Characteristics of Gas-Phase Partial Oxidation of Nascent Tar from the Rapid Pyrolysis of Cedar Sawdust at 700-800 °C

Volatiles from the rapid pyrolysis of cedar sawdust were subjected in situ to gas-phase thermal cracking in the presence and absence of O-2. It was found that O-2 influenced the resulting product distribution in completely different manners at 700 and 800 degrees C. O-2 was consumed at 700 degrees C mainly by the oxidation of tar and light oxygenates, forming CO, CO2, and H2O, unless the ratio of O-2/carbon involved in the nascent volatiles (O/C) exceeded 0.7 mol of O/mol of C. On the other hand, the oxidation at 800 degrees C consumed H-2 and lower hydrocarbons selectively, leaving the residual tar yield nearly unchanged. Such different behaviors of O-2 were mainly due to the difference in characteristics of thermal cracking of the tar and light oxygenates between 700 and 800 degrees C. The thermal cracking at 700 degrees C left the tar with a molecular mass (MM) range up to > 1000, which was decomposed in the presence of O-2 but incompletely. At 800 degrees C, the tar was decomposed quickly even in the presence of O-2, forming refractory aromatics with a MM range up to 400 together with soot, while light gases were oxidized. A portion of the tar from the cracking at 700 degrees C was deposited inside a ceramic-fiber filter downstream of the reactor, the temperature of which was 350-500 degrees C, and the deposit could not be removed mechanically. The entire part of the tar from the cracking at 800 degrees C was allowed to pass through the filter regardless of O/C. It was thus found that the gas-phase temperature of 700-800 degrees C was critical to the tar property relevant to dust removal at 350-500 degrees C.