Evaluation of Vermiculite in Reducing Ash Deposition during the

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Evaluation of Vermiculite in Reducing Ash Deposition during the Combustion of High-Calcium and High-Sodium Zhundong Coal in a Drop-Tube Furnace Yuxiang Yao, Jing Jin,* Dunyu Liu, Yongzhen Wang, Xuesen Kou, and Yuyu Lin School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, People’s Republic of China ABSTRACT: The additive injection technology has been effective in alleviating slagging and fouling during coal combustion. In this study, experiments were conducted in a drop-tube furnace to understand the influence of vermiculite injection on ash deposition during the combustion of Zhundong coal. The interaction between coal ash and vermiculite was investigated by inductively coupled plasma optical emission spectrometry (ICP−OES), X-ray diffraction (XRD), scanning electron microscopy− energy-dispersive spectrometry (SEM−EDS). Results show that deposits formed at 1100 °C with vermiculite additive are more friable and polyporous than those without vermiculite additive, and these deposits can be easily removed. The mechanisms behind this phenomenon can be explained as follows: (1) The agglomeration of ash particles is inhibited by the partition of lowviscosity slices formed through vermiculite swelling after heat treatment. (2) Intermetallic reactions occur on the surface of vermiculite particles, resulting in the formation of frizzle sheets, and these sheets are stacked in microscale to produce a macroscale polyporous structure. (3) The surface of sticky and spherical ash particles are covered by high-melting-point and less sticky minerals, including forsterite and enstatite. (4) The decomposition of anhydrite in ash to produce free CaO is inhibited by vermiculite injection, and therefore, the formation of a relevant low-temperature eutectic is reduced.

1. INTRODUCTION Coal combustion in China meets more than 70% of the total energy demand, and this situation will not change in the near future. The Zhundong coal field, located in Xinjiang province, has an estimated reserve of 390 billion tons and has been identified as the largest coal field in the world.1 Zhundong coal is characterized as a high-quality coal considering its medium moisture content, low-to-medium ash yields, high volatile matters, and low S content.2 At this stage, Zhundong coal combustion causes severe slagging and fouling in the off-design boilers, and it is generally believed that the high sodium content is the main cause.3 The majority of sodium in coal volatilizes before 800 °C and is then entrained by the flue gas. Volatile sodium in the flue gas could condense on the heat exchanger and react with other minerals, including silica and calcia, to form sticky particles, which can capture more ash particles to form thick deposits. Currently, several measures have been proposed and applied in coal-fired power plants, including reduction of furnace volume heat release, slag, and ash deposit cleaning.4 However, these measures are expensive to use and reduce the efficiency of power generation. The additive injection technology has been effective in alleviating slagging and fouling during coal combustion. Most studies aims to reduce the release of alkali metals during combustion. Previous studies have proven that clay minerals, including kaolin, alumina, and bauxite, rich in Si and Al contents are effective additives.5−7 Dou et al.6 tested seven sodium sorbents for adsorption of alkali metal, and activated Al2O3 provides the highest capture capacity for sodium. Vuthuluru et al.8 investigated the effects of alumina additive on the combustion of Loy Yang brown coal. The introduction of alumina during combustion reduces the sodium level in © XXXX American Chemical Society

sodium silicates by ion exchange, and therefore, the stickiness caused by sodium silicates on the surface of coal ash is reduced. Kosminski et al.9 reported that Na2Si2O5 is the major reaction product of sodium salts and silica. Xu et al.10 investigated the effects of the Kaolin content, reaction temperature, and particle sizes of both kaolin and coal on the sodium capture efficiency in coal, and it was found that the sodium capture efficiency is the highest with 6 wt % kaolin injection. The capture mechanism of kaolin on sodium is due to the reaction between metakaolinite and sodium to form Na2O·Al2O3·SiO2. Two types of ash deposition mechanisms have been found in coal combustion:11,12 one is the homogeneous condensation of metal vapor to form sub-micrometer particles (around 0.1 μm), and the other is heating and oxidation of mineral matter to form large particles (10−20 μm). Several studies13−16 show that the contents of both sodium and sodium-bearing minerals are low in deposits during Zhundong coal combustion, while Si, Al, and Ca are the main elements in deposits. Meanwhile, Zhou et al.17 found that agglomeration and sintering occur in the ash deposits at 1200 °C and sintering may be related to the contents of Ca and Si in ash. Xinjiang province has a large reserve for vermiculite, which is a clay mineral enriched with Si and Al.18 Vermiculite has remarkable physical and chemical properties, including swelling and refractory behaviors, but has not been applied widely to reduce slagging and fouling during coal combustion. Lu et al.19 found that vermiculite could improve the contact between CaCO3 and SO2 because of its expansion characteristics during Received: January 14, 2016 Revised: March 8, 2016

A

DOI: 10.1021/acs.energyfuels.6b00078 Energy Fuels XXXX, XXX, XXX−XXX

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methods of desanding, crushing, and filtering. The chemical composition of vermiculate is presented in Table 2. The original coal and vermiculite samples were dried and crushed in a fan-type disk mill separately, and an automatic sieving machine was used to collect the powders with an approximate size range of 10−150 μm. The coal and vermiculite powders were fully mixed in a mixer, with the mass ratio of vermiculite/coal being (0.02−0.1):1. In addition, the vermiculite and raw coal were also included in experiments for comparison. 2.2. Experiments on Ash Deposition. Two types of experiments were used to investigate the effects of vermiculite additive on ash deposition and Na capture, respectively. The ash deposition experiments were conducted in a 5000 W drop-tube furnace (Figure 1). The experimental system mainly consisted of a coal sample

desulfurization, and in addition, the ash formation process is also influenced by vermiculite injection. Therefore, vermiculite has the potential to be a new additive to prevent ash deposition. In this study, the influences of vermiculite on both ash deposition and sodium volatilization are investigated in a labscale drop-tube furnace. The deposits and fly ash were collected and analyzed by inductively coupled plasma optical emission spectrometry (ICP−OES), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). The aim of this study is to assess the effectiveness of vermiculite as a kind of coal additive.

2. EXPERIMENTAL SECTION 2.1. Experimental Materials. Shaerhu coal as a high-calcium type of Zhundong coal was used in this study. The property of the coal sample is detailed in Table 1. The tested coal had good combustion

Table 1. Summary of Coal and Ash Analyses for Shaerhu Coal Proximate Analysis M (wt %, ad) A (wt %, ad) V (wt %, ad) FC (wt %, ad) heating value (kJ/kg) Ultimate Analysis C (wt %, ar) H (wt %, ar) O (wt %, ar) N (wt %, ar) S (wt %, ar) Na (mg/kg) Ash Composition Na2O (wt %) K2O (wt %) CaO (wt %) Fe2O3 (wt %) MgO (wt %) Al2O3 (wt %) SiO2 (wt %) SO3 (wt %) Ash Fusion Temperature deformation (°C) soft (°C) hemispherical (°C) flow (°C)

18.81 6.32 33.43 41.44 20329.75 57.99 2.88 13.13 0.83 0.05 7237.7 5.93 1.47 38.6 5.86 3.87 15.94 22.16 5.37

Figure 1. Schematic diagram of the drop-tube furnace system. injector, induced draft fan, a main tube reactor (2400 mm total height of heating zone × 100 mm inner diameter), a deposit probe, and an ash collecting unit. In particular, the deposition disc (700 mm length × 5 mm inner diameter) was made by cutting a ceramic tube and positioned at the bottom of the constant temperature zone. Before experiments, the temperature distribution along the furnace height was calibrated with a thermocouple, and the constant temperature zone was marked with a red line in Figure 1. In combustion experiments, pulverized coal samples were fed at the top of the furnace with a screw feeder in conjunction with primary air, and then injected coal particles reacted with secondary air in the furnace. The excess air ratio was set to 1.2 to ensure the complete combustion. The flow rate of air passing through the furnace was 49−52 L/min, and the residence time of the particles was around 1.7−1.8 s. In the ash deposition tests, vermiculite, raw coal, and their mixture (6 wt % vermiculite in coal) were fed into the furnace separately with the temperature kept at 1100 °C, where the mineral composition of ash deposits was totally changed13 and severe

1181 1231 1253 1340

characteristics as a result of a relatively low ash content and high volatile yield. It should be mentioned that sodium and calcium contents were over 5 and 38%, respectively, and this may result in severe ash deposition problems during combustion. The vermiculite used for the experiment was also taken from Xinjiang province, near Zhundong coal mine, which was convenient and cost-effective to the pithead power plant. Its chemical formula was (Mg,Ca)0.7(Mg,Fe3+,Al)6[(Al,Si)8O20](OH4·8H2O). The gangue and impurities in raw ore of vermiculite were removed through the

Table 2. Chemical Analysis of Vermiculite composition (wt %)

MgO

CaO

Fe2O3

SiO2

Al2O3

Na2O

K2O

H2O

weight loss

sum

common vermiculite tested

11−23 17.60

1−2 1.82

3.5−15 6.57

37−42 40.28

9−17 16.24

minor 1.35

minor 2.56

5−11 6.28

6.72

99.42

B


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Energy & Fuels deposition problems occurred in our previous tests. The feeding rate was about 8 g/min for both raw coal and the mixture (raw coal and vermiculite), while about 0.48 g/min was used for vermiculite only. During each experiment, the samples were continuously fed for 4 h, when they were heated and transformed in the reaction section of the furnace, and finally the deposits were collected from a deposition disc. Both microcosmic and mineralogical analyses of deposits were performed by SEM−EDS and XRD, respectively, to identify the microstructure and crystalline present in the examined deposits. For each experiment, a 0.1 g deposit was taken and completely dissolved in hydrofluoric, perchloric, and hydrochloric acids step by step and the solution was diluted to one tenth with deionized water for the elemental analysis by ICP−OES. 2.3. Experiments on Na Capture. To understand the temperature dependence of sodium capture by vermiculite, a mixture of raw coal with 6 wt % vermiculite was fed continuously for 30 min at furnace temperatures of 800, 900, 1000, and 1100 °C, with tests of raw coal being performed as a comparison. To understand the dependence of sodium capture upon the amount of vermiculite additive, the tests of raw coal with 2, 4, 6, 8, and 10 wt % vermiculite additive were also performed at 1100 °C. Before each test, the sample was fed for about 10 min to make conditions stable and then the collecting units were installed to collect the ash. The feeding rate for coal samples, excess air ratio, flow rate of air, and residence time were set to 8 g/min, 1.2, 47− 52 L/min, and 1.7−2.4 s, respectively. The collected ash was dissolved by the same method used on deposits, and then the sodium content was tested by ICP−OES. The sodium capture was calculated by comparing the sodium content in collected ash and the total sodium in the combusted coal and vermiculite, as defined in eq 1

Na capture rate (%) = sodium content in the collected ash /total sodium in the combusted coal and vermiculite

(1) Figure 2. Top view and SEM pictures (500×) of ash deposition at 1100 °C: (a) vermiculite, (b) untreated coal, and (c) treated coal.

in which the sodium content in the collected ash is the sodium content in 0.1 g of ash × 10 × ash weight and the total sodium in the combusted coal and vermiculite is the sodium content in 1 g of raw × total coal combusted + sodium content in 1 g of vermiculite × total vermiculite injected.

an amorphous chunk, and spherical particles are observed adhering to it. This phenomenon indicates that liquid slag is formed at 1100 °C, and surprisingly, this temperature is lower than the ash fusion point given in Table 1. The reasons for this are detailed in section 3.2. Figure 2c shows the macromorphology and relevant SEM images of deposits after combustion of a mixture of coal and vermiculite at 1100 °C. The deposit on the target disc from treated coal (with 6 wt % vermiculite) differs markedly from the untreated coal deposit. A loose and friable deposit is partly formed on the target disc in the presence of the vermiculite additive, indicating that the vermiculite additive plays an important role in alleviating ash deposition. The SEM image of the deposit from treated coal shows that the majority of the deposit exists as frizzy sheets with a few floccules on the edge. A lower proportion of spherical particles and many pores are also observed, and these may be caused by the interaction between coal ash and vermiculite. As a result of the large increase of the specific area of vermiculite during heating, the ash particles are more likely to interact with exfoliation, which has a low density and low viscosity,21 inhibiting the direct contact of ash particles. Three round holes can be noticed on a big slice, and the diameter of the holes is approximately equal to the spherical particles (Figure 2b), suggesting that spherical particles may be located in these holes initially and then drop during ash deposition or the movement of the disc. A comparison between panels a and c of Figure 2 indicates that the interaction between coal ash and vermiculite makes the slices frizzle, and the

3. RESULTS AND DISCUSSION 3.1. Morphological Change of Ash Deposition with Vermiculite Additive in a Drop-Tube Furnace. Ash deposition experiments were performed at 1100 °C in a drop furnace to assess the effects of vermiculite on ash deposition after combustion of Zhundong coal. Results are presented in Figure 2. The macromorphology and relevant SEM image of vermiculite particles on the deposition disc after heating at 1100 °C are shown in Figure 2a. A small amount of loose deposit is observed on the target disc, and it is easily removed by shaking. The SEM image shows that the vermiculite deposit exists in the form of thin slices with sharp edges. It has been reported by Marcos et al.20 that swelling and exfoliating occurred when vermiculite was heated at around 1000 °C as a result of the release of water from the gap between slices. As a result, the vermiculite particles swell to 20 or 30 times their original size to form a basal−cleavage−plane type of structure and the exfoliation stacks loosely on the target disc. Figure 2b shows the macromorphology and relevant SEM images of ash deposits on the target disc after untreated coal (without vermiculite) combustion at 1100 °C. The target disc is totally covered by ash deposit, which is more compact than the vermiculite deposit shown in Figure 2a. The SEM image shows that the deposit appears to be compactly agglomerated to form C


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Energy & Fuels stacking of these frizzled planes contributes to the formation of a loose and friable deposit. Although the deposit is still likely to form in a drop-tube furnace, it appears to be more friable and, hence, would be easily removed by traditional methods, such as frequent soot blowing. 3.2. Mineral Composition of Ash Deposits. XRD analysis was conducted to qualitatively identify the major and minor mineral components in deposits shown in Figure 3.

Figure 4. Elemental compositions of different ash samples at 1100 °C.

quantitative decrease of Ca and minor quantities of Na and K.20 CaSO4 identified in the deposit indicates that vermiculite has an inhibiting effect on the decomposition of CaSO4 through the co-combustion of coal and vermiculite, which is also reported elsewhere.25 3.3. Elemental Composition of Ash Deposits. The elemental compositions for all of the deposit samples collected on each deposition disc are displayed as stacked columns in Figure 4. It can be seen that the most significant differences in element content of deposits between untreated coal and vermiculite are calcium, magnesium, silicon, and sulfur. The calcium and sulfur contents in untreated coal deposit are relatively high, while magnesium and silicon contents in the vermiculite deposit are relatively high. In comparison to untreated coal, the calcium content decreases from 36 to 10% with vermiculite injection, which is much lower than the reference value (22%). The magnesium content increases from 4 to 24%, which is much higher than the reference value (10%). XRD tests shown in Figure 3c also show that a large number of minerals rich in magnesium are identified, indicating that magnesium tends to remain in the ash with vermiculite injection. Although the sulfur content is very low in the vermiculite deposit (0.17%), the sulfur content in untreated coal is about 9.50%, while that in treated coal is around 8.14%, and they appear to be similar; both are much higher than the ash sulfur content in Table 1. The sodium contents in all deposits are under 3.6% and do not change much. In the micropresentation of deposits of untreated and treated coal, different morphological ash particles are obviously observed, as shown in Figure 2. However, the elemental and mineral differences on the surface of samples are still not clearly demonstrated. Thus, SEM−EDS analysis was also conducted to explore the melting and agglomeration behavior of ash in the deposit. The micromorphology and EDS results of the deposit from untreated coal combustion at 1100 °C are shown in Figure 5a. The spherical particle and amorphous chunk are clearly observed in this picture, marked as target 1 and target 2, respectively. The elemental content of each target was analyzed for more than three points to ensure a similar distribution of elements in the same targets. It can be observed from the appearance of the spherical particle that this ash particle seems to be covered with a “liquid” layer of molten phases, and in addition, the spherical particle adheres to the amorphous chunk

Figure 3. XRD patterns of ash deposition at 1100 °C [(1) SiO2 (quartz), (2) Mg2SiO4 (forsterite), (3) Al2O3·SiO2 (mullite), (4) Ca2MgSi2O7 (akemanite), (5) Ca2Al2SiO7 (gehlenite), (6) CaMgSiO6 (diopside), (7) MgSiO3 (enstatite), and (8) CaSO4 (calcium sulfate)].

Figure 3a shows that the strong peaks identified in the vermiculite deposit are Mg2SiO4 (forsterite) and SiO2 (quartz) at 1100 °C, and a minor amount of Al2O3·SiO2 (mullite) is also found. A high fusion point is the common property for these main minerals; therefore, the vermiculite deposit has low stickiness at 1100 °C. Figure 3b shows that Ca2MgSi2O7 (akemanite) is the principal phase present in the untreated coal deposit at 1100 °C. SiO2 and Ca2Al2SiO7 (gehlenite) are also observed. Previous studies have proven that CaCO3 (calcite) decomposes between 800 and 1000 °C, while CaSO4 (calcium sulfate) decomposes at around 1100 °C.22 Neither CaCO3 or CaSO4 is found in this deposit, and the calcium content is particularly high in both ash (Table 1) and deposit (Figure 4), suggesting that a large amount of free CaO exists in the ash deposition process, which may contribute to the formation of a lowtemperature eutectic.23 Nankervis and Furlong22 studied the formation mechanism of calcium silicates and found that CaO formed from the decomposition of CaSO4 may react with MgO, SiO2, and Al2O3 at temperatures above 850 °C to form gehlenite and akermanite. Vassileva and Vassilev24 found that both CaO and MgO are stable at temperatures up to 1200 °C, but they commonly take part in the formation of Ca−Mg silicates at lower temperatures. Consequently, in this study, the phenomenon of ash melting at 1100 °C is closely related to the interaction among Ca, Mg, and Si. Figure 3c shows that akemanite and gehlenite disappear in the deposit for treated coal, while a large amount of CaMgSiO6 (diopside) and Mg2SiO4 (forsterite) is identified. MgSiO3 (enstatite) and CaSO4 (calcium sulfate) are also observed with the vermiculite additive. The XRD pattern implies that Mg turns out to be the major exchangeable cation as a result of the D


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Figure 5. Microstructures (5000×) and elemental contents of ash deposits at 1100 °C: (a) raw coal and (b) mixture of raw coal and vermiculite.

3.4. Na Capture Efficiency. In the previous section, the influence of vermiculite injection on ash deposition has been fully investigated. However, the sodium capture efficiency of vermiculite is still not clearly understood when vermiculate is also regarded as a kind of sodium sorbent. Experiments on combustion of the mixtures of raw coal with 6% vermiculate in the temperature range of 800−1100 °C were conducted in the drop-tube furnace to investigate the temperature dependence of sodium capture efficiency, with untreated coal being presented as a comparison. Figure 6 shows

seamlessly, suggesting the property of high viscosity. EDS data show that Ca, Si, Mg, and Al are the major elements in target 1, and this suggests that the formation of complex molten phases occurs on silica particles. Only the Ca content is very high in target 2. Vuthaluru and Wall26 and Ninomiya and Sato23 pointed out that the spherical particles of the melting phase result from co-melting silicate with other oxides, some of which provide elements capable of replacing Si in the polymeric network. This may also occur in this study because more Mg and Al contents are found in target 1. Mg is especially high, and the content is almost equal to Si in target 1. However, a very low Mg content is found in the deposit as a whole (Figure 4) and in target 2, indicating that Mg is preferentially enriched in the spherical particles. The phase Ca2MgSi2O7 has been identified by XRD in the untreated coal deposit, indicating that the final mineral forms after co-melting behavior. The micromorphology and EDS results of the ash deposit from treated coal combustion at 1100 °C are shown in Figure 5b. The spherical particle is also observed in the picture, marked as target 3, indicating that melting also occurred during the combustion of treated coal. However, in comparison to target 1, the spherical particle of target 3 is nearly separated from the surrounding material, which seems to be rough and full of bumps. EDS data show that the Ca content in target 3 diminishes, while Mg and Si contents are relatively high. XRD analysis in Figure 3 confirms the existence of Mg2SiO4 and MgSiO3, suggesting that these high-melting-temperature minerals tend to cover on the spherical particles, making them less viscous. As a result of the dropping of these spherical particles, round holes can be formed in Figure 2c. Besides the spherical particle, layered and loose structures are also observed, marked as target 4. EDS data show that the sequence of elemental content in the layered structure is Si, Ca, Al, and Mg from high to low. The high Ca content in target 4 indicates that a Ca-based intermetallic reaction occurs on the surface of vermiculite, and the mineral forms have changed with the vermiculite additive from Ca2MgSi2O7 to CaMgSiO6, as shown by XRD.

Figure 6. Sodium capture efficiency changes with the temperature for untreated and treated coal.

that the sodium content in treated coal decreases as the temperature increases. Approximately 38% sodium remains in the ash at 800 °C, while only under 10% sodium remains in the ash at 1100 °C, and the gap between treated and untreated coal reduces as the temperature increases, suggesting the low capture rate of Na for vermiculite at a higher temperature. The physical and chemical adsorption mechanisms of Na vapor by additives in different temperature sections have been reported by Dou et al.6 The sodium capture rate with the vermiculite additive is deemed to be associated with the physical adsorption E


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When raw coal is used for combustion, a large number of particles is formed from inorganic minerals in the coal particles. Owing to the co-melting behavior of metal oxides occurring on the surface of these particles, a high viscosity film is formed on the surface of these particles. Therefore, particles are easily accumulated on the surface of the deposition disc. Then, melting and agglomeration occur to form an amorphous chunk, which can be observed as a compact slag on the deposition disc. When vermiculite is used for combustion, the stacked vermiculite slices are transformed into separate thin slices as a result of lose association between slices. Because the viscosity of vermiculite is low, these slices are barely deposited on the disc and sintering is not observed either. At the same time, plenty of minerals of high melting point, including forsterite and enstatite, are also formed during the heating of vermiculite slices. When a mixture of raw coal and vermiculite is used for combustion, the interaction mechanism is complicated. As shown in section 3.4, vermiculite has little effect on sodium capture at a higher temperature, especially at 1100 °C. However, it has significant influence on the process of ash deposition. Round ash particles are formed on the surface of vermiculite slices, which make the thin slices frizzle. These slices prevent contact of the sticky particles to form agglomerate directly. At the same time, forsterite and enstatite formed during the heat treatment of vermiculite tend to insert on round particles to make them less sticky. In addition, the main Ca-bearing minerals transform from gehlenite to diopside after injection of vermiculite, and these minerals mainly exist on the surface of vermiculite slices. Therefore, these round particles drop easily by gravity and flow field, indicating the positive effect of the vermiculite additive on reducing deposits.

of Na that only occurs at low temperatures. A sharper fall is observed after 900 °C, and this could also be related to the absorption of sodium by vermiculite. Thus, the Na capture efficiency of vermiculite can be neglected, probably because Si and Al are not activated, although the concentrations of them are high.21 The Na content in the deposit is quite low and does not change much with the vermiculite additive, shown in Figure 4, suggesting that the Na element has little effect on the ash deposition. Experiments were also conducted at 1100 °C to investigate the effects of the vermiculite content on sodium capture. The results are shown in Figure 7. Approximately 4% sodium is

Figure 7. Na capture efficiency changes with the vermiculite additive.

retained in the ash when 2 wt % vermiculite is injected. The Na capture rate increases slightly with the vermiculite additive, and about 12 wt % sodium remains in the ash with 10 wt % vermiculite injected. These results indicate that vermiculite has little effect on Na capture. 3.5. Mechanism of Reducing Ash Deposition by Vermiculite Additive in Zhundong Coal Combustion. On the basis of the results from ash deposition and sodium capture experiments, the interaction mechanism between coal ash and vermiculite during combustion is proposed in Figure 8.

4. CONCLUSION The influence of vermiculite on ash deposition and Na capture during the combustion of high-calcium and high-sodium Zhundong coal was studied in a lab-scale drop-tube furnace.

Figure 8. Schematic of the mechanism of the interactions between vermiculite and coal ash. F


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(21) Balek, V.; Pérez-Rodríguez, J. L.; Pérez-Maqueda, L. a.; Šubrt, J.; Poyato, J. J. Therm. Anal. Calorim. 2007, 88 (3), 819−823. (22) Nankervis, J. C.; Furlong, R. B. Fuel 1980, 59 (6), 425−430. (23) Ninomiya, Y.; Sato, A. Energy Convers. Manage. 1997, 38 (10), 1405−1412. (24) Vassileva, C. G.; Vassilev, S. V. Fuel Process. Technol. 2006, 87 (12), 1095−1116. (25) Geng, M.; Shi, L.; Xu, W. Non-Met. Mines 2006, 29 (4), 28−30. (26) Vuthaluru, H. B.; Wall, T. F. Fuel Process. Technol. 1998, 53 (3), 215−233.

The blending effect is assessed, and then the interaction mechanism is proposed. The following conclusions can be drawn from this work: (1) Vermiculite could efficiently reduce ash deposition during Zhundong coal combustion. The compact structure of deposits at 1100 °C from raw coal combustion is transformed into a friable and polyporous structure with vermiculite injection, and deposits are therefore easily removed by shaking and soot blowing. (2) The agglomeration behavior of ash particles is inhibited by the partition of low-viscosity slices, which are formed through vermiculite swelling when heated. Besides, intermetallic reaction occurs on the surface of vermiculite particles, making the sheets frizzle. They stack in the microscale; thus, a polyporous deposit is observed in the macroscale. (3) The sticky and spherical particles are covered by high-melting-point minerals, including forsterite and enstatite, making these particles less sticky. (4) The decomposition of anhydrite is restrained to produce less free CaO, thus reducing relevant formation of a low-temperature eutectic during coal combustion.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13651975399. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support received for this research from funds of the National “12th Five-Year” Plan for Science and Technology Support.

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REFERENCES

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Evaluation of Vermiculite in Reducing Ash Deposition during the

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