Process simulation of bio-dimethyl ether synthesis from

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Energy 175 (2019) 36e45

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Energy

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Process simulation of bio-dimethyl ether synthesis from tri-reformingof biogas: CO2 utilization

Dang Saebea a, b, *, Suthida Authayanun c, Amornchai Arpornwichanop d

a Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailandb Research Unit of Developing Technology and Innovation of Alternative Energy for Industries, Burapha University, Chonburi 20131, Thailandc Department of Chemical Engineering, Faculty of Engineering, Srinakharinwirot University, Nakorn Nayok 26120, Thailandd Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok10330, Thailand

a r t i c l e i n f o

Article history:Received 26 May 2018Received in revised form16 February 2019Accepted 10 March 2019Available online 11 March 2019

Keywords:Biogas tri-reformingDME synthesisCO2 utilizationCO2 recirculation ratioCO2 removalH2O removal

* Corresponding author. Department of Chemicalneering, Burapha University, Chonburi 20131, Thailan

E-mail address: [email protected] (D. Saebea).

https://doi.org/10.1016/j.energy.2019.03.0620360-5442/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

The main contributions of this work are to study the suitable condition of biogas tri-reforming for DMEsynthesis process and to design the systems of the biogas tri-reforming process coupling with the DMEsynthesis. The effects of operating parameters in terms of boundary of carbon formation, steam to carbonratio, and oxygen to carbon ratio on the biogas reforming process are firstly investigated. To utilize moreCO2 in the system, CO2 produced from the DME synthesis is recycled to use in the biogas tri-reformingprocess. The H2 and CO yields of the tri-reforming process increase with increasing the CO2 recirculationratio while the DME yield and system efficiency decrease. The requirement of gas cleaning unit for theDME synthesis coupling with the biogas tri-reforming system is also analyzed. The results indicate thatthe system with CO2 removal from syngas has more impact on the DME yield than that with H2Oremoval. On the contrary, the total CO2 emission intensity of the system with H2O removal is lower thanthat with CO2 removal. When comparing all cases, the system with both H2O and CO2 removals achievesthe highest DME yield and system efficiency.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Increasing the energy demand of transportation sector results ina raising and fluctuating oil price. An environmental problem hasbeen also extremely concerned. Dimethyl ether (DME) has beenconsidered as a promising fuel instead of diesel to use forcompression ignition engines because it does not emit harmfulgases during the combustion, i.e., NOX, SOX, carcinogenic compo-nent, smoke or particles [1]. Its prominent properties are high ce-tane number, volatile compound, non-toxic compound, and lowignition temperature [2]. For the DME synthesis, there are tworoutes: the two-step and single-step methods. In the first route, thesynthesis gas is firstly converted to methanol and then the meth-anol dehydration occurs to produce DME [3]. This method needstwo reactors: a catalyst for the methanol synthesis, i.e., CuO-ZnO-Al2O3, packed in the first reactor and the second reactor for the

Engineering, Faculty of Engi-d.

methanol dehydration using a solid acidic catalyst, i.e., zeolite, g-Al2O3, or ferrierite [4]. Another route of the DME production is thesimultaneous occurrence of methanol synthesis and dehydrationreactions in one reactor, with bifunctional, hybrid, or physicallymixed catalysts [5]. The single-step route is more preferred to two-step route because of its low investment, operating cost, andthermodynamic limitation [6].

In the conventional process, the DME is produced from naturalgas [7]. Due to the climate-changing, carbon emissions, and fossilfuel depletion, the syngas production from renewable sources forthe DME synthesis is an interesting choice for reducing CO2 emis-sions and substituting for using fossil fuel. Biogas is an interestingbiofuel instead of natural gas owing to its low CO2 emissions [8]. Itis generated by anaerobic digestion of organic matter [9]. Its char-acteristic is similar to the natural gas due to methane as maincomponent of biogas. Themethane concentration in the natural gasis higher than that in the biogas. CO2 is another main compositionof biogas about 30e40% [10]. In general, the biogas is combusted touse for heating or generating steam. However, the combustion ofbiogas has low efficiency due to its high CO2 content [11]. Biogascan produce hydrogen-rich syngas by reforming process, which is

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CH4

Tri-reforming of

biogas

CO2

H2O

O2

TreatmentBiogas

H2SDimethyl ether

synthesisPurification

Dimethyl ether

CO2 recycle

H2

COCO2

H2O

(a)

Separator

CH4

Tri-reforming of

biogas

CO2

H2O

O2

TreatmentBiogas

H2SDimethyl ether

synthesisPurification

Dimethyl ether

CO2 recycle

H2

COCO2

H2O

CO2 or/and H2ORemoval

(b)

Fig. 1. Configuration of DME synthesis process from biogas via the tri-reforming (a)using unpurified syngas and (b) using purified syngas.

D. Saebea et al. / Energy 175 (2019) 36e45 37

more efficient approach than the combustion. Additionally, thestorage of biogas produced excessively is rather difficult. It isburned and discharged. To deal with this problem, the conversionof biogas to liquid fuel is the efficient storage of excess biogasproduced.

There are various reforming processes of hydrocarbon for thesyngas production, i.e., steam reforming, partial oxidation, and dryreforming [12]. In order not to add the separation process of CO2from biogas, CO2 in the biogas can be used as agent to produce H2and CO via dry reforming. This process is the CO2 utilization whichcan reduce the amount of CO2 emissions [13]. Nevertheless, the dryreforming requires high energy consumption and easily occurscarbon deposition on the catalyst [14]. To avoid these problems, thetri-reforming process as an interesting process for syngas produc-tion from biogas involves the steam reforming, partial oxidation,and dry reforming [15]. The addition of steam and oxygen in thereforming process alleviates the carbon formation on the catalystsurface. Using the steam as agent can also obtain higher hydrogenyield, while the partial oxidation occurringwith feeding oxygen candecrease the energy input required in the reformer due to theexothermic reaction.

The DME production has been extensively studied in the liter-ature [16,17]. The suitable operating condition of DME production isin range between 200 and 300 �C and high pressure [18,19]. The H2/CO ratio in the syngas is significant parameter on the DME yield.The excess H2 in syngas is preferred in the methanol synthesisreaction [20]. Conversely, themethanol dehydration reaction favorsin high CO concentration in the syngas [21]. Most research hasfocused on the influence of H2/CO ratio on the DME yield. Thesynthesis gas from the reforming process composes of other gases,such as H2O and CO2, which affect the DME yield as well. Chen et al.[22] reported that co-feeding of CO2 with H2 and CO leads to thereduction of the DME yield in the single-step process. Zhang et al.[23] investigated the optimum heat exchange network for theprocess of DME production from methane. The CO2 in syngas isseparated from other gases via the membrane separator in thisprocess. Luu et al. [24] studied the DME production in the single-step process from the syngas produced from the dry methanereforming. In their process, the syngas from the reformer is dehy-drated before feeding to the DME synthesis unit. From literaturereviews, the studies on the suitability of direct feeding of the syn-thesis gas from reformer to the DME synthesis and the necessary ofpurified-syngas unit with the removals of H2O and/or CO2 on theDME yield and the system efficiency have not been clarified yet.

CO2 can be consumed and produced in the single-step process ofthe DME synthesis. Recently, the requirement of efficient CO2 uti-lization to reduce the CO2 emissions has motivated in severalstudies. Ateka et al. [25] investigated the influence of CO2 utiliza-tion by CO2 co-feeding with CO and H2 to produce the DME. Theirresults indicated that more CO2 contents in the feed stream canincrease the CO2 conversion. Nevertheless, the high CO2 in the feedhas detrimental effects on the oxygenate yield and selectivity. Theproper trade of CO2 utilization and the reduction of DME yieldshould be concerned. In general, the amount of CO2 consumed inthe DME synthesis is less than that of CO2 produced. The recyclingof CO2 to use in the CO2 required unit can reduce the net CO2

emissions from the system. In the combined unit of the tri-reforming and the DME synthesis, CO2 produced from the DMEsynthesis can be used as the co-feeding with hydrocarbon to pro-duce the syngas. However, the biogas already consists of high CO2

content. The suitability of adding CO2 in the biogas tri-reformingprocess should be studied to achieve the highest CO2 utilizationand syngas yield. Moreover, the operating parameters of bothbiogas tri-reforming and DME synthesis processes have direct ef-fect on their performance and system efficiency. The study of

parameter relation between the tri-reforming of biogas and DMEsynthesis processes has been limited.

The aims of this work are to study and design the system of theDME synthesis from syngas produced via the biogas tri-reforming.The parameter relation between the biogas tri-reforming and theDME synthesis processes is considered. The proper condition of thebiogas tri-reforming, i.e., carbon formation boundary, steam tocarbon ratio, and oxygen to carbon ratio is firstly analyzed. To un-derstand the depth of the effect of syngas components on the DMEsynthesis process, the performance of DME synthesis usingunpurified syngas and purified syngas with the removal of CO2 and/or H2O is studied in this work. The suitability of CO2 recycling fromthe DME synthesis process to use in the biogas tri-reforming is alsotaken into consideration in terms of the system efficiency and CO2emission intensity. These results can indicate the conceptual designof DME synthesis system from biogas, which can help and improvethe system efficiency of the DME synthesis couplingwith the biogastri-reforming.

2. Process description

This section shows the process configuration and the method-ology used to evaluate and analyze this work. Fig. 1 presents anoverview of process for the DME production from biogas. In theintegrated process of the biogas tri-reforming and DME synthesis,there are three main units, such as syngas production, DME syn-thesis, and purification, as shown in Fig. 1(a). Unpurified syngasobtained from the tri-reforming unit is directly used to produce theDME. To improve the DME yield, the separators for removing CO2and/or H2O from the syngas are added before the DME synthesisunit (Fig. 1(b)). To analyze the integrated process, the four config-urations for the DME production are investigated. In the firstconfiguration, the unpurified syngas is directly fed to the DMEproduction without the CO2 and H2O separation steps. The secondand third configurations are rectified by adding the separation unitto remove H2O or CO2 from syngas before feeding to the DMEsynthesis unit. Finally, the system coupling with both H2O and CO2separation steps from syngas to produce the DME is considered.

Fig. 2 shows the detailed flowsheet of the integrated processbetween the DME synthesis and tri-reforming process. The biogaspurified by removing the H2S is used as a feedstock for the tri-reforming process. Biogas containing a mixture of 60% CH4 and40% CO2 is fed with steam and oxygen to the reformer. Biogas,

TRI

COMDME

CO-2

HE1BIO

COMBIO

CO-1

PUMP

MIX-TRI

HE1H2O

COMOXYHE1O2

RDME

HE-DIS

D1

SEP

B6

HE-R

B8

CO-3

MEM

COOL4

FLASH

B5D2

BIO1

SYN1

SYN3

MIX1-BIO

SYN4

HE1-BIO

SYN2

WATER1HE1-H2O

MIX1-H2O

INTRI

OXY1

HE1-O2 MIX1-O2

P1

P6BOT1

DIS1

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OUT

R2R3

P5

P2

RCO2

P3 P4

S5

S6

S7

DIS2

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(a)

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CO-2

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SEPCO2

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PUMP

MIX1-TRI

HE1H2O

COMOXYHE1O2

RDME

SEPH2O

CO-3 D1

D2

SEP-R

V-R

HE-R

MEM

FLASH

HE-FLASH

VDIS2VDIS1

HE-DIS

BIO1

SYN4

SYN1

SYN5

MIX1-BIO

CO2OUT

SYN6

HE1-BIO

SYN2

HE1-H2O MIX1-H2O

INTRIOXY1

HE1-O2MIX1-O2

P1

SYN3

H2O

P2

DIS1

BOT1

DIS2

BOT2

R1

OUT

R2

R3

RCO2

P3

OUT1

P5

P4

P8

P6P7

(b)Fig. 2. Flowsheet of combined system between tri-reforming process and DME synthesis (a) system without both removals and (b) system with CO2 and H2O removals.

D. Saebea et al. / Energy 175 (2019) 36e4538

steam, and oxygen are compressed and preheated to the operatingcondition of reformer. The syngas produced via the tri-reformingconsists of high H2 and CO contents. The syngas is cooled andconsequently compressed before introducing to the DME synthesisunit. DME is directly produced from the unpurified syngas withoutthe separation step, as shown in Fig. 2(a). The product stream ob-tained from the DME synthesis unit is sent through the membraneseparator to separate CO2 [21] and then purified by the flash andtwo distillations. In the flash unit, the mixed gases are separatedfrom a liquid stream which contains DME, methanol, and water.The liquid stream from the flash is sent to the first distillationcolumn in order to separate the DME from the mixed liquid.Consequently, a condensate as the mixture of methanol and waterfrom the first column is also separated in the last distillationcolumn.

For the second system coupling with the separation process of

H2O, the syngas at the reformer outlet is cooled and compressed at25 �C and 50 bars. Then, the syngas is fed to the gas cleaning unit.The water is condensed and separated from the light gases in theflash drum. The dehydrated syngas is fed to the compressor to in-crease pressure at the operating condition for the DME synthesis.For the third system coupling with the CO2 separation process, CO2in the syngas is separated through the membrane separator [23]. Inthis work, the fraction of CO2 removal from syngas is fixed at 0.95.The syngas with CO2 removal is compressed and cooled beforeintroducing to the DME synthesis unit. Fig. 2(b) shows the fourthsystem coupling with the separation unit of both H2O and CO2. H2Ois separated from syngas before introducing to CO2 separator.Finally, the purified syngas with the CO2 or/and H2O removals is fedto the DME synthesis. The DME purification process of all systems issimilar.

In the process, the reactions for the biogas tri-reforming and


DME synthesis are considered as equilibrium state of phase andcomposition. The most common method used for an analysis tocalculate equilibrium composition is Gibbs free energyminimization.

2.1. Tri-reforming of biogas

In the tri-reforming of biogas, methane reacted with H2O, O2,and CO2 is converted to H2, CO, CO2, and H2O via the reformingreactions concluded in Table 1. Peng-Robinson equation of statethat can exactly predict for hydrocarbon mixtures is used tocalculate thermodynamic parameters. To evaluate the proper con-dition of biogas tri-reforming, the consumption of H2O and CO2 isconsidered in terms of H2O and CO2 conversions calculated by thefollowing equations:

% CO2 conversion�xCO2 ;r

� ¼FrCO2;in

� FrCO2;out

FrCO2;in� 100 (1)

% H2O conversion�xH2O;r

� ¼ FrH2O;in � FrH2O;out

FrH2O;in� 100 (2)

The amounts of H2 and CO produced are important indicator ofsyngas quality from biogas tri-reforming. H2 and CO yields arecalculated as expressed in Eqs. (3) and (4).

H2 yield ð%Þ : YH2;r ¼FrH2;out

2FrCH4;inþ FrH2O;in

(3)

CO yield ð%Þ : YCO;r ¼ FrCO;outFrCH4 ;in þ FrCO2;in

(4)

where Fri;in and Fri;out aremolar flow rates of species i at the reformerinlet and outlet, respectively.

2.2. DME synthesis

DME is produced from the syngas that mainly composes ofhydrogen and carbon monoxide. The single-step process as a directsynthesis of DME from the syngas by combining the methanolsynthesis reaction and methanol dehydration reaction in the singlereactor is considered. All reactions of the single step for the DMEsynthesis can be expressed by the following reactions [26].

Methanol synthesis reaction: CO þ 2H2 4 CH3OH (5)

Methanol synthesis reaction: CO2 þ 3H2 4 CH3OH þ H2O (6)

Methanol dehydration reaction: 2CH3OH4 CH3OCH3 þ H2O (7)

Table 1Reactions of biogas tri-reforming process.

Reaction

Steam reforming CH4 þ H2O 4 CO þ 3HDry reforming CH4 þ CO2 4 2CO þ 2Partial oxidation CH4 þ 0.5O2 4 CO þ 2Combustion of methane CH4 þ2O2 4CO2 þ 2HMethane decomposition CH4 4 C þ 2H2

Boudouard reaction 2CO 4 C þ CO2

Hydrogenation of carbon dioxide CO2 þ 2H2 4 C þ 2H2

Hydrogenation of carbon monoxide CO þ H2 4 C þ H2OCombustion of carbon C þ O2 4 CO2

Water gas shift reaction: COþ H2O4 CO2 þ H2 (8)

For the DME synthesis unit, Soave-Redlich-Kwong equation ofstate is applied to explain non-ideal behavior for polar mixtures,i.e., methanol and dimethyl ether at high pressure condition.

To identify the performance of the DME synthesis, DME yield isevaluated by the following equation:

% DME yield ðYDMEÞ ¼2FDME;out

FCO;in þ FCO2;in� 100 (9)

In the single-step process of DME synthesis, the DME andmethanol are produced. Thus, the DME selectivity is considered,according to Eq. (10).

DME selectivity ðSDMEÞ ¼2FDME;out

2FDME;out þ FMeOH;out� 100 (10)

The efficiency of the DME synthesis system coupling with thebiogas tri-reforming is defined:

systemefficiency�hsystem

�¼ FDME;outLHVDME

FCH4;inLHVCH4þQusedþPused

�100

(11)

where LHVi is lower heating value of species i, Qused is thermalenergy used for heater, and Pused is power consumed for pumps andcompressors.

Total CO2 emissions of each system are determined as followingequation:

Total CO2 emissions ¼ CO2;outlet þ CO2; energy

FDME(12)

where CO2;outlet is CO2 emissions in gas stream at the outlet process,CO2; energy is CO2 emissions associated with the energy consump-tion of utilities, and FDME is molar flow rate of DME.

3. Results and discussion

3.1. Sensitivity analysis of biogas tri-reforming

In the syngas production via the tri-reforming of biogas, thesuitable condition to prevent the carbon formation in process isinvestigated due to the carbon formation as a main cause of rapidcatalyst degradation. The influence of steam to carbon ratio atvarious temperatures, with specifying the oxygen to carbon ratio of0.1, on the mole fraction of carbon is presented in Fig. 3(a). It showsthat the tendency of carbon formation decreases with increasingthe steam to carbon ratio. The increase of steam results in thereverse of carbonmonoxide hydrogenation (R8) and carbon dioxidehydrogenation (R7) reactions. When considering the influence of

Heat of reaction

2 DН ¼ þ206 kJ/mol (R1)H2 DН ¼ þ247 kJ/mol (R2)H2 DН ¼�36 kJ/mol (R3)2O DН ¼�880 kJ/mol (R4)

DН ¼ þ75 kJ/mol (R5)DН ¼�172 kJ/mol (R6)

O DН ¼�90 kJ/mol (R7)DН ¼�131 kJ/mol (R8)DН ¼�394 kJ/mol (R9)

H2O/CH4 ratio

(a)

O2/CH4 ratio

(b)

Fig. 3. Effects of temperature and (a) H2O/CH4 ratio and (b) O2/CH4 ratio on molefraction of carbon formation in tri-reforming process.

Table 2Operating condition of the DME production processfrom biogas.

Parameters Value

Biogas tri-reformingTemperature (�C) 850Pressure (bar) 20O/C ratio (�) 0.1S/C ratio (�) 0.5DME synthesisTemperature (�C) 280Pressure (bar) 50


inlet oxygen to carbon ratio, the steam to carbon ratio is specified at0.5, as shown in Fig. 3(b). It can be seen that the mole fraction of

)a(

Fig. 4. Effects of inlet steam and oxygen to carb

carbon reduces at higher oxygen feeding. The increase in theamount of oxygen feeding results in driving forward of carboncombustion reaction (R9), and leading to more CO2 production. Forthe effect of operating temperature, the mole fraction of carbonobviously decreases when the operating temperature increases upto 800 �C. Because Boudouard reaction (R6) which is main reactionof carbon formation in reforming process is an exothermic reaction,the carbon formation will decrease at high temperature. However,an increase in temperature above 800 �C has an insignificant effecton the amount of carbon formation. The suitable condition ofbiogas tri-reforming should be operated above 800 �C, inlet steamto carbon of 0.5, and inlet oxygen to carbon of 0.1 to avoid theoccurrence of carbon.

Fig. 4 shows the influence of inlet steam and oxygen to carbonratios on H2 and CO yields. The addition of inlet oxygen to thebiogas tri-reforming causes a decrease in the H2 and CO yields asillustrated in Fig. 4(a) and (b). This can be explained that the excessoxygen in the tri-reforming process affects the combustion of H2and CO. The H2O and CO2 are further produced, leading to thedecrement of H2O and CO2 consumptions. For an increase in theinlet steam, the H2 and CO yields slightly reducewith increasing thesteam to carbon ratio. The proper operating condition of the biogastri-reforming to produce syngas for DME synthesis is the oxygen tocarbon ratio of 0.1 and the steam to carbon ratio of 0.5. This con-dition can achieve the highest H2 and CO yields of 84.46 and 90.87%.

3.2. Comparison of four combined systems

To utilize more CO2, CO2 produced in the DME synthesis can berecycled to use in the biogas tri-reforming. The effect of CO2recirculation ratio from DME synthesis unit to the reformer on the

)b(

on ratios on (a) H2 yield and (b) CO yield.


system performance should be investigated. For DME synthesis,more CO2 and H2O might have negative effect on the DME yield.The addition of gas cleaning unit to remove CO2, H2O, and both CO2and H2O from syngas before introducing to the DME synthesis unitis considered. In this work, there are four systems; (1) systemwithout both CO2 and H2O removals; (2) systemwith H2O removal;(3) system with CO2 removal; and (4) system with both CO2 and

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H2

yiel

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Recirculation ratio (-)

Removal of CO2Removal of H2ORemoval of CO2 and H2ONo removal of H2O and CO2No removal of CO2 and H2O

Removal of CO2 and H2ORemoval of H2ORemoval of CO2

)a(

-100

-80

-60

-40

-20

0

20

0 0.2 0.4 0.6 0.8

H2O

conv

ersi

on(%

)


Removal of CO2Removal of H2ORemoval of CO2 and H2ONo removal of H2O and CO2No removal of CO2 and H2ORemoval of CO2 and H2ORemoval of H2ORemoval of CO2

)c(

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8

H2/C

O




(e)Fig. 5. Effect of CO2 recirculation ratio to reformer on (a) H2 yield, (b) CO yield, (c) H2

H2O removals. The operating condition of the DME productionprocess from the biogas tri-reforming is concluded in Table 2.

3.2.1. Biogas tri-reforming performanceFig. 5(a)-(e) illustrate the effect of CO2 recirculation ratio on the

performance of biogas tri-reforming for four systems. From Fig. 5(a)and (b), the H2 and CO yields raise with increasing the CO2

56

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yiel

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)b(

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2co

nver

sion

(%)



)d(

O conversion, (d) CO2 conversion, and (e) H2/CO ratio from biogas tri-reforming.


recirculation ratio. This can be explained that the molar flow rate ofCO2 is higher, resulting in driving the dry reforming reaction for-ward. Consequently, moremethane is converted to H2 and CO. FromFig. 5(c) and (d), the CO2 and H2O conversions decrease when CO2recirculation ratio increases. The decrement of H2O conversion ishigher than that of CO2 conversion at higher ratio of CO2 recircu-lation. The H2O conversion is negative because the amount of H2Oconsumption is less than that of H2O production. On the otherhand, the consumed CO2 is higher than the generated CO2. This isbecause the dry reforming reaction dominates over the steamreforming in an environment with high CO2. Moreover, the highCO2 drives the reverse water-gas shift reaction forward. Observably,the increment of CO yield is higher than that of H2 yield. The H2/COratio reduces when the recirculation ratio of CO2 increases as seenin Fig. 5(e).

The influence of CO2 recirculation ratio on performance ofbiogas tri-reforming in four systems is similar tendency, except theH2 and CO yields. As can be seen in Fig. 5(a) and (b), the H2 and COyields in the systemwithout both CO2 and H2O removals increase atthe CO2 recirculation ratio in the range of 0e0.5, and considerablysubside at the CO2 recirculation ratio above 0.5. At high recircula-tion ratio of CO2, the amount of CO2 in systemwithout both CO2 and

0

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0 0.2 0.4 0.6 0.8

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Eyi

eld

(%)



()a(

60

65

70

75

80

85

90

95

0 0.2 0.4 0.6 0.8

CO

conv

ersi

on(%

)




()c(Fig. 6. Effect of CO2 recirculation ratio on (a) DME yield, (b) D

H2O removals is plentifully accumulated. This results in the over-abundant amount of CO2 for dry reforming reaction and reducesthe occurrence of steam reforming reaction. Meanwhile, the H2 andCO yields of three systems coupling to H2O and CO2 separationsteps from syngas before feeding to the DME synthesis remain upwith increasing the CO2 recirculation ratio. The H2 and CO yields ofsystem with H2O removal are higher than those of systems withCO2 removal and with both CO2 and H2O removals. Whencomparing four systems, the system without both removals at lowCO2 recirculation ratio in the range of 0e0.5 achieves the highest H2and CO yields. At CO2 recirculation ratio above 0.5, the H2 and COyields of system coupling to only water separation step are thehighest.

3.2.2. DME synthesis performanceThe effect of CO2 recirculation ratio from DME synthesis unit to

tri-reforming on the performance of DME synthesis is shown inFig. 6. From Fig. 6(a), the DME yield of all systems reduces withincreasing the CO2 recirculation ratio. The rise of CO2 content in thefeed stream of the DME synthesis process due to the enhancementof CO2 accumulation in the system has detrimental effect on theDME yield. This is because the increase of CO2 in the DME synthesis

8990919293949596979899

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ivity

(%)




)b

0102030405060708090

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)dME selectivity, (c) CO conversion, and (d) H2 conversion.

0

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ienc

y(%

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Removal of CO2Removal of H2ORemoval CO2 and H2ONo removal of H2O and CO2No removal of CO2 and H2ORemoval of CO2 and H2ORemoval of H2ORemoval of CO2

(a)

(b)

Fig. 7. (a) Effect of CO2 recirculation ratio on the system efficiency and (b) Distributionof energy consumption of four systems (R¼ 0).


process promotes the reverse water-gas shift reaction, resulting inthe raise of H2O in environment. Consequently, the CO2 hydroge-nation andmethanol dehydrogenation reactions reverse, leading tothe decrease in the DME yield. Moreover, the increase of the CO2recirculation ratio to reformer causes the increment of H2O pro-duced from the reverse water-gas shift reaction in tri-reformingprocess. The combined system between the biogas tri-reformingand DME synthesis processes can be improved with the additionof separation unit for the removal of CO2 and H2O.

As can be seen in Fig. 6(a), the DME yield of systems with CO2removal and H2O removal can enhance about 9.49 and 8.42%,compared to the systemwithout both removals at CO2 recirculationratio of 0. The removal of CO2 from syngas can promote themethanol synthesis from CO, leading to the rise of the DME yield, asobserved in Fig. 6(c). CO conversion of the systemwith CO2 removalis upper than that without both removals.

For the system with H2O removal, the methanol dehydrationreaction is increasingly driven forward. Also, the DME synthesisfrom a dehydrated syngas can promote the utilization of CO2 via themethanol synthesis from CO2 in Eq. (6). This can be considered inFig. 6(c) and (d). The H2 conversion of the systemwith H2O removalis higher than that of the systemwith CO2 removal. Conversely, theCO conversion of the systemwith H2O removal is lower than that ofthe system with CO2 removal. The CO2 produced from the DMEsynthesis of this system is rather low, compared to other systems.When considering the increase in the recirculation ratio of CO2 inthe system with H2O removal, the CO conversion decreases whilethe H2 conversion increases. As seen in Fig. 6(b), the DME selectivityin the system with H2O removal remains slightly up at a higherratio of CO2 recirculation. This indicates that the system with H2Oremoval can increase the CO2 utilization as increasing the amountof CO2 to the DME synthesis unit.

The system with both H2O and CO2 removals can achieve thehighest DME yield about 51.82%, which is higher than that withoutboth removals about 21.54%, as shown in Fig. 6(a). When increasingthe CO2 recirculation ratio from DME synthesis to the reformer inthe systemwith both H2O and CO2 removals, the decrement of DMEyield decreases while the DME selectivity slightly increases,compared to the system without both removals.

3.2.3. Efficiency of combined systemFig. 7(a) shows the efficiency of four systems of the DME syn-

thesis coupled with the biogas tri-reforming process. It can observethat the separation of H2O from syngas before producing the DMEcan considerably improve the system efficiency, increasing about19.83e24.73% compared to the system without both removals. Forthe systemwith CO2 removal, its system efficiency is slightly higherthan the system without both removals, in range of 0.67e4.49%.Although the DME yield of systemwith CO2 removal is higher thanthat of systemwith H2O removal, it has an inverse characteristic interms of the system efficiency. Fig. 7(b) indicates that the heat inputof the system with CO2 removal is not different from that of thesystemwithout both removals. The heat consumption of heater anddistillation in the system with the H2O removal obviously reduces.This results from decreasing in the amount of H2O and CO2 prod-ucts from the DME synthesis process, and leading to the reductionof heat duty for the purification step, as supplying dehydratedsyngas. The efficiency of system with H2O removal is superior tothat with CO2 removal. When considering all cases, the efficiency ofthe system with both H2O and CO2 removals is the highest about65.65e65.98%. For the effect of CO2 recirculation ratio, the effi-ciency of system without both removals reduces with increasingthe recirculation ratio of CO2. The increase of the CO2 recirculationratio has an insignificant effect on the efficiency of system withboth H2O and CO2 removals.

3.2.4. CO2 emission intensityCO2 emissions to a mole of DME product for four integrated

systems are shown in Fig. 8. From Fig. 8(a), it can be seen that thesystem with CO2 removal shows the highest total CO2 emissionintensity about 2.51e2.69mol of CO2/a mole of DME. Table 3 in-dicates that the amount of CO2 used in the systemwith CO2 removalis negative. This is because CO2 produced from the DME synthesis ishigher than CO2 in the feed. However, the CO2 used in the systemwith CO2 removal increases with increasing the CO2 recirculationratio. Although CO2 emissions from the energy consumption ofutilities in the system without both removals are the highest, itsCO2 emission intensity is lower than that of the system with CO2removal, as seen in Fig. 8(b). This can be explained that the systemwithout both removals has more CO2 consumption than the systemwith CO2 removal at a higher recirculation ratio of CO2, as shown inTable 3.

The removal of H2O from syngas before introducing the DMEsynthesis can assist the reduction of total CO2 emissions, which areless than 0.92e1.14mol of CO2/a mole of DME, compared to thesystem without both removals, as seen in Fig. 8(a). This is due tohigh CO2 utilization in this system and low CO2 produced in DMEsynthesis unit, as shown in Table 3. The DME synthesis fromdehydrated syngas results in more CO2 converted to methanol viathe CO2 hydrogenation reaction, leading to low CO2 produced in theDME synthesis unit. Moreover, the CO2 emissions associated with

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8

Tota

lCO

2em

issi

ons

(mol

esof

CO

2/am

ole

ofD

ME)



(a)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8

CO2

emiss

ion

fr om

ener

gyus

ed(m

oles

ofCO

2/am

ole

ofD

ME)



(b)

Fig. 8. Effect of CO2 recirculation ratio on (a) total CO2 emission intensity and (b) CO2

emission associated with energy consumption of utilities.

Table 3The amount of CO2 used in system and produced in the DME synthesis unit.

System moles of CO2 to a mole of DME

CO2 used in system CO2 produced inDME synthesis

R¼ 0 R¼ 0.5 R¼ 0 R¼ 0.5

No removal of H2O and CO2 0.04 0.30 0.94 1.73Removal of CO2 �0.07 �0.05 0.97 1.32Removal of H2O 0.45 0.67 0.28 0.42Removal of H2O and CO2 0.37 0.42 0.33 0.38

*R ¼ CO2 recirculation ratio.


energy consumption of all utilities in the systemwith H2O removalare low. As the results, the CO2 emission intensity of the systemwith H2O removal is the lowest, about 1.25e1.47mol of CO2/a moleof DME.

The DME synthesis from the systemwith CO2 removal results inhigh total CO2 emission intensity. The removals of both CO2 andH2O from syngas can reduce total CO2 emissions, compared to thesystem with CO2 removal. Total CO2 emission intensity of the sys-tem with both CO2 and H2O removals is in the range of1.39e1.46mol of CO2/a mole of DME. This can be expressed that theDME synthesis in the system with both CO2 and H2O removalsachieves high DME yield and the amount of CO2 produced from theDME synthesis unit of this system is low, as expressed in Table 3.

4. Conclusions

The DME synthesis system from syngas obtained from thebiogas tri-reforming process has been simulated and studied in thiswork. The simulation results can be drawn:

- To avoid coke formation, the biogas tri-reforming processshould be operated above 800 �C, steam to carbon ratio of 0.5,and oxygen to carbon ratio of 0.1. The increase of the inlet ox-ygen to carbon ratio in the biogas tri-reforming causesdecreasing the H2 and CO yields, while the H2 and CO yieldsslightly reduce with increasing the steam to carbon ratio.

- The recycling of CO2 from DME synthesis unit to biogas tri-reforming has a positive effect on the H2 and CO yields ofbiogas tri-reforming process in the systems with removal of CO2or/and H2O. At the high CO2 recirculation ratio, the H2 and COyields of the system without both removals decrease.

- The system with CO2 removal from syngas has more influenceon the increment of the DME yield than the system with H2Oremoval. However, the total CO2 emission intensity of the sys-tem with H2O removal is the lowest while that of system withCO2 removal shows the highest.

- The system with both H2O and CO2 removals achieves thehighest DME yield and system efficiency. The efficiency of thissystem slightly increases owing to increasing the CO2 recircu-lation ratio. Its CO2 emission intensity is low, which is close tothe system with the H2O removal.

Acknowledgements

Support from the Thailand Research Fund and the Office of theHigher Education Commission (MRG6280043) and the BuraphaUniversity, is gratefully acknowledged.

References

[1] Panigrahy S, Mishra SC. The combustion characteristics and performanceevaluation of DME (dimethyl ether) as an alternative fuel in a two-sectionporous burner for domestic cooking application. Energy 2018;150:176e89.

[2] Ying W, li H, Longbao Z, Wei L. Effects of DME pilot quantity on the perfor-mance of a DME PCCI-DI engine. Energy Convers Manag 2010;51:648e54.

[3] Zhu Y, Wang S, Ge X, Liu Q, Luo Z, Cen K. Experimental study of improved twostep synthesis for DME production. Fuel Process Technol 2010;91:424e9.

[4] Azizi Z, Rezaeimanesh M, Tohidian T, Rahimpour MR. Dimethyl ether: a re-view of technologies and production challenges. Chem Eng Process ProcessIntensif 2014;82:150e72.

[5] Falco MD, Capocelli M, Giannattasio A. Membrane Reactor for one-step DMEsynthesis process: industrial plant simulation and optimization. J CO₂ Util2017;22:33e43.

[6] Arcoumanis C, Bae C, Crookes R, Kinoshita E. The potential of di-methyl ether(DME) as an alternative fuel for compression-ignition engines: a review. Fuel2008;87(7):1014e30.

[7] Salkuyeh YK, Adams II TA. A new power, methanol, and DME polygenerationprocess using integrated chemical looping systems. Energy Convers Manag2014;88:411e25.

[8] Kirchbacher F, Biegger P, Miltner M, Lehner M, Harasek M. A new methanationand membrane based power-to-gas process for the direct integration of rawbiogas - feasability and comparison. Energy 2018;146:34e46.

[9] Herz G, Reichelt E, Jahn M. Design and evaluation of a Fischer-Tropsch processfor the production of waxes from biogas. Energy 2017;132:370e81.

[10] Khan IU, Othman MHD, Hashim H, Matsuura T, Ismail AF, Rezaei-DashtArzhandi M, et al. Biogas as a renewable energy fuel e a review of biogasupgrading, utilization and storage. Energy Convers Manag 2017;150:277e94.

[11] Montenegro Camacho YS, Bensaid S, Lorentzou S, Vlachos N, Pantoleontos G,Konstandopoulos A, et al. Development of a robust and efficient biogas pro-cessor for hydrogen production. Part 1: modelling and simulation. Int JHydrog Energy 2017;36:1e15.

[12] Pino L, Vita A, Lagana M, Recupero V. Hydrogen from biogas: catalytic tri-reforming process with Ni/La CeO mixed oxides. Appl Catal B2014;148e149:91e105.

[13] Zhang Y, Zhang S, Gossage JL, Lou HH, Benson TJ. Thermodynamic analyses oftri-reforming reactions to produce syngas. Energy Fuels 2014;28(4):2717e26.

[14] Izquierdo U, Barrio VL, Requies J, Cambra JF, Guemez MB, Arias PL. Tri-reforming: a new biogas process for synthesis gas and hydrogen production.

http://refhub.elsevier.com/S0360-5442(19)30466-9/sref1






















































Int J Hydrog Energy 2013;38:7623e31.[15] Zou H, Chen S, Huang J, Zhao Z. Effect of additives on the properties of nickel

molybdenum carbides for the tri-reforming of methane. Int J Hydrog Energy2016;41:16842e50.

[16] Ju F, Chen H, Ding X, Yang H, Wang X, Zhang S, Dai Z. Process simulation ofsingle-step dimethyl ether production via biomass gasification. BiotechnolAdv 2009;27:599e605.

[17] Huang MH, Lee HM, Liang KC, Tzeng CC, Chen WH. An experimental study onsingle-step dimethyl ether (DME) synthesis from hydrogen and carbonmonoxide under various catalysts. Int J Hydrog Energy 2015;40:13583e93.

[18] Bonura G, Cordaro M, Cannillaa C, Mezzapicaa A, Spadaroa L, Arena F, et al.Catalytic behaviour of a bifunctional system for the one step synthesis of DMEby CO2 hydrogenation. Catal Today 2014;228:51e7.

[19] Allahyari S, Haghighi M, Ebadi A, Saeedi HQ. Direct synthesis of dimethyl etheras a green fuel from syngas over nanostructured CuOeZnOeAl2O3/HZSMe5catalyst: influence of irradiation time on nanocatalyst properties and catalyticperformance. J Power Sources 2014;272:929e39.

[20] Zeng C, Sun J, Yang G, Ooki I, Hayashi K, Yoneyama Y, et al. Highly selectiveand multifunctional Cu/ZnO/Zeolite catalyst for oneestep dimethyl ether

synthesis: preparing catalyst by bimetallic physical sputtering. Fuel 2013;112:140e4.

[21] Peng XD, Toseland BA, Tijm PJA. Kinetic understanding of the chemical syn-ergy under LPDME™ conditions-once-through applications. Chem Eng1999;54:2787e92.

[22] Chen WH, Hsu CL, Wang XD. Thermodynamic approach and comparison oftwo-step and single step DME (dimethyl ether) synthesis with carbon dioxideutilization. Energy 2016;109:326e40.

[23] Zhang Y, Zhang S, Benson T. A conceptual design by integrating dimethylether (DME) production. Fuel Process Technol 2015;131:7e13.

[24] Luu MT, Milani D, Wake M, Abbas A. Analysis of di-methyl ether productionroutes: process performance evaluations at various syngas compositions.Chem Eng Sci 2016;149:143e55.

[25] Ateka A, P�erez-Uriarte P, Gamero M, Ere~na J, Aguayo AT, Bilbao J.A comparative thermodynamic study on the CO2 conversion in the synthesisof methanol and of DME. Energy 2017;120:796e804.

[26] Manenti F, Garzon ARL, Ardebili ZR, Pirola C. Systematic staging designapplied to the fixed-bed reactor series for methanol and one-step methanol/dimethyl ether synthesis. Appl Therm Eng 2014;70:1228e37.






























































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