Waste Heat Recovery from Aluminium Production

Miao Yu

Thesis of 60 ECTS credits Master of Science in Sustainable Energy Engineering

January 2018

Miao Yu

Thesis of 60 ECTS credits submitted to the School of Science and Engineering at Reykjavík University in partial fulﬁllment of the requirements for the degree of Master of Science in Sustainable Energy Engineering

January 2018 Supervisor(s): Dr. Gudrun Saevarsdottir Professor, Reykjavík University, Iceland

Dr. Maria S. Gudjonsdottir Assistant Professor, Reykjavík University, Iceland

Dr. Pall Valdimarsson Adjunct Professor, Reykjavík University, Iceland

Examiner(s): Mr. Gestur Valgarðsson Engineer, EFLA Engineering company, Iceland

ABSTRACT

Around half of the energy consumed in aluminum production is lost as waste heat. Approximately 30-45% of the total waste heat is carried away by the exhaust gas from the smelter which is the most easily accessible waste heat stream. Alcoa Fjardaal in east Iceland produces 350 000 tons annually, emitting the 110 °C exhaust gas with 88.1 MW of heat. This work concentrates on creating and comparing models of low-temperature energy utilization from the aluminum production. Three scenarios, including organic Rankine cycle (ORC) system for electric power production, heat supply system and combined heat and power (CHP) system, were proposed to recover waste heat from the exhaust gas. The electric power generation potential is estimated by ORC models. The maximum power output was found to be is 2.57 MW for an evaporation temperature of 61.22°C and R-123 as working fluid. 42.34 MW can be produced by the heat supply system with the same temperature drop of the exhaust gas in the ORC system. The heat requirement to supply heat to a district heating system for the local community can be fulfilled by the heat supply system. There is a potential opportunity for agriculture, snow melting and other industrial applications with the heat supply system. The CHP system is more comprehensive. 1.156 MW power and 23.55MW heating capacity can be produced by CHP system. The highest energy efficiency is achieved by the heat supply system and the maximum power output can be obtained with the ORC system. In the power generation system, the efficiency of organic Rankine cycle is significantly limited by the low temperature of the exhaust gas. It would be possible to improve the efficiency of the ORC by increasing the heat source temperature. The temperature of the heat source can be raised by reducing the dilution air from the environment into the pot. The heat balance of the smelter is studied. The positive result is found, that the efficiency of the cycle and power output are effectively improved, and a good trend of the system performance can be observed. The maximum 9.174 MW power can be produced at the exhaust gas temperature of 150 °C, and the system exergy efficiency of 54.38% can be achieved in these conditions. This work shows there is a good potential for heat and power production by recovering the waste heat from aluminum production. The efficiency of energy utilization in aluminum production can be effectively improved as studied. KEYWORDS: Low-temperature energy recovery; Aluminum production; Organic Rankine cycle; Combined heat and power; waste heat recovery

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ABSTRAKT

Um það bil helmingur orku sem notuð er í framleiðslu áls í álverum tapast sem afgangsvarmi. U.þ.b. 30-45% af þessum glatvarma er í gasútblástri sem jafnframt er aðgengilegasta varmaflæðið frá álveri. Alcoa Fjardaal á austurhluta Íslands framleiðir 350.000 tonn af áli árlega og flytur frá sér um 110 °C heitu gasi sem samsvarar 88,1 MW af varma. Í þessu verkefni, voru gerð líkön og samaburður fyrir lághitakerfi sem geta nýtt afgangsvarma frá álverum. Þjár sviðsmyndir voru skoðaðar: Rafmagnsframleiðsla með Organic Rankine vinnuhring (ORC), heitavatnsframleiðsla, og samtíma framleiðsla vatns og rafmagns. (CHP) Hægt er að framleiða að hámarki 2.57 MW af raforku við uppgufunarhitastig 61.22 °C og R-123 sem vinnuvökva í ORC vinnuhring. Heitavatnsframleiðsla nýtir orkuna betur þar sem það nær 42.34 MW af varma úr gufuni með sama hitastigsmun og ORC kefið. Þetta orkumagn getur auðveldlega annað hitaveitueftirspurn í næsta bæjafélagi með möguleika á að umfram magn gæti nýst í hitun gróðurhúsa, snjóbræðslu, og í öðrum iðnaði. CHP kerfið er meira alhliða þar sem það gefur frá sér bæði heitt vatn og rafmagn. Það gefur frá sér að hámarki 1.156 MW af rafmagni og 23.55MW af heitu vatni við hagkvæmustu skilyrði. Þegar þessar þrjár sviðsmyndir eru bornar saman, er orkunýtnin best í heitavatnsframleiðslu og mesta raforkuframleiðslan í ORC kerfinu. Nýtnin á ORC kerfinu takmarkast mest af hitastigi útblástursgassins en það væri hægt að auka nýtni þess með því að hækka hita útblástursins, sem er gert með því að minnka þynningu afgassins úr kerskálunum með lofti. Þegar þessari aðferð er beitt, eykst nýtni kerfisins og orkuafköstin hækka. Rafmagnsframleiðsla væri um 9.174 MW þegar útblástursgasið er við 150 °C, og exergíunýtnin væri 54.38%. Við þessar aðstæður geta komið upp áskoranir varðandi varmajafnvægi í álverinu. Þetta verkefni synir að það er möguleiki á að nýta útblástur álvers í rafmagns- og heitavatnsframleiðslu, til þess að bæta nýtingu orku í álverum.

Lykilorð: Lághita orkunýting, Álframleiðsla, organic Rankine cycle, Samtíma framleiðsla hita og rafmagns

摘要

铝生产中大约一半的能源未被利用，在这些热量之中，大约 30-45％的总废热量被冶炼厂的烟气带走，这部分能量是最容易被回收的。冰岛东部的 Alcoa Fjardaal 铝厂每年生产 35 万吨铝，其烟气的温度为 110°C，所携带的热量达到 88MW。这篇论文专注于铝生产过程中的余热回收利用技术，通过 Engineering Equation Solver (EES)软件模拟，建立数学模型。本文提出了有机朗肯循环（ORC）系统，供热系统和热电联产（CHP）系统三种方案来回收烟气中的余热。在最优工况下，ORC 系统可发电 2.57MW，其对应的最优设计参数为工质 R-123，蒸发温度 61.22°C，冷凝温度 16.33°C，蒸发器夹点温差 9.469°C，冷凝器夹点温差 3.501°C。供热系统如果采用和 ORC 系统相同的烟气温度降，可实现供热量 42. 34MW。供热系统可满足当地集中供热需求，农业，融雪等工业应用潜力巨大。热电联产系统更为均衡全面,在最优工况下可生产 1.156MW 的电力和 23.55MW 的供热量。在发电系统中，有机朗肯循环的效率受到排气温度的限制。提高 ORC 效率的有效措施是提高热源温度。电解铝槽会吸收大量的空气进入电解槽中，这些吸入的空气降低了铝槽的烟温。通过减少空气吸入量来升高热源的温度。结果发现，循环效率和功率输出得到有效改善，系统性能良好。在 150℃的排气温度下可以产生最大的 9.174MW 的功率输出，在这种情况下可以达到 54.38％的系统的火用效率。如若改变吸入电解槽的空气量，电解槽内部热平衡也随之改变。稀释空气较少，环境热量损失减少。为了保持热平衡并避免热量积累，电解槽的热阻布置需要改变，减少底部及壁侧热阻。在这项研究中，通过回收电解铝过程中产生的废热，铝生产过程中的能源利用效率可有效提高。关键词：低温热能利用，电解铝，有机朗肯循环，热电联产

I dedicate this work to my family.

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Acknowledgement

Primarily I wish to thank my supervisor, Professor Gudrun Saevarsdottir, for her kind support in my study and my life in Iceland. She is a serious scientist and researcher. The scientific altitude and research methods are the valuable courses she taught me. The acknowledgement I also wish to send to Maria S. Gudjonsdottir, my kind and smart supervisor. She always showed me the correct direction to solve the problems. Thanks for her patient guiding. I also would like to acknowledge my supervisor, Pall Valdimarsson. He taught me a lot, coding, plotting, writing and thinking. I liked talking to him about not only thesis and project, also everything, from the earth to the sun, from Iceland to China. The most important, I wish to gratefully thank my parents, Jinfang Yu and Fengying Wang. I could not have finished this work without their tireless efforts and supports. I would like to thank Professor Haiyan Lei and Halla Hrund Logadóttir. They assisted me coming to Iceland. Many thanks to my XiaoHuoBan in Iceland, Shu Yang, Romain Metge, TingTing Zheng, Yuxi Li, et al. Thanks for their company. Also thank Svava, Angela and Gabi. I also would like to acknowledge Alcoa Foundation for the financial support of this work. Also, acknowledgements go to Jón Steinar Garðarsson Mýrdal at Austurbrú, Hilmir Ásbjörnsson, Unnur Thorleifsdottir at Alcoa Fjardaal and Þorsteinn Sigurjónsson at Fjarðabyggð Municipal Utility for their assistance. I will never forget the wonderful and fantastic time in Iceland.

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Contents

ABSTRACT ...... iii

Acknowledgement ...... viii

Contents ...... x

List of Tables ...... xiii

List of Figures ...... xiv

Nomenclature ...... xv

1 Introduction ...... 1

1.1 Main concerns of the study ...... 1

1.2 An overview of the present work ...... 1

2 Survey of Background Literature ...... 1

2.1 Energy and environment in Iceland ...... 1

2.1.1 Geothermal resource ...... 1

2.1.2 Electricity production ...... 2

2.1.3 Power-intensive industries in Iceland ...... 3

2.1.4 Space heating...... 3

2.2 Utilization of low-temperature energy ...... 4

2.3 Power generation technologies ...... 5

2.3.1 Organic Rankine cycle ...... 5

2.3.2 Kalina cycle ...... 5

2.3.3 Thermoelectric generator(TEG) ...... 6

2.4 Direct utilization ...... 7

2.4.1 District heating ...... 7

2.4.2 Snow melting...... 8

2.4.3 Agriculture applications ...... 8

2.4.4 Fish farming ...... 9

2.4.5 Drying and dehydration industrial...... 10

3 Alcoa Aluminum Smelter ...... 11

3.1 Aluminum production...... 11

3.1.1 Bayer Process ...... 11

3.1.2 Hall-Héroult Process ...... 11

3.2 Energy balance of an aluminum reduction cell ...... 12

3.3 Cell emissions ...... 13

3.4 Heat recovery potential of exhaust gas ...... 14

3.5 Alcoa Fjardaal aluminum plant ...... 15

3.5.1 Thermal energy content ...... 15

3.5.2 Local energy demand ...... 15

3.5.3 Dew point temperature ...... 16

4 Heat Recovery System...... 18

4.1 Scenarios ...... 18

4.2 Assumptions ...... 20

4.3 Models ...... 21

4.4 Optimization of ORC...... 24

4.4.1 Objective function ...... 24

4.4.2 Design variables ...... 26

4.4.3 Flowchart ...... 29

4.5 Conditions of Alcoa smelter ...... 29

4.5.1 Heat source condition ...... 29

4.5.2 Ambient condition ...... 29

4.5.3 Heat transfer coefficient and pressure drop...... 30

4.5.4 Heating system conditions ...... 32

4.5.5 Equipment conditions ...... 32

4.6 Result ...... 32

4.6.1 Power generation system ...... 32

4.6.2 Heat supply system...... 39

4.6.3 CHP system ...... 40

4.7 Summary ...... 41

5 Power Generation with Enhanced Heat Source ...... 43

5.1 Setting dilution air flow rate ...... 43

5.2 Heat balance influence ...... 45

5.3 The optimal working performance of ORC at various conditions ...... 46

5.4 The optimal design variables in different conditions ...... 46

5.5 Summary ...... 47

6 Conclusion and Future Work ...... 48

6.1 Conclusion ...... 48

6.2 Recommendation for further research ...... 49

References ...... 50

Appendix ...... 54

List of Tables

Table 3-1 Summary of dew point temperature of the sulfuric acid ...... 17 Table 4-1 System module component ...... 18 Table 4-2 Parameters of exhaust gas ...... 29 Table 4-3 The monthly sea surface temperature in 2016 ...... 29 Table 4-4 Parameters in evaporator...... 31 Table 4-5 Parameters in preheater ...... 31 Table 4-6 Parameters in heat exchanger in CHP system...... 31 Table 4-7 Conditions of heating system ...... 32 Table 4-8 Parameters of basic equipment ...... 32 Table 4-9 Properties of various working fluids ...... 33 Table 4-10 Setting of evaporation parameter ...... 34 Table 4-11 Setting of condensation parameters ...... 35 Table 4-12 Setting of parameters in PPTD in evaporator ...... 36 Table 4-13 Setting of parameters in PPTD in condenser ...... 38 Table 4-14 Parameters in heat supply system ...... 39 Table 4-15 setting of parameters in middle temperature selection ...... 40 Table 4-16 Iteration results of optimal parameters ...... 42 Table 4-17 System performance of three scenarios ...... 42 Table 5-1 Parameters of dilution air ...... 44 Table 5-2 Summary of power generation system with an enhanced heat source . 47

xii

List of Figures

Figure 2-1 Distribution of geothermal fields in Iceland ...... 1 Figure 2-2 Installed capacity in Iceland from 1913 to 2014 ...... 2 Figure 2-3 Electricity consumption in Iceland in 2016 ...... 3 Figure 2-4 Energy consumption of space heating in Iceland from 1970 to 2013 .. 4 Figure 2-5 Sketch of an ORC system ...... 5 Figure 2-6 Sketch of a Kalina system ...... 6 Figure 2-7 Thermo-Electric method...... 7 Figure 2-8 Snow smelting system in Reykjavik, Iceland ...... 8 Figure 2-9 Heating system in greenhouse ...... 9 Figure 3-1 Flow diagram of aluminum production ...... 11 Figure 3-2 Electrolytic smelter of Hall-Héroult process ...... 12 Figure 3-3 Temperature distribution of electrolytic cell thermal model ...... 13 Figure 3-4 typical Hall-Heroult cell loss distribution ...... 14 Figure 3-5 Temperature distribution of the cell gas ...... 14 Figure 3-6 Geographical condition of Alcoa Smelter in east Iceland ...... 16 Figure 4-1 System sketch ...... 20 Figure 4-2 T-S diagram of power generation system ...... 21 Figure 4-3 Three types of working fluids: dry, isentropic and wet ...... 26 Figure 4-4 Iteration procedure of optimization ...... 28 Figure 4-5 Temperature distribution of world’s ocean ...... 30 Figure 4-6 Arrangement of heat exchanger ...... 30 Figure 4-7 Working fluid selection ...... 33 Figure 4-8 T-S diagram of R-123 ...... 33 Figure 4-9 Evaporation parameter selection ...... 35 Figure 4-10 Condensation parameter selection ...... 36 Figure 4-11 PPTD in evaporator ...... 38 Figure 4-12 PPTD in condenser ...... 39 Figure 4-13 Performance of heat supply system ...... 40 Figure 4-14: Middle gas temperature selection ...... 41 Figure 5-1 Origination of heat of exhaust gas ...... 43 Figure 5-2 Relationship between flow rate and temperature of exhaust gas ...... 44 Figure 5-3 Performance of ORC system in various conditions...... 46 Figure 5-4 Optimal design variables in various conditions ...... 46

xiii

Nomenclature

Letters c heat capacity kJ/kg·K Ex exergy kW g gravitational acceleration 9.81 m/s2 h specific enthalpy kJ/kg m mass flow rate kg/s P pressure bar Q heat transfer rate kW S entropy kJ/kg·K T temperature °C U heat transfer coefficient kW/m2·K v specific volume m3/kg w Power kW

Greek letters

Δ difference η efficiency ρ density Kg/m3 λ thermal conductivity W/K·m φ weighting factor of objective function ψ weighting factor of objective function

Subscripts amb ambient cw cooling water cond condenser eva evaporator fan fan hex heat exchanger gas state of gas gen generator gral general xiv

mid middle net net amount pin pinch point pre preheater sup superheater sys system turb turbine wf working fluid

Abbreviations

EES Engineering Equation Solver GTC Gas Treatment Center KC Kalina Cycle ORC Organic Rankine Cycle TEM Thermo-Electric Method TEG Thermoelectric Generator

Superscripts

′ ideal state

CHPATER 1 Introduction

Chapter 1 Introduction

1.1 Study outline

This work concentrates on comparing options for low-temperature energy utilization from the aluminum production using modelling as a tool. Reasonable and effective utilization of low-temperature energy from the pot in aluminum production is the central issue in the present work. Power generation and direct utilization of heat are the main approaches to utilize the low-temperature energy. Therefore, the emphasis is on the design and optimization of 1. an ORC system 2. a heat supply system 3. a combined heat and power (CHP) system, which are matching with the heat source of the aluminum production and meeting the local energy requirement. As an option to improve efficiency, modification of the heat source was proposed by reducing the dilution air and thus raising the temperature of pot gas.

1.2 An overview of the present work

This study is about the mathematical models of the power generation system, heat supply system and CHP system. A background literature survey is offered in Chapter 2. In Chapter 3, the process of aluminum production is described. The possibility and potential to utilize the heat from exhaust gas are analyzed. The emphasis is on the heat balance of the pot and the chemical components of the exhaust gas. The heat content of the exhaust gas is calculated. In Chapter 4, the mathematical models and the optimization methods of the power generation system, heat supply system and CHP system are proposed. The multi- objective function used for optimization is described. Four design variables are selected. The optimal design variables and the system performance of each system are pointed out. In Chapter 5, The optimal working performance and the design variables are figured with different conditions the temperature of the exhaust gas is raised by controlling the flow rate of dilution air into the pot. The heat balance influence is discussed. The conclusion of this work is outlined in Chapter 6 together with a recommendation for the future work.

CHPATER 1 Introduction

Chapter 2 Survey of Background Literature

2.1 Energy in Iceland

Iceland was one of Europe's energy-starved countries a hundred years ago. Peat and imported coal were the main energy sources in Iceland. At present, Iceland is a country with a high standard of living and the majority of energy is from renewable resources. In 2014, around 85% of primary energy consumption is from local renewable resources in Iceland. 66% of this energy is geothermal [1].

Figure 2-1 Distribution of geothermal fields in Iceland [1]

2.1.1 Geothermal resource

Geologically, Iceland is a young country. One of the earth's major fault lines, the Mid-Atlantic Ridge, crosses Iceland. This ridge is the boundary between the North American and Eurasian tectonic plates. The two plates are breaking apart at a rate of around 2 cm/year [1]. Iceland is an anomalous part of the ridge. The deep mantle material lifts up and unusually creates a hot spot of great volcanic productivity. This makes Iceland a geothermally active place where an active spreading ridge above sea level can be observed. Because of the location of Iceland, it is one of the most tectonically active places, resulting in a considerable number of volcanoes and hot springs. Earthquakes are frequent in Iceland but rarely cause serious damage. In the active volcanic zone, more than 200 volcanoes are located. This volcanic zone is shown in Fig. 2-1, which

CHPATER 2 Survey of Background Literature

stretches through the country from the southwest to the northeast. Since the country was settled, more than 30 of them have erupted. There are more than 20 known high-temperature areas in this volcanic zone containing steam fields with reservoir temperatures around 250°C within 1 km depth. The active volcanic systems are directly linked with these areas. About 250 separate low-temperature areas, whose temperatures are not exceeding 150°C at the depth of 1 km, are found mostly on the sides of the active zone and there are more than 600 hot springs have been located, whose temperature is over 20°C.

2.1.2 Electricity production

Majority of electrical energy in Iceland is produced by renewable energy resources, including hydropower of 75.5% and geothermal energy of 24.5%. Only the islands of Grimsey and Flatey are not connected to the national grid and their electricity is produced using diesel generators.

Figure 2-2 Installed capacity in Iceland from 1913 to 2014 [2] All power stations larger than 1 MW have to be connected to the national grid. Some smaller station owners sell electricity into the grid. Power stations connected to the national grid are shown in Fig. 2-2. Landsvirkjun is the National Power Company and the largest producer of electricity in Iceland. Landsvirkjun produces amount to 12 469 GWh corresponding to 75% of the total. Reykjavik Energy is following Landsvirkjun and it produces 2138 GWh corresponding to 12% of the total. The third company is HS Orka, which produces 1431 GWh corresponding or 9% of the total national production.

CHPATER 2 Survey of Background Literature

2.1.3 Power-intensive industries in Iceland

In 2016, the total electricity of 18 060 GWh was consumed in Iceland. 12 595 GWh of electricity was consumed by aluminum industrial. The requirement of electricity in Iceland increases considerably, owing to the rapid expansion of energy-intensive industry. Fig. 2-3 shows electricity consumption in Iceland in 2016. The figure indicates that the use of electricity for aluminum production accounts for a majority of the consumption. Around 80% of all electricity produced was used by the power intensive industry in Iceland. The residential use only accounts for 4.6% of total electricity.

Figure 2-3 Electricity consumption in Iceland in 2016 [3]

2.1.4 Space heating

From the 1950s, the space heating system has been developed in Iceland. Nowadays around 89% of Icelandic homes are connected to a district heating system driven by geothermal energy. Therefore, the price of space heating is generally low and the dependence on imported energy is thus reduced. 47 years ago, more than half of energy used for space heating was oil. At present, only around 1% of energy supplying heat is oil. Geothermal energy accounts for near 89 % and keeps growing. The remaining part is electric heating, as shown in Fig. 2- 4.

CHPATER 2 Survey of Background Literature

Figure 2-4 Energy consumption of space heating in Iceland from 1970 to 2013 [4] 2.2 Utilization of low-temperature energy

Most of the heat recovery systems are designed at medium temperature (200–350 °C) and high temperature (350–500 °C) levels. In general, low-temperature heat energy is in the temperature range of 100–150 °C [5]. It contains several different kinds of energy including solar energy, geothermal energy, biomass or waste heat from industrial processes, etc. In Iceland, more than 250 low-temperature geothermal area have been identified and over 2/3 electricity production was consumed by power-intensive industries, such as silica and aluminum industries, which generate a considerable amount of low- temperature waste heat [6]. The potentiality of using low-temperature energy is enormous in Iceland. It is a challenging work to utilize low-temperature thermal energy compared with high-temperature energy. The power generation and direct heat utilization are two directions to utilize the low-temperature thermal energy. It is hard to drive the steam Rankine cycle by the low-temperature heat source, because of the high boiling temperature of water. The technologies of organic Rankine cycle (ORC), Kalina cycle and Thermo-Electric Method (TEM) provide the possibility to generate power from low-temperature heat source [7]. For direct thermal utilization, a number of approaches can be taken to utilize low-temperature energy, including district heating, snow melting, fish farming, greenhouses and some industrial use[8]. From Kiyarash Rahbar [9], all over the world approximately half of the energy is waste owing to the limitation of energy conversion processes.

CHPATER 2 Survey of Background Literature

2.3 Power generation technologies

2.3.1 Organic Rankine cycle

An organic Rankine cycle is a Rankine cycle using an organic fluid as a working fluid to instead of steam. The organic fluids have lower boiling temperatures and make it possible to convert low-temperature heat to power, a process from low-grade energy to high-grade energy. Energy from various kinds of heat sources can be utilized by the ORC system, such as solar energy, biomass energy, geothermal energy and waste heat from industry [10-15].

21 T 4 3 5 2 5 evaporator 3 4 1 turbine generator 1s 6 pump 6s 8 7 3 condenser 2

S (a) Systemic sketch (b) Temperature-entropy diagram Figure 2-5 An ORC system The working fluid is the main difference between ORC and normal Rankine cycle. The unique feature of ORC system is the organic working fluid, comparing with the conventional Rankine cycle. These organic materials always have the property of low-boiling temperature, which makes the low-temperature energy can be absorbed by the boiling process [16]. The ORC system consists of four main components: an evaporator, a turbine or an expander, a condenser, and a pump as shown in Fig. 2-5. The organic working fluid absorbs low-temperature heat in the evaporator, releases heat in the condenser, outputs the power in the turbine or the expander and is compressed in the pump [10], shown in Fig. 2-5.

2.3.2 Kalina cycle

The unfixed evaporation temperature is a feature of the zeotropic working fluid. The mixed working fluid of water and ammonia is used in Kalina cycle, which is a typical binary cycle [17-22]. A number of Kalina cycle systems were designed from the early1980s. KCS 34g is a typical kind of Kalina cycle, which is suitable for a low- temperature heat source, below around 120 °C [23].

CHPATER 2 Survey of Background Literature

separator generator

turbine

evaporator

condenser

Recuperator pump

Figure 2-6 Sketch of a Kalina system The mixture experiences five processes in a cycle: adiabatic compression in the pump, isobaric heating in the recuperator and evaporator, separation in the separator, expansion in turbine and then isobaric cooling in the condenser. Comparing with the ORC, an additional component is a separator and regenerator in this Kalina cycle (KCS 34g), shown in Fig. 2-6. In a Kalina cycle, the ammonia evaporates before the water when the working fluid is heated. A small temperature gap in the heat exchanger is enabled by the evaporation with raised saturation evaporation, because the ammonia concentration drops in the liquid [23]. The saturated vapor and liquid are separated in the separator. Low ammonia concentration is contained in the liquid. The vapor expands in the turbine and generates power in the turbine [24]. A number of advantages are attributed to the Kalina cycle because the mixture of ammonia-water is used as working fluid. It offers good heat transfer coefficients and is environmentally friendly. The fluid circulated is around half of ORC. The irreversibility of heat exchange is relatively small due to a small temperature difference during the heat transfer, owing to the varying evaporation temperature.

CHPATER 2 Survey of Background Literature

2.3.3 Thermoelectric generator(TEG)

(a) Peltier and Seebeck effect of TEG for cooling and power generation

(b) thermoelectric generator with three coupled devices Figure 2-7 Thermo-Electric method. [7] TEG is used to recover low-temperature energy in various applications. The thermal energy can be converted into electricity by the thermoelectric generator with Seebeck effect [25-31], that the electromotive force can be conducted with a temperature differential between hot and cold ends, shown in Fig. 2-7. The temperature difference influences the magnitude of potential difference Δ푉 [27].

2.4 Direct heat utilization

Comparing with power generation technologies by low-temperature energy, the high efficiency is an advantage of direct utilization, because the efficiency of power cycle cannot exceed the efficiency of Carnot cycle. The major field of direct utilization is space heating and cooling, including district heating, agriculture applications, industrial use, snow melting and swimming pool. The heat source temperatures from 50 to 100°C is acceptable for district heating. The temperature in the range of 25 to 90°C can be utilized for agriculture and aquaculture. The higher temperature is required for the cooling and industrial processing [32].

CHPATER 2 Survey of Background Literature

2.4.1 District heating

A heat balance of the room is maintained by the district heating system, to keep the temperature of the room in a stable condition. The heat supply from the district heating system should be basically equal to the heat loss from the room to ambient. The district heating system can be powered by various kinds energy, such as geothermal, solar energy or industrial waste heat. [33-41]. For the waste-heat district heating system, there is a loop connecting the consumers and heating station. Water cycles in the closed loop to deliver the heat. For the system using geothermal energy as a heat source, it has only a supply network, and the consumers dispose of the heat carrying water after it has been used [42].

2.4.2 Snow melting

Slippery pavement and streets due to ice formation or snow accumulation is an important problem in many northern countries, including Iceland. Melting snow and ice by low-temperature thermal energy is a good solution for this problem. The heating pipeline is installed underground and supplies heat to the surface. These installations include sidewalks, roadways, bridges and parking space.

Figure 2-8 Snow smelting system in Reykjavik, Iceland Currently in Iceland, the snow melting system can be supplied by the return water of district heating system, whose temperature is around 35°C [8]. The system temperature can also be adjusted by mixing the return water with the incoming water, whose temperature is around 80°C, when the heat load is high. About two-thirds of

CHPATER 2 Survey of Background Literature

the energy is from return water from space heating systems in a year [8]. An example of a snow melting system in Iceland, shown in Fig. 2-8.

2.4.3 Agriculture applications

Low-temperature energy has high potential on the agriculture applications. Many agriculture applications can be proposed, such as: greenhouse heating, aquaculture, biogas generation and so on [32]. The task of greenhouse construction is to create a convenient environment for the production of out-of-season or non-local fresh vegetable and flowers products. By the artificial creation of the closed room, the optimal temperature for the plant can be supplied continuously.

Figure 2-9 Heating system in greenhouse [32] There are many kinds of heating system in the greenhouse. Classification of geothermal heating installations: a) Aerial pipe heating systems; b) Bench heating; c) Vegetative heating; d) Soil heating; e) Convectors; f) Forced convection air heaters; g) Fan-jet air heating; h) Low positioned air heating, shown in Fig. 2-9. In Iceland, the heating system for a greenhouse has been utilizing geothermal energy since 1924, and most of the greenhouses are in southern Iceland.

CHPATER 2 Survey of Background Literature

2.4.4 Fish farming

Depending on fish species, the growing period and the optimal temperature is different. In Iceland, the salmon, cod, and halibut are the most popular fish in the fish farm, which is heated by geothermal energy [8].

2.4.5 Drying and dehydration industrial

The potential of low-temperature energy utilization can be found in drying and dehydration. Comparing with traditional drying system, the only difference is that instead of using fossil fuels for heating the drying air, a water/air heat exchanger is used [32].

CHPATER 2 Survey of Background Literature

Chapter 3 Alcoa Aluminum Smelter

3.1 Aluminum production

Aluminum is extracted from ore with typical steps described in Fig. 3-1, which includes bauxite mining, bauxite refining and electrochemical reduction of alumina in the smelter. Bayer and Hall-Héroult processes are two basic processes in the aluminum production. Bauxite Alumina Smelting mining refining

Figure 3-1 Flow diagram of aluminum production

3.1.1 Bayer Process

Bauxite, containing 30-60 % alumina, is the main ore used for aluminum production. Alumina can be refined out from bauxite by Bayer process, which was invited by the Karl Josef Bayer 200 years ago. In Bayer process, impurities are removed through the following reactions.

Alumina hydrate (퐴푙2푂3 ∙ 푥퐻2푂) in the bauxite is dissolved by sodium hydroxide (푁푎푂퐻) solution.

퐴푙2푂3 ∙ 푥퐻2푂 + 2푁푎푂퐻 → 2푁푎퐴푙푂2 + (푥 + 1)퐻2푂 Then alumina hydrate precipitates in the reverse reaction.

2푁푎퐴푙푂2 + 4퐻2푂 → 퐴푙2푂3 ∙ 3퐻2푂 + 2푁푎푂퐻 The alumina is obtained by calcination of the alumina hydrate, in which the water of crystallization is taken off by heating the alumina trihydrate to 1000°C [43].

퐴푙2푂3 ∙ 3퐻2푂 → 퐴푙2푂3 + 3퐻2푂

3.1.2 Hall-Héroult Process

Alumina is reduced to aluminum in the Hall-Héroult process which was invented around 1886 by Charles Hall in America and Paul Héroult in France. The cell is shown in Fig. 3-2. In this process, alumina is dissolved in an electrolyte based on cryolite (Na3AlF6) with an addition of AlF3 to reduce the electrolyte liquid so that the process temperature is around 940-980°C. Other components are present as well in

CHPATER 3 Alcoa Aluminum Smelter smaller quantities. Currently, the mechanism of electrolysis is not fully understood, 3− most scientists agree that cryolite ionizes to form hexafluoroaluminate ( 퐴푙퐹6 ): + 3− 푁푎3퐴푙퐹6 → 3푁푎 + 퐴푙퐹6 3− − − 퐴푙퐹6 → 퐴푙퐹4 + 2퐹 4−2푛 Alumina dissolves by forming oxyfluoride ions 퐴푙2푂퐹2푛 , at low 1−푛 concentrations and oxyfluoride ions 퐴푙푂퐹푛 , at higher alumina concentrations [44]: 3− 2− − 퐴푙2푂3 + 4퐴푙퐹6 → 3퐴푙2푂퐹6 + 6퐹 3− − 퐴푙2푂3 + 퐴푙퐹6 → 3퐴푙푂퐹2 Cathodic reaction 푁푎+ + 푒− → 푁푎

3푁푎 + 퐴푙퐹3 → 퐴푙 + 3푁푎퐹 Anodic reaction 2− − 2퐴푙푂퐹3 → 2퐴푙퐹3 + 푂2 + 4푒 Overall reaction

2퐴푙2푂3 + 3퐶 → 4퐴푙 + 3퐶푂2

Figure 3-2 Electrolytic smelter of Hall-Héroult process [45] 3.2 Heat balance of an aluminum reduction cell

The solubility of 퐴푙2푂3 depends on electrolyte composition and temperature.

Alumina and Na3AlF6 form a eutectic at 960°C with 11% 퐴푙2푂3. The higher solution of 퐴푙2푂3 with higher temperature makes electrolysis better. By contrast, the material, which conducts the current to electrodes, cannot withstand a too high temperature. Industrial cells usually operate in the range 940-980°C. The melting point of aluminum is 660°C [47] and always lower than the operating temperature. The simulation of the temperature distribution in the cell is shown in Fig. 3-4.

CHPATER 3 Alcoa Aluminum Smelter

Figure 3-3 Temperature distribution of electrolytic cell thermal model[45] 3.3 Gas emissions

More than 95% of the gases collected by the cell exhaust system is originally pot room air sucked into the cell by the negative pressure under the cell hood. 5% is produced inside the cell, mainly in the bath [45]. The chemical reactions involving gaseous species that are considered in this work as of importance for the heat and mass balance are listed below: Overall reaction for Alumina reduction to produce metal:

2퐴푙2푂3 + 3퐶 → 3퐶푂2(푔) + 4퐴푙 Aluminium reoxidation reaction, causing loss of current efficiency:

3퐶푂2(푔) + 2퐴푙 → 퐴푙2푂3 + 3퐶푂(푔) Anode sulfur content is responsible for the appearance of SO2 in the cell emissions. First COS is formed in the anode at higher temperatures. Subsequently, COS is burnt under the cell hood.

퐴푙2푂3 + 3퐶 + 3푆 → 2퐴푙 + 3퐶푂푆(푔)

2퐶푂푆(푔) + 3푂2(푔) → 2퐶푂2(푔) + 2푆푂2(푔) Anode carbon air burn: Part of anode surface is exposed to air and it is hot enough to combust.

퐶 + 푂2(푔) → 퐶푂2(푔) Hydrogen fluoride evolution from bath, alumina-water content is responsible to provide hydrogen.

2퐴푙퐹3 + 3퐻2푂 → 퐴푙2푂3 + 6퐻퐹(푔) Sodium tetrafluoraluminate evolution from the bath. The water involved here comes from air humidity.

푁푎3퐴푙퐹6 + 2퐴푙퐹3 → 푁푎퐴푙퐹4(푔)

3푁푎퐴푙퐹4(푔) + 3퐻2푂(푔) → 퐴푙2푂3 + 푁푎3퐴푙퐹6 + 6퐻퐹(푔)

CHPATER 3 Alcoa Aluminum Smelter

3.4 Heat recovery potential of exhaust gas

Exhaust gas 30%

Crust 10%

Deck 7% Anode Anode Bath Sidewall 35% Aluminum

Cathode Block Cathode 5%

Bottom 7% Figure 3-4 Typical Hall-Heroult cell heat loss distribution Around 30-45%[48] of the total waste heat is carried away by the exhaust gas which is drawn from the cell. The other waste heat is the conduction by sidewall around 35%, by cathode rods around 5%, bottom around 7%, deck around 7% and crust around 10%, shown in Fig. 3-5. Developing utilization of waste heat is an opportunity and development for aluminum production. The temperature distribution of the exhaust gas is shown in Fig. 3-6.

Figure 3-5 Temperature distribution of the cell gas [45] 3.5 Alcoa Fjardaal aluminum plant

Alcoa Fjardaál aluminum plant, located in Reyðarfjörður in the east of Iceland, began operations in 2007. It is one of the most modern and technologically advanced plants in the world. The production capacity is 350,000 tons of aluminum annually [49].

CHPATER 3 Alcoa Aluminum Smelter

Aluminum electrolysis is one of the most energy-intensive processes known in the industrial world. The high energy consumption is inherent. The theoretical minimum power requirement is 9,03 kWh/kg aluminium while the actual situation is between 13 and 15 kWh/kg aluminium. In 2016, the electricity consumption of Fjardaál was 4699 GWh indicating close to 13 kWh/kg aluminium. From this large amount of energy, about half of it lost as heat during production. As mentioned above, heat is carried by the exhausted gas from the smelting pot. From the measurement, around 910 푁푚3/푠 gas comes into the gas treatment centre (GTC) at approximate 110°C after dilution by ambient air. Hydrogen fluoride is removed from the flue gas in the GTC and particulates removed by a baghouse filter before it is released to the atmosphere. Results from emission measurement done in September to October 2016, the total particular content was found on average to be 0.37 푚푔/푁푚3. The fluoride gas was on average 0.225 푚푔/푁푚3. The concentration 3 of 푆푂2 was 109.5 푚푔/푁푚 and the humidity was 0.545%.

3.5.1 Thermal energy content

From Gusberti V et al. [45], 95% of the exhaust gas is original from the air in potroom. Thermodynamic properties of air are taken for the calculation instead of the exhaust gas. In this case, the thermal energy potential of the exhaust gas is 88.080 MW, calculated by

푄̇푔푎푠 = 퐶푝,푔푎푠휌푔푎푠푉푔푎푠̇ (푇푔푎푠,𝑖푛 − 푇̅푎푚푏) (3-1) The hourly average dry bulb temperature is used as the ambient temperature. It is 3.929 °C in east Iceland. ∑8760 푇 푇̅ = 1 𝑖 (3-2) 푎푚푏 8760 The average heat capacity for the temperature range from 4 °C to 110 °C is 1.0009KJ/Kg, given as 푡2 푏 푐푝| = 푎 + (푡1 + 푡2) (3-3) 푡1 2 Where the factors a is 0.7088; b is 0.000186[50]. The density of exhaust gas is 0.9023Kg/푚3, calculated by the EES.

3.5.2 Local energy demand

The location of Alcoa smelter is close to the towns of Reydarfjordur and Eskifjordur, shown in Fig. 3-7. The population of Eskifjordur is 1034 and the heat demand for space heating is around 3.5 MWth, including the heat used by households, companies and institution such as swimming pool. The heating system in Eskifjordur is powered by geothermal energy, but the heat-supply is insufficient to completely meet the needs of the community.

CHPATER 3 Alcoa Aluminum Smelter

Despite some effort, a geothermal resource has not been found in Reyðarfjörður. Currently, the electrical heating is applied in Reydarfjordur. The heat demand of

Reydarfjordur is estimated as approximate 4.05 MWth, 15% more than Eskifjordur, corresponding to the population of Reydarfjordur at 1195, 15% more than Eskifjordur. The line distance between Reydarfjordur and Eskifjordur is 9.91 km. The weather condition and the reference outdoor temperature used in the calculations of the heating load for these two towns are considered the same, owing to the short distance. The structure and material of the building in Reydarfjordur and Eskifjordur are similar. It is possible to supply the heating system in Reydarfjordur by the waste heat from the exhausted gas. The path distance from Alcoa smelter to Reydarfjordur is around 6 km.

Eskifjörður

Reyðarfjörður

Alcoa Smelter

Figure 3-6 Geographical condition of Alcoa Smelter in east Iceland

3.5.3 Dew point temperature

From the thermodynamic view, a low temperature of the exhaust gas is expected at the outlet of the heat exchanger, owing to that enables more thermal energy to be recovered by the waste heat recovery unit. However, the temperature is also limited from below by the acid dew point temperature.

The sulfur enters the reduction cell with the anodes and is oxidized to 푆푂2 in the exhaust gas. A small part of the 푆푂2 is further oxidized to 푆푂3. Formation of 푆푂3 can occur through two mechanisms: homogeneous gas-phase reactions and heterogeneous reactions. For exhaust gas, the homogeneous gas-phase reaction is dominated. The reaction may occur by direct oxidation of 푆푂2 according to reactions:

푆푂2 + 푂2 ⇌ 푆푂3

푆푂2 + 퐻2푂 ⇌ 푆푂3 + 푂퐻

Sulphur trioxide can also be formed in the presence of 푂퐻 radical by 퐻푂푆푂2 as an intermediate:

푆푂2 + 푂퐻 ⇌ 퐻푂푆푂2

CHPATER 3 Alcoa Aluminum Smelter

퐻푂푆푂2 + 푂2 ⇌ 푆푂3 + 푂퐻

When the temperature of exhaust gas drops, 푆푂3 stars to react with water vapor to form gaseous sulphur acid. As the temperature in the exhaust gas drops further, the gaseous sulphuric acid starts to form an acid mist or condenses on the surfaces and corrodes it. The temperature, in which the acid mist appears, is called acid dew point temperature [51]. From the emission measurement by the exhaust dust sampling in September 2016, the humidity of the pot gas is 0.67% at 100°C. The average concentration of 푆푂2 is 3 112 mg/m . The conversion rate from 푆푂2 to 푆푂3 is assumed as 3%. The pressure in the duct is 90 kPa. The saturated vapor pressure can be calculated by the correlation 2 3991.11 푝 = exp [18.5916 − ] (3-4) 푠 15 푡−233.84 Where the partial pressure, ps, is expressed in mm Hg and the temperature, t, is in degree Celsius. The dew point temperature of the sulfuric acid can be calculated by:

푇푑푒푤 = 365.6905 + 11.9864푙푛(푝퐻2푂) + 4.70336 ln(푝푆푂3)

2.19 +(0.446 ln(푝푆푂3) + 5.2572) (3-5)

The dew point temperature of the sulfuric acid 퐻2푆푂4 in the exhaust gas is around

90.9 °C. The partial pressure of the 푆푂2 , 푆푂3 and 퐻2푂 in the exhaust gas are 0.02842 mmHg, 0.0008526 mmHg and 5.108 mmHg respectively, shown in Table 3-1. This constitutes a practical lower limit on the temperature of the flue-gas in the system as higher than 90.9°C . Table 3-1 Summary of dew point temperature of the sulfuric acid Items Value Unit

Partial pressure of 푆푂2 0.02842 mmHg

Partial pressure of 푆푂3 0.0008526 mmHg

Partial pressure of 퐻2푂 5.108 mmHg Dew point temperature 90.9 °C

CHPATER 4 Heat Recovery System

Chapter 4 Heat Recovery System

Three scenarios are proposed to recover the heat from the exhaust gas in this chapter, which is power generation system, heat supply system and CHP system. The mathematic model of the systems is built up and the calculation is conducted in the software of Engineering Equation Solver (EES). The multi-objective function for the optimization of the system is figured out to obtain the optimal working condition for each scenario.

4.1 Scenarios

The objective of this system is to extract energy from the low-temperature exhaust gas. To utilize this thermal energy, three scenarios are considered: 1. power generation scenario 2. combined heat and power scenario and 3. direct utilization of heat scenario, as illustrated in Fig. 4-1. Three scenarios are composed of four modules, which are the heat source module, the power module, the cooling module and the heating module, shown in Table 4-1. The exhaust gas is collected from the smelting pots and releases heat to a working fluid in an evaporator and preheater proper order in the cases where power generation is considered. In the case of a district heating module, the heat for that application is extracted after the preheater. After releasing heat, the gas passes through a fan to maintain flowing. The processes of hydrogen and dust removal happen in the GTC. Finally, the exhaust gas is emitted into the atmosphere through the chimney. The merits of placing the heat exchanger before the gas fan is less power consumption of fan module, because the density of the gas is smaller after cooling in the heat exchanger. Also, the flue gas will be hotter before it passes through the GTC. In the power module: the working fluid absorbs heat in the heat exchangers of preheater and evaporator. Then the heated working fluid outputs power in the turbine, releases heat in the condenser and is compressed in the pump to finish a power cycle.

Table 4-1 System module component Heat source Power Cooling Heating System module module module module Power generation system ✓ ✓ ✓ Heat supply system ✓ ✓ CHP system ✓ ✓ ✓ ✓

CHPATER 4 Heat Recovery System

The cooling module serves the power module as a cold sink. In case of the smelter in Reydarfjordur, seawater would be used to cool down the working fluid in the power module. The sea water is pumped into the condenser and absorbs heat from working fluid. Then the sea water carrying the heat returns into the ocean. An interface for the heating system can be provided by the heating module. Turbine

Condenser Sea level

Cycle pump

Water pump Stack Evaporator Waste heat Preheater recovery unit

Gas fan

Dry scrubber

Smelter (a) Power generation system

Waste heat Stack recovery unit

Gas fan

Dry scrubber

Smelter District Snow Swimming Fish heating melting pool farming (b) Heat supply system

CHPATER 4 Heat Recovery System

Turbine

Condenser Sea level

Cycle pump

Heat exchanger Water pump Evaporator Waste heat Preheater Stack recovery unit

Gas fan

Dry scrubber

Smelter District Snow Swimming Fish heating melting pool farming (c) CHP system

Figure 4-1 System sketch 4.2 Assumptions

To simplify the model, the following assumptions are made: • The system and heat source are in a steady state. The temperature of the exhaust gas is 110 °C and the volumetric flow rate is 910 푚3/푠. • The heat sink of the cycle is the sea water, which is close to the aluminum smelter. The temperature is assumed at 6 °C, the mean temperature of the local sea surface. • 2 °C of superheat and 2 °C of subcooling are assumed at the outlet and inlet of the evaporator. • On the working fluid side, the pressure drops and heat losses to the environment in the evaporator, condenser, turbine, and pump are neglected. • The thermal resistance of conduction in heat exchanger pipes/walls is neglected. • The pressure drop in the heat source module after waste heat recovery unit is considered constant.

CHPATER 4 Heat Recovery System

4.3 Models

21 T 4 3 5 2 5 3 4 1 1s 6 6s 8 7 3 2

1 S Figure 4-2 T-S diagram of power generation system The T-s diagram of ORC system is shown in Fig. 4-2. The thermal energy content of the exhaust gas can be calculated by

푄̇푔푎푠 = 퐶푝,푔푎푠휌푔푎푠푉푔푎푠̇ (푇푔푎푠,1 − 푇̅푎푚푏) (4-1)

The 퐶푝,푔푎푠 is the heat capacity of the gas. The 휌푔푎푠 is the density of the gas. The heat transfer through preheater is given by the following equations.

푄̇푝푟푒 = 푚̇ 푤푓(ℎ푤푓,2 − ℎ푤푓,1) (4-2)

푄̇푝푟푒 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,4 − 푇푔푎푠,5) (4-3) Heat exchanger area can be calculated by 푄̇ 푎푟푒푎 = (4-4) 푈Δ푇푚 The logarithmic mean temperature difference can be calculated by

Δ푇푚푎푥−Δ푇푚𝑖푛 Δ푇푚 = Δ푇 푙푛 푚푎푥 (4-5) Δ푇푚𝑖푛 In general, the overall heat transfer coefficient can be calculated by 1 1 훿 1 = + + (4-6) 푈 푈𝑖푛푠𝑖푑푒 휆 푈표푢푡푠𝑖푑푒

훿 The heat resistance of conduction, , is ignored, then 휆 1 1 1 = + (4-7) 푈 푈𝑖푛푠𝑖푑푒 푈표푢푡푠𝑖푑푒 The minimum temperature difference between the hot and cold stream in the heat transfer unit is referred to as the pinch point temperature difference (PPTD). The pinch position in a heat exchanger emerges at the place where the heat transfer is the most constrained. Therefore, the PPTD is the critical parameter in an evaporator and

CHPATER 4 Heat Recovery System condenser. The heat transfer area and cycle performance are influenced by PPTD. The PPTD in evaporator is

Δ푇푝𝑖푛,푒푣푎 = 푇푔푎푠,3 − 푇푤푓,3 (4-8) The heat transfer in the evaporator can be given by following equations.

푄̇푒푣푎 = 푚̇ 푤푓(ℎ푤푓,5 − ℎ푤푓,2) (4-9)

푄̇푒푣푎 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,1 − 푇푔푎푠,4) (4-10)

Where the 푄̇푟ℎ푠 refer to the heat transfer in the evaporator excluding the preheat section. The power of turbine is given as

푊̇ 푡푢푟푏 = 푚̇ 푤푓(ℎ푤푓,5 − ℎ푤푓,6) (4-11) The efficiency of the turbine is

ℎ푤푓,5−ℎ푤푓,6 η푡푢푟푏 = (4-12) ℎ푤푓,5−ℎ푤푓,6푠 The generator power output is given as

푊̇푔푒푛 = η푔푒푛푊̇ 푡푢푟푏 (4-13) The heat transfer in condenser can be given as following equations.

푄̇푐표푛푑 = 푚̇ 푤푓(ℎ푤푓,6 − ℎ푤푓,8) (4-14)

푄̇푐표푛푑 = 푚̇ 푐푤푐푝,푐푤(푇푐푤,3 − 푇푐푤,1) (4-15)

Where 푄̇푐표푛푑,푐표푛 is the heat transfer rate in the condensation section in condenser. The power of working fluid pump is calculated as

푊̇ 푤푓,푝푢푚푝 = 푚̇ 푤푓Δℎ푤푓,푝푢푚푝 (4-16)

ℎ푤푓,1푠−ℎ푤푓,8 휂푝푢푚푝 = (4-17) ℎ푤푓,1−ℎ푤푓,8 The power of the cooling seawater pump can be calculated by

푚̇ 푐푤(휌푐푤푔Δℎ+Δ푝) 푊̇푐푤,푝푢푚푝 = (4-18) 휌푐푤휂 The increase in fan power load due to the pressure drop as the flue gas passes through the heat recovery unit

푣̇푔푎푠(푝푔푎푠,1−푝푔푎푠,5) 푊̇ 푔푎푠,푓푎푛 = (4-19) 휂푔푎푠,푓푎푛 For a constant mass flow, the volume flow changes as the temperature drops in the exhaust gas. The power consumption of original fan module is proportional to the volume flow and will therefore be reduced in comparison to the present case with no heat recovery. On the other hand, the power demand is increased because of pressure drop through the heat exchange units. The less volume flow after cooling, which

CHPATER 4 Heat Recovery System originally exists in the GTC. The flow rate of the exhaust gas at the outlet of waste heat recovery unit can be calculated by the following equation:

푇푔푎푠,표푢푡 푃푔푎푠,𝑖푛 푉푔푎푠̇ ,표푢푡 = 푉푔푎푠̇ ,𝑖푛 (4-20) 푇푔푎푠,𝑖푛 푃푔푎푠,표푢푡 The original fan power is saved by the volume flow rate descending:

푉̇ 푔푎푠,𝑖푛Δ푃푔푎푠 푉푔푎푠̇ ,표푢푡Δ푃푔푎푠 푊̇ 푓푎푛,푠푎푣푒 = − (4-21) 휂푓푎푛,표푟𝑖 휂푓푎푛,표푟𝑖 The net power output is considered as

푊̇ 푛푒푡 = 푊̇푔푒푛 − 푊̇ 푔푎푠,푓푎푛 − 푊̇푐푤,푝푢푚푝 − 푊̇ 푤푓,푝푢푚푝 (4-22) The saving power of the gas fan module is considered as a part of power output by the ORC system. Therefore, the general net power output can be defined as

푊̇ 푛푒푡,푔푟푎푙 = 푊̇푔푒푛 − 푊̇ 푔푎푠,푓푎푛 − 푊̇푐푤,푝푢푚푝 − 푊̇ 푤푓,푝푢푚푝 + 푊̇ 푓푎푛,푠푎푣푒 (4-23) The general thermal efficiency of the system is

푊̇ 푛푒푡,푔푟푎푙 η푠푦푠,푡ℎ푒푟푚푎푙 = (4-24) 푄̇푔푎푠 In order to evaluate the entire system, the range of study is extended. The entire system including heat source module, power module and the cooling module is chosen as the closed system for exergy analysis. 1 푑퐸 = 푑ℎ − 푇푑푠 + 푐2 (4-25) 푥 2 푓 The term of kinetic energy is neglected. 푐 푅푔 푑푠 = 푝 푑푇 − 푑푝 (4-26) 푇 푝 Inserted into equation (4-25), and then integrated from environmental state to current state. The enthalpy exergy of gas is 푝 푇 퐸̇푥,푔푎푠 = 푚̇ 푔푎푠(푐푝(푇 − 푇0) + 푅푔푇0푙푛 − 푐푝푇0푙푛 ) (4-27) 푝0 푇0 Exergy efficiency of system

푊̇ 푛푒푡,푔푟푎푙 휂푠푦푠,푒푥푒푟푔푦 = (4-28) 퐸̇푥,푔푎푠 For the heat supply system, the energy balance in the waste heat recovery unit is considered as

푄̇ℎ푒푎푡 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,5 − 푇푔푎푠,표푢푡) (4-29)

푄̇ℎ푒푎푡 = 푚̇ 푤푎푡푒푟푐푝,푤푎푡푒푟(푇푤푎푡푒푟,𝑖푛 − 푇푤푎푡푒푟,표푢푡) (4-30) The power input for the heat supply system is

푊̇ 𝑖푛푝푢푡 = 푊̇ 푓푎푛,푠푎푣푒 − 푊̇ 푔푎푠,푓푢푛 − 푊̇ 푤푎푡푒푟,푝푢푚푝 (4-31) The thermal efficiency of heat supply system is

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푄̇ ℎ푒푎푡 ηℎ푒푎푡 = (4-32) 푄̇푔푎푠 4.4 Optimization of ORC

4.4.1 Objective function

4.4.1.1 Single objective functions

The optimization of the system can be done by optimizing a set of objective functions that represent desirable characteristics of the system. Various objective functions can be used to optimize the system and different optimal working conditions would be resulted in. Various single objective functions are shown below. The capacity of power generation can be evaluated by the general net power output, 푊̇ 푛푒푡,푔푟푎푙. The recovery of available work in energy conversion process can be evaluated by the exergy efficiency of system, 휂푠푦푠,푒푥푒푟푔푦. The performance of ORC can be evaluated by the thermal efficiency of system, η푠푦푠,푡ℎ푒푟푚푎푙. The cost of the heat exchanger including preheater, evaporator and condenser accounts for around 80% of system investment. The cost of the heat exchanger is proportional to the area of the heat exchanger. Therefore, the economy of the system can be evaluated by the area of the heat exchanger, Area [52]. The system performance and initial investment can be shown by the objective function of the ratio between heat transfer area and systemic power output, APR [53].

4.4.1.2 Multi-objective optimization

The single-objective optimization problem is the most classical optimization problem. A basic single-objective optimization problem can be formulated as follows: 푚푎푥 푓(푥) (4-33)

푥 ∈ 푠 Where 푓 is a scalar function and 푠 is the accessibility domain of 푓. However, the problem to face the engineering application sometime is multi- objective optimization. The multi-objective optimization problem can be described in the mathematical term as flow:

푚푎푥[푓1(푥), 푓2(푥), … , 푓푛(푥)] (4-34) 푛 > 1, 푥 ∈ 푠 The space in which the objective vector belongs is called the objective space. The scalar concept of “optimality” does not apply directly in the multi-objective setting.

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The scalarization method is an efficient approach to solve the multi-objective optimization problem. The objective functions are combined into one single- objective scalar function. This approach, in general, is also known as the weighted- sum method, shown as follow: 푛 푚푎푥 ∑𝑖=1 훾𝑖 ∙ 푓𝑖(푥) (4-35) 푛

∑ 훾𝑖 = 1 𝑖=1

훾𝑖 > 0, 𝑖 = 1, … , 푛 푥 ∈ 푠 That represents a new optimization problem with a unique objective function[54]. In this study, the optimization for ORC system is a multi-objective optimization. The objective function and direction are shown below:

푚푎푥: 푊̇ 푛푒푡 (4-36)

푚푎푥: 휂푠푦푠,푒푥푒푟푔푦 (4-37) 푚𝑖푛: 퐴푟푒푎 (4-38) Three objective functions are integrated into two functions and keep the same direction. 푊̇ . 푚푎푥: 퐹 = 푛푒푡 (4-39) 1 퐴푟푒푎

푚푎푥: 퐹2 = 휂푠푦푠,푒푥푒푟푔푦 (4-40)

These two objective functions are combined into a single objective function 퐹(푥) with the weighting factors 휑, 휓.

푚푎푥: 퐹(푥) = 휑퐹1(푥) + 휓퐹2(푥) (4-41) Therefore, the objective function is proposed as 푊̇ 푚푎푥: 퐹(푥) = 휑 푛푒푡,푔푟푎푙 + 휓(100 ∙ 휂 ) (4-42) 퐴푟푒푎 푠푦푠,푒푥푒푟푔푦 From [55], the determination of weighting factor was proposed 1 2 퐹2 −퐹2 휑 = 2 1 1 2 (4-43) (퐹1 −퐹1 )+(퐹2 −퐹2 )

2 1 퐹1 −퐹1 휓 = 2 1 1 2 (4-44) (퐹1 −퐹1 )+(퐹2 −퐹2 )

1 2 Where 퐹1 is the maximum value of 퐹1(푥); 퐹2 is the maximum value of 퐹2(푥); 2 1 퐹1 is the value of 퐹1(푥) when 퐹2(푥) reaches the maximum; 퐹2 is the value of 퐹2(푥) when 퐹1(푥) reaches the maximum.

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4.4.2 Design variables

4.4.2.1 Working fluids selection

The physical properties, chemical stability, environmental impact, safety and economic performance of the working fluid are all important issue and have considerable influence on working fluid selection. Some criteria to select the working fluid for the cycle can be summarized as follows: 1. Thermodynamic and physical properties a) Types of the working fluids A working fluid can be classified as a dry, isotropic, or wet fluid corresponding with the slope of the saturation vapor curve on the T-s diagram, shown in Fig. 4-3. 푑푡 푑푡 When is positive, the working fluid is dry. When is negative, the working fluid 푑푠 푑푠 푑푡 is wet. When is trending to infinity, the fluid is considered isentropic. 푑푠 Low vapor quality lowers the efficiency of the turbine. To avoid low vapor quality of the working fluid at the end of the turbine, dry or isentropic working fluids are expected. On the other hand, the cooling load is higher if the working fluid is too dry.

Figure 4-3 Three types of working fluids: dry, isentropic and wet [56] b) Influence of latent heat, density and specific heat The latent heat of the working fluid is expected to be high, so that less working fluid is required in the cycle [57]. High density and specific heat of the working fluid are beneficial properties in the ORC. c) Critical point of the working fluids If the critical temperature is lower than the ambient temperature, the condensation process cannot be realized. The critical temperature also should not be too high. 2. Stability of the fluid and compatibility with materials in contact Chemical stability is an important property for the working fluid, at extreme condition such as high temperature. The fluid should not be aggressive or cause corrosion problem in equipment and not be soluble with machine oil. 3. Environmental aspects

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As to the environmental aspects, the main concerns include the ozone depletion potential (ODP), global warming potential (GWP) and the atmospheric lifetime (ALT). The ODP and GWP represent substance’s potential to contribute to ozone degradation and global warming if released to the atmosphere. These properties should not be ignored when selecting a working fluid. 4. Safety The ASHRAE refrigerant safety classification is a good indicator of the fluid’s level of danger. Generally, characteristics like noncorrosive, non-flammable, and non-toxic are expected. However, some flammable substances are still considered as acceptable if there is no ignition source around. 5. Availability and cost The availability and cost of the working fluids are among the considerations when selecting working fluids. Traditional refrigerants used in organic Rankine cycles are expensive. This cost could be reduced by a more massive production of those refrigerants, or by selecting low-cost hydrocarbons [56].

4.4.2.2 Evaporation parameter selection

The evaporation parameter is a chief parameter of the ORC system. The efficiency of the cycle and the system performance significantly change with various evaporation temperatures or pressures. The heat absorption of the system would be influenced by the evaporation parameters as well.

4.4.2.3 Condensation parameter selection

The performance of the cycle is influenced by the condensation parameter. It affects the performance of the cycle and cooling system. The efficiency and the power output of the cycle are included in the cycle performance and would change with various condensation parameter. The flow rate of the cooling water and power consumption of the pump in the cooling system are related to the condensation parameter.

4.4.2.4 Pinch point temperature difference selection

The minimum temperature difference between the hot and cold stream in the heat transfer unit is referred to as the pinch point temperature difference (PPTD). The pinch position in a heat exchanger emerges at the place where the heat transfer is the most constrained [58]. Therefore, the PPTD is the critical parameter in an evaporator and condenser. The heat transfer area and cycle performance are influenced by PPTD.

4.4.2.5 Middle temperature selection

A critical parameter, the gas temperature at the outlet of evaporator, is defined as the middle temperature, 푇푔푎푠,푚𝑖푑. The middle temperature is a design variable only for the

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CHP system. The distribution ratio of the thermal energy, which is absorbed into the ORC part and heat supply part, is decided by the middle temperature.

4.4.3 Flowchart

As mentioned above, the design variables for power generation system are 푇푤푓,푒푣푎,

푇푤푓,푐표푛푑 , Δ푇푝𝑖푛,푒푣푎 and Δ푇푝𝑖푛,푐표푛푑 . When the differences between the input and results of design variables are less than 0.05%, convergence condition is met and then output the result. Otherwise, the next iteration will be conducted, shown in Fig. 4-4.

For CHP system, the design variables are 푇푔푎푠,푚𝑖푑, 푇푤푓,푒푣푎, 푇푤푓,푐표푛푑 and Δ푇푝𝑖푛,푐표푛푑.

Initial data input Working fluid, Teva,Tcond,TPPTD_eva,TPPTD_cond

Working fluid, Teva,Tcond,TPPTD_eva,TPPTD_cond

Weighting factor φ,ψ

Working fluid

Evaporation temperature Teva

Condensation temperature

Tcond

PPTD in evaporator

TPPTD,eva

PPTD in condenser

TPPTD,cond

Δ<0.005 No

Yes

Results output Working fluid, Teva,Tcond,TPPTD_eva,TPPTD_cond Figure 4-4 Iteration procedure of optimization

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4.5 Conditions of Alcoa smelter

4.5.1 Ambient condition

The Alcoa aluminum smelter is close to the sea and the sea water can be used as the cooling water for the cooling module. The Alcoa smelter locates around 64.875E, 12.125W in east Iceland where the temperature of the sea surface is relatively low, as shown in Fig. 4-5. The monthly sea surface temperature in 2016 is shown in Table 4-2. For the condenser, the inlet temperature on the water side is assumed as the mean sea surface temperature in 2016.

Table 4-2 The monthly sea surface temperature in 2016 [59]

Month January February March April May June Temperature/°C 4.581 3.557 3.265 3.850 4.289 6.482 Month July August September October November December Temperature/°C 8.383 9.553 8.968 7.652 6.628 5.138 weighted average: 6.029 °C

Figure 4-5 Temperature distribution of world’s ocean The mean local atmospheric pressure is calculated by the hourly record in 2016. ∑8760 푃 푃̅ = 1 𝑖 = 1.003496 푏푎푟 (4-45) 푎푡푚,푙표푐푎푙 8760 4.5.2 Heat source condition

The temperature of the exhaust gas from the aluminum smelter fluctuate from 100 °C to 120 °C and the range of pressure is from -980 Pa to -1250 Pa. For the current analysis, a stable condition of the heat source is assumed, assuming that the exhaust

CHPATER 4 Heat Recovery System gas temperature is 110 °C and pressure is -1100Pa. The flow rate of exhaust gas is 910푚3/푠. Other parameters of the exhaust gas are shown in Table 4-3. Table 4-3 Parameters of exhaust gas Parameters Value unit Temperature 110 °C Flow rate 910 m3/s Pressure 0.99250 bar Density 0.9182 Kg/m3 Heat capacity (4 to 110 °C) 1.0009 KJ/Kg

4.5.3 Heat transfer coefficient and pressure drop

The tube arrangement of the staggered bank is designed in the heat recovery unit, shown in Fig. 4-6.

Heat exchanger

Evaporator

Preheater Gas inlet

(a) Heat recovery unit

(b) Staggered bank Tube [60]

Figure 4-6 Arrangement of heat exchanger The working fluid pass through inside the tube and the gas outside the tube. In the heat recovery unit, evaporator, preheater and heat exchanger for CHP arrange in sequence by streamwise. The average heat transfer coefficient and total pressure drop over the staggered bank of tubes are calculated by the EES model External Flow Staggered Bank. The heat transfer coefficient outside the tube is 129.3 W/푚2퐾, 129.5 3W/푚2퐾 and 130.0 3W/푚2퐾 in evaporator, preheater and CHP heat exchanger separately. Gas

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pressure drop is 0.01040 bar for evaporator and 0.01061 bar for preheater. Other parameters are shown in Table 4-4, Table 4-5, Table 4-6. Table 4-4 Parameters in evaporator

Parameters Value Unit Inlet absolute pressure 0.99250 Bar Velocity of gas 8 m/s Internal diameter (3/8’’) 0.01715 m Number of rows 50 Distance between the center of tubes 0.05145 m Length 6 m Temperature of tube surface 77 °C Heat transfer coefficient (outside) 129.3 W/m2·K Pressure drop 0.01040 Bar

Table 4-5 Parameters in preheater Parameters Value Unit Inlet absolute pressure 0.98210 Bar Velocity of gas 8 m/s Internal diameter (3/8’’) 0.01715(3/8’’) m Number of rows 45 Distance between the center of tubes 0. 05145 m Length 6 m Temperature of tube surface 45 °C Heat transfer coefficient(outside) 129.5 W/m2·K Pressure drop 0.01061 Bar

Table 4-6 Parameters in heat exchanger in CHP system Parameters Value Unit Inlet absolute pressure 0.97149 Bar Velocity of gas 8 m/s Internal diameter (3/8’’) 0.01715 m Number of rows 50 Distance between the center of tubes 0. 05145 m Length 6 m Temperature of tube surface 60 °C Heat transfer coefficient(outside) 130.3 W/m2·K Pressure drop (assume) 0.02 Bar

4.5.4 Heating system conditions

The supply water temperature for the district heating system is commonly 80 °C in Iceland and the return temperature is 40 °C. The water pressure drop in the heat exchanger is assumed as 2 bar, shown in Table 4-7.

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Table 4-7 Conditions of heating system Items Value Unit Water pressure drop 2 Bar Inlet temperature of water 40 °C Outlet temperature of water 80 °C Inside heat transfer coefficient 2.00 Kw/m2K

4.5.5 Equipment conditions

The basic parameters of the equipment are assumed, including turbine, generator, cooling pump, gas fan, etc., shown in Table 4-8. Table 4-8 Parameters of basic equipment Parameters Value Unit Efficiency of circulation pump 0.80 Efficiency of gas fan 0.70 Efficiency of cooling pump 0.82 Efficiency of turbine 0.85 Efficiency of generator 0.96[53] Temperature difference between preheater outlet and boiling 2.00 °C temperature Superheat temperature in evaporator 2.00 °C Inside heat transfer coefficient of preheater 0.60 Kw/m2K Inside heat transfer coefficient of evaporator (boiling section) 1.00 Kw/m2K Inside heat transfer coefficient of evaporator (superheat section) 0.50 Kw/m2K Heat transfer coefficient of condenser (condensation section) 1.2 Kw/m2K Heat transfer coefficient of condenser (superheat section) 1.00 Kw/m2K Gas pressure drop in heat exchangers 0.25 bar

4.6 Results

4.6.1 Power generation system (Scenario 1)

4.6.1.1 Working fluid selection

For the power generation system optimization, a good systemic performance was not obtained with the weight factors 휑, 휓, which are calculated by the equations 4-43 and 4-44. The best performance is obtained with the condition of 휑 = 0.3 푎푛푑 휓 = 0.7.

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Figure 4-7 Working fluid selection (14 working fluids) Fig. 4-7 shows the variation of the objective function with the various evaporation temperature for 14 different working fluids. Five working fluids, R-236fa, R-245fa, R-236ea, R-123 and R-600a, have good thermodynamic performance. The environmental issue and safety of the working fluid are considered as well. R-123 has environmentally friendly properties, comparing with R-236fa, R-245fa, and R- 236ea, shown in Table 4-9. High flammability makes R-600a a safety hazard. All things considered, R-123 is selected as working fluid in the iteration. The optimal parameters of the power generation system and CHP system are shown in Table 4- 10. The T-S property diagram of R123 is shown in Fig. 4-8.

Table 4-9 Properties of various working fluids [61] Working fluid R236fa R245fa R236ea R123 R600a

Structural formula CF3CH2CF3 CF3CH2CHF2 CF3CHFCHF2 CHCl2CF3 CH(CH3)3 ODP 0 0 0 0.012 0 GWP 9800 1030 1370 120 3

250 R123

0.0038 200

0.086

0.4 1.93 9.18 150 21.62 bar 45 m3/kg

100 5.872 bar

T [°C] T 50

0.8541 bar 0

0.03343 bar -50 0.2 0.4 0.6 0.8

-100 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 s [kJ/kg-K] Figure 4-8 T-S diagram of R-123

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4.6.1.2 Evaporation parameter selection

Table 4-10 Setting of evaporation parameter Design variables Value Range of evaporation temperature 50~100°C Working fluid R123 Condenser temperature 30°C PPTD in evaporator 5°C PPTD in condenser 10°C

The optimization of the evaporation parameter conducts under the condition shown in Table 4-10. As Fig. 4-9 shown, the peak values of two different single objective functions are found with different evaporation temperatures. The multi-objective function of F(x), which combines the features of two single objective functions, gives an optimal evaporation temperature between the values given by the two single objective functions. Results of Fig. 4-9 shows the peak thermal and exergy efficiency found around 60 °C. The increment on the left-hand side of the peak is owing to the increased thermal efficiency of the cycle with the higher evaporation temperature. The decrement on the right-hand side of the peak is owing to the declining heat absorption from the pot gas with higher evaporation temperature, although the thermodynamic efficiency of the cycle keeps growing with higher evaporation temperature.

0.20 0.04

0.15 0.03

0.10 0.02

0.05

0.01

exergy thermal  0.00  0.00

-0.05  exergy -0.01

-0.10 thermal -0.02 -0.15 50 60 70 80 90 100 Evaporation temperature (°C) (a) (b)

100 30000 65 2.2 90

25000 )

2 0 80 )

1.1 2 20000

-65 (MW) 70

gral

Ratio (W/m Ratio W 0.0 (m Area -130 Ratio 15000 60 Area

Gas(°C) temperature outlet 50 -195 Wgral -1.1 10000 40 -260 5000 50 60 70 80 90 100 110 50 60 70 80 90 100 Evaporation temperature (°C) Evaporation temperature (°C) (c) (d)

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4 Wgral

Wgen 3 Wnet

1 Power (MW) 0

-1

-2 50 60 70 80 90 100 Evaporation temperature (°C) (e)

Figure 4-9 Evaporation parameter selection: (a) multi-objective function; (b) efficiency; (c)ratio of power to area; (d) gas outlet temperature; (e)power output at various evaporation temperature. From, the ratio between general power output and heat transfer area shown in Fig. 4-9 (c). A peak of the curve appears, corresponding with the power output curves in Fig. 4-9 (e)and the efficiency curves in Fig. 4-9 (b). The gas outlet temperature increases with the increased evaporation temperature.

4.6.1.3 Condensation parameter selection

Table 4-11 Setting of condensation parameters Design variables Value Evaporation temperature 66°C Working fluid R123 Range of condenser temperature 20~40°C PPTD in evaporator 5°C PPTD in condenser 10°C

Optimization of the condensation parameter was conducted under the conditions shown in the Table 4-11. From Fig. 4-9 (a), the similar trends of three objective functions can be observed. The maximum value of F(x) can be obtained with various condensation temperatures. On one hand, the large water flow rate can be lead by the low condensation temperature, so that the power for cooling pump increases and net power output decreases. On the other hand, the low efficiency of the cycle can be caused by the high condensation temperature, therefore the power output declines. The gas outlet temperature increases with the increasing condensation temperature.

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(a) (b)

57 Gas(°C) temperature outlet

10 15 20 25 30 Condensation temperature (°C) (e) (f)

Figure 4-10 Condensation parameter selection: (a) multi-objective function (b)ratio of power to area; (c)efficiency; (d)power; (e)gas outlet temperature; (f)flow rate of cooling water at various condensation temperature

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4.6.1.4 PPTD in evaporator

Table 4-12 Setting of parameters in PPTD in evaporator Design variables Value Evaporation temperature 66°C Working fluid R123 condenser temperature 21.84°C Range of PPTD in evaporator 1~20°C PPTD in condenser 10°C

The optimization of the PPTD in the evaporator was conducted assuming the condition shown in Table 4-12. An optimal value of PPTD in the evaporator can be found from Fig. 4-11 (a). The uneven increment of heat transfer area with the decreasing PPTD in evaporator makes the ratio between power output and heat transfer area in a convex curve. The thermal efficiency, exergy efficiency, power output and gas outlet temperature are in the straight linear relation with the PPTD in the evaporator, shown in Fig. 4-11. 80 105 105 0.21 70 2.8 40000 90 90

60 )

) 32000 2

2 2.1 )

75 75 2 0.14

exergy

F(x) (MW)  24000

60 60 1.4 gral

Area (m

W Ratio (W/m

Ratio (W/m 40 0.07 45 16000 45 Ratio Ratio 0.7 30 Area exergy 30 Wgral 30 F(x) 8000 0.00 20 0.0 0 5 10 15 20 25 0 5 10 15 20 25 PPTD in evaporator (°C) PPTD in evaporator (°C) (b) (a)

0.30 5 0.032 W exergy gral W 0.24 thermal 4 gen

Wnet 0.024 3 0.18

thermal 2

exergy 0.016   0.12 1

0.008 (MW) output Power 0.06 0

0.00 0.000 0 5 10 15 20 -1 0 5 10 15 20 PPTD in evaporator (°C) PPTD in evaporator (°C) (c) (d)

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60 Gas outlet temperature (°C) 55

50 0 5 10 15 20 PPTD in evaporator (°C) (e)

Figure 4-11 PPTD in evaporator: (a)multi-objective function; (b)ratio of power to area; (c)efficiency; (d)power output; (e)gas outlet temperature with different PPTD in evaporator

4.6.1.5 PPTD in condenser

Table 4-13 Setting of parameters in PPTD in condenser Design variables Value Evaporation temperature 66°C Working fluid R123 Condenser temperature 21.84°C PPTD in evaporator 7.59°C Range of PPTD in condenser 1~15°C

The optimization of the PPTD in condenser was conducted under the condition shown in Table 4-13. The anomalies can be seen from the Fig. 4-12 (a)-(d). The mechanism of the anomalies is similar to the condensation parameter optimization, which the high flow rate of water and high-power consumption of pump caused by the low PPTD in the condenser.

0.21 150 100 150 3 22500

0.00 0 0 21600

0 0

)

) 2 -0.21 -100 -150 20700

-150 -3

(MW)

exergy

F(x)



gral

Area (m Area

W Ratio (W/m Ratio

Ratio (W/m Ratio 19800 Ratio -0.42 -200 -300 -300 -6  Ratio exergy Area F(x) -0.63 -300 -450 18900 Wgral -450 -9

0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 PPTD in condenser (°C) PPTD in condenser (°C) (a) (b)

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0.10 4 0.2 2 0.05 0.0 0 -2 -0.2

0.00 -4

exergy thermal   -6 -0.4 Power output (MW) Power output W -0.05 -8 gral Wgen exergy -0.6 -10 Wnet thermal -0.10 -12 -0.8 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 PPTD in condenser (°C) PPTD in condenser (°C) (d) (c)

Figure 4-12 PPTD in condenser: (a)multi-objective function; (b)ratio of power to area; (c)efficiency; (d)power output with different PPTD in condenser

4.6.2 Heat supply system (Scenario 2)

Table 4-14 Parameters in heat supply system

푻풐풖풕풍풆풕 푸̇ 풉풆풂풕 휼풕풉풆풓풎풂풍 푴̇ 풘풂풕풆풓 Area 푾풊풏풑풖풕 푾풑풖풎풑 °C MW Kg/s 퐦ퟐ MW MW 90 16.57 0.1881 98.93 3460 1.405 0.01484 85.56 20.25 0.2299 120.9 4446 1.408 0.01814 81.11 23.93 0.2717 142.9 5548 1.411 0.02144 76.67 27.62 0.3135 164.9 6795 1.414 0.02473 72.22 31.3 0.3553 186.9 8227 1.418 0.02803 67.78 34.98 0.3971 208.9 9903 1.421 0.03133 63.33 38.66 0.4389 230.8 11914 1.424 0.03463 58.89 42.34 0.4807 252.8 14412 1.428 0.03792 54.44 46.03 0.5226 274.8 17678 1.431 0.04122 50 49.71 0.5644 296.8 22321 1.434 0.04452

0.65 25000 50 50 0.60 45 45 0.55 20000

40 0.50 40 15000

35 0.45 35 ) 2 0.40 30 thermal 30

 10000 Area (m Area 0.35

25 25 Heat rate (MW) flow

0.30 (MW) rate Heatflow 20 Heat flow rate 20 5000 0.25  Heat flow rate 15 thermal Area 0.20 15 0 50 60 70 80 90 50 60 70 80 90 T (°C) T (°C) gas,outlet gas,outlet (a) (b)

CHPATER 4 Heat Recovery System

0.042

0.036

0.030

0.024

Pump power (MW) Pump power

Mass (Kg/s) flow rate 0.018 Mass flow rate Pump power 0.012 50 60 70 80 90

T (°C) gas,outlet (c)

Figure 4-13 Performance of heat supply system From Fig. 4-13 (a), it can be seen that the heat absorption from exhaust gas decreases with the growing gas outlet temperature. Therefore, the thermal efficiency, heat transfer area and the flow rate of water decrease with the growing gas outlet temperature. Around 20.25 MWth heat can be obtained at the gas outlet temperature of 85 °C, which is higher than the temperature of gas dew point. The detailed data is shown in Table 4-14.

4.6.3 CHP system (Scenario 3)

Table 4-15 setting of parameters in middle temperature selection Design variables Value Working fluid R123 Range of middle temperature 83~95°C Evaporation temperature 61°C Condensation temperature 15.86°C PPTD in condenser 3.286°C

The optimization of the middle temperature was conducted under the conditions shown in Table 4-15 and Fig. 4-14 (a) shows the optimal middle gas temperature given by the F(x). Different optimization directions are offered by two single objective functions. The effective combination of the two optimization functions is realized by the multi-optimization method (4-6). The design variable of the middle gas temperature indicates the proportion of the heat absorption, offering to ORC part and heating part. More energy is obtained by the heating system with the increasing middle gas temperature and less energy by the ORC system. The flow rate curves, efficiency curves and power output curves are matching with this mechanism, shown by Fig. 4-14 (b)-(d).

CHPATER 4 Heat Recovery System

180

Working fluid 160 Water

140

120

100 Flow rate (Kg/s) Flow rate 80

40 82 84 86 88 90 92 94 96 T (°C) gas,mid (a) (b)

3.0 W 0.34 gral 0.020 2.5 Wgen

0.32 Wnet 0.018 2.0 0.30 1.5

0.016 0.28

1.0

exergy exergy  0.26 

0.014 0.5 Power output (MW) Power output

0.24 0.0 exergy 0.012 thermal 0.22 -0.5

0.010 0.20 82 84 86 88 90 92 94 96 82 84 86 88 90 92 94 96 T (°C) gas,mid T (°C) gas,mid (d) (c)

Figure 4-14: Middle gas temperature selection: (a)multi-objective function; (b)flow rate of heated water and working fluid; (c)efficiency; (d)power output at various middle gas temperature 4.7 Summary

The optimal parameters for the system are shown in Table 4-16. From Table 4-17, the highest exergy efficiency of 19.22% is obtained and 2.573MW electricity is produced by the power generation system. The thermal efficiency of 48.1% and a heating capacity of 42.34MW are obtained by the heat supply system. The power output and heating capacity of the CHP system are 1.156MW and 23.55MW respectively. The gas fan power consumption in the three systems are considerable. More heat exchanger area is required by CHP system and less is required by heat supply system. The performance of ORC is significantly influenced by the selection of working fluid and design parameters. Comparing various scenarios, the highest exergy efficiency, 19.22%, can be obtained by power generation system and the highest energy efficiency of 48.1% can be obtained by the heat supply system. The efficiency of organic Rankine cycle is limited by the low temperature of the exhaust gas.

CHPATER 4 Heat Recovery System

Table 4-16 Iteration results of optimal parameters Working Evaporation Condensation PPTD in PPTD in System fluid temperature temperature evaporator condenser °C °C °C °C Power generation R-123 61.22 16.33 9.469 3.501 system Middle CHP system R-123 76.02 17.10 temperature 3.857 86.43

In engineering application, the temperature of exhaust gas should stay above the acid dew point generally. In this case, the dew point temperature of sulfur acid is around 90.9 ℃ as given in Chapter 3. Although the emission temperature of the exhaust gas is lower than the acid dew point temperature, the potential of waste heat utilization from aluminum production can be shown in this study.

Table 4-17 System performance of three scenarios Parameters ORC Heat supply CHP

푊̇ gral 2.573MW -0.104 MW 1.156MW

푄̇heat 0 42.34 MW 23.550 MW

휂exergy 19.220% 0 8.63%

휂thermal 2.921% 48.1% 28.05% 퐴푟푒푎 21115m2 14412m2 25509m2

푊̇ net 1.248 MW 0 -0.195MW

푊̇gen 4.17 MW 0 2.344MW

푊̇ gas,fan -2.417 MW -1.39 MW -2.295 MW

푊̇ fan,save 1.325 MW 1.324 MW 1.351MW

푊̇cw,pump -0.465 MW 0 -0.194MW

푊̇ wf,pump -0.0403 MW -0.03792 MW -0.0291 MW

푇gas,outlet 58.84°C 58.89°C 58°C

푚̇ wf 207.4Kg/s 0 91.97Kg/s

푚̇ cw 1278Kg/s 0 533.1Kg/s

푚̇ heat 0 252.81Kg/s 140.61Kg/s

CHPATER 5 Power Generation with Enhanced Heat Source

Chapter 5 Power Generation with Enhanced Heat Source

The efficiency of organic Rankine cycle is significantly limited by the low temperature of the exhaust gas. One way to improve the efficiency of the ORC is to increase the heat source temperature, which should be possible as the temperature is controlled by the amount of ambient air sucked into the electrolysis cell hood. In this chapter, the heat balance of the pot is studied. The temperature of the heat source is uplifted by reducing the dilution air from the environment into the pot.

5.1 Setting dilution air flow rate

The exhaust gas from the pot is composed of gas emission from the process, which produced inside the pot, and dilution air drawn into the pot. The main component of the gas emission is 퐶푂2 at the process temperature and it only accounts for around 5% of the total exhaust gas. More than 95% [45]of the gases collected by the cell exhaust system are originally pot room air at ambient temperature drawn into the cell by the negative pressure present under the cell hood, shown Fig. 5-1.

Figure 5-1 Origination of heat of exhaust gas The flow rate of the dilution air is reduced in order to obtain a higher temperature exhaust gas. The heat content in the exhaust gas is from the pot and through two ways,

CHPATER 5 Power Generation with Enhanced Heat Source which are carried by the emission gas and heat transfer from pot to gas. The heat content can be calculated by

푄̇푔푎푠 = 푄̇푐푎푟푟푦 + 푄̇ℎ푒푎푡 푡푟푎푛푠푓푒푟 (5-1) The assumption is made that 5% of the total exhaust gas is the original pot gas and the remaining 95% is dilution air. The emission gas from the pot is assumed as only containing 퐶푂2 and other components are ignored. The heat carried by the emission gas from the pot can be calculated by ̇ 푄푐푎푟푟푦 = 0.05 ∙ ρV퐶푝,퐶푂2(푇푝표푡 − 푇푒푚) (5-2)

Where 퐶푝,퐶푂2 = 0.9287 퐾퐽/퐾푔 ∙ 퐾 at 110°C 푇 −푇̅ 푄̇ = 퐴 푝표푡 푔푎푠 (5-3) ℎ푒푎푡 푡푟푎푛푠푓푒푟 푅 Where A is the heat transfer area, R is the thermal resistance. The influence on the thermal resistance by the changing gas temperature is ignored. The mean endothermic temperature of the gas 푇 +푇 푇̅ = 𝑖푛 표푢푡 (5-4) 푔푎푠 2 The heat contained in the exhaust gas with various temperature are shown Fig. 5- 2. The gas temperature increases with decreasing mass flow of dilution air and the total flow rate of the exhaust gas is reduced as well, as shown in Table 5-1. When the flow rate of the exhaust gas reduces to around 700 푚3/푠, the gas temperature achieves 150 °C.

Figure 5-2 Relationship between flow rate and temperature of exhaust gas

CHPATER 5 Power Generation with Enhanced Heat Source

Table 5-1 Parameters of dilution air Exhaust gas Heat content Exhaust gas Dilution air temperature °C MW 풎ퟑ/풔 풎ퟑ/풔 110 83.022 910 864.5 115 82.792 866.8 823.4 120 82.562 827.3 785.9 125 82.333 791 751.5 130 82.103 757.7 719.8 135 81.873 726.9 690.5 140 81.643 698.3 663.4 145 81.413 671.8 638.2 150 81.183 647.1 614.7

5.2 Heat balance influence

Although the temperature of the exhaust gas increases with less dilution air, the heat transfer from pot to the gas decreases. The heat balance of the cell is altered with this change. The cell heat balance obeys the first law of thermodynamics. The difference between heat inputs and outputs is the heat accumulation, Qac. The equation below presents the heat balance:

푄푎푐 = 푄𝑖푛 − 푄표푢푡 (5-5) In order to maintain the heat balance and avoid the heat accumulation, with the constant heat input, the heat loss to ambient should be increased to the original value. Majority of the heat loss to the ambient is through three directions: top, sides and bottom. When the heat loss through the top decline, the measure to weaken the thermal resistance of the bottom and wall sides is necessary.

10 0.6  Wgral exergy W 0.5  8 net thermal Wgen 0.4 6 0.3  4

Power (MW) 0.2

2 0.1

0.0 0 112 120 128 136 144 152 110 120 130 140 150 T (°C) T (°C) gas gas (a) (b)

CHPATER 5 Power Generation with Enhanced Heat Source

240 T 70 gas,outlet Ratio 24000 Working fluid 400 Area

230 22000

) 3

) 300

) 2 20000

(°C) 220

Area (m Area

gas,outlet Ratio (W/m Ratio

T 200 18000 Flow rate (Kg/m Flow rate 55 210 16000 100 50 200 110 120 130 140 150 110 120 130 140 150

Tgas (°C) T (°C) gas (d) (c)

Figure 5-3 Performance of ORC system in various conditions 5.3 The optimal working performance of ORC at various conditions

Fig. 5-3 (a)-(d) shows the influence of the changing gas temperature on the performance of the ORC system. From (a), the increase in power output is shown as a function of temperature of the exhaust gas, owing to that both heat absorption and cycle efficiency are growing as shown in (b). The maximum 9.174 MWe power can be produced at the gas exhaust gas temperature of 150 °C, and the systemic exergy efficiency of 54.38% can be achieved at these conditions. The thermal efficiency keeps increasing as well with an increasing gas temperature. The gas outlet temperature increases with increasing gas temperature but is still under the temperature of sulfuric acid dew point.

5.4 The optimal design variables in different conditions

Fig. 5-4 shows the influence of various gas temperature on the optimal design variables. From Fig. (a), the evaporation temperature increases with the increasing gas temperature and other design variables keep a stable situation, shown Fig. 5-4 (b). The maximum optimal evaporation temperature is 84.53°C at the gas temperature of 150°C.

CHPATER 5 Power Generation with Enhanced Heat Source

Tcond 85 20 PPTDeva

PPTDcond 80 16

75 12

(°C)

eva T

70 8 Temperature (°C)

65 4

60 0 110 120 130 140 150 110 120 130 140 150

Tgas (°C) T (°C) gas (a) (b)

Figure 5-4 Optimal design variables in various conditions 5.5 Summary

In this chapter, the effect on heat recovery potential by raising the temperature of the heat source by reducing the dilution air is evaluated. The aim of this is to improve the power generation system. The optimization method is the same as the optimization for the power generation system in Chapter 4. The result is that the efficiency of the cycle and power output are effectively improved with the increasing temperature of the exhaust gas and a good trend of the systemic performance can be observed, shown in Table 5-2. 9.174 MW power output at 150 °C is significantly higher than the maximum 2.573 MW power output at 110°C. Therefore, a good potential of power generation is found by the strategy of reducing the dilution air and raising the gas temperature. The heat balance is an issue that must be considered in this case. The heat loss to the ambient is reduced with less dilution air. The arrangement of the thermal resistance of the pot has to be redesigned in order to keep the heat balance and avoid overheating of the cell.

Table 5-2 Summary of power generation system with an enhanced heat source

Tgas V̇gas Wnet,gral ηexergy ηthermal Tgas,outlet Tdew,SO2 ṁ wf Area °C m3/s MW °C °C Kg/s m2 115 867 3.555 0.2564 0.04071 59.97 91.35 210.9 21096 120 827.3 4.387 0.3062 0.05103 62.21 91.79 209.5 20319 125 791 5.076 0.3436 0.05996 65.12 92.21 206 18993 130 757.7 5.91 0.3885 0.07088 65.97 92.63 209.5 19091 135 726.9 6.879 0.4397 0.08374 65.36 93.03 216.5 20431 140 698.3 7.542 0.4696 0.09319 66.99 93.42 216.5 19835 145 671.8 8.399 0.5099 0.1053 66.57 93.8 221.7 20866 150 647.1 9.174 0.5438 0.1167 65.98 94.17 227.7 21587

CHAPTER 6 Conclusion and Future Work

Chapter 6 Conclusion and Future Work

6.1 Conclusion

This work concentrates on evaluating the heat recovery potential from the flue-gas from the aluminum production. Three scenarios, including organic Rankine cycle (ORC) system, heat supply system and combined heat and power (CHP) system, were proposed to recover waste heat from the exhaust gas, and the recovery potential was quantified through modelling for the different scenarios. The electric power generation potential is estimated by ORC models. A multiple-objective function and a new optimization method were proposed for the optimization of the ORC system. The optimal parameters and systemic performance for each system are figured out. In the power generation system, the efficiency of the organic Rankine cycle is significantly limited by the low temperature of the exhaust gas. An effective measure to improve the efficiency of the ORC is to increase the heat source temperature. The temperature of the heat source is raised by reducing the dilution air from the environment into the pot. It was found that the efficiency of the cycle and power output are effectively improved, and a good trend of the systemic performance can be observed. From the result, the main conclusion can be summarized as follows:

1. At the optimal condition, 2.573 MWe electricity can be produced by the

power generation system. For CHP system, 1.156 MWe power and 23.55

MWth heating capacity can be produced. 42.34 MWth heating capacity can be produced by heat supply system. It is thus clear that there is considerable potential for waste heat recovery from aluminum production. The performance of ORC is significantly influenced by the selection of working fluid and design parameters. 2. Comparing various scenarios, the highest exergy efficiency, 19.22%, can be obtained by power generation system and the highest energy efficiency of 48.1% can be obtained by the heat supply system. The efficiency of organic Rankine cycle is limited by the low temperature of the exhaust gas. The highest energy efficiency can be obtained by heat direct utilization. 3. The preferable performance for the power generation system is found by using the enhanced heat source, which produces the exhaust gas at a higher temperature. Up to 9.174 MW electric power can be produced if

CHAPTER 6 Conclusion and Future Work

the gas exhaust gas temperature is raised to 150 °C by less dilution with air, and the systemic exergy efficiency of 54.38% can be achieved in this case.

6.2 Recommendation for further research

The acid dew point issue is analysed in Chapter 3. The acid components in the exhaust gas might cause corrosion in the duct. The dew point temperature of H2SO4 is around 90 °C in the exhaust gas. The corrosion of steel is probably caused by HF and CO2 in the exhaust gas. The influence of HF and CO2 on corrosion of steel is recommended to be figured out in the future. In Chapter 5, enhanced heat source model of the smelter is proposed. The temperature range of the exhaust gas is from 110 to 150 °C. The upper limit of the temperature is not exactly known. The upper limit is an important work to be figured out. The heat balance is an issue in this promotion. The heat loss from the cell carried with the flue gas is reduced with less dilution air. The arrangement of the thermal resistance of the cell has to be redesigned in order to keep the heat balance and avoid the heat accumulation and overheating of the cell. The system selection should be integrated with the local demand. For the town of Reydarfjordur in east Iceland, the basic space heating load can be met by the heat supply scenario or CHP scenario. Snow melting on the pavement can be taken by supplying the heat from aluminum production. Agriculture, fishing farming, drying, dehydration and other industries can also be conducted with the recyclable heat from aluminum production. The waste heat recovery project can play a vital role on improving life quality and promote the development of local economy.

搞定！

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Appendix

Appendix Technical paper

Waste heat recovery from aluminum production

Miao Yu1,2, Maria S. Gudjonsdottir2, Pall Valdimarsson2, Gudrun Saevarsdottir2 1.- Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, MOE, School of Mechanical Engineering, Tianjin University, Tianjin 300072, China; Email: [email protected]; Tel: +86 131 3226 3660 2.- School of Science and Engineering, Reykjavik University, Menntavegi 1, Reykjavik 101, Iceland Abstract Around half of the energy consumed in aluminum production is lost as waste heat. Approximately 30-45% of the total waste heat is carried away by the exhaust gas from the smelter and is the most easily accessible waste heat stream. Alcoa Fjarðaál in east Iceland produces 350 000 tons annually, emitting the 110 °C exhaust gas with 88.1 MW of heat, which contains 13.39 MW exergy. In this study, three scenarios, including organic Rankine cycle (ORC) system, heat supply system and combined heat and power (CHP) system, were proposed to recover waste heat from the exhaust gas. The electric power generation potential is estimated by ORC models. The maximum power output was found to be is 2.57 MW for an evaporation temperature of 61.22°C and R-123 as working fluid. 42.34 MW can be produced by the heat supply system with the same temperature drop of the exhaust gas in the ORC system. The heat requirement for local district heating can be fulfilled by the heat supply system, and there is a potential opportunity for agriculture, snow melting and other industrial applications. The CHP system is more comprehensive. 1.156 MW power and 23.55MW heating capacity can be produced by CHP system. The highest energy efficiency is achieved by the heat supply system and the maximum power output can be obtained with the ORC system. The efficiency of energy utilization in aluminum production can be effectively improved by waste heat recovery as studied in this paper. Keywords Aluminum production, Waste heat, ORC, District heating, Combined heat and power Introduction in east Iceland close to the town of Reyðarfjörður. The space heating demand of Reyðarfjörður is Aluminum production is a power-intensive estimated at approximately 4.05 MWth. industry. Around 72.3% of produced electricity in Power generation and direct heat utilization are Iceland was consumed by aluminum production in two approaches to utilize the low-temperature 2016 [1]. Approximately half of the energy energy. The conventional Rankine cycle is widely consumed in aluminum production is lost as waste used in power generation from high-temperature heat and the 30-45% of total waste heat is carried heat sources. The ORC system is considered as a away by the exhaust gas from the aluminum smelter low-temperature heat recovery technology for as seen in Fig. 1. Alcoa Fjarðaál is an aluminum power generation, using an organic substance with plant, producing 350 000 tons aluminum per year, a low boiling point as the working fluid.

Appendix

Nomenclature Ex exergy (kW) cond condenser h specific enthalpy (kJ/kg) eva evaporator m mass flow rate (kg/s) fan fan P pressure (bar) hex heat exchanger Q heat transfer rate (kW) gas state of gas S entropy (kJ/kg K) gen generator T temperature (°C) gral general heat transfer coefficient U mid middle (kW/m2K) v specific volume (m3/kg) net net amount w Power (kW) pin pinch point η efficiency pre preheater sup superheater Subscripts sys system amb ambient turb turbine cw cooling water wf working fluid

From the view of the first law of thermodynamics, A temperature-entropy diagram of the ORC higher energy efficiency can be obtained by direct system is shown in Fig. 2 The Kalina Cycle is a heat utilization. Various approaches are possible to power generation cycle which converts low- utilize thermal energy directly, such as space temperature thermal energy to mechanical power, heating, snow melting, swimming pools, using a mixture of ammonia and water as the agriculture applications and industrial use [3]. Power working fluid [2]. Thermoelectric generator (TEG) generation by the Rankine cycle and Brayton cycle can convert thermal energy from a temperature to recover the waste heat from aluminum smelter gradient into electric energy by Seebeck effect. exhaust gas was studied by Ladam Y. et al [4]. An Comparing the three methods for power generation, ORC system was designed by Ke (2009) to recover the technology of ORC is considered to be the most the heat from aluminum production [5]. Most feasible one. research focuses on electricity generation from the As a consequence of the second law of waste heat in aluminum production, but ignore the thermodynamics, the conversion of low- heating potential and the combination of electricity temperature heat to electricity has low efficiency. generation and heat utilization.

Table 1 System module component Heat source Power Cooling Heating module module module module Power generation system (Fig. ✓ ✓ ✓ 3a) Heat supply system (Fig. 3b) ✓ ✓ CHP system (Fig. 3c) ✓ ✓ ✓ ✓

Appendix

Exhaust gas 30~45%

T 21 4 3 5 2 5 3 4

Anode Anode 1 1s 6 Bath Sidewall 35% Aluminum 6s 8 7 Cathode Block 3 2

1 Bottom 7% Fig. 1 Heat loss distribution in aluminum S smelter [6] Fig. 2. Temperature-entropy diagram of an ORC system

Turbine

Condenser Sea level

Condenser Sea Waste heat Stack Cycle pump level recovery unit Heat exchanger Water pump

Evaporator Waste heat Preheater Stack recovery unit Cycle pump Gas fan

Water pump Stack Gas fan

Evaporator Waste heat Preheater recovery unit Dry scrubber Dry scrubber Gas fan

Dry scrubber

Smelter District Snow Swimming Fish Smelter District Snow Swimming Fish Smelter heating melting pool farming heating melting pool farming (a) (b) (c)

Fig. 3. Sketch of (a) ORC system. (b) Direct heating system. (c) CHP system. In this study, three scenario studies were Model carried out, concentrating on both electricity To simplify the model, the assumptions are generation and heat utilization. A proper multiple- made: objective target function was proposed for the • The system and heat source is at a steady state. power generation scenario and CHP scenario. The The temperature of the exhaust gas is 110 °C optimal parameters for each scenario were found by and the volumetric flow rate is 910 푚3/푠. optimization. • The heat sink of the cycle is the sea water, System description which is close to the aluminum smelter. The temperature is assumed at 6 °C, the mean The three scenarios are composed of four temperature of the local sea surface [7]. modules, which are the heat source module, the • 2 °C of superheat and 2 °C of subcooling are power module, the cooling module and the heating assumed at the outlet and inlet of the module (Table 1). The heat source module is the evaporator. same module in all scenarios. The cooling module • On the working fluid side, the pressure drops serves the power module as the cold sink. An and heat losses to the environment in the evaporator, condenser, turbine, and pump are interface for the heating system can be provided by neglected. the heating module.

Appendix

• The thermal resistance of conduction in heat The heat transfer in the evaporator can be given exchanger pipes/walls is neglected. by following equations. • The pressure drop in the heat source module 푄̇푒푣푎 = 푚̇ 푤푓(ℎ푤푓,5 − ℎ푤푓,2) (3.9) after waste heat recovery unit is considered ̇ (3.10) constant. 푄푒푣푎 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,1 − 푇푔푎푠,4) The T-s diagram of ORC system is shown in Where the 푄̇ refer to the heat transfer in the Fig. 2. The thermal energy content of the exhaust 푟ℎ푠 evaporator excluding the preheat section. gas can be calculated by The power of turbine is given as 푄̇푔푎푠 = 퐶푝,푔푎푠휌푔푎푠푉푔푎푠̇ (푇푔푎푠,1 (3.1) 푊̇ 푡푢푟푏 = 푚̇ 푤푓(ℎ푤푓,5 − ℎ푤푓,6) (3.11) − 푇̅ ) 푎푚푏 The efficiency of turbine is The 퐶푝,푔푎푠 is the heat capacity of the gas. The ℎ푤푓,5 − ℎ푤푓,6 (3.12) 휌 is the density of the gas. η푡푢푟푏 = 푔푎푠 ℎ푤푓,5 − ℎ푤푓,6푠 The heat transfer through preheater can be The generator power output is given as given as following equations. 푊̇푔푒푛 = η푔푒푛푊̇ 푡푢푟푏 (3.13) ̇ (3.2) 푄푝푟푒 = 푚̇ 푤푓(ℎ푤푓,2 − ℎ푤푓,1) The heat transfer in condenser can be given as 푄̇푝푟푒 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,4 (3.3) following equations.

− 푇푔푎푠,5) 푄̇푐표푛푑 = 푚̇ 푤푓(ℎ푤푓,6 − ℎ푤푓,8) (3.14)

Heat exchanger area can be calculated by 푄̇푐표푛푑 = 푚̇ 푐푤푐푝,푐푤(푇푐푤,3 (3.15) 푄̇ (3.4) − 푇 ) 푎푟푒푎 = 푐푤,1 푈Δ푇푚 Where 푄̇푐표푛푑,푐표푛 is the heat transfer rate in the The logarithmic mean temperature difference condensation section in condenser. can be calculated by The power of working fluid pump is calculated Δ푇푚푎푥 − Δ푇푚𝑖푛 (3.5) Δ푇푚 = as Δ푇푚푎푥 푙푛 푊̇ = 푚̇ Δℎ (3.16) Δ푇푚𝑖푛 푤푓,푝푢푚푝 푤푓 푤푓,푝푢푚푝 In general, the overall heat transfer coefficient ℎ푤푓,1푠 − ℎ푤푓,8 (3.17) 휂푝푢푚푝 = can be calculated by ℎ푤푓,1 − ℎ푤푓,8 1 1 훿 1 (3.6) The power of cooling seawater pump can be = + + 푈 푈𝑖푛푠𝑖푑푒 휆 푈표푢푡푠𝑖푑푒 calculated by 훿 The heat resistance of conduction, ,is ignored, 푊̇푐푤,푝푢푚푝 (3.18) 휆 푚̇ (휌 푔Δℎ + Δ푝) then = 푐푤 푐푤 1 1 1 (3.7) 휌푐푤휂 = + The power of gas fan can be calculated by 푈 푈𝑖푛푠𝑖푑푒 푈표푢푡푠𝑖푑푒 ̇ The minimum temperature difference between 푊푔푎푠,푓푎푛 (3.19) the hot and cold stream in the heat transfer unit is 푣푔푎푠̇ (푝푔푎푠,1 − 푝푔푎푠,5) = referred to as the pinch point temperature difference 휂푔푎푠,푓푎푛 (PPTD). The pinch position in a heat exchanger The volume flow was changed by the emerges at the place where the heat transfer is the temperature drop of the exhaust gas. The power most constrained [8]. Therefore, the PPTD is the consumption of original fan module reduces with critical parameter in an evaporator and condenser. the less volume flow after cooling, which originally The heat transfer area and cycle performance are exists in the gas treatment center (GTC). The flow influenced by PPTD. The PPTD in evaporator is rate of the exhaust gas at the outlet of waste heat

Δ푇푝𝑖푛,푒푣푎 = 푇푔푎푠,3 − 푇푤푓,3 (3.8) recovery unit can be calculated by the following equation:

Appendix

푉푔푎푠̇ ,표푢푡 (3.20) The enthalpy exergy of gas can be calculated

푇푔푎푠,표푢푡 푃푔푎푠,𝑖푛 by = 푉푔푎푠̇ ,𝑖푛 푇푔푎푠,𝑖푛 푃푔푎푠,표푢푡 퐸̇푥,푔푎푠 = 푚̇ 푔푎푠(푐푝(푇 − 푇0) (3.25) The original fan power is saved by the volume 푝 + 푅푔푇0푙푛 flow rate descending: 푝0 ̇ 푇 푊푓푎푛,푠푎푣푒 (3.21) − 푐 푇 푙푛 ) 푝 0 푇 푉̇ Δ푃 푉̇ Δ푃 0 = 푔푎푠,𝑖푛 푔푎푠 − 푔푎푠,표푢푡 푔푎푠 The general exergy efficiency of system 휂푓푎푛,표푟𝑖 휂푓푎푛,표푟𝑖 푊̇ 푛푒푡,푔푟푎푙 (3.26) The net power output is considered as 휂푠푦푠,푒푥푒푟푔푦 = 퐸̇푥,푔푎푠 푊̇ = 푊̇ − 푊̇ (3.22) 푛푒푡 푔푒푛 푔푎푠,푓푎푛 For the heat supply system, the heat transfer in − 푊̇ 푐푤,푝푢푚푝 the waste heat recovery unit is considered as − 푊̇ 푤푓,푝푢푚푝 푄̇ℎ푒푎푡 = 푚̇ 푔푎푠푐푝,푔푎푠(푇푔푎푠,5 (3.27) The saving power of the gas fan module is − 푇푔푎푠,표푢푡) considered as a part of power output by the ORC 푄̇ℎ푒푎푡 (3.28) system. Therefore, the general net power output = 푚̇ 푐 (푇 can be defined as 푤푎푡푒푟 푝,푤푎푡푒푟 푤푎푡푒푟,𝑖푛 − 푇푤푎푡푒푟,표푢푡) 푊̇ 푛푒푡,푔푟푎푙 = 푊̇푔푒푛 − 푊̇ 푔푎푠,푓푎푛 (3.23) The power input for the heat supply system is − 푊̇푐푤,푝푢푚푝 푊̇ 𝑖푛푝푢푡 (3.29) − 푊̇ 푤푓,푝푢푚푝 = 푊̇ 푓푎푛,푠푎푣푒 − 푊̇ 푔푎푠,푓푢푛 + 푊̇ 푓푎푛,푠푎푣푒 − 푊̇ 푤푎푡푒푟,푝푢푚푝 The general thermal efficiency of the system is The thermal efficiency of heat supply system is 푊̇ 푛푒푡,푔푟푎푙 (3.24) η푠푦푠,푡ℎ푒푟푚푎푙 = 푄̇푔푎푠

푄̇ℎ푒푎푡 (3.30) 푚푎푥: 휂푠푦푠,푒푥푒푟푔푦 (4.2) ηℎ푒푎푡 = 푄̇푔푎푠 푚𝑖푛: 퐴푟푒푎 (4.3) Optimization Three objective functions are integrated into two functions and keep the same direction. These Objective function two objective functions can be combined into A multi-objective function, which combines 퐹(푥)with the weighting factors 휑, 휓. ̇ various single objective functions, can make the 푊푛푒푡. (4.4) 푚푎푥: 푓1 = optimization comprehensive. The scalarization 퐴푟푒푎 (4.5) method is an efficient approach to obtain the multi- 푚푎푥: 푓2 = 휂푠푦푠,푒푥푒푟푔푦 objective function from multiple individual 푚푎푥: 퐹(푥) = 휑퐹1(푥) + 휓퐹2(푥) (4.6) objective functions [9]. For the power generation Therefore, the objective function is proposed as ̇ system, the objective function and direction are 푊푛푒푡,푔푟푎푙 푚푎푥: 퐹(푥) = 휑 + 휓(100 ∙ 휂푠푦푠,푒푥푒푟푔푦) (4.7)) shown below: 퐴푟푒푎

푚푎푥: 푊̇ 푛푒푡 (4.1)

Appendix

Fig. 4 Iteration procedure of 1 optimizationWhere 퐹1 is the maximum value of 2 2 퐹1(푥); 퐹2 is the maximum value of 퐹2(푥); 퐹1 is

the value of 퐹1(푥) when 퐹2(푥) reaches the 1 maximum; 퐹2 is the value of 퐹2(푥) when 퐹1(푥) reaches the maximum. Iteration procedure The design variables for power generation

system are 푇푤푓,푒푣푎 , 푇푤푓,푐표푛푑 , Δ푇푝𝑖푛,푒푣푎 and Δ푇 . When the differences between the input Fig. 5. Working fluid selection for ORC system 푝𝑖푛,푐표푛푑 and results of design variables are less than 0.05%, By the similar principle, the objective function convergence condition is met and then output the of optimization for CHP system is result. Otherwise, the next iteration will be 푊̇ 푄̇ (4.8) 퐹(푥) = 휑 푛푒푡,푔푟푎푙 + 휓 ℎ푒푎푡 conducted, shown in Fig. 4. For CHP system, the 퐴 퐴 푂푅퐶 ℎ푒푥 design variables are 푇 , 푇 , 푇 and In the condition, the dimension and magnitude 푔푎푠,푚𝑖푑 푤푓,푒푣푎 푤푓,푐표푛푑 Δ푇 , shown in Table 4. A critical parameter, of two terms are same. The weighting factor can be 푝𝑖푛,푐표푛푑 decided by following equations [10]. the gas temperature at the outlet of evaporator, is 1 2 defined as the middle temperature 푇푔푎푠,푚𝑖푑. 퐹2 − 퐹2 (4.9) 휑 = 2 1 1 2 (퐹1 − 퐹1 ) + (퐹2 − 퐹2 ) Result and discussion 2 1 퐹1 − 퐹1 (4.10) Fig. 5 shows the variation of the objective 휓 = 2 1 1 2 (퐹1 − 퐹1 ) + (퐹2 − 퐹2 ) function with the various evaporation temperature Initial data input Working fluid, for 14 different working fluids. Five working fluids, Teva,Tcond,TPPTD_eva,TPPTD_cond

Working fluid, Teva,Tcond,TPPTD_eva,TPPTD_cond R-236fa, R-245fa, R-236ea, R-123 and R-600a, have good thermodynamic performance. The Weighting factor φ,ψ environmental issue and safety of the working fluid are considered as well. R-123 has good

Working fluid environmental friendly property, comparing with R-236fa, R-245fa, and R-236ea, shown in Table 2. The high flammability makes R-600a a low safety. Evaporation temperature Teva Therefore, R-123 is selected as working fluid in the iteration. The optimal parameters of the power

Condensation temperature generation system and CHP system are shown in Tcond Table 3. In the condition of the same temperature drop PPTD in evaporator

TPPTD,eva for the exhaust gas, the optimal performance of each system is shown in Table 4. A considerable

PPTD in condenser difference of the evaporation temperature between TPPTD,cond the power generation system and CHP system, owing to the different heat source conditions for the No Δ<0.005 two systems. Yes

Results output Working fluid, Teva,Tcond,TPPTD_eva,TPPTD_cond

Appendix

Table 2 Properties of working fluids [11] R-236fa R-245fa R-236ea R-123 R-600a

Molecular 퐶퐹3퐶퐻2퐶퐹3 퐶퐹3퐶퐻2퐶퐻퐹2 퐶퐹3퐶퐻퐹퐶퐻퐹2 퐶퐻퐶푙2퐶퐹3 퐶퐻(퐶퐻3)3 formula ODP 0 0 0 0.02 0 GWP(100 years) 9810 1030 1370 77 3

Table 3 Iteration results of optimal parameters Working Evaporation Condensation PPTD in PPTD in System fluid temperature temperature evaporator condenser °C °C °C °C Power generation R-123 61.22 16.33 9.469 3.501 system Middle CHP system temperature R-123 76.02 17.10 86.43 3.857

Table 4 System performance of three scenarios Parameters ORC Heat supply CHP

푊̇ 푔푟푎푙 2.573푀푊 -0.104 푀푊 1.156푀푊

푄̇ℎ푒푎푡 0 42.34 푀푊 23.550 푀푊

휂푒푥푒푟푔푦 19.220% 0 8.63%

휂푡ℎ푒푟푚푎푙 2.921% 48.1% 28.05% 퐴푟푒푎 21115푚2 14412푚2 25509푚2

푊̇ 푛푒푡 1.248 푀푊 0 -0.195푀푊

푊̇푔푒푛 4.17 푀푊 0 2.344푀푊

푊̇ 푔푎푠,푓푎푛 -2.417 푀푊 -1.39 푀푊 -2.295 푀푊

푊̇푓푎푛,푠푎푣푒 1.325 푀푊 1.324 푀푊 1.351푀푊

푊̇푐푤,푝푢푚푝 -0.465 푀푊 0 -0.194푀푊

푊̇ 푤푓,푝푢푚푝 -0.0403 푀푊 -0.03792 푀푊 -0.0291 푀푊

푇푔푎푠,표푢푡푙푒푡 58.84°C 58.89°C 58°C

푚̇ 푤푓 207.4퐾푔/푠 0 91.97퐾푔/푠

푚̇ 푐푤 1278퐾푔/푠 0 533.1퐾푔/푠

푚̇ ℎ푒푎푡 0 252.81퐾푔/푠 140.61퐾푔/푠

The highest exergy efficiency of 19.22% is The gas fan power consumption in the three obtained and 2.573MW electricity is produced by systems are considerable. More heat exchanger area the power generation system. The thermal is required by CHP system and less is required by efficiency of 48.1% and heating capacity of heat supply system. 42.34MW are obtained by the heat supply system. From the thermodynamic view, the low The power output and heating capacity of CHP emission temperature of the gas is the more thermal system are 1.156MW and 23.55MW separately. energy is recovered by the waste heat recovery unit.

Appendix

However, the temperature is also limited by the acid desalinization, drying industry and other dew point temperature in the exhaust gas and industrial applications. cannot reach too low. The dew point temperature is influenced by the partial pressure of the acid in the Acknowledgements exhaust gas. The absolute pressure of exhaust gas is Gratefully acknowledge Alcoa Foundation for as low as 0.96 bar. Therefore, a low acid dew point the financial support of this work. The authors also temperature is possible, which will pose an wish to acknowledge Jón Steinar Garðarsson effective lower temperature limit on exhaust gas Mýrdal at Austurbrú, Hilmir Ásbjörnsson, Unnur temperature The further quantitative analysis Thorleifsdottir at Alcoa Fjarðaál and Þorsteinn should be taken in the future. Sigurjónsson at Fjarðabyggð Municipal Utility for Conclusion their assistance. In this study, the low-temperature waste heat of References exhaust gas from the aluminum smelter is [1] Orkustofnun (2017). OS-2017-T016-01: recovered the three different scenarios. The ORC Development of electricity consumption in Iceland (2016) technology is used in the power generation system http://os.is/gogn/Talnaefni/OS-2017-T016- and CHP system. A multiple-objective function and 01.pdf. Accessed 30 August 2017. a new optimization method were proposed for the [2] Zhang, X., He, M., & Zhang, Y. (2012). A optimization of the ORC system. Four design review of research on the Kalina cycle. variables were selected by the optimization Renewable and sustainable energy iteration procedures. From the result, the main reviews,16(7), 5309-5318. conclusion can be summarized as follows: [3] Popovski, K., Andritsos, N., Fytikas, M., Sanner, B., Sanner, S., & Valdimarsson, P. 1. At the optimal condition, 2.573 MW electricity e (2010). Geothermal energy. Skopje. can be produced by the power generation system. [4] Ladam, Y., Solheim, A., Segatz, M., & For CHP system, 1.156 MW power and 23.55 e Lorentsen, O. A. (2011). Heat recovery from MW heating capacity can be produced. 42.34 th aluminum reduction cells. In Light Metals MW heating capacity can be produced by heat th 2011: 393-398. supply system. The considerable potential of [5] Ke W.(2009), Research on waste heat power waste heat recovery from aluminum production generation of aluminum reduction cells flue was established. based on ORC, MSc thesis, Central South 2. The performance of ORC is significantly University Changsha. influenced by selection of working fluid and [6] Fleer, M., Lorentsen, O. A., Harvey, W., design parameters. Palsson, H., & Saevarsdottir, G. (2010). Heat 3. Comparing various scenarios, the highest exergy recovery from the exhaust gas of aluminum efficiency, 19.22%, can be obtained by power reduction cells.Minerals, Metals and generation system and the highest energy Materials Society/AIME, 420 efficiency of 48.1% can be obtained by the heat Commonwealth Dr., P. O. Box 430 supply system. The efficiency of organic Rankine Warrendale PA 15086 USA.[np]. 14-18 Feb. cycle is limited by the low temperature of the [7] Sea surface temperature 1998+ (1 Month exhaust gas. A high energy efficiency can be MWOI). NASA. obtained by heat direct utilization. https://neo.sci.gsfc.nasa.gov/servlet/Render 4. The system selection should be integrated with Data?si=1710746&cs=gs&format=SS.CSV the local demand. For the town of Reyðarfjörður &width=250&height=125. Accessed 4 in east Iceland, the basic space heating load can September 2017. be amply met by the heat supply scenario or CHP [8] Dimian, A. C., Bildea, C. S., & Kiss, A. A. scenario. More direct thermal utilizations can be (2014). Integrated design and simulation of developed with this considerable heat potential, chemical processes(Vol. 13). Elsevier. such as snowing melting, greenhouse,

Appendix

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