Plastic Properties of West Virginia Coal: Deterioration During Storage Due to Temperature, Top Size, and Exposure--a View to Further Research
Carl J. Smith, Hobart M. King, II
Coal moves through several stages during coking. Coal loses moisture and volatile matter, and in the process the coal softens, swells, and passes through a plastic phase. Finally, the coal becomes a solid, termed "coke." Without the plastic phase, in which the coal "melts" and the fluid portion fuses with the inert matter, a coke could not be formed. Rarely do individual coals make an adequately strong coke that will support the charge in the blast furnace. Normally, a blending of two or more coals is used to balance the coals that may be too fluid with the ones that are not fluid enough. The plasticity of the blend determines the coke quality. To measure plastic properties of coal, a Giesler Plastometer is used. This device has a crucible filled with coal in which a stir rod is imbedded. The stir-rod is placed under a constant torque while the crucible of coal is heated. As the coal heats and softens, the stir-rod begins to move. The rate of movement is measured as dial divisions per minute (DDPM). Three temperatures are recorded: the initial-softening (stir-rod begins movement), maximum-fluid (highest stir-rod movement), and solidification (end of stir-rod movement). The DDPM of the maximum fluidity is also recorded. Plastic properties of coal samples have been reported to diminish over time.
An experiment was devised to evaluate the change of plastic properties of a stockpile sample of highly fluid Sewickley coal (greater than 30,000 DDPM). Because of the high fluidity the torque of the plastometer was adjusted to a very low level, and the sample was analyzed periodically over 8 months. The sample was subdivided into six "splits." Half of the splits were of a top size of less than 1/2 in. (termed "chip") and the remainder was crushed to 40 mesh. One split of each size was stored in an open pan, one in a plastic bag, and one frozen. The plastic properties did diminish during the experiment. The greatest decline was seen in the 40-mesh size in open-pan storage and the least in the larger frozen "chip" sample. The results of this study indicate that further work should be co ducted on other West Virginia coals to determine the rate of change of plasticity as related to seams of different rank. The experiment should be run at normal torques, over an extended time, and under a variety of top sizes and storage methods.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.