1390_ftp

8/8/2019 1390_ftp

1/13

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2008; 32 :722734Published online 27 November 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/er.1390

System study on natural gas-based polygeneration systemof DME and electricity

Chen Bin 1,2, *,y, Jin Hongguang 1 and Gao Lin 1

1 Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080, Peoples Republic of China2 Graduate School of the Chinese Academy of Sciences, Beijing 100080, Peoples Republic of China

SUMMARY

An innovative system for the polygeneration of dimethyl ether (DME) and electricity was proposed in this paper. Thesystem uses natural gas as the raw material. Polygeneration is sequential, with one-step and once-through DMEsynthesis. Syngas is made to react to synthesize DME rst, and then the residual syngas is sent to the power generationunit as fuel. The exergy analysis from the view of cascade utilization was executed for individual generation and forpolygeneration. The analysis results showed that both chemical energy and thermal energy in polygeneration wereeffectively utilized, and both chemical exergy destruction and thermal exergy destruction in polygeneration weredecreased. The cause of the decrease in exergy destruction was revealed. The analysis showed that hydrogen-rich(natural gas-based) polygeneration was as desirable as carbon-rich (coal-based) polygeneration. The energy saving ratioof polygeneration was about 10.2%, which demonstrated that high efficiency natural gas-based polygeneration isattainable, and the cascade utilizations of both chemical energy and thermal energy are key contributors to theimprovement of performance. Copyright # 2007 John Wiley & Sons, Ltd.

KEY WORDS : natural gas based; DME/electricity polygeneration; cascade utilization of chemical energy

1. INTRODUCTION

Natural gas is a clean fuel for combined cycle(CC). According to BP company report, in 2005,the worldwide consumption of natural gas grew by2.3%, accounting for 23.5% of the consumption of total primary energy [1]. In CC tremendous exergy

destruction is caused by the large level of difference between the fuel and combustiontemperatures.

Dimethyl ether (DME) is regarded as analternative and clean fuel of the 21st century.DME has potential applications as a dieselsubstitute or as a domestic fuel. Ohno et al .

*Correspondence to: Chen Bin, Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080,Peoples Republic of China.

y E-mail: [email protected]

Contract/grant sponsor: National Key Projects Fund; contract/grant number: 90210032Contract/grant sponsor: National Basic Research Program; contract/grant number: 2005CB221207.

Received 28 March 2007 Revised 18 October 2007

Accepted 21 October 2007 Copyright # 2007 John Wiley & Sons, Ltd.

8/8/2019 1390_ftp

2/13

regarded DME as a clean fuel that does notproduce toxic gases or particulate matter (PM) onburning [2]. NKK company researched in themanufacture of a catalyst for DME preparation

and manufacture of DME [3]. Haldor Topsoecompany (1996) proposed a process for thepreparation of fuel grade DME [4]. Gogate et al .point out that favorable results have been obtainedfor liquid phase dimethyl ether (LPDME) synth-esis in laboratory experiments [5]. Paul et al .concluded research progress in LPDME of airproducts company [6]. Wang et al . found thatalthough the energy consumption has been sig-nicantly reduced, it is difficult to make furtherimprovements with traditional methods [7].

The same materials provide the possibility of

integrating DME generation and power genera-tion. It is clear that polygeneration, whichintegrates DME production and the CC, is anadvantageous method. Polygeneration uses syn-gas, which is yielded from coal, heavy oil, andother hydrocarbons, to produce electricity andchemical products simultaneously. Research of Jackson et al . shows that polygeneration provideshigher efficiency and lower pollution [8]. In areport to the president, the US Department of Energy (DOE) proposed the Vision 21 energyprogram [9]. Shell Company (1999) has proposed

the idea of a Syngas Park [10]. Eric and Ren foundthat once-through polygeneration has the highestefficiency for the polygeneration of both methanoland DME [11]. Ni et al. reported that the energyconsumption of recycled syngas is decreased in

polygeneration [12]. Gao et al. found that thermalenergy matching and energy utilization are betterin polygeneration [13].

Jin et al . indicated that both chemical energy

cascade utilization and thermal energy cascadeutilization exist in polygeneration [14]. It isnecessary to take both chemical energy cascadeutilization and thermal energy cascade utilizationinto consideration in polygeneration research. Andthe differences are so great that it is necessary tostudy natural gas-based polygeneration as ser-iously as coal-based polygeneration.

The purpose of this paper is to propose aninnovative once-through natural gas-based DME-power polygeneration system to analyze thesystem with exergy destruction analysis and

EUD method and to reveal the reason for thehigh efficiency of the system.

2. DME INDIVIDUAL GENERATION ANDDMECC POLYGENERATION

A vapor phase one-step natural gas DME synth-esis owchart is presented in Figure 1. Reformingnatural gas (NGR) is mixed with reforming steam(RS). The mixture is reformed in REFO. FLAMEprovides heat for reforming. Fresh syngas is mixed

with recycled unreacted syngas. Mixed syngas (10)is fed into the synthesis reactor (SYN) and isconverted to DME with a bi-functional catalyst.Methanol synthesis and methanol dehydrationtake place in the reactor simultaneously. Released

Figure 1. Flow sheet of DME individual generation.

NATURAL GAS-BASED POLYGENERATION SYSTEM OF DME AND ELECTRICITY 723

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res . 2008; 32 :722734DOI: 10.1002/er

8/8/2019 1390_ftp

3/13

heat generates the steam for distillation. Product(11) from the synthesis reactor is a mixture of methanol, DME, water, and unreacted syngas.The recycling of unreacted syngas improves the

conversion of syngas, but decreases the concentra-tion of DME in the product. The mixture isdivided into gas phase (12) and liquid phase (13).The gas phase is sent to an absorption unit (ABS),where most of the DME and CO 2 in the gas areabsorbed. The concentration decrease of DME inthe product results in an increase in the quantity of the absorbent. Most of the unreacted syngas (15) isreturned to the SYN, and a small part of it is sentto FLAME as fuel. Liquid phase (13) andabsorbed phase (18) are separated in the distilla-tion towers (DIST1, DIST2, and DIST3). CO 2 and

other light components, such as DME, water, andmethanol, are separated in turn, and the absorbentis regenerated. Methanol is dehydrated in thedehydration reactor (DEH), and then sent toDIST1. Part of the unreacted syngas (16), naturalgas (NGC), and air are combusted in FLAME.The heat from the high-temperature gas (30) isrecovered to generate steam (36).

A owchart of the CC is presented in Figure 2.Air is compressed in COMP. High-pressure air (1)and NG are mixed and sent to COMB to becombusted. High-temperature and high-pressure

gas (2) is expanded to generate power in TURG.The heat from the middle temperature exhaust gas(3) is recovered in the heat recovery steam

generator (HRSG) to generate steam. The steamfrom the HRSG is sent to the two steam turbines(TUR1 and TUR2) to generate power. The steamsystem is a dual pressure reheat HRSG.

The characteristics of individual generation canbe distinctly revealed from the view of cascadeutilization. Natural gas is reformed to syngas,syngas is made to react in DME synthesis, andunreacted syngas is combusted nally. The level of fuel is decreased stage by stage in DME produc-tion. The aim of heat exchange is to meet the needsof DME production, and the utilization of combustion heat is badly arranged. This leads totremendous thermal exergy destruction. In CC, theutilization of combustion heat is excellent, whilethe direct combustion of natural gas results in

huge loss of fuel exergy. It is logical to design apolygeneration method that combines the virtuesof DME production and those of CC. With thiscombination, the advantages of the two systemsare integrated and their shortcomings are avoided.This improvement is shown clearly in Figure 3.

The polygeneration system, which achievedchemical energy cascade utilization and thermalenergy cascade utilization, is shown in Figure 4.Polygeneration is a one-step once-through DMEsynthesis. The syngas preparation is almost thesame as in individual generation, except for the

heat recovery. The heat of hot syngas is exchangedwith RS and NG in HX4, and then is recovered inHX2. The recycled unreacted syngas is not

Figure 2. Flow sheet of CC individual generation.Figure 3. Sketch map of natural gas-based

polygeneration.

C. BIN, J. HONGGUANG AND G. LIN724


8/8/2019 1390_ftp

4/13

presented here; only fresh syngas (6) enters theSYN to synthesize DME. The product from SYNis divided into gas and liquid phases (8). Theseparation process is similar to individual genera-tion. The unreacted syngas, which is larger inquantity than in individual generation, is notrecycled. It is divided into two parts: one part(25) is sent to the combustor (COMB) of the gasturbine (GT) as fuel; the other is mixed withstream 37, and then is sent to FLAME as fuel.

Heat from the SYN reactor is sent to BO toevaporate water, and the pressure is higher than inindividual generation. Steam for distillation (36)and reforming (RS) are drawn from TUR1 at thecorresponding pressure. The heat of hot syngasand ue gas is recovered in HX1, HX2, HX3, andHX4. Compared with individual generation, thetemperature differences of heat exchange aredecreased.

3. COMPARISON OF DME INDIVIDUAL

GENERATION AND DMECCPOLYGENERATION

The integration of DME production and CCfollows the principles of cascade utilization of chemical energy and of thermal energy. Individualgeneration and polygeneration are simulated withthe assumptions listed in Table IV. The ow rates

and compositions of DME individual generationsystem, CC, and polygeneration system are listedin Tables IIII, respectively. The study is based ona large-scale individual DME generation of approximately two hundred thousand tons peryear. The large-scale application providedfull opportunity for system integration betweenchemical production and power generation.

There are several differences between polyge-neration and individual generation. First, the

conversion of syngas in individual generation ishigher than that in polygeneration. Natural gas-based syngas is hydrogen rich, so a key componentof the conversion is carbon. In individual genera-tion, the carbon conversion is kept as high aspossible to acquire the highest product quantity.In order to obtain this goal, the cycle ratio of syngas is increased. The cycle ratio is 0.552 and thecarbon conversion is 0.855. In polygeneration, theunreacted syngas can be utilized on the power side,so a moderate conversion is preferred. Theconversion of once-through polygeneration is

0.632, and unreacted syngas is not recycled.Secondly, the fuel of the power side in individualgeneration is different from the one in polygenera-tion. In individual generation, the fuel of thepower side is natural gas; in polygeneration, thefuel of the power side is unreacted syngas. Thirdly,the heat exchange in individual generation is not asgood as in polygeneration. The different carbon

Figure 4. Flow sheet of DMECC polygeneration.



8/8/2019 1390_ftp

5/13

conversion results in different concentrations of DME in the product, so separation is different inindividual generation and in polygeneration. Allthese differences inuence the system efficiency,and result in the variation of system performance.

The general results of individual generationand of polygeneration are listed in Table V. Theenergy consumption of DME in individualgeneration was 48 : 4 GJ t 1 : The thermal efficiencyof the CC was 56.5%. The energy saving ratio of

Table I. Stream ow rates and compositions in DME individual generation.

Mole fraction

Temperature8C

Pressure(bar)

Mole owkmol h 1 H 2 N 2 CO CO 2 CH 4O H 2O DME O 2 CH 4 C 2H 6

1 38.2 10 1496.9 0 0 0 0 0 1 0 0 0 02 184.9 10 1496.9 0 0 0 0 0 1 0 0 0 03 179.9 10 1496.9 0 0 0 0 0 1 0 0 0 04 219.8 25.8 4028.9 0 0 0 0 0 0.767 0 0 0.209 0.0235 510 25.8 4028.9 0 0 0 0 0 0.767 0 0 0.209 0.0236 950 23.478 5962.9 0.516 0 0.117 0.046 0 0.311 0 0 0.011 07 163 23.478 5962.9 0.516 0 0.117 0.046 0 0.311 0 0 0.011 08 38 23.478 4120.2 0.747 0 0.169 0.066 0 0.003 0 0 0.015 09 46.4 25.331 4120.2 0.747 0 0.169 0.066 0 0.003 0 0 0.015 0

10 32.9 25.331 9359.4 0.84 0 0.078 0.05 0 0.002 0 0 0.029 011 85 40 9359.4 0.84 0 0.078 0.05 0 0.002 0 0 0.029 012 35 34.9 6908.4 0.867 0 0.006 0.042 0.002 0.001 0.042 0 0.039 013 35 34.9 747.8 0.003 0 0 0.015 0.137 0.728 0.114 0 0.003 014 22.3 25.331 6549.0 0.913 0 0.007 0.038 0.001 0.001 0.001 0 0.04 015 22.3 25.331 5239.2 0.913 0 0.007 0.038 0.001 0.001 0.001 0 0.04 016 160 20 1541.5 0.785 0 0.006 0.069 0.001 0.001 0.001 0 0.129 0.0117 39.8 25.331 5391.8 0.002 0 0 0.008 0.085 0.85 0.053 0 0.002 018 165.7 20 6057.6 0 0 0 0 0.093 0.846 0.061 0 0 019 149.8 7 5813.7 0 0 0 0 0.104 0.895 0 0 0 020 20 7 4347.4 0 0 0 0 0.104 0.895 0 0 0 021 20 7 1466.4 0 0 0 0 0.104 0.895 0 0 0 022 164.3 7 1288.2 0 0 0 0 0.003 0.997 0 0 0 023 164.3 7 685.2 0 0 0 0 0.003 0.997 0 0 0 024 164.3 7 602.9 0 0 0 0 0.003 0.997 0 0 0 025 125.8 7 178.2 0 0 0 0 0.835 0.165 0 0 0 026 174 20 178.2 0 0 0 0 0.251 0.457 0.292 0 0 027 20.7 25.331 5032.6 0 0 0 0 0.091 0.909 0 0 0 028 300 1 7087.0 0 0.763 0 0 0 0.034 0 0.203 0 029 950 1 8027.9 0 0.674 0 0.043 0 0.237 0 0.047 0 030 784.4 1 8027.9 0 0.674 0 0.043 0 0.237 0 0.047 0 031 114.8 1 8027.9 0 0.674 0 0.043 0 0.237 0 0.047 0 032 39.4 110 3884.9 0 0 0 0 0 1 0 0 0 033 303.1 110 3884.9 0 0 0 0 0 1 0 0 0 034 328.1 110 3884.9 0 0 0 0 0 1 0 0 0 035 440 110 3884.9 0 0 0 0 0 1 0 0 0 036 273.5 30 793.0 0 0 0 0 0 1 0 0 0 037 38 23.478 1842.711 0 0 0 0 0 1 0 0 0 0AIR 25 1 7087.0 0 0.763 0 0 0 0.034 0 0.203 0 0CO 2 29.8 20 82.0 0.166 0 0.01 0.684 0 0 0.001 0 0.139 0DME 30.9 7 422.1 0 0 0 0 0 0 1 0 0 0NGC 38 25.8 149.7 0 0 0 0 0 0 0 0 0.9 0.1NGR 38 25.8 937.0 0 0 0 0 0 0 0 0 0.9 0.1RS 287.1 30 3091.9 0 0 0 0 0 1 0 0 0 0



8/8/2019 1390_ftp

6/13

polygeneration was 10.2%. The performance of polygeneration was enhanced obviously. Exergydestructions were classied for further analysis.

Table VI illustrates the categorized exergydestruction. For the convenience of comparison,the outputs of DME and electricity in individualgeneration were assumed to be the same as inpolygeneration. The categorized exergy destruc-tions were listed, and the improvement in poly-generation was shown. The exergy destruction

decrease of reforming accounted for 29.2% of thetotal decrease. Separation, synthesis, GT combus-tion, and heat recovery on the chemicalside accounted for 23 : 5 ; 4 : 3 ; 15 : 8 ; and 27.8% of the total exergy destruction decrease, respectively.Exergy destruction of the compressor and theturbine were slightly increased. Exergy destructiondecreases of the other parts were relatively small.Heat recovery on the thermal side was responsiblefor 1.2% of the exergy destruction decrease.

The system can be divided into two sides byfunction: the chemical production side and the

power generation side. Exergy destruction is alsoclassied under two categories: the destruction onthe chemical side and the destruction on thethermal side. According to the categorization, thereforming (including the reforming reaction andcombustion for the reforming reaction) and thesynthesis heat recovery are on the chemical side,and the separation also belongs to the chemical

side. Heat recovery in the HRSG, compressor andturbine, the combustion in GT and other partsbelong to the thermal side.

In this DME polygeneration system, the sum of exergy destruction decrease on the chemical sideaccounts for 84.8% of the total decrease, while thesum of exergy destruction decrease on the thermalside accounts only for 15.2% of the total decrease.It was shown that the performance enhancementof polygeneration was the result of both the

chemical production side integration and of thepower generation side integration, and the cascadeutilization on the chemical production side was theprimary factor.

Table VI shows a rough description of theexergy destruction decrease. It shows that naturalgas-based DME polygeneration was highly effi-cient, but the reasons for the exergy destructiondecrease are not illustrated clearly. It is necessaryto introduce another exergy analysis method forfurther analysis.

4. GRAPHICAL EXERGY ANALYSIS OFTHE SYSTEMS

The energy utilization diagram (EUD) is aconvenient exergy analysis method. Ishida andNakagawa proposed the EUD method [15]. Zhenget al . applied the EUD method for two types of

Table II. Stream ow rates and compositions in CC individual generation.

Mole fraction

Temperature 8C Pressure (bar) Mole ow kmol h 1 N 2 O2 CO 2 H 2O CH 4 C 2H 6

1 436.0 16.03 10620.3 0.79 0.21 0 0 0 02 1260.0 16.03 11016.3 0.762 0.129 0.038 0.072 0 03 611.5 1.03 11016.3 0.762 0.129 0.038 0.072 0 04 40.1 120 2063.9 0 0 0 1 0 05 39.1 5 802.6 0 0 0 1 0 06 535.0 120 2063.9 0 0 0 1 0 07 369.5 39 2063.9 0 0 0 1 0 08 535.0 39 2063.9 0 0 0 1 0 09 265.1 5 2063.9 0 0 0 1 0 010 260.0 5 802.6 0 0 0 1 0 011 39.0 0.07 2866.5 0 0 0 1 0 0AIR 25.0 1 10620.3 0.79 0.21 0 0 0 0FLUE 108.1 1.03 11016.5 0.762 0.129 0.038 0.072 0 0NG 38.0 25.8 377.2 0 0 0 0 0.9 0.1



8/8/2019 1390_ftp

7/13

LNG power-generation systems [16]. Jin andIshida applied the EUD method to complex cycles[17]. The abscissa of EUD is enthalpy change, andthe ordinate is A ; which stands for the energy level.The parameter A is the ratio of the exergy change

to the enthalpy change:

A D E D H

1

The enthalpy change refers to any kind of energy changes, such as thermal energy, power

Table III. Stream ow rates and compositions in polygeneration.

Mole fraction

Temperature8C

Pressure(bar)

Mole owkmol h 1 H 2 N 2 CO CO 2 CH 4O H 2O DME O 2 CH 4 C 2H 6

1 279.8 23.22 5652 0 0 0 0 0 0.767 0 0 0.209 0.0232 900 23.22 5652 0 0 0 0 0 0.767 0 0 0.209 0.0233 950 20.898 8365.2 0.516 0 0.117 0.046 0 0.311 0 0 0.011 04 38 20.898 2585.1 0 0 0 0 0 1 0 0 0 05 38 20.898 5780.1 0.747 0 0.169 0.066 0 0.003 0 0 0.015 06 115.7 40 5780.1 0.747 0 0.169 0.066 0 0.003 0 0 0.015 07 134.2 35 4066.735 0.658 0 0.012 0.111 0.022 0.081 0.094 0 0.022 08 35 34.9 576.92 0.003 0 0 0.049 0.144 0.565 0.236 0 0.002 09 45.1 25.331 2941.694 0.002 0 0 0.023 0.082 0.81 0.083 0 0.001 0

10 150.2 20 3410.614 0 0 0 0 0.095 0.794 0.111 0 0 011 148.9 7 3148.514 0 0 0 0 0.114 0.886 0 0 0 012 60 1 2069.555 0 0 0 0 0.114 0.886 0 0 0 013 99.6 1 918.959 0 0 0 0 0 1 0 0 0 014 99.6 1 549.568 0 0 0 0 0 1 0 0 0 015 67.7 1 160 0 0 0 0 0.77 0.23 0 0 0 016 174 20 160 0 0 0 0 0.231 0.499 0.27 0 0 017 38.7 30 1250 0 0 0 0 0 1 0 0 0 018 358.9 30 1250 0 0 0 0 0 1 0 0 0 019 26.6 120 4100 0 0 0 0 0 1 0 0 0 020 245 120 1930 0 0 0 0 0 1 0 0 0 021 535 120 2170 0 0 0 0 0 1 0 0 0 022 38.7 45 2900 0 0 0 0 0 1 0 0 0 023 412.4 45 2900 0 0 0 0 0 1 0 0 0 024 24.5 25.331 3167.228 0.842 0 0.015 0.112 0.001 0.001 0.001 0 0.027 025 300 25.331 1326.428 0.842 0 0.015 0.112 0.001 0.001 0.001 0 0.027 026 458.6 16.029 10112.22 0 0.79 0 0 0 0 0 0.21 0 027 614.6 1.05 10872.32 0 0.735 0 0.019 0 0.11 0 0.136 0 028 94.6 1.05 10872.32 0 0.735 0 0.019 0 0.11 0 0.136 0 029 58.4 1.03 1948.8 0.799 0 0.015 0.155 0.001 0.001 0.002 0 0.028 030 300 1 4750 0 0.79 0 0 0 0 0 0.21 0 031 950 1 5909.178 0 0.635 0 0.066 0 0.284 0 0.014 0 032 99.6 1 5909.178 0 0.635 0 0.066 0 0.284 0 0.014 0 033 162.7 5 1581.662 0 0 0 0 0 1 0 0 0 034 260 5 1581.662 0 0 0 0 0 1 0 0 0 035 39 0.07 1581.662 0 0 0 0 0 1 0 0 0 036 194.8 7 2330.838 0 0 0 0 0 1 0 0 0 037 164.9 7 2330.838 0 0 0 0 0 1 0 0 0 0AIR1 25 1 4750 0 0.79 0 0 0 0 0 0.21 0 0AIR2 25 1 10112.22 0 0.79 0 0 0 0 0 0.21 0 0CO 2 20.7 20 108 0.067 0 0.01 0.877 0 0 0.005 0 0.041 0DME 25 7 422.1 0 0 0 0 0 0 1 0 0 0NG 38 25.8 1314.5 0 0 0 0 0 0 0 0 0.9 0.1RS 358.2 30 4337.5 0 0 0 0 0 1 0 0 0 0



8/8/2019 1390_ftp

8/13

consumption or generation, and energy change inchemical reaction, etc. Energy transformationprocesses can be illustrated as the energy donatingprocess and energy accepting process. Exergydestruction is the area that is enclosed by the

energy donating line and energy accepting line.EUD can help in explaining the internal reasonsfor the exergy destruction decrease.

4.1. EUDs for the reforming process

EUDs for reforming are shown in Figure 5. Thequantity of natural gas and reforming steam in

polygeneration 1314 : 5 kmol h 1 is larger thanthat in individual generation 937 : 0 kmol h 1; andthe heat for the reforming reaction in polygenera-tion (77.7 MW) is higher than that in individualgeneration (55.4MW), but the exergy destruction

decrease for reforming in polygeneration is sig-nicant. There are two reasons for the decrease.First, in polygeneration, considerable preheatingheat is provided by hot output syngas rather thanfuel combustion. The combustion heat for pre-heating in individual generation is 76.0 MW, whilein polygeneration it is 43.4 MW. This results in aremarkable decrease in fuel for preheating in

Table V. Energy savings ratio of polygeneration system.

Individual generationPolygeneration

CC DME DMECC

NG input kg h 1 17 445 23 456 22 933

DME output kg h1

27 280 19 446Work output (kW) 135 790 51 216DME consumption GJ t 1 48.4Electricity efficiency (%) 56.5Energy savings ratio (%) 10.2

Table IV. Parameters of polygeneration and individual generation systems.

Chemical production sideDME synthesis temperature 8C 260DME synthesis pressure (bar) 35Methanol dehydration temperature 8C 280Methanol dehydration pressure (bar) 20Reforming temperature 8C 950Reforming pressure (bar) 23.22

Gas turbineTurbine inlet temperature 8C 1260Pressure ratio 16.029Isentropic efficiency, turbine (%) 89Isentropic efficiency, compressor (%) 84.6

Individual generation Polygeneration

Steam systemTurbine inlet temperature ( 8C) 535

Turbine inlet pressure bar 120Turbine pressure high/middle/low bar 120/30/5 120/45/5Isentropic efficiency of turbine (%) 87/89/95 87/89/95Pinch temperature of HRSG ( 8C) 20



8/8/2019 1390_ftp

9/13

polygeneration and leads to a signicant exergyloss decrease. Secondly, the fuel level in poly-generation is lower than that in individual genera-tion. The fuel for individual generation is a

mixture of natural gas and unreacted syngas,while all the fuel for polygeneration is unreactedsyngas. Unreacted syngas was utilized in thesynthesis reactor, so the level of unreacted syngaswas lower than that of natural gas. The resultsshowed that the chemical energy level of naturalgas was cascade utilized in polygeneration.

4.2. EUDs of GT combustion

EUDs of GT combustion are shown in Figure 6.The exergy destruction of combustion was de-

creased signicantly. Figure 6 shows that theexergy destruction decreases have two causes.The rst cause is the decline of the differencebetween the energy donator and the energyacceptor. It is obvious that the fuel level in Figure6(a) is higher than that in Figure 6(b). Inindividual generation, natural gas is directly sentto the GT as fuel. In polygeneration, the fuel for

the GT is the unreacted syngas. In reforming, onlypart of the fuel is natural gas. However, in GTcombustion, all of the fuel is natural gas, so theexergy loss decrease is bigger than in reforming

combustion. The second reason for the decrease isthe decrease in fuel quantity. In individualgeneration, the combustion heat for the GT is91.0MW. In polygeneration, the combustion heatis 88.1 MW. It is clear that the combustion heat forGT in individual generation is higher than that inpolygeneration.

4.3. Separation energy consumption

Figure 7 shows the variation of the quantity of absorbent and the energy consumption of separa-

tion with DME concentration. The separationexergy destruction is related to absorbent quantity,and the absorbent quantity is the function of theDME concentration in the syngas, so the separa-tion exergy destruction is related to the DMEconcentration in the syngas. There are two lines inFigure 7. The line with open squares represents theratio of absorbent quantity to DME quantity and

Table VI. Exergy destruction distribution (kW).

Individualgeneration

PolygenerationDecreasein exergy

Proportion of decrease

CC DME DMECC destruction (%)

Input NG exergy 94 708 272 921 330 133Air exergy 67 0 94

DestructionChemical side Heat recovery 28 454 18 633 9821 27.8

Reforming 46 744 36 422 10 322 29.2Synthesis 5752 4239 1513 4.3Separation (kW) 14 686 6399 8287 23.5

Thermal side Heat recovery 6639 6205 433 1.2GT combustion 28 076 22 495 5581 15.8Compressor andturbine

7656 1946 10 425 823 2.3

Other 5193 4998 194 0.6

Output DME 166 540Work 51 216 51 216Waste 1694 3945 1694

Total exergy input 94 776 272 921 330 227Total exergy destruction 52 911 170 485 220 111Total exergy output 42 370 102 775 109 818Exergy balance error (%) 0.53 0.12 0: 09



8/8/2019 1390_ftp

10/13

reects the variation of the ratio with DMEconcentration. The line with lled squares repre-sents the variation of the separation energyconsumption with DME concentration. TheDME concentration in individual generation was0.054. The ratio of absorbent quantity to DMEquantity was 13.8, and the separationenergy consumption was 12 : 9 kJ mol 1 DME.The DME concentration in polygeneration was0.071. The ratio of absorbent quantity to

DME quantity was 10.8, and the separation energyconsumption was 9 : 97 kJ mol 1 DME. It is ob-vious that the absorbent quantity and the separa-tion energy consumption in individual generationwere larger than in polygeneration. The DMEconcentration was inuenced by synthesis, so theexergy destruction decrease was also inuenced bysynthesis.

The analysis showed that the exergy destructiondecrease of synthesis was small, but it had aninuence on reforming, GT combustion, and

Figure 5. EUD of NG reforming in individual genera-tion (a) and in polygeneration (b).

Figure 6. EUD of combustion for GT in individualgeneration (a) and in polygeneration (b).

Figure 7. Variation of absorbent with DME.



8/8/2019 1390_ftp

11/13

separation. It played an important role in poly-generation.

4.4. EUD of heat exchange

The exergy destruction of heat exchange is shownin Figure 8. Figure 8(a) shows the heat exchange inindividual generation. The left side of the gureshows the heat exchange in DME production; theright side shows the heat exchange in the CC. It isclear that heat utilization in the CC was effective,but the heat utilization in DME production wasnot well arranged. Figure 8(b) shows the heatexchange in polygeneration. The left part of Figure8(b) shows the exergy destruction decrease of heatexchange on the chemical production side, and theright part shows the exergy destruction decrease of heat exchange on the power generation side. Here,

the heat exchange was carefully arranged, andresulted in high thermal efficiency in polygenera-tion. As a whole, the exchanged heat in individualgeneration was 165.1 MW, while in polygeneration

it was 131.6 MW. It can be concluded that inpolygeneration both the quality and the quantityof heat exchange are decreased, so the overall heatutilization is enhanced.

It is clear that in polygeneration, the decrease inthe chemical production side was remarkablyreduced both in quality and in quantity. However,the decrease in the power generation side wasrelatively small. The differences in the decreases of the two sides proved that the potential of thechemical production side in individual generationis great, and this potential is exploited in

polygeneration. The results are consistent withthe results in Table VI.The analysis above showed that combining the

chemical production side and the power genera-tion side in polygeneration is an interesting andimportant focus of study. The key to high systemperformance is the realization of cascade utiliza-tion both on the chemical production side and onthe power generation side. The exergy decrease of synthesis is relatively small, but it plays animportant role in energy utilization. It acts as abridge for chemical energy utilization and thermal

energy utilization, and other exergy destructiondecreases are interrelated with synthesis. Only withsynthesis, could the chemical energy of syngas bepartly converted into DME, and also the level of unreacted syngas decreased. The proportion of syngas converted into DME is the crucial para-meter for polygeneration performance. It isimportant to select an appropriate syngas conver-sion to obtain high polygeneration performance.An insufficient syngas conversion or excessivesyngas conversion is contradictory to the principleof cascade utilization, so it is important to select

the appropriate synthesis parameters.

5. CONCLUSIONS

With the combination of a CC and DME synthesis,natural gas-based DME/power polygeneration was

Figure 8. EUD of heat exchange in individual genera-tion (a) and in polygeneration (b).



8/8/2019 1390_ftp

12/13

integrated, and high-performance polygenerationwas achieved.

1. The energy saving ratio was 10.2% in poly-generation, proving that natural gas polyge-

neration is desirable.2. The general exergy analysis showed that the

decrease in exergy destruction in chemicalenergy utilization contributed the most to thedecrease in exergy destruction; the decrease inexergy destruction in thermal energy utilizationwas a secondary factor.

3. Further graphical analysis provided more de-tails about the cascade utilization of chemicalenergy and of thermal energy; cascade utiliza-tion is the essential reason the performanceimproved.

The above analysis shows that the key tosuccessful polygeneration was the cascade utiliza-tion of chemical exergy and of thermal exergy,and synthesis plays an important role in associat-ing power generation and chemical production.It is obvious that the decrease in exergy destruc-tion in chemical energy utilization is important,and the decrease in exergy destruction in thermalenergy utilization is also needed. The success of natural gas-based polygeneration, which wasproposed and described in this paper, was theresult of implementing the cascade utilizationprinciple.

NOMENCLATURE

A energy levelH enthalpy (MW)M mass owrate kg s 1Q heat duty (MW)S entropy kJ kg 1 K 1T temperature (K)

X concentration (%)

Subscript

A energy acceptorABS property of absorbentD energy donatorDME property of DME

Abbreviation

CC combined cycleDME dimethyl ether

EUD exergy utilization diagramIGCC integrate gasication combined cycle

ACKNOWLEDGEMENTS

This work has been supported by the National KeyProjects Fund (Grant No. 90210032) and by NationalBasic Research Program (2005CB221207).

REFERENCES

1. Quantifying energy, BP statistical review of world energy.2006. BP press center, London. Available from http://www.bp.com/liveassets/bp internet/globalbp/globalbp ukenglish/reports and publications/statistical energy review

{2006}/STAGING/ local assets/downloads/pdf/statisticalreview of world energy full report 2006.pdf (accessed 25

September 2006).2. Ohno Y, Yoshida M, Shikada T, Inokoshi O, Ogawa T,

Inoue N. New direct synthesis technology for DME(dimethyl ether) and its application technology. JFE Technical Report 2006; 8 :3440.

3. Manufacture of catalyst for DME preparation andmanufacture of DME. 1998. Patent Number in China:CN 1169888A.

4. Preparation of fuel grade dimethyl ether. 1996. Interna-tional Application Number : PCT/DK96/00047 .

5. Gogate MR, Vijayaraghavan P, Lee S, Kulik CJ. Single-stage, liquid-phase dimethyl ether synthesis process fromsyngas. IV. Thermodynamic analysis of the LPDMEprocess system. Fuel Science and Technology International 1992; 10 :281311.

6. Paul ND, Edward CH, James CS, Dennis MB, Peter GG.Air products new technology programs in syngas genera-tion and conversion. GPA (Global Programme of Action )Annual Conference 27th29th , Barcelona, 2000.

7. Wang Y, Ren S, Wang S. Analysis of energy consumptionfor manufacturing methanol from natural gas. Applied Chemical Industry 2003; 32 :5759 (in Chinese).

8. Jackson RG. Polygeneration system for power andmethanol based on coal gasication. Coal Conversion1989; 3 :6064.

9. Report to the president on federal energy research anddevelopment for the challenges of the twenty-rst century.1997. Available from http://www.ostp.gov/Energy/index.html (accessed 16 January 2004).

10. Brian A. Coal gasication an available and economicalternative for China. The China Council for International Cooperation on the Environment and Development Meeting ,Beijing, 1999.

11. Eric DL, Ren TJ. Synthetic fuel production by indirect coalliquefaction. Energy for sustainable Development 2003;7 :79102.


http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-

8/8/2019 1390_ftp

13/13

12. Ni WD, Li Z, Xue Y. Polygeneration on energysystem based on coal gasication-integrated optimizationand sustainable development of resources, energyand environment. Engineering Science 2000; 2 :5968 (inChinese).

13. Gao L, Jin HG, Liu ZL, Zheng DX. Exergy analysis of coal-based polygeneration system for power and chemicalproduction. Energy 2004; 29 :1215.

14. Jin HG, Hong H, Wang BQ, Han W, Lin RM. Thenew principle: synthetic cascade utilization of chemical

energy and physical energy. Science in China 2005; 48 :117.

15. Ishida M, Nakagawa N. Exergy analysis of pervaporationsystem based on an energy utilization diagram. Journal of Membrane Science 1985; 24 :271283.

16. Zheng DX, Uchiyama Y, Ishida M. Energy-utilizationdiagrams for two types of LNG power-generation systems.Energy 1986; 11 :631639.

17. Jin HG, Ishida M. Graphical exergy analysis of complexcycles. Energy } The International Journal 1993; 18 :615625.



1390_ftp

Documents