Control of Molecular Composition of Tar by Secondary Reaction in Fluidized-Bed Pyrolysis of a Subbituminous Coal

Changes in the molecular structure of tar are investigated in a fluidized-bed reactor divided into two regions: a dense bed for the primary reaction and a freeboard for the secondary reaction in the gas phase. The temperature in the dense bed is kept at 600 °C and that in the freeboard is varied in the range of 600–900 °C. A pulverized subbituminous coal (C = 76 wt %) is continuously fed into the dense bed, and tar is recovered at the reactor outlet. The tar is separated by solvent extraction into preasphaltene and asphaltene. The asphaltene is further classified by column chromatography into saturates, aromatics, phenolic ethers, nitrogen-containing compounds, and hydroxylic compounds. The aromatics are fractionated by HPLC on the basis of the number of double bonds per molecule (db) into dicyclic (5 db), dicyclic (6 db), tricyclic (7 db), tetracyclic (8 db), tetracyclic (9 db), pentacyclic (10 db), and pentacyclic and greater (+11 db) PAHs (polycyclic aromatic hydrocarbons). Each homologue is characterized by FIMS and ¹H NMR to determine molar yield, number-based distribution of aliphatic carbons per molecule, and number-based distribution of α-, β-, and γ-carbons per molecule. At freeboard temperatures of 600–700 °C, detachment of aliphatic substituents proceeds with bond cleavage at aryl-α positions rather than α-β and remote positions. Decomposition of α-methyl groups is not observed below 700 °C. At a freeboard temperature of 600 °C, PAHs with three to four aliphatic carbons per molecule are most abundant, and the mole fraction of unsubstituted PAHs is only ca. 5% for each homologue. At 900 °C, however, the fraction of unsubstituted PAHs is more than 50%. The structural distribution in pyrolysis products is thus controlled by changing the freeboard temperature.