Primary and secondary fragmentation of coals in a circulating fluidized bed combustor

Twenty-FifthSymposium(International)on CombustionYFheCombustionInstitute,1994/pp.219-226

PRIMARY A N D SECONDARY F R A G M E N T A T I O N O F COALS IN A CIRCULATING F L U I D I Z E D B E D C O M B U S T O R UMBERTO ARENA,* ANTONIO CAMMAROTAANDRICCARDO CHIRONE Istituto di Ricerche sulla Combustione National Research Council (CNR) Piazzale Tecchio 80, 80125 Napoli, Italy Primaly and secondary fragmentation of two Kentucky No. 9 coals, having similar proximate and ultimate analysesbut different swellingindexes (2.5 and 9, respectively),were studied in a laboratory scale circulating fluidized bed combustor (CFBC). The apparatus, having a 41-mm i.d. and 1.92-m-high riser, was operated keeping fixedthe gas velocity and the size of inert bed material at valuesof practical interest. Two experimental procedures were used to separately investigateprimary and secondary fragmentation effeets taking place during fluidized bed combustion of coals. Particle multiplication factor, i.e., the number of particles generated per one mother particle, was used to quantify these effects. Statistical functions of fragmentation (the probability of breakage by primary fragmentation, the probability density that a shrinking particle of a given size breaks into fragments, and the size distribution of subpartieles produced by secondary fragmentation) were also determined and embodied into an availablemodel for circulatingfluidizedbed combustion of coals. On the basis of this mathematical model, the relevance of primary fragmentation on some output variables chosen to characterize the performance of a circulating fluidized bed combustor was quantified.

Introduction

dicated that the effect of the gas velocity and bed solids size was almost negligible within the range of these variables of practical interest. On the contrary, the extent of secondary fragmentation appeared strongly connected to char structure, whose mechanical properties are determined by the devolatilization process [81. A scope of this paper is that of acquiring information about the correlation between initial size and structure of coal/char particles and phenomena of primary and secondary fragmentation. To this end, two coals having an almost similar chemical composition but a strongly different free-swelling index were tested. Experimental procedures were set up in order to isolate primary and secondary fragmentation effects from those of the other bed carbon comminution phenomena. Attention is focused on a coal particle from its sudden injection in a CFBC until its almost complete burnout. The relevance of primary fragmentation on the performance of a circulating fluidized bed (CFB) combustor continuously operated is estimated by means of an available mathematical model, evaluating carbon loading and size distribution of carbon particles in the riser.

The modelling, design, and performances of circulating fluidized bed combustors (CFBC's) are affected by the comminution phenomena, which contribute to size reduction of coal and char particles [1-3]. Massimilla and his coworkers [4] recently reviewed phenomenology of comminution in fluidized bed combustion, pointing out that this can be seen as a result of at least four phenomena occurring in series parallel with each other and with combustion, namely, the primary fragmentation of coal, the secondary fragmentation of char, the fragmentation by percolation of relatively fine char, and the abrasive attrition among chars, bed inert solids, and combustor walls. Several papers [4-7] analyzed the role of comminution phenomena for what concerns operations of bubbling fluidized bed combustors (BFBC's), More recently, some studies [1,2] have been carried out to extend information to operative conditions typical of CFBC's, analyzing phenomenology and relevance of abrasive attrition and secondary fragmentation during combustion of a char particle. These studies showed that when moving from bed captive regimes typical of BFBC's to bed transport regimes proper of CFBC's, the relevance of secondary fragmentation appears to increase [2]. Results in-

Apparatus:

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The circulating fluidized bed combustor (Fig. 1) consists of a riser 41-mm i.d. and 1.92-m high, a solids collecting system, and a 41-mm i.d. recirculation

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Experimental

219

PRACTICALASPECTS OF COMBUSTION

220

Injection point of char particles

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small stream (less than 1% of the overall gas flow rate) of 4.5% O2 oxygen-nitrogen mixture was injected immediately above the elbow connecting the recirculation column and inclined pipe (gas stream 4). An undervacuum vessel was connected to the bottom of the riser to collect solids at a given time. Solids were quenched by nitrogen to avoid further reaction between air and char. The riser was operated at 850 ~ while the rest of the loop was at about 700 ~ A probe placed in the middle of the inclined recirculation pipe was used for on-line gas analysis. Electronic transducers gave pressure profiles in each section of the loop. Solids concentrations along the riser were calculated using a pressure gradient technique [9].

Materials:

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column. The apparatus is not equipped with a device able to control solids circulation in the CFB loop. To avoid bypass through the recirculation column, fluidizing gas was divided into two streams: gas stream 1, consisting of nitrogen, maintained solids at the bottom of the riser in a state of bubbling fluidization; gas stream 2, preheated at a temperature of 700 ~ circulated solids in the loop. This latter stream consisted of nitrogen during primary fragmentation tests and of an oxygen-nitrogen mixture (4.5% 02) during secondary fragmentation tests (such a low-oxygen concentration was used to expand the timescale of secondary fragmentation phenomena). Solids separated by a high-efficiency cyclone were conveyed to the reeirculation column and transferred to the riser by means of a 41-ram-i.d. inclined pipe. Nitrogen (gas stream 3) was injected downstream the cyclone to wash out solids from any oxygen coming from the riser. During secondary fragmentation tests, a very

Properties of coals are reported in Table 1. Both are classified as Kentucky No. 9 coal, and their almost equal chemical compositions confirm this affinity. In the following, that having the low free-swelling index is indicated as Kentucky No. 9-L (or briefly, KgL); the other, having a high index, is indicated as Kentricky No. 9-H (KgH). Both Kentucky No. 9 coals were used for primary fragmentation experiments, i.e., devolatilized by injection in the CFB eombustor operated under inert atmosphere at 850 ~ The different swelling properties of parent coals strongly affected the generation of relative chars. Table 2 reports their chemical composition and physical properties, whereas micrographs of Figs. 2a and 2b give an idea of macroscopic differences in their mechanical structures. A comparative analysis of micrographs and of Table 2 data leads to the following considerations: (1) Char partieles from K9H coal present several large pores: the structure appears kept together by thin bridges of carbonaceous material. (2) Several silica sand particles are visible in the structure of char obtained from KgH coal. These particles, which are probably captured at the plasticity stage during the devolatilization process and embodied by the char particle as it resolidificates at the end of devolatilization, are responsible for the higher ash content of K9H char particles compared to that of the parent coal. Char particles obtained from both coals were sieved in order to obtain narrow cuts, which were then used to prepare batches of char particles of different initial sizes (Table 2). Silica sand with a particle size distribution between 0.3 and 0.4 mm and a density of 2540 kg/ma was used as bed material.

Primary Fragmentation Test Procedure: A batch of five coal particles, 9-10 mm as initial size, was injected at the middle of the inclined pipe by means of a pulse of nitrogen (Fig. 1) and fiuidized

PRIMARY AND SECONDARY FRAGMENTATION OF COALS

221

TABLE 1 Characteristics of coals used Coal

Kentucky No. 9 L

Kentucky No. 9 H

Free-swelling index (ASTM D720) Proximate analysis (%) (as received): Moisture Ash Volatile matter Fixed carbon Ultimate analysis (%) (dry basis): Carbon Hydrogen Nitrogen Sulphur Oxygen Ash Coal particle size (mm)

2-2.5

8-9

3.3 6.7 41.5 48.5

3.8 7.1 38.2 50.9

70.9 5.4 1.6 3.4 11.8 6.9 9-10

71.6 5.4 2.0 3.2 10.4 7.4 9-10

TABLE 2 Characteristics of chars used Parent coal

Kentucky No. 9 L

Kentucky No. 9 H

Apparent particle density (kg/m3) Ultimate analysis (%) (dry basis) Carbon Hydrogen Nitrogen Sulphur Oxygen Ash Initial char particle size ranges (mm)

815

436

76.9 1.2 0.3 0.5 2.2 18.9 4~5; 5-6; 7-8

57.2 0.5 0.2 0.3 1.3 40.5 4-5; 5-6

at an overall gas velocity of 2.7 m/s by nitrogen streams 1 and 2. Devolatilizing coal particles mainly stayed in the dense zone of the riser because of their density and size; just occasionally, some of them ran along the loop. The experiment was stopped after a time interval (about 180 s) that was estimated by parallel investigation to correspond to the devolatilization time. At this point, all char and sand particles in the loop were collected in the undervacuum vessel and "gently" sieved. Fragments larger than 0.425 mm were analyzed to measure their size and number. Char particles collected from the bed were used for secondary fragmentation tests in successive runs. Devolatilization time was evaluated by using the same apparatus and the same operating conditions of primary fragmentation tests. Batches of 9-10-mm coal particles were fluidized in the CFB combustor for a fixed time interval, then collected, sieved, and weighed to measure the losses due to volatile emissions. This procedure was reiterated with new

batches, fluidizing for progressively longer time intervals, until the weight no longer diminished, thereby indicating the end of volatile release. The yield of volatile was estimated as being equal to about 43 and 43.5%, respectively, for K9L and K9H.

Secondary Fragmentation Tests Procedure: A batch of ten char particles was injected into the loop at the top of the recirculation column. As partides reached the elbow upstream the inclined pipe, they contacted oxygen contained in gas stream 4. CO2 detection made immediately downstream was used to determine when char particles were being recycled. They entered into the riser and reacted in the oxidizing atmosphere until they were entrained by the gas and conveyed to the cyclone. Particles were circulated in the CFB loop for a fixed time interval. Then combustion was stopped by switching the oxygen-nitrogen stream to nitrogen. All char and

222

PRACTICAL ASPECTS OF COMBUSTION

sand particles in the loop were collected in the undervacnum vessel and sieved. The degree of carbon conversion was evaluated by weighing particles before and after the rnn and assuming carbon content of each char particle to be constant during burnoff. Chemical analyses validated this assumption, showing that carbon content varies less than 10% by weight. The particles of a batch that in a rnn did not undergo secondary fragmentation were those whose size was larger than or equal to the average size, corresponding to the assessed degree of conversion. Char particles collected from the bed were tested for further secondary fragmentation in successive runs. Particles smaller than 3 mm were not injected at the top of the recireulation column but in the receiver downstream the cyclone leg (Fig. 1) in order to avoid their entrainment by the outlet gas leaving the cyclone.

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Experimental Results and Discussion

Primary Fragmentation: Primary fragmentation of coal occurred just after injection of particles into the fluidized bed as a consequence of internal stresses due to devolatilization and thermal shock. Volatile and moisture release, which is responsible for devolatilization stresses, takes place at rates influenced by coal properties and by rates of heat exchange between bed and coal particles [10]. It is expected that primary fragmentation in a CFB eombustor occurs while coal particles are still in the dense zone at the bottom of the riser. In this so-called "primary zone," commercial boilers generally operate with values of gas velocity close to 2.7 m/s at which experiments in the present work were carried out. Figure 3 reports particle size distribution on mass basis of fragments generated by primary fragmentation of K9L and K9H coal particles. The same figure also gives the probability of particle breakage Sf, i.e., the ratio between the number of particles tha-t undergo fragmentation and that of particles injected into the bed; the particle multiplication factor Nout/ Nin, i.e., the ratio between the number of particles collected at the end of devolatilization time and the number of injected particles; and the Sauter mean particle diameter of fragments d,,e,evaluated on mass basis. High-swelling K9H coal produced a number of subparticles larger than that of KgL coal (Nout/Nin = 115 compared to Nout/Ni, = 22) and characterized by a smaller average size (dp = 3.1 mm compared t o d = 3 6 m m ) Analysis of size distributions of Fig. 3 indicates that no particles larger than 6 mm were found in tests with K9H coal, whereas fragments up to 9 mm were collected using K9L coal. Moreover, in the case of p

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FIG. 2. Micrographs of char particles from (a) Kentucky No. 9 L coal and (b) Kentucky No. 9 H coal. 1, 2, and 3: tentative identification of carbon bridges connecting elements of the char particles. K9L coal, there was only a negligible fraction (less than 1%) of fragments below 1 mm, while the presence of a large number of fragments (which represents, however, only the 4% on mass basis) was found in the size range 0.8-0.425 mm using K9H coal. These different contributions to the finest range, limited on mass basis but remarkable on numerical basis, mainly determined a so-much-larger value of Nout/ Nin obtained when K9H coal was tested.

Relevance of Primary Fragmentation on CFBC Performance: Relevance of primary fragmentation on the performance of a CFBC continuously operated was evaluated on the basis of a recently published mathematical model [11]. Output variables chosen to characterise the performance of the combustor are carbon loading and carbon particle size distribution in the riser. Model calculations were carried out assuming the following set of input variables: coal feed rate = 0.565 kg/h; solids mass flux = 40 kg/(sm~);

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Secondary Fragmentation: Secondary fragmentation of char particles results from the weakening (by combustion) and breaking

(by collisions) of bridges (indicated by arrows in Figs. 2a and 2b) connecting the various elements of a char particle [4]. According to this definition, the breakup occurs only when combustion has produced a sufficiently deep weakening of char structure, i.e., when carbon conversion of char particles is sufficiently greater than zero [2]. Data of Figs. 5a and 5b confirm the influence of the degree of conversion on the particle multiplication factor N o u t / N i n (i.e., the number of particles generated per one char particle by secondary fragmentation), whatever the type and the initial particle size of char tested. No,t/Nin becomes greater than 1, i.e., secondary fragmentation occurs, only for a conversion larger than about 40%. Comparative analysis of Figs. 5a and 5b also indicates that particle multiplication factor relative to secondary fragmentation of K9H is about one order of magnitude larger than that of K9L. This indicates that char particles from K9H undergo a stronger secondary fragmentation phenomenon. The higher fragility of char mechanical structure (Fig. 2b), to some extent taken into account

TABLE 3 Results of model calculations Kentucky No. 9 H

Kentucky No. 9 L Without primary fragmentation Carbon loading in theriser (g) Santermean carbon particle size, mm

46 2.2

With primary fragmentation 20 1.2

Relative Without With difference primary primary (%) fragmentation fragmentation - 56 - 45

22 2.2

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0.9

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FIc. 5. The influence of the degree of conversionon the particle multiplication factor for different initial size of char particles from K9L and K9H. in the swelling index, might be considered as responsible of this different behaviour. Data in Figs. 5a and 5b show that the initial char particle size, do, affects secondary fragmentation. Nout/Nin considerably increases when do increases. This behaviour might b e related to the increase of weak points in a nonhomogeneous structure when a larger char particle was tested [13]. Figures 6 and 7 give the statistical functions of secondary fragmentation to be embodied in particle size population balances involved in coal combustor modelling. These functions are the probability F (db) that a shrinking particle breaks into fragments when its size crosses the size db, and the particle size distributionfw(df/d b) of fragments of size df generated by the breal(ing of char particles of size db. The derivation of these functions from experimental data was described in previous works [2,7]. Figures 6a and 6b give the probability F(db) obtained starting with several narrow cut batches of K9L and K9H chars of different initial sizes. The F (d/~) curves present a maximum at a value of db

FIe. 6. The probability that a particle of size d~, is subjected to secondary fragmentation. ranging between 3.5 and 5 mm. For larger db, i.e., at a low degree of conversion, bridges connecting elements of a burning particle are not sufficiently weakened to break it into pieces. For smaller db, i.e., at a high degree of conversion, the particle becomes too small to react to collisions with fragmentation [2]. Experiments with d/~ < 3 mm were carried out only for K9H char, because in this case, the amount of char particles that still underwent secondary fragmentation was not negligible. Data reported in Figs. 6a and 6b confirm that char particles of a larger initial size and those obtained from K9H coal have a larger probability of being subjected to secondary fragmentation. The fragment size distributions on mass basis, fw(df/dg), are reported in Figs. 7a and 7b. Each line in these figures is the best fitting curve among those that, for the fixed set of operating variables, satisfy the condition thatfw(df/db) is equal to zero for df/db = 0 and df/db = 1. Generated fragments show a unimodal size distribution, whatever the values of initial char particle size and the type of parent coal. The most probable value of df/db varies, under the different operating conditions t-ested, in the relatively narrow interval between 0.85 and 0.95.

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FIC. 7. Fragment size distributionf,,(dy/db), based on the mass of breaking particles. Comparative analysis of curves relative to different initial char particle size indicates that, for both K9L and KgH chars, the similarity hypothesis, i.e., the possibility of expressing data relative to different initial sizes by means of a singlef~(dy/d b) curve, is not strictly fulfilled. This finding greatly complicates char particle population balances involved in coal combustor modelling, where an integro-differential equation, embodying information on secondary fragmentation, has to be soIved. A single relationship between fragment size distribution and the size of the breaking particles, whatever the size, cannot be used. Modelling work presently in progress is focused on the numerical solution of char particle population balances of the above-cited model [11], already modified to take into account differentfw(df/db). Conclusions

Primary fragmentation behaviour of 9-10 mm particles of a low- and a high-swelling coal was characterized by using a CFB combustor operated under the same experimental conditions. The high-swelling coal showed a sharper tendency to undergo primary fragmentation, with a production of a large number of relatively small fragments. Taking into account the similarity of chemical composition of the two coals,

225

it should be permissible to refer the different primary fragmentation behaviour to properties of the mechanical structure of each coal and its char, to some extent taken into account by the swelling index. Model calculations indicate that primary fragmentation should be taken into account in CFBC modelling. Neglecting this phenomenon might lead to an error as high as 63% in the evaluation of carbon loading and to a misleading size distribution of carbon particles in the riser. Both the coals produced chars, which are candidates for undergoing secondary fragmentation. The high-swelling coal showed a value of particle multiplieation factor of about one order of magnitude larger than that of low-swelling coal. The higher fragility of char mechanical structure obtained from high-swelling coal might be eonsidered as responsible for this different behaviour. Initial char particle size do affects secondary fragmentation. The particle multiplication factor considerably increases when do increases. This behaviour, which might be related to the increase of weak points in a nonhomogeneous structure, fails to strictly fulfil the similarity hypothesis, i.e., the possibility of expressing data relative to different initial size by means of a singlef,~(dy/db) curve. This greatly complicates the solution of the char particle population balances involved in a coal combustor model. Acknowledgments

Professor Leopoldo Massimilla gave an incomparable contribution to define the research program of this work and to anal~e initial experimental results. The authors are indebted to Dr. A. Malandrino (ENIRicerehe-ENI Group) for his help in model calculations and to Mr. Giovanni D'Anna and Miss Olimpia De Robbio (FISIA-FIATIMPRESIT Group) for their help in performing experimental runs. REFERENCES 1. Arena, U., Cammarota, A., Massimilla, L. Siciliano,L., and Basu, P., Combust. Sci. Technol. 73:383 (1990). 2. Arena, U., Cammarota, A., Chirone, R., and Massimilla, L., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1341-1348. 3. Engstrom, F., and Lee, Y. Y., Circulating Fluidized Bed Technology III (P. Basu, M. Horio, and M. Hasatani, Eds.), Pergamon Press, 1991, p. 15. 4. Chirone, R., Massimilla, L., and Salatino, P., Prog. Energy Combust. Sci. 17:297 (1991). 5. Sundback,C. A., Be6r, J. M., and Sarofim,A. F., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, p. 1495. 6. Stubington, J. F., and Linjewile, T. M., Fuel 68:155 (1989).

226

PRACTICAL ASPECTS OF COMBUSTION

7. Chirone, R., Massimilla, L., and Salatino, P., Combust. Flame 77:79 (1989). 8. Street, P. J., Weight, R. P., Lightman, P., Fuel 48:343365 (]969). 9. Arena, U., Marzocchella, A., Massimilla, L., and Malandrino, A., Powder Technol. 70:237; 71:116 (1992). 10. Pechana, R. P., and Gibbs, B. M., Proceedings of 3rd

International Fluidized Bed Combustion Conference, London, DISC/9/65, 1984.

11. Arena, U., Malandrino, A., and Massimilla, L., Can. J. Chem. Eng. 69:860--868 (1991). 12. Malandrino, A., Arena, U., and Massimilla, L., in Pro-

ceedings of the 11th International Conference on Fluidized Bed Co~rd~ustion (E. J. Anthony Ed.), ASME, 1991, pp. 841-848. 13. Chirone, R., and Massimilla, L., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 267-277.

COMMENTS Wei-Yin Chen, University of Mississippi, USA. In our study of primary fragmentation, we observed quite a lot of particles with the size of about 100/~m. Did you observe similar particles?

Author's Reply. The amount of fragments smaller than 425/zm resulting from primary fragmentation of the coals tested was negligible. It reached a maximum value of about 1% on a mass basis in the case of tests with Kentucky No9H coal.