Energy Conversion and Management 103 (2015) 73–81

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Energy Conversion and Management

journal homepage: www.elsevier .com/ locate /enconman

Conceptual designs of hydrogen production, purification, compressionand carbon dioxide capture

http://dx.doi.org/10.1016/j.enconman.2015.06.0460196-8904/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +886 6 2757575; fax: +886 6 2344496.E-mail address: weiwu@mail.ncku.edu.tw (W. Wu).

Wei Wu ⇑, Po-Chih KuoDepartment of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

a r t i c l e i n f o

Article history:Received 22 March 2015Accepted 16 June 2015Available online 26 June 2015

Keywords:Hydrogen productionOxy combustionHeat exchanger networkProcess integration

a b s t r a c t

The design of hydrogen production, purification, compression and carbon dioxide capture is developed astwo types of heat-integrated processes. The SWP (SMR + WGS + PCC) is mainly composed of thesteam methane reforming (SMR) reactor, the low temperature water–gas-shift (WGS) reactor and theprocess of hydrogen purification, compression and carbon dioxide capture (PCC), and the SCWP(SMR + CO2R + WGS + PCC) primarily consists of the SMR reactor, the carbon dioxide reforming ofmethane (CO2R) reactor, the WGS reactor and the PCC. From economic aspects, it is expectable thatthe SWP process is superior to the SCWP process due to lower energy demand and less equipment.From aspects of energy utilization and CO2 capture, it is verified that the SCWP process is superior tothe SWP process.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction post-combustion capture system could deal with separating out

Cost-effective hydrogen production and CO2 capture arerequired for a hydrogen economy [1]. About 95% of the hydrogenproduced in the U.S. is obtained using a thermal process with nat-ural gas as the feedstock. The conventional hydrogen productionprocess primarily consists of a steam methane reforming (SMR)and a water gas shift (WGS) reaction [2]. Since the use of hydrogenin fuel cell applications requires a high purity by 99.99 + %, thepressure swing adsorption (PSA) process is widely used for hydro-gen purification and separation [3]. In addition, a small amount ofcarbon monoxide may poison fuel cell electrodes. The WGS reactorcan effectively reduce CO amount in the hydrogen stream [4].However, the most hydrogen production processes would accom-pany with a large CO2 emissions due to the WGS reactions.

The CO2 capture is a well-developed technique to reduce theiremissions of CO2 [5]. Some schemes such as a monoethanolamine(MEA) CO2 scrubbing process and cardo polyimide hollow fibermembrane capture [6,7] are applied to reduce the emissions ofgreenhouse gases including methane and carbon dioxide.Koumpouras et al. [8] introduced a low-temperature hydrogenproduction with in situ CO2 capture. CO2 adsorbent particles arepassed through a stationary SMR catalyst monolith, but adsorbentregeneration was carried out in an external unit. Nord et al. [9] pro-posed pre-combustion CO2 capture to reduce greenhouse gas emis-sions but the complexity of the plant increases. The advantage for a

CO2 from flue gases at a low pressure. To consume or suppressthe CO2 emissions of hydrogen production processes, Song andPan [10] presented a configuration for tri-reforming methane,which involved a carbon dioxide reforming of methane (CO2R),steam reforming and the partial oxidation of methane, to enhanceCO2 conversion and utilization. Farniaei et al. [11] proposed a newsystem configuration in which the steam reforming reaction wasproceeded by excess generated heat from tri-reforming reactioninstead of huge fired-furnace in conventional steam reformer.Fan et al. [12] utilized greenhouse gases as the feed of the catalyticcarbon dioxide reforming process to improve hourly space velocityand hydrogen production. Fan et al. [13] also studied the optimiza-tion of hydrogen production from the CO2 reforming of methane.Wang and Cao [14] studied the simulation of the hydrogen produc-tion by the ethanol steam reforming process and the carbon diox-ide reforming unit. Recently, Wu et al. [15] developed the designfor a combination of the SMR and CO2R reactors to achieve thestand-alone syngas production process. Although CO2 emissionscan be effectively suppressed, the syngas yield is low due to theinternal combustion to recovering the energy demands. Manysteam reforming processes in refineries are usually connected tothe process for hydrogen purification and compression in orderto store the pure hydrogen [16]. The heat integration using pinchanalysis is a feasible method to utilize the waste heat, such asindustrial chemical plants [17,18]. Synthesis of heat exchanger net-works (HENs) was successfully implemented to the steam reform-ing process for producing high-pressure hydrogen [19,20].

(a)

(b)

Fig. 1. Hydrogen production system configurations: (a) SMR + WGS; (b) SMR + CO2R + WGS.

Table 1Specifications of major process units.

Equipment Aspenmodel

Specification/configuration

SMR RPLUG Heat duty required, reactor length = 5 m, reactordiameter = 1.5 m, pressure drop = 50 kPa, andvoid fraction = 0.6

CO2R RPLUG, Heat duty required, reactor length = 5 m, reactor

74 W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81

To develop the process of hydrogen production, purification andcompression, the heat and power integration is necessary. The syn-thesis of HENs is an effective approach for waste heat recovery, butCO2 emissions of the processes cannot be suppressed. In ourapproach, an extension of the steam reforming process is pre-sented to pursue the hydrogen production process withnear-zero carbon emissions. A carbon dioxide reforming ofmethane (CO2R) reactor is added into the SMR process to producethe hydrogen by consuming greenhouse gases with methane andcarbon dioxide. Since the process of hydrogen purification andcompression would release heat and waste gas, the oxy combus-tion technique, which was successfully implemented to the cleancoal-fired power plant [21], is added to increase the CO2 concentra-tion of exhausted flue gas such that the energy penalty of the CO2

capture is dramatically reduced. To address near-zero carbon emis-sions of high-pressure hydrogen process, the performance of CO2

reduction and energy efficiency improvements are verified by theAspen Plus simulator.

REquil diameter = 1.5 m, pressure drop = 60 kPa, andvoid fraction = 0.6

WGS RPLUG No heat duty, reactor length = 3 m, reactordiameter = 0.15 m, pressure drop = 50 kPa, andvoid fraction = 1

PSA Componentsplitter

No heat duty and 99.95% purity of H2

Combustor RStoic Stoichiometry reactorSeparator Flash2 Two phase flash drum

2. Hydrogen production processes

2.1. Kinetics and process design

In general, the conventional hydrogen production process is acombination of SMR and WGS reactors. The SMR reactor is

considered as a plug-flow reactor where three exother-mic/endothermic reactions are shown as follows:

CH4 þH2O$ COþ 3H2ðr1Þ;DH0298 ¼ 206:2 kJ mol�1 ð1Þ

COþH2O$ CO2 þH2ðr2Þ;DH0298 ¼ �41:2 kJ mol�1 ð2Þ

CH4 þ 2H2O$ CO2 þ 4H2ðr3Þ;DH0298 ¼ 165:0 kJ mol�1 ð3Þ

(a)

H2O in (kmol/hr)

H2

mol

eflo

wra

te(k

mol

/hr)

Dut

y(kW

)

10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

35

40

45

50

0

500

1000

1500

2000

2500

3000

SMR+WGSSMR+CO2R+WGS

(b)

CH4in (kmol/hr)

H2

mol

eflo

wra

te(k

mol

/hr)

Dut

y(kW

)

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

35

40

45

50

0

500

1000

1500

2000

2500

3000

SMR+WGSSMR+CO2R+WGS

Fig. 2. Comparisons of SMR + WGS and SMR + CO2R + WGS (no CO2,in) by adjusting(a) H2Oin and (b) CH4,in.

CO2,in (kmol/hr)

H2

Mol

eflo

wra

te (k

mol

/hr)

Dut

y (k

W)

0 1 2 3 4 5 6 7 8 9 10 1120

22

24

26

28

30

700

750

800

850

900

950

FCO2R,outFWGS,outDuty

Optimalvalue

Fig. 3. Sensitivity analysis of SMR + CO2R + WGS by adding CO2,in.

W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81 75

Referring the kinetics of these reactions on a Ni/MgAl2O4 catalyst[2,19] in the temperature range of 650–850 �C, the correspondingrate expressions are described as

r1 ¼4:225� 1015

Den2 exp240:1

RT

� �PCH4 PH2O

P2:5H2

�P0:5

H2PCO

K1

!ð4Þ

r2 ¼1:955� 106

Den2 exp67:13

RT

� �PCOPH2O

PH2

� PCO2

K2

� �ð5Þ

r3 ¼1:02� 1015

Den2 exp243:9

RT

� �PCH4 P2

H2O

P3:5H2

�P0:5

H2PCO2

K1K2

!ð6Þ

where K1 ¼ expð�26830=T þ 30:114Þ, K2 ¼ expð4400=T � 4:036Þ,and

Den¼1þ8:23�10�5 exp�70:65

RT

� �PCO

þ6:12�10�9 exp�82:90

RT

� �PH2 þ6:65�10�4 exp

�38:28RT

� �PCH4

þ1:77�105 exp88:68

RT

� �PH2O

PH2

� �ð7Þ

Moreover, the H2-rich syngas produced from the SMR reactor isdirectly fed into the WGS reactor in order to reduce CO andincrease the amount of hydrogen. We consider a plug-flow reactorwith the low-temperature WGS reaction over CuO/ZnO/Al2O3 cat-alysts in the temperature range of 200–350 �C range. The corre-sponding rate of reaction is expressed by [2]

rWGS ¼ 82:2 exp �47400RT

� �PCOPH2O �

PCO2 PH2

KWGS

� �ð8Þ

where the equilibrium constant KWGS can be found by

lnðKWGSÞ ¼5693:5

Tþ 1:077 lnðTÞ þ 5:44� 10�4T

� 1:125� 10�7T2 � 49170T2 � 13:148 ð9Þ

In the conventional hydrogen production process, the processflow diagram of the SMR + WGS process is depicted in Fig. 1(a).Notably, a heater (H1) with heating rate QH1 is used to adjust theinlet temperature of the SMR reactor from 25 �C to 700 �C, and acooler (C1) with cooling rate QC1 is utilized to adjust the outlettemperature of the SMR reactor from 700 �C to 300 �C. The heat-ing/cooling rates of QR1 and QR2 are added to keep the SMR andWGS reactors with constant temperature, respectively.

The CO2R reactions are complex due to carbon and water for-mations [22,23]. The simplified reaction networks are as shown:

CO2 þ CH4 $ 2COþ 2H2 ðrCO2RÞ; DH0298 ¼ 247:2 kJ mol�1 ð10Þ

CH4 þH2O$ COþ 3H2; DH0298 ¼ 206:2 kJ mol�1 ð11Þ

CO2 þH2 $ COþH2O; DH0298 ¼ 41:2 kJ mol�1 ð12Þ

CH4 ! Cþ 2H2; DH0298 ¼ 75:6 kJ mol�1 ð13Þ

Eq. (10) is the main reaction of the CO2R. The kinetics of the cat-alytic carbon dioxide reforming reaction in Eq. (10) on a highlyactive Ni/La2O3 catalyst is described as follows [24]:

rCO2R ¼ 1:35� 10�7 exp529:2

RT

� �PCH4 PCO2 þ 2:61� 10�3 exp

�517:2RT

� �PCH4

�

þ 2:77� 10�5 exp144:3

RT

� �PCO2

��1

� 7:22� 10�8 exp�372:9

RT

� �PCH4 PCO2 ð14Þ

(a)

Length (m)

Mol

eflo

wra

te (k

mol

/hr)

0 1 2 3 4 5 6 7 80

5

10

15

20

25

30

H2COCO2CH4H2O

SMR WGS

(b)

Length (m)

Mol

eflo

wra

te (k

mol

/hr)

0 1 2 3 4 5 6 7 8 9 10 11 12 130

5

10

15

20

25

30

H2COCO2CH4H2O

SMR CO2R WGS

Fig. 4. Composition profiles of each reactor at (a) SMR + WGS and (b)SMR + CO2R + WGS.

76 W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81

Eqs. (11)–(13) are the accompanied reactions. Referring the previ-ous study [15], the methane cracking reaction in Eq. (13) isrestricted for carbon formation and accompanied reactions by Eqs.(11) and (12) proceed to achieve equilibrium if the operating tem-perature is larger than 900 �C. A combination of two Aspen mod-ules, RPLUG and REquil, shown in Table 1 is used to describe theCO2R process. To develop the hydrogen production process withlow carbon emissions, the process flow diagram of theSMR + CO2R + WGS process is depicted in Fig. 1(b). Notably, theCO2R reactor is added between the SMR and WGS reactors. The sec-ond cooler (C2) with cooling rate QC2 is added to adjust the outlettemperature of the SMR reactor from 700 �C to 25 �C. The additionalseparator unit is used to remove the water in the stream.Sequentially, the second heater (H2) with heating rate QH2 is addedto adjust the outlet temperature of the separator from 25 �C to1100 �C. The additional CO2 flow is mixed with thehigh-temperature stream, then it flows into the CO2R reactor.Similarly, another heating rate QR3 is added to keep the CO2R

reactor with constant temperature. The CO2R reactor producesthe CO-rich syngas and it flows into the WGS reactor before it iscooled down to 300 �C by the first cooler (C1).

For Aspen Plus simulations, the specifications of major units inboth processes are shown in Table 1. Notably, all reactors are con-sidered as catalytic reactors, the thermodynamic properties ofsome species are evaluated using the Peng–Robinson equationof state, and the signs of heating/cooling are taken aspositive/negative.

2.2. Comparisons

According to above process designs for hydrogen production,the CO2R reactor in the SMR + CO2R + WGS process consumesunreacted methane and extra carbon dioxide but the externalenergy supply, QH2 and QR3, are also required. If both feeds ofCH4 (CH4,in) and H2O (H2Oin) are adjustable, comparisons of theSMR + WGS process and the SMR + CO2R + WGS process withregard to hydrogen production rate (H2,out) and total energy duty

Q need ¼P

i¼1Q Hi þP

j¼1QCj þP

k¼1QRk

� �are addressed as follows.

(i) If the feed of CO2 in the SMR + CO2R + WGS process is closed,i.e. CO2,in = 0, and CH4,in is fixed at 10 kmol/h, Fig. 2(a) showsthat the SMR + WGS process produces the more hydrogenthan the SMR + CO2R + WGS process while H2Oin increasesfrom 15 kmol/h to 100 kmol/h. It is owing that the WGS reac-tor in the SMR + WGS process dominates the hydrogen pro-duction if S/C (H2Oin/CH4,in) is larger than 1.5. When theexcess water, i.e. S/C > 3, is taken into consideration, thetotal energy duty of the SMR + WGS process is dramaticallyincreased but it cannot effectively increase hydrogen yieldbecause CO effluent from the SMR is insufficient. For theSMR + CO2R + WGS process, the excess water cannotincrease additional energy duty because water is almostremoved in the front of the CO2R reactor.

(ii) If CO2,in = 0 and H2Oin = 10 kmol/h, Fig. 2(b) shows that thehydrogen production rate of both processes significantlyincreases by increasing CH4,in from 0 to 20 kmol/h. WhenCH4,in > 20 kmol/h, i.e. S/C < 0.5, the hydrogen productionrate of the SMR + CO2R + WGS process is higher than theSMR + WGS process, but the corresponding energy demandsare almost the same. It implies that the CO2R reactor is aidedto consume the unreacted CH4 from the SMR and produce alittle hydrogen.

To address the performance of hydrogen production, theSMR + WGS process is superior to the SMR + CO2R + WGS processwhen 1.5 < S/C < 3, but both processes with respect to hydrogenyield and total energy duty is similar when S/C < 0.5. To addressthe performance of CO2 reduction, the SMR + CO2R + WGS processis specified as follows.

(iii) If CH4,in = H2Oin = 10 kmol/h, Fig. 3 shows that the hydrogenproduction rate of the SMR + CO2R + WGS process graduallyincreases by adding CO2,in from 1 to 4 kmol/h, but the corre-sponding energy duty also increases from about 700 kW to800 kW. It shows that the CO2R reactor could completely con-sume the unreacted CH4 from the SMR. However, the excessCO2, i.e. CO2,in > 4 kmol/h, cannot effectively increase thehydrogen production rate due to the reverse WGS reaction.

Compared to the conventional SMR + WGS process, theSMR + CO2R + WGS process can consume extra CO2. Based on theoptimal condition, CO2,in = 4 kmol/h, by Fig. 3, we found that thehydrogen production rate is improved by 17.9% and the energy

Fig. 5. The process of hydrogen purification, compression and carbon dioxide capture.

Table 2Hot/cold streams data.

Utility Mass flow rate(kg/h)

Inlet temperature(�C)

Outlettemperature (�C)

DH (kW)

SWPH1 341.59 18 700 300.22H3 1413.67 25 200 70.13C1 341.59 700 300 �129.25C3 38.33 1359 300 �172C4 38.33 784 300 �76.03C5 38.33 519 27 �77.15C6 1716.93 2050 30 �1429.86

SCWPH1 341.59 18 700 300.22H2 302.26 25 1100 326.54H3 1413.67 25 200 70.13C1 341.59 700 25 �227.83C2 478.30 700 300 �239.14C3 45.42 1359 300 �253.47C4 45.42 784 300 �112.04C5 45.42 519 27 �91.42C6 60.45 2088 30 �1068.79

W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81 77

duty also increases by 14.2%. Moreover, composition profiles ofeach reactor for both hydrogen production processes at prescribedinlet conditions, CH4,in = H2Oin = 10 kmol/h and CO2,in = 4 kmol/h,are shown in Fig. 4(a) and (b), respectively. Fig. 4(a) shows thatthe SMR + WGS process produces H2-rich syngas but the CH4 con-version is about 75%. Fig. 4(b) shows that CH4 conversion of theSMR + CO2R + WGS process is close to 100% and the correspondingCO2 conversion achieves 62.5%.

3. Results and discussion

To meet requirements for the hydrogen storage as well as solvethe high energy demand of above processes, the process design forhydrogen purification, compression and carbon dioxide capture isaddressed as follows.

3.1. Hydrogen purification, compression and carbon dioxide capture

Since the products of both SMR + WGS and SMR + CO2R + WGSprocesses contain unreacted CH4 and/or a plenty of CO, the inter-nal combustor is regularly utilized to produce thehigh-temperature flue gas for heat recovery. In our approach,the process of hydrogen purification, compression and carbondioxide capture (PCC) shown in Fig. 5 is developed to meet theobjectives of the hydrogen storage and CO2 capture. It is directlyconnected to the SMR + WGS and the SMR + CO2R + WGS pro-cesses, respectively. The abbreviations of new processes arenamed as the SWP (SMR + WGS + PCC) process and SCWP(SMR + CO2R + WGS + PCC) process. Regarding the process ofhydrogen purification and compression, the outlet stream ofhydrogen production processes is fed into a pressure swingadsorption (PSA), which utilizes different loading capacities ofadsorbent at different pressures, to purify hydrogen and separatewaste gas. The high-purity hydrogen (99.95%) is compressed up to300 atm by a series of three compressors, which is the typicalstorage pressure for hydrogen powered vehicles. The waste gasfrom the PSA is fed into the combustor with the air at the pre-scribed air temperature (Tair) and flowrate (Fair) to produce hightemperature flue gas that is usually composed of CO2, H2O andN2. The flue gas at the outlet of the combustor is cooled downto 30 �C by the cooler (C6) with cooling rate QC6. To address theCO2 capture technique, the oxy combustion is taken into account.A separator is added to remove the water of the flue gas, and thepreheated O2-rich air via the air pretreatment separator is mixedwith the recycled flue gas. Notably, the split ratio is fixed by thesplitter. A comparison of the CO2 capture for the SWP and SCWPprocesses is depicted in Fig. 6(a). It shows that the SCWP processcan capture a higher concentration of CO2 than the SWP processwhile the purity of oxygen in the air stream is higher than 95%.Since the air pretreatment separator need consume extra energyto adjust the O2 concentration, it implies that the energy penaltyfor the CO2 capture in the SCWP process is lower than it in theSWP process. The corresponding heat release rate (QC6) and the

(a)

Oxygen (%)

CO 2

(mol

%)

94 95 96 97 98 99 10090

92

94

96

98

100

SMR+WGSSMR+CO2R+WGS

(b)

Oxygen (%)

QC

6(k

W)

94 95 96 97 98 99 1001350

1375

1400

1425

1450

1475

1500

1525

1550

SMR+WGSSMR+CO2R+WGS

(c)

Oxygen (%)

Tem

pera

ture

(o C

)

94 95 96 97 98 99 100

1300

1350

1400

1450

1500

1550

1600

1650

1700

SWPSCWP

SWPSCWP SWPSCWP

SWPSCWP

Fig. 6. Comparisons of SWP and SCWP using oxy combustion: (a) the outlet CO2

mol% of the flue gas, (b) heat release rate, and (c) the outlet temperature of thecombustor.

78 W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81

outlet temperature of the combustor are depicted inFig. 6(b) and (c), respectively. Notably, the waste heat from theSWCP process is less than it from the SWP process because the

SWP process provides a few unreacted CH4 as the fuel of theinternal combustor.

Since both SWH and SCWH processes depend on severalhot/cold streams, the heat integration design is regularly appliedto minimize the costs of cold/hot utilities and improve the energyefficiency.

3.2. Heat integration

By Figs. 1(a) and 5, there are two heaters and five coolers in theSWP process. By Figs. 1(b) and 5, there are three heaters and sixcoolers in the SCWP process. The corresponding values of massflow rates and inlet/outlet temperatures of hot/cold streams areshown in Table 2. The heat integration (HI) design using the heatexchanger networks (HENs) is a typical approach to reduce theoperating (energy) cost. To address the minimum energy con-sumption or maximum heat recovery using the pinch technology,the HENs of the SWP and SCWP processes are obtained by usingAspen Energy Analyzer, which are depicted in Fig. 7(a) and (b),respectively. The corresponding specifications of HENs are shownin Table 3. Based on the HENs, the heat-integrated system config-urations of the SWP and SCWP processes are shown inFig. 8(a) and (b), respectively. Moreover, the comparisons of bothprocesses with and without use of the heat integration are statedas follows.

(i) Two heaters (H1, H3) of the SWH process and three heaters(H1, H2, H3) of the SCWP process are all replaced by fourheat exchangers (HE-1, HE-2, HE-3, HE-4).

(ii) The HI design for the SWP process contributes to save370.35 kW of the hot utility duty and the correspondingtotal area of heat exchangers is 157.58 m2. It also saves440.55 kW of the cold utility duty from 1884.29 kW to1443.74 kW.

(iii) The HI design for the SCWP process contributes to save696.89 kW of the hot utility duty and the correspondingtotal area of heat exchangers is 572.07 m2. It also saves674.61 kW of the cold utility duty from 1992.69 kW to1318.08 kW.

According to above analysis, the waste heat in the SCWP pro-cess is recovered more than it in the SWP process, but the corre-sponding heat transfer area is 3.63 times of the area used in theSWP process. Similarly, the duty of the cold utility in the SCWPprocess is saved more than it in the SWP process. By Fig. 8(a),the heat release of the SWP process is estimated 2.17 times ofthe external heat demand by 695.02 kW. By Fig. 8(b), the heatrelease of the SCWP process is estimated 1.23 times of the externalheat demand by 1073.21 kW. Apparently, the heat-integratedSCWP process is difficult to become a stand-alone energy systembecause the heat demand is too large. The capital cost of theSCWP cannot be evaluated because the CO2R process is currentlynot commercialized. By a comparison of the performance of HIdesign for both processes, the operating cost of the SCWP is higherthan the SWP. From economic aspects, the SWP process is superiorto the SCWP process.

3.3. Efficiencies

From the viewpoint of improving the hydrogen yield(H2,out/CH4,in) and the ratio of CO2 emissions (CO2,out/H2,out),Fig. 9(a) shows that the hydrogen yield of the SCWP process with2.82 is higher than the SWP process with 2.49, and the ratio ofCO2 emissions of the SCWP process with 0.066 is lower than the

Fig. 7. Heat integration networks for the process of (a) SWP and (b) SCWP.

W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81 79

SWP process with 0.132. From the viewpoint of CO2 capture, it hasbeen verified that the SCWP process is superior to the SWP process.Furthermore, we provide two types of energy efficiencies, thermaland hydrogen efficiencies [25], to make a comparison of the SWPand SWP processes.

gT ¼LHVH2

LHVCH4 þ Q needð15Þ

gH2¼ LHVH2

LHVCH4 þ Q need þWcð16Þ

Table 3Specifications of heat exchanger networks.

Heat exchanger/utility Duty (kW) Area (m2)

SWPHE-1 37.72 14.84HE-2 118.89 34.59HE-3 143.62 50.57HE-4 70.13 14.37CU-1 1059.50 20.38CU-2 129.25 18.67CU-3 172.00 2.21

(a)

(b)

Fig. 8. Heat-integrated system configurations: (a) SWP; (b) SCWP.

80 W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81

where gT and gH2represents the thermal and hydrogen efficiencies,

respectively. The lower heating value (LHV) of hydrogen andmethane are 242 kJ/mol and 801.36 kJ/mol, respectively. Wc repre-sents the total duty for hydrogen compression. The comparisonsof both heat-integrated processes with respect to thermal andhydrogen efficiencies are shown in Fig. 9(b). It shows that theSCWP process increases the thermal efficiency by 14.7% and thehydrogen efficiency by 5.4% as compared to the SWP process.From the viewpoint of energy utilization, it is verified that theSCWP process is superior to the SWP process.

(a)

H2

yiel

d(m

ol/m

olC

H4)

CO

2em

issi

ons

(mol

eC

O2/

mol

H2

prod

uced

)

0

0.5

1

1.5

2

2.5

3

3.5

0

0.03

0.06

0.09

0.12

0.15H2 yieldCO2 emissions

(b)

Effi

cien

cy(%

)

00

20

40

60

80Thermal efficiencyHydrogen efficiency

SWP SCWP

SWP SCWP

Fig. 9. Comparisons of SWP and SCWP: (a) hydrogen yield vs. CO2 reduction; (b) gT

vs. gH2.

Table 3 (continued)

Heat exchanger/utility Duty (kW) Area (m2)

CU-4 76.02 0.72CU-5 41.65 0.80CU-6 31.84 0.43

SCWPHE-1 239.14 5.20HE-2 61.09 0.79HE-3 326.54 300.07HE-4 70.13 37.16CU-1 192.39 1.32CU-2 112.04 0.48CU-3 113.70 1.29CU-4 227.83 45.81CU-5 660.94 78.97CU-6 11.18 100.98

W. Wu, P.-C. Kuo / Energy Conversion and Management 103 (2015) 73–81 81

4. Conclusion

The novel design, heat integration and simulation of SWP andSCWP processes is carried out in the Aspen Plus environment.Both process designs can capture CO2 via the oxy combustionmechanism. The SCWP process not only paid lower energy penaltyfor the CO2 capture than the SWP process, but also it can increaseCO2 consumption by feeding CO2 as the reactant of the CO2R reac-tion. Since the CO2R reaction is strongly endothermic, it is seen

that the additional energy consumption of the SCWP process mustbe higher than the SWP process. It is verified that the thermal andhydrogen efficiencies of the SCWP is higher than the SWP process.Eventually, the SCWP process is superior to the SWP process if theexternal energy supply uses the renewable energy.

Acknowledgments

The authors would like to thank the Ministry of Science andTechnology of the Republic of China for its partial financial supportof this research under Grant MOST 103-2221-E-006-251.

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