The Granular Nature of High Rank Coals
- PDF / 380,616 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 57 Downloads / 188 Views
THE GRANULAR NATURE OF HIGH RANK COALS George D. Cody Jr.*, John W. Larsen**, Michael Siskin** and George D. Cody Sr.** *The Pennsylvania State University, Dept. of Geosciences, University Park, PA 16802 "**Corporate Research Laboratory, Exxon Research and Engineering Co., Annandale, NJ 08801
ABSTRACT We present optical evidence in the visible and mid-infrared for the coal to graphite transformation proceeding via a nucleation mechanism. In the visible region, we show that changes in reflectivity that occur with increasing coal rank can be understood by an increase in the volume fraction of sub-microscopic "graphite-like" nuclei and a subsequent increase of the complex part of the dielectric constant with increasing coal rank. Optical phenomena associated with the granular nature of coal is also observed in the mid-infrared; specifically, measurements of absorption across the mid-infrared show a significant increase of absorption with increasing coal rank. This increase can be ascribed to absorption arising from a plasma resonance for the "graphite-like" material of the granular composite. A broad range of the optical properties of high rank coals in the visible and mid-infrared can thus be readily understood and quantitatively modeled in terms of a granular medium consisting of nuclei of "graphite-like" material imbedded in an insulating organic matrix. If validated by further experimentation, this model should greatly expand current capabilities in the optical characterization of high rank carbonaceous materials.
INTRODUCTION Geologic processes metamorphose terrestrial organic matter through a transformation defined as coalification. This transformation starts at the early stages with biochemical alteration of detrital plant remains and culminates at a final carbonization stage resulting in a graphite-like material. Coals are ranked based on the extent of coalification; the distinction between adjoining coal ranks being the gain or loss of a rank characteristic property. Each coal rank, therefore, defines a specific range of physical properties and lies within specific compositional limits. In this paper, we are concerned only with the last stage of coalification, i.e. that stage which lies between high rank bituminous coals and graphite. Within this stage coals are ranked as semi anthracites, anthracites, and meta-anthracites. Changes which occur during this stage define the process of anthracitization. Although it has long been accepted that during anthracitization coals become more graphite-like, the mechanism by which this transformation takes place remains poorly understood. We propose that the transformation occurs through a nucleation mechanism, where the mean diameter of the nuclei is extremely small, i. e. on the scale of 10's of angstroms. This mechanism is consistent with the results of small angle X-ray scattering [1] which detect the presence of dense carbonaceous bodies with diameters on the order of 2 nm. Based on this model anthracites are, therefore, composites whose apparent homogeneity at lar
Data Loading...
