Research PaperMoisture adsorption rate and durability of commercial softwood pellets in a humid environment
Introduction
Wood pellet, a uniform renewable solid biofuel, is increasingly used in Western Europe and East Asia. Wood pellets are increasingly being consumed in two sectors: the industrial sector as a substitute for coal to combat climate change, and the residential sector to substitute heating oil and natural gas where these fuels are expensive. The consumption of wood pellets has therefore grown rapidly in the past decade, reaching 35 million tonnes in 2018 (Proskurina, Junginger, Heinimö, Tekinel, & Vakkilainen, 2019). Wood pellets offer advantages such as high bulk density, uniformity in quality, good flowability, and low moisture content. Pellets occupy less volume per mass unit and thus reduce the storage required for logistic purposes. However, pellets are hygroscopic and adsorb moisture from a humid environment during storage and handling. The moisture sorption induces volumetric expansion that leads to a drop in durability (McMullen, Fasina, Wood, & Feng, 2005). Fasina and Sokhansanj (1992) and Colley, Fasina, Bransby, and Lee (2006) reported that the density of alfalfa and switchgrass pellets showed a slight increase at a moisture content of 7–8%, followed by a drop with the adsorbed moisture. Hartley and Wood (2008) observed that the relationship between the swelling of wood pellets and the relative humidity could be described using a power–law equation. These studies were carried out on bulk amounts of 100–1000 g pellets and the applicability of their data to the industrial handling of pellets was not discussed.
Peng, Bi, Lim, and Sokhansanj (2013) compared the moisture uptake of steam-treated and untreated wood pellets in an environmental chamber and showed that the steam-treated pellets had less propensity for moisture adsorption. Deng, Alzahrani, and Bradley (2019) measured the sorption isotherm and durability of pine wood pellets and recycled wood over a range of relative humidity and found that durability only decreased 5% at relative humidity (RH) above 80%. Wang et al. (2016) observed that cedar wood pellets, stored at 60 °C and 90% RH for five days, had a moisture content of 29% wb, while the hardness of pellets decreased from 5.45 to 0.15 N mm−2.
During a study of the long term impact of weathering (10–20 months) on the mechanical and chemical properties of biomass pellets during storage, Graham, Eastwick, Snape, and Quick (2017) noted that the mechanical degradation of the pellets resulting from moisture uptake was more substantial than both chemical or biological degradation. After one month of storage RH > 90%, untreated wood pellets disintegrated and their durability was essentially zero. Steam-treated wood pellets were much more durable, with a durability rating of around 87%. None of these studies addressed specifically the rate of moisture uptake and its effect on durability and density of softwood (spruce, pine, fir) pellets. Jensen, Temmerman, and Westborg (2011) identified particle size distribution within individual pellets and the local temperature gradient in the die as two significant factors influencing the durability of pellets. Lee, Sokhansanj, Lim, Lau, and Bi (2019) discussed the handling history of the pellets and their varying swelling responses that affect the physical qualities of a wood pellet.
Previous research has not addressed the time-dependent moisture adsorption and the effects the adsorption rate has on the durability and density of wood pellets. Storage conditions for pellets can be characterised as being active or passive. In the active case, the interstitial air within the pile circulates around the pellets, whereas in the passive case the interstitial air may be stagnant. These two cases can represent stored pellets in an active ventilated silo, or in a ship's hold or a rail car with no forced airflow. The objective of this research was to expose softwood pellets to two different sets of environmental conditions, one in a humidity chamber with active air circulation and the other over a number of salt solutions with no active airflow. The durability and density of the pellets were then examined.
Section snippets
Materials
Wood pellets used in these experiments were manufactured in British Columbia, Canada, using a mix of softwood species, primarily pine but mixed with spruce and fir (SPF). The authors purchased the pellets from a hardware store in 18.2 kg bags.
ImageJ software (National Institutes of Health, version 1.52s December 2019) was used to measure the lengths and diameters of the individual wood pellets. The dimensions were used to calculate pellet volume and density. Individual pellets were photographed
Surface conditions and internal structure
Figure 2 shows the conditions of pellets before and after exposure to humid conditions in the humidity chamber. The expansion of wood particles within the pellets increased with moisture adsorption, causing the surface cracks to widen. Small cracks appeared mostly along the length of the pellets after seven hours. The wood particles on the surface of pellets swelled and this swelling pushed the particles away from the main pellet matrix. That forced small flakes to separate from the pellet
Discussion
This study suggests that decreases in the durability of individual pellets due to moisture adsorption are caused by the volumetric swelling of pellets. We believe the variability in the data is due to variability in the internal structure of pellets. The saturated salt solution method influences M of pellets by altering the relative humidity of the atmosphere that pellets are exposed to. In the humidity chamber method, M of pellets was controlled by setting the exposure time. During the longer
Conclusion
This study examined the effect of moisture adsorption on single pellet durability, volumetric swelling, and pellet density.
At M above 0.15 (db), slower moisture adsorption by pellets above salt solutions resulted in durabilities above 96.8%, whereas the faster moisture adsorption in a humidity chamber was associated with a more severe decrease in the durability to ~87%. However, in both adsorption tests, the average durability dropped below 80% at M above 0.15. The variability in pellet
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The Natural Sciences and Engineering Research Council of Canada supported this research under the Discovery Grant programme. Professor Philip Evans of Wood Science Department at the University of British Columbia provided the μ-CT images of a wood pellet.
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