Ci a ion: O ˇcaˇcíko á, H.; Veliˇcka, M.;
Vlˇcek, J.; Topinko á, M.; Klá o á, M.;
Bu da, J. Co osi e E ec o Wood
Ash P oduced by Biomass
Combus ion on Re ac o y Ma e ials
in a Bina y Al–Si Sys em. Ma e ials
2022,15, 5796. h ps://doi.o g/
10.3390/ma15165796
Academic Edi o : Panos
Tsaki opoulos
Recei ed: 31 July 2022
Accep ed: 18 Augus 2022
Published: 22 Augus 2022
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Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
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ma e ials
A icle
Co osi e E ec o Wood Ash P oduced by Biomass
Combus ion on Re ac o y Ma e ials in a Bina y Al–Si Sys em
Hana O ˇcaˇcíko á* , Ma ek Veliˇcka , Joze Vlˇcek, Michaela Topinko á, Mi osla a Klá o áand Jiˇ íBu da
Depa men o The mal Enginee ing, Facul y o Ma e ials Science and Technology, VSB-Technical Uni e si y o
Os a a, 17. lis opadu 2172/15, 708 00 Os a a, Czech Republic
*Co espondence: [email p o ec ed]; Tel.: +4-205-9732-1523
Abs ac :
In e ms o i s chemical composi ion, biomass is a e y complex ype o uel. I s combus ion
leads o he o ma ion o ma e ials such as alkaline ash and gases, and he e is e idence o he co osi e
e ec his p ocess has on e ac o y linings, hus sho ening he se ice li e o he combus ion uni .
This equen ly encoun e ed p ocess is known as “alkaline oxida i e bu s ing”. Co osion is e y
complex, and i has no been comple ely desc ibed ye . Alkaline co osion is he mos common cause
o u nace-lining deg ada ion in agg ega es ha bu n biomass. This a icle deals wi h an expe imen
in es iga ing he co osion esis ance o 2 ypes o e ac o y ma e ials in he Al
2
O
3
-SiO
2
bina y
sys em, o he ollowing composi ions: I. (53 w .% SiO
2
/42 w .% Al
2
O
3
) and II. (28 w .% SiO
2
/
46 w .% Al
2
O
3
/12 w .% SiC). These we e exposed o se en ypes o ash ob ained om one biomass
combus ion company in he Czech Republic. The chemical composi ion o he ash is a good indica o
o he p oblema ic na u e o a ype o biomass. The ashes we e analyzed by X- ay di ac ion and
X- ay luo escence. Analysis con i med ha ash composi ion a ies. The expe imen also included he
calcula ion o he so-called “slagging/ ouling index” (I/C, TA, S , B/A, Fu, e c.), which can be used
o es ima e he p obabili y o slag o ma ion in combus ion uni s. The co osi e e ec on e ac o y
ma e ials was e alua ed acco ding o he no m ˇ
CSN P CEN/TS 15418, and a s a ic co osion es was
used o in es iga e sample co osion.
Keywo ds: co osion; e ac o y; biomass; he mal p ocessing; wood ash
1. In oduc ion
Wo ldwide, 80% o elec ici y is p oduced using ossil uels. Acco ding o he In e -
na ional Ene gy Agency (IEA), elec ici y p oduc ion eached app oxima ely 25.8 T-kWh
in 2020, and an inc ease o 36.5 T-kWh is expec ed by 2040 [
1
]. As s a ed by he Wo ld
Bioene gy Associa ion, 59.2 NPP/yea , i.e., 10.3% o he global ene gy supply, comes om
biomass [
2
]. Biomass is becoming a popula sou ce o ene gy which can be used in a ious
ways. Elec ici y p oduced om biomass cu en ly co esponds o 493 TWh, which is
app oxima ely 2% o he wo ld’s elec ici y p oduc ion [
2
]. Using biomass as a aw ma e ial
o powe plan s is ce ainly in e es ing and use ul; howe e , his echnology also has ce ain
disad an ages. The use o biomass in powe plan s leads o he o ma ion o esidue, called
biomass ash. I is es ima ed ha a ound 480 million ons o ash a e p oduced e e y yea
by biomass powe plan s wo ldwide. This is simila o coal ash, wi h 780 million ons pe
yea [2].
The mos equen ly bu ned ma e ial is wood (64%), ollowed by ce eals and plan
esidue om ag icul u al p oduc ion. In gene al, i can be said ha he a e age pe cen age
o ash p oduced by bu ning biomass anges be ween 1 and 6%; o wood, i is 0.6–1.6%;
o ba k, i a ely exceeds 3%; s aw p oduces an ash con en o a ound 5%, while g ass
p oduces 7%. A he o he end o he spec um, he ash con en p oduced by black coal
is signi ican ly highe , eaching 20–30%, and om b own coal, his amoun can be e en
g ea e [3]. Ash ep esen s a a iable composi ion o mine al and ino ganic componen s.
Ma e ials 2022,15, 5796. h ps://doi.o g/10.3390/ma15165796 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2022,15, 5796 2 o 15
Du ing he combus ion p ocess, ash con inuously changes i s physical and chemical
p ope ies, he inal p oduc being a mol en mix u e o o iginal mine als, a ious eu ec ics,
and elemen s. Ash causes a ious p oblems, especially co osion, e osion, s icke s, e c. [
4
]
I he mel ing empe a u e o ash du ing combus ion is
ash
<
lame
, hen he g a e o
he hea h can become clogged. Ash laye s on he walls o he u nace di use in o he
lining, which hen peels o in hin laye s. The combus ion chambe o he boile mus
he e o e be s uc u ally adjus ed such ha he lame empe a u e d ops below he ash
mel ing empe a u e, i.e., he empe a u e on he g a e should be lowe han he mel ing
empe a u e o he biomass ash [5].
The majo p oblem om a chemical poin o iew is co osion, which comes om he
in e ac ion be ween a e ac o y and a co osi e medium: gas, mol en me als, mol en glass,
mol en sal s, o slag. I esul s in a loss o mass and hickness and in he deg ada ion o he
ma e ial p ope ies [
6
]. The co osion o e ac o y ma e ials is a combina ion o ex e nal
and in e nal physical and chemical in luences.
The p ocess is basically a chemical eac ion be ween he e ac o y ma e ial and he
slag o me al. Reac an s a e anspo ed o he in e ace o he e ac o y ma e ial, and, in
u n, he p oduc eac s and is anspo ed o he liquid phase. The dissolu ion o e ac o y
ma e ials in he mel is con olled by di usion. Th ee ypes o co osion ha e been de ined:
su ace, dimple, and unde su ace co osion [7].
Alkaline co osion, o “alkaline oxida i e bu s ing”, is ex emely common, e ec i e,
and pa icula ly ha m ul o alumina–silicon (Al–Si) lining sys ems, and i is usually ob-
se ed in he empe a u e ange o agg ega es o 800–1000
◦
C. Du ing biomass combus ion,
damage o he e ac o y lining is obse ed (Figu e 1) as he peeling o su ace laye s,
c acking, he bending o indi idual pa s o he lining, he bulging o en i e walls, and
e en ually hei collapse [8,9].
Ma e ials 2022, 15, x FOR PEER REVIEW 2 o 16
signi ican ly highe , eaching 20–30%, and om b own coal, his amoun can be e en
g ea e [3]. Ash ep esen s a a iable composi ion o mine al and ino ganic componen s.
Du ing he combus ion p ocess, ash con inuously changes i s physical and chemical
p ope ies, he inal p oduc being a mol en mix u e o o iginal mine als, a ious eu ec-
ics, and elemen s. Ash causes a ious p oblems, especially co osion, e osion, s icke s,
e c. [4] I he mel ing empe a u e o ash du ing combus ion is ash < lame, hen he g a e o
he hea h can become clogged. Ash laye s on he walls o he u nace di use in o he
lining, which hen peels o in hin laye s. The combus ion chambe o he boile mus
he e o e be s uc u ally adjus ed such ha he lame empe a u e d ops below he ash
mel ing empe a u e, i.e., he empe a u e on he g a e should be lowe han he mel ing
empe a u e o he biomass ash [5].
The majo p oblem om a chemical poin o iew is co osion, which comes om he
in e ac ion be ween a e ac o y and a co osi e medium: gas, mol en me als, mol en
glass, mol en sal s, o slag. I esul s in a loss o mass and hickness and in he deg ada ion
o he ma e ial p ope ies [6]. The co osion o e ac o y ma e ials is a combina ion o
ex e nal and in e nal physical and chemical in luences.
The p ocess is basically a chemical eac ion be ween he e ac o y ma e ial and he
slag o me al. Reac an s a e anspo ed o he in e ace o he e ac o y ma e ial, and, in
u n, he p oduc eac s and is anspo ed o he liquid phase. The dissolu ion o e ac-
o y ma e ials in he mel is con olled by di usion. Th ee ypes o co osion ha e been
de ined: su ace, dimple, and unde su ace co osion [7].
Alkaline co osion, o “alkaline oxida i e bu s ing”, is ex emely common, e ec i e,
and pa icula ly ha m ul o alumina–silicon (Al–Si) lining sys ems, and i is usually ob-
se ed in he empe a u e ange o agg ega es o 800–1000 °C. Du ing biomass combus-
ion, damage o he e ac o y lining is obse ed (Figu e 1) as he peeling o su ace laye s,
c acking, he bending o indi idual pa s o he lining, he bulging o en i e walls, and
e en ually hei collapse [8,9].
Figu e 1. Deg ada ion o e ac o y ma e ials in boile s a e combus ion o di e en ypes o bio-
masses [8,9]: (1) he damage o e ac o y ma e ials a e 1 yea o he combus ion o wood chips; (2)
he u nace aul a e 1.5 yea s o he combus ion o chipboa d; (3) he co oded pa o e ac o y
samples a e 2 yea s o combus ion o plan biomass, and (4–9) he p esen a ion o he de ec s o
he e ac o y lining a e he combus ion o biomass o 10 yea s o ope a ion.
The main di e ence be ween coal ash and biomass ash is ha coal ash con ains highe
amoun s o SiO2 and Al2O3, bu i con ains lowe amoun s o K2O and Na2O. The eu ec ic
o Al–Si o ming ly ash lies abo e 1200 °C, while he eu ec ic o plan ly ash is much
lowe . Eu ec ic empe a u es o mix u es o alkali me als oge he wi h silica o phospho-
us ha e a low mel ing poin : Na2O.2SiO2 (874 °C), K2O.4SiO2 (770 °C), and 2CaO.3P2O5
(774 °C) [10].
Figu e 1.
Deg ada ion o e ac o y ma e ials in boile s a e combus ion o di e en ypes o
biomasses [
8
,
9
]: (
1
) he damage o e ac o y ma e ials a e 1 yea o he combus ion o wood chips;
(
2
) he u nace aul a e 1.5 yea s o he combus ion o chipboa d; (
3
) he co oded pa o e ac o y
samples a e 2 yea s o combus ion o plan biomass, and (
4
–
9
) he p esen a ion o he de ec s o he
e ac o y lining a e he combus ion o biomass o 10 yea s o ope a ion.
The main di e ence be ween coal ash and biomass ash is ha coal ash con ains highe
amoun s o SiO
2
and Al
2
O
3
, bu i con ains lowe amoun s o K
2
O and Na
2
O. The eu ec ic
o Al–Si o ming ly ash lies abo e 1200
◦
C, while he eu ec ic o plan ly ash is much lowe .
Eu ec ic empe a u es o mix u es o alkali me als oge he wi h silica o phospho us
ha e a low mel ing poin : Na
2
O.2SiO
2
(874
◦
C), K
2
O.4SiO
2
(770
◦
C), and 2CaO.3P
2
O
5
(774 ◦C) [10].
Aluminosilica e e ac o ies a e based on he SiO
2
-Al
2
O
3
sys em. The equilib ium
diag am o his sys em is gi en in Figu e 2, ma king a ious e ac o ies. The main phase
Ma e ials 2022,15, 5796 3 o 15
in he Al–Si bina y diag am is mulli e (3Al
2
O
3
.2SiO
2
) [
11
], which inc eases he esis ance
o he e ac o y ma e ial agains he co osi e e ec s o ash [11].
Ma e ials 2022, 15, x FOR PEER REVIEW 3 o 16
Aluminosilica e e ac o ies a e based on he SiO2-Al2O3 sys em. The equilib ium di-
ag am o his sys em is gi en in Figu e 2, ma king a ious e ac o ies. The main phase in
he Al–Si bina y diag am is mulli e (3Al2O3.2SiO2) [11], which inc eases he esis ance o
he e ac o y ma e ial agains he co osi e e ec s o ash. [11].
Figu e 2. Ca ego iza ion o basic e ac o y ma e ials in bina y diag am o SiO2-Al2O3. No e: * he
amoun o Al2O3.
In he AL–SI sys em, new phases a e o en o med as a esul o di e en chemical
eac ions, g adually deg ading he sys em. The newly o med p oduc s ha e a la ge ol-
ume han he o iginal ma e ial, wi h expansion being epo ed be ween 7 and 30%. This
c ea es compounds in he lining o on i s su ace ha ha e chemical composi ions and
physical pa ame e s di e en om he lining i sel [12]. Te na y diag ams o he ypes
Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 also desc ibe he o ma ion o indi idual phases in
he gi en sys em (see Figu e 3).
Figu e 3. Te na y diag am o Na2O-SiO2-Al2O3 [13] and K2O-Al2O3-SiO2 [14], ma king he indi idual
phases o med du ing alkaline co osion.
The co osion mechanism in he Na–Al–Si sys em includes he o ma ion o albi e
(NaAlSi3O8), nosean (Na8Al6Si6O28S) [15,16], and na osili e (Na2Si2O5) by Equa ion (1),
which u he eac s wi h mulli e (Al6Si2O13) o o m albi e (NaAlSi3O8) and aluminum
oxide acco ding o Equa ion (2). Nepheline (NaAlSiO4) can also be o med acco ding o
Equa ion (3). Nosean is a ely epo ed in he li e a u e as a co osion p oduc . Howe e ,
Figu e 2.
Ca ego iza ion o basic e ac o y ma e ials in bina y diag am o SiO
2
-Al
2
O
3
. No e: * he
amoun o Al2O3.
In he AL–SI sys em, new phases a e o en o med as a esul o di e en chemical
eac ions, g adually deg ading he sys em. The newly o med p oduc s ha e a la ge
olume han he o iginal ma e ial, wi h expansion being epo ed be ween 7 and 30%. This
c ea es compounds in he lining o on i s su ace ha ha e chemical composi ions and
physical pa ame e s di e en om he lining i sel [
12
]. Te na y diag ams o he ypes
Na
2
O-Al
2
O
3
-SiO
2
and K
2
O-Al
2
O
3
-SiO
2
also desc ibe he o ma ion o indi idual phases in
he gi en sys em (see Figu e 3).
Ma e ials 2022, 15, x FOR PEER REVIEW 3 o 16
Aluminosilica e e ac o ies a e based on he SiO2-Al2O3 sys em. The equilib ium di-
ag am o his sys em is gi en in Figu e 2, ma king a ious e ac o ies. The main phase in
he Al–Si bina y diag am is mulli e (3Al2O3.2SiO2) [11], which inc eases he esis ance o
he e ac o y ma e ial agains he co osi e e ec s o ash. [11].
Figu e 2. Ca ego iza ion o basic e ac o y ma e ials in bina y diag am o SiO2-Al2O3. No e: * he
amoun o Al2O3.
In he AL–SI sys em, new phases a e o en o med as a esul o di e en chemical
eac ions, g adually deg ading he sys em. The newly o med p oduc s ha e a la ge ol-
ume han he o iginal ma e ial, wi h expansion being epo ed be ween 7 and 30%. This
c ea es compounds in he lining o on i s su ace ha ha e chemical composi ions and
physical pa ame e s di e en om he lining i sel [12]. Te na y diag ams o he ypes
Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 also desc ibe he o ma ion o indi idual phases in
he gi en sys em (see Figu e 3).
Figu e 3. Te na y diag am o Na2O-SiO2-Al2O3 [13] and K2O-Al2O3-SiO2 [14], ma king he indi idual
phases o med du ing alkaline co osion.
The co osion mechanism in he Na–Al–Si sys em includes he o ma ion o albi e
(NaAlSi3O8), nosean (Na8Al6Si6O28S) [15,16], and na osili e (Na2Si2O5) by Equa ion (1),
which u he eac s wi h mulli e (Al6Si2O13) o o m albi e (NaAlSi3O8) and aluminum
oxide acco ding o Equa ion (2). Nepheline (NaAlSiO4) can also be o med acco ding o
Equa ion (3). Nosean is a ely epo ed in he li e a u e as a co osion p oduc . Howe e ,
Figu e 3.
Te na y diag am o Na
2
O-SiO
2
-Al
2
O
3
[
13
] and K
2
O-Al
2
O
3
-SiO
2
[
14
], ma king he indi id-
ual phases o med du ing alkaline co osion.
The co osion mechanism in he Na–Al–Si sys em includes he o ma ion o albi e
(NaAlSi
3
O
8
), nosean (Na
8
Al
6
Si
6
O
28
S) [
15
,
16
], and na osili e (Na
2
Si
2
O
5
) by Equa ion (1),
which u he eac s wi h mulli e (Al
6
Si
2
O
13
) o o m albi e (NaAlSi
3
O
8
) and aluminum
oxide acco ding o Equa ion (2). Nepheline (NaAlSiO
4
) can also be o med acco ding o
Equa ion (3). Nosean is a ely epo ed in he li e a u e as a co osion p oduc . Howe e ,
Ma e ials 2022,15, 5796 4 o 15
due o i s s uc u al simila i y o nepheline, i can also be expec ed o p oduce swelling.
The eac ion can be desc ibed by Equa ion (4) [17]:
Na2SO4+ 2SiO2= Na2Si2O5+ SO2+ 1/2O2(1)
Na2Si2O5+ 2Al6Si2O13 = 2NaAlSi3O8+ 5Al2O3(2)
2NaAlSi3O8+ Al6Si2O13 + 3Na2SO4= 8NaAlSiO4+ 3SO2+ 3/2O2(3)
4Na2SO4+ 3Al6Si2O13 = Na8Al6Si6O28S + 6Al2O3+ 3SO2+ 3/2O2(4)
In he case o high-alumina e ac o ies (>45% Al
2
O
3
) con aining mulli e (A
3
S
2
) and
c is obali e (SiO
2
), eac ion wi h NaO
2
abo e 1000
◦
C o ms nepheline (NaS
2
) and
α
-Al
2
O
3
acco ding o Equa ion (5). As can be deduced om he e na y diag am K
2
O-Al
2
O
3
-SiO
2
, a
a lowe con en o Al
2
O
3
< 30%, o hoclase KAS
6
is o med, and a a con en o
Al2O3> 30%
,
new phases o leuci e (KAS4) a e o med acco ding o Equa ion (6):
3Al2O3·2SiO2+ Na2O→Na2O·Al2O3·2SiO2+ 2Al2O3(5)
K2O·Al2O3·6SiO2→K2O·Al2O3·4SiO2+ 2SiO2(6)
Since he composi ion o biomass ash encou ages he o ma ion o eu ec ic mel s, i is
ad isable o use high-alumina e ac o y ma e ials wi h an Al
2
O
3
con en > 80% o o add
silicon ca bide o hese linings. The compound, aluminosilica e-based ma e ials mainly
include p oduc s con aining oxide-less cons i uen s—g aphi e and silicon ca bide
Ma e ials in Al
2
O
3
-SiO
2
-SiC sys ems combine he high he mal conduc i i y and chem-
ical ine ness o silicon ca bide wi h he chemical and he mal s abili y o aluminosilica e
and co undum. The p oduc s a e he e o e highly esis an o co osion by liquid me als, as
well as o sudden changes in empe a u e.
SiC oxidizes acco ding o Equa ions (7) and (8) and c ea es an amo phous SiO
2
ilm
on he su ace [18,19]:
SiC + 1.5O2→SiO2+ CO (7)
SiC + 2O2→SiO2+ CO2(8)
To p e en g aphi e oxida ion, i ing is ca ied ou wi hou any con ac be ween he
i ed p oduc and oxygen. The i ing empe a u e is chosen o c ea e a ce amic bond in he
p oduc s. A p esen , he p ocess o quick i ing is used, ensu ing a educing a mosphe e in
he kilns a highe empe a u es and while cooling he p oduc s.
K
2
O and Na
2
O, in he o m o alkaline apo s, a e capable o di using in o he e ac-
o y ma ix, and hen hey eac wi h Al
2
O
3
and SiO
2
componen s o o m K-aluminosilica e
and Na-aluminosilica e phases [
20
]. In he AL–SI bina y sys em, po assium pai s eac
acco ding o Equa ions (9)–(11). The mos ha m ul is he p esence o ee SiO
2
and Na
2
O,
which inc ease he eac ion a e a high empe a u es and suppo he o ma ion o eac i e
glassy phases, acco ding o Equa ion (12) [20]:
K2O + SiO2→K2O.SiO2(9)
3 (K2O.2SiO2) + 3Al2O3.2SiO2→3 (K2O Al2O3.2SiO2) + 2SiO2(10)
K2O.Al2O3.2SiO2+ 2SiO2→K2O.Al2O3.4SiO2(11)
2SiO2+ Na2O→Na2O·2SiO2(12)
The so-called slagging/ ouling index can be used o es ima e he p obabili y o slag
o ma ion in combus ion uni s du ing biomass combus ion. Slagging/ ouling means he
o ma ion o laye s (s icky, mel ed, o so ) o ash pa icles on hea exchange su aces. A
summa y o slagging and ouling indices and hei calcula ion a e p esen ed in Table 1.
Ma e ials 2022,15, 5796 5 o 15
Table 1. Ash cha ac e iza ion indices [10,21–24].
Index Equa ion Tendency Slagging/Fouling
Low Middle High Ex. High
SiO2(%) - <20 20–25 >25
Cl (%) - <0.2 0.2–0.3 0.3–0.5 >0.5
B/A B
A=Fe2O3+CaO+MgO+Na2O+K2O
SiO2+Al2O3+TiO2<0.5 0.5–1 1–1.75 >1.75
S/A S/A=SiO2
Al2O3<0.31 - 0.3–3 -
I/C I
C=Fe2O3
CaO <0.31 0.3–3 >3 -
Fu Fu =B
A·(Na2O+K2O)<0.6 0.6–40 >40 -
TA TA =Na2O+K2O <0.3 0.3 < TA < 0.4 >0.4 -
S S =SiO2
SiO2+Fe2O3+CaO+MgO ·100 >72 65–72 <65
The SiO
2
index is o en he p edominan elemen in biomass samples and causes he
o ma ion o mel , o “s icke s”, he e o e gi ing i he cha ac e is ic o being slag- o ming.
The chlo ine index Cl ac s as an accele a o o he eac ion be ween K and SiO2, which
leads o he o ma ion o used glass deposi s and he o ma ion o slag a boile ope a ing
empe a u es o 800–900 ◦C [23].
Ash-deposi ion po en ial may be e alua ed in e ms o base- o-acid (B/A). The basici y
index B/A (base/acid a io) is based on he gene al ule ha basic oxide compounds lowe
he mel ing poin , and acidic compounds aise i . The B/A a io is an indica ion o he
usion and slagging po en ial o ash. I/C (i on/calcium a io) s ands o Fe
2
O
3
/CaO, e.g.,
ash wi h a a io o Fe2O3/CaO = 0.3/3.0 con aining eu ec ics ha inc ease slag o ma ion.
The Fouling index Fu ( ouling index) is he B/A a io, also aking in o accoun he
alkali con en (Na
2
O + K
2
O). Fouling e e s o he d y deposi ion o ash pa icles o he
condensa ion o ola ile ino ganic componen s on hea ans e su aces. The no mal
pe cen age o alkali in biomass ash is be ween 25 and 35%, and i o ms a eu ec ic in
combina ion wi h silica.
Ash has a high iscosi y (S ) alue, slag iscosi y index S [
24
], so i will ha e a low
endency o slag. The TA ( o al alkali) index assesses he uel’s abili y o o m ash laye s.
Values o indi idual ash samples, de ined based on he abo e-men ioned indices, a e
summa ized in Sec ion 3.2.
The chemical composi ion o ash is a good indica o o he p oblema ic na u e o
biomass. Fo biomass uels, massi e slagging o hea exchange su aces o boile s occu s
du ing combus ion. Ash composi ion and a mosphe e in a combus ion chambe in luence
he ash-mel ing empe a u e [
10
]. Indica o s ell us o he cha ac e is ics o ash in e ms
o hei in luence on he o ma ion o he glassy phase, and hus hei endency o slag
and clog linings, hea exchange su aces, and gas low ou es. These indices a e based on
chemical composi ion o biomass and i s combus ion. The equa ions a e mainly based on
uel e alua ion. Howe e , since he e is no speci ic index o biomass, i is possible o apply
hese indices o his ype o uel as well.
2. Ma e ials and Me hods
2.1. Ashes om Wood Biomass Combus ion
Se en di e en ypes o ash om di e en ypes o wood biomass we e used o he
expe imen al po ion o ou s udy. All o hese we e ob ained om he Czech Republic,
mainly om he Mo a ian–Silesian Region, bu one was om he Cen al Bohemian Region.
Ashes u ilized du ing he expe imen al po ion we e used in he o iginal o m o he
c ucible es . he g anulome y was no adjus ed. Mo e in o ma ion abou he ash samples
is p esen ed in Table 2.

Ma e ials 2022,15, 5796 6 o 15
Table 2. Cha ac e iza ion o wood ash used o expe imen .
Type o Wood Biomass Disposal Me hod Labeled
Sp uce pelle s combus ion P019
Woodchips combus ion P020
Woodchips combus ion P031
Woodchips, woodba k, sawdus , pelle s, sc aps combus ion P032
Woodchips, woodba k, sawdus , pelle s, sc aps combus ion P033
Woodchips gassi ica ion P059, P060
Ma e ials 2022, 15, x FOR PEER REVIEW 6 o 16
he c ucible es . he g anulome y was no adjus ed. Mo e in o ma ion abou he ash sam-
ples is p esen ed in Table 2.
Table 2. Cha ac e iza ion o wood ash used o expe imen .
Type o Wood Biomass Disposal Me hod Labeled
Sp uce pelle s combus ion P019
Woodchips combus ion P020
Woodchips combus ion P031
Woodchips, woodba k, sawdus , pelle s, sc aps combus ion P032
Woodchips, woodba k, sawdus , pelle s, sc aps combus ion P033
Woodchips gassi ica ion P059, P060
2.2. Re ac o y Ma e ials
Tes ed e ac o y ma e ials we e manu ac u ed by one o he la ges p oduce s and
supplie s o e ac o y p oduc s and aw ma e ials in he Czech Republic. Two ypes o
shaped e ac o y ma e ials, belonging o he silica–aluminum g oup, we e selec ed o
he co osion expe imen .
The i s ype was quali y labeled as STV. I is a shaped e ac o y ma e ial classi ied
as s anda d i e clay. The second ype was quali y labeled as ARS60N and is classi ied
high alumina. The pa ame e s o he men ioned es ed ma e ials wi h hei p ope ies a e
shown in Table 3.
Table 3. Chemical composi ion and p ope ies o e ac o y ma e ials.
Oxides w .% STV ARS60N
SiO2 53.5 28.40
Al2O3 40.5 46.60
TiO2 2.1 -
Fe2O3 2.1 0.88
CaO 0.3 0.2
MgO 0.3 0.27
K2O+ Na2O 0.8 + 0.2 0.5
SiC - 13.2
Bulk densi y (kg/m3) 2150 2700
Appa en po osi y (%) 18.0 15
Cold c ushing s eng h (MPa) 30 70
Re ac o y quali ies unde load (RUL) T0.5 (°C) 1360 >1500
2.3. Co osion C ucible Tes and E alua ion Me hod
The c ucible es gi es only app oxima e esul s. The e ac o y cube was illed wi h
co oden and hea ed o he es ing empe a u e o a speci ied pe iod. The es ing condi-
ions ( empe a u e and co oden composi ion) may e lec he expec ed se ice condi-
ions, bu in some si ua ions, a mo e agg essi e co oden and/o high empe a u e may
be used o speed up he a ack o de e mine he esis ance o he e ac o y o he co osi e
liquid in a ela i ely sho ime. The c ucible es is desc ibed s ep by s ep in Figu e 4. The
e ac o y cuboid sample wi h a cylind ical hole in he cen al po ion was illed wi h co -
2.2. Re ac o y Ma e ials
Tes ed e ac o y ma e ials we e manu ac u ed by one o he la ges p oduce s and
supplie s o e ac o y p oduc s and aw ma e ials in he Czech Republic. Two ypes o
shaped e ac o y ma e ials, belonging o he silica–aluminum g oup, we e selec ed o he
co osion expe imen .
The i s ype was quali y labeled as STV. I is a shaped e ac o y ma e ial classi ied
as s anda d i e clay. The second ype was quali y labeled as ARS60N and is classi ied high
alumina. The pa ame e s o he men ioned es ed ma e ials wi h hei p ope ies a e shown
in Table 3.
Table 3. Chemical composi ion and p ope ies o e ac o y ma e ials.
Oxides w .% STV ARS60N
SiO253.5 28.40
Al2O340.5 46.60
TiO22.1 -
Fe2O32.1 0.88
CaO 0.3 0.2
MgO 0.3 0.27
K2O + Na2O 0.8 + 0.2 0.5
SiC - 13.2
Bulk densi y (kg/m3)2150 2700
Appa en po osi y (%) 18.0 15
Cold c ushing s eng h (MPa) 30 70
Re ac o y quali ies unde load (RUL) T0.5 (◦C) 1360 >1500
2.3. Co osion C ucible Tes and E alua ion Me hod
The c ucible es gi es only app oxima e esul s. The e ac o y cube was illed
wi h co oden and hea ed o he es ing empe a u e o a speci ied pe iod. The es ing
condi ions ( empe a u e and co oden composi ion) may e lec he expec ed se ice
condi ions, bu in some si ua ions, a mo e agg essi e co oden and/o high empe a u e
may be used o speed up he a ack o de e mine he esis ance o he e ac o y o he
co osi e liquid in a ela i ely sho ime. The c ucible es is desc ibed s ep by s ep in
Figu e 4. The e ac o y cuboid sample wi h a cylind ical hole in he cen al po ion was
illed wi h co osi e, medium/powde ed ash wi h a hea ing empe a u e o 1200
◦
C o 2 h.
A e cooling, he es ed sample was cu h ough along he e ical axis, and he co oded
po ion was measu ed.
Ma e ials 2022,15, 5796 7 o 15
Ma e ials 2022, 15, x FOR PEER REVIEW 7 o 16
osi e, medium/powde ed ash wi h a hea ing empe a u e o 1200 °C o 2 h. A e cool-
ing, he es ed sample was cu h ough along he e ical axis, and he co oded po ion
was measu ed.
Figu e 4. Schema ic diag am o he c ucible es o e ac o y ma e ials.
A e he co osion es , samples we e isually checked o compac ness, po en ial
c acks, and holes in he sample and walls. The ČSN P CEN/TS 15418 me hod [25] and he
in e nal egula ion me hod o P-D Re ac o ies CZ a.s. [26] we e used o es e alua ion.
The classi ica ion used o epo ing he condi ion o he c ucible wi h de ined pa-
ame e s [25] U: una ec ed/no isible a ack; LA: ligh ly a acked/mino a ack; A: a -
acked/clea ly a acked and C: co oded/comple ely co oded. In addi ion o he abo e-
men ioned e alua ion egula ions, ano he in e nal egula ion me hod o P-D Re ac o ies
CZ was also used [26].
Table 4 shows he pa ame e s o he classi ica ion a e he co osion es . Two e al-
ua ion me hods may some imes be eques ed by a cus ome o company es ing labo a-
o y, and he pa ame e s can be used o compa ison.
Table 4. Alkali es classi ica ion a e in e nal egula ion o P-D Re ac o ies CZ [26].
Class Classi ica ion
Co osion In il a ion C acks
A no a acked no co osion and/o in il a ion No
B sligh a ack <6 mm co osion and/o in il a ion No
C dis inc i e a ack >7 mm co osion and/o in il a ion Sligh
D se e e a ack >9 mm co osion and/o in il a ion la ge, clea ly isible c acks
2.4. Cha ac e iza ion Me hods
The chemical composi ion (XRF) o he ash was de e mined by ene gy-dispe si e X-
Ray luo escence spec oscopy (ED-XRF) on he SPECTRO XEPOS (Spec o Analy ical In-
s umen s, Kle e, Ge many). Powde ed samples we e shaped/p essed in o able s o
XRD measu emen .
The mine alogical composi ion (XRPD) o he samples was e alua ed using X-Ray
di ac ion analysis on he X-Ray di ac ome e MiniFlex 600 (Rigaku, Tokyo, Japan)
equipped wi h a 0Co ube and a D/ eX Ul a 250 de ec o . XRD pa e ns we e eco ded in
a 5–90° 2θ ange wi h a scanning a e o 5° min−1.
3. Resul s and Discussion
3.1. Ash Cha ac e iza ion
Chemical analysis is a good indica o o de e mining he p oblema ic na u e o bio-
mass. The chemical composi ion o all o he ash ypes is p esen ed in Figu e 5. Biomass
Figu e 4. Schema ic diag am o he c ucible es o e ac o y ma e ials.
A e he co osion es , samples we e isually checked o compac ness, po en ial
c acks, and holes in he sample and walls. The ˇ
CSN P CEN/TS 15418 me hod [
25
] and he
in e nal egula ion me hod o P-D Re ac o ies CZ a.s. [26] we e used o es e alua ion.
The classi ica ion used o epo ing he condi ion o he c ucible wi h de ined pa-
ame e s [
25
] U: una ec ed/no isible a ack; LA: ligh ly a acked/mino a ack; A: a -
acked/clea ly a acked and C: co oded/comple ely co oded. In addi ion o he abo e-
men ioned e alua ion egula ions, ano he in e nal egula ion me hod o P-D Re ac o ies
CZ was also used [26].
Table 4shows he pa ame e s o he classi ica ion a e he co osion es . Two e alua-
ion me hods may some imes be eques ed by a cus ome o company es ing labo a o y,
and he pa ame e s can be used o compa ison.
Table 4. Alkali es classi ica ion a e in e nal egula ion o P-D Re ac o ies CZ [26].
Class Classi ica ion
Co osion In il a ion C acks
A no a acked no co osion and/o in il a ion No
B sligh a ack <6 mm co osion and/o in il a ion No
C dis inc i e a ack >7 mm co osion and/o in il a ion Sligh
D se e e a ack >9 mm co osion and/o in il a ion la ge, clea ly isible c acks
2.4. Cha ac e iza ion Me hods
The chemical composi ion (XRF) o he ash was de e mined by ene gy-dispe si e
X- ay luo escence spec oscopy (ED-XRF) on he SPECTRO XEPOS (Spec o Analy ical
Ins umen s, Kle e, Ge many). Powde ed samples we e shaped/p essed in o able s o
XRD measu emen .
The mine alogical composi ion (XRPD) o he samples was e alua ed using X- ay
di ac ion analysis on he X- ay di ac ome e MiniFlex 600 (Rigaku, Tokyo, Japan)
equipped wi h a 0Co ube and a D/ eX Ul a 250 de ec o . XRD pa e ns we e eco ded in
a 5–90◦2θ ange wi h a scanning a e o 5◦min−1.
3. Resul s and Discussion
3.1. Ash Cha ac e iza ion
Chemical analysis is a good indica o o de e mining he p oblema ic na u e o
biomass. The chemical composi ion o all o he ash ypes is p esen ed in Figu e 5. Biomass
ash almos always con ains ca bona es, especially calci e, and e y o en po landi e, as
well as a p opo ion o o ganic ca bon.
Ma e ials 2022,15, 5796 8 o 15
Ma e ials 2022, 15, x FOR PEER REVIEW 8 o 16
ash almos always con ains ca bona es, especially calci e, and e y o en po landi e, as
well as a p opo ion o o ganic ca bon.
Figu e 5. Concen a ions o majo elemen s in ash a e wood biomass combus ion.
Oxides in biomass ash can be di ided in o acidic (SiO2, Al2O3, TiO2, e c.) and basic
(K2O, CaO, MgO, Na2O, Fe2O3, P2O5, e c.). Acidic oxides inc ease he mel ing poin o ash.
The highe he con en o acidic oxides, he highe he mel ing poin . On he o he hand,
basic oxides lowe he mel ing poin o he ash.
The p edominan oxides a e SiO2 and CaO. A high le el o CaO is ypical o wood.
The highe he con en o basic oxides, he lowe he mel ing poin . SiO2 plays an im-
po an ole as a glass- o ming oxide, while CaO and K2O educe he iscosi y o he e-
sul ing glass- o ming mel . The na u e o he oxides and hei ep esen a ion de e mines
he o ma ion o o he compounds and he beha io o he e ac o y ma e ial in con ac
wi h he co osi e agen . Ash was analyzed by XRDF, and his showed a iable sample
composi ion. The pe cen age o single oxides is as ollows: SiO2 9.13–55.17 w .%, CaO
16.33–41.79 w .%, Al2O3 0.98–10,14 w .%, Fe2O3 1.80–13.16 w .%. Fo alkali oxides i is
Na2O 0.38–12.23 w .% and K2O 6.11–19.17 w .%. The amoun o Cl is a ound 0.6 w .%.
In e ms o chemical composi ion, ash esembles low-mel ing glass. The a iabili y
o chemical composi ion complica es accu a e ep esen a ion in a e na y diag am. An
app oxima e composi ion based on he la ges con en o w .% o oxides is shown in he
diag am. Fou ash ypes, labeled P020, P033, P059 and P060, a e ma ked in he CaO-Al2O3-SiO2
e na y diag am, and wo ypes, labeled P031 and P019 a e ma ked in he K2O-SiO2-CaO
sys em, as p esen ed in Figu e 6.
Figu e 5. Concen a ions o majo elemen s in ash a e wood biomass combus ion.
Oxides in biomass ash can be di ided in o acidic (SiO
2
, Al
2
O
3
, TiO
2
, e c.) and basic
(K
2
O, CaO, MgO, Na
2
O, Fe
2
O
3
, P
2
O
5
, e c.). Acidic oxides inc ease he mel ing poin o ash.
The highe he con en o acidic oxides, he highe he mel ing poin . On he o he hand,
basic oxides lowe he mel ing poin o he ash.
The p edominan oxides a e SiO
2
and CaO. A high le el o CaO is ypical o wood.
The highe he con en o basic oxides, he lowe he mel ing poin . SiO
2
plays an impo an
ole as a glass- o ming oxide, while CaO and K
2
O educe he iscosi y o he esul ing glass-
o ming mel . The na u e o he oxides and hei ep esen a ion de e mines he o ma ion o
o he compounds and he beha io o he e ac o y ma e ial in con ac wi h he co osi e
agen . Ash was analyzed by XRDF, and his showed a iable sample composi ion. The
pe cen age o single oxides is as ollows: SiO
2
9.13–55.17 w .%, CaO 16.33–41.79 w .%,
Al
2
O
3
0.98–10.14 w .%, Fe
2
O
3
1.80–13.16 w .%. Fo alkali oxides i is Na
2
O 0.38–12.23 w .%
and K2O 6.11–19.17 w .%. The amoun o Cl is a ound 0.6 w .%.
In e ms o chemical composi ion, ash esembles low-mel ing glass. The a iabili y
o chemical composi ion complica es accu a e ep esen a ion in a e na y diag am. An
app oxima e composi ion based on he la ges con en o w .% o oxides is shown in
he diag am. Fou ash ypes, labeled P
020
, P
033
, P
059
and P
060
, a e ma ked in he CaO-
Al
2
O
3
-SiO
2
e na y diag am, and wo ypes, labeled P
031
and P
019
a e ma ked in he
K2O-SiO2-CaO sys em, as p esen ed in Figu e 6.
Ma e ials 2022, 15, x FOR PEER REVIEW 9 o 16
Figu e 6. App oxima e posi ion o ash ypes P020, P033, P059, P060, in e na y diag am CaO–Al2O3–SiO2
[27] and P031 and P019 in e na y diag am K2O–Al2O3–SiO2 [28].
The nex me hod o ash cha ac e iza ion was X- ay powde di ac ion phase analysis
(XRPD). The samples we e compa ed o he e e ence di ac og am da abase published
by ICDD (PDF-2) in he ange o 5–90° 2 he a. The esul s o he analyzed samples a e
p esen ed in Table 5, whe e he e is an o e iew o he phases in he samples.
Table 5. Phase composi ion o analyzed biomass ash samples.
Phase Composi ion Labeled o Sample
P019 P020 P031 P032 P033 P059 P060
qua z (SiO2) x x x x x
calci e (CaCO3) x x X x x x X
g aphi e C x
CaO x x X x
magnesi e (MgCO3) X
MgO X x
ano hi e (CaAl2Si2O8) x
mic ocline (KAlSi3O8) x x x x
a cani e (K2SO4) X
anhyd i e (CaSO4) x
ano hoclase x
leuci e (KAlSi2O6) x x
o hoclase (KAlSi3O8) x
syl i e (KCl) x
po landi e Ca(OH)2 x x
hema i e (Fe2O3) x x
mulli e (Al4.59Si1.41O0.97) x
analcime (NaAlSi2O6) x
As con i med by he analysis, he mos equen ly ecu ing phases a e qua z, ano -
hi e, calcium silica e, hema i e, anhyd i e, and mic ocline. In ash samples P059 and P060,
he e we e se en phases iden i ied as po landi e; mic ocline, leuci e, and po landi e oc-
cu in bo h. Samples P019 and P032, we e especially ich in he glass phase.
3.2. Calcula ion o he Slagging and Fouling Indices
To p edic slagging/ ouling in a combus ion u nace, i is possible o use indices o
he SiO2, basic/acid a io, silica/alumina a io, ouling, i on/calcium a io, and o al alkalis,
Figu e 6.
App oxima e posi ion o ash ypes P
020
, P
033
, P
059
, P
060
, in e na y diag am CaO-Al
2
O
3
-
SiO2[27] and P031 and P019 in e na y diag am K2O-Al2O3-SiO2[28].
Ma e ials 2022,15, 5796 9 o 15
The nex me hod o ash cha ac e iza ion was X- ay powde di ac ion phase analysis
(XRPD). The samples we e compa ed o he e e ence di ac og am da abase published
by ICDD (PDF-2) in he ange o 5–90
◦
2 he a. The esul s o he analyzed samples a e
p esen ed in Table 5, whe e he e is an o e iew o he phases in he samples.
Table 5. Phase composi ion o analyzed biomass ash samples.
Phase Composi ion Labeled o Sample
P019 P020 P031 P032 P033 P059 P060
qua z (SiO2) x x x x x
calci e (CaCO3) x x X x x x X
g aphi e C x
CaO x x X x
magnesi e (MgCO3) X
MgO X x
ano hi e (CaAl2Si2O8) x
mic ocline (KAlSi3O8) x x x x
a cani e (K2SO4) X
anhyd i e (CaSO4) x
ano hoclase x
leuci e (KAlSi2O6) x x
o hoclase (KAlSi3O8) x
syl i e (KCl) x
po landi e Ca(OH)2x x
hema i e (Fe2O3) x x
mulli e (Al4.59Si1.41O0.97) x
analcime (NaAlSi2O6) x
As con i med by he analysis, he mos equen ly ecu ing phases a e qua z, ano -
hi e, calcium silica e, hema i e, anhyd i e, and mic ocline. In ash samples P
059
and P
060
,
he e we e se en phases iden i ied as po landi e; mic ocline, leuci e, and po landi e occu
in bo h. Samples P019 and P032, we e especially ich in he glass phase.
3.2. Calcula ion o he Slagging and Fouling Indices
To p edic slagging/ ouling in a combus ion u nace, i is possible o use indices o
he SiO
2
, basic/acid a io, silica/alumina a io, ouling, i on/calcium a io, and o al alkalis,
as summa ized in Table 6. A special index only o biomass does no exis , bu many
au ho s ha e calcula ed hese indices wi h ega d o he p obabili y o slag o ming in
combus ion uni s.
Table 6. Calcula ion o slagging and ouling indices o indi idual ash ypes.
Ash Index
SiO2(%) Cl (%) B/A S/A I/C Fu TA S
P019 9.1 l0.21 s8.1 ex 9.3 h0.0 l240.1 h29.7 h15.8 h
P020 46.5 h0.41 h0.7 m5.6 h0.2 l7.7 m11.5 h67.1 m
P031 16.2 l0.16 l3.7 ex 6.4 h0.1 l72.1 h19.3 h25.3 h
P032 19.1 l1.74 ex 2.3 ex 3.3 h0.1 l46.4 h20.0 h35.2 h
P033 55.1 h0.10 l0.5 l5.4 h0.4 m3.6 m7.6 h71.7 m
P059 41.7 h-1.2 h11.6 h0.1 l10.2 m8.05 h 49.8 h
P060 33.1 h-1.3 h3.4 h0.5 m6.9 m5.3 h41.0 h
No e: X l: low alue; X m: middle alue; X h: high alue; X ex: ex eme alue.
In he case o SiO
2
con en in P
020
, P
033
, P
059
and P
060,
hey ha e a high inclina ion
owa ds slagging. The high le els o silica in wood biomass ashes may ha e been caused
by con amina ion wi h di e en elemen s (clay, sand, e c.); also, each pa o he wood
plan may con ain di e en amoun s o oxides. Acco ding o chlo ide con en , ex emely
high ouling inclina ions we e obse ed in samples P
032
= 1.74 and P
020
= 0.47, while a low
ouling inclina ion wi h a alue > 0.2 was calcula ed o P033 = 0.1.