Method to produce hot metal in a blast furnace

Abstract

The invention relates to a method to produce hot metal (12) in a blast furnace (1) wherein a natural gas (18) is subjected to a cracking step in a cracking unit (6) to produce a cracked gaseous stream (31), said cracked gaseous stream (31) comprising at least 20% in volume of hydrogen, less than 80% in volume of unreacted natural gas and a suspension of solid carbon black, and being injected into the blast furnace at the tuyere level (2), a hot blast (13) being also injected at said tuyere level (2). The invention also relates to a plant allowing to implement such a method.

Description

[0001] Method to produce hot metal in a blast furnace

[0002]

[0001] Currently, more than 75% of the steel in the world is produced through the blast furnaces (BF) and basic oxygen furnaces (BOF) production route which include raw material preparation plants, such as coke and sinter plants. Cokemaking, iron ore pelletizing and sintering generate about 20% of total CO2 emissions, and blast furnace ironmaking generates about 70%. At present, a blast furnace (BF) of good performance produces about 1300kg of CO2 per tonne of hot metal (kg / thm). The unit kg / thm stands for kilograms per tonne of hot metal, wherein a tonne, also known as metric ton, is equal to 1000kg.

[0003]

[0002] Technically, the steel industry could minimize its CO2 emissions by switching to the so called DRI+EAF Route wherein iron ore is reduced to iron using a Direct Reduction Process and then melted in an Electric Arc Furnace (EAF). However, worldwide certain types of iron ores pellets are not compatible in terms of quality with the DRI+EAF route, and some steel products have specific properties which are not yet reachable with the DRI+EAF route. Therefore, there is a need to reduce the CO2 footprint of the BF-BOF route, and especially of the Blast Furnace process itself.

[0004]

[0003] One of the considered solutions it the injection of natural gas or methane CH4 into the blast furnace. However, injecting natural gas into blast furnace would reduce the raceway adiabatic flame temperature (RAFT) significantly.

[0005]

[0004] Indeed, with the method according to prior art, the injected natural gas, will be cracked inside the raceways of the blast furnace. This reaction is an endothermic reaction (Eq. 1). It consumes heat from the raceways, heat which is generated by carbon combustion with the oxygen from hot blast. In the raceways, the carbon is oxidized by the oxygen to CO according to Equation 2.

[0006] Equation 1

[0007]

[0008] Equation 2

[0009]

[0010] >

[0005] If the effect of the sensible heat from the hot blast is not considered, according to above reactions 1 and 2, the cracking of 1 mole of CH4 in the raceways would consume 0.678 mol of carbon, i.e., the cracking of 1 kg of natural gas would consume 0.508 kg of carbon. Assuming the coke contains 90% carbon, the pyrolysis of 1 kg of natural gas would consume about 0.565 kg of coke in the raceways.

[0011]

[0006] For the blast furnaces with gaseous fuel injection only, the RAFT needs to be 1650 °C at least. For a given hot bast temperature, increasing the natural gas injection rate to a certain level will lead to the increase of the oxygen enrichment of the blast for maintaining the minimum RAFT. Increasing its oxygen content will reduce the sensible heat from the hot blast (air + added oxygen). This reduction of the sensible heat must be compensated by burning more coke in the raceway. Consequently, the coke rate would be increased accordingly which is detrimental to the CO2 footprint of the process.

[0012]

[0007] The aim of the present invention is therefore to remedy the drawbacks of the prior art by providing a method to produce hot metal in a blast furnace wherein coke consumption is reduced and thus is the CO2 footprint of the process.

[0013]

[0008] For this purpose, a first object of the present invention consists in a method to produce hot metal in a blast furnace wherein a natural gas is subjected to a cracking step in a cracking unit to produce a cracked gaseous stream, said cracked gaseous stream comprising at least 20% in volume of hydrogen, less than 80% in volume of unreacted natural gas and a suspension of solid carbon black, and being injected into the blast furnace at the tuyere level, a hot blast being also injected at said tuyere level.

[0014]

[0009] The method according to the invention may also have the optional features listed below, considered individually or in combination:

[0015] the cracked gaseous stream is injected into the blast furnace at a temperature from 25°C to 1500°C,

[0016] the cracking step is performed at a temperature from 900°C to 1800°C,

[0017] the hot blast comprises more than 21 % in volume of 02,

[0018] the hot blast comprises from 21 to 31% in volume of 02,

[0019] the cracked gaseous stream comprises from 20 to 99% in volume of H2, from 1 to 80% of unreacted natural gas, and from 49g / Nm3 of gas to 265 g / Nm3 of gas of solid carbon black,

[0020] after cracking the cracked gaseous stream is cooled down to a temperature below 700°C,

[0021] the cracking step is performed using one technology among molten salt pyrolysis, microwave pyrolysis, molten metal pyrolysis, plasma pyrolysis, catalytic pyrolysis or fluidized bed pyrolysis.

[0022]

[0010] The invention is also related to a plant allowing to produce a method according to any one of the previous embodiments comprising a blast furnace able to produce hot metal and equipped with tuyeres located at a tuyere level able to inject gas within the blast furnace, a cracking unit able to produce a cracked gaseous stream comprising at least 20% in volume of hydrogen, less than 80% in volume of unreacted natural gas and a suspension of solid carbon black, a gas circuit allowing to bring the cracked gaseous stream to the tuyeres of the blastfurnace so as to inject the cracked gaseous stream through said tuyeres.

[0011] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication, and which is in no way restrictive, with reference to the appended figures in which:

[0023] Figure 1 illustrates a blast furnace allowing to implement a method according to the invention,

[0024] Figure 2 illustrates the amount of CO2 in the Blast Furnace top gas according to the rate of gaseous stream injected in the blast furnace for different temperatures of injection, Figure 3 illustrates the 02 enrichment of the blast needed according to rate of gaseous stream injected in the blast furnace for different temperatures of injection.

[0025]

[0012] First, it is noted that on the figures, the same references designate the same elements regardless of the figure on which they feature and regardless of the shape of these elements. Similarly, should elements not be specifically referenced in one of the figures, their references may be easily found by referring to another figure.

[0026]

[0013] It is also noted that the figures represent mainly one embodiment of the object of the invention but other embodiments which correspond to the definition of the invention may exist. Elements in the figures are illustration and may not have been drawn to scale.

[0027]

[0014] Figure 1 illustrates an ironmaking plant allowing to perform a method according to one embodiment of the invention. This plant comprises at least one blast furnace 1 wherein an iron-containing charge 10 such as sintered iron ore, iron ore pellets, lump iron ore is loaded, optionally with fluxes such as limestone or dolomite, in the throat of the blast furnace 1 . A first carbon-based material 11 is also charged, preferably independently from the other raw materials, into the throat of the blast furnace 1. This first-carbon based material 11 may be traditional metallurgical coke and can preferentially be at least partly substituted by a non-fossil-based carbon material such as artificial coke, biomass, biochar, biocoal or waste plastics.

[0028]

[0015] By biochar or biocoal it is meant a charcoal that is produced by pyrolysis of biomass in the absence of oxygen. Biomass is renewable organic material that comes from plants and animals. Biomass sources for energy include notably wood and wood processing wastes — firewood, wood pellets, and wood chips, lumber and furniture mill sawdust and waste, and black liquor from pulp and paper mills, agricultural crops and waste materials — corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues, biogenic materials in municipal solid waste, paper, cotton, and wool products, and food, yard, and wood wastes and animal manure and human sewage.

[0016] The plant also comprises a cracking furnace 6 to allow cracking of natural gas or methane 18 to produce a cracked gaseous stream 31 comprising hydrogen, unreacted natural gas and carbon black. In the rest of the text both terms of natural gas and methane will be used indifferently, as natural gas is a gas comprising methane as main component (more than 97% in volume). Methane cracking is the process in which thermal energy is applied to methane (CH4) to crack the chemical bond between carbon and hydrogen, generating hydrogen gas and a solid carbon product, called carbon black, with no CO2 emissions. A small portion of hydrocarbon remains unreacted depending on the reactor's efficiency and / or of the pyrolysis rate, this is the residual hydrocarbon. Cracking may also be defined as the result of the pyrolysis treatment of alkanes.

[0029]

[0017] The cracked gaseous stream 31 preferably comprises from 18 to 99% by volume of H2, from 1 to 82% of unreacted natural gas and from 49g / Nm3 to 265 g / Nm3 of carbon black.

[0030]

[0018] The cracked gaseous stream 31 is transported and injected into the blast furnace 1 through its tuyeres 2. There is no prior separation of the carbon black from the gaseous components of the cracked gaseous stream 31. The tuyeres also allow to inject a hot blast 13. The first carbon-based material 11 and the cracked gaseous stream 31 are oxidized by the oxygen from the hot blast in the raceways 17. The hot blast consists mainly of air, optionally enriched in oxygen. In a preferred embodiment, the hot blast comprises from 21 to 31% in volume of 02.

[0031]

[0019] For clarity sake, reference 2 designate both the tuyeres and the level of injection of the gaseous stream through said tuyeres. It is further noted that even if only two tuyeres 2 are represented in the figure, it is only for illustration purposes and that there are preferentially several tuyeres 2 located at the same level around the whole circumference of the blast furnace 1 and injecting the same gaseous stream.

[0032]

[0020] The oxidation of the first carbon-based material 11 and of the carbon black from the injected cracked gaseous stream 31, together with the H2, also from the cracked gaseous stream 31, generates hot reducing gas that consists of CO, H2 and N2 as well as some impurities vaporized from the iron ore, carbon-based reductants, and fluxes. This hot reducing gas flows upward to reduce the iron ores into iron and to heat up all the materials that are charged into the blast furnace from its top. After the heating and reduction, the reducing gas becomes top gas 15 and exits trough the top of the blast furnace.

[0033]

[0021] According to the invention, the carbon black is not removed from the cracked gaseous stream 31 before its injection into the blast furnace 1. This allows to advantageously replace the pulverized coal usually injected together with the hot blast 13 so as to further reduce the carbon footprint.

[0022] After cracking of the natural gas, the cracked gaseous stream 31 is preferably cooled down quickly at temperature below 700°C, or even preferably to ambient temperature. This prevents the recombination of C and H2 back to methane. This step is required notably when the cracking unit 6 is located outside of the ironmaking plant which implies the transportation of the cracked gaseous stream over a long distance.

[0034]

[0023] The cracked gaseous stream 31 may be reheated before its injection into the blast furnace. This reheating is preferably performed using electrically powered heating device, using even more preferably green electricity.

[0035]

[0024] In the hearth of the blast furnace 1 there is also a deadman 3. The iron ores together with the fluxes form molten slag 14 and hot metal 12 (liquid iron). The slag 14 and hot metal 12 are collected in the hearth of the blast furnace, then drained out through the taphole 14.

[0036]

[0025] As shown in Figure 1 , in this invention, the natural gas is cracked in a cracking furnace 6 that is located outside the ironmaking blast furnace 1. The cracking temperature is preferably from 900°C to 1800°C and varies according to the technology used for the cracking and to the flow rate of natural gas 18 to be cracking. The cracking step may be done using any kind of known technologies such as molten salt pyrolysis, microwave pyrolysis, molten metal pyrolysis, plasma pyrolysis, catalytic pyrolysis or fluidized bed pyrolysis. In the figure the cracking unit 6 is represented near the blast furnace 1 but it could be located further away in the plant or event in another distant location and the cracked gaseous transported by appropriate means to be injected into the blast furnace 1.

[0037]

[0026] In the prior art natural gas is injected into the blast furnaces without pre-cracking. As explained in the preamble, that method limits the flow rate of natural gas injection per ton of hot metal due to the lower-limit of the adiabatic raceway flame temperature (RAFT).

[0038]

[0027] With the method according to the invention there is no longer a RAFT lower-limit.

[0039] Example 1 - Influence of temperature

[0040]

[0028] Figure 2 illustrates the CO2 content in the top gas as a function of the flow rate of cracked gaseous stream injected into the blast furnace using a a method according to the invention wherein a 100% methane gas was cracked at a cracking rate of 95% in volume and the resulting cracked gaseous stream was injected through the tuyeres of the blast furnace at different temperatures of injection, respectively at 25°C, 900°C, and 1200°C. The cracked gaseous stream contains 95% in volume of H2 and 5% in volume of unreacted methane and contains also solid carbon black.

[0041]

[0029] Calculations of CO2 content in the top gas were done using heat and mass balance models.

[0030] Chemical compositions of raw materials loaded in the blast furnace considered for these calculations are indicated in below tables 1 and 2.

[0042] Table 1. The Chemical Compositions of Iron Ore Pellet and Sinter

[0043]

[0044]

[0031] Other includes, the water content, the binder used in the manufacturing process but also the elements constituting the gangue of the iron ore which are not considered in the rest of the compositions.

[0045] Table 2. The Chemical Compositions of Coke

[0046]

[0047]

[0032] In the case studies, the raceway adiabatic flame temperature (RAFT) is maintained at 1700 °C. The RAFT is a commonly known parameter for the person skilled in the art of blast furnace operations. The method of heat and masse balance, as well as the definition of RAFT can notably be found in the following reference--J.G. Peacey and W.G. Davenport, “The Iron Blast Furnace - Theory and Practice”, Pergamon Press, ISBN 0-08-023218-3, 1979, p 45-211.

[0048]

[0033] Figure 2 clearly illustrates that the CO2 content of the gas decreases as soon as cracked methane gas is injected into the blast furnace. This decrease is faster when the temperature of injection of the cracked gaseous stream increases.

[0049] Example 2 - Influence on 02 enrichment of the blast

[0050]

[0034] Figure 3 shows the oxygen enrichments that are needed to maintain the RAFT at 1700°C when using a method according to prior art without cracking of the natural gas before injection and with a method according to the invention with injection of the cracked gaseous stream resulting from the cracking of natural gas injected at different temperatures.

[0051]

[0035] Same models and raw materials compositions as for first example were used for the method according to the invention.

[0052]

[0036] In the case of direct natural gas injection according to the prior art, oxygen enrichment has to be started when the natural gas injection rate is beyond 111.0 kg / thm; in the case of injecting cracked natural gas at 25°C according to the invention, the oxygen enrichment is only needed when its rate is greater than 144.0 kg / thm. When injecting the cracked natural gas with a temperature of 900°C or 1200°C, oxygen enrichment would not be needed.

[0053]

[0037] This illustrates that with the method according to the invention less heat from the RAFT is consumed and there is thus less need to inject 02 and to add carbon to maintain the required RAFT temperature.

[0054] Example 3 - CO2 savings

[0055]

[0038] Calculations were done to determine CO2 emissions of blast furnaces operated with a method according to the invention. Temperature of injection of the cracked gaseous stream 31 is varied but also the coke rate. Indeed, depending on the size of the blast furnace there might be a different required minimum coke rate to maintain the permeability of the blast furnace. Gas to be cracked is a 100% methane gas.

[0056]

[0039] The savings in coke consumption are based on comparison with a classical blast furnace with injection of hot blast and pulverized coal through tuyeres, charging of coke in the throat of the blast furnace.

[0057]

[0040] Same models and raw materials compositions as for first example were used. In addition, chemical composition considered for the pulverized coal is indicated in Table 3.

[0058] Table 3. The Chemical Compositions of Injection Coal

[0059]

[0060]

[0041] Table 4 shows considered classical blast furnace operations and its specific CO2 emission from its top gas. The typical specific CO2 emission is 1345.6 kg per tonne of hot metal.

[0061] Table 4. Reference - Prior art classical Blast Furnace Operations and Its Specific CO2 Emission

[0062]

[0063]

[0042] Table 5 summarizes the percentages of reduction of CO2 emissions when using methods according to the invention when compared with a classical prior art blast furnace.

[0064] Table 5. CO2 emissions reductions with methods according to the invention

[0065]

[0066]

[0043] With this invention, and considering an injection temperature of the cracked gaseous stream in the blast furnace of 1200°C, the specific CO2 emission of hot metal production in a blast furnace could be reduced by 26.2% if the bottom limit of coke rate was set to 250 kg / thm, by 32.4% if the bottom limit of the coke rate was set to 200 kg / thm, and by 38.6% if the bottom limit of coke rate was set to 150 kg / thm.

Claims

CLAIMS1 ) Method to produce hot metal (12) in a blast furnace (1 ) wherein a natural gas (18) is subjected to a cracking step in a cracking unit (6) to produce a cracked gaseous stream (31 ), said cracked gaseous stream (31 ) comprising at least 20% in volume of hydrogen, less than 80% in volume of unreacted natural gas and a suspension of solid carbon black, and being injected into the blast furnace at the tuyere level (2), a hot blast (13) being also injected at said tuyere level (2).2) Method to produce hot metal according to claim 1 wherein the cracked gaseous stream (31 ) is injected into the blast furnace (1 ) at a temperature from 25°C to 1500°C.3) Method to produce hot metal according to any one of claims 1 or 2 wherein the cracking step is performed at a temperature from 900°C to 1800°C.4) Method to produce hot metal according to any one of claim 1 to 3 wherein the hot blast (13) comprises more than 21 % in volume of 02.5) Method to produce hot metal according to claim 4 wherein the hot blast (13) comprises from 21 to 31 % in volume of 02.6) Method to produce hot metal according to any one of the claims 1 to 5 wherein the cracked gaseous stream (31 ) comprises from 20 to 99% in volume of H2, from 1 to 80% of unreacted natural gas, and from 49g / Nm3 of gas to 265 g / Nm3 of gas of solid carbon black.7) Method to produced hot metal according to any one of the claims 1 to 6 wherein after cracking the cracked gaseous stream 31 is cooled down to a temperature below 700°C8) Method according to any one of the claims 1 to 7 wherein the cracking step is performed using one technology among molten salt pyrolysis, microwave pyrolysis, molten metal pyrolysis, plasma pyrolysis, catalytic pyrolysis or fluidized bed pyrolysis.9) Plant allowing to produce a method according to any one of the claims 1 to 8 comprising:a. A blast furnace (1 ) able to produce hot metal (12) and equipped with tuyeres located at a tuyere level (2) able to inject gas within the blast furnace (1 ),b. A cracking unit (6) able to produce a cracked gaseous stream (31 ) comprising at least 20% in volume of hydrogen, less than 80% in volume of unreacted natural gas and a suspension of solid carbon black,c. A gas circuit allowing to bring the cracked gaseous stream (31 ) to the tuyeres of the blast furnace so as to inject the cracked gaseous stream (31 ) through said tuyeres.

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