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Energy 32 (2007) 1334–1342

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A novel combined cycle with synthetic utilization of coal and natural gas

Wei Han, Hongguang Jin�, Wei Xu

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100080, China

Received 10 April 2006

Abstract

In this paper, a novel combined cycle with synthetic utilization of coal and natural gas is proposed, in which the burning of coal

provides thermal energy to the methane/steam reforming reaction. The syngas fuel, generated by the reforming reaction, is directly

provided to the gas turbine as fuel. The reforming process with coal firing has been investigated based on the concept of energy level, and

the equations has been derived to disclosing the mechanism of the cascade utilization of chemical energy of natural gas and coal in the

reforming process with coal firing. Through the synthetic utilization of natural gas and coal, the exergy destruction of the combustion of

syngas is decreased obviously compared with the direct combustion of natural gas and coal. As a result, the overall thermal efficiency of

the new cycle reaches 52.9%, as energy supply by methane is about twice as much as these of coal. With the same consumption of natural

gas and coal the new cycle can generate about 6% more power than the reference cycles (the combined cycle and the steam power plant).

The promising results obtained here provide a new way to utilize natural gas and coal more efficiently and economically by synthetic

utilization.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Methane/steam reforming; Dual fuel; Combined cycle; Exergy; Energy level

1. Introduction

The world energy demand has been increasing steadilyand will continue to increase in the future [1]. As coal isoverwhelmingly abundant and more widely distributed insome countries in comparison with other fossil fuels, suchas oil and natural gas, it is important to utilize the coalefficiently and cleanly [2].

Several thermal cycles with coal as fuel have beenproposed [2,3], such as the Externally Fired Humid AirTurbine (EFHAT), the Pressurized Fluidized Bed Combus-tion (PFBC) and the Integrated Gasification CombinedCycle (IGCC). EFHAT is an indirectly fired combinedcycle [3], which has the potential for significantly higherefficiency. However, the higher efficiency depends on thehigher inlet temperature of the turbine, and the heatexchanger materials are currently limited to 1100K. PFBC[2] also has the potential to reach higher efficiency,especially the second-generation system. In PFBC systems,

e front matter r 2006 Elsevier Ltd. All rights reserved.

ergy.2006.10.012

ing author. Tel.: +8610 82543032; fax: +86 10 82622854.

ess: hgjin@mail.etp.ac.cn (H. Jin).

solid wastes and SO2 have to be removed efficiently fromhigh-temperature flue gas or fuel gas before it is providedto the gas turbine. But with the current technology, this isvery difficult. IGCC offers a coal-based power technologywith low emission and high thermal efficiency. The thermalefficiency of IGCC has reached 39–45% [4–6] and manyIGCC demonstration plants are operating throughout theworld. An IGCC power plant is a gasification facilitycoupled to a gas-fired combined-cycle unit, and gasificationis a key step for advanced conversion of coal to electricity.As coal gasification is very complex, requiring a gasifier, airseparation unit and fuel gas cleanup unit, the specificinvestment cost of the IGCC is extremely high. The lack ofeconomic competition restricts the application of theIGCC.Natural gas (NG) is another main fossil fuel for power

generation in common uses and currently the powergeneration depends on the capacity of NG-fired combinedcycles [7]. In a combined cycle, NG is directly burned in thecombustion chamber, and the exergy destruction ofcombustion accounts for about half of the total exergydestruction of the combined cycle [8]. Researchers have

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Nomenclature

A energy levelA1 energy level of reactant of reforming reactionA3 energy level of syngasAc energy level of coalAfl energy level of flue gasT absolute temperatureAT energy level of combustion products at T

Tref absolute temperature of reforming reactionT0 absolute ambient temperatureZref efficiency of Carnot cycle with high-tempera-

ture Tref

Zst average energy level of water evaporation inboiler

DSref entropy change of the reforming processDGref Gibbs function change of the reforming processDEref exergy change of the reforming processE1 exergy of the reactant of reforming reaction

E3 exergy of the reactant of reforming reactionH1 energy of reactant of reforming reactionH3 energy of syngasHc energy of coalDH2 reaction enthalpy of reforming processTofg absolute temperature of output flue gas of pre-

reformerTIT turbine inlet temperatureTpa absolute temperature of preheated aire pressure ratio of compressorZnet net thermal efficiency of coal to powerWnet net work output of coalWnc work output of the new combined cycleWcc work output of the combined cycleIs,net net specific investment of coal to powerInet net investment of coal to powerIcc investment of combined cycleInc investment of new combined cycle

W. Han et al. / Energy 32 (2007) 1334–1342 1335

attempted to reduce the exergy destruction by usingnew kinds of combustion of natural gas, such as theChemically Rcuperated Gas Turbine (CRGT) cycle [9–11].Refs. [10,11] disclosed the cascade utilization of chemicalenergy of NG in the CRGT cycle, and concluded that thetemperature and the molar ratio of steam to methane ofreforming reaction are the key factors that affected theperformance of the CRGT cycle. However, it is verydifficult to improve the performance of the CRGT further,because the CRGT cycle cannot provide the optimalconditions for the reforming reaction by it self. Thereexists a possibility to integrate the utilization of coal andnatural gas. Through integration, the optimal conditions ofthe reforming reaction can be obtained, and natural gasand coal can be utilized more efficiently.

The objectives of this study were: (1) to develop a newapproach to utilize coal and natural gas synthetically andefficiently; (2) to discover the internal phenomenon of thedual fuel reforming process; and (3) to synthesize a novelpower plant with coal and natural gas as fuel.

reactor

combustor

natural gas

steam

syngas

thermalenergy

natural gasflue gas

+air

Fig. 1. Conventional methane/steam reforming.

2. New approach of synthetic utilization of coal and natural

gas

2.1. Concept of methane–steam reforming with coal firing

Methane–steam reforming is widely used in the chemicalindustry, and it is the main method for producing syngasfrom natural gas. Since the main component of natural gasis methane, we assume that the phrase ‘‘natural gas’’ refersto methane. The reforming process is based on twoindependent equilibrium-limited reactions, as follows:

CH4 þH2O! 3H2 þ CO ðDH0298 ¼ 206 kJ=molÞ (A)

COþH2O! CO2 þH2 ðDH0298 ¼ �41 kJ=molÞ: (B)

Reaction (A) is the endothermic steam reformingreaction, and the typical reaction temperature is around900 1C. The second reaction, often referred to as thewater–gas shift reaction, is a lightly exothermic reaction.As a whole, the reforming of methane by steam will absorba large amount of high-temperature thermal energy.Fig. 1 shows the process of conventional methane/steam

reforming in the chemical industry. The dashed arrowrepresents the flow of thermal energy. The thermal energyfor the reforming reaction comes from the burning ofnatural gas or purge gas of a chemical producing process.A tube is filled with the catalyst, and the reactants also flowinto the interior of the tube. The methane/steam reformingtakes place on the surface of the catalyst, and at the sametime, a large amount of thermal energy is absorbed. Thefuel (natural gas or purge gas) is burned at the outside ofthe tube and the high-temperature thermal energy istransferred through the tube. As we know, the naturalgas or purge gas is very clean and can be used by the gasturbine directly. Currently, the TIT of the gas turbine has

ARTICLE IN PRESS

reactor

combustor

natural gas

steam

syngas

thermalenergy

coal+ flue gasair

Fig. 2. Methane–steam reforming with coal firing.

steam

reforming

natural gas

and steam

syngas

thermal energy

H1, E

1

H3, E

3

combustorcoal and air flue gas

Hc, EcHfl, Efl

heat

reserviorΔ H

2, ΔE

2

Fig. 3. Sketch map of the dual fuel reforming process.

W. Han et al. / Energy 32 (2007) 1334–13421336

reached 1430 1C [12], which is much higher than that of thereforming reaction (Tref ¼ 900 1C). Although the naturalgas is burned both in the conventional reformer and thecombustor of the gas turbine, the exergy destruction of theformer is much higher than that of the latter. The fuel ofnatural gas or purge gas has the potential to be used moreefficiently. Fig. 2 is a sketch map of the methane–steamreforming with burning of coal. The new reforming processintegrates the methane–steam reforming and the burning ofcoal. It should be noted that in the new reforming processthe thermal energy absorbed by the reforming reactioncomes from the burning of coal instead of the burning ofnatural gas. We named the new reforming process as ‘‘dualfuel reforming’’; as two types of fossil fuel (coal and naturalgas) are input to generate syngas.

2.2. Internal phenomenon of the dual fuel reforming process

The dual fuel reforming process was studied by makinguse of the concept of the energy level to reveal the internal

workings of the process. Energy level A proposed by Ishidaand Kawamura [13] was used as the property thatrepresents the quality of released or accepted energy for agiven process. It is defined as the ratio of exergy change DE

to energy change DH during a process, as

A ¼DE

DH¼ 1�

T0DS

DH. (1)

For example, for a heat reservoir with a temperature T

releasing heat into the environment, the energy level of thereleased heat can be simplified as AT ¼ 1�T0/T, which isequal to the Carnot cycle efficiency (ZT).Fig. 3 illustrates the process of dual fuel reforming in

more detail. A heat reservoir at Tref (the temperature of thereforming reaction) is assumed to absorb the thermalenergy from the combustion of coal and release the thermalenergy to the steam reforming reaction. Hence, the dualfuel reforming process has been separated into twoprocesses: the methane/steam reforming and the combus-tion of coal.

2.2.1. Efficient utilization of natural gas in the dual fuel

reforming process

We assume that the input and output flows of the dualfuel reforming process are steady-flows. As for themethane/steam reforming (Fig. 3), the change of the exergyand Gibbs energy can be evaluated as

DEref ¼ DH2 � T0DSref (2)

DGref ¼ DH2 � Tref DSref , (3)

where Tref and T0 are, respectively, the absolute tempera-tures of the heat reservoir and environment.By substituting Eq. (3) into Eq. (2) and rearranging it,

the following equation is derived:

DEref

DH2¼ 1�

T0

Tref

� ��

DGref

DH2

T0

Tref

� �. (4)

Since the energy level of the thermal energy forreforming is Zref ¼ 1–T0/Tref, Eq. (4) may be reduced to

DEref

DH2¼ Zref �

DGref

DH21� Zref

� �. (4a)

In the methane/steam reforming process, DEref and DH2

can be expressed by the exergy and the energy of reactantsand products:

DEref ¼ E1 � E3, (5)

DH2 ¼ H1 �H3. (6)

In Eq. (6) H1 and H3 are, respectively, the energy of thereactants and the products, which is the sum of the physicalinternal energy (associated with temperature, pressure andphase changes) and the chemical internal energy (asso-ciated with the destruction and formation of chemicalbonds between atoms). We defined the parameter bref asthe ratio of the reaction enthalpy of the reforming process

ARTICLE IN PRESSW. Han et al. / Energy 32 (2007) 1334–1342 1337

to the energy of reactants. The term bref is expressed as

bref ¼DH2

H1. (7)

Substituting Eqs. (1), (5), (6) and (7) into Eq. (4a) yields

1þ bref

� �A3 ¼ A1 þ bref Zref �

�DGref

DH21� Zref

� �� �. (8)

In Eq. (8), A1 is the energy level of the mixture of naturalgas and steam and A3 is the energy level of syngas. Thevalue of (�DGref) is equal to the reversible work of thereforming process, as the reforming process is reversible.Since there is no work output during an irreversiblereforming process, �DGref represents the exergy destruc-tion of the reforming process on the reforming conditions.If we study the reforming process in an environment at25 1C and 1 atm, the exergy destruction of the reformingprocess changes to �DGref(1�Zref). Here, we recognize�DGref/DH2 as the energy level of Gibbs energy of thereforming process and it is denoted as Bref. Thereby, Eq. (8)may be rearranged and expressed as

A1 � A3 ¼ bref A3 � Zref

� �þ bref Bref 1� Zref

� �. (9)

Eq. (9) denotes the relationship between the energy levelof natural gas (A1), syngas (A3), thermal energy for thereaction (Zref) and Gibbs energy of the reforming process(Bref). As the energy level of syngas (A3) is higher than thatof thermal energy for the reforming process (Zref), the righthand side of Eq. (9) is positive (A1�A340), which meansthat the energy level of syngas (A3) is lower than that ofnatural gas (A1). Through the reforming process, theenergy level of natural gas is decreased to that of syngasand the difference of the energy level (A1�A3) acts as adriving force to upgrade the energy level of thermal energyfor the reforming process (Zref) to the energy level of syngas(A3). In other words, the thermal energy for the reformingprocess transformed into the chemical energy of syngas,and the driving force of the process is the degradation ofthe energy level of natural gas.

Both sides of Eq. (9) multiply the energy of the reactantof the reforming process (H1), and the exergy-balanceequation based on the energy level is obtained:

H1 1þ bð Þ � A3 ¼ H1 � A1 þH1bref � Zref

�H1bref � Bref 1� Zref

� �. ð10Þ

Substituting Eqs. (6) and (7) into Eq. (10) yields

H3 � A3 ¼ H1 � A1 þ DH2 � Zref � DH2 � Bref 1� Zref

� �.

(11)

In Eq. (11) the product of the energy level of syngas (A3)and the energy of syngas (H3) is the exergy of the syngas.Similarly, the product of the energy level of the reactant(A1) and the energy of the reactant (H1) is the exergy of thereactant. The second item on the right hand side of Eq. (11)is the exergy of thermal energy for the reforming process,

whose energy level and energy are, respectively, Zref andDH2. The last item on the right hand side of Eq. (11) is theexergy destruction of the reforming process. From Eqs.(10) and (11), we can find that the product’s energy(H3 ¼ H1(1+bref)) is increased by bref �H1 through thereforming process compared with the reactant’s energy(H1). The thermal energy for the reaction, which comesfrom the chemical energy of coal, is transformed into thechemical energy of the syngas; therefore, we can considerthat the coal is somewhat gasified indirectly.The syngas produced by the reforming process can be

provided to the gas turbine as fuel. Through the com-bustion the chemical energy of syngas is changed intothermal energy, and the combustion of natural gas afterthe reforming process is referred to the indirect combus-tion of natural gas. We assume that the direct and indirectcombustion of natural gas proceeds at the same tem-perature T, and the reactions act as the energy donor.The combustion products are considered as the energyacceptor, and the energy levels of the acceptors duringthe combustion of the syngas and natural gas are iden-tical to each other. The term AT is used to representthe energy level of the combustion products. Thus, basedon the exergy destruction of combustion processesDE ¼ DHea(Aed�Aea) [14], the exergy destruction in thecombustion of syngas is

DE3 ¼ H3ðA3 � AT Þ. (12)

When Eqs. (9), (6) and (7) are substituted into Eq. (12), ityields

H1 A1 � ATð Þ �H3 A3 � ATð Þ ¼ DH2Bref 1� Zref

� �

þ DH2 AT � Zref

� �. ð13Þ

The term H1(A1�AT) is the exergy destruction of thedirect combustion process of natural gas, and H3(A3�AT)is the exergy destruction of the indirect combustion processof natural gas. Then, the left hand side of Eq. (13)represents the difference of the exergy destruction betweenthe direct and indirect combustion of natural gas. Be-cause the value of DH2Bref(1�Zref) (the exergy destructionof the reforming process) is positive and the value ofDH2(AT�Zref) is also positive, the exergy destruction of theindirect combustion of natural gas is lower than that of thedirect combustion of natural gas. From Eq. (13) we candetermine that the energy level change of the fuels from A1

to A3 causes the exergy destruction of combustion to bedifferent. The decrease of exergy destruction means that wecan get more thermal exergy (DH2(AT�Zref)) from theindirect combustion of natural gas.

2.2.2. Efficient utilization of coal in the dual fuel reforming

process

Fig. 3 also illustrates the process of the burning of coal.Similar to the reforming process, we can get the exergy-balance equation of the combustion of coal based on the

�,

ARTICLE IN PRESS

1

a

j1

e

f

g

2

4

10

11

12

13

9

3

5

6

7

8

14

15

b

d

j2

a: reformer b: pre-reformer c: heat exchanger

d: steam turbine e: condenser f: pump g: compressor

h: combustor i: gas turbine j1, j2: power generator

16 1718

c

h

i

19

20

20

power generation subsystemdual fuel reforming subsystem

21

Fig. 4. Flow diagram of the dual fuel combined cycle.

W. Han et al. / Energy 32 (2007) 1334–13421338

energy level, as follows:

Hcbc � Zref ¼ Hc � Ac �Hc 1� bc

� �� Afl

�Hcbc � Bcc 1� Zref

� �, ð14Þ

where Ac and Afl are, respectively, the energy levels of coaland flue gas and bc is the ratio of thermal energy absorbedby the heat reservoir to the chemical energy of coal. Theleft hand side of Eq. (14) is the thermal exergy absorbed bythe heat reservoir of the reforming process. The first itemon the right hand side (Hc �Ac) is the exergy of coal, thesecond item is the exergy of the flue gas and the third itemis the exergy destruction of the combustion of coal. Thedefinition of bc is

bc ¼DH2

Hc

. (15)

During dual fuel reforming, the thermal energy releasedfrom coal combustion is absorbed by the heat reservoir asshown in Fig. 3. In the conventional boiler the thermalenergy is used to vaporize water. The exergy-balanceequation of combustion of coal in the boiler is

Hcb0c � Zst ¼ Hc � Ac �Hc 1� b0c

� �� A0fl �Hcb

0c � B

0cc 1� Zref

�(16)

where Zst represents the average energy level of the waterevaporation process. Here we assume that the temperatureof flue gas of the boiler is equal to that of the reformer, andthen Afl ¼ A0fl and bc ¼ b0c. From Eqs. (14) and (16), wecan get

Hcbc � B0cc 1� Zref

� ��Hcbc � Bcc 1� Zref

� �

¼ Hcbc � Zref � Zst

� �. ð17Þ

As the temperature of the reforming process is higher thanthat of the steam, the energy level of thermal energy for thereforming reaction (ZrefE0.75) is higher than that of thesteam evaporation, superheating and reheating(ZstE0.54). Therefore, the exergy destruction of coalcombustion in the dual fuel reforming process is smallerthan that in the boiler.

Coal and natural gas are used synthetically by means ofthe dual fuel reforming process. The chemical energyof natural gas is used efficiently and the exergy destructionof the process from chemical energy of natural gas tothermal energy is decreased. The dual fuel reformingreaction acts as the energy acceptor during the coalcombustion process. Because the energy level of thereforming process is higher than that of the processes ofwater evaporation, the exergy destruction during theburning of coal is diminished obviously. In other words,the chemical energy of coal and natural gas can be utilizedmore efficiently by use of the dual fuel reforming method.

3. A novel combined cycle with dual fuel reforming

3.1. Description of a new combined cycle with dual fuel

reforming

The proposed combined cycle with synthetic utilizationof coal and natural gas, shown in Fig. 4, consists of twomajor subsystems: the dual fuel reforming subsystem andthe power generation subsystem. The dual fuel reformingsubsystem includes a methane/steam reformer with burningof coal (a) and a pre-reformer (b). The power generationsubsystem consists of a gas turbine and a steam turbinepower plant. On one hand, the coal (stream 1) and air(stream 19) are preheated in the heat exchanger (c), andthen enter the interior of the reformer where the coal isburned with the air (20). The thermal energy from theburning of coal is absorbed by the high-temperaturereforming reaction (the reaction temperature between 600and 850 1C). The high-temperature flue gas dischargedfrom the reformer enters the pre-reformer (b), providingthermal energy for the low-temperature reforming reaction(the reaction temperature below 600 1C), and then it is usedto generate the steam (stream 6) in the heat exchanger (c).Finally the low-temperature flue gas (140 1C) is dischargedinto the atmosphere through a stack (stream 4). On theother hand, the steam (stream 7) drawn from a steamturbine (d) is mixed with the natural gas (stream 10), andthen the mixture gas enters the pre-reformer (b), where apart of the natural gas is reformed with the steam, andconverted into syngas. Then most of the remaining naturalgas is further changed into syngas in the reformer at ahigher temperature by absorbing the thermal energy fromthe burning of coal. At last the syngas (stream 14) is burnedin the combustor (h) together with the compressed air

ARTICLE IN PRESS

Table 2

Temperature and pressure data for the novel combined cycle

Point Temperature (1C) Pressure (bar)

2 900.0 1.06

3 487.2 1.03

4 140.0 1.0

5 40.1 130.0

6 535.0 120.0

7 457.2 23.4

8 39.0 0.07

9 36.0 0.07

10 25.0 23.4

11 299.5 23.4

12 550.0 21.1

13 596.0 1.04

14 850.0 19.0

15 25.0 1.0

16 420.4 16.0

17 1290 15.52

18 609.6 1.03

19 25.0 1.15

20 200.0 1.1

21 110.0 1.0

W. Han et al. / Energy 32 (2007) 1334–1342 1339

(stream 16) from the compressor (g). Then the combustionproduct (stream 17) is expanded in a gas turbine (i). Thesurplus heat of the exhaust gas of the gas turbine (stream18) is recovered by the heat exchanger (c) to generatesteam. Finally the exhaust gas (110 1C) is also dischargedinto the atmosphere.

3.2. Evaluation of the new combined cycle

The thermal efficiency is widely used to evaluate theperformance of thermal cycle with one type of fuel input.Since the new power plant cycle utilize natural gas and coalsimultaneously, the evaluation of this cycle becomes morecomplex. We cannot estimate directly whether the naturalgas and coal is used more efficiently or not in the new cyclejust from the thermal efficiency. Based on the comparisonof the new combined cycle and the reference cycles(the conventional combined cycle and the steam powerplant), a new criterion is defined to evaluate the perfor-mance of the new cycle more directly and clearly. The newcriterion is a net thermal efficiency of coal to power, andthe definition is as

Znet ¼W net

Hc

¼W nc �W cc

mc � qc

, (18)

where Wnet denotes the net work generated from coal andHc denotes the energy of coal consumed in the newcombined cycle. The net work output of coal is equal to thedifference of the work output of the new combined cycle(Wnc) and the work output of the conventional gas turbinecombined cycle (Wcc) with the same natural gas consump-tion. Here, we assume that the new combined cycle and theconventional gas turbine combined cycle are in the sametechnology class. In a word the net efficiency of coal topower can divide the contributions of natural gas and coalclearly. Comparing the net efficiency of coal to power andthe thermal efficiency of the conventional steam powerplant with coal as fuel, we can judge the performance of thenew cycle easily and clearly. If the net efficiency of coal topower is higher than the thermal efficiency of the steampower plant, we may deduce that the new cycle use thenatural gas and coal more efficiently, otherwise, the new

Table 1

Simulation conditions

TIT of Gas turbine 1290 1C

Pressure ratio of gas turbine 16

Polytropic gas turbine efficiency 0.91

Polytropic compressor efficiency 0.88

Pressure loss ratio of combustor 0.03

Live steam pressure 12MPa

Live/reheated steam temperature 5351C

Reheated steam pressure 3.9MPa

Pump efficiency 0.8

Ambient temperature 25 1C

Ambient pressure 0.1MPa

Min temperature approach 30 1C

cycle has no advantage comparing with the referencecycles.The new combined cycle was evaluated using ASPEN

PLUS code. The most relevant assumptions are summar-ized in Table 1. The pressure ratio of the gas turbine is 16and the turbine inlet temperature was 1290 1C. Thecompressor and turbine efficiency was, respectively, 0.88and 0.91 and about 9% of compressed air was used for theblade cooling. The temperature of the live steam andthe reheated steam of the bottom cycle was 535 1C and thepressures were respectively 12 and 3.9MPa. The tempera-ture of the exhaust gas at the outlet of the HRSG remained110 1C. In addition, the reaction conditions of reformingwere a temperature of 850 1C and an s/c (mole ratio ofsteam to methane) of 3. Coal and preheated air of 200 1Cwas burnt in the reformer and the temperature of flue gasat the output of the pre-reformer was 900 1C. The mainsimulation results were presented in Table 2.The reference cycles include two conventional power

plants: the combined cycle and steam power plant. As forthe combined cycle, the pressure ratio, TIT of the gasturbine, compressor and turbine efficiency were the same asthat of the new combined cycle. The ratio of compressedair for the blade cooling was also about 9%. Thetemperature and pressure of the live steam and the reheatedsteam of the bottom cycle in the combined cycle were alsothe same as that of the new combined cycle. Thetemperature of exhaust gas remained 110 1C too. In thesteam power plant, the circulated fluidized bed boiler wasused to generate live steam with temperature of 540 1C andpressure of 13.7MPa to high-pressure steam turbine, andreheated steam with temperature of 540 1C and pressure of2.65MPa to a second turbine. At last there is a condensingturbine, characterized by a final pressure of 0.007MPa.

ARTICLE IN PRESS

Table 3

Exergy analysis of the new combined cycle and the reference cycles

Item (kJ) New cycle Reference cycles Total of reference cycles

Combined cycle Steam power plant

Exergy input

Chemical exergy of NG 100.00 100.00 — 100.00

Chemical exergy of coal 47.38 — 47.38 47.38

Exergy destruction

Reaction subsystem 45.10 29.61 20.22 49.83

Combustion of NG — 29.61 — 29.61

Combustion of syngas 28.97 — — —

Combustion of coal 16.13 — 20.22 20.22

Power subsystem 11.28 8.69 2.06 10.75

Gas turbine 6.05 4.46 — 4.46

Steam turbine 2.65 2.18 1.94 4.12

Compressor 2.52 2.02 — 2.02

Pump 0.06 0.03 0.12 0.15

Heat exchange and exhaust 15.53 8.18 7.24 15.42

Heat exchangers 7.47 3.92 3.98 7.90

Condenser 1.66 1.52 1.40 2.92

Exhaust 6.40 2.74 1.86 4.60

Total of exergy loss 71.92 46.48 29.52 76.00

Exergy output

Work 75.47 53.52 17.86 71.38

Exergy efficiency 51.3% 53.52% 37.70% —

Reforming temperature Tref (K)

900 950 1000 1050 1100 115041

42

43

44

45

46

47

Net th

erm

al effic

iency o

f coal to

pow

er � n

et (

%)

s/c=4.0

s/c=3.0

s/c=2.0

Tpa=473K, Tofg=1173K, TIT=1563K, �=16

Fig. 5. Variation of the net thermal efficiency of coal to power with

reforming temperature (Tref) and ratio of steam to methane (s/c).

W. Han et al. / Energy 32 (2007) 1334–13421340

There are seven steam extractions for regenerative feed-water preheating; the pressure in the deaerator is at about0.8MPa. The temperature of the exhaust flue gas was140 1C.

As a result, the thermal efficiency of the new cycle is52.86%. When the new cycle and the reference cyclesconsume the same quantity of natural gas and coal, thenew cycle can generate about 6% more power than that ofthe reference cycles. Correspondingly, the net thermalefficiency of coal to power is 46.3%, which is about 8percentage points higher than that of the reference steampower plant. The synthetic utilization of natural gas andcoal is contributing to the performance improvement of thenew system just as the theoretic analysis in the second partof the paper.

Based on the simulation, the exergy balance of the newcombined cycle and the reference cycles were determined,and the results were presented in Table 3. With the samefuel input, the work output of the new cycle is 75.47 kJ, andthe total work output of the reference cycles is 71.38 kJ.The work output of the new cycle is about 6% greater thanthat of the reference cycles. The exergy destruction of thereaction subsystem of the new cycle is 4.73 kJ lower thanthat of the reference cycles, and this is the main factor thatmakes an increase of the output work of the new combinedcycle. Just as predicted by the foregoing theoreticalanalysis, the combustion exergy destruction of naturalgas and coal of the new combined cycle are, respectively,decreased by 0.64 and 4.09 kJ. Hence, the synthetic

utilization of coal and natural gas has great potential toimprove the performance of the thermal cycle.

4. Advanced thermodynamic performance

Fig. 5 illustrates the effect of the reaction temperature ofthe reforming process (Tref) and the ratio of steam tomethane (s/c) on the net thermal efficiency of coal to powerof the new combined cycle. As the temperature of the

ARTICLE IN PRESS

10 12 14 16 18 20

46

47

48

49

50

51Tref=1123K, s/c=2, Tpa=773K, Tofg=873K

Net

therm

al eff

icie

ncy o

f coa t

o p

ow

er � n

et (

%)

Pressure Ratio of compressor �

TIT=1473K

TIT=1573K

TIT=1673K

Fig. 7. Variation of the net thermal efficiency of coal to power with

pressure ratio of compressor (e) and TIT of gas turbine.

W. Han et al. / Energy 32 (2007) 1334–1342 1341

reforming process increased from 650 to 850 1C as shownby the curves in Fig. 5, the net thermal efficiency isincreased from 43.5% to 46.3%, when the s/c is equal to 3.When the temperature of reforming rises with the same s/c,the conversion of natural gas to syngas, the quantity (DH2)and the energy level (Zref) of thermal energy for reformingreaction are increased and the benefit of indirect combus-tion of natural gas (right hand side of Eq. (13)) andcombustion of coal (right hand side of Eq. (17)) is alsoincreased. Therefore, the net thermal efficiency of coal topower is accordingly increased. The net thermal efficiencydecreases as the parameter of s/c rises from 2 to 4 with thesame reforming temperature as shown in Fig. 5. Theincrement of the s/c requires more steam to be extractedfrom the steam turbine (d in Fig. 4) and the output work ofthe steam turbine decreases rapidly. At the same time theincrement of the s/c also makes the endothermic reactionmore complete and required more coal burned in thereformer to provide thermal energy to the reaction. Asthere is more steam in the flue gas, the exergy destruction ofexhaust is also increased. Affected by the aspects above, theperformance of the new cycle can be improved according tothe decrement of s/c. But the steam to methane ratio (s/c)should have a value somewhat larger than 2. If the ratio islower than 2, carbon deposition will take place and foulingof the catalyst will occur.

Fig. 6 illustrates the effects of the temperature ofpreheated air (Tpa) burned with coal and the temperatureof output flue gas of the pre-reformer (Tofg) on the netthermal efficiency of coal to power of the new combinedcycle. As the term Tpa increases, less thermal energy isabsorbed by the air heating process during the burning ofcoal, which means that more thermal energy released fromthe burning of coal is transformed into the chemical energyof syngas through the reforming process. Therefore, theperformance of the new cycle is improved as thetemperature of preheated air rises. The decrement of Tofg

means more thermal energy of the flue gas to be absorbed

450 500 550 600 650 700 750 800

46.8

47.4

48.0

48.6

49.2

49.8

50.4 TIT=1563K, �=16, s/c=2, Tref=1123K

Net

therm

al eff

icie

ncy o

f coal to

pow

er � n

et (

%)

Temperature of preheated air Tpa (K)

Tofg= 873K

Tofg= 973K

Tofg= 1073K

Tofg= 1173K

Fig. 6. Variation of the net thermal efficiency of coal to power with

temperature of preheated air (Tpa) and flue gas (Tofg).

in the pre-reformer (b in Fig. 4) and consumption of coal inthe main reformer (a in Fig. 4) decreases at the same time.Hence, the net thermal efficiency of the new combined cycleis increased with the decrement of the Tofg.Fig. 7 shows the effects of the pressure ratio (e) and

turbine inlet temperature (TIT) on the net thermalefficiency of the new combined cycle. As the turbine inlettemperature is increased, the efficiency of the gas turbineraises accordingly, which makes the relative efficiency ofthe new combined cycle rises. The net thermal efficiency isfirstly increased with the increase of the pressure ratio ofcompressor, then, at a proper pressure ratio the relativeefficiency is decreased gradually. The proper pressure ratioalso rises with the increase of the turbine inlet temperature.If the efficiency of syngas to power can be improved, theperformance of the new combined cycle will be improvedfurther.

5. Features of the proposed combined cycle

Compared with the reference combined cycle, the newcombined cycle adds an additional reformer before the gasturbine and the investment of the new combined cycle willbe increased. Similar as the concept of the net thermalefficiency of coal to power, the net specific investment ofcoal to power is defined as follows:

Is;net ¼Inet

W net

¼Inc � Icc

W nc �W cc

,

where Inet is the net investment of coal to power and Wnet isthe net work output of coal. The net investment for coalInet is the difference between the investment of the newcombined cycle (Inc) and the reference combined cycle (Icc).Similarly, the work output of coal (Wnet) is the differencebetween the work output of the new combined cycle (Wnc)and the reference combined cycle (Wcc). The investment ofthe new combined cycle is the summation of the investment

ARTICLE IN PRESSW. Han et al. / Energy 32 (2007) 1334–13421342

of a conventional combined cycle and the additional dualfuel reformer. We assume that the specific investment ofcombined cycle is 400$/kW power output. Since thestructure of the dual fuel reformer is similar to thetraditional reformer, the specific investment of the dualfuel reformer is assumed about 20% higher than that of thetraditional methane/steam reformer. In China the specificinvestment of the conventional reformer is about 200$ asper kW NG burned in the reformer, and then the specificinvestment of the dual fuel reformer is 240$ as per kW coalburned in the dual fuel reformer. Based on the results inTable 3, we can calculate the investment of the newcombined cycle (Inc) as 41559$ (400$/kW� 75.47 kW+240$/kW-coal� 47.38 kW-coal) and the investment of thereference combined cycle (Icc) is 21408$ (400$/kW�53.52 kW). The net investment of coal to power is 20151$(41559$�21408$). The work output of the new combinedcycle and reference combined cycle are, respectively, 75.47and 53.52 kW, and the net work output of coal is 21.95 kW.Hence, the net specific investment of coal to power is 920$/kW (20151$/21.95 kW), which is decreased about 24%comparing with the specific investment of IGCC (1212$/kW [15]). The high investment cost is the fatal problem forthe development of the IGCC. The new combined cycle hasthe potential to solve this problem very well, as the specificinvestment is decreased. The result obtained here alsoindicates that cascaded and synthetic use of the chemicalenergy of fossil fuel is a promising way to improve theperformance of thermal cycle.

Dual fuel reforming can also be regarded as a newapproach to generate syngas efficiently and economically.Making use of the new approach, we can integrateother energy systems, such as chemical processes withmethanol, hydrogen or DME as products and a poly-generation system generating chemical products and powersimultaneously.

6. Conclusions

The synthetic utilization of natural gas and coal isimplemented in the proposed combined cycle through anovel method named as dual fuel reforming. In the dualfuel reforming process the thermal energy released from theburning of coal is absorbed by the reforming reaction, andis transformed into chemical energy of syngas. Since thechemical energy of natural gas and coal is used moreefficiently, the net thermal efficiency of coal to power canreach 43.5–46.3%, which is almost equal to the efficiency ofthe integrated gasification combined cycle (IGCC). Basedon the results of sensitivity analysis, the performance of thenew cycle can be improved further to about 50%. Here, we

had better notice that the natural gas contribution topower production is much higher than that of coal, sincethe energy consumption of natural gas is almost twice asmuch as that of coal. The net specific investment of coal topower and operating cost of the new combined cycle aredecreased obviously compared with IGCC. The newcombined cycle is also suit for the alteration of theconventional combined cycle for the country like Chinawhere both of the natural and coal are widely used. Thepromising results obtained here give a new approach to usecoal and natural efficiently and economically.

Acknowledgment

This research was supported by the National NaturalScience Fundamental Research Program (Nos. 90210032and 50520140517).

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