Visual Investigation of the Growth of Ash Deposits and

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Visual Investigation of the Growth of Ash Deposits and Characteristics of Fly Ash with CaO Additives Hao Zhou,* Weichen Ma, 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: Experiments were conducted in a pilot-scale test furnace to investigate the influence of CaO additive on the growth of ash deposits and the characteristics of fly ash. The amounts of CaO additive mixed with Datong coal were 0 and 3.5 wt %, and the operating temperature of the furnace was 1573 K. An oil-cooled probe was inserted in the furnace, and a chargecoupled device (CCD) monitoring system was equipped with a water-cooling system to obtain the variation of thickness of the deposits with time. This system was used to evaluate the buildup of the deposits. Online monitoring of flue gas was carried out for the emissions of NOx and SO2 using the flue gas analyzer. The fly ash was collected from the bottom of the cyclone separator, and its characteristics were determined using a range of analytical techniques, including polarization microscopy, physisorption analysis, X-ray diffraction (XRD), and X-ray fluorescence (XRF). Frequent shedding of ash deposits was observed for pure coal. Nevertheless, under the condition with additive, serious slagging with a thickness of the deposits up to 66 mm occurred. The addition of CaO resulted in a reduction of SO2 but an increase of NOx emission. The fly ash produced by burning pure coal had a greater specific surface area (SSA). The XRD and XRF analysis results revealed that the CaO additive had a significant impact on the mineral and chemical composition of the fly ash.

1. INTRODUCTION Ash depositing on heat-exchange tube surfaces may cause serious problems in coal-fired power plants, such as the reduction of thermal conductivity of the heat exchanger and boiler tube corrosion.1,2 Researchers over the past several years have focused on ash-related problems, including slagging, fouling, and corrosion. The growth of ash deposits is a highly complex process and is influenced by the physical and chemical characteristics of fuel and operating conditions. Researchers have investigated these characteristics through both simulation and experimental methods. Theoretical studies, concentrated on the development of proper models to simulate the ash growth, take into consideration the operating conditions and fuel composition. Richards et al. proposed a model with a comprehensive combustion code to investigate the influence of deposition rates, operating conditions, and chemical composition on the characteristics of deposits. The simulation results were compared to the experimental data, and the operating conditions were found to significantly influence the formation of deposits.3 Garba et al. developed a computational fluid dynamics model to predict the deposition rate. The model combined the viscosity and melting behavior of the ash particles to calculate the sticking probabilities of the particles. They also proposed a numerical slagging index to estimate the sintering degree of the deposits.4 In experimental research, studies reported the use of air/water-cooled probes to sample the ash deposits.5−9 Additionally, the online weighing technique was used to monitor the buildup of ash deposits.10 However, quantifying the growth of the ash deposits visually has been rarely reported. Many researchers focused on the chemical composition of ash and investigated the conversion mechanism of different types of compounds. The alkali metals (K and Na) and alkaline earth metals (Ca and Mg) play significant roles in the © 2015 American Chemical Society

formation of ash deposits. Volatile alkali mineral matter reacts with fly ash particles in the flue gas and condensates at the surface of tubes. This is a major reason for fouling, and the elements responsible for fouling include volatile sodium, volatile calcium, chlorine, sulfur, and phosphorus.11 Several studies also reported calcium as a factor influencing the viscosity of ash deposits. At high temperatures, the addition of CaO to SiO2 caused a reduction in viscosity. Nevertheless, a contrary effect occurred at low temperatures because calcium coordinates the non-bridging oxygen bonds and reinforces the structure. A similar effect was observed when CaO was added to an alkali silicate. For blends of SiO2 and Na2O, the influence of temperature on viscosity was most pronounced when CaO is added, followed by SrO, BaO, and MgO.12 Arvelakis et al. noted that the higher proportion of calcium resulted in a more inhomogeneous ash melt and the non-Newtonian behavior influenced the melt flow at a higher temperature.13 The reaction involving CaO and Fe2O3 formed eutectics, which have lower melting temperatures on the receding char surface.14 CaO or CaSO4 reacted with SiO2 and Al2O3 to generate gehlenite (Ca2Al2SiO7) at a low temperature.15 Nevertheless, few studies investigated the online recording of the growth of ash deposits with the addition of CaO. Along with the characteristics of the fuels, the operation conditions, such as oxygen enrichment and in-furnace turbulent and temperature fluctuation, also considerably influence the process of ash deposition. Wang et al. reported that the ash Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 28, 2015 Revised: November 30, 2015 Published: December 14, 2015 1792

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Figure 1. Schematic diagram of the pilot-scale test rig.24 analyzer. The analyzer was equipped with a gas preprocessor to avoid blocks and decrease the temperature of the gas sample effectively. At the end of each experiment, fly ash was collected from the bottom of the cyclone separator. 2.2. Deposition Probe System. The formation of the ash deposits was observed online on a specially designed probe that was made of stainless steel. The deposit sampling system consisted of two key components: the probe and the oil-cooling system. The hollowcylinder deposition probe had a length of 76 mm, and its inner and outer diameters were 27 and 40 mm, respectively. The structure of the probe is illustrated in Figure 2.

deposition propensity was higher under oxy-combustion than under conventional conditions. Additionally, more K2O and Na2O were found in the deposits under oxy-combustion.16 Eddy diffusion occurred in turbulent systems, and the turbulent eddies near the tube transported fine particles toward the tube surface.17 The influence of the temperature on the ash deposits was studied by many researchers,18−20 and the ash deposition propensity was usually higher under higher temperatures. Fly ash particles are widely investigated as a product of burning coal, are highly diverse and inhomogeneous, and possess different morphologies.21 The characteristics of fly ash directly affect the buildup of ash deposits. Zheng et al. compared the compositions of fly ash and ash deposits and found that, in comparison to fly ash, the deposits had the same amount of K, a lower proportion of Cl, and higher sulfur.22 Wu et al. studied the influence of the addition of coal fly ash on ash deposition during wood combustion and found that adding the coal fly ash reduced ash deposition and corrosion.23 This paper conducted a visual investigation of the growth of ash deposits on an oil-cooling probe through a charge-coupled device (CCD) monitoring system and examined the effects of the addition of CaO on ash deposition. We also analyzed the emissions of pollutants and fly ash to further study the influence of the additive.

Figure 2. (a) Schematic diagram and (b) image of the deposition probe.24

2. EXPERIMENTAL SECTION 2.1. Combustion Facility. Figure 1 shows the schematic diagram of the pilot-scale experimental facility. The main components include the vertical furnace, the feeding system, the swirl burner, the temperature monitoring system, the ash deposition probe equipped with the oil-cooling system, and the image monitoring system. The vertical furnace surrounded by refractory material had an inner diameter of 350 mm and a length of 3950 mm. The feeding rate of fuel could be regulated from 10 to 50 kg/h by adjusting the feeding electromotor, and a rate of 45 kg/h was selected as the probe was inserted into the furnace. The temperature of each stage of the furnace was continuously measured by several S-type thermocouples, as shown in Figure 1. The vertical furnace consisted of four stages, and the first stage was on the top. The ash deposition probe was inserted into the third stage at a distance of 1910 mm from the burner outlet. Flue gas was sampled from the bottom of the furnace, and the concentrations of O2, SO2, and NOx were measured by the TESTO 350 flue gas

The heat conducting oil is circulated from a 9 KWe oil temperature control unit to the probe to regulate the surface temperature of the deposition probe and simulate the water wall tube in the coal-fired boiler to make the process of ash deposition similar to that under real conditions.25 In this study, the inlet temperature of the cooling oil was 503 K and the outlet temperature fluctuated around 513 K. 2.3. Image Acquisition System and Data Processing. The growth of the ash deposits was observed through a CCD camera fixed immediately opposite the probe. A schematic diagram of the image acquisition system is shown in Figure 3. A special lens was fixed in front of the CCD camera to bring the object into focus. The camera and lens were fixed in a protective container to avoid any accidental damage during the experiment. A protection tube, with cooling water circulating in the outer channel, was used to protect the camera lens from the high-temperature flue gas. Additionally, pressurized air was released from the inner channel of the tube to avoid the fly ash 1793

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chamber and burned to heat the furnace, and S-type thermocouples were used to simultaneously measure the temperatures of each stage. The variations of the temperature with time for the first three stages of the furnace for the two cases are shown in panels a and b of Figure 5,

Figure 3. (a) Schematic diagram and (b) image of the CCD monitoring system.24 deposition and damage to the image quality. As a result of the high temperature, the view of the field was red. The three color channels (red, green, and blue) and the exposure time of the camera could be adjusted to obtain high-quality images of the ash deposits. In this paper, the growth of the ash deposit was evaluated by the change in thickness obtained by the digital image processing illustrated in Figure 4. With the help of MATLAB software, the edge of the

Figure 4. Digital image processing: (left) original image and (right) edge image. deposits could be extracted from the background. Then, the edge of the probe was detected to determine the location of the circle point and the diameter. The maximum height of the deposits was assumed to represent the thickness (h). The pixels of the probe diameter (PD) and the thickness of the deposit (Pt) were calculated. The outer diameter (D) of the probe was 40 mm, as shown in Figure 2. Then, the thickness h was calculated by the following equation:

h=D

PD Pt

Figure 5. Temperature of each stage of the furnace for (a) pure coal and (b) additive blend. respectively. After approximately 3 h of the heating process, the temperature of the third-stage furnace became more stable and fluctuated around approximately 1573 K. Then, the probe was inserted into the third-stage furnace through the fire hole. The inlet temperature of the oil was set at 503 K. At the end of the experiment, the fly ash was collected from the cyclone separator. The fuel properties and experimental conditions are shown in Tables 1 and 2.

(1)

With respect to the measurement accuracy, the deflection of distance on the image occurred as a result of the system error of the camera. The calibration of distance on the image was performed, and the deflection of distance in the vertical direction was so small that it could be ignored. In fact, measuring the actual size of each pixel was not required because the thickness is calculated by the ratio listed above and the actual outer diameter of the probe that is already known. Moreover, each thickness data point was obtained from at least two pictures as a result of the frame rate of 3 frames/s of the CCD camera, which further reduced the errors. 2.4. Experimental Procedure. The growth of deposits, the emission of pollutants, and the characteristics of fly ash were investigated in the third stage and at the bottom of the vertical furnace with 0 and 3.5% additive shares mixed with Datong coal. Diesel oil and pulverized coal were first fed into the combustion

3. RESULTS AND DISCUSSION 3.1. Growth of the Deposits. During the experiments, the morphology of the ash deposition on the probe was monitored by a CCD camera. We used the thickness of deposits to evaluate the growth of the deposits on the probe. The frame rate of the CCD camera was set as 3 frames/s. We extracted more than 70 images from the videos for each case to obtain the change in the thickness of the deposit. A bright flame appeared in the field of view as a result of the existence of combustion oscillations and resulted in bad images, where the edge of the deposit could not be extracted from the background. The combustion oscillations also resulted in an 1794

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Energy & Fuels Table 1. Fuel Properties fuel moisture (wt %, ad) proximate analysis (wt %, ad)

ultimate analysis (wt %, db)

HV (MJ/kg, db) ash fusion temperature (K)

Datong coal volatile matter fixed carbon ash carbon hydrogen nitrogen sulfur oxygen HHV IT ST HT FT

7.22 28.56 54.4 9.82 68.28 4.19 0.91 0.75 8.83 26.89 1642 1671 1681 1708

Table 2. Experimental Conditions fuel excess air ratio CaO additive content (wt %) primary air velocity (m/s) furnace temperature (K)

Datong coal case 1 case 2

second stage third stage oxygen concentration at the furnace outlet (%) exposure time of the ash deposit probe (min)

1.2 0 3.5 ∼2.8 ∼1643 ∼1573 4−5 120

in-furnace temperature fluctuation (see Figure 5) and a nonuniform distribution of the thermal field. The temperature gradients transported the small particles to the tube surface through the process of thermophoresis and promoted the ash deposition. Researchers have reported that the moderate or intense low-oxygen dilution (MILD) combustion achieved lowtemperature fluctuations and a uniform in-furnace thermal field.26,27 Figure 6 shows the variation of deposit thickness on the probe with time for pure coal and the additive blend. Frequent shedding of deposits was observed in Figure 6a for the pure coal case. The thickest deposit of 16.8 mm appeared at 46 min. A few images between 10 and 35 min were unavailable as a result of the bright flame and the ash clogging the tube in front of the CCD camera. The growth curve of the deposit could be divided into six parts. Each part consisted of a rising curve and a declining curve and corresponded to a shedding process of the deposit. The images of the deposits before and after the shedding were captured by the CCD camera and are shown in Figure 7. The slopes of the each rising curve were 1.62, 0.71, 0.77, 0.75, 0.99, and 0.95 mm/min, and the highest thicknesses observed for each part were 16.8, 8.9, 8.28, 6.2, 7.8, and 7.9 mm, respectively. All of the curves, except the first, showed the same trend, which implied the same deposition process. It is speculated that the growth and shedding processes of the ash deposits on the probe act as a habitus and the highest thickness fluctuates around approximately 8 mm. Datong coal has a low slagging propensity and causes the frequent shedding of deposits. The ash has a very high fusion temperature. Also, the existence of combustion oscillations aggravates the shedding process. Figure 6b shows a succession of the thickness increase of ash deposits for the additive blend. Considerable ash deposition

Figure 6. Deposit thickness as a function of time for (a) pure coal and (b) additive blend.

was observed in the image. After 105 min, the thickness became stable and the value was approximately 66 mm, which was much greater than that in the pure coal case. There are two rapid growth stages of the curve, with the first lasting 12 min and the other ranging from 38 to 57 min. The slopes of the two stages were 1.04 and 0.94 mm/min, which are smaller than those mentioned previously for pure coal. This is due to the change in the physical characteristics of fly ash particles, which will be discussed in the following section. During the image processing, a bright flame also appeared in the video and represented the occurrence of combustion oscillations. However, no shedding occurred during the 120 min. Thus, the additive increased the thickness of the deposits and was inferred to react with the ash particles to change the structure and viscosity of the deposits, especially the initial layer of the deposit, so that the deposits could firmly stick to the surface of the probe. In fact, the ash deposit had several layers. As discussed in our previous work, the several stages of the growth curve with different slopes were consistent with the three-layer structure of the ash deposits. The collected ash deposits with and without additive could be divided into three layers, namely, the initial layer, the sintered layer, and the slag layer.24,25 1795

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Figure 8. Emission of NOx and SO2 with time: (a) pure coal and (b) additive blend.

CaSO4 started dissociating. Certain types of sulfates decompose in the presence of other inorganic matter to form minerals with lower melting temperatures. For example, CaSO4 reacted with Na2SO4 in the presence of silicates and formed a melilite that had a low melting temperature of 1025 K, as per the reaction listed below.11

Figure 7. Shedding images captured by the CCD camera for pure coal.

3.2. Emission of NOx and SO2. During the experiments, the TESTO 350 flue gas analyzer monitored the emission of NOx and SO2 in the flue gas. As a result of the high amount of fly ash in the flue gas, the analyzer and the filter bags were often blocked; hence, they required frequent cleaning. Subsequently, we connected the data of several time periods occurring in a case together and plotted them. The emission of NOx and SO2 with time for the two cases is shown in Figure 8. In comparison to the emissions for burning pure coal, a sharp decrease in the SO2 emission was observed. The quicklime tended to react with SO2, and the common reactions are shown below. CaO + SO2 = CaSO3

(2)

2CaSO3 + O2 = 2CaSO4

(3)

[x Na 2SO4 + yCaSO3] + Al 2O3gzSiO2 → x Na 2OgyCaOgAl 2O3gz SiO2 + (x + y)SO3

(4)

Calcium-sulfate-bonded deposits have drawn attention since the 1960s. It was reported that calcium promotes the initiation of deposit growth. The initial layer of the deposit is rich in anhydrite, whereas the fireside outer layer is plagioclase-rich.11 From the discussion above, it is known that sulfur plays a significant role in CaO additive by promoting the growth of ash deposits, which was demonstrated in section 3.1. As illustrated in the two panels, the emission of NOx increased with CaO mixed in the coal because CaO promoted the conversion of fuel N to nitric oxide and increased the selectivity to NO in the conversion of HCN and CN radicals, thereby decreasing the concentration of N2O.28

Most sulfur remained in the ash as sulfate, and its fusion temperature was low. It should be noted that, although the melting temperature of CaSO4 was reported as 1721 K, at approximately 1283 K (which is far below the melting point), 1796

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fine particles with spherical shapes were found in panel b. The shape distribution was obtained from the morphologic analysis. The fly ash particles ranged in size from 5 to 100 μm, for both cases. The results of the morphologic analysis are given in Table 3, and they confirm that burning coal mixed with a CaO additive produced more spherical fly ash particles.

3.3. Fly Ash. 3.3.1. Physical Characteristics. At the end of the experiments, fly ash was collected from the bottom of the cyclone separator. Subsequently, a series of analytical tools, including a polarization microscope, a specific surface area meter, and a pH meter, were used to determine the physical characteristics of the fly ash for the two cases. The density of the fly ash for the two cases was measured according to the standard GB/T1713-2008. As a result of the higher molecular weight of the CaO additive, the fly ash collected in the additive case had a higher density of 2134 kg/ m3, which is 1.12 times that of pure coal case (which is 1901 kg/m3). The microstructures of fly ash particles from burning coal with or without a CaO additive are shown in Figure 9. More

Table 3. Morphologic Analysis of Fly Ash Particles particle shape distributions case pure coal additive blend

spherical (%)

non-spherical (%)

particle size distribution range (μm)

49.4 55.2

50.6 44.8

5−100 5−100

This result could be explained by the formation of calciumsulfate-bonded deposits. As a result of the sulfidation of calcium fumes formed by sub-micrometer particles scavenged by fly ash, large crystals of calcium sulfate grow and the sphere becomes encapsulated with the crystals. The crystals of calcium sulfate attach to the adjacent particle and, thereby, promote the bonding of the fly ash. As the deposit grows, the outer layer may become fused as a result of the decomposition of CaSO4 at 1223 K. In some cases, calcium could form a single sphere.11 A specific surface area is an important parameter for determining the physical and chemical characteristics of fine particles and porous solids. In this work, an ASAP 2020 physisorption analyzer characterized the surface features of fly ash particles, and the Brunauer−Emmett−Teller (BET) specific surface area was obtained for the two cases. When CaO was added to the coal, the BET specific surface area of fly ash particles was reduced from 15.732 to 9.7052 m2/ g. This could be because the crystals of calcium sulfate decompose to form the fused phase and the pore structures on the particle surface would be reduced. Conversely, the crystals of calcium sulfate promote the bonding of the fly ash to form larger and more spherical particles. As shown in section 3.1, the ash deposition growth rate of the pure coal was greater than that of the additive blend. The larger specific surface area of the fly ash particles in the pure coal case may be a significant reason for this growth rate. To verify the phenomenon, a precise distribution of the particle size needs to be measured further. According to the results, the addition of CaO leads to a big change in the physical characteristics of fly ash. It was reported that oxygen enrichment led to the higher fine particulate formation.29 Chen et al. made a comparison of the particle size in O2/CO2 and O2/N2 atmospheres and found that more small ash particles were generated in an O2/CO2 atmosphere.30 It is also of significant interest to further investigate the influence of the additive on ash deposition under oxy-combustion. The MILD oxy-combustion would be a good choice as a result of its fairly uniform temperature distribution. For the MILD oxycombustion, Li et al. provided three burner configurations and investigated the difference with the conventional combustion.31 The pH value of fly ash was also measured in this work according to the standard SB/T10322-1999. With 3.5% mass of additive content mixed with coal, the pH value of the generated fly ash increased from 12.5 to 13. 3.3.2. Chemical Composition by X-ray Fluorescence (XRF) Analysis. XRF analysis was applied to determine the chemical composition of the collected fly ash for the two cases. The oxide composition of the fly ash produced from different fuel

Figure 9. Microstructure of the fly ash particles for (a) pure coal and (b) additive blend. 1797

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Energy & Fuels blends was compared in the form of weight percentage in Figure 10.

Figure 10. XRF results of ash composition.

It is evident that certain iron oxides, such as Fe2O3 and Al2O3, do not differ much in terms of relative proportions between the pure coal and additive blend cases because the XRF only provides information on relative proportions of elements in the form of oxides rather than their species. The majority of Fe and Al remain in the ash. The high aluminum content promotes the formation of the mullite. As a result of added CaO in the fuel blend, the proportion of CaO in the fly ash increased by more than twice that of the pure coal case. An interesting observation was that the proportions of alkali metals (such as K and Na) and alkaline earth metal (Mg) were reduced with the addition of CaO. 3.3.3. Mineralogy by X-ray Diffraction (XRD) Analysis. XRD analysis was conducted to investigate the mineralogy of the fly ash, as shown in Figure 11. The XRD patterns of the two cases showed similar results in the matching phase. The major phases of the ash were mullite, quartz, and lime. A quantitative analysis was also conducted to study the effect of the addition of CaO on the mineralogy of the fly ash, and the results are shown in Table 4. The mullite occupied more than half of the fly ash. This phenomenon agrees with the XRF results shown in section 3.3.2. Larger proportions of Si and Al promoted the formation of the mullite. The mass fraction of lime increased considerably as a result of the additive. An increase in the quartz content and a decrease in the mullite were also observed. Additionally, free lime analysis of the fly ash was conducted to further investigate the influence of the additive. The mass fraction of free lime in the fly ash for the pure coal case was 1.69%, which was much lower than the mass fraction (3.44%) in the additive blend case. The larger proportion of the free lime increased the probability of the reaction between the lime and SO2 to form more crystals of calcium sulfate, which promoted the formation of the calcium-sulfate-bonded deposits.

Figure 11. XRD patterns of the fly ash for (a) pure coal and (b) additive blend.

Table 4. Quantitative Analysis of the Mineralogy (%) phase

quartz

lime

mullite

amorphous

pure coal additive blend

3.35 5

6.7 10

56.95 50

33 35

thickness of the ash deposit was obtained through the digital imaging technique. The emissions of major pollutants, namely, NOx and SO2, were measured by the TESTO 350 flue gas analyzer. A series of analytical techniques, such as polarization microscope, physisorption analyzer, XRD, XRF, and free lime test, were conducted to determine the characteristics of the fly ash. In comparison to the frequent shedding of ash deposits in the pure coal case, the addition of CaO resulted in significant ash deposition with a maximum thickness of 66 mm. The disappearance of the shedding indicated that CaO reacts with the ash particles to change the structure and viscosity of the deposits. The reduction of emissions indicated that the additive promoted the reaction between SO2 and lime to form crystals of calcium sulfate, which played a significant role in the

4. CONCLUSION The influence of the CaO additive on the ash deposit and fly ash in a pilot-scale test rig was investigated by measuring the ash deposit growth and emission of pollutants and analyzing the fly ash. A CCD camera monitored the morphology of the ash deposit growth on the oil-cooled probe, and the change in 1798

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(17) Tomeczek, J.; Waclawiak, K. Fuel 2009, 88, 1466−1471. (18) Laursen, K.; Frandsen, F.; Larsen, O. H. Energy Fuels 1998, 12, 429−442. (19) Namkung, H.; Kang, T.; Xu, L.; Jeon, Y.; Kim, H. Korean J. Chem. Eng. 2012, 29, 464−472. (20) Zhou, H.; Zhou, B.; Li, L.; Zhang, H. Energy Fuels 2014, 28, 5756−5765. (21) Mehra, A.; Farago, M. E.; Banerjee, D. K. Environ. Monit. Assess. 1998, 50, 15−35. (22) Zheng, Y. J.; Jensen, P. A.; Jensen, A. D.; Sander, B.; Junker, H. Fuel 2007, 86, 1008−1020. (23) Wu, H.; Bashir, M. S.; Jensen, P. A.; Sander, B.; Glarborg, P. Fuel 2013, 113, 632−643. (24) Zhou, H.; Zhou, B.; Zhang, H.; Li, L.; Cen, K. Ind. Eng. Chem. Res. 2014, 53, 7233−7246. (25) Zhou, H.; Zhou, B.; Qu, H.; Lin, A.; Cen, K. Energy Fuels 2012, 26, 6824−6833. (26) Li, P.; Dally, B. B.; Mi, J.; Wang, F. Combust. Flame 2013, 160, 933−946. (27) Mei, Z.; Li, P.; Mi, J.; Wang, F.; Zhang, J. Flow, Turbul. Combust. 2015, 95, 803. (28) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 105, 511−527. (29) Li, G.; Li, S.; Dong, M.; Yao, Q.; Guo, C.; Axelbaum, R. L. Fuel 2013, 106, 544−551. (30) Chen, Y.; Wang, G.; Sheng, C. Energy Fuels 2014, 28, 136−145. (31) Li, P.; Wang, F.; Tu, Y.; Mei, Z.; Zhang, J.; Zheng, Y.; Liu, H.; Liu, Z.; Mi, J.; Zheng, C. Energy Fuels 2014, 28, 1524−1535.

formation of calcium-sulfate-bonded deposits. Although the thickness of the ash deposit with the additive was much bigger, the slope of the growth curve is smaller than that of the pure coal case as a result of the changes in the physical characteristics of fly ash particles. The fly ash produced by burning coal with the addition of CaO had a less specific surface area and a greater amount of spherically shaped particles presumably because the crystals of calcium sulfate decomposed to form the fused phase; thus, the pore structure on the particle surface was reduced. The small specific surface area could explain why the corresponding slopes of the two rapid growth stages were smaller than those for the pure coal case. The XRD results and XRF analysis indicated that a large amount of mullite was generated as a result of the high aluminum content of the coal. The results also show that the greatest effect of the additive was to increase the fraction of lime in the fly ash, which was reinforced by the free lime test. The changes caused by additive in the physical and chemical characteristics of fly ash particles had a significant impact on the more serious ash deposition.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87952598. Fax: +86-571-87951616. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51476137), the National Basic Research Program of China (2015CB251501), and the Key Technologies R&D Program of China (2012BAA12B03).

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REFERENCES

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