The cooling of pellets is necessary because pellets reach 70 to 90 degrees C after the pellet press. The reduction in temperature solidifies the pellets, which increases the pellet quality and reduces the risk of self-heating during storage. Industrially, pellet plants use outdoor air in counterflow coolers and cooling ends when the pellet temperature is approximately 5 degrees C above ambient temperature. Cooling performed in the summer could result in high temperatures in the pellet stacks during storage, and cooling at low temperatures and high airflows in the winter could cause quality problems. Therefore, the aim was to determine how cooling air temperature, airflow, and storage time impact the durability, hardness, and off-gassing of the pellets. The results showed that the highest durability (97.7%) and hardness (310 N) were achieved when cooling with low-temperature air and low airflow. Additionally, durability and hardness stabilized at high values (98.9% and 640 N) after 30 to 40 days of storage, regardless of the airflow and cooling air temperature used. Furthermore, it was found that high airflows reduce off-gassing regardless of the cooling air temperature. It is recommended that the industry reduce airflow during the winter and increase it during the summer to produce high-quality pellets and minimize the risk of self-heating.

Hornification of cellulose-rich materials, particularly wood pulps, occurs when chemical bonds form between cellulose surfaces, along with intermolecular forces created during dewatering and drying, preventing the material from reswelling in water to its original structure. Hornification of pulps results in a reduced ability to form effective fiber networks and therefore weaker paper products. The objective of this work was to investigate the role of hornification in pelletized cellulosic biomass and materials in general to provide more information than can be obtained by measuring standard wet state properties, such as water retention. Pellets were produced from chemical pulps with different degrees of hornification, as indicated by the water retention value (WRV), and their mechanical performance was evaluated. The chemical pulps served as a model material for investigating hornification. Pulps with higher hornification produced pellets with inferior mechanical properties, which has not been shown before by such a test. This effect is attributed to increased fiber stiffness and reduced surface flexibility, which limits fiber-fiber bonding. In addition, high drying temperatures prior to pelletizing, and thus higher hornification, will increase compression energy and friction in the pelletizing process. A novel connection was observed between WRV and mechanical performance, highlighting the impact of hornification on the surface interactions of cellulose-based materials.

This study investigated self-heating and off-gassing of Scots pine (Pinus sylvestris) wood pellets made from sawdust generated from separated mature and juvenile wood. The pellets were produced at an industrial scale and stored in large piles of about 7.2 tonnes. The production process involved drying the sawdust using three different methods and to varying moisture contents. The results indicated significant influences of both raw material type (F(6) = 61.97, p < 0.05) and drying method (F(2) = 65.38, p < 0.05) on the self-heating of the pellets. The results from the multiple regression analysis further showed that both the raw material type and pellet moisture content significantly influenced the temperature increase, with strong correlations observed for pellets produced using low-temperature drying (F(3, 14) = 83.52, multiple R2 = 0.95, p < 0.05), and medium temperature drying (F(3, 13) = 62.05, multiple R2 = 0.93, p < 0.05). The pellets produced from fresh mature wood sawdust were found to be more prone to self-heating and off-gassing while steam drying the sawdust at high temperature and pressure led to a significant reduction in heat and gas generation across all materials. The heightened self-heating and off-gassing in mature wood pellet can be attributed to a higher proportion of sapwood in the raw material. The probable explanations to the observed differences are in line with biological mechanisms for self-heating and off-gassing, as well as the chemical oxidation of fatty and resin acids.

In the production of wood fuel pellets, starch is frequently used as an additive to enhance bonding and durability. This study investigated the effectiveness of four different kinds of starches as additives, each at a concentration of 5% (dry basis), when combined with sawdust from Scots pine (Pinus sylvestris). The starches tested included plain wheat flour, hydrothermally treated wheat starch, wheat starch with amylose-like properties, and nearly pure amylopectin obtained from waxy rice flour. All pellets were produced at a die temperature of 100 °C using a Single Pellet Press, with varying moisture contents of 5%, 8%, 11%, and 14% (wet basis). The pellets were evaluated for compression work, back pressure, physical density, hardness, and moisture content. Additionally, chemical bonding was assessed using FT-IR spectroscopy. Compression energy was found to be influenced by moisture content, irrespective of starch utilization, and it decreased with increasing moisture levels, especially between 5 to 8% (wb). The inclusion of starch led to notably higher pellet hardness, with amylose yielding the hardest pellets, 34±3 kg when the moisture content was 11%. Based on this study, it is recommended to use hydrothermally treated wheat flour, as it consistently produced high-quality pellets. 

Wood pellets produced from fresh sawdust can form and release uncontrolled gases during bulk storage, a tendency referred to as off-gassing. This study investigated the off-gassing tendencies of Scots pine wood pellets made from separated sapwood and heartwood sawdust. The effects of drying temperature, raw material storage, as well as varying proportions of sapwood and heartwood were also investigated. There was a strong linear correlation between off-gassing and sapwood content, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases. An increase in sapwood content of the feedstock led to a significant increase in off-gassing of CO2, CO, and CH4, and O2 consumption. The drying temperature of the raw material had a significant effect on off-gassing of both sapwood (F (8, 26) = 51.32, p < 0.05) and heartwood (F (8, 26) = 334.1, p < 0.05) pellets. Increasing the drying temperature for heartwood resulted in increased off-gassing, while for sapwood, the off-gassing reduced. Storage of sapwood raw material before pelletization reduced the off-gassing of wood pellets, whereas for heartwood, it had no significant impact. Based on the results, it is suggested that a biological process, in combination with the chemical oxidation of fatty acids, lay behind the off-gassing of wood pellets. 

A global increase in the wood fuel pellet market requires knowledge of new biomasses pelleting abilities. As large-scale industrial tests of new materials are costly, tests in e.g., a single pellet press (SPP) are desirable. SPPs have many different configurations and it typically produces one pellet at a time and can give results of its pelletability. This review has surveyed the research that has been carried out of SPPs to ascertain the feasibility of comparing their obtained data and the results. The results show that it is almost impossible to compare the data and results of the various different SPP studies, e.g., some information from the data used was missing, resulting in that only 27 out of 70 papers were comparable. One solution could be the introduction of a common SPP testing method using a determined set of data that enables a reference pellet to be produced in every study.

Off-gassing and self-heating are the major challenges when it comes to transportation and storage of wood pellets. The heat generated due to self-heating poses a fire risk while off-gassing of toxic gasses such as carbon monoxide (CO) and some volatile organic compounds (VOCs) is an environmental and human health risk. With the increase in production volumes of wood pellets which has subsequently increased the amounts of wood pellets in transportation and storage, there is need to find lasting solutions to off-gassing and self-heating of wood pellets. The objective of this study was to test the off-gassing abilities of Scots pine wood pellets produced from sawdust with varying amounts of extractives. The aim is to come up with raw material pre-treatment measures so as to produce wood pellets that are not liable to off-gassing. Six (6) types of sawdust raw materials namely; fresh pine sawdust (FPS), stored pine sawdust (SPS), sawdust plus pine rosin (PRS), sawdust plus linseed oil (LOS), sawdust plus tall oil (TOS) and acetone extracted sawdust (AES) were used to produce the pellets. The produced pellets were then subjected to off-gassing tests under controlled conditions using the ECOM J2KN analyser. The concentrations of carbon monoxide, carbon dioxide and methane increased with storage time but slowed down towards the end of the nine days test period. The formation of these gasses were largely dependent on the type of extractives present in the raw material and not the total extractive content. The formation of methane started later than the other gases and coincided with the time when residual oxygen was depleted.

Fuel wood pellets have the tendency of undergoing self-heating and off-gassing during storage and transportation. Self-heating can lead to spontaneous combustion and cause fires while toxic gasses such as carbon monoxide and some volatile organic compounds released due to off-gassing are a human health and environmental hazard. Previous research suggests that the self-heating and off-gassing of wood pellets are as a result of the oxidation of wood extractives. The aim of this study was to identify the extractives, i.e., fatty and resin acids that are responsible for the emissions of carbon monoxide, carbon dioxide and methane from wood pellets by testing the off-gassing tendencies of pellets produced from synthetic microcrystalline cellulose and different additive oils. The additive oils were intentionally selected to represent different types of wood extractives (mainly fatty and resin acids) and they included: tall oil, pine rosin, linseed oil and coconut oil. The highest mean concentrations of carbon monoxide, carbon dioxide and methane were recorded from cellulose pellets with added linseed oil. The concentrations of carbon monoxide and methane for the other four pellet types were negligible and there was no carbon dioxide emission. Pellets with added linseed oil had high off-gas emissions due to the high content of unsaturated fatty acids compared to other pellet types.