ene gies
A icle
Viabili y E alua ion o Th ee G ass Bio uels:
Expe imen al S udy in a Small-Scale Combus o
A aceli Reguei o 1,* , Lucie Jeze ská2, Raquel Pé ez-O ozco 1, Da id Pa iño 1,
Jiˇ íZegzulka 2and Jan Neˇcas 2
1Indus ial Enginee ing School, Uni e si y o Vigo, Campus Lagoas-Ma cosende, s/n, 36310 Vigo, España;
[email p o ec ed] (R.P.-O.); [email p o ec ed] (D.P.)
2VSB-TU Os a a, Cen e ENET, Bulk Solid Cen e, 17. lis opadu 15, 708 33 Os a a, Czech Republic;
[email p o ec ed] (L.J.); [email p o ec ed] (J.Z.); [email p o ec ed] (J.N.)
*Co espondence: a [email p o ec ed]; Tel.: +34-986-818-624
Recei ed: 2 Janua y 2019; Accep ed: 30 Ma ch 2019; Published: 9 Ap il 2019


Abs ac :
This expe imen al s udy ocusses on he iabili y o h ee low-g ade bio uels in small-scale
biomass uni s o 7–12 kW The es ed ma e ials we e homemade g ass pelle s (gp1 and gp2) and
a mix u e o lea es and woody pelle s (lp50) ha we e made wi hou binde s and comple ely
cha ac e ized in e ms o hei chemical and physical p ope ies (p oxima e analysis, elemen a y
composi ion, ash con en , lowabili y, e c.). The esul s ob ained wi h he non-comme cial uels
we e compa ed wi h comme cial wood pelle s (wp). The iabili y s udy comp ised he ope a ional
pa ame e in luences, such as p ima y and seconda y ai low, uel consump ion, e c., oge he wi h
he impac o hose pa ame e s on he main p oblems in combus ion (concen a ion and pa icle
dis ibu ion, ouling and slagging). The esul s e ealed ha he biomass whose beha io du ing
combus ion induced ewe pa icula e emissions and deposi s was he mixed uel, made o 50% lea +
50% wood pelle (lp50). Con a y o he g ass-based pelle s, lp50 has he possibili y o being used
in comme cial de ices wi h he inco po a ion o p ocesses du ing he manu ac u ing ha imp o e
hei p ope ies.
Keywo ds: biomass combus ion; low-g ade bio uels; ouling; slagging; lowabili y
1. In oduc ion
Rising ossil uel p ices, inc eased conce n abou he en i onmen and clima e change, and
inc eased dependence on ene gy [
1
] ha e all led o a g owing in e es in he p oduc ion o sus ainable
ene gy. Biomass is one o he mos impo an sou ces and i is widely used o hea and powe
p oduc ion, especially in he EU coun ies [
1
]. The inc ease in demand o aw ma e ial o pelle
p oduc ion has inc eased he p ice o sawdus and gi en ise o a e y compe i i e si ua ion. To
cope wi h hese s ong demands, people ha e begun o look o new sou ces o biomass o uel
p oduc ion [
2
]. In ecen yea s, small scale combus ion echnologies ha use pelle s as uel ha e
become a comme cial al e na i e, pa icula ly in Eu ope [
3
,
4
]. Cu en ly he e a e companies ha
p oduce boile s exclusi ely designed o bu n g ass pelle s, and o he s ha e been adap ed o di e en
aw ma e ials [5].
The inc ease in biomass demand has led he sea ch o new p oduc ion sou ces and o gua an ee
hei a ailabili y and supply. The a ailabili y o biomass om wood is inc easingly limi ed and
his si ua ion has caused he hun o new ag o o es y esidues [
6
,
7
]. Nume ous s udies ha e been
ocused on he in es iga ion o new uel p ope ies o use as biomass. Royo e al. u ilized ou
di e en ag opelle s made o esidual ag icul u al biomass o s udying he combus ion p ope ies
using o each hem in a labo a o y ixed bed eac o , ob aining sa is ac o y esul s. This esea ch
Ene gies 2019,12, 1352; doi:10.3390/en12071352 www.mdpi.com/jou nal/ene gies
Ene gies 2019,12, 1352 2 o 19
wo k helps esea che s make choices ela ed o he design and ope a ion o acili ies o he u iliza ion
o ag icul u al esidues biomass [
8
]. Ve ma e al., due o a g owing ma ke o sun lowe husks in
Belgium, Uk aine, and Poland, decided o compa e hem wi h o he uels (wood, apple pomace, pea
. . .
) in a esiden ial pelle boile , eaching e iciency alues o up o 92.4% in he bes case scena io [
5
,
9
].
Sampson e al. used biomass om ag icul u e, such as swi chg ass and ene gy c ops, and hey e i y
he iabili y o hei p oduc ion wi h low cos and educ ion o g eenhouse e ec because hey ha e an
impo an po en ial o displace ossil uels [
10
]. Gonzalez e al. in es iga ed he combus ion o pelle s
made wi h di e en ag icul u al esidues ( o es , oma o plan , ca doon, and oli e bone) o domes ic
hea p oduc ion. They also in es iga ed he mix u e o hese uels in di e en pe cen ages. In his
s udy some combus ion pa ame e s ha e been analyzed and i was de e mined ha he beha io o he
esidues is simila o ha o he o es pelle . Besides, hey e i ied ha he op imum esidue mix u e
was o es (25%) and oma o (75%) [
11
]. Reguei o e al. analyzed he iabili y o h ee non-comme cial
uels (ba ley pelle , lea pelle (100%), and a mix u e wi h lea pelle (25%) and wood pelle (75%))
h ough he s udy o di e en combus ion pa ame e s. They concluded ha he i s wo op ions we e
a bad al e na i e. Ne e heless, he las one could be a good an op ion, al hough i s comme cializa ion
i needs he inco po a ion o addi i es o imp o e he uel p ope ies. The pa icula e ma e (PM)
emissions o he low-g ade bio uels we e be ween 257 mg/Nm
3
and 325 mg/Nm
3
, 17 mg/Nm
3
and 43 mg/Nm
3
, and 22 mg/Nm
3
and 53 mg/Nm
3
o pelle made o ba ley, lea , and a mix u e
o 25% lea /–75% wood, espec i ely [
12
]. And easen and La sen s udied s aw pelle combus ion
and compa ed hei esul s wi h wood pelle s. They e i ied ha hese kinds o pelle s wi hou
an islag agen s cause impo an p oblems wi h slag o ma ion, he amoun o dus in he uel causes
ope a ional p oblems and inc ease he emissions, making he s aw pelle s unsui able o some o
he es ed boile s [
13
]. Olsson e al. [
14
] in es iga ed combus ion wi h pea and whea s aw pelle s.
Resul s we e compa ed wi h hose o wood pelle s, he la e being he one ha p esen ed he bes
e iciencies. Fu he mo e, s aw pelle a e - lame smoulde ing esul ed eleased high concen a ions
o compounds ha a e haza dous o en i onmen and human being heal h, such as ca bon monoxide
and ni ogen oxides. In some s udies ca ied ou i has been e i ied ha he desi ed p ope ies o
any uel a e low ash con en , low slagging endency, and high calo i ic alue [
15
–
17
]. Ne e heless,
delimbing and de olia ion in ees a e used o p oducing uels o be e quali y because o he ash
con en educ ion [18].
The boile e iciency, gaseous and pa icle emissions, o ouling and slagging a y acco ding o
he uel quali y, ope a ing condi ions, and he echnologies used [
6
,
7
,
17
,
19
–
21
]. Focusing on uel, he e
a e a numbe o p ope ies wi h impo an ele ance, such as, physical and chemical ea u es, mass
and ene gy densi y, calo i ic alue, chemical composi ion, and o al amoun o ash and mois u e [
22
].
Nume ous s udies ha e ocused on di e en ope a ional pa ame e s and hei in luence on combus ion.
Some au ho s e i ied ha he uel quali y is uly impo an o gua an eeing low emissions and
a good bu ning p ocess [
3
,
23
]. Eskilsson e al. e i ied ha he design and con ol o a biomass
combus o depends on he uel calo i ic alue [
24
]. Mohon Roy e al. es ed g ass pelle s and
p o ed ha he emissions we e always less han 400 mg/Nm
3
. In addi ion, hey ound a co ela ion
be ween he NOx emissions and he ni ogen uel con en [
25
]. O he s udies conduc ed in small scale
boile s and combus o s wi h comme cial uels p o ided concen a ions be ween 13 mg/Nm
3
o 34
mg/Nm
3
[
26
–
29
]. Finally, nume ous s udies ha e ound ha high concen a ions o silicon (an elemen
wi h a low mel ing poin ) along wi h alkali me als, especially po assium, a e he p e equisi es o he
onse and p og ession o slagging and ouling phenomena [
9
,
18
,
20
,
30
,
31
]. In o he s udies, i was
ound ha Si, Ca, Mg, K, and P had a highe endency o slagging because o hei low mel ing poin s.
In cases in which alkaline me als a e mixed wi h Cl and S, a new p oblem appea s in he acili ies,
co osion [32,33].
P e ious wo ks ha e bu n non-comme cial biomass in speci ic boile s o modi ied exis ing
boile s. In his pa icula s udy, h ee ne e be o e used g ass-based ypes o uels (gp1, gp2, and lp50)
a e es ed in an expe imen al bu ne . The objec i e is o ca y ou a comple e s udy o he sui abili y o
Ene gies 2019,12, 1352 3 o 19
each one o be used in comme cial de ices, comp ising he in luence o he main ope a ing condi ions
oge he wi h he e alua ion o esul s ela i e o he mos impo an combus ion p oblems (pa icula e
ma e —PM concen a ion and dis ibu ion, slagging and ouling). Th ough a deep in es iga ion, he
au ho s aim o e alua e he comme cial iabili y o hese no el and low-g ade bio uels.
2. Ma e ials and Me hods
2.1. Fuels
In his s udy, ou di e en uels we e used (Figu e 1). One is a comme cial wood pelle
ma ked as wp (Biomac L d., Unico , Czech Republic, pelle diame e 6 mm) and he o he h ee
a e non-comme cial pelle s ha ha e no been used so a . Two o hem a e made o g ass: gp1- all
oa g ass (A ena sa i a) and gp2-ben g ass (Ag os is bacilla a) and he o he consis s o a mix u e o
50% u ban p uning esidue (lea es) and 50% sp uce sawdus (lp50). The inpu aw ma e ials o he
e alua ed wood pelle s (wp) we e sp uce sawdus .
Ene gies 2019, 12, x 3 o 18
p oblems (pa icula e ma e —PM concen a ion and dis ibu ion, slagging and ouling). Th ough a
deep in es iga ion, he au ho s aim o e alua e he comme cial iabili y o hese no el and low-g ade
bio uels.
2. Ma e ials and Me hods
2.1. Fuels
In his s udy, ou di e en uels we e used (Figu e 1). One is a comme cial wood pelle ma ked
as wp (Biomac L d., Unico , Czech Republic, pelle diame e 6 mm) and he o he h ee a e non-
comme cial pelle s ha ha e no been used so a . Two o hem a e made o g ass: gp1- all oa g ass
(A ena sa i a) and gp2-ben g ass (Ag os is bacilla a) and he o he consis s o a mix u e o 50% u ban
p uning esidue (lea es) and 50% sp uce sawdus (lp50). The inpu aw ma e ials o he e alua ed
wood pelle s (wp) we e sp uce sawdus .
Figu e 1. Raw inpu ma e ials used in pelle iza ion, designed as: wp-wood pelle s; gp1- all oa g ass;
gp2-ben g ass; lp50-mixed pelle s made o 50% lea es and 50% wood.
The g asses we e g own sepa a ely on single-b eeding sowing a eas in Mo a ian-Silesian
Region o he Czech Republic. These a eas a e he sandy-loam soil ( ype b own-soil). Ni ogen
e iliza ion was ca ied ou in he au umn and in he sp ing. The samples we e ha es ed in ha es
ma u i y by he combine ha es e in one phase in Augus (2017). Tall oa g ass is a s u dy, pe ennial
cul u al g ass g owing in sepa a e bunches, eaching a heigh o e 150 cm. I is a b oadly widesp ead
species, g owing especially in lowland o oo hills, meadows, and pas u es. Ben g ass is a medium-
heigh (100 cm) win e pe ennial g ass wi h sho unde g ound s ems. I is used as a supplemen a y
species in ex ensi e pe manen meadows and pas u e g assland on hea ie soils and we lands. These
g asses we e d ied o 15% mois u e o pelle iza ion (see below in he ex ). T ee lea es om he pa ks
loca ed in Os a a in he Czech Republic (Mo a ian-Silesian Region) we e used o he nex sample
(Figu e 1—ma ked as lp50—50% u ban lea es and 50% sp uce sawdus om sawmill ope a ions).
The lea es we e mainly om ees such as maple (Ace pseudopla anus), Eu opean ash (F axinus
excelsio ), and ches nu (Aesculus hippocas anum) and we e collec ed by he lea collec o machine
( acuum sweepe s) o OZO L d., Os a a, Czech Republic in Au umn 2017. These lea es and sp uce
sawdus we e d ied o 14% mois u e ( o pelle iza ion).
Table 1 p o ides a de ailed p oxima e, elemen al, and ash analysis, and di e en ouling indexes
ha e been calcula ed. Obse ing he da a con ained in he able, in ela ion o he p oxima e analysis,
he pe cen age o mois u e, ola iles, and cha a e qui e simila in all uels, highligh ing no iceable
a ia ions among he ash pe cen age. Whe eas wp uel akes a alue o 0.57% in acco dance wi h
p e ious s udies [34–37], non-comme cial uels (gp1, gp2 and lp50) p esen highe amoun s han
o he sou ces [20], e en eaching 5.97% o gp1. Independen ly o he used uel, he analyzed ash is
o med undamen ally by silicon, calcium, and po assium [18]. In addi ion, i con ains impo an
amoun s o phospho us, Sulphu , Chlo ine, and I on.
In his esea ch, di e en heo e ical indexes ha e been calcula ed o make an app oxima ion o
he acili y beha io wi h each o he es ed- uels. The indexes do no gi e de ini i e esul s bu an
app oxima ion o ha e a gene al idea. Based on he alkaline index, he possibili y o ouling wi h wp
is eminen because i s alue in all cases is highe han 0.17 [38]. The acid o basic a ios a ied be ween
0.42 o 2.37; he e o e, he endency o o m slagging will be medium o non-comme cial and low o
comme cial uels [39]. Howe e , based on he slag a io [40], he endency o o m slagging is high
Figu e 1.
Raw inpu ma e ials used in pelle iza ion, designed as: wp-wood pelle s; gp1- all oa g ass;
gp2-ben g ass; lp50-mixed pelle s made o 50% lea es and 50% wood.
The g asses we e g own sepa a ely on single-b eeding sowing a eas in Mo a ian-Silesian Region
o he Czech Republic. These a eas a e he sandy-loam soil ( ype b own-soil). Ni ogen e iliza ion
was ca ied ou in he au umn and in he sp ing. The samples we e ha es ed in ha es ma u i y
by he combine ha es e in one phase in Augus (2017). Tall oa g ass is a s u dy, pe ennial cul u al
g ass g owing in sepa a e bunches, eaching a heigh o e 150 cm. I is a b oadly widesp ead species,
g owing especially in lowland o oo hills, meadows, and pas u es. Ben g ass is a medium-heigh
(100 cm) win e pe ennial g ass wi h sho unde g ound s ems. I is used as a supplemen a y species
in ex ensi e pe manen meadows and pas u e g assland on hea ie soils and we lands. These g asses
we e d ied o 15% mois u e o pelle iza ion (see below in he ex ). T ee lea es om he pa ks
loca ed in Os a a in he Czech Republic (Mo a ian-Silesian Region) we e used o he nex sample
(Figu e 1—ma ked as lp50—50% u ban lea es and 50% sp uce sawdus om sawmill ope a ions). The
lea es we e mainly om ees such as maple (Ace pseudopla anus), Eu opean ash (F axinus excelsio ),
and ches nu (Aesculus hippocas anum) and we e collec ed by he lea collec o machine ( acuum
sweepe s) o OZO L d., Os a a, Czech Republic in Au umn 2017. These lea es and sp uce sawdus
we e d ied o 14% mois u e ( o pelle iza ion).
Table 1p o ides a de ailed p oxima e, elemen al, and ash analysis, and di e en ouling indexes
ha e been calcula ed. Obse ing he da a con ained in he able, in ela ion o he p oxima e analysis,
he pe cen age o mois u e, ola iles, and cha a e qui e simila in all uels, highligh ing no iceable
a ia ions among he ash pe cen age. Whe eas wp uel akes a alue o 0.57% in acco dance wi h
p e ious s udies [
34
–
37
], non-comme cial uels (gp1, gp2 and lp50) p esen highe amoun s han o he
sou ces [
20
], e en eaching 5.97% o gp1. Independen ly o he used uel, he analyzed ash is o med
undamen ally by silicon, calcium, and po assium [
18
]. In addi ion, i con ains impo an amoun s o
phospho us, Sulphu , Chlo ine, and I on.
Ene gies 2019,12, 1352 4 o 19
Table 1.
P oxima e analysis, ash analysis, elemen al analysis, and ouling index o he ou pelle s
used (wp, gp1, gp2, and lp50).
P oxima e Analysis * (w %) Ash Analysis (w .% o D y Ash)
wp gp 1 gp 2 lp50 wp gp 1 gp 2 lp50
Mois u e 6.41 5.59 6.91 10.44 Na2O 2.94 0.17 0.19 0.10
Vola ile 68.80 66.32 65.01 61.98 MgO 8.29 2.43 2.29 4.01
Cha 24.23 22.13 22.31 21.65 Al2O36.96 0.86 1.13 10.04
Ash 0.57 5.97 5.77 5.94 SiO217.12 35.08 62.58 51.96
Elemen al Analysis ** (w %) P2O53.65 7.20 3.82 1.83
C 48.24 46.71 47.51 49.70 SO3 9.76 5.02 3.89 2.41
H 6.94 6.34 6.67 6.71 Cl 3.27 8.98 1.22 0.25
N 0.26 0.83 0.55 0.48 K2O 20.32 33.55 17.95 4.77
Oa44.56 46.12 45.26 43.11 CaO 23.35 5.57 5.69 19.28
Sb- - - TiO20.39 0.07 0.06 0.38
Fouling Index C 2O3- - 0.02 -
Alkali index 7.56 1.20 0.61 0.18 MnO 1.08 0.21 0.19 0.62
Fe2O32.18 0.78 0.78 4.16
Base o acid a io 2.37 1.18 0.42 0.52 ZnO 0.17 0.05 0.04 0.08
Rb2O - - 0.01 -
Slag a io 33.60 79.98 87.72 65.44 S O - 0.03 0.03 0.12
Z O2- - 0.08 -
Fouling a io 55.15 39.79 7.66 2.52 BaO 0.50 - - -
PbO - - 0.01 -
* (we basis); ** (d y ash ee basis); ade e mined by di e ence; bou o de ec ion limi .
In his esea ch, di e en heo e ical indexes ha e been calcula ed o make an app oxima ion o
he acili y beha io wi h each o he es ed- uels. The indexes do no gi e de ini i e esul s bu an
app oxima ion o ha e a gene al idea. Based on he alkaline index, he possibili y o ouling wi h wp
is eminen because i s alue in all cases is highe han 0.17 [
38
]. The acid o basic a ios a ied be ween
0.42 o 2.37; he e o e, he endency o o m slagging will be medium o non-comme cial and low o
comme cial uels [
39
]. Howe e , based on he slag a io [
40
], he endency o o m slagging is high o
wp and lp50, (slag a io < 65) and low o gp1 and gp2, (slag a io > 72). In ela ion wi h he ouling
a io, he endency o o m ouling is high o all uels excep wp. Fo wood pelle , his index is e y
high acco ding o P onobis e al. [41].
2.2. Pa icle Size Dis ibu ion, Flowabili y and Shea P ope ies o Raw Inpu Ma e ials and Pelle iza ion
The basic cha ac e is ics such as lowabili y and shea p ope ies we e de e mined o aw inpu
g asses, lea es, and sp uce sawdus mix u es om which he pelle s we e made o p edic hei
p ocessing possibili ies.
Fo pa icle size analysis, a Re sch (Haan, Ge many) AS 200 Con ol ib a ing machine (20 min,
ampli ude 4 mm) wi h s anda d sie es (DIN ISO 3310-1) was used. The mois u e con en was
de e mined by mois u e analyze Excellence Plus HX204 (Me le Toledo, G ei ensee, Swi ze land).
Op imum lowabili y o al e na i e uels in inpu bulk o m is c ucial in he manu ac u ing
p ocess. The pelle iza ion p ocess i sel can be di ided in o se e al s eps. The i s is he discha ge o
bulk ma e ials om he hoppe , low in o he eed ame by sc ew con eyo , die illing, comp ession,
and cu ing pelle s. Each o hese s eps has a speci ic se o condi ions such as low a e, s esses,
equipmen su ace, e c. The lowabili y o bulk inpu ma e ials is he e o e one o he impo an
p ocess pa ame e s. In his s udy, h ee me hods o lowabili y e alua ion we e used. The adi ional
lowabili y me hodology measu emen o angle o epose (AoR), as well as Ca ’s index [
42
] and he
Hausne a io [43] calcula ed om bulk and apped densi ies we e de e mined. The hi d lowabili y
me hod was he shea es [
44
]. The compa ison o lowabili y o hose al e na i e ma e ials p o ides
c i ical insigh in o hei use o he pelle izing p ocess.
Ene gies 2019,12, 1352 5 o 19
A pa en ed de ice (Zenege o) o angle o epose measu emen was used [
45
]. The inpu aw
ma e ials we e pou ed in o he cell while g adually c ea ing he pile. Then, he d i e makes he cell
o a e a ound i s e ical axis and he came a successi ely eco ds a pile o all eigh sides. The esul s
we e e alua ed by so wa e. The classi ica ion sys em o he bulk ma e ials lowabili y acco ding o
he angle o epose is shown in Table 2[46].
Table 2. Flow p ope ies and angle o epose.
Angle o Repose [◦] Flow P ope y
25–30 Excellen
31–35 Good
36–40 Fai -aid no needed
41–45 Passable-may hang up
46–55 Poo -mus agi a e, ib a e
56–65 Ve y poo
>66 Ve y, e y poo
Ca ’s index (CI) and he Hausne a io (HR) a e gi en by Equa ions (1) and (2). Whe e
ρT
is
apped densi y and
ρB
is he bulk densi y o aw ma e ials. Classi ica ion o bulk ma e ials lowabili y
acco ding o he calcula ed alues CI and HR is shown in Table 3.
CI =ρT−ρB
ρT
·100 [%](1)
HR =ρT
ρB
[–](2)
Table 3.
Flow cha ac e is ics e alua ed by Ca ’s index (CI) and he Hausne a io (HR) o aw inpu
ma e ials [42,43].
CI [%] HR [–] Flow Diagnosis
≤10 1.00–1.11 Excellen / e y ee low
11–15 1.12–1.18 Good/ ee low
16–20 1.19–1.25 Fai
21–25 1.26–1.34 Passable
26–31 1.35–1.45 Poo /cohesi e
32–37 1.46–1.59 Ve y poo / e y cohesi e
>38 >1.60 Ve y, e y poo /app ox. non- low
When apped densi y was e alua ed he cylinde was illed wi h a ce ain mass o aw ma e ials.
The ini ial olume was measu ed in h ee pa allels and pou ed densi ies we e calcula ed. A e 10, 250,
500, and 1250 aps he co esponding olume was ead. As he di e ence in all cases be ween V
500
and
V1250 was smalle han 2 mL, he V1250 was used o calcula e apped densi y.
The low index ( c) was calcula ed om in e nal ic ion measu emen s. The ic ion pa ame e s
(e ec i e angle o in e nal and wall ic ion) we e measu ed by he Schulze ing shea es e (RST
01, Wol enbu el, Ge many) acco ding o ASTM D6773 [
44
]. The low index ( c) is he a io o majo
consolida ion s ess (
σ1
) o uncon ined yield s eng h (
σc
). Ma e ial was classi ied acco ding o ules
de eloped by Jenike, as pe o med in p e ious wo k [
12
]. Flow cha ac e is ics we e de e mined a
oom empe a u e 23 ◦C±1◦C and he ela i e humidi y was 46% ±2%.
All inpu ene gy g asses, lea es, and sp uce sawdus we e d ied in Binde o en a 40
◦
C (Model
FED 400, Tu lingen, Ge many) and hen c ushed in a hamme g inde (G een Ene gy 9FQ 50, Uhe sky
B od, Czech Republic). A labo a o y pelle p ess wi h a la die was used o pelle iza ion (KAHL
14-175, Amandus Kahl GmbH & Co. KG, Reinbek, Ge many). One and he same die was used in

Ene gies 2019,12, 1352 6 o 19
iden ical condi ions o all he pelle s. Pelle s we e made wi h a diame e o 6 mm, aiming o ha e he
same dimensions ha he comme cial wood biomass. No binde s we e added in his s udy.
2.3. Expe imen al Biomass Combus o and Measu emen Equipmen
The gene al scheme o he combus o wi h i s di e en sec ions is ep esen ed in Figu e 2.
In addi ion, Figu e 2emphasizes ce ain pa s o he ins alla ion impo an o de elopmen o
he es s. The bu ning plan is a biomass small-scale combus o wi h unde eed ixed bed and ai
s aging (wi h p ima y and seconda y ai inle s), supplying he mal powe be ween 5 kW and 12 kW
(300 kW/m2–900 kW/m2).
Ene gies 2019, 12, x 7 o 18
Figu e 2. Scheme o he main acili y wi h de ails o he di e en auxilia y measu emen equipmen .
To de e mine he gas composi ion, a SERVOMEX 4900 gas analyze was used ha acili a es
%O2, %CO, %CO2, and pm NOx measu emen s. The sampled gas- low has o be condi ioned be o e
being sen o he analyze o educe mois u e and emo e soo pa icles [47].
In his esea ch, a s udy o combus ion esidue deposi ion o e he hea exchange s was ca ied
ou . Wi h his pu pose, he acili y coun s wi h a deposi ion p obe, which ac s as a wa e -cooled
mono- ube exchange , loca ed inside he deposi ion module. Apa om his, he slagging o ma ion
has also been in es iga ed. The me hodology o collec ion and ea men o ouling and slagging
deposi ions has been explained in p e ious wo ks [35,36]; hese me hodologies dis inguish he
“a ached mass” om he “deposi ed mass” in hea exchange deposi s, and he “g a e ash” om he
“sin e ed deposi s” o slagging samples.
Figu e 2. Scheme o he main acili y wi h de ails o he di e en auxilia y measu emen equipmen .
Ene gies 2019,12, 1352 7 o 19
The acili y, wi h modula s uc u e, has wo ai inle s: he p ima y en ance (
.
m00
1
) loca ed a he
bo om, and he seconda y inle (
.
m00
2)
be ween he bu ne and he deposi ion sampling module. The
p ima y ai goes in o he bu ne h ough a g a e, being esponsible o ini ia ing combus ion and
allowing he uel eac ion in he bed. The access o seconda y ai is h ough a module ha has 44 holes
each wi h a diame e o 4.4 mm uni o mly dis ibu ed along i s inne pe ime e . The mass low a e is
con olled by a closed loop ci cui in bo h cases. The scheme o he eeding ai egula ion is ep esen ed
in de ail in Figu e 2(de ail 1): he ai is supplied by a cen i ugal an and he low is measu ed wi h a
low me e senso (VPF.R200.100); he an speed is con olled by a a iable equency d i e (Al i a 31),
connec ed o a digi al PID con olle (Om om E5CK) ha allows one o se he desi ed low.
The uel en e s he combus ion chambe h ough a sc ew ac i a ed wi h an elec ical mo o , which
can wo k bo h manually o au oma ically, using a ime . A load cell is used o de e mine he mass.
The ouling module is si ua ed jus abo e he seconda y ai inle and allows one o s udy deposi ion
o e a p obe ha simula es a boile hea exchange . In Figu e 2(de ail 2), he deposi ion sampling
module wi h i s e ige a ion ci cui has been ep esen ed. The wa e sampling-p obe is loca ed inside
he module and i s diame e is 25 mm. The cooling ci cui coun s wi h wo PT100 a e esponsible o
measu ing he inle and ou le sampling p obe empe a u e. A de ailed desc ip ion o he ins alla ion
has been ca ied ou in a p e ious s udy [36].
Jus abo e he ouling module, along he chimney module, he e a e wo gas ou le s o sampling:
one leads o a pa icle impac o (Deka i DLPI wi h 13 scales) and he o he o he gas analyze
(SERVOMEX 4900). These wo measu emen sys ems ha e been ep esen ed in Figu e 2(de ails 3 and
4). A i s op, he acili y coun s wi h a dilu ion module, whe e he exhaus gases a e mixed wi h a
s eam o ambien ai o educe hei empe a u e, ensu ing i is less han 200 ◦C.
2.4. Expe imen al Me hodology
To de e mine he s abili y pe iod and he op imum pa ame e s o he acili y, a se ies o
p elimina y es s we e ca ied ou . The o al du a ion o each is ela ed o he uel cha ac e is ics,
especially ash con en [
12
]. The o al du a ion o he es s ca ied ou in his s udy was 2 h and 30 min,
consis ing o an ini ial ansien pe iod o 45 min, a s a iona y pe iod o 1 h and 30 min, and a shu down
o 15 min o each combus ion. Du ing he es s, he ype o uel used (wp, gp1, gp2, lp50) and he ai
s aging a ios be ween p ima y and seconda y ai (30–70%; 40–60% and 50–50%) we e modi ied.
A hi een-s age Deka i DLPI low-p essu e impac o , was used o measu e he concen a ion and
dis ibu ion o eleased pa icula e ma e (PM). To a oid condensa ion in he gas s eam, he sampling
lines we e hea ed o 130
◦
C. Aluminium il e s we e used in each s age o PM collec ion, and hey
ecei ed a he mal ea men be o e and a e each es acco ding o he s anda d no m UNE-303-5:2012.
In hese es s, a o aliza o was no employed [36].
To de e mine he gas composi ion, a SERVOMEX 4900 gas analyze was used ha acili a es %O
2
,
%CO, %CO
2
, and pm NOx measu emen s. The sampled gas- low has o be condi ioned be o e being
sen o he analyze o educe mois u e and emo e soo pa icles [47].
In his esea ch, a s udy o combus ion esidue deposi ion o e he hea exchange s was ca ied ou .
Wi h his pu pose, he acili y coun s wi h a deposi ion p obe, which ac s as a wa e -cooled mono- ube
exchange , loca ed inside he deposi ion module. Apa om his, he slagging o ma ion has also been
in es iga ed. The me hodology o collec ion and ea men o ouling and slagging deposi ions has
been explained in p e ious wo ks [
35
,
36
]; hese me hodologies dis inguish he “a ached mass” om
he “deposi ed mass” in hea exchange deposi s, and he “g a e ash” om he “sin e ed deposi s” o
slagging samples.
Apa om deposi ion, slagging o ma ion has also been in es iga ed. Fo his, he acili y
was ho oughly cleaned be o e each es . Once inished, a ep esen a i e amoun o ash ha had
accumula ed in he combus ion chambe and in he ash ay was collec ed. The esidues we e sepa a ed
in o wo ac ions as well. On one hand, “g a e ash” conside s he sum o all collec ed ash in he
ash ay oge he wi h some o he ash le in he combus ion chambe apped be ween he slagging.
Ene gies 2019,12, 1352 8 o 19
On he o he hand, “sin e ed deposi s” a e collec ed en i ely in he combus ion chambe and consis o
mol en ash. Once samples we e sepa a ed, hey we e in oduced o a u nace o 200 min a 550
◦
C.
Bo h masses we e measu ed be o e and a e being in oduced i in he u nace. Once he he mal
ea men has been ca ied ou , all o ganic ma e is ola ilized. This way, he o al ino ganic ma e
can be de e mined.
3. Resul s and Discussion
This sec ion is di ided in i e subheadings. I p o ides a concise and p ecise desc ip ion o he
expe imen al esul s, hei in e p e a ion as well as he expe imen al conclusions ha can be d awn.
3.1. Pa icle Size Dis ibui on, Flowabili y and Shea P ope ies o Raw Ma e ials
A se ies o expe imen s we e ca ied ou o e alua e he lowabili y and shea p ope ies
o al e na i e biomass ma e ials. Se e al impo an aspec s we e in es iga ed h ough hese
cha ac e is ics when he ma e ials in e ac wi h s o age, c ushing, pelle izing, and ano he handling
equipmen . The inpu aw ma e ials ( uels) we e cha ac e ized by pa icle size dis ibu ions, which a e
shown in Figu e 3.
Ene gies 2019, 12, x 8 o 18
Apa om deposi ion, slagging o ma ion has also been in es iga ed. Fo his, he acili y was
ho oughly cleaned be o e each es . Once inished, a ep esen a i e amoun o ash ha had
accumula ed in he combus ion chambe and in he ash ay was collec ed. The esidues we e
sepa a ed in o wo ac ions as well. On one hand, “g a e ash” conside s he sum o all collec ed ash
in he ash ay oge he wi h some o he ash le in he combus ion chambe apped be ween he
slagging. On he o he hand, “sin e ed deposi s” a e collec ed en i ely in he combus ion chambe
and consis o mol en ash. Once samples we e sepa a ed, hey we e in oduced o a u nace o 200
min a 550 °C. Bo h masses we e measu ed be o e and a e being in oduced i in he u nace. Once
he he mal ea men has been ca ied ou , all o ganic ma e is ola ilized. This way, he o al
ino ganic ma e can be de e mined.
3. Resul s and Discussion
This sec ion is di ided in i e subheadings. I p o ides a concise and p ecise desc ip ion o he
expe imen al esul s, hei in e p e a ion as well as he expe imen al conclusions ha can be d awn.
3.1. Pa icle Size Dis ibui on, Flowabili y and Shea P ope ies o Raw Ma e ials
A se ies o expe imen s we e ca ied ou o e alua e he lowabili y and shea p ope ies o
al e na i e biomass ma e ials. Se e al impo an aspec s we e in es iga ed h ough hese
cha ac e is ics when he ma e ials in e ac wi h s o age, c ushing, pelle izing, and ano he handling
equipmen . The inpu aw ma e ials ( uels) we e cha ac e ized by pa icle size dis ibu ions, which
a e shown in Figu e 3.
F om he pa icle size dis ibu ion esul s, i is e iden ha he sample o all oa g ass (gp1)
sligh ly di e s om he ben g ass sample (gp2). All samples ha e he la ges p opo ion o pa icles
in he ange o 0.5 mm–1.0 mm. Fine pa icles con ain samples o lea es (pl50) and sp uce sawdus
(wp). Mois u e con en o aw inpu all oa g ass (gp1) was 8.3% ± 1.5%, he ben g ass (gp2) 7.5% ±
1.8%, he sp uce sawdus (wp) 8.6% ± 1.9%, and mix u e 50% lea es and 50% sp uce sawdus (lp50)
7.9% ± 1.8%.
Table 4 summa izes lowabili y measu emen s acco ding o he angles o epose and densi ies
(CI and HR) o aw inpu ma e ials. AoR alues a e be ween 49° and 53°. Lowe AoR alues
gene ally indica e be e low. All aw ma e ial samples we e in one mode in o al. The e we e no
di e ences in lowabili y egime acco ding he AoR. Poo low which needs o be subjec ed o
ib a ion, was p edic ed.
Figu e 3. Pa icle size ange dis ibu ion o aw inpu ma e ials.
Figu e 3. Pa icle size ange dis ibu ion o aw inpu ma e ials.
F om he pa icle size dis ibu ion esul s, i is e iden ha he sample o all oa g ass (gp1)
sligh ly di e s om he ben g ass sample (gp2). All samples ha e he la ges p opo ion o pa icles
in he ange o 0.5 mm–1.0 mm. Fine pa icles con ain samples o lea es (pl50) and sp uce sawdus
(wp). Mois u e con en o aw inpu all oa g ass (gp1) was 8.3%
±
1.5%, he ben g ass (gp2)
7.5%
±
1.8%, he sp uce sawdus (wp) 8.6%
±
1.9%, and mix u e 50% lea es and 50% sp uce sawdus
(lp50) 7.9% ±1.8%.
Table 4summa izes lowabili y measu emen s acco ding o he angles o epose and densi ies
(CI and HR) o aw inpu ma e ials. AoR alues a e be ween 49
◦
and 53
◦
. Lowe AoR alues
gene ally indica e be e low. All aw ma e ial samples we e in one mode in o al. The e we e no
di e ences in lowabili y egime acco ding he AoR. Poo low which needs o be subjec ed o ib a ion,
was p edic ed.
Ene gies 2019,12, 1352 9 o 19
Table 4.
Flowabili y o aw inpu ma e ials acco ding o angle o epose (AoR), Ca ’s index (CI), and
he Hausne a io (HR).
Raw
Inpu Ma e ial AoR [◦]Flowabili y
-AoR CI [–] Flowabili y
-CI
HR
[–]
Flowabili y
-HR
wp 52.1 ±1.1 Poo -mus
be agi a ed,
ib a ed
35.6 Ve y poo / e y cohesi e 1.6 Ve y poo /app ox. non- low
gp1 52.8 ±0.8 24.8 Passable 1.3 Passable
gp2 50.2 ±0.9 30.9 Poo /cohesi e 1.4 Poo /cohesi e
lp50 51.1 ±1.2 20.1 Fai 1.3 Passable
The lowabili y es s show ha ene gy g ass (gp1 and gp2) has ela i ely good low compa ed o
adi ional sp uce sawdus . F om he esul s i is clea ha he CI and HR me hods could be sui able as
al e na i e pelle izing ma e ial lowabili y es s.
Figu e 4shows Ca ’s index o aw ma e ials wi h angle o epose pho os o he es ed samples.
The high angles o epose alues a e ob ious. Due o low sensi i i y, he AoR me hod is a he
inapp op ia e o he lowabili y classi ica ion mode ( o hese ypes o al e na i e ma e ials).
Ene gies 2019, 12, x 9 o 18
Table 4. Flowabili y o aw inpu ma e ials acco ding o angle o epose (AoR), Ca ’s index (CI), and
he Hausne a io (HR).
Raw
Inpu Ma e ial
AoR [°] Flowabili y
-AoR CI [–] Flowabili y
-CI HR [–] Flowabili y
-HR
wp 52.1 ± 1.1 Poo -mus be
agi a ed,
ib a ed
35.6 Ve y poo / e y cohesi e 1.6 Ve y poo /app ox. non- low
gp1 52.8 ± 0.8 24.8 Passable 1.3 Passable
gp2 50.2 ± 0.9 30.9 Poo /cohesi e 1.4 Poo /cohesi e
lp50 51.1 ± 1.2 20.1 Fai 1.3 Passable
The lowabili y es s show ha ene gy g ass (gp1 and gp2) has ela i ely good low compa ed
o adi ional sp uce sawdus . F om he esul s i is clea ha he CI and HR me hods could be sui able
as al e na i e pelle izing ma e ial lowabili y es s.
Figu e 4 shows Ca ’s index o aw ma e ials wi h angle o epose pho os o he es ed samples.
The high angles o epose alues a e ob ious. Due o low sensi i i y, he AoR me hod is a he
inapp op ia e o he lowabili y classi ica ion mode ( o hese ypes o al e na i e ma e ials).
Figu e 4. Ca ’s index o aw inpu ma e ials wi h angle o epose images.
In con as , acco ding o he CI classi ica ion, each o he samples is in a di e en mode. Sp uce
sawdus exhibi s he wo s low (wp), and he bes mix u e is lp50. Samples gp1 and lp50 showed
passable and ai low acco ding o he CI and HR me hods. This mean ha hose ma e ials we e
well designed o pelle iza ion. Ene gy g asses (especially gp1) could he e o e imp o e sp uce
sawdus low i al e na i e mix u es we e c ea ed.
Table 5 summa izes he ic ion pa ame e s o he aw inpu ma e ials and low index. The alue
o he e ec i e angle o in e nal ic ion o gp2 is lowe han o o he ma e ials. This is closely ela ed
o he low index, which is he highes o gp2 and he e o e shows ee lowing ma e ial as well as
gp1. The wall ic ion angle is ano he impo an ic ion pa ame e . I indica es si ua ions when he
al e na i e ma e ial is in con ac wi h s ainless-s eel su aces (silos, hoppe , anspo ou es, e c.).
The esul s show ha he lowes wall ic ion angle is o gp2, and he highes is o wp. gp2 and wp
mixing would con ibu e o easie low o ma e ial in con ac wi h s ainless-s eel su aces.
Table 5. F ic ion pa ame e esul s. δe-e ec i e angle o in e nal ic ion, ϕ-wall ic ion angle, c-
low index.
Raw Inpu Ma e ial δe [°] Φ [°] c [-] Flowabili y Classi ica ion
wp 46.7 13.3 7 Easy lowing
gp1 33.0 11.8 15 F ee lowing
gp2 27.5 9.0 17 F ee lowing
lp50 45.0 12.6 6 Easy lowing
Figu e 4. Ca ’s index o aw inpu ma e ials wi h angle o epose images.
In con as , acco ding o he CI classi ica ion, each o he samples is in a di e en mode. Sp uce
sawdus exhibi s he wo s low (wp), and he bes mix u e is lp50. Samples gp1 and lp50 showed
passable and ai low acco ding o he CI and HR me hods. This mean ha hose ma e ials we e well
designed o pelle iza ion. Ene gy g asses (especially gp1) could he e o e imp o e sp uce sawdus
low i al e na i e mix u es we e c ea ed.
Table 5summa izes he ic ion pa ame e s o he aw inpu ma e ials and low index. The alue
o he e ec i e angle o in e nal ic ion o gp2 is lowe han o o he ma e ials. This is closely ela ed
o he low index, which is he highes o gp2 and he e o e shows ee lowing ma e ial as well as
gp1. The wall ic ion angle is ano he impo an ic ion pa ame e . I indica es si ua ions when he
al e na i e ma e ial is in con ac wi h s ainless-s eel su aces (silos, hoppe , anspo ou es, e c.). The
esul s show ha he lowes wall ic ion angle is o gp2, and he highes is o wp. gp2 and wp
mixing would con ibu e o easie low o ma e ial in con ac wi h s ainless-s eel su aces.
Table 5.
F ic ion pa ame e esul s.
δ
e-e ec i e angle o in e nal ic ion,
φ
-wall ic ion angle,
c- low index.
Raw Inpu
Ma e ial δe [◦]Φ[◦] c [-] Flowabili y
Classi ica ion
wp 46.7 13.3 7 Easy lowing
gp1 33.0 11.8 15 F ee lowing
gp2 27.5 9.0 17 F ee lowing
lp50 45.0 12.6 6 Easy lowing
Ene gies 2019,12, 1352 16 o 19
Au ho Con ibu ions:
A.R. and R.P.-O. pe o med he es s and collabo a ed in he manusc ip w i e up. L.J.
p epa ed he non-comme cial pelle s o es ing. D.P. supe ised he es and he a iche w i ing. J.Z. designed
and pe o med he expe imen s wi h aw ma e ials. J.N. analyzed he da a.
Acknowledgmen s:
This esea ch was unded by he p og amme INTERREG V-A España-Po ugal 2014-2020
(POCTEP) and co- inanced by he Eu opean Regional De elopmen Fund (FEDER) numbe (0015_BIOMASA
AP_1_E). Also suppo ed by he Minis y o Educa ion, You h and Spo s o he Czech Republic unde OP RDE
g an numbe CZ.02.1.01/0.0/0.0 /16_019/0000753 “Resea ch cen e o low-ca bon ene gy echnologies” and i
was conduc ed wi hin he amewo k o he p ojec LO1404: Sus ainable de elopmen o ENET Cen e).
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Abb e ia ions
The ollowing abb e ia ions and symbols a e used in his a icle.
AoR angle o epose [◦]
CI Ca ’s index [–]
DLPI Deka i low p essu e impac o
c low index
gp1 all oa g ass pelle
gp2 ben g ass pelle
HR Hausne a io [–]
lp50 mixed pelle (50% lea es–50% wood)
.
m”
1p ima y ai mass low [kg/m2s]
.
m”
2seconda y ai mass low [kg/m2s]
PM pa icula e ma e
RST 01 Schulze ing shea es e
wp wood pelle
δe e ec i e angle o in e nal ic ion
λai excess [-]
ρBbulk densi y [kg/m3]
ρT apped densi y [kg/m3]
σ1majo consolida ion s ess [kPa]
σcuncon ined yield s eng h [kPa]
Φangle o wall ic ion [◦]
ϕai s aging [%]
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