Investigation of the Anisotropic Behavior of Wood Char Particles during Gasification

Abstract

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.

Gasification of woodchip particles: Experimental and numerical study of char–H2O, char–CO2, and char–O2 reactions

“…Groeneveld and van Swaaij (1980) compared flake and spherical particles during gasification and concluded that at constant particle volume, spherical particles were less reactive. A few other authors conducted experiments to determine the influence of particle shape on gasification (Henriksen et al, 2006;Moilanen et al, 1993;De Diego et al, 2002). Henriksen et al (2006) investigated the behavior of parallelepiped pieces of wood and revealed the influence of the dimensions on the rate of conversion.…”

“…A few other authors conducted experiments to determine the influence of particle shape on gasification (Henriksen et al, 2006;Moilanen et al, 1993;De Diego et al, 2002). Henriksen et al (2006) investigated the behavior of parallelepiped pieces of wood and revealed the influence of the dimensions on the rate of conversion. Experimentally, he showed that char reactivity was considerably higher when the particle was positioned so that the fibers were in the direction of the reactant gas flow.…”

Influence of temperature and particle size on the single and mixed atmosphere gasification of biomass char with H 2 O and CO 2

“…The chars were kept for 1 h at the final temperature, cooled under nitrogen and stored afterwards in a sealed container. The low heating rate is expected to ensure good temperature uniformity in the wood particle and to lead to a quite homogeneous wood char, from the structural and chemical viewpoints, as demonstrated by [24] and pointed out by [25] and [26]. During the pyrolysis reaction, the char par ticles shrink and get an ovoid form.…”

Simulation Investigation of Single Oil Shale Particles Drying in Superheated Steam and Experimental Validation

“…Further, the coefficient of moisture diffusion as a function of temperature and moisture content was determined. The anisotropy of effective diffusion, permeability and thermal conductivity of moisture in different directions was determined as well [18,19]. The experiments were carried out under atmospheric pressure at different temperatures (463-483 K).…”