Article pubs.acs.org/EF
Volatilization of Arsenic in Coal during Isothermal Oxy-Fuel Combustion Huimin Liu,† Chunbo Wang,*,† Xin Sun,‡ Yue Zhang,† and Chan Zou† †
Department of Energy Power & Mechanical Engineering, North China Electric Power University, Baoding 071003, China College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063009, China
‡
ABSTRACT: Arsenic volatilization characteristics of SJS bituminous coal were carried out in a customized isothermal thermogravimetric experimental system at 600−1400 °C under different oxy-fuel atmospheres. The mineralogical and morphological characterization of ash samples were analyzed using XRD and SEM instruments. Different from conventional nonisothermal mass loss curves by TG analyzer, the isothermal mass loss curves of coal did not show a clear process of moisture removal and devolatilization. With the increase of combustion temperature, the isothermal mass loss curve of SJS coal shifted to the left gradually and the burnout time shortened at the same time, whereas the mass loss curves of arsenic showed different tendencies at 900 °C stages. At the 900 °C stage, the volatilization of arsenic was delayed in oxyfuel condition but with a higher release rate; thus the volatility ratio of arsenic in oxy-fuel combustion was even larger than that of conventional air/flue gas combustion.
1. INTRODUCTION Coal is a major source of the primary energy, contributing to approximately 40% of the total electricity produced globally.1 This scenario is expected to continue stably in the foreseeable future.2,3 However, coal combustion has huge environmental implications as the vast total amounts of coal combusted release undesirable quantities of hazardous 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.4,5 Nearly all types of arsenic compounds are toxic, and As3+ is 50 times more toxic than As5+.6 Other than information on the emission of arsenic in power plants, knowledge of its volatilization characteristics during coal combustion is equally important as it strongly influences the migration of arsenic to the environment and the arsenic-related health risks associated with coal utilization.7,8 The volatilization behavior of arsenic during coal combustion is significantly affected by the mode of occurrence of arsenic, temperature, coal rank, and combustion atmosphere as well. Oxy-fuel combustion is a process which uses high-purity oxygen mixed with flue gas recirculation for coal combustion to deliver a CO2-rich flue gas for direct sequestration and/or storage;9 it is being fast realized under the pressures of carbon taxes and global warming. Compared with conventional air combustion, CO2 was largely captured in oxy-fuel conditions and other air pollutants such as SO2 and NOx were dropped at the same time. Therefore, it has a good possibility to achieve near-zero emission of air pollutants, to meet the stringent emission regulations. The high CO2 partial pressure in an oxyfuel boiler is supposed to promote the generation of CO,10,11 which would form a reducing atmosphere on the surface of carbon particles. Since oxy-fuel combustion is quite different from air condition, the vaporization and migration behaviors of © 2016 American Chemical Society
trace elements such as arsenic during oxy-fuel combustion are also affected. Zheng and Furimsky12 compared the environmental assessment of coal combustion in oxy-fuel atmosphere with that in air; F*A*C*T software predicted that combustion medium had little effect on the amount and type of the trace-elementcontaining emissions in the vapor phase. Yan et al.13 simulated the migration of trace elements at 400−1800 K with various excess air ratios from 0.6 to1.2; they found that trace elements were more likely to generate unstable compounds such as volatile suboxides and sulfides under a reducing atmosphere, while the experimental results of Lu et al.14 showed that high CO2 concentration inhibited the formation of simple substance or suboxides. The inner reason was that the oxy-fuel condition decreased the temperature on the particle surface and temperature presented a greater effect than atmosphere. Wang et al.15 studied the distribution of trace elements in a 1500 °C drop furnace and found that, compared with O2/N2, O2/CO2 atmosphere contained a higher concentration of arsenic in fine particulate matters; thus arsenic in flue gas was improved to a certain extent. Low and Zhang16 conducted drop-tube furnace experiments, and combined with XANES analysis, it was concluded that the volatile proportion of arsenic in 27O2/73CO2 atmosphere was slightly higher than in air condition. However, Contreras et al.17 used HSC chemistry 6.1 software to simulate the distribution of trace elements in the oxy-fuel combustion process and found the volatilization ratio of arsenic at 850 °C in 30O2/70CO2 atmosphere had no significant change compared with air condition. Other literature18−20 reported the speciation of arsenic in bottom Received: January 9, 2016 Revised: March 29, 2016 Published: March 30, 2016 3479
DOI: 10.1021/acs.energyfuels.6b00057 Energy Fuels 2016, 30, 3479−3487
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Energy & Fuels Table 1. Properties of Coal Sample wada/% ultimate analysis
proximate analysis b
C
H
O
N
60.07
4.31
9.78
0.63
S 0.49 Ash Composition/%
M
V
A
FC
1.06
22.51
24.72
51.71
Al2O3
CaO
Fe2O3
K2O
Na2O
MgO
MnO
SiO2
21.09
6.24
7.92
3.75
1.55
1.62
0.11
49.87
Arsenic in Coal mode of occurrence of arsenic in coal/%
a
amount/(μg·g−1)
exchangeable
organic
sulfide
residual
4.26
0
31
42
27
wad, air-dried weight. bBy difference
Figure 1. Isothermal thermogravimetric experimental system of coal combustion.
ash and fly ash in oxy-coal power plants rather than the volatilization of arsenic in the furnace. Several studies have been reported on arsenic volatilization in oxy-enriched atmosphere; while a majority of the results were predicted by thermodynamic simulation, only limited experimental data of arsenic releasing are available. Meanwhile researchers had different opinions on whether an oxy-enriched atmosphere played a positive or negative role on arsenic volatilization, and the impact of CO2 was still unknown. In addition, oxy-fuel results obtained at low-temperature zones (750−950 °C) and high-temperature conditions (1500 °C) varied significantly. The present work aims to examine the difference of arsenic volatilization at an oxy-combustion environment and an airfired environment. To further explore the effects of temperature and atmosphere on arsenic volatility, a typical bituminous coal was combusted in a variety of gas compositions, including air and O2/CO2 mixtures with a temperature range of 600−1400 °C. A customized isothermal thermogravimetric experimental system was used to simulate the realistic combustion conditions with high heating rate. For the quantification of arsenic in the samples, atomic fluorescence spectrometer (PSA10.055 Millennium Excalibur by PS Analytical) was employed with microwave-assisted digestion being the selected sample preparation method. The X-ray diffraction (XRD) and scanning electronic microscopy (SEM) analysis were also used to characterize the mineralogical and morphological transformation of minerals in coal ashes.
2. EXPERIMENTAL SECTION 2.1. Parameters and Apparatus. With the average arsenic content in Chinese coals being 4 μg/g and with sulfide binding being the main chemical speciation of arsenic in coal, a typical SJS bituminous coal which accounts for nearly one-third of the total coal usage in China21 was selected to conduct combustion experiments. The arsenic content of SJS coal is 4.26 μg/g, and sulfide-bound arsenic accounts for 42% of the total arsenic. Coal samples were air-dried, crushed, and sieved to 100−150 μm. The ultimate/proximate analysis, major ash composition, total arsenic in coal and mode of occurrence of arsenic in coal are listed in Table 1. The customized isothermal thermogravimetric experimental system, with a high-temperature tube furnace, two temperature controllers, a thermocouple, a gas mixture, and a data acquisition system, is presented in Figure 1. The furnace with the size of 50 mm diameter and 120 mm length could be moved along the horizontal smooth rail. After heating to a set temperature and remaining stable for 30 min, the furnace was quickly pushed along the rail to the set position, where a corundum boat containing 0.50 ± 0.01 g of coal was located on a horizontal balance for combustion. The real-time weight signals were continuously recorded by the data acquisition system to monitor the weight changes of coal samples. After each combustion test, the ash sample was cooled and collected for further study. Various combustion parameters were shown in Table 2. 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.22,23 The thermocouple was calibrated regularly to ensure the accuracy of the temperature measurement. The errors of the thermocouple were found to be within ±3 °C.24 The burning experiments were repeated at least three times to verify the reproducibility of the experimental results, and the mean values were used with the deviation less than 2%.25 Since the buoyancy effect may affect the test results in this experimental setup, additional experiments were conducted with an 3480
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where y is the mass loss of arsenic, %; r is the mass loss rate of arsenic, %/°C; w1 and w2 refer to the mass loss of arsenic at temperatures T1 and T2 separately, %.
Table 2. Experimental Parameters of Coal Combustion Experimental Atmospheres flue gas based on oxy21
oxy21 21O2/79CO2 600
air
5O2/95CO2 21O2/79N2 Experimental Temperatures/°C 800
900
1100
flue gas based on air
3. RESULTS AND DISCUSSIONS 3.1. Isothermal Mass Loss Characteristics of Coal. Effect of Temperature. The isothermal mass loss curves of coal samples at various temperatures under oxy21 (21O2/79CO2) condition are shown in Figure 2. Different from conventional
5O2/16CO2/79N2 1300
1400
empty sample boat at each condition. The effect of buoyancy can be eliminated by subtraction of values of the empty boat from the actual experiment results. 2.2. Sample Analysis Method. 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 of hydrofluoric acid (both acids were trace metal grade) in the PTFE digestion container which was subsequently heated at 20 °C/min 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 fortified blanks and laboratory fortified 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%. The mineral compositions in the coal and ashes were determined using an Ultima IV X-ray diffractometer (Rigaku Co., Tokyo, Japan). The X-ray beam was made to intersect the sample at glancing angles from 10° to 100° with a scanning speed of 20°/min. Morphological characterization of ash samples was performed by ZEISS EVO18 canning electronic microscopy (ZEISS Co., Oberkochen, Germany), aiming to provide reference for arsenic volatilization. 2.3. Thermodynamic Equilibrium Modeling. Thermodynamic equilibrium calculations have been widely adopted to predict the vaporization and transformation behaviors of trace elements in coal combustion and gasification, based on a principle of minimizing the total Gibbs free energy of the system,26,27 calculation results give the most stable chemical species and phases with changing parameters such as pressure, temperature, and elemental composition. In this work, thermodynamic equilibrium calculations, using HSC-Chemistry 6.0 software, were performed to predict the equilibrium compounds of arsenates in oxidized or reductive atmospheres. It would provide reference for the decomposition and transformation of arsenates during oxy-fuel combustion. 2.4. Data Analysis. The arsenic contents in the coal and ash samples were used to determine the volatilization characteristics of arsenic. mcoal refers to the arsenic content in initial unburned coal, μg/ g; cash represents the arsenic concentration in ash obtained at temperature T, μg/g; η is the corresponding ash yield per mass unit at the cited temperature; and 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= cashη. The residual percentage of arsenic in the coal ash based on the initial coal mass during coal combustion at a temperature T, w is calculated as m ηc w = ash = ash × 100% mcoal mcoal (2-1)
Figure 2. Isothermal weight loss of SJS coal at 800−1400 °C under 21O2/79CO2 atmosphere.
nonisothermal mass loss curves by TG analyzer, the isothermal mass loss curves of coal do not show a clear process of moisture removal and devolatilization due to high heating rates (the heating rate could be (0.5−1.0) × 104 °C/s). With the increase of combustion temperature, the mass loss curve of SJS coal shifts to left gradually and the burnout time shortens. The burnout time and corresponding residual proportion of SJS coal at 600−1400 °C under 21O2/79CO2 atmosphere are shown in Table 3. From Table 3 we can see that the burnout time decreases from 350 to 170 s during the 600−1400 °C period and the residual proportion of coal ash becomes smaller at the same time. In addition, combined with Figure 2 and Table 3, it is seen that the shifting extent of the mass loss curves reduces as temperature rises. At the same temperature interval of 200 °C, the time of burnout is advanced by 90 s from 600 to 800 °C, while only 20 s of burnout time is advanced in the 900−1100 °C zone. It indicates that the effect of temperature becomes less significant with increasing temperature. Effect of Atmosphere. The isothermal mass loss characteristics of SJS coal under different atmospheres (21O2/79N2, 21O2/79CO2, 5O2/16CO2/79N2, and 5O2/95CO2) were investigated in this work. The residual proportion of coal mass at oxy-fuel and air conditions at 800 °C are shown in Figure 3. The burnout characteristics varied at different atmospheres. The corresponding time and residual proportion at the burnout point are shown in Table 4. From Table 4 it is found that the burnout time of SJS coal at 21% inlet O2 content conditions (21O2/79N2 and 21O2/ 79CO2) are 240s and 260 s, respectively, significantly lower than that of 5% inlet O2 content conditions (5O2/16CO2/79N2 and 5O2/95CO2, 790 and 900 s, respectively). It is mainly because, with higher O2 concentration, elevated O2 partial pressure promotes the diffusion of O2 and thus more intensive
The mass loss ratio of arsenic (y), which equals the volatility proportion of arsenic, is expressed as
y = (1 − w) × 100%
(2-2)
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
(2-3) 3481
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Energy & Fuels Table 3. Burnout Characteristics of SJS Coal under 21O2/79CO2 Atmosphere temperature/°C parameter
600
800
900
1100
1300
1400
burnout time/s burnout residual proportion/%
350 22.60
260 22.34
230 22.17
200 21.23
180 20.95
170 20.87
Figure 4. Mass loss curves of arsenic during oxy-fuel combustion.
Figure 3. Isothermal mass loss curves of SJS coal at different atmospheres at 800 °C.
fitting based on the mass loss scatters of arsenic at different temperatures, so as to reflect the mass loss characteristics of arsenic during coal combustion to a certain extent. As can be seen from Figure 4, SJS coal shows a significant decrease in the residual proportion of arsenic at four different atmospheres with temperature increases. The volatilization ratio of arsenic at 1400 °C is over 83%. The main reason is because, with the increasing of temperature, the evaporation rate of moisture and volatile matter in coal rises gradually, which accelerates the volatilization rate of arsenic on the surface of the particles. In this case, organic and inorganic arsenic compounds in solid phase are more likely to evaporate into a gaseous state, thus improving the release proportion of arsenic. Second, higher temperature enlarges the internal porosity of char particles and reduces the diffusion resistance of arsenic in particle pores; thus more arsenic compounds could release to the atmosphere. In addition, with temperature increases, the diffusion coefficient of arsenic compounds from the interior of the molten body to the surface of the molten body becomes larger,30 leading to a higher releasing amount of arsenic with the burning of coal. Arsenic Volatility at 25−900 °C. From Figure 4 it is also found that, in the temperature range lower than 900 °C, the residual proportion of arsenic at different conditions in ascending order are 21O2/79N2 < 21O2/79CO2 < 5O2/ 16CO2/79N2 < 5O2/95CO2. That is, regardless of the concentration of CO2, the residual ratio of arsenic at 21% O2 content is less than that of 5% O2 content in the 900 °C). From Figure 5 we can see that the shape of the second mass loss peak of arsenic under air atmosphere (21O2/79N2) and conventional flue gas atmosphere (5O2/ 16CO2/79N2) is flat, while the peak is sharp for O2/CO2 atmospheres (21O2/79CO2 and 5O2/95CO2). Moreover, as seen in Table 5, the average peak temperature of arsenic in O2/ CO2 condition is 1358 °C, higher than the peak temperature of conventional air/flue gas atmosphere (∼1245 °C). It shows that volatilization behavior of arsenic at O2/CO2 atmosphere is significantly delayed compared with a conventional air/flue gas mode. However, the maximum mass loss rate of arsenic under O2/CO2 atmospheres is 0.20%/°C, approximately three times higher than that of air atmosphere (0.08%/°C), which indicates that the volatilization of arsenic in oxy-fuel conditions is more intensive in high-temperature zones. 3.3. Mineral Transition Characteristics. To further analyze the volatility difference of arsenic at high temperature (>900 °C) under different atmospheres, the mineralogical characterization of SJS ashes under air (21O2/79N2) and oxy21 (21O2/79CO2) conditions at 900−1400 °C were performed using X-ray powder diffractometer (XRD), and the results are presented in Figure 6. As seen in Figure 6, the major components of SJS coal ash at 900 °C are quartz, hematite, anhydrite, and a little bit of montmorillonite. Due to the decomposition of calcite to calcium oxide (CaO) in the reaction eq 3-1,35 the peak intensities of calcite (CaCO3) disappear completely at 900 °C under air condition, while calcite is still detected under oxy21 atmosphere. The explanation for this phenomenon is that a high concentration of CO2 in 21O2/79CO2 atmosphere discourages the decomposition of CaCO3. Since anhydrite (CaSO4) starts to decompose at 1000 °C (reaction 3-235), the characteristic peaks of anhydrite decrease significantly at 1100 °C and completely disappear at 1300 °C. Besides, CaO, which is generated by the decomposition of calcite and anhydrite, again reacts with Al2O3 and SiO2 in the ash to form anorthite (CaO·Al2O3·2SiO2, reaction 3-335) and mullite (3Al2O3·2SiO2, reaction 3-435); thus mullite and anorthite are observed in the 1100 °C ash. With temperature raised to 1300 °C, the peak intensities of mullite and anorthite increase gradually, while the peak intensities of quartz decrease sharply, which demonstrates that quartz has participated in reactions 3-3 and 3-4. As
temperatures. While arsenic in the residual form (arsenic into clay mineral lattice, mainly for arsenates) usually has relatively high-decomposition points due to their chemical stability.33 It is concluded that the volatility proportion of arsenic at relatively low temperatures is mainly due to the combined effects of organic-bound and sulfide-bound arsenic, while at relatively high temperatures the decomposition of residual arsenic, mainly for arsenate, dominates. At temperatures lower than 900 °C, the evaporation of organic-bound and the decomposition/oxidation reactions of sulfide-bound arsenic are the main cause of arsenic volatilization. With higher O2 content in the atmosphere (21% O2 vs 5% O2), more intensive coal combustion takes place, which makes a stronger evaporation degree of organic-bound arsenic and enlarges the extent of endothermic decomposition of sulfidebound arsenic. Besides, larger O2 concentration promotes the oxidation reaction of sulfide-bound arsenic and more gas phase arsenic is released. As a result, the average residual proportion of arsenic based on the initial coal decreases with the increasing of O2 content. However, at the same O2 concentration, the substitute of N2 by CO2 in oxy-fuel condition leads to a high CO2 partial pressure. Together with the larger specific heat capacity of CO2, O2 diffusion from atmosphere to coal particle surfaces is blocked, which reduces the intensity of coal combustion. In this case, in comparison with air condition, the arsenic volatilization proportion of oxy-fuel combustion is smaller. Arsenic Volatility at >900 °C. At temperature greater than 900 °C, the volatilization characteristics of arsenic in SJS coal under different atmospheres were not strictly in accordance with the tendency of the 25−900 °C range. It can be seen from Figure 4 that the volatility ratio of arsenic in 21O2/79CO2 atmosphere at 1400 °C is 90.03%, much higher than the proportion of 21O2/79N2 condition (83.76%). Meanwhile, the mass loss curve of arsenic at 5O2/16CO2/79N2 condition steepened downward in the >900 °C zones and crossed with the curve of the 21O2/79N2 condition at about 1200 °C. There are significant differences in the volatile behavior of arsenic in different atmospheres in high-temperature zones (>900 °C). By taking the derivative of the mass loss curves of arsenic with respect to temperature, the mass loss rate of arsenic was obtained as shown in Figure 5. It is shown in Figure 5 that the mass loss rate of arsenic varies at different temperatures. And no matter whether it is in the air atmosphere or in the oxy-fuel atmosphere, two obvious mass loss peaks of arsenic are observed. The first mass loss peak
Figure 5. Mass loss rate of arsenic during oxy-fuel combustion. 3483
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Energy & Fuels Table 5. Mass Loss Peak of Arsenic during Oxy-fuel Combustion peak one
peak two
atmosphere
temperature/°C
mass loss rate/(%·°C−1)
temperature/°C
mass loss rate/(%·°C−1)
21O2/79N2 5O2/16CO2/79N2 21O2/79CO2 5O2/95CO2
829 841 838 846
0.13 0.14 0.13 0.15
1257 1234 1355 1362
0.08 0.13 0.19 0.20
Figure 6. X-ray diffraction patterns of SJS ash under different combustion temperatures: (a) air condition; (b) oxy21 condition. Minerals indentified are as follows: Ah, anhydrite; An, anorthite; C, calcite; D, dolomite; H, hematite; La, labradorite; Mo, montmorillonite; M, mullite; Q, quartz.
Figure 7. SEM micrographs of SJS ash at different atmospheres and temperatures: (a) air, 1100 °C; (b) oxy21, 1100 °C; (c) air, 1400 °C; (d) oxy21, 1400 °C.
characteristic peaks. It shows that more minerals are melting and aluminosilicate crystals gradually convert to amorphous bodies.
temperature continues rising, however, the content of anorthite decreases due to the melting problem. At 1400 °C, the major component of SJS coal ash is mullite, with a small amount of anorthite and quartz. The reactions among minerals tend to generate mullite which has a high stability in thermodynamics. In addition, the X-ray diffraction peaks become less significant at 1400 °C, along with the disappearance of large amounts of 3484
CaCO3 → CaO + CO2
(3-1)
CaSO4 → CaO + SO3
(3-2) DOI: 10.1021/acs.energyfuels.6b00057 Energy Fuels 2016, 30, 3479−3487
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Figure 8. Equilibrium composition of arsenates at oxidation and reduction atmospheres: Ca3(AsO4)2 at (a) oxidation and (b) reduction atmosphere; FeAsO4 at (c) oxidation and (d) reduction atomosphere.
CaO + Al 2O3 ·2SiO2 → CaO·Al 2O3 ·2SiO2
(3-3)
Al 2O3 + SiO2 → 3Al 2O3 ·2SiO2
(3-4)
powder, proving the stronger combustion in air condition. With temperature increasing from 1100 to 1400 °C, ash particles become bigger with a spherical shape, which indicates that ash melting occurs at 1400 °C. However, compared with the ash samples under air condition at 1400 °C (panel c), it is observed that ash particles under oxy21 condition (panel d) all stick together with a much bigger size, demonstrating that the melting problem under oxy21 condition is worse than air at 1400 °C. In fact, the even higher melting characteristic of oxy-fuel combustion at 1400 °C is mainly caused by the gasification reaction between char and CO2 at high temperature. During 21O2/79CO2 combustion, a large amount of CO2 reacting with char promotes the formation of a reducing atmosphere. At temperature lower than 1300 °C, the gasification rate of CO2 reacting with char is limited with a weak reducing atmosphere, when the inhibition of CO2 dominates. As temperature reaches 1300 °C, the content of CO increases significantly since intense gasification takes place (according to the standard operating temperature of coke oven, the gasification temperature usually needs to reach about 1300 °C). High partial pressure of CO enhances the reducing atmosphere, thus increasing the char burning rate in oxy-fuel combustion,36,37 leading to a worse melting phenomenon in oxy21 ash at 1400 °C. It can also be found that the mass loss rate of arsenic in oxy-fuel combustion is larger than in air condition at 1400 °C (as seen in Figure 5). Since the volatilization of arsenic at high-temperature condition is mainly caused by the decomposition of arsenates, it is suggested that the reduction atmosphere under high temper-
Through comparison of the XRD patterns under different atmospheres, it is found that oxy21 combustion shows a weaker peak intensity of anorthite at 1100 °C than air combustion, which indicates that the content of anorthite in oxy21 ash is less generated. This is mainly because, under 21O2 /79CO 2 atmosphere, a high concentration of CO2 with larger specific heat capacity constrains the transfer of heat to coal particles. Thus, higher temperature is required to generate the same amount of anorthite. At the same time, arsenates with high thermal stability gradually decompose to form arsenic oxides. Due to the resistance of CO2 compared to N2, higher temperature is required for the decomposition of arsenates. As a result, the volatilization behavior of arsenic at O2/CO2 atmosphere at >900 °C stage is significantly delayed. Besides, from Figure 6 we can find that the characteristic peaks of minerals at 1400 °C are less significant in oxy21 atmosphere compared with air combustion, which shows that the ash fusion degree of 21O2/79CO2 atmosphere is more serious than in 21O2/79N2 condition. In this case, the morphological characteristics of SJS ashes under two atmospheres at 1100 and 1400 °C are further analyzed using scanning electronic microscopy (SEM), and the results are shown in Figure 7. From Figure 7 it is found that the air ashes at 1100 °C (panel a) are in the flake form, while the ash samples under oxy21 condition at 1100 °C (panel b) are mainly in the form of 3485
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Energy & Fuels ature promotes the decomposition of arsenates and in turn the volatilization rate is enhanced. 3.4. Thermodynamic Equilibrium Analysis. To further explore the decomposition and transformation of arsenates under different atmospheres, HSC chemistry 6.0 software was used to calculate the equilibrium components of calcium arsenate (Ca3(AsO4)2) and ferric arsenate (FeAsO4) in an oxidation atmosphere and a reduction atmosphere. The As/O species of As(g), As2(g), As3(g), As4(g), AsO(g), AsO2(g), As2O3(g), As4O6(g), As4O7(g), As4O8(g), As4O9(g), and As4O10(g) were included in the calculation.38,39 The temperature range was set at 200−2000 °C, where O2/C molar ratio = 1.2 (oxidation atmosphere) or 0.8 (reduction atmosphere). The simulation results are shown in Figure 8. Panels a and b of Figure 8 are the equilibrium compositions of Ca3(AsO4)2 at oxidation and reduction atmospheres; panels c and d of Figure 8 are the equilibrium compositions of FeAsO4 at the two atmospheres. Under the oxidation atmosphere, Ca3(AsO4)2 starts to decompose at temperature higher than 1200 °C and AsO(g) is the main gaseous product. FeAsO4 starts to decompose at 900 °C with the generation of As4O6(g) and As4O7(g). With the increase of temperature, As4O6(g) and As4O7(g) transfers to As2O3(g), AsO2(g), and finally stable as AsO(g). In the oxidation condition, Ca3(AsO4)2 and FeAsO4 are the most stable arsenic compounds; meanwhile +2 oxides of arsenic are the major decomposition products at 1400 °C. Under the reduction atmosphere, Ca3(AsO4)2 first decomposes into Ca(AsO2)2 with a lower arsenic valence of three. Ca(AsO2)2 is less stable than Ca3(AsO4)2, and it starts to break down into As2(g) at 900 °C. The concentration of As2(g) reaches maximum at ∼1500 °C. With temperature increases, AsO(g) becomes the most stable species. As for FeAsO4, first it is reduced to FeAs2 and FeAs and then the Fe−As compounds further decompose into As4(g) and As2(g), at last AsO(g). In the reduction condition, Ca(AsO2)2 in As3+, FeAs2 in As1−, and FeAs in As1− are the most stable arsenic compounds; zerovalent arsenic is the major decomposition product at 1400 °C. Compared with the oxidation atmosphere, arsenates in reduction atmosphere tend to transmit into arsenic compounds with lower arsenic valence, along with the gaseous decomposition products such as As2(g) rather than As2O3(g). This is mainly because gasification between CO2 and char enhances the reduction atmosphere inside/around the particles; thus arsenic compounds with high chemical valence are reduced to compounds with lower chemical valence. The report of Yan et al.13 showed that, under reductive condition, arsenates with high thermal stability such as Ca3(AsO4)2 and FeAsO4 tended to be reduced to Ca(AsO2)2 and FeAs. However, the arsenic compounds with lower valence state have poor thermal stability; they can easily decompose into secondary oxides or single arsenic at high-temperature condition,40 leading to the rapid volatilization of arsenic. Thus, the second mass loss peak of arsenic under oxy-fuel combustion is much higher than the peak under conventional air/flue gas condition. The possible reactions at reduction atmosphere are as follows (eqs 3-5, 3-6, and 3-8, ref 13; eqs 3-9 and 3-10, ref 40). step 1: Ca3(AsO4 )2 + 2CO(g) = Ca(AsO2 )2 + 2CaO + 2CO2 (g)
(3‐5)
6FeAsO4 + 20CO(g) = 3FeAs 2 + Fe3O4 + 20CO2 (g)
(3‐6)
step 2: Ca(AsO2 )2 + 3CO(g) = As2 (g) + CaO + 3CO2 (g)
(3‐7)
3FeAs 2 + Fe3O4 + 4CO(g) = 6FeAs + 4CO2 (g)
(3‐8)
4FeAs = 4Fe + As4 (g)
(3‐9)
As4 (g) = 2As2 (g)
(3‐10)
Besides, worth noting is that, at temperature higher than 1000 °C, the gap between conventional flue gas atmosphere (5O2/16CO2/79N2) and air atmosphere (21O2/79N2) in the mass loss rate becomes wider as temperature increases (as seen in Figure 5). Higher mass loss rate causes the mass loss curve of arsenic at 5O2/16CO2/79N2 condition steepening downward and crossing with the curve of 21O2/79N2 condition at about 1200 °C (as seen in Figure 4). Moreover, the mass loss rate of arsenic under 5O2/16CO2/79N2 condition is much larger than the other three scenarios at ∼1000 °C. The possible explanation is that 1000 °C is a transition temperature when the effect of O2 weakens while the incomplete combustion reaction of C + (1/2)O2(g) → CO(g) carries out to generate reducing gas CO steadily. However, the gasification reaction of C + CO2(g) → 2CO(g) hardly takes place due to limitation of reaction temperature. In this case, the effect of CO generated by incomplete combustion reaction dominates the vaporization of arsenic; thus the mass loss rate of arsenic at 5% O2 (with 16CO2/79N2) atmosphere shows the largest value compared to all of the other conditions at about 1000 °C. Furthermore, it is seen in Figure 5 that the mass loss rate peak of arsenic in 5% O2 condition (with 16O2/79N2 or 95% CO2) is higher than that in the 21% O2 condition (with 79%N2 or 79% CO2) . This is mainly because, with temperature increases, gasification reaction takes place gradually. Compared to 21% O2 condition, deficient O2 content in 5% O2 condition leads to a higher partial pressure of CO. The enhanced reducing atmosphere in 5% O2 condition promotes the volatilization of arsenic; thus the second mass loss peak of arsenic in 5% O2 condition is higher than that in the 21% O2 condition.
4. CONCLUSIONS Arsenic volatilization characteristics during oxy-coal combustion were studied in a customized isothermal TG experimental system at 600−1400 °C. XRD and SEM analyses were used to characterize the mineral transition of ash samples. The major conclusions are drawn as follows: (1) The releasing proportion of arsenic increases with temperature both in oxy-fuel and air combustion; two arsenic mass loss peaks are observed at 900 °C stages, separately. (2) At temperature lower than 900 °C, oxygen content is the major factor. The greater the O2 ratio, the larger is the volatile proportion of arsenic. Under the same O2 ratio condition, a larger CO2 concentration hampers the emission of arsenic and the volatile proportion is decreased. (3) At temperature higher than 900 °C, the content of carbon dioxide is the major factor; a larger CO2 concentration in oxy-fuel atmosphere hinders heat transfer, and thus the intensive volatilization of arsenic is delayed compared to that in air condition. The corresponding mass loss peak of arsenic in oxy-fuel and air conditions occurred at 1358 and 1245 °C, respectively. However, the peak mass loss rate of arsenic in oxyfuel combustion is much higher: the volatile ratio of arsenic in 3486
DOI: 10.1021/acs.energyfuels.6b00057 Energy Fuels 2016, 30, 3479−3487
Article
Energy & Fuels 21O2/79CO2 atmosphere reaches 90% at 1400 °C, even higher than the proportion of 84% in air.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from The National Natural Science Foundation of China (Grant No. 51276064).
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DOI: 10.1021/acs.energyfuels.6b00057 Energy Fuels 2016, 30, 3479−3487
