Influences of environmental humidity on physical properties and attrition of wood pellets

Abstract

Wood pellets have been widely used for power generation and domestic heating recently to substitute fossil fuels. For health and safety reasons, particle degradation and dusts generation in material storage and handling processes are always a serious concern for anyone involved. Also the level of fines in wood pellets can strongly influence fuel milling and combustion. For previous study, wood pellets attrition subjected to its mechanical properties has been reported. However pellet mechanical properties can be inconsistent and influenced by environmental conditions such as humidity, temperature and storage time. In this paper, attrition behaviour and dust generation of two types of wood pellets have been studied, which the pellets were stored in a constant relative humidity environment for a period of time (4 days). Physical properties such as moisture contents, bulk density and flexural stress of the pellets were measured for the pellets conditioned in a RH chamber, which was believed that equilibrium condition was achieved. Correspondingly, the conditioned pellets for a number of different humidity levels were tested for both impact test and dustiness test. The study shows that the pellets can lose structural strength due to in taking moisture from environment for a high relative humidity. Also a high humidity can result in an increased level of pellet degradation and dusts generated.

Determination of mechanical properties for wood pellets used in DEM simulations

“…Graham et al (2017) investigated the influence of different storage conditions on several properties of wood pellets. Wood pellets stored outdoors may be subjected to freezing and defrosting processes, and they usually exhibit a higher moisture content, even if they are placed in roof-covered locations (Graham et al, 2017), because increased relative humidity leads to an increase in the moisture content of the pellet (Deng et al, 2019). The increase in wood pellet moisture content reduces their durability and mechanical properties, e.g.…”

“…The increase in wood pellet moisture content reduces their durability and mechanical properties, e.g. the modulus of elasticity and flexural strength, increases in pellet attrition and dust content or decreases in bulk density (Deng et al, 2019;Graham et al, 2017).…”

Microstructural degradation during the storage of biomass pellets

“…high length-to-cross-section ratio of pellets 9 . Thus, to monitor the expansion of cracks, we performed a three-dimensional (3D) surface profile analysis of pellets at different storage times using digital microscopy.…”

“…Depending on the transportation type, transport distance and storage conditions, biomass pellets are being stored for a period from up to one week or to three months in a fluctuating temperature and humidity environment 7 . Storage during extended periods of time, especially at high temperatures [8][9][10][11] , accelerates the degradation of pellets, reducing the quality of the fuel and consequently the fuel conversion behavior 12 . Recently, coupling advanced pretreatment processes 4 , such as torrefaction with pelletization, have opened new frontiers to enhance the physicochemical properties of raw biomass to stand the current environmental conditions during transport and storage 4,13 .…”

“…However, similar problems as those observed in untreated biomass pellets, such as particle breakage and moisture-induced damage, are still present during logistics of torrefied pellets, leading to structural degradation 13 . Biomass degradation, especially when dealing with large volume of pellets, can lead to selfheating, spontaneous combustion, pellet attrition, equipment fouling, and numerous health threats to workers exposed to dust particles 9,[14][15][16][17] .…”