Ash In e ac ion wi h Two Cu-Based Magne ic Oxygen Ca ie s
du ing Biomass Combus ion by he Chemical Looping wi h Oxygen
Uncoupling P ocess
Published as pa o Ene gy &Fuels special issue “2024 Pionee s in Ene gy Resea ch: Juan Adanez”.
A. Filsou , I. Adánez-Rubio,*T. Mendia a, A. Abad, and J. Adánez
Ci e This: Ene gy Fuels 2024, 38, 19548−19558
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ABSTRACT: Chemical-looping combus ion (CLC) s ands ou as
a p omising me hod o ca bon cap u e and s o age o he pu pose
o mi iga ing clima e change. The p ocess in ol es he con e sion
o uel acili a ed by an oxygen ca ie , wi h he esul ing CO2
inhe en ly sepa a ed om o he ai componen s. No ably, when
applied o biomass combus ion his p ocess o e s a pa hway o
achie ing nega i e CO2emissions. Howe e , a signi ican challenge
o CLC, pa icula ly in i s applica ion o biomass, is he
managemen o in e ac ions be ween ash and oxygen ca ie s.
Biomass-de i ed ashes ypically con ain subs an ial quan i ies o
eac i e ash- o ming subs ances, such as alkaline and alkali ea h
elemen s. These in e ac ions can impac he pe o mance and
longe i y o he oxygen ca ie , necessi a ing ca e ul conside a ion
and mi iga ion s a egies in CLC sys ems u ilizing biomass eeds ocks. This s udy examined he in e ac ion be ween biomass ash
componen s and wo ecen ly de eloped oxygen ca ie s, Cu30MnFekao7.5 and Cu30MnFe, du ing combus ion in a 1.5 kW h
con inuous uni . Bo h oxygen ca ie s achie ed 100% combus ion e iciency and a CO2cap u e e iciency o 95% a 900 °C.
Al hough he coppe in bo h oxygen ca ie s did no exhibi any no iceable in e ac ion wi h ash componen s, he accumula i e
p esence o po assium and magnesium in Cu30MnFekao7.5 was iden i ied by induc i ely coupled plasma and scanning elec on
mic oscopy wi h ene gy dispe si e X- ay analysis, indica ing an inc ease in he amoun o bo h elemen s in he pa icles a e
combus ion ope a ion. No p oblems o agglome a ion o luidiza ion we e obse ed in any o he expe imen s.
1. INTRODUCTION
The goal o limi ing global wa ming o below 2 °C has led o
he se ing o s ic limi s on ca bon emissions,
1
al hough hese
a e highly likely o be exceeded conside ing he ongoing ise in
CO2emissions, wi h a cu en alue o 425 ppm epo ed in
Ma ch 2024.
2
To adhe e o he Pa is ag eemen plan, and
pa icula ly he mo e s ingen a ge o keeping wa ming
below 1.5 °C, he implemen a ion o ca bon dioxide emo al
echnologies om he a mosphe e becomes e iden ly neces-
sa y.
3
Nume ous nega i e emission echnologies ha e been
sugges ed, among which bioene gy wi h ca bon cap u e and
s o age (BECCS) is g adually eme ging as a highly p omising
op ion, demons a ing conside able po en ial in e ms o i s
cos -e ec i eness and capaci y o emo e ca bon om he
a mosphe e.
4,5
Chemical-looping combus ion (CLC) s ands
ou as a p omising echnology ha enables he sepa a ion o
CO2 om he combus ion p ocess a ela i ely low cos and
wi h a low e iciency penal y.
6
In CLC, he oxygen needed o
he uel combus ion is supplied by a solid oxygen ca ie (OC),
he e o e a oiding gas mixing. The oxygen ca ie , ypically a
me al oxide, ci cula es be ween wo in e connec ed eac o s:
he uel and ai eac o s. The uel eac o (FR) is whe e he
uel is bu ned o CO2and H2O, causing he oxygen ca ie o
be educed. The educed oxygen ca ie is hen ans e ed o
he ai eac o (AR), whe e is eoxidized wi h ai . The
combina ion o CLC wi h biomass as uel can be conside ed a
p omising BECCS echnology cu en ly unde de elopmen .
Solid uels commonly u ilized in CLC encompass biomass
7
and solid was es, such as sewage sludge.
8
Chemical looping
wi h oxygen uncoupling (CLOU) is a me hod o enhance he
kine ics o solid−solid eac ions in ol ing uels and OCs. A
ecen e iew
9
ou lines he key p ope ies o cu en ly
conside ed oxygen ca ie s, wi h a pa icula emphasis on he
Recei ed: May 24, 2024
Re ised: Sep embe 17, 2024
Accep ed: Sep embe 19, 2024
Published: Oc obe 4, 2024
A iclepubs.acs.o g/EF
© 2024 The Au ho s. Published by
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CLOU ea u e, which appea s o be c ucial o di ec solid uel
combus ion. The key idea behind CLOU is he u iliza ion o
ce ain oxygen ca ie s, such as CuO/Cu2O, Mn2O3/Mn3O4,
and Co3O4/CoO, and hei mixed oxides, which ha e he
unique abili y o elease oxygen inside he FR.
10
As a esul ,
his eleased oxygen akes pa in he combus ion p ocess and
p omo es e icien uel oxida ion. The educed oxygen ca ie s
a e hen ans e ed o he AR, whe e hey unde go eoxida ion
h ough in e ac ion wi h ambien ai , allowing he now
eoxidized oxygen ca ie s o be cycled back o he FR o
euse in subsequen cycles.
11
Mos o he s udies ega ding
CLOU oxygen ca ie s a e ocused on coppe -based oxides. Cu
p esen s an economically iable p ice poin , en i onmen ally
iendly a ibu es, he highes oxygen uncoupling capaci y and
no able eac i i y, making i a good al e na i e o be
conside ed in u he esea ch.
12−16
The p oo o concep o
CLOU echnology was demons a ed in a con inuous 1.5 kW h
uni u ilizing a coppe oxide-based oxygen ca ie .
17
In ha
plan , ull combus ion o biomass was achie ed using a coppe
oxide-based oxygen ca ie a a empe a u e o 935 °C, along
wi h a CO2cap u e e iciency o 100%.
18
Adanez-Rubio e al.
de eloped a Cu−Mn mixed oxide oxygen ca ie and assessed
i s pe o mance in a CLOU uni . Rema kably, hey achie ed
comple e combus ion o he uel and a ained a high CO2
cap u e e iciency o o e 95% e en a he ela i ely low
empe a u e o 850 °C.
19
In he same au ho s’ p e ious s udy,
a Cu-based magne ic Fe−Mn-suppo ed oxygen ca ie
(Cu30MnFe) de eloped by hem was es ed in a 1.5 kW h
CLOU uni . Besides achie ing comple e combus ion in he FR,
i s magne ic p ope ies enhanced i s e iciency o sepa a ion
om he ashes, wi h an achie ed alue o 98% epo ed.
20
The
magne ic pe meabili y o he oxygen ca ie dec eased om 3.4
(−) o 3.2 (−) a e 63 h o ho ci cula ion in plan . The
magne ic p ope ies o oxygen ca ie s p esen an oppo uni y
o enhance hei sepa a ion om ashes, acili a ing hei euse
wi hin he plan . In o de o analyze he e ec o doping wi h
small amoun o di e en ine oxides o imp o e he
p ope ies o Cu-based oxygen ca ie s, Abuelgasim e al.
21
syn hesized di e en Cu-based oxygen ca ie s by adding 5 w
% Na2O, K2O, CaO, and MgO, espec i ely, by means o sol−
gel me hod es ing in he mog a ime ic analysis. They ound
ha addi ion o Na2O and K2O had a nega i e e ec on he
uncoupling a e. In con as , CaO and MgO addi ion inc eased
he uncoupling a e. Howe e , i is necessa y o know hei
e ec wi h ega d o he mechanical beha io o he oxygen
ca ie . In addi ion, ano he s udy
22
pe o med a sc eening
p ocedu e on magne ic CuO ma e ials enhanced wi h kaolin o
p o ide hem wi h highe mechanical esis ance o he
pu pose o he CLOU p ocess, and a selec ed oxygen ca ie
was es ed in a 1.5 kW h CLOU uni . In e ms o mechanical
s eng h, kaolin addi ion inc eased c ushing s eng h om 1.7
o 3.3 N, while he a i ion a e o he kaolin-enhanced oxygen
ca ie e ained almos cons an h oughou he campaign.
Cu30MnFekao7.5 showed high eac i i y, esul ing in ull
combus ion e iciency e en a he low empe a u e o 800 °C
wi h di e en biomasses wi hou no p oblems o agglome -
a ion. The oxygen ca ie e ained i s magne ic p ope ies a e
80 h o ho ci cula ion ope a ion. Ini ially, he esh oxygen
ca ie exhibi ed a magne ic pe meabili y o 3.6. A e ex ended
combus ion, his alue dec eased o 1.6, indica ing ha he
Cu30MnFekao7.5 oxygen ca ie emained su icien ly mag-
ne ic o sepa a ion om biomass ashes. As a esul , 99.3% o
he oxygen ca ie pa icles could be success ully sepa a ed
om he ashes, demons a ing i s sui abili y o euse in he
plan .
23
To make he chemical looping (CL) p ocess and CLC wi h
biomass iable o BECCS, mo e knowledge abou he
in e ac ions be ween biomass ashes and oxygen ca ie s is
needed, as i is well-known ha biomass ash can be agg essi e
and co osi e du ing he mal con e sion a high empe a u e.
In e ac ion be ween ash and he oxygen ca ie can dec ease
solid con e sion and inc ease agglome a ion in he bed
ma e ial.
24
Dai and Whi y
25
s udied he eac ion be ween
ash-de i ed Al2O3/Fe2O3and coppe oxide as he oxygen
ca ie du ing he combus ion o coal. The e ec o coal ash on
agglome a ion was ound o be s ongly ela ed o ash
composi ion, empe a u e and he ash o CuO a io, and ha
IL6 coal ash deac i a ed a ound 15% o he coppe oxide a
900 °C. The ex en o sin e ing and deac i a ion inc eased by
inc easing he empe a u e. Se e al p e ious s udies in he
li e a u e a emp ed o shed ligh on he complex in e ac ions
be ween oxygen ca ie s and ash cons i uen s in biomass
combus ion p ocesses. Da wish e al.
26
examined he
in e ac ion be ween coppe oxide-based oxygen ca ie s and
ash componen s. They obse ed ha a subs an ial po ion o
coppe oxides emained in ac wi hou unde going any
in e ac ion wi h he ash. Howe e , he o ma ion o silica e-
based compounds, pa icula ly po assium silica es, was ound
o con ibu e signi ican ly o s ong agglome a ion phenomena.
Ano he s udy by S anicice al.
27
conside ed he solid−s a e
eac ion o hema i e, hausmanni e, and syn hesized ilmeni e
oxygen ca ie s wi h di e en ash componen s, such as calcium
ca bona e, silica, and po assium ca bona e a 900 °C. The
indings indica ed ha he oxygen ca ie s, speci ically
hausmanni e and hema i e, exhibi ed a mo e p onounced
in e ac ion compa ed o he syn hesized ilmeni e in e ms o
bo h physical a ibu es and disce nible phases. No ably,
syn hesized ilmeni e only o med new phases in sys ems
inco po a ing po assium. Yilmaz and Leion
28
in es iga ed he
in e ac ion be ween i on-based oxygen ca ie s and alkaline
sal s ound in biomass-de i ed ash. Thei indings e ealed ha
calcium deposi s p ima ily o med on he ou e laye o he
i on-based oxygen ca ie . Addi ionally, he p esence o
po assium and silicon esul ed in he o ma ion o b idges
be ween ash componen s and he oxygen ca ie , leading o
inc eased agglome a ion. The e ec o po assium sal on
ilmeni e was in es iga ed. In edox expe imen s conduc ed a
850 °C, Hildo e al.
29
ound ha po assium sal s, such as
K2CO3, K2SO4, and KCl, inc eased he eac i i y o ilmeni e,
and ha KPO3 o med a laye a ound he oxygen ca ie ,
leading o agglome a ion. This KPO3laye is almos
impe meable o CO and dec eases eac i i y owa d H2.
Ano he wo k
30
examined he in e ac ion be ween wo oxygen
ca ie s dilu ed wi h silica sand, one LD slag and he o he
ilmeni e, and po assium sal s in a ixed bed a 900 °C. The
in e ac ion o he po assium sal wi h he oxygen ca ie
esul ed in agglome a ion in he se up. The in e ac ion
be ween i on oxygen ca ie s and biomass ash- o ming
compounds was s udied.
31
The esul s showed ha du ing
educ ion and oxida ion eac ions a 950 °C, calcium and
magnesium ni a e and phospha e caused he agglome a ion o
i on-based oxygen ca ie s. Mei e al.
32
in es iga ed he e ec
o he alkali componen s o po assium and he sodium om
biomass on a b auni e manganese o e oxygen ca ie in a ba ch
luidized bed eac o . Pa ial agglome a ion and de luidiza ion
we e expec ed and ound o occu on he su ace o pa icle
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exac ly whe e, based on EDX esul s, he alkali componen s o
he sodium and po assium we e loca ed. In ac , he
accumula ion o po assium can lead o so agglome a ion
wi hin he combus ion bed.
33
I was obse ed ha he ash
elemen s in biomass, pa icula ly componen s wi h low mel ing
poin s, such as sodium and po assium, as well as compounds
wi h high mel ing poin s, such as calcium and magnesium,
exhibi inhibi o y p ope ies on oxygen ca ie s.
34
In ano he
s udy conduc ed by Liu e al.,
35
i was disco e ed ha
po assium can induce agglome a ion wi hin he bed. Ne e -
heless, i was also obse ed ha po assium has a posi i e
impac on expedi ing he con e sion o biomass. Pu nomo e
al.
36
s udied he in e ac ion be ween po assium sal s,
ep esen a i e o he mos impo an biomass ash compounds,
and an i on-based oxygen ca ie . The expe imen s in ol ed
mixing he oxygen ca ie ma e ial wi h a po assium sal
(K2CO3, KH2PO4, and K2SO4, espec i ely) in an alumina
c ucible a high empe a u e unde educing condi ions. The
po assium con en in he mix u e (4 w %) app oached he
lowe de ec ion limi o he di ac ome e , posing challenges
o de ini i ely iden i ying any po assium phases in he samples
ia X- ay di ac ion (XRD). Acco ding o he scanning
elec on mic oscopy wi h ene gy dispe si e X- ay analysis
(SEM−EDX) esul s, he expe imen s in ol ing i on sand
showed ha po assium om all h ee di e en po assium sal s
consis en ly di used in o he i on sand pa icles. Among he
es ed ma e ials, i is e iden ha i on sand consis en ly
exhibi ed some deg ee o agglome a ion a e exposu e o any
po assium sal . The high silica con en in i on sand likely
con ibu es o i s agglome a ion, as silicon and po assium ha e
a s ong a ini y o each o he , o ming po assium silica e,
which has a low mel ing poin .
Ande sson e al.
37
conside ed gaseous alkali in e ac ions
wi h ilmeni e, manganese oxide, and calcium mangani e in he
luidized bed a 900 °C. The ype o oxygen ca ie plays a
c ucial ole in alkali up ake, wi h ilmeni e showing almos
comple e up ake o alkali, pa icula ly unde educ ion
condi ions. O e all, he expe imen al esul s showed he
complex beha io o alkali and alkaline ma e ials in CLC.
The e o e, i is c ucial o conside he in e ac ion be ween uel
ash and CLOU oxygen ca ie s. This wo k s udied he
in e ac ion o wo magne ic coppe -based oxygen ca ie s,
Cu30MnFe and Cu30MnFekao7.5, wi h pine sawdus biomass
du ing combus ion in a 1.5 kW h con inuous uni unde CLOU
condi ions.
2. EXPERIMENTAL SECTION
2.1. Oxygen Ca ie P epa a ion. Two dis inc oxygen ca ie s
we e syn hesized using a spou ed luidized bed sp ay g anula o (Gla
W51530 + OPP1). Raw componen s we e used, such as CuO
(Pan eac), Mn3O4(Mic omax, Elkem), Fe2O3(Ac os O ganics), and
kaolin (Sumi omo Seika), along wi h di e en addi i es including
poly(e hylene oxide) (PEO-1, Sumi omo), dispe sing agen s and
de locculan s. Bo h oxygen ca ie s we e p oduced wi h he same
amoun o CuO as he ac i e phase (30 w % CuO). In addi ion o
CuO, he i s comp ised 30.55 w % Mn3O4, 31.95 w % Fe2O3, and
7.5 w % kaolin, whe eas he second con ained 34 w % Mn3O4and
36 w % Fe2O3. The oxygen ca ie s a e deno ed as Cu30MnFekao7.5
and Cu30MnFe, espec i ely. The Cu30MnFekao7.5 g anules we e
subjec ed o calcina ion a 1050 °C o 4 h, while he Cu30MnFe was
calcined a 1100 °C o 4 h. The wo calcined oxygen ca ie s we e
sie ed o ob ain pa icles wi hin he desi ed ange o 100−300 μm.
2.2. Oxygen Ca ie and Pine Sawdus Cha ac e iza ion and
Da a E alua ion. Key p ope ies o he syn hesized oxygen ca ie s
a e p esen ed in Table 1. The magne ic pe meabili y (μ) lis ed in he
able was calcula ed based on he magne ic olume ic suscep ibili y
alue χ (m3/kg), which was measu ed using a Ba ing on single
equency MS2G senso connec ed o a magne ic suscep ibili y MS3
me e .
( ) 1 eal
= +
(1)
A Shimpo FGN-5 digi al o ce gauge appa a us was employed o
de e mine he c ushing s eng h o he oxygen ca ie s, measu ed as
he o ce necessa y o ac u e a pa icle. The epo ed c ushing
s eng h alues in his s udy ep esen he a e age o a minimum o 20
measu emen s conduc ed on andomly selec ed pa icles alling wi hin
he speci ied ange o 100−300 μm.
In his s udy, XRD was employed o de e mine he c ys alline solid
phases p esen in ou designa ed samples. The co esponding spec a
we e cap u ed using a B uke D8 Ad ance X- ay powde
di ac ome e , ou i ed wi h a Cu anode X- ay sou ce ope a ing a
40 kV and 40 mA, alongside an ene gy-dispe si e one-dimensional
de ec o . The di ac ion pa e n was ob ained wi hin he 2θ ange o
10−80°, wi h a s ep size o 0.019°. To iden i y he phases wi hin he
oxygen ca ie s, we u ilized DIFFRAC.EVA so wa e, augmen ed by a
comp ehensi e e e ence pa e n da abase sou ced om he
C ys allog aphy Open Da abase (COD) and he Powde Di ac ion
File (PDF). This app oach acili a ed p ecise phase iden i ica ion,
enabling a ho ough analysis o ou samples.
Induc i ely coupled plasma op ical emission spec ome y (ICP-
OES) was pe o med by means o an Xpec oblue-EOP-TI FMT26
(Spec o) o de e mine he elemen s in he ash and p epa ed oxygen
ca ie s in a ange o quan i iable concen a ions.
Selec ed samples we e obse ed unde a ield emission scanning
elec on mic oscope (SEM, Ca l Zeiss MERLIN wi h a ho ca hode
ield emission elec on gun) and equipped wi h an ene gy-dispe si e
X- ay spec oscopy (EDX) de ec o o analysis o he ene gy o
sca e ed X- ays X-max (20 mm2) wi h silicon d i de ec o om
Ox o d ins umen s. The samples we e embedded in an epoxy mold,
a e which he epoxy su ace was me iculously polished o p oduce
he c oss sec ions o he pa icles.
A h ee-hole ATTRI-AS ai je a i ion esis ance es e (MaTec
Ma e ials Technologies Snc) was u ilized o e alua e he a i ion
esis ance o each oxygen ca ie in e ms o i s a i ion je index
(AJI) as pe he Ame ican s anda d es me hod D5757-22.
38
In
acco dance wi h his p o ocol, weigh losses ( ines) om 50 g o each
oxygen ca ie we e eco ded unde an ai low o 10 L/min a e 5 h
o ope a ion. This acili a ed he calcula ion o AJI as he pe cen age
o ines a e 5 h, whe e AJI is calcula ed using he ollowing o mula
m mAJI ( / ) 100
5 h s
= ×
(2)
whe e m5 h ep esen s he mass o ines sized less han 40 μm collec ed
a e 5 h, and ms ep esen s he mass o he sample loaded in o he
appa a us.
Helium pycnome y is he me hod used o de e mine he skele al
densi y o solid ma e ials by measu ing he amoun o helium
displaced by he sample, using a mic ome i ics ACCUPYC II
ins umen . Di e en wo king olumes, a e a ailable, allowing o
Table 1. P ope ies o F esh and Used Cu30MnFekao7.5
and Cu30MnFe
Cu30MnFe Cu30MnFekao7.5
esh used esh used
CuO con en (w %) 30 30 30 30
oxygen anspo capaci y,
ROC (w %) 2.4 2.8 2.3 2.4
c ushing s eng h (N) 1.49 1.12 2.3 1.43
magne ic pe meabili y μ(−) 4.7 2.4 3.6 2.3
skele al densi y (kg/m3) 5125 5070 4720 4617
po osi y (%) 6.9 26.39 29.5 26.10
ai je a i ion index (AJI)
(%) 0.4 2.8 0.2 3.4
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he analysis o a ying sample quan i ies. The empe a u e o he
analysis chambe can be con olled, enabling densi y de e mina ion a
di e en empe a u es which was 30 °C.
In he speci ic su ace a ea es conduc ed on bo h oxygen ca ie s
using he B unaue −Emme −Telle me hod, bo h esh and in
pos expe imen condi ions, he measu ed alues we e ound o be
below 0.5 m2/g.
ROC e e s o he maximum amoun o oxygen ha an oxygen
ca ie can anspo du ing a CL p ocess. In his s udy, coppe oxide
and i s mixed oxides we e assumed o be he ac i e phase esponsible
o oxygen uncoupling. I is impo an o no e ha unde he speci ied
condi ions,
39
he oxida ion o MnFe mixed oxide o he bixbyi e
phase, which also possesses oxygen uncoupling capabili y, is hinde ed.
The e o e, any oxygen uncoupling capabili y obse ed in he p epa ed
ma e ials was assumed o come om he CuO o a mixed oxide
con aining coppe , manganese and i on.
Pine sawdus (Pinus syl es is), milled and sie ed 0.5−2 mm, was
used as uel biomass du ing combus ion. Table 2 shows he p ope ies
o he pine sawdus a e analysis.
Combus ion e iciency was calcula ed based on he amoun o
oxygen equi ed o consume unbu n gases om he FR, and he
oxygen equi ed o comple e combus ion o solid uel in he FR, as
ollows
1oxygen equi ed by unbu n gases
oxygen equi ed by solid uel con e ed in uel eac o
comb,FR
=
(3)
CO2cap u e e iciency, ηcc, was de ined as he a io o CO2exi ing
he FR and he ca bon exi ing as CO2a he FR and AR ou le s.
CO in gases om FR
ca bon in gases om FR and AR
CC
2
=
(4)
2.3. Expe imen al Se up and Expe imen al Condi ions. The
1.5 kW h CLOU uni consis ed o wo in e connec ed bubbling
luidized beds, se ing as FR and AR, along wi h a loop seal designed
o p e en he mixing o gases be ween hese eac o s. I also ea u ed
a cyclone and a solid con ol al e. An ai −N2mix u e was u ilized a
a a e o 2100 LN/h inside he AR, while N2was used inside he FR
and loop seal a a es o 180 LN/h and 100 LN/h, espec i ely. A
de ailed desc ip ion o he plan can be ound in p e ious esea ch
wo ks.
17
The aim o he p esen expe imen al campaign was o analyze he
possible in e ac ion be ween biomass ashes and he oxygen ca ie s
used. In his espec , he expe imen al condi ions we e con igu ed o
allow he co esponding expe imen s o achie e high combus ion
e iciency and CO2cap u e e iciency, and hen o emain unchanged
h oughou he whole expe imen al campaign. The expe imen al se up
main ained a cons an uel eeding a e o 120 g/h h oughou he
expe imen s. Simul aneously, he oxygen ca ie ci cula ion a e
emained s eady a 25 kg/h. The empe a u e wi hin he sys em
was egula ed and kep cons an a 900 °C in bo h he FR and AR
h oughou he expe imen s.
Fu he mo e, he in en o y o oxygen ca ie ma e ial p esen in
he plan was 4 kg. In o de o analyze possible in e ac ions ash-
oxygen ca ie , expe imen s should be un o e a long pe iod. In he
case o Cu30MnFekao7.5, he expe imen s in ol ed 56 h o
combus ion ope a ion wi h pine sawdus as uel and 105 h o ho
luidiza ion. The expe imen using Cu30MnFe comp ised 16 h o
combus ion and 35 h o ho ci cula ion.
3. RESULTS AND DISCUSSION
3.1. Expe imen al Pa ame e s. In all he pine sawdus
combus ion expe imen s pe o med in he 1.5 kW h CLOU
con inuous uni , a combus ion e iciency o 100% was achie ed
wi h bo h o he oxygen ca ie s es ed in his wo k, con i ming
hei high eac i i y.
Figu e 1 illus a es he CO2cap u e pe cen age du ing he
hou s o combus ion o Cu30MnFekao7.5 and Cu30MnFe. I
is e iden ha in bo h cases, he CO2cap u e pe cen age
emained nea ly cons an a a high alue o app oxima ely 95%.
Fu he mo e, Figu e 1 depic s he cha con e sion achie ed
by he wo oxygen ca ie s. Simila ly, o he s able CO2cap u e
pe cen age obse ed du ing combus ion ope a ion, cha
con e sion o bo h oxygen ca ie s emained s eady a
app oxima ely 0.8. The alues shown in Figu e 1 a e in line
wi h hose p e iously ob ained in pine sawdus combus ion
wi h he same oxygen ca ie s in p e ious wo ks by he
au ho s.
20,23
3.2. Analysis o Ash-Oxygen Ca ie In e ac ion. The
pine sawdus ash componen s we e i s de e mined using ICP-
OES, whose esul s a e shown in Table 3. The biomass ash
analyzed was ob ained om he p oxima e analysis and hen
diges ed wi h Li2B4O7in o de o analyze by ICP-OES.
The main ash componen s in pine sawdus ash we e Ca, K,
Si, and Mg. The me hodology used o analyze possible
in e ac ions be ween hese ash componen s and he oxygen
ca ie s was he ex ac ion o used oxygen ca ie samples a e
di e en combus ion imes and hei subsequen cha ac e -
iza ion by ICP-OES o de e mine hei composi ion and
Table 2. Main P ope ies o Pine Saw Dus
(w %) (w %)
humidi y 6.9 C 47.5
ola iles 73.6 H
a
5.6
ixed ca bon 17.9 N 0.3
ash 1.6 S <0.11
LHV (kcal/kg) 4384
a
H ee o mois u e. Figu e 1. CO2cap u e pe cen age and cha con e sion o bo h
oxygen ca ie s Cu30MnFekao7.5 and Cu30MnFe du ing combus ion
ope a ion ime.
Table 3. Pine Sawdus Ash Componen
a
pine sawdus ash
componen w % pine sawdus ash
componen w %
Ca 22.8 Al 1.34
K 7 P 0.67
Si 6.33 Na 0.21
Mg 2.61 Mn 0.15
Fe 1.56 Ti 0.09
a
O is he elemen equi ed o comple e he equilib ium.
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SEM−EDX o analyze hei changes in mo phology and/o
dis ibu ion o hei cons i uen elemen s. In o de o analyze
possible in e ac ion be ween oxygen ca ie wi h ashes, oxygen
ca ie pa icles we e sepa a ed om independen ash pa icles
in he bed by magne ic sepa a ion, p io o analysis wi h ICP-
OES and SEM−EDX, as can be seen in Figu e S1, and only
oxygen ca ie pa icles can be ound in he analyzed sample.
The ICP-OES composi ion ob ained o he used oxygen
ca ie s was expec ed o p o ide a i s indica ion o any
in e ac ion o p esence in case any o hese elemen s
signi ican ly inc eased hei p esence in he used oxygen
ca ie pa icles. Table 4 shows he ICP-OES esul s o bo h
Table 4. ICP Resul s o F esh and Used Cu30MnFe and CuMnFekao7.5 Oxygen Ca ie s (w %)
samples Ca K Si Mg
Cu30MnFe- esh 0.12 ±0.04 <0.04 0.4 ±0.03 0.06 ±0.07
Cu30MnFe-used_15 h <0.05 <0.04 0.48 ±0.03 0.05 ±0.07
Cu30MnFekao7.5- esh 0.12 ±0.04 0.12 ±0.01 2.53 ±0.03 0.18 ±0.07
Cu30MnFekao7.5-used_15 h 0.12 ±0.04 0.15 ±0.01 2.50 ±0.03 0.25 ±0.07
Cu30MnFekao7.5-used-56 h 0.10 ±0.04 0.28 ±0.01 2.7 ±0.03 0.38 ±0.07
Figu e 2. SEM−EDX mapping o used Cu30MnFe.
Figu e 3. SEM−EDX backsca e ed elec on (BSE) images o a c oss-sec ion o a used Cu30MnFe pa icles including composi ion a di e en
poin s.
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esh and used oxygen ca ie s, which ocus on he main
elemen s iden i ied in he biomass ashes acco ding o, i.e. Ca,
K, Si, and Mg. In all cases P was ound o be below he ICP-
OES quan i a i e le el (<0.04%). Di e ences in he
composi ion o he esh oxygen ca ie s can be obse ed in
Table 4 ega ding he amoun s ound o elemen s K, Si, and
Mg. This is due o he ac ha hese elemen s a e componen s
o he kaolin used in he p epa a ion o he Cu30MnFekao7.5
ma e ial. The ICP-OES analysis pe o med on he kaolin
sample indica ed 2.68 and 19.13 w % o K and Si,
espec i ely.
In he case o he Cu30MnFe oxygen ca ie , no signi ican
inc ease was obse ed in he amoun o Ca, Si, Mg, and K a e
15 h o pine sawdus combus ion, see Table 4, and he e o e
no sign o in e ac ion could be assumed. This ac , oge he
wi h he mapping esul o he used Cu30MnFe, Figu e 2, and
di e en elemen al poin s composi ion on SEM−EDX image
conduc ed, Figu e 3, i can be concluded ha no in e ac ion
was ound o ake place be ween he oxygen ca ie and
alkaline me al.
As shown in Figu e 3 and in he accompanying able, which
p esen s da a om a ious poin s on di e en pa icles, no ash
elemen s, such as Mg o K, we e de ec ed on di e en pa s o
he Cu30MnFe pa icles used. This obse a ion co obo a es
he quan i a i e ICP-OES esul s, con i ming ha he o al
amoun o ash elemen s in he oxygen ca ie was almos
negligible and ha he e was no in e ac ion be ween he
Cu30MnFe oxygen ca ie and ash componen s. Fu he mo e,
Figu es S2 and S3 show SEM−EDX mapping o he esh and
used Cu30MnFe, espec i ely, o compa e elemen dis ibu ion
in bo h cases.
Because o he uns able ope a ion wi h Cu30MnFe obse ed
in he con inuous uni and a ibu ed o he high gene a ion o
ines, see Figu e S1b, he expe imen al campaign was only able
o con inue wi h he Cu30MnFekao7.5 oxygen ca ie .
Howe e , in he case o he Cu30MnFekao7.5 and a e he
same ime o pine sawdus combus ion, he esul s shown in
Table 4 could indica e he accumula i e p esence o biomass
ashes, gi en ha he e is an inc ease in he amoun o K and
Mg in he oxygen ca ie . A e 56 h o pine sawdus
combus ion wi h his ma e ial, no signi ican changes we e
obse ed in he amoun o Ca ound in he used oxygen ca ie ,
which is he main elemen p esen in he ashes. The e o e, he
absence o in e ac ion be ween he oxygen ca ie and Ca, as
he main componen o ash, indica e ha he g ea e pa o
he ashes can be sepa a ed wi hou p oblems owing o he
magne ic p ope ies o he Cu30MnFekao7.5, as was
p e iously obse ed by Adanez-Rubio e al., who ob ained a
magne ic sepa a ion e iciency o he oxygen ca ie om ash o
99.3%.
23
On he o he hand, an inc ease was obse ed in he
p esence o Mg om 0.18 o 0.38 w %. The amoun o K also
inc eased om 0.12 o 0.28 w %. These esul s could indica e
ha an accumula i e p esence o pine sawdus ashes was aking
place in he Cu30MnFekao7.5 oxygen ca ie . Consequen ly
SEM−EDX analysis was pe o med o look o u he
e idence o his p esence.
Figu e 4 shows he dis ibu ion o di e en elemen s on one
used Cu30MnFekao7.5 oxygen ca ie pa icle a e 56 h o
pine sawdus combus ion. Di e en a eas can be obse ed on
he pa icle, some o which a e en iched wi h Cu−Mn−Fe and
o he s wi h kaolin. Wi h ega d o he la e , he ac ha kaolin
al eady con ains Si, Al, K, and Mg makes i di icul o obse e
any changes a ibu ed o he in e ac ion be ween he pa icles
and biomass ashes. Accumula ions o hese elemen s on he
pa icle su ace could only be in e ed in he case o Mg. In
addi ion, i seems ha K can pene a e in o he oxygen ca ie
pa icles, di using in he kaolin- ich a eas, wi hou in e ac ion
wi h he ac i e phase. The p esence o K in egions de oid o
Figu e 4. SEM−EDX elemen al mapping o used Cu30MnFekao7.5 oxygen ca ie a e 56 h o pine sawdus combus ion.
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coppe oxide, manganese oxide, and i on oxide, ye con aining
Si, is illus a ed in Figu e 4. Addi ionally, a small amoun o Mg
can be de ec ed, mainly on he su ace o he pa icle.
Fu he mo e, Figu e 5a shows BSE images o esh
Cu30MnFekao7.5 oxygen ca ie , Figu e 5b,c shows he
same oxygen ca ie a e 56 h o combus ion. Acco ding o
he EDX esul s, accumula i e K could be obse ed in he da k
a eas o Figu e 5, whe e he e was a high kaolin concen a ion.
As obse ed a poin s 12, and 13, whe e Si and Al a e p esen
inside he used oxygen ca ie Cu30MnFekao7.5, K also
appea s. Con e sely, a poin 14, whe e he concen a ion o Si
and Al, indica i e o kaolin, is low, he e is no ace o K. Figu e
S6 p esen s addi ional poin s in he used pa icles. As can be
seen in Figu es 5b and in S6a,b he p esence o K in he su ace
is simila o e en lowe (due o he su ace was en iched in
Mn, Fe, and Cu, see Figu e 6a,b) han he ound in he inne o
he pa icle. The a e age alue o 2.4 mol % o K inside he
used pa icles is calcula ed based on he da a om he used
oxygen ca ie shown in Figu es 5 and S6. This esul
demons a es an accumula ion o K ela ed o he kaolin in
he used pa icles compa ed o he esh oxygen ca ie , which
has an a e age alue o 1.5 mol %.
Figu e 6a,b shows he elemen al mapping o esh and used
Cu30MnFekao7.5 wi h ega d o he main elemen s ound in
he ashes. In he esh oxygen ca ie , he p esence o ash
componen s, such as Mg, Ca, and K, is di use, and hey a e
de ec ed o e he en i e pa icle; only a small highe p esence
o hese can be seen in a eas en iched wi h Si and Al.
Con e sely, he SEM−EDX esul s in Figu e 6b o he used
oxygen ca ie e eal ha K is p edominan ly ound in egions
associa ed wi h kaolin, e en in he cen e o he pa icle,
in ol ing he elemen s Al and Si. I can he e o e be said ha
some kind o in e ac ion has aken place, al hough gi en he
small change in composi ion, i is no possible o indica e
whe he his was due o chemical in e ac ion o simply
di usion o K owing o g ea e solubili y in he a eas o kaolin
inside he pa icles. Mo eo e , Mg is mainly ound o be mo e
p esen on he su ace o he pa icle oge he wi h only Si.
Figu e 5. SEM−EDX BSE images o (a) c oss-sec ion o esh Cu30MnFekao7.5 and (b) su ace-sec ion o used Cu30MnFekao7.5, and (c) c oss-
sec ion o used Cu30MnFekao7.5 a e 56 h o pine sawdus combus ion, including able o main componen changes a di e en poin s. *The
a e age alue o each componen is calcula ed based on he concen a ions o he componen a he poin s in he esh and used oxygen ca ie , as
indica ed in Figu es 5b and S6.
Figu e 6. Resul o SEM−EDX elemen al mapping o he main elemen s ound in he ashes a e pine sawdus combus ion: (a) esh
Cu30MnFekao7.5, (b) used Cu30MnFekao7.5.
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Figu e 7 shows ano he SEM−EDX elemen al mapping o
used Cu30MnFekao7.5 and indica es he p esence o K in
egions con aining kaolin. Simila K pene a ion in o he
oxygen ca ie , as shown in he expe imen al esul s, was
p e iously obse ed by Pu nomo e al.
36
in hei expe imen s
using i on sand, an oxygen ca ie en iched wi h silica (16 w
%) and po assium sal s (K2CO3, KH2PO4, and K2SO4), hey
ound ha K di used in o he indi idual pa icles o he oxygen
ca ie . The i on sand exhibi ed agglome a ion a e exposu e
o any po assium sal , which can be a ibu ed o he
in e ac ion o K wi h i s high silica con en . In he p esen
s udy, al hough K was p esen wi hin he pa icles,
agglome a ion did no occu du ing he pine sawdus
combus ion. Figu es S4 and S5 p esen SEM−EDX elemen al
mappings o addi ional pa icles o esh and used Cu30Mn-
Fekao7.5, espec i ely.
In o de o analyze he p esence o K in he su ace o he
pa icle, Figu e 8 illus a es he SEM−EDX elemen al mapping
o he su ace o he used oxygen ca ie , Cu30MnFekao7.5.
The analysis e eals he p esence o small amoun s o Al and Si
on he pa icle su ace, while no no iceable accumula ion o K
is obse ed. Simila ly, Figu e S8 p esen s he SEM−EDX
Figu e 7. Resul o SEM−EDX elemen al mapping o used Cu30MnFekao7.5 a e pine sawdus combus ion.
Figu e 8. SEM−EDX elemen al mapping image o he su ace o used Cu30MnFekao7.5 a e pine sawdus combus ion.
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elemen al mapping o ano he pa icle o he same oxygen
ca ie , exhibi ing an elemen al dis ibu ion pa e n consis en
wi h ha shown in Figu e 8. As i can be seen he su ace is
en iched in Mn, Fe, and Cu, obse ing less p opo ion o
kaolin in he su ace and he e o e low amoun o K in he
su ace. So, i can be concluded ha he e was no a su ace
accumula ion o K due o he low p esence o kaolin in i .
To u he elucida e he implica ions ha he in e ac ion
be ween biomass ashes and he used Cu30MnFekao7.5 could
ha e, XRD es s we e pe o med o esh and used oxygen
ca ie samples (a e 15 h and a e 56 h o combus ion wi h
pine sawdus ). The XRD pa e ns shown in Figu e 9 indica e
ha he esh and used oxygen ca ie s exhibi simila phases
and ha only ee SiO2appea s in he Cu30MnFekao7.5 a e
se e al hou s o ope a ion. Thus, he possibili y o new
c ys alline phases o ma ion owing o in e ac ion be ween he
oxygen ca ie and Mg o K canno be con i med by he
analysis pe o med. Cu does no seem o in e ac wi h he ash
cons i uen s.
P e ious s udies indica ed ha he accumula ion o K can
lead o so agglome a ion wi hin he combus ion bed,
33
wi h
he p esence o K and Si esul ing in he o ma ion o b idges
be ween ash componen s and he oxygen ca ie , leading o
inc eased agglome a ion. In a s udy on he in e ac ion be ween
coppe oxide-based oxygen ca ie s and ash componen s,
26
he
o ma ion o po assium silica es was ound o be he cause o
agglome a ion. Howe e , i should be highligh ed ha du ing
he 56 h o pine combus ion wi h Cu30MnFekao7.5, no signs
o de luidiza ion o agglome a ion p oblems we e ound.
3.3. E olu ion o O he Oxygen Ca ie P ope ies. I
should be men ioned ha he magne ic p ope ies a ibu ed o
he oxygen ca ie s can in ac be a ibu ed o he p esence o
he spinel phase.
40
These magne ic p ope ies aid he e ec i e
sepa a ion o ash om pa icles and he eco e y o he used
oxygen ca ie . Acco ding o Table 1, i is no ewo hy ha
al hough he magne ic p ope ies o he wo oxygen ca ie s
dec eased o e he cou se o he expe imen s, bo h oxygen
ca ie s s ill exhibi ed su icien magne ic p ope ies by he end
o he ope a ion. This cha ac e is ic enabled he e ec i e
sepa a ion o he oxygen ca ie s om he ashes, acili a ing
hei eco e y and po en ial euse in subsequen cycles.
20,23
The mechanical s eng h o oxygen ca ie s a e ope a ion
hou s was e alua ed, as shown in Figu e 10.
The mechanical s eng h o bo h oxygen ca ie s dec eased
du ing ope a ion, as indica ed by he esul s in Figu e 10. The
dec ease obse ed in he case o Cu30MnFe can be a ibu ed
o an inc ease in po osi y; howe e , o Cu30MnFekao7.5, his
dec ease canno be di ec ly linked o he e olu ion o po osi y
o e ime, as shown in Table 1. Ne e heless, by he end o he
expe imen s, mechanical s eng h alues we e abo e alues o 1
N, which is conside ed accep able o ope a ion.
41
As can be
obse ed, he addi ion o kaolin imp o ed he mechanical
s eng h o he oxygen ca ie .
22
An inc ease in oxygen
anspo capaci y was obse ed a e he combus ion p ocess
Figu e 9. XRD pa e ns o esh Cu30MnFekao7.5 and used a e 15 and 56 h o pine biomass combus ion.
Figu e 10. E olu ion o mechanical s eng h o Cu30MnFe and
Cu30MnFekao7.5 du ing ope a ion.
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