Effect of Five Different Additives on the Sintering Behavior of Coal Ash

Article pubs.acs.org/EF

Effect of Five Different Additives on the Sintering Behavior of Coal Ash Rich in Sodium under an Oxy-fuel Combustion Atmosphere Hao Zhou,* Jianyang Wang, and Bin Zhou State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: This paper was addressed to investigate the influence of different additives on the sintering behavior of coal ash under an oxy-fuel combustion (O2/CO2 combustion) atmosphere. A lignite (Zhundong coal) rich in sodium has been used as the fuel material. The raw ash mixed with additive was heat-treated at 1350 °C under an oxy-firing (30 vol % O2/70 vol % CO2) atmosphere. In this study, five kinds of oxides have been applied as the additive: Al2O3, SiO2, kaolin, CaO, and Fe2O3, while the pure ash acts as a reference. At the same time, the shape shifting of the sintered ash samples during the sintering process was monitored by a charge-coupled device (CCD). The image processing system was used for analyzing the photos captured by the CCD to obtain the shrinkage of area and height of sintered samples. Moreover, the microstructure of the sintered samples was observed by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDX), which can analyze the chemical compositions. In addition, the distribution of mineral phases in the sintered samples was identified by X-ray diffraction (XRD). It turned out that SiO2 obviously promoted the melting process, but CaO suppressed the sintering and melting processes greatly. Furthermore, Al2O3, Fe2O3, and kaolin also affected the quantity of molten substances to some extent. Meanwhile, all of the samples are characterized by a layer structure with different colors. The results also show that the mineral phases and microstructure changed dramatically with different additives from the XRD and SEM analyses.

1. INTRODUCTION Coal is the main constitution of energy consumption in China. Because of the national energy policy, low-rank coal is normally used in coal-fired utility boilers and industrial boilers burning. However, these low-rank coals contain relatively higher alkali, sulfur, and ash. In consequence, it can result in fouling, slagging, erosion, abrasion, and other ash-related problems in the heattransfer surface in the boiler. Investigations1−3 have indicated that ash depositing and slagging are some of the most important problems among those matters. It can lead to the reduction of heat-transfer efficiency and the corrosion of boiler water wall. A previous study4 has proven that the slagging behavior includes three stages, and the sintering behavior is the main stage of slagging. Furthermore, the sintering characteristic is remarkably influenced by the mineral matters of coal ash, which affects the steady run and high-efficient operation of the boiler as well.5 Thus, as the properties of coal ash change with the mineral matters by adding additives, the sintering and melting mechanisms vary as well.6 Studies7−9 have shown that fuel additive technology is an effective and economic method to solve the ash-related problems. The additives chosen for sintering preventatives are various, and different oxides are usually used, such as acid oxide, basic oxide, amphoteric oxide, compound oxide, etc.10,11 Additionally, the sintering behavior of coal ash is also related to the burning atmosphere.12 As is well-known, the oxy-fuel combustion technology has been recognized as one of the most promising technologies for reducing CO2 emission in a coal-fired boiler,13 because oxy-fuel combustion (O2/CO2 combustion) can contribute to an easier recovery of CO214 and a lower cost of CO2 capturing than conventional air-firing combustion.15,16 However, this brings about a significantly high concentration of CO2 in flue gas under the oxy-fuel conditions.17−21 Moreover, previous studies have © 2015 American Chemical Society

shown that oxy-fuel combustion could give rise to substantial reduction of NOx and Hg emissions and unburned carbon in fly ash.22−24 Nevertheless, oxy-fuel combustion may cause high concentrations of SO2 and alkali in flue gas and change the sintering behavior of coal ash.25−27 In recent years, many studies have been carried out that cover experimental and engineering issues on the application of the oxy-firing technology, for instance, combustion characteristics,28 emissions of pollutant,23 heat-transfer characteristics,29 etc. Only a few studies have been conducted on ash-related and additive-associated topics in an oxy-fuel combustion atmosphere. Zheng et al.30 investigated the fly ash deposition under an oxyfiring atmosphere in a bench-scale fluidized-bed combustor. Wang et al.31 investigated the effect of additives on the sintering behaviors of biomass ash. Additionally, Jing et al.10,12 used a highpressure thermogravimetric analyzer apparatus to research the influence of chemical composition on the sintering behavior of coal ash in combustion and gasification atmospheres. Moreover, fuel additive technology has been studied in coal ash deposition and slagging,9,32−34 burning high-alkali coal safely,27 preventing ash fouling and sintering,11,35 NOx and PM2.5 reduction,7,8 co-combustion,36 combustion efficiency,7 etc. However, these investigations cannot monitor the shape shifting of ash samples online and simulate the ash sintering on the surface of the water wall tube in a pulverized coal boiler. Meanwhile, to date, there is still a lack of experimental research with regard to the influence of additives on ash sintering and melting behaviors in oxy-firing conditions. Consequently, fundamental research is needed to be Received: March 9, 2015 Revised: July 31, 2015 Published: August 3, 2015 5519

DOI: 10.1021/acs.energyfuels.5b00500 Energy Fuels 2015, 29, 5519−5533

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Energy & Fuels conducted to understand the changes of ash sintering and melting behaviors with different additives under an oxy-fuel combustion atmosphere. The purpose of this study is to evaluate the effect of different additives on sintering behaviors of ash particles on an oil-cooled deposition probe in an oxy-fuel combustion atmosphere. Five additives (acid oxide, SiO2; basic oxide, CaO and Fe2O3; amphoteric oxide, Al2O3; and compound oxide, kaolin) were used for investigation to screen out sintering preventatives. These additives were different types and usually used in studies and applications.8,11,32,37 Moreover, the charge-coupled device (CCD) was applied to observe the shape shifting of the ash samples online for different additives. Meanwhile, the method used by this paper is a limited applicability as far as sintering in pulverized coal boilers.

2. EXPERIMENTAL SECTION 2.1. Preparation and Analysis of Ash Samples. A representative lignite coal [Zhundong (ZD) coal] rich in sodium was selected as the experimental material for this study. Before the experiments, the coal was ground to less than 70 μm. Because of the high level of sodium and chlorine in the coal ash, the coal samples were ashed at 550 °C for 10 h in a muffle furnace. Table 1 presents the coal ash composition.

Table 1. Proximate Analysis, Ultimate Analysis, Gross Heating Value, Ash Melting Temperature, and Chemical Composition of ZD Coal ZD coal moisture (wt %, ad) proximate analysis (wt %, db)

ultimate analysis (wt %, db)

HV (MJ/kg) ash melting temperature (°C)

ash composition (as oxides, wt %, dry basis)

volatile matter ash fixed carbon carbon hydrogen nitrogen sulfur oxygen HHV IT ST HT FT Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO TiO2 MnO Fe2O3

15.6 32.79 12.3 52.91 64.07 3.58 0.65 0.18 19.22 24.01 1213 1218 1221 1231 7.60 2.90 14.50 27.43 0.03 3.82 10.72 0.33 27.461 0.87 0.07 4.26

Figure 1. (a) XRD patterns of coal ash samples prepared at 550 °C and (b) cubical ash block before sintering (15 × 15 × 25 mm). a mortar for homogeneous mixing. To evaluate the change of sample dimension before and after sintering, the ash samples (with and without additives) were pressed into cubical ash blocks (length, 25 mm; width, 15 mm; and height, 15 mm), as depicted in Figure 1b. The preparation process for ash blocks included three steps: (1) a total of 8 g of ash samples were weighed; (2) ash samples were placed in the same mold; and (3) the mold was pressed in the tablet machine for compressing with the same pressure. 2.2. Experimental Setup and Procedure. Figure 2a presents the schematic diagram of the sintering furnace system, showing that it mainly consists of a gas system, an image sampling system, a hightemperature tube furnace, a sintering probe system, and a probe-driving and -reversing system. The image sampling system is composed of a CCD camera, optical lenses, protective tube, and camera shield. This system was used for monitoring the shape shifting of the ash sample online during the sintering process. The ash sample was placed on the sintering probe made of stainless steel. Inside the probe, the conduction oil was keeping at the temperature of 230 °C (see Figure 2b) to simulate the temperature of the water wall tube. As a result, the temperature distribution inside the ash sample increased from bottom to top. In this study, two ratios (10 and 20 wt % of total ash mixed with additive) were chosen for each additive and pure ash was selected as a reference. As for the oxy-fuel combustion atmosphere, the flow rates of O2 and CO2 controlled by the mass flow meters were 1.5 and 3.5 min−1, respectively. When the furnace temperature achieved 1350 °C from room temperature at an 8 °C/min heating rate, the cubical ash block was placed on the sintering probe and then inserted into the center of the horizontal tube furnace by the advancing and retracting device. An hour later, the sintered ash sample was withdrawn from the furnace by the advancing and retracting device. Then, the liquid nitrogen was used to quench the sintered ash sample. After that, the sample was cut vertically by the cutting machine in the middle of the longest side (25 mm). One half was embedded into epoxy resin and then polished to obtain a

Meanwhile, the major crystalline phases in the coal ash are shown in Figure 1a. It can be seen that the ZD coal ash contains a mass of halite, which is corresponding to the relatively high sodium concentration (7.60 wt %) in ash given in Table 1. Additionally, the coal ash is also rich in anhydrite, calcite, and quartz. Less hematite is found in the X-ray diffraction (XRD) pattern. Coal ash with additives (Al2O3, SiO2, kaolin, CaO, and Fe2O3) was made by mixing the pure oxide powders (alumina−Al2O3, quartz−SiO2, kaolin−Al2Si2O7•, lime−CaO, and hematite−Fe2O3) into the ash samples and then grinding them into 5520


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Figure 2. (a) Schematic diagram of the sintering furnace system and (b) close-up view of the sintering probe. smooth cross-section. Subsequently, the cross-section was coated with platinum and analyzed by scanning electron microscopy (SEM) to obtain the microstructure images. In addition, the distribution of elements was determined by energy-dispersive X-ray spectrometry (EDX). Meanwhile, the other half was divided into three parts based on three layers. Each part was ground into powder for XRD analysis. 2.3. Image Processing System. During the sintering process, the shape shifting of the ash sample was monitored by the CCD. An image processing system based on MATLAB was applied to analyze the photos captured by the CCD to obtain the shrinkage rates of the area and height for the ash sample. In the first place, the edge detection was conducted by the image processing system to acquire the edge image of the ash sample, as presented in Figure 3b. Consequently, the number of pixels for the height in the edge image was easy to calculate. Then, the shrinkage rate of the height of the ash sample can be calculated by the following equation:

ηH = 1 − H /H0

Figure 3. Description of the image processing system. oxy-fuel combustion. The calculations were carried out for a temperature range between 1000 and 1500 °C. Furthermore, the solution species included ash constituents and the selected additive (10 and 20 wt %), while pure ash was selected as a reference. The molten slag fraction in the sample was calculated with 5 °C increments. Additionally, the coal ash compositions were further applied in the isothermal phase diagrams of the ternary system at 1350 °C by FactSage to evaluate the solid−liquid phase equilibrium of the sample. The main chemical compositions of the coal ash were normalized and plotted on the ternary equilibrium phase diagrams.

(1)

where H represents the height of the ash sample at any time and H0 represents the height of the ash sample at initial moments. In addition, the image of the ash sample also underwent binarization by program processing, as illustrated in Figure 3c. The number of pixels for the area of the observed sample could be obtained. Thus, the shrinkage rate of the area of the ash sample can be formulated

ηA = 1 − A /A 0

3. RESULTS AND DISCUSSION 3.1. Sintering Behavior of Pure Coal Ash. Figure 4a shows the sintered sample of pure coal ash, from which it can be observed that the top of the sample becomes significantly different from the bottom. This is because the molten ash exists at the topside because of the high temperature. The corresponding cross-section photo is depicted in Figure 5a. Obviously, it can be seen that the ash sample is characterized by a layer structure with different colors for the reason that the temperature increases from the inner side to the outer side. Because the sintered sample comprises of three layers, layer 1 (outer layer), layer 2

(2)

where A represents the area of the ash sample at any time and A0 represents the area of the ash sample at initial moments during the sintering process. 2.4. Thermodynamic Calculation. To have a better understanding of coal ash melting characteristics with additives, chemical equilibrium calculations were conducted using the FactSage (version 5.2) equilibrium module software. The calculations included the ash constituents listed in Table 1 and the oxidant, being 30 vol % O2/70 vol % CO2 for 5521


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condensates result from the vaporization of alkali species in the raw ash, which is rich in sodium and chlorine. The change of area and height with time is shown in Figure 7. Apparently, the area and height reduced with time. The ash sample shrank dramatically in the first 25 min and slightly in the remaining 35 min. However, an abrupt change appeared from 9 to 10 min because of the fracture of bubble inside the sample, as illustrated in Figure 8. This result is also consistent with the sintering image shown in Figure 4a. Furthermore, the explanation can also account for the sudden change from 23 to 24 min. In addition, the values of ηH and ηA are 0.14 and 0.20, respectively, as listed in Table 2. Panels a−e of Figure 9 show the slag−liquid formation with the temperature for different additives in an oxy-fuel combustion atmosphere. It was apparent that the pure ash started to fuse at 1140 °C and was totally molten above 1280 °C. Besides, according to the phase diagram depicted in Figure 10a, the pure ash is in the liquid state and only one crystalline phase, gehlenite (Ca2Al2SiO7), can be found. The SEM micrographs of the sintered ash sample are depicted in Figure 11. It can be seen that the structure of each layer varies distinctly as a result of the decrease of the sintering degree from layer 1 to layer 3. Interestingly, layer 1 presents a porous structure, and layer 2 has a compact structure because of a little sintering involved; however, layer 3 appears to be loose and tough. Additionally, it shows some similarities with the slagging behavior.38 The chemical composition of the sintered pure ash sample analyzed by EDX is given in Figure 12a. It can be found that the proportion of main elements differs obviously in each layer. The main elements contained in the sintered ash sample are silicon, aluminum, calcium, sodium, and chlorine, except for oxygen. This is in accordance with the components of raw ash listed in Table 1. The sodium and chlorine constituents decrease from layer 3 to layer 1 because of the vaporization of NaCl at a high temperature. Nevertheless, the silicon, aluminum, and calcium contents have the opposite trend. This is on account of the more depletion of alkali species with the rise of the temperature. Figure 13 shows the XRD patterns of the sintered pure ash sample. It can be noted that layers 3 and 2 are rich in NaCl, but layer 1 contains none. This is consistent with the result of EDX mentioned above. In comparison to the original sample (the sample before experiment), hematite is not identified in layer 3, which is instead identified by the formation of andradite (Ca3Fe2Si3O12). The involved reaction can be given as follows:

Figure 4. Photos of the sintered ash samples with different additives (wt %).

Figure 5. Cross-sections of the sintered ash samples with different additives.

3CaO + Fe2O3 + 3SiO2 → Ca3Fe2SiO12 (andradite)

(3)

Moreover, diopside (CaMgSi2O6) is observed in layer 2 but disappears in layer 1. This is attributed to the certain temperature of formation of diopside. The transformation can be described as CaO + MgO + 2SiO2 → CaMgSi2O6 (diopside)

(4)

Meanwhile, layers 2 and 1 are rich in gehlenite, especially for layer 1. This can be explained by the high temperature that the formation of gehlenite needs. The chemical equation can be expressed as

Figure 6. (a) Condensate on the sintering probe and (b) XRD analysis results of the condensate.

2CaO + Al 2O3 + SiO2 → Ca 2Al 2SiO7 (gehlenite)

(middle layer), and layer 3 (inner layer) are named from top to bottom. Besides, some white condensates deposited on the probe (see Figure 6a) after the experiments were analyzed by XRD. The result (Figure 6b) shows that there exists only one crystalline phase, halite (NaCl), in the condensate. These NaCl

(5)

However, anhydrite (CaSiO4) and andradite do not exist in layer 1, where only gehlenite and the amorphous phase are found. It can be concluded that they are low-temperature minerals, which can also account for the disappearance of the peaks of calcite in layers 2 and 1. 5522


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Figure 7. Changes of area and height of all of the samples with time in an oxy-fuel combustion atmosphere.

pores can be found in layer 1 of the samples with Al2O3 compared to the pure ash sample (see Figure 5a). It can be seen that the sample with 10% Al2O3 contains pore structures of a larger size than those of the 20% Al2O3 sample. Figure 7 presents the shrinkage of the sintered sample with 10% Al2O3. It showed that the height had a rise in the first 5 min and then decreased in the next 10 min but interestingly increased slowly from 15 to 30 min. After that, the height remained unchanged and so did the area. This is attributed to the pore structures formed in layer 1. However, for the sample with 20% Al2O3 shown in Figure 7, a steep decline of the area and height appeared in the first 5 min and was kept stable afterward. Meanwhile, it can be noted that the shrinkage rates of area and height (see Table 2) for the sintered samples with Al2O3 are smaller than those of the pure ash sample. From Figure 9a, it can be noticed that, at the same temperature, the slag−liquid reduces obviously by adding Al2O3 and 20% Al2O3 has a greater effect than 10% Al2O3. This is consistent with the photos of cross-sections in Figure 5b. The samples with

Figure 8. Photos of the deposit sample captured by the CCD from 9 to 10 min.

3.2. Additive Effects on the Sintering Behavior of Coal Ash. 3.2.1. Effect of Alumina (Al2O3) on the Sintering Behavior. The photos of sintered samples with Al2O3 are given in panels b and c of Figure 4, and Figure 5b shows the corresponding cross-sections. It can be noticed that less glass exists in the sintered samples with Al2O3. Furthermore, more Table 2. Shrinkage Rates of the Sintered Samples samples (wt %)

pure ash

10% Al2O3

20% Al2O3

10% SiO2

20% SiO2

10% kaolin

20% kaolin

10% CaO

20% CaO

10% Fe2O3

20% Fe2O3

ηH ηA

0.14 0.21

0.05 0.11

0.12 0.13

0.35

0.45

0.17 0.15

0.10 0.14

0.08 0.11

0.02 0.01

0.19 0.22

0.01 0.21

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Figure 9. Calculated results of the molten slag fraction in ash by adding different additives.

Figure 10. Phase diagrams of a ternary system at 1350 °C.

Figure 11. SEM micrographs of the cross-sections of the sintered pure ash sample.

Al2O3 are totally molten above 1350 °C, which is higher than the fully melted temperature of the pure ash sample. Moreover, from the phase diagram illustrated in Figure 10a, it can be found that, by adding Al2O3, anorthite (CaAl2Si2O8) emerges and liquid and gehlenite still exist.

The SEM images of the sintered samples with Al2O3 (see Figure 14) indicate that layer 1 is characterized by pore structures as well. Unlike the spherical pores in the sintered pure ash sample, these pores are in irregular shape. Besides, it can be found that layers 2 and 1 of both samples show basically the same 5524


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Figure 12. Chemical compositions of all of the sintered ash samples obtained by EDX analysis (wt %).

structure as that of the sintered pure ash sample. In layer 3 of both samples, quantities of crystal particles can be found and prove to be NaCl, which has a high degree of crystallinity. Furthermore, the EDX results of the sintered samples with Al2O3 shown in panels b and c of Figure 12 reveal that the change of sodium, aluminum, silicon, and chlorine appear to have the same trend as that of the sintered pure ash sample. However, calcium in layer 2 is more than that in layer 1 for both samples. Nevertheless, chlorine appears in layer 1 of the two samples,

which differs from the sintered pure ash sample. Also, the two samples are rich in aluminum in each layer because of the addition of Al2O3. Panels a and b of Figure 15 show the XRD patterns of the two samples, and variations of main minerals in different layers are presented in Figure 15c. It can be seen that the intensity of major minerals, such as anhydrite, andradite, calcite, gehlenite, halite, and quartz, shows a similar variation trend to that of the pure ash sample. From original sample to layer 1, there is an obvious decline of the intensity of quartz, suggesting that 5525


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3.2.2. Effect of Quartz (SiO2) on the Sintering Behavior. The sintered samples with SiO2 differ markedly from the sintered pure ash sample, as illustrated in panels d and e of Figure 4. It can be noted that the top of samples has melted apparently, meaning that SiO2 strongly promotes the melting process.10 As a result, layer 1 is seen to be thinner, as depicted in Figure 5c. However, because of the slag liquid, only the shrinkage of the height was analyzed in Figure 7. In comparison to the sintered pure ash, the height of the two samples with SiO2 changed sharply and some abrupt change points appeared in consequence. The corresponding images of the points are given in panels a−g of Figure 16. For the sample with 10% SiO2, the height had a rise in the beginning 7 min and shrank dramatically in the next 6 min. This can be confirmed by the fact that the molten liquid flowed down from time A (see Figure 16a) to time B (see Figure 16b). After that, the height increased slowly for 10 min, dropped sharply in 1 min, and then remained unchanged until the end. This is attributed to the breakup of the bubble from time C to time D, as shown in panels c and d of Figure 16. On the other hand, the sample with 20% SiO2 shrank quickly in the first 20 min and slowly in the next 20 min and then was in steady state in the remaining time. However, an abrupt change appeared from time E to time G within the first 20 min, which can also be explained by the bubble breakup, as illustrated in panels e−g of Figure 16. The values for ηH of the two samples are extremely larger than that of the sintered pure ash sample, as listed in Table 2. Figure 9b presents the calculated results of the molten slag fraction in ash for SiO2, showing that SiO2 significantly lowers the initial melting temperature for the reason that SiO2 may play the role of flux in the reactions. From Figure 10a, it can be seen that the samples with SiO2 are in the liquid region of the phase diagram. The microstructures of different layers obtained by SEM are given in panels a and b of Figure 17 for the samples with SiO2. Apparently, no pore structures can be found in layer 1 of both samples with SiO2. Interestingly, it can be observed that some crystalline substance appears in layer 1 of the sample with 10% SiO2. By elemental analysis of EDX, this crystal phase proves to be NaCl. On the other hand, only some small particles of NaCl can be found in layer 1 of the sample with 20% SiO2. Moreover, it can be noted that layers 2 and 3 of the sintered sample with 10% SiO2 are loose, while the sintered sample with 20% SiO2 appears to be compact in layers 2 and 3. From EDX analysis of chemical compositions in panels d and e of Figure 12, it can be observed that the variation trend of the main elements in the two samples is almost the same as that of the sintered pure ash sample. Nevertheless, there is more chlorine in layer 2 of the sample with 10% SiO2 in comparison to the samples with 20% SiO2 and no SiO2. This agrees with the loose structure caused by NaCl, as shown in Figure 17a. In addition, a little chlorine is contained in layer 1 of both samples. This is consistent with the crystal phases found in the SEM images (see panels a and b of Figure 17). Panels a and b of Figure 18 present the XRD patterns of the minerals of the sintered ash samples with SiO2, showing that the peaks of quartz are strengthened in each layer and the original sample as a result of the addition of SiO2. It can be seen from Figure 18c that the intensity of quartz reduces from the original sample to layer 1, suggesting that SiO2 transformed into aluminosilicate. Meanwhile, the intensity of gehlenite reduces sharply in layer 1 for the two samples in contrast with the sintered pure ash sample. Additionally, in accordance with the SEM and EDX results, halite can also be found in layer 1 of the two samples by XRD analysis. Furthermore, in the XRD patterns for both

Figure 13. XRD patterns of the sintered pure ash sample.

Figure 14. SEM micrographs of the cross-sections of sintered ash samples with Al2O3 (wt %).

quartz reacted with Al2O3 and CaO to produce gehlenite as the temperature rose. It can be confirmed by the increase of gehlenite from layer 3 to layer 1. However, the intensity of halite in layer 3 is the strongest, indicating that NaCl crystallized in big crystal particles of good crystallinity. This agrees with SEM images of layer 3 shown in panels a and b of Figure 14. In addition, the peaks of hauyne and sodalite appear in layers 2 and 1, respectively, for both samples. The reactions involved can be expressed as follows: 3Na 2O + 2CaSO4 + 3Al 2O3 + 6SiO2 → Na6Ca 2Al 6Si6O24 [SO4 ]2 (hauyne)

(6)

2NaCl + 3Na 2O + 3Al 2O3 + 6SiO2 → Na8Al 6Si6O24 Cl 2 (sodalite)

(7)

At the same time, chlorine in sodalite found in layer 1 is in accordance with the EDX results in panels b and c of Figure 12. Moreover, in layer 1, albite (NaAlSi3O8) can be observed for the 10% Al2O3 sample, while anorthite (CaAl2Si2O8) and grossularite (Ca3Al2Si3O12) can be noted for the 20% Al2O3 sample. This indicates that more mineral phases of aluminosilicate can be formed at a high temperature with the increase of Al2O3. The transformations of aluminosilicate are shown below. Na 2O + Al 2O3 + 6SiO2 → 2NaAlSi3O8 (albite)

(8)

CaO + Al 2O3 + 2SiO2 → CaAl 2Si 2O8 (anorthite)

(9)

3CaO + Al 2O3 + 3SiO2 → Ca3Al 2Si3O12 (grossularite)

(10) 5526


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Figure 15. XRD patterns and changes to the main mineral phases of the sintered ash samples with Al2O3 (wt %).

rates of the two samples are given in Table 2. The values of ηH are 0.17 and 0.10 for the samples with 10 and 20% kaolin, respectively. However, the values of ηA are almost the same and smaller than that of the sintered pure ash sample. According to the calculations in Figure 9c, the melting behavior of the sample with 10% kaolin is roughly similar to that of the pure ash sample. However, beyond that, it can be found that the fully melted temperature of the sample with 20% kaolin is much higher than that of the samples with 10% kaolin and no kaolin. However, all three samples start to melt at the same temperature. The molten slag fraction decreases with the addition of kaolin at the same temperature. Figure 10a presents that both samples with kaolin are in the region of liquid and anorthite. Images obtained by SEM (see panels a and b of Figure 19) show that the microstructures of the samples with kaolin are similar to those of the sintered pure ash sample. Layer 1 is identified as porous; layer 2 is identified as compact; and layer 3 is identified as loose. The difference is that pores in layer 1 of the sample with 20% kaolin are in irregular shape. From panels f and g of Figure 12, it can be noted that the variations of chemical compositions appear to have the same trend as that of the sintered pure ash sample. Beyond that, a little chlorine can be found in layer 1 of both samples with kaolin. From panels a−c of Figure 20,

samples, the peaks of anorthite and hauyne appear in layers 1 and 2, respectively, and the peaks of diopside appear in both layers. From above, it can be concluded that anorthite, hauyne, and diopside are prone to be formed with the increase of SiO2 as well as Al2O3. Besides, wollastonite (CaSiO3) can be found in layer 1 of the sample with 20% SiO2, and the formation can be expressed as CaO + SiO2 → CaSiO3 (wollastonite)

(11)

The variations of the other minerals appear to have a similar tendency within the two samples with SiO2 and the sintered pure ash sample. 3.2.3. Effect of Kaolin (Al2Si2O7) on the Sintering Behavior. The sintered ash samples with kaolin (see panels f and g of Figure 4) are more like samples with Al2O3 rather than with SiO2, because both aluminum and silicon are contained in kaolin. Figure 5d shows the corresponding photos of cross-sections, from which it can be seen that, with the increase of kaolin, the molten fraction decreases in layer 1. Figure 7 shows the variations of area and height of the kaolin samples versus time. It can be found that lines of the kaolin samples are not smooth, indicating that bubbles formed and broke in the first 40 min and then were kept stable until the end. On the contrary, the sample with 20% kaolin shrank moderately from the beginning to the end and remained steady in the last 20 min. Meanwhile, the shrinkage 5527


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3.2.4. Effect of Lime (CaO) on the Sintering Behavior. From panels h and i of Figure 4, it can be seen that the physical characteristics of the two samples with CaO appear to be different with the sintered pure ash sample. Apparently, the sample with 10% CaO only sintered but was not molten, and the sample with 20% CaO that barely sintered was too loose to crack. Also, the cross-sections depicted in Figure 5e reveal the distinctions. It can be found that layer 1 of the sample with 20% CaO is extremely thin in contrast with other samples. Figure 7 presents the shrinkage of the sintered samples with CaO, showing that the sample with 10% CaO shrank slowly in 40 min and remained steady in the time left. However, the sample with 20% CaO scarcely changed during the experiment. Furthermore, the values of ηH and ηA for the samples with CaO are given in Table 2, implying that the shrinkage rates of height and area decline sharply in contrast to the sintered pure ash sample. Figure 9d shows the molten slag fraction in ash with CaO by the FactSage calculation. It can be observed that not only the molten slag fraction reduces dramatically with the addition of CaO at the same temperature but also the initial melting and fully melted temperatures rise accordingly. This is consistent with the experimental results. From the phase diagram in Figure 10a, it can be noted that the sample with 10% CaO is in the region of liquid and gehlenite at 1350 °C, while liquid, gehlenite, and larnite are found in the region where the sample with 20% CaO lies. The SEM micrographs of cross-sections for the two samples are given in panels a and b of Figure 21, showing that the biggest difference is the pore structures in layer 1 for both samples with CaO. The sample with 10% CaO has similar spherical pores in common with the sintered pure ash sample, while the sample with 20% CaO contains irregular pores in layer 1. In addition, layers 2 and 3 of the sample with 10% CaO can be identified as compact and loose, respectively. However, the sample with 20% CaO has a compact structure in layers 2 and 3. Panels h and i of Figure 12 present the major elements in the two samples with CaO, suggesting that sodium and chlorine decrease and others increase as a result of NaCl vaporization from layer 3 to layer 1. This is the same as the sintered pure ash sample. However, much calcium can be observed in the two samples because of CaO added. Panels a and b of Figure 22 show the XRD patterns of the sintered samples with CaO. It can be seen that gehlenite is the only mineral component in layer 1 of the sample with 10% CaO. As for the sample with 20% CaO, gehlenite, larnite, and bredigite (Ca7MgSi4O16) can be found in layer 1. The related reactions can be expressed as follows:

Figure 16. Photos of deposit samples with SiO2 by the CCD.

Figure 17. SEM micrographs of the cross-sections of sintered ash samples with SiO2 (wt %).

it can be found that, with the increase of kaolin, the peaks of anorthite are strengthened and the peaks of gehlenite are weakened.27 In addition, gehlenite could be barely found in layer 1 of the 20% kaolin sample. This can also be confirmed by the variations of intensity in Figure 20c. Therefore, taken together, it can be inferred that anorthite is prone to be formulated when there is much Al2O3 and SiO2 in the sample. Besides, kyanite (Al2SiO5) can be found in layer 2 of the sample with 10% kaolin. Sodalite as the dominant mineral has the highest peak in the XRD pattern of layer 2 for the 20% kaolin sample. The transformation of kyanite can be described as Al 2Si 2O7 → SiO2 + Al 2SiO5 (kyanite)

2CaO + SiO2 → Ca 2SiO4 (larnite)

(13)

7CaO + MgO + 4SiO2 → Ca 7MgSi4O16 (bredigite)

(14)

Larnite can also be found in layer 2 of the sample with 20% CaO. The experimental results are consistent with the phase diagram calculated by FactSage too. Therefore, it can be concluded that CaO has a great effect on the formulations of gehlenite, larnite, and bredigite. Larnite and bredigite are prone to be formed with more CaO. Figure 22c presents the variations of major mineral phases in different layers. It can be noticed that the intensity of gehlenite is the strongest in layer 1 of the sample with 10% CaO. Moreover, andradite cannot be found in layer 2 of the sample with 20% CaO, but the intensity of andradite is the strongest for the sample with 10% CaO, indicating that the formulation of andradite had a close relationship with the amount of CaO contained. Meanwhile, the intensity of calcite in layer 1 increases

(12)

Moreover, davyne, a complex mineral component expressed as (Na,K)5Ca2(AlSiO4)6(SO4)0.5Cl2, can be found in layer 2 of the sample with 20% kaolin as well. In addition, the other chemical compositions vary similarly to those of the sintered pure ash sample, as depicted in Figure 20c. 5528


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Figure 18. XRD patterns and changes to the main mineral phases of the sintered ash samples with SiO2 (wt %).

3.2.5. Effect of Hematite (Fe2O3) on the Sintering Behavior. In panels j and k of Figure 4, photos of the sintered samples with Fe2O3 are presented. It can be obviously seen that the samples with Fe2O3 look darker than the sintered pure ash sample as a result of Fe2O3 added. Cross-sections of the two samples shown in Figure 5f reveal that less glass was formed in layer 1 of the samples with Fe2O3. Figure 7 presents the shrinkage of the samples with Fe2O3, suggesting that both height and area of the sample with 10% Fe2O3 reduce moderately. Moreover, the area of the sample with 20% Fe2O3 varies the same as that with 10% Fe2O3, but its height remains almost unchanged. In addition, both samples changed significantly in the beginning 15 min. Table 2 indicates that the shrinkage rates ofthe area for the two samples are 0.22 and 0.21, which are basically the same as that of the sintered pure ash sample. Values of ηH are 0.19 and 0.01, respectively. Calculated results of the molten slag fraction in ash with Fe2O3 are shown in Figure 9e. It can be see that the melting behavior of the samples with Fe2O3 is similar to that of the pure ash sample, except for some tiny differences, meaning that the effect of Fe2O3 is not obvious under an oxy-fuel combustion atmosphere.

Figure 19. SEM micrographs of the cross-sections of sintered ash samples with kaolin (wt %).

with the addition of CaO, and this is because CaO reacted with CO2 to produce CaCO3.37 Other minerals, such as anhydrite, halite, and quartz, of the two samples with CaO vary similarly to those of the sintered pure ash sample. 5529


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Figure 20. XRD patterns and changes to the main mineral phases of the sintered ash samples with kaolin (wt %).

two samples by EDX analysis, interpreting an obvious increase of iron because of the Fe2O3 additive. In layer 2, sodium and chlorine of the sample with 10% Fe2O3 are higher than those of the sample with 20% Fe2O3. Meanwhile, calcium is the most in layer 2 among three layers of the sample with 20% Fe2O3. Other elements, such as aluminum, silicon, and iron, increase from layer 3 to layer 1 for both samples, while sodium and chlorine have the opposite tendency. Consequently, the variations are the same as those of the sintered pure ash sample. Panels a and b of Figure 24 show the XRD patterns of the sintered samples with Fe2O3. It can be seen that, in layers 1 and 2 for both samples, a complex mineral component, diopside subsilic ferrian, which can be described as Ca0.991(Mg0.641Fe0.342)(Si1.6Fe0.417)O6, appears. It seems that the replacement of some magnesium and silicon atoms by iron atoms can account for the formation. This agrees with the complex effect of the iron ion on the mineral compositions under different atmospheres.39 The corresponding variations of intensity are depicted in Figure 24c. It can be observed that the peaks of andradite in layers 3 and 2 are strengthened with the increase of Fe2O3. This can be easily explained by reaction 3 and more reactant added. In addition, gehlenite cannot be found in layer 2 of the sample with 20% Fe2O3. The intensity values of gehlenite in layer 1 for both samples are smaller than that of the sintered pure ash sample, which means that Fe2O3 suppressed the formation of gehlenite. Meanwhile, hauyne exists in layer 2 of both samples. Anhydrite, calcite, halite, and quartz in the two

Figure 21. SEM micrographs of the cross-sections of sintered ash samples with CaO (wt %).

Figure 10b shows the phase diagram of SiO2−CaO−Fe2O3, and it can be found that the two samples are in the region of liquid and wollastonite, indicating that Fe2O3 is in the liquid state. From panels a and b of Figure 23, it can be noticed that pore structures can also be found inside the matrix of layer 1 of both samples. Additionally, in layer 2 of the two samples, a more compact structure can be observed than that of the sintered pure ash sample. Layer 3 of both samples can be identified as rough and loose, which resembles that of the sintered pure ash sample. Panels j and k of Figure 12 present the main compositions of the 5530


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Figure 22. XRD patterns and changes to the main mineral phases of the sintered ash samples with CaO (wt %).

structure with different colors. Outer layers of samples with different additives differ markedly because of the sintering and melting degree of the sample surface. The photos of the sintered samples and the corresponding cross-sections also show that, with the increase of Al2O3, Fe2O3, and kaolin, the molten reduced but SiO2 significantly promoted melting. CaO had a strongly negative influence on the sintering and melting processes of the ash. Additionally, kaolin acted like Al2O3, and the effect of Fe2O3 was not as obvious as other additives. (2) Sintering, melting, and vaporization are three key factors that affect the shrinkage of the area and height. That is to say, sintering makes the area reduce, but melting and bubbles by vaporization make the area increase; melting makes the height decline, but bubbles make the height rise, and sintering has a tiny effect. Panels c and d of Figure 7 present the shrinkage rates of all of the samples, showing that the height shrinkage rates of the sintered samples with SiO2 are much more than those of the others. The samples with CaO shrank the least in height. Moreover, the area shrinkage rates of the pure ash sample and Fe2O3 samples ranked the most, while the area of the sample with 20 wt % CaO changed little. (3) Calculated results by FactSage are in accordance with the change of the samples with respect to the molten slag faction. SiO2 significantly lowered the initial melting temperature, and CaO, Al2O3, and kaolin raised the fully melted temperature. Besides, at the same temperature,

Figure 23. SEM micrographs of cross-sections of sintered ash samples with Fe2O3 (wt %).

samples vary similarly to the sintered pure ash sample from the original sample to layer 1.

4. CONCLUSION The aim of this study is to investigate the effect of five different additives on the sintering behavior of ZD coal ash at 1350 °C under an O2/CO2 atmosphere. The following conclusions can be drawn: (1) All of the samples are characterized by the layer 5531


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Figure 24. XRD patterns and changes to the main mineral phases of the sintered ash samples with Fe2O3 (wt %).

Notes

the molten slag fraction decreased with the increase of Al2O3, kaolin, and CaO, among which CaO had the most significant effect. In addition, the influence of Fe2O3 is not obvious. (4) The EDX results reveal that the relative contents of silicon, aluminum, calcium, and iron have a close relationship with the added additives. Meanwhile, sodium and chlorine decreased from the inner layer to the outer layer as a result of the vaporization of alkali, and other elements increased accordingly. Furthermore, the SEM images show that the sintering degree of the layers increases from the inner layer to the outer layer. In addition, additives can result in different pore structures in the outermost layer. (5) The XRD analysis results indicate that the mineral components of the samples are strongly influenced by the additives. However, some are contained in most samples, i.e., andradite, anhydrite, calcite, gehlenite, halite, and quartz. What can also be found is that the change of mineral components with additives appears markedly in the outer layer, which is at a high temperature.

■

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2015CB251501).

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REFERENCES

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Effect of Five Different Additives on the Sintering Behavior of Coal Ash

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