Patent Application
20140272736
This 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.
Certain industrial processes, such as heating a load in a furnace or generating steam in a boiler, rely on heat produced by the combustion of fuel and oxidant in a combustion chamber. The fuel is typically natural gas. The oxidant is typically air, vitiated air or air enriched with oxygen. Combustion of the fuel and oxidant in the combustion chamber causes NOx to result from the combination of oxygen and nitrogen. It may be desirable to suppress the production of NOx.
A method of low NOx combustion includes steps of injecting reactants into a combustion chamber. A primary reactant stream, including fuel and combustion air premix, is injected from a premix burner port into the combustion chamber. A staged fuel stream is injected into the combustion chamber from a staged fuel injector port adjacent to the premix burner port. A stream of recirculated flue gas is injected into the combustion chamber from a flue gas injector port adjacent to the premix burner port and the staged fuel injector port. In this manner, the stream of recirculated flue gas is injected into the combustion chamber unmixed with the primary reactant stream and unmixed with the staged fuel stream.
Preferably, at least a portion of the stream of recirculated flue gas is injected into the combustion chamber between the primary reactant stream and the staged fuel stream, whereby the stream of recirculated flue gas shields the staged fuel stream from combustion temperatures at the primary reactant stream.
Summarized differently, a method of low NOx combustion includes injecting primary fuel, staged fuel, and combustion air into a combustion chamber in streams. The streams include a primary reactant stream containing primary fuel and combustion air premix, and a staged reactant stream containing staged fuel. Combustion air is injected into the combustion chamber in a total amount and at a total flow rate. Recirculated flue gas also is injected into the combustion chamber in a total amount. The total amount of recirculated flue gas is injected separately from the primary fuel, and is injected at a total flow rate greater than the total flow rate of combustion air.
The apparatus shown schematically in the drawings can be operated in steps that are examples of the elements recited in the method claims, and has parts that are examples of the elements recited in the apparatus claims. The illustrated apparatus thus include examples of how a person of ordinary skill in the art can make and use the claimed invention. It is described here to meet the enablement and best mode requirements of the patent statute without imposing limitations that are not recited in the claims. The various parts of the illustrated apparatus, as shown, described and claimed, may be of either original and/or retrofitted construction as required to accomplish any particular implementation of the invention, and all or part of each embodiment can be used in combination with all or part of any one or more of the others.
The apparatus 10 that is shown for example in
The reactants delivered to the combustion chamber 15 include fuel and combustion air. The combustion air is preferably delivered in a single stage. The fuel is delivered in primary and secondary stages simultaneously with delivery of the combustion air. Recirculated flue gas is delivered to the combustion chamber 15 to vitiate other reactants and thereby to suppress the production of NOx.
A premix burner 40 delivers the combustion air and primary fuel to the combustion chamber 15. As shown in
A plurality of secondary fuel injectors 44 deliver the secondary fuel. The secondary fuel injectors 44, two of which are shown in
As shown, the secondary fuel injectors 44 are preferably located at the first end wall 20, and are preferably arranged in a circular array centered on the longitudinal axis 19. Each secondary fuel injector 44 has a port 45 facing into the chamber 15 along a respective axis 47. The axes 47 of the fuel injector ports 45 are preferably parallel to the axis 19, but one or more could be inclined to the axis 19 to inject secondary fuel in a skewed direction.
Flue gas recirculation (FGR) injectors 50 are located adjacent to the premix burner 40 and the secondary fuel injectors 44. In this example, the FGR injectors 50 are arranged in a circular array concentric with, and radially inward of, the array of secondary fuel injectors 44. Each FGR injector 50 has a port 51 facing into the chamber 15 along a respective axis 53 that is preferably parallel to the adjacent fuel injector axis 47.
As further shown in
The combustion air supply line 68 extends directly to the premix burner 40, and has a combustion air control valve 70. A first branch line 72 extends from the fuel supply line 66 to the premix burner 40, and has a primary fuel control valve 74. A second branch line 76 has a secondary fuel control valve 78, and extends from the fuel supply line 66 to a fuel distribution manifold 80. The manifold 80 communicates with the secondary fuel injectors 44 through fuel distribution lines 82.
The reactant supply and control system 60 further includes an FGR line 84 reaching from the flue 24 to an FGR manifold 86. The FGR line has a blower 88 and a control valve 90. FGR distribution lines 92 communicate the FGR manifold 86 with the FGR injectors 50.
A controller 100 in the reactant supply and control system 60 is operatively associated with the blower 64 and the valves 70, 74, 78 and 90 to initiate, regulate and terminate flows through the valves 70, 74, 78 and 90. Specifically, the controller 90 has combustion controls in the form of hardware and/or software for actuating the blower 64 and the valves 70, 74, 78 and 90 in a manner that causes combustion of the reactants to proceed axially downstream through the chamber 15 in generally distinct stages that occur in the generally distinct zones identified in
In operation, the controller 100 actuates the combustion air control valve 70 and the primary fuel control valve 74 to provide the premix burner 40 with a stream of combustion air and a stream of primary fuel. Those reactant streams mix together inside the premix burner 40 to form premix. The premix is delivered to the combustion chamber 15 as a primary reactant stream directed from the port 41 along the longitudinal central axis 19. Ignition of the premix occurs within the premix burner 40. This causes the primary reactant stream to form a primary combustion zone that expands radially outward from the port 41 as combustion proceeds downstream along the axis 19.
The controller 100 actuates the secondary fuel control valve 78 to provide the secondary fuel injectors 44 with streams of secondary fuel. The secondary fuel streams are injected from the secondary ports 45 which, as described above, are located radially outward of the primary port 41. This causes the unignited streams of secondary fuel to form a combustible mixture with reactants and products of combustion that recirculate in the upstream corner portions of the combustion chamber 15. Auto-ignition of that combustible mixture creates a secondary combustion zone that surrounds the primary combustion zone at the upstream end portion of the chamber 15.
The controller 100 also actuates the FGR control valve 90 to provide the FGR injectors 50 with streams of recirculated flue gas. The streams of recirculated flue gas are then injected from the FGR ports 51 into the secondary combustion zone to mix with the gaseous contents of the secondary zone. This helps to suppress the production of NOx by vitiating the products of combustion in the secondary zone.
Each stream of recirculated flue gas is injected from an FGR port 51 in a condition not yet mixed with the stream of secondary fuel injected from the adjacent secondary fuel port 45. Additionally, as noted above and shown partially in
Other examples of FGR port arrangements are shown in
In addition to providing the generally distinct combustion zones within the combustion chamber 15, and providing the NOx-suppressing FGR streams, the controller 100 can further suppress the production of NOx by maintaining fuel-lean combustion throughout the two zones. For example, the controller 100 can actuate the valves 70, 74, and 78 to deliver fuel and combustion air to the combustion chamber 15 at target rates of delivery that together have a target fuel to oxidant ratio, with the target rate of oxidant being provided entirely by the combustion air in the primary reactant stream, and with the target rate of fuel being provided at first and second partial rates in the primary reactant stream and the secondary fuel streams, respectively. The first partial target rate of fuel is higher than the second partial target rate, but is preferably low enough to ensure that the premix, and consequently the primary reactant stream, is fuel-lean. This helps to ensure that combustion in the primary zone is fuel-lean.
The second partial target rate also is preferably low enough to ensure that the resulting combustion is fuel-lean rather than fuel-rich. This helps to avoid the production of NOx that would occur if the secondary fuel were to form a fuel-rich mixture with the relatively low concentration of oxidant in the gasses that recirculate in the secondary zone. Fuel-lean conditions in the secondary zone also help to avoid the high temperature production of NOx that can occur at the interface between the primary and secondary zones when fuel from the secondary zone forms a combustible mixture with oxidant from the primary zone. However, the NOx-suppressing effects of the recirculated flue gas, as described above, may permit the second partial target rate of fuel injection to have a fuel to oxidant ratio that is higher than it might be otherwise.
Parts of an alternative heating system 200 are shown schematically in
Each of the multiple separate arrays 210, 212 and 214 of reactant delivery structures is oriented transversely across an elongated combustion chamber 215, and is operatively interconnected with a reactant supply and control system 220 (shown partially) in the same manner as the single array described above. Accordingly, each array 210, 212 and 214 is operative with reference to corresponding primary and secondary combustion zones 221 and 223 that extend across the combustion chamber 215, as shown schematically in
The controller 226 for the heating system 200 is preferably configured for each array 210, 212 and 214 to deliver respective target rates of fuel and oxidant that together define a respective fractional portion of an overall combined target rate of fuel and oxidant delivered to the combustion chamber 215. The controller 226 also provides streams of recirculated flue gas at each array 210, 212 and 214 of reactant delivery structures in the manner described above. The overall combined target rate of reactant delivery is provided by simultaneous operation of all of the multiple arrays 210, 212 and 214.
Further regarding the single array of reactant delivery structures in the heating apparatus 10, combustion air is injected into the combustion chamber 15 in a total amount. The total amount of combustion air is injected at a total flow rate. The recirculated flue gas also is injected into the combustion chamber 15 in a total amount. However, the total amount of recirculated flue gas is preferably injected into the combustion chamber 15 at a total flow rate that is greater than the total flow rate of combustion air injection. The total flow rate of recirculated flue gas injection can exceed the total flow rate of combustion air injection because, unlike the combustion air, the recirculated flue gas is not injected within a primary reactant stream, and therefore can have a flow rate that would blow off the flame at a burner if provided in a primary reactant stream. Recirculated flue gas can thus be injected into the combustion chamber 15 as desired to suppress NOx without affecting flame stability. Similarly, 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 structural or method elements that do not differ from the literal language of the claims, or if they have equivalent structural or method elements with insubstantial differences from the literal language of the claims.
