Volatilization of Arsenic During Coal Combustion Based on Isothermal

Article pubs.acs.org/EF

Volatilization of Arsenic During Coal Combustion Based on Isothermal Thermogravimetric Analysis at 600−1500 °C Huimin Liu,† Wei-Ping Pan,†,‡ Chunbo Wang,*,† and Yue Zhang† †

Department of Energy Power & Mechanical Engineering, North China Electric Power University, Beijing 102206, China Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, Kentucky 42101, United States

‡

ABSTRACT: The volatilization characteristics of arsenic in different coals at 600−1500 °C were studied on an isothermal reaction system. Through thermogravimetric analysis similar to that used for coal analysis, the mass loss and mass loss rate of arsenic for coal samples were determined. Sequential chemical extraction method was used to measure the mode of occurrence of arsenic in the coals. TG-MS techniques were also carried out to study the relationship between sulfur and arsenic. Results show that the volatilization proportion of arsenic increases with temperature and 53− 99% of the total arsenic in coal is vaporized in the combustion zone at PC conditions (1500 °C). Coals with higher arsenic concentrations (As > 4 μg/g) tend to have larger arsenic volatility proportions than coals with lower arsenic concentrations (As < 4 μg/g) at a given temperature. In addition, three volatility zones with two mass loss rate peaks of arsenic are observed in all coals during coal combustion. Before 600 °C, the evaporation of organic-bound arsenic dominates; then the first arsenic mass loss peak at 800−900 °C is mainly from the decomposition or oxidation of arsenic in sulfide forms. The second peak after 1000 °C is probably generated through the decomposition of arsenates. For As > 4 μg/g coals, the first peak is higher than the second peak due to the larger proportion of sulfide-bound arsenic, while the second mass loss peak of arsenic is higher in As < 4 μg/g coals due to the larger proportion of arsenates. Furthermore, thermodynamic analysis by HSC chemistry6.0 software were carried out to prove that arsenates, like Ca3(AsO4)2, FeAsO4, and Mg3(AsO4)2, are thermally stable and could only decompose at relatively high temperatures.

1. INTRODUCTION With increasing public awareness of the environmental impact of coal combustion, serious concerns have been raised regarding the emissions of various hazardous trace elements, such as mercury, arsenic, lead, and cadmium, from power plants. Among the more harmful elements, arsenic (As) is garnishing greater attention due to its toxicity, volatility, bioaccumulation in the environment, and potential carcinogenic propensities.1 Nearly all types of arsenic compounds are toxic and As3+ is 50 times more toxic than As5+.2 As the major anthropogenic emission source of arsenic, coal-fired power plants account for 2−6% of the total arsenic emission, of which 0.7−52% was reported as As3+ in the vapor phase during the combustion process.3,4 Studying the volatility characteristics of arsenic during coal combustion shows an important guiding significance for arsenic emission and removal in coal-fired power stations. During combustion, trace elements diffuse from the pores of coke particles and volatilize into the gas phase. The volatilization behavior of arsenic during coal combustion is significantly affected by the mode of occurrence of arsenic, temperature, atmosphere, coal rank, minerals, and other factors.4,5 Clarke6 pointed out that temperature was the main factor affecting arsenic volatilization in coal. Wang7 showed that arsenic was significantly volatilized during rapid-heating combustion at a heating rate of 500 °C/min, whereas minimal quantities were emitted during slow-heating combustion of 5− 10 °C/min. Senior8 reported that for bench scale combustion experiments using bituminous and sub-bituminous pulverized coals, 40−80% of the initial arsenic content was vaporized at a © XXXX American Chemical Society

high temperature (1423K). Similar positive correlations between arsenic volatilization and temperature in a combustion atmosphere are available in the literature.9−11 Only a few scientists theorized that arsenic volatilization did not always increase with temperature.12,13 Numerous studies have focused on the volatilization of arsenic during coal combustion, but most studies focused on the release of arsenic as a function of temperature without studying the associated cause. Dai14 studied the volatilization characteristics of arsenic in coals with high arsenic concentrations in the temperature range of 600−1000 °C. Dai concluded that the volatilization ratio of arsenic increased with combustion temperature, while the volatilization rate of arsenic varied at different temperature ranges and arsenic showed a significant increase in volatilization at 700−900 °C. This study indicates that temperature would not only affect the volatilization ratio of arsenic, but also the volatilization rate of arsenic. Unfortunately, there is limited similar research available for comparison and no detailed explanations have been presented so far. Besides, the mode of occurrence of arsenic in coals changes greatly as it transfers from solid phase to vapor phase during combustion. Many scholars have studied the migration characteristics of arsenic under power station conditions and measured the proportions of arsenic in raw coals, bottom ashes, and fly ashes.2,4,11,15−18 However, these analyses were usually limited to the phase transition and the Received: April 7, 2016 Revised: July 12, 2016

A

DOI: 10.1021/acs.energyfuels.6b00816 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Ultimate/Proximate Analysis and Arsenic Concentration of Coal Samples ultimate analysis wada/%

a

b

sample

C

H

O

A1 A2 JL SJS GZ L1 KY

79.02 75.62 44.5 60.07 44.78 42.59 44.74

3.36 1.28 3.74 4.31 2.48 6.82 5.33

7.90 2.35 12.09 9.78 9.74 38.77 32.94

ad:

proximate analysis wada/% N

S

M

V

A

FC

As μg/g

1.51 1.26 0.77 0.63 0.45 0.66 1.23

0.99 0.60 0.32 0.49 4.32 0.45 1.58

1.01 0.54 1.58 1.06 1.31 5.97 4.59

9.39 3.45 22.50 22.51 22.27 38.46 39.59

7.22 18.89 38.58 24.72 38.23 10.71 14.18

82.38 77.12 37.34 51.71 38.19 44.86 41.64

0.36 3.19 0.96 4.26 10.22 4.51 68.35

air-dry. bBy difference.

Table 2. Major Compounds in Coal Ash ash composition, % sample

Al2O3

CaO

Fe2O3

K2O

Na2O

MgO

MnO

SiO2


35.17 24.04 52.75 21.09 18.48 19.09 20.67

3.82 1.05 2.83 6.24 5.02 15.55 18.21

8.02 12.30 1.83 7.92 4.98 5.69 6.61

2.04 3.25 1.41 3.75 4.70 4.13 1.28

0.25 0.39 0.35 1.55 1.13 5.58

1.33 1.29 0.27 1.62 3.45 7.50 3.10

0.14 0.23 0.06 0.11 0.03 0.16 0.04

49.20 55.12 28.41 49.87 55.10 33.39 63.25

Figure 1. Experimental system of arsenic volatilization during coal combustion.

distribution of total arsenic content, without considering the change of specific arsenic compounds in raw coal during the combustion process. For example, under burning conditions, sulfide-bound arsenic in the coals could generate arsenic trioxide through oxidation or decomposition, meanwhile minerals could react with arsenic to generate stable arsenates. These internal changes may be the main reason causing arsenic migration and the volatilization difference of coals at different temperatures. The volatilization characteristics of arsenic during coal combustion was studied in our previous work,19 in which the combustion temperature was limited to 1100 °C due to limitation of the conventional tube furnace. However, the chamber temperature in a PC (pulverized coal) boiler is relatively high, usually above 1400 °C. Thus, a further study on the arsenic volatilization under higher temperature conditions is urgently needed to better understand the actual conditions in power plants. In this study, isothermal combustion experiments of seven coals in three ranks were carried out in a hightemperature tube furnace in the temperature range of 600 to 1500 °C to further study the migration and volatilization characteristics of arsenic. Experiments combined with isothermal thermogravimetric analysis, the goal of this study was

to examine the effects of temperature and arsenic content on the mass loss characteristics of arsenic during coal combustion. The knowledge obtained from this work should provide a basis for understanding the volatility characteristics of arsenic and hopefully lead to developing techniques for removing arsenic in large scale power plants.

2. EXPERIMENTAL SECTION 2.1. Description of Coal Samples. Seven typical Chinese coals representing three ranks were used in this study. They were two anthracites (A1, A2), three bituminous coals (JL, SJS, GZ), and two lignites (L1, KY). The GZ and KY coals had relatively high sulfur content and the KY coal possessed the highest arsenic content. Coal samples were air-dried, crushed, and sieved to 120−150 μm. The ultimate/proximate analysis and arsenic concentrations of the seven coals are shown in Table 1. Major composition of the ash is shown in Table 2. 2.2. Combustion Testing. The combustion experiments were carried out using isothermal conditions in a horizontal hightemperature tubular reactor system as shown in Figure 1. The furnace (Φ50 mm × L120 mm) could move along the horizontal smooth rail. After heating to a set temperature and keeping stable for 30 min, the furnace was quickly pushed along the rail to the set position, where about 0.5 g coal sample was weighted and loaded on the corundum B


Article

Energy & Fuels

g; cash represents the arsenic content in ash obtained at temperature T, μg/g; and η is the corresponding ash yield per mass unit at cited temperature; mash refers to the amount of arsenic in ash which is denoted on the basis of the initial coal mass, μg/g. Thus, mash could be represented as mash = cash × η. The residual percentage of arsenic (w) in coal ash during coal combustion at temperature T is calculated as follows.

boat for combustion. Data collection system and computer were used to record the ash weight loss. After each combustion test, ash sample was cooled and collected for further study. The mixed gas flow rate was constant at 0.14 m3/h, preliminary experiments showed that the flow rate could eliminate the influence of external diffusion, which means that little effect was made on the isothermal kinetics of coal combustion with a higher gas flow rate.20,21 A temperature range of 600−1500 °C was selected in this study. To minimize the ash quality difference caused by weighting error, airflow fluctuations, heating rate, and other factors, each condition was repeated three times and the average quality of ash sample was calculated as the final value. Postcombustion ash samples were digested in a microwave system (SpeedWaveMWS-4, German). The digestion involved placing 200 mg of the ash sample with 10 mL of nitric acid and 3 mL hydrofluoric acid (both acids were trace metal grade) in the polytetrafluoroethylene (PTFE) digestion container, which was subsequently heated at 20 °C per minute to 150 °C, followed by isothermal conditions for 12 min. The concentration of arsenic in the coal and ash samples was measured using an atomic fluorescence spectrometer (PSA10.055 Millennium Excalibur by PS Analytical). Laboratory blank and spiked samples were measured with each sample batch for quality control purposes. Each sample was analyzed in triplicate and the relative standard deviation for all results was 5−10%. 2.3. Sequential Chemical Extraction. Sequential chemical extraction was used to determine the mode of occurrence of arsenic in coals. The mode of occurrence of arsenic in coal was divided into exchangeable, sulfide-bound, organic-bound, and residual fractions.22 The extraction procedures are listed in Table 3, where F1 is mainly

w=

solid phases

F1 F2

water-soluble and exchangeable sulfide-bound

F3

organic-bound

F4

residual

extraction (0.5 g of dry solid)

v = (1 − w) × 100%

8 h at room temperature

40 mL of 12.5% HNO3 (w/w) 30 mL of DI water (pH2) + 10 mL of 30% H2O2 10 mL of HNO3 and 3 mL HF

0.5 h at 95 °C with intermittent oscillation 5 h at 80−85 °C, with intermittent oscillation

(2)

According to the definition of the mass loss rate of coal, the mass loss rate of arsenic (r) during coal combustion refers to the changing speed of the arsenic concentration with temperature in the ash based on the initial coal mass. Then the expression of r is r=

w − w1 d(1 − w) dw =− =− 2 dT dT T2 − T1

(3)

Where w is the residual proportion of arsenic, %; r is the mass loss rate of arsenic, %/°C; and w1 and w2 refer to the mass loss of arsenic at temperatures T1 and T2 separately, %.

3. RESULTS AND DISCUSSION 3.1. Arsenic Mass Loss as a Function of Temperature. Arsenic volatilization characteristics of different coals during isothermal coal combustion are shown in Figure 2. Since the combustion experiments were carried out at different temperature points, several residual proportions of arsenic (w) were obtained as shown by scatters in Figure 2, where w = 100% at 25 °C. Through B-spline curve fitting, the curves of residual proportion of arsenic versus temperature were obtained. Similar to the mass loss of coal, this kind of curve is identified as the mass loss of arsenic. It is worth noting that, (1) not all of the coals were heated to 1500 °C in the experiments because of a melting problem of ash on the sample holder due to their various AFTs (ash fusion temperature). In this case, the L1 coal was heated to 1000 °C, GZ and KY coals were heated to 1200 °C, and the A1, A2, and SJS coals were heated to 1400 °C. (2) Under the premise of AFT limitations, arsenic mass loss curves of all the coals were extended to 1500 °C based on the experimental results and trends, which are indicated by the dotted lines shown in Figure 2. (3) Due to the trace content of arsenic in coal, the mass loss and mass loss rate of arsenic through coal combustion could not directly obtained by TG analyzer or TG-MS analyzer. In this case, the mass loss of arsenic was indirectly determined in this paper utilizing the B-

shaking time and temperature

40 mL of 1N MgCl2

(1)

The mass loss ratio of arsenic (v) which equals to the volatility proportion of arsenic is expressed as below.

Table 3. Sequential Chemical Extraction Procedure Used for Arsenic Speciation step

mash η × cash = × 100% mcoal mcoal

drying for 12 h at 60 °C, then microwave digestion

water-soluble and exchangeable fractions; F2 presents arsenic dissolved in dilute nitric acid, including minority carbonates, sulfates, iron and manganese oxides, and the vast majority of arsenic in the form of sulfides; F3 presents arsenic combined with organic matter; and F4 is for arsenic into clay mineral lattice, mainly for arsenates. 2.4. Data Analysis. The arsenic content in the coal and ash samples was used to determine the volatilization characteristics of arsenic. mcoal refers to the arsenic content in initial unburned coal, μg/

Figure 2. Arsenic mass loss curves of different coals at 25−1500 °C. C


Article

Energy & Fuels

content (>4 μg/g) tend to have a higher arsenic volatilization at the same conditions. This is mainly because the four coals with high arsenic concentrations (>4 μg/g) are all young coals with a relatively low degree of coalification; the molecules in the coals are held together by relatively weak bonds, thus arsenic is easily vaporized under heating conditions, increasing the mass loss. While the coals with lower arsenic concentrations ( 4 μg/g coals are all larger than 1, indicating that these coals tend to release arsenic

spline curve fitting based on the mass loss scatters of arsenic at different temperatures. As shown in Figure 2, the arsenic mass of all the coal samples decrease as temperature increases, indicating that a large amount of arsenic is released during combustion. The higher combustion temperatures lead to more complete coal decomposition, allowing the trace elements originally contained in coal to escape into the atmosphere.10 From Figure 2 we can see that the residual arsenic proportion of all coals at 1500 °C varies from 1 to 47%, which means that approximately 53− 99% of the arsenic present in coals would be vaporized at the temperature conditions (1400−1500 °C) in a PC power plant combustion zone. These results are strikingly similar to the model results of Senior et al.23 Though the concentration of arsenic decreases with increasing temperature, not all seven of the coals exhibit mass loss to the same extent, the volatility proportions of seven coals at different temperatures are shown in Table 4. Table 4. Arsenic Volatility Proportions (v) of Coals at Different Temperatures volatility proportion (v)/% coal

name

600 °C

900 °C

1500 °C

anthracite


2 2 14 19 19 17 14

17 17 36 54 62 43 53

60 54 53 88 95 90 99

bitumite

lignite

At 600 °C, 2% of the total arsenic in A1 and A2 coals and 14−19% of the total arsenic in the other coals escaped into the vapor phase. At 900 °C, 17−36% of the total arsenic in coals A1, A2, and JL escaped into the vapor phase. In comparison, vaporization in coals SJS, GZ, L1, and KY was elevated at 43− 62%. At 1500 °C, coals A1, A2, and JL had less than 60% mass loss in arsenic, whereas the last four coals had almost 100% arsenic mass loss. Clearly the volatility proportions of arsenic in A1, A2, and JL coals are generally lower than the other four coals at the same temperature. Since the arsenic concentrations in A1, A2, and JL coals are relatively low (lower than the average value of China, 4 μg/ g24), while the arsenic contents in the other four coals are all higher than 4 μg/g. It is indicated that coals with higher arsenic

Figure 3. Arsenic mass loss rate of different coals at 25−1500 °C. D


Article

Energy & Fuels Table 5. Peak Values of the Mass Loss Rate Curves of Arsenic peak one

peak two

coal

name

temperature T1/°C

mass loss rate r1/%·°C1−

temperature T2/°C

mass loss rate r2/%·°C1−

r1/r2

anthracite


867 858 850 829 850 873 850

0.09 0.09 0.11 0.13 0.22 0.14 0.21

1355 1362 not completed 1355 1173 1227 1166

0.11 0.11 not completed 0.09 0.13 0.12 0.13

0.82 0.82

bitumite

lignite

1.44 1.69 1.17 1.62

exchangeable species. It shows that As > 4 μg/g coals tend to have a larger proportion of weak bonded arsenic, such as sulfides and organic-bound forms. 4 μg/g coals, the first mass loss peak of arsenic is higher than the second one, which means that As > 4 μg/g coals own more weak-bonded arsenic and tend to release arsenic at lower temperatures. In this section, sequential chemical extraction results in Figure 4 show that As > 4 μg/g coals own a larger proportion of sulfide-bound arsenic, which is weakly bonded and will continuously generate arsenic oxides through decomposition and oxidation with increasing temperature. Mass loss rate of arsenic (Section 3.2) combined with the sequential chemical extraction experiments (Figure 4), it is indicated that the main reason for the first mass loss peak of arsenic is the decomposition and oxidation of sulfide-bound forms. The detailed transformation of the occurrence mode of arsenic in coal ashes at 600−1000 °C was reported elsewhere.19 >1000 °C Analysis. At temperature greater than 1000 °C, the second mass loss peak of arsenic is observed while the peak temperature varies from coal to coal (as seen in Figure 3). As discussed in Section 3.2, (1) the second mass loss peak at the 1100−1500 °C temperature zone represents the loss of various arsenic compounds with strong intermolecular bonding; (2) for As < 4 μg/g coals, the second mass loss peak of arsenic is higher than the first one, which means that As < 4 μg/g coals own more strong-bonded arsenic and tend to release arsenic at higher temperatures. In this section, sequential chemical extraction results in Figure 4 show that As < 4 μg/g coals own a larger proportion of residual arsenic, mainly arsenates which are strong bonded and have relatively high decomposition points due to their chemical stabilities.26 Since there

at lower temperatures. While the r1/r2 values of the coals with As < 4 μg/g are smaller than 1, indicating that these coals tend to release arsenic at higher temperatures. The r1/r2 value for the JL coal was not calculated since the second mass loss peak of arsenic in JL coal did not appear at the end of the experiment. 3.3. Effect of Mode of Occurrence on Volatilization of Arsenic. To further analyze the effect of temperature and arsenic content on the volatilization of arsenic, sequential chemical extraction was used to measure the occurrence of arsenic in raw coals as shown in Figure 4.

Figure 4. Mode of occurrence of arsenic in coal samples.

In this study, the mode of occurrence of arsenic in coal was divided into four forms of exchangeable, organic-bound, sulfidebound, and residual. In general, arsenic in organic-bound and sulfide-bound forms are easily oxidized and vaporized into gas phase due to their low thermal stabilities, or in other words, relatively weak bonds.25 While arsenic in residual form presents arsenic that into clay mineral lattice, mainly arsenates which have high thermal stabilities with strong intermolecular bonding energy.26 Extraction results in Figure 4 show that, (1) there is little to no arsenic present in the exchangeable form in the coals. (2) Organic-bound arsenic is relatively low in the range of 10−30%, except for the anthracites with no organic-bound portion. (3) The proportion of residual arsenic varies from 1 to 50%, older coals (A1, A2, JL) with low arsenic content ( sulfide-bound > exchangeable and organic-bound form (not found). For As > 4 μg/g coals (SJS, GZ, L1, KY), the decreasing order is sulfide-bound > organic-bound > residual > E


Article

Energy & Fuels

Figure 5. Releasing curves of arsenic and SO2 during GZ coal combustion. (a) SO2 emission intensity vs arsenic volatilization rate at various temperatures (b) further enlargement of (a) at 800−900 °C.

hexagonal pyrrhotit (FeS0.3−0.4),29 it is possible that arsenopyrite (FeAsxSy) exists in the form of a class like state. And also, the substitution of As would not affect the behavior of hexagonal pyrrhotit.29 Thus, it is considered that arsenopyrite (FeAsxSy) decomposes and further oxides in the same temperature zone of 800−900 °C as hexagonal pyrrhotit. As a result, SO2 and arsenic oxides are released at the same time as shown in Figure 5b. However, due to the limitation of instruments and techniques, the chemical formula of FeAsxSy could not be determined at present. Further studies should be carried out to explore the specific arsenic compounds at different temperature stages. 3.5. Interactions Between Minerals and Arsenic. Compared to simple sulfide compounds, various residual arsenates with different decomposition temperatures resulted in the fluctuation of second mass loss peak (Figure 3), showing that arsenate species could affect the volatilization of arsenic. Apart from the arsenates in raw coal, arsenates formed through the combination of mineral elements and gaseous arsenic play a certain role as well. Mineral matters such as Al2O3, MgO, CaO, and Fe2O3 could provide chemical absorption sites for arsenic to form relatively stable inorganic arsenates.30,31 The possible reactions can be represented as below.

are several kinds of arsenates, the second mass loss peak occurs at different temperatures. Mass loss rate of arsenic (Section 3.2) combined with the sequential chemical extraction experiments (Figure 4), it is indicated that the main reason for the second mass loss peak of arsenic is the decomposition of arsenates. Because of the difference in the mode of occurrence of arsenic, seven coals exhibit various volatilization characteristics of arsenic at 1500 °C. For coals with As > 4 μg/g, weak bonded arsenic (organic + sulfide) stands for 73−98% of the total arsenic in coal. During combustion, weak bonded arsenic compounds could easily vaporize into gas phase, thus high volatilization ratio of arsenic was observed (88−99%). While for As < 4 μg/g coals, the proportions of weak bonded arsenic are much smaller than As > 4 μg/g coals. Meanwhile, the decomposition of residual arsenic is incomplete due to high thermal stability. Thus, much lower volatilization of arsenic was observed in As < 4 μg/g coals (53−60%). 3.4. Relationship Between Sulfur and Arsenic. To further study the volatilization of sulfide-bound arsenic, GZ coal, which possesses the highest sulfur content of all the coals, was studied using TG-MS techniques. In this research, CO2 and SO2 emissions were monitored during GZ coal combustion. Combined with the experimental fitting mass loss rate of arsenic in GZ coal (Figure 3b), several gas emission results during GZ coal combustion are shown in Figure 5. As seen in Figure 5a, nearly all of the CO2, SO2 flue gases are emitted before 900 °C. In the same region, a majority of arsenic with weak bonds in GZ coal is volatilized with char combustion. Due to the very small amount of the mass loss rate of arsenic in the coal combustion, further enlargement of Figure 5a between 800 and 900 °C was carried out to investigate the species of the weak bonded arsenic, as shown in Figure 5b. It can be observed from Figure 5b that there are two small peaks on the mass loss rate curves of arsenic during GZ coal combustion at 800−900 °C, with two peaks appearing on the emission curves of SO2 at the same time. This indicates that arsenic vapor emits along with the SO2 emission at 800−900 °C, proving that the decomposition/oxidation of arsenic in sulfide form takes place within this interval. According to the work of Li,28 pyrite (FeS2) transformed into hexagonal pyrrhotit (FeS0.3−0.4) at temperature higher than 800 °C. Due to the poor thermal stability of hexagonal pyrrhotit, it decomposed to generate SO2 and no diffraction peaks of hexagonal pyrrhotit were detected at 900 °C. Since arsenic could substitute for some of the S in the crystal lattice of

As 2 O3(s) ↔

1 As4 O6 (g ) 2

(9)

3CaO(s) +

1 As4 O6 (g ) + O2 → Ca3(AsO4 )2 (s) 2

(10)

Fe2O3(s) +

1 As4 O6 (g ) + O2 → 2FeAsO4 (s) 2

(11)

3MgO(s) +

1 As4 O6 (g ) + O2 → Mg 3(AsO4 )2 (s) 2

(12)

Al 2O3(s) +

1 As4 O6 (g ) + O2 → 2AlAsO4 (s) 2

(13)

In order to study the correlation properties between mineral compounds and arsenic volatilization, the Ca, Fe, Mg, and Al contents in seven coal ashes at 1000 °C were tested by ICPAES and compared to the arsenic content in the corresponding ashes with linear fitting method (choosing 1000 °C because it is the dividing point between the first and the second mass loss peak of arsenic. At 1000 °C most sulfide arsenic has released while large amounts of residual arsenic compounds remain in F


Article

Energy & Fuels

Figure 6. Correlation between mineral elements and arsenic content in coal ashes.

Arsenic considering minerals tend to generate arsenates at low temperatures, which are chemically stable even at very high temperature conditions. In the combustion conditions, Ca3(AsO4)2, FeAsO4, Fe3(AsO4)2, Mg3(AsO4)2, and AlAsO4 are proved to be the most possible arsenate compounds. Ca3(AsO4)2 is the most stable Ca−As species (Figure 7a) which only starts to decompose at temperature higher than 1200 °C. For the Fe−As−O system (Figure 7b), FeAsO4 dominates before 1000 °C, followed by full decomposition in the 1000−1700 °C range, along with the generation of a small amount of Fe3(AsO4)2. MgO and As2O3 react to form Mg3(AsO4)2 (Figure 7c), which decomposes at temperatures higher than 1400 °C. It is also seen in Figure 7 that AlAsO4 (Figure 7d) owns the highest decomposition temperature of ∼1700 °C and only a small amount of AsO(g) is generated even at 2000 °C. However, due to that physical adsorption plays a major role for the retention of Al2O3 on arsenic,32 Al2O3 and As2O3 are difficult to generate AlAsO4 in actual combustion process. Thermodynamic equilibrium models have become a relevant tool to predict elements behavior in most reports. In spite of the limitations that this method does not take into account all the necessary factors (such as rate of reactions, heat and mass transfer issues),34 thermodynamic analysis could provide useful information for the transformation trends of arsenic compounds. Thermodynamic calculations combined with experimental results, this article presents strong evidence for the possible arsenates, along with their formation and decomposition temperatures. In this case, it is concluded that mineral matters react with arsenic to form arsenates at low temperatures and decompose at higher temperatures, thus the second mass loss peak at >1000 °C zone is observed. Table 5 (peak values of

the ash). Due to the large difference of arsenic content between KY ash and others (for KY 1000 °C ash, the arsenic content is 216 μg/g; while for the others, the range of arsenic content is 2−30 μg/g), the fitting curves drawn without KY ash are given in Figure 6. It can be seen from Figure 6 that Ca, Fe, and Mg have positive correlations with arsenic in coal ash, which means that stable Ca−As compounds (e.g., Ca3(AsO4)2), Fe−As compounds (e.g., FeAsO4), and Mg−As compounds (e.g., Mg3(AsO4)2) are formed. The Ca−As and Fe−As results are in agreement with the previous study of Zhang,32 who found that the adsorption of gas-phase arsenic by CaO and Fe2O3 is mainly chemical adsorption. Meanwhile, MgO was proved to be a good additive on the removal of arsenic during coal combustion.33 According to the R2 value, the descending order of correlation of Ca, Fe, Mg, and As is Ca > Mg > Fe. However, aluminum has a negative correlation with arsenic, indicating that the reaction in eq 13 is not favored. The possible explanation is that the retention mechanism for alumina sorbents on arsenic is mainly physical adsorption rather than chemical adsorption.32 To better understand the formation and decomposition of arsenates, thermodynamic equilibrium calculations by HSC chemistry6.0 software based on minimum Gibbs free energy of the system9,34,35 were performed to evaluate the effects of mineral compounds (CaO, Fe2O3, MgO, and Al2O3) on arsenic volatilization during coal combustion. The As/O species of AsO(g), AsO2(g), As2O3(g), As4O6(g), As4O7(g), As4O8(g), As4O9(g), and As4O10(g) were considered in the calculation.36,37 The equilibrium compositions for As considering Ca, Fe, Mg, and Al are present in Figure 7. G


Article

Energy & Fuels

Figure 7. Equilibrium compositions for arsenic considering minerals during combustion.

the mass loss rate of arsenic, for the second mass loss peak, the peak temperatures are 1100−1400 °C) combined with Figure 7 (decomposition temperatures of arsenates), it is suggested that the second mass loss peak may be caused by the decomposition of FeAsO4, Fe3(AsO4)2, and a small amounts of Ca3(AsO4)2. In addition, based on the close relationship between minerals and arsenic, it is found that CaO, MgO, and Fe2O3 are possible sorbents for arsenic retention, which shows an important guidance for the development of techniques on As removal in large scale power plants.

4. CONCLUSIONS Seven Chinese coals in three ranks were chosen to study the volatilization of arsenic during isothermal coal combustion. The results show that • The volatilization proportion of arsenic in coal increases with temperature. For pulverized coal combustion (PC, 1400−1500 °C), the releasing proportion of arsenic is between 53% and 99%. Among the coals, As > 4 μg/g coals tend to have a higher arsenic vaporization than As < 4 μg/g coals at a given temperature. • The mass loss rate of arsenic varies at different temperature zones. Three volatility zones with two mass loss rate peaks of arsenic are observed in all coals during coal combustion. At < 600 °C zone, the evaporation of organic-bound arsenic dominates, the

■

first mass loss peak of arsenic at 800−900 °C is mainly caused by the decomposition and oxidation of arsenic in sulfide forms, while the second peak at >1000 °C zone is probably generated through the decomposition of arsenates. • For As > 4 μg/g coals with a larger proportion of sulfidebound arsenic, the first mass loss peak of arsenic is higher than the second peak, As > 4 μg/g coals tend to release arsenic at lower temperature zones. While for As < 4 μg/ g coals with a larger proportion of arsenates, the first mass loss peak of arsenic is lower than the second peak, As < 4 μg/g coals tend to release arsenic at higher temperature zones. • Ca, Fe, and Mg have positive correlations with arsenic in coal ash. Arsenates, such as Ca3(AsO4)2, FeAsO4, and Mg3(AsO4)2, could be formed at low temperatures and start to decompose at relatively high temperatures due to their thermal stabilities.

AUTHOR INFORMATION

Corresponding Author

*[email protected]. Notes

The authors declare no competing financial interest. H


Article

Energy & Fuels

■

(34) Contreras, M. L.; Arostegui, J. M.; Armesto, L. Fuel 2009, 88 (3), 539−546. (35) Dıaz-Somoano, M.; Martınez-Tarazona, M. Fuel 2003, 82 (2), 137−145. (36) Miller, B.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2003, 17 (5), 1382−1391. (37) Monahan-Pendergast, M. T.; Przybylek, M.; Lindblad, M.; et al. Atmos. Environ. 2008, 42 (10), 2349−2357.

ACKNOWLEDGMENTS Financial support from the National High Technology Research and Development Program of China (863 Project, No.2013AA065404) and High School Subject Innovation Engineering Plan of China (111 Project, B12034) are gratefully acknowledged.

■

REFERENCES

(1) Locating and estimating air emissions from sources of arsenic and arsenic compounds. Final Report; PB−98-163132/XAB CNN; EPA-68D7-0068; TRN: 82541524; Environmental Protection Agency: United States, 1998. (2) Shah, P.; Strezov, V.; Prince, K.; et al. Fuel 2008, 87 (10), 1859− 1869. (3) Helble, J. J. Fuel Process. Technol. 2000, 63 (2−3), 125−147. (4) Tang, Q.; Liu, G.; Yan, Z.; et al. Fuel 2012, 95, 334−339. (5) Liu, H.; Wang, C.; Sun, X.; et al. Energy Fuels 2016. (6) Clarke, L. B. Fuel 1993, 72 (6), 731−736. (7) Wang, J.; Tomita, A. Energy Fuels 2003, 17 (4), 954−960. (8) Senior, C. L.; Bool, L. E.; Morency, J. R. Fuel Process. Technol. 2000, 63 (2−3), 109−124. (9) Furimsky, E. Fuel Process. Technol. 2000, 63 (1), 29−44. (10) Zhou, C.; Liu, G.; Yan, Z.; et al. Fuel 2012, 97 (0), 644−650. (11) Zhao, Y.; Zhang, J.; Huang, W.; et al. Energy Convers. Manage. 2008, 49 (4), 615−624. (12) Lu, H.; Chen, H.; Li, W.; et al. Fuel 2004, 83 (6), 645−650. (13) Folgueras, M. B.; Díaz, R. M.; Xiberta, J.; et al. Energy Fuels 2007, 21 (2), 744−755. (14) Caisheng, D.; Fangwen, L. J. China Coal Soc. 2005, 30 (1), 109− 113. (15) Shah, P.; Strezov, V.; Stevanov, C.; et al. Energy Fuels 2007, 21 (2), 506−512. (16) Tian, H.; Lu, L.; Hao, J.; et al. Energy Fuels 2013, 27 (02), 601− 614. (17) Kang, Y.; Liu, G.; Chou, C.-L.; et al. Sci. Total Environ. 2011, 412, 1−13. (18) Luo, Y.; Giammar, D. E.; Huhmann, B. L.; et al. Energy Fuels 2011, 25 (7), 2980−2987. (19) Liu, H.; Wang, C.; Zhang, Y.; et al. J. Chem. Ind. Eng. (in Chinese) 2015, 66 (11), 4643−4651. (20) Wang, C.; Shao, H.; Lei, M.; et al. Appl. Therm. Eng. 2016, 93, 438−445. (21) Wang, C.; Zhou, X.; Jia, L.; et al. Ind. Eng. Chem. Res. 2014, 53 (42), 16235−16244. (22) Jing, L.; Chuguang, Z.; Junying, Z. J. Combust. Sci. Techno. 2003, 9, 295−299. (23) Senior, C. L.; Lignell, D. O.; Sarofim, A. F.; et al. Combust. Flame 2006, 147 (3), 209−221. (24) Wang, M.; Zheng, B.; Wang, B.; et al. Sci. Total Environ. 2006, 357 (1−3), 96−102. (25) Yudovich, Y. E.; Ketris, M. Int. J. Coal Geol. 2005, 61 (3), 141− 196. (26) Hu, H.; Liu, H.; Chen, J.; et al. Proc. Combust. Inst. 2015, 35, 2883−2890. (27) Bolanz, R. M.; Majzlan, J.; Jurkovič, L.; et al. Fuel 2012, 94 (1), 125−136. (28) Li, P.; Chen, T.; Yan, Y.; et al. J. Chin. Ceram. Soc.(in Chinese) 2013, 41 (11), 1564−1570. (29) Ruppert, L. F.; Hower, J. C.; Eble, C. F. Int. J. Coal Geol. 2005, 63 (1−2), 27−35. (30) Sterling, R. O.; Helble, J. J. Chemosphere 2003, 51 (10), 1111− 1119. (31) Li, Y.; Tong, H.; Zhuo, Y.; et al. Environ. Sci. Technol. 2007, 41 (8), 2894−2900. (32) Zhang, Y.; Wang, C.; Li, W.; et al. Energy Fuels 2015, 29 (10), 6578−6585. (33) Tresintsi, S.; Simeonidis, K.; Katsikini, M.; et al. J. Hazard. Mater. 2014, 265, 217−225. I


Volatilization of Arsenic During Coal Combustion Based on Isothermal

Recommend Documents