Quantifying the heats of coal devolatilization
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I. INTRODUCTION
COAL devolatilization is a complex process of the decomposition of coal macromolecular structure. When heated under inert conditions, the weaker primary bonds in the coal start to break, which may lead to softening and formation of metaplast. The decomposition of the metaplast phase continues up to the resolidification temperature. At higher temperatures, stronger aromatic bonds are broken producing heavy tars and liquids, which continue to decompose and reform secondary volatiles.[1] The final solid product is coke with similar structure and properties to graphite. These complex changes are associated with heats of reactions, as well as changes in the thermal properties. Consequently, the available data on thermal properties of coal are limited to temperatures below the decomposition point.[2–6] The measurements of specific heat have generally been conducted at temperature intervals of 50 8C to 100 8C, lacking in resolution.[8,9] Heats of devolatilization have primarily been measured using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). While DTA provides only a qualitative analysis, estimating the baseline in a DSC measurement remains a major difficulty,[10,11,12] leaving the quality of the results uncertain. The more recent inverse numerical technique, successfully used in the area of solidification of metals,[13] has been applied to the coal devolatilization processes.[14] This method provides continuous measurement of specific heat and thermal conductivity at elevated temperatures with one second time resolution. In the present work, the application of the VLADIMIR STREZOV, Research Associate, and JOHN A. LUCAS, Senior Lecturer, are with the Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia. LES STREZOV, Principal Research Engineer, is with BHP Minerals Development, Wallsend, NSW 2287, Australia. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
inverse method in the measurement of specific heat and thermal conductivity of five Australian coals is described. II. EXPERIMENTAL TECHNIQUE The experimental apparatus used in this work was a modified infrared image furnace and is shown in Figure 1. The coal sample was placed in a silica glass tube and heated with radiation from a surrounding graphite cylinder. The heating rate of the furnace was controlled by a type K thermocouple embedded in the graphite and was typically maintained at 10 8C/min. To keep the sample under inert atmosphere, 5 mL/min of argon was flowed through the glass tube. The coal sample temperatures were measured by thermocouples positioned at the surface and in the center of the sample. The measured data were acquired at a frequency of 1 Hz. The heat flux at the surface of the sample was calculated by applying the heat-transfer equation for radiation: Q 5 F122 s (T 4g 2 T 4S) [1] where
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