Energy & Fuels 2006, 20, 2137-2141
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Investigation of Bonding Mechanism of Coking on Semi-coke from Lignite with Pitch and Tar Vedat Arslan Dokuz Eylul UniVersity Engineering Faculty Mining Engineering Department, 35160 Buca, Izmir, Turkey ReceiVed June 19, 2006. ReVised Manuscript ReceiVed July 21, 2006
In coking, the bonding ability of inert macerals by reactive macerals is dependent on various parameters and also is related to the wettability of the inert macerals. In this study, the effect of carbonization temperature on the wettability of semi-cokes produced at various temperatures has been investigated. Soma and Yatagˇan semicokes represent inert macerals, and pitch was used as a reactive structure in the experiments. The briquetted pitch blocks were located on the semi-cokes and heated from the softening temperature of pitch (60 °C) to 140 °C to observe the wettability. In addition, liquid tar was also used to determine the wettability of semi-cokes. From the standpoint of wettability, the temperature of 900 °C was determined to be the critical point for coke produced from sub-bituminous coals.
1. Introduction Coals that have 18%-38% volatile matter on a dry ash free (daf) basis can be used in classic coke-making processes. Other processes, especially briquette charge methods and coke-forming methods, can widen the coal range.1-4 However, using high volatile coals in metallurgical coke production has caused several problems. Especially, using high volatile bituminous coals and sub-bituminous coals in coal blends for making metallurgical coke, the parameters that must be taken into account have changed. In the coking process, reactive macerals are softened and fluidized at 350-500 °C, and these liquefied materials can bond inert macerals. Maceral types related with coalification are observed to be very important for this bonding mechanism.5-7 The main factors that affect the bonding mechanism during coking are fluidity of the reactive macerals and surface wettability of the inert materials. Fluidized reactive macerals spread on the inert coal particles, forming coke walls between them. Insufficient wettability of inert particles can weaken the coke structure. Good wettability of inert materials and sufficient quantity of reactive macerals that cause caking of coal is necessary for a strong coke structure.6,8,9 * Author to whom correspondence should be addressed. Tel.: 90-2324127518. E-mail:
[email protected]. (1) Callcott, G. Principles for Blending Coals. BHP Tech. Bull. 1979, 23 (November), 49. (2) Tsuyuguchi, M. Recent advances on metallurgical cokemaking. In Proceedings, Cokemaking International; 1979; Vol. 38. (3) Bayraktar, K. N.; Lawson, G. J. Properties of formed cokes derived from two Turkish lignites by carbonization of binderless briquettes. Fuel 1984, 63 (9), 1221-1225. (4) Kosˇina, M. Effects of the properties and composition of coal blends on coke mechanical properties. Fuel 1988, 67 (3), 431-436. (5) Vogt, B. D. et al. Effect of Rank and Interactions of Coals on Experimental Coke Properties. Ironmaking Conf. Proc. 1986, 45, 211219. (6) Kilau, H. W.; Pahlman, J. E. Capillary Wetting Response of Coal after Exposure to Ambient Air Atmosphere; United States Department of the Interior, Bureau of Mines: Pittsburgh, PA, 1989. (7) Gray, R.; Champagne, P. Petrographic Characteristics Impacting the Coal to Coke Transformation. Ironmaking Conf. Proc. 1988, 47, 313324. (8) Bexley, K.; Green, P. D.; Thomas, K. M. Interaction of mineral and inorganic compounds with coal: The effect of caking and swelling properties. Fuel 1986, 65 (1), 47-53.
Surface wettability in coking differs from the wettability of solid materials in liquids. There is no liquid phase in coking process. Thus, the behaviors of carbonized inert coal particles and fluidized materials are usually examined in coking.9-11 Wettability of a carbonized coal surface by various oils (originated from fossil fuels) provides important data on coal coking properties. In coking, plasticity occurs at 350-500 °C, and reactive macerals are fluidized and swollen and they cover inert materials. At ∼500 °C, this matrix resolidifies again. The strength of this matrix (coke) is strongly dependent on the wettability of inert matrix elements. Decreased wettability of the inert materials necessitates more fluidity from the reactive macerals.9,11,12 The wettability of inert materials is also important in the blends of coking coal. Generally, sub-bituminous coals are added in the blend in the form of semi-coke. Semi-coke is an inert substance in a blend for a strong coke structure; it must be wellwetted by the fluidized reactive macerals. When reactive macerals do not possess enough fluidity, pitch is added to the blend to support the proportion of fluidized reactive macerals.10,13,14 For good bonding between the inert materials and fluidized reactive macerals, there must be a balance between the wettability of the inert materials and the fluidity of the reactive (9) Kemal, M.; Yıldırım, K. E.; Klose, W. The effect of carbonization temperature on the wettability of lignite surface. Braunkohle 1984, 41 (6), 200-201. (In Ger.) (10) Born, W.; Scichter, A. Determining the wettability of coke by tar. VEB Deutscher Verlag fu¨r Grundstoffindustrie; 1979; pp 17-85. (In Ger.) (11) Zhu, T. et al. Study on the coking mechanism of coal and coal tar pitches. In 1st International Cokemaking Congress, September 13-18, 1987, Essen; Vol. 1-B. (12) Lin, M.-F.; Hong, M.-T. The effect of coal blend fluidity on the properties of coke. Fuel 1986, 65 (3), 307-311. (13) Maroto-Valer, M. M.; Andre´sen, J. M.; Snape, C. E. In situ 1H NMR study of the fluidity enhancement for a bituminous coal by coal tar pitch and a hydrogen-donor liquefaction residue. Fuel 1988, 77 (9/10), 921926. (14) Bujnowska, B.; Collin, G. Co-carbonization of coals with coal-tar pitch. Cokemaking Int. 1994, 6 (2), 25-32. (15) Hower, J. C.; Lloyd, W. G. Petrographic observations of Gieseler semi-cokes from high volatile bituminous coals. Fuel 1999, 78 (4), 445451.
10.1021/ef060281h CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006
2138 Energy & Fuels, Vol. 20, No. 5, 2006
Arslan
Figure 1. Photographs of melted pitch tablets formed at various oven temperatures: (a) 60, 70, 80, and 90 °C (from left to right); and (b) 100, 120, and 140 °C (from left to right). Table 1. Results of Analysis of Samples Received from Yatagˇ an and Soma Coalsa Proximate Analysis coal
moisture (%, ar)
ash (%, ar)
sulfur (dry basis)
volatile matter (%, daf)
AID (kcal/kg, ar)
Soma Yatagˇan
10.38 27.59
1.72 6.45
1.12 1.26
41.16 52.30
5966 3604
a
Legend: ar, as received; daf, dry ash free.
macerals. The decreasing wettability should be compensated by increasing the fluidity and also the amount of reactive macerals.11,12 Kemal et al. tested a serious of coking coal and lignite blends and concluded that, above a coking temperature of 900 °C, the strength of coke and the wettability of lignite cokes were decreased. It was also deduced that these high-temperature lignite cokes absorb more coal oil as pitch than normal carbonized coking coal.10
Figure 2. Photographs of the instrument used for computer-supported investigation.
2. Materials and Methods
Table 2. Results of Pitch Analyses Used in Tests
2.1. Materials. For the investigation of the bonding mechanism, the coal pitch was used as reactive material in these tests. Two different lignite semi-cokes obtained from two different pits were used as inert substances; these are Soma lignite and Yatagˇan lignite. Short analyses of these coals are presented in Table 1. A sample of Soma coal was taken from raw coal by hand sorting; therefore, its ash ratio is very low. The moisture content is also low, on an air-dried basis. A Yatagˇan lignite sample was taken from the coal seam carefully as a clean part. Considering their general characteristics and the analysis given below, the Soma coal can be classified in the group of sub-bituminous-A coal and the Yatagˇan coal can be classified in the group of sub-bituminous-B coal, as stated by ASTM standards. Because of the high moisture ratio, Yatagˇan coal dries rapidly under atmospheric conditions, resulting in cracking. Therefore, Yatagˇan coal sample was preserved in the form of lumps. Coal pitch, representing the reactive macerals, was used to investigate the wettability of the coke surface under various temperatures. Coal tar was also used to determine wettability in a different type of test. The characteristics of pitch are given in Table 2. The tar used in these tests had a 60% pitch ratio after distillation. 2.2. Methods. 2.2.1. Wettability Tests with Pitch. In new coking technologies where noncoking coals are used in blends, coal is added to the blend as carbonized coal. Therefore, the Soma and Yatagˇan coal samples were carbonized separately a various tem-
parameter
value
moisture content sulfur content gross calorific value density softening temperature
3% 1.32% 9662 kcal/kg 1.18 g/cm3 60 °C
peratures from 100 °C to 1000 °C. In a carbonization process with Soma coal, controlled heating was used below 400 °C to hinder the occurrence of excessive fissures. This limit is 150 °C for Yatagˇan coal, because of its higher moisture ratio. For carbonization, the samples were heated in a drying oven with a stainless steel box at >200 °C. Above 200 °C, specially designed retorts and jenkner retorts were used. Nitrogen gas was used to produce an inert atmosphere in the retort. In the wettability experiments, small pitch tablets (∼1 g each) were prepared using a small hand press and then were placed on the top of the semi-cokes (see Figure 1a). For every semi-coke produced at various temperatures, 14 pieces were prepared for wettability tests. The tests were conducted at 7 different temperature intervals, in the temperature range of 60-140 °C. In these tests, pitch tablets were melted and spread on the surface of semi-cokes. Because of fluidity, some melted pitch filled the cracks in the samples, depending on the wettability of the semi-cokes. Semi-cokes treated with pitch at various temperatures were taken from the oven after 30 min, cooled under atmospheric conditions,
Coking on Semi-coke from Lignite with Pitch and Tar
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Figure 3. Polarized-light photographs of various coal samples: (a) S 400-100b, (b) S 800-120c, (c) Y 400-100a, (d) Y 1000-14a, and (e) Y 900-120c. Features labeled “k” represent coal, whereas those labeled “z” represent pitch.
and placed in polyester molds. These molds were then cut perpendicularly to form surfaces, and they were polished for microscopic observation. A Leitz Wetzlar Orthoplan polarized-lighttype microscope was used for the observations (Figure 2). 2.2.2. Wettability Tests with Tar. To produce numerical values for the surface wetting of semi-cokes, a different type of wettability test was performed, using only Soma coals. Semi-cokes and cokes were produced at various temperatures (100-1000 °C), as done previously. After the semi-cokes and cokes were produced, all of the samples were crushed to
