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Carbon attrition in the fluidized combustion of a metallurgical coke
COMBUSTION A N D F L A M E 57:123-125 (1984)
123
BRIEF COMMUNICATION Carbon Attrition in the Fluidized Combustion of a Metallurgical Coke U. ARENA, R. CHIRONE, M. D'AMORE, and L. MASSIMILLA lstituto di Ricerche sulla Combustione, C.N.R., lstituto di Chimica Industriale e Impianti Chimici, Universit~t di Napoli, Naples, Italy
INTRODUCTION The number of carbon particles collected at the cyclone of a fluidized combustor is by orders of magnitude larger than that of feed coal particles. This suggests that elutriated fines are formed by mechanisms different from the simple one to one passage from an individual coal particle into its elutriable carbon residue [1, 2]. Encoding this concept into a fluidized combustion model removed some inconsistencies between predicted and experimental carbon losses, particularly in the case of coarse coal feed [3]. The multiplication of carbon particles inside the bed of a fluidized coal combustor can be related to fragmentation and attrition. Fragmentation leads to a number of relatively coarse subparticles, which burn in the bed until they shrink to the size of elutriable residues. Attrition generates finer carbon, which is rapidly elutriated from the bed. The first of these two phenomena is mainly associated with coal devolatilization; the second, to the rubbing of carbon particles against bed inert material. Previous works on attrition [1, 4] were carried out using a South African, nonswelling coal. The present paper aims at extending to a metallurgical coke information on the attrition behavior of carbon during fluidized combustion. With respect to the coal previously tested, metallurgical coke is not subject to fragmentation and is less reactive. This allowed attrition to be examined separately and at higher
bed carbon content for given fluidizing velocity and air excess.
EXPERIMENTAL The combustor, 140 mm, id, and the experimental technique used were essentially those described by Donsi et al. [1] and Arena et al. [4]. A few modifications were made to test in- and above-bed fuel feeding and determine that contributions of attrition in the grid jet region and in the cyclone were negligible with respect to attrition related to solids-to-solids interaction inside the bed. In particular, the combustor could be operated (i) with a two-stage cyclone (A) of 2.6 cm2 entrance cross section or a three-stage cyclone (B) by adding an initial unit of 10.6 cm 2 entrance cross section, (ii) inserting one or other of two pipe type air distributors. Both had 19 pipes, each with four 1.5 mm holes (C) or one 6.25 mm hole (D). All experiments were made with a metallurgical coke of nominal sizes 0.4-1, 1-3, and 3-6 mm. Carbon and ash contents of the coke were 81 and 19% (by weight), respectively. Sand, 0.2-0.4 mm, was used as bed solids. Quiescent bed heights Hb were 24, 33, and 67 cm. Bed temperatures T were 750, 850, and 950°C. Fluidizing velocity U was 75 cm s- i at these temperatures. The excess air factor e varied from 0.70 to 1.35. Steady state operation was possible even at e < 1 because of carbon elutriation. Oxygen concentration in exit gas varied in
Copyright © 1984 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
O010-2180/841503.00
124
U. ARENA ET AL.
fact between 0.71 and 7.73% within the range of e tested.
2 suggests that, particularly with 1-3 and 36 mm coke, most elutriated carbon is generated by attrition. Whatever the experimental conditions, about 50 % of the fines collected at the cyclone were in fact below 50 /~m, 80% below 125 #m, and 100% below 212 /~m. Fig. 1A shows in particular that the influence of feed coke particle size, bed temperature, and air excess factor on particle size distribution of attrited carbon is small. The low sensitivity of such distributions to changes of T and e suggests that postcombustion of attrited carbon fines is negligible. Changes in distributor design and, as shown by Fig. 1B, in cyclone set do not appreciably affect attrited carbon particle size distribution, which confirms that fines generated by attrition takes place inside the bed. . Bed carbon particle size distribution (Fig. 2): Bed carbon particle size distribution is negligibly affected by changes in air excess factor, bed temperature, and height. It depends on feed coke particle size distribution. The spread between feed coke and bed carbon particle size distribution increases with the coke feed size. . Elutriation rates o f attrited carbon: E~" ' is affected by feed coke particle size, bed temperature, and air excess factor. It is
RESULTS AND DISCUSSION
1. Attrited carbon particle size distribution (Fig. 1): The rates E~ of elutriated carbon collected at the cyclone may be considered as the sum of three contributions: (i) E~', made of fines directly charged into the bed with the feed coke; (ii) E~' ', made of residues of size d* from combustion and attrition of coke feed particles, d* being the maximum elutriable coke particle size; and (iii) E¢" ', made of attrited carbon of size smaller than d*. Under the experimental conditions tested, d* is 150 /~m, i.e., smaller than the finer feed coke particles. Since, therefore, the contribution E~' of fines directly fed into the bed is zero, Ec = E~ ~ + E¢" '. E~" is given by the relationship
(1)
Xa¢o,
E~"=F~. ~ dc~n
where Fc is the rate of carbon injected into the bed with the feed coke, Xa~o is the mass fraction of the component of size d¢o in the feed, and d~oma~and dcominare the upper and lower feed sizes, respectively. Evaluation of E¢" using feed coke size distribution in Fig.
~t
.t=
•~' 30 .~I00 A E =
cyclone A cvclor~ 8
.~
•
Hb= a3 orll__ ~b = 33 ~m • 67 cm 24 cm •
--
p
e=O93
,:95o°c/ . ,feed coke
/
• bed carbon /
i 3 mm coke T= 751TC
,W • lie :ll • ~•
~50~
r:85o°c/ : ,feed
coke
/,
I
~ v bed carbon#
I
/
!e=o.
I
I
,feedcoke 0 obed carbon/
/
/
0
/
*
r
v +
•
/
~r=85o°c/
I: I
65
~
1
~
Hb=33 om O79.te~oge Toc 750 850 950 04-1rnrn coke • : t
•
30 0
e.og,
z:
100 fl
65
10004-1,n,~coke =?~ r=_3.,n coke /7 13-Sran,coke 0/~
v
3-6 turn 100
0
200
[]
r:
I
0
, 0
0.5
particle diameter pm
Fig. 1. Cumulative particle size distributions of carbon fines collected at cyclone.
I
v
/+ 0
Ii
1.5
/
O
particle diameter, mm Fig. 2.
Cumulative
bed carbon
particle
I
/o
I I~o +_÷,+ 30 3
size distributions.
I 6
COMBUSTION OF COKE
125
also somewhat affected by the air distributor design and bed height. Moreover, for any variation found in E¢ ~ ' as a result of the change of one of these variables there is a corresponding change in bed carbon content We. This trend was also found in experiments with coal [4], except that, under comparable conditions, W~ for coal was about one order of magnitude smaller, due to the higher reactivity of char with respect to that of metallurgical coke. A simple proportionality relationship applies between E~ ~ ' and W~ for each size range of the feed coke regardless of bed height, air excess factor, type of distributor, and bed temperature. Values of Ec" ' for all the three coke sizes can be expressed as a function of exposed surface of carbon, which is proportional to W J a , a being the average bed carbon size. In spite of the fact that all the experiments have been carried out at constant excess of fluidizing velocity with respect to minimum fluidizing velocity (U - Uo), E ~ ' ' is correlated in Fig. 3 with (U - Uo)WJd in order to simplify the comparison with the relationship
(2)
Ec" " = k ( U - Uo) Wc/~!
previously reported for coal. The straight line in 7,,, 16! ~n
-
•
i
0 8~ ;
~
'
.T ='V / "• T j •
~,,~ y/o ° ~' v/'v []
4'/
i, •
o
•' 04-1ramcoke 1-3 • ' D 3"6
,;/ :
v ~ _ [] /ILl 0//(DL/ .
o
o
io
v
0
" 24on, , distributor O .~r
above-bedfeeding
- 8
1. Donsi, G., Massimilla, L., and Miccio, M., IRFF Chemical Panel, Pisa, April 1979; Combust. Flame 41:57 (1981). 2. Ross, I. B., Ph.D. Dissertation, University of Cambridge, 1979. 3. Be6r, J. M . , Massimilla, L . , and Sarofim, J. F., Paper presented at Int. Conf. on Fluidized Combustion: Systems and Application, Institute of Energy Symp. Ser. No. 4, London, November 1980. 4. Arena.U., D'Amore, M., and Massimilla, L., AIChE Journal 29:40 (1983).
T°C750 850 950
. /
-
-
(u- uo).~.lO~
The study was sponsored by the Progetto Finalizzato Energetica, C.N.R., Rome. This paper was prepared with the support o f the U.S. Department o f Energy, Grant No. DEFG22-81PC40796. However, any opinion, findings, conclusions, or recommendations expressed herein are those o f the authors and do not necessarily reflect the views o f DOE. The authors are indebted to Mr. A. Cammarota and to Ing. B. Farina f o r their assistance. REFERENCES
Hb: 33cm distributor C
vv
this figure has been obtained by using the least squares method with the condition that for (U Uo) W~/a = O, Ec"' = 0. Dotted lines include 80% of the experimental results. The slope k of the continuous line, which has the meaning of an attrition rate constant, is 1.29 x 10 -7. Inspection of Fig. 3, however, shows that data for different coke sizes are better represented by separate lines with k = 0.807 x 10 -7, 1.41 x 10 -7, and 2.88 x 10 -7 for 0.4-1, 1-3, and 3-6 mm coke, respectively. This suggests that the effect of particle size is not entirely expressed by the l/~/dependence in Eq. (2). The lower reactivity of coke allowed this equation to be tested in a range of (U - Uo) Wc/d larger than that investigated in experiments with coal. Compared with k = 1.57 x 10 -7 for 1-3 mm South African coal, the attrition rate constant for coke of the same size was about 1.12 smaller.
$ g ~'
d Fig. 3. Attrited carbon rates as a function o f ( U
-
U0)
WJa~.
Received I June 1983;revised 30 January 1984
123
BRIEF COMMUNICATION Carbon Attrition in the Fluidized Combustion of a Metallurgical Coke U. ARENA, R. CHIRONE, M. D'AMORE, and L. MASSIMILLA lstituto di Ricerche sulla Combustione, C.N.R., lstituto di Chimica Industriale e Impianti Chimici, Universit~t di Napoli, Naples, Italy
INTRODUCTION The number of carbon particles collected at the cyclone of a fluidized combustor is by orders of magnitude larger than that of feed coal particles. This suggests that elutriated fines are formed by mechanisms different from the simple one to one passage from an individual coal particle into its elutriable carbon residue [1, 2]. Encoding this concept into a fluidized combustion model removed some inconsistencies between predicted and experimental carbon losses, particularly in the case of coarse coal feed [3]. The multiplication of carbon particles inside the bed of a fluidized coal combustor can be related to fragmentation and attrition. Fragmentation leads to a number of relatively coarse subparticles, which burn in the bed until they shrink to the size of elutriable residues. Attrition generates finer carbon, which is rapidly elutriated from the bed. The first of these two phenomena is mainly associated with coal devolatilization; the second, to the rubbing of carbon particles against bed inert material. Previous works on attrition [1, 4] were carried out using a South African, nonswelling coal. The present paper aims at extending to a metallurgical coke information on the attrition behavior of carbon during fluidized combustion. With respect to the coal previously tested, metallurgical coke is not subject to fragmentation and is less reactive. This allowed attrition to be examined separately and at higher
bed carbon content for given fluidizing velocity and air excess.
EXPERIMENTAL The combustor, 140 mm, id, and the experimental technique used were essentially those described by Donsi et al. [1] and Arena et al. [4]. A few modifications were made to test in- and above-bed fuel feeding and determine that contributions of attrition in the grid jet region and in the cyclone were negligible with respect to attrition related to solids-to-solids interaction inside the bed. In particular, the combustor could be operated (i) with a two-stage cyclone (A) of 2.6 cm2 entrance cross section or a three-stage cyclone (B) by adding an initial unit of 10.6 cm 2 entrance cross section, (ii) inserting one or other of two pipe type air distributors. Both had 19 pipes, each with four 1.5 mm holes (C) or one 6.25 mm hole (D). All experiments were made with a metallurgical coke of nominal sizes 0.4-1, 1-3, and 3-6 mm. Carbon and ash contents of the coke were 81 and 19% (by weight), respectively. Sand, 0.2-0.4 mm, was used as bed solids. Quiescent bed heights Hb were 24, 33, and 67 cm. Bed temperatures T were 750, 850, and 950°C. Fluidizing velocity U was 75 cm s- i at these temperatures. The excess air factor e varied from 0.70 to 1.35. Steady state operation was possible even at e < 1 because of carbon elutriation. Oxygen concentration in exit gas varied in
Copyright © 1984 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
O010-2180/841503.00
124
U. ARENA ET AL.
fact between 0.71 and 7.73% within the range of e tested.
2 suggests that, particularly with 1-3 and 36 mm coke, most elutriated carbon is generated by attrition. Whatever the experimental conditions, about 50 % of the fines collected at the cyclone were in fact below 50 /~m, 80% below 125 #m, and 100% below 212 /~m. Fig. 1A shows in particular that the influence of feed coke particle size, bed temperature, and air excess factor on particle size distribution of attrited carbon is small. The low sensitivity of such distributions to changes of T and e suggests that postcombustion of attrited carbon fines is negligible. Changes in distributor design and, as shown by Fig. 1B, in cyclone set do not appreciably affect attrited carbon particle size distribution, which confirms that fines generated by attrition takes place inside the bed. . Bed carbon particle size distribution (Fig. 2): Bed carbon particle size distribution is negligibly affected by changes in air excess factor, bed temperature, and height. It depends on feed coke particle size distribution. The spread between feed coke and bed carbon particle size distribution increases with the coke feed size. . Elutriation rates o f attrited carbon: E~" ' is affected by feed coke particle size, bed temperature, and air excess factor. It is
RESULTS AND DISCUSSION
1. Attrited carbon particle size distribution (Fig. 1): The rates E~ of elutriated carbon collected at the cyclone may be considered as the sum of three contributions: (i) E~', made of fines directly charged into the bed with the feed coke; (ii) E~' ', made of residues of size d* from combustion and attrition of coke feed particles, d* being the maximum elutriable coke particle size; and (iii) E¢" ', made of attrited carbon of size smaller than d*. Under the experimental conditions tested, d* is 150 /~m, i.e., smaller than the finer feed coke particles. Since, therefore, the contribution E~' of fines directly fed into the bed is zero, Ec = E~ ~ + E¢" '. E~" is given by the relationship
(1)
Xa¢o,
E~"=F~. ~ dc~n
where Fc is the rate of carbon injected into the bed with the feed coke, Xa~o is the mass fraction of the component of size d¢o in the feed, and d~oma~and dcominare the upper and lower feed sizes, respectively. Evaluation of E¢" using feed coke size distribution in Fig.
~t
.t=
•~' 30 .~I00 A E =
cyclone A cvclor~ 8
.~
•
Hb= a3 orll__ ~b = 33 ~m • 67 cm 24 cm •
--
p
e=O93
,:95o°c/ . ,feed coke
/
• bed carbon /
i 3 mm coke T= 751TC
,W • lie :ll • ~•
~50~
r:85o°c/ : ,feed
coke
/,
I
~ v bed carbon#
I
/
!e=o.
I
I
,feedcoke 0 obed carbon/
/
/
0
/
*
r
v +
•
/
~r=85o°c/
I: I
65
~
1
~
Hb=33 om O79.te~oge Toc 750 850 950 04-1rnrn coke • : t
•
30 0
e.og,
z:
100 fl
65
10004-1,n,~coke =?~ r=_3.,n coke /7 13-Sran,coke 0/~
v
3-6 turn 100
0
200
[]
r:
I
0
, 0
0.5
particle diameter pm
Fig. 1. Cumulative particle size distributions of carbon fines collected at cyclone.
I
v
/+ 0
Ii
1.5
/
O
particle diameter, mm Fig. 2.
Cumulative
bed carbon
particle
I
/o
I I~o +_÷,+ 30 3
size distributions.
I 6
COMBUSTION OF COKE
125
also somewhat affected by the air distributor design and bed height. Moreover, for any variation found in E¢ ~ ' as a result of the change of one of these variables there is a corresponding change in bed carbon content We. This trend was also found in experiments with coal [4], except that, under comparable conditions, W~ for coal was about one order of magnitude smaller, due to the higher reactivity of char with respect to that of metallurgical coke. A simple proportionality relationship applies between E~ ~ ' and W~ for each size range of the feed coke regardless of bed height, air excess factor, type of distributor, and bed temperature. Values of Ec" ' for all the three coke sizes can be expressed as a function of exposed surface of carbon, which is proportional to W J a , a being the average bed carbon size. In spite of the fact that all the experiments have been carried out at constant excess of fluidizing velocity with respect to minimum fluidizing velocity (U - Uo), E ~ ' ' is correlated in Fig. 3 with (U - Uo)WJd in order to simplify the comparison with the relationship
(2)
Ec" " = k ( U - Uo) Wc/~!
previously reported for coal. The straight line in 7,,, 16! ~n
-
•
i
0 8~ ;
~
'
.T ='V / "• T j •
~,,~ y/o ° ~' v/'v []
4'/
i, •
o
•' 04-1ramcoke 1-3 • ' D 3"6
,;/ :
v ~ _ [] /ILl 0//(DL/ .
o
o
io
v
0
" 24on, , distributor O .~r
above-bedfeeding
- 8
1. Donsi, G., Massimilla, L., and Miccio, M., IRFF Chemical Panel, Pisa, April 1979; Combust. Flame 41:57 (1981). 2. Ross, I. B., Ph.D. Dissertation, University of Cambridge, 1979. 3. Be6r, J. M . , Massimilla, L . , and Sarofim, J. F., Paper presented at Int. Conf. on Fluidized Combustion: Systems and Application, Institute of Energy Symp. Ser. No. 4, London, November 1980. 4. Arena.U., D'Amore, M., and Massimilla, L., AIChE Journal 29:40 (1983).
T°C750 850 950
. /
-
-
(u- uo).~.lO~
The study was sponsored by the Progetto Finalizzato Energetica, C.N.R., Rome. This paper was prepared with the support o f the U.S. Department o f Energy, Grant No. DEFG22-81PC40796. However, any opinion, findings, conclusions, or recommendations expressed herein are those o f the authors and do not necessarily reflect the views o f DOE. The authors are indebted to Mr. A. Cammarota and to Ing. B. Farina f o r their assistance. REFERENCES
Hb: 33cm distributor C
vv
this figure has been obtained by using the least squares method with the condition that for (U Uo) W~/a = O, Ec"' = 0. Dotted lines include 80% of the experimental results. The slope k of the continuous line, which has the meaning of an attrition rate constant, is 1.29 x 10 -7. Inspection of Fig. 3, however, shows that data for different coke sizes are better represented by separate lines with k = 0.807 x 10 -7, 1.41 x 10 -7, and 2.88 x 10 -7 for 0.4-1, 1-3, and 3-6 mm coke, respectively. This suggests that the effect of particle size is not entirely expressed by the l/~/dependence in Eq. (2). The lower reactivity of coke allowed this equation to be tested in a range of (U - Uo) Wc/d larger than that investigated in experiments with coal. Compared with k = 1.57 x 10 -7 for 1-3 mm South African coal, the attrition rate constant for coke of the same size was about 1.12 smaller.
$ g ~'
d Fig. 3. Attrited carbon rates as a function o f ( U
-
U0)
WJa~.
Received I June 1983;revised 30 January 1984