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15
Particle Generation
Particle generation (entrainment or erosion) on a coal stockpile occurs when aerodynamic forces
from wind exceed the resisting forces (gravity, inter-particle cohesion, and moisture effects) holding
particles in place. This is governed by the surface friction velocity
(derived from shear stress =
,
where is air density). Erosion begins when
surpasses the particle-specific threshold friction velocity
, which depends on particle diameter, density, shape, and surface moisture.
The CFD assumes wind-driven erosion only. Mechanical activities (stacker/reclaimers at 5,900 tph,
bulldozers) generate dust independent of wind threshold. Wind fences still provide benefit by reducing
transport of mechanically generated dust.
Wind load contributes through:
Mean dynamic pressure (
) and local acceleration of flow over the stockpile’s windward
slope (often triangular or conical geometry).
Turbulent fluctuations in pressure and velocity, which impart lift and drag on surface particles.
Surface shear stress peaks on the windward face and crest due to flow impingement and
separation.
For coal (similar to granular materials like sand), generation is intermittent and strongest on exposed
and contribute to fugitive dust emissions.
Dust movement after generation follows three classic modes (observed via flow visualization in wind-
tunnel studies of triangular piles):
Surface creep
Saltation
–
accounting for most mass transport.
Suspension
distances, creating plumes and off-site pollution.
On stockpiles, saltation dominates near the surface, while suspension disperses dust downwind.
Turbulence intensity amplifies both lift-off and transport.
Porous wind fences (typically 30–40% porosity) mitigate these effects by creating a sheltered zone of
reduced wind speed and turbulence downstream, without forming large recirculating eddies (which could
otherwise increase local erosion). The fence exerts drag on the oncoming flow, lowering mean velocity
and
in the protected area. It can also physically intercept some saltating or suspended particles.
In summary, wind load drives erosion via elevated shear and turbulence on the stockpile; porous wind
-50% porosity) lower the effective wind load, raise the erosion threshold, suppress
particle generation, and reduce dust transport—yielding substantial emission cuts. These findings have
informed practical designs at coal facilities worldwide. Actual performance also depends on local wind
roses, pile shape, moisture, and fence height/orientation relative to the dominant wind.
The CFD study looks how porous wind fences (or windbreaks) influence the dispersion and emission of
fugitive dust from coal stockpiles or open storage areas at a terminal, with a specific focus on two particle
size classes:
(often approximating PM10, respirable/inhalable fraction) and
(coarser
particles more prone to saltation and short-range settling).
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