Release Behaviors of Arsenic in Fine Particles Generated from a

Jul 13, 2016 - ... of arsenic in air combustion, CO2 gasification, and N2 pyrolysis were 85, 65, ... Shilin ZhaoYufeng DuanCong ChenHao WuDeye LiuMeng...
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Release Behaviors of Arsenic in Fine Particles Generated from a Typical High-Arsenic Coal at a High Temperature Chong Tian,*,† Rajender Gupta,‡ Yongchun Zhao,† and Junying Zhang*,† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074 People’s Republic of China ‡ Department of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada ABSTRACT: To make an accurate assessment on transformation behaviors of arsenic in coal at a high temperature, a typical high-arsenic coal collected from southwest China has been chosen in the study and a series of high-temperature experiments in different atmospheres have been conducted with the help of a lab-scale drop-tube furnace. Fine particulate matter were collected by a low-pressure impactor, for obtaining their mass size distributions and further quantifying for arsenic distributions and speciation in fine particles of different sizes. The results indicated that the bleeding ratios of arsenic in air combustion, CO2 gasification, and N2 pyrolysis were 85, 65, and 45%, respectively, at 1300 °C. The ratio was found to remain relatively constant when the temperature increased from 1200 to 1400 °C. Organically associated arsenic would be more inclined to vaporize in N2 pyrolysis, while both organic and inorganic associated arsenic would vaporize in CO2 gasification and air combustion. Pyriteassociated arsenic would vaporize together with sulfur in pyrite inclusions. The decomposition of pyrite followed the principle of an unreacted core model and was mostly controlled by the surface sulfur vapor pressure. Mass size distributions of fine particulate matter generated from coal gasification presented a bimodal distribution, and two major peaks appeared at 0.4 and 5 μm. Particles in the size range of 5 μm were presented as a round shape with pores and cracks on the surface, while particles in the size range of 0.4 μm were confirmed to be soot. Arsenic was obviously enriched in fine particles with a size of around 0.1−0.2 μm in both combustion and gasification. The major speciation of arsenic identified in fine particles generated from coal combustion was As2O5 and Ca3(AsO4)2, while that in fine particles generated from coal gasification was As2O5, As, AsO, and Ca3(AsO4)2.

1. INTRODUCTION Arsenic is known to be of concern for the environment and public health because of its high toxicity and potential carcinogenic properties.1−5 Many cases of incidents on arsenic contamination in the environment with resultant adverse health risks have been reported extensively.6 Combustion of arseniccontaining coal is considered to be one of the large contributors for arsenic contamination in the environment.7 Attention on arsenic emissions in a coal-fired power plant has been gradually facing increasingly stringent emission standards for power generation. Arsenic is classified as a semi-volatile element during coal combustion, and a large fraction of arsenic in coal is expected to be vaporized at a high temperature. However, the actual vaporization behaviors and distributions in coal combustion products (CCPs) during combustion have a strong link to the modes of occurrences of arsenic in coal.8−10 It has been reported that arsenic associated with organic and/or pyrite are expected to volatilize during combustion, which subsequently transform into gas-phase As2O3 and/or arsenic particles, while the fraction of arsenic that is contained in the slicate matrix would not vaporize and would remain in residual ash.11−13 However, apart from the occurrences of arsenic in coal, the occurrences of minerals that host arsenic in coal would also affect the vaporization behavior of arsenic. Hence, understanding of the occurrences of arsenic and its host minerals in a coal is critical to precisely predict its transformation behavior and, consequently, ascertain a better assessment on its toxicity in the environment. © XXXX American Chemical Society

Even though most arsenic in coal is volatile and, subsequently, escapes into the gas phase in coal combustion, more than 90% gas-phase arsenic would be captured by the CCPs in flue gas.14 Fine particles generated from coal combustion are the most prone sites for arsenic precipitation as a result of their large surface area and number of active cation sites on the surface. Hence, most gas-phase arsenic would deposit on the surface of fine particles in cooling flue gas downstream.15 Those arsenic-containing fine particles are captured less efficiently by the electrostatic precipitator (ESP) in air pollution control devices (APCDs), thus finally being emitted into the environment.16−18 Consequently, these fine particles laden with arsenic would be easily inhaled and deposited in lungs of humans and inevitably cause detrimental health risks. The condensation and deposition of arsenic on fine particles are simultaneously occurring along with the growth of fine particles in flue gas. Arsenic precipitation from vapor may take two paths: one is heterogeneous condensation, and the other is homogeneous condensation following nucleation. Because the arsenic concentration is extremely low compared to the major species in coal, it is highly possible that gas-phase arsenic molecules would contact with ash particles with a consequence of heterogeneous partition on the surface of the fine particles prior to nucleation.19,20 Hence, the pathway by which arsenic transforms in flue gas from the vapor phase to Received: February 2, 2016 Revised: July 7, 2016

A

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pyrolysis and gasification experiments were conducted at different temperatures (1200, 1300, and 1400 °C), while the combustion experiments were conducted at 1300 °C. The coal feeding rate was 18 g/h in all of the experiments. In the N2 pyrolysis and CO2 gasification conditions, the gas flow rate was accurately controlled by mass flow controllers at 4 L/min. While in the combustion experiments, the flow rate of air was 10 L/min. The ratio of primary gas and secondary gas was 1:9, and the secondary gas was heated to 400 °C before injecting to the reactor. The residence time was estimated to be around 2 s. The sample collection probe was water-cooled. Coarse ash samples were collected by means of a cyclone with a cut-off size of 10 μm. The fine particulate matter were further trapped by polycarbonate foil and aluminum foil installed in the DLPI. The particles were segregated into 13 stages with size ranging from 0.03 to 10 μm. The outlet flue gas was monitored by an Agilent micro gas chromatograph (CP-4900). 2.2. Analytical Technique. The amount of fine particles collected on the aluminum foil was weighted by a microbalance. Mineral phases present in ashes generated from coal combustion and gasification were identified by X-ray diffraction (XRD) measurement. The XRD investigations were carried out on an X’Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator. The accelerating voltage was 40 kV, and the current was 40 mA. XRD patterns were recorded over a 2θ interval of 5−80° using Cu Kα radiation with a step size of 0.017°/10 s. Investigations of field emission scanning electron microscopy (FE-SEM) for revealing the micromorphologies of ashes and fine particles were carried out on a Sirion200 microscope equipped with GENESIS energy-dispersive Xray spectroscopy (EDX). The surface elemental analysis of fine particles and bottom ash were determined by X-ray photoelectron spectroscopy (XPS) with an AXIS 165 spectrometer (Kratos Analytical). Arsenic concentrations in coal, ash residuals, and fine particles were determined using high-resolution inductively coupled plasma mass spectrometry (ICP−MS), and the solid samples for ICP− MS measurement were acid-digested in HF, HNO3, and HClO4 in the low-pressure sealed vessel prior to detection. Time of flight secondary ion mass spectrometry (TOF-SIMS) examinations were conducted for determining the arsenic speciation in the fine particles.

particle surface deposition is suspected to affect the speciation of arsenic in fine particles. However, as a result of the extremely low concentration of arsenic in coal, it is still a great challenge to ascertain the pathway of arsenic transformation and quantify the speciation of arsenic. An accurate assessment on the arsenic speciation in fine particles generated from coal utilization is still lacking. To make an accurate assessment on vaporization of arsenic and speciation of arsenic in fine particles in coal utilization at a high temperature, a typical high-arsenic coal (arsenic concentrations > 260 μg/g) was chosen in this study. A series of experiments, including N2 pyrolysis, CO2 gasification, and air combustion, have been conducted in a lab-scale drop-tube furnace (DTF) for systematically understanding the arsenic emission behaviors in coal utilization in different gas atmospheres at a high temperature. The effect of arsenic occurrences in the coal and gas atmospheres on its vaporization behavior has been investigated in detail. Possible mechanisms on arsenic partitioning in fine particles has also been proposed. The distributions and speciation of arsenic in fine particles generated from the combustion and gasification of typical higharsenic coal have also been investigated.

2. EXPERIMENTAL SECTION 2.1. DTF Experiment. A typical high-arsenic coal collected from southwestern China is chosen in this study, where arseniasis, an extensively found disease in the local area, is caused by the utilization of high-arsenic coals.2,6,21 The experiments of the high-arsenic coal at high temperatures, including N2 pyrolysis, air combustion, and CO2 gasification, were conducted in a lab-scale DTF (Figure 1). The details of the setup has been described in our previous studies,22,23 while the Dekati low-pressure impactor (DLPI) for fine-particle capture was installed extra to the setup for fine particulate matter collection. The

3. RESULTS AND DISCUSSION 3.1. Coal Properties. Ultimate and proximate analyses of the high-arsenic coal studied are listed in Table 1. On the basis of the volatile content (12.17 wt %) of the coal, the coal is classified into anthracite. The coal contains relatively high ash (45.94 wt %) and sulfur (20.31 wt %) contents. The major chemical compositions of the high-temperature ash of the coal studied are shown in Table 2. The concentration of SO3 is as high as 21.46 wt %. Sulfur in coal occurs as both organic and inorganic forms, with inorganic sulfur usually recognized as sulfides and sulfates, of which pyrite is the most common sulfur-containing mineral in most coals.24 Pyrite is also considered to be the major host of arsenic.25 It is expected that the high-ash and high-sulfur coal would be enriched in pyrite and arsenic. Our previous studies2 on the typical higharsenic coal indicated that the arsenic content in the coal was as high as 267 μg/g and arsenic occurs as both organic associated (approaching 30%) and inorganic associated, of which pyriteassociated arsenic dominated. 3.2. Vaporizarion of Arsenic in High-Temperature Treatment. The bleeding ratio (BR)26 has been adopted to qualify arsenic in terms of its volatility observed from arsenic concentrations in coal and ashes. BR = 1 −

arsenic content in ash × ash yield arsenic content in coal

The BRs of arsenic in high-temperature treatment of the coal are shown in Figure 2. It is found that BRs of arsenic in air

Figure 1. Schematic diagram of the laboratory-scale DTF. B

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Energy & Fuels Table 1. Ultimate and Proximate Analyses of the Coal Studieda ultimate analysis (wt %)

a

proximate analysis (wt %)

Cdaf

Hdaf

Ndaf

St

Odaf

Mad

Ad

Vad

FCad

arsenic concentration (μg/g)

75.29

1.56

0.79

20.31

3.03

1.05

45.94

12.17

40.84

267

daf, dry and ash free; St, sulfur total; ad, air dried; d, dry; M, moisture; A, ash; V, volatile; and FC, fixed carbon.

into consideration, and the modes of occurrence of arsenic in coal would eventually impact on its vaporization behaviors. Herein, the remaining arsenic in the ash residuals at 1300 °C is probably resulting from alumina-silicate-bound arsenic in the coal, which would be fused and assimilated as arsenate in alumina silicate in the residual ashes.9,13 Further, the discrepancies of BRs in the three gas ambience at 1300 °C are highly suspected to link to the distinct vaporization behavior of different arsenic occurrences in the coal at a high temperature. For instance, inorganic-associated arsenic in coal would vaporize together with the transformations of minerals at a high temperature. Three consecutive processes of the vaporization of inorganicassociated arsenic in coal combustion have been welldocumented,9 which include arsenic diffusion through the mineral melt, arsenic vaporization at the interface of the melt and gas, and arsenic transportation through the fissures of the char. In comparison, the major resistance for organic-associated arsenic in vaporization is the binding energy of C−As and/or S−As and the transportation of the vapor phase in fissures of char particles, which is apparently much simpler than that of inorganic-associated arsenic. The BR of arsenic in N2 pyrolysis is about 40% and remains constant in the temperature ranging from 1200 to 1400 °C. It can also be found that BRs of arsenic in N2 pyrolysis are very close to the amount of organicassociated arsenic in the coal, which indicates that only organicassociated arsenic would easily vaporize in N2 pyrolysis at a high temperature, while BRs of arsenic in CO2 gasification increase from 60 to 80% when the temperature rises from 1200 to 1400 °C, which indicates that not only is organic-associated arsenic vaporized but also part of inorganic-associated arsenic would also be vaporized in CO2 gasification. It can also be concluded that both organic- and inorganic-associated arsenic would vaporize in air combustion, because the initial

Table 2. Major Chemical Compositions of the HighTemperature Ash of the Coal Studied (wt %) Na2O

MgO

Al2O3

SiO2

Fe2O3

SO3

K2O

CaO

TiO2

0.6

0.7

36.8

24.93

9.76

21.46

2.34

0.58

2.6

Figure 2. BRs of arsenic in different atmospheres as a function of the temperature.

combustion, CO2 gasification, and N2 pyrolysis are about 85, 65, and 40% in 1300 °C, respectively, which indicates that a certain part of arsenic in the coal studied is not releasable, even when the temperature reaches 1300 °C. The experimental results seem to be different with the thermodynamic equilibrium calculation, indicating that almost all of the arsenic would vaporize and present as gas-phase AsO(g) when the temperature is higher than 1000 °C, despite variations in the gas atmosphere.27 In fact, thermodynamic equilibrium calculation could not take the modes of occurrence of arsenic in coal

Figure 3. SEM observations of ashes from coal pyrolysis/combustion/gasification. C

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Energy & Fuels decomposition behavior of pyrite in combustion is similar to that in gasification.28−30 However, a relatively higher BR of arsenic in air combustion than that in CO2 gasification is probably ascribed to the secondary reactions between gas-phase arsenic and ash residuals in flue gas. It is widely accepted that secondary reactions between gas-phase arsenic and fly ash in flue gas would result in the retention of arsenic. The residual ashes generated from the three ambiences have been characterized by FE-SEM−EDX examinations for supporting that point. As shown in Figure 3, obvious discrepancies in micromorphologies of ashes generated in the three different ambiences at a high temperature can be observed. Massive particles (panel 1 of Figure 3), which are similar to mineral grains in morphologies in raw coal, are extensively observed in N2 pyrolysis. Fragments are mostly formed as a result of fragmentation led by a rapid release of volatile matter in coal at a high temperature, leaving some cracks on the surface of the particle (panel M1 of Figure 3). While ash residuals (panel 2 of Figure 3) generated from air combustion show typical melting and coalescence (panel M2 of Figure 3), the EDX results indicate that most of the particles are alumina silicates. In contrast, morphology of the ash residuals (panel 3 of Figure 3) generated from coal gasification is different in comparison to that in pyrolysis and combustion conditions. Significant unburned carbon (panel M3 of Figure 3) has been identified in ash residues as a result of the low carbon conversion rate in gasification. Further, both secondary reactions with Ca/Fe-bearing compositions in flue gas31 and the interactions between gas-phase arsenic and residual unburned carbon in cooling flue gas downstream may result in the retention of arsenic in ash residuals.32 Especially, the unburned carbon in the ash residuals generated from CO2 gasification is highly suspected to be capable of absorbing gasphase arsenic in cooling flue gas downstream in gasification conditions. 3.3. Mineral Transformations and Decompositions of Pyrite. In CO2 gasification and/or air combustion conditions, apart from organically associated As, most arsenic associated with minerals in the coal would also vaporize, together with mineral transformations.9 Pyrite-associated arsenic dominates in the coal studied. Arsenic substitutes for sulfur in the pyrite structure.2 Hence, the process of mineral transformations, such as decompositions of pyrite, at a high temperature would have a significant impact on the vaporization of arsenic. Detailed analyses on decompositions of pyrite are critical for interpreting the vaporization of pyrite-associated arsenic. The XRD spectra in Figure 4 help in identifying different mineral phases in ash generated from both air combustion and CO2 gasification of the coal at a high temperature. Major mineral phases are quartz and mullite in the two gas ambiences. Quartz is probably generated from the phase transformation of silica in the coal or original quartz in coal. Mullite is formed by dewatering of kaolinite in the coal, and the whole transformation process is as shown below.

Figure 4. XRD patterns of ashes from coal combustion and gasification.

transformation of pyrite in temperatures higher than 1300 °C. However, the process of transformation from pyrite to hematite might become weak with a lack of oxygen or in a low temperature. The transformation process is as follows: FeS2 → Fe0.887S → Fe2O3

The SEM−EDX identifications of phase mineral compositions are shown in Figure 5, and the corresponding EDX results are listed in Table 3. The SEM−EDX observations are identical to the results from XRD analysis. Aluminosilicate and iron oxides are commonly identified. However, some particles, which mostly contain sulfur and iron (Table 4), are also identified by the SEM observation, as indicated in Figure 6, which are probably generated from decompositions of pyrite at a high temperature and represent typical transition forms of pyrite in decomposition. Previous publications28−30 have also indicated that pyrite would undergo multi-steps in a reductive and/or an inert atmosphere in sequence of pyrite−pyrrhotite−troilite− iron and pyrrhotite would be oxidized to produce a stable FeO−FeS melt phase under a reductive atmosphere. The whole decomposition process follows the unreacted core model and is controlled majorly by the temperature and total sulfur vapor pressure.29 Panel 1 of Figure 6 shows the original massive pyrite particle with a size of around 15 μm identified in the coal studied. In the high-temperature treatment in a reductive and/ or an inert atmosphere, the FeS2 starts to decompose and an overall chemical reaction28−30,33 is supposed to occur.

Al 2SiO5(OH)4 (kaolinite) → Al 2O3 · 2SiO2 (metakaolin) → Al 2O3 · SiO2 (spinel) → Al 6Si 2O13 (mullite)

FeS2(s) → FeSx (s) + (1 − 0.5x)S2(g)

Hematite can be identified in ashes from coal combustion at 1300 and 1400 °C. It is probably formed from the

Panel 2 of Figure 6 shows the initial decomposition process of FeS2 at a high temperature. It can be observed that massive D

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Figure 5. SEM images of ashes generated from coal combustion and gasification.

Table 3. Elemental Compositions of Phase Mineral Compositions Corresponding to SEM Observation in Figure 5 (Atomic %) number 1-1 1-2 1-3 1-4 1-5 2-1 2-2 2-3 2-4

S 1.68 0.69 11.08 1.58

Fe

Al

Si

K

Ti

Mg

O

17.59 50.76 6.17 0.53 5.37 0.96 27.94 43.89

13.38 3.12 20.33 2.03 11.72 21.97 10.24 1.73 0.46

21.4 6.25 19.02 2.80 24.52 21.55 10.14 3.36 41.17

1.68

0.24

0.37

7.93 0.48 4.46 2.69 1.68

7.43

45.33 38.19 38.42 42.63 51.98 51.83 49.71 51.02 58.17

0.2

0.36 0.5 0.28

0.52

C

40.46

phase compositions aluminosilicate + iron oxides hematite aluminosilicate unburned carbon + SO3 aluminosilicate aluminosilicate aluminosilicate + iron oxides hematite mullite

Table 4. Elemental Compositions of Pyrite Particles Corresponding to SEM Observation in Figure 6 (Atomic %) number

S

Fe

Al

Si

O

1 2 3 4 5 6 7 8

38.59 25.07 51.09 14.34 5.13 1.7

26.72 42.35 48.91 41.61 33.96 43.85 43.24 0.82

2.69

3.18

27.45 32.59

2.43 4.68 1.73 2.6 3.96

2.32 6.79 1.72 2.6 4.46

39.31 49.18 51 51.22 42.16

45.13

pyrite particles (panel 1 of Figure 6) are transforming into a sphere particle, which results from thermal expansion at a high temperature. Meanwhile, sulfur starts to vaporize into the vapor phase, thus forming the FeSx phase. EDX analysis indicates sulfur appearing on the surface of the sphere particle. A high temperature would bring about the vaporization and release of sulfur from the inside layer of pyrite, thus resulting in the porosity surface of the sphere particle. However, sulfur vapor might condense when the temperature falls, with the result of sulfur adhering to the surface of the sphere particles. The sulfur vapor pressure on the interface of the gas phase and solid FeSx particles has a significant impact on the process of pyrite decomposition. 29 However, with the chemical reaction continuing, sulfur vapor accumulating at the interface of the gas and solid phase would be depleted because of the influence of the gas flow field, thus causing a diminishing sulfur gas pressure locally. Meanwhile, sulfur from the inner layer of FeSx would vaporize and release from the inside layer of pyrite, thus forming a more porous spherical particle with a higher Fe content and lower S content, as indicated in panels 3 and 4 of Figure 6. In a reductive atmosphere, FeSx would finally form Fe

Figure 6. SEM observations of decompositions of pyrite in coal at a high temperature. E

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distribution.34 It suggests that formation pathways of the PM10 [particular matter (Da < 10 μm)] in coal gasification are different from that in coal combustion. Fine particles generated from coal combustion are considered to be formed by the inorganic element vaporization in coal particles burning at high temperatures, followed by condensation in cooler regions, and a high temperature is in favor of fine-particle formation. Coarse particles generated from coal combustion are present as a round shape with a smooth surface. SEM observations of fine particles generated from coal gasification are shown in Figure 8. Panel 1 of Figure 8 illustrates the fine particles from the 11th stage of the DLPI; most of particles appear to be in a round shape with a size of about 5−10 μm with rough surfaces, which are definitely different from the coarse particles from coal combustion with a smooth surface. The result indicates that the coarse particles have not undergone a complete melting and condense process. However, some round particles with a smooth surface were also identified. Most of the particles are composed of small strips adhering together with cracks and pores on the surface; it is likely that these have undergone an incomplete melting process, while the cracks and pores are probably caused by the release of the volatile matter in the coal particles. Uniform particles with a size of about 0.5 μm from particles on the 5th stage (0.3 μm) from coal gasification can be observed (panel 2 of Figure 8). Particles seem to agglomerate with each other. However, round-shaped particles can still be observed distinctly. The surface elemental analysis determined by XPS of ashes from bottom ash (BA, collected from the cyclone) and the fine particular matter (PM) (5th stage) is presented in Table 5. It can be seen that elements on the surface of BA and PM are mostly composed of carbon and oxygen, which is identical with the findings from Morris et al.35 Especially in the PM, the carbon content is approaching 70%, which indicates that fine particles are mostly soot, which is probably generated from the condensation of products from incomplete combustion. A comparison of the elemental distributions in BA and PM illustrates that refractory elements, including Fe, Si, and Al, are obviously enriched in BA, while Fe and Al are not identified on the surface in the depth of 0−10 nm of PM. A large fraction of carbon is observed on the surface of PM, which is possibly caused by the superficial deposit of soot on the nucleating center, which is composed of Si or Al. A certain amount of arsenic is also identified on the surface of both BA and PM. 3.5. Speciation of Arsenic in Fine Particles. The arsenic concentrations and distributions in fine particles generated from coal combustion and gasification are shown in Figure 9. It is observed that arsenic is enriched in fine particles, and the concentration of arsenic in the particles changes with the particle size. The arsenic concentrations in the fine particles range from 1.0 × 104 to 12 × 104 μg/g. A common finding in coal combustion and gasification is that arsenic is enriched in the particles with a size of about 0.1−0.2 μm. A high temperature would promote the accumulation of arsenic in the fine particles. On the basis of that, arsenic is obviously enriched in fine particles with a size of 0.2 μm. We have conducted the examinations of arsenic speciation in fine particles generated from coal combustion and gasification by TOF-SIMS. Our results obtained from the TOF-SIMS examinations are shown in Figure 10. It can be identified that only As2O5 and Ca3(AsO4)2 can be identified in fine particles generated from coal combustion, while As2O5, As, AsO, and

(panel 5 of Figure 6) with a relatively dense surface in comparison to the transition form, as indicated in panel 4 of Figure 6. In an oxygen-containing atmosphere, the process of thermal decomposition of pyrite in the initial step is analogous to that in a reductive or an inert atmosphere.29 However, differences on transformation of pyrite between coal combustion and gasification would occur in the final step. For instance, elemental Fe would be oxidized to form Fecontaining oxides in an oxygen atmosphere (panel 6 of Figure 6), with the relative melting surface in comparison to FeSx (panel 3 of Figure 6), indicating that it may have been through an semi-melting process at a high temperature. Pyrite wrapping on the surface of alumina silicon is also identified (panel 7 of Figure 6), and such pyrite would probably melt together with the intimately mixed alumina silicon at a high temperature. Typical char particles are shown in panel 8 of Figure 6. It could be observed that these char particles were larger than 50 μm and highly porous, and melting of aluminosilicate was also observed on the surface. Further, a certain amount of sulfur is also identified in the residual carbon by EDX measurement, which was probably generated from the decomposition of pyrite during the process of coal gasification and adsorbed by the unburned carbon in the flue gas. Arsenic in the coal studied substitutes for sulfur in the pyrite structure.2 Hence, arsenic vaporization is expected to occur along with sulfur vaporization, and the vaporization behavior is probably similar to that of sulfur from pyrite at a high temperature, which is mostly controlled by the temperature and sulfur gas pressure.29 3.4. Fine-Particle Emissions in the Coal Combustion and Gasification. The particle size distributions of fine particles generated from coal gasification and combustion are illustrated in Figure 7. It can be found that the mass size

Figure 7. Mass size distributions of fine particles generated from coal gasification.

distributions of fine particles generated from coal gasification are present as a bimodal distribution, and the two major peaks appear at 0.4 and 5 μm. The temperature has a relatively small impact on the bimodal mass size distribution in coal gasification. However, the yields of PM1 [sub-microparticular matter (Da < 1 μm)] decrease with the temperature increasing, which suggests that a high temperature inhibits the formation of PM1 during coal gasification. This finding is also contrary to that in coal combustion conditions. The distribution obtained from coal combustion is different and has as a trimodal F

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Figure 8. SEM images of fine particles generated from coal gasification.

Table 5. Elemental Compositions in BA and PM (5th Stage) (Mass %)

a

samplea

Fe

BA PM

5.45

N

O

K

C

S

Si

Al

As

28.5 21.11

1.98

37.54 67.5

1.98 3.00

13.18 2.74

8.61

1.04

0.31 0.02

BA, bottom ash; PM, particulate matter.

As2O5 in fine particles generated from coal combustion and gasification is probably formed in the process of chemical adsorption of As3+ on activated calcium-bearing compositions.38 Hence, it is deduced that chemical reactions between gas-phase arsenic and calcium-bearing compounds with products of Ca3(AsO4)2 are expected to be part of a partitioning mechanism of arsenic on the surface of fine particles in both coal combustion and gasification. However, As and AsO, which are of chemical instabilities, can only be identified in fine particles generated from coal gasification. These two arsenic-containing species are probably formed by homogeneous condensation, following nucleation in a reducing atmosphere, and quickly adsorbed by soot prior to getting in touch with the other major species in the flue gas, thus partitioning on the surface of soot by physical absorption.39 In general, the vaporization and partitioning of arsenic in coal in a high temperature are supposed to be governed by multiple factors, including coal properties, arsenic concentration and occurrence in coal, reaction temperature, reaction ambience, residence time, carbon conversion rate, chemical compositions of fine particles in flue gas, etc. General rules can be achieved that the partitioning of arsenic would be dominated by the extent of arsenic vaporization and the surface reactions between gas-phase arsenic and available active sites on the surface of fine particles, which is determined by the coal properties and reaction conditions. Minor variations of arsenic speciation in fine particles in flue gas might be identified in high-temperature treatment of different coals. Hence, a comprehensive understanding of the coal properties would be conductive to predict the possible chemical speciation of arsenic after high-temperature treatment. 3.6. Possible Solutions To Reduce Arsenic Emissions in Coal Utilization. The speciation of arsenic can determine the level of toxicity and carcinogenic potency,18,40 and it is reported that As3+ is much more toxic than As5+.41,42 To eliminate arsenic toxicity in the environment caused by coal utilization, the following suggestions are available for consideration: First of all, the easily vaporized modes of

Figure 9. Arsenic concentrations and distributions in fine particles.

Ca3(AsO4)2 are the major arsenic-containing species in the fine particles generated from coal gasification. Even though most of arsenic is vaporized at a high temperature, the total amount of arsenic in the gas phase is extremely small in comparison to other major species in coal.36 Hence, it is more likely that gas-phase arsenic would get in touch with the surface of fine particles prior to achieving nucleation conditions as a result of a high number density of fine particles in flue gas.37 Therefore, homogeneous condensation may not be a critical mechanism for arsenic enrichment in fine particles. The discrepancy of arsenic species in coal combustion and gasification is highly suspected to be linked to the exterior surface reaction between gas-phase arsenic and available active sites on the surface of fine particles in flue gas.20 Ca3(AsO4)2 identified in both fine particles generated from coal combustion and gasification is probably formed by the chemical reactions between activated calcium cations and arsenic oxides, while G

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Figure 10. Arsenic speciation in fine particles generated from coal combustion and gasification.

investigating the emissions of arsenic-containing fine particles at a high temperature with the help of a lab-scale DTF. The results indicate that most arsenic in coal is released at high temperatures, while modes of occurrence of arsenic and gas atmospheres would affect the release behavior of arsenic. BRs of arsenic in air combustion, CO2 gasification, and N2 pyrolysis are 85, 65, and 45%, respectively, at 1300 °C and would remain constant with the temperature increasing from 1200 to 1400 °C. Organic-associated arsenic would be more inclined to vaporize in N2 pyrolysis, while both organic- and inorganicassociated arsenic would be vaporized in CO2 gasification and air combustion. Arsenic remaining in coal ashes was probably ascribed to the aluminosilicate association of arsenic in the coal. Pyrite-associated arsenic would be vaporized along with sulfur in pyrite inclusions. The decompositions of pyrite follow the principle of the unreacted core model and are mostly controlled by the surface sulfur vapor pressure. Mass size distributions of fine particulate matter generated from coal gasification are present as a bimodal distribution, and two major peaks appear at 0.4 and 5 μm. Particles with a size of 5 μm are presented as a round shape with pores and cracks on the surface, while particles with a size of 0.4 μm are found to be soot. Arsenic is obviously enriched in the fine particles with a size of around

occurrence of arsenic in raw coal should be removed before utilization. It has been found that organic-associated arsenic and pyrite-associated arsenic would be easily vaporized at a high temperature. Hence, it is possible to reduce and/or avoid the escape of arsenic into the flue gas by removing these two modes of arsenic in raw coal by chemical treatment before utilization. Second, arsenic should be induced to transform to less toxicity and water-insoluble arsenic compounds in flue gas. It has been found that As5+ compounds are more stable under oxidizing conditions and As3+ compounds are more likely to exist in reducing conditions.43 To avoid the formation of As3+ compounds in flue gas at a high temperature, it is effective to increase the oxygen content in the reaction system to increase the number of oxyanions on the surface of fine particles, which would be conducive to transformation of arsenic to Ca3(AsO4)2 and As5+. Lastly, calcium-based sorbents should be injected to increase the active cation sites for arsenic accommodation, and arsenic should be transformed to arsenic calcium in the flue gas, which would also be helpful for arsenic stabilization.

4. CONCLUSION A typical high-arsenic coal (arsenic concentration > 260 μg/g of coal) from southwestern China has been chosen for H

DOI: 10.1021/acs.energyfuels.6b00279 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels 0.1−0.2 μm in both coal combustion and gasification. The arsenic species of arsenic in fine particles generated from coal combustion are As2O5 and Ca3(AsO4)2, while species in fine particles generated from coal gasification are As2O5, As, AsO, and Ca3(AsO4)2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Key Basic Research and Development Program (2014CB238904), the National Natural Science Foundation of China (51506066 and 51376074), the China Postdoctoral Science Foundation (2015M582222), the Open Research Fund of the State Key Laboratory of Environment Geochemistry (SKLEG2016908), and the Helmholtz Alberta Initiative (HAI), Lab of Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5MPT), in Canada. The authors thank anonymous reviewers for the critical comments.

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NOMENCLATURE CCP = coal combustion product ESP = electrostatic precipitator APCD = air pollution control device DTF = drop-tube furnace DLPI = Dekati low-pressure impactor XRD = X-ray diffraction FE-SEM = field emission scanning electron microscopy XPS = X-ray photoelectron spectroscopy ICP−MS = inductively coupled plasma mass spectrometry TOF-SIMS = time of flight secondary ion mass spectrometry EDX = energy-dispersive X-ray spectroscopy BR = bleeding ratio PM1 = sub-microparticular matter (Da < 1 μm) PM10 = particular matter (Da < 10 μm)

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DOI: 10.1021/acs.energyfuels.6b00279 Energy Fuels XXXX, XXX, XXX−XXX