Patent Application
20120213684
This invention relates to treatment of offgas streams produced by steel furnaces.
During steel production in an electric arc furnace (EAF) a large volume of high temperature off-gas, which contains high concentrations of combustible gases such as carbon monoxide (CO) and hydrogen (H2), is produced inside the EAF vessel. This off-gas can reach a temperature of 3000 F or higher, and CO and H2 peak concentrations in the off-gas can reach up to 60% and 35%, respectively. To reduce electrical energy consumption, the thermal energy contained in the EAF off-gas needs to be recovered as much as possible inside the EAF vessel.
One way is to recover chemical heat of CO and H2 combustion using oxygen lances for combustion of the combustibles inside the vessel. Another way to recover the energy from the off-gas stream is to use the heat of the stream to preheat scrap that is going to be fed into the EAF. In one example, the EAF off-gas is piped into a conduit where the scrap is moving counter current to the direction of the off-gas flow, and the hot off-gas preheats the scrap by direct heat transfer. Air is injected into the scrap preheating conduit to provide oxygen for the combustion of the CO and H2. The conduit may have auxiliary burners installed to augment the heating of the scrap to a desired preheat temperature.
If scrap preheating is not used then once the off-gas leaves the EAF vessel, the remaining energy contained in the unburned CO, H2, and other combustibles may or may not be recovered. For safe operation of the furnace and for environmental reasons, the remaining gaseous combustibles in the off-gas are burned to sufficiently complete levels, so that the thus-treated off-gas can then be further cleaned downstream in the duct and discharged into the atmosphere. For example, high temperature combustibles such as CO and H2 in the off-gas are typically burned with air introduced from an “air gap” in a water-cooled exhaust duct connecting to the “fourth hole” of the EAF furnace (herein called “EAF exhaust duct”). Downstream of the EAF exhaust duct, a “dropout box” may also function as a post combustion chamber for additional CO and H2 combustion. Combustion air for the dropout box may be introduced upstream of the box or in the box by lancing. Part of the flue gas stream exiting from the dropout box may be recycled back into the EAF vessel, if the operator wishes to recover the flue gas energy for melting purpose.
Despite the above efforts, the presence of small amounts of CO in the off-gas (termed “CO slip”) is still an operational issue for many EAF operators. This is because during an EAF heat cycle, there are large variations in off-gas composition, volume, and temperature. These variations in off-gas properties are further complicated by the varying amount of the air infiltration and the changing composition of the scrap being fed. Thus, keeping the bag house CO emissions in check during the whole EAF heat cycle represents challenges to the furnace operators. EAF operators face heavy financial penalties should the CO emissions exceed regulatory limits.
In order to avoid exceeding emissions limits for CO and other combustibles, some EAF operators may opt to operate their furnaces conservatively in order to meet the emissions limits, but at the expense of furnace energy efficiency. For example, an operator may set the furnace or the exhaust duct pressures low to induce excessive amount of air infiltration for completing CO burnout. This excess amount of air can increase the total flue gas volume and the heat loss associated with the off-gas, thus decreasing the furnace thermal efficiency. In other instances, the production capacity of an EAF furnace can be substantially limited by the ability of the downstream ducts or dropout box to complete the combustion of the CO, H2, and other combustibles. If scrap preheating is used, EAF operators may also face the issue of containing fugitive odors and limiting the formation of undesired byproducts such as dioxins in the flue gas ducts.
The foregoing concerns are also applicable to other apparatus used in steelmaking, such as basic oxygen furnaces (BOF), ladle refining furnaces, and argon-oxygen decarburizing furnaces.
In one aspect of the invention, a method for treating an offgas stream from a steel furnace comprises
(A) providing an offgas stream having a temperature lower than 2000 F by obtaining offgas from the atmosphere above the surface of molten steel in a furnace which obtained offgas contains carbon monoxide in an amount higher than 500 ppm, admitting air into said obtained offgas and cooling said obtained offgas to the extent necessary so that it is at a temperature lower than 2000 F;
(B) mixing fuel and oxygen and combusting a portion of the oxygen in the mixture with said fuel in a chamber to form a hot oxygen stream emerging from an outlet in said chamber that contains oxygen, wherein the residence time of said combustion in said chamber is long enough that said hot oxygen stream has a temperature higher than the temperature of the offgas stream to which it is added in step (C) and said residence time is short enough that said hot oxygen stream contains products of said combustion including radicals selected from the group consisting of radicals corresponding to the formulas O, H, OH, C2H, CH2, CjH2j+1 or CjH2j−1 wherein j is 1-4, and mixtures of two or more of such radicals;
(C) feeding the hot oxygen stream formed in step (B) into the offgas stream provided in step (A) to raise the temperature of the offgas in said provided stream to a temperature higher than 1100F that is higher than the temperature of the offgas to which said hot oxidant stream is added, wherein the hot oxygen stream is added at a rate sufficient to convert carbon monoxide in the offgas to which it is added to carbon dioxide, thereby lowering the carbon monoxide content of said offgas.
In a preferred aspect of the foregoing method, metal such as scrap metal is fed into one of the aforementioned furnaces, and before said metal is fed into the furnace it is contacted in direct heat transfer with said offgas obtained from said furnace to heat the metal and to cool said obtained offgas.
In another preferred aspect of the foregoing method, said scrap metal contains organic material that is volatilized into said obtained offgas by said heat transfer with said obtained offgas stream, and said hot oxygen stream is mixed together with said cooled offgas stream at a rate sufficient to convert said organic material to carbon dioxide and water vapor.
This invention involves injecting a stream or streams of high-momentum hot oxygen to destroy low concentration levels of off-gas combustibles, particularly where the off-gas temperature is already below the spontaneous ignition temperature of the off-gas combustibles. Because the injected oxygen is hot and the jet momentum is high, the hot oxygen will mix rapidly with the offgas, thus enhancing the ability of the radicals in the hot oxygen stream to destroy combustibles in the offgas even though the offgas temperature is below the combustibles' ignition temperatures. These streams of high momentum hot oxygen are produced by the hot oxygen generator described herein.
While the following description of the present invention refers to the Figures, the invention is not to be considered to be confined to the embodiments illustrated in the Figures.
Referring to
The offgas stream downstream of air gap 27 passes into dropout box 50. Larger particulate material entrained in the offgas is separated from the offgas in the dropout box 50. Stream 52 is the offgas stream exiting from the dropout box 50. The dropout box 50 may be used as a combustion chamber for combustion of combustible products in the offgas, if lances for combustion oxidant injection and/or burners are installed (not shown).
The offgas is preferably at a temperature of 2000 F or lower when a hot oxygen stream is fed into the offgas as described herein. If the offgas as formed is already 2000 F or cooler, then the offgas does not need to be cooled. If the offgas temperature is above 2000 F, then cooling of the offgas is necessary. Typically, offgas as recovered from steel furnace atmospheres such as an EAF is hotter than 2000 F. The admission of air into the offgas stream via the air gap 27 and any other air inlets cools the offgas. Additional cooling as necessary can be provided by passing the duct in which the offgas is flowing through a water jacket or other equivalent mechanism that withdraws heat from the offgas through and away from the wall of the duct. Cooling of the offgas can also be provided by radiative and/or convective cooling of the duct to the ambient atmosphere.
Hot oxygen stream 56, which can be produced by hot oxygen generator 54 as described herein, is fed into stream 52 to form stream 53.
Offgas 10 formed in furnace 20 can also be recovered by rising into canopy 30 and into canopy duct 31. In this embodiment, air is drawn into duct 31 from the surrounding atmosphere. Hot oxygen stream 33, which can be produced by hot oxygen generator 32 as described herein, is fed into stream 31 to form stream 34.
Offgas streams that are advantageously treated by this invention can also come from other sources in a steel plant, such as ladle refining furnaces and/or argon-oxygen decarburizing furnaces. Stream 42 represents any such offgas stream, which can also be a stream formed by combining the offgas streams from two or more ladle refining furnaces, two or more argon-oxygen decarburizing furnaces, or from both types of furnaces. Hot oxygen stream 48, which can be produced by hot oxygen generator 46 as described herein, is fed into stream 42 to form stream 49.
As stated above, any one of these hot oxygen generators and streams can be employed, or several can be employed to treat CO content of several different offgas streams. In addition, a second (or additional) hot oxygen generator can be employed to feed a hot oxygen stream into an offgas stream downstream from a location where a hot oxygen stream has already been fed. For example, hot oxygen stream 36 from hot oxygen generator 35 can be fed even where a hot oxygen stream such as streams 33 and/or 48 have been fed upstream. A hot oxygen stream that is fed as stream 36 is, downstream from where another hot oxygen stream is fed, can be fed continuously, or can be turned off and on intermittently as a backup source of hot oxygen where needed to provide sufficient CO destruction during a specific time period of the heat cycle where CO contents are elevated above their normal levels. In the embodiment shown in
The streams formed upon addition of the hot oxygen into the offgas stream or streams, such as streams 39 and 53 in the case of
The formation of a hot oxygen stream, such as streams 56, 33, 48 and/or 36, formed from its corresponding hot oxygen generator, is described with reference to
Stream 204 of fuel is provided to hot oxygen generator 54 through a suitable fuel nozzle which may be any suitable nozzle generally used for fuel injection. The fuel may be any suitable combustible fluid examples of which include natural gas, methane, propane, hydrogen, refinery fuel gas, landfill offgas, syngas, carbon monoxide, and coke oven gas. The presence of hydrogen in the fuel fed to the hot oxygen generator 54 is advantageous in assisting conversion of CO to CO2 evidently because the combustion that forms the hot oxygen stream promotes the formation of (nonionic) OH and O radicals in the hot oxygen stream. Preferably the fuel is a gaseous fuel. Liquid fuels such as number 2 fuel oil may also be used, although it would be harder to maintain good mixing and reliable and safe combustion with the oxygen with a liquid fuel than with a gaseous fuel.
The fuel 204 provided into the hot oxygen generator 54 combusts there with oxidant to produce heat and combustion reaction products such as carbon dioxide and water vapor. Preferably, no more than about 35 percent of the oxygen of the oxidant combusts with the fuel. If more than about 35 percent of the oxygen combusts with the fuel in the hot oxygen generator, then appropriate measures should be taken such as using refractory materials of construction and/or employing a heat removal feature such as a water wall to keep the temperature of the remaining oxygen from increasing to undesirable levels.
The combustion reaction products generated in the hot oxygen generator 54 may mix with some of the remaining oxygen of the oxidant 202, thus providing heat to the remaining oxygen and raising its temperature. Preferably, the fuel is provided into the hot oxygen generator 54 at a high velocity, typically greater than 200 fps and generally within the range of from 500 to 1500 fps. The high velocity serves to entrain oxidant into the combustion reaction products thus promoting combustion of the fuel in the chamber.
Generally the temperature of remaining oxidant within the oxidant supply duct is raised by at least about 500 F, and preferably by at least about 1000 F. It is preferred that the temperature of the remaining oxygen not exceed about 3000 F to avoid overheating problems with supply ducts and nozzles.
As the temperature of the remaining oxygen within the hot oxygen generator 54 is increased, the requisite supply pressure of the oxygen to achieve any given oxygen injection velocity into the offgas decreases. For example, for injection of the oxygen at ambient temperature the requisite pressure exceeds 7 pounds per square inch gauge (psig) in order to inject the oxygen into the offgas at a velocity of 800 fps. As the oxygen temperature increases, the requisite pressure decreases sharply. At a temperature of 1500 F the requisite pressure is 1.65 psig and at a temperature of 3000 F the requisite pressure is only 0.91 psig. At temperatures exceeding 3000 F there is little additional benefit, thus providing another reason for not exceeding 35 percent oxygen combustion with the fuel. Thus, generation of hot oxygen in this manner can provide a high velocity hot oxygen stream 56 to the offgas without the need for a high supply pressure thus reducing or eliminating the need for compressing oxygen prior to passing it into the offgas which would otherwise be necessary if the oxygen source pressure is not high.
The combustion that occurs in hot oxygen generator 54 should be carried out in a manner such that the hot oxygen stream 56 that emerges from generator 54 contains one or more radicals corresponding to the formulas O, H, OH, C2H, CH2, CjCH2j+1 or CjH2j−1 wherein j is 1-4, and mixtures of two or more of such radicals. This can be achieved by providing that the residence time of the reactants (fuel and oxygen) within the hot oxygen generator is long enough to enable combustion reaction of fuel and oxygen to occur in the hot oxygen generator producing a stream having a temperature higher than the temperature of the offgas into which the stream is to be fed, and simultaneously providing that said residence time is short enough that at least some of the above-mentioned radicals are present. The residence time, in turn, is determined by the volume of the space within generator 54, by the feed rates of fuel stream 204 and of oxidant stream 202 into generator 54, and by the size of the exit orifice through which the hot oxygen stream 56 emerges from generator 54. Preferred residence times are about 1 to 2 msec.
Referring to
The hot oxygen stream 56 preferably contains at least 75% (volume) O2. A typical composition for this stream is about 80% O2, 12% H2O, 6% CO2, some highly reactive radicals such as (nonionic) OH, O, and H which are particularly effective to initiate and oxidize CO to CO2, and the aforementioned radicals. The hot oxygen stream 56 exits through orifice 201 and is fed into the offgas at high velocity and momentum, which results in accelerated mixing between the hot gas and the offgas.
The hot oxygen stream 56 (as well as streams 33, 36 and/or 48, and other streams generated and used in the practice of this invention) obtained in this way typically has a temperature of at least 1600 F and preferably at least 2000 F. Generally the velocity of the hot oxygen stream will be within the range of from 500 to 4500 feet per second (fps), preferably 800 to 2000 or to 2500 fps, and will exceed the initial velocity by at least 300 fps. In a preferred embodiment this velocity is at Mach 1.
The description in U.S. Pat. No. 5,266,024, the content of which is hereby incorporated herein by reference, further describes formation of the high momentum hot oxygen stream.
The high velocity hot oxygen stream is believed to entrain the offgas into which it is fed through jet boundaries by velocity gradients or fluid shear, and by turbulent jet mixing. The gaseous stream that is formed upon combining the offgas and the hot oxygen stream, which mixture may include reaction products of the hot oxygen and the offgas, has a temperature of at least 1000 F, preferably at least 1250 F, although advantages can be realized when the temperature of this mixture is above 1400 F.
In each use of the hot oxygen stream, such as where hot oxygen stream 56 is fed into offgas stream 52, stream 56 is fed at high momentum into the steel furnace offgas. The desired reaction of the hot oxygen with the offgas is enhanced by increasing the intimacy of mixing between the hot oxygen and the offgas. The intimate mixing can be promoted by dividing the hot oxygen into a plurality of streams and feeding these streams into the offgas, or by feeding the hot oxygen across or countercurrent to the offgas. Preferably, the intimate mixing is promoted by providing physical structure, within the duct such as the duct carrying offgas stream 52, that promotes contact between the hot oxygen and the offgas. Examples of such structure include wire mesh that the gases have to flow through, or baffles. The hot oxygen and the offgas mix, during which the hot oxygen burns CO in the offgas to CO2. The resulting gas mixture such as stream 53 comprises the products of these reactions between the hot oxygen and the offgas.
The preferred location to place a hot oxygen generator is so that the hot oxygen stream is fed into the offgas where the temperature of the offgas is too low for injection of air alone to be effective in destroying sufficient amounts of CO in the offgas. For instance, the hot oxygen generator can effectively be used where it feeds the hot oxygen stream to regions where the temperature of the offgas is up to 1800 F, such as 1000 F to 1800 F, preferably 1100 F to 1600 F or even up to 1500 F or even 1400 F. Even at these temperatures, destruction of CO can occur to lower the CO content of the offgas to less than 500 ppm and even to 100 ppm or less. After injection of the hot oxygen stream, the combined stream preferably has a temperature of 1350 to 1450 F. Typical injection velocities of the hot oxygen stream can be 500 to 3500 fps, preferably 1000 to 2800 fps.
As shown in
Hot oxygen generator 450, configured and operated as described herein with respect to generator 54, is installed to feed hot oxygen stream 452 into stream 437 downstream of preheater 440. The hot oxygen contained in hot oxygen stream 452 mixes rapidly with stream 437, and destroys CO in stream 437. The resulting stream 439 combines with stream 438 to form stream 454, which in turn can optionally be combined with stream 31 from the EAF canopy 30, to form stream 58. Off-gas streams 454 and 58 can be processed by downstream pollution control equipment, as described herein.
In any of these embodiments, or any other embodiments in which offgas from a steelmaking vessel is used to preheat feed material for the vessel, the feed material may contain organic material such as residues or waste material. When organic material is present in or on the feed material, the preheating of the feed material as described herein may create gasified emissions, by which is meant the gaseous state of organic material that was in or on the feed material, or gaseous organic byproducts formed by exposing said organic material to the temperatures encountered in bringing the feed material into heat transfer contact with the offgas, or mixtures thereof. These gaseous emissions may contain chemical components which can cause unpleasant odors, or which can be harmful if the components include hazardous substances such as dioxins or if the components can form into hazardous substances when exposed to the usual preheating conditions.
Feeding the hot oxygen stream formed as described herein either into the offgas stream within the apparatus in which the offgas preheats feed material, or downstream of the apparatus in which the off gas preheated feed material, has the additional benefit of removing or even completely eliminating chemical (organic) components by converting them into carbon dioxide and water.
To achieve most satisfactory CO conversion to carbon dioxide, as well as removal or elimination of potentially hazardous chemical substances that enter the offgas from feed material that is preheated by the offgas, the following three criteria are important: (1) achieve good mixing between the hot oxygen stream and the offgas; (2) the temperature of the mixture that is formed of the hot oxygen stream and the offgas is in the proper range; and (3) the mixture has enough residence time for completing CO burnout to CO2. Operating within the conditions described herein satisfies these criteria.
Compared to conventional lancing of oxygen at cold or ambient temperatures, or to installing and using post combustion burners in the offgas duct as the offgas exits the furnace, the use of the hot oxygen stream as described herein has the following advantages:
Using the hot oxygen generator and the hot oxygen stream as described herein to destroy CO in the offgas stream provides the following process advantages:
This invention enables the capacity of an existing furnace to be boosted if the production rate of that furnace is constrained by high off-gas CO emissions. If an EAF furnace increases its production rate, more offgas volume will be generated. Due to this increased offgas volume, air-based CO burnout measures installed upstream of the ducts may not be able to combust CO to a level that meets emissions limits at the baghouse. The other possible outcome with the increased production rate is that the EAF ducts may become limited by the capacity of the fan, meaning that there is a maximum amount of air that an existing fan can process for CO combustion. With this invention, an operator can let the existing air-based CO burnout system operate at its maximum ability to destroy CO. He can then use a hot oxygen generator installed downstream of the air systems for final burnout of the CO, where air based systems are no longer effective for CO destruction.
Many furnace operators control their furnaces in very conservative ways to try to be sure that all CO and other combustibles in plant effluent meet regulatory emissions limits. For example, an operator may increase duct suction pressure to introduce excessive amounts of air into the duct for post combustion of CO, H2, and other combustibles. By supplying high percentages of excess air for post combustion, the furnace may meet CO emissions limits but at the expense of its thermal efficiency. This is because high air infiltration increases offgas volume and its total heat contents. Without external flue gas recirculation, the available heat contained in the offgas is a heat loss. Higher offgas volume also increases operating costs in downstream off-gas cleaning equipments.
This invention provides a simple and effective way for furnace operators to meet their specific CO emissions goals. The hot oxygen generator becomes an assuring device for the furnace where the operating environment is dynamic and transient in nature. With this invention, operators can operate their furnaces with more flexibility to meet changing process needs, knowing that any CO slip to downstream ducts will be destroyed by the hot oxygen stream whether that stream is fed continuously or periodically when the need arises.
This application is a continuation of and claims priority from U.S. application Ser. No. 12/602,627 filed Dec. 21, 2009, which was a National Stage Entry of application PCT/US09/48211, filed Jun. 23, 2009, which claims priority from U.S. Provisional Patent Application No. 61//075,958, filed Jun. 26, 2008, which is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61075958 | Jun 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12602627 | Dec 2009 | US |
| Child | 13458423 | US |
