Production of Bark Briquettes from Wood Pellet Factory Waste

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. 

Stages in the Utilization of Empty Fruit Bunches at Indonesian Palm Oil Mills

In addition to palm kernel shells (PKS) and mesocarp fiber, another solid biomass waste product available in large quantities at palm oil mills is the empty fruit bunch (EFB). EFB represents the largest volume of biomass waste generated by these mills. Mesocarp fiber, along with a portion of the palm kernel shells (PKS), is typically consumed as fuel for boilers. In fact, the volume of EFB produced exceeds that of the mill's primary product, crude palm oil (CPO). Despite this, EFB—which accounts for approximately 20% of the fresh fruit bunch (FFB) weight—remains largely unutilized. Given its substantial volume, it also poses a significant potential for environmental pollution. 

Many mills are striving to achieve a "zero waste" status, which necessitates the processing of empty fruit bunches (EFB). However, the challenges lie in determining the appropriate processing methods, the resulting products, and the overall economic viability. Ideally, this waste should not only cease to be an environmental pollutant but also generate additional profit—ideally, by produce products that maximize returns. A realistic approach, therefore, involves processing EFB in a way that prevents environmental pollution while minimizing the capital investment required for the necessary processing equipment. These factors are critical considerations, grounded in both current realities and future developments.

Empty Fruit Bunches (EFB) have significant potential for use in various bioenergy and biochar products. Through a process of biomass densification, EFB can be converted into pellets, which—much like palm kernel shells (PKS)—can be utilized within the energy sector. Given the massive volume of EFB available, the potential for EFB pellet production is substantial. EFB pellets are expected to emerge as a future trend, following in the footsteps of palm kernel shells (PKS), primarily due to their abundant availability and the fact that the vast majority remains unutilized. When compared to other potential raw materials for pellets—such as wood waste from forestry or the wood working industry—the volume of EFB is highly competitive, if not greater. Furthermore, unlike wood pellets derived from energy plantation (which require the cultivation of species such as calliandra or gliricidia), the production route for EFB pellets is significantly faster.

Another use for EFB (Empty Fruit Bunches) is the production of biochar. Biochar can serve as a soil amendment in the agricultural sector—specifically within oil palm plantations, whether company-owned or managed by smallholders. The application of biochar enhances nutrient use efficiency (NUE) by effectively turning fertilizers into slow-release products. Furthermore, biochar producers can generate revenue through carbon credits by adhering to methodologies established by recognized carbon standards, such as Puro, Verra, or Isometric. Industrial-scale slow pyrolysis equipment used for biochar production also generates excess energy—such as syngas—which can be utilized as fuel for palm oil mill boilers (via co-firing) or for other applications.

Currently, many palm oil mills utilize empty fruit bunches (EFB) by burning them in simple incinerators (as shown in the photo), which yields EFB ash—a substance rich in potassium that can be used as fertilizer. However, these simple incinerators not only generate smoke pollution but also fail to harness the heat produced during combustion. Upgrading to more efficient incineration systems—such as those employing reciprocating grate combustors—and incorporating pollution control equipment, like water scrubbers, would render the utilization of EFB environmentally friendly. Furthermore, the heat generated from combustion (waste heat recovery) can be harnessed for various purposes, including cogeneration with palm oil mill boilers, electricity generation, or other applications.