Wood is a strongly anisotropic material, and likewise, the char produced by pyrolysis of wood is characterized by a strong anisotropy. This anisotropic behavior allows relatively easy transport of gas in the longitudinal (L) direction of the wood, but the transport is much less easy in the radial (R) and tangential (T) directions. Despite this, this property has normally not been included in mathematical model descriptions of gasification of thermally thick particles. The present paper describes a study of the influence of the anisotropy on the reactivity of thermally thick char particles during gasification of wood using macro TGA equipment. The char particles, in the form of slabs (approximately 50 × 70 × 10 mm), were produced by pyrolysis of wood slabs that had been cut from the trunk of beech trees. The char slabs were grouped into three categories according to the orientation of the normal to the greater surface of the slabs L, R, or T (see Figure 8). When the smaller surfaces were coated with alumina silicate, the gasification agent could only enter the interior of the slabs through the greater surfaces. Thermally thick char particles from beech and pine reacted more slowly if the gas was transported in the R and T directions than in the L direction. In the reported study, the difference was between 25 and 35%. For lower values of conversion, the difference in reactivity was considerably greater, but for higher values of conversion, the reactivity was almost the same in all directions. For increasing conversion, a considerable cracking was seen and it was concluded that the gasification agent could penetrate through the cracks with an increased reactivity as a result.
Energy and Fuels, 2006, Vol 20, Issue 5, p. 2233-2238