Time-Dependent Layer Formation Process on Quartz Bed Particles during the Fast Pyrolysis Process of Wood

TL;DR

This study examines how layers form on quartz particles during wood pyrolysis, revealing thinner layers and less calcium compared to combustion processes.

Contribution

The study provides new insights into the time-dependent formation of bed particle layers during fast pyrolysis, including their morphology and elemental composition.

Findings

Bed particle layers during fast pyrolysis are thinner and contain less calcium compared to combustion processes.

Layer formation begins with Ca-rich ash deposition on convex surfaces, followed by Ca2+ diffusion into quartz.

Over time, layer thickness approaches a limit of ~4 μm due to reduced Ca2+ diffusion.

Abstract

Understanding the characteristics and formation process
of bed
particle layers resulting from interactions between ash-forming matter
and bed material during fast pyrolysis is crucial for optimizing fast
pyrolysis bio-oil (FPBO) production. However, research on this topic
remains limited. In this study, the evolution of the bed particle
layers formed on quartz bed particles during fast pyrolysis of wood
was investigated across bench-, pilot-, and industrial-scale units.
Bed material samples with different exposure times were characterized
using scanning electron microscopy coupled with energy-dispersive
spectroscopy (SEM/EDS), and focused ion beam-SEM (FIB-SEM) to assess
the morphology, elemental composition, and thickness of the bed particle
layers. Overall, the time-dependent formation and characteristics
of the quartz bed particle layers were similar to those reported for
the fluidized-bed combustion of woody biomass. The key difference,
however, was that the layers formed during fast pyrolysis were significantly
thinner and contained less Ca. The bed particle layer formation began
early in the process through the deposition of Ca-rich ash particles,
primarily on convex surfaces, likely followed by solid–solid
diffusion of Ca2
+ into the quartz core, forming
a Ca-silicate-rich inner layer. The inner layer developed later and
more sparsely in concave regions, resulting in thinner layers. After
approximately 1 day, an outer layer developed on the convex surfaces
due to continued ash particle deposition, while a K–Si-rich
inner-inner layer (likely composed of K-rich silicates and associated
with gaseous alkali diffusion) formed primarily in concave regions.
Over time, the bed particle layer thickness approached a limiting
value of approximately 4 μm, likely due to reduced growth of
the inner layer, which may be attributed to diminished inward transfer
of Ca2
+ as the diffusion distance increased.

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