Wood pellets are heating fuels made from compressed biomass. Wood pellets are generally made from compacted sawdust or other waste streams from saw mills or other wood products manufacture. Other sources of feedstock include rice husks, peanut shells, palm and coconut shells, groundnut shell, sugarcane biogases, crop stalk, coffee husk, paddy straw, sunflower stalk, cotton stalks, tobacco waste, mustard stalk, soybeans husk and many other Agro wastes.
There are of course wood pellets made from complete trees, tree tops and branches from logging operations. A range of grasses like switchgrass can also be made into wood pellets. In contrast to wood based pellets which are increasing in cost due to growing scarcity of forest residues, grasses offer a more cost effective and renewable option. To avoid removal of whole trees being and deforestation to supply wood pellets market, new solutions like faster growing grasses will need to be developed. A grass based biofuel industry also has the potential to inject new cash into rural economies without depleting forest resources. However attention will need to paid to soil enrichment otherwise soil health will decline on these farm lands.
Wood Pellets Processing
Wood pellets are manufactured in several types and grades as fuels for electric power plants, home furnaces, wood stoves and other gasifier heating and power applications. Wood pellets are quite dense, easily handled and can be produced with a low moisture content which allows them to burn with relatively high combustion efficiency.
The first part of the process is to reduce chunky biomass (wood, straw, grass) from large pieces down to chip a size. Of course this step would not be necessary if sawdust is being used to make wood pellets. There are two main types of chipping units, a disc wood chipper and drum chippers which can be stationery or mobile units. The ultimate choice of disc or drum along with sizing the unit will depend on the material being chipped.
Once the chunky biomass is chipped the material can be fed into a hammer mill. A hammer mill can be powered by an electric or diesel motor and grinds or hammers the straw, switchgrass, wood chips, or corn stalks ready for the pellet mill. Hammer mills are an important part of creating high quality wood pellets as the raw material must be consistent and uniform before being fed into the pellet mill.
Wood pellets are popular as they are a dry and uniform fuel with a moisture content below 10%. If the biomass has a moisture content above 15% a flash, or a rotary drum dryer will be required. Drying costs can add significant costs due to the due to the high heat demand, capital and overall operating costs. Cost savings can be achieved by air drying the wood biomass or circulating air through wood stored in bins or racks. However this is time-intensive and depending on output volume may require large storage areas.
The dry hammered material is then transported to the pellet mill through the cyclone where dust and raw material is separated before being fed directly into the pellet mill. The main purpose of the cyclone is to reduce the dust and separate other non combustible particles from the biomass.
The pellet mill or press is the heart of the wood pellet process. Small scale mills are often simple screw or hydraulic presses, although many small units are now offering flat or ring die mills. Units can be powered by combustion engines in smaller mills or 3 phase electricity in larger systems. Poor quality pellets can crumble to sawdust, or produce more ash, so it is important that pellets have been manufactured to a recognised standard. More information on wood pellets mills is available on our dedicated pellet mill page.
This often just a vibrating sieve or conveyor belt with directed airflow to cool the pellets. Generally the costs for cooling are quite small don’t add that much to total pellet production costs.
Packaging and Storage
Good packaging and storage are important to protect the pellets from absorbing moisture and breaking apart. If the pellets are exposed to humid environments they will absorb moisture, which will impact on the combustion efficiency.