Temperature regime of secondary char forming reactions in the pyrolysis of radiata pine

Intraparticle and extraparticle secondary char forming reactions have been investigated in the slow pyrolysis regime using radiata pine. A laboratory thermogravimetric analyser (TGA) and a Macro-TGA were used to cover a large range of sizes, extending from thin slices to large cylindrical specimens in the micrometer and centimeter ranges respectively. Extraparticle reactions were studied by pyrolysing sawdust, particle size <1 mm, in crucibles with and without a lid. All experiments were done at 5 degreesC/min up to 700 degreesC in nitrogen. Under these conditions, transfer limitations for cylindrical samples with axial grain direction were found to be negligible for diameters <=2 cm. In this regime secondary reactions begin to occur at 140 degreesC, contrary to previous suggestions that range from 280 degreesC to above 380 degreesC. Temperature measurements inside the large cylinders indicate that intraparticle secondary reactions similarly occur from around 180 degreesC. Yield increases with particle size and plateaus at 2 cm. The maximum yield that could be achieved was 22.66 +/- 0.82 % (wt/wt) on an air dry basis at 700 degreesC. Larger sizes did not cause an increase in yield due to the formation of cracks as a result of increased internal pressure gradients. These findings have large implications for the manufacture of char and the char formation mechanism itself, in particular as they suggest that secondary char forming reactions take place parallel to primary reactions.