Preparation of activated carbon from western Canadian high rank coals

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology 41 (1995) 89-99

Preparation of activated carbon from western Canadian high rank coals G. Kovacik a, B. Wong a, E. Furimsky b'* aAlberta Research Council, Coal and Hydrocarbon Processing Department, P.O. Box 1310, Devon, Alberta, TOC lEO, Canada bNatural Resources CANADA, Canada Center .for Mineral and Energy Technology, Energy Research Laboratories, 555 Booth Street. Ottawa, Ontario, KIA OGI, Canada

Received 14 January 1994; accepted in revised form 28 April 1994

Abstract Partial steam gasification of Mt. Klappan anthracite and Cascade semianthracite with char conversion greater than 60%, produced activated carbons with surface areas greater than 1000 m2/g. The pore structures of the activated carbons were predominantly microporous and mesoporous. The proportions of macropores were on the order of 2%. Fuel gas produced during steam activation of chars contained predominantly combustible gases, i.e., 45-55% H2 and 30-40% CO whereas the amount of CO2 ranged between 5 and 15%. Correlations of char conversion with operating parameters and surface areas were developed and used to predict the activation process. Selected samples of activated carbons were characterized for the water and wastewater treatment as well as for gold recovery.

1. Introduction Activated carbon is a m o n g the important carbon based materials, finding wide industrial applications. The consumption of activated carbon has been steadily increasing mainly in various environmentally related systems such as water treatment and gas purification. It is produced in a powder and granular form. A wide range of organic solids and semisolids has been used for production of activated carbon. In most cases, the procedure comprises carbonization of the organic base followed by the activation of the residual char. The activation involves either partial gasification or a heat treatment in the presence of various chemical agents. Rather extensive information on various aspects of activated carbon is available in the literature. This information has been periodically reviewed. Thus, the preparation, application and

* Corresponding author. 0378-3820/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 7 8 - 3 8 2 0 ( 9 4 ) 0 0 0 5 5 - X

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characterization aspects of activated carbon were reviewed in detail by Wigmans [1] and Rodriguez-Reinoso [2]. The activated carbon was part of the review on various advanced carbon materials prepared from coal, published recently by Walker [31. It appears that all ranks of coal have been used for preparation of activated carbon. However, the information on the use of high rank coals such as anthracite and semianthracite is the least extensive. In the present study, such high rank coals were used as potential feedstocks for the preparation of activated carbon. The aim was to develop a procedure and to establish the optimal parameters of the process. Also, an attempt was made to develop correlations for predicting the activation process. Two particle size fractions of each anthracite were used to simulate the preparation of a powder and a granular form of activated carbon.

2. Experimental 2.1. Feedstocks

The Mt. Klappan anthracite from British Columbia and Cascade semianthracite from Alberta were used as feedstocks. For the experiments, the feedstocks were air dried, crushed and screened to obtain fractions of 20-40 and 100-150 mesh. Proximate and ultimate analyses of these fractions are shown in Table 1. 2.2. Reactor

A schematic of the reactor is shown in Fig. 1. The coal/char bed within the reactor was contained in a 30 mm diameter quartz basket. The reactor was externally heated using a single zone Lindberg furnace. The steam to the reactor was supplied by converting water in a coil in the furnace. Water was supplied by a Gilson model 304 high accuracy, positive displacement pump. The steam flow could be controlled by the pump over a range of 0.0025-50.0 ml/min. The internal reactor temperature was measured by a type K thermocouple placed near the coal sample. Pressure was monitored using a Tescom 0-250 psig pressure regulator and Matheson 0-220 psig test gauges. 2.3. Procedure

A 5.0 g sample of coal was placed into the porous basket which was then dropped into the preheated reactor zone under the flow of N2 (0.5 l/min). The sample was held at the desired temperature for 10 min to remove volatiles (carbonization step). The weight losses occurring during the carbonization step are shown in Table 2 together with the sum of moisture and volatiles from proximate analysis. At the end of the carbonization step, the gas was switched from N2 to steam flowing at a rate of 1.0 g/min (steam gasification/activation step). The reaction time varied from 30 to 90 min. At the conclusion of the experiment, the unreacted residue was removed from the reactor and its weight was determined.

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Table 1 Properties of coals Mt. Klappan

Cascade

20-40

100-150

20-40

100 150

Proximate Moisture Ash Volatiles Fixed carbon

3.1 7.4 4.5 85.0

3.1 7.3 3.8 85.8

2.1 4.5 12.1 81.3

2.9 5.5 12.7 79.0

Ultimate Carbon Hydrogen Nitrogen Sulfur

94.5 3.1 1.4 0.5

95.0 3.1 1.4 0.5

91.3 4.3 1.6 0.9

93.4 4.2 1.6 0.9

productgasout

1 -- ~coal

sample

~

~ porous basketceramic

t

steamin Fig. 1. Gasification reaction schematic.

To minimize the number of experiments required to find the optimum conditions, statistical, three-level Box-Behnken experimental design [4] was selected for one case, i.e., Mt. Klappan anthracite (20-40 mesh). This was based on the experiments carried out during the system commissioning and other previous experience. The variables considered for the investigations were temperature, pressure and residence time. The ranges of variables to be used for the selected case are in Table 3. a

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Table 2 Weight losses during carbonization step (wt%) Proximate analysis (moisture + volatiles)

Carbonization temperature (°C)

Coal

850

900

950

Mt. Klappan 20-40 100-150

9.2 11.2

9.6 11.6

10.0 12.1

7.6 6.9

Cascade 20-40 100-150

nd nd

16.9 20.2

17.6 20.8

14.2 15.6

nd

not determined.

Table 3 Experimental design for Mt. Klappan anthracite (20--40 mesh) Parameter

Symbol

Low ( - 1)

Middle (o)

High ( + 1)

Unit

Temperature Residence time Pressure

xl x2 x3

850 30 127

900 60 427

950 90 815

°c min kPa

Three-variable Box-Behnken design X1

+1 +1 -1 -1 +1 +l -1 -1 0 0 0 0 0 0 0

X2

+1 -1 +l -1 0 0 0 0 +1 +1 -1 -1 0 0 0

X3

0 0 0 0 +1 -1 +1 -1 +1 -1 +1 -1 0 0 Center point 0

2.4. Analysis T h e gas p r o d u c e d d u r i n g the s t e a m gasification step was collected in a gas T h e total v o l u m e of the gas was calculated using the k n o w n v o l u m e of the pressure a n d t e m p e r a t u r e . T h e gas c o m p o s i t i o n was then d e t e r m i n e d with a l e t t - P a c k a r d 5790a gas c h r o m a t o g r a p h using m o l e c u l a r sieve an d P o r o p a c Q r a t i o n c o l u m n s a n d a t h e r m o c o n d u c t i v i t y detector.

tank. tank, Hewsepa-

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Surface areas and porosities of activated carbons were determined by an automated Micromeritics Digisorb 2600 system. For the determinations, about 0.2 g of samples was taken. The sample was degassed at 300°C in a vacuum of about 10-3 Torr. The BET surface area was calculated for adsorption at relative pressures between 0.05 and 0.21. A Langmuir surface area was calculated for the BET intercepts having negative values. The porosity parameters were determined for selected samples. Attempts were also made to use the Kelvin equation and the assumption of straight, parallel cylindrical pores opened to both ends was used to obtain additional porosity data. For this purpose, the values of surface tension and contact angle of liquid N2 were 8.85 erg/cm 2 and 0, respectively. The pore size distribution was calculated according to the methodology of Barret, Joyner and Halenda, the so-called BJH method [5] . Selected activated carbons, characterized for water and wastewater treatment, were reduced to a top size of 325 mesh. The standard American Waste Works Association (AWWA) and Environmental Protection Agency (EPA) methods were used for the characterization. Thus, the iodine number (IN), modified phenol value (MPV) and tannine number (TN) were determined by the AWWA B600-78 method whereas the methylene blue number (MBN) by the EPA 625/1-71 method. The gold sorption capacity was determined using the 30-50 mesh samples. In this case, the activated carbons were immersed in a gold solution for 4 h and the residual gold concentrations of the solution were determined at various time intervals for sorption rate and equilibrium sorption capacity.

3. Results and discussion

3.1. Process modelling Correlation of char conversion with experimental parameters showed that a quadratic model gave a good fit based on least square regression analysis (r 2 = 0.9864). The correlation is summarized as follows: C % = 10-a( - 0.08 - 6 . 6 1 T - 282.16R - 89.33P + 0.44TR + 0.12TP + 0.14RP + 0.006T 2 - 0.42R z - 0.09p2), where C% is the char conversion (DAF), T is temperature (°C), P is pressure (psi) and R is residence time. The model equation was used to calculate surface plots for three pressures, i.e., 127, 471 and 815 kPa. An example of such a surface plot is shown in Fig. 2 for Mt. Klappan anthracite (20-40 mesh) at 471 kPa. An inspection of these plots showed that pressure increase from 127 to 471 resulted in only about a 12% conversion increase at the maximum temperature and residence time. Further pressure increase to 815 kPa had no measurable effect on the conversion. It appears that the combined effects of temperature and residence time had the largest influence on conversion.

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Converaion

[%]

100

880 80

940 Temperature

Fig. 2. Surface plot of conversion as a function of temperature and residence time: P = 471 kPa, 20-40 mesh Mt. Klappan.

A quadratic model was also developed to correlate the surface areas and conversion. For char conversion less than 90%, the model exhibits the following equation for BET and Langmuir surface area: BET

SA = - 1237.51C% 2 + 2742.12C% - 108.7,

LANGM

SA = - 1198.79C% 2 + 2292.16C% - 134.8,

where C % is char conversion (DAF) expressed in fractions. For Mt. Klappan anthracite (20-40 mesh), these correlations are shown in Fig. 3. The C % in the surface areas equations can be replaced by the equation correlating the C % with experimental parameters. This then offers an opportunity for correlating the surface area of activated carbons with experimental parameters.

G. Kovacik et al./Fuel Processing Technology 41 (1995) 89-99 15

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95

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14 13

12

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U ¢/

n

9

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8

!

~

°n •

//o

7

n

s

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o.6

o'.7 o'.s o'.9 l.o

daf [- ]

n B~I" N~ o ~ _ , , ~ , ~ Nffi .... - I ~ 7 . N 8 9 4 " X " 2 + 27,G.123N3"X - 1 ~ . 7 0 1 0 ~ --I198.788~19"X'2 + 2292.16222"X - 134.18.3847

Fig. 3. Surfacearea as a function of conversion 20~0 mesh Mt. Klappan.

3.2. Properties o f activated carbons

Surface properties of activated carbons, such as N2 BET surface area, pore volume and pore size distribution, are shown in Table 4. The results are for char conversions ranging from 58 to 85%. As the results in Fig. 3 show, this conversion range is optimal for achieving high surface areas of activated carbons. This observation was made for both coals and both particle size fractions. The results in Table 4 show that the activated carbons from both coals are predominantly microporous. However, the data also suggest that as char conversion increased, the proportion of mesopores increased at the expense of micropores. The decrease of surface area with increasing proportions of mesopores is evident as well. A close examination of the results in Table 4 suggests that at comparable conversion of a particular char and particle fraction, the increase of mesopores led to a decrease of the surface area.

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Table 4 Surface properties of activated carbons Temperature (°C)

Residence time (min)

Conversion (%)

BET SA (m2/g)

Pore volume (ml/g)

Pore distribution (%) Micro

Meso

Macro

< 20A

20-200A

200-1000A

Mt. Klappan 20-40 95(P 950 900

60 60 90

85 64 65

906 872 920

0.44 0.38 0.40

67 75 82

32 23 17

1 1 1

60 60 60

81 84 58

1184 1188 987

0.50 0.50 0.35

67 64 86

31 34 12

2 2 2

60 90 60 60

82 83 60 61

1243 1378 1037 1082

0.59 0.65 0.35 0.41

59 64 81 79

39 35 18 20

2 1 1 1

45 45 75 60

83 87 84 71

884 1030 1240 1104

0.54 0.59 0.56 0.43

35 52 60 72

61 46 38 25

4 2 2 3

Mt. Klappan 100-150 950 b 950 900

Cascade 20-40 950 900 900 900

Cascade 100-150 950 b 950 900 900

a Experiment performed at 815 kPa pressure.

b Experiment performed with no charring step.

The proportions of mesopores and pore volume of activated carbons prepared from the Cascade chars were consistently larger than those from the Mt. Klappan anthracite. This can be attributed to a greater combustion and gasification reactivity of the former I-6]. Also, more volatile matter removed from the Cascade semianthracite during the charring step resulted in a more reactive char compared with that from the Mr. Klappan anthracite. This is supported by the properties of chars obtained from the coals without the charring step (Table 4). Thus, little difference between properties was observed for the activated carbons prepared from the Mt. Klappan anthracite (100-150 mesh) and its char whereas for the same particle fraction of Cascade semianthracite and its char the difference in surface area, pore volume and pore distribution was quite evident. Nevertheless, for both coals, a good quality activated carbon can be prepared directly, without the charring step. The predictable effect of particle size was observed. Thus, for both chars the conversion and pore volume for 100-150 mesh particles were larger than those for 20-40 mesh particles under similar operating conditions.

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Table 5 Composition of gaseous products (mol% dry) Operating conditions

Pressure

Composition

High heating value (MJ/m 3 )

(kPa)

Temperature (°C)

Residence time (min)

CO

H2

CO2

815 127 815 127

29 40 35 37

56 54 53 55

15 6 12 8

10.2 11.3 10.6 11.0

60 30

127 127

42 39

52 53

6 8

11.3 11.0

60 60

127 127

39 37

53 54

8 9

11.0 10.9

60 60

127 127

38 35

53 54

9 11

10.9 10.7

Mt. Klappan 20-40 950 950 850 850

60 60 60 68

Mt. Klappan 100-150 950 900

Cascade20-40 950 900

Cascade lO0-150 950 900

3.3. Composition of gaseous products Typical compositions of gaseous products obtained during the activation step are shown in Table 5. The products have relatively high heating value and as such may contribute to the overall energy balance of the plant. The compositions suggest that C + H 2 0 reactions played a key role during the activation step. For experiments performed at 127 kPa, concentrations of CO2 were relatively small. However, the CO2 concentration increased with increasing pressure, suggesting an increased contribution from the CO + H 2 0 reaction.

3.4. Characterization of activated carbons The characterized samples of activated carbons were prepared from Mt. Klappan anthracite. Two commercial samples, i.e., ACA (for water treatment) and ACB (for gold recovery), were tested concurrently for comparison. The water and wastewater treatment results are shown in Table 6. The IN indicates the adsorption capacity for low molecular weight compounds whereas the MBN that of high molecular weight compounds. The MPV indicates the adsorption capacity for polar and ionic low molecular weight compounds whereas the TV that of aquatic humic substances and color-imparting organic compounds. The results show that all samples prepared from the Mt. Klappan anthracite exceeded the minimum IN value of 500 mg/g, suggested by the AWWA. The MBN is surface area dependent. In this

98

G. Kovacik et al./ Fuel Processing Technology 41 (1995) 89-99

Table 6 Summary of water and wastewater characterization data Sample

Surface area (m2/g)

IN (mg/g)

MBN (g/100g)

MPV (g/l)

TV (mg/1)

A B C D ACA

772 871 1239 870 1018

740 780 1050 800 1090

16.8 17.7 24.7 17.5 24.6

4.0 3.9 2.7 3.3 1.7

1375 1032 1525 1376 273

Table 7 Summary of gold recovery results Sample

Surface area (m2/g)

Adsorption rate (h- x)

Adsorption capacity (mg/g)

A B C D ACB

772 871 1239 870 1050

5.94 5.28 14.65 6.27 3.95

18.8 20.6 28.8 24.2 36.6

regard, an activated carbon which is comparable in its performance to the commercial sample can also be prepared from the anthracite. The M P V and TV represent the amounts of activated carbon required to remove 90% of phenol and tannic acid from a solution, respectively. The results show that activated carbons having suitable M P V (max. 3.5 g/l) can also be obtained. However, all four activated carbons exceeded the TV suggested by the A W W A (max. 270 mg/1). Then, the activated carbon prepared from Mt. K l a p p a n anthracite is less efficient in the sorption of humic molecules. The adsorption rate and adsorption capacity are perhaps the most important parameters in selecting activated carbons for gold recovery. It is generally accepted that in an actual operation, the former is more important. The results shown in Table 7 were determined using the methods described by Faulkner et al. [7]. For all four samples prepared from the Mt. Klappan anthracite, the adsorption rates were higher than that of the commercial ACB sample in spite of lower surface areas. However, the adsorption capacity of the commercial sample was better. It is noted that the commercial sample was of a coconut origin.

4. Conclusions Both the Mt. Klappan anthracite and Cascade semianthracite are suitable for preparation of activated carbon. Thus, products with surface areas exceeding

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1000 m2/g could be r e a d i l y p r e p a r e d from b o t h 2 0 - 4 0 a n d 100-150 mesh particle fractions. T h e m a j o r i t y of p o r e s p r o d u c e d d u r i n g a c t i v a t i o n were in a m i c r o p o r e size range. The p r o p o r t i o n of m e s o p o r e s increased with increasing conversion. F o r b o t h coals, g o o d q u a l i t y a c t i v a t e d c a r b o n s were p r e p a r e d even w i t h o u t a c a r b o n i z a t i o n step, which is usually e m p l o y e d before the a c t i v a t i o n step in the p r o d u c t i o n of a c t i v a t e d c a r b o n from coal. T h e results of the B o x - B e h n k e n test m a t r i x on the 2 0 - 4 0 mesh Mt. K l a p p a n a n t h r a c i t e s h o w e d t h a t residence time a n d t e m p e r a t u r e h a d a m a j o r effect on conversion whereas the effect of pressure was small. U s i n g these results, the c o r r e l a t i o n s were d e v e l o p e d for c h a r c o n v e r s i o n as a function of o p e r a t i n g c o n d i t i o n s a n d surface a r e a as a function of the conversion. The a c t i v a t i o n process c o u l d be p r e d i c t e d using these correlations. The c h a r a c t e r i z a t i o n d a t a show t h a t p r e p a r e d a c t i v a t e d c a r b o n s are suitable for r e m o v i n g s o m e c o n t a m i n a n t s from the w a t e r a n d w a s t e w a t e r as well as for the r e c o v e r y of g o l d from a solution.

References [1] Wigmans, T., 1986. Fundamentals and practicalimplicationsofactivatedcarbon production by partial gasification of carbonaceous materials. In: J.L. Figueiredo and J.A. Moulijn (Eds.), Carbon and Coal Gasification, NATO ASI Series E, No. 105, pp. 559-600. [2] Rodriguez-Reinoso, F., 1986. Preparation and characterization of activated carbons. In: J.L. Figueiredo and J.A. Moulijn (Eds.), Carbon and Coal Gasification, NATO ASI Series E, No. 105, pp. 601-643. [3] Walker, P.L., Jr., 1990. Carbon: An old but new material revisited. Carbon, 28: 261-271. [4] Box, G.E.P. and Behnken, D.W., 1960. Some new three level designs for the study of quantitative variables. Technometrics, 2: 455-465. [5] Barret, E.P., Joyner, L.G. and Halenda, P.P., 1951. Determination of pore volume and area distributions in porous substances. I. Computation from nitrogen isotherms. J. Am. Chem. Soc., 73: 373-380. [6] Furimsky, E., Palmer, A.D., Kalkreuth, W.D., Cameron A.R. and Kovacik, G. 1990. Prediction of coal reactivity during combustion and gasification by using petrographic data. Fuel Proc. Technol., 25: 135 151. [7] Faulkner, W.D., Urbanic, J.E. and Ruckel, R.W., 1987. Activated carbon for precious metals recovery. In Proc. of Mining and Metallurgical Society, 110th Annual Meeting, Denver, CO, 23-27, February.