Common materials for pelletization
Wood and its derivatives are the raw materials most widely used to create pellets. Forestry operations often establish pellet plants to make use of byproducts that might otherwise be discarded as waste. Debris from the harvesting of timber is collected for economic use. Cleared brush on tree farms is another source. However, the most common sources of material for the manufacture of wood pellets are the by-products of primary wood processing at sawmills and plywood plants: bark, sawdust, woodchips and shavings. Initial wood processing involves removal of the tree bark, which then becomes a by-product. Edging and trimming the lumber creates sawdust as the log is converted into boards. Whole slabs can be reduced into woodchips and mulch. Shavings are created as the lumber is then smoothed. These materials are superior for the pelletization process in that fewer processing steps are necessary before entering the pellet mill.
In locations without large-scale forestry operations the focus has been on the conversion of agricultural waste—notably straw—into biomass energy. This is due to its relative abundance and fast rate of rejuvenation. In Europe alone, it is estimated that an estimated 23 million dry tons is produced annually—double the weight of its current pellet demand. Denmark has been one of the leading countries in converting this agricultural product into energy.
However, it is less often used than wood for several reasons. First, the material is characterized by low density. This creates problems in transport, as greater volumes of straw must be shipped and processed in order to match the calorific value of more dense materials such as wood. There are also higher traces of impurities in straw than in wood, such as potassium and sodium, which lead to slagging when burned. Specially designed boilers must be used to counter the effects of these impurities.
Nonetheless, a number of agricultural residues are viable for making biomass pellets.
Reed canary grass
Reed canary grass is harvested annually in spring. Its dry yield varies between 5 to 12 tons per hectare with a moisture content of between 10 and 15 percent. The plant is advantageous in that it can grow in the same location for several years without the need for rotation, and does so without the need of extensive amounts of water and fertilizer. Harvested reed canary grass is baled for more convenient transport, although the baled straw must be stored in a way to avoid gathering moisture.
Miscanthus can exceed heights of 3.5 meters in a single growing season. Its rapid growth, low mineral content and high biomass yield make miscanthus a viable source of biofuel. Additionally, the plant exhibits greater photosynthetic efficiency and lower water use requirements than other plants. Miscanthus has low nutritional requirements and is highly efficient in the processing of nitrogen. These properties allow the plant to grow on barren land without the need for heavy fertilization.
Miscanthus has the potential to yield 15-25 metric tons per hectare, dry weight, on an annual basis. Upon harvest it can be converted into biofuel products such as pellets.
Switchgrass is another option for pelletization, although at a yield of only 7-11 metric tons per hectare it is outperformed by miscanthus. This grass is also called “elephant grass,” and grows in the prairies of North America.
Cardoon is another herbaceous species that has been identified as a feasible energy crop for countries in southern Europe. Its dry yield varies between 3-11 metric tons per hectare. Cardoon grows in areas with limited rainfall. Although the plant’s calorific value is high at 15 megajoules per kilogram its 13.9 percent ash content is comparatively high. The greatest potential for cardoon may be as a supplement to other biomass sources to make blended biofuel.
Olive and rapeseed
Olive pits may be recovered as a by-product in the production of olive oil. These then compete with pellets as a source of biomass energy in domestic boilers or via cogeneration at large industrial plants. Unlike pellets, olive pits need not be manufactured, only conditioned. Whereas olive oil, and hence olive pits, is more prevalent in southern Europe, rapeseed is more common in the north. Here, rape residues are mixed with wood or other straw materials to make pellets. In Denmark, four pellet plants use rape residues as a raw material.
Residues from sunflowers may also be used to make pellets in the Ukraine and in some other European countries pellets.
Grapes and citrus fruits
Nut shells and fruit seeds are generally unavailable in significant quantities to utilize in large-scale pellet production, but may hold potential on a local basis.
Numerous other agricultural materials have also shown potential for use in pellet production. Of particular interest to the biomass community have been various by- products of industrial production—materials such as coffee and corn waste. The abundance of such residual materials enhances the appeal of such niche pellet production, although most have shown poor combustion characteristics.
Mixing raw materials remains rare, although it has been shown than mixing one with natural binding qualities to a base feed improves pellet durability. Potential additives include bark to a hardwood base, brewers spent grains and beech dust. Digestate— a by-product of biogas production—may also be used to make pellets.
For the prospective pellet plant operator, the agricultural sources which can be utilized are nearly limitless; although so too are the variations in quality from using different raw materials. Substantial differences exist between softwoods and hardwoods, different species, and even different parts of plants. Also impacting the quality of the raw material are the climatic and seasonal variations, as well as the duration and method of storage.
The standard pelleting process
This guide will follow the process to produce quality fuel pellets, and will state the equipment used at each stage, the principles of the process and other key points. The process is as follows:
- Material transportation: fans, cyclone separators and screw augers
- Drying: rotary/drum dryers, pipe dryers
- Mixing: batch mixers
- Conditioning: water and steam addition, binders
- Pellet production: round and flat die pellet mills
- Sieving: removing fines
- Cooling: counter flow coolers
- Pellet transportation: bucket elevators
- Bagging and storage: bags, sacks and silos
Moisture in the biomass facilitates starch gelatinization, protein denaturization, and fiber solubility during densification. Steam heat-treated biomass is superior to raw biomass since the additional heat modifies these physiochemical properties to the extent that binding between particles is enhanced, resulting in improved physical pellet quality. Moisture in the biomass during the densification process increases bonding, thereby increasing the contact area for each feedstock particle. Pellets tend to become fragile in a few days if the moisture content is less than 4 percent due to moisture absorption from the environment. Feed material, which contains higher proportions of starch and protein, will produce more durable and higher quality pellets than biomass containing only cellulosic material. The optimum moisture content for pelleting cellulosic materials is 8 to 12 percent, whereas for starch and protein materials (mostly animal feeds), the optimum moisture content can range up to 20 percent.
Particle size, shape and distribution
In general, the density and durability of pellets is inversely proportional to particle size because the smaller particles have a greater surface area exposed during densification. Medium or fine-ground feed constituents are desirable in pelleting because these sizes have greater surface area for moisture addition during steam conditioning, which increases starch gelatinization and promotes better binding. A certain percentage of fines to medium-sized particles may be required to improve pelleting efficiency and reduce pelleting costs. However, very small particle sizes can lead to jamming of the pellet mills and affect production capacity. The compaction and stabilization of straw may be different than grasses due to the fact that straw tends to have a significantly smaller leaf content. Particle size and pellet durability must be balanced against the energy required to accomplish size reduction.
Lignin is a complex carbohydrate found in the cell walls of all cellulosic plant material. Lignin is central to plants’ mechanical strength. Therefore, generally the more lignin present, the stronger the plant. Different cellulosic materials have different lignin compositions. For example, wood has a greater amount than straw, which makes sense in relation to mechanical strength.
During pellet production, lignin is what binds the material together to form a pellet. Therefore it makes sense that the more lignin present, the mechanically stronger the pellet. Wood pellets are generally harder and stronger than straw pellets. A raw material with a higher percentage of lignin is less likely to require a binder and more likely to produce mechanically strong pellets.
Under the heat and pressure of the pellet mill the lignin melts and binds the material together to form a pellet. Once the pellet leaves the mill, the lignin begins to solidify and cool, conferring strength and density.
If the amount of lignin present in the raw material is insufficient, additional binders can be used. Many different binders are used in the pellet industry depending on the purpose of the pellet. For example binders suitable for feed pellet production may not be suitable for fuel pellets as the binder may interfere with the combustion process.
Some binders are used purely to increase pellet density, while some are also used to improve lubrication through the pellet mill, reducing wear and increasing productivity.
The amount of binder to add to the raw material varies depending on its intended purpose—whether to purely bind or to also increase productivity. When used in quantities below 10 percent of the feedstock, the binder serves purely for binding. Anything above this may increase productivity by reducing resistance as the material passes through the pellet mill die. The simplest additional binders include vegetable oil, molasses, starch, gluten, dried distiller’s grain and rape cake. Many organizations constantly experiment using different materials as pellet binders, using by-products from other processes and waste products. Generally if it’s sticky and oily it may be function as a viable binder.
Mixing materials together can also address the binder issue. For example, mixing wood with straw and then placing it into the pellet mill can raise quality beyond that of a pure straw pellet. This can have other benefits, particularly for fuel pellets. Straw pellets compared to wood produce more corrosion and ash, and therefore require more maintenance. A wood/straw pellet has better combustion characteristics and extends limited wood resources.
A: The material first enters the pellet mill and comes into contact with the roller. B: Some of the material will be compressed under the roller through the die holes. Material which lacks binder may struggle to do even this. It may display properties more like a dry material, as the material cannot bind together under compression.
C: Due to the lack of a binder the material may be unable to form a carpet, and again it may display similar properties to a dry material.
D: Even with sufficient moisture to give the required pressure and heat, the material may not compress at all. Generally though some compression and binding will take place, but not to the levels required.
E: If the lack of binder is extreme, the material may not form a pellet at all. In many cases small layers of compressed material will emerge, but not a complete pellet. A short pellet is not detrimental to its purpose, however it should be checked that once the pellets cool they do not crumble into dust too easily. If this occurs the lack of a binder needs to be addressed.
Ideally to save costs the plant should be situated as close to the supply of raw material as possible. However, this is obviously not always possible. Location will also be dictated by access to a power supply if three-phase electricity is needed.
Heavy lifting equipment
Pellet production equipment is heavy, particularly the pellet mill. Therefore heavy lifting equipment such as large forklifts may be required. This should be considered when choosing a location, so that there is sufficient access to the site.