PCT/EP2014/074319 / 2012P20220WO 1 Description Title of the invention Method and device for providing reducing gas under constant conditions Field of technology The application relates to a method for producing metallic sponge, wherein metal-oxide-containing starting materials are reduced to metallic sponge in a reduction unit by means of a reducing gas. The application further relates to a plant for performing such a method. Prior art In the case of combined plants which use export gas from a pig-iron production plant, e.g. a COREX@ plant, FINEX@ plant, blast furnace, or in particular an oxygen blast furnace, for the purpose of reducing iron-oxide-containing feed materials in a further reduction unit, e.g. a direct reduction shaft, the quantity of export gas that occurs per time unit fluctuates by +/- 10% depending on operational factors. The export gas is normally used as a reactant gas for the purpose of producing the reducing gas for the further reduction unit, wherein top gas from the further reduction unit is also added. The operation of the further pig-iron production unit requires a delivery of reducing gas under largely constant conditions, e.g. without pressure fluctuations or fluctuations in the quantity of reducing gas delivered per time unit, and it is therefore necessary to equalize the fluctuations in the quantity of export gas that occurs per time unit. This is PCT/EP2014/074319 / 2012P20220WO 2 achieved by regulating the quantity of the added top gas correspondingly, and by storing excess gas in a gas reservoir, usually a low-pressure gas reservoir. Top gas that is not otherwise required is often introduced into said gas reservoir likewise, such that the gas reservoir generally contains a stored gas mixture of at least these two components. The gas which is present in a low-pressure gas reservoir is held at a pressure that is too low to allow its introduction into gas flows that are used to produce the reducing gas. It is therefore not possible to utilize its energy content and reduction potential in the further reduction unit, and this impairs the economic efficiency and environmental balance of the operation of the combined plant. Summary of the invention Technical problem The object of the present invention is to provide a method, which does not have the aforementioned disadvantages, for producing metallic sponge. Technical solution This object is achieved by a method for producing metallic sponge, wherein metal-oxide-containing starting materials are reduced to metallic sponge in a reduction unit by means of a reducing gas, and reducing gas thereby consumed is drawn off from the reduction unit as top gas, and wherein the reducing gas entering the reduction unit is PCT/EP2014/074319 / 2012P20220WO 3 produced - by mixing at least a partial amount of the top gas with at least one reactant gas having reduction potential to form a mixed gas or - from pure reactant gas having reduction potential, and wherein reactant gas having reduction potential is stored together with at least a partial amount of the top gas in a gas reservoir as a stored gas mixture, characterized in that at least a partial amount of the stored gas mixture - is introduced into the top gas before mixing the top gas with the reactant gas having reduction potential, and/or - is introduced into the reactant gas, and/or - is introduced into the mixed gas. In this context, pure reactant gas is understood to be the reactant gas before it becomes part of a mixed gas. According to an embodiment, the metallic sponge is iron sponge. The metal oxide is preferably iron oxide. The reduction unit is e.g. a reduction shaft, preferably a direct reduction shaft, or a fluidized-bed reactor system. According to a preferred variant of the method, the reactant gas having reduction potential is export gas from a pig-iron production plant used in combination with the reduction unit.
PCT/EP2014/074319 / 2012P20220WO 4 According to a preferred variant of the method, the pig-iron production plant used in combination with the reduction unit is - a COREX@ plant, or - a FINEX@ plant, or - a blast furnace, preferably an oxygen blast furnace. The reactant gas having reduction potential can therefore be export gas from a COREX@ plant, export gas from a FINEX@ plant, or export gas from a blast furnace, in particular an oxygen blast furnace. In this case, export gas is understood to be a gas which is drawn off from a pig-iron production plant, e.g. a plant for producing liquid pig iron, after purification of the top gas from the production of raw iron. Excess gas from e.g. a melter gasifier of a plant for producing liquid pig iron can also be added to the top gas from the production of raw iron; such a mixture is also called export gas following purification. According to the claimed method, metal-oxide-containing starting materials are reduced to metallic sponge in the reduction unit by means of a reducing gas, and reducing gas thereby consumed is drawn off from the reduction unit as top gas. The reducing gas entering the reduction unit is produced at least in part by mixing at least a partial amount of the top gas with at least one reactant gas having reduction potential to form a mixed gas; the entirety of the reducing gas can also be produced thus. The reducing gas entering the reduction unit can also be produced solely from the reactant gas having reduction potential, i.e. from pure reactant gas having reduction potential. As part of the claimed method, reactant gas having reduction PCT/EP2014/074319 / 2012P20220WO 5 potential is stored together with at least a partial amount of the top gas in a gas reservoir, preferably a low-pressure gas reservoir, as a stored gas mixture. The method according to the invention is characterized in that at least a partial amount of the stored gas mixture - is introduced into the top gas before mixing the top gas with the reactant gas having reduction potential, and/or - is introduced into the reactant gas and/or - is introduced into the mixed gas. A low-pressure gas reservoir is understood to be a gas reservoir having a pressure of less than 15 kPa (g). The stored gas mixture is inventively introduced into gases that are used in the production of the reducing gas. If a stored gas mixture is introduced into the mixed gas, the introduction into the mixed gas can take place before or after the mixed gas has undergone further treatment stages such as cooling or compression or removal of C02, for example. A stored gas mixture is therefore used for the production of reducing gas, thereby exploiting its reduction potential and rendering the method for producing metallic sponge more environmentally friendly and more economical than if this reduction potential is not used. The stored gas mixture is preferably introduced into the top gas or into the reactant gas or into the mixed gas after an increase in pressure, this being effected by means of one or more compressors, for example. In this case, the stored gas mixture is preferably raised to a pressure which corresponds to or exceeds the pressure of the gas into which it is PCT/EP2014/074319 / 2012P20220WO 6 introduced. For example, a stored gas mixture which is held in a low-pressure gas reservoir at a pressure of 7 kPa(g) is raised to a pressure of 20 - 40 kPa(g) of the top gas of a direct reduction shaft, or to 270 - 720 kPa(g) of the mixed gas, or to 80 - 250 kPa(g) of the reactant gas. If this top gas is mixed with the reactant gas, it is itself first compressed to the pressure of the reactant gas 80 - 250 kPa(g); the stored gas mixture can also be raised to a pressure of 80 - 250 kPa(g) for the purpose of introduction into the top gas which has been compressed thus. According to a preferred variant of the method, the production of the reducing gas entering the reduction shaft includes removal of CO 2 from the mixed gas and heating of the product gas after the CO 2 removal, wherein the stored gas mixture is introduced into the mixed gas before the CO 2 removal. In this way, CO 2 contained in the mixed gas is likewise removed. The present application also relates to a plant for performing a method according to the invention, comprising - a reduction unit for reducing metal-oxide-containing starting materials to metallic sponge, - a top gas line for drawing off consumed reducing gas from the reduction unit as top gas, - a reducing gas supply line for supplying reducing gas into the reduction unit, - a mixed gas line for supplying mixed gas to the reducing gas supply line, - a reactant gas line for supplying reactant gas having reduction potential to the mixed gas line, wherein a recycle gas line leads from the top gas line and flows into the reactant gas line, - a gas reservoir, preferably a low-pressure gas reservoir, PCT/EP2014/074319 / 2012P20220WO 7 wherein a top gas reservoir line for introducing top gas into the gas reservoir, preferably a low-pressure gas reservoir, leads from the top gas line, and wherein a reactant gas reservoir line for introducing reactant gas into the low-pressure gas reservoir leads from the reactant gas line. The plant is characterized in that one or more storage gas recycling lines are provided for the purpose of supplying stored gas from the gas reservoir into - the top gas line, and/or - the recycle gas line and/or - the reactant gas line and/or - the mixed gas line. According to a preferred embodiment, at least one device for increasing the pressure, e.g. a compressor or a blower, is provided in at least one stored gas recycle line. According to a preferred embodiment, one or more devices for removing CO 2 from the mixed gas are provided in the mixed gas line, and at least one device for heating the product gas emerging from the device or devices for removing CO 2 is provided in the mixed gas line, wherein at least one stored gas recycle line flows into the mixed gas line upstream of the devices for removing C0 2 , as viewed from the outlet of the recycle gas line into the reactant gas line in the direction of the reducing gas supply line. According to a preferred embodiment, the reactant gas line PCT/EP2014/074319 / 2012P20220WO 8 leads from a pig-iron production plant, most preferably from a pig-iron production plant which is used in combination with the reduction unit. According to a preferred variant of the method, the pig-iron production plant which is preferably used in combination with the reduction unit is - a COREX@ plant, or - a FINEX@ plant, or - a blast furnace, preferably an oxygen blast furnace. The quantity of stored gas which is introduced into the top gas or into the reactant gas or into the mixed gas is preferably controlled continuously, this being effected e.g. by means of flow control valves downstream of the devices for increasing the pressure as viewed in the gas flow direction, or by controlling the devices for increasing the pressure. Advantageous effects of the invention Brief description of the drawings Figure 1 shows a schematic exemplary illustration of an embodiment of a plant according to the invention for performing a method according to the invention. Description of the embodiments Figure 1 shows a plant 1 for performing a method according to the invention, comprising - a reduction unit - here a direct reduction shaft 2 - for reducing iron-oxide-containing starting materials 3 to iron sponge 4, PCT/EP2014/074319 / 2012P20220WO 9 - a top gas line 5 for drawing off consumed reducing gas from the direct reduction shaft 2 as top gas, - a reducing gas supply line 6 for supplying reducing gas into the direct reduction shaft 2, - a mixed gas line 7 for supplying mixed gas to the reducing gas supply line 6, - a reactant gas line 8 for supplying reactant gas having reduction potential to the mixed gas line 7, wherein a recycle gas line 9 leads from the top gas line 5 and flows into the reactant gas line 8, - a low-pressure gas reservoir 10, wherein a top gas reservoir line 11 for introducing top gas into the low-pressure gas reservoir 10 leads from the top gas line 5, and wherein a reactant gas reservoir line 12 for introducing reactant gas into the low-pressure gas reservoir leads from the reactant gas line 8. In the variant illustrated here, the reactant gas reservoir line 12 flows into the top gas reservoir line 11 before the latter flows into the low pressure gas reservoir 10. Along a certain section, therefore, reactant gas is carried together with top gas into the low pressure gas reservoir 10. The plant has a plurality of storage gas recycling lines 13a, 13b, 13c, 14 for supplying stored gas from the low-pressure gas reservoir 10 into - the top gas line 5, - the recycle gas line 9, - and the mixed gas line 7. A compressor 15 is provided in the stored gas recycle line 13a as a device for increasing the pressure. A blower 16 is provided in the stored gas recycle line 14 as a device for increasing the pressure.
PCT/EP2014/074319 / 2012P20220WO 10 Top gas is supplied via the recycle gas line 9 to the reactant gas flowing in the reactant gas line 8 in order thereby to produce a mixture. A compressor 17 is provided in the recycle gas line 9 and raises the top gas to the pressure level of the reactant gas, e.g. from 20 - 40 kPa(g) to 80 - 250 kPa(g). A pressure of 7 kPa(g) is present in the low-pressure gas reservoir. A device (a VPSA plant here) for removing CO 2 from the mixed gas is provided in the mixed gas line 7, said device consisting of a plurality of trains 18a, 18b, 18c, 18d, 18e, 18f. Downstream of said trains, as viewed from the outlet of the recycle gas line 9 into the reactant gas line 8 in the direction of the reducing gas supply line 6, devices for heating the product gas emerging from the trains 18a, 18b, 18c, 18d, 18e, 18f of the VPSA are provided in the mixed gas line, specifically here a furnace 19 which operates by means of indirect heat exchange and a burner 20 which operates by means of partial combustion with oxygen. The stored gas recycle line 13c flows into the mixed gas line 7 upstream of the devices for removing C02, comprising the trains 18a, 18b, 18c, 18d, 18e, 18f, as viewed from the outlet of the recycle gas line 9 into the reactant gas line 8 in the direction of the reducing gas supply line 6. Compressors 21a, 21b and coolers 22a, 22b are provided in the mixed gas line upstream of the VPSA plant, as viewed from the outlet of the recycle gas line 9 into the reactant gas line 8 in the direction of the reducing gas supply line 6, and bring the mixed gas to a pressure, e.g. 270 - 720 kPa(g), and temperature that are suitable for the operation of the (V)PSA plant. The reactant gas line 8 leads from a COREX@ plant, which is PCT/EP2014/074319 / 2012P20220WO 11 not shown for reasons of clarity, and carries export gas of said plant. Iron-oxide-containing starting materials 3 are reduced to metallic sponge 4 in the direct reduction shaft 2 by means of a reducing gas. Reducing gas thereby consumed is drawn off from the direct reduction shaft 2 via the top gas line 5 as top gas. The reducing gas entering the direct reduction shaft 2 is produced by mixing a partial amount of the top gas with the export gas of the COREX@ plant as reactant gas having reduction potential to form the mixed gas. Reactant gas having reduction potential is stored together with a partial amount of the top gas in the low-pressure gas reservoir as a stored gas mixture. The stored gas mixture is introduced both into the top gas, before the top gas is mixed with the reactant gas having reduction potential, and into the mixed gas. Although the invention is illustrated and described in detail with reference to the preferred exemplary embodiments, it is not restricted to the examples disclosed herein, and other variations may be derived therefrom by a person skilled in the art without thereby departing from the scope of the invention.
PCT/EP2014/074319 / 2012P20220WO 12 List of reference signs 1 Plant for performing method according to the invention 2 Direct reduction shaft 3 Iron-oxide-containing starting materials 4 Iron sponge 5 Top gas line 6 Reducing gas supply line 7 Mixed gas line 8 Reactant gas line 9 Recycle gas line 10 Low-pressure gas reservoir 11 Top gas reservoir line 12 Reactant gas reservoir line 13a, 13b, 13c Stored gas recycle line 14 Stored gas recycle line 15 Compressor 16 Blower 17 Compressor 18a, 18b, 18c, 18d, 18e, 18f Trains of a device for removing
CO
2 19 Furnace 20 Burner 21a,21b Compressors 22a, 22b Coolers