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Milling and handling Cynara Cardunculus L. for use as solidbiofuel: Experimental tests

Miguel Gil*, Inmaculada Arauzo, Enrique Teruel, Carmen Bartolome

Centre of Research for Energy Resources and Consumptions, University of Zaragoza, Mariano Esquillor 15, E-50018 Zaragoza, Spain

a r t i c l e i n f o

Article history:

Received 28 February 2011

Received in revised form

21 February 2012

Accepted 27 February 2012

Available online 21 March 2012

Keywords:

Cynara Cardunculus L.

Energy crop

Milling

Handling

Particle size analysis

Specific energy requirement

* Corresponding author. Tel.: þ34 976 761863E-mail addresses: miguelgc@unizar.es (M

zar.es (C. Bartolome).0961-9534/$ e see front matter ª 2012 Elsevdoi:10.1016/j.biombioe.2012.02.023

a b s t r a c t

The behaviour of Cynara Cardunculus L., for use as a solid biofuel, is evaluated during

industrial grinding in a hammer mill and handling. Three distinct presentations are

considered: whole Cynara, stems, or pellets. The evolution of performance figures of the

milling process (specific energy requirement and drying effect) and the obtained product

(particle size distribution and bulk density) are described as a function of the target particle

size, from (5e0.5) mm, and the milling strategy, open or with external sieving and recir-

culation. Energy requirements, on dry ton basis, for (1 and 2) mm target sizes are about

20 kWh t�1 for pellets and (50e60) kWh t�1 for stems or whole Cynara. For target size 5 mm,

the specific energy to grind pellets is as low as 6.12 kWh t�1, and for stems or whole Cynara

is below 15 kWh t�1. The handling behaviour is also analysed, mainly the tendency to

bridge in three different hoppers, before and after milling, showing that milling improves

the handling behaviour in all cases. A major cause of problems was the presence of hairs

from capitula in ground whole Cynara, so it is recommended to separate each fraction for

the use as biofuel.

ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction The separative scenario consists on having a full and

One of themain issues in using biomass for energy production

is the supply problem, particularly in those countries where

the climate conditions do not favour plant growing. In order to

increase the use of biomass, new energy crops with flexibility

and low cost of exploitation are needed. Cardoon (Cynara

Cardunculus L.) is an herbaceous perennial plantwith a relative

high yield, above 10 tonnes of biomass per hectare and year in

dry climate conditions [1,2], like Mediterranean countries [3].

The aerial components of the cardoon plant, which reaches

3 m in height, are stem, leaves and capitula, where seeds or

achenes, hairs and pappi are found. In their energy applica-

tion two major scenarios are considered [4]: separative and

non-separative.

; fax: þ34 976 732078.. Gil), iarauzo@unizar.es

ier Ltd. All rights reserved

value-optimized use of each fraction of the cardoon plant. In

this case it is mandatory to separate the different fractions,

during harvest using specific machinery yet in prototype

status [5] or in the processing plant upon reception [6]. The

lignocellulosic biomass would be used for energy production,

the seeds for oil production, and subsequently for biodiesel

production [3,4,7], while hairs and pappi from capitula could

be used to produce paper pulp [8]. Stems can also be used,

supplied in the form of pellets, for combustion in domestic

grid boilers for heating or in fluidised bed co-firing [9]. In the

non-separative scenario, the whole cardoon plant is used as

pulverised solid biofuel in power generation facilities [10].

Non-separative scenario presents advantages like conven-

tional and more efficient harvesting [6] with low moisture

(I. Arauzo), eteruel@unizar.es (E. Teruel), carmen.bartolome@uni-

.

Nomenclature

AoR Angle of repose

RBH As-received biomass hopper

CYN Whole Cynara

CYNPEL Cynara pellets

CYNST Cynara stems

dgm Geometric mean diameter

dp Particle diameter

dsieve Opening size in sieve classifier

dtarget Target particle size

dx Size such that for x% mass dp < dxEs Specific energy requirement (kWh t�1, where 1

t ¼ 1 Mg)

GBB Ground biomass bin

hi Input moisture content

ho Output moisture content

l Rosin-Rammler size parameter

LWFS Loss-in-weight feeding system

Kg Geometric kurtosis

m Rosin-Rammler distribution parameter

Q Feed rate

Sgm Geometric standard deviation

Skg Geometric skewness

rb Bulk density

b i om a s s an d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6146

content of cardoon at harvest (wH2O ¼ 11%e15%), but can also

present drawbacks such as high contents in potassium or

chlorine, partially originated by soil contamination and by

fertilizers [11]. These corrosion problems can be mitigated by

co-firing with protective coals [12].

Both scenarios require some handling processes from the

harvesting to the final energy conversion, particularly tricky

with herbaceous resources such as cardoon. The harvested

product is usually very heterogeneous, both in particle sizes

and shapes. Particles are commonly elongated and with low

bulk density. Few studies have explored the biomass handling

problems after harvesting. Mattson et al. [13e17] analysed

relative bridging tendencies of chipped biomass, reporting

a better behaviour with forest resources with lower moisture

content and lower particle size and the negative influence of

the proportion of overlong particles.

Particle size reduction processes have a significant positive

effect on handling parameters such as angle of repose,

internal friction coefficient, wall friction coefficient, or bulk

density due to size decrease and shape and size homogene-

isation. Ileleji and Zhou [18] confirmed the influence of

moisture content and particle size for corn stover, and high-

lighted the great positive influence of milling process on

flowability behaviour between chipped and milled material

with particle sizes (6.4, 3.2 and 1.6) mm, although not enough

to consider it as “free flowing”. The reduction size pretreat-

ment includes particle breaking processes from a first chip-

ping, crushing or shredding to finer milling. Even so, the main

objective of these processes is to adapt the particle size to the

final application requirements. In particular, milling is

a highly energy-consuming process, with a considerable

impact in the biomass global energetic and economic balance.

For this reason, it is necessary to find a compromise between

the efficiency improvement in the ultimate energy conver-

sion, which is generally better for finer particles, and the

energy consumption required to reduce the size. Pulverised

biomass is used for co-firing with upper particle size around

(1e6) mm [19e24], for the production of densified biofuel for

use in domestic boilers with maximum requirements around

(3e5) mm for pellets and (5e10)mm for briquettes [25], and for

bioconversion to ethanol with agricultural biomass with

nominal particle sizes of about 1 mm [26].

High costs on milling pretreatment have promoted

a research line in this area. Miu et al. [27] established that

hammer mill, knife mill and disc mill were the most appro-

priate equipment for biomass processing. Several studies

were performed to compare these three types of mills with

different biomass [28e31]. As a conclusion from these studies,

hammer mill can be widely considered the most appropriate

option. For this case, authors investigated the influence on

mill electrical consumption and the final biomass physic

characteristics of different operational and design variables

like operational speed, (2000e3600) min�1, with 90� and 30�

hammer edges [26], hammers thickness [32], hammer tip

speed or outlet mill pressure [29]. Esteban et al. performed

different milling strategies for forest biomass [33] and olive-

kernel oil extraction in [34], and Mani et al. [35] obtained the

milling energy requirements of wheat and barley straw, corn

stover and switchgrass with different moisture contents.

Our experience with different preparations of Cynara Car-

dunculus L. in an industrial-size hammer mill is reported here,

regarding to particle size distribution, drying effect, densifi-

cation and specific energy requirements as a function of the

target particle size, and including visual observation of

tendency to bridge on several experimental hoppers.

2. Materials and methods

Samples were taken from as-received and ground biomass.

The sampleswere collected under the CEN/TS 14778-1:2005 EX

standard specifications. Subsequently, the samples were

analysed in a laboratory to obtain the data of bulk density

(CEN/TS 15103:2007), moisture content (CEN/TS 14774-1:2007),

and particle size (CEN/TS 15149-2:2007). Average particle sizes,

diameters andmoments fromparticle sizes distributionswere

calculated according to ISO 9276-2:2001.

2.1. Biomass products

Three different types or preparations of Cynara Cardunculus L.

have been considered for this study: Cynara pellets (CYNPEL),

Cynara stems (CYNST), and whole Cynara (CYN). The first one

was harvested in Alcala de Henares (close to Madrid, Spain,

Fig. 1 e Pellets of Cynara (CYNPEL).

b i om a s s a n d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6 147

UTM coordinates 30N 495948 4511955) in 2003, stored in the

field, and stems and side branches were subsequently sepa-

rated for pelletizing in 2004. Pellets were indoor stored until

they were supplied to CIRCE in 2006. With regard to Cynara

stems, it came from Badajoz (Southwest of Spain, UTM coor-

dinates 29N 674996 4299012) and was harvested and supplied

to CIRCE in September 2006. Bothmaterialswere indoor stored

until the experimental tests in June 2007. These preparations

are considered particularly interesting, since capitula are

often separated to produce biodiesel [4,7,36,37] or paper [8],

therefore stems alone would be used as solid biofuel. Finally,

whole Cynara was also cultivated in Alcala de Henares and

harvested in 2007. It was then stored in the field from

September 2007 until it was chipped with a blade crusher and

delivered to CIRCE in January 2008. Milling experiments were

performed in March 2008, up to this date the material was

indoor stored.

Pellets were indoor stored without significant moisture

content variations and therefore without strength or dura-

bility disturbances [38], and the fungal degradation of

Fig. 2 e Crushed stems of Cynara (CYNST).

a CYNST and CYN stored was negligible with a moisture

content below fibre saturation point or around 20% [39]. More

information about harvesting and subsequent treatments of

the plants was undertaken in several research and develop-

ment projects (e.g., EU project BIOCARD and see [6]).

2.1.1. Cynara pellets (CYNPEL)Pellets are mainly used in domestic boilers, but also in co-

firing utility boilers, when the biomass is traded [40,41]. The

additional energy cost of milling the pellets is far lower than

the transportation cost, which reaches about 15% in overseas

trade [42].

The size of pellets ranges from (2e5) cm in length, with

a diameter about 5 mm. The particle size distribution shows

that 90.31% of the material is composed of compact pellets,

while the remainder is powder smaller than 3 mm, from

partial or total breakage of some pellets, as it is shown in Fig. 1.

CYNPEL initial moisture content was above wH2O ¼ 12.5%, and

bulk density was 550 kg m�3.

2.1.2. Cynara stems (CYNST)Stem and branches were separated from capitula and crushed

after the harvest. Initial moisture content was

wH2O ¼ (10.5e14) % and bulk density was 114 kg m�3. Three

kinds of particles could be distinguished, see Fig. 2:

� Small chips and bits, around (1 or 2) cm in size, with a square

or rectangular-shape.

� Fibrous longer particles, up to 15 cm long.

� Particles smaller than 2 mm, due to the crushing process.

2.1.3. Whole Cynara (CYN)The third kind of biomass considered in our study was

crushed whole Cynara. The different parts of the cardoon

plant had not been sorted or separated, so the main compo-

sition is capitula, stems and leaves. As it can be appreciated in

Fig. 3, the product is very different from CYNST due to the

presence of capitula, with white filaments called hairs [8]. In

average, these hairs are 45 mm long with a 0.2 mm diameter

section.

Fig. 3 e Crushed whole Cynara (CYN).

Fig. 4 e Diagram of the milling facility, showing Configurations A1 and A2.

b i om a s s an d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6148

Whole Cynara was supplied with very heterogeneous

particle sizes. Besides a significant amount of particles smaller

than 2 mm, due to crushing, whole capitula with character-

istic size 4 cm and long stems up to 15 cm were found. Initial

moisture content was wH2O ¼ (12e14) % and the bulk density

was 69.7 kg m�3.

2.2. Biomass facility

2.2.1. Milling pilot plantThe milling facility [43], sketched in Fig. 4, comprises

a hammermill (11 kWand 3000min�1), a screen classifier, and

conveying and collecting ancillary equipments, a belt

conveyor feeder, a cyclone and bag filter to retain dust, and

a bin for the ground biomass (GBB, in Section 2.2.2). The

conveyor belt velocity was regulated to keep the electric

current to the motor around 18 A, with a safety margin below

its 21 A nominal value in order to allow reaction to mill clog-

ging. The electric consumption was sampled each second

using network analysers.

2.2.2. Description of the silosFor handling tests, three feeding systems were used, one for

as-received materials and two for milled biomass (main

geometric dimensions in Table 1 and Fig. 5):

Table 1e Geometric dimensions of the three feeding systems. F

Silo H (m) h (m) a (�) L (m) FD (m

RBH 0 1.5 25 1.9 15

GBB 1 1 24 1.2 10

LWFS 0 0.5 15 B 0.25 7

� As-received biomass hopper (RBH): a wedge-shape hopper

with capacity for 2.15 m3 provided with a screw feeder.

� Ground biomass bin (GBB): a wedge-shape with upper

rectangular-shape hopper with capacity for 1.8 m3 provided

with an a screw feeder.

� Loss-in-weight feeder system (LWFS), Fig. 6: the trunk-conic

hopper, with capacity for 50 l, is provided with a bridge

breaker and sweeping blade rotating around its axis, so the

discharge outlet and screw are placed eccentric with respect

to the axis of the cone, see [44].

2.3. Design of experiments

The goal of this studywas to quantify the impact in themilling

process in particle size reduction and also inmoisture content

or bulk density, and to evaluate the electric consumption of

this pretreatment as a function of the type of Cynara and the

final target particle size, dtarget. The three types (CYNPEL,

CYNST, CYN) and four target sizes, (5, 2, 1 and 0.5) mm, led to

twelve tests. Each test consisted in the essentially stationary

continuous grinding of 15 kg of biomass, approximately.

Due to the observed problems during handling labours as

gravity discharge in ducts, screw feeding, sieving and in

milling operations, it was decided to develop an additional

feeding discharge using three different silos to characterize

or LFWS, the lower diameter of the trunk-cone is given in L.

m) SD (mm) Pitch (mm) Velocity (min�1)

0 40 150 10

5 60 50 54

0 30 65 10e50

Fig. 5 e Characteristic dimensions of the hopper/bin (left)

and discharge screw (right).

Fig. 6 e Dual loss-in-weight feeding system with bridge

breaker.

b i om a s s a n d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6 149

the tendency to bridge of these materials and to prove the

significant influence of milling on the handling behaviour.

2.3.1. Milling tests and configurationsDepending on dtarget a different milling strategy (Fig. 4) was

applied:

� The first milling strategy (Configuration A1) is a continuous

milling in open circuit for dtarget¼ (5 or 2) mm. Particle size is

achieved with the metal screen surrounding the grinding

chamber,withopeningsizedtarget, preventing theparticles to

leaveuntil twoof their dimensions are smaller than this size.

� The second milling strategy (Configuration A2) is a contin-

uous milling in closed circuit for dtarget ¼ (1 or 0.5) mm. After

a first hammermill grindingwith a screenwith opening size

of 2 mm, identical to Configuration A1, the product was

subsequently sieved with a mesh size dsieve ¼ dtarget. The

fines (dp < dsieve) were collected as final product while the

fraction of coarse particles (dsieve < dp < 2 mm) were recir-

culated to the mill for additional grinding. The combination

between hammer mill and screen classifier was necessary

to avoid screen blinding during the grinding process: it is not

advisable to grind biomass in a hammer mill with internal

screen opening sizes much smaller than 2 mm.

In order to minimise the cost of tests and to reach quickly

a stationary regime, since the industrial sieving process is not

amatter of studyhere,Conf.A2was simulated replacing for the

recirculation mass flow a direct feeding of a blend of crushed

and coarse groundmaterial. The coarseparticleswereobtained

form a previous classification of ground material from an

analogous open milling process (Conf. A1, dtarget ¼ 2 mm). The

coarse mass flow was calculated as a function of the coarse

percentage, considering a 90% sieving efficiency.

With regard to ground CYN, it was remarkable the particle

agglomeration due to filaments from capitula (see Section

3.1.3) and, consequently the operational problems with

Configuration A2, because the vibrating screening stage fav-

oured the formation of agglomerates, and they generated

blockages, a problem that was particularly severe in the case

of the most demanding sieving, dtarget ¼ 0.5 mm. This sieving

problem invalidated the data for this milling strategy at these

conditions, therefore neither specific energy consumption nor

drying data are given.

2.3.2. Handling test planingThe basic tests consist on the discharge from hoppers using

screw feeders (Section 2.2.2). The tests are intended to show

the influence of several factors:

� The kind of cardoon (stems, whole Cynara, or pellets).

� The previous storage time of the biomass in the hopper

(except for the LWFS, which incorporates bridge breakers). A

week guarantees the biomass settlement.

� Whether the product is chipped or milled.

� The particle size distribution of the milled biomass, which

depends on the milling strategy and the target particle size.

The tests plan (Table 2) includes discharges of crushed and

milled biomass. For as-received biomass the RBH was used,

Table 2 e Discharge experiments.

Biomass Silo Size Storage Moisture Code Observed behaviour

CYNPEL

RBH PelletNo

12.3%PR1

Discharge without problems

A week PR2

GBB

5 mmNo

8.5%

PG1

A week PG2

1 mmNo PG3

A week PG4

LWFS5 mm

NoPL1

1 mm PL2

CYNST

RBH CrushedNo

14.2%SR1 Discharge without problems

A week SR2 Initial bridges & feeding interruption

GBB

5 mmNo

10.7%

SG1

Discharge without problems

A week SG2

1 mmNo SG3

A week SG4

LWFS5 mm

NoSL1

1 mm SL2

CYN

RBHCrushed

No14%

CR1

Continuous bridges & feeding interruptionA week CR2

GBB

5 mmNo

12%

CG1

A week CG2

1 mmNo CG3 Frequent bridges & feeding interruption

A week CG4 Continuous bridges & feeding interruption

0.5 mmNo CG5 Occasional discharge channels

A week CG6 Frequent discharge channels

LWFS5 mm

NoCL1

Discharge without problems1 mm CL2

b i om a s s an d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6150

varying the storage time. For ground products the two feeding

systems (GBB and LWFS), different particle sizes, and different

storage times could be combined (each experiment is identi-

fied by a code for reference in the next section). Initially, RBH

and GBB were partially full (z1.3 m3 and the inferior wedge-

shape hopper full, 0.6 m3, respectively) and LWFS was totally

full. Since it is clear that flowability is better with smaller

particle sizes, the largest sizes for eachmilling strategy, that is

dtarget ¼ 5 mm for Configuration A1 and dtarget ¼ 1 mm for

Configuration A2, were selected for the initial tests plan. Only

whenever problems arose, additional tests were carried out

with the smallest particle size, that is dtarget ¼ 0.5 mm.

3. Results and discussion

3.1. Handling experimental results

Cynara Cardunculus L. tendency to bridge was determined by

visual observation for the three kinds of cardoon when dis-

charging fromdifferent feeding systems. In addition, potential

blockage problems during vibrating sieving or gravity

discharge and pneumatic transport through ducts were

investigated.

3.1.1. Cynara pellets (CYNPEL)Eight tests (PR1-PL2, see Table 2) with pellets and ground

CYNPEL were fed continuously and homogeneously, without

formation of bridges or preferential discharge channels. The

use of bridge-breaking systems was not required. The storage

time previously to the discharge did not affect the behaviour

either. The behaviour during gravity discharges and pneu-

matic transport was also excellent. Sieving of ground CYNPEL

was performed without problems both in the industrial and

laboratory vibrating sieves.

3.1.2. Cynara stems (CYNST)Ground CYNST (SG1-4, SL1-2, see Table 2) did not show any

tendency to bridge or feeding interruption problems which-

ever the feeding system, particle size, and storage time.

On the contrary, the discharge of crushed CYNST, partic-

ularly when it has had time to settle while stored in the

hopper (SR2), showed an initial tendency to bridge that

provoked a complete feeding interruption. The blockages had

to be broken to resume the discharge, and then it proceeded

without further problems. Recalling the description of chip-

ped CYNST (see x2.1.2), overlong particle (with cylindrical or

hook shape) tend to intertlock, and it is only the relatively low

fraction of these particles what avoids more severe and

frequent arching problems. In any case, it is strongly recom-

mended to use a bridge-breaking devicewhen feeding chipped

CYNST.

3.1.3. Whole Cynara (CYN)This was the most difficult product to handle. The causes of

the problems were again the interlocking elongated particles

from stems crushing and, more significantly, the hairs in

capitula. These hairs, with very small diameter and great

length, showed a dramatic tendency to entangle, building up

structures that trap other particles of small size, and these

agglomerates impeded the flow. Fernandez et al. [6] observed

also severe drawbacks during harvesting originated by

Fig. 7 e Cummulative particle size distributions for the

different milled biomass and target particle sizes.

b i om a s s a n d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6 151

suspension of low density particles, which might provoke

obstruction of the radiator, of the air filter and the engine or

powder-hairs spontaneous combustion.

Crushed CYN presented continuous problems during

discharge (CR1-2, see Table 2) with reiterated feeding inter-

ruptions. In this case the interlock of elongated particles from

stems were apparently the major problem, producing block-

ages, although also problems from hairs entanglement

occurred, exacerbating the former blockages and preventing

besides the discharge of smaller particles. All in all, a feeding

system such as RBH is not considered feasible, and probably it

wouldn’t be either although it included a bridge-breaking

device, due to the serious flowability issues observed.

Less expected, hence more interesting, was the behaviour

observedwith ground CYN,whose behaviour depended on the

particle size and storage time. In the discharge tests using GBB

with particle size 5 mm (CG1-2 in Table 2) continuous feeding

interruptions due to bridge formation were observed, irre-

spective of the storage time. For particle size 1mm (CG3-4) the

problems were continuous after storage, and frequent when

the product was immediately discharged. On the contrary, no

problem arose feeding the same products with LWFS, thanks

to its bridge-breaking device, that is therefore considered

necessary. With particle sizes of 0.5 mm, in CG6, after a week

of storage, the material had settled and arches formed

initially, interrupting the feeding. When they were broken the

feeding proceeded without interruption, although exhibiting

preferential discharge channels. In CG5 (i.e., without storage)

the discharge was performed without problems, even though

preferential discharge channels were observed again.

The reason for the different behaviour lies in the propor-

tion and size of the hairs from capitula and subsequently of

the agglomerates (Section 3.2). This proportion depends

mainly of the milling strategy. In milling configuration A2,

dtarget¼ (1 and 0.5)mm, the external vibrating sieving allows to

retain a higher percentage of these agglomerates. However,

the problemswith hairs are not solved by sieving, but they are

shifted. During the sieving the following problems were

observed:

� Overlong thin particles might (or might not) pass through

sieves with holes greater than their cross-section but much

smaller than their length.

� The low density of hairs [8] makes gravity sieving more

difficult, a minor resistance prevents passing.

� The vibration facilitates the contact between hairs on the

sieve surface, which easily entangle and build up agglom-

erates which trap fine particles from the stems and leaves.

� A significative fraction of particles within the target size are

not retrieved but discarded as oversize together with the

agglomerates, therefore reducing the throughput.

� With time, the sieve might appear virtually lined, provoking

blockages, which are also common in the outlet for oversize

material.

These problems occurred also when using the laboratory

sieves following the standard to analyse the particle size

distribution. The hairs retained in the upper sieve formed

a layer, which retained a small part of fines, leading to a slight

underestimation of the fines.

b i om a s s an d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6152

Finally, problems with hair agglomerates were observed in

other handling operations: blockages in outlets and ducts.

3.2. Milling results

3.2.1. Particle size analysisThe ultimate application of the pulverised fuel strongly

depends on the particle sizes, both in terms of performance

and constraints to be fulfilled, so a detailed particle size

analysis is especially important.

The cumulative distributions, both the experimental data

and the Rosin-Rammler fit [45], FðdpÞ ¼ 1� expð�ðdp=lÞmÞ, areplotted in Fig. 7. This function is generally adequate for

skewed particle distributions, such as ground products. For

the particular case of biomass, this has been confirmed for

hammer-milled switchgrass, wheat straw and corn stover

[26], and knife-milled corn stover [46] or switchgrass [47]. For

Cynara we note that the fit is also excellent, with R2 z 0.99 in

all cases. The Rosin-Rammler distribution and size parame-

ters, m and l, are given in Table 3 wherem generally increases

for smaller dtarget, meaning that the diversity in sizes is

reduced, particularly when dtarget is 0.5 mm and l decreases,

showing a inverse tendency to m. Table 3 also report other

standard size distribution parameters:

� Size-related parameters, such as the median diameter, d50,

or the geometric mean diameter, dgm (for log-normal

distributions). It is remarkable to observe the relevant

differences betweenmean diameters, which are considered

a representative size of the distribution, and dtarget as

maximum particle size of distribution and as operational

parameter.

� Distribution-related parameters, such as the geometric

standard deviation, Sgm, uniformity index, d95=d5, unifor-

mity coefficient, d60=d10 (values below 4 indicate a uniform

mix of particles), mass relative span, ðd90 � d10Þ=d50 (values

above 1 indicate a wide distribution), or coefficient of

gradation, d230=ðd60,d10Þ (values between 1 and 3 indicate

well graded particle sizes). In general, the analysed distri-

butions are wide, according to the values of the mass rela-

tive span, particle sizes are always well graded, according to

the values of the coefficient of gradation, and the mix of

Table 3 e Particle size analysis results.

Biomass dtarget (mm) l (mm) m dmedian (mm) dgm (m

CYNPEL 5 0.65 1.40 0.50 0.43

2 0.37 1.34 0.28 0.25

1 0.42 1.44 0.33 0.28

0.5 0.31 1.86 0.25 0.20

CYNST 5 0.98 1.55 0.77 0.70

2 0.45 1.73 0.36 0.31

1 0.43 1.78 0.35 0.30

0.5 0.32 2.22 0.27 0.23

CYN 5 0.90 1.62 0.71 0.69

2 0.59 1.23 0.43 0.42

1 0.41 1.78 0.34 0.30

0.5 0.29 2.12 0.24 0.21

particles can be considered uniform, according to the values

of the uniformity coefficient.

� Skewness, Skg, which measures the asymmetry of a distri-

bution, and kurtosis, Kg, which measures the peakedness of

the distribution, relative to the log-normal. In our case,

skewness is generally negative, which indicates a relative

predominance of fines, except for CYN and dtarget � 2 mm.

Kurtosis is always very negative, which indicates that the

distribution is relatively flat with respect to the log-normal.

It is observed grossly similar sizes for CYNST and CYN,

while the sizes for CYNPEL are significantly smaller, with

a large amount of fines (50% or 35% of ground CYNPEL was ten

times smaller than dtarget for (5 and 2) mm, respectively).

The differences between CYN and CYNST are greater for

Configuration A1 (that is, dtarget ¼ 5 mm or 2 mm), because

hairs in CYN produce agglomerates, increasing the coarsest

fractions, although this fact is not reflected in the values of

dgm, because the presence of agglomerates in CYN is

compensated by a greater amount of coarse particles in

CYNST. In closed circuit, Configuration A2, agglomerates are

more effectively avoided in the final product, and the sizes are

essentially identical.

The dependency of the particle size distributions with the

kind of biomass and dtarget is shown in Fig. 7. The distributions

shift somehow towards smaller sizes when dtarget is reduced,

and this tendency is consistent for all the size figures, with

l > dmedian > dgm due to negative skewness of the distribution.

It is remarkable that this shift is not proportional to dtarget, in

fact the shift between dtarget ¼ (2 and 1) mm is almost negli-

gible for CYNST and CYNPEL, and quite small for CYN. This is

because for dtarget ¼ 1 mm the milling stage is as for

dtarget ¼ 2 mm, except that the coarse fraction

1 mm < dp < 2 mm is recirculated to the mill, and for

CYNST and CYNPEL this is a little fraction, around 3%. For

dtarget ¼ 0.5 mm the milling stage is also as for dtarget ¼ 2 mm,

but now the coarse fraction 0.5 mm < dp < 2 mm is greater,

29% for CYNST for instance. In the case of CYN, due to the

presence of agglomerates, the coarse fraction recirculated to

the mill is much greater, namely it reaches 17% for 1 mm and

40% for 0.5 mm, what justifies greater differences in the

product obtained in closed or open circuit.

m) Sgmd95d5

d60d10

d90 � d10d50

d230d60,d10

Skg Kg

2.56 18.27 4.69 2.10 1.22 �0.25 �2.21

2.43 20.62 5.00 2.20 1.23 �0.25 �2.56

2.38 16.95 4.50 2.04 1.21 �0.24 �2.40

2.12 8.95 3.21 1.55 1.16 �0.21 �2.59

2.28 13.70 4.02 1.87 1.19 �0.21 �2.42

2.23 10.57 3.50 1.67 1.17 �0.24 �2.40

2.18 9.80 3.37 1.61 1.17 �0.26 �2.43

2.05 6.25 2.65 1.29 1.13 �0.29 �2.48

2.50 12.31 3.80 1.79 1.19 0.01 �2.35

2.50 27.30 5.80 2.44 1.25 0.00 �2.50

1.94 9.83 3.37 1.62 1.17 �0.08 �2.80

1.93 6.81 2.77 1.35 1.14 �0.16 �2.78

Table 4 e Bulk density, moisture content, and specific energy requirement.

Biomass dtarget (mm) dgm (mm) rb (kg m�3) hi (%) ho (%) Abs. drying (%) Rel. drying (%) Es (kWh t�1)

CYNPEL 5 0.43 444 12.10 10.59 1.93 13.96 6.12

2 0.25 506 12.22 11.14 1.38 9.95 18.47

1 0.28 514 12.15 11.14 1.29 9.35 19.69

0.5 0.20 529 12.20 11.49 0.91 6.58 35.00

CYNST 5 0.70 163 10.60 9.95 0.81 6.81 14.96

2 0.31 216 10.40 9.23 1.44 12.42 51.22

1 0.30 228 10.40 9.00 1.72 14.79 53.27

0.5 0.23 250 10.40 8.98 1.74 15.03 79.92

CYN 5 0.69 220 12.47 11.70 1.00 6.99 14.16

2 0.42 229 13.78 10.72 3.98 24.87 57.68

1 0.30 248 13.10 10.35 3.53 23.42 60.76

0.5 0.21 304 e e e e e

b i om a s s a n d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6 153

3.2.2. Bulk density and densificationAt this range of particle sizes, the milling process generally

increases the bulk density when the particle size distributions

are wide, as it is has been shown to be the case in the reported

results, because the finer particles produced bymilling occupy

the free space between the larger particles.

The bulk density (rb) of the different ground products is

included in Table 4, and it is shown together with the bulk

density of the as-received biomass in Fig. 8. Following Mani

et al. [35], we also represent bulk density versus geometric

mean diameter, see Fig. 9, which reflects the general tendency

of bulk density to increase as the particles become finer by

grinding.

The most noticeable bulk density increase is observed for

CYN, where the increase ranges from 216% to 336% of the bulk

density of the crushed biomass (Section 2.1), see Fig. 8. This

outstanding increase is not only due to the general effect of

finer particles but also to the presence, or absence, of

agglomerates. This is why the increase is relatively higher for

the milling in closed circuit (Configuration A2), and particu-

larly for the sieve size of 0.5 mm: even the relatively small

agglomerates produced by hair entanglement which escape

from themill are retained by the sieve and recirculated. This is

clearly observed in the evolution of rb with dgm, which departs

from the generally linear relationship observed in other cases,

see Fig. 9. For instance, in the case of CYNST, with increases

Fig. 8 e Bulk density for as-received and milled products

for different dtarget.

from 43% to 119% with respect to the crushed biomass, the rb

evolves linearlywith respect to the dgm, and the same happens

for CYNPEL. It must be pointed out, though, that for pellets

bulk density firstly decreases by milling, because in this

peculiar case the process is partly breaking up the particle

binding developed during pelletizing [38], and not only proper

particle fracture, but when only ground products are

compared bulk density increases as the particle size is

progressively reduced, as usual.

These data confirm the reported trends by other authors.

Mani et al. [35] showed generalized increasing trends for corn

stover, switchgrass, wheat and barley straw. In all cases, rbwas lower than our three types of Cynara. Esteban and Car-

rasco [33] ground pine, poplar and pine bark with a hammer

mill. Pine results showed a 27% increase of the rb in relation to

chipped material, with a final average bulk density around

330 kg m�3. Ground poplar and bark pine gained more than

50%, and poplar final bulk density was around 225 kg m�3,

close to that of CYN and CYNST.

3.2.3. Moisture content and drying effectMoisture reduction was observed in all experiments, the

initial moisture being above 10%e14% over wet basis. This

drying effect varied depending on the material and

Fig. 9 e Bulk density versus geometric mean diameter.

Fig. 10 e Specific energy requirement versus geometric

mean diameter.

b i om a s s an d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6154

operational parameters, as it is shown in Table 4. In each case

the moisture content of the input and output biomass (hi and

ho, respectively) is given over wet basis and the drying is

calculated, both in absolute terms (over dry basis), that is,

water mass removed with respect to the total dry mass, and

relative to the total initial moisture content.

The most notorious moisture reduction was registered for

CYN, up to 25% of the initial moisture content. For CYNST and

CYNPEL the largest drying observed was around 15%. In all

cases, the least moisture reductions were about 7%. Esteban

and Carrasco [33] also observed the same drying effect in

forest biomass, although it was quantitatively still more

important: Working with pine and poplar, with initial mois-

ture around 12%, they reported reductions around 30%.

3.2.4. Specific energy requirementData on the specific energy requirement, Es, are shown in

Table 4. The energy required to run the hammer mill empty

was not subtracted because it is considered a necessary,

hence computable, cost. The data confirm that a greater size

reduction requires higher milling energy requirement. Es for

grinding CYNST or CYN were similar, while CYNPEL deman-

ded appreciably lower values, as expected, since in this case

the process consists partly in breaking up the pellets, made of

compacted previously ground biomass, and not only a proper

particle fracture.

Comparing to the values reported by [35] and [26], where

no-load power was subtracted, Es for CYN and CYNST appears

to be somehow higher than for corn stover ((11.04, 19.84 and

34.3) kWh t�1 at 12% over wet basis, for (3.2, 1.8 and 0.8) mm,

respectively, in [35], or 13 kWh t�1, at 3.2 mm and 2000 min�1

with 90� -edge hammers in [26]), and lower than for wheat

straw ((24.66, 43.56 and 45.32) kWh t�1 in [35]) and switchgrass

(27.63, 58.47 and 56.57 kWh t�1 in [35], or 16 kWh t�1 in [26]).

Cadoche y Lopez [28] and Himmel et al. [29] reported the

energy requirement with hammer and knife mills for wheat

straw and corn stover at lower moisture content (4%e7% over

wet basis), with similar results.

For CYNPEL the energy requirement was the lowest, close

to that for corn stover, and slightly lower than that for pine

bark chips, according to data from [33] (about 19.7 kWh t�1 for

target size 1.5 mm, as a mean of different processes, without

subtracting no-load power). These authors reported appre-

ciably higher Es to grind other forest biomass, namely poplar

and pine chips, (85.4 and 118.5) kWh t�1 respectively. In other

work [34], they estimated Es from (10.4e28.8) kWh t�1 for dry

residues from olive-kernel oil extraction, in a hammer mill

with (3e1) mm opening size screen, respectively.

The dependency of Es with the particle size of the ground

biomass is illustrated in Fig. 10, where dgm is selected as the

representative figure to indicate the final particle size. Expo-

nential curve fitting of the data show very good regression,

with R2 z 0.9. It is interesting to observe that the dependency

with respect to the target size, dtarget, is not so direct, in fact it

is quite apparent that the consumption for dtarget¼ (1 or 2)mm

is essentially identical, reflecting the fact that the particle size

distribution is in both cases very similar, as discussed in

Section 3.2.1. The cause why this increase is relatively low is

the change from open to closed circuit configuration. For

CYNST and CYNPEL, the amount of recirculated biomass is

very low, about 4%, therefore most of the biomass reaches

sizes below 1 mm after a first grinding with a 2 mm screen,

with the same consumption required to obtain dtarget ¼ 2 mm.

In the case of CYN,where the recirculation rate ismuch larger,

17.4%, a correspondingly higher increment in the consump-

tion would be expected. Nevertheless it is not the case,

because most of the recirculation is due to agglomerates,

which need to be disintegrated rather than fractured in order

to pass the 1 mm sieve, and this is done without much

additional energy, by the mere fact of recycling them to the

mill.

4. Conclusions

The grinding process, in an industrial hammer mill with or

without recirculation, and handling tests, on three different

feeding systems, of three distinct types of cardoon (Cynara

Cardunculus L.) with similar moisture content, wH2O ¼ 10%e

14% over wet basis, to different target sizes, from 5 to 0.5 mm,

has been analysed in terms of the product particle size

distribution, densification,moisture reduction, specific energy

requirement and tendency to bridge.

Cynara pellets (CYNPEL), a preparation with growing rele-

vance due to biomass trade, were the easiest to mill, as

expected, particularly in terms of energy requirement,

(6.12e35) kWh t�1, since the process is partly breaking up the

particle binding developed during pelletizing, and not only

proper particle fracture. This peculiarity is reflected also by

the fact that bulk density decreases when pellets are ground.

Even so, CYNPEL showed excellent handling behaviour before

and after the milling process.

Cynara stems (CYNST) and whole Cynara (CYN) presented

a similar milling behaviour, with similar specific energy

requirements, (14.96e79.92) kWh t�1 in the case of CYNST.

While the energy requirement increased noticeably, in

a factor of 5.3, when the target size decreased ten times, the

final product particle size, in terms of geometric mean

b i om a s s a n d b i o e n e r g y 4 1 ( 2 0 1 2 ) 1 4 5e1 5 6 155

diameter, decreased only three times, dgm (0.7e0.23) mm in

the case of CYNST. In both cases, as expected, bulk density

increased and moisture decreased when the target size

decreased. The moisture reduction, up to 25% in the case of

CYN, and 15% in the case of CYNST, should be taken into

account to optimize the overall exploitation of Cynara Car-

dunculus L. Particle size distributions were similar with

differences in the coarse fraction due to CYN agglomerates.

Ground CYN agglomerates were favoured by the hair from

capitula and were the main reason of risk of arching, block-

ages and feeding interruptions, both in silo discharge and in

other handling operations such as pneumatic conveying

through ducts or sieving, what hinders or prevents its energy

use as pulverised biofuel. Only a costly very fine grinding with

sieving and recirculation, as in tests CG5-6, can effectively

reduce the size of these hairs and avoid problems.

Biomass milling pretreatment showed a significant

improvement on handling behaviour due to homogenization

of particles sizes and shapes. Crushed CYN and CYNST

showed mainly tendency to bridge due to overlong particle

with cylindrical or hook shapes, while ground CYNST can be

handled without significant problems and ground CYN pre-

sented problems, as an exception, due to mentioned

agglomerates. Anyway, eventual feeding difficulties can be

solved by the use of an advanced system such as LWFS, which

is able to deliver continuously a wide range of flows of diverse

pulverised materials with great accuracy [44].

Concerning the particle size characterization of milled

products, in all cases cummulative distributions could be

fitted to Rosin-Rammler distributions, with R2 > 0.99. It was

observed that the size and distribution parameters and

kurtosis were reducedwith dtarget, while skewness was always

negative, that is, by reducing dtarget more uniform (with

predominance of relative fines) and finer particles were

obtained, as expected. Nevertheless, the change of dtarget from

(2e1) mm resulted in minor differences in the particle size

distribution, and the energy requirement. This is due to the

change in the milling configuration, from open to closed

circuit. Either the fraction of recirculated product in the latter

case is low (for CYNST and CYNPEL), or the recirculated

product consists mainly of agglomerates (for CYN), so the

external classification plus recirculation does neither imply

a significant increase in the energy consumption nor changes

substantially the size distribution besides removing the

coarse fraction. In conclusion, a closed circuit configuration is

recommended to effectively achieve dtarget values below the

size of the internal screen of the hammer mill.

From the observed behaviours we conclude that a separa-

tive scenario, either during harvest or in reception, is more

appropriate for the energy use of cardoon, otherwise frequent

problems are expected in grinding, sieving, discharge and

transport.

Acknowledgements

This work was partially supported by projects “Global Process

to Improve Cynara Cardunculus Exploitation for Energy Appli-

cations”, funded by the European Union, and ENE2008-03358/

ALT, funded by the Ministry of Education in Spain. The

authors are grateful to Mr. Oscar Puyo for his extensive

support in the laboratory work.

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