Application of Percolation Model to Ash Formation Process in Coal

Energy & Fuels 2004, 18, 1077-1086

1077

Application of Percolation Model to Ash Formation Process in Coal Combustion Ryoichi Kurose,† Hisao Makino,*,† Hiromitsu Matsuda,† and Akira Suzuki§ Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan Received January 28, 2004

The percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to the ash formation process in coal combustion, and its validity is examined by comparison with the experimental results obtained using the drop tube furnace facility (DTF). The characteristics of a burning coal particle, such as particle diameter and specific surface area, are investigated in detail. Newlands and Plateau coals with different fuel ratios and ash contents are tested. The ambient temperature is set at 850 or 1400 °C, at which temperature fluidized-bed or pulverized coal combustion occurs. The relationship between particle temperature and conversion of coal required in the percolation model is obtained by performing a numerical simulation of a combustion field in the DTF. The results show that the characteristics of the burning coal particle obtained through the computations of the percolation model are generally in agreement with the experimental data. The particle diameter of Newlands coal with a higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustiondominant process. For both Newlands and Plateau coals, compared to the particle diameter of the lower ambient temperature case of 850 °C, that of the higher ambient temperature case of 1400 °C becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. These behaviors can be explained in terms of swelling due to devolatilization and ash agglomeration.

Introduction Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. Currently, coal-fired power plants involve pulverized or fluidized-bed coal combustion, for which the utilization of various types of coal is desired to diversify fuel sources and reduce cost and to operate utility boilers with higher efficiency and lower pollutant emission. Recently, the utilization of ash exhausted from coal combustion is also becoming a significant issue in Japan.1 Because high-quality ash that has characteristics such as a low unburned carbon fraction, small particle diameter, and high particle circularity may be purchased as a mixture of cement and concrete, it is of great importance, in terms of cost reduction, for coal combustion power plants to generate high-quality ash. In addition, particulate matter, which consists of unburned carbon and ash, causes slugging, fouling, and erosion in combustors, so methods of their prediction and protection are strongly desired. To develop such new technologies, it is essential to understand the effects of coal properties and combustion conditions on the characteristics of the particulate matter during combustion. * Author to whom correspondence should be addressed. Fax: +81 46 856 3346. E-mail address: [email protected]. † Now with Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI). § Now with CD-adapco Japan Co., Ltd. (1) Makino, H. J. Soc. Powder Technol., Jpn. 1997, 34, 361-366 (in Jpn.).

Monte Carlo simulation that is based on percolation theory for char combustion processes (the percolation model), which has been under development since the 1980s,2,3 is a useful tool for clarifying the effects of various factors on the characteristics of the particulate matter, because it can simulate the time variations of burning char in detail by taking into account its heterogeneous structure and fragmentation behavior. Recently, Suzuki et al.4 extended this model to predict particle swelling due to coal devolatilization, and Kurose et al.5 studied the effects of coal properties, as well as ambient temperature and pressure, on the characteristics of the burning particulate matter such as particle diameter and specific surface area, using Suzuki et al.’s model4 and comparison with the results of experiments. However, some issues concerning this model must be resolved. The main issue concerns the temperature of the coal temperature. In our previous study,5 the temperature of a coal particle was assumed to approximately increase linearly with increasing conversion and to reach the gas temperature when the conversion is 0.3, because it is difficult to give the actual particle temperature variation experimentally and theoretically. (2) Kerstein, A. R.; Niksa, S. In 20th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 941949. (3) Kerstein, A. R.; Edwards, B. F. Chem. Eng. Sci. 1987, 42, 16291634. (4) Suzuki, A.; Yamamoto, T.; Aoki, H.; Miura, T. In 29th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 2002; pp 459-466. (5) Kurose, R.; Matsuda, H.; Makino, H.; Suzuki, A. Adv. Powder Technol. 2003, 14, 673-694.

10.1021/ef0400159 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004

1078 Energy & Fuels, Vol. 18, No. 4, 2004

Kurose et al.

Moreover, because the given maximum particle temperature was lower than the melting point of ash, ash agglomeration was not taken into consideration. In this study, the modified percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to ash formation process in coal combustion, and its validity is examined by comparison with the experimental results obtained using a drop tube furnace facility (DTF). Also, the characteristics of a burning coal particle are investigated in detail. The variation of the particle temperature is determined by performing a numerical simulation of a combustion field in the DTF. Percolation Model General Features. In the percolation model, only one coal particle is considered, because we assume that the particles are far enough apart to behave independently. It is also assumed that the initial coal particle is a simple three-dimensional cube of size L × L × L arranged in smaller cubic lattices of size l × l × l. Hence, the total number of smaller cubic lattices is (L/l)3. The coal particle is arranged in Cartesian cubic coordinates of LM × LM × LM (LM > L) to represent the swelling behavior of the coal particles. The lattice components are distinguished between fixed carbon (while burning, this is also called “char”), volatile matter, ash, and pore. Each lattice is located uniformly within the coal particle, according to the weight fraction of each component, as determined by the proximate analysis of the coal. Moisture is neglected, because of its small quantity. The coal reaction process is classified into a homogeneous combustion process in the gas phase after devolatilization and a heterogeneous char combustion process. The devolatilization and char combustion are simulated in this percolation model, under the assumption that they proceed simultaneously. Devolatilization Model. To predict the devolatilization process, the first-order reaction model by Van Krevelen et al.,6

dV ) kV(V* - V) dt

(1)

is used. Here, kV is given by the Arrhenius equation kV ) AV exp[-EV/(RT)], and V* and V indicate the total volatile matter content in the coal (in kilograms, kg) and volatile mass released from the coal (in kg). T is the gas temperature (in Kelvin, K), and R is the universal gas constant (R ) 8.31 J (mol K)-1). Kinematic parameters of the Arrhenius equation, i.e., the pre-exponential factor AV and the activation energy EV, are 2021 s-1 and 3.11 × 104 J/mol, respectively.7 As a result of the transformation of eq 1, the time required to consume an arbitrary volatile lattice, tV0, is obtained as

tV0 )

F Vl 3 kV(V* - V)

(2)

where FV is the volatile density (given in units of kg/ m3). It is well-known that coal particles swell during the devolatilization process because their internal pressure increases and viscosity decreases as a result of the

Figure 1. Image of swelling behavior of coal in devolatilization process. Legend is as follows: V, volatile matter; C, char; A, ash; and D, devolatilization. Arrows represent the swelling direction.

presence of metaplast in the coal particles. An image illustrating the swelling behavior of coal in the devolatilization process is shown in Figure 1. In this model, a volatile lattice chosen as a reaction lattice is lost, and all six lattices surrounding the lost lattice are assumed to be shifted outward by one lattice length. This increases the particle diameter and the porosity. The devolatilization process continues until all volatile lattices are lost. Char Combustion Model. The char surface reaction is given as follows:

C + O2 f CO/CO2

(3)

The char combustion rate is determined by both the surface chemical reaction rate and the diffusion rate to the particle surface and the internal pore for the reactive gas. The surface chemical reaction rate is obtained by Field’s equation:8

dC ) k′CxPO2l2 dt

(4)

where

( )

k′C ) Aa exp -

Ea RTp

(5)

In eqs 4 and 5, C is the mass of the char lattice (in kg), PO2 the partial pressure of oxygen (given in pascals, Pa), and Tp the particle temperature (in K). Kinematic parameters of the Arrhenius equation in eq 5si.e., the pre-exponential factor Aa and the activation energy Eas are 8 × 10-3 kg (m2 s Pa0.5)-1 and 5.0 × 104 J/mol, respectively.7 Hence, the chemical reaction time of one lattice, tcr, is given by

tcr )

FCl P k′C x O2

(6)

On the other hand, the diffusion rate is calculated using the diffusion-limited reaction model, which simulates the reaction by losing a surface char lattice that has reacted with oxygen.9 The diffusion process is expressed by taking a broad view of the random walk (6) Van Klevelen, D. W.; Van Heerden, C.; Huntgens, F. J. Fuel 1951, 30, 253-258. (7) Kurose, R.; Makino, H.; Suzuki, A. Fuel 2004, 83 (6), 693. (8) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. The Combustion of Pulverized Coal; British Coal Research Association: Leatherhead, Surrey, U.K., 1967. (9) Miccio, F.; Salatino, P. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1145-1151.

Modeling Ash Formation Process in Coal Combustion

Energy & Fuels, Vol. 18, No. 4, 2004 1079

Figure 2. Image of random walk of oxygen lattices around a coal particle. Legend is as follows: V, volatile matter; C, char; A, ash; and O2, oxygen. Arrows represent the direction of movement of the oxygen lattice.

motion for molecules.10 According to this model, the average diffusion time for a oxygen lattice to pass N lattices, tN, is obtained by

tN )

(Nl)2 6DO2

(7)

where the diffusion coefficient for oxygen, DO2, is given by11

DO2 ) c0D0O2

( ) T T0

1.75P0

P

(8)

Here, D0O2 is the diffusion coefficient for oxygen at 273.15 K and 1.013 × 105 Pa (D0O2 ) 1.78 × 10-5 m2/s), P is the gas pressure (in Pa), T0 and P0 are the normal gas temperature (T0 ) 273.15 K) and normal pressure (P0 ) 1.013 × 105 Pa), and c0 is the morphological coefficient (c0 ) 0.2). When the reaction defined by reaction 3 occurs, there must be at least the same number of oxygen lattices, m, as half the mole number of carbons organized in a char lattice to burn out a char lattice. Here, m is given as

m)

3 1 FCl /mC 2Pf l3/(RT) O2

(9)

where fO2 is the volume fraction for oxygen in air. Consequently, the char lattice reaction time, tr, is determined using the chemical reaction rate and the diffusion rate for oxygen, as

tr ) max[tcr,tm]

(10)

where “max[a,b]” means the larger value of the parameters a and b within the brackets. Figure 2 shows an image of a random walk of oxygen lattices. In this model, the oxygen lattices are arranged outside the surface lattice for the coal particle on the basis of the assumption that oxygen is distributed uniformly around the coal particle. Each oxygen lattice moves randomly to one of the six neighbors unless it is another oxygen lattice, volatile lattice, or ash lattice. (10) Shewmon, P. G. Diffusion in Solids, 2nd ed.; Minerals, Metals & Materials Society: Warrendale, PA, 1989. (11) Washburn, E. W.; Clarence, C. J. International Critical Tables of Numerical Data, Physics, Chemistry and Technology, Prepared under the Auspices of the International Research Council and the National Academy of Sciences (1st ed.); McGraw-Hill: New York, 1929; Vol. 5, p 62.

Figure 3. Image of ash transfer. Legend is as follows: A, ash; C, char; V, volatile matter; and X, center of calculation domain. Arrows represent the direction of ash transfer.

According to eq 7, the more lattices the oxygen lattice has passed, the longer the average time required to pass one lattice becomes. It is assumed that if an oxygen lattice reaches a char lattice, the oxygen included in the oxygen lattice reacts with the char and then the CO/ CO2 generated by the reaction is quickly removed. When a char lattice consumes the first oxygen lattice, the reaction starts, the chemical kinetic reaction time (tcr) of which is calculated using eq 6. When the passage time of a char lattice becomes longer than the time of both oxygen diffusion and the chemical kinetic reaction, the char lattice is lost. Char combustion proceeds simultaneously in multiple char lattices and continues until all char lattices are lost or only char lattices surrounded by ash lattices remain because no oxygen can reach the char surface. Ash Agglomeration Model. This percolation model can also be applied to ash agglomeration, although it is limited to one coal particle and does not represent the interaction among coal particles. Ash agglomeration occurs when ash melts and then transfers to a neighboring lattice. Each ash lattice has a certain transfer time, and after this ash transfer time has elapsed, the lattice can move by one lattice length. The direction of ash transfer is selected randomly from among directions toward the center of the calculation domain. The ash transfer pattern is distinguished into two patterns. Figure 3 shows the image of ash transfer. In pattern 1, when an ash lattice is surrounded by pore lattices, the ash lattice randomly transfers in the direction toward the center of the calculation domain. In pattern 2, when an ash lattice connects with volatile, char, or other ash lattices, the ash lattice moves so that the present connection is not severed. According to Miccio et al.,9 the transfer time is expressed as

tc ≈

( )

µash l σash

(11)

where σash is the surface tension of ash (given in units of N/m) and µash is the ash viscosity (given in units of Pa s). The ash viscosity is dependent on the ash content of the coal. The Urbain model12,13 is applied to predict the ash viscosity of various types of coal. According to the Urbain model,12,13 the ash viscosity is represented (12) Urbain, G.; Cambier, F.; Deletter, M.; Anseau, M. R. Trans. J. Br. Ceram. Soc. 1981, 80, 139-141. (13) Urbain, G. Steel Res. 1987, 58, 111-116.


Kurose et al.

Figure 4. Schematic of the drop tube furnace facility (DTF).

by the Weymann equation:

µash ) AwTp exp

( ) 103Bw Tp

Table 1. Coal Properties Value

(12)

where Aw (given in units of Pa s K-1) and Bw (in K) are parameters governed by the ash contents. Tp is the particle temperature (in K). On the other hand, there is no model for predicting the ash surface tension. However, because Miccio et al.9 assumed that the ash surface tension is constant (equal to 0.1 N/m), because the ash surface tension is not sensitive to temperature, compared with the ash viscosity, we use the same value in this study. It is assumed that ash melts when the ash viscosity is lower than the critical viscosity of µash ) 105 Pa s, as reported in previous papers.14-16 Numerical Conditions. The computations are performed at the ambient temperature of Ts ) 850 or 1400 °C. Newlands and Plateau coals are tested, the properties of which are shown in Table 1. The notable difference between the two coals is observed in the fuel ratio (equal to fixed carbon divided by volatile matter) and ash content. Because of its higher fuel ratio and higher ash content, the combustibility of Newlands coal is lower and its ash exhaust is higher, compared to Plateau coal. It is assumed that the densities of volatile matter and char are 1200 kg/m3, and that of ash is 2500 kg/m3. The gas temperature is assumed to correspond to the particle temperature. The initial diameter and porosity of the coal particle are 50 µm and 0.2, respectively. The initial and maximum lattice numbers, L/l × L/l × L/l and LM/l × LM/l × LM/l, respectively, are 40 × 40 × 40 and 100 × 100 × 100, respectively. DTF Experiment and Numerical Simulation DTF Experiment. For comparison, the characteristics of the particulate matter, i.e., burning coal particles (14) Walsh, P. M.; Sayre, A. N.; Loehden, D. O.; Monroe, L. S.; Beer, J. M.; Sarofim, A. F. Prog. Energy Comb. Sci. 1990, 16, 327-346. (15) Wang, H.; Harb, J. N. Prog. Energy Comb. Sci. 1997, 23, 267282. (16) Costen, P. G.; Lockwood, F. C.; Siddique, M. M. 28th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 2000; pp 2243-2250.

property proximate analysis (wt %) moisturea volatile matterb fixed carbonb ashb fuel ratio ultimate analysisb (wt %) carbon hydrogen nitrogen oxygen combustible sulfur heating value (high)b (MJ/kg) melting point (K) a

Newlands coal

Plateau coal

(2.6) 27.7 54.9 14.8 1.98

(5.0) 43.4 42.1 9.6 0.97

71.8 4.45 1.59 6.44 0.48 28.2 1830

70.1 5.86 1.26 12.37 0.48 28.0 1623

As received. b Dry basis.

that contain ash, in coal combustion are experimentally investigated using a DTF. The DTF facility has an simple electric furnace, whose central temperature can be varied up to 1773 K at atmospheric pressure. In addition, the variation of the coal particle temperature during combustion required for the percolation model is obtained by a numerical simulation of a combustion field in DTF. The schematic of the DTF is shown in Figure 4. The pulverized coal is fed to the reaction tube from the feed tube of the screw feeder. The reaction tube is made of alumina and has a diameter of 50 mm and a length of 1500 mm. A flat temperature profile is observed in the reaction zone (800 mm length) when using an electric heater. The particulate matter in the combustion process is sampled with a water-cooled traversable sampling probe, and the residence time, which is the time required for the coal particles to move from the feed tube to the sampling tube, is controlled by vertically traversing the sampling probe inserted upward from the bottom of the furnace. Detailed descriptions of this facility are given in other papers.17-20 Similar to the computations of the percolation model, the temperature on the central axis at the center of the reaction zone (which, hereafter, is referenced as the “ambient temperature”) before particle injection is set



Figure 8. Time variation of conversion CV. Figure 5. Schematic of the computational domain of the DTF.

Newlands and Plateau coals have been tested. The coal feed rate, which is determined to maintain the concentration of coal particles, is set at a value in the range of 25-40 g/h. The air ratio is 1.24. Conversion and particle size are investigated for the coal particles with a residence time in the range of 1.0-4.0 s. The particle size is evaluated using a laser diffraction particle size analyzer. The initial mass base median diameter, dp50, for Newlands coal is 32.6 µm, and that for Plateau coal is 48.7 µm. The particle shape is evaluated using scanning electron microscopy (SEM). Numerical Simulation. A schematic of the computational domain for the combustion field in the DTF facility is shown in Figure 5. Axisymmetric twodimensional coordinates are used. The shape of the drop tube is designed and the combustion conditions are set to match the aforementioned experiments. The computations are performed using the STAR-CD code.21 The turbulence is represented by a k- two-equation model.22 The numerical method and models used for the pulverized coal combustion are essentially the same as those described in our previous paper.6,23 The number of grid points given in this simulation is about 2200. Results and Discussion

Figure 6. Axial distributions of (a) temperature (T) and (b) O2 concentration in computations and experiments.

Figure 7. Relationship between the temperature of the coal particle (Tp) and conversion (CV).

at Ts ) 850 or 1400 °C (note that this temperature includes the effect of radiation from coal particles and the walls, because it is measured with a thermocouple).

Temperature of the Coal Particles. The variation of the temperature of the coal particles during combustion required for the percolation model is obtained through numerical simulations of the combustion field in the DTF facility. To verify the accuracy of the numerical simulations, the axial distributions of the gas temperature T and O2 concentration in the drop tube in computations and experiments are compared for Newlands and Plateau coals at the ambient temperatures of Ts ) 850 and 1400 °C, as shown in Figure 6. (17) Kajitani, S.; Matsuda, H.; Hara, S.; Ashizawa, M.; Takahashi, T. In Proceedings of the 13th Annual International Pittsburgh Coal Conference; Pittsburgh, PA, 1996; p 976. (18) Matsuda, H.; Kajitani, S.; Shindo, M.; Makino, H. In Proceedings of the 10th International Conference on Coal Science; Taiyuan/ China, 1999; pp 367-370. (19) Matsuda, H.; Shindo, M.; Kajitani, S. CRIEPI Report No. W98029, 1999 (in Jpn.). (20) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81, 539-546. (21) Computational Fluid Dynamics Software STAR-CD Version 3.15, Methodology, CD adapco Group, Computational Dynamics Limited, 2001. (22) Launder, B. E.; Spalding, D. B. Comput. Methods Appl. Mech. Eng. 1974, 3 (2), 269. (23) Kurose, R.; Ikeda, M.; Makino, H.; Kimoto, M.; Miyazaki, T. Pulverized Coal Combustion Characteristics of High-Fuel-Ratio Coals. Fuel, in press.

1082 Energy & Fuels, Vol. 18, No. 4, 2004 Kurose et al.

Figure 9. Depiction of the behavior of the coal particles: (a) Newlands coal and (b) Plateau coal. Green, red, and white lattices indicate char, volatile, and ash lattices, respectively.


The computed results are found to be in general agreement with the experimental results. As Ts increases, the profile of T becomes higher and the consumption of O2 becomes faster for both Newlands and Plateau coals. Compared to Newlands coals, T is higher and the consumption of the O2 concentration is faster for Plateau coal in the upstream region. This is because the volatile matter content in Plateau coal is higher than that in Newlands coal, and the consumption and reaction of the volatile matter are much faster than the char reaction. Although the differences in the T profile between computations and experiments are somewhat marked in the upstream region, this is considered to be due to the fact that the temperature obtained in the experiments using a thermocouple is underestimated by radiation heat from the side wall. For the percolation model, the variation of the temperature of the coal particle is often related to conversion. Figure 7 shows the relationship between the temperature of the coal particle (Tp) and the conversion (CV) (see Appendix). Tp is observed to increase as Ts increases. Compared to Newlands coal, the rise of Tp for Plateau coal shifts to the higher conversion side for both cases of Ts ) 850 and 1400 °C. This is because Plateau coal has a higher volatile matter content, which delays char reaction against conversion. In the computations of the percolation model described below, these temperature-conversion relationships are used. Time Variation of Conversion. Figure 8 shows the time variations of the conversion CV for Newlands and Plateau coals at the ambient temperatures of Ts ) 850 and 1400 °C. The reaction rate at Ts ) 1400 °C was so fast that we could not measure the CV value with time in the DTF experiments. Therefore, only the experimental results for Ts ) 850 °C are plotted in the figure. The computed profiles for Ts ) 850 °C agree with the experimental profiles. For both the computed and experimental profiles, the CV value for Plateau coal is higher than that for Newlands coal at a fixed time, because the amount of volatile matter, whose reaction rate is much faster than the char reaction rate, is greater for Plateau coal than that for Newlands coal. The reason CV for Newlands coal increases faster than that for Plateau coal at early times (t e0.5 s) is that the particle temperature of Newlands coal increases faster than that of Plateau coal, as shown in Figure 7. The marked differences between computations and experiments appearing at t ) 1 s are considered to be due to the initial condition of the char lattices arranged along the outer face of the coal particle. The char reaction rate at early times is strongly dependent on whether external char contains oxygen before starting the computation. Although the external char is assumed to contain oxygen abundantly in this study, the increment of CV has a tendency to be delayed when it did not contain oxygen.5 To improve the conformity at this early time, further examination is needed. On the other hand, the CV for Newlands and Plateau coals for Ts ) 1400 °C reaches a value of almost 1.0 before t ) 1 s. Similarly to the case of Ts ) 850 °C, the CV value for Plateau coal is higher than that for Newlands coal in the case of Ts ) 1400 °C. General Behavior of Coal Particles. Figure 9 shows the behavior of coal particles of Newlands and


Figure 10. Scanning electron microscopy (SEM) micrographs for Newlands coal for (a) Ts ) 850 °C and (b) Ts ) 1400 °C.

Plateau coal at the ambient temperatures of Ts ) 850 and 1400 °C. Three-dimensional and sectional features are given. Green, red, and white lattices indicate char, volatile, and ash lattices, respectively. It is observed that, for both Newlands and Plateau coals, the disappearance of volatile matter is much faster than that of char, and there are devolatilization-dominant and charcombustion-dominant processes. Also, ash at the lower ambient temperature of Ts ) 850 °C remains stationary, whereas ash at the higher ambient temperature of Ts ) 1400 °C agglomerates at the center. According to the ash agglomeration model used in this study, ash starts to melt at the coal temperature of Tp ≈ 1300 °C (ash viscosity reaches the critical viscosity of µash ) 105 Pa s at this temperature). Hence, only for the case of Ts ) 1400 °C, in which the maximum Tp value greatly exceeds the ash-melting temperature (see Figure 7), does the ash agglomeration appear. The validity of this computed result is verified in Figure 10, which shows SEM micrographs for Newlands coal at Ts ) 850 and 1400 °C. Compared to the case of the lower ambient temperature of Ts ) 850 °C, more particulate matters are spherical in the case of the higher ambient temperature case of Ts ) 1400 °C, which suggests that ash melts only in the case of Ts ) 1400 °C. Comparison between the behaviors of coal particles of Newlands and Plateau coals shows that the disappearance of volatile matter is slower, the swelling of coal is more marked, and the ash size for Ts ) 1400 °C at CV ≈ 1.0 is smaller for Plateau coal than for Newlands coal. These observations are caused by the Plateau coal having higher


Kurose et al.

Figure 11. Radial distributions of the number fractions of char, volatile matter, and ash: (a) Newlands coal, Ts ) 850 °C; (b) Newlands coal, Ts ) 1400 °C; (c) Plateau coal, Ts ) 850 °C; and (d) Plateau coal, Ts ) 1400 °C.


Figure 12. Effects of ambient temperature and coal properties on (a) maximum particle diameter (dmax), (b) specific surface area (As), and (c) porosity, relative to conversion (CV).

volatile matter and lower ash contents. It is also observed that, for Ts ) 1400 °C, there is a discrepancy in conversion, where ash starts to agglomerate, between Newlands and Plateau coals, despite the similar profiles of the coal particle temperature (see Figure 7). Ash in Newlands coal starts to agglomerate at CV ≈ 0.3-0.4, whereas ash agglomeration in Plateau coal occurs at CV ≈ 0.5-0.6. The reason for this will be discussed later. The behavior of the coal particles, which is apparent in Figure 9, is quantitatively given in Figure 11, which shows the radial distributions of the number fractions of char, volatile matter, and ash. This figure explicitly shows that the amount of volatile matter decreases much faster than that of char, and ash at Ts ) 1400 °C aggregates at the center. Particle Diameter, Specific Surface Area, and Porosity. Figure 12 shows the effects of the ambient temperature and coal properties on the maximum particle diameter (dmax), specific surface area (As), and porosity, relative to conversion (CV). Generally, as char combustion proceeds from the particle surface, some particles are separated from the central large particle. Hence, dmax corresponds to the diameter of the central particle. In this figure, the mass base median diameter, dp50, obtained from the DTF experiments for only Ts ) 850 °C, are plotted. Because the initial dp50 values for Newlands and Plateau coals in the experiments are different (32.6 µm and 48.7 µm, respectively), the


experimental results cannot be compared to the computed results directly. Therefore, the experimental values are normalized by the initial dp50 value and then multiplied by the initial computational diameter (50 µm). Figure 12a shows that swelling of the coal particle is observed in the devolatilization-dominant process, whereafter it begins to shrink, because of the char reaction (also see Figure 9). The maximum value of dmax is as large as 1.40 times greater than the initial diameter. Compared to Newlands coal, the dmax value for Plateau coal has a tendency to have a maximum value at higher CV values, because of its higher volatile matter content, and to decrease faster for both the cases of Ts ) 850 and 1400 °C. Similar trends are evident in the experimental data for Ts ) 850 °C. In the charcombustion-dominant process, the experimental particle diameter is larger for Newlands coal than that for Plateau coal. This agreement verifies the validity of the present percolation model. The reason dmax for Plateau coal decreases faster than that for Newlands coal in the char-combustion-dominant process is considered to be the fact that the specific surface area As for Plateau coal becomes larger than that for Newlands coal, because of the greater swelling, which enhances the fragmentation of the particle from outside during char combustion (see Figure 12b). In contrast, because particle fragmentation decreases the porosity, the porosity of Plateau coal decreases faster than that of Newlands coal, although it temporarily becomes higher at the end of the devolatilization-dominant process, because of the greater swelling (see Figure 12c). It is also found that, for both Newlands and Plateau coals, dmax at Ts ) 1400 °C is small in the early stage of the char-combustion-dominant process but subsequently becomes large, compared to that at Ts ) 850 °C. These are caused by ash agglomeration. That is, dmax becomes smaller for Ts ) 1400 °C in the early stage of the charcombustion-dominant process, because molten ash aggregates at the center. However, the dmax value at Ts ) 850 °C decreases monotonically to zero as the char reaction proceeds, whereas that at Ts ) 1400 °C approaches a constant value determined by the amount of ash in coal. As a result, the trends of dmax at Ts ) 850 and 1400 °C reverse in the char-combustiondominant process, as shown in Figure 12a. The reason the effect of ash agglomeration is more marked for Plateau coal is that ash can move more easily, because the density of char in Plateau coal is lower, because of the greater swelling. As described previously in Figure 9, the conversions where ash starts to agglomerate are different between Newlands and Plateau coals, despite the similar profiles of Tp (see Figure 7). Ash in Newlands coal starts to agglomerate at CV ≈ 0.3-0.4, whereas ash agglomeration in Plateau coal starts at CV ≈ 0.50.6. These values coincide with CV, showing the maximum dmax value. This suggests that the ash agglomeration behavior is affected not only by the ambient temperature but also by swelling. Ash agglomeration has a tendency to proceed prominently after swelling. In addition, Figures 12b and 12c show that the As value and the porosity at Ts ) 1400 °C become lower than those at Ts ) 850 °C in the char-combustion-dominant process, because of ash agglomeration.


Kurose et al.

Conclusions The percolation model, which can account for swelling that is due to devolatilization and ash agglomeration, was applied to the ash formation process in coal combustion, and its validity was examined by comparison with the experimental results obtained using a drop tube furnace facility (DTF). The characteristics of a burning coal particle, such as particle diameter and specific surface area, were investigated in detail. Newlands and Plateau coals with different fuel ratios and ash contents were used as the test coals. In this study, the relationship between particle temperature and conversion required in the percolation model was obtained by performing a numerical simulation of a combustion field in the DTF. The characteristics of the burning coal particle obtained through computations of the percolation model were in general agreement with the experimental data. The particle diameter of Newlands coal with a higher fuel ratio and ash content was observed to be larger than that of Plateau coal in the char-combustion-

dominant process. Also, for both Newlands and Plateau coals, compared to the case of the lower ambient temperature of 850 °C, the particle diameter in the case of the higher ambient temperature of 1400 °C becomes small in the early stage of the char-combustiondominant process, but becomes large afterward. These behaviors were explicitly explained in terms of swelling that was due to devolatilization and ash agglomeration. Therefore, it can be said that the present percolation model is useful for predicting the characteristics of particulate matter (i.e., coal and ash particles) in coal combustion. Acknowledgment. The last author (A. S.) is grateful to Professors T. Miura and H. Aoki of Tohoku University for their help in developing the numerical code of the percolation model. This work was supported by NEDO (New Energy and Industrial Technology Development Organization) International Joint Research Grant Program. Appendix

Calculated Coal Particle Temperature against Conversion Newlands Coal

Plateau Coal

Newlands Coal

Plateau Coal

Ts ) 850 °C

Ts ) 1400 °C

Ts ) 850 °C

Ts ) 1400 °C

Ts ) 850 °C

Ts ) 1400 °C

Ts ) 850 °C

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

CV

Tp (°C)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.311 0.321 0.331 0.341 0.351 0.396 0.4 0.413 0.422 0.435 0.444 0.451 0.46 0.473 0.482 0.492 0.501

323.00 345.33 370.48 398.23 428.37 460.77 495.09 531.21 568.94 608.09 648.58 690.06 732.44 775.56 819.28 863.51 907.94 952.52 997.10 1041.55 1085.84 1129.67 1173.00 1215.73 1257.84 1299.04 1339.34 1378.63 1416.84 1453.96 1489.75 1524.23 1557.33 1588.99 1619.22 1647.83 1698.72 1714.44 1736.93 1765.47 1784.60 1799.18 1814.68 1829.94 1840.51 1848.57 1856.94 1864.14

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.389 0.391 0.402 0.414 0.421 0.431 0.444 0.452 0.464 0.473 0.481 0.494

123.50 225.93 307.51 371.31 420.13 456.56 482.72 500.75 512.48 519.51 523.28 525.00 525.75 526.45 527.85 530.58 535.14 541.90 551.16 563.10 577.84 595.35 615.61 638.50 663.94 691.60 721.29 752.75 785.68 819.82 854.73 890.12 925.67 961.04 995.98 1030.04 1085.37 1151.45 1172.06 1201.79 1219.58 1240.26 1257.04 1269.07 1281.36 1287.74 1292.87 1296.46

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.371 0.381 0.391 0.401 0.411 0.421 0.431 0.441 0.451 0.461 0.471

414.00 411.81 414.81 420.62 431.09 445.24 462.79 483.55 507.25 533.65 562.53 593.58 626.73 661.68 698.24 736.20 775.30 815.49 856.52 898.21 940.39 982.81 1025.48 1068.16 1110.71 1152.98 1194.76 1236.09 1276.76 1316.65 1355.66 1393.60 1430.52 1466.27 1500.75 1533.87 1565.52 1595.74 1624.41 1651.47 1676.87 1700.52 1722.49 1742.68 1761.07 1777.65 1792.37 1805.29

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47

52.00 143.09 221.39 288.00 344.13 390.95 429.48 460.91 486.12 506.03 521.50 533.29 542.19 548.85 553.90 557.93 561.43 564.89 568.73 573.32 578.98 585.98 594.58 604.96 617.28 631.64 648.09 666.73 687.53 710.47 735.49 762.44 791.31 821.91 854.08 887.63 922.28 957.95 994.34 1031.18 1068.21 1105.10 1141.70 1177.66 1212.69 1246.52 1278.78 1309.33

0.511 0.523 0.532 0.545 0.55 0.56 0.57 0.581 0.591 0.6 0.611 0.621 0.631 0.641 0.651 0.661 0.671 0.681 0.691 0.701 0.711 0.721 0.731 0.741 0.751 0.761 0.771 0.781 0.791 0.801 0.811 0.821 0.832 0.841 0.851 0.861 0.871 0.881 0.891 0.901 0.911 0.921 0.931 0.941 0.951 0.961 0.971 0.981

1868.93 1872.31 1873.65 1873.33 1871.24 1867.49 1862.40 1855.91 1847.94 1838.85 1828.26 1816.61 1803.99 1790.80 1775.97 1760.60 1745.00 1728.09 1710.68 1693.44 1675.39 1657.63 1639.79 1622.37 1605.05 1588.96 1571.93 1556.84 1542.27 1528.44 1516.31 1505.78 1496.74 1489.46 1484.36 1481.28 1480.74 1475.96 1470.50 1465.14 1459.64 1454.22 1448.79 1443.40 1438.00 1432.54 1427.12 1421.69

0.503 0.512 0.521 0.542 0.559 0.575 0.605 0.655 0.669 0.669 0.669 0.671 0.681 0.691 0.701 0.711 0.721 0.731 0.741 0.751 0.761 0.771 0.781 0.791 0.801 0.811 0.821 0.831 0.841 0.851 0.861 0.871 0.881 0.891 0.901 0.911 0.921 0.931 0.941 0.951 0.961 0.971 0.981 0.991

1296.77 1295.15 1291.20 1286.12 1272.11 1252.69 1231.59 1185.63 1104.15 1083.46 1083.30 1081.27 1066.67 1052.12 1038.01 1023.82 1009.18 993.75 976.94 958.22 936.90 910.77 908.44 905.74 903.21 900.41 897.76 895.12 892.48 889.79 887.20 884.48 881.88 879.18 876.53 874.01 871.22 868.57 865.92 863.27 860.61 857.96 855.31 852.66

0.481 0.491 0.501 0.511 0.521 0.531 0.559 0.569 0.579 0.589 0.599 0.61 0.619 0.629 0.639 0.649 0.659 0.669 0.679 0.69 0.7 0.709 0.72 0.729 0.739 0.75 0.759 0.77 0.779 0.789 0.8 0.81 0.82 0.83 0.839 0.85 0.86 0.869 0.879 0.89 0.899 0.91 0.92 0.93 0.94 0.949 0.96 0.969

1816.36 1825.60 1833.01 1838.61 1842.44 1844.51 1843.62 1840.65 1836.03 1830.03 1822.85 1813.64 1803.61 1792.01 1779.58 1766.46 1754.02 1737.98 1720.81 1703.67 1686.48 1669.58 1650.92 1633.18 1615.21 1598.53 1582.21 1564.67 1549.52 1535.13 1520.42 1508.43 1497.08 1488.62 1480.82 1475.63 1472.80 1467.83 1463.01 1458.20 1453.39 1448.72 1443.67 1438.82 1434.00 1429.16 1424.33 1419.49

0.48 0.49 0.5 0.51 0.52 0.53 0.556 0.565 0.574 0.584 0.594 0.604 0.614 0.624 0.634 0.644 0.654 0.664 0.674 0.684 0.694 0.704 0.714 0.724 0.734 0.744 0.754 0.764 0.774 0.784 0.794 0.804 0.814 0.824 0.834 0.844 0.854 0.864 0.874 0.884 0.894 0.904 0.914 0.924 0.934 0.944 0.954 0.964

1337.83 1364.02 1387.64 1408.44 1426.26 1440.87 1458.47 1463.42 1464.15 1460.68 1453.97 1442.42 1427.21 1409.31 1386.63 1362.57 1334.70 1302.94 1269.66 1234.11 1197.22 1158.74 1121.87 1084.89 1048.98 1015.01 985.85 959.46 938.58 924.69 919.16 915.85 912.56 909.35 905.96 902.68 899.38 896.08 892.8 889.52 886.25 882.96 879.69 876.41 873.11 869.90 866.56 863.28

EF0400159

Ts ) 1400 °C

Application of Percolation Model to Ash Formation Process in Coal

Recommend Documents