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The effect of swelling pressure during coal carbonization on coke porosity
ELSEVIER
0016-2361(95)00238-3
FuelVol. 75 No. 2, pp. 187-194, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96/$15.00 + 0.00
The effect of swelling pressure during coal carbonization on coke porosity Seiji Nomura and K. M a r k Thomas* Nippon Steel Corporation, Ironmaking Process Laboratory, Process TechnologyResearch Laboratories, 20-1, Shintomi, Futtsu, Chiba 293, Japan *Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK (Received 7 December 1994; revised 23 June 1995) In cokemaking, the generation of swelling pressure is an important consideration since excessive pressures result in problems with pushing the coke and in extreme cases, damage to coke ovens may occur. In this investigation, the effect of swelling pressure generated during coal carbonization in coke ovens on coke porosity was studied. The swelling pressure was measured using a movable-wall test oven. The gas pressure in the plastic layer and the swelling pressure were monitored during the carbonization of coals in the test coke ovens. The porous structure of the resultant coke was assessed. The coke macropore structure was characterized using an optical microscope coupled with an image analyser and the micropore structure was characterized using a volumetric gas adsorption method. A method to determine the extent to which the macropores were distorted under unit pressure during the thermoplastic stage was also developed. It was shown that the macropores were compressed against the coke oven wall by the gas pressure in the plastic layer, resulting in the distortion of their shape; the extent of distortion varied greatly with the type of coal. The results suggested that the macropore distortion depended not only on the swelling pressure but also on the amount of distortion per unit pressure during the thermoplastic stage. Furthermore, the characteristics of the coke pore structure from a dangerously swelling coal varied considerably over the oven width. As the oven centre, where the gas pressure was extremely high, was approached, the macroporosity and the mean macropore diameter decreased, while the microporosity increased. The changes in the macroporosity and microporosity over the oven width are discussed in relation to the gas pressure in the plastic layer and the swelling pressure developed during carbonization. (Keywords: coal; swelling pressure; coke pore structure)
In the cokemaking industry one of the most significant problems to be solved is how to avoid damage to coke oven walls caused by dangerously high coking pressure, or swelling pressure, which is the pressure imposed on the coke oven wall as a result of swelling of the coal during the carbonization process 1. Experience in the USA 2'3 and in Europe 4'5 has shown that coke oven walls can be badly damaged as a result of excessive swelling pressure caused by certain coals present in blends. These coals are described as dangerously swelling coals. Such damage to coke ovens during their working life must be avoided because it not only lowers the productivity but also necessitates costly replacement or major repairs of coke ovens. An understanding of the cause of the swelling pressure is essential to avoid oven wall damage. There have been a n u m b e r o f investigations into this 6-~3. It is k n o w n that the swelling pressure is due to the pressure of the gas in the plastic layer. During carbonization in the coke oven, as its temperature reaches 350°-400°C the coal begins to soften to form the plastic layer. The permeability of the plastic layer is 1"4 16 low and the volatiles released are trapped in the layer. The pressure in the layer therefore increases as
more gases are evolved. This gas pressure in the plastic layer is transmitted to the walls of the coke oven. To understand the phenomenon of swelling pressure in commercial coke ovens, a method of measuring swelling pressure on the laboratory scale was developed 17. The effects of coal characteristics and carbonization conditions, e.g. bulk density, particle size, flue temperature, moisture, on swelling pressure have been studied 7-11,13,18 and recently some prediction models for swelling pressure have been proposed 19'2°. On the other hand, very little research has been carried out on the effect of swelling pressure on the development of porosity during the carbonization of coal. Generally, coke pores are classified into three types21: macropores > 50 nm wide, mesopores 2 - 5 0 n m wide, and micropores < 2 nm wide. Recently Patrick e t al. 22 have reported a relation between the gas pressure generated in the plastic layer and the macroporosity of coke. Furthermore, it has been found that the macropores in the coke made in coke ovens are distorted or 'compressed' in the direction perpendicular to the oven wall and it has been suggested that this might be caused by the gas pressure in the plastic layer 23-26. However, the factors affecting the
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The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas extent to which the macropores are distorted or compressed are still unknown. Many studies have been carried out on the effect of heat treatment on the changes in microporosity during the carbonization of coals 27-33 and it is known that microporosity changes drastically as a function of heat treatment temperature; however, the relation between swelling pressure and microporosity has not been discussed. Since the pore structure is one of the most important factors determining the mechanical strength of coke 34'35 and its strength after reaction with CO236,37,which are considered by some workers to play significant roles in blast furnace operation 38'39, it is of importance to study the effect of swelling pressure on coke microporosity as well as macroporosity. The objectives of this study were to examine the effect of swelling pressure generated during coal carbonization in coke ovens on the macroporosity and microporosity of coke, to examine the factors determining the distortion and the compression of macropores during the thermoplastic stage, and to understand the characteristics of the porous structure of cokes prepared from dangerously swelling coals. To achieve these objectives, the macropore and micropore structures of cokes were studied in relation to measurements of the gas pressure in the plastic layer and the swelling pressure. Furthermore, a method for measuring the extent to which the macropores are distorted during the thermoplastic stage by unit external pressure was also developed and the amount of macropore distortion per unit pressure was investigated in relation to the effect of swelling pressure on the macropore structure.
EXPERIMENTAL
Coals used The characterization data for the coals used in this study are given in Table 1. Coal HP is of high swelling pressure (dangerously swelling), coal MP is of medium swelling pressure and coal LP is of low swelling pressure.
Gas pressure in plastic layer The coals were crushed to 85% < 3mm. The coals were charged into a steel box measuring 410mm high, 619 mm long and 425 mm wide. The box was put into an electrically heated small test coke oven with two heating walls 4° and the sample was carbonized. The moisture content of the coal was 3 wt% and the bulk density was 860 kg m -3 (db). The gas pressure in the plastic layer was measured by steel pipes with 1 mm i.d. 41, each of which was connected to an electrical manometer. The pipes were arranged in a horizontal plane 125 mm from the bottom of the box at five different places, 25, 50, 100 and 150mm from the wall and at the oven centre (i.e. 215.5mm from the wall). A thermocouple of l mm diameter was attached to each pipe so that the changes in the temperature as well as the pressure were measured. It is known that the gas pressure reaches a maximum when the plastic layer reaches the probe 6, therefore at each measuring point the pressure peak when the temperature of the probe reached the thermoplastic temperature range was defined as the gas pressure in the plastic layer.
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Table 1 Characterizationfor the coals used
Coal
HP
Proximate analysis (wt% db) Volatile matter Ash Caking and swellingproperties Dilatometry Total dilatation (%) Softening temperature (°C) Temperature of maximum contraction (°C) Resolidificationtemperature(°C) Plastometry Maximum fluidity(log d.d.p.m.) Softening temperature (°C) Maximum fluiditytemperature (°C) Resolidificationtemperature(°C) Petrographic analysis Reflectance(av.) (%) Maceral analysis (vol. % mmf) Vitrinite Inertinite
18.6 10.0
MP
LP
20.6 9.8
20.6 9.9
59 432
90 405
22 404
464 497
445 494
456 488
1.65 444 481 494
2.69 419 465 497
1.62 422 466 494
1.48
1.32
1.39
73.7 26.3
73.4 26.6
62.4 37.6
After carbonization, the coke was quenched to room temperature with nitrogen and taken out of the oven. These cokes were examined for macropore and micropore structure.
Swelling pressure The swelling pressure was measured using the movable-wall oven at Nippon Steel Corporation. This has a height of 600 ram, a length of 593 mm and a width of 400 mm 2°. The coals were crushed to 85% < 3 mm and charged through the charge hole at the top of the oven. The bulk density was 800 k g m -3 (db). The changes in swelling pressure imposed on the movable wall during carbonization were measured with a load cell mounted on the outside of the wall.
Macropore structure of coke Five samples were cut from the coke lump prepared by carbonization in the small test oven. The position of each cut corresponded to the position where the gas pressure was measured. To investigate the distortion of the macropores in the direction of the oven width by the
z
(oven heightt direction) ~
Y
x(oven width direction)
vertical cross-section s. . . .
ove~ wall
width part to w a l l width perpendicular to
wall
horizontal cross-section (x-y plane)
Figure I
Measurement of macropore structure of coke
i...~
The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas gas pressure in the plastic layer, as shown in Figure 1 both the plane perpendicular to the oven wall (the horizontal cross-section, x - y plane) and the plane parallel to the oven wall (the vertical cross-section, y - z plane) were studied. For the vertical cross-section, only three samples (25 and 100mm from the wall and at the oven centre) were studied. Coke samples were moulded in slow-gelling epoxy resin (Buehler resin SW). The moulded sample was evacuated for 30 min in a desiccator and kept overnight while the resin was hardening, and the sample was then polished. The surface of the polished sample was analysed with an optical microscope combined with an image analyser (Optomax V). The magnification of the objective lens was × 4. The pore area was measured for each pore and the number of pores was counted for each field. The small macropores of diameter 12 #m were not included in these measurements, since the resolution of the image analyser was 6#m. As shown in Figure 1, for the horizontal cross-section, the pore width parallel to the oven wall, which was defined as the longest projection of the pore in the axis parallel to the oven wall (y-axis), and the pore width perpendicular to the oven wall, which was defined as the longest projection of the pore in the axis perpendicular to the oven wall (x-axis), were measured for each pore. For the vertical cross-section, the pore widths y and z, which were defined as the longest projections of the pore in the y- and z-axes respectively, were measured for each pore. These measurements were used to calculate the following parameters: Macropore distortion ratio for the horizontal crosssection (x-y plane) = (width parallel to wall)/(width perpendicular to wall) Macropore distortion ratio for the vertical cross-section ( y - z plane) = (width y)/(width z) Macroporosity = (Epore area)/(field area) Mean macropore diameter for the horizontal crosssection (x-y plane) = (Epore area)/{[(Ewidth perpendicular to wall) + (Ewidth parallel to wall)]/2} The macropore distortion ratio for the horizontal crosssection shows to what extent the macropore is distorted as a result of being pressed against the plane parallel to the oven wall O'-z plane). The greater the macropore distortion ratio, the more the macropore is distorted. The macropore distortion ratio for the cross-section of a spherical pore is 1.0. The mean macropore diameter represents the average length of all horizontal and vertical scan lines that traverse the macropores in a field of view. Since coke is a very heterogeneous material, to get representative values the measurements were made on > 10 fields of view randomly spread over the polished block, and > 600 macropores were counted for each sample.
piston
0 0 0 O' 0 0 0
dilatometer retort macropores
o~...[ O~
l
1
heater
height
] ~ x t width
X
coal sample
Figure 2 Measurement of the amount of macropore distortion per unit pressure during the thermoplastic stage
1000
100
lO
~
•
- -&
. . . .
i
0
. ~
. . . . .
•
i
. . . . . . .
i
J
50 100 150 200 Distance from oven wall [mm]
Figure 3 Gas pressure in the plastic layer across the oven width: II, coal HP; O, coal MP; A, coal LP
oven. The coal samples were crushed to a particle size of < 1 mm and 0.4 g of the sample was put into the retort of a dilatometer 42 of 8.0 mm i.d. In this experiment the coal sample was carbonized as < I mm particles to simulate the phenomena in commercial coke ovens. The retort was placed in an electric furnace and heated at ] 3 K m i n - . At the desired temperature, the piston of the dilatometer, which had been suspended in the retort without touching the coal sample, was lowered slowly onto the coal sample, i.e. at this temperature the coal sample began to be compressed mechanically by the piston. Thereafter, the sample was carbonized under the 80 O.
-~60
= ¢0
P40 e-
~20
Macropore distortion per unit pressure during thermoplastic stage The extent to which the macropores were distorted during the thermoplastic stage under unit external pressure was measured in laboratory-scale experiments, to examine the factors affecting the distortion of the macropores generated during carbonization in the coke
0=" 0
M
5 10 Time [hrs.]
15
Figure 4 Changes in swelling pressure with time for carbonization in the movable-wall test oven: II, coal HP; O, coal MP; &, coal LP
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The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas pressure of the piston to 600°C, as shown in Figure 2, and kept at that temperature for 30 min before cooling. To examine the extent to which the macropores were distorted during the thermoplastic stage under constant pressure, all the experiments were carried out under a pressure of 20 kPa. The coke sample taken out of the retort was cut in half along the vertical cross-section (x-z plane in Figure 2) after moulding in the epoxy resin; it was then moulded again and polished so that the vertical cross-section (x-z plane) was examined using the optical microscope combined with the image analyser. The region near the top of the sample where the piston had touched was observed so as to obtain information about macropore distortion by the pressure of the piston. As shown in Figure 2, the pore width, which was defined as the longest projection of the pore in the x-axis parallel to the surface of the piston, and the pore height, which was defined as the longest projection of the pore in the z-axis perpendicular to the surface of the piston, were measured for each pore. The macropore distortion ratio per unit pressure, defined as (pore width)/(pore height) was obtained.
Micropore structure of coke The specific surface area of the coke samples was measured by a volumetric gas adsorption method43. Nitrogen was used as the adsorbate at 77 K and the adsorption data were analysed by the BET method43. The coke samples were cut from the lump prepared by carbonization in the small test coke oven, at the positions 25 and 100 mm from the wall and at the oven centre. The samples were crushed to a particle size of < 600 #m and a sample of 0.3 g was used in the experiment.
1.40
/\
.2 2 1.30 c 0
~1.20
P
0
°1.10
o
A"
0
A ......
1.00
i
i
i
50
100
150
200
Distance from oven wall [turn]
Figure 5 Macropore distortion ratio across the oven width for the horizontal cross-section (x-y plane). II, coal HP; O, coal MP; A, coal LP
1.20 0
~ 1.10 c
.9° 1.00 "0
0
,',0.90 2 o
0.80
i
i
i
50
100
150
200
Distance from oven wall [mrn]
Figure 6 Macropore distortion ratio across the oven width for the vertical cross-section (y-z plane), m, coal HP; O, coal MP; A, coal LP
RESULTS AND DISCUSSION The three coals chosen for this study had similar rank. The vitrinite reflectance covered the range 1.32-1.48% and the volatile matter was in the range 18.6-20.6 wt% (db). However, the carbonization characteristics in the coke oven were markedly different; in particular, the swelling and gas pressures differed considerably. The generation of gas and swelling pressures will affect the development of porosity in the coal. The generation of pressure may compress the macropores, leading to a loss of macroporosity. Also, since the pressure is applied in one direction, the pressure may distort the shape of the macropores. These two aspects, i.e. the compression of the macropores and the distortion of the macropore shape, have been investigated on the three coals using a movable wall oven and also by simulating the pressure observed in the coke ovens in laboratory-scale experiments using pressures applied by mechanical methods.
Gas pressure and swelling pressure Figure 3 shows the gas pressure changes across the oven width. The gas pressure generated in the carbonization of coal LP in the test oven is less than that of coal MP. The gas pressure generated in the carbonization of coal LP decreases slightly as the oven centre is approached, while that of coal MP increases. The gas pressure generated in the carbonization of coal HP is similar to those of the other two coals near the oven wall, but increases steeply as the oven centre is approached
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and is much higher at the oven centre than for the other two coals. Figure 4 shows the changes in swelling pressure with time. The swelling pressure of coal HP increases as the carbonization proceeds and goes through a maximum when the two plastic layers meet in the middle of the coke oven. The swelling pressure of coal MP increases slightly in the initial stage of carbonization, after which it remains constant virtually until the plastic layers meet at the centre of the oven, producing a sharp rise in pressure. The swelling pressure of coal LP is much lower than that of the other two coals and no peak is observed. It is clear that the swelling pressures and the gas pressure in the plastic layer of these three coals differ considerably. The phenomenon of the swelling pressure versus time graph reaching a maximum has been reported previously8'11'44. This swelling pressure peak appears when the two plastic layers travelling from both oven walls meet at the oven centre. When the plastic layers which join together at the oven centre change into coke, the swelling pressure decreases to zero. Considering that the plastic layer proceeds from the oven wall to the oven centre as carbonization proceeds, the pattern of swelling pressure resembles that of gas pressure. This can be explained by the theory that the swelling pressure generated during carbonization in the coke oven is caused by the gas pressure in the plastic layer6-13.
The effect of swelling pressure during carbonization: S. Nomura and K. M. Thomas D&tortion of the macropores in the coke prepared & the test oven Figure 5 shows the macropore distortion ratio across the oven width for the horizontal cross-section. The ratio varies greatly with the type of the coal and lies between 1.05 and 1.40. Figure 6 shows the macropore distortion ratio across the oven width for the vertical cross-section. This ratio is close to 1.0 and changes little with distance from the oven wall or with coal type. It should be noted that in the case of the horizontal cross-section, the macropores are distorted in the direction perpendicular to the oven wall and the distortion ratio differs greatly for each coal, while in the case of the vertical cross-section, it is constant, i.e. ,,~ 1.0, in spite of different coals with different swelling pressures being used. This result suggests that there is a force perpendicular to the oven wall to compress the macropores against the oven wall resulting in the distortion of the macropore shape and there is little or no force parallel to the oven wall to compress the macropores. The gas pressure in the plastic layer is considered as the only force which operates in the direction perpendicular to the oven wall. Therefore, this result supports the theory that macropores developing in the plastic layer in the coke oven are compressed against the oven wall and distorted by the gas pressure, as implied by some previous studies23-26. Furthermore it should be noted that the macropore distortion ratio for the horizontal cross-section of coke LP, which is between 1.05 and 1.17, is lower than that of coke MP (1.29-1.40) and coke HP (1.17-1.29). The fact that the macropore shape of the coal of low swelling pressure is distorted to a smaller extent suggests that the extent to which the macropore shape is distorted depends on the swelling pressure. However, the difference in the macropore distortion ratio for the horizontal crosssection cannot be explained by the difference in the gas pressure in the plastic layer alone. This is shown by the observation that although near the oven centre the gas pressure in the plastic layer of coal MP is lower than that of coal HP, the macropores of coke MP are distorted to a greater extent than those of coke HP. This leads to the hypothesis that the amount of the macropore distortion per unit pressure varies with the type of coal. The extent to which the macropores are distorted during the carbonization of coals in the test oven is considered to depend both on the gas pressure in the plastic layer and on the amount of macropore distortion per unit pressure; this aspect is discussed later. It has been shown45 that macropore shape affects the tensile strength of coke. Therefore, as suggested previously25, it is expected that mechanical properties of coke such as the tensile strength and Young's modulus differ between the directions parallel and perpendicular to the oven wall. Macropore distortion per unit pressure during thermoplastic stage To assess the stages of pore distortion, laboratory experiments were carried out involving the introduction of external pressure during the thermoplastic phase. Figure 7 shows the relation between the temperature at which the external mechanical pressure was applied to the coal by the piston and the macropore distortion ratio per unit pressure defned previously. The macropore
1.40 Q.
o ..= P Z 1.30 A--.._
o ~ 1.20
e"
--..
~. I
2 1.00 0
i
i
i
i
100
200
300
400
500
Temperature at which pressure was applied [°C]
Figure 7 Changes in macropore distortion ratio per unit pressure with temperature at which external mechanical pressure was applied to the coal. B, coal HP; Q, coal MP; &, coal LP
distortion ratio per unit pressure changes little up to 300°C. It starts to increase a little in the range 300400°C. Thereafter it goes through a maximum in the range 400-470°C, which is the thermoplastic temperature range of the coals, and decreases sharply to ~ 1.0 in the range 470-500°C, which is the resolidification temperature of the coals. This decrease in the macropore distortion ratio per unit pressure near the resolidification temperature means that the macropores are distorted to a smaller extent as the resolidification temperature is approached and that the shape of the macropores does not change beyond the resolidification temperature, because the coal sample loses its thermoplasticity. The coals have different temperatures at which the macropore distortion ratio per unit pressure reaches a maximum, and coal HP reaches a maximum at the highest temperature. It should be noted that the macropore distortion per unit pressure varied with the type of coal. The maximum macropore distortion ratio per unit pressure of coal HP (1.17 at 470°C) is lower than that of coal MP (1.30 at 425°C) and coal LP (1.30 at 400°C), which means that the macropores developed during the carbonization of coal HP are distorted in the thermoplastic stage to a smaller extent than those for coals MP and LP under unit pressure. It is considered that macropores in highly viscous material such as the plastic mass of coal HP are distorted to a smaller extent under unit pressure. Since the shape of the macropores does not change beyond the resolidification temperature, the distortion of macropores is considered to take place only in the plastic layer. There are two possible stages in which the macropores are distorted in the plastic layer during the carbonization process in the coke oven. The first is the initial stage of the plastic layer development, where the coal starts to soften and macropores begin to develop; the second is the latter stage of the plastic layer, just before the coal starts to resolidify. In the initial stage, the swelling of the macropores is restricted by the coal mass on one side and the adjacent slightly hotter plastic layer on the other side, which might result in the generation of distorted macropores. However, the uncarbonized coal layer is considered to be less rigid than the semicoke layer and may be compressed to some extent by external pressure. Therefore the macropore distortion is expected
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The effect of swelling pressure during carbonization. S. Nomura and K. M. Thomas to be lower, since some expansion into the voids in the coal bed is possible. On the other hand, in the latter stage the macropores are considered to be pressed further against the rigid semicoke by the pressure caused by the swelling of the adjacent slightly cooler plastic layer, resulting in distortion of the macropores. This conjecture is supported by the suggestion made previously26 that the size of the macropores decreases in the final stage of the plastic layer partly because they are compressed and compacted. It is interesting to compare the macropore distortion ratio for the horizontal cross-section of the coke prepared by carbonization in the test oven with the macropore distortion ratio per unit pressure obtained in the laboratory experiments, from the viewpoint of the effect of the gas pressure in the plastic layer and the macropore distortion per unit pressure on the macropore shape. In the case of coke prepared by carbonization in the test oven, it was observed that the macropore distortion ratio for the horizontal cross-section of coke LP was much lower than that of coke MP, but the results obtained here have shown that under the same pressure the macropores generated in coke LP are distorted to the same extent as those in coke MP. This suggests that the difference in the macropore distortion ratio for the horizontal cross-section between cokes MP and LP in the case of coke prepared in the test oven can be ascribed to the difference in pressure, i.e. the gas pressure in the plastic layer. Furthermore, in the case of the coke prepared in the test oven, it was observed that the macropores in coke MP were distorted to a larger extent than those in coke HP in spite of the gas pressure of coal MP being smaller than that of coal HP. Since it has been shown that the macropore distortion ratio per unit pressure for coke MP is larger than that for coke HP, this observation can be rationalized by assuming that the macropore distortion ratio in coke prepared in the coke oven depends not only on the gas pressure in the plastic layer but also on the macropore distortion ratio per unit pressure during the thermoplastic stage. In the case of coke HP prepared by carbonization in the test oven, the amount ofmacropore distortion per unit pressure is so low that its high gas pressure in the plastic layer does not distort the macropore shape to such an extent as in the case of coke MP. Therefore from this experiment the factors determining the distortion of macropores during the thermoplastic stage are considered to be the gas pressure generated in the plastic layer and the amount of macropore distortion per unit pressure, which is related to the rate of release of volatiles in the thermoplastic phase, the permeability of the plastic layer and the thermoplasticity. The macropore distortion ratio for the horizontal cross-section of coke MP prepared in the test oven (average 1.34) is close to the maximum macropore distortion ratio per unit pressure for coke MP obtained in this experiment (1.30 at 425°C). This result suggests that the process of distortion of macropores in the test oven under the gas pressure in the plastic layer is quite similar to that occurring in the dilatometer retort under the pressure of the piston. In fact, the pressure imposed on the coal with the piston, 20 kPa, is as high as the gas pressure in the plastic layer, 10-15 kPa. The macropore distortion ratio for the horizontal cross-section of coke LP prepared by carbonization in the test coke oven
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Fuel 1996 Volume 75 Number 2
140 --t
~130 ...... E 120 "O
.¢ 110 O O. O
~100 M E "
90
8O
50
100
150
200
Distance from oven wall [mm]
Figure 8 Mean macropore diameter across the oven width for the horizontal cross-section (x-y plane). II, coal HP; 0, coal MP; A, coal LP
65
60
E ~55 2 O
t~
45
40
0
r
n
50
100
150
200
Distance from oven wall [mm]
Figure 9 Macroporosity across the oven width for the horizontal cross-section (x-y plane). II, coal HP; 0, coal MP; A, coal LP
(average 1.11) is lower than the maximum macropore distortion ratio per unit pressure for coke LP obtained in this experiment (1.30 at 400°C). This is considered to be because the gas pressure in the plastic layer generated during the carbonization is lower than the pressure provided mechanically by the piston in the dilatometer retort. The macropore distortion ratio for the horizontal cross-section of coke HP prepared by carbonization in the test coke oven (average 1.24) is higher than the maximum macropore distortion ratio per unit pressure for coke HP obtained in this experiment (1.17 at 470°C). This is considered to be because the gas pressure in the plastic layer is higher than the pressure provided mechanically by the piston.
Size of macropores and macroporosity Figure 8 shows the changes in the mean macropore diameter with the distance from oven wall for the horizontal cross-section. The mean macropore diameters of cokes LP and MP change little across the oven width and the mean macropore diameter of coke LP is nearly equal to that of coke MP. On the other hand, in the case of coke HP, the mean macropore diameter decreases markedly as the oven centre is approached. Furthermore, the mean macropore diameter of coke HP near the oven centre is smaller than that of cokes LP and MP.
The effect of swelling pressure during carbonization: S. Nomura and K. M. Thomas
Figure 9 shows the changes in the macroporosity measured on the horizontal cross-section across the oven width. The macroporosity of coke LP decreases slightly as the oven centre is approached, while that of coke MP increases slightly. On the other hand, the macroporosity of coke HP decreases markedly as the oven centre is approached, from 57% near the oven wall to 43% near the oven centre. Near the oven wall the macroporosity of coke HP is higher than that of cokes MP and LP, but near the oven centre it is lower than that of cokes MP and LP. This remarkable decrease in the mean macropore diameter and in the macroporosity of coke HP across the oven width could be explained by reference to the increase in the gas pressure during the formation of coke HP. When the plastic layer is situated near the oven wall, many small macropores with high pressure are formed in the plastic layer at first. These pores are unlikely to coalesce, owing to the high viscosity of the plastic mass of coal HP. Instead of coalescing, however, they could expand and finally reach a similar size to the macropores in coals MP and LP, because at this stage there exists an uncarbonized coal layer in the oven centre, which can be compressed and provides voids for the plastic coal mass to expand. On the other hand, when the plastic layer is situated near the oven centre, macropores with high pressure formed in the plastic layer cannot expand to the same extent, since no uncarbonized coal layer exists and the plastic layer is sandwiched by the rigid semicoke layers, which cannot be compressed and do not provide space for the plastic mass to expand. Therefore the gas pressure in the plastic layer remains high and the size of the macropores and the macroporosity decrease. Furthermore, it has been shown that the size of the macropores decreases after passing through a maximum in the plastic layer, and this is partly ascribed to the compression and compaction of the macropores 26. In the case of coal HP, as shown in Figure 5, the macropore distortion ratio for the horizontal cross-section changes little with the distance from the wall, while the gas pressure in the plastic layer increases markedly and both the size of the macropores and the macroporosity decrease considerably. Therefore it is reasonable to say that the changes in the macroporosity and the macropore size are modified by the gas pressure in the plastic layer. It is considered that the extremely high gas pressure in the plastic layer generated during the carbonization of coal HP in the test oven compresses the macropores, leading to the loss of macroporosity and decrease in macropore size but the low fluidity developed during the thermoplastic phase prevents distortion of the pores. Considering that the tensile strength increases exponentially with decrease in macroporosity46 , in the case of dangerously swelling coals such as HP, the large change in the macroporosity from the oven wall to the oven centre implies that the strength of the coke near the oven wall is quite different from that of the coke near the oven centre. Since one of the most important aims in commercial cokemaking is to supply blast furnaces with coke having a uniform quality, it is considered that dangerously swelling coals not only damage the oven wall but also are detrimental to control of coke quality. Microporosity Figure 10 shows the changes in the specific surface area across the oven width. The surface area of coke MP
E tg ~p 0
,e-
o k~
.g Q.
ffl 0
i
i
L
50
100
150
200
Distance from oven wall [ram]
Figure 10 Specific surface area across the oven width. I1, coal HP; O, coal MP; A, coal LP
increases slightly from 1.4 to 1.9 m 2 g-~ as the oven centre is approached, while that of coke LP decreases slightly from 2.0 to 1.9 m 2 g-l. On the other hand, the surface area of coke HP increases markedly from 1.6m 2 g-I near the oven wall to 3.0m 2 g-1 near the oven centre. The changes in microporosity across the oven width can be explained by considering the changes in the macroporosity in relation to the swelling pressure. In the previous section, it has been shown that the mean macropore diameter and the macroporosity of coke HP decrease drastically as the oven centre is approached. Therefore, as far as the characteristics of the pore structure of coke from dangerously swelling coal are concerned, it is reasonable to say that the microporosity increases while the macroporosity decreases as the oven centre is approached and the gas pressure in the plastic layer increases. It is apparent that the macroporosity and the microporosity across the oven width are modified by the gas pressure in the plastic layer. CONCLUSIONS A study of the effect of swelling pressure generated during coal carbonization in coke ovens on coke porosity has shown that the macropores are compressed against the coke oven wall by the swelling pressure, resulting in modification of the size of the macropores and their distortion. The distortion of the macropores occurs in the thermoplastic phase during the carbonization of coals in a coke oven. The extent to which the macropores are distorted appears to depend not only on the swelling pressure but also on the amount of macropore distortion per unit pressure during the thermoplastic stage. In the coke prepared from coal of high swelling pressure in the test oven, the macropore distortion was less than that for a medium-swelling coal, whereas the macroporosity was much reduced in the regions where the gas pressure in the plastic layer and the swelling pressure were greatest. The macropore distortion per unit pressure during the thermoplastic stage for the highswelling coal was the lowest. As the oven centre, where the gas pressure was extremely high, was approached, the macroporosity and the mean macropore diameter decreased, while the microporosity increased. The macroporosity and the microporosity along the oven width are modified by the gas pressure in the plastic layer
Fuel 1996 Volume 75 Number 2
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The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas
and the resulting swelling pressure developed during carbonization in the coke oven. ACKNOWLEDGEMENTS S.N. is grateful to N i p p o n Steel C o r p o r a t i o n for f i n a n c i a l s u p p o r t to u n d e r t a k e this s t u d y as a p o s t g r a d u a t e s t u d e n t in the N o r t h e r n C a r b o n R e s e a r c h L a b o r a t o r i e s o f the University of Newcastle upon Tyne. The authors would like to t h a n k N i p p o n Steel C o r p o r a t i o n for p r o v i s i o n o f the coals a n d p e r m i s s i o n to p u b l i s h this p a p e r .
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
194
Tucker, J. and Everitt, G. In '2nd International Cokemaking Congress', Vol. 2, Institute of Materials, London, 1992, p. 40 Addes, V. I. and Kaegi, D. D. AIME Ironmaking Conf. Proc. 1990, 49, 663 McDermott, J. F. and Rueckl, R. L. AIME Ironmaking Conf. Proc. 1986, 45, 443 Tilma, J., Vander, T. and Vas, R. A. AIME Ironmaking Conf. Proc. 1987, 46, 277 Vergote, H. and Deroo, H. AIMElronmaking Conf. Proc. 1989, 48, 677 Russell, C. C., Perch, M. and Smith, H. B. AIME Blast Furn., Coke Oven Raw Mater. Proc. 1953, 12, 197 Harris, H. E., Glowacki, W. L. and Mitchell, J. AIME Blast Furn., Coke Oven Raw Mater. Proc. 1960, 19, 336 Brysch, O. P. and Ball, W. E. Research Bulletin 11, Institute of Gas Technology, Chicago, 1951 Gransden, J. F., Khan, M. A. and Price, J. T. AIME Ironmaking Conf. Proc. 1985, 44, 305 Khan, M. A., Gransden, J. F. and Price, J. T. AIME Ironmaking Conf. Proc. 1985, 44, 299 Loison, R., Foch, P. and Boyer, A. 'Coke: Quality and Production' Butterworths, London, 1989, pp. 353-416 Latshaw, G. M., McCollum, H. R. and Stanley, R. W. AIME Ironmaking Conf. Proc. 1984, 43, 373 British Coke Research Association. Technical Paper No. 5, London, 1952 FoxweU, G. E. Fuel 1924, 3, 122 Klose, W. and Heckmann, H. In '1989 International Conferences on Coal Science'. NEDO, Tokyo, 1989, p. 1063 Nogami, H., Sato, H., Nakajima, H. and Miura, T. J. Fuel Soc. Japan 1991, 70, 847 Koppers, H. and Jenkner, A. Fuel 1931, 10, 232 Benedict, L. G. and Thompson, R. R. AIME Ironmaking Conf. Proc. 1976, 35, 276 Osinski, E. J., Brimacombe, J. K., Barr, P. V. and Khan, M. A.
Fuel 1996 Volume 75 Number 2
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
In '2nd International Cokemaking Congress', Vol. 2, Institute of Materials, London, 1992, p. 374 Nomura, S., Arima, T. and Okuhara, T. Current Advances in Materials and Processes, Iron and Steel Institute of Japan, 1992, 5, 1077 Gregg, S. J. and Sing, K. S. W., 'Adsorption, Surface Area and Porosity', Academic Press, London, 1982, p. 25 Patrick, J. W., Psomiadou, E. and Walker, A. In 'Tth International Conference on Coal Science', Vol. 2, Canadian Organizing Committee, Devon, Alberta, 1993, p. 393 Alpern, B. In Comptes Rendus du 70e Congr~s de rlndustrie du Gaz en France, 1953, p. 942 Gibson, J. and Gregory, D. H. Yearb. Coke Oven Mgrs' Ass. 1975, 137 Taits, E. M. Coke Chem. USSR 1963, (8), 31 Hays, D., Patrick, J. W. and Walker, A. Fuel 1976, 55, 297 Chiche, P., Durif, S. and Pregermain, P. Fuel 1965, 44, 5 Cameron, A. and Stacey, W. O. Aust. J. Appl. Sci. 1958, 9, 283 Toda, Y., Hatami, M., Toyoda, S., Yoshida, Y. and Honda, H. Carbon 1970, 8, 565 Nandi, S. P., Ramadas, V. and Walker, P. L., Jr. Carbon 1964, 2, 199 Jenkins, R. G., Nandi, S. P. and Walker, P. L., Jr. Fuel 1973, 52, 288 Miura, S. and Silveston, P. L. Carbon 1980, 18, 93 Singla, P. K., Miura, S., Hudgins, R. R. and Silveston, P. L. Fuel 1983, 62, 645 Patrick, J. W., Sims, M. J. and Stacey, A. E. J. Phys. D: Appl. Phys. 1980, 13, 937 Grant, M. G. K., Cbaklader, A. C. D. and Price, J. T. Fuel 1991, 70, 181 Nishioka, K. and Yoshida, S. Tetsu-to-Hagane (J. Iron Steel Inst. Japan) 1984, 70, 343 Yoshida, S. and Nishioka, K. Coke Circ. 1985, 34, 114 Mayers, M. A. In 'Chemistry of Coal Utilization' (Ed H. H. Lowry), Vol. 1, Wiley, New York, 1945, pp. 863-895 Ishikawa, Y., Kase, M., Abe, Y., Ono, K., Sugata, M. and Nishi, T. AIME Ironmaking Conf. Proc. 1983, 42, 357 Shiraishi, K., Yamaguchi, T., Nishi, T. and Arima, T. Coke Circ. 1981, 30, 239 Nomura, S., Shiraishi, K., Arima, T. and Aramaki, T. Coke Circ. 1992, 41, 25 British Standards Institution. 'Determination of Swelling Properties Using a Dilatometer', BS 1016: Section 107.3:1990 Mahajan, O. P. and Walker, P. L., Jr. In 'Analytical Methods for Coal and Coal Products' (Ed. C. Karr, Jr), Vol. 1, Academic Press, New York, 1978, pp. 125-162 Gransden, J. F., Price, J. T. and Khan, M. A. AIME Ironmaking Conf. Proc. 1988, 47, 155 Patrick, J. W. and Walker, A. In Proceedings of the 4th Conference on Carbon, Baden Baden, 1986, p. 156 Patrick, J. W. and Stacey, A. E. Trans. Iron Steel Soc. AIME 1983, 3, 1
0016-2361(95)00238-3
FuelVol. 75 No. 2, pp. 187-194, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96/$15.00 + 0.00
The effect of swelling pressure during coal carbonization on coke porosity Seiji Nomura and K. M a r k Thomas* Nippon Steel Corporation, Ironmaking Process Laboratory, Process TechnologyResearch Laboratories, 20-1, Shintomi, Futtsu, Chiba 293, Japan *Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK (Received 7 December 1994; revised 23 June 1995) In cokemaking, the generation of swelling pressure is an important consideration since excessive pressures result in problems with pushing the coke and in extreme cases, damage to coke ovens may occur. In this investigation, the effect of swelling pressure generated during coal carbonization in coke ovens on coke porosity was studied. The swelling pressure was measured using a movable-wall test oven. The gas pressure in the plastic layer and the swelling pressure were monitored during the carbonization of coals in the test coke ovens. The porous structure of the resultant coke was assessed. The coke macropore structure was characterized using an optical microscope coupled with an image analyser and the micropore structure was characterized using a volumetric gas adsorption method. A method to determine the extent to which the macropores were distorted under unit pressure during the thermoplastic stage was also developed. It was shown that the macropores were compressed against the coke oven wall by the gas pressure in the plastic layer, resulting in the distortion of their shape; the extent of distortion varied greatly with the type of coal. The results suggested that the macropore distortion depended not only on the swelling pressure but also on the amount of distortion per unit pressure during the thermoplastic stage. Furthermore, the characteristics of the coke pore structure from a dangerously swelling coal varied considerably over the oven width. As the oven centre, where the gas pressure was extremely high, was approached, the macroporosity and the mean macropore diameter decreased, while the microporosity increased. The changes in the macroporosity and microporosity over the oven width are discussed in relation to the gas pressure in the plastic layer and the swelling pressure developed during carbonization. (Keywords: coal; swelling pressure; coke pore structure)
In the cokemaking industry one of the most significant problems to be solved is how to avoid damage to coke oven walls caused by dangerously high coking pressure, or swelling pressure, which is the pressure imposed on the coke oven wall as a result of swelling of the coal during the carbonization process 1. Experience in the USA 2'3 and in Europe 4'5 has shown that coke oven walls can be badly damaged as a result of excessive swelling pressure caused by certain coals present in blends. These coals are described as dangerously swelling coals. Such damage to coke ovens during their working life must be avoided because it not only lowers the productivity but also necessitates costly replacement or major repairs of coke ovens. An understanding of the cause of the swelling pressure is essential to avoid oven wall damage. There have been a n u m b e r o f investigations into this 6-~3. It is k n o w n that the swelling pressure is due to the pressure of the gas in the plastic layer. During carbonization in the coke oven, as its temperature reaches 350°-400°C the coal begins to soften to form the plastic layer. The permeability of the plastic layer is 1"4 16 low and the volatiles released are trapped in the layer. The pressure in the layer therefore increases as
more gases are evolved. This gas pressure in the plastic layer is transmitted to the walls of the coke oven. To understand the phenomenon of swelling pressure in commercial coke ovens, a method of measuring swelling pressure on the laboratory scale was developed 17. The effects of coal characteristics and carbonization conditions, e.g. bulk density, particle size, flue temperature, moisture, on swelling pressure have been studied 7-11,13,18 and recently some prediction models for swelling pressure have been proposed 19'2°. On the other hand, very little research has been carried out on the effect of swelling pressure on the development of porosity during the carbonization of coal. Generally, coke pores are classified into three types21: macropores > 50 nm wide, mesopores 2 - 5 0 n m wide, and micropores < 2 nm wide. Recently Patrick e t al. 22 have reported a relation between the gas pressure generated in the plastic layer and the macroporosity of coke. Furthermore, it has been found that the macropores in the coke made in coke ovens are distorted or 'compressed' in the direction perpendicular to the oven wall and it has been suggested that this might be caused by the gas pressure in the plastic layer 23-26. However, the factors affecting the
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The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas extent to which the macropores are distorted or compressed are still unknown. Many studies have been carried out on the effect of heat treatment on the changes in microporosity during the carbonization of coals 27-33 and it is known that microporosity changes drastically as a function of heat treatment temperature; however, the relation between swelling pressure and microporosity has not been discussed. Since the pore structure is one of the most important factors determining the mechanical strength of coke 34'35 and its strength after reaction with CO236,37,which are considered by some workers to play significant roles in blast furnace operation 38'39, it is of importance to study the effect of swelling pressure on coke microporosity as well as macroporosity. The objectives of this study were to examine the effect of swelling pressure generated during coal carbonization in coke ovens on the macroporosity and microporosity of coke, to examine the factors determining the distortion and the compression of macropores during the thermoplastic stage, and to understand the characteristics of the porous structure of cokes prepared from dangerously swelling coals. To achieve these objectives, the macropore and micropore structures of cokes were studied in relation to measurements of the gas pressure in the plastic layer and the swelling pressure. Furthermore, a method for measuring the extent to which the macropores are distorted during the thermoplastic stage by unit external pressure was also developed and the amount of macropore distortion per unit pressure was investigated in relation to the effect of swelling pressure on the macropore structure.
EXPERIMENTAL
Coals used The characterization data for the coals used in this study are given in Table 1. Coal HP is of high swelling pressure (dangerously swelling), coal MP is of medium swelling pressure and coal LP is of low swelling pressure.
Gas pressure in plastic layer The coals were crushed to 85% < 3mm. The coals were charged into a steel box measuring 410mm high, 619 mm long and 425 mm wide. The box was put into an electrically heated small test coke oven with two heating walls 4° and the sample was carbonized. The moisture content of the coal was 3 wt% and the bulk density was 860 kg m -3 (db). The gas pressure in the plastic layer was measured by steel pipes with 1 mm i.d. 41, each of which was connected to an electrical manometer. The pipes were arranged in a horizontal plane 125 mm from the bottom of the box at five different places, 25, 50, 100 and 150mm from the wall and at the oven centre (i.e. 215.5mm from the wall). A thermocouple of l mm diameter was attached to each pipe so that the changes in the temperature as well as the pressure were measured. It is known that the gas pressure reaches a maximum when the plastic layer reaches the probe 6, therefore at each measuring point the pressure peak when the temperature of the probe reached the thermoplastic temperature range was defined as the gas pressure in the plastic layer.
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Table 1 Characterizationfor the coals used
Coal
HP
Proximate analysis (wt% db) Volatile matter Ash Caking and swellingproperties Dilatometry Total dilatation (%) Softening temperature (°C) Temperature of maximum contraction (°C) Resolidificationtemperature(°C) Plastometry Maximum fluidity(log d.d.p.m.) Softening temperature (°C) Maximum fluiditytemperature (°C) Resolidificationtemperature(°C) Petrographic analysis Reflectance(av.) (%) Maceral analysis (vol. % mmf) Vitrinite Inertinite
18.6 10.0
MP
LP
20.6 9.8
20.6 9.9
59 432
90 405
22 404
464 497
445 494
456 488
1.65 444 481 494
2.69 419 465 497
1.62 422 466 494
1.48
1.32
1.39
73.7 26.3
73.4 26.6
62.4 37.6
After carbonization, the coke was quenched to room temperature with nitrogen and taken out of the oven. These cokes were examined for macropore and micropore structure.
Swelling pressure The swelling pressure was measured using the movable-wall oven at Nippon Steel Corporation. This has a height of 600 ram, a length of 593 mm and a width of 400 mm 2°. The coals were crushed to 85% < 3 mm and charged through the charge hole at the top of the oven. The bulk density was 800 k g m -3 (db). The changes in swelling pressure imposed on the movable wall during carbonization were measured with a load cell mounted on the outside of the wall.
Macropore structure of coke Five samples were cut from the coke lump prepared by carbonization in the small test oven. The position of each cut corresponded to the position where the gas pressure was measured. To investigate the distortion of the macropores in the direction of the oven width by the
z
(oven heightt direction) ~
Y
x(oven width direction)
vertical cross-section s. . . .
ove~ wall
width part to w a l l width perpendicular to
wall
horizontal cross-section (x-y plane)
Figure I
Measurement of macropore structure of coke
i...~
The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas gas pressure in the plastic layer, as shown in Figure 1 both the plane perpendicular to the oven wall (the horizontal cross-section, x - y plane) and the plane parallel to the oven wall (the vertical cross-section, y - z plane) were studied. For the vertical cross-section, only three samples (25 and 100mm from the wall and at the oven centre) were studied. Coke samples were moulded in slow-gelling epoxy resin (Buehler resin SW). The moulded sample was evacuated for 30 min in a desiccator and kept overnight while the resin was hardening, and the sample was then polished. The surface of the polished sample was analysed with an optical microscope combined with an image analyser (Optomax V). The magnification of the objective lens was × 4. The pore area was measured for each pore and the number of pores was counted for each field. The small macropores of diameter 12 #m were not included in these measurements, since the resolution of the image analyser was 6#m. As shown in Figure 1, for the horizontal cross-section, the pore width parallel to the oven wall, which was defined as the longest projection of the pore in the axis parallel to the oven wall (y-axis), and the pore width perpendicular to the oven wall, which was defined as the longest projection of the pore in the axis perpendicular to the oven wall (x-axis), were measured for each pore. For the vertical cross-section, the pore widths y and z, which were defined as the longest projections of the pore in the y- and z-axes respectively, were measured for each pore. These measurements were used to calculate the following parameters: Macropore distortion ratio for the horizontal crosssection (x-y plane) = (width parallel to wall)/(width perpendicular to wall) Macropore distortion ratio for the vertical cross-section ( y - z plane) = (width y)/(width z) Macroporosity = (Epore area)/(field area) Mean macropore diameter for the horizontal crosssection (x-y plane) = (Epore area)/{[(Ewidth perpendicular to wall) + (Ewidth parallel to wall)]/2} The macropore distortion ratio for the horizontal crosssection shows to what extent the macropore is distorted as a result of being pressed against the plane parallel to the oven wall O'-z plane). The greater the macropore distortion ratio, the more the macropore is distorted. The macropore distortion ratio for the cross-section of a spherical pore is 1.0. The mean macropore diameter represents the average length of all horizontal and vertical scan lines that traverse the macropores in a field of view. Since coke is a very heterogeneous material, to get representative values the measurements were made on > 10 fields of view randomly spread over the polished block, and > 600 macropores were counted for each sample.
piston
0 0 0 O' 0 0 0
dilatometer retort macropores
o~...[ O~
l
1
heater
height
] ~ x t width
X
coal sample
Figure 2 Measurement of the amount of macropore distortion per unit pressure during the thermoplastic stage
1000
100
lO
~
•
- -&
. . . .
i
0
. ~
. . . . .
•
i
. . . . . . .
i
J
50 100 150 200 Distance from oven wall [mm]
Figure 3 Gas pressure in the plastic layer across the oven width: II, coal HP; O, coal MP; A, coal LP
oven. The coal samples were crushed to a particle size of < 1 mm and 0.4 g of the sample was put into the retort of a dilatometer 42 of 8.0 mm i.d. In this experiment the coal sample was carbonized as < I mm particles to simulate the phenomena in commercial coke ovens. The retort was placed in an electric furnace and heated at ] 3 K m i n - . At the desired temperature, the piston of the dilatometer, which had been suspended in the retort without touching the coal sample, was lowered slowly onto the coal sample, i.e. at this temperature the coal sample began to be compressed mechanically by the piston. Thereafter, the sample was carbonized under the 80 O.
-~60
= ¢0
P40 e-
~20
Macropore distortion per unit pressure during thermoplastic stage The extent to which the macropores were distorted during the thermoplastic stage under unit external pressure was measured in laboratory-scale experiments, to examine the factors affecting the distortion of the macropores generated during carbonization in the coke
0=" 0
M
5 10 Time [hrs.]
15
Figure 4 Changes in swelling pressure with time for carbonization in the movable-wall test oven: II, coal HP; O, coal MP; &, coal LP
Fuel 1996 Volume 75 Number 2
189
The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas pressure of the piston to 600°C, as shown in Figure 2, and kept at that temperature for 30 min before cooling. To examine the extent to which the macropores were distorted during the thermoplastic stage under constant pressure, all the experiments were carried out under a pressure of 20 kPa. The coke sample taken out of the retort was cut in half along the vertical cross-section (x-z plane in Figure 2) after moulding in the epoxy resin; it was then moulded again and polished so that the vertical cross-section (x-z plane) was examined using the optical microscope combined with the image analyser. The region near the top of the sample where the piston had touched was observed so as to obtain information about macropore distortion by the pressure of the piston. As shown in Figure 2, the pore width, which was defined as the longest projection of the pore in the x-axis parallel to the surface of the piston, and the pore height, which was defined as the longest projection of the pore in the z-axis perpendicular to the surface of the piston, were measured for each pore. The macropore distortion ratio per unit pressure, defined as (pore width)/(pore height) was obtained.
Micropore structure of coke The specific surface area of the coke samples was measured by a volumetric gas adsorption method43. Nitrogen was used as the adsorbate at 77 K and the adsorption data were analysed by the BET method43. The coke samples were cut from the lump prepared by carbonization in the small test coke oven, at the positions 25 and 100 mm from the wall and at the oven centre. The samples were crushed to a particle size of < 600 #m and a sample of 0.3 g was used in the experiment.
1.40
/\
.2 2 1.30 c 0
~1.20
P
0
°1.10
o
A"
0
A ......
1.00
i
i
i
50
100
150
200
Distance from oven wall [turn]
Figure 5 Macropore distortion ratio across the oven width for the horizontal cross-section (x-y plane). II, coal HP; O, coal MP; A, coal LP
1.20 0
~ 1.10 c
.9° 1.00 "0
0
,',0.90 2 o
0.80
i
i
i
50
100
150
200
Distance from oven wall [mrn]
Figure 6 Macropore distortion ratio across the oven width for the vertical cross-section (y-z plane), m, coal HP; O, coal MP; A, coal LP
RESULTS AND DISCUSSION The three coals chosen for this study had similar rank. The vitrinite reflectance covered the range 1.32-1.48% and the volatile matter was in the range 18.6-20.6 wt% (db). However, the carbonization characteristics in the coke oven were markedly different; in particular, the swelling and gas pressures differed considerably. The generation of gas and swelling pressures will affect the development of porosity in the coal. The generation of pressure may compress the macropores, leading to a loss of macroporosity. Also, since the pressure is applied in one direction, the pressure may distort the shape of the macropores. These two aspects, i.e. the compression of the macropores and the distortion of the macropore shape, have been investigated on the three coals using a movable wall oven and also by simulating the pressure observed in the coke ovens in laboratory-scale experiments using pressures applied by mechanical methods.
Gas pressure and swelling pressure Figure 3 shows the gas pressure changes across the oven width. The gas pressure generated in the carbonization of coal LP in the test oven is less than that of coal MP. The gas pressure generated in the carbonization of coal LP decreases slightly as the oven centre is approached, while that of coal MP increases. The gas pressure generated in the carbonization of coal HP is similar to those of the other two coals near the oven wall, but increases steeply as the oven centre is approached
190
Fuel 1996 Volume 75 Number 2
and is much higher at the oven centre than for the other two coals. Figure 4 shows the changes in swelling pressure with time. The swelling pressure of coal HP increases as the carbonization proceeds and goes through a maximum when the two plastic layers meet in the middle of the coke oven. The swelling pressure of coal MP increases slightly in the initial stage of carbonization, after which it remains constant virtually until the plastic layers meet at the centre of the oven, producing a sharp rise in pressure. The swelling pressure of coal LP is much lower than that of the other two coals and no peak is observed. It is clear that the swelling pressures and the gas pressure in the plastic layer of these three coals differ considerably. The phenomenon of the swelling pressure versus time graph reaching a maximum has been reported previously8'11'44. This swelling pressure peak appears when the two plastic layers travelling from both oven walls meet at the oven centre. When the plastic layers which join together at the oven centre change into coke, the swelling pressure decreases to zero. Considering that the plastic layer proceeds from the oven wall to the oven centre as carbonization proceeds, the pattern of swelling pressure resembles that of gas pressure. This can be explained by the theory that the swelling pressure generated during carbonization in the coke oven is caused by the gas pressure in the plastic layer6-13.
The effect of swelling pressure during carbonization: S. Nomura and K. M. Thomas D&tortion of the macropores in the coke prepared & the test oven Figure 5 shows the macropore distortion ratio across the oven width for the horizontal cross-section. The ratio varies greatly with the type of the coal and lies between 1.05 and 1.40. Figure 6 shows the macropore distortion ratio across the oven width for the vertical cross-section. This ratio is close to 1.0 and changes little with distance from the oven wall or with coal type. It should be noted that in the case of the horizontal cross-section, the macropores are distorted in the direction perpendicular to the oven wall and the distortion ratio differs greatly for each coal, while in the case of the vertical cross-section, it is constant, i.e. ,,~ 1.0, in spite of different coals with different swelling pressures being used. This result suggests that there is a force perpendicular to the oven wall to compress the macropores against the oven wall resulting in the distortion of the macropore shape and there is little or no force parallel to the oven wall to compress the macropores. The gas pressure in the plastic layer is considered as the only force which operates in the direction perpendicular to the oven wall. Therefore, this result supports the theory that macropores developing in the plastic layer in the coke oven are compressed against the oven wall and distorted by the gas pressure, as implied by some previous studies23-26. Furthermore it should be noted that the macropore distortion ratio for the horizontal cross-section of coke LP, which is between 1.05 and 1.17, is lower than that of coke MP (1.29-1.40) and coke HP (1.17-1.29). The fact that the macropore shape of the coal of low swelling pressure is distorted to a smaller extent suggests that the extent to which the macropore shape is distorted depends on the swelling pressure. However, the difference in the macropore distortion ratio for the horizontal crosssection cannot be explained by the difference in the gas pressure in the plastic layer alone. This is shown by the observation that although near the oven centre the gas pressure in the plastic layer of coal MP is lower than that of coal HP, the macropores of coke MP are distorted to a greater extent than those of coke HP. This leads to the hypothesis that the amount of the macropore distortion per unit pressure varies with the type of coal. The extent to which the macropores are distorted during the carbonization of coals in the test oven is considered to depend both on the gas pressure in the plastic layer and on the amount of macropore distortion per unit pressure; this aspect is discussed later. It has been shown45 that macropore shape affects the tensile strength of coke. Therefore, as suggested previously25, it is expected that mechanical properties of coke such as the tensile strength and Young's modulus differ between the directions parallel and perpendicular to the oven wall. Macropore distortion per unit pressure during thermoplastic stage To assess the stages of pore distortion, laboratory experiments were carried out involving the introduction of external pressure during the thermoplastic phase. Figure 7 shows the relation between the temperature at which the external mechanical pressure was applied to the coal by the piston and the macropore distortion ratio per unit pressure defned previously. The macropore
1.40 Q.
o ..= P Z 1.30 A--.._
o ~ 1.20
e"
--..
~. I
2 1.00 0
i
i
i
i
100
200
300
400
500
Temperature at which pressure was applied [°C]
Figure 7 Changes in macropore distortion ratio per unit pressure with temperature at which external mechanical pressure was applied to the coal. B, coal HP; Q, coal MP; &, coal LP
distortion ratio per unit pressure changes little up to 300°C. It starts to increase a little in the range 300400°C. Thereafter it goes through a maximum in the range 400-470°C, which is the thermoplastic temperature range of the coals, and decreases sharply to ~ 1.0 in the range 470-500°C, which is the resolidification temperature of the coals. This decrease in the macropore distortion ratio per unit pressure near the resolidification temperature means that the macropores are distorted to a smaller extent as the resolidification temperature is approached and that the shape of the macropores does not change beyond the resolidification temperature, because the coal sample loses its thermoplasticity. The coals have different temperatures at which the macropore distortion ratio per unit pressure reaches a maximum, and coal HP reaches a maximum at the highest temperature. It should be noted that the macropore distortion per unit pressure varied with the type of coal. The maximum macropore distortion ratio per unit pressure of coal HP (1.17 at 470°C) is lower than that of coal MP (1.30 at 425°C) and coal LP (1.30 at 400°C), which means that the macropores developed during the carbonization of coal HP are distorted in the thermoplastic stage to a smaller extent than those for coals MP and LP under unit pressure. It is considered that macropores in highly viscous material such as the plastic mass of coal HP are distorted to a smaller extent under unit pressure. Since the shape of the macropores does not change beyond the resolidification temperature, the distortion of macropores is considered to take place only in the plastic layer. There are two possible stages in which the macropores are distorted in the plastic layer during the carbonization process in the coke oven. The first is the initial stage of the plastic layer development, where the coal starts to soften and macropores begin to develop; the second is the latter stage of the plastic layer, just before the coal starts to resolidify. In the initial stage, the swelling of the macropores is restricted by the coal mass on one side and the adjacent slightly hotter plastic layer on the other side, which might result in the generation of distorted macropores. However, the uncarbonized coal layer is considered to be less rigid than the semicoke layer and may be compressed to some extent by external pressure. Therefore the macropore distortion is expected
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The effect of swelling pressure during carbonization. S. Nomura and K. M. Thomas to be lower, since some expansion into the voids in the coal bed is possible. On the other hand, in the latter stage the macropores are considered to be pressed further against the rigid semicoke by the pressure caused by the swelling of the adjacent slightly cooler plastic layer, resulting in distortion of the macropores. This conjecture is supported by the suggestion made previously26 that the size of the macropores decreases in the final stage of the plastic layer partly because they are compressed and compacted. It is interesting to compare the macropore distortion ratio for the horizontal cross-section of the coke prepared by carbonization in the test oven with the macropore distortion ratio per unit pressure obtained in the laboratory experiments, from the viewpoint of the effect of the gas pressure in the plastic layer and the macropore distortion per unit pressure on the macropore shape. In the case of coke prepared by carbonization in the test oven, it was observed that the macropore distortion ratio for the horizontal cross-section of coke LP was much lower than that of coke MP, but the results obtained here have shown that under the same pressure the macropores generated in coke LP are distorted to the same extent as those in coke MP. This suggests that the difference in the macropore distortion ratio for the horizontal cross-section between cokes MP and LP in the case of coke prepared in the test oven can be ascribed to the difference in pressure, i.e. the gas pressure in the plastic layer. Furthermore, in the case of the coke prepared in the test oven, it was observed that the macropores in coke MP were distorted to a larger extent than those in coke HP in spite of the gas pressure of coal MP being smaller than that of coal HP. Since it has been shown that the macropore distortion ratio per unit pressure for coke MP is larger than that for coke HP, this observation can be rationalized by assuming that the macropore distortion ratio in coke prepared in the coke oven depends not only on the gas pressure in the plastic layer but also on the macropore distortion ratio per unit pressure during the thermoplastic stage. In the case of coke HP prepared by carbonization in the test oven, the amount ofmacropore distortion per unit pressure is so low that its high gas pressure in the plastic layer does not distort the macropore shape to such an extent as in the case of coke MP. Therefore from this experiment the factors determining the distortion of macropores during the thermoplastic stage are considered to be the gas pressure generated in the plastic layer and the amount of macropore distortion per unit pressure, which is related to the rate of release of volatiles in the thermoplastic phase, the permeability of the plastic layer and the thermoplasticity. The macropore distortion ratio for the horizontal cross-section of coke MP prepared in the test oven (average 1.34) is close to the maximum macropore distortion ratio per unit pressure for coke MP obtained in this experiment (1.30 at 425°C). This result suggests that the process of distortion of macropores in the test oven under the gas pressure in the plastic layer is quite similar to that occurring in the dilatometer retort under the pressure of the piston. In fact, the pressure imposed on the coal with the piston, 20 kPa, is as high as the gas pressure in the plastic layer, 10-15 kPa. The macropore distortion ratio for the horizontal cross-section of coke LP prepared by carbonization in the test coke oven
192
Fuel 1996 Volume 75 Number 2
140 --t
~130 ...... E 120 "O
.¢ 110 O O. O
~100 M E "
90
8O
50
100
150
200
Distance from oven wall [mm]
Figure 8 Mean macropore diameter across the oven width for the horizontal cross-section (x-y plane). II, coal HP; 0, coal MP; A, coal LP
65
60
E ~55 2 O
t~
45
40
0
r
n
50
100
150
200
Distance from oven wall [mm]
Figure 9 Macroporosity across the oven width for the horizontal cross-section (x-y plane). II, coal HP; 0, coal MP; A, coal LP
(average 1.11) is lower than the maximum macropore distortion ratio per unit pressure for coke LP obtained in this experiment (1.30 at 400°C). This is considered to be because the gas pressure in the plastic layer generated during the carbonization is lower than the pressure provided mechanically by the piston in the dilatometer retort. The macropore distortion ratio for the horizontal cross-section of coke HP prepared by carbonization in the test coke oven (average 1.24) is higher than the maximum macropore distortion ratio per unit pressure for coke HP obtained in this experiment (1.17 at 470°C). This is considered to be because the gas pressure in the plastic layer is higher than the pressure provided mechanically by the piston.
Size of macropores and macroporosity Figure 8 shows the changes in the mean macropore diameter with the distance from oven wall for the horizontal cross-section. The mean macropore diameters of cokes LP and MP change little across the oven width and the mean macropore diameter of coke LP is nearly equal to that of coke MP. On the other hand, in the case of coke HP, the mean macropore diameter decreases markedly as the oven centre is approached. Furthermore, the mean macropore diameter of coke HP near the oven centre is smaller than that of cokes LP and MP.
The effect of swelling pressure during carbonization: S. Nomura and K. M. Thomas
Figure 9 shows the changes in the macroporosity measured on the horizontal cross-section across the oven width. The macroporosity of coke LP decreases slightly as the oven centre is approached, while that of coke MP increases slightly. On the other hand, the macroporosity of coke HP decreases markedly as the oven centre is approached, from 57% near the oven wall to 43% near the oven centre. Near the oven wall the macroporosity of coke HP is higher than that of cokes MP and LP, but near the oven centre it is lower than that of cokes MP and LP. This remarkable decrease in the mean macropore diameter and in the macroporosity of coke HP across the oven width could be explained by reference to the increase in the gas pressure during the formation of coke HP. When the plastic layer is situated near the oven wall, many small macropores with high pressure are formed in the plastic layer at first. These pores are unlikely to coalesce, owing to the high viscosity of the plastic mass of coal HP. Instead of coalescing, however, they could expand and finally reach a similar size to the macropores in coals MP and LP, because at this stage there exists an uncarbonized coal layer in the oven centre, which can be compressed and provides voids for the plastic coal mass to expand. On the other hand, when the plastic layer is situated near the oven centre, macropores with high pressure formed in the plastic layer cannot expand to the same extent, since no uncarbonized coal layer exists and the plastic layer is sandwiched by the rigid semicoke layers, which cannot be compressed and do not provide space for the plastic mass to expand. Therefore the gas pressure in the plastic layer remains high and the size of the macropores and the macroporosity decrease. Furthermore, it has been shown that the size of the macropores decreases after passing through a maximum in the plastic layer, and this is partly ascribed to the compression and compaction of the macropores 26. In the case of coal HP, as shown in Figure 5, the macropore distortion ratio for the horizontal cross-section changes little with the distance from the wall, while the gas pressure in the plastic layer increases markedly and both the size of the macropores and the macroporosity decrease considerably. Therefore it is reasonable to say that the changes in the macroporosity and the macropore size are modified by the gas pressure in the plastic layer. It is considered that the extremely high gas pressure in the plastic layer generated during the carbonization of coal HP in the test oven compresses the macropores, leading to the loss of macroporosity and decrease in macropore size but the low fluidity developed during the thermoplastic phase prevents distortion of the pores. Considering that the tensile strength increases exponentially with decrease in macroporosity46 , in the case of dangerously swelling coals such as HP, the large change in the macroporosity from the oven wall to the oven centre implies that the strength of the coke near the oven wall is quite different from that of the coke near the oven centre. Since one of the most important aims in commercial cokemaking is to supply blast furnaces with coke having a uniform quality, it is considered that dangerously swelling coals not only damage the oven wall but also are detrimental to control of coke quality. Microporosity Figure 10 shows the changes in the specific surface area across the oven width. The surface area of coke MP
E tg ~p 0
,e-
o k~
.g Q.
ffl 0
i
i
L
50
100
150
200
Distance from oven wall [ram]
Figure 10 Specific surface area across the oven width. I1, coal HP; O, coal MP; A, coal LP
increases slightly from 1.4 to 1.9 m 2 g-~ as the oven centre is approached, while that of coke LP decreases slightly from 2.0 to 1.9 m 2 g-l. On the other hand, the surface area of coke HP increases markedly from 1.6m 2 g-I near the oven wall to 3.0m 2 g-1 near the oven centre. The changes in microporosity across the oven width can be explained by considering the changes in the macroporosity in relation to the swelling pressure. In the previous section, it has been shown that the mean macropore diameter and the macroporosity of coke HP decrease drastically as the oven centre is approached. Therefore, as far as the characteristics of the pore structure of coke from dangerously swelling coal are concerned, it is reasonable to say that the microporosity increases while the macroporosity decreases as the oven centre is approached and the gas pressure in the plastic layer increases. It is apparent that the macroporosity and the microporosity across the oven width are modified by the gas pressure in the plastic layer. CONCLUSIONS A study of the effect of swelling pressure generated during coal carbonization in coke ovens on coke porosity has shown that the macropores are compressed against the coke oven wall by the swelling pressure, resulting in modification of the size of the macropores and their distortion. The distortion of the macropores occurs in the thermoplastic phase during the carbonization of coals in a coke oven. The extent to which the macropores are distorted appears to depend not only on the swelling pressure but also on the amount of macropore distortion per unit pressure during the thermoplastic stage. In the coke prepared from coal of high swelling pressure in the test oven, the macropore distortion was less than that for a medium-swelling coal, whereas the macroporosity was much reduced in the regions where the gas pressure in the plastic layer and the swelling pressure were greatest. The macropore distortion per unit pressure during the thermoplastic stage for the highswelling coal was the lowest. As the oven centre, where the gas pressure was extremely high, was approached, the macroporosity and the mean macropore diameter decreased, while the microporosity increased. The macroporosity and the microporosity along the oven width are modified by the gas pressure in the plastic layer
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The effect of swelling pressure during carbonization." S. Nomura and K. M. Thomas
and the resulting swelling pressure developed during carbonization in the coke oven. ACKNOWLEDGEMENTS S.N. is grateful to N i p p o n Steel C o r p o r a t i o n for f i n a n c i a l s u p p o r t to u n d e r t a k e this s t u d y as a p o s t g r a d u a t e s t u d e n t in the N o r t h e r n C a r b o n R e s e a r c h L a b o r a t o r i e s o f the University of Newcastle upon Tyne. The authors would like to t h a n k N i p p o n Steel C o r p o r a t i o n for p r o v i s i o n o f the coals a n d p e r m i s s i o n to p u b l i s h this p a p e r .
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