Index for True Dilatation of Coal and Its Usefulness*

Transcription

1 Index for True Dilatation of Coal and Its Usefulness* By Kunihiko NISHIOKA** and Shuhei YOSHIDA** Synopsis Caking property is one of the most meaningful indices for the evaluation of coking coal, and is a basis for the coking mechanism analysis. The index given by the Audibert-Arnu dilatometer widely employed at present is re-evaluated in this paper. The results suggest that present indices of contraction and dilatation have little physical meaning, and are not suitable for the absolute evaluation of the caking property of coals; An index for a true dilatation is newly proposed. This true dilatation index (DT) is defined as the ratio of the apparent coal volume after dilatation (VD) to that before dilatation (Vs) and expressed as, DT = 100(VD-VG)IVc. It was confirmed that this index was not affected by test conditions, and was suitable for the absolute evaluation of caking coal with less deviation. Furthermore, the effects of coal size and heating rate are quantitatively evaluated by the true dilatation index. I. Introduction The coke industry has made a great progress mutually with the iron industry by blast furnace, since the coke was first utilized in the blast furnace by Abraham Darby in the 18th century. Especially in the recent decades, productivity and coke quality have been improved tremendously with larger scale coke ovens, mechanization of auxiliary facilities and automation. Furthermore it is understood that the importance of coke must be greater in the future, considering the trend of energy supply and consumption situation; and therefore a development of the coking technology to achieve more economical and more stable coke quality must be required strongly. Coking mechanism has, however, been analyzed very little, even though the coke industry has such a long history and is supposed to have important feature. It may be said that coke ovens are still operated and controlled mostly with techniques based on experiences. In order to achieve further development, the coke production technology should be based on the coking theory rather than the experiences, and the analysis of the coking mechanism will be of unavoidable important step. The authors have discussed and analyzed the basic mechanism of coking from the first principles. What mechanisms are considered for the coking mechanism? Many mechanisms may possibly be proposed but the following two mechanisms are considered here and have been investigated. They are (1) the bonding mechanism of coal particles and (2) the generation mechanism of coke strength. First, the coal dilatation, by which coal particle bonding is explained, will be discussed. Caking property is one of the most meaningful indices for the evaluation of coking coal. The dilatation measured by the Audibert-Arnu dilatometer is a parameter for the classification of bituminus coal based on coalification rank adopted by EEC. This method is widely applied together with the fluidity measurement by the Gieseler plastometer. Many discussions on this method have been reported,l-s~ and have been introduced as an ISO Recommendation in December, The contraction and dilatation measured by this method have little physical meaning as Von W. Lange et a1.2~ and van Krevelen et a1.3~ suggested and are not suitable for an absolute evaluation of caking property, though they are referred to as relative evaluation indices. Dilatation should be a physical quantity and should indicate inherent dilatation of coal in an absolute evaluation of caking property by True dilatation index obtained by the coal dilatation measurement is introduced here as it is a useful physical value. The measurement is based on Ruhr method described in JIS M8801.7~ II. Expression of Coal Dilatation 1. Inconsistency of.present Indices In the Audibert-Arnu method, coal specimen is a pencil shape briquette of length 60 mm. This is inserted, as illustrated in Fig. 1, into a tube with 8 mm inner diameter, and contraction (a), dilatation (b) and Fig. 1. Audibert-Arnu dilatometer test. * * * Presented to the 102nd ISIJ Meeting, November 1981, S791 and S792, at Kyoto-fu Chusho Kigyo Kaikan in Kyoto. Manuscript received April 28, ISIJ Hasaki Center, Central Laboratories, Sumitomo Metal Industries, Ltd., Hasaki-machi, Kashima-gun, Ibaraki (381)

2 (382) Transactions ISIJ, Vol. 23, 1983 total dilatation (a+b) are determined by the displacement of the piston observed during heating at a constant rate. The physical meaning of those indices is discussed below. 1. Contraction : a (%) The pencil shaped specimen deforms by softening and fills the spaces surrounding the pencil, tube and piston during the heating. This results in the sinking of the piston. To confirm above arrangement, correlation between the softening temperature given by this method and the temperature at which thermal decomposition started (starting temperature of thermal weight loss measured by the thermal gravity analysis) of several coals with different coalification rank was investigated. As shown in Fig. 2, the results indicate that the softening temperature is generally lower than the thermal decomposition temperature for several coalification ranks expressed by the mean reflectance of vitrinite (R0), and suggest that gas is not yet generated at the time when coal softening starts. Therefore, it can be considered that the piston load fills of the space with coal during the primary period of the coal softening. The sinking ratio of piston to the initial pencil length before heating is calculated by Eq. (1), supposing the complete melting of coal pencil. Notations used in the calculation are shown in Fig. 3. The ratio a' is given by; a' =100-1 OOE VP/ VT...(1) where, VT 2r410 (cm3) VVP =1 /3 rrlo (ri +rb+rtr3) (cm3) E=PA/PT PA: apparent density of coal pencil (g/cm3) PT: true specific gravity (g/cm3). This ratio is greatly affected by the diameter of the coal pencil. Table 1 shows the calculated sinking ratio a' using the actually manufactured pencil size, weight and coal density. LV2O/32/01 mold of Ruhr method,7~ which was one of the two standard molds illustrated in Fig. 4, was employed. Fig. 3. Dimensions of coal pencil and tube Eq. (1). appeared in Fig. 2. Decomposition coal. and softening temperatures of each Fig. 4. Types of the mold. Table 1. The dimension of the coal pencil and comparison of the calculated space volume ratio with the contraction.

3 Transactions Is", Vol. 23, 1983 (383) Note the larger diameter of pencil than mold inner diameter. The difference of diameters at both side is approximately 1 mm, while that of the mold is 1.2 mm with the taper of 1/50. This suggests that the side of a smaller diameter brings a greater increase of pencil diameter because of a larger stamping effect. The sinking ratio (a') obtained by Eq. (1) is approximately 33 % which differs from the measured contraction by approximately 8 %. That is, the piston has not sunk down to the theoretical position because of the immediate start of dilatation and existence of void in the specimen after the start of softening/ melting. This is interpreted that the contraction value (a), indicated at present, does not give a proper physical contraction. It only gives an approximate sinking ratio of the piston during the softening of coal, though not as low as the theoretical position. 2. Dilatation: b (%) It must be considered that the dilatation index (b) which is the ratio to the initial 60 mm pencil length also gives a relative indication, since the contraction index (a) does not properly indicate the contraction of coal and its physical meaning is not clear. Proper dilatation of coal should mean the coal volume after dilatation in comparison to that before dilatation and this should be calculated to give the apparent volume ratio after dilatation to the theoretical coal volume given in Fig. 5. The present method for indicating both contraction and dilatation is not physically meaningful and is not appropriate for the purpose of evaluating absolute caking property of coal, even though it may be useful for its relative evaluation. 2. Improved Dilatation Index True dilatation ratio (DT) shown in Fig. 5 is proposed as a preferable index, since it clearly indicates the physical meaning of the phenomenon, unlike the presently used index. This is expressed as, DT =100(VD - V)/ V0...(2) where, Vc : coal volume before dilatation (cm3) VD : apparent coal volume after dilatation (cm). In Eq. (2) Vc is given by the coal weight W(g) and density pt(g/cm3), as Vc=W/pT, and VD is calculated from the tube section area S(cm2) and piston position ld(cm) as VD =S X 1D. Therefore, Eq. (2) can be rewritten as follows: DT= 100(pTSID/W-1)...(3) If present dilatation index (b) based on 60 mm (100 %) is used, DT =100 {pts(lo-f6b/100)/ W-1 }...(4) where, to : specimen height before dilatation (cm). Figure 6 illustrates the situation described by Eqs. (2) to (4). This idea has been proposed by van Krevelen et al.3~ However, in their proposal, PT and W in Eq. Fig. 5. The schematic expression of the true Fig. 6. Coal volume before notations used in Eq (4) are treated as constants. The applicability of their proposal may be restricted, since PT and W vary depending on the kinds of coal even if the same size of pencils is used in the test. The dilatation index given above properly indicates the physical meaning of coal dilatation, and it is not necessarily a requirement to manufacture the shape of the coal pencil to ISO standard and/or Ruhr method. III. Usefulness of True Dilatation Index DT 1. Absolute Evaluation of Caking Property 1. Piston Load and Dilatation and. (2). after dilatation and the Influence of the piston load on the total dilatation in the Ruhr method test and the true dilatation index proposed above are investigated. The results in Fig. 7 show that present total dilatation increases greatly as the caking property is improved, when piston load is increased from 100 to 150 g, whilst the true dilatation index gives a stable figure within acceptable deviation. The reason why the piston load affects the total dilatation is that the dilatation is not greatly influenced by the load and the contraction is small with less load as shown in Table 2. That is to say, less load causes less sinking of piston resulting in less total

4 (384) Transactions ISIJ, Vol. 23, 1983 Fig. 7. Relation between the weight of the piston and the Fig. 8. Relation between the length of the pencil and the true Table 2. Influence of the weight of the piston on the 2. Coal Pencil Length and Dilatation Ruhr method and ISO standard specify a 60 mm of coal pencil length, and a careful operation of pencil manufacturing is required. Especially, the coal pencil is so brittle that it can be easily broken and/or has defects. This requires reproduction of the pencil. On the other hand, as shown in Fig. 8, true dilatation index is not affected by pencil length, and different pencil length also gives the same value of the index, resulting in an easy manufacturing of the pencil. 3. Packing Density and Dilatation The effect of the bulk density of the specimen on the dilatation was investigated by charging the coal sample without briquetting into the tube. The results illustrated in Fig. 9 show that the total dilatation is affected by packing density: The higher the bulk density, the greater is the total dilatation, while the higher bulk density lowers the true dilatation index. This low true dilatation index caused by higher bulk density may be explained by the higher bulk density (less void before dilatation) which causes less void in the specimen during From the preceding discussions it can be concluded that present test method has the following problems : Piston load, pencil length and bulk density of specimen affect the dilatation index resulting in a great fluctuation. In addition the information given by the index does not bring proper evaluation of coal. The Fig. 9. Relation between the packing the density of coal and true dilatation index is, on the other hand, hardly affected by those parameters. Therefore, the true dilatation index is a suitable measure to evaluate an absolute caking property of each coal. 2. Coal Particle Size and Dilatation The particle size of coal specimen is specified to be below 100 mesh, while it is below approximately 3 mm for the coal used in industrial process. In order to find the behavior in an industrial oven, the effect of the coal size to the dilatation index was investigated. Seven kinds of coal with 3 levels of particle size were prepared, and the bulk density of specimen was adjusted to be 0.80 g/cm3 in the tube. True dilatation index linearly increases with the mean coal size, as shown in Fig. 10. This agrees with the previous findings by Ida.8~ The improvement is that the true dilatation index does not depend on the kind of coal. The index can be expressed as; Ddx = Do+38.7 log (dx/do)...(5) where, Ddx : true dilatation index for a coal with mean particle size ofd x (%) Do : true dilatation index for the coal prepared with standard test method with

5 Transactions Is", Vol. 23, 1983 (385) Fig. 10. Relation and the between the mean true particle size of coal mean particle size of do (%) dx: particle size of a coal (mm) do: particle size of the coal prepared with standard test method (mm). Three reasons for the larger dilatation with coarser coal particle are considered. (1) Finer particles are easily oxidized yielding lower (2) Coarser particles result in the displacement due to voids during dilatation, increasing the total volume after (3) Dilatation increases with the coarser particle size due to the increased intra-particle diffusion resistance of thermally decomposed gas. The coal is carbonized in an industrial oven with the result of the better dilatation properties than the values obtained by the test, since the charged coal is coarser than that used in the test. Fig. 11. Relation between the heating rate. dilatation of coal and the 3. Heating Rate and Dilatation The heating rate of 3 C/min is normally applied to the measurement of dilatation index. It is well known that the heating rate greatly affects the 5'9~ The quantitative discussion on the heating rate is, however, not yet published. Several coals were tested with different heating rate by the authors. Figure 11 shows some examples of the test results which confirm previous data-higher the heating rate, greater the Such an effect of heating rate on the dilatation index can be reasonably explained for each coal. However it is difficult to have an equation for the index generally applicable to any kinds of coal, as shown in Fig. 12. With the true dilatation index, however, effect of heating rate is clearly expressed, as shown in Fig. 13, by Eq. (6) below. From this result, it is concluded that the true dilatation index gives the inherent dilatation of coal as a physical quantity better than the total dilatation index, and is expressed as follows : where, Da: Da =1.114Do(log a+o.42)...(6) true dilatation at the heating rate of a (%) Fig. 12. Relation between the Fig. 13. Relation between the heating Do: true dilatation at the 3 C/min (%) a: heating rate ( C/min). heating rate and the total rate and the true heating rate of

6 (386) Transactions ISU, Vol. 23, 1983 The true dilatation index at any heating rate can be calculated from the data obtained at 3 C/min which is usually applied. This true dilatation at any heating rate can support the analysis of coking mechanism in the industrial oven. Iv. Conclusions Caking property is one of the most important items for the evaluation of coking coal and, simultaneously, the basis of analysis of coking mechanism. The dilatation test method with the Audibert-Arnu dilatometer is widely employed at present. This method of caking property evaluation was re-evaluated and the following results were obtained. (1) Previous indices for contraction and dilatation hardly describe physical meaning and is not preferable to the absolute evaluation of caking property. (2) True dilatation index (DT) defined as the ratio of the apparent coal volume after dilatation (VD) to that before dilatation (V0), DT =100(VD- VC)/VC, was proposed. This index is hardly affected by the test conditions and the deviation of the data from the equation is small. This is most reliable to evaluate caking property of coal absolutely. (3) The effects of coal particle size and/or heating rate to the dilatation are estimated quantitatively by the true dilatation index. The true dilatation proposed here is considered to be a widely useful index, but the factors which influence the dilatation have not been surveyed completely, and will be the subject of the future research. REFERENCES 1) Von Helmut Pitchler and Duk Hi Lee : Brennstqf Chemie, 42 (1961), 15. 2) Von W. Lange, W. Radmacher and H. Vierneisel : Brennstqff-Chemie, 42 (1961), 312, 385; 43 (1962), 39. 3) D. W. van Krevelen, H.A.G. Chermin, H.N.M. Dormans and F. J. Huntiens: Brennstoff-Chemie, 39 (1958), ) Von K. Hohne and H. Tschamler: Brennstoff-Chemie, 43 (1962), ) Von W. Schuhnknecht and H. Schinkel : Brennstof-Chemie, 44 (1963), ) W. Simonis: Gluckauf-Forschungshefte, 29 (1968), ) JIS M8801, (1980), 12. 8) S. Ida, T. Okuhara and T. Yamaguchi: Presented at the 54th Meeting of the Special Committee on Coke, The Fuel Soc. Japan, Tokyo, (April, 1973). 9) D. W. van Krevelen : Coal, Elsevier Pub. Co., Amsterdam, (1961), 276.