Internal Gas Pressure Characteristics Generated during Coal

The internal gas pressure and its corresponding temperature at the coal charge center were measured using the probe with thermocouple. Internal gas pr...
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Energy & Fuels 2001, 15, 618-623

Internal Gas Pressure Characteristics Generated during Coal Carbonization in a Coke Oven Woon-Jae Lee* and Yong-Kuk Lee Energy/Coal and Chemical Process Research Team, Research Institute of Industrial Science and Technology, Pohang, 790-330, Korea Received August 13, 1999. Revised Manuscript Received November 18, 2000

Coal carbonization for various kinds of single coals, coal blends, and operating conditions has been carried out in a movable-wall test coke oven (0.18 mW × 0.4 mH × 0.45 mL) to investigate the coking pressure behavior. The internal gas pressure and its corresponding temperature at the coal charge center were measured using the probe with thermocouple. Internal gas pressure of single coals increases with an increase of coking capacity and a decrease of volatile matter content. The internal gas pressure and the coke strength of coal blends mixed with single coals mainly depend on the blending ratio of a given coal in coal blends. With decreasing moisture content of coal blends, heating rate at the coal charge center decreases and internal gas pressure exponentially increases. Bulk density of coal blends exponentially increases with decreasing moisture content. Also, coke strength linearly increases with a decrease of coal moisture content. With increasing heating-wall temperature, internal gas pressure of coal blends increases, but coke strength decreases.

Introduction One of the most critical aspects encountered during the coking process is the development of high coking pressure in the industrial coke ovens. The force exerted by the charge on the oven walls should not exceed the limits of resistance of the refractory structure.1 The prediction and control of coking pressure in coke oven is extremely important, especially for aged coke batteries with deteriorated walls. In general, it is accepted that coking pressures originate from volatile matter released from the coal in the plastic stage.2 Furthermore, there is agreement that coking pressure depends both on the rate of gas evolution from the coal and the decrease of the gas permeability during the plastic stage,3 which offers resistance to the escape of the gases. When coals are carbonized in slot-type ovens, two principal layers of plastic coal are formed parallel to the oven walls and linked near the sole and the top of the charge by two secondary plastic layers.4 As carbonization proceeds, the plastic layers move progressively inward eventually meeting at the oven center. Usually, for some coals with volatile matter contents between 17 and 25% (daf), these processes are accompanied by the generation of a measurable gas pressure within the plastic layer which attains a maximum level when the two layers meet at the oven center.4,5 The pressure is * Author to whom correspondence should be addressed at Energy/ Coal & Chemical Research Team, RIST, #32 Hyoja-Dong, Nam-Ku, Pohang City 790-330 Kyungbuk, Korea. Fax: 82-54-279-6309. E-mail: [email protected]. (1) Monson, J. R. Cokemaking Int. 1992, 4, 3-4. (2) Koch, A.; Gruber, R.; Cagniant, D.; Pajack, J.; Krzton, A.; Duchene, J. M. Fuel Process. Technol. 1995, 45, 135-153. (3) Tucker, J.; Everitt, G. 2nd International Cakemaking Congress 1992, 40-61. (4) Loison, R.; Foch, P.; Boyer, A. Coke Quality and Production; Butterworth: London, 1989: pp 353-416.

transmitted to the oven walls and able to cause damage if sufficiently high. Though coals have the similar characteristics in terms of chemical analysis and swelling properties, coking pressure behavior can be quite different. The problem has lately become a matter of great importance, mainly associated with the introduction of the coal pretreatment technology and widespread acceptance of taller and wider ovens. These factors increase the bulk density of the coal charge, which is directly related to the coking pressure and to the risk of shortening coke oven life. Many test methods have been used for the evaluation of coals with regard to their expansion and contraction behavior in the coking process.6-9 The use of the movable-wall oven has been the most widely accepted and has also been fundamental in explaining the mechanism of development of coking pressure.10-14 Measurements of pressure inside the coal charge by means of probes introduced through doors or charging holes result in determination of internal gas pressure.3,4,15-19 (5) Latshaw, G. H.; McCollum, H. R.; Stanley, R. W. Ironmaking Proc. ISS-AIME 1984, 43, 373-380. (6) Nishioka, K.; Yoshida, S. J. Fuel Soc. Jpn. 1989, 68, 210-216. (7) Alvarez, R.; Miyar, E. A.; Canga, C. S.; Pis, J. J. Fuel 1990, 69, 1511-1516. (8) Marzec, A.; Alvarez, R.; Casal, D. M.; Schulten, H. R. Energy Fuels 1990, 9, 834-840. (9) Alvarez, R.; Pis, J. J.; Diez, M. A.; Marzec, A.; Czajkowska, S. Energy Fuels 1997, 11, 978-986. (10) Gransden, J. F.; Price, J. T.; Khan, M. A. Ironmaking Conf. Proc. 1988, 155-162. (11) Tucker, J.; Everitt, G. Ironmaking Conf. Proc. 1989, 599-617. (12) Benedict, L. G.; Thompson, R. R. Ironmaking Proc. ISS-AIME, 1976, 35, 276-288. (13) Lee, W. J.; Kim, J. Y. HWAHAK KONGHAK 1998, 36, 576583. (14) Geny, J. F.; Duchene, J. M. Cokemaking Int. 1992, 4, 21-25. (15) Nomura, S.; Thomas, K. M. Fuel 1996, 75, 801-808. (16) Barriocanal, C.; Hays, D.; Patrick, J. W.; Walker, A. Fuel 1998, 77, 729-733.

10.1021/ef990178a CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001

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Table 1. Analyses of Various Coals Used in Coal Blending coal analysis proximate analysisdb V.M F.C ash ultimate analysis C H N S Oa fluidity (LMF) total dilatation FSI RI calorific value (cal/g) a

CW

PD

GP

DP

MX

30.70 61.84 7.46 87.28 5.45 1.90 0.91 4.46 4.04 247 8.0 0.97 7884

19.98 70.37 9.65 80.80 4.46 2.02 0.62 12.10 2.53 93 8.5 1.24 7782

25.62 65.99 8.39 80.47 4.67 1.59 0.71 12.56 2.52 95 7.6 0.98 7775

33.85 57.19 8.96 75.74 4.84 1.56 0.41 17.45 3.39 92 4.1 0.7 7527

35.03 56.47 8.50 82.26 5.44 1.89 0.46 9.95 1.55 17 3.5 0.71 7435

Difference [100 - (C + H + N + S)]. Table 2. Blending Ratios of Various Coal Blends Composed of 5 Single Coals

Coal blend

CW (wt %)

PD (wt %)

GP (wt %)

DP (wt %)

MX (wt %)

Coal blend1 Coal blend2 Coal blend3 Coal blend4 Coal blend5 Coal blend6 Coal blend7 Coal blend8 Coal blend9 Coal blend10 Coal blend11 Coal blend12 Coal blend13 Coal blend14 Coal blend15 Coal blend16 Coal blend17 Coal blend18 Coal blend19 Coal blend20

5 10 15 20 11.7 10.8 10 9.2 11.4 10.7 10 9.4 10.5 10 9.5 8.9 10.6 10 9.4 8.8

42.2 40 37.8 35.6 30 35 40 45 45.7 42.9 40 37.1 42.1 40 37.9 35.8 42.2 40 37.6 35.3

31.7 30 28.3 26.7 35 32.5 30 27.5 20 25 30 35 31.6 30 28.4 26.9 31.8 30 28.2 26.5

5.3 5 4.7 4.4 5.8 5.4 5 4.5 5.7 5.3 5 4.6 5 10 15 5.4 5 4.8 4.4

15.8 15 14.2 13.3 17.5 16.3 15 13.8 17.2 16.1 15 13.9 15.8 15 14.2 13.4 10 15 20 25

In this study, coal carbonization in a movable-wall test coke oven (30 kg of coal) has been carried out to investigate the internal gas pressure behavior generated during the coking in the coke oven. The effects of blending ratio of coal, coal moisture content (4∼9 wt %), and heating-wall temperature (900∼1050 °C) on internal gas pressure at the coal charge center have been investigated. Experimental Section Coals Used. Five kinds of coals to prepare the coal blend were used. These coals are classified with volatile matter content and coking capacity. The ultimate and proximate analyses are shown in Table 1. The blending ratio of coal blends composed of single coals is shown in Table 2. The charging coal used to make the coke at the coke oven in POSCO was also studied. The charging coal was generally composed of 10 kinds of single coal. The size of coals used was 83 wt % less than 3 mm. Movable-Wall Test Coke Oven. Schematic diagram of movable-wall test coke oven that has the capacity of 30 kg of (17) Jordan, P.; Patrick, J. W.; Walker, A. Cokemaking Int. 1992, 4, 12-15. (18) Lindert, M. T.; van der Velden, B. Ironmaking Conf. Proc. 1994, 115-124. (19) Grimley, J. J.; Radley, C. E. Ironmaking Conf. Proc. 1995, 415420.

Figure 1. Schematic diagram of lab-scale movable-wall coke oven. (A) Test coke oven; 1. movable-wall, 2. coal charging port, 3. evolved gas exit pipe, 4. electric heater, 5. gas probe, 6. load cell, 7. thermocouple. (B) Details of gas probe. coal used in this study is shown in Figure 1. The test coke oven largely consists of three parts: oven, measuring system, evolved gas treatment system. The oven had a dimension of 0.18 mW × 0.45 mL × 0.4 mH, was fabricated by CA16 refractory and consisted of two parts: movable wall and remainder part. To heat the coal charged in the oven, an electric heater (SiC) was mounted in both oven walls (0.4 mH × 0.45 mL), as the similar heating method of the industrial coke oven. To prevent the electric heater from reacting with evolved gas generated from coal, the electric heater was put within aluminum tube. One of the two heating oven walls is movable to measure the force exerted on the wall during carbonization and to discharge the coke produced. The movable wall with wheels is fabricated to move on the rail. The charging hole (0.1 mW × 0.2 mL) was mounted on the oven roof to charge the coal into oven. Gas generated during carbonization was evolved through the pipe (50 A) mounted on the oven roof. To reduce the heat loss from the oven, the oven wall was insulated by ceramic wool. To measure the internal gas pressure within the charged coal, 1/4” SUS tube was used as probe. A probe has a slit-type hole (2 × 10 mm) near its closed tip and a 1/8” K-type thermocouple within it, as shown in Figure 1(B). The probes are horizontally placed, parallel to the oven walls. Three rows of three probes evenly located across the oven width are placed in a center and off-centers of the charge, at 0.09 m, 0.045 m from the oven wall, respectively. The gas pressure is continuously recorded with pressure transducer. A load cell (500 kg) was mounted on the movable wall to measure the force exerted on the wall during carbonization. The load cell is supported by a screw steel bar connected to the buttress. The data from load cell, pressure transducer, and thermocouple are stored in a personal computer through the data acquisition system. Evolved gas from the coal is passed into the primary tar separator (0.6 m dia. × 1 mH) filled with water and the

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Figure 2. Effect of coal rank on internal gas pressure. secondary tar separator sprayed the water through nozzle by circulation pump and then discharged into the atmosphere through I.D fan. The pressure in gas discharging pipe of the oven top is controlled about -2 to 0 mm H2O using the bypass valve fitted in the I.D fan. If the heating wall reaches the required temperature, a movable wall is pressed against the fixed wall by a screw steel bar. Water is sprayed in the secondary tar separator through a nozzle by operating the circulation pump. The I.D fan is on and pressure in the gas discharging pipe of the oven top is controlled about -2 to 0 mm H2O by a bypass valve. The weighted coal in the hopper is charged into the oven through the charging hole using the hoist. After charging the coal, gas pressure probes are inserted into the oven and data are stored in the PC. If the temperature of the coal charge center reaches approximately 850 °C, coal carbonization is finished and hot coke is discharged into the coke discharge box from the oven. Discharged hot cokes are extinguished using water. Coke Strength (DI15015). The drum test was used to assess the coke produced from the test coke oven. The drum index of coke strength was measured according to JIS:20 10 kg of >25 mm coke composed of size fraction ratio (+25, +38, +75, +100 mm) was rotated in a drum of 1500 mm dia. for 150 revolutions. The percentage by mass of coke remaining >15 mm sieve was defined as DI15015.

Results and Discussion The Effect of Single Coal and Coal Blending. The effect of coal rank on internal gas pressure at the coal charge center is shown in Figure 2. As can be seen, internal gas pressure increases with increasing coal rank. Coals with low volatile matter content in the similar reflectance index have shown higher internal gas pressure. Results obtained from the movable-wall test coke oven have been reported that between 18 and 25% volatile matter the risk is too great and between 25 and 28% the risk is moderate.4 Results of this study for internal gas pressure behavior seem to tend an agreement with other works. Alvarez et al. reported that coals of more than 24% volatile matter (daf) were not dangerous at normal bulk density.21 Generally, it is very difficult to forecast and assess the coking pressure of coal and coal blend using the conventional laboratory determinations such as volatile matter and so the danger of coking pressure of coal is determined by direct experiment with the movable-wall (20) Japan Industrial Standard 2151. (21) Alvarez, R.; Pis, J. J.; Lorenzana, J. J. Fuel Process. Technol. 1990, 24, 91-97.

Lee and Lee

Figure 3. Effect of blending ratio of CW coal on internal gas pressure and coke strength at 950 °C.

Figure 4. Effect of blending ratio of PD coal on internal gas pressure and coke strength at 950 °C.

coke oven. Various coal blends were prepared from the combination of the single coals to assess the internal gas pressure characteristics of coal blend as shown in Table 2. The effect of the blending ratio (5-20 wt %) of CW coal in coal blend on internal gas pressure and coke strength is shown in Figure 3. CW coal has low internal gas pressure (33 mmH2O) and increases the volatile matter of coal blend with increasing blending ratio of CW. As can be seen, internal gas pressure of the coal blend shows a drastic decrease in the initial stage with addition of CW. Addition of above 10 wt % of CW does not largely influence the internal gas pressure of the coal blend. This result is in good agreement with the other works that blending of coal with high volatile matter content can reduce the coking pressure of dangerous coking coal.4,12 The calculated internal gas pressure from the blending ratio is higher than the measured one at > 10 wt % of CW. Coke strength of coal blend tends to increase up to 10 wt % of CW and then decrease above 10 wt % of CW. The effect of the blending ratio (30-45 wt %) of PD coal in the coal blend on internal gas pressure and coke strength is shown in Figure 4. PD coal has low volatile matter content, the high coking property, and the highest internal gas pressure (167 mm H2O) among the coals used. As can be seen, internal gas pressure of coal blend is not consistent with increasing of blending ratio of PD. The calculated internal gas pressure from the

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Figure 5. Effect of blending ratio of GP coal on internal gas pressure and coke strength at 950 °C.

Figure 7. Effect of blending ratio of MX coal on internal gas pressure and coke strength at 950 °C.

Figure 6. Effect of blending ratio of DP coal on internal gas pressure and coke strength at 950 °C.

Figure 8. Typical profiles of the internal gas pressure and its corresponding temperature at the coal charge center of coal blend5 with moisture content at 950 °C.

blending ratio is much higher than the measured one and increases with increasing of blending ratio PD. Coke strength of coal blend increases with addition of PD due to improvement of coking property. The effect of blending ratio (20-35 wt %) of GP coal in coal blend on internal gas pressure and coke strength is shown in Figure 5. GP coal has the medium coking property and the internal gas pressure of 63 mm H2O. Volatile matter content of coal blend is nearly constant with variation of blending ratio of GP. As can be seen, with increasing blending ratio of GP, the measured internal gas pressure of coal blend increases, but the calculated internal gas pressure from the blending ratio decreases. Also the measured internal gas pressure is lower than the calculated one. In the case of increasing the blending ratio of GP in coal blend, the coking pressure of coal blend should be assessed by experiment in the test coke oven. Coke strength of coal blend tends to slightly increase with an increase of blending ratio of GP. The effect of blending ratio (0-15 wt %) of DP coal in the coal blend on internal gas pressure and coke strength is shown in Figure 6. DP coal has high volatile matter content, the internal gas pressure of 32 mm H2O, and is classified as weak-coking coal. As can be seen, internal gas pressure of coal blend steeply increases up to 10 wt % of DP and then decreases with increasing blending ratio of DP. The measured value is lower than the calculated one. The calculated internal gas pressure

decreases with increasing the blending ratio of DP. Coke strength is improved up to 5 wt % addition of DP, but tends to decrease with more addition of DP. The effect of blending ratio (10-25 wt %) of MX coal in coal blend on internal gas pressure and coke strength is shown in Figure 7. MX coal has very high volatile matter content, is classified as non-coking coal, and does not show the internal gas pressure as shown in Figure 2. As can be seen, internal gas pressure of coal blend slightly increases up to 20 wt % addition of MX and then drastically decreases with an increasing blending ratio of MX. The calculated internal gas pressure linearly decreases with increasing the blending ratio of DP and higher than the measured one. Coke strength linearly decreases with increasing the blending ratio of MX due to the reduction of the coking property. The Effect of Coal Moisture Content. The typical profiles of the internal gas pressure and its corresponding temperature at the coal charge center for coal blend5 with moisture content during coal carbonization at 950 °C are shown in Figure 8. As can be seen, the temperature at the charge center remains at 100 °C until the moisture has been evaporated before rising progressively. Pressure associated with steam at the initial coking stage for wet coal charge is not observed, but for dried coal charge (4.4 wt %) shows a high value. The temperature occurring at the maximum internal gas pressure is about 430-460 °C. Internal gas pressure

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Figure 9. Effect of coal moisture content on internal gas pressure and coke strength at 950 °C.

Figure 10. Effect of coal moisture content on bulk density.

drastically increases with decreasing moisture content due to the increase of evolved gas flow and the reduction of gas permeability in the plastic layer and the increase of simultaneous mergence of two plastic layers at the oven center by uniform heating. The heating rate at the coal charge center in the temperature range 200-500 °C decreases 4.5 °C/min, 4.7 °C/min, 3.7 °C/min, and 3.3 °C/min with decreasing moisture content due to the increase of bulk density. The effect of coal moisture content of coal blends on internal gas pressure and coke strength at 950 °C is shown in Figure 9. As can be seen, the internal gas pressure of coal blends exponentially increases with decreasing moisture content due to the increase of gas evolved and resistance of gas flow in the plastic layer by the increase of bulk density. In particular, the internal gas pressure shows much higher increase at low moisture content than at high one. The coke strength linearly increases with decreasing moisture content regardless of coal blends due to the increase of bulk density. The effect of coal moisture content on bulk density of various coal blends is shown in Figure 10. As can be seen, bulk density exponentially increases with decreasing moisture content due to the compaction of coal by the reduction of moisture on the coal surface, but nearly keeps constant at above 8 wt %. The increase of bulk density with decreasing moisture content shows similar trends regardless of the measuring methods and coal blends. Also, the bulk density of charging coal measured

Lee and Lee

Figure 11. Typical profiles of the internal gas pressure and its corresponding temperature at the coal charge center of coal blend5 with heating-wall temperature.

at shutter test with the dropping height of 1.8 m was slightly higher than that of the hopper test with the dropping height of 1 m due to the compaction by gravitation. Wakuri et al. reported that the bulk density measured in the industrial coke oven increased by 6.9% as coal moisture content was decreased from 9 to 5 wt %.22 Therefore, if the dried coal charging does not cause the dangerous coking pressure in the industrial coke oven, the reduction of moisture content of charging coal has the merits of the improvement of coke strength and the increase of coke productivity. The Effect of Heating-Wall Temperature. The typical profiles of internal gas pressure and corresponding temperature at the coal charge center for coal blend5 with heating-wall temperature during coal carbonization at nearly constant moisture content are shown in Figure 11. As can be seen, the temperature at the charge center remains at 100 °C until the moisture has been evaporated and after evaporating moisture linearly increases. The heating rate in the temperature range 200-500 °C increases 3.7 °C/min, 3.8 °C/min, 4.9 °C/ min, and 6 °C/min with increasing heating-wall temperature. Pressure associated with steam at the initial coking stage is not observed. Temperature occurring at the peak internal gas pressure is about 440-480 °C and slightly decreases with increasing heating-wall temperature due to the fast formation of semicoke. The internal gas pressure increases with an increase of heating-wall temperature due to the increase of gas evolution rate in plastic layer. The increase of internal gas pressure with heating-wall temperature can be explained as follows; an increase in heating-wall temperature increases the heating rate and consequently the flow of gas evolved in the plastic layer. However, it reduces the viscosity and thickness of the plastic layer. These effects act in opposite directions for internal gas pressure. Nevertheless, the overall known effect is that an increase in heating-wall temperature tends to increase the internal gas pressure.4 The effect of heating-wall temperature for coal blends on internal gas pressure and coke strength at nearly constant moisture content is shown in Figure 12. As can (22) Wakure, S.; Ohno, M.; Hosokawa, K.; Nakagawa, K.; Takanohashi, Y.; Ohnishi, T.; Kushioka, K.; Konno, Y. Trans. ISIJ 1985, 25, 1111-1115.

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strength of cell walls, fissure development, and size and distribution of microcrystallites.23 Conclusions

Figure 12. Effect of heating-wall temperature on internal gas pressure and coke strength.

be seen, the internal gas pressure of coal blends increases with increasing heating-wall temperature due to the interaction of gas evolution rate and gas permeability in plastic layer. Also coke strength of coal blends tends to slightly decrease with increasing heating-wall temperature. This may be attributed to the reduction of abrasion resistance of coke due to the strong contraction and the fissure development by rapid heating. In general, coke strength has been known to be associated with the various factors such as the extent of adhesion of the coal particles, pore size and its distribution,

1. Internal gas pressure for single coals increased with an increase of coking capacity and a decrease of volatile matter content. 2. Internal gas pressure and coke strength for coal blends mainly depended on the blending ratio of a given coal in coal blends. Internal gas pressure of coal blend decreased with an increase of CW, was nearly constant with PD, increased with GP, and showed the maximum value with DP and MX. 3. With decreasing moisture content of coal blends, heating rate at the coal charge center decreased and internal gas pressure exponentially increased. Bulk density of coal blends exponentially increased with decreasing moisture content. Also coke strength linearly increased with a decrease of moisture content. 4. With increasing heating-wall temperature, internal gas pressure for coal blends increased, but coke strength decreased. Acknowledgment. The authors gratefully acknowledge the financial support of POSCO. EF990178A (23) Amamoto, K. Fuel 1997, 76, 17-21, 133-136.