Microstructural degradation during the storage of biomass pellets

Storage of biomass pellets can compromise their properties, due to fluctuating temperature and humid environments. In this study we show that extended storage of pellets results in higher pellet porosity, weight gain, increased inclusion body formation and creation of cracks.

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The use of biomass pellets has increased in recent times as a source of renewable energy. Data on pellet trade indicate that biomass pellets are consumed more than ever and travel long distances, until they reach their final destination. Storing pellets either in ships, trucks or in the garage of the most secluded house on earth, influences the properties of this sensitive and high-quality fuel.

Researchers have dedicated tremendous amount of effort towards understanding the effects of moisture and storage degradation on wood and, more recently, on biomass pellets. Indeed, the notion of some mechanisms involved in pellet degradation have opened the door to researchers to advance the theory of biomass. Recent studies have shown that pellets face enhanced degradation when they are exposed to fluctuating temperature and humid environments. Storage degradation has a great impact on pellet’s properties, such as particle breakage, moisture induced damage, self-heating, spontaneous combustion, pellet attrition, equipment fouling and numerous health threats to workers exposed to dust particles. Surely, the environment matters but so does the physical and chemical properties of biomass. To explain these phenomena, something at the heart of the pellets, at the microscale level, must have happened and here is where our contribution to the pellet’s world took place.

We decided to put ourselves in the "pellet's shoes" and imagined ourselves trapped in a room with sauna conditions for a long period of time. We decided to create this experiment and stored some pellets during a period of one month at constant condition of 40 °C and 85% relative humidity. Conditions such as this are not uncommon during pellet storage. We performed the following characterization techniques to quantify their degradation on micro-scale: digital microscopy, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray micro-computed tomography (micro-CT). We were particularly excited the first time we put a biomass pellet in a micro-CT scanner to see its “body” and how it changed during storage. Below is a video, extracted from micro-CT scans that shows inclusion and crack growth processes at the surface and within the pellets over the course of 1 month of storage.

Our work adds to the call of previous studies that storage conditions are critical in the supply chain of pellets to maintain their quality. Even if by human eye the changes on pellets as result of storage are not evident, their microstructural properties are attacked, which can compromise their quality. We believe that without the development of stronger policies to avoid premature degradation of biomass pellets, they may not realize their full potential as a bioenergy source.

For more information about this study, please check out our recent publication in Communications Materials titled: "Microstructural degradation during the storage of biomass pellets".

https://www.nature.com/articles/s43246-020-00113-y

Dr. Luis Cutz (left) and Dr. Urša Tiringer (right) at the Micro-computed tomography scanner at Delft University of Technology (TU Delft).

Luis Cutz

Postdoctoral Researcher, Delft University of Technology

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