Effect of Biomass Size and Aspect Ratio on Intra-Particle Tar Decomposition during Wood Cylinder Pyrolysis

Abstract

The effect of biomass size and aspect ratio on intra-particle tar decomposition has been investigated both numerically and experimentally to achieve a high rate of intra-particle tar decomposition. In one experiment, wood cylinders with a diameter of 8 mm and lengths of 2, 5, and 9 mm were pyrolyzed in an infrared reactor in an argon environment. The final reactor temperature was 973 K and the heating rate was 30 K/s. To make a calculation, a two-dimensional, unsteady state, single particle model was used, and the same convective and radiative heat fluxes were given to the top and side surface of all wood cylinders. Both calculation and experiment showed that tar yield when the length of the biomass was increased and the diameter was kept constant. The calculation showed that, first, tar was formed in the wood cylinder, and then it moved outwards during decomposition. To find an effective aspect ratio of the wood cylinder for further tar decomposition, calculations were also performed in which the aspect ratio (D/L) varied from 0.4 to 6.9 and the wood volume was fixed. As a result, a low aspect ratio was suited for intra-particle tar decomposition because of the difference in the thermal conductivity along the grain and radial directions, although there is an optimum aspect ratio because of the change of residence time. It is well known that the thermal conductivity of unpyrolyzed wood in the radial direction is much lower than that along the grain. By decreasing the aspect ratio, the ratio of the side surface area to total surface area increases. This means that more heat entered from side surface, and low thermal conductivity in the radial direction caused a temperature gradient in the cylinder. When the intra-particle temperature gradient was large, primary tar, which has been formed in the biomass with a relatively low temperature, passed through the side surface layer at high temperatures, enough to advance intra-particle tar decomposition before the tar was released. This resulted in the enhancement of intra-particle tar decomposition.