Analysis of production routes for silicon carbide using air as carbon source empowering negative emissions

Mühlbauer, Andreas; Keiner, Dominik; Galimova, Tansu; Breyer, Christian Article — Published Version Analysis of production routes for silicon carbide using air as carbon source empowering negative emissions Mitigation and Adaptation Strategies for Global Change Provided in Cooperation with: Springer Nature Suggested Citation: Mühlbauer, Andreas; Keiner, Dominik; Galimova, Tansu; Breyer, Christian (2024) : Analysis of production routes for silicon carbide using air as carbon source empowering negative emissions, Mitigation and Adaptation Strategies for Global Change, ISSN 1573-1596, Springer Netherlands, Dordrecht, Vol. 29, Iss. 1, https://doi.org/10.1007/s11027-023-10100-6 This Version is available at: https://hdl.handle.net/10419/315333 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. http://creativecommons.org/licenses/by/4.0/ Vol.:(0123456789) Mitig Adapt Strateg Glob Change (2024) 29:4 https://doi.org/10.1007/s11027-023-10100-6 1 3 ORIGINAL ARTICLE Analysis ofproduction routes forsilicon carbide using air ascarbon source empowering negative emissions AndreasMühlbauer1 · DominikKeiner2 · TansuGalimova2 · ChristianBreyer2 Received: 19 September 2022 / Accepted: 20 December 2023 / Published online: 9 January 2024 © The Author(s) 2024 Abstract A rapid defossilisation of the industry sector is required to stop further greenhouse gas emissions and to curb global warming. Additionally, to avoid irreversible consequences caused by climate change, the deployment of negative emission technologies is required to reduce the carbon dioxide (CO2) concentration in Earth’s atmosphere to a sustainable level. A novel approach to store gaseous CO2 from direct air capture facilities in solid silicon carbide (SiC) is presented. A chain of established processes to produce SiC from renewable electricity and air is evaluated in terms of energy and mass balances. Furthermore, possible fields of SiC utilisation are considered. Electricity-based SiC (e-SiC) can serve the growing global market for technical ceramics and can possibly be used to tackle increasing construction sand shortages in the construction industry by partially substituting sand. Calculations of the levelised cost of carbon dioxide removal show that storing ambient CO2 in solid SiC that can be subsequently sold on the world market can eventually create profit. In 2050, a net benefit of 259 €/tCO2 or 631 €/tSiC can be realised if the SiC product is sold at the world market with additional carbon compensation. Therefore, the proposed SiC production chain might be able to challenge conventionally produced SiC, while empowering negative emissions. In 2050, the net CO2 emission potential is limited to about 290 MtCO2/a for technical ceramics, but may reach up to 13.6 GtCO2/a for construction sand. Results show that e-SiC production is economically feasible for technical ceramics but not for construction sand without further process cost decrease. Alternative processes to produce e-SiC are described and evaluated. Future research opportunities are discussed. Keywords Negative emissions· Silicon carbide· Carbon dioxide removal· Defossilisation· Industry· Power-to-X * Dominik Keiner [email protected] 1 Faculty ofElectrical Engineering andInformation Technology, OTH Regensburg, Prüfeninger Straße 58, 93049Regensburg, Germany 2 School ofEnergy Systems, LUT University, Yliopistonkatu 34, 52850Lappeenranta, Finland Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 2 of 25 1 Introduction Mitigating the effects of global warming requires urgent action to limit the atmospheric concentration of carbon dioxide (CO2) to not exceed a global average temperature rise of 1.5°C as agreed upon in the Paris Agreement of 2015 (UNFCC 2015). To limit global warming to a level at which humanity will be able to adapt, a radical transformation to 100% renewable energy systems is needed and proven to be economically feasible (Bogdanov et al. 2021; Breyer et al. 2022b). To keep global warming at 1.5 °C or even below, future energy systems will require negative emission technologies (NETs) (Fuss etal. 2018) in addition to a rapid energy transition (Breyer etal. 2020). Proposed NETs such as direct air capture (DAC) with carbon capture and storage (DACCS) (Breyer etal. 2019; Chen and Tavoni 2013; Realmonte etal. 2019) or bioenergy with carbon capture and storage (BECCS) (Kemper 2015) capture gaseous CO2 to store it on the long term. Carbon capture and utilisation (CCU) (Mertens etal., 2023) aims to utilise captured CO2 as a renewable carbon source for fuels and chemicals (Bui etal. 2018; Galán- Martín etal. 2021; Galimova etal. 2022). While CCU can be realised with net-zero emissions (Bogdanov etal. 2021; Breyer etal. 2019), the re-emission of CO2 at the end of the products lifetime hinders the approach to be net-negative (Hepburn etal. 2019). In contrast, chemically inert and long-term carbon capture and storage (CCS) can, in combination with a defossilised energy system, enable negative emissions needed for mitigating global warming (Gabrielli etal. 2020). Currently, one possible method to store CO2 is to sequester it in its gaseous phase in sub-surficial aquifers or other geological formations. However, this method bears the risk of potential leakage over the long storage duration that is required (Aminu etal. 2017). A major leakage event could bring significant risks for both humanity and ecosystems (Vinca etal. 2018). To avoid such risks, a method was investigated to store gaseous CO2 in a solid-state product that shows a high combustion point as well as chemical inertness in order to provide options for safe and long-term storage of ambient CO2 and to empower effective negative emissions. Silicon carbide (SiC) was identified to fulfil these criteria. The material can be used for power electronic applications or as a technical ceramic (Mukasyan 2017). SiC shows a high chemical inertness that makes it attractive for utilisation in chemical industry. Also, it has a decomposition temperature of around 2830°C (Guichelaar 1996). Today, SiC is mainly produced via the Acheson process (Guichelaar 1996; Mukasyan etal. 2013), where carbon black and silicon dioxide (SiO2) are processed at elevated temperature above 1557°C to SiC (Guichelaar 1996). Whereas today, the utilised carbon black comes from fossil sources such as petroleum (Fyven 2022), or fossil methane (Boretti 2021), process routes that can substitute fossil carbon sources with ambient CO2 can be identified. Other approaches avoiding fossil resources mainly focus on biomass as carbon source for the SiC production (Chiew and Cheong 2011; Thomas etal. 2021). This study aims to present a novel production route using atmospheric CO2 as the carbon source, thus enabling negative emissions, for which the mass and energy balances, as well as cost assumptions are provided. The results are suitable for including the novel power-to-SiC (PtSiC) option in future energy system modelling and assessment of the new NET option regarding its mitigation potential within the conventional NET portfolio. Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 3 of 25 4 2 Methods anddata Value chains of chemical processes to produce SiC from atmospheric CO2 are identified to create an effective carbon sink in a material that shows outstanding resistance to wear off, as well as to chemical and thermal stress. Subsection2.1 deals with the conventional SiC production. Subsection2.2 introduces the integrated pyrolysis and Acheson (IPA) process to produce electricity-based silicon carbide (e-SiC). Subsection2.3 describes an evaluation of IPA economics to produce e-SiC. In subsection2.4, alternative processes are introduced for a possible substitution of sub-processes within the IPA value chain if the technology readiness level (TRL) increases. All energy and mass balances were either derived from theoretical chemical reaction equations or calculated from numbers given in literature. The data are normalised to 1 t of output products. In this study, electric heating for high-temperature levels and heat supply via heat pump for temperatures not higher than 100°C are assumed. Therefore, all heat required for the processes will be satisfied with electricity in the latter process chain models. The resulting energy balances for all the processes used in this study are shown in the Appendix (Table3). 2.1 Conventional silicon carbide production Currently, the majority of SiC is produced via the Acheson process (Fyven 2022; Guichelaar 1996; Mukasyan 2017). In fact, this process was first proposed by Edward Acheson as early as 1893 (Acheson 1895) and was initially meant to produce a crystal from the materials carbon and alumina (Guichelaar 1996). However, this process is still the main way to produce SiC (Fyven 2022). SiO2 is reduced with carbon to synthesise the crude SiC, with carbon monoxide (CO) as a byproduct as shown in Eq. (1) (Chiew and Cheong 2011). The input carbon black for the Acheson process is usually derived from fossil petroleum (Fyven 2022) or from fossil methane (Boretti 2021). The process is conducted at elevated temperatures around 1700–2500°C and is endothermic. The high-temperature level in the Acheson furnace is reached with electric heaters. The coke is placed together with SiO2 in the Acheson furnace unit with the heating rod placed in the centre and a plastic cover to capture produced CO (Guichelaar 1996). The theoretical energy requirement per t of SiC produced is 5.74 MWh (Guichelaar 1996). A realistic assumption is that the production of 1 t of SiC requires 6.5 MWh of energy (Guichelaar 1996). This energy is needed in the form of electricity to heat the arc furnace with an electric heating system (Guichelaar 1996). Usually, the feedstock utilised shows a SiO2/C mass ratio of 1.7 (Guichelaar 1996). This is in line with the stoichiometric mass balance calculated from Eq. (1). About 22.5 wt% of carbon and SiO2 input mass are reacted to SiC in a single Acheson furnace run (Guichelaar 1996). (1) SiO2(s)+3C(s)+618.5 kJmol − 1 → SiC(s)+2CO(g) Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 4 of 25 2.2 Electricity‑based silicon carbide production Gaseous CO2 can be captured from the atmosphere via low-temperature DAC (Fasihi etal. 2019). The required heat at a temperature level of 100°C for the regeneration of the solid sorbent can be supplied by a heat pump. Electricity-based methane (e-methane), the feedstock for methane pyrolysis (Parkinson etal. 2021) to produce solid carbon, can be produced in a methanation process (Thema et al. 2019). Methanation is seen as an integral process of various power-to-gas (PtG) approaches (Götz etal. 2016; Peters etal. 2019; Sterner and Specht 2021). Therefore, its techno-economic specifications can be derived thoroughly. The theoretical reaction from gaseous CO2 to methane is given in Eq. (2). The catalytic methanation reaction typically takes place in adiabatic fixed bed reactors at a pressure level of 1–100 bar and at temperatures ranging from 200 to 550°C (Götz etal. 2016). The process is exothermic. However, the practical reaction equation differs from theory. Since the conversion process is not ideal, some amount of CO2 and hydrogen will remain in the product (Götz etal. 2016). Overall, the process requires an energy input of about 423 kWhel of electricity for the methanation of CO2 to 1 t of e-methane, according to DVGW (2013). This e-methane consists of 96 v% methane, 2 v% hydrogen and 2 v% CO2. For simplification, it is assumed that for 1 t of methane, 2.86 t of CO2 is reacted with 0.51 t of hydrogen. Therefore, the carbon conversion efficiency, i.e. the share of carbon converted, is assumed to be 100% for the methanation process. The required hydrogen for the methanation unit can be partly fed from methane pyrolysis (Boretti 2021; Parkinson etal. 2017), which is described in detail below. The remaining amount of hydrogen is produced via water electrolysis. Water electrolysis is a mature and commercialised process. There are various designs proposed that require different catalysts, electrolytes and temperature levels. In the present study, an alkaline water electrolyser is used (Fasihi and Breyer 2020). Around 95% of global carbon black is produced from non-catalytic methane pyrolysis (Parkinson etal. 2019). The solid carbon powder can be used for several applications, such as rubber tire production, utilisation as catalyst or as structural material (Pérez etal. 2021). To achieve negative emissions and to produce carbon black for the SiC production, no fossil methane must be used. Methane pyrolysis attracts attention mainly because of the possibility to produce fossil methane-based hydrogen (H2). However, some studies also acknowledge the idea of selling the side product carbon to reduce overall hydrogen production costs (Parkinson etal. 2019). The endothermic pyrolysis reaction to produce solid carbon and gaseous hydrogen from gaseous methane is given in Eq. (3) (Boretti 2021; Parkinson etal. 2018). The pyrolysis can employ various catalysts such as carbon black itself (Boretti 2021). The pyrolysis reaction, according to the reaction equation, theoretically requires around 1285 kWh of heat at a temperature of 1000°C and at a pressure of 35 bar for the splitting of 1 t of methane (Parkinson etal. 2017). One t of carbon black and 0.3 t of hydrogen are pyrolysed from 1.3 t of methane (Boretti 2021). As described by Sánchez-Bastardo etal. (2020), unreacted methane can be looped back to the input methane. Therefore, no carbon losses and subsequently a carbon conversion efficiency of 100% are assumed for methane pyrolysis. (2) CO2 +4H 2 →CH 4 +2H 2 O+165 kJmol −1 (3) CH4 + 74 kJmol−1 → 2H2 + C Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 5 of 25 4 The process utilised to produce SiC from carbon black remains the Acheson process, since this is the only process identified at a high TRL and wide commercialisation. The conversion efficiency within an Acheson furnace mentioned in subsection2.1 does not affect the modelling of the production of e-SiC. Since all unreacted input material is recycled and used for another run of the furnace (Guichelaar 1996), the mass balance for modelling the production of e-SiC does not have to be adjusted regarding the conversion efficiency. However, unreacted by-products increase the throughput and therefore energy demand of the intermediate processes methanation and methane pyrolysis (cf. Fig.1). As shown by Sun etal. (2019), the carbon purity for SiC production is of lesser relevance, as SiC can be synthesised from low-grade educts. The Acheson process produces CO as a by-product, which is a synthesis gas of the methanation and can be fed back to the respective process step. However, since the molar masses of CO2 and CO differ, the CO fed back to the methanation reduced the net CO2 demand and the methanation process is modelled in a simplified way, the CO must be converted to a CO2 equivalent by applying Eq. (4). wherein mCO,CO2eq represents the mass of CO when accounted for as CO2, mCO is the mass of CO, MCO is the molar mass of CO of 28 g/mol, and MCO2 is the molar mass of CO2 of 44 g/mol. (4) m CO,CO2eq =mCO ⋅ M CO MCO2 Fig. 1 Simplified schematic visualisation of the integrated e-SiC production route Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 6 of 25 2.3 Economics ofelectricity‑based silicon carbide There were no sufficiently reliable numbers available in the literature considering cost of processes for combustion synthesis from elements or by SiO2 reduction (Mukasyan 2017) as well as for CO2 electrolysis in molten lithium carbonate (Laasonen etal. 2022). Therefore, these processes were not evaluated in terms of costs. Only e-SiC production via methane pyrolysis could be fully evaluated regarding economic aspects: The schematic model of the e-SiC production chain based on air, water and electricity is shown in Fig.1. The levelised cost of carbon dioxide removal (LCOCDR) of this process chain is calculated according to Eq. (5): wherein LCOPp is the levelised cost of each process p, mp is the mass output produced from each process required to store 1 t of CO2 in solid SiC, HLT is the heat demand on a low-temperature level (max. 100°C), LCOHLT is the levelised cost of low-temperature heat, Eel is the electricity demand, HHT is the high-temperature heat demand which is covered via direct electric heating, and finally, costel describes the cost of electricity. The LCOP of specific processes is calculated applying Eq. (6): wherein capex are the capital expenditures, opexfix are the fixed operational expenditures, opexvar are the variable operational expenditures, and crf is the capital recovery factor. Process output outp is defined in Eq. (7) including the annual capacity and availability factor τ that is set to 95% in this work. The crf is defined as in Eq. (8). The weighted average cost of capital WACC is assumed to 7% as a global average for all years. LCOP for transformers with a given capex based on installed capacity or energy unit output are calculated with Eq. (9). The LCOHLT is calculated using Eq. (10). The COP describes the coefficient of performance for the heat pump. (5) LCOCDR = proc ∑ p (LCOPp⋅mp)+HLT ⋅LCOHLT +(Eel +HHT)⋅cost el (6) LCOP = ( capex ⋅ ( crf + opexfix )) ⋅capacity out p + opex var (7) outp= capacity ⋅ τ (8) crf =WACC ⋅(1+WACC) N (1+WACC) N −1 (9) LCOP = capex ⋅ ( crf +opexfix ) FLH ⋅𝜏 +opex var (10) LCOH LT = capex ⋅ ( crf +opexfix ) FLH ⋅𝜏 +opexvar +cost el COP Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 7 of 25 4 Table 1 Available economic input data for all processes used for modelling costs of carbon removed and SiC produced for 2030, 2040 and 2050 Process Parameter Unit 2030 Value 2040 2050 Reference Acheson1Production cost €/tSiC 195 195 195 (Guichelaar 1996) Methane pyrolysis Capex €/(tCB∙a) 921.8 921.8 921.8 (Parkinson etal. 2018) Opexfix % of capex 5 5 5 Lifetime Years 30 30 30 Capacity tCB/a 272,000 272,000 272,000 Methanation (incl. CO2 compressor) Oapex €/kWCH4,HHV 143 96 96 based on (Fasihi etal. 2017) Opexfix % of capex 4 4 4 Opexvar €/kWhCH4,HHV 0.0023 0.0023 0.0023 Lifetime Years 30 30 30 Efficiency % 78 78 78 Alkaline electrolyser Capex €/kWH2,HHV 417 241 181 (Fasihi and Breyer 2020) Capex €/kWel 547 304 221 Opexfix % of capex 3.5 3.5 3.5 Opexvar €/kWhH2,HHV 0.0012 0.0012 0.0012 Lifetime years 30 30 30 Efficiency, HHV % 76.2 79.2 82.1 Storage cost €/MWH2,LHV 555 DAC Capex €/(tCO2∙a) 338 237 199 (Fasihi etal. 2019) Opexfix % of capex 4 4 4 Lifetime Years 25 30 30 Capacity tCO2/a 360,000 360,000 360,000 Heat pump Capex €/kWth 590 554 530 (Fasihi etal. 2019) Opexfix % of capex 2 2 2 Opexvar €/kWhth 0.00170 0.00163 0.00160 Lifetime Years 25 25 25 COP - 3.26 3.41 3.51 Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 8 of 25 Table 1 (continued) Process Parameter Unit 2030 Value 2040 2050 Reference Electricity Cost €/MWhel 20 15 10 (Fasihi etal. 2021) SiC sales Income €/tSiC 833.3 833.3 833.3 (Businesswire 2022) Construction sand sales2Income €/tSiC 15.8 32.6 67.2 (Statista 2022) CO2 pricing Compensation €/tCO2135 220 220 based on (IEA 2021) 1 Adapted from Guichelaar with an assumed electricity price of 20 €/MWh (Guichelaar 1996) 2 Price was estimated using the forecasted growth rate (Wrede 2019) Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 15 of 25 4 does not seem economically viable unless significant cost reductions in production processes or a significant increase in construction sand price occur in the future. A possible negative CO2 emission potential in construction sand used for concrete production of 1.3 GtCO2/a in 2030, 5.3 GtCO2/a in 2040 and 13.6 GtCO2/a in 2050 can be estimated at given market values and substitution rates. Further evaluation of these assumptions will be necessary to reduce the uncertainties related to the assumptions made. Since the proposed production chain might be considered NET option from a climate change mitigation perspective and from a SiC material perspective, the results are also presented as LCOSiC considering SiC sales and CO2 pricing, as shown in Fig.5. The CO2 pricing assumed for 2030, 2040 and 2050 was normalised to 1 t of e-SiC produced and results in a deductible CO2 pricing equivalent of 329 €/tSiC in 2030, 536 €/tSiC in 2040 and 536 €/tSiC in 2050. The possible economic benefit per t of e-SiC produced from atmospheric CO2 if current SiC sales prices and future CO2 pricing is applied is 37 €/tSiC in 2030, 471 €/tSiC in 2040 and 631 €/tSiC in 2050. If produced SiC is used to substitute construction sand and sold for the respective sales price and Fig. 4 LCOCDR of e-SiC production with energy cost including SiC sales (left) and to substitute construction sand (right) and CO2 pricing for 2030, 2040 and 2050. Abbreviation: CS, construction sand Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 16 of 25 future CO2 pricing is considered, the LCOSiC is 781 €/tSiC in 2030, 330 €/tSiC in 2040 and 135 €/tSiC in 2050. 3.3 Discussion andresearch outlook As described above, alternative processes, especially for the substitution of the costly Acheson process, increasingly attract attention. Additionally, process substitution would solve the problem of the low molar carbon efficiency of the Acheson process, since no CO is produced during combustion synthesis. Improved carbon efficiency would increase the net throughput of carbon from atmospheric CO2 to e-SiC. However, the combustion synthesis would also bring some drawbacks, especially regarding input materials. The combustion synthesis using elementary carbon and silicon requires pure silicon that needs refining. The other possible alternative to the Acheson process is combustion synthesis from carbon, SiO2 and pure magnesium (Aminu etal. 2017). Although the future supply of sustainable biomass will be limited (Creutzig etal. 2015) and bio-based energy is desired by various sectors (Reid etal. 2020), biomethane Fig. 5 LCOSiC for e-SiC production with energy cost including SiC sales (left) and to substitute construction sand (right) and CO2 pricing for 2030, 2040 and 2050 Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 17 of 25 4 and biochar are potential alternative carbon sources to e-methane and carbon black from integrated pyrolysis, respectively. Biomethane can be obtained at cost as low as 464 €/t (33.7 €/MWhth,LHV for an energy density of e-methane of 13.75 kWhth,LHV/kg) (Bose etal. 2022). The static techno-economic framework used in this work results in cost for e-methane production including renewable energy of 557.5 €/t (40.5 €/MWhth,LHV), 401.5 €/t (29.2 €/MWhth,LHV) and 300.9 €/t (21.9 €/MWhth,LHV) in 2030, 2040 and 2050, respectively. Therefore, using biomethane could reduce the cost of CO2 storage in e-SiC by about 9.9% in 2030 but would increase the cost of CO2 storage by 8.3% and 26.4% in 2040 and 2050, respectively. Similarly, biochar can potentially substitute the carbon black produced in the integrated pyrolysis. The production cost of biochar is generally estimated to be on the order of 1000 €/t, and one study specifies the cost of biochar from date palm biomass as 883 €/t (assuming a long-term exchange rate of 1.2 USD/€) (Shahen etal. 2022). While the cost of biochar is subjected to uncertainty in, e.g. the cost of biomass feedstock, the cost are comparable to the cost of carbon black produced of about 892.0 €/t in 2030 as derived in this study. The potential reduction of the total cost of CO2 storage is therefore negligible at about 0.7% in 2030, while the cost would increase by 20.7% and 42.6% in 2040 and 2050, respectively, if the cost of biochar is assumed to remain constant. There is significant uncertainty in the estimation of future cost of biomass-based products. Therefore, future developments in this domain should be monitored to assess the potential use of biomass-based products for e-SiC production. Furthermore, CO2 electrolysis might be an interesting option for future carbon black production. The stoichiometric reaction equation implies an energy demand of 2494 kWhel/ tCO2 split into solid carbon and gaseous CO2. Licht etal. (2019) claim that an energy demand of 2 MWh is required to process one t of gaseous CO2 into solid carbon (Licht etal. 2019). However, no specific cost numbers regarding CO2 electrolysis to solid carbon in molten lithium salt are provided, which implies a relatively low TRL. However, the potential of this technology should be emphasised. For e-SiC production, CO2 electrolysis could possibly substitute water electrolysis, methanation and methane pyrolysis. Therefore, it would concur with an electricity demand of 7414 kWhel as for 2030. As mentioned above, CO2 electrolysis might require about 2 MWh per t of CO2 split. Also, CO2 electrolysis cost would concur with 110 €/tCO2 for production cost via water electrolysis, methanation and methane pyrolysis excluding energy cost. Therefore, if large-scale CO2 electrolysis in molten lithium carbonate will be technically feasible in the future, the process will be very interesting to include in the process chain producing e-SiC from atmospheric CO2. Also, water electrolysis will become increasingly more established as hydrogen will play a key role for hydrogen-to-X processes in the defossilisation of hard-to-abate energy sectors, as an integral part of the arising Power-to-X Economy (Breyer etal. 2022a). For an electricity-based production of hydrogen powered by 100% renewable electricity, only the electrolysis efficiency limits the overall process efficiency, as one advantage of renewable energy sources is a 100% conversion efficiency from primary energy to electricity (Kraan etal. 2019, Keiner etal. 2023). However, a massive rollout of solar PV and electrolysers might help to increase these efficiencies further by accelerated research and development activities. Also, novel approaches such as direct air electrolysis proposed and studied by Guo etal. (2022) discuss the decreasing cost and energy demand of hydrogen production with theoretic solar-to-hydrogen efficiencies of up to 32%, while highest realised efficiencies are around 20% (Wang 2021), compared to about 15% for the separated solar PV plus electrolyser route. Direct air electrolysis could also overcome the necessity of freshwater supply of conventional electrolysers by utilising the air’s humidity and, therefore, enable Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 18 of 25 hydrogen production in arid and semi-arid locations with best solar PV potentials (Guo 2022). Even though practical deployment must show feasibility, SiC production via IPA is a sequence of established standard processes. Therefore, storing atmospheric CO2 in solid SiC can be a valuable option for safe and long-term storage. However, negative CO2 emissions in Gt-scale remain unlikely to be realised in the form of e-SiC with the presented cost of the process chain hardly being profitable by 2050. Nevertheless, the amount of CO2 that is required to fulfil the global demand for SiC via these production routes might be an interesting approach for the future. Therefore, a potential market of up to 3.3 MtSiC in 2027 (Businesswire 2022) could enable a flux carbon removal potential of up to 8.1 MtCO2/a. With an expected compound annual growth rate of 16.8% p.a. (Businesswire 2022) for the years to come, a possible carbon removal potential of up to 289.4 MtCO2/a could be enabled in 2050. However, considering the second option of construction sand substitution with 50% e-SiC, a negative emission potential of 13.6 GtCO2/a can be enabled by 2050. While the CDR potential for SiC in its conventional application areas is lower than the CDR potential of afforestation and reforestation, BECCS, biochar, enhanced weathering, DACCS, ocean fertilisation and soil carbon sequestration at 0.5–5 GtCO2/a (Fuss etal. 2018), widening the application area of e-SiC to the substitution of construction sand could cover about 65% of the total 21 GtCO2/a of carbon removal requirement estimated for mid-century by Fuss etal. (2018). Since the conventional SiC production is energy and cost-intensive, the phase-in of the production routes presented in this study to replace fossil sources for carbon black production seems feasible. This can potentially lower production cost and therefore may result in faster growing SiC markets. e-SiC furthermore offers the combination of CCS and CCU, while both concepts should normally be strictly separated (Mertens etal. 2023, Bruhn etal. 2016). A similar concept to e-SiC is the production of electricity-based carbon fibres (e-CF), with an estimated CDR potential of 0.7 GtCO2/a by 2050 (Keiner etal. 2024). Therefore, both approaches are situated within the CCUS nexus combining the storage and utilisation of captured atmospheric CO2. While dedicated CCU will most probably be necessary to defossilise hard-to-abate energy sectors in a 100% renewable energy system, it offers no long-term storage of atmospheric CO2 (Galimova etal. 2022, Mertens etal. 2023). In contrast to liquid or gaseous products of CCU approaches, SiC cannot be combusted and therefore can be seen as a permanent carbon sink without a carbon cycle as it is present in CCU applications. However, the proposed approach is strongly interlinked with other CCU approaches via DAC, water electrolysis, methanation and methane pyrolysis being also applied for producing different energy carriers such as e-fuels or e-hydrogen (Mertens etal. 2023, Boretti etal. 2021). These common processes can reduce cost of CCU, CCS and e-SiC production as well, by providing a common basis for technology learning. Further research and development for processes such as SiC production via combustion synthesis and Acheson process are required to enable large-scale rollout. Also, technological specifications must be made to ensure the actual viabilities of the processes. As an example, the necessary carbon purity for SiC production in the proposed process chain must be determined, even though current work showed SiC synthesis from low-grade educts (Sun etal. 2019). In addition to the global demand in SiC for various technical applications, in particular ceramics and semiconductors, other sectors could use e-SiC to replace crucial materials. In particular, the ever-increasing demand for construction sand in the civil engineering sector draws increasingly more attention. Sand from deserts is not suitable due to the round shape of the sand grain (WWF 2022). In contrast, river and coastal sand is typically very well Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 19 of 25 4 suited for concrete production. The global scarcity of construction sand brought up a phenomenon called sand robbery (WWF 2022). Because crude SiC from the Acheson process shows grain sizes large enough to cover the whole range needed (Guichelaar 1996) and the microstructure seems to be suitable and porous enough, the idea of storing e-SiC produced from atmospheric CO2 in concrete is proposed. The cost calculated in this study shows that this approach is not economically viable from today’s perspective, but close, as a CO2 pricing of 300 €/tCO2 would be sufficient in 2050. However, the implementation of alternative processes such as combustion synthesis to produce SiC or CO2 electrolysis to split gaseous CO2 and produce solid carbon might give this approach the required boost. This research aims to advance research and discussion on how new energy-industry-CDR systems and integrated modelling of the latter can promote climate change mitigation (Breyer etal. 2022b). The large-scale deployment of NETs required for future CDR applications like DACCS or BECCS still brings drawbacks regarding the safe and long-term sequestration of captured CO2. The production of e-SiC from air and renewable electricity might be one possible solution to this challenge. 4 Conclusion Defossilisation of the industry sector plays a crucial role in mitigating global warming. In this study, a process chain to produce solid SiC from gaseous CO2 captured from the atmosphere to empower negative CO2 emissions with safe long-term storage was presented. The integrated pyrolysis and Acheson process are a value chain consisting of established processes linked to store atmospheric CO2 in solid SiC. A total of 10.8 MWhel in 2030, 10.2 MWhel in 2040 and 9.9 MWhel in 2050 is required to store 1 t of atmospheric CO2 in solid SiC. The LCOCDR of producing electricity-based SiC was estimated to be a net loss of 120 €/tCO2 in 2030, 27 €/tCO2 in 2040 and a net profit of 39 €/tCO2 in 2050, if the produced SiC is sold on the SiC world market, without factoring in the value of permanently sequestered CO2. Other applications for electricity-based SiC such as construction sand substitution were discussed. Since construction sand is becoming an increasingly scarce resource, the idea to utilise electricity-based SiC as a construction sand substitute was elaborated. However, the calculated net cost including the value of the output material shows this approach is not economically viable. The LCOCDR of the integrated pyrolysis and Acheson process with subsequent SiC utilisation for construction sand is 455 €/tCO2 in 2030, 355 €/tCO2 in 2040 and 275 €/tCO2 in 2050, without considering income from permanent CO2 sequestering. The production of 1 t of electricity-based SiC requires a total of 26.2 MWhel in 2030, 24.9 MWhel in 2040 and 24.2 MWhel in 2050. The LCOSiC if produced SiC that is sold at the world market is profitable with of 37 €/tSiC in 2030, 471 €/tSiC in 2040 and 631 €/ tCO2 in 2050 if income form CO2 pricing is accounted for. Future research opportunities were identified. Alternative processes that can possibly lower the energy demand as well as the overall production cost were presented and described. Also, additional future applications of electricity-based SiC were discussed. Electricity-based SiC can be considered an attractive production option that can enable safe and long-term negative CO2 emissions. Electricity-based SiC contributes to defossilising the industry sector while simultaneously acting as a long-term and safe carbon sink. Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 20 of 25 Appendix Table 3 Mass and energy balances used for IPA process modelling 1 As 2030/2040/2050 values CB carbon black Process Reference material Input material Input value Output material Output value Electricity demand LT heat demand HT heat demand HT heat generation Reference Unit - - t/tout - t/tout kWhel/tout kWhth/tout kWhth/tout kWhth/tout DAC CO2Air n/a CO21 225/203/18211500/1286/11021(Fasihi etal. 2019) Methanation incl. CO2 compressor CH4CO22.78 CH41 422.7 (Fasihi and Breyer 2020) Hydrogen 0.51 H2O 2.27 Methane pyrolysis CB CH41.33 CB 1 1713 (Parkinson etal. 2017) Hydrogen 0.33 Acheson SiC CB 0.90 SiC 1 6500 (Guichelaar 1996) SiO21.50 CO 1.40 Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 Page 21 of 25 4 Table 4 Alternative processes to the IPA process chain 1 Partly needed for direct electric heating Process Reference material Input material Input value Output material Output value Electricity demand LT heat demand HT heat demand HT heat generation Reference Unit - - t/tout - t/tout kWhel/tout kWhth/tout kWhth/tout kWhth/tout CO2 electrolysis Carbon CO23.66 Carbon 1 91341(Laasonen etal. 2022) Oxygen 2.66 Combustion synthesis— elementary Si SiC Carbon 0.30 SiC 1 1710 1204 (Mukasyan etal. 2013; Narayan etal. 1994) Silicon 0.70 Combustion synthesis— elementary Mg SiC Carbon 0.30 SiC 1 1710 865 (Mukasyan etal. 2013; Narayan etal. 1994) SiO21.50 MgO 2.01 Magnesium 1.21 Mitig Adapt Strateg Glob Change (2024) 29:4 1 3 4 Page 22 of 25 Acknowledgements The authors would like to thank Mahdi Fasihi, Gabriel Lopez and Mai ElSayed for their valuable insights and interesting discussions. The authors would like to thank Gabriel Lopez for proofreading. Author contributions AM: conceptualisation, methodology, investigation, resources, writing—original draft, writing—review and editing and visualisation; DK: conceptualisation, methodology, investigation, resources and writing—review and editing; TG: validation and writing—review and editing and CB: conceptualisation, methodology, validation, writing—review and editing and supervision Funding Open Access funding provided by LUT University (previously Lappeenranta University of Technology (LUT)). The authors gratefully acknowledge the public financing of Business Finland for the ‘P2XENABLE’ project under the number 8588/31/2019, Academy of Finland for the ‘Industrial Emissions & CDR’ project under the number 329313 and ‘DAC 2.0’ project under the number 329313, and LUT University Research Platform ‘GreenRenew’, which partly funded this research. Dominik Keiner would like to thank the Jenny and Antti Wihuri foundation for the valuable grant. Data availability Not applicable. Declarations Competing interests The authors declare no competing interests. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. 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