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Steam-activated carbons from a bituminous coal in a continuous multistage fluidized bed pilot plant
Carbon Vol.34, No. 12,pp. 1515~1520,1996 Copyright 0 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0008~6223/96 $15.00+ 0.00
Pergamon PII: SOOOS-6223(96)00106-6
STEAM-ACTIVATED CARBONS FROM A BITUMINOUS COAL IN A CONTINUOUS MULTISTAGE FLUIDIZED BED PILOT PLANT I. MARTEN-GULL~N, M. ASENSIO, R. FONT and A. MARCILLA Chemical Engineering Department, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spain (Received 7 December 1995; accepted in revisedform 30 May 1996) Abstract-A
vertical three-stage fluidized bed pilot plant, with downcomers, was designed and built in order to study the continuous process of the production of activated carbons from a high-volatile bituminous coal from the Puertollano basin (Spain), by steam activation. The pilot plant can operate with a production of up to 40 kg per day. Very good activated carbons were produced at the selected operating conditions. The effect of the following operating conditions on the reactivity and adsorption characteristics of the activated carbons was analyzed: (1) carbonization conditions (one- and two-step activation), (2) activation temperature (SOO&SSOYZ), and (3) steam gas velocity (1.5-3 times the minimum fluidization velocity). Carbonization conditions considerably affect the reactivity of the chars obtained; the faster the carbonization process, the higher the reactivity. Nevertheless, the effect of this variable on the development of porosity is not very relevant, and consequently the direct activation process is an attractive alternative to the two-step (carbonization and activation) process. On the other hand, both temperature and steam flow rate (affecting the reaction rate) have a marked effect on the development of porosity. Copyright 0 1996 Elsevier Science Ltd Key Words-A.
activated carbon, B. coal, C. activation, D. carbonization.
1. INTRODUCTION
Any material with a high carbon content is susceptible of being transformed into activated carbons. In fact, coals, wood and coconut shells are the most common raw materials in worldwide industries. Moreover, many other precursors have been also investigated, such as almond shells [l-3], olive stones [4,5], peach stones [3], cherry stones [6], etc. Around one third of the world production of activated carbon comprises coal-origin activated carbons, and this ratio is bigger in both the European Union and the United States [7]. Although Spain is the third largest coal producer in Europe, all the activated carbon needed must be imported, because there is no factory producing activated carbon in Spain. Low rank coals are, in general, better precursors to be converted into activated carbons than anthracites. Mufioz-Guillena et al. [S] pointed out that the ash content does not seem to affect porosity development of the activated carbons, but the ash content must also be as low as possible [8,9]. A review of the literature shows that experimental conditions cannot be extrapolated from one given raw material to others; with regard to the oxidizing agent, some authors reveal that carbon dioxide produces more micropores that steam [ 5, lo], while other researchers have found the opposite [ 11,121. Carbonization conditions, temperature and heating rate may have different effects, and using a one- or two-step steam activation process may lead to opposite results. In addition, the type of reactor, gas flow, etc., can also 1515
lead to different results, and consequently, a specific study must be carried out for any particular raw material. The University of Alicante is studying the possibility of obtaining good-quality activated carbons from Spanish coals. First of all, a preliminary study [9,13,14] was carried out in order to select the precursor from nine different Spanish coals, from anthracites to lignites. A bituminous coal (from the Maria Isabel Mine, Puertollano) with a low ash content was the best precursor found. A wide research project, funded by CECA and by OCICARBON has been carried out in order to study the production of activated carbons at both laboratory and pilot-plant scale from the bituminous coal previously selected. For the lab-scale study, the influence of the oxidizing agent (carbon dioxide and steam), the type of reactor (horizontal and vertical fixed bed, and fluidized bed), the carbonization conditions (final temperature, heating rate, direct activation), and the reaction kinetics were analyzed. Two continuous reactors were designed and built to carry out the pilot-plant scale study: a moving-bed pilot plant [ 151 and a multistage fluidized-bed pilot plant. In addition, a previously designed rotary kiln pilot plant was also used to complete this pilot-plant scale study [16]. This paper deals with the results obtained in the three-stage fluidized-bed pilot plant, where the influence of the carbonization conditions, activation temperature and gas velocity are analyzed.
I. MARTIN-GULL~Net al
1516 2. EXPERIMENTAL
2.1 Raw material As mentioned above, the raw material used in this work is a high-volatile bituminous coal (28% volatiles) from the Maria Isabel Mine (TECSA company), Puertollano (Spain), which has been already used in previous works [17], with the following maceral analysis: 58-61% vitrinite, lo-12% exenite, and 23-25% inertite. It should be noted that this coal does not present thermoplastic behaviour, and consequently no air preoxidation treatment at low temperature was needed to avoid coke formation [ 141. In addition, two chars previously obtained from the same coal have been used in different experiments: (1) char MB850, obtained in a continuous movingbed reactor at 850°C [15]. This char was almost completely devolatilized, and the soaking time was around 1 hour. (2) Char FB500, obtained in a continuous fluidized-bed reactor at 500°C. This char was not completely devolatilized, with 10% of volatiles remaining. The soaking time was around 30 minutes. All experiments were carried out using coal/char particles of 1.5-0.84 mm (mean 1.13 mm) particle size. The minimum fluidization velocity of the char obtained from the carbonization of this coal was around 0.18 m s-l at the activating conditions (85OC and water vapor as activating agent). This velocity decreased continuously as the gasification process continued to around 0.13 m s-l, depending on the burn-off attained.
2.2 Experimentalpilotplant A continuous fluidized-bed reactor behaves as a back-mix reactor, which is not desired if a homogeneous solid product is required. In order to overcome this problem, several fluidized beds can be used in series to narrow the residence-time distribution of the solids. In this work, it was decided to design a three-stage fluidized-bed reactor, with the stages connected by downcomers or standpipes (flat tubes). The pilot-plant scheme is shown in Fig. 1. Depending on the experimental run, coal or char initially placed in the hopper is fed by a screw feeder to the upper fluidized bed, flows through the downcomers to the intermediate and lower beds, and finally exits by a weir to an activated-carbon tank at the bottom of the plant. At the top of this tank, there is a system for the sampling of solids. Steam (the oxidizing agent) is fed by the bottom of the reactor, crosses a preheating stage (with ceramic balls inside), and then the three stages countercurrently to solids, and finally exits off the reactor, passes through a cyclone and through a burner. All stages are identical, being 0.147 m in internal diameter and 0.75 m in height, with a fluidized-bed height of 0.15 m. In addition, these stages include a baffle in order to avoid a complete back-mixing of solids in each fluidized bed, and contribute to narrow the overall residence-time distri-
k
#!@
9
Fig. 1. Multistage fluidized bed pilot plant scheme: (a) hopper; (b) screw feeder; (c) fluidized bed; (d) downcomer; (e) activated carbon reservoir; (f) pre-heating stage; (g) cyclone; (h) burner; (i) electric furnaces; (j) thermocouples; (k) steam generator; (1) pressure reductor; (m) regulation valve; (n) orifice; (0) steam pre-heater; (p) sampler; (q) steam purge; (r) N, supply; (s) control; (t) manometers.
bution. The downcomers have an internal diameter of 0.021 m, and allow the transfer of solids from an upper to a lower bed. Perforated plates were used as distributors. The heat required is supplied by four 10 kW electric furnaces, controlled by corresponding thermocouples placed at the fluidized beds. Hydrodynamically, this reactor can operate between two limiting velocities, the minimum fluidization velocity (below this value there is no solid movement) and the velocity at which the solids’ height inside the downcomer (in the slugging regime) equals the total downcomer height. Further details of the pilot plant and the hydrodynamic model previously developed in order to determine this upper velocity limit can be found elsewhere [18]. At the activating conditions (85O”C, steam as the fluidizing gas and char particle size of 1.13 mm) the gas velocity stability range is wide enough to assure stable conditions at up to five times the minimum fluidization velocity. The residence-time distribution of solids was previously determined at room temperature, using 1.13 mm coal particles, air as the fluidizing agent and under hydrodynamic conditions similar to those used previously [ 191. The experimental residence-time distribution function vs dimensionless time is reproduced in Fig. 2, along with the best fit to the “tanks in series” model [20]. It can be seen that this reactor behaves almost as if an additional stage were present,
Steam-activated carbons from a bituminous coal
1517
DA7-850, MB850A850 and FB5OOA850: experiments were carried out at different carbonization conditions but activated at 850°C and 1.5u,,. In the run DA7-850 coal was activated directly but with the upper bed at 700°C. Runs MB850A850 and FB5OOA850 were carried out at 850°C using two previously carbonized chars in a moving-bed reactor at 850°C (char MBSSO) and in a one-stage fluidizedbed reactor at 500°C (char FB850), respectively.
0.8
Fig. 2. Experimental and best fit to “tanks in series” model residence-time distribution for the three-stage fluidized-bed pilot plant (dimensionless residence-time distribution &, dimensionless residence time 0).
due to the combined downcomers.
effects of the baffles and
2.3 Activation runs The results of 12 experiments carried out with this pilot plant are reported here, analyzing the influence of the following variables: (1) residence time of solids (solid feed rate), (2) number of activation steps (10 runs with direct activation and two runs with activation of a previously devolatilized char), (3) activation temperature (eight runs at 850°C and four runs at SOOC), and (4) steam velocity (10 runs at 1.5-2~~~ and two runs at 3-4u,r). Table 1 shows the operating conditions of each run. These runs are classified in the following series: DA850: including experiments 2, 3, 4 and 5, direct activation runs at 850°C and 1.5u,, varying only the feed rate (and consequently the residence time). DA800: including experiments 6, 7 and 8, direct activation runs at 800°C and 1.5u,r, varying the feed rate. DA850-3 and DA800-3: experiments 9 and 10, direct activation runs similar to those mentioned above, at 850 and 8OO”C, respectively, but at 3u,r steam velocity.
2.4 Activated carbon characterization 2.4.1 Burn-off The burn-off was indirectly determined from the ash content of the activated carbons obtained and that of the initial raw material. 2.4.2 Residence time Residence time was determined from the experimental hold-up (obtained from the experimental pressure drops) and the solid feed rate (on a dry and volatile-free basis). 2.4.3 Adsorption capacity The micropore volume was characterized by both 77 K nitrogen and 273 K carbon dioxide adsorption isotherms with a Quantacrome Autosorb 6 sorption apparatus. The following parameters were obtained: total micropore volumes (sum of ultramicropore and supermicropore volumes) from the application of the DI equation to the O-O.4 relative pressure range nitrogen isotherm data [21], and the ultramicropore volumes applying the DR equation to the carbon dioxide isotherm data. All volumes are expressed by DAF mass unit. 3. RESULTS All the runs shown in Table 1 were successful, and the reactor operated smoothly. In addition, the hydrodynamic behaviour of the process was monitored by the absolute pressure at each stage, showing therefore that a continuous process at stationary conditions was being carried out. Experiments were stopped after more than 2.5 times the residence time corresponding to each experiment. Samples were collected in each run at different times, showing for each particular run a very low dispersion in the micropore pore volumes (about 2%),
Table 1. Activation runs carried out in this work with the pilot plant. T,,=nominal temperature of the upper bed, T,= actual temperature of the upper bed, T,,,= temperature of the medium and lower beds, F=feed flow rate, u=fluidization velocity
Run 2 3 4 5 6 7 8 9 10 16 24
Series
MB850A850 DA850
DA800 DA850-3 DA800-3 DA7-850 FB5OOA850
L
(“C)
850 810 800 820 835 800 800 800 850 800 700 850
T, (“C)
850 850 850 850 850 800 800 800 850 800 700 850
T,,I (“C) 850 850 850 850 850 800 800 800 8.50 800 850 850
F (kg per day)
u (m s-l)
30.4 30.4 35.1 25.6 18.3 36.8 15.2 8.2 23.4 10.0 25.6 26.6
0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.54 0.54 0.32 0.32
I. MARTIN-GULL~N
1518
indicating that stationary conditions were also attained. The dispersions of the nominal burn-off degrees obtained were a little higher (about 6%), as expected, but indicated a stationary state.
3.1 Burn-ojJ Figure 3 shows the mean nominal burn-off degrees vs nominal residence times for the direct activation series (DASOO, DA800-3, DA850 and DA850-3). It can be seen, as expected, that the reactivity at 850°C was higher than that at 800°C. It can be observed that the reactivity for the two DA850 and DA800 series (at 1.5~~~) behaves linearly vs residence time. To obtain a 50% burn-off activated carbon, a residence time of 2 hours is needed at 850°C; at SOO’C, 5 hours are required to obtain the same burn-off. Increasing the steam flow rate to 3~,,,~ does not seem to influence the reaction rate with respect to 1.5~~~ at 85O”C, whilst at 800°C the reaction rate increases markedly. This fact can be explained by considering the strong inhibition effect of the reaction products, competing with the non-inhibited reaction. At 800°C the effect of the dilution of the products dominates, and consequently the reaction rate is sensitive to the steam flow rate, whereas at 850°C the rate seems to be unaltered, showing that the non-inhibited reaction dominates. Figure 4 shows the 850°C activation runs with
70
0’
1
1
I
I
I
2
3
4
5
Residence
Fig. 3. Burn-off
70
different carbonization processes, as well as those corresponding to the DA850 series, included as a reference for comparing the results at the same temperature and gas velocity. Char FB500, in the run FB500DA850, underwent a higher conversion, although it should be less reactive than the direct activation chars obtained, probably as a consequence of the fact that less time is required for devolatilization and a lesser amount of volatiles was produced. Nevertheless, the MB850 char, in the run MB850A850, presents equal or perhaps lower reactivity than the DA850 activated carbons, which can be explained by considering that these chars were prepared in a moving-bed reactor at a much lower heating rate than those produced in the fluidized-bed reactor, which would result in them being less reactive in the subsequent activation step. This shows the marked effect of the carbonization-step heating rate on the reactivity of the chars obtained, as stated previously [ 171. Activated carbons obtained in the DA7-850 run, with the first stage at 700°C (where only carbonization took place), obviously presented a lower reactivity than those prepared in the DA850 series. In the DA7-850 run, the material is only activated in the two lower fluidized beds, and consequently the char should be less reactive. In section 3.2, the adsorption characteristics of these processes will be analyzed.
3.2 Adsorption
Burn-off (%)
,
0
eta/
6
7
time (h)
vs residence-time plot for direct activation runs at 800 and 850°C.
capacity
Figures 5 and 6 show the 71 K nitrogen adsorption data for activated carbons obtained in the 850 and 800°C direct activation runs, respectively. The last number added to the name of the run indicates the burn-off degree. It can be seen that all samples correspond to microporous carbons, with the isotherms more open as the burn-off increases. At 85O”C, the isotherm of the DA85-3 sample seems to present the same tendency as those of DA85 series (as in the case of reactivity), whereas at 800°C the DA800-3/48 sample has nearly the same capacity as the DA800/28, with only a 28% burn-off. It seems that when the reaction rate is high, less pore volume is developed. Figure 7 shows the total Nz micropore volume (sum
Burn-off (%)
60 50 40 I
/ .
30 20 10
0 MB850A850 I
0 1
1.5
n
DA850
l
DA7-850
I 2 Residence
Fig. 4. Burn-off vs residence-time 850°C and 1.5u,, with different
v FBSOOA850 I 2.5
3
time(h)
plot for activation runs at carbonization treatments.
Fig. 5. 77 K nitrogen
adsorption isotherms activation samples.
for 850°C direct
Steam-activated carbons from a bituminous coal n (mmollg)
25
l
DA8CWlO
’ DABOOf28
DA800/64
l
* DABOO-3/48
I 20 . 15 10
0.
.
H’ .+t
_.
l
+v: . . . . . .
-
l
’
:
:
. .
.
:
,
.
v
.
.
.
.
.
.
1
l
.*
.I
.
.
.
l
50’
I
1
0.2
0
,
0.4
I
I
0.6
0.8
1
P/P”
Fig. 6. 77 K nitrogen adsorption isotherms for 800°C direct activation samples.
0.5
Vmicro
(cm’lg)
m DA850
0 DA850-3
01
7 DA800
I
0
10
20
A DASOO-3
I
I
I
1
30
40
50
80
70
Burn-Off(%)
Fig. 7. Total micropore volumes vs burn-off for direct activation runs at 850 and 800°C.
o.35; CWcm’kO
,
1519
of supermicropore and ultramicropore volumes of the DI equation [21]), in DAF basis vs burn-off plot. The total micropore volumes increase linearly with burn-off, showing that the DA800 series has a higher adsorption capacity than the DA850 series for the same burn-offs, with good values of 0.35 and 0.27 cm3 g-’ for 50% burn-off, respectively. Comparing the DA800-3 series, carried out at 3u,r, with the DA800 series, carried out at 1.5u,, it can be seen that the porosity developed is lower in the DASOO-3 series, probably due to the fact that the reaction rate is high, because the product inhibition effect is small (as noted earlier). Figure 8 shows the volume of ultramicropores obtained from the COZ 273 K isotherms for the same series. It can be seen that the activated carbon produced with a 3~,.,.,~steam flow rate presented lower ultramicroporosity than the corresponding one at 1.5U,, as obtained from both nitrogen and carbon dioxide isotherms. It can be seen that the volume of ultramicropores at high burn-off levels out or decreases slightly (in accordance with the findings of Rodriguez-Reinoso et al. [S]). At low burn-off these volumes increase slightly, especially for the series at 850°C. Figure 9 shows total micropore volumes vs burnoff for series MB850A850, FB500A850 and DA7-850, as well as the DA850 series as a reference, in order to study the influence of the carbonization process. It can be seen, as stated earlier, that as the reaction rate increases (i.e. DA850 > FB5OOA850 > MB850A850 > DA7-850), the micropore volume developed decreases.
0.3 0.25
4. CONCLUSIONS ---T==
0.2 1 0.15 +DASOO
0 DASOO-3
0.1 0
10
20
‘DA850 I
30
40
A DA850-3 I 50
I 80
70
Burn-off(%)
Fig. 8. Ultramicropore volumes (from CO,-273 K adsorption isotherm) vs burn-off for 800 and 850°C series of direct activation runs.
micro
0.4
(cm3/g)
0.3
0.2
0.1 n
20
DA850 30
0 MB850A850 40
7 FB5OOA850 50
Good-quality activated carbon (i.e. up to 1700 m* g-’ DAF) can be obtained by steam activation from a bituminous coal in a three-stage fluidized-bed pilot plant which is similar to those produced in a laboratory-scale discontinuous reactor. The process can be carried out in one single step, without carbonization and activation separation, obtaining similar adsorption results when the carbonization stage is carried out previously or by direct activation. Analyzing the effect of the temperature and the gas velocity on the reactivity and the total micropore volume, as the reactivity of the process increases, the micropore development decreases. In addition, it has been observed that in the fluidized-bed reactor, steam seems to activate the material, widening the narrow micropore structure formed in the carbonization process.
A DAI-850 80
70
REFERENCES
Burn-off(%)
Fig. 9. Total micropore volumes vs burn-off for activation runs at 850°C and 1.5~~~ with different carbonization conditions.
1. F. Ruiz BeviP, D. Prats Rico and A. F. Marcilla-Gomis, Ind. Eng. Chem. Prod. Rex Dev. 23, 266 (1984). 2. F. Rodriguez-Reinoso and M. Molina-Sabio, Carbon 30, 1111 (1992).
1520
I. MARTIN-GULL~N et al.
3. K. Gergova, A. Galushko, N. Petrov and V. Minkova, Carbon 30, 721 (1992). 4. J. de D. Lopez-Gonzalez, F. Martinez-Vilchez and F. Rodriguez-Reinoso, Carbon 18, 413 (1980). 5. F. Rodriguez-Reinoso, M. Molina-Sabio and M. T. Gonzalez, Carbon 33, 15 (1995). 6. M. G. Lussier, J. C. Shull and D. J. Miller, Carbon 32, 1493 (1994). 7. The Economics ofActivated Carbon. Roskill Information, London (1990). 8. M. J. Muiioz-Guillena, M. J. Illan-Gbmez, J. M. MartinMartinez, A. Linares-Solano and C. Salinas-Martinez de Lecea, Energy and Fuels 6, 9 (1992). 9. G. R. Couch, Advanced Coal Cleaning Technology, p. 86. IEA Coal Research, London (1991). 10. K. Tomkov, T. Siemieniewska, F. Czechowski and A. Jankowska, Fuel 56, 121 (1977). 11. T. Wigmans, Carbon 27, 13 (1989). 12. H. Ktihl, M. M. Kashani-Motlagh, H. J. Mtilhen and K. H. van Heek, Fuel 71, 879 (1992). 13. M. J. Mufioz Guillena, M.S. Thesis, University of Alicante (1989).
14. B. Serrano, MS. Thesis, University of Alicante (1993). 15. A. Marcilla, I. Martin, M. Asensio, R. Font and A. Linares-Solano, Extended Abstracts of Carbon 94, Grupo Espaiiol de1 Carbon, Granada, Spain, Paper C56, p. 302 (1994). 16. J. Rodriguez-Mirasol, E. Gonzalez-Serrano, T. Corder0 and J. J. Rodriguez. Proc. I Meeting GEC, Zaragoza (edited by J. Adanez and J. V. Ibarra) pp. 93-94 (1992). 17. A. Marcilla, M. Asensio and I. Martin-Gullon, Carbon 34, 449 (1996). 18. I. Martin-Gull&i, A. Marcilla. R. Font and M. Asensio, Powder Technol. 85, 193 (1995). 19. A. Marcilla, I. Martin, R. Font, A. Linares-Solano and M. Asensio. Proc. European Meeting on Fluidization, Las Palmas (Edited by A. Macias-Machin and E. Winter) p. 115 (1994). 20. B. A. Buffham and L. G. Gilifaro, AIChE J. 14, 805 (1968). 21. T. I. Izotova and M. M. Dubinin, Zh. Fiz. Khim. 39, 2796 (1965).
Pergamon PII: SOOOS-6223(96)00106-6
STEAM-ACTIVATED CARBONS FROM A BITUMINOUS COAL IN A CONTINUOUS MULTISTAGE FLUIDIZED BED PILOT PLANT I. MARTEN-GULL~N, M. ASENSIO, R. FONT and A. MARCILLA Chemical Engineering Department, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spain (Received 7 December 1995; accepted in revisedform 30 May 1996) Abstract-A
vertical three-stage fluidized bed pilot plant, with downcomers, was designed and built in order to study the continuous process of the production of activated carbons from a high-volatile bituminous coal from the Puertollano basin (Spain), by steam activation. The pilot plant can operate with a production of up to 40 kg per day. Very good activated carbons were produced at the selected operating conditions. The effect of the following operating conditions on the reactivity and adsorption characteristics of the activated carbons was analyzed: (1) carbonization conditions (one- and two-step activation), (2) activation temperature (SOO&SSOYZ), and (3) steam gas velocity (1.5-3 times the minimum fluidization velocity). Carbonization conditions considerably affect the reactivity of the chars obtained; the faster the carbonization process, the higher the reactivity. Nevertheless, the effect of this variable on the development of porosity is not very relevant, and consequently the direct activation process is an attractive alternative to the two-step (carbonization and activation) process. On the other hand, both temperature and steam flow rate (affecting the reaction rate) have a marked effect on the development of porosity. Copyright 0 1996 Elsevier Science Ltd Key Words-A.
activated carbon, B. coal, C. activation, D. carbonization.
1. INTRODUCTION
Any material with a high carbon content is susceptible of being transformed into activated carbons. In fact, coals, wood and coconut shells are the most common raw materials in worldwide industries. Moreover, many other precursors have been also investigated, such as almond shells [l-3], olive stones [4,5], peach stones [3], cherry stones [6], etc. Around one third of the world production of activated carbon comprises coal-origin activated carbons, and this ratio is bigger in both the European Union and the United States [7]. Although Spain is the third largest coal producer in Europe, all the activated carbon needed must be imported, because there is no factory producing activated carbon in Spain. Low rank coals are, in general, better precursors to be converted into activated carbons than anthracites. Mufioz-Guillena et al. [S] pointed out that the ash content does not seem to affect porosity development of the activated carbons, but the ash content must also be as low as possible [8,9]. A review of the literature shows that experimental conditions cannot be extrapolated from one given raw material to others; with regard to the oxidizing agent, some authors reveal that carbon dioxide produces more micropores that steam [ 5, lo], while other researchers have found the opposite [ 11,121. Carbonization conditions, temperature and heating rate may have different effects, and using a one- or two-step steam activation process may lead to opposite results. In addition, the type of reactor, gas flow, etc., can also 1515
lead to different results, and consequently, a specific study must be carried out for any particular raw material. The University of Alicante is studying the possibility of obtaining good-quality activated carbons from Spanish coals. First of all, a preliminary study [9,13,14] was carried out in order to select the precursor from nine different Spanish coals, from anthracites to lignites. A bituminous coal (from the Maria Isabel Mine, Puertollano) with a low ash content was the best precursor found. A wide research project, funded by CECA and by OCICARBON has been carried out in order to study the production of activated carbons at both laboratory and pilot-plant scale from the bituminous coal previously selected. For the lab-scale study, the influence of the oxidizing agent (carbon dioxide and steam), the type of reactor (horizontal and vertical fixed bed, and fluidized bed), the carbonization conditions (final temperature, heating rate, direct activation), and the reaction kinetics were analyzed. Two continuous reactors were designed and built to carry out the pilot-plant scale study: a moving-bed pilot plant [ 151 and a multistage fluidized-bed pilot plant. In addition, a previously designed rotary kiln pilot plant was also used to complete this pilot-plant scale study [16]. This paper deals with the results obtained in the three-stage fluidized-bed pilot plant, where the influence of the carbonization conditions, activation temperature and gas velocity are analyzed.
I. MARTIN-GULL~Net al
1516 2. EXPERIMENTAL
2.1 Raw material As mentioned above, the raw material used in this work is a high-volatile bituminous coal (28% volatiles) from the Maria Isabel Mine (TECSA company), Puertollano (Spain), which has been already used in previous works [17], with the following maceral analysis: 58-61% vitrinite, lo-12% exenite, and 23-25% inertite. It should be noted that this coal does not present thermoplastic behaviour, and consequently no air preoxidation treatment at low temperature was needed to avoid coke formation [ 141. In addition, two chars previously obtained from the same coal have been used in different experiments: (1) char MB850, obtained in a continuous movingbed reactor at 850°C [15]. This char was almost completely devolatilized, and the soaking time was around 1 hour. (2) Char FB500, obtained in a continuous fluidized-bed reactor at 500°C. This char was not completely devolatilized, with 10% of volatiles remaining. The soaking time was around 30 minutes. All experiments were carried out using coal/char particles of 1.5-0.84 mm (mean 1.13 mm) particle size. The minimum fluidization velocity of the char obtained from the carbonization of this coal was around 0.18 m s-l at the activating conditions (85OC and water vapor as activating agent). This velocity decreased continuously as the gasification process continued to around 0.13 m s-l, depending on the burn-off attained.
2.2 Experimentalpilotplant A continuous fluidized-bed reactor behaves as a back-mix reactor, which is not desired if a homogeneous solid product is required. In order to overcome this problem, several fluidized beds can be used in series to narrow the residence-time distribution of the solids. In this work, it was decided to design a three-stage fluidized-bed reactor, with the stages connected by downcomers or standpipes (flat tubes). The pilot-plant scheme is shown in Fig. 1. Depending on the experimental run, coal or char initially placed in the hopper is fed by a screw feeder to the upper fluidized bed, flows through the downcomers to the intermediate and lower beds, and finally exits by a weir to an activated-carbon tank at the bottom of the plant. At the top of this tank, there is a system for the sampling of solids. Steam (the oxidizing agent) is fed by the bottom of the reactor, crosses a preheating stage (with ceramic balls inside), and then the three stages countercurrently to solids, and finally exits off the reactor, passes through a cyclone and through a burner. All stages are identical, being 0.147 m in internal diameter and 0.75 m in height, with a fluidized-bed height of 0.15 m. In addition, these stages include a baffle in order to avoid a complete back-mixing of solids in each fluidized bed, and contribute to narrow the overall residence-time distri-
k
#!@
9
Fig. 1. Multistage fluidized bed pilot plant scheme: (a) hopper; (b) screw feeder; (c) fluidized bed; (d) downcomer; (e) activated carbon reservoir; (f) pre-heating stage; (g) cyclone; (h) burner; (i) electric furnaces; (j) thermocouples; (k) steam generator; (1) pressure reductor; (m) regulation valve; (n) orifice; (0) steam pre-heater; (p) sampler; (q) steam purge; (r) N, supply; (s) control; (t) manometers.
bution. The downcomers have an internal diameter of 0.021 m, and allow the transfer of solids from an upper to a lower bed. Perforated plates were used as distributors. The heat required is supplied by four 10 kW electric furnaces, controlled by corresponding thermocouples placed at the fluidized beds. Hydrodynamically, this reactor can operate between two limiting velocities, the minimum fluidization velocity (below this value there is no solid movement) and the velocity at which the solids’ height inside the downcomer (in the slugging regime) equals the total downcomer height. Further details of the pilot plant and the hydrodynamic model previously developed in order to determine this upper velocity limit can be found elsewhere [18]. At the activating conditions (85O”C, steam as the fluidizing gas and char particle size of 1.13 mm) the gas velocity stability range is wide enough to assure stable conditions at up to five times the minimum fluidization velocity. The residence-time distribution of solids was previously determined at room temperature, using 1.13 mm coal particles, air as the fluidizing agent and under hydrodynamic conditions similar to those used previously [ 191. The experimental residence-time distribution function vs dimensionless time is reproduced in Fig. 2, along with the best fit to the “tanks in series” model [20]. It can be seen that this reactor behaves almost as if an additional stage were present,
Steam-activated carbons from a bituminous coal
1517
DA7-850, MB850A850 and FB5OOA850: experiments were carried out at different carbonization conditions but activated at 850°C and 1.5u,,. In the run DA7-850 coal was activated directly but with the upper bed at 700°C. Runs MB850A850 and FB5OOA850 were carried out at 850°C using two previously carbonized chars in a moving-bed reactor at 850°C (char MBSSO) and in a one-stage fluidizedbed reactor at 500°C (char FB850), respectively.
0.8
Fig. 2. Experimental and best fit to “tanks in series” model residence-time distribution for the three-stage fluidized-bed pilot plant (dimensionless residence-time distribution &, dimensionless residence time 0).
due to the combined downcomers.
effects of the baffles and
2.3 Activation runs The results of 12 experiments carried out with this pilot plant are reported here, analyzing the influence of the following variables: (1) residence time of solids (solid feed rate), (2) number of activation steps (10 runs with direct activation and two runs with activation of a previously devolatilized char), (3) activation temperature (eight runs at 850°C and four runs at SOOC), and (4) steam velocity (10 runs at 1.5-2~~~ and two runs at 3-4u,r). Table 1 shows the operating conditions of each run. These runs are classified in the following series: DA850: including experiments 2, 3, 4 and 5, direct activation runs at 850°C and 1.5u,, varying only the feed rate (and consequently the residence time). DA800: including experiments 6, 7 and 8, direct activation runs at 800°C and 1.5u,r, varying the feed rate. DA850-3 and DA800-3: experiments 9 and 10, direct activation runs similar to those mentioned above, at 850 and 8OO”C, respectively, but at 3u,r steam velocity.
2.4 Activated carbon characterization 2.4.1 Burn-off The burn-off was indirectly determined from the ash content of the activated carbons obtained and that of the initial raw material. 2.4.2 Residence time Residence time was determined from the experimental hold-up (obtained from the experimental pressure drops) and the solid feed rate (on a dry and volatile-free basis). 2.4.3 Adsorption capacity The micropore volume was characterized by both 77 K nitrogen and 273 K carbon dioxide adsorption isotherms with a Quantacrome Autosorb 6 sorption apparatus. The following parameters were obtained: total micropore volumes (sum of ultramicropore and supermicropore volumes) from the application of the DI equation to the O-O.4 relative pressure range nitrogen isotherm data [21], and the ultramicropore volumes applying the DR equation to the carbon dioxide isotherm data. All volumes are expressed by DAF mass unit. 3. RESULTS All the runs shown in Table 1 were successful, and the reactor operated smoothly. In addition, the hydrodynamic behaviour of the process was monitored by the absolute pressure at each stage, showing therefore that a continuous process at stationary conditions was being carried out. Experiments were stopped after more than 2.5 times the residence time corresponding to each experiment. Samples were collected in each run at different times, showing for each particular run a very low dispersion in the micropore pore volumes (about 2%),
Table 1. Activation runs carried out in this work with the pilot plant. T,,=nominal temperature of the upper bed, T,= actual temperature of the upper bed, T,,,= temperature of the medium and lower beds, F=feed flow rate, u=fluidization velocity
Run 2 3 4 5 6 7 8 9 10 16 24
Series
MB850A850 DA850
DA800 DA850-3 DA800-3 DA7-850 FB5OOA850
L
(“C)
850 810 800 820 835 800 800 800 850 800 700 850
T, (“C)
850 850 850 850 850 800 800 800 850 800 700 850
T,,I (“C) 850 850 850 850 850 800 800 800 8.50 800 850 850
F (kg per day)
u (m s-l)
30.4 30.4 35.1 25.6 18.3 36.8 15.2 8.2 23.4 10.0 25.6 26.6
0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.54 0.54 0.32 0.32
I. MARTIN-GULL~N
1518
indicating that stationary conditions were also attained. The dispersions of the nominal burn-off degrees obtained were a little higher (about 6%), as expected, but indicated a stationary state.
3.1 Burn-ojJ Figure 3 shows the mean nominal burn-off degrees vs nominal residence times for the direct activation series (DASOO, DA800-3, DA850 and DA850-3). It can be seen, as expected, that the reactivity at 850°C was higher than that at 800°C. It can be observed that the reactivity for the two DA850 and DA800 series (at 1.5~~~) behaves linearly vs residence time. To obtain a 50% burn-off activated carbon, a residence time of 2 hours is needed at 850°C; at SOO’C, 5 hours are required to obtain the same burn-off. Increasing the steam flow rate to 3~,,,~ does not seem to influence the reaction rate with respect to 1.5~~~ at 85O”C, whilst at 800°C the reaction rate increases markedly. This fact can be explained by considering the strong inhibition effect of the reaction products, competing with the non-inhibited reaction. At 800°C the effect of the dilution of the products dominates, and consequently the reaction rate is sensitive to the steam flow rate, whereas at 850°C the rate seems to be unaltered, showing that the non-inhibited reaction dominates. Figure 4 shows the 850°C activation runs with
70
0’
1
1
I
I
I
2
3
4
5
Residence
Fig. 3. Burn-off
70
different carbonization processes, as well as those corresponding to the DA850 series, included as a reference for comparing the results at the same temperature and gas velocity. Char FB500, in the run FB500DA850, underwent a higher conversion, although it should be less reactive than the direct activation chars obtained, probably as a consequence of the fact that less time is required for devolatilization and a lesser amount of volatiles was produced. Nevertheless, the MB850 char, in the run MB850A850, presents equal or perhaps lower reactivity than the DA850 activated carbons, which can be explained by considering that these chars were prepared in a moving-bed reactor at a much lower heating rate than those produced in the fluidized-bed reactor, which would result in them being less reactive in the subsequent activation step. This shows the marked effect of the carbonization-step heating rate on the reactivity of the chars obtained, as stated previously [ 171. Activated carbons obtained in the DA7-850 run, with the first stage at 700°C (where only carbonization took place), obviously presented a lower reactivity than those prepared in the DA850 series. In the DA7-850 run, the material is only activated in the two lower fluidized beds, and consequently the char should be less reactive. In section 3.2, the adsorption characteristics of these processes will be analyzed.
3.2 Adsorption
Burn-off (%)
,
0
eta/
6
7
time (h)
vs residence-time plot for direct activation runs at 800 and 850°C.
capacity
Figures 5 and 6 show the 71 K nitrogen adsorption data for activated carbons obtained in the 850 and 800°C direct activation runs, respectively. The last number added to the name of the run indicates the burn-off degree. It can be seen that all samples correspond to microporous carbons, with the isotherms more open as the burn-off increases. At 85O”C, the isotherm of the DA85-3 sample seems to present the same tendency as those of DA85 series (as in the case of reactivity), whereas at 800°C the DA800-3/48 sample has nearly the same capacity as the DA800/28, with only a 28% burn-off. It seems that when the reaction rate is high, less pore volume is developed. Figure 7 shows the total Nz micropore volume (sum
Burn-off (%)
60 50 40 I
/ .
30 20 10
0 MB850A850 I
0 1
1.5
n
DA850
l
DA7-850
I 2 Residence
Fig. 4. Burn-off vs residence-time 850°C and 1.5u,, with different
v FBSOOA850 I 2.5
3
time(h)
plot for activation runs at carbonization treatments.
Fig. 5. 77 K nitrogen
adsorption isotherms activation samples.
for 850°C direct
Steam-activated carbons from a bituminous coal n (mmollg)
25
l
DA8CWlO
’ DABOOf28
DA800/64
l
* DABOO-3/48
I 20 . 15 10
0.
.
H’ .+t
_.
l
+v: . . . . . .
-
l
’
:
:
. .
.
:
,
.
v
.
.
.
.
.
.
1
l
.*
.I
.
.
.
l
50’
I
1
0.2
0
,
0.4
I
I
0.6
0.8
1
P/P”
Fig. 6. 77 K nitrogen adsorption isotherms for 800°C direct activation samples.
0.5
Vmicro
(cm’lg)
m DA850
0 DA850-3
01
7 DA800
I
0
10
20
A DASOO-3
I
I
I
1
30
40
50
80
70
Burn-Off(%)
Fig. 7. Total micropore volumes vs burn-off for direct activation runs at 850 and 800°C.
o.35; CWcm’kO
,
1519
of supermicropore and ultramicropore volumes of the DI equation [21]), in DAF basis vs burn-off plot. The total micropore volumes increase linearly with burn-off, showing that the DA800 series has a higher adsorption capacity than the DA850 series for the same burn-offs, with good values of 0.35 and 0.27 cm3 g-’ for 50% burn-off, respectively. Comparing the DA800-3 series, carried out at 3u,r, with the DA800 series, carried out at 1.5u,, it can be seen that the porosity developed is lower in the DASOO-3 series, probably due to the fact that the reaction rate is high, because the product inhibition effect is small (as noted earlier). Figure 8 shows the volume of ultramicropores obtained from the COZ 273 K isotherms for the same series. It can be seen that the activated carbon produced with a 3~,.,.,~steam flow rate presented lower ultramicroporosity than the corresponding one at 1.5U,, as obtained from both nitrogen and carbon dioxide isotherms. It can be seen that the volume of ultramicropores at high burn-off levels out or decreases slightly (in accordance with the findings of Rodriguez-Reinoso et al. [S]). At low burn-off these volumes increase slightly, especially for the series at 850°C. Figure 9 shows total micropore volumes vs burnoff for series MB850A850, FB500A850 and DA7-850, as well as the DA850 series as a reference, in order to study the influence of the carbonization process. It can be seen, as stated earlier, that as the reaction rate increases (i.e. DA850 > FB5OOA850 > MB850A850 > DA7-850), the micropore volume developed decreases.
0.3 0.25
4. CONCLUSIONS ---T==
0.2 1 0.15 +DASOO
0 DASOO-3
0.1 0
10
20
‘DA850 I
30
40
A DA850-3 I 50
I 80
70
Burn-off(%)
Fig. 8. Ultramicropore volumes (from CO,-273 K adsorption isotherm) vs burn-off for 800 and 850°C series of direct activation runs.
micro
0.4
(cm3/g)
0.3
0.2
0.1 n
20
DA850 30
0 MB850A850 40
7 FB5OOA850 50
Good-quality activated carbon (i.e. up to 1700 m* g-’ DAF) can be obtained by steam activation from a bituminous coal in a three-stage fluidized-bed pilot plant which is similar to those produced in a laboratory-scale discontinuous reactor. The process can be carried out in one single step, without carbonization and activation separation, obtaining similar adsorption results when the carbonization stage is carried out previously or by direct activation. Analyzing the effect of the temperature and the gas velocity on the reactivity and the total micropore volume, as the reactivity of the process increases, the micropore development decreases. In addition, it has been observed that in the fluidized-bed reactor, steam seems to activate the material, widening the narrow micropore structure formed in the carbonization process.
A DAI-850 80
70
REFERENCES
Burn-off(%)
Fig. 9. Total micropore volumes vs burn-off for activation runs at 850°C and 1.5~~~ with different carbonization conditions.
1. F. Ruiz BeviP, D. Prats Rico and A. F. Marcilla-Gomis, Ind. Eng. Chem. Prod. Rex Dev. 23, 266 (1984). 2. F. Rodriguez-Reinoso and M. Molina-Sabio, Carbon 30, 1111 (1992).
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I. MARTIN-GULL~N et al.
3. K. Gergova, A. Galushko, N. Petrov and V. Minkova, Carbon 30, 721 (1992). 4. J. de D. Lopez-Gonzalez, F. Martinez-Vilchez and F. Rodriguez-Reinoso, Carbon 18, 413 (1980). 5. F. Rodriguez-Reinoso, M. Molina-Sabio and M. T. Gonzalez, Carbon 33, 15 (1995). 6. M. G. Lussier, J. C. Shull and D. J. Miller, Carbon 32, 1493 (1994). 7. The Economics ofActivated Carbon. Roskill Information, London (1990). 8. M. J. Muiioz-Guillena, M. J. Illan-Gbmez, J. M. MartinMartinez, A. Linares-Solano and C. Salinas-Martinez de Lecea, Energy and Fuels 6, 9 (1992). 9. G. R. Couch, Advanced Coal Cleaning Technology, p. 86. IEA Coal Research, London (1991). 10. K. Tomkov, T. Siemieniewska, F. Czechowski and A. Jankowska, Fuel 56, 121 (1977). 11. T. Wigmans, Carbon 27, 13 (1989). 12. H. Ktihl, M. M. Kashani-Motlagh, H. J. Mtilhen and K. H. van Heek, Fuel 71, 879 (1992). 13. M. J. Mufioz Guillena, M.S. Thesis, University of Alicante (1989).
14. B. Serrano, MS. Thesis, University of Alicante (1993). 15. A. Marcilla, I. Martin, M. Asensio, R. Font and A. Linares-Solano, Extended Abstracts of Carbon 94, Grupo Espaiiol de1 Carbon, Granada, Spain, Paper C56, p. 302 (1994). 16. J. Rodriguez-Mirasol, E. Gonzalez-Serrano, T. Corder0 and J. J. Rodriguez. Proc. I Meeting GEC, Zaragoza (edited by J. Adanez and J. V. Ibarra) pp. 93-94 (1992). 17. A. Marcilla, M. Asensio and I. Martin-Gullon, Carbon 34, 449 (1996). 18. I. Martin-Gull&i, A. Marcilla. R. Font and M. Asensio, Powder Technol. 85, 193 (1995). 19. A. Marcilla, I. Martin, R. Font, A. Linares-Solano and M. Asensio. Proc. European Meeting on Fluidization, Las Palmas (Edited by A. Macias-Machin and E. Winter) p. 115 (1994). 20. B. A. Buffham and L. G. Gilifaro, AIChE J. 14, 805 (1968). 21. T. I. Izotova and M. M. Dubinin, Zh. Fiz. Khim. 39, 2796 (1965).