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Energy & Fuels 1997, 11, 978-981
Studies on Generation of Excessive Coking Pressure. 1. Semicoke Contraction versus Thermoplastic Properties of Coals Ramon Alvarez, Jose J. Pis, and Maria A. Diez Instituto National del Carbon INCAR, C.S.I.C, Apartado 73, Oviedo 33080, Spain
Anna Marzec* and Sylwia Czajkowska Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-102 Gliwice, Poland Received December 18, 1996X
There are two views on the mechanism of generation by some coals of excessive coking pressure and on laboratory testing methods for coking coals. According to one of them, the excessive pressure is generated in a coal thermoplastic layer if its viscosity is too high. Another view is that some properties of a semicoke layer are responsible for generating high pressure. The two layers, i.e., semicoke and plastic layers, follow each other when they move from coke oven wall to the oven center plane during the coking process. The aim of our work was to seek an answer to whether thermoplastic properties of coals and volume contraction (or expansion) observed for their semicoke layers are interrelated. The following characteristics were measured for a set of 42 coals: (i) Gieseler thermoplastic properties determined in two independent laboratories using two different standard methods and (ii) contraction values of carbonized coals by the KoppersINCAR test. In general, rather diffused or no relationships were found between the contraction on one hand and the thermoplastic properties on the other. It is concluded that the above two views might be synthesized on the condition that causative links could be found between more specific characteristics of thermal decomposition during coal plastic state and contraction of semicoke layer. This issue is a subject of part 2 of the paper.
Introduction on1-7
for some years that A debate has been going addresses two questions. (1) What is the nature of physical and chemical phenomena that are responsible for high coking pressure observed when some coals are used for coke production? (2) What laboratory method could be used for testing coking coals with the aim of differentiating the coals that do not produce excessive coking pressure (i.e., “safe” coals) from others that would generate such pressure (i.e., “dangerous” coals) during industrial coking? Coking pressure is often believed to be generated in the coal plastic layer.1,4 More specifically, it is asssumed that volatile species generated on heating coal are entrapped as bubbles inside the plastic layer, which is highly viscous in some coals and shows too low permeability for vapors. However, the view has not been supported by direct experimental data. There is no * Corresponding author. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Coking Pressure Seminar. Cokemaking Int. 1992, 4, 1-40. (2) Marzec, A.; Alvarez, R.; Casal, D. M.; Schulten, H.-R. Energy Fuels 1995, 9, 834-840. (3) Barriocanal, C.; Patrick, J. W.; Walker, A.; Walker, A. R. The Identification of Dangerously Coking Coals. In Coal Science; Pajares, J. A., Tascon, J. M. D., Eds; Elsevier: Amsterdam, 1995; Vol. 1, pp 989-992. (4) Koch, A.; Gruber, R.; Cagniant, D.; Krzton, A.; Duchene, J. M. Fuel Process. Technol. 1995, 45, 135-153. (5) Steyls, D. Cokemaking Int. 1992, 4, 31-33. (6) Geny, J. F.; Duchene, J.-M.; Isler, D.; Yax, E. Proc. Ironmaking Conf. Proc. 1991, 169-175. (7) Alvarez, R.; Pis, J. J.; Barriocanal, C.; Sirgado, M. Cokemaking Int. 1992, 4, 16-18.
S0887-0624(96)00226-5 CCC: $14.00
evidence that coking pressure of coals increases with an increase of their thermoplastic layer viscosity. No experimental data were found that would indicate a decreasing trend between permeability of the plastic layer and the coking pressure. However, some experiments showed that plastic layer permeabilities of safe coals are definitely lower compared with the permeabilities found for dangerous coals.4 Moreover, other observations2,5,6 make questionable the concept of a coal plastic layer as a source of excessive coking pressure and imply that in dangerous coals such pressure is generated in a space between the semicoke layer and the coke oven walls. The Spanish National Coal Institute (INCAR) has developed and patented a laboratory method for measuring the expansion and contraction behavior of coking coals. The test is known as the Koppers-INCAR test.7-10 The results of the test application were confirmed by coking experiments carried out on a semiindustrial scale.7,11 The results of the test application for numerous coals were summarized: coals giving a contraction (shrinkage) greater than 10 mm are not dangerous during coking.7,12 The test refers to a characteristic feature of the semicoke layer that is (8) Alvarez, R.; Pis, J.; Barriocanal, C.; Lazzaro, M. Cokemaking Int. 1991, 1, 37-42. (9) Escudero, J. B.; Alvarez, R. Fuel 1981, 60, 251-253. (10) Alvarez, R.; Pis, J. J.; Lorenzana, J. J. Fuel Process. Technol. 1990, 24, 91-97. (11) Alvarez, R.; Miyar, E. A.; Canga, C. S.; Pis, J. J. Fuel 1990, 69, 1511-1516. (12) ECSC Research Project 7220-EB/756 Final Report, 1995.
© 1997 American Chemical Society
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Energy & Fuels, Vol. 11, No. 5, 1997 979 Table 1. Properties of the Coals thermoplastic
coal symbol, origin 1
TS 2
F(max) ddpm 3
F, Spain Pe, USA NP, Australia Pi, USA W, USA EG, USA A, USA S, Australia Pd, Australia CR, Canada R, Australia Po, Poland Wtz, USA M, USA L, Canada
438 430 448 449 434 441 405 426 421 409 406 407 395 387 406
21 78 6 10 61 48 1120 84 117 205 639 599 6198 10233 646
TF(max) 4
TR 5
481 475 476 483 478 480 466 474 468 459 455 458 442 441 452
504 506 498 507 505 507 504 500 497 495 489 490 480 479 486
TR - TS 6
Koppers-INCAR test, mm 7
VM wt % daf 8
ash wt % db 9
S wt % db 10
66 76 50 58 71 66 99 74 76 86 83 83 85 92 80
+8 -1 -2 -4 -5 -7 -10 -12 -16 -20 -20 -20 -25 -26 -28
20.1 19.7 19.1 17.5 19.6 18.9 23.6 20.9 23.8 25.4 25.7 29.5 35.2 36.3 30.5
8.0 4.8 9.6 4.7 7.9 8.3 4.6 9.7 9.6 9.5 9.9 6.6 7.2 6.8 9.0
1.0 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.4 0.6 0.9 0.3
mean reflectance Ro st dev 11 12 1.63 1.45 1.53 1.53 1.54 1.60 1.29 1.39 1.27 1.26 1.14 1.05 0.92 0.90 0.97
0.16 0.13 0.08 0.15 0.14 0.15 0.11 0.13 0.15 0.11 0.15 0.15 0.12 0.17 0.12
obtained by heating coal charge in the 650-820 °C temperature range. At the end of the test, the temperature is well over temperatures of coal thermoplasticity. One cannot exclude, however, the possibility that the contraction is influenced in a way by some phenomena occurring in the plastic state and preceding the semicoke formation. The objective of the study presented in part 1 is a systematic examination of contraction and Gieseler thermoplastic properties for a set of numerous coals with the aim of finding out the presence (or absence) of links between them. Experimental Section Coals. Forty two coals covering a wide variety of thermoplastic properties were studied. The set of coals includes 28 carboniferous coals from Poland, 7 coals from the USA, 4 from Australia, 2 from Canada, and 1 from Spain. Properties of the Polish coals such as ultimate, proximate, and maceral analyses were already reported (see Table 1 in ref 13 ). Characteristics of the other coals are included in the present paper (Table 1). Briefly, properties of the whole set of 42 coals are in the following ranges: 82-92% C daf; 17-42% volatile matter (VM) daf; 4-10% ash db (except a few Polish coals that contained higher ash content). Thermoplastic Properties of the Coals. The properties were determined using constant-torque Gieseler plastometers in two laboratories that applied two different standard methods. The details of measurements following the Polish standard method (PN62/04536) as well as the measurements results were already described.13 In the other laboratory (INCAR, Spain) the measurements were carried out according to ASTM 2639-89. The values of the thermoplastic properties determined with the use of the two standard methods are different, since some criteria and measures used by the standards are different. For example, one of them expresses fluidity in angle units, the other in dial division units. It is worth pointing out that mass spectrometric and chemometric studies were carried out on the Gieseler thermoplastic properties of the same Polish coals that are included in the present work. The studies revealed that temperatures of maximum fluidity and resolidification (TF(max) and TR) were significantly correlated to each other (r ) 0.97)13 and with individual composition of coal thermal degradation products identified with the use of pyrolysis field ionization mass spectrometry.14,15 (13) Marzec, A.; Czajkowska, S.; Moszynski, J. Energy Fuels 1992, 6, 97-103.
Figure 1. Contraction or expansion for 42 coals measured with the use of Koppers-INCAR method (see Experimental Section) versus Gieseler temperature of softening (ASTM 263989) of the coals. Volume change on heating the coal sample compared to the initial charge volume is recorded in millimeters (-mm for contraction, +mm for expansion). Contraction Measurements of Coals. The measurements were carried out with the use of the Koppers-INCAR test. A detailed description of the test can be found elsewhere.7,8 Briefly, 80 g of coal sample, ground to 485 °C; VM daf is from 30 to 17.5) is characterized by a steep TR-contraction relationship. The dissemination of points shown in the Figures 2 and 3 does not allow for a prediction of contraction values on the basis of TF(max) or TR measurement. However, when the two thermoplastic properties are used simultaneously, there is an opportunity of selecting coals that show a high degree of contraction (much greater than -10 mm) and thus are safe in coking. As Figure 4 indicates, this refers to coals that have TF(max) < 475 °C and TR < 495 °C. Contraction versus the Range of Thermoplasticity (TR - TS). The data in question are shown in Figure 5. As in the case of Figure 3, one may discriminate the same two families of coals. For one of them (contraction in the -20 to -30 mm range; VM daf > 30), there is no effect of the thermoplasticity range on contraction. For the lower volatile coals (below about 30% VM daf), there is a dispersed trend of a contraction increase upon an increase of thermoplasticity range. Figure 5 shows that there is no possibility of discriminating safe (contraction greater than -10 mm) and
Contraction versus Temperature of Softening (TS ). The data in question are shown in Figure 1. One can hardly say there is any correlation that would make it possible to predict contraction on the basis of TS . However, one might say that coals characterized by TS below 430 °C are safe, since their contractions are greater than -10 mm. And coals characterized by TS >440 °C should be expected to be dangerous, since their contractions are less than -10 mm. Contraction versus Temperature of Maximum Fluidity (TF(max)) and Temperature of Resolidification (TR). The data are presented in Figures 2 and 3. In general, the figures show a nonlinear trend: the
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Energy & Fuels, Vol. 11, No. 5, 1997 981
the intermediate expansion peak. In contrast, safe coals (final contraction greater than -10 mm) do not show any intermediate expansion peak. Summary of the Results. (1) No links have been found between the Koppers-INCAR contraction values on one hand and the temperatures of softening on the other (Figure 1). (2) An increase of the temperature of maximum fluidity has a negative effect on the degree of contraction (Figure 2). (3)Data referring to the Koppers-INCAR contractions versus the temperatures of resolidification (Figure 3), the thermoplasticity ranges (Figure 5), and the maximum fluidities (Figure 6) illustrate that the whole set of 42 coals could be devided into 2 coal families (subsets). (4) For the family of coals characterized by VM daf > about 30 wt %, no effect of the three Gieseler properties (TR; TR - TS; log Fmax ) on the contraction is observed. (5) For the second family (VM daf < about 30%) an enhancement of the contraction is observed (i) when TR decreases, (ii) when TR TS increases, and (iii) when Fmax increases. However, the three relationships are rather diffused and cannot be used for predicting contraction values. (6) Simultaneous use of TF(max) and TR measurements offers an opportunity of selecting coals characterized by highdegree contraction and thus safe coals in coking. (7) Koppers-INCAR curves for coals characterized by poor contraction (less than -10 mm), and thus are dangerous coals, show an intermediate expansion peak at a temperature about the resolidification point. Conclusions
Figure 7. Typical Koppers-INCAR curves for coals showing low contraction or expansion and high wall pressure (as an example, the curves are presented for coals F, Pe, and EG from the table) and for coals showing high contraction and low wall pressure (the curves for coals S and Po).
dangerous (contraction less than -10 mm) coals on the basis of their thermoplasticity range. Contraction versus Maximum Fluidity (Fmax). Figure 6 presents logarithmic values of maximum fluidity and the contractions. Once again, no correlation exists between them for the family of coals characterized by the contraction in the -20 to -30 mm range and VM daf > 30. For the second family of coals, a diffuse trend of the contraction increase upon a log Fmax increase can be observed. Figure 6 also shows that there is no possibility of discriminating safe and dangerous coals on the basis of maximum fluidity measurements. In addition, the inverses of Fmax (1/Fmax), which provide some measure of viscosity of the plastic layer, were calculated. No reliable relationship was found between the inverses and the contractions. Koppers-INCAR Curves: Relation between Intermediate Expansion and Final Contraction. Figure 7 shows some representative curves of selected coals. For coals classified as dangerous (final contraction less than -10 mm) the Koppers-INCAR curve always shows an intermediate expansion peak (see three upper curves in the figure) in the region corresponding to a final temperature of thermoplasticity. After that, a continuous decrease of the curve is observed until the final contraction is reached. It should be pointed out that the final contraction is always around 9 mm below
Some relationships found between the KoppersINCAR contractions on one hand and some Gieseler thermoplastic properties on the other indicate that phenomena occurring in a coal thermoplastic state may influence semicoke contraction and generation of excessive pressure in coke ovens. The same conclusion may be also derived from observations of Koppers-INCAR curves, which show an intermediate expansion peak generated in dangerous coals during the final stage of thermoplasticity. No correlation or diffused correlation (depending on the volatile matter content of the coals) between the Koppers-INCAR contraction values and the Gieseler maximum fluidities rules out the possibility that both properties (i.e., contraction and fluidity) can be strongly correlated with wall pressure. In other words, if the wall pressure of coals depends on their Koppers-INCAR contraction values, then the pressure cannot be strongly correlated with the Gieseler fluidity (or viscosity) of coals. Our experimental data do not include measurements of permeability of the plastic layer. However, such data can be found elsewhere (see Figure 10 in ref 4). The data contradict the view that low permeability of the plastic layer is a causative factor of high coking pressure. In summary, though there are some links between phenomena occurring in the plastic state of coals and contraction of their semicoke layer, nevertheless, there are no direct links between fluidity or permeability of the plastic layer and semicoke contraction. Acknowledgment. Financial support by ECSC Grant EUR 7220-EB 756 is gratefully acknowledged. EF960226G
