In a circular economy, the efficient utilization of all materials as valuable resources, with a focus on minimizing waste, is paramount. This study shows the possibilities of upgrading the lowest-valued residuals from the forest industry into a new product with both liming and fertilizing properties on forest soil. Hydrothermal carbonized sludge mixed with bark and ash in the proportions of 45:10:45 was densified into fertilizer pellets that meet the nutrient requirements of 120 kg N per hectare when 7 tons of pellets is spread in forests. The pellets met a high-quality result according to durability and density, which were above 95% and 900 kg/m3. However, pellets exposed to wet and cold conditions lost their hardness, making the pellets dissolve over time. Small amounts, <5‰, of nutrients, alkali ions, and heavy metals leached out from the pellets under all conditions, indicating good properties for forest soil amendment. The conclusion is that it is possible to close the circle of nutrients by using innovative thinking around forest industrial residual products.

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. 

The supply-chain logistics – storage and transportation over long distances – and downstream processes in biofuel production are adversely impacted by the moisture content in the biomass feedstock. Most woody, herbaceous, low-cost biomass resources such as municipal organic solid wastes and forest residues have moisture content over 30% (of the wet-biomass mass). This makes them less amenable to thermochemical biomass-to-biofuel conversion technologies like pyrolysis and gasification. If pyrolyzed or gasified, the resulting biofuels have a higher moisture content, which truncates their calorific values. During storage, there is a loss of dry matter owing to a tendency to compost aerobically/anaerobically, which is detrimental to the quality of the biomass as a potential source of biofuel. Beyond that, fire hazards due to the spontaneous combustion of wet biomass are not uncommon, necessitating storage in a dry condition. However, drying high-moisture biomass is energy-intensive. The quality of the product and the efficiency of drying are affected by particle sizes and drying technologies adopted. Within this chapter, the authors focus on managing and controlling the moisture content of the biomass utilized in the biofuels sector by resorting to drying and torrefaction technologies. The chapter dwells on drying principles, models and media in drying systems, types of drying systems, mechanical dewatering and torrefaction, the impact of drying, dewatering, and torrefaction on the physical and chemical properties of the end-product, and techno-economic analysis of torrefaction.

Future pellet plants will need to be more flexible regarding the need to vary the feedstock species and increase the areas for biomass. By blending biomass species, pellets can be a solution that meets new desirable specifications for different conversion pathways, such as biochemical, thermochemical, and biopower. In this chapter, the authors address opportunities for blending and densification to homogenize the chemical composition of biomaterials and improve the potential for other processing and transformation methods. The advantages of blending biomasses are many: increasing the potential supply of biomass, creating new markets, reducing raw material costs, and improving biomass flow and pelletizing properties. Important densification parameters to keep in mind are highlighted, and five different studies are summarized; these include investigations from a single pellet press study to briquette applications. Some conclusions are that blending woody and herbaceous biomasses yields products with uniform product quality in terms of physical properties and chemical composition. Furthermore, blending pine and switchgrass yields a compressed product with higher density and durability while reducing energy consumption.

Different biomaterials have different chemical compositions and since the densification properties are strongly related to feedstock, it is important to increase knowledge about biomass relationship to densification properties. The purpose of this chapter is to give a brief introduction to the development of the plant kingdom, the chemical composition of biomasses, and how different components can affect the densification characteristics. A study where 11 different pure substances (cellulose, hemicelluloses, lignin, etc.), added to pine and beech, are pelletized in a single pellet press and results are presented showing that polysaccharides can play an important role when biomasses are densified as single sources or blends solutions in a densification process. 

Climate change concerns have goaded countries towards seeking renewable energy options (bio-energy being one of them), to replace/supplant the conventional fossil-fuel alternatives (coal, oil and natural gas) in vogue. Fuel-pellets - at the confluence of the forestry, agriculture, waste management and bio-energy sectors - when produced from biomass-residues, serve the dual purpose of ensuring energy security and environmental sustainability.  By valorising more and more organic wastes to bio-energy products, one could, to use the old adage, ‘kill two birds with one stone’.  Social-LCA is a method used to analyse a very wide range of social issues associated with the stakeholders in a value-chain – workers, local community dwellers, society, global consumers, banks, investors, governments, researchers, international organizations and NGOs. In this analysis, the authors commence with a highly-focused, niche literature review on the social dimension of sustainability in the African energy / bio-energy sector. The streamlined social footprint analysis inspired by the relatively lesser number of such studies for this sector in Africa, is not a novel addition per se to the S-LCA knowledge base. The purpose of application is to shed light on something in Zambia, which must be understood better, known and studied better, in order so as to bring about much-needed alterations in the direction of sustainable development. While the questions addressed to four different groups of stakeholders encompass a clutch of sustainable development goals, gender equality (SDG 5) and the need for greater interest on the part of governments and investors (SDG 9) to look at sustainable alternatives to the status quo, stand out as concerns which need to be tided over. This paper and the streamlined social footprint analysis carried out, is all the more relevant and timely, when one considers some key changes which have happened in Zambia over the last five years – The implementation of the National Energy Policy in 2019, and the creation of the Ministry of Green Economy in 2021.These are verily  in Zambia, are harbingers of positive change auguring well for future developments in the  supporting developments in the bio-energy (and bio-pellets) sector, not just in Zambia, but by way of emulating and learning, in other countries on the continent.    

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.