Method for operating an iron- or steelmaking- plant

Abstract

A method of operating an ironmaking or steelmaking plant with low CO.sub.2-emissions is provided. Hydrogen and oxygen are generated by water decomposition and at least part of the generated hydrogen is injected as a reducing gas into one or more ironmaking furnaces with off-gas decarbonation and reinjection into the furnaces of at least a significant part of the decarbonated off-gas and at least part of the generated oxygen is injected as an oxidizing gas in the one or more ironmaking.

Claims

1. A method of operating an ironmaking or steelmaking plant comprising an ironmaking furnace set comprising one or more furnaces in which iron ore is transformed into liquid hot metal by means of a process which includes iron ore reduction, melting and off-gas generation, the ironmaking or steelmaking plant, the method comprising the steps of: a. charging the ironmaking furnace set with iron ore and coke, b. injecting oxidizing gas into the ironmaking furnace set, c. producing an off-gas and decarbonating the off-gas downstream of the ironmaking furnace set thereby obtaining a CO.sub.2-enriched tail gas stream and a decarbonated off-gas stream containing not more than 10% vol CO.sub.2, d. injecting at least 50% of the decarbonated off-gas stream back into the ironmaking furnace set as a reducing gas recycle stream, e. generating hydrogen and oxygen by means of water decomposition, f. injecting at least part of the hydrogen generated in step in step (e) combined with at least a part of the decarbonated off-gas into the ironmaking furnace set, and g. injecting at least part of the generated oxygen into the ironmaking furnace set and/or a converter as oxidizing gas.

2. The method according to claim 1, whereby at least part of the hydrogen generated in step (e) which is injected into the ironmaking furnace set is mixed with the reducing gas recycle stream before the gas mixture so obtained is injected into the ironmaking furnace set.

3. The method according to claim 1, wherein: h. the gas recycle stream or the mixture of hydrogen generated in step (e) with the gas recycle stream is heated upstream of the ironmaking furnace set to a temperature between 700° C. and 1300° C.

4. The method according to claim 3, wherein: i. a low-heating-value gaseous fuel having a heating value of from 2.8 to 7.0 MJ/Nm.sup.3 is produced containing (i) at least a portion of the tail gas stream and (ii) a second part of the hydrogen generated in step (e), said low-heating-value gaseous fuel being used to heat hot stoves used for heating the gas recycle stream.

5. The method according to claim 1, whereby a ratio between: (i) the hydrogen generated in step (e) and injected into the ironmaking furnace set and (ii) the oxygen generated in step (e) and injected into the ironmaking furnace set and/or the converter in step (g) is between 1.50 and 2.50.

6. The method according to claim 1, whereby a ratio between: (i) the hydrogen generated in step (e) and injected into the ironmaking furnace set and (ii) the oxygen generated in step (e) and injected into the ironmaking furnace set in step (g) is between 1.75 and 2.25.

7. The method according to claim 1, wherein pulverized coal and/or another organic combustible substance is injected into the blast furnace by means of tuyeres.

8. The method according to claim 1, wherein all or part of the generated hydrogen which is injected into the ironmaking furnace set is injected into the ironmaking furnace set via tuyeres.

9. The method according to claim 1, wherein all or part of the oxygen generated in step (e) is mixed with oxygen-containing gas not generated in step (e) so as to obtain a mixture which is injected as oxidizing gas into the ironmaking furnace set.

10. The method according to claim 1, wherein the oxidizing gas which is injected into the ironmaking furnace set in step (b) consists of oxygen generated in step (e).

11. The method according to claim 1, wherein in step (e), hydrogen and oxygen are generated by biological and/or electrolytic water decomposition.

12. The method of claim 11, wherein in step (e), hydrogen and oxygen are generated by electrolytic water decomposition at a pressure above atmospheric pressure and/or at a temperature above ambient temperature.

13. The method according to claim 1, wherein the reducing gas is injected into the ironmaking furnace set via tuyeres.

14. The method according to claim 1, wherein the ironmaking furnace set comprises one or more blast furnaces.

15. The method according to claim 1, wherein the hydrogen generated in step (e) consists of at least 70% vol of H.sub.2 molecules.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1 schematically illustrates a prior art steelmaking plant, and

(3) FIG. 2 schematically illustrates an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) The present invention and its advantages are further clarified in the following example, reference being made to FIGS. 1 and 2, whereby FIG. 1 schematically illustrates a prior art steelmaking plant whereby the IFS consists of one or more non-TGRBFs (only one blast furnace is schematically represented and in the corresponding description reference is made to only one non-TGRBF) and FIG. 2 schematically illustrates an embodiment of the method according to the invention applied to a steelmaking plant whereby the IFS consists of one or more TGRBFs (only one TGRBF is represented and in the corresponding description reference is also made to only one TGRBF), whereby identical reference numbers are used to indicate identical or analogous features in the two figures.

(5) FIG. 1 which shows a prior art conventional blast furnace 1 without top gas decarburization or recycling. Blast furnace 1 is charged from the top with coke and iron ore 2 which descend in the blast furnace 1.

(6) Air 28 is preheated in hot stoves 20 before being injected into blast furnace 1 via hearth tuyeres 1b. Substantially pure oxygen 22 can be added to blast air 28 via the hearth tuyeres 1b or upstream of the hot stoves 20.

(7) Pulverized coal (or another organic combustible substance) 23 is typically also injected into the blast furnace 1 by means of hearth tuyeres 1b.

(8) The air 28, and, if added, the substantially pure oxygen 22 and the pulverized coal (or another organic fuel) 23 combine inside the blast furnace so as to produce heat by combustion and reducing gas 1d (in contact with the coke present in solid charge 2). Reducing gas 1d ascends the inside of blast furnace 1 and reduces the iron oxides contained in the ore to metallic iron. This metallic iron continues its descent to the bottom of the blast furnace 1 where it is removed (tapped) 1a along with a slag containing oxide impurities.

(9) The off-gas, better known as blast furnace gas (BFG), 3 exits the blast furnace 1 and travels to an initial dust removal unit 4 where large particles of dust are removed. It continues to a second dust removal system 5 that removes the fine dust particles to produce a “clean gas” 6. The clean gas 6 is optionally dewatered before entering the BFG distribution system 7a where part of the clean gas 6 can be sent distributed to the hot stoves 20, where it is used as a fuel, and part 8 of the clean gas 6 can be sent to other locations 8a of the steel plant for various uses. The flow of BFG to the one or more other locations 8a is controlled by control valve system 8b.

(10) Hydrogen, CO or a mixture of hydrogen and CO may be also be injected into the blast furnace 1 via hearth tuyere 1b as additional reducing gas. (A single tuyere is schematically represented in the figure, whereas in practice, a blast furnace comprises a multitude of tuyeres)

(11) In order to limit the carbon footprint of the known blast furnace operation, the hydrogen, CO or the mixture of hydrogen and CO can be sourced from environmentally friendly sources, such as biofuel partial combustion or reforming.

(12) As indicated earlier, in order to limit CO.sub.2 emissions by the blast furnace, hydrogen could appear to be the preferred additional reducing gas. Unfortunately, the cost of substantially pure hydrogen gas is usually inhibitive for this kind of industrial application.

(13) A further technical problem related to hydrogen (and CO) injection into a blast furnace relates to the thermodynamics of the blast furnace process, namely the fact that the efficiency of hydrogen (and CO) usage in the blast furnace rarely exceeds 50%. 50% of the hydrogen injected in the blast furnace thus exits the top of the blast furnace without participating in the reactions. This limits the use of hydrogen in a conventional blast furnace.

(14) Table 1 presents a theoretical comparison, based on process simulation, between operations of a conventional blast furnace injecting 130, 261 and 362 Nm.sup.3 hydrogen/tonne hot metal (thm) into a standard blast furnace with powdered coal injection (PCI) when that hydrogen is used to replace coal while keeping the coke rate constant. Also presented in Table 1 are the cases when 130 and 197 Nm3 of hydrogen are replacing coke while keeping the coal injection (PCI) rate constant.

(15) TABLE-US-00001 TABLE 1 11.72 Kg H2 11.72 Kg H2 17.7 Kg H2 23.44 Kg H2 33.61 Kg H2 Period (Enter the name Reference Replacing Replacing Replacing Replacing Replacing of the period) Units Final Coal Coke Coke Coal Coal Reductant Consumption Coke rate (small + big) Kg/thm 293 293 265 253 293 293 Fuel Injection Rate Kg/thm 197 179 209 215 164 153 Coal Injection Rate Kg/thm 197 167 197 197 141 120 Hydrogen Injection Rate Kg/thm 0 11.72 11.72 17.70 23.44 32.61 Hydrogen Injection Rate Nm3/thm 0 130 130 197 281 362 Total Fuel Rate Kg/thm 490 471 474 468 457 445 Tuyeres Blast Volume (Air Only) Nm3/thm 832 828 827 814 814 801 Blast Temperature ° C. 1176 1176 1176 1176 1176 1176 Oxygen Volume Calculated Nm3/thm 82.0 76.8 79.7 80.4 75.7 75.1 Oxygen in the cold blast % 27.6 27.2 27.4 27.5 27.2 27.2 Water Vapour added to Blast g/Nm3 12.23 5.00 5.00 5.00 5.00 5.00 Raceway Gas Volume Nm3/thm 1311 1396 1413 1470 1496 1573 (Gosh Gas Volume) Bosh Reducing Gas Nm3/thm 633 723 739 803 833 920 (CO2/(CO + CO2) RAFT (Raceway Adiabatic ° C. 2251 2124 2089 2006 1992 1901 Flame Temperature) Top Gas Volume (dry) Nm3/thm 1441 1453 1459 1469 1467 1477 Temperature ° C. 128 154 176 200 181 200 CO % 24.5 22.6 22.6 21.7 20.9 19.7 CO2 % 24.1 22.4 22.3 21.5 20.9 19.6 H2 % 4.3 8.5 8.9 11.4 13.0 16.5 N2 % 47.1 46.4 46.2 45.4 45.2 44.2 CO2/(CO + CO2) % 0.496 0.499 0.497 0.497 0.499 0.499 BF Operational Results Gas Utilization at FeO Level % 93.0 93.0 93.0 93.0 93.0 93.0 Calculated Heat Losses MJ/thm 408.7 408.7 408.7 408.7 408.7 408.7 % of Heat Losses in the Lower BF % 80.7 80.7 80.7 80.7 80.7 80.7 Global Direct Reduction Rate % 30.8 26.1 25.4 22.2 20.6 16.2 Direct Reduction Degree of % 29.7 24.9 24.1 20.9 19.2 14.8 Iron Oxides Reduction of CO2 Emission (per tonne HM) Carbon Consumption Kg/thm 423 398 399 388 376 359 CO2 Emissions Kg/thm 1550 1459 1461 1421 1378 1315 CO2 Savings Kg/thm — 92 89 130 172 235 % CO2 Savings % — 5.9 5.7 8.4 11.1 15.2 Relative Production Rate % 100 100 100 100 100 100 CO2 for electricity @ 600 g CO2/kWh Kg/thm 24.0 24.0 24.0 24.0 24.0 24.0 (not including oxygen) O2 for electricity @ 600 g CO2/kWh Kg/thm 27.1 25.3 26.3 26.5 25.0 24.8 (oxygen) Total CO2 saved Kg/thm 0 93 90 130 174 237 % CO2 saved % — 5.8 5.6 8.1 10.9 14.8 Hydrogen to Oxygen Ratio 1.7 1.64 2.45 3.44 4.83

(16) TABLE-US-00002 TABLE 2 Total CO2 Iron Oxygen Volume Saved With Additional Production Coke Charge Coal Injection Required in CO2 Respect to % CO2 Hydrogen Rate Rate Rate Blast Furnace Produced Conventional BF Saved Injected Units tonne/d Kg/thm Kg/thm Nm3/thm kg/thm Tonnes/year % Nm3/h Reference 5784 293 146 92.2 1510 — — — Conventional w/PCL 5784 300 189 58.1 1550 — — — Conventional w/NG 5784 303 0 173.4 1402 308971 9.8 — Conventional 100 Nm3 H2/thm 5784 270 189 63.7 1467 242922 7.7 24098 Conventional 200 Nm3 H2/thm 5784 240 189 69.8 1385 483163 15.4 48197 Conventional 300 Nm3 H2/thm 5784 210 189 74.9 1259 814611 26.0 72295 ULCOS Version 4 6383 209 190 239.6 1258 903884 26.1 — ULCOS 100 Nm3/t H2 Injection 7019 185 190 227.5 1180 1258836 33.1 29246 ULCOS 100 Nm3/t H2 Injection 6344 263 74 203.9 1082 1138784 33.1 26432 74 Kg/thm PCL ULCOS 200 Nm3/t H2 Injection 7506 169 190 219.3 1127 1539163 37.8 62546 ULCOS 200 Nm3/t H2 Injection 6812 291 1 177.4 947 1463335 39.6 56764 No PCL ULCOS 300 Nm3/t H2 Injection 7866 170 164 206.0 1053 1810700 42.4 98319 ULCOS 300 Nm3/t H2 Injection 7526 258 1 160.6 840 2006584 49.2 94071 NO PCL ULCOS 400 Nm3/t H2 Injection 8197 167 151 197.2 1003 2041574 45.9 136624  w 151 Kg PCL ULCOS 400 Nm3/t H2 Injection 8188 195 94 180.0 920 2176259 49.0 136472  w 94 Kg PCL Total Oxygen Requirements Total O2 Additional Additional (80% hot metal/ Requirement O2 O2 Additional H2 20% Scrap 93% yield) For BF Surplus/Deficit Surplus/Deficit Produced/Additional Blast L-D Converter and from H2O from H2O O2 Required Furnace (55 Nm3/thm) LD Converter Decomp Decomp Units H2/O2 Ratio Nm3/h Nm3/h tonnes/day NmS/h tonnes/day Reference — 22211 15408 1289 Conventional w/PCL — 13996 15408 1008 Conventional w/NG — 41791 15408 1960 Conventional 100 Nm3 H2/thm 1.57 15348 15408 1054 −18707 −641 Conventional 200 Nm3 H2/thm 2.87 16816 15408 1104 −8125 −278 Conventional 300 Nm3 H2/thm 4.01 18050 15408 1147 2690 92 ULCOS Version 4 — 63714 17004 2766 ULCOS 100 Nm3/t H2 Injection 0.44 66532 18699 2921 −70608 −2420 ULCOS 100 Nm3/t H2 Injection 0.49 53894 16900 2426 −57578 −1973 74 Kg/thm PCL ULCOS 200 Nm3/t H2 Injection 0.91 68582 19995 3036 −57304 −1964 ULCOS 200 Nm3/t H2 Injection 1.13 50347 18147 2347 −40112 −1375 No PCL ULCOS 300 Nm3/t H2 Injection 1.46 67516 20954 3032 −39310 −1347 ULCOS 300 Nm3/t H2 Injection 1.87 50347 20049 2412 −23360 −801 NO PCL ULCOS 400 Nm3/t H2 Injection 2.03 67352 21838 3057 −20879 −716 w 151 Kg PCL ULCOS 400 Nm3/t H2 Injection 2.22 61406 21814 2852 −14984 −514 w 94 Kg PCL

(17) Table 2 demonstrates the reduced requirement for external oxygen at the blast furnace and at the L-D Converter as illustrated in FIG. 2 when oxygen from the water decomposition process is used in the steelmaking plant.

(18) As shown in Table 2, if oxygen from the water decomposition process is used for the blast furnace and the L-D converter, the need for external oxygen, typically from an air separation plant, to meet the oxygen requirement of the steel plant is greatly reduced or non-existent.

(19) For most of the embodiments illustrated in Table 2, the use of water decomposition to meet the entire requirement of the blast furnace for additional hydrogen results in a generation of oxygen which is insufficient to meet the (additional) oxygen requirement of the blast furnace and the converter. Consequently, additional oxygen must be obtained from a further oxygen source, such as an ASU, in order to meet said requirement. However, the amount of oxygen to be obtained from said further oxygen source is drastically reduced.

(20) However, when the use of water decomposition to meet the entire requirement of the blast furnace and/or for the converter (if present) results in the generation of oxygen in excess of the additional oxygen requirement of the blast furnace (and, if applicable, the converter), surplus generated oxygen may advantageously be used in other processes/installations of the iron- or steelmaking plant and/or be sold to generate revenue. The present invention thus provides a method for reducing CO.sub.2 emissions from an iron- or steelmaking plant comprising an iron furnace set (IFS) by means of the injection into the IFS of a non-carbon-based reducing agent and this at lower overall cost. It also greatly reduces the amount of external oxygen produced by ASU, VSA, VPSA or any other method to complete the oxygen requirement of the iron- or steelmaking plant. In doing this the amount of indirect CO.sub.2 emissions from oxygen production are also avoided or reduced. The carbon footprint of the iron- or steelmaking plant can be further reduced by using low-carbon-footprint electricity as described above.

(21) A method according to the present invention is illustrated in FIG. 2 with respect to an IFS containing one or more TGRBFs. Again, blast furnace 1 is charged from the top with coke and iron ore 2 which descend in the blast furnace 1. Substantially pure oxygen 22 and pulverized coal (or another organic fuel) 23 are injected into blast furnace 1 via hearth tuyeres 1b. The blast furnace gas (BFG) 3 exits the blast furnace 1 and travels to an initial dust removal unit 4 for course dust particles, followed by a second dust removal system 5 that removes the finer dust particles to produce a “clean gas” 6.

(22) Clean gas 6 is optionally dewatered before entering the CO2-removal system 7. The CO2-removal system 7 can be a vacuum pressure swing adsorption system (VPSA), a pressure swing adsorption system (PSA) or a chemical absorption system such as an amines-based absorption system or any other type of system that removes most of the CO2 from the (dean) BFG 6. Typically, less than 15% vol; preferably less than 10% vol and more preferably less than 3% vol CO2 will remain in the decarbonated BFG 9. CO2-removal system 7 thus splits the dean gas stream 6 into two streams: a CO2-enriched tail gas 8 and a CO2-lean product gas 9.

(23) The CO2-rich tail gas 8 is removed from the blast furnace operation process through evacuation line 8a equipped with control valve 8b. The CO2-lean product gas stream (decarbonated BFG) 9 exits the CO2-removal system 7 at elevated pressure (typically 4-8 bar). The decarbonated BFG $ is sent to hot stoves 20, where it is heated before being sent to hearth tuyeres 1b for injection into the blast furnace 1. In accordance with the invention, water 10 and suitable electrolyte 10a are mixed to produce an aqueous solution 11 that has an optimum electrical potential for water dissociation into hydrogen and oxygen when a suitable electrical potential (voltage) is applied to the solution 11, i.e. for water electrolysis.

(24) Pump 12 generates a pressurized flow 13 of solution 11 towards electrolysis installation 14 (high-pressure electrolysis). As a consequence, the generated hydrogen 15 and oxygen 22a streams leaving electrolysis installation 14 are likewise pressurized, rendering said gas streams suitable for downstream use without compression or with reduced additional compression of the hydrogen 15, respectively the oxygen 22a.

(25) After electrolysis of solution 13 to hydrogen 15 and oxygen 22a, the hydrogen 15 is mixed with decarbonated BFG 9 so as to fortify the latter. The oxygen 22a is injected as oxygen stream 22c into blast furnace 1 where it is used as a combustion oxidizer and/or as oxygen stream 22d into converter 50 also present in the plant, where it is used as a decarburization agent.

(26) Depending on the pressure at which hydrogen 15 and oxygen 22a streams leave electrolysis installation 14, said gases may or may not need to be pressurized or depressurized to an appropriate pressure for combination with decarbonated BFG stream 9 and/or for injection into the blast furnace 1 and/or converter 50. Gas pressurization may be achieved in a compressor, gas depressurization in an expander.

(27) FIG. 2 shows an embodiment whereby both hydrogen stream 15 and oxygen stream 22a need to be depressurized. Hydrogen stream 15 is depressurized using gas expander 17, Oxygen stream 22a is depressurized using further gas expander 22b.

(28) It will be appreciated that when generated oxygen 22a is divided to be injected in multiple installations of the steelmaking plant, e.g. in a blast furnace and in a converter or in an EAF for melting scrap, pressurization or depressurization may be required for only some of said installations or may apply differently to different installations, in which case separate pressurization or depressurization equipment may be provided for the different installations.

(29) Depending on the pressure drop between the entrance and exit of the two expanders 17 and 22b, energy from the expander 17 and expander 22b could be used to generate electricity, thus further improving the (energy) efficiency of the plant. Fortified gas stream 19 is obtained by mixing of decarbonated BFG stream 9 with depressurized hydrogen stream 18.

(30) In the illustrated embodiment, hot stoves 20 are heated by the combustion of a diverted portion 25 of the CO2-rich tail gas 8 with air stream 28. Valves 8b and 25a control the portion 25 of the CO2-rich tail gas 8 which is thus diverted.

(31) A portion 26 of fortified gas stream 19 may, as shown, be diverted for making a “mixed gas” 27 that can be used as a low-heating-value fuel for heating the stoves as such or in combination with other fuels, such as coke oven gas. In that case, portion 26 (if needed) of fortified gas stream 19 used in the mixed gas 27 is regulated using valve 26a. Care is taken so that mixed gas 27 has a heating value appropriate for heating stoves 20. The heating value of mixed gas 27 is typically arranged to be low (5.5-6.0 MJ/Nm3) and the mixed gas preferably has (a) a low content of hydrocarbons to prevent vibration in the stove combustion chamber and (b) a significant content of CO and H2 for facilitating smooth combustion.

(32) As shown, another portion of fortified gas stream 19 (stream 16) can be used as fuel to heat electrolysis installation 14 if higher electrolysis temperatures are needed (high-temperature electrolysis), though other means may (also) be provided to that effect. The flow rate of stream 16 is regulated using valve 26b. Air stream 28 is used as an oxidant to combust stream 27 for heating the stoves 20. In addition, air stream 24 is used as an oxidant to combust stream 16 for heating electrolysis installation 14, if necessary.

(33) Fortified gas stream 19 is heated in stoves 20 to create gas streams 21 and optionally 29 having a temperature greater than 700° C. and as high as 1300° C. However, the preferred temperature of stream 21 is between 850° C. and 1000° C. and more preferably 880° to 920° C. in order to have a sufficiently high temperature to promote rapid iron ore reduction while having a sufficiently low temperature to prevent possible reduction of the oxide refractory lining the pipeline to the blast furnace.

(34) Optionally a portion 29 of heated fortified gas stream 19 (containing recycled product gas 9 and generated hydrogen 18) is injected into the shaft tuyere 1c to combine inside the blast furnace with the gases produced at the hearth tuyeres to produce a reducing gas 1d that ascends the inside of blast furnace 1, contacts the iron ore and coke 2 and reduces the iron oxides contained in the ore to metallic iron. Gas stream 29 may or may not be used depending on the configuration of the particular TGRBF. The distribution of flow rates between streams 21 and 29 are governed by valve 30.

(35) Oxygen stream 22c may provide all of the oxygen injected into blast furnace 1. The oxygen injected into blast furnace 1 may also entirely or partially come from an external oxygen supply, for example, an Air Separation Unit (ASU), such as a Vacuum Swing Adsorption (VSA) unit, a Vacuum Pressure Swing Adsorption (VPSA) unit, an oxygen pipeline etc.

(36) Preferably, at least part of the oxygen stream 22a produced on-site (i.e. inside the iron- or steelmaking plant) by water decomposition (more specifically by water electrolysis in installation 14) is injected into the blast furnace 1 as oxygen stream 22c.

(37) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.