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Biofuel pellets

Tuesday, 09 June 2015 10:59

Biofuel pellets have increased tremendously in popularity as a heating fuel during recent years, with many homeowners and commercial facilities choosing pellet stoves or boilers over traditional wood-fired equipment due to their relative ease of use. Wood pellets are a refined and densified biomass fuel that is formed when wood residues are compressed into a uniform diameter under high pressure. Wood pellets have a uniform shape, size and density and are ideal for automatic combustion heating systems such as pellet stoves and boilers. By pelletizing wood residues from sustainably harvested biomass and quality waste wood, millions of tons of biomass can be put to work for the local economy while at the same time preserving the environment. There are approximately 1,000,000 homes in the U.S. using wood pellets for heat, in freestanding stoves, fireplace inserts, furnaces and boilers. Pellet fuel for heating can also be found in such large-scale environments as schools and prisons.

However, wood is not the only suitable feedstock for manufacturing pellet fuel. A wide array of biomass materials can be used to manufacture pellets, most notably perennial grasses such as switchgrass or miscanthus. By engineering crops and waste such as cornstalks, straw, and residual forest waste, pellets can utilize millions of tons of waste and put them to work.

Why not simply burn raw biomass?

First, the moisture content of pellets is substantially lower (4% to 8% water–compared to 20% to 60% for raw biomass). Less moisture means higher BTU value and easier handling especially in freezing situations with green raw biomass materials. Second, the density of pellet fuel is substantially higher than raw biomass (40 lbs. per cubic foot verses 10-25 lbs. per cubic foot in raw material form). More fuel can be transported in a given truck space, and more energy can be stored at your site. Third, pellets are more easily and predictably handled. Their uniform shape and size allows for a smaller and simpler feed system that reduces costs. This high density and uniform shape can be stored in standard silos, transported in rail cars and delivered in truck containers.


Wood pellets are:
• Efficient | Biomass pellets are an efficient source of heat because they are heavily compressed and contain very low levels of moisture and ash, when compared to woodchips or cordwood. Virtually all of the material is burned and converted to heat. Wood pellets burn longer and more intensely due to their high density compared to firewood. The calorific value of wood pellets is 17.5 MJ / kg, which is 1.5 times greater than that of wood and comparable to coal. Wood pellet boilers are now extremely advanced and can operate up to a 95% efficiency rate and meet the strictest2222 environmental regulations.
• Cost-effective | Biomass pellets are economically competitive with home fossil fuel options and electric heat. Relative to other home heating alternatives, pellet fuel prices are less volatile.
• Convenient | Bags of pellets are easy to store. A ton of pellets consists of fifty 40-lb bags. Maximum safety during storage and transportation (not explosive, not subject to spontaneous combustion). Bags of pellets pour directly into a stove hopper, and regulating the rate at which the fuel flows into the hopper is easy given the small, uniform size of the pellets. The pellets do not freeze together at low temperatures. Boilers are low maintenance and easy to control, through automatic settings. Wood pellets can be used by those who are not currently on the national grid.
• Environmentally responsible | Biomass pellets are a sustainable fuel source, and burning pellets is carbon neutral. Reduced carbon emissions – much less than oil, log, coal or gas-fired boilers. Wood pellets burn cleanly and are more convenient than logs or wood chip. Cozy and pleasant heating, looks and smells like fireplace.
• Renewable | Wood pellets are a biomass product made of wood waste or other forest-thinning byproducts. According to data collected by the Pellet Fuels Institute, wood pellets are an unlimited fuel source.
• Locally produced | Biofuel pellet mills provide jobs, support local economies and can lead to energy independence. Wood pellets can be used by those who are not currently on the national grid.

In 2012 statistics show global wood pellet production was 19 million metric tons, with about half, or 9.3 million metric tons, being traded internationally. Europe and North America account for nearly all global production at 66 percent and 31 percent, respectively, while Europe consumed 80 percent and North American 17 percent.

A Description of the Pelleting Process

The process of manufacturing fuel pellets involves placing ground biomass under high pressure and forcing it through a round opening called a “die.” When exposed to the appropriate conditions, the biomass “fuses” together, forming a solid mass. This process is known as “extrusion.” Somebiomass (primarily wood) naturally forms high-quality fuel pellets, while other types of biomass may need additives to serve as a “binder” that holds the pellet together. However, the creation of the pellets is only a small step in the overall process of manufacturing fuel pellets. These steps involve feedstock grinding, moisture control, extrusion, cooling, and packaging. Each step must be carried out with care if the final product is to be of acceptable quality.

Feedstock Grinding
Standard-sized pellet mills generally require biomass that is ground to particles that are no more than 3 millimeters in size. Several types of equipment are available to carry out this task. If the biomass is quite large and dense (e.g., wood), the material is first run through a “chipper,” and then run through a hammer mill or similar device to reduce the particles to the required size. Smaller and softer biomass (e.g., straw) can be fed directly into the hammer mill without first being chipped.

Moisture Control
Maintaining an appropriate moisture level in your feedstock is vital for overall quality of the final pellets. For wood, the required moisture level of the feedstock is at or near 15 percent. Other types of biomass have other requirements—you may need to experiment a bit. Moisture can be removed from the feedstock by oven-drying or by blowing hot air over or through the particles. If the feedstock is too dry, moisture can be added by injecting steam or water into the feedstock.

The pellet is actually created in this step. A roller is used to compress the biomass against a heated metal plate called a “die.” The die includes several small holes drilled through it, which allow the biomass to be squeezed through under high temperature and pressure conditions. If the conditions are right, the biomass particles will fuse into a solid mass, thus turning into a pellet. A blade is typically used to slice the pellet to a predefined length as it exits the die. Some biomass tends to fuse together better than other biomass. Sawdust is an especially suitable feedstock for pelleting because the lignin that is naturally present in the wood acts as a glue to hold the pellet together. Grasses tend to not fuse nearly as well, and the resulting pellets are less dense and more easily broken. The proper combination of input material properties and pelleting equipment operation may minimize or eliminate this problem. It is also possible to add a “binder” material to the biomass to help it stick together, or to mix a fraction of sawdust, with similar results. Distillers Dry Grains (a product of the corn ethanol industry) are reported to improve the binding properties of some biomass.

Pellets, as they leave the die, are quite hot (~150°C) and fairly soft. Therefore, they must be cooled and dried before they are ready for use. This is usually achieved by blowing air through the pellets as they sit in a metal bin. The final moisture content of the pellets should be no higher than 8 percent.