Table 1. Properties of coals used.

Transcription

1 Hydrogen Transfer Reaction during Carbonization of Coal and Pitch* By Tetsuro YOKONO, * * Takeo UNO, * * * Toshi yuki OBARA * * * * and Yuzo SANADA** Synopsis The extent of hydrogen donor and acceptor ability of the respective coals and pitches was evaluated experimentally. The hydrogen donor ability (Da) o f coal depends on coal rank (reflectance) and correlates with the development of optical texture in cokes. The mild oxidations of coal and coal blends were also characterized with respect to hydrogen donor ability. The extent of anisotropic texture in cokes from co-carbonization of Akabira coal with various pitch systems was assessed by using hydrogen donor and acceptor ability of coals and pitches. Materials with a value of DMA produce fine grained mosaic texture I Mf l. I. Introduction It has been established that the optically anisotropic texture of carbonization products derived from coal and pitch is very useful for evaluating the metallurgical and petroleum. cokes. The size of the optical texture varies from less than 0.5 pm to about 500 im for starting material. During the carbonization reaction, the pyrolysis chemistry of starting materials is important in coke formation. The authors1 3) have previously reported that inter- and intramolecular hydrogen transfer reactions in pitch and heavy oil play an important role in determining the size and shape of the optical texture during the early stage of carbonization. It was suggested that the pitches which change to cokes with large units of optical texture exhibit a pronounced hydrogen donor ability. It is thought that the process of stabilization of the thermally induced radicals by hydrogen transfer is important for the development of optical texture in the carbonization reaction. Simultaneously, hydrogen acceptor ability of coals is also an important factor controlling the carbonization system. In this paper, the relation between the hydrogen donor ability of coal and its coking properties in singleand co-carbonization systems is examined in order to improve the understandings with the making of metallurgical coke. The extent of development of anisotropic texture in co-carbonization system is also discussed in terms of the hydrogen donor (Da) and acceptor (Aa) ability of coal and pitch. II. Experimental 1. Goal and Pitch Eight different ranks of coal and six types of pitches were selected as samples. The characteristics of the coals and the pitches used are shown in Tables 1 to 3. The coals were crushed to pass through 100 Tyler mesh and dried before use. 2. Measurement of Hydrogen Donor Ability and Hydrogen Acceptor Ability In order to evaluate the hydrogen donor ability [Da] of coal and pitch, anthracene was used as a hydrogen acceptor com.pound.4) Heat-treatment of coal or pitch with anthracene (1: 1 by weight) was carried out in a sealed glass tube with the rate of 10 K min-1 at various temperatures and for various soaking times. The CDCl3 soluble-fraction in the resultant products was examined by 1H-NMR. The amount of hydrogen transferred from the sample to anthracene was evaluated from the peak intensity appearing at the 9,10-protons (3.9 ppm) in 9,10-dihydroanthracene (9,10-DHA). A typical 1H-NMR spectrum of CDC13 Table 1. Properties of coals used. * ** *** **** This article was presented to the 17th Biennial Conference on Carbon, American Carbon Society and University of Kentucky, Lexington, June 1985, by the financial support of the Huga Fund. Manuscript received on July 25, 1985: accepted in the final form on November 6, ISIJ Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060. R & D Laboratories-III, Nippon Steel Corporation, Edamitsu, Yahatahigashi-ku, Kitakyushu 805. Faculty of Engineering, Tohoku University, Aramaki Aza Aoba, Sendai 980. (512)

2 Transactions ISIJ, Vol. 26, 1986 (513) Table 2. Properties of coal used. Table 3. Ultimate analysis of coals and pitches. Fig. 1. 1H-NMR spectrum of CDC13 soluble from products of the heat-treatment of coal with anthracene. Fig. 2. Diagram coal. of hydrogen transfer from 9, l0-dha to solution from heat-treatment of coal and anthracene is shown in Fig. 1. For quantitative measurements of Da, the intensity at 13,9 was calibrated by the intensity at 3.4 ppm due to 1,2 hydrogens of acenaphthene as an internal standard.* As for the evaluation of hydrogen acceptor ability [Aa] of coal and pitch, 9,10-dihydroanthracene was assessed as a hydrogen donor molecule (Fig. 2). Aa was evaluated from the intensity at 8.4 ppm (18.4) due to the 9,10 positions of anthracene generated in the resultant product. The peak appearing at 3.9 ppm is isolated far from the peaks due either to aromatic or aliphatic protons in the sample. The peak at 8.4 ppm sits in the range of chemical shift due to aromatic protons. However, the 8.4 ppm peak was easily separated and its intensity measurement did not interfere with the parent spectrum. The unit used for Da and Aa is (mg-h2/g-coal). 3. Gas Evolution during Heat-treatment Heat-treatment of coal with anthracene was carried out in a pyrex glass tube reactor sealed with a rubber cap. A vertical infrared image furnace was used to heat the reactor. After reaching the desired temperatures, a gas sampling syringe was inserted through the rubber cap and the hydrogen gas and methane that had evolved was measured by TCD gas chromatography. Values for hydrogen evolution shown in Figs. 4 and 5 represent hydrogen gas (H2 only) evolution. 4. Carbonization and Optical Microscopic Observation Single and co-carbonization reactions were carried out in a pyrex glass tube (diam. 10 mm and length 510 mm) under nitrogen gas flow. A vertical infrared image furnace was used to heat the reactor in which the sample was placed. The heating rate, the soaking temperature and the soaking period were 10 K/min, 823 K and 30 min, respectively. The resultant cokes were mounted in resin and the surfaces were polished in the usual way. Optical textures were assessed with a Nikon Apophoto optical microscope with polarized light. The optical textures of the cokes, classified according to Marsh et al.5j are listed in Table 4. Identifications of the optical textures was performed quantitatively by 400 pointscount analysis under microscope. III. Results and Discussion 1. Hydrogen Donor Ability (Da) and Gas Evolution of Coal Figure 3 shows the effect of reaction time on hydrogen donor ability (Da) of coal at 673 K. Figure 4 shows CH4 and H2 evolution vs, reaction time at the same temperature of 673 K. Comparing Fig. 3 with Fig. 4, it is clear that the amount of transferable hydrogen captured by anthracene, Da, is about 10 to c Acenaphtene is sometimes a component of coal tar. However, when our samples were carbonized without anthrecene to 743 K and the carbonization system was extracted with CDC13 and the ih nmr was obtained, no signal due to 1,2 positions of acenaphthene was observed. Therefore no acenaphthene was abtained at 743 K, and it is an acceptable internal standard.

3 (514) Transactions ISIJ, Vol. 26, 1986 Table 4. The c lassification of optical texture in cokes. Fig. 4. CH4 and H2 evolution res. reaction time at 673 K. Fig. 3. The effect of reaction 673 K for coals. time on the value of Dry at 100 times larger than that of hydrogen gas evolved. The Da value reaches a maximum and then decreases with reaction time for Sufco and Newdell coal (see Fig. 3). This decrease of Da with reaction time might be due to an increase in retrogressive reaction or CH4 evolution (see Fig. 4). Sufco and Newdell coals, exhibiting a significant amount of CH4 evolution on heating, show that the value of Da decreases markedly with time. Figures 5 and 6 show the temperature dependence of hydrogen evolution and hydrogen donor ability (Da), respectively. Although the amount of hydrogen evolution in coals increases with increasing temperature, no substantial difference is found among ranks of coal, except for Sufco coal. On the other hand, a good correlation is found between the value of Da and coal rank. Da value in coals with higher reflectance of vitrinite increases considerably with temperature, reaches a maximum and then decreases. A relatively small increase in Da value was observed for most of the coals with lower reflectance. 2. Hydrogen Donor Ability (Da) and Characteristics of Coal Figure 7 shows the relation between the several characteristics and hydrogen donor ability of coals Fig. 5. Hydrogen evolution vs, heat-treatment temperature.

4 Transactions ISIJ, Vol. 26, 1986 (515) Fig. 7. Relation between the properties MF, Ro) soaking). and the value of Da of coal (VM, FC, (673 K, without Fig. 6. Temperature dependence of Do values for coals. (Da) evaluated at 673 K with no soaking period. It appears that the Da value decreases with increasing volatile matter. A good correlation is found between FG (Fixed carbon), Ro (Reflectance) and the value of Da. The value of Da increases with increasing reflectance and reaches a maximum at Ro=1.4. The extent of hydrogen transfer is correlated with not only ash in coal, but also the various kinds of macerals. Assuming that the transferable hydrogen is not provided by volatile matter and inert macerals, one can obtain the following equation: Da (corr) Da [1-(VM+Inert+Ash)] (1) Figure 8 shows also the value of Da (corr) vs. the reflectance Ro of the coals. The correlation between Da (corr) and Ro is better than that between Da and Ro. The values of Da for the each macerals and components in coal will be expected in the successive experiments. 3. Relation between Hydrogen Donor Ability (Da) and Optical Texture, and the Effect of Blending, and Mild Oxidation The optical texture measured by point-count analysis of the size and extent of anisotropic components in coke are represented as percentages of an inert-free basis in the histograms in Fig. 9. A good correlation was found between Da and size of optical Fig. 8. Relation corrected between value reflectance of Da. of coal (Ro) and the texture as shown in Figs. 8 and 9(a). For Newdell coal (Ro=0.69) that produces mainly coke with an isotropic texture (Iso), the value of Da is small. The value of Da for Goonyella coal (Ro=0.75) which produce cokes with fine-grained mosaics (Mf) is

5 (516) Transactions ISIJ, Vol. 26, 1986 Fig. 9. Point-counting analysis of cokes from coals and coal blending systems (1: 1). greater than that of Newdell coal. Peakdown coal (Ro=1.38), which forms coke with a comparatively large optical texture, gives a larger value of Da. Although the size of anisotropic units of coke from Beatrice coal (Ro=1.57) is greater than that of the coke from Peakdown coal, there is less Da in Beatrice, coal. The coke from Beatrice coal showed large isochromatic regions, which are defined as domain or leaflet texture. It is known that cokes of coals over 90 % C show originally an optical anisotropic texture before carbonization.6~ The development of domain texture during the carbonization of high rank coal such as Beatrice coal can be explained in terms of the transformation of pre-ordered structural units in the original coal. It seems, therefore, that the mechanism of formation of anisotropic texture for Beatrice coal differs from that for coals below Ro= 1.4. Figure 9(b) shows the histograms for various optical textures in cokes obtained from. coal blends. Taking account of the distribution of optical textures from single coal carbonization, we can calculate the value of optical distribution for blended systems. The calculated and observed values are compared in Fig. 9(b). The experimental values for the content of isotropic texture of blended systems were lower than the calculated values for each coal, except for the case of Peakdown-Sufco (P-S). For the content of fine mosaic texture, in all cases the experimental values were higher than the calculated ones. Figure 10 shows the variation of Da with the blending ratio of the different coals. It is interesting to note the fact that the experimental values for Da in blended systems of coals are higher to a greater or lesser extent than those calculated from additivity of blending. Also, for most cases in Fig. 10, Da exhibited a maximum value at a given blending ratio. In the (P-N) blended system, for which the experimental values for Da are markedly higher than the calculated values, the experimental values for the content of fine mosaic texture in the cokes were also higher than the calculated values as shown in Fig. 9(b). While the experimental values for the isotropic content were higher than the calculated values in the (P-S) blended systems, the experimental values for Da were not significantly higher. Mochida et

6 Transactions ISIJ, Vol. 26, 1986 (517) Fig. 10. Variation of Da with blending ratio of coals (673 K, without soaking). Table 5. Change of hydrogen coal with weathering. donor ability (Da)y of anisotropic texture. Recently, it has been reported that Da and Aa for pitch and coal are important factors governing mesophase development in the cocarbonization of low rank coal and pitch system.9j In order to define the extent of hydrogen transfer in the co-carbonization system, we have already proposed the following (D/A) parameter, which takes into account of the fact that hydrogen transfer occurs from coal to coal, pitch to pitch and pitch to coal.lo~ al.7 introduced the concept of "compatibility" of components in co-carbonization systems. If the change in size and content of the optical texture of resultant cokes in such co-carbonization systems is dependent upon the "compatibility " of coal type pairs. A preliminary evaluation of the compatibility may be found from the hydrogen donor ability (Da) in the coal blending systems. It is well known that a reduction in the coking properties of coal can be caused by atmospheric oxidation. Therefore, the influence of oxidation on Da in coal was studied. Authors have previously reported that the value of Da decreases with increasing duration of oxidation at 383 K and a simultaneous reduction in the development of the optical texture resulted for Yubarishinko coal.8~ Table 5 shows the effect of mild oxidation (weathering) at room temperature on Da in coal. Mild oxidation of coal also decreases the value of Da except for Sufco coal. This reduction of Da by oxidation appears to be connected with a decrease in fluidity at high temperatures. In other words, the decrease in Da becomes a sensitive indicator of the degree of oxidation of coal. 4. Development of Anisotropic Texture in Co-carbonization of Coal with Pitch Co-carbonization of low grade coal with pitch is one of the ways to produce coke having optically D/A = (mpitch/mcoal' (A (Da)pitch) + (Da)eoal a)coai...(2) where, (Da)plteh,(Da)Coai : the hydrogen donor ability of pitch and coal, respectively (Aa)epal: the hydrogen acceptor ability of coal mpltch f mcoal : the blending ratio of pitch to coal (in weight). Potential difference between (Da)plteh or coal and (Aa)coai is thought to be much larger than that between (Da) and (Aa)plteh, considering the fact that coal has a lot of hydrogen accepting sites such as oxygen containing groups. Therefore, (Aa)plteh is ignored in the denominator of Eq. (2). The extent of development of anisotropic texture in cokes from Akabira coal (Ro = 0.78) with pitches having different hydrogen donor ability was assessed by using the D /A parameter proposed in Eq. (2). Table 6 summarizes the value of DMA, the blending ratio for the co-carbonization systems and the extent of anisotropic texture of resultant cokes. It is apparent that the correlation is fairly good between the D /A values and the size of optical texture of resultant cokes for various kinds of pitches except for A240. Akabira coal itself produces isotropic coke. For

7 (518) Transactions ISIJ, Vol. 26, 1986 Table 6. Hydrogen donor ability (Da) of pitch, blending ratio, D/A values for co-carbonization of Akabira coal with various pitches and optical texture of resultant cokes. (Aa) of coal and pitch were evaluated quantitatively, using anthracene and 9,10 dihydroanthracene as hydrogen acceptor and donor compounds. (2) Good correlation was obtained between hydrogen donor ability (Da) and rank of coal (reflectance). The value of Da increased with increasing reflectance and reached a maximum at Ro=1.4. (3) The D/A parameter would find its place in a manual describing how to select and add pitch as a modifier into low grade coal used as a raw material for coke manufacturing. Akabira coal/pitch systems, materials with a value of DMA= produce fine-mosaic texture [Mf whatever kind of pitch is employed. For A240 pitch, DMA value corresponding to Mf texture is smaller than that of the other pitches used. A240 pitch is considered to have not only powerful hydrogen donor ability but is also a good solvent for coal. From the above results the balance between the hydrogen donorr ability of pitch and coal and the acceptor ability of coal is one of the crucial factors controlling the co-carbonization reaction. Iv. Conclusions (1) Hydrogen donor (Da) and acceptor ability 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) REFERENCES T. Yokono, H. Marsh and M. Yokono : Fuel, 60 (1981), 607. T. Obara, T. Yokono, K. Miyazawa and Y. Sanada : Carbon, 19 (1981), 263. T. Yokono and H. Marsh : Coal Liquefaction Products, I, John Wiley & Sons, New York, (1983), Chap. 6. T. Yokono, T. Obara, S. Iyama, J. Yamada and Y. Sanada : J. Fuel Soc. Japan, 63 (1984), 239. H. Marsh, I. Mochida and E. Scott: Fuel, 59 (1980), 514. I. Mochida, Y. Korai, H. Fujitsu, K. Takeshita, K. Komatsubara and K. Koba : Fuel, 60 (1981), I. Mochida, K. Amamoto, K. Maeda and K. Takeshita : Fuel, 56 (1977), 49. T. Obara, K. Miyazawa, T. Yokono and Y. Sanada: Cokes Circular, 32 (1983), 113. T. Yokono, T. Obara, S. Iyama and Y. Sanada: Carbon, 22 (1984), 623. S. Iyama, T. Yokono and Y. Sanada: to be published in Carbon, 24 (1986).