BIOMASS DIRECT REDUCED IRON

Abstract

A method and an apparatus for recycling a significant amount of heat within a linear hearth furnace by means of an endless conveyor that transports briquettes through the furnace from an inlet (briquette feed) end to an outlet (DRI discharge) end and then returns to the inlet end and transfers a significant amount of heat from the outlet end to the inlet end of the furnace.

Claims

1. A method for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in a linear hearth furnace including a refractory-lined chamber having the following zones along the length of the furnace between an inlet for briquettes of iron ore fragments and biomass and an outlet for direct reduced iron: a feed zone that includes the inlet, a preheat zone, a final reduction zone and a discharge zone that includes the outlet, and an endless conveyor that moves through the zones from the inlet to the outlet and then returns to the inlet, with the conveyor having a base for receiving and supporting briquettes, the method including: a) feeding briquettes onto the base of the conveyor in the feed zone; b) transporting briquettes on the conveyor through the preheat zone and heating briquettes and reducing iron ore in briquettes and releasing volatiles in biomass in briquettes, with heating including generating heat by burning combustible gases in a top space of the preheat zone via a plurality of air or oxygen-enriched air fed burners, with the combustible gases including combustible gases generated within the furnace; c) transporting heated briquettes on the conveyor from the preheat zone through the final reduction zone, with the final reduction zone being an anoxic environment, and supplying electromagnetic energy, such as microwave energy, into the final reduction zone and heating briquettes and reducing iron ore in briquettes and forming DRI; d) transporting DRI on the conveyor to the discharge zone at the outlet and discharging DRI from the discharge zone; and e) returning the unloaded conveyor from the outlet to the inlet of the furnace and repeating step (a) of feeding briquettes onto the base of the conveyor, with the base of the conveyor having a hot face temperature of at least 500 C. when it returns to the feed zone and before it is loaded with briquettes and thereby returning heat to the furnace and contributing to heating briquettes.

2. The method defined claim 1 wherein step (a) includes forming a relatively uniform bed of briquettes on the conveyor.

3. The method defined in claim 1 wherein the hot face temperature of the base of the conveyor in the feed zone before it is loaded with briquettes in the feed zone is at least 800 C. so as to recover heat therefrom.

4. The method defined in claim 3 wherein the hot face temperature of the unloaded base of the conveyor in the feed zone before it is loaded with briquettes in the feed zone is at least 900 C. so as to recover heat therefrom.

5. The method defined in claim 1 includes returning the conveyor to the inlet through a thermally-insulated chamber in a lower region of the furnace to maintain heat or minimise heat loss in the base of the conveyor between the outlet and the inlet of the furnace.

6. The method defined in claim 4 includes heating the lower region of the furnace and therefore the base of the conveyor using hot gases from the furnace.

7. The method according to claim 6 includes heating the base of the conveyor by transferring at least a part of hot gases produced by combusting combustible gases in the preheat zone to the lower region of the furnace and/or by transferring at least a part of hot gases produced in the final reduction zone to the lower region of the furnace and/or by electrical heating.

8. (canceled)

9. (canceled)

10. The method defined in claim 1 wherein the mass percentage of biomass in briquettes is 20-45%, typically 30-45%, by weight on a wet (as-charged) basis.

11. The method defined in claim 10 wherein the biomass includes a significant lignocellulosic component.

12. The method defined in claim 9 wherein the balance of the composition of briquettes is (a) iron ore fragments (b) optionally flux/binder materials and (c) optionally additional carbonaceous material, such as coal or pre-charred biomass, in an amount of <5% by weight of the total weight of individual briquettes.

13. The method defined in claim 1 includes controlling the method so that the bulk temperature of briquettes is at least 500 C. when briquettes leave the preheat zone and pass to the final reduction zone.

14. The method defined in claim 1 wherein step (c) includes electromagnetic energy heating briquettes by at least 250 C. in the final reduction zone.

15. The method defined in claim 1 includes releasing at least 90% of volatiles in biomass in the briquettes as a gas in the preheat zone.

16. The method defined in claim 1 wherein step (c) includes generating a higher pressure of gases in the final reduction zone compared to gas pressure in the preheat zone and thereby causing gases generated in the final reduction zone to flow counter-current to the direction of movement of briquettes on the conveyor through the furnace.

17. The method defined in claim 1 includes generating heat in step (b) by burning combustible gases in a plurality of burners that are spaced apart along the length of the top space of the preheat zone of the furnace and/or spaced across the width of the preheat zone of the furnace.

18. The method defined in claim 17 includes generating heat in step (b) by burning combustible gases in a plurality of burners that are spaced across the width of the preheat zone of the furnace.

19. The method defined in claim 17 includes adjusting the amount of air or oxygen-enriched air fed to each burner in step (b) to compensate for variations in combustible gas in the top space of the preheat zone.

20. (canceled)

21. An apparatus for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass, the apparatus including a linear hearth furnace that includes a refractory-lined chamber having: (a) an inlet for briquettes of iron ore and biomass at one end and an outlet for direct reduced iron at the other end, (b) the following zones: (i) a feed zone that includes the inlet for briquettes; (ii) a preheat zone for heating briquettes and reducing iron ore in briquettes and releasing volatiles in biomass in briquettes, the preheat zone including a plurality of air or oxygen-enriched air fed burners for generating heat by burning combustible gases in a top space of the preheat zone, with the combustible gases including combustible gases originating within the furnace, (iii) a final reduction zone for heating briquettes and reducing iron ore in briquettes and forming DRI, the final reduction zone including a means for supplying electromagnetic energy, such as microwave energy, into the final reduction zone for heating briquettes; and (iv) a discharge zone that includes the outlet, the discharge zone being configured for discharging DRI from the furnace; and (c) an endless conveyor having a base for receiving and transporting briquettes through the zones from the inlet to the outlet and then returning to the inlet, the apparatus being configured to return the conveyor from the outlet to the inlet of the furnace with the base having a hot face temperature of at least 500 C. when it returns to the feed zone and before it is loaded with briquettes thereby recovering heat from the furnace and then returning heat to the furnace and contributing to heating briquettes.

22. The apparatus defined in claim 21 wherein the conveyor includes a return leg that extends through a thermally-insulated chamber in a lower region of the furnace configured to maintain heat or minimise heat loss in the base of the conveyor between the outlet and the inlet of the furnace.

23. (canceled)

24. (canceled)

25. The apparatus defined in claim 21 configured to generate a higher pressure of gas in the final reduction zone compared to gas pressure in the preheat zone to cause gases generated in the final reduction zone to flow counter-current to the direction of movement of briquettes on the conveyor through the furnace.

26. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0140] The present invention is described further by way of example with reference to the accompanying drawings, of which:

[0141] FIG. 1 is: [0142] (a) a schematic diagram of one embodiment of an apparatus for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in accordance with the invention, [0143] (b) a temperature profile along the length of the furnace of the apparatus of FIG. 1 for an embodiment of a method for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in accordance with the invention, and [0144] (c) a plot of off-gas volumetric flow rate of gas produced along the length of the furnace during the course of the method;

[0145] FIG. 2 is a flowsheet diagram illustrating one embodiment of a method for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in accordance the invention in the apparatus of FIG. 1; and

[0146] FIGS. 3 and 4 are schematic diagrams of another, although not the only other, embodiment of an apparatus in accordance with the invention, with FIG. 3 showing a vertical cross-section and FIG. 4 showing a transverse cross-section of the apparatus diagrammatically shown in these Figures.

DESCRIPTION OF EMBODIMENTS

[0147] As noted above, in broad terms, the present invention is a method and an apparatus for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass that includes transporting briquettes through, typically continuously through, a furnace having an inlet for briquettes and an outlet for DRI and a feed zone, a preheat zone, a reduction zone, and a discharge zone between the inlet and the outlet.

[0148] FIG. 1 is a schematic diagram of an embodiment of an apparatus of the present invention in the form of a linear hearth furnace.

[0149] The invention is not confined to a single linear hearth furnace and, by way of example, may extend to two linear hearths working in tandem. i.e. where an endless conveyor passes through both. The invention extends to other arrangements.

[0150] With reference to FIG. 1, the furnace, generally identified by the numeral 3, includes an elongated thermally-insulated, typically refractory-lined, chamber that has the following zones along its length: [0151] (a) a feed zone 10 that includes an inlet 14 to the chamber and is configured to receive briquettes 120 (see FIG. 2) of iron ore and biomass, [0152] (b) a preheat zone 20 for heating briquettes and reducing iron ore in briquettes and releasing volatiles in biomass in briquettes as a gas, with the volatiles being combusted in the preheat zone, [0153] (c) a final reduction zone 30 for heating briquettes and reducing iron ore in briquettes and forming DRI; [0154] (d) a discharge zone 40 that includes an outlet 46 of the chamber and is configured to discharge DRI, [0155] (e) an endless conveyor 50 having a refractory or metallic material base that moves through the chamber, typically continuously, from the inlet to the outlet and transports briquettes through the chamber from the inlet and discharges DRI from the outlet and then returns to the inlet to be re-loaded with briquettes; and [0156] (f) a flue gas outlet 70 in the preheat zone 20 for discharging gas produced in the furnace by heating and/or combustion within the furnace.

[0157] The feed zone 10 is configured in this embodiment to continuously feed briquettes 120 into the feed zone 10 via the inlet 14 to form a relatively uniform bed of briquettes on the moving conveyor 50 in the feed zone 10 of the chamber, while restricting outflow of furnace gases via the inlet 14. The feed zone 10 includes a feed chute 12 that can receive and direct briquettes 120 onto the conveyor 50.

[0158] The discharge zone 40 is configured to continuously discharge DRI from the discharge zone 40 via the outlet, while restricting the inflow of oxygen-containing gases into the final reduction zone 30 of the chamber. The discharge zone 40 includes an enclosed discharge chute 42 that has a downwardly-directed outlet 46 that has a flow control valve 44 that can be selectively operated to allow DRI to flow through the outlet 46.

[0159] The furnace may have any suitable dimensions.

[0160] The relative lengths of the feed zone 10, the preheat zone 20, the final reduction zone 30, and the discharge zone 40 may be selected as required having regard to the iron ore and biomass in the feed briquettes, the required characteristics (such as metallisation) of the DRI product, and the required operating conditions in the furnace.

[0161] The preheat zone 20 has a plurality of air or oxygen-enriched air fed burners 22 for generating heat by burning combustible gases in a top space of the preheat zone 20. The burners 22 are spaced along the length and across the width of the preheat zone 20. The optimal spacing can be readily determined by a skilled person for any given operating conditions, such as the amount and type of biomass and the amount and type of iron ore in the feed briquettes and the required metallisation and other characteristics of the DRI product. The spacings along the length and across the width may be constant or may vary depending on the operating requirements for the furnace.

[0162] The combustible gases generated in the furnace include combustible gases originating within the furnace. The combustible gases include: [0163] (a) volatiles in biomass in briquettes moving through the preheat zone 20; and [0164] (b) combustible gases, such as CO, generated by reduction of iron ore in briquettes in: [0165] (i) the preheat zone 20; and [0166] (ii) the final reduction zone 30, with the combustible gases generated in the final reduction zone 30 flowing from the final reduction zone 30 to the preheat zone 20, as described further below.

[0167] There may be additional combustible gases supplied to the burners 22 depending on the required operating conditions in the furnace.

[0168] In use, the final reduction zone 30 is maintained as an anoxic environment.

[0169] The final reduction zone 30 includes a plurality of electromagnetic energy input units 32 (including waveguides 36 and hoods 38) in a top space thereof for heating briquettes. The electromagnetic energy input units 32 are operatively connected to an electromagnetic energy generator 34 (see FIG. 2in which the generator is a microwave energy generator).

[0170] FIG. 1 shows how the bulk temperature of briquettes and gases generated in the furnace in the described embodiment of the method vary (in a qualitative form) along the length of the furnace.

[0171] The bulk temperature of briquettes is contributed to in part by the amount of heat in the conveyor 50 that is retained when the conveyor 50 returns to the feed zone 10 and how much heat is then transferred to briquettes as briquettes move through the furnace.

[0172] In the described embodiment the apparatus is configured, and the method conditions are selected, so that the base of the conveyor 50 has a hot face temperature of at least 500 C., at typically least 550 C., and more typically at least 600 C., when the conveyor returns to and is in the feed zone 10. This retained, i.e. recycled, heat contributes to heating briquettes and therefore contributes to the bulk temperature of briquettes. The temperature of at least 500 C. is considered to be a minimum temperature to make a significant contribution to the heat requirements of the method/apparatus.

[0173] In use of the apparatus, gases generated in the final reduction zone 30 flow into the preheat zone 20 counter-current to the direction of movement of briquettes on the conveyor 50 through the furnace from the inlet to the outlet.

[0174] The counter-current flow of gas from the final reduction zone 30 into the preheat zone 20 is caused by a higher gas pressure in the final reduction zone 30 compared to gas pressure in the preheat zone 20.

[0175] The higher gas pressure is a result of several structural and operational factors in the described embodiments of the method and the apparatus of the invention.

[0176] One factor is that the transverse cross-sectional area of the final reduction zone 30 is less than that of the preheat zone 20. In this regard, the final reduction zone 30 includes an additional elongate upper wall section 60 that makes the height of the preheat zone 20 lower than that of the preheat zone 20.

[0177] Another factor is injection of nitrogen gas (or any other suitable gas) into the final reduction zone 30 which, in addition to contributing to generating and maintaining the higher pressure, contributes to generating the anoxic environment in the final reduction zone 30.

[0178] Another factor is the volume of gas generated via reduction of iron ore in the briquettes in the final reduction zone 30 which, in addition to contributing to generating and maintaining the higher pressure, contributes to generating the anoxic environment in the final reduction zone 30. The volume of reduction gas generated in the final reduction zone 30 is illustrated by the plot of off-gas volumetric flow rate along the length of the chamber shown in FIG. 1.

[0179] A final factor is a suction effect of an exhaust fan at the end of the off-gas train (heat exchanger 90 and boiler 100see FIG. 2) connected to the flue gas outlet 70 of the furnace; which depending on its size may have a significant influence.

[0180] The counter-current flow of gas from the final reduction zone 30 to the preheat zone 20 transfers combustible gases, such as CO, that are generated in reactions that reduce iron ore in the final reduction zone 30 to the preheat zone 20. The combustible gases in the gas flow from the final reduction zone 30 are combusted by the plurality of air or oxygen-enriched air fed burners 22 spaced along the length and across the width of the preheat zone 20. The combustion profile may be 35-45% at a hot end of the preheat zone 20, i.e. at the end adjacent the final reduction zone 30, increasing to around 85-90% at a cold end of the preheat zone 20, i.e. at the end adjacent the feed zone 10.

[0181] The combustion of (a) combustible gases generated in the final reduction zone 30, (b) combustion of volatiles released from biomass in the preheat zone, and (c) combustion of combustible gases generated by reduction of iron ore in the preheat zone 20 provides an important component of the heat requirements for the method.

[0182] The temperature profile shown in FIG. 1 is an example of a suitable temperature profile along the length of the furnace. With reference to the Figure, the temperature in the furnace steadily increases in the feed zone 10 and the preheat zone 20 with distance from the inlet, with the temperature reaching 800 C. at the end of the preheat zone 20, noting that the temperature may be higher or lower in other embodiments depending on operational and DRI requirements, with a typical range of 600-900 C. The temperature remains substantially constant around 1100 C. in the final reduction zone 30, thereby allowing time for the required metallisation to be achieved, noting again that the temperature may be higher or lower in other embodiments depending on operational and DRI requirements.

[0183] In use, the conveyor 50 transports briquettes (not shown) successively and continuously through the zones 10, 20, 30, 40 in a sequential manner and eventually circles back in its endless path so that each portion of the refractory or metallic base material of the conveyor 50 eventually presents itself at the feed zone 10 to be loaded with more briquettes.

[0184] The refractory or metallic base material has residual heat from the chamber when the conveyor 50 returns to the feed zone 10 and this heat contributes to heating briquettes loaded onto the conveyor 50 in the feed zone 10. In other words, the conveyor 50 is a means of recycling heat of the furnace.

[0185] Depending on the selection of the materials and the size of the conveyor 50, the conveyor can recycle significant thermal mass to the furnace and make a significant contribution to heating briquettes in the feed zone 10. The above description refers to the conveyor 50 having a refractory or metallic material base. One particular option is a conveyor 50 with a lower section formed form a refractory material and an upper section formed from stainless steel or other heat conductive material.

[0186] In use, gases generated in the chamber are discharged as a flue gas via the flue gas outlet 70 in the preheat zone 20.

[0187] As described above in relation to the term briquettes, it is important for the invention that iron ore fragments and biomass be in quite close contact. Any approach to achieving this close contact may be used. Ore-biomass mixing followed by compaction of the materials to form briquettes between two rolls in which there are naturally aligning pockets, is one example. Alternative such compaction option is ore-biomass mixing followed by roll pressing using rolls without pockets into compressed slabs containing the iron ore fragments and biomass that break up naturally (or are deliberately broken up) prior to feeding into the feed station zone.

[0188] 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) such as a Eirich mixer and the drum arms rotated to form a homogeneous mixture.

[0189] Thereafter, the mixture may be transferred to a suitable briquette-making apparatus (not shown) and cold-formed into briquettes.

[0190] 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.

[0191] 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.

[0192] Either way, with further reference to FIG. 1, the DRI is fed into an insulated vessel (not shown) which is configured to transport the DRI (hot) to a downstream electric melting furnace (not shown). Here a feed system (not shown) can accept the hot DRI from the vessel and pass the DRI through a system of (for example) pushers and breaker bars (not shown) in order to feed the DRI into the electric melting furnace, including any furnace bath, for the production of steel.

[0193] It is noted that those structural components that are not specifically shown in FIG. 1 are generally standard components within the iron industry and the skilled person would be able to make an appropriate selection of the components.

[0194] As an example of a selection that could be made, reference is made to the linear heath system shown in U.S. Pat. No. 7,413,592 in the name of Nu-iron Technology, LLC. FIG. 1 discloses a conveyor that has a segmented base in the form of a plurality of individual trays (as further detailed in FIG. 4 and FIG. 5A) that could form a segmented base of an endless conveyor and shows how the conveyor returns from an outlet of the furnace to the inlet of the furnace and holds residual heat. To be clear, DRI cooling zone 34 in FIG. 1 effectively teaches a forced cooling of the segment trays before their return to the feed zone 27 (that accordingly teaches away from the subject invention).

[0195] FIG. 2 is a process flowsheet diagram illustrating one embodiment of a method for producing direct reduced iron (DRI) according to the invention from cold-formed briquettes of iron ore and biomass in the furnace of FIG. 1.

[0196] The data in the diagram of FIG. 2 is derived from a model developed by the applicant and illustrates an embodiment of the method carried out in the linear hearth furnace arrangement of FIG. 1.

[0197] With further reference to FIG. 2, in the described embodiment, cold-formed briquettes are continuously fed through a feeding device (not shown) onto a refractory or metallic base of a conveyor travelling at around 5 m/min, with the briquettes forming a bed depth of around 60 mm. The feed system delivers around 80 tonnes per hour of briquettes into the furnace. The effective width of the base is four (4) metres.

[0198] The briquettes comprise 37% elephant grass at 20% water, 5% limestone and 58% Pilbara Blend iron ore fines.

[0199] The length of the preheat zone 20 is 140 metres and is divided into 4 sections for ease of processing controls.

[0200] The length of the final reduction zone 30 is 60 metres with 50 microwave energy input units 32 extending downwardly into the top space thereof.

[0201] As described above in relation to FIG. 1, briquettes are heated as they are transported through the preheat zone 20, with volatiles being released as a gas and combusted in the preheat zone 20 and iron ore in the briquettes being partially reduced in the preheat zone 20.

[0202] The residence time of briquettes in the preheat zone 20 is 26.4 mins.

[0203] With reference to FIG. 2, the briquettes leave the preheat zone 20 at 900 C. at a rate of 42.6 t/h and a metallisation of 67.5%.

[0204] With further reference to FIG. 2, the briquettes are heated further in the final reduction zone 30 via the microwave energy input units 32. The iron ore in the briquettes is reduced further and produces 138.7 t/h DRI, with a composition of 95.3 wt. %, Fe, 5.89 wt. % C, 0.179 wt. % P, and 0.022 wt. % S at a temperature of 1150 C. The residence time of briquettes in the final reduction zone 30 is 11.3 mins. The reduction of iron ore in the briquettes generates gas that includes combustible gases such as CO. The FIG. 2 model assumes that 2.0 kNm.sup.3/h tramp air entering the final reduction zone 30. The tramp air post-combusts a portion of the combustible gases in the gas generated in the final reduction zone 30, resulting in a post combustion degree of 22.5%.

[0205] The DRI is discharged continuously from the conveyor 50 at the discharge zone 40. As shown in FIG. 1, the discharge zone 40 may be configured with an enclosed discharge chute 42 that has a downwardly-directed outlet 46 that has a flow control valve 44 that can be selectively operated to allow DRI to flow through the outlet 46.

[0206] The hot DRI is transported for use as a feed material in an open arc furnace (not shown) that produces molten iron at a rate of 109 tph, with a C concentration of 3.0 wt. %, S concentration of 0.012 wt. %, and a P concentration of 0.032 wt. %

[0207] A gas flow restriction is created between the two zones 20, 30 by the baffle wall 60 shown in FIG. 1 that changes the top space heights between the two zones, with the top space height and the overall transverse cross-sectional area of the final reduction zone 30 being less than that of the preheat zone 20.

[0208] In the FIG. 2 embodiment, gas flows from the final reduction zone 30 to the preheat zone 20. In the FIG. 2 model this gas has a post combustion degree of around 22.5% in the final reduction zone. The amount of post combustion will vary as a function of the amount of tramp air (if more than negligible) that flows into the final reduction zone 30, such as from the discharge zone 40. Therefore, there is considerable combustible gas in this gas as it flows into the preheat zone 20.

[0209] In the FIG. 2 embodiment, the amount of gas flowing from the final reduction zone 30 to the preheat zone 20 is 9.3 kNm.sup.3/h at a gas velocity of 5 m/s.

[0210] Typically, the operating range is 200-300 Nm.sup.3/tonne of DRI discharged from the furnace, and the gas velocity at the interface between the final reduction zone 30 and the preheat zone 20 is around 4-10 m/s (nominally 5 m/s).

[0211] As described in relation to FIG. 1, the gas flows into and along the preheat zone 20, counter-current to the movement of briquettes through the furnace, and the gas is subjected to incremental combustion as it passes through the plurality of air or oxygen-enriched air fed burners 22 which, in this embodiment, receive preheated (and/or oxy-enriched) air.

[0212] Typically, the post-combustion profile in the preheat zone 20 is typically 35-45% at the hot end (i.e. the final reduction zone 30 end), increasing gradually to around 85-90% at the flue gas outlet 70 end. The preheat zone top space is therefore maintained in a bulk reducing condition all the way along its length in the embodiment, with feed oxygen being consumed rapidly in the vicinity of each burner 22 (in a small localised region).

[0213] Off-gas at the flue gas outlet 70 end is then ducted (hot, around 1100-1300 C.) to an afterburning chamber 80, where final combustion of combustible gas in the gas is performed.

[0214] The gas from the afterburning chamber 80 is then used (in this embodiment) to preheat air for the burners 22 in the preheat zone 20 via a heat exchanger 90, before passing to a boiler 100 for final heat recovery via heat exchange in the boiler and then discharge as a flue gas to the atmosphere. FIG. 2 indicates that the flue gas has a temperature of 202 C.

[0215] After the hot DRI is discharged from the conveyor 50 at the discharge zone 40, the conveyor 50 circles back in its endless path to the inlet end of the furnace so that the conveyor 50 can be re-loaded with new briquettes in the feed zone 10 and transport the briquettes through the chamber. The refractory or metallic base material of the conveyor 50 has residual heat from the chamber when the conveyor 50 returns to the feed zone 10 and this recycled heat contributes to heating the feed briquettes.

[0216] There is considerable data in FIG. 2 in addition to that described above. The data in FIG. 2 describes the operating conditions for one embodiment of the invention based on a model developed by the applicant. The model is one of a number of models that could be developed as a basis for determining operating conditions for embodiments of the invention in a range of different embodiments of apparatus in accordance with the invention. The invention does not include the model.

[0217] The data in FIG. 2 necessarily contains multiple assumptions regarding kinetic parametersprecise details may shift as a result of different kinetics. However, the principles are not expected to change. Although the current example is based on preheated air, additional oxygen could be added to the air mixture prior to heating so that the ratio of air to oxygen could be varied as an additional control parameter to further optimise the process.

[0218] It is evident from FIG. 2 that the method and apparatus of the invention are a viable option for effective and efficient production of direct reduced iron (DRI) from iron ore and biomass, with the recycled heat of the furnace 3 making a significant contribution to the overall heat requirements of the method/apparatus.

[0219] The embodiment of the invention shown in FIGS. 1 and 2 does not include any measures to minimise heat loss from the unloaded conveyor 50 as it travels in a return path from the outlet in the discharge zone 40 to the inlet in the feed zone 10.

[0220] FIGS. 3 and 4 are schematic diagrams of an embodiment of the apparatus of the invention that includes specific measures to minimise heat loss and thereby maintain heat in the unloaded conveyor 50 when it returns to the feed zone 10 of the furnace.

[0221] FIG. 3 is a vertical cross-section and FIG. 4 is a transverse cross-section of the apparatus diagrammatically shown in these Figures.

[0222] The furnace shown in FIGS. 3 and 4 includes the same basic structure of the embodiment shown in FIG. 1, and the same reference numerals are used to describe the same features.

[0223] With reference to FIGS. 3 and 4, the furnace 3 also includes a thermally-insulated, typically refractory-lined, housing 130 that defines a lower chamber 140 through which the conveyor 50 passes in its return path from the outlet 46 in the discharge zone 40 to the inlet 14 in the feed zone 10. The housing 130 may be of any suitable construction and be made from any suitable materials.

[0224] In addition, the furnace 3 is configured so that at least a part of the hot gases generated in the chamber of the furnace 3 as briquettes 120 move successively through the preheat zone 20 and the final reduction zone 30 flow into the lower chamber 140 and heat the chamber to a temperature in a range of 1100-1200 C. and thereby at least minimise heat loss from the unloaded conveyor on its return path and thereby maintain heat in the unloaded conveyor 50 when it returns to the feed zone 10 of the furnace.

[0225] In modelling work, the applicant has determined that it is possible to minimise heat loss to an extent that the base of the conveyor 50 has a hot face temperature of at least 500 C. when it returns to the feed zone 10 and before it is loaded with briquettes and thereby returns heat to the furnace and contributes to heating briquettes.

[0226] The furnace 3 is also configured to use furnace gases beneficially to generate heat. Specifically, the furnace 3 is configured so that at least a part of the gases that are discharged via the flue gas outlet 70 in the preheat zone 20 are subsequently post-combusted in the afterburning chamber 80 (see FIG. 2, as well as FIGS. 3 and 4). The heat gas can be used in the furnace 3 or in other applications.

[0227] Many modifications may be made to the embodiment described above without departing from the spirit and scope of the invention.

[0228] By way of example, whilst the embodiment shown in FIG. 2 includes a 80 tonnes per hour briquette fed furnace has an effective width of 4 m wide by 200 m long (with a bed depth of 60 mm), 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.

[0229] By way of further example, whilst the conveyor 50 in the above embodiments has a refractory or metallic material base, the invention is not limited to this arrangement and extends to any suitable conveyor including a base formed from any suitable material.

[0230] By way of further example, whilst the above embodiments include the use of nitrogen gas injection to generate and maintain the anoxic environment in the final reduction zone, the invention is not limited to this particular gas. In addition, the invention is not confined to such gas injection at all if the gas generated via reduction of iron ore in the final reduction zone 30 is sufficient to maintain the required anoxic environment.

[0231] By way of further example, whilst the above embodiments include continuous operation, the invention is not so limited.

REFERENCES

[0232] 1. Vogl, V et al, Assessment of hydrogen direct reduction for fossil-free steelmaking, Journal of Cleaner production 203 (218) 736-745 [0233] 2. Strezov, V, Iron ore reduction using sawdust: experimental analysis and kinetic modelling, renewable Energy 31(12) 1892-1905, October 2006