Bark vs Wood – Size, Shape and Flowability of Particles

In recent years, the power plants adopted regulations to phase out coal firing and investigate the feasibility of utilizing lignocellulosic biomass in order to reduce GHG emission [1]. Woody biomass has a lower ash content than agricultural biomass and consequently is a preferred feedstock for thermal conversion applications. A tree as a forest-sourced biomass is divided into the internal white wood, bark that covers the outer surface of the tree, and branches. For pulp and paper applications, de-barking is an essential process to obtain clean and high-quality wood chips. De-barking includes removal of bark and small branches that are not desirable in the pulp-making process. The bark is a waste for pulp and paper industry and consequently is a low-price feedstock.

Due to the more exposure of tree surface to the environmental dirt and also the higher mineral content of the bark [2]; such as Ca, K, Si, Mg, S, Mn, P, and Zn; the ash content of bark is more than the white wood. Chow et al. [3] measured the ash content of wood and bark for seven tree species and showed that generally bark of the tested species has an ash content of 3-7 times of wood. In addition to the chemical properties, wood and bark are different with respect to their physical and thermal properties. White wood and bark are two potential biomass feedstocks for the power stations. For power generation applications, biomass feedstocks should be ground below 2 mm to prevent a partial combustion phenomenon [4, 5]. Ground particles flow pneumatically with the re-circulated hot gas in the pipe lines leading to the conversion reactors. Particles dry in the exposure of hot air. The size, shape, and density of the biomass particles are known to affect the flow properties in the lines and feeding systems, drying rate and kinetics of thermal decomposition [5-10].

A research at the University of British Columbia (UBC) in Biomass and Bioenergy Research Group (BBRG) investigated the size, shape, density and their effects on the flowability properties of ground bark (GB) and ground wood (GW) particles. Both white wood and bark wood are ground using a laboratory-scale hammer mill grinder installed with a 3.2 mm grinder screen to produce the particles up to 2 mm. During the size reduction stage, bark produces more dust than wood chips. Around 12% of GB particles are smaller than 125 μm. On the other hand, the percentage of GB particles in the size range of 0.5-2 mm is slightly lower than GW particles. Therefore, the GB particles show a more distributed PSD than GW particles.

The digital image analysis of single particles (procedure is explained by Rezaei et al. [8]) helped to measure the real dimensions (width and length) of the GB and GW particles and calculate two shape factors of aspect ratio and sphericity. The shape factors quantify the deviation of particles’ shape from a sphere. Visual microscopic observation (Figure 1) shows that the GW particles are long and thin (needle shape), but GB particles are more irregular round and have comparable width and length. The further analysis, presented in Figure 2, shows that GW particles are about 3.5 times longer than GB particles. The average aspect ratio of GW and GB particles are 0.22 and 0.61, respectively.

Figure 1 – Microscopic picture of ground bark (GB) and ground wood (GW) particles.

The differences in size and shape of GB and GW particles influences their flow properties such as compressibility and angle of repose. Briefly, compressibility and angle of repose represents the tendency of the particles to interlock and compress. The higher values of Hausner Ratio (HR), Compressibility index (CCI) and Angle of Repose (AOR) means the poorer flowability of the material (procedure is explained by Rezaei et al. [8]).  Table 1 lists the particle density, loose and tapped bulk densities and the flowability indexes. The bulk of GB particles is denser than GW particles. The first reason is that GB particles have a higher individual particles density than GW particles that shows GB particles have a less internal void fraction. The second reason is that more-spherical GB particles make a less-porous bulk in a random settlement than needle shape GW particles.

Table 1 – Flow properties of ground bark (GB) and ground wood (GW) particles.


Particle density, ρp (kg/m3) Bulk density, ρb (kg/m3) Bulk porosity, ε Hausner ratio, HR Compressibility index, CCI (%) Angle of repose, AOR (degree) Flow class








292.68 341.56 0.78 0.74















The compressibility of GB particles is much less than GW particles. During a tapping process, the bulk of GB particles compress up to 17%, whereas the bulk of GW particles compress up to around 57%. Due to a mechanical handling of material, the needle shape GW particles fill the void volumes of the bulk more than the spherical particles and compress to a higher extend. More compression means the higher level of interlocking that prevents a free flow of material. The numerical values of the HR and CCI represent the higher compression of needle shape particles. Regarding the categorized range of HR and CCI indexes that Rezaei et al. [8] explained, GW particles are cohesive and GB particles have excellent flow properties.

Another flowability index is the angle of repose (AOR). AOR indicates the relative cohesiveness of the particles under a normal gravity force [11]. The experimental AOR data in Table 1 shows that GB particles tend to flow on each other more than GW particles. Similar to the results pertinent to compressibility, the AOR test shows that needle shape GW particles are cohesive and do not flow easily compared to the irregular round GB particles.

Figure 2 – Actual dimensions and shape factors of ground bark (GB) and ground wood (GW) particles.


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