ene gies
A icle
Expe imen al In es iga ion on CO2Me hana ion
P ocess o Sola Ene gy S o age Compa ed o
CO2-Based Me hanol Syn hesis
Bea ice Cas ellani 1,* , Albe o Ma ia Gambelli 1, Elena Mo ini 1, Benede o Nas asi 2,
And ea P esciu i 1, Mi ko Filipponi 1, And ea Nicolini 1and Fede ico Rossi 1
1Enginee ing Depa men , Uni e si y o Pe ugia, CIRIAF, Via G. Du an i 67, 06125 Pe ugia, I aly;
[email p o ec ed] (A.M.G.); [email p o ec ed] (E.M.); and ea.p [email p o ec ed] (A.P.);
[email p o ec ed] (M.F.); [email p o ec ed] (A.N.); ede ico. [email p o ec ed] (F.R.)
2Depa men o A chi ec u al Enginee ing & Technology, En i onmen al & Compu a ional Design Sec ion,
TU Del Uni e si y o Technology, Julianalaan 134, 2628 BL Del , The Ne he lands;
[email p o ec ed]
*Co espondence: [email p o ec ed]
Recei ed: 18 May 2017; Accep ed: 21 June 2017; Published: 27 June 2017
Abs ac :
The u iliza ion o he cap u ed CO
2
as a ca bon sou ce o he p oduc ion o ene gy
s o age media o e s a echnological solu ion o o e coming c ucial issues in cu en ene gy sys ems.
Sola ene gy p oduc ion gene ally does no ma ch wi h ene gy demand because o i s in e mi en
and non-p og ammable na u e, en ailing he adop ion o s o age echnologies. Hyd ogen cons i u es
a chemical s o age o enewable elec ici y i i is p oduced by wa e elec olysis and is also he
key eac an o CO
2
me hana ion (Saba ie eac ion). The u iliza ion o CO
2
as a eeds ock o
p oducing me hane con ibu es o alle ia e global clima e changes and seques a ion ela ed p oblems.
The p oduced me hane is a ca bon neu al gas ha i s in o exis ing in as uc u e and allows issues
ela ed o he a o emen ioned in e mi ency and non-p og ammabili y o sola ene gy o be o e come.
In his pape , an expe imen al appa a us, composed o an elec olyze and a ubula ixed bed
eac o , is buil and used o p oduce me hane ia Saba ie eac ion. The objec i e o he expe imen al
campaign is he e alua ion o he p ocess pe o mance and a compa ison wi h o he CO
2
alo iza ion
pa hs such as me hanol p oduc ion. The in es iga ed p essu e ange was 2–20 ba , ob aining a
me hane olume ac ion in ou le gaseous mix u e o 64.75% a 8 ba and 97.24% a 20 ba , wi h
con e sion e iciencies o , espec i ely, 84.64% and 99.06%. The me hanol and me hane p ocesses
we e compa ed on he basis o an ene gy pa ame e de ined as he spen ene gy/s o ed ene gy. I is
highe o he me hanol p ocess (0.45), wi h espec o he me hane p oduc ion p ocess (0.41–0.43),
which has a highe ene gy s o age capabili y.
Keywo ds:
CO
2
me hana ion; ca bon ecycling; ene gy s o age; sola ene gy; P2G Powe o Gas;
syn he ic uels
1. In oduc ion
The en i onmen al and economic sus ainabili y o u u e ene gy sys ems needs o deal wi h
se e al challenges and echnical aspec s. The inc ease o he sha e o Renewable Ene gy Sou ces
(RES) o e came 25% o ene gy gene a ion, implying new issues ela ed o balancing, secu i y, and
p oduc ion-consump ion p o iles ma ching [
1
]. While he exis ing elec ici y ne wo k esul s we e
la gely unchanged, sola and wind powe ha e seen a la ge expansion du ing he las decade.
This means ha he cu en elec ical ene gy sys em is un-op imized and needs adjus men s.
The elec ici y ansmission sys em needs o be adap ed om he la ge scale p oduc ion acili ies
used oday o smalle and locally dis ibu ed ene gy p oduc ion si es. Ene gy s o age solu ions a e
Ene gies 2017,10, 855; doi:10.3390/en10070855 www.mdpi.com/jou nal/ene gies
Ene gies 2017,10, 855 2 o 13
being implemen ed o compensa e o he luc ua ions in in e mi en ene gy p oduc ion [
2
]. Recen
in es iga ions indica e ha he in oduc ion o ene gy s o age echnologies [
3
,
4
] and ou es can help o
s abilize powe ou pu while also enhancing he eliabili y o enewable ene gy p oduc ion [5].
Then, excess elec ici y om enewable ene gy sou ces (RES) has o be s o ed in di e en ways
such as in ba e ies o be used o p oduce hyd ogen (H
2
) [
6
]. Sola ene gy is he i s enewable
sou ce associa ed wi h H
2
p oduc ion as a s o age medium, especially o he shi om day ime
p oduc ion o nigh ime demand o he seasonal shi om summe o e p oduc ion o win e needs [
7
].
Hyd ogen-based ene gy s o age sys ems ha e p o en o be one o he o he mos p omising ene gy
s o age echniques [
8
]. A u he p oo is he ex ensi e esea ch in s and-alone applica ion, such as
in [9].
Resea ch on hyd ogen echnologies has been acing many challenges, including p oduc ion,
s o age, and anspo a ion in as uc u e [
10
,
11
]. I is no ewo hy ha hyd ogen is al eady p o en
in cu en gas g id pipelines and e en, in ce ain ac ions, used in well-es ablished end-use de ices
such as boile s and cooke s [
12
]. Indeed, new measu emen p ocedu es o con en ional sys ems we e
s udied [
13
]. Thus he e a e nume ous e o s unde way o de elop be e ways o p oducing H
2
,
especially ia he enhanced elec olysis o wa e [14].
Mo eo e , conce ns abou clima e change as well as ossil uel usage mo i a e CO
2
seques a ion
echnologies in o de o limi he emissions wi hou subs an ially changing he well-p o en
echnologies al eady in use [
15
]. Ne e heless, he main d awback is o handle he cap u ed and s o ed
CO2as a was e o be disposed o [16].
To o e come his ision, eco e ing and combining wo was e ma e ials such as enewable excess
elec ici y and CO
2
is he key d i e o u he os e RES p oduc ion and inc ease i s sha e in he
na ional ene gy mix as well as o make ou ene gy ca bon oo p in smalle by means o syn he ic uel
p oduc ion. RES capaci y i ming can be mi iga ed and sol ed by means o eeding elec olyze s o
p oduce H
2
o add ess a single ene gy ne wo k o o c ea e syne gies among elec ici y, hea ing [
17
],
and e en anspo pu poses. Thus H
2
can be combined by he mo-chemical p ocesses wi h he CO
2
coming om exis ing powe plan s equipped wi h Ca bon Cap u e and S o age (CCS) solu ions. In his
way, he esul ing uels could again eed he ene gy gene a ion side and, consequen ly, educe he
ene gy- ela ed emissions [
18
]. Thus, H
2
-CO
2
combina ion-de i ed uels would, by connec ing di e en
ene gy sec o s, make he sys em mo e lexible and ecycle CO
2
as a ca bon sou ce and, by implemen ing
elec olyze s, help balance he G id and acili a e high RES powe in eg a ion owa ds sma ene gy
sys ems [19].
Mo eo e , CO
2
can be cap u ed by a biomass gasi ica ion p ocess and ecombined wi h he
H
2
- ich gas [
20
] o ob ain uels and ole ins in poly-gene a ion sys ems [
21
]. Among hose p oduc s,
me hanol is ecei ing a en ion by esea che s as a possible ca bon ecycling s a egy [
22
], bu u he
analysis is equi ed when he p oduc ion and he ene gy gene a ion is e alua ed om an economic
pe spec i e wi hin he new mul i-ene gy sys ems layou [23].
The main issue o his is ela ed o he absence o dedica ed in as uc u es, while, o na u al gas
hese a e well es ablished and capilla y in many coun ies. Hence, he in e es in syn he izing me hane
by he combina ion o H
2
-CO
2
is suppo ed by he chance o use i as a aw ma e ial o o he uses o
p oduc s [24].
This pape ocuses on he abo e-men ioned s a egy; he u iliza ion o CO
2
as a eeds ock, oge he
wi h enewable hyd ogen, o syn he ic uel p oduc ion. An expe imen al appa a us was assembled
and used o an in es iga ion on he CO
2
me hana ion o syn he ic me hane p oduc ion. Me hane is
al eady a co e supply o ene gy sys ems wo ldwide, wi h huge amoun s o physical and knowledge
in as uc u e [
25
]; i is easy o s o e and boas s an ene gy densi y a ound h ee imes g ea e han
hyd ogen. The expe imen al esul s we e used o compa e he me hane p oduc ion p ocess wi h he
me hanol p oduc ion p ocess. Fo bo h he ou es, he a io be ween he ene gy spen h oughou he
p ocess o p oduce he uel and he ene gy s o ed in he uel is calcula ed.
Ene gies 2017,10, 855 3 o 13
This pa ame e is in oduced o quan i y he pe o mances o he abo e s o age sys ems wi h
he aim o de e mining he bes a ailable echnology h oughou he whole chain, om hyd ogen
p oduc ion o he deli e y o uel o inal use.
Fo eseeable oppo uni ies a e ela ed o he mixing o syn he ic uel coming om di e en
p oduc ion chains so as o in eg a e enewable elec ici y excess, lue gas, and Ca bon Cap u e
and S o age solu ions oge he wi h o hcoming bio- e ine ies concep s and p omising algae
ea men s [26].
2. P ocess Desc ip ion
2.1. Me hane P oduc ion
The me hane-based p ocess is based on he ans o ma ion o elec ical ene gy o syn he ic
na u al gas by means o an elec olyze and c ea ing he condi ions sui able o he Saba ie eac ion.
The Saba ie eac ion combines ca bon dioxide and hyd ogen o p oduce me hane and wa e in he
eac ion (Equa ion (1)):
CO2+4H2→CH4+2H2O∆H=−165 kJ/mol (1)
This eac ion is exo he mic and is limi ed by he he modynamic equilib ium. The ope a ing
empe a u es a e ypically a ound 250–400
◦
C. The Saba ie eac ion has been widely in es iga ed
because o i s po en ial o allow CO
2
o be cap u ed and ecycled, o s o e ene gy om enewable
sou ces used o eed he elec olyze o p oducing H
2
, and, subsequen ly, o CH o be injec ed and
s o ed in exis ing na u al gas g ids o used as a uel o anspo a ion.
The p ocess is mainly composed by wo s ages; (i) an elec olyze , supplied by he elec ic ene gy
and (ii) a Saba ie eac o o he con e sion o hyd ogen in o me hane (Figu e 1).
Figu e 1. Scheme o he CO2me hana ion p ocess.
2.2. Me hanol P oduc ion
Me hanol is a p ima y liquid pe ochemical ha has a ac ed conside able a en ion in he
chemical and ene gy indus ies. The p oduc ion o me hanol used as a uel and a chemical has been
p oposed in he me hanol economy by [27].
Two main aspec s o his new economy a e unde lined. CO
2
can be cap u ed and ans o med in o
me hanol allowing he cos s o CO
2
seques a ion o be a oided and CO
2
o be e ec i ely ecycled.
The p ocess also has g ea indus ial signi icance because he p oduc , me hanol, can se e as a liquid
uel as well as a aw ma e ial o he syn hesis o o he o ganic compounds [28].
Ene gies 2017,10, 855 4 o 13
The eac ion o he con e sion o he CO2 o CH3OH is as in Equa ion (2):
CO2+H2→CH3OH +2H2O∆H=−49.5 kJ/mol (2)
The me hanol-based p ocess is based on he use o hyd ogen in he con e sion o elec ical ene gy
o me hanol by he use o CO
2
. Elec ical ene gy and wa e a e included in he p ocess o p oduce
hyd ogen ha eac s wi h CO2and esul s in me hanol.
The eac ion is highly exo he mic; he e o e i is acili a ed a a high p essu e and low empe a u e.
CO
2
is con e ed o me hanol in he empe a u e ange o 250–300
◦
C and p essu e ange o 5–10 MPa
using CuO/ZnO/Al
2
O
3
as a ca alys . The p oduced me hanol CH
3
OH is liquid a ambien empe a u e
and can be sa ely s o ed and handled.
The p ocess is mainly composed by wo s ages: (i) an elec olyze , supplied by elec ical ene gy
and (ii) a eac o o he con e sion o CO2and hyd ogen in o me hanol (Figu e 2).
Figu e 2. Scheme o me hanol-based p ocess.
3. Expe imen al Sec ion
This sec ion desc ibes he expe imen al se up and gi es da a abou he used ma e ials.
3.1. Expe imen al Se up
The expe imen al appa a us includes he ollowing sec ions; an elec olyze , a CO
2
-H
2
mixing
sec ion, a hea ing sec ion, a Saba ie eac o , and a wa e sepa a ion sec ion. Hyd ogen is p oduced by
an elec olyze , supplied by E edue S l (G Se ies H0
3
, E edue S l, Li o no, I aly), and is hen mixed
wi h CO
2
acco ding o he s oichiome ic p opo ion (Equa ion (1)). The elec olyze , shown in Figu e 3,
ope a es a a p essu e o 2.5 ba and p oduces a maximum hyd ogen low a e o 2.5 Nm
3
/h. H
2
/CO
2
mixing occu s h ough a T piping. Gas lows a e measu ed and con olled by digi al he mal mass low
me e s (El- low Se ies), supplied by B onkho s . Mass low me e s ha e he ollowing cha ac e is ics:
low a e in he ange om 0.08 o 4 Nm
3
/h; maximum inle p essu e 30 ba ; con ol s abili y lowe
han
±
0.1% Full Scale (FS); and empe a u e sensi i i y lowe han 0.05% FS/
◦
C. The gaseous mix u e
lows wi h a a e o 0.63 m
3
/h h ough he wo chambe s (p e-hea e and eac o ). The hea ing sec ion
is cons i u ed by a ho izon al ube wi h a nominal diame e o 25.4 mm (1 inch) and a leng h o 300 mm
on which h ee mine al insula ed band hea e s a e moun ed. I is equipped wi h in e nal empe a u e
and p essu e senso s and is connec ed o he Saba ie eac o h ough a ga e al e (supplied by
Swagelok, model SS-6nbs 10 mm-G), sui able o a maximum empe a u e o 640 ◦C.
Ene gies 2017,10, 855 5 o 13
Figu e 3. Elec olyze G Se ies H03supplied by E edue S l.
The Saba ie eac o is a cylind ical AISI 304 s ainless mono ubola ixed bed eac o wi h a
diame e o 1 inch (25.4 mm) and a leng h o 300 mm. I is illed wi h Ni ca alys pelle s. The eac o is
equipped wi h one p essu e senso and one empe a u e senso . The p essu e senso is a Bou don Tube
p essu e gauge (NG100 se ies, Kobold Mess ing GmbH, Ho heim am Taunus, Ge many), supplied by
Kobold (accu acy class 1), wi h a p essu e ange om 0 o 40 ba . The empe a u e senso s a e mine al
insula ed ype K he mocouples (accu acy class 1).
Fo educing hea loss and o sa e y easons, he wo chambe s we e coa ed wi h he mo-insula ing
ma e ial and an ex e nal p o ec i e aluminium coa ing. The band hea e s a e p o ided by Wa low and
ha e he ollowing echnical cha ac e is ics: leng h o 63.5 mm, in e nal diame e o 32 mm, powe
o 700 W, and in e nal K he mocouple. The band hea e s a e con olled by P opo ional–In eg al–
De i a i e (PID) egula o s moun ed on a con ol panel.
The eac ion p oduc s pass hen h ough he wa e sepa a ion sec ion, cons i u ed by a 6 mm
s ainless s eel cooling coil imme sed in a he mos a ic ba h. In his sec ion, wa e apo condensa ion
occu s, and he gaseous incondensable p oduc s a e collec ed o gasc oma og aphic analysis.
The ins alla ion o he expe imen al appa a us is shown in Figu e 4. Vol age signals om he senso s
a e collec ed by so wa e o da a acquisi ion on a pe sonal compu e e e y 5 s.
Figu e 4. Expe imen al appa a us o CO2me hana ion p ocess.
Ene gies 2017,10, 855 6 o 13
Gas-ch oma og aphic analyses a e ca ied ou wi h a gas-ch oma og aphe supplied by VARIAN
(model VARIAN CP 4900 Mic o-GC, VARIAN Inc., Middelbu g, The Ne he lands). I is con igu ed
o use helium (He) and A gon (A ) as ca ie s, wi h a minimum pu i y o 99.995%. The
gas-ch oma og aphe has: (i) ou columns, including Molsie e MS5A, wo king wi h A gon and
Po aplo PPU, CPSil 5CB, Molsie e MS5A, wo king wi h Helium; and (ii) a he mal conduc ibili y
de ec o (TCD). This de ec o esponds o di e ences in he mal conduc i i y be ween he ca ie gas
and each componen o he analyzed gas mix u e. When a pa icula componen goes h ough he
TCD, he signal shows a de ia ion, he en i y o which is p opo ional o ha componen concen a ion.
The measu emen s, pe iodically ca ied ou wi h ce i ied mix u es, show ha he unce ain ies
a e always lowe han 1%, wi h an excellen epea abili y on sequences o samples [16].
3.2. Ma e ials
The Saba ie eac ion is ca alyzed by Ni-based ca alys supplied by BASF (G1-85 T5x5, BASF
Co po a ion, E ie, PA, USA). BASF G1-85 Ni/Z /Al
2
O
3
Ca alys con ains me allic nickel and zi conium
on an alumina suppo : he nickel con en in weigh is 50%. The ca alys is supplied in 1 mm
×
4 mm
cylind ical pelle s, wi h a speci ic su ace a ea o 123 m
2
/g. The ca alys is sui able o he me hana ion
o syn hesis gas ecei ed om coal o biomass gasi ica ion o p oduce Subs i u e Na u al Gas (SNG).
Unde p ope condi ions, li e imes o mo e han ou yea s can be achie ed.
The ca alys mus be con e ed in o he ac i e o m o u n nickel o he me allic s a e, emo ing
he oxide laye om he ca alys su ace. Ac i a ion was achie ed by he ollowing ac i a ion p ocess,
consis en wi h [
29
]. Ini ially, pu e ni ogen was luxed inside he eac o a a p essu e o 1.5 ba , wi h
he aim o emo ing all he ai p esen inside he wo cylinde s. The in e nal empe a u e was b ough
o a alue o 220
◦
C, wi h a hea ing a e o 60
◦
C/h. Then, pu e hyd ogen was luxed a a p essu e o
2 ba h ough he ca alys , and he sys em was hea ed o 430
◦
C wi h a hea ing a e equal o 120
◦
C/h.
Ni ogen and hyd ogen (99.999% Alphagaz 1) we e supplied by Ai Liquide I alia Se ice (Ai Liquide
I alia S.p.A., Milan, I aly). The ca alys ’s ac i a ed s a e can be isually checked by a colo change om
black, ypical o nickel oxide, o da k g ey, ypical o molecula nickel.
4. Expe imen al Resul s
The expe imen al campaign consis s o 14 es s. The ope a ing pa ame e s such as he CO
2
/H
2
a io, low condi ions, empe a u e, and p essu e a e shown in Table 1. Tes s 1 o 10 we e ca ied ou
using he s oichiome ic H
2
/CO
2
a io o ou , while Tes s 11 o 14 wi h a H
2
/CO
2
a io equal o one.
Tes s 1 o 6 and Tes s 11 o 14 we e ca ied ou con inuously, wi h a hou ly space eloci y, de ined
as he a io be ween he low a e and he eac o olume, o 414.4 h
−1
. Tes s 7 o 10, ins ead, we e
ca ied ou in ba ch condi ions; his means ha he inle CO
2
/H
2
mix u e was uploaded inside he
eac o un il he expe imen al condi ions o empe a u e and p essu e we e eached. The eac o was
hen sealed, and pa ame e s we e moni o ed. The expe imen al es s we e ca ied ou wi h an ini ial
in e nal empe a u e in he ange om 250
◦
C and 451
◦
C and an ini ial in e nal p essu e om 2 ba o
20 ba . The empe a u e and p essu e alues in Table 1a e e e ed o as he in e nal ini ial condi ions,
measu ed inside he eac o a he s eady s a e. The eac ion ime, which is conside ed o s a om he
empe a u e inc ease, was equal o 30 min o all he es s.
To be e unde s and he empe a u e end du ing he expe imen al es s, he p o ile o he
in e nal empe a u e du ing Tes 1 is shown in Figu e 5. The hea ing phase is ep esen ed in he i s
pa o he g aph (0–80 min), du ing which he eac o eaches he ini ial expe imen al empe a u e o
250
◦
C. A ha poin , a sha p inc ease in empe a u e occu s. The empe a u e passes om 268
◦
C
o 403
◦
C in one minu e because o he exo he mici y o he Saba ie eac ion. A e 30 min in which
empe a u e is in he ange om 350 ◦C o 450 ◦C, i s a s o dec ease un il 300 ◦C.
Ene gies 2017,10, 855 7 o 13
Table 1. Expe imen al es .
Tes CO2/H2Ra io Ba ch o In-Con inuo (B-C) Tempe a u e [◦C] P essu e [ba ]
1 1/4 C 250 2
2 1/4 C 317 2
3 1/4 C 363 4
4 1/4 C 490 4
5 1/4 C 422 8
6 1/4 C 451 20
7 1/4 B 250 10
8 1/4 B 271 10
9 1/4 B 350 10
10 1/4 B 353 15
11 1/1 C 371 16
12 1/1 C 325 17
13 1/1 C 329 17
14 1/1 C 378 17
Figu e 5. Tempe a u e p o ile du ing Tes 1.
In all es s, a e he hea ing phase, he e is a 1-minu e apid empe a u e inc ease when he
eac ion occu s. A e 30 min since he empe a u e inc ease, he con e sion p ocess is conside ed
comple ed and he ou le gaseous mix u e is collec ed o be analyzed a he gas-ch oma og aphe .
In Table 2, o each es , he composi ion o he gaseous mix u e collec ed a he end o he p ocess is
shown, oge he wi h he con e sion pe cen age o he eac an s.
Compa ing he esul s o Tes s 1 o 6, which we e ca ied ou in-con inuo wi h he s oichiome ic
CO
2
/H
2
a io, he e is an inc ease in he me hane olume ac ion when p essu e has he same end.
In pa icula , a 2 ba , he me hane olume ac ion is equal o 8.55%, while Tes 5 a 8 ba shows he
bes esul a low p essu es (64.75%). Tes 6 a high p essu e (20 ba ) eaches 97.24% me hane con en .
The highes con e sion e iciencies a e ob ained in Tes s 5 and 6, wi h, espec i ely 84.64% and 99.06%.
Tes s 7 o 10 lead o negligible me hane p oduc ion because o ba ch condi ions, wi h consequen
low con e sion e iciencies ( om 6.51% o 31.63%). Fo his g oup, he bes esul s we e ob ained
in Tes s 9 (13.36%) and 10 (12.52%), ca ied ou a espec i ely 350
◦
C and 353
◦
C, wi h con e sion
e iciencies o 31.63% and 30.04% espec i ely.
Ene gies 2017,10, 855 8 o 13
Table 2. Expe imen al esul s.
Tes P essu e
[ba ]
Tempe a u e
[◦C]
Final % / Con e sion
E iciency [%]
H2CH CO2
1 2 250 85.98 8.55 5.47 21.90
2 2 317 15.53 37.80 46.68 64.58
3 4 363 56.33 29.13 14.54 55.22
4 4 490 72.61 19.47 7.92 42.04
5 8 422 24.58 64.75 10.67 84.64
6 20 451 1.74 97.24 1.02 99.06
7 10 250 58.56 5.99 35.44 16.05
8 10 271 60.57 2.27 37.16 6.51
9 10 350 17.93 13.36 68.71 31.63
10 15 353 29.75 12.52 57.73 30.04
11 16 371 90.65 8.79 0.56 22.43
12 17 325 33.11 32.82 34.08 59.44
13 17 329 11.68 36.60 51.72 63.39
14 17 378 3.96 15.79 80.25 36.00
The las ou es s (11–14) we e ca ied ou wi h a CO
2
/H
2
a io equal o one. The me hane olume
ac ion eaches a alue o 36.6%, wi h a empe a u e o 329
◦
C. Signi ican con e sion e iciency is
ob ained a 325–329
◦
C (59.44% and 63.39% espec i ely). Tes s 11 and 14, ins ead, esul in a lowe
me hane con en wi h a highe empe a u e alue.
The esul s, shown in Table 2, allow a discussion o he in luence o he ope a ing pa ame e s on
he me hana ion p ocess. The in-con inuo ope a ion o he appa a us is clea ly he undamen al elemen
o ob ain no ewo hy esul s. Tes s 11–14 demons a ed he s ong con e sion dec ease caused by a
de ia ion om he ideal alue o he CO
2
/H
2
a io. Since he CO
2
me hana ion eac ion occu s wi h a
dec ease in he numbe o moles, p essu e is he key pa ame e o os e he con e sion; i accele a es he
eac ion and is di ec ly p opo ional o he me hane p oduc ion a e. Figu e 6clea ly shows he posi i e
e ec o p essu e on he p ocess, wi h an inc easing eg ession line. The eg ession line was buil on
he esul s o Tes s 2, 4, 5, and 6, ca ied ou a di e en empe a u es bu in he highes in es iga ed
ange (300–400
◦
C). Figu e 7 ep esen s he CH
4
olume con en as a unc ion o empe a u es, using
he esul s o Tes s 1 o 6, ca ied ou a p essu es om 2 ba o 20 ba . The empe a u e ep esen s an
aid only wi hin i s op imal ange and has a bene icial ac ion only wi hin he ange om 400 o 450
◦
C.
Figu e 6. P essu e in luence on me hane olume ic ac ion.
Ene gies 2017,10, 855 9 o 13
Figu e 7. Tempe a u e in luence on me hane olume ic ac ion.
5. Ene gy E alua ion
The expe imen al me hane-based p ocess was compa ed o he me hanol-based p ocess desc ibed
in Sec ion 2.2 on he basis o an ene gy pa ame e de ined in Equa ion (3) [30]:
Spen Ene gy
S o ed Ene gy (3)
The conside ed pa ame e is an adimensional numbe calcula ed as he a io be ween he speci ic
ene gy spen in all he sec ions o he p ocess o p oduce he uel in i s s o ing o m and he speci ic
ene gy s o ed in he uel, co esponding o he lowe hea ing alue.
The o al ene gy in ol ed in he wo p ocesses o p oduc ion o he inal uel in i s anspo able
o m was calcula ed. The o al ene gy in ol ed in he p ocesses consis s o :
•hyd ogen and ca bon dioxide comp ession o injec ion in he syn hesis eac o ,
•ene gy consump ion o hea ing he eac o o he syn hesis empe a u e,
• emo al o eac ion hea ,
•ene gy consump ion o b inging he uel o he s o ing condi ions.
The p ocess condi ions o bo h he analyzed cases a e summa ized in Table 3. The da a o elec olysis
used in he calcula ions a e hose o in-lab G Se ies H03Elec olyze .
Table 3. Compa ison o expe imen al esul s.
Pa ame e Me hane P ocess Me hanol P ocess
Elec olyze p essu e (ba ) 2.5 2.5
Syn hesis eac o p essu e (ba ) 20 70
Syn hesis eac o empe a u e (K) 723 673
Reac ion hea (kJ/mol) 165 49.5
Con e sion e iciency 0.95 0.95
S o age p essu e (ba ) 200 1
S o age empe a u e (K) 298 298
Lowe hea ing alue (kJ/kg) 50,000 19,700
The ene gy consump ion o he elec olyze is equal o 3.75 kWh/Nm
3
o hyd ogen
(13,500 kJ/Nm
3
). Fo he me hane p ocess, he p essu e and empe a u e o he syn hesis eac o
