Direct reduction plant and process thereof

Abstract

A direct reduction plant for a direct reduction of iron oxides provided with a gas circulation circuit comprising: - a reactor (1) having a reduction area (2) adapted to be charged with said iron oxides; - an external source (20) of make-up gas; - a recovery and treatment line (10), arranged downstream of the reactor (1), to recover and treat the exhaust gas that comes out of the reactor (1); - a treatment and feeding line (11), arranged upstream of the reactor (1), to treat a process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line (10), and to feed the reduction area (2) of the reactor (1) with said process gas; wherein the recovery and treatment line (10) communicates downstream with said treatment and feeding line (11); wherein, in its end stretch, proximal to said treatment and feeding line (11), the recovery and treatment line (10) comprises a carbon dioxide removal device (50) to remove carbon dioxide from the exhaust gas, arranged upstream of a compressor (42) to compress the exhaust gas towards said treatment and feeding line (11).

Description

[0001] DIRECT REDUCTION PLANT AND PROCESS THEREOF

[0002] Field of the invention

[0003] The present invention relates to a direct reduction plant of the zero-reformer type, or alternatively of the type with a reformer outside the reactor, and to the process thereof, adapted, in particular, to produce metallic iron by direct reduction of iron oxides using reducing gas.

[0004] Background art

[0005] The systems for the production of direct reduced iron (DRI) ore of the known type comprise a reactor, into which iron oxide in the form of pellets and / or lumps is charged, and a line for treating and supplying reducing gas, comprising hydrogen and / or carbon monoxide and adapted to reduce said iron oxide in the reactor. The reducing gas is injected into a reaction chamber, or reactor, at high temperature. The reactor can be of the fixed bed type, the moving bed type, the fluidized bed type, or of the rotary or kiln type. In a moving bed reactor, the reducing gas is typically introduced into the central part of the reactor, made to rise in counterflow through the iron oxide, and then extracted, reprocessed, and recycled in a reduction circuit. The exhaust gas coming out of the reactor is dedusted, deprived of reaction products (H2O and CO2), and then compressed; it is then mixed with a make-up gas (natural gas, COG, gas obtained in a reformer, Corex Gas, syngas, hydrogen, etc.). The flow of gas, defined by the mixture of the new make-up gas and the exhaust gas recycled after appropriate treatment, is sent to a heating unit, which brings it to the temperature required by the reduction process, normally higher than 850 °C.

[0006] The heated flow of reducing gas, into which oxygen can be injected to increase the temperature thereof further, is sent to the reactor, in which the iron oxides to be reduced are introduced from above in the form of pellets and / or lumps and flow downwardly there-through, while the DRI (reduction product) is extracted at the opposite end of said reactor and sent, by means of a pneumatic conveying system or by gravity or by means of belts, to a blast furnace or an electric arc furnace or an oxygen converter or any device capable of melting the produced DRI. In greater detail, in the iron oxide direct reduction process, oxygen is removed from iron ore by means of chemical reactions with hydrogen and carbon monoxide to obtain DRI with a high level of metallization (ratio of metallic iron to total iron contained in the DRI). The overall reduction reactions involved in the process are well-known and are shown below:

[0007] Fe2O3+ 3H2-> 2Fe + 3H2O (1 )

[0008] Fe2O3+ 3CO -> 2Fe + 3CO2(2).

[0009] Hydrogen and carbon monoxide react with the oxygen of the iron oxide and are transformed into water and carbon dioxide according to the reactions (1 ) and (2). In addition to H2O and CO2, unreacted H2and CO are also present in the exhaust gas coming out of the reactor. The exhaust gas is treated as described above to recover these reductants.

[0010] Disadvantageously, the known direct reduction plants have a large number of devices along the gas circulation circuit. Furthermore, because of the high flow of gas to be processed and the pressures involved, some of these devices are large in size and thus energy-intensive. Adequate instruments are also needed for control and safety.

[0011] Figure 1 diagrammatically shows a direct reduction plant in which a recovery and treatment line 10, arranged downstream of the reactor 1 , recovers and treats the exhaust gas coming out of the reactor, and communicates downstream with a treatment and feeding line 11 , arranged upstream of reactor 1 , which treats the process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line 10, to feed the reduction area 2 of the reactor 1 with said process gas.

[0012] The recovery and treatment line 10 comprises in succession in the end stretch thereof, proximal to said treatment and feeding line 11 , a compressor 42, a cooling device 43 to cool the gas flow heated by compression, and a carbon dioxide removal device 50.

[0013] Instead, the treatment and feeding line 11 comprises, in the initial stretch thereof, proximal to the carbon dioxide removal device 50, a humidifier 51 required to increase the water content of the process gas, thereby preventing carbon deposition inside both the heating unit 18 of the process gas and the conduits defining the path of the hot process gas to the reactor 1 .

[0014] A further example of direct reduction plant is disclosed in US8940076B2 (see, for example, Figure 2 of US8940076B2) where, into addition to the carbon dioxide absorption removal system 38 / 40, along the recovery and treatment line there are provided two compressors 34, 54 and a further adsorption unit 140 of the PSA (pressure swing adsorption) type for separation of the gases of large molecules, whereas in the treatment and feeding line it is necessary to provide a humidifier 66 to humidify the process gas.

[0015] In the recovery and treatment line, the cleaned and cooled resulting spent gas stream 32 is divided, with a small first portion 33 containing CO, CO2, H2, and methane, being purged from the system as tail gas (which may be used as fuel in the gas heater 72).

[0016] The other major portion 35 of the cool gas stream 32 is subsequently pressurized by compressor 34 before being fed, as stream 36, to an absorber column 38 of the CO2 chemical absorption removal system, for example using an amine solvent.

[0017] In this way, the CO2 produced by the reduction reactions is selectively separated from said gas stream 36 and chemically absorbed in the lean amine solution 130. Although not illustrated in Figure 2 of US8940076B2, a cooling device is necessary between the compressor 34 and the carbon dioxide removal device 38 / 40, in order to cool the gas flow heated by compression.

[0018] Downstream of said carbon dioxide removal device 38 / 40, the upgraded reducing gas effluent 44 is subsequently divided into two streams.

[0019] A first stream 58 is directed to humidifier 66 and is recycled to the reduction zone 12 of reactor 10, while a second stream 46 is further split into two portions.

[0020] In particular, a first portion 48 is treated in the physical adsorption unit 140 of the PSA type for separation of the gases of large molecules, mostly carbon monoxide and methane from the lighter molecules contained in said gas stream e.g. hydrogen, nitrogen and water.

[0021] Therefore, a hydrogen rich stream 74 is produced and subsequently fed as fuel to burners of heater 72. A small second portion 50 of stream 46 is purged from the system as tail gas regulated by valve 51 and is burned in the process gas heater 72.

[0022] The remaining portion of stream 48, after being dehydrogenated, is removed from the PSA unit 140 as stream 52 mainly composed of CO and CH4 and is subsequently compressed by compressor 54 before being added directly to the upgraded recycled gas stream 58 coming from absorber column 38.

[0023] Compressed stream 56, after being combined with upgraded reducing gas 58 as stream 60 and further combined with the make-up stream of natural gas 64, is fed to humidifier 66 where its water content is adjusted so that the amount of water present ranges between 3% and 10% by volume of stream 70.

[0024] Therefore, disadvantageously the plant of US8940076B2 needs a large number of devices along the gas circulation circuit.

[0025] Furthermore, because of the high flow of gas to be processed and the pressures involved, some of these devices are large in size and thus energy-intensive.

[0026] In the diagram of a process currently in use, CO2 emissions are generated by the selective removal of CO2 from the recycled exhaust gas from the reactor, after the iron oxide reduction reactions, in addition to the emissions released through the flue of the heating unit 18 of the reducing gas. Compared to other known direct reduction processes, the process described above, which is supplied with natural gas to facilitate the methane reforming reactions inside the reduction reactor, still ensures a good H2 / CO ratio in the composition of the reducing gas which is introduced into the reactor.

[0027] However, at present, a further reduction of CO2 emissions is extremely difficult.

[0028] Furthermore, since in the plant in Figure 1 of this patent application there is provided a pipe 34 which connects a stretch of the recovery and treatment line 10, upstream of compressor 42, to the burners of the heating unit 18, to send part of the exhaust gas (also called tail gas) as fuel gas for said burners, sending this gas with carbon dioxide and sulfides to the burners further increases the atmospheric emissions of CO2 and SO2.

[0029] Therefore, the need is felt to develop a direct reduction plant and the related process which can overcome the above drawbacks. Summary of the invention

[0030] It is the object of the present invention to develop a direct reduction plant and process thereof which allow a reduction in the number of devices along the gas circulation circuit.

[0031] It is another object of the present invention to allow the use of smaller devices and thus reduce both the related power consumption and the corresponding control and safety instruments.

[0032] It is a further object of the present invention to enable a further reduction of carbon dioxide emissions compared to the known plants.

[0033] The present invention achieves these and other objects, which will be apparent in light of the present description, by means of a direct reduction plant for a direct reduction of iron oxides which comprises a gas circulation circuit comprising:

[0034] - a reactor having a reduction area adapted to be charged with said iron oxides;

[0035] - an external source of make-up gas;

[0036] - a recovery and treatment line, arranged downstream of the reactor, to recover and treat exhaust gas exiting the reactor;

[0037] - a treatment and feeding line, arranged upstream of the reactor, to treat a process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line, and to feed the reduction area of the reactor with said process gas; wherein the recovery and treatment line communicates, downstream of the reactor, with said treatment and feeding line; wherein the treatment and feeding line comprises a heating unit, upstream of the reactor, to heat the process gas to a temperature suitable for introduction into the reactor; wherein the recovery and treatment line comprises only one compressor; wherein the recovery and treatment line, in the end stretch thereof proximal to said treatment and feeding line, comprises

[0038] - a carbon dioxide removal device to remove carbon dioxide from the exhaust gas,

[0039] - and said only one compressor, arranged between said carbon dioxide removal device and said heating unit, to compress the exhaust gas free of carbon dioxide towards said treatment and feeding line, preferably directly towards the heating unit.

[0040] In this description, the term "process gas" indicates the gas mixture obtained by mixing the make-up gas, containing gaseous hydrogen and / or gaseous hydrocarbons, with the exhaust gas treated in the recovery and treatment line.

[0041] According to a further aspect of the invention, a direct reduction process for a direct reduction of iron oxides is provided, which can be carried out by means of the aforesaid plant, and which comprises the following steps when fully operational: a) recovering and treating the exhaust gas coming out of the reactor by means of the recovery and treatment line; b) feeding process gas into a reduction area of the reactor by means of the treatment and feeding line, said process gas being obtained by mixing the makeup gas, coming from the external source, with the exhaust gas treated in the recovery and treatment line; wherein, before the exhaust gas is introduced into the treatment and feeding line, there are provided in succession a removal of carbon dioxide from said exhaust gas by means of the carbon dioxide removal device and a compression of the exhaust gas free of carbon dioxide, by means of the only one compressor, towards said treatment and feeding line.

[0042] Preferably, the catalytic reforming of the process gas is carried out in situ inside the reactor, so that the process gas undergoes no additional catalytic reaction beyond the catalytic reactions which take place in the reactor, or inside an external reductant generation unit.

[0043] Further features and advantages of the invention will be more apparent in light of the detailed description of the preferred, but not exclusive embodiments.

[0044] The dependent claims describe particular embodiments of the invention.

[0045] Brief description of the figures

[0046] The description of the invention refers to the accompanying drawings, which are provided by way of non-limiting examples, in which:

[0047] Figure 1 is a diagram of an embodiment of a direct reduction plant according to the prior art; Figure 2 is a diagram of an embodiment of a direction reduction plant according to the invention.

[0048] Description of illustrative embodiments of the invention

[0049] An example of a direct reduction plant, which is the object of the present invention, is shown in Figure 2 and comprises a gas circulation circuit provided with:

[0050] - a reactor 1 having a reduction area 2 adapted to be charged with iron oxides;

[0051] - an external source 20 of make-up gas;

[0052] - a recovery and treatment line 10, arranged downstream of the reactor 1 , to recover and treat the exhaust gas coming out of the reactor 1 ;

[0053] - a treatment and feeding line 11 , arranged upstream of the reactor 1 , to treat a process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line 10, and to feed the reduction area 2 of the reactor 1 with said process gas.

[0054] The recovery and treatment line 10 communicates downstream with the treatment and feeding line 11 , this latter comprising a heating unit 18, upstream of the reactor 1 , to heat the process gas to a temperature suitable for introduction into the reactor 1 .

[0055] Preferably, in a known way, this plant is configured to carry out a process of the zero-reformer type, in which the catalytic reforming of the process gas is carried out in situ inside the reactor 1 , so that the process gas undergoes no additional catalytic reaction beyond the catalytic reactions which take place inside the reactor 1.

[0056] Alternatively, the plant can be configured to carry out a process in which the catalytic reforming of the process gas is carried out inside an external reductant generation unit.

[0057] Advantageously, the recovery and treatment line 10 comprises in its end stretch, proximal to said treatment and feeding line 11 , a carbon dioxide removal device 50 to remove carbon dioxide from the exhaust gas, said removal device being arranged upstream of a compressor 42, associated to the carbon dioxide removal device 50, preferably directly connected to said carbon dioxide removal device 50, to compress the exhaust gas towards said treatment and feeding line 11 . More in detail, the whole recovery and treatment line 10 comprises only one compressor 42 and, in said end stretch thereof proximal to said treatment and feeding line 11 , comprises

[0058] - only one carbon dioxide removal device 50 to remove carbon dioxide from the exhaust gas,

[0059] - and said only one compressor 42, arranged between said carbon dioxide removal device 50 and the heating unit 18, to compress the exhaust gas, free of carbon dioxide, towards the treatment and feeding line 11 , preferably directly towards the heating unit 18.

[0060] Preferably, as shown in Figure 2, the portion of the treatment and feeding line 11 , arranged between the only one compressor 42 and the heating unit 18, has no additional components along the extension thereof. In particular, neither humidifier nor further heat exchange devices are provided in the treatment and feeding line 11.

[0061] Providing the only one carbon dioxide removal device 50 upstream of the only one compressor 42 has the advantage of not needing a post-cooling device, which is instead provided between the compressor and the carbon dioxide removal device in the plants of the prior art in order to make efficient the carbon dioxide removal process, because the temperature of the exhaust gas immediately upstream of the only one compressor 42 is already quite low, e.g., about 40-60°C, allowing the most proper working conditions for the carbon dioxide removal device 50. This configuration, without any post-cooling device after the compressor 42, allows also achieving an energy saving since the temperature increase given by the compression effect (typically bringing the temperature to 110-130°C) has not to be abated down like in the prior art plants, but the gas can go directly to the heating unit or Process Gas Heater 18 that will have to provide a lower temperature gradient thanks to the higher temperature of the inlet gas. This aspect turns directly into a fuel or electricity saving inside the heating unit 18.

[0062] Furthermore, the compressor 42 can be reduced in size because the gas flow it must treat is less diluted, having removed upstream its portion of carbon dioxide (typically in the range of 8-12%), leading to a reduction in electricity consumption of up to 10%. Preferably, the compressor 42 is arranged immediately upstream of the treatment and feeding line 11 , for example immediately upstream the connection between the external source 20 and the treatment and feeding line 11 , or between the external source 20 and the recovery and treatment line 10, or between the external source 20 and the junction connecting said recovery and treatment line 10 to said treatment and feeding line 11 (Figure 2).

[0063] An oxygen injection device 30 can be provided downstream of said heating unit 18 to inject oxygen into the process gas flow.

[0064] Preferably, the carbon dioxide removal device 50 is of the capture type with amine solvents to remove hydrogen sulfide, in addition to carbon dioxide.

[0065] The fact of also providing the selective separation of hydrogen sulfide prevents sulfur salts from accumulating in the compressor 42, as a result of the compression which evaporates water, corroding its components over time.

[0066] This is achieved immediately and effortlessly once the carbon dioxide removal device 50 is installed upstream of the compressor 42 itself because the CO2 absorbent solutions have the same affinity for H2S; consequently, a gas from which CO2 is removed by means of amine solvents also automatically is deprived of all the sulfur contained in the gas.

[0067] This also means that the compressor can be made from less expensive materials because the acid attacks decrease.

[0068] By way of example only, the carbon dioxide removal device 50, which involves the capture of carbon dioxide by means of chemical absorption with amine-based solvents, can comprise in succession at least one absorption column, a recovery heat exchanger, and a regeneration column. A reboiler and a gas-liquid separator can cooperate with said regeneration column.

[0069] Downstream of the carbon dioxide removal device 50, in particular downstream of the absorption column, a washing column 8 can be provided, in which the gas from which carbon dioxide was removed is washed, preferably by contact with demineralized water, in order to remove and recycle any leakage of amine solution back into the system. Preferably, the washing column 8 is arranged between said carbon dioxide removal device 50 and said only one compressor 42. A further advantage of the washing column 8 is that there is no chloride deposition in the compressor 42, avoiding phenomena such as stress corrosion cracking which instead remains a problem in the configuration of the prior art.

[0070] Alternatively, the carbon dioxide removal device 50 is of the pressure-cycle adsorption type on zeolite beds (PSA or VPSA Systems).

[0071] In a first preferred variant of the invention, the recovery and treatment line 10 comprises, downstream of reactor 1 , at least one heat exchange device 3, preferably only one heat exchange device 3, so that the heat is transferred from the exhaust gas to a heat transfer fluid.

[0072] Preferably, the at least one heat exchange device 3 is proximal to the reactor 1 , such as immediately downstream of the reactor.

[0073] A pipe 75, capable of transporting the heat transfer fluid exiting from said at least one heat exchange device, can advantageously connect the heat exchange device 3 to the carbon dioxide removal device 50, in order to provide energy for carbon dioxide absorption by means of amine solution, and / or connect the heat exchange device 3 to the feeding line 11 whereby the heat and composition of the heat transfer fluid (typically steam) can be transferred directly to the process gas, increasing the efficiency of the plant by virtue of the increased temperature of the process gas which is directed towards the heating unit 18, thereby decreasing the energy consumption, and providing the most preferred humidity content to the stream inside feeding line 11 for the correct operation of reactor 1 .

[0074] More in detail, pipe 75 connects the at least one heat exchange device 3 to the treatment and feeding line 11 in a zone preceding the heating unit 18, considering the flow direction of the process gas, so that the heat and mass transfer fluid is added directly to the process gas flow in the treatment and feeding line 11 .

[0075] Preferably, pipe 75 connects the at least one heat exchange device 3 to the treatment and feeding line 11 in a zone between the only one compressor 42 and the heating unit 18.

[0076] In a variant, there is provided a direct connection between the pipe 75 and the treatment and feeding line 11 , possibly, but not necessarily, by means injection nozzles or lances.

[0077] For example, by using water to cool the exhaust gas in the heat exchange device 3, the heat transfer fluid in the pipe 75 is dry steam. Therefore, when pipe 75 connects the first heat exchange device 3 to the carbon dioxide removal device 50, this steam is the energy carrier for the absorption by means of amine solution.

[0078] When pipe 75 connects the first heat exchange device 3 to the treatment and feeding line 11 , since the exhaust gas that is compressed by the compressor 42 is no longer cooled after compression and is strongly superheated, the process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line 10, can be humidified by adding said dry vapor directly to the process gas flow, thus eliminating the need to provide a special humidifier along the treatment and feeding line 11. This humidification operation makes it possible to adjust the water content of the process gas in case of high content of heavy gaseous hydrocarbons in said process gas.

[0079] The heating unit 18 is arranged downstream of the connection between the pipe 75 and said treatment and feeding line 11 .

[0080] In a second preferred variant of the invention, a pipe 34 advantageously connects a stretch of the recovery and treatment line 10, comprised between the carbon dioxide removal device 50 and the compressor 42, to burners of the heating unit 18, in order to send part of the exhaust gas free of carbon dioxide and hydrogen sulfide as fuel gas for said burners.

[0081] An adjusting device, e.g., a pressure control valve, is preferably provided along the pipe 34.

[0082] The fact of providing the sending of this gas, known as tail gas, already free of carbon dioxide and hydrogen sulfide, to the burners of the heating unit 18 has the advantage of reducing atmospheric emissions of CO2 and SO2, maximizing instead the selective recovery in the carbon dioxide removal device 50.

[0083] In general, the heating unit 18 is supplied by the combustion in part of a fuel coming directly from the external source 20 and in part of said exhaust gas, free of carbon dioxide and hydrogen sulfide, coming from the pipe 34.

[0084] The technical features of the first variant and the second variant can be combined together into a third variant of the plant of the invention.

[0085] In all embodiments of the invention, at least one knockout drum 7 can be provided in the recovery and treatment line 10, upstream of the carbon dioxide removal device 50, to remove liquid particles from the exhaust gas which can contaminate the solution circulating in the device 50.

[0086] In particular, the knockout drum allows liquid particles to be removed from the gas by the use of a vane demister. The gas enters the drum and passes through the vane demister. The sudden changes in direction push the liquid particles against metal sheet walls. The droplets, joining into larger droplets, precipitate by gravity to the part below where they are collected and drained.

[0087] In a preferred, but not limiting, variant, the recovery and treatment line 10 comprises, or consists of, in sequence

[0088] - at least one heat exchanger 3 to cool the exhaust gas coming out of the reactor 1 ;

[0089] - at least one washing and cooling unit 4 to remove dust from the exhaust gas, obtaining a dedusted gas, i.e. , cleaned from the dust content drawn out from the reactor;

[0090] - a separator 5 to remove the washing water from the dedusted gas flow;

[0091] - a cooling unit 6, e.g., a hairpin cooler or a non-contact exchanger or a water / gas direct contact cooling column, to cool the dedusted gas;

[0092] - said at least one knockout drum 7 to remove liquid particles from the already dedusted and cooled gas;

[0093] - the carbon dioxide removal device 50;

[0094] - a possible washing column 8;

[0095] - and said compressor 42.

[0096] A further advantage is that downstream of the carbon dioxide removal device 50 the gas from which the carbon dioxide has been removed is preferably washed with demineralized water in the washing column 8, whereby when the gas flow is then compressed by means of the compressor 42 and the temperature takes the water to evaporation there is no chloride deposition in the compressor, avoiding phenomena such as stress corrosion cracking which instead remains a problem in the configuration of the prior art, where the compressor 42 is located upstream of the removal device 50 and downstream of the cooling unit 6 that, also having to cool the gas, inevitably makes use of large quantities of industrial water. A further advantage is that arranging the carbon dioxide removal device 50 upstream of the single compressor 42 and downstream of the cooling unit 6 makes it possible to eliminate the post-cooler 43, which is present in the configuration of the prior art (Figure 1) and whose purpose is solely to cool the gas in order to make the performance of the absorption column more efficient.

[0097] Further advantages of the configuration of the invention are given below.

[0098] 1 ) In general, the carbon dioxide removal systems work under pressure because the pressure promotes removal thereof.

[0099] In amine solution-based removal systems, the two main columns, i.e., the absorption column (Absorber), which removes carbon dioxide from the gas, and the regeneration column (Stripper), which removes carbon dioxide from the amine solution, operate at two distinct pressures.

[0100] The regeneration column works at the lowest possible pressure, generally 0.3- 0.5barg, and the absorption column works at the pressure of the direct reduction process, which is about 6barg if the carbon dioxide removal device 50 is arranged upstream of the compressor 42, or 10barg if arranged downstream of the compressor 42.

[0101] These two columns are connected by means of pumps, generally medium voltage, which in the configuration of the prior art will have to provide a head proportional to 10bar, while in the configuration proposed by the present invention, they will have to provide a head proportional to 6bar, allowing the adoption of smaller pumps with lower electrical consumption.

[0102] 2) An amine solution absorption column is also provided with a demineralized water make-up pump adapted to inject said demineralized water into the washing column downstream of the absorption column to restore losses, which occur during plant operation. The make-up pump will also benefit from a lower head because the carbon dioxide removal device 50 is arranged upstream of the compressor 42.

[0103] 3) The power reduction of the above-mentioned pumps as well as the energy consumption reduction of the process compressor 42 (which treats a smaller amount of gas and has to overcome a lower pressure drop by having removed some equipment from the processing circuit, i.e., post-cooler and humidifier) allow for an estimated energy consumption reduction of around 10-15%.

[0104] In all the embodiments of the plant of the invention, the external source 20 is connected to said treatment and feeding line 11 or to said recovery and treatment line 10.

[0105] In particular, the external source 20 is connected to a stretch of the circuit between the compressor 42 of the recovery and treatment line 10 and the heating unit 18 of the treatment and feeding line 11 , preferably between the compressor 42 and the connection of the pipe 75 to the treatment and feeding line 11.

[0106] The first choice in reducing gas for the external source 20 is Natural Gas.

[0107] Alternatively, the external source 20 can provide a stream, containing hydrogen and carbon monoxide, generated in an external unit (Reformed Gas or Synthetic Gas) by hydrocarbon reaction.

[0108] Alternatively, the external source 20 of make-up reducing gas can be a commercially pure gaseous hydrogen source (at least 99% by volume) or a gas source with a gaseous hydrogen content of at least 65% by volume, preferably from 70 to 98% by volume.

[0109] In the case of make-up gas containing gaseous hydrogen with a content of at least 80% by volume of gaseous hydrogen, the rest of the composition can comprise carbon monoxide, water, carbon dioxide, methane, and nitrogen.

[0110] By way of example only, a make-up gas composition containing gaseous hydrogen can be as follows in volume percentage: gaseous hydrogen in the range of 92-96%; carbon monoxide in the range of 1 .5-2.5%; water 0.2-0.6%; carbon dioxide 0.0-0.4%; methane 0.3-0.9%; nitrogen 2.0-4.0%.

[0111] Alternatively, the external source 20 of make-up reducing gas is a source of a gas containing gaseous hydrocarbons with a gaseous hydrocarbon content of at least 25% by volume, such as coke oven gas, Corex Gas, hydrogenated gases released from other industrial activities, and so on. At least one gas injection device, adapted to inject gas containing gaseous hydrocarbons, such as natural gas or coke oven gas, into a lower area of the reactor 1 arranged below the reduction area 2, or directly into a transition zone of the reactor 1 between the reduction area 2 and a reactor discharge area, can also be provided.

[0112] An example of a process, at full capacity, for the direct reduction of iron oxides, carried out by means of the plant just described, is described below. This process comprises the following steps at full capacity: a) recovering and treating the exhaust gas coming out of reactor 1 by means of the recovery and treatment line 10; b) feeding process gas into a reduction area 2 of the reactor 1 by means of the treatment and feeding line 11 , said process gas being obtained by mixing the make-up gas, coming from the external source 20, with the exhaust gas treated in the recovery and treatment line 10; wherein, before the exhaust gas is introduced into the treatment and feeding line 11 , there are provided in succession a removal of carbon dioxide from said exhaust gas, by means of the carbon dioxide removal device 50, and a compression of the exhaust gas free of carbon dioxide, by means of the only one compressor 42 of the recovery and treatment line 10, towards said treatment and feeding line 11 .

[0113] Preferably, the removal of carbon dioxide from the exhaust gas is carried out by means of capture with amine solvents to remove, e.g., by chemical absorption, both carbon dioxide and hydrogen sulfide.

[0114] In a first variant of the process of the invention, heat is transferred from the exhaust gas coming out of the reactor 1 to a heat transfer fluid, by means of the at least one first heat exchange device 3 provided in the recovery and treatment line 10 downstream of the reactor 1 .

[0115] The heated heat transfer fluid is transferred at least partially, by means of the pipe 75, from said at least one first heat exchange device 3 to the treatment and feeding line 11 , and possibly from said at least one first heat exchange device 3 to the carbon dioxide removal device 50, with the purpose of providing energy for carbon dioxide absorption by amine solution. More in detail, the heated heat transfer fluid is transferred in a zone of the treatment and feeding line 11 preceding the heating unit 18, preferably between the only one compressor 42 and the heating unit 18, by means of a direct addition of said heat transfer fluid to the process gas flow.

[0116] Preferably, said heated heat transfer fluid is dry steam which is added directly to the process gas flow in the treatment and feeding line 11 to humidify said process gas without the need for a dedicated vessel for this purpose as in the configuration of the prior art.

[0117] In a second variant of the process of the invention, part of the exhaust gas free of carbon dioxide is sent as fuel gas to the burners of the heating unit 18 by means of the pipe 34 which connects the stretch of the recovery and treatment line 10, comprised between the carbon dioxide removal device 50 and the compressor 42, to said burners.

[0118] The features of the first process variant and second process variant can be combined into a third process variant.

[0119] In all these process variants, the exhaust gas free of carbon dioxide can be washed, preferably by contact with demineralized water, by means of the washing column 8, possibly arranged between the carbon dioxide removal device 50 and the only one compressor 42, in order to remove and recycle any leakage of amine solution.

[0120] In a further variant of the process of the invention, the elimination of liquid particles from the exhaust gas is provided by means of at least one knockout drum 7, e.g., a single knockout drum 7, immediately upstream of the carbon dioxide removal device 50. Advantageously, the amount of gas leaving said at least one knockout drum 7 is greater than the amount of gas entering the compressor 42, which is, therefore, less charged with respect to the configuration of the prior art.

[0121] The catalytic reforming of the process gas can be carried out in situ inside the reactor 1 , so that the process gas undergoes no additional catalytic reaction beyond the catalytic reactions which take place in the reactor 1 , or inside an external reductant generation unit, i.e. an external catalytic reforming unit.

Claims

CLAIMS1. A direct reduction plant for direct reduction of iron oxides, comprising a gas circulation circuit comprising:- a reactor (1 ) having a reduction area (2) adapted to be charged with said iron oxides;- an external source (20) of make-up gas;- a recovery and treatment line (10), arranged downstream of the reactor (1 ), to recover and treat exhaust gas exiting the reactor (1 );- a treatment and feeding line (11 ), arranged upstream of the reactor (1 ), to treat a process gas, obtained by mixing the make-up gas with the exhaust gas treated in the recovery and treatment line (10), and to feed the reduction area (2) of the reactor (1 ) with said process gas; wherein the recovery and treatment line (10) communicates, downstream of the reactor (1 ), with said treatment and feeding line (11 ); wherein the treatment and feeding line (11 ) comprises a heating unit (18), upstream of the reactor (1 ), to heat the process gas to a temperature suitable for introduction into the reactor (1 ); wherein the recovery and treatment line (10) comprises only one compressor (42); wherein the recovery and treatment line (10), in the end stretch thereof proximal to said treatment and feeding line (11 ), comprises- a carbon dioxide removal device (50) to remove carbon dioxide from the exhaust gas, and said only one compressor (42), arranged between said carbon dioxide removal device (50) and said heating unit (18), to compress the exhaust gas free of carbon dioxide towards said treatment and feeding line (11 ).

2. A plant according to claim 1 , wherein the portion of the treatment and feeding line (11 ), arranged between the only one compressor (42) and the heating unit (18), has no additional components along the extension thereof.

3. A plant according to claim 1 or 2, wherein said only one compressor (42) is associated to the carbon dioxide removal device (50), preferably connected to said carbon dioxide removal device (50).

4. A plant according to any one the preceding claims, wherein the carbon dioxide removal device (50) is of the capture type with amine solvents or of the adsorption type with pressure cycling on zeolite beds (PSA or VPSA Systems).

5. A plant according to claim 4, wherein there is provided a washing column (8), arranged between said carbon dioxide removal device (50) and said only one compressor (42), so that the exhaust gas free of carbon dioxide is washed, preferably by contact with demineralized water, in order to remove and recycle any leakage of amine solution.

6. A plant according to any one the preceding claims, wherein the recovery and treatment line (10) comprises, downstream of the reactor (1 ), at least one first heat exchange device (3) so that heat is transferred from the exhaust gas to a heat transfer fluid; preferably wherein a pipe (75), capable of transporting the heat transfer fluid exiting from said at least one first heat exchange device (3), connects the at least one first heat exchange device (3) to the treatment and feeding line (11 ) in a zone preceding the heating unit (18), preferably between the only one compressor (42) and the heating unit (18), so that the heat transfer fluid is added directly to the process gas flow in the treatment and feeding line (11 ); preferably wherein there is provided a direct connection between the pipe (75) and the treatment and feeding line (11 ); preferably wherein said pipe (75) also connects the at least one first heat exchange device (3) to the carbon dioxide removal device (50) of the capture type with amine solvents.

7. A plant according to any one of the preceding claims, wherein an additional pipe (34) is provided, which connects a stretch of the recovery and treatment line (10), comprised between the carbon dioxide removal device (50) and the compressor (42), to burners of the heating unit (18), to send part of the exhaust gas, free of carbon dioxide, as fuel gas for said burners.

8. A plant according to any one of the preceding claims, wherein at least one knockout drum (7) is provided in the recovery and treatment line (10), upstream of said carbon dioxide removal device (50), to remove liquid particles from the exhaust gas.

9. A plant according to claim 8, wherein the recovery and treatment line (10) comprises in sequence- at least one first heat exchange device (3) to cool the exhaust gas coming out of the reactor (1 );- at least one washing and cooling unit (4) to remove dust from the exhaust gas, resulting in a dedusted gas;- a separator (5) to remove the washing water from the dedusted gas flow;- a cooling unit (6) to cool the dedusted gas;- said at least one knockout drum (7) to eliminate liquid particles from the already dedusted and cooled gas;- said carbon dioxide removal device (50);- an optional washing column (8);- and said only one compressor (42).

10. A direct reduction process for a direct reduction of iron oxides, implementable by a plant according to any one of the preceding claims, the process comprising the following steps when fully operational: a) recovering and treating the exhaust gas coming out of the reactor (1) by means of the recovery and treatment line (10); b) feeding process gas into a reduction area (2) of the reactor (1 ) by means of the treatment and feeding line (11 ), said process gas being obtained by mixing the make-up gas, coming from the external source (20), with the exhaust gas treated in the recovery and treatment line (10); wherein, before the exhaust gas is introduced into the treatment and feeding line (11 ), there are provided in succession a removal of carbon dioxide from said exhaust gas by means of the carbon dioxide removal device (50) and a compression of the exhaust gas free of carbon dioxide, by means of the only one compressor (42), towards said treatment and feeding line (11 ).

11. A process according to claim 10, wherein the removal of carbon dioxide from said exhaust gas is achieved by means of capture with amine solvents or is achieved by pressure cycling adsorption on zeolite beds (PSA or VPSA Systems).

12. A process according to claim 11 , wherein the exhaust gas free of carbon dioxide is washed, preferably by contact with demineralized water, by means of awashing column (8), arranged between said carbon dioxide removal device (50) and said only one compressor (42), in order to remove and recycle any leakage of amine solution.

13. A process according to any one of claims 10 to 12, wherein heat is transferred from the exhaust gas coming out of the reactor (1) to a heat transfer fluid by means of at least one first heat exchange device (3) provided in the recovery and treatment line (10) downstream of the reactor (1 ); and wherein the heated heat transfer fluid is at least partially transferred by means of a pipe (75) from said at least one first heat exchange device (3) to the treatment and feeding line (11 ) in a zone preceding the heating unit (18), preferably between the only one compressor (42) and the heating unit (18), by means of a direct addition of said heat transfer fluid to the process gas flow in the treatment and feeding line (11 ); preferably wherein the heated heat transfer fluid is also at least partially transferred by means of the pipe (75) from said at least one first heat exchange device (3) to the carbon dioxide removal device (50) of the capture type with amine solvents.

14. A process according to claim 13, wherein said heated heat transfer fluid is steam, which is added directly to the process gas flow in the treatment and feeding line (11 ) to humidify said process gas without the need for a dedicated device; preferably wherein said steam is also supplied to the carbon dioxide removal device (50) for the absorption of carbon dioxide by amine solution.

15. A process according to any one of the claims from 10 to 14, wherein part of the carbon dioxide-free exhaust gas is sent as fuel gas to burners of a heating unit (18), provided in the treatment and feeding line (11 ), by means of a further pipe (34) which connects a stretch of the recovery and treatment line (10), comprised between the carbon dioxide removal device (50) and the only one compressor (42), to said burners.

16. A process according to any one of the claims from 10 to 15, wherein said make-up gas contains gaseous hydrogen with a gaseous hydrogen content of at least 80% by volume or contains gaseous hydrocarbons, preferably with a gaseous hydrocarbon content of at least 25% by volume.

17. A process according to any one of the claims from 10 to 16, wherein the catalytic reforming of the process gas is carried out in situ inside the reactor (1 ), so that the process gas undergoes no additional catalytic reaction beyond the catalytic reactions which take place in the reactor (1 ), or inside an external reductant generation unit.

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