Chemical s udy o ly ash deposi ion in combus ion o 1
pelle ized esidual ag icul u al biomass 2
Ja ie Royo; Paula Canalís; Da id Quin ana; 3
Uni e si y o Za agoza, c/ Ma ía de Luna 3, E-50018 Za agoza, Spain 4
5
ABSTRACT 6
Ag icul u al esidual biomass has g ea po en ial as an ene gy sou ce, bu is used only o a limi ed 7
ex en mainly because o he cha ac e is ics o i s ash (quan i y and composi ion), which can lead o 8
p oblema ic phenomena du ing combus ion, among hem ly ash deposi ion, he ocus o his s udy. 9
A p e ious wo k p esen ed he esul s o labo a o y expe imen s ca ied ou using a ixed-g a e 10
eac o and in ol ing ou di e en ag opelle s unde di e en ope a ing condi ions; he a iables 11
es ed we e deposi ion a e, bo om ash p opo ion and sin e ing deg ee du ing combus ion. Based 12
on hese esul s, he analysis has been aken u he and he ly ash deposi s collec ed du ing hese 13
es s ha e been cha ac e ized by SEM-EDS and XRD. A me hodology o di e en ia e be ween 14
deposi s caused by condensa ion (including he mopho esis and u bulen di usion) and by ine ial 15
impac o coa se ly ash en ained om he bed has been p oposed. Deposi ion by condensa ion has 16
been ound o dec ease o highe alues o excess ai a io in all cases. Con e sely, deposi ion by 17
ine ial impac does no show a common beha io , due o he in luence o bo om ash sin e ing 18
deg ee and uel composi ion. The ul ima e aim o his s udy is o gain a be e unde s anding o ly 19
ash deposi ion, in o de o de elop be e uel blends, boile design and ope a ing pa ame e s, 20
enhancing he ma ke pene a ion o ag icul u al esidual biomass. 21
22
KEY WORDS: 23
Ag icul u al esidual biomass; Combus ion; Fixed bed eac o ; Fly ash deposi ion; SEM-EDS; XRD 24
25
1. INTRODUCTION 26
The main con ibu ion o biomass o he gene a ion o enewable ene gy in he EU is ound in he 27
hea ing and cooling sec o [1], whe e impo an g ow h is expec ed in coming yea s; he a ge o 28
2020 ha ing been se a 3785 PJ [1]. New uses o o es biomass ([2], [3]), in addi ion o he 29
adi ional ene gy p oduc ion, make impe a i e o ind new esou ces wi h which o mee he 30
p edic able ise in demand o he mal ene gy. The bigges g ow h in supply should come om he 31
ag icul u al sec o , whe e an inc ease o o e 150% compa ed wi h 2006 is expec ed [1]. 32
In addi ion o ene gy c ops and some ypes o esidual ag o-indus ial biomass, hese new esou ces 33
mainly comp ise ag icul u al c op esidues: he baceous c op esidues and p uning esidues o 34
pe manen woody c ops. In pa icula , his pape ocuses on h ee esidual ag icul u al biomasses: 35
ineya d p uning esidues, co n s o e and ba ley s aw. These we e selec ed due o hei po en ial 36
as sou ces o ene gy bo h in Eu ope and he es o he wo ld. FAOSTAT da a (a ailable a [4]) 37
indica e ha he a ea co e ed by ines and maize and ba ley c ops in he EU in 2017 was nea ly 23.4 38
Mha. Using conse a i e a ailabili y indices (50% o ineya d p uning esidue and co n s o e , and 39
10% o ba ley s aw), his ansla es in o an ene gy po en ial o o e 500 PJ/y o he EU. In 40
consequence, hei use could con ibu e signi ican ly o achie e he objec i es se . 41
The mal con e sion o ag icul u al biomass, mainly o he he baceous ype, shows clea di e ences 42
compa ed wi h o es biomass. This is mainly due o he cha ac e is ics o he ash (quan i y and 43
composi ion), which can lead o ce ain p oblems in con e sion acili ies. 44
Du ing combus ion, ash unde goes physical and chemical ans o ma ions which cause ac ioning. 45
Pa o he componen s o he ash emain as a solid ac ion which accumula es in he g a e (bo om 46
ash) and in some cases can sin e , a ec ing con e sion in he bed, es ic ing e iciency o he g a e 47
and nega i ely a ec ing he con ol o gaseous emissions: ca bon monoxide, ni ogen oxides, and 48
ola ile o ganic compounds ([5]-[9]). 49
O he pa , mainly ela ed wi h alkali me al compounds, is ola ilized. A e complex and no always 50
well known mechanisms ([10], [11]), hese compounds can di ec ly condense o a e o ming 51
ae osols be deposi ed by he mopho esis and/o u bulen di usion ([11]-[13]) on he su aces o he 52
equipmen used o hea exchange, in he o m o small c ys als (e.g., po assium chlo ide -KCl-, 53
po assium sul a e -K2SO4- and po assium ca bona es -K2CO3 and KHCO3-). An ash en ainmen o solid 54
pa icles (coa se ly ash) in gas combus ion low om he bed can also be gene a ed and, in some 55
condi ions, hese pa icles can be deposi ed on con ec i e a eas by ine ial impac . These 56
phenomena ( ola iliza ion and ash en ainmen ) a e esponsible, alongside deposi ion, o co osion 57
and e osion, which educe equipmen pe o mance and use-li e ([5], [9]). 58
In ecen decades, se e al p es igious esea ch cen e s ha e been wo king owa ds iden i ying key 59
ac o s in he con e sion o bio uels, as well as in he ans o ma ion o hei ash, in o de o 60
unde s and p oblems caused by he la e ([8], [14]-[23]). In all cases, he c i ical in luence o ash 61
chemical composi ion, especially he concen a ion o Na, Mg, Al, Si, P, S, Cl, K and Ca [24], is 62
ecognized in issues associa ed wi h he mal con e sion (e.g. sin e ing, deposi ion, co osion, e osion 63
and emissions). Howe e , chemical composi ion is no he only ac o , since ash beha io is also 64
a ec ed by combus ion condi ions in he bed, which a e hemsel es ela ed o design ([23]-[26]) and 65
ope a ional pa ame e s [27]. 66
Owing o he complexi y o he phenomena ha con ibu e o ash ac ioning, combus ion es s a e 67
o en unde aken in labo a o y eac o s, mos o which ope a e wi h a ixed-g a e in o de o keep 68
combus ion condi ions unde con ol ([23],[28]-[32]). This ype o eac o enables, in he simples 69
way, he collec ion o impo an in o ma ion conce ning he beha io o uels unde di e en 70
ope a ing condi ions. I allows he e alua ion o uel eac i i y (igni ion on eloci y and igni ion 71
a e [33]), quan i ying o bo om ash in he bed and de e mining i s p opensi y o sin e ing, as well as 72
quan i ying he amoun o solid esidue deposi ed on hea exchange su aces pe ime and uni a ea 73
(deposi ion a e) [31]. Fu he mo e, hese eac o s allow samples o be aken o he 74
cha ac e iza ion o solid esidues (bo om ash ac ion and ly ash deposi s), allowing a be e
75
unde s anding o he phenomena d i ing ash ac ioning. In addi ion, he analysis o gaseous (e.g. 76
CO, NOx o ola ile o ganic compounds) ([34]-[37]) and pa icle emissions ([38]-[40]), is also possible. 77
In a p e ious wo k [41], au ho s p esen ed he esul s analysis o he ou i s poin s ( eac i i y, 78
bo om ash quan i y, sin e ing deg ee and deposi ion a e) o di e en pelle s made o esidual 79
ag icul u al biomass (ag opelle s). These pelle s we e e alua ed unde a ange o ope a ing 80
condi ions in a labo a o y ixed-g a e eac o . In his pape , i is in ended o go a s ep u he and 81
cha ac e ize ly ash deposi ion samples collec ed in combus ion es s by means o scanning elec on 82
mic oscopy (SEM) wi h ene gy dispe si e X- ay spec ome y (EDS), and powde X- ay di ac ome y 83
(XRD). These me hods a e widely used o iden i y and cha ac e ize ash compounds ([9],[13], [42]-84
[48]). 85
SEM-EDS p o ides de ailed imaging in o ma ion abou mo phology, as well as de ining he elemen al 86
chemical composi ion o samples. This echnique is bo h easy and highly p ecise. Al hough elemen s 87
which a e p esen in concen a ions below 0.1-0.5% a e below de ec ion limi s [42], in gene al i 88
does no a ec he de ec ion o he p e iously commen ed mos signi ican ash- o ming elemen s 89
esponsible o ash- ela ed ope a ional p oblems du ing combus ion. 90
The XRD me hod is applied o iden i y and quan i y c ys alline phases p esen in he sample by 91
measu ing hei concen a ions, as well as de e mining he amo phous ac ion [42]. 92
These a e complemen a y echniques. On he one hand, XRD allows o a be e unde s anding o 93
how chemical elemen s de ec ed by SEM-EDS a e associa ed. On he o he hand, he iden i ica ion o 94
mino mine als in a mul icomponen sys em by means o XRD is unce ain due o such issues as 95
de ec ion limi s, peak o e lapping and unknown amo phous ma e . SEM-EDS esul s acili a e he 96
iden i ica ion o phases and can p o ide con i ma ion o XRD esul s [49]. 97
F om he esul s ob ained by means o SEM-EDS and XRD, a me hodology is p oposed o 98
di e en ia e be ween deposi s caused by condensa ion (including he mopho esis and u bulen 99
di usion) and by ine ial impac o coa se ly ash en ained om he bed. 100
The ul ima e aim is o gain a be e unde s anding o deposi ion phenomena a ec ing ag icul u al 101
esidual biomass. This will, i is hoped, help esea che s and echnologis s o make be e decisions 102
ega ding uel blends, boile s design and op imum ope a ing pa ame e s, inc easing he ma ke 103
pene a ion o his impo an ype o biomass. 104
2. MATERIAL AND METHODS 105
2.1 Fuels 106
Fly ash deposi ion chemis y o ou di e en ag opelle s (ag icul u al esidual pelle s) is s udied in 107
his pape : 108
• Woody ag opelle : 100% Vineya d p uning pelle (PV)1 109
• Mixed ag opelle s (Vineya d p uning blended wi h an he baceous componen ): 110
o 70% Vineya d p uning + 30% Ba ley s aw (PVB) 111
o 70% Vineya d p uning + 30% Co n s o e (PVC) 112
o 60% Vineya d p uning + 20% Co n s o e + 20% Ba ley s aw (PVCB). 113
The main he mochemical p ope ies o selec ed uels a e ep oduced om [41] and shown in Tables 114
1 and 2. 115
Table 1 116
Fuel p ope ies (% m/m: mass pe cen age; d.b.: d y basis; w.b.: we basis). 117
PV PVB PVC PVCB
Bulk densi y (kg·m-3)a
599
562
556
546
P oxima e analysis
(% m/m d.b.)
Vola ile
ma e
b
76.5 72.4 72.1 72.3
Fixed ca bonc
20.5
21.7
18.6
21.2
Ashd
3.1
5.9
9.3
6.5
To al mois u e (% m/m w.b.)e
9.0
9.1
9.2
9.0
Ul ima e analysis
(% m/m d.b.)
Ca bon
48.9
46.36
46.01
46.36
Hyd ogen
5.8
5.77
5.64
5.55
Ni ogen
0.55
0.56
0.55
0.60
Sul u g
0.09
0.055
0.050
0.094
Chlo ine g
0.03
0.047
0.080
0.090
Oxygen c
41.6
41.29
38.33
40.58
HHV (d.b. a p=cons an ) (MJ·kg-1)h
19.11
18.54
18.06
18.36
LHV (w.b. a p=cons an ) (MJ·kg-1)h
16.01
15.48
15.06
15.40
a EN 15103:2009 b EN-ISO 18123:2016 c Calcula ed d EN-ISO 18122:2016 e EN-ISO 18134:2016 118
EN-ISO 16948:2015 g EN-ISO 16994:2015 h EN-ISO 14918:2011 119
1 Vineya d p uning esidues used o p oduce his ag opelle we e no he same as hose used o mixed ag opelle s.

120
Table 2 121
Ash p ope ies (% m/m: mass pe cen age; d.b.: d y basis). 122
PV PVB PVC PVCB
Chemical ash
composi ion
(% m/m d.b.)a
Al2O3
0.91
2.72
2.19
2.30
CaO
42.39
45.77
48.17
40.54
Fe
2
O
3
0.71
2.22
1.98
1.27
K2O
30.09
14.88
15.79
19.43
MgO
10.45
8.64
7.64
11.01
Na2O
0.62
0.41
0.39
0.38
P
2
O
5
7.35
4.45
4.00
4.36
SO3
3.95
2.32
3.24
4.39
SiO2
2.65
17.70
15.31
15.22
TiO
2
0.07
0.17
0.18
0.16
Cl
0.12
0.21
0.57
0.54
Ash mel ing
poin s in
oxidizing
condi ions
(oC)b
Ini ial de o ma ion
empe a u e (DT)
1240 1130 1310 1330
Hemisphe e
empe a u e (HT)
> 1500 1310 1460 1460
Flow empe a u e
(FT)
> 1500 1370 1480 1470
a EN-ISO 16967:2015 b CEN/TS 15370-1:2006 123
2.2 Reac o 124
As no ed, esea ch o ash- ela ed phenomena du ing combus ion is gene ally ca ied ou wi h he aid 125
o labo a o y eac o s. In he case o ixed-bed eac o s, simpli ied geome ies a e used o being able 126
o conside one-dimensional beha io [50]. 127
In o de o pe o m he combus ion es s, an expe imen al ixed-g a e eac o was used (see Fig. 1). 128
In his eac o , inle ai is injec ed h ough he g a e om he bo om by means o a an equipped 129
wi h a a iable- equency d i e which allows ai low o be egula ed. Since expe imen s equi e inle 130
ai empe a u e o emain unde con ol, he eac o is equipped wi h a e ige a o and an elec ical 131
esis o ei he o cool he ai o hea i as needed. This allowed wo di e en ypes o es s o be 132
unde aken: wi hou p ehea ing (inle ai a 25oC) and wi h p ehea ing (inle ai a 80oC). The eac o 133
is i ed wi h i een N he mocouples o moni o empe a u e bo h a he bed and he eeboa d. 134
In addi ion, he acili y includes a deposi ion p obe, wi h a emo able sampling ing in he chimney o 135
he eac o [51]. This is a common de ice used o simula e ly ash deposi ion in u nace pipes and 136
hea exchange s [49]. P io o he expe imen , he emo able sampling ing is cleaned, d ied, 137
measu ed and weighed. Du ing he s able combus ion pe iod, he deposi ion p obe is inse ed inside 138
he chimney and he ing is cooled by comp essed ai , keeping i s su ace a an app op ia e 139
empe a u e o s udying deposi ion [51]. Fo he es s p esen ed he e, comp essed inle ai was 140
adjus ed o keep an a e age empe a u e o 335±25oC. Once ex ac ed, he di y ing is d ied and 141
weighed again o de e mine he mass o deposi s, allowing deposi ion a e (DR, g·m-2·h-1) o be 142
calcula ed and, hus, he di e en p opensi y o each uel used o deposi ion o be assessed ([31], 143
[41], [52]-[56]). 144
145
Fig. 1. Scheme o he expe imen al es acili y [41]. 146
Finally, once combus ion is comple ed and he eac o cools down, bo om ash is collec ed om he 147
su ace o he g a e o weighing and classi ica ion, which allows he sin e ing endency o each uel o 148
be de e mined ([5], [21], [41], [57], [58]). Th ee ac ions we e conside ed: S1, which passes h ough a 149
3.15 mm sie e and is conside ed o be no sin e ed; S2, which does no pass h ough a 3.15 mm sie e, 150
bu is easily disagg ega ed by hand and p esen s a low sin e ing deg ee; S3, which does no pass he 151
3.15 mm sie e, is di icul o disagg ega e by hand and p esen s a high sin e ing deg ee. Since he 152
di e ence be ween S2 and S3 is subjec i e, a ac ion S2/3 encompassing bo h classes was used. 153
2.3 Ash analysis 154
In all he es s, once deposi s had been weighed and deposi ion a e calcula ed, a sample was aken 155
om he on ace o he emo able sampling ings, ha is, om he side acing and pe pendicula 156
o he low o combus ion gases. Samples o S1 bo om ash ac ions we e also collec ed. All samples 157
we e glued on o me al pla es wi h ca bon ape and coa ed wi h ca bon be o e being analyzed by 158
SEM-EDS. The equipmen used was a Ca l Zeiss Me lin elec onic ield emission mic oscope equipped 159
wi h Gemini Column, wi h accele a ion ol ages be ween 0.02 and 30 kV, i ed wi h an EDS X-MAS 160
de ec o by Ox o d Ins umen s wi h a window o 20 mm2 and ene gy esolu ion be ween 127 eV 161
and 5.9 keV. Fo each sample, h ee 1 mm2-zones we e selec ed, and images aken wi h he e o-162
dispe sed de ec o (asb). A e age elemen al composi ion was ob ained h ough EDS, using a ol age 163
o 15 kV. INCA so wa e was used o p ocess he esul s. Majo pa icipa ing elemen s in he mos 164
impo an ash ans o ma ion p ocesses -namely Na, Mg, Al, Si, P, S, Cl, K, Ca and Fe- we e included 165
in he analysis. 166
In addi ion, ou combus ion expe imen s wi hou ai p ehea ing we e selec ed o each uel. These 167
es s we e chosen o co e e enly he common ange o excess ai a io (λ) o each uel (see Table 168
3). A p ehea ed expe imen was also selec ed o each ag opelle , all ou wi h an almos iden ical 169
excess ai a io alue (λ≈1.3). Fo all hese es s he c ys alline ma e composi ion o he ly ash 170
deposi ion samples collec ed in he ing was de e mined by XRD. S anda d X- ay di ac ion pa e ns 171
we e collec ed a oom empe a u e using a Rigaku D/max ins umen wi h a coppe o a ing anode 172
and a g aphi e monoch oma o o selec CuKα wa eleng h. The measu emen s we e pe o med a 173
40 kV and 80 mA, in he angula ange om 5° o 80° on 2θ, applying a s ep size o 0.03° and a 174
coun ing a e o 1 s/s ep. X- ay pa e ns we e analyzed wi h JADE so wa e, wi h access o he JCPDS-175
In e na ional Cen e o Di ac ion Da abase (2000) and p o ile-based RIR analyses. 176
3. RESULTS AND ANALYSIS 177
3.1. Tes s and esul s 178
A o al o 68 combus ion es s we e ca ied ou ollowing he same p o ocol wi h he ou uels. As 179
al eady no ed es s bo h wi h and wi hou p ehea ing (“ph” expe imen s) – a ying inle ai 180
empe a u e (Ta) – we e unde aken o e e y uel. In he es s he excess ai a io anged om 1.1 181
o 2.3 (o e -s oichiome ic condi ions), in o de o ep oduce he combus ion condi ions ound in 182
small domes ic equipmen . Table 3 summa izes he main ea u es o he expe imen s pe o med. 183
Table 3 184
Ou line o es ea u es. 185
PV PVB PVC PVCB
Numbe o
es s
pe o med
Wi hou p ehea ing (Ta=25 °C)
10
10
10
12
P ehea ed es s (-ph-; Ta=80 °C) 8 6 6 6
λ
Min
1.15
1.21
1.18
1.23
Max
2.04
2.30
2.29
2.07
Fed uel (kg)
4.03
3.78
3.74
3.67
186
Table 4 shows mean alues (and ange) o elemen al composi ion ob ained by SEM-EDS o each o 187
he ou uels o ly ash deposi s, exp essed as a pe cen age o he o al mass o measu ed elemen s 188
(Na, Mg, Al, Si, P, S, Cl, K, Ca and Fe). 189
Fu he mo e, Fig. 2 plo s he alues summa ized in Table 4 agains excess ai a io, keeping ou he 190
elemen s wi h a concen a ion signi ican ly lowe han 10 % in all he samples (Al, Si, P and Fe) o ly 191
ash deposi s collec ed in he combus ion expe imen s (wi h and wi hou p ehea ing). Due o hei 192
chemical simila i y and he almos iden ical ole hey play in he eac ions ha ake place in ash 193
ans o ma ion p ocesses, he concen a ions o K and Na [24] as well as Ca and Mg [59] ha e been 194
agg ega ed in Fig. 2. 195
The al e na e combina ion o deposi s by condensa ion and by ine ial impac esul s in he 269
cons uc ion o an o e lapping mul i-laye ed s uc u e ([12], [60]). 270
In he ollowing subsec ion, i is p oposed a me hodology ha allows es ima ing he pe cen age and 271
he amoun o ash deposi ed by each o hese wo p ocesses. 272
3.3.1. Me hodology desc ip ion 273
The wo mechanisms in ol ed in ash deposi ion a e highly complex, and i is help ul o s ablish 274
se e al simpli ica ions in o de o acili a e he analysis o esul s. 275
A i s se o assump ions is ela ed o bo om ash and he en ainmen o coa se ly ash. I is 276
assumed ha Si o ms compounds (silica es, aluminosilica es and oxides) ha emain solid ega dless 277
o combus ion empe a u es ([12], [24]). Consequen ly, all Si p esen in he sampling ing (subsc ip 278
“P obe”) is assumed o ha e been deposi ed by ine ial impac (subsc ip “Imp.”)2: 279
SiP obe = SiImp. (1) 280
I has been epo ed ha he chemical composi ion o coa se ly ash en ained om he bed 281
esembles ha o bo om ash ([52], [64]). Fo he de elopmen o his me hodology i is only 282
necessa y o conside ha Si/K p opo ion in S1 ac ion (subsc ip “S1”, a ac ion ha is cons i u ed 283
by pa icles ha can be easily d agged) emains he same in en ained ash (which can subsequen ly 284
be deposi ed by ine ial impac ). This assump ion, oge he wi h equa ion (1), leads o he ollowing 285
equa ion (2): 286
(K/Si)S1 = (K/Si)Imp. = KImp. /SiP obe (2) 287
Following equa ion (2), and conside ing ha , ou o he o al amoun o K ound in sampling ing 288
deposi s, a ac ion can be asc ibed o ine ial impac o solids en ained di ec ly om he bed, wi h 289
he emainde asc ibed o condensa ion (subsc ip “Cond.”), equa ion (3) ollows: 290
KCond. = KP obe - KImp. = KP obe – (K/Si)S1 · (Si)P obe (3) 291
The a gumen a ion ha leads o equa ions (2) and (3) would also wo k o Na ins ead o K. 292
2 In equa ions (1) o (3) “Si” and “K” can be mass o mola con en s o each elemen .

A second se o conside a ions is ela ed o he compounds p esen in deposi s. Based on XRD esul s 293
(see Table 5), c ys alline phases ela ed chlo ides, sul a es and ca bona es o alkali me als 294
(condensa ion) a e KCl, K2SO4, KHCO3, while hose ela ed o Mg, Si and Ca (ine ial impac ) a e 295
CaCO3, Ca(OH)2, MgO, SiO2 and Ca2SiO4. 296
Bea ing in mind he molecula mass o he a o emen ioned compounds, i can be no ed ha : 297
• 1 kg o KCond. p esen in he p obe implies 1.9 kg o KCl, 2.23 kg o K2SO4 o 2.56 kg o 298
KHCO3. Tha is o say, 1 kg o KCond. implies deposi s o he o de o 2 kg o ming KCl, 299
K2SO4 and/o KHCO3. O he chlo ides, sul a es and ca bona es o alkali me als which can 300
be ola ilized and hen deposi ed by condensa ion, e en hose which do no appea in 301
hese XRD esul s (e.g. K2CO3 and K3Na(SO4)2), also ollow, in o de o magni ude, he 302
p opo ion o 2 kg o deposi s pe kg o K+Na3. 303
• 1 kg o Ca p esen in he p obe implies 1.85 kg o Ca(OH)2 o 2.5 kg o CaCO3; 1 kg o Mg 304
implies 1.67 kg o MgO; 1 kg o Si implies 2.14 kg o SiO2; 1 kg o Ca+Si (0.74 kg o Ca + 305
0.26 kg o Si) implies 1.59 kg o Ca2SiO4. Tha is o say, 1 kg o Ca+Mg+Si also implies 306
deposi s o he o de o 2 kg in he o m o MgO, CaCO3, Ca(OH)2, SiO2 and/o Ca2SiO4. 307
Likewise, he compounds o Ca, Mg, Si, P, Al and/o Fe, which a e ypically o med in 308
combus ion and can be en ained om he bed (mainly silica es, aluminosilica es, 309
phospha es, oxides, ca bona es, sul a es, and hyd oxides, whe e K and Na can also be 310
p esen [65]), also ollow, in o de o magni ude, he p opo ion o 2 kg o deposi s pe 311
kg o Ca+Mg+Si+P+Al+Fe+K+Na4. 312
In iew o he ac ha , on one side, each mass uni o KCond.+NaCond. and, on he o he side, o 313
Ca+Mg+Si+P+Al+Fe+ KImp.+NaImp. p oduces app oxima ely (in e ms o o de o magni ude) he same 314
amoun o deposi s, i is possible o app oxima ely de e mine he mass a io o deposi s due o 315
3 I he 7 compounds o his ype men ioned in [65] wi h a p esence o e 1 % a e conside ed, i is ob ained a maximum
alue o 2.52, minimum o 1.77 and mean (unweigh ed) o 2.19 kg o deposi s pe each kg o K+Na (deposi ed by
condensa ion).
4 I he 45 compounds o his ype men ioned in [65] wi h a p esence o e 1 % a e conside ed, i is ob ained a maximum
alue o 3.40, minimum o 1.40 and mean (unweigh ed) o 1.93 kg o deposi s pe each kg o Ca+Mg+Si+P+Al+Fe+K+Na
( o alkalis, he ac ion om deposi s by ine ial impac is only included).
condensa ion (DMCond., kg) and ine ial impac (DMImp, kg) acco ding o equa ion (4)5: 316
DMCond./DMImp.= (K+Na)Cond./((Ca+Mg+Si+P+Al+Fe)P obe + (K+Na)Imp.) (4) 317
Taking in o accoun he ela ionship shown in equa ion (4), he mass ac ion o deposi s caused by 318
condensa ion (m _deposi sCond.) and ine ial impac (m _deposi sImp.) can be accoun ed o , as shown 319
in equa ions (5) and (6): 320
m _deposi s Cond. = DMCond./(DMCond. + DMImp.) = 321
= (K+Na)Cond./((K+Na)Cond. + ((Ca+Mg+Si+P+Al+Fe)P obe + (K+Na)Imp.)) = 322
= (K+Na)Cond./(K+Na+Ca+Mg+Si+P+Al+Fe)P obe (5) 323
324
m _deposi sImp. = DMImp./(DMCond.+ DMImp.) = 325
= ((Ca+Mg+Si+P+Al+Fe)P obe + (K+Na)Imp.)/( (K+Na)Cond. + ((Ca+Mg+Si+P+Al+Fe)P obe + (K+Na)Imp.)) = 326
= ((Ca+Mg+Si+P+Al+Fe)P obe + (K+Na)Imp.)/(K+Na+Ca+Mg+Si+P+Al+Fe)P obe (6) 327
By mul iplying each o hese mass ac ions by he deposi ion a e i is possible o sha e ou he o al 328
mass o deposi s be ween bo h mechanisms, ob aining he deposi ion a e by condensa ion (DRCond., 329
g·m-2·h-1) and by ine ial impac (DRImp., g·m-2·h-1), equa ions (7) and (8): 330
DRCond. = m _deposi sCond. · DR (7) 331
DRImp. = m _deposi sImp. · DR (8) 332
Na u ally, his me hodology only p o ides app oxima e alues, bu based on some easonable 333
hypo heses and simpli ica ions, i allows ob aining conclusions abou he mechanisms o deposi ion 334
(condensa ion o ine ial impac ), as can be seen in nex subsec ion. 335
3.3.2. Me hodology applica ion and discussion 336
The combina ion o he elemen al composi ion (SEM-EDS) o ing deposi s (Fig. 2 and Table 4) and S1 337
bo om ash ac ions (Fig. 3), he deposi ion a e [41] and equa ions (3) o (8), leads o Fig. 4, which
338
exp esses deposi ion a es by condensa ion and by ine ial impac as a unc ion o excess ai a io o 339
each o he ou uels analyzed ( es s wi h and wi hou p ehea ing a e shown). 340
5 In equa ions (4) o (6) “Ca”, “Mg”, “Si”, “P”, “Al”, “Fe”, “K” and “Na”, a e mass con en s o each elemen .
341
Fig. 4. Deposi ion a es (DR) by condensa ion and by ine ial impac agains excess ai a io (λ) o PV, PVB, PVC and PVCB.342
Rega ding es s wi hou inle ai p ehea ing, i can be e i ied ha , o all uels analyzed, deposi ion 343
by condensa ion clea ly dec eases as excess ai a io inc eases ( o all uels and wi hin he ange o λ 344
analyzed, he e is a ac o o abou 3 be ween he highes and he lowes condensa ion deposi ion 345
a e alues). The eason o his lies in he ac ha a g ea e excess ai a io leads o a educ ion o 346
combus ion empe a u e, limi ing he ola ili y o he main eac i e ash elemen s ha play a ole in 347
deposi ion by condensa ion (mainly K, Cl and S) ([66], [67]). 348
In addi ion, he subs an ial alues o deposi ion by condensa ion p esen ed by PVB, especially a low 349
excess ai a ios, a e no ewo hy. This is con i med by he chemical analysis o he deposi s: 350
• XRD analysis (see Table 5): PVB has a high pe cen age o KCl (37.4% on a e age in he ou 351
samples analyzed, much highe han he o he pelle s) and K2SO4 (30.1% on a e age in he 352
ou samples analyzed, only below PVCB). 353
• SEM-EDS analysis (see Fig.2 and Table 4): PVB p esen s he highes pe cen ages o K and Cl, 354
and also o S. 355
Howe e , he e is no ob ious co ela ion be ween hese esul s and ash p ope ies ob ained in uel 356
analysis (see Tables 1 and 2), as PVB p esen s lowe concen a ions o K and Cl han he o he mixed 357
pelle s, and lowe concen a ion o S han PVCB. This ac co obo a es ha ying o p edic he 358
pe o mance o ash biomass based only on indices ob ained om uel analysis is no always accu a e; 359
in ac , he use ulness o hese indices has been ques ioned in o he esea ch wo ks (e.g., [48], [56], 360
[68]-[70]). 361
Con inuing wi h he esul s o es s wi hou p ehea ing, hey also e eal ha PV p esen s sligh ly 362
lowe alues o deposi ion a e by condensa ion han mixed pelle s (≤ 13 g·m-2·h-1 in all cases), 363
especially when excess ai a ios a e low. This ac is ela ed o he di e en composi ion o hese 364
deposi s, which p esen low pe cen ages o K2SO4 and, abo e all, o KCl de ec ed by XRD (Table 5), 365
al hough his is pa ially compensa ed by he high pe cen age o KHCO3 (25.3%, whe eas in he es 366
o uels i is only de ec ed in one PVB sample). The low concen a ion o KCl in he deposi s can be 367
explained by he low concen a ion o Cl in his uel (Tables 1 and 2). Conce ning K2SO4, al hough PV 368
p esen s a high pe cen age o K and S (Tables 1 and 2), K has a g ea e a ini y o P (i is ound in e y 369
signi ican amoun s in his uel (Table 2)), wha could acili a e he o ma ion o K-phospha es be o e 370
K-sul a es [24]. 371
Conce ning deposi ion by ine ial impac , no common endency has been ound o apply o all uels 372
in es s wi hou p ehea ing. Whe eas in PV i s alue clea ly dec eases when excess ai a io is highe , 373
in mixed pelle s i emains p ac ically cons an . These di e en endencies among uels may be 374
caused by he ac ha an inc ease in excess ai a io leads o wo opposi e e ec s ha in e ac wi h 375
di e en weigh s: on he one hand, inc easing he ai low aises i s speed in he bed, encou aging 376
he en ainmen o coa se ly ash; on he o he hand, he adhesion o solid pa icles is discou aged, 377
as s icky deposi s in he ing (alkali me al sul a es and chlo ides) become less subs an ial, owing o 378
educed apo iza ion and subsequen condensa ion. 379
To deepen he analysis o he beha io p esen ed by each uel, Table 6 shows o al deposi ion a e, 380
by condensa ion and by ine ial impac , oge he wi h bo om ash p opo ion and sin e ing deg ee 381
( ac ion S2/3) as e lec ed by he expe imen al esul s p esen ed in [41]. 382
Table 6 383
Bo om ash p opo ion, sin e ing deg ee and deposi ion a es ( o al, by condensa ion and by ine ial impac ) 384
mean alues ( ange) o all es s wi hou inle ai p ehea ing. 385
Bo om ash
p opo ion a
Sin e ing
deg ee
( ac ion
S2/3)
a
DR DRCond. DRImp.
%
%
g·m-2·h-1
g·m-2·h-1
%
g·m-2·h-1
%
PV
25.3
(18.1-31.3)
1.6
(0.3-3.1)
16.6
(11.0-21.3)
8.1
(3.7-13.0)
49.0
(31.0-62.5)
8.5
(5.5-12.8)
51.0
(37.5-69.0)
PVB
74.7
(72.8-77.7)
51.8
(26.7-62.2)
20.9
(10.5-29.9)
17.3
(8.2-24.3)
82.9
(73.4-90.8)
3.6
(1.9-5.6)
17.1
(9.2-26.6)
PVC
50.0
(48.3-50.7)
33.8
(23.5-40.7)
19.4
(14.2-25.0)
10.1
(5.4-15.5)
52.0
(32.9-74.1)
9.3
(5.3-14.0)
48.0
(25.4-67.1)
PVCB
59.5
(58.4-60.9)
40.1
(26.4-49.8)
19.2
(13.5-23.8)
9.7
(4.3-16.1)
54.0
(30.1-68.8)
8.3
(6.2-10.8)
46.0
(31.2-69.9)
a % wi h ega d o o al mass o ash in oduced wi h he uel. 386
Table 6 seems o indica e ha , in mixed pelle s, sin e ing p e en s deposi ion by ine ial impac by 387
discou aging he en ainmen o pa icles om he bed. As a esul , PVB, which p esen s high 388
sin e ing alues, yields a much lowe deposi ion a e by ine ial impac alues han PVC and PVCB. 389

The esul s o inle ai p ehea ing es s ( ep esen ed by un illed ma ke s in Fig. 4) can be used o 390
co obo a e his beha io o mixed pelle s. Inle ai p ehea ing inc eases ai eloci y in he bed, 391
encou aging en ainmen , bu does no lead o a signi ican inc ease o combus ion empe a u e 392
[41]: 393
• Due o he ac ha combus ion empe a u e emains p ac ically unchanged, ash 394
apo iza ion and he e o e deposi ion by condensa ion is no a ec ed subs an ially by 395
p ehea ing o any o he mixed pelle s. 396
• In con as , i may be obse ed ha ai p ehea ing la gely inc eases deposi ion by ine ial 397
impac in PVC and PVCB, bu much less in he case o PVB. In o he wo ds, an inc ease in ai 398
eloci y causes a much bigge impac in pa icle en ainmen and i s subsequen deposi ion 399
in uels which a e less suscep ible o sin e ing. 400
The case o PV is somewha di e en , because high en ainmen ( he e is e y li le ash e en ion in 401
he bed) does no di ec ly ansla e, in es s wi hou p ehea ing, in o a g ea e amoun o deposi s by 402
ine ial impac compa ed o ha o o he uels, possibly owing o he lowe quan i y o s icky 403
deposi s. In ac , i seems ha gi en ha his uel p esen s low concen a ions o KCl and K2SO4, he 404
sampling ing was sa u a ed by deposi s om ine ial impac . As a esul o his sa u a ion, ollowing 405
an inc ease o excess ai a io and hence a dec ease in s icky deposi s due o condensa ion, he 406
capaci y o PV o e ain deposi s by ine ial impac dec eases (Fig. 4). I is wo h analyzing he esul s 407
o he es s wi h p ehea ing o his uel. Fi s , a sha p dec ease in deposi ion by condensa ion can be 408
no iced compa ed wi h es s wi hou p ehea ing. This is due o he lowe combus ion empe a u es 409
eached in expe imen s wi h p ehea ing [41], which discou ages he e apo a ion o alkali me als 410
om he bed, as expec ed in iew o Table 5. The ac ha KOH has g ea e a ini y o SO2/SO3 and 411
HCl han o CO2 [24] explains a leas pa ly he a o emen ioned non-appea ance o K-ca bona es in 412
he p ehea ed es . In addi ion, p ehea ing o inle ai en ails no signi ican inc ease o deposi ion by
413
ine ial impac , which ein o ces he idea o sa u a ion. 414
415
3.3.3. (K+Na)/(Cl+2S) mola a ios 416
To comple e he compa a i e analysis o he a ious uels, Table 7 shows (K+Na)/(Cl+2S) mola a ios, 417
calcula ed om SEM esul s o deposi samples (Fig. 2), equa ion (3) and he ini ial analysis o he 418
uels (Tables 1 and 2). 419
Table 7 420
(K+Na)/(Cl+2S) mola a ios (mean alues o es s wi h and wi hou p ehea ing). 421
(K+Na)P obe/(Cl+2S)
deposi s SEM
(K+Na)Cond./(Cl+2S)
deposi s SEM
(K+Na)/(Cl+2S)
Fuel analysis
PV 2.13 1.71 3.16
PVB 1.26 1.21 4.08
PVC 1.11 1.01 6.02
PVCB 1.24 1.09 3.29
422
Rega ding (K+Na)P obe/(Cl+2S) mola a io using o al alkali me al concen a ion in deposi s by SEM, i 423
can be obse ed ha all uels p esen alues highe han 1, i.e., he e is an excess o alkali me als 424
compa ed wi h Cl and S. The easons o his a e wo old: 425
• Pa o K and Na ha e apo ized as hyd oxides and condensed as ca bona es (mainly KHCO3, 426
which was de ec ed by XRD in PV and in one PVB sample, see Table 4). 427
• Ash en ainmen o solid pa icles con aining K and Na om he bed occu s. 428
I he second a io is conside ed, (K+Na)Cond./(Cl+2S) -a e discoun ing alkali me als compounds 429
deposi ed in he sampling ing by ine ial impac ( ollowing equa ion (3))- i can be no ed ha , in he 430
case o PVCB and PVC, he alue ob ained is e y close o 1 (p ac ically all alkali me al ha e 431
condensed as chlo ides o sul a es), while in PVB and especially in PV i is highe , owing o he 432
a o emen ioned p esence o KHCO3. The cohe ence o hese a ios wi h XRD esul s co obo a es 433
ha he hypo heses and assump ions on which equa ion (3) was based we e sound. 434
Finally, i should be s essed ha i is no possible o easily p edic he alues o hese mola a ios 435
(ob ained by analyzing he deposi s) no o explain di e ences in he beha io o he a ious uels on 436
he basis o a ios calcula ed wi h uels p elimina y analysis (las column o Table 7), since he e is no 437
di ec co espondence be ween hem. 438
4. CONCLUSIONS 439
This s udy has p esen ed he esul s o chemical analysis o deposi s ob ained in combus ion es s 440
ca ied ou wi h ou a ie ies o ag opelle in a labo a o y ixed-g a e eac o . The analyses we e 441
ca ied by elec on mic oscopy (SEM) wi h ene gy dispe si e X- ay spec ome y (EDS) and X- ay 442
di ac ome y (XRD). 443
In o de o ake he esul s u he , a simple me hodology was de eloped ha allows o deposi s 444
p oduced by condensa ion (including he mopho esis and u bulen di usion) and by ine ial impac 445
o coa se ly ash en ained om he bed o be dis inguished. 446
This me hodology, alongside he esul s o chemical analysis and he da a o deposi ion a es 447
p esen ed in a p e ious wo k [41], has yielded impo an esul s conce ning he deposi ion 448
phenomena a ec ing he ou ag opelle s unde s udy. 449
I was con i med ha an inc ease in excess ai leads o a dec ease in deposi ion by condensa ion, 450
owing o a educ ion in combus ion empe a u es, which limi s he ola ili y o K, Cl and S. The lowe 451
deposi ion a es a es ed o PV could be ela ed o i s high P con en . 452
Howe e , conce ning deposi ion by ine ial impac , no common beha io has been ound, p obably 453
because an inc ease in excess ai a io leads o wo opposi e e ec s. Fi s , an inc ease in excess ai 454
a io also inc eases he ai low, encou aging he en ainmen o coa se ly ash. This e ec becomes 455
less acu e as sin e ing inc eases; al hough sin e ing unde mines he ope a ion o he g a e, i also 456
discou ages ash en ainmen . Second, an inc ease in ai excess a io leads o a dec ease o deposi s 457
by condensa ion, some o which ake he shape o a s icky laye (mainly alkali me al sul a es and 458
chlo ides), and hus he adhesion o coa se ly ash en ained om he bed. In ac , a leas 459
conce ning PV, i is a gued ha he adhesion o solid pa icles o s icky deposi s can esul in 460
sa u a ion. 461
The quan i ica ion o deposi s p oduced by condensa ion and by ine ial impac , al hough achie ed 462
h ough a se ies o simpli ica ions and assump ions, p o ides use ul in o ma ion which, i is hoped, 463
will con ibu e o inding solu ions o he p oblem posed by high deposi ion a es in he combus ion 464
o ag icul u al esidual biomass, leading o bo h be e uel blends and boile design and ope a ional 465
pa ame e s, inc easing he ma ke pene a ion o his impo an kind o biomass. 466
As no ed, ly ash deposi ion and bo om ash sin e ing a e ela ed; sin e ing and he ela ionship 467
be ween bo h phenomena will be add essed in dep h om bo om ash chemical cha ac e iza ion, as 468
pa o complemen a y u he esea ch s udies. 469
5. ACKNOWLEDGEMENTS 470
The au ho s g ea ly acknowledge he Spanish Minis y o Science, Inno a ion and Uni e si ies o 471
unding he p ojec “MHWPelle : Mixed pelle s based on ag icul u al c ops esidues (he baceous and 472
woody) o hei use in he esiden ial sec o : op imiza ion o hei composi ion and con e sion 473
pa ame e s” ( e . ENE2015-68809-R (MIMECO/FEDER, UE)). 474
Au ho s also would like o acknowledge he use o Se icio Gene al de Apoyo a la In es igación-SAI, 475
Uni e sidad de Za agoza. 476
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