Energy and Exergy Analyses of a Zero Emission Power Plant for Coproduction of Electricity and...

Energy and Exergy Analyses of a Zero Emission PowerPlant for Coproduction of Electricity and Methanol 13Canan Acar and Ibrahim Dincer

Abstract

In this study, we study and evaluate a zero emission integrated system, as taken from the literature, for coproduction of

electricity and methanol. The investigated integrated system has three subsystems: water electrolysis, Matiant power

plant (oxy-fuel combustion of pure methane), and methanol production unit. The system and its components are analyzed

energetically and exergetically. The rates of exergy destructions, relative irreversibilities, and sustainability indexes of

each subunit of each subsystem, as well as the overall system are analyzed to identify the greatest exergy losses and

possible future research directions. The total rate of exergy destruction of the overall system is calculated to be around

280 MW. The greatest rate of exergy destruction, therefore the greatest irreversibility, occurs within the power plant unit

(about 60 % of the total rate of exergy destruction). The energy efficiencies of electrolysis, power plant, and methanol

synthesis unit are found to be 30 %, 76 %, and 41 %, respectively. The exergy efficiencies of electrolysis, power plant,

and methanol synthesis unit are found to be 30 %, 64 %, and 41 %, respectively. Depending on the utilization of the heat

rejected from the different units of each subsystem, the overall system could have energy and exergy efficiencies up to

68 % and 47 %, respectively.

Keywords

Methanol � Carbon capture � Cogeneration � Electricity � Energy � Exergy � Efficiency

Introduction

During the twentieth and the beginning of the twenty-first century, world’s energy consumption has increased steadily due to

global rise in population and standards of living. The increasing trend in world’s energy need is expected to continue in the

future. As a result, a growth in energy generation capacity will be needed [1]. One of the main sources of CO2 emissions is

power generation [2]. Therefore, reduction of fossil fuel utilization by increasing efficiency and decreasing emissions by

converting CO2 into valuable products offer some potential solutions to world’s current energy-related problems.

There have been several studies focusing on coproduction of fossil fuels with CO2 utilization. A comparative study of

electricity and hydrogen production systems by CO2 capture and storage has been presented by Damen et al. [3]. Minutillo and

Perna [4] studied a tri-reforming process in fossil fired power plants to generate a synthesis gas for production of chemicals

including methanol. Energy analysis, economy, and policy scenarios of electricity/methanol synthesis coproduction systems’

are taken into account in Guang-jian et al. and Chen et al.’s [5, 6] work. Katayama and Tamaura [7] and Takeuchi et al. [8]

C. Acar (*) � I. DincerFaculty of Engineering and Applied Science, University of Ontario Institute of Technology,

2000 Simcoe Street North, Oshawa, ON, Canada L1H 7K4

e-mail: [email protected]; [email protected]

I. Dincer et al. (eds.), Progress in Exergy, Energy, and the Environment,DOI 10.1007/978-3-319-04681-5_13, # Springer International Publishing Switzerland 2014

145

mailto:[email protected]

mailto:[email protected]

considered methanol synthesis via CO2 recovery and renewable hydrogen from water electrolysis unit by renewable energy

sources as a zero emission integrated system. Mignard et al. [9] proposed an integrated system for coproduction of electricity

and methanol in small scale/partial carbon capture in common fossil fuel power plants. The utilization of renewable hydrogen

for methanol production is investigated by Cifre and Badr [10].

This chapter aims to study and evaluate a zero emission integrated system for coproduction of electricity and methanol

as originally proposed and studied by Soltanieh et al. [11]. The characteristics and performance of electricity–methanol

cogeneration system with water electrolysis, Matiant power cycle, and methanol synthesis unit are energetically and

exergetically evaluated. Energy, exergy, and sustainability analyses of every component of the system as well as the overall

system are performed. The performance of the system is assessed for various environmental parameters such as relative

irreversibility and sustainability index. In the end, recommendations for better performance in terms of environmental and

sustainability issues are provided.

System Description

Overall System

The integrated system, as originally developed and published by Soltanieh et al. [11], is presented in Fig. 13.1. It has three

subsystems: water electrolysis for H2 and O2 production, the matiant-cycle power plant to produce electricity via oxy-

combustion, and methanol synthesis unit fed by the CO2 from power plant and renewable H2. In this study, all processes are

assumed to be steady state and steady flow (SSSF), where potential and kinetic energy changes are neglected. It is also

assumed that no chemical reactions occur except the water electrolysis modules, combustion chambers, and methanol

reactor. Heat loss and pressure drop in tubing, valves, and connections are ignored. All equipments except the heat

exchangers are assumed to be adiabatic. Compressors and turbines are assumed to operate at an isentropic efficiency of

85 %, and the efficiency of heat exchangers is assumed to be 90 %.

Water Electrolysis Unit

Figure 13.2 shows the water electrolysis unit. The electrolysis unit uses wind (renewable) energy to provide the necessary

energy for the electrolysis reaction is 3H2O ! 3/2O2 + 3H2 (ΔH ¼ 286 kJ/mol H2) [12]. The technical criteria used for the

unit is selected based on Soltanieh et al.’s [11] study. The water feed (stream 1) is assumed to undergo complete electrolysis.

The product H2 (streams 2–10) is either stored or sent to the methanol synthesis unit. The product O2 (streams 11–19) is

either stored or sent to the matiant power plant. The inlet molar flow rate of water to the electrolysis unit is 1.4 kmol/s at

Fig. 13.1 Integrated system

block diagram (modified

from [11])

146 C. Acar and I. Dincer

80 �C and 1 bar. 0.75 kmol/s of this amount comes from the power plant and methanol synthesis units while 0.65 kmol/s is

the fresh feed. H2 and O2 are desired to be stored at 140 �C and 10 bar. O2 is sent to the power plant at 160 �C and 40 bar.

Then, H2 is sent to the methanol synthesis at 180 �C and 50 bar.

Matiant-Cycle Power Plant

The Matiant-cycle power plant is presented in Fig. 13.3. In this subsystem, the fuel (pure methane) is burned with pure oxygen

from the electrolysis unit and undergoes complete combustion. The combustion reaction is CH4 + 2O2 ! CO2 + 2H2O

(ΔH ¼ �800 kJ/mol CH4) [12]. The technical criteria used for the plant is selected based on Soltanieh et al.’s [11] study. The

products of two combustion chambers (CO2 and H2O) are separated; the H2O is sent back to the electrolysis unit, and about

92 % of the CO2 is used as a thermodynamic fluid within the Matiant cycle while the remaining part is sent to the methanol

synthesis unit. 70 % of the O2 from the electrolysis unit is sent to the combustion chamber H, and the remaining is sent to the

combustion chamber K. The CH4 inlet temperature and pressure are 30 �C and 2 bar, respectively. The output temperature and

pressure of water from the flash drum are 80 �C and 2 bar. CO2 is sent to the methanol synthesis unit at 180 �C and 50 bar.

Methanol Synthesis Unit

Figure 13.4 presents the methanol synthesis unit. The methanol reactor converts CO2 to CH3OH according to the following

reaction: CO2 + 3H2 ! CH3OH + H2O (ΔH ¼ �49.51 kJ/mol CH3OH). The technical criteria used for the unit are

selected based on Soltanieh et al.’s [11] study. 0.25 kmol/s CO2 is fed to the unit from the power plant. The CO2 to

CH3OH conversion ratio in the methanol reactor is 20 %. Methanol and water are leaving the distillation column at 1 bar and

40 �C and 80 �C, respectively.

Storage Compressor

8

3 4 5

67

1615

14

1312

1718 19

2

1

11

9 10Heat Exchanger

HydrogenStorageTank

OxygenStorageTank

Heat Exchanger

Storage Compressor

Oxygen

Electrolysis Modules

Water

Hydrogen

O2 Compressors

H2 to Methanol Reactor

O2 to The Matiant Cycle

H2 Compressors

Fig. 13.2 Process flow diagram of the water electrolysis unit (modified from [11])

13 Energy and Exergy Analyses of a Zero Emission Power Plant for Coproduction of Electricity and Methanol 147

Compressor

CH4

11

12

10

CombustionChamber K

CombustionChamber H

Expander G

Expander LExpander I

Compressor14

8

4

5

76

O2

CH4

Pre Heater

Compressor Stage

19

20

PumpFlashDrum

18CO2

CO2

117

16

15

2

3

13

Water

HeatExchanger

Synthesis UnitCO2 to

O2

9

Fig. 13.3 Process flow diagram of the matiant-cycle power plant (modified from [11])

Compressor

14

5

1 2

4

3FlashDrum

MethanolReactor

ReactorOutput

ReactorFeed

Valve

7

6

8

9

10 11

12 13 Water

DistillationTower

Methanol

Fig. 13.4 Process flow diagram of the methanol synthesis unit (modified from [11])


Analysis

The operating conditions and production capacities of each subsystem as well as the overall system are selected based on the

literature work [11]. The general mass, energy, entropy, and exergy balance equations can be summarized as:

Mass Balance Equation (MBE):

X_min ¼

X_mout ð13:1Þ

Energy Balance Equation (EBE):

X_Ein ¼

X_Eout ð13:2Þ

Entropy Balance Equation (EnBE):

X_Sin þ _Sgen ¼

X_Sout ð13:3Þ

Exergy Balance Equation (ExBE):

X_Exin ¼

X_Exout þ _Exdest ð13:4Þ

Specific exergy of a stream is comprised of two components, namely physical and chemical exergy, and defined as

follows [13, 14]:

ex ¼ exph þ exch ð13:5Þ

exph ¼ h� hoð Þ � T0 � s� soð Þ ð13:6Þ

exch ¼X

xi exch0 � RT0 �

Xxiln xið Þ ð13:7Þ

Ex: ¼ _m � ex ð13:8Þ

where T0 is the ambient temperature, R is the specific gas constant, h/h0 and s/s0 are the specific enthalpy and entropy of a

stream at a given state/at ambient state, and xi is the mol fraction of component i. Relative irreversibilities (RI) and

sustainability indexes (SI) of each unit of each subsystem as well as the subsystems themselves and the overall system are

calculated based on the following equations:

RIi ¼_Ex

dest

i

_Exdest

total

ð13:9Þ

SIi ¼ 1

1� Ψ ið13:10Þ

In order to study the system energetically and exergetically, each stream is defined by its state, components, temperature,

and pressure. After the states were defined, the specific enthalpy, entropy, exergy, and energy and exergy flow rates of each

stream is calculated based on the specified inlet data given elsewhere [11] mentioned in section “System Description” or via

balance equations. Ambient state is taken to be 20 �C and 1 bar, respectively. The thermodynamic properties of H2O, H2, O2,

CH4, CO2, and CH3OH are found using the Engineering Equation Solver (EES) software package.


Results and Discussion

Water Electrolysis Unit

Overall, the electrolysis unit needs 1.4 kmol/s of water (in total 0.75 kmol/s of this water comes from the matiant power

cycle and methanol production units). After the complete electrolysis, 0.75 kmol/s of H2 is sent to the methanol production

unit while 0.65 kmol/s of H2 is sent to the storage tank. Similarly, 0.35 kmol/s of O2 is sent to the matiant power cycle unit

while 0.35 kmol/s of O2 is sent to the storage tank.

Table 13.1 summarizes the energy and exergy efficiencies as well as the energy input/output (either work or heat) of

electrolysis modules and compressors and heat exchangers and the overall subsystem itself. The empty cells can be assumed

to be 0 or “not applicable.”

The electrolysis unit’s total work input is around 46 MW; this amount is required to drive the electrolysis reaction and

compress the products H2/O2. Heat exchangers’ load indicates the total amount of cooling required following the compres-

sion processes. The system has an energy and exergy efficiency of 30 %. The overall sustainability index of the water

electrolysis subsystem is 1.43. These energy and exergy efficiencies are calculated by the following equations:

η ¼_EO2

þ _EH2

_Ewater þ _Welectrolysis þ _Wcompressors

ð13:11Þ

Ψ ¼_ExO2

þ _ExH2

_Exwater þ _Welectrolysis þ _Wcompressors

ð13:12Þ

Figure 13.5 shows that 49 % of the system irreversibility, therefore the exergy destruction, occurs in the electrolysis

modules. This means when the heat exchangers and compressors are combined together, they have about the same exergy

destruction rates with the electrolysis modules alone. When compared, the total exergy destruction rate in compressors is

very close to the one of heat exchangers in total. However, on average, compressors are exergetically more efficient than the

heat exchangers. This might be due to the high temperature differences between the inlet and outlet of heat exchangers.

Initially, the ambient state is taken to be 20 �C and 1 bar, respectively. In order to find how the electrolysis unit performs

exergetically under different ambient conditions, the system is run under various ambient temperatures. The exergy

efficiency and exergy destruction rate of the electrolysis modules, compressors, and heat exchangers as well as the overall

electrolysis unit at different ambient temperatures are shown in Fig. 13.6.

Table 13.1 Summary of the major components and the overall electrolysis unit

Description η Ψ _Q(kW) _W (kW)

Electrolysis modules 51% 38% 7,514

Heat exchangers 90% 64% 27,904

Compressors 85% 80% 38,652

Overall electrolysis unit 30% 30% 27,904 46,166

Compressors26%

HeatExchangers

25%

ElectrolysisModules

49%

Fig. 13.5 Relative irreversibilities of the major components of the electrolysis subsystem


Figure 13.6 shows that, with increasing ambient temperature, a slight increase is observed in the exergy destruction rate of

heat exchangers and compressors. This increase is less than 6 % when the ambient temperature is increased from 2 to 28 �C.However, there is a slight decrease in the exergy destruction rate of electrolysis modules. This decrease is less than 2 % when

the ambient temperature is increased from 2 to 28 �C. The overall system’s exergy destruction rate increases with increasing

ambient temperature. This increase is less than 2 % as the ambient temperature is increased from 2 to 28 �C.Furthermore, it can be seen from Fig. 13.6 that an increase in the ambient temperature has a negligible effect on the

exergy efficiencies of the electrolysis modules and the compressors (1 % change as the ambient temperature increased from

2 to 28 �C). However, the exergy efficiency of the heat exchangers decreases with increasing ambient temperature (3 %

decrease as the ambient temperature increased from 2 to 28 �C), and the exergy efficiency of the overall electrolysis unit

increases with increasing ambient temperature (2 % increase as the ambient temperature increased from 2 to 28 �C). Takingthe small percentage of the change of rate of exergy destructions and the exergy efficiencies, it can be concluded that the

ambient temperature doesn’t have a dramatic effect on exergetic performance of the system within the studied temperature

interval.

Matiant-Cycle Power Plant

In power plant, 0.5 kmol/s of the O2 from the electrolysis unit is used for complete combustion of 0.25 kmol/s of natural gas

(pure methane). As a result, 0.50 kmol/s of H2O is produced and sent back to the electrolysis unit. About 0.25 kmol/s of

the CO2 is sent to the methanol synthesis unit while about 3 kmol/s of CO2 is used as thermodynamic working fluid in

the system.

Table 13.2 summarizes the energy and exergy efficiencies, as well as the energy input/output (either work or heat) of the

major components and the overall system itself. The empty cells can be assumed to be 0 or “not applicable.” From

25%

35%

45%

55%

65%

75%

85%

2

7

12

17

22

27

32

0 5 10 15 20 25 30

Exer

gy E

ffic

ienc

y

Rate

of E

xerg

y D

estr

uctio

n (M

W)

Ambient Temperature (°C)

Electrolysis ExergyDestruction Rate

Compressors ExergyDestruction Rate

HEX ExergyDestruction Rate

Overall ExergyDestruction Rate

Electrolysis ExergyEfficiency

Compressors ExergyEfficiency

HEX Exergy Efficiency

Overall ExergyEfficiency

Fig. 13.6 The effect of ambient temperature on exergy destruction rates and exergy efficiencies of major components of the electrolysis unit

Table 13.2 Summary of the major components and the overall matiant-cycle power plant



Expanders (turbines) 85% 80% 132,588

Combustion chambers 79% 69%

Compressors and pumps 85% 75% 11,426 16,710

Flash drum 95% 78%

Overall unit 76% 64% 47,005 115,878


Table 13.2, it can be seen that the matiant-cycle power plant has an overall energy efficiency of 76 % and an exergy

efficiency of 64 %. The sustainability index of the overall power plant is around 2.76. The total heat removal (from the heat

exchanger used before the flash drum and the compressor stage) is about 47 MW, and the network output (the work output of

the turbines—the work required by compressor and pumps) is around 115 MW. The total exergy destruction rate is about

168 MW. The energy and exergy efficiency of the matiant-cycle power plant unit is calculated based on the following

equations:

η ¼_Wturbines

_Emethane þ _Wcompressors,pumps

ð13:13Þ

Ψ ¼_Wturbines

_Exmethane þ _Wcompressors,pumps

ð13:14Þ

Figure 13.7 shows that the greatest irreversibilities occur at combustion chambers (combined together, 58 % of the exergy

destruction happens at the combustion chambers). The temperature and pressure difference in combustion chambers is

significant; also the chemical exergy destruction has a considerable effect on the irreversibility at the combustion chambers.

Following the combustion chambers, turbines, compressors and pumps, and heat exchangers have similar relative irreversi-

bilities, between 11 and 15 %. Flash drum has a small effect on the system’s irreversibility. One of the reasons of this result

might be because almost no temperature/pressure change is observed between the inlet/outlet streams of the flash drum.

From Fig. 13.8, it can be seen that as ambient temperature is increased from 2 to 28 �C, the total exergy destruction rate ofthe turbines increases by 2.7 %. Combustion chambers’ total exergy destruction rate also increases by 5.9 %. Overall, the

HeatExchangers

11%

Expanders(Turbines)

15%

CombustionChambers

58%

Compressorsand Pumps

13%

Flash Drum3%

Fig. 13.7 Relative

irreversibilities of the major

components of the matiant-cycle

power plant

60%

65%

70%

75%

80%

85%

30405060708090

100110120130140150160170180

0 5 10 15 20 25 30Ex

ergy

Eff

icie

ncy

Rate

of E

xerg

y D

estr

uctio

n (M

W)

Ambient Temperature (�C)

Turbines ExergyDestruction Rate

CombustionChambers ExergyDestruction Rate


Turbines ExergyEfficiency

CombustionChambers ExergyEfficiency


Fig. 13.8 The effect of ambient temperature on exergy destruction rates and exergy efficiencies of major components of the power plant


total exergy destruction rate of the power plant system increases by 1.6 %. The different effect of the ambient temperature on

exergy destruction rates of various major components can be explained by the different temperature and pressures of inlet/

outlet streams of each component. Since each stream’s exergetic potential depends on how close/far they are to/from the

ambient (dead) state, some streams’ exergetic potential increases as the ambient temperature is increases, while some of

them decrease. A decrease in exergy efficiency with increasing ambient temperature occurs in combustion chambers. Within

the defined temperature range, the decrease in exergy efficiency of combustion chambers is about 1 %. The turbines’ exergy

efficiency decreases by about 0.4 % as the temperature increased within the defined range. Overall, the power plant’s exergy

efficiency decreases by 2 % within the defined ambient temperature range.

Methanol Synthesis Unit

The methanol synthesis unit uses the H2 from the water electrolysis unit and the CO2 from the power plant. Overall, the

methanol synthesis unit needs 0.25 kmol/s of CO2 (from power plant unit) and 0.75 kmol/s H2 (from electrolysis unit). As a

result, 0.25 kmol/s of H2O is produced and sent back to the electrolysis unit. The methanol synthesis unit produces

0.25 kmol/s of CH3OH.

Table 13.3 summarizes the energy and exergy efficiencies, as well as the energy input/output (either work or heat) of the

major components and the overall system itself. The empty cells can be assumed to be 0 or “not applicable.”

Overall, the methanol synthesis unit requires about 2 MW of heat and about 2.2 MW of work input. At given ambient

conditions (20 �C and 1 bar), the synthesis unit has an energy and exergy efficiency of around 41 %. As a result, the

sustainability index of the methanol synthesis unit is 1.7. The energy and exergy efficiency of methanol synthesis unit is

calculated based on the following equations:

η ¼_Emethanol þ _Ewater

_ECO2þ _EH2

þ _Wcompressor þ _Qinput

ð13:15Þ

Ψ ¼_Exmethanol þ _Exwater

_ExCO2þ _ExH2

þ _Wcompressor þ _Qinput 1� T0

Tavg

� � ð13:16Þ

During the process, the rate of exergy destruction is about 80 MW, and Fig. 13.9 shows that 25 % of the irreversibility

occurs at the methanol reactor. However, combined together, heat exchangers have a significant effect on exergy destruction

rate of the overall system, which means the greatest irreversibility occurs at heat exchangers, combined. Following the heat

exchangers, methanol reactor has the second highest irreversibility. Compressors, distillation tower, and the flash drum have

similar relative irreversibilities, around 12–14 %. One of the reasons of the low irreversibility at flash drum and distillation

column is almost no temperature/pressure change is observed between the inlet/outlet streams of the flash drum and

distillation tower. The compressor’s pressure difference is not significant either. The overall system requires additional

heat, which is provided by the rejected heat and is not mentioned here.

Figure 13.10 shows that ambient temperature has different effects on different major units of the methanol synthesis unit.

For instance, as ambient temperature is increased from 2 to 28 �C, the exergy destruction rate of heat exchangers (combined)

decreases by about 6 %. The exergy destruction of the reactor increases by 12 %. Overall, the exergy destruction rate of

the methanol synthesis unit increases by about 1 % within the given ambient temperature range. The different effect of the

ambient temperature on exergy destruction rates of various major components can be explained by the different temperature

and pressures of inlet/outlet streams of each component. Since each stream’s exergetic potential depends on how close/far

Table 13.3 Summary of the major subunits and the overall methanol synthesis unit



Methanol reactor 68% 68%

Flash drum 90% 82%

Compressor 85% 76% 2,284

Distillation tower 89% 90%

Overall synthesis unit 41% 41% 1,934 2,284


they are to/from the ambient (dead) state, some streams’ exergetic potential increases as the ambient temperature increases,

whereas some of them decrease. From Fig. 13.10, it can be seen that the decrease in exergy efficiency of heat exchangers

is about 2 %. The reactor’s exergy efficiency decreases by about 0.11 % as the temperature increased within the defined

range. Overall, the methanol synthesis unit’s exergy efficiency increases by about 0.2 % within the defined ambient

temperature range.

Overall System Analysis

Overall, the integrated system has three major units: the electrolysis, the matiant-cycle power plant, and methanol synthesis

units. In Soltanieh et al.’s [11] study wind energy is proposed as the required power input required for water electrolysis, and

the work required by compressors is provided by the output of the power plant unit. The system has ten compressors, nine

heat exchangers that require heat removal by a different process, three turbines, two heat exchangers that use different

streams of the same unit to provide heat/cold to one another, two combustion chambers, two flash drums, two storage tanks

(one for H2 and one for O2), one electrolysis unit, one heat exchanger that requires heat input from a different process, one

methanol reactor, one distillation tower, and one pump. In this study, each unit is examined within its boundary limits, in

HeatExchangers

37%

MethanolReactor

25%

Flash Drum12%

Compressor14%

DistillationTower12%

Fig. 13.9 Relative

irreversibilities of the major

components of the methanol

synthesis unit

40%

45%

50%

55%

60%

65%

70%

75%

80%

15

20

25

30

35

40

45

50

55

60

65

70

75

80

0 5 10 15 20 25 30

Exer

gy E

ffic

ienc

y

Rate

of E

xerg

y D

estr

uctio

n (M

W)

Ambient Temperature (�C)

HEX ExergyDestruction Rate

Reactor ExergyDestruction Rate


HEX ExergyEfficiency

Reactor ExergyEfficiency


Fig. 13.10 The effect of ambient temperature on exergy destruction rates and exergy efficiencies of major components of the methanol

synthesis unit


order to see where the greatest irreversibilities occur and how the ambient temperature affects the system exergetically; the

overall system is also investigated.

Figure 13.11 shows that the greatest irreversibility in the overall system occurs at the power plant; this is mainly because

of the effect of two combustion chambers in the power plant. The combustion chambers’ effect on exergy destruction rates is

discussed earlier in section “Matiant-Cycle Power Plant”. Temperature difference has an important effect on exergy

destruction rates; therefore, any improvement to reduce the temperature difference between inlet and outlet streams

would improve the system’s overall performance by reducing the exergy destruction rates, therefore irreversibilities.

The energy and exergy efficiencies of the system can be defined in several ways. Table 13.4 summarizes the results based

on the three different methods mentioned below:

1. Heat rejected from the system is not considered as a useful output, but some of this heat is used to provide the necessary

input required for the distillation column input of the methanol synthesis unit. The work input required for the

compressors and pumps are provided by the work output of turbines. The work input required for the electrolysis

modules is provided externally:

η1 ¼_EH2, stored þ _EO2, stored þ _EMethanol þ _Wturbines � _Wcompressors,pumps

_Ewater þ _EMethane þ _Welectrolysis

ð13:17Þ

Ψ 1 ¼Ex:H2, stored

þ _ExO2, stored þ _ExMethanol þ _Wturbines � _Wcompressors,pumps

_Exwater þ _ExMethane þ _Welectrolysis

ð13:18Þ

2. Heat rejected from the system is considered as a useful output; some of this output is used to provide the necessary input

required for the distillation column input of the methanol synthesis unit. The work input required for the compressors and

pumps are provided by the work output of turbines. The work input required for the electrolysis modules is provided

externally:

η2 ¼_EH2, stored þ _EO2, stored þ _EMethanol þ _Wturbines�compressors,pumps þ _Qrejected � _Qinput

_Ewater þ _EMethane þ _Welectrolysis

ð13:19Þ

Ψ 2 ¼_ExH2, stored þ _ExO2, stored þ _ExM þ _Wturbines�compressors,pumps þ _Ex Q, rejected�Q, inputð Þ

_Exwater þ _ExMethane þ _Welectrolysis

ð13:20Þ

Electrolysis11%

Power Plant60%

Methanol29%

Fig. 13.11 Relative

irreversibilities of the

electrolysis, power plant, and

methanol synthesis unit

Table 13.4 Summary of different scenarios with corresponding energy and

exergy efficiencies

Case η Ψ1 35% 42%

2 64% 46%

3 68% 47%


Case Study Soltanieh et al. [11] propose to use the work output of the turbines for the work required in compressors and

pumps. The paper uses wind energy to run the electrolysis unit. If electrolysis unit also used the work output of the turbines,

the energy and exergy efficiencies become

η3 ¼_EH2, stored þ _EO2, stored þ _EMethanol þ _Wturbines�compressors,pumps�electrolysis þ _Qrejected�input

_Ewater þ _EMethane

ð13:21Þ

Ψ 3 ¼_ExH2

þ _ExO2þ _ExMethanol þ _Wturbines�comp,pumps�electrolysis þ _Ex Q, rejected�Q, inputð Þ

_Exwater þ _ExMethane

ð13:22Þ

Conclusions

The integrated system, as modified from a literature work, electrolyzes water using wind energy and stores 46 % of the

produced H2 and 28 % of the produced O2. The remaining O2 is sent from the electrolysis unit to the Matiant-cycle power

plant and goes into complete combustion with natural gas (pure methane). The combustion product water is sent back to the

electrolysis unit, and CO2 is sent to the methanol synthesis unit. In the methanol synthesis unit, CO2 reacts with the H2

coming from the electrolysis unit and produces methanol. The overall system does not release CO2 to the environment, and

the heating requirement of the system is met by the rejected heat within the integrated system. Exergy efficiency of the

overall system increases by 2 % if the rejected heat is used as a product, such as water or space heating. This amount

increases by 1 % if the work requirements of not only the compressor and pumps but also the electrolysis modules are met by

the work output of the turbines. The integrated system’s outputs are about 133 MW of electricity energy (60 MW, if the

pumps, compressors, and electrolysis modules’ input is provided) and 0.25 kmol/s methanol. By converting the CO2 to

methanol, the methanol synthesis unit avoids the release of 0.25 kmol/s CO2 into the atmosphere. If the system is assumed to

work 8,000 h, the avoided CO2 emissions could reach around 160 tons.

References

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