Understanding the mechanisms behind coking pressure: Relationship to pore structure John J. Duffy a , M. Castro Dı ´az a , Colin E. Snape a , Karen M. Steel a, * , Merrick R. Mahoney b a Nottingham Fuel and Energy Centre, School of Chemical, Environmental and Mining Engineering, The University of Nottingham, Nottingham, NG7 2RD, United Kingdom b BHP Billiton Technology, Newcastle Technology Centre, P.O. Box 188, Wallsend 2287, Australia Received 31 October 2006; received in revised form 21 March 2007; accepted 26 March 2007 Available online 4 May 2007 Abstract During carbonisation coal undergoes both physical and chemical changes that result in the generation of gas and tar and the forma- tion of an intermediate plastic state. This transformation is known to generate high internal gas pressures for some coals during carboni- sation that translate to high pressures at the oven wall. In this study, three low volatile coals A, B and C with oven wall pressures of 100 kPa, 60 kPa and 20 kPa respectively were investigated using high-temperature rheometry, 1 H NMR, thermogravimetric analysis and SEM, with the primary aim to better understand the mechanisms behind the coking pressure phenomenon. Rheometer plate dis- placement measurements (DL) have shown differences in the expansion and contraction behaviour of the three coals, which seem to cor- relate with changes in rheological properties; while SEM images have shown that the expansion process coincides with development of pore structure. It is considered that the point of maximum plate height (DL max ) prior to contraction may be indicative of a cell opening or pore network forming process, based on analogies with other foam systems. Such a process may be considered important for coking pressure since it provides a potential mechanism for volatile escape, relieving internal gas pressure and inducing charge contraction. For coal C, which has the highest fluidity DL max occurs quite early in the softening process and consequently a large degree of contrac- tion is observed; while for the lower fluidity coal B, the process is delayed since pore development and consequently wall thinning pro- gress at a slower rate. When DL max is attained, a lower degree of contraction is observed because the event occurs closer to resolidification where the increasing viscosity/elasticity can stabilise the expanded pore structure. For coal A which is relatively high flu- idity, but also high coking pressure, a greater degree of swelling is observed prior to cell rupture, which may be due to greater fluid elas- ticity during the expansion process. This excessive expansion is considered to be a potential reason for its high coking pressure. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Coke; Coal; Viscoelastic; Rheometry; Coking pressure 1. Introduction When coal is heated in the absence of oxygen it under- goes physical and chemical changes that cause both gas and tar to be released and an intermediate plastic state to be formed. The plastic state is considered to be the result of both physical melting and pyrolysis [1,2], with initial softening being observed around 400 °C depending on the rank of the coal. Decomposition reactions progress both fluidity and the rate of volatile generation, which reach maximum values, causing the formation of a swollen, vis- cous mass. With increasing temperature condensation reac- tions become dominant as the availability of transferable hydrogen required for free radical stabilisation diminishes; fluidity then decreases and the whole mass eventually reso- lidifies into semi-coke at a temperature close to 500 °C. Further heating above this temperature causes secondary gasification, mainly with the loss of hydrogen, and semi- coke contraction leading to the final coke product [3,4]. 0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.03.040 * Corresponding author. Tel.: +44 115 9514078; fax: +44 115 9514115. E-mail address: karen.steel@nottingham.ac.uk (K.M. Steel). www.fuelfirst.com Fuel 86 (2007) 2167–2178