower temperaturesat about 558 Ksand the devolatilization was complete at about 923 K, while the range for Polish coal was 598-993 K. This is also believed to contribute to slower reduction of VOCs in the case of Polish coal. The type of coal was found to influence the levels of VOCs released as Polish coal (PC) appeared to give rise (8) Gulyurtlu, I.; Boavida, D.; Lockwood, F.; Godoy, S. The ECSC Contract CECA-7220/ED/031, “Understanding of the mechanisms of the combustion of chars and volatiles with regard to their contribution to the levels of NOx and N2O formed in both atmospheric and pressurised fluidised bed coal combustors”, Final Report, 1997.
Figure 3. Results obtained during co-combustion of 60% Polish coal (PC) + 40% pine sawdust (PS).
Figure 4. Results obtained during co-combustion of 65% Polish coal (PC) + 35% sewage sludge.
to higher amounts of VOCs both alone or during co-combustion as shown in Figure 3. Although PC has lower volatile matter in comparison with CC, and considering that the bed temperature was about the same when each coal was burned alone, the yield of VOCs was 50% greater and this behavior was maintained during co-combustion. When pine sawdust (PS) was added to PC, there was a sudden release of volatile matter which helped to increase VOCs formed just above the distributor plate. This was not observed with other waste materials used although they were also high volatile fuels, as clearly demonstrated in Figure 4. This could be due to the fact that the amount of volatiles released from pine sawdust for the same weight composition of blends prepared from coal and wastes was much higher in comparison with other wastes because of the small amount of ash present in PS. It appeared that most of the volatiles were entrained from the bed to the freeboard where the temperature was more than 473 K above the bed due to the combustion of the volatiles. This high temperature combustion in fact reduced VOCs significantly in the case of PC as other cases involving PC produced greater quantities of VOCs. The sewage sludge addition to PC reduced the formation of VOCs in the bed; however, the temperature in the freeboard was found to be lower during co-combustion than burning only PC which then gave rise to higher VOCs. The behavior of carbocol was found to be significantly different and it resulted in much lower VOC emissions, as shown in Figure 5. When burned alone, CC was found to liberate more VOCs than when co-combusted with sewage sludge or urban waste. VOCs were, however, found to be destroyed by the volatiles
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the amount of VOCs decreased from 476 to 191 mg/nm3 when the temperature increased from 1123 to 1173 K. The release of VOCs, following the entry to the combustor was very fast and this was confirmed with high levels of concentrations measured. In the case of PS addition, the VOC amount was more than 4000 mg/ nm3. In this zone, due to the devolatilization, the fuel/ air ratio is high and the combustion is incomplete and this resulted in relatively high VOC amounts. However, with the introduction of secondary air, the combustion in the freeboard was promoted to have high temperature, thus causing rapid depletion of VOCs. There was not observed a clear synergy between different fuels during co-combustion as given in Table 5. The levels of VOCs released during mono-combustion of PS and SS decreased when they were burned with coals which could be due to the dilution effect. However, at higher heights of the freeboard, there was no significant reduction in VOC concentration which was expected due to the combustion of char particles elutriated from the bed which continued burning in the freeboard. It could be that the combustion of these particles reduced the O2 levels, thus preventing the breakdown of VOCs. The only exception was when CC was co-fired with SS and the synergy leading to the decrease in VOC took place. The possible explanation could be that the char particles from CC were more reactive and their combustion locally increased temperature and caused a thermal destruction of VOC. The efficiency of combustion was found to be high, varying between 91% and 98% because ashes from the cyclone were recirculated. However, the unburned content in ashes from the bed was quite low and this suggests that the combustion in the bed was almost complete. Both coals had a tendency to produce very fine char particles which could be due to the attrition and that is why the unburned carbon amounts were highest when they were burned alone. The CO levels were low with the exception of the case in which sewage sludge was co-fired with Polish coal. The origin of this CO in this case was the bed as the carbon loading in the bed was largest with Polish coal which could have originated in the formation of CO. It appears that most of this CO then by-passed the freeboard, possibly due to inefficient mixing. This observation was verified by the authors in other studies and this phenomenon is related with the combustor geometry. Because of insignificant lateral mixing along the freeboard height, the gases leaving the bed do not mix well which then prevents the combustion of CO. This could be avoided with the addition of secondary air to the freeboard with enough turbulence to give rise to mixing that is required for CO destruction. The presence of VOCs is basically correlated with incomplete gas-phase combustion which is the same for CO. The causes of incomplete combustion are then the same for both CO and VOCs, and their combustion is
Figure 5. Results obtained during co-combustion of 50% carbocoal + 50% sewage sludge.
Figure 6. Results obtained during co-combustion of 70% carbocoal (CC) + 30% urban waste (UW) at two different temperatures.
combustion in the freeboard because at the exit of the second cyclone, they almost disappeared. In all runs with CC, the temperature in the freeboard was found to be higher near the top of the bed, thus helping the thermal destruction of VOCs. The importance of the bed temperature was clearly demonstrated in Figure 6 as
Table 5. Summary of the Results of VOCs Emitted emission average (mg CH4/nm3)_8% O2 sampling height
1 PC
2 CC
3 PS
4 SS
5 PC+PS
6 PC+SS
7 CC+SS
8 CC+UW
0.55 m 1.6 m 4.9 m 2nd cyclone exit
652 156 27 36
414 4 1 1
13264 26 12 14
1340 49 43 20
4041 126 84 12
151 77 88 71
209 6 3 1
191 4 1 0
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Table 6. Unburned Carbon Content of Ashes Collected from the Bed and Cyclones and the Combustion Efficiency LOI in ash streams (%) second cyclone
efficiency (%)
0.7
30.9
98.2
0.8
33.8
96.0
combustible
bed
60% Polish coal + 40% pine sawdust 65% Polish coal + 35% sewage sludge 50% carbocoal + 50% sewage sludge 70% carbocoal + 30% urban waste Polish coal carbocoal
first cyclone
combustion
0.6
21.9
21.0
93.6
0.6
20.6
15.0
93.9
0.7 0.6
45.0
42.3 44.0
96.7 91.9
encouraged by higher temperature and efficient mixing with air. Table 6 gives a summary of the results. In general, the additional observations were made: ‚ The excess air levels caused a reduction in VOCs; however, it is important that the excess air is introduced gradually and in small amounts. If not, it gave rise to local cooling, thus freezing the destruction reactions of VOCs due to the temperature. ‚ The mixing of air in the freeboard with volatiles was found to have great influence on combustion efficiency, thus for the elimination of VOCs through thermal destruction. ‚ There was found to be some relationship between CO, unburned carbon, and VOCs since both increased due to the incomplete combustion. ‚ Coal appeared to be mainly responsible for the emissions of VOCs as waste fuels helped to destroy VOCs.
Conclusions ‚ The parameters that are most important in controlling the emissions of VOCs are temperature and the excess air levels which influence the rate of combustion. ‚ Independent of the nature of fuel, there is a correlation between the levels of CO and VOCs as both give an indication of whether the combustion is complete or not. ‚ The mixing of fuel with air is highly influential in achieving a high degree of complete combustion, thus reducing levels of VOCs. ‚ The nature of fuels did not appear to influence the levels of VOCs released. ‚ For high volatile fuels, there is a considerable combustion in the freeboard and the presence of secondary air is very important in reducing the amounts of VOCs released. ‚ The volatile combustion in the freeboard is found to help to reduce VOCs originating from coal which appears to be the main source of VOCs. Acknowledgment. Part of this work was financed by the European Community through ECSC program and the authors recognize the importance of this support. The grant of Mr. P. Abelha was financed by the Fundac¸ a˜o para a Cieˆncia e a Tecnologia through Praxis XXI program and the authors also recognize this support. EF0340155
