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Studies on pore size and reactivity of cokes
Fuel Processing Technology, 1 (1977/1978) 242--246 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Short communication STUDIES ON PORE SIZE AND REACTIVITY OF COKES
A.K. BOSE, P.K. MITRA, D.K. AS, K. RAJA and K.A. KINI Central Fuel Research Institute, P.O.F.R.I., Dhanbad, Bihar (India) (Received November 9th, 1977)
R e a c t i v i t y o f c o k e s t o w a r d s c a r b o n d i o x i d e is an i m p o r t a n t p a r a m e t e r det e r m i n i n g t h e i r c o n s u m p t i o n in c u p o l a s as well as in blast furnaces. I f the reactivity is high, t h e c o n s u m p t i o n of c o k e s is higher and vice versa. With the increasing use o f o x y g e n e n r i c h e d blast in blast f u r n a c e s f o r reasons o f econo m y in c o k e c o n s u m p t i o n , t h e r e a c t i v i t y is b e c o m i n g m o r e i m p o r t a n t . A f e w a s p e c t s o f the r e a c t i v i t y of c o k e p r o d u c e d f r o m I n d i a n coals, w i t h s t e a m , c a r b o n d i o x i d e etc., h a v e b e e n studied b y R a o a n d Das G u p t a [ 1 ] . F u r t h e r investigations o n the r e a c t i v i t y of c o k e s w i t h c a r b o n d i o x i d e were carried o u t b y Banerjee e t al. [2] a n d an a l t e r n a t i v e m e t h o d f o r its measurem e n t has b e e n described. M o r e r e c e n t l y , Kini [3] has also e x a m i n e d t h e rel a t i o n s h i p b e t w e e n r e a c t i v i t y a n d t h e surface area o f cokes. F u r t h e r investigation o f the results has s h o w n t h a t t h e r e is a c o r r e l a t i o n b e t w e e n t h e reactivity a n d t h e p o r o s i t y o f cokes, t h e f o r m e r increasing w i t h t h e square r o o t o f the p o r o s i t y , as d e t e r m i n e d b y specific v o l u m e s in m e r c u r y . T h e results are given in T a b l e 1. TABLE 1 Specific gasification rates and total porosity of gasified cokes from demineralised (Buller) New Zealand and Young Wallsend (Australian) coals Material
20% burn-off Specific gasification rate
40% burn-off Specific volume (ml/g)(dry)
(g/sec/g) • 104
Coke from demineralised New Zealand (Buller) coal Coke from demineralised Young Wallsend Vitrain (Australian)
Specific gasification rate
Specific volume (ml/g)(dry)
(g/sec/g) . 104
0.5
1.30
0.6
1.71
1.1
1.30
1.3
1.44
243
To the knowledge of the authors, no a t t e m p t has been made so far to compare the reactivities of cokes with Thiele's theory [4], as elaborated by Wheeler [ 5]. The relevant relationship is: h tan h ; 18D
PBVg l°ge 1--~e
where a is the pellet diameter in em, D is the bulk diffusion coefficient for a single pore in cm2/sec, FR is the feed rate to the reactor in molecules/ml/sec, CA is the inlet concentration of reactant in molecules/ml, PB is the bulk density in g/ml, Vg is the specific volume in ml/g, C~c is the fraction of solid reactant and h is a dimensionless quantity given by the relation:
h = L ~D where L = length of the pore, k = reaction constant, r = radius of the pore, and D = diffusion constant. An a t t e m p t has therefore been made to examine the applicability of this theory to a few coke samples prepared from Indian coals. Along with these, the results from cokes prepared from Australian and New Zealand coals are also discussed. EXPERIMENTAL PROCEDURE
Preparation of coke samples Coals of varying rank were selected, and cokes were prepared from them in the high temperature carbonization pilot plant of this Institute. The coals were crushed to 80% through 3 mm, and after adjusting the moisture c o n t e n t to around 5%, they were carbonized by the conventional top-charging technique in the 400 mm wide oven at an average flue temperature of 1250°C. At the end of coking, the h o t coke was discharged and water-quenched. Later, representative samples were drawn for chemical analysis and reactivity measurements. Indian Standard methods [6--9] were employed for the analyses. The properties of the coals used for coking are given in Table 2, and the properties of the cokes and the corresponding reactivity values are shown in Table 3. The pore entrance diameters of the respective cokes, as measured by electron microscopy (described below), are also given in Table 3.
Measurement of pore entrance diameters by electron microscopy A Metropolitan-Vickers Model EM 3A transmission type electron microscope was used for this study. It was calibrated using Dow polystyrene latex particles. A two-stage replica (collodion/carbon) m e t h o d was used for specimen preparation. The final (carbon) replica was shadow-cast and examined in the electron microscope.
244
TABLE 2 Properties of Indian coals Particulars o f the sample
V.M. (%)
L.T.G.K. assay***
C a r b o n (%)
(on d.m.m.f, basis)*'* *
coke type
(on d.m.m.f. basis)
Hydrogen (%) (on d . m . m . f . basis)
20--22
D--F
91.7
4.4
XIV seam
29--31
G+
89.2
5.0
Methani Colliery Dishergarh seam
39--43
F--G +
84.0
5.6
Bastacolla Colliery ' O ' seam Lodna Colliery
• V.M. = volatile matter. • * d . m . m , f . = dry mineral matter free basis. • ** L . T . G . K . A s s a y = Low Temperature Gray King Assay.
RESULTS AND DISCUSSION
It is seen that there is a rough correlation between the reactivity and the square root of the reciprocal of the pore radius. A complete calculation o f the right-hand side of Thiele's relationship using the values a = 0.11 cm, D = 0.65 cm 2/sec, F R = 3.23 × L~schmidt number*, C A = 3 • 1019 molecules/ml, ~c = 0.2 (i.e. 20%), gives a theoretical value of 6.7 • 10- 4 g/sec/g for the specific gasification rate at 20% burn-off on coke from a demineralised New Zealand coal. This is to be compared with an experimental value of 0.5 • 10- 4 g/sec/g for the specific gasification rate**. A similar calculation on Lodna coke (for 15% conversion) gives a figure of 1.3 • 10- 3 g/sec/g which is about four times higher than the theoretical value o f 3.2 • 10 -4 g/sec/g. It is thus seen that the agreement is better for Indian coals, the slightly higher experimental value being possibly due to iron present in the mineral matter. Analysis of the ash samples from a number of Indian coals reveals that the Fe2 0 3 c o n t e n t in these cases varies from 14% to as high as 24%. It is known from the work of Walker et al. [10] that traces of iron act as a catalyst in the CO2 + C = 2 CO reaction. As stated earlier, Table 3 also gives the values for reactivities and pore diameter for cokes from different Indian coals. The values for the product of the reactivity and the square root of the mean pore radius are also given. The values, however, are n o t constant. This is to be expected as changes in the • The LSschmidt number refers to the number of molecules per unit volume of an ideal gas at NTP. • *The large difference between theoretical and experimental value may be due to the fact that a linear velocity and not the space velocity was substituted in the equation.
22.7 28.3 25.5
2.1
(%)
Ash
2.3 0.8
Moisture (%)
Proximate analysis (air-dried basis)
2.0
2.3 0.2
Volatile matter (%)
**
* F.R.S. = Fuel Research Station (London). r m = Mean of radius.
Bastacolla coke Lodna coke Methani coke
Particulars of the sample
70.4
72.7 70.7
Fixed carbon (%)
123
106 90
Reactivity (R) towards CO2, F.R.S. method (ml CO)*
150--200
100--150 50--100
Pore entrance diameter (A)
1150
840 550
R r~mm**
Proximate analyses, reactivities (F.R.S. method) and pore diameter of cokes from Bastacolla, Lodna and Methani coals
TABLE 3
b~ e~
246
length of pores and in the diffusional constant (brought a b o u t by variation in oxygen in the coke samples) were not taken into consideration. A calculation by the present authors of the theoretical and experimental specific gasification rates in hydrogasification results obtained by Tomita et al. [11] on a coke from an American coal of 89.6% and 5.0% H, also confirms the applicability of Wheeler's theory, values of 0.021 and 0.017 g/sec/g having been obtained for the theoretical and experimental values, respectively. CONCLUSION
The results of the present study show that Thiele's theory, as elaborated by Wheeler, can be applied for obtaining specific gasification rates on coke samples, provided that interfering factors such as the presence of impurities do not invalidate the results. ACKNOWLEDGEMENT
The authors thank Dr. M.G. Krishna, Director, Central Fuel Research Institute, for his helpful criticisms. Thanks are also due to the staff involved at both the Carbonization pilot plant and the laboratory, for preparing the coke and providing analytical data, respectively. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
Rao, V.V. and Das Gupta, N.N., 1956. J. Proc. Inst. Chem. Calcutta, 28: 281. Banerjee, N.G., Mitra, P.K., As, D.K. and Raja, K., 1976. J. Mines, Metals Fuels, 24, 6: 206. Kini, K.A., 1977. Fuel, 56: 341. Thiele, E.W., 1939. Ind. Chem., 31: 916. Wheeler, A., 1951. Advances in Catalysis. Academic Press, New York, Vol. 3, p. 249. Methods of test for coal and coke, Pt. I, IS: 1350--1969, Indian Standards Institution, New Delhi. Methods of test for coal and coke, Pt. IV, IS: 1350--1974, Indian Standards Institution, New Delhi. Methods of test for coal carbonization, IS: 1353--1959, Indian Standards Institution, New Delhi. Industrial coke, IS: 439--1976, Indian Standards Institution, New Delhi. Walker, Jr., P.L., Shelef, M. and Anderson, R.A., 1968. In" P.L. Walker, Jr. (Editor), Chemistry and Physics of Carbon. V01.4, Marcel Dekker, New York, p. 294. Tomita, A., Mahajan, O.P. and Walker, Jr., P.L., 1977. Fuel, 56: 137.
Short communication STUDIES ON PORE SIZE AND REACTIVITY OF COKES
A.K. BOSE, P.K. MITRA, D.K. AS, K. RAJA and K.A. KINI Central Fuel Research Institute, P.O.F.R.I., Dhanbad, Bihar (India) (Received November 9th, 1977)
R e a c t i v i t y o f c o k e s t o w a r d s c a r b o n d i o x i d e is an i m p o r t a n t p a r a m e t e r det e r m i n i n g t h e i r c o n s u m p t i o n in c u p o l a s as well as in blast furnaces. I f the reactivity is high, t h e c o n s u m p t i o n of c o k e s is higher and vice versa. With the increasing use o f o x y g e n e n r i c h e d blast in blast f u r n a c e s f o r reasons o f econo m y in c o k e c o n s u m p t i o n , t h e r e a c t i v i t y is b e c o m i n g m o r e i m p o r t a n t . A f e w a s p e c t s o f the r e a c t i v i t y of c o k e p r o d u c e d f r o m I n d i a n coals, w i t h s t e a m , c a r b o n d i o x i d e etc., h a v e b e e n studied b y R a o a n d Das G u p t a [ 1 ] . F u r t h e r investigations o n the r e a c t i v i t y of c o k e s w i t h c a r b o n d i o x i d e were carried o u t b y Banerjee e t al. [2] a n d an a l t e r n a t i v e m e t h o d f o r its measurem e n t has b e e n described. M o r e r e c e n t l y , Kini [3] has also e x a m i n e d t h e rel a t i o n s h i p b e t w e e n r e a c t i v i t y a n d t h e surface area o f cokes. F u r t h e r investigation o f the results has s h o w n t h a t t h e r e is a c o r r e l a t i o n b e t w e e n t h e reactivity a n d t h e p o r o s i t y o f cokes, t h e f o r m e r increasing w i t h t h e square r o o t o f the p o r o s i t y , as d e t e r m i n e d b y specific v o l u m e s in m e r c u r y . T h e results are given in T a b l e 1. TABLE 1 Specific gasification rates and total porosity of gasified cokes from demineralised (Buller) New Zealand and Young Wallsend (Australian) coals Material
20% burn-off Specific gasification rate
40% burn-off Specific volume (ml/g)(dry)
(g/sec/g) • 104
Coke from demineralised New Zealand (Buller) coal Coke from demineralised Young Wallsend Vitrain (Australian)
Specific gasification rate
Specific volume (ml/g)(dry)
(g/sec/g) . 104
0.5
1.30
0.6
1.71
1.1
1.30
1.3
1.44
243
To the knowledge of the authors, no a t t e m p t has been made so far to compare the reactivities of cokes with Thiele's theory [4], as elaborated by Wheeler [ 5]. The relevant relationship is: h tan h ; 18D
PBVg l°ge 1--~e
where a is the pellet diameter in em, D is the bulk diffusion coefficient for a single pore in cm2/sec, FR is the feed rate to the reactor in molecules/ml/sec, CA is the inlet concentration of reactant in molecules/ml, PB is the bulk density in g/ml, Vg is the specific volume in ml/g, C~c is the fraction of solid reactant and h is a dimensionless quantity given by the relation:
h = L ~D where L = length of the pore, k = reaction constant, r = radius of the pore, and D = diffusion constant. An a t t e m p t has therefore been made to examine the applicability of this theory to a few coke samples prepared from Indian coals. Along with these, the results from cokes prepared from Australian and New Zealand coals are also discussed. EXPERIMENTAL PROCEDURE
Preparation of coke samples Coals of varying rank were selected, and cokes were prepared from them in the high temperature carbonization pilot plant of this Institute. The coals were crushed to 80% through 3 mm, and after adjusting the moisture c o n t e n t to around 5%, they were carbonized by the conventional top-charging technique in the 400 mm wide oven at an average flue temperature of 1250°C. At the end of coking, the h o t coke was discharged and water-quenched. Later, representative samples were drawn for chemical analysis and reactivity measurements. Indian Standard methods [6--9] were employed for the analyses. The properties of the coals used for coking are given in Table 2, and the properties of the cokes and the corresponding reactivity values are shown in Table 3. The pore entrance diameters of the respective cokes, as measured by electron microscopy (described below), are also given in Table 3.
Measurement of pore entrance diameters by electron microscopy A Metropolitan-Vickers Model EM 3A transmission type electron microscope was used for this study. It was calibrated using Dow polystyrene latex particles. A two-stage replica (collodion/carbon) m e t h o d was used for specimen preparation. The final (carbon) replica was shadow-cast and examined in the electron microscope.
244
TABLE 2 Properties of Indian coals Particulars o f the sample
V.M. (%)
L.T.G.K. assay***
C a r b o n (%)
(on d.m.m.f, basis)*'* *
coke type
(on d.m.m.f. basis)
Hydrogen (%) (on d . m . m . f . basis)
20--22
D--F
91.7
4.4
XIV seam
29--31
G+
89.2
5.0
Methani Colliery Dishergarh seam
39--43
F--G +
84.0
5.6
Bastacolla Colliery ' O ' seam Lodna Colliery
• V.M. = volatile matter. • * d . m . m , f . = dry mineral matter free basis. • ** L . T . G . K . A s s a y = Low Temperature Gray King Assay.
RESULTS AND DISCUSSION
It is seen that there is a rough correlation between the reactivity and the square root of the reciprocal of the pore radius. A complete calculation o f the right-hand side of Thiele's relationship using the values a = 0.11 cm, D = 0.65 cm 2/sec, F R = 3.23 × L~schmidt number*, C A = 3 • 1019 molecules/ml, ~c = 0.2 (i.e. 20%), gives a theoretical value of 6.7 • 10- 4 g/sec/g for the specific gasification rate at 20% burn-off on coke from a demineralised New Zealand coal. This is to be compared with an experimental value of 0.5 • 10- 4 g/sec/g for the specific gasification rate**. A similar calculation on Lodna coke (for 15% conversion) gives a figure of 1.3 • 10- 3 g/sec/g which is about four times higher than the theoretical value o f 3.2 • 10 -4 g/sec/g. It is thus seen that the agreement is better for Indian coals, the slightly higher experimental value being possibly due to iron present in the mineral matter. Analysis of the ash samples from a number of Indian coals reveals that the Fe2 0 3 c o n t e n t in these cases varies from 14% to as high as 24%. It is known from the work of Walker et al. [10] that traces of iron act as a catalyst in the CO2 + C = 2 CO reaction. As stated earlier, Table 3 also gives the values for reactivities and pore diameter for cokes from different Indian coals. The values for the product of the reactivity and the square root of the mean pore radius are also given. The values, however, are n o t constant. This is to be expected as changes in the • The LSschmidt number refers to the number of molecules per unit volume of an ideal gas at NTP. • *The large difference between theoretical and experimental value may be due to the fact that a linear velocity and not the space velocity was substituted in the equation.
22.7 28.3 25.5
2.1
(%)
Ash
2.3 0.8
Moisture (%)
Proximate analysis (air-dried basis)
2.0
2.3 0.2
Volatile matter (%)
**
* F.R.S. = Fuel Research Station (London). r m = Mean of radius.
Bastacolla coke Lodna coke Methani coke
Particulars of the sample
70.4
72.7 70.7
Fixed carbon (%)
123
106 90
Reactivity (R) towards CO2, F.R.S. method (ml CO)*
150--200
100--150 50--100
Pore entrance diameter (A)
1150
840 550
R r~mm**
Proximate analyses, reactivities (F.R.S. method) and pore diameter of cokes from Bastacolla, Lodna and Methani coals
TABLE 3
b~ e~
246
length of pores and in the diffusional constant (brought a b o u t by variation in oxygen in the coke samples) were not taken into consideration. A calculation by the present authors of the theoretical and experimental specific gasification rates in hydrogasification results obtained by Tomita et al. [11] on a coke from an American coal of 89.6% and 5.0% H, also confirms the applicability of Wheeler's theory, values of 0.021 and 0.017 g/sec/g having been obtained for the theoretical and experimental values, respectively. CONCLUSION
The results of the present study show that Thiele's theory, as elaborated by Wheeler, can be applied for obtaining specific gasification rates on coke samples, provided that interfering factors such as the presence of impurities do not invalidate the results. ACKNOWLEDGEMENT
The authors thank Dr. M.G. Krishna, Director, Central Fuel Research Institute, for his helpful criticisms. Thanks are also due to the staff involved at both the Carbonization pilot plant and the laboratory, for preparing the coke and providing analytical data, respectively. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
Rao, V.V. and Das Gupta, N.N., 1956. J. Proc. Inst. Chem. Calcutta, 28: 281. Banerjee, N.G., Mitra, P.K., As, D.K. and Raja, K., 1976. J. Mines, Metals Fuels, 24, 6: 206. Kini, K.A., 1977. Fuel, 56: 341. Thiele, E.W., 1939. Ind. Chem., 31: 916. Wheeler, A., 1951. Advances in Catalysis. Academic Press, New York, Vol. 3, p. 249. Methods of test for coal and coke, Pt. I, IS: 1350--1969, Indian Standards Institution, New Delhi. Methods of test for coal and coke, Pt. IV, IS: 1350--1974, Indian Standards Institution, New Delhi. Methods of test for coal carbonization, IS: 1353--1959, Indian Standards Institution, New Delhi. Industrial coke, IS: 439--1976, Indian Standards Institution, New Delhi. Walker, Jr., P.L., Shelef, M. and Anderson, R.A., 1968. In" P.L. Walker, Jr. (Editor), Chemistry and Physics of Carbon. V01.4, Marcel Dekker, New York, p. 294. Tomita, A., Mahajan, O.P. and Walker, Jr., P.L., 1977. Fuel, 56: 137.