CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

used to convey the particles from mill outlet to other
equipments of handling, storing, treatmeant or directly to
consume milled biomass.
Usually, mills are composed by rotating elements
inside the grinding chamber and at high revolutions The
material circulates around the mill along paths parallel to
the screen surface making the openings are only partially
effective [13].
This rotating flow produces an oblique particle
trajectory to the screen (e.g. Fig. 2). Therefore the
probability of particle evacuation through screen
openings decreases. Higher angle between normal to the
screen and particle trajectory causes higher probability of
impact against the screen and bounce back into the
impact zone where non necessary impacts may happen.
Working under negative pressure in the mill exit
modifies the air stream and the trajectory of fine particles
against the internal screen, increasing the particle
evacuation probability of grinding chamber. Inertial
effect predominates on coarser particles due to their
higher mass and high velocity after impact.
Figure.2: Particle impact trajectory against the internal
screen.
On the other side, high negative pressure may
produce screen blinding since the particles leave so far
the impact zone blocking the screen. After the impact,
high negative pressure hinder the bounce particle,
remaining against the screen and decreasing the screen
effective surface. Under the most unfavorables
conditions, biomass can totally blind the screen and cause
the stop of mill drive and the stop of the process.
Negative pressure is controlled by means of variable
speed drive installed in fans. These electronic devices
vary the rotary revolutions of fan blades, increasing or
decreasing the suction in the mill outlet.
To assess the influence of this parameter in the
properties of milled material and milling energy
consumption it has been necessary to install pressure
transducers in both mills outlets. Negative pressure is
obtained, registred and saved for data processing.
The parameter study allows to determine:
Upper limit of negative pressure without
blinding
Optimal pressure to decrease the milling
energy requirement
Their influence in the pulverized biomass
properties
3.1.2 Speed of rotation
Speed of rotation influences residence time in two
aspects:
Released energy in impacts
Rotating air-particles flow inside the grinding
chamber
3.1.2.1 Impact energy
Residence time depends on breaking velocity, i.e. the
velocity at which the particle is fractured until it achieves
a lower size than the screen opening size. In the case of
hammer mill, the fragmentation (creation of new
surfaces) is influenced by the energy released in impact.
The impact energy is entirely in the form of the relative
kinetic energy between the particle and the point of
hammer where the impact happens.
2
r
I = mν
/ 2 = m ⋅ ( ν −ν
) / 2 = m ⋅ ( r ⋅ω
− v
m
p
m
p
) / 2
Where m is the particle mass, vr the relative lineal
velocity between the particle velocity (vp), and velocity
of the impact point in hammer (vm),r radius from mill
axis to impact point in hammer and ωm the hammers
angular velocity.
Lower angular velocity ωm implies lower impact
energy and lower particle fragmentation. Therefore, the
original particle needs a higher number of impacts until
the suitable size is achieved, increasing the residence
time of particle in grinding chamber.
3.1.2.2 Rotating air-particle flow inside the grinding
chamber
Higher angular velocity of internal elements produces
higher rotating air-particle flow effect inside the chamber.
As it is explained in section 3.1.1, this effect increases the
angle between the particle trajectory and normal to the
screen and decreases the probability of particle
evacuation of the chamber.
3.2 Drying effect during milling
Grinding process causes significant secondary effects
in final milled product. One of those is the mositure
content reduction.
Drying effect is caused by the heat released through
two processes:
Global drying: particles are in air-particles
turbulent atmosphere receiving constantly heat
flows from friction process between the mobile
elements and between this elements and the airparticle
flow.
Local drying: It is caused by the evaporation of
water content in particle due to released heat in
impact. As the particle strikes and compresses,
some of the kinetic energy is converted to strain
energy in the compressed particle, with the sum
of strain energy and kinetic energy equal to the
impact energy. When the strain energy reaches
that required to cause fracture, some of the
stored strain energy is used to create new
surfaces and the rest is converted to heat by
relaxation of the fragments of breakage [14].
The released heat as from friction process as from
particle relaxation after impact is difficult to quantify. For
this reason, a global energy balance in mill is raised. The
objective is to determinate quantitevily the disiped and
inverted energy in drying effect by means of the
numerical value determination of implicit variables
regarding as air as biomass:
Input and output temperature
Input and output air humidity and biomass
moisture content
Biomass feed rate and air mass flow
3.2.1 Input and output temperature
In practice, air-particle flow temperature inside the
grinding chamber is increased during the milling.