Article pubs.acs.org/EF
Microwave Absorption Capability of High Volatile Bituminous Coal during Pyrolysis Zhiwei Peng,+ Jiann-Yang Hwang,*,+ Byoung-Gon Kim,‡ Joe Mouris,§ and Ron Hutcheon§ +
Department of Materials Science and Engineering, Michigan Technological University, Houghton, Michigan 49931, United States Mineral Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea § Microwave Properties North (MPN), Deep River, Ontario K0J1P0, Canada ‡
ABSTRACT: The microwave absorption capability of an eastern high volatile bituminous coal from West Virginia was investigated by measuring the dielectric properties from room temperature to ∼900 °C in ultrahigh purity (UHP) argon, at both 915 and 2450 MHz. The dielectric properties remain relatively constant below 500 °C. As the temperature continues to increase, however, the relative dielectric constant and loss factor increase dramatically. This is due to the release of volatiles, resulting in the increased electrical conductivity and higher microwave loss. The calculation of microwave penetration depth shows that the pyrolysis process significantly improves the microwave absorption capability of the coal at high temperatures. The calculated reflection losses of the coal sample suggest that the maximum microwave absorption with the reflection losses of −41.25 dB and −32.54 dB can be obtained for the coal having thicknesses of 0.14 and 0.20 m at 915 and 2450 MHz, respectively. The sample dimension has a significant effect on the overall performance of microwave absorption of coal during pyrolysis and thus on the efficiency of microwave coal pyrolysis.
1. INTRODUCTION Coal pyrolysis is an important high temperature process for production of activated carbon and coke that has attracted much attention within the scientific community in recent years. The effects of temperature,1 heating rate,1,2 pressure,3 particle size,4,5 coal rank,4,6 additives,7,8 and heating method9 on pyrolysis and its product composition were extensively investigated, whereas the microwave absorption capability of coal during the process was rarely studied.10−12 Owing to increasing applications of microwave irradiation in the processing of coal and coal-based materials, the investigation of microwave absorption capability of coal is absolutely necessary for microwave coal pyrolysis and the related research (e.g., microwave coal desulfurization).9,13−15 Microwave absorption of coal is known to be dependent on its dielectric properties. Several studies have attempted to characterize the dielectric properties of coal. It is demonstrated that the organic fraction of coal presents low dielectric loss, and it is essentially transparent to microwaves at room temperature. Some components with higher dielectric loss in the coal matrix, such as moisture and some minerals including pyrite, may increase the microwave absorption of coal.15 It is also reported that the microwave absorption of coal can be significantly affected by the heating temperature. However, most published works only considered a relatively narrow temperature range (generally
