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Carbonization of coals into anisotropic cokes
Carbonization
of coals into anisotropic
cokes
6. Formed cokes of high density and optical anisotropy from non-fusible and slightly fusible coals lsao Mochida,
Akira
Shiraki,
Yozo Korai and Toshiaki
Okuhara*
Research institute of Industrial Science, Department of Molecular Engineering, Graduate School of Engineering Sciences, Kyushu University S6, Kasuqa, Fukuoka, 816 Japan * Research and Development laboretor~es-Ill Nippon Steel Corp., ~a~ata-~jqash~* Kitakyushu, 805 Japan (Received f2 September f9%3; revised 13 March f984)
A procedure for the preparation of solid formed coke of enough adhesion and anisotropic development for use in the blast furnace has been studied, using non-fusible and slightly fusible coals with petroleum cocarbonizing additives. The coke precursor was prepared through the copreheat-treatment of coal and a suitable additive in adequate quantity under stipulated conditions. The desired coke was produced by carbonization after forming with a press. The condjtionsforthe copreheat-treatment have been carefully examined in terms of the temperature, time and heating devices. The behaviour of coals during copreheat-treatment and carbonization were discussed in terms of coal ranks, comparing this behaviour to the liquefaction reactivity and thermal stability of their liquefied product. (Keywords: coal: formed coke; cocarbonization;
anisotropic development)
Increased price and possible shortage of the coking coals in the near future are encouraging studies to find methods of producing good blast furnace cokes from non-coking low rank coals’. Such cokes are expected to have both high mechanical strength and chemical resistivity against carbon dioxide at the elevated temperatures of blast furnace operatior?. Thus, the cokes should be of solid form, high density and optical anisotropy3*4. Thecocarbonization procedure has been revealed to be excellent for ensuring the fusibility5*6 for adhesion and optical anisotropy of low rank coals. However, the resultant coke tends to swell too much, leading to porous coke of low density. On the other hand, the formed coking procedure has been recognized to produce a solid coke of high density, by selecting the compositions of coals and additives’. However, the optical texture derived from the low rank coals remained isotropic, exhibiting high reactivity in carbon dioxide*. To satisfy the requirements of a high mechanical strength and chemical resistance to CO,, the present authors adopted a series of coking steps, which consist of copreheat-treatment of a coal with an additive, grinding, forming (moulding), and carbonization. In a previous Paper9 a fusible coal was successfully carbonized into a solid formed coke through copreh~t-tr~tment of the coal with an additive. EXPERIMENTAL The coals used in this study were Emery (a slightly fusible coal) and Coal Valley coals (a non-fusible coal). These coals were ground to pass a 100 US mesh (149 pm) sieve, The additives used in this study were A240 pitch (Petroleum pitch from Ashland Petroleum Co.) and
hydrogenated A240 (HA240) pitch. The latter was prepared using lithium and ethylenediamine” ‘. Analyses of these materials are presented in Table I. Procedures
Carbonization procedures are summarized in Figure 1 (procedures A-D). Procedure A, A powdered mixture (at the prescribed mixing ratio of coal and additives) was carbonized in a Pyrex glass tube (30 mm dia., 300 mm in length} under nitrogen flow in a vertical electric furnace. The heating rate and soaking time were @WC h - ’ and 1 h, respectively. Procedwe 8. A coal and additive mixture was pressed before carboni~tion into a disc form (20 mm dia., 5 mm in thickness) under 400 kg cm -2. The disc was carbonized in the same way as Procedure A. Procedure C. A coal and additive mixture was copreheated at variable temperature in a Pyrex glass tube under nitrogen flow in an electric furnace (air bath). The heating rate and soaking time were 600°C h - 1 and 1 h, respectively. The copreheated mixture was ground in a mortar and then formed before carbonization. The forming and carbonization were carried out in the same way as Procedure B. Procedure I?. To copreheat the mixture of coal and additive more homogeneously at the prescribed temperature, the mixture was transferred to a stainless steel tube (19 mm dia., 200 mm in length) to be immersed in a moltentin bath of prescribed temperature under nitrogen flow”‘. The copreheat temperatures were attained within 6 min. After the prescribed copreheat-treatment time (1 h), the
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45
Carbonization of coals. 6: I. Mochida et al. Table 1 Properties of coals and additives Ultimate analysis (wt %, daf) Sample
C
H
N
0
s
Ash
CSN
TR
Emery Coal Valley A240 HA240
81.3 78.8 91.3 86.3
5.9 5.1 5.5 8.3
1.2 1 .O 0.2 0.8
10.8 15.0 0.9 4.6
0.8 0.1 -
10.2 9.8 -
1 0
63.6 64.4 -
CSN = Crucible swelling number TR = total reactive: TR = V (R > 2.2%) + E + SF
Procedure
A
COPREHEATTREATMENT Procedure C (in an air bath) Procedure Figure
1
Experimental procedures
13
, (in a molten
tin
tube was removed from the bath and rapidly cooled by imme~ing in cold water to quench the reaction. The preheated mixture was ground, formed and carbonized in the same way as Procedure C. The resultant cokes were examined in terms of their shape, the adhesion extent of their coal grains and their anisotropic texture. Optical texture of resultant coke was examined using an optical microscope (Leitz, OrthoplanPol). After calcination at 1200°C for 1 h under argon flow, tensile strength of the coke was measured using an autograph (H-560, Shimazu). RESULTS The photographs of non-formed (Procedure A) and formed (Procedure B) cokes prepared from Emery and Coal Valley coals are shown in Figure 2, where A240 and HA240 were used as the additive, respectively. Emery coal produced a coke of disc form without the additive or press. However, the coke was very weak and easily broken in the hand (Figure 2~). Addition of A240 increased the coke strength, although the resultant coke deformed significantly (Figure 2b). The addition of more additive significantly increased swelling, which continued to increase with increasing amounts of additive (Figure 2~). The effects of forming were observable at lower amounts of additive, where the formed shape was maintained after carbonization (Figure 2d and e). The addition of more additive, however, diminished the effects of forming, allowing marked swelling (Figure 2f). Coal Valley produced similar cokes when it was formed with HA240. The effects of forming were observable when the additive (10 or 2Owt%) was added (Figure 2h). The effect of forming was destroyed with additive levels > 30 wt% (Figure 2i). Optical micrographs of formed and non-formed cokes from the Emery and Coal Valley coals are shown in Figure 3. Emery coal, when carbonized singly, exhibited isotropic texture, the coal particles slightly fusing regardless of forming (Figure 3a and d). By increasing the amount of
46
FUEL, 1985, Vol 64, January
bath)
‘9-
a
d
e
g
tcm
I
Figure 2 Photographs of formed and non-formed cokes prepared from Emery (a-f) and Coal Valley (g-i) coals. a-c, Non-formed cokes; d-i, formed cokes. Additive: A240 for Emery coal; HA240 for Coal Valley coal. Amount of additive (wt%): a, d and g. no additive; b, e and h, 20; c, f and i, 30
A240 pitch to 20 wt%, the coal particles fused much better, although resultant cokes were still isotropic (Figure 3b and e). With 30 wt% of additive, the resultant coke exhibited very fine-gmined mosaic in the major region with some isotropic areas (Figure 3c and f), no coal particle being distinguishable any more. Above 40 wt%, the resultant cokes exhibited very fine and fine-grained mosaic textures over the entire surface, the size of anisotropic unit increasing with increased amount of additive. Thus, the forming hardly influenced the extent of anisotropic developement in the carbonized coke. Coal Valley coal, when carbonized after forming, exhibited complete isotropic texture, and the coal
Carbonization of coals. 6: I. Mochida et al. (Figure 3h). At 30 wt%, the resultant coke exhibited very
tine-, tine- and medium-grained mosaic texture over the coke surface (Figure 3i). HA240 at > 35 wt% was enough to develop complete anisotropy in the resultant coke. Size increased with increasing amounts of additive. Copreheat-treatment of coal with additive
Figure 3 Photomicrographs of formed and non-formed cokes prepared from Emery (a-f) and Coal Valley (g-i) coals. a-c, non-formed cokes; d-i, formed cokes. Additive: A240 for Emery coal: HA240 for Coal Valley coal. Amount of additive (wt%): a, d and g, without additive; b, e and h, 20; c, f and i, 30
a
b
C
m
Shapes and optical micrographs of formed cokes from copreheat-treated Emery coal with A240 pitch (A240, 40 wt%) at different high temperature treatments (HTT) for 1 h are shown in Figure 4. Thecopreheated mixture at 430°C produced a solid hard coke (Figure 4b), which exhibited very thick pore walls and anisotropic texture over the entire surface, no grains being distinguishable (Figure 4e). Lower HTT (420°C) allowed the deformation and swelling of the formed shape (Figure 4a), although anisotropic development and fusion were achieved (Figure 4d). The copreheat-treatment at temperatures >44o”C reduced the fusibility of grains, facilitating the loss of adhesion and maintaining the angular shapes of the initial coal particles (Figure 4f). Although the resultant coke maintained formed shape (Figure 4c), no high strength was obtained. The extent of anisotropic development in the resultant cokes after carbonization at 600°C were similar regardless of preheat-treatment temperature. The shapes and optical micrographs of formed cokes from copreheat-treated Coal Valley coal with HA240 pitch (HA240, 35 wt%) are shown in Figure 5. The copreheat-treatment at variable temperatures in an air bath failed to achieve the objectives of good adhesion and no swelling at the same time, although all cokes exhibited very fine- and fine-grained mosaic textures. For example, the coke obtained from copreheated mixture at 430°C was deformed and swollen. Although the cokes from copreheated mixtures at 435 and 440°C maintained the formed shape, they failed to show high strength. The preheated grains did not fuse, staying angular (Figure .5e and f ). Copreheat-treatment using a tube bomb in a molten-tin bath ascertains more homogeneous heating of the mixture of coal and additive. The shapes and optical micrographs of resultant formed cokes are shown in Figure 6.
L
Icm
b
Figure 4 Effects of copreheat-treatment in an air bath on the shape and optical texture of formed coke prepared from Emery coal with A240 (4Owt%). HTf of preheat-treatment: a and d, 420X, 1 h; b and e, 43o”C, 1 h; c and f, 440°C. 1 h
particles maintained the same angular shapes as those before the carbonization (Figure 3g). Partial fusion of coal particles was observable with 10 wt% HA240. The extent of fusion was further improved by the addition of 20 wt%, no coal particles being distinguished at this stage. However, resultant cokes still exhibited isotropic texture
Figure 5 Effects of copreheat-treatment in an air bath on the shape and optical texture of formed coke prepared from Coal Valley with HA240 (35 wt%). HlT of preheat-treatment: a and d, 43o”C, 1 h; b and e, 435’C. 1 h; c and f, 44o’C, 1 h
FUEL, 1985, Vol64,
January
47
Carbonization of coals. 6: I. Mochida et al.
ible coals with additive is very effective in producing a formed coke of excellent adhesion and anisotropic development. A coal, with a suitable additive in certain quantities, is dissolved or liquefied at the initial stage of the copreheat-treatment into pitch-like material, which is then converted to the coke precursor during the later stage of copreheat-treatment by liberating the excess volatile matter. The depolymerization, dealkylation and/ or deoxygenation reactions in the initial stage enable the modified coal to form the nuclei of the anisotropic mesophase in the coke precursor” via the liquid-phase carbonization, through reduced viscosity. The coke precursor, when heat treated under unsuitable conditions, still maintains enough fusibility to allow its ground grains to adhere to each other completely during the following carbonization stage, after tight packing by the press. Such a procedure is very similar to that applied to produce moulded carbons without external binder’l. The three coals studied in the present Paper have different carbonization properties as indicated by their CSN tests. Their behaviours in the copreheat-treatment may reflect such properties. Non-fusible coal requires hydrogen donating HA240 to be liquefied to obtain fusibility and optical anisotropy. Highly aromatic A240 can modify enough the slightly fusible coals. A larger amount of the additive is necessary for higher rank coal. Coal rank also appears to influence the range of appropriate copreheat-treatment conditions required. Hunter Valley coal required treatment in an air bath between 410 and 430°C whereas 430°C was the required temperature to produce a hard formed coke from Emery coal. Coal Valley coal produced a hard coke only when it was heat treated in a molten-tin bath. The non-fusiblecoal requires very homogeneous heating under strict conditions. These properties of coals may be intimately related to the thermal stability of the coke precursor produced by copreheat-treatment with the additive. The precursors from lower rank coals tend to be highly reactive because of their many oxygen and alkyl groups, which may cause rapid condensation reactions unless they are properly eliminated”. The behaviour of the precursor in the successive carbonization stage can, in principle, be related to its structure. The content of PS (very little benzene-soluble fraction included in the present case) appears to be a measure of fusibility. However, the nature of PI can never be neglected since it should be affected by PS to a considerable extent during the carbonization. Further work is required in this area. Information obtained on copreheat-treatment suggests some ideas for preparing better coke precursors. The following Paper describes such ideas further.
Copreheat-treatment at 420°C for 1 h provided the precursor for a solid hard coke with good anisotropic development. However, lower or higher copreheattemperature failed to produce the desired coke by allowing either slight swelling or loss of fusibility. Analyses of copreheated coals and pitches
Yields of pyridine solubles (PS) obtained from the copreheated materials of Emery and Coal Valley coals with the respective additive at various temperatures are summarized in Table 2, where those of Hunter Valley coal (a slightly fusible coal, CSN = 2) with A240 pitch studied in a previous study’ are also included for comparison. When the copreheat-treated materials contained PS in a certain range [20-26 wt% for Hunter Valley coal (A240, 30 wt%) and 28 wt% for Emery coal (A240, 40 wt%)], solid formed cokes of good adhesion and anisotropy could be produced by formed-coking after the preheattreatment. In contrast, the amount of PS decreased drastically at 400 and 430°C in the case of Coal Valley coal (HA240, 35 wt%). Although both copreheat-treatments at 400 and 420°C produced 18 and 17 wt% of PS, respectively, the treatment at 420°C could only produce an aimed coke, which suggests some essential role for the pyridineinsoluble (PI) fraction. DISCUSSION The copreheat-treatment
of non-fusible and slightly fus-
Figure 6 Effects of copreheat-treatment in a molten-tin bath on the shape and the optical texture of formed coke prepared from Coal Valley coal with HA240 (35 wt%). HTT of preheattreatment: a and d, 410°C; b and e, 420°C; c and f, 430°C Table 2 Yields after copreheat-treatment
HTT of preheat-treatment (“C, 1 h)
c (wt%, Coal
daf)
CSN
Additive fwt %)
350
380
390
400
410
420
430
450
480
Hunter Valley Emery Boal Valley b
83.1 al .3 78.8
1.5 1 .o 0
A240 A240 HA240
46 -
31
31 -
39 36 16
26 -
17
20 26 9
12 20 6
0 -
f~ By molten-tin bath
48
FUEL, 1985, Vol 64, January
(30) (40) (35)
Carbonization
REFERENCES 1 2
3 4
Elliott, M. A. and Yohe, R. ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), Wiiey-Interscience, NY, 1981, p. 81 Patrick, J. W. and Wilkinson, H. C. ‘Analytical Methods for Coal and Coal Products’ (Ed C. Karr, Jr), Academic Press, NY, 1978, p. 339 Mochida, I., Maeda, K., Korai, Y. and Takeshita, K. J. Fuel Sot. Jpn. 1982,61,986 Marsh, H. and Smith, J. ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr, Jr), vol. 2, Academic Press, NY, 1978, p. 371
5
6 7 8 9 10 11
of coals. 6: I. Mochida
et al.
Mochida, I., Matsuoka, H., Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1982,61, 587 Mochida, I., Matsuoka, H., Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1982,61, 595 Okuhara, T., Nakama, H. and Miura, Y. Fuel 1981,60, 1091 Mochida, I., Korai, Y.,Takeshita, K., Komatsubara,Y., Koba, K. and Marsh, H. Fuel 1984,63, 136 Mochida,I., Shiraki, A., Korai, Y. and Okuhara,T. Coke Circular 1983,32,468 Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1981, 60, 117 Mochida, I., Korai, Y., Fujitsu, H., Takeshita, K., Mukai, K. and Nagino, H. J. Mat. Sci. 1982, 17, 525
FUEL, 1985, Vol 64, January
49
of coals into anisotropic
cokes
6. Formed cokes of high density and optical anisotropy from non-fusible and slightly fusible coals lsao Mochida,
Akira
Shiraki,
Yozo Korai and Toshiaki
Okuhara*
Research institute of Industrial Science, Department of Molecular Engineering, Graduate School of Engineering Sciences, Kyushu University S6, Kasuqa, Fukuoka, 816 Japan * Research and Development laboretor~es-Ill Nippon Steel Corp., ~a~ata-~jqash~* Kitakyushu, 805 Japan (Received f2 September f9%3; revised 13 March f984)
A procedure for the preparation of solid formed coke of enough adhesion and anisotropic development for use in the blast furnace has been studied, using non-fusible and slightly fusible coals with petroleum cocarbonizing additives. The coke precursor was prepared through the copreheat-treatment of coal and a suitable additive in adequate quantity under stipulated conditions. The desired coke was produced by carbonization after forming with a press. The condjtionsforthe copreheat-treatment have been carefully examined in terms of the temperature, time and heating devices. The behaviour of coals during copreheat-treatment and carbonization were discussed in terms of coal ranks, comparing this behaviour to the liquefaction reactivity and thermal stability of their liquefied product. (Keywords: coal: formed coke; cocarbonization;
anisotropic development)
Increased price and possible shortage of the coking coals in the near future are encouraging studies to find methods of producing good blast furnace cokes from non-coking low rank coals’. Such cokes are expected to have both high mechanical strength and chemical resistivity against carbon dioxide at the elevated temperatures of blast furnace operatior?. Thus, the cokes should be of solid form, high density and optical anisotropy3*4. Thecocarbonization procedure has been revealed to be excellent for ensuring the fusibility5*6 for adhesion and optical anisotropy of low rank coals. However, the resultant coke tends to swell too much, leading to porous coke of low density. On the other hand, the formed coking procedure has been recognized to produce a solid coke of high density, by selecting the compositions of coals and additives’. However, the optical texture derived from the low rank coals remained isotropic, exhibiting high reactivity in carbon dioxide*. To satisfy the requirements of a high mechanical strength and chemical resistance to CO,, the present authors adopted a series of coking steps, which consist of copreheat-treatment of a coal with an additive, grinding, forming (moulding), and carbonization. In a previous Paper9 a fusible coal was successfully carbonized into a solid formed coke through copreh~t-tr~tment of the coal with an additive. EXPERIMENTAL The coals used in this study were Emery (a slightly fusible coal) and Coal Valley coals (a non-fusible coal). These coals were ground to pass a 100 US mesh (149 pm) sieve, The additives used in this study were A240 pitch (Petroleum pitch from Ashland Petroleum Co.) and
hydrogenated A240 (HA240) pitch. The latter was prepared using lithium and ethylenediamine” ‘. Analyses of these materials are presented in Table I. Procedures
Carbonization procedures are summarized in Figure 1 (procedures A-D). Procedure A, A powdered mixture (at the prescribed mixing ratio of coal and additives) was carbonized in a Pyrex glass tube (30 mm dia., 300 mm in length} under nitrogen flow in a vertical electric furnace. The heating rate and soaking time were @WC h - ’ and 1 h, respectively. Procedwe 8. A coal and additive mixture was pressed before carboni~tion into a disc form (20 mm dia., 5 mm in thickness) under 400 kg cm -2. The disc was carbonized in the same way as Procedure A. Procedure C. A coal and additive mixture was copreheated at variable temperature in a Pyrex glass tube under nitrogen flow in an electric furnace (air bath). The heating rate and soaking time were 600°C h - 1 and 1 h, respectively. The copreheated mixture was ground in a mortar and then formed before carbonization. The forming and carbonization were carried out in the same way as Procedure B. Procedure I?. To copreheat the mixture of coal and additive more homogeneously at the prescribed temperature, the mixture was transferred to a stainless steel tube (19 mm dia., 200 mm in length) to be immersed in a moltentin bath of prescribed temperature under nitrogen flow”‘. The copreheat temperatures were attained within 6 min. After the prescribed copreheat-treatment time (1 h), the
FUEL,
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45
Carbonization of coals. 6: I. Mochida et al. Table 1 Properties of coals and additives Ultimate analysis (wt %, daf) Sample
C
H
N
0
s
Ash
CSN
TR
Emery Coal Valley A240 HA240
81.3 78.8 91.3 86.3
5.9 5.1 5.5 8.3
1.2 1 .O 0.2 0.8
10.8 15.0 0.9 4.6
0.8 0.1 -
10.2 9.8 -
1 0
63.6 64.4 -
CSN = Crucible swelling number TR = total reactive: TR = V (R > 2.2%) + E + SF
Procedure
A
COPREHEATTREATMENT Procedure C (in an air bath) Procedure Figure
1
Experimental procedures
13
, (in a molten
tin
tube was removed from the bath and rapidly cooled by imme~ing in cold water to quench the reaction. The preheated mixture was ground, formed and carbonized in the same way as Procedure C. The resultant cokes were examined in terms of their shape, the adhesion extent of their coal grains and their anisotropic texture. Optical texture of resultant coke was examined using an optical microscope (Leitz, OrthoplanPol). After calcination at 1200°C for 1 h under argon flow, tensile strength of the coke was measured using an autograph (H-560, Shimazu). RESULTS The photographs of non-formed (Procedure A) and formed (Procedure B) cokes prepared from Emery and Coal Valley coals are shown in Figure 2, where A240 and HA240 were used as the additive, respectively. Emery coal produced a coke of disc form without the additive or press. However, the coke was very weak and easily broken in the hand (Figure 2~). Addition of A240 increased the coke strength, although the resultant coke deformed significantly (Figure 2b). The addition of more additive significantly increased swelling, which continued to increase with increasing amounts of additive (Figure 2~). The effects of forming were observable at lower amounts of additive, where the formed shape was maintained after carbonization (Figure 2d and e). The addition of more additive, however, diminished the effects of forming, allowing marked swelling (Figure 2f). Coal Valley produced similar cokes when it was formed with HA240. The effects of forming were observable when the additive (10 or 2Owt%) was added (Figure 2h). The effect of forming was destroyed with additive levels > 30 wt% (Figure 2i). Optical micrographs of formed and non-formed cokes from the Emery and Coal Valley coals are shown in Figure 3. Emery coal, when carbonized singly, exhibited isotropic texture, the coal particles slightly fusing regardless of forming (Figure 3a and d). By increasing the amount of
46
FUEL, 1985, Vol 64, January
bath)
‘9-
a
d
e
g
tcm
I
Figure 2 Photographs of formed and non-formed cokes prepared from Emery (a-f) and Coal Valley (g-i) coals. a-c, Non-formed cokes; d-i, formed cokes. Additive: A240 for Emery coal; HA240 for Coal Valley coal. Amount of additive (wt%): a, d and g. no additive; b, e and h, 20; c, f and i, 30
A240 pitch to 20 wt%, the coal particles fused much better, although resultant cokes were still isotropic (Figure 3b and e). With 30 wt% of additive, the resultant coke exhibited very fine-gmined mosaic in the major region with some isotropic areas (Figure 3c and f), no coal particle being distinguishable any more. Above 40 wt%, the resultant cokes exhibited very fine and fine-grained mosaic textures over the entire surface, the size of anisotropic unit increasing with increased amount of additive. Thus, the forming hardly influenced the extent of anisotropic developement in the carbonized coke. Coal Valley coal, when carbonized after forming, exhibited complete isotropic texture, and the coal
Carbonization of coals. 6: I. Mochida et al. (Figure 3h). At 30 wt%, the resultant coke exhibited very
tine-, tine- and medium-grained mosaic texture over the coke surface (Figure 3i). HA240 at > 35 wt% was enough to develop complete anisotropy in the resultant coke. Size increased with increasing amounts of additive. Copreheat-treatment of coal with additive
Figure 3 Photomicrographs of formed and non-formed cokes prepared from Emery (a-f) and Coal Valley (g-i) coals. a-c, non-formed cokes; d-i, formed cokes. Additive: A240 for Emery coal: HA240 for Coal Valley coal. Amount of additive (wt%): a, d and g, without additive; b, e and h, 20; c, f and i, 30
a
b
C
m
Shapes and optical micrographs of formed cokes from copreheat-treated Emery coal with A240 pitch (A240, 40 wt%) at different high temperature treatments (HTT) for 1 h are shown in Figure 4. Thecopreheated mixture at 430°C produced a solid hard coke (Figure 4b), which exhibited very thick pore walls and anisotropic texture over the entire surface, no grains being distinguishable (Figure 4e). Lower HTT (420°C) allowed the deformation and swelling of the formed shape (Figure 4a), although anisotropic development and fusion were achieved (Figure 4d). The copreheat-treatment at temperatures >44o”C reduced the fusibility of grains, facilitating the loss of adhesion and maintaining the angular shapes of the initial coal particles (Figure 4f). Although the resultant coke maintained formed shape (Figure 4c), no high strength was obtained. The extent of anisotropic development in the resultant cokes after carbonization at 600°C were similar regardless of preheat-treatment temperature. The shapes and optical micrographs of formed cokes from copreheat-treated Coal Valley coal with HA240 pitch (HA240, 35 wt%) are shown in Figure 5. The copreheat-treatment at variable temperatures in an air bath failed to achieve the objectives of good adhesion and no swelling at the same time, although all cokes exhibited very fine- and fine-grained mosaic textures. For example, the coke obtained from copreheated mixture at 430°C was deformed and swollen. Although the cokes from copreheated mixtures at 435 and 440°C maintained the formed shape, they failed to show high strength. The preheated grains did not fuse, staying angular (Figure .5e and f ). Copreheat-treatment using a tube bomb in a molten-tin bath ascertains more homogeneous heating of the mixture of coal and additive. The shapes and optical micrographs of resultant formed cokes are shown in Figure 6.
L
Icm
b
Figure 4 Effects of copreheat-treatment in an air bath on the shape and optical texture of formed coke prepared from Emery coal with A240 (4Owt%). HTf of preheat-treatment: a and d, 420X, 1 h; b and e, 43o”C, 1 h; c and f, 440°C. 1 h
particles maintained the same angular shapes as those before the carbonization (Figure 3g). Partial fusion of coal particles was observable with 10 wt% HA240. The extent of fusion was further improved by the addition of 20 wt%, no coal particles being distinguished at this stage. However, resultant cokes still exhibited isotropic texture
Figure 5 Effects of copreheat-treatment in an air bath on the shape and optical texture of formed coke prepared from Coal Valley with HA240 (35 wt%). HlT of preheat-treatment: a and d, 43o”C, 1 h; b and e, 435’C. 1 h; c and f, 44o’C, 1 h
FUEL, 1985, Vol64,
January
47
Carbonization of coals. 6: I. Mochida et al.
ible coals with additive is very effective in producing a formed coke of excellent adhesion and anisotropic development. A coal, with a suitable additive in certain quantities, is dissolved or liquefied at the initial stage of the copreheat-treatment into pitch-like material, which is then converted to the coke precursor during the later stage of copreheat-treatment by liberating the excess volatile matter. The depolymerization, dealkylation and/ or deoxygenation reactions in the initial stage enable the modified coal to form the nuclei of the anisotropic mesophase in the coke precursor” via the liquid-phase carbonization, through reduced viscosity. The coke precursor, when heat treated under unsuitable conditions, still maintains enough fusibility to allow its ground grains to adhere to each other completely during the following carbonization stage, after tight packing by the press. Such a procedure is very similar to that applied to produce moulded carbons without external binder’l. The three coals studied in the present Paper have different carbonization properties as indicated by their CSN tests. Their behaviours in the copreheat-treatment may reflect such properties. Non-fusible coal requires hydrogen donating HA240 to be liquefied to obtain fusibility and optical anisotropy. Highly aromatic A240 can modify enough the slightly fusible coals. A larger amount of the additive is necessary for higher rank coal. Coal rank also appears to influence the range of appropriate copreheat-treatment conditions required. Hunter Valley coal required treatment in an air bath between 410 and 430°C whereas 430°C was the required temperature to produce a hard formed coke from Emery coal. Coal Valley coal produced a hard coke only when it was heat treated in a molten-tin bath. The non-fusiblecoal requires very homogeneous heating under strict conditions. These properties of coals may be intimately related to the thermal stability of the coke precursor produced by copreheat-treatment with the additive. The precursors from lower rank coals tend to be highly reactive because of their many oxygen and alkyl groups, which may cause rapid condensation reactions unless they are properly eliminated”. The behaviour of the precursor in the successive carbonization stage can, in principle, be related to its structure. The content of PS (very little benzene-soluble fraction included in the present case) appears to be a measure of fusibility. However, the nature of PI can never be neglected since it should be affected by PS to a considerable extent during the carbonization. Further work is required in this area. Information obtained on copreheat-treatment suggests some ideas for preparing better coke precursors. The following Paper describes such ideas further.
Copreheat-treatment at 420°C for 1 h provided the precursor for a solid hard coke with good anisotropic development. However, lower or higher copreheattemperature failed to produce the desired coke by allowing either slight swelling or loss of fusibility. Analyses of copreheated coals and pitches
Yields of pyridine solubles (PS) obtained from the copreheated materials of Emery and Coal Valley coals with the respective additive at various temperatures are summarized in Table 2, where those of Hunter Valley coal (a slightly fusible coal, CSN = 2) with A240 pitch studied in a previous study’ are also included for comparison. When the copreheat-treated materials contained PS in a certain range [20-26 wt% for Hunter Valley coal (A240, 30 wt%) and 28 wt% for Emery coal (A240, 40 wt%)], solid formed cokes of good adhesion and anisotropy could be produced by formed-coking after the preheattreatment. In contrast, the amount of PS decreased drastically at 400 and 430°C in the case of Coal Valley coal (HA240, 35 wt%). Although both copreheat-treatments at 400 and 420°C produced 18 and 17 wt% of PS, respectively, the treatment at 420°C could only produce an aimed coke, which suggests some essential role for the pyridineinsoluble (PI) fraction. DISCUSSION The copreheat-treatment
of non-fusible and slightly fus-
Figure 6 Effects of copreheat-treatment in a molten-tin bath on the shape and the optical texture of formed coke prepared from Coal Valley coal with HA240 (35 wt%). HTT of preheattreatment: a and d, 410°C; b and e, 420°C; c and f, 430°C Table 2 Yields after copreheat-treatment
HTT of preheat-treatment (“C, 1 h)
c (wt%, Coal
daf)
CSN
Additive fwt %)
350
380
390
400
410
420
430
450
480
Hunter Valley Emery Boal Valley b
83.1 al .3 78.8
1.5 1 .o 0
A240 A240 HA240
46 -
31
31 -
39 36 16
26 -
17
20 26 9
12 20 6
0 -
f~ By molten-tin bath
48
FUEL, 1985, Vol 64, January
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Carbonization
REFERENCES 1 2
3 4
Elliott, M. A. and Yohe, R. ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), Wiiey-Interscience, NY, 1981, p. 81 Patrick, J. W. and Wilkinson, H. C. ‘Analytical Methods for Coal and Coal Products’ (Ed C. Karr, Jr), Academic Press, NY, 1978, p. 339 Mochida, I., Maeda, K., Korai, Y. and Takeshita, K. J. Fuel Sot. Jpn. 1982,61,986 Marsh, H. and Smith, J. ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr, Jr), vol. 2, Academic Press, NY, 1978, p. 371
5
6 7 8 9 10 11
of coals. 6: I. Mochida
et al.
Mochida, I., Matsuoka, H., Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1982,61, 587 Mochida, I., Matsuoka, H., Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1982,61, 595 Okuhara, T., Nakama, H. and Miura, Y. Fuel 1981,60, 1091 Mochida, I., Korai, Y.,Takeshita, K., Komatsubara,Y., Koba, K. and Marsh, H. Fuel 1984,63, 136 Mochida,I., Shiraki, A., Korai, Y. and Okuhara,T. Coke Circular 1983,32,468 Korai, Y., Fujitsu, H. and Takeshita, K. Fuel 1981, 60, 117 Mochida, I., Korai, Y., Fujitsu, H., Takeshita, K., Mukai, K. and Nagino, H. J. Mat. Sci. 1982, 17, 525
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