KR20020001772A - A method and apparatus for reducing a feed material in a rotary hearth furnace - Google Patents

Abstract

The present application relates to a rotary hearth furnace for reducing feedstock. The rotary hearth furnace 10 has a rotatable hearth 12 mounted for rotational movement and disposed on the enclosure. The enclosure includes an annular inner wall 14, an annular outer wall 16, and a ceiling 18. The enclosure is sealed in a furnace and divided into a plurality of zones having at least a loading zone 28, a treatment zone 24 and a discharge zone 22. The furnace additionally comprises a flue 30 and a rotary hearth disposed in the reduction zone of the furnace between the discharge zone 22 and the preheating zone 26 to exhaust the gases produced by the feedstock processing and the combustion gases from the burners. It is further provided with a plurality of burners 20 arranged on at least an outer wall of the enclosure to provide a control temperature in the furnace. The flue has a space for combustion and precipitation of unburned particulates. Water spray quench lowers the temperature of the furnace off-gas so that subsequent combustion with the inflow of air or oxygen does not produce significant nitrogen oxides.

Description

A METHOD AND APPARATUS FOR REDUCING A FEED MATERIAL IN A ROTARY HEARTH FURNACE

Typical rotary hearth furnaces have an annular hearth disposed between the annular inner fire wall, the annular outer fire wall and the inner and outer walls. The hearth is movably supported by a roller arranged around its circumference. A fixed ceiling is placed on the roadbed and between the inner and outer walls. The plurality of burners are disposed along the inner and / or outer walls, ignited in an annular space above the rotary hearth in a fixed ceiling and conveyed to the rotary hearth through various zones, for example, the loading zone, the treatment zone, and the discharge zone. Heat the feedstock.

In operation, the heated feedstock is placed directly on the furnace in the loading zone, where the feedstock is subjected to radiant heating and conductive process gases for chemical reactions as the feed is conveyed around the hearth passage. The treated feedstock is then removed from the rotary hearth in the discharge zone.

In rotary hearth furnaces, as shown in US Pat. Nos. 4,597,564 and 4,622,905, the disclosures of which are incorporated herein by reference in their entirety, the gas produced in the rotary hearth furnace is remote from the furnace's discharge zone and disposed adjacent to the loading zone. Exhaust from the flue. The year is located adjacent to the loading zone and remotely from the discharge zone of the furnace to maximize the exposure time to the countercurrent flow system, for example the feedstock being heated, withdrawing gas from the discharge zone to the loading zone. This creates a system in which the effluent flows in the reverse direction to the direction of rotation of the furnace. Even if the flow system is satisfactorily implemented, the gas produced by the rotary hearth furnace near the discharge zone in the general counterflow type flows directly into the flue due to the intentional obstruction of the gas flow through the watertight tunnel, resulting in the treatment zone and the loading. Flow through the area. In addition, if the year is located adjacent to either the loading or discharge zone, it can be foreseen that a pressure differential is obtained in the loading and discharge zones to help the emissions of the furnace gas from the rotary hearth furnace through the loading and discharge zones. have.

In addition, in the direct reduction of iron process, a high CO / CO ₂ ratio in the final zone of the furnace is needed to prevent back oxygen from the direct reduced iron (DRI). In order to maintain a high CO / CO ₂ ratio, it can be predicted that the burners should be operated with significantly lower air-to-fuel ratios (below 6.2 to 1). This low air-to-fuel ratio translates from an unacceptable low available heat values of, for example, 84.5 BTU / ft ³ natural gas at a 6.12 air / fuel ratio that translates into high fuel use. Three alternatives are available to improve thermal issues. The first is to preheat combustion air to recover energy from the furnace exhaust, the second to replace some or all combustion air with oxygen, and the third is to combine oxygen enrichment and preheat combustion air. . Table 1 summarizes the effects of preheated combustion air and oxygen enrichment on pounds of natural gas consumed per pound of DRI produced and on combustible thermal heating.

Table 1: Heat of combustion and oxygen enrichment at about 2 CO / CO ² ratio

In view of the foregoing, it can be envisaged that there is an increasing need for an improved rotary hearth furnace that faces the problems of the prior art. It is an object of the present invention to provide a rotary hearth furnace with improved process gas flow. Another object of the present invention is to provide a rotary hearth furnace which prevents process gas from being avoided in the treatment zone through either the discharge zone and / or the loading zone of the furnace. Another object of the present invention is to provide a rotary hearth furnace which efficiently utilizes energy useful for reducing feedstock to a rotary hearth furnace. It is another object of the present invention to provide a rotary hearth furnace that effectively reduces the amount of laminated gas passing through the year of the rotary hearth furnace so that the size of the exhaust equipment is reduced. It is a further object of the present invention to provide an aerator away from the loading zone so that there is no release of potential toxic vapors from the organic or carbonaceous binder used to prepare the feedstock through the slot of the feeder. This fact can be expected to allow the process gas to be combined with oxygen from air and incinerated to release heat in the preheating zone of the furnace. Another object of the present invention is to provide a rotary hearth furnace which is simple and economical to manufacture.

The present invention relates to a process for reducing feedstock to a rotary hearth furnace and a rotary hearth furnace. More specifically, the present invention relates to a rotary hearth furnace with an improved flue system and a process method for reducing feedstock to a rotary hearth furnace.

1 is a plan view of a rotary hearth furnace.

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1; FIG.

In accordance with the present invention, a rotary hearth furnace for reducing feedstock is provided. The rotary hearth furnace of the present invention has a rotatable hearth mounted for rotational operation and disposed in a refractory lined enclosure. The enclosure has an annular inner wall, an annular outer wall and a ceiling. The enclosure is operably sealed on the furnace and is divided into a plurality of zones having at least a loading zone, a preheating zone, a treatment zone and a discharge zone. The furnace additionally comprises a plurality of burners arranged on at least the outer wall of the enclosure to provide a controlled temperature in the rotary hearth furnace, and the treatment zone of the furnace to evacuate the process gases generated in the processing of the feed material and the combustion gases from the burners. And the year disposed between and the preheating zone.

Other features, objects, and advantages of the present invention will be described below with reference to the accompanying drawings.

In the rotary hearth furnace 10 shown in the drawing, similar features have been given the same numbers. In the drawings, it should be noted that the detailed parts that are well understood by those skilled in the art are not shown for the purpose of clearly describing the structure of the rotary hearth furnace 10 of the present invention. For example, burners, blowers, piping and ductwork, such as those required for manipulating gaseous and specific solid materials, are required for the entire system of the present invention as described herein by those skilled in the art. It is a known commercially available useful ingredient except that it can be adapted accordingly. For reference, the Chemical Engineer's Handbook, 1984, McGraw Hill, New York, USA; Kelly, Lee at 1982 John Willy & Sons Incorporated. The introduction to Mineral Processing, by Kelly, E., G., is a chemical industry document detailing various devices, process structures, and conditions.

The rotary hearth furnace 10 shown in the drawing has a hearth 12 mounted to rotate about its center in the direction of the system. The hearth 12 is placed in a donut shaped enclosure supported by a roller arranged around its circumference as is well known in the art. The enclosure has an annular inner wall 14 and an annular outer wall 16. The annular sidewalls 14 and 16 are preferably vertically arranged and made of refractory materials well known in the art. The fixed ceiling 18 is disposed and connected between the top ends of the inner and outer side walls 14 and 16. The enclosure is operably sealed to the hearth 12 by a watertight body (not shown) as is known in the art.

Suitable burners 20 of conventional design are arranged on the vertical outer wall 14 and / or the inner wall 16 of the enclosure. Burner 20 is supplied with appropriate fuel such as oil, coal dust and / or gas and combusted with air. Burner 20 is operatively fired to provide a controlled temperature in rotary hearth furnace 10 for the reduction of feedstock. In treating feedstock containing volatile coal, the selected burners are utilized by providing different air inlets in the furnace enclosure for the purpose of burning the combustion gases. As further described herein, when operating with gases having a very high reduction in the final quadrant of the treatment zone, only air is introduced into the selected burners to partially incinerate the combustion gases.

In general, the rotary fireside furnace 10 is divided into a plurality of zones having at least a loading zone 28, a preheating zone 26, a treatment zone 24 and a discharge zone 22. Each zone is one that is well known in the art and is separated from the adjacent zone by a barrier film (not shown) composed of an alloy or ceramic fiber suitable to withstand high temperature and corrosive atmospheres within the zone. As used herein, the term "zone" refers to a rotary hearth, in which the basic action that occurs is different from the basic action that occurs in different compartments of the furnace, such as, for example, loading, preheating, processing, and discharging. An artificial division of the furnace is referred to. Each zone may additionally be divided into sequential quadrants. The term "quadrant" as used herein refers to the separation zone of each zone of the furnace. As shown in the figure, the quadrants are of the same size.

In the loading zone 28, the reduced feed material is distributed to the rotary operation hearth 12 of the rotary hearth furnace 10. The feed material is a material suitable for being reduced by a heating operation or exposed to the process gas at a controlled atmospheric pressure. Feedstocks include carbonaceous materials, such as coal ash, coal ash containing mixtures, petroleum coke and / or petroleum coke containing mixtures. The feedstock is also virgin, such as untreated or untreated, such as hematite, limonite, magnetite, taconite, rhomborite, pyrite and chromite, usually collected as a result of metal fabrication or mixing thereof. Metal oxide condensates and natural ore fines and / or metallized abrasive waste, electric arc furnace dust, rolled abrasive flakes or the like. The feedstock may contain volatiles such as coal ash or coal ash containing mixtures and the like, or the feedstock may be nonvolatile, such as coke ash. The feedstock is a compact or pellet shaped particulate as known in the art.

The feedstock is preferably the hearth of the rotary hearth furnace 10 by a conventional feeder such as an electric vibratory feeder on the profiled star wheel which extends through the outer wall 16 of the furnace at a suitable distance above the surface of the hearth. It is evenly distributed to (12). In a preferred embodiment, the feed material is placed one layer deep directly on the rotary operation hearth 12 to facilitate uniform processing of the feed material, and the feed material which is more exposed and the less exposed feed material. This prevents the reactivity from changing.

After the feed material is filled in the loading zone 28, the feed material is conveyed to the preheating zone 26 in the enclosure formed by burning the hearth passage, and then to the processing zone 24. The preheating zone 26 operates at temperatures lower than the treatment zone 24, for example 1800 ° F and 2200 ° F versus 2300 ° F and 2600 ° F, thus minimizing the adverse reaction of heat transfer that leads to different crushing of the feedstock. . As shown in FIG. 1, the preheating zone 26 extends from the downstream end of the loading zone 28 to the inlet of the treatment zone 24.

Treatment zone 24 is subdivided into three sequential sequential quadrants identified as 1,2,3. Each quadrant has an inlet zone and an outlet zone. The first quadrant extends from the downstream end of the preheating zone 26 to the upstream inlet of the second quadrant, the second quadrant extends from the outflow zone of the first quadrant to the inlet of the third quadrant, and the third quadrant It extends from the outlet of the quadrant to the upstream end of the discharge zone.

In the preheating zone 26, the burner 20 is fired to achieve a predetermined zone temperature between 1800 ° F and 2200 ° F with an air-to-fuel ratio sufficient to incinerate the volatile organics emitted from the feedstock as the main fuel source. do. In the treatment zone 24, the burner 20 is fired to obtain the temperature of the furnace between 2300 ° F. and 2600 ° F. with the air atmosphere reducing the feedstock. The feed material receives the action and radiant heat of the combustion gas from the burner 20 and, depending on the feed material, receives the processing gas emitted from the processing of the feed material while traveling around the hearth passage and is reduced. In addition, air is introduced into the treatment zone 24 of the furnace as needed to combust with some excessive carbon monoxide and hydrocarbons in the combustion process to form carbon dioxide and water vapor so as to maintain a predetermined furnace temperature for handling feedstock in the treatment zone. Releases heat.

After the feedstock is reduced in the treatment zone 24, the reduced feedstock is removed from the rotary hearth in the discharge zone 22. For example, reduced feedstock may be discharged from the discharge zone 22 by helical screws disposed spaced across the hearth. The reduced feedstock is then discharged into the soaking pit for additional processing processes known in the art.

In another embodiment, the rotary hearth furnace 10 also includes a warming zone (not shown). The warming zone of the rotary hearth furnace 10 is placed just before the loading zone for the introduction of feedstock. Warming zones without feedstock may be heated to the required temperature prior to loading the feedstock. The heating of the hearth without feedstock immediately before filling the feedstock comprises the operations of warming the entire upper surface of the hearth furnace, radiant heating of the feedstock sequentially introduced from the top, and from the bottom. The operation of conducting radiant heating of the feed material is effected. Dedicated warming of the zone of the rotary hearth furnace with no feed material causes the furnace to heat up the rotary hearth furnace, resulting in the cooling effect caused by the continuous charging of the cooling feedstock in the rotary hearth furnace. It can be expected to achieve a constant loading zone temperature as opposed to a hearth furnace.

The ceiling 18 of the rotary hearth furnace 10 has a flue 30 disposed in the zone of the treatment zone 24 of the furnace between the preheating zone 26 and the discharge zone 22. The flue 30 is disposed in the treatment zone 24 so that feed material, treatment gas and combustion gas flow in the preheating zone 26 and combine with oxygen from the combustion air to dissipate heat in the preheating zone 26. Combustion may cause the process gas and the combustion gas to flow out of the discharge zone 22 to be combined with oxygen from air and / or oxygen enriched air to combust and release heat in the furnace 10.

The year 30 is disposed at any position between the inlet zone of the third quadrant of the treatment zone and the outlet zone of the preheat zone 26. In a preferred embodiment of the present invention, the rotary hearth furnace 10 has a flue 30 disposed approximately in the outflow zone of the second quadrant of the treatment zone or in the outflow zone of the preheating zone 26. By arranging the flue 30 approximately at the outlet of the preheating zone 26, the furnace loading and discharge zones are maintained at the same pressure as the atmospheric pressure, so that the gas from the furnace leaks through the loading or discharge mechanism in a positive pressure situation. Anticipate and prevent unnecessary air from entering the furnace in a negative pressure situation.

As shown in FIG. 2, flue 30 includes a series of interconnected horizontally extending reburn burner chambers 32 and one or more vertically extending stacks 34. The bottom of the chamber 32 is conical and additionally operates to exit the chamber to a collection site as described below. A foreign matter removal valve 36 of the type known in the art is operatively connected to the bottom of each chamber 32. The foreign matter removal valve 36 is easy to access and remove the foreign matter and particulate falling, and is collected in the reburn burner chamber (32).

It can be foreseen that the hot exhaust stack gas in the flue 30 leaves the rotary hearth furnace 10 containing waste sensitive and chemical energy. Waste sensitive energy is in the form of heating, and chemical energy is in the form of organic volatiles of carbon monoxide and hydrogen. Stack gas also carries particulates made of fine metal oxides and / or carbonaceous materials. In order to achieve a standard exhaust that can accommodate the stack gas from the rotary hearth furnace 10, combustion air and / or oxygen are introduced into the reburn burner chamber 32 via the air pipe 38, and from the rotary hearth furnace, And combustion with other combustion gases and carbonaceous particulates. The particulates that are not incinerated in the stack gas are then precipitated in the collection zone of the downstream reburn burner chamber 32 for removal through the debris removal valve 36.

It is foreseen that the gas leaves the rotary hearth furnace 10 at a temperature in the range of 1800 ° F to 2350 ° F and rises above 2500 ° F in the combination of combustion air and / or oxygen. Temperatures above 2500 ° F. are advantageous for the formation of nitric oxide. Therefore, there is a need to control the combustion temperature in the reburn burner chamber 32 to facilitate low particulate transport, acceptable hydrocarbon exhaust, acceptable carbon monoxide exhaust, and acceptable low NOx exhaust. The temperature of the reburn burner chamber 32 is controlled for the reburn process below about 1800 ° F. to prevent oxidation of nitrogen from forming nitrogen oxides. The temperature in the reburn burner chamber 32 is controlled by water spray quench. Water spray cooling involves spraying water droplets to cool the temperature in the reburn burner chamber 32 and the flue 30 to inject moisture into the stream of combustion air and / or oxygen. In a preferred embodiment, moisture is injected by the plurality of fluid nozzles 40 in the flue 30. The fluid nozzle 40 is a suitable nozzle, such as the Flo Max air spray nozzle from Spraying Systems Ltd.

In the case of a typical rotary hearth furnace, the gas produced while treating the combustion gas and the feed material from the burner 20 is exhausted near the loading area 26 of the furnace, thereby maximally exposing the feed material to the consumed gas and the process gas. You can foresee it. In addition, it has been found that a significant portion of the exhaust gas and the combustible hatching gas generated in the general rotary hearth furnace flows directly into the flue to avoid the reduction zone.

Various aspects of the present invention will be more clearly understood through the following examples, which are made for the purpose of describing the present invention only.

Yes

The rotary hearth furnace is subdivided into two zones, the preheating zone 26 and the treatment zone 24. The treatment zone 24 of the rotary hearth furnace is subdivided into three identical quadrants 1, 2 and 3, as shown in FIG. 1, to numerically indicate the effect of flue position on stack gas flow and energy consumption. Stack gas flow is the metered flow rate of combustion and process gas products from the furnace. Energy consumption is the metered energy required to reduce the feedstock.

In an example, about 105,032 lbs / hr of a feedstock comprising about 21 wt% low volatile bituminous coal and about 79 wt% agglomerated low silica hematite ore. It appears to be reduced in this rotary hearth furnace. The rotary hearth furnace is maintained at a constant operating temperature of about 2 to 3 ° F and a working CO / CO ₂ ratio between about 2-3 to maintain a reduced barometric pressure within the third quadrant of the furnace. Combustion air distributed over the entire quadrant is preheated to about 1200 ° F prior to entering the furnace.

The amount of natural gas fuel supplied to the rotary hearth furnace to maintain a constant furnace temperature of 2350 ° F. is determined when the outlet zone of the preheating zone 26 and the filling zone of the charging zone are arranged in the outlet zones of the first, second and third quadrants. Determined as a function of year position. Table 2 shows these results.

Table 2

As shown in Table 2, the maximum natural gas requirements arise when the year is placed in the outflow zone of the third quadrant. The minimum natural gas requirement also occurs when the flue is disposed in the outflow zone of the preheating zone 26.

The amount of stack gas from the rotary hearth furnace is determined as a function of the position of the year in the first, second and third quadrants and the outflow zone of the preheating zone 26 and the position of the immediate downstream of the filling zone. Tables 3 to 5 are tables showing the results.

Table 3

Table 4

Table 5

As shown in Table 5, the flow rate ratio of the maximum stack gas occurs when the flue is located in the outflow zone of the third quadrant. In addition, the flow rate ratio of the lowest stack gas occurs when it is located in the outflow zone of the preheat zone 26.

Coal volatilization is released from the preheating zone 26 of the furnace with weak metallicity and burned at a low CO / CO ₂ ratio. In the first and second quadrants, the CO / CO ₂ ratio becomes more important to limit the reoxidation of reduced iron in the feedstock. In the third quadrant, the CO / CO ₂ ratio is between about 1.5 and 3.5 to inhibit re-oxidation of the reduced iron. CO and other reducing gases produced in the third quadrant are combusted in the first and second quadrants to provide energy to the reduction process. Air is added to the furnace and burned with CO and H ₂ to reduce the amount of stack gas.

In another embodiment of the process of the invention, 95% pure oxygen is added to the preheated combustion air for the third quadrant. Oxygen content is generally increased by about 21% to 30%. This increase is nominally improving fuel flammability from 218 to 265 BTU / ft ³ .

It is to be understood that the embodiments described herein may be practiced differently within the scope of the appended claims since they are described by reference.

Claims (23)

In a rotary hearth furnace for reducing feedstock, the furnace is: Rotational movements, consisting of an annular inner wall, an annular outer wall and a ceiling, arranged in an enclosure divided into a plurality of zones operatively sealed on the furnace and having at least a loading zone, a treatment zone and a discharge zone; Roadbed; Disposed between the inlet zone of the first quadrant of the treatment zone and the outlet zone of the preheating zone and comprising flue enclosures for exhausting combustion gases from the rotary hearth furnace; The treatment zone is further divided into three sequential quadrants (1, 2, 3), each quadrant having an inlet zone and an outlet zone, The preheating zone operates in a temperature range of 1800 ° F. to 2200 ° F., The treatment zone operates in a temperature range of 2300 ° F. to 2600 ° F., A plurality of burners are disposed on at least an outer wall or an inner wall of the enclosure to provide a controlled temperature in the rotary hearth furnace; And The flue comprises at least one reburn burner chamber for the collection of particulates from the rotary hearth furnace and for further combustion of the exhaust flue gases, The reburn burner chamber is provided with a water spray quench to reduce the combustion temperature for NOx control rotary hearth furnace. The rotary hearth furnace of claim 1, wherein the feedstock is selected from virgin metal oxide concentrates, natural metal light spectroscopy, metallized abrasive waste, electric arc furnace dust, rolled abrasive flakes and mixtures thereof. . 3. The rotary hearth furnace according to claim 2, wherein the feed material additionally contains a carbonaceous material. 4. The rotary hearth furnace according to claim 3, wherein the carbonaceous material is selected from coal material, coal material containing mixture, petroleum coke and petroleum coke containing mixture. The rotary hearth furnace of claim 1, wherein the feedstock is evenly distributed on the furnace at approximately one layer deep to facilitate uniform processing thereof. 3. The rotary hearth furnace according to claim 2, wherein the feedstock is reduced under feed of radiant heating and under the action of combustion gases from the burner. 5. The rotary hearth furnace according to claim 4, wherein the feed material is reduced under the action of radiant heating and under the action of combustion gases from burners and process gases contained in volatiles. 2. The rotary hearth furnace of claim 1, wherein the burner includes an air inlet for introducing air into the enclosure for combustion with combustible gas. The rotary hearth furnace of claim 1, wherein the year of the rotary hearth furnace is disposed in the outflow zone of the first quadrant. 10. The rotary hearth furnace of claim 1, wherein the year of the rotary hearth furnace is located in the outflow zone of the preheat zone. The treatment zone is further divided into three sequential quadrants 1, 2 and 3, each quadrant having an inlet zone and an outlet zone; Rotational movements, consisting of an annular inner wall, an annular outer wall and a ceiling, arranged in an enclosure divided into a plurality of zones operatively sealed on the furnace and having at least a loading zone, a treatment zone and a discharge zone; A method of reducing a feedstock to a rotary hearth furnace comprising a hearth, the method comprising: Filling the feed material into the loading zone of the rotary hearth furnace; Conveying feedstock from the loading zone through the treatment zone to a discharge zone along the hearth passage in the enclosure; Arson burning the burner to obtain the required furnace temperature; Reducing the feedstock in the rotary hearth furnace; Removing the reduced feedstock from the furnace; Spraying a water spray quench into the reburn burner chamber to reduce the combustion temperature in the reburn burner chamber, wherein the combustion gases are inlet of the second quadrant of the treatment zone and outlet of the preheating zone. Exhausted from the burner through a flue operatively disposed between the flue and at least one reburn burner chamber for the collection of particulates from the rotary hearth furnace and for further combustion of the exhaust flue gases. How to. 12. The method of claim 11, wherein the feed material is reduced in response to the action of combustion gases from a burner and radiant heating. 12. The furnace of claim 11, wherein the furnace combusts with excess carbon monoxide and hydrogen in a combustion process to form carbon dioxide and volatiles contained in the feedstock and releases heat to maintain a predetermined furnace temperature for the treatment of the feedstock. Injecting combustion air into the preheating zone. 12. The method of claim 11, wherein only combustion air is introduced through the burner in the first quadrant of the treatment zone to partially combust the combustible gas. The method of claim 14, wherein the combustion air is preheated to about 800 ° F. to 1400 ° F. 15. The method of claim 15 wherein the combustion air is preheated to about 1200 ° F. 16. 15. The method of claim 14, wherein the combustion air is enriched with oxygen. 15. The method of claim 14, wherein the combustion air is replaced with oxygen. 17. The method of claim 16, wherein the preheated combustion air for the third quadrant is enriched with oxygen. 20. The method of claim 19, further comprising enriching the preheated combustion air with about 21% -30% oxygen. 12. The feedstock of claim 11 wherein the feedstock comprises virgin metal oxide concentrate, natural metal light spectroscopy, metallized abrasive waste, electric arc furnace dust, rolled abrasive flakes, coal material, coal material containing mixture and A method selected from the mixtures. 20. The method of claim 19, wherein the year is disposed about the outflow zone of the first quadrant. 20. The method of claim 19, wherein the year is disposed around an outlet of the preheat zone.