The present invention pertains to a method and a direct reduction plant for producing direct reduced iron.

Generation of electric energy by coal fired power stations and the production of iron and steel are both industrial processes with high emission levels of carbon dioxide (C02). It is well known that the emission of C02 into the atmosphere presently leads to an ever-increasing content of this gas in the atmosphere which may lead to global warming.

Direct reduced iron (DRI), produced by direct reduction of iron ore, is often used in the production of iron and steel and is therefore an important feedstock in these industries.

US 4,701,214 A describes the original direct reduction process in a rotary hearth furnace, also known under the brand name of „Fastmet“. US 5,730,775 A describes a further improvement of the „Fastmet“ process.

US 5,685,524 A describes the use of self-reducing pellets in a rotary hearth to produce DRI for use in iron- or steelmaking. In an effort to save energy the pellets are preheated with hot off gas from other iron- or steelmaking processes.

US 5,066,325 A describes a three-stage process in which coal is treated in a first stage to drive off volatiles and to form a very low volatile char. The volatiles can be collected and utilized as a valuable oil product, such as for diesel fuel or the like. The char material is utilized as a reductant in a second stage of direct iron reduction. The hot product gases generated in the reduction process are utilized to generate electricity in a third stage.

Also, US 6,519,942 B2 describes the usage of hot off gases from a direct iron production process to generate electricity.

WO 1994/008055 A1 describes a method to use Petroleum Coke (petcoke) to create first a reducing gas which can be used to reduce iron ore. Due to its high sulfur content petcoke is often an unwanted material, but in the context of direct reduction where a de-sulfurization process must be employed anyway it can be economically to use it as reductant and/or fuel.

US 7,632,330 B2 describes a method by which iron ore can be reduced using biomass particles and resinous material. US 2013/0081516 A1 describes a direct reduction of iron ore with biomass and coal. By using an excess of biomass synthetic gas is produced which can be used to produce electricity.

In general, the off gas from the direct reduction process for producing DRI contains pollutants such as dust, oxides of sulfur (SOx), oxides of nitrogen (NOx) etc. It is state of the art and also a legal requirement to remove these pollutants from the off gas before venting it to the atmosphere. The process for removing the pollutants can be carried out in such a way that the reaction products will be in the form of a solution („wet scrubbing “) or dry solids („dry scrubbing").

US 7,625,527 B2, for example, describes a method to treat a hot off gas with slurry of lime in such a way that a dry reaction product is obtained. Dry scrubbing is also described in US 4,019,444 A. Dry scrubbing products are favourable because no waste liquid has to be treated further and often the by-product can be used for other applications.

Although pollutants in the off gas can be removed, e.g. with dry or wet scrubbing, it nevertheless still comprises significant amounts of C02 that reach the atmosphere when the off gas is vented into the atmosphere.

Carbon capture and storage (CCS) technology has been widely described in recent years for recovering of C02 gases. This technology is seen as a potential solution to diminish or even reduce the ever-increasing content of C02 in the atmosphere which may lead to a global warming of the atmosphere.

CCS may be used advantageously in industrial processes where off gases with relatively high carbon dioxide concentrations are generated. For off gases with low C02 content the technology cannot be used economically due to the high costs of treating high off gas volumes. The higher the C02 concentration in the off gas, the better the efficiency of a CCS process becomes.

It is usually a prerequisite that the off gas entering the CCS process is purified from all other pollutants such as dust, SOx, NOx etc. This is usually achieved by a gas cleaning system using wet scrubbing or dry scrubbing with basic absorbents such as lime, magnesia, soda ash and similar compounds.

One of the established technologies of CCS is amine absorption. In this case C02 in off gas is absorbed into a liquid solution of alkanolamines and thereby removed from the off gas. Absorbents require a regeneration step (desorption) where the C02 is removed in order for the sorbent or solution to be reused. The removed C02 may be used further or stored. US 4,112,052 A describes such a method in more detail.

The first step of C02 absorption takes place at temperatures of typically around 40°C to 60°C, the second step of desorption takes place at a higher temperatures, typically about 80°C to 150°C. This second step of the process is the main energy consumer since a relatively large volume of C02-rich absorbent must be heated from the lower to the higher temperature.

The recovered C02 can be stored or used in other industrial processes, for example for enhanced oil recovery. EP 1,258,595 A2 describes a method to inject a gas mixture of preferably more than 70% C02 into a wellbore to improve the recovery of oil from the well or underground formation.

An object of the present invention is the efficient and economical production of DRI with low C02 emissions. This is achieved according to the features of the independent claims.

By providing the heat for the direct reduction process primarily electrically, and not by a combustion process, essentially no nitrogen is introduced into the off-gas of the direct reduction process and, thus, the C02 concentration in the off-gas can be increased (as compared to conventional DRI processes with combustion). This allows an efficient and economical removal of C02 from the off-gas.

When the heat of the hot off-gas is used for producing steam and using at least part of the produced steam to drive a steam turbine and an electrical generator for producing electrical energy, then at least part of this electrical energy may advantageously be used for electrically heating the direct reduction unit or may be used in other industrial processes or plants. This makes the DRI process even more economical.

The very high concentrations of C02 in the off-gas allows the efficient C02 recovery using a carbon capture and storage process. Efficiency may even be increased when the residual heat of the steam downstream of the steam turbine is used as heat source for the carbon capture and storage process. In that way, the most energy consuming step in the CCS process, namely the desorption step, may be supported by this residual heat and less additional heat must be provided.

When the off-gas contains water or water vapour, especially when steam or an oxy-fuel burner is used for purging off-gas from the direct reduction unit, the carbon dioxide final gas can simply be provided by cooling the purified off-gas for condensing water comprised in the off-gas and by removing the condensed water.

The present invention is explained in the following in more detail referring to Figures 1 to 4, which show exemplary, schematic and non-limiting advantageous embodiments of the invention. In the drawings

Fig.1 shows a direct reduction unit with electrical heating,

Fig.2 shows a direct reduction process in combination with the generation of electrical energy and C02 recovery from the off-gas,

Fig.3 shows using steam for purging off-gas from a direct reduction unit and Fig.4 shows using oxy-fuel burners for purging off-gas from a direct reduction unit.

It has been mentioned above that the removal of carbon dioxide from an industrial off gas stream is more economic if the C02 content in the off gas is as high as possible. Most of the C02 worldwide is generated by combustion processes where carbonaceous material, like coal or hydrocarbons like oil or gas, is burned with air. Thus, it is inevitable that the biggest portion of the off gas is nitrogen from the air, according to the following formula,

C + 02 + 4 N2 = C02 + 4 N2

Therefore, the theoretical highest content of C02 in such combustion processes is 1/5 (20%). In practice however due to combustion with excess air it is considerably lower.

The C02 content could be increased with chemical looping combustion (CLC) which increases the efficiency of combustion processes.

In a direct reduction process for producing DRI an iron ore is heated in a hearth or furnace at a temperature high enough to burn off its carbon and oxygen content (a process called reduction) but below iron's melting point, i.e. not higher than 1200°C. The objective of the process is to drive off the oxygen contained in the various forms of iron ore, in order to convert the ore to metallic iron, without melting it. The required heat for this process is provided by burning carbonaceous material, like coal or hydrocarbons like oil or gas, in a furnace with air. In the direct reduction process about 30% of the carbonaceous material is consumed for supplying the necessary heat of reaction and is consumed in a combustion reaction using air as an oxidant. The rest is used for the reduction of the iron ore. Combined, the reactions (reduction and combustion) in the direct reduction process with coal as carbonaceous material could be written as

1,4 Fe203 + 2,4 C + 0,302 + 1,2 N2 = 2,8 Fe + 2,4 C02 + 1,2 N2 resulting in C02 concentration in the off gas of 2, 4/3, 6 (67%).

This C02 concentration in the off-gas is more than 3 times higher than in a normal combustion process and, therefore, it is suited for combination with a C02 removal process, like a CCS process, e.g. removal of C02 by an amine washing process.

By using at least partly biomass in the direct reduction process as a reductant for the iron ore and/or as a source to provide the necessary reaction energy the whole process can even become „carbon negative", i.e. the carbon dioxide content in the atmosphere can actually be reduced from present levels.

It is a goal of the invention to increase the C02 concentration in the off gas of the direct reduction process, in order to make the subsequent C02 removal process even more economical.

In direct reduction of iron oxide, the oxygen which is reacting with carbon is originating from the iron oxide and thus no nitrogen is involved, according to the formula

2Fe203 +3 C = 4Fe + 3 C02. In reality, this is a multi-stage process with an intermediate formation of CO gas but the overall formula can be used for this theoretical calculation. Theoretically, the C02 content in the off gas could be 100%, again in practical installations the C02 content is lower.

The required energy (heat) for driving this process is now provided by electrical heating and not by combustion of carbonaceous material. In that way no carbonaceous material has to be burned with air to drive the reaction and thus no nitrogen is contained in the off gas. By this method an off gas is generated with a carbon dioxide content of more than 80%, and up to 95%, typically up to 90%, whereas the remainder being mainly pollutants like CO, SOx, NOx and possibly water (H20).

Electrical heating can be realised by arranging electrically heated heating elements 11 in the direct reduction unit 1, like a rotary hearth, in the vicinity of the reduction bed 12 onto which the materials 13 of the direct reduction process (iron ore, carbonaceous material like coal or petcoke, self reducing pellets (as explained below)) are placed, as shown in Fig.1. The heating elements 11 may be arranged above and/or surrounding the reduction bed 12, for example. The direct reduction unit 1 is heated to a temperature of typically 1000°C to 1200°C by the heating elements 11 to drive the direct reduction process. The direct reduction process takes preferably place at temperatures between 1000°C and 1200°C and typically for 15 to 60 minutes.

The C02 (and polluting constituents) containing off gas produced in the direct reduction process is continuously removed from the direct reduction unit 1, for example with removal by suction or by continuously purging the direct reduction unit 1 (as explained in more detail below).

The hot off gas from the direct reduction unit 1 may also be used as energy source for an electricity production process, es explained with reference to Fig.2. In the embodiment of Fig.2, the hot off gas is used for steam production that is used in a steam turbine 4, that drives an electric generator 5. The generated electric energy may be used in the direct reduction unit 1 for electrically heating, as indicated in Fig.2 with dashed line. In that way at least part of the electrical energy for electrically heating the direct reduction unit 1 may be provided.

This allows the combination of a direct reduction process for producing DRI and an electricity production process that uses hot off gas from the direct reduction process, which improves the efficiency of the direct reduction process.

The direct reduction process may also, and independently from combination with electricity production, be combined with a C02 obtaining process for obtaining C02 from the off-gas. In an advantageous embodiment, the direct reduction process is combined with an electricity generation process and with a C02 obtaining process, as is described in the following with reference to Fig.1 in more detail.

In a well-known direct reduction unit 1, e.g. a rotary hearth furnace, tube furnace or similar device, a direct reduction process takes place for producing DRI. The direct reduction unit 1 is fed with raw material 13 comprising iron oxide containing material, like iron ore, and carbonaceous material, like coal, petcoke, wood, biomass, oil etc. The carbonaceous material serves as reductant in the direct reduction process. The direct reduction unit 1 is heated electrically by the heating element 11 for driving the chemical reaction for producing DRI. The electrical energy for the electrical heating element 11 may be provided by an electricity generation process using the hot off gas as energy source.

In a possible embodiment self-reducing pellets are produced as raw material 13 for the direct reduction process. To this end iron oxide, like iron ore, and the carbonaceous material, like petcoke, coal, wood pellets, biomass, or mixtures thereof, are ground to a submicron particle size, whereas at least 90% of the ground particles are submicron. The ground iron oxide and carbonaceous particles are thoroughly mixed in a dry mixer. A suspension of a binder and a solvent, like water, is mixed with the mix of the ground iron oxide and ground carbonaceous material to form a slurry. The slurry is then pelletized, e.g. in a drum pelletizer, by forming pellets and drying the pellets. The size of the formed pellets can be between 10 and 20mm (diameter). The such formed pellets are dried in a furnace to evaporate the solvent of the binder suspension. At which temperatures and for how long the pellets are dried may depend on the binder. Pellets can also be formed by extruding the slurry and cutting disks of the extrudate and then drying the disks. The heat for drying the pellets may be provides by the hot off gas from the direct reduction unit 1.

Such pellets are self-reducing in that they already comprise the required reductant (carbonaceous material) for the direct reduction process.

In an example, iron ore and petcoke (e.g. from an oil refining process) are ground to at least 90% submicron particles. The composition of the used iron ore was Fe total 64,1% as 91,6% Fe203, Si02 5,2%, AI203 1,6%, MnO 1,1%, MgO 0,4%. The composition of the petcoke was carbon 88,0%, hydrogen 3,6%, sulfur 5,1%, ash 0,8%. A binder suspension of 10% bentonite, 20% dextrin and 70% water was prepared. The ground iron ore, ground petcoke and the binder suspension was mixed in the ratio 60:30:10. Pellets of diameter between 10 and 20mm of the such produced slurry were formed and dried in a furnace at 110°C for 3 hours.

The dried pellets were introduced as raw material 13 for the direct reduction process into an electrically heated tube furnace and the temperature of the furnace was set to 1100°C by electrically heating the direct reduction unit 1. After 20 minutes a metallized solid product (DRI) containing 87% elemental iron (Fe) was obtained.

The hot off-gas 14 of the direct reduction process in the direct reduction unit 1 is typically at temperatures of 1150 to 1250°C, depending on the temperature in the direct reduction unit 1, and is led to an electricity production unit 2 and to a purification unit 3.

The electricity production unit 2 comprises a steam turbine 4 that drives a generator 5 for generating electric energy. The generated electric energy may be provided to an electric grid or could also be used for electrically heating the direct reduction unit 1 or supplying electrical energy to another industrial plant, like a steel production plant. The steam for the steam turbine 4 is produced in a heat exchanger 6. The heat exchanger can be of any suitable type, like a parallel-flow, counter-flow or cross-flow heat exchanger. In the heat exchanger 6 the heat in the hot off gas of the direct reduction unit 1 is, directly or indirectly, transferred to the working media of the steam turbine, e.g. water, to vaporize the working media for producing the steam.

The hot off-gas 14 is cooled down in the heat exchanger 6 to temperatures of about 100°C to 150°C, typically around 120°C. The such cooled off-gas is led to an off-gas cleaning device 7 that implements cleaning process, for example a wet scrubbing or dry scrubbing cleaning process as explained above, for removing pollutants in the off-gas and for purifying the off gas. Gaseous or solid pollutants in the off-gas, others than C02, such as dust, SOx, NOx etc. and other possible trace impurities, are removed from the off-gas in the off-gas cleaning device 7.

Downstream of the off-gas cleaning device 7, the off-gas comprises high concentrations of C02, because of the implemented direct reduction process with electrical heating, as explained above. The high concentrations of C02 in the off-gas, typically 80 to 90%, allows efficiently obtaining the C02 in a C02 obtaining unit 8. The C02 obtaining unit 8 implements a carbon capture and storage (CCS) process (like explained above) or by membrane separation, for example. In the C02 obtaining unit 8 the C02 final gas is obtained from the off-gas.

The residual heat in the steam downstream or in the low-pressure section of the steam turbine 4 may advantageously be used in the C02 obtaining unit 8 as indicated in Fig.1. For example, the residual heat of the steam may be used in a CCS process for heating or supporting heating of the C02 enriched absorbent from the lower temperature after the absorption step to the required higher temperature for regeneration of the absorbent and desorption of C02 from the absorbent.

To this end the output steam of the steam turbine 4 could be led to the C02 obtaining unit 8 where the heat of the steam could be directly used for the desorption step. The heat in the steam downstream of the steam turbine 4 could, however, also be transferred to another working media, for example in a further heat exchanger (not shown in Fig.1). Then the heat of this working media could be used in the C02 obtaining unit 8.

The C02 final gas obtained in the C02 obtaining unit 8 may be stored in a C02 storage 9, e.g. an underground storage, or could be used in other industrial processes and could also be liquified before further usage.

Because of the achieved very high concentrations of C02 in the off-gas, it is even possible to simply directly use the purified off-gas from the off-gas cleaning device 7 as the C02 final gas, e.g. in other industrial processes or for storage in a storage unit 9. In this case, the C02 obtaining unit 8 could be omitted.

When the off-gas comprises water, it is also possible to cool the purified off-gas from the off gas cleaning device 7, e.g. by indirect cooling, whereby the contained water in the off-gas is condensed and can be removed. The resulting C02 final gas with a C02 content of >98% can then directly be used, e.g. can be compressed and/or liquified for further transport or usage or can be stored in a storage unit 9. Also, in this case, a C02 obtaining unit 8 could be omitted or could be replaced by a condensing unit for cooling the off-gas.

The C02 final gas has a C02 content of at least 95%, up to 100%.

By using petcoke as carbonaceous material in the direct reduction process, a cycle between oil production and refining and the production of iron or steel could be established. The generated C02 could be used for oil production. Petcoke, a by-product of the oil refining process, could be used for producing the self-reducing pellets for the direct reduction process. The produced DRI is used for steel or iron production. The generated electric energy could be used in the direct reduction unit 1, but also in oil production or refining plants or in a steel production plant.

In a further advantageous embodiment, the hot steam upstream of the steam turbine 4 may also be used in the direct reduction unit 1, as is explained with reference to Fig.3.

Part of the hot (superheated) steam produced in the heat exchanger 6 is branched off and led to the direct reduction unit 1. The main part of the hot steam may be used in a steam turbine 4 as explained above. The branched off steam is used to generated a directional flow in the direct reduction unit 1 for purging the off-gas. To this end, the steam is advantageously fed to the direct reduction unit 1 from below and flows through openings in the reduction bed 12 (as indicated in Fig.3), that may be designed like a grating. The steam therefore also flows through the material 13 on the reduction bed 12.

The steam serves three purposes. Firstly, it supports the reduction process as the water steam reacts with the carbon in the material 13 according to the formula H20 + C = H2 + CO.

Both, H2 and CO, will again react with the iron oxide, e.g. Fe203, in the material 13 leading to reduced iron and the reaction products H20 and C02. Secondly, the heat of the steam supports heating of the direct reduction unit 1 to the reduction temperature. Therefore, less electrical heating is required. And last but not least, the directional flow of the steam in the direct reduction unit 1, may be used for purging the direct reduction unit 1 for removing the hot off-gas from the direct reduction unit 1.

For producing a directional flow in the direct reduction unit 1 for purging the C02, also oxy- fuel burners 15 may be used in the direct reduction unit 1, as shown in Fig.4. An oxy-fuel burner 15 burns fuel, e.g. gas like methane (CH4), using pure oxygen, e.g. according to the formula

CH4 + 202 = C02 + 2H20.

The gaseous combustion products are used for producing the directional flow for purging the direct reduction unit 1. The H20 as reaction product of the oxy-fuel combustion may also support the reduction process as explained above. Furthermore, the heat generated by the oxy-fuel combustion also supports heating the direct reduction unit 1 to the required reduction temperatures. The primary heat source for the direct reduction process is, however, still the electrical heating with the heating element 11. Typically, only 10% to 30% of the required heat for heating the direct reduction unit 1 is provided by the oxy-fuel burner 15.

Using steam and oxy-fuel burners 15 in the direct reduction unit 1 may also be combined.