Characterization of Coke Formed during Thermal Reaction of Tar

摘要

Coking of volatiles generated from coal in pyrolysis has been a focal issue in coal pyrolysis and upgrading of coal tar, but limited work can be found in the literature on evolution of coke in composition and structure under the pyrolysis conditions. This work characterizes the coke formed in reaction of a subbituminous coal tar at 300, 400, and 500 degrees C in 40 min in a semibatch system which allows natural evaporation of light fractions. The coke is categorized into two types, the one suspended in tetrahydrofuran (THF), coke-S, and the one deposited on the wall of tube reactor, coke-D. It is found that coke-D accounts for 70-85% of total coke. With increasing tar reaction temperature and time the quantity of coke increases from 1.0% to 16.3 wt % and the particle size of coke-S increases from a most probable size of approximately 0.1 to 700-800 um. This change is accompanied by reduction in alkyl side chains and heteroatoms (0, N, and 5), as well as the enrichment in the aromatic C-ar-C-ar bond, which lead to a decrease in H/C ratio from 0.9 to 0.6 and increase in aromaticity f(a) from 0.70 to 0.86. The carbon distribution in coke-S is similar to that in bituminous coals and is composed of 3-7 fused aromatic rings. The changes in coke-S also include increase in radical concentration and decreases in the radicals' g value and line width, indicating continued pyrolysis and condensation of the coke due to the removal of oxygen atoms and side chains on the aromatic structure. When compared with coke-S, coke-D formed under the same conditions is more condensed as indicated by higher radical concentration and lower g value and line width. The morphological change in coke-D includes transformation of small irregular particles to spherical-like particles and to coke film that crack in 30 min at 300 degrees C or 10 min at 500 degrees C.