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A hydrogen and oxygen combined cycle with chemical-looping combustion
Energy Conversion and Management xxx (2014) xxx–xxx
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A hydrogen and oxygen combined cycle with chemical-looping combustion Xiaosong Zhang ⇑, Sheng Li, Hui Hong, Hongguang Jin Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Chemical looping Hydrogen generation H2–O2 combined cycle System integration
a b s t r a c t In the current paper, new systems integrating chemical-looping hydrogen (CLH) generation and the hydrogen (H2) and oxygen (O2) combined cycle have been proposed. The new methane-fueled cycle using CLH has been investigated with the aid of the exergy principle (energy utilization diagram methodology). First, H2 is produced in the CLH, in which FeO and Fe3O4 are used as the looping material. The H2 and O2 combined cycle then uses H2 as fuel. Two types of these combined cycles have been analyzed. Waste heat from the H2–O2 combined cycle is utilized in the CLH to produce H2. The advantages of CLH and the H2 and O2 combined cycle have resulted in a breakthrough in performance. The new system can achieve 59.8% net efficiency with CO2 separation when the turbine inlet temperature is 1300 °C. Meanwhile, the cycle is environmentally superior because of the recovery of CO2 without an energy penalty. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction At present, we face a potentially serious problem of rapid climate change attributed to anthropogenic emissions of greenhouse gases (e.g., CO2). One option to control greenhouse gas emission is the use of CO2 capture technologies from flue gases. In a fossil fuelfired power plant, CO2 capture can be carried out mainly through three available technologies: pre-combustion, post-combustion, and oxy-fuel combustion. The progress in this field has been addressed by Abu-Khader [1]. The main disadvantages of these techniques are the substantial addition to the power generation costs and the large amount of energy required for the CO2 separation, which amounts to a relative reduction of 15–20% in the overall efficiency of a power plant [2,3]. A new method of separating CO2 from flue gases in power plants with a negligible energy penalty is therefore urgently needed. Chemical-looping combustion (CLC) with inherent separation of CO2 is a promising technology proposed by Ishida and Jin in 1994 [4,5]. It is the most attractive, energy-efficient method of CO2 capture from fuel conversion using the combustion process. CLC involves the use of a metal oxide as an oxygen (O2) carrier, which transfers O2 from the combustion air to the fuel, thereby avoiding direct contact between the fuel and the mixture of fuel and air. In this way, CO2 and H2O are inherently separated from the other components of flue gases, so that no energy is needed for CO2 separation. This novel CO2 capture technology simultaneously re⇑ Corresponding author. E-mail address: [email protected] (X. Zhang).
solves both energy and environmental problems in combustion processes because the conversion of fuel-based chemical energy into thermal energy in traditional combustion not only results in the largest irreversibility in the power system, but also has serious environmental impact. In recent years, several researchers have investigated and contributed to the development of the CLC technology [6,7]. For example, Mattisson and Lyngfelt [8] designed and proposed a 10 kW fluidized-bed boiler using CLC [8]. Korea [9] developed a 50 kW CLC for future industrial application. A project for a novel CO2 separation system using CLC has been initiated by the Department of Energy of the United States [10]. A recent 1 MWth chemical looping plant is reported by Ströhle et al. [11]. In addition, the use of the chemical-looping process was recently proposed for the production of hydrogen (H2). There are two kinds of processes to product hydrogen with CLC, the chemical looping reforming (CLR) and chemical looping hydrogen generation (CLH). The CLR produces hydrogen using the CLC principle [12][13]. The main reactions in CLR are as follows, with nickel used as an example:
NiO þ CH4 ! Ni þ 2H2 þ CO DH ¼ 211 kJ=mol 2NiO þ CH4 ! 2Ni þ 2H2 þ CO2 CH4 þ Ni ! C—Ni þ 2H2
DH ¼ 148 kJ=mol
DH ¼ 191 kJ=mol
The first reaction is considered as the primary pathway, whereas the other two reactions are possible reactions. Natural gas is used as fuel in CLR research. To supply enough heat to drive the reforming reaction on the fuel reactor side, part of the natural
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Nomenclature CC combined cycle CLC chemical-looping combustion CLH chemical-looping hydrogen generation HO-CLH H2 and O2 combined cycle with CLH RHO-CLH reheat H2 and O2 combined cycle with CLH TIT turbine inlet temperature (°C) p pressure ratio g thermal efficiency (LHV) DE exergy change
DH
enthalpy change
Subscripts Opt optimal value Ox oxidation reaction Re reduction reaction Comp compressor ST steam turbine
gas is burned directly to support the reaction. Another method to produce hydrogen using CLC is water splitting applied to chemical looping (chemical looping hydrogen generation, CLH) [14]. The CLH process is based on two reactors. The CLH fuel reactor is similar to that used in CLC, but the CLC air reactor in is replaced with a steam reactor, in which steam reacts with the metal to produce hydrogen. In CLH, both sides of the reactions are endothermic. These reactions are as follows:
Ni þ H2 O ! NiO þ H2
DH ð900 CÞ ¼ 186 kJ=mol
CH4 þ 4NiO ! Ni þ CO2 þ 2H2 O DH ð800 CÞ ¼ 58 kJ=mol Approximately 10–20% of natural gas is burned directly to supply the heat for the reactions in SMR and CLH systems. Sung et al. [15] reported that 3.7 L of H2 per kilogram was generated through the reaction between the fully reduced copper-based oxide and steam. Kang et al. [16] investigated H2 production using the chemical-looping of methane (CH4) in a fluidized-bed reactor using an iron-based O2 carrier. The H2 and O2 combined cycle was first proposed by Cai and Fang [17]. A fairly high efficiency has been obtained based on the stoichiometric reaction of H2 and O2. The combination of CLH and the H2 and O2 combined cycle can capture CO2 without an energy penalty and with high efficiency.
Oxidation:
H2 O þ 3FeO ! Fe3 O4 þ H2
2. Description of the novel systems Fig. 1 shows the plant scheme for the integration of the mixing H2 and O2 combined cycle with CLH (HO-CLH). The plant consists of three main parts, namely, the CLH subsystem, the H2 and O2 combined cycle subsystem, and the CO2 separation subsystem. 2.1. CLH subsystem In the CLH subsystem, two separate reactors, CH4 with metal oxide (reduction) and the resulting metal with H2O (oxidation), are used. In the current study, ferric oxide (Fe3O4) is used as a solid metal oxide (i.e., looping material) in the chemical-looping hydrogen generation. Oxygen is transferred between the two reactors through an O2 carrier. CH4 is first reacted with the solid Fe3O4 [reaction (1)] in a reduction reactor, producing solid ferrous oxide (FeO) and steam. When 95–98% of Fe3O4 is reduced, the equilibrium temperature of reaction (1) is approximately 600–800 °C. To achieve a higher Fe3O4 conversion ratio, the temperature of the reduction reaction should be higher than 600 °C. In the oxidation reactor, H2O is reacted with solid FeO [reaction (2)] in the high temperature produced from the first reactor, yielding Fe3O4 and H2 through strong exothermic oxidation. Reduction:
CH4 þ 4Fe3 O4 ! 12FeO þ 2H2 O þ CO2
Fig. 1. Mixing H2 and O2 combined cycle with chemical-looping hydrogen (HOCLH).
DH0Red ¼ 406 kJ=mol
ð1Þ
DH0Ox ¼ 60 kJ=mol
ð2Þ
2.2. H2 and O2 combined cycle subsystem H2 produced by the CLH subsystem then enters the H2 and O2 combined cycle subsystem. The reaction per mole of H2 and a half mole of O2 produce one mole of H2O, with a very large amount of energy released compared to any other conventional type of fuel. The oxygen comes from a low pressure air separate unit (ASU) outlet and is further compressed by a compressor. After the combustion with O2 from the ASU, steam from the combustor can be used as the working fluid to generate power in the steam turbines. Finally, steam from the steam turbines releases heat in the reduction reaction, which is then used to supply the endothermic reaction. 2.3. CO2 separation The CO2 separation subsystem is used to separate CO2 from the CO2/H2O mixture. The mixture is cooled using liquid H2O from the steam turbine, releasing heat in the reduction reactor. The mixture is then cooled to 70–90 °C in the condenser. At this temperature, H2O turns into its liquid form and CO2 is separated. After heating using the CO2/H2O mixture, the liquid H2O is separated and flowed to the oxidation reactor and combustor.
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2.4. Steam reheating Fig. 2 shows the reheat H2 and O2 combined cycle with CLH (RHO-CLH), which is an improved HO-CLH cycle. In RHO-CLH, the steam first enters the high-pressure steam turbine, reheated in the combustor, and finally goes into the low-pressure steam turbine. If the turbine inlet temperature is allowed to be high enough in the future until the condensate feed H2O recirculation is eliminated, the cycle efficiency will reach its maximum value (this process will be discussed further in the subsequent section).
3. Evaluation of the system performance The current paper presents an evaluation of the system performance using the ASPEN PLUS software. The pressure in the reactors was 40 bar. The cooling air fraction was specified for each cooled stage, which was approximately 7% of the air compressor inlet flow. The temperature of H2O and CO2 in the condenser was 70– 90 °C. STEAM-TA method is used in the simulation of steam cycle, while SOLIDS and PENG-ROB method is selected in solid cycle and gas cycle. Chemical reactors were simulated in Aspen Plus using RGibbs blocks, which simultaneously model the chemical and phase equilibrium by minimizing the Gibbs free energy of the reactor products, subject to atomic balance constraints. The effectiveness of all heat exchangers was 90%, and the pressure loss across the heat exchangers was ignored. The latter assumption is widely used in publications related to the thermodynamic analysis of hydrogen production processes because the introduction of pressure losses in the heat exchangers increases the complexity of the analysis without significantly affecting the results and conclusions. In the CLH subsystem, FeO and Fe2O3 oxide were selected as oxygen carrier. The minimum temperature approach was 40 °C in the heat exchanger. The data of the unit operation are listed in Table 1. Tables 2 and 3 give the parameters of the main points of the two systems and Table 4 shows the exergy destruction in the new systems. The turbine inlet temperature of the reference system was set at 1300 °C, and the approach temperature difference of the heat exchanger and the efficiency of the corresponding assembly are the same. The thermal efficiency of RHO-CLH would be expected at 59.8%, which is higher than that of the HO-CLH system (55.5%). A CH4-fueled combined cycle was simulated under the same condi-
Table 1 Main assumptions for evaluation.
Turbine inlet temperature (°C) Turbine inlet pressure (bar) Reduction temperature (°C) Pressure of the reduction reactor (bar) Pressure loss of heat exchangers (%) Isentropic efficiency of steam turbine Pressure loss of heat exchanger (gas side) (%) Condensation pressure (bar)
HO-CLH
RHO-CLH
1300 40.00 650 40.00 3.00 0.88 3 0.08
1300 40.00 760 40.00 3.00 0.88 3 0.08
Table 2 Parameters of the main points in HO-CLH. Point
T (°C)
P (bar)
Component
Flowrate (K mol/s)
1 2 3 4 5 6 7 8 9 10 11 12 13
25 660 177 605 650 60 25 200 177 1300 650 25 405
40 40 40 40 40 0.2 0.2 40 40 40 40 40 0.20
CH4 (100%) Fe3O4 (100%) H2O (100%) H2 (100%) FeO (100%) H2O (100%) H2O (100%) O2 (100%) H2O (100%) H2O (100%) CO2 (33%), H2O (66%) H2O (100%) H2O (100%)
1 4 4 4 12 13.84 13.84 2 9.84 13.84 3 13.84 13.84
Table 3 Parameters of the main points in RHO-CLH. Point
T (°C)
P (bar)
Component
Flowrate (K mol/s)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
25 780 244 640 760 42 25 200 244 1300 780 25 783 1300 602
40 40 40 40 40 0.08 0.08 40 40 40 40 40 0.08 1.00 1.03
CH4 (100%) Fe3O4 (100%) H2O (100%) H2 (100%) FeO (100%) H2O (100%) H2O (100%) O2 (100%) H2O (100%) H2O (100%) CO2 (33%) H2O (66%) H2O (100%) H2O (100%) H2 O (100%) H2O (100%)
1.00 4.00 4.00 4.00 12.00 10.17 10.17 2.00 6.17 10.17 3.00 10.17 10.17 10.17 10.17
tions and an efficiency of 54.6% was obtained. The comparison of the combined cycles will be discussed in Section 5. The comparison of the exergy distribution of these two systems in Table 4 shows that the exergy destruction of the combustor in RHO-CLH was 121.45 kJ/mol-CH4, which is 31.36 kJ/mol-CH4 lower than that in the HO-CLH system and is the primary factor in the improvement of the thermodynamic performance of RHO-CLH.
4. Graphical exergy analysis of the two methanol gas turbine cycles
Fig. 2. Reheat H2 and O2 combined cycle with CLH (RO-CLH).
With the development of complex cycles, including complicated chemical/thermal processes, many researchers have paid close attention to the exergy principle for the analysis, optimization, and synthesis of thermal/chemical systems. To reveal the mechanism of the key processes in the new system, the graphical exergy analysis (EUD methodology) proposed by Ishida and Kawamura [18] was adopted. The EUD methodology used in the
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Table 4 Exergy analysis results. Items
HO-CLH
RHO-CLH
Ratio (%) Exergy Ratio (%) Exergy (kJ/mol-CH4) (kJ/mol-CH4) Lower heating value
814
Input exergy of fuel (HHV) 839.58
814 100
839.58
100
55.60
6.62
55.60
6.62
45.01 152.81 21.29 31.53 46.80 35.10 388.13
5.36 18.20 2.54 3.76 5.57 4.18 46.23
36.5 121.45 19.58 34.79 49.78 35.10 352.80
4.35 14.47 2.33 4.14 5.93 4.18 42.02
Output exergy (net power) 451.45 Sum of exergy 839.58
53.77 100
486.78 839.58
57.98 100
Thermal efficiency
55.5
Exergy destruction Air separation unit (ASU) Turbine Combustor Oxidation reaction Reduction reaction Heat exchanger Exhaust gas loss Total exergy destruction
59.8
current study graphically illustrates the energy level difference in an energy donor and energy acceptor pair. Both the variation in the energy level and energy quantity are graphically shown using A—DH coordinates. In the current study, the energy level, A, is a dimensionless criterion (A = DE/DH = 1 T0 DS/DH, representing the ratio of the exergy change to the energy change), and the energy quantity, DH, refers to any kind of energy change, such as thermal energy, power consumption or generation, and energy change in a chemical reaction. The x-coordinate in the EUD is the energy change and the ycoordinate is the energy level, A. An energy transformation process involves an energy donor and an energy acceptor; the exergy destruction is illustrated by the shaded areas between the curves of the energy donor and the energy acceptor. The particular advantage of the EUD methodology over conventional exergy methods is in showing (i) the energy level degradation in each process, instead of only magnitudes of exergy losses obtained from the exergy value difference between the unit output and input; (ii) the variation in the driving force by dividing the entire process into infinitive processes; and (iii) both global and special information on different phenomena such as work, thermal, and chemical processes. Hence, the EUD methodology may provide information on the feasibility of the process, as well as the driving force, defect points, and potential improvement from intuitive and global viewpoints [19]. The detailed exergy analysis is also studied on absorption chiller [20], co-generation plants [21] and other processes [22]. All these researches make a better understanding of the internal processes and can modify the system more efficiency.
Fig. 3a. EUD for the HO-CLH combustion.
Fig. 3b. EUD for the RHO-CLH combustion.
To increase the performance, a reheat combustor, which operates at 1300 °C, is adopted in RHO-CLH. The area between Aed and Aea2 in Fig. 3b is smaller than that in Fig. 3a; this difference may be due to the reheating of steam. Hence, the overall exergy destruction in the combustion of H2 in HO-CLH is 121.41 kJ/molCH4, which is 31.36 kJ/mol-CH4 higher than that in RHO-CLH.
4.1. Combustion process
4.2. Oxidation reaction
Based on the exergy analysis, the efficiency of RHO-CLH is 4% points higher than that of HO-CLH, in which the highest exergy destruction reduction is from the combustion process. Figs. 3a and 3b show that the width of curve Aed;H2 corresponds to the heat released by the combustion of H2 and O2; the heights of the curves indicate the energy level degradation caused by the transformation of chemical energy into thermal energy. The heating processes of gas (H2 and O2) and steam act as the energy acceptor, which are illustrated by the Aea1 and Aea2 curves, respectively. Aea1 is the energy acceptor (H2 and O2) and Aea2 is the energy acceptor (H2O). The area between Aed and Aea corresponds to the exergy destruction caused by H2 combustion with O2 to the thermal energy at the turbine inlet temperature of 1300 °C.
The oxidation reactions of HO-CLH and RHO-CLH are shown in Figs. 4a and 4b. In the oxidation reaction, the Aed,Ox curve acts as the energy donor. Oxidation occurs at 660 °C in HO-CLH and 780 °C in RHO-CLH at a pressure of 40 bar. The heating processes of Fe3O4 and H2 act as the energy acceptor and are illustrated by the Aea;Fe3 O4 and Aea;H2 curves, respectively. The shaded area between the energy donors and the energy acceptors represent the exergy destruction in the oxidation reaction process. In these two systems, the oxidation reaction is the same; thus, the curves for the energy donor in Figs. 4a and 4b are similar. However, because the temperature of H2 and Fe3O4 (steams 2 and 4) are higher in RHO-CLH, the energy acceptor level in RHO-CLH is higher than that in HO-CLH, resulting in the reduced area in Fig. 4b. As a result, the
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Fig. 4a. EUD for the oxidation reaction in HO-CLH.
Fig. 5a. EUD for the reduction reaction in HO-CLH.
Fig. 4b. EUD for the oxidation reaction in RHO-CLH.
Fig. 5b. EUD for the reduction reaction in RHO-CLH.
total exergy destruction of the oxidation reaction in HO-CLH is 21.29 kJ/mol-CH4, which is 8% higher than that in RHO-CLH.
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tively, which are much lower than that of the combined cycle (221.24 kJ/mol-CH4). 4.4. Heat exchanger subsystem
4.3. Reduction reaction The areas in Figs. 5a and 5b represent the reduction of Fe3O4 by CH4, in which the Aed;Fe3 O4 and Aed;H2 O curves represent the energy level of Fe3O4 and H2O acting as the energy donor; the released thermal energy is used to supply the heat of the endothermic reaction. In addition, the energy level of the reduction reaction is represented by Aea. In the CH4-fueled CLC, the reduction of Fe3O4 by CH4 is an endothermic reaction. The exergy destruction in the reduction process is 31.53 kJ/mol-CH4, corresponding to 3.76% based on the input CH4 exergy in HO-CLH. In RHO-CLH, the temperature of the steam, which supplies heat to the reduction reaction, is 780 °C, which is 380 °C higher than that in HO-CLH. Thus, the level of the energy donor in RHO-CLH is high, thereby increasing the exergy destruction to 34.79 kJ/mol-CH4. Finally, the overall exergy destruction in the reaction subsystem, including the combustion, reduction, and oxidation, are 205.63 and 175.82 kJ/mol-CH4 in HO-CLH and RHO-CLH, respec-
Fig. 6a shows the exergy destruction in the heat recovery process and the exergy waste of flue gases. The Aed curve represents the energy level of CO2/H2O from the reduction reaction acting as the energy donor. The CO2/H2O mixture is cooled below 100 °C to separate CO2, and the liquid H2O is heated to 140 and 244 °C in HO-CLH and RHO-CLH, respectively. The total exergy destruction in the heat subsystems in HO-CLH and RHO-CLH are 46.8 and 49.78 kJ/mol-CH4, respectively. The total exergy destruction in the RHO-CLH subsystem is 6% higher because the CO2/H2O temperature in RHO-CLH is higher than that in HO-CLH, resulting in the higher level of the energy donor (Fig. 6b). The natural gas is usually used to generate electricity through combined cycle. The heat exchanger often causes more exergy destruction in the heat recovery boiler (HRSG). Because of the physical property of water, there is a phase change process causing a big shadow area (Fig. 6c). The curve of Aed1 represents the energy level of the N2/O2 from the turbine acting as the energy donor. The total exergy destruction in the heat subsystems is
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Fig. 6a. EUD for the HO-CLH heat exchanger.
Fig. 6c. EUD for the heat exchanger in combined cycle.
Fig. 6b. EUD for the RHO-CLH heat exchanger. Fig. 7a. Variation in the thermal efficiency of HO-CLH.
114.46 kJ/mol-CH4, which is about twice as much as RHO-CLH system. In RHO-CLH system, the steam reactor is used to recover the heat of the work fluid from the turbine, that the key reason why the exergy destruction decreased.
5. Results and discussion 5.1. Upgrading the energy level of the thermal energy The mid-grade thermal energy from the steam turbine at around 500–700 °C can be upgraded to the high-grade chemical energy of solid FeO via a reduction reaction. It can then be transferred into H2 via an oxidation reaction and finally released as a high-temperature thermal energy to produce electricity in the combustion process with a high-efficiency steam cycle. According to the EUDs, the reduction reaction absorbs the middle temperature thermal energy from steam. The heat is first stored in FeO through CH4 reduction and is then turned into the chemical energy of H2. The average energy level of steam heat is approximately 0.6, whereas that of H2 is approximately 0.9. Hence, the redox reaction of CH4 increases the energy levels of the heat from Fe2O3. This result is attributed to the energy-level degradation in HOCLH from CH4 combustion to H2 combustion; that is, degradation
acts as a ‘‘driving force’’ that raises the level of mid-temperature steam thermal energy. The mid-temperature steam heat is then absorbed by the reduction reaction. The average reaction level, A, of the reduction reaction is approximately 0.5; this level absorbs the mid-temperature thermal energy. Mid-temperature energies are finally released in the H2 reactor as high-temperature thermal energy. The decrease in exergy destruction in the CH4 reduction reactions has led to the attainment of the cascade utilization of chemical energy. Table 3 shows that the net efficiency in the new cycle could reach 59.8%, which is a competitive figure compared with that of the current advanced reference system. 5.2. Advanced thermodynamic performance Figs. 7a and 7b show the variations in the overall thermal efficiency (g) of the system with varying initial temperatures (TIT) and pressure ratios (p). The pressure ratio is the total ratio of compression. The expansion ratio in the cycle is much higher than the pressure ratio, which is entirely different from the commonly used gas turbine pressure ratio. Figs. 7a and 7b show that the cycle efficiency of the two cycles sharply increases with the temperature ratio. The effect of the
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Fig. 8a. Process diagram of steam in HO-CLH.
Fig. 7b. Variation in the thermal efficiency of RHO-CLH.
temperature ratio on efficiency is more evident in the heat recovery steam generator (HRSG) combined cycle compared with that of the system in a previous study [23]. In addition, for a given p, g increases with the increase in TIT. Each TIT has a corresponding popt to achieve the maximum thermal efficiency, which is similar to that of the conventional combined cycle, which has a popt of about 40. Thus, the two proposed systems have popt values different from that of the conventional combined cycle. The effect of the pressure ratio on efficiency can be explained as follows: at the same temperature ratio, the turbine exit temperature decreases when the pressure ratio increases, as long as the exit point is outside the saturated region. In turn, the expansion work per unit mass flow increases. On the other hand, the recirculation condensate feed H2O entering the reactor for the reduction of regenerative heat is decreased. These two factors contribute to the maximum efficiency. As shown in Fig. 7a, the effects of the pressure ratio on the efficiency between p = 20–40 appear flat; however, the curves are evident in Fig. 7b. In HO-CLH, more turbine exhaust heat can be taken back to the combustor using steam 9 when the pressure ratio decreases. Mass flow increases, and the output work of the turbine is almost kept constant. However, when the pressure ratio is lower than 20, the superfluous heat is released to keep the temperature of the reduction reactor below 900 °C, that causes the changes of the efficiency. On the other hand, the mechanism of the RHOCLH system is different. By adjusting the pressure ratios of the high-pressure and low-pressure steam turbines, the output power increases when p increases, and the exergy destruction in the reduction reaction is reduced because of the increased compatibility of the energy donor and the energy acceptor as the temperature of the turbine outlet steam decreases (Fig. 5b). When the pressure ratio exceeds 40 to supply enough heat to the reduction reactor, the temperature of the turbine outlet steam should be kept constant to increase the outlet pressure, subsequently lowering the output power of the turbine. The popt in RHO-CLH is around 40. Figs. 7a and 7b show that the HO-CLH system has better applicability because of its varying pressure ratios; however, the RHO-CLH system has higher thermal efficiency if the turbine inlet is allowed to reach a high enough temperature. Figs. 8a and 8b show that the reason for the high efficiency of RHO-CLH compared with that of HO-CLH is the reheating of the steam, which not only increases the output power of the turbine, but also decreases the exergy destruction in the reduction reactor through the use of the turbine exhaust heat. The thermal efficiency of RHO-CLH with
Fig. 8b. Process diagram of steam in RHO-CLH.
CO2 capture at the pressure ratio of 40 and at a turbine inlet temperature of 1400 °C is expected to reach approximately 61%. 5.3. Significant role of decreasing energy penalty for CO2 capture Removing undesired byproducts using various separation processes often require dealing with a great amount of exhaust gas and entails a high energy cost [24]. The goal of future generation systems will be to eliminate undesirable products from the upstream process. In the current study, CLC and CLH showed good performance in CO2 recovery and thus, can be used in the elimination of environmental pollution caused by greenhouse gas emissions. The proposed novel system is intended to combined the HO system and CLH system more effectively. The chemical looping hydrogen production unit is the key equipment in the whole system. This equipment not only produced hydrogen with low energy penalty, but also capture the CO2 without energy penalty, because the products of the reduction reactor in the CLH are very simple, consisting only of H2O and CO2. Table 5 compares the CO2 capture performance of the three systems (combined cycle, HO-CLH, and RHO-CLH) with and without CO2 capture. Given the characteristics of CLH, the thermal efficiencies of the two proposed systems would still be 1–4% higher than that of the combined cycle, respectively, even without CO2 capture.
Table 5 Comparison of the thermal efficiencies of the systems with the combined cycle.
Thermal efficiency (CC without CO2 capture) Thermal efficiency (with CO2 capture)
CC (%)
HO-CLH (%)
RHO-CLH (%)
54.6
55.5
59.8
46–48
55.5
59.8
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If CO2 capture is included in the three systems, the thermal efficiency of the combined cycle will decrease by 8%, significantly reflecting the advantages of the two proposed cycles. The thermal efficiency will be 8–12% higher than that of the combined cycle with CO2 capture. 6. Conclusions
[7]
[8]
[9]
A novel system integrating CLH with the H2 and O2 combined cycle has been proposed, and two types of such combined cycle have been discussed. The performances of HO-CLH mainly depend on the temperature ratio. The pressure ratio has little effect on the cycle efficiency of HO-CLH, but has significant effect on that of RHO-CLH. The cycle efficiency of HO-CLH can reach 55.5% at a TIT of 1300 °C, whereas that of RHO-CLH is 59.8% under the same condition; these values are at least 8–12% higher than that of the combined cycle with CO2 capture. RHO-CLH is selected for the integration system, and the best cycle performance has been obtained via simulation. At lower temperature ratios, this type of cycle has higher efficiency than HOCLH. Moreover, this cycle is easier to develop from an engineering point of view.
[10]
Acknowledgment
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A hydrogen and oxygen combined cycle with chemical-looping combustion Xiaosong Zhang ⇑, Sheng Li, Hui Hong, Hongguang Jin Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Chemical looping Hydrogen generation H2–O2 combined cycle System integration
a b s t r a c t In the current paper, new systems integrating chemical-looping hydrogen (CLH) generation and the hydrogen (H2) and oxygen (O2) combined cycle have been proposed. The new methane-fueled cycle using CLH has been investigated with the aid of the exergy principle (energy utilization diagram methodology). First, H2 is produced in the CLH, in which FeO and Fe3O4 are used as the looping material. The H2 and O2 combined cycle then uses H2 as fuel. Two types of these combined cycles have been analyzed. Waste heat from the H2–O2 combined cycle is utilized in the CLH to produce H2. The advantages of CLH and the H2 and O2 combined cycle have resulted in a breakthrough in performance. The new system can achieve 59.8% net efficiency with CO2 separation when the turbine inlet temperature is 1300 °C. Meanwhile, the cycle is environmentally superior because of the recovery of CO2 without an energy penalty. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction At present, we face a potentially serious problem of rapid climate change attributed to anthropogenic emissions of greenhouse gases (e.g., CO2). One option to control greenhouse gas emission is the use of CO2 capture technologies from flue gases. In a fossil fuelfired power plant, CO2 capture can be carried out mainly through three available technologies: pre-combustion, post-combustion, and oxy-fuel combustion. The progress in this field has been addressed by Abu-Khader [1]. The main disadvantages of these techniques are the substantial addition to the power generation costs and the large amount of energy required for the CO2 separation, which amounts to a relative reduction of 15–20% in the overall efficiency of a power plant [2,3]. A new method of separating CO2 from flue gases in power plants with a negligible energy penalty is therefore urgently needed. Chemical-looping combustion (CLC) with inherent separation of CO2 is a promising technology proposed by Ishida and Jin in 1994 [4,5]. It is the most attractive, energy-efficient method of CO2 capture from fuel conversion using the combustion process. CLC involves the use of a metal oxide as an oxygen (O2) carrier, which transfers O2 from the combustion air to the fuel, thereby avoiding direct contact between the fuel and the mixture of fuel and air. In this way, CO2 and H2O are inherently separated from the other components of flue gases, so that no energy is needed for CO2 separation. This novel CO2 capture technology simultaneously re⇑ Corresponding author. E-mail address: [email protected] (X. Zhang).
solves both energy and environmental problems in combustion processes because the conversion of fuel-based chemical energy into thermal energy in traditional combustion not only results in the largest irreversibility in the power system, but also has serious environmental impact. In recent years, several researchers have investigated and contributed to the development of the CLC technology [6,7]. For example, Mattisson and Lyngfelt [8] designed and proposed a 10 kW fluidized-bed boiler using CLC [8]. Korea [9] developed a 50 kW CLC for future industrial application. A project for a novel CO2 separation system using CLC has been initiated by the Department of Energy of the United States [10]. A recent 1 MWth chemical looping plant is reported by Ströhle et al. [11]. In addition, the use of the chemical-looping process was recently proposed for the production of hydrogen (H2). There are two kinds of processes to product hydrogen with CLC, the chemical looping reforming (CLR) and chemical looping hydrogen generation (CLH). The CLR produces hydrogen using the CLC principle [12][13]. The main reactions in CLR are as follows, with nickel used as an example:
NiO þ CH4 ! Ni þ 2H2 þ CO DH ¼ 211 kJ=mol 2NiO þ CH4 ! 2Ni þ 2H2 þ CO2 CH4 þ Ni ! C—Ni þ 2H2
DH ¼ 148 kJ=mol
DH ¼ 191 kJ=mol
The first reaction is considered as the primary pathway, whereas the other two reactions are possible reactions. Natural gas is used as fuel in CLR research. To supply enough heat to drive the reforming reaction on the fuel reactor side, part of the natural
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Nomenclature CC combined cycle CLC chemical-looping combustion CLH chemical-looping hydrogen generation HO-CLH H2 and O2 combined cycle with CLH RHO-CLH reheat H2 and O2 combined cycle with CLH TIT turbine inlet temperature (°C) p pressure ratio g thermal efficiency (LHV) DE exergy change
DH
enthalpy change
Subscripts Opt optimal value Ox oxidation reaction Re reduction reaction Comp compressor ST steam turbine
gas is burned directly to support the reaction. Another method to produce hydrogen using CLC is water splitting applied to chemical looping (chemical looping hydrogen generation, CLH) [14]. The CLH process is based on two reactors. The CLH fuel reactor is similar to that used in CLC, but the CLC air reactor in is replaced with a steam reactor, in which steam reacts with the metal to produce hydrogen. In CLH, both sides of the reactions are endothermic. These reactions are as follows:
Ni þ H2 O ! NiO þ H2
DH ð900 CÞ ¼ 186 kJ=mol
CH4 þ 4NiO ! Ni þ CO2 þ 2H2 O DH ð800 CÞ ¼ 58 kJ=mol Approximately 10–20% of natural gas is burned directly to supply the heat for the reactions in SMR and CLH systems. Sung et al. [15] reported that 3.7 L of H2 per kilogram was generated through the reaction between the fully reduced copper-based oxide and steam. Kang et al. [16] investigated H2 production using the chemical-looping of methane (CH4) in a fluidized-bed reactor using an iron-based O2 carrier. The H2 and O2 combined cycle was first proposed by Cai and Fang [17]. A fairly high efficiency has been obtained based on the stoichiometric reaction of H2 and O2. The combination of CLH and the H2 and O2 combined cycle can capture CO2 without an energy penalty and with high efficiency.
Oxidation:
H2 O þ 3FeO ! Fe3 O4 þ H2
2. Description of the novel systems Fig. 1 shows the plant scheme for the integration of the mixing H2 and O2 combined cycle with CLH (HO-CLH). The plant consists of three main parts, namely, the CLH subsystem, the H2 and O2 combined cycle subsystem, and the CO2 separation subsystem. 2.1. CLH subsystem In the CLH subsystem, two separate reactors, CH4 with metal oxide (reduction) and the resulting metal with H2O (oxidation), are used. In the current study, ferric oxide (Fe3O4) is used as a solid metal oxide (i.e., looping material) in the chemical-looping hydrogen generation. Oxygen is transferred between the two reactors through an O2 carrier. CH4 is first reacted with the solid Fe3O4 [reaction (1)] in a reduction reactor, producing solid ferrous oxide (FeO) and steam. When 95–98% of Fe3O4 is reduced, the equilibrium temperature of reaction (1) is approximately 600–800 °C. To achieve a higher Fe3O4 conversion ratio, the temperature of the reduction reaction should be higher than 600 °C. In the oxidation reactor, H2O is reacted with solid FeO [reaction (2)] in the high temperature produced from the first reactor, yielding Fe3O4 and H2 through strong exothermic oxidation. Reduction:
CH4 þ 4Fe3 O4 ! 12FeO þ 2H2 O þ CO2
Fig. 1. Mixing H2 and O2 combined cycle with chemical-looping hydrogen (HOCLH).
DH0Red ¼ 406 kJ=mol
ð1Þ
DH0Ox ¼ 60 kJ=mol
ð2Þ
2.2. H2 and O2 combined cycle subsystem H2 produced by the CLH subsystem then enters the H2 and O2 combined cycle subsystem. The reaction per mole of H2 and a half mole of O2 produce one mole of H2O, with a very large amount of energy released compared to any other conventional type of fuel. The oxygen comes from a low pressure air separate unit (ASU) outlet and is further compressed by a compressor. After the combustion with O2 from the ASU, steam from the combustor can be used as the working fluid to generate power in the steam turbines. Finally, steam from the steam turbines releases heat in the reduction reaction, which is then used to supply the endothermic reaction. 2.3. CO2 separation The CO2 separation subsystem is used to separate CO2 from the CO2/H2O mixture. The mixture is cooled using liquid H2O from the steam turbine, releasing heat in the reduction reactor. The mixture is then cooled to 70–90 °C in the condenser. At this temperature, H2O turns into its liquid form and CO2 is separated. After heating using the CO2/H2O mixture, the liquid H2O is separated and flowed to the oxidation reactor and combustor.
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2.4. Steam reheating Fig. 2 shows the reheat H2 and O2 combined cycle with CLH (RHO-CLH), which is an improved HO-CLH cycle. In RHO-CLH, the steam first enters the high-pressure steam turbine, reheated in the combustor, and finally goes into the low-pressure steam turbine. If the turbine inlet temperature is allowed to be high enough in the future until the condensate feed H2O recirculation is eliminated, the cycle efficiency will reach its maximum value (this process will be discussed further in the subsequent section).
3. Evaluation of the system performance The current paper presents an evaluation of the system performance using the ASPEN PLUS software. The pressure in the reactors was 40 bar. The cooling air fraction was specified for each cooled stage, which was approximately 7% of the air compressor inlet flow. The temperature of H2O and CO2 in the condenser was 70– 90 °C. STEAM-TA method is used in the simulation of steam cycle, while SOLIDS and PENG-ROB method is selected in solid cycle and gas cycle. Chemical reactors were simulated in Aspen Plus using RGibbs blocks, which simultaneously model the chemical and phase equilibrium by minimizing the Gibbs free energy of the reactor products, subject to atomic balance constraints. The effectiveness of all heat exchangers was 90%, and the pressure loss across the heat exchangers was ignored. The latter assumption is widely used in publications related to the thermodynamic analysis of hydrogen production processes because the introduction of pressure losses in the heat exchangers increases the complexity of the analysis without significantly affecting the results and conclusions. In the CLH subsystem, FeO and Fe2O3 oxide were selected as oxygen carrier. The minimum temperature approach was 40 °C in the heat exchanger. The data of the unit operation are listed in Table 1. Tables 2 and 3 give the parameters of the main points of the two systems and Table 4 shows the exergy destruction in the new systems. The turbine inlet temperature of the reference system was set at 1300 °C, and the approach temperature difference of the heat exchanger and the efficiency of the corresponding assembly are the same. The thermal efficiency of RHO-CLH would be expected at 59.8%, which is higher than that of the HO-CLH system (55.5%). A CH4-fueled combined cycle was simulated under the same condi-
Table 1 Main assumptions for evaluation.
Turbine inlet temperature (°C) Turbine inlet pressure (bar) Reduction temperature (°C) Pressure of the reduction reactor (bar) Pressure loss of heat exchangers (%) Isentropic efficiency of steam turbine Pressure loss of heat exchanger (gas side) (%) Condensation pressure (bar)
HO-CLH
RHO-CLH
1300 40.00 650 40.00 3.00 0.88 3 0.08
1300 40.00 760 40.00 3.00 0.88 3 0.08
Table 2 Parameters of the main points in HO-CLH. Point
T (°C)
P (bar)
Component
Flowrate (K mol/s)
1 2 3 4 5 6 7 8 9 10 11 12 13
25 660 177 605 650 60 25 200 177 1300 650 25 405
40 40 40 40 40 0.2 0.2 40 40 40 40 40 0.20
CH4 (100%) Fe3O4 (100%) H2O (100%) H2 (100%) FeO (100%) H2O (100%) H2O (100%) O2 (100%) H2O (100%) H2O (100%) CO2 (33%), H2O (66%) H2O (100%) H2O (100%)
1 4 4 4 12 13.84 13.84 2 9.84 13.84 3 13.84 13.84
Table 3 Parameters of the main points in RHO-CLH. Point
T (°C)
P (bar)
Component
Flowrate (K mol/s)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
25 780 244 640 760 42 25 200 244 1300 780 25 783 1300 602
40 40 40 40 40 0.08 0.08 40 40 40 40 40 0.08 1.00 1.03
CH4 (100%) Fe3O4 (100%) H2O (100%) H2 (100%) FeO (100%) H2O (100%) H2O (100%) O2 (100%) H2O (100%) H2O (100%) CO2 (33%) H2O (66%) H2O (100%) H2O (100%) H2 O (100%) H2O (100%)
1.00 4.00 4.00 4.00 12.00 10.17 10.17 2.00 6.17 10.17 3.00 10.17 10.17 10.17 10.17
tions and an efficiency of 54.6% was obtained. The comparison of the combined cycles will be discussed in Section 5. The comparison of the exergy distribution of these two systems in Table 4 shows that the exergy destruction of the combustor in RHO-CLH was 121.45 kJ/mol-CH4, which is 31.36 kJ/mol-CH4 lower than that in the HO-CLH system and is the primary factor in the improvement of the thermodynamic performance of RHO-CLH.
4. Graphical exergy analysis of the two methanol gas turbine cycles
Fig. 2. Reheat H2 and O2 combined cycle with CLH (RO-CLH).
With the development of complex cycles, including complicated chemical/thermal processes, many researchers have paid close attention to the exergy principle for the analysis, optimization, and synthesis of thermal/chemical systems. To reveal the mechanism of the key processes in the new system, the graphical exergy analysis (EUD methodology) proposed by Ishida and Kawamura [18] was adopted. The EUD methodology used in the
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Table 4 Exergy analysis results. Items
HO-CLH
RHO-CLH
Ratio (%) Exergy Ratio (%) Exergy (kJ/mol-CH4) (kJ/mol-CH4) Lower heating value
814
Input exergy of fuel (HHV) 839.58
814 100
839.58
100
55.60
6.62
55.60
6.62
45.01 152.81 21.29 31.53 46.80 35.10 388.13
5.36 18.20 2.54 3.76 5.57 4.18 46.23
36.5 121.45 19.58 34.79 49.78 35.10 352.80
4.35 14.47 2.33 4.14 5.93 4.18 42.02
Output exergy (net power) 451.45 Sum of exergy 839.58
53.77 100
486.78 839.58
57.98 100
Thermal efficiency
55.5
Exergy destruction Air separation unit (ASU) Turbine Combustor Oxidation reaction Reduction reaction Heat exchanger Exhaust gas loss Total exergy destruction
59.8
current study graphically illustrates the energy level difference in an energy donor and energy acceptor pair. Both the variation in the energy level and energy quantity are graphically shown using A—DH coordinates. In the current study, the energy level, A, is a dimensionless criterion (A = DE/DH = 1 T0 DS/DH, representing the ratio of the exergy change to the energy change), and the energy quantity, DH, refers to any kind of energy change, such as thermal energy, power consumption or generation, and energy change in a chemical reaction. The x-coordinate in the EUD is the energy change and the ycoordinate is the energy level, A. An energy transformation process involves an energy donor and an energy acceptor; the exergy destruction is illustrated by the shaded areas between the curves of the energy donor and the energy acceptor. The particular advantage of the EUD methodology over conventional exergy methods is in showing (i) the energy level degradation in each process, instead of only magnitudes of exergy losses obtained from the exergy value difference between the unit output and input; (ii) the variation in the driving force by dividing the entire process into infinitive processes; and (iii) both global and special information on different phenomena such as work, thermal, and chemical processes. Hence, the EUD methodology may provide information on the feasibility of the process, as well as the driving force, defect points, and potential improvement from intuitive and global viewpoints [19]. The detailed exergy analysis is also studied on absorption chiller [20], co-generation plants [21] and other processes [22]. All these researches make a better understanding of the internal processes and can modify the system more efficiency.
Fig. 3a. EUD for the HO-CLH combustion.
Fig. 3b. EUD for the RHO-CLH combustion.
To increase the performance, a reheat combustor, which operates at 1300 °C, is adopted in RHO-CLH. The area between Aed and Aea2 in Fig. 3b is smaller than that in Fig. 3a; this difference may be due to the reheating of steam. Hence, the overall exergy destruction in the combustion of H2 in HO-CLH is 121.41 kJ/molCH4, which is 31.36 kJ/mol-CH4 higher than that in RHO-CLH.
4.1. Combustion process
4.2. Oxidation reaction
Based on the exergy analysis, the efficiency of RHO-CLH is 4% points higher than that of HO-CLH, in which the highest exergy destruction reduction is from the combustion process. Figs. 3a and 3b show that the width of curve Aed;H2 corresponds to the heat released by the combustion of H2 and O2; the heights of the curves indicate the energy level degradation caused by the transformation of chemical energy into thermal energy. The heating processes of gas (H2 and O2) and steam act as the energy acceptor, which are illustrated by the Aea1 and Aea2 curves, respectively. Aea1 is the energy acceptor (H2 and O2) and Aea2 is the energy acceptor (H2O). The area between Aed and Aea corresponds to the exergy destruction caused by H2 combustion with O2 to the thermal energy at the turbine inlet temperature of 1300 °C.
The oxidation reactions of HO-CLH and RHO-CLH are shown in Figs. 4a and 4b. In the oxidation reaction, the Aed,Ox curve acts as the energy donor. Oxidation occurs at 660 °C in HO-CLH and 780 °C in RHO-CLH at a pressure of 40 bar. The heating processes of Fe3O4 and H2 act as the energy acceptor and are illustrated by the Aea;Fe3 O4 and Aea;H2 curves, respectively. The shaded area between the energy donors and the energy acceptors represent the exergy destruction in the oxidation reaction process. In these two systems, the oxidation reaction is the same; thus, the curves for the energy donor in Figs. 4a and 4b are similar. However, because the temperature of H2 and Fe3O4 (steams 2 and 4) are higher in RHO-CLH, the energy acceptor level in RHO-CLH is higher than that in HO-CLH, resulting in the reduced area in Fig. 4b. As a result, the
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X. Zhang et al. / Energy Conversion and Management xxx (2014) xxx–xxx
Fig. 4a. EUD for the oxidation reaction in HO-CLH.
Fig. 5a. EUD for the reduction reaction in HO-CLH.
Fig. 4b. EUD for the oxidation reaction in RHO-CLH.
Fig. 5b. EUD for the reduction reaction in RHO-CLH.
total exergy destruction of the oxidation reaction in HO-CLH is 21.29 kJ/mol-CH4, which is 8% higher than that in RHO-CLH.
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tively, which are much lower than that of the combined cycle (221.24 kJ/mol-CH4). 4.4. Heat exchanger subsystem
4.3. Reduction reaction The areas in Figs. 5a and 5b represent the reduction of Fe3O4 by CH4, in which the Aed;Fe3 O4 and Aed;H2 O curves represent the energy level of Fe3O4 and H2O acting as the energy donor; the released thermal energy is used to supply the heat of the endothermic reaction. In addition, the energy level of the reduction reaction is represented by Aea. In the CH4-fueled CLC, the reduction of Fe3O4 by CH4 is an endothermic reaction. The exergy destruction in the reduction process is 31.53 kJ/mol-CH4, corresponding to 3.76% based on the input CH4 exergy in HO-CLH. In RHO-CLH, the temperature of the steam, which supplies heat to the reduction reaction, is 780 °C, which is 380 °C higher than that in HO-CLH. Thus, the level of the energy donor in RHO-CLH is high, thereby increasing the exergy destruction to 34.79 kJ/mol-CH4. Finally, the overall exergy destruction in the reaction subsystem, including the combustion, reduction, and oxidation, are 205.63 and 175.82 kJ/mol-CH4 in HO-CLH and RHO-CLH, respec-
Fig. 6a shows the exergy destruction in the heat recovery process and the exergy waste of flue gases. The Aed curve represents the energy level of CO2/H2O from the reduction reaction acting as the energy donor. The CO2/H2O mixture is cooled below 100 °C to separate CO2, and the liquid H2O is heated to 140 and 244 °C in HO-CLH and RHO-CLH, respectively. The total exergy destruction in the heat subsystems in HO-CLH and RHO-CLH are 46.8 and 49.78 kJ/mol-CH4, respectively. The total exergy destruction in the RHO-CLH subsystem is 6% higher because the CO2/H2O temperature in RHO-CLH is higher than that in HO-CLH, resulting in the higher level of the energy donor (Fig. 6b). The natural gas is usually used to generate electricity through combined cycle. The heat exchanger often causes more exergy destruction in the heat recovery boiler (HRSG). Because of the physical property of water, there is a phase change process causing a big shadow area (Fig. 6c). The curve of Aed1 represents the energy level of the N2/O2 from the turbine acting as the energy donor. The total exergy destruction in the heat subsystems is
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Fig. 6a. EUD for the HO-CLH heat exchanger.
Fig. 6c. EUD for the heat exchanger in combined cycle.
Fig. 6b. EUD for the RHO-CLH heat exchanger. Fig. 7a. Variation in the thermal efficiency of HO-CLH.
114.46 kJ/mol-CH4, which is about twice as much as RHO-CLH system. In RHO-CLH system, the steam reactor is used to recover the heat of the work fluid from the turbine, that the key reason why the exergy destruction decreased.
5. Results and discussion 5.1. Upgrading the energy level of the thermal energy The mid-grade thermal energy from the steam turbine at around 500–700 °C can be upgraded to the high-grade chemical energy of solid FeO via a reduction reaction. It can then be transferred into H2 via an oxidation reaction and finally released as a high-temperature thermal energy to produce electricity in the combustion process with a high-efficiency steam cycle. According to the EUDs, the reduction reaction absorbs the middle temperature thermal energy from steam. The heat is first stored in FeO through CH4 reduction and is then turned into the chemical energy of H2. The average energy level of steam heat is approximately 0.6, whereas that of H2 is approximately 0.9. Hence, the redox reaction of CH4 increases the energy levels of the heat from Fe2O3. This result is attributed to the energy-level degradation in HOCLH from CH4 combustion to H2 combustion; that is, degradation
acts as a ‘‘driving force’’ that raises the level of mid-temperature steam thermal energy. The mid-temperature steam heat is then absorbed by the reduction reaction. The average reaction level, A, of the reduction reaction is approximately 0.5; this level absorbs the mid-temperature thermal energy. Mid-temperature energies are finally released in the H2 reactor as high-temperature thermal energy. The decrease in exergy destruction in the CH4 reduction reactions has led to the attainment of the cascade utilization of chemical energy. Table 3 shows that the net efficiency in the new cycle could reach 59.8%, which is a competitive figure compared with that of the current advanced reference system. 5.2. Advanced thermodynamic performance Figs. 7a and 7b show the variations in the overall thermal efficiency (g) of the system with varying initial temperatures (TIT) and pressure ratios (p). The pressure ratio is the total ratio of compression. The expansion ratio in the cycle is much higher than the pressure ratio, which is entirely different from the commonly used gas turbine pressure ratio. Figs. 7a and 7b show that the cycle efficiency of the two cycles sharply increases with the temperature ratio. The effect of the
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Fig. 8a. Process diagram of steam in HO-CLH.
Fig. 7b. Variation in the thermal efficiency of RHO-CLH.
temperature ratio on efficiency is more evident in the heat recovery steam generator (HRSG) combined cycle compared with that of the system in a previous study [23]. In addition, for a given p, g increases with the increase in TIT. Each TIT has a corresponding popt to achieve the maximum thermal efficiency, which is similar to that of the conventional combined cycle, which has a popt of about 40. Thus, the two proposed systems have popt values different from that of the conventional combined cycle. The effect of the pressure ratio on efficiency can be explained as follows: at the same temperature ratio, the turbine exit temperature decreases when the pressure ratio increases, as long as the exit point is outside the saturated region. In turn, the expansion work per unit mass flow increases. On the other hand, the recirculation condensate feed H2O entering the reactor for the reduction of regenerative heat is decreased. These two factors contribute to the maximum efficiency. As shown in Fig. 7a, the effects of the pressure ratio on the efficiency between p = 20–40 appear flat; however, the curves are evident in Fig. 7b. In HO-CLH, more turbine exhaust heat can be taken back to the combustor using steam 9 when the pressure ratio decreases. Mass flow increases, and the output work of the turbine is almost kept constant. However, when the pressure ratio is lower than 20, the superfluous heat is released to keep the temperature of the reduction reactor below 900 °C, that causes the changes of the efficiency. On the other hand, the mechanism of the RHOCLH system is different. By adjusting the pressure ratios of the high-pressure and low-pressure steam turbines, the output power increases when p increases, and the exergy destruction in the reduction reaction is reduced because of the increased compatibility of the energy donor and the energy acceptor as the temperature of the turbine outlet steam decreases (Fig. 5b). When the pressure ratio exceeds 40 to supply enough heat to the reduction reactor, the temperature of the turbine outlet steam should be kept constant to increase the outlet pressure, subsequently lowering the output power of the turbine. The popt in RHO-CLH is around 40. Figs. 7a and 7b show that the HO-CLH system has better applicability because of its varying pressure ratios; however, the RHO-CLH system has higher thermal efficiency if the turbine inlet is allowed to reach a high enough temperature. Figs. 8a and 8b show that the reason for the high efficiency of RHO-CLH compared with that of HO-CLH is the reheating of the steam, which not only increases the output power of the turbine, but also decreases the exergy destruction in the reduction reactor through the use of the turbine exhaust heat. The thermal efficiency of RHO-CLH with
Fig. 8b. Process diagram of steam in RHO-CLH.
CO2 capture at the pressure ratio of 40 and at a turbine inlet temperature of 1400 °C is expected to reach approximately 61%. 5.3. Significant role of decreasing energy penalty for CO2 capture Removing undesired byproducts using various separation processes often require dealing with a great amount of exhaust gas and entails a high energy cost [24]. The goal of future generation systems will be to eliminate undesirable products from the upstream process. In the current study, CLC and CLH showed good performance in CO2 recovery and thus, can be used in the elimination of environmental pollution caused by greenhouse gas emissions. The proposed novel system is intended to combined the HO system and CLH system more effectively. The chemical looping hydrogen production unit is the key equipment in the whole system. This equipment not only produced hydrogen with low energy penalty, but also capture the CO2 without energy penalty, because the products of the reduction reactor in the CLH are very simple, consisting only of H2O and CO2. Table 5 compares the CO2 capture performance of the three systems (combined cycle, HO-CLH, and RHO-CLH) with and without CO2 capture. Given the characteristics of CLH, the thermal efficiencies of the two proposed systems would still be 1–4% higher than that of the combined cycle, respectively, even without CO2 capture.
Table 5 Comparison of the thermal efficiencies of the systems with the combined cycle.
Thermal efficiency (CC without CO2 capture) Thermal efficiency (with CO2 capture)
CC (%)
HO-CLH (%)
RHO-CLH (%)
54.6
55.5
59.8
46–48
55.5
59.8
Please cite this article in press as: Zhang X et al. A hydrogen and oxygen combined cycle with chemical-looping combustion. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.03.013
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If CO2 capture is included in the three systems, the thermal efficiency of the combined cycle will decrease by 8%, significantly reflecting the advantages of the two proposed cycles. The thermal efficiency will be 8–12% higher than that of the combined cycle with CO2 capture. 6. Conclusions
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[8]
[9]
A novel system integrating CLH with the H2 and O2 combined cycle has been proposed, and two types of such combined cycle have been discussed. The performances of HO-CLH mainly depend on the temperature ratio. The pressure ratio has little effect on the cycle efficiency of HO-CLH, but has significant effect on that of RHO-CLH. The cycle efficiency of HO-CLH can reach 55.5% at a TIT of 1300 °C, whereas that of RHO-CLH is 59.8% under the same condition; these values are at least 8–12% higher than that of the combined cycle with CO2 capture. RHO-CLH is selected for the integration system, and the best cycle performance has been obtained via simulation. At lower temperature ratios, this type of cycle has higher efficiency than HOCLH. Moreover, this cycle is easier to develop from an engineering point of view.
[10]
Acknowledgment
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Please cite this article in press as: Zhang X et al. A hydrogen and oxygen combined cycle with chemical-looping combustion. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.03.013