Modification of coke properties as a consequence of coal preheating

Fuel Processing Technology, 36 (1993) 307-312 Elsevier Science Publishers B.V., Amsterdam

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Modification of coke properties as a consequence of coal preheating R. Alvareza, J.J. Pis a, M.A. Dfeza, J.A. Men6ndeza, J.B. Parraa, E. Alvareza, C. Sudreza and M. Sirgadob aInstituto Nacional del Carb6n (INCAR), CSIC, Apartado 73. 33080-Oviedo (SPAIN). bENSIDESA, Apartado 93. Avilds (SPAIN).

Abstract The influence of coal preheating on the properties of the resultant cokes has been studied using a wide range of coking coals which were carbonized, wet and after preheating, at the INCAR Coking Test Plant operating on a semi-industrial scale. Coke mechanical strength improved by using the preheating process rather than wet charging, except for some cokes produced from coals with a content of volatile matter between 19.5 and 25 wt% db. The most significant differences between cokes from preheated and wet coals were observed in terms of microporosity. Increase in microporosity as a consequence of preheating is accompanied by an increase in coke reactivity. As regards the optical texture of cokes, a slight increase in the total anisotropy and the size of mosaic features was observed.

1. INTRODUCTION Preheating of coal previous to carbonization in coke ovens is a technology that was used by the coking industry at a number of commercial plants around the world in the 1970s [1], but the trend declined in the 1980s. Most of the information in the literature on preheating was provided by the companies selling their process and for this reason they emphasized the advantages and minimized the drawbacks. A review of the advantages and drawbacks of coal preheating has recently been published [2]. Nevertheless, little information was published about the causes leading to the closure of preheating plants. Preheating is again being considered, in combination with dry coke quenching, in an ambitious European Eureka Project started in 1991 under German leadership and with the participation of other European countries. The Project attempts to develop a new coking system that has been named Jumbo Coking Reactor. It is thought that this Reactor will be ready for a trial run in the spring of 1993 [3]. The Spanish National Coal Institute (INCAR) together with the Spanish National Steel Company (ENSIDESA) has lately been studying fundamental and practical aspects of preheating technology using the 2 t/h preheating pilot plant, Precarbon process [4-5], built on-line with the INCAR Experimental Coking Test Plant [6]. In this paper, a comparative study of the carbonization of a wide range of coals, between 19.5 and 36 wt% V.M., wet and previous preheating, was carried out using the 6 t coal

0378-3820/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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capacity oven at the INCAR Experimental Coking Test Plant. Taking advantage of the special characteristics of the INCAR ovens that are reinforced and can stand higher coking pressures than commercial ovens, it was possible to test some potentially dangerous coals. 2. EXPERIMENTAL The characteristics of the coals used are summarized in Table 1. Carbonization tests were carded out in the 6 t coal capacity oven (6.5 x 2.8 x 0.45 m) of the INCAR coking test plant. For coal preheating, a 2 t/h pilot plant, Precarbon process, was used. The details of the coking plant and the Precarbon pilot plant at INCAR were described elsewhere [2,7]. Coke mechanical strength was assessed by the Micum and Irsid tests following standard procedures, based on ISO recommendations. Coke reactivity was determined by the NSC method [8], using 200 g of coke with a particle size of between 19-21 mm at 1100 + 3 °C for 2 hours in an atmosphere of CO2 with a flow rate of 5 1/min. Coke reactivity (CRI index) is expressed as the percentage of weight loss. For the determination of helium densities a Micromeritics Autopycnometer 1320 was used. Apparent densities were determined in a Carlo Erba Macropores Unit, with mercury at 0.1 MPa. Pore volume distribution in pores with a diameter from 7.4 to 7500 nm were evaluated with a mercury porosimeter Carlo Erba 2000. Coke porosities were calculated from real (helium) and apparent (mercury) densities. In all textural determinations, coke samples ground to a diameter of 1-3 mm were used. The topography of the coke samples (1-3 mm in size) was monitored by SEM. The coke samples were also subjected to microscopic analysis for optical texture determinations. Quantitative assessments of the optical texture of the coke samples were made by means of a point-counting technique, based on 700 points. The different optically identifiable features were classified into isotropic, mosaic type anisotropy of various grain sizes, domains, granular flow anisotropy, flow anisotropy and inerts [9]. The optical texture index (OTI) was calculated from quantitative data and the individual factors given for each anisotropic structural feature according to its size [9]. 3. RESULTS AND DISCUSSION Table 1 shows the main characteristics of the coals used, which include a wide variety of international coals employed by the coking industry in the preparation of the industrial coal blends for producing metallurgical coke for the blast furnace. The preheating temperature used was 200 + 10 °C. An increase in bulk density of the charge (690 + 14 kg/nP db for wet charges and 753 + 27 kg/m 3 db for preheated charges) and a reduction in coking time (18 hours for wet and from 12 h 30 min to 14 h 30 min for preheated charges) is always produced using the preheating process. No relevant modifications of the mean flue temperature of heating take place and the same criterion for ending the coking process, -lh after reaching 1000 °C at the centre of the charge-, was maintained. One of the advantages proposed for coal preheating was an improvement in coke strength. Table 2 summarizes the Micum and Irsid indices of all the cokes produced from the wet and preheated charges. It can be observed that the improvement in coke strength is specially evident when high-volatile poor coldng coals, such as coals referred to as Am5 and Sp, are used. In this case, the resultant coke (Sp coke) from the preheated charge exhibits an I20

309 index improved by 10 points and a M4o index by 17 points. By increasing the rank of the parent coal and using the preheating process again the M4o and 120indices increased and the Mr0 and Ilo decreased, but the extent of the improvement in coke strength was not so great. However, in the case of some coals, generally those with a volatile matter content in the range of 19.5-25 wt% db, the mechanical strength of the cokes produced was not improved but impaired. Similar behaviour in an other study has been reported recently [10]. Coke reactivity (CRI) and post-reaction strength (CSR) indices were always impaired by preheating, even if high-volatile bituminous coals were used (Figure 1). Table 1 Analyses of coals used, as Coal code and origin a Moisture (wt%) Ash (wt% d.b.) V.M. (wt% d.b.) Sulphur (wt% d.b.) Swelling index Gieseler fluidity (°C) log ddpm Mean Reflectance (Ro, %) Standard deviation of Ro

received Aul Ge 7.3 9.5 9.8 8.1 19.5 22.4 0.67 1.07 71,~ 8

Au2 Aml Am2 Po Am3 Am4 Am5 Sp 6.5 6.7 7.1 6.4 4.2 5.9 6.4 8.3 10.6 6.5 6.4 7.0 5.0 7.8 6.9 6.6 23.7 24.1 28.2 28.8 30.6 33.8 34.4 36.2 0.60 0.90 0.90 0.75 0.86 0.85 0.79 1.22 71/2 8 8 8 8 71/2 7¾ 7¾

429 442 398 404 391 398 396 1.69 1.76 3.18 2.50 3.85 3.26 4.27 1.41 1.18 1.09 1.15 1.08 1.00 1.00 0.13 0.27 0.15 0.26 0.10 0.11 0.07

395 386 386 4.11 4.06 4.07 0.82 0.90 0.84 0.09 0.17 0.07

" Au: Australian; Ge: German; Am: American; Po: Polish; Sp: Spanish. CRI 8O

CSR

O oh .Owct

6O 4O 2O OAul Ge Au2AmlAm2 Po AndAm4Arn5 Sp Coke reference

Go A u 2 A m l A m 2 Po Am3Am4Am5 Sp Coke reference

Figure 1. Variation of CRI and CSR indices for cokes from wet and preheated charges The structure within the pore-wall material and porosity are two essential factors affecting the technological properties of the metallurgical cokes. In general, real (helium) densities of cokes from preheated coals exhibit nearly the same values as those of the cokes produced from wet charges. On the other hand, some notable variations in the apparent (mercury) densities from the wet and preheated cokes were observed. Apparent densities decrease by

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Porosity (%) 21]

Pore volume (mm3/g) I-qWet D Preheated

15

mr>2Jm Fir < 3.7 am 80604020" 0

0Aul Ge Au2AmlAm2 Po Am3Am4Am5 Sp

Figure 2. Variation of porosity for cokes from wet and preheated charges.

Wp WP WP WP Wl) WI~ WI~ WI: W p W1~

Aul Ge Au2AmlAm2 Po AndAm4Am5 Sp

Figure 3. Pore volume distribution for cokes from wet and preheated charges.

Figure 4. SEM micrographs of cokes produced from: a) coal Po, wet charging; b) coal Po, preheated charging; c) coal Aml, wet charging and d) coal Aml, preheated charging.

311 Table 2 Micum and Irsid indices of cokes produced from wet and preheated charges. Coke reference Aul-W Au 1-P Ge-W Ge-P Au2-W Au2-P Aml-W Aml-P Am2-W Am2-P Po-W Po-P Am3-W Am3-P Am4-W Am4-P Am5-W Am5-P Sp-W Sp-P

M4o 79.6 74.8 75.8 68.2 73.0 77.1 76.8 75.8 70.4 70.0 66.2 73.7 70.6 76.3 55.5 60.0 57.1 62.9 42.5 60.2

Mlo

I~0

7.3 7.8 7.8 7.8 11.1 8.1 6.8 7.3 7.8 7.5 9.0 8.2 7.8 7.2 9.2 8.6 10.0 8.9 10.2 8.3

77.6 75.5 76.6 76.8 72.9 76.2 78.9 77.6 74.8 76.3 71.2 75.0 75.8 77.2 71.1 74.4 67.2 73.3 62.2 72.8

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Ii0 21.0 22.6 21.7 21.4 25.4 22.1 19.5 20.2 21.9 21.0 25.2 22.9 21.2 20.9 22.9 22.1 25.3 23.1 24.0 22.4

W: coke from wet charge; P: coke from preheated charge. preheating, the most important differences being for cokes from coking coals with a volatile matter content in the range of 19.5 to 30 wt%, with the exception of coal Ge. As a consequence of densities differences (similar real helium density and lower apparent mercury density values), porosity increases for cokes produced by the preheating process, especially for cokes obtained from the coking coals mentioned above (Figure 2). In addition, pore volume distribution for different pore radius size was also performed in order to gain insight into the changes detected in coke porosity by the preheating process. An enhancement of the volume of pores was observed in all cokes as a consequence of preheating. Pore volume of pores with a radius of between 3.7 and 25 nm present the smallest variation. Coal preheating facilitates the development of pores with a radius of less than 3.7 nm (Figure 3). The differences in porosity of the resultant cokes, wet and preheated, were also detected by SEM. Higher porosity and a greater number of pores of relatively smaller size can be clearly observed for cokes produced from preheated charges (Figure 4b) in comparison to cokes from wet charges (Figure 4a) with the exception of the Aml coal (Figures 4c and 4d). These findings were especially evident when high- or medium-volatile bituminous coals were used to produce the cokes. Improvement in cold strength of all the cokes studied can be explained by SEM with the exception of Au 1. In this particular case no explanation has been found.

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Coke microscopy revealed that the main effects of coal preheating were a slight increase in total and average size of anisotropic optical texture. This is reflected in the slight increase in the optical textural index -OTI- (Figure 5). OTI 12 10

S 6 4 2 0Aul Ge Au2AmlAm2 Po Am3Am4Am5 Sp Coke reference

Figure 5. Variation of OTI for cokes from wet and preheated charges 4. CONCLUSIONS Mechanical strength of cokes from the preheated charges was always improved, except for some cokes produced from coals with a volatile matter content between 19.5 and 25 wt%. The preheating process produces notable modifications in the coke porosity development, a reduction in pore size in terms of macroporosity and an increase in the number of pores accompanied by an increase in microporosity (pore radius < 3.7 nm). ACKNOWLEDGEMENTS The authors would like to thank the European Coal and Steel Community (ECSC), Research Project EUR 7220-EB/754, for financial support. 5. REFERENCES 1. W. Eisenhut, in "Chemistry of Coal Utilization", Second Supplementary Volume, John Wiley and Sons, New York, 1981, pp. 889-892. 2. M.A. Dfez, R. Alvarez, M. Sirgado and H. Marsh, ISIJ Int., 31(5) (1991) 449. 3. G. Nashan, Cokemaking International, 2 (1990) 19. 4. K.G. Beck, in "The COMA Year-Book", Mexborough, U.K., 1974, pp. 230-242. 5. J. Dartnell, "The preheating of coal in cokemaking", International Iron and Steel Institute, Committee on Technology, Vol. I, Brussels, 1980, pp. 69-99. 6. R. Alvarez, E. Alvarez, C. Su~ez, E M6ndes de And6s, E. Ferugmdez and M. Sirgado, 1st Int. Cokemaking Congress, Essen, 1985, E5. 7. R. Alvarez, E. Alvarez, C. Barriocanal, M.A. Dfez, J.J. Pis, C. Su~ez and M. Sirgado, Steel Times, 220 (12) (1992) 566. 8. British Carbonization Research Assoc. (BCRA), Chesterfield (UK), Report 87, (1980). 9. O. Ruiz, E. Romero-Palaz6n, M.A. Dfez and H. Marsh, Fuel, 69 (1990) 456. 10. R. Alvarez, E. Alvarez, C.S. Canga, M.A. Dfez, A.I. Gonz~ilez and H. Marsh, Fuel Processing Technology, 33 (1993) 117.