Patent Application
20240210115
The present invention relates a steel making plant with an electric arc furnace.
Typically, the direct melting of materials which contain iron, such as scrap, is performed in electric arc furnaces (EAF).
The primary feedstock for EAFs is ferrous scrap, which can consist of scrap coming from within the steel mill, scraps, waste from mechanical industries (e.g., vehicle manufacturers), and demolition or post-consumer scrap (e.g., end-of-life products such as cars, buildings).
Direct reduced iron (DRI) is also increasingly being used as a feedstock for EAFs because of its low gangue content, lower content of undesired metals (e.g. copper), and low CO2 footprint in the manufacturing process.
Finally, liquid pig iron could also be used in the material mix to feed an electric arc furnace.
An electric arc furnace is usually charged with scrap and/or DRI and/or liquid pig iron by:
Secondary metallurgy is performed on the molten steel after melting in the EAF to the casting point. It is typically performed at ladle treatment stations, with the molten steel remaining in the ladle itself. These treatment stations generally consist of an arc heating unit, called a ladle furnace (LF), which allows the final temperature of the liquid steel for the casting operation to be adjusted. The treatment involves the addition of scarifying agents and binding elements to regulate the chemical composition of the finished steel. In some cases, the vacuum processing units are used to achieve special gas content requirements.
A simplified diagram of a steel plant provided with an electric arc furnace and a ladle furnace is shown in
A steel making plant generally comprises an emission collection system, which in particular can suck the emissions generated during the melting process, and convey them to a treatment system.
Each electric arc furnace (EAF) and ladle furnace (LF) is provided with its own suction system. In
In the EAF, primary suction may be performed either through an appropriate hole in the furnace roof (called the 4th hole or 2nd hole) or through the material feed channel into furnaces with continuous charging systems. In the latter case, the fumes are sucked through the continuous charging system and preheat the scrap before it is charged into the EAF.
Furthermore, the EAF electric arc furnace is provided with a hood C located on the roof of the building containing the furnace. The function of the hood C is to ventilate the building during the melting step and to collect the fumes generated inside the building following the opening of the furnace roof during the basket loading step. This additional suction system of the EAF is referred to as secondary suction and is indicated by S1 in
The gases emitted in the basket loading step are diffused inside the building and are strongly diluted before being collected by hood C. The secondary suction S1 must thus treat much larger volumes of fumes than the primary suction P1. For this reason, the suction capacity of the secondary suction system is much greater than that of the primary suction. Furthermore, due to dilution, the fumes treated by the secondary suction system S1 are much cooler than those treated by the primary suction.
The fumes collected from the suction systems of the EAF and the LF contain dust, nitrogen oxides and sulfur oxides, carbon monoxide, and organic pollutants, e.g., such as volatile organic compounds (VOCs), chlorobenzenes, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dioxins (PCDDs), and furans (PCDFs). The presence of organics in emissions depends primarily on the quality of the scrap used.
As shown in the diagram in
The fume collection and treatment system comprises a main duct L1 into which all the suction systems P1, P2, S1, A1, A2 and A3 discharge; all the fumes collected by the suction systems are led to a fume treatment system, which will be explained in greater detail below, through the main duct L1.
Primary suction P1 from EAF electric arc furnace: the hot fumes from the EAF furnace are collected from the furnace roof through a water-cooled elbow, and conveyed to a cooled chamber CH, where the post-combustion of the CO generated in the melting process is completed; the primary fumes are sucked in at a high temperature (over 1000° C.) and are then cooled by means of water-cooled ducts CH, and then by water cooling towers QT or convection exchangers (natural or forced) to reduce the temperature so that they can be treated downstream in a bag filter BF installed in the main duct L1 and part of the fume treatment system.
Primary suction P2 from ladle furnace LF: the fumes are collected from the LF furnace roof with a single-wall (uncooled) pipe and conveyed into the main duct L1 of the fume system. The temperature of the fumes sucked from the LF is below 180° C.
Secondary suction S1 from EAF electric arc furnace: the hood C, installed on top of the building, captures the fumes during the EAF charging steps and allows ventilation during the melting steps; it also allows the suction of the air necessary to further cool the fumes collected by the primary suction P1 before being processed in the bag filter BF.
Auxiliary suction points A1, A2, and A3: the fume collection system may comprise auxiliary suction points which depend on site-specific plant configurations, which may include, for example, material or additive handling, tundish ladle demolition, tundish ladle tipping, EAF refractory material demolition, etc.
The collected fumes are treated in a bag filter BF and then dispersed into the atmosphere.
Substantially, the bag filters capture dust, including all heavy metals present as particulate matter at the filtering temperature, as well as some organic compounds.
Generally, adsorbing materials (e.g. activated carbon, pulverized activated lignite coke or mixtures of these with lime, clay) are dosed in the fume main duct L1 upstream of the bag filter by means of an appropriate dosing device ADS to reduce persistent organic pollutants, in particular, to control the content of PCDD dioxins and PCDF furans. The adsorbent material is retained by the filter bags BF and, after absorbing dioxins and furans, is disposed of with the dust collected by the filter.
Currently, the fume treatment systems of a steel making plant are configured to abate the following pollutants:
However, the current fume treatment systems in a steel making plant do not allow the abatement of NOx in the gaseous emissions.
NOx is contained upstream through combustion control techniques. To date, however, technologies for the abatement of NOx from post-combustion emissions have not yet been successfully implemented in steel making plants with electric arc furnaces, despite the wide availability of such technologies applied to plants, such as fossil fuel boilers, incinerators, etc.
NOx formation occurs through several mechanisms.
In the case of an EAF, NOx is formed primarily by thermal dissociation and the successive reaction of nitrogen and oxygen molecules in the combustion air, referred to as “thermal” NOx. The other NOx formation mechanisms, i.e., “fuel NOx” (due to the evolution and reaction of nitrogen compounds in fuels with oxygen) and “prompt NOx” (due to the formation of hydrogen cyanide HCN followed by oxidation to NOx) make minor contributions to NOx emissions from an EAF.
Post-combustion NOx abatement and control systems include:
In greater detail, SCR units utilize a nitrogen-based reagent, such as ammonia (NH3) or urea, to chemically reduce NOx into molecular nitrogen and water vapor. The reagent is injected through a system of injectors into the fume stream, upstream of a catalytic bed or reactor. The exhaust gas mixes with the reactant and enters a reactor module containing the catalyst. Hot combustion gas and the reactant diffuse through the catalyst, in which the reactant selectively reacts with NOx; the reactions occur if fume temperatures are within a specific range. Generally, operating temperatures comprised between 220° C. (430° F.) and 420° C. (800° F.) of the gas stream are required in the catalytic bed for the catalytic reduction process to occur efficiently. The reaction between NH3 and NOx is promoted by the presence of excess oxygen (greater than 1%).
Below the optimal temperature range, the catalyst activity is greatly reduced, potentially allowing direct emission of unreacted ammonia (known as “ammonia slip”) into the atmosphere. The SCR systems may also be subject to catalyst deactivation over time, due to physical deactivation and/or chemical poisoning.
For an SCR system to effectively reduce NOx emissions, the exhaust gas stream must thus be fed with relatively stable gas flow rates, NOx concentrations, and temperature.
On the other hand, it is known that the operating conditions vary widely during the melting cycle in terms of gas flow rates, temperatures, and NOx concentrations in EAF fume treatment systems, making denox systems inapplicable.
In particular, the SCR system cannot be installed after particulate removal due to low fume temperatures (90° ° C./195° F. to 150° C./300° F.), well outside the effective operating range.
There are currently no known applications of SCR technology to control NOx emissions in steel plants with EAFs. Indeed, NOX abatement (denox) systems are considered technically unfeasible due to the unresolved technical problems outlined above.
Thus, in the reference technical field, the need to abate NOx from the gaseous emissions of a steel plant with an electric arc furnace is still completely unsatisfied.
Therefore, it is the main object of the present invention to eliminate the drawbacks of the aforementioned prior art either entirely or in part by providing an electric arc furnace steel plant which is equipped with a fume collection and treatment system 5 capable of efficiently abating NOx by means of SCR-type denox apparatus.
It is a further object of the present invention to make available an electric arc furnace steel making plant which is provided with a fume collection and treatment system capable of efficiently abating NOx through SCR-type denox apparatus while being operationally reliable and simple to operate.
It is a further purpose of the present invention to make available a method of collecting and treating the fumes generated by an electric arc furnace steel plant which allows efficiently abating NOx from the emissions generated by the plant itself.
The technical features of the invention according to the aforesaid objects may be clearly found in the contents of the claims hereinbelow and the advantages thereof will become more apparent from the following detailed description, given with reference to the accompanying drawings which show one or more embodiments merely given by way of non-limiting example, in which:
The electric arc furnace steel making plant according to the invention is indicated as a whole by reference numeral 1 in
According to a general embodiment of the invention, the steel making plant 1 comprises at least one electric arc furnace 10 and a fume collection and treatment system 100 suitable to collect and treat gaseous emissions produced by said steel making plant 1.
In this description and the appended claims, the expressions “gaseous emissions,” “emissions,” or “fumes” are synonymous and, unless expressly stated otherwise, refer generically to gas and dust mixtures generated during the operation of steel making plant 1. The composition of said gaseous emissions varies according to the zone of steel making plant 1. In some zones, said emissions may contain primarily only dust, such as in the dust collecting apparatus of the additive transport systems, the ladle and tundish h refractory material demolition and lining areas, or in the slag handling area. In the case of the electric arc furnace, said emissions contain, in addition to dust, combustion products such as nitrogen oxides and sulfur oxides, carbon monoxide and organic pollutants, e.g., such as volatile organic compounds (VOC), chlorinated benzenes, polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAH), dioxins (PCDD) and furans (PCDF). The presence of organics in emissions depends mainly on the quality of the feedstock used. On the other hand, in the case of the ladle furnace, said emissions mainly contain only NOx and dust.
According to the aforesaid general embodiment, the fume collection and treatment system 100 comprises:
Preferably, the electric arc furnace 10 is installed inside a building (not illustrated in the diagram in
Furthermore, as shown in
The aforesaid at least one filtration apparatus 130 may be of any type suited for the purpose. Preferably, the filtration apparatus 130 is a bag filter.
According to the invention, the electric arc furnace is fed by a continuous charging system 11.
By virtue of this contrivance, it is not necessary to open the furnace roof and thus the direct emission of fumes from the electric arc furnace 10 into the surrounding environment is avoided during the charging steps of the electric arc furnace 10. Thus, the air sucked from the secondary suction line 120 through the suction hood 121 is not substantially contaminated by fumes directly from the electric arc furnace 10. Indeed, the fumes remain substantially confined to the furnace 10 and/or the continuous charging system 11, and emissions are limited to only small amounts due to unavoidable leaks. In particular, the air sucked from the secondary suction line 120 contains no NOx or negligible concentrations thereof. Operationally, the NOx generated by the electric arc furnace 10 is thus substantially all sucked from the primary suction line 110.
In particular, the continuous charging system 11 of the electric arc furnace 10 is of the type connectable to a furnace wall 10 or the furnace roof 10.
In particular, the first primary suction line 110 may be fluidically connected to the electric arc furnace through a hole made in the furnace roof 10 or through a material feed channel of the continuous charging system 11.
According to the invention, the following are arranged in sequence along said primary suction line 110 starting from the electric arc furnace 10:
Again according to the invention, the secondary suction line 120 flows into the first primary suction line 110 downstream of the denox selective catalytic reduction 113 and upstream of said at least one filtration apparatus 130.
By virtue of the invention, the fumes collected from the primary suction line 110 are fed to the denox SCR apparatus 113 before said fumes are combined with fumes collected from the secondary suction line 120 and then sent to the filtration apparatus 130. Thus, the fumes sucked from the electric arc furnace 10 are not diluted and cooled with the fumes drawn from the secondary suction line 120. Thus, the denox SCR apparatus can be operated under stable and controllable conditions; in particular, the denox SCR apparatus 113 can treat uncooled fume and undiluted NOx. Thus, it is possible to efficiently abate NOx generated in a steel making plant 1 using a denox SCR apparatus 113.
Furthermore, by virtue of the fact that upstream of the denox SCR apparatus 113, the first primary suction line 110 comprises a fume cooling apparatus 111 and a dust collecting device 112, the fumes collected from the first primary suction line 110 can be preventively:
The dust collecting device 112 may be any dust filtration device suited for the purpose and capable of treating the fumes generated by the electric arc furnace 10.
Preferably, said dust collecting device 112 is an electrofilter, also known as an electrostatic precipitator. The electrostatic precipitator is a device without mechanical filtration which removes particles, such as dust and fumes, from a gaseous stream using the force of an electrostatic charge induced on the dust.
Preferably, as shown in
In particular, the dosing means 113b are suitable to inject the reagent upstream or downstream of the dust collecting device 112. Preferably, as shown in
Advantageously, the dosing means 113b are controlled by a control system to adjust the dosed amount of the nitrogen-based reagent as a function of:
The operation of a denox SCR apparatus is well known in itself to a person skilled in the art and will thus not be described in detail.
The denox SCR apparatus uses a nitrogen-based reagent, such as ammonia (NH3) or urea, to chemically reduce NOx nitrogen oxides into molecular nitrogen and water vapor. The reagent is injected into the fume stream either before or after the dust collecting device 112, upstream of the catalytic bed. The reagent flow is automatically controlled by the automation/control system by means of gas analyzers and flow meters installed in the fume streams, which make it possible to measure the amount of pollutants and the reagent “slip”.
Advantageously, the catalyst with nitrogen-based reagent injection abates down NOx, but it also can chemically reduce dioxins, furans, and carbon monoxide CO. By virtue of this capacity, it is possible to avoid the insertion in the fume collection and treatment system of an injection apparatus of adsorbing materials (e.g. activated carbon, pulverized activated lignite coke or mixtures of these with lime, clay) traditionally provided in the known fume collection and treatment systems to abate dioxins and furans and CO from the fumes. The elimination of the adsorbent injection apparatus leads to a reduction in operating costs for material injected and for the amount of dust produced and collected in the filtration apparatus 130.
Operationally, the choice was made to abate NOx through denox SCR systems and not through Non-Selective Catalytic Reduction (NSCR) denox systems for the following reasons.
In NSCR non-selective catalytic reduction systems, CO, NOx and hydrocarbons are converted to CO2 and N2 through a catalyst. This technique does not require the injection of additional reagents because unburned hydrocarbons are used as reducing agents. However, the gases must have very low oxygen contents. NOx removal occurs in two sequential steps:— in step 1, the reactions remove excess oxygen, because the latter reacts better with CO and hydrocarbons than with NOx;— in step 2, the hydrocarbons react with the NOX in the mixture, reducing them. This is why the concentration of oxygen in the fume must be very low, in particular below 0.5%. Therefore, NSCR systems can only be used with fuel-rich, oxygen-poor mixtures. However, said limitation does not apply to denox SCR systems, which can thus also treat oxygen-rich mixtures, such as those that characterize the fumes extracted from an electric arc furnace.
Preferably, as shown in
The production of steel in an EAF is a discontinuous process in which melting steps alternate with molten steel tapping and/or charging steps. During the tapping steps, the fumes generated by the electric arc furnace are cold and the production of NOx emissions is essentially negligible. If the fumes captured during the tapping steps were conveyed to the denox SCR apparatus, they could cool the catalytic bed below the operating temperature and damage the system.
For this reason, the first primary suction line 110 preferably comprises a by-pass line 110a, which fluidically connects the section of the first primary suction line 110 upstream of the denox selective catalytic reduction 113 with the section of the first primary suction line 110 downstream of the denox SCR apparatus 113.
In particular, as shown in
Advantageously, the first primary suction line 110 is provided with one or more by-pass 110b, 110c, which are suitable to adjust the passage of fumes through the by-pass 110a, and the actuation of which is controlled by a control system as a function of the temperature of the fumes coming out from the fume cooling apparatus 111, measured by at least one temperature sensor 110d.
Operationally, when temperature sensor 110d detects a temperature of the fume coming out from the cooling apparatus 111 below a predetermined value, said one or more by-pass valves 110b, 110c are actuated to allow the fumes to pass through the by-pass line 110a, preventing passage through the catalytic bed 113a at the same time.
The melting process in an EAF generates significant amounts of carbon monoxide CO. For such reason, a fume post-combustion chamber 114 is preferably arranged upstream of the fume cooling apparatus 111 in the aforesaid first primary suction line 110. In said post-combustion chamber it is possible to burn CO and significantly reduce its concentration in the fumes.
Advantageously, the fume cooling apparatus 111 is suitable to generate an adjustable cooling capacity such that the fume exiting said apparatus 111 has a temperature within a predetermined temperature range as a function of the operating requirements of the denox SCR apparatus 113. Typically, the operating requirements of the denox SCR apparatus 113 may require a fume temperature range comprised between 220° C. and 350° C.
Preferably, the fume cooling apparatus 111 is feedback-controlled by a control system as a function of the temperature of the fume exiting the apparatus 111 as measured by at least one temperature sensor 110d.
According to a preferred embodiment, the aforesaid fume cooling apparatus 111 may comprise a shell and tube exchanger and a plurality of fans 111a, the actuation of which is controlled by the control system to modulate the cooling air flow rate over the shell and tube exchanger as a function of the cooling capacity required by the fume cooling apparatus 111.
Alternatively, said fume cooling apparatus 111 may comprise a water cooling tower. However, the shell-and-tube heat exchanger system is preferable to the cooling tower because it avoids the introduction of water into the fumes.
As shown in
As mentioned above, the emissions generated by a ladle furnace contain NOx. The fumes sucked from said second primary suction line 140 may be combined with the fumes collected from the first primary suction line 110 and then be treated together in the denox SCR apparatus 113.
However, the fumes generated by ladle furnace 20 and sucked in by said second primary suction line 140 are too cold to be efficiently treated in a denox SCR apparatus. For this reason, they are combined with the fumes generated by the electric arc furnace 10 so as to be heated.
Due to the high temperatures of the fumes generated by the electric arc furnace 10 (further elevated by the eventual post-combustion), the mixture of the two fume streams (from EAF 10 and from LF 20) is still too hot for the denox SCR apparatus. For this reason, the fumes sucked in from said second primary suction line 140 are joined to the fumes sucked in from the first primary suction line 110 upstream of the fumes cooling apparatus 111, to make it possible to effectively control the temperature thereof.
Preferably, the second primary suction line 140 comprises at least one fan 141 the actuation of which is controlled by a control system as a function of the pressure inside the ladle furnace 20 as measured by at least one pressure sensor 142.
As shown in
The connection zone between continuous charging system 11 of the furnace and the electric arc furnace 10, being a connection between two moving parts, cannot be closed mechanically and is thus a source of spurious air inputs into the furnace, resulting in increased production of nitrogen oxides from the furnace.
Advantageously, as diagrammatically shown in
Preferably, the sealing suction line 160 comprises at least one fan 161 the actuation of which is controlled as a function of the pressure within the containment casing 12 as measured by at least one pressure sensor 162.
Said sealing system of the connection zone between the furnace and the charging system can be defined as “active” because the suction capacity is regulated based on the pressure measured in the charging zone of the furnace, guaranteeing the correct degree of suction with reference to the operating pressure in the furnace.
It is an object of the present invention to provide a method for collecting and treating the fumes generated by a steel making plant.
The method according to the invention applies to a steel making plant with an electric arc furnace, in particular, such as the one which is the object of the present invention and in particular as described above. For this reason, the method is described below using the same reference numerals used to describe steel making plant 1.
In general, the steel making plant 1 comprises at least one electric arc furnace 10 and a fume collection and treatment system 100 suitable to collect and treat gaseous emissions produced by said steel making plant 1.
The aforesaid fume collection and treatment system 100 comprises:
The method according to the invention comprises:
Furthermore, according to the invention, before treating the fumes collected by the first primary suction line 110 in the denox catalytic reduction apparatus 113 said fumes are freed from dust in a dust collecting device 112 and are cooled in a fume cooling apparatus 111, to take the temperature of said fumes within a predetermined temperature range as a function of the operating requirements of the denox apparatus 113.
Preferably, during tapping phases of the molten steel from the electric arc furnace 10 the fumes collected by the first primary suction line 110 are sent to the filtration apparatus 130 by-passing the denox selective catalytic reduction apparatus 113, so as not to send to the denox selective catalytic reduction apparatus 113 cold fumes having temperatures below a predetermined temperature range as a function of the operating requirements of the denox apparatus 113.
Preferably, said steel making plant 1 comprises at least one ladle furnace 20. Said fume collection and treatment system 100 comprises a second primary suction line 140 which is fluidically connected to said ladle furnace 20 to suck in fume generated in said ladle furnace 20. The fumes collected by said second primary suction line 140 are combined with fumes collected by said first primary suction line 110 before cooling them in the fume cooling apparatus 111.
Preferably, the aforesaid steel making plant 1 may comprise one or more auxiliary stations 51, 52, 53, 54 which are intended to operationally support the steel production activity and are likely to generate emissions containing primarily dust. The fume collection and treatment system 100 comprises for each auxiliary station 51, 52, 53, 54 an auxiliary suction line 151, 152, 153, 154 which is fluidically connected to the respective auxiliary station to suck in emissions generated by said station. The emissions collected from each auxiliary suction line 151, 152, 153, 154 are sent directly to the filtration apparatus 130.
The invention provides numerous advantages, some of which have already been described.
The electric arc furnace steel plant 1 according to the invention is provided with a fume collection and treatment system capable of efficiently abating NOx by means of SCR-type denox apparatuses.
The steel making plant 1 with an electric arc furnace according to the invention is provided with a fume collection and treatment system capable of efficiently abating NOx by means of SCR-type denox apparatus, and is at the same time operationally reliable and simple to operate.
The method of collecting and treating the fumes generated by a steel making plant with an electric arc furnace makes it possible to efficiently abate NOx from the emissions generated by the plant itself.
Therefore, the invention thus devised achieves the set objects.
Obviously, in the practice, it may also take shapes and configurations different from the one disclosed above, without because of this departing from the present scope of protection.
Furthermore, all details may be replaced by technically equivalent elements, and any size, shape, and material may be used according to needs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102021000012065 | May 2021 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/054162 | 5/5/2022 | WO |
