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Exergy analysis and performance evaluation of a newly developed integrated energy system for quenchable generation
Energy 179 (2019) 1191e1204
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Exergy analysis and performance evaluation of a newly developed integrated energy system for quenchable generation H. Ishaq*, I. Dincer Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000, Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2018 Received in revised form 23 April 2019 Accepted 6 May 2019 Available online 7 May 2019
This paper presents an innovative use of waste heat recovered from a cement slag for multigeneration purposes, including power, ammonia, heat, hot water and oxygen production. A novel approach of ammonia production is employed in this study. The proposed system consists of four major subsystems; copper-chlorine (Cu-Cl) cycle, cryogenic Air Separation Unit (ASU), ammonia synthesis reactor and two steam Rankine cycles. The Aspen plus 9.0 version is employed for modeling and simulation of the proposed system. A comparative study is also conducted considering the different CuCl cycle based integrated systems for multigenerational purpose with numerous energy sources. The parametric studies are carried out by varying parameters, namely flow rate, compressor and turbine discharge pressures and ammonia reactor conversion rate. The exergy analysis is comprehensively conducted for each of the system components through exergy balance equations and exergy efficiencies. The overall exergy efficiency of the designed system is found to be 36.1%. Further results are also presented and discussed. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Exergy analysis Hydrogen Heat recovery Ammonia synthesis Cryogenic air separation Exergy efficiency
1. Introduction Energy plays a significant role in economic and industrial development of a country and its importance is clearly recognized by many that the ones who control the energy will control their future. Energy demand and supply are the two core components to meet energy requirement and availability. As energy has an intimate connection with the environment, a society should carefully look for the energy resources which cause less environmental impact [1]. Since the cement, steel and glass industries consume huge portions of energy and end up with high-temperature process- and waste-heat, there is a clear opportunity to recover heat and use for producing useful commodities are required by the sectors and applications which is really the heart of this study as aiming to recover heat and use it through and integrated system for multigenerational purposes. The maximum useful work achievable from a system at a specified state is known as exergy. The amount of work which is wasted during a process due to the irreversibilities is lost work or exergy destruction. Exergy carries a distinctive property that it remains conserved under reversible process condition taking place
* Corresponding author. E-mail address: [email protected] (H. Ishaq). https://doi.org/10.1016/j.energy.2019.05.050 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
within a system and environment and it is destroyed in any irreversible process [2]. To overcome the effects accompanying the fossil fuels practice, renewable energy resources are gaining the attraction and being employed [3]. Nevertheless, solar energy resources are irregular in natural surroundings. Thus, these described resources can be better utilized with the help of integrating with the storage media. Hydrogen is one of the most auspicious storage media as the hydrogen utilization does not eject damaging pollutants but hydrogen does not carry high volumetric density which possesses a drawback in hydrogen transportation as well as storage. In regards to carbon-free options, hydrogen offers great opportunities which carry low energy density at the ambient temperature high pressure. With the aim of increasing the volumetric density of hydrogen, it can be liquefied for transportation and storage purposes. However, liquefying hydrogen obliges a very low temperature. Liquefied ammonia has considerably higher volumetric density than hydrogen. In addition, ammonia carries higher energy density as compared to liquid hydrogen while stored in the form of liquid at 25 C and 10 atm pressure. Ammonia delivers as an alternative fuel source in energy generation process in order to attain inferior environmentally harmful emissions, free of carbon contaminants as well as cost-effective [4]. Rabah et al. [5] developed a novel cryogenic air separation unit (ASU) including the flash separator. In the air separation process, a
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flash separator is employed replacing the turbine to the conventional low and high-pressure distillation columns. The turbine is replaced by a flash separator which was set up to generate an energy share in the low and high-pressure distillation column air separation process. Employed flash separator assisted the double objective of separation and throttling. The new flash separator based air separation unit was simulated with Aspen Plus. The flow rate of the model air was considered as 50000 Nm3/h o and feed compositions were considered as 20.9% O2 and 79.1% N2. The newly designed method enhances productivity and reduces energy consumption. The improvement of the efficiency of cryogenic ASU is studied in the paper [6]. Barati et al. [7] presented a paper on the energy recovery from slag. The slag waste heat amounts at very high temperatures range of 1200e1600 C, offers the chance to recover the energy. The steam or hot air recovery, thermoelectric power production and converting chemical energy to fuel are the three under development technologies currently for consuming slag thermal energy. The prior method is technologically advanced for large scale establishing recovery efficiencies of 65%. The latter two types are developing as subsequent generation waste heat recovery methods [8,9]. Tafone et al. [10] designed an integrated system by linking a waste of enriched oxygen with ASU and cryogenic engines to energy plant. Barbooti and Al-Ani [11] conducted a study of water splitting Copper-chlorine cycle for hydrogen production. Copper dissolves with HCl and formats hydrogen and cuprous chloride. This cuprous chloride further disproportionates by giving cupric chloride and copper metal and cupric chloride works as chlorine source in next step. Van der Ham and Kjelstrup [9] considered two different cryogenic systems is this study and conducted exergy analysis. The cryogenic air separation unit (ASU) supplied nitrogen to a gas turbine while nitrogen and oxygen to the gasifier. The two designed processes differed in the distillation columns. Third column addition dropped the distillation section exergy destruction by 31%. Thus, the second design reduced the overall exergy destruction by 12%. Exergy destruction calculated revealed that nearly half of total exergy destruction existed in compressor placed after the coolers. Avery [12] undertook a study on ammonia role in hydrogen economy. Ammonia gives nitrogen and water while undergoing combustion and results as a non-contaminating fuel. Thus, ammonia has the capability of becoming a hydrogen substitute intended for vehicle motive power. In order to achieve this purpose, electrolytic ammonia was prepared with the catalytic arrangement of nitrogen and electrolytic hydrogen. Naterer et al. [13] presented a paper on the progress in copperchlorine (Cu-Cl) hydrogen production cycle. Recent developments were stated by international research and development team on hydrogen production Cu-Cl cycle. Fresh experimental and mathematical results were provided in this paper for several cycle processes. Experimental findings of electrolysis and its integration in Cu-Cl cycle were presented. Some studies were conducted to emphasize the idea of industrial heat recovery and clean energy solutions [14,15]. Giddey et al. [16] presented a review paper on the electrochemical base ammonia production advancements. Globally produced ammonia is used in fertilizer at a large percentage of ammonia, it is utilized for space heating and transport vehicles fuel. Ammonia is considered as an outstanding media for energy storage and infrastructure is already established for its distribution and transportation in numerous countries. A comprehensive exergy analysis of the proposed system is conducted in this paper. Currently, ammonia is formed by the Haber-Bosch process commercially and that is very capital and
energy intensive. Several new ammonia production techniques are under consideration for further economical and efficient process and considering the growth in ammonia production forecast. A new approach is employed in this paper by synthesizing ammonia from cryogenic ASU based nitrogen separation and thermochemical CuCl cycle based hydrogen. The entire system is employed by the industrial waste heat, also, heat released by the ammonia synthesis reactor during exothermic process is employed for power generations. This paper aims to develop this particular system for industrial applications and analyze it thermodynamically through exergy approaches. It also studies the performance of the present system through exergy efficiency and discusses the possible improvements. 2. System description The exergy analysis of an integrated system recovering the cement slag waste heat for running Cu-Cl cycle for clean hydrogen production and further converting it into ammonia. The slag from the cement industry is usually released at a very high temperature of 1200e1600 C which offers the bright chance to recover the energy. Two furnaces are considered for heat recovery purpose. The major subsystems employed in the designed system are named as cryogenic air separation unit, heat recovery by thermal management of cement slag, water splitting Cu-Cl cycle, two steam Rankine cycles and ammonia synthesis reactor. Fig. 1 presents a schematic sketch of the proposed system. The present system is modeled and simulated in Aspen plus 9.0. The Aspen Plus simulation illustration is shown in Figs. 2 and 3 which represents all the streams and blocks numbers which are referred during the thermodynamic analysis of the individual components and overall system. The system description is explained comprehensively in this section which refers to Figs. 2 and 3. All the blocks are named with B while the streams are named with S in the Aspen plus model. The recovered heat from the cement slag is linked with the designed system via the heat exchanger B1. This heat exchanger plays a vital role in transferring the heat from flue gas to the water for multiple purposes. The high temperature of the flue gas is more adequately utilized for power production as the designed CuCl cycle requires maximum temperature of 550 C. This heat exchanger is designed to provide the CuCl cycle with the required flow rate of steam at 500 C and additional heat is set to be utilized in steam Rankine cycle to produce power The stream S3 carries water which is converted into steam and leaves at 550 C which is employed to the Cu-Cl cycle. Another water stream S5 is passed through the heat exchanger carrying water as working fluid and employed in steam Rankine cycle. The major inlet for Cu-Cl cycle is water which is entered in the form of steam after recovering heat from the source via streams S4 which further splits into S10 and S11. The water reacts with cupric chloride in hydrolysis reactor while in thermolysis, it is utilized to provide with heat. The four major steps are occurring in Block B7 (hydrolysis), B10 (thermolysis), B14 (electrolysis) and B18 (drying). Numerous heat exchangers are integrated within the system. Heat exchangers B6, B9, B11 and B17 are integrated to provide with the required heat while heat exchanger B13 and B15 are targeted to recover heat from the reaction. One of the final product is oxygen which is separated from copper-oxychloride in thermolysis reactor and achieved via stream S22. Another final commodity is hydrogen which is produced in electrolysis reactor and separated via stream S29. Hydrogen and oxygen are the final products of the Cu-Cl cycle and water is the only input excluding heat and electric work. All other compounds like cupric chloride, hydrogen chloride, cuprous chloride and copper-oxychloride are recycled within the same
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Fig. 1. Schematic illustration of the proposed system.
cycle. The four-step water splitting Cu-Cl cycle built in Clean Energy Research Lab (CERL) at UOIT is employed in this designed system and the four key steps of hydrolysis, thermolysis, electrolysis and drying are organized in Table 2 in an arranged manner. This novel CuCl cycle was built to utilize the waste heat from the industrial heat sources and this novel heat dependent hydrogen production cycle was revealed. Four steps of the Cu-Cl cycle are arranged in Table 1.
The cryogenic air separation unit is employed in this system for achieving nitrogen. Air is separated into 95% pure oxygen and nitrogen. The input considered for cryogenic ASU is the air at ambient conditions. Air first ranges through a compressor B19 where it is compressed at a high pressure of 6 bar. The temperature also rises with the increase in pressure and high-temperature stream is passed through a heat exchanger B20 to recover this heat for producing hot water. Air then approaches the turbine B21 and it is
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Fig. 2. Aspen plus illustration of steam Rankine cycle.
expanded to 1.3 bar pressure to harvest some power and turbine is integrated to lower the pressure for being utilized in high-pressure separation column. At the exit of the turbine, its temperature drops down to 50.9. Air then enters the high-pressure separation column, where nitrogen is separated from the top as it is lighter and oxygen comes out through the bottom stream. The nitrogen separated through cryogenic ASU and hydrogen produced via Cu-Cl cycle are brought in contact via ammonia synthesis reactor B23. In this study, a unique system for ammonia synthesis is introduced and comprehensively investigated with parametric studies. This ammonia production is an exothermic reaction. The air passes through the air separation unit and nitrogen is separated via stream S40 to be employed to the ammonia synthesis reactor. The CuCl cycle is integrated with the designed system which utilizes the waste heat to produce clean hydrogen. The hydrogen reaches to the ammonia synthesis reactor via stream S29, reacts with nitrogen and ammonia is collected via stream S42. As to be shown in the results and discussion section, the effect of air flow rate in ASU is investigated on the ammonia production temperature, heat rate of ammonia reactor and steam Rankine cycle and study reveals that the air flow rates show positive effect on the ammonia production temperature and heat rate of ammonia reactor. Ammonia synthesis is one of the novel integrations in the designed system, thus, it is significant to investigate the ammonia synthesis under different conversion rates. Furthermore, it is to be presented that the effect on ammonia conversion rate on the ammonia capacity, heat released by exothermic reaction and power of the steam turbine employed by the heat recovery from ammonia synthesis. This released heat is further linked with the steam Rankine cycle via stream S26 to generate the electric power. The operating conditions and the power output of this steam Rankine cycle are thus investigated under different conditions of air flow rates and different ammonia capacities.
3. Modeling and analysis The following provide the assumptions made for the present system analysis and assessment. The integrated system is operated under steady-state conditions. The insignificant variations in kinetic and potential energies take place in the system components. The temperature T0 ¼ 25 C.and Pressure P0 ¼ 101.325 Pa are taken as reference state. The SOLID property method is employed in Aspen Plus 9.0 to deal with real gases and fluids [17]. The compressor efficiency is taken as 95% [18]. The heat losses in turbines, heat exchangers and compressors are neglected. The isentropic efficiency for turbines is taken as 72% [19]. The compressors and turbines operate adiabatically. The general entropy balance equation is:
X Q_ X k _ i si þ S_ gen þ m Tk i
i
!
X Q_ X k _ e se þ ¼ m Tk e e
! (1)
The general exergy balance equation is:
X X _ Q þ Ex _ w¼ _ w þ Ex _ Q þ Ex _ _ i exi þ Ex _ e exe þ Ex m m d
(2)
e
i
_ In the presented equations, mass flow rate is represented by m, _ work rate is denoted by W _ and exergy heat rate is signified by Q, _ . destruction rate is symbolized by Ex d
_ Ex Q ¼
1
_ _ W ¼W Ex
To _ Q T
(3)
(4)
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Fig. 3. Aspen Plus simulation model of the designed integrated energy system including air separation unit, CuCl cycle, steam Rankine cycle and ammonia synthesis unit. Table 1 Four-steps; hydrolysis, thermolysis, electrolysis and drying. Reaction Step-1
400 C
2CuCl2 ðsÞ þ H2 OðgÞ ! Cu2 OCl2 ðsÞ þ 2HClðgÞ 500 C
Step-2
Cu2 OCl2 ðsÞ ! 0:5 O2 ðgÞ þ 2CuClðlÞ
Step-3
2CuClðaqÞ þ 2HClðaqÞ ! H2 ðgÞ þ 2CuCl2 ðaqÞ
Step-4
_ Q signifies heat exergy and Ex _ W denotes work exergy. Here, Ex The subscript “i” characterizes input while “e” embodies exit stream. The correlation for physical exergy is given in equation (5) while chemical exergy is described in equation (6).
25 C
80 C
CuCl2 ðaqÞ ! CuCl2 ðsÞ
exph ¼ h h0 T0 ðs s0 Þ
(5)
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Table 2 Exergy balance equation and exergy efficiency of each component in Aspen Plus flowsheet. Component
Energy balance equation
Exergy balance equation
Exergy efficiency
B1 (heat exchanger)
m_ s1 hs1 þ m_ s3 hs3 þ m_ s5 hs5 ¼ m_ s2 hs2 þ m_ s4 hs4 þ m_ s6 hs6
m_ s1 exs1 þ m_ s3 exs3 þ m_ s5 exs5 ¼ m_ s2 exs2 þ m_ s4 exs4 þ _ m_ s6 exs6 þ Ex d
jB16 ¼
B2 (turbine)
_ out m_ s6 hs6 ¼ m_ s7 hs7 þ W
_ out þ Ex _ m_ s6 exs6 ¼ m_ s7 exs7 þ W d
B3 (condenser)
m_ s7 hs7 ¼ m_ s8 hs8 þ Q_ out
_ _ m_ s7 exs7 ¼ m_ s8 exs8 þ Ex Q
B4 (pump)
_ out ¼ m_ s5 hs5 m_ s8 hs8 þ W
_ out ¼ m_ s5 exs5 þ Ex _ m_ s8 exs8 þ W d
B6 (heat exchanger)
m_ s10 hs10 þ Q_ in ¼ m_ s13 hs13
_ _ m_ s10 exs10 þ Ex Q
B7 (hydrolysis reactor)
m_ s13 h13 þ m_ s32 h32 ¼ m_ s14 hs14
_ m_ s13 ex13 þ m_ s32 ex32 ¼ m_ s14 exs14 þ Ex d
B8 (separator)
m_ s14 hs14 ¼ m_ s17 hs17 þ m_ s16 hs16
_ m_ s14 exs14 ¼ m_ s17 exs17 þ m_ s16 exs16 þ Ex d
in
out
_ þ Ex d
_ ¼ m_ s13 exs13 þ Ex d
m_ s2 exs2 þ m_ s4 exs4 þ m_ s6 exs6 m_ s1 exs1 þ m_ s3 exs3 ¼ m_ s5 exs5 _ out m_ ex þ W jB17 ¼ s7 s7 m_ s6 exs6 _ _ m_ s8 exs8 þ Ex Q out jB22 ¼ m_ s7 exs7 m_ s5 exs5 jB23 ¼ _ out m_ s8 exs8 þ W m_ s13 exs13 jB2 ¼ _ _ þ Ex m_ ex s10
B10 (thermolysis reactor)
m_ s18 hs18
þ m_ s20 hs20 þ Q_ in ¼ m_ s21 hs21
m_ s18 exs18
_ _ þ m_ s20 exs20 þ Ex Q
in
_ ¼ m_ s21 exs21 þ Ex d
out
_ þ Ex d
jB5 jB7
m_ s18 m_ s23 hs23 ¼ m_ s24 hs24 þ Q_ out
_ _ m_ s23 exs23 ¼ m_ s24 exs24 þ Ex Q
_ e ¼ m_ h m_ s24 hs24 þ m_ s27 hs27 þ W s28 s28
_ e ¼ m_ ex þ Ex _ m_ s24 exs24 þ m_ s27 exs27 þ W s28 s28 d
B18 (Dryer)
m_ s31 hs31 ¼ m_ s32 hs32 þ m_ s33 hs33
_ m_ s31 exs31 ¼ m_ s32 exs32 þ m_ s33 exs33 þ Ex d
B19 (compressor)
_ out ¼ m_ h m_ s34 hs34 þ W s35 s35
_ out ¼ m_ ex þ Ex _ m_ s34 exs34 þ W s35 s35 d
B10 (heat exchanger)
m_ s35 hs35 þ m_ s37 hs37 ¼ m_ s36 hs36 þ m_ s38 hs38
_ m_ s35 exs35 þ m_ s37 exs37 ¼ m_ s36 exs36 þ m_ s38 exs38 þ Ex d
B21 (turbine)
_ out m_ s36 hs36 ¼ m_ s39 hs39 þ W
_ out þ Ex _ m_ s36 exs36 ¼ m_ s39 exs39 þ W d
B22 (low-pressure separation column) B23 (ammonia production reactor)
m_ s39 hs39 ¼ m_ s40 hs40 þ m_ s41 hs41
_ m_ s39 exs39 ¼ m_ s40 exs40 þ m_ s41 exs41 þ Ex d
B24 (heat exchanger)
_ m_ s43 hs43 ¼ m_ s44 hs44 þ Q_ out þ Ex d
_ _ m_ s43 exs43 ¼ m_ s44 exs44 þ Ex Q
B25 (turbine)
_ out m_ s44 hs44 ¼ m_ s45 hs45 þ W
_ out þ Ex _ m_ s44 exs44 ¼ m_ s45 exs45 þ W d
B26 (condenser)
_ m_ s45 hs45 ¼ m_ s46 hs46 þ Q_ out þ Ex d
_ _ m_ s45 exs45 ¼ m_ s46 exs46 þ Ex Q
B13 (heat exchanger) B14 (electrolysis reactor)
m_ s29 hs29
þ m_ s40 hs40 þ Q_ in ¼ m_ s42 hs42
m_ s29 exs29
_ _ þ m_ s40 exs40 þ Ex Q
in
_ ¼ m_ s42 exs42 þ Ex d
out
_ þ Ex d
B27 (pump)
exch ¼
m_ s46 hs46
X X xj ex0ch þ RT0 xj ln xj
_ out ¼ m_ h þW s43 s43
m_ s46 exs46
(6)
Here, xj implies mole fraction whereas ex0ch specifies standard specific chemical exergy. The calculation of total exergy is carried out by following equation:
ex ¼ exph þ exch
(7)
The energy and exergy analyses of the whole system are conducted in this paper. The energy and exergy balance equations and exergy efficiencies for each component are tabulated in Table 2. This study explains some brief data representing the energy analysis in terms of energy balance equations for each component, enthalpy values for each state point and energy efficiency of the overall system as this study majorly comprises of the exergy analysis of the proposed novel integrated system. The state points linked with each stream associated with Aspen Plus model are arranged in Table 3. The significant parameters like phase, temperature, pressure, mass flow, enthalpies, mass exergy and exergy flow rate are presented in the table. The stream number
_ þ Ex d
_ out ¼ m_ ex þ Ex _ þW s43 s43 d
s18
jB9 ¼
Q in
s20
s20
_ _ m_ s24 exs24 þ Ex Q m_ s23 exs23
jB10 ¼
Q in
out
m_ s28 exs28 _e m_ s24 exs24 þ m_ s27 exs27 þ W m_ s32 exs32 þ m_ s33 exs33 jB14 ¼ m_ s31 exs31 m_ s35 exs35 jB15 ¼ _ out þW m_ ex s34
s34
m_ s36 exs36 þ m_ s38 exs38 m_ s35 exs35 þ m_ s37 exs37 _ out m_ ex þW ¼ s39 s39 m_ s36 exs36 þ m_ s41 exs41 m_ ex ¼ s40 s40 m_ s39 exs39 ¼ m_ s42 exs42 _ _ ex þ m_ ex þ Ex
jB16 ¼ jB17 jB18 jB19 m_ s29
out
s10
m_ s14 exs14 m_ s13 ex13 þ m_ s32 ex32 þ m_ s16 exs16 m_ ex ¼ s17 s17 m_ s14 exs14 ¼ m_ s21 exs21 _ _ ex þ m_ ex þ Ex
jB3 ¼
s29
jB20 ¼ jB21 ¼ jB22 ¼ jB23
s40
s40
_ _ m_ s44 exs44 þ Ex Q
Q in
out
m_ s43 exs43 _ out m_ s45 exs45 þ W m_ s44 exs44 _ _ m_ s46 exs46 þ Ex Q
out
m_ s45 exs45 m_ s43 exs43 ¼ _ out m_ s46 exs46 þ W
refers to Fig. 2 presenting the Aspen plus model of the designed system. The following equation is used to calculate total input of heat exergy flow rate for Cu-Cl cycle and equation (9) presents the net work rate of the designed system.
_ _ _ _ _ _ _ Ex Q CuCl ¼ ExQ B2 þ ExQ B3 þ ExQ B5 þ ExQ B6 þ ExQ B7 þ ExQ B13
(8)
_ _ _ _ _ _ _ _ net ¼ W W B2 þ WB21 þ WB25 Welec WB19 WB4 WB27 (9) The subsequent equation provided us with the total amount of heat recovered from the successive condensers.
_ _ _ Ex Q heat ¼ ExQ cond1 þ ExQ cond2 The overall energy and exergy efficiencies are written as
(10)
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Table 3 State point table of the designed system considering exergy analysis. State point
Phase
Substance
T ( C)
P (bar)
m_ (kg/s)
h (kJ/kg)
ex (kJ/kg)
_ (kW) Ex
S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S13 S14 S16 S17 S18 S20 S21 S22 S23 S24 S27 S28 S29 S30 S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46
Vapor Vapor Liquid Vapor Liquid Vapor Mixed Liquid Vapor Vapor Vapor Mixed Liquid Vapor Liquid Vapor Mixed Vapor Mixed Mixed Vapor Mixed Vapor Liquid Liquid Liquid Liquid Vapor Vapor Vapor Liquid Liquid Vapor Vapor Vapor Vapor Liquid Vapor Mixed Liquid
Flue gas Flue gas Water Water Water Water Water Water Water Water Water Cu2OCl2 þ HCl Cu2OCl2 HCl Cu2OCl2 H2O CuCl þ O2 Oxygen CuCl CuCl HCl CuCl2 þ H2 Hydrogen CuCl2 CuCl2 CuCl2 Water Air Air Air Hot water Hot water Air Nitrogen Oxygen Ammonia Water Water Water Water
1300.0 600.0 25.0 550.0 27.5 438.9 45.8 25.0 550.0 550.0 550.0 400.0 400.0 400.0 500.0 550.0 500.0 500.0 500.0 430.0 25.0 25.0 25.0 25.0 80.0 80.0 80.0 25.0 296.4 25.0 25.0 39.6 50.9 50.9 50.9 25.0 25.5 271.5 45.8 25.0
1.0 1.0 1.0 1.0 50.0 50.0 0.1 0.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 6.0 6.0 1.0 1.0 1.3 1.3 1.3 1.0 6.0 6.0 0.1 0.1
16.0 16.0 0.0 0.0 4.0 4.0 4.0 4.0 0.0 0.0 0.4 5.6 4.2 1.4 4.2 0.4 4.5 0.3 4.2 4.2 1.4 5.6 0.0 5.6 5.6 5.2 0.4 0.2 0.2 0.2 1.1 1.1 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.2
1420.9 602.2 15864.3 12373.7 15853.7 12610.1 13465.7 15864.3 15864.3 15864.3 12373.7 1901.4 1789.5 2229.9 1791.6 12373.7 490.2 466.5 561.4 617.5 2531.8 699.6 0.0 708.8 645.6 358.6 15634.3 0.0 278.7 0.0 15864.3 15803.1 76.7 78.9 69.3 2695.0 15862.3 12950.8 13461.4 15864.3
883.1 268.8 0.0 926.7 0.1 1320.5 154.0 0.0 926.7 926.7 926.1 25.1 2.7 105.8 3.9 926.1 425.7 189.6 453.8 402.9 0.9 0.1 16.2 0.0 254.6 273.0 19.7 1.1 233.7 152.8 0.0 1.5 33.2 34.2 30.0 1.9 0.0 850.0 153.7 0.0
14129.6 4300.0 0.0 33.4 0.4 5282.2 616.2 0.0 16.7 16.7 325.3 140.2 11.4 150.5 16.4 325.3 1925.8 59.1 1911.6 1697.1 1.3 0.6 0.6 0.0 1424.6 1431.5 6.9 0.3 55.5 36.3 0.0 1.7 7.9 6.2 1.7 0.4 0.0 170.0 30.7 0.0
_ net _ O2 hO2 þ m _ S37 ðhS38 hS37 Þþ Q_ heating þ W _ NH3 LHVNH3 þ m m hov ¼ _ _ Q þW in
electrolyzer
(11)
jov ¼
_ _ _ O2 exO2 þ m _ S27 ðexS28 exS27 Þþ Ex _ NH3 exNH3 þ m m Q heat þ Wnet _Ex _ Q in
(12)
rate in overall system is linked with heat exchanger B1, in Cu-Cl cycle, it is calculated in thermolysis reactor while in cryogenic ASU, the maximum exergy destruction rate is carried out by ASU turbine and in Rankine cycle, turbine gives the highest exergy destruction rate and one reason is the high temperature difference. The effect of turbine B2 discharge pressure is plotted against the turbine output temperature and turbine work in Fig. 4. This steam Rankine cycle is one of the major subsystems within the proposed system, thus, investigation of the power produced by steam turbine is significant for this study. This steam Rankine cycle is investigated under different operating conditions to observe the operational
4. Results and discussion A new ammonia synthesizing approach is employed in this paper. The slag from the cement industry is employed as the heat source which has a very high temperature of 1200e1600 C when released and offers the heat recovery potential. Ammonia, oxygen, hot water and electricity are the major commodities carried out of the system. The designed system comprises of four major subsystems; thermochemical Cu-Cl cycle for clean hydrogen production, cryogenic ASU for nitrogen separation, ammonia synthesis reactor and Rankine cycle. Numerous parametric studies are directed to investigate and reveal the system performance. The exergy destruction rates of the major components are calculated and arranged in Table 4. The topmost exergy destruction
Table 4 Exergy destruction rates associated with major components. Exergy destruction rate
Unit (kW)
B1 (Heat exchanger) B2 (Steam Rankine cycle turbine) B7 (Hydrolysis reactor) B10 (Thermolysis reactor) B14 (Electrolysis reactor) B18 (Dryer) B19 (ASU compressor) B20 (Heat exchanger) B21 (ASU turbine) B23 (Ammonia synthesis reactor) B24 (Rankine cycle turbine)
4514.4 1244.0 1616.7 7454.1 2768.9 1424.6 10.4 17.5 46.4 6.0 37.1
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Fig. 4. Turbine discharge pressure vs turbine discharge temperature and turbine work.
parameters. The graphical representation shows that turbine output temperature rises with an increase in turbine discharge pressure and turbine work decreases simultaneously at the same time. The cryogenic air separation unit is a major component which is investigated in detail under different flow rates of input air, as
nitrogen is supplied to the ammonia synthesis reactor by cryogenic air separation unit. The flow rate of nitrogen and oxygen by the cryogenic ASU is depicted in Fig. 5. The cryogenic air separation unit is taken as the source of nitrogen for the designed system which is directly affected by the air flow rate. The graphical representation gives the variation in nitrogen and oxygen flow rate with change in
Fig. 5. Air flow rate vs nitrogen and oxygen flow rate.
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Fig. 6. Variation in ammonia reactor heat rate, Rankine cycle turbine power and ammonia production temperature by the change in air inlet flow rate.
air flow rate. At 29.6 kmol/h of air flow rate, 6.5 mol/s of nitrogen and 1.7 mol/s of oxygen is separated. A new approach for ammonia synthesis is introduced in this study which is mandatory to be investigated comprehensively. Flow rate of inlet air is investigated against the heat rate of ammonia reactor, ammonia production temperature and steam Rankine cycle employed by the heat recovery from ammonia synthesis. The effect of air flow rate in cryogenic ASU is studied against the ammonia reactor heat rate, Rankine cycle turbine power and
ammonia production temperature in Fig. 6. The graphical representation depicts that ammonia reactor heat rate and Rankine cycle turbine power rises rapidly with increase in air flow rate while ammonia production temperature remains constant till the time to reach the required flow rate of components and once it is established, it also starts rising with other parameters. The conversion ratio of ammonia reactor is plotted against the reactor heat rate, turbine power and ammonia flow rate in Fig. 7. The significance of this study is to investigate the novel integration
50 45
600
40
500
35 30
400
25
300
20 15
200
10
100
5
0
0
0.3
0.5
0.7
0.9
Conversion ratio of ammonia reactor Heat duty
Turbine power
Ammonia flow rate
Fig. 7. Ammonia reactor conversion ratio effect on the reactor heat rate, turbine power and ammonia flow rate.
Ammonia flow rate (kmol/h)
Heat rate and turbine power (kW)
700
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of ammonia synthesis reactor under different operating conditions. The conversion rate of ammonia synthesis depends upon several parameters like operating temperature, pressure and catalyst, thus, conversion rate is studied against the heat duty, turbine power and ammonia flow rate. Different conversion rates are considered in ammonia reactor from 0.3 to 1 and its effect is studied on other parameters like turbine power, heat rate and ammonia flow rate. Figure exhibits that with the rise in conversion rate, unreacted products are reduced which results in increased ammonia flow rate, increased heat rate associated with ammonia reactor and increased turbine power. Gibbs free energy of pure components associated with the stream numbers is tabulated in Table 5. The purpose of this parametric study is to investigate the room for heat recovery from the heat exchangers within the CuCl cycle. The electrolysis reactor is set to operate at 25 C temperature, thus, both inlet streams to the electrolysis reactor are processed through the heaters to recover the available heat. Fig. 8 exhibits the water flow rate effect on the heat recovered by the heat exchangers B13 and B15. Heat is covered from the B13 heat exchanger as soon as it comes out of the thermolysis reactor and reaches to B13 so it has an increasing pattern while for heat exchanger B15, heat duty decreases first to achieve the reaction condition and then starts increasing. This parametric study investigates the effects of compressor pressure employed to the cryogenic air separation unit on the work
rates of compressor and turbine and turbine inlet temperature. This study is significant to be investigated as cryogenic air separation unit is one of the major subsystems in this proposed study. The compressor pressure is varied in order to see its effect of the subsequent parameters. Fig. 9 exhibits the effect of compressor pressure on compressor work consumption, compressor output stream temperature and turbine power. The results revealed that rise in compressor pressure causes an increase in compressor work consumption, compressor output stream temperature and turbine power. The solid line represents the turbine power, narrow dotted line exhibits compressor work consumption and wide dotted line presents compressor output stream temperature. At the compressor pressure of 6 bar, the compressor work consumption is 66.1 kW, 39.6 C is the output stream S38 temperature and 18.2 kW is the turbine power. Fig. 10 shows the turbine discharge pressure effect on the turbine power and outlet stream temperature. This particular study becomes important for the fact that the turbine employed to the cryogenic air separation unit is set to cover some amount of work required by the compressor. This study investigates the effect of turbine discharge pressure effect on the turbine exhaust temperature and power. The solid line is representing the turbine B21 power and a dotted line shows the turbine output stream S39 temperature. The graphical representation depicts that with the rise in turbine B21 pressure, working fluid is not expanded to the
Table 5 Gibbs free energy of pure compounds in each stream. Unit
H2O
H2
O2
HCl
CuCl
CuCl2
Cu2OCl2
Air
N2
NH3
kJ/mol S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S13 S14 S16 S17 S18 S20 S21 S22 S23 S24 S27 S28 S29 S30 S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46
41.0 10.8 237.2 216.1 236.7 194.2 233.8 237.2 216.1 216.1 216.1 89.5
559.4 1236.5
104.2 1363.3 216.1 192.4
17.7
23.2 8.1
188.3 194.1
22.3 17.1 95.3
235.0
10.1
17.7 0.0
237.1 228.5
123.8 199.0 199.0
228.5 0.0 5.5 4.4
0.0 5.6 4.4
0.2
0.2 0.2
237.1 234.8
0.2 0.0 237.1 212.3 233.8 237.1
16.4
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Fig. 8. Heat recovery is depicted against the water flow rate (for details, see Ref. [20]).
Fig. 9. Compressor pressure effect on turbine and compressor work and stream S38 temperature.
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Fig. 10. Turbine discharge pressure effect on turbine power and output stream temperature.
desired pressure which results in reduced turbine power and increased output temperature. The effect of turbine B21 discharge pressure is plotted against the stream S40 temperature, turbine B25 power and ammonia reactor heat rate in Fig. 11. This study investigates the operating parameters for three major components distillation column, ammonia synthesis reactor and steam turbine. The exit stream temperature for the distillation column, heat released by the ammonia synthesis reactor and power produced by the steam
turbine are investigated against the turbine discharge temperature. The blue line shows the stream S40 temperature, grey line represents the ammonia reactor heat rate and third colored line signifies the turbine B25 power production. As the same stream from the turbine turbine B21 passes through the ammonia reactor and Rankine cycle, so it effects reactor heat rate and turbine power as well. This study shows that with the rise in turbine B21 discharge pressure, all three parameters stream S40 temperature, ammonia reactor heat rate and turbine B25 power production increases.
Fig. 11. Effect of turbine discharge pressure on stream S30 temperature, ammonia reactor heat rate and turbine B21 power.
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Fig. 12. A comparative study of the CuCl cycle based integrated systems with the proposed integrated system.
4.1. System validation A comparative study is conducted considering the different studies based on the integrated systems including the CuCl cycle and presented in Fig. 12. Al-Zareer et al. [21] designed a nuclear energy based integrated system for hydrogen production and achieved the exergy efficiency of 40.6%. Ishaq et al. [20] conducted a study on the integration of industrial waste heat with the CuCl cycle for multiple commodities and the exergy efficiency was concluded as 31.8%. DinAli and Dincer [22] and Zamfirescu and Dincer [23] proposed different configurations for a solar based integrated system including the CuCl cycle and achieved the exergy efficiencies of 55.2% and 20%. Some of these studies achieved higher exergy efficiencies as compared to the proposed system but these studies employed nuclear and renewable energy while this proposed study utilized industrial waste heat and achieved competitive efficiency. Wu et al. [24] published a study on the CuCl cycle based system for multigeneration purpose and the exergy efficiency was found to be 31.97%. 5. Conclusions A novel idea of ammonia production utilizing the industrial waste heat is presented in this paper. The quenchable useful outputs produced by the proposed system includes ammonia, oxygen, power, heating and hot water. For hydrogen, the Cu-Cl cycle is employed utilizing the waste heat from cement slag while for nitrogen, cryogenic ASU is installed. Hydrogen and nitrogen are brought together in the ammonia synthesis reactor. Ammonia synthesis is an exothermic process carried out on ambient conditions. Ammonia reactor is also employed with a number of conversion ratios to investigate of effect on ammonia production rate and heat released. Exergy destruction of all major components is tabulated in a separate table and exergy flow rates of each stream are a plotted is graph. The Gibbs free energy of pure compound associated with each stream is also arranged in a table. The
hydrogen production rate of the designed system is 19.5 mol/s and the ammonia production rate is carried out as 13 mol/s. Steam Rankine cycle electric power is finalized as 3433 kW, and hence overall energetic and exergetic efficiencies are found to be 30.1% and 36.1%. Nomenclature B _ En ex _ Ex
Block number energy rate (kW) specific exergy (kJ/kg) exergy rate (kW) specific enthalpy (kJ/kg) mass flow rate (kg/s) molar flow rate (mol/s) pressure (kPa) ambient pressure lower heating value (kJ/kg) Stream number specific entropy (kJ/kg K) temperature ( C) ambient temperature
h _ m N P P0 LHV S s T T0 Q_
heat rate (kW)
_ W
power or work rate (kW)
Greek letters h energy efficiency j exergy efficiency Subscripts 0 ch e i d
reference conditions chemical exit input destruction
1204
NH3 O2 ov W ph
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Ammonia Oxygen overall work physical
Acronyms ASU Air separation unit CERL Clean energy research lab Cu-Cl Copper-chlorine cycle References [1] Dincer I, Zamfirescu C. Sustainable energy systems and applications. New York Dordrecht Heidelberg London: Springer; 2011. https://doi.org/10.1007/978-0387-95861-3. [2] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. Amsterdam, Netherlands: Elsevier; 2013. [3] Ishaq H, Dincer I, Naterer GF. Performance investigation of an integrated wind energy system for co-generation of power and hydrogen. Int J Hydrogen Energy 2018;43:9153e64. [4] Siddiqui O, Dincer I. A review and comparative assessment of direct ammonia fuel cells. Therm. Sci. Eng. Prog. 2018;5:568e78. [5] Rabah AA, Khalel ZAM, Rabah AA, Barakat TAM. A new cryogenic air separation process with flash separator a new cryogenic air separation process with flash separator. 2013. https://doi.org/10.1155/2013/253437. [6] Aneke M, Wang M. Potential for improving the energy efficiency of cryogenic air separation unit (ASU) using binary heat recovery cycles. Appl Therm Eng 2015;81:223e31. [7] Barati M, Esfahani S, Utigard TA. Energy recovery from high temperature slags. Energy 2011;36:5440e9. [8] Arzbaecher C, Fouche E, Parmenter K, Partners GE. Industrial waste-heat Recovery : benefits and recent advancements in technology and applications definition of waste heat for this paper quantity. Quality and Temporal Availability of Waste Heat Heat-Recovery Potential in US Manufacturing Industry 2007;1e13. [9] Qin Y, Lv X, Bai C, Qiu G, Chen P. Waste heat recovery from blast furnace slag by chemical reactions. JOM (J Occup Med) 2012;64:997e1001.
[10] Tafone A, Dal Magro F, Romagnoli A. Integrating an oxygen enriched waste to energy plant with cryogenic engines and Air Separation Unit: technical, economic and environmental analysis. Appl Energy 2018;231:423e32. [11] Barbooti MM, Al-Ani RR. The copper-chlorine thermochemical cycle of water splitting for hydrogen production. Thermochim Acta 1984;78:275e84. [12] AVERY W. A role for ammonia in the hydrogen economy. Int J Hydrogen Energy 1988;13:761e73. [13] Naterer GF, Suppiah S, Stolberg L, Lewis M, Wang Z, Rosen MA, Dincer I, Gabriel K, Odukoya A, Secnik E, Easton EB, Papangelakis V. Progress in thermochemical hydrogen production with the copper-chlorine cycle. Int J Hydrogen Energy 2015;40:6283e95. [14] Gao L, Wu H, Jin H, Yang M. System study of combined cooling, heating and power system for eco-industrial parks. Int J Energy Res 2008;32:1107e18. [15] Dincer I, Acar C. A review on clean energy solutions for better sustainability. Int J Energy Res 2015;39:585e606. [16] Giddey S, Badwal SPS, Kulkarni A. Review of electrochemical ammonia production technologies and materials. Int J Hydrogen Energy 2013;38: 14576e94. [17] Ishaq H, Dincer I. A comparative evaluation of three Cu-Cl cycles for hydrogen production. Int J Hydrogen Energy 2019;44:7958e68. [18] J.M.C.. Gas conditioning and processing: the equipment modules. Campbell Petroleum Series 1984;2. €fer H. Rotordynamics of automotive turbochargers. New York [19] Nguyen-Scha Dordrecht Heidelberg London: Rotordynamics Automot. Turbochargers. Springer; 2012. https://doi.org/10.1007/978-3-642-27518-0. [20] Ishaq H, Dincer I, Naterer GF. Exergy and cost analyses of waste heat recovery from furnace cement slag for clean hydrogen production. Energy 2019. https://doi.org/10.1016/j.energy.2019.02.026. [21] Al-Zareer M, Dincer I, Rosen MA. Assessment and analysis of hydrogen and electricity production from a Generation IV lead-cooled nuclear reactor integrated with a copper-chlorine thermochemical cycle. Int J Energy Res 2018;42:91e103. [22] DinAli MN, Dincer I. Development of a new trigenerational integrated system for dimethyl-ether, electricity and fresh water production. Energy Convers Manag 2019;185:850e65. [23] Zamfirescu C, Dincer I. Assessment of a new integrated solar energy system for hydrogen production. Sol Energy 2014;107:700e13. [24] Wu W, Hsu FT, Chen HY. Design and energy evaluation of a stand-alone copper-chlorine (Cu-Cl) thermochemical cycle system for trigeneration of electricity, hydrogen, and oxygen. Int J Energy Res 2018;42:830e42.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Exergy analysis and performance evaluation of a newly developed integrated energy system for quenchable generation H. Ishaq*, I. Dincer Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000, Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2018 Received in revised form 23 April 2019 Accepted 6 May 2019 Available online 7 May 2019
This paper presents an innovative use of waste heat recovered from a cement slag for multigeneration purposes, including power, ammonia, heat, hot water and oxygen production. A novel approach of ammonia production is employed in this study. The proposed system consists of four major subsystems; copper-chlorine (Cu-Cl) cycle, cryogenic Air Separation Unit (ASU), ammonia synthesis reactor and two steam Rankine cycles. The Aspen plus 9.0 version is employed for modeling and simulation of the proposed system. A comparative study is also conducted considering the different CuCl cycle based integrated systems for multigenerational purpose with numerous energy sources. The parametric studies are carried out by varying parameters, namely flow rate, compressor and turbine discharge pressures and ammonia reactor conversion rate. The exergy analysis is comprehensively conducted for each of the system components through exergy balance equations and exergy efficiencies. The overall exergy efficiency of the designed system is found to be 36.1%. Further results are also presented and discussed. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Exergy analysis Hydrogen Heat recovery Ammonia synthesis Cryogenic air separation Exergy efficiency
1. Introduction Energy plays a significant role in economic and industrial development of a country and its importance is clearly recognized by many that the ones who control the energy will control their future. Energy demand and supply are the two core components to meet energy requirement and availability. As energy has an intimate connection with the environment, a society should carefully look for the energy resources which cause less environmental impact [1]. Since the cement, steel and glass industries consume huge portions of energy and end up with high-temperature process- and waste-heat, there is a clear opportunity to recover heat and use for producing useful commodities are required by the sectors and applications which is really the heart of this study as aiming to recover heat and use it through and integrated system for multigenerational purposes. The maximum useful work achievable from a system at a specified state is known as exergy. The amount of work which is wasted during a process due to the irreversibilities is lost work or exergy destruction. Exergy carries a distinctive property that it remains conserved under reversible process condition taking place
* Corresponding author. E-mail address: [email protected] (H. Ishaq). https://doi.org/10.1016/j.energy.2019.05.050 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
within a system and environment and it is destroyed in any irreversible process [2]. To overcome the effects accompanying the fossil fuels practice, renewable energy resources are gaining the attraction and being employed [3]. Nevertheless, solar energy resources are irregular in natural surroundings. Thus, these described resources can be better utilized with the help of integrating with the storage media. Hydrogen is one of the most auspicious storage media as the hydrogen utilization does not eject damaging pollutants but hydrogen does not carry high volumetric density which possesses a drawback in hydrogen transportation as well as storage. In regards to carbon-free options, hydrogen offers great opportunities which carry low energy density at the ambient temperature high pressure. With the aim of increasing the volumetric density of hydrogen, it can be liquefied for transportation and storage purposes. However, liquefying hydrogen obliges a very low temperature. Liquefied ammonia has considerably higher volumetric density than hydrogen. In addition, ammonia carries higher energy density as compared to liquid hydrogen while stored in the form of liquid at 25 C and 10 atm pressure. Ammonia delivers as an alternative fuel source in energy generation process in order to attain inferior environmentally harmful emissions, free of carbon contaminants as well as cost-effective [4]. Rabah et al. [5] developed a novel cryogenic air separation unit (ASU) including the flash separator. In the air separation process, a
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flash separator is employed replacing the turbine to the conventional low and high-pressure distillation columns. The turbine is replaced by a flash separator which was set up to generate an energy share in the low and high-pressure distillation column air separation process. Employed flash separator assisted the double objective of separation and throttling. The new flash separator based air separation unit was simulated with Aspen Plus. The flow rate of the model air was considered as 50000 Nm3/h o and feed compositions were considered as 20.9% O2 and 79.1% N2. The newly designed method enhances productivity and reduces energy consumption. The improvement of the efficiency of cryogenic ASU is studied in the paper [6]. Barati et al. [7] presented a paper on the energy recovery from slag. The slag waste heat amounts at very high temperatures range of 1200e1600 C, offers the chance to recover the energy. The steam or hot air recovery, thermoelectric power production and converting chemical energy to fuel are the three under development technologies currently for consuming slag thermal energy. The prior method is technologically advanced for large scale establishing recovery efficiencies of 65%. The latter two types are developing as subsequent generation waste heat recovery methods [8,9]. Tafone et al. [10] designed an integrated system by linking a waste of enriched oxygen with ASU and cryogenic engines to energy plant. Barbooti and Al-Ani [11] conducted a study of water splitting Copper-chlorine cycle for hydrogen production. Copper dissolves with HCl and formats hydrogen and cuprous chloride. This cuprous chloride further disproportionates by giving cupric chloride and copper metal and cupric chloride works as chlorine source in next step. Van der Ham and Kjelstrup [9] considered two different cryogenic systems is this study and conducted exergy analysis. The cryogenic air separation unit (ASU) supplied nitrogen to a gas turbine while nitrogen and oxygen to the gasifier. The two designed processes differed in the distillation columns. Third column addition dropped the distillation section exergy destruction by 31%. Thus, the second design reduced the overall exergy destruction by 12%. Exergy destruction calculated revealed that nearly half of total exergy destruction existed in compressor placed after the coolers. Avery [12] undertook a study on ammonia role in hydrogen economy. Ammonia gives nitrogen and water while undergoing combustion and results as a non-contaminating fuel. Thus, ammonia has the capability of becoming a hydrogen substitute intended for vehicle motive power. In order to achieve this purpose, electrolytic ammonia was prepared with the catalytic arrangement of nitrogen and electrolytic hydrogen. Naterer et al. [13] presented a paper on the progress in copperchlorine (Cu-Cl) hydrogen production cycle. Recent developments were stated by international research and development team on hydrogen production Cu-Cl cycle. Fresh experimental and mathematical results were provided in this paper for several cycle processes. Experimental findings of electrolysis and its integration in Cu-Cl cycle were presented. Some studies were conducted to emphasize the idea of industrial heat recovery and clean energy solutions [14,15]. Giddey et al. [16] presented a review paper on the electrochemical base ammonia production advancements. Globally produced ammonia is used in fertilizer at a large percentage of ammonia, it is utilized for space heating and transport vehicles fuel. Ammonia is considered as an outstanding media for energy storage and infrastructure is already established for its distribution and transportation in numerous countries. A comprehensive exergy analysis of the proposed system is conducted in this paper. Currently, ammonia is formed by the Haber-Bosch process commercially and that is very capital and
energy intensive. Several new ammonia production techniques are under consideration for further economical and efficient process and considering the growth in ammonia production forecast. A new approach is employed in this paper by synthesizing ammonia from cryogenic ASU based nitrogen separation and thermochemical CuCl cycle based hydrogen. The entire system is employed by the industrial waste heat, also, heat released by the ammonia synthesis reactor during exothermic process is employed for power generations. This paper aims to develop this particular system for industrial applications and analyze it thermodynamically through exergy approaches. It also studies the performance of the present system through exergy efficiency and discusses the possible improvements. 2. System description The exergy analysis of an integrated system recovering the cement slag waste heat for running Cu-Cl cycle for clean hydrogen production and further converting it into ammonia. The slag from the cement industry is usually released at a very high temperature of 1200e1600 C which offers the bright chance to recover the energy. Two furnaces are considered for heat recovery purpose. The major subsystems employed in the designed system are named as cryogenic air separation unit, heat recovery by thermal management of cement slag, water splitting Cu-Cl cycle, two steam Rankine cycles and ammonia synthesis reactor. Fig. 1 presents a schematic sketch of the proposed system. The present system is modeled and simulated in Aspen plus 9.0. The Aspen Plus simulation illustration is shown in Figs. 2 and 3 which represents all the streams and blocks numbers which are referred during the thermodynamic analysis of the individual components and overall system. The system description is explained comprehensively in this section which refers to Figs. 2 and 3. All the blocks are named with B while the streams are named with S in the Aspen plus model. The recovered heat from the cement slag is linked with the designed system via the heat exchanger B1. This heat exchanger plays a vital role in transferring the heat from flue gas to the water for multiple purposes. The high temperature of the flue gas is more adequately utilized for power production as the designed CuCl cycle requires maximum temperature of 550 C. This heat exchanger is designed to provide the CuCl cycle with the required flow rate of steam at 500 C and additional heat is set to be utilized in steam Rankine cycle to produce power The stream S3 carries water which is converted into steam and leaves at 550 C which is employed to the Cu-Cl cycle. Another water stream S5 is passed through the heat exchanger carrying water as working fluid and employed in steam Rankine cycle. The major inlet for Cu-Cl cycle is water which is entered in the form of steam after recovering heat from the source via streams S4 which further splits into S10 and S11. The water reacts with cupric chloride in hydrolysis reactor while in thermolysis, it is utilized to provide with heat. The four major steps are occurring in Block B7 (hydrolysis), B10 (thermolysis), B14 (electrolysis) and B18 (drying). Numerous heat exchangers are integrated within the system. Heat exchangers B6, B9, B11 and B17 are integrated to provide with the required heat while heat exchanger B13 and B15 are targeted to recover heat from the reaction. One of the final product is oxygen which is separated from copper-oxychloride in thermolysis reactor and achieved via stream S22. Another final commodity is hydrogen which is produced in electrolysis reactor and separated via stream S29. Hydrogen and oxygen are the final products of the Cu-Cl cycle and water is the only input excluding heat and electric work. All other compounds like cupric chloride, hydrogen chloride, cuprous chloride and copper-oxychloride are recycled within the same
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Fig. 1. Schematic illustration of the proposed system.
cycle. The four-step water splitting Cu-Cl cycle built in Clean Energy Research Lab (CERL) at UOIT is employed in this designed system and the four key steps of hydrolysis, thermolysis, electrolysis and drying are organized in Table 2 in an arranged manner. This novel CuCl cycle was built to utilize the waste heat from the industrial heat sources and this novel heat dependent hydrogen production cycle was revealed. Four steps of the Cu-Cl cycle are arranged in Table 1.
The cryogenic air separation unit is employed in this system for achieving nitrogen. Air is separated into 95% pure oxygen and nitrogen. The input considered for cryogenic ASU is the air at ambient conditions. Air first ranges through a compressor B19 where it is compressed at a high pressure of 6 bar. The temperature also rises with the increase in pressure and high-temperature stream is passed through a heat exchanger B20 to recover this heat for producing hot water. Air then approaches the turbine B21 and it is
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Fig. 2. Aspen plus illustration of steam Rankine cycle.
expanded to 1.3 bar pressure to harvest some power and turbine is integrated to lower the pressure for being utilized in high-pressure separation column. At the exit of the turbine, its temperature drops down to 50.9. Air then enters the high-pressure separation column, where nitrogen is separated from the top as it is lighter and oxygen comes out through the bottom stream. The nitrogen separated through cryogenic ASU and hydrogen produced via Cu-Cl cycle are brought in contact via ammonia synthesis reactor B23. In this study, a unique system for ammonia synthesis is introduced and comprehensively investigated with parametric studies. This ammonia production is an exothermic reaction. The air passes through the air separation unit and nitrogen is separated via stream S40 to be employed to the ammonia synthesis reactor. The CuCl cycle is integrated with the designed system which utilizes the waste heat to produce clean hydrogen. The hydrogen reaches to the ammonia synthesis reactor via stream S29, reacts with nitrogen and ammonia is collected via stream S42. As to be shown in the results and discussion section, the effect of air flow rate in ASU is investigated on the ammonia production temperature, heat rate of ammonia reactor and steam Rankine cycle and study reveals that the air flow rates show positive effect on the ammonia production temperature and heat rate of ammonia reactor. Ammonia synthesis is one of the novel integrations in the designed system, thus, it is significant to investigate the ammonia synthesis under different conversion rates. Furthermore, it is to be presented that the effect on ammonia conversion rate on the ammonia capacity, heat released by exothermic reaction and power of the steam turbine employed by the heat recovery from ammonia synthesis. This released heat is further linked with the steam Rankine cycle via stream S26 to generate the electric power. The operating conditions and the power output of this steam Rankine cycle are thus investigated under different conditions of air flow rates and different ammonia capacities.
3. Modeling and analysis The following provide the assumptions made for the present system analysis and assessment. The integrated system is operated under steady-state conditions. The insignificant variations in kinetic and potential energies take place in the system components. The temperature T0 ¼ 25 C.and Pressure P0 ¼ 101.325 Pa are taken as reference state. The SOLID property method is employed in Aspen Plus 9.0 to deal with real gases and fluids [17]. The compressor efficiency is taken as 95% [18]. The heat losses in turbines, heat exchangers and compressors are neglected. The isentropic efficiency for turbines is taken as 72% [19]. The compressors and turbines operate adiabatically. The general entropy balance equation is:
X Q_ X k _ i si þ S_ gen þ m Tk i
i
!
X Q_ X k _ e se þ ¼ m Tk e e
! (1)
The general exergy balance equation is:
X X _ Q þ Ex _ w¼ _ w þ Ex _ Q þ Ex _ _ i exi þ Ex _ e exe þ Ex m m d
(2)
e
i
_ In the presented equations, mass flow rate is represented by m, _ work rate is denoted by W _ and exergy heat rate is signified by Q, _ . destruction rate is symbolized by Ex d
_ Ex Q ¼
1
_ _ W ¼W Ex
To _ Q T
(3)
(4)
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1195
Fig. 3. Aspen Plus simulation model of the designed integrated energy system including air separation unit, CuCl cycle, steam Rankine cycle and ammonia synthesis unit. Table 1 Four-steps; hydrolysis, thermolysis, electrolysis and drying. Reaction Step-1
400 C
2CuCl2 ðsÞ þ H2 OðgÞ ! Cu2 OCl2 ðsÞ þ 2HClðgÞ 500 C
Step-2
Cu2 OCl2 ðsÞ ! 0:5 O2 ðgÞ þ 2CuClðlÞ
Step-3
2CuClðaqÞ þ 2HClðaqÞ ! H2 ðgÞ þ 2CuCl2 ðaqÞ
Step-4
_ Q signifies heat exergy and Ex _ W denotes work exergy. Here, Ex The subscript “i” characterizes input while “e” embodies exit stream. The correlation for physical exergy is given in equation (5) while chemical exergy is described in equation (6).
25 C
80 C
CuCl2 ðaqÞ ! CuCl2 ðsÞ
exph ¼ h h0 T0 ðs s0 Þ
(5)
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Table 2 Exergy balance equation and exergy efficiency of each component in Aspen Plus flowsheet. Component
Energy balance equation
Exergy balance equation
Exergy efficiency
B1 (heat exchanger)
m_ s1 hs1 þ m_ s3 hs3 þ m_ s5 hs5 ¼ m_ s2 hs2 þ m_ s4 hs4 þ m_ s6 hs6
m_ s1 exs1 þ m_ s3 exs3 þ m_ s5 exs5 ¼ m_ s2 exs2 þ m_ s4 exs4 þ _ m_ s6 exs6 þ Ex d
jB16 ¼
B2 (turbine)
_ out m_ s6 hs6 ¼ m_ s7 hs7 þ W
_ out þ Ex _ m_ s6 exs6 ¼ m_ s7 exs7 þ W d
B3 (condenser)
m_ s7 hs7 ¼ m_ s8 hs8 þ Q_ out
_ _ m_ s7 exs7 ¼ m_ s8 exs8 þ Ex Q
B4 (pump)
_ out ¼ m_ s5 hs5 m_ s8 hs8 þ W
_ out ¼ m_ s5 exs5 þ Ex _ m_ s8 exs8 þ W d
B6 (heat exchanger)
m_ s10 hs10 þ Q_ in ¼ m_ s13 hs13
_ _ m_ s10 exs10 þ Ex Q
B7 (hydrolysis reactor)
m_ s13 h13 þ m_ s32 h32 ¼ m_ s14 hs14
_ m_ s13 ex13 þ m_ s32 ex32 ¼ m_ s14 exs14 þ Ex d
B8 (separator)
m_ s14 hs14 ¼ m_ s17 hs17 þ m_ s16 hs16
_ m_ s14 exs14 ¼ m_ s17 exs17 þ m_ s16 exs16 þ Ex d
in
out
_ þ Ex d
_ ¼ m_ s13 exs13 þ Ex d
m_ s2 exs2 þ m_ s4 exs4 þ m_ s6 exs6 m_ s1 exs1 þ m_ s3 exs3 ¼ m_ s5 exs5 _ out m_ ex þ W jB17 ¼ s7 s7 m_ s6 exs6 _ _ m_ s8 exs8 þ Ex Q out jB22 ¼ m_ s7 exs7 m_ s5 exs5 jB23 ¼ _ out m_ s8 exs8 þ W m_ s13 exs13 jB2 ¼ _ _ þ Ex m_ ex s10
B10 (thermolysis reactor)
m_ s18 hs18
þ m_ s20 hs20 þ Q_ in ¼ m_ s21 hs21
m_ s18 exs18
_ _ þ m_ s20 exs20 þ Ex Q
in
_ ¼ m_ s21 exs21 þ Ex d
out
_ þ Ex d
jB5 jB7
m_ s18 m_ s23 hs23 ¼ m_ s24 hs24 þ Q_ out
_ _ m_ s23 exs23 ¼ m_ s24 exs24 þ Ex Q
_ e ¼ m_ h m_ s24 hs24 þ m_ s27 hs27 þ W s28 s28
_ e ¼ m_ ex þ Ex _ m_ s24 exs24 þ m_ s27 exs27 þ W s28 s28 d
B18 (Dryer)
m_ s31 hs31 ¼ m_ s32 hs32 þ m_ s33 hs33
_ m_ s31 exs31 ¼ m_ s32 exs32 þ m_ s33 exs33 þ Ex d
B19 (compressor)
_ out ¼ m_ h m_ s34 hs34 þ W s35 s35
_ out ¼ m_ ex þ Ex _ m_ s34 exs34 þ W s35 s35 d
B10 (heat exchanger)
m_ s35 hs35 þ m_ s37 hs37 ¼ m_ s36 hs36 þ m_ s38 hs38
_ m_ s35 exs35 þ m_ s37 exs37 ¼ m_ s36 exs36 þ m_ s38 exs38 þ Ex d
B21 (turbine)
_ out m_ s36 hs36 ¼ m_ s39 hs39 þ W
_ out þ Ex _ m_ s36 exs36 ¼ m_ s39 exs39 þ W d
B22 (low-pressure separation column) B23 (ammonia production reactor)
m_ s39 hs39 ¼ m_ s40 hs40 þ m_ s41 hs41
_ m_ s39 exs39 ¼ m_ s40 exs40 þ m_ s41 exs41 þ Ex d
B24 (heat exchanger)
_ m_ s43 hs43 ¼ m_ s44 hs44 þ Q_ out þ Ex d
_ _ m_ s43 exs43 ¼ m_ s44 exs44 þ Ex Q
B25 (turbine)
_ out m_ s44 hs44 ¼ m_ s45 hs45 þ W
_ out þ Ex _ m_ s44 exs44 ¼ m_ s45 exs45 þ W d
B26 (condenser)
_ m_ s45 hs45 ¼ m_ s46 hs46 þ Q_ out þ Ex d
_ _ m_ s45 exs45 ¼ m_ s46 exs46 þ Ex Q
B13 (heat exchanger) B14 (electrolysis reactor)
m_ s29 hs29
þ m_ s40 hs40 þ Q_ in ¼ m_ s42 hs42
m_ s29 exs29
_ _ þ m_ s40 exs40 þ Ex Q
in
_ ¼ m_ s42 exs42 þ Ex d
out
_ þ Ex d
B27 (pump)
exch ¼
m_ s46 hs46
X X xj ex0ch þ RT0 xj ln xj
_ out ¼ m_ h þW s43 s43
m_ s46 exs46
(6)
Here, xj implies mole fraction whereas ex0ch specifies standard specific chemical exergy. The calculation of total exergy is carried out by following equation:
ex ¼ exph þ exch
(7)
The energy and exergy analyses of the whole system are conducted in this paper. The energy and exergy balance equations and exergy efficiencies for each component are tabulated in Table 2. This study explains some brief data representing the energy analysis in terms of energy balance equations for each component, enthalpy values for each state point and energy efficiency of the overall system as this study majorly comprises of the exergy analysis of the proposed novel integrated system. The state points linked with each stream associated with Aspen Plus model are arranged in Table 3. The significant parameters like phase, temperature, pressure, mass flow, enthalpies, mass exergy and exergy flow rate are presented in the table. The stream number
_ þ Ex d
_ out ¼ m_ ex þ Ex _ þW s43 s43 d
s18
jB9 ¼
Q in
s20
s20
_ _ m_ s24 exs24 þ Ex Q m_ s23 exs23
jB10 ¼
Q in
out
m_ s28 exs28 _e m_ s24 exs24 þ m_ s27 exs27 þ W m_ s32 exs32 þ m_ s33 exs33 jB14 ¼ m_ s31 exs31 m_ s35 exs35 jB15 ¼ _ out þW m_ ex s34
s34
m_ s36 exs36 þ m_ s38 exs38 m_ s35 exs35 þ m_ s37 exs37 _ out m_ ex þW ¼ s39 s39 m_ s36 exs36 þ m_ s41 exs41 m_ ex ¼ s40 s40 m_ s39 exs39 ¼ m_ s42 exs42 _ _ ex þ m_ ex þ Ex
jB16 ¼ jB17 jB18 jB19 m_ s29
out
s10
m_ s14 exs14 m_ s13 ex13 þ m_ s32 ex32 þ m_ s16 exs16 m_ ex ¼ s17 s17 m_ s14 exs14 ¼ m_ s21 exs21 _ _ ex þ m_ ex þ Ex
jB3 ¼
s29
jB20 ¼ jB21 ¼ jB22 ¼ jB23
s40
s40
_ _ m_ s44 exs44 þ Ex Q
Q in
out
m_ s43 exs43 _ out m_ s45 exs45 þ W m_ s44 exs44 _ _ m_ s46 exs46 þ Ex Q
out
m_ s45 exs45 m_ s43 exs43 ¼ _ out m_ s46 exs46 þ W
refers to Fig. 2 presenting the Aspen plus model of the designed system. The following equation is used to calculate total input of heat exergy flow rate for Cu-Cl cycle and equation (9) presents the net work rate of the designed system.
_ _ _ _ _ _ _ Ex Q CuCl ¼ ExQ B2 þ ExQ B3 þ ExQ B5 þ ExQ B6 þ ExQ B7 þ ExQ B13
(8)
_ _ _ _ _ _ _ _ net ¼ W W B2 þ WB21 þ WB25 Welec WB19 WB4 WB27 (9) The subsequent equation provided us with the total amount of heat recovered from the successive condensers.
_ _ _ Ex Q heat ¼ ExQ cond1 þ ExQ cond2 The overall energy and exergy efficiencies are written as
(10)
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Table 3 State point table of the designed system considering exergy analysis. State point
Phase
Substance
T ( C)
P (bar)
m_ (kg/s)
h (kJ/kg)
ex (kJ/kg)
_ (kW) Ex
S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S13 S14 S16 S17 S18 S20 S21 S22 S23 S24 S27 S28 S29 S30 S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46
Vapor Vapor Liquid Vapor Liquid Vapor Mixed Liquid Vapor Vapor Vapor Mixed Liquid Vapor Liquid Vapor Mixed Vapor Mixed Mixed Vapor Mixed Vapor Liquid Liquid Liquid Liquid Vapor Vapor Vapor Liquid Liquid Vapor Vapor Vapor Vapor Liquid Vapor Mixed Liquid
Flue gas Flue gas Water Water Water Water Water Water Water Water Water Cu2OCl2 þ HCl Cu2OCl2 HCl Cu2OCl2 H2O CuCl þ O2 Oxygen CuCl CuCl HCl CuCl2 þ H2 Hydrogen CuCl2 CuCl2 CuCl2 Water Air Air Air Hot water Hot water Air Nitrogen Oxygen Ammonia Water Water Water Water
1300.0 600.0 25.0 550.0 27.5 438.9 45.8 25.0 550.0 550.0 550.0 400.0 400.0 400.0 500.0 550.0 500.0 500.0 500.0 430.0 25.0 25.0 25.0 25.0 80.0 80.0 80.0 25.0 296.4 25.0 25.0 39.6 50.9 50.9 50.9 25.0 25.5 271.5 45.8 25.0
1.0 1.0 1.0 1.0 50.0 50.0 0.1 0.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 6.0 6.0 1.0 1.0 1.3 1.3 1.3 1.0 6.0 6.0 0.1 0.1
16.0 16.0 0.0 0.0 4.0 4.0 4.0 4.0 0.0 0.0 0.4 5.6 4.2 1.4 4.2 0.4 4.5 0.3 4.2 4.2 1.4 5.6 0.0 5.6 5.6 5.2 0.4 0.2 0.2 0.2 1.1 1.1 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.2
1420.9 602.2 15864.3 12373.7 15853.7 12610.1 13465.7 15864.3 15864.3 15864.3 12373.7 1901.4 1789.5 2229.9 1791.6 12373.7 490.2 466.5 561.4 617.5 2531.8 699.6 0.0 708.8 645.6 358.6 15634.3 0.0 278.7 0.0 15864.3 15803.1 76.7 78.9 69.3 2695.0 15862.3 12950.8 13461.4 15864.3
883.1 268.8 0.0 926.7 0.1 1320.5 154.0 0.0 926.7 926.7 926.1 25.1 2.7 105.8 3.9 926.1 425.7 189.6 453.8 402.9 0.9 0.1 16.2 0.0 254.6 273.0 19.7 1.1 233.7 152.8 0.0 1.5 33.2 34.2 30.0 1.9 0.0 850.0 153.7 0.0
14129.6 4300.0 0.0 33.4 0.4 5282.2 616.2 0.0 16.7 16.7 325.3 140.2 11.4 150.5 16.4 325.3 1925.8 59.1 1911.6 1697.1 1.3 0.6 0.6 0.0 1424.6 1431.5 6.9 0.3 55.5 36.3 0.0 1.7 7.9 6.2 1.7 0.4 0.0 170.0 30.7 0.0
_ net _ O2 hO2 þ m _ S37 ðhS38 hS37 Þþ Q_ heating þ W _ NH3 LHVNH3 þ m m hov ¼ _ _ Q þW in
electrolyzer
(11)
jov ¼
_ _ _ O2 exO2 þ m _ S27 ðexS28 exS27 Þþ Ex _ NH3 exNH3 þ m m Q heat þ Wnet _Ex _ Q in
(12)
rate in overall system is linked with heat exchanger B1, in Cu-Cl cycle, it is calculated in thermolysis reactor while in cryogenic ASU, the maximum exergy destruction rate is carried out by ASU turbine and in Rankine cycle, turbine gives the highest exergy destruction rate and one reason is the high temperature difference. The effect of turbine B2 discharge pressure is plotted against the turbine output temperature and turbine work in Fig. 4. This steam Rankine cycle is one of the major subsystems within the proposed system, thus, investigation of the power produced by steam turbine is significant for this study. This steam Rankine cycle is investigated under different operating conditions to observe the operational
4. Results and discussion A new ammonia synthesizing approach is employed in this paper. The slag from the cement industry is employed as the heat source which has a very high temperature of 1200e1600 C when released and offers the heat recovery potential. Ammonia, oxygen, hot water and electricity are the major commodities carried out of the system. The designed system comprises of four major subsystems; thermochemical Cu-Cl cycle for clean hydrogen production, cryogenic ASU for nitrogen separation, ammonia synthesis reactor and Rankine cycle. Numerous parametric studies are directed to investigate and reveal the system performance. The exergy destruction rates of the major components are calculated and arranged in Table 4. The topmost exergy destruction
Table 4 Exergy destruction rates associated with major components. Exergy destruction rate
Unit (kW)
B1 (Heat exchanger) B2 (Steam Rankine cycle turbine) B7 (Hydrolysis reactor) B10 (Thermolysis reactor) B14 (Electrolysis reactor) B18 (Dryer) B19 (ASU compressor) B20 (Heat exchanger) B21 (ASU turbine) B23 (Ammonia synthesis reactor) B24 (Rankine cycle turbine)
4514.4 1244.0 1616.7 7454.1 2768.9 1424.6 10.4 17.5 46.4 6.0 37.1
1198
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Fig. 4. Turbine discharge pressure vs turbine discharge temperature and turbine work.
parameters. The graphical representation shows that turbine output temperature rises with an increase in turbine discharge pressure and turbine work decreases simultaneously at the same time. The cryogenic air separation unit is a major component which is investigated in detail under different flow rates of input air, as
nitrogen is supplied to the ammonia synthesis reactor by cryogenic air separation unit. The flow rate of nitrogen and oxygen by the cryogenic ASU is depicted in Fig. 5. The cryogenic air separation unit is taken as the source of nitrogen for the designed system which is directly affected by the air flow rate. The graphical representation gives the variation in nitrogen and oxygen flow rate with change in
Fig. 5. Air flow rate vs nitrogen and oxygen flow rate.
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Fig. 6. Variation in ammonia reactor heat rate, Rankine cycle turbine power and ammonia production temperature by the change in air inlet flow rate.
air flow rate. At 29.6 kmol/h of air flow rate, 6.5 mol/s of nitrogen and 1.7 mol/s of oxygen is separated. A new approach for ammonia synthesis is introduced in this study which is mandatory to be investigated comprehensively. Flow rate of inlet air is investigated against the heat rate of ammonia reactor, ammonia production temperature and steam Rankine cycle employed by the heat recovery from ammonia synthesis. The effect of air flow rate in cryogenic ASU is studied against the ammonia reactor heat rate, Rankine cycle turbine power and
ammonia production temperature in Fig. 6. The graphical representation depicts that ammonia reactor heat rate and Rankine cycle turbine power rises rapidly with increase in air flow rate while ammonia production temperature remains constant till the time to reach the required flow rate of components and once it is established, it also starts rising with other parameters. The conversion ratio of ammonia reactor is plotted against the reactor heat rate, turbine power and ammonia flow rate in Fig. 7. The significance of this study is to investigate the novel integration
50 45
600
40
500
35 30
400
25
300
20 15
200
10
100
5
0
0
0.3
0.5
0.7
0.9
Conversion ratio of ammonia reactor Heat duty
Turbine power
Ammonia flow rate
Fig. 7. Ammonia reactor conversion ratio effect on the reactor heat rate, turbine power and ammonia flow rate.
Ammonia flow rate (kmol/h)
Heat rate and turbine power (kW)
700
1200
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of ammonia synthesis reactor under different operating conditions. The conversion rate of ammonia synthesis depends upon several parameters like operating temperature, pressure and catalyst, thus, conversion rate is studied against the heat duty, turbine power and ammonia flow rate. Different conversion rates are considered in ammonia reactor from 0.3 to 1 and its effect is studied on other parameters like turbine power, heat rate and ammonia flow rate. Figure exhibits that with the rise in conversion rate, unreacted products are reduced which results in increased ammonia flow rate, increased heat rate associated with ammonia reactor and increased turbine power. Gibbs free energy of pure components associated with the stream numbers is tabulated in Table 5. The purpose of this parametric study is to investigate the room for heat recovery from the heat exchangers within the CuCl cycle. The electrolysis reactor is set to operate at 25 C temperature, thus, both inlet streams to the electrolysis reactor are processed through the heaters to recover the available heat. Fig. 8 exhibits the water flow rate effect on the heat recovered by the heat exchangers B13 and B15. Heat is covered from the B13 heat exchanger as soon as it comes out of the thermolysis reactor and reaches to B13 so it has an increasing pattern while for heat exchanger B15, heat duty decreases first to achieve the reaction condition and then starts increasing. This parametric study investigates the effects of compressor pressure employed to the cryogenic air separation unit on the work
rates of compressor and turbine and turbine inlet temperature. This study is significant to be investigated as cryogenic air separation unit is one of the major subsystems in this proposed study. The compressor pressure is varied in order to see its effect of the subsequent parameters. Fig. 9 exhibits the effect of compressor pressure on compressor work consumption, compressor output stream temperature and turbine power. The results revealed that rise in compressor pressure causes an increase in compressor work consumption, compressor output stream temperature and turbine power. The solid line represents the turbine power, narrow dotted line exhibits compressor work consumption and wide dotted line presents compressor output stream temperature. At the compressor pressure of 6 bar, the compressor work consumption is 66.1 kW, 39.6 C is the output stream S38 temperature and 18.2 kW is the turbine power. Fig. 10 shows the turbine discharge pressure effect on the turbine power and outlet stream temperature. This particular study becomes important for the fact that the turbine employed to the cryogenic air separation unit is set to cover some amount of work required by the compressor. This study investigates the effect of turbine discharge pressure effect on the turbine exhaust temperature and power. The solid line is representing the turbine B21 power and a dotted line shows the turbine output stream S39 temperature. The graphical representation depicts that with the rise in turbine B21 pressure, working fluid is not expanded to the
Table 5 Gibbs free energy of pure compounds in each stream. Unit
H2O
H2
O2
HCl
CuCl
CuCl2
Cu2OCl2
Air
N2
NH3
kJ/mol S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S13 S14 S16 S17 S18 S20 S21 S22 S23 S24 S27 S28 S29 S30 S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46
41.0 10.8 237.2 216.1 236.7 194.2 233.8 237.2 216.1 216.1 216.1 89.5
559.4 1236.5
104.2 1363.3 216.1 192.4
17.7
23.2 8.1
188.3 194.1
22.3 17.1 95.3
235.0
10.1
17.7 0.0
237.1 228.5
123.8 199.0 199.0
228.5 0.0 5.5 4.4
0.0 5.6 4.4
0.2
0.2 0.2
237.1 234.8
0.2 0.0 237.1 212.3 233.8 237.1
16.4
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Fig. 8. Heat recovery is depicted against the water flow rate (for details, see Ref. [20]).
Fig. 9. Compressor pressure effect on turbine and compressor work and stream S38 temperature.
1201
1202
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Fig. 10. Turbine discharge pressure effect on turbine power and output stream temperature.
desired pressure which results in reduced turbine power and increased output temperature. The effect of turbine B21 discharge pressure is plotted against the stream S40 temperature, turbine B25 power and ammonia reactor heat rate in Fig. 11. This study investigates the operating parameters for three major components distillation column, ammonia synthesis reactor and steam turbine. The exit stream temperature for the distillation column, heat released by the ammonia synthesis reactor and power produced by the steam
turbine are investigated against the turbine discharge temperature. The blue line shows the stream S40 temperature, grey line represents the ammonia reactor heat rate and third colored line signifies the turbine B25 power production. As the same stream from the turbine turbine B21 passes through the ammonia reactor and Rankine cycle, so it effects reactor heat rate and turbine power as well. This study shows that with the rise in turbine B21 discharge pressure, all three parameters stream S40 temperature, ammonia reactor heat rate and turbine B25 power production increases.
Fig. 11. Effect of turbine discharge pressure on stream S30 temperature, ammonia reactor heat rate and turbine B21 power.
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Fig. 12. A comparative study of the CuCl cycle based integrated systems with the proposed integrated system.
4.1. System validation A comparative study is conducted considering the different studies based on the integrated systems including the CuCl cycle and presented in Fig. 12. Al-Zareer et al. [21] designed a nuclear energy based integrated system for hydrogen production and achieved the exergy efficiency of 40.6%. Ishaq et al. [20] conducted a study on the integration of industrial waste heat with the CuCl cycle for multiple commodities and the exergy efficiency was concluded as 31.8%. DinAli and Dincer [22] and Zamfirescu and Dincer [23] proposed different configurations for a solar based integrated system including the CuCl cycle and achieved the exergy efficiencies of 55.2% and 20%. Some of these studies achieved higher exergy efficiencies as compared to the proposed system but these studies employed nuclear and renewable energy while this proposed study utilized industrial waste heat and achieved competitive efficiency. Wu et al. [24] published a study on the CuCl cycle based system for multigeneration purpose and the exergy efficiency was found to be 31.97%. 5. Conclusions A novel idea of ammonia production utilizing the industrial waste heat is presented in this paper. The quenchable useful outputs produced by the proposed system includes ammonia, oxygen, power, heating and hot water. For hydrogen, the Cu-Cl cycle is employed utilizing the waste heat from cement slag while for nitrogen, cryogenic ASU is installed. Hydrogen and nitrogen are brought together in the ammonia synthesis reactor. Ammonia synthesis is an exothermic process carried out on ambient conditions. Ammonia reactor is also employed with a number of conversion ratios to investigate of effect on ammonia production rate and heat released. Exergy destruction of all major components is tabulated in a separate table and exergy flow rates of each stream are a plotted is graph. The Gibbs free energy of pure compound associated with each stream is also arranged in a table. The
hydrogen production rate of the designed system is 19.5 mol/s and the ammonia production rate is carried out as 13 mol/s. Steam Rankine cycle electric power is finalized as 3433 kW, and hence overall energetic and exergetic efficiencies are found to be 30.1% and 36.1%. Nomenclature B _ En ex _ Ex
Block number energy rate (kW) specific exergy (kJ/kg) exergy rate (kW) specific enthalpy (kJ/kg) mass flow rate (kg/s) molar flow rate (mol/s) pressure (kPa) ambient pressure lower heating value (kJ/kg) Stream number specific entropy (kJ/kg K) temperature ( C) ambient temperature
h _ m N P P0 LHV S s T T0 Q_
heat rate (kW)
_ W
power or work rate (kW)
Greek letters h energy efficiency j exergy efficiency Subscripts 0 ch e i d
reference conditions chemical exit input destruction
1204
NH3 O2 ov W ph
H. Ishaq, I. Dincer / Energy 179 (2019) 1191e1204
Ammonia Oxygen overall work physical
Acronyms ASU Air separation unit CERL Clean energy research lab Cu-Cl Copper-chlorine cycle References [1] Dincer I, Zamfirescu C. Sustainable energy systems and applications. New York Dordrecht Heidelberg London: Springer; 2011. https://doi.org/10.1007/978-0387-95861-3. [2] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. Amsterdam, Netherlands: Elsevier; 2013. [3] Ishaq H, Dincer I, Naterer GF. Performance investigation of an integrated wind energy system for co-generation of power and hydrogen. Int J Hydrogen Energy 2018;43:9153e64. [4] Siddiqui O, Dincer I. A review and comparative assessment of direct ammonia fuel cells. Therm. Sci. Eng. Prog. 2018;5:568e78. [5] Rabah AA, Khalel ZAM, Rabah AA, Barakat TAM. A new cryogenic air separation process with flash separator a new cryogenic air separation process with flash separator. 2013. https://doi.org/10.1155/2013/253437. [6] Aneke M, Wang M. Potential for improving the energy efficiency of cryogenic air separation unit (ASU) using binary heat recovery cycles. Appl Therm Eng 2015;81:223e31. [7] Barati M, Esfahani S, Utigard TA. Energy recovery from high temperature slags. Energy 2011;36:5440e9. [8] Arzbaecher C, Fouche E, Parmenter K, Partners GE. Industrial waste-heat Recovery : benefits and recent advancements in technology and applications definition of waste heat for this paper quantity. Quality and Temporal Availability of Waste Heat Heat-Recovery Potential in US Manufacturing Industry 2007;1e13. [9] Qin Y, Lv X, Bai C, Qiu G, Chen P. Waste heat recovery from blast furnace slag by chemical reactions. JOM (J Occup Med) 2012;64:997e1001.
[10] Tafone A, Dal Magro F, Romagnoli A. Integrating an oxygen enriched waste to energy plant with cryogenic engines and Air Separation Unit: technical, economic and environmental analysis. Appl Energy 2018;231:423e32. [11] Barbooti MM, Al-Ani RR. The copper-chlorine thermochemical cycle of water splitting for hydrogen production. Thermochim Acta 1984;78:275e84. [12] AVERY W. A role for ammonia in the hydrogen economy. Int J Hydrogen Energy 1988;13:761e73. [13] Naterer GF, Suppiah S, Stolberg L, Lewis M, Wang Z, Rosen MA, Dincer I, Gabriel K, Odukoya A, Secnik E, Easton EB, Papangelakis V. Progress in thermochemical hydrogen production with the copper-chlorine cycle. Int J Hydrogen Energy 2015;40:6283e95. [14] Gao L, Wu H, Jin H, Yang M. System study of combined cooling, heating and power system for eco-industrial parks. Int J Energy Res 2008;32:1107e18. [15] Dincer I, Acar C. A review on clean energy solutions for better sustainability. Int J Energy Res 2015;39:585e606. [16] Giddey S, Badwal SPS, Kulkarni A. Review of electrochemical ammonia production technologies and materials. Int J Hydrogen Energy 2013;38: 14576e94. [17] Ishaq H, Dincer I. A comparative evaluation of three Cu-Cl cycles for hydrogen production. Int J Hydrogen Energy 2019;44:7958e68. [18] J.M.C.. Gas conditioning and processing: the equipment modules. Campbell Petroleum Series 1984;2. €fer H. Rotordynamics of automotive turbochargers. New York [19] Nguyen-Scha Dordrecht Heidelberg London: Rotordynamics Automot. Turbochargers. Springer; 2012. https://doi.org/10.1007/978-3-642-27518-0. [20] Ishaq H, Dincer I, Naterer GF. Exergy and cost analyses of waste heat recovery from furnace cement slag for clean hydrogen production. Energy 2019. https://doi.org/10.1016/j.energy.2019.02.026. [21] Al-Zareer M, Dincer I, Rosen MA. Assessment and analysis of hydrogen and electricity production from a Generation IV lead-cooled nuclear reactor integrated with a copper-chlorine thermochemical cycle. Int J Energy Res 2018;42:91e103. [22] DinAli MN, Dincer I. Development of a new trigenerational integrated system for dimethyl-ether, electricity and fresh water production. Energy Convers Manag 2019;185:850e65. [23] Zamfirescu C, Dincer I. Assessment of a new integrated solar energy system for hydrogen production. Sol Energy 2014;107:700e13. [24] Wu W, Hsu FT, Chen HY. Design and energy evaluation of a stand-alone copper-chlorine (Cu-Cl) thermochemical cycle system for trigeneration of electricity, hydrogen, and oxygen. Int J Energy Res 2018;42:830e42.