Utilizing Waste Heat in EU Industries

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Supercritical Carbon Dioxide Brayton Cycle for Power

Generation

Utilizing Waste Heat in EU Industries

BJÖRN J. THORSSON HADY R. SOLIMAN

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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www.kth.se

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Abstract

The industrial sector accounts for approximately 30% of the global total energy consumption and up to 50% of it is lost as waste heat. Recovering that waste heat from industries and utilizing it as an energy source is a sustainable way of generating electricity. Supercritical CO₂(sCO₂) cycles can be integrated with various heat sources including waste heat. Current literature primarily focuses on the cycle’s performance without investigating the economics of the system.

This is mainly due to the lack of reliable cost estimates for the cycle components. Recently developed cost scaling models have enabled performing more accurate techno-economic studies on these systems. This enables a shift in focus from plant efficiency to economics as a driver for commercialization of sCO₂ technology. This work aims to develop a techno-economic model for these waste-heat-to-power systems. Based on the literature, waste heat from different industries is calculated, showing that the four industries with the greatest potential for waste heat recovery are cement, iron and steel, aluminum and gas compressor stations. Six different sCO₂ cycle configurations were developed and simulated for these four industries. The techno-economic model optimizes for the highest Net Present Value (NPV) using an Artificial Bee Colony algorithm. The optimization variables are the pressure levels, split ratios, recuperators effectiveness, condenser temperature and the turbine inlet temperature limited by the heat source. The results show a vast potential for industries to cut down costs using this system. Out of the four industries modeled, a waste heat recovery system in an iron and steel factory yielded the highest NPV. Results show that the integration of sCO₂ cycle in the cement industry could help reduce their waste heat by 60%, whilst simultaneously enabling them to cover up to 56% of their electricity demand. The payback period for the four industries varies between 6 to 9 years. Furthermore, simple recuperated sCO₂ cycles with preheating are more economical than recompression cycles. Even though recompression cycles have higher thermal efficiency, they are limited by the temperature glide in the waste heat exchanger.

This analysis could help investors and engineers take more informed decisions to increase the efficiency and economic return on investment for sCO₂ cycles and heat recovery at industrial sites. To encourage adoption of supercritical CO2cycles, a demo is needed along with more research for higher temperature applications with special attention to mechanical integrity.

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Sammanfattning

Industrisektorn står för cirka 30% av den globala totala energiförbrukningen och upp till 50% av den går förlorad som spillvärme. Återskapa att spillvärme från industrier och använda det som energikälla är ett hållbart sätt att produce- ra el. Superkritiska CO₂ (sCO₂) cykler kan integreras med olika värmekällor inklusive spillvärme. Nuvarande litteratur fokuserar främst på cykelens pre- standa utan att undersöka systemets ekonomi. Detta beror främst på bristen på tillförlitliga kostnadsberäkningar för cykelkomponenterna. Baserat på nyli- gen utvecklade kostnadsskalningsmodeller är det möjligt att utföra mer exakta teknikekonomiska studier på dessa system. Detta möjliggör en förskjutning i fokus från cykeleffektivitet till ekonomi som drivkraft för kommersialisering av sCO₂ teknologi. Detta arbete syftar till att utveckla en teknisk ekonomisk modell för dessa avfall-värme-till-kraftsystem. Baserat på litteraturen beräk- nas spillvärme från olika industrier, vilket visar att de fyra industrierna med störst potential för återvinning av spillvärme är cement, järn och stål, aluminium och gaskompressorstationer. Sex olika sCO₂ konfigurationer utvecklades och simulerades för dessa fyra industrier. Den teknisk-ekonomiska modellen optimerar för det högsta Net Present Value (NPV) med hjälp av en artificiell bi-kolonialgoritm. Optimeringsvariablerna är pressure levels, delade förhål- landen, recuperatorseffektivitet, kondensortemperatur och turbininloppstem- peraturen begränsad av värmekällan. Resultaten visar en stor potential för industrier att sänka kostnaderna med detta system. Av de fyra modellerna in- dustrin gav ett återvinningssystem i en järn och stålfabrik den högsta NPV.

Resultaten visar att integrationen av sCO₂cykeln i cementindustrin kan bidra till att minska deras spillvärme med 60%, samtidigt som de gör det möjligt för dem att täcka upp till 56% av deras elbehov. Återbetalningsperioden för de fyra branscherna varierar mellan 6 till 9 år. Dessutom är simple recuperated sCO₂ cykler med förvärmning mer ekonomiska än recompressioncykler.

Trots att recompressioncykler har högre termisk effektivitet, begränsas de av temperaturglidningen i spillvärmeväxlaren. Denna analys kan hjälpa investe- rare och ingenjörer att fatta mer informerade beslut för att öka effektiviteten och ekonomiska avkastningen på investeringar för sCO₂ cykler och värmeå- tervinning på industriområden. För att uppmuntra antagandet av superkritiska CO2 cykler krävs en demo tillsammans med mer forskning för högre tempe- raturapplikationer med särskild uppmärksamhet på mekanisk integritet.

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Acknowledgments

First off, we would like to thank Rafael Guédez, our supervisor. He played a huge role in inspiring us towards this topic and pushing us forward. He is one of the best professors at KTH, passionate about his students and leading a transformation through inspiring the next generation. We would also like to extend our gratefulness towards Silvia Trevisan, who gave us a lot of resources and guided our way of thinking. We are also grateful for our friends and family, who believed in us and encouraged us along the way.

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1 Introduction 1

1.1 Objectives . . . 2

1.2 Limitations . . . 2

1.3 Methodology . . . 4

1.4 Division of Work . . . 5

2 Literature Review 6 2.1 Waste Heat Recovery Potential . . . 6

2.2 Waste Heat Recovery Systems . . . 10

2.2.1 Organic Rankine Cycle . . . 11

2.2.2 Kalina Cycle . . . 12

2.3 Supercritical CO₂Cycle . . . 13

2.3.1 Cycle Configurations . . . 17

2.3.2 Cycle Equipment . . . 22

2.3.3 Emissions Trading System . . . 25

3 Industries and Waste Heat Estimation 28 3.1 Iron and Steel . . . 28

3.2 Gas Compression . . . 30

3.3 Cement . . . 32

3.4 Aluminum . . . 34

3.4.1 Hall Heroult . . . 35

3.4.2 Bayer Process . . . 36

3.4.3 Anode Baking . . . 36

3.4.4 Furnace Chimney . . . 36

3.5 Waste Heat Estimation Summary . . . 39

4 Performance Model 40 4.1 System Optimization . . . 40

4.2 Design Point Model . . . 44

IV

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4.2.1 Turbomachinery . . . 44

4.2.2 Heat Exchangers (HEXs) . . . 44

4.2.3 Cycle Model . . . 48

4.2.4 Fluid Properties . . . 51

4.2.5 Economic Model . . . 51

4.3 Heat Pump Model . . . 53

4.4 Packed Bed thermal Storage Model . . . 54

4.5 Key Performance Indicators . . . 55

4.5.1 Thermodynamic KPIs . . . 56

4.5.2 Economic KPIs . . . 57

4.5.3 Environmental KPI . . . 58

5 Results and Discussion 59 5.1 Different Industries . . . 59

5.1.1 Heat Pumps . . . 61

5.2 Different Cycles . . . 63

5.3 Effect of Split Ratio on NPV . . . 63

5.4 Effect of Turbine Inlet Temperature . . . 64

5.5 Effect of Recuperator Effectiveness . . . 65

5.6 System Costs . . . 66

5.7 Sensitivity Analysis . . . 67

5.8 Model Verification . . . 68

5.8.1 Cycle Model Verification . . . 69

5.8.2 Heat Exchanger Verification . . . 69

6 Conclusion 72

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2.1 Global Waste Heat as Estimated by Forman et al. (2016) . . . 7

2.2 EU Waste Heat as estimated by Papapetrou et al. (2018). . . . 8

2.3 Ts Diagrams of (a) ORC , (b) Kalina Cycle (Jouhara et al. 2018) . . . 12

2.4 Temperature profiles of heat source with (a) pure fluid, (b) zeotropic fluid, and (c) supercritical fluid. (Liu et al. 2019) . . . 15

2.5 Density of sCO2at Different Pressures and Temperatures (Cunha et al. 2018). . . 16

2.6 Simple Recuperated Cycle. . . 18

2.7 sCO2Recompression Cycle . . . 18

2.8 CO2 specific heat capacity variation in the recuperators (Liu et al. 2019). . . 19

2.9 Recompression as Topping Cycle. . . 20

2.10 Simple Recuperated as Topping Cycle. . . 20

2.11 Preheating Cycle . . . 21

3.1 Coke Production Process . . . 29

3.2 Combined 2 Cycle for Coke Production . . . 29

3.3 Simple sCO2 Cycle for Coke Production . . . 30

3.4 Distribution of Gas Compressor Stations over Europe . . . 31

3.5 Variation of Flow Rate throughout the Year . . . 32

3.6 Clinker Forming Process . . . 33

3.7 Upstream sCO2 Cycle in Cement Industry . . . 33

3.8 Cement sCO2 Heat Pump . . . 34

3.9 Cement sCO₂ Heat Engine . . . 35

3.10 Schematic of Anode Baking . . . 37

3.11 Temperatures along the Anode Baking Furnace . . . 37

3.12 Variations along the Furnace . . . 38

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3.13 Aluminum Upstream sCO₂Cycle . . . 39

4.1 sCO2 Recompression Cycle . . . 40

4.2 Optimization Model . . . 43

4.3 Iterative process logic flow for the cycle model. . . 49

4.4 Recuperated Storage . . . 54

5.1 NPVs of the Different Industries . . . 60

5.2 DPBs of the Different Industries . . . 60

5.3 Electric Demand Sufficiency of Different Industries . . . 61

5.4 The Effect of Difference in Prices on a Cement Heatpump NPV 62 5.5 The Effect of Difference in Prices on an Aluminum Heatpump NPV . . . 62

5.6 NPVs of the Different Cycle Configurations . . . 63

5.7 NPVs of Different Split Ratio in Recompression Cycle . . . . 64

5.8 Effect of TIT on NPV and Efficiency . . . 65

5.9 Effect of Recuperator Effectiveness on NPV and Efficiency . . 66

5.10 CAPEX Share Among Main Plant Components . . . 66

5.11 Share of Different Components in Powerblock Cost . . . 67

5.12 Sensitivity Analysis of Change in Power Block Price . . . 68

5.13 Sensitivity Analysis of Change in Electricity Price . . . 68

5.14 The Influence of Number of Subsections on Error of Main Heater Calculations . . . 70

5.15 The Influence of Number of Subsections on Error of Recuper- ator Calculations . . . 70

5.16 The Influence of Number of Subsections on Error of Preheater Calculations . . . 71

6.1 Sensitivity Analysis of Change in Compressor Cost . . . 94

6.2 Sensitivity Analysis of Change in Turbine Cost . . . 94

6.3 Sensitivity Analysis of Change in Recuperator Cost . . . 95

6.4 Sensitivity Analysis of Change in Condenser Cost . . . 95

6.5 Sensitivity Analysis of Change in Discount Rate . . . 96

6.6 Sensitivity Analysis of Change in DownTime . . . 96 6.7 Sensitivity Analysis of Change in Waste Heat Exchanger Price 97 6.8 Some Survey Answers as Received from the Cement Industry . 98

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2.1 Number of Plants and Audits from the Campana et al. (2013) Study . . . 8 2.2 Summary of Results from Campana et al. (2013). . . 9 2.3 Recoverable heat in Exhausts of Various Steelmaking Processes

(McKenna & Norman 2010). . . 9 3.1 Summary of Aluminum Industry and extrapolating to Europe . 38 3.2 Summary of Waste Heat Estimations in the EU . . . 39 4.1 Information needed for the waste heat flow . . . 42 4.2 Ranges of design variables for recompression cycle optimization 42 4.3 Operating assumptions for the sCO₂Waste Heat Recovery cy-

cles. . . 48 4.4 Economic Assumptions Used in Calculating KPIs . . . 51 4.5 Industrial Electricity Prices in EU Countries . . . 52 4.6 Thermodynamic Properties of Packed Bed Material (Tiskati-

nee et al. 2017, Tiskatine et al. 2017, Becattini et al. 2017, Haenel et al. 2012). . . 55 4.7 Values of the main parameters used for validation of the packed

bed model by Trevisan et al. (2019). . . 56 5.1 Model verification for cycle thermal efficiency using data re-

ported by Manente & Lazzaretto (2014). . . 69 6.1 Components’ Reference Prices and Exponents . . . 92 6.2 Storage Component Costs . . . 92 6.3 Labor and Material Costs’ share of Component Costs (Wei-

land 2019) . . . 93

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Introduction

The amount of CO₂ in the air has been increasing steadily with human activity. This has led to global warming, an increase of temperatures on the Earth. The power and industry sectors produce around 60% of global CO2

emissions (Lead n.d.). A lot of solutions have been addressing this problem, by trying to reduce energy demand, reusing elements in a circular economy or using renewables. However, a solution that addresses the two sources simultaneously is utilizing waste heat from industries to produce power. There are different methods to generate power from waste heat such as Kalina and Organic Rankine Cycles. However, they are limited by their low efficiencies.

A novel emerging technology is the supercritical Carbon Dioxide (sCO₂) Cy- cle. When using medium temperature waste heat, sCO2 is preferred due to its compactness, cost and high thermal efficiency (Wang & Dai 2016, Santini et al. 2016). It is a closed Brayton cycle that can achieve efficiencies, higher than a traditional steam Rankine cycle at similar temperatures. An important question is can sCO2 cycles be designed in such a way that it delivers higher efficiencies at feasible costs and therefore yielding a high NPV for industrial waste heat recovery?

The work hereby presented aims at studying this question in different industries and seeing the financial benefit. The methodology to answer the question is to first model the sCO2cycle based on the available waste heat, followed by a techno-economic optimization of different parameters and systems. The study also compares the results with other research and aforementioned technologies.

The waste heat can be used in the same industrial plant where it is produced. To which degree waste heat is recovered relies on different technical and economic factors; heat source temperature, mass flow of the waste heat,

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dust content, chemical composition and intermittency of the source.

There seems to be a great future for sCO2cycles. It has mostly been investigated coupled with heat sources from nuclear, coal or solar plants. This work aims at investigating sCO₂Cycles for waste heat recovery from EU industries.

1.1 Objectives

The purpose of this Thesis is to further develop the model of a sCO₂ cycle, in order to identify its potential role in the waste heat recovery market. This is achieved through performing an optimization analysis to be able to find the best set of different parameters that yields a financially attractive sCO2 cycle, using a number of cost variables (KPIs). It is followed by a sensitivity analysis to be able to recognize the most important factors that need further research.

The specific objectives are as follows:

• Identify best Industries and best integration schemes

• Develop a flexible model that can be applied to different industries

• Implement these models using real-life data

• Investigate the techno-economic performance of these systems and financial feasibility

• Identify the market conditions that make these models financially attractive.

• Perform sensitivities to electricity prices and advances in technology

1.2 Limitations

This thesis is limited to optimizing an sCO2 cycle for the highest NPV in 4 different industries using 6 different configurations. It solely focuses on the EU, and results may vary in other locations. There are some possible limitations in this study. First is that the cooler component was simplified to save computing power as is discussed in Section 4.2.2. For further studies, more computing power will yield more accurate results. Secondly, there is a possibility of error in waste heat estimations carried out by other researchers and the authors of this paper. Another problem was that the survey sent out was filled by a few industry players. Coupled with the fact that there is a limited

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access to real data from industries. The aforementioned three limitations can be overcome in the future by partnering with industries that are interested in this technology. Thirdly, The cost model for the waste heat recovery heat exchanger is not as details as the model used for other components, which may lead to misleading results. Development of a detailed cost model for that component would enhance the accuracy of the economic analysis. Last, there is a limited knowledge of sCO2 cycle component performance. However, as time progresses and the technology evolves, experience will increase.

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1.3 Methodology

1. Literature Review of the different topics presented here such as the sCO2cy- cle, recompression cycle, different industries such as Steel and Cement. These are presented in Chapter 2. It consists of the following steps:

• Understanding sCO₂Cycles and components

• Gathering info about waste heat in EU

• Gathering info about major industries and how they operate

• Contacting one industry and getting real data

• Analyzing the industries and calculating waste heat. For each industry, this is the general process:

– Check EU production of the desired industry.

– Analyze different existing factory sizes and their heat streams (flow rate, temperature and specific heat capacity)

– Figure out which heat streams can be utilized (some are very im- portant for the industry and some might be contaminated)

– Check their future plans and the vision for 2050

– Calculate a factor of Waste Heat per tonne of production – Multiply Heat factor with EU production capacity – Validate using research and existing literature review

• Researching Waste Heat Recovery Technologies

• Investigating Emission Trading Scheme (ETS)

2. Model Development and Implementation. This part is two-fold. The first part aims at modeling waste heat recovery for the different industries. Second part aims at developing a MATLAB model of the sCO₂ cycle along with its complex recuperators and incorporating the heat from different industries. The Model Development is explained in Chapter 4. The process is as follows:

• With the gathered data, the industries are analyzed. Therefore it is feasible to see where the sCO2 cycle can add value to the plant owners. The plant size to be used in the model will be an average plant size in the EU.

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• Designing the sCO₂Cycle and the parameters to use

• Modeling the sCO2Cycle

• Verification by comparing the efficiency to other studies

3. Techno-economic optimization of different industries using the developed model of sCO2Cycle. The process is explained in further detail in Chapter 4.

• Gathering Costs of different components

• Profitability analysis using NPV and payback period

• Optimizing the cycle using a Metaheuristic algorithm for the highest NPV

4. Analyzing Results. The final optimal results of each industry are analyzed.

• Calculating exergy efficiency and waste heat utilized

• Calculating CO2 mitigated

• Comparing results of different industries

• Figuring out obstacles different industries have to overcome and which industry is most suited to start implementing the sCO₂Cycle.

1.4 Division of Work

The work load was equally divided among the two authors. Both were respon- sible for deeply understanding the cycle and how it works. Both also worked on the literature review and data gathering about the different industries. Both authors worked on developing the model structure for the power cycle, i.e., how it would calculate all thermodynamic states through the cycle. While Björn went into the deep details of the cycle components, optimization methods, the calculations of the heat exchangers, speeding up the code, and running calculations. Concurrently, Hady went through all the data gathered and performed the estimations of the waste heat potential for EU industries and began writing the literature review and the report. Both authors worked on analyzing the results once the optimization was done and then both worked on finishing the writing of the thesis.

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Literature Review

This chapter contains the information gathered at the beginning of the work. It also provides the foundation on which the modeling will be performed. In specific, this chapter will focus on the amount of unutilized industrial waste heat in the European Union, the major industries, waste heat recovery technologies and Emission Trading Scheme.

2.1 Waste Heat Recovery Potential

Forman et al. (2016) calculated that there are approximately 68 PWh of wasted heat every year globally. The industrial high grade heat at temperatures above 300^◦C was estimated to be 3.4 PWh. Their methodology was checking how much energy sources are being used (such as coal and petroleum), and how much useful energy each sector actually consumes. The remainder is the waste heat. This is illustrated in Figure 2.1.

Firth et al. (2019) continued building on Forman’s research and delved into the industrial sector . They estimated the global waste heat from industries in 2030 under different scenarios. The biggest industrial potential was in non- metallic minerals (e.g. cement and glass) and Iron & Steel industries.

Some researchers focused on low grade waste heat in the EU. For example, Kosmadakis (2019) investigated how heat pumps can be used to upgrade low- grade waste heat industry utilization. Cayer et al. (2010) investigated using a transcritical CO₂(tCO₂) cycle to generate electricity from industrial low grade heat. However, the reported thermal efficiencies are low between 6-9%.

Hammond & Norman (2014) calculated the waste heat coming out of industries in the UK. It was based on research and data gathered by McKenna

& Norman (2010). Their main data source was the UK National Allocation

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Figure 2.1: Global Waste Heat as Estimated by Forman et al. (2016)

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Plan for the EU Emissions Trading Scheme, which has data of 90% of energy intensive industries. They estimate their error in calculations to be ± 33%. Pa- papetrou et al. (2018) took Hammond’s research and scaled it up, accounting for different energy intensities and different energy efficiency improvements.

They showed that for the EU alone, the potential of industrial waste heat is 314 TWh/year, with 33% at temperatures of 100-200^◦C (100 TWh/year), 25% between 200-500^◦C (78 TWh/year) and the rest above 500^◦C (124 TWh/year).

Figure 2.2: EU Waste Heat as estimated by Papapetrou et al. (2018).

Figure 2.2 shows that there is a huge potential in the Iron & Steel Industry.

It is followed by non-metallic minerals (cement and glass) and then the Non- ferrous metals (Aluminum industry).

Campana et al. (2013) focused on four main industries in the EU; Steel, Gas Compression, Cement and Glass. They analyzed 44 audits and feasibility studies of different factories. The number of plants modeled and audits investigated is shown in Table 2.1 and a summary of their results is shown in Table 2.2.

Table 2.1: Number of Plants and Audits from the Campana et al. (2013) Study

Industry Number of Number of

plants considered audits analyzed

Iron and Steel 399 8

Cement 241 21

Glass 58 5

Gas Transmission 613 10

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Table 2.2: Summary of Results from Campana et al. (2013).

Industry T Mass flow rates EU Energy recovery

(^◦C) (kg/s) (GWh)

Steel 200-250 3-8

Steel 300-320 8-20 5984

Cement 260-420 15-110 4592

Float Glass 300-470 20-45 628

Gas Compression 300-500 30-70

10432

Gas Storage 300-500 30-70

McKenna & Norman (2010) concluded that even though Aluminum has a lot of waste heat, but most of it is low grade waste heat. However, they have showed that there is high grade waste heat in the steel industry as can be seen in Table 2.3.

McBrien et al. (2016) also confirm this. They mention the importance of upgrading the waste heat recovery systems installed in steel mills to be able to utilize it more.

Table 2.3: Recoverable heat in Exhausts of Various Steelmaking Processes (McKenna & Norman 2010).

Process

Process Temp.

(^◦C)

Exhaust Temp.

(^◦C)

Recoverable heat in exhaust

(GJ/t steel)

Exhaust Stream (gas/solid) Coke ovens 1100 1100 0.12 - 0.24 Hot coke (s)

Sintering 1350 350 0.49 - 0.97 Cooler and

exhaust gas (g) Blast

furnace 1500 150 0.16 - 0.31 Blast furnace

exhaust gas (g) Basic

oxygen furnace

1600 1600 0.10 - 0.20

Basic oxygen furnace gas

(g) Continuous

casting 980 800 0.25 - 0.50 Cast slabs (s)

Hot rolling 900 900 0.31 - 0.62 Steel (s)

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Table 2.3: Continued.

Process Technology for heat recovery Coke ovens Dry quenching

Sintering Advanced sintering Blast

furnace

Top-pressure recovery turbine,

dry cleaning Basic

oxygen furnace

Gas recovery/

Boiler Continuous

casting

Radiant heat boilers Hot rolling Water spraying,

Heat pumps

Lu et al. (2016) have shown that the glass industry has the highest waste heat to power capacity per ton compared to other industries . Johnson et al.

(2008) have showed that for the glass industry in the US, there are different grades of unrecovered waste heat. However, Campana et al. (2013) showed that this is not the case in the EU. In the EU, the heat is utilized better. Since there is a small number of plants, it has a small potential, therefore it will not be considered in this work. This work will focus on Iron & Steel, Gas Compressor stations, Cement and Aluminum industries.

2.2 Waste Heat Recovery Systems

Waste heat recovery methods involve the capture and transfer of excess heat from a process and using it as an extra heat source. The energy from this heat source can be used directly as heat, either within the same process or in some other process. Another option is to utilize the heat source to generate electrical and mechanical power (Naik-Dhungel 2012).

There are various heat recovery systems available, which are used for the capture and recovery of waste heat. These systems primarily consist of com- mon heat exchangers technologies such as air preheaters, recuperators, furnace regenerators, rotary regenerators, heat wheels, run around coils, regenerative

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and recuperative burners, plate heat exchangers, economisers, heat pipe heat exchangers, waste heat boilers, and direct electrical conversion devices. These technologies all function by the same principle of capturing, recovering and exchanging heat with a potential energy content in a process (Jouhara et al.

2018).

The use of thermodynamic cycles enables heat recovery from waste sources to be conducted to produce electrical energy and improve the energy efficiency of a process (Costiuc et al. 2015). Conventionally, this has been performed using water as the working fluid in the steam Rankine cycle. However, a compar- ative thermodynamic analysis by Nemati et al. (2017) suggested that the use of thermodynamic cycles employing organic working fluids is a promising way of waste heat recovery from low or medium grade heat sources. Therefore, in this chapter these thermodynamic cycles for waste heat recovery will be reviewed.

2.2.1 Organic Rankine Cycle

The Organic Rankine Cycle functions under the same working principle as the conventional Steam Rankine Cycle. However, instead of using water as the working fluid the system uses organic fluids with low boiling points and high vapor pressures (Quoilin et al. 2013). The use of an organic fluid in a Rankine cycle makes the system suitable for utilizing low and medium grade waste heat along with other energy sources such as biomass, geothermal and solar applications (Rentizelas et al. 2009, Song et al. 2020, Freeman et al.

2015).

A typical ORC system consists of a set of heat exchangers (preheater and evaporator), turbine, recuperator, pump, and condenser. The organic fluid cycles through the preheater and evaporator where it is heated by the energy source, vaporizes and becomes superheated vapor.

The vaporised organic fluid then passes through a turbine where it expands causing the turbine to spin and generate electricity. The vapor then enters the recuperator to reduce its temperature and utilizing that energy to preheat the organic fluid at a later stage in the cycle.

The fluid is cooled further in the condenser where the organic vapor con- denses back into a liquid. Finally, the condensed organic fluid is then re- pressurized in the pump before entering the recuperator to get heated and the cycle restarts.

The design and performance of an ORC system is highly dependent on the working fluid that is selected and that fluid’s thermodynamic properties

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along with its environmental and safety criteria (Saleh et al. 2007). A thermodynamic analysis conducted by Douvartzides & Karmalis (2016), which considered 37 different working fluids, showed that the appropriate selection of a working fluid can increase the overall plant efficiency by 5.5% and reduce fuel consumption by 12.7%.

2.2.2 Kalina Cycle

The Kalina cycle is very similar to the ORC. However, it uses a mixture of water and ammmonia as its working fluid (Mlcak n.d.). It has the same components as an ORC, but with an extra recuperator and separator. The added benefit of using a Kalina cycle is that heat exchange does not happen at constant temperature during boiling, but rather there is a temperature glide as seen in Figure 2.3. This results in a higher cycle efficiency (Rogdakis & Lolos 2015). In single fluid cycles such as ORC, the working fluid rises to the boiling temperature and then it reaches supercritical or superheated stage. However, in binary fluid cycles, each fluid’s temperature rises independently since each have different boiling points. This leads to better thermal matching with the heat exchangers as the heat/cold sources do not have satisfy one set of working fluid parameters (Martínez 1992) . It is more efficient at medium grade temperatures above 200^◦C (Milewski & Krasucki 2018).

Figure 2.3: Ts Diagrams of (a) ORC , (b) Kalina Cycle (Jouhara et al. 2018)

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2.3 Supercritical CO

₂

Cycle

Steam cycles have been in use since the 18th century. They have reached effi- ciencies of 40% (SWEPCO Fact Sheet n.d.) and expected to rise to 50% using advanced ultrasupercritical cycles (Weiland & Shelton 2016).

An alternative cycle to steam which has been gaining attention over the last years is the sCO2 cycle. First mention of sCO2 cycle is in 1948, when Sulzer bros patented a CO2 Brayton cycle (Schmidt 1970). Since then many countries began investigating it (Gokhshtein & Verkhivker 1969, Gokhshtein et al. 1971) Angelino in Italy (Angelino 1968, 1969) , Feher (1968) in the United States , Sulzer Brown – Boveri in Switzerland (Strub & Frieder 1970).

Feher (1968) proposed a cycle above the critical temperature and pressure.

He concluded that this cycle can be highly efficient and have small component sizes. Angelino (1969) focused on condensation cycles. However, he concluded that this cycle is more efficient than steam cycles. He found that recompression cycles are very efficient. He also concluded that CO2 is ther- mally stable up to 1500^◦C and 40MPa. In addition, he recognized how CO₂ has lower specific volume and the importance of minimizing the effect of heat capacity changes.

Feher (1968) designed a 150kW supercritical cycle. However, to achieve part-time load they used a parasitic load bank, as they could not find a suitable bypass valve.

In the following decades, different layouts were designed and modeled.

It was found that the recompression achieved the highest efficiencies without increasing complexity (Watzel 1971, Pfost & Seitz 1971). In 1976, General Electric compared many cycles for use in applications using coal and coal de- rived fuel. Unfortunately, they concluded that sCO2 is very expensive. How- ever, this was due to using way higher pressures and temperatures than needed.

None of these studies were taken further though due to the technical limitations of turbomachinery and heat exchangers at that time along with the lack of heat sources. Recently, there was a revival of interest in sCO2 again. In 1997 the Czech Technical University and in 1999 (Petr & Kolovratnik 1997, Petr et al. 1999), 2000 in MIT in USA (Dostal et al. 2002) then in 2001 the Tokyo Institute of Technology (Yasuyoshi et al. 2001).

Many pilot projects have been implemented to further study this cycle.

Sandia has implemented a 1MW cycle (Conboy et al. 2012, 2013, Fleming et al. 2012). China also made some progress (Wang et al. 2014). They initially focused on the thermotechnical calculations of the components used in the Demo project in Czech (Vesely et al. 2014).

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tCO₂cycles, which operate both at subcritical and supercritical conditions have also been investigated (Zhang et al. 2007, 2005, 2006, Zhang & Yam- aguchi 2008, 2011, Pan et al. 2016, Ge et al. 2018, Li et al. 2018, Shi, Shu, Tian, Huang, Chen, Li & Li 2017, Li et al. 2017, Shi, Shu, Tian, Huang, Chang, Chen & Li 2017). However, they have lower efficiencies than sCO₂ Cycles.

Support continued for sCO2. US Department of Energy (DoE) has been supporting it. Several companies such as Echogen, GE and Netpower along with different research facilities (Sandia, Oak Ridge, KAIST (Korea Advanced Institute of Science and Technology)) have all been working on developing the sCO2cycle. Within the EU, the EC has funded two projects (HeRo and Flex) (sCO2-flex 2019). They aim at developing a small scale Brayton sCO₂ cycle and a modular flexible coal plant based on sCO₂cycle.

Recently General Electric, led by Vinnemier published a paper talking about using sCO2 cycles as storage in thermal plants. Their modeling shows that the cycle can have a round trip efficiency of 60% (Vinnemeier et al. 2016).

However, this had no economic modeling in it. Therefore it might be very expensive and not financially attractive. They also compare using different fluids such as air and Argon and the benefits of each. They show that tCO2 is beneficial for inputs of low grade temperatures. According to their study, current reasonable design temperatures are 300-600^◦C and recuperation 0.25-0.6 (due to technical and efficiency factors for heat pumps).

Carbon dioxide enters supercritical conditions at temperatures higher than 31.1^◦C and pressures above 74 bars. There it does not behave as an ideal gas.

Its behavior is very sensitive to the changes in pressure and temperature. sCO₂ Cycles have the potential of having a higher thermal efficiency than Rankine- Steam cycles. When used for waste heat recovery, it performs better than the steam cycle (Kizilkan 2020). In the steam cycle, the heat exchange is limited by the narrow range of allowable temperatures. However, the sCO₂Cycle can recover heat at different waste temperatures and utilize it efficiently.

Its low critical temperature, makes it easier to use air as a cooling medium.

Using air instead of water will reduce the impact that the system has on the environment (Liu et al. 2019). Furthermore, CO₂ is non-flammable, currently in excess in nature and inexpensive. It has a superior thermal stability.

sCO2 cycles achieve high efficiencies over a broad range of temperatures from different heat sources. It is made of compact components and therefore smaller and cheaper than steam cycles. This is because it has a relatively higher working pressure. It does not require water conditioning or conden- sate control. This is necessary in steam generation to avoid corrosion, fouling

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and scaling. Dry CO₂ is non-corrosive, non-fouling and non-scaling. It usually does not require multiple stages of energy exchange (boiler, superheater, several turbines, heat recovery steam generator, economizer). Therefore the system is smaller and cheaper. sCO₂ cycle has a relatively shorter start-up time. Even when operating at lower temperatures of around 204^◦C, they generate 13% more energy than an HRSG and 48% more than an ORC system (Persichilli et al. 2011).

In the Brayton cycle, sCO₂avoids the pinch problem in the heat exchanger.

As shown in Figure 2.4, there is no pinch point for an sCO₂heating process.

Figure 2.4: Temperature profiles of heat source with (a) pure fluid, (b) zeotropic fluid, and (c) supercritical fluid. (Liu et al. 2019)

This pinch point problem is usually present for other working fluids, such as pure and zeotropic working fluids used in ORCs and Kalina cycles. Avoid- ing this problem reduces the irreversibility of heat exchangers. It also allows for more heat absorption.

When compared with Helium or other ideal gases, CO₂requires less compressor work. It takes around 30% of the turbine output work rather than 45%

as is the case with Helium (Dostal et al. 2004). The properties of CO2 approach that of an incompressible fluid when operating near the critical point, which means that it requires less work for compression (Liu et al. 2019). In turn, one stage compressors can be used. Turbines and heat exchangers are also more compact. It can achieve 46% thermal efficiency at 550 ^◦C, which helium achieves at 800^◦C (Dostal et al. 2004). Compression is usually done close to the critical point as shown in Figure 2.5, to keep the compressor power minimum.

The turbine power is not affected by the operating pressures. The pressure ratio and temperature determine the output. However, the compressor is affected by the operating pressures (Dostal et al. 2004). This is because the

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Figure 2.5: Density of sCO₂at Different Pressures and Temperatures (Cunha et al. 2018).

density of CO2changes around the critical point. It decreases drastically after it passes the critical point. This is why sCO2 cycles are more efficient than ideal gas Brayton cycles. The cost of CO₂fluid is 90 % less than Helium cycle and 98% less than the organic fluid R-134a (Liu et al. 2019).

The density is not the only property that varies with pressure and temperature changes. The specific heat capacity also changes greatly. So within the heat exchanger/recuperator, the heat exchange varies greatly, and the minimum temperature difference happens within the recuperator rather than the inlet/outlet of the recuperator (Dostal et al. 2002). Therefore, it is essential to evaluate the properties along the heat exchanger because a simplified analysis will not catch that behavior. For Helium and other ideal gases, it does not change significantly. Therefore the efficiency depends only on the temperature and pressure ratio. However for sCO₂ the operating pressures have a signif-

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icant impact. Therefore, this has to be analyzed to determine the optimum cycle efficiency and recuperator size. Some disadvantages are material corrosion at temperatures exceeding 500^◦C and requiring high pressures to attain high efficiency (Liu et al. 2019).

Another beneficial point of sCO₂cycles is that the start-up time is currently below 5 minutes, which is very fast for a thermodynamic system (Mercangöz et al. 2012).

2.3.1 Cycle Configurations

Turbine work can be increased by recovering heat from the recuperator or by adding a reheating process. Compression work can be decreased by reducing the compressor inlet temperature along with adding intercooling. However, the complexity and the difficulty in controlling the system might be increased substantially when adding additional coolers or heaters (Liu et al. 2019). Re- searchers have carried out research on different cycle configurations. Exam- ples are not limited to simple recuperated, condensation, precompression, recompression, split expansion, partial cooling, cascade, preheating, simple reheat, and double reheat cycles. This work uses a more detailed heat exchanger model than other work to compare 6 different cycle configurations.

Simple Recuperated Cycle

Adding a recuperator to recover heat can increase the efficiency of the cycle.

However, the greater thermal capacity of sCO₂ on the high pressure side can lead to an internal pinch point within the recuperator, which can lower the cycle efficiency (Liu et al. 2019). The internal pinch point happens due to the large difference between thermal capacities on either sides of the recuperator.

The cycle schematic can be seen in Figure 2.6.

Recompression Cycle

The pinch point problem present in the simple recuperated cycle can be avoid by using split flow cycles such as the recompression cycle. The cycle configuration (shown in Figure 2.7) is more complex, having two compressors and two recuperators.

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Figure 2.6: Simple Recuperated Cycle.

Figure 2.7: sCO₂Recompression Cycle

The CO₂ leaving the low temperature recuperator (LTR) is split into two streams. The majority of the flow enters the cooler and is compressed by the main compressor whereas the remaining flow is directly compressed in the recompressor. The stream leaving the main compressor is heated in the LTR before mixing with the stream from the recompressor and being further heated in the high temperature recuperator (HTR) (Liu et al. 2019).

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As the CO₂ approaches its critical point, its heat capacity increases drastically. Therefore the hot CO2 after the turbine has a lower thermal capacity than the cold CO2on the high pressure side of the recuperator. This decreases the cycle efficiency because the temperature the CO₂ rises to is limited. The recompression cycle helps in overcoming this obstacle (Baldwin et al. 2015).

As can be seen in Figure 2.8, the difference between thermal capacities is much greater in the LTR, than in the HTR. Therefore, having only a fraction of the fluid pass through the cold side of the LTR, the pinch point problem can be avoided. This creates a better match of thermal capacities between the hot and cold streams, giving a lower temperature pinch point in the recuperator, which improves the heat exchange (Trevisan et al. 2019).

Figure 2.8: CO2 specific heat capacity variation in the recuperators (Liu et al.

2019).

Cascaded Cycle Configurations

A big limitation of both the simple and recompression sCO₂cycles is that they utilize heat from an external heat source over a small temperature interval.

The CO2is typically heated up to temperatures above 300^◦C in the recuperator, meaning that all available heat from the heat source at lower temperatures cannot be used in the power cycle. Adding a bottoming sCO₂ cycle with a lower TIT than the topping cycle would allow further utilization of the avail-

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able heat from the heat source. This type of cascaded cycle configurations was first proposed by Johnson et al. (2013) to minimize the thermal storage salt inventory in solar power applications. Manente & Lazzaretto (2014) investigated the cascaded cycle configurations for sCO₂ cycles for electricity production from biomass. Their research looks at the feasibility of either having a simple recuperated cycle or recompression cycle as the topping cycle, with a simple recuperated cycle as the bottoming cycle. The cycle configurations can be seen in Figures 2.9 and 2.10.

Figure 2.9: Recompression as Topping Cycle.

Figure 2.10: Simple Recuperated as Topping Cycle.

Preheating Cycle

The preheating cycle is essentially the simple recuperated cycle except a fraction of the CO2flow coming out the the compressor is sent to a preheater with the remainder of the flow goes through the recuperator. The two flows are then directly combined and enter the primary heater. The cycle configuration can be seen in Figure 2.11. This approach has two main benefits. Firstly, it can ameliorate the pinch point problem in the recuperator by having different mass flow rates on each side in the heat exchanger. Secondly, having both a primary

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heater and preheater enables the heat source temperature to be lowered further compared to the simple recuperated cycle. This benefit being especially pertinent in relation to waste heat recovery applications, increasing the energy recovery efficiency at high thermal power conversion efficiency (Wright et al.

2016).

Figure 2.11: Preheating Cycle Heat Pumps

Another use of sCO₂ systems is heat pumping. A sCO₂ cycle can be used to take low grade heat and pump it to a higher temperature when electricity prices are low. When electricity prices rise again, the high grade heat can be transformed into electricity.

In a conventional heat pump, the operating temperature range is limited by low critical temperatures. In other words, heat cannot be delivered at a temperature higher than the critical temperature. With sCO2, the operation is not limited by the low critical temperature. This is due to the operating pressures being higher than the critical pressure and therefore the operating temperature is not limited by the critical temperature (Singh & Dasgupta 2017).

Starting from compressor. The CO2 is compressed to a higher pressure and temperature. Then it is used to heat the storage medium (which is at a

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lower temperature), so its temperature glides down in a heat exchanger in near constant pressure. Then it goes into an expander/turbine, where some energy can be recovered to drive the compressor. The fluid expands and temperature drops slightly. After that it goes into the recuperators to give off heat for heat recovery followed by the condenser. When electricity prices are high, the pre- vious process is reversed. The low pressure sCO2 is compressed to a higher pressure. Then the fluid receives heat from the the storage. Therefore, its temperature rises significantly. It then expands in a turbine to generate electricity.

Then it goes into a condenser to discard the low grade heat before restarting the cycle.

2.3.2 Cycle Equipment

There are many components that make up the sCO2 cycle. The main components are the turbines, compressors and heat exchangers. The cycle efficiency is highly affected by the turbine and compressor efficiencies. Therefore, it is necessary to accurately model these efficiencies. On the other hand, heat exchangers make up the recuperators, condenser and the waste heat exchanger.

The effectiveness of the heat exchangers greatly affects the cycle performance.

These components are briefly presented here in regards to the sCO₂cycle.

Turbines

Turbines deal with working fluids at very high temperatures and pressures.

The internal flow inside turbines is a complex phenomenon due to the viscos- ity and compressibility properties of sCO2. Therefore more research using ex- periments or complex numerical computations is needed. Not much research has gone into centrifugal turbines where sCO₂ is the working fluid. Different manufacturers have went different paths to develop them. Toshiba has been working on axial turbines (Allam et al. 2017). Axial turbines are mostly used in large scale applications (Zhang et al. 2015). Axial turbines can withstand higher temperatures and pressures (Weiland et al. 2019). While NET power is working on developing a radial turbine, so that it would be simpler and with fewer stages (El Samad et al. 2020).

There are a few technical limitations for sCO2 turbines. First, the density of sCO₂ is high and the size of the turbine can be relatively small, the heat exchanger needs to be compact to match. Second, the turbine operates in a high pressure and high speed environment, which can cause large frictional losses (Beucher et al. 2010).

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Highest isentropic efficiency was found to be 87% using a centripetal turbine (Liu et al. 2019). However, it is only suitable for small flows. There are different sCO2 turbines currently being tested, some of which have an efficiency of up to 86% (Liu et al. 2019).

The choice of turbine type will depend on the size, application but mostly economics. Axial turbines are generally cheaper (Weiland et al. 2019).

Compressors

Compressors are a vital part of the sCO₂ cycle. They are the main source of energy loss, especially in tCO2 cycles (Vinnemeier et al. 2016). In a sCO2

recompression cycle the recompressor can account for around 40% of the total work input to the system (McTigue et al. 2019). Compressors are classified into axial and radial categories. Radial compressors can be either centrifugal or centripetal compressors. Axial compressors are known for handling large flows and having high efficiencies. Radial compressors are characterized by having single-stage pressure ratio, compact structure and low cost (Liu et al.

2019).

Since compressors deal with sCO2close to the critical point, condensation should be avoided as two-phase flow is harmful to the equipment. Different researchers have seen that condensation most likely occurs in the leading edge of the main impeller blades, the blade tip at the leading edge of main blade, the trailing edge of both of the main and splitter blades, and the leading edge of the vaned diffuser (Pecnik & Colonna 2011, Pecnik et al. 2012, Rinaldi et al.

2015, 2014, 2013).

Highest isentropic efficiency was found 84% for a centrifugal compressor. The pressure ratio is generally around 1.8 (Liu et al. 2019). Currently, air compressors are limited to temperatures of 600^◦C. Whereas, compressors for CO₂are available only up to 450^◦C (Vinnemeier et al. 2016). Therefore much research is needed to further develop CO₂compressors to match air compressors.

Turbomachinery for CO2 has higher efficiency and lower costs than those used for air. This is because CO₂has higher density, which additionally allows for more compact equipment (Mercangöz et al. 2012). Its low surface tension allows the turbomachinery to operate near saturation curve as cavitation has a smaller effect.

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Heat Exchangers

Heat exchangers in sCO₂ cycles deal with fluids at very high temperatures and pressures. they face major mechanical, thermo-mechanical, and thermal- hydraulic challenges(Carlson 2014). Recuperators enable high cycle efficiencies, and avoids the pinch point in the low-temperature end of the recuperation process (Dostal et al. 2004). They can be a tubular type with a staggered tube bundle (Seo et al. 2020), or recent research for this type has been focused on printed circuit heat exchangers (PCHEs) and cast metal heat exchangers (CMHEs) (Carlson 2014). PCHEs are the most commonly used for sCO2 cycles. They are manufactured by chemically milling channels roughly 1mm x 0.5 mm into plate swith a size of 600mm x 1500 mm (Musgrove et al. 2014).

This produces compact heat exchangers that can withstand the high pressures required by the sCO2cycle.

Another parameter that impacts the cycle efficiency is the condenser power consumption. As mentioned above, one of the benefits of using an sCO₂ cycle is the possibility to have dry cooling. However, dry cooling might require huge fan power. It is also affected by the cooling air maximum discharge temperature. Rankine cycles discharge waste heat air at around 120^◦C (Saadaoui 2020). For Industrial refrigeration, the cooling air has a maximum temperature of 45^◦C in the EU (Koelet & Gray 2017).

API standard 661 regulates heat exchanger design for use in refinery (Stan- dard & Edition 2002). However, it only specifies a difference between inlet and outlet without setting a maximum discharge temperature. It also states that the temperature should not exceed 60^◦C due to the technical limitations of the fan.

This is also confirmed by KLM technology group (KLM 2011).

Sandia has been using heat exchangers with temperatures that reached 482^◦C (Shiferaw 2017). The effectiveness of the heat exchangers used depends on the economics as heat exchangers form a significant part of the overall CAPEX.

Electric Heaters

Since there are technical limitations on the temperature of the sCO2 reached by the compressor, electric heaters can be used to further heat sCO2. This is particularly useful for heat pumps. Using electric heaters lowers the round trip efficiency, but reduces the technical challenges of designing high temperature heat pumps (Vinnemeier et al. 2016). The use of electric heaters depends on the application and the economics.

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2.3.3 Emissions Trading System

For the purpose of this thesis, several industry players were contacted. One of the biggest gas compression industries expressed worries of losing free emission allowances due to electricity generation. To confirm the findings, the EU ETS commission was contacted and confirmed the conclusions. There- fore this section, explores how the sCO₂ cycle affects the Emission Trading Scheme (ETS).

The EU ETS Handbook gives guidance in this matter. It states that "For any allocation to electricity generators the linear reduction factor (LRF) is applied to the total allocation. The LRF reduces the total allocation annually by 1.74%

compared to the allocation for 2013. Article 3(u) of the EU ETS Directive defines an installation as ‘electricity generator’ if “on or after 1 January 2005, it has produced electricity for sale to third parties, and in which no activity listed in Annex I (of the EU ETS Directive) is carried out other than the combustion of fuels”" (EU Commission 2015).

However, to be identified as an electricity generator, these 4 conditions must apply:

• An electricity generator has to be an installation (see also Article 3e of the revised EU ETS Directive).

• An electricity generator has to produce electricity.

• An electricity generator has produced or produces electricity for sale at any time from 1 January 2005.

• An electricity generator must not carry out any activity listed in the An- nex other than the "combustion of fuels". (EU Commission 2019a) Therefore, if the gas compressor station produces electricity and uses it to drive an electric compressor, it does not qualify as an electricity generator.

This means that its emission allowances should not decrease.

Another article of interest is "No free allocation shall be made in respect of any electricity production, except for cases falling within Article 10c and electricity produced from waste gases" (EU Court of Justice 2019).

The definition of a waste gas in Art.2(11) of the FAR states that:" ‘waste gas’ means a gas containing incompletely oxidised carbon in a gaseous state under standard conditions which is a result of any of the processes listed in point (10), where ‘standard conditions’ means temperature of 273 K and pressure conditions of 101,325 Pa defining normal cubic meters (Nm³) according

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to Article 3(50) of Commission Regulation (EU) No 601/2012 " (EU Com- mission 2019b). Therefore the waste heat does not qualify as waste gas.

Many installations produce more than one product. In these cases an installation can be divided into a number of ‘sub-installations’. "The boundaries of a sub-installation are determined by the benchmark being applied . Use a product benchmark, a heat benchmark and a fuel benchmark. The allocation would need to be calculated separately for each sub-installation." (EU Com- mission 2015)

Since heat is what is being utilized here. The waste heat can be treated as a product Benchmark Heat. This means that they will get an extra 224.3 Allowances /GWh of heat consumed. Heat should meet the conditions below in order to qualify as a heat benchmark sub-installation:

• The heat is used for a purpose (production of products, mechanical energy, heating, cooling)

• The heat is not used for the production of electricity

• The heat is not consumed within the system boundaries of a product benchmark

• Heat is consumed within the ETS installation’s boundaries and produced by an ETS-installation (EU Commission 2019a) (article 3(c)).

The definition of electricity production is not clear whether it is the same as above or not. However, to gain those extra allowances the turbine can turn the high pressure into mechanical power that drives a gas compressor right away, rather than generate electricity.

The EU ETS Handbook states that "installations where the emissions are so small that the administrative costs per unit of emissions might be dispro- portionately high are allowed to opt-out from the EU ETS as long as they are subject to equivalent measures. Installations are considered small emitters if they emit less than 25 ktCO2e annually and, if they are combustion installations, have a thermal rated input below 35MW. Hospitals may also opt-out if they are subject to equivalent measures" (EU Commission 2015).

Sweden doesn’t opt-out installations with low total input or low fossil emissions. Sweden has instead chosen to opt in installations as long as they are connected to a district heating network where the total input to the network is greater than 20 MW. Sweden has not chosen to opt any installations out of the ETS. On the contrary, Sweden applies opt-in, thereby including installations with a thermal rated input of less than 20 MW. In short Sweden has chosen to

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apply the threshold of 20 MW, which applies on installation level in the EU, to each distribution network for municipal heating. Any combustion unit, no matter how small on installation level, is included in the ETS if it is connected to a network where the total thermal rated input of all installations connected exceeds 20 MW (Swedish Energy Agency 2019).

In conclusion, the ETS won’t be affected if the electricity is used on site.

The industry can opt out of deductions if input thermal energy is less than 35MWth, and another policy is in place (Aschenbruck et al. 2004). It also can be part of the compressor station, not all of it. For example, it can be installed for one compressor, rather than all the compressors in the plant. An- other option is that the plant can be divided into two sub-installations. The first compressor will have no change and is now a separate sub-installation.

The Energy-generator is another one, with different allocations. According to GD2, they can have a sub-installation of heat, as long as mechanical energy is used right away instead of electricity. They will receive an extra free allocation of 224.3 Allowances/GWh of heat consumed (EU Commission 2019a). So us- ing this alternative actually gives them extra allowances rather than decrease allowances as was feared.

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Industries and Waste Heat Esti- mation

3.1 Iron and Steel

The production of steel is an energy intensive industry and consists of many subprocesses. It involves sintering, coke making, furnace and casting. Each of these processes involves different alternative methods. A lot of research has went into heat recovery (Jouhara et al. 2018). Most heat is utilized well in the industry, except in coke production. Coke production is around 10% of the total energy demand in the industry. The process involves heating coking coals to 1100^◦C for about 21 hours in the coke oven to drive off volatile compounds.

About 3.3% of the total energy used is electricity, 7.4% is steam and 89.3%

is thermal energy from Coke Oven Gas (COG) combustion (Qin & Chang 2017). To cool down the coke it undergoes a quenching process, the industry uses either coke wet quenching (CWQ) or coke dry quenching (CDQ). For the heat to be recovered, it must be CDQ. Of the 62 coke production facilities, only five have CDQ (Wortler et al. 2013). Consequently, 57 facilities can switch to CDQ to reduce their carbon emissions and water usage. The coke production process can be seen in Figure 3.1. There are two high grade heat streams available for power production.

For CDQ, the ratio of waste gas to coke is 1.23 (Qin & Chang 2017). Using this ratio, a medium sized plant’s flow can be estimated. The EU produces 37 Million tons of steel per year in 62 facilities (Eurostat 2020). This is equivalent to coke production of 19 kg/s in each facility resulting in 23.2 kg/s of waste gas flow. The waste heat in the EU is 15 TWh per year. For each facility, it can produce approximately 9 MWe using an sCO2cycle. As for the COG, the

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Figure 3.1: Coke Production Process

average coke battery produces 365 m³/tcoal of COG (Ndlovu et al. 2017, Tran et al. 2016). For an average size facility this is equivalent to 3.65 kg/s with a density of 0.5 kg/m³ (Satyendra 2015). It has a high specific heat capacity of 3.3 KJ/kg.K (Zhang 2019). For the EU this is waste heat of 4.6 TWh. For each facility, it can produce approximately 2.5 MW using an sCO2cycle. There are two alternatives for how electricity can be produced. It can be a simple sCO2

cycle as shown in Figure 3.3. Otherwise, it can be a cascaded cycle, where more waste heat is utilized as seen in Figure 3.2.

Figure 3.2: Combined 2 Cycle for Coke Production

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Figure 3.3: Simple sCO2Cycle for Coke Production

3.2 Gas Compression

There is no official aggregate data for the gas compression industry in the EU.

However, a group of researchers gathered data about the industry (Campana et al. 2013). The distribution of gas compressor stations in the EU is shown in Figure 3.4. For the work of this thesis, the group is contacted to see how they gathered the data. They went through different companies’ databases one by one to gather their data. Therefore, their data is used as it is. Bianci et al investigated a an average size compressor station and the amount of waste heat there. They concluded that the flue gas had a flow rate of 69 kg/s and temperature of 540^◦C (Bianchi et al. 2017). According to Campana et al, the average useful waste from a station is 84.1MW_th. However, the researchers multiply by a correction factor of 0.65 to account for backup units. Therefore the average useful waste is 55MWth. This means that the flow rate for a medium sized station is 126 kg/s. There are approximately 254 stations with different sizes. The total available waste heat in Europe is 127 TWh. However, only 22.7 TWh are useful. If the gas compressor station has a thermal efficiency of 0.4, then 5.5 GW of electricity can be generated from all the stations.

However, gas compression isn’t a constant load. As can be seen in Figure 3.5, it changes throughout the year. This is also confirmed by other researchers (Bianchi et al. 2017, ROSSIN 2013). This variation is accounted for in the results. Similar to the coke production process, this can either be a simple cycle or a combined cycle.

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Figure 3.4: Distribution of Gas Compressor Stations over Europe (Campana et al. 2013)

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Figure 3.5: Variation of Flow Rate throughout the Year (Gómez-Aláez et al. 2017)

3.3 Cement

For the Cement industry, a survey was sent out to different factories over Eu- rope. The survey is attached in Appendix C. Data from 3 main industrial players was used. Each industrial player represent a different factory size. The factor of heat per production is almost equal in all of them. However, different factories use different compositions of fuel. Their numbers are compared and confirmed using an IFC report (Doğan et al. 2018). The waste heat comes out at 300^◦C with a flow rate of 74 kg/s from the kiln and 40 kg/s from the clinker cooler. It has a specific heat capacity of 1300 (J/kg K ). The cement manufacturing process is seen below in Figure 3.6. The EU produces 125-200 Mtons of clinker per year (de Vet et al. 2018, Cembureau 2019). This is equivalent to 120 medium sized factories. They have a cumulative energy of 27 TWh of waste heat energy.

As seen from the figure above, the waste heat is utilized in the process.

So there are several methods that the sCO2 cycle can add value here. It can take the waste heat from the process at 285^◦C, and use it as the heat source.

However, this will have a low thermal efficiency since the Turbine inlet temperature is relatively low. This can generate 0.5 GW in Europe. Several plants have been implemented globally, the first was in Germany and then 120 more power plants were built in China with a capacity of 700 MW (Moya et al.

2010). Another alternative is that before the heat goes into the preheaters, it passes through a heat exchanger that absorbs some of the high grade waste heat into the sCO2 cycle and then the rest of the heat goes into preheating.

This schematic can be seen in Figure 3.7. Through this process, the EU can generate 5 GW of electricity.

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Figure 3.6: Clinker Forming Process

Figure 3.7: Upstream sCO2Cycle in Cement Industry

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Figure 3.8: Cement sCO2Heat Pump

There are two ways to ensure the heat taken does not adversely affect the preheating process. First approach is to burn more fuel in the rotary kiln.

Second approach is to replace the 4-stage preheater with a 6-stage preheater so that the kiln feed absorbs more heat. This would costAC5-8M (CSI 2017).

Ma et al. (2019) have shown that using the existing preheaters, there is a lot of room for better thermal exchange and heat utilization.

The last alternative is to use the cycle as a heat pump and engine. When the electricity prices are cheap, the cycle pumps up the low grade heat and stores it in a backed bed. Then when the prices are high, the cycle is reversed, and the heat stored is used to generate electricity using the sCO₂ cycle. This process can be seen in Figure 3.8 and 3.9.

3.4 Aluminum

Aluminum has several different waste streams (Yu et al. 2018). According to (World Aluminium 2020), the EU produces 7.5 Million tons of Aluminum every year. The production of Aluminum is a complex process, but it mainly consists of two main subprocesses; Bayer and Hall-Heroult processes. In the Bayer process, the Bauxite is crushed and changed into Alumina using heat mainly that can go up to 1100^◦C. The Hall-Heroult process is fundamentally electrolysis at 940-980 ^◦C in molten cryolite, to produce molten aluminum.

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Figure 3.9: Cement sCO₂Heat Engine

Molten aluminum is then cast or mixed with other elements to form alloys, it operates 24 hours. Recycling Aluminum is less energy-intensive, this is why using recycled Aluminum is one of the main goals of 2050. There is also research into alternative manufacturing methods but nothing will be commer- cialized before 2030 (CTCN 2012).

3.4.1 Hall Heroult

The Hall Heroult process requires mainly electricity and keeps the heat constant. Smelters consumer 13MWh of electricity per ton of Aluminum in electrolysis (Nowicki & Gosselin 2012). There are two heat streams in this process; the exhaust flue gas and the pot surface. The exhaust flue gas has a temperature of 110^◦C but can be slowed down to increase the temperature up to 150^◦C (Yu et al. 2018). For example, a smelter in Iceland of 350,000 t/annum capacity can have a flow rate of 230 kg/s at that higher temperature (Yu et al.

2018). Zhao has analyzed slowing the pot cooling air to increase its temperature (Zhao 2015). The main concern is this could melt the frozen electrolyte and jeopardize pot integrity. Therefore, it is not recommended because of the subtle thermal balance between the electrolytic bath and the heat loss from sidewalls. Sørhuus & Wedde (2016) developed a heat exchanger which has a good trade-off between heat recovery and cost efficient cooling of pot gas.