Energy & Fuels 2009, 23, 2111–2117
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Use of Oscillatory Shear Rheometry and Thermogravimetric Analysis To Examine the Microstructural Changes during Coal Pyrolysis/Carbonization for the Prediction of IRSID Strength Indices Karen M. Steel,*,† Miguel Castro Dı´az, John J. Duffy, and Colin E. Snape Department of Chemical and EnVironmental Engineering, The UniVersity of Nottingham, Nottingham NG72RD, United Kingdom ReceiVed NoVember 10, 2008. ReVised Manuscript ReceiVed January 25, 2009
During pyrolysis and carbonization of coal, the viscoelastic properties vary across a wide range, with complex viscosity (η*) decreasing to as low as 100 Pa s before increasing to approximately 108 Pa s and phase angle (δ) varying from close to 90° (Newtonian liquid) down to 0° (Hookean solid). A new rheometry method has been developed that combines tests using 25 and 8 mm plates to enable measurements of the entire resolidification process. When combined with thermogravimetric analysis, the method has provided new insights into the mechanisms leading to high and low IRSID I40 strength indices. Although coals with very different volatile matter contents have similar rates of volatile release above 475 °C, viscoelastic properties above this temperature are highly variable. From a study of 13 coals, all coals for which δ < 55° at 475 °C had an I40 index < 44%, while all coals for which δ > 65° at 475 °C had an I40 index > 44%. It is thought that, when δ < 55°, the material is less able to deform and accommodate the loss of mass/volume, causing it to crack/ fissure. However, when the δ > 65° and stays high until higher temperatures, the material is able to contract as volatiles are released without fissuring, ultimately leading to a higher I40 index. A relationship between the final storage modulus (G′) of the material and the I10 index was also found, whereby a low G′ corresponded to a high I10 index. Greater understanding of the relationships between viscoelasticity and pore/fissure network development could enable more precise relationships to be developed, ultimately leading to improved methods for predicting coke quality and devising strategies to make high-quality coke from various sources.
Introduction When coal is heated in the absence of oxygen, it softens and volatiles are released as a result of phase changes and pyrolysis reactions, causing the material to become viscoelastic and foam. At higher temperatures, carbonization reactions dominate, leading to the formation of a porous solid, called coke, which is used to reduce iron oxide. Besides acting as a heat source and reducing agent, coke must also act as a permeable support within the blast furnace. Strength is therefore an important criterion, both at the beginning and during the life of the coke in the furnace. A comprehensive review of the work that has been performed to relate various coke quality indices with coal properties and carbonization conditions has been made by Dı´ez et al.1 One indice is coke size distribution. The continued release of volatiles during carbonization causes the material to contract, which leads to the formation of fissures that in turn affect the size distribution of the coke. There is a preferred “lump” size approximately the size of a fist, as well as a narrow lump size distribution. This paper focuses on predicting the degree of fissuring for a given coal, with a view to better understand coke size distribution. * To whom correspondence should be addressed. Telephone: +61-733653977. Fax: +61-7-33654361. E-mail:
[email protected]. † Current Address: Division of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia. (1) Dı´ez, M. A.; Alvarez, R.; Barriocanal, C. Coal for metallurgical coke production: Predictions of coke quality and future requirments for cokemaking. Int. J. Coal Geol. 2002, 50, 389–412.
Because coke is subjected to considerable mechanical forces as it is transported and charged into a blast furnace, the coke size distribution is assessed after subjecting it to forces that are similar to what it would receive. This involves placing the coke inside a drum that is rotated many times, causing it to knock against the drum wall and against itself. The result is a bimodal coke size distribution curve with one peak generally occurring between 0 and 10 mm and the other occurring above 20 mm2. The two peaks reflect the fact that there are two basic breakage mechanisms. The position of the peak at >20 mm largely depends upon the degree of fissuring in the coke, which acts as weak spots, while the peak at 65° at 475 °C, the I40 index > 44%. Despite this overall observation, no precise relationships between viscoelastic properties and I40 have yet been obtained. It is thought that this requires a better understanding of the fissuring process. A greater understanding of heat transfer and temperature gradients could be elucidatory. A relationship between the final storage modulus (G′) of the material and the I10 index was also found, whereby a low G′ corresponded to a high I10 index. The techniques explored in this paper could prove useful as a cheap and quick method of predicting coke quality and for providing a fundamental understanding of the development of coke properties from which to devise strategies to make highquality coke from various sources. Tests on a greater range of coals in addition to coal blends will be explored in the future. Acknowledgment. The authors thank Karl Pilz of voestalpine and Paul Pernot of CPM for supplying the coals and IRSID strength indices. The authors also thank the Research Fund for Coal and Steel for financial support (Project MAXICARB: RFCR-CT-200600002, coordinated by Ruth Poultney of Corus). EF800977Q
