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Energy & Fuels 2005, 19, 1962-1970
Elastic and Optical Anisotropy of the Single-Coal Monolithic High-Temperature (HT) Carbonization Products Obtained on a Laboratory Scale Marta Krzesin´ska,*,†,‡ Sławomira Pusz,† and Andrzej Koszorek§ Institute of Coal Chemistry, Polish Academy of Sciences, Sowin´ skiego 5, 44-121 Gliwice, Poland, Institute of Physics, Academy of Jan Dlugosz, Armii Krajowej 13/15, 42-200 Cze¸ stochowa, Poland, and Department of Chemical Technology of Coal and Petroleum, Silesian Technical University, Krzywoustego 3, 44-100 Gliwice, Poland Received August 19, 2004. Revised Manuscript Received June 14, 2005
The aim of the present study was to investigate the directional dependences of the elastic and optical properties of monolithic single-coal high-temperature (HT) carbonization products obtained on a laboratory scale (with very slow heating rate) from coals of different caking propensity. Sixteen monolithic HT carbonization products, mainly cokes, were produced in the Jenkner retort furnace using 16 various types of coals of varying rank (from 83.1 wt % carbon to 98.3 wt % carbon) with a Roga index (RI) in the range of 0-76. Coals were carbonized in the form of monolithic blocks. The physical parameters such as true density, porosity, ultrasonic velocity, and dynamic elastic moduli, as well as optical reflectance parameters (Rmax, Rmin, Ram), were determined for the resultant products. The elastic and optical properties of the HT carbonization products were related to their porosity and the rank of the parent coals. It was determined that the HT carbonization products exhibit the different directional properties of the studied parameters, and they can be divided into three groups, with respect to the observed differences. The properties of these groups were related to the parent coal rank and the caking propensity (i.e., to the RI value). Anisotropy of the coke matrix structure was determined to be important for discussion about the anisotropic properties of cokes.
Introduction Coal carbonization processes are customarily regarded as low-temperature (LT) operations ( 1 are anisotropic. The higher the value of Emax/ Emin, the larger the elastic anisotropy. Optical Parameters. Optical texture and reflectance values of all samples studied were determined with a reflected light microscope (Axioskop MPM 200, Opton-Zeiss, Germany), using monochromatic polarized light (with a wavelength of λ ) 546 nm), with immersion oil. The maximum (Rmax) and minimum (Rmin) reflectance values were automatically measured for no less than 300 measurement points per sample, on randomly oriented grains of fused coke, on the lowest porous surface. Each measurement point was placed on various sample particles. An optical anisotropy parameter Ram was calculated using the Kilby method.36,37 The Ram value denotes a distance from the middle of the reflectance, indicating the surface classification chart-ternary diagram (RIS), and it is given in eq 4:
Ram ) xx2 + y2
(4)
where x and y are complicated functions of Rmax and Rmin.36 Optically isotropic material is described by a value of Ram ) 0. Materials with Ram > 0 are optically anisotropic. The higher the value of Ram, the larger the optical anisotropy of the material.
Results and Discussion Fourteen various bituminous coals of wide rank (83.1-93.1 wt % carbon) and two anthracites were carbonized at high temperature. Not all coals will produce coke, because not all coals produce a fluid phase. Coke is a highly carbonaceous product of the pyrolysis of organic material, at least parts of which have transitioned through a liquid or liquid-crystalline state during the carbonization process. As in the case of anthracites, the lowest-rank bituminous coals and lignites do not fuse. This is due to extensive covalent linkages within the parent coal, which prevent fusing. With increasing rank, the oxygen-dominated crosslinkages diminish to be replaced by hydrogen-bonding arrangements within the macromolecular networks. Hydrogen bonding is weaker than covalent bonding and the coals exhibit fluidity/plasticity on heating. With further increases in rank, this type of bonding is lost, with the development of aromaticity and carbon-carbon cross-linkages producing anthracites, which also have nonfusible structures.7 Thus, among the 16 HT carbonization products studied, 14 were expected to be cokes. The properties of the solid matrix and the porosity character of the HT carbonization products can be expected to affect their mechanical and optical properties (i.e., dynamic elastic moduli and optical reflectance).
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Figure 2. Plot of the bulk porosity of the HT carbonization products versus the rank of the parent coals. Solid line denotes the fitting of the experimental data (r ) 0.6).
Elastic and optical parameters of the HT carbonization products determined in this study were discussed in terms of their porosity, as well as of the parent coal properties (i.e., of coal rank and caking propensity (the RI value)). Coal Rank Dependence of Elasticity and Optical Reflectance of the HT Carbonization Products. Figure 2 shows the plot of the bulk porosity P (see eq 1) of the HT carbonization products versus the rank of the parent coals. The data are scattered (r ) 0.6), but it can be seen from this figure that the higher the rank of the raw coals, the lower the porosity of the resultant products. The most porous cokes are produced from lowrank coals. Panels a and b in Figure 3 show the plots of optical parameters (maximal reflectance Rmax and optical anisotropy Ram, respectively) of the HT carbonization products versus the rank of the parent coals. Cokes produced from coals with a carbon content of ∼90 wt % are attributed to a maximum value of Rmax. It is known that coals with a carbon content of ∼88-90 wt % exhibit the lowest degree of cross-linking,42-45 which correlates with the position of a maximum of the dependence Rmax(Cdaf) obtained for cokes (where Cdaf denotes the dry ash-free carbon content). The optical anisotropy Ram (Figure 3b) exhibits a similar course with a sharper maximum for a carbon content of ∼90 wt %. Thus, cokes produced from coals with =90 wt % carbon and a low degree of cross-linking have the greatest values of optical parameters Rmax and Ram. Figure 4 shows a plot of elastic parameters of the HT carbonization products related to rank of original coals. Figure 4a shows the dependence of Emax of both the HT carbonization products and the parent coals versus coal rank. The dependence of Emax on the carbon content of the coals has a minimum for carbon contents of =90 wt %. It can be seen that heat treatment mostly increases Emax for coals with a carbon content of =90 wt %. The elastic properties of low- and high-rank coals are almost (42) Larsen, J. W.; Kovac, J. In Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series No. 71; Washington, DC, 1978; pp 36-49. (43) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 199-282. (44) Sanada, Y. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 2000, 45, 216-220. (45) Krzesin´ska, M. Energy Fuels 2001, 15, 930-935.
Figure 3. Plot of optical parameters: (a) maximal reflectance (Rmax) and (b) optical anisotropy (Ram) of the HT carbonization products, each versus the rank of the parent coals (r ) 0.63).
the same after heating, even at temperatures as high as 1000 °C. Thus, the greatest change is observed for coals of the lowest cross-linking degree. Figure 4b shows that the elastic anisotropy Emax/Emin has a distinct minimum for carbon contents of =90 wt %. Cokes from coals with carbon contents of =87-92 wt % are almost isotropic (Emax ≈ Emin), i.e., elastic properties are independent of the direction of the material. Elastic parameters of cokes exhibit completely different behavior, with respect to the rank of the parent coals, than those of optical parameters. Porosity Dependences of Elasticity and Optical Reflectance. Figure 5a shows a plot of maximal dynamic elastic modulus of the HT carbonization products versus their porosity. The higher porosity, the lower the elastic modulus. With increasing porosity, many solid-state properties, as well as elastic properties, change drastically.46 Knudsen47 proposed, first on an empirical basis, and then on an analytical basis, an expression that is valid for any physical property; e.g., for the elastic modulus:
E ) E0 exp(-bP)
(5)
where E and E0 are the elastic moduli at porosity P and zero porosity, respectively, and b is a parameter that is determined by the porosity character. This expression has been widely used for porosity studies in the past for the lower- and higher-P region. An advantage of this (46) Czeremskoy, P. G.; Slezov, W. W.; Betechtin, W. I. In Pores in Solids (in Russ.); Energoatomizdat: Moscow, 1990; p 306. (47) Knudsen, F. P. J. Am. Ceram. Soc. 1959, 42, 376.
Anisotropy of Monolithic HT Carbonization Products
Figure 4. Plot of elastic parameters of the HT carbonization products related to the rank of the parent coals: (a) maximal dynamic elastic modulus (Emax) of both original coals (the Emax value for raw anthracite B was not determined) and the resultant products, versus coal rank; (b) elastic anisotropy of the HT carbonization products. Solid and dashed lines denote the fitting of the experimental data. Notation: (O) the HT carbonization products (r ) 0.9) and (b) coals (r ) 0.7).
exponential expression is that it provides a single parameter b, which can be correlated with the pore character and can be readily adapted for pore combinations via the weighted average of the b values. The value of b gives information about the shape of pores (see refs 20, 48, 49, and 50). Low b values (from 1.4 to 2.7) are associated with, respectively, cylindrical pores (aligned with the direction of measurement) and spherical pores in a matrix, both extending to very high levels of porosity. Low b values are consistent with expected elongated pores, whereas higher b values are consistent with denser particle packing. We fitted the experimental data using the Knudsen equation (eq 5). Figure 5a shows the results of the fitting for three different approaches. The data were fitted (i) for all samples studied, (ii) for products from bituminous coals (two anthracite products were excluded), and (iii) for cokes from coking coals (in addition to the products from the two anthracites, one nontypical coke from a coal of type 36 (Polish Classification), which exhibited extremely low coking yield, was excluded). The results of the fitting are presented in Table 3. The fitting parameters Emax0 and b that have been derived for all HT carbonization products and for the set of cokes from (48) Rice, R. W. J. Mater. Sci. 1996, 31, 1509. (49) Krzesin´ska, M.; Celzard, A.; Mareche, J. F.; Puricelli, S. J. Mater. Res. 2001, 16, 606-614. (50) Krzesin´ska, M. Mater. Chem. Phys. 2004, 87, 336-344.
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Figure 5. Plot of (a) maximal dynamic elastic modulus Emax and (b) maximal reflectance Rmax of the HT carbonization products, each versus their porosity. Lines denote the fitting of the experimental data: (s) coking coals (r ) 0.9), (- - -) bituminous coals (r ) 0.89), and (- ‚ -) all coals (r ) 0.95). Table 3. Parameters of the Fitting of the Experimental Data, Using the Knudsen Equation number of fitted points 16 14 13
type of parent coals
Emax0 (GPa)
b
correlation coefficient, r
all coals studied bituminous coals coking coals
30.80 23.35 31.16
3.27 2.73 3.28
0.95 0.89 0.90
coking coals are Emax0 ) 30.8 and 31.2, and b ) 3.27 and 3.28. Both parameters are slightly higher for coking coal products. The value of Emax0 is the elastic modulus of the solid matrix of the HT carbonization product. A clear dependence of Emax on porosity gives evidence that the elastic modulus is strongly dependent mainly on porosity. Thus, the elastic modulus of the HT carbonization product matrix is approximately the same: ∼31 GPa. The parameter b for cubic pores is known to vary from 2 to 4.6, depending on the angle between the stress and the basic directions of a pore cube (i.e., b ) 2 if the stress is perpendicular to the face of a pore cube, 3.3 if the stress is parallel to the diagonal of a pore cube face, and 4.6 for the case of stress parallel to the space diagonal of a pore cube.48-50 Our results show that the value of b is ∼3.3, which is characteristic for cubic pores of a material in which the stress is parallel to the diagonal of the cube face. This value of b indicates that pores in cokes are not elongated. For the metallurgical cokes, which were produced in large-scale (10- and 17ton) and smaller-scale test ovens, Patrick and Walker20 obtained the lower values of b, which varied over a range of 2.62-2.76. Figure 5b shows Rmax as a function
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Figure 6. Relationship between elastic anisotropy (Emax/Emin) and optical anisotropy (Ram) of the studied HT carbonization products. Notation: (b) group A, (0) group B, and (9) group C. Table 4. Elastic and Optical Anisotropy for Three Various Groups of the High-Temperature (HT) Carbonization Products Related to the Roga Index (RI) carbon content (%, daf)
Emax/Emin
Ram
RI
85.3 87.2 88.2 88.6 89.1 89.4 89.5 90.5 90.6 93.1
Group A 2.30 1.68 1.48 1.61 1.39 1.14 2.11 1.15 1.50 1.50
0.280 0.303 0.276 0.317 0.323 0.277 0.297 0.277 0.307 0.313
69 30 69 65 76 54 67 38 51 53
83.1 83.4 84.7 87.2
Group B 3.72 1.31 1.86 1.58
0.087 0.125 0.079 0.184
21 53 51 44
94.9 98.3
Group C 30.64 11.50
0.198 0.251
0 0
of porosity. No clear relationship between porosity and reflectance of the HT carbonization products was observed, which could mean that reflectance is indicative of the properties of a solid matrix of cokes, whereas elastic parameters reflect the bulk characteristics of a combination of the continuous matrix and the porosity of coke. Relationship between Elastic and Optical Anisotropies. Figure 6 shows the mutual relationships between elastic and optical anisotropies of the studied samples. The plot consists of three various groups of data that are dependent on the rank of the parent coals. They are denoted as group A (cokes from mid-rank coals), group B (cokes from low-rank coals), and group C (carbonization products from high-rank coals (anthracites)). Values of Emax/Emin and Ram related to the parameters of parent coals, i.e., carbon content and the RI that characterize these three groups, are presented in Table 4. Figures 7-9 show optical microscopy images of the HT carbonization products that are characteristic for the aforementioned three groups (A, B, and C). Figures 7a, 8a, and 9a describe the pore system (magnification of 80×), whereas Figures 7b, 8b, and 9b reflect the matrix structure (magnification of 500×).
Figure 7. Optical microscopy images of coke produced from the coal sample denoted JM/35 (from group A): (a) 80× magnification and (b) 500× magnification.
The most optically anisotropic cokes (Ram ≈ 0.280.32) attributed to group A are produced from mid-rank coals with the highest RI value. The elasticity of these cokes is almost independent of direction in a sample. It can be seen from Figure 7b that, indeed, the coke matrix has an anisotropic structure. However, Figure 7a shows that the pores are packed randomly and, therefore, the bulk elastic properties are almost isotropic. The elastic anisotropy parameter generally is ∼1.5, whereas a purely isotropic structure is described by parameter that is equal to 1. The most mechanically anisotropic HT carbonization products (elastic anisotropy of =12 and 31) (group C) were obtained from anthracites with RI ) 0. These samples have not passed through the fluid stage during the carbonization process. Figure 9 shows the structure of carbonization products from the coals from group C. These products exhibit very low porosity (
