Methods for Preheating Metal-Containing Pellets

Abstract

A method for preheating metal pellets before charging into a melting furnace, wherein the pellets are transported by a conveyor belt to a chute and discharged from the chute into the melting furnace, the method including heating the pellets by direct flame impingement from two or more banks of burners, wherein the two or more banks of burners comprise an upstream bank of burners and a downstream bank of burners; and controlling the upstream bank of burners to operate oxygen-rich so as to create an oxidizing zone and the downstream bank of burners to operate fuel-rich so as to create a reducing zone.

Claims

1. A method for preheating metal pellets before charging into a melting furnace, wherein the pellets are transported by a conveyor belt to a chute and discharged from the chute into the melting furnace, the method comprising: heating the pellets by direct flame impingement from two or more banks of burners, wherein the two or more banks of burners comprise an upstream bank of burners and a downstream bank of burners; and controlling the upstream bank of burners to operate oxygen-rich so as to create an oxidizing zone and the downstream bank of burners to operate fuel-rich so as to create a reducing zone.

2. The method of claim 1, wherein the two or more banks of burners comprise a conveyor bank of burners positioned to direct flames into contact with the pellets being transported by the conveyor belt and a chute bank of burners positioned to direct flames into contact with the pellets discharged from the chute into the melting furnace.

3. The method of claim 1, further comprising contacting the pellets with an inert fluid selected from the group consisting of an inert gas, an inert liquid, and a combination of an inert gas and an inert liquid to enable at least one of cooling the pellets and fire suppression of the pellets.

4. The method of claim 1, further comprising convectively heating the pellets by flowing a hot flue gas from the melting furnace over the pellets in a direction from the chute toward the conveyor.

5. The method of claim 1, further comprising mixing the pellets using one or more ploughs on the conveyor belt.

6. The method of claim 1, wherein the two or more banks of burners combust fuel with one or more of air, oxygen-enriched air having greater than 23% molecular O2, and industrial-grade oxygen having at least 70% molecular O2.

7. The method of claim 1, further comprising: measuring at least one process condition; and controlling the two or more banks of burners based on the at least one process condition; wherein when the at least one process condition is a concentration of one or more gases in the hot flue gas, adjusting the two or more banks of burners based on the concentration of one or more gases in the hot flue gas; wherein when the at least one process condition is one or more temperatures selected from the list of the gas temperature, pellet temperature, or belt temperature, adjusting the two or more banks of burners based on the one or more temperatures; and wherein when the at least one process condition is a safety condition, shutting down the two or more banks of burners in the event the safety condition is detected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The present invention will hereinafter be described in conjunction with the appended figures wherein like numerals denote like elements:

[0074] FIG. 1 is a schematic side view showing an arrangement of a system for charging metal pellets into a furnace including a preheater.

[0075] FIG. 2 is a schematic side view of an embodiment of a preheating system showing a fully covered refractory hood preheater using strategically positioned direct flame impingement (DFI) burners over the chute only.

[0076] FIG. 3 is a schematic side view of an embodiment of a preheating system showing a fully covered refractory hood preheater using strategically positioned DFI burners over the chute and the conveyor (including heated and unheated zones).

[0077] FIG. 4 is a schematic side view of an embodiment of a preheating system showing a partially covered refractory hood preheater using strategically positioned DFI burners over the chute and a portion of the conveyor.

[0078] FIG. 5 is a schematic side view of an embodiment of a preheating system showing a fully covered refractory hood preheater using strategically positioned DFI burners to create oxygen-rich and fuel-rich zones over the conveyor.

[0079] FIG. 6 is a schematic side view of an embodiment of a preheater system showing a fully covered refractory hood preheater using strategically positioned DFI burners in the chute only, with oxidizing and reducing zones.

[0080] FIG. 7 is a graph showing pellet temperature vs. time when put under DFI burner at different firing rates.

[0081] FIG. 8 is a graph showing pellet heat up rate vs. time when put under DFI burner at different firing rates.

[0082] FIG. 9 is a schematic side view showing a first arrangement of a system with a rotary furnace configured for DRI preheating.

[0083] FIG. 10 is a schematic side view showing a second arrangement of a system with a rotary furnace configured for DRI preheating.

[0084] FIG. 11 is a schematic side view showing a third arrangement of a system with a rotary furnace configured for DRI preheating.

DETAILED DESCRIPTION

[0085] The present inventors propose a system and method of preheating the cold DRI/HBI as it is being transported or conveyed from (local) storage at a steel mill to be charged into steel operations such as EAFs (and other relevant processes). Use of a preheater furnace, and preferably a rotary type preheater furnace, is expected to provide higher heat transfer efficiency and reduced firing rate requirements (due to increased residence time) as compared to inline heating.

[0086] Two important factors when heating DRI are heat transfer efficiency and atmosphere control to avoid oxidation of the pellets.

[0087] Direct Reduced Iron (DRI) and/or Hot Briquetted Iron (HBI) are being increasingly used as the charge material into steel operations such as EAFs (and BOFs), in some cases as much as 30-50% of the charge. DRI plants are also fast replacing traditional forms of iron ore processing such as blast furnaces because of higher usage of natural gas in DRI making process. Natural gas is preferred because it is a lower-carbon containing, more economically available fuel source compared to coal. DRI plants are usually located closer to mining operations and not close to steel mill operations. As a result, a majority of the DRI produced today is transported cold to steel mills, before being stored and eventually charged cold into steel making operations.

[0088] The present inventors propose a system and method of preheating the cold DRI/HBI as it is being transported or conveyed from (local) storage at a steel mill to be charged into steel operations such as EAFs (and other relevant processes). Use of direct flame impingement (DFI) is expected to be relatively less capitally intensive and less wasteful in energy practices. The time required to increase the temperature of the pellets can be optimized using firing rate modulations, engaging and disengaging different banks of burners; so as to heat the pellets in shortest amount of time possible to reduce any oxidation. As used herein, the terms “pellets” and “metal pellets” are understood to include DRI pellets as well as HBI briquettes or metal-containing granules or other unitized elements of metal-containing materials.

[0089] An arrangement for a DRI preheater system 10 is shown in FIG. 1, and various specific embodiments of the arrangement are shown in FIGS. 2-4 and 7-8 (discussed in detail below). A conveyor belt 42 transports pelleted metal-containing materials 99 from a storage location (not shown) and then down a chute 28 into a melting furnace 90. The conveyor belt 42 may be flat or sloped upward toward the chute 28, but typically includes an upward sloped portion 43 that feeds the chute 48. A refractory-lined chute hood 20 positioned above the chute 28 partially or completely covers the chute 28. The chute 28 itself may also be refractory-lined to aid in resisting the heat of combustion generated by the melting furnace 90 and the preheater system 10. The chute 28 and chute hood 20 together form a passage 24 that serves to exhaust some of the hot flue gases from the melting furnace 90.

[0090] A preheater 12 is configured to provide combustion heating to the pellets 99 before the pellets 99 are charged into the furnace 90. As shown in FIGS. 2-4 and 7-8, the preheater 12 includes both the chute hood 20 and a refractory-lined conveyor hood 40. The conveyor hood 40 is positioned above the conveyor 42 and partially or fully covers at least a lengthwise portion of the conveyor 42. The conveyor 42 and conveyor hood 40 together form a passage 46 for the exhaust gases that leave the melting furnace 90 and flow through the passage 24 formed by the chute 28 and chute hood 20.

[0091] For reference purposes, the orientation of the conveyor 42, conveyor hood 40, and/or passage 46 may be described as having an entrance end 44 where the pellets 99 enter and an exit end 48 where the pellets 99 exit to the chute 28. A flow P of the pellets 99 moves from the entrance end 44 to the exit end 48, while a flow F of gaseous exhaust or combustion products moves in generally the opposite direction, from the exit end 48 toward the entrance end 44.

[0092] A flue duct 60 is located at or near the entrance end 44 of the conveyor hood 40 to exhaust combustion products (flue gases) either out of the building, toward the canopy, or elsewhere as determined by local requirements.

[0093] One or more banks 50 of burners 52 are housed at one or more locations in the chute hood 20 and/or the conveyor hood 40. The burners 52 in each bank 50 are positioned strategically along the length and width of the conveyer 42 and emits a flame 54 that impinges the pellets 99. Additionally, the hoods 20 and 40 will be heated by the burners 52 and radiation from the hoods 20 and 40 will assist efficient heating.

[0094] The preheater 12 utilizes hot flue gases F flowing in a direction that is countercurrent with respect to a direction of the flow P of pellets 99, thereby assisting with preheating in the same manner as a counterflow heat exchanger.

[0095] Preferably, the preheater 12 is lined with special refractory coatings to reflect and re-radiate energy back to the pellets 99.

[0096] The pellets 99 may be mixed by positioning one or more ploughs (not shown) or other mechanism for bringing pellets from the bottom to the top along the length of the belt so that heat may be able to contact all pellets.

[0097] The DFI burners 52 can use oxidizers from 20.9% (all air) to 100% (all oxygen) oxygen-content-in-oxidizer and any fuel including natural gas, propane, COG, BFG, or the like. Preferably, the burners are oxy-fuel burners that use an oxidant having at least 23% molecular O2, more preferable at least 30% molecular O2, and still more preferably industrial-grade oxygen having at least 70% molecular O2.

[0098] The DFI burners 52, positioned along the length of a portion or entirety of conveyor belt 42 or over multiple belts for heating (cold or warm) DRI pellets to be charged continuously into any process including a melting furnace 90 such as an electric arc furnace. The DFI burner position, height from the conveyor belt, spacing, angle relative to vertical, flame shape, number of and intensity may be adjusted based on pellet density (e.g., pellet depth, width, height), type of pellet, and speed of the conveyor belt. Belt material and shape may be modified to accommodate burner. Preferably a high temperature belt material is used. Preferably a belt type that enables maximum surface exposure of DRI pellets to the heat is used, e.g., a belt that provides for a shallow and broad distribution of the pellets.

[0099] Various embodiments of the preheater 12 are shown in FIGS. 2-6. In the embodiment of FIG. 2, a burner bank 50 including at least one burner 52, and preferably a plurality of burners 52, is positioned in the chute hood 20 over the chute 28. Just prior to dropping the heated DRI pellets into the desired process (e.g., melting furnace 90), the pellets 90 tumble through the flames 54 emanating from the burners 52. The embodiments of FIGS. 3-6 also use a bank 50 of burners 52 in the chute hood 2. This configuration enables individual and intimate contact between the flames 54 and each pellet 99 for enhancement of preheating.

[0100] Limiting a zone of direct combustion heating to the chute 28 that carries the pellets 99 from conveyor 42 to the melting furnace 90, alleviates the need for an expensive high temperature conveyor belt and any damage that may be caused by interaction of flames and hot combustion products with the belt. The burners 42 in the chute hood 20 can be configured to deliver intense heat, enabling a high heat-up rate of the pellets 99 so that the pellets 99 can take on significant heat in the chute 28 just before they fall into the melting furnace 90. Additionally, hot combustion products from the burners 52 in the chute hood 29 are routed upstream through the conveyor hood 40, over at least a portion of the conveyor 42 and the pellets 99 being transported toward the chute 28, so as to transfer some of the residual heat in those combustion products to the pellets 99 before they reach the chute 28. And if the chute 40 is lined with refractory as described herein, that refractory can help capture most of the heat from the burners 52 and reradiate some of that heat onto the pellets 99 while also preventing any damage to the surrounding structure.

[0101] In some embodiments, it may be beneficial to use a combination of fired zones and non-fired zones to control rate of preheating. Non-fired zones can be accomplished either by the absence of burners 52 or can be accomplished periodically as needed by selectively turning on and off banks 50 of burners 52 or even individual burners 52.

[0102] In the embodiment of FIG. 3, a burner bank 50 is positioned in the chute hood 20 as in the embodiment of FIG. 2, and in addition, another burner bank 50 is positioned in a downstream portion 30 of the conveyor hood 40, while an upstream portion 32 of the conveyor hood 40 is devoid of a burner bank 50 (or has a burner bank 50 that is turned off. Thus, the bank 50 of burners 52 in the downstream portion 30 forms a heated zone, while the absence of a burners 52 in the upstream portion 32 forms an unheated zone. In the embodiment of FIG. 4, the conveyor hood 40 can be shortened to only cover an upstream portion of the conveyor 42, with a burner bank 50 in the conveyor hood, so that only the covered portion becomes a heated zone. As used herein, the terms “upstream” and “downstream” are with reference to the flow P of pellets 99.

[0103] DRI and other metal-containing pellets 99 may be prone to oxidization, so in some embodiments it may be beneficial to create fuel-rich and/or oxygen-rich zones along the length of the preheating furnace.

[0104] In the embodiment of FIG. 5, in addition to a burner bank 50 in the chute hood 20 and a burner bank 50 in the downstream portion 30 of the conveyor hood 40, a further burner bank 50 is positioned in the upstream portion 32 of the conveyor hood 40. With this arrangement, the preheater 12 can be configured and operated such that the burner bank 50 in the upstream portion 32 farther from the furnace 90 operates oxygen-rich (i.e., more oxygen than is stoichiometrically necessary to full combustion the fuel) so as to create an oxygen-rich or oxidizing zone, while the burner bank 50 in the downstream portion 30 nearer the furnace 90 operates fuel-rich (i.e., with insufficient oxygen to fully combust the fuel) so as to create a fuel-rich or reducing zone.

[0105] Similarly, in the embodiment of FIG. 6, oxygen lances 84 can be used to create an oxygen-rich or oxidizing zone 33 over the conveyor 42 farther upstream from furnace 90, while a burner bank 50 in the chute hood 20 (or in a conveyor hood 40 nearer the furnace, not shown) can be operated fuel-rich to create a fuel-rich or reducing zone in the chute 20.

[0106] A benefit of a downstream fuel-rich zone as the pellets 99 increase in temperature is that exposing the pellets 99 to a reducing environment will reduce decarburization and protecting the pellets 99 from oxidation (FeO, Fe.sub.3O.sub.4, Fe.sub.2O.sub.3 and the like). A benefit of an upstream oxygen-rich zone closer to the combustion products exhaust where the pellets 99 are cooler is that the oxidizing environment can consume undesired CO and extract additional energy release from the combustion process prior to exhaust.

[0107] Operation of the burner banks 50 and individual burners 52, including such parameters as firing rate, number of burners operating and sequence of firing, and stoichiometry of the burners, is controlled based on requirement to achieve a target average heat content/temperature of the pellets 99 being charged into the furnace 90. Strategically located sensors 82 in the preheater 12, in conjunction with a controller (not shown) can be used to facilitate this control.

[0108] For example, the sensors 82 may be or may include composition sensors to measure composition of combustion products or flue gases along the length of the conveyor hood 40 and at the exit of the preheater 19 (i.e., at the flue duct 60) to modify and control operation of the burner banks 50 to create of desired atmospheres in different zones. In addition, or alternatively, the sensors 82 may be or may include temperature and imaging sensors (at the same or different locations as the composition sensors 82) can be used to measure temperature along the length of the conveyor hood 40 and at the flue duct 60 to control the energy input rates from the various burner banks 50.

[0109] FIG. 7 shows increase in temperature of a pellet with time for three different firing rates. It is observed that the slope of the curve becomes more steep with increasing firing rate suggesting increase in heat-up rate.

[0110] FIG. 8 shows heat-up rate as a function of different firing rates. When the firing rate is increased by 3 times the heat up rate is increased by ˜2 times. Thus, DFI burners can be used to heat the pellets in very short durations (e.g., approximately 8-10 seconds.)

[0111] The burner banks 50 or individual burners 52 can be shut down instantaneously should the conveyor belt 42 fail or for other safety critical reasons. Additionally, an emergency inert cooling system (using an inert gas as nitrogen or argon and/or an inert liquid such as liquid nitrogen or liquid argon) can be installed along the length of the conveyor hood 40 and in-between the burners 52 should quick cooling of the pellets 99 be necessary (for example, if there is a belt stoppage), to alleviate the risk of fire or harm to equipment.

[0112] Various other arrangements of a DRI preheater 110 are shown in FIGS. 9-11. Each arrangement has some common elements or features.

[0113] In the depicted embodiments of FIGS. 9-11, a conveyor transports pelleted DRI/HBI (or other metal-containing pellets) from a storage location (not shown) up a ramped portion and into a preheater furnace. It is understood that any other known supply apparatus for transporting the pellets may be used, such as a hopper or vessel moved by an overhead crane. In the preheater furnace, a burner, or in some cases multiple burners, are fired to provide heating of the pellets, and a flue exhaust combustion products of the burner or burners from the furnace. From the preheater furnace, preheated DRI pellets are supplied to a melting furnace (such as an electric arc furnace or EAF).

[0114] As shown in FIGS. 9-11, the preheater furnace 120 is a refractory-lined substantially cylindrical furnace defined by an axis (extending lengthwise) and having two end walls 122 and 124, each end wall having an opening or door through at least a portion thereof. An inlet end wall 122 corresponds to the end of the furnace through which the DRI pellets 99 enter the furnace 120, and an exit end wall 124 corresponds to the end of the furnace 120 through which the DRI pellets exit the preheating furnace 120. The inlet wall 122 is opposite the exit wall 124. A substantially cylindrical side wall 126 joins the inlet end wall 122 and the exit end wall 124, and a central axis is defined by the cylindrical side wall 126. The preheater furnace 120 is mounted so that it can be rotated on its axis. Preferably, the speed of rotation can be controlled. Preferably, the preheater furnace 120 is lined with one or more special refractory coatings to reflect and re-radiate energy back to the DRI pellets 99. Flue gases will be directed either out of the building, toward the canopy, or elsewhere as determined by local requirements.

[0115] Preheating the DRI pellets requires at least one burner 130 to supply heat to the furnace 120 and at least one flue 160 to exhaust combustion products from the furnace 120.

[0116] In a first embodiment (FIG. 9), at least one burner 130 is mounted in the inlet end wall 122 of the furnace 120 and a flue 160 is mounted in the exit end wall 124 of the furnace 120; this embodiment results in a single-pass co-flow arrangement.

[0117] Alternatively, in a second embodiment (FIG. 10), at least one burner 130 is mounted in the exit end wall 124 of the furnace 120, and a flue 160 is also mounted in the exit end wall 124; this embodiment results in a double-pass arrangement that is initially counter-flow.

[0118] Alternatively, in a third embodiment (FIG. 11), at least one burner 130 is mounted in the inlet end wall 122 of the furnace 120, and a flue 160 is also mounted in the inlet end wall 122; this embodiment results in a double-pass arrangement that is initially co-flow.

[0119] In a fourth embodiment (not shown), at least one burner 130 is mounted in the exit end wall 124 of the furnace 120 and a flue 160 is mounted in the inlet end wall 122 of the furnace 120; this embodiment results in a single-pass counter-flow arrangement.

[0120] The pellets 99 on the moving conveyor 42 are inputted into the preheater furnace 120 through an opening in the inlet end 122 or door of the furnace 120 and discharged from the preheater furnace 120 through an opening in the exit end 124 or door. The process can operate in a batch mode or semi-continuous mode. The term “semi-continuous” is used to denote: (i) a mode that could be operated continuously, in which the feed rate through the inlet end is nominally equal to the discharge rate through the exit end, for an indefinite period of time, or for as long as necessary to charge the melting furnace; and/or (ii) a mode in which there are disruptions of flow at one end or the other, and in which the preheater furnace serves as a buffer to either accumulate pellets (e.g., when the inlet feed cannot be stopped but the melting furnace is not capable of immediately receiving heated pellets) or disperse pellets (e.g., when the inlet feed is stopped, whether by plan or unintentionally, and it is desired to continue charging the melting furnace). In this way, a preheater furnace is superior to merely heating on a continuous conveyor due to the added buffering capacity.

[0121] In a batch mode, a predetermined amount of pellets are loaded in the preheater furnace (e.g., by mass or volume or quantity of pellets) and are heated for a period of time, or until a desired average pellet temperature is reached, or until some other parameter or criteria is attained, and then the pellets are discharged as a batch into the melting furnace. The pellets can be heated to any desired temperature that is less than their melting temperature.

[0122] In a semi-continuous operation, pellets are loaded from the moving conveyor into the preheater furnace at a feed rate and are heated as they move axially through the preheater furnace. Heated pellets are discharged from the preheater furnace at a discharge rate into the melting furnace. The feed rate of the pellets is at least as large as the discharge rate, and preferably the feed rate is somewhat greater than discharge rate so as to ensure a sufficient residence time of the pellets in the preheater furnace and to ensure a substantially continuous stream of pellets being discharged from the furnace. There is expected to be a rough correlation of the feed rate and the discharge rate such that adjusting the feed rate will, with a time lag, cause a resultant adjustment in the discharge rate. In the semi-continuous operation mode, the pellets are contained in the furnace for a residence time on the order of minutes. Particularly in the semi-continuous mode, the exit opening through which pellets are discharged from the preheater furnace into the melting furnace is preferably located at the opposite end of the preheater furnace from the inlet opening through which pellets are fed into the preheater furnace The at least one burner and the inlet opening can be located on the same end or on opposite ends of the preheater furnace.

[0123] The pellets are mixed thoroughly by the rotating motion of the furnace. In addition, a screw conveyor arrangement could be placed inside the furnace to efficiently mix the pellets and to ensure substantially uniform exposure of the pellets to the radiation and hot combustion gases produced by the at least one burner. Alternatively, or in addition, at least one baffle may be positioned on at least a portion of the substantially cylindrical side wall to function to urge the pellets to move from the inlet end to the exit end. In one embodiment, the at least one baffle is a helical baffle configured to function as a screw conveyor on the interior of the preheater furnace side wall. In addition to or separately from the at least one baffle, a screw conveyor may be positioned within the preheater furnace for urging or compelling the pellets to move from the inlet end to the exit end.

[0124] Alternatively, or in addition to a screw conveyor or baffles, the preheater furnace may be a tilted furnace or capable of tilting to help better contain the charge material while batch process as well as accomplishing input and discharge of pellets. The preheater furnace may be mounted at a fixed angle α with respect to horizontal, with the inlet end higher than the exit end. In addition, or alternative, the preheater furnace may be pivotable so that it can move to any angle α from horizontal to near vertical during charging, heating, and/or discharging, as required by the process. For a batch process, the inlet and outlet can be through the same end of the furnace and tilting can be used to facilitate both holding the pellets in the furnace and discharge. For a continuous process, the inlet and exit are preferably at opposite ends, but a modest tilt angle α through the operation can still assist in both retaining pellets and encouraging a flow from feed to discharge.

[0125] A controller may be used to operate the at least one burner, for example to control the heating profile in the furnace, the atmosphere in the furnace, and/or the discharge temperature of the pellets. In some embodiments, a single burner is utilized. In other embodiments, two or more burners are used in order to control the amount of heat provided to one or more zones or regions in the furnace.

[0126] The burner firing rate and residence time in preheater furnace can be controlled based on requirement to achieve an aim average heat content/temperature of the charged pellets using sensors in the preheater furnace. Further, as noted above, the preheater furnace can serve as a buffer so that pellets an continue to be fed into the melting furnace for a period of time even if is a stoppage of the input conveyor belt.

[0127] In some embodiments, it may be beneficial to modulate the firing rate of the burner to control the preheating temperature if needed.

[0128] DRI pellets tend to oxidize, so in some embodiments it may be beneficial to control the atmosphere in the furnace to be slightly fuel rich (an equivalence ratio of 1 to 1.3, or preferably an equivalence ratio of 1 to 1.1). Equivalence ratio indicates the amount of fuel provided as compared with the amount of fuel that would be completely combusted to CO.sub.2 and H.sub.2O by the available oxygen). A skilled person would understand that equivalence ratio is the inverse of stoichiometry, wherein stoichiometric combustion uses the theoretical amount of oxygen required to completely combust the fuel, super-stoichiometric or fuel-lean (equivalence ratio less than 1) uses excess oxygen, and sub-stoichiometric or fuel-rich (equivalence ratio greater than 1) uses insufficient oxygen. In addition, flue gas sensors could be used to measure composition of flue gases along the length and at the exit of the preheater to modify and control the generation of desired atmospheres. In addition, or alternatively, temperature and imaging sensors could be used to measure temperature along the length and at the exit of the preheater to control the energy input.

[0129] While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention.