Nanosilica coatings are considered a simple physical
treatment
to alleviate the effect of cohesion on powder flowability. In limestone
powders, these coatings buffer the rise in cohesion at high temperatures.
Here, we investigate the role of particle size in the efficiency (and
resilience) of these layers. To this end, this work examines a series
of four limestone powders with very sharp particle size distributions:
average particle size ranged from 15 to 60 μm. All the samples
were treated with nanosilica at different concentrations from 0 to
0.82 wt %. Powders were subjected to short- and long-term storage
conditions in calcium looping based systems: temperatures that vary
from 25 to 500 °C and moderate consolidations (up to 2 kPa).
Experiments monitored powder cohesion and its ability to flow by tracking
the tensile strength of different samples while fluidized freely.
Fluidization profiles were also used to infer variation in packings
and the internal friction of the powder bed. Interestingly, for particle
sizes below 50 μm, the nanosilica treatment mitigated cohesion
significantlythe more nanosilica content, the better the flowability
performance. However, at high temperatures, the efficiency of nanosilica
coatings declined in 60 μm samples. Scanning electron microscopy
images confirmed that only 60 μm samples presented surfaces
barely coated after the experiments. In conclusion, nanosilica coatings
on limestone are not stable beyond the 50 μm threshold. This
is a critical finding for thermochemical systems based on the calcium
looping process, since larger particles can still exhibit a significant
degree of cohesion at high temperatures.
Understanding the flowability of cohesive powders at high temperature is of great importance for many industrial applications where these materials are handled at harsh thermal conditions. For instance, the Calcium-Looping (CaL) process, involving the transport, storage and fluidization of limestone powders at high temperature, is being considered nowadays as a promising technology for thermochemical energy storage (TCES) in concentrated solar power plants (CSP). In this context, the High Temperature Seville Powder Tester (HTSPT) is presented in this work as a useful tool to analyze how the flow behavior of cohesive powders changes with temperature. The manuscript reviews the main results obtained so far using this novel apparatus. The change of powder cohesiveness and therefore of powder flowability as depending on temperature, particle size, material properties and nanosilica surface coating is illustrated.
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