Biomass Pelletization Process

Biomass pellets are a popular type of biomass fuel, generally made from wood wastes, agricultural biomass, commercial grasses and forestry residues. In addition to savings in transportation and storage, pelletization of biomass facilitates easy and cost effective handling. Dense cubes pellets have the flowability characteristics similar to those of cereal grains. The regular geometry and small size of biomass pellets allow automatic feeding with very fine calibration. High density of pellets also permits compact storage and rational transport over long distance. Pellets are extremely dense and can be produced with a low moisture content that allows them to be burned with very high combustion efficiency.

Biomass pelletization is a standard method for the production of high density, solid energy carriers from biomass. Pellets are manufactured in several types and grades as fuels for electric power plants, homes, and other applications. Pellet-making equipment is available at a variety of sizes and scales, which allows manufacture at domestic as well industrial-scale production. Pellets have a cylindrical shape and are about 6-25 mm in diameter and 3-50 mm in length. There are European standards for biomass pellets and raw material classification (EN 14961-1, EN 14961-2 and EN 14961-6) and international ISO standards under development (ISO/DIS 17225-1, ISO/DIS 17225-2 and ISO/DIS 17225-6).

Process Description

The biomass pelletization process consists of multiple steps including raw material pre-treatment, pelletization and post-treatment. The first step in the pelletization process is the preparation of feedstock which includes selecting a feedstock suitable for this process, its filtration, storage and protection. Raw materials used are sawdust, wood shavings, wood wastes, agricultural residues like straw, switchgrass etc. Filtration is done to remove unwanted materials like stone, metal, etc. The feedstock should be stored in such a manner that it is away from impurities and moisture. In cases where there are different types of feedstock, a blending process is used to achieve consistency.

The moisture content in biomass can be considerably high and are usually up to 50% – 60% which should be reduced to 10 to 15%. Rotary drum dryer is the most common equipment used for this purpose. Superheated steam dryers, flash dryers, spouted bed dryers and belt dryers can also be used. Drying increases the efficiency of biomass and it produces almost no smoke on combustion. It should be noted that the feedstock should not be over dried, as a small amount of moisture helps in binding the biomass particles. The drying process is the most energy intensive process and accounts for about 70% of the total energy used in the pelletization process.

Schematic of Pelletization of Woody Biomass

Before feeding biomass to pellet mills, the biomass should be reduced to small particles of the order of not more than 3mm.  If the pellet size is too large or too small, it affects the quality of pellet and in turn increases the energy consumption. Therefore the particles should have proper size and should be consistent. Size reduction is done by grinding using a hammer mill equipped with a screen of size 3.2 to 6.4 mm. If the feedstock is quite large, it goes through a chipper before grinding.

The next and the most important step is pelletization where biomass is compressed against a heated metal plate (known as die) using a roller. The die consists of holes of fixed diameter through which the biomass passes under high pressure. Due to the high pressure, frictional forces increase, leading to a considerable rise in temperature. High temperature causes the lignin and resins present in biomass to soften which acts as a binding agent between the biomass fibers. This way the biomass particles fuse to form pellets.

The rate of production and electrical energy used in the pelletization of biomass  are strongly correlated to the raw material type and processing conditions such as moisture content and feed size. The The average energy required to pelletize biomass is roughly between 16 kWh/t and 49kWh/t. During pelletization, a large fraction of the process energy is used to make the biomass flow into the inlets of the press channels.

Binders or lubricants may be added in some cases to produce higher quality pellets. Binders increase the pellet density and durability. Wood contains natural resins which act as a binder. Similarly, sawdust contains lignin which holds the pellet together. However, agricultural residues do not contain much resins or lignin, and so a stabilizing agent needs to be added in this case. Distillers dry grains or potato starch is some commonly used binders. The use of natural additives depends on biomass composition and the mass proportion between cellulose, hemicelluloses, lignin and inorganics.

Due to the friction generated in the die, excess heat is developed. Thus, the pellets are very soft and hot (about 70 to 90oC). It needs to be cooled and dried before its storage or packaging. The pellets may then be passed through a vibrating screen to remove fine materials. This ensures that the fuel source is clean and dust free.

The pellets are packed into bags using an overhead hopper and a conveyor belt. Pellets are stored in elevated storage bins or ground level silos. The packaging should be such that the pellets are protected from moisture and pollutants. Commercial pellet mills and other pelletizing equipment are widely available across the globe.

Eco-friendly Benefits of Fiberglass Insulation For Metal Buildings

Fiberglass insulation was first put on the market in 1938, and in all the years since, no alternative has really challenged its preeminent position as the most effective choice for insulation on both commercial and residential construction projects. Fiberglass insulation improves a structure’s energy efficiency, reduces heating and cooling costs, and makes occupants more comfortable. These are just a few of the advantages that make it the insulator of choice, even in the latest eco-friendly projects. Below are  additional benefits of fiberglass insulation:

1) Moisture Resistance

Fiberglass insulation does not absorb or retain water according to www.cyclonebuildings.com who utilise it in some instances. It can still be contaminated or compromised by moisture; insulation that has gotten wet needs to be inspected and dried to ensure that it does not lose its insulating properties.

Wet insulation can be successfully re-installed and deliver its full R-value as intended by the manufacturer so long as installers confirm that the insulation and the area around it in the structure have not been compromised by water.

In order to provide full insulating value, fiberglass insulation requires a vapor barrier. When properly selected and installed, a vapor barrier catches condensation before it can penetrate the building envelope and reach the insulation. The vapor barrier’s perm rating must be appropriate to the structure and the local climate, and it must be sealed into place with a proper adhesive so that it does not leak.

2) Fire Resistance

Fiberglass insulation is inherently non-combustible because the materials from which it is made – sand and/or recycled glass – are non-combustible themselves. Fiberglass insulation does not need to be treated with chemicals to make it fire-resistant, and it does not become any more combustible as it ages.

In many areas, local building codes even allow the use of fiberglass insulation as an effective fire stop in wall assemblies made of wood or steel.

3) Sound Dampening

Fiberglass insulation absorbs sound, and this means it reduces sound transmission through walls, ceilings, floors, and HVAC ducts where it is used. As a general rule of thumb, one inch of fiberglass insulation increases the sound transmission class, or STC, of a building assembly by three or even four points. Additional inches of fiberglass insulation each add two more points to the STC rating.

4) Use Of Recycled Materials

The manufacture of fiberglass insulation has come to rely on incorporating a significant amount of recycled material. Between 1992 and 2000, insulation manufacturers used over 8 billion pounds (3.6 billion kg) of recycled glass from pre and post-consumer sources. Using this material productively saved millions of cubic feet in landfill space.

The total amount of recycled material used in fiberglass insulation varies from brand to brand and product to product, but some products are made with as much as 80 percent recycled glass. Fiberglass insulation also requires the use of silica sand, which is an abundant and naturally-renewing resource.

Bottom Line

Fiberglass insulation remains a highly competitive and attractive insulation option, even when considered according to environmentally-friendly “green” priorities. In the decades it has been used, it has proven time and again to be a reliable and effective material.

Biomass Storage Methods

Sufficient storage for biomass is necessary to accommodate seasonality of production and ensure regular supply to the biomass utilization plant. The type of storage will depend on the properties of the biomass, especially moisture content. For high moisture biomass intended to be used wet, such as in fermentation and anaerobic digestion systems, wet storage systems can be used, with storage times closely controlled to avoid excessive degradation of feedstock. Storage systems typically used with dry agricultural residues should be protected against spontaneous combustion and excess decomposition, and the maximum storage moisture depends on the type of storage employed.

Moisture limits must be observed to avoid spontaneous combustion and the emission of regulated compounds. Cost of storage is important to the overall feasibility of the biomass enterprise. In some cases, the storage can be on the same site as the source of the feedstock. In others, necessary volumes can only be achieved by combining the feedstock from a number of relatively close sources. Typically, delivery within about 50 miles is economic, but longer range transport is sometimes acceptable, especially when disposal fees can be reduced.

Storage of biomass fuels is expensive and increases with capacity.

Agricultural residues such as wheat straw, rice husk, rice straw and corn stover are usually spread or windrowed behind the grain harvesters for later baling. Typically these residues are left in the field to air dry to moisture levels below about 14% preferred for bales in stacks or large piles of loose material. After collection, biomass may be stored in the open or protected from the elements by tarps or various structures. Pelletizing may be employed to increase bulk density and reduce storage and transport volume and cost.

Biomass Storage Options

  • Feedstock is hauled directly to the plant with no storage at the production site.
  • Feedstock is stored at the production site and then transported to the plant as needed.
  • Feedstock is stored at a collective storage facility and then transported to the plant from the intermediate storage location.

Biomass Storage Systems

The type of biomass storage system used at the production site, intermediate site, or plant can greatly affect the cost and the quality of the fuel. The most expensive storage systems, no doubt, are the most efficient in terms of maintaining the high fuel quality. Typical storage systems, ranked from highest cost to lowest cost, include:

  • Enclosed structure with crushed rock floor
  • Open structure with crushed rock floor
  • Reusable tarp on crushed rock
  • Outside unprotected on crushed rock
  • Outside unprotected on ground
  • Subterranean

The storage of biomass is often necessary due to its seasonal production versus the need to produce energy all year round. Therefore to provide a constant and regular supply of fuel for the plant requires either storage or multi-feedstocks to be used, both of which tend to add cost to the system.

Reducing the cost of handling and stable storage of biomass feedstocks are both critical to developing a sustainable infrastructure capable of supplying large quantities of biomass to biomass processing plants. Storage and handling of biomass fuels is expensive and increases with capacity. The most suitable type of fuel store for solid biomass fuel depends on space available and the physical characteristics of the fuel.