fuel processing technology · in view of carbon formation was performed with aspen plus based on...

Fuel Processing Technology 92 (2011) 678–691

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Fuel Processing Technology

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Thermodynamic analysis of carbon dioxide reforming of methane in view of solidcarbon formation

M. Khoshtinat Nikoo a,b, N.A.S. Amin a,⁎a Chemical Reaction Engineering Group (CREG), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysiab Dept of Catalyst and Process, R&D of Bandar Imam Petrochemical Complex (BIPC), Mahshahr, Iran

⁎ Corresponding author. Tel.: +60 7 553 5579; fax: +E-mail addresses: [email protected] (M.K. Nik

(N.A.S. Amin).

0378-3820/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.fuproc.2010.11.027

a b s t r a c t
a r t i c l e i n f o
Article history:Received 23 July 2010Received in revised form 16 November 2010Accepted 26 November 2010Available online 30 December 2010

Keywords:Thermodynamic equilibriumGibbs free energyReformingCarbon formationMethaneCarbon dioxide

A thermodynamic equilibrium analysis on themulti-reaction system for carbon dioxide reforming of methanein view of carbon formation was performed with Aspen plus based on direct minimization of Gibbsfree energy method. The effects of CO2/CH4 ratio (0.5–3), reaction temperature (573–1473 K) and pressure(1–25 atm) on equilibrium conversions, product compositions and solid carbon were studied. Numericalanalysis revealed that theoptimalworking conditions for syngasproduction in Fischer–Tropsch synthesiswere attemperatures higher than 1173 K for CO2/CH4 ratio being 1 at which about 4 mol of syngas (H2/CO=1) could beproduced from 2 mol of reactants with negligible amount of carbon formation. Although temperatures above973 K had suppressed the carbon formation, the moles of water formed increased especially at higher CO2/CH4

ratios (being 2 and 3). The increment could be attributed to RWGS reaction attested by the enhanced number ofCOmoles, declined H2 moles and gradual increment of CO2 conversion. The simulated reactant conversions andproduct distributionwere comparedwith experimental results in the literatures to study thedifferences betweenthe real behavior and thermodynamic equilibrium profile of CO2 reforming of methane. The potential ofproducing decent yields of ethylene, ethane, methanol and dimethyl ether seemed to depend on active andselective catalysts. Higher pressures suppressed the effect of temperature on reactant conversion, augmentedcarbon deposition and decreased CO and H2 production due to methane decomposition and CO disproportion-ation reactions. Analysis of oxidative CO2 reforming of methane with equal amount of CH4 and CO2 revealedreactant conversions and syngas yields above 90% corresponded to the optimal operating temperature and feedratio of 1073 K and CO2:CH4:O2=1:1:0.1, respectively. The H2/CO ratio was maintained at unity while waterformation was minimized and solid carbon eliminated.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Numerous efforts are being directed at limiting CO2 and CH4

emissions to halt global green-house warming. Utilization of landfillgas (LFG) and a CO2-rich fossil natural gas (NG) to produce higher-value added chemicals seems promising to reduce green houseeffects, since CH4 and CO2 are the predominant constituents in bothLFG and fossil natural gas. The CO2/CH4 ratio in LFG varies widelydepending on the type of waste, age of landfill and its extractionsystems. Landfill gas commonly consists of 40–70% CH4 and 30–60%CO2. Similarly the compositions of fossil NG vary from one field toanother. For example, the CO2/CH4 ratio for NG in Natuna's field(located in the South China Sea) [1] and zone D of the Dalan Formation(located in south of Iran) are 71/28 and 85/2.5 [2], respectively.However, only several industrial processes involving CO2 as reactantgas have been developed due to the inert and stable characteristics of

CO2 and circumvent the thermodynamic constraints [3–6]. Numerousefforts have been and are being conducted to activate and utilize CO2

[7].Traditionally, steam reforming of methane to syngas (H2 and CO)

[6–8] produces light hydrocarbons or oxygenates including methanol.But, an inevitable drawback of this process is the H2/CO ratio being 3:1is higher than the needed ratio for Fischer–Tropsch process [11,12].Furthermore, a considerable amount of CO2 (the greenhouse gas) isbeing produced in syngas and higher hydrocarbons production [13].The feasible utilization of CH4 and CO2 to higher value-added productssuch as higher hydrocarbons, syngas and liquid oxygenates are beinginvestigated [3,14,15]. Besides being exploited as fuel, higherhydrocarbons form basic materials for industries including petro-chemical, rubber and plastics. Syngas is the building block for liquidfuel production via Fischer–Tropsch process and also amajor source ofhydrogen in the refinery processes. Oxygenates such as methanol notonly represents an easy and safe way to store and transport theenergy, but methanol mixed with dimethyl ether (DME) yield anexcellent fuel for diesel engines with high cetane number andvaluable combustion properties [16].

Nomenclature

aik Number of atoms of the kth element present in eachmolecule of species i

Ak Total mass of kth element in the feedˆfg

i Fugacity of species i in gas systemˆf si Fugacity of species i in the solid phasefi°s Standard-state of fugacity of pure component i in the

solid phasefi°g Standard-state of fugacity of pure component i in the

gas phaseG(T, P)tg Total Gibbs free energy in gas phase (kJ)

G(T, P)ts Total Gibbs free energy in solid phase (kJ)

Ggl Partial molar Gibbs free energy of species i in a gas

phase (kJ/mol)Gsl Partial molar Gibbs free energy of species i in a solid

phase (kJ/mol)Gc°s StandardGibbs free energy of pure solid carbon(kJ/mol)

Gi°g Standard Gibbs free energy of species i in a gas phase

(kJ/mol)Gi°s Standard Gibbs free energy of species i in a solid phase

(kJ/mol)G(T, P)t Total Gibbs free energy of two phase (kJ)

Gi, e°g Standard Gibbs free energy of elements i in a gas phase

(kJ/mol)GgC Partial molar Gibbs free energy of carbon in a gas state

(kJ/mol)GSC Partial molar Gibbs free energy of carbon in a solid

state (kJ/mol)ΔGfi

°g Standard Gibbs free energy of formation for species i inthe gas phase (kJ/mol)

ΔGfC°S Standard Gibbs energy of formation for solid graphite

carbon (kJ/mol)ΔGr

° Gibbs free energy change of reaction (kJ/mol)ni Mole of species i (mole)N Number of species in a reaction systemP° Standard-state pressure (1 atm)P Pressure of the reaction system (atm)R Molar gas constant (kJ/mole.K)T Temperature of the reaction system (K)yi Mole fraction of species i in a gas phase

Greek Symbolλk Lagrange multiplierμiS Chemical potential of species i in a solid phaseμig Chemical potential of species i in a gas phase

μCS Chemical potential of carbon in a gas phaseucs Chemical potential of carbon in a solid phase

yi Stoichiometric coefficient of species iϕ̂i Fugacity coefficient of species i

SuperscriptG Gas phaseS Solid phaseT Total

Subscripti Component in the mixturek Element in each molecule

679M.K. Nikoo, N.A.S. Amin / Fuel Processing Technology 92 (2011) 678–691

The major advantage of CO2 reforming of methane is given thatthe H2/CO ratio being close to 1 makes it suitable for the synthesis ofoxygenated chemicals [15,16]. Extensive investigations have beenlured towards the development of active and selective catalysts forthe CO2 reforming of methane with Ni containing [17–21] and noblemetal-supported catalyst such as Pt, Rh, Ru and Pd [22–25] catalystsbeing commonly used to produce syngas. However, carbon depositionis still the main cause for catalyst deactivation due to deep cracking ofCH4 and CO2 as well as sintering of the metallic active phase.Therefore, determination of the optimum operating condition byconsidering carbon formation boundary provides a fundamentalremedy to prolong the catalyst lifetime and to surpass the perfor-mance of the reaction system. Thermodynamic analysis is usuallyhelpful to determine the constraints governed on a reaction systemand provide the range of operating conditions.

Research on thermodynamic behaviors of reaction systems bycalculating equilibriumcompositionshave beenutilized inunderstandingthe feasibility of a vast variety of reactions. Garcia and Laborde [26]analyzed the thermodynamics of steam reforming of ethanol in order toinvestigate the possibility of superseding methanol by ethanol as afeedstock without consideration on the existence of solid carbon inequilibrium. After estimating the equilibrium product distributions of thegaseous species, they checked for the formation of carbon. In anotherresearch [27], the equilibrium compositions of CO and CO2 calculated bytheGibbs free energyminimizationmethodwere furtherused to estimatethe presence of carbon deposition in all possible reactions. Maggio et al.[28] used the same models to compare steam reforming of methane,methanol and ethanol in a molten carbonate fuel cell, but carbonformation was not considered in their calculations. Thermodynamiccalculations for obtaining equilibriumcompositions of steamreformingofDME was recently reported, but again carbon formation had not beentaken into account in the calculation [29]. In another development,thermodynamic equilibrium computation of DME steam reforming in anexternal reformer for fuel cell applications was performed [30] in whichGibbs free energy of formation for solid carbonwas considered in the totalGibbs free energy minimization equation. Recently, the dry reforming ofglycerol for synthesis gas production by considering Gibbs free energy ofcarbon in the total Gibbs energyminimization equation was investigated[31].

Pertaining to CO2 reforming of methane, Gibbs free energyminimization without considering carbon deposition was applied toinvestigate the influence of operating parameters on the methaneconversion and syngas production for reforming reactor design [12].Also, Yaw and Amin [32] conducted a thermodynamic equilibriumanalysis on the syngas production from CO2 reforming and partialoxidation of methane at the atmospheric pressure, but had notconsidered carbon deposition. Apparently, previous investigators hadnot considered carbon formation or often used the principle ofequilibrated gas to compute the carbon formation in reaction systems.

A thermodynamic analysis of CO2 reforming of methane to the co-generation of C2 hydrocarbons and syngas has been reported [14]. Theeffect of various conditions, i.e. temperature, CO2/CH4 feed ratio andsystempressureon chemical equilibriumusingChemkinCollectionR3.7.1were investigated. In the paper, the regions of carbon and no carbonformation in the equilibrium system were demonstrated, but not thenumerical trends of carbon formation over the investigated range oftemperature, CO2/CH4 ratios and pressure. Therefore, in this paperthermodynamic equilibrium compositions for the co-generation of C2hydrocarbons, syngas andoxygenates (methanol andDME) fromCH4 andCO2 in view of coke formation were computed by employing total Gibbsfree energy minimization method with Aspen plus. Effects of CO2/CH4

feed ratio, temperature and pressure on carbon deposition, waterformationandH2/COratio in the reaction systemas importantparametersfor producing various products are discussed. Meanwhile, a thermody-namic analysis for oxidative CO2 reforming of methane is studied toprovide a deep insight into syngas production as the main product of

Table 1Reactions in CO2 reforming of methane.

Reaction number Reaction ΔH298 (kJ/mol)

1 CH4+CO2↔2CO+2H2 2472 CO2+H2↔CO+H2O 413 2CH4+CO2↔C2H6+CO+H2O 1064 2CH4+2CO2↔C2H4+2CO+2H2O 2845 C2H6↔C2H4+H2 1366 CO+2H2↔CH3OH −90.67 CO2+3H2↔CH3OH+H2O −49.18 CH4↔C+2H2 74.99 2CO↔C+CO2 −172.410 CO2+2H2↔C+2H2O −9011 H2+CO↔H2O+C −131.312 CH3OCH3+CO2↔3CO+3H2 258.413 3H2O+CH3OCH3↔2CO2+6H2 13614 CH3OCH3+H2O↔2CO+4H2 204.815 2CH3OH↔CH3OCH3+H2O −3716 CO2+4H2↔CH4+2H2O −16517 CO+3H2↔CH4+H2O −206.2

680 M.K. Nikoo, N.A.S. Amin / Fuel Processing Technology 92 (2011) 678–691

methane reforming. Appropriate temperature and CO2:CH4:O2 ratio forsyngas production from oxidative CO2 reforming of methane are alsopresented.

2. Methodology

The accurate function to specify the equilibrium composition of areaction system can be defined by Gibbs free energy minimization.The total Gibbs free energy of a single-phase only for a gas or a solidsystem at certified temperature and pressure is a function ofequilibrium composition of compounds and can be respectivelyrepresented as Eqs. (1) and (2):

GtgT;Pð Þ = ∑N

i=1ni Ggl = ∑N

i=1ni μgi = ∑N

i=1niG-gi

+ RT∑Ni=1ni ln

ˆf gi = f-gi

� �ð1Þ

GtsT;Pð Þ = ∑N

i=1ni Gsl = ∑N

i=1ni μsi = ∑N

i=1ni G-si

+ RT∑Ni=1ni ln

ˆf si = f-si

� �:

ð2Þ

Hence, the total Gibbs free energy of a two-phase system can bewritten as Eq. (3):

GtT;Pð Þ = ∑N

i=1ni G-gi + RT∑N

i−1ni lnˆf gi = f

-gi

� �

+ ∑Ni=1ni G

-si + RT∑N

i=1ni lnˆf si = f

-si

� �:

ð3Þ

Since the standard state is defined as the pure ideal gas state at1 atm, fi°g=P°=1 atm and since Gi, e

°g equals to zero for each elementin its standard state; hence, Gi

°g=ΔGfi°g [33]. Meanwhile, the solid

phase is assumed as a pure solid carbon. As the reference state for thesolid phase is at atmospheric pressure and 25 °C, the partial fugacitycan be written as shown in Eq. (4):

ˆf si = f-si : ð4Þ

Considering equality of chemical potential of carbon in anequilibrated gas to solid phase and applying Eq. (2) along with theabove assumption for solid carbon species, the equation for solidphase obtained is shown in Eq. (5) [33]:

μgC = μs

C = GgC = GS

C = G-sC = ΔG

-sfC ≅0: ð5Þ

By substituting ˆfgi = yi

ˆϕi P; f-gi = P - = 1 and Gi

°g=ΔGfi°g in Eq. (1)

and using the Lagrange multiplier method, the Gibbs free energyminimization of gaseous species can be expressed as Eq. (6):

ΔG-gfi + RT ln

��y�i ϕ̂i P

��P -

+ ∑

kaikλk = 0: ð6Þ

In the same manner, by substituting ˆfgi = yi

ˆϕi P, fi°g=P°=1,

Gi°g=ΔGfi

-g and Eq. (5) into Eq. (3) the Gibbs free energy minimi-zation with considering solid carbon can be expressed as Eq. (7):

∑N

i=1ni ΔG-g

fi + RT ln��

y�i ϕ̂i P

��P-

+ ∑

kaikλk

+ nCΔG

-

fC Sð Þ = 0: ð7Þ

The above equations (Eqs. (6) and (7)) along with the constraintshown in Eq. (8) are simultaneously solved to obtain the equilibriumcomposition of the reaction system.

∑ini aik = Ak ð8Þ

The equilibrium computations employing the R-GIBBS reactorwere accomplished with the Aspen plus, Aspen Tech™ with thecapability of simulating a maximum of 9 phases of multi-componentsat equilibrium in the reaction system. The total moles of the reactantsincluding methane and carbon dioxide were 2. In order to simulateboth LFG and fossil NG with high CO2 content, the CO2/CH4 ratio wasvaried between 0.5 and 3. The pressure was varied in the rangeof 1–25 atm while the temperature in the range of 573 to 1473 K.Soave–Redlich–Kwong (SRK)model was used as the equation of state,since some polar components such as methanol and DME wereconsidered as products in the reaction system. Types of reactants,products, temperature, pressure, moles of reactants, composition ofreactants and reaction phases have to be clarified to perform theequilibrium composition calculations. The possible products in CO2

reforming of methane were considered to be the following tencomponents: CH4, C2H6, C2H4, CO, CO2, H2, DME, water and methanolas gas species. Pure solid carbon (graphite) was in chemical and phaseequilibrium with the gas species.

The Gibbs free energy change of reaction (ΔGr°) for each reaction at

different temperatures was calculated by Eq. (9):

ΔG-

r = ∑iγiΔG

-gfi : ð9Þ

where γi is the stoichiometric coefficient of species i [34]. Theequilibrium constant (K), obtained from Eq. (10), determined thefeasible range of the spontaneous occurrence of the reactions.Thermodynamic data such as Gibbs free energy of formation ΔGfi

□g

for all compounds were obtained elsewhere [35].

K = exp −ΔG-

r = RT� �

ð10Þ

3. Results and discussion

3.1. Feasible reactions

The main reactions which may occur in CO2 reforming of methaneare considered in Table 1. The equilibrium constants of all reactionsthat are supposed to occur are exhibited as a function of temperaturein Fig. 1. According to the thermodynamic principles [36], when theGibbs free energy change of reaction (ΔGr) is negative, the reaction isspontaneous. Conversely, for positive ΔGr, the reaction is thermody-namically limited [36]. The equilibrium constant (K) (Eq. (10))determines the extent to which the reaction occurs. The reactionscannot be shifted to the opposite side by changing the molar ratio of

Fig. 1. Equilibrium constants of reactions involving in CH4–CO2 reaction at differenttemperatures and atmospheric pressure.

Fig. 2. CH4 equilibrium conversion as a function of temperature and CO2/CH4 ratio at1 atm n (CH4+CO2)=2 mol.


reactants when K is much higher than 1. But for K in the vicinity of 1,varying the molar ratio of the reactants has considerable influence onthe distribution of the products [31]. Whenever ΔGr is negative, alarger Ln(K) indicates a spontaneous reaction is more feasible tooccur. As shown in Fig. 1, it can be deduced that CO2 reforming ofmethane (reaction 1: CH4+CO2↔2CO+2H2) to form syngas is afavorable reaction, particularly at a temperature N1000 K, consistentwith the suggested temperature range in the previous study[14]. Reverse water gas-shift (RWGS) as numbered reaction 2(CO2+H2↔CO+H2O) is much affected by equilibrium within theentire investigated temperature range. In general, CO2 reforming ofmethane is typically accompanied by simultaneous occurrence ofRWGS reaction.

The high negative values of Ln(K) for CO2 OCM (Oxidative couplingof methane) reactions (reactions 3 and 4: 2CH4+CO2↔C2H6+CO+H2O and 2CH4+2CO2↔C2H4+2CO+2H2O) illustrate that thesereactions are not feasible to occur except at a very high temperature.Dehydrogenation of ethane (reaction 5: C2H6↔C2H4+H2) has theenough tendency to occur at higher temperature for ethyleneproduction, although the reaction can be also affected by equilibriumlimitations. Meanwhile, reaction 5 takes place along with reactions 3and 4 [14]. Hydrogenation of CO2 and CO (reactions 6 and 7: CO+2H2↔CH3OH and CO2+3H2↔CH3OH+H2O) is much favorabletowards the reverse side, especially at high temperatures, as their Ln(K) are negative.

Carbon may be formed via methane decomposition (reaction 8:CH4↔C+2H2), disproportionation (reaction 9: 2CO↔C+CO2) [37],hydrogenation of carbon dioxide (reaction 10: CO2+2H2↔C+2H2O) and hydrogenation of carbon monoxide (reaction 11: H2+CO↔H2O+C), although these reactions are affected by the change inmolar ratio of reactants due to their low Ln(K) within the investigatedtemperature range. Reaction 8 is more plausible for carbon formationat the higher temperature, whereas all three reactions [9–11] tend togenerate carbon at lower temperature (b800 K) and can be influencedby equilibrium limitations at the higher temperature. Reactions 12(CH3OCH3+CO2↔3CO+3H2), 13 (3H2O+CH3OCH3↔2CO2+6H2)and 14 (CH3OCH3+H2O↔2CO+4H2) however, can be improvedtowards the right-hand side within the whole considered tempera-ture range, while reaction 15 (2CH3OH↔CH3OCH3+H2O) is easilyinfluenced by equilibrium limitations. The composition of theproducts can be greatly varied for those reactions whose Ln(K) arein the vicinity of zero within the considered temperature range.Reactions 16 (CO2+4H2↔CH4+2H2O) and 17 (CO+3H2↔CH4+

H2O), namely methanation can occur at lower temperature (b800 K)with positive magnitude of Ln(K), but are restricted at the hightemperature due to their negative Ln(K) and both reactions beingexothermic.

3.2. Effect of temperature and CO2/CH4 ratio on equilibrium reactantconversion and product distribution

3.2.1. Methane conversionThe effect of operating temperature at atmospheric pressure on

the equilibrium state is shown in Fig. 2 for methane gas. For all CO2/CH4 ratios, CH4 conversion almost quickly increases with increasingtemperature up to 1000 K, beyond which the conversion goes upsmoothly to attain unity. Meanwhile, CH4 conversion increases withCO2/CH4 ratio implying the CO2 gas as a soft oxidant has a positiveeffect on CH4 conversion in the temperature range of interest. Thepositive effect is more pronounced at temperatures lower than 973 K;therefore, adding more CO2 to CH4 as an active oxidant brings about ahigher activity for methane molecules. Nevertheless, exothermicreactions 16 and 17 may be involved in decreasing methaneconversion at the lower temperature. For example, the equilibriumconversion reported in [14] was about 42% when CO2/CH4 ratio andtemperature were 1 and 873 K, respectively, while our work reporteda value of 82%. It is supposed thatmethane decomposition (reaction 8)is themain reason for the increase in methane conversion. At CO2/CH4

ratio being 0.5 to 1 for reaction 1, CO2 gas acts as a limiting reactantand is not able to convert CH4 completely. Therefore, high conversionof CH4 at higher temperatures can be ascribed to reaction 8 as thepredominant reaction (see Fig. 1) to form hydrogen and carbon.

From the experimental results reported by Khalesi et al. [38]displayed in Table 2, it can be observed that the values of CH4

conversions using Sr0.8La0.2Ni0.3Al0.7O2.6 catalyst are less than those ofequilibrium CH4 conversions at lower than 873 K. Owing to the factthat the thermodynamic equilibrium calculations were performedconsidering solid carbon, the values of CH4 conversions less than thatof equilibrium conversions may imply CH4 decomposition cannotimprove easily; therefore, the resistance of the catalyst to carbondeposition. Although, for temperatures N923 K, the presence of thecatalyst caused a higher CH4 conversion compared to equilibriumcondition, it may be due to the small susceptibility of the catalyst tocarbon deposition at higher temperatures, evident by the presence ofsolid carbon on the surface of the catalyst. For the experimentalresults of Jablonski et al. [39], all CH4 conversions were belowequilibrium, suggesting higher resistance to carbon depositioncompared to the results of Khalesi et al. [38] in the whole investigatedtemperature range.

Table 2Reactant conversions (X) and product yield (Y) for CO2 reforming of methane with CO2/CH4 ratio of 1 at atmospheric pressure.

Temperature (K) XExpCH4 (%) XEq

CH4 (%) XExpCO2 (%) XEq

CO2 (%) YExpH2 (%) YEqH2 (%) YExpCO (%) YEqCO (%) H2/COExp H2/COEq

773 52.5a

–

70.9 –

–

45.9 47a

–

28.5 31.1a

–

3.5 1.5a

–

8.1

823 71a

20.4b76.6 –

37.1b44.8 64a

–

40.1 45.3a

–

8.3 1.4a

0.41b4.8

873 81.9a

40.6b82.3 –

59.1b47.2 78.8a

–

52.5 61.6a

–

17.4 1.28a

0.49b3

923 90.7a

58b87.4 –

78.4b54.5 87.7a

–

64.9 75.6a

–

31.9 1.16a

0.55b2

973 95.4a72.4b

91.5 –

91.7b66.3 91.1a

–

75.9 80.6a

–

50.8 1.13a

0.6b1.5

1023 97.9a

82.6b94.4 –

95.7b79.1 91.5a

–

84.6 83.2a

–

69.3 1.1a

0.63b1.2

1073 100a

85.1b96.3 –

96.5b88.6 92.8a

–

90.6 84.4a

–

82.9 1.1a

0.65b1.1

Exp and Eq are representatives of experimental data by the other researchers and equilibrium values calculated in our work, respectively.a The values are corresponding to Sr0.8La0.2Ni0.3Al0.7O2.6 taken from a research by Khalesi et al. [38].b The values are corresponding to Pt/Na(0.3 wt.%)–Al2O3 taken from a research by E. L. Jablonski et al. [39].


3.2.2. Carbon dioxide conversionSince CO2 as a soft oxidant has a positive effect on CO2 reforming of

methane, conversion of CO2 needs to be considered and discussed.Although methane conversion increases within the consideredtemperature range and CO2/CH4 ratios, Fig. 3 depicts two trends ofCO2 conversion versus temperature for all CO2/CH4 ratios. Initially,CO2 conversion gradually decreases with temperature beginning from573 K to about 823–873 K (depending on the CO2/CH4 ratios). Thefirst decreasing trend can be mainly described by reaction 10, whichconverts CO2 and hydrogen to a large quantity of carbon and water.The exothermic reaction spontaneously occurs at the low tempera-ture, but diminishes as the equilibrium constant decreases andreduces CO2 conversion. Reaction 16 that is exothermic and favorableat the lower temperature may be also attributed to the samedecreasing trend. The trend can also be verified by decreasing molesof water in the mentioned temperature range (Fig. 8). None of theother reactions are significantly involved in CO2 conversion, sincethey have a negative value of Ln(K) within the mentioned temper-ature range.

The following trend illustrates that CO2 conversion begins toenhance, as endothermic reactions 1 and 2 are favorable at highertemperatures. This result is consistent with what has been obtainedfrom previous calculations regardless of the amount of the CO2

conversion which is due to the reactions involved in carbon formation[12,14]. Temperatures above 873 K favor the conversion of CO2 as

Fig. 3. CO2 equilibrium conversion as a function of temperature and CO2/CH4 ratio at1 atm for n (CH4+CO2)=2 mol.

depicted in Fig. 3. Equilibrium conversion of CO2 reaches a maximumbetween 1273 K and 1473 K for all CO2/CH4 ratios as similarlyreported [40]. CO2 is completely converted at CO2/CH4 ratio of 0.5and temperature of 1273 K better than when CO2/CH4 has a largerproportion since CO2 is the limiting reactant. Whenever the CO2/CH4

ratio is greater, CO2 conversion is less, as methane more intensivelyplays the role of a limiting reactant to the extent that at CO2/CH4 ratioshigher than 1, equilibrium conversion of CO2 cannot be completed.Table 2 displays that the values of CO2 conversions reported byJablonski et al. [39] are higher than the equilibrium ones for allinvestigated temperatures, except for 823 K where experimental CO2

conversion is lower than its equilibrium value. Besides, equilibriumconversions of CO2 are lower than those of CH4 within the wholeinvestigated temperature range. Conversely, the experimental CO2

conversions were greater than CH4 conversions. It is worth mention-ing that the variation trends of experimental conversions of CO2 andCH4 are closer to the results of the previous researches [12,14]obtainedwithout considering the solid carbon formation, suggesting alower carbon deposition in this catalytic reaction compared to theequilibrium condition. The dissociation of CO2 followed by CO and O isthought to bemore preferable than CH4 decomposition by the catalystleading to higher conversion of CO2 than that of CH4. This resultpresents a good agreement with the research byWisniewski et al. [41]in which conversions of CO2 exceeded those of CH4 while no carbondeposition was observed in CO2 reforming of methane over Ir/CGOcatalysts.

3.2.3. Syngas productionThe region for H2 and CO production with respect to CO2/CH4

ratios can be divided to two: CO2/CH4 N1 and CO2/CH4 b1.Fig. 4 depicts the production of H2 gas as a function of different

temperatures and CO2/CH4 ratios at the atmospheric pressure. WhenCO2/CH4 ratios b1, the amount of H2 produced enhances within thewhole investigated temperature, as CO2 is the limiting reactant andthe RWGS reaction cannot simultaneously improve along withreaction 1, as much as when the CO2/CH4 is higher than 1. Meanwhile,the number of H2 moles produced decreases with increasing CO2/CH4

ratio from 0.5 to 1 at a specified temperature. As RWGS is not beingmuch involved, whenever CO2/CH4 ratio is larger, reaction 1 proceedsbetter and faster suppressing reaction 8, which causes the loweramount of H2 production.

In the case of CO2/CH4 ratios N1, the number of moles of H2 stepsup with increasing temperature, attains a maximum around 973–1023 K (depending on the CO2/CH4 ratio), and then slightly decreasesat higher temperatures. The declining trend of H2 either for specifiedCO2/CH4 ratios (N1) versus different temperatures or for specified

Fig. 4. Moles of H2 as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.

Fig. 5. Moles of CO as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.


temperature (N973 K) versus different CO2/CH4 ratios (N1) arepresumably ascribed to RWGS reaction (reaction 2) in which H2

produced reacts with CO2 to form water and CO. Generally, H2

production becomes lower with increasing CO2/CH4 ratio due to CH4

being a more intensive limiting reactant restricted the source ofhydrogen atoms. This result is compatible with the results of thethermodynamic analysis reported previously [12,14]. In comparison,the number of moles of H2 produced in CO2 reforming of methane atCO2/CH4 ratio of 1 equals to those reported in the past [42]. Moreover,from an investigation on CO2 reforming of methane using Ni/Al2O3 at1000 K, H2 mole fractions decreased as the feed ratio increased from0.5 to 3, which had a good agreement with the thermodynamiccalculations without considering carbon deposition [43]. Consequent-ly, this catalyst seems to be carbon resistant as the values of H2 molefractions were fitted by the equilibrium curve. The H2 mole fractionsobtained from our thermodynamic calculations also were almostidentical to the previous thermodynamic calculation without consid-ering carbon formation except for CO2/CH4 ratio of 0.5. The H2 molefraction reached 0.6 for CO2/CH4 ratio of 0.5 with respect to ourcalculations, but 0.4 for thermodynamic calculations without consid-ering carbon deposition (the mole fractions are not shown)[12,14,43]. The greater amount of H2 mole fractions obtained fromthermodynamic calculations considering solid carbon can be attrib-uted to the occurrence of CH4 decomposition. Comparing theexperimental results of Khalesi et al. [38] with the equilibriumcalculations in Table 2, the experimental H2 yields exceeded theequilibrium yields of H2 at all investigated temperatures. The highervalues of H2 yields than the equilibrium ones at lower 873 K cannot bejustified by the occurrence of CH4 decomposition as the CH4

conversions attained lower values than thermodynamic calculations,as mentioned before. It may be due to the significant ability of thecatalyst in improving CO2 reforming of methane (reaction 1).

Fig. 5 depicts the number of moles of CO versus CO2/CH4 ratio andtemperature at atmospheric pressure. Higher temperatures favor COproduction for any CO2/CH4 ratios, since all the reactions involved inCO production are endothermic (see Table 1). At CO2/CH4 ratio b1 andspecified temperature, CO production enhances along with increasingCO2/CH4 ratio, as CO2 is the limiting reactant. However, increasingCO2/CH4 ratio N1 at higher temperatures brings about a decrease in COproduction, as CH4 becomes the limiting reactant and CO2 conversionattains a lower quantity. These variation trends of CO production areconsistent with the experimental results of CO2 reforming of methaneon Ni/Al2O3 at 1000 K by Takano et al. [43].

Contrary to the decreasing trend of H2 production for a specifiedCO2/CH4 ratio (N1) versus temperature (N973), CO production is

favorable at the above mentioned range due to RWGS reaction(reaction 2). In RWGS reaction, H2 reacts with CO2 to produce CO andwater. Hence, it is inevitable to see the opposite trends in H2 and COproduction at temperatures above 973–1023 K and CO2/CH4 ratios N1.With reference to Table 2, higher experimental CO and H2 yields thantheir equilibrium ones along with high methane conversions couldprovide evidence for the significant ability of the catalyst indissociation of CO2 followed by CO and O production as well asimproving CO2 reforming of methane. Furthermore, both ascendingexperimental CO and H2 yields with temperature confirm that RWGSreaction is not involved when CO2/CH4 ratio is 1.

Comparing this work with the preceding study which did notconsider methane decomposition (reaction 8) in the reaction system[14], contradict trends of moles of CO and H2 for CO2/CH4 b1 at aspecified temperature is observed. The number of moles of COincreases opposite to that of H2 with increasing CO2/CH4 ratio up to 1in our work, but both increased in the previous study [14]. As a matterof fact, this contradict arises from reaction 8 which just as mentioned,is preferable alongwith reaction 1 and plays a key role in H2 (and solidcarbon) production when CO2/CH4 b1, or in other words, when CO2 isa limiting reactant. Just due to this reason, CH4 conversions reportedin our work attain higher magnitudes than previous study at the sametemperature when CO2/CH4 b1 [14]. Besides, in the present study dueto the same said reason, moles of CO and H2 achieved their maximumat CO2/CH4 being 1 and 0.5, respectively, but both at 1 in the formerwork. It is interesting to note that according to the experimentalresults of Takano et al. [43], both CO and H2 productions reached totheir maximum at CO2/CH4 ratio of 1, while fitting the equilibriumcurve without considering solid carbon deposition.

Fig. 6 portraits the H2/CO ratios produced from CO2 reforming ofmethane as a function of temperature and CO2/CH4 ratio atatmospheric pressure. Furthermore, this figure provides a broadrange of operating parameters for production of syngas for variousindustrial applications. The desirable amount of H2/CO ratio varieswith different industrial applications. The ratio of H2/CO decreaseswith increasing either temperature or CO2/CH4 ratio. As can be seen inFig. 6, the values for H2/CO ratios are high; especially at lower CO2/CH4

ratios and temperatures less than 823 K. Hydrogen produced in thisarea is more suitable for application in industries with demand formore concentrated H2.

A H2/CO ratio of unity is needed for FTS to produce liquid fuelsuch as DME [17,44]. From Fig. 6, a H2/CO ratio in the order of 1 canbe obtained at higher than 1173 K for CO2/CH4 ratio being 1 at whichabout 4 mol of syngas can be produced from 2 mol of reactants(CO2+CH4) with a CO2 conversion of more than 98%. The

Fig. 6.H2/CO ratio as a function of temperature (573–823 K) and CO2/CH4 ratio at 1 atmfor n (CH4+CO2)=2 mol.

Fig. 7. Moles of carbon as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.


experimental results taken from a research by Khalesi et al. [38] asshown in Table 2, revealed a H2/CO ratio close to unity in alltemperature range studied, demonstrating the considerable activityand selectivity of the catalyst towards CO2 reforming of methane(reaction 1). It is more pronounced at temperatures higher than 923 Kwhere H2/CO ratio remained at the constant value of 1.1, representingthe preference of CO2 reforming of methane. Furthermore, equilibri-um H2/CO ratios greater than the experimental ones at a lowertemperature are a sign of more carbon formed due to the preferableoccurrence of CH4 decomposition. In another research by Jablonski etal. [39] higher CO2 conversions along with lower H2/CO ratioscompared to the corresponding equilibrium ones are associatedwith the RWGS reaction (see Table 2). In other words, the exploitedcatalyst in their work was selective towards RWGS reaction, althoughthe CO2/CH4 was 1.

A H2/CO ratio of about 2 is needed to produce methanol fromsyngas. Up to now, Figs. 4–6 reveal that the suitable condition forsyngas to methanol production is a temperature higher than 1123 K,CO2/CH4 ratios of 0.5 at which about 3.5 mol of syngas can beproduced. In the next section, it will be discussed this conditioncannot be acceptable due to a significant amount of carbon depositionin the reaction system.

3.2.4. Carbon productionAll reactions involve in carbon formation (reactions 8–11) can be

relatively affected by operational parameters due to their lowequilibrium constants. From Fig. 7, carbon formation decreases withincreasing temperature, as higher temperature is a barrier forimproving the exothermic reactions of 9, 10 and 11 (2CO↔C+CO2,CO2+2H2↔C+2H2O and H2+CO↔H2O+C) which involved car-bon formation. When the CO2/CH4 is higher than 1, moles of carbondisappear at the lower temperature. However, a considerable andnearly constant amount of carbon still remained for CO2/CH4 ratioequals to 0.5 at temperatures higher than 1073 K as reaction 8becomes more plausible. Moreover, it can be explained by reverse ofdisproportionation (reverse of reaction 9: C+CO2↔2CO) which isfavorable at the higher temperature (especially at CO2/CH4 N1), butthe rate reduces when CO2 is the limiting reactant [45,46]. This isconsistent with the existence of less amount of CO at CO2/CH4 ratio of0.5 compared to higher CO2/CH4 ratios. In the case of increasingcarbon formation for CO2/CH4 ratio of 0.5 at low temperatures up to873 K while CO2 is a limiting reactant, it can be probably due toendothermic reaction of methane decomposition (reaction 8) whichimproves with increasing temperature. Meanwhile, the number ofmoles of carbon for CO2/CH4 ratio of 0.5 is lower than that of CO2/CH4

ratio of 1 at temperatures less than 723 K. It may be mainly due toreaction 10, which is not plausible when CO2 is the limiting reactant.Also, carbon formation decreases with increasing CO2/CH4 ratio (N1)at a constant temperature, since CH4 becomes more intensively alimiting reactant and the amount of H2 available for reactions 10 and11 are less, which results in a decrease of carbon formation. Moreover,in this case, reaction 8 is not much involved in carbon formation dueto CH4 being a limiting reactant.

According to thermodynamic calculations presented in Figs. 6 and7, negligible amount of carbon would be formed to achieve a H2/COratio of unity at temperature higher than 1173 K for CO2/CH4 ratio of1. However, about 0.7 mol solid carbon is produced in order to obtaina H2/CO ratio of about 2 for methanol production at a temperaturehigher than 1123 K and CO2/CH4 ratio of 0.5. Although CO2 reformingof methane seemed to be a suitable reaction system for production ofsyngas to methanol production significant amount of carbon for-mation lowers the process efficiency and increases the operationalcost. Moreover, CO2 reforming of methane would be a useful methodfor syngas production with a negligible amount of carbon in DMEproduction, since production of DME needs a H2/CO ratio of unity [18].

Choudhary et al. [25] compared the rate of carbon depositionover different Ni-, Co-, or noble metal containing catalysts at 1123 Kduring the CO2 reforming of methane with CO2/CH4 ratio of 1.1. TheRu (5 wt.%) /Al2O3 achieved methane conversion of 97.8%, 1.9 mol ofH2, and H2/CO ratio of 0.96. They reported 0.106 mol of solid carbon,approximately identical to their corresponding equilibrium quanti-ties. The moles of the products have been calculated based on 2 molof reactants containing CO2 and CH4 for comparison with our work.Meanwhile, no carbon was formed when CoNdOx (Co/Nd=1) wasused as a catalyst [25]. Resistance of this catalyst to carbon depositionwas attributed to the strong interaction between metallic Co andNd2O3 particles, which assisted in the uniform dispersion of themetallic Co on the Nd2O3 support [25]. However, NiMgOx (Ni/Mg=1)showed a higher amount of carbon deposition in the order of 0.35 molat the same temperature (1123 K). Practically, it may suggest applyinga higher temperature than the equilibrium limiting temperature (thetemperature above which carbon formation is avoided from the re-action equilibrium) to prevent carbon deposition. On the other hand,using higher operation temperature leads to consuming more energyand even catalyst sintering. It has been proven that graphitic carbon isinevitable to form using the nickel catalyst, especially for CO2/CH4

ratio b1. The greater carbon deposition on the latter catalyst may beascribed to the inherence of the kinetic mechanism for CO2 reformingof methane on Ni catalysts. Ni catalysts kinetically catalyze CO


disproportionation, CH4 decomposition and CO dissociation [47,48].As a matter of fact, carbon deposition arises from both CH4 de-composition and intensive CO2 dissociation on the catalyst surface[46,47], which causes excess carbon formation compared to thermo-dynamic equilibrium quantity. The initial step of CO2 reforming ofmethane is methane decomposition to solid carbon (and H2) which isthen oxidized with CO2 (via reverse disproportionation reaction) inthe reaction system or kinetically by the reducible oxides on thecatalyst surface to produce CO [46,48–50]. In the absence of sufficientreducible oxides on the catalyst structure, carbon would be deposited.In the following, CO2 dissociates to CO and O on the catalyst surface;therefore, O reoxidizes the catalyst surface and enables it to be com-petent for ceaseless redox reactions to remove solid carbon.When therate of methane decomposition surpasses the CO2 dissociation, solidcarbon would be deposited [48]. Hence, the presence of CO2 in theextent to remove solid carbon formed can be determined by the redoxcapability of the catalyst structure in addition to the carbon-freetemperature of the equilibrium reaction system.

Noble metals such as Pt, Ru, Rh and Pd are reported to be superiorresistant towards carbon formation [23,48], in some cases even forCO2/CH4 b1 [51]. For example, no carbon formation was seen on theRh/γ-Al2O3 monolithic catalyst for CO2/CH4 ratio of 1 and 0.71;however, solid carbon was observed over the investigated tempera-ture range based on our thermodynamic equilibrium calculations.Moreover, the carbon deposition mechanism on Rh-containingcatalysts may be different from Ni-containing catalyst [47]. Therefore,different types and preparation methods for CO2 reforming catalystsmake diverse type of deviation from the thermodynamic equilibriumprofile.

3.2.5. Water productionThe number of moles of water produced at different temperatures

and CO2/CH4 ratios are exhibited in Fig. 8. Although similar reducingtrends can be seen before 973–1023 K (depending on considered CO2/CH4 ratio), two different trends are observed after that. Accordingto the above discussion, exothermic reactions 10 and 11 (CO2+2H2↔C+2H2O and H2+CO↔H2O+C) are associated with carbonformation as well as water production at lower temperatures. Hence,with increasing temperature and decreasing equilibrium constants forreactions 10 and 11 (see Fig. 1); the number of moles of water reduceslike carbon as shown in Fig. 8. Furthermore, exothermic reactions 16and 17 limit water production when temperature increases. Mean-while, the amount of water formation for CO2/CH4 equals to 0.5 waslower than that for CO2/CH4 ratio of 1, possibly due to reaction 10,

Fig. 8. Moles of water as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.

which is not plausible when CO2 is the limiting reactant, just as wasdiscussed for carbon formation. Increasing CO2/CH4 ratio ( N1) atspecified temperature (b973 K) causes a decrease of water formationsince fewer hydrogen atoms would be available to form water andeven H2 as CH4 is the limiting reactant. This can be corroborated byFig. 4 where H2 production decreases at CO2/CH4 ratios larger than 1at the identical temperatures.

Raising the CO2/CH4 ratio from 0.5 to 3 increases the moles ofwater at higher temperature (N1000 K). It is in good agreement withthe results of equilibrium composition of water that did not considersolid carbon irrespective of the amount of water formed [14,43]. Themoles of water formed considering carbon deposition exceeded theequilibrium ones when carbon formation was not taken into accountdue to participation of CH4 decomposition, carbon monoxide andcarbon dioxide hydrogenation reactions associated with solid carbonand water production while carbon deposition is considered. FromFig. 8, the increasing number of moles of water at CO2/CH4 ratios being2 and 3 within the temperature range of higher than 973–1023 K(depending on the reactant molar ratios) is reasonably owed to RWGSreaction (reaction 2) which is confirmed by enhanced moles of CO,declined moles of H2 and smooth increase of CO2 conversion. Theseoutcomes are compatible with preceding research regardless of a littledifference in the value of the temperature at which water formationincreased [14,44]. However, exploiting Ir/CGO catalyst in CO2

reforming of methane reduced water production at higher than700–800 °C depending on the CO2/CH4 ratios due to improvingpreferable reaction between water formed and methane (steamreforming of methane) which lowered water concentration [41].

3.2.6. Ethylene and ethane productionThe number of moles of C2H6 and C2H4 produced at different

temperatures and CO2/CH4 ratios are depicted in Figs. 9 and 10. Thesefigures illustrate that the number of moles of C2H6 and C2H4 formed inthe reaction system decreases at higher CO2/CH4 ratio as fewer Hatoms from methane are available to react for C2H6 and C2H4

production.As can be seen from Figs. 9 and 10, moles of C2H6 and C2H4 initially

enhance with increasing temperature as both reactions 3 and 4are endothermic. Moles of C2H6 go through a maximum around723–773 K (depending on reactants molar ratio) but decreases at thehigher temperature as reaction 5 consumes moles of C2H6 to formC2H4. The temperature at which moles of C2H6 start to reduce almostcoincides with the temperature at which reaction 5 occurs (see Fig. 1).

Fig. 9. Moles of C2H6 as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.

Fig. 10. Moles of C2H4 as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.

Fig. 11. Moles of methanol as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.

Fig. 12. Moles of DME as a function of temperature and CO2/CH4 ratio at 1 atm forn (CH4+CO2)=2 mol.


From Fig. 10, two different trends for C2H4 formation are observeddepending on the reactant molar ratio. When CO2/CH4 ratio b1, thenumber of moles of C2H4 increases with increasing temperature. Thisresult resembles that of previouswork [14] regardless of the values forC2H4 formed. In this case, considering CO2 as the limiting reactant,there are sufficient H atoms available from CH4 that can contribute inthe reactions leading to C2H4 production. Moreover, moles of C2H4

increases with a swift incline owing to reaction 5 which multipliesC2H4 formation. From Fig. 10, a considerable increase of moles of C2H4

is exhibited relative to that of C2H6 (see Fig. 9) at temperatures higherthan 1073 K due to the higher equilibrium constant in reaction 4compared to reaction 3 (see Fig. 1).

From Fig. 10 the trends for CO2/CH4 ratios being 2 and 3, are similarto those for CO2/CH4 ratio b1, except at a temperature above923–973 K (depending on the reactant molar ratio) at which thenumber of moles of C2H4 decreases. The decreasing trend of C2H4 forCO2/CH4 ratios being 2 and 3 reveals that there is a relation betweenreaction temperature and contribution of H atom from methane atequilibrium system leading to a notable effect on C2H4 formation[14]. As a result, temperatures lower than 1073 K and CO2/CH4 ratiosbelow 1 is more in favor of C2H6 production than C2H4 althoughtemperatures above 1073 K and similar CO2/CH4 ratio are morefavorable to C2H4 production as the predominant product relative toC2H6. As CO2/CH4 ratio b1 produces a large amount of carbon (seeFig. 7) which causes clogging in the reactor, CO2/CH4 ratio of 1 andtemperature of 1273 K are suggested for C2H4 production at whichcarbon deposition is negligible. Meanwhile, due to production of thelarge amount of solid carbon at CO2/CH4 ratio b1, it seems CO2/CH4

ratio of about 2 and temperature of about 1000 K is suitable for higherC2H6 production while eliminating solid carbon. Since the number ofmoles of C2H6 and C2H4 formed are very low, an active and selectivecatalyst with decreasing operational temperature is strongly neces-sary. The catalysts performance using CaO/MnO/CeO2 to produce C2(ethylene and ethane) has shown the optimal factors of CO2/CH4 ratioand reactor temperature at 2 and 1127 K, respectively, correspondingtomaximumCH4 conversion of 5.1% and C2 hydrocarbons yield of 3.9%[52]. However, the value of equilibrium CH4 conversion is 99.5%, whilean only negligible amount of C2 equilibrium yield is calculated. Theselarge differences from the thermodynamic equilibrium values arisefrom the synergistic effect between the catalyst basicity andreducibility, enhancing the activity towards CO2 OCM (reactions 3and 4) [52,53]. Some preceding researches for C2 production fromCO2 reforming of methane over different metal oxide catalysts such

as CeO2–ZnO, MnO2–SrCO3 and MnO/CaO/CeO2 have been reported[54–57]. Although the conditions were similar with the simulation, amaximum yield of about 4.5% was achieved revealing a more efficientcatalyst is needed for the process.

3.2.7. Methanol and DME productionMoles of methanol and DME produced at different temperatures

and CO2/CH4 ratios are depicted in Figs. 11 and 12, respectively. Thesefigures illustrate that the amount of methanol and DME in the reactionsystem increases with enhancing temperature and reaches amaximum around 923–973 K (depending on the reactant molarratio) before declining.

From Fig. 1 and reactions 6 and 7, methanol can be produced fromsyngas (CO, H2 and CO2) at the lower temperature than ourinvestigated temperature range. Furthermore, reaction 1 is a slowreaction at the low temperature. Only small amounts of H2 and COpresent are in the reaction system and therefore, a negligible amountof methanol can be produced at low temperatures as shown in Fig. 11.Also, occurrence of quite exothermic methanation reactions (reac-tions 16 and 17) restrict methanol production at the low temperature.As temperature increases, due to increasing syngas production from

Fig. 13. The effect of pressure on a) equilibrium conversion of reactants and productsdistribution for CO2/CH4=1, 1173 K and n (CH4+CO2)=2 mol and on b) carbondeposition as a function of temperature.


reaction 1, the number of moles of methanol produced fromreactions 6 and 7 increases to a maximum. RWGS, an endothermicreaction is much faster at higher temperatures and seems to be themain competitive reaction for methanol formation [56]. Hence, RWGSreaction dominates and decreases methanol at a temperature N900 K.The amount of methanol for CO2/CH4 ratio equals to 0.5 is higherrelative to that for greater ratios, since CO2 would be the limitingreactant and more H atoms are available owing to the existence ofmore CH4 than CO2. Meanwhile, methanol production is diminishedwith the increase of CO2/CH4 ratio, especially at higher than 1 as CH4

more intensively plays the role of a limiting reactant. Although Fig. 11shows that moles of methanol have the highest value at CO2/CH4 ratioof 0.5 and temperature in the order of 923 K, moles of carbon formedare significant. Therefore, a temperature in the order of 1000 K andCO2/CH4 ratio of 2 seem to be appropriate for methanol productionwith negligible carbon deposition. However, the amount of methanol isstill very low for the mentioned temperature and a highly selective andactive catalyst is desirable for improving the methanol production.

Similar trends for DME and methanol production as a function oftemperature revealed in Figs. 11 and 12 are elucidated by reaction 15(an exothermic reaction) at whichmethanol is consumed to formDME.DME production is supported by reaction 15 at the lower temperature,reach a maximum and start to decline at a temperature of about 973 K,almost the same atwhichmethanol begins to decrease. Reactions 12, 13and 14 are active in the whole investigated temperature range andreduce the number of moles of DME. Similar to methanol production,CO2/CH4 ratio being 0.5 and temperature around 973 K is not a suitablecondition for DME production due to the presence of a considerableamount of carbon. Fig. 12 reveals that temperature about 1173 K andCO2/CH4 ratio being in unity are suitable for DME production withoutconsiderable carbon formation provided that an active and selectivecatalyst is available to overcome the thermodynamic limitation on DMEformation.

3.3. Effect of pressure on reactant conversion and product distributionduring CO2 reforming of CH4

Effect of pressure is presented only for the main products: H2, CO,water and solid carbon, as the analysis of the equilibrium data for CO2

reforming of CH4 showed a slight quantity of C2H6, C2H4, DME andmethanol as byproducts. Fig. 13a portraits the effect of systempressure on CH4 and CO2 conversions, the main products distributionand H2/CO ratio at 1173 K for CO2/CH4 ratio of 1, where syngasproduction and reactant conversion attained the optimum with anegligible amount of carbon at the atmospheric pressure. CO2 and CH4

conversions are always higher at lower pressures than those at higherpressures. This suggests that at such a high temperature, greaterpressures can suppress the effect of temperature on increasedreactant conversion. These decreased trends can be expressed byendothermic CO2 reforming of methane (reaction 1), which tends toshift to the left (reactant) side, according to LeChatelier's principles.Besides, methane decomposition and CO disproportionation assist inlowering CH4 and CO2 conversions, as well as decreasing CO and H2

formation at the higher pressures. Moreover, reactions (16) and (17)contribute to lower CH4 conversion, the moles of H2 and CO, as well asto augment water formation. At 1173 K, water disappears atatmospheric pressure, but increases as the pressure increases. Thegreater quantity of water at higher pressures can be mainly expressedby RWGS [45] and in less degrees by reactions (10), (11), (16) and(17).

Contrary to the preceding study [14] which did not consider solidcarbon in the equilibrium reaction system, the presence of solidcarbon in our thermodynamic equilibrium calculations resulted ingreater moles of H2 than those of CO within the whole investigatedrange of pressure; hence, H2/CO ratio N1. As one can see in Fig. 13a,H2/CO ratio increases from 1 at 1 atm to 1.15 at 25 atm, but in

previous work [14], H2/CO ratio was less than 1. This contradictmainly arises frommethane decomposition (reaction 8) which assistsCO2 reforming of CH4 to produce the higher amount of H2. Theseequilibrium data are compatible with the thermodynamic equilibriumcalculations by S-W. Hong et al. [40].

Fig. 13b shows the effect of pressure on carbon deposition atvariable temperatures during CO2 reforming of CH4. Themoles of solidcarbon go upwith increasing pressure to attain the maximum. Shamsiand Johnson [45], performing thermodynamic calculations on the COdisproportionation and methane decomposition (as the two majorreactions of carbon formation) showed that the carbon deposited bymethane decomposition decreased with increasing the pressure,while increased by CO disproportionation. These results suggest thatdisproportionation reaction is the most favorable contributor tocarbon formation at a higher pressure, particularly when the CO2/CH4

ratio is higher than 1. Meanwhile, due to disproportionation reaction,CO decreases with a steeper incline than H2 when increasing thepressure. On the contrary, as said before methane decomposition is afavorable reaction when CO2 is a limiting reactant (CO2/CH4 b1).


3.4. The effect of O2 addition in CO2 reforming of CH4

As discussed in the previous section, a carbon-free CO2 reformingof methane is practically possible by increasing the temperaturehigher than 1173 K at the atmospheric pressure, yielding a consid-erable amount of syngas and H2/CO ratio of 1 suitable for downstreamFischer–Tropsch synthesis of different chemical products. However,producing such a high temperature for endothermic CO2 reforming ofmethane requires intensive energy consumption. In order to reducethe energy requirements and to control the thermal behavior of thereaction system, coupling exothermic reaction of methane oxidationwith the endothermic CO2 reforming of methane has attracted muchattention as a promising solution for methane reforming to syngas inrecent years [46]. To obtain an understanding of syngas productionwith no carbon deposition and with minimum loss of syngas at alower temperature, a thermodynamic analysis was performed foroxidative CO2 reforming of methane with an identical amount of CH4

and CO2, since the optimum condition for syngas production withnegligible carbon deposition was achieved for this ratio in CO2

reforming of methane. The main reactions involved in methaneoxidation are considered in Table 3 numbered as reactions 1 and 2.

The effects of adding different O2 contents to CO2 reforming ofmethane and temperature on the equilibrium reactant conversions,moles of H2, CO, H2O and H2/CO ratio are depicted in Fig. 14 for CO2/CH4 ratio of 1. In comparing Fig. 2 and 14a, introducing different O2

contents to CO2 reforming of methane caused higher CH4 conversionsrelative to those of CO2 reforming of methane within the wholeconsidered temperature range (N973 K). The increment was morepronounced at higher O2/CH4 ratios since CH4 became more intensivelimiting reactant while being consumed in CO2 reforming, partial andtotal oxidation of methane (reactions 1 and 2 in Table 3). In themeantime, O2 was fully consumed for all O2/CH4 ratios, while CH4

conversion via partial or total oxidation of methane depending on O2/CH4 ratios was augmented. Against CH4 conversion, increasing O2/CH4

reduced CO2 conversion due to contribution of O2 in the totaloxidation of methane leading to more CO2 production.

It can be deduced from previous research [46] total oxidation ofmethane is significantly predominant reaction over dry reforming ofmethane when O2/CH4 ratio reaches minimum 0.75 at 973 K and evengreater than that at higher temperatures. Therefore, total oxidation ofmethane did not prevailingly govern on our reaction system with thedefined operational conditions: O2/CH4 ratio b0.4 and temperatureN973 K. These results are consistent with the results obtained by theother researchers [46,58,59].

According to Fig. 14a and b, the moles of CO and H2 decreased withaugmenting O2/CH4 ratios, as more moles of O2 would participate inthe partial and total oxidation of methane resulting in plausibleburning H2 and CO into CO2 and water. Although the variation ofequilibrium moles of H2 presents a descending trend with increasingO2 contents and is in good agreement with the previous studies [32],practically, it may differ in some cases depending on various catalysts.For example, according to a research on oxidative CO2 reforming ofmethane over Co–Ni/Al2O3 catalysts [46], initial increase appeared foroverall H2 production before its decrement, probably due to oxidationof carbon residue (CxH1− x species) produced from CO2 reforming ofmethane to H2 and CO.

From Fig. 14c, d and e, greater temperatures (N1023 K) favor lowerH2, higher CO and water production implying the considerable

Table 3Oxidation reactions in oxidative CO2 reforming of methane.

No Name of reaction Reaction formulae ΔH 298 (kJ/mol)

1 Partial oxidation of methane CH4+1/2O2↔CO+2H2 −362 Total oxidation of methane CH4+2O2↔CO2+2H2O −8023 Carbon oxidation C (s)+1/2O2↔CO −110

contribution of RWG in the combined oxidation and CO2 reformingof methane. Avila-Neto et al. [60] have proven that for CO2 reformingof methane with CO2/CH4 ratio of 1, WGS reaction is the mainresponsible for increasing the production of H2 and fading that ofwater higher than 973 K. However, it seems that addition of O2 to CO2

reforming of methane reverses the WGS direction and reinforcesRWGS. It is more pronounced at higher O2/CH4 ratios and tempera-tures, which can be substantiated by a steeper decrease of H2

simultaneously, along with a steeper increase of water and CO. It maybe attributed to the availability of more CO2 produced from totaloxidation of methane.

With reference to Fig. 14e, the decreasing trend of water productionbelow 1023 K may be ascribed to exothermic reactions 10 and 11associated with carbon formation as well as reactions 16 and 17(methanation reactions). Nevertheless, the variation trend of waterproduction changed to a linear functionality over O2/CH4 ratio of 0.3,since total oxidation of methane proceeds rather in the reaction systemwith burningmore H2 formed to produce a significant amount ofwater.

The effect of different O2 contents on H2/CO ratios as a function oftemperaturehas beendepicted in Fig. 14f. H2/CO ratios decreased as lowas unity at a temperature in the order of 1050 K and almost did not varyfrom 1050 to 1273 K. Comparing the H2/CO ratios profile of our workwith those of Amin and Yaw [32] revealed a descending trend for H2/COratios as O2/CH4was incremented at lower 1073 K in ourwork,whereasan ascending trend was observed for the results of Amin and Yaw [32].This is due to not considering solid carbon formation by Amin and Yawin the reaction system. In fact, solid carbon formed at lowertemperatures would be burned by oxygen via reaction 3 (mentionedin Table 3) (C+1/2O2↔CO) and reverse disproportionation(C+CO2↔2CO) leading to a higher amount of CO and lower H2/CO,particularly at greaterO2/CH4 ratios. The increment of themoles of COatgreater O2 contents at lower temperature (b1000 K) in Fig. 14d as anevidence for lower H2/CO ratio at less than 1000 K can be considered.

Fig. 15 presents the moles of carbon formed in the reformateproduct of oxidative CO2 reforming of methane in chemicalequilibrium as a function of temperature with a pressure of 1 atmand CO2/CH4 ratio of 1. Carbon formation decreased while raising thetemperature in oxidative CO2 reforming of methane just as justifiedfor decreasing carbon deposition regarding CO2 reforming of methanein previous section. However, the employment of O2 in CO2 reformingof methane caused a steeper diminish of carbon deposition comparedto CO2 reforming, which is due to the effective role of co-oxidant of O2

in burning solid carbon. When the temperature increased over1073 K, no carbon deposition was formed for all O2/CH4 ratiosstudied. The O2/CH4 ratio of 0.1 at the temperature of 973 K inoxidative CO2 reforming of methane decreased the carbon depositiononly 28% less than in CO2 reforming ofmethane.Moreover, conversionof CH4, that of CO2 and the amount of syngas obtained under thiscondition were as low as 92%, 61% and 2.5 mol, respectively, but theamount of water was as great as 0.31 mol. However, the employmentof the same value of O2/CH4 at higher temperature, 1073 K, caused adecrement in carbon formation by 93% less than in CO2 reforming ofmethane, while conversion of CH4, that of CO2 and the amount ofsyngas reached to 96.5%, 87% and 3.5 mol with H2/CO ratio of 1,respectively. Meanwhile the water formation attained its minimum inthe whole range of temperature and O2/CH4 studied. It is alsoobserved that although the temperature higher than 1073 K favorsthe reactants conversion, caused a slight decrease of H2 productionand an increase of water formation.

Although higher O2 contents eliminated solid carbon at lowertemperatures, there is a significant decrease in the amount of syngasproduction and an increase in water formation. Previous studies [32]proposed O2/CH4 ratios from 0.1 to 0.2 were suitable for oxidative CO2

reforming of methane at 1073 K, But referring to Fig. 14 in the presentwork, ratios greater than 0.1 caused about 7% decrease of H2 and 59%increment of water productions.

Fig. 14. Equilibrium conversion of reactants and product distribution as a function of temperature and O2/CH4 ratio at 1 atm and CO2/CH4=1 for n (CH4+CO2+O2)=2 mol: (a) CH4

conversions; (b) CO2 conversions; (c) moles of H2; (d) moles of CO; (e) moles of water; and (f) H2/CO ratios.


In a previous literature [32], feed ratio of CH4:CO2:O2=1:1:0.2wasreported as the optimal condition at a temperature below 1000 Kwhen solid carbon deposition was not considered in thermodynamiccalculations. However, referring to Figs. 14 and 15, a minimum valueof solid carbon in the order of 0.16 mol and that of water in the orderof 0.25 mol still remained in the oxidative CO2 reforming reactionsystem. Accordingly, based on our results only the optimal operatingtemperature of 1073 K and feed ratio of CO2:CH4:O2=1:1:0.1 havebeen justified to obtain reactant conversions and syngas yields of

higher than 90% as well as to maintain H2/CO ratio of unity, whileminimizing water formation and eliminating solid carbon.

4. Conclusion

The thermodynamic chemical equilibrium analysis of CO2 reform-ing of methane to form syngas, C2, methanol and DME consideringthe reactions involved in carbon deposition was studied using Gibbsfree energy minimization. The CO2/CH4 reactant ratio and reaction

Fig. 15.Moles of solid carbon as a function of temperature for different O2/CH4 ratios at1 atm and n (CH4+CO2+O2)=2 mol.


temperature had a considerable influence on the equilibrium of thereactants conversion, equilibrium composition of the products andsolid carbon formation. Four reactions regarding carbon formationwere considered: hydrogenation of carbon dioxide, hydrogenation ofcarbon monoxide, methane decomposition and disproportionation.Methane decomposition, an endothermic reaction, is the key reactionfor carbon deposition when CO2/CH4 b1 leading to increased moles ofH2 and constant moles of solid carbon even at a very hightemperature. Adversely, hydrogenation of carbon dioxide is notmuch involved in carbon deposition when CO2/CH4 b1, and it can beattested by lower carbon deposition for CO2/CH4 in the order of 0.5relative to that of 1 at a temperature less than 723 K. At CO2/CH4 N1,three hydrogenation reactions, hydrogenation of carbon dioxide,hydrogenation of carbon monoxide, and disproportionation are morepertinent to carbon formation than methane decomposition, as CH4 isthe limiting reactant. This was verified by a gradual decrease of water,hydrogen, solid carbon formation, and a decrease of CO2 conversion athigher CO2/CH4 (N1). Meanwhile, the number of moles of carbon forCO2/CH4 ratio of 0.5 is lower than that of CO2/CH4 ratio being 1 attemperatures less than 723 K. It may be mainly due to reaction 10,which is not plausible when CO2 is the limiting reactant. RWGSreaction was not fast and active in low temperature and CO2/CH4 b1,but favorable at temperature N973 and CO2/CH4 ratio N1 attested bythe decreasing trend of H2 production contrary to increasing trend ofCO production within the mentioned ranges. From the foregoingthermodynamic equilibrium calculations and analysis, we have founda considerable range of H2/CO ratios regarding CO2 reforming ofmethane that can facilitate industrial downstream application ofsyngas. An optimal working condition for syngas (with H2/CO in theorder of 1) using FTS can be chosen at a temperature higher than1173 K for CO2/CH4 ratio being 1 at which about 4 mol of syngas canbe produced from 2 mol of reactants (CO2+CH4) with a CO2

conversion of more than 98% and only a negligible amount of carbonformation. Choosing CO2/CH4 ratio b1 is not suggested as the solidcarbon formation is quite high within the entire consideredtemperature range (573–1473 K). Thermodynamic chemical equilib-rium analysis and standard Gibbs free energy calculations reveal thatreactions 3, 4 and 5 are less favorable to ethylene and ethaneformation. Therefore, the yields of C2H4 and C2H6 production may beincreased using a highly active and selective catalyst. CO2/CH4 ratio of1 and temperature of about 1273 K are suggested for C2H4 productionat which carbon deposition is negligible. Meanwhile, CO2/CH4 ratiobeing 2 and temperature 1000 K are appropriate for higher C2H6

production while eliminating solid carbon. Numerical results show

that a temperature around 1000 K and CO2/CH4 ratio of 2 aresuggested for methanol production with considerable carbon elimi-nation. Meanwhile, the temperature about 1173 K and CO2/CH4 ratioof 1 seem to be suitable for DME production with a negligible amountof solid carbon. Since the numbers of moles of methanol and DMEformed were very small, an active and selective catalyst is highlydesired to improve the yield of methanol and DME production. Effectof pressure on CO2 reforming of methane revealed that methanedecomposition and CO disproportionation assisted in lowering CH4

and CO2 conversions, as well as decreasing CO and H2 formation athigher pressures. At a higher pressure, disproportionation reaction isthe most favorable contributor to carbon formation especially whenthe CO2/CH4 ratio N1, but methane decomposition was the maincontributor when CO2/CH4 b1. Increasing O2 contents in CO2

reforming of methane had increased CH4 conversion while the riskof carbon formation at lower temperatures was reduced. Additionally,it has a significant effect on decreasing CO2 conversion, CO and H2

yield but water yield was observed to increase. From thermodynamicequilibrium analysis, optimal operating temperature of 1073 K andfeed ratio of CO2:CH4:O2=1:1:0.1 at atmospheric pressure gaveconversions and syngas yields higher than 90%. At the optimumcondition, H2/CO ratio was maintained in unity, water formationminimized and interestingly carbon deposition eliminated.

Acknowledgements

The authors express their sincere gratitude to Professor Dr. Jahanmiri(Shiraz University-Iran), Professor Dr. Zainuddin Manan (Director ofPROSPECT, UTM) and Dr. Sirus Ghotbi (Sharif University-Iran) for theirkindadviceduring thesimulationprocess. Calculationswereperformedatthe Simulation Lab, Department of Chemical Engineering in UTM. Theauthors also thank the Ministry of Science, Technology and Innovation(MOSTI) Malaysia and Exxon-Mobil (Malaysia) for sponsoring thisresearch under Project no: 03-01-06-SF0210 andVot 68729, respectively.

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