To reduce the ash content of wood pellets and thereby improve their quality, a process known as "debarking" (removing the tree bark) is commonly employed. Debarking allows for the production of premium-quality wood pellets, which are typically used for space heating—read more details here. Tree bark has a high ash content, which can reach as much as 10%, and it accounts for approximately 5–20% of the total mass of the tree trunk, with the remaining 80–95% consisting of the inner wood. Factors that influence this proportion include the tree species, the thickness of the bark, and the age of the tree.
Naturally, tree bark has a significantly higher ash content—potentially up to 10 times greater—than the inner wood (the wood core or material used for wood pellets), which typically contains less than 1% ash. This occurs because bark serves as an outer protective layer that absorbs and accumulates inorganic minerals, sand, and dust from the surrounding environment.
Despite its relatively high ash content, tree bark remains suitable for use as fuel due to its high calorific value. The calorific value of dry bark ranges from 3,500 to 4,000 calories per gram (approximately 14.6 to 16.7 MJ/kg). In general, the calorific value of bark is lower than that of the inner wood core, which, when dry, can reach an average of 4,200 to 4,700 calories per gram (17.5–19.6 MJ/kg). The high ash content is the primary factor contributing to the lower calorific value of the bark. Beyond being processed into pellets—known as "bark pellets"—tree bark can also be manufactured into briquettes; the choice between these forms typically depends on market demand.
From a technical standpoint, producing briquettes is easier than producing pellets. The requirements for particle size and moisture content are less stringent for briquettes than for pellets. Furthermore, there are several types of briquettes, depending on the machinery used. Three types of machines are employed for briquette production: screw presses, mechanical presses, and hydraulic presses. For large-scale briquette production intended for industrial fuel, a mechanical press is the most suitable type of machine.
Furthermore, given the high ash content and large volume of bark, a mechanical press is the most suitable choice. Another technical reason is that the friction generated within a mechanical press is lower than that in a screw press. Hydraulic presses, on the other hand, are generally designed for smaller production capacities and are better suited for household or small-scale industrial use. The density of the output from a mechanical press is also higher than that from a hydraulic press. Because the density of the product from a hydraulic press is not excessively high, it can be easily ignited with a standard lighter—making it well-suited for the aforementioned applications—whereas the higher-density product from a mechanical press requires a high-temperature combustion chamber to ignite, rendering it more appropriate for industrial applications.
In the combustion technology sector, reciprocating grate and fluidized bed systems are the most suitable for biomass fuels with high ash content, such as tree bark. In a fluidized bed system, the particle size must be reduced to a very fine level to allow for fluidization—meaning that briquettes would need to be crushed down to that size—whereas a reciprocating grate system can effectively accommodate larger particle sizes. Furthermore, when biomass has a high ash content, conventional combustion technologies—such as static grate or chain grate systems—are prone to issues like slagging (the formation of glassy deposits caused by melting ash) and the obstruction of airflow.
In Europe, bark briquettes are used for domestic heating during the winter, specifically as "night briquettes" (or "over-night logs"). European consumers typically burn standard wood briquettes during the day to warm their homes quickly. However, before going to bed, they place one or two bark briquettes into their wood stove and reduce the airflow. Due to the inherent characteristics of bark, these briquettes smolder for a very long time—lasting anywhere from 8 to 10 hours—without producing large flames. By morning, embers remain in the stove, sparing the residents the trouble of starting a fire from scratch and ensuring the home stays warm throughout the night.
By producing bark briquettes, wood pellet manufacturers can not only achieve a zero-waste operation but also generate additional revenue from this line of business. In general, bark waste from any type of wood processing industry can be utilized to produce these briquettes—as well as bark pellets.
