IN DEGREE PROJECT TECHNOLOGY,FIRST CYCLE, 15 CREDITS

, STOCKHOLM SWEDEN 2017

Carbon capture and utilisation in the steel industryA study exploring the integration of carbon capture technology and high-temperature co- electrolysis of CO2 and H2O to produce synthetic gas

JULIA SJOBERG ELF

KRISTOFER WANNHEDEN ESPINOSA

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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Bachelor of Science Thesis EGI-2017

Carbon capture and utilisation in the steel industry

A study exploring the integration of carbon capture technology and high-temperature co-electrolysis of CO2 and H2O to produce synthetic gas

Koldioxidåtervinning inom stålindustrin

En studie av möjligheterna till syntesgasproduktion genom integration av kolavskiljningsteknik och co-elektrolys av CO2 och H2O

Julia Sjöberg Elf

Kristofer Wannheden Espinosa

Approved

Examiner

Andrew Martin

Supervisor

Vera Nemanova

Commissioner

Contact person

Abstract The present thesis studies the potential for introducing the technology of co-electrolysis of carbon dioxide (CO2) and water (H2O) through a Solid Oxide Electrolyser Cell (SOEC) in a top gas recycling blast furnace (TGR-BF) in a steel plant. TGR-BF, commonly presented in literature as a promising carbon capture and storage (CCS) pathway for the steel industry, can drastically decrease these emissions by successively recycling up to 90 % of the top gas from a blast furnace (EU, 2014) and sequestering the CO2 from the highly carbon concentrated remaining top gas. Blast furnaces (BF) represent about 20 % of the total carbon dioxide emissions of a steel plant (Carpenter, 2012). Based on the current research status of SOEC, this report aims at exploring the utilisation of carbon dioxide captured from TGR-BF through a simultaneous electrolysis of CO2 and H2O, a novel and highly efficient pathway of producing valuable synthetic gas (syngas), used in chemical and industrial applications. It is important to note that neither of the technologies is yet in commercialisation phase, and that the suggested installation would presently not be possible, but nevertheless provides an interesting pathway towards closing the carbon cycle of steelmaking. To give an idea of the magnitude of the SOEC installation and its syngas production if combined with TGR-BF, an analysis of existing case studies of each technology was made. The SOEC system modelled by Fu et al. (2010) was scaled to fit the CO2 emissions of Ruukki Metals steel plant in Raahe, Finland, for which data is abundant and reliable. To highlight the integration potential of the two separate technologies, a conceptual process flow chart was designed and a literature review of the respective technologies performed, allowing the identification of integration challenges, presented in the analysis. The literature study reveals that challenges for the system include: gas purity requirements, gas composition requirements, scalability, life-time compatibility, plant complexity and high variation of plant infrastructure. In the discussion, difficulties related to a technology shift in a traditional industry are considered. For further research, mathematical modelling of thermodynamics of the system as well as an economic assessment are recommended.

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Sammanfattning Följande studie utforskar potentialen att implementera co-elektrolys av koldioxid (CO2) och vatten (H2O) genom en fastoxid elektrolyscell (SOEC) i en masugn där återvinning av masugnsgasen tillämpas genom s.k. Top-Gas Recycling Blast Furnace (TGR-BF). Masugnen representerar omkring 20 % av de totala koldioxidutsläppen från ett stålverk (Carpenter, 2012) varför TGR-BF i flera studier beskrivs som en lovande teknik för avskiljning och lagring av koldioxid (CCS) i stålindustrin. TGR-BF har potentialen att drastiskt minska utsläppen genom att återvinna upp till 90 % av masugnsgasen (BFG) och avskiljning av koldioxid från den CO2-rika gasen som återstår. Genom att kartlägga den senaste forskningen inom SOEC och analysera resultat från försöksanläggningar som tillämpar TGR-BF syftar denna studie att utforska möjligheten för ett kombinerat system där koldioxiden från masugnsgasen, genom en simultan co-elektrolys av CO2 och H2O, används för syntesgasproduktion; en viktig gas i många kemiska och industriella tillämpningar. Det är viktigt att poängtera att ingen av de två teknikerna idag är kommersialiserade och att en integration av dessa för tillfället därför inte är genomförbar, men att studien tillhandahåller en intressant möjlighet för minskade koldioxidutsläpp för stålindustrin. För att undersöka skalbarheten mellan de två teknikerna genomfördes en fallstudie på Ruukki Metal’s stålverk i Raahe, Finland kombinerat med ett SOEC-system som tillämpats av Fu m.fl. (2010) i deras modellering av syntesgas genom co-elektrolys. Fallstudien uppskattar att 2838 ton syntesgas per dag skulle kunna produceras från den infångade koldioxiden i stålverket Raahe, Finland. Ett konceptuellt flödesschema utformades för att åskådliggöra integrationspunkterna för de två teknikerna. En litteraturstudie gjordes i syfte att förstå vilka utmaningar en sådan integration skulle innebära. Dessa utmaningar, tillsammans med utmaningar för de två enskilda teknikerna, presenteras i analysen. Litteraturstudien påvisade att utmaningar för det integrerade systemet inkluderar: krav på gasernas renhet samt sammansättning, systemens skalbarhet, livstid samt komplexiteten och variationen mellan olika stålverk. Analysen och diskussionen behandlar svårigheterna med stora teknikskiften i en traditionell industri. För vidare studier rekommenderas en matematisk modellering av systemet där termodynamiska och ekonomiska aspekter behandlas.

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Table of contents Abstract....................................................................................................................................... 2 Sammanfattning ......................................................................................................................... 3 Table of contents ........................................................................................................................ 4 List of figures and tables ............................................................................................................ 5 List of acronyms and abbreviations ........................................................................................... 6 1 Introduction .............................................................................................................................. 7

1.1 Problem formulation ......................................................................................................... 8 1.2 Aim and objectives ........................................................................................................... 8

2 Methodology ............................................................................................................................ 8 2.1 Methodological basis ........................................................................................................ 8 2.2 Research purpose ............................................................................................................ 9 2.3 Literature study................................................................................................................. 9 2.4 Process flow diagram ..................................................................................................... 10 2.5 Case study...................................................................................................................... 10 2.6 Limitations ...................................................................................................................... 11

3 Literature review .................................................................................................................... 12 3.1 Carbon dioxide emissions .............................................................................................. 12 3.2 Carbon capture, storage and utilisation ......................................................................... 15 3.3 Steel industry .................................................................................................................. 17 3.4 Carbon capture through top gas recycling blast furnace............................................... 20 3.5 H2O/CO2 Co-electrolysis ................................................................................................ 23

4 Analysis ................................................................................................................................. 36 4.1 System description ......................................................................................................... 37 4.2 Case study...................................................................................................................... 38 4.3 Challenges ..................................................................................................................... 39

5 Discussion ............................................................................................................................. 42 6 Conclusion ............................................................................................................................. 43 7 References ............................................................................................................................ 45 Appendix ................................................................................................................................... 52

Appendix 1: Number of publications on Scopus, since 2003 .............................................. 52 Appendix 2: Evolution of the number of publications on Scopus, since 2005 .................... 52

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List of figures and tables Figure 1: Carbon dioxide emissions per sector ....................................................................... 13 Figure 2: Carbon dioxide emissions from industrial sectors .................................................... 13 Figure 3: Process scheme integrated steel mill. ...................................................................... 18 Figure 4: Schematic diagram of blast furnace ......................................................................... 19 Figure 5: Schematic diagram of TGR-BFG ............................................................................. 21 Figure 6: Operating principle of SOEC water electrolysis ....................................................... 26 Figure 7: Operating principle of SOEC co-electrolysis ............................................................ 27 Figure 8: SOEC co-electrolysis process .................................................................................. 28 Figure 9: Inversely proportional electrical energy and thermal energy demands ................... 28 Figure 10: Temperature dependency energy demand of H2O and CO2 reduction reactions 29 Figure 11: Cell configuration .................................................................................................... 34 Figure 12: Integrated system ................................................................................................... 37 Table 1: Table of carbon conversion technologies .................................................................. 16 Table 2: Reforming reactions for syngas production ............................................................... 17 Table 3: Comparison of electrolysis technologies ................................................................... 25 Table 4: Key takeaways from literature study .......................................................................... 36 Table 5: Description of integration points ................................................................................ 37 Table 6: Reference steel plant used in case study .................................................................. 38 Table 7: Syngas production based on reference steel plant and SOEC ................................ 38 Table 8: TGR-BF, SOEC and integration challenges .............................................................. 39

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List of acronyms and abbreviations Full word Abbreviation Air separation unit ASU Alkaline Electrolysis AEL Balance of Plant BoP Basic Oxygen Furnace BOF Basic Oxygen Furnace Gas BOFG Blast Furnace BF Blast Furnace Gas BFG Blast Furnace/Basic Oxygen Furnace BF-BOF Capital Expense CAPEX Carbon capture and storage CCS Carbon capture and utilisation CCU Carbon capture, utilisation and sequestration CCUS Carbon dioxide CO2 Carbon monoxide CO Coke Oven Gas COG Dymethil Ether DME Electric Arc Furnace EAF Enhanced Oil Recovery EOR European Commission EC European Union EU European Union Emissions Trading Scheme EU-ETS Exajoule EJ Gigatonne carbon dioxide equivalent GtCO2eq Greenhouse gas GHG Growth Domestic Product GDP Heat exchanger HX Hot Rolled Coil HRC Hydrogen H2 International Energy Agency IEA International Panel on Climate Change IPCC Kilowatt kW Megatonne Mt Megatonne per year Mt/yr Megawatt MW Megawatt thermal MWth Methane Steam Reforming MSR Nitrogen N2 Nitrous Oxide N2O Operating Expense OPEX Operation and Maintenance O&M Organisation for Economic Co-operation and Development OECD Part per million by volume ppmv Perflurocarbons PFCs Proton Exchange Member PEM Solid Oxide Electrolyser Cell SOEC Solid Oxide Fuel Cell SOFC Solid Oxide Fuel-assisted Electrolyser Cell SOFEC Top gas recycling TGR Top gas recycling blast furnace TGR-BF Triple Phase Boundary TPB Ultra-Low CO2 Steelmaking ULCOS Vacuum Pressure Swing Absorption VPSA Water H2O

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1 Introduction By signing the Paris Agreement, 195 parties agreed on a long-term goal of keeping the increase in global average temperature to below 2°C above pre-industrial levels (EU, 2017). To reach this target, global carbon dioxide (CO2) emissions must be reduced by 50 % by 2050 compared to levels in 1990. Representing 6-7 % of global CO2 emissions, the steel industry needs to find solutions for reducing its carbon footprint. Although minimising energy consumption offers the greatest measure for cutting emissions in the short term, it can only contribute to an emission reduction of 15-20 % (Carpenter, 2012). The necessity of additional measures to decarbonise the steelmaking process is thus widely accepted. Significant advancements have been made through recycling scrap steel through an electric arc furnace (EAF), excluding the need for iron processing and avoiding 70-80 % of carbon emissions (Birat, 2010). However, the lifecycle of steel is long, and its demand continuously exceeds the availability of scrap steel. The IEA Clean Coal Centre (2012) estimated the emissions of CO2 from Electric Arc Furnaces (EAF) and direct reduction of iron to be about five times lower (0.4 tCO2/t crude steel) than the conventional blast furnace and basic oxygen furnace (BF-BOF) (1.8 tCO2/t crude steel). Nevertheless, 70 % of the world’s steel plants still utilise the conventional BF-BOF process. The organisation Ultra-Low Carbon Dioxide Steelmaking (ULCOS), supported by the European Commission (EU), consisting of engineering partners, research institutes, universities and all major European Union steel plants, is actively assessing the possibilities for reducing the carbon emissions from the steel industry. SSAB, LKAB and Vattenfall have also recently come together in a project, called HYBRIT (Hydrogen Breakthrough Ironmaking Technology) aiming at producing entirely carbon-free steel by replacing the blast furnace process with a direct reduction of iron ore with hydrogen obtained by electrolysis (SSAB, 2016). While this process would represent an unprecedented shift in steelmaking, widespread adoption throughout the world would be lengthy, as the development of a hydrogen infrastructure as well as a complete retrofit of the steel plant represent major economic barriers. The International Energy Agency (IEA), among others, identify Carbon Capture and Storage (CCS) as an interesting pathway for drastically reducing carbon footprint in steelmaking without refurbishing the whole plant. Accounting for 20 % of total plant emissions, the blast furnace is a crucial source of carbon emission, thus adapted for carbon capture. Top gas recycling blast furnace (TGR-BF) is one of many initiatives of ULCOS to decarbonise the industry, considered the most promising technology to significantly reduce CO2 emissions (EU, 2014). Through TGR-BF, 90 % of the exhaust gas from the blast furnace can be recycled into the combustion, while the remaining 10 %, highly concentrated in carbon dioxide, can be compressed for storage or utilisation. Co-electrolysis of carbon dioxide and water through Solid Oxide Electrolyser Cells (SOEC) is a promising pathway for the utilisation of excess carbon, due to its high efficiency. It relies on a simultaneous high temperature electrolysis of water and carbon dioxide. This process produces syngas, useful in many chemical or industrial processes, or for conversion into fuels. While TGR-BF is in demonstration mode through various pilot plants, SOEC is still in research stage. However, as TGR-BF rolls out, the utilisation of the captured carbon on-site will emerge not only as a way of avoiding the cost and infrastructural issues related to compression, transportation and storage, but also as carbon valorisation activity. There is little research on the integration of TGR-BF and SOEC, although it could present interesting benefits due to their high individual performance. This work explores the conceptual challenges and benefits from such a system and lays the foundation for further research focusing on the economic and technical aspects of the integration.

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1.1 Problem formulation Around the world, measures are taken to decrease carbon emissions from steel plants. While reducing energy consumption offers the greatest measure for cutting emissions in the short term, it is not alone sufficient as mitigation strategy (Carpenter, 2012). The necessity of additional measures to decarbonise the steelmaking process is thus recognised. According to the International Panel on Climate Change (IPCC, 2005), carbon capture and storage could contribute with 15-55 % of global mitigation efforts until 2100. Presently, an implementation of CCS is expensive, inhibiting the widespread use of the technology, despite economical penalties for carbon emissions. On-site utilisation is thus an opportunity for heavy emitters to valorise their captured CO2. Out of the potential utilisations, an emerging pathway is the co-electrolysis of CO2 and H2O, for its efficient syngas production. CCS through TGR-BF is currently being tested at industrial scale, and SOEC is a lab-scale co-electrolysis technology. Both technologies are promising separately, but what would be the implications of combining them? More specifically, what aspects need to be considered and resolved before a commercialisation of an SOEC into a TGR-BF can occur?

1.2 Aim and objectives To answer the questions above, this study reviews the SOEC technology to preliminarily assess its potential as a carbon capture and utilisation pathway for the steel industry, and especially its combination potential with a TGR-BF system in an integrated steel plant. The objective to explore the possibility of implementing a SOEC co-electrolysis system using recovered CO2 from the blast furnace in a BF-BOF steel factory is achieved in several steps. Despite accrued research in the fields of carbon capture and storage for the steel industry and in SOEC co-electrolysis (see Appendix 1), there is a research gap on the integration in a single system (see Appendix 2), calling for an exploratory, interdisciplinary study. As such, the sub-objectives identified below serve as a foundation for further detailed studies in this field:

1. Explain the importance of CO2 abatement in the steel industry 2. Review the status of TGR-BF and the ongoing research on SOEC 3. Map a conceptual co-electrolysis/TGR-BF system in a BF-BOF steel plant 4. Evaluate opportunities and challenges with the implementation of such a system

2 Methodology

2.1 Methodological basis The initial research objective was to identify a potential market application for SOEC and assess its economic opportunities. The first phase of the study, consisting in a literature review of current research status, gave a more thorough understanding of opportunities and challenges in the commercialisation of the technology. As for market considerations, SOEC provides a promising pathway for grid regulation and hydrogen production, but the rapid advancements of batteries respectively the lack of hydrogen infrastructure caused a shift in the research to an interesting peculiarity of the SOEC: CO2/H2O co-electrolysis for syngas production. While many studies point to this simultaneous electrolysis of CO2 and H2O as a promising CO2 mitigation pathway (Stoots et al., 2008; Stempien et al., 2012), few

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investigated the technical implications and the economic potential of installing a SOEC device for a CO2

emitting industry. This knowledge gap introduced the second part of this study, namely the identification of an adaptable industry and the depiction of the potential process. The iron and steel industry is actively exploring ways to deploy CCS technology through programs such as ULCOS. The idea of combining SOEC with CCS was supported by studies by Shi et al. (2015) and Varone et al. (2015), suggesting the integration of SOEC co-electrolysis with an oxygen-based combustion process (oxyfuel) for coal plants, much more efficient than air-fuelled combustion, and valorising the oxygen emitted by the SOEC. This made the case for the study of an electrochemical conversion of CO2 in the context of top gas recycling blast furnace, resting on the same concept of oxyfuel combustion and widely spoken about in CCS technologies adapted for steelmaking, e.g. by ULCOS. The interest of such a study was further confirmed by a modelling of a SOEC CO2 electrolysis of blast furnace gas, realised by Nakagaki et al. in 2015, the difference here being the introduction of the co-electrolysis, having some advantages over CO2 electrolysis.

2.2 Research purpose The research purpose is often categorised as exploratory, descriptive, explanatory, or predictive. A study of exploratory character, adopted for this thesis, intends to explore a research field and does not intend to offer any conclusive solutions to an existing problem. Exploratory research acts as a basis for further study and intends to investigate a research field rather than to give a conclusive solution to an existing problem (Saunders et al., 2007). The aim of this study was to review two separate research areas (SOEC and TGR-BF) to explore the bridging possibilities. Thus, the purpose was to lay the groundwork for further analysis of the specific technical and economic integration opportunities and barriers highlighted in this study. To understand the purpose of such inter-disciplinary studies, a great part of this study consists in setting the global context for carbon dioxide emissions and highlight the responsibility of heavy industries, namely iron and steel, as well as the considerable role of carbon capture and storage.

2.3 Literature study To accomplish the purpose of highlighting integration opportunities and challenges between SOEC and TGR-BF, a thorough literature review was necessary. It was performed on scientific publications within the area of SOEC, co-electrolysis and carbon capture technology. The review of SOEC consisted in identifying recurring sources, frequently cited, such as research conducted at the Idaho National Laboratory as well as Stoots et al., Mogensen et al., Ebbesen et al., Mougin, and Ni, amongst others. For the field of steelmaking and the TGR-BF process, more focus was put on market realities from the industry as well as current decarbonisation initiatives from industrial organisations and institutions, amongst others ULCOS. This literature from ULCOS was validated by frequently cited research such as Maria Pérez-Fortes (2016) from the Joint-Research Centre of the European Commission (218 citations).

2.3.1 Databases The scientific articles were found via databases including DiVA, ScienceDirect, Research Gate, Royal Society of Chemistry and scientific journals such as Journal Energy Storage, Journal of Power Technologies, International Journal of Hydrogen Energy. The search engine Scopus was also used to

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find relevant sources. The sections concerning CO2 emissions, carbon capture and storage and steelmaking extracted information principally from recognised international scientific or energy institutions such as the IEA, the IPCC, the Global Carbon Capture and Storage Institute (CCS Institute), and ULCOS.

2.3.2 Search terms The search was two-fold. First, each technology was studied independently, for which keywords included, amongst others, “SOEC”, “Solid Oxide Electrolyser Cell”, “CCS”, “CCU”, “CCUS”, “co-electrolysis”, “TGR-BF”, “BF-BOF” etc. Second, a combination of these keywords was used for identifying literature recouping the technologies, such as “SOEC and TGR-BF”, “SOEC and BF-BOF”, “co-electrolysis and BF-BOF” etc. The articles containing these keywords were analysed for relevance, then used for the literature study.

2.3.3 Time frame According to Saunders et al. (2007), time horizons are needed for the research design independent of the research methodology used. Due to the level of immaturity of SOEC technology, the publication date of the sources used was critical when performing the literature study. The number of publications using a certain keyword can be used as measure of output in a certain research field and was thus used to decide on the time frame for the literature review. The number of publications with keyword “SOEC” on e.g. the database ScienceDirect has increased every year since 2005 (with year 2011 as only exception) which indicates that the technology is still subject of research. As the performance of SOEC is still improving, experimental results around cell durability, efficiency, costs, were drawn from studies published after 2013 in priority.

2.4 Process flow diagram The system was illustrated through a schematic process flow diagram in which the system’s elements are represented using conceptual, graphic symbols rather than realistic pictures. The schematic system is highly simplified and all details irrelevant for the study have been omitted, due to the uniqueness of every steel plant and the technological immaturity of both technologies considered. Rather, emphasis was put on the interconnection between the SOEC and the TGR-BF unit to highlight possible synergies. The system limits are drawn to fit the scope of the study and does not take into consideration the source of electricity nor the end use of syngas. The source of electricity is assumed to be carbon-free, premise for a considerable reduction of carbon footprint of the established system.

2.5 Case study A case study was performed to exemplify the magnitude of syngas production the integrated system could benefit from as well as determine whether the two technologies could be scaled to the same order of magnitude. The case study was conducted based on data from two separate studies: data concerning the SOEC system were extracted from Fu et al. (2010) and the carbon capture potential was drawn from a TGR-BF system, modelled by Arasto et al. (2015). The TGR-BF system by Arasto et al. (2015) used Ruukki Metals Ltd’s steel mill situated in Raahe, on the coast of the Gulf of Bothnia, Finland, as reference plant. Ruukki in Raahe is the largest integrated

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steel mill in the Nordic countries, producing 2.8 Mt steel yearly (Goski and Smith, 2013). Several studies of CCS have been performed on the Ruukki plant and information on the carbon capture potential through TGR-BF was therefore available in literature and was thus chosen as source of data for the case study. The carbon capture potential was further used as input to a theoretical SOEC system. The conversion rates for the SOEC system used in the case were based on a SOEC system examined by Fu et al. (2010) who modelled a H2O/CO2 co-electrolysis for syngas production, using ASPEN Plus. The modelled system by Fu et al was chosen due to the scale of the system being closer to the size needed in a large system than in most research. Fu is a recurring author in the field of syngas production via high temperature steam/CO2 co-electrolysis and the referenced article was cited by 120 other researchers. One of the co-writers is Annabelle Brisse, also frequently cited in literature concerning SOEC.

2.6 Limitations There are two types of limitations to the study: the system’s conceptual nature and the system boundaries. The first limitation implies that no mathematical modelling was performed on neither the economics nor the thermodynamics of the system. The co-electrolysis process not yet fully understood nor commercialised, as well as the uniqueness of steel plants would make such calculations highly speculative and potentially misleading. Focus was made on the compatibility between the technologies through an assessment of the gas flow rates. The TGR-BF system comprises many different components, with a lot of a priori research compared with the SOEC. Therefore, the system components were considered as black boxes, to draw attention on bottlenecks to the potential implementation, namely the research status of the SOEC and the integration benefits and challenges. The case study relies on data extracted from the literature and was performed as a gauge for the magnitude of the system. The gas composition, stack size and performance vary widely in literature and the case study should therefore be contemplated as applicable only for the reference steel plant in combination with the SOEC studied by Fu et al (2010). The overall limitation of the system depends on the specific assumptions and peculiarities of the publications reviewed, such as:

- The age of the studies. The one conducted by Fu et al. dates to 2010. - Fu et al., estimated a conversion rate of CO2 of about 90 %, much higher than the 60 % usually

considered (see section about co-electrolysis for more detailed information). - The steel plant considered is the biggest steel plant in the Nordic countries, thus is not

necessarily representative of most steel plants worldwide. - The size of the SOEC stack is subject to variations, depending on its efficiency, the materials

used and the stack configuration.

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3 Literature review

3.1 Carbon dioxide emissions

3.1.1 Overview Total anthropogenic greenhouse gas (GHG) emissions (principally carbon dioxide, methane, nitrous oxide, fluorinated gases) have increased over the years 1970 to 2010 with larger absolute increases during the latter decade. GHG emissions increased on average by 1 gigatonne carbon dioxide equivalent (GtCO2eq) (2.2 %) per year from 2000 to 2010 compared to 0.4 GtCO2eq (1.3 %) per year from 1970 to 2000. The total GHG emissions reached the highest levels (49 GtCO2eq/yr) in human history year 2010, 76 % of these being CO2 emissions. The emissions decreased only temporarily (by 1.5 %) in the aftermath of the economic crisis 2007/2008 (IPCC, 2014). CO2 emissions from fossil fuel combustion and industrial processes contributed about 78 % of the total GHG emission increase from 1970 to 2010, with the same percentage contribution for the last decade (2000-2010). CO2 emissions from fossil fuels globally grew about 3 % between 2010 and 2011 and by 1-2 % between 2011-2012. In OECD countries (Organisation for the Economic Co-operation and Development), emissions grew at a slower rate but OECD countries still emit far more CO2 that other regions on a per-capita basis (OECD, 2011).

The most important drivers of the increase of CO2 emissions from fossil fuel combustion globally are the economic and population growths (IPCC, 2014). These drivers outpaced emission reduction mitigation initiatives, despite a growing number of climate policies. Middle income countries experienced the largest increase, in part due to rapid economic development and infrastructure expansion. A significant share of the emissions from middle income countries come from the production of goods exported to high income countries (IPCC, 2014). The 2014 IPCC report accounts for production-based CO2 emissions and does not allocate emissions according to their end-use (territorial emissions versus the carbon footprint (Minx, 2008)). This results in a deceptive picture of the emissions by country and is believed to be the reason behind the slower pace of emission increase in high income countries (OECD, 2011).

3.1.2 Carbon dioxide emissions in the industry In 2013, power generation was the largest emitting sector globally, contributing with 42 % of total CO2 emissions, followed by transport (23 %) and industry (19 %) (Figure 1) (IEA, 2015).

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Figure 1: Carbon dioxide emissions per sector (IEA, 2015)

In 2015, the iron and steel industry was the second largest industrial user of energy, consuming 25.8 EJ, and is the largest industrial source of direct CO2 emissions with 2.87 MtCO2/tonne crude steel (Figure 2) (Todurut, 2016). Overall, iron and steel production accounts for around 22 % of the world manufacturing industry’s final energy use and around 30 % of its direct CO2 emissions (IEA, 2015). Thus, steelmaking accounts for 6-7 % of world anthropogenic CO2 emissions. The crude steel production is expected to grow by almost 2 % per year until 2025 (IEA, 2015). Emissions per tonne of steel vary widely between countries due to the production routes used, product mix, production energy efficiency, fuel mix, carbon intensity of the fuel mix, and electricity carbon intensity (Carpenter, 2012). As mentioned in 3.1.1, energy consumption is correlated to population and economic growth. Studies also show that there is a strong positive correlation between economic growth, namely Gross Domestic Product (GDP) per capita, and crude steel production. This correlation follows from the development of infrastructure and industry as well as growing consumption of goods when wealth increases (Dobrotă and Căruntu, 2013). In other words, the steel industry face an increasing demand and stronger pressure for cleaner steelmaking at the same time.

Figure 2: Carbon dioxide emissions from industrial sectors (IEA, 2016)

Power and heat generation 42%

Other 5%Manufacturing industry 19%

Transport 23%

Residential and buildings 11%

Aluminium2%

Cement26%

Chemicals17%

Pulp and paper2%

Other23%

Iron and steel30%

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3.1.3 Outlook and mitigation pathways Without any additional efforts to reduce CO2 emissions, a continued increase is to be expected due to growth in population and economic activities (IPCC, 2014) and CO2 emissions are projected to remain the largest contributor to global GHG emissions (OECD, 2011). The International Energy Agency (IEA) estimated that 80 % of projected emissions in 2020 are already locked-in due to the long lifetime of the power plants currently in place or under construction (IEA, 2011). The mitigation scenarios involve a wide range of technological, socioeconomic, and institutional trajectories. Policy tools for mitigation include price-based instruments, command and control regulations, technology support policies and information and voluntary approaches (OECD, 2011). According to the OECD, carbon taxes and trading schemes are the cheapest ways of reducing CO2 emissions. Two price-based instruments are EU-Emission Trading Scheme (EU-ETS) and carbon taxes, discussed in the section below.

3.1.3.1 EU-ETS

An example of price-based instruments is the EU-ETS. The EU-ETS is a “cap and trade” system operating in all 28 EU countries plus Iceland, Liechtenstein and Norway (EC, 2017) making it the world’s largest emissions trading system (OECD, 2011). It is based on the principle of capping the total amount of GHG emissions from power plants, industries and other installations covered by the system. This cap was set to 2,084,301,856 allowances in 2013 and is further reduced by 38,264,246 allowances annually 2013-2020. The allowances are tradable and must cover all emissions, otherwise the company will face heavy fines. The allowances are allocated through auctioning, meaning that the companies must buy the allowances. Some allowances are allocated for free but the share of free allowances decreases each year. Manufacturing industry received 80 % of its allowances free of charge in 2013. This proportion will decrease gradually down to 30 % in 2020 (EC, 2017). The gases covered in the trading scheme are carbon dioxide (CO2), nitrous oxide (N2O) and perfluorocarbons (PFCs) and the sectors include power and heat generation, energy-intensive industry sectors, such as commercial aviation, oil refineries, steel works and production of iron, aluminium, metals, cement, lime, glass, ceramics, pulp and paper, etc. (EU, 2017). The price per EU Allowance (EUAs) in the EU ETS fell from almost 30€/tCO2 in mid-2008 to less than 5€/tCO2 in mid-2013 (Koch et al., 2014) and were back at €7.6/tCO2 in 2015 (EEA, 2016). This price drop is due to the decrease in demand, following the economic crisis (which reduced emissions more than anticipated). This price drop has led to a surplus in the number of allowances and contributing to the already weak incentives to invest in low-carbon technology, increasing the risk for a generalised carbon lock-in (Koch et al., 2014).

3.1.3.2 Carbon taxes

Carbon tax is a tax levied on the carbon content of fuels. If set high enough, carbon taxes become a powerful monetary disincentive that motivates switches to clean energy. Carbon taxes are currently used in 10 OECD countries, with Denmark, Finland, the Netherlands, Norway, Sweden and the United Kingdom leading these efforts since the early 1990s. Sweden was one of the first countries to introduce a carbon tax in 1991, with the general level of the tax increasing over the years to reach €131/tCO2 in

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2016 (World Bank, 2016). Price and policies for carbon taxes vary from country to country. In Sweden, many heavy industries or products are not subject to carbon tax, in order for them not to lose their competitiveness, and to not risk carbon leakage. The excluded industries include the CO2 rich gas from the blast furnaces in the steel industry (Skatteverket, n.d.).

3.2 Carbon capture, storage and utilisation

3.2.1 The role of CCS in climate change mitigation As described in chapter 2.1, the use of fossil fuels for energy supply and chemical conversions will contribute to global warming and will ultimately lead to the depletion of these limited resources. While the energy sector can decrease its emissions by using alternative fuels, the potentially most significant alternative in the heavy industry sector is to capture the CO2 using CCS. Numerous projections for the global energy system emphasise the importance of CCS in strategies for reducing greenhouse gases (IEA, 2011). In, most climate change mitigation scenarios in the CCS Special Report conducted by the IPCC (2005), CCS could be the single biggest reduction measure worldwide. The IEA also projects a significant role for CCS in their Blue Map scenario, with around 30.24 MtCO2/yr captured in 2020, rising to 822.6 MtCO2/yr in 2050 (IEA, 2011). This can be compared to the emissions from an average steel mill at about 4.5 MtCO2/yr. The IEA Blue Map scenario, in which global energy-related CO2 emissions are halved from current levels by 2050, assumes that policies are in place to provide strong incentives for CCS and other low-carbon technologies (Carpenter, 2012). Recently CCS has been expanded and the interest in utilisation of CO2 as feedstock has emerged, this system is named carbon capture and utilisation (CCU) or, the combination between CCS and CCU; carbon capture, utilisation and sequestration (CCUS). CCUS includes CO2 capture, compression, transportation, utilisation and sequestration. While the goal of a CCS supply chain network is to reduce the CO2 emissions, CCUS aims at maximising the revenue or profit from CO2 utilisation since it can be used or sold as feedstock. Due to the potential to provide CO2 as feedstock to synthesise materials, chemicals and fuels, CCUS plays an important role in CO2 reduction and should be complementary to the geological storage in a CCS system. Presently, 0.4 % of emitted carbon is re-used (Pérez-Fortes, 2016). CCUS covers a broad range of technologies allowing for the use carbon dioxide emissions from fossil fuel to be used as industry feedstock. CCUS could represent a new economy for CO2 and has the potential of reducing CO2 emissions and the depletion of fossil fuels (Hasan et al, 2011). The recovered CO2 can either be used as feedstock in the same industry where it was captured or sold on the free market. The CCUS potential is mainly limited by the market size of CO2-based products (von der Assen et al, 2014). It is also dependent on its energy efficiency of the CCUS since this process requires substantial amounts of energy both for capturing and converting the CO2. Von der Assen et al. highlight that the amount of utilised CO2 is not equal to the amount of CO2 avoided and that the amount avoided should include the emissions used during capture, transport, CO2 transformation, and CO2 product consumption. It is recommended that such system be powered by renewable energy sources such as wind or solar power.

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3.2.3 Carbon utilisation As for any chemical compound, there is a market for CO2. There is a strong supply surplus, however, with an estimated 500 million tonnes emitted from high-concentration sources, and 18 gigatonnes annually of dilute CO2 from power, steel and cement plants, compared with 80 Mt/yr on the demand side (Global CCS Institute, 2011). The low demand can be explained by the limited numbers of technological pathways for carbon utilisation. The biggest demand comes from Enhanced Oil Recovery (EOR), a technique for extracting the remaining oil after conventional methods have reached their limits (Maersk, 2016). EOR is considered a tertiary recovery process in oil field development, after the primary depletion and the water flood. The demand from the EOR industry is satisfied mostly from carbon reservoirs and not from captured CO2 (Joos, 2016).

Table 1: Table of carbon conversion technologies (Global CCS Institute, 2011)

There are various other end products constituted from CO2: fuels, fertilizers, chemicals etcetera. Mineralisation, biological- and chemical processes are the three main pathways of converting CCS into useful products (Global CCS Institute, 2011). Their characteristics are compared in Table 1, highlighting their potential in Mt/yr, their permanency (whether the technology avoids any eventual reemission of the utilised CO2) as well as their respective advantages and disadvantages.

Technology Description Products Potential Permanency Advantage Disadvantages

Carbon mineralisation

Reaction with a mineral or industrial waste

Compound used in construction

>300 Mt/yr

Yes Abundance of minerals and industrial waste

High energy and feedstock demand

Concrete curing

CO2 stored as unreactive limestone within concrete

Concrete 30-300 Mt/yr

Yes Cost competitive direct use of flue gas,

Quality standards need to be met, cost of retrofitting curing process

Algae cultivation

Absorption from microalgae

Proteins, fertilizers, biomass

>300 Mt/yr

No Competitive source of biofuels

Sensitivity to impurities, low efficiency

Fuels Catalyst-based reaction or redox

Liquid or gaseous fuels (methanol, syngas, etc.)

>300 Mt/yr

No Energy-carrier with a wide range of uses

Requires renewable source of electricity, CO2 purification cost

Chemical feedstock

Synthesis of polycarbonates

Polycarbonates used in chemical industry

5-30 Mt/yr

No Existing infrastructure direct use of flue gas without purification wide range of uses

Quick re-emission of CO2

Urea yield boosting

Ammonia and CO2

conversion Urea fertilizer 5-30

Mt/yr No Mature

technology Quick re-emission of CO2

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Syngas (H2 + CO) is called an intermediate gas, as it can be separated or used directly in the synthesis of various end products, especially in the chemical industry and for fuel synthesis. Fuel production is particularly interesting due to its energy storage characteristic, highly demanded with the increasing share of renewable energy sources (IASS, 2014). The global consumption of syngas and derivatives in 2015 was 115 000 MWth, and is anticipated to rise at a 9.40 % growth rate by 2024 to reach more than 250 000 MWth (Transparency Market Research, 2017). The growing market for syngas supports the idea of a syngas producing system. The high versatility of syngas makes it a building block for many different applications. The various chemical or industrial applications include production of synthetic diesel, methanol, dimethyl ether (DME) or hydrocarbons (IRENA, 2013). As illustrated in Table 2, synthetic gas can be synthesized in different ways, sometimes in multiple steps, including methane reformation, the reverse water-gas shift reaction, carbon gasification, and co-electrolysis (Styring and Jansen, 2011). Although there is no single best fuel or process, syngas production from co-electrolysis is interesting for its high efficiency and since it does not require to purchase large quantities of hydrogen.

Table 2: Reforming reactions for syngas production

Methane reformation 𝑪𝑶𝟐 + 𝑪𝑯𝟒 ↔ 𝟐𝐂𝐎 + 𝑯𝟐 Reverse water-gas shift reaction 𝐶𝑂2 + 𝐻2 ↔ 𝐶𝑂 + 𝐻2𝑂 Carbon gasification 𝐶𝑂2 + 𝐶 ↔ 2𝐶𝑂 Co-electrolysis 𝐻20 + 𝐶𝑂2 ↔ 𝐻2 + 𝐶𝑂 + 𝑂2

3.3 Steel industry The steel sector is associated with a complex industrial structure but with two dominating production routes (Rootzén, 2015):

• Integrated steel mills (Figure 3). This production route involves interconnected production units, processing iron ore and scrap metal to crude steel. The production units involve coking ovens, sinter plants, pelleting plants, blast furnaces, basic oxygen furnaces and continuous casting units.

• Mini-mills. Crude steel is produced in an electric arc furnace by processing scrap metal, direct reduced iron, and cast iron.

Although steel production in mini-mills emits less CO2 than the integrated steel mill (0.4 tCO2/t crude steel and 1.8 tCO2/t crude steel respectively) (Carpenter, 2012), the integrated steel mill with blast furnace and basic oxygen furnace (BF-BOF) still dominates steel production, accounting for 73.7 % of world steel production (Todurut, 2016). Over the three last decades, steel production from BF-BOF has been steady (Carpenter, 2012) and the increase in steel production was represented by an increasing share of EAF production, indicating a gradual shift from BF-BOF production. However, the EAF production route is limited to scrap availability which, together with long lifetime of blast furnaces (20 years), signal that a shift can be lengthy and only BF-BOF has thus been considered in this study.

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Figure 3: Process scheme integrated steel mill (Rootzén, 2015).

3.3.1 Fundamentals of BF-BOF steelmaking The production in an integrated steel plant using BF-BOF process (Figure 3) can be categorised into four steps: 1) Raw material preparation: coke making and iron ore preparation, 2) Ironmaking: iron ore is reduced into hot metal in a blast furnace, 3) Steelmaking: the hot metal is converted into liquid steel, and 4) Manufacturing of steel: through casting, rolling and finishing. The full steelmaking process is complex and as the focus area of this study is the blast furnace, steps 1, 3 and 4 will be omitted in the literature review. Step 2: Ironmaking This study focuses on the ironmaking process in the integrating steel plant, taking place in the blast furnace. During this step, iron is extracted from iron ore (containing iron oxide) through a reduction reaction, under high temperatures of 900-1600 °C. The iron ore, coke and limestone are charged into the top of the blast furnace. The iron ore is used as source of iron, the coke is used to burn the air and heat is provided by the highly exothermic coke combustion reaction. It also reacts to form carbon monoxide, used to reduce the iron oxide. Limestone reacts with acidic impurities forming molten slag, which can then be removed from the process. A hot air blast and a reductant are blown trough the tuyeres from the bottom of the blast furnace (Figure 4). Generally, coal is used as reductant due to the reactive characteristics of carbon, reducing iron oxide to metallic iron through the following equation (Onarheim et al., 2015):

2𝐹𝑒2𝑂3 + 3𝐶 → 4𝐹𝑒 + 3𝐶𝑂2 (1) Due to the high temperatures in the blast furnace, CO can also be used as reductant, namely:

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𝐹𝑒2𝑂3 + 3𝐶𝑂 → 2𝐹𝑒 + 3𝐶𝑂2 (2)

It takes 6 to 8 hours for the raw material to descend to the bottom of the furnace where it becomes liquid slag and liquid iron, called molten iron. From the top of the BF the blast furnace gas (BFG) exits. The BFG contains around 17–25 % of CO2, 50-55 % N2, 20-28 % CO, and 1-5 % H2 (Carpenter, 2012). It also contains impurities such as sulphur, cyanide compounds, and dust. The hot air blast blown through the tuyeres can be changed for oxygen, termed oxyfuel-BF. Oxyfuel-BF avoids the accumulation of nitrogen in the exiting blast furnace gas and increases the CO2 concentration in the BFG, enabling CO2 capture (Carpenter, 2012). The oxygen is provided from air separation units, already present in the steel plant due to the high oxygen consumption in other parts of the steel plant, namely in the oxygen blast furnace. Oxyfuel-BF does, however, increase the production need of oxygen, which represents a major capital investment (Zheng et al., 2014).

Figure 4: Schematic diagram of blast furnace

3.3.2 Options for CO2 emission reduction in the iron and steel industry There are several technologies and measures available to abate direct and process CO2 emissions from the different iron- and steelmaking processes. These involve fuel shift, improved energy efficiency, new steelmaking processes and CCS. The improvement in energy efficiency offer the greatest measure for cutting CO2 emissions in the short term. However, it has been pointed out that these measures can only attain a 15 % to 20 % reduction, thus recognising the necessity of additional measures of decarbonising the industry (Carpenter, 2012). Fuel shift The blast furnace accounts for 18.1 % of the total carbon emissions. A fuel shift in the furnace, being the single most energy-consuming process in the production process, can work as abatement option. Presently, coke (derived from coal) is used both as fuel and reducing agent. A shift towards alternative fuels such as natural gas or bio-coke might reduce the CO2 emissions from the blast furnace (Rootzén, 2015).

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Energy efficiency Over the years the iron and steel industry has made significant efforts to reduce energy consumption and lower CO2 emission. Such measures have allowed manufacturers to reduce energy consumption by 50 % compared to 1975 levels for North America, EU and Japan (Carpenter, 2012). Novel technology ULCOS has selected four technologies that could provide a drastic reduction of CO2 emissions by more than 50 % compared to the present best practices. These four technologies are (ULCOS, 2011; SETIS, 2011):

- Top Gas Recycling Blast Furnace with CCS: used in this study and further described in section 3.4

- HIsarna with CCS: technology combining a cyclone converter furnace for the ore melting and a smelting reduction vessel where the reduction to liquid iron takes place. HIsarna does not require iron ore agglomerates and has therefore lower carbon emissions than the traditional BF-BOF plant.

- ULCORED: Using an EAF, where direct-reduced iron is produced from direct reduction of iron ore by a reducing gas from natural gas.

- ULCOWIN/ULCOLYSIS. In these initiatives, the iron is produced by electrolysis, the blast furnace is no longer required. In the ULCOS project, this electrolysis is carried out through alkaline electrolysis.

Carbon Capture and Storage Kundak et al. (2009), among others reinforce the fact that deep cuts in CO2 emissions demand carbon capture and storage in the steel industry. The opportunities for CO2 capture in steel production vary depending on the process and feedstock. The direct emission sources in integrated steel plants from which CO2 could be removed are the flue gases (gas emissions) from the lime kilns, sinter plants, coke ovens, hot stoves, BFs, and BOFs. Capturing CO2 from stack gases is considered advantageous for retrofitting since this would not require fundamental changes in the iron and steel making process. Since BFG is a large source of carbon dioxide, most of the effort to develop CCS for integrated steelworks is concentrating on the application of CCS to the BF through top gas recycling (TGR-BF) in combination with oxyfuel-BF. (Rootzén, 2015). TGR-BF is described in detail in the section below.

3.4 Carbon capture through top gas recycling blast furnace A promising technology for significantly reducing the CO2 emissions from the blast furnace is TGR-BF, a technology including: the injection of reducing top gas components CO and H2 in the shaft and tuyeres, lower fossil carbon input due to lower coke rates, the usage of pure oxygen instead of hot blast air at the tuyere, and recovery of pure CO2 from the top gas for storage. In TGR-BF, the conventional blast furnace is replaced or retrofitted with a top gas recycling blast furnace where the CO2 is separated from the blast furnace gas and stored or utilised. The remaining CO2-stripped gas is fed back into the blast furnace (Figure 5). Recycling the CO2-stripped gas into the blast furnace reduces direct carbon emissions from the blast furnace by 26 % and an additional 52 % can be avoided by combining the top gas recycling with carbon capture and storage or utilisation. The total CO2 reduction potential of the blast furnace is thus 78 % (van der Stel et al., 2013).

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Figure 5: Schematic diagram of TGR-BFG

3.4.1 Process The blast furnace gas exits the top of the blast furnace and is led into a shift reactor followed by a CO2 capture unit, in this case VPSA, separating the CO2 from the CO2-rich BFG. The CO2 stripped gas is then recycled back into the tuyeres of the blast furnace, after passing through a heater (Figure 5). Below follows a more detailed description of the process steps. STEP 1 The BFG leaves the blast furnace at a temperature of 125-350 °C and at 0.5-1.5 bar. The flow rate varies depending on the blast furnace capacity, but identified at 240 m3/s by Carpenter (2012) and 104.1 m3/s by Arasto et al (2013). The TGR-BF gas must be de-dusted before entering the Vacuum Pressure Swing Absorption (VPSA) since it contains pollutants such as Coarse PM, cyanids, NH3 and H2S. It is cleaned in the gas cleaner in two stages; Coarse PM is removed in the first stage, and PM including zinc oxide and carbon, cyanide and NH3 are removed in the second stage by wet scrubbing or wet electrostatic precipitation (IPCC, 2011). STEP 2 The role of the shift reactor is to enable the removal of carbon in the BFG. The shift process is accomplished in two reactors; a high temperature shift reactor at 400°C and a low temperature shift reactor at 250 °C (Carpenter, 2012). In the reactors, the CO in the BFG is reacted with H2O to produce H2 and CO2 (water gas shift reaction), increasing the amount of CO2 in the BFG. This allows for a drastic increase in the total carbon captured from the blast furnace from 50 % (without shift reactor) to 85-99.5 % (Gielen, 2003; Kuramochi et al, 2011). Heat is recovered from the off gas of the second reactor, used to preheat the feedstock and steam for the first reactor, and some surplus steam can be reused elsewhere in the steel factory (Carpenter, 2012). STEP 3 Following the shift reactor, the gas is led into the carbon capturing unit. There are presently several commercial technologies to capture carbon dioxide from gases, including absorption (chemical or physical), adsorption (PSA, VPSA), membranes, gas hydrates, and mineral carbonation (MacElroy, 2015). These CO2 capture technologies each have their optimal field of application, and their own advantages and disadvantages. Only VPSA is discussed in this study since it has been proven to work without failure in ULCOS steel plant in Luleå where it processed up to 90 % of blast furnace top (EU, 2014). VPSA has the lowest energy consumption (Quader et al, 2016) and was chosen in the ULCOS steel plant in Luleå as the simplest and cheapest solution (Carpenter, 2012; Kuramochi et al, 2011).

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VPSA involves passing the BFG through a bed of a solid sorbent which adsorbs the CO2. When the bed is fully loaded, the BFG is sent to another bed. By reducing the pressure to below atmospheric pressure, the fully loaded bed is regenerated. VPSA operates under ambient temperature and is suitable for flue gases containing more than 15 % CO2 (Wang et al., 2012). The CO2-rich gas exiting the VPSA has a composition of 87.2 % CO2, 10.7 % CO, 1.6 % N2 and 0.6 % H2 and is further led for utilisation or storage. In a TGR-BF with carbon capture and storage, the VPSA is followed by a cryogenics unit due to the high level of purity required when storing carbon dioxide. In the cryogenics unit, the CO2 can be separated from other gases by cooling and condensation (Carpenter, 2012). STEP 4 In STEP 4, the CO2-stripped gas from the VPSA is either fed direct into the blast furnace or passed through a heater. The CO2 stripped gas leaves the VPSA consisting of mainly CO and H2 (van der Stel et al., 2013). The injection of the CO2-stripped BFG leads to the reduction of iron oxides, lowering the demand for coke and thus reducing carbon emissions (Carpenter, 2012; van der Stel et al., 2013). The temperature of the gas has been proved to affect the performance of the blast furnace, and while no optimal temperature has been found, three versions of TGR-BF have been tested by ULCOS; hereby named version 1, 2, and 3 (Quader et al, 2016). The third version resulted in a higher reduction of CO2 emissions. However, more analysis most be put into the system benefits from each version, including costs of heating and adding new tuyeres, as well as the retrofitting possibilities of each version. In version 1, the cold (25°C), recycled gas from the VPSA was fed into the blast furnace through additional tuyeres. This resulted in a 22 % reduction of direct carbon emissions. In version 2, the CO2 stripped gas from the VPSA was heated and recycled through the main tuyeres, resulting in a 24 % reduction of direct carbon emissions. In version 3, the recycled gas was heated to 1250°C and fed into the blast furnace through additional tuyeres. This resulted in a 26 % reduction of direct carbon emissions.

3.4.2 Energy Electricity consumption for the carbon capture using VPSA is, according to Birat (2010b) 0.38 GJ/tCO2 captured and 0.94 GJ/tCO2 captured according to Kuramochi (2011). Arasto (2015) estimates the energy consumption to 0.41 GJ/tCO2 captured. The VPSA uses only electricity for CO2 removal and does not contribute to any additional emissions given that the electricity is generated from renewable sources (Kuramochi, 2011). Water usage increase for CCS is estimated to 108 kg/tCO2 captured (Tsupari et al, 2013). Finally, In 2014, it was estimated that the ASU consumes about 200kWh/tO2, meant to decline to 150kWh/tO2 by 2017 according to Air Liquide (Zheng, 2017).

3.4.3 Costs There are few long-term cost analyses of TGR-BF. Most studies agree that the total cost of TGR-BF implementation is complicated to assess and depends on factors such as location of the plant, energy and materials prices, carbon pricing, capital cost estimation etcetera, all differing significantly from plant to plant. The costs related to TGR-BF include electricity purchase, operational expenses, capital expenses as well as CO2 transportation and storage. On the upside, savings from cost abatement related to emission penalties such as EU-ETS are taken into consideration.

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Carpenter estimated CO2 capture costs for TGR-BF at 40-65 €/tCO2. Kuramochi et al. (2012) estimated the costs to 50–80 €/tCO2 and Tsupari et al. (2013) to 31.1-77.0 €/tCO2 depending on capacity. These cost assessments, however, include the compression, transportation and storage of CO2 and the costs for a TGR-BF system where the CO2 is utilised is therefore likely to be less. Tsupari et al. (2013) assessed the transportation and storage costs to 46.6-63.9 €/tCO2.

A major capital expenditure in oxyfuel combustions is the ASU, which often represents the major barrier to investment (Varone, 2015).

3.4.4 Recent progress The ULCOS TGR-BF with CCS is included in five of ULCOS:s programs and has been validated in the ULCOS program and demonstrated on LKAB:s blast furnace in Luleå. In an early phase of ULCOS experimental blast furnace in Luleå, the heat and mass balance as well as the tuyeres conditions were mathematically modelled, modifications were made and two test campaigns were then conducted. During the campaigns, the results obtained were not far from the calculated ones (EU, 2014). Commercial deployment is expected in 2020 (Quader, 2016).

3.5 H2O/CO2 Co-electrolysis The simultaneous electrolysis of H2O and CO2 into a synthetic gas through a Solid Oxide Electrolysis Cell (SOEC) is a promising pathway to reuse carbon dioxide. This section will review the current research of the H2O/CO2 co-electrolysis (thus forth simply called co-electrolysis). Firstly, the operating principle of a classical electrolysis will be explained. Then, the characteristics of a co-electrolysis will be detailed, such as its difference with a single electrolysis, the underlying thermodynamics and the material used. Finally, a short review of the recent research advancements will be discussed, to provide more insight in its future possibilities and applications.

3.5.1 Electrolysis The word electrolysis was first introduced by Michael Faraday in the 20th century. Electrolysis is the chemical process of separating elements through an electrical current. The system, called electrolytic cell, is composed of two electrodes (the anode and the cathode) separated by an ionic conducting electrolyte. The passage of a direct current from the anode to the cathode forces a non-spontaneous chemical reaction, whereby atoms and ions interchange from the oxidation reaction at the anode, and from the reduction reaction at the cathode (Shi et al., 2015). The reduction-oxidation reaction is most commonly referred to as redox. The most common electrolysis is of water. It consists in splitting water (H2O) with electricity to form hydrogen (H2) and oxygen (O2). At the negatively charged cathode, surplus electrons combine with H+ ions from the H2O molecule (reduction reaction) to form H2 and OH- molecules. The electrolyte will allow for the passage of the latter to the anode, where an electron will be released from the oxidation reaction, thereby closing the electrical circuit, and forming O2 and H2O molecules. The two simultaneous half reactions at the cathode and the anode, called reduction and oxidation respectively are characterised by the following equations (Stempien et al., 2013):

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Reduction 2𝐻+ + 2𝑒− → 𝐻2 (3) Oxidation 2𝑂𝐻− → 1

2⁄ 𝑂2 + 𝐻2𝑂 + 2𝑒− (4)

Overall 𝐻2𝑂 → 𝐻2 + 12⁄ 𝑂2 (5)

Hydrogen, produced from electrolysis, is the most abundant element on earth, but cannot be found in natural form but rather as elements in molecules. The feed of electricity in an electrolysis cell separates the hydrogen from the molecules, thus creating an energy storage medium. The hydrogen can then be converted back into electricity through the reverse process, in a fuel cell (Ferrero, 2016). The efficiency of the power-to-hydrogen-to-power process, called round-trip-efficiency, was predicted from an electrochemical model by Klotz to be 0.68 (in the case of the SOEC), to which losses in heat management and from the balance of plant (BoP) system would have to be added (Klotz et al., 2014). The three main electrolysis techniques available presently are at different developmental phases. While Alkaline electrolysis (AEL) and Proton Exchange Membrane (PEM) are mature technologies, Solid Oxide Electrolyser Cells (SOEC) is not yet commercialised. The latter technology, by operating in very high temperatures (between 650 and 1000°C), has a very high electricity-to-hydrogen efficiency rate – theoretically up to 100 % if the conditions are met (thermoneutral voltage, perfect insulation) (Ferrero, 2016). Accounting for system losses, such as in electrical converters and in thermal auxiliaries, Mougin (2015) reports an 89 % SOEC system efficiency. A brief comparison of the three electrolysers is made in Table 3. By using a liquid electrolyte (alkaline), a solid electrolyte in a cold environment (PEM) or a solid electrolyte at high temperatures (SOEC), various advantages and disadvantages emerge. AEL and PEM have a relatively high capacity and durability, but their maturity levels give little hope for efficiency breakthroughs. Conversely, the high theoretical performance of the SOEC comes at the cost of its durability. Since only the latter technology is well-suited for a co-electrolysis of CO2 and H2O (for thermodynamic issues, as will be seen later), the two other technologies will be further omitted in this study.

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Table 3: Comparison of electrolysis technologies (Götz et al., 2016)

3.5.1.1 Solid Oxide Electrolyser Cell

Solid Oxide Electrolyser Cells have attracted a lot of attention in recent years because of their high electrical efficiency, that can even exceed 100 % during endothermal mode, thanks to the high operating temperatures (see section Energy) (Brisse et al., 2008). Reducing the electrical energy requirement is the fundamental goal, since it represents the main operational cost (Carmo et al., 2013). The SOEC consists of porous solid electrodes, allowing for the passage of gas, separated by a solid ion-conducting electrolyte. Electrolysis in SOEC can be identified by the following three steps (Figure 6) (Stoots, 2008):

1. H2O in form of steam is fed into the SOEC to reduce the cathode, represented by the following half-equation:

𝐻2𝑂 + 2𝑒− → 𝐻2 + 𝑂2− (6)

2. The liberated H2 is recuperated at the porous cathode, while the O2- passes through the electrolyte towards the anode.

3. The oxidation reaction at the anode closes the electrical loop by dissociating the oxide ion (O2-) into electrons and oxygen elements. It can be represented by the following half-equation:

Technology Charge carrier

Temperature range and pressure

Electrolyte & Electrode

Characteristics

Alkaline Electrolysis

OH- 40-90°C < 30 bar

Liquid alkaline and Ni/Fe electrodes

Technology: mature Cost: Cheapest and effective

Efficiency: 70% Lifetime: ~ 100,000 h (up to 15-

20 years) Stack: MW range

Cold start time: minutes PEM

electrolysis H+ 20-100°C

< 200 bar Solid acid

polymer and noble materials

(Platinum, Iridium,…)

Technology: New and partially established

Cost: medium Efficiency: 70-80 % Lifetime: ~ 40,000 h (up to 10-15 years)

Stack: Below MW range Cold start time: seconds

SOEC O2- 700-1000°C Atm.

Ceramic metal compound and

Ni doped ceramic

Technology: In laboratory phase Cost: high

Efficiency (theoretical): 100 % Lifetime: ~ 5,000 h

Stack: lab-scale Cold start time: hours

Possibility of co-electrolysis and reverse operation (SOFC)

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𝑂2− → 12⁄ 𝑂2 + 2𝑒− (7)

The by-products of the electrolysis are H2, used as energy storage, and O2, a non-polluting gas. The overall reaction of water-electrolysis in SOEC can be written as (Nguyen and Blum, 2015):

𝐻2𝑂 + 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦 + ℎ𝑒𝑎𝑡 ↔ 𝐻2 + 12⁄ 𝑂2 (8)

Figure 6: Operating principle of SOEC water electrolysis

3.5.1.2 Co-electrolysis

The co-electrolysis follows the same principle as SOEC, except two electrolyses occur at the same time: H2O is converted into H2, and CO2 is converted into CO. It is essentially adding a CO2 electrolysis reaction to the previously illustrated H2O electrolysis, for which the reaction can be written as (Nguyen and Blum, 2015):

𝐶𝑂2 + 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦 + ℎ𝑒𝑎𝑡 ↔ 𝐶𝑂 + 12⁄ 𝑂2 (9)

Thus, added to the H2O electrolysis, the final overall reaction is:

𝐻20 + 𝐶𝑂2 ↔ 𝐻2 + 𝐶𝑂 + 𝑂2 (10)

The reactions are illustrated in Figure 7.

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Figure 7: Operating principle of SOEC co-electrolysis

CO2 can be electrolysed by itself, just as water. However, a co-electrolysis presents several benefits, compared to successive electrolysis of H2O and CO2. Firstly, the presence of steam effectively avoids the risk of carbon deposition from the CO2 electrolysis. Secondly, the performance of the Ni-based cathode is notably lower in the case of CO2 electrolysis than in the H2O electrolysis (Stoots et al., 2009). In the presence of even smaller amounts of steam in the case of the co-electrolysis, the cell performance almost reaches that of pure H2O electrolysis. Finally, a single-step process is economically less costly than two successive steps (Stoots et al., 2007). The bigger picture of the co-electrolysis process (Figure 8) can be summarised in the following three steps: STEP 1 The inlet gases pass through mass flow controllers (X) to obtain the right gas composition, and are then mixed. STEP 2 The mixed gas is first heated by a heat exchanger (HX), recovering the high heat from the SOEC outlet gases. The SOEC is placed in an insulated environment, meaning that the outlet gases lose very little heat in the process. Much of this heat can be transferred to the inlet gases through efficient cross-flow heat exchangers. The rest of the heat necessary is gained by superheating the inlet gases. STEP 3 The co-electrolysis takes place, by feeding electrical energy to the cell. The syngas and the oxygen are recovered separately and cooled in the heat exchanger. Note that hydrogen is necessary for improved cell performance, and some of the outlet syngas can be recycled into the inlet mix after the heat exchange.

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Figure 8: SOEC co-electrolysis process

3.5.1.3 Energy

In this section, some theoretical aspects of the thermodynamic and kinetic reactions of a co-electrolysis will be explained. Although this study does not aim at predicting results but is rather based on empirical results, it is important to understand the underlying concepts introduced below, as they explain the reason behind the usage of SOECs and sheds light on their performance and limitations. As explained earlier, energy is needed to split the H2O and the CO2 to produce syngas. The energy demand, ∆𝐻, for the SOEC is expressed by:

∆𝐻 = ∆𝐺 + 𝑇∆𝑆 (11) Where ∆𝐺 is the Gibbs free energy and represents the electrical demand, and 𝑇∆𝑆 is the thermal energy demand (J/mol H2) (Shi et al., 2015).

Gibbs free energy decreases significantly with higher temperatures: 1.23 eV (237 kJ mol-1) for ambient temperature, and 0.95 eV at 900°C (183 kJ mol-1). The molar enthalpy, however, remains fairly constant, at 1.3 eV or 249 kJ mol-1 at 900°C) (Larmine et al., 2003). In other words, a large amount of energy necessary for the electrolysis can be provided by heat. This is illustrated by the inversely proportional electrical energy and thermal energy demands from H2O electrolysis in Figure 9.

Figure 9: Inversely proportional electrical energy and thermal energy demands (Ni et al., 2008)

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Presented in Figure 10, at temperatures between 750°C and 850°C, the Gibbs free energy (i.e., the electrical energy demand) of CO2 and H2O respective electrolyses are close, which is the optimal window for syngas production, given an equal applied voltage (Nguyen and Blum, 2015).

Figure 10: Temperature dependency of energy demand of H2O and CO2 reduction reactions (Nguyen and Blum,

2015)

The minimum electrical energy required to initiate the reaction splitting H2O/CO2 is the Nernst potential 𝑉𝑁, and is largely determined by the operating temperature and the inlet gas partial pressures. Ideally, the optimal cell voltage would be the Nernst potential, but the operating voltage of the cell (or open cell voltage) needs to account for three losses (Shi et al., 2015):

1. The ohmic overpotential (𝜂𝑜ℎ𝑚), due to ionic and electronic charge transport resistances. 2. The activation overpotential (𝜂𝑎𝑐𝑡), due to the irreversibility of electrochemical reactions. 3. The concentration overpotential (𝜂𝑐𝑜𝑛𝑐), due to the mass transport resistance in the electrode.

Thus, the operating voltage 𝑉𝑜𝑝 can be written as:

𝑉𝑜𝑝 = 𝑉𝑁 + 𝜂𝑜ℎ𝑚 + 𝜂𝑎𝑐𝑡 + 𝜂𝑐𝑜𝑛𝑐 = 𝑉𝑁 + 𝑖 ∗ 𝐴𝑆𝑅 (12) Where ASR is the Area Specific Resistance (Ωcm-2), and accounts for the total losses of the cell. Essentially, the ASR determines the performance of the cell, i.e. the electrical energy demand W, by the following relationship:

�� = 𝑉𝑜𝑝 ∗ 𝐼 = 𝑉𝑜𝑝 ∗ 𝑖 ∗ 𝐴𝑐𝑒𝑙𝑙 (13) Where I is the total current, 𝑖 is the current density (Acm-2), and Acell the total area of the cell. The current density depends on the operating conditions, namely the electrode potential and the concentration of reactant and product, by the Butler-Volmer equation. Closely related to the gas composition, temperature, electrode materials and the effective triple phase boundary area (TPB – where the reaction occurs on the electrode), the current density is a critical performance indicator. Note that there are three modes of operation for the electrolysis: thermoneutral, exothermic and endothermic (Wendel et al., 2015). A thermoneutral voltage implies that the process is adiabatic, and no heat is exchanged with the environment. In other words, the electrical energy input equals enthalpy of

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reaction. The entropy necessary to split the water is equal to the heat generated by the loss reactions in the cell and the electrical efficiency is 100 %. The operating voltage in this mode depends on the heating value of the reactant (∆𝐻) divided by the number of Faradays of charge needed to completely consume the reactant (nF) (Milobar et al., 2015):

𝑉𝑡𝑛 = ∆𝐻𝑛𝐹 (14)

In the exothermic mode, the input electricity exceeds the enthalpy of reaction (efficiency <100 %). The endothermic mode is the opposite: cell voltage is below the thermoneutral one, thanks to the supplied heat. The efficiency is then higher than 100 %. The operating mode largely depends on the availability of external heat and electricity. It is often desirable to operate around thermoneutral efficiency, however, to avoid costs for supplying more electricity (in exothermic mode) or avoiding thermal stress on the cell by cycling the temperature (Graves et al., 2010). The final energy equation for the co-electrolysis is expressed by (Stoots, 2009):

�� − �� = ∑ ��𝑖𝑃 [Δ𝐻𝑓𝑖0 + 𝐻𝑖(𝑇𝑃) − 𝐻𝑖

0] − ∑ ��𝑅 [Δ𝐻𝑓𝑖0 + 𝐻𝑖(𝑇𝑅) − 𝐻𝑖

0] (15) Where:

• �� is the external heat transfer rate to the electrolyser • �� is the supplied electrical energy to the electrolyser • ��𝑖 is the molar flow rate of each reactant or product • Δ𝐻𝑓𝑖

0 is the standard-state molar enthalpy of each reactant or product • 𝐻𝑖(𝑇𝑃) − 𝐻𝑖

0 is the sensible enthalpy for each reactant or product. Note that a 100 % conversion of CO2 into CO does not occur. Rather, the electrochemical conversion rate of the CO2 varies depending on the various operating conditions, especially after the current density. Generally, a conversion rate of about 60 % is found (Sun et al., 2013). During operation, there is a concurring reverse water gas shift reaction (rWGS) contributing to the formation of CO:

𝐶𝑂2 + 𝐻2 ↔ 𝐶𝑂 + 𝐻20 (16) Despite being a reaction of major importance in the overall conversion of CO2, the exact contribution of the rWGS has not been clearly identified, and has brought much complexity in the co-electrolysis. By comparing the ASR of different modes (H2O electrolysis, CO2 electrolysis and co-electrolysis), studies have found that almost no CO2 is converted to CO by CO2 electrolysis. The ASR of co-electrolysis was found to be very close to the ASR of the H2O electrolysis: 0.70 Ωcm-2 to 0.79 Ωcm-2 for co-electrolysis, close to 0.75 Ωcm-2 exhibited by H2O electrolysis, and much lower than the 1.06 Ωcm-2 for CO2 electrolysis (Chen et al., 2015). This confirms the initial findings by Stoots et al. (2009), in earlier experiments: 1.38 Ωcm-2 for co-electrolysis, 1.36 Ωcm-2 for H2O electrolysis and 3.84 Ωcm-2 for CO2 electrolysis. Since the ASR essentially represents the performance of the electrolytic cell, this explains the accrued interest for co-electrolysis above CO2-electrolysis, despite not yet being fully understood.

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At low temperature (below 700°C) or high pressure operation, methane can be formed through the following equation (Jensen et al, 2003):

𝐶𝑂 + 3𝐻2 ↔ 𝐶𝐻4 + 𝐻20 (17) Even if the syngas is meant to be further processed into methane through a subsequent Fischer-Tropsch reaction, this first methanation reaction is not desirable as it consumes H2 and CO, thus decreasing the syngas concentration (Graves et al., 2010). It is even possible that some CO is reconverted into CO2 through the water gas shift reaction, blurring the actual amount of CO produced by the process (Alenazey et al., 2015). Many different types of gas compositions have been tested, and no single optimum has been found. Several parameters play a role in finding an optimal one (operating temperature, material used, desired outlet-gas composition, etc.). Chen et al. (2015), found a maximum utilisation of H2O and CO2 of 77 % and 76 % respectively at 1.0Acm-2 with an inlet gas composition of H2/H2O/CO2 of 20/40/40. A large part of the heat from the product gas stream can be recovered to heat the feed stream. In a proposed SOEC plant by Doenitz et al. (1980), significant heat could be recovered from the product gas to the feedstock water through a heat exchanger. While their energy exergy analysis showed that more than half of the exhaust heat from the system could be recovered, superheating would be necessary to reach the SOEC’s needed temperature. However, this value can vary with the complexity of the system and the performance of the heat exchangers used, as well as the sealing of the SOEC and the overall plant design (Zheng et al., 2017).

3.5.2 Material and system configuration The SOEC operates in very harsh conditions (high temperatures, reduction/oxidation environments), the choice of material is thus paramount for a high-performance SOEC. The materials used should fulfil a complex list of requirements, detailed in the following section. It is important to remember some fundamentals of the process (Zheng et al., 2017):

• The reduction of H2O (and CO2 in the case of the co-electrolysis) occur at the fuel electrode, while the oxidation (also called oxygen evolution reaction) occurs in the oxygen electrode. Thus, the electrodes must provide a pathway for the electrons and the produced ions (good electronic and ionic conductivity).

• The electrolyte allows for the passage of oxide ions between the fuel cell and the oxygen cell. This ionic conductivity mainly determines the cell resistance. However, no electrons should be able to pass, as this would bypass the redox reactions.

• As explained in more detail in the section Cell Configuration, the SOECs are modular, and can be stacked. The quality and stability of the sealing and the material connecting the cells should not be overlooked.

3.5.2.1 Electrolyte

For the electrolyte, a solid, ceramic ion conducting electrolyte in high temperatures is commonly used. Generally, a Zirconia-based material is used, for its cost advantage over noble materials (Singhal et al, 2003). However, due to the many constraints on the electrolyte, no single optimal material has been found. Rather, each exhibits some advantages and disadvantages regarding certain requirements. A few important characteristics are (Zheng et al., 2017):

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• Good ionic conductivity and poor electronic conduction, to allow for the passage of O2- and avoid any electron directly going from the fuel electrode to the oxygen electrode.

• The material should be as thin as possible, to reduce ohmic resistance. • Gas tightness, to totally separate the synthetic gas and O2 to prevent any leakage. • Compatible thermal expansion with electrodes, to avoid mechanical failures. This is also true

for the interconnect material and sealing (see section Cell Configuration). • Chemical stability in highly unfavourable environment (reduction, oxidation).

To obtain these properties, Zirconia is doped with different oxides (Y2O3, CaO, MgO and Sc2O3, Yb2O3). The ionic conductivity of the electrolyte depends on its concentration of dopants. According to Arashi et al. (1991), an optimal dopant concentration for oxygen ions is 8 mol% Y2O3, called Yttria-based Zirconia. There are several ways to fabricate the electrolyte, amongst others tape casting, dry pressing, plasma spraying and chemical vapour deposition (Zheng et al., 2017).

3.5.2.2 Electrodes

The microstructure and material of the two electrodes are key aspects to consider, and need to be constantly optimised for better cell performance (Zheng et al, 2017). Besides their compatibility with the electrolyte, general functionalities looked upon are good ionic and electronic conduction, as well as a good catalytic activity (Irvine et al., 2006). The main objective is to increase the reactive area of the triple phase boundary, while allowing for passage of ions for easy recollection of the by-product. Metals, fluorite and perovskites are materials with interesting such abilities, while porous microstructures such as crystals are favoured.

3.5.2.2.1 Fuel Electrode

To fulfil the requirements above, electronic conducting Nickel (Ni) doped with YSZ, for the conduction of ions, is the most common material for the cathode, despite its high degree of degradation (Hauch, 2008). It is relatively cheap and has high electrochemical reactivity. Noble, costly metals, such as platinum (Pt), are not necessary. Moreover, they cause the formation of volatile oxides and aging of porous structures at high temperatures (Doenitz, 1980). While good performance was recorded in the reverse mode, Solid Oxide Fuel Cell, (SOFC), this material suffers from high overpotentials in SOEC mode (Eguchi et al., 1996). Ni particles were oxidised into NiO or NiOH, resulting in a less active layer, thus a low activity (Utz et al., 2011). To prevent this, hydrogen is needed, though it can result in incongruousness. Reduction and oxidation of Ni particles lead to volume changes in the Ni-YSZ electrode. As mentioned earlier, an important material selection criteria for the electrode is its volume compatibility with the electrolyte. Thus, the redox essentially cause mechanical instability, leading to lower electrode activity. Impurities such as SiO2, even at a very low level, causes deposits on the fuel electrode, significantly affecting the performance of the SOEC, in addition to the carbon deposition from the CO2. As stated earlier, the carbon deposition can be largely avoided by the supply of steam, which is why co-electrolysis can be preferred over CO2-electrolysis (Zheng et al., 2017). Microstructures are degraded under high steam partial pressure. Note that this issue is true for SOEC operation, and not in SOFC mode (Hauch et al., 2008). Microstructures at the TPB can also change due

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to the relocation of Ni. The deposition of a dense layer of Ni on the electrolyte results from the following reaction:

𝑁𝑖(𝑂𝐻)2(𝑔) + 4𝑒− → 𝑁𝑖(𝑠) + 2𝑂2− + 𝐻2(𝑔) (18) This could block the passage of oxygen from the cathode to the electrolyte (Zheng, 2017).

3.5.2.2.2 Oxygen electrode

The oxygen electrode will operate in a highly oxidizing environment. Strontium doped Lanthanum Manganite (LSM) is widely used, for its good electronic conductivity, compatible thermal expansion coefficient and low chemical reactivity with YSZ. Due to its poor ionic conductivity, it is enhanced to a composite, with YSZ. LSF, LSCuF and LSCoF are other candidates, more catalytically active, thus more performant. The choice consists in finding the right trade-off between material price and availability, durability and efficiency. In terms of microstructures, perovskite crystals are also interesting for their high ionic and electronic conductivity (Zheng et al., 2017). The oxygen electrode is generally the main source of stack degradation/failure (Zheng et al., 2017). The triple phase boundary is weakened by the oxygen evolution, potentially resulting in delamination. Jacobsen, Mogensen and Virkar confirm that high oxygen pressures in this electrode is responsible for this (Virkar, 2010; Jacobsen, 2008). The precise reason of the delamination has not been found, and highly depends, again, on the operating conditions. Several studies point to mechanical stress, and cracking of the electrode/electrolyte interface. Chen et al. (2016) attributed it to the formation of nanoparticles on the electrolyte/electrode interface and the migration of oxygen ions from the electrolyte to the LSM grains. The result is that LSM lattice shrinks, increasing local tension and causing cracks at the interface. Mawdsley et al. (2009) attributed the cracking to localised pressure caused by the mismatch between different ion conductions of the electrode/electrolyte. Finally, poisoning also affects the oxygen electrodes, notably through the presence of boron, sulphur and chromium, causing deposits on the electrode, reducing the reaction surface (Zheng et al., 2017). The fabrication of the electrolytic cell is also delicate. For example, over-sintering the electrode could also close the porosity, blocking the passage of oxygen away from the TPB, resulting in local pressure.

3.5.2.3 Cell configuration

Cells can be configured either in planar or in tubular form (Ebbesen et al., 2012). Since the cells are modular, the production rate can be increased by stacking the cells, allowing for a higher active area for the electrolysis (Carmo et al., 2013). The advantages of a planar configuration are the high volumetric gas production rate and the ease of manufacturing and transporting them. Moreover, this facilitates the collection of outlet gases. As shown in Figure 11, the stacks have other components, each important in cell lifetime and performance (e.g. interconnects, seals, flow channels, end plates). Tubular stacks have higher mechanical and thermal stabilities, as well as easier sealing. However, as opposed to the planar configuration, they are inconvenient for manufacturing and current collecting, and

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have a low gas volumetric production rate (Zheng et al., 2017). Thus, a combination of both (flat tubular) aims at reaping the benefits from the planar configuration (stack piling for increased dimensioning), and from the tubular (long-term stabilities for increased durability).

Figure 11: Cell configuration (Zheng et al., 2017)

3.5.3 Recent progress Research is highly focused on testing different materials to increase ionic and electronic conductivities while reducing the cell degradation. Additionally, as performance is highly dependent on operational factors, more experience is needed with varying operating temperatures and voltages, inlet gas compositions, gas flow rates, etcetera. Some new findings and examples of research directions are explained below. CO2 conversion rate is one area of major importance for improving cell performance. Chen et al. (2015) have reported maximum utilisation rates of CO2 and H2O of 76 % and 77 % respectively, at 1.0 Acm-2 and inlet gas compositions of H2/H2O/CO2 at 20/40/40. H2O and CO2 conversion of 100 % was even found by a recent study by Luyi Song et al. (2017), with ideal H2/CO molar ratio of 2 at 800°C and 1.0 Acm-2 using micro-tubular solid oxide of 15 cm length. They also found that a constant molar ratio of H2 to CO of 2 could be achieved at 817.5°C, and that reducing the inlet gas flow rate would increase the conversion rate of inlet gases – although decreasing the syngas production rate. Recent research from DTU reports significant improvements in the cell durability and cell performance. Long term degradation of a single cell SOEC was decreased from ca. 40 %/1000h in 2005 to 0.4 %/1000h in 2015. Over the same period, initial ASR was brought down to 0.15 Ωcm2, from 0.44 Ωcm2 (at a current density of -0.5 A/cm2, a temperature of 750°C and gas flows H2O/H2 of 50/50. This study focused on the removal of impurities, material enhancement in the anode and microstructure modifications in the cathode. Another study by DTU found that operational temperature could be carefully varied as a tool to regain stack performance, suggesting the feasibility of long-term SOEC operation (Chen et al., 2016). In terms of material, proton-conducting electrolytes have attracted some attention, as it can effectively split water in hydrogen and oxygen. This includes cerate-, ziconate- and titanate-based perovskite materials, although cerate-based are preferred for their high protonic conductivity and stability. However, due to the lack of protons in CO2 splitting, proton-conducting electrolytes are not well fitted for co-electrolysis. (Zheng et al., 2017).

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Experiences from SOFC can help finding suitable solutions for the SOEC mode of operation. For instance, tests of carbon resistant anodes in SOFC mode fuelled with hydrocarbon fuels conducted by Mocoteguy and Brisse (2013), can contribute to the development of CO2 electrolysis. New approaches to further improve the electricity-to-syngas efficiency include a fuel-assisted co-electrolysis (SOFEC), whereby fuel is injected and oxidised in the anode. This improves the performance of the SOEC, by reducing the Nernst Potential (Shi et al., 2015). In other words, with similar overpotentials than conventional SOEC, the injection of cheaper fuels can reduce the electrical energy need. Further, Wang et al. (2014) have found the methane oxidation reaction to be exothermic, contributing to the energy demand of the co-electrolysis process. This was supported by Sun et al. in November 2016, who reported on a promising solid oxide cell using a bifunctional electrolysis cell configuration. Assisted by natural gas, the cell would effectively consume CO2 in the cathode, while producing syngas on the anode side. This could be done by combining the steam methane reforming (MSR) and the partial oxidation of methane (POM), much faster and cost-effective processes, in a single step. As such, the ratio H2/CO produced syngas could be adjustable.

3.5.4 Costs An accurate cost assessment of syngas production by co-electrolysis is not available since the technology is still in development, and there is no standardised production method. Electricity represents the major expense, which explains the need for compensating it with thermal power. Mougin (2015) compared the levelised costs of hydrogen production between SOEC, PEM and Alkaline electrolyses, and found a price of 11.2k€/Nm3/h for SOEC, 11k€/Nm3/h for PEM and 6.5k€/Nm3/h for alkaline. However, the cost of SOEC is expected to fall to 5k€/Nm3/h with the technological progress (in durability, performance, stack design). Mogensen et al. (2012) estimated the cost per surface area at 5140€/m2. Fu et al. (2010) estimate it to be in the range of 500-5000 €/m2 cell area. The costs of all system components also need to be assessed (for example power converters, heat exchangers, compressors, etc.). By 2020, the Technology Data for Energy Plants predicts a 590 €/kW in capital costs, including installation and balance of plant costs, and an Operation and Maintenance (O&M) cost of 15000€/MW/yr (Energinet DK, 2012). The variable costs can be categorised in costs of feedstock (CO2, H2O, H2) and energy costs (thermal and electrical). Most of the heat can be saved in the process, and electrical energy represents the major energy costs. Redissi and Bouallou (2013) estimated the electrical energy demand to be almost 2/3 of total energy consumption (62 %). In their investment calculation, Fu et al. (2010), found a contribution of 57.3 % from electricity costs in total annual costs of the SOEC. The cost of CO2 makes up most of the feedstock price. They, use a price of 160€/tCO2 in their economic assessment of syngas production through co-electrolysis, as they considered CO2 transportation as well. With improving CO2 capturing technologies, this price is expected to drop. According to their calculation, water costs are negligible in the syngas production costs, as it is merely 1.15€/tonne. In their economic assessment, Samavati et al. (2017), estimate the impact of CO2 feedstock and electrical energy are much bigger than the fixed costs, making them almost be negligible. This strengthens the idea that co-electrolysis is particularly interesting in the case of high CO2 emitting plants or with excess electricity from renewables.

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The overall cost of production is generally expressed in cost per kg syngas produced. There are a few studies estimating final production costs, which vary case study to case. Redissi and Bouallou (2013) estimated the production costs at around 1.30€/kg syngas.

4 Analysis In the previous section, the comprehensive review of the global carbon dioxide situation, the role of steel industry and its carbon abatement initiatives, the process description of TGR-BF and the research advancements in SOEC co-electrolysis provide important tools to understand the purpose and the technical implications of the proposed system. Before going into the analysis, Table 4 summarises the key takeaways from the literature review.

Table 4: Key takeaways from literature study

Topic Key takeaway

Global carbon dioxide emissions

Increasing population and industrial development throughout the world calls for drastic measures of reducing the pace of CO2 emissions, while putting pressure on continued growth in steel production. Sustainable development initiatives should not be restricted to clean electricity generation, but also cutting the emissions from the industry, particularly steel, major contributor to global CO2 emissions.

The steel industry and TGR-BF

Steelmakers are actively testing ways to reduce their carbon footprint, notably through top gas recycling blast furnace, avoiding a complete plant modification and reducing the need of coke with the injection of the BFG, composed of H2 and CO. By enhancing the blast furnace combustion with oxygen, TGR-BF further increases the CO2 concentration of the BFG for easier subsequent sequestration (removal of inert N2).

SOEC co-electrolysis

With high operating temperatures, co-electrolysis through SOEC is an efficient CO2 conversion technology for syngas production, only emitting oxygen.

SOEC is still in research phase, and many technical challenges lie ahead (in terms of understanding the reactions, cell-durability, operating conditions optimisation, etc.), as well as many opportunities (different inlet and outlet gas compositions, stack compositions for improved CO2 conversion, etcetera).

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4.1 System description The carbon capture TGR-BF system is combined with the SOEC co-electrolysis system in Figure 12.

Figure 12: Integrated system

The critical interaction points are identified at VPSA to SOEC and SOEC to BF, described in Table 5.

Table 5: Description of integration points

Integration point Description VPSA to SOEC

Benefit

CO2-rich air leaving the VPSA is sent to a mass flow controller in the SOEC system, to calibrate the desired inlet gas composition of CO2, H2O and H2 for the co-electrolysis. Reduced costs of compression and storage/transport. The integration of the SOEC system allows for immediate on-site utilisation. The CO2 is already highly purified according to conventional TGR-BF practice.

SOEC to BF Benefit

The O2 from the SOEC is fed into the blast furnace to enhance the combustion. Allowing a much more efficient combustion (oxy-fuel combustion), the O2

must not be purchased or separated from an ASU to the same degree as without an SOEC system, providing savings for the steel plant.

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4.2 Case study The analysis of two separate, existing case studies for each technology considered was made. The steel plant Raahe Steel Works in Finland was used as reference case in the present study. Mathematical modelling of CCS has been performed by Arasto et al. (2015) on the same reference plant. Arasto et al. estimated the CO2 captured from TGR-BF with VPSA and oxyfuel BF to 1.418 Mt/yr. The relevant data from the reference steel plant is presented in Table 6.

Table 6: Reference steel plant used in case study (Arasto 2015; Goski and Smith, 2013)

Steel production 2.8 Mt/yr CO2 emissions 4 Mt/yr CO2 capture potential 1.418 Mt/yr O2 to blast furnace 0.753 Mt/yr

The CO2 captured is used as feedstock in the SOEC (Figure 12). The amount of feedstock CO2 is therefore determined by the CO2 capture potential: 1.418 Mt/yr (or 3885 t/day) for the reference steel plant. By using the same gas composition as the reference case, this feedstock of CO2 determines the amount of the other gases, H2 and H2O needed. The result in t/day is presented in Table 7 and shows that the syngas and oxygen production based on the reference gas composition is ca. 500 times larger than in the reference case, namely 2838 t/day and 3896 t/day respectively. This would require about 37 000 cells of 1x1m, with the size ratio used from Fu et al. (2010). This illustrates the importance of the stacking possibility. If we consider 5x5m stacks of 3m height, with a vertical separation of 10cm between each cell, the system would be composed of 50 stacks. Fewer stacks could be achieved given smaller steel plants and considering the progress of the SOEC technology.

Table 7: Syngas production based on reference steel plant and SOEC

Gas Ref. SOEC Case based on ref. SOEC

Unit

Input CO2 H2O H2 CO

7.665 6.703 0.084 0.549

3885 3345 42 274

t/day t/day t/day t/day

Output Syngas

O2

H2O H2 CO2 CO

7.808 0.734 0.752 0.891 4.936 7.313

3896 366.3 375.3 444.6 2463 2838

t/day t/day t/day t/day t/day t/day

Stack size

74.8

37327

m2

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4.3 Challenges The main technological challenges with TGR-BF and SOEC are summarised in Table 8 and more extensively described in the section below. The section provides a selection of challenges and is not intended to be exhaustive.

Table 8: TGR-BF, SOEC and integration challenges

Carbon capture through TGR-BF challenges

High energy and steam consumption Material and feedstock issues Low economic incentives

Co-electrolysis through SOEC challenges

CO2 conversion/activation SOEC degradation issues Instability in face of fluctuating, intermittent electrical energy

Integration challenges Gas purity requirements Gas composition requirements Scalability Life-time compatibility Plant complexity and high variation of plant infrastructure

4.3.1 Carbon capture through TGR-BF challenges TGR-BF has not yet been implemented at a large-scale and there are still a few challenges that can inhibit deployment. The most challenging aspects identified from the literature study are listed below.

4.3.1.1 High energy and steam consumption

As mentioned in the literature review, carbon capture requires significant energy and steam supply, 0.38-0.94 GJ/tCO2 captured and 108 kg/tCO2 captured respectively. TGR-BF has the clear advantage of retrofit possibility (Pérez-Fortes et al., 2016). The modification of the steel plant may, however, be challenging due to the high complexity of the plant infrastructure as well as the high energy and steam consumption, requiring space in the steel plant. Challenges related to energy supply also arise from the complexity of the plant and from the different points of treatment in the facility, requiring the ability to supply extra energy to each facility. The high energy consumption is considered a challenge due to its counter-productive effect on the overall CO2 reductions, in literature the use of carbon free electricity is therefore stressed.

4.3.1.2 Material and feedstock challenges

The temperature and gas composition of the recycled gas injected into the blast has to be evaluated since the composition has an impact on the performance, as shown by ULCOS:s three versions in section 3.4.1. An advantage of TGR-BF is the reduction potential of coke due to the injection of the recycled gas. The reduction of coal makes the process more cost competitive. Critical to the reduction of coke is the permeability and mechanical strength of the coke since it physically supports the iron burden in the BF. There is no other satisfactory reductant that can yet fulfil this last physical role and so it is not possible to replace all the coke in BFs.

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4.3.1.3 Low economic incentives

From the literature review, it appears that applying carbon capture on blast furnace is expensive compared to costs of emitting CO2. As mentioned in the literature review, the estimated CO2 capture costs for TGR-BF vary between 40-80 €/tCO2 with high uncertainties due to fluctuations in energy prices among other factors. The price per allowance in EU-ETS was 7.6 €/tCO2 in 2015. This lack of economic incentives is assumed to prohibit the commercial deployment.

4.3.2 SOEC co-electrolysis challenges The current technical maturity of co-electrolysis for CO2 utilisation does not allow for any implementation or commercialisation yet. The main challenges are the activation/conversion efficiency of CO2 and the SOEC cell durability (Zheng et al., 2017). Generally, there are two ways of tackling these challenges: either by changing operating conditions, or by altering the cell composition. Since the co-electrolysis process is not yet well-understood, research is currently focusing on developing predictive mathematical models and making verification experiments (Zheng et al., 2017). The difficulty lies in the many influencing parameters.

4.3.2.1 Activation/conversion of CO2

The high kinetic and chemical stability of the CO2 molecule is a natural hinder for its separation by electrolysis, increasing the required energy for the activation of the reaction, i.e. the conversion of CO2 into CO. Despite being a competitive CO2 conversion technology in both cost and energy efficiency (Zheng et al., 2017), SOEC can generally not convert CO2 beyond 80 % (Chen et al., 2015) – and is generally often found to be around 60 % (Wang et al., 2016, Takao et al., 2015). It is recognised that the inlet gas CO2 concentration has little impact on the conversion rate. Rather, the operation temperature, the applied cell voltage and the current density are major parameters. Some studies consider the capture and reuse of the CO2 after the electrolysis (Fu et al., 2010), or even purifying the outlet gas, increasing the cost of operation. Co-electrolysis is much more complicated to understand than steam- or dry CO2-electrolysis, due to the contribution from the water gas shift reaction, the extent of which authors are not agreeing (Kim et al., 2012). Although co-electrolysis has shown an area specific resistance (ASR) close to that of pure steam electrolysis, partly why co-electrolysis is preferred to dry CO2 electrolysis, fundamental understanding about the process is required before commercialisation.

4.3.2.2. Long-term stability of SOEC cells

The long-term stability of the SOEC refers to the mechanical and chemical stabilities of the material used during operation. The environment in the SOEC operation is aggressive, and causes a degradation on the electrodes, leading to higher overall cell resistance. This not only rapidly decreases the system performance over-time, but also limits its lifetime to a few thousand hours (Ferrero, 2016). More time is needed to evaluate the longer-term effects of the degradation rate of the SOEC stack has been reported to be between 2 and 5 %/1000h (Ferrero, 2016). Under practical conditions and for 3600 hours, literature has reported that it has been as low as 1.7 %/1000h (at -1A/cm2). But this is still twice as much as similar cells under SOFC mode (Klotz et al., 2014). It is estimated that at least 1 %/1000h should be reached for commercialisation (O’Brien et al., 2013). Giglio et al. (2015) considered that the target value for state-of-the-art technology could be brought down to 0.2 %/1000h.

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The principal sources of cell degradation reported in the literature are (Mougin, 2012): • Operating temperature • Current density • Cell polarization • Purity and type of gas

Graves et al. (2015) have demonstrated that SOEC/SOFC cyclic operations can ensure long term stability of the system. This has been confirmed by several other authors. Studies have showed, however, that the scale-up of an SOEC/SOFC kW-class stack showed a 10 times higher decay than a kW-class stack, as it magnified the thermo-mechanical issues associated with cyclic operation (Tang et al., 2012). As such, adapting the size of the cell-stack to industrial processes depends on the scalability advancements of the technology.

4.3.2.3 Instability in case of fluctuating, intermittent electrical energy

The SOEC needs to be kept at very high temperatures. If the electricity is fed from intermittent renewable sources, such as wind and solar, the SOEC would also operate intermittently. This poses problems concerning the operation of the electrolysis, due to the high ramp-up time of the process activation, due to the high thermal energy needs. The intermittency of renewable energy sources of electricity can be a challenge in the implementation of the SOEC. While Petipas et al. (2013) recently discovered that there is no experienced cell degradation or instability increase during transient operation, the SOEC cell needs to be kept at its optimal temperature to provide the flexibility needed in a power-to-gas operation (Ferrero, 2016). The cold start-up is lengthy (usually hours), and thermal cycling can further damage the SOEC as the stack assembly materials can have different thermal expansion coefficients (interconnects, seals). In conclusion, frequent start-and-stop should be avoided, whether in operation or not, complicating the integration of the SOEC with RES. An accurate optimization of operation based on the potential intermittency of the electrical energy input is necessary.

4.3.3 Integration challenges

4.3.3.1 Gas purity requirements

As discussed above, reasons for degradation of the SOEC is not yet fully described but impurities such as sulphur, chromium, boron, silicon dioxide, etc. have been proved to, even at low levels, affect the performance of the SOEC. The sensitivity of the SOEC to impurities makes it particularly challenging to directly combine to such an intensive industrial process since the BFG contains impurities, namely sulphur. The outlet gas from the blast furnace is purified from dust in a gas cleaner, catching dust particles. However, depending on the dust catching method used, the efficiency ranges between 50-80 % for different particle sizes (Lajtonyi and Corus, 2011). The TGR-BF includes purification steps, but additional purification could be necessary for the outlet gas from the VPSA before it is fed into the SOEC system.

4.3.3.2 Gas composition requirements

According to Carpenter, the outlet gas composition from the VPSA is 87.2 % CO2, 10.7 % CO, 1.6 % N2 and 0.6 % H2. The optimal inlet gas composition to the SOEC is not fully determined. The literature review reveals that the CO2 concentration used in experiments varies between 25-80 % (Kim et al., 2012;

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Chen, 2015), mixed with varying amounts of H2O and H2, In the present system, the right amount of H2O and H2 must thus be added to the CO2-rich gas to satisfy the operation requirements of the SOEC. Dependent of the amount of H2 and H2O required, an economic assessment needs to be performed to examine if the feedstock cost is reasonable.

4.3.3.3 Scalability

The case study reveals that the size of the SOEC for the present case is 37327 m2 (total area, stacked). This size of a SOEC system seems unrealistic and indicates that the scaling of the two systems might pose a problem to the integration. However, large SOEC is being developed and tested and since the SOECs are stackable, their size can be adjusted, though not yet tested on such large scale. The scalability issue stresses optional ways to utilize the CO2 such as a solution where utilisation is combined with storage. The case study is based on the biggest steel plant in the Nordic countries which is also contributing to the unrealistic size of the Ruukki SOEC system. The case plant is not necessarily representative for the typical steel plant size.

4.3.3.4 Life-time compatibility

Both technologies are at different technological development stages, so the life-time of each technology will probably greatly vary in the years to come. While the life-span of a steel-plant is quite long – 20 years or more (Manning et al., 2001) – research is needed before an accurate estimation of the potential life-time of an SOEC can be done. Most probably, the SOEC would need several replacements of the cells or other stack material (sealing, interconnects) during the life-time of the steel plant. A lifetime study by Hendriksen et al. (2015), at DTU University, estimated the lifetime of the SOEC cell to 2-3 years at a temperature of 800°C and a current density up to -1Acm2, which is below the commercialisation requirement of 5 years of stable performance As discussed earlier, long-term stability of the cell highly depends on current density: to achieve 5 years of continuous operation, the current density would need to be brought down to 0.45Acm2, which would however not satisfy the efficiency expectations (Hansen, 2015). This entails that the SOEC needs to be replaceable without a shut-down of the steel-plant. 4.3.3.5 Plant complexity and high variation of plant infrastructure

The plant complexity also poses a problem to the installation of the SOEC system. Moreover, the capture facility as well as the steam and energy network, also requires adequate space. Even though a limited number of processes are applied in iron and steelmaking worldwide, a complex industrial structure exists, incorporating a variety of process technologies with different plant layouts (Carpenter, 2012). One carbon capture technology standard for all blast furnaces is therefore not possible which complicates the implementation of TGR-BF since its success must be validated for the plant in question. This can inhibit its widespread commercial deployment. Demonstration of TGR-BF has not yet been done on large scale. The results from the ULCOS facility in Luleå validated modelling but a large scale TGR-BF should be able to operate and demonstrate a full cycle of the BF operation, it should operate at least 10 years (IEAGHG, 2013).

5 Discussion While this study intends to rely on factual elements, several other considerations need to be discussed before such an implementation of the evaluated system occurs. More specifically, several other integration benefits should be looked upon and alternative investments considered.

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Additional integration synergies. The system studied highlighted the compatibility of TGR-BF and SOEC at its most fundamental level, strictly in terms of input and output gas flows. A customised mathematical modelling for a specific steel plant is necessary to account for the realities of the actual gas flows and plant layout, but also to identify further optimization possibilities. We suggest there are more opportunities to be harnessed, namely:

- The produced syngas can be recycled within the steel plant, either directly in the blast furnace or in other processes requiring a reduction or a fuel. Moreover, there are several other conversion possibilities for the syngas (Fischer-Tropsch for methanation, for example).

- The captured CO2 could be directly injected into the SOEC to avoid costs of compression. - The high temperatures from the blast furnace and its flue gas could be recovered to pre-heat the

inlet gas composition for the SOEC. - Certain steel plants harness the heat from the blast furnace to produce electricity with turbines,

which could be used for the SOEC. - There is excess steam being produced by the shift reactor in the TGR-BF system, which could

be used as feedstock for the SOEC. - The recent new fields of development for the SOEC could be interesting applications (SOFEC,

bi-functional electrolysis cell configuration assisted by natural gas). Alternative investments. The proposed system is one of many ways of reducing carbon emissions while valorising carbon capture. Alternative investment possibilities, both for the various carbon abatement or efficiency improvement pathways, and for other utilisations of the suggested products from the SOEC (syngas and oxygen), were not studied. For decision-makers within the steel-industry, a comprehensive comparison of all investments and their associated risks would ground a rational choice. The proposed system is also subject to a less rational decision-making. Many traditional, heavy industries are under pressure from external constraints, such as environmental regulations, international competition and even popular feeling. The suggested system is not only a carbon abatement pathway, but also a diversification potential. Just as hydrogen can be considered a building block for a new future economy, syngas can also represent a development block, as Dahmén (1950) puts it. Being such a flexible intermediate product, it could allow a steel plant to harness new resources emerging from their waste in a frugal way, compared to the hydrogen paradigm shift studied by LKAB, SSAB and Vattenfall. In short, by focusing on what potential new resources a steel plant could possess instead of being product-oriented, innovative industrial transformations could occur. While this does not overweigh the importance of a rational investment calculation, such considerations are becoming paramount in underlying long-term strategic orientations, in a dynamic competitive and political environment.

6 Conclusion In this thesis, the SOEC technology was studied to assess its potential in carbon capture and utilization for a steel plant. The literature review also pointed out the important role of the steel industry in global CO2 emission mitigation efforts, due to its correlation to population and economic growths. A range of initiatives for decarbonising the steel industry were identified but focus was put on reduction of CO2 emissions from the blast furnace, namely TGR-BF, for the prevalence of BF-BOF in global steelmaking, and its possibility to be retrofitted on existing plants.

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The study showed that the SOEC in co-electrolysis, being a promising electrolysis pathway, can potentially be applied together with a TGR-BF system of an integrated steel plant. The single system would use the CO2 captured in the TGR-BF system as feedstock in the SOEC co-electrolysis for syngas production. The case study performed on Ruukki Metals steel plant in Raahe showed that 2838 tonnes of syngas could be produced from the feedstock of 1.418 Mt/yr of CO2. This CO2 feedstock was set to the potential carbon captured in Ruukki Metals, calculated by Arasto et al (2015). The mapping of the system illustrated the direct integration points between the two technologies. The mapping together with an extensive literature study helped reveal potential additional benefits and challenges, the benefits being:

- Reuse of waste heat from the TGR-BF in the SOEC - Injection of SOEC oxygen by-product in the blast furnace, for an oxy-fuel combustion. - The syngas could be reused in other processes within the steel plant.

The challenges with the system mainly arise from:

- Gas purity requirements - Gas compositions - Scalability of the SOEC - Life-time compatibility - Plant complexity and high variation of plant infrastructure

The study showed that additional challenges may include the current lack of incentives for carbon abatement, heavy investment costs, and the low understanding of the co-electrolysis process. For further research, a model of the system with recent, actual results from TGR-BF and SOEC systems is recommended, and a comparison with alternative investment possibilities will give critical information for any decision-maker.

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Appendix

Appendix 1: Number of publications on Scopus, since 2003

Keywords Publications

SOEC SOEC AND solid oxide AND cell 1079 Coelectrolysis OR co-electrolysis 706

Steel industry Steel industry AND carbon capture 239 Top gas recycling AND blast furnace 261

Integration of both technologies SOEC AND carbon capture 38 SOEC AND oxyfuel 6 SOEC AND blast furnace 3 Top gas recycling AND SOEC 0

Appendix 2: Evolution of the number of publications on Scopus, since 2005

0

50

100

150

200

250

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017

Evolution of number of publications on Scopus

(SOEC) and (solid oxide) and (cell) (Coelectrolysis) OR (co-electrolysis)

(Steel industry) and (carbon capture) (Top gas recycling) and (blast furnace)

(SOEC) and (carbon capture)