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Effect of coal blending on the leaching characteristics of arsenic in fly ash from fluidized bed coal combustion
Fuel Processing Technology 106 (2013) 769–775
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Effect of coal blending on the leaching characteristics of arsenic in fly ash from fluidized bed coal combustion Facun Jiao a, b,⁎, Yoshihiko Ninomiya b, Lian Zhang c, Naoomo Yamada b, Atsushi Sato b, Zhongbing Dong a, d a
National Center of Coal Chemical Products Quality Supervision & Inspection, Huainan, Anhui 232001, PR China Department of Applied Chemistry, Chubu University, Kasugai, Aichi 487-8501, Japan Department of Chemical Engineering, Monash University, PO Box 36, Clayton Campus, Victoria 3800, Australia d School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China b c
a r t i c l e
i n f o
Article history: Received 23 August 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 Available online 19 November 2012 Keywords: Arsenic Leaching behavior Coal blending Fluidized bed
a b s t r a c t The leaching characteristics of arsenic (As) in fly ash collected from lab-scale fluidized bed reactor have been systematically investigated through the combustion of two bituminous coals (A and B) and their mixture with different blending ratio. Leaching tests were conducted according to Japanese Industrial Standard (JIS).The results indicate that, the fly ash derived from the combustion of coal B, which contains abundant calcium, shows a larger capture ability for arsenic vapor than that from coal A, due to the chemical reaction of arsenic with CaO. This reaction is however competed by the sulfation of CaO at coal combustion temperature, therefore, a nonlinear increase was observed with increasing the blending ratios of high-calcium coal B with coal A. Leaching performance of arsenic from fly ash is largely dependent on the finally pH of the leachate. CaO in fly ash preferentially generates a high-pH leachate during leaching test and subsequently promotes the combination of calcium with arsenic to form precipitate. Improving Ca/S ratio through the combustion of blending coal is a promising method to prevent the emission of arsenic into ambient and reduce its leachability from fly ash. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Arsenic (As) released from coal combustion is of environmental concern, due to its toxic and potential carcinogenic propensities [1]. It undergoes an initial vaporization in flame zone during coal combustion and then retains in fly ash through complex chemical/physical reactions and nucleation/condensation upon flue gas cooling [2]. Moreover, since of its high volatility, a portion of the arsenic can escape the air pollutant control system as vapor, the release of which to air is harmful to human and the nature [3]. Even for the arsenic retained in fly ash, the subsequent treatment of fly ash through landfill can cause the leaching of arsenic into the waters. With the increase in awareness of the public in environment impact of the combustion of coal, it is vital to control the emission of arsenic during coal combustion and its leaching during fly ash treatment. It has been reported that the mode of occurrence of arsenic in a coal exists as pyritic, organic and arsenate [4]. Regardless of its species in coal, at the temperature of coal combustion, arsenic preferentially
⁎ Corresponding author at: National Center of Coal Chemical Products Quality Supervision & Inspection, Huainan, Anhui 232001, PR China. Tel.: +86 554 2694896; fax: 86 554 2694896. E-mail address: [email protected] (F. Jiao). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.10.015
vaporized as oxides/sub-oxides, which will finally distribute between fly ash particles and gas phase as flue gas temperature decreases. This partitioning is dependent on a number of factors such as the initial concentration of arsenic in parent coal, combustion conditions and ash composition [5]. All the arsenic in fly ash appears to be present as arsenate, possibly combined with alumino-silicate slag and calcium ortho-arsenate, as determined by X-ray absorption fine structure (XAFS) spectroscopy [6–8]. Consequently, the emission of arsenic during coal combustion is frequently controlled by the use of sorbent such as calcium oxide, calcium hydroxide and calcium silicate. Lime has proven to possess the highest capture ability to arsenic vapor at a moderate temperature of around 600–1000 °C than the other sorbents [9–11]. This process is attributed to the dominant mechanism of irreversible chemical reaction rather than physical adsorption [12]. In light of this, Adjusting ash composition especially for Ca content in fly ash via the combustion of blending coal, a ubiquitous technique adopted by coal-fired power plants, is one of the possible and practicable methods to improve the retention ability of fly ash to arsenic vapor. In general, the fly ash derived from coal combustion is subjected to disposal and reuse. The leaching of arsenic from fly ash will cause potential hazard on aquifer systems and soil, which thus causes a critical environmental problem. The leaching of arsenic from fly ash during fly ash–water interaction is dependent on many factors, e.g. the phases associated with arsenic in fly ash, pH of leachate, ash
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properties (e.g. acidic or alkaline) and the leaching environment [13–16]. Arsenic combined with soluble phase is readily released at a much higher rate. The increase in the pH of leachate exhibits an inhibiting effect on the release of arsenic during leaching test [17]. The pH of ash–water system highly lies on the concentration of alkaline earth elements such as Ca and Mg in ash and the proportion of potentially acid-deriving SO3 [13,18]. The presence of abundant calcium probably induces secondary precipitation reactions, and in turn limits arsenic mobility. In addition, the temperature of leaching system also affects the leaching of arsenic from fly ash into water resources [14,15]. At the temperatures beyond 30 °C, arsenic in fly ash can be stabilized completely, regardless of the pH of the system. Based on above literature review, the leaching behavior of arsenic is obviously influenced by the ash chemical properties. Therefore, combustion of blending coal has potential to result in the shift of ash chemical composition and is expected to in turn alter the leaching behavior of arsenic. Such a hypothesis needs to be confirmed by experimental observation. The present paper aims to examine the effect of coal blend combustion on the retention of arsenic in fly ash and its leaching characteristics. For this purpose, the combustion of two Australian bituminous coals and their mixtures has been first carried out in a lab-scale fluidized bed reactor. The resulting fly ash was then subjected to a leaching test. Arsenic concentration in parent coals and fly ash, as well as in leaching solution from leaching test was determined using hydrated generation inductively coupled plasma optical emission spectroscopy (HG-ICP-OES). Energy dispersive X-ray spectroscopy (EDX) mapping was adopted to describe the distribution of arsenic in fly ash particulates. The modes of occurrence of arsenic in fly ash and its leaching mechanisms were clarified systematically with the aid of theoretical prediction. The new knowledge is expected to help optimize the coal combustion process to reduce the emission of arsenic into ambient and its leachability from fly ash.
2. Experimental 2.1. Coal samples Two Australian bituminous coals, namely coals A and B hereafter, were selected for combustion experiments. The coal samples were air-dried and sieved to a size range of 106–300 μm prior to use. Table 1 shows the properties of coal samples used in this experiment. Ash content of the two coal samples is 6.4% for coal A and 9.5% for coal B, respectively. The composition of inorganic elements detected using X-ray fluorescence (XRF) in two coal samples with the size bin of 106–300 μm is quite different. As shown in Table 2, calcium content in coal B is 20.7% whereas only 0.5% calcium is in coal A. The analysis using computer controlled scanning electron microscope (CCSEM) suggests that the predominant species of calcium in coal B are calcite,
Table 2 Composition of inorganic elements in raw coal (wt.%).
SiO2 Al2O3 CaO Fe2O3 K2O MgO TiO2 SO3 As, mg/kg-coal
A
B
36.1 23.9 0.5 8.7 1.0 – 7.4 20.7 10.1
38.3 14.5 20.7 10.4 2.3 1.97 – 11.5 8.6
dolomite and Ca–Al-silicate, as shown in Table 3. Arsenic concentration in coals A and B is10.1 mg/kg-coal and 8.6 mg/kg-coal, respectively. 2.2. Characterization of samples The concentration of arsenic in fly ash was analyzed using HGICP-OES with the assistance of acid digestion. Briefly, about 0.1 g ash sample was weighted and moved into PTFE bottle. A microwave digester was adopted to digest ash sample using the acid of HNO3 + HF (9:3 v/v) by a thermal program. After cooling, the residue was dissolved and diluted to 50 mL using 1 mol/L HCl. CCSEM analysis (JEOL JSM5600 SEM coupled with an Exford EDAX detector) was carried out to characterize mineral species in raw coal. Around 3000 individual particles were analyzed for each sample under three magnifications covering a particle size range of 0.5– 211.0 μm. The modes of occurrence of the elements of interest were determined with the use of a variety of classification categories [19]. Prior to analysis, coal sample was first grinded to less than 200 μm and then mixed with resin to make a pellet. The detailed procedure of sample preparation for CCSEM analysis has been described elsewhere [20]. Speciation of Ca in fly ash samples was determined using X-ray photoelectron spectroscopy (XPS, Shimadzu ESCA-3300KM) at Ca2p.C 1s at 284.0 eV was used for peak shift calibration. Major elemental compositions in coal were quantified using X-ray fluorescence (XRF, Rigaku 2100). For XRF analysis of a certain sample, a few amount of it (approximately 500 mg) was first mixed with the similar amount of resin. The resulting mixture was subsequently poured into an Al-made O-ring with an outer diameter of 10 mm, and pelletized using a compressor. Prior to analysis, a standard reference coal ash sample (SRM 1632c) was used to validate XRF results. 2.3. Combustion facility Combustion experiments were conducted in a lab-scale fluidized bed reactor, the schematic of which is shown in Fig. 1. This reactor was made of a quartz tube with a length of 1000 mm and an inner diameter of 85 mm. Coal sample at a feeding rate of around 0.5 g/min through a piezo feeder was pneumatically conveyed by 2 L/min
Table 1 Properties of coal samples tested here. Coal A
Coal B
Proximate analysis, air-dried, wt.% Moisture Ash Volatile matter Fixed carbon
6.7 6.4 29.0 57.9
3.7 9.5 34.2 52.7
Ultimate analysis, daf, wt.% C H N S O (by difference)
71.8 3.45 1.64 0.71 22.4
74.1 3.15 1.76 0.50 20.5
Table 3 Mineral species in raw coal quantified by CCSEM. Category
A
B
Quartz Calcite Dolomite Ca–Al-silicate Kaolinite Montmorillonite Fe–S Iron oxide Unknown
15.31 0.07 0.01 0.09 69.4 1.05 1.03 0.9 12.1
22.14 7.40 2.68 7.17 45.60 1.80 1.62 1.77 9.82
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the combustion of coal A mixed with different amount of Ca(OH)2 (analytical reagent, purity > 98%) to match the Ca/S mole ratio of 1 and 3 was also carried out. Here, the Ca/S mole ratio was defined as
Ca=Sðmol=molÞ ¼
Ca in additives : S in coal
ð2Þ
2.4. Leaching test
Fig 1. Schematic of fluidized bed reactor.
secondary gas through an injector installed at the top of the reactor. The primary gas of 6 L/min was introduced from the bottom for coal combustion and bed material fluidization (quartz sand with a diameter of 300–500 μm). Around 120 g coal sample was fed for each run. The furnace temperature was kept at 860 °C and residence time was ~ 4 s, comparable with industrial fluidized bed boiler. The resulting ash was entrained by flue gas out of the reactor and subsequently separated by two cyclones. The tail gas composition including O2, CO2, SO2, NOx and CO was analyzed on-line by a gas analyzer on the real time basis. The ash collected from two series of cyclones was mixed together (termed as fly ash hereafter) and stored in fridge prior to analysis. Three replicas were made for each condition to ensure a satisfactory accuracy and reproducibility for ash yields throughout this study. Apart from two coal samples, the combustion of their mixture with five different blending ratios (A:B, wt/wt = 87.5:12.5, 75:25, 62.5:37.5, 50:50, 25:75) were also carried out. Variation of ash yield and arsenic concentration in collected fly ash under all conditions was shown in Table 4. In these experiments, the fly ash yields, defined as the weight ratio of the fly ash collected in cyclone to the total ash fed with coal, vary in the range of 67.5%–72.2%. Regarding the arsenic content in fly ash, adding 50% of coal B in coal blends causes the maximum arsenic concentration in fly ash, 152.4 mg/kg, reflecting significant effects of blending coal combustion on the fate of arsenic. To quantify the retention extent of arsenic in fly ash, the retention ability was evaluated by an enrichment factor (EF), which was defined as follows.
EF ¼
C M−in−ash ash content in % 100 C M−in−coal
ð1Þ
Leaching behavior of arsenic in fly ash was quantified using leaching test according to Japanese Industrial Standard (JISK0102-61.2). Relative to the leaching method suggested by American society of testing and materials (ASTM), toxicity characteristic leaching procedure (TCLP) and European Committee for Standardization (CEN) [21], The liquid– solid ratio, initial pH and leaching time proposed in JIS method are strictly monitored to mimic the natural circumstance of land fill of solid waste in Japan. Regarding the leaching procedure used in this work, fly ash of 3 g was mixed with 30 mL preconditioned deionized water (pH = 6.3)and the slurry was stirred for 6 h at room temperature. Filtration was performed to separate the fly ash using a filter (Advantec No.5C). The concentration of arsenic in filtrate was measured by HGICP-OES. 2.5. Thermodynamic equilibrium calculation The thermodynamic equilibrium software, FactSage 6.2, was employed to theoretically predict the possible As-bearing compounds in fly ash and its distribution in water solution under different pH during leaching test. The elemental compositions of ash were used as calculation input. The databases used include ELEM, FACT and Fact53. Aqueous and solid species are chosen as output. 3. Results and discussion 3.1. Coal combustion To monitor coal combustion performance in fluidized bed reactor, flue gas composition was analyzed online by a gas analyzer. Fig. 2 shows the composition of flue gas produced from the combustion of coal A. The lines of all gas concentration as a function of experimental time are almost flat, suggestive of a stable combustion of coal in the period when ash sampling was conducted. O2 concentration in flue gas remained at approximately 11% to achieve a carbon conversion yield > 90%. SO2 concentration in flue gas derived from the combustion of coal A is approximate 150 ppm whereas its concentration in flue gas during coal B combustion is nearly zero since of higher calcium content relative to coal A.
where, CM-in-ash denotes the concentration of arsenic in fly ash; CM-in-coal denotes the concentration of arsenic in coal. Moreover, to confirm the role of free calcium on the fate of arsenic during coal combustion,
35 30
Ash yield, wt.% (Aver. ± STDa)
Arsenic in ash, mg/kg (Aver. ± STD)
0 12.5 25 37.5 50 75 100
6.4 6.5 6.8 7.2 7.6 8.4 9.5
72.2 ± 1.0 70.4 ± 2.5 71.9 ± 2.4 68.7 ± 1.1 71.0 ± 0.9 67.5 ± 2.2 72.0 ± 1.3
104.2 ± 4.0 112.2 ± 5.1 116.7 ± 3.7 143.9 ± 4.3 152.4 ± 4.7 139.2 ± 3.1 119.5 ± 5.5
a
Standard deviation.
O2/CO2,vol%
Ash content, wt.%
CO 20
10 O2
15
1
10 5
CO2
SO2/NO/CO, ppm
Ratio of coal B in coal blends, wt.%
100
SO2
25 Table 4 Variation of ash yield and arsenic in ash with experimental condition.
1000
NO
0.1
0 0
20
40 Time, min
60
80
Fig. 2. Flue gas composition as a function of experimental time during combustion of coal A.
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3.2. Retention of arsenic in fly ash Fig. 3 demonstrates the EF of arsenic in fly ash derived from the combustion of coals A and B as well as their mixture with different blending ratio. As can be seen, for single coal combustion, coal B shows a higher EF of 1.3, relative to 0.65 in coal A, indicating a higher retention of arsenic in fly ash generated from the combustion of coal B. Regarding the combustion of coal blends, the EF of arsenic in the resulting fly ash falls between the results for two single coals, as expected. Nevertheless, the variation trend of EF with blending ratio of coal B is quite interesting. The EF of blending coal show a rapid increase when the blending ratio of coal B is between 30% and 50%, which, however, was smoothly increase as the blend fraction of coal B moves out of above range, suggestive of more complex interaction of arsenic with fly ash during blending coal combustion. In order to compare the impacts of coal blending, the theoretical values of EF under different blending ratio, based on an assumption of no additional reactions between the two fly ashes during blending coal combustion, are also depicted as dashed line in Fig. 3. The experimental observation is not in line with their corresponding theoretical values, indicating a nonlinear effect of coal blending on the retention of arsenic in fly ash. Thermodynamic equilibrium calculation was conducted to predict the formation of arsenic-bearing species during coal combustion. As demonstrated in Fig. 4, it indicates that calcium is the most influential factor on arsenic transformation. The formation of Ca3(AsO4)2 affects the partitioning of arsenic between solid phase and gas phase. Calcium is beneficial to associate with arsenic during coal combustion and in turn reduce the emission of arsenic vapor. This is consistent with the experimental observation that the retention of arsenic in coal B ash is higher than that in coal A ash since of higher calcium content in coal B (see Table 2). EDX mapping result also confirmed the combination of arsenic with calcium. Two particles marked A and B with quite different chemical composition are shown in Fig. 5. As can be seen, arsenic is potentially enriched in the particle B with calcium content of around 48% according to EDX analysis. In contrast, its abundance in particle A, dominated by Si and Al, is lower through the comparison of the characteristic signal, indicative of a preferential chemical association of arsenic with calcium. Although the aluminumsilicates in fly ash produced from the decomposition of clay minerals (e.g. kaolinite), to some extent, can capture arsenic vapor, its influence is insignificant in comparison to calcium [10]. Here, it should be also noted that the effect of physical adsorption on the retention of arsenic in fly ash can be neglected [3,12]. As the retention ability of different calcium compounds for arsenic vapors is diverse [10–12], calcium-bearing compounds in fly ash were detected and quantified using XPS Ca (2p). As shown in Fig. 6, four calcium species, CaCO3, CaO, Ca-silicate and CaSO4, are detectable.
1.4 1.3 1.2
EF,-
1.1 1.0 0.9 0.8
Experimental Calculation
0.7 0.6 0
20 40 60 80 100 Ratio of coal B in blending coal, wt%
Fig. 3. EF of arsenic in the fly ash collected from single coal and blending coal combustion. Calculation was conducted based on the assumption that no extra reaction between the two kinds of fly ashes during blending coal combustion.
Among of them, CaO possesses the highest reaction activity with arsenic vapor through Eq. (3) [11,22]. CaSO4 in the fly ash is produced from the sulfation of CaO with SO2 in terms of Eq. (4). 3CaO þ
1
. 2
As4 O6 ðg Þ þ O2 →Ca3 ðAsO4 Þ2
CaO þ SO2 þ
1
. 2
O2 →CaSO4 :
ð3Þ ð4Þ
Clearly, CaO, derived from the decomposition of calcite (CaCO3) during coal combustion, was subjected to the reaction of sulfur and arsenic vapor at high temperature. In terms of above mechanisms, it is undoubted that the competition between reactions (3) and (4) affects the capture ability of CaO to arsenic vapor. Although Ca3(AsO4)2, once formed, is a stable compound at the temperature up to 1400 °C [10], its formation rate is slower than that of the sulfation of CaO under the same conditions over a temperature range of 600–1000 °C [11]. Consequently, CaSO4 are preferentially formed and in turn lead to extensive pore plugging/ blocking of the active site of CaO and retard the reaction with gaseous arsenic. Based on the above discussion, the retention ability of fly ash to arsenic vapor is greatly dependent on the available CaO content in fly ash. Quantification of CaO in fly ash from the combustion of blending coals was shown in Fig. 7. Increase in the blending ratio of coal B, CaO content in fly ash shows an initially slow increase and then exhibits a rapid increase once the blending ration of coal B over 30%. Such observation is related to SO2 concentration in flue gas which was nearly close to zero when more than 30% of coal B was added into blending coal, as shown in Fig. 7, further substantiating the faster reaction rate of Eq. (4) than Eq. (3). Once the blending ratio of coal B exceeds 30%, the excess CaO in fly ash has the potential to react with arsenic vapor, resulting in a rapid increase in the retention ability of fly ash to arsenic. However, as the blending ratio of coal B was above 50%, the EF in Fig. 3 shows a marginal increase as their error bars overlap greatly. A plausible reason is that the arsenic vapor was entirely captured by fly ash when more than 50% of coal B was added in the coal blends, which was confirmed by the experimental result that the total amount of arsenic in fly ash is nearly close to that fed with coal. 3.3. Leaching behavior of arsenic in fly ash According to the above results, the retention ability of fly ash to arsenic vapor was promoted via the combustion of blending coal. The subsequent treatment of the resulting fly ash through landfilling will cause secondary pollution due to relative high concentration of arsenic in fly ash and its mobilized propensity. Clarifying the leaching characteristics of arsenic in such fly ash generated from the combustion of blending coal is highly significant. Fig. 8 shows the leaching ratio of arsenic in the fly ash collected from the combustion of single coal and mixed coal. Here the leaching ratio was defined as the percentage of total mass of arsenic in leachate to its mass in fly ash. Clearly, the leaching ratio of arsenic in the fly ash from combustion of coal B is much lower than that in coal A. Regarding blending coal combustion-derived fly ash, the leaching ratio of arsenic was reduced with the addition of coal B to coal blend, as expected. Here again, the theoretical values under different blending ratio are also displayed for comparison. The leaching ratio of arsenic in blending coal is quite lower than calculation results, indicating that the leaching of arsenic from fly ash is reduced via the combustion of blending coal. The leaching ratio of arsenic as a function of final pH of leachate was demonstrated in Fig. 9. It obviously indicates that the leachability of arsenic is largely dependent on the pH of leachate at a wide pH range, which was further affected by the blending ratio
F. Jiao et al. / Fuel Processing Technology 106 (2013) 769–775
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120
120
Ca3(AsO4)2(s) 100
80
Mole fraction, %
Mole fraction, %
100
As2O5(s)
60
As4O6(g)
40
Coal A
20
Ca3(AsO4)2(s)
80 60 40 20
0
Coal B
0 0
200
400 600 Temperature, °C
800
1000
0
200
400 600 Temperature, °C
800
1000
Fig. 4. Prediction of As-bearing compounds during the combustion of coals A and B. Arsenic preferentially combines with calcium to form Ca–As compound.
Fig. 5. SEM picture and EDX mapping of arsenic in fly ash collected from blending coal combustion (50/50, wt/wt). The characteristic signal of arsenic in particle B containing abundant calcium is higher than that in particle A.
of coal B. Increase in the fraction of coal B in coal samples caused the increase of pH of leachate and in turn reduced the leaching ratio of arsenic. For instance, leaching ratio of arsenic decreased from 57% under the pH of 3.8 to 0.05% under the pH of 12.8. To better understand the leaching mechanisms of arsenic from fly ash, the possible chemical species of arsenic in solution were predicted by a thermodynamic equilibrium model, as depicted in Fig. 9. Apparently, the variation trend of leaching ratio of arsenic against the pH of leachate was consistent with the experimental observations. Regarding the concentration of arsenic in solution, the leaching ratio of arsenic in solution determined experimentally
is far lower than that from prediction. For instance, at the pH of 3.8, the arsenic leaching ratio of 57% was achieved, compared to about 100% as predicted. A possible reason is the kinetic control during leaching test due to a short leaching duration tested here [23]. The prediction of arsenic species in solution shift with pH value is also shown as dot line. Three types of arsenic species, H2AsO4−, HAsO42 − and AsO43 −, in turn appeared along with the increase of pH, which is consistent with the fact that the arsenic acid possesses three acidity constants of 2.26, 6.76 and 11.29, respectively. As the pH above 11, AsO43 − was the dominating species, which can form precipitates with Ca [24], as indicated in Fig. 9.
Fig. 6. Ca (2p) XPS of the fly ash collected from the combustion of blending coal. The ratio of coal B in coal blends in panel (a) and (b) are 50% and 12.5%, respectively. The major calcium-bearing compounds are CaCO3, CaO, Ca-silicate and CaSO4.
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6 160 CaO SO2
4
120
3
80
2 40 1 0 100
0 0
SO2 concentration, ppm
CaO in fly ash, %
5
20 40 60 80 Ratio of coal B in blending coal, wt%
Fig. 7. Relationship of CaO in fly ash with SO2 concentration in flue gas.
100
Leaching ratio, %
A 10
1
0.1
Experimental Calculation
B
0.01 0
20 40 60 80 100 Ratio of coal B in blending coal, wt.%
Fig. 8. Leaching ratio of arsenic in fly ash collected from single coal and blending coal combustion. Increase in the ratio of coal B in coal blends causes the leaching ratio of arsenic to reduce.
Based on the above discussion, it is clear that the final pH of leachate is influential on the leaching of arsenic from fly ash. Acidic leachate produced from acidic fly ash leaching test promotes the release of arsenic. Extra leaching test of coal A fly ash, a typical acidic fly ash, was carried out with different leaching time to investigate the leaching progress of arsenic. As shown in Fig. 10(a), the leaching ratio of arsenic increased with increasing pH of leachate, as expected. Conversely, alkaline leachate derived from alkaline fly ash leaching has suppressing effect on the mobilization of arsenic. There are two possible mechanisms governing the leaching progress of arsenic during alkaline fly ash leaching test. One is that the solubility of As-bearing compounds
1000
Total As (aq)
Leaching ratio, %
H2AsO4- (aq)
1
HAsO42- (aq)
0.1
AsO43- (aq) 0.01 2
4
6
4. Conclusions The combustion of two Australian bituminous coals and their mixture with different blending ratio was carried out in a lab-scale fluidized bed reactor. Apart from the effect of blending coal on the retention of arsenic in fly ash, its leaching characteristics from fly ash were also examined systematically. The main conclusions were drawn as follows.
Ca3(AsO4)2(s)
100 10
in fly ash under alkaline condition is lower than that in acidic leachate. Another one is that the As-bearing compounds in fly ash was subjected to initial dissolution and then combined with calcium through secondary reaction with the increase of pH of leachate. To confirm which route is dominant in the leaching progress of arsenic, extra leaching test of coal B fly ash, a typical alkaline fly ash, was conducted with different leaching time. As shown in Fig. 10(b), the leaching ratio of arsenic shows a drastic increase in the short term (less than 2 min) and then decreases with leaching time. Such observation is a clear sign that the latter mechanism is the major route for the leaching of arsenic. Abundant of calcium in coal B fly ash preferentially associates with arsenic to form solid compound and thus reduce the leaching of arsenic. Calcium in fly ash seemingly still plays an important role on the leaching of arsenic, especially for free CaO, which possesses a higher solubility during leaching test than other species such as CaSO4 and Ca-silicate [25]. To further verify such a hypothesis, combustion of coal A mixed with Ca(OH)2 for matching the Ca/S mole ratio of 1 and 3 was carried out. The resulting fly ash was also subjected to leaching test. As shown in Fig. 11, leaching ratio of arsenic in the fly ash decreased with the increase of Ca/S. Through comparing the leaching ratio of arsenic in the fly ash collected from the combustion of blending coal and Ca(OH)2 addition, as tabulated in Table 5, the effects of different calcium compounds on the leaching of As vary noticeably. As shown in the second row from below in Table 5, although the calcium content in the coal with adding Ca(OH)2 is comparable with that in blending coal, the leaching ratio of arsenic is quite lower. This observation clearly indicates free CaO in coal is favorable for reducing the leachability of arsenic. Another direct evidence to confirm the role of free CaO on the leaching of arsenic is attained form the experimental observation of the combustion of coal A mixed with CaSO4 (Ca/S =3). The leaching ratio of arsenic in the resulting fly ash is greatly higher than that in the fly ash from combustion of coal A mixed with Ca(OH)2 (Ca/S = 3). CaO is the most important Ca-containing species both in arsenic vapor capture during coal combustion and prevention of its leaching from fly ash. In fluidized bed coal combustion, limestone is usually employed as desulfurization agent. Through adjusting Ca/S to increase the CaO content in fly ash can efficiency inhibit the leaching of arsenic. On the other hand, although this work was carried out in a fluidized bed reactor, it also can shed new lights into reducing the leaching of arsenic in fly ash from pulverized coal combustion. Combustion of blending coal is a promising method to shift the ash properties and in turn alters the leaching behavior of arsenic.
8 pH,-
10
12
14
Fig. 9. Comparison of experimental results with prediction. Black round point denotes experimental results, line denotes predicted results. The variation of experimental results is in line with that of prediction.
1. CaO content in parent coal affects the retention of arsenic in fly ash through the chemical reactions of arsenic vapor with CaO. This reaction is largely influenced by the sulfation of CaO during coal combustion temperature. Combustion of 50% coal A mixed with 50% coal B caused a significant reduction on the emission of arsenic, in comparison with the combustion of single coal A. 2. Ash property is influential on the leaching behavior of arsenic from fly ash. Acidic leachate favors the leaching of arsenic from fly ash. Conversely, alkaline leachate prevents the leaching of arsenic. 3. Abundance of free CaO in coal B-derived fly ash generates an alkaline leachate during leaching test and successively reduces arsenic leaching since the precipitate was preferentially occurred via the reaction of arsenic with calcium at high pH leachate. Adjusting Ca/S in feedstock via coal blending is a promising method to shift the ash properties and in turn alters the leaching behavior of arsenic.
F. Jiao et al. / Fuel Processing Technology 106 (2013) 769–775
80
(a)
100
8
(b)
Coal B 7 6
Leaching ratio pH
5 20
13
10 Leaching ratio
11 10
pH
pH,-
40
14
12 Leaching ratio, %
60
pH,-
Leaching ratio, %
Coal A
775
9
1
8
4
7 0 0
10
20
30 40 50 Time, min
3 180 240 300
0.1 0
10
20
30 180 Time, min
240
6 300
Fig. 10. Leaching ratio of arsenic and pH of leachate as a function of leaching time. In alkaline condition, arsenic-bearing compounds are subjected to an initial dissolution and then combine with calcium to form solid phase with the increase of pH.
100
Ca/S=0 Leaching ratio, %
10
1
Ca/S=1 0.1
Ca/S=3 0.01 2
4
6
8 pH,-
10
12
14
Fig. 11. Leaching of arsenic in the fly ash collected from the combustion of coal A mixed with Ca(OH)2.
Table 5 Comparison of leachability of arsenic in the fly ash collected from the combustion of blending coal and Ca(OH)2 addition. Blending coal
Ca(OH)2 addition
Ca content in coal, wt.%
Leaching ratio,%
Ca content in coal, wt.%
Leaching ratio,%
0.03 (A) 0.77 (A:B, 62.5/37.5) 2.0 (B)
57.5 8.4 0.05
0.03 (Ca/S = 0) 0.72 (Ca/S = 1) 2.1 (Ca/S = 3)
57.5 0.06 0.02
Acknowledgment The financial support from Grant-in-aid for Scientific Research on Priority Areas (B), 20310048, Ministry of Education, Science, Sports and Technology in Japan was gratefully acknowledged. References [1] P. Shah, V. Strezov, C. Stevanov, P.F. Nelson, Speciation of arsenic and selenium in coal combustion products, Energy & Fuels 21 (2007) 506–512. [2] C.L. Senior, J.J. Helble, Emissions of mercury, trace elements, and fine particles from stationary combustion sources, Fuel Processing Technology 65–66 (2000) 263–288. [3] X. Gou, C.-G. Zheng, M.-H. Xu, Characterization of arsenic emission from a coal-fired power plant, Energy & Fuels 18 (2004) 1822–1826. [4] Y. YaE, M.P. Ketris, Arsenic in coal: a review, International Journal of Coal Geology 61 (2005) 141–196. [5] M.L. Contreras, J.M. Arostegui, L. Armesto, Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations, Fuel 88 (2009) 539–546.
[6] G.P. Huffman, F.E. Huggins, N. Shah, J. Zhao, Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy, Fuel Processing Technology 39 (1994) 47–62. [7] F.E. Huggins, N. Shah, G.P. Huffman, J.D. Robertson, XAFS spectroscopic characterization of elements in combustion ash and fine particulate matter, Fuel Processing Technology 65–66 (2000) 203–218. [8] F.E. Huggins, G.P. Huffman, X-ray absorption fine structure (XAFS) spectroscopic characterization of emission from combustion of fossil fuels, International Journal of the Society of Materials Engineering for Resources 10 (2002) 1–13. [9] B.K. Gullett, K. Raghunathan, Reduction of coal-based metal emission by furnace sorbent injection, Energy & Fuels 8 (1994) 1068–1076. [10] S. Mahuli, R. Agnihotri, S. Chauk, A. Ghosh-Dastidar, L.-S. Fan, Mechanism of arsenic sorption by hydrated lime, Environmental Science and Technology 31 (1997) 3226–3231. [11] R.O. Sterling, J.J. Helble, Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates, Chemosphere 51 (2003) 1111–1119. [12] Y. Li, H. Tong, Y. Zhou, Y. Li, X. Xu, Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture, Environmental Science and Technology 41 (2007) 2894–2900. [13] J. Jankowski, C.R. Ward, D. French, S. Groves, Mobility of trace elements from selected Australian fly ashes and its potential impact on aquatic ecosystems, Fuel 85 (2006) 243–256. [14] A. Baba, G. Gurdal, F. Sengunalp, O. Ozay, Effect of leachant temperature and pH on leachability of metals from fly ash. A case study: can thermal power plant, province of Canakkale, Turkey, Environmental Monitoring and Assessment 139 (2008) 287–298. [15] A. Baba, G. Gurdal, F. Sengunalp, Leaching characteristics of fly ash from fluidized bed combustion thermal power plant: case study: can (Canakkale — Turkey), Fuel Processing Technology 91 (2010) 1073–1080. [16] A.G. Kim, The effect of alkalinity of Class F PC fly ash on metal release, Fuel 85 (2006) 1403–1410. [17] E.E. Van Der Hoek, P.A. Bonouvire, R.N.J. Comans, Sorption of As and Se on mineral components of fly ash: relevance for leaching processes, Applied Geochemistry 9 (1994) 403–412. [18] C.O. Plank, D.C. Martens, Boron availability as influenced by application of fly ash to soil, Soil Science Society of America Proceedings 38 (1974) 974–977. [19] L. Zhang, F. Jiao, E. Binner, S. Bhattacharya, Y. Ninomiya, C.-Z. Li, Experimental investigation of the combustion of bituminous coal in air and O2/CO2 mixtures: 2. Variation of the transformation behaviour of mineral matter with bulk gas composition, Fuel 90 (2011) 1361–1369. [20] L. Zhang, A. Sato, Y. Ninomiya, CCSEM analysis of ash from combustion of coal added with limestone, Fuel 81 (2002) 1499–1508. [21] A. Baba, A. Kaya, Leaching characteristics of solid wastes from thermal power plants of western Turkey and comparison of toxicity methodologies, Journal of Environmental Management 73 (2004) 199–207. [22] P. Shah, V. Strezov, K. Prince, P.F. Nelson, Speciation of As, Cr, Se and Hg under coal fired power station conditions, Fuel 87 (2008) 1859–1869. [23] A. Ugurlu, Leaching characteristics of fly ash, Environmental Geology 46 (2004) 890–895. [24] T. Wang, J. Wang, Y. Tang, H. Shi, K. Ladwig, Leaching characteristic of arsenic and selenium from coal fly ash: role of calcium, Energy & Fuels 23 (2009) 2959–2966. [25] A.P. Khodadoust, P. Naithani, T.T. Theis, I.P. Murarka, Leaching characteristics of arsenic from aged alkaline coal fly ash using column and sequential batch leaching, Industrial and Engineering Chemistry Research 50 (2011) 2204–2213.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Effect of coal blending on the leaching characteristics of arsenic in fly ash from fluidized bed coal combustion Facun Jiao a, b,⁎, Yoshihiko Ninomiya b, Lian Zhang c, Naoomo Yamada b, Atsushi Sato b, Zhongbing Dong a, d a
National Center of Coal Chemical Products Quality Supervision & Inspection, Huainan, Anhui 232001, PR China Department of Applied Chemistry, Chubu University, Kasugai, Aichi 487-8501, Japan Department of Chemical Engineering, Monash University, PO Box 36, Clayton Campus, Victoria 3800, Australia d School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China b c
a r t i c l e
i n f o
Article history: Received 23 August 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 Available online 19 November 2012 Keywords: Arsenic Leaching behavior Coal blending Fluidized bed
a b s t r a c t The leaching characteristics of arsenic (As) in fly ash collected from lab-scale fluidized bed reactor have been systematically investigated through the combustion of two bituminous coals (A and B) and their mixture with different blending ratio. Leaching tests were conducted according to Japanese Industrial Standard (JIS).The results indicate that, the fly ash derived from the combustion of coal B, which contains abundant calcium, shows a larger capture ability for arsenic vapor than that from coal A, due to the chemical reaction of arsenic with CaO. This reaction is however competed by the sulfation of CaO at coal combustion temperature, therefore, a nonlinear increase was observed with increasing the blending ratios of high-calcium coal B with coal A. Leaching performance of arsenic from fly ash is largely dependent on the finally pH of the leachate. CaO in fly ash preferentially generates a high-pH leachate during leaching test and subsequently promotes the combination of calcium with arsenic to form precipitate. Improving Ca/S ratio through the combustion of blending coal is a promising method to prevent the emission of arsenic into ambient and reduce its leachability from fly ash. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Arsenic (As) released from coal combustion is of environmental concern, due to its toxic and potential carcinogenic propensities [1]. It undergoes an initial vaporization in flame zone during coal combustion and then retains in fly ash through complex chemical/physical reactions and nucleation/condensation upon flue gas cooling [2]. Moreover, since of its high volatility, a portion of the arsenic can escape the air pollutant control system as vapor, the release of which to air is harmful to human and the nature [3]. Even for the arsenic retained in fly ash, the subsequent treatment of fly ash through landfill can cause the leaching of arsenic into the waters. With the increase in awareness of the public in environment impact of the combustion of coal, it is vital to control the emission of arsenic during coal combustion and its leaching during fly ash treatment. It has been reported that the mode of occurrence of arsenic in a coal exists as pyritic, organic and arsenate [4]. Regardless of its species in coal, at the temperature of coal combustion, arsenic preferentially
⁎ Corresponding author at: National Center of Coal Chemical Products Quality Supervision & Inspection, Huainan, Anhui 232001, PR China. Tel.: +86 554 2694896; fax: 86 554 2694896. E-mail address: [email protected] (F. Jiao). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.10.015
vaporized as oxides/sub-oxides, which will finally distribute between fly ash particles and gas phase as flue gas temperature decreases. This partitioning is dependent on a number of factors such as the initial concentration of arsenic in parent coal, combustion conditions and ash composition [5]. All the arsenic in fly ash appears to be present as arsenate, possibly combined with alumino-silicate slag and calcium ortho-arsenate, as determined by X-ray absorption fine structure (XAFS) spectroscopy [6–8]. Consequently, the emission of arsenic during coal combustion is frequently controlled by the use of sorbent such as calcium oxide, calcium hydroxide and calcium silicate. Lime has proven to possess the highest capture ability to arsenic vapor at a moderate temperature of around 600–1000 °C than the other sorbents [9–11]. This process is attributed to the dominant mechanism of irreversible chemical reaction rather than physical adsorption [12]. In light of this, Adjusting ash composition especially for Ca content in fly ash via the combustion of blending coal, a ubiquitous technique adopted by coal-fired power plants, is one of the possible and practicable methods to improve the retention ability of fly ash to arsenic vapor. In general, the fly ash derived from coal combustion is subjected to disposal and reuse. The leaching of arsenic from fly ash will cause potential hazard on aquifer systems and soil, which thus causes a critical environmental problem. The leaching of arsenic from fly ash during fly ash–water interaction is dependent on many factors, e.g. the phases associated with arsenic in fly ash, pH of leachate, ash
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properties (e.g. acidic or alkaline) and the leaching environment [13–16]. Arsenic combined with soluble phase is readily released at a much higher rate. The increase in the pH of leachate exhibits an inhibiting effect on the release of arsenic during leaching test [17]. The pH of ash–water system highly lies on the concentration of alkaline earth elements such as Ca and Mg in ash and the proportion of potentially acid-deriving SO3 [13,18]. The presence of abundant calcium probably induces secondary precipitation reactions, and in turn limits arsenic mobility. In addition, the temperature of leaching system also affects the leaching of arsenic from fly ash into water resources [14,15]. At the temperatures beyond 30 °C, arsenic in fly ash can be stabilized completely, regardless of the pH of the system. Based on above literature review, the leaching behavior of arsenic is obviously influenced by the ash chemical properties. Therefore, combustion of blending coal has potential to result in the shift of ash chemical composition and is expected to in turn alter the leaching behavior of arsenic. Such a hypothesis needs to be confirmed by experimental observation. The present paper aims to examine the effect of coal blend combustion on the retention of arsenic in fly ash and its leaching characteristics. For this purpose, the combustion of two Australian bituminous coals and their mixtures has been first carried out in a lab-scale fluidized bed reactor. The resulting fly ash was then subjected to a leaching test. Arsenic concentration in parent coals and fly ash, as well as in leaching solution from leaching test was determined using hydrated generation inductively coupled plasma optical emission spectroscopy (HG-ICP-OES). Energy dispersive X-ray spectroscopy (EDX) mapping was adopted to describe the distribution of arsenic in fly ash particulates. The modes of occurrence of arsenic in fly ash and its leaching mechanisms were clarified systematically with the aid of theoretical prediction. The new knowledge is expected to help optimize the coal combustion process to reduce the emission of arsenic into ambient and its leachability from fly ash.
2. Experimental 2.1. Coal samples Two Australian bituminous coals, namely coals A and B hereafter, were selected for combustion experiments. The coal samples were air-dried and sieved to a size range of 106–300 μm prior to use. Table 1 shows the properties of coal samples used in this experiment. Ash content of the two coal samples is 6.4% for coal A and 9.5% for coal B, respectively. The composition of inorganic elements detected using X-ray fluorescence (XRF) in two coal samples with the size bin of 106–300 μm is quite different. As shown in Table 2, calcium content in coal B is 20.7% whereas only 0.5% calcium is in coal A. The analysis using computer controlled scanning electron microscope (CCSEM) suggests that the predominant species of calcium in coal B are calcite,
Table 2 Composition of inorganic elements in raw coal (wt.%).
SiO2 Al2O3 CaO Fe2O3 K2O MgO TiO2 SO3 As, mg/kg-coal
A
B
36.1 23.9 0.5 8.7 1.0 – 7.4 20.7 10.1
38.3 14.5 20.7 10.4 2.3 1.97 – 11.5 8.6
dolomite and Ca–Al-silicate, as shown in Table 3. Arsenic concentration in coals A and B is10.1 mg/kg-coal and 8.6 mg/kg-coal, respectively. 2.2. Characterization of samples The concentration of arsenic in fly ash was analyzed using HGICP-OES with the assistance of acid digestion. Briefly, about 0.1 g ash sample was weighted and moved into PTFE bottle. A microwave digester was adopted to digest ash sample using the acid of HNO3 + HF (9:3 v/v) by a thermal program. After cooling, the residue was dissolved and diluted to 50 mL using 1 mol/L HCl. CCSEM analysis (JEOL JSM5600 SEM coupled with an Exford EDAX detector) was carried out to characterize mineral species in raw coal. Around 3000 individual particles were analyzed for each sample under three magnifications covering a particle size range of 0.5– 211.0 μm. The modes of occurrence of the elements of interest were determined with the use of a variety of classification categories [19]. Prior to analysis, coal sample was first grinded to less than 200 μm and then mixed with resin to make a pellet. The detailed procedure of sample preparation for CCSEM analysis has been described elsewhere [20]. Speciation of Ca in fly ash samples was determined using X-ray photoelectron spectroscopy (XPS, Shimadzu ESCA-3300KM) at Ca2p.C 1s at 284.0 eV was used for peak shift calibration. Major elemental compositions in coal were quantified using X-ray fluorescence (XRF, Rigaku 2100). For XRF analysis of a certain sample, a few amount of it (approximately 500 mg) was first mixed with the similar amount of resin. The resulting mixture was subsequently poured into an Al-made O-ring with an outer diameter of 10 mm, and pelletized using a compressor. Prior to analysis, a standard reference coal ash sample (SRM 1632c) was used to validate XRF results. 2.3. Combustion facility Combustion experiments were conducted in a lab-scale fluidized bed reactor, the schematic of which is shown in Fig. 1. This reactor was made of a quartz tube with a length of 1000 mm and an inner diameter of 85 mm. Coal sample at a feeding rate of around 0.5 g/min through a piezo feeder was pneumatically conveyed by 2 L/min
Table 1 Properties of coal samples tested here. Coal A
Coal B
Proximate analysis, air-dried, wt.% Moisture Ash Volatile matter Fixed carbon
6.7 6.4 29.0 57.9
3.7 9.5 34.2 52.7
Ultimate analysis, daf, wt.% C H N S O (by difference)
71.8 3.45 1.64 0.71 22.4
74.1 3.15 1.76 0.50 20.5
Table 3 Mineral species in raw coal quantified by CCSEM. Category
A
B
Quartz Calcite Dolomite Ca–Al-silicate Kaolinite Montmorillonite Fe–S Iron oxide Unknown
15.31 0.07 0.01 0.09 69.4 1.05 1.03 0.9 12.1
22.14 7.40 2.68 7.17 45.60 1.80 1.62 1.77 9.82
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the combustion of coal A mixed with different amount of Ca(OH)2 (analytical reagent, purity > 98%) to match the Ca/S mole ratio of 1 and 3 was also carried out. Here, the Ca/S mole ratio was defined as
Ca=Sðmol=molÞ ¼
Ca in additives : S in coal
ð2Þ
2.4. Leaching test
Fig 1. Schematic of fluidized bed reactor.
secondary gas through an injector installed at the top of the reactor. The primary gas of 6 L/min was introduced from the bottom for coal combustion and bed material fluidization (quartz sand with a diameter of 300–500 μm). Around 120 g coal sample was fed for each run. The furnace temperature was kept at 860 °C and residence time was ~ 4 s, comparable with industrial fluidized bed boiler. The resulting ash was entrained by flue gas out of the reactor and subsequently separated by two cyclones. The tail gas composition including O2, CO2, SO2, NOx and CO was analyzed on-line by a gas analyzer on the real time basis. The ash collected from two series of cyclones was mixed together (termed as fly ash hereafter) and stored in fridge prior to analysis. Three replicas were made for each condition to ensure a satisfactory accuracy and reproducibility for ash yields throughout this study. Apart from two coal samples, the combustion of their mixture with five different blending ratios (A:B, wt/wt = 87.5:12.5, 75:25, 62.5:37.5, 50:50, 25:75) were also carried out. Variation of ash yield and arsenic concentration in collected fly ash under all conditions was shown in Table 4. In these experiments, the fly ash yields, defined as the weight ratio of the fly ash collected in cyclone to the total ash fed with coal, vary in the range of 67.5%–72.2%. Regarding the arsenic content in fly ash, adding 50% of coal B in coal blends causes the maximum arsenic concentration in fly ash, 152.4 mg/kg, reflecting significant effects of blending coal combustion on the fate of arsenic. To quantify the retention extent of arsenic in fly ash, the retention ability was evaluated by an enrichment factor (EF), which was defined as follows.
EF ¼
C M−in−ash ash content in % 100 C M−in−coal
ð1Þ
Leaching behavior of arsenic in fly ash was quantified using leaching test according to Japanese Industrial Standard (JISK0102-61.2). Relative to the leaching method suggested by American society of testing and materials (ASTM), toxicity characteristic leaching procedure (TCLP) and European Committee for Standardization (CEN) [21], The liquid– solid ratio, initial pH and leaching time proposed in JIS method are strictly monitored to mimic the natural circumstance of land fill of solid waste in Japan. Regarding the leaching procedure used in this work, fly ash of 3 g was mixed with 30 mL preconditioned deionized water (pH = 6.3)and the slurry was stirred for 6 h at room temperature. Filtration was performed to separate the fly ash using a filter (Advantec No.5C). The concentration of arsenic in filtrate was measured by HGICP-OES. 2.5. Thermodynamic equilibrium calculation The thermodynamic equilibrium software, FactSage 6.2, was employed to theoretically predict the possible As-bearing compounds in fly ash and its distribution in water solution under different pH during leaching test. The elemental compositions of ash were used as calculation input. The databases used include ELEM, FACT and Fact53. Aqueous and solid species are chosen as output. 3. Results and discussion 3.1. Coal combustion To monitor coal combustion performance in fluidized bed reactor, flue gas composition was analyzed online by a gas analyzer. Fig. 2 shows the composition of flue gas produced from the combustion of coal A. The lines of all gas concentration as a function of experimental time are almost flat, suggestive of a stable combustion of coal in the period when ash sampling was conducted. O2 concentration in flue gas remained at approximately 11% to achieve a carbon conversion yield > 90%. SO2 concentration in flue gas derived from the combustion of coal A is approximate 150 ppm whereas its concentration in flue gas during coal B combustion is nearly zero since of higher calcium content relative to coal A.
where, CM-in-ash denotes the concentration of arsenic in fly ash; CM-in-coal denotes the concentration of arsenic in coal. Moreover, to confirm the role of free calcium on the fate of arsenic during coal combustion,
35 30
Ash yield, wt.% (Aver. ± STDa)
Arsenic in ash, mg/kg (Aver. ± STD)
0 12.5 25 37.5 50 75 100
6.4 6.5 6.8 7.2 7.6 8.4 9.5
72.2 ± 1.0 70.4 ± 2.5 71.9 ± 2.4 68.7 ± 1.1 71.0 ± 0.9 67.5 ± 2.2 72.0 ± 1.3
104.2 ± 4.0 112.2 ± 5.1 116.7 ± 3.7 143.9 ± 4.3 152.4 ± 4.7 139.2 ± 3.1 119.5 ± 5.5
a
Standard deviation.
O2/CO2,vol%
Ash content, wt.%
CO 20
10 O2
15
1
10 5
CO2
SO2/NO/CO, ppm
Ratio of coal B in coal blends, wt.%
100
SO2
25 Table 4 Variation of ash yield and arsenic in ash with experimental condition.
1000
NO
0.1
0 0
20
40 Time, min
60
80
Fig. 2. Flue gas composition as a function of experimental time during combustion of coal A.
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3.2. Retention of arsenic in fly ash Fig. 3 demonstrates the EF of arsenic in fly ash derived from the combustion of coals A and B as well as their mixture with different blending ratio. As can be seen, for single coal combustion, coal B shows a higher EF of 1.3, relative to 0.65 in coal A, indicating a higher retention of arsenic in fly ash generated from the combustion of coal B. Regarding the combustion of coal blends, the EF of arsenic in the resulting fly ash falls between the results for two single coals, as expected. Nevertheless, the variation trend of EF with blending ratio of coal B is quite interesting. The EF of blending coal show a rapid increase when the blending ratio of coal B is between 30% and 50%, which, however, was smoothly increase as the blend fraction of coal B moves out of above range, suggestive of more complex interaction of arsenic with fly ash during blending coal combustion. In order to compare the impacts of coal blending, the theoretical values of EF under different blending ratio, based on an assumption of no additional reactions between the two fly ashes during blending coal combustion, are also depicted as dashed line in Fig. 3. The experimental observation is not in line with their corresponding theoretical values, indicating a nonlinear effect of coal blending on the retention of arsenic in fly ash. Thermodynamic equilibrium calculation was conducted to predict the formation of arsenic-bearing species during coal combustion. As demonstrated in Fig. 4, it indicates that calcium is the most influential factor on arsenic transformation. The formation of Ca3(AsO4)2 affects the partitioning of arsenic between solid phase and gas phase. Calcium is beneficial to associate with arsenic during coal combustion and in turn reduce the emission of arsenic vapor. This is consistent with the experimental observation that the retention of arsenic in coal B ash is higher than that in coal A ash since of higher calcium content in coal B (see Table 2). EDX mapping result also confirmed the combination of arsenic with calcium. Two particles marked A and B with quite different chemical composition are shown in Fig. 5. As can be seen, arsenic is potentially enriched in the particle B with calcium content of around 48% according to EDX analysis. In contrast, its abundance in particle A, dominated by Si and Al, is lower through the comparison of the characteristic signal, indicative of a preferential chemical association of arsenic with calcium. Although the aluminumsilicates in fly ash produced from the decomposition of clay minerals (e.g. kaolinite), to some extent, can capture arsenic vapor, its influence is insignificant in comparison to calcium [10]. Here, it should be also noted that the effect of physical adsorption on the retention of arsenic in fly ash can be neglected [3,12]. As the retention ability of different calcium compounds for arsenic vapors is diverse [10–12], calcium-bearing compounds in fly ash were detected and quantified using XPS Ca (2p). As shown in Fig. 6, four calcium species, CaCO3, CaO, Ca-silicate and CaSO4, are detectable.
1.4 1.3 1.2
EF,-
1.1 1.0 0.9 0.8
Experimental Calculation
0.7 0.6 0
20 40 60 80 100 Ratio of coal B in blending coal, wt%
Fig. 3. EF of arsenic in the fly ash collected from single coal and blending coal combustion. Calculation was conducted based on the assumption that no extra reaction between the two kinds of fly ashes during blending coal combustion.
Among of them, CaO possesses the highest reaction activity with arsenic vapor through Eq. (3) [11,22]. CaSO4 in the fly ash is produced from the sulfation of CaO with SO2 in terms of Eq. (4). 3CaO þ
1
. 2
As4 O6 ðg Þ þ O2 →Ca3 ðAsO4 Þ2
CaO þ SO2 þ
1
. 2
O2 →CaSO4 :
ð3Þ ð4Þ
Clearly, CaO, derived from the decomposition of calcite (CaCO3) during coal combustion, was subjected to the reaction of sulfur and arsenic vapor at high temperature. In terms of above mechanisms, it is undoubted that the competition between reactions (3) and (4) affects the capture ability of CaO to arsenic vapor. Although Ca3(AsO4)2, once formed, is a stable compound at the temperature up to 1400 °C [10], its formation rate is slower than that of the sulfation of CaO under the same conditions over a temperature range of 600–1000 °C [11]. Consequently, CaSO4 are preferentially formed and in turn lead to extensive pore plugging/ blocking of the active site of CaO and retard the reaction with gaseous arsenic. Based on the above discussion, the retention ability of fly ash to arsenic vapor is greatly dependent on the available CaO content in fly ash. Quantification of CaO in fly ash from the combustion of blending coals was shown in Fig. 7. Increase in the blending ratio of coal B, CaO content in fly ash shows an initially slow increase and then exhibits a rapid increase once the blending ration of coal B over 30%. Such observation is related to SO2 concentration in flue gas which was nearly close to zero when more than 30% of coal B was added into blending coal, as shown in Fig. 7, further substantiating the faster reaction rate of Eq. (4) than Eq. (3). Once the blending ratio of coal B exceeds 30%, the excess CaO in fly ash has the potential to react with arsenic vapor, resulting in a rapid increase in the retention ability of fly ash to arsenic. However, as the blending ratio of coal B was above 50%, the EF in Fig. 3 shows a marginal increase as their error bars overlap greatly. A plausible reason is that the arsenic vapor was entirely captured by fly ash when more than 50% of coal B was added in the coal blends, which was confirmed by the experimental result that the total amount of arsenic in fly ash is nearly close to that fed with coal. 3.3. Leaching behavior of arsenic in fly ash According to the above results, the retention ability of fly ash to arsenic vapor was promoted via the combustion of blending coal. The subsequent treatment of the resulting fly ash through landfilling will cause secondary pollution due to relative high concentration of arsenic in fly ash and its mobilized propensity. Clarifying the leaching characteristics of arsenic in such fly ash generated from the combustion of blending coal is highly significant. Fig. 8 shows the leaching ratio of arsenic in the fly ash collected from the combustion of single coal and mixed coal. Here the leaching ratio was defined as the percentage of total mass of arsenic in leachate to its mass in fly ash. Clearly, the leaching ratio of arsenic in the fly ash from combustion of coal B is much lower than that in coal A. Regarding blending coal combustion-derived fly ash, the leaching ratio of arsenic was reduced with the addition of coal B to coal blend, as expected. Here again, the theoretical values under different blending ratio are also displayed for comparison. The leaching ratio of arsenic in blending coal is quite lower than calculation results, indicating that the leaching of arsenic from fly ash is reduced via the combustion of blending coal. The leaching ratio of arsenic as a function of final pH of leachate was demonstrated in Fig. 9. It obviously indicates that the leachability of arsenic is largely dependent on the pH of leachate at a wide pH range, which was further affected by the blending ratio
F. Jiao et al. / Fuel Processing Technology 106 (2013) 769–775
773
120
120
Ca3(AsO4)2(s) 100
80
Mole fraction, %
Mole fraction, %
100
As2O5(s)
60
As4O6(g)
40
Coal A
20
Ca3(AsO4)2(s)
80 60 40 20
0
Coal B
0 0
200
400 600 Temperature, °C
800
1000
0
200
400 600 Temperature, °C
800
1000
Fig. 4. Prediction of As-bearing compounds during the combustion of coals A and B. Arsenic preferentially combines with calcium to form Ca–As compound.
Fig. 5. SEM picture and EDX mapping of arsenic in fly ash collected from blending coal combustion (50/50, wt/wt). The characteristic signal of arsenic in particle B containing abundant calcium is higher than that in particle A.
of coal B. Increase in the fraction of coal B in coal samples caused the increase of pH of leachate and in turn reduced the leaching ratio of arsenic. For instance, leaching ratio of arsenic decreased from 57% under the pH of 3.8 to 0.05% under the pH of 12.8. To better understand the leaching mechanisms of arsenic from fly ash, the possible chemical species of arsenic in solution were predicted by a thermodynamic equilibrium model, as depicted in Fig. 9. Apparently, the variation trend of leaching ratio of arsenic against the pH of leachate was consistent with the experimental observations. Regarding the concentration of arsenic in solution, the leaching ratio of arsenic in solution determined experimentally
is far lower than that from prediction. For instance, at the pH of 3.8, the arsenic leaching ratio of 57% was achieved, compared to about 100% as predicted. A possible reason is the kinetic control during leaching test due to a short leaching duration tested here [23]. The prediction of arsenic species in solution shift with pH value is also shown as dot line. Three types of arsenic species, H2AsO4−, HAsO42 − and AsO43 −, in turn appeared along with the increase of pH, which is consistent with the fact that the arsenic acid possesses three acidity constants of 2.26, 6.76 and 11.29, respectively. As the pH above 11, AsO43 − was the dominating species, which can form precipitates with Ca [24], as indicated in Fig. 9.
Fig. 6. Ca (2p) XPS of the fly ash collected from the combustion of blending coal. The ratio of coal B in coal blends in panel (a) and (b) are 50% and 12.5%, respectively. The major calcium-bearing compounds are CaCO3, CaO, Ca-silicate and CaSO4.
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6 160 CaO SO2
4
120
3
80
2 40 1 0 100
0 0
SO2 concentration, ppm
CaO in fly ash, %
5
20 40 60 80 Ratio of coal B in blending coal, wt%
Fig. 7. Relationship of CaO in fly ash with SO2 concentration in flue gas.
100
Leaching ratio, %
A 10
1
0.1
Experimental Calculation
B
0.01 0
20 40 60 80 100 Ratio of coal B in blending coal, wt.%
Fig. 8. Leaching ratio of arsenic in fly ash collected from single coal and blending coal combustion. Increase in the ratio of coal B in coal blends causes the leaching ratio of arsenic to reduce.
Based on the above discussion, it is clear that the final pH of leachate is influential on the leaching of arsenic from fly ash. Acidic leachate produced from acidic fly ash leaching test promotes the release of arsenic. Extra leaching test of coal A fly ash, a typical acidic fly ash, was carried out with different leaching time to investigate the leaching progress of arsenic. As shown in Fig. 10(a), the leaching ratio of arsenic increased with increasing pH of leachate, as expected. Conversely, alkaline leachate derived from alkaline fly ash leaching has suppressing effect on the mobilization of arsenic. There are two possible mechanisms governing the leaching progress of arsenic during alkaline fly ash leaching test. One is that the solubility of As-bearing compounds
1000
Total As (aq)
Leaching ratio, %
H2AsO4- (aq)
1
HAsO42- (aq)
0.1
AsO43- (aq) 0.01 2
4
6
4. Conclusions The combustion of two Australian bituminous coals and their mixture with different blending ratio was carried out in a lab-scale fluidized bed reactor. Apart from the effect of blending coal on the retention of arsenic in fly ash, its leaching characteristics from fly ash were also examined systematically. The main conclusions were drawn as follows.
Ca3(AsO4)2(s)
100 10
in fly ash under alkaline condition is lower than that in acidic leachate. Another one is that the As-bearing compounds in fly ash was subjected to initial dissolution and then combined with calcium through secondary reaction with the increase of pH of leachate. To confirm which route is dominant in the leaching progress of arsenic, extra leaching test of coal B fly ash, a typical alkaline fly ash, was conducted with different leaching time. As shown in Fig. 10(b), the leaching ratio of arsenic shows a drastic increase in the short term (less than 2 min) and then decreases with leaching time. Such observation is a clear sign that the latter mechanism is the major route for the leaching of arsenic. Abundant of calcium in coal B fly ash preferentially associates with arsenic to form solid compound and thus reduce the leaching of arsenic. Calcium in fly ash seemingly still plays an important role on the leaching of arsenic, especially for free CaO, which possesses a higher solubility during leaching test than other species such as CaSO4 and Ca-silicate [25]. To further verify such a hypothesis, combustion of coal A mixed with Ca(OH)2 for matching the Ca/S mole ratio of 1 and 3 was carried out. The resulting fly ash was also subjected to leaching test. As shown in Fig. 11, leaching ratio of arsenic in the fly ash decreased with the increase of Ca/S. Through comparing the leaching ratio of arsenic in the fly ash collected from the combustion of blending coal and Ca(OH)2 addition, as tabulated in Table 5, the effects of different calcium compounds on the leaching of As vary noticeably. As shown in the second row from below in Table 5, although the calcium content in the coal with adding Ca(OH)2 is comparable with that in blending coal, the leaching ratio of arsenic is quite lower. This observation clearly indicates free CaO in coal is favorable for reducing the leachability of arsenic. Another direct evidence to confirm the role of free CaO on the leaching of arsenic is attained form the experimental observation of the combustion of coal A mixed with CaSO4 (Ca/S =3). The leaching ratio of arsenic in the resulting fly ash is greatly higher than that in the fly ash from combustion of coal A mixed with Ca(OH)2 (Ca/S = 3). CaO is the most important Ca-containing species both in arsenic vapor capture during coal combustion and prevention of its leaching from fly ash. In fluidized bed coal combustion, limestone is usually employed as desulfurization agent. Through adjusting Ca/S to increase the CaO content in fly ash can efficiency inhibit the leaching of arsenic. On the other hand, although this work was carried out in a fluidized bed reactor, it also can shed new lights into reducing the leaching of arsenic in fly ash from pulverized coal combustion. Combustion of blending coal is a promising method to shift the ash properties and in turn alters the leaching behavior of arsenic.
8 pH,-
10
12
14
Fig. 9. Comparison of experimental results with prediction. Black round point denotes experimental results, line denotes predicted results. The variation of experimental results is in line with that of prediction.
1. CaO content in parent coal affects the retention of arsenic in fly ash through the chemical reactions of arsenic vapor with CaO. This reaction is largely influenced by the sulfation of CaO during coal combustion temperature. Combustion of 50% coal A mixed with 50% coal B caused a significant reduction on the emission of arsenic, in comparison with the combustion of single coal A. 2. Ash property is influential on the leaching behavior of arsenic from fly ash. Acidic leachate favors the leaching of arsenic from fly ash. Conversely, alkaline leachate prevents the leaching of arsenic. 3. Abundance of free CaO in coal B-derived fly ash generates an alkaline leachate during leaching test and successively reduces arsenic leaching since the precipitate was preferentially occurred via the reaction of arsenic with calcium at high pH leachate. Adjusting Ca/S in feedstock via coal blending is a promising method to shift the ash properties and in turn alters the leaching behavior of arsenic.
F. Jiao et al. / Fuel Processing Technology 106 (2013) 769–775
80
(a)
100
8
(b)
Coal B 7 6
Leaching ratio pH
5 20
13
10 Leaching ratio
11 10
pH
pH,-
40
14
12 Leaching ratio, %
60
pH,-
Leaching ratio, %
Coal A
775
9
1
8
4
7 0 0
10
20
30 40 50 Time, min
3 180 240 300
0.1 0
10
20
30 180 Time, min
240
6 300
Fig. 10. Leaching ratio of arsenic and pH of leachate as a function of leaching time. In alkaline condition, arsenic-bearing compounds are subjected to an initial dissolution and then combine with calcium to form solid phase with the increase of pH.
100
Ca/S=0 Leaching ratio, %
10
1
Ca/S=1 0.1
Ca/S=3 0.01 2
4
6
8 pH,-
10
12
14
Fig. 11. Leaching of arsenic in the fly ash collected from the combustion of coal A mixed with Ca(OH)2.
Table 5 Comparison of leachability of arsenic in the fly ash collected from the combustion of blending coal and Ca(OH)2 addition. Blending coal
Ca(OH)2 addition
Ca content in coal, wt.%
Leaching ratio,%
Ca content in coal, wt.%
Leaching ratio,%
0.03 (A) 0.77 (A:B, 62.5/37.5) 2.0 (B)
57.5 8.4 0.05
0.03 (Ca/S = 0) 0.72 (Ca/S = 1) 2.1 (Ca/S = 3)
57.5 0.06 0.02
Acknowledgment The financial support from Grant-in-aid for Scientific Research on Priority Areas (B), 20310048, Ministry of Education, Science, Sports and Technology in Japan was gratefully acknowledged. References [1] P. Shah, V. Strezov, C. Stevanov, P.F. Nelson, Speciation of arsenic and selenium in coal combustion products, Energy & Fuels 21 (2007) 506–512. [2] C.L. Senior, J.J. Helble, Emissions of mercury, trace elements, and fine particles from stationary combustion sources, Fuel Processing Technology 65–66 (2000) 263–288. [3] X. Gou, C.-G. Zheng, M.-H. Xu, Characterization of arsenic emission from a coal-fired power plant, Energy & Fuels 18 (2004) 1822–1826. [4] Y. YaE, M.P. Ketris, Arsenic in coal: a review, International Journal of Coal Geology 61 (2005) 141–196. [5] M.L. Contreras, J.M. Arostegui, L. Armesto, Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations, Fuel 88 (2009) 539–546.
[6] G.P. Huffman, F.E. Huggins, N. Shah, J. Zhao, Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy, Fuel Processing Technology 39 (1994) 47–62. [7] F.E. Huggins, N. Shah, G.P. Huffman, J.D. Robertson, XAFS spectroscopic characterization of elements in combustion ash and fine particulate matter, Fuel Processing Technology 65–66 (2000) 203–218. [8] F.E. Huggins, G.P. Huffman, X-ray absorption fine structure (XAFS) spectroscopic characterization of emission from combustion of fossil fuels, International Journal of the Society of Materials Engineering for Resources 10 (2002) 1–13. [9] B.K. Gullett, K. Raghunathan, Reduction of coal-based metal emission by furnace sorbent injection, Energy & Fuels 8 (1994) 1068–1076. [10] S. Mahuli, R. Agnihotri, S. Chauk, A. Ghosh-Dastidar, L.-S. Fan, Mechanism of arsenic sorption by hydrated lime, Environmental Science and Technology 31 (1997) 3226–3231. [11] R.O. Sterling, J.J. Helble, Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates, Chemosphere 51 (2003) 1111–1119. [12] Y. Li, H. Tong, Y. Zhou, Y. Li, X. Xu, Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture, Environmental Science and Technology 41 (2007) 2894–2900. [13] J. Jankowski, C.R. Ward, D. French, S. Groves, Mobility of trace elements from selected Australian fly ashes and its potential impact on aquatic ecosystems, Fuel 85 (2006) 243–256. [14] A. Baba, G. Gurdal, F. Sengunalp, O. Ozay, Effect of leachant temperature and pH on leachability of metals from fly ash. A case study: can thermal power plant, province of Canakkale, Turkey, Environmental Monitoring and Assessment 139 (2008) 287–298. [15] A. Baba, G. Gurdal, F. Sengunalp, Leaching characteristics of fly ash from fluidized bed combustion thermal power plant: case study: can (Canakkale — Turkey), Fuel Processing Technology 91 (2010) 1073–1080. [16] A.G. Kim, The effect of alkalinity of Class F PC fly ash on metal release, Fuel 85 (2006) 1403–1410. [17] E.E. Van Der Hoek, P.A. Bonouvire, R.N.J. Comans, Sorption of As and Se on mineral components of fly ash: relevance for leaching processes, Applied Geochemistry 9 (1994) 403–412. [18] C.O. Plank, D.C. Martens, Boron availability as influenced by application of fly ash to soil, Soil Science Society of America Proceedings 38 (1974) 974–977. [19] L. Zhang, F. Jiao, E. Binner, S. Bhattacharya, Y. Ninomiya, C.-Z. Li, Experimental investigation of the combustion of bituminous coal in air and O2/CO2 mixtures: 2. Variation of the transformation behaviour of mineral matter with bulk gas composition, Fuel 90 (2011) 1361–1369. [20] L. Zhang, A. Sato, Y. Ninomiya, CCSEM analysis of ash from combustion of coal added with limestone, Fuel 81 (2002) 1499–1508. [21] A. Baba, A. Kaya, Leaching characteristics of solid wastes from thermal power plants of western Turkey and comparison of toxicity methodologies, Journal of Environmental Management 73 (2004) 199–207. [22] P. Shah, V. Strezov, K. Prince, P.F. Nelson, Speciation of As, Cr, Se and Hg under coal fired power station conditions, Fuel 87 (2008) 1859–1869. [23] A. Ugurlu, Leaching characteristics of fly ash, Environmental Geology 46 (2004) 890–895. [24] T. Wang, J. Wang, Y. Tang, H. Shi, K. Ladwig, Leaching characteristic of arsenic and selenium from coal fly ash: role of calcium, Energy & Fuels 23 (2009) 2959–2966. [25] A.P. Khodadoust, P. Naithani, T.T. Theis, I.P. Murarka, Leaching characteristics of arsenic from aged alkaline coal fly ash using column and sequential batch leaching, Industrial and Engineering Chemistry Research 50 (2011) 2204–2213.