Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 415

The hopper angle should be adequately steep to force solids to flow at the
walls.
The hopper outlet must be large enough to prevent arching.
The hopper outlet must be adequately large to achieve the maximum dis-
charge rate.
The required smoothness and sloping angle for mass flow in a hopper
depends on the friction between the particles and the hopper surface. This
friction can be measured in a laboratory using a standard test (ASTM, 2000).
Factors that affect wall friction for a given fuel are:
Wall material
Surface texture or roughness of the wall
Moisture content and variations in solids composition and particle size
Length of time solids remain unmoved
Corrosion of wall material due to reaction with solids
Scratching of wall material caused by abrasive materials.
To enhance the smoothness of the surface, sometimes the hopper is
coated or a smooth lining is applied to it. Lining materials that can be used
include polyurethane sheets, TIVAR-88, ultra high molecular weight poly-
ethylene plastic, and krypton polyurethane.
Mass flow can be adversely affected by the narrowness of the hopper out-
let. A too-narrow outlet (compared to particle size) permits the particles to
interlock when exiting and form an arch over the outlet. The probability of
this happening increases when:
The particles are large compared to the outlet width.
There is high moisture in the solids.
The particles are of a low shape factor and have a rough surface texture.
The particles are cohesive.
Wedge-shaped hoppers require a smaller width than conical hoppers do,
in order to prevent bridging. Slotted outlets must be at least three times as
long as they are wide.
Negative angle of outlet walls could also help avoid solids flow problem
through a hopper. The edges of the hopper could be flared (positive angle)
on one side while it is tapered (negative angle) on other side.
12.3.2.5 Hopper Design for Mass Flow
The design of the hopper outlet significantly affects the flow of solids. When
solids flow through the hopper, air or gas enters, dilating the particles. It is
essential for powder solids to flow freely through the outlet. Air drag, which
is proportional to surface area, must be balanced by gravitational force that
is equal to the weight of the particle. Fine particles have a lower ratio of