IN
DEGREE PROJECT TECHNOLOGY,
FIRST CYCLE, 15 CREDITS STOCKHOLM SWEDEN 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
JULIA SJOBERG ELF
KRISTOFER WANNHEDEN ESPINOSA
KTH ROYAL INSTITUTE OF TECHNOLOGY
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
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.
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
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
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
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.
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
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
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
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.
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).
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%
Aluminium 2%
Cement 26%
Chemicals 17%
Pulp and paper 2%
Other 23%
Iron and steel 30%
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
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.
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
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.
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:
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).
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).
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).
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
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 H
2O/CO
2Co-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):
Reduction 2𝐻++ 2𝑒−→ 𝐻2 (3)
Oxidation 2𝑂𝐻−→ 1 2⁄ 𝑂2+ 𝐻2𝑂 + 2𝑒− (4) Overall 𝐻2𝑂 → 𝐻2+ 1 2⁄ 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.
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)
