1
Hyd ogen- ich gas p oduc ion by con inuous py olysis and in- 1
line ca aly ic e o ming o pine wood was e and HDPE 2
mix u es 3
Ai o A egi, Maide Amu io, Ga zen Lopez*, Mai e A e xe, Jon Al a ez, Ja ie 4
Bilbao and Ma in Olaza 5
Depa men o Chemical Enginee ing, Uni e si y o he Basque Coun y UPV/EHU, 6
P.O. Box 644 - E48080 Bilbao (Spain). [email p o ec ed]7
Abs ac 8
The con inuous py olysis-ca aly ic s eam e o ming o di e en mix u es o biomass 9
and high densi y polye hylene (25, 50 and 75 w % HDPE) has been ca ied ou in a 10
wo-s ep eac ion sys em, p o ided wi h a conical spou ed bed eac o (CSBR) and 11
luidized bed. The in luence o HDPE co- eeding on he con e sion, e o ming p oduc s 12
yields and composi ion and ca alys deac i a ion has been s udied a a e o ming 13
empe a u e o 700 ºC, wi h 16.7 gca min g eeding-1 and s eam/(biomass+HDPE) mass 14
a io o 4, compa ing he esul s wi h hose ob ained by eeding pu e biomass and 15
HDPE. The co- eeding o plas ics enhances he p oduc ion o hyd ogen, which 16
inc eases om 10.9 g o H2 pe 100 g o biomass o 37.3 g o H2 pe 100 g o HDPE 17
ed. Ca alys deac i a ion by coke is a enua ed when HDPE is co- ed due o he lowe 18
con en o oxygena ed compounds in he eac ion en i onmen . The highe yield o 19
hyd ogen achie ed wi h his wo-s ep (py olysis- e o ming) s a egy, i s lexibili y o 20
join ly alo ise biomass and plas ic mix u es and he lowe empe a u es equi ed in 21
ela ion o gasi ica ion, makes his p ocess p omising o p oduce H2 om enewable aw 22
ma e ials and was es. 23
This is he accep ed manusc ip o he a icle ha appea ed in inal o m in Ene gy Con e sion and Managemen 136 : 192-201 (2017), which
has been published in inal o m a h ps://doi.o g/10.1016/j.enconman.2017.01.008. © 2017 Else ie unde CC BY-NC-ND license (h p://
c ea i ecommons.o g/licenses/by-nc-nd/4.0/)
2
Keywo ds: hyd ogen; py olysis; e o ming; biomass; was e plas ics; conical spou ed 24
bed, ca alys deac i a ion 25
1. In oduc ion 26
The en i onmen al awa eness associa ed wi h he use o adi ional esou ces (na u al 27
gas, pe oleum and coal) gi es way o he de elopmen o new ou es o sus ainable 28
hyd ogen p oduc ion, whose demand is g owing due o i s in e es as ene gy ca ie and 29
eac an in e ine y hyd op ocessing uni s [1]. In his scena io, biomass can play an 30
impo an ole as an al e na i e eeds ock, gi en ha is a CO2 neu al enewable sou ce 31
and chemicals o uels p oduced om i a e conside ed sus ainable [2]. 32
Amongs he di e en he mochemical ou es, di ec s eam gasi ica ion [3-6] and he 33
indi ec ou e o bio-oil e o ming [7-10] a e he mos s udied ou es o hyd ogen 34
p oduc ion om biomass. Ne e heless, he gasi ica ion p ocess is di ec ed o p oduce 35
syngas and he a o ma ion is an issue o i s indus ial applica ions [11,12]. On he 36
o he hand, he indi ec ou e o bio-oil e o ming has se e al p oblems ela ed o bio-oil 37
p ope ies and i s apo iza ion and e-polyme iza ion [13,14]. The e o e, he wo-s ep 38
py olysis-ca aly ic s eam e o ming p ocess, in which is no necessa y o condensa e 39
and e- apo ized he bio-oil, is gaining a en ion las yea s [15-19]. This p ocess, in 40
which each s ep is ca ied ou in di e en eac o s, in ol es some ad an ages in ela ion 41
o one-s ep py olysis p ocess wi h a e o ming ca alys in-si u. On he one hand, he 42
empe a u e o each s ep can be op imized in o de o maximize he p oduc ion o 43
hyd ogen [20] and on he o he hand, he ca alys is mo e e ec i e o ola iles 44
ans o ma ion and he p ocess is mo e e sa ile in o de o es ablish he desi ed 45
ca alys / eeding a io. The e o e, a mo e homogeneous p oduc s eam will be ob ained, 46
due o he highe e iciency o he ca alys in o de o a enua e seconda y eac ions. 47
3
Ne e heless, he low con en o hyd ogen and high con en o oxygen o he biomass 48
eeds ock is a d awback o ob ain high p oduc ion o hyd ogen. Mo eo e , he ca alys 49
has a conside able deac i a ion by coke [17]. In his wo k, he imp o emen o H2 50
p oduc ion and he a enua ion o ca alys deac i a ion by he alo isa ion o biomass 51
and HDPE mix u es has been s udied. 52
The imp o emen on hyd ogen p oduc ion has been epo ed by se e al au ho s in he 53
co-gasi ica ion o biomass and HDPE mix u es [21-24]. The co- eeding sol es he 54
seasonal limi a ions o biomass a ailabili y and con ibu es o a enua e he 55
en i onmen al p oblems associa ed o he was e plas ics managemen . E en hough he 56
py olysis is conside ed a sui able ou e o he alo isa ion o was e plas ics on a la ge 57
scale, and pa icula ly o polyole ins [25-27], he s udies conce ning py olysis and in-58
line ca aly ic s eam e o ming o biomass and plas ic mix u es a e e y sca ce. Al a ez 59
e al. [20] s udied he co- eeding o polyp opylene in py olysis- e o ming o biomass in 60
ba ch labo a o y scale eac o , ob aining highe gas yield and highe hyd ogen 61
p oduc ion in ela ion o eeding pu e biomass. In he same expe imen al uni Kumagai 62
e al. [28] es ed a Ni-Mg-Al-Ca ca alys syn hesized by a co-p ecipi a ion me hod o 63
py olysis- e o ming o a biomass/polyp opylene mix u e, ob aining a maximum 64
hyd ogen p oduc ion o 6.0 g o H2 pe 100 g o eeding when he ca alys was calcined 65
a low empe a u es, 500 ºC. 66
The aim o his wo k is o inc ease he p oduc ion o hyd ogen by plas ics co- eeding, 67
using a con inuous wo-s ep p ocess (Figu e 1). The equipmen combines he excellen 68
pe o mance o he conical spou ed bed eac o (CSBR) o he py olysis o biomass 69
[29] and plas ics [30] wi h he sui abili y o he luidized bed eac o o he s eam 70
e o ming p ocess [31,32]. The cyclic igo ous mo emen o he sawdus and sand 71
pa icles coa ed wi h mel ed plas ic in he CSBR minimizes he seg ega ion p oblems 72
4
and a oids he de luidiza ion o he bed. On he o he hand, he luidized bed ca aly ic 73
eac o allows con olling he empe a u e o he endo he mic e o ming eac ion and 74
delays he blocking o he bed by coke o ma ion. This wo s ep con igu a ion has been 75
desc ibed in p e ious pape s o he py olysis-ca aly ic s eam e o ming o biomass [17] 76
and plas ics [33], in which he good pe o mance o he p ocess wi hou ope a ional 77
p oblems and high hyd ogen yields we e epo ed. 78
Figu e 1 ( al a es e ha emos un esquema del p oceso) 79
2. Ma e ials and Me hods 80
2.1. Ma e ials 81
Pine sawdus (pinus insignis) was e has been c ushed, g ound and sie ed o a pa icle 82
size be ween 1 and 2 mm, which is a sui able pa icle diame e in o de o gua an ee he 83
good pe o mance o he solid eeding sys em, and d ied a oom empe a u e o a 84
mois u e con en below 10 w %. The high densi y polye hylene (HDPE) was p o ided 85
by Dow Chemical (Ta agona, Spain) in he o m o chippings (4 mm), wi h he 86
ollowing p ope ies: a e age molecula weigh , 46.2 kg mol-1; polydispe si y, 2.89 and 87
densi y, 940 kg m-3. The highe hea ing alue (HHV) o bo h eeds ocks has been 88
measu ed in a Pa 1356 isope ibolic bomb calo ime y. Mo eo e , he ul ima e and 89
p oxima e analyses ha e been de e mined in a LECO CHNS-932 elemen al analyze 90
and in a TGA Q5000IR he mog a ime ic analyze , espec i ely and he esul s o he 91
cha ac e iza ion o biomass and HDPE used in his s udy a e summa ized in Table 1. 92
Table 1. Cha ac e iza ion o he biomass and HDPE used. 93
Ul ima e analysis (w %)
Biomass
HDPE
Ca bon
49.33
85.71
5
Hyd ogen
6.06
14.29
Ni ogen
0.04
0
Oxygen
44.57
0
P oxima e analysis (w %)
Vola ile ma e
73.4
99.7
Fixed ca bon
16.7
0.3
Ash
0.5
-
Mois u e
9.4
-
HHV (MJ kg-1)
19.8
43.1
94
A comme cial Ni e o ming ca alys (G90-LDP) p o ided by Süd Chemie (Ge many) 95
has been used o he e o ming s ep. The o iginal ca alys (in he o m o pe o a ed 96
ings 19 x 16 mm) has been g ound and sie ed be ween 0.4-0.8 mm, which is he 97
sui able pa icle size in o de o gua an ee he luid dynamic condi ions o he luidized 98
bed. The me al con en (p o ided by he supplie ) and physical p ope ies o he ca alys 99
a e summa ized in Table 2. The adso p ion-deso p ion iso he m o he ca alys has been 100
measu ed by N2 adso p ion-deso p ion (Mic ome i ics ASAP 2010). As obse ed, he 101
ca alys shows low BET su ace a ea and low po osi y. 102
Table 2. Me al con en and physical p ope ies o he ca alys . 103
Ca alys
NiO con en (w %)
SBET (m2 g-1)
Vpo ous (cm3 g-1)
dpo ous (Å)
G90-LDP
14
19
0.04
122
104
Mo eo e , he ca alys has been educed in-si u in o de o ensu e i s ac i i y. The 105
educ ion has been conduc ed o 4 h unde 10 ol % H2 a 710 ºC acco ding o he 106
esul s ob ained by empe a u e p og ammed educ ion. Bo h he adso p ion-deso p ion 107
iso he m and he TPR p o ile o his ca alys can be ound elsewhe e [34,35]. 108
6
2.2. Equipmen and eac o s 109
Figu e 2 shows he scheme o he expe imen al equipmen . The plan is p o ided wi h 110
wo eac o s in-line: (i) a CSBR o py olysis s ep and (ii) a luidized bed eac o o he 111
e o ming s ep o py olysis ola iles. 112
113
Figu e 2. Scheme o he bench scale uni . 114
The plan is p o ided wi h wo independen eede s o biomass and HDPE as 115
seg ega ion p oblems ook place when bo h ma e ials we e mixed in a single uni . Each 116
eede consis s o a essel equipped wi h a e ical sha connec ed o a pis on placed 117
below he ma e ial bed. The ma e ial is ed in o he eac o by aising he pis on a he 118
same ime as he whole sys em is ib a ed by an elec ic engine. The pipe ha connec s 119
7
he eede s wi h he eac o is cooled wi h ap wa e . Mo eo e , a e y small ni ogen 120
low is in oduced in o he essel, which a oids he condensa ion o s eam in he 121
eeding essel. 122
A pump (Gilson 307) has been used in o de o eed he wa e in o he py olysis eac o , 123
which has been p e iously apo ized by an elec ic ca idge placed inside he o ced 124
con ec ion o en. Ni ogen, ai o hyd ogen can also be in oduced o he CSBR eac o 125
and hei lows a e con olled by mass low con olle s, which allow eeding up o 20 L 126
min-1 o ni ogen and ai , and up o 5 L min-1 o hyd ogen. The empe a u e o he s eam 127
and he gases is inc eased up o eac ion condi ions in a gas p ehea e loca ed in he 128
lowe sec ion o he eac o , which is illed wi h s ainless s eel pipes ha inc ease he 129
su ace a ea o hea ans e . 130
The py olysis s ep has been ca ied ou in a CSBR. This eac o has been success ully 131
used in he py olysis and gasi ica ion o di e en was e ma e ials, such as biomass 132
[5,36], plas ics [37,38] and y es [39,40]. The de ailed design and main dimensions o 133
he CSBR can be ound elsewhe e [17,33]. The empe a u e o he eac o is con olled 134
by wo he mocouples loca ed inside he eac o , one in he bed annulus and he o he 135
one close o he wall. P io en e ing he e o ming eac o he p oduc s eam is cleaned 136
by a high-e iciency cyclone, o e aining he ine sand and cha pa icles en ained 137
om he CSBR. 138
In o de o a oid he blocking o he low due o he coke deposi ion, which has been 139
obse ed in a ixed bed eac o [34], he e o ming s ep has been ca ied ou in a 140
luidized bed eac o , whose dimensions a e 38.1 mm o diame e and 440 mm o 141
leng h. The empe a u e o he luidized bed eac o is con olled by a he mocouple 142
placed inside he ca alys bed. The ola iles om luidized bed eac o ci cula e h ough 143
8
a sin e ed s eel il e (5 µm) o e ain ca alys ines elu ia ed om he luidized bed, 144
wi h his amoun being bellow 5% o he ca alys used in he expe imen s pe o med. 145
All he in e connec ion pipes, cyclone, il e and bo h eac o s a e loca ed inside an o en 146
kep a 270 ºC, which ensu es ha he s eam and p oduc s a e no condensed in he 147
connec ions be ween he eac o s. 148
Finally, he condensa ion sys em o he plan ensu es o al condensa ion and e en ion o 149
non- eac ed s eam and biomass and HDPE de i ed p oduc s, which consis s o a 150
condense and a coalescence il e . 151
2.3. Expe imen al condi ions 152
The luid dynamic equi emen s o he wo eac o s in-line wi h a common gas low 153
ha e condi ioned he s eam low and he pa icle size o he sand in he CSBR and he 154
pa icle size o bo h ca alys and sand in he luidized bed eac o . Thus, 3 mL min-1 o 155
wa e low has been es ablished, which co esponds o a s eam low o 3.73 NL min-1, 156
and he bed consis s o 50 g o sand in he py olysis s ep, wi h pa icle size being 157
be ween 0.3-0.35 mm. The uns ha e been ca ied ou in con inuous egime by eeding 158
0.75 g min-1 o biomass and HDPE mix u es. Mo eo e , he py olysis s ep has been 159
ca ied ou a 500 ºC, which has been p o ed in p e ious s udies o be a sui able 160
empe a u e o biomass [29] and HDPE py olysis [30] in a CSBR. 161
In he same way, a e luid dynamic es s, a bed o 25 g o ca alys and sand mix u e 162
has been es ablished o he luidized bed eac o , wi h pa icle size being be ween 0.4-163
0.8 mm o he ca alys and 0.3-0.35 mm o he sand, in o de o wo k wi h a ela i e 164
eloci y 3 o 4 imes highe han minimum luidiza ion eloci y. These condi ions 165
gua an ee he comple e luidiza ion o he bed, e en when he coke con en o he 166
ca alys is high. 167
9
The e ec o eeding di e en HDPE/biomass mass a ios in he py olysis- e o ming 168
p ocess has been s udied. Thus, HDPE/biomass mix u es o 25/75, 50/50 and 75/25 w 169
% ha e been es ed and he esul s ha e been compa ed wi h hose o pu e biomass and 170
plas ic eeds. The empe a u e o he e o ming s ep was 700 ºC, gi en ha is he 171
minimum empe a u e needed o he comple e con e sion o ola iles om HDPE 172
py olysis [33]. The o he ope a ing condi ions o he p ocess a e he ollowing: 16.7 gca 173
min g eeding-1 (co esponding o 12.5 g o ca alys ) and s eam/(biomass+HDPE) mass 174
a io o 4. The uns ha e been epea ed se e al imes (a leas 3) unde he same 175
condi ions in o de o gua an ee ep oducibili y o he esul s. 176
2.4. P oduc analysis 177
The ola ile p oduc s o he e o ming s ep ha e been analysed on-line by means o a 178
GC Agilen 6890 p o ided wi h a HP-Pona column and a lame ioniza ion de ec o 179
(FID). The sample has been ans e ed om he eac o o he GC by means o a 180
he mos a ed line a 280 ºC, in o de o a oid he condensa ion o hea y compounds. 181
Mo eo e , he non-condensable gases ha e been analyzed on-line in a mic o GC 182
(Va ian 4900) once he gases we e comple ely ee o s eam and non- eac ed liquid 183
p oduc s. 184
The coke con en deposi ed on he e o ming ca alys has been de e mined a he end o 185
con inuous expe imen s by empe a u e p og ammed oxida ion (TPO) in a 186
he mobalance TGA Q5000 (TA Ins umen s), which was connec ed on-line o a mass 187
spec ome e The mos a (Balze s Ins umen s), gi en ha he Ni o he ca alys is 188
oxidized oge he wi h he ca bonaceous coke, and acco dingly, he ca bon dioxide 189
o ma ion mus be moni o ed o de e mine TPO cu es. The ollowing p ocedu e has 190
been ca ied ou : (i) signal s abiliza ion wi h He s eam (10 mL min-1) a 100 ºC, (ii) 191
16
con en o bo h eeds ocks. Gi en he ela ion be ween he esul s o he e o ming s ep 313
and he composi ion o he inle s eam o he eac o , he lineal inc ease o he eac ion 314
indexes plo ed in Figu e 3 shows ha he e is no a signi ican syne ge ic e ec o he 315
HDPE co- eeding o he py olysis eac o on e o ming p oduc s composi ion. 316
Ne e heless, his e ec has been obse ed by o he au ho s, who ha e e i ied ha he 317
co-py olysis o plas ics and biomass ha e a no iceable e ec on bio-oil composi ion 318
[50,51]. 319
In o de o compa e he esul s in Figu e 3 wi h he li e a u e, i should be poin ed ou 320
ha he py olysis and in-line s eam e o ming o biomass and plas ic mix u es is limi ed 321
o he s udies o he esea ch g oup headed by p o . Williams. Thus, he in luence o 322
biomass/polyp opylene a io ( eeding be ween 5 and 20 w % o PP) has been s udied 323
by Al a ez e al. [20] in he ba ch py olysis- e o ming p ocess on a Ni/Al2O3 ca alys , 324
ob aining a maximum hyd ogen p oduc ion o 5.5 w %, when 20 w % o PP was used. 325
Kumagai e al. [28] s udied a Ni-Mg-Al-Ca ca alys wi h di e en Ca con en s and 326
calcina ion empe a u es and he highes hyd ogen p oduc ion o 6.0 w % was ob ained 327
using a calcina ion empe a u e o 500 ºC. These esul s a e signi ican ly lowe in 328
ela ion o hose ob ained in his s udy, which is a consequence o he con inuous mode 329
used in his wo k. On he o he hand, he esul s epo ed by o he au ho s o he 330
indi idual alo iza ion o biomass and plas ics by con inuous py olysis and in-line 331
e o ming a e in he same ange o hose ob ained in he p esen s udy [15,16,52,53]. 332
The s eam co-gasi ica ion o biomass and polyole ins s udies e eal he exis ence o 333
posi i e and e en syne ge ic e ec s o e he hyd ogen and gas p oduc ion and a 334
con en in he gas p oduc [21-24,54]. Howe e , he hyd ogen p oduc ion ob ained in 335
his wo k is highe han hose epo ed in he s eam gasi ica ion p ocesses, be ween 4 336
17
and 7 w % o biomass gasi ica ion [5,55,56] and in he 6-15 w % ange in he 337
gasi ica ion o polyole ins [37,57]. 338
339
Figu e 3. E ec o HDPE co- eeding in he biomass py olysis and in-line 340
e o ming o e gas and hyd ogen p oduc ions and eac ed s eam. 341
Figu e 4 shows he e ec o HDPE con en in he eed on indi idual p oduc yields 342
(g aph a) and gas composi ion (g aph b). As i can be obse ed, he e a e no able 343
di e ences in H2, CO2 and CO yields dis ibu ion. In his way, H2 and CO2 yields 344
dec ease when HDPE con en is inc eased om 0 o 100 w %, om 93.2 o 85.7 % and 345
87.2 o 67.4 %, espec i ely. Ne e heless, CO yield inc eases om 12.5 o 29.3 %. 346
These esul s e idence he e ec o he highe ca bon con en o HDPE and he e o e, 347
he highe amoun o ca bon o be e o med. Mo eo e , as he same space ime is used 348
o all expe imen s, highe yield o CO and lowe yield o CO2 a e ob ained when 349
18
HDPE con en in he eed is inc eased, due o he lowe ex en o WGS eac ion (eq. 350
10). I can also be obse ed ha CH4 and C2-C4 yields a e e y low in all cases s udied, 351
al hough he e is a sligh inc ease o hese yields when HDPE con en is inc eased om 352
0 o 100 w %, om 0.2 o 0.8 % o CH4 and om 0.1 o 0.7 % o C2-C4 ac ion. 353
I can be poin ed ou ha H2 concen a ion inc eases wi h HDPE con en in he eed 354
un il 72 ol % when 75 w % o HDPE is used (Figu e 4b), due o he highe con en o 355
hyd ogen and lack o oxygen in he plas ics composi ion. Ne e heless, he lowe 356
e ec i e space ime when HDPE is co- ed gi es way o lowe CO2 and highe CO 357
concen a ions in he gaseous ac ion, which change om 30.2 o 20.9 % and om 4.3 358
o 9.1 %, espec i ely, in he ange o HDPE con en s udied. The hyd ogen 359
concen a ions epo ed by Al a ez e al. [20] in he py olysis- e o ming o biomass/PP 360
mix u es we e below hose ob ained in he p esen s udy, wi h he maximum alue 361
being o 52.1 ol. % o 20 w % o PP in he eed. The hyd ogen concen a ions 362
ob ained in he s eam co-gasi ica ion o hese eeds ocks a e also below, in he 40 o 55 363
% ange [21,23,58]. 364
19
365 366
Figu e 4. E ec o HDPE co- eeding in he HDPE/biomass mix u e ed o e he 367
indi idual p oduc s yields (a) and gaseous p oduc concen a ions (b), in he py olysis 368
and in-line e o ming p ocess. 369
3.2.2. Ca alys deac i a ion 370
In o de o s udy he e ec o eed composi ion on he e o ming ca alys deac i a ion, 371
he e olu ion wi h ime on s eam o con e sion (Figu e 5) and gas composi ion (Figu e 372
6) in he e o ming s ep has been analyzed. Figu e 5 shows ha he deac i a ion 373
beha iou s ongly depends on he eed composi ion. Thus, he con e sion in e o ming 374
s ep is below 60 % o pu e biomass a e 120 min o con inuous ope a ion, whe eas is 375
highe han 90 % wi h pu e HDPE o he same ime on s eam. Mo eo e , i is 376
no ewo hy he linea decay o ca alys ac i i y o HDPE, while in he case o biomass, 377
he ac i i y is main ained o he i s 60 minu es and ollows an acu e dec easing end 378
abo e 75 minu es. 379
The ini ial s able con e sion pe iod obse ed wi h pu e biomass canno be ela ed o he 380
lowe deac i a ion wi h his eed, and can be explained due o he space ime alue in 381
excess wi h espec o he equilib ium one. Consequen ly, he highe dec ease o he 382
ac i i y obse ed in Figu e 5 o biomass is especially ele an , aking in o accoun he 383
20
highe e ec i e space ime (a ound 2.5 imes highe ) o his eeding. Thus, hese 384
esul s clea ly shows ha he oxygena ed compounds and a oma ic ings con aining 385
compounds (as phenols) o med in biomass py olysis p o oked a much as e 386
deac i a ion han ha caused by long chain hyd oca bons om HDPE py olysis. In he 387
same line, Cze nik e al. [49] ema ked ha oxygena ed compounds ha e mo e ma ked 388
endency han ha o hyd oca bons o o m ca bonaceous deposi s on he ca alys 389
su ace and acco dingly, p o oke a as e ca alys deac i a ion. In ac , he se e e 390
e o ming ca alys deac i a ion has been p e iously epo ed by o he au ho s in he 391
e o ming o biomass de i ed oxygena es [59-61]. 392
When di e en HDPE/biomass mass a ios a e used, he con e sions e olu ions 393
obse ed a e be ween hose o wo pu e eedings, which con i ms ha he plas ic co-394
eeding has a no able e ec on a enua ion o ca alys deac i a ion. 395
396
21
Figu e 5. E ec o HDPE con en in HDPE/biomass mix u e ed o e he e olu ion 397
wi h ime on s eam o con e sion in he e o ming s ep in py olysis and in-line 398
e o ming p ocess. 399
Figu e 6 shows he e olu ion o gas composi ion wi h ime on s eam o h ee di e en 400
eedings: pu e biomass (a and b), a HDPE/biomass mix u e o 50/50 w % (c), and pu e 401
HDPE (d). When pu e biomass is used, H2 concen a ion dec eases om 65 o 55 ol % 402
in 120 min on s eam (Figu e 6a), whe eas i is main ained a ound 70 ol % when pu e 403
HDPE is ed (Figu e 6d). Mo eo e , o pu e biomass alo iza ion CO concen a ion 404
inc eases om 5 o 16 ol % and he opposi e occu s o ha o CO2, which dec eases 405
om 30 o 24 ol % a e 120 min on s eam. This e olu ion o CO and CO2 406
concen a ions e eal a signi ican deac i a ion o he ca alys owa ds he WGS 407
eac ion (eq. 10). On he o he hand, he concen a ions o CO and CO2 emain cons an 408
o pu e HDPE (Figu e 6d) and only change abo e 75 min on s eam o di e en 409
HDPE/biomass mix u es (Figu e 6c). The e o e, when HDPE is co- ed, he deac i a ion 410
o he WGS eac ion is conside ably a enua ed. 411
As discussed p e iously, he concen a ion o main gaseous p oduc s o med by 412
seconda y c acking eac ions, i.e. CH4 and C2-C4 ac ion (e hylene, e hane, p opylene 413
and p opane, mainly), a e e y low o di e en eeds s udied due o he ini ial ca aly ic 414
ac i i y o bo h oxygena ed compounds and hyd oca bons e o ming (eq. (8-9)) and 415
WGS eac ion (eq. (10)). Howe e , when he ca alys is deac i a ed, CH4 and C2-C4 416
ac ion concen a ions inc ease sligh ly, which is shown in de ail in Figu e 6b o 417
biomass alo iza ion. A simila al hough less ma ked end can be seen when HDPE is 418
co- ed ( esul s no shown). 419
22
420
421
422
423
Figu e 6. E olu ion wi h ime on s eam o gas composi ion in he py olysis-424
e o ming o pu e biomass (a and b), HDPE/biomass mix u e o 50/50 w % (c) and 425
pu e HDPE (d). 426
3.3.4. Cha ac e iza ion o he coke deposi ed 427
In o de o explain he e ec o eeding composi ion on he e olu ion o con e sion 428
wi h ime on s eam, he coke deposi ed on he ca alys has been cha ac e ized by 429
empe a u e p og ammed oxida ion (TPO) and ansmission elec on mic oscopy (TEM) 430
images. Figu e 7 shows he TPO p o iles o deac i a ed ca alys o h ee eedings: pu e 431
biomass, a mix u e o HDPE/biomass o 50/50 w % and pu e HDPE. In he 432
alo iza ion o pu e biomass, a main peak a a ound 600 ºC is obse ed, which 433
23
co esponds o a polya oma ic and s uc u ed coke, wi h a small shoulde a a ound 425 434
ºC ela ed o coke whose combus ion is ac i a ed by he Ni me allic si es. This coke can 435
be ela ed o he ca bon whiske s epo ed by T ane-Res up and Jensen [2] in he s eam 436
e o ming o u u al and guaiacol a 600 ºC. On he o he hand, in he s eam e o ming 437
o he py olysis p oduc s o pu e HDPE, a main peak a 580 ºC wi h a shoulde a 450 438
ºC was obse ed. The main peak co esponds o a s uc u ed and ilamen ous coke 439
simila o ha ob ained by Wu and Williams [62] and Acomb e al. [63] in he 440
e o ming o polyp opylene (PP), as i could be obse ed in TEM images which will be 441
discussed la e . The sligh di e ence o maximum empe a u e (605 ºC o hese 442
au ho s) can be a ibu ed o he highe po osi y o he ca alys used by hem, which 443
complica es he combus ion o he coke ac ion which blocks he po es o he ca alys . 444
The peaks o he mix u e o biomass and HDPE a e be ween he TPO p o iles ob ained 445
o pu e biomass and HDPE. 446
447
24
Figu e 7. Compa ison o TPO p o iles o coke deposi ed in he ca alys o pu e 448
biomass, HDPE/biomass mix u e o 50/50 w % and pu e HDPE alo iza ion. 449
Figu e 8 shows TEM images o he deac i a ed ca alys o di e en eedings: pu e 450
biomass (a), HDPE con en in he eed o 25 w % (b), 50 w % (c), 75 w % (d) and 451
pu e HDPE (e). As obse ed, di e en s uc u e and na u e o he coke can be 452
dis inguished, which is a consequence o he di e en composi ion o he ola iles ed 453
in o he e o ming s ep. In he images, Ni ac i e si es can be iden i ied as da ke a eas 454
and Figu e 8a shows ha he coke deposi ed is mainly non-s uc u ed o pu e biomass 455
alo iza ion, co e ing comple ely he Ni c ys als (encapsula ing coke). The p esence o 456
amo phous and non-s uc u ed coke has also been obse ed in he ca aly ic s eam 457
e o ming o me hane [64] di e en hyd oca bons [62,65] and oxygena ed compounds 458
[2,66]. The high combus ion empe a u e o his coke obse ed in TPO p o ile (600 ºC) 459
shows ha i is a e y condensed coke. 460
Howe e , he s uc u e o he coke changes when HDPE is co- ed, wi h i s na u e being 461
mo e ilamen ous as HDPE con en in he eed is inc eased. This na u e o ilamen ous 462
coke has been obse ed p e iously in he e o ming o polyole ins py olysis p oduc s 463
[62,63]. 464
Consequen ly, he as e deac i a ion obse ed o biomass and he a enua ion o he 465
deac i a ion when HDPE is co- ed can be a ibu ed o he di e en na u e o he coke. 466
The amo phous coke o med in he e o ming o oxygena ed compounds de i ed om 467
biomass py olysis encapsula es he Ni cen es, causing as deac i a ion o he ca alys , 468
whe eas he s uc u ed and ilamen ous coke o med mainly in he e o ming o 469
hyd oca bons de i ed om HDPE py olysis do no block he Ni ac i e cen es, al hough 470
i s p og essi e deposi ion complica es he gas low o he eac an s in o Ni pa icles 471
25
[65,67]. This in e p e a ion o he deac i a ion esul s is consis en wi h he as 472
deac i a ion o he ca alys in he e o ming o oxygena ed compounds (DME, e hanol 473
and bio-oil), a ibu ed o he encapsula ion o Ni cen es by he amo phous coke o med 474
by condensa ion o in e media e oxygena es [8,66,68,69]. The deac i a ion is lowe in 475
he e o ming o hyd oca bons p oduced in he py olysis o polyole ins, whe e he coke 476
is mainly s uc u ed [62,63]. 477
32
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