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.

Material

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

(HR/CCI)

Loose

Tapped

Loose

Tapped

GB

1446.04

292.68 341.56 0.78 0.74

1.17

14.29

42.5

Excellent/excellent

GW

1337.26

137.82

216.67

0.90

0.85

1.57

36.39

61.0

Cohesive/poor

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.

References

  1. Marshall, L. and D. Gaudry, The Application of the Dedicated Milling Concept for 100% Wood Firing at Atikokan Generating Station. 2011, Ontario Power Generation, Atikokan GS OPG.
  2. Barta-Rajnai, E., G. Várhegyi, L. Wang, Ø. Skreiberg, M. Grønli, and Z. Czégény, Thermal Decomposition Kinetics of Wood and Bark and Their Torrefied Products. Energy & Fuels, 2017, 31(4) 4024-4034.
  3. Chow, P., F.S. Nakayama, B. Blahnik, J.A. Youngquist, and T.A. Coffelt, Chemical constituents and physical properties of guayule wood and bark. Industrial Crops and Products, 2008, 28(3) 303-308.
  4. Bridgwater, A.V., Review of fast pyrolysis of biomass and product upgrading. Biomass and Bioenergy, 2012, 38(0) 68-94.
  5. Rezaei, H., F. Yazdanpanah, C.J. Lim, A. Lau, and S. Sokhansanj, Pyrolysis of ground pine chip and ground pellet particles. The Canadian Journal of Chemical Engineering, 2016, 94(10) 1863-1871.
  6. Baxter, L., Biomass-coal co-combustion: opportunity for affordable renewable energy. Fuel, 2005, 84(10) 1295-1302.
  7. Lam, P.S., S. Sokhansanj, X. Bi, C.J. Lim, L.J. Naimi, M. Hoque, S. Mani, and A.R. Womac, Bulk density of wet and dry wheat straw and switch grass particles. Applied Engineering in Agriculture, 2008, 24(3) 351-358.
  8. Rezaei, H., C.J. Lim, A. Lau, and S. Sokhansanj, Size, shape and flow characterization of ground wood chip and ground wood pellet particles. Powder Technology, 2016, 301(737-746.
  9. Rezaei, H., C.J. Lim, A. Lau, X. Bi, and S. Sokhansanj, Development of Empirical Drying Correlations for Ground Wood Chip and Ground Wood Pellet Particles. Drying Technology, 2016, null-null.
  10. Rezaei, H., S. Sokhansanj, X. Bi, C.J. Lim, and A. Lau, A numerical and experimental study on fast pyrolysis of single woody biomass particles. Applied Energy,
  11. Guo, Z., X. Chen, Y. Xu, and H. Liu, Effect of granular shape on angle of internal friction of binary granular system. Fuel, 2015, 150(298-304.

Processing Municipal Solid Waste to Produce Refuse-Derived Fuel (RDF)

Waste to Energy Approach

Municipal governments throughout the world are facing choices about how to manage the unending stream of waste generated by their residents and businesses. In some places, landfills and dumpsites are filling up, and all landfills and dumpsites leak into the environment. As populations continue to grow, the issue of waste becomes more urgent and more complicated. Although enhancing recycling technologies reduces a significant fraction of waste, still a large portion of municipal solid waste ends up in landfills. Landfilling waste means the loss of resources and landfill sites. Also, risk toxins leach into soil and water and produce emissions of methane (CH4) and carbon dioxide (CO2) that contribute to climate change.

Using a combustible fraction of solid wastes that cannot be recycled as an energy source is one efficient way to reduce the amount of landfilled waste. The combustible fraction of waste is known as “Refuse Derived Fuel (RDF)”. RDF covers a wide range of waste materials which have been processed to fulfill guidelines, regulatory or industry specifications mainly to achieve a high calorific value. RDF production technology contributes to the “waste to energy” approach, reduces the carbon footprint and is essential for diverting waste from landfill. RDF captures the energy in non-recyclable and combustible waste and turns it into a replacement for fossil fuels like coal or oil. RDF is a renewable solid fuel that is used to generate energy. The RDF could be used in the cement industry; steel furnaces; power stations, substituting coal and oil; or be incinerated in energy-from-waste plants.

The benefits of harnessing this otherwise wasted energy are clear. Energy harvesting from RDF eliminates a huge amount of carbon dioxide equivalent gases (mostly carbon dioxide and methane) from being emitted every year from the burning of fossil fuels. Not only that but also every tonne of waste that is diverted from landfill eliminates 0.54 tonnes of carbon dioxide equivalent from being emitted in landfill gas. This is a significant saving of greenhouse gas emissions and we’re proud to be investing in and using this technology.

The further processing of RDF to enhance its physical and chemical properties strongly depends on the following application. Even taking into account transport costs, this often offers a cheaper alternative to landfill.  It also reduces emissions of the air pollutants that contribute to climate change, as the waste is burnt cleanly and efficiently to extract as much energy as possible.

Processing MSW to produce RDF:

To produce a standard solid fuel, waste goes through a range of processes. Municipal solid waste (MSW) and commercial and industrial waste (C&I) contains many different materials that need to be separated mechanically into a high- and a low-calorific fraction. This process involves shredding, screening and classifying of the material. Metals, inerts, and organics are removed; light fractions with high-calorific value (e.g. plastics, textile, and paper) remain. Table 1lists the typical components in input and output streams of an RDF production facility.

 

Table 1 – Typical components in input and output streams of an RDF production facility.

Input Output
Municipal solid waste (MSW)

Commercial and industrial waste (C&I)

Construction and demolition waste

Pulp and paper industry rejects

Other waste fractions, bulky waste, organic waste, sorting residue

RDF

Glass

Paper

Plastic

Mineral

Textile

PVC

Wood

 

RDF production starts with the separating non-combustible wastes such as metal and glass from combustibles. The larger items must be broken into smaller pieces. Nest stage is the collection of un-segregated municipal waste, including organic waste (primarily food waste) and materials like paper, cloth, plastic, and wood that provide the calorific value required to burn. Ideally, during the separation stages, hazardous materials would be removed completely, but unfortunately, this is rarely possible. Another serious challenge in making RDF, particularly in less developed or tropical countries, is moisture. Since organic materials are not separated out at the source, MSW has a very high moisture content. Many RDF plants separate out some of the organic matter and sell it as compost.

The production of RDF includes a series of steps. The steps are taken and their sequence – as well as the specific machinery used – may differ depending on the waste characteristics, climatic conditions, technologies available, and final treatment(s) planned in a given location. The separation of waste mostly happens based on their physical properties such as size, weight (moisture content), electromagnetic properties etc.

In a more detailed process flow diagram, Figure 2 shows the sequences of a process in an RDF-production facility developed by ANDRITZ (Stattegger, Austria). For the sake of more clarification, the following clips, Figure 1 and Figure 2, show the industrial MSW treatment processes to shred, sort, separate, dry and produce final RDF either in bale or pellet form.

Figure 1 – A video showing various stages in MSW processing such as shredding, sorting and separation and baling.

Figure 2 – A video showing processes taken to produce RDF by ANDRITZ.

 

In continue, some of the processes that are used in the processing of MSW are explained. Obviously, the existence and sequence of the described stages depend on the waste characteristics and final product quality/application.

Manual separation

Bulky items such as large pieces of wood, rocks, long pieces of cloth, etc. are removed by hand before mechanical processing begins. Equipment involved in manual separation usually includes a sorting belt or table. Handpicking of refuse is perhaps the most prevalent MSW handling technique; it is also the only technique for removal of PVC plastics.

Air separation

In this step, fans are used to create a column of air moving upwards. Low-density materials are blown upwards, and dense materials fall. The air carrying light materials, like paper and plastic bags, enters a separator where these items fall out of the air stream. The quality of air separation depends on the strength of the air currents and how materials are introduced into the column. Moisture content is also critical as water may weigh down some materials or cause them to stick together.

Size reduction

Two types of devices are commonly used for this process: hammer mills and shear shredders. Hammer mills consist of rotating sets of swinging steel hammers through which the waste is passed, and shear shredders are used for materials that are difficult to break apart such as tires, mattresses, plastics, etc. The hammers need frequent resurfacing or replacement. Both are energy and maintenance-intensive. Hammer mills shatter items such as fluorescent light bulbs, compact fluorescent lamps, and batteries.

Trommel screening

A trommel screen, also known as a rotary screen, is a mechanical screening machine used to separate materials, mostly solid-waste processing industries. It consists of a perforated cylindrical drum that is normally elevated at an angle at the feed end. For an inclined drum, objects are being lifted and then dropped with the help of lifter bars to move it further down the drum; otherwise, the objects roll down slower. Furthermore, the lifter bars shake the objects to segregate them. Lifter bars will not be considered in the presence of heavy objects as they may break the screen.

Physical size separation is achieved as the feed material spirals down the rotating drum, where the undersized material smaller than the screen apertures passes through the screen, while the oversized material exits at the other end of the drum.

In municipal solid waste industry, trommel screens classify sizes of solid waste. By removing inorganic materials such as moisture and ash from the air-classified light fraction segregated from shredded solid waste, trammel screening improves the fuel-derived solid waste.

Figure 3 – Schematic of a trommel screening apparatus to classify materials based on their dimensions.

 

Another available design of trommel screen is concentric screens with the coarsest screen located at the innermost section. It can also be designed in parallel in which objects exit one stream and enter the following. A trommel in series is a single drum whereby each section has different apertures size arranged from the finest to the coarsest.

Advantages and limitations of competitive processes

One of the competitors in the screening process is vibrating screens. Trommel screens are vibration-free which cause less noise than vibrating screens. Trommel screens are cheaper to produce than vibrating screens, too. Trommel screens are more mechanically robust than vibrating screens allowing it to last longer under mechanical stress.However, the trammel screen has a lower capacity of processing material than vibrating screens. This is because only one part of the screen area of the trommel screen is utilized during the screening process whilst the entire screen is used for a vibrating screen.

Drying

Drying process reduces the moisture content of waste and prevents the leachate production. Dried materials are inactive biologically and are easier to store. The result is a homogeneous refuse-derived fuel. The partially decayed waste should be dried, either under the sun, by hot air, or by a combination of both. This important step in the process differs in each facility depending on the investment or land availability. Solar drying is not possible during rainy seasons, and most facilities run at a fraction of their capacity during the rains, sending most of the waste to landfills. Mechanical drying, on the other hand, requires significant amounts of energy that could easily render RDF plants unprofitable without huge government subsidies.

Metal separation

Ferrous metal separation (Magnetic separation)

Electro-magnets are used in this step so they can be switched on or off to allow removal of collected metals. However, not all metals can be removed by magnets. Non-ferrous metals do not have iron and do not respond to the magnetic field. Stainless steel, copper, and aluminum, for example, are only weakly magnetic or are not magnetic at all. A further limitation of this technique is that small magnetic item will not be picked up if they are buried in non-magnetic materials, and larger magnetic items can drag unwanted items like paper, plastic, and food waste along with them.

Non-ferrous metal separation (Eddy current separator)

The clip showing in Figure 4 displays a separation of non-ferrous metals from inert materials in an eddy current separator. Eddy current separators, or non-ferrous separators, use the current induced in little swirls (“eddies”) on a large conductor and separate non-magnetic metals. An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. If a large conductive metal plate is moved through a magnetic field which intersects perpendicularly to the sheet, the magnetic field will induce small “rings” of current which will actually create internal magnetic fields opposing the change.

Eddy current separators handle high capacities because the conveyor belt separates and carries away non-ferrous metals continuously and fully automatically. An important factor for good separation is an even flow of material, supplied by a vibrating feeder or conveyor belt, for example, to provide a uniform monolayer of materials across the belt. It is especially important with smaller fraction sizes.

 

Figure 4 – A video showing the eddy current separator operating mechanism.

 

Producing the final product

Once all of the separating and size reduction steps are complete, the final RDF product can be formed into bricks or pellets or can be left as fluff. Each form is derived from material separated at a particular stage in the process. Large pieces that escape the trommel screening stage and lighter materials like plastic bags that get blown off during air separation are baled together as RDF bricks. The shredded material from the hammer/flail mill and medium-size rejects from the trommel screens are used for the RDF fluff. Finally, the residual waste is mixed with binders like agricultural husk and passed through a pelletizing machine that converts the waste into pellets.

Differences Between Torrefaction & Carbonization

As the reduced burning of coal, the global demand for biomass energy is increasing rapidly in recent decades of years. As the largest global source of renewable energy, the biomass energy contributes an estimate 10% of global primary energy production. Given the rising concern about the global warming and sustainable development, this share is expected to rise.

The most common use of the biomass energy is direct combustion, followed by gasification, carbonization, torrefaction etc. Even though emission out of the direct biomass combustion is much lower than the coal, people are seeking the much cleaner way to reduce the carbon emission. Having gone through the carbonization and torrefaction, the biomass products have higher heating calorific value and lower flue gas emission.

Carbonization that produces charcoal from biomass was widely practiced for extraction of iron from iron ore in ancient India and China ( 4000 BCE ). Charcoal is still being used in many parts of the world as a smokeless fuel as well as a medium for filtration of water or gas.

Torrefaction (French word for “roasting”), a relatively new biomass conversion option, is similar to carbonization that produces solid fuels from biomass but has some important differences. In any case, this option is also attracting much attention especially in its near term application in co-firing biomass in coal-fired power plants and possibly for replacement of coke in metallurgy.

What is torrefaction?
It is a thermochemical process in an inert or limited oxygen environment, where biomass is slowly heated to within a specified temperature range and retained there for a stipulated time so that it results in near complete degradation of its hemicellulose content while maximizing mass and energy yield of solid product. Typical temperature range for this process is between 200℃ and 300℃. When the temperature is higher than 300℃, it would cause the loss of lignin in biomass, which could make it difficult to form pellets from torrefied products.

What is carbonization?
Carbonization (or carbonisation) is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. The temperature range of this process is above 300℃.

What are the differences between the the carbonization and torrefaction?
The torrefaction process is sometimes confused with carbonization, but the motivation, process conditions and usages of this two processes are not necessarily the same.

1. Major Objective:
A major objective of torrefaction is to increase the energy density of the biomass by increasing its carbon content while decreasing its oxygen and hydrogen content. This objective is similar to that of carbonization that produces charcoal but with an important difference that the latter does not retain maximum amount of energy of the biomass, and thereby gives low energy yield.

2. Process Condition:
Carbonization is similar to torrefaction in many respects, both carbonization and torrefaction require relatively slow rates of heating. Carbonization takes place at higher temperatures with a certain level of oxygen that allows sufficient combustion to supply the heat for the process. The torrefaction process on the other hand tries to avoid oxygen as well as combustion. Torrefaction is a thermal decomposition that takes place at low temperature and within a narrow temperature range of 200~300℃, while carbonization is 600℃ destructive distillation process. Carbonization produces more energy dense fuel than torrefaction, but it has a much lower energy yield.

3. Usages:
Both of the torrefaction pellets and carbonization pellets can be used as fuel. In addition, the carbonization pellets can be used in the following industries:

- Manufacture of carbon disulfide, sodium cyanide, and carbides;
- Smelting and sintering of iron ores, case hardening of steel, and purification in smelting of nonferrous metals;
- The carbonization pellets can be converted to activated carbon, which apply to water purification, gas purification, solvent recover, and waste water treatment;
- Carbon sequestration and soil remediation. Torrefied chicken manure and biomass can also be used in soil improvement.

SIMEC has started the R&D of torrefaction & carbonization technology since 2004. It is the technology that upgrade the value of biomass. Torrefaction and pelletization can be combined to maximize the commercial value of biomass. The final product is also called Black Pellets. Accompany with the increasing market demand of Black Pellets, we believe SIMEC technology will benefit more and more clients and investors.
Torrefaction is the future of biomass!

Pinnacle Renewable Energy building Alberta pellet plant

Pinnacle Renewable Energy Inc. will soon build its first wood pellet plant outside of British Columbia.

Parkland County officials announced on May 3 that the company will build a new, $85 million pellet plant near Entwistle in central Alberta. The new facility will bring Pinnacle’s total number of pellet plants to eight, and add 400,000 metric tons to its existing capacity of more than 1.5 million metric tons.

All of Pinnacle’s plants, located in Houston, Burns Lake, Meadowbank, Quesnel, Williams Lake, Armstrong and Lavington, B.C., run 24-7, according to the company. Pinnacle also owns and operates the Westview port in Prince Rupert, B.C., the first dedicated wood pellet terminal in the world capable of handling Panamax vessels.

Most of Pinnacle’s output are pellets that are exported to overseas markets, but the company also manufacturers a range of other products, including softwood pellet fuel for home heating, animal bedding and natural sorbent.

"We are thrilled to welcome Pinnacle Renewable Energy Inc. to Parkland County and the Hamlet of Entwistle," said Entwistle Mayor Rod Shaigec. "The positive economic impact this investment will have on our community is tremendous. We look forward to a long-term relationship with Pinnacle."

“Parkland County is excited to work with Pinnacle Renewable Energy Inc. on its expansion into Alberta,” said Mike Heck, chief administrative officer of Parkland County. “With the recent launch of our Major Business Attraction Program, welcoming Pinnacle to the Entwistle community shows the opportunities and success this program can create for our communities.”

The most recent Pinnacle plant to come online was the Lavington plant, built about a year and a half ago, according to CEO Rob McCurdy. He said that as the company expanded throughout B.C., it saw opportunity in Alberta. "We've got a strong bench strength of talent, we had the team for building the plant, and as we learned more about the fiber basket, it all came together," he said. "We’re quite happy to be in Alberta—it’s a pretty natural growth for our company."

McCurdy said dirt and heavy equipment work began at the site on May 1. "We're underway, and we anticipate to have most of the equipment assembled and start commissioning by the end of the year, with a phased commissioning through the first quarter of next year," he said.

Once in production, the plant will create approximately 70 full-time positions in the community of roughly 30,000 people.

Source: Biomass Magazine

Size, shape and flow characterization of ground chip and pellet particles of woody biomass

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.