Study on the Fragmentation Behaviors of Deposited Particles on

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Study on the Fragmentation Behaviors of Deposited Particles on the Molten Slag Surface and Their Effects on Gasification for Different Coal Ranks and Petroleum Coke Zhongjie Shen,† Qinfeng Liang,† Jianliang Xu,† Haifeng Liu,*,† and Kuangfei Lin‡ †

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Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education and Shanghai Engineering Research Center of Coal Gasification and ‡State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Post Office Box 272, Shanghai 200237, People’s Republic of China ABSTRACT: Coal char particles were deposited on the molten slag surface and continued to react with the gas near the gasifier wall slag surface in an entrained flow gasifier, which significantly affected the total carbon conversion and slag flow properties. The current research work is to investigate the fragmentation and gasification characteristics of coal char particles, which were captured on the molten slag surface. A high-temperature stage microscope was used to study and analyze the particle evolution and char gasification process on the molten slag surface with the effects of different coal ranks. Results indicated that char particles first shrank and then broke into several fragments during the gasification on the molten slag surface. Char particles with fragmentation behaviors on the molten slag surface had higher reaction rates and carbon conversions than the particles in the shrinking period. The initial fragmentation time of a char particle showed an increasing linear relationship with the initial particle size and coal rank. The number of fragments for a char particle displayed a relationship of Gaussian function distribution with the reaction time. A fragmentation intensity factor defined in this study increased with the contents of moisture and volatiles in the raw samples, which means that char particles with a low coal rank could easily break into fragments on the molten slag surface. The ash content in the raw sample also affected fragmentation and gasification behaviors of char particles when reacted with CO2 on the slag surface.

1. INTRODUCTION The entrained flow gasification technology is widely used in the chemical industry and power generation, with its high conversion efficiency and multi-coal.1,2 The operating temperature in an entrained flow gasifier is controlled above the ash fusion temperature to create slagging conditions with the fusion behaviors of deposited ash particles. Then, the melting behavior of the coal ash layer caused the slagging condition and the consequent high deposition rate of coal ash and slag particles.3 The slagging condition is of importance to the stable operation of an entrained flow gasifier. Yong et al.4,5 simulated the slag flow behavior with a particle capture model, and the simulation results showed that the particle with a slower velocity could deposit on the slag layer and continue to build up the layer. Pednekar et al.6 also proposed a dynamic model with the deposition fluxes of coal char and slag particles, and they also predicted the effect of deposited particles on the thickness of the slag layer. The deposition behaviors of coal ash, slag, and char particles were studied and found to be of importance to the slag flow properties.7−10 Researchers have used experimental methods,9,11−14 modeling methods,6,7,15,16 or numerical simulation10,17−19 to study the particle deposition behaviors for optimizing the gasifier structure and achieving the high-efficient carbon conversion. However, residual carbon was still found in the slag from the industrial entrained flow gasifier, which mainly came from the partly gasified or unreacted particles.20 Therefore, the deposition and consequent reaction of the unreacted particles on the molten slag © XXXX American Chemical Society

layer had an important influence on the total conversion of carbon in an entrained flow gasifier system. The impaction of char particles on the gasifier wall was dominant in the mode of slag deposition at the beginning region of a gasifier.6 Montagnaro et al.21,22 and Ambrosino et al.23,24 found that char particles would be entrapped inside the slag melt, and then char particles covered the slag layer from dispersing to dense. With the dissipation of momentum, char particles impacted and promoted the formation of the densedispersed layer near the wall of an entrained flow gasifier.25 Walsh et al.11 found that the probability of char deposition was dependent upon the viscosity of molten slag, while a probability of char deposition considered as a parameter was related to the slag viscosity and char particle.26 The diverse chemical compositions of coal slag, various slag viscosities, and char particles with the sticky or non-sticky properties increased the complexities of deposition behaviors. Troiano et al.25,27−29 carried out the deposition experiments of char particles and developed a phenomenological model for particle deposition. The deposition rates of coal char, ash, or slag particles were found dependent upon the carbon conversion, particle size, gas flow field, density, etc.6,25,28−30 More importantly, char particles on the molten slag layer continued the reaction with the gasification agents in the nearwall region. A slag flow model combined with a wall reaction Received: June 13, 2018 Revised: August 2, 2018 Published: August 3, 2018 A

DOI: 10.1021/acs.energyfuels.8b02053 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of Experimental Samples Used in This Study proximate analysis (wt %, air-dried basis) lignite coal bituminous coal anthracite coal petroleum coke lignite char bituminous char anthracite char

ultimate analysis (wt %, air-dried basis)

Mad

Vad

Aad

FC

C

H

O

N

S

3.100 2.490 1.280 0.780 1.960 0.950 0.900

30.33 20.55 8.010 7.420 5.640 3.240 1.080

15.37 8.480 20.23 2.960 30.00 10.60 23.41

51.20 68.48 70.48 88.84 62.40 85.21 74.61

57.92 69.21 67.61 88.92 64.60 84.96 72.66

4.160 4.520 2.510 3.152 0.230 0.427 0.382

18.11 11.78 5.080 0.337 1.937 0.503 0.717

0.880 1.010 0.560 1.410 0.880 0.820 0.480

0.460 2.510 2.730 2.441 0.393 1.740 1.451

Table 2. Chemical Components and Contents of Coal Ash Samples Used in This Study composition (wt %)

SiO2

Al2O3

CaO

MgO

Fe2O3

K2O

TiO2

Na2O

bituminous coal lignite coal anthracite coal

43.87 26.77 53.77

15.43 42.53 23.73

22.86 10.27 12.53

0.840 1.280 0.560

12.44 17.12 6.660

0.970 0.620 0.740

0.900 0.510 0.460

2.690 0.900 1.550

were also measured and analyzed. The effects of different particle sizes and the contents of moisture and volatiles on the fragmentation behaviors of particles on the molten slag surface were also studied. Finally, the carbon conversion and gasification rate were calculated and compared between the fragmentation period and non-fragmentation period.

model for the char−slag interaction was developed by Wang et al.31 Researchers also found that the carbon conversions and gasification rates of coal/char particles were sensitive to the slag and were higher than those without the slag.32−34 Shen et al.35 found that the molten slag promoted the char gasification while hindered the char combustion during the reaction of captured chars on the slag layer. According to the research of Xu et al.,36 there were about 22.5% of char particles reacted on the molten slag wall in an entrained flow gasifier. A large amount of the carbon particle reacted with near-wall gas on the molten slag layer, especially in the industrial gasifier, with larger quantities of coal per day. The gasification reaction and carbon conversion of deposited char particles would affect the overall carbon conversion in the whole gasification system. In addition, the particle size distribution of the pulverized coal sample used in the gasifier was not homogeneous, although the size was usually below 75 μm, and several large particles were still found more than 100 μm inside the milled coal sample. With a large capacity of energy storage, larger particles additionally affected the fuel transportation, carbon conversion, and time. Therefore, the effect of a larger particle size needs to be studied for the gasification of coal/char particles on the slag layer. For the fragmentation behaviors of different coals, the effect of devolatilization (volatiles and moisture in the raw coal sample) on the development of the pore structure, which differed from the coal ranks, and its subsequent impact on the fragmentation behavior on the molten slag surface are seldom in the literature. The pore structure and ash content are important parameters, which would affect the diffusion of reactant gases, fragmentation behavior, etc. Besides, the high surface tension of molten slag and its effect on the char gasification and particle evolution (e.g., the fragmentation behavior) also need to be considered and studied. Therefore, the gasification of deposited particles on the molten slag surface with the effects of different coal ranks and structures acquires comprehensive and in-depth research. This work is to investigate the fragmentation and gasification characteristics of deposited char particles on the molten slag surface with the effects of coal ranks compared to the experimental results for the petroleum coke particles. The particle evolution and gasification process were recorded and analyzed. The fragmentation behaviors of different types of particles on the molten slag surface, including the number of fragments, initial fragmentation time, and fragment intensity,

2. EXPERIMENTAL SECTION 2.1. Experimental Material. Three coal samples from China, including a lignite coal, a bituminous coal, and an anthracite coal, were chosen to prepare the coal char samples in this study. Herein, the lignite coal char (LC), bituminous coal char (BC), anthracite coal char (AC), and petroleum coke (PC) were denoted as abbreviations for convenience. The fast pyrolysis of raw coal samples was carried out in a drop-tube furnace (DTF) in the Ar atmosphere. The gas flow rate of Ar was set to 0.2 L/min, and the feed rate of the coal sample was set to 2 g/min. The temperature for pyrolysis was set to 1300 °C in the DTF. The furnace contained a corundum tube (2.2 m length, Al2O3, purity of 99.9%) with a length of 1.2−1.3 m in a constant temperature zone. According to the free-fall formula, the residence time for a single char particle is about 0.7 s. With the effect of gas flow, the residence time of a char particle would be larger than 1 s. From the study of Cui et al.,37 when the pyrolysis time was above 100 ms, the mass loss of coal particles kept a nearly constant value at 1300 °C, which means that the rapid pyrolysis happened below 100 ms at a high temperature. These results were also found in different types of coals (e.g., lignite coal, bituminous coal, and anthracite coal). This residence time in the DTF could approve the pyrolysis of the raw coal sample in the DTF. The proximate and ultimate analyses of experimental samples used in this study are given in Table 1. From Table 1, there were a certain content of moisture and residual volatiles, which were difficult to remove, and similar results were also found in the literature.25,38,39 However, when the char samples were heated to the setting temperature in the high-temperature stage microscope, the residual volatiles were also reduced to less before the gasification with CO2 on the molten slag surface. Therefore, the residual content of volatiles in the char samples were ignored in this study. The coal ash sample, which was prepared in the muffle furnace at 815 °C, was melted in the ceramic crucible to create the slagging condition. The ash fusion temperatures (AFTs) of three coal ash samples were detected by the ash fusion point determination meter, which was produced by Changsha Kaiyuan, China. The chemical compositions of coal ash samples were detected by X-ray fluorescence, which was made by Thermo Fisher Scientific, Waltham, MA, U.S.A. The results of XRF analysis were given in Table 2, and the data of the AFTs were given in Table 3. From Table 2, three different coal samples used in the experiment contained different contents of the B


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could ensure the accuracy of the calculation of the reaction time. The video of the gasification process was shifted to a different number of images using a format software based on the reaction time differing from coal chars and petroleum coke. Finally, the ImageJ software40 was applied in this study to measure the parameters for the evolution of experimental particles (e.g., cross-sectional area of the particle, particle diameter, and fragment number). Particles were assumed to be spherical particles because the residual coal ash after gasification on the surface actually melted into the subjacent liquid slag. The evolution of the cross-sectional area could show the shrinkage of a char particle during gasification on the molten slag surface. From a shrinking line for the cross-sectional area of a char particle, the time when the area started to decrease was the initial reaction time. Finally, when the gasification reaction completed and the particle disappeared on the liquid slag surface, the total reaction time was calculated.

Table 3. AFTs of Coal Ash Samples Used in This Study temperature bituminous coal lignite coal anthracite coal

deformation temperature (DT, °C)

softening temperature (ST, °C)

hemisphere temperature (HT, °C)

flow temperature (FT, °C)

1164

1171

1193

1214

1248 1173

1298 1246

1302 1257

1308 1270

chemical components. However, the coal ash sample with a higher AFT was not suitable for the melting experiment and the subsequent gasification experiment for observation and recording via a video as a result of the requirement of a high melting temperature. Therefore, the bituminous coal ash with a lower AFT was chosen as the experimental sample for the slagging condition. 2.2. Experimental Method. The high-temperature stage microscope (HTSM) was used to carry out the gasification experiments of char particles on the molten slag surface, and the diagram of HTSM was shown in Figure 1. The detailed introduction of the HTSM was

3. RESULTS AND DISCUSSION 3.1. Gasification on the Molten Slag Surface. The whole gasification processes of the particles for different coal ranks and petroleum coke on the molten slag surface are shown in Figure 2. The completed reaction time for the lignite coal char was about 25 s, while the conversion time decreased to 20 s for the bituminous char. For the coal char particles of high coal rank, the reaction time of anthracite coal char was much longer than both the lignite char particle and bituminous char particle. Similarly, the reaction time of petroleum coke was obviously prolonged, which was much longer than the other three coal char samples. In combination with the scanning electron microscopy (SEM) images of coal chars and petroleum coke in Figure 3, it was found that the apparent pores of lignite char and bituminous char were more obvious than the pores of anthracite coal char and petroleum coke. Pore structures could also promote the gasification and reduce the reaction time. Therefore, the reaction time of char particles with lower coal ranks was shorter than the time for char particles of higher coal rank. However, the gasification reaction was also hindered by the ash layer on the char particle surface, owing to the high ash content in the lignite char. From Figure 2, it was also found that both the char particles and petroleum coke particles on the liquid slag layer first shrank and then broke into fragments at different reaction times. The fragmentation behavior was found for all of the samples during the gasification on the liquid slag layer. Accordingly, particle evolution for the coal char particle and petroleum coke on the molten slag surface was divided into two parts: particle shrinking and particle fragmentation. The fragmentation behavior of coal usually started during the devolatilization as a result of the release of moisture and volatiles.41−44 Herein, the fragmentation behaviors on the molten slag surface for coal char and petroleum coke were both found during the reaction with CO2, which were related to pore structures, coal ash content, and slag surface. The coal char particles with lower coal ranks had a higher possibility for the fragmentation behavior. On the basis of the morphology of SEM images in Figure 3, coal char of lower coal rank had more and larger pores, while the char with a higher coal rank had a dense structure. The high content of coal ash in the raw coal sample also affected the fragmentation behavior during gasification on the molten slag surface. The reaction of a char particle above the liquid slag layer was not similar to the tradition condition in the space, because the molten slag surface with a high surface tension might cause a dragging force to the char particle and promote the fragmentation behavior. Therefore, the primary fragmentation of a coal

Figure 1. Diagram of the HTSM used in this study. referred in our previous work.35 The HTSM contained a hightemperature stage and a camera. The high-temperature stage was made by Linkam, U.K. The microscope and camera was made by Leica, Germany. In addition, the reaction gas system and watercooling system were connected with the stage. The setting temperature of the furnace in the HTSM was also calibrated with the melting temperature of silver (purity of 99.9%) 2 or 3 times, and the error was controlled below 0.03%. About 0.5 mg of the experimental sample (coal char or petroleum coke) was put on the coal ash layer with a sapphire slip (Al2O3, purity of 99.9%). The mass of coal ash on the sapphire slip was about 5 mg. First, the corundum crucible furnace with coal char particles and coal ash in the HTSM was preheated to 100 °C at a heating rate of 30 °C/ min, and this temperature was held for 1 min. Next, the temperature of experimental samples (coal ash and char particles) was increased to 1300 °C with the heating rate of 100 °C/min. Then, the samples were held for 10 min when the coal ash sample was melted to liquid slag. When the samples were heated and held for ash melting, an Ar gas flow of 0.1 L/min (purity of 99.9%), was injected into the corundum crucible as a protection gas, removing air. When the liquid slag formed under the coal ash sample, char particles floated on the liquid slag layer. Then, the gas flow of CO2 (purity of 99.9%) was injected into the corundum crucible, replacing Ar as the reactant gas, and reacted with the particles. The flow rate of CO2 gas was set to 0.1 L/ min. Char particles or petroleum coke particles in the HTSM reacted with CO2 on the molten slag surface at 1300 °C. Meanwhile, the digital camera with the combination of a microscope recorded the whole gasification processes of particles via a video from the injection of CO2 to the end of the reaction. Therefore, the time for the injection of CO2 and start of the video recording was the same, which C


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Figure 2. Images of the evolution and gasification processes of char particles for different coal ranks and petroleum coke at 1300 °C.

Figure 3. SEM images of char particles with different coal ranks and petroleum coke.

D


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Figure 4. Relationship between the initial particle size, initial fragmentation time (tf), and total reaction time (tc) for char particles of different coal ranks and petroleum coke.

Figure 5. Relationship between the initial fragmentation time (tf) and total reaction time (tc) for char particles of different coal ranks and petroleum coke.

particle in the gasifier occurred as a result of the devolatilization. When the deposited char particles continued to react with gases near the slag wall, the fragmentation behavior would also occur as a result of the molten slag surface and pore structures, which were also affected by the raw coal sample. 3.2. Total Reaction Time and Initial Fragmentation Time. The total reaction time and initial fragmentation time of

particles on the molten slag surface for different coal ranks and petroleum coke with the effects of different particle sizes are given in Figure 4. The straight lines denoted the fitted lines from the points of experimental data. The total reaction time (tc) was prolonged as a result of the increase of the initial particle size and mass, which differed from the coal char types and petroleum coke. An initial fragmentation time (tf) was defined in this study, which was the initial time that the E


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Energy & Fuels fragmentation behavior of a particle started in the gasification process when a fragment was found and separated from the char particle on the molten slag surface. The initial fragmentation time (tf) similarly showed a linear relationship with the increase of the initial particle size, which was also measured and given as the red hollow points in Figure 4. During gasification, particles floated above the liquid slag surface. Then, the reaction of a particle mainly occurred above the liquid level. When a char particle was consumed and became smaller, the surface tension of molten slag would affect the reacted particle, and this effect was also strengthened by the consumption of a particle, which found the difference of the initial fragmentation time between the coarse and fine particles. However, it was found that the interval of the fragmentation time, namely, the difference of the reaction time between black and red lines, increased obviously with increasing the initial particle size for both the lignite char and bituminous char, while these differences for anthracite char and petroleum coke decreased. The relationship between the initial fragmentation time and total reaction time for different coal chars and petroleum coke is given in Figure 5. The red line denoted the fitted line of the initial fragmentation time from the points of measured experimental data. Results showed that the initial fragmentation time had a linear relationship with the total reaction time. This indicated that the fragmentation behavior occurred when the particle was consumed by gasification for a certain mass. The consumed mass of carbon in the particle was also verified with the types of coal char and petroleum coke. Results also showed that the initial fragmentation time was prolonged with the total reaction time, which means that the fragmentation behavior of larger particles began later than the fine particles. Char particles or petroleum coke particles floated on the molten slag surface as a result of the smaller density, and then these particles continued to react with CO2. Therefore, a majority of carbon for a single particle above the liquid level of molten slag was consumed by CO2. When a particle was consumed for a certain mass, depending upon the gasification rate, the fragmentation behavior began. The fragmentation behavior was also related to the surface tension of molten slag and the pore structure of the reacted particle. When a particle became smaller as a result of the carbon consumption, the effect of surface tension from the molten slag on the fragmentation behavior of particles was also enhanced. For example, the lignite coal char had more pore structures inside, and the fragmentation behavior began earlier than other coal chars or petroleum coke. The petroleum coke had a dense structure with fewer pore structures, and hence, these particles were not easily broken into fragments. Besides, the high ash content in a particle also affected the fragmentation behavior via the melting behavior, such as the lignite char and anthracite coal, and the porous structure could promote the fragmentation behavior from the experimental results. The ratio of the initial fragmentation time to the total reaction time also increased with the coal rank and petroleum coke from 0.634 to 0.943 in Figure 6. The values of the ratios for different coal ranks and petroleum coke mean that the fragmentation behaviors of particles on the molten slag surface occurred when the particles were consumed for a certain level. A high ash content and porous structure could promote the fragmentation behavior from the results of lignite char and bituminous char. However, the internal dense structure (e.g.,

Figure 6. Ratio of the initial fragmentation time (tf) to the total reaction time (tc) for char particles of different coal ranks and petroleum coke.

petroleum coke) would hinder the fragmentation behavior during gasification on the molten slag surface. 3.3. Number of Fragments. In this study, the number of fragments in the particle-fragmentation period on the molten slag surface was measured by ImageJ software and given in Figure 7, with the effects of different coal ranks and particle sizes. The number of fragments increased obviously and then decreased with the carbon consumption and the coal ash melting into slag for all of the experimental samples. The colored lines in Figure 7 denoted the fitted lines for the evolution of the fragment number in the particle-fragmentation period. The fitted equations of the colored lines revealed that the evolution of the fragment number (N) during the particlefragmentation period had a relationship of Gaussian function distribution with the reaction time (t). 2

N = Nmax e−(t − tmax)

/2β 2

(1)

The parameters in the fitted equation of the fragment number for different char particles of coal ranks and particle sizes are displayed in Table 4. The maximum number of particle fragments decreased when the initial particle size was increased for all of the coal chars, while this maximum value decreased for petroleum coke. It was also found that a high ash content in the raw coal sample (e.g., lignite char and anthracite char) would obviously affect the maximum number of fragments compared to the particles with a lower coal ash content. Results in Table 4 also showed the time that the maximum fragment number (tmax) increased for all of the experimental samples. The parameter β, which denoted the total fragmentation period, was increased with the increasing particle size and coal ranks. 3.4. Fragmentation Characteristics. A fragmentation intensity (I) was defined in this study to present the fragmentation and gasification properties of deposited particles on the molten slag surface. From eq 1, the total fragmentation time length (tc − tf) and the time for full width at half maximum (Δth) were calculated and given in Figure 8. Herein, the full width was the total fragmentation time length. Then, the ratio of time for full width at half maximum (Δth) to the total fragmentation time length (tc − tf) was expressed as

τ=

Δth tc − tf

(2)

where τ was the non-dimensional number for the time ratio, tc was the total reaction time, and tf was the initial fragmentation F


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Figure 7. Evolution of the number of fragments for char particles of different coal ranks and petroleum coke.

surface. Therefore, the fragmentation intensity (I) in this study was denoted as

Table 4. Parameters in the Fitted Equation of the Fragment Number for Different Particle Types and Sizes sample

initial particle size (μm)

Nmax

tmax (s)

β

LC

189 376 595 89 242 438 131 259 444 114 321 415

43 42 26 29 28 26 20 10 5 5 5 6

21 43 59 10 19 29 94 205 468 95 630 1140

2.323 4.158 4.62 1.46 1.992 2.1 4.2 13.65 20.16 10.5 75.6 100.8

BC

AC

PC

I∝

1 τ

(3)

The fragmentation behavior of a coal/char particle was frequently observed in the combustion and gasification reactions, which were related to the carbon content, ash content, moisture, and volatiles.42 The volatile matter was the key factor to affect the primary fragmentation behavior of coal samples with the effects of the release from pyrolysis.43−46 A large pressure inside the particle was caused by the release of volatile matter, which would also cause fragile porous structures, especially for the high contents of moisture and volatiles in the raw coal sample.47,48 In addition, the coal rank affected the contents of moisture and volatiles and finally caused various char structures and mechanical strengths.49,50 Thus, the fragmentation behavior of a char particle corresponded to the contents of moisture and volatiles. Herein, the sum of the contents of moisture and volatiles was associated with the fragmentation intensity as the following relationship, which was given in Figure 9. The error of the fragmentation intensity was also given. Herein, the values of the error lines were calculated from the standard deviation between the actual intensity and the average intensity of fragmentation. In this study, three different particle sizes from coarse to fine particles were chosen to analyze the fragmentation intensity for each coal char or petroleum coke. The fragmentation intensity for each particle size was calculated, and then the average value of intensity for three particle sizes of a coal char or petroleum was then calculated accordingly. On the basis of the standard deviation formula, the error was calculated and added to Figure 9. The maximum value of fragmentation intensity for a coal char corresponded to the coarse particles, and the minimum values corresponded to the fine particles, because the particle sizes had effects on the fragmentation behaviors of coal chars or petroleum coke.

Figure 8. Diagram of the evolution of the number of particle fragments.

time. From eq 2, the increasing value of τ denoted the increasing time for full width at half maximum but revealed the decreasing fragment number of a particle on the molten slag G


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m 0 − mt m zy 1 ijj = jj1 − t zzz j m0 − ma 1 − φa k m0 z{

(5)

In the particle-shrinking period, the mass of a particle at time t was equal to the mass sum of the unreacted core and the residual ash layer on the particle surface, and this equation was mt = mc + ma ′

(6)

where mc was the mass of the unreacted core π mc = ρc dc 3 6

and ma′ was the mass of the residual ash on the particle surface π ma ′ = ρ0 (d p3 − dc 3)φa (8) 6

Figure 9. Relationship between the fragmentation intensity (I) and the sum of the mass fraction of moisture and volatiles (w) in the raw samples.

where φa was the ash content and given in Table 1. However, the actual particle diameter (dt) measured in this study was larger than the diameter of the unreacted core (dc) as a result of the ash layer that accumulated outside the unreacted core. Then, the actual diameter of the particle on the molten slag surface was calculated.

Therefore, for each coal char or petroleum coke, the standard deviation was used to calculate the error, and these errors were added to compare the relationship between the fragmentation intensity between the sum of the mass fraction of moisture and volatiles in Figure 9. With increasing the mass contents of moisture and volatiles in the raw coal samples, the subsequent structures of char particles obviously increased the fragmentation intensity from petroleum coke to lignite char. Therefore, the fragmentation intensity was related to the internal structure and mass contents of moisture and volatiles (w). 1 I∝ ∝w τ

(7)

dt = dc + 2δa

(9)

The mass of the residual ash layer on the particle surface could also be calculated as ma ′ = ρa Va′ = ρa Scδa(1 − εa)

(10)

where εa was the porosity of the ash layer around the unreacted core. The near optimal values of εa ranged from 0.16 to 0.25, which was different from various coals, and the mean value was about 0.21,51 which was used in this study. With the combinations of eqs 5−10, the diameter of the unreacted core (dc) was calculated. The carbon conversion in the particle-shrinking period was given as

(4)

3.5. Carbon Conversion. Particles used in this study were assumed to have no reaction with the minerals of slag, and the mineral matter was uniformly distributed in these particles. The evolution of a particle under the gasification reaction on the molten slag surface from Figure 2 was divided into two parts: particle shrinking and particle fragmentation. The fragmentation behaviors of char particles and petroleum coke on the molten slag surface increased the difficulties to calculate the carbon conversion, which also differed from the calculation of carbon conversion in the particle-shrinking period. Therefore, in this study, the carbon conversion for a single particle on the molten slag surface should be calculated from two parts: the carbon conversion in the particle-shrinking period and the carbon conversion in the particle-fragmentation period. The diagram of the reaction of a single particle during gasification on the slag layer was displayed in Figure 10. Actually, the carbon conversion equation of a single particle herein was expressed as

x=1−

dc 3 d p3

(11)

In the particle-fragmentation period, the total carbon conversion was calculated as n

x=

m0 − ∑i = 1 mt , i m0 − ma

n ∑i = 1 mt , i yzz 1 ijj j zz, = j1 − 1 − φa jk m0 z{

n > 1, t > 0

(12)

where ∑ni=1mt,i was the mass sum of all of the unreacted particle fragments at time t and was denoted as n

∑ mt ,i = mt ,1 + mt ,2 + ... + mt ,n ,

n > 1, t > 0

i=1

(13)

Similarly, the mass sum of all of the unreacted particle fragments at time t was equal to the mass sum of the unreacted core and the residual ash layer. n

n

n

∑ mt , i =

∑ mc,i +

∑ ma,i′,

i=1

i=1

i=1

n>1 (14)

For the ith particle fragment, the actual diameter of the ith fragment was equal to the sum of the diameter of the unreacted core and the thickness of the ash layer.

Figure 10. Diagram of the evolution of a single particle during gasification on the molten slag surface. H


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Figure 11. Carbon conversions of char particles for different coal ranks and petroleum coke at different particle sizes.

Figure 12. Average reaction rates of char particles for different coal ranks and petroleum coke in the shrinking period (rs) and fragmentation period (rf).

dt , i = dc, i + 2δa, i

Finally, the carbon conversion for a single particle when reacted with CO2 on the molten slag surface was combined with the above two parts and denoted as

(15)

For the ith particle fragment, the mass of the ash layer could also be given similar to eq 10. ma, i′ = ρa Va, i′ = ρa Sc, iδa, i(1 − εa)

n

(16)

x=1−

Then, the diameter of the unreacted core (dc,i) for the ith particle fragment was calculated from the combinations of eqs 12−16. The carbon conversion in the particle-fragmentation period was given as ∑i = 1 dc, i 3 d p3

,

d p3

,

l o o n = 1, particle shrinking i=m o o n > 1, particle fragmentation n

(18)

Figure 11 gives the carbon conversions of particles with different sizes, which were calculated from eq 18. Obviously, a sharp increase of the carbon conversion line was found for all of the experimental samples, which corresponded to the occurrence of the particle fragmentation behavior on the

n

x=1−

∑i = 1 dc, i 3

n>1 (17) I


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molten slag surface. Without the effect of the fragmentation behavior, the carbon conversion lines first increased and then continued as the fitted dashed lines as a result of the diffusion resistance from the ash layer. The fragmentation behavior of a char particle caused the coal ash layer to melt into the molten slag, then reduced the diffusion resistance of the ash layer, and increased the gasification area. These series of behaviors promoted the carbon conversion when a particle was reacting with gases on the molten slag surface under the same reaction time. For a particle with lower reactivity, such as anthracite char and petroleum coke, the fragmentation behavior could also increase the carbon conversion similarly. 3.6. Reaction Rate. In this study, the average reaction rates of deposited char particles in both the particle-shrinking period and the particle-fragmentation period were calculated as

r=

dx dt

Haifeng Liu: 0000-0002-2572-8693 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (U1402272), the Shanghai Engineering Research Center of Coal Gasification (18DZ2283900), the Fundamental Research Funds of the Central Universities (222201817004 and 222201814052).

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NOMENCLATURE dc = diameter of the unreacted core (m) dp = initial particle diameter (m) dt = actual diameter of the particle at time t (m) I = fragmentation intensity defined in this study ma = mass of the coal ash layer (kg) mc = mass of the unreacted core (kg) mt = char particle mass at time t (kg) m0 = char mass at initial time (kg) N = number of fragments r = reaction rate (s−1) Sc = surface area of the unreacted core (m2) t = time (s) tf = initial fragmentation time (s) tc = total reaction time (s) tmax = time at the maximum fragment number (s) Δth = time for full width at half maximum Va = volume of the ash layer (m3) w = mass faction of volatiles and moisture x = carbon conversion

(19)

Results of the average reaction rates calculated from eq 19 for the particles with different coal ranks and petroleum coke are shown in Figure 12. The average reaction rate of a particle on the molten slag surface in the particle-fragmentation period was higher than the rate in the particle-shrinking period. In addition, the average reaction rate decreased when the initial particle size was increased in the study. On the basis of the above analyses, a char particle or a petroleum coke on the molten slag surface with the fragmentation behavior had a higher gasification rate than the particle without a fragmentation behavior.

4. CONCLUSION During the gasification of deposited coal char particles on the molten slag surface, the fragmentation behavior was found and investigated with different particle sizes in this study. Particle evolution and total gasification processes for the particles of different coal ranks and petroleum coke were analyzed and compared. Results showed that the evolution of a particle on the molten slag surface could be divided into two parts during the gasification, including particle shrinking and particle fragmentation. The char particle with a fragmentation behavior on the molten slag surface had a higher gasification rate than the char particle without fragmentation, which means that the fragmentation behavior that occurred on the molten slag layer could promote the gasification rate obviously. The initial fragmentation time had an increasing linear relationship with the initial particle size. Coal char particles with a lower coal rank, such as the lignite coal chars, were easily broken into fragments on the molten slag surface with a longer fragmentation period. The number of fragments first increased and then decreased, acting as a relationship of the Gaussian function distribution with the reaction time. High contents of moisture and volatile matter in the raw coal sample would cause the porous structures and promote the occurrence of the fragmentation behavior. A high ash content in the char particle also affected the fragmentation behavior through the fragment number.

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Greek Symbols

β = fitted constant δ = thickness of the ash layer (m) ε = porosity ρa = coal ash density (kg m−3) ρc = unreacted core density (kg m−3) ρ0 = initial particle density (kg m−3) φa = content of coal ash Subscripts

0 = initial time a = coal ash c = unreacted core or total reaction time i = ith fragment p = particle t = time t

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Study on the Fragmentation Behaviors of Deposited Particles on

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