TGA and DMA studies of blends from very good coking Zofiówka coal and various carbon additives: Weakly coking coals, industrial coke and carbonized plants

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Abstract

The aim of this work is to study the effects of various carbon additives, blended with very good coking coal, on the thermal decomposition of the blends. The blends possess fixed content (50 wt.%) of very good coking coal from the Zofiówka Mine. The remaining components of the blends are worse coking coals collected from the Janina, Krupiński, Szczygłowice, Jas-Mos mines (coals of carbon content ranging from 73 up to 92 wt.%), and very porous carbons: coke (from the coking plant Zdzieszowice), as well as woody stems of bamboo and yucca carbonized at 400 °C. The content of porous carbon in a blend does not exceed 20 wt.%. Thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) are used in the study. The weight loss during low-temperature pyrolysis (< 600 °C), and storage/loss elastic moduli measured as a function of the increasing temperature are related to the kind and concentration of additives. The temperature dependences of elastic moduli determined for binary coal blends differ clearly from those of ternary coal blends. The consumption of energy during the interaction of the components in binary blends was found to be distinctly bigger than the one observed for ternary blends. Non-softening additives such as carbonized plants and low rank coal, containing many functional groups, diminish both moduli of the blends distinctly. However, the addition of coke does not reduce the value of the elastic moduli but increases the width of the maximum occurring in the temperature dependence of the moduli. The influence of the coke additive on rheological properties of the blends, different in comparison with the remaining additives studied, was assigned with different number of functional groups and radicals.

Introduction

Recently various kinds of waste of different structures and origins (agricultural, municipal, electronic, automobile) have been proposed as additives to coal blends for metallurgical coke manufacture in order to contribute to the protection of the environment and natural resources. The use of chemical additives in the processing of cokes is one of the ways to influence coal thermal decomposition with possible effects such as: (i) catalytic, (ii) change in the quantity and quality of the products of pyrolysis, (iii) improvement of the thermoplastic properties of coal (generally, additives reduce viscosity of the plastic phase and broaden the plastic range) and (iv) improvement of coking ability of coals (Shevkoplyas, 2002). An adequate development of coal fluidity in high-temperature carbonization is a prerequisite for producing graphitizable carbon material of high quality. Therefore, a number of studies on thermo-chemical conversion of coals blended with various additives were carried out (Alvarez and Diez, 2000).

The effect of various plastics (polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC), polystyrene (PS), poly(ethylene terephthalate) (PET), terephthalic acid (TFA), polyacrylonitrile, polyphenylene sulfide, etc.) on coal during heating was widely studied (Nomura et al., 2003, Sakurovs, 2003a, Vivero et al., 2005, Nomura and Kato, 2005, Nomura and Kato, 2006, Orinak et al., 2006, Zubkova, 2006). The effect of plastic addition on coal caking property varied with the types of plastic because of the different interactions between thermal decomposition products of plastics and hydrogen in coal. For example, the addition of: PS, PET, PP, TFA and polyphenylene sulfide reduced fluidity and total dilatation dependently on coal rank, the amount, the structure and the thermal behavior of the plastic waste added. However, polyacrylonitrile appeared to increase the fluidity of the coals at temperatures near the softening temperature of the coals (Nomura et al., 2003, Sakurovs, 2003a, Vivero et al., 2005, Zubkova, 2006). Nomura et al. (2003) suggested that the radicals formed as a result of PS or PET thermal decomposition abstracted hydrogen from coal, which resulted in the decrease of coal caking property. The change of electric and dielectric properties of co-carbonization products of coals with PET showed that PET reacted with coals with high fluidity in a different way than in the case it was blended with coals of low fluidity because of the formation of cross-links between the macromolecules (coals with low fluidity) or the increase in length of macromolecular chains (coals with high fluidity) (Zubkova, 2006).

Apart from plastic waste, petroleum coke (PCA) was widely used as a component of blends with coals (Barriocanal et al., 1996, Zubkova, 1999, Alvarez and Diez, 2000, Pis et al., 2002). Petroleum based additives are the most available. Zubkova (1999) found that the PCA introduction did not promote the initiation of thermo-chemical transformations in the organic coal mass through the mechanism of intra-structural plasticization. The mechanism of the influence of PCA on the coke-making process consists in the external plasticization of the surface of coal grains. Such plasticization promotes moderate swelling and gluing together the grains of the poorly caking coal. Barriocanal et al. (1996) and Pis et al. (2002) investigated the effects of the addition of petroleum coke on the texture and reactivity of the resultant metallurgical cokes. It was observed that the addition of petroleum coke to coking blends caused the reduction of the micropore volume and the reactivity of metallurgical cokes. A relation between those two parameters was found, suggesting that the decrease in reactivity was the consequence of the decrease in the surface area.

Agricultural residues (biomass), e.g., pine chips (Pan et al., 1996) and poor (waste) coals as well as other non-coking coals were also blended and pyrolysed (Barriocanal et al., 1996, Barranco et al., 2007). Pan et al. (1996) investigated the pyrolytic behavior of biomass-poor coal blends. Studies showed that no interaction took place between biomass and coal in the blend during pyrolysis. Barriocanal et al. (1996) studied microscopically the quality of interfaces between textural components in cokes produced from coals with added calcinated anthracite. They evaluated the ability of coal to bind various inerts into the coke structure. The calcinated anthracite was found to have the highest interface quality index (IQI) among the inerts studied.

Carbonaceous additives co-carbonized with coal are expected to increase the coke quality. The primary function of the additives is to reduce the development of cross-links during heating so that large aromatic structures can develop. The most effective additives should be high-boiling, highly aromatic substances and should contain functional groups and mobile hydrogen (Gray, 1989). The most available petroleum based additives are not renewable and they commonly contain a lot of sulfur. It would be useful to apply carbon aromatic substances produced from renewable resources. Carbonized plants are carbon materials of aromatic structure with a carbon content of about 80 wt.%. They are expected to contain functional groups when they are carbonized at lower temperatures, but over the temperature of the primary thermal decomposition, e.g. at 400 °C.

The aim of this work is to study the effects of various carbon additives blended with very good coking coal, on the thermal decomposition of the blends. The studies were performed on the blends that possessed a fixed content (50 wt.%) of very good coking coal from the Zofiówka Mine, commonly used in Polish coke-making industry. The remaining part of every blend consisted of worse coking coals and very porous carbons, i.e., coke and carbonized stems of plants. This work is a continuation of our previously published paper Krzesińska et al. (2009) dealing with the thermal decomposition of three Polish bituminous coking coals and their blends by means of techniques such as DSC, TGA and DMA.

Section snippets

Samples used

Five Polish bituminous coals of rank ranging from 73.4 up to 91.7 wt.% of carbon content were collected from the Janina, Krupiński, Szczygłowice, Zofiówka, and Jas-Mos mines. All mines are located in the Upper Silesian Coal Basin (Poland). The coke additive (Ck) was manufactured at the coking plant Zdzieszowice from the single very poor caking coal supported by the Bielszowice Mine. The carbonization products of bamboo (B) and yucca (Y) were obtained by pyrolysis at 400 °C, at inert atmosphere (

Pyrolysis behavior of coals, porous carbons and their blends

Fig. 2 shows the derivative of the weight loss (DTG) curves as a function of the pyrolysis temperature of the single coals and carbon additives used for the preparation of the blends. Fig. 2 implies that the temperature of the single coal primary decomposition (minimum of the DTG curve), Tpeak, depends distinctly on the coal rank, i.e., on the carbon content as follows: the higher the carbon content in coal, the higher the Tpeak in the DTG curves. The thermal decomposition of the coke additive

Conclusions

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

The course of the temperature dependence and values of moduli E′ and E″ were found to be considerably different between the binary and ternary coal blends. The consumption of energy during the interaction of components in binary blends was distinctly greater than that found for single coals and for the ternary blends.

Non-softening porous carbon additives such as carbonized bamboo and yucca as well as low rank Janina coal, that

Acknowledgment

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

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