Investigation of Gasification Chemical Looping Combustion

The compressed air from the gas turbine compressor is fed to the CLC AR, ..... to 9 MPa in four stages using centrifugal compressors whose pressure ra...
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Energy & Fuels 2008, 22, 961–966

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Investigation of Gasification Chemical Looping Combustion Combined Cycle Performance Wenguo Xiang* and Sha Wang Key Laboratory of Clean Coal Power Generation and Combustion Technology of the Ministry of Education, Southeast UniVersity, Nanjing 210096, China

Tengteng Di Sichun Electric Vocational and Technical College, Chengdu 610072, Sichuan, China ReceiVed NoVember 22, 2007. ReVised Manuscript ReceiVed January 5, 2008

A novel combined cycle based on coal gasification and chemical looping combustion (CLC) offers a possibility of both high net power efficiency and separation of the greenhouse gas CO2. After pressurization, a coal slurry enters a pipe-type gasifier immersed in the CLC air reactor and takes in the heat released from the air reactor. After removal of particulates, the syngas is used as the fuel of the CLC fuel reactor or the supplementary firing. The technique involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from the combustion air to the fuel, and the avoidance of direct contact between fuel and combustion air. The fuel gas is oxidized by an oxygen carrier, an oxygen-containing compound, in the fuel reactor. The oxygen carrier in this study is NiO. The reduced oxygen carrier, Ni, in the fuel reactor is regenerated by the air in the air reactor. In this way, fuel and air are never mixed, and the fuel oxidation products CO2 and water vapor leave the system undiluted by air. All that is needed to get an almost pure CO2 product is to condense the water vapor and to remove the liquid water. When the technique is combined with gas turbine and heat recovery steam generation technology, a new type of combined cycle is formed which gives a possibility of obtaining high net power efficiency and CO2 separation. The performance of the combined cycle is simulated using the ASPEN software tool in this paper. The influence of the water/coal ratio on the gasification and the influence of the CLC process parameters such as the air reactor temperature, the turbine inlet supplementary firing, and the pressure ratio of the compressor on the system performance are discussed. Results show that, assuming an air reactor temperature of 1200 °C, a gasification temperature of 1100 °C, and a turbine inlet temperature after supplementary firing of 1350 °C, the system has the potential to achieve a thermal efficiency of 44.4% (low heating value), and the CO2 emission is 70.1 g/(kW h), 90.1% of the CO2 captured.

Introduction Observation of the increased concentration of carbon dioxide in the atmosphere and the thereto connected global warming effect has made prevention of carbon dioxide emission from power plants an important field of research. Today, most fuels used in thermal power plants are fossil fuels, which on combustion release CO2 to the atmosphere. To decrease the emission of CO2 from fossil fueled power plants, an increase in the power conversion efficiency is necessary. There is also a possibility of separating the CO2 from the other exhaust gases and disposing of the carbon dioxide in an environmentally safe way. However, conventional gas separation techniques like membrane separation and absorption have been estimated to be rather costly because of the large volume of dilute gas that needs to be treated. The energy consumed in the separation processes has also been estimated to decrease the net power efficiency by about 10 percentage points. There are many kinds of technologies to separate CO2, one of which is through chemical looping combustion (CLC). The technique involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from the combustion air to the fuel, * To whom correspondence should be addressed. E-mail: wgxiang@ seu.edu.cn.

Figure 1. CLC process.

and the avoidance of direct contact between fuel and combustion air. A simple schematic of a CLC system is shown in Figure 1. Here a hydrocarbon fuel is oxidized by a metal oxide, MexOy, in the fuel reactor (FR). The fuel is oxidized to CO2 and H2O while the metal oxide is reduced to a metal, Me (or to a metal oxide with a lower oxidation number). To regenerate the oxygen carrier, the Me is transported to the air reactor (AR) where it is reoxidized with oxygen in the air according to the oxidation reaction. In this way, the combustion products CO2 and H2O leave the system undiluted by excess air. All that is needed to separate CO2 is to condense the water vapor and to remove the

10.1021/ef7007002 CCC: $40.75  2008 American Chemical Society Published on Web 02/19/2008

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liquid water. The CO2 can then be stored or utilized in an environmentally friendly way. The metal and metal oxide is just circulated between the two reactors and never leaves the system. Thermodynamically, the reaction between metal and oxygen is normally exothermic; that is, heat equal to ∆Hox is released. The reaction between fuel and metal oxide is normally endothermic; that is, heat equal to ∆Hred is consumed (if the reaction between metal oxide and fuel is exothermic, heat ∆Hred is released and ∆Hred < 0). According to the energy balance, the sum of the heat of reaction for the two reactions is equal to the heat of fuel combustion, ∆Hc. Generally, ∆Hox must be greater than ∆Hc. Accordingly, in a CLC system it is possible to generate more heat at a high temperature compared with conventional combustion. The principles of CLC were first introduced by Richter and Knoche in 1983.1 Since then studies have been performed on selection and preparation of oxygen carriers. Ishida and Jin studied the hydrogen-fueled CLC characteristics using NiO as the oxygen carrier.2–4 Cho et al. and others have investigated Fe-, Ni-, and Cu-based oxygen carriers5–9 and designed the CLC reactors as a fluidized bed.10,11 Combining CLC with a gas turbine combined cycle offers a novel power generation technique with high efficiency and CO2 separation. Researchers have also paid attention to the possibilities of CLC based power generation. Ishida et al. investigated a CLC power generation system with exergy analysis and discussed the possibility of a coal fired CLC combined cycle.12–15 Anheden and Svedberg performed an exergy analysis of a natural gas-fired CLC (1) Ritcher, H.; Knoche, K. Reversibility of combustion process. ACS Symp. Ser. 1983, 235, 71–85. (2) Jin, H.; Ishida, M. Reactivity study on a novel hydrogen fueled chemical-looping combustion. Hydrogen Energy 2001, 26, 889–894. (3) Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemicallooping combustion: Synthesis of a looping material with a double metal oxide of CoO-NiO. Energy Fuels 1998, 12, 1272–1277. (4) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel ChemicalLooping Combustion: Synthesis of a Solid Looping Material of NiO/ NiAl2O4. Ind. Eng. Chem. Res. 1999, 3, 126–132. (5) Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 2004, 83, 1215–1225. (6) Consonni, S.; Lozza, G.; Pelliccia, G.; et al. Chemical looping combustion for combined cycles with CO2 capture. Proceedings of ASME Turbo Expo 2004, Power for Land, Sea, and Air, Vienna, Austria, June 14–17, 2004; GT2004-53503. (7) Mattisson, T.; Lyngfelt, A.; Cho, P. The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2. Fuel 2001, 80, 1953–1962. (8) Abad, A.; Adánez, J.; García-Labiano, F. Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion. Chem. Eng. Sci. 2007, 62, 533–549. (9) Corbella, B. M.; Palacios, J. M. Titania-supported iron oxide as oxygen carrier for chemical-looping combustion of methane. Fuel 2007, (86), 113–122. (10) Johansson, E.; Mattisson, T.; Lyngfelt, A. A 300W laboratory reactor system for chemical- looping combustion with particle circulation. Fuel 2006, (85), 1428–1438. (11) Lyngfelt, A.; Leckner, B.; Mattisson, T. A fluidized-bed combustion process with inherent CO2 separation: application of chemical-looping combustion. Chem. Eng. Sci. 2001, 56, 3101–3113. (12) Ishida, M.; Jin, H.; Akehata, T. Evaluation of a chemical-loopingcombustion power-generation system by graphic exergy analysis. Energy 1987, 12 (2), 147–154. (13) Ishida, M.; Jin, H. A New Advanced Power-Generation System Using Chemical-looping Combustion. Energy 1994, (4), 415–422. (14) Jin, H.; Ishida, M. A new advanced IGCC power plant with chemical-looping combustion. Proceedings of the International Symposium on Thermodynamics Analysis and ImproVement of Energy System; Beijing World Publishing Corporation: Beijing, China, 1997; pp 548–553. (15) Jin, H.; Ishida, M. Investigation of a Novel Gas Turbine Cycle with Coal Fueled Chemical-looping Combustion. Proc. ASME ASE DiV. 2000, 40, 548–553.

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Figure 2. Schematic diagram of a GCLC-CC.

system.16 Brandvoll and Bolland simulated the performance of a natural gas fired CLC power cycle.17–21 Wolf and Yan studied a CLC based trigeneration of hydrogen, heat, and electrical power with CO2 capture.22 Those papers mainly focus on the system configuration and the overall efficiency prediction using natural gas as fuel. In this paper, a novel cycle configuration using coal as the fuel is proposed. Because the system is highly integrated, it is important to study the effects of parameters on the performance of the system. Key parameters of the process are identified, and their impacts on the efficiencies of producing electrical power and the CO2 emission properties are investigated. System Description The system studied in this paper is shown in Figure 2. The combined cycle system is based on coal gasification, CLC, and gas turbine combined cycle (namely, GCLC-CC). It offers a possibility of both high net power efficiency and separation of the greenhouse gas CO2 for coal combustion. Direct reaction between the coal and the oxygen carrier in the CLC is not expected to be feasible because the reaction rate is likely to be too slow. There is a risk of coal and ash covering the metal particle surface and thereby hindering the CLC reactions. It is also likely that a large part of the coal fed to the reduction reactor will be entrained with the metal stream and combusted with oxygen in the air in the oxidation reactor instead of reacting with the oxygen carrier in the reduction reactor. Thus, the advantage of easy CO2 separation is lost. To (16) Anheden, M.; Svedberg, G. Exergy Analysis of Chemical-Looping Combustion Systems. Energy ConVers. Manage. 1998, 39 (16–18), 1967– 1980. (17) Brandvoll, Ø.; Bolland, O. Inherent CO2 capture using chemicallooping combustion in a natural gas fired power cycle. Proceedings of ASME Turbo Expo 2002, Amsterdam, The Netherlands, June 3–6, 2002; GT200230129. (18) Kvamsdal, H. M.; Jordal, K.; Bolland, O. A quantitative comparison of gas turbine cycles with CO2 capture. Energy 2007, 32, 10–24. (19) Naqvi, R.; Bolland, O.; Brandvoll, Ø.; et al. Chemical looping combustion—Analysis of natural gas fired power cycle with inherent CO2 capture, Proceedings of ASME Turbo Expo 2004, Power for Land, Sea, and Air, Vienna, Austria, June 14–17, 2004; GT2004-53359. (20) Naqvi, R.; Wolf, J.; Bolland, O. Part-load analysis of a chemical looping combustion (CLC) combined cycle with CO2 capture. Energy 2007, 32 (4), 360–370. (21) Wolf, J.; Anheden, M.; Yan, J. Comparison of nickel- and ironbased oxygen carriers in chemical looping combustion for CO2 capture in power generation. Fuel 2005, 84, 993–1006. (22) Wolf, J.; Yan, J. Parametric study of chemical looping combustion for tri-generation of hydrogen, heat, and electrical power with CO2 capture. Int. J. Energy Res. 2005, 29, 739–753.

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create an acceptable reaction scheme, the coal is first gasified, and then the resulting syngas is fed to the CLC reduction reactor where it is oxidized. Coal, sorbent, and water are prepared to be a coal slurry. After pressurization, the coal slurry is led to a pipe-type pressurized coal gasifier immerged in the CLC AR and takes in the heat released from the CLC AR. The coal slurry is turned to raw syngas under high temperature. Meanwhile, more than 95% of the sulfides are captured as CaS, and the remaining sulfides will be captured by the metal oxides and released from the AR. After passing through a cyclone and removing the ash particles, the raw syngas is cleaned and can be used as the fuel gas of the CLC system and as supplementary firing fuel. The syngas makes it possible to get a high concentration of CO2 and H2O in the gaseous products from the CLC reduction reactor and, therefore, enables an easier separation of carbon dioxide. The compressed air from the gas turbine compressor is fed to the CLC AR, where the oxygen carrier reacts with oxygen. It is also heated to a very high temperature and then expands in the gas turbine. The heat released from the oxidant reaction is used in two ways: one is used to heat the compressed air, and the other is used as the heat of coal slurry gasification. The heat in the gas turbine exhaust is recovered in a three pressure reheat heat recovery steam generator (HRSG). In this paper Ni/NiO is selected as the oxygen carrier in order to keep the AR at a relatively high temperature where the coal gasification can take place. When Ni/NiO is the oxygen carrier, the heat released from the syngas FR is quite different from that of Fe/FeO/Fe3O4/Fe2O3 as the oxygen carrier. In the syngas FR 4NiO + CH4 ) CO2 + 2H2O(g) + Ni

∆H ) 398.8 kJ/mol (1)

NiO + H2 ) H2O(g) + Ni

∆H ) -2.1 kJ/mol

(2)

NiO + CO ) CO2 + Ni

∆H ) -43.3 kJ/mol

(3)

H2O + CO ) CO2 + H2

∆H ) -41.2 kJ/mol

(4)

Because the content of CH4 in the syngas is small, there is much heat released in the FR. The total reaction of eqs 4 and 2 is the same as eq 3, so the water-gas shift reaction eq 4 is not included in the simulation. And in the AR, there is a large amount of heat released: 2Ni + O2 ) 2NiO

∆H ) -479.8 kJ/mol

(5)

The reactions in the two reactors are exothermic. The main purpose of this paper is to determine whether or not it is possible to use coal as a fuel in a CLC system which remains at high efficiency. Simulation results and a detailed heat balance analysis of the new systems are presented. The components of the syngas are estimated according to the mass equations and kinetics. The influences of the water/coal mass ratio on the gasification, the AR temperature, the turbine inlet supplementary firing, and the pressure ratio of the compressor on the system performance are also discussed. Assumptions Because this paper focuses on the coal fired CLC combined cycle performance for power generation, we will not deal with the details of the reactor chemistry and kinetics nor with the issues related to the thermo-fluid dynamic and structural design of the reactors and their components (cyclones, feeding systems, etc.). Here the CLC system is modeled on the basis of simplifying assumptions which are insufficient to define the

Table 1. Input Illinois No. 6 Coal Characteristics (wt %) wt %

C

H

O

N

S

H2O

ash

FC

VM

61.2

4.7

8.8

1.1

3.4

12.0

8.8

42.85

36.35

Table 2. Air Composition (vol %) N2

O2

CO2

H2O

Ar

77.3

20.74

0.03

1.01

0.92

actual design of the CLC components but are fully adequate to predict the heat/mass balances and thus the overall performance of the integrated CLC-CC system. The whole CLC combined cycle power plant has been studied with the ASPEN PLUS software. The gasifier, AR, and FR have been simulated using the Gibbs models available in the ASPEN library, which determines equilibrium conditions by minimizing the Gibbs energy.23 More specifically, we have assumed the following: (1) Coal (Table 1) is wet ground. The resulting coal slurry is fed to the gasifier at a pressure of 2.5 MPa. (2) Both AR and FR are adiabatic. (3) Ni/NiO is selected as the oxygen carrier, carrying out the chemical loop, and the reactions in the two reactors are the reactions 1, 2, 3, and 5. (4) The composition at the outlet of both reactors is at equilibrium; that is, the reactant residence times are higher than the characteristic times of chemical kinetics and of heat/mass transport. A catalyst may be needed. (5) Excess oxygen, that is, excess NiO, in the reduction reactor is 15-20%. Maintaining some excess oxygen is essential to warrant the full oxidation of the syngas. (6) Inert solid material (NiAl2O4) is 50% of the total flow of solids entering the oxidation reactor. This inert material is necessary to achieve appropriate physical characteristics of the solid particles and to realize a favorable temperature distribution across the reactors. (7) The air separation unit is not modeled here; rather, we adopt the specific work at 0.4 kW h/kg of 98% O2.24,25 (8) Ambient conditions are 15 °C, 1 bar, and 60% relative humidity (RH) air content as shown in Table 2. Table 3 exemplifies the CLC operating conditions corresponding to these assumptions. The table also reports the operating conditions obtained under the same assumptions with nickel oxide. Because of the lower molecular weight of its reduced and oxidized form, nickel gives a dramatic reduction of the mass flow circulating across the reactors. Define the whole power plant efficiency as ηnet )

WGT1 + WGT2 + WST1 + WST2 - WC - WCO2 m ˙ coalQycoal

(6)

The specific work of the system is defined as w)

WGT1 + WGT2 + WST1 + WST2 - WC - WCO2 m ˙ air

(7)

(23) Wstkinson, A. P.; Lucas, J. P.; Lim, C. J. A Predication of Performance of Commercial Coal Gasifiers. Fuel 1991, 70, 519–527. (24) Wang, B.; Jin, H.; Han, W.; Zheng, D. IGCC system with integration of CO2 recovery and the cryogenic energy in air separation unit. Proceedings of ASME Turbo Expo 2004, Power for Land, Sea, and Air, Vienna, Austria, June 14–17, 2004; GT2004-53723. (25) De Lorenzo, L.; Kreutz, T. G.; Chiesa1, P.; Williams, R. H. Carbonfree Hydrogen and Electricity from Coal: Options for Syngas Cooling in Systems Using a Hydrogen Separation Membrane Reactor. Proceedings of GT 2005, ASME Turbo Expo 2005: Power for Land, Sea and Air, RenoTahoe, NV, U.S.A., June 6–9, 2005; GT2005-68572.

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Table 3. Assumptions Adopted for the Performance Simulation assumption temperature 950 °C pressure loss 6% heat loss 1% syngas temperature 870 °C gasification pressure 25 bar Ca/S mole ratio 1.6 carbon conversion rate 98% NiO/NiAl2O4 ) 3:2 1200 °C pressure loss 5% 1000 °C pressure loss 5% air flow 634 kg/s pressure ratio 17 TIT 1350 °C air cooling fraction 0.12 compressor polytropic efficiency 88% turbine polytropic efficiency 90% turbine discharge pressure 1.047 bar mechanical/generator efficiency: 99.5%/98.5% approach ∆T ) 15 °C, 15 °C, 8 °C (HP/IP/LP) pinch ∆T ) 10 °C, 10 °C, 8 °C (HP/IP/LP) three pressure levels 12.5 MPa/ 535 °C (HP) 2.86 MPa/535 °C (IP) 0.72 MPa/232 °C (LP) pressure loss 3.4 kPa exhaust gas temperature from the HRSG: 80 °C HRSG thermal losses 0.7% of thermal input polytropic efficiency 90% mechanical/generator efficiency: 99%/99% condensing pressure 0.05 bar stage compression ratio of CO2 compressor 3.5, 3.5, 3.5, 2.5 compressor stage isentropic efficiency (%): 85 cooling water inlet temperature 15 °C pressure losses in the heat exchanger (%): 3 temperature at intercooler outlet 30 °C mechanical/electric efficiency 99%/99% liquid CO2 to disposal 30 °C, 85 bar pump efficiency 75%

gasification process

CLC reactors AR FR gas turbine

HRSG

steam turbine

CO2 compression

balance of plant (BOP)

To measure the CO2 capture performance of a system, we define the CO2 capture efficiency as ηCO2 )

m ˙ sep m ˙ tot

(8)

Figure 3. Effects of water/coal ratio and gasification temperature on the gasification efficiency. Table 4. Syngas Composition at the Gasifier Outlet vol %

H2

CO

H2O

CH4

CO2

N2, H2S, NH3, COS

50.43

37.13

6.70

2.39

2.99

trace

and the Ca/S mole ratio is 1.6, a best cold gasification efficiency is reached, and 95.4% of sulfur is captured as CaS. Because water is the only gasification oxidant, the hydrogen volume fraction in the syngas is relatively higher, as shown in Table 4, and the syngas lower heat value (LHV) is 11670.13 kJ/Nm3, with the volume 2.18 Nm3/kg (coal). The raw syngas is cooled down to 870 °C by a waste heat boiler to generate superheat steam. After removal of the particulate, the clean syngas is used as the fuel of CLC FR and supplementary firing. 2. CLC Process. To meet the oxygen needs of the CLC reactors, the minimum oxygen carrier flow Mmin and the minimum air flow Mair for 1 kg of coal base are simulated under different AR temperatures as shown in Figure 4 when the water/ coal ratio is 0.6. Because the syngas composition nearly remains the same, there are only slight changes in the value in response to the AR temperature change. To ensure higher carbon and hydrogen conversion, the FR excess oxygen ratio is selected as 1.25, and the oxygen carrier flow rate is 15 kg/kg coal. Assuming that the excess oxygen ratio in the AR is 1.8 and the AR temperature is 1200 °C, the FR outlet temperature is 1136 °C, and the composition at the outlet of the AR and the FR is shown in Tables 5 and 6. To prevent sintering of the oxygen carrier, the AR operation temperature is below 1200 °C. 3. Turbine Inlet Supplementary Firing. The temperature at the outlet of the AR and the FR is relatively lower than 1350

Results and Discussion 1. Gasification Process. We define the cold gasification efficiency ηg as ηg )

QygVg m ˙ coalQycoal + Qgf

(9)

The effects of gasification temperature, pressure, and water/ coal mass ratio on the gasification performance are simulated, and the effects of the sorbent (mainly CaCO3) fraction on the desulfuration are also included. Figure 3 shows the effects of the gasification temperature and the water/coal ratio on the cold gas efficiency. When the gasification temperature is in the range of 1000-1100 °C, the water/coal ratio is in the range 0.6-0.7,

Figure 4. Minimum air and minimum oxygen carriers as a function of the AR temperature.

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Table 5. Depleted Air Composition at the AR Outlet vol %

N2

O2

H2O

Ar

others

85.59

12.14

1.12

1.02

0.16

Table 6. CO2 Rich Stream Composition at the FR Outlet vol %

H2O

CO2

CO

N2

H2

SO2

58.87

40.19

0.36

0.31

0.24

0.03

°C. Because increasing the turbine inlet temperature (TIT) can raise the system efficiency, supplementary firing is used to increase the depleted air temperature of GT1, which had remained at 1350 °C. To obtain the supplementary firing of the CO2 rich stream, pure oxygen and catalytic combustion techniques are needed so as to increase the carbon and hydrogen conversion and to decrease the chemical unburned loss in the FR. GT2 needs to be cooled by the compressed CO2 rich stream. The AR temperature is a key parameter for the combined cycle plant. The temperature affects the carbon conversion rate of the pipe-type gasification process and the CO2 emission. If the temperature is lower, the carbon conversion rate and the system efficiency will go down. Maintaining the TIT, the CO2 emission will increase. Assuming that supplementary firing is used only to the GT1 and the TIT of GT1 is kept at 1350 °C, the system net efficiency is decreased when the AR temperature goes up (Figure 5). Because the supplementary firing fuel is diminished, the mass flow to the gas turbine (GT1) goes down and the average temperature of turbine inlet stream (GT1 and GT2) decreases, which leads to the decrease of the cycle efficiency. But the gasification temperature (900-1100 °C) and the cold gasification efficiency will increase as the AR temperature rises assuming that the gasification temperature is 100 °C lower than the AR temperature, as shown in Figure 3. Thus, the system net efficiency is only 0.35% (LHV) lower as the AR temperature is changed from 1000 to 1200 °C. Meanwhile, the 77% (CO2 emission 162 g/(kW h)) and the 90.1% (CO2 emission 70 g/(kW h)) of the CO2 can be separated at the AR temperature of 1000 and 1200 °C, respectively. However, keeping the AR outlet temperature at 1200 °C, when the supplementary firing temperature increases, the net efficiency goes up and the CO2 capture efficiency goes down, as shown in Figure 6. As the TIT of GT1 is changed from 1200 to 1500 °C, the net efficiency rises from 43.2% (LHV) to 45.5% (LHV) and the CO2 capture efficiency from 99% (CO2 emission 5 g/(kW h)) to 83% (CO2 emission 116 g/(kW h)). When the AR temperature is taken to 1200 °C and the TIT of GT1 to 1350 °C, the FR flue gas temperature is 1135 °C. The system net efficiency is increased by 0.5% (LHV) if the FR flue gas temperature is raised to 1350 °C by supplementary firing.

Figure 5. Influence of the AR outlet temperature on system efficiency and CO2 capture efficiency.

Figure 6. Net efficiency and CO2 capture rate as a function of TIT. Table 7. Final CO2 Stream Composition (vol %) CO2

H2O

CO

N2

H2

SO2

H2 S

97.6

0.16

0.91

0.81

0.51

0.028

7.4 × 10-5

4. CO2 Capture and Storage. The CO2 rich stream is cooled and then compressed to 9 MPa in four stages using centrifugal compressors whose pressure ratio is 3.5 and isentropic efficiency is 85%. Dehydration takes place after compression and cooling. The composition of the final stream is shown in Table 7. 5. Effect of Compressor Pressure Ratio on the Net Efficiency. Figure 7 presents the net cycle efficiency and CO2 emission as a function of specific power output with increasing pressure ratio and TIT (after the supplementary firing) under the constant maximum CLC AR temperature. Three different TIT values (1200 °C, 1300, and 1350 °C) are simulated with the cooling air fraction of 4%, 8%, and 12%. Under certain TIT, the increase of the pressure ratio results in the decrease of the specific power output, and there exists an optimum pressure ratio for the system net efficiency. For TIT ) 1350 °C, the fired GCLC-CC plant reached a net efficiency of 44.4% (LHV) at the optimum pressure ratio of 19. The optimum pressure ratio tends to increase as the TIT rises. At the TIT of 1200 °C, 1300 °C, and 1350 °C, the optimum pressure ratios are 15, 17, and 19, respectively. The system net efficiency improvement brought about by supplementary firing can result in the increase of CO2 emission. The CO2 emission at different pressure ratios and TIT is also shown in Figure 7. As the TIT or pressure ratio increases, the CO2 emission increases. At the optimum pressure ratio of 19 with a TIT of 1350 °C, CO2 emission is 74.5 g/(kW h). Conclusions Gasification integrated CLC-CC plants offer the potential for coal-fired power generation with near zero emission. We have

Figure 7. Efficiency vs specific work.

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mainly concentrated on the simulation of a coal fired CLC-CC system. The results show several merits of this system. The pipetype gasification process takes in the heat from the CLC AR, and oxygen used for gasification is not needed. So, the cold gas efficiency is relatively high, and the integrated combined cycle plants have better efficiency. Most of the CO2 is separated with CLC technology. There is no energy consumption on separation, and only CO2 compression energy is needed. The efficiency of the system is improved with the increase of the supplementary firing temperature, but the emission of CO2 increases. A limitation of this study is the pipe-type gasification process. Although there are some difficulties in realizing this type of gasification at present, it is possible to discuss this system performance. We have also simulated the coal fired CLC-CC plants with commercial-ready gasification technologysTexaco gasification. Its net efficiency is 41.7% (LHV), and the CO2 emission is 125 g/(kW h) at the same conditions as discussed in this paper. Acknowledgment. The authors wish to express thanks to the National Natural Science Foundation of China (90410009, 50776018) and the Special Fund of the National Priority Basic Research of China (2007CB210101) for financial support of this project.

Nomenclature AR ) air reactor C ) compressor CLC ) chemical looping combustion C.W. ) circulating water FR ) fuel reactor GT ) gas turbine

Xiang et al. HP ) high pressure HRSG ) heat recovery steam generator IP ) intermediate pressure LP ) low pressure M ) mass flow (kg/s) m ) mass flow rate (kg/s) PR ) pressure ratio Q ) gasification heat absorbing from the AR (kJ/s) Qy ) lower heating value (kJ/kg for coal, kJ/m3 for syngas) RH ) relative humidity ST ) steam turbine TIT ) turbine inlet temperature (°C) V ) volume flow (m3/s) W ) work (kW) w ) specific work (kW/kg) η ) efficiency (%) ∆H ) chemical reaction heat at 25 °C (kJ/mol) Subscripts c ) combustion C ) compressor CO2 ) CO2 compression g ) gas gf ) gasification GT ) gas turbine min ) minimum ox ) oxidation reaction red ) reduction reaction sep ) separated CO2 ST ) steam turbine sf ) supplementary firing tot ) total generated CO2 EF7007002