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A field study on the trace metal behaviour in atmospheric circulating fluidized-bed coal combustion
Twenty-Fifth Symposium (International) on Combustion/The Combustion Institute, 1994/pp. 201-209
A F I E L D STUDY O N T H E TRACE METAL BEHAVIOUR IN ATMOSPHERIC CIRCULATING F L U I D I Z E D - B E D COAL C O M B U S T I O N TERTFALIISA LIND, ESKO I. KAUPPINEN ANDJORMA K. ]OKINIEMI VTT Aerosol Technology Group P.O. Box 1401, FIN-02044 VTT, Finland AND
WILLY MAENHAUT University of Gent, Institute for Nuclear Sciences Proeftuinstraat 86, B-9000 Gent, Belgium
Trace element behaviour in atmospheric circulating fluidized-bed combustion (CFBC) of Venezuelan bituminous coal was studied by determining particle size distributions in the CFBC flue gas. The size distributions of calcium, iron, aluminium, and 21 trace elements, Sc, V, Cr, Mn, Co, Ni, Zn, Ga, As, Se, Sr, Cd, Sb, Cs, Ba, La, Ce, Sm, Lu, Pb, and Th, in the size range 0.01-70/zm, were determined by collecting aerosols with a low-pressure impactor-cyclone sampling train from the flue gases of an 80MW(th) CFBC boiler upstream of the electrostatic precipitator. The collected samples were analyzed gravimetrically and with instrumental neutron activation analysis (INAA), particle-induced x-ray emission analysis (PIXE), and inductively coupled plasma mass spectrometry (ICP-MS). The number size distributions of the aerosols were determined with a differential electrical mobility method in the size range 0.01-0.8/tin. In the ultrafine particle mode, i.e., D1, < 0.1/*rn, the CFBC number concentrations varied strongly during the experiments, being one to two orders of magnitude lower than those observed in pulverized coal combustion. For all of the elements studied, 75% or more were found in particles larger than 5 pm. None of the studied elements showed significant vaporization and subsequent chemical surface reaction or condensation in the CFBC. The Sr, Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th size distributions resembled those of aluminium, suggesting their occurrence in aluminosilicate-rieh particles in the fly ash. The association of the trace elements with aluminium in the fly ash particles may result from reactions of the trace elements with the aluminosilicate mineral particles inside the burning coal particles, or their initial occurrence'in association with these minerals.
Introduction Coal combustion is a major source of toxic species emitted to the atmosphere. During combustion, trace elements contained in the coal are transformed and either retained in the boiler bottom ash, captured by the gas cleaning devices, or released to the atmosphere as particles or gases. The total emission to the atmosphere depends on the transformations of the noncombustible matter in the coal during combustion, and on the gas cleaning equipment employed at the combustion unit. The combustion behaviour of noncombustible matter in coal, including trace elements, is determined by their occurrence in the coal, the size and temperature history of the coal particle, the atmosphere the coal particle is exposed to, and the flow conditions in the combustion unit. The form of occurrence in the coal is important when connecting the combustion behaviour with the coal characteristics. Many studies on the occurrence of trace ele201
ments in the coal have revealed that they are found in various forms, as reviewed by Finkelman [1] and Swaine [2]. The trace elements are found in association with, e.g., pyrite and accessory sulphides, carbonates, clays, selenides, and organic matter. For many trace elements, the occurrence can only be determined speculatively, as most studies are based on indirect measurement, e.g., leaching behaviour or density determination (float and sink method) [2]. In a recent study, x-ray absorption fine structure spectroscopy (XAFS) method was used to study the occurrence of arsenic and chromium in several U.S. coals directly [3]. In many field and laboratory studies on pulverized coal combustion (PCC), the trace elements have been found to be enriched in the submicron particles [4-8]. The enrichment is believed to result from the vaporization of these species during the combustion, followed by condensation of the vaporized species on the surfaces of the pre-existing particles entrained in the flue gas [9]. During the pulverized coal combus-
202
PRACTICAL ASPECTS OF COMBUSTION
tion, high number concentrations of fine particles are formed when the vaporized fraction of the refractory oxides (e.g., Fe, Mg, Si, A1, Ca) condenses homogeneously prior to the condensation of the trace metals. The condensation of the trace metals is proportional to the surface area of the particles, and hence occurs predominantly on the fine particle surfaces, resulting in their enrichment with condensed species [10]. Also, chemical reactions of trace element vapours at the fly ash particle surfaces would result in their enrichment in submicron ash particles [11]. This is particularly harmful to the environment, as particles in the size range 0.1-1.0/2m have been found to penetrate readily particle removal equipment [12]. Also, some of the condensed species on the emitted fly ash particles may be easily leached in dilute acids, enabling the transport of the trace elements into the surface water, soil, and finally into the groundwater. The trace elements that are not vaporized and thus retained in the residual ash particles during combustion are usually not easily removed from the ash particles by leaching in dilute acids. While in the ambient air, however, the fly ash particles may also be exposed to concentrated acids. This may occur either within accumulation mode ambient aerosol particles (0.1-1/2m in diameter, only submicron fly ash particles), within fog droplets (all emitted ash particles smaller than a few microns), or within cloud droplets (all emitted ash particles). As a result, the solubility of trace elements may increase prior to the dry and wet deposition. Several elements have been found to partially vaporize in the pulverized coal combustion studies. Profound laboratory studies conducted by Quann et al. [7] showed large differences in the composition of the submicron particles for the coals with varying characteristics regarding rank and origin. The major components of the submicron fume were SiO2 for bituminous coals, and MgO, CaO, and FeO for lower-rank coals. Enrichment in the fine particles was seen for, e.g., arsenic, antimony, cobalt, chromium, and manganese. Despite the enrichment of the trace elements, the major fraction of the total mass in the fine particles consisted of coal matrix elements. Also, major fractions of the trace elements were found in the supermicron fly ash particles, indicating that only minor fractions were released to the gas phase during combustion. In the field studies, vaporization and condensation behaviour has been observed for, e.g., arsenic, antimony, mercury, zinc, nickel, vanadium, copper, cadmium, strontium, barium, and selenium [4-6,13]. According to our recent studies on the submicron particle size distributions from the real-scale PCC [14], the submicron particles form two distinctive modes. Ultrafine mode was observed in the range 0.03-0.1 Izm, being formed from the vaporized ash matrix species via nucleation. The intermediate mode was found between 0.3 and 0.7/tin, consisting
mainly of coal matrix species. Accordingly, the assumption that all the submicron-sized particles have been formed from the gaseous ash species is not necessarily valid, and this has to be taken into account while interpreting the results of any previous studies on the ash matrix and trace element combustion behaviour. Most of the studies on the trace element behaviour in coal combustion have concentrated on PCC. Fluidized-bed combustion (FBC) differs significantly from PCC. In FBC, the temperature history of the particles is different from that of a PCC because of large coal feed particle size and the dense bed the coal particles are fed to. The combustion times are longer [15], and the peak temperatures and heating rates lower than in the PCC, with gas temperatures of up to 1150 K and particle peak temperatures of up to 1700 K [16,17]. In FBCs, the gas atmosphere encountered by the coal particles varies with the position in the furnace [18]. In the circulating FBC (CFBC), the density of the bed decreases rapidly while going to the upper parts of the furnace, and there is more oxygen available for the combustion. All these parameters are expected to affect the behaviour of ash-forming constituents, including trace elements, during combustion. Several studies have addressed the trace element partitioning in different sized particles in the fluidized bed combustion [e.g., 19,20,21]. However, no detailed data have been published on the submicron particle size distributions and composition. Consequently, the fraction of the trace metals released to the gas phase and later condensed during FBC is not known. In this study, we approached the problem by measuring, for the first time, detailed fly ash size distributions from a real-scale circulating fluidized-bed boiler flue gases upstream of the electrostatic precipitator using advanced aerosol measurement techniques. The particle characteristics, including physical (number and mass) as well as elemental size distributions, were determined in the particle size range 0.01-70/lm. The coal and limestone sorbent were also carefully characterized with regard to their mineral and trace metal contents. Size-classified fly ash particle samples were analyzed with instrumental neutron activation analysis (INAA), particle-induced x-ray emission analysis (PIXE) and inductively coupled plasma mass spectrometry (ICP-MS) methods for up to 50 elements. Detailed size distribution data for trace elements associated with both coal and limestone are compared with size distributions of elements abundant in coal and in limestone. Tile trace element vaporization and condensation as well as possible reactions inside the burning coal particles are discussed on the basis of experimental fly ash size distribution data. We have earlier reported data on the alkali metal behaviour in CFBC [22] and presented the analytical procedures used in INAA and PIXE analyses [26,27].
BEHAVIOURIN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION
Process Description The experiments were carried out at an 80MW(th) steam-generating atmospheric circulating fluidized-bed combustion unit during two one-week periods in March and August 1990, hereby marked as 3/90 and 8/90, respectively [22]. Venezuelan bituminous coal was burned during the experiments, and limestone was used as a sulphur sorbent. In the experimental period 8/90, some English, higher-sulphur coal was accidentally mixed in the coal feed silo at the beginning of the experimental week. The process parameters are described in Ref. 22.
Methods
Aerosol Sampling: The mass and elemental size distributions were determined by collecting size-classified coal combustion aerosol samples with an ll-stage multijet compressible flow Berner-type low-pressure impactor (BLPI) [4,23,24]. Because of high particle concentrations in the CFBC flue gas, double stages were used in the impactor in stages 7, 8, and 9 to prevent overloading of these stages during the latter experimental period, i.e., 8/90. Thin aluminium (A1) and polycarbonate (poreless Nuclepore, NP) films greased with Apiezon L vacuum grease were used as impaction substrates [24]. A cyclone with Stokes cut diameter of 5.4/~m was used as a precutter sampling device to prevent overloading of the upper BLPI stages. The cyclone-collected particles were size classified with the Bahco-sieve method. The BLPI and cyclone sampling system is described in more detail in Refs. 14 and 22. The combustion aerosol number size distributions in the size range 0.01-0.8 #m were determined by sampling in-stack through a precyclone having a Stokes cut diameter of 2.5 ~tm, followed by dilution with a two-stage, ejector-based dilution system [25], and measuring the diluted aerosol with a differential mobility analyser (DMA, TSI model 3071) by using a condensation nucleus counter (CNC, TSI model 3020) as a number concentration sensor [14,22].
Analytical Techniques: The BLPI samples were weighed carefully on a microbalance before and after the sampling. The cyclone-collected samples were weighed to determine the total mass of the samples and size classified with a Bahco-sieve method. The size fractions were weighed and stored for elemental analyses. Up to about 50 elements were determined in the samples with INAA, ICP-MS, and PIXE. Only polycarbonate substrates were used for elemental analysis of the BLPI samples. Sample preparation,
203
analysis procedures, and analytical errors are discussed in Refs. 26 and 27, respectively. For the inductively coupled plasma mass spectrometry (ICPMS) analysis, the BLPI samples were digested in 1.5 mL of concentrated suprapure HNO3 kept in an ultrasonic bath in a Tefon bomb for 4 h at 150 ~ and then diluted into 10 mL with 1% HNO3. Cyclonecollected and size-classified samples were digested in 10 mL of concentrated suprapure HNO3 and kept in an ultrasonic bath in a Teflon bomb for 8 h at 150 ~ After digestion, the solutions were filtered and analyzed with ICP-MS. Coal and limestone sorbent were analyzed with INAA and ICP-MS, coal also with PIXE. For ICPMS, the coal and limestone samples were prepared similarly as the cyclone-collected fly ash. Coal and limestone samples were analyzed for both experimental periods. The coal minerals were analyzed at the University of Kentucky using computer-controlled scanning electron microscopy (CCSEM).
Results and Discussion
Coal and Limestone Analysis: The average concentrations for 21 trace elements, and Ca, Fe, and A1, in coal and limestone are presented in Table 1. The concentrations of Ni and Sr in the limestone and Cd in the coal were under the detection limit of INAA and PIXE. For these elements, the values given in Table i have to be considered as the highest concentration limit. The most abundant trace elements in this coal were Sr, Ba, and Mn (which was also abundant in limestone). For all the elements considered, the concentration in the coal was higher in the 8/90 sample than in the 3/90 sample. This can be explained by the addition of the English coal in the Venezuelan coal used during the 8/90 experimental period, but also, the composition of the same coal is not necessarily uniform. The chlofine content in the coal was 220 ppm. During the experiments, the average limestone feed in the CFBC was 0.06 kg/s in 3/90 and 0,11 kg/s in 8/90. Coal feed was 2.60 kg/s during both experimental periods. Average feed particle sizes for coal and limestone were 2 mm and 200/lm, respectively. The higher limestone feed in the 8/90 experimental period was needed because of the high-sulphur English coal that was burned along with the Venezuelan coal. The ratio of the coal feed rate to the limestone feed rate was 43 and 27 for 3/90 and 8/90, respectively. Hence, considering the trace element concentrations in the coal and limestone from Table 1, we see that the trace elements are mostly fed to the CFBC in the coal. The amount of the trace elements that is fed in the limestone is typically few percent of the total trace element concentration. Limestone contributed more than 20% only for Mn and Ni.
204
PRACTICAL ASPECTS OF COMBUSTION
TABLE 1 The trace element and calcium, iron, and aluminium concentrations in the Venezuelan coal and sorbent limestone.
Element Ca Fe AI Sc V Cr Mn Co Ni Zn Ga As Se Sr Cd Sb Cs Ba La Ce Sm Lu Pb Th
Coal (ppm)
Limestone (ppm)
2,720 2,060 5,470 0.8 8.7 4.7 28 1.0 3.7 8.6 1.4 1.1 7.9 23 <0.5 0.3 0.2 35 1.6 2.8 0.3 0.03 1.1 0.4
346,000 3,240 6,410 1.0 2.9 4.2 540 0.7 <39 21 2.0 0.3 0.3 < 139 1.1 0.1 0.5 41 3.7 7.4 0.6 0.1 4.4 0.9
According to the CCSEM results, the mineral matter in the coal was mostly quartz (20% of the mineral matter), miscellaneous silicates (20%), and calcite (22%) [22]. Iron was found mainly as pyrite (8% of the mineral matter), as confirmed by M6ssbauer spectroscopy [28].
Physical Size Distributions: The submicron aerosol particle number size distributions showed two modes at about 0.02 and 0.3 /~m [22]. The number concentrations below 0.1/lm varied strongly during the experiments, being one to two orders of magnitude lower than those reported for pulverized coal combustion [14]. The mass size distributions peaked at about 25 tzm. Ninety-three percent of the particle mass was collected in the precutter cyclone, which had a cut diameter of 5.4 #m. Less than 0.1% of the mass was found in the particles smaller than 0.1/~m, and less than 1% in the particles between 0.1 and 1.0 pm. [221.
Differential Elemental Size Distributions: We present the size distributions for Ca, A1, Mn, and Sr in Fig. 1. The major fraction of all these elements is found in the particles collected in the cyclone, i.e., with diameters larger than 5.4 pm. The relative amount of the particles in the cyclone vs BLPI is highest for Ca and Mn. A1 and Sr show clear tendency towards finer particles. The largest particle mode peaks at 15-20/lm instead of 20-30 ~tm as in the case of Ca. No detectable amount of any of these elements was found in particles smaller than 0.1/~m, as can be seen from the small figure of Sr in the size 0.01-1/~m. No Sr is detected in particles <0.1 jzm. Sr concentration increases at 0.1 pm, and detectable mass is observed in the size range 0.1-1/xm, where a particle mode is seen in number size distributions [22]. The figure for Sr shows the effect of the double stages used in the BLPI during the experimental period 8/90. The concentrations from the period 3/90 are higher in the stages 5 and 6 because of the particle bounce and re-entrainment from the upper stages. In the measurements of 8/90, the particle bounce was minimized by using double stages in the stages 7, 8, and 9 to prevent overloading, and hence, the mass collected in the subsequent stages, i.e., 5 and 6, is smaller.
Elemental Mass Fraction Size Distributions: Mass fraction size distributions present the mass of a certain element compared with the total mass of the particles in each particle size class. Mass fraction size distributions were determined for CFBC fly ash particles collected in the BLPI and cyclone. In Fig. 2, we present the mass fraction size distributions for Ca, A1, Mn, and Sr. The errors due to elemental and gravimetrical analyses were smaller than 20%. None of the elemental concentrations presented in Fig. 2 show (1/Dp) n dependence on the particle size, indicative of vaporization followed by condensation or surface reaction [29,30]. The concentration is size independent only for very porous particles (e ~-. 1), which is not the case in the CFBC fly ash, as verified by SEM micrographs. In Fig. 2, we see small increase in the mass fraction size distributions with decreasing particle size for aluminium and strontium, but the increase starts in the supermicron particle size range, and no increase is seen in particles <1 /xm. Hence, this increase is not due to the condensation or surface reaction process. The similar behaviour of aluminium and strontium mass fraction and differential size distributions suggests the same transformation mechanisms for these elements. Other elements that follow the size distribution of aluminium are Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th. This can result from the reactions of these elements with aluminosilicate particles, or these el-
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION Ca
205
Mn
8000
3000000
6000
~" 2000000
4000
.~E
l OOOOOO
0
0.01
2OOO
9 , II1" " : "'"---0.1
1
10
0 0,01
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0.1 Sr
.~000 4OOOOO
'!!
1
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IC~
.....
2 000 i
300 000 2~M)ooo
1000
21. I
1000OO 0
0
0.01
0,1
1
10
100
Dp, p m
0.01
0.1
1
10
100
Dp, pm
FIG. 1. The CFBC fly ash differential elemental size distributions for Ca, M, Mn, and Sr determined with BLPI and precutter cyclone and analyzed with INAA and PIXE. Open markers denote the 3/90 and black markers the 8/90 experiments.
ements may be originally bound in the aluminosilicate minerals in the coal. Finkelman et al. [31] suggest the occurrence of strontium in the bituminous coals as partly organically bound and partly as celestite, but the leaching behaviour of strontium can also result from an association with the clays. Swaine [2] reports the occurrence of Sr in bituminous coals also in phosphate minerals and calcite. As phosphorus is low in this coal, the probable modes of Sr are the association with organic matter and calcite minerals. Sc, V, Zn, Ga, Cs, and rare earth elements (La, Sin, Lu) have been found in clay minerals [2]. The organically bound magnesium has been found to react with the aluminosilicate minerals inside or on the surface of the coal particles during PCC [14], so the size distributions of Sr may indicate reactions of the organically bound St, or Sr released from calcite, with the aluminosilicate minerals, even though, in FBC, the combustion conditions are different from PCC. Calcium mass fraction size distributions show a clear increase when increasing particle size. In the largest size ranges, calcium constitutes almost 30% of the total particle mass. This is due to the large limestone feed particle size, with average particle size of 200 #m. Even though 33-42% of the manganese in the CFBC fly ash derives from the hme-
stone, the manganese mass fraction size distribution is significantly different from the mass fraction size distribution of calcium. While calcium content in the fly ash particles increases with increasing particle size, there seems to be only slight increase in the fly ash size distributions of manganese in the largest particle sizes. Swaine [2] reports the occurrence of manganese in the bituminous coals predominantly in carbonate minerals and clays, minor amounts organically bound, and in pyrite. No clear conclusions can be made as to the occurrence of manganese in the CFBC fly ash, although the differential size distributions of calcium and manganese resemble each other,
The Distribution of the Trace Elements in Different Size Classes: In Table 2, we show the distribution of the total particle mass, the trace elements, and calcium, iron, and aluminium in different size classes. The concentrations presented in Table 2 are average values for the two experimental periods. The concentration values for each size fraction from the two experimental periods were consistent (within 20%) for all the other elements but As. For arsenic, the 3/90 value was only
206
PRACTICAL ASPECTS OF COMBUSTION Ca
Mn 0.2
o.15 20 o.1
0.05
o
0
0.01
0.1
1
10
100
O.Ol
AI
0.1
1
10
1
10
Sr 0.12
15 0.08
,~ ~o 0.04
E 5
0
0.01
0.1
1
10
100
D p , txm
0.01
0.1
100
D p , lam
FIG. 2. The CFBC fly ash elemental mass fraction size distributions for Ca, AI, Mn, and Sr determined with BLPI and precutter cyclone and analyzed with INAA and PIXE. Open markers denote the 3/90 and filled markers the 8/90 experiments.
37% of the 8/90 value as a result of higher As concentration in the 8/90 coal. The finest particle fraction consists of the particles with diameters 0.01~0.07/zm, corresponding to the ultrafine mode at number size distribution [22]. It contains less than 0.1% of the total particle mass collected in the BLPI and the cyclone. The trace element concentrations in this size range are low. For most elements, the concentration in Table 2 is expressed as the upper limit of the concentration (<), because the concentrations are too low to be detected with INAA, PIXE, and ICP-MS. The amount of the trace elements in this size fraction is less than 1% of the total amount of the given element collected. The second particle size range is 0.07-1.4/zm, corresponding to the intermediate mode in the number size distribution at 0.3/zm. The particles in this size range contain 0.5-7.1% of the total concentration of the trace elements and 0.6% of the total mass of the particles collected. Three trace elements, Mn, Ni, and As, have less than 1% of the total concentration in this size range. A significant fraction of Mn and Ni (23-42%) enter the CFBC within the limestone feed, and in the distribution to the size classes, they seem to follow the behaviour of calcium. The third size range is 1.4-5/tm and contains the rest of the particles collected in the BLPI. This size
range consists mostly of the residual ash and limestone particles and contains 6.3% of the total particle mass collected. Here we can distinguish the depletion of the same elements as in the previous size class. Mn and Ni show very low percentages in this size range, following the behaviour of Ca. Sb and As show lower than average fractions in this size range. All the other trace elements have 11-18% of the total concentration in this size range. The largest particle size range contains the particles that were collected in the precutter cyclone. The particles in this size range contain 93% of the total mass collected. They are abundant with Ca species, i.e, CaSO4 and CaO. Twenty percent of the total particle mass in th e cyclone-collected particles consists of Ca. A major fraction of all the trace elements was found in this large particle fraction, with more than 75% of the total concentration of each element in the cyclone-collected particles. When studied with a SEM, the particles in this size fraction were irregular in shape and consisted of large solid Ca-rich particles, and agglomerates containing several elements, e.g., Al, Si, Fe, and Ca. Conclusions In this study, we determined the distributions of 21 trace and three coal matrix elements in the fly ash
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION
207
TABLE 2 The CFBC fly ash total mass, trace element, and calcium, iron, and aluminium concentrations in different size fractions determined according to the various particle modes
Element
Total (/tm/Nm3)
Mass Ca Fe A1 Sc V Cr Mn Co Ni Zn Ga As Se Sr Cd Sb Cs Ba La Ce Sm Lu Pb Th
16,130,000 2,990,000 260,000 598,000 94 785 441 6,259 122 2,112 956 148 152 1,076 3,310 56 29 20 3,046 185 387 33 2.8 256 56
0.01-0.07/xm (%) 0.1 0.01 0.06 0.05 0.2 0.1 <4.4 <0.2 0.8 <0.5 0.4 <0.1 <0.2 <0.3 <0.3 <4.5 <0.2 < 1.9 < 1.9 <0.1 <1.3 0.1 <2.3 <1.1 <0.6
of an 80-MW(th) circulating fluidized-bed unit during the combustion of Venezuelan coal. The experiments were conducted upstream of the electrostatic precipitator. A limestone sorbent was used in the C F B C to capture SO2 during the experiments. The trace elements in the C F B C originated mainly from coal. The detailed fly ash particle size distributions for the trace and matrix elements were determined from the flue gases in the size range 0.01-70/2m. None of the studied elements showed (1/Dp) n dependence on the particle size, indicative of sighificant vaporization followed by condensation or surface reaction in the CFBC. According to the data obtained in this study, only partial vaporization of the trace elements is possible. The St, Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th size distributions resembled those of aluminium, suggesting their occurrence in aluminosilicate-rich particles in the fly ash. The similar size distributions of the trace elements with aluminium may result from reactions of the trace elements with the mineral particles, or their initial occurrence in association with the minerals. More work is needed on the occurrence of the trace ele-
0.07-1.4 #m (%) 0.6 0.5 1.1 2.0 4.1 3.8 3.5 0.9 3.2 0.5 1.9 2.6 0.6 3.4 3.0 7.1 1.8 2.0 <2.6 3.4 3.7 3.4 2.5 5.0 3.2
1.4-5/tm (%) 6.3 4.7 8.6 14 16 17 <8.9 6.4 17 3.0 13 11 4.1 14 11 14 8.9 13 15 16 14 17 12 16 18
5-100/zm (%) 93 95 91 85 80 79 83 93 79 96 84 87 95 82 85 75 89 83 80 81 81 80 83 78 78
ments in the coal to connect the trace metal behaviour in combustion with the coal characteristics, and also on the determination of the trace metal reactions with the limestone sorbent particles.
Acknowledgments The authors want to thank the Ministry of Trade and Industry of Finland and companies IVO, Ahlstr~m, and Tampella for funding the study through the combustion research program LIEKKI. We are grateful to Professor Frank Huggins and Mr. Anup Shah from the University of Kentucky for CCSEM analyses. Willy Maenhaut is grateful to the Belgian "Nationaal Fonds voor Wetenschappelijk Onderzoek" and the "Interuniversitair Instituut voor Kernwetenschappen" for research support.
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208
PRACTICAL ASPECTS OF COMBUSTION
2. Swaine, D. J., Trace Elements in Coal, Butterworth & Co. Ltd., Tiptree, UK, 1990. 3. Huggins, F. E., Shah, N., Zhao, J., Lu, F., and Huffman, G. P., Energy Fuel 7:482-489 (1993). 4. Kauppinen, E. I., and Pakkanen, T. A., Environ. Sci. Technol. 24:1811-1818 (1990). 5. Markowski, G. R., and Filby, R., Environ. Sci. Technol. 19:796-804 (1985). 6. McElroy, M. W., Carr, R. C., Ensor, D. S., and Markowski, G. R., Science 215:13-19 (1982). 7. Quann, R. J., Neville, M., and Sarofim, A. F., Combust. Sci. Technol. 74:245-265 (1990). 8. Quann, R. J., Neville, M., Janghorbani, M., Mires, C. A., mad Sarofim, A. F., Environ. Sci. TechnoL 16:776781 (1982). 9. Flagan, R. C., and Seinfeld, J. H., Fundamentals of Air Pollution Engineering, Prentice-Hall, Englewood Chffs, NJ, 1988, pp. 358-372. 10. Jokiniemi, J. K., Lazaridis, M., Lehtinen, K. E. J., and Kauppinen, E. I., "Numerical Simulation of VapourAerosol Dynamics in Combustion Processes," J. Aerosol Sci. 25:429-446 (1994). 11. Linak, W. P., and Wendt, J. O. L., Prog. Energy Combust. Sci. 19:145-185 (1993). 12. Kauppinen, E. I., Yl~italo, S. I., Joutsensaari, J., and Jokiniemi, J. K., Tenth Particulate Control Symposium and Fifth International Conference on Electrostatic Precipitation, April 5-8, 1993, Washington, DC. 13. Davidson, R. L., Natusch, D. F. S., and Wallace, J. R., Environ. Sci. Technol. 8:1107-1113 (1974). 14. Joutsensaari, J., Kauppinen, E. I., Jokiniemi, J. K., and Helble, J. j., in The Impact of Ash Deposition on Coal Fired Plants: Proceedings of the Engineering Foundation Conference (J. Williamson and F. Wigley, Eds.), Taylor & Francis, 1994, pp. 613-624. 15. Sarofim, A. F., Beer, J. M., Baron, R. E., and Hodges, J. L., in AFBC Technical Source Book (S.-E. Tung and G. C. Williams, Eds.), Final Report, DOE/MC/145362544, 1987. 16. Tang, J. T., and Taylor, E. S., in AFBC Technical Source Book (S.-E. Tung and G. C. Williams, Eds.), Final Report, DOE/MC/14536-2544, 1987.
17. Hernberg, R., Stenberg, J., and Zethraeus, B., Cornbust. Flame 95:191-205 (1993). 18. Agarwal, P. K., and La Nauze, R. D., Chem. Eng. Res. Des. 67:457-480 (1989). 19. Weissman, S. H., Carpenter, R. L., and Newton, G. J., Environ. Sci. Technol. 17:65-71 (1983). 20. Clarke, L. B., Fuel 72:731-736 (1993). 21. Mojtahedi, W., Nieminen, M., Hulkkonen, S., and Jahkola, A., Fuel Proc. Technol. 26:83-97 (1990). 22. Lind, T. M., Kauppiuen, E. I., Jokiniemi, J. K., Maenhaut, W., and Pakkanen, T. A., in The Impact of Ash Deposition on Coal Fired Plants: Proceedings of the Engineering Foundation Conference (J. williamson and F. Wigley, Eds.), Taylor & Francis, 1994, pp. 7788. 23. Kauppinen, E. 1., Aerosol Sci. Technol. 16:171-197 (1992). 24. Hillamo, R. E., and Kauppinen, E. I., Aerosol Sci. Technol. 14:33-47 (1991). 25. Koch, W., L6dding, H., M61ter, W., and Munzinger, F., Staub -Reinhaltung der Luft 48:341-344 (1990). 26. Maenhaut, W., Kauppinen, E. I., and Lind, T. M., J. Radioanalyt. Nucl. Chem. 167:259-269 (1993). 27. Maenhaut, W., Royset, O, Vadset, M., Kauppinen, E. I., and Lind, T. L., Nucl. Instrum. Meth. Phys. Res. B 75:266-272 (1993). 28. Huggins, F. E., and Huffman, G. P., in MssbauerAnalysis of the Iron-Bearing Phases in Coal, Coke and Ash, Analytical Mcthods for Coal and Coal Products (C. Karr, Jr., Ed.), Academic Press, New York, 1979, Vol. III. 29. Flagan, R. C., and Friedlander, S. K., Recent Developments in Aerosol Science (D.T. Shaw, Ed.), Wiley, New York, 1978, pp. 25-59. 30. Hinds, W. C., Aerosol Technology: Properties, Behaviour, and Measurement of Airborne Particles, Wiley, New York, 1982, pp. 249-262. 31. Finkehnan, R. B., Palmer, C. A., Krasnow, M. R., Aruscavage, P. J., Sellers, G. A., and Dulong, F. T., Energy Fuels 4:755-767 (1990).
COMMENTS Dino Musmarra, CNR-Instituto di Ricerche Scilla Combustione, Italy. The mercury emissions from the combustion of coal are considered to be responsible for more than 50% of the total anthropogenic emissions. While your paper takes into account numerous metals, why is mercury not considered? Author's Reply. In coal combustion, mercury has been found to be highly volatile, and a major fraction of it is expected to be found in the gas phase in the flue gas upstream of the particle removal equipment. In order to study
the mercury behaviour in combustion, both gas phase and particulate phase mercury should be determined. In this study, however, we concentrated only on particulate phase in the flue gas, and no mercury was detected with any of the analysis methods used.
j. A. Kozinski, McGill University, Canada. 1. Do you have any idea how metals you have analyzed were distrib-
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION uted in the particle? Were they concentrated in the surface layers or inside ash particles? 2. What was the accuracy of metal concentration measurements in particles as small as 10 nm in diameter?
Author's Reply. We did not measure the distribution of the metals in individual fly ash particles. We collected the
209
particles from the flue gas with a low-pressure impactor, which collects the particles according to their aerodyaaamic mobility. The elemental analyses were then carried out for these narrow, well-defined particle size-fractions collected in the impactor, not individual ash particles. From these analyses, no conclusions can be made on the distribution of the elements in individual ash particles.
A F I E L D STUDY O N T H E TRACE METAL BEHAVIOUR IN ATMOSPHERIC CIRCULATING F L U I D I Z E D - B E D COAL C O M B U S T I O N TERTFALIISA LIND, ESKO I. KAUPPINEN ANDJORMA K. ]OKINIEMI VTT Aerosol Technology Group P.O. Box 1401, FIN-02044 VTT, Finland AND
WILLY MAENHAUT University of Gent, Institute for Nuclear Sciences Proeftuinstraat 86, B-9000 Gent, Belgium
Trace element behaviour in atmospheric circulating fluidized-bed combustion (CFBC) of Venezuelan bituminous coal was studied by determining particle size distributions in the CFBC flue gas. The size distributions of calcium, iron, aluminium, and 21 trace elements, Sc, V, Cr, Mn, Co, Ni, Zn, Ga, As, Se, Sr, Cd, Sb, Cs, Ba, La, Ce, Sm, Lu, Pb, and Th, in the size range 0.01-70/zm, were determined by collecting aerosols with a low-pressure impactor-cyclone sampling train from the flue gases of an 80MW(th) CFBC boiler upstream of the electrostatic precipitator. The collected samples were analyzed gravimetrically and with instrumental neutron activation analysis (INAA), particle-induced x-ray emission analysis (PIXE), and inductively coupled plasma mass spectrometry (ICP-MS). The number size distributions of the aerosols were determined with a differential electrical mobility method in the size range 0.01-0.8/tin. In the ultrafine particle mode, i.e., D1, < 0.1/*rn, the CFBC number concentrations varied strongly during the experiments, being one to two orders of magnitude lower than those observed in pulverized coal combustion. For all of the elements studied, 75% or more were found in particles larger than 5 pm. None of the studied elements showed significant vaporization and subsequent chemical surface reaction or condensation in the CFBC. The Sr, Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th size distributions resembled those of aluminium, suggesting their occurrence in aluminosilicate-rieh particles in the fly ash. The association of the trace elements with aluminium in the fly ash particles may result from reactions of the trace elements with the aluminosilicate mineral particles inside the burning coal particles, or their initial occurrence'in association with these minerals.
Introduction Coal combustion is a major source of toxic species emitted to the atmosphere. During combustion, trace elements contained in the coal are transformed and either retained in the boiler bottom ash, captured by the gas cleaning devices, or released to the atmosphere as particles or gases. The total emission to the atmosphere depends on the transformations of the noncombustible matter in the coal during combustion, and on the gas cleaning equipment employed at the combustion unit. The combustion behaviour of noncombustible matter in coal, including trace elements, is determined by their occurrence in the coal, the size and temperature history of the coal particle, the atmosphere the coal particle is exposed to, and the flow conditions in the combustion unit. The form of occurrence in the coal is important when connecting the combustion behaviour with the coal characteristics. Many studies on the occurrence of trace ele201
ments in the coal have revealed that they are found in various forms, as reviewed by Finkelman [1] and Swaine [2]. The trace elements are found in association with, e.g., pyrite and accessory sulphides, carbonates, clays, selenides, and organic matter. For many trace elements, the occurrence can only be determined speculatively, as most studies are based on indirect measurement, e.g., leaching behaviour or density determination (float and sink method) [2]. In a recent study, x-ray absorption fine structure spectroscopy (XAFS) method was used to study the occurrence of arsenic and chromium in several U.S. coals directly [3]. In many field and laboratory studies on pulverized coal combustion (PCC), the trace elements have been found to be enriched in the submicron particles [4-8]. The enrichment is believed to result from the vaporization of these species during the combustion, followed by condensation of the vaporized species on the surfaces of the pre-existing particles entrained in the flue gas [9]. During the pulverized coal combus-
202
PRACTICAL ASPECTS OF COMBUSTION
tion, high number concentrations of fine particles are formed when the vaporized fraction of the refractory oxides (e.g., Fe, Mg, Si, A1, Ca) condenses homogeneously prior to the condensation of the trace metals. The condensation of the trace metals is proportional to the surface area of the particles, and hence occurs predominantly on the fine particle surfaces, resulting in their enrichment with condensed species [10]. Also, chemical reactions of trace element vapours at the fly ash particle surfaces would result in their enrichment in submicron ash particles [11]. This is particularly harmful to the environment, as particles in the size range 0.1-1.0/2m have been found to penetrate readily particle removal equipment [12]. Also, some of the condensed species on the emitted fly ash particles may be easily leached in dilute acids, enabling the transport of the trace elements into the surface water, soil, and finally into the groundwater. The trace elements that are not vaporized and thus retained in the residual ash particles during combustion are usually not easily removed from the ash particles by leaching in dilute acids. While in the ambient air, however, the fly ash particles may also be exposed to concentrated acids. This may occur either within accumulation mode ambient aerosol particles (0.1-1/2m in diameter, only submicron fly ash particles), within fog droplets (all emitted ash particles smaller than a few microns), or within cloud droplets (all emitted ash particles). As a result, the solubility of trace elements may increase prior to the dry and wet deposition. Several elements have been found to partially vaporize in the pulverized coal combustion studies. Profound laboratory studies conducted by Quann et al. [7] showed large differences in the composition of the submicron particles for the coals with varying characteristics regarding rank and origin. The major components of the submicron fume were SiO2 for bituminous coals, and MgO, CaO, and FeO for lower-rank coals. Enrichment in the fine particles was seen for, e.g., arsenic, antimony, cobalt, chromium, and manganese. Despite the enrichment of the trace elements, the major fraction of the total mass in the fine particles consisted of coal matrix elements. Also, major fractions of the trace elements were found in the supermicron fly ash particles, indicating that only minor fractions were released to the gas phase during combustion. In the field studies, vaporization and condensation behaviour has been observed for, e.g., arsenic, antimony, mercury, zinc, nickel, vanadium, copper, cadmium, strontium, barium, and selenium [4-6,13]. According to our recent studies on the submicron particle size distributions from the real-scale PCC [14], the submicron particles form two distinctive modes. Ultrafine mode was observed in the range 0.03-0.1 Izm, being formed from the vaporized ash matrix species via nucleation. The intermediate mode was found between 0.3 and 0.7/tin, consisting
mainly of coal matrix species. Accordingly, the assumption that all the submicron-sized particles have been formed from the gaseous ash species is not necessarily valid, and this has to be taken into account while interpreting the results of any previous studies on the ash matrix and trace element combustion behaviour. Most of the studies on the trace element behaviour in coal combustion have concentrated on PCC. Fluidized-bed combustion (FBC) differs significantly from PCC. In FBC, the temperature history of the particles is different from that of a PCC because of large coal feed particle size and the dense bed the coal particles are fed to. The combustion times are longer [15], and the peak temperatures and heating rates lower than in the PCC, with gas temperatures of up to 1150 K and particle peak temperatures of up to 1700 K [16,17]. In FBCs, the gas atmosphere encountered by the coal particles varies with the position in the furnace [18]. In the circulating FBC (CFBC), the density of the bed decreases rapidly while going to the upper parts of the furnace, and there is more oxygen available for the combustion. All these parameters are expected to affect the behaviour of ash-forming constituents, including trace elements, during combustion. Several studies have addressed the trace element partitioning in different sized particles in the fluidized bed combustion [e.g., 19,20,21]. However, no detailed data have been published on the submicron particle size distributions and composition. Consequently, the fraction of the trace metals released to the gas phase and later condensed during FBC is not known. In this study, we approached the problem by measuring, for the first time, detailed fly ash size distributions from a real-scale circulating fluidized-bed boiler flue gases upstream of the electrostatic precipitator using advanced aerosol measurement techniques. The particle characteristics, including physical (number and mass) as well as elemental size distributions, were determined in the particle size range 0.01-70/lm. The coal and limestone sorbent were also carefully characterized with regard to their mineral and trace metal contents. Size-classified fly ash particle samples were analyzed with instrumental neutron activation analysis (INAA), particle-induced x-ray emission analysis (PIXE) and inductively coupled plasma mass spectrometry (ICP-MS) methods for up to 50 elements. Detailed size distribution data for trace elements associated with both coal and limestone are compared with size distributions of elements abundant in coal and in limestone. Tile trace element vaporization and condensation as well as possible reactions inside the burning coal particles are discussed on the basis of experimental fly ash size distribution data. We have earlier reported data on the alkali metal behaviour in CFBC [22] and presented the analytical procedures used in INAA and PIXE analyses [26,27].
BEHAVIOURIN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION
Process Description The experiments were carried out at an 80MW(th) steam-generating atmospheric circulating fluidized-bed combustion unit during two one-week periods in March and August 1990, hereby marked as 3/90 and 8/90, respectively [22]. Venezuelan bituminous coal was burned during the experiments, and limestone was used as a sulphur sorbent. In the experimental period 8/90, some English, higher-sulphur coal was accidentally mixed in the coal feed silo at the beginning of the experimental week. The process parameters are described in Ref. 22.
Methods
Aerosol Sampling: The mass and elemental size distributions were determined by collecting size-classified coal combustion aerosol samples with an ll-stage multijet compressible flow Berner-type low-pressure impactor (BLPI) [4,23,24]. Because of high particle concentrations in the CFBC flue gas, double stages were used in the impactor in stages 7, 8, and 9 to prevent overloading of these stages during the latter experimental period, i.e., 8/90. Thin aluminium (A1) and polycarbonate (poreless Nuclepore, NP) films greased with Apiezon L vacuum grease were used as impaction substrates [24]. A cyclone with Stokes cut diameter of 5.4/~m was used as a precutter sampling device to prevent overloading of the upper BLPI stages. The cyclone-collected particles were size classified with the Bahco-sieve method. The BLPI and cyclone sampling system is described in more detail in Refs. 14 and 22. The combustion aerosol number size distributions in the size range 0.01-0.8 #m were determined by sampling in-stack through a precyclone having a Stokes cut diameter of 2.5 ~tm, followed by dilution with a two-stage, ejector-based dilution system [25], and measuring the diluted aerosol with a differential mobility analyser (DMA, TSI model 3071) by using a condensation nucleus counter (CNC, TSI model 3020) as a number concentration sensor [14,22].
Analytical Techniques: The BLPI samples were weighed carefully on a microbalance before and after the sampling. The cyclone-collected samples were weighed to determine the total mass of the samples and size classified with a Bahco-sieve method. The size fractions were weighed and stored for elemental analyses. Up to about 50 elements were determined in the samples with INAA, ICP-MS, and PIXE. Only polycarbonate substrates were used for elemental analysis of the BLPI samples. Sample preparation,
203
analysis procedures, and analytical errors are discussed in Refs. 26 and 27, respectively. For the inductively coupled plasma mass spectrometry (ICPMS) analysis, the BLPI samples were digested in 1.5 mL of concentrated suprapure HNO3 kept in an ultrasonic bath in a Tefon bomb for 4 h at 150 ~ and then diluted into 10 mL with 1% HNO3. Cyclonecollected and size-classified samples were digested in 10 mL of concentrated suprapure HNO3 and kept in an ultrasonic bath in a Teflon bomb for 8 h at 150 ~ After digestion, the solutions were filtered and analyzed with ICP-MS. Coal and limestone sorbent were analyzed with INAA and ICP-MS, coal also with PIXE. For ICPMS, the coal and limestone samples were prepared similarly as the cyclone-collected fly ash. Coal and limestone samples were analyzed for both experimental periods. The coal minerals were analyzed at the University of Kentucky using computer-controlled scanning electron microscopy (CCSEM).
Results and Discussion
Coal and Limestone Analysis: The average concentrations for 21 trace elements, and Ca, Fe, and A1, in coal and limestone are presented in Table 1. The concentrations of Ni and Sr in the limestone and Cd in the coal were under the detection limit of INAA and PIXE. For these elements, the values given in Table i have to be considered as the highest concentration limit. The most abundant trace elements in this coal were Sr, Ba, and Mn (which was also abundant in limestone). For all the elements considered, the concentration in the coal was higher in the 8/90 sample than in the 3/90 sample. This can be explained by the addition of the English coal in the Venezuelan coal used during the 8/90 experimental period, but also, the composition of the same coal is not necessarily uniform. The chlofine content in the coal was 220 ppm. During the experiments, the average limestone feed in the CFBC was 0.06 kg/s in 3/90 and 0,11 kg/s in 8/90. Coal feed was 2.60 kg/s during both experimental periods. Average feed particle sizes for coal and limestone were 2 mm and 200/lm, respectively. The higher limestone feed in the 8/90 experimental period was needed because of the high-sulphur English coal that was burned along with the Venezuelan coal. The ratio of the coal feed rate to the limestone feed rate was 43 and 27 for 3/90 and 8/90, respectively. Hence, considering the trace element concentrations in the coal and limestone from Table 1, we see that the trace elements are mostly fed to the CFBC in the coal. The amount of the trace elements that is fed in the limestone is typically few percent of the total trace element concentration. Limestone contributed more than 20% only for Mn and Ni.
204
PRACTICAL ASPECTS OF COMBUSTION
TABLE 1 The trace element and calcium, iron, and aluminium concentrations in the Venezuelan coal and sorbent limestone.
Element Ca Fe AI Sc V Cr Mn Co Ni Zn Ga As Se Sr Cd Sb Cs Ba La Ce Sm Lu Pb Th
Coal (ppm)
Limestone (ppm)
2,720 2,060 5,470 0.8 8.7 4.7 28 1.0 3.7 8.6 1.4 1.1 7.9 23 <0.5 0.3 0.2 35 1.6 2.8 0.3 0.03 1.1 0.4
346,000 3,240 6,410 1.0 2.9 4.2 540 0.7 <39 21 2.0 0.3 0.3 < 139 1.1 0.1 0.5 41 3.7 7.4 0.6 0.1 4.4 0.9
According to the CCSEM results, the mineral matter in the coal was mostly quartz (20% of the mineral matter), miscellaneous silicates (20%), and calcite (22%) [22]. Iron was found mainly as pyrite (8% of the mineral matter), as confirmed by M6ssbauer spectroscopy [28].
Physical Size Distributions: The submicron aerosol particle number size distributions showed two modes at about 0.02 and 0.3 /~m [22]. The number concentrations below 0.1/lm varied strongly during the experiments, being one to two orders of magnitude lower than those reported for pulverized coal combustion [14]. The mass size distributions peaked at about 25 tzm. Ninety-three percent of the particle mass was collected in the precutter cyclone, which had a cut diameter of 5.4 #m. Less than 0.1% of the mass was found in the particles smaller than 0.1/~m, and less than 1% in the particles between 0.1 and 1.0 pm. [221.
Differential Elemental Size Distributions: We present the size distributions for Ca, A1, Mn, and Sr in Fig. 1. The major fraction of all these elements is found in the particles collected in the cyclone, i.e., with diameters larger than 5.4 pm. The relative amount of the particles in the cyclone vs BLPI is highest for Ca and Mn. A1 and Sr show clear tendency towards finer particles. The largest particle mode peaks at 15-20/lm instead of 20-30 ~tm as in the case of Ca. No detectable amount of any of these elements was found in particles smaller than 0.1/~m, as can be seen from the small figure of Sr in the size 0.01-1/~m. No Sr is detected in particles <0.1 jzm. Sr concentration increases at 0.1 pm, and detectable mass is observed in the size range 0.1-1/xm, where a particle mode is seen in number size distributions [22]. The figure for Sr shows the effect of the double stages used in the BLPI during the experimental period 8/90. The concentrations from the period 3/90 are higher in the stages 5 and 6 because of the particle bounce and re-entrainment from the upper stages. In the measurements of 8/90, the particle bounce was minimized by using double stages in the stages 7, 8, and 9 to prevent overloading, and hence, the mass collected in the subsequent stages, i.e., 5 and 6, is smaller.
Elemental Mass Fraction Size Distributions: Mass fraction size distributions present the mass of a certain element compared with the total mass of the particles in each particle size class. Mass fraction size distributions were determined for CFBC fly ash particles collected in the BLPI and cyclone. In Fig. 2, we present the mass fraction size distributions for Ca, A1, Mn, and Sr. The errors due to elemental and gravimetrical analyses were smaller than 20%. None of the elemental concentrations presented in Fig. 2 show (1/Dp) n dependence on the particle size, indicative of vaporization followed by condensation or surface reaction [29,30]. The concentration is size independent only for very porous particles (e ~-. 1), which is not the case in the CFBC fly ash, as verified by SEM micrographs. In Fig. 2, we see small increase in the mass fraction size distributions with decreasing particle size for aluminium and strontium, but the increase starts in the supermicron particle size range, and no increase is seen in particles <1 /xm. Hence, this increase is not due to the condensation or surface reaction process. The similar behaviour of aluminium and strontium mass fraction and differential size distributions suggests the same transformation mechanisms for these elements. Other elements that follow the size distribution of aluminium are Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th. This can result from the reactions of these elements with aluminosilicate particles, or these el-
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION Ca
205
Mn
8000
3000000
6000
~" 2000000
4000
.~E
l OOOOOO
0
0.01
2OOO
9 , II1" " : "'"---0.1
1
10
0 0,01
IOO
0.1 Sr
.~000 4OOOOO
'!!
1
10
IC~
.....
2 000 i
300 000 2~M)ooo
1000
21. I
1000OO 0
0
0.01
0,1
1
10
100
Dp, p m
0.01
0.1
1
10
100
Dp, pm
FIG. 1. The CFBC fly ash differential elemental size distributions for Ca, M, Mn, and Sr determined with BLPI and precutter cyclone and analyzed with INAA and PIXE. Open markers denote the 3/90 and black markers the 8/90 experiments.
ements may be originally bound in the aluminosilicate minerals in the coal. Finkelman et al. [31] suggest the occurrence of strontium in the bituminous coals as partly organically bound and partly as celestite, but the leaching behaviour of strontium can also result from an association with the clays. Swaine [2] reports the occurrence of Sr in bituminous coals also in phosphate minerals and calcite. As phosphorus is low in this coal, the probable modes of Sr are the association with organic matter and calcite minerals. Sc, V, Zn, Ga, Cs, and rare earth elements (La, Sin, Lu) have been found in clay minerals [2]. The organically bound magnesium has been found to react with the aluminosilicate minerals inside or on the surface of the coal particles during PCC [14], so the size distributions of Sr may indicate reactions of the organically bound St, or Sr released from calcite, with the aluminosilicate minerals, even though, in FBC, the combustion conditions are different from PCC. Calcium mass fraction size distributions show a clear increase when increasing particle size. In the largest size ranges, calcium constitutes almost 30% of the total particle mass. This is due to the large limestone feed particle size, with average particle size of 200 #m. Even though 33-42% of the manganese in the CFBC fly ash derives from the hme-
stone, the manganese mass fraction size distribution is significantly different from the mass fraction size distribution of calcium. While calcium content in the fly ash particles increases with increasing particle size, there seems to be only slight increase in the fly ash size distributions of manganese in the largest particle sizes. Swaine [2] reports the occurrence of manganese in the bituminous coals predominantly in carbonate minerals and clays, minor amounts organically bound, and in pyrite. No clear conclusions can be made as to the occurrence of manganese in the CFBC fly ash, although the differential size distributions of calcium and manganese resemble each other,
The Distribution of the Trace Elements in Different Size Classes: In Table 2, we show the distribution of the total particle mass, the trace elements, and calcium, iron, and aluminium in different size classes. The concentrations presented in Table 2 are average values for the two experimental periods. The concentration values for each size fraction from the two experimental periods were consistent (within 20%) for all the other elements but As. For arsenic, the 3/90 value was only
206
PRACTICAL ASPECTS OF COMBUSTION Ca
Mn 0.2
o.15 20 o.1
0.05
o
0
0.01
0.1
1
10
100
O.Ol
AI
0.1
1
10
1
10
Sr 0.12
15 0.08
,~ ~o 0.04
E 5
0
0.01
0.1
1
10
100
D p , txm
0.01
0.1
100
D p , lam
FIG. 2. The CFBC fly ash elemental mass fraction size distributions for Ca, AI, Mn, and Sr determined with BLPI and precutter cyclone and analyzed with INAA and PIXE. Open markers denote the 3/90 and filled markers the 8/90 experiments.
37% of the 8/90 value as a result of higher As concentration in the 8/90 coal. The finest particle fraction consists of the particles with diameters 0.01~0.07/zm, corresponding to the ultrafine mode at number size distribution [22]. It contains less than 0.1% of the total particle mass collected in the BLPI and the cyclone. The trace element concentrations in this size range are low. For most elements, the concentration in Table 2 is expressed as the upper limit of the concentration (<), because the concentrations are too low to be detected with INAA, PIXE, and ICP-MS. The amount of the trace elements in this size fraction is less than 1% of the total amount of the given element collected. The second particle size range is 0.07-1.4/zm, corresponding to the intermediate mode in the number size distribution at 0.3/zm. The particles in this size range contain 0.5-7.1% of the total concentration of the trace elements and 0.6% of the total mass of the particles collected. Three trace elements, Mn, Ni, and As, have less than 1% of the total concentration in this size range. A significant fraction of Mn and Ni (23-42%) enter the CFBC within the limestone feed, and in the distribution to the size classes, they seem to follow the behaviour of calcium. The third size range is 1.4-5/tm and contains the rest of the particles collected in the BLPI. This size
range consists mostly of the residual ash and limestone particles and contains 6.3% of the total particle mass collected. Here we can distinguish the depletion of the same elements as in the previous size class. Mn and Ni show very low percentages in this size range, following the behaviour of Ca. Sb and As show lower than average fractions in this size range. All the other trace elements have 11-18% of the total concentration in this size range. The largest particle size range contains the particles that were collected in the precutter cyclone. The particles in this size range contain 93% of the total mass collected. They are abundant with Ca species, i.e, CaSO4 and CaO. Twenty percent of the total particle mass in th e cyclone-collected particles consists of Ca. A major fraction of all the trace elements was found in this large particle fraction, with more than 75% of the total concentration of each element in the cyclone-collected particles. When studied with a SEM, the particles in this size fraction were irregular in shape and consisted of large solid Ca-rich particles, and agglomerates containing several elements, e.g., Al, Si, Fe, and Ca. Conclusions In this study, we determined the distributions of 21 trace and three coal matrix elements in the fly ash
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION
207
TABLE 2 The CFBC fly ash total mass, trace element, and calcium, iron, and aluminium concentrations in different size fractions determined according to the various particle modes
Element
Total (/tm/Nm3)
Mass Ca Fe A1 Sc V Cr Mn Co Ni Zn Ga As Se Sr Cd Sb Cs Ba La Ce Sm Lu Pb Th
16,130,000 2,990,000 260,000 598,000 94 785 441 6,259 122 2,112 956 148 152 1,076 3,310 56 29 20 3,046 185 387 33 2.8 256 56
0.01-0.07/xm (%) 0.1 0.01 0.06 0.05 0.2 0.1 <4.4 <0.2 0.8 <0.5 0.4 <0.1 <0.2 <0.3 <0.3 <4.5 <0.2 < 1.9 < 1.9 <0.1 <1.3 0.1 <2.3 <1.1 <0.6
of an 80-MW(th) circulating fluidized-bed unit during the combustion of Venezuelan coal. The experiments were conducted upstream of the electrostatic precipitator. A limestone sorbent was used in the C F B C to capture SO2 during the experiments. The trace elements in the C F B C originated mainly from coal. The detailed fly ash particle size distributions for the trace and matrix elements were determined from the flue gases in the size range 0.01-70/2m. None of the studied elements showed (1/Dp) n dependence on the particle size, indicative of sighificant vaporization followed by condensation or surface reaction in the CFBC. According to the data obtained in this study, only partial vaporization of the trace elements is possible. The St, Sc, V, Zn, Ga, Cs, Ba, La, Sm, Lu, and Th size distributions resembled those of aluminium, suggesting their occurrence in aluminosilicate-rich particles in the fly ash. The similar size distributions of the trace elements with aluminium may result from reactions of the trace elements with the mineral particles, or their initial occurrence in association with the minerals. More work is needed on the occurrence of the trace ele-
0.07-1.4 #m (%) 0.6 0.5 1.1 2.0 4.1 3.8 3.5 0.9 3.2 0.5 1.9 2.6 0.6 3.4 3.0 7.1 1.8 2.0 <2.6 3.4 3.7 3.4 2.5 5.0 3.2
1.4-5/tm (%) 6.3 4.7 8.6 14 16 17 <8.9 6.4 17 3.0 13 11 4.1 14 11 14 8.9 13 15 16 14 17 12 16 18
5-100/zm (%) 93 95 91 85 80 79 83 93 79 96 84 87 95 82 85 75 89 83 80 81 81 80 83 78 78
ments in the coal to connect the trace metal behaviour in combustion with the coal characteristics, and also on the determination of the trace metal reactions with the limestone sorbent particles.
Acknowledgments The authors want to thank the Ministry of Trade and Industry of Finland and companies IVO, Ahlstr~m, and Tampella for funding the study through the combustion research program LIEKKI. We are grateful to Professor Frank Huggins and Mr. Anup Shah from the University of Kentucky for CCSEM analyses. Willy Maenhaut is grateful to the Belgian "Nationaal Fonds voor Wetenschappelijk Onderzoek" and the "Interuniversitair Instituut voor Kernwetenschappen" for research support.
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PRACTICAL ASPECTS OF COMBUSTION
2. Swaine, D. J., Trace Elements in Coal, Butterworth & Co. Ltd., Tiptree, UK, 1990. 3. Huggins, F. E., Shah, N., Zhao, J., Lu, F., and Huffman, G. P., Energy Fuel 7:482-489 (1993). 4. Kauppinen, E. I., and Pakkanen, T. A., Environ. Sci. Technol. 24:1811-1818 (1990). 5. Markowski, G. R., and Filby, R., Environ. Sci. Technol. 19:796-804 (1985). 6. McElroy, M. W., Carr, R. C., Ensor, D. S., and Markowski, G. R., Science 215:13-19 (1982). 7. Quann, R. J., Neville, M., and Sarofim, A. F., Combust. Sci. Technol. 74:245-265 (1990). 8. Quann, R. J., Neville, M., Janghorbani, M., Mires, C. A., mad Sarofim, A. F., Environ. Sci. TechnoL 16:776781 (1982). 9. Flagan, R. C., and Seinfeld, J. H., Fundamentals of Air Pollution Engineering, Prentice-Hall, Englewood Chffs, NJ, 1988, pp. 358-372. 10. Jokiniemi, J. K., Lazaridis, M., Lehtinen, K. E. J., and Kauppinen, E. I., "Numerical Simulation of VapourAerosol Dynamics in Combustion Processes," J. Aerosol Sci. 25:429-446 (1994). 11. Linak, W. P., and Wendt, J. O. L., Prog. Energy Combust. Sci. 19:145-185 (1993). 12. Kauppinen, E. I., Yl~italo, S. I., Joutsensaari, J., and Jokiniemi, J. K., Tenth Particulate Control Symposium and Fifth International Conference on Electrostatic Precipitation, April 5-8, 1993, Washington, DC. 13. Davidson, R. L., Natusch, D. F. S., and Wallace, J. R., Environ. Sci. Technol. 8:1107-1113 (1974). 14. Joutsensaari, J., Kauppinen, E. I., Jokiniemi, J. K., and Helble, J. j., in The Impact of Ash Deposition on Coal Fired Plants: Proceedings of the Engineering Foundation Conference (J. Williamson and F. Wigley, Eds.), Taylor & Francis, 1994, pp. 613-624. 15. Sarofim, A. F., Beer, J. M., Baron, R. E., and Hodges, J. L., in AFBC Technical Source Book (S.-E. Tung and G. C. Williams, Eds.), Final Report, DOE/MC/145362544, 1987. 16. Tang, J. T., and Taylor, E. S., in AFBC Technical Source Book (S.-E. Tung and G. C. Williams, Eds.), Final Report, DOE/MC/14536-2544, 1987.
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COMMENTS Dino Musmarra, CNR-Instituto di Ricerche Scilla Combustione, Italy. The mercury emissions from the combustion of coal are considered to be responsible for more than 50% of the total anthropogenic emissions. While your paper takes into account numerous metals, why is mercury not considered? Author's Reply. In coal combustion, mercury has been found to be highly volatile, and a major fraction of it is expected to be found in the gas phase in the flue gas upstream of the particle removal equipment. In order to study
the mercury behaviour in combustion, both gas phase and particulate phase mercury should be determined. In this study, however, we concentrated only on particulate phase in the flue gas, and no mercury was detected with any of the analysis methods used.
j. A. Kozinski, McGill University, Canada. 1. Do you have any idea how metals you have analyzed were distrib-
BEHAVIOUR IN ATMOSPHERIC CIRCULATING FLUIDIZED-BED COAL COMBUSTION uted in the particle? Were they concentrated in the surface layers or inside ash particles? 2. What was the accuracy of metal concentration measurements in particles as small as 10 nm in diameter?
Author's Reply. We did not measure the distribution of the metals in individual fly ash particles. We collected the
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particles from the flue gas with a low-pressure impactor, which collects the particles according to their aerodyaaamic mobility. The elemental analyses were then carried out for these narrow, well-defined particle size-fractions collected in the impactor, not individual ash particles. From these analyses, no conclusions can be made on the distribution of the elements in individual ash particles.