Biomass pellet combustion: Cavities and ash formation characterized by synchrotron X-ray micro-tomography

• Published in: Fuel processing technology 2018-07, Vol.176, p.211-220
• Main Authors: Strandberg, Anna, Thyrel, Mikael, Skoglund, Nils, Lestander, Torbjörn A., Broström, Markus, Backman, Rainer
• Format: Article
• Language: English
• Subjects: Ash composition; Ashes; Biodiesel fuels; Bioenergi; Bioenergy; Biomass; Biomass burning; Conversion; Density; Devolatilization; Energy dispersive X ray spectroscopy; Fuels; Gas transport; Heat conductivity; Holes; Liquefaction; Operational problems; Pellet; Pellets; Poplar; Scanning electron microscopy; SEM-EDS; Straw; Synchrotron radiation; Thermochemical conversion; Void fraction; Wheat; Wheat straw; X ray microtomography
• ISSN: 0378-3820; 1873-7188; 1873-7188
• DOI: 10.1016/j.fuproc.2018.03.023

Ash formation during thermochemical conversion of biomass-based pellets influences both char conversion rates and ash-related operational problems. The objective of the present study was to provide detailed insights into changes in fuel and ash properties during fuel conversion. Pellets of poplar wood and wheat straw were used as model biofuels, representing vastly different compositions of ash-forming elements. Pellet samples at different char conversion phases were analyzed by synchrotron-based 3D X-ray micro-tomography, to map and visualize the development of cracks, internal cavities, and ash layers during conversion. The analysis of ash layers was complemented by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. The results provide new insights into how large cracks and internal cavities are developed already during devolatilization, for example, the poplar wood pellets had a 64% void fraction after the devolatilization stage. As expected, there were large variations between the ash layer properties for the two fuels. A porous, low density, and calcium-rich ash was formed from the poplar fuel, whereas the wheat straw ash was a high-density silicate melt that developed into bubbles on the surface. As the conversion proceeded, the wheat straw ash covered parts of the active char surface area, but without blocking the gas transport. [Display omitted] •Investigation of charred pellets by synchrotron based X-ray micro-tomography.•Large cracks and internal cavities were formed already during devolatilization.•Poplar stem wood formed a porous, permeable, low density and calcium rich ash.•For wheat straw high density silicate melts developed into bubbles on the surface.•The results can be used to increase accuracy and flexibility of fuel conversion models.