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Gasification reactivities of metallurgical cokes with carbon dioxide, steam and their mixtures
Gasification reactivities of metallurgical cokes with carbon dioxide, steam and their mixtures Keiichiro
Koba and Shiro Ida
Department of Production Administration, Wakamatsu, Kitakyushu, Japan 808 (Received 29 May 1979)
Mitsui-Kozan Coking industry Company, Hibiki- 1,
Gasification reactivities of cokes with carbon dioxide, steam and their mixtures at 1200°C were investigated, using seven single cokes made in a laboratory furnace from single coals of different properties, and also one commercial coke. The reactivity with steam was significantly higher than that with carbon dioxide. The reactivities of the mixture increased proportionally with increasing steam content above 10%; however, the reactivity below 10% steam content was smaller than that with pure carbon dioxide, the minimum reactivity being observed at a steam content of 2-4%. The reactivities of the compounds corresponding to optical texture were determined by analysing the coke microscopically before and after gasification. In general, except for one coke, the inert texture was the most reactive. Increasing size of the optical unit decreased the gasification reactivity, fibrous and leaflet textures were the least reactive. Gasification with steam developed pores within the inner region of the coke, whereas carbon dioxide reacted with the coke at or near the surface, producing a relatively smooth surface. Some proposals are suggested for better coke production.
Strength is the most important quality required in a blast furnace coke. Although mechanical strength has been measured at room temperature to evaluate coke quality, the coke strength is found to be significantly reduced by reaction with oxidizing gases in a blast furnace; subsequently, strength after the reaction is now assumed to be the better parameter’f. Carbon dioxide is usually used as the oxidizing agent for testing reactivity ‘-’ because it is the dominant constituent in the stack of a blait fumace4. However, a significant amount of steam may also exist in a blast furnace. It can be produced through reactions of Hz-CO2 and of H2-iron ore and also the combustion of fuel oil. In the present study, the gasification reactivities of a series of metallurgical cokes were observed at 12OO’C using the mixed gases of steam and carbon dioxide in variable mixing ratios in order to study gasification under similar conditions to those in a blast furnace. Reactivities of coke were related to optical texture and the properties of the
Table 1 Properties
parent coal. Based on such a study, some proposals for coke production were possible.
EXPERIMENTAL Materials
Seven single cokes and one commercial coke were used in the present study. The parent coals selected (Table I) had broad coking properties. Inert contents and mean maximum reflectances of the coals varied from 6.6 to 37.8% and from 0.74 to 1.78%, respectively. The single coals were carbonized in a laboratory furnace. The soaking temperature, time, rate of carbonization and the size of the feed coal were 9.50°C, 3 h, 2OO’C h-l and under 3 mm, respectively. The commercial coke was produced from blended coals using a conventional coke oven where the flue temperature and the coking time were 1200°C and 22 h, respectively.
of coals
No.
Coal
Ash
Volatile matter
Fixed carbon
content
Mean max. reflectance
Max. fluidity by Gieseler plastometer (Log ddpml
1 2 3 4 5 6 7 8
Miike (Japan) Lemington (Australia) Caribbean (America) Peak Downs (Australia) Balmer (Canada) Saraji (Australia) Beatrice (America) Blended coal for commercial
8.2 8.7 5.0 9.9 9.5 9.7 4.1 8.6
43.6 33.4 28.5 21.2 20.6 19.5 17.5 27.9
48.2 57.9 66.5 68.9 69.9 70.4 78.4 63.5
6.6 24.7 9.5 27.2 37.8 25.8 16.0 26.7
0.74 0.78 1.26 1.44 1.42 1.57 1.78 1.16
4.5 0.7 3.5 2.2 0.9 2.1 0.7 -
Proximate
analysis fwt %)
Microscopic
Inert
analysis f%)
Total dilatation by Audibert-Arnu dilatometer f%) 320 20 310 80 30 100 60 _
coke
0616-2361/80/010059-05S2.00 0 1980 IPC Business Press
FUEL, 1980, Vol 59, January
59
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida co2100 H,O 0
98 2 I
92 6 I
9L 6 I
96 L I
go( 10 %1
1 r ,O
9
8
of the mixture proportionally increased with increasing steam content above 10%; but below this the reactivity was smaller than that with pure carbon dioxide, as shown in Figure 1. At a steam content of 2-4%, the minimum reactivity was 10% less than that of pure carbon dioxide. Reactivities of a series of single cokes The reactivities of the laboratory cokes with carbon dioxide, steam and their mixtures are shown in Figure 2. The gasification rate proportionally increased with increasing content of steam for all the cokes (the rate varying with the type of coke). The relative reactivities of the steam and the mixtures are shown in Figure 3. Reactivities of cokes with carbon dioxide and steam may be correlated with suitable properties of the parent coal. The reactivities of the cokes are plotted against the mean maximum reflectance of the parent coal in Figure 4. Both reactivities varied according to the reflectance in a similar
7
H-0
0
cb,100
I 20
80
I
I
LO 60
60 LO
Composition
of reactant
I 80 20
%)
gas (~01%)
figure 1 Gasification reactivities of steam, carbon dioxide and their mixtures with a commercial coke. Reaction temperature, 1200°C; 0, Concentration of Hz0 = O-100%; 0, Concentration of H,O = O-l 0%
The resultant cokes were examined optically with a Leitz Ortholux microscope using reflected polarized light after ordinary polishing. The optical texture was analysed by a point-counting technique. Apparatus and procedure for reactivity measurement The reactivity of the coke was assessed from weight losses. The coke sample (10 g, 3-6 mm size) was held in a platinum-gauze container suspended from a balance into a vertical furnace. Steam was supplied using a pump (10% or more in the reactant gas) or a saturator (below 10%). After the furnace reached 1200°C the nitrogen flow was replaced by reactant gas at atmospheric pressure flowing at 21 min-1. The reactivity of coke (v) was defined as the reciprocal of the time required for a half weight loss. The relative reactivity of the steam and steam-carbon dioxide mixtures are described by the following equation:
x
,x .> z B
lx
V Relative reactivity = vco, where: V, reactivity of steam or mixture; activity of carbon dioxide.
and Vco,, re-
RESULTS i Gasification reactivity of carbon dioxide, steam and their mixtures Gasification reactivities of carbon dioxide, steam and their mixtures with the commercial coke are plotted against composition of oxidizing gas in Figure 1. The reactivity of steam was higher than that of carbon dioxide. The reactivity
60
FUEL, 1980, Vol 59, January
H,O 0
co*100
I 20 80
I LO 60 Composition
I 60 LO of reactant
I
80 __ 2u gas (~01%)
Gasification reactivities of steam, carbon dioxide and figure 2 their mixtures with single cokes. Reaction temperature, 12OO’C; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
1 0
Gasification reactivities of metallurgical cokes: K. Koba ar,d S. Ida I
I
I
I
I
+
*t
n,d0 co,100
Y#
20
(
LO
60
,
80
60 LO 20 80 Composition of reactant gas fvol%j
disappeared almost completely from the cokes after the gasification reaction with steam. The isotropic texture of Miike and commercial cokes coming from the reactive macerals disappeared along with the inert, whereas the isotropy of Lemington coke increased its percentage, indicating a lowered reactivity compared to that of the inert material. The behaviour of a fine mosaic texture depends upon the presence of coexisting textures in the same coke. The cokes which had no other anisotropic regions (ex. Miike and Lemington) increased their percentage of fine mosaic, however other cokes which contained larger anisotropic units lost their fine mosaic content. The coarse mosaic content in the coke from the high rank coats (Beatrice and Saraji) increased only slightly after the gasification, whereas the same unit in the coke from the lower rank coals (Balmer and Peak Downs) increased considerably. The contents of flow and leaflet textures increased significantly after the gasification. The gasification pattern of Caribbean coke was exceptional with no significant change in texture distribution
J
100 0
figure 3 Relative reactivities of steam, carbon dioxide and their mixtures. Reaction temperature, 1200°C; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
cz
I
0.6
manner. Except for Caribbean which showed an extraordinarily high reactivity, the reactivity decreased with increasing reflectance to 1.4% and then slightly increased, thus giving a minimum reactivity at the reflectance of 1.4%. The reactivity can be similarly correlated with volatile matter and futed carbon, but maximum fluidity and dilatometry failed to give a correlation. The relative reactivity of steam to carbon dioxide with a series of cokes is plotted with the reflectance of the parent coal in Figure 4. The relative reactivity is reversed when compared with the reactivity (with the exception of Caribbean coal). The relative reactivity is rectilinearly related to the inert content (Figure 5). These relations commonly mean that the least reactive coke shows the largest relative reactivity. Microscopic analyses of the coke before and after the gasification Optical texture of the cokes were related to the gasification reactivity. Changes in optical texture by gasification were analysed statistically with a reflective microscope. The classification of optical texture is shown in Table 2; other classifications however, have been reported’-*. The optical textures of cokes before and after reaction are shown in Figure 6. Among the textures, inert, especially micrinite, was most reactive. Except for Caribbean coke, it
0.6
I
I
1.0 I.2 Mean maximum
I I 1.6 I.8 reflectance I% I I
1.L
2.0
Correlation between reactivities of cokes and mean max. Figure 4 reflectance of their parent coals. 0, Reactivity with CO,; 0, Reactivity with H,O; 0, Relative reactivity; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
L
I
0
I
10
I
20 Inert content
I I 30 LO of parent coal (%)
I 50
Correlation between relative reactivities of cokes with Figure 5 steam and inert contents of their parent coals. 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
FUEL, 1980, Vol 59, January
61
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida Table 2 Classification
of optical textures
Textures
Definition
The textures from reactive macerals I: Isotropic optical isotropy Mf: Fine mosaic mosaic anisotropy,
unit grain below
1 Ctm mosaic anisotropy,
unit grain above
MC:
The extent of anisotropic development has been recognized to be approximately correlated with the rank of the coa16. The volatile matter, carbon content, and reflectance may describe coal rank. The pore structure of the coke may be determined, in principle, by a balance between the liberation of volatile matter and the fluidity of the fused coal during the carbonization and the thermal shock accompanied with gas liberation after the resolidification”. Thus, the reactivity of the cokes is approximately correlated with various parameters for the rank of coals. The higher gasification reactivity of steam than carbon dioxide should be considered in terms of the chemical oxidation and diffusion at 12OO’C. The gasification of coke with steam and carbon dioxide are compared in equations (1) and (2).
Coarse mosaic
Fi:
Fibrous
L:
Leaflet
1 pm flow-type anisotropy, unit grain length/width > 3 and length XI firn flat and featureless anisotropy, unit grain shortest-width >20 pm
The inert textures from inert macerals Sf: Semifusinitelike weakly anisotropic unit from semifusinite F: Fusinitelike isotropic unit from fusinite Mi: Micrinitelike isotropic unit from micrinite
C+H20---CO+H2 c + co2 ---
I
Mf
t-4,
Mf
MC
FI
LSfF
Coke ‘0
60
2CO
(2)
Mi
Bolmel
Beatrice
Peak Downs 0 20 LO
60
60
100
I%I
Commercial 0 20
60
100
I V. I
Figure 6 Contents of optical textures in coke before and after gasification. Reaction temperature, 1200° C; Extent of gasification, weight loss; Upper row, Before gasification; Middle row, Aftar gasification with COz; Lower row, After gasification with Hz0
before and after reaction indicating that the different optical textures reacted equally with steam or carbon dioxide. Microphotographs of the cokes after the reaction with steam or carbon dioxide are shown in Figure 7. Pores developed within the inner region of the coke when reacted with steam, whereas the coke after the reaction with carbon dioxide showed a relatively smooth surface, probably due to the preferential gasification of the surface region. These photographs may indicate the different extent of the oxidant diffusion during reaction.
50%
a
DISCUSSION Some mechanistic consideration of gasification Gasification of coke at 12OO’C with steam or carbon dioxide is influenced by chemical and physical properties of the coke and oxidant. Among the chemical properties, the optical anisotropy of the coke is associated with the ordered arrangement of graphite-like layers, which may be less reactive than isotropic, material, as exhibited in microscopic analyses of the present paper and as reported in the literatureg. However, the correlation in Figure 4 may also indicate the importance of pore structure for the reactivity of the coke because the Beatrice coke which had the most developed optical texture was not the least reactive.
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FUEL,
1980, Vol 59, January
b Microphotographs of cokes after reaction with CO2 and Figure 7 HzO. (a) After reaction with COz, (b) After reaction with H20. Reaction temperature, 12OO’C; Coke, Commercial coke
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida
The higher reactivity of steam has been well documented in the literature”. Its smaller size which should be favourable for the diffusion in the micropores of the coke may be one of the reasons for higher reactivity. Higher diffusibility of steam is experimentally shown in Figure 7, where the gasification occurred at the inner region of the coke. The lowered gasification rate due to the addition of 2-4% steam into carbon dioxide may be related to equation (1) where hydrogen is produced. Hydrogen molecules of high diffusibility may be absorbed on the coke, to some extent, and may retard the reactions of equations (1) and
than the coke by water quenching. Rough surfaces may indicate severe gasification during the quenching”. The quality of coke must be greatly influenced by the composition of internal heating gas in the continuous coking process of formed coke. If the heating gas contains Hz0 or CO2, formed coke will be gasified during the coking process. Especially, the high concentration of Hz0 leads to violent gasification. The use of dry and reductive heating gas may be strongly recommended in the coking process.
(2)‘2_‘4.
ACKNOWLEDGEMENTS
Assuming that the reactivity of the mixture can be described by equation (3) in the mixed chemical and physical control zone, the reactivity may have a minimum at a certain concentration of steam, because the amount of hydrogen may be proportional to the partial pressure of steam.
v= kl f’co2 + W’H,o 1 + KPH2
We would like to thank Mr T. Shimizu, Mr. M. Kumagai and Mr K. Sakata, Mitsui-Kozan Coking Industry Co. for their assistance in experiments and analyses. We are also grateful to Professors K. Takeshita and I. Mochida, Research Institute of Industrial Science, Kyushu University, for helpful suggestions and critical reading of the manuscript.
(3)
where kl and k2 are rate constants and K is the equilibrium constant of hydrogen adsorption. Further kinetic study is now in progress. Some proposals ORthe blast furnace operation and coke production From these results on the gasification reactivity of the coke some suggestions as to better blast-furnace operation and coke production can be proposed. The retarded gasification by a small content of steam may be ascribed to the hydrogen produced, so that additional hydrogen in the blast air may be advantageous. The use of fuel oil injection causes an increase in the hydrogen content of gases in the bosh. A coke with a fibrous texture has previously been considered to be the best texture for use as a blast furnace coke, based on the reactivity with dry CO2 16. The present investigation indicated that the leaflet as well as fibrous were the most stable textures against steam. Therefore, lowvolatile coking coals, which produce a leaflet texture, to a certain extent, can be used as a favourable blending coal. A coke dry quenching process (CDQ) has been adopted in a commercial plant in Japan. It was pointed out that C.D.Q. improves the coke quality. Since the cooling gas was reportedly composed of CO, 5-10%; Hz, l-3%; 02, O-0.1%; CO2, 10-l 5%; and N2, the rest; such a dry gas can be expected to be less reactive than water. This assumption was supported by the microscopic investigation. The coke produced by dry quenching had a smoother surface
REFERENCES
10 11 12 13 14 15
16 17
Ida, S., Nishi, T. and Nakama, H. Coke Circular (Japan) 1972,21,252 Murakami, S., Hara, Y. and Ishikawa, I. Coke Circular (Japan) 1974,23,82 French, M. and Marsh, H. 5th Int. Conf on Carbon and Graphite London, 1978, 123; Mochida, I., French, M. and Marsh, H. ibid. 155 Hedden, K. Gas und Wasserfach 1976, 117,409 Sugimura, H., Kumagai, M., Kimura, H. and Honda, H. J. Fuel Sot. Japan 1969,48,920 Patrick, J. W., Reynolds, M. J. andShaw, F. M. Fuel 1973, 52,198 Grint, A., Swietlik, U. and Marsh, H. 5th Int. Conf on Carbon and Graphite London, 1978,226 Miyagawa, T., Saga, M. and Tanihara, H. J. Fuel Sot. Japan 1977,56,337 Akamatsu, K., Nire, H., Miyazaki, T. and Nishioka, K. Conf. on Coal, Coke and Blastfurnace Paper 6, Middlesbrough, 1977 Hays, D., Patrick, J. W. and Walker, A. Fuel 1976, 55, 297 Walker, P. L. Jr. in ‘Chemistry and Physics of Carbon’ Marcel Dekker, New York, 1965 Long, F. J. and Sykes, K. W. Proc. Roy. Sot. l948,193A, 377 Kornev, V. K. and Shavrin, S. V. Steel in the U.S.S.R. 197 1, 96 Blackwood, J. D. Coke and Gas 1960, May, 190 Shigemi, A., Nakamura, T. and Fuzimoto, M. Japan Society for the Promotion of Science, 54 Committee, Progress Report of the Section of Blast furnace reaction 26 September 1972 14th Conf of Coke Sec. of the Iron and Steel Institute of Japan, Progress Report of Sumitomo Metal Ind. 18 May 1917 Haraguchi, H., Nishi, T., Miura, Y. and Komaki, I. JIST Japan 1977,63, S524
FUEL, 1980, Vol 59, January
63
Koba and Shiro Ida
Department of Production Administration, Wakamatsu, Kitakyushu, Japan 808 (Received 29 May 1979)
Mitsui-Kozan Coking industry Company, Hibiki- 1,
Gasification reactivities of cokes with carbon dioxide, steam and their mixtures at 1200°C were investigated, using seven single cokes made in a laboratory furnace from single coals of different properties, and also one commercial coke. The reactivity with steam was significantly higher than that with carbon dioxide. The reactivities of the mixture increased proportionally with increasing steam content above 10%; however, the reactivity below 10% steam content was smaller than that with pure carbon dioxide, the minimum reactivity being observed at a steam content of 2-4%. The reactivities of the compounds corresponding to optical texture were determined by analysing the coke microscopically before and after gasification. In general, except for one coke, the inert texture was the most reactive. Increasing size of the optical unit decreased the gasification reactivity, fibrous and leaflet textures were the least reactive. Gasification with steam developed pores within the inner region of the coke, whereas carbon dioxide reacted with the coke at or near the surface, producing a relatively smooth surface. Some proposals are suggested for better coke production.
Strength is the most important quality required in a blast furnace coke. Although mechanical strength has been measured at room temperature to evaluate coke quality, the coke strength is found to be significantly reduced by reaction with oxidizing gases in a blast furnace; subsequently, strength after the reaction is now assumed to be the better parameter’f. Carbon dioxide is usually used as the oxidizing agent for testing reactivity ‘-’ because it is the dominant constituent in the stack of a blait fumace4. However, a significant amount of steam may also exist in a blast furnace. It can be produced through reactions of Hz-CO2 and of H2-iron ore and also the combustion of fuel oil. In the present study, the gasification reactivities of a series of metallurgical cokes were observed at 12OO’C using the mixed gases of steam and carbon dioxide in variable mixing ratios in order to study gasification under similar conditions to those in a blast furnace. Reactivities of coke were related to optical texture and the properties of the
Table 1 Properties
parent coal. Based on such a study, some proposals for coke production were possible.
EXPERIMENTAL Materials
Seven single cokes and one commercial coke were used in the present study. The parent coals selected (Table I) had broad coking properties. Inert contents and mean maximum reflectances of the coals varied from 6.6 to 37.8% and from 0.74 to 1.78%, respectively. The single coals were carbonized in a laboratory furnace. The soaking temperature, time, rate of carbonization and the size of the feed coal were 9.50°C, 3 h, 2OO’C h-l and under 3 mm, respectively. The commercial coke was produced from blended coals using a conventional coke oven where the flue temperature and the coking time were 1200°C and 22 h, respectively.
of coals
No.
Coal
Ash
Volatile matter
Fixed carbon
content
Mean max. reflectance
Max. fluidity by Gieseler plastometer (Log ddpml
1 2 3 4 5 6 7 8
Miike (Japan) Lemington (Australia) Caribbean (America) Peak Downs (Australia) Balmer (Canada) Saraji (Australia) Beatrice (America) Blended coal for commercial
8.2 8.7 5.0 9.9 9.5 9.7 4.1 8.6
43.6 33.4 28.5 21.2 20.6 19.5 17.5 27.9
48.2 57.9 66.5 68.9 69.9 70.4 78.4 63.5
6.6 24.7 9.5 27.2 37.8 25.8 16.0 26.7
0.74 0.78 1.26 1.44 1.42 1.57 1.78 1.16
4.5 0.7 3.5 2.2 0.9 2.1 0.7 -
Proximate
analysis fwt %)
Microscopic
Inert
analysis f%)
Total dilatation by Audibert-Arnu dilatometer f%) 320 20 310 80 30 100 60 _
coke
0616-2361/80/010059-05S2.00 0 1980 IPC Business Press
FUEL, 1980, Vol 59, January
59
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida co2100 H,O 0
98 2 I
92 6 I
9L 6 I
96 L I
go( 10 %1
1 r ,O
9
8
of the mixture proportionally increased with increasing steam content above 10%; but below this the reactivity was smaller than that with pure carbon dioxide, as shown in Figure 1. At a steam content of 2-4%, the minimum reactivity was 10% less than that of pure carbon dioxide. Reactivities of a series of single cokes The reactivities of the laboratory cokes with carbon dioxide, steam and their mixtures are shown in Figure 2. The gasification rate proportionally increased with increasing content of steam for all the cokes (the rate varying with the type of coke). The relative reactivities of the steam and the mixtures are shown in Figure 3. Reactivities of cokes with carbon dioxide and steam may be correlated with suitable properties of the parent coal. The reactivities of the cokes are plotted against the mean maximum reflectance of the parent coal in Figure 4. Both reactivities varied according to the reflectance in a similar
7
H-0
0
cb,100
I 20
80
I
I
LO 60
60 LO
Composition
of reactant
I 80 20
%)
gas (~01%)
figure 1 Gasification reactivities of steam, carbon dioxide and their mixtures with a commercial coke. Reaction temperature, 1200°C; 0, Concentration of Hz0 = O-100%; 0, Concentration of H,O = O-l 0%
The resultant cokes were examined optically with a Leitz Ortholux microscope using reflected polarized light after ordinary polishing. The optical texture was analysed by a point-counting technique. Apparatus and procedure for reactivity measurement The reactivity of the coke was assessed from weight losses. The coke sample (10 g, 3-6 mm size) was held in a platinum-gauze container suspended from a balance into a vertical furnace. Steam was supplied using a pump (10% or more in the reactant gas) or a saturator (below 10%). After the furnace reached 1200°C the nitrogen flow was replaced by reactant gas at atmospheric pressure flowing at 21 min-1. The reactivity of coke (v) was defined as the reciprocal of the time required for a half weight loss. The relative reactivity of the steam and steam-carbon dioxide mixtures are described by the following equation:
x
,x .> z B
lx
V Relative reactivity = vco, where: V, reactivity of steam or mixture; activity of carbon dioxide.
and Vco,, re-
RESULTS i Gasification reactivity of carbon dioxide, steam and their mixtures Gasification reactivities of carbon dioxide, steam and their mixtures with the commercial coke are plotted against composition of oxidizing gas in Figure 1. The reactivity of steam was higher than that of carbon dioxide. The reactivity
60
FUEL, 1980, Vol 59, January
H,O 0
co*100
I 20 80
I LO 60 Composition
I 60 LO of reactant
I
80 __ 2u gas (~01%)
Gasification reactivities of steam, carbon dioxide and figure 2 their mixtures with single cokes. Reaction temperature, 12OO’C; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
1 0
Gasification reactivities of metallurgical cokes: K. Koba ar,d S. Ida I
I
I
I
I
+
*t
n,d0 co,100
Y#
20
(
LO
60
,
80
60 LO 20 80 Composition of reactant gas fvol%j
disappeared almost completely from the cokes after the gasification reaction with steam. The isotropic texture of Miike and commercial cokes coming from the reactive macerals disappeared along with the inert, whereas the isotropy of Lemington coke increased its percentage, indicating a lowered reactivity compared to that of the inert material. The behaviour of a fine mosaic texture depends upon the presence of coexisting textures in the same coke. The cokes which had no other anisotropic regions (ex. Miike and Lemington) increased their percentage of fine mosaic, however other cokes which contained larger anisotropic units lost their fine mosaic content. The coarse mosaic content in the coke from the high rank coats (Beatrice and Saraji) increased only slightly after the gasification, whereas the same unit in the coke from the lower rank coals (Balmer and Peak Downs) increased considerably. The contents of flow and leaflet textures increased significantly after the gasification. The gasification pattern of Caribbean coke was exceptional with no significant change in texture distribution
J
100 0
figure 3 Relative reactivities of steam, carbon dioxide and their mixtures. Reaction temperature, 1200°C; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
cz
I
0.6
manner. Except for Caribbean which showed an extraordinarily high reactivity, the reactivity decreased with increasing reflectance to 1.4% and then slightly increased, thus giving a minimum reactivity at the reflectance of 1.4%. The reactivity can be similarly correlated with volatile matter and futed carbon, but maximum fluidity and dilatometry failed to give a correlation. The relative reactivity of steam to carbon dioxide with a series of cokes is plotted with the reflectance of the parent coal in Figure 4. The relative reactivity is reversed when compared with the reactivity (with the exception of Caribbean coal). The relative reactivity is rectilinearly related to the inert content (Figure 5). These relations commonly mean that the least reactive coke shows the largest relative reactivity. Microscopic analyses of the coke before and after the gasification Optical texture of the cokes were related to the gasification reactivity. Changes in optical texture by gasification were analysed statistically with a reflective microscope. The classification of optical texture is shown in Table 2; other classifications however, have been reported’-*. The optical textures of cokes before and after reaction are shown in Figure 6. Among the textures, inert, especially micrinite, was most reactive. Except for Caribbean coke, it
0.6
I
I
1.0 I.2 Mean maximum
I I 1.6 I.8 reflectance I% I I
1.L
2.0
Correlation between reactivities of cokes and mean max. Figure 4 reflectance of their parent coals. 0, Reactivity with CO,; 0, Reactivity with H,O; 0, Relative reactivity; 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
L
I
0
I
10
I
20 Inert content
I I 30 LO of parent coal (%)
I 50
Correlation between relative reactivities of cokes with Figure 5 steam and inert contents of their parent coals. 1, Miike; 2, Lemington; 3, Caribbean; 4, Peak Downs; 5, Balmer; 6, Saraji; 7, Beatrice; 8, Commercial coke
FUEL, 1980, Vol 59, January
61
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida Table 2 Classification
of optical textures
Textures
Definition
The textures from reactive macerals I: Isotropic optical isotropy Mf: Fine mosaic mosaic anisotropy,
unit grain below
1 Ctm mosaic anisotropy,
unit grain above
MC:
The extent of anisotropic development has been recognized to be approximately correlated with the rank of the coa16. The volatile matter, carbon content, and reflectance may describe coal rank. The pore structure of the coke may be determined, in principle, by a balance between the liberation of volatile matter and the fluidity of the fused coal during the carbonization and the thermal shock accompanied with gas liberation after the resolidification”. Thus, the reactivity of the cokes is approximately correlated with various parameters for the rank of coals. The higher gasification reactivity of steam than carbon dioxide should be considered in terms of the chemical oxidation and diffusion at 12OO’C. The gasification of coke with steam and carbon dioxide are compared in equations (1) and (2).
Coarse mosaic
Fi:
Fibrous
L:
Leaflet
1 pm flow-type anisotropy, unit grain length/width > 3 and length XI firn flat and featureless anisotropy, unit grain shortest-width >20 pm
The inert textures from inert macerals Sf: Semifusinitelike weakly anisotropic unit from semifusinite F: Fusinitelike isotropic unit from fusinite Mi: Micrinitelike isotropic unit from micrinite
C+H20---CO+H2 c + co2 ---
I
Mf
t-4,
Mf
MC
FI
LSfF
Coke ‘0
60
2CO
(2)
Mi
Bolmel
Beatrice
Peak Downs 0 20 LO
60
60
100
I%I
Commercial 0 20
60
100
I V. I
Figure 6 Contents of optical textures in coke before and after gasification. Reaction temperature, 1200° C; Extent of gasification, weight loss; Upper row, Before gasification; Middle row, Aftar gasification with COz; Lower row, After gasification with Hz0
before and after reaction indicating that the different optical textures reacted equally with steam or carbon dioxide. Microphotographs of the cokes after the reaction with steam or carbon dioxide are shown in Figure 7. Pores developed within the inner region of the coke when reacted with steam, whereas the coke after the reaction with carbon dioxide showed a relatively smooth surface, probably due to the preferential gasification of the surface region. These photographs may indicate the different extent of the oxidant diffusion during reaction.
50%
a
DISCUSSION Some mechanistic consideration of gasification Gasification of coke at 12OO’C with steam or carbon dioxide is influenced by chemical and physical properties of the coke and oxidant. Among the chemical properties, the optical anisotropy of the coke is associated with the ordered arrangement of graphite-like layers, which may be less reactive than isotropic, material, as exhibited in microscopic analyses of the present paper and as reported in the literatureg. However, the correlation in Figure 4 may also indicate the importance of pore structure for the reactivity of the coke because the Beatrice coke which had the most developed optical texture was not the least reactive.
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b Microphotographs of cokes after reaction with CO2 and Figure 7 HzO. (a) After reaction with COz, (b) After reaction with H20. Reaction temperature, 12OO’C; Coke, Commercial coke
Gasification reactivities of metallurgical cokes: K. Koba and S. Ida
The higher reactivity of steam has been well documented in the literature”. Its smaller size which should be favourable for the diffusion in the micropores of the coke may be one of the reasons for higher reactivity. Higher diffusibility of steam is experimentally shown in Figure 7, where the gasification occurred at the inner region of the coke. The lowered gasification rate due to the addition of 2-4% steam into carbon dioxide may be related to equation (1) where hydrogen is produced. Hydrogen molecules of high diffusibility may be absorbed on the coke, to some extent, and may retard the reactions of equations (1) and
than the coke by water quenching. Rough surfaces may indicate severe gasification during the quenching”. The quality of coke must be greatly influenced by the composition of internal heating gas in the continuous coking process of formed coke. If the heating gas contains Hz0 or CO2, formed coke will be gasified during the coking process. Especially, the high concentration of Hz0 leads to violent gasification. The use of dry and reductive heating gas may be strongly recommended in the coking process.
(2)‘2_‘4.
ACKNOWLEDGEMENTS
Assuming that the reactivity of the mixture can be described by equation (3) in the mixed chemical and physical control zone, the reactivity may have a minimum at a certain concentration of steam, because the amount of hydrogen may be proportional to the partial pressure of steam.
v= kl f’co2 + W’H,o 1 + KPH2
We would like to thank Mr T. Shimizu, Mr. M. Kumagai and Mr K. Sakata, Mitsui-Kozan Coking Industry Co. for their assistance in experiments and analyses. We are also grateful to Professors K. Takeshita and I. Mochida, Research Institute of Industrial Science, Kyushu University, for helpful suggestions and critical reading of the manuscript.
(3)
where kl and k2 are rate constants and K is the equilibrium constant of hydrogen adsorption. Further kinetic study is now in progress. Some proposals ORthe blast furnace operation and coke production From these results on the gasification reactivity of the coke some suggestions as to better blast-furnace operation and coke production can be proposed. The retarded gasification by a small content of steam may be ascribed to the hydrogen produced, so that additional hydrogen in the blast air may be advantageous. The use of fuel oil injection causes an increase in the hydrogen content of gases in the bosh. A coke with a fibrous texture has previously been considered to be the best texture for use as a blast furnace coke, based on the reactivity with dry CO2 16. The present investigation indicated that the leaflet as well as fibrous were the most stable textures against steam. Therefore, lowvolatile coking coals, which produce a leaflet texture, to a certain extent, can be used as a favourable blending coal. A coke dry quenching process (CDQ) has been adopted in a commercial plant in Japan. It was pointed out that C.D.Q. improves the coke quality. Since the cooling gas was reportedly composed of CO, 5-10%; Hz, l-3%; 02, O-0.1%; CO2, 10-l 5%; and N2, the rest; such a dry gas can be expected to be less reactive than water. This assumption was supported by the microscopic investigation. The coke produced by dry quenching had a smoother surface
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10 11 12 13 14 15
16 17
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