CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

milled biomass characteristics analysis: particle size
distribution, moisture content, bulk density, particle
shape factor and chemical composition as function
particle size. The properties variations permit to assess
the influence of the parameters mentioned in final milled
material.
2 BIOMASS MILLING PILOT PLANT
Experimental facility characterization should analyse
the design and operational parameters needed to satisfy
the particle size requeriment with the lowest milling
energy consumption.
The size requirements are fixed by the final energetic
conversion technology and can be expressed by range of
particle sizes, or by more specific granulometric
regulations.
Biomass shows a high heterogeneity in connection
with inherent mechanical and physical properties, and in
reception conditions (moisture content, raw particle
size…). Flexibility in experimental comminution facility
permit to carry out several tests varying:
Type of mill
Equipment combinations
Operational parameters
Biomass mechanical properties represent a relevant
parameter to select the adequate type of mill. Biomass is
mainly composed of non-brittle materials [11] in contrast
with other energetic resources as coal. Several studies
show three types of mills as the most appropriate for
biomass milling: hammer mills, knife mills and also disc
mills in specific conditions [11].
Studied facility is composed of two mills: a hammer
mill and duplex mill [8]. Hammer mill with horizontal
axis and a drive of 11kW comprises 6 hammers arranged
on 3 radial lines. Duplex mill with a drive of 4kW consist
of three hammers in revolution that carry out two
different milling actions: impact breakage of hammers
against particles and breakage by shearing stress between
the hammer, particles and a fixed element inside the
milling chamber.
Grinders are the main equipment in any milling
facility. However, classifying equipments are
incorporated to separate the particles as function their
size. The classifier has operational flexibility which
increases the number of possible mills configuration
allowing the most suitable solid treatment.
Classifying systems select those particles whose size
satisfies the final requirement. Classifier can be used as a
previous stage to separate the particles which satisfy the
size requirement, as a final stage to guarantee the particle
size of final product or as an intermediate stage to convey
the coarse particle again to mill (recirculation flow).
When the upper limit size is fixed, coarse particle can be
eliminated or can be passed by the mill a number of times
necessary to obtain the desired size. In case of lower size
limit, the fine particles are eliminated of the final milled
product.
Milling facility also incorporates auxiliary solid
handling equipment to convey the particles between the
main equipments. At CIRCE milling biomass facility
(e.g. Fig. 1), three different systems are installed to
convey solids. First of all, a belt conveyor is used to feed
the biomass to the mills. Subsequently, pneumatic
conveying, composed of two fans to generate the air
flow, and two equipments (cyclone and bag filter) to
separate the air-particles flow, carries the pulverized
biomass from mills to the final bin. Finally, coarse
particle recirculation is conveyed to mills using an screw
conveyor.
The different combinations between hammer mill,
duplex mills and screen classifier provide five milling
configurations:
1. Configuration A.1: hammer mill
2. Configuration A.2: hammermill and screen
classifier
3. Configuración B.1: duplex mill
4. Configuración B.2: duplex mill and screen
classifier
5. Configuración C: hammer mill, screen
classifier and duplex mill.
Experimental tests in A.1 and A.2 configurations are
focused on the hammer mill analysis:
A.1. Hammer mill.
Particle size is fixed by the internal screen opening
size. Particles pass trough the hammer mill and are
directly carried by pneumatic conveying to the bin. .
A.2. Hammer mill with recirculation to the same mill.
Milled biomass particles in hammer mill are
conveyed to the screen classifier where fines are
separated from the coarse ones. Fine particles are
directly transported to the bin, whereas coarse
particles are conveyed again to the hammer mill
until a suitable particle size is achieved. In this
configuration, the goal particle size is fixed by the
screen opening size in the classifier.
Third and fourth configurations allow the study to be
focused on duplex mill:
B.1. Duplex mill.
As in the A.1 configuration, particles pass trough the
duplex mill before they are being transported by
pneumatic conveying to the bin. Internal screen
determines the final particle size.
B.2. Duplex mill with recirculation to the same mill.
This configuration is similar to A.2. In case finer
particle sizes are needed, this configuration allows
the particle flow to pass several times trough the
mill until the suitable size is reached.
The last configuration is focused on the combination
between both mills with a screening intermediate stage:
C. Hammer mill with recirculation to the duplex mill.
In this configuration (e.g. Fig. 1) the milling and
classifying process are the same as in A.2 configuration,
except for the coarse recirculation. In this case, the coarse
particles are redirected to the duplex mill. The facility
allows both recirculation types.
Configuration changes can be made by means of
removable pneumatic conveying ducts. The combinations
between configurations and internal screens allow to
obtain the suitable particle size in the process.
To control the equipment of the facility, an control
automatic system and data acquisition have been
implemented. The main elements are:
Programable logic Control (PLC): it is the main
control unit. It collets all plant information, and
it take the action on the plant.
Distributed input/output units: they are remote
device, where signals from sensor an signals to
actuators are connect to the PLC
Variable speed drivers: It is the main actuator
in the mill plant. With this we can regulate the
angular speed and the power supply to mill, fan
and others devices.