Relationships between the optical reflectance of coal blends and the microscopic characteristics of their cokes

https://doi.org/10.1016/j.coal.2008.06.003Get rights and content

Abstract

The aim of the present study was to investigate the relationships between optical properties of various coal blends and their resulting cokes. Twenty-seven various cokes were produced in laboratory scale from three Polish coals of different caking ability and from their binary and ternary blends. Mean random reflectance of the parent coals and their blends was compared to the reflectance parameters and microtexture of their produced cokes. The results show a strong positive correlation between the reflectance of the initial coal blends and the optical properties of their cokes. This relationship is stronger for cokes from the binary than from the ternary blends. It was also revealed that the degree of optical anisotropy of coke depended not only on the rank but also on the coking properties of parent coal. Close relations between the reflectance parameters of cokes and the nature of the microtexture of their binder phase were observed.

This study also proposes a new method of coke classification which includes the simplified description of coke microtexture combined with the measurement of bireflectance value of its binder phase.

Introduction

Coke is a solid, carbon-rich residue with a specific microstructure and microtexture, which is produced by the coking of coal. Microstructure refers to the spatial relation of the coke material (pore sizes and shapes, pore wall thickness etc.), whereas microtexture describes the nature of the carbon in coke, its crystallite development and degree of optical anisotropy (Coin, 1987).

The relationships between basic parameters of parent coals and microtexture of their produced cokes have been extensively studied (e.g., Ammosov et al., 1957, Qian et al., 1983, Moreland et al., 1988, Patrick and Walker, 1991, Gentzis and Chambers, 1993, Jasieńko et al., 1997, Menendez et al., 1997). Gray (1976) established that coke consists of a binder phase and a filler phase. The binder phase was formed from reactive materials in the parent coal and its microtexture varies from apparently isotropic to a mosaic of varied anisotropic domains, while the filler phase consists of isotropic chars from inert macerals. The dimensions of the mosaic units depend on the properties of the parent coals. The higher the rank of parent coal, the larger and more elongate the isochromatic areas in the coke (Patrick et al., 1973). Beginning in the 1950s, optical parameters including reflectance, refractive and adsorptive indices, and bireflectance (Rbi = Rmax  Rmin) were studied for coals of different rank as well as for their carbonization products. Following these pioneering studies, the influence of the temperature, heating rate, pressure, oxidation, and other physical and chemical factors on optical properties of coal were examined (Marsh and Smith, 1978, Murchison, 1978 and references therein; Goodarzi and Murchison, 1978, Goodarzi, 1985, Patrick et al., 1989, Ross and Bustin, 1997, Pusz et al., 2003a, Pusz et al., 2003b, Suarez-Ruiz and Garcia, 2007).

The bireflectance reflects the degree of ordering of the molecular structure of organic constituents, which defines the microtexture of the coke (Marsh and Smith, 1978, Murchison, 1978, Rouzaud and Oberlin, 1990, Rouzaud et al., 1991). Thus, through the determination of coke microtexture, one can predict its technological properties (e.g., reactivity and strength), which are important for industrial applications (Marsh and Smith, 1978, Gill and Coin, 1981, Patrick and Walker, 1985, Marsh and Clarke, 1986, Rouzaud et al., 1988, Moreland et al., 1989, Gransden et al., 1991, Barriocanal et al., 1994, Rosenberg et al., 1996, Chang et al., 1998, Gentzis and Rahimi, 2003). Reactivity of coke has a tendency to increase with decreasing degree of anisotropy, while the coke strength shows the opposite trend (Vogt and Depoux, 1990, Buø et al., 2000).

The nomenclature used to describe coke microtexture has been developed over many years. There are many various classification schemes and terminologies regarding coke optical texture used in different laboratories (Coin, 1987 and references therein; Marsh, 1989, Kwiecinska and Petersen, 2004). Most of these classifications are very complex and the results of coke microtexture analyses often depends on the skills and experience of the researchers who perform the investigations. Therefore, the idea of using reflectance parameters of coke as a complementary element of microscopic classification of cokes has gained favour, especially since optical methods for studying coal and coal products have been improved in recent years (Kilby, 1988, Kilby, 1991, Duber and Rouzaud, 1999, Duber et al., 2000). This quantitative measurement of the optical character of coke would facilitate comparison among various cokes as well as comparison between optical parameters of coke and its physical, chemical, and technological factors. Up to the present, only few attempts to use reflectance values of cokes to study their optical texture have been undertaken (Bellot et al., 1992, Eilertsen et al., 1996, Krzesińska et al., 2002, Pusz et al., 2003a, Pusz et al., 2003b, Krzesińska et al., 2005).

Multi-component coal blends are commonly used in coke-making technologies, so it is important to know the relationships between the optical properties of initial coals and cokes produced from their blends. The behaviour of coal blends during coking process is not always the weighted mean of the behaviour of individual components (Nomura et al., 2004, Diaz-Faes et al., 2007, Krzesińska et al., 2009). Sakurovs (2003) showed that the softening of two-component blend was more complex than it was expected. Because the softening of coal significantly influences the nature of coke microtexture it is not easy to predict the optical properties of coke produced from blend based on the properties of coal components of the blend.

To our knowledge, no papers described systematic studies of the optical reflectance parameters of cokes in relation to the optical properties of parent coals and their blends. The relationships between the nature of microtexture and reflectance parameters of cokes made from various coal blends also were not studied.

The aim of the presented study was to investigate the relationships between the reflectance of parent coals and their blends, and the microscopic characteristics of their resulting cokes. Additionally, new microscopic classification of cokes combining simplified microtextural description with bireflectance of coke is proposed.

Section snippets

Experimental

Three bituminous coals of different rank and caking ability from the Upper Silesian Coal Basin (Poland) were chosen as parent coals: (i) the Zofiówka (Z), a very good coking coal; (ii) the Szczygłowice (S), a moderately coking coal; and (iii) the Krupiński (K), a poor coking coal. Proximate and ultimate analyses, and coking indices of these coals are presented in Table 1, Table 2. Proximate analyses were performed following the Polish Standard procedures for moisture (Wa), PN-G-4512:1980 (ISO

Coal blends

The proximate and ultimate parameters, petrographic composition, and mean reflectance of vitrinite are additive. Thus, the value of each parameter in a coal blend is, approximately, the weighted mean parameter of the individual coal components (Fig. 1, Fig. 2). Fig. 1 shows the carbon content (Cdaf), volatile matter content (VMdaf) and the Roga Index (RI) of binary blends with various proportion of the Zofiówka and the Krupiński coals. The results show, the greater the proportion of Z coal, the

Conclusions

The results of the present study can be summarized as follows:

  • Reflectance parameters of cokes distinctly depend on mean reflectance of coal blends. These correlations were found to be stronger for the binary than for the ternary blends.

  • The coking properties of coal blend components play an important role in the development of coke microtexture.

  • Although the microtexture of coke is affected with the rank as well as coking properties of initial coal or blend, the rank remains the most important

Acknowledgements

This study was financed by the Ministry of Education and Science (Poland), under Grant No. 4 T12B 044 29.

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