Experimental investigation of intraparticle secondary reactions of tar during wood pyrolysis

This study experimentally investigated intraparticle secondary reactions of tar during the pyrolysis of woody biomass. For this, pyrolysis products (gas, char, and tar) derived from Japanese cypress sawdust (particle size < 1 mm) and Japanese cypress wood cylinders (8 mm in diameter and 9 mm long) were compared. The samples were pyrolyzed in a thermobalance under argon atmosphere. The final reactor temperature was 600 °C, and the heating rate was 0.5 K/s. Under these conditions, the difference between the sawdust and wood cylinder is the intraparticle secondary reactions of tar. The tar yield of the wood cylinder decreases, whereas its total gas and char yields increase when compared with those of the sawdust. These results indicate that intraparticle secondary reactions of tar, which include intraparticle tar decomposition to form gases and polymerization to form char, occur during the pyrolysis of the wood cylinder. It is found that the intraparticle tar decomposition progresses between 400 and 500 °C, which is lower than the homogeneous cracking temperature of tar. This observation suggests that the intraparticle tar decomposition can occur heterogeneously on the surface of the microporous char which acts as a catalyst. This study shows that the intraparticle secondary reactions of tar play an important role in the pyrolysis of large wood particles. Moreover, these reactions have the potential to achieve tar reduction in biomass gasification without any additional tar removal process.