Patent Application
20240240275
The invention is related to a method for manufacturing Direct Reduced Iron (DRI) and to a DRI manufacturing equipment.
Steel can be currently produced through two main manufacturing routes. Nowadays, most commonly used production route consists in producing pig iron in a blast furnace, by use of a reducing agent, mainly coke, to reduce iron oxides. In this method, approx. 450 to 600 kg of coke, is consumed per metric ton of pig iron; this method, both in the production of coke from coal in a coking plant and in the production of the pig iron, releases significant quantities of CO2.
The second main route involves so-called “direct reduction methods”. Among them are methods according to the brands MIDREX, FINMET, ENERGIRON/HYL, COREX, FINEX etc., in which sponge iron is produced in the form of HDRI (Hot Direct Reduced Iron), CDRI (cold direct reduced iron), or HBI (hot briquetted iron) from the direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI, and HBI usually undergo further processing in electric arc furnaces.
There are three zones in each direct reduction shaft with cold DRI discharge: Reduction zone at top, transition zone at the middle, cooling zone at the cone shape bottom. In hot discharge DRI, this bottom part is used mainly for product homogenization before discharge.
Reduction of the iron oxides occurs in the upper section of the furnace, at temperatures up to 950° C. Iron oxide ores and pellets containing around 30% by weight of Oxygen are charged to the top of a direct reduction shaft and are allowed to descend, by gravity, through a reducing gas. This reducing gas is entering the furnace from the bottom of reduction zone and flows counter-current from the charged oxidised iron. Oxygen contained in ores and pellets is removed in stepwise reduction of iron oxides in counter-current reaction between gases and oxide. Oxidant content of gas is increasing while gas is moving to the top of the furnace.
The reducing gas generally comprises hydrogen and carbon monoxide (syngas) and is obtained by the catalytic reforming of natural gas. For example, in the so-called MIDREX method, first methane is transformed into a reformer according to the following reaction to produce the syngas or reduction gas:
and the iron oxide reacts with the reduction gas, for example according to the following reactions:
At the end of the reduction zone the ore is metallized.
A transition section is found below the reduction section; this section is of sufficient length to separate the reduction section from the cooling section, allowing an independent control of both sections. In this section carburization of the metallized product happens. Carburization is the process of increasing the carbon content of the metallized product inside the reduction furnace through following reactions:
Injection of natural gas in the transition zone is using sensible heat of the metallized product in the transition zone to promote hydrocarbon cracking and carbon deposition. Due to relatively low concentration of oxidants, transition zone natural gas is more likely to crack to H2 and Carbon than reforming to H2 and CO. Natural gas cracking provides carbon for DRI carburization and, at the same time adds reductant (H2) to the gas that increases the gas reducing potential.
In view of the considerable increase in the concentration of CO2 in the atmosphere since the beginning of the last century and the subsequent greenhouse effect, it is essential to reduce emissions of CO2 where it is produced in a large quantity, and therefore in particular during DRI manufacturing.
Based on the above, there is a need for a method of manufacturing Direct Reduced Iron that is CO2-neutral, environmentally friendly and easy to implement, while showing a good yield.
This problem is solved by a method for manufacturing Direct Reduced Iron wherein iron ore is reduced in a DRI shaft by a reducing gas comprising hydrogen obtained by extraction from coke oven gas through a hydrogen separation unit, the remaining part of such coke oven gas being at least partly injected in the transition section of said DRI shaft to set the carbon amount of said Direct Reduced Iron from 0.5 to 3 wt. %.
The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations:
In the frame of the present invention, Direct Reduced Iron covers so-called DRI, but also hot briquetted iron (HBI), Cold Direct Reduced iron (CDRI) and Hot Direct Reduced Iron (HDRI). Such material can be later used in different processes, like, for example, processes to produce pig iron in a blast furnace or steel in a BOF or in an electric arc furnace. It can be also used as a combustible or as an electrode in a battery.
The invention is also related to a DRI manufacturing equipment including a DRI shaft and a hydrogen separation unit, wherein said hydrogen separation unit inlet is connected to a coke oven gas supply and includes a first outlet connected to the DRI shaft to inject hydrogen separated from said coke oven gas and a second outlet connected to the transition section of said DRI shaft to inject at least part of the remaining part of such coke oven gas.
The equipment may also comprise the following optional characteristics considered separately or according to all possible technical combinations:
Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication and which is in no way restrictive, with reference to the appended FIGURES in which:
Elements in the FIGURE are illustration and may not have been drawn to scale.
On top of the shaft, the top gas exiting the DRI shaft is collected in a pipe 20 which can optionally be connected to a scrubber 2 located on the top gas outlet of the DRI shaft. The top gas exiting from the DRI shaft usually comprises H2, CO, CH4, H2O, CO2 and N2 in various proportions. The top gas scrubbing operation allows removing water vapor from the rest of the stream to improve its reduction potential.
In a preferred embodiment, after scrubbing, the top gas comprises from 40 to 75 vol % of H2, from 0 to 30 vol % of carbon monoxide CO, from 0 to 10 vol % of methane CH4, from 0 to 25 vol % of carbon dioxide CO2, up to 5 vol % of H2O, the remainder being nitrogen N2. It is preferred to have, after scrubbing, a ratio of H2/N2 from 1.5 to 3 in such top gas.
Once the top gas exits the scrubber 2, it can optionally be compressed and can either be sent back to the DRI shaft or sent to one of the inlets of a mixer 4 through a connecting pipe 21.
Another inlet of said mixer 4 can be connected to a reduction gas supply 3. Such reduction gas can consist in hydrogen or in a hydrocarbon gas, like methane for example. In a preferred embodiment, the hydrogen supply is fed with green hydrogen produced without CO2 emission, for example by water or steam electrolysis that can be powered with CO2 neutral electricity.
CO2 neutral electricity includes notably electricity from renewable source but can encompass the use of electricity coming from nuclear sources as it is not emitting CO2 to be produced. CO2 from renewable source is defined as energy that is collected from renewable resources, which are naturally replenished on a human timescale, including sources like sunlight, wind, rain, tides, waves, and geothermal heat.
In another embodiment, the reductant gas supply consists in a biogas, which is a renewable energy source that can be obtained by the breakdown of organic matter in the absence of oxygen inside a closed system called bioreactor. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, food waste or any biodegradable materials. A preferred bio gas is for example biomethane.
A third inlet of the mixer 4 is connected to the outlet of a separation unit 5. Such separation unit 5 is connected to a Coke Oven gas supply 6. Coke oven gas composition is usually comprising 3 to 6 vol % of CO, 1 to 5 vol % of CO2, 36 to 62 vol % of H2, 16 to 27 vol % of CH4, the remainder being nitrogen. Coke oven is produced as a by-product of the coke production and is usually used to fire the coke oven battery or simply burned. In most cases, its further use results in CO2 emissions in the atmosphere.
The separation unit 5 allows extracting hydrogen from such stream and sending such hydrogen to the mixer 4 through a connecting pipe 50.
The separation unit can be based on any suitable industrial process of separation of gases, like physical and chemical absorption processes, adsorption processes or membrane processes.
In a preferred embodiment, the separation unit is a Pressure Swing Adsorption (PSA).
In another embodiment, the separation unit is a membrane, preferably a ceramic microporous membrane.
The reduction gas produced in the mixer 4 through the addition of the top gas, additional reductant gas and hydrogen from Coke Oven Gas, can optionally be heated through heating means provided to the mixer, such heating means being for example preferably powered by CO2 neutral electricity or by burning a part of the coke oven gas. In a preferred embodiment, the temperature of the reduction gas is set to a range from 700° ° C. to 1000° C., preferably from 800 to 1000° C.
This reduction gas is then sent back to the DRI shaft, preferably in its reduction section through a pipe 11.
Coming back to the separation unit 5, the remaining part of the gas obtained after extraction of the hydrogen, is being sent back to the transition section of the DRI shaft 1 through a connecting pipe 51.
The injection of this gas is made to increase the carbon content of the Direct Reduced Iron to a range from 0.5 to 3 wt. %, preferably from 1 to 2 wt. % which allows getting a Direct Reduced Iron that can be easily handled and that keeps a good combustion potential for its future use.
The DRI manufacturing equipment may further comprise a recycling loop in the cooling section that allows extracting part of the gas present at that level to send it in a scrubber 30 and in a compression unit 31 before reinjecting it in the shaft 1.
In a preferred embodiment, part of the gas transported in the connection pipe 51 can be injected in such recycling loop of the cooling section after the compression unit to allow increasing the carbon content of the Direct Reduced Iron in the cooling section as well.
It is also possible to inject part of the gas transported in the connection pipe 51 in the reduction section of the DRI shaft, as such gas has a reduction power thanks to its content in CO and remaining H2.
By using the method according to the invention, Direct Reduced Iron can be manufactured with the appropriate quality and yield, while remaining CO2 neutral and taking optimal advantage of the gas co-product from coke manufacturing. It also allows decreasing the use of fossil energy like natural gas.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/054252 | 5/18/2021 | WO |
