METHOD FOR HEATING PROCESS GASES FOR DIRECT REDUCTION SYSTEMS

Abstract

A method for reducing iron ore in the direct reduction method, in which the iron ore to be reduced is conveyed through a reduction unit such as a reduction shaft and is brought into contact with a reduction gas; the reduction gas is brought into the reduction unit and flows through the unit; after flowing through the unit, it is taken from the unit; after exiting the unit, the gas is prepared and possibly enriched with new gas components and is fed back again; and the generated gas is heated before entry into the reduction unit, characterized in that the heating of the reduction gas prior to the entry into the unit is carried out in an electrical fashion.

Description

FIELD OF THE INVENTION

The invention relates to a method for heating process gases for direct reduction systems.

BACKGROUND OF THE INVENTION

Steel production is currently carried out in a variety of ways. Classic steel production is carried out by producing pig iron in the hot furnace process, primarily out of iron oxide carriers. In this method, approx. 450 to 600 kg of reducing agent, usually coke, is consumed per metric ton of pig iron; this method, both in the production of coke from coal and in the production of the pig iron, releases very significant quantities of CO₂. In addition, so-called “direct reduction methods” are known (methods according to the brands MIDREX, FINMET, ENERGIRON/HYL, etc.), in which the sponge iron is produced primarily from iron oxide carriers in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron), or so-called HBI (hot briquetted iron).

There are also so-called smelting reduction methods in which the melting process, the production of reduction gas, and the direct reduction are combined with one another, for example the methods of the brands COREX, FINEX, HiSmelt, or HiSarna.

Sponge irons in the form of HDRI, CDRI, and HBI usually undergo further processing in electric furnaces, which is extraordinarily energy-intensive. The direct reduction is carried out using hydrogen and carbon monoxide from natural gas (methane) and possibly synthesis gas as well as coke oven gas. For example, in the so-called MIDREX method, first methane is transformed according to the following reaction:

CH₄+CO₂=2CO+2H₂

and the iron oxide reacts with the reduction gas, for example according to the following formula:

Fe₂O₃+6CO(H₂)=2Fe+3CO₂(H₂O)+3 CO(H₂).

This method also emits CO₂.

DE 198 53 747 C1 has disclosed a combined process for the direct reduction of fine ores in which the reduction is to be carried out with hydrogen or another reduction gas in a horizontal turbulence layer.

DE 197 14 512 A1 has disclosed a power station with solar power generation, an electrolysis unit, and an industrial metallurgical process; this industrial process relates either to the power-intensive metal production of aluminum from bauxite or is intended to be a metallurgical process with hydrogen as a reducing agent in the production of nonferrous metals such as tungsten, molybdenum, nickel, or the like or is intended to be a metallurgical process with hydrogen as a reducing agent using the direct reduction method in the production of ferrous metals. The cited document, however, does not explain this in detail.

WO 2011/018124 has disclosed methods and systems for producing storable and transportable carbon-based energy sources using carbon dioxide and using regenerative electrical energy and fossil fuels. In this case, a percentage of regeneratively produced methanol is prepared together with a percentage of methanol that is produced by means of non-regenerative electrical energy and/or by means of direct reduction and/or by means of partial oxidation and/or reforming.

In the direct reduction method, the gas emerging downstream of the reduction shaft—after it is purified and the water has been separated out and additional CO₂ separation in the HYL method or optional additional CO₂ separation in the HYL MIDREX method—is predominantly fed back into the process as recycling gas. As a rule, this gas is in turn enriched with natural gas in order to supply fresh reduction gas. In the HYL method, the gas, which the gas purification has cooled from approximately 105° C., is heated again to approximately 700 to 1100° C. and then a partial oxidation with oxygen is performed.

In the MIDREX method, CO₂ and water are transformed with natural gas into H₂ and CO in a heated reformer in a temperature range from approximately 700 to 1100° C. Both methods share the fact that a partial flow of the gas that has been purified and is exiting the reduction shaft is introduced and is enriched with natural gas.

The reduction process can be expressed with the following equation:

Fe₂O₃+6CO(H₂)=2Fe+3CO₂(H₂O)+3CO(H₂)  (1)

In the MIDREX method, the following reactions take place in the reformer:

CH₄+CO₂→2CO+2H₂  (2)

CH₄+H₂O→CO+3H₂  (3)

In the HYL method, the following reaction takes place:

CH₄+½O₂→CO+2H₂  (4)

In both methods, the additionally used fossil fuel, namely natural gas, is used to heat the process gases and to heat the reformer.

One object of the invention is to create a method for heating process gases for direct reduction systems with which the heating of process gases can be better and more flexibly adapted to and optimized for an overall process that is adapted to the energy demand and to the available energy.

Another object of the invention is to reduce CO₂ emissions.

SUMMARY OF THE INVENTION

In order to make the heating process more flexible, according to the invention, the heating of the reduction gases and of the reformer is changed to an electrical heating.

Preferably, the electrical energy can be produced from renewable resources, thus replacing fossil fuels.

This advantageously increases the flexibility of the process with regard to the energy sources used; this is achieved through combined heating by means of a variable use of fossil fuels and electrical energy.

In this regard, the invention has the advantage that electrical current can be considered to be 100% energy so that it can be completely converted into high temperature heat. The direct convertibility of electrical energy into heat permits the addition of a high degree of flexibility, particularly also with regard to the use of current peaks that are inexpensively available on the market.

It is also advantageous that current from renewable energy sources such as hydroelectric, wind power, or solar energy does not cause any CO₂ emissions when it is produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained by way of example in conjunction with the drawings. In the drawings:

FIG. 1 shows as an example the HYL Energiron method according to the prior art, with a natural gas-powered process gas heating;

FIG. 2 shows the HYL Energiron method according to the invention, with an electrically-powered process gas heating;

FIG. 3 is a very schematic depiction of the MIDREX method;

FIG. 4 is a very schematic depiction of an expensive and complex CO₂-optimized MIDREX method according to the prior art, with a CO₂-removal unit (e.g. VPSA—vacuum-pressure swing adsorption).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The HYL method is shown by way of example in FIG. 2 on the basis of a capacity of two million metric tons of direct reduced iron (DRI) per year, including an electric arc furnace (EAF). The process gas from the shaft in which the iron ore is reduced is first conveyed through a water separation and then through a CO₂ separation. The circulating gas volume flow in this case is approximately 500,000 m³ per hour. Approximately 72,000 m³ of natural gas per hour is added to this gas flow, 56,000 m³ of which is used for the reduction and approximately 16,000 m³ of which is diverted for heating the process gas from 105 to 970° C. Next, oxygen is added to the heated process gas and this is then fed back into the reduction shaft.

In a method according to the invention (FIG. 2), the reduction gas is likewise taken from the shaft and conveyed through a water separation and a CO₂ separation. Thanks to the electrical heating of the process gas heating, it is only necessary to add a quantity of approximately 56,000 m³ of natural gas per hour, which is split with oxygen into CO and hydrogen in accordance with the above-mentioned formulas. The table in FIG. 2 shows that this achieves a 21% reduction in CO₂ per ton of reduced iron. In addition, because of the electric heating, the process can be used in an exactly controllable and flexible way.

FIG. 3 shows the MIDREX method in which the exhaust gas is likewise withdrawn in the reduction shaft and divided into a process gas flow and a heating gas flow. The process gas flow is conveyed through a process gas compressor until natural gas is added to it—particularly in a system that is likewise designed for 2 million metric tons of reduced iron per year—in a quantity of approximately 63,000 m³ of natural gas per hour. This process gas passes through a heat exchanger, in which it is preheated by the exhaust gases from the reformer to 600° C. and then passes through the reformer and in so doing, is heated to 980° C. and is conveyed back to the shaft as process gas, which is enriched with additional natural gas and oxygen. The heating gas is likewise taken from the shaft furnace, enriched with natural gas, and conveyed into the reformer together with preheated combustion air. The total required quantity of natural gas is approximately 68,200 m³ per hour; by heating the reformer electrically, it is possible to compensate for approximately 5,100 m³ of exhaust gas per hour with 52 Megawatts of electric power. As a result of this, it is possible on the one hand to achieve a 7.5% reduction of CO₂ per metric ton of reduced iron ore. In addition, this process can also be controlled in a more flexible, precise fashion thanks to the electric heating.

The invention has the advantage of achieving a simple and quickly implementable option for replacing fossil fuels with electrical power from renewable energies. CO₂ emissions from direct reduction systems are also reduced. The invention also makes it possible to successfully operate direct reduction systems in an effective and flexible way. In particular, in a steel production that is adapted to the availability of regenerative energies with an electrically-powered preheating of process gas, particularly one with heating based on renewable energies, it is possible to achieve an improvement and reciprocal adaptation.

It is also advantageous that such a system can inexpensively make use of available current peaks.

Claims

1. A method for reducing iron ore in a direct reduction method, comprising:

conveying the iron ore to be reduced through a reduction unit such as a reduction shaft and bringing the iron ore into contact with a reduction gas;

bringing the reduction gas into the reduction unit to flow through the unit;

after flowing through the unit, taking the reduction gas from the unit;

after exiting the unit, preparing the gas and possibly enriching the gas with new gas components and feeding the gas back again into the reduction unit; and

heating the generated gas mixture or the reduction gas products from the generated gas mixture to 700 to 1100 before entry into the reduction unit, wherein the heating is carried out in a predominantly electrical fashion.

2. The method according to claim 1, comprising using electrical power from regenerative energy sources for the electric heating.

3. The method according to claim 1, further comprising, after the gas has exited the unit, enriching the gas with natural gas, coke oven gas, or a synthesis gas from biomass or coal.

4. The method according to claim 1, comprising enriching the gas mixture with oxygen.

5. The method according to claim 1, comprising enriching the gas taken from the reduction shaft with natural gas, coke oven gas, or a synthesis gas from biomass or coal and then heating the enriched gas.

6. The method according to claim 1, comprising enriching the gas taken from the reduction shaft with natural gas, coke oven gas, or a synthesis gas from biomass or coal and then transforming the enriched gas in a reformer.

7. The method according to claim 1, comprising ensuring a cost-optimized use of energy sources through a continuous evaluation of gas prices and electricity prices.