Moisture adsorption rate and durability of commercial softwood pellets in a humid environment

Highlights

• CT scans of the cross-section of a wood pellet exhibit axial and radial cracks.

• These cracks may be responsible for humid air penetration into a pellet.

• Pellet durability drops drastically when the adsorbed moisture exceeds 12%.

• Pellet durability drops more severely in ventilated than in unventilated pellets.

• The ventilation time with humid air should be short to lessen the durability loss.

Durability, a crucial parameter in determining the quality of a wood pellet, is affected by moisture adsorption from the air. The conditions in a confined space in which pellets are stored can be characterised as 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 is stagnant. These two cases represent stored pellets in a ventilated silo (active) or in a ship's hold (passive). Commercial wood pellets were exposed to humid air in a humidity chamber with circulating air set at 95% relative humidity (RH) and 30 °C for up to 24 h. The wood pellets were also exposed to a range of RH values (11–90%) for four weeks, created using saturated salt solutions. The durability of pellets, volumetric swelling, and pellet density were measured. For pellets exposed to 95% RH, the durability of pellets, initially at 99%, dropped to below 80% after 10 h. A similar drop in durability was observed for pellets stored above saturated salt solutions but after a longer storage period. The density of pellets initially increased when pellets had a moisture content of 0.05–0.08 decimal dry basis (db). The pellets expanded by 20–80% volumetrically when the moisture content exceeded 0.15 (db), resulting in a marked decrease in pellet density.

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.