Impact of Bed Particle Size Distribution on the Distribution of

Combust. Sci. Technol., 184: 811–828, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0010-2202 print=1563-521X online
DOI: 10.1080/00102202.2012.669802
IMPACT OF BED PARTICLE SIZE DISTRIBUTION ON
THE DISTRIBUTION OF HEAVY METAL DURING
DEFLUIDIZATION PROCESS IN FLUIDIZED BED
INCINERATOR
Min-Hao Wu,1 Kaimin Shih,2 and Chiou-Liang Lin1
1
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Department of Civil and Environmental Engineering, National University of
Kaohsiung, Kaohsiung, Taiwan
2
Department of Civil Engineering, The University of Hong Kong, Hong Kong
In this study, artificial waste was used to investigate the impact of bed particle-size
distributions (narrow, flat, and Gaussian) on heavy-metal distributions in the particles in
the bottom ash during defluidization in a fluidized bed incinerator. When the particle size
was less than 0.500 mm, the heavy-metal concentration within the particles tended to
increase, and when the particle size was greater than 0.850 mm, the heavy-metal concentration showed a substantial increase. With regard to heavy-metal capture, the formation
of the low-melting eutectic complexes was produced by the combination of heavy metals
with Na. The capturing effect of the liquid eutectic material may be a more important mechanism than adsorption. The comparison of heavy-metal concentrations at different
particle-size distributions showed that heavy-metal concentrations in large and small particles with narrow particle distributions and their total retention rates were higher than the
corresponding values in the case of flat and Gaussian particle distributions.
Keywords: Agglomeration; Bottom ash; Cadmium; Chromium; Lead
INTRODUCTION
Fluidized bed reactors are widely used for various purposes, such as for waste
incineration, gasification, pyrolysis, and biomass fuel combustion (Arena et al., 2010;
Chen et al., 2007; Srinivasa Rao and Venkat Reddy, 2007). However, during the
operation of fluidized bed reactors using complex mixtures of feed materials, sticky
substances may accumulate. The gradual accumulation of these substances can cause
the bed material to agglomerate into large blocks. Previous studies have demonstrated that many kinds of elements can cause stickiness, and the results of these studies are summarized in Table 1. It was found that alkali group elements such as Na
and K are among the major materials that cause agglomeration. The generation of
agglomerated materials can affect the operating conditions of the fluidized bed by
Received 6 July 2011; revised 22 February 2012; accepted 22 February 2012.
Address correspondence to Chiou-Liang Lin, Department of Civil and Environmental Engineering,
National University of Kaohsiung, 700, Kaohsiung University Rd., Nanzih District, 811, Kaohsiung,
Taiwan. E-mail: [email protected]
811
812
M.-H. WU ET AL.
Table 1 Possible elements inducing agglomeration in the fluidized bed
Na
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Ghaly et al., 1993
Conn, 1995
Mikami et al., 1996
Steenari et al., 1998
Wang et al., 1999
Lin et al., 2003
Atakül et al., 2005
Lin et al., 2010a
K
Mg
Ca
Si
Cl
S
Fe
V
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
affecting parameters such as the minimum fluidization velocity, bubble size, bubble
frequency, and rise velocity (Tardos and Pfeffer, 1995). Agglomeration can even
cause the bed to stop functioning properly as a result of defluidization. Therefore,
agglomeration issues are among the most difficult and perplexing problems in the
use of fluidized bed reactors.
Many researchers have explored the causes of fluidized bed agglomeration.
Skrifvars et al. (1992, 1994) pointed out that the phenomena of agglomeration
and defluidization originate from the stickiness of bed materials, and that in addition
to the properties of the particles, the most common reasons for stickiness include (1)
the presence of sticky sintered plastic material and the production of vitrified
material and (2) the production of liquid substances by chemical reactions or by
the melting of materials. Therefore, alkali group elements, when present in waste,
can react with the bed material or other elements in the waste to produce liquid
eutectic materials that adhere to the surface of the bed, causing the bed material
to become sticky and leading to agglomeration and defluidization (Lin and Wey,
2004).
Additionally, operating parameters are also important factors in causing
agglomeration and defluidization. Langston and Stephens (1960), Moseley and
O’Brien (1993), and Wank et al. (2001) have pointed out that the surface area of
the bed material; operating temperature; gas velocity; the density, size, surface area,
particle-size distribution, etc., of the bed particles; and other parameters are all
related to the occurrence of agglomeration and defluidization. Among these factors,
particle-size distribution has a particularly high impact on bed fluidization quality
and the conversion rate of chemical reactions, and also indirectly affects bed agglomeration and defluidization. Ray et al. (1987) and Pell (1990) have shown that the
bed-material particle-size distribution affects the minimum fluidization velocity,
terminal velocity, elutriation velocity, rates of chemical reactions, etc.; further,
particle-size distribution also affects the dynamic properties of fluidized beds.
Gauthier et al. (1999) indicated that a narrow range of particle diameters can be
employed to increase the operational stability of fluidized bed reactors by, for
example, reducing the occurrence of bed-material separation. A wide range of particle diameters can increase parameters such as mobility and chemical conversion
rates.
Although Na metal in the waste can cause bed agglomeration and defluidization, at high combustion temperatures, other metals such as Cd, Pb, and Cr may
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HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
813
evaporate and form metal vapor or particles, which may then be released into the
environment along with the exhaust gas or may get attached to fly ash or bottom
ash. The transport behavior of different heavy metals varies depending on their
physical properties and the forms of the compounds in which they are present. In
general, heavy-metal compounds with high volatility are mainly found in fly ash
or flue gases, while heavy-metal compounds with high boiling points are mainly
found in bottom ash or in the large particles of fly ash. Studies by Fournier et al.
(1991) and Reimann (1989) have also suggested that the distribution of heavy metals
during combustion is related to heavy-metal compound properties and boiling
points. Metals with high boiling points such as Cr are mainly found in bottom
ash, whereas volatile heavy metals such as Cd mainly form vapor and leave the incineration system with the exhaust gas.
Apart from the properties of the heavy metal, the operating conditions of
incinerators comprise another important set of factors that affect the distribution
of heavy metals. Studies by Hiraoka and Takeda (1980) and Gerstle and Albrinck
(1982) have indicated that combustion temperature can affect the heavy-metal distribution ratio in bottom ash. Increasing the temperature results in a reduction in the
zinc, lead, and cadmium contents in bottom ash, while increasing the arsenic, cadmium, mercury, zinc, and lead contents in the emitted exhaust gas. Wey et al.
(1996) pointed out that the lead, chromium, and cadmium contents in fluidized sand
beds decreased in the order Pb>Cr>Cd. The distribution of various heavy-metal
compounds is related to the properties of the heavy metal; it is also strongly related
to the combustion temperature, operating gas-flow rate, feed ingredient load, and the
compositions of other elements (such as oxygen, chlorine, and sulfur).
The emission of heavy-metal pollutants is related to the operating conditions.
Furthermore, changes in the particle-size distribution of the fluidized bed material
affect fluidization. Altered particle-size distributions may directly cause changes in
the fluidized bed operating parameters, while at the same time indirectly causing
changes to the heavy-metal pollutant distribution. Few previous studies on fluidized
bed incineration have reported the effects of bed particle-size distribution on
agglomeration=defluidization and heavy-metal pollutant emission. Therefore, in
the current study, artificial waste with different compositions to mimic conditions
that promote agglomeration is used to investigate the effects of three different
bed-material particle-size distributions (narrow, flat, and Gaussian) on the deposition of heavy metals in bottom ash during agglomeration and defluidization.
Experimental conditions include the presence or absence of Na, changes in Na
concentration, and the addition of Ca and other elements. The results of this work
can be used as a reference for the operation of fluidized bed combustion reactors.
EXPERIMENTAL
Apparatus
Figure 1 depicts the laboratory-scale fluidized bed incinerator used in these
experiments. The main furnace was a stainless steel pipe with a diameter of 0.09 m
(AISI 310) and a height of 1.2 m. The bottom of the incinerator was a porous plate
made of stainless steel, and the pore area was 15.2%. A temperature-feedback
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M.-H. WU ET AL.
Figure 1 The bubbling fluidized bed reactor. (1) PID controller, (2) blower, (3) flow meter, (4) thermocouple, (5) pressure transducer, (6) electric resistance, (7) sand bed, (8) feeder, (9) cyclone, (10) filter,
and (11) induced fan.
control system together with a thermocouple was used to control the system temperature. A cyclone dust collector was installed at the gas exhaust, which was connected to a bag-filter-type dust collector to control particulate pollutants.
Artificial Wastes
Since most wastes contain alkali group elements (such as Na), which may cause
agglomeration and defluidization during the fluidized bed combustion process,
elemental Na was added to the artificial waste during experiments to form
low-melting-point eutectic materials. In addition, Ca was added in some experiments
to examine its effect on agglomeration and on the heavy-metal distribution. The
heavy metals added to the waste were primarily chromium, lead, and cadmium.
The addition of these metals was accomplished by dissolving the metal nitrates in
deionized water, which was then added to the artificial waste. The artificial waste
comprised primarily wood sawdust (1.6 g) and polypropylene (PP) (0.35 g). The
wood sawdust was willow that was obtained from a sawmill. To this material,
1 mL of the heavy-metal-nitrate aqueous solution was added, and the resulting mixture was wrapped in a polyethylene (PE) plastic bag (0.29 g). Each bag of artificial
waste had a final weight of 3.24 g. The artificial waste was enclosed in a PE bag
and had a cylindrical shape with a diameter of 1.2 cm and a length of 3.0 cm. The
weight percentage of Pb, Cd, and Cr was 0.7% in each artificial waste bag (3.24 g).
The elemental analysis of the sawdust, polypropylene, and polyethylene was
conducted using an elemental analyzer (EA), and the results are listed in Table 2.
Table 3 shows the composition of the artificial waste and the experimental
conditions.
HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
815
Table 2 Elemental analysis of different wastes by weight
Species
C (%)
H (%)
N (%)
O (%)
Sawdust
Polypropylene (PP)
Polyethylene (PE)
43.12
86.16
85.71
5.80
12.20
13.04
5.01
1.12
0.86
46.07
0.52
0.39
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Bed Materials
The bed material used in the experiment was silica sand with a density of
2,600 kg=m3. The height of the bed material was 18 cm, approximately twice the
diameter of the incinerator (H=D ¼ 2), and the silica sand was sieved according to
the method of Gauthier et al. (1999) with an ASTM standard sieve. Taking the
desired particle-diameter ratios into consideration, three different particle-size distributions (PSD) were prepared while maintaining dsv for each at about 0.725 mm. The
value of dsv is calculated using the following equation, and Table 4 presents a
composite of the three PSD values.
1
dsv ¼ P xi
i
dpi
where xi is the ratio of weight (%) xi and dpi is the average diameter (mm).
Experimental Procedure
Before the experiment, the minimum fluidization velocity was measured
according to the method of Lin et al. (2002). According to the results, the minimum
Table 3 Operating conditions for each experiment
Type of powder
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Temp.
( C)
Gas velocity
(m=s)
Na conc.
(%)
Ca conc.
(%)
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
0.163
–
–
–
0.7
0.7
0.7
0.5
0.5
0.5
0.9
0.9
0.9
0.7
0.7
0.7
–
–
–
–
–
–
–
–
–
–
–
–
0.7
0.7
0.7
Narrow
Flat
Gaussian
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Defluidization
time (sec)
—
—
—
1,840
1,300
1,740
2,840
2,100
2,500
1,240
1,040
1,220
2,160
1,960
2,020
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M.-H. WU ET AL.
Table 4 Particle size distributions of different powder types
Type of powder
Narrow
Gaussian
Flat
Sieves no.
Weight (%) xi
100
8
25
35
23
9
17
17
19
23
24
30–20
45–35
35–25
25–20
20–18
18–16
45–35
35–25
25–20
20–18
18–16
dsv ¼ 725.0
Sieves (mm)
Average diameter di (mm)
600–850
355–500
500–710
710–850
850–1000
1000–1180
355–500
500–710
710–850
850–1000
1000–1180
725.0
427.5
605.0
780.0
925.0
1090.0
427.5
605.0
780.0
925.0
1090.0
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The dsv calculation formula is: dsv ¼ P1 xi .
i
dpi
fluidization velocities were different for different PSDs. The values of the minimum
fluidization velocity for the three PSDs at 800 C were 0.125 m=s (narrow), 0.126 m=s
(flat), and 0.113 m=s (Gaussian). The excess air was approximately 40% (62 L=min)
during the combustion process, and the operating temperature was maintained at
800 C. Operating conditions are shown in Table 3. When the sand bed was heated
to a stable preset temperature, the blower was turned on to establish airflow at
the desired rate. Air was preheated by passing it through a preheating chamber. Artificial waste was delivered to the combustion chamber through the feed inlet at a feed
rate of 3.24 g=(20 s). During experimental operation, in addition to the visual observation of defluidization, the pressure change between the sand bed and the freeboard
area was monitored as an indicator of the occurrence of defluidization using two
pressure probes connected to a pressure transmitter with a range of 0 to 1,000 mm
H2O. When defluidization occurred, the artificial waste feed was stopped and
allowed to cool down. The bed material was removed and analyzed with an
ASTM standard sieve to measure changes in the bed material particle-size distribution. The particle-size screening intervals were >1.180 mm, 1.180–1.000 mm, 1.000–
0.850 mm, 0.850–0.710 mm, 0.710–0.600 mm, 0.600–0.500 mm, 0.500–0.355 mm, and
<0.355 mm.
The total weight of the bed material for each particle diameter was recorded,
and samples were taken from each particle-size fraction to analyze the heavy-metal
concentrations. In order to analyze heavy-metal concentrations, solid samples were
first treated using microwave digestion to completely release heavy metals from the
particulate material. For digestion, the bottom ash sample (0.5 g), 9 mL concentrated
nitric acid, and 3 mL concentrated hydrofluoric acid were added to the vessel. The
oven was set to reach a temperature of 180 5 C over 10 min and was then left at
180 5 C for a duration of 10 min. The heavy-metal concentration of this digested
sample was then analyzed using an inductively coupled plasma spectrometer (ICP).
In addition, the agglomerated material was analyzed using scanning electron microscopy=energy dispersive spectrometry (SEM=EDS) to examine the agglomeration
status of the bed material particles.
HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
817
RESULTS AND DISCUSSION
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Changes in Bed Particle Distribution
During operation of the fluidized bed, collisions between bed particles and the
friction between the bed and the furnace wall cause a gradual decline in bed particle
size. Figure 2 shows the particle distribution after agglomeration=defluidization in
the presence of different additives. The figure shows that the distribution range of
bed particles widened. The widening was especially significant in the case of the narrow distribution. In the absence of Na, there was a large increase in the number of
particles smaller than 0.600 mm, suggesting a gradual decrease in particle size caused
by fluidization. Vaux and Fellers (1981) and Shamlou et al. (1990) have proposed
that thermal, chemical, static mechanical, and kinetic stresses during fluidization
all cause bed particle attrition, leading to a decrease in particle size. Moreover, the
inside of a combustion chamber is a high-temperature environment, and thermal
stress may therefore also have an impact on changes in particle size. Chirone et al.
(1985) and Lin and Wey (2003) pointed out that the thermal shock generated in
the incineration process within the thermal fluidized bed also causes a decrease in
particle size. Hence, the bed particle size tends to decrease after thermal fluidization.
When agglomeration occurs within the reactor, the changes in particle size are
more complicated. From Figure 2, we see that in addition to the general tendency
towards a decline in particle sizes, some large particles have a tendency to increase
in size. The resulting amount of large particles was greater when Na and Ca were
added simultaneously than when only Na was added, but the amount of large particles was higher in both these cases than when neither Na nor Ca were added. On
Figure 2 Particle size distribution after agglomeration=defluidization with the addition of different
additives.
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M.-H. WU ET AL.
the basis of these results, we speculate that low-melting eutectic compounds
are formed from Na and the impurities in the silica sand during agglomeration=
defluidization. These eutectic compounds have lower melting points and therefore
become viscous molten substances that adhere to the bed particles. If the kinetic
energy (separation force) of the bed particles themselves is not sufficient in order
to overcome the adherent forces of the liquid substance, the bed particles will
agglomerate (Lin et al., 2010b). Subsequently, the diameter of bed particles will
increase.
According to the literature, the presence of Ca generates high-melting-point
compounds and therefore increases the melting point of the overall resultant compounds and mixtures (Atakül et al., 2005). Lin and Wey (2004) and Lin et al.
(2009) reported a delayed effect of defluidization under different operating conditions with the addition of Ca. Therefore, the bed material remains fluidized for
a long time and contributes to an increased ratio of small particles owing to attrition.
However, Figure 2 shows that with the addition of Ca, the concentration of small
particles became lower than that corresponding to the other two operating conditions, whereas the ratio of large particles was higher than that found without Ca
addition. This suggests that in spite of the attrition effect, the bed particles do not
tend to decrease in particle size due to agglomeration.
Heavy-Metal Distribution in Bed Particles Under Different Operating
Conditions
In order to understand the rates of heavy-metal retention corresponding to different bed-particle sizes in the experiment, we collected bed particles after agglomeration=defluidization and analyzed them. We divided these particles into eight size
ranges and analyzed the concentrations of three heavy metals. Figure 3 shows the
heavy-metal concentration distributions for different particle-size fractions in the
bottom ash under different additive conditions. Figure 4 shows the heavy-metal concentration distributions for the different particle sizes in the bottom ash for different
Na concentrations. From the results shown in Figures 3 and 4, we find that heavymetal concentrations were highest in the largest and smallest particle-size fractions
measured. In general, when the particle size was smaller than 0.500 mm, the
heavy-metal concentration tended to increase, and when the particle size was larger
than 0.850 mm, the heavy-metal concentration also increased substantially.
Small particles are primarily generated from attrition during fluidized bed
operation. Small particles have a high relative surface area and thus readily adsorb
heavy metals. Therefore, the smaller the bed particles become, the greater the tendency for heavy metals to adsorb onto them, and thus the heavy-metal concentration
in the bed particles increases. However, the generation of large particles is caused by
low-melting-point eutectic compounds forming molten liquid materials during
high-temperature operation. The viscosity of this liquid material causes agglomeration of bed particles and therefore increases the bed particle size. Although the
surface area of these large bed particles is rather small, the heavy-metal concentration is still fairly high, suggesting that adsorption may not be the only mechanism
of heavy-metal capture by bed particles. Another possibility is that heavy metals
and Na may combine to form low-melting eutectic compounds. Alternatively,
819
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HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
Figure 3 Heavy metal concentration distribution with the addition of different additives.
incoming heavy metals may come into contact with and stick to or cover molten Nacontaining eutectic compounds formed under thermal melting conditions, and
thereby increase the heavy-metal concentrations in bed particles. Figure 5 shows a
SEM=EDS analysis of the surface properties of agglomerated bed particles. The
results show that in addition to Na, heavy metals (Cd, Pb, and Cr) also exist in
the agglomerate. This finding suggests that heavy metals may form eutectic
M.-H. WU ET AL.
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820
Figure 4 Heavy metal concentration distribution with the addition of different Na concentrations.
compounds with Na, or may stick to or cover the Na-containing liquid eutectic
material, thereby causing heavy-metal retention. Furthermore, a comparison of
the different additive conditions (Figure 3) shows that the heavy metal concentrations within small bed particles did not differ significantly between the no-additives,
Na-addition, and Na þCa-addition conditions. Thus, the adsorption ability of
small bed particles was not significantly different in the presence or absence of
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HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
821
Figure 5 FE-SEM=EDS analysis results of agglomerated material. (Figure is provided in color online.)
Na. However, for large bed particles, heavy-metal concentrations under the Na or
Na þCa addition conditions were much greater than the concentrations in the
absence of Na. This suggests that adsorption is not the main mechanism of
heavy-metal capture for large bed particles, but that the formation of low-meltingpoint eutectic heavy-metal=Na compounds or the capturing effect of liquid eutectic
materials may be more important mechanisms.
Figures 3 and 4 show that the heavy-metal concentration in bed particles is
generally lowest when no Na is added. After Na is added, heavy-metal concentrations in different-sized particles tend to increase. Although adding Na-containing
eutectic materials during the fluidization process produces particle agglomeration
and therefore increases the risk of defluidization, the increased heavy-metal concentration within bed particles (caused by the formation of low-melting eutectic compounds by heavy metals and Na, or by the adherence on contact of heavy metals
to molten Na-containing eutectic compounds liquefied under thermal melting
conditions) decreases the heavy-metal emission volume. Therefore, the large agglomerated bed particles have high heavy-metal concentrations.
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M.-H. WU ET AL.
Heavy-Metal Retention Rate of Bed Particles Under Different
Operating Conditions
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Figures 6 and 7 show heavy-metal retention rates corresponding to bed particles of different sizes under different operating conditions. These rates were calculated by dividing the total heavy-metal quantities retained by each particle-size
fraction by the total incoming metal quantity. These results show that in spite of
Figure 6 Heavy metal retention rate distribution with the addition of different additives.
823
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HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
Figure 7 Heavy metal retention rate distribution with the addition of different Na concentrations.
the high heavy-metal concentrations in the larger bed particles, the total quantities of
heavy metals they retained were lower than those of other particles owing to the low
abundance of large particles in the reactor bed. Meanwhile, mid-sized particles (ranging from 0.500–0.850 mm) retained more heavy metals owing to their higher abundance, although these particles had relatively low heavy-metal concentrations. The
abundance of small particles was lower than that of mid-sized particles but higher
than that of large particles, and small particles had high heavy-metal concentrations.
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M.-H. WU ET AL.
Figure 8 Heavy metal retention rate with the addition of different additives.
Therefore, the total amount of heavy metals retained by small particles was
relatively high.
Figures 8 and 9 show the ratio of total heavy-metal content in bed particles
to the total incoming heavy-metal content under different operating conditions.
The differences between the defluidization times in different experimental trials
indirectly causes differences between the total incoming amounts of heavy metals
Figure 9 Heavy metal retention rate with the addition of different Na concentrations.
HEAVY METAL DISTRIBUTION DURING DEFLUIDIZATION
825
corresponding to these trials. If we compare the total heavy-metal retention in the
silica sand with the total amount of incoming heavy metals, Cr generally has the
highest retention rate, followed by Pb and Cd. The three heavy metals Cd, Pb,
and Cr have high, intermediate, and low volatilities, respectively. Hence, Cr is the
heavy metal with the highest boiling point, and it also had the highest concentration
within the bed particles. According to Chen et al. (1997), when silica sand is used as
the bed, it adsorbs large amounts of heavy metals during operation. The adsorption
efficiencies observed in that study were in the order Cr>Pb>Cd, which followed the
order of the boiling points of the metals. Therefore, on the basis of both literature
studies and our current experiments, the relative rates of heavy-metal adsorption
onto sand are suggested to correspond to the relative boiling points of the heavy
metals.
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Heavy-Metal Distribution Within Bed Particle-Size Distributions
Figures 3 and 4 show heavy-metal concentration distributions for the three different particle-size fractions considered. In general, under the condition of a narrow
particle-size distribution, the heavy-metal concentrations for both large particles and
small particles were higher than those for particles under the flat or Gaussian distribution conditions, immediately followed by the Gaussian distribution with intermediate concentrations. The comparison of heavy-metal retention rates in bed
particles in Figures 8 and 9 shows that, under most conditions, heavy-metal retention
rates were higher for the narrow particle-size distribution than for the other two distributions studied, with the results of the Gaussian distribution being the second
highest. Gauthier et al. (1999) indicated that separation behavior generally occurred
in flat distribution fluidization. The proportion of main particles was greater than
that of extreme particles for the Gaussian distribution, such that the influence of
the extreme particles was not significant. Therefore, the Gaussian distribution behavior was similar to and tended to co-occur with the narrow distribution. Narrow and
Gaussian distributions showed better mixing results, and problems such as uneven
mixing or bed particle detention were less likely to occur in the case of these distributions; consequently, particle agglomeration was less probable in the case of these distributions. Meanwhile, the degree of mixing in the case of narrow and Gaussian
distributions was higher than that in case of flat distribution. Therefore, in the incineration process, bed particles with narrow and Gaussian distributions were more
likely to come into contact with the waste than particles with the flat distribution,
and this increased contact corresponds to increasing adsorption rates.
However, in some of the tests, the differences between the adsorption rates for
the three bed-particle distributions were not obvious. In the case of the flat distribution, since there were no significant differences in the amounts of differently sized
particles, the proportion of small particles was higher than that in the narrow and
Gaussian distributions. Although the fraction of small particles in the narrow and
Gaussian distributions showed a tendency to increase after the fluidization process,
the amount of small bed particles was still lower under these conditions than in the
flat distribution. The higher concentration of small bed particles in the flat distribution greatly enhanced the rate of heavy-metal adsorption. Therefore, despite the
lowest degree of bed fluidization and a decreased degree of contact between bed
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M.-H. WU ET AL.
particles and waste for the flat distribution, the increase in adsorption by small particles led to heavy-metal retention rates for this particle distribution that were not
significantly different from the other two distributions in some tests.
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CONCLUSIONS
In this study, we used simulated waste with different compositions to
investigate the impact of three different particle-size distributions (narrow, flat,
and Gaussian) on the heavy-metal distribution in bottom ash during defluidization.
The results showed that when no Na was added, the particle size gradually
decreased, owing to bed-material attrition and thermal shock during the incineration
process. However, when Na was added, low-melting eutectic compounds would form
a molten liquid material that then became sticky and caused bed agglomeration,
thereby increasing particle size.
Additionally, the large and small particles had the highest concentrations of
heavy metals. Generally, heavy-metal concentrations increased with decreasing size
when the particle size was less than 0.500 mm, and heavy-metal concentrations also
showed a dramatic increase when the particle size was greater than 0.850 mm. In the
large particle bed, the elevated heavy-metal concentrations resulting from Na or Na
þCa addition were much higher than those observed when no Na was added. These
results suggest that the capturing effects of the low-melting eutectic compounds generated from heavy metals and Na, or by the liquid eutectic material, are likely to be a
more important mechanism. The heavy-metal concentrations within large and small
particles and their total retention rates were higher for the narrow particle distribution than for the flat or Gaussian particle distributions, with the Gaussian distribution having the second highest retention rates. Since mixing effects were greater
with narrow and Gaussian distributions, the opportunity for bed-waste contact during the incineration process was greater for the narrow and Gaussian distributions
than for the flat distribution, and therefore resulted in increased retention rates.
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