Background:
The goal of this study is to characterize the ground chip and ground pellet particles in terms of their grindability, size, shape and flow properties. Pulp-quality wood chips and commercial wood softwood pellets are ground using a lab-scale hammer mill equipped with screens ranging from 3.2 mm to 25.4 mm perforations. The power input and the flow of biomass through the grinder was measured. The ground particles from each screen size were analyzed for specific energy for grinding, analyzing shape and geometry of single ground particles, flow properties like bulk density, compressibility, and angle of repose.
The present study contributes to generate engineering data and to improve the understanding of particle handling and transporting in the feeding systems of thermochemical and biochemical conversion plants. The produced knowledge increases the operational efficiency and reduces the probability of stoppage in feeding system; thus, contributes to a reduction in the produced liquid fuel.
Grindability and Particle Size Distribution (PSD):
Pelletization enhances grindability. Wood pellets consumed significantly less grinding energy than wood chips. To produce the similar size distribution that is desirable for thermal conversion (<1 mm), wood pellet needed 1/7 of energy to grind wood chip (Figure 1).
Chip particles have a wide spread PSD which significantly change with the grinder screen size (Figure 2). But, PSD of the pellet particles have a narrow spread. Pellet particles are smaller and are less variable with grinder screen size. Regardless of opening size of grinder screen, more than 90% of pellet particles pass through a 2 mm sieve that makes the ground pellet appropriate for thermal conversion applications. It comes from the fact that internal particles of pellets were ground using a 3-5 mm grinder screen before the pelleting process. Wood chips should be ground with grinder screens of <6 mm to produce particles of smaller than 2 mm.
Particle Shape Analysis:
A representative number of single chip and pellet particles are pictured using a high resolution microscope and the pictures are processed using ImageJ software. Chip particles are rectangular, whereas pellet particle are irregular round and more circular. Figure 3 shows the shape of single chip and pellet particles in the microscopic pictures. Chip particles have a small aspect ratio (AR=W/L) of 0.21-0.22. On the other hand, pellet particles are shorter and have the AR of 0.62-0.64. It means that for a similar particle width, pellet particles were 3 times shorter than chip particles.
Even though the mechanical sieving process separates the particles based on their width, measurement of actual dimensions of particles by an image processing technique show that mechanical sieving underestimates the actual dimensions of the particles, since width of chip particles is about 1/2 of sieving average particle size and width of pellet particles is about 1/3 of sieving average particle size.
Flow Characterization
Flow properties of particles are analyzed using two dimensionless indexes of carr-compressibility (CCI) and Hausner ratio (HR) and also the angle of repose (AOR). HR and CCI shows how much compression happens in a bulk of sample during tapping.
Figure 4 depicts that larger particles had less tendency to make a compact bulk. Smaller particles were able to fill very small pores in the bulk and increase the density and consequently the compressibility index. The same trend was also observed for the pellet particles, though the variation in the compressibility of pellet particles was less than chip particles. In addition to the particle size, the effects that particle shape has on bulk compressibility is significant. Greater length appears to promote the compression of the particles. Chip and pellet particles subject to the 3.2 mm grinder screen have similar PSD. Yet, the chip particles compact by 50%, which is significantly more than the pellet particles which compact by 26%. Upon comparing the compressibility results, particle shape was confirmed to be more important than particle size in tapping compressibility.
The circular shape of pellet particle also enhances their tendency to flow over each other in angle of repose tests. Pellet particles roll over on each other and flow easier than thinner and longer chip particles.
