COAL DEVOLATILIZATION DURING RAPID TRANSIENT HEATING .2. SECONDARY PYROLYSIS

This study utilizes the radiant coal flow reactor described in part 1 in an operating mode which independently regulates the extent of secondary pyrolysis. Product distributions for transient secondary pyrolysis at nominal heating rates of 10(4) K/s are reported for subbituminous and hvA bituminous coals, to characterize the transformations involving oxygen, nitrogen, and aliphatic functional groups as coal volatiles are pyrolyzed into soot. They show that the sum of the yields of tar, oils, and soot remains invariant at the ultimate yields of tar plus oils for primary devolatilization. Ultimate soot yields can reach one-third of the coal mass for hvA bituminous coals. The aggregate amounts of C1-C3 hydrocarbons change very little while all species are eliminated except acetylene. Among oxygenated gases, CO yields increase steadily and then surge while H2O and CO2 yields pass through weak maxima. Tar aromaticities increase dramatically, reaching ultimate values greater than two-thirds, and tar H/C ratios decrease, reaching 0.75. Soot is continuously graphitized, and its C/H ratio covers the same range as in flames, from 2 to 10. Up to one-fourth of the coal nitrogen expelled with volatiles during primary devolatilization is incorporated into soot during secondary pyrolysis for three different coals. All nitrogen is incorporated very early, and the total amount of coal nitrogen in soot remains constant even while soot yields increase dramatically. Nitrogen incorporation into soot traces the importance of direct conversion of tar into soot during the initial stages. Thereafter, substantial soot mass is added from C1-C3 hydrocarbons, probably acetylene. Since heteroatoms are expelled from tar and the tar/oils plus soot mass is invariant, the light addition species cannot come entirely from a tar decomposition product.