BIOMASS DIRECT REDUCED IRON

Abstract

A method for producing direct reduced iron (“DRI”) from iron ore and biomass is disclosed. The method includes heating a batch of iron ore and biomass in each oven chamber of a non-recovery batch oven by a combination (i) the thermal mass of a lining of the oven chamber and (ii) combustion of a fuel gas from at least one other oven chamber and at least partially reducing the iron ore and forming DRI. The method also includes discharging gases from the oven chamber through passageways in a wall and a floor of the oven chamber and further combusting combustible gases and transferring heat to the wall and the floor of the oven chamber as the gases move through the passageways. The method also includes discharging at least a portion of gases from the oven chamber, without passing the gases through passageways in the floor of the oven chamber, and using these gases as a fuel gas in subsequent combustion heating in other batch oven chambers when a first predetermined trigger point is reached. A non-recovery batch oven is also disclosed.

Claims

1. A method for producing direct reduced iron (DRI) from iron ore and biomass using a batch oven in a batch cycle mode of operation, with the oven having a plurality of separate batch ovens, with each batch oven having a chamber defined by a refractory-lined wall and floor having a thermal mass and a plurality of burners, and with the oven chambers having shared fuel gas and off-gas offtakes, with the method including the following steps in at least one batch oven: a) charging a batch of composite of iron ore and biomass into the batch oven chamber; b) heating the charged iron ore and biomass in each oven chamber by a combination of heat from (i) the thermal mass of the lining of the oven chamber and (ii) combusting a fuel gas from at least one other oven chamber in a top space of the oven chamber in a flame of at least one oxygen-enriched burner in the oven chamber and at least partially reducing the iron ore and forming DRI and discharging gases from the oven chamber through passageways in a wall and a floor of the oven chamber and further combusting combustible gases in the discharged gases and transferring heat to the wall and the floor of the oven chamber as the gases move through the passageways and thereby contributing to the thermal mass of the non-recovery oven and heat transfer to the oven chamber; c) on reaching a first predetermined trigger point, discharging at least a portion of gases from the oven chamber, without passing the gases through passageways in the floor of the oven chamber, and using the gases as a fuel gas in subsequent combustion heating in other batch oven chambers; and d) on reaching a second predetermined trigger point in the batch oven, stopping discharging gases from the oven chamber in step c) and re-commencing step b); and e) at the end of the batch cycle discharging DRI from the oven chamber.

2. The method defined in claim 1 includes discharging gases from the passageways through a flue gas system to the atmosphere.

3. The method defined in claim 1 wherein step c) includes discharging at least a portion of the gases from the oven chamber without passing the gases through passageways in the wall and the floor of the oven chamber and using the gases as a fuel gas in subsequent combustion heating in other batch oven chambers.

4. The method defined in claim 1 wherein step b) includes operating the oxygen-enriched burner(s) with a nominally cold oxygen-air mixture with a minimum of 25% oxygen in the air-oxygen mixture (calculated as a mixed stream regardless of whether or not air and oxygen are (a) actually pre-mixed or (b) fed independently as two individual streams to the gas burners).

5. The method defined in claim 1 wherein the batch of the composite of iron ore and biomass charged into the batch oven in step a) includes 20-50% by weight biomass on a wet (as-charged) basis of the total weight of the batch.

6. The method defined in claim 5 wherein the balance of the batch of the composite of iron ore and biomass charged into the batch oven in step a) includes (i) iron ore and (ii) flux/binder materials and (iii) optionally carbonaceous material, which may be coal or pre-charred biomass, in an amount of <5% by weight of the total weight of the batch.

7. The method defined in claim 1 wherein the batch of composite of iron ore and biomass charged into the batch oven in step a) includes 30-40% by weight on a wet (as-charged) basis of the total weight of the batch.

8. The method defined in claim 1 wherein step b) includes heating iron ore and biomass to a temperature in a range of 800-1300° C. in the batch cycle time.

9. The method defined in claim 1 wherein the batch cycle time in step b) is in a range of 30-60 hours.

10. The method defined in claim 1 includes forming the composite of iron ore and biomass for the batch for step a) by roll pressing an iron ore biomass mix into slab form (whether now remaining in such slab form or in broken pieces thereof).

11. The method defined in claim 1 wherein step a) includes forming some of the biomass charged into the batch oven as a layer or a sheet.

12. The method defined claim 1 wherein step a) includes forming the composite of iron ore and biomass on a discrete layer of biomass in the batch oven.

13. The method defined claim 1 wherein the composite of iron ore and biomass in the batch charged into the batch oven in step a) includes briquettes.

14. The method defined in claim 1 wherein step e) includes discharging DRI from the oven into a product handling system that is configured to prevent bulk ingress of oxygen-containing gases and allows transportation in a hot state away from the non-recovery oven.

15. A non-recovery oven for producing direct reduced iron (DRI) from iron ore and biomass comprising a plurality of separate batch ovens, with each batch oven having a chamber defined by a refractory-lined wall and floor having a thermal mass and a plurality of oxygen-enriched burners, with the wall and the floor of each oven chamber having a plurality of passageways for transferring gases from the oven chamber and heating the refractories in the wall and the floor as the gases pass through the passageways, a gas collection and gas sharing assembly interconnecting the batch ovens, the gas collection and sharing assembly including a communal header and pipes extending between the batch ovens and the header for supplying fuel gas from the oven chambers to the header and for supplying fuel gas from the header to the oven chambers.

16. The non-recovery oven defined in claim 15 wherein the plurality of the burners is spaced along the length of the oven chamber.

17. The non-recovery oven defined in claim 16 wherein the plurality of the burners is spaced also across the width of the oven chamber.

18. The non-recovery oven defined in claim includes a product handling system for DRI discharged from the batch oven chamber that is configured to prevent bulk ingress of oxygen-containing gases and allow transportation in a hot state away from the non-recovery oven.

19. A method for producing direct reduced iron (“DRI”) from iron ore and biomass includes heating a batch of iron ore and biomass in each oven chamber of a non-recovery batch oven by a combination (i) the thermal mass of a lining of the oven chamber and (ii) combustion of a fuel gas from at least one other oven chamber and at least partially reducing the iron ore and forming DRI, discharging gases from the oven chamber through passageways in a wall and a floor of the oven chamber and further combusting combustible gases and transferring heat to the wall and the floor of the oven chamber as the gases move through the passageways, and discharging at least a portion of gases from the oven chamber, without passing the gases through passageways in the floor (and optionally the wall) of the oven chamber, and using these gases as a fuel gas in subsequent combustion heating in other batch oven chambers when a first predetermined trigger point is reached.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The present invention is described further by way of example with reference to the accompanying drawings, of which:

[0076] FIG. 1 is a schematic diagram of one embodiment of a process and apparatus for producing direct reduced iron (“DRI”) from iron ore and biomass which includes a plurality of batch ovens; and

[0077] FIGS. 2, 3 and 4 are process flowsheet diagrams illustrating an embodiment of the method for producing direct reduced iron (“DRI”) from iron ore and biomass in one of the batch chambers of FIG. 1 by capturing the process at each of the different gas handling stages within the method.

DESCRIPTION OF EMBODIMENTS

[0078] As noted above, the invention is based on a realisation that an adapted form of a non-recovery coke oven is an opportunity to provide an efficient way of heating and reducing ore-biomass on a batch basis.

[0079] As noted above, in broad terms, the present invention provides a method for producing direct reduced iron (“DRI”) from a composite of iron ore and biomass as described herein that has been charged into a chamber of a batch oven in a battery of linked batch oven chambers of a plurality of batch ovens. The method includes heating iron ore and biomass in the oven chamber of the batch oven using a plurality of oxygen enriched burners, as described herein, in a top space of the chamber to a temperature range of 800-1300 □C in a batch cycle time of typically in a range of 30-60 hours and reducing iron ore and forming DRI having a metallisation of at least 85% and generating a fuel gas. The method includes combusting at least some of the fuel gas and heating the iron ore and biomass in the oven chamber. The method includes diverting some of the fuel gas for subsequent use (such as for use in oxygen enriched burners in other batch oven chambers). The method also includes discharging the DRI at the end of the batch cycle into a product handling system in the form of a container that can be sealed to prevent substantial ingress of oxygen and discharging off-gas that has been fully combusted during the course of the batch cycle.

[0080] FIG. 1 is a schematic diagram of one embodiment of apparatus used for producing direct reduced iron (DRI) according to the method from composite of iron ore and biomass based on a plurality of batch ovens.

[0081] With reference to FIG. 1, the apparatus, generally identified by the numeral 3, includes (a) a plurality of batch ovens 5 arranged in a line (i.e. forming a battery) and (b) gas collection and sharing assembly, interconnecting the batch ovens 5, with each batch oven 5 having a batch oven chamber (not shown in FIG. 1 but evident from FIGS. 2-4) defined by a refractory-lined wall and floor having a thermal mass and a plurality of oxygen enriched burners 23 (see FIGS. 2-4) in a top space of the chamber.

[0082] The gas collection and sharing assembly includes a communal header 7 and pipes 9, 11 extending between the batch ovens 5 and the communal header 7 for supplying fuel gas to the header and supplying fuel gas from the header 7 to the chamber of the batch ovens 5 as required. The pipes 11 can supply fuel gas from the batch ovens 5 to the communal header 7. The pipes 9 can supply fuel gas from the communal header 7 to the batch ovens 5. The above described gas collection and sharing assembly will collectively hereafter be called the gas exchange system.

[0083] In use, by way of overview of a typical embodiment of the method, the batch ovens 5 that are in early (and also possibly in late) parts of a batch cycle receive fuel gas from other batch ovens 5 via the communal header 7 and pipes 9 and generate heat via the oxygen-enriched burners. This fuel gas from other batch ovens is necessary to meet the heating requirements for the early (and also possibly in the late) stages of a batch cycle. In other words, insufficient heat is typically generated from biomass and other reactions in a batch oven 5 to achieve/maintain a required minimum temperature, i.e. thermal mass, in the batch oven 5 during the early (and also possibly in the late) stages of the batch cycle.

[0084] In addition, in use, batch ovens 5 that are in middle (fuel-rich) stages of the cycle transfer fuel gas from the batch ovens 5 via the pipes 11 to the header 7. In this stage of the cycle, there is excess fuel gas to that required to generate sufficient heat to achieve/maintain a required minimum temperature in the batch oven 5. This fuel gas will be hot at the extraction point, and therefore the gas collection and sharing equipment includes a cooling element 13 that cools the fuel gas before it is admitted into the communal header 7.

[0085] The cooling element 13 may be any suitable cooling element. By way of example, the cooling element may be in the form of a wet scrubber or an indirect heat exchanger (e.g. long pipes with water or air cooling on the outside). Typically, the communal header 7 and heat exchanger include systems to manage condensation and corrosion issues in such a way that they do not interfere with the process.

[0086] It is noted that FIG. 1 illustrates a line of seven (7) batch ovens 5. The invention is not confined to this number of batch ovens 5. Typically, the number of batch ovens 5 is 6-10 ovens. However, in any given situation, the number of batch ovens 5 in a cluster will be a function of oven size and physical constraints of arranging batch ovens and gas collection and sharing equipment in an efficient arrangement.

[0087] The batch ovens 5 may be any suitable form.

[0088] By way of example, the batch ovens 5 may be an adapted non-recovery coke oven, with (a) refractory-lined wall and floor defining an oven chamber and providing thermal mass for the oven and (b) passageways in the wall and the floor that, in use transfer gases from the chamber to an external flue gas system, with resultant heat transfer to the refractories in the wall and the floor, thereby contributing to the thermal mass of the non-recovery oven and heat transfer to the oven chamber. As noted above, U.S. Pat. No. 5,318,671 in the name of Sun Coal Company describes a non-recovery coke oven.

[0089] While there are a number of different design variations for non-recovery coke ovens, with U.S. Pat. No. 5,318,671 being one design variation, those skilled in such art would be able to adapt such designs to utilize the invention described herein, after the invention has been disclosed to them.

[0090] In use of a batch oven 5, a batch of the composite of ore and biomass as described herein is charged into the oven chamber of the batch oven 5 prior to the commencement of a batch cycle and is pushed out of the chamber (in the form of DRI) at the end of the batch cycle. In such style of batch ovens, typically there is a charger car and a separate pusher, both on the same rails with the rails extending beyond the ends of the battery to enable the charger car and the pusher to move out of the way of each other when carrying out their function on the batch ovens at the end of the battery.

[0091] Ore and the majority of biomass should preferably be in quite close contact with one another for the method to work efficiently. Any method of achieving this composite of ore and biomass may be used, briquetting being just one example. Other options may involve ore-biomass mixing followed by roll pressing into slabs that break up naturally (or are deliberately broken up) prior to charging. It may also be possible to use some form of non-agglomerated charge into the ovens such as alternate layering of ore and biomass (somewhat akin to stamp-charging). It may also be advantageous in having a separate layer of biomass as part of the charging process that sits on the floor of the oven chamber. This may over time assist in the reduction of wear to the floor through the charging of the composite of iron ore and biomass. Likewise, it may assist (through some residual thereof remaining) in the reduction of wear to the floor through the discharge of DRI when pushing that material from the chamber.

[0092] For illustration purposes the following description uses composites in the form of ore-biomass briquettes.

[0093] The briquettes may be manufactured by any suitable method. By way of example, measured amounts of iron ore fines and biomass and water (which may be at least partially present as moisture in the biomass) and optionally flux is charged into a suitable size mixing drum (not shown) and the drum rotated to form a homogeneous mixture. Thereafter, the mixture may be transferred to a suitable briquette-making apparatus and cold-formed into briquettes.

[0094] In one embodiment of the invention, the briquettes are roughly 20 cm.sup.3 in volume and contain 30-40% biomass (e.g. elephant grass at 20% moisture). A small amount of flux material (such as limestone) may be included, with the balance comprising iron ore fines.

[0095] In one embodiment of the invention, the process begins with a layer of (typically) 800 mm deep ore-biomass briquettes charged into an oven chamber of a batch oven 5. While charging may be done in any manner, a suitable manner would be that akin to how stamp charging of a non-recovery coke oven is carried out.

[0096] During an initial heating stage of the method in a batch oven 5, heat in the oven chamber produces water and no or insufficient combustible gases, i.e. fuel gases, to support a flame for the plurality of oxygen enriched burners in the top space of the chamber of the batch oven 5. However, at a later stage in the process, the briquette bed will produce fuel gas to that required to generate heat required to maintain a required minimum temperature for the batch oven 5. At this point, which is a trigger point, excess fuel gas may be harvested (for example, from wall downcomers of the batch oven 5) as described above via pipes 11 transferring a portion of the fuel gas to the communal header 7 for use in other batch ovens 5 at different, fuel gas-deficient stages of the process.

[0097] It is noted that, in use, in a typical embodiment of the method, a batch oven 5 will be pre-heated to an extent before a batch of composite of iron ore and biomass is charged into the chamber of the batch oven 5 so that there is at least some thermal mass in the lining of the chamber.

[0098] A typical embodiment of the method includes the following steps in at least one batch oven: [0099] a) charging a batch of composite of iron ore and biomass into the chamber of one of the batch ovens 5 shown in FIG. 1 under a negative pressure, noting that the pressure may be any suitable pressure; [0100] b) heating the charged iron ore and biomass in the oven chamber by a combination of heat from (i) the thermal mass of a lining of the oven chamber and (ii) combusting a fuel gas from at least one other oven chamber in a top space of the oven chamber in a flame of at least one oxygen-enriched burner in the oven chamber and at least partially reducing the iron ore and forming DRI and discharging gases from the oven chamber through passageways in a wall and a floor of the oven chamber and further combusting combustible gases in the discharged gases and transferring heat to the wall and the floor of the oven chamber as the gases move through the passageways and thereby contributing to the thermal mass of the non-recovery oven and heat transfer to the oven chamber; [0101] c) on reaching a first predetermined trigger point in the batch oven 5, discharging gases from the oven chamber, without passing the gases through passageways in the floor of the oven chamber, and using the gases as a fuel gas in subsequent combustion heating in other batch oven chambers of the battery; and [0102] d) on reaching a second predetermined trigger point in the batch oven, stopping discharging gases from the oven chamber in step c) and re-commencing step b); and [0103] e) at the end of the batch cycle discharging DRI from the oven chamber.

[0104] The “trigger point” may be based on one or more of a number of factors. For example, the “trigger point” may be temperature-based, heating time-based, off gas flow-based or any other relevant measure that correlates to a decision to capture excess fuel gas from an oven chamber for use elsewhere, for example in another oven chamber, or a decision to increase the amount of fuel gas fed into the oven chamber to supplement the amount of fuel gas already available for combustion in the chamber. Typically, the “trigger point” is set by changes in the off-gas flow. The above-mentioned “operational signal” may be any signal relating to operational parameters, such as temperature, heating time, off gas flow, or other parameter at any suitable location.

[0105] Once the batch process cycle is complete in a batch oven, the DRI is removed from the batch oven 5 via a product handling system. For example, the DRI is pushed out of the batch oven into an insulated container (not shown) that prevents ingress of oxygen-containing gases for transportation of the DRI in a hot state away from the batch oven 5.

[0106] The physical structure of the DRI at the end of the process is not critical. The physical structure may be friable and break easily or it could resemble a robust 3D “chocolate bar”.

[0107] Typically, the insulated container is transported (hot) to a downstream electric melting furnace 17. Here, a feed system (not shown in FIG. 1) will accept the hot container and pass the DRI through a system of (for example) pushers and breaker bars (not shown in FIG. 1) in order to feed the DRI into the furnace, including any furnace bath.

[0108] It is noted that those structural components that are not specifically shown in FIG. 1 are generally standard components and the skilled person would be able to make an appropriate selection of the components.

[0109] It is noted that there is no requirement to break up the DRI completely to supply to the electric melting furnace 17—only into lumps small enough to constitute more or less steady feed into the furnace from a metallurgical control point of view. It is expected that fairly large lumps (e.g. 20-30 briquettes clumped together) could pass through such a system without causing any issues.

[0110] FIGS. 2-4 are process flowsheet diagrams illustrating different parts of one embodiment of a process and apparatus for producing direct reduced iron (DRI) according to the method of the invention from cold-formed briquettes of iron ore and biomass in one of the batch ovens 5 of FIG. 1.

[0111] The process flowsheet diagrams of FIGS. 2-4 also illustrate transferring the DRI from the batch oven 5 to an electric melting furnace 17 and operating the furnace to produce molten metal, in accordance with one embodiment of a process and apparatus for producing molten metal (such as cold pig iron or steel) from DRI.

[0112] The data in the diagrams of FIGS. 2-4 is derived from a model developed by the applicant.

[0113] The process and apparatus shown in FIG. 2 illustrates the start of the embodiment of an oven heating cycle according to the method (the first 3 hours of a 48-hour cycle) for one batch oven 5.

[0114] It is noted that the oven heating cycle of FIGS. 2-4 may apply to any one of the batch ovens 5 in the array shown in FIG. 1. It is also noted that the start times of the oven heating cycles for the batch ovens 5 shown in FIG. 1 may be staggered to match the fuel gas generation and fuel gas supply requirements across the batch ovens 5. It is also noted that different oven heating cycles may be used in the batch ovens 5 in FIG. 1 to optimise operational efficiency in relation to fuel gas utilisation (or other factors, such as upstream briquette production and supply factors and downstream hot metal production factors).

[0115] With further reference to FIG. 2, in the described embodiment, a 59-tonne cold-formed briquette bed is charged into a chamber of a hot batch oven 5 (where “hot” means that the batch oven 5 has at least 50% of the residual heat of processing the last batch of DRI) that is 4 m wide by 15 m deep (800 mm bed depth). The briquettes comprise 38% elephant grass at 20% water, 5% limestone and 57% Pilbara Blend iron ore fines. The charge is cold and only water vapour is released during the 3-hour period. Fuel gas largely originating from other batch ovens 5 (see FIG. 1) in the “fuel production” stages of their cycle is drawn from the gas exchange system 10. The fuel gas, supplied to batch oven 5 via a line 9, is burned with an air-oxygen mixture containing 41% oxygen in burners 23 in batch oven 5. Oxygen is produced via cryogenic air separation in an oxygen plant 19 in a conventional way and supplied to the burners via a line 27. Off-gas generated in the batch oven 5 is discharged from the batch oven and transferred via a line 21 for downstream processing and release to the atmosphere.

[0116] Downstream processing of DRI briquettes produced in the batch oven 5 involves melting the DRI in an electric furnace (OAF) 17 to produce hot metal, followed by conversion to steel in a BOF. Both the OAF and the BOF generate combustible fuel gas streams—although small in terms of overall energy demand—these gas streams are nevertheless used in the batch oven burners as supplementary fuel by feeding those gas streams into gas exchange system 10.

[0117] In this 3-hour period (as shown in FIG. 2) 6570 Nm.sup.3 fuel gas originating from other batch ovens is imported through gas exchange system 10, augmented by 72.9 Nm.sup.3 OAF gas and 54.0 Nm.sup.3 BOF gas.

[0118] FIG. 3 shows a 3-hour period in the middle of the 48-hour batch cycle when fuel gas is being produced in the batch oven 5 shown in the Figure is more than the amount required to generate sufficient heat to maintain a required minimum temperature, i.e. maintain a thermal balance, for the batch oven 5. At this stage, the bed is around 800° C. and fuel gas production exceeds heat requirements by 3380 Nm.sup.3/3 h. This excess fuel gas is exported to the communal header 7 for use by other batch ovens 5. Typically, the gases generated in the chamber are transferred to the header 7 via the pipe 11 and the cooling element 13 shown in FIG. 1 without passing through the passageways in the wall and the floor of the oven chamber. In an alternative arrangement, typically, the gases discharged from the chamber pass through passageways in the wall of the chamber and then to the header 7 via the pipe 11 and the cooling element 13 and bypass only the passageways in the floor of the batch oven 5.

[0119] FIG. 4 shows the final 3 hours of the 48-hour batch cycle. At this point the bed has reached 956° C. and metallisation is around 98-99%. In this instance, a small amount of imported fuel gas (310 Nm.sup.3/3 h) is needed to sustain a thermal balance.

[0120] This example necessarily contains multiple assumptions regarding kinetic parameters—precise details may shift as a result of different kinetics. However, the principles are not expected to change substantially—in particular, the sharing of fuel gas between batch ovens 5 within an oven cluster (see FIG. 1) such that each oven 5 produces and receives fuel gas, typically around the same amount of fuel gas, in the overall integrated cycle. Although the current example is based on a constant air-oxygen blend to the gas burners (41% oxygen by volume), it is expected that the ratio of air to oxygen could be varied as an additional control parameter to further optimise the process.

[0121] Around 60-70% of the required plant electric power (including power needed for the electric melting furnace and the oxygen plant) is generated from residual heat in the flue gas (and fuel gas) from the ovens.

[0122] After the final 3 hours of the 48-hour batch cycle has elapsed, the bed of DRI is pushed out from the batch oven 5 and transferred to the OAF unit 17 (which may operate in either submerged-arc or open-arc mode, the name notwithstanding). Flux and coke breeze are added in the OAF 17 to control metal carbon and slag chemistry. Hot metal (molten pig iron in this embodiment) is produced. This may be cooled and cast into pigs or passed directly (in liquid form) to a steelmaking vessel (BOF or EAF).

[0123] Many modifications may be made to the embodiment described above without departing from the spirit and scope of the invention.

[0124] By way of example, whilst the embodiment shown in FIGS. 2-4 includes a 59-tonne cold-formed briquette bed that is charged into a batch oven 5 that is 4 m wide by 15 m deep (800 mm bed depth), with the briquettes comprising 38% elephant grass at 20% water, 5% limestone and 57% Pilbara Blend iron ore fines, it can readily be appreciated the invention is not confined to this size briquette bed with this composition of the briquettes.

REFERENCES

[0125] 1. Vogl, V et al, Assessment of hydrogen direct reduction for fossil-free steelmaking, Journal of Cleaner production 203 (218) 736-745 [0126] 2. Strezov, V, Iron ore reduction using sawdust: experimental analysis and kinetic modelling, renewable Energy 31(12) 1892-1905, October 2006