Effect of Blast Furnace Sludge on SO2 Emissions from Coal

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Effect of Blast Furnace Sludge on SO2 Emissions from Coal Combustion Zhifang Gao,*,† Zhaojin Wu,‡ and Mingdong Zheng§ †

School of Metallurgy and Resource, ‡Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, and §School of Chemistry & Chemical Engineering, Anhui University of Technology, Maanshan 243002, China ABSTRACT: Blast furnace sludge (BFS) exhibits a strong sulfur-fixing ability due to the large amount of metal oxides, such as Fe2O3, CaO, SiO2, and Al2O, that it contains. In this study, the influence of BFS on SO2 emissions and the sulfur-fixing mechanism is studied in the coal combustion process. The results show that the SO2 emission peak values decrease with the increase of BFS mass percentage and that the sulfur-fixing ratio increases to 89% when the BFS mass percentage reaches 10% at 900 °C. The sulfur-fixing mechanism predicts that the multicomponents of BFS favor the formation of a stable sulfate (3CAO3· A12O3·CaSO4) and reduce the rapid decomposition of CaSO4 coated by thermally stable sulfur compounds such as CaFe3(SiO4)2OH. These results indicate that BFS, as a sulfur-fixing agent, is cheap and highly efficient in controlling SO2 emissions during coal combustion processes.

1. INTRODUCTION Because of the increasing usage of high-sulfur content coals, atmospheric pollution is becoming increasingly serious. Serious environmental pollution resulting from coal combustion has led to a strict control of the SO2 emission level. To reduce SO2 emission, inhibiting SO2 emissions during coal combustion processes has been the focus to protect the atmospheric environment.1 Coal sulfur-fixing agents, such as CaCO3, CaO, Ca(OH)2, MgCO3, MgO, Na2CO3, and NaOH, are crucial for limiting sulfur dioxide emissions during coal combustion.2,3 Sulfur-fixing agents include calcium-based sorbents (CaCO3, CaO, and Ca(OH)2) and alkaline residues (carbide slag, waste paper, boron mud, natural minerals, and other industrial waste salt and limestone).7−9 The most widely used and cheap sulfurfixing agents are calcium-based sorbents (CaCO3, CaO, and Ca(OH)2).4−6 Because calcium-based sulfur-fixing products (CaSO4) decompose easily at high temperatures, many scholars have studied the methods for reducing CaSO4 decomposition at high temperatures and found that the addition of alkali metal oxides or alkaline have efficient effects. For example, Sm-based and Mn-/SBA-15 sulfur-fixing agents can effectively inhibit SO2 emissions from coal combustion.10,11 In addition, based on the properties of calcium-based sulfur-fixing agents, SiO2, CuO, ZnO, Fe2O3, and other substances have shown certain synergistic effects on sulfur-fixing processes.12,13 Therefore, the studies on the compounds in sulfur-fixing agents have received significant interest in recent years; however, synthetic or natural sulfurfixing agents have high costs. Thus, the use of solid wastes instead of synthetic sulfur-fixing agents during coal combustion processes to reduce SO2 emissions become the research hotspots. Blast furnace sludge (BFS) is a type of metallurgical ironbearing dust from steel industrial solid waste that contains not only a high mass weight of Fe (30−80%) but also Al (5−10%), Si, and a variety of small amounts of trace transition metals (e.g., Mn, Zn, Ni, V, Mo, and Cr), which are valuable metallurgical secondary resources.14,15 If the BFS is improperly applied, it will not only be a waste of secondary resources but can also cause © XXXX American Chemical Society

serious environmental pollution. At present, most of the metallurgical dust and sludge is returned to steel production for Fe recovery. However, this type of usage can cause the enrichments of ZnO, PbO, Na2O, K2O, and other harmful impurities, a high energy consumption, poor operating conditions, and other problems; thus, the recycling of BFS is greatly restricted.16,17 In recent years, BFS has been used as an adsorbent for the adsorption of Pb2+ and other harmful heavy metal ions in wastewater, but the more valuable components in BFS have not been fully utilized.18−20 In addition, the application of BFS as building materials requires extensive work, and the added value is very low.21−23 Therefore, a key challenge is to simultaneously recover certain amounts of the valuable components in BFS during its application. A previous study has indicated that a large amount of metal elements, i.e., alkaline metal oxides, iron oxides, transition metal oxides, and salts, plays an advantageous role in coal combustion.24 Therefore, in a multicomponent system, iron oxide (approximately 20%) can be used as a sulfur-fixing agent, carbon (23%) can be involved in coal combustion to increase the heat value, and other compounds (50%) can contain auxiliary sulfur-fixing functions. For example, CaO is a calcium-based sorbent, MgO is a magnesium-based sulfur-fixing agent, SiO2 and Al2O3 represent additives of a calcium-based sulfur-fixing agent, and other trace transition metal oxides, such as Zn, Mn, and Ni, have characteristics that promote sulfur-fixing reactions. In this study, X-ray diffraction (XRD), scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM-EDS), and tthermogravimetry-mass spectrometry (TG-MS) are used to investigate the synergy of a multicomponent system in affecting the sulfur-fixing mechanism in the coal combustion process. The results are beneficial for describing the potential of solid wastes as a sulfur-fixing agent to reduce SO2 emissions and can further the Received: December 23, 2015 Revised: February 16, 2016

A

DOI: 10.1021/acs.energyfuels.5b02986 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Particle Size and Characteristic Diameters of BFS and Coalsa particle size (μm)

a

sample

grinding time (min)

D10

D25

D50

D75

D90

Dav

specific surface area (m2/g)

BFS coarse coal fine coal

5 40 120

6.75 25.39 19.34

10.71 36.94 28.25

15.80 49.87 38.64

22.71 64.36 53.64

32.39 80.12 75.23

17.93 51.01 26.31

12.38 9.67 18.99

Note: D10, D25, D50, D75, D90 are the cumulation reached 10%, 25%, 50%, 75%, 90%, respectively, Dav is average diameter.

Table 2. Chemical Composition of BFS component

TFe

CaO

MgO

Al2O3

SiO2

C

Zn

Pb

others

content (%, ω)

26.57

10.14

5.76

5.54

12.67

24.33

4.17

1.00

9.82

Table 3. Industrial, Ultimate, and Ash Analysis of Coal (%, ω)a industrial analysis ultimate analysis ash analysis a

Mt

Atd

Std

Vtd

Vdaf

FCdaf

8.77 Cdaf

7.48

1.81

33.19

36.03

58.94

SiO2

85.58 TiO2

42.27

0.84

Hdaf

Odaf

Ndaf

3.88 Al2O3

5.34 Fe2O3

MgO

1.46 CaO

25.34

11.43

1.13

5.88

caloric value (MJ/kg) 31.56

Std

C/H

C/O

0.37 K2O

21.19 Na2O

SO3

14.72 others

0.38

0.705

7.89

1.20

Note: Mt, A, td, daf are denoted as the total moisture of coal, ash of coal, all and dry basis, respectively.

Figure 1. Schematic diagram of coal combustion in a fixed reactor device. spectrometer (Thermo Elemental-IRIS Intrepid). A LecoCS-344 determinator was used for carbon and sulfur analyses. A PE-5100 atomic absorption spectrophotometer was used to detect the Pb content. TFe was measured by titrating dichromate. The mineral phase composition was determined by X-ray diffraction (XRD, Bruker D8 Advance). The XRD pattern was recorded from 20° to 80° (2θ) with a step size of 0.02° using a counting time of 0.4 s per step. Morphological analysis of BFS was performed by scanning electron microscopy (SEM, Hitachi S-3400N II). Particle size analysis of the coal samples was performed using a laser particle size analyzer (LS-C (1), China). To analyze the SO2 emissions, the samples were first placed in a 1 kWth drop tubular furnace (GSL-1700X, Hefei Branch Crystal) (GSL-1700X, Hefei Branch Crystal shown in Figure 1) in an air atmosphere (30 mL/

understanding of the effect of BFS combustion on SO2 emission characteristics.

2. EXPERIMENTAL PROCEDURES 2.1. Materials and Instruments. 2.1.1. Materials. The BFS used in the experiment was obtained from Ma’anshan Iron and Steel Trade Co. Ltd. in Ma’anshan City, China, and the coal was selected from Shangxi Province, China. Proximate analysis, ultimate analysis, sulfur forms, and ash constituents of coals are given in Table 3. The chemical compositions and densities of BFS are listed in Table 2. 2.1.2. Instruments. The main contents (CaO, MgO, Al2O3, SiO2, P, and Zn) of the samples were determined by an ICP emission B


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Energy & Fuels min flow rate) with 10 °C/min heating rate from room temperature to 100 °C for 30 min. Then, the samples were heated at a 10 °C/min heating rate for 20 min to ensure that the samples were completely dry. In accordance with the tubular furnace combustion conditions, the characteristics of SO2 emissions were determined using thermogravimetry-mass spectrometry (TG-MS, France Dorset RAM). 2.2. Characterization and Analysis. First, the coal was mixed with 2%, 6%, 8%, and 10% BFS by mass. The combustion reaction experiments were carried out in the TG-MS system under an air atmosphere (30 mL/min flow rate) with a 10 °C/min heating rate from room temperature (approximately 25 °C) to 100 °C for 30 min. Then, a 10 °C/min heating rate was applied for 20 min to ensure that the samples were completely dry. To ensure the full combustion of coal, parallel experiments under the same burning conditions were carried out in the tubular furnace, and the microstructure and mineral phase of the sulfur-fixing residues were identified and analyzed via XRD and SEM. The total sulfur formed was estimated using the Eschka method, and the sulfate sulfur (SS) was analyzed gravimetrically using BaSO4. The sulfur-fixing ratio is calculated by eq 1: Rs =

AR SA × 100% CSR

reaction) of fixed coal decreased, and the ignition temperature of raw coal was 452 °C, but the ignition temperature of sample decreased to 425 °C after adding 6% BFS. When the added BFS mass percentage reached 10% in mixed sample, the ignition temperature of decreased to 410 °C. This phenomenon can be attributed to the components of BFS, such as Na, Ca, Fe, Mg, Zn, Al, and some trace metal elements (Cr, Ni, Co, Mn, K, Cu), which have catalytic properties.25 The catalytic properties of metal ions in BFS can promote the formation and strengthening of O−C bonds. The complex COM (where M is a metal ion) is formed by an O−C bond and a metal, which increases the complex reaction as an active center. In addition, a large number of Fe2O3 particles and a small amount of Al2O3 particles in BFS, which play an important oxygen carrying role, can easily react with H2 and CO, which are released from coal pyrolysis due to their high activities in the combustion processes.26,27 Thus, the TG and DTA curves showed that the effect of BFS on the sample combustion temperature were significant. The burned temperature (when the combustion reaction processing is over) of raw coal was 645 °C, whereas the combustion temperatures (the corresponding temperature of peak value in the curve) of the mixed samples decreased with increasing mass percentage of BFS. That is the fixed carbon and organic minerals decreased in mixed samples. However, fixed carbon and organic minerals, which are difficult to decompose at low temperatures and need to be kept at the high temperature stage, take part in multiphase reactions that are limited from the inside to the outside, which can lead to slow burning of the samples and prolong the overall burnout time and temperature. Figure 4 shows the SO2 emissions curves from raw coal and mixed coal combustion. The vertical axis is denoted as change-

(1)

where RS is the sulfur-fixing ratio of coal (%); AR is the ash content of raw coal (%); SA is the sulfur content in the residues (%); SR is the sulfur content in the raw coal (%); and C is the raw coal mass percentage in the mixed coal sample (%).

3. RESULTS AND DISCUSSION 3.1. TG-MS Analysis. The TG curves of the combustion samples (Figure 2) show that the weight loss rate is obviously

Figure 2. TG curve of coal combustion blended with BFS.

reduced after adding BFS. According to Figure 3, the ignition temperature (the temperature of beginning to combustion

Figure 4. Effect of BFS on SO2 emission from samples in coal combustion processing.

Figure 3. DTA curve of coal combustion blended with BFS.

quantity mass ratio of ions in Figure 4. SO2 was released in the coal combustion processes at a temperature of 290 °C, and the SO2 concentration reached a peak value at 450 °C. The SO2 concentration returned to its original value at 600 °C. When 2% BFS was added to the coal in the combustion process, SO2 was released at 300 °C; the release amount reached a peak value at 400 °C, and the SO2 concentration returned to its original state at 600 °C. When 6% BFS was added to the coal, SO2 started its release at 350 °C; the SO2 emission reached a peak value at 450 °C, where its concentration was clearly lower than that of the 2% BFS sample. When the BFS mass percentage reached 10% in the mixed sample, SO2 started its release at 400 °C, and its concentration reached a peak value at 500 °C, but the SO2 emission time was obviously shorter than that of the coal sample with added 2% BFS. Thus, when BFS was added to the coal in the C


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and the molten slag no longer exhibited any further sulfur-fixing capacity at high temperatures. 3.3. XRD and SEM Analyses. 3.3.1. Effect of BFS on the Sulfur-Fixing Ratio. According to the requirements of the heating value in the coal combustion process, the influence of the combustion temperature on the sulfur-fixing ratio of the mixed samples was studied by comparison tests based on previous studies.24 (a). XRD Analyses. Figure 6 shows the XRD analysis of a sulfur-containing slag at 900 °C during coal combustion. Figure 6a shows the mineral phase compositions of raw coal, i.e., αFe2O3, SiO2, CaCO3, 3CaO·Al2O3 and other amorphous phases. Figure 6b shows the mineral phase compositions of the residues containing sulfur from test samples blended with 2% BFS by mass weight: Fe9TiO15, SiO2, α-Fe2O3, CaSO4, Ca3Al2Si2O8, and other amorphous phases. CaSO4 was not detected in the raw coal residue but was detected in the residue from the test sample blended with 2% BFS. The sulfur-fixing residues from raw coal included γ-2Fe2O3, SiO2, C, CaCO3, 3CaO·Al2O3 and other amorphous phases at 900 °C. CaCO3 was reduced with the increase of BFS, but SiO2 was still clearly detected with the increase of BFS. SiO2 in the raw coal ash easily reacted with silicates and other metal oxides to produce low melting point eutectics in the combustion process, such as SiO2−Al2O3−K2O, with a melting point of 750 °C; SiO2−CaO-Na2O, with a melting point of 720 °C; and SiO2− CaO-K2O, with a melting point of 710 °C. Thus, the mass percentage of the amorphous phase in the residues reached 37% at 900 °C. Ideally, if all of the sulfur from the raw coal existed in the residue samples, the conversion ratio of calcium could reach approximately 70% based on existing research (e.g., see ref 8).8 However, in reality, organic sulfur functional groups with poor thermal stabilities, such as thiols (RSH), sulfides (RSR′), and sulfur anthracene (RSSR′), began to release at a temperature range of 300 to 400 °C, and at the same time, the decomposition ratio of CaCO3 was still very low; thus, a large amount of organic sulfur could escape from residues before reacting with CaO. Another reason was that the surface pores of the CaO particles was blocked with CaSO4 to prevent the salinization reaction of sulfur from entering the interior of the residue particles, which resulted in a large amount of CaO remaining in the unreacted cores. Therefore, a large amount of calcium particles not reacted with sulfur can generate residual CaCO3 and the CaO crystalline phase; however, because the residues had good reaction activities, there was a great potential for sulfur fixation. Figure 6c shows the mineral phase compositions of the residues produced from coals blended with 6% BFS, including Fe9TiO15, SiO2, γ-2Fe2O3, CaSO4, Ca3Al2Si2O8, and others. When the mass percentage of BFS increased to 10%, the desulfurization phase in the residues contained Fe9TiO15, SiO2, γ-2Fe2O3, CaSO4, Ca3Al2Si2O8, and others. It can be seen that the content of CaCO3 in the sulfur-fixing residues gradually decreased with the increase of BFS mass percentage. The increase in the CaSO4 mass percentage and the main reaction mechanism of CaCO3 with SO2 to generate CaSO4 in the combustion can be simplified to the following reactions:29

combustion process, the SO2 emission concentrations were significantly reduced, thus increasing the release temperature of SO2 significantly and shortening the SO2 emission time in the burning process. To detail the quantitative change of SO2 emissions with added BFS, the sulfur-fixing ratios of the residues of the mixed samples are analyzed and described in Section 3.2. 3.2. Sulfur-Fixing Ratio. The total sulfur content was estimated using the Eschka method, and the sulfate sulfur was analyzed gravimetrically using BaSO4. The sulfur-fixing ratio is obtained from eq 1. Figure 5 shows the effect of adding BFS on the sulfur-fixing ratios, which are calculated by the sample residues obtained after

Figure 5. Sulfur-fixing ratios of coals blended with BFS at temperature of 800−1200 °C.

combustion at 800−1200 °C. The sulfur fixation ratio clearly increased significantly with an increasing BFS mass percentage, though the rate of increase showed a decreasing trend. When the BFS in the mixed samples increased from 2% to 10% by mass weight, the sulfur-fixing ratio increased from 57% to 89% at a temperature of 900 °C, meeting the industry requirement compared with traditional Ca-based sulfur-fixing agents. With an increasing temperature, the sulfur-fixing ratio was reduced when the BFS mass percentage was kept constant, indicating that the combustion temperature was not beneficial for inhibiting SO2 emission. Previous works have predicted that the weight ratio of Ca to S was an important factor for the sulfur-fixing ratio of samples and that the sulfur-fixing ratio did not improve when the weight ratio of Ca to S increased.28 However, because the BFS contains many calcium oxides and other alkaline metal oxides, which can easily react with alkaline metal oxides, a larger mass percentage of BFS is in favor of increasing the sulfur-fixing ratio; hence, the curves (Figure 5) indicated that the sulfur-fixing ratios of the samples increased with increasing BFS mass percentage. Another characteristic (Figure 5) of the sulfur-fixing ratio of samples with high BFS mass weights was that the ratio was lower than that of samples with small BFS mass weights at 1200 °C, but the ratio was higher than that at low reaction temperatures for samples with high BSF mass weights. The reason was that CaO or CaCO3 in BFS exhibited good reactivity and CaSO4 in the sulfur-fixing residue was difficult to decompose at low temperatures, whereas high temperatures were beneficial for decomposing CaSO4 in the sulfur-fixing residue, which reduced the sulfur-fixing ratio. In addition, a small part was transformed into a new calcium feldspar phase; therefore, the crystalline anorthite exhibited a strong inertia at high temperatures, so the sulfurfixing ratio of raw coal decreased at temperatures below 1200 °C, D

CaCO3 → CaO + CO2

(2)

CaO + SO2 → CaSO3

(3)

4CaSO3 → CaS + 3CaSO4

(4)

CaS + 2O2 → CaSO4

(5) DOI: 10.1021/acs.energyfuels.5b02986 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. XRD analyses of sulfur-fixing residues from mixed samples with (a) 0% BFS, (b) 2% BFS, (c) 6% BFS, (d) 8% BFS, and (e) 10% BFS at 900 °C.

Figure 7. SEM images of sulfur-fixing residues from mixed samples with (a) 0% BFS, (b) 2% BFS, (c) 6% BFS, (d) 8% BFS, and (e) 10% BFS at 900 °C.

E


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Figure 8. EDX spectra of sulfur-fixing residues from mixed samples with (a) 0% BFS, (b) 2% BFS, (c) 6% BFS, (d) 8% BFS, and (e) 10% BFS at 900 °C.

with particle sizes ranging from 0.3 to 1 μm. These particles exhibited a flocculent surface structure with micropores and were uniformly and densely covered. These characteristics are beneficial for the particles to enter inside the diffusion channels and surfaces, which can cause the reaction of CaO with SO2 to produce a sulfosalt. With the increase in the BFS mass percentage, the alkaline metal oxide and free SiO2 mass percentages increased. Increasing the alkaline oxide mass percentage was beneficial in producing CaSO4 from SO2; in addition, Ca3Al2Si2O8 appeared. Because of Al2O3 in the BFS, it can promote mullite and anorthite in coal combustion, as shown in the following reactions:31

The above reactions are noncatalytic gas−solid reactions and not pure surface chemical reactions, which include three stages: the reaction gas diffusion in the hole of the limestone, the diffusion of the reaction gas in the product layer, and the surface chemical reaction process. The pore structure of the solid reactant changed with the reaction, and a large number of micropores appeared on the surfaces of the calcined CaO particles. In the process of gas (SO2 and O2) reaction with the CaO particles, the reaction gas had to overcome the diffusion resistance of the outside particles to reach the CaO particle surface, and the reaction gas then diffused into the micropores of the particles. SO2 diffused through the product layer to reach the CaO surface, but this process did not conform with the initial CaO reaction. Subsequently, the SO2 reacted with CaO to form CaSO4. Because of the different forms of sulfur in coal, the form from its precipitation is different, and SO2 generally precipitates earlier than CaCO3.30 (b). SEM-EDS Analysis of the Sulfur-Fixing Residues. Figures 7 and Figure 8 show the SEM-EDS analysis of sulfur-fixing residues of the mixed samples at a reaction temperature of 900 °C. Figures 7 and Figure 8 show that when the added mass percentage of BFS was low, the surfaces of residues appeared thin fluffy blocks and loose structures containing more pores and without sinters. The morphology of the ash changed with the increase of BFS, in which the surface appeared as a melting slag with particle segregation and was flaky, smooth, slightly folded. The surface also had fewer pores, and some small particles were scattered on the surface. The energy spectrum analysis of the larger caking shows that when the mass percentage of BFS in the mixed coal reached 6%, sulfur-fixing compounds appeared, and combined with the XRD analysis, the major components of the particles were 3CaO·Al2O3, CaSO4, and other compounds. The rest of the small scattered particles were mainly SiO2 and Fe2O3,

SiO2 + 3Al 2O3 → SiO2 ·3Al 2O3

(6)

CaO + 2SiO2 + Al 2O3 → CaO·2SiO2 ·Al 2O3

(7)

The surfaces of CaSO4 particles were covered or enclosed by these products to inhibit CaSO4 decomposition in producing more SO2 emissions. The SEM images clearly show that many particles were bonded to each other in large chunks, in which the surfaces exhibited obvious swelling from sintering and melting at high temperatures. Except for the particles existing between some of the large holes, no pores existed in a single particle surface, which indicated that the slag samples had basically lost the sulfur salt gas−solid reaction properties for producing pores. The contrasting phenomenon illustrated that an increase of the BFS mass percentage could change the structures of the reaction products; however, BFS added to coal was very important to the sulfur-fixing reactions during the combustion process and inhibited SO2 emissions from sulfur-fixing residues. 3.3.2. Effect of Combustion Temperature. According to the industrial requirements for the heat values of coal and the previous results,32 the effect of 10% BFS on SO2 emissions in F


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Figure 9. XRD analysis of sulfur-fixing residues from (a) raw coal and (b) 10% BFS at 900 °C.



CaSO4 in the residues could reach 26%.33 However, in reality, the organic sulfur functional groups with poor thermal stabilities, such as thiols (RSH), sulfides (RSR′), and sulfur anthracene (RSSR′), began to decompose at 300−400 °C, and the decomposition ratio of CaCO3 was still very low; thus, a large amount of organic sulfur could escape from residues before reacting with CaO.. Figure 10 shows the XRD analysis of the sulfur-fixing residues of raw coal and mixed coal at 1000 °C. Figure 10b shows the mineral phase compositions of the residues, including γ-2Fe2O3, 2CaO·SiO2, 3CaO·3Al2O3·CaSO4, and 2CaO·Al2O3·SiO2. However, 3CaO·3Al2O3·CaSO4 and 2CaO·Al2O3·SiO2CaCO3 do not appear in Figure 10a. The 1000 °C reaction condition favored CaO, A12O3, and SiO2 to produce 2CaO·Al2O3·SiO2, and the residues of the raw coal still contained a medium amount of active calcium (CaCO3). Thus, the residues still had a further

mixed coal combustion was studied by comparison with raw coal combustion. (a). XRD Analysis of Sulfur-Fixing Residues. Figure 9 shows the XRD analysis of the sulfur-fixing residues of raw coal and mixed coal at 900 °C. Figure 9a shows the mineral phase compositions of the raw coal including γ-2Fe2O3, SiO2, C, CaCO3/, 3CaO·Al2O3, and other amorphous phases. However, the compositions of the residues (Figure 9b) were relatively simple, and the main components were α-2Fe2O3, SiO2, CaSO4, and other compounds. SiO2 in the raw coal residue easily reacted with silicates and other metal oxides to produce low melting point eutectics in the combustion process, such as SiO2−Al2O3− K2O, with a 750 °C melting point; SiO2−CaO-Na2O, with a 720 °C melting point; and SiO2−CaO−K2O, with a 710 °C melting point. If all of the sulfur of raw coal existed in the residues, the calcium conversion ratio could reach 70%, and the content of G


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Figure 13. SEM of sulfur-fixing residues of coal samples (10% BFS) at (a) 900 °C, (b) 1000 °C, (c) 1100 °C, and (d) 1200 °C.

CaSO4, so the sulfur-fixing compound of 3CaO·3Al2O3·CaSO4 generated in Figure 11b. The reaction can be attributed to a noncatalytic gas−solid reaction and not a single surface chemical reaction, which contains three surface chemical reactions: gas diffusion in the limestone pores, layer diffusion of the reaction gas, and a surface chemical reaction. The pore structure of the reactants changes with the reaction. Some micropores existed on the surfaces of the CaO particles, and O2 and SO2 overcame the diffusion resistance to react with CaO particles by entering the macrospores of the CaO particles. O2 and SO2 diffused into the product layer and the CaO surface to produce CaSO4. Because of the different forms of sulfur in coal, the release of sulfur was generally earlier than the CaCO3 calcination reaction.35 At the same time, CaO began to react with SiO2 to produce 2CaO·SiO2 and CaO, and Al2O3 reacted with SiO2 to produce 2CaO·Al2O3·SiO2. Meanwhile, CaO and A12O3 reacted with CaSO4 indirectly to produce

sulfur-fixing potential. However, because of the low residue melting point of coal, the softening temperature was approximately 1000 °C, so the liquid phase melting occurred at 1000 °C, and the contents of the melting glass in the amorphous phase increased with increasing combustion temperature. Additionally, CaSO4 was covered by the melting silicates to generate 3CaO·3Al2O3·CaSO4, which had a high melting point and density specific surface area, thus disfavoring both its decomposition and SO2 emissions. Figure 11 shows the XRD analysis of the sulfur-fixing residues of raw coal and mixed coal with 10% BFS at 1100 °C. Figure 11a shows the mineral phase compositions of the slag, including Fe9TiO15, SiO2, γ-2Fe2O3, CaSO4, and Ca3Al2Si2O8. The CaCO3 content reduced with increasing temperature, and when the temperature reached 1100 °C, the CaCO3 content reached its lowest point and could be generally described using the reactions from eqs 2−5.34 Finally, CaS could be oxidized to produce H


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Figure 14. EDX spectra of sulfur-fixing residues (10% BFS) at (a) 900 °C, (b) 1000 °C, (c) 1100 °C, and (d) 1200 °C.

3CaO·3A12O3·CaSO4 and a small amount of 2CaO·A12O3·SiO2, CaO, and SiO2. The active calcium base compositions, such as CaCO3 or CaO, no longer existed in the 1100 °C residue because CaSO4 decomposed easily at 1100 °C. Therefore, a large amount of Ca2+ in the raw coal ash still existed in the residues, but most of the calcium base compositions were melted into a glassy phase and completely lost their activities. In addition, a small part was translated into a new calcium feldspar phase. Therefore, the crystalline anorthite exhibited a strong inertia at high temperatures, so the sulfur-fixing ratio of raw coal decreased at temperatures below 1100 °C, and the molten slag no longer exhibited any further sulfur-fixing capacity at high temperatures. Figure 12 shows the XRD analysis of residues at 1200 °C, and Fe2O3, 2CaO·SiO2, 3CaO·3Al2O2·CaSO4, 4Ca·2SiO2·CaSO4, CaO·Al2O3·SiO2, Fe9TiO15 and others appear in Figure 12a. However, some solid sulfur components, such as CaSO4, 3CaO· 3Al2O2·CaSO4 and 4Ca·2SiO2·CaSO4, were not observed. Obviously, the silicate composition reacting with metal oxides produced low melting point eutectics, such as SiO2·2Al2O3· 2Fe2O3 (1073 °C), CaO·2FeO·2SiO2 (1093 °C), and 2FeO· SiO2 (1065 °C) when the coal residues melted at 1200 °C, which led to an increase of the amorphous phase existing as a molten glass form. Because CaSO4 was unstable at 1200 °C, no CaSO4 crystalline phase existed in the residues. Only a small amount of stable CaSO4 was observed, and the remaining Si and Al had been transformed into 4Ca·2SiO2·CaSO4 and 3CaO·3Al2O2· CaSO4 (C4A3S̅), which kept the CaSO4 stable at higher temperatures. In addition, active CaCO3 or CaO did not exist in the residue samples because a large amount of free Ca2+ had melted into the glass matter phase, which led to a complete loss of reactivity and eliminated the sulfur-fixing capacities of the slag samples.25 XRD analysis shows that 2CaO·A12O3·SiO2 (C2AS) and CaO completely disappeared in the raw coal residues. Although the already generated sulfur aluminum acid calcium (C4A3S̅) and silicate (4CaO·2SiO2·CaSO4) appeared gradually in mixed coal with10% BFS at high temperatures (Figure 12b), sulfur did not completely exist in the compounds, and a small amount of SiO2 particles still existed in the residues.

(b). SEM-EDS Analysis of Sulfur-Fixing Residues. Figure 13 shows the SEM analysis of the combustion residues of coal blended with 10% BFS by mass. The results show that the sulfur-fixing residue particles were mostly irregularly shaped and loose at 900 °C. The morphology of the residues changed from 1000 to 1100 °C, mainly due to the chemical reaction of the aluminum silicon compound at a higher temperature, which accelerated the growth of flat large and smooth particles. CaSO4 and 2CaO·A12O3·SiO2 (C2AS) were the main components of sulfur-fixing residue particles, although a small amount of CaO·2SiO2 (C2S), 3CaO·3Al2O3·CaSO4 (C4A3S̅), CaO·Al2O3·2SiO2 (CAS2), free calcium and silicon oxide appeared (Figure 13). When the combustion temperature increased to 1200 °C, the residue was composed of flat bulk solid melt particles dotted by small Fe2O3 particles. According to the EDX (Figure 14) and XRD (Figure 12) analyses, these planar bulk solid melt particles were mainly C2AS and C4A3S.̅ These sulfur-fixing residues appeared as flat, angular, flat type melt particles, dense solid melts, and large smooth particles. In addition, there were some cylindrical large holes formed in the melt condensation, and the surfaces of the whole particles and the interior did not appear porous. This type of dense structure can prevent the decomposition of C2AS and C4A3S̅ to a certain extent. CaSO4 was not only the main phase of sulfur-fixing residues at 900 °C, which also included free SiO2 and calcium compounds, and many particles with sizes ranging from 0.3 to 1 μm had flocculent structures (Figure 13a). The interiors of the particles contained many tiny uniform and dense pores, which allowed SO2 to enter the diffusion channels inside the particle. Furthermore, the large surface areas were in favor of the reaction between CaO and SO2. With the increase in the combustion temperature, the solid phase reaction accelerated, and the free SiO2 decreased, CA2 increased, and more C2AS and CAS2 was produced. The peak value of CaSO4 became weaker at 1200 °C, while the peaks of SiO2 and CA2 disappeared, and free A12O3 and C4A3S̅ appeared. The SEM image analysis clearly shows that many particles are bonded with each other to form large chunks, and the surfaces have obvious swollen sintered structure and high temperature melting traces. In addition to some of the large holes that existed between the particles, almost no pores existed in a I


Article

Energy & Fuels

Previous studies have shown that carbonate additives (K2CO3, Na2CO3, and SrCO3) and alkali metal carbonates have good sulfur-fixing functions and that the sulfur-fixing ratio increased from 20% to 60−70%. First, alkali carbonates in the form of calcium-based sorbents can improve the porosity of the sorbent. Second, because of the ion effect of alkali metal ions in the reaction process, the conversion ratio of calcium oxide can be greatly increased, which shows that alkali metal salts can improve sulfur fixation.39,40 For example, the addition of NaCO3 not only makes the distribution and size of the hole beneficial for sulfurfixing but also aids sulfur fixation by producing Na2SO3 and Na2SO4. The results indicate that the multiple components in BFS can cooperate with each other to a certain extent to produce stable sulfur-fixing products or produce certain materials to disfavor CaSO4 decomposition and SO2 emission. They can also greatly increase the decomposition temperature of the solid sulfur products, thus playing a role in inhibiting SO2 emission. Generally, the high-temperature thermal stability of sulfur-fixing compounds has two sides. One is that the CaO or CaCO3 coated by the compounds can possibly reduce the SO2 adsorption rate and decrease the sulfur-fixing ratio. The other is that CaSO4 coated by thermally stable sulfur compounds, such as CaFe3(SiO4)2OH, can delay and prevent CaSO4 decomposition, which releases SO2.

single particle, indicating that the residues had basically lost the gas−solid reaction pores for the sulfur salt. C4A3S̅, C2AS, and other minerals mixed into the dense clusters of CaSO4 were generated. This phenomenon showed that the structure of the reaction product changed with the increased temperature and that the BFS added to coal was beneficial for controlling SO2 emission and residue decomposition in the coal combustion process; hence, the sulfur-fixing ratio was improved at a high temperature. 3.3.3. Effect of the Multiple Components of BFS on SO2 Emission. Fe2O3, Al2O3, Fe−Si compounds, and Al−Si compounds have assistive roles in sulfur-fixing during the combustion process and can generate high temperature thermally stable compounds or low temperature eutectics to prevent or delay the decompositions of sulfate components at high temperatures and thereby reduce SO2 emissions. Fe2O3 added to CaO can accelerate the reaction of SO2(g) + CaO(S) → CaSO3(S) and favor the generation of CaSO4. Fe2O3 and SiO2 as catalysts can accelerate the reactions of SO2(g) + 1/2O2(g) → SO3 and SO2(g) + CaO(S) → CaSO3(S). The reactions accelerate the oxidation of CaSO3 to produce CaSO4, which can improve the reaction process and control SO2 emission.36 It can be seen that the synergistic effect of the multicomponents accelerates the process of the sulfur-fixing reaction and greatly inhibits SO2 emission. Thereby, the addition of Fe2O3 leads to a fluxing action in the combustion process of the residues and decreases the melting point of the slag in the silicates, which produce a series of eutectics.37 The XRD diagram shows that the surfaces of the CaO and CaSO4 particles are encapsulated by a glassylike material formed from a comolten material at low temperatures, which disfavors the release of SO2 by CaSO4 and increases the sulfur-fixing ratio. In addition, the Fe−Si component of the BFS may generate a new Ca−Fe−Si−O system to produce CaFe3(SiO4)2OH in the coal combustion process. CaSO4 coated by CaFe3(SiO4)2OH can delay and prevent the decomposition of CaSO4, which releases SO2. The thermodynamic analysis shows that there is a dynamic reaction equilibrium of the CaO−SiO2−SO3 system at a high temperature. On the one hand, the addition of SiO2 can decrease the decomposition temperature of CaSO4 by approximately 200 °C and increase the decomposition ratio of CaSO4 in the final reaction equilibrium, leading to an increased SO2 emission. On the other hand, CaSO4 can react with SiO2 and CaO to produce a high-temperature stable ternary compound, Ca5(SiO4)2SO4, which eliminates sulfur in the activities of CaSO4 at a high temperature, thus reducing SO2 emission. In addition, the interaction between Al2O3 and CaO can modify the lattice structure of CaO, leading to larger pore sizes and more pores, and can improve the sintering of CaO, which increases the ability of gas to diffuse into CaO particles, improves the utilization ratio of CaO and increases the efficiency of the sulfur reaction during combustion.38 According to the XRD analysis, the Al−Si system can be transformed into 2CaO·A12O3·SiO2 existing in the molten state in the coal combustion process at a high temperature, which can effectively encapsulate CaSO4 to prevent the decomposition of CaSO4 and block the escape path of SO2. The Mg−Al component can accelerate SO2 oxidation to produce SO3 at temperatures ranging from 400 to 750 °C. According to the SEM and XRD analyses of the combustion ash, the results show that Si and Fe in the sulfur ash can closely associate with CaSO4 to produce silicate compounds that attach onto the surface of the slag to delay or prevent thermal decomposition of CaSO4 at a high temperature.

4. CONCLUSIONS (1) With the increase of in mass percentage of BFS in coal combustion processing, the corresponding temperature of SO2 initial emission increased, but the peaks values of the SO2 emissions decreased significantly, and the time of SO2 emissions were significantly shortened. Comparing the values of the sulfurfixing ratio calculated by residues in the combustion processing, the sulfur-fixing ratio increased from 57% to 89% when the mass percentage of BFS increased from 2% to 10% at 900 °C. (2) The multicomponents of BFS, such as Fe2O3, Al2O3, SiO2, and Al−Si compounds, exhibited assistive sulfur-fixing roles in coal combustion process, which generated high temperature thermally stability sulfur compounds, such as 3CAO3·A12O3· CaSO4. In addition, CaSO4 was coated by the high temperature thermally stable sulfur compounds, such as CaFe3(SiO4)2OH, which can delay and prevent the decomposition of CaSO4 to release SO2 under a high temperature combustion process.

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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 This work was financially supported by National Natural Science Foundation of China (Grant No. 51404003), the Anhui Science & Technology Department Foundation of China (KJ2013Z017) and the Anhui Provincial Natural Science Foundation of China (1508085ME69).

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