The invention relates to a method of producing pig iron in a shaft furnace which is charged in an upper region of the shaft furnace with raw materials which fall within the shaft furnace under the influence of gravity, wherein a portion of the raw materials is melted and/or at least partly reduced under the action of the atmosphere that exists within the shaft furnace, and a hot gas stream which is introduced in a lower region of the shaft furnace flows through, especially in countercurrent, and influences the atmosphere that exists within the shaft furnace in terms of chemical composition and/or temperature, wherein a cold gas stream, especially upstream of the shaft furnace process, is fed to at least one heat exchanger in which the cold gas stream is heated to a temperature higher than 700° C. to give a hot gas stream.
The production of pig iron in shaft furnaces, for example in blast furnaces, that are operated in a sustained manner is globally the most common and standard method, by which far more than 80% of the global demand for pig iron is produced. Conventionally, in the reduction method, the upper region of the shaft furnace is charged via what is called the furnace top with raw materials, known as the burden, comprising iron ore and optionally lime, coke and/or coal and, if required, further additives or other oxidic or metallic starting materials/feedstocks. By virtue of gravity and batchwise charging, especially as a result of discontinuous tapping and hence removal of the liquid substances, the raw materials drop down. By virtue of the chemical composition of the atmosphere (reducing condition), and as a result of the temperature increasing in the direction of the lower region, the iron ore is reduced. In the lower region of the shaft furnace, especially in the region of introduction of hot gas streams, called hot blast, and optionally further additives, such as carbon and/or oxygen, the temperature is such as to cause the iron ore or reduced iron to melt, and different liquid phases are established in what is called the hearth region, which collects at different levels into liquid iron and a molten slag that covers the liquid iron on account of its lower density. In particular, the liquid phases can each be drawn off via different discharges/tapping points, and these may be sent to further processes.
Conventionally, the hot blast, also called hot air, is ambient air which is sucked in as a cold gas stream, called cold blast, via a starting bar, compressed to a defined pressure and fed to at least one blast heater in which the compressed ambient air (cold blast) is heated to a temperature of at least 700° C., and this is then introduced or blown into the blast furnace via what are called blast tuyeres in the lower region of the shaft furnace. Heat exchangers in the form of blast heaters, also called Cowper stoves, are alternately flooded with a hot gas, where the hot gas remains in the blast heater until a defined temperature is attained within the blast heater, then the hot gas is removed and flooded with compressed ambient air (cold blast) and remains in the blast heater until a predefined temperature, generally between 700 and 1400° C., has been attained, such that the hot ambient air is then drawn off as hot blast (hot blast stream) and fed to the blast furnace. An economically viable method of charging the blast heater with hot gas that has become established is the utilization of the waste heat stream which is drawn off as what is called top gas from the furnace top, which is reincinerated with further gases (air/natural gas) in the blast heater. In order to maintain sustained operation and depending on the size of the blast furnace, at least two, especially at least three, blast heaters are connected in parallel and can be appropriately switched on and off, such that, for example, one of the blast heaters is being flooded with hot gas and utilized for heating of the blast furnace with release of heat, another blast heater is being flooded with cold blast and heated with absorption of heat to give hot blast, and a third blast heater is operational and is supplying the blast furnace with hot blast. In principle, it is also possible to use two blast heaters that are flooded in alternating operation.
The carbon-based production of pig iron gives rise to an enormous CO2 output which pollutes the environment in that increasing the CO2 boosts the greenhouse effect in the atmosphere and hence adversely affects the climate. For many years, operators of shaft furnaces have been contemplating how this output can be reduced. Exacerbated by national/international stipulations associated with CO2 constraints or certificates, the operators of these plants are being forced to take measures that still permit operation of these plants under particular prerequisites. Economic operation should also still be ensured.
The prior art describes numerous approaches for achieving a positive and reducing effect on CO2 output by means of controlled addition of media (solid, liquid and gaseous) to a shaft furnace. By way of example, publications CN 101871026 A1 and WO 2019/057930 A1 are mentioned.
As well as CO2, the conventional shaft furnace process also results in other, in particular “harmful”, gas compounds and liquids within a flow of matter that can have adverse effects not just in the shaft furnace process but also in downstream applications. The use of air (cold gas stream, cold blast), which consists of nitrogen to an extent of about 79% by volume and is used as hot gas stream or hot blast for blowing into the blast furnace, depending on temperature and pressure, contributes to the formation of nitrogen oxide emissions (NOx), hydrogen cyanide (HCN) and potassium cyanide compounds in the atmosphere of the shaft furnace and in the top gas, optionally in the blast heater. Nitrogen is additionally inert, and its presence does not result in optimization in the process. Instead, nitrogen contributes, for example, to a reduced calorific value in the top gas and affects the Wobbe index (WI). Efficiency in the blast furnace process (including up- and downstream) is nonoptimal as a result of the presence of nitrogen. Furthermore, the nitrogen compounds formed, especially hydrogen cyanide (HCN) and potassium cyanide compounds, lead to elevated nitrogen oxide emissions (NOx) on utilization, especially in the combustion of the top gas and the gas mixtures formed therefrom. A high level of economic detriment (deNOx plants) is necessary, especially in the power plant or sintering plant, in order to be able to comply with relevant offgas limits.
It is therefore an object of the invention to specify a method of producing pig iron in a blast furnace, with which an optimal process can be provided, and nitrogen oxide emissions and other nitrogen compounds, such as hydrogen cyanide or potassium cyanide compounds, can be reduced or essentially prevented.
The object is achieved by the features of claim 1.
The inventors have found that air (cold gas stream, cold blast) which is sucked in in the existing conventional process can be partly or fully replaced by supplying CO2. According to the invention, the cold gas stream, before being introduced into the at least one heat exchanger, comprises a CO2 component of at least 5% by volume, wherein the cold gas stream, as well as impurities, may contain air and/or pure oxygen as residual component. The cold gas stream may especially comprise at least 10% by volume, preferably at least 20% by volume, more preferably at least 30% by volume, especially preferably at least 40% by volume, of CO2, in order to reduce or to partly or fully replace the air component in the cold gas stream.
Impurities, especially unavoidable impurities, in the context of the invention are components or accompanying elements in the composition of the cold gas stream/hot gas stream, which may be present up to 2.0% by volume, especially up to 1.5% by volume, preferably up to 1.0% by volume, but do not make any significant contribution or are of no consequence and hence do not have any influence on the process. Using the example of air, the main components are oxygen and nitrogen, and impurities include noble gas (argon) and carbon dioxide, adding up to about 1% by volume.
Depending on the shaft furnace and/or mode of operation of the shaft furnace, proportions of the air in the cold gas stream may be effectively substituted or displaced by CO2, such that less nitrogen is fed in or circulated in the volume flow or in the flow of matter throughout the process. As a result, firstly, a reduction in NOx, HCN and potassium cyanide compounds is ensured and, secondly, use of CO2 in the cold gas stream has a positive effect on the CO2 balance over the entire process. CO2 has a higher specific heat capacity than air, which is about 26% higher, such that the efficiency of the heat exchanger or of the blast heater (Cowper stove) can be increased, since the same volume flow rate can achieve a higher calorific power density. Moreover, the viscosity (kinematic, dynamic) of CO2 is lower compared to air, which can have an advantageous effect in the heat exchanger/blast heater and in the shaft furnace, including up- and downstream.
The CO2 heated to the temperatures that are customary nowadays in the heat exchanger (blast heater) is unstable within this temperature range, >700 to 1400° C., meaning that it would break down to CO after being blown in in the lower region of the blast furnace in what is called the raceway in contact with a (substitute) reducing agent with consumption of tangible heat. In the case of reaction with carbon, for example, about 172 kJ/mol is consumed. In the case of reaction with hydrogen, for example, only about 30.9 kJ/mol. This heat consumption is essentially covered by the heat released simultaneously in the combustion with oxygen, such that there can still be a considerable energy surplus in net terms, depending on the mixing ratio. Thus, a metallurgically effective (meaning that it is available as a reducing agent) optimized hot gas stream (hot blast) is available from the start in the raceway, which can additionally increase the efficiency of the blast furnace.
Carbon is introduced directly with the CO2 in the hot gas stream (hot blast) via the blast tuyeres, such that it is possible to reduce the amount of raw material, especially the feed of coke, for example, but also the additional injection of carbon, compared to conventional operation.
Further advantageous configurations and developments will be apparent from the description that follows. One or more features from the claims, the description and the drawings may also be combined with one or more other features thereof to give further configurations of the invention. It is also possible for one or more features from the independent claims to be linked by one or more other features.
In one configuration of the method of the invention, the cold gas stream (cold blast) contains CO2, air and optionally pure oxygen in addition to impurities, where the proportion of air is limited to not more than 50% by volume, especially to not more than 40% by volume, preferably to not more than 30% by volume, more preferably to not more than 20% by volume, especially preferably to not more than 10% by volume. The partial use of air has the advantage, for example, that the cold gas stream or hot gas stream has a certain “moisture content” in the form of water or water vapor in the stream of matter (hot blast), which depends on the proportion of air in the cold/hot gas stream and on ambient conditions, such that it is either sufficient or is adjustable once again in a controlled manner by the introduction of, for example, water vapor into the hot gas stream prior to introduction of the hot gas stream (hot blast) into the shaft furnace. The “blast moisture content” may be advantageous for a calm and uniform mode of operation in the shaft furnace. The regulation of the blast moisture content can control the combustion temperature in the raceway, called the RAFT, via the endothermic properties. At the same time, blast moisture content can also affect the hydrogen content in the top gas.
The pure oxygen optionally introduced into the cold gas stream may be brought to temperature, by using it as oxidizing agent for release of heat and as reducing agent, especially carbon monoxide, in order thus to increase the efficiency by increasing the power density. This effect is partly based on the fact that the ratio of oxygen to nitrogen is increased. When CO2 is used, it is possible to partly or completely dispense with a further separate injection of oxygen. Alternatively, the optional pure oxygen can also first be introduced into the hot gas stream (hot blast) before the introduction of the hot gas stream into the shaft furnace, in order to prevent any possible reaction with air or the nitrogen in the air to give NOx, HCN, especially in the heat exchanger. What is meant by “optionally” in this connection is that no pure oxygen is supplied either, either to the cold gas stream or the hot gas stream.
In an alternative configuration of the method of the invention, the cold gas stream (cold blast) contains CO2 and optionally pure oxygen in addition to impurities, where the proportion of CO2 is at least 70% by volume, especially at least 75% by volume, preferably at least 80% by volume, more preferably at least 85% by volume, especially preferably at least 90% by volume. The cold gas stream is effectively air- or nitrogen-free, such that no NOx emissions can be released in the shaft furnace at least by the hot gas stream, and the top gas is thus also free of nitrogen and nitrogen compounds, such that the top gas has a better calorific value and better emissions values compared to the conventional derived top gas. On account of the essentially nitrogen-free top gas, the use thereof is suitable not just for increasing the efficiency of the heat exchanger (blast heater), but also for direct separation of CO2, or downstream separation of CO2 especially in what are called oxyfuel processes.
The pure oxygen optionally introduced into the cold gas stream can be brought to temperature, wherein it oxidizing agent is used for release of heat and the formation of reducing agent, especially carbon monoxide (CO), in order thus to increase the efficiency by increasing the power density. This effect is partly based on the fact that the ratio of oxygen to nitrogen is increased. When CO2 is used, it is possible to partly or completely dispense with a further separate injection of oxygen. Alternatively, the optional pure oxygen may also be introduced only into the hot gas stream before introduction of the hot gas stream into the shaft furnace. What is meant by “optionally” in this connection is that no pure oxygen is supplied either, either to the cold or to the hot gas stream. The cold gas stream, and possibly also the hot gas stream, may in the specific case consist solely of CO2 together with impurities.
In a preferred configuration of the method of the invention, CO2 for the cold gas stream is provided from a CO2 separation, which is either separated out of an offgas combusted in the heat exchanger or blast heater or can be produced or deposited from other processes which especially in the immediate proximity in the metallurgy plant. Other processes are, for example, the use/separation of CO2 from a direct reduction (DR), which may be coupled to the shaft furnace process (integrated metallurgy plant). Alternatively, it is also conceivable to provide CO2 as a pure industrial gas or else with low demands on purity.
In one configuration of the method of the invention, hydrogen is additionally introduced in the lower region of the shaft furnace. Hydrogen as what is called a replacement reducing agent in conjunction with the CO2 introduced from the hot gas stream (hot blast) can contribute to lowering the demand for coal and coke and hence to an increase in economic viability. The use of hydrogen may, for example, be 0.005% to 0.1%, especially 0.01% to 0.08%, preferably 0.015% to 0.07%, per tonne of pig iron produced.
In one configuration of the method of the invention, pure oxygen is additionally introduced in the lower region of the shaft furnace. If the optional pure oxygen introduced via the hot gas stream (hot blast) is insufficient for combustion or no additional pure oxygen is introduced with the hot gas stream, separate introduction of oxygen may be required in order to ensure the necessary energy for operation of the shaft furnace. The use of additional oxygen may, for example, be 0.1% to 13% per tonne of pig iron produced. In the case of operation without additional oxygen, a half mole of oxygen is released from the hot CO2-containing gas stream per mole of CO2.
In one configuration of the method of the invention, carbon is additionally introduced in the lower region of the shaft furnace. If the carbon from the CO2 of the hot gas stream introduced via the hot gas stream (hot blast) and/or the input of coal/coke via the top is insufficient, separate introduction of carbon may be required to generate heat input into the shaft furnace, in order that the temperature level that takes place for the reduction reactions in the furnace is attained. Target temperatures in the raceway are in the range between 1800° C. and 2500° C.
In one configuration of the method of the invention, the hydrogen and the oxygen are produced and provided from a (water or chloralkali) electrolysis. Also conceivable are other hydrogen sources, especially the hydrogen present in the blast furnace gas or the coking furnace gas mixture per se. In addition, a high-temperature electrolysis is also conceivable, since this can be integrated efficiently with the wide variety of different heat sources in an integrated metallurgy plant. The oxygen may also come from other sources, for example air fractionation plants.
In one configuration of the method of the invention, if air is to be supplied to the cold gas stream (cold blast), at least the air component of the cold gas stream is compressed to a pressure above ambient pressure before being combined with the other components, before being combined with the other components and before the introduction of the cold gas stream into the at least one heat exchanger. Introduction of compressed air into the heat exchanger (blast heater) corresponds to the conventional course of action. The CO2 and/or optionally pure oxygen components may already be provided with a pressure above ambient pressure as a result of the process, such that this/these component(s) is/are supplied to the cold gas stream after compression, and hence the throughput through the compressor and thus the operating costs of the compressor can be reduced.
If the components or component of the cold gas stream are provided in unpressurized form or at the level of ambient pressure, the cold gas stream is compressed, especially completely, to a pressure above ambient pressure, before the cold gas stream is introduced into the at least one heat exchanger (blast heater).
There follows a detailed elucidation of specific configurations of the invention with reference to the drawing. The drawing and accompanying description of the resulting features should not be read as being restricted to the respective configurations, but instead serve for illustration of exemplary configuration. In addition, the respective features may be utilized together with one another or else together with features of the above description for further possible developments and improvements of the invention, specifically in the case of additional configurations that are not shown. Identical parts are always given the same reference numerals.
The drawing shows:
The invention is also implementable with proportions of air and/or pure oxygen in the cold gas stream (cold blast), since at least 5% by volume, especially at least 10% by volume, preferably at least 20% by volume, more preferably at least 30% by volume, especially preferably at least 40% by volume of CO2 in the cold gas stream (cold blast) and hence partial to complete replacement of the air can lead to a reduction in NOx emissions caused by nitrogen, and this can, for example, also increase/improve the efficiency of the (overall) process.
The invention is applicable to any type of shaft furnace, i.e. not just restricted to blast furnaces, but is also implementable in cupola furnaces, primary energy furnaces etc. that work by the principle of action described.
Number | Date | Country | Kind |
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10 2020 212 806.5 | Oct 2020 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/075844 | 9/21/2021 | WO |