CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

CHARACTERIZATION OF A BIOMASS MILLING PILOT PLANT
M. Gil*, I. Ramos, I. Arauzo, J. Román
CIRCE, Centre of Research for Energy Resources and Consumption
C/ Maria de Luna, 3 50018, Zaragoza, Spain
*Corresponding author: miguelgc@unizar.es; tel: +34 976762562; Fax:+34 976732078
ABSTRACT: Biomass milling pretreatment is a necessary process to adapt particle size to the requirements of
different energy conversion technologies. The objectives of biomass milling experimental tests are to optimize the
high energy requirement and to characterize the different milling variables to produce a high quality solid fuel and to
decrease the electrical consumptions.
The characterization process of milling facility at CIRCE cofiring laboratory includes the analysis, instrumentation,
control and data acquisition of the main factors that have an influence in size distribution and shape of milled
particles, in drying and densification effects, and in energy requirement of the whole process. The facility is fully
instrumented and a control and data acquisition system has been installed. These features will allow to obtain on line
information about the working conditions and energy requirements in mills and pneumatic conveying system. Also
several sample points has been installed to take a representative sample of milled biomass. A study has been
developed to assess the feasibility of isokinetic sampling methods in pneumatic conveying ducts to obtain a
representative sample of milled biomass and to analyse and characterize the influence of the grinding variables in the
milled product.
Keywords: characterization, pretreatment, sampling, control systems.
1 INTRODUCTION
Biomass is one of renewable energy with highest
potential and implantation capacity due to the great
variety of biomass species and the wide possibilities of
energetic transformation. Biomass energy conversion
technologies include solids combustion in power stations
to electricity generation (cofiring or biomass exclusive
combustion in pulverized or grate boilers), in district
heating generation or by combustion of densified biomass
solids in domestic boilers. Other technologies transform
the biomass to liquid fuels, mainly for automobile
engines, or to gaseous fuels for later combustion.
Most of these technologies require biomass
pretreatment to adequate the physic and chemical
properties to the final transformation requirements. One
of these pretreatment is size reduction. It is considered
grinding or milling when the final particle size is lower
than 80 mm [1]. Milling objective is to provide a milled
product, whose characteristics satisfy the requirements,
with the lowest possible milling energy requirements.
Usually, biomass is supplied with a previous chipping or
crushing pretreatment. The size reduction results in an
increase of the particle surface-to-volume ratio (particle
specific surface) and, subsequently, in an increase of the
outer surface that will promote further treatments or
chemical reactions. This size reduction is required to
optimize the combustion of biomass in pulverized-fuel
burners or the production of high-density fuels [2-4] or
bio-ethanol [4-6]. However, the milling process brings
along remarkable energy consumptions, within the range
0.5-10% of biomass heating value and depending on
multiple factors [7]. The decrease of the related costs
could represent the key step to warrant the feasibility for
using biomass as the raw material for all those processes.
Aside from the particle size reduction, the milling
process includes several advantages that strengthen the
added value of the product:
Easy sorting of material by size through
separation
Significant biomass bulk density increase. A
reduction size stage is necessary to carry out
intensive densification process as briquetting or
pelletezing [8-10]
Easy handling and drying of bulk material
Reduced transportation costs
Drying effect during milling process [8-10]
Physical properties homogenization of milled
material.
Increase the efficiency energetic conversion
Although publications in this subject are scarce,
several studies show the influence of different variables
in milling energy requirement. These variables can be
classified in input and output biomass characteristics and
operational parameters of comminution process. Physical
and mechanical properties of biomass species and
varieties are very important to select the appropriate type
of mill. Conditions of supply biomass as initial particle
size, moisture content or impurities play an essential role.
In modelling of pulverization process, three aspects are
fundamental to achieve the established goals. Facility
design requires: a suitable selection of type of mill and
the appropriate combinations between mills and auxiliary
equipments. Mills can be arranged in series or in parallel,
with one or more number of mill stages by production
line, batch or continuous milling and the possibility to
include previous, intermediate or later particle size
classifier equipment. Finally, operational parameters
should be studied to decrease the global operational costs
of pretreatment.
Identification and study of different physical process
carried out during milling may allow to determinate the
influential variables. Instrumentation and automation
permit its variation, registration, acquisition and
visualization to evaluate the influence on enumerated
objectives.
By means of the control and monitoring systems of
the variable, it is possible to regulate automatically the
main principal operational parameters, as mills angular
speed, air mass flow in pneumatic conveying and
biomass feed rate to the mills. The system presents other
advantages as the signals treatments, visualization and
observation of variables trends on line. With this
information it is possible to take decisions during test
based on the actual system state.
Process characterization is completed by several

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.

Belt conveyor
B.1
A.1.Configuration
A.2 Configuration
B.1 Configuration
B.2 Configuration
C Configuration
Duplex mill
C
Coarse
recirculation
Hammer
mill
l
Hammer
mill
l
A.2
Fig 1: Milling configurations of experimental pilot plant
Comunication network: It is used to
communicate the PLC with the others devices
in the network. The industrial fielbus used in as
plant are CANOpen and Ethernet.
Supervisor Control And Data Acquisition
(SCADA): It is the Human Machine Interface
(HMI) and offers the function of monitoring
and control the operation in the plant.
Several control loops have been implemented to
regulate and optimize the equipment electrical
consumption. Used control loops are:
Biomass feed rate control: this control loops
optimize the mills electrical consumption
regulating the biomass feed rate by means to
control the feeding belt conveyor velocity. It
exits an optimal input biomass feed rate for
which milling energy requirement (kWh/t)
decreases [12].
Pneumatic conveying control. Control is carried
out varying the angular speed of fans blades.
By means of this control, the pressure at mill
outlet can be modified to observe the influence
in the particle residence time in grinding
chamber (section 3.1.1).
Finally, operational parameters should be obtained,
registered and analyzed to study their influence in the
process and in the milled product. A study of the
instrumentation has been carried out to obtain these
variables.
3 OPERATIONAL PARAMETERS
Pilot plant characterization requires the inherent
processes identification during the milling to determinate
the variables that have significant influence in the
pulverized biomass properties or/and in the milling
A.1
B.2
Cyclone
+
Screen
classifier
Biomass bin
Bag filter
Primary air
Secondary air
Screw feeder
energy requirement.
Two influential processes inside the milling chamber
are analyzed:
Residence time of the particles
Drying effect during milling
3.1 Residence time
Describes the amount of time a particle could spend
inside the grinding chamber. Consequently, the particle
can be impacted and fractured by the mobile elements
inside the mill.
Usually, the final maximum particle size in the
milling chamber is fixed by a metal screen that encloses
the chamber, or by pneumatic system.
The final goal consists in determining the optimal
operational conditions for which the particle evacuates
the chamber as far as possible when its size is smaller as
desired size. Residence time increase involves higher
possibility of new breaking actions which entails milling
energy consumption increment in creating new particle
surfaces. This over-consumption would not be necessary
because the particle would have had a suitable size.
Obviously, the screen openings sizes and mechanical
properties of biomass are fundamental factors in
connection with residence time. Others aspects with no
direct dependency have been analyzed to check his
influence on residence time.
3.1.1 Mill outlet pressure
Particle removal can be by gravity or pneumatic
discharge. Gravity discharge takes place when other
pneumatic system is not connected to mill outlet. The
mill fan effect generates a slight overpressure in the
outlet region. Rotating hammers in the grinding mill act
as fan and built up air pressure against the screen and mill
outlet.
On the other side, pneumatic discharge is commonly

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.

Input and output temperature measurement of air and
biomass allow to assess the heat flow spent in
temperature increase.
3.2.2 Input and output air humidity and biomass
moisture content
Moisture content is analyzed by means of initial and
final biomass representative sampling [15]. Input and
output air humidity are also instrumented.
Water evaporation flow per mass unity is calculated
considering the difference between input and output
moisture content of biomass.
On the other side, it is necessary to know the initial
air humidity to assess the heat flow to increase the water
vapour temperature.
3.2.3 Biomass feed rate and air mass flow
An average biomass feed rate is calculated as ratio
between total raw biomass weigtht and the measured
feeding time.
Air volumetric flow is instrumented with an annubar
flowmeter located in a pipe stretch in ausence of
particles.
4 MILLED BIOMASS SAMPLING
Milling process characterization includes the analysis
and influence of several variables and operational
parameters in the final properties of milled biomass.
First of all, different sampling methodologies to
obtain a representative sample of the total comminuted
biomass in milling experimental pilot plant are reviewed.
4.1 Coning and quartering procedure.
Once the total volume is milled, a representative
sample can be obtained using the coning and quartering
procedure. It is a progressive process, from the bulk
volume (A) two quarters are taken (C and D) and they are
divided into quarters again. The final sample is taken
from the last one (G). The process is shown in figure 3.
Figure 3: Coning and quartering procedure [16]
It is a static sampling method where powder is
confined or heaped. In this situation, size segregation
occurs. Fine particles tend to remain at the center of the
heap and coarse ones congregate at the periphery and the
particle size analyse of the sample is disturbed.
4.2 Gravity discharge sampling.
Sampling from flowing powder such as discharging
flow by gravity, the entire stream of powder should be
sampled by traversing the stream, and the sampling
should continue for a long series of short time intervals.
The sampling point is located between cyclone outlet
and screen classifier (e.g. Fig. 6, GS.1). Particles from
grinders are conveyed until cyclone where are collected
and precipitated to screen classifier. Two-way distributor
(e.g. Fig. 4) has been introduced between both
equipments to change the direction flow on discharge to
the sampling outlet.
This sampling method is simple, economic and
allows taking a representative sample of collected
biomass in cyclone. However, finer particles not
collected by cyclone are not represented in this sample.
Figure 4: Two-way distributor
4.3 Isokinetic sampling
The principal standard for direct measurement of
local particle mass flux in most gas-solid flows is
provided by the isokinetic sampling system. The
isokinetic sampling principle requires that the sampling
probe which is aligned with the flow (isoaxial) extracts
airborne particulates at the sampling velocity matching
the original undisturbed local flow velocity.
In practice, the isokinetic sampling is closely
approached but almost impossible to be rigorously
realized. Several problems as the determination of flow
velocity in the presence of significant amount of
particles, the elimination of intrusive effect of the
sampling probe, the interactions between particles and
carrying fluid, the loss of particles to the wall deposition
and particle bounce/reentrainment in the sampling tube
are frequently observed. By these reasons, three types of
sampling (e.g. Fig. 5) can be carried out: isokinetic
(sampling velocity=stream velocity) or two kind of
anisokinetic sampling: over-sucking (sampling
velocity>stream velocity), under-sucking (sampling
velocity< stream velocity).
Figure 5: Isokinetic and non-isokinetic sampling [17]

Other important factors in this sampling method are
the size and concentration of particles. A study of several
isokinetic sampling points with radical different
conditions is presented to overcome these problems
4.3.1 Sampling points
Milled biomass from Mills (IS.1)
Sampling point is located between mill outlet and
cyclone inlet, previous to intermediate screen classifier
stage. Dilute phase air-solid flow is characterized by high
solids concentration of full range of particle size (

4.4 Characterization of sampling point
The extracted sample at IS.1 from mills is a
representative of full range of milled biomass particles,
however, this localization presents several problems to
achieve the isokinetic conditions and the sampling can
not be representative of this particle flow.
The obtained samples in GS.1 and IS.2 are not
representative of full range of milled particles. At GS.1
point, it is sampling the collected particles by cyclone. At
IS.2, the particles flow is from not collected in cyclone,
in other words, the very fines particles of the full milled
biomass particles. Both samples are not representative of
full range, but the feasibility of a right sampling is higher
due to, in the first place, GS.1 is a correct system to
obtain a sample in discharge, and in the second place, at
IS.2 presents less problems to achieve the isokinetic
conditions.
The possibility of three sampling point allows:
To obtain a representative sample of the whole
amount of milled biomass using two different
methods.
In the first place, to obtain a representative
sampling of full range at IS.1
In second place, to estimate a representative
sampling of full range as composition of the
obtained sample at GS.1 and IS.2:
χt
=
m
m
m = m + m
t
GS1
IS 2
m
· χ IS
GS1
IS 2
χGS1
+ ·
GS1
+ mIS
2 mGS1
+ mIS
2
where:
χ: analyzed property (granulometry, moisture
content, chemical composition or shape factor).
Note: bulk density can not calculate as
composition of both samples.
m GS1: mass flux solids at GS.1
mIS2: mass flux solids at IS.2
Validation of extracted sample at IS.1 through
the comparison his properties with the
properties of the composed sample from GS.1
and IS.2
4.3.2 Milled biomass sample analysis
Milled biomass characterization involves:
Particle size distribution:
It represents the mass percentage for each
particle size interval. It is the most important
parameter in analysis of milled product.
Shape factor.
The spherical approximation is a used
commonly in powder handling. However, the
milled biomass presents usually a cylindrical
shape. Shape factor allows to asses the final
particle shape.
Moisture content
This parameter is expressed as:
Ww
−Wd
M = 100
Ww
where M is the moisture content expressed in
percentage of wet matter, Ww is the weight of
the wet sample and Wd the weight of the dry
sample.
The difference between inlet and outlet
moisture content allows to asses the drying
2
effect during comminution. Several studies [8-
10] have observed a decrease between 10 and
30% of the initial moisture content as function
different factors.
Bulk density.
Miling is ones of solids densification process
due to bulk density increase with particle size
decrease.
Chemical composition
It is observable that the particle size shows
influence on chemical properties [21].
5 CONCLUSIONS
Several parameters have been instrumented to assess
the influence on particle residence time on grinding
chamber and on drying effect:
Pressure, temperature, humidity and mass flow
air at mill inlet and outlet.
Moisture content, temperature and feed rate of
biomass at mills inlet and outlet.
Variable speed drives have been installed in
mills to control the energy impact, in fans to
control the mill outlet pressure and in belt
conveyor to control the biomass feed rate.
Milled biomass properties characterization has been
realized to analyse the parameters influence on the final
product properties.
Three sampling methods have been studied to obtain
a representative sample of milling process:
Coning and quartering sampling can show size
segregration due to the sample is taken out
form stored milled biomass.
Two-way distributor has been installed to
obtain a representative sample in gravity
discharge. Particle flow is in motion and size
segregation is avoided.
Two isokinetic sampling points have been
analyzed where the air-particle flow conditions
are extremely different. At first point, high
solids concentration of all range of milled
particle sizes in horizontal ducts, isokinetic
sampling presents inertial effect of coarse
particle, possible asymmetry in the solids
concentration profiles as function sampling
point height from the bottom of the pipe in the
vertical plane of the cross section and finally,
erroneous solid concentration measurement in
case of anisokinetic sampling.
At second point, low air-particles concentration
of fines particles, isokinetic sampling can
realize suitable, even the concentration
measurement and representative sample can be
obtained in anisokinetic conditions.
Possibility to obtain two representative
sampling of full range of milled biomass size:
isokinetic sampling at first point and as
composition of isokinetic sample at second
point (fines particles representative) and gravity
discharge sample (coarse particles
representative).
First isokinetic sample validation in
comparison with a composition sample of
gravity discharge sample and second isokinetic
sample.

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