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Application of percolation model to particulate matter formation in pressurized coal combustion
Powder Technology 172 (2007) 50 – 56 www.elsevier.com/locate/powtec
Application of percolation model to particulate matter formation in pressurized coal combustion Ryoichi Kurose a,⁎, Hisao Makino a , Nozomu Hashimoto a , Akira Suzuki b a
b
Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan CD-adapco Japan Co., Ltd., Yokohama Landmark Tower 37F, 2-2-1-1 Minato Mirai, Nishi-ku, Yokohama, Kanagawa 220-8137, Japan Received 27 January 2006; accepted 17 October 2006 Available online 10 November 2006
Abstract 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. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN® model [Niksa, S., Combust. Flame, 100, (1995) 384–394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration. © 2006 Elsevier B.V. All rights reserved. Keywords: Percolation model; Coal combustion; Particulate matter; Ash
1. 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. At present, coal-fired power plants involve pulverized or fluidized bed coal combustion, for which the utilization of various types of coal is desired in order to diversify fuel sources and lower cost and to operate utility boilers with higher efficiency and lower pollutant emission. The technology that is attracting the most attention as a means to greatly increase the efficiency of coal-utilized thermal power generation is a combined cycle power generation system, which ⁎ Corresponding author. Present address: Department of Mechanical Engineering and Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. E-mail address: [email protected] (R. Kurose). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.10.030
utilizes gas turbines or fuel cells. However, in this system, gas turbines or fuel cells are operated at very high temperatures and high pressure, and there are particles in the exhaust gases causing abrasion. Therefore, making dust collection at high temperatures and high pressure is essential. Filtration type dust collectors including ceramic filters and metallic filters hold much promise as they can perform at high temperatures and have both considerable resistance to corrosion and high collection efficiency. Nevertheless, since dust cake, which is found on filters by continuous use, raises the pressure loss, it is needed to remove the dust cake from the filters at an adequate interval. A technology that enables the removal of the accumulated dust is thus key to the successful long-term use of ceramic filters. Generally, stickiness and removal behaviors of the dust are believed to depend on the properties of particulate matter including ash such as particle diameter, shape and other physical features. These properties are also influenced by the type of coal used, combustion conditions and other
R. Kurose et al. / Powder Technology 172 (2007) 50–56
51
Fig. 1. Schematic of DTF.
variables. Hence, to develop the technologies that can easily remove the dust cake from filters, it is essential to understand the influence of these factors on the particulate matter. Monte-Carlo simulation based on percolation theory for char combustion processes called percolation model, which has been under development since the 1980s [1,2], 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. [3] extended this model to predict particle swelling due to coal devolatilization. Kurose et al. [4,5] employed an ash agglomeration model for it and studied the effects of coal properties, ambient temperature and pressure on the characteristics of a burning coal (particulate matter). Concerning the ambient pressure, however, the effect has not been explicitly clarified yet, since universal devolatilization model under the arbitrary pressurized conditions has not been proposed. Effect of initial coal size has not been studied, either. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion in a drop tube furnace facility, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are investigated. The difference in the numerical method from the previous papers [4,5] is that the devolatilization rate of coal is predicted by Van Krevelen et al.'s model [6] with FLASHCHAIN® model developed by Niksa [7]. This method is validated in the present study. 2. Percolation model 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 3D-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 N L) to represent coal particle swelling behavior. The lattice components are distinguished between fixed carbon (while burning, this is also referred as to “char”), volatile matter, ash and pore. Each lattice is located uniformly within the coal particle. 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. The details of used models for devolatilization, char combustion, and ash agglomeration are described in other papers [3–5]. The numerical conditions are determined on the basis of the drop tube furnace facility (DTF) tests, as shown in Fig. 1 [8– 11]. 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 length of 1500 mm, respectively. A flat temperature profile is observed in the reaction zone Table 1 Cases performed in this study Case
Coal
Tw [°C]
P [MPa]
dp [μm]
1 2 3 4 5 6 7 8
Newlands Newlands Newlands Newlands Newlands Newlands Plateau Plateau
1400 1400 850 850 1400 1400 1400 850
0.1 1 0.1 1 0.1 0.1 0.1 0.1
40 40 40 40 20 50 40 40
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Table 2 Coal properties Coal type Proximate analysis [wt.%] Moisture⁎1 Volatile matter⁎2 Fixed carbon⁎2 Ash⁎2 Fuel ratio [–] Ultimate analysis⁎ [wt.%] Carbon Hydrogen Nitrogen Oxygen Combustible sulfur Heating value (high)⁎2 [MJ/kg] Melting point [°C]
Newlands
Plateau
(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 1560
70.1 5.86 1.26 12.37 0.48 28.0 1350
⁎1 As received; ⁎2 dry basis.
of about 800 mm length when using an electric heater. The cases performed are listed in Table 1. The properties of used coals, i.e., Newlands and Plateau coals, are shown in Table 2. Here, Tw is DTF wall temperature. The notable difference between the two coals is in the fuel ratio (= fixed carbon/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 coal particle size, dp, is varied 20, 40 and 50 μm. Initial porosity of the coal particle is 0.2. The initial and maximum lattice numbers, L / l × L / l × L / l and LM / l × LM / l × LM / l, are 40 × 40 × 40 (20 × 20 × 20 for 20 μm, 50 × 50 × 50 for 50 μm) and 100 × 100 × 100, respectively. 3. Results and discussion 3.1. Devolatilization behavior by FLASHCHAIN® model Generally, devolatilization rate required for the percolation model is estimated by the first-order reaction model by Van Krevelen et al. [6]: dV ¼ kV V ⁎ −V ; dt
kV ¼ AV expð−EV =RT Þ:
ð1Þ
Here, V⁎ and V indicate the total volatile matter content in the coal [kg] and volatile mass released from the coal [kg]. T is the gas temperature [K], and R is the universal gas constant (=8.31 J/(mol K)). Although total volatile matter, V⁎, and kinematic parameters for the Arrhenius equation, i.e., the preexponential factor AV [1/s] and activation energy EV [J/mol], are often determined to meet the experimental data [12–15], these values are obtained using FLASHCHAIN® model (DTF type is selected) here. The numerical conditions and results are listed in Table 3. In computations of FLASHCHAIN® model, temperatures of ambient gas and DTF wall are required. These are assumed to be the same values of Ta = 1300 °C or 1900 °C, which corresponds to the peak coal temperature obtained by numerical
simulations of combustion fields in DTF [5] (it should be noted that this value is higher than that of the wall temperature, Tw, of 850 °C or 1400 °C in Table 1, because FLASHCHAIN® model itself cannot account for the gaseous combustion (this is offered as an optional tool in FLASHCHAIN®), and hence the coal temperature obtained from FLASHCHAIN® model approaches to the given ambient gas temperature at the highest). Initial coal temperature is set at 27 °C. In numerical simulations of pulverized coal combustion fields, analyzed total volatile matter amount given in Table 2 (and sometimes multiplied by Q factor, e.g., Q = 1.2) and Van Krevelen et al.'s values [6] for AV and EV, i.e., AV = 2.021 × 103 s− 1 and EV = 3.11 × 104 J/mol, are often used regardless of coal properties [12–15]. The computed total volatile matter amounts for both Newlands and Plateau coals are found to be higher than analyzed ones in Table 2, and increase with increasing ambient temperature and with decreasing ambient pressure and initial coal size. This effect of the pressure agrees well with experimental results by Okumura et al. [16]. On the other hand, although computed EV are of the same order as Van Krevelen et al.'s value [6], computed AV are larger than that by two orders of magnitude. The computation of the percolation model also needs the time variation of coal particle temperature, Tp. This is shown in Fig. 2 with the variation of total weight, i.e. the weight of char, volatile and ash. It is found that the effect of ambient pressure on Tp is small, whereas those of ambient temperature and coal size are marked. As the ambient temperature increases and the coal size decreases, Tp becomes to ascend rapidly. For all cases, Tp tends to approach to the given ambient gas (DTF wall) temperature, Ta. It is, therefore, assumed that for the percolation model, the coal temperature keeps the ambient gas temperature after the devolatilization (during the char combustion). Fig. 3 shows the relationship between coal temperature, Tp, and conversion, CV. Left and right figures show the effects of coal properties and ambient temperature and those of ambient pressure and coal size, respectively. It is found from the left figure that compared to Newlands coal, the rise of Tp for Plateau coal shifts to the higher conversion side for both cases of Ta = 1300 °C and 1900 °C. This is because Plateau coal has a higher volatile matter content, which delays char reaction against conversion. As Ta increases, Tp becomes higher. The inclination of Tp becomes remarkable nearly at CV, where the devolatilization finishes. On the other hand, since the total Table 3 Devolatilization properties obtained by FLASHCHAIN® model Case
Coal
Ta [°C]
P [MPa]
dp [μm]
V⁎ [wt.%]
AV [1/s]
EV [J/mol]
1 2 3 4 5 6 7 8
Newlands Newlands Newlands Newlands Newlands Newlands Plateau Plateau
1900 1900 1300 1300 1900 1900 1900 1300
0.1 1 0.1 1 0.1 0.1 0.1 0.1
40 40 40 40 20 50 40 40
47.6 38.2 45.7 36.7 49.5 46.4 60.7 58.7
1.86 × 103 9.84 × 104 6.50 × 103 3.50 × 104 1.97 × 105 1.07 × 105 2.45 × 105 7.63 × 104
4.85 × 104 4.23 × 104 4.44 × 104 3.87 × 104 4.13 × 104 4.77 × 104 4.64 × 104 4.18 × 104
R. Kurose et al. / Powder Technology 172 (2007) 50–56
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Fig. 2. Time variations of temperature and total weight of coal particle.
volatile matter content decreases for the pressurized condition (see Table 3), Tp for P = 1.0 MPa tends to increase earlier than that for P = 0.1 MPa against CV for both Ta = 1300 °C and 1900 °C. It can be seen that the effect of coal size is negligibly small on the Tp profile (in particular, the profiles after devolatilization overlap).
3.2. General features of particulate matters Fig. 4 shows the coal particle behaviors for all cases. Threedimensional (upper) and sectional (lower) features are given. Green, red and white lattices indicate char, volatile and ash lattices, respectively. It is found that for all cases the
Fig. 3. Relationship between coal temperature and conversion.
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Fig. 4. Depiction of the behaviors of coal particles.
disappearance of volatile matter is much faster than that of char, and there are devolatilization-dominant and char-combustiondominant processes. Also, ash at the lower ambient temperature of Ta = 1300 °C remains stationary, whereas ash at the higher ambient temperature of Ta = 1900 °C agglomerates at the center. According to the ash agglomeration model used in this study, ash starts to melt at the coal temperature, Tp, of approximately 1300 °C (ash viscosity reaches the critical viscosity of μash = 105 Pa s at this temperature). Hence, only for the case of Ta = 1900 °C, in which the maximum Tp greatly exceeds the ashmelting temperature, the ash agglomeration appear. 3.3. Effects of coal properties and ambient temperature Fig. 5 shows the effects of coal properties and ambient temperature on the maximum particle diameter, dmax, specific
surface area, As, and porosity with 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 Tw = 850 °C, are plotted. Since the initial dp50 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 ones directly. Therefore, the experimental values are normalized by the initial dp50 and then multiplied by the initial computational diameter (= 40 μm). In figure of dmax, swelling of the coal particle is observed in the devolatilization-dominant process, whereafter it begins to shrink due to the char reaction. The maximum value of dmax is as large as 1.40 times the initial diameter. Compared to
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agglomeration. That is, dmax becomes smaller for Ta = 1900 °C in the early stage of the char-combustion-dominant process due to the faster reaction rate, but whereafter molten ash aggregates at the center. On the other hand, dmax at Ta = 1300 °C decreases monotonically to zero as the char reaction proceeds (compare cases 1 and 3 and cases 7 and 8 in Fig. 4). 3.4. Effects of ambient pressure and initial coal size Effects of ambient pressure and initial coal size on the maximum particle diameter, dmax, specific surface area, As, and porosity with conversion, CV, are shown in Fig. 6. The effect of ambient pressure is observed to be prominent in the charcombustion-dominant process, rather than the devolatilizationdominant process. At a fixed CV in the char-combustiondominant process, dmax for P = 1.0 MPa tends to be larger than that for P = 0.1 MPa. The reason why dmax for P = 1.0 MPa is larger than that for P = 0.1 MPa is considered to be that since the diffusion coefficient of the oxygen decreases with increasing P,
Fig. 5. Effects of coal properties and ambient temperature on the maximum particle diameter, specific surface area and porosity for initial coal size of 40μm and ambient pressure of 1.0 MPa.
Newlands coal, dmax for Plateau coal tends to decrease faster and more remarkable for both cases of Ta = 1300 °C and 1900 °C. Similar trends are evident in the experimental data for Tw = 850 °C. In the char-combustion-dominant process, the experimental particle diameter is larger for Newlands coal than that for Plateau coal. The reason why dmax for Plateau coal decreases faster and more remarkable than that for Newlands coal in the char-combustion-dominant process is considered to be that the specific surface area, As, for Plateau coal becomes larger than that for Newlands coal due to higher volatile matter content. This enhances the fragmentation of the particle from outside during char combustion. In contrast, because particle fragmentation decreases the porosity, the porosity of Plateau coal decreases faster than that of Newlands coal. It is also found that for both Newlands and Plateau coals, dmax for Ta = 1900 °C is small in the early stage of the charcombustion-dominant process, but subsequently becomes large, compared to that for Ta = 1300 °C. These are caused by ash
Fig. 6. Effects of ambient pressure and coal size on the maximum particle diameter, specific surface area and porosity for Newlands coal.
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the diffusion of the oxygen to the central large particle with high char concentration is suppressed. This inference is supported by the fact that porosity is also higher for P = 1.0 MPa than those for P = 0.1 MPa in the char-combustion-dominant region. That is, at a fixed conversion, the suppression of the diffusion to the central large particle induces the enhancement of the diffusion to the outer small particles, which causes the delays of decreases in porosity. As illustrated in Fig. 4, the comparison of the behavior for CV = 0.99 between case 1 and case 2 shows that the unreacted char lattices for P = 1.0 MPa concentrate on the center region further than that for P = 0.1 MPa. This means that the diameter of the center particle is larger for P = 1.0 MPa than that for P = 0.1 MPa. It is found that as the initial coal size decreases, the profile of dmax with CV tends to monotonically shift lower. While there appears no marked difference in the profile of As, the explicit difference is observed in the char-combustion-dominant process in the profile of porosity. The porosity decreases with decreasing the initial coal size, and the difference is marked between the initial coal sizes of 40 μm and 20 μm. This is considered to be that since the distance between the coal center and surface is short for the small coal particle, ash lattices easily move to the central region and consume the porous. 4. Conclusions The percolation model was applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle were investigated. The devolatilization rate of coal was predicted by Van Krevelen et al.'s model [6] with FLASHCHAIN® model [7]. In the atmospheric pressure condition, the characteristics of the burning coal particle obtained through computations of the percolation model were found to be in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. Also, as the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward, because of the ash
agglomeration. Furthermore, the effect of the initial coal size on the porosity was observed mainly in the char-combustiondominant process, too. Since ash easily arrives at the central region for the small particle, the porosity decreases with decreasing the initial coal size. It was also found that the effect of ambient pressure is prominent in the char-combustion-dominant process. Since the diffusion of the oxygen to the char particle is suppressed, the particle diameter in the pressurized condition becomes larger than that in the atmospheric pressure condition. Acknowledgements The authors would like to thank Prof. Hidehiro Kamiya of Tokyo University of Agriculture and Technology for his helpful suggestions on this study. References [1] A.R. Kerstein, S. Niksa, Proc of The Combust. Inst., vol. 20, 1984, pp. 941–949. [2] A.R. Kerstein, B.F. Edwards, Chem. Eng. Sci. 42 (1984) 1629–1634. [3] A. Suzuki, T. Yamamoto, H. Aoki, T. Miura, Proc. Combust. Inst., vol. 29, 2002, pp. 459–466. [4] R. Kurose, H. Matsuda, H. Makino, A. Suzuki, Adv. Powder Technol. 14 (2003) 673–694. [5] R. Kurose, H. Makino, H. Matsuda, A. Suzuki, Energy Fuels 18 (2004) 1077–1086. [6] D.W. Van Krevelen, C. Van Heerden, F.J. Huntgens, Fuel 30 (1951) 253–258. [7] S. Niksa, Combust. Flame 100 (1995) 384–394. [8] S. Kajitani, H. Matsuda, S. Hara, M. Ashizawa, T. Takahashi, Proc. of 13th Annual International Pittsburgh Coal Conference, Pittsburgh/U.S.A., 1996, p. 976. [9] H. Matsuda, S. Kajitani, M. Shindo, H. Makino, Proc. of 10th International Conference on Coal Science, Taiyuan/China, 1999, pp. 367–370. [10] H. Matsuda, M. Shindo, S. Kajitani, CRIEPI report, 1999, p. W98029, (in Japanese). [11] S. Kajitani, S. Hara, H. Matsuda, Fuel 81 (2002) 539–546. [12] R. Kurose, H. Tsuji, M. Makino, Fuel 80 (2001) 1457–1465. [13] R. Kurose, M. Ikeda, H. Makino, Fuel 80 (2001) 1447–1455. [14] R. Kurose, H. Makino, A. Suzuki, Fuel 83 (2004) 693–703. [15] R. Kurose, M. Ikeda, H. Makino, M. Kimoto, T. Miyazaki, Fuel 83 (2004) 1177–1785. [16] Y. Okumura, Y. Sugiyama, K. Ozaki, Fuel 81 (2002) 2317–2324.
Application of percolation model to particulate matter formation in pressurized coal combustion Ryoichi Kurose a,⁎, Hisao Makino a , Nozomu Hashimoto a , Akira Suzuki b a
b
Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan CD-adapco Japan Co., Ltd., Yokohama Landmark Tower 37F, 2-2-1-1 Minato Mirai, Nishi-ku, Yokohama, Kanagawa 220-8137, Japan Received 27 January 2006; accepted 17 October 2006 Available online 10 November 2006
Abstract 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. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN® model [Niksa, S., Combust. Flame, 100, (1995) 384–394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration. © 2006 Elsevier B.V. All rights reserved. Keywords: Percolation model; Coal combustion; Particulate matter; Ash
1. 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. At present, coal-fired power plants involve pulverized or fluidized bed coal combustion, for which the utilization of various types of coal is desired in order to diversify fuel sources and lower cost and to operate utility boilers with higher efficiency and lower pollutant emission. The technology that is attracting the most attention as a means to greatly increase the efficiency of coal-utilized thermal power generation is a combined cycle power generation system, which ⁎ Corresponding author. Present address: Department of Mechanical Engineering and Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. E-mail address: [email protected] (R. Kurose). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.10.030
utilizes gas turbines or fuel cells. However, in this system, gas turbines or fuel cells are operated at very high temperatures and high pressure, and there are particles in the exhaust gases causing abrasion. Therefore, making dust collection at high temperatures and high pressure is essential. Filtration type dust collectors including ceramic filters and metallic filters hold much promise as they can perform at high temperatures and have both considerable resistance to corrosion and high collection efficiency. Nevertheless, since dust cake, which is found on filters by continuous use, raises the pressure loss, it is needed to remove the dust cake from the filters at an adequate interval. A technology that enables the removal of the accumulated dust is thus key to the successful long-term use of ceramic filters. Generally, stickiness and removal behaviors of the dust are believed to depend on the properties of particulate matter including ash such as particle diameter, shape and other physical features. These properties are also influenced by the type of coal used, combustion conditions and other
R. Kurose et al. / Powder Technology 172 (2007) 50–56
51
Fig. 1. Schematic of DTF.
variables. Hence, to develop the technologies that can easily remove the dust cake from filters, it is essential to understand the influence of these factors on the particulate matter. Monte-Carlo simulation based on percolation theory for char combustion processes called percolation model, which has been under development since the 1980s [1,2], 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. [3] extended this model to predict particle swelling due to coal devolatilization. Kurose et al. [4,5] employed an ash agglomeration model for it and studied the effects of coal properties, ambient temperature and pressure on the characteristics of a burning coal (particulate matter). Concerning the ambient pressure, however, the effect has not been explicitly clarified yet, since universal devolatilization model under the arbitrary pressurized conditions has not been proposed. Effect of initial coal size has not been studied, either. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion in a drop tube furnace facility, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are investigated. The difference in the numerical method from the previous papers [4,5] is that the devolatilization rate of coal is predicted by Van Krevelen et al.'s model [6] with FLASHCHAIN® model developed by Niksa [7]. This method is validated in the present study. 2. Percolation model 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 3D-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 N L) to represent coal particle swelling behavior. The lattice components are distinguished between fixed carbon (while burning, this is also referred as to “char”), volatile matter, ash and pore. Each lattice is located uniformly within the coal particle. 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. The details of used models for devolatilization, char combustion, and ash agglomeration are described in other papers [3–5]. The numerical conditions are determined on the basis of the drop tube furnace facility (DTF) tests, as shown in Fig. 1 [8– 11]. 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 length of 1500 mm, respectively. A flat temperature profile is observed in the reaction zone Table 1 Cases performed in this study Case
Coal
Tw [°C]
P [MPa]
dp [μm]
1 2 3 4 5 6 7 8
Newlands Newlands Newlands Newlands Newlands Newlands Plateau Plateau
1400 1400 850 850 1400 1400 1400 850
0.1 1 0.1 1 0.1 0.1 0.1 0.1
40 40 40 40 20 50 40 40
52
R. Kurose et al. / Powder Technology 172 (2007) 50–56
Table 2 Coal properties Coal type Proximate analysis [wt.%] Moisture⁎1 Volatile matter⁎2 Fixed carbon⁎2 Ash⁎2 Fuel ratio [–] Ultimate analysis⁎ [wt.%] Carbon Hydrogen Nitrogen Oxygen Combustible sulfur Heating value (high)⁎2 [MJ/kg] Melting point [°C]
Newlands
Plateau
(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 1560
70.1 5.86 1.26 12.37 0.48 28.0 1350
⁎1 As received; ⁎2 dry basis.
of about 800 mm length when using an electric heater. The cases performed are listed in Table 1. The properties of used coals, i.e., Newlands and Plateau coals, are shown in Table 2. Here, Tw is DTF wall temperature. The notable difference between the two coals is in the fuel ratio (= fixed carbon/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 coal particle size, dp, is varied 20, 40 and 50 μm. Initial porosity of the coal particle is 0.2. The initial and maximum lattice numbers, L / l × L / l × L / l and LM / l × LM / l × LM / l, are 40 × 40 × 40 (20 × 20 × 20 for 20 μm, 50 × 50 × 50 for 50 μm) and 100 × 100 × 100, respectively. 3. Results and discussion 3.1. Devolatilization behavior by FLASHCHAIN® model Generally, devolatilization rate required for the percolation model is estimated by the first-order reaction model by Van Krevelen et al. [6]: dV ¼ kV V ⁎ −V ; dt
kV ¼ AV expð−EV =RT Þ:
ð1Þ
Here, V⁎ and V indicate the total volatile matter content in the coal [kg] and volatile mass released from the coal [kg]. T is the gas temperature [K], and R is the universal gas constant (=8.31 J/(mol K)). Although total volatile matter, V⁎, and kinematic parameters for the Arrhenius equation, i.e., the preexponential factor AV [1/s] and activation energy EV [J/mol], are often determined to meet the experimental data [12–15], these values are obtained using FLASHCHAIN® model (DTF type is selected) here. The numerical conditions and results are listed in Table 3. In computations of FLASHCHAIN® model, temperatures of ambient gas and DTF wall are required. These are assumed to be the same values of Ta = 1300 °C or 1900 °C, which corresponds to the peak coal temperature obtained by numerical
simulations of combustion fields in DTF [5] (it should be noted that this value is higher than that of the wall temperature, Tw, of 850 °C or 1400 °C in Table 1, because FLASHCHAIN® model itself cannot account for the gaseous combustion (this is offered as an optional tool in FLASHCHAIN®), and hence the coal temperature obtained from FLASHCHAIN® model approaches to the given ambient gas temperature at the highest). Initial coal temperature is set at 27 °C. In numerical simulations of pulverized coal combustion fields, analyzed total volatile matter amount given in Table 2 (and sometimes multiplied by Q factor, e.g., Q = 1.2) and Van Krevelen et al.'s values [6] for AV and EV, i.e., AV = 2.021 × 103 s− 1 and EV = 3.11 × 104 J/mol, are often used regardless of coal properties [12–15]. The computed total volatile matter amounts for both Newlands and Plateau coals are found to be higher than analyzed ones in Table 2, and increase with increasing ambient temperature and with decreasing ambient pressure and initial coal size. This effect of the pressure agrees well with experimental results by Okumura et al. [16]. On the other hand, although computed EV are of the same order as Van Krevelen et al.'s value [6], computed AV are larger than that by two orders of magnitude. The computation of the percolation model also needs the time variation of coal particle temperature, Tp. This is shown in Fig. 2 with the variation of total weight, i.e. the weight of char, volatile and ash. It is found that the effect of ambient pressure on Tp is small, whereas those of ambient temperature and coal size are marked. As the ambient temperature increases and the coal size decreases, Tp becomes to ascend rapidly. For all cases, Tp tends to approach to the given ambient gas (DTF wall) temperature, Ta. It is, therefore, assumed that for the percolation model, the coal temperature keeps the ambient gas temperature after the devolatilization (during the char combustion). Fig. 3 shows the relationship between coal temperature, Tp, and conversion, CV. Left and right figures show the effects of coal properties and ambient temperature and those of ambient pressure and coal size, respectively. It is found from the left figure that compared to Newlands coal, the rise of Tp for Plateau coal shifts to the higher conversion side for both cases of Ta = 1300 °C and 1900 °C. This is because Plateau coal has a higher volatile matter content, which delays char reaction against conversion. As Ta increases, Tp becomes higher. The inclination of Tp becomes remarkable nearly at CV, where the devolatilization finishes. On the other hand, since the total Table 3 Devolatilization properties obtained by FLASHCHAIN® model Case
Coal
Ta [°C]
P [MPa]
dp [μm]
V⁎ [wt.%]
AV [1/s]
EV [J/mol]
1 2 3 4 5 6 7 8
Newlands Newlands Newlands Newlands Newlands Newlands Plateau Plateau
1900 1900 1300 1300 1900 1900 1900 1300
0.1 1 0.1 1 0.1 0.1 0.1 0.1
40 40 40 40 20 50 40 40
47.6 38.2 45.7 36.7 49.5 46.4 60.7 58.7
1.86 × 103 9.84 × 104 6.50 × 103 3.50 × 104 1.97 × 105 1.07 × 105 2.45 × 105 7.63 × 104
4.85 × 104 4.23 × 104 4.44 × 104 3.87 × 104 4.13 × 104 4.77 × 104 4.64 × 104 4.18 × 104
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Fig. 2. Time variations of temperature and total weight of coal particle.
volatile matter content decreases for the pressurized condition (see Table 3), Tp for P = 1.0 MPa tends to increase earlier than that for P = 0.1 MPa against CV for both Ta = 1300 °C and 1900 °C. It can be seen that the effect of coal size is negligibly small on the Tp profile (in particular, the profiles after devolatilization overlap).
3.2. General features of particulate matters Fig. 4 shows the coal particle behaviors for all cases. Threedimensional (upper) and sectional (lower) features are given. Green, red and white lattices indicate char, volatile and ash lattices, respectively. It is found that for all cases the
Fig. 3. Relationship between coal temperature and conversion.
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Fig. 4. Depiction of the behaviors of coal particles.
disappearance of volatile matter is much faster than that of char, and there are devolatilization-dominant and char-combustiondominant processes. Also, ash at the lower ambient temperature of Ta = 1300 °C remains stationary, whereas ash at the higher ambient temperature of Ta = 1900 °C agglomerates at the center. According to the ash agglomeration model used in this study, ash starts to melt at the coal temperature, Tp, of approximately 1300 °C (ash viscosity reaches the critical viscosity of μash = 105 Pa s at this temperature). Hence, only for the case of Ta = 1900 °C, in which the maximum Tp greatly exceeds the ashmelting temperature, the ash agglomeration appear. 3.3. Effects of coal properties and ambient temperature Fig. 5 shows the effects of coal properties and ambient temperature on the maximum particle diameter, dmax, specific
surface area, As, and porosity with 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 Tw = 850 °C, are plotted. Since the initial dp50 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 ones directly. Therefore, the experimental values are normalized by the initial dp50 and then multiplied by the initial computational diameter (= 40 μm). In figure of dmax, swelling of the coal particle is observed in the devolatilization-dominant process, whereafter it begins to shrink due to the char reaction. The maximum value of dmax is as large as 1.40 times the initial diameter. Compared to
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agglomeration. That is, dmax becomes smaller for Ta = 1900 °C in the early stage of the char-combustion-dominant process due to the faster reaction rate, but whereafter molten ash aggregates at the center. On the other hand, dmax at Ta = 1300 °C decreases monotonically to zero as the char reaction proceeds (compare cases 1 and 3 and cases 7 and 8 in Fig. 4). 3.4. Effects of ambient pressure and initial coal size Effects of ambient pressure and initial coal size on the maximum particle diameter, dmax, specific surface area, As, and porosity with conversion, CV, are shown in Fig. 6. The effect of ambient pressure is observed to be prominent in the charcombustion-dominant process, rather than the devolatilizationdominant process. At a fixed CV in the char-combustiondominant process, dmax for P = 1.0 MPa tends to be larger than that for P = 0.1 MPa. The reason why dmax for P = 1.0 MPa is larger than that for P = 0.1 MPa is considered to be that since the diffusion coefficient of the oxygen decreases with increasing P,
Fig. 5. Effects of coal properties and ambient temperature on the maximum particle diameter, specific surface area and porosity for initial coal size of 40μm and ambient pressure of 1.0 MPa.
Newlands coal, dmax for Plateau coal tends to decrease faster and more remarkable for both cases of Ta = 1300 °C and 1900 °C. Similar trends are evident in the experimental data for Tw = 850 °C. In the char-combustion-dominant process, the experimental particle diameter is larger for Newlands coal than that for Plateau coal. The reason why dmax for Plateau coal decreases faster and more remarkable than that for Newlands coal in the char-combustion-dominant process is considered to be that the specific surface area, As, for Plateau coal becomes larger than that for Newlands coal due to higher volatile matter content. This enhances the fragmentation of the particle from outside during char combustion. In contrast, because particle fragmentation decreases the porosity, the porosity of Plateau coal decreases faster than that of Newlands coal. It is also found that for both Newlands and Plateau coals, dmax for Ta = 1900 °C is small in the early stage of the charcombustion-dominant process, but subsequently becomes large, compared to that for Ta = 1300 °C. These are caused by ash
Fig. 6. Effects of ambient pressure and coal size on the maximum particle diameter, specific surface area and porosity for Newlands coal.
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the diffusion of the oxygen to the central large particle with high char concentration is suppressed. This inference is supported by the fact that porosity is also higher for P = 1.0 MPa than those for P = 0.1 MPa in the char-combustion-dominant region. That is, at a fixed conversion, the suppression of the diffusion to the central large particle induces the enhancement of the diffusion to the outer small particles, which causes the delays of decreases in porosity. As illustrated in Fig. 4, the comparison of the behavior for CV = 0.99 between case 1 and case 2 shows that the unreacted char lattices for P = 1.0 MPa concentrate on the center region further than that for P = 0.1 MPa. This means that the diameter of the center particle is larger for P = 1.0 MPa than that for P = 0.1 MPa. It is found that as the initial coal size decreases, the profile of dmax with CV tends to monotonically shift lower. While there appears no marked difference in the profile of As, the explicit difference is observed in the char-combustion-dominant process in the profile of porosity. The porosity decreases with decreasing the initial coal size, and the difference is marked between the initial coal sizes of 40 μm and 20 μm. This is considered to be that since the distance between the coal center and surface is short for the small coal particle, ash lattices easily move to the central region and consume the porous. 4. Conclusions The percolation model was applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle were investigated. The devolatilization rate of coal was predicted by Van Krevelen et al.'s model [6] with FLASHCHAIN® model [7]. In the atmospheric pressure condition, the characteristics of the burning coal particle obtained through computations of the percolation model were found to be in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. Also, as the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward, because of the ash
agglomeration. Furthermore, the effect of the initial coal size on the porosity was observed mainly in the char-combustiondominant process, too. Since ash easily arrives at the central region for the small particle, the porosity decreases with decreasing the initial coal size. It was also found that the effect of ambient pressure is prominent in the char-combustion-dominant process. Since the diffusion of the oxygen to the char particle is suppressed, the particle diameter in the pressurized condition becomes larger than that in the atmospheric pressure condition. Acknowledgements The authors would like to thank Prof. Hidehiro Kamiya of Tokyo University of Agriculture and Technology for his helpful suggestions on this study. References [1] A.R. Kerstein, S. Niksa, Proc of The Combust. Inst., vol. 20, 1984, pp. 941–949. [2] A.R. Kerstein, B.F. Edwards, Chem. Eng. Sci. 42 (1984) 1629–1634. [3] A. Suzuki, T. Yamamoto, H. Aoki, T. Miura, Proc. Combust. Inst., vol. 29, 2002, pp. 459–466. [4] R. Kurose, H. Matsuda, H. Makino, A. Suzuki, Adv. Powder Technol. 14 (2003) 673–694. [5] R. Kurose, H. Makino, H. Matsuda, A. Suzuki, Energy Fuels 18 (2004) 1077–1086. [6] D.W. Van Krevelen, C. Van Heerden, F.J. Huntgens, Fuel 30 (1951) 253–258. [7] S. Niksa, Combust. Flame 100 (1995) 384–394. [8] S. Kajitani, H. Matsuda, S. Hara, M. Ashizawa, T. Takahashi, Proc. of 13th Annual International Pittsburgh Coal Conference, Pittsburgh/U.S.A., 1996, p. 976. [9] H. Matsuda, S. Kajitani, M. Shindo, H. Makino, Proc. of 10th International Conference on Coal Science, Taiyuan/China, 1999, pp. 367–370. [10] H. Matsuda, M. Shindo, S. Kajitani, CRIEPI report, 1999, p. W98029, (in Japanese). [11] S. Kajitani, S. Hara, H. Matsuda, Fuel 81 (2002) 539–546. [12] R. Kurose, H. Tsuji, M. Makino, Fuel 80 (2001) 1457–1465. [13] R. Kurose, M. Ikeda, H. Makino, Fuel 80 (2001) 1447–1455. [14] R. Kurose, H. Makino, A. Suzuki, Fuel 83 (2004) 693–703. [15] R. Kurose, M. Ikeda, H. Makino, M. Kimoto, T. Miyazaki, Fuel 83 (2004) 1177–1785. [16] Y. Okumura, Y. Sugiyama, K. Ozaki, Fuel 81 (2002) 2317–2324.