Tar Emission during Pyrolysis of Low Rank Coal in a Circulating Fluidized Bed Reactor

A circulating fluidized bed (CFB) reactor composed of a pyrolyzer and combustor was developed to observe tar emission during pyrolysis of low rank coal. Tar emission in the CFB pyrolyzer was examined under a wide range of operating conditions. Emissions of light tar substances (e.g., benzene, toluene, naphthalene, etc.) could be suppressed at 973 K by enhancement of contact between tar and resultant char in the pyrolyzer (i.e., enhancement of the volatile–char interaction (VCI)). It was also confirmed that about 50% of the heavy tar fraction emissions could be suppressed by the enhancement of VCI at 973 K. These trends were also observed at higher temperature (1173 K). A certain amount of heavy tar was emitted even after enhancement of VCI, so the mechanism of tar elimination was qualitatively determined using Spiral-type TOF-MS. The heavy tar was homogeneously deposited on the char and then was cracked to form lighter fractions by enhanced contact between tar and resultant char during pyrolysis.