NOx Suppression Techniques for an Indurating Furnace
Techniques for suppressing NOx in an indurating furnace include the use of a premix burner, the use of a staged fuel injector structure to enhance fuel lean conditions in the downdraft, and the use of a Venturi mixture structure in the downcomer.
This application claims the benefit of provisional U.S. patent application 61/180,235, filed May 21, 2009, and provisional U.S. patent application 61/162,853, filed Mar. 24, 2009, both of which are incorporated by reference.
TECHNICAL FIELDThis technology relates to a heating system in which combustion produces oxides of nitrogen (NOx), and specifically relates to a method and apparatus for suppressing the production of NOx in an indurating furnace.
BACKGROUNDCertain industrial processes, such as heating a load in a furnace, rely on heat produced by the combustion of fuel and oxidant. The fuel is typically natural gas. The oxidant is typically air, vitiated air, oxygen, or air enriched with oxygen. Combustion of the fuel and oxidant causes NOx to result from the combination of oxygen and nitrogen.
An indurating furnace is a particular type of furnace that is known to produce high levels of NOx. Large quantities of pelletized material, such as pellets of iron ore, are advanced through an indurating process in which they are dried, heated to an elevated temperature, and then cooled. The elevated temperature induces an oxidizing reaction that hardens the material. When cooled, the indurated pellets are better able to withstand subsequent handling in storage and transportation.
The indurating furnace has sequential stations for the drying, heating, and cooling steps. Pelletized material is conveyed into the furnace, through the sequential stations, and outward from the furnace. Air shafts known as downcomers deliver downdrafts of preheated air to the heating stations. Burners inject fuel and combustion air into the downdrafts, and the resulting combustion provides heat for the reaction that hardens the pelletized material.
An example of a pelletizing plant 10 with an indurating furnace 20 is shown schematically in
A blower system 50 drives air to circulate through the furnace 20 along the flow paths indicated by the arrows shown in
As shown for example in
The burner 44 of
The invention provides techniques for suppressing the production of NOx in an indurating furnace. The furnace has a downcomer with a vertical passage that directs a downdraft toward a heating station. A burner injects fuel gas and combustion air into the downcomer for combustion to occur in the downdraft. The NOx suppression techniques include the use of a premix burner, which injects a premix of fuel gas and combustion air into the downcomer. This avoids the production of NOx that would occur upon interaction of unmixed fuel gas with preheated downdraft air. The NOx suppression techniques further include the use of a staged fuel injector structure to enhance fuel-lean conditions in the downdraft, and the use of a Venturi mixture structure in the downcomer. Any one or more of these techniques may be used to suppress the production of NOx.
Each of
The structures shown schematically in
As shown partially in
The burners 102 are preferably configured as premix burners with the structure shown in
Mixer tubes 148 are located within the oxidant plenum 141. The mixer tubes 148 are preferably arranged in a circular array centered on a longitudinal axis 149. Each mixer tube 148 has an open inner end that receives a stream of combustion air directly from within the oxidant plenum 141. Each mixer tube 148 also receives streams of fuel from fuel conduits 150 that extend from the fuel plenum 143 into the mixer tube 148. These streams of fuel and combustion air flow through the mixer tubes 148 to form a combustible mixture known as premix.
An outer portion 160 of the burner 102 defines a reaction zone 161 with an outlet port 163. The premix is ignited in the reaction zone 161 upon emerging from the open outer ends of the mixer tubes 148. Ignition is initially accomplished by use of an igniter before the reaction zone 161 reaches the auto-ignition temperature of the premix. Combustion proceeds as the premix is injected from the outlet port 163 into the downcomer 110 to mix with the downdraft 113. The fuel in the premix is then burned in a combustible mixture with both premix air and downdraft air. By mixing the fuel with combustion air to form premix, the burner 102 avoids the production of interaction NOx that would occur if the fuel were unmixed or only partially mixed with combustion air before mixing into the downdraft air.
As further shown in
The controller 186 has hardware and/or software that is configured for operation of the burner 102, and may comprise any suitable programmable logic controller or other controlled device, or combination of controlled devices, that is programmed or otherwise configured to perform as recited in the claims. As the controller 186 carries out those instructions, it operates the valves 188 and 190 to initiate, regulate, and terminate flows of reactant streams that cause the burner 102 to fire the premix flame 119 into the downcomer 110. The controller 186 is preferably configured to operate the valves 188 and 190 such that the fuel and combustion air are delivered to the burner 102 in amounts that form premix having a lean fuel-to-oxidant ratio. The fuel-lean composition of the premix helps to avoid the production of interaction NOx in the downdraft 113.
Although the premix produces less interaction NOx upon combustion of the fuel-air mixture in the high temperature downdraft 113, this has an efficiency penalty because it requires more fuel to heat the cold atmospheric air in the premix. The efficiency penalty is greater if the premix has excess air to establish a lean fuel-to-oxidant ratio. However, the efficiency penalty can be reduced or avoided by using an embodiment of the invention that includes preheated air in the premix. For example, in the embodiment shown in
The embodiment of
An additional NOx suppression feature of the invention appears in
In the embodiment of
Additional suppression of interaction NOx can be achieved with differently staged fuel injection ports along with a recessed combustion zone. As shown for example in
The embodiment of
In the embodiment of
This written description sets forth the best mode of carrying out the invention, and describes the invention so as to enable a person skilled in the art to make and use the invention, by presenting examples of elements recited in the claims. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they have equivalent elements with insubstantial differences from the literal language of the claims.
Claims
1. An apparatus comprising:
- an indurating furnace structure defining drying, heating, and cooling stations;
- a conveyor to convey pelletized material through the drying, heating and cooling stations;
- a downcomer configured to direct a downdraft toward the heating station; and
- a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft.
2. An apparatus as defined in claim 1 further comprising a reactant supply and control system configured to supply the premix burner with fuel gas and combustion air in amounts for premix to have a lean fuel-to-oxidant ratio.
3. An apparatus as defined in claim 1 further comprising a reactant supply and control system configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air.
4. An apparatus as defined in claim 3 wherein the reactant supply and control system is configured to supply the premix burner with the fuel gas, the unheated air, and the heated downdraft air in amounts for the premix to have a lean fuel-to-oxidant ratio.
5. An apparatus as defined in claim 1 wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the burner is configured to inject the premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage.
6. An apparatus as defined in claim 5 further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage.
7. An apparatus as defined in claim 5 further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone.
8. An apparatus comprising:
- an indurating furnace structure defining drying, heating, and cooling stations;
- a conveyor to convey pelletized material through the drying, heating and cooling stations;
- a downcomer configured to direct a downdraft toward the heating station;
- a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft; and
- a reactant supply and control system configured to supply fuel gas and combustion air to the premix burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the premix injected by the premix burner.
9. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the premix burner.
10. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the premix burner.
11. An apparatus as defined in claim 8 wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the premix burner is configured to inject premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage.
12. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage.
13. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone.
14. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the recessed combustion zone.
15. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the recessed combustion zone.
16. An apparatus as defined in claim 8 further comprising a Venturi mixer structure that has a mixer flow passage and is arranged for the injected premix to entrain staged fuel gas into the mixer flow passage.
17. An apparatus as defined in claim 16 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure.
18. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to supply fuel gas and combustion air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air at a rate that is stoichiometric with the premix supplied to the premix burner.
19. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air.
20. An apparatus as defined in claim 19 wherein the reactant supply and control system is configured to supply the fuel gas, the unheated air, and the heated downdraft air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air into the downcomer at a rate that is stoichiometric with the premix supplied to the premix burner.
21. An apparatus as defined in claim 8 wherein the premix burner port has a central axis and is configured to inject the premix into the downdraft along the axis, and the reactant supply and control system comprises an array of secondary fuel ports that are spaced radially from the burner port and configured to inject streams of second stage fuel gas into the downdraft in directions parallel to or inclined toward the axis.
22. An apparatus comprising:
- an indurating furnace structure defining drying, heating, and cooling stations;
- a conveyor to convey pelletized material through the drying, heating and cooling stations;
- a downcomer configured to direct a downdraft toward the heating station;
- a burner configured to inject fuel gas and combustion air into the downcomer for combustion in the downdraft;
- a reactant supply and control system configured to supply fuel gas and combustion air to the burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the burner; and
- a Venturi mixer structure that has a mixer flow passage and is arranged for the injected fuel gas and combustion air to entrain staged fuel gas into the mixer flow passage.
23. An apparatus as defined in claim 22 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure.
24. An apparatus as defined in claim 22 wherein the burner is a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer, and the reactant supply and control system is configured to supply the burner with fuel gas and combustion air in amounts for the premix to have a lean fuel-to-oxidant ratio while simultaneously injecting the staged fuel gas at a rate that is stoichiometric with the premix supplied to the burner.
Type: Application
Filed: Sep 2, 2009
Publication Date: Sep 30, 2010
Inventors: Bruce E. Cain (Akron, OH), Thomas F. Robertson (Medina Township, OH), John J. Nowakowski (Valley View, OH)
Application Number: 12/552,515
International Classification: C22B 1/00 (20060101);