Influence of tarry material deposition on low-strength cokes or pyrolyzed chars of low rank coals on the strength

摘要

In this fundamental laboratory scale investigation, carbon/carbon (C/C) composites were prepared from a mixture of pyrolyzed-chars of low-rank coals (lignite and sub-bituminous coals) or low-strength cokes (drum index: DI6150 = 52 and 73) and coke-oven gas (COG) tar to investigate the possibility of producing high-strength coke for blast furnace use from low-grade coals or cokes. In addition, the optimum conditions for composite preparation and the influence of the tarry material on the strength of the prepared C/C composite was examined. The C/C composites were prepared as follows: the tar was first added over particles of coke or char at room temperature, subsequently, the mixture was pyrolyzed at a predetermined temperature (500-900 degrees C) in an inert atmosphere. The indirect tensile strength of the C/C composite prepared at a pyrolysis temperature of 500 degrees C increased with increasing weight ratio of tar to cokes (3.5-4.5 MPa) or chars (2.0-3.0 MPa) up to a ratio of 2.0; at this point, the indirect tensile strength reached approximately 6.0 MPa. The indirect tensile strength of the C/C composites prepared with the weight ratio of tar to cokes/chars of 2.0 also increased with increasing pyrolysis temperature, reaching approximately 7.0-7.5 MPa at 800-900 degrees C, which was comparable to the ca. 7.0 MPa strength of the high-grade cokes (DI6150 = 87) used in commercial blast furnaces. Based on pore size distribution measurements using the mercury intrusion porosimetry, N-2 adsorption, and polarized optical microscope methods, it was likely that the increase in the indirect tensile strength of the prepared C/C composite occurred because of the vapor infiltration of tar-derived carbonaceous materials into the micro-, meso-and macro-pores in the low-grade cokes and pyrolyzed chars during the pyrolysis of the mixtures.