Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion

Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion

Fuel 79 (2000) 627–633 www.elsevier.com/locate/fuel Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion...

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Fuel 79 (2000) 627–633 www.elsevier.com/locate/fuel

Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion G. Liu*, H. Wu, R.P. Gupta, J.A. Lucas, A.G. Tate, T.F. Wall Cooperative Research Centre for Black Coal Utilization, Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia Received 27 April 1999; received in revised form 19 July 1999; accepted 3 August 1999

Abstract Char fragmentation during pulverized coal combustion was studied using an Australian bituminous coal. The coal was combusted with air in a drop tube furnace at a gas temperature of 13008C. The char samples were collected at different levels of char burnout, and their structure was examined using scanning electron microscopy. Approximately 40% of the char particles formed after pyrolysis were cenospheres with a highly non-uniform porous structure and a large central void. A large number of fine particles were also observed in the char samples with burnout levels between 30 and 50 vol%, which suggests that significant fragmentation occurs during the early combustion stage. A mathematical model was developed relating the fragmentation of cenospherical char particles with the macropores in the particle shell. The formation of these macropores partially results because of the carbon removal from the surface of the thin shell due to surface oxidation. A percolation model was used to simulate the char structural changes during combustion in regime III, and the predicted particle size distributions qualitatively agreed with the experimental measurements. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fragmentation; Bituminous coal char; Coal combustion; Modeling

1. Introduction Char fragmentation is an important phenomenon during pulverized coal combustion, and has a significant impact on ash formation [1–4] and conversion efficiency [5–7]. The fragmentation can reduce the initial particle mass of the pulverized coal by around 20–30% [5,8], therefore decreasing the burnout time of the coal particles. In addition, the size distribution of fly ash [1–3] produced from the coal combustion is significantly influenced by char fragmentation. For bituminous coal chars, for example, a large number of ash particles with a power-law distribution are formed due to extensive fragmentation [4]. The percolation theory [1–3,6,7] allows an understanding of char combustion and char fragmentation, and has been extensively applied to the studies of fragmentation. A range of critical porosity [6] for various materials have been determined for char combustion under chemical controlled conditions (regime I), above which the char samples disintegrate into fine fragments. Further attempts [7] have been made to study char fragmentation during combustion in regimes II and III, where the non-uniform reactivity and reactant gas diffusion throughout the particle have to be * Corresponding author. Tel.: 161-2-4921-7442; fax: 161-2-4921-8692. E-mail address: [email protected] (G. Liu).

taken into consideration. Consequently, the derived critical porosity is consistent with that obtained for combustion in regime I. In regimes II and III perimeter fragmentation dominates which can also be predicted by the percolation theory. Char fragmentation has been found to be associated with the porous structure of char particles. Macropores present in char particles have been shown to play a significant role in char fragmentation during diffusion controlled reactions [2,9]. Recent findings on char morphology [10] and its linkage to char reactivity [11,12] and ash formation [13] have motivated the study of the relationship between char structure and fragmentation. Fig. 1 shows three typical char types formed during high heating rate pyrolysis of an Australian bituminous coal. Among them, Group I char (cenosphere) has a high porosity (0.7–0.9), while Group III char (called dense char) is of low porosity (,0.4) and high density [10]. The remaining char particles with a structure between Group I and Group III have an intermediate porosity, and are classed as Group II. The number proportion of cenospherical char particles correlates well with the pressure of pyrolysis and the vitrinite content of the coal [2,10]. Previous studies have shown that cenospherical char particles are very reactive during combustion [14] producing a large number of ash particles after burnout [2,4], which implies that extensive fragmentation occurs during the

0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00186-6

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The objective of this paper is to further investigate the fragmentation of cenospherical char during coal combustion using both experimental and theoretical methods. An Australian bituminous coal was combusted in a drop tube furnace at a gas temperature of 13008C. Char samples were collected at different levels of burnout, and were examined using scanning electron microscopy (s.e.m.). The fragmentation of a cenospherical char particle was modeled by the percolation theory, and the predicted particle size distribution for char samples qualitatively compared with the experimental measurements. 2. Experimental

Fig. 1. Typical coal char types formed during high heating rate pyrolysis of an Australian bituminous coal [11]. (a) Group I char, cenosphere; (b) Group II char, mixed porous; and (c) Group III char, dense char.

char combustion. The mechanisms of ash formation from char particles with different porous structures have recently been proposed and a significant correlation between ash formation and char fragmentation has been noted [13]. The fragmentation of cenospherical char particles has been investigated for the purpose of ash formation and has been modeled using the percolation theory [2]. The cenospherical char structure was represented by varying the porosity along with the particle radial distance. The model allowed a better understanding of the ash formation from cenospherical char particles; however, no detailed structural changes were provided during char oxidation.

An Australian bituminous coal was used in the present study. The coal was ground and sieved to a size range of 163–90 mm. The excluded minerals were separated by float/sink separation at a specific gravity of 2.0. The floated coal sample contained 2.9 vol% moisture, 9.2 vol% mineral matter, 28.1 vol% volatile matter, and 59.8 vol% fixed carbon (air dry basis). The maceral composition of this coal sample was 37.7 vol% vitrinite, 57.8 vol% inertinite, and 4.6 vol% liptinite (mineral free), respectively. A drop tube furnace (d.t.f.) was used to produce the char samples. The furnace had a tube diameter of 50 mm and length of 250 mm. The air feed rate was approximately 1600 ml/min, allowing a laminar flow in the tube and a residence time of 1–2 s. The coal samples were fed into the furnace through a water-cooled probe at a rate of approximately 5 g/h with 20 vol% excess air. Particles enter the furnace just above the top of the furnace hot zone. The furnace temperature was set to 13008C. The collection system consisted of a water-cooled high efficiency quench probe connected to a cyclone (.2 mm) followed by a Millipore aerosol filter (,2 mm). The position of the quench probe was adjusted so as to control the particle residence time in the furnace, and hence the level of burnout. The char particles were collected from both the cyclone and the filter. Full details of the experimental setup can be found elsewhere [15]. A s.e.m. with an attached energy-dispersive X-ray detection system was used to examine the char samples for surface characterization. The s.e.m. operated at an accelerating voltage of 15 keV and a working distance of 15 mm. Char samples were prepared for s.e.m. examination by a procedure described in detail elsewhere [16]. A Malvern Mastersizer was used to measure the particle size distribution of the char samples. 3. Percolation model description Percolation theory was used to model the fragmentation of cenospherical char particles, and to qualitatively display the development of the porous structure of the char during combustion. An initial porous structure of a char particle

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Fig. 2. (a) A 2D schematic representation of a cenospherical char particle used in the percolation simulation. The char is generated from measured particle size, porosity and surface area using the discrete pore model [17]; (b) structural changes of a cenospherical char particle during combustion in regime III simulated by the percolation model.

was generated in two dimensions. For the cenospherical char, the shell was represented by a ring with an external diameter and thickness determined by measurement, with the interior of the ring representing the void of the cenospherical char. Pores greater than 1 mm have been found to play an important role in char fragmentation [2,9], and were considered to make up the porous structure in the present investigation. The macropores were distributed in the shell randomly so as to approximately match the measured porosity and surface area. The pore size and the number of pores in the shell can be calculated using the discrete pore model [17] which has been used to predict gas–solid reactions [18]. In the discrete pore model, the macropores were considered to be parallel cylinders, therefore a three-dimensional (3D) structure can be represented in a 2D simulation. The macropore size and number can be calculated by integrating the following equations [17]:

1 ˆ 1 2 F N …j†

…1†

rP Sg h ˆ NF N21 …j†G…j†

…2†

F…j† ˆ

1 1 4j ; 1 1 4j 1 p j 2

G…j† ˆ

2pj…1 1 2j† …1 1 4j 1 pj2 †2

…3†

where 1 is the shell porosity (a ratio of pore volume in the shell to total volume of the shell), Sg is the surface area covered by macropores in the shell, N and j represent the number and the dimensionless radii of macropores (j ˆ r=h, r is the radius of the macropore and h is the characteristic length of the shell), respectively, and rP is the shell apparent density. In the above equations, 1, Sg and rP can be obtained

from measurements. With a known value of h, the variables F, G, N and j can be calculated by Eqs. (1)–(3). The macropores are randomly distributed and may overlap [17]. Fig. 2(a) shows the 2D schematic representation of a generated cenospherical char particle. Those pores directly exposed to ambient atmosphere allow oxygen to penetrate into the pores. The carbon in the char particle was represented by a number of clusters occupied by carbon sites on a 2D lattice. Two adjacent sites are defined as being connected, with all occupied sites containing an equal mass of the carbon in the initial char particle. The unoccupied sites represent pores, and the surrounding environment is defined to be pores. Length and width of each carbon site was determined so that the representation in the 2D lattice matches the physical structure of the particle. A more detailed description can be found elsewhere [1,7]. Those carbon sites exposed to oxygen are combusted, simulating reaction in the external diffusion controlled regime III [2]. Char combustion is represented by the removal of carbon sites. When at least three neighboring sites around a carbon site are occupied by the oxygen gas, the carbon site is removed. The carbon sites on the internal surface are not removed until a pore connecting the internal void with the external environment is established by its growth during the reaction. The reaction proceeds until all the carbon sites are removed. The number of removed carbon sites during each scanning represents the relative reaction rate. Carbon conversion was calculated by dividing the total loss of carbon sites after each scan by the initial number of carbon sites. The connectivity of neighboring sites was checked by

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Fig. 3. S.e.m. images of char samples generated at various burnout levels at a gas temperature of 13008C in a drop tube furnace under atmospheric condition. The scale bar is 500 mm.

scanning the lattice, and carbon sites or clusters surrounded by unoccupied (empty) sites were treated as a fragment, hence the number of fragments was determined. In the calculation of their sizes, the fragments were considered to be circular with an equivalent fragment diameter estimated from the total area occupied by the carbon sites within each fragment. After each scan, the size distribution of the fragments was obtained based on the number and the mass of the fragments. The bins representing size distribution had a size interval of 1 mm.

4. Results and discussion 4.1. S.e.m. overview images The char samples were examined using a s.e.m. The char

burnout was calculated from the coal burnout which was determined using ash as a tracer. It is believed that there is a negligible loss between mineral matter in the coal and ash in residual char during char combustion, as the coal contained Al2 O3 1 SiO2 as high as 95 vol%, therefore this technique can allow an accurate estimation of burnout. The s.e.m. images for the char samples collected at char burn-off levels of 1.6, 30.2, 54.4 and 80.8 vol% are shown in Fig. 3. Fig. 3(a) …X ˆ 1:6 wt%† shows the intact char particles of different sizes; no fine particles were found in the char samples, implying that no fragmentation had occurred during pyrolysis. This is inconsistent with the results of Mitchell et al. [5] who found significant fragmentation occurring in a synthetic char during devolatilization. At a burnout level of 1.6% approximately 30–40% of the particles by number were found to have a cenospherical structure

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level, and continued as combustion proceeded. A large number of small particles were formed due to fragmentation at burnout levels of approximately 54 and 80 vol% (Fig. 3(c) and (d)). A number of ash particles were observed in the char samples. A decrease in average particle size was found as burnout increases. 4.2. Particle size distributions

Fig. 4. The sentivity of the particle size and porosity on fragment size distribution of a cenospherical char at a burnout level of 55 vol%. (1) Base case: dpexternal ˆ 150 mm; d ˆ 7:5 mm; 1total ˆ 0:8; Sg ˆ 1 m2 =g: (2) Increased porosity 1total ˆ 0:85: (3) Increased wall thickness d ˆ 20 mm: (4) Increased area Sg ˆ 2 m2 =g:

through examination of images of the individual chars in cross section [16]. The remaining 60–70% of char particles were dense chars. The cenospherical char had a high porosity consisting of non-uniformly distributed macropores, with a large central void in the particle surrounded by a thin shell. SEM images showed a number of macropores (10–40 mm in diameter) opening at the particle surface [16]. During the process of combustion and gasification, this char type behaves differently from uniformly porous char particles and will significantly influence the char reactivity. The surface macropores provide channels for the transport of adequate reactant gas from the outside surface to the internal void surface. The large void in the particle center and the macropores in the particle shell reduce the connectivity of the porous network of the particle. A number of small particles resulting from fragmentation of cenospherical char particles were observed at an approximate char burnout of 30 vol% (Fig. 3(b)). The individual char particles at this burnout level consist of cenospherical particles and dense particles (without a large void). It indicates that char fragmentation occurred prior to this burnout

The cenospherical char structure was approximated as shown in Fig. 2(a) for the percolation model simulation. The external diameter of 150 mm and shell thickness of 7.5 mm for the original cenospherical particle were derived from measurements using s.e.m. image analysis. The macroporosity of the particle was assumed to be 0.8, including the void porosity of 0.75. The surface area occupied by the macropores was taken as 1 m 2/g based on experimental measurements. The irregular thickness of the thin shell was represented by the macropores embodied in the shell. Combustion of the cenospherical char was simulated using the percolation model described above. Carbon was removed from the sites on the particle surface and the pore walls that were exposed to oxygen. Fig. 2(b) shows char structural changes during combustion simulated using the percolation model. It is worthwhile to mention that the 2D simulation can represent 3D measurements. The 2D percolation model has been used in char fragmentation and ash formation modeling during pulverized coal combustion [1,2,7]. It has been indicated previously that the particle size obtained by the 2D simulation is close to that obtained from a 3D calculation [2]. Sensitivity studies of the model. A sensitivity study of cenospherical char fragmentation has been performed. The effects of the particle and pore structure on the fragment size distribution have been obtained at a burnout level of 55 vol%, as shown in Fig. 4. An increase in the macroporosity and the surface area of the shell results in a slight reduction in the fragment size. This is because the high porosity and large numbers of pores (subsequent to high surface area) lead to a high disconnectivity for the carbon

Table 1 Comparison of characteristics between the cenospherical char and the dense char particles Characteristics

Cenospherical char

Dense char

Volumetric porosity a Particle size b Apparent reactivity b Critical porosity for fragmentation Estimated burnout at fragmentation threshold Number and size of fragment d

0.7–0.9 ,1.5 3 N/A c

0.1–0.3 1 1 0.7–0.8

0.5

0.7–0.8

Large number but small size

Medium

a

Measured by s.e.m. analysis[16]. Relative to dense char. c N/A: not available. d Also depending on reaction conditions. b

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Fig. 5. (a) Experimental measurements and (b) model predictions of particle size distributions of char samples at different burnout level. The predictions were performed using the measured particle size distribution at burnout level of 1.6 vol% as an initial distribution.

matrix, as illustrated previously [11]. However, the shell thickness has a significant effect as indicated in Fig. 4. The fragment size for a thick shell is much larger than that of thin shells, and fragmentation of the cenospherical char with a thick shell occurs later than chars with a thin shell. Comparison with measurements. The measured particle size distribution at a burnout level of 1.6 vol% was used as an initial distribution in the simulation. In the initial char particles, 40 vol% of the particles were treated as cenospherical char particles; the remainder were regarded as dense (Group III) particles. Calculations were performed to simulate fragmentation of cenospherical char particles of varying size and number proportions. For dense particles, a uniform pore structure was assumed so that a shrinking core model could be applied to combustion under external diffusion controlled conditions. The fragmentation of dense particles by perimeter percolation also occurred during the later stages of combustion where particles approached the critical porosity [6] (see Table 1). The measured and predicted particle size distributions of char samples collected at the different levels of burnout are shown in Fig. 5, indicating a qualitative agreement. For the measured distributions, most particles are within a size range of 270 to 1120 mm for unburnt char—many show an increased size (compared to the coal sample) due to swelling during high heating rate pyrolysis. A slight shift towards smaller sizes was observed at a char burnout level of 30%, which was partially attributed to the cenospherical char fragmentation. Significant fragmentation was indicated by a large shift in the size distribution between the burnout level of 30 and 54 vol%. Approximately 20% of particles by volume at 54 vol% burnout were under 20 mm which consisted mostly of ash particles (Fig. 5(a)). This was not reflected in the simulation, as ash is not considered in the present model. A large number of ash particles were observed at a burnout level of 81 vol%. The decrease in the range of the size distribution in the predicted results between initial and 30 vol% burnout (Fig. 5(b)) suggests that fragmentation was over-estimated. Further work on fragmentation needs to include the effect of ash inclusions. 4.3. Mechanisms of char fragmentation

Fig. 6. S.e.m image of fragmented char particle generated at about 30 vol% burnout level at a gas temperature of 13008C in a drop tube furnace under atmospheric condition. The scale bar is 50 mm.

A typical fragmented particle observed in the char sample generated at a char burnout level of 30.2 vol% is shown in Fig. 6. It is apparent that a small fragment has detached from the particle, which is similar to the small particles seen in Fig. 3(b). A narrow fault can be seen on the particle surface indicating that it likely to form another fragment during further reaction. It is suggested that the fault in the shell of the particle results from the connection of neighboring macropores on the particle surface that connect the external environment to the internal void. It is also suggested that the macropores in the shell play an important role in the char fragmentation. This is consistent with the previous studies

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[2,9] on fragmentation of uniformly porous char particles. However, the development of macropores and their contributions to the fragmentation of cenospherical char is different. There are two mechanisms for the formation of macropores on the char surface. One is that the macropores originate from devolatilization. A number of surface macropores have been observed on the particle surface of unburnt char in a previous study [16]. Obviously these evolve during combustion and partially contribute to the macropores on the char particle surface up to a certain level of burnout. The other mechanism is the formation of surface macropores during the combustion. The gas temperature of 13008C implied that the char was combusted in the external diffusion controlled regime III [2], where oxidation took place on the particle surface. The irregular thickness of the shell provides some weak parts, which may be consumed rapidly, contributing to the formation of the surface pores. It is suggested that mechanisms for fragmentation of cenospherical char and dense char are consistent. 4.4. Approximation of char burnout at the fragmentation threshold From the above discussion it appears that cenospherical char particles fragment at an overall char burnout level of about 30 vol%. According to the previous studies [12,14], cenospherical char particles have an apparent reaction rate approximately three times higher than dense chars under the same combustion conditions. Therefore the burnout level for cenospherical char fragmentation is estimated to be 0.5 by considering 40% cenospherical char particles by number in the char sample. The critical porosity derived for the dense char particle is 0.7–0.8 at which fragmentation of the particle commences [6]. For a char particle with an initial porosity of 0.1, the burnout level at the fragmentation threshold was estimated to be 0.7–0.8. As discussed earlier, thin shells lead to a high probability of disconnection in the carbon matrix. A comparison of cenospherical char and dense char particles is presented in Table 1. 5. Conclusions and implications Coal particles undergo complicated structural changes during combustion. Devolatilization of a coal at 13008C under atmospheric conditions resulted in approximately 40% of the char particles with cenospherical characteristics, i.e. a highly porous structure and a large central void surrounded by a thin shell. Experimental results presented in this study show that significant fragmentation takes place during the early stages of char combustion, resulting in a large number of small char particles. Fragmentation begins below an overall char burnout level of 30 vol%, and becomes significant at char burnout levels between 30 and

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50 vol%. Size distributions predicted using the percolation model qualitatively agreed with experimental measurements. The fragmentation of cenospherical char particles appears to be associated with the macropores in the particle shell, which provide weak points from which a fragment can detach. Macropores in the shell are formed during both devolatilization and combustion. The conventional percolation model can explain cenospherical char fragmentation by incorporating a non-uniform pore structure. The fragmentation of cenospherical char particles during the early stages of combustion also has a significant influence on ash formation and char reactivity. The ash particle size distribution is expected to shift towards fine ash particles. Fragmentation will also significantly increase the reaction rate of char combustion in regime III due to the increased external particle surface area.

Acknowledgements The authors wish to acknowledge the financial support provided by the Cooperative Research Centre for Black Coal Utilization, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. References [1] Kang S-G, Helble JJ, Sarofim AF, Beer JM. 22nd Symposium (International) on Combustion. The Combustion Institute, 1988. p. 231. [2] Kang SG, Sarofim AF, Beer JM, 24th Symposium (International) on Combustion. The Combustion Institute, 1992. p. 1153. [3] Helble JJ, Sarofim AF. Combustion and Flame 1989;76:183. [4] Baxter LL. Combustion and Flame 1992;90:174. [5] Mitchell RE, Akanetuk AEJ. 26th Symposium (International) on Combustion. The Combustion Institute, 1996. p. 173. [6] Kerstein AR, Niska S. 21st Symposium (International) on Combustion. The Combustion Institute, 1984. p. 941. [7] Salatino P, Micco F, Massimilla L. Combustion and Flame 1993;95:501. [8] Hargrave G, Pourkashanian M, Williams A. 21st Symposium (International) on Combustion. The Combustion Institute, 1986. p. 221. [9] Kantorovich II, Bar-Ziv E. Combustion and Flame 1998;113:532. [10] Benfell KE, Bailey JG. Eighth Australian Coal Science Conference, 1998. p. 157. [11] Liu GS, Benyon P, Benfell KE, Bryant GW, Tate AG, Boyd RK, Harris DJ, Wall TF. Submitted for publication. [12] Wall TF, Tate AG, Bailey JG, Jenness LG, Mitchell RE, Hurt RH. 24th Symposium (International) on Combustion. The Combustion Institute, 1992. p. 1207. [13] Wu H, Bryant GW, Wall TF. Submitted for publication. [14] Morley C, Jones RB. 21st Symposium (International) on Combustion. The Combustion Institute, 1986. p. 269. [15] Bailey CW. PhD Thesis, The University of Newcastle, 1999 [16] Wu H, Bryant GW, Benfell KE, Wall TF. Submitted for publication. [17] Ballal G, Zygourakis K. Ind Engng Chem Res 1987;26:911. [18] Ballal G, Zygourakis K. Ind Engng Chem Res 1987;26:1787.