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Effect of ash composition on the partitioning of arsenic during fluidized bed combustion
Fuel 204 (2017) 91–97
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Full Length Article
Effect of ash composition on the partitioning of arsenic during fluidized bed combustion Chuncai Zhou a,b, Guijian Liu a,b,⇑, Zhongyu Xu a, Hao Sun a, Paul Kwan Sing Lam c a
CAS Key Laboratory of Crust-Mantle Materials and the Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, The Chinese Academy of Sciences, Xi’an, Shaanxi 710075, China c State Key Laboratory in Marine Pollution and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b
h i g h l i g h t s The transformation behavior of arsenic were affected by ash composition. The retention efficiency was affected by combustion temperature and addition ratio. Chemical adsorption may be the primary mechanism between arsenic and ash components.
a r t i c l e
i n f o
Article history: Received 11 January 2017 Received in revised form 8 May 2017 Accepted 11 May 2017
Keywords: Arsenic Ash composition Redistribution Chemical adsorption Coal combustion
a b s t r a c t The effect of the ash composition on the redistribution of arsenic in a coal combustion system was determined via both experimental laboratory simulations and thermodynamic equilibrium calculations. The experimental study was performed in a fluidized bed reactor in combustion temperature range of 500–1000 °C. The results show that arsenic redistribution is strongly related to the chemical composition of coal in a combustion system. The presence of Fe, Ca, Mg, Al, Na and K favour arsenic retention, while Si causes an opposing effect. The arsenic retention on ash compositions is both temperature and amount dependent. With rising combustion temperature, the arsenic retention efficiency of Fe, Ca and Mg oxides augmented up to a maximum and then decreased, while those of Al, Na and K compounds continued expanding in the range of 500–1000 °C. Chemical adsorption may be the primary mechanism for Fe, Ca, Mg, K and Na, while the adsorption of arsenic by Al may be ascribed to both physical and chemical adsorption. Arsenic is captured in ashes as a result of the formation of stable arsenate compounds arising from interactions with the inorganic matrix. The negative arsenic retention effect by silica is attributed to reactions with ash compositions which promote arsenic capture and the reduction of the solid arsenic compounds. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal used for energy generation is identified as one of the largest anthropogenic contributors to environmental sensitively trace elements [1]. Toxic heavy metal elements such as As, Hg, Se, Pb and Cd are at least partially volatilized at high temperature and cannot be captured by atmospheric pollution control devices [2–5]. The total emissions of As, Hg, Se, Pb and Cd from coal combustion by energy generation in 2012 were estimated at approximately 406.4, 139.4, 538.6, 833.0 and 15.5 tons in China, respectively [6].
⇑ Corresponding author at: CAS Key Laboratory of Crust-Mantle Materials and the Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.fuel.2017.05.048 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
As a result of emerging environmental and health concerns, a series of environmental legislations and technologies are related with reducing these toxic elements in environment [7]. Arsenic and its compounds are known to be of greatly environmental and health concern due to its high toxicity and carcinogenicity [8,9]. Many studies have addressed that arsenic is associated with sulfide minerals, clay minerals, carbonate minerals and organic matter [10–16]. It has been reported that arsenic associated with sulfides and organic matter is volatilized during combustion, and subsequently released as vapor phase or particulate phase [2,17–19]. The elemental arsenic (As) and arsenic trioxide (As2O3) are supposed to the two probable forms of arsenic in the oxidizing flue gas environment [7,20]. For the chemical reactivity of elemental arsenic, As2O3 is regarded as the only species in the combustion environment [7]. Gas phase arsenic is preferred to
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deposit on fine particle which contains larger surface area and active cation sites in the cooling flue gas [2,19,21]. These arsenicenriched fine particles are reduced limit by currently widely applied atmospheric pollution control devices (electrostatic precipitator and wet flue gas desulfurization), and partially emitted into environment. The arsenic removal efficiencies of electrostatic precipitator and wet flue gas desulfurization are estimated at about 86.2% and 80.4%, respectively [6]. Consequently, these emitted arsenic could be inhaled and deposited in lungs of humans and resulted in adverse health risks [8]. Meanwhile, many studies addressed that As2O3 in flue gas could react with active V2O5 sites and block the catalyst pores, and are prejudicial to selective catalytic reduction (SCR) installations [22]. Therefore, the study on gaseous arsenic control is of great environmental and industrial urgency. The redistribution behavior of arsenic during combustion is dependent on many factors, including the association in coal, the interaction with other element both during and after combustion, the combustion conditions (temperature and boiler type) [18,23,24]. It has been reported that the partitioning behavior of arsenic is not only determined by the volatility of arsenic, but also affected by the interactions with different mineral components [25,26]. Thus, any improvement to the process that favour a more efficient retention of arsenic by mineral components is worthwhile. Recycling fly ashes or some of mineral components for the retention of mercury, arsenic and selenium has been addressed, and a positive relationship is found between fly ash calcium and iron concentrations and the corresponding arsenic retention concentration in the fly ash [27–30]. The ability of various mineral sorbents, including silica, alumina, kaolinite, limestone, bauxite for gaseous arsenic retention is well established in the literature, and illustrated that Ca-, Al and Fe-based sorbents are more efficient in arsenic retention during combustion process [20,27,31]. Although many previous studies have been conducted on gaseous arsenic retention by Ca, Al and Fe-based materials, the interactions between arsenic with other typical ash compositions (Si, Mg, Na and K) are extremely scarce [25]. Meanwhile, many previous studies determined the retention behaviors of arsenic by using pure arsenic compounds (As2O5 standard liquid and As2O3) [7,20,27]. These simulating experiments may not reflect the real retention behavior of arsenic during coal combustion due to the complex chemical reactions among fuel compositions [25]. Therefore, study on the interaction mechanism of arsenic with the mineral components of fly ash by co-combustion coal with ash compositions in a fluidized bed reactor may be useful for identifying the optimum technology for gaseous arsenic removal. Thermodynamic equilibrium modelling could provide useful information for the interactions among two compounds, and have been widely applied to investigate the partitioning behavior of arsenic during combustion [31,32]. Nevertheless, the equilibrium modelling is calculated based on a particular condition (temperature, pressure and atmosphere), and are not considered about the complexity (association of arsenic in different type coal, size distribution of coal, mass transfer, interactions with other elements) in real combustion system [25]. Therefore, the thermodynamic equilibrium calculations were combined with experimentation to evaluate the possible reaction mechanisms between arsenic and different mineral components. The main objectives of this study are to: (1) ascertain the effect of combustion temperature, ash composition type and amount on arsenic retention during combustion; (2) evaluate the arsenic retention mechanism by different ash compositions. The ultimate aim will try to identify the ash compositions and combustion conditions which could optimize the gas-phase arsenic control.
2. Experimental 2.1. Materials The bituminous coal used for power generation in Pingwei Coalfired Plant was selected and collected. The physico-chemical property of the selected coal including ash, proximate and ultimate analysis were determined by our previous study and summarized in Table 1 [3]. The ash yield of the selected bituminous coal was approximately 13.1 wt.% and labeled as medium-to-low ash. The concentration of arsenic was 3.24 mg/kg, which was similar to the weighted average value of Chinese coal (3.79 mg/kg) [33]. The selected coal was air-dried, grinded and sieved into 100– 150 lm for subsequent analysis. 2.2. Combustion procedures The laboratory-scale fluidized bed furnace with a height of 1500 mm and an inside diameter of 50 mm was used to investigate the retention behavior of arsenic during combustion with various ash compositions. The details of the experimental equipment and procedure were found elsewhere [3,24]. The furnace was made by stainless steel and heated by a batch of electrical resistances packed with ceramic fibers to thermal insulation. The fuel feeding rate, the excess air ratio (relative to the theoretical air), the fluidizing air flow, the secondary air flow and the fuel/air ratio were 0.2 kg/h, 40%, 1 Nm3/h, 0.5 Nm3/h and 0.714, respectively. The details of the combustion conditions were presented in Table 2. The admixture of coal and additives was pneumatically placed in a screw feeder, equipped with an agitation chamber and an insulated injector to ensure the uniform fuel feeding rate. The addition ratios of ash compositions were 1 wt.%, 3 wt.% and 5 wt.% of the selected coal. For the chemical instability of alkali metal oxides (K2O and Na2O) in environment, the oxides were substituted by the alkali metal nitrates (NaNO3 and KNO3). The weight percentages of the added ash compositions were calculated as oxides content. Six combustion temperature points including 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C were conducted. The surface area, pore volume, pore diameter and particle size of the added ash chemicals were measured by an adsorption volumetric analyzer ASAP 2020 and Nano S laser particle size analyzer, respectively. The structure characteristics of the selected additives could be found in Table 3. The feedstock was fed into the reactor when the temperature reached the desired level and attained at a steady state. The combustion procedure was maintained approximately 2 h. The bottom ash and fly ash were collected at each site simultaneously. In addition, in order to investigate the distribution behavior of arsenic in different particle size during different addition ratios, the fly ash were sieved by the standard Tyler sieve into three parts of particle size distribution (<38 lm, 38–200 lm, and >200 lm). In order to ensure the precision and accuracy of the experimental results, the sampling for each experimental condition were repeated in triplicate. Once the relative deviations of the data from these samples were more than 20%, another samplings were required. The retention efficiency (RE) could illustrate the capture behavior of additives during combustion effectively. Generally, arsenic may be captured by ash compositions via both physisorption and chemical reactions. The retention efficiency was calculated as described below:
RE ð100%Þ ¼
Vc Vad 100 Vc
ð1Þ
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C. Zhou et al. / Fuel 204 (2017) 91–97 Table 1 The physical-chemical properties of the selected coal [3]. Proximate analysis, db (wt.%) Ultimate analysis, db (wt.%) Ash analysis (wt.%)
Moisture 1.80 C 71.5 SiO2 53.1
Al2O3 32.7
Ash 13.1 H 4.75 Fe2O3 4.31
N 1.38 CaO 3.13
MgO 1.18
Volatile matter 29.0 O 8.34 K2O Na2O 1.32 1.71
Fixed carbon 56.1 S 0.93 TiO2 SO3 1.53 0.07
P2O5 0.05
Table 2 Experimental conditions. Conditions
Value(s)
Feeding rate of feedstock The excess air ratio Fluidizing air flow Secondary air flow Fuel/air ratio Ash composition Addition of ash composition Combustion Temperature Combustion duration
0.2 kg/h 40% 1 Nm3/h 0.5 Nm3/h 0.714 SiO2, Al2O3, Fe2O3, MgO, CaO, NaNO3, KNO3 1%, 3% and 5% of coal 500, 600, 700, 800, 900, 1000 °C 2h
where Vc represents the volatilization ratio of arsenic during coal combustion (%), Vad represents the volatilization ratio of arsenic during combustion with additive (%). 2.3. Arsenic analysis
Fig. 1. The mineral phase of the selected coal.
The feedstock, bottom ash, fly ash and particles in different size distributions were acidic digested by using an acid mixture (HCl: HNO3:HF = 3:3:2). Details of the digestion procedure were given elsewhere [16,34,35]. The standard reference material NIST 1632b (coal) was used to calibrate the accuracy of analytical method. The concentration of arsenic was determined by inductively coupled plasma mass spectrometry (ICP-MS). The concentration of arsenic for each sample was obtained by the average value of three parallel sampling. Meanwhile, the modified sequential chemical extraction procedure described by Zheng et al. (2008) was applied to investigate the association of arsenic in coal. Arsenic could be partitioned into six fractions, including waterleachable bound (B1), ion-changeable bound (B2), organic-bound (B3), carbonate-bound (B4), silicate-bound (B5) and sulfidebound (B6) [36]. The fractions obtained during each step of the analytical procedure were measured for arsenic by ICP-MS. 3. Results and discussion 3.1. The volatilization behavior of arsenic during combustion The physico-chemical property of the selected bituminous coal was described in our previous study [3]. The selected coal was regarded as typical medium to low bituminous coal, and was suitable for power generation. The mineral composition of the selected coal consists of kaolinite, quartz, calcite, pyrite, illite, dolomite and chlorite (Fig. 1). Many studies have addressed that arsenic was
associated with both organic matter and inorganic minerals (clay minerals, sulfides), mostly associated with inorganic matrix [10– 14,16,19]. Arsenic was mainly associated with sulfide bound (54.3%), silicate bound (20.8%), organic bound (9.7%) and carbonate bound (9.6%) (Fig. 2a), indicates that arsenic in selected coal was associated with sulfides, clay minerals, carbonate minerals, and organic matter. The organic matter and inorganic minerals (including sulfides, clay and carbonate minerals) undergo various physico-chemical reactions, including decomposition, carbonization, oxidation, hydration, dehydration and dehydroxylation during high temperature combustion [18]. Accompanied with these complex reactions, arsenic in coal was released and redistributed among coarse particles, fine particles and flue gas. The redistribution behavior of arsenic was attributed to the physico-chemical property of coal, association of arsenic, reaction of inorganic mineral and organic matter, boiler type, combustion temperature and atmosphere [19,34,37]. The volatilization ratio of arsenic at different combustion temperatures was presented in Fig. 2b. It was obviously found that the volatilization ratio of arsenic increases from 15.2% to 21.6% with the increasing combustion temperature from 500 °C to 1000 °C. The volatilization behavior of arsenic had been widely addressed and could be found elsewhere [2,16,19]. The release of arsenic below 700 °C was attributed to the decomposition of organic matter and sulfide minerals, while the emission of arsenic at 700–1000 °C may be explained by the decomposition of clay and carbonate minerals. Winter et al. reported that arsenic
Table 3 The structure properties of ash compositions. Ash composition
Surface area (m2/g)
Particle size (lm)
Pore volume (cm3/g)
Pore diameter (nm)
SiO2 Al2O3 Fe2O3 MgO CaO
45.58 85.32 14.27 68.76 7.44
2.645 1.783 5.016 6.735 4.652
0.64 0.72 0.55 0.16 0.22
14.4 7.24 23.65 8.27 13.68
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Fig. 2. The association (a) and volatilization ratio (b) of arsenic in coal. B1- water-leachable bound, B2- ion-changeable bound, B3- organic-bound, B4- carbonate-bound, B5silicate-bound, B6- sulfide-bound.
trioxide (As2O3) was the most likely form of gaseous arsenic in flue gas in an oxidizing environment [38].
3.2. Effect of combustion temperature and ash composition on arsenic retention The retention efficiency of arsenic captured at temperature in the range of 500–1000 °C by addition ratio of 5% ash compositions were presented in Fig. 3. As illustrated in Fig. 3, the retention efficiencies were various among different additives and different temperatures. According to the retention behaviors, the selected additives could be clustered into three groups. For Fe2O3, CaO and MgO, the retention efficiencies increased maximum at 600 °C, 600 °C and 700 °C, respectively, and then decreased with increasing combustion temperature. For Al2O3, NaNO3 and KNO3, the retention efficiencies increased with the increasing combustion temperatures in the range of 500–1000 °C. For SiO2, the gaseous
arsenic increased with the increasing temperature, indicated that Si causes the contrary effect of gaseous arsenic capture. In the temperature range of 600–1000 °C, the amounts of arsenic captured by F2O3, CaO and MgO were decreased from 63.2 to 37.5%, 25.3 to 12.4% and 7.84 to 6.48%, respectively. The retention efficiency of Fe2O3 was greatly higher than that of CaO and MgO at each temperature range. These results were presented well reproducibility and consistent for previous studies [27,30]. Zhang et al. suggest that mineral components including Fe and Ca could provide reactive sites and act as a catalyst and reactant in the retention processes [20]. The elevated retention efficiency of Fe2O3 indicated that Fe may provide more reactive sites and have better reactivity per unit mass than that of Ca and Mg. Many studies have reported that Fe2O3 has excellent ability to remove arsenic in water [39]. Ca-based materials were not only exhibited excellent capabilities for SO2 removal but also have shown greatly potential for the capture of gaseous elements (As, Cd and Se) emitted during coal combustion [28]. The drop in arsenic capture after 600 °C may be attributed to the decomposition of the reaction product and the sintering-induced loss in the surface area of CaO at high temperatures [7]. The arsenic retention efficiencies of Al2O3, NaNO3 and KNO3 were increased from 5.48 to 13.9%, 5.85 to 15.3%, and 4.62 to 17.6%, respectively. The arsenic retention mechanisms may different among these ash compositions. The increase in arsenic captured by alkali metals (K and Na) with increasing temperature may be due to the chemical reaction between alkali metal (K and Na) oxides and arsenic [25,40]. The reaction mechanisms were discussed in detail in Section 3.4. Physisorption may be the mainly arsenic sorption mechanism for Al2O3 due to the great surface area (85.32 m2/g). This result was fairly agreement with previous studies, which suggested that the great surface area may provide more active sites for gaseous arsenic [20,41]. 3.3. Effect of addition ratio of ash composition on arsenic retention
Fig. 3. Effect of temperatures and ash composition on arsenic capture at addition ratio of 5% ash composition.
The effect of addition ratio of ash composition on arsenic retention efficiency at 1000 °C was presented in Fig. 4. The arsenic reten-
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Fig. 4. Effect of addition ratio of ash composition on arsenic capture at combustion temperature of 1000 °C.
tion efficiencies were increased with increasing of ash compositions (except for SiO2). The results were agreement with previous studies on single gas-phase arsenic adsorption [21,27,30]. The arsenic retention efficiency increases from 11.1% to 37.5% when the addition ratio of Fe2O3 increases from 1% to 5%. The leastsquares linear fitting on arsenic retention efficiency by different ash compositions at 1000 °C was summarized in Table 4. The slope of each ash composition (except for SiO2) were higher than 1, suggesting that the retention efficiency were greatly relation with the addition ratio of these components. The slope of each ash composition also indicates that the impact of addition ratio decreases with the order of Fe2O3 > NaNO3 KNO3 > Al2O3 CaO > MgO > SiO2. The difference changes in arsenic retention quantity of the different ash compositions at various addition ratios were resulted from the different physico-chemical properties of the ash compositions and the relative reactions between arsenic and ash compositions [31]. For SiO2, the arsenic retention efficiency decreases with the increasing of SiO2, which may be attributed to the complex Si interactions [25]. The Si interaction mechanism was described in detail below. 3.4. Mechanism of arsenic and ash composition interaction The redistribution of arsenic in a combustion system was not only highly dependent on volatility and association of arsenic during combustion, but also determined by its interactions with gas phase species and ash compositions [19,21]. Among them, the interactions of arsenic with ash compositions may reduce the emission of gaseous arsenic due to the formation of thermally stable arsenates. Laboratory experimentation combined with thermodynamic equilibrium calculations using HSC-Chemistry 6.0 software were conducted to predict the possible interactions between arsenic and ash compositions.
Table 4 Linear fitting information. R2
Slope
SiO2 Al2O3 MgO Fe2O3 CaO NaNO3 KNO3
Value
Standard error
2.7778 1.9676 1.3889 6.5972 1.8519 2.4306 2.1991
1.2028 0.0668 0.2673 0.4678 0.2887 0.2005 0.0668
0.6842 0.9977 0.9286 0.9900 0.9526 0.9865 0.9982
95
Fe2O3 exerts a strong influence on redistribution of arsenic in a combustion system, the retention efficiency increases greatly with the increasing Fe in combustion processes. Lopez-Anton et al. applied both LA-ICP-MS and SEM/EDX to ascertain the potential for fly ash to capture arsenic and selenium, identified that arsenic in particles had a good association with Fe [30]. The arsenic distribution by the effect of the addition of ash compositions and the ash composition of fly ash during combustion at 1000 °C were shown in Fig. 5 and Table 5, respectively. The results indicate that the proportion of arsenic in fine particle (<38 lm) increased with the addition of Fe2O3. According to Table 5, it could be found that the added Fe2O3 was mainly existed in fly ash. The abovementioned results suggesting that Fe2O3 could react with gaseous arsenic easily in the combustion process, and then form as fine particle with the decreasing of flue gas temperature. Thermodynamic equilibrium calculations suggesting that FeAsO4 was the major stable specie during Fe-As interaction. Physisorption was regarded to be negligible due to the less surface area of selected Fe2O3 (14.27 m2/g). Meanwhile, Fe2O3 was known as catalytic oxidation, which may promote the formation of Fe arsenates [39]. Therefore, the Fe2O3 may be regarded as suitable additive for arsenic intention in a combustion system. The comprehensive reaction mechanism between arsenic species and Fe2O3 should be further investigations. Ca-based materials were extensively applied for the removal of sulfur during coal combustion [28]. Meanwhile, CaO was also supposed to have good effectiveness in toxic element capture by adding into fluidized bed combustion, or injecting into the flue gas during coal processing. The ability of CaO to capture gaseous arsenic increased with the increasing of temperature in the range of 500–600 °C, while the arsenic capture decreased with the increasing of temperature in the range of 600–1000 °C (Fig. 3). The result was consistent with previous study by Jadhav and Fan (2001). Meanwhile, this trend was also confirmed that the interaction mechanism between CaO and gaseous arsenic was temperature dependence. Thermodynamic equilibrium calculations suggesting that calcium arsenate (Ca3(AsO4)2), a non-volatile specie in flue gas was regarded as the most probably specie as a result of gas-solid reactions between gaseous arsenic and CaO. The decrease of arsenic retention in 600–1000 °C may be attributed to the formation of unstable specie (Ca2As2O7) in the 700–900 °C range [27,32]. Many studies suggesting that the arsenic retention 500 °C was ascribed to a combination of physical and chemical adsorption, while the chemical adsorption may be the primary arsenic retention mechanism for CaO at high temperature [42]. It could be observed that the added CaO was mainly found in fly ash (Table 5). The ratio of arsenic in small particles (<200 lm) increased gradually with adding CaO, indicating that calcium arsenate was mainly existed in small particles (Fig. 5). The arsenic retention efficiency of Mg oxides augmented up to a maximum and then decreased with the rising combustion temperature, which was similar to that of Fe2O3 and CaO. However, the retention efficiency of MgO was much lower than that of Fe2O3 and CaO. Like As-Ca interaction, Mg3(AsO4)2 was the most probably phase resulted from As-Mg interaction. Meanwhile, the proportion of arsenic in fine particle increased with adding MgO (Fig. 5). The presence of Al2O3 may result in the formation of AlAsO4, which was a very stable compound even at 1400 °C. Therefore, the adsorption efficiency of Al2O3 increased with the increasing of combustion temperature and adding ratio. Meanwhile, physisorption may be also suggested for Al2O3 due to the great surface area (85.32 m2/g) and increasing ratio of arsenic in coarse particle (Fig. 5 and Table 5). Arsenic and alkali metal (K and Na) oxides interact to form new species including K3AsO4, KH2AsO4, KAs3O8, Na3AsO4, NaH2AsO4, and NaAs3O8 [25]. Among them, K3AsO4 and Na3AsO4 were pre-
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Fig. 5. The effect of the addition of ash composition on arsenic distribution in fly ash during combustion at 1000 °C.
Table 5 The ash composition of fly ash by adding 5% of different ash components during combustion at 1000 °C (wt.%).
5% 5% 5% 5% 5% 5% 5%
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
54.7 52.2 51.9 51.5 51.8 51.6 51.7
32.3 34.6 31.8 31.7 31.9 31.6 31.8
4.25 4.26 7.72 3.98 4.21 4.19 4.33
3.09 2.87 3.05 7.09 3.21 3.04 3.14
1.16 1.18 1.23 1.14 4.57 1.15 1.22
1.30 1.37 1.29 1.32 1.22 5.07 1.28
1.64 1.67 1.54 1.57 1.67 1.66 5.36
dicted as the most probably compounds. The sizes of most particles decreased during combustion as a result of particle collisions caused attrition [43]. However, the existence of Na and K may occur agglomeration, and resulted in the increase of particle size. It could be found that the arsenic in the large particles (>38 lm) increased with the increasing of Na and K (Fig. 5). The addition of SiO2 in combustion system resulted in a higher amount of arsenic volatilization, which may be attributed to the various reactions among Si and ash compositions. Thermodynamic equilibrium calculations indicating that Si can react with Ca, K and Al. With the formation of silicates (CaSiO3, KAlSi2O8, KAlSi2O6, and KAlSiO4), SiO2 conduces a decrement in arsenic retention. Meanwhile, Si could promote the solid arsenic oxides (As2O3, As2O4 and As2O5) translate into volatile arsenic compounds including AsO (g), AsH (g) and As2 (g) [25]. Therefore, arsenic present in coal with a high concentration of Fe, Ca, Mg, Al, K and Na may have an important reduce in flue gas, while the existence of Si could cause a contrary effect on arsenic retention.
4. Conclusions Arsenic in coal was associated with sulfides, clay minerals, carbonate minerals and organic matter. With the decomposition of
these matrix at different temperatures, the volatilization of arsenic increased with the increasing of combustion temperature. The investigation on arsenic and ash compositions interactions via both laboratory experimentation and thermodynamic equilibrium calculations, indicating that arsenic behavior was strongly related to the chemical composition of coal in a combustion system. The interactions among arsenic and ash compositions were both temperature and amount dependence. The arsenic retention efficiency decreased with the order of Fe2O3 > NaNO3 KNO3 > Al2O3 CaO > MgO > SiO2. The retention efficiency increased with the adding ash compositions (except for SiO2). The arsenic retention efficiencies of Fe2O3, CaO and MgO increased maximum and then decreased with increasing combustion temperature, while the retention efficiencies of Al2O3, NaNO3 and KNO3 increased with the increasing of combustion temperature. The chemical adsorption may be the primary mechanism for Fe2O3, CaO, MgO, NaNO3 and KNO3, the adsorption of arsenic by Al2O3 may be ascribed to both physical and chemical adsorption. Arsenic may be captured with the formation of solid arsenic compounds (FeAsO4, AlAsO4, Ca3(AsO4)2, Mg3(AsO4)2, NaAs3O8, KAs3O8 and K2AsO4). SiO2 causes a contrary effect on arsenic retention by reacts with ash compositions which promote arsenic capture and reduce in the solid arsenic compounds.
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Full Length Article
Effect of ash composition on the partitioning of arsenic during fluidized bed combustion Chuncai Zhou a,b, Guijian Liu a,b,⇑, Zhongyu Xu a, Hao Sun a, Paul Kwan Sing Lam c a
CAS Key Laboratory of Crust-Mantle Materials and the Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, The Chinese Academy of Sciences, Xi’an, Shaanxi 710075, China c State Key Laboratory in Marine Pollution and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b
h i g h l i g h t s The transformation behavior of arsenic were affected by ash composition. The retention efficiency was affected by combustion temperature and addition ratio. Chemical adsorption may be the primary mechanism between arsenic and ash components.
a r t i c l e
i n f o
Article history: Received 11 January 2017 Received in revised form 8 May 2017 Accepted 11 May 2017
Keywords: Arsenic Ash composition Redistribution Chemical adsorption Coal combustion
a b s t r a c t The effect of the ash composition on the redistribution of arsenic in a coal combustion system was determined via both experimental laboratory simulations and thermodynamic equilibrium calculations. The experimental study was performed in a fluidized bed reactor in combustion temperature range of 500–1000 °C. The results show that arsenic redistribution is strongly related to the chemical composition of coal in a combustion system. The presence of Fe, Ca, Mg, Al, Na and K favour arsenic retention, while Si causes an opposing effect. The arsenic retention on ash compositions is both temperature and amount dependent. With rising combustion temperature, the arsenic retention efficiency of Fe, Ca and Mg oxides augmented up to a maximum and then decreased, while those of Al, Na and K compounds continued expanding in the range of 500–1000 °C. Chemical adsorption may be the primary mechanism for Fe, Ca, Mg, K and Na, while the adsorption of arsenic by Al may be ascribed to both physical and chemical adsorption. Arsenic is captured in ashes as a result of the formation of stable arsenate compounds arising from interactions with the inorganic matrix. The negative arsenic retention effect by silica is attributed to reactions with ash compositions which promote arsenic capture and the reduction of the solid arsenic compounds. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal used for energy generation is identified as one of the largest anthropogenic contributors to environmental sensitively trace elements [1]. Toxic heavy metal elements such as As, Hg, Se, Pb and Cd are at least partially volatilized at high temperature and cannot be captured by atmospheric pollution control devices [2–5]. The total emissions of As, Hg, Se, Pb and Cd from coal combustion by energy generation in 2012 were estimated at approximately 406.4, 139.4, 538.6, 833.0 and 15.5 tons in China, respectively [6].
⇑ Corresponding author at: CAS Key Laboratory of Crust-Mantle Materials and the Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.fuel.2017.05.048 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
As a result of emerging environmental and health concerns, a series of environmental legislations and technologies are related with reducing these toxic elements in environment [7]. Arsenic and its compounds are known to be of greatly environmental and health concern due to its high toxicity and carcinogenicity [8,9]. Many studies have addressed that arsenic is associated with sulfide minerals, clay minerals, carbonate minerals and organic matter [10–16]. It has been reported that arsenic associated with sulfides and organic matter is volatilized during combustion, and subsequently released as vapor phase or particulate phase [2,17–19]. The elemental arsenic (As) and arsenic trioxide (As2O3) are supposed to the two probable forms of arsenic in the oxidizing flue gas environment [7,20]. For the chemical reactivity of elemental arsenic, As2O3 is regarded as the only species in the combustion environment [7]. Gas phase arsenic is preferred to
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deposit on fine particle which contains larger surface area and active cation sites in the cooling flue gas [2,19,21]. These arsenicenriched fine particles are reduced limit by currently widely applied atmospheric pollution control devices (electrostatic precipitator and wet flue gas desulfurization), and partially emitted into environment. The arsenic removal efficiencies of electrostatic precipitator and wet flue gas desulfurization are estimated at about 86.2% and 80.4%, respectively [6]. Consequently, these emitted arsenic could be inhaled and deposited in lungs of humans and resulted in adverse health risks [8]. Meanwhile, many studies addressed that As2O3 in flue gas could react with active V2O5 sites and block the catalyst pores, and are prejudicial to selective catalytic reduction (SCR) installations [22]. Therefore, the study on gaseous arsenic control is of great environmental and industrial urgency. The redistribution behavior of arsenic during combustion is dependent on many factors, including the association in coal, the interaction with other element both during and after combustion, the combustion conditions (temperature and boiler type) [18,23,24]. It has been reported that the partitioning behavior of arsenic is not only determined by the volatility of arsenic, but also affected by the interactions with different mineral components [25,26]. Thus, any improvement to the process that favour a more efficient retention of arsenic by mineral components is worthwhile. Recycling fly ashes or some of mineral components for the retention of mercury, arsenic and selenium has been addressed, and a positive relationship is found between fly ash calcium and iron concentrations and the corresponding arsenic retention concentration in the fly ash [27–30]. The ability of various mineral sorbents, including silica, alumina, kaolinite, limestone, bauxite for gaseous arsenic retention is well established in the literature, and illustrated that Ca-, Al and Fe-based sorbents are more efficient in arsenic retention during combustion process [20,27,31]. Although many previous studies have been conducted on gaseous arsenic retention by Ca, Al and Fe-based materials, the interactions between arsenic with other typical ash compositions (Si, Mg, Na and K) are extremely scarce [25]. Meanwhile, many previous studies determined the retention behaviors of arsenic by using pure arsenic compounds (As2O5 standard liquid and As2O3) [7,20,27]. These simulating experiments may not reflect the real retention behavior of arsenic during coal combustion due to the complex chemical reactions among fuel compositions [25]. Therefore, study on the interaction mechanism of arsenic with the mineral components of fly ash by co-combustion coal with ash compositions in a fluidized bed reactor may be useful for identifying the optimum technology for gaseous arsenic removal. Thermodynamic equilibrium modelling could provide useful information for the interactions among two compounds, and have been widely applied to investigate the partitioning behavior of arsenic during combustion [31,32]. Nevertheless, the equilibrium modelling is calculated based on a particular condition (temperature, pressure and atmosphere), and are not considered about the complexity (association of arsenic in different type coal, size distribution of coal, mass transfer, interactions with other elements) in real combustion system [25]. Therefore, the thermodynamic equilibrium calculations were combined with experimentation to evaluate the possible reaction mechanisms between arsenic and different mineral components. The main objectives of this study are to: (1) ascertain the effect of combustion temperature, ash composition type and amount on arsenic retention during combustion; (2) evaluate the arsenic retention mechanism by different ash compositions. The ultimate aim will try to identify the ash compositions and combustion conditions which could optimize the gas-phase arsenic control.
2. Experimental 2.1. Materials The bituminous coal used for power generation in Pingwei Coalfired Plant was selected and collected. The physico-chemical property of the selected coal including ash, proximate and ultimate analysis were determined by our previous study and summarized in Table 1 [3]. The ash yield of the selected bituminous coal was approximately 13.1 wt.% and labeled as medium-to-low ash. The concentration of arsenic was 3.24 mg/kg, which was similar to the weighted average value of Chinese coal (3.79 mg/kg) [33]. The selected coal was air-dried, grinded and sieved into 100– 150 lm for subsequent analysis. 2.2. Combustion procedures The laboratory-scale fluidized bed furnace with a height of 1500 mm and an inside diameter of 50 mm was used to investigate the retention behavior of arsenic during combustion with various ash compositions. The details of the experimental equipment and procedure were found elsewhere [3,24]. The furnace was made by stainless steel and heated by a batch of electrical resistances packed with ceramic fibers to thermal insulation. The fuel feeding rate, the excess air ratio (relative to the theoretical air), the fluidizing air flow, the secondary air flow and the fuel/air ratio were 0.2 kg/h, 40%, 1 Nm3/h, 0.5 Nm3/h and 0.714, respectively. The details of the combustion conditions were presented in Table 2. The admixture of coal and additives was pneumatically placed in a screw feeder, equipped with an agitation chamber and an insulated injector to ensure the uniform fuel feeding rate. The addition ratios of ash compositions were 1 wt.%, 3 wt.% and 5 wt.% of the selected coal. For the chemical instability of alkali metal oxides (K2O and Na2O) in environment, the oxides were substituted by the alkali metal nitrates (NaNO3 and KNO3). The weight percentages of the added ash compositions were calculated as oxides content. Six combustion temperature points including 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C were conducted. The surface area, pore volume, pore diameter and particle size of the added ash chemicals were measured by an adsorption volumetric analyzer ASAP 2020 and Nano S laser particle size analyzer, respectively. The structure characteristics of the selected additives could be found in Table 3. The feedstock was fed into the reactor when the temperature reached the desired level and attained at a steady state. The combustion procedure was maintained approximately 2 h. The bottom ash and fly ash were collected at each site simultaneously. In addition, in order to investigate the distribution behavior of arsenic in different particle size during different addition ratios, the fly ash were sieved by the standard Tyler sieve into three parts of particle size distribution (<38 lm, 38–200 lm, and >200 lm). In order to ensure the precision and accuracy of the experimental results, the sampling for each experimental condition were repeated in triplicate. Once the relative deviations of the data from these samples were more than 20%, another samplings were required. The retention efficiency (RE) could illustrate the capture behavior of additives during combustion effectively. Generally, arsenic may be captured by ash compositions via both physisorption and chemical reactions. The retention efficiency was calculated as described below:
RE ð100%Þ ¼
Vc Vad 100 Vc
ð1Þ
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C. Zhou et al. / Fuel 204 (2017) 91–97 Table 1 The physical-chemical properties of the selected coal [3]. Proximate analysis, db (wt.%) Ultimate analysis, db (wt.%) Ash analysis (wt.%)
Moisture 1.80 C 71.5 SiO2 53.1
Al2O3 32.7
Ash 13.1 H 4.75 Fe2O3 4.31
N 1.38 CaO 3.13
MgO 1.18
Volatile matter 29.0 O 8.34 K2O Na2O 1.32 1.71
Fixed carbon 56.1 S 0.93 TiO2 SO3 1.53 0.07
P2O5 0.05
Table 2 Experimental conditions. Conditions
Value(s)
Feeding rate of feedstock The excess air ratio Fluidizing air flow Secondary air flow Fuel/air ratio Ash composition Addition of ash composition Combustion Temperature Combustion duration
0.2 kg/h 40% 1 Nm3/h 0.5 Nm3/h 0.714 SiO2, Al2O3, Fe2O3, MgO, CaO, NaNO3, KNO3 1%, 3% and 5% of coal 500, 600, 700, 800, 900, 1000 °C 2h
where Vc represents the volatilization ratio of arsenic during coal combustion (%), Vad represents the volatilization ratio of arsenic during combustion with additive (%). 2.3. Arsenic analysis
Fig. 1. The mineral phase of the selected coal.
The feedstock, bottom ash, fly ash and particles in different size distributions were acidic digested by using an acid mixture (HCl: HNO3:HF = 3:3:2). Details of the digestion procedure were given elsewhere [16,34,35]. The standard reference material NIST 1632b (coal) was used to calibrate the accuracy of analytical method. The concentration of arsenic was determined by inductively coupled plasma mass spectrometry (ICP-MS). The concentration of arsenic for each sample was obtained by the average value of three parallel sampling. Meanwhile, the modified sequential chemical extraction procedure described by Zheng et al. (2008) was applied to investigate the association of arsenic in coal. Arsenic could be partitioned into six fractions, including waterleachable bound (B1), ion-changeable bound (B2), organic-bound (B3), carbonate-bound (B4), silicate-bound (B5) and sulfidebound (B6) [36]. The fractions obtained during each step of the analytical procedure were measured for arsenic by ICP-MS. 3. Results and discussion 3.1. The volatilization behavior of arsenic during combustion The physico-chemical property of the selected bituminous coal was described in our previous study [3]. The selected coal was regarded as typical medium to low bituminous coal, and was suitable for power generation. The mineral composition of the selected coal consists of kaolinite, quartz, calcite, pyrite, illite, dolomite and chlorite (Fig. 1). Many studies have addressed that arsenic was
associated with both organic matter and inorganic minerals (clay minerals, sulfides), mostly associated with inorganic matrix [10– 14,16,19]. Arsenic was mainly associated with sulfide bound (54.3%), silicate bound (20.8%), organic bound (9.7%) and carbonate bound (9.6%) (Fig. 2a), indicates that arsenic in selected coal was associated with sulfides, clay minerals, carbonate minerals, and organic matter. The organic matter and inorganic minerals (including sulfides, clay and carbonate minerals) undergo various physico-chemical reactions, including decomposition, carbonization, oxidation, hydration, dehydration and dehydroxylation during high temperature combustion [18]. Accompanied with these complex reactions, arsenic in coal was released and redistributed among coarse particles, fine particles and flue gas. The redistribution behavior of arsenic was attributed to the physico-chemical property of coal, association of arsenic, reaction of inorganic mineral and organic matter, boiler type, combustion temperature and atmosphere [19,34,37]. The volatilization ratio of arsenic at different combustion temperatures was presented in Fig. 2b. It was obviously found that the volatilization ratio of arsenic increases from 15.2% to 21.6% with the increasing combustion temperature from 500 °C to 1000 °C. The volatilization behavior of arsenic had been widely addressed and could be found elsewhere [2,16,19]. The release of arsenic below 700 °C was attributed to the decomposition of organic matter and sulfide minerals, while the emission of arsenic at 700–1000 °C may be explained by the decomposition of clay and carbonate minerals. Winter et al. reported that arsenic
Table 3 The structure properties of ash compositions. Ash composition
Surface area (m2/g)
Particle size (lm)
Pore volume (cm3/g)
Pore diameter (nm)
SiO2 Al2O3 Fe2O3 MgO CaO
45.58 85.32 14.27 68.76 7.44
2.645 1.783 5.016 6.735 4.652
0.64 0.72 0.55 0.16 0.22
14.4 7.24 23.65 8.27 13.68
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Fig. 2. The association (a) and volatilization ratio (b) of arsenic in coal. B1- water-leachable bound, B2- ion-changeable bound, B3- organic-bound, B4- carbonate-bound, B5silicate-bound, B6- sulfide-bound.
trioxide (As2O3) was the most likely form of gaseous arsenic in flue gas in an oxidizing environment [38].
3.2. Effect of combustion temperature and ash composition on arsenic retention The retention efficiency of arsenic captured at temperature in the range of 500–1000 °C by addition ratio of 5% ash compositions were presented in Fig. 3. As illustrated in Fig. 3, the retention efficiencies were various among different additives and different temperatures. According to the retention behaviors, the selected additives could be clustered into three groups. For Fe2O3, CaO and MgO, the retention efficiencies increased maximum at 600 °C, 600 °C and 700 °C, respectively, and then decreased with increasing combustion temperature. For Al2O3, NaNO3 and KNO3, the retention efficiencies increased with the increasing combustion temperatures in the range of 500–1000 °C. For SiO2, the gaseous
arsenic increased with the increasing temperature, indicated that Si causes the contrary effect of gaseous arsenic capture. In the temperature range of 600–1000 °C, the amounts of arsenic captured by F2O3, CaO and MgO were decreased from 63.2 to 37.5%, 25.3 to 12.4% and 7.84 to 6.48%, respectively. The retention efficiency of Fe2O3 was greatly higher than that of CaO and MgO at each temperature range. These results were presented well reproducibility and consistent for previous studies [27,30]. Zhang et al. suggest that mineral components including Fe and Ca could provide reactive sites and act as a catalyst and reactant in the retention processes [20]. The elevated retention efficiency of Fe2O3 indicated that Fe may provide more reactive sites and have better reactivity per unit mass than that of Ca and Mg. Many studies have reported that Fe2O3 has excellent ability to remove arsenic in water [39]. Ca-based materials were not only exhibited excellent capabilities for SO2 removal but also have shown greatly potential for the capture of gaseous elements (As, Cd and Se) emitted during coal combustion [28]. The drop in arsenic capture after 600 °C may be attributed to the decomposition of the reaction product and the sintering-induced loss in the surface area of CaO at high temperatures [7]. The arsenic retention efficiencies of Al2O3, NaNO3 and KNO3 were increased from 5.48 to 13.9%, 5.85 to 15.3%, and 4.62 to 17.6%, respectively. The arsenic retention mechanisms may different among these ash compositions. The increase in arsenic captured by alkali metals (K and Na) with increasing temperature may be due to the chemical reaction between alkali metal (K and Na) oxides and arsenic [25,40]. The reaction mechanisms were discussed in detail in Section 3.4. Physisorption may be the mainly arsenic sorption mechanism for Al2O3 due to the great surface area (85.32 m2/g). This result was fairly agreement with previous studies, which suggested that the great surface area may provide more active sites for gaseous arsenic [20,41]. 3.3. Effect of addition ratio of ash composition on arsenic retention
Fig. 3. Effect of temperatures and ash composition on arsenic capture at addition ratio of 5% ash composition.
The effect of addition ratio of ash composition on arsenic retention efficiency at 1000 °C was presented in Fig. 4. The arsenic reten-
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Fig. 4. Effect of addition ratio of ash composition on arsenic capture at combustion temperature of 1000 °C.
tion efficiencies were increased with increasing of ash compositions (except for SiO2). The results were agreement with previous studies on single gas-phase arsenic adsorption [21,27,30]. The arsenic retention efficiency increases from 11.1% to 37.5% when the addition ratio of Fe2O3 increases from 1% to 5%. The leastsquares linear fitting on arsenic retention efficiency by different ash compositions at 1000 °C was summarized in Table 4. The slope of each ash composition (except for SiO2) were higher than 1, suggesting that the retention efficiency were greatly relation with the addition ratio of these components. The slope of each ash composition also indicates that the impact of addition ratio decreases with the order of Fe2O3 > NaNO3 KNO3 > Al2O3 CaO > MgO > SiO2. The difference changes in arsenic retention quantity of the different ash compositions at various addition ratios were resulted from the different physico-chemical properties of the ash compositions and the relative reactions between arsenic and ash compositions [31]. For SiO2, the arsenic retention efficiency decreases with the increasing of SiO2, which may be attributed to the complex Si interactions [25]. The Si interaction mechanism was described in detail below. 3.4. Mechanism of arsenic and ash composition interaction The redistribution of arsenic in a combustion system was not only highly dependent on volatility and association of arsenic during combustion, but also determined by its interactions with gas phase species and ash compositions [19,21]. Among them, the interactions of arsenic with ash compositions may reduce the emission of gaseous arsenic due to the formation of thermally stable arsenates. Laboratory experimentation combined with thermodynamic equilibrium calculations using HSC-Chemistry 6.0 software were conducted to predict the possible interactions between arsenic and ash compositions.
Table 4 Linear fitting information. R2
Slope
SiO2 Al2O3 MgO Fe2O3 CaO NaNO3 KNO3
Value
Standard error
2.7778 1.9676 1.3889 6.5972 1.8519 2.4306 2.1991
1.2028 0.0668 0.2673 0.4678 0.2887 0.2005 0.0668
0.6842 0.9977 0.9286 0.9900 0.9526 0.9865 0.9982
95
Fe2O3 exerts a strong influence on redistribution of arsenic in a combustion system, the retention efficiency increases greatly with the increasing Fe in combustion processes. Lopez-Anton et al. applied both LA-ICP-MS and SEM/EDX to ascertain the potential for fly ash to capture arsenic and selenium, identified that arsenic in particles had a good association with Fe [30]. The arsenic distribution by the effect of the addition of ash compositions and the ash composition of fly ash during combustion at 1000 °C were shown in Fig. 5 and Table 5, respectively. The results indicate that the proportion of arsenic in fine particle (<38 lm) increased with the addition of Fe2O3. According to Table 5, it could be found that the added Fe2O3 was mainly existed in fly ash. The abovementioned results suggesting that Fe2O3 could react with gaseous arsenic easily in the combustion process, and then form as fine particle with the decreasing of flue gas temperature. Thermodynamic equilibrium calculations suggesting that FeAsO4 was the major stable specie during Fe-As interaction. Physisorption was regarded to be negligible due to the less surface area of selected Fe2O3 (14.27 m2/g). Meanwhile, Fe2O3 was known as catalytic oxidation, which may promote the formation of Fe arsenates [39]. Therefore, the Fe2O3 may be regarded as suitable additive for arsenic intention in a combustion system. The comprehensive reaction mechanism between arsenic species and Fe2O3 should be further investigations. Ca-based materials were extensively applied for the removal of sulfur during coal combustion [28]. Meanwhile, CaO was also supposed to have good effectiveness in toxic element capture by adding into fluidized bed combustion, or injecting into the flue gas during coal processing. The ability of CaO to capture gaseous arsenic increased with the increasing of temperature in the range of 500–600 °C, while the arsenic capture decreased with the increasing of temperature in the range of 600–1000 °C (Fig. 3). The result was consistent with previous study by Jadhav and Fan (2001). Meanwhile, this trend was also confirmed that the interaction mechanism between CaO and gaseous arsenic was temperature dependence. Thermodynamic equilibrium calculations suggesting that calcium arsenate (Ca3(AsO4)2), a non-volatile specie in flue gas was regarded as the most probably specie as a result of gas-solid reactions between gaseous arsenic and CaO. The decrease of arsenic retention in 600–1000 °C may be attributed to the formation of unstable specie (Ca2As2O7) in the 700–900 °C range [27,32]. Many studies suggesting that the arsenic retention 500 °C was ascribed to a combination of physical and chemical adsorption, while the chemical adsorption may be the primary arsenic retention mechanism for CaO at high temperature [42]. It could be observed that the added CaO was mainly found in fly ash (Table 5). The ratio of arsenic in small particles (<200 lm) increased gradually with adding CaO, indicating that calcium arsenate was mainly existed in small particles (Fig. 5). The arsenic retention efficiency of Mg oxides augmented up to a maximum and then decreased with the rising combustion temperature, which was similar to that of Fe2O3 and CaO. However, the retention efficiency of MgO was much lower than that of Fe2O3 and CaO. Like As-Ca interaction, Mg3(AsO4)2 was the most probably phase resulted from As-Mg interaction. Meanwhile, the proportion of arsenic in fine particle increased with adding MgO (Fig. 5). The presence of Al2O3 may result in the formation of AlAsO4, which was a very stable compound even at 1400 °C. Therefore, the adsorption efficiency of Al2O3 increased with the increasing of combustion temperature and adding ratio. Meanwhile, physisorption may be also suggested for Al2O3 due to the great surface area (85.32 m2/g) and increasing ratio of arsenic in coarse particle (Fig. 5 and Table 5). Arsenic and alkali metal (K and Na) oxides interact to form new species including K3AsO4, KH2AsO4, KAs3O8, Na3AsO4, NaH2AsO4, and NaAs3O8 [25]. Among them, K3AsO4 and Na3AsO4 were pre-
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Fig. 5. The effect of the addition of ash composition on arsenic distribution in fly ash during combustion at 1000 °C.
Table 5 The ash composition of fly ash by adding 5% of different ash components during combustion at 1000 °C (wt.%).
5% 5% 5% 5% 5% 5% 5%
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
54.7 52.2 51.9 51.5 51.8 51.6 51.7
32.3 34.6 31.8 31.7 31.9 31.6 31.8
4.25 4.26 7.72 3.98 4.21 4.19 4.33
3.09 2.87 3.05 7.09 3.21 3.04 3.14
1.16 1.18 1.23 1.14 4.57 1.15 1.22
1.30 1.37 1.29 1.32 1.22 5.07 1.28
1.64 1.67 1.54 1.57 1.67 1.66 5.36
dicted as the most probably compounds. The sizes of most particles decreased during combustion as a result of particle collisions caused attrition [43]. However, the existence of Na and K may occur agglomeration, and resulted in the increase of particle size. It could be found that the arsenic in the large particles (>38 lm) increased with the increasing of Na and K (Fig. 5). The addition of SiO2 in combustion system resulted in a higher amount of arsenic volatilization, which may be attributed to the various reactions among Si and ash compositions. Thermodynamic equilibrium calculations indicating that Si can react with Ca, K and Al. With the formation of silicates (CaSiO3, KAlSi2O8, KAlSi2O6, and KAlSiO4), SiO2 conduces a decrement in arsenic retention. Meanwhile, Si could promote the solid arsenic oxides (As2O3, As2O4 and As2O5) translate into volatile arsenic compounds including AsO (g), AsH (g) and As2 (g) [25]. Therefore, arsenic present in coal with a high concentration of Fe, Ca, Mg, Al, K and Na may have an important reduce in flue gas, while the existence of Si could cause a contrary effect on arsenic retention.
4. Conclusions Arsenic in coal was associated with sulfides, clay minerals, carbonate minerals and organic matter. With the decomposition of
these matrix at different temperatures, the volatilization of arsenic increased with the increasing of combustion temperature. The investigation on arsenic and ash compositions interactions via both laboratory experimentation and thermodynamic equilibrium calculations, indicating that arsenic behavior was strongly related to the chemical composition of coal in a combustion system. The interactions among arsenic and ash compositions were both temperature and amount dependence. The arsenic retention efficiency decreased with the order of Fe2O3 > NaNO3 KNO3 > Al2O3 CaO > MgO > SiO2. The retention efficiency increased with the adding ash compositions (except for SiO2). The arsenic retention efficiencies of Fe2O3, CaO and MgO increased maximum and then decreased with increasing combustion temperature, while the retention efficiencies of Al2O3, NaNO3 and KNO3 increased with the increasing of combustion temperature. The chemical adsorption may be the primary mechanism for Fe2O3, CaO, MgO, NaNO3 and KNO3, the adsorption of arsenic by Al2O3 may be ascribed to both physical and chemical adsorption. Arsenic may be captured with the formation of solid arsenic compounds (FeAsO4, AlAsO4, Ca3(AsO4)2, Mg3(AsO4)2, NaAs3O8, KAs3O8 and K2AsO4). SiO2 causes a contrary effect on arsenic retention by reacts with ash compositions which promote arsenic capture and reduce in the solid arsenic compounds.
C. Zhou et al. / Fuel 204 (2017) 91–97
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