FR2848123A1 - Recuperation of blast furnace gas with carbon dioxide and nitrogen purification to produce an enriched gas for recycling to the blast furnace or for export to other gas users - Google Patents

Abstract

The recuperation of blast furnace gas, in which the gases from the blast furnace are recovered, the carbon dioxide present in the gas is at least partially withdrawn during a first stage of carbon dioxide purification in a manner to create a blast furnace gas poor in carbon dioxide, the gas being either re-injected into the blast furnace, or recuperated and sent towards another point of use, after purification. After this first stage of carbon dioxide purification, it is provided with a second stage of purification of a substantial quantity of the nitrogen contained in the gas, in a manner to recuperate an enriched gas, before re-injection or recuperation to another point of use, for which the nitrogen content is less than 10 % by volume and preferably less than 5 % vol.

Description

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Process for recovering blast furnace gas and its use for the production of pig iron
The present invention relates to a process for recovering blast furnace gas, in which the gases from the blast furnace are recovered, the carbon dioxide present in the gas is at least partially removed during a first stage of purification of CO2 , so as to create a blast furnace gas poor in carbon dioxide, said gas being either reinjected into the blast furnace, or recovered and sent to another point of use. It also relates to the use of this purified blast furnace gas for the manufacture of pig iron in the blast furnace.

 Today, steel fabrication is predominantly carried out by the so-called integrated die, the heart of which is the blast furnace, in which cast iron is first manufactured, the carbon content of which will be subsequently reduced by operations of injection of oxygen, in particular and of various additives in order to produce steel of different grades. At the level of the blast furnace, the process is said to be open: hot air, possibly enriched with oxygen, is injected at the base of the blast furnace in order to generate a reducing gas necessary for the reduction of iron and to bring the oxidizer necessary for melting the metal. The gas generated, called blast furnace gas (abbreviated as GHF) after passing through the blast furnace is exported and is used partially in the various workshops of the steelworks as a source of energy and / or heat for example to heat the recuperators (called hot stoves in English) heat exchangers, responsible for preheating the air (cold wind) which is then injected at the base of the blast furnace (hot wind), or partially sent to an energy power station.

 As with any industrial process, the skilled person is constantly confronted with the problems of increasing the production and productivity of the blast furnace, while the legislation requires him to reduce carbon dioxide emissions, such emissions being harmful for the environment.

 It is known that the increase in production and the reduction in costs have been achieved by injecting auxiliary fuels into the fuel injection nozzles, in particular coal in the form of pulverized powder, injected using injection. under air pressure (process known as PCI: Pulverized Coal Injection), as well as over-oxygenation of the wind, that is to say by injecting pure oxygen and / or oxygen-enriched air into the air cold so as to obtain a cold wind containing more than 21% vol. oxygen. The injection of this pulverized coal allows the reduction of the quantity of coke necessary and therefore substantial savings as well as the reduction of C02 emissions at the coking plant.

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 Injecting coal or other types of fuel reduces CO2 emissions by lowering CO and CO2 concentrations and increasing the concentration of hydrogen. The reduction in coke consumption results in a reduction in coke oven gases and all the pollutants associated with this production.

 The rapid successes obtained with the injection of coal have led the skilled person to increase the injection rates of coal. To achieve high injection rates, high wind oxygenation is required, or even a wind comprising substantially 100% vol. oxygen. Above 60% vol. around oxygen in the wind, the quantity of gas formed in the wind injection nozzles decreases so much that it becomes insufficient to maintain the upper zone of the blast furnace at temperature. In order to circumvent this difficulty, a person skilled in the art has suggested compensating for the quantity of missing heat by injecting hot gases into the blast furnace using auxiliary nozzles, which in particular allows the use of pure oxygen at the place of the wind of air or of oxygenated air.

 The process described in DE-A-2261766 and in the article by F. FINK entitled The floss furnace process: a new process to smelt iron in blast furnace @ ist European lronmaking Congress - Aachen, 1986, VDEh, PV / 3, consists compressing the blast furnace gas, removing the carbon dioxide present therein and possibly recycling all or part of the gas thus purified at two different levels of the blast furnace, adding to this gas pure or substantially pure oxygen and a fuel.

 It is known from the article Heat and Mass Balance of Oxygen enriched and Nitrogen Free Blast Furnace Operations with Coal Injection, Dongke Ma, Linutan Kong, WK LU, 1988, Ironmaking Conférence Procedings pp 595-609, a process in which the blast furnace gas after CO2 purification, then it is recycled (at least partially) after heating by injecting it halfway up the blast furnace, another part of blast furnace gas, generally not purified of CO2, being recycled after injection into it of fuel and substantially pure oxygen (more than about 90% oxygen).

 He is also known from the article The New Blast Furnace Technique with Ultra High Rate of Coal Injection Qin Mensheng, Yang Yong Yi, 1st International Symposium on Ironmaking, Sept. 1985 and the article The Full Oxygen Blast Fuanece (FOBP) Process by Qin Mensheng, Qi Baoming, Proceedings of the Sixth international Iron and Steel Congress 1990, Nagoya, ISIJ, pp 589-595, a process which consists of compressing and then purifying the blast furnace gas into CO2, to recycle it the base of the blast furnace, after adding oxygen and fuel.

 In general, the double injection of air possibly enriched in oxygen (one at the base, the other halfway up the blast furnace), makes it possible to increase the

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 blast furnace productivity. A significant part of the gases no longer passing through the melting zone which is the bottleneck of the counter-current gas and liquid flows, this makes it possible to increase the liquid flow rates, and therefore the production of pig iron.

 All of these processes have in common the recycling of blast furnace gas after a first CO 2 purification step and operation with pure oxygen. None of the processes described above have yet reached an industrial stage, the modifications to be made to the blast furnace process are complex, expensive, risky financially and require extensive experimentation.

 The methods known at the present time therefore do not make it possible to compensate for the reduction in the gas flow, due to the enrichment of wind with oxygen, said reduction causing an insufficient temperature in the upper area of the furnace: it arises So today the problem of improving the blast furnace process by enriching the hot wind even more with oxygen while maintaining a sufficiently high temperature in the upper part of the blast furnace, a simple process to install in the blast furnace, which does not does not affect the operation of the blast furnace.

 The invention makes it possible to solve the problem posed.

 It consists, after the first stage of purification of carbon dioxide, to provide for a stage of purification of nitrogen from the blast furnace gas so as to remove a substantial quantity of nitrogen and deliver a gas comprising essentially carbon monoxide and at most 10% vol., preferably at most 5% vol. nitrogen. This gas can also contain a small amount of hydrogen and very small amounts of water and / or residual CO2.

 Preferably, the initial step of removing the CO2 is carried out by washing with amines (mono, di or methyldi-ethanol amine, preferably).

 According to the technology used in the subsequent step according to the invention of separation of nitrogen and carbon monoxide, in particular when the latter is carried out by cryogenic separation, a purification is preferably carried out generally by adsorption to remove moisture. residual as well as the residual carbon dioxide after washing with amines, so as to bring this concentration preferably below a value of the order of 50 ppm more preferably below a value of 10 ppm.

 The blast furnace with recycling of blast furnace gas and N2-C02 purification according to the invention is an intermediate solution between the conventional, open blast furnace and the all-oxygen blast furnace with recycling and CO2 purification.

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 According to the invention, the blast furnace continues to be supplied with hot wind, possibly enriched, and possibly with an injection of auxiliary fuels.

 The blast furnace gas which escapes from the blast furnace is notably charged with C02 and nitrogen. In fact, it has been found that the simple purification of CO2 from this blast furnace gas does not make it possible to obtain a gas which is sufficiently rich and leads to an excessively large inert nitrogen transport which is harmful to the reactions of the blast furnace. By substantially eliminating the CO2, preferably completely and by conserving at most only 10% vol of nitrogen, preferably less than 5% vol. nitrogen, a rich fuel is obtained which can be reinjected into the blast furnace and / or exported to other points of use.

This solution has the following advantages: allowing wind enrichment levels higher than the current level while maintaining a gas flow rate sufficient for heat exchange in the blast furnace; limit the changes in the operation of the blast furnace to the reinjection of a rich gas and allow a gradual transition from the conventional operation of the blast furnace to operation with recycling according to the invention; in the case of a blast furnace without auxiliary fuel injection equipment, the injection of rich gas from recycling offers an alternative solution to conventional techniques (injection of natural gas, coal
PCI) without jeopardizing the entire operation of the blast furnace; limit the quantities of blast furnace gas to be exported to another point of use. In the case of an increase in the capacity of the blast furnace, recycling makes it possible, for example, to avoid investing in an energy plant necessary to process the excess of exported gas; - supply a rich gas which can be used in the rest of the steelworks, in particular when natural gas (or other fuel) is not available on site or if there is no coking plant gas available (closed coking plant) or non-existent).

 The gas from a conventional blast furnace typically contains 23% CO, 22% CO2, 3% H2 and 52% N2. Depending on the oxygen enrichment chosen and the fuel injection, the blast furnace gas used according to the invention will preferably comprise from 20 to 40% vol. C02, from 10 to 55% vol. nitrogen, the balance being CO and a little hydrogen to recover.

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According to the invention, the treatment of recycled gas or preferably used on site, is done in two phases:
The first phase consists in totally or partially eliminating the C02 in order to obtain a gas essentially composed of CO and nitrogen (and possibly H2). According to a variant of the invention, part of this gas (containing nitrogen) can be reheated and re-shipped in the blast furnace in the upper injection of the blast furnace (to achieve the necessary thermal equilibria explained above).

 The second phase is a total or partial elimination of N2 in order to obtain a gas composed essentially of CO. The residual nitrogen content is then generally less than 10% vol., Preferably less than 5% vol. All or part of this rich gas is returned to the blast furnace either like any other auxiliary fuel, at the level of the blast furnace tank, or by a double injection system in order to satisfy the thermal needs of the upper blast furnace area .

 For example, for a blast furnace supplying a gas of conventional composition (23% CO, 20% CO2, 1% H2, 1% H2O, 55% N2), recycling will be carried out as follows: - compression of blast furnace at 5 or 6 bars; purifying the CO2 using a process of the amine washing type, in order to obtain a gas containing less than 600 ppm of CO 2; passage over a purification at the head to completely eliminate the C02 (preferably less than 10 ppm; cryogenic separation of nitrogen and CO; the cryogenic column is kept cold by a nitrogen cycle compressor.

 The gas obtained is CO having a content of 3 to 4% vol. in nitrogen.

 The invention will be better understood with the aid of the following exemplary embodiments given without limitation, together with the figures which represent: - Figure 1, the process for manufacturing blast furnace using thermal energy blast furnace gas to heat the air entering the base of the blast furnace; - Figure 2, the process of Figure 1 modified by injection of pulverized coal at the base of the blast furnace; - Figure 3, the method described in DE-A-2 261 766 and in the article by F. Fink; - Figure 4, the so-called FOBF process described in the articles by Qin Mensheng et al. ; Figure 5, the process described by W-K. Lu et al. ; - Figure 6, the method according to the invention with recycling and purification N2 / C02;

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 - Fig 7, the method according to the invention with recycling and purification N2 / C02 in a first variant of double injection; - Figure 8, the method according to the invention with recycling and purification N2 / C02 in a second variant of double injection; - Figure 9, the method according to the invention with production of rich gas (energy); - Figure 10, an embodiment of a purification of CO2 using an amine wash; - Figure 11, an example of production of gas rich in CO after cryogenic separation N2 / CO.

 In Figure 1 is shown a bottom furnace manufacturing process with the use of thermal energy from the blast furnace gas to heat the air entering the base thereof. The blast furnace 4 is supplied with coke and agglomerated by line 5 at point 8. The blast furnace gas is withdrawn at 7 via line 9 on which line 11 allows direct use of the top gas - furnace in another part of the steel manufacturing plant and via line 10 allows all or part of this blast furnace gas to be recycled to heat exchanger 2 (called hot stoves in English) -saxon) the cold wind, that is to say the cold air 1 is heated in this exchanger to an adequate temperature and is sent to the base of the blast furnace as hot wind by the pipe 3 to point 6.

 In the other figures, the same elements bearing the same references have the same meaning.

 In FIG. 2, we find the method and device of FIG. 1 in which the injection of PCI is provided at point 14 via line 13 and the over-oxygenation in 12 of the cold wind 1 before preheating in the exchanger 2.

 FIG. 3 schematically describes the method described in DE-A-22611766 and in the article by F. Fink. The blast furnace gas taken from line 9 is sent to a compressor 20 so as to bring the blast furnace gas to a sufficiently high pressure to then be purified of carbon dioxide on the unit 21 so as to separate d 'firstly the carbon dioxide taken at 22 and the rest of the blast furnace gas which leaves this purification unit by line 23 and can either be used via line 11 in another device on the site or else be recycled wholly or partially in the blast furnace, either at the base thereof via the line 24 injected at the low point of the blast furnace or after possible injection of oxygen at 25 and fuel at 26 and / or in a higher part of the blast furnace via the pipe 27 at point 31 after injection of oxygen at 28 and / or fuel at 29.

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 FIG. 4 succinctly describes the so-called FOBF method described in the articles by Qin Mensheng et al. The blast furnace gas from line 9 is partially compressed by the compressor 20 in order to be purified into CO2 to 21, this being recovered in 22, the purified gas via 23 being preheated in the device 31 so as to be reinjected hot halfway up the blast furnace via line 32 at point 33. A portion of blast furnace gas from line 29 can be compressed by compressor 30 to be sent via line 34 directly to the base of the blast furnace at point 37 after possible injection of oxygen at 35 and / or fuel at 36.

 FIG. 5 succinctly represents the method described by W. K. Lu et al. In this figure, the blast furnace gas 9 is wholly or partially compressed in the compressor 20 purified of CO2 in the device 21, the CO2 being recovered at 22, the purified gas at 23 being directly recycled to the base of the blast furnace at point 40 after injection of oxygen in 41 and / or of fuel in 42.

 FIG. 6 and the following represent different variant embodiments of the invention.

 In Figure 6 is shown the process according to the invention with recycling and purification of CO2 and nitrogen. At point A of line 9 in which the blast furnace gas circulates, part of it is withdrawn to be compressed in the compressor 50 purified of CO2 in the device 51, the carbon dioxide being recovered at 52, the purified gas being sent via 53 into a nitrogen purification unit 54 from which there emerges at 55 residual nitrogen (that is to say impure nitrogen) and by line 46 of the rich gas (that is ie a gas containing essentially carbon monoxide and possibly a little hydrogen and water vapor). This rich gas is either used via 57 at various points in the steel manufacturing plant or via line 58 recycled in the process by being added to the hot wind at 59, this being injected at the base of the blast furnace in 64 after possibly the addition of oxygen in 60. The hot wind comes from the exchanger 61 heated by part of the blast furnace gas, as before, while the cold wind 62 can possibly be oxygenated by pure oxygen 63.

 In FIG. 7, there is another alternative embodiment of the invention with recycling and purification of CO 2 / nitrogen in a first variant of double injection into the blast furnace. The gas leaving 56 from the nitrogen purification is either recovered as a rich gas via 57 or via 58 returned on the one hand through the heater 70 to the injection point 71 at mid-height of the top - furnace either via the pipe 69 mixed with the hot wind at 59 to be injected at 73 at the base of the blast furnace after possible addition of oxygen at 72.

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FIG. 8 represents the process according to the invention similar to that of FIG. 7 but with a second variant of double injection. According to this variant, the blast furnace gas purified in C02 but still containing nitrogen is taken off on line 53 and via the pipe 82 and the heater 83 injected at 84 into the blast furnace so provide a sufficient volume of gas as explained above. A second part of the blast furnace gas purified in carbon dioxide is sent via 81 in the nitrogen purification 54 so as on the one hand to recover residual nitrogen at 55 and on the other hand a gas rich in 56 which on the one hand, is used as a rich gas via line 57 in another device of the steel plant, either via the line 85 mixed with the hot wind coming from the exchanger
61 to be injected at the base of the blast furnace 73 after optionally an additional injection of fuel and / or oxygen at 72. As before, the exchanger 61 is heated using blast furnace gases via line 64 .

 FIG. 9 represents an alternative embodiment of the invention in which the blast furnace gas is essentially recovered in order to produce gas rich in CO and hydrogen (rich in energy) so as to be stored or used in another device for the steel plant, part of the blast furnace gas however being via the pipe 64 used in the exchanger 61 as described above. The other part of the blast furnace gas is therefore via the compressor 50 purified of carbon dioxide in the unit 51 and then purified of nitrogen in the unit 54 so as to obtain a rich gas in the pipe 57.

 FIG. 10 shows an exemplary embodiment of the step of eliminating the CO2 present in the blast furnace gas using an amine wash (process well known per se by those skilled in the art for removing CO2 from a gas mixture by the action of mono-ethanol-amine (MEA), diethanolamine (DEA) and / or methyl-diethanol-amine (MDEA)).

 The blast furnace gas is introduced at A into compressor 20.

 The absorption of C02 in the amine solution is carried out using two columns 22 and 45, column 22 being used for the adsorption of CO2 and of the gaseous mixture rising against the current of the amine solution descending, column 45 being used to regenerate the amine used and to provide the necessary supplement.

 The process in the adsorption column uses countercurrent circulation to achieve an optimum mixture of the gas and the amine. An amino solution poor in acid is introduced at 24 into column 22, through line 29 connected to pump 30 which gives the necessary pressure to the liquid. The liquid descends in rain in the column through the adsorbents 46. The blast furnace gas enters the lower part of the

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 column 22 at point 23 (base of the adsorbents) after compression in 20 and via the line 21. The gas rises through the adsorbent 46 and the CO2 reacts with the amine solution on the adsorbent lining 46. Taking into account the temperature of the gas, part of the water and of the amines vaporize: washing with water makes it possible to recondense these gases so as to minimize the consumption of solvent. The amino acid-rich solution (C02) is recovered from the bottom of the tank 26 of the column 22 and via the valve 27 and the pipe 28 injected at about halfway up the column 45, at point 30. The pipe 28 passes through a heat exchanger 47 in order to raise the temperature of this acid-rich solution before it penetrates into the stripping column 45 in order to minimize the energy required for this stripping, during which the acid gas (i.e. CO2) is separated and escapes to the top of the column via the pipe 44, after having passed through a cooling zone 43 supplied with cooling water 42. The amino solution poor in CO2 accumulates in the tank of the column 45 and returns to the reboiler 48 where it is heated by steam 36, vaporizes and returns to the column 45 at 34, where it rises to the top of the said column, against the current of the solution rich in CO2. The CO2 entrained by this vapor upwards (CO2 vaporized by heat exchange) is cooled and the vapor of amino solution is condensed (in contact with 43) and returns to the bottom of the column in the form of liquid in order to thus wash the trays 49 of the stripping column 45. The hot solution containing very little C02 contained in the tank 45 returns via 31 and 29, through the exchanger 47 (cited above), to the pump 30, this clean and cold solution being sent by the latter towards the top 24 of the column 22. An additional amine supply system is provided at 40, the vapor 41 mixed in the tank 39, and sent by means of the pump 50 and the pipe 38 in the tank from 45 to point 37.

 In Figure 11 is shown an example of cryogenic separation of nitrogen and CO.

 The purpose of this separation is to remove nitrogen from a mixture essentially composed of CO, nitrogen and a little hydrogen. The wet mixture N2 CO H2 to be treated (B in FIG. 11) comes from the MDEA unit described in FIG. 10 in which the C02 initially contained in the gas has been eliminated.

 The gas to be treated is first sent via line 25 on a so-called conventional overhead purification in order to eliminate the humidity of the gas and the traces of CO 2. For this, there are two bottles 106,107 containing the usual absorbents used to remove H20 and CO2, these two bottles alternately purifying the gas from 25 which is sent via 108 into the exchanger 116 where it is cooled. When a bottle 106 or 107 is not used, it is regenerated by residual nitrogen from column 121 via the line 125 and the purge gas is released into the open air at 150. The lines in

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 dashed lines 151,152, 153,154 represent the alternative of using the bottle 107 in purification and the regeneration of the bottle 106.

 Once dried, the gas to be separated is sent via 108 to the main exchanger 116 where it is cooled to cryogenic temperatures by heat exchange with the cold pure CO gas in line 130, the cold nitrogen gas waste in line 125 and cold nitrogen in the cycle via 115 passing in the opposite direction in exchanger 116.

 The gas to be separated enters 125 into the cryogenic distillation column 121 in the middle of the column. In the column, the rising gas becomes progressively poorer in CO.

At the top of the column, the gas now contains only nitrogen (residual nitrogen) and hydrogen. The waste gas via 125 passes through the main exchanger 116 and heats up to a temperature close to the temperature of the gas entering via 108 in the separation unit. All or part of the waste gas is used via 109 for the regeneration of bottles 106 and 107 of the overhead treatment. According to a variant of the invention, it is possible to recover the hydrogen from the waste gas, the treatment of the waste gas can possibly be carried out at the outlet of the main exchanger (C in the figure) by installing, for example, a membrane-based unit. , in which the hydrogen in the condenser 123 will generally pass through the membrane (permeate) while the nitrogen will not pass through it (not permeate).

At the top of the separation column 121, part of the waste gas is recondensed to ensure the reflux of the distillation column 121. This condensation takes place by exchange with liquid nitrogen coming from the nitrogen cycle at 124 via 141. The two valves 138 and 139 connected at 140 allowing the expansion of the gas and its liquefaction
In the tank 129 of the distillation column 121, the liquid essentially contains CO. Part of this liquid is heated in the exchanger 142 by cycle nitrogen passing through 117. The CO thus vaporized returns via 143 in the tank 129 and allows fractional distillation at the bottom of the column. The nitrogen content in the CO bath is typically between 0 and 10% vol. Part of the liquid CO, corresponding to the production requested by the customer in its factory, is sent to the main exchanger via 130 for vaporization and leaves at a temperature close to the temperature of the N2 / CO / H2 gas entering via 108.

 A nitrogen cycle is used to keep the cryogenic unit cold and provide the liquid necessary for the condensation at 123 of the gas at the head of the distillation column 121. The cycle nitrogen is compressed in a compressor 111 and cooled via 113 in l 'main exchanger. A first part via 114 is sent to the expansion turbine 117 in order to ensure the refrigeration production, heated via 118 in the exchanger

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 main and returned via 112 to the suction of the cycle compressor. A second part via 115 is expanded isenthalpically, liquefies and ensures via 115 the supply of liquid nitrogen necessary for the condensation at the head of distillation column 121. A third part via 117 ensures the supply of energy necessary in 142 for reboiling the Tank CO before being also expanded via 138 and liquefied. The liquid nitrogen used at the head of the column for the condensation of the waste gas is vaporized by heat exchange. The nitrogen gas via 122,120 is sent to the main exchanger to be reheated before returning to the suction of the cycle compressor 111.

Claims (4)

1. Process for recovering blast furnace gas, in which the gases from the blast furnace are recovered, the carbon dioxide present in the gas is at least partially removed during a first stage of purification of C02, so as to create a blast furnace gas poor in carbon dioxide, said gas being either reinjected into the blast furnace, or recovered and sent to another point of use, after purification, characterized in that after the first step for purifying carbon dioxide, a second step is provided for purifying a substantial amount of the nitrogen contained in said gas, so as to recover a rich gas, before reinjection or recovery to another point of use , whose nitrogen concentration is less than 10% vol. preferably less than 5% vol.  2. Method according to claim 1, characterized in that the nitrogen purification of the gas is carried out using a cryogenic distillation column, an additional CO2 purification step being provided to bring the concentration of C02 gas at a value less than or equal to 50 ppm, preferably 10 ppm.  3. Method according to claim 2, characterized in that the additional step is a step called purification at the head of the gas, in which the residual CO 2 and / or the water vapor present in the gas are substantially eliminated above all cooling of said gas.  4. Use of the method according to one of claims 1 to 3, for the manufacture of cast iron by recycling all or part of the purified gas into CO2 and / or nitrogen in the blast furnace.