Patent Application
20090061366
The present invention relates to combustion of fuel in a furnace, and especially in a furnace used to heat solid and liquid materials and/or to melt solid materials, as the materials are held in or passing through the furnace.
Many industrial processes require heating material to elevated temperatures, on the order of 1000° F. or higher. Examples are numerous but include heating or reheating steel prior to its being worked in a mill, and melting glassmaking materials to form a glassmelt from which glass products are formed.
In many of these applications the heat is applied to the material in a furnace in which the material has been placed, or through which the material is passed. The heat is obtained by combustion within the furnace, at one or more burners where fuel is burned to produce heat of combustion.
In many furnaces the burner or burners combust fuel with air, which of course contains the oxygen needed for the combustion. Such combustion is termed “air-fuel combustion” and burners at which air-fuel combustion occurs are termed “air-fuel burners”. In many other applications the burner or burners combust fuel with a gaseous oxidant that contains oxygen in a concentration higher than that of air, ranging from 25 vol. % to 99 vol. % depending on the application and other considerations such as (but not limited to) economics, the higher temperature at which the combustion (termed “oxy-fuel combustion”) occurs, and the opportunity to generate a smaller amount of nitrogen oxides. Oxy-fuel combustion often requires the use of burners (termed “oxy-fuel burners”) that are adapted for oxy-fuel combustion, in particular in their ability to withstand the higher combustion temperatures obtained in oxy-fuel combustion.
Some applications attempt to use both air-fuel combustion and oxy-fuel combustion. One example occurs in steel reheating furnaces, in which a piece (slab, bloom or billet) of steel is passed through a furnace wherein the piece is heated first by the heat provided from one or more air-fuel burners and then (as it continues its passage through the furnace) by heat provided from one or more oxy-fuel burners. In addition, in some industrial heating processes the advantages of oxy-fuel combustion have led operators to remove air-fuel burners and replace them with oxy-fuel burners or add additional zones composed of oxy-fuel burners.
There remains a need, however, to be able to selectively and alternatingly obtain the benefits of air-fuel combustion and oxy-fuel combustion, without having to undergo the expense and lost time that would be encountered in repeatedly removing air-fuel burners, replacing them with oxy-fuel burners, and then replacing the oxy-fuel burners with air-fuel burners, and continuing to repeat the cycle.
The present invention, in one aspect, is combustion apparatus comprising
(a) a furnace enclosing a combustion zone and having at least one burner through a wall of the furnace to which air is fed through an air conduit and fuel is fed through a burner fuel conduit from outside the furnace to be combusted at the burner within the combustion zone;
(b) an oxidant conduit through which oxidant can be fed into the furnace from outside the furnace; and
(c) control means that regulates the flow of oxidant through the oxidant conduit and the flow of air through the air conduit such that the ratio of air flow to oxidant flow can be controlled;
wherein the oxidant conduit and the burner fuel conduit are oriented with respect to each other so that the oxidant conduit feeds oxidant into an oxidant mixing zone in the combustion zone and the burner fuel conduit feeds fuel into a fuel reaction zone in the combustion zone which is segregated from the oxidant mixing zone.
Another aspect of the present invention is a burner apparatus comprising
(a) a burner to which air is fed through an air conduit and fuel is fed through a burner fuel conduit to be combusted at the burner;
(b) an oxidant conduit through which oxidant can be fed to the burner; and
(c) control means that regulates the flow of oxidant through the oxidant conduit and the flow of air through the air conduit such that the ratio of air flow to oxidant flow can be controlled;
wherein the oxidant conduit and the burner fuel conduit are oriented with respect to each other so that the oxidant conduit feeds oxidant into an oxidant mixing zone in the combustion zone and the burner fuel conduit feeds fuel into a fuel reaction zone in the combustion zone which is segregated from the oxidant mixing zone.
Another aspect of the present invention is a method for retrofitting an air-fired furnace, comprising
(a) providing a furnace enclosing a combustion zone and having at least one burner through a wall of the furnace to which air is fed through an air conduit and fuel is fed through a burner fuel conduit from outside the furnace to be combusted at the burner within the combustion zone;
(b) providing an oxidant conduit through which oxidant can be fed into the furnace from outside the furnace;
(c) providing control means that regulates the flow of oxidant through the oxidant conduit and the flow of air through the air conduit such that the ratio of air flow to oxidant flow can be controlled; and
(d) orienting the oxidant conduit with respect to the burner fuel conduit so that the oxidant conduit feeds oxidant into an oxidant mixing zone in the combustion zone and the burner fuel conduit feeds fuel into a fuel reaction zone in the combustion zone which is segregated from the oxidant mixing zone.
The invention can be practiced in any furnace of conventional design, which will typically comprise an enclosure within which combustion at high temperature takes place. The enclosure is typically lined with material such as refractory furnace brick or the equivalent that can withstand temperatures of several thousand degrees which are generated within the furnace enclosure. Preferably, the floor, all sides, and the roof of the furnace are all lined with such material. Examples of furnaces with which this invention can be practiced include steel reheating furnaces and other furnaces through which solid material is passed to be heated, as well as glass melting furnaces and other furnaces in which material fed to the furnace is to be melted or to be maintained in a molten state.
The desired high temperature is established within the furnace by combustion carried out at one or more burners.
Suitable fuels for this air-fuel combustion include gaseous hydrocarbons, such as natural gas and methane, byproduct gases produced in steel mills, such as coke oven gas and blast furnace gas, mixtures of these gaseous fuels, as well as liquid fuels such as atomized fuel oil, and solid fuels such as pulverized coal. The fuel and the air are supplied through their respective passages 4 and 5 by suitable means connected to sources thereof, all by conventional technology quite familiar to those of ordinary skill in this field.
Apparatus, indicated schematically as 13 in
The present invention can add to burners that combust fuel in an air-fuel mode of combustion the capability to selectively combust fuel in an oxy-fuel mode of combustion. This capability can be added by, among other things, providing a way to feed oxidant having a higher oxygen content than the oxygen content of air into combustion zone 3. Preferably, the oxygen has an oxygen concentration of at least 25 vol. %, and more preferably at least 90 vol. %. A preferred manner of carrying out this feeding is shown in
The present invention can be operated so that in the oxy-fuel combustion mode the fuel that is combusted is the same as the fuel that is combusted in the air-fuel combustion mode. In such cases, the fuel can be supplied through fuel passage 4. Alternatively, such as when the fuel that is combusted in the oxy-fuel combustion mode is different from the fuel that is combusted in the air-fuel combustion mode, or when the fuel fed in the oxy-fuel combustion mode must be fed at a higher flow rate, the fuel for oxy-fuel combustion is fed through a second fuel conduit. One such second fuel conduit is shown in
As is described further below, in the oxy-fuel combustion mode the relative momentum of the fuel flow and the oxidant flow needs to be managed. In most cases where the oxidant conduit is within the burner, the second fuel conduit will be required that is capable of feeding the fuel into the combustion zone 3 at the requisite higher velocity. If NOx formation from combustion in the furnace is not an issue, then the existing fuel conduit can be employed with the oxidant conduit described herein. If NOx formation is an issue, then the second fuel conduit could be integrated into the air-fuel burner through its fuel conduit if it is suitably sized, or through a hole leading into the combustion air conduit, or outside the burner through a hole in the wall of the furnace as shown in
Other embodiments that accomplish the same objectives of the invention can also be employed. Indeed, depending on the configuration of the air-fuel burner, and depending on the available space in the immediate area outside the burner, other configurations may be preferable for ease of construction and operation.
The lance or other apparatus by which fuel is to be fed into combustion zone 3 in the oxy-fuel mode of operation, and the lance or other device through which oxidant is fed in to combustion zone 3 or the oxy-fuel mode of operation, must be oriented with respect to each other so that the oxidant mixing zone, into which the oxidant is fed as described hereinbelow, and the fuel reaction zone, into which the fuel is to be fed, are segregated (i.e., physically distinct from each other) within combustion zone 3. The feeding of the oxygen and fuel, and the operation of the burner when it is in the oxy-fuel mode of operation, should be carried out in accordance with the description contained in U.S. Pat. No. 5,076,779, the entire content of which is hereby incorporated herein by reference. In particular, the oxidant is injected into combustion zone 3 with velocity sufficient to entrain or mix furnace gases that are in combustion zone 3 with the injected oxidant. The furnace gases comprise ambient gases which infiltrate into the combustion zone, and gases from the oxidant mixture and fuel reaction mixture. Generally the velocity of the oxidant will be at least 200 feet per second and preferably is within the range of 250 to sonic velocity (1,070 feet per second at 70° F.). The velocity of the oxidant is such that sufficient furnace gases mix with the injected oxidant to dilute the oxygen concentration of the injected oxidant so that an oxidant mixture is produced within the oxidant mixing zone having an oxygen concentration of not more than 10 vol. % and preferably not more than 5 vol. %. When pure oxygen or oxygen-enriched air is used as the oxidant, higher entrainment of the furnace gas is required to reduce the oxygen concentration to the desired lower levels. No combustion reaction takes place in this zone because the furnace atmosphere entrained into the oxidant jet is substantially free of fuel.
The furnace gases mix with or are entrained into the oxidant due to the turbulence or the aspiration effect caused by the high velocity of the oxidant stream being fed into the oxidant mixing zone. The resulting oxidant mixture, containing a significantly lower concentration of oxygen than was present in the injected oxidant, flows out from the oxidant mixing zone and serves to form part of the atmosphere within combustion zone 3. That is, the oxidant mixture provides additional furnace gases to combustion zone 3.
When fuel is injected into combustion zone 3 during the oxy-fuel mode of operation of the invention, furnace gases from the atmosphere within combustion zone 3 flow into and mix with the fuel stream due to the turbulence caused by the fuel stream injection, and the oxygen within the furnace gases combusts with the fuel in the fuel reaction zone. Depending on the amount of air delivered through air conduit 5 and the relative location of fuel lance 11, a small amount of fuel may react with the air supplied via air conduit 5 in a combustion zone of the furnace prior to the main combustion zone 3.
The temperature within the combustion zone 3 should exceed 1400° F. as temperatures below 1400° F. can result in flame instabilities. The fuel reacts with oxygen molecules in the furnace gases spontaneously, as the temperature of the furnace gas is above the auto-ignition temperature of the fuel and oxygen. However, since the oxygen concentration is relatively low, the flame temperature is kept relatively low due to the presence of large amounts of non-reacting molecules such as carbon dioxide, water vapor, and molecular nitrogen in the fuel reaction zone. The combustion under these conditions in the fuel reaction zone produces heat of combustion and combustion reaction products such as carbon dioxide and water vapor but produces very little nitrogen oxides. The actual amount of nitrogen oxides produced varies with each particular situation and will depend on factors such as the furnace gas temperature, nitrogen concentration in the combustion zone and the residence time.
The resulting fuel mixture including the combustion reaction products flows out of the fuel reaction mixture and serves to form part of the atmosphere within combustion zone 3 thus providing additional furnace gases to the combustion zone. Within the fuel reaction zone, the fuel undergoes substantially complete combustion so that there is no significant amount of uncombusted or incompletely combusted fuel in the combustion zone outside of the fuel reaction zone.
It is important in the practice of the oxy-fuel combustion mode of this invention that the oxidant mixing zone and the fuel reaction zone are maintained separate from each other (or “segregated”) within combustion zone 3. In this way, combustion is restricted primarily to the fuel reaction zone and under conditions which dampen formation of nitrogen oxides (“NOx”). Although various steps of this mode of combustion are described in sequence, those skilled in the art will appreciate that the steps of this method are conducted simultaneously and continuously.
The oxidant mixing zone and the fuel reaction zone can be maintained segregated as desired, by positioning the injection points (that is, the ends of lances 11 and 14, for example) and orienting the injection directions, of the fuel and oxidant so as to avoid integration and overlap thereof prior to the requisite dilution of the oxidant within the oxidant mixing zone and the requisite substantially complete combustion of the fuel within the fuel reaction zone.
The fuel and the oxidant are fed into the combustion zone 3 in a manner to achieve sufficient mixing within combustion zone 3 so that the combustion zone atmosphere outside of the oxidant mixing zone and of the fuel reaction zone is substantially homogeneous. In a particularly preferred embodiment, the fuel and the oxidant are injected into combustion zone 3 in a manner to promote a recirculating pattern of furnace gases within combustion zone 3. This recirculating pattern contributes to improved temperature distribution and gas homogeneity within the combustion zone 3 and improves the mixing within the oxidant mixing zone and within the fuel reaction zone, resulting in smoother combustion and retarding formation of NOx. With optimum furnace gas recirculation within combustion zone 3, the composition of the flue gas taken out of the combustion zone is substantially the same as the composition of the atmosphere at points within combustion zone 3 outside of the oxidant mixing zone and fuel reaction zone. This recirculation pattern also promotes the entrainment of the furnace gases downstream of the fuel reaction zone into the oxidant stream and the entrainment of the furnace gases downstream of the oxidant mixing zone into the fuel stream.
It is particularly preferred to feed the oxidant stream and the fuel stream, in the oxy-fuel combustion mode of operation of the invention, at high velocities and away from each other so that the oxidant mixing zone and the fuel reaction zone do not overlap. Preferably, the ratio of the fuel stream momentum flux to the oxidant stream momentum flux should be within 1:5 to 5:1 when injected from relatively close proximity, such as in the embodiments depicted in
The adaptation of an air-fuel burner into a burner which is capable of selectively carrying out air-fuel combustion and oxy-fuel combustion is aided by providing suitable controls so that the operator can controllably switch between an air-fuel combustion mode and an oxy-fuel combustion mode at the same burner. Providing this capability requires controls which can controllably minimize or in the limit, shut off or turn on, the flow of air through the air passages, and which can controllably shut off or turn on the flow of oxidant through the oxidant lance or other unit by which oxidant is fed to combustion zone 3. Preferably, the controls also permit regulation of the flow rates of the combustion air, and the flow rate of oxidant, through their respective conduits. In its simplest mode, the control mechanism can comprise simply a regulating valve controlling the flow of oxidant to combustion zone 3, and a regulating valve controlling the flow of air to the air passages of the burner. In most embodiments, one will desire to shut off one such flow completely when the other such flow is to be turned on. Commercially available oxygen supply equipment typically has double block valves (for safety), flow measurement devices, pressure switches and other instrumentation with which this level of control can be facilitated.
In addition, in those embodiments in which the same fuel is used whether the combustion is air-fuel or oxy-fuel, no additional controls need to be provided so long as controls were already present to regulate the flow rate of fuel through the burner into combustion zone 3. However, in those embodiments wherein a different fuel, or a different fuel feed conduit, is provided depending on whether the combustion is air-fuel or oxy-fuel, then controls should be provided that permit the operator to shut off the flow of fuel associated with the air-fuel combustion when the oxy-fuel combustion mode is to be operated, and to shut off the flow of fuel associated with the oxy-fuel combustion when the air-fuel combustion mode is to be operated. However, even when the same fuel is combusted in the air-fuel and oxy-fuel modes, the oxy-fuel mode usually requires a higher velocity fuel flow rate. Accordingly, the fuel supplied from the fuel delivery and metering system that is in place for supplying fuel to the fuel conduit for feeding fuel to the air-fuel burner for air-fuel combustion (e.g. typically, low velocity fuel supply) is switched to the second fuel conduit that is used for feeding fuel for oxy-fuel combustion (i.e. to the burner, or to a conduit 11, or to a separate opening 11 as shown for instance in
The controls preferably permit a base flow of air through the air conduit, even in the oxy-fuel combustion mode wherein oxidant is being fed and combusted. The controls give the operator the ability to gradually, controllably increase the ratio of the oxidant flow rate to the air flow rate until the desired combustion conditions are established.
When the air-fuel burner has been fitted as described herein, to provide the capability to controllably carry out oxy-fuel combustion and air-fuel combustion at the same burner, and to controllably alternate as desired between air-fuel combustion and oxy-fuel combustion at the same burner, the resultant apparatus and its capability provide several significant advantages to the operator. One such advantage is that energy efficiency can be improved. That is, fuel consumed for a given amount of furnace output is improved, and the fuel costs can be reduced even taking into account the cost of the oxygen in the oxidant that is consumed. Another advantage is that productivity, in the sense of the amount of furnace output (such as the amount of steel that is reheated) in a given unit of time), is improved. Depending on the characteristics of the furnace before retrofitting as described herein, this improvement can be attributed to the fact that combustion with oxidant having an elevated oxygen content relative to air can overcome the furnace's limitations in the amount of combustion air that it could be fed in the air-fuel combustion mode, and/or to the reduction in the volume of flue gas that must be discharged through the flue (since this flue gas will contain less nitrogen than flue gas generated in air-fuel combustion).
This application is a continuation of, and claims priority from, application Ser. No. 11/395,141, filed Apr. 3, 2006.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11395141 | Apr 2006 | US |
| Child | 12261100 | US |
