WO2019219340A1

WO2019219340A1 - Method for the direct reduction of iron ore

Info

Publication number: WO2019219340A1
Application number: PCT/EP2019/060487
Authority: WO
Inventors: Christiaan Johannes Kooij
Original assignee: Tata Steel Nederland Technology B.V.
Priority date: 2018-05-15
Filing date: 2019-04-24
Publication date: 2019-11-21

Abstract

The invention relates to a method for the direct reduction of iron ore using a reducing agent generated in an assembly of solid oxide electrolysis cell modules wherein the direct reduction of the iron ore is controlled dependent on the availability of energy sources to generate electrical energy for the assembly of solid oxide electrolysis.

Description

METHOD FOR THE DIRECT REDUCTION OF IRON ORE

Field of the invention

The invention relates to a method for the reduction of iron ore using hydrogen gas.

Background of the invention

In the steel industry most pig iron is produced by means of the blast furnace (BF) process. In the BF process iron ore together with coal and flux is continuously supplied through the top of a furnace while a hot blast of air or oxygen enriched air is blown into the lower section of the furnace resulting in the reduction and smelting of the iron ore.

In the BF-process there is an absolute minimum amount of carbon necessary per produced ton of pig iron / liquid metal. In the end all this carbon will be emitted as C02 and this gives the minimum amount of C02 for the most optimised process conditions, assuming also zero heat losses. It can easily be calculated that the minimum C02 emission for any carbon based iron making process without C02 sequestration or off-gas recirculation is always more than 1892 kg C02/ton liquid metal. Although the amount of C02 calculated in this manner is far too optimistic in comparison with the actual amount it provides an absolute lower limit on C02 associated with a carbon based steelmaking process without C02 sequestration and without the addition of biomass to the BF process instead of cokes.

In order to try control climate changes one of the measures is to considerably reduce worldwide C02 emissions in the near future, with reductions up to 50% already agreed on. With the above indicated theoretical minimum C02 emission per ton liquid metal, it is impossible to realise drastic reductions of C02 emission in the steel industry without switching at least partially to completely different technologies in the steelmaking industry. In the very near future C02 emissions could be reduced by C02 capture and storage and/or by the use of biomass in the iron- and steelmaking processes but these technologies will not result in the aimed at reduction of C02 emissions and certainly not in a 50% reduction because of the limited possibilities of C02 storage and the availability biomass.

The reduction of iron ore by having it react with hydrogen gas is a well-known process. Most known iron making processes using hydrogen, for instance the MIDREX® process, make use of natural gas where the natural gas is first reformed into a mixture of CO and H2. In this process, part of the iron ore is reduced by carbon monoxide while the other part is reduced by hydrogen. Because large amounts of natural gas are consumed in this process, the plants applying the process are predominantly found where relatively cheap natural gas is available. In many areas of the world natural gas is a by-product from the oil industry, for that reason most of these plants are found in the oil producing countries. Because natural gas is used in the process, still one third of the off gas volume is C02 gas.

Objectives of the invention

It is an objective of the present invention to provide a method for direct reduction of iron ore using hydrogen gas as reducing agent, wherein the hydrogen gas is obtained by electrolysis of water in an assembly of solid oxide electrolysis cells.

It is another objective of the present invention to provide a method for direct reduction of iron ore wherein no C02 is emitted.

It is another objective of the present invention to provide a method for direct reduction of iron ore wherein the energy for the process is obtained from renewable energy resources.

It is another objective of the present invention to provide a method for direct reduction of iron ore wherein the off gas from the iron ore reduction unit is reused in the assembly of solid oxide electrolysis cells.

It is another objective of the present invention to provide a method for direct reduction of iron ore wherein the heat available from the assembly of solid oxide electrolysis cells is used to heat the iron ore reduction unit.

It is still another objective of the present invention to provide a method for direct reduction of iron ore wherein part of the gasses resulting from the electrolysis are used to heat the iron ore reduction unit.

Description of the invention

The invention relates to a method as defined in claims 1 -17. One or more of the objectives of the invention are realized by providing a method for the direct reduction of iron ore using a reducing agent generated in an assembly of solid oxide electrolysis cell modules wherein the direct reduction of the iron ore is controlled dependent on the availability of energy sources to generate electrical energy for the assembly of solid oxide electrolysis cell modules.

A solid oxide electrolysis cell uses electricity for the electrolysis of water wherein the products of the electrolysis, oxygen and hydrogen, are formed on each side of the electrolysis cell. The solid oxide electrolysis cell operates at elevated temperatures with high efficiency.

The advantage of using solid oxide electrolysis cell modules to generate H2 as a reducing agent over known processes is that water is used as source material and not a natural gas as in the known processes. In the known processes the reducing agent is obtained by reforming natural gas into CO and H2 components. However, there is no environmental benefit from the method according the invention compared with the known direct reduction processes if the electrical energy needed to operate the solid oxide electrolysis cell modules is obtained from fossil fuel-fired power stations, like coal-, natural gas- or petroleum-fired power stations.

For that reason it is provided that the electrical energy for the solid oxide electrolysis cell modules is obtained at least partly from renewable energy sources. By operating the solid oxide electrolysis cell modules at least partly on renewable energy a large reduction in C02 emission can be achieved, which is of course dependent on the amount of renewable energy used.

Preferably the solid oxide electrolysis cell modules are operated only on electrical energy from renewable energy sources. However, with the almost unavoidable high and lows in the supply of renewable electrical energy measures are required to control the reduction process.

According to an aspect of the invention it is provided that dependent on the available amount of electrical energy one or more of the solid oxide electrolysis cell modules are switched on or switched off. A certain minimal power is needed to operate a solid oxide electrolysis cell module so that in order to overcome changes in the availability of electrical energy the number of solid oxide electrolysis cell modules used in the process can be adapted to the availability of electrical energy.

The solid oxide electrolysis cell modules can be operated within a certain power range so that instead of switching modules on or off as the situation may be it is alternatively provided that the direct reduction of the iron ore is adjusted to the amount of reducing agent generated in the solid oxide electrolysis cells modules. Adjustment in this sense means the amount of iron ore per unit of time that is reduced according to the method. The amount of reducing agent generated in the modules is directly proportional to the amount of renewable energy available and with that the amount of iron ore that can be reduced.

In case of periods wherein no renewable energy is available at all for whatever reason, the reduction process can simply be interrupted. At the moment that renewable energy is available again the reduction process can be continued. Alternatively the high and lows in the supply of renewable electrical energy could at least partly be overcome by using electricity from the grid, but that means that the process still has a certain contribution to the total C02 emission, although less than without using renewable energy sources.

Another advantage of the control possibilities of the solid oxide electrolysis cell modules is that these can also be used to stabilise the grid by balancing out the peaks and troughs in the electricity supply and demand. Since a lot of electrical energy is needed in ironmaking adjusting the control of the solid oxide electrolysis cell modules can be quite effective in stabilising the grid.

With the above possibilities in the control of the reduction process makes the reduction process according to the invention a very flexible process. Moreover, the resulting product, direct reduced iron, can easily be stored and kept for a considerable period of time, which allows to keep sufficient stock to be able to guarantee continuous steel production further down the line.

It is even possible to reverse the process by oxidising direct reduced iron by supplying steam to the direct reduced iron and using the hydrogen resulting from the oxidising process as fuel for the solid oxide electrolysis cell modules which are then run as a solid oxide fuel cell modules. In this manner electrical energy can be generated if needed for whatever reason.

According to a further aspect of the invention it is provided that the method comprises the steps of:

generating hydrogen gas by means of electrolysis of water in an assembly of solid oxide electrolysis cells,

supplying iron ore to an iron ore reduction unit,

passing the generated hydrogen gas as reducing agent to the iron ore reduction unit,

removing the reduced iron ore from the iron ore reduction unit,

returning the off gas of the reduction unit containing hydrogen gas and steam to the assembly of solid oxide electrolysis cells.

The solid oxide electrolysis cell modules working in regenerative mode are specially suitable for high temperature electrolysis of water, or rather steam since the working temperature of the solid oxide electrolysis cell modules is in a range of about 400 - 1000°C.

Each of the solid oxide electrolysis cell modules in the assembly contains multiple solid oxide electrolysis cells, wherein a cell is composed of a ceramic layer acting as a electrolyte at high temperature to transport oxygen ions. On the one side of the electrolyte layer is another layer of anode material and on the other side of the electrolyte layer is a layer of cathode material. Steam is dissociated at the SOEC cathode to produce oxygen and hydrogen, wherein the oxygen is transported across the cell to oxidize the anode fuel and wherein the hydrogen is separated from the effluent stream by condensation of the residual steam.

The chemical potential from the fuel, that is the high temperature steam significantly reduces the electrical input required for hydrogen production relative to traditional electrolysis processes, resulting in high overall system efficiency and reduced energy costs. The hydrogen produced is 99% pure or better, which is far better than possible with steam reformation.

By using separate modules each having stacks of solid oxide electrolysis cells the system can easily be adapted to the availability of electrical energy, which allows to run the modules at least partly on electrical energy obtained from renewable energy sources. The reduction process can easily be operated in a non-continuous mode which allows to use only renewable energy sources. In a non-continuous mode sufficient stock of direct reduced iron should be build up if other continuous processes are dependent on the availability of the direct reduced iron produced according to the method.

Accordingly, it is provided that the iron ore is transported either continuously or batch wise through the iron ore reduction unit.

According to a further aspect the iron ore is supplied as iron pellets, the manufacturing of which is well-known in the art. The heat needed in the pelletizing process can be supplied by means of fuel burners, an electric furnace or by means of plasma torches (US9752206), wherein the last two methods could use renewable energy sources. By first processing the iron ore into iron pellets has the advantage of easy handling of the iron ore, not only in the reduction process but also in temporarily storage and supply to a further steelmaking process. The direct reduced iron can be supplied to a basic oxygen furnace in a blast furnace steelmaking route or to an electric arc furnace, as an addition to or replacement of steel scrap.

The iron ore or the iron pellets are fed into the reduction unit either continuous, semi-continuous or batch wise wherein the reduction unit is provided with a charge and discharge device designed to close off the interior of the reduction unit from the surrounding space. The hydrogen gas is supplied to the iron ore reduction unit either in counter current flow and/or top down flow.

The reduction process is an endothermal process working at an elevated temperature. According the invention it is provided that the energy needed for the reduction process is at least partly provided by transferring heat generated in the assembly of solid oxide electrolysis cells to the iron ore reduction unit by thermal conductivity and/or radiation. However this is dependent on the operation mode of the solid oxide electrolysis cell modules.

The following operating modes can be distinguished for the high temperature electrolysis process using the solid oxide electrolysis cell modules: equilibrium, thermo-neutral, endothermal, and exothermal. The electrolysis process operates at thermal equilibrium when the electrical energy input equals the total energy demand and the electrical-to-hydrogen conversion efficiency is 100 %. In the thermo-neutral mode, the heat demand necessary for the water splitting equals the heat released by the joule heating (ohmic losses) within a solid oxide electrolysis cell. In the exothermal mode, the electric energy input exceeds the enthalpy of reaction, corresponding to an electrical efficiency below 100 %. In this mode, heat is generated from the cell and can be reused in the system to preheat the inlet gases and/or the reduction unit. With the invention the solid oxide electrolysis cell modules are typically operated in exothermal mode. This mode has also the advantage to operate at higher current density allowing decreasing the size of the system. However, it could be a source of prematurely ageing of the system components. Finally, in the endothermal mode the electric energy input stays below the enthalpy of reaction which means a cell voltage below the theoretical thermo-neutral voltage of 1 .286 V at 800°C. Therefore, heat must be supplied to the system to maintain the temperature. This mode means electrical-to-hydrogen conversion efficiencies of above 100 %. This operation mode also allows minimal long-term degradation rates, since it is achieved at the lowest power densities.

If the heat transferred from the solid oxide electrolysis cell modules is not enough to keep the operating temperature of the reduction process in the reduction unit at the required or at a predefined level other energy sources have to be used or have to be used in addition.

According to a further aspect it is provided that part of the hydrogen gas generated in the solid oxide electrolysis cell modules is used in a heater for heating of the iron ore reduction unit. The hydrogen can be burned using the oxygen present in the ambient air. Alternatively, it is provided that the oxygen gas generated in the solid oxide electrolysis cell modules is supplied to the heater to react with the hydrogen gas.

According to a further aspect the solid oxide electrolysis cell modules and the reduction unit are operated at the same temperature or at about the same temperature and with the modules and the reduction unit positioned for heat transfer from the modules to the reduction unit the heater for the reduction unit only needs to supply a limited amount of additional heat. Typically it will be sufficient to supply 5 - 25 vol% of the hydrogen gas generated in the solid oxide electrolysis cell modules to the heater. In most cases it will be sufficient to supply 5 - 10 vol% of the hydrogen gas to the heater. The solid oxide electrolysis cell modules and the iron ore reduction unit are operated in a temperature range of 400 - 1000°C, more in particular in a temperature range of 600 - 900°C.

The hydrogen gas supplied from the solid oxide electrolysis cell modules to the reduction unit is only partly used in the reduction process in the reduction unit which is due to thermodynamic limitations of the reduction process. The off gas from the reduction unit has an equilibrium composition of steam with hydrogen gas and for that reason still has a large amount of unreacted hydrogen gas next to the produced steam.

According to the invention it is provided that the off gas from the iron ore reduction unit is returned to the solid oxide electrolysis cell modules by means of a circulation pump. By feeding the off gas back into the solid oxide electrolysis cell modules, the steam in the off gas is converted into additional hydrogen gas while the original hydrogen gas in the supply gas to the reduction unit gas remains in the gas stream and is again supplied to the reduction unit. Feeding the off gas from the reduction unit to the solid oxide electrolysis cell modules can easily be done by means of a circulation pump. The oxygen gas produced in the solid oxide electrolysis cell modules is a separate stream, which does not come in contact with the circulating stream of hydrogen and steam.

The off gas from the reduction unit may contain other components than steam and hydrogen, for instance carbon monoxide and/or sulphur components. The solid oxide electrolysis cell is tolerant for these components which is a clear advantage over low temperature type electrolysis cells which are very sensitive for both components and will immediately be damaged when in contact with these components.

It is further provided that water and/or steam is supplied to of solid oxide electrolysis cell modules in an amount to compensate for losses between the amount of hydrogen gas and steam supplied to the iron ore reduction unit and the amount of the hydrogen gas and steam returned to the solid oxide electrolysis cell modules.

By feeding the off gas containing hydrogen gas and steam from the reduction unit back into the solid oxide electrolysis cell modules a very efficient system is realised wherein all the hydrogen gas obtained from the high temperature electrolysis process is used, if not in the first circulation it will be in a next circulation through the reduction unit. In this way only a limited amount of water and/or steam have to be fed into the system to compensate for the used hydrogen gas.

Finally, by operating the solid oxide electrolysis cell modules and the reduction unit at the same or about the same temperature, wherein heat from the solid oxide electrolysis cell modules is transferred to the reduction unit, the efficiency of the process is optimised.

Brief description of the drawings

The invention will be further explained on hand of the example shown in the drawing, in which:

fig. 1 shows schematically a system with solid oxide electrolysis cell modules and a reduction unit for the direct reduction of iron ore , and fig. 2 shows schematically the system of fig.1 with additional components.

Detailed description of the drawings

In fig.1 schematically a system is shown with solid oxide electrolysis cell modules 1 and a reduction unit 2 for the direct reduction of iron ore. The electrical energy 3 generated by renewable energy source(s) is supplied to the solid oxide electrolysis cell modules 1 and is utilised in the solid oxide electrolysis cell modules 1 for the electrolysis of water, generating two separated streams of respectively hydrogen 4 and oxygen 5.

The solid oxide electrolysis cell modules 1 are built up of solid oxide electrolysis cells (SOEC), which is an electrolyser with ceramic plates that transport oxygen atoms when electrical power is applied. The working temperature of these cells is above 600°C and maximal 1 100°C. This type of electrolyser is much more efficient than a standard low temperature electrolyser because it operates at high temperature where part of the energy needed to split water comes from heat. The heat can be applied from an external heating system or generated in the solid oxide electrolysis cell modules 1 when an overvoltage is applied.

The reduction unit 2 is a container for iron ore agglomerates 6 for instance supplied as iron pellets. The hydrogen containing stream 4 is the reducing agent for the iron ore pellets contained in the reduction unit 2. The hydrogen containing stream typically has a hydrogen content in the range of 75 - 95 vol%. The iron ore pellets are reduced into metallic iron 7 (DRI) in the reduction unit 2. Iron ore pellets 6 are continuously charged into the reduction unit 2 while produced DRI 7 is removed avoiding leakage of process gas by standard charge/discharge devices used in various iron making processes. This DRI is a well-known commodity material used as scrap replacement in the steel industry.

The off gas 8 from the reduction unit 2 has an equilibrium composition of H20 (steam) with H2. Because of the thermodynamic equilibrium of the reduction process only part of the hydrogen can be utilised for reduction. As a result the off gas of the reduction unit is still rich in hydrogen. The off gas at the aimed working temperature of 600-900°C has a hydrogen content in a range of 55 - 75 vol%, the remainder being steam. By recycling the off gas back into the electrolyser 1 the steam is converted into additional H2 while the original H2 in the input gas remains in the circulating gas stream 4,8. For the recycling of the off-gas 8 from reduction unit 2 a recirculation pump 12 is used as shown schematically in fig.2. The amount of gas transported must be sufficient to maintain maximal reaction rates in both units 1 ,2. The circulation pump 12 head has one inlet and on outlet of the gas stream and should be operated at similar temperatures as both units 1 ,2 without leakage. Only the pump head should be at high temperature and can be separated with a longer shaft to connect to an electromotor. The rate of circulation is controlled by controlling this electromotor.

Both the solid oxide electrolysis cell modules 1 and reduction unit 2 operate in the same temperature range. The reduction unit 2 consumes energy needed for the reduction process and at least part of the heat for the reduction process can be supplied by excess heat generated by the solid oxide electrolysis cell modules 1. This can be realized by building the system in close vicinity of each other such that heat can be transferred by conduction and/or radiation. This also avoids that temperature changes in both units 1 ,2 will occur or at least that large temperature changes will occur.

In case more heat is required for the reduction process a heat generator 9 is provided, see fig.2, to supply heat to the reduction unit 2. The heat generator 9 is supplied with a part of the circulating gas stream 4 wherein the hydrogen is supplied through a supply line to the heat generator 9. The hydrogen is burned with air or some of the pure 02 stream 5 from solid oxide electrolysis cell modules 1 which is supplied through supply line 1 1 to the heat generator 9. The off gas from the heater exits the heat generator 9 through duct 13. The very pure oxygen stream 5,1 1 produced by the solid oxide electrolysis cell modules 1 is in a separate stream 1 , which is not in contact with the circulating H2/H20 stream 4,8. When a part of the circulating gas stream 4,8 is consumed for heat generation, an additional amount of water must be supplied to the solid oxide electrolysis cell modules 1 to compensate for the hydrogen used in the heat generator 9.

Claims

1. Method for the direct reduction of iron ore using a reducing agent generated in an assembly of solid oxide electrolysis cell modules characterised in that the direct reduction of the iron ore is controlled dependent on the availability of energy sources to generate electrical energy for the assembly of solid oxide electrolysis cell modules.

2. Method according to claim 1 , wherein electrical energy for the solid oxide electrolysis cell modules is obtained at least partly from renewable energy sources.

3. Method according to claim 1 or 2, wherein dependent on the available amount of electrical energy one or more of the solid oxide electrolysis cell modules are switched on or switched off.

4. Method according to any of the previous claims, wherein the direct reduction of the iron ore is adjusted to the amount of reducing agent generated in the solid oxide electrolysis cells modules.

5. Method according to any of the previous claims, wherein the method comprises the steps of:

supplying iron ore to an iron ore reduction unit,

removing the reduced iron ore from the iron ore reduction unit,

- returning the off gas of the reduction unit containing hydrogen gas and steam to the assembly of solid oxide electrolysis cells.

6. Method according to claim 5, wherein the iron ore is supplied as iron pellets.

7. Method according to any of the previous claims, wherein each of the solid oxide electrolysis cell modules in the assembly contains multiple solid oxide electrolysis cells.

8. Method according to claim 6 or 7, wherein the iron ore is transported either continuously or batch wise through the iron ore reduction unit.

9. Method according to any of the previous claims 6-8, wherein the hydrogen gas is supplied to the iron ore reduction unit either in counter current flow and/or top down flow.

10. Method according to any of the previous claims 6-9, wherein heat generated in the assembly of solid oxide electrolysis cells is transferred to the iron ore reduction unit by thermal conductivity and/or radiation.

1 1. Method according to any of the previous claims 6-10, wherein part of the hydrogen gas generated in the solid oxide electrolysis cell modules is used in a heater for the heating of the iron ore reduction unit.

12. Method according to claim 1 1 , wherein oxygen gas generated in the solid oxide electrolysis cell modules is supplied to the heater to react with the hydrogen gas.

13. Method according to claim 11 or 12, wherein 5 - 25 vol% of the hydrogen gas generated in the solid oxide electrolysis cell modules is supplied to the heater, more in particular 5 - 10 vol% of the hydrogen gas.

14. Method according to one or more of the previous claims, wherein the off gas from the iron ore reduction unit is returned to the solid oxide electrolysis cell modules by means of a circulation pump.

15. Method according to any of the previous claims, wherein the solid oxide electrolysis cell modules and the iron ore reduction unit are operated in a temperature range of 400 - 1000°C, more in particular in a temperature range of 600 - 900°C.

16. Method according to claim 14, wherein the solid oxide electrolysis cell modules and the iron ore reduction unit are operated at the same temperature.

17. Method according to any of the previous claims, wherein water and/or steam is supplied to of solid oxide electrolysis cell modules in an amount to compensate for losses between the amount of hydrogen gas and steam supplied to the iron ore reduction unit and the amount of the hydrogen gas and steam returned to the solid oxide electrolysis cell modules.