Carbon recovery from carbon dioxide

Abstract

A process and an apparatus for iron production comprises converting CO₂ in a CO₂-containing gas to solid carbon and oxygen in a CO₂ conversion process and using solid carbon from the carbon formation step in an ironmaking process and producing iron. Iron from the ironmaking process can be used as a feed for a steelmaking process. The CO₂ conversion process comprises (i) reacting CO₂ to CO in an electrolysis step, (ii) reacting CO to solid carbon and CO₂ in a carbon formation step and (iii) transferring CO₂-containing gas from the carbon formation step to the electrolysis step. The process effectively recycles CO and CO₂ to extinction.

Description

[0001] CARBON RECOVERY FROM CARBON DIOXIDE TECHNICAL FIELD The present invention relates to a process and apparatus for converting carbon dioxide (CO₂) into oxygen and solid carbon. The present invention also relates to an at least substantially CO₂ emission-free high temperature metal production process and apparatus involving at least some oxidation of carbon, wherein carbon dioxide generated in the process and apparatus is collected and converted back into solid carbon via gas-phase electrolysis with CO as an intermediate species. The term “solid carbon” as used herein encompasses all morphologies of solid-state carbon such as graphite, amorphous carbon and graphene-like “hard carbon” structures. The term also specifically includes carbon that is bonded to iron in the form of iron carbides. BACKGROUND Climate change is driving a fundamental re-evaluation of future options for energy-intensive industries such as power generation, steel and cement production. Renewable energy is seen as key part of this - according to a recent IEA report, installed capacity of wind and solar PV will become dominant over all other forms of power generation by about 2024 (1). The intermittent nature of wind and solar PV power generation is a problem that will need to be managed. Large-scale energy storage systems (e.g., pumped hydro, static batteries etc) will contribute significantly but are unlikely to provide anything approaching complete coverage. This means renewable energy will, at times, be produced in excess relative to demand (including satiated local energy storage). An agile, scalable technology that is able to utilise this off-peak (hence low cost) power is therefore highly attractive (at least in principle). One option under intense study (particularly in Europe) is green hydrogen production and utilisation. The concept is that off-peak green power can be used to electrolyse water, and the resulting hydrogen used for a variety of tasks including green steel production via the DRI- EAF route (2). Water electrolysis plants can (in general) ramp up or down in a few minutes and are therefore agile enough to (substantially) follow the availability of off-peak renewable energy. However, on the hydrogen consumption side, DRI plants (in particular) run at elevated temperatures and need to operate in a steady-state mode. To bridge this, large amounts of hydrogen buffer storage are needed. Hydrogen storage as a pressurised cryogenic liquid is difficult, though not impossible (e.g., LH2 storage site at Kobe Port, Japan, 3). Extremely low temperatures are needed (within about 20-40 K of absolute zero), implying significant energy losses and relatively high cost. Alternatives involving pressurised hydrogen storage in depleted gas reservoirs and salt caverns are currently preferred, although this necessarily implies a need for favourable local geological storage structures. There is little doubt that hydrogen produced via electrolysis using off-peak power will become a significant part of the future energy landscape. However, hydrogen storage issues are challenging - this will most likely limit its use to certain parts of the world where favourable geological structures are available. The concept of an electrolysis cell for CO₂ (similar in principle to an electrolysis cell for making hydrogen from water) is known. Information exists relating to the production of either formic acid or carbon monoxide gas via electrolysis of CO₂ (4-6). Whilst the production of synthetic fuels and chemicals via this route will no doubt contribute, the applicant’s view is that this does not represent a scalable solution to the overall problem. In particular, the demand for synthetic chemicals is likely to dominate how much can be produced. The synthetic fuel route is more open in terms of demand-side tonnage, but CO₂ will still ultimately be released into the atmosphere. This approach, when based on fossil carbon dioxide, is one of “re-purposing” CO₂ to use it a second time and delay its release, rather than actually dealing with the core problem. The applicant is not aware of any industrially established technology for electrolysing CO₂ directly to solid carbon and oxygen. A significant body of fundamental (lab-scale) work has shown that molten carbonates operating in the range 450-900 °C can, at least in a chemical sense, electrolyse CO₂ to oxygen and solid carbon with high faradic efficiency (7, 8). Carbon is typically deposited on a solid cathode for removal and product recovery. Various parties have been actively pursuing this for some time and filed patents in this regard (9-11). Although the molten carbonate electrolysis process is considered technically feasible, some development challenges remain. Cost analyses for green steel production using this approach suggest marginal economics, due in part to (expected) high capital intensity associated with molten carbonate electrolysis systems. It is no surprise that, for something as fundamental as CO₂ being split into O₂ plus solid carbon, there are diverse applications across many fields. Space travel is one such field, where CO₂ (from astronauts breathing) needs to be converted back into oxygen. NASA has developed a process for this (12), based on a solid oxide electrolysis cell (SOEC) and a carbon formation reactor (CFR). The concept revolves around first electrolysing CO₂ to CO at about 850 °C in the SOEC, followed by a step wherein a resulting CO-CO₂ mixture from a cathode in the SOEC reacts at about 450 °C in a second reactor, the CFR, (using, for example, the Boudouard reaction) to produce solid carbon plus CO₂. Solid carbon (typically deposited on steel plates) is removed periodically and offgas (typically CO₂ and CO) from this stage is recycled. In this case the primary product from an anode in the SOEC is oxygen, which is returned to the astronaut breathing cycle. It is noted that the method of deposition of carbon on plates in the NASA approach does not appear to be a suitable option for scaling to large tonnages. The above description and the following description is not an admission of the common general knowledge in Australia or elsewhere. SUMMARY OF THE DISCLOSURE The applicant has considered whether the NASA approach to converting CO₂ into oxygen and solid carbon can be modified and scaled up to be an industrial process and apparatus. In broad terms, the invention is a process and an apparatus for iron production that comprises converting CO₂ in a CO₂-containing gas to solid carbon and oxygen in a CO₂ conversion process and using solid carbon from the process in an ironmaking process and producing iron. Iron from the ironmaking process can be used as a feed for a steelmaking process in accordance with the invention. The CO₂ conversion process comprises (i) reacting CO₂ to CO in an electrolysis step, (ii) reacting CO to solid carbon and CO₂ in a carbon formation step and (iii) transferring CO₂-containing gas from the carbon formation step to the electrolysis step. The process effectively recycles CO and CO₂ to extinction. In broad terms, the invention provides a process for iron production wherein a reductant for use in the ironmaking process is solid carbon derived from CO₂ which has been recovered from an offgas from an ironmaking process, with the process being characterised in that: (a) CO₂ from the offgas is converted to solid carbon (for use in the iron making process) and oxygen in a CO₂ conversion process, for example using at least substantially renewable energy as a source of power; and (b) the CO₂ conversion process comprises (i) an electrolysis step, for example in a solid oxide electrolysis cell, wherein CO₂ is at least partially reacted to CO, (ii) a carbon formation step wherein CO is at least partially reacted to solid carbon and CO₂ and (iii) a recycling step wherein offgas from the carbon formation step is transferred to the electrolysis step such that, typically, both CO and CO₂ are effectively recycled to extinction. In broad terms, the invention provides a process for iron production that comprises: (a) converting CO₂ in a CO₂-containing gas to solid carbon (for use in the ironmaking process) and oxygen in a CO₂ conversion process comprising (i) reacting CO₂ to CO in an electrolysis step, (ii) reacting CO to solid carbon and CO₂ in a carbon formation step and (iii) transferring CO₂-containing gas from the carbon formation step to the electrolysis step and thereby effectively recycling CO and CO₂ to extinction; and (b) using solid carbon from the carbon formation step in an ironmaking process. In broad terms, the invention provides an apparatus for iron production wherein a reductant comprises solid carbon derived from a CO₂-containing offgas from an ironmaking process, with the apparatus being characterised by an apparatus for conversion of CO₂ into oxygen and solid carbon comprising: (a) an electrolysis cell, such as a solid oxide electrolysis cell, for converting CO₂ from a CO₂-containing offgas produced in an ironmaking process into CO, with the electrolysis cell configured to operate for example using at least substantially renewable energy as a source of power; and (b) a carbon formation reactor for converting CO to solid carbon and an offgas that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed. The invention also provides a process for steel production comprising using iron from the above-described iron production process as a feed material for the steel production process. The invention also provides an apparatus for steel production comprising the above-described iron production apparatus for producing iron and a steel production apparatus for producing steel from iron. The invention is based on the realisation that the NASA approach to converting CO₂ into oxygen and solid carbon can be modified and scaled-up to be an industrial process and apparatus. In particular, the invention is based on a realisation that a different approach is needed for the carbon formation reactor used in the NASA approach to provide an industrially scalable process and apparatus for converting CO₂ into oxygen and solid carbon. The invention is also based on a realisation that the modified and scaled-up process and apparatus for converting CO₂ into oxygen and solid carbon can be advantageously coupled with carbon-based green iron and steel production, with solid carbon from the modified and scaled-up process and apparatus being used in iron and steel production. This type of coupling may comprise (i) carbon from the modified and scaled-up process and apparatus being oxidized to CO₂ in a process for iron and steel production from iron ore, and (ii) at least most of the resulting CO₂ being captured and converted back to carbon in the modified and scaled up process and apparatus, typically using renewable energy as a source of energy for the modified and scaled up process and apparatus. This type of coupling provides an opportunity for at least substantially emission-free steel production. The different approach of the applicant is the use of a carbon formation reactor that comprises at least one of (a) a fluidized bed reactor, such as a spouted fluidised bed reactor or (b) a continuously or intermittently moving packed bed. In broad terms, the modified and scaled-up process for converting CO₂ into oxygen and solid carbon of the invention comprises: (a) converting CO₂ into CO in an electrolysis step, for example carried out in a solid oxide electrolysis cell; and (b) converting CO to solid carbon and an offgas in a carbon formation reactor that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed. In broad terms, the modified and scaled up apparatus for converting CO₂ into oxygen and solid carbon of the invention comprises: (a) an electrolysis cell, such as a solid oxide electrolysis cell, for converting CO₂ into CO; and (b) a carbon formation reactor for converting CO to solid carbon and an offgas that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed. The solid oxide electrolysis cell may use at least substantially renewable energy as a source of power for the cell. It is noted that the invention is not confined to the use of renewable energy as a source of power for the cell. The term "fluidized bed reactor” is understood herein to mean a reactor into which a fluid is passed upwardly through a solid particulate material at a velocity above that needed to lift the bed of particles (“minimum fluidization velocity”). The fluid causes solids to lift and separate, with the bed as a whole behaving as though it was a dense fluid (containing gas bubbles or otherwise). The term “spouted bed reactor” is understood herein to mean a type of reactor wherein particles rest upon one another (as packed a bed with slow bulk downward movement) and fluid up flow in most of the cross-section. In a small zone (typically 10-20% of the cross- section) the upward fluid velocity is much higher, such that particles are entrained from the bottom of the bed and lifted to the top, where they are deposited on the upper surface to begin their slow downward journey. The term “continuously or intermittently moving packed bed” reactor is understood herein to mean a reactor wherein solid particles rest upon one another, with fluid flow through the bed in any convenient direction. Solids may be withdrawn from a bottom of the reactor vessel via an appropriate solids flow regulation system (such as a rotating star valve) and moved to another vessel. In this second vessel some type of surface shear may be applied (e.g, a rotating drum containing significantly larger steel balls or a static vessel with a mechanical paddle system). Carbon product is removed from the surface of the particles, and cleaned particles are then returned to the first vessel via a second solids flow regulation system. In another option, the catalyst bed is fixed and is swapped out from time to time with the catalyst particles then being placed in a second vessel (which may be at a different temperature) where the carbon is attritted off the catalyst by fluidisation or tumbling or by lifters or stirrers. The electrolysis step may be carried out in a solid oxide electrolysis cell that operates in the temperature range 700-950 °C. The carbon formation step may be carried out in a temperature range 350-650 °C. The carbon formation reactor, in fluidized bed and spouted bed forms, may contain 0.3-3 mm metallic or dense ceramic particles as sites for carbon. The carbon formation reactor, in moving packed bed form, may contain 2-20 mm metallic or dense ceramic particles as sites for carbon. If the metallic particles are steel particles, they may be alloyed as necessary to promote catalysis of carbon formation. The carbon formation reactor may contain an amount of a carbon formation catalyst. The carbon formation catalyst may be an iron oxide-containing material. By way of example, the iron oxide-containing material may be iron ore (e.g. hematite or goethite) screened to a suitable particle size distribution or in the form of a finely divided powder. Fine calcined iron ore is one particular example. This material provides small catalytic iron oxide / carbide seeds for enhanced catalysis of carbon product formation. The carbon formation catalyst may be dosed with catalytic materials such as alkali metals. When the carbon formation reactor is a fluidized bed, the fluidised bed may be arranged such that natural attrition of carbon from the surface of the bed particles occurs, and carbon is entrained in gas leaving the bed. The carbon may then be collected as a fine carbon product in a suitable gas cleaning and filtration device. There may be some entrained iron in the carbon product, for example arising from (i) iron- based metallic particle erosion in the bed and (ii) iron derived from added catalyst micro- particles. The total amount of iron in carbon from the carbon formation reactor is typically < 5%, but it could also be higher. It is noted that when the carbon is intended for use for ironmaking, its presence in the carbon product is of no serious practical concern. One embodiment of the carbon formation reactor includes a fluidized bed in which the bed comprises primarily particles of iron ore, optionally pre-treated with alkali catalytic promotors. In this case the carbon product is likely to comprise (at least partially) iron carbide rather than carbon. It is noted that when the carbon product is intended for use in an iron ore smelting process, the presence of iron carbide may be acceptable. Another, but not the only other, embodiment of the carbon formation reactor is an intermittently moving packed bed wherein larger-sized catalytically active bed particles are used (for example 2-20 mm). Carbon generation occurs in packed bed mode (in a batch cycle) with large beds and high residence times. Once a packed bed is considered “loaded” with carbon, it may then be agitated by (for example) operating it as a spouted bed for a period or transporting loaded particles to another vessel in order to shear off product carbon as dust into the gas for collection. Another option, depending on particle size and size range, is to collect carbon by a separate screening or by pick up into a gas phase. When the above-described modified and scaled-up process and apparatus for converting CO₂ into oxygen and solid carbon is coupled with the above-described process and apparatus for iron and steel production, the net result is a CO₂ electrolysis process, coupled with green steel manufacturing, that performs essentially the same task as that described in relation to its molten carbonate electrolysis counterpart. However, there are some important differences: 1. The modified and scaled up process and apparatus is a simple gas-solid reaction system which avoids multiple complexities associated with high temperature molten phases. Examples of these challenges include long-term electrode stability, process containment / corrosion issues, difficulties with carbon recovery from molten electrolyte and residual impurities in carbon product. 2. The carbon formation reactor of the modified and scaled up process and apparatus is inherently scale-friendly and can be implemented using conventional (high volumetric efficiency) fluidized bed, spouted bed or intermittently moving packed bed reactor design principles. 3. The solid oxide electrolysis cell of the modified and scaled-up process and apparatus requires surface area scale-up to achieve the large capacities required for green steel applications. However, the relatively simple gas-gas nature of the system and the absence of anything other than CO and CO₂ in the gas loop makes it easier to manage, compared to systems involving additional chemical species such as N₂, H₂ and H₂O. 4. Capital intensity has been calculated to be significantly lower than that of the molten carbonate process and hydrogen-based DRI systems. BRIEF DESCRIPTION OF THE DRAWINGS The invention is described further by way of example only with reference to the accompanying Figures, of which: Figure 1 is an embodiment of the CO₂ to carbon conversion process and apparatus (referred to herein as Carbelec™ Mk III) in accordance with the present invention, Figure 2 is an embodiment of an integrated blast furnace based green steel manufacturing process and apparatus incorporating Carbelec™ Mk III in accordance with the invention, and Figure 3 is an embodiment of a HIsarnaTMbased green steel manufacturing process and apparatus incorporating Carbelec™ Mk III in accordance with the invention. DESCRIPTION OF EMBODIMENTS The invention provides a process and an associated apparatus for iron or steel production wherein a reductant is solid carbon derived from CO₂ which has been recovered from an offgas from an ironmaking process, with the process and apparatus being characterised by the use of the above-mentioned Carbelec™ Mk III process and apparatus for CO₂ to carbon conversion. The Carbelec™ Mk III process and apparatus is characterised in that: (a) CO₂ from the offgas is converted to solid carbon (for use in the iron making process) and oxygen in a CO₂ conversion process, for example using at least substantially renewable energy as a source of power; and (b) the CO₂ conversion process comprises (i) an electrolysis step, for example in a solid oxide electrolysis cell, wherein CO₂ is at least partially reacted to CO, (ii) a carbon formation step wherein CO is at least partially reacted to solid carbon plus CO₂ and (iii) a recycling step wherein offgas from the carbon formation step is recycled to the electrolysis step such that, typically, both CO and CO₂ are effectively recycled to extinction. In broad terms, the process for iron production of the invention comprises: (c) converting CO₂ in a CO₂-containing gas to solid carbon (for use in the ironmaking process) and oxygen in a CO₂ conversion process comprising (i) reacting CO₂ to CO in an electrolysis step, (ii) reacting CO to solid carbon and CO₂ in a carbon formation step and (iii) transferring CO₂-containing gas from the carbon formation step to the electrolysis step and thereby effectively recycling CO and CO₂ to extinction; and (d) using solid carbon from the carbon formation step in an ironmaking process. With reference to Figure 1, in the embodiment of the Carbelec™ Mk III process and apparatus of the invention shown in the Figure, technically pure CO₂ 101 from a cryogenic storage tank or other suitable CO₂ source at approximately ambient temperature is fed through heat exchanger 102 where it is heated to about 600 °C. Warm CO₂ 125 from heat exchanger 102 is then mixed with recycle gas 103 (comprising about 57% CO₂, 43% CO on a molar basis). The flow rate of stream 103 in kNm³ / h is about 3-5, typically, 3.5 times greater than that of feed stream 101. The mixed stream 127 (streams 101 and 103) is then passed through and heated in (electric heater) HTR 104 to about 800 °C. Heater HTR 104 consumes electric power (123) at approximately 390 kW per tonne of CO₂ feed in stream 101. HTR 104 may be engineered to use conventional electric heating elements or it may utilise a plasma heating device or any other suitable heat source. Heated mixed stream 129 from heater HTR 104 is fed into a solid oxide electrolysis cell (SOEC) electrolyzer 105. The electrolyzer 105 may be any suitable solid oxide electrolysis cell (SOEC) electrolyzer. For example, see the cell disclosed in (6). Electrolyzer 105 draws a large amount of power from power source 122 (typically 3410 kWh / t CO₂ feed in feed stream 101). It operates at about 850 °C and roughly 80% of the CO₂ is converted to CO at an cathode (per pass) of electrolyzer 105. The resulting CO-CO₂ mixture generated at the cathode leaves as hot SOEC offgas 107 (typically 89% CO, 11% CO₂ on a molar basis). A commensurate amount of oxygen 106 is released at the anode (essentially pure oxygen at about 415 Nm³ / tonne of CO₂ feed in stream 101. This hot oxygen is safely vented, optionally using cold air dilution to cool it down and manage any associated safety issues. Hot CO-CO₂ 107 from the electrolyzer 105 passes through heat exchanger HX102 where it is cooled down to about 680 °C via heat exchange with feed CO₂ stream 101. From here, it passes through heat exchanger HX 108 where it is further cooled to about 630 °C and then passes through waste heat boiler WHB 109 where it is cooled to about 250 °C by virtue of steam-raising for power generation. Compressor 110 raises the pressure of the cooled CO₂-CO gas from WHB 109 by about 1 bar (without intercooling), thereby raising temperature to about 390 °C. Power consumed in this compressor is about 130 kWh / t of CO₂ in feed stream 101. Compressed gas 131passes through HX 108, where its temperature is increased to 450 °C via heat exchange with hot CO-CO₂ 107. From here, the heated compressed gas enters carbon formation reactor CFR 111 in the form of a fluidized bed reactor via fluidizing nozzles in a bottom of the bed. A small amount of catalyst 112 (typically comprising high surface area powdered, calcined iron ore which may be optionally doped with alkalis) is added to CFR 111 to promote carbon formation. This iron ore catalyst provides nano-particles of catalytically active iron in the bed for promotion of carbon formation. The main fluidized bed of the CFR 111 operates in bubbling or spouted bed mode with particles of steel shot in a size range 0.3-3 mm. Carbon forms within the fluidized bed of CFR 111 with around 50% of the CO reacting to C (on a per-pass basis). This carbon forms predominantly on the surface of bed particles and is subject to in-bed attrition by virtue of particle motion inside the bed (especially in the tuyere inlet region near the bottom). Carbon can also grow on the bed particles, but typically is rubbed off by particle collisions and predominantly reports to dust in the outgoing gas stream. It is noted that carbon also be recovered by other options, for example by recirculation of particles from and to the bed. CFR 111 is equipped with a freeboard space above the bed, optionally containing internal cyclones of the type found in FinmetTMor FinexTMiron ore reduction reactors (13). The bulk of the carbon produced in CFR 111 is carried in CFR offgas 113 to gas filter 114 where it is separated and removed as product carbon product 115. The amount of carbon produced is about 0.27 tonnes per tonne of CO₂ in feed stream 101 (very close to the amount expected on the basis of stoichiometry). Carbon 115 is typically cooled in a water-cooled screw feeder before being stored and (ultimately) re-used. Recycle gas 103 from gas filter 114 is returned to SOEC 105 via heater HTR 104 as shown. The end result is that offgas from CFR 111 is recycled to SOEC 105 such that both CO and CO₂ are effectively recycled to extinction. With gas recycle systems such as this, typically it is necessary to bleed off a percentage of gas in order to avoid buildup of inert species (such as N₂). However, because this system operates with essentially just two gas types (CO and CO₂), both of which are reactive, there is very little “inert” material present. If the SOEC of the electrolyzer 105 is engineered to run at slight positive pressure (for example, plus 20-30 mbar g) and the CFR 111 (top space) operates at about 50-80 mbar g, it becomes possible to eliminate in-leakage of air by design. Typically, purge gas will be needed at various locations (for example, keeping pressure sensor nozzles open in CFR 111). For this purpose, cold CO₂ may be used for this in place of standard nitrogen purge. The key is that this system is designed to remain clean, with essentially no contamination from gas in-leakage. Some recycle purge bleed may still be needed, but it will represent a lower than normal percentage of the recycle gas. This small bleed stream may optionally be treated in a pressure swing absorber (or alternative) system to recover CO and CO₂ for return to the process. A further alternative is to duct this bleed stream back to the flue of the ironmaking process (upstream of the final CO₂ collection step) and recapture the CO₂ in this manner. The overall gas loop will therefore function very much like an ideal “recycle to extinction” system. The embodiment of the Carbelec™ Mk III process and apparatus shown in Figure 1 consumes a large amount of power, mostly in SOEC 105. Typical power balances is as follows: SOEC 105, Power 122: 3410 kWh / t CO₂ in feed 101 HTR 104, Power 123: 390 kWh / t CO₂ in feed 101 Compressor 110, Power 124: 130 kWh / t CO₂ in feed 101 Power Plant 120, Power 121: -150 kWh / t CO₂ in feed 101 Ancillary Power Users: 20 kWh / t CO₂ in feed 101 Total Plant Power 3800 kWh / t CO₂ in feed 101 The applicant has calculated that, if the theoretical minimum power for CO₂ conversion to carbon is assumed to be 9.76 GJ / t CO₂, this equates to 2710 kWh / t CO₂. It is then instructive to calculate a so-called “system efficiency” which is defined as the percentage of total plant power input that ends up as chemical energy on the products (carbon in this case). For the flowsheet shown in Figure 1 and described in relation to the Figure, this system efficiency may be calculated as: Plant system efficiency = 100*(2710 / 3800) = 71% This is comparable with system efficiencies for typical alkaline electrolysis plants for green hydrogen production. It is also about 10 percentage points higher than efficiencies calculated (by the applicant) for equivalent molten carbonate electrolysis systems. It shows that, despite the presence of a large gas recycle (3.5 times the volume of the CO₂ feed stream), the SOEC-based process still achieves a high level of thermodynamic efficiency. Figure 2 is an embodiment of an integrated green steel process and apparatus which utilises the embodiment of the Carbelec™ Mk III as described above in relation to Figure 1. The embodiment of the steelmaking apparatus shown in Figure 2 comprises a conventional arrangement of sub-units including coke ovens 208, sinter plant 210, blast furnace 211, power plant 212, and BOF steelmaking plant 213. In use, flue streams 201, 202, 203 respectively from a boiler of the power plant 212, the coke ovens 208, and stoves of the blast furnace 211 are fed to amine scrubber 204. Approximately 90-95% of the CO₂ is removed from the combined flue stream in the scrubber 204, yielding recovered CO₂ stream 205. The recovered CO₂ stream 205 is transferred to the embodiment of the Carbelec™ Mk III plant shown in Figure 1, identified by the numeral 206 in Figure 2. Depending on the nature of the Carbelec™ Mk III plant operation (intermittent to balance renewable energy availability or not), stream 205 may be collected and stored as CO₂ liquid under mild cryogenic conditions (not shown). The recovered CO₂ is separated into carbon and oxygen in the Carbelec™ Mk III plant 206 as described in relation to Figure 1. The recovered carbon 214 produced in the Carbelec™ Mk III plant 206 is transferred for use in the blast furnace 211 and the coke ovens 208. Around 27% of the recovered carbon is injected into tuyeres of the blast furnace as a PCI coal substitute 207 and the remainder 215 of the recovered carbon is fed to the coke plant 208. A small amount of fresh coal 209, comprising about 20% of the total feed to the coke ovens 208, is added to the coke ovens 208 for the purpose of carbon balance and binding in the coke making process. Additional binding agents such as bio-oil may optionally be added to the coke ovens 208 to improve coke strength. The overall result is a blast furnace plant 211 running more or less normally, with around 80- 90% of its energy demand shifted from coal to renewable energy. A key advantage of this arrangement is that it allows existing (large, efficient) blast furnaces to keep running in a climate-friendly manner. Electric power consumption is high – typically around 5500-6000 kWh / t steel (as is the case with all green steel processes using renewable energy). Cost of power is therefore a key factor, and it will be typically necessary to operate the Carbelec™ Mk III plant 206 when renewables are available (at low cost), then stop and wait at other times. This is not a concern for the plant, as it is for other green steel options such as H2-DRI. Storage of liquid CO₂ at the feed end of the Carbelec™ Mk III plant 206, together with safe (wet) carbon storage at the product end, will most likely be part of any such implementation. Figure 3 is an alternative embodiment of an integrated green steel involving Carbelec™ Mk III plant shown in Figure 1 in conjunction with a HIsarnaTMplant operating in accordance with a HIsarnaTMprocess. HIsarnaTMtechnology is described in the technical and patent literature including a-revolutionary-iron-making- process / and https: / / www.gem.wiki / HIsarna_process. One of the attractive features of the HIsarnaTMprocess is that it naturally produces concentrated CO₂ without the need for any type of gas scrubber (such as an amine scrubber). This CO₂ may be easily upgraded to technically pure CO₂ by simple distillation in a cold box, yielding CO₂ that is readily stored in liquid form. With reference to Figure 3, HIsarnaTMplant 301 produces CO₂ stream 302 which is (optionally, not shown) stored as liquid under mild cryogenic conditions. This CO₂ is then fed to the Figure 1 embodiment of Carbelec™ Mk III plant 303 where renewable energy is used as an energy source to separate it into oxygen and solid carbon as per the embodiment described in relation to Figure 1. Recovered carbon 304 is then optionally stored, before being fed back into the HIsarnaTMcarbon injection lances to perform smelting in the HIsarnaTMplant 301. A small amount of supplementary carbon is typically needed to make to make up for carbon losses in the loop, and for this purpose about 4-5% of the total carbon feed may comprise dried biomass (not shown). The HIsarnaTM-Carbelec™ Mk III combination is an exceedingly simple arrangement wherein each of the two processes leverages strengths of the other. Electric power consumption is again high (around 6800-6900 kWh / t steel) which is comparable to the blast furnace combination in Figure 2 when the residual 20% of coal feed to the coke ovens is factored in. The HIsarnaTM-Carbelec™ combination is totally green, whereas the blast furnace combination in Figure 2 is “substantially” green. Nevertheless, comments around CO₂ storage and Carbelec™ product carbon storage (to accommodate the availability of low-cost renewable power) are common to both green steel implementation examples as described. Many modifications may be made to the embodiment of the Carbelec™ Mk III process and apparatus shown in Figure 1 without departing from the spirit and scope of the invention. By way of example, the invention is not confined to the temperature and flow rates and other specific operational conditions in the embodiment of the Carbelec™ Mk III process and apparatus shown in Figure 1. The specific data provided in relation to the Figure 1 embodiment is derived from calculations of the applicant. This is an example of a broad range of operational conditions within the scope of the invention. Other operational conditions within the scope of the invention and could be calculated by a skilled person. In addition, many modifications may be made to the embodiments of combinations of (a) the Carbelec™ Mk III process and apparatus shown in Figure 1 and (b) the blast furnace and the HIsarnaTMprocesses and apparatus shown in Figures 2 and 3, respectively without departing from the spirit and scope of the invention. By way of example, the invention is not confined to the specific operational data in the embodiments shown in Figures 2 and 3. The specific conditions provided in relation to the Figures 2 and 3 embodiments is derived from calculations of the applicant. These are examples of broad ranges of operational conditions within the scope of the invention. Other operational conditions within the scope of the invention and could be calculated by a skilled person.

[0002] References 1. IEA Renewables 2020 Fuel Report (November 2020) Hyperlink: https: / / www.iea.org / reports / renewables- 2020?utm_campaign=IEA%20newsletters&utm_source=SendGrid&utm_medium =Email 2. SEI, 2019. Hydrogen Breakthrough Ironmaking Technology (HYBRIT) is a ground-breaking effort to reduce CO₂ emissions and de-carbonise the steel industry. Hyperlink: https: / / www.sei.org / projects-and-tools / projects / hybrit / 3. Hydrogen Storage, Wikipedia Hyperlink: https: / / en.wikipedia.org / wiki / Hydrogen_storage 4. J Kaczur et al, Process for High Surface Area Electrodes for the Electrochemical reduction of carbon Dioxide, US Patent 8,858,777 B2, Oct 14, 2014 (Liquid Light Inc, NJ) 5. C A Oloman and H Li, Continuous Electrochemical Reduction of Carbon Dioxide, Canadian patent 2,625,656, October 2006 (Mantra Energy Alternatives, CA) 6. N B Jakobsson et al, Process for Producing CO from CO2 in a Solid Oxide Electrolysis Cell, US Patent 10,494,728 B2, Dec 2019 (Haldor Topsoe) 7. E Laarsonen et al, Insights into carbon production by CO2 reduction in molten salt electrolysis in a coaxial-type reactor, J Env Chem Eng, 10 (2022) 106933, Insights into carbon production by CO2 reduction in molten salt electrolysis in coaxial-type reactor - ScienceDirect 8. J Ren et al, The Minimum Electrolytic Energy Needed To Convert Carbon Dioxide to Carbon by Electrolysis in Carbonate Melts, J. Phys. Chem. C 2015, 119, 41, 23342–23349, https: / / pubs.acs.org / doi / 10.1021 / acs.jpcc.5b07026 9. Licht et al, Methods and Systems for Carbon nanofiber Production, US Patent 2018 / 0044183A1, Feb 2018 10. Elgammal et al, Electrolytic Generation of Graphite, Saratoga Energy, US Patent 9,290,853 B2, March 2016 11. Reid, et al, Electrolytic Generation and Purification of Carbon, Saratoga Energy, US Patent 10,900,134 B2, Jan 2021 12. Green et al, Development Status for a Combined Solid Oxide Co-Electrolyzer and Carbon Formation Reactor System for Oxygen Regeneration (nasa.gov), American Institute of Aeronautics and Astronautics, 2016 13. Plaul et al, Fluidized-bed technology for the production of iron products for steelmaking, The Journal of The Southern African Institute of Mining and Metallurgy Vol 108 p121, Feb 2009

Claims

CLAIMS 1. A process for iron production wherein a reductant for use in the ironmaking process is solid carbon derived from CO₂ which has been recovered from an offgas from an ironmaking process, with the process being characterised in that: (a) CO₂ from the offgas is converted to solid carbon (for use in the ironmaking process) and oxygen in a CO₂ conversion process, for example using at least substantially renewable energy as a source of power; and (b) the CO₂ conversion process comprises (i) an electrolysis step wherein CO₂ is at least partially reacted to CO, (ii) a carbon formation step wherein CO is at least partially reacted to solid carbon and CO₂ and (iii) a recycling step wherein offgas from the carbon formation step is transferred to the electrolysis step such that both CO and CO₂ are typically effectively recycled to extinction.

2. A process according to claim 1 wherein the CO₂ electrolysis step is carried out in a solid oxide electrolysis cell and operates in the temperature range 700-950 °C.

3. A process in accordance with any claim 1 or claim 2 wherein the carbon formation step is carried out in a temperature range 350-650 °C.

4. A process in accordance with any one of the preceding claims wherein the carbon formation step comprises operating with a carbon formation catalyst.

5. A process in accordance with claim 4 wherein the carbon formation catalyst comprises an iron oxide-containing material, such as (a) iron ore (e.g. hematite or goethite) for example screened to a suitable particle size distribution or a finely divided powder or (b) calcined iron ore, all optionally dosed with catalytic materials such as alkali metals.

6. A process in accordance with any one of the preceding claims wherein the carbon formation step produces product carbon as a fine dust into offgas, whereafter this product dust is collected in a suitable gas filtration device.

7. A process in accordance with any one of the preceding claims wherein the carbon formation step is carried out in a fluidized or spouted bed and operates with 0.3-3 mm particles of steel shot or a dense oxide (such as alumina, hematite, magnetite or iron ore) or a combination thereof.

8. A process in accordance with any one of the preceding claims wherein the carbon formation step is carried out in an intermittently moving packed bed reactor system.

9. A process in accordance with any one of claims 1 to 6 wherein the carbon formation step is carried out in a carbon formation reactor that operates as a bubbling or spouted fluidized bed with a well-defined upper bed surface and a freeboard for disengagement of bed particles from the up-flowing gas.

10. A process in accordance with claim 9 wherein the freeboard region comprises internal cyclones to ensure that bed material remains in the bed.

11. A process in accordance with any one of the preceding claims wherein cold CO₂ gas is used in place of nitrogen for services such as instrument purges, such that no inert gases of any type enter the gas loop.

12. A process in accordance with any one of the preceding claims wherein the electrolysis step is carried out in a solid oxygen electrolysis cell (SOEC) that is pressurised and controlled at a pressure sufficiently above ambient (in a range 5-50 mbar g) to guarantee no air ingress.

13. A process in accordance with any claim 12 wherein the carbon formation step is operated at a top space (freeboard) pressure in a range 20-200 mbar aboveSOEC pressure in order to provide necessary gas flow driving forces for gas filtration and heat transfer trains.

14. A process in accordance with any one of claims 1 to 11 wherein the electrolysis step is carried out in a solid oxygen electrolysis cell (SOEC) that is pressurised and controlled to a pressure in a range of 1-30 bar g.

15. A process in accordance with any claim 14 the carbon formation step operates at marginally higher pressure than that in the SOEC.

16. A process for steel production comprising using iron from the iron production process defined in any one of the preceding claims as a feed material for the steel production process.

17. An apparatus for iron production wherein a reductant for use in the ironmaking process is solid carbon derived from CO₂ which has been recovered from an offgas from an ironmaking process, with the apparatus being characterised by an apparatus for conversion of CO₂ into oxygen and solid carbon comprising: (a) an electrolysis cell, such as a solid oxide electrolysis cell, for converting CO₂ produced in ironmaking into CO, with the electrolysis cell configured to operate for example using at least substantially renewable energy as a source of power; and (b) a carbon formation reactor for converting CO to solid carbon and an offgas that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed.

18. An apparatus for steel production comprising the iron production apparatus for producing iron defined in claim 17 and a steel production apparatus for producing steel from the iron.

19. A process for converting CO₂ into oxygen and solid carbon comprises: (a) converting CO₂ into CO in an electrolysis cell, such as a solid oxide electrolysis cell; and (b) converting CO to solid carbon and an offgas in a carbon formationreactor that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed.

20. An apparatus for converting CO₂ into oxygen and solid carbon comprises: (a) an electrolysis cell, such as a solid oxide electrolysis cell, for converting CO₂ into CO; and (b) a carbon formation reactor for converting CO to solid carbon and an offgas that comprises at least one of (i) a fluidized bed reactor, such as a spouted bed reactor or (ii) a continuously or intermittently moving packed bed.

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