This work investigates the proportion of generated fines in a pilot-scale experiment using a belt conveyor and commercial fuel pellets. For this, a belt conveyor with a length of 3.1 m was used and operated at varying conditions: speeds, percentages of material loading on the belt, two combinations of the inclination angle of the belt and the falling height, and a different number of handling steps. We considered a design of experiments approach based on response surface methodology to investigate the effect of different conditions on the potential of fines generation. Moreover, a comparison between the results of the belt conveyor and three common benchmark experimental approaches (tumbling box, rotary impact tester, and mechanical compression test) was made. Results show that the number of handling steps and the combined effect of drop height and inclination angle directly affected the fines generation. However, the tested belt speed range and the level of loading were of lower significance. A polynomial quadratic model was derived based on the regression analysis and showed a high accuracy to predict the proportion of fines. Moreover, the tumbling box method showed good potential to predict the proportion of fines in a belt conveyor when transported several times.

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The use of biomass pellets as a source of renewable energy has increased in recent times. However, pellet storage during transportation can compromise their properties, due to fluctuating temperature and humid environments. Here, we show that extended storage of one month at 40 °C and 85% relative humidity causes significant biomass pellet degradation. This was evidenced by higher pellet porosity, weight gain, increased inclusion body formation and creation of an internal network of cracks. We quantify the inclusion and pore growth processes at the surface and within the pellets, which has implications for subsequent thermochemical conversion. The global bioenergy transition may depend upon biomass pellets, and this study shows that storage conditions are critical in the supply chain, so to maintain their quality. Without the development of stronger policies to avoid premature degradation of biomass pellets, they may not realize their full potential as a bioenergy source.

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The change in physical properties of wood pellets, with a focus on particle size distributions due to pellet breakage and attrition, was studied in a large-scale (∼450 ton/h) transportation system. Critical locations with a high probability of breakage through the whole transportation system were chosen and sampled to study the effect of transportation system design and operation on the mechanical properties of pellets. Bulk density, mechanical durability, moisture content, and particle size distribution of pellets were characterized for each sample. Analysis of variance showed that there were significant differences between the percentages of small particles (< 5.6 mm) in the samples taken at different locations, especially at one with a vertical free fall of 7.8 m. On average, this relatively long drop increased the proportion of particles < 5.6 mm in the samples from 8.73% to 14.09%, and that of particles < 3.15 mm from 4.82% to 9.01%. Moreover, the measurements showed a wide deviation in the mechanical durability values, between a minimum of 90.8% and a maximum of 98.7%, which were not correlated to the sampling points but related to pellet properties. It can be concluded that pellet transportation systems require more dedicated design strategies to prevent breakage and attrition.

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Presently, biomass pellets play a significant role in energy transition scenarios worldwide. Due to the lack of local supplies, many countries import their pellets from countries with enormous resources. For instance, in Europe, a big share of pellets is imported from the USA, Canada, and Asian countries. Pellets are normally transferred in bulk using ocean vessels with a capacity of up to 40,000 metric tons. Due to mechanical forces and environmental changes throughout the transport and storage steps, pellets are prone to degradation. This may degrade pellets physically or chemically. As a result, fines and dust are generated. Moreover, as pellets absorb and adsorb moisture from the environment, the moisture content and the heating value of pellets may change, and this may also weaken the physical structure because of swelling. The presence of fines and dust may lead to self-ignition and dust explosion, material loss, equipment fouling, and environmental and health issues. The goal of this dissertation is to investigate to what extent biomass pellets degrade during transport and storage. To achieve this, first, we conducted an extensive literature review to reveal the factors that affect the extent of degradation of pellets. Moreover, we studied the commonly used methods to assess the quality parameters and the degradation behavior of pellets in detail. Then, we carried out a series of experiments on physical and chemical degradations of pellets from laboratory to large-scale and analyzed them in the operational and environmental context. By conducting these experiments, we unveiled the relationship between the laboratory test results and the pilot or large-scale transport impact on the proportion of generated fines. Furthermore, a model in the discrete element method (DEM) was developed and used to simulate the breakage pattern of individual pellets under the compression test. The model shows high fidelity in simulating the breakage behavior of pellets under compressive forces in two directions. @en

With the recent increase in biomass pellet consumption, the mechanical degradation of pellets during transport and handling has become more important. ISO standard 17831-1 is an accepted global standard that is commonly used amongst researchers and industries to determine the mechanical durability of pellets. However, the measured mechanical durability sometimes fails to match the certificate accompanying the shipment. In such cases, pellet length specifications are suspected to play a role. This paper studies the effect of pellet length on mechanical durability for various types of commercially produced biomass pellets. In addition, the effect of test conditions and torrefaction on the mechanical durability of biomass pellets has been investigated. To study the effect of pellet length, pellets were classified into three groups: shorter than 15 mm, 15 to 30 mm, and longer than 30 mm, and their length distributions were measured using an in-house image processing tool. Then, the mechanical durability of pellets was measured using ISO standard 17831-1. The mechanical durability results were compared to random-sized pellet samples. To study the effect of test conditions, the mechanical durability test was operated at different time intervals to elucidate the effect of tumbling at different conditions. The results show that the mechanical durability depends highly on the length distribution of the pellets, with a difference between categories of up to 13%. It was also observed that the mechanical durability remains relatively constant after a specific time interval. Based on the results, we highly recommend modifying the current ISO standard to account for the pellet length distribution (PLD)@en

Biomass pellets provide a pivotal opportunity in promising energy transition scenarios as a renewable source of energy. A large share of the current utilization of pellets is facilitated by intensive global trade operations. Considering the long distance between the production site and the end-user locations, pellets may face fluctuating storage conditions, resulting in their physical and chemical degradation. We tested the effect of different storage conditions, from freezing temperatures (−19 °C) to high temperature (40 °C) and humidity conditions (85% relative humidity), on the physicochemical properties of untreated and torrefied biomass pellets. Moreover, the effect of sudden changes in the storage conditions on pellet properties was studied by moving the pellets from the freezing to the high temperature and relative humidity conditions and vice versa. The results show that, although storage at one controlled temperature and RH may degrade the pellets, a change in the temperature and relative humidity results in higher degradation in terms of higher moisture uptake and lower mechanical strength. @en

The presence and generation of fines and dust in the bulk of biomass pellets have inflicted several problems in the supply chain during transportation and storage, and the breakage behavior of pellets has been scarcely studied so far. Fines and dust are the consequences of impact and abrasive forces through the whole supply chain; however, the breakage happens at the particle level. Therefore, to study the fines generation, first, the breakage behavior of individual pellets should be understood, and then, the behavior of the bulk materials in operational conditions can be investigated. This paper aims to investigate the breakage behavior of individual pellets under experimental compression tests and to introduce a calibrated numerical model using discrete element method (DEM) in order to pave the way for further studies on pellet breakage. For that purpose, seven different types of biomass pellets were studied experimentally, and then, a calibrated model was introduced via the Timoshenko–Ehrenfest beam theory using DEM. Results show that the model could reasonably predict the breakage behavior of pellets under uniaxial and diametrical compressions. The findings could help to develop a new design of the equipment for transportation and handling of biomass pellets with the aim to reduce the amount of generating fines and dust.@en

The breakage behaviour of biomass pellets with a diameter of 6 mm under uniaxial compression test was studied experimentally and numerically using the discrete element method (DEM). Two types of the available bonding contact models in EDEM software were used to compare the macroscopic properties including the maximum stress at failure, strain at failure, and the pellet Young’s Modulus. The models are based on 1) the Timoshenko beam theory and 2) a bonded particle model. The results show that both models reasonably predict the maximum stress values, however, the bonded contact model is not able to predict the strain at failure and the Young’s Modulus while the results show a big deviation from the experimental results.@en

Densification has been carried out for many years, mostly in biomass processing, animal feed production, and pharmaceutical industries. During the years, researchers and engineers attempted to improve the product quality and minimize the production costs. The most important quality parameters of solid bio-materials are the compressive strength, abrasion resistance, impact resistance, moisture adsorption, and density. Various studies used different standard and non-standard methods to characterize these quality parameters. The objective of this paper is twofold: (1) to investigate the state-of-the-art methods and devices used in the quality assessment of densified bio-materials, including a comparison between non-standard and standard methods. (2) to discuss the effect of different factors on the properties of densified bio-materials using an integrated approach. The results show a lack of standard methods for the quality assessment of bio-materials and therefore, there is an emerging need for development of dedicated standards for bio-materials. Moreover, the use of dissimilar methods and devices in the quality assessment of bio-materials gives risk to uncertainties about the effect of different factors on the product quality.

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The transportation, handling, and storage of biomass pellets are challenging due to pellet breakage and fines generation. The amount of fines generated during transportation and handling of biomass pellets is mostly measured in the laboratory using mechanical durability testers such as tumbling can method according to ISO 17831-1. This standard sets requirements on the amount of pellets, but not on the pellet length distribution (PLD) to be used for the mechanical durability measurements. This study aims to investigate the effect of PLD on the mechanical durability of biomass pellets. Two different types of torrefied biomass of Poplar and Mixed wood were used in this study. Samples were classified into three different groups of shorter than 15 mm, between 15 and 30 mm, and longer than 30 mm before durability test. In addition, a random PLD was considered as a reference case. The results show that the mechanical durability strongly depends on the PLD and increases from 83.6% to 96.4% for torrefied Poplar and from 92.7% to 98.5% for torrefied Mixed wood by increasing the PLD from shorter than 15 mm to longer than 30 mm. Therefore, it is highly recommended to consider the PLD of the pellet samples before durability tests.@en

The poster investigates the mechanical properties (Particle and bulk densities, compressive strength, bending strength, and the angle of repose) of a variety of torrefied and non-torrefied biomass pellets and shows the relationship between different properties. Moreover, a novel and quick method to determine the particle size distribution of cylindrical pellets based on image processing is introduced.@en

The large-scale storage of biomass pellets is challenging due to health issues, material loss, and the risk of fire and explosions. The thermal conductivity is a material property which plays a key role in determining the self-heating properties of biomass. Meanwhile, understanding the thermal conductivity and the affecting factors help to better understand and reduce the risk of self-heating and fire explosions. This poster investigates the effect of particle size distribution and bulk density on the thermal conductivity of biomass pellets.@en