Ash Deposit Shedding during Co-combustion of Coal and Rice Hull

In this study, an online digital image technique was used to investigate the process of spontaneous shedding of ash deposits when burning Da Tong (DA)...
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Ash Deposit Shedding during Co-combustion of Coal and Rice Hull Using a Digital Image Technique in a Pilot-Scale Furnace Hao Zhou,* Hailong Zhang, Letian Li, and Bin Zhou State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: In this study, an online digital image technique was used to investigate the process of spontaneous shedding of ash deposits when burning Da Tong (DA) coal mixed with 20 wt % rice hull in a pilot-scale furnace. The morphology of shedding deposit, the variation in deposit thickness, and the heat flux during the spontaneous deposit shedding process were all obtained by combining the digital image technique with an oil-cooled probe. Ash samples were analyzed by scanning electron microscopy linked with energy-dispersive X-ray analysis (SEM−EDX) and X-ray diffraction (XRD) to obtain the microstructure, semiquantitative chemical composition, and mineralogy of the ash deposits. Commercial thermochemical software (FactSage) was used to calculate the thermal equilibrium and obtain the molten slag fraction and viscosity of the deposit. The process of ash deposition can be divided into four stages according to different growth rates. The shedding deposit from the probe at high temperature (probe 1) belonged to ductile deposits, while the shedding deposits from the probe at low temperature (probe 2) were brittle deposits. The duration of deposit shedding from probe 1 was about 4 min, while it lasted a few seconds for probe 2. For probe 2, the time required to build up a new deposit and shed it was 45 min. The stable heat flux through probe 2 was 180 kW/m2, and the decreasing heat flux of the initial layer was about 100 kW/m2. It was important to remove the initial layer when conducting artificial shedding. Different sootblowing media were used for removal of ductile deposits and brittle deposits. The SEM−EDX results showed that the shedding deposit originated in the sintered layer, which was confirmed by the XRD analysis results. The main shedding mechanism at high temperature was falling of molten ash deposits. Erosion and thermal shock were the main reasons for deposit shedding at a low temperature. The viscosity of the deposit was a key factor in the deposit shedding process.

1. INTRODUCTION Coal is the main fossil fuel for steam generation, as fired in coalfired power plants; the current situation will not change over a period of time in the future. This results in severe environmental problems associated with the emission of acid gases and greenhouse gases. It is extremely urgent to develop and use new and renewable energy. One expedient option is to substitute biomass fuel for fossil fuel in pulverized-fired (PF) boilers. Biomass fuel, which is considered to be CO2-neutral, provides an effective means of reducing the net emission of CO2 and SO2 because of its low contents of sulfur in biomass.1,2 Biomass fuels have some drawbacks (low energy density, low heating value, variable chemical composition, instability of biomass feedstock supply, and high investment cost) that limit their applications.3,4 It is therefore a better option to use biomass by co-firing coal with biomass, which can ensure boiler efficiency at the same time as it improves the economical and environmental performance.5 The main inorganic components in biomass are potassium, silicon, calcium, sulfur, and chlorine, resulting in a propensity to generate high-ash residue. Biomass containing the inorganic constituents, such as alkali oxides and salts, further aggravates the ash deposition problem, as well as fouling and slagging.6 If it is not removed, ash deposited on the heat-transfer surface reduces both convective and radiative heat-transfer efficiency, raising the flue gas temperature, resulting in the reduction of the system efficiency and increasing corrosion problems in the boiler, which may result in reduced generating capacity and unscheduled shutdown.7,8 © 2013 American Chemical Society

Therefore, it is important to guarantee the efficiency of ash deposit removal. “Deposit shedding” is the term used to describe the process of removing the ash deposit from the heat-transfer surface, which may be performed artificially or may occur spontaneously. The main causes of spontaneous deposit shedding are erosion, gravity, or thermal shock.9 Erosive shedding occurs when the solid deposit is impacted by solid particles with high speed, causing it to fall.10 Gravity shedding occurs when the weight of the deposit exceeds its adhesion force.11 Thermal shock, which is the main cause of deposit shedding, occurs when the load of fuel in the boiler undergoes a sudden change.12 Artificial shedding techniques, such as sootblowing, are the main means of removing deposits online. However, the high consumption of steam for sootblowing causes a significant reduction in the efficiency of the boiler.13 The mechanical strength of deposits is an important parameter in determining the deposit removal efficiency. The strength of the deposits is obtained principally through solidification of molten or partially molten particles and sintering of solid particles, and it is determined by the chemical composition, flue gas temperature, and porosity of the deposit.14 Consequently, an understanding of the deposit shedding mechanism for boiler operation is essential. Several studies of ash deposit shedding behavior have been reported. Mathematical models have been developed to help in the understanding of ash deposit shedding behavior and Received: September 10, 2013 Revised: October 22, 2013 Published: October 24, 2013 7126

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Figure 1. Schematic diagram of the slagging test rig.22

Figure 2. Method of digital image processing: (left) original image and (right) edge image.

then used for optimizing operation parameters. Oka et al.15 proposed an erosion model for estimating the extent of damage caused by solid-particle impact erosion and showed that the effective parameters, including impact velocity, impact angle, particle characteristics, and strength of the deposit material, were key causes of erosion damage. Theoretical predictions of a dynamic mechanistic model of deposit shedding on a horizontal probe in a straw-fired grate boiler developed by Zhou et al.16 were supported by experiments, in which the weight of the deposit on the probe, the heat uptake, and the shape of the deposit predicted by the mechanistic model were consistent with experimental results. Although these models predicted the deposit shedding behavior to some extent, their scope and

complexity limited the application and influenced the accuracy. For a better understanding, a precise measurement experiment was needed to monitor the process of deposit shedding online. Kaliazine et al.17 reported an investigation in which a scaled laboratory apparatus was used to investigate how the strength and thickness of the deposit and the sootblower jet characteristics affect deposit removal. They presented a simple quantitative criterion related to the tensile strength of the deposit and the sootblower peak impact pressure (PIP) required to remove a deposit. A newly developed cooleddeposit probe has been applied to study the deposit shedding process by video recording in a straw-fired grate boiler. The heat uptake and deposit mass were investigated during the 7127

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circulation, and flue gas analysis. Four type S thermocouples were installed in the furnace, positioned at 410, 1160, 1910, and 2600 mm from the burner exit, to monitor the temperature at these different locations. On the basis of the different temperatures, the furnace was divided into four stages (vertically downward), referred to below as the first, second, third, and fourth stages. When the furnace temperature stabilized, two stainless-steel deposition probes were inserted into the center of the furnace, one 1910 mm from the burner nozzle exit and one vertically below it 2660 mm from the burner nozzle exit. Two CCD cameras were positioned opposite and parallel to the two probes for the purpose of monitoring the deposit shedding online. Each probe consisted mainly of a deposit sampling section screwed into an oil-cooled tubular section. The deposit sampling section, cooled by heat-conducting oil, was a hollow cylinder (length, 76 mm; inner diameter, 27 mm; and outer diameter, 40 mm). The deposit sampling section was fitted with two thermocouples in the radial direction to measure the temperature of its inner and outer surfaces and, thus, estimate the heat flux through the probe. DA coal was chosen as the main fuel, with rice hulls as an additive fuel. The probes were carefully removed and cooled after 4 h. Each

process.18 Bashir et al.19 studied the deposit shedding process using an advanced online deposit probe and a sootblowing probe in a 350 MWth suspension-fired boiler, using a straw and wood fuel. They also quantified the influences of feedstock, flue gas temperature, probe surface temperature, and probe exposure time on deposit shedding. A laboratory study of the mechanism of deposit debonding from the tube surfaces has been carried out, in which the effects of major aerodynamic and geometric parameters on the debonding mechanism were also investigated.20 Another study found that the jet must drill into the deposit and quickly fracture it to effectively break a deposit and, in addition, that the sootblower jet flow and penetration between platens were strongly affected by any interaction between the jet and the first tube of the platen.21 While no shedding experiments have been conducted for a coal−biomass mix, some studies have been conducted on a biomass-fired boiler. Researchers mentioned above studied deposit shedding by simulation or experiment. Little research on deposit shedding has been performed using a digital imaging technique, such as a charge-coupled device (CCD) camera. The goal of this work was to investigate the morphology of the shedding deposit, the variation of deposit thickness, and the heat flux during the spontaneous deposit shedding process when firing a mixture of Da Tong (DA) coal and rice hull in a pilot-scale furnace. The process was monitored online using a digital imaging technique. Ash deposition probes cooled by heat-conducting oil were used to simulate the water wall. Samples of the shed ash deposit were collected during the shedding process and subsequently analyzed for mineralogy, quantitative chemical composition, and microstructure by scanning electron microscopy linked with energy-dispersive X-ray analysis (SEM−EDX) and X-ray diffraction (XRD) to help explain the spontaneous shedding mechanism.

2. EXPERIMENTAL SECTION 2.1. Combustion Facility. The pilot-scale slag formation test arrangement is shown in Figure 1. The experimental rig consisted of a vertical furnace with systems for fuel feed, oil ignition, temperature measurement, image acquisition, sampling the deposited ash, oil

Figure 3. Crushed rice hull after milling.

Table 1. Properties of Fuels Tested fuel

DA coal rice hull

proximate analysis (wt %, ad)

moisture volatile matter fixed carbon ash ultimate analysis (wt %, db) carbon hydrogen nitrogen sulfur oxygen heating value (MJ/kg, db) high heating value (HHV) ash fusion temperature (°C) initial deformation temperature (IT) softening temperature (ST) hemispherical (HT) flow temperature (FT)

7.22 28.56 54.40 9.82 68.28 4.19 0.91 0.75 8.83 26.89 1369

10.07 59.98 14.34 15.61 36.73 4.13 0.41 0.07 15.61 14.758 >1500

1398 1408 1435

>1500 >1500 >1500

Figure 4. Size distribution of the rice hull.

Table 2. Chemical Composition of Ash (wt %) DA coal rice hull

Al2O3

SiO2

Fe2O3

CaO

MgO

Na2O

K2O

P2O5

MnO2

TiO2

19.4954 0.1607

48.0782 94.713

11.4912 0.08136

16.602 0.5592

1.3075 0.6179

0.6185 0.104

1.0757 2.4438

0.4906 1.1658

0.126 0.1534

0.7149 0.0009

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probe to the inner thermocouple, 15 mm; λ is the thermal conductivity of the metal probe, 22.5 W m−1 K−1; and r0 is the outer radius of the probe, 20 mm. 2.4. Fuels. The proximate analysis, ultimate analysis, ash fusion temperature, and calorific value of DA coal and rice hull were given in Table 1. Both of the two fuels had a high ash fusion temperature, especially the rice hull. The ash fusion test of the rice hulls was conducted on the same basis as the standard test for coal. The volatilization of alkali metal elements in the process of preparation for ash samples resulted in a rise of the ash fusion temperature. The chemical compositions of both the coal ash and rice hull ash were evaluated using X-ray fluorescence (XRF) spectroscopy; the results are listed in Table 2. The DA coal was rich in Al, Si, Fe, and Ca, which caused its high ash fusion temperature. The ash content for air-dried samples of rice hulls was high (15.61 wt %), as was the volatile content (almost 60 wt %). Silicon was the dominating element in the ash of rice hull (almost 95 wt % calculated as oxide in the ash). Before being introduced into the furnace, the rice hulls were ground and sieved to fine particles less than 0.6 mm (see Figure 3). The size distribution of the rice hulls is shown in Figure 4. The coal was pulverized with 85% in the mass fraction less than 70 μm measured by a Malvern particle size analyzer, as shown in Figure 5. 2.4. Experimental Condition for the Pilot-Scale Experiment. A fuel mixture comprising 80:20% by weight coal/rice hull was studied. In all experiments, the maximum thermal load of the furnace was 350 kW and the excess air ratio was 1.2 to ensure an oxidizing atmosphere. In addition, the furnace temperatures of the third and fourth stages, where the ash deposition probes were inserted, were

deposit was then gold-coated to avoid charge effects and subjected to SEM−EDX and XRD analyses. A more detailed description of the experimental arrangement has been given in a previous study.22 2.2. Calculation of Deposits Thickness. The process of deposit shedding was recorded by the CCD cameras; the large number of images were most conveniently handled by MATLAB software. The maximum height of deposits (h) on the deposition probe was chosen to represent the thickness of deposits. A series of images were extracted by MATLAB and converted into 24-bit images. The Canny operator was used to detect the edges of the images. The center and radius of the metal probe, which showed as a circle in the edge image, were extracted by the Hough transform digital image-processing algorithm. The actual value of the deposition probe outer diameter (D1) can be measured. If thermal deformation is neglected, the outer diameter of the probe was constant throughout the experiment. The number of pixels representing the probe diameter (PD) and the maximum deposit thickness (Ph) were then calculated. Consequently, h was obtained from

h = D1Ph /PD

(1)

Figure 2 illustrates the method of digital image processing, showing the original slag formation image (left) and the edge image (right). 2.3. Calculation of Heat Flux. The deposit built up the radial direction and formed a cylinder. Consequently, a cylindrical heat conduction equation was appropriate for calculating the heat flux. The inner and outer surface temperatures, Tinner and Touter, were measured by thermocouples. The heat flux through the outer surface was selected to represent the heat-transfer efficiency. The heat flux, q, is given by q=

λ(Touter − Tinner) r0 ln(router /rinner)

(2)

where router is the distance from the center of the probe to the outer thermocouple, 19 mm; rinner is the distance from the center of the

Figure 5. Size distribution of the DA coal.

Table 3. Experimental Conditions coal blended with rice hull

fuel thermal load (kW) excess air ratio gas velocity (m/s) furnace temperature (°C) oxygen concentration at the furnace outlet (%) exposure time for ash deposition (min)

third stage fourth stage

350 1.2 ∼2.8 ∼1200 ∼1050 4.0−5.0 240

Figure 6. Photo of ash deposits from different deposition probes. 7129

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Figure 7. Cross-sections of slags collected from different probes. maintained at 1200 and 1050 °C within a 2% inaccuracy margin, respectively. The experiment continued for 240 min. Detailed parameters were given in Table 3.

and layer 3 (the slag layer), according to its colors, thickness, hardness, and porosity (see Figure 7a). Layer 1 was thin, less than 0.5 mm, and was loose, with low porosity, and unsintered. Layer 2 (sintered layer) was thin and not molten. The slag layer (layer 3) was partially molten, dense, and difficult to remove. Many pores at the surface of the slag layer were filled. The shedding deposits of probe 2 had a similar morphology but without a layered structure and was loose, easily removed, and partially sintered. The deposit on probe 2 showed a two-layered structure: layer 1 (the initial layer) and layer 2 (the sintered layer). There was no layer 3 (the slag layer) because the low ambient temperature was insufficient to the particles. It can be concluded that the melt of the sintered layer formed the slag layer. The deposit showed a propensity to be shed at the interfaces of the layers because of their different properties. 3.2. Shedding Process of the Deposits from Ash Deposition Probes. A digital image technique was applied to obtain the deposit shedding process online. In this study, the frame rate of the CCD camera was set to 3 frames/s, which produced a film during the experiment. Consequently, one image every 5 min was extracted from the film to calculate the thickness of the deposit by the digital image processing technique based on MATLAB. The different slopes of the deposit growth curves implied that the growth process of the deposit on the deposition probe occurred in four stages:22 (1) initial layer formation, (2) rapid

3. RESULTS AND DISCUSSION 3.1. Visual Evaluation of the Slagging. The morphology of the ash deposits from the two probes was shown in Figure 6. The shedding deposit from probe 1 was not found. The surface of slag on probe 1 was sintered and partially molten, as shown in Figure 6a. However, the temperature of the slag surface was less than the ash fusion temperature. It was found that significant partial melting of the ash occurred at temperatures as low as 200−400 °C, less than the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test.23 Figure 6b showed the deposits collected from probe 2. The order of the five deposits from left to right was as follows: deposit from first shedding, deposit from second shedding, deposit from third shedding, deposit from fourth shedding, and deposit on probe 2. There were no significant differences between the four slags, which were all loose, easy removable, and partially sintered. There were obvious differences between deposits collected from probes 1 and 2 because of the different ambient temperatures at the location of the probes. Figure 7 showed the cross-sections of the deposits in Figure 6. The deposit on probe 1 appeared as a clear, three-layered structure: layer 1 (the initial layer), layer 2 (the sintered layer), 7130

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Figure 8. CCD image of the ash deposit shedding process of probe 1.

Figure 9. Calculated results of viscosity as function of the temperature.

Figure 10. Calculated results of the molten slag fraction as function of the temperature.

growth stage, (3) slow growth stage, and (4) stable (no growth) stage. The thickness of the deposits collected from probe 1 with increasing time has been analyzed before. Deposit shedding process images were recorded by the CCD camera, as shown in Figure 8. Deposit shedding began to occur at 52 min and 15 s (52′ 15″) after commencement of the experiment. The deposit was in the molten state, began to slide from the

probe, and then fell off in one piece without any liquid ash drop being observed, possibly because of its high viscosity. The viscosity was negatively correlated with the temperature. It indicated that the ambient temperature was not high enough to melt deposit into the ash liquid drop. Consequently, the deposit 7131

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FactSage calculation was 0.1 MPa. The initial and final temperatures for the FactSage calculation were 1000 and 1600 °C, respectively, in steps of 10 °C. Figure 9 showed the calculated results of viscosity as function of the temperature. The viscosity of the ash deposit dropped sharply between 1000 and 1300 °C, implying that most of the deposit melted within this temperature range. The viscosity then decreased slowly and finally reached equilibrium when the deposit was completely melted. The molten slag fraction remained stable below 1100 °C, as shown in Figure 10 (slag 1 + slag 2), and then increased rapidly until the temperature reached 1300 °C, which was confirmed by the results of viscosity. Liquid immiscibility was taken into consideration in the molten slag fraction calculations. The molten slag fraction of slag 1 showed the same trend as the total molten slag fraction. Slag 2 began to appear at 1075 °C and increased to its maximum value at 1025 °C. Subsequently, the molten slag fraction of slag 2 decreased to zero at about 1575 °C. It can be concluded that slag 1 played the dominant role in the melting process. The viscosity of deposit was about 800 Pa s, and the molten slag fraction was about 70% at 1200 °C, which was the temperature at which probe 1 was inserted. Consequently, the deposit was classed as a high-viscosity ductile material, for which no liquid ash drop formed. Figure 11 showed the variation of deposit thickness with time for probe 2. The thickness decreased rapidly in stage CO1, stage C1O2, stage C2O3, and stage C3O4, corresponding to the first deposit shedding, the second deposit shedding, the third deposit shedding, and the fourth deposit shedding. The first deposit shedding occurred at 75 min and 27 s (75′ 27″), as shown in Figure 12a. The deposit dropped from probe 2 abruptly, and the process took only 1 s. The duration was so short that the CCD camera did not capture it. The deposit was not melted as a result of the low ambient temperature. The main reason of the first deposit shedding may be erosion caused by non-sticky particles (mainly quartz, which was abundant in the rice hull ash) colliding with a non-sticky deposit surface. The second deposit shedding occurred at 117 min and 16 s (117′ 16″) and lasted for 4 s (Figure 12b). The third deposit shedding occurred at 159 min and 23 s (159′ 23″) and lasted for 2 s (see Figure 12c). The fourth deposit shedding occurred at 205 min and 32 s (205′ 32″) and lasted for 2 s (see Figure 12d). The three latter deposits had an “exploded” appearance, from which it was inferred that the main mechanism in these three cases was thermal shock because of the differential in thermal expansion between the deposit and the metal probe. The frequency of deposit shedding was roughly one event per 45 min; this was presumed to be related to the ash deposition rate, the properties of the deposit material, and the ambient temperature. The four shedding deposits were brittle and fell off suddenly under low strain. Consequently, the deposits would easily be removed by sootblowing. 3.3. Heat Flux of the Deposits on Probe 2. The plot of heat flux through the second probe versus time for probe 2 was

dropped gradually from the probe, and the deposit shedding process lasted for 4 min. However, the shedding deposit from probe 1 was not collected. Parts of the deposit fell off, leaving the initial layer on the probe (see panel M-c of Figure 8), explaining why the ash deposit grew sharply after shedding. The adhesion force between the initial layer and the probe was larger than the intermolecular force between the initial layer and the sintered layer. The deposits can be divided into ductile (plastic) deposits and brittle deposits according to the fracture behavior. The first melting temperature (FMT) was the temperature at which the deposit began to melt and the pellets began to form. For deposits that contain no chloride, the brittle−plastic transition occurred near the FMT, which was 200−400 °C lower than the IDT.14 The shedding deposit from probe 1 was classed as ductile and underwent considerable deformation before rupturing (Figure 8). The main factor of deposit shedding was the removal of molten deposits because of the balance of the gravity, adhesion strength, and tensile strength acting upon the surface of the deposit. Viscosity was the key factor for the flow of the deposit when molten. The main shedding mechanism at high temperature was the falling of molten ash deposits as a result to gravity. We tried to evaluate the flow properties of the deposit by chemical equilibrium calculations using FactSage software. The ash composition of Al2O3 (15.75 wt %), SiO2 (57.84 wt %), Fe2O3 (9.28 wt %), CaO (13.50 wt %), MgO (1.18 wt %), Na2O (0.52 wt %), K2O (1.36 wt %), and TiO2 (0.57 wt %) was input in the table of the software. The oxidizing atmosphere was employed in the FactSage calculation. The pressure for the

Figure 11. Deposit thickness as function of time on probe 2.

Table 4. Porosity of Deposits Collected from Probe 2 first shedding

second shedding

third shedding

fourth shedding

layer 1

layer 2

3.069

2.695

3.635

2.736

1.952

2.532

Table 5. Chemical Composition of Ash Deposits Collected on Deposition Probe 2 Analyzed by SEM−EDX sample probe 2

first shedding second shedding third shedding fourth shedding deposit on probe layer 1 deposit on probe layer 2

Na2O (wt %) MgO (wt %) 0.5 0.6 0.6 0.6 0.6 0.4

1.2 1.1 0.9 0.9 1.1 1.1

Al2O3 (wt %) SiO2 (wt %) 10.6 10.4 10.1 9.9 15.1 10.6 7132

62.1 62.8 64.1 64.4 55.1 63.6

K2O (wt %)

CaO (wt %)

TiO2 (wt %)

Fe2O3 (wt %)

1.6 1.5 1.5 1.5 1.8 1.5

9.9 9.8 9.3 9.3 11.3 9.4

1.2 1.0 1.0 1.1 1.0 1.0

12.8 12.7 12.6 12.2 13.9 12.4

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Figure 12. CCD image of the ash deposit shedding process of probe 2.

shown in Figure 13. The heat flux peaked at 350 kW/m2 when the probe was first inserted into the furnace, that is, when the thermal resistance (equal to the thermal resistance of the metal probe) was at its lowest value. The heat flux then decreased with time to a steady value, which fluctuated through a range consistent with the growth of the ash deposits. It was clear that the decreasing heat flux indicated three modes of deposition: (1) The initial stage, stage OA, during which the heat flux

decreased sharply; this process continued for about 10 min. This was because the initial layer had the greatest resistance, even though it was quite thin. Once the initial layer has formed, further growth of the deposit had little effect on the loss in absorption of thermal energy from flue gas.24 (2) The slow decrease stage, stage AB, during which the heat flux showed only a moderate decrease with time. (3) The stable stage, stage BC, during which the heat flux achieved a stable state and 7133

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retained inside the ash deposit, together with the differential shrinkage of the mineral phases in the ash.28 Also, melted or partially melted individual particles became attached to adjacent particles, causing an increase in thermal conductivity. The sintered layer (layer 2) was similar to the shedding deposit. It implied that the shedding deposits were from the sintered layer. Large amounts of mineral phases were transformed into amorphous phases, which was confirmed by XRD analysis results. In the SEM images, the pores appeared black and the deposit particles were light gray. It was assumed that the porosity was randomly distributed. The surface area porosity can present the volume porosity in accordance with Delesse’s principle.29 The porosity was thus equal to the percentage of black pixels in the whole image. The images were converted to greyscale images, and a gray threshold was set such that areas exceeding a gray value of 245 were regarded as pores. The porosity of the deposits was determined using MATLAB software; see Table 4. The porosity of deposits increased from layer 1 to layer 2. The main potassium-containing substance was KCl at a low temperature. At a high temperature, KCl reacted with SO2, H2O, and O2 to form K2SO4, releasing HCl. HCl was trapped in the interior of deposits to form pores. The temperature of the deposit particles also increased from layer 1 to layer 2, which facilitated the sulfuration reaction and increased the porosity of layer 2. The porosity influenced the mechanical properties and the erosional performance of the deposit. It was found that, in a low porosity deposit (2.5%), the erosion removal occurred mainly by intergranular cracking and chipping. Mass erosional removal occurred at higher porosity (e.g., 17.8%).30 The SEM−EDX analysis results normalized to 100% were presented in Table 5, which showed that the deposits comprised mainly of SiO2, Al2O3, CaO, and Fe2O3, which accounted for about 95% of the total. The results indicated that the deposit might be rich in calcium aluminosilicate. This was confirmed by the XRD analysis results. The ash of rice hull was rich in K2O, which dominated the process of ash deposition to form the initial layer. The outer layer agglomerated through a process involving inertial impact of large particles containing Si, Al, Ca, and Fe compounds. Some potassium compounds, mainly KCl or K2SO4, acted as a bonding agent between separate ash particles31 and led to the increased viscosity of the ash deposit discussed at some length above. Therefore, rice hull caused severe ash deposition. Figure 15 showed the composition of the ash deposits from probe 2. The initial layer (layer 1) formed by the volatilization and condensation of K2O. From layer 1 to layer 2 in the radial direction of the ash deposit growth, the contents of CaO, Fe2O3, Al2O3, and K2O decreased, while the contents of SiO2 and MgO increased slightly. The selective deposition of elements resulted in the layered structure. The sintered layer and slag layer contained more metal elements, which increased the thermal conductivity. Consequently, the heat flux can be divided into three modes of deposition. The composition of the shedding deposit from probe 2 was similar to that of the sintered layer (layer 2) of the deposit on probe 2. It indicated that the shedding deposits were from the sintered layer. The different compositions of the two layers of deposit resulted in differential thermal expansion and viscosity. The initial layer closest to the metal probe was cooler than the sintered layer close to the hot flue gas. The viscosity of the

Figure 13. Heat flux through the second deposition probe versus time.

fluctuated at a certain value. The stable heat flux was 180 kW/m2. Deposit shedding occurred at stage A1C, stage A2B1, stage A3B2, and stage A4B3 during the experiment. The heat flux rose to about 250 kW/m2 after deposit shedding. The decreasing heat flux of the initial layer was about 100 kW/m2. The heat flux graph appears as a sawtoothed curve because of successive shedding of the deposit. This resulted in a fluctuating of thermal load, and it was necessary to take measures to ensure the stability of the boiler load. Sootblowing was the most common way of removing the deposit, but it can consume a large amount of steam and, thus, reduce the efficiency of the boiler. The time, pressure, and angle of sootblowing should ideally be optimized to ensure a maximum continuous boiler rating, but these factors are different for ductile deposits and brittle deposits. Different sootblowing media are therefore used. Most ductile deposits were molten, sintered, and strongly adhered to the surface. Water was the most effective medium for removing heavily sintered or slagged deposits. Liquid water sprayed from a nozzle quickly expands to steam, creating a pressure wave.14 Brittle deposits, on the other hand, are easily removed by sootblowing using a jet of high-pressure compressed air, which initiates brittle failure on the deposit surface and causes it to break apart.25 Detonation-wave sootblowers have also been reported to be effective.26 In addition, a useful technique is to treat the coal with an additive to reduce the ash fusion temperarure. 3.4. Microstructure and Chemical Composition of the Shedding Deposits. The shedding deposits from probe 1 were not found; only the shedding deposits from probe 2 were analyzed. Each layer of the ash deposits collected from probe 2 was analyzed by SEM−EDX to determine its microstructure and distribution of elements, with the aim of contributing to an explanation of the spontaneous deposit-shedding mechanism. None of the SEM micrographs of cross-sections of the shedding deposits from probe 2 (Figure 14) showed any layering but varied only in porosity. The surfaces were rough and partially sintered. The deposit on probe 2 appeared as a clearly layered structure. The initial layer (layer 1) in contact with the metal probe consisted mainly of fine particles with a diameter of