Patent Application
20140272737
This technology relates to a heating system in which reactants are injected into a combustion chamber in primary and staged reactant streams.
Certain industrial processes, such as generating steam in a boiler or heating a load in a furnace, 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 reactants in the combustion chamber causes NOx to result from the combination of oxygen and nitrogen. The production of NOx can be suppressed by injecting the fuel into the combustion chamber partially in a primary reactant stream and partially in staged reactant streams. It may be desirable to control staged combustion conditions such as, for example, a heat release profile developed by combustion of primary and staged reactant streams, or the length of a flame developed by combustion of primary and staged reactant streams.
A primary reactant stream is injected from a burner port into a combustion chamber along an axis. Staged reactant streams are injected into the combustion chamber adjacent to the primary reactant stream in differing configurations. The differences between the configurations of the staged reactant streams include differences in radial distance from the burner port, axial distance from the burner port, and direction relative to the axis. The reactant streams are shifted between differing modes, with differences between the modes including, for example, the presence or absence of a flame at the burner port, differences in fuel flow rates, and differences in combustion air flow rates. A heat release profile developed by combustion of the primary and staged reactant streams, or the length of a flame developed by combustion of the primary and staged reactant streams, can be controlled by shifting between differing modes with reference to the heat release profile or the length of the flame.
The structures shown schematically in the drawings can be operated in steps that are examples of the elements recited in the method claims, and have parts that are examples of the elements recited in the apparatus claims. The illustrated structures thus include examples of how a person of ordinary skill in the art can make and use the claimed invention. They are 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 structures, 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 of the multiple embodiments can be used in combination with all or part of any one or more of the others.
The structure 10 shown in
The reactants delivered to the combustion chamber 15 include oxidant and fuel. The oxidant is preferably delivered entirely in a primary reactant stream. The fuel is delivered simultaneously with the oxidant, but is delivered partially as primary fuel in the primary reactant stream and partially as staged fuel in staged reactant streams. The staged reactant streams in the illustrated example include secondary and tertiary reactant streams.
A premix burner 40 delivers the oxidant and the 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
A fuel injection manifold 50 adjacent to the premix burner 40 delivers the tertiary fuel. The fuel injection manifold 50 is preferably centered on the longitudinal axis 19 within the combustion chamber 15 and, in this particular implementation, is closer to the second end wall 22 than to the first end wall 20. Tertiary fuel injection ports 51 face radially outward from the manifold 50 along respective axes 53 that are perpendicular to the longitudinal axis 19.
As further shown in
The oxidant supply line 68 extends directly to the premix burner 40, and has an oxidant control valve 70. The fuel supply line 66 has a fuel shut-off valve 71. 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. That manifold 80 communicates with the secondary fuel injectors 44 through fuel distribution lines 82. A third branch line 84 with a tertiary fuel control valve 86 extends from the fuel supply line 66 to the tertiary fuel injection manifold 50.
The reactant supply and control system 60 further includes a controller 90 that is operatively associated with the blower 64 and the valves 70, 74, 78 and 86 to initiate, regulate and terminate flows of reactants through the valves 70, 74, 78 and 86. 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 86 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 90 actuates the oxidant control valve 70 and the primary fuel control valve 74 to provide the premix burner 40 with a stream of oxidant 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 as combustion proceeds downstream along the axis 19.
The controller 90 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, as indicated by the arrows shown in
The controller 90 also actuates the tertiary fuel control valve 86 to provide the downstream manifold 50 with tertiary fuel. The tertiary fuel is delivered to the combustion chamber 15 in streams that are injected from the tertiary ports 51 in directions extending radially outward along the axes 53. The tertiary fuel is thus injected into the combustion chamber 15 at locations within the primary combustion zone. This causes the streams of tertiary fuel to form a combustible mixture with the contents of the primary combustion zone. Auto-ignition of that combustible mixture creates a tertiary combustion zone that extends downstream from the primary zone as combustion in the chamber 15 proceeds downstream toward the second end wall 22.
In addition to providing the generally distinct combustion zones within the combustion chamber 15, the controller 90 can further control the reactant streams in a manner that suppresses the production of NOx. This is accomplished, for example, by maintaining fuel-lean combustion throughout the three zones.
For example, the controller 90 can actuate the valves 70, 74, 78 and 86 to deliver fuel and oxidant 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 in the primary reactant stream, and with the target rate of fuel being provided at first, second and third partial rates in the primary reactant stream, the secondary fuel streams, and the tertiary fuel streams, respectively. Preferably, the first partial target rate of fuel is the highest of the three partial target rates, but is 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 of fuel delivery may be greater than, less than, or equal to the third partial target rate. Suitable values for the first, second and third partial rates could be, for example, 65%, 15%, and 20%, respectively, of the target rate. However, the second partial 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.
The target fuel-to-oxidant ratio is maintained by injecting the tertiary fuel at a third partial rate equal to the balance of the target rate. As the tertiary fuel is injected from the manifold 50, it encounters the fuel-lean conditions in the primary combustion zone. This helps to avoid the fuel-rich and thermal conditions that could increase the production of NOx if the tertiary fuel were injected directly into the secondary combustion zone along with the secondary fuel. The production of NOx is further suppressed by injecting the tertiary fuel streams at locations that are far enough downstream for combustion in the primary zone to have consumed oxidant sufficiently to prevent the formation of fuel-rich conditions upon delivery of the tertiary fuel into the primary zone.
As shown schematically in
As indicated in
As the streams of fuel and oxidant continue to flow to the burner 40, the controller 90 provides flame supervision in accordance with the corresponding flame supervisory controls 110. Flame supervision is one of several supervisory functions the controller 90 performs by monitoring sensors that can indicate system malfunctions. Such sensors are known in the art, and are omitted from the drawings for clarity of illustration. If a malfunction occurs, the controller 90 can respond by closing the shut-off valve 71 as a safety precaution.
If a malfunction does not present an unsafe condition, the controller 90 can bypass the supervisory function for the corresponding sensor, and can allow combustion to continue. The controller 90 thus monitors the flame detector 104 in readiness to close the shut-off valve 71 if the flame detector 104 indicates the absence of a flame. This would occur if the flame were inadvertently extinguished by a system malfunction. However, the controller 90 monitors the temperature sensor 100 also, and is operative to compare the sensed combustion chamber temperature to a predetermined auto-ignition temperature of the fuel supplied to the burner 40. If the sensed combustion chamber temperature is not less than the auto-ignition temperature, and if the flame supervision bypass function 112 is enabled, the flame supervisory controls 110 are bypassed. With the flame supervisory controls 110 bypassed, indication by the flame detector 104 of the absence of a flame projecting from the burner port 41 will not result in the controller 90 closing the shut-off valve 71. Rather, the controller 90 then holds the shut-off valve 71 open so that the fuel can continue to flow through the burner 40 to enter the combustion chamber 15 through the port 41. This results in diffuse combustion of the fuel upon auto-ignition in the combustion chamber 15 in the absence of a flame at the port 41.
As shown schematically in 4, the controller 90 can have additional control functions 114 for initiating diffuse combustion in a controlled manner, i.e. for inducing diffuse combustion. These additional control functions 114 can direct the controller 90 to induce diffuse combustion either automatically without intervention by an operator of the heating apparatus 10, or if the operator provides corresponding actuating input. The additional control functions 114 could direct the controller 90 to receive the actuating input either before, during, or after a flame is ignited at the burner 40.
If diffuse combustion is to be induced, the controller 90 compares the sensed combustion chamber temperature with the predetermined auto-ignition temperature of the fuel to determine whether or not the sensed temperature is below the auto-ignition temperature. This is preferably accomplished by comparing the combustion chamber temperature to a specified bypass setpoint temperature that is predetermined to be above the auto-ignition temperature of the fuel. If the combustion chamber temperature is less than the specified temperature, and if the flame detector 104 then indicates the absence of a flame at the burner 40, the controller 90 closes the shut-off valve 71. On the other hand, if the flame detector 104 indicates the continued presence of a flame, the controller 90 continues to monitor the sensed combustion chamber temperature with reference to the specified temperature. When the sensed combustion chamber temperature increases from a level below the specified temperature to a level at or above the specified temperature, the controller 90 responds by enabling the flame supervision bypass function 112 so that absence of a flame at the burner 40 will not result in closing of the shut-off valve 71. The controller 90 then begins to execute the diffuse combustion function 114, closing the shut-off valve 71. This extinguishes the flame at the burner 40.
The controller 90 continues to monitor the sensed combustion chamber temperature during a short delay, such as about five seconds, after closing the shut-off valve 71. This ensures that the flame has been fully extinguished, which can be confirmed by the flame detector 104. If the sensed combustion chamber temperature drops below the specified temperature during the delay, the shut-off valve 71 will remain closed until the process is restarted. However, if the sensed combustion chamber temperature remains at or above the specified temperature, as it was when the shut-off valve 71 was closed, the controller 90 will reopen the shut-off valve 71. The fuel stream will then flow once again through the burner 40 to enter the combustion chamber 15 through the port 41. Although the igniter 102 is ordinarily actuated if the shut-off valve 71 is opened from a closed condition, it is not actuated in response to reopening of the shut-off valve 71 in this manner. Auto-ignition of the fuel then occurs in the combustion chamber 15 to provide diffuse combustion in the absence of a flame at the burner 40. The controller 90 continues to monitor the sensed combustion chamber temperature during operation in the diffuse combustion mode, and will close the shut-off valve 71 if the temperature falls below the specified temperature.
As described above, the flame is extinguished by closing the shut-off valve 71. Instead, the flame could be extinguished by operating the valves 70 and 74, and/or the blower 64, so as to cause the reactant supply and control system 60 to provide the reactants to the burner 40 in a fuel to oxidant ratio that does not sustain the flame at the burner 40. This could be accomplished by interrupting, decreasing or increasing the flow of oxidant to the burner 40 while maintaining the flow of fuel. Recirculated flue gas also could be delivered to the burner 40 to extinguish the flame by diluting the fuel to oxidant ratio in a similar manner. The reactant supply and control system 60 can further be provided with a source of compressed air, or an inert gas such as nitrogen, for creating a pulse which would extinguish the flame.
Another embodiment of the invention is shown schematically in
Two arrays of reactant delivery structures 160 and 162 deliver reactants to the combustion chamber 155. Each array 160 and 162 includes a single respective premix burner 164. Each array 160 and 162 further includes multiple secondary fuel injectors 166 that are adjacent to the respective premix burner 164, i.e. located closer to that burner 164 than to the other burner 164, and multiple tertiary fuel injectors 168 that are likewise adjacent to the respective premix burner 164 and the respective secondary fuel injectors 166.
The premix burners 164 are arranged to project flames into the combustion chamber 155 over the loads 156 in directions from a first end wall 170 of the refractory structure 152 toward a second end wall 172. The secondary fuel injectors 166 in the first array 160 are located at the first end wall 170, and have ports 173 that face into the combustion chamber 155 in directions skewed toward the axis 175 of the respective burner 164. The tertiary fuel injectors 168 in the first array 160 are located at an adjacent side wall 176 of the refractory structure 152. Those injectors 168 have ports 177 at locations that are spaced progressively downstream from the secondary fuel injector ports 173. The tertiary ports 177 also face into the combustion chamber 155 in directions skewed toward the adjacent burner axis 173. As viewed from above in
A reactant supply and control system, which is omitted from
As shown in
Also shown in
In each embodiment, the primary reactant streams can be injected in differing modes. Differences between the modes of primary reactant injection can include, for example, the presence or absence of a flame at a premix burner port, which distinguishes between the stable flame and diffuse modes of combustion. Other differences between primary injection modes can include differences in flow rates of fuel and combustion air, which can affect the fuel to oxidant ratio of premix in a primary reactant stream. The flow rates can also affect the length of a flame projecting from a burner in the stable flame mode, as well as the length and temperatures the heat release profile of a primary reactant stream combusting in either the stable flame mode or the diffuse mode.
The staged reactant streams are injected in differing configurations as well as differing modes. The configurations of the staged reactant streams can differ in radial and axial distance from the adjacent premix burner port, and also in the direction in which a staged stream is injected relative to the adjacent primary stream. Differences between the modes of staged reactant injection can include differences in flow rates of secondary and tertiary fuel. Those flow rates can affect the extent to which combustion of the staged fuel streams contributes to the heat release profile of combustion along the axis of a burner, and can also affect the length of a flame projecting along the axis. Specifically regarding flame length, the controller 90 preferably operates the spaced-apart tertiary fuel injectors 168 in each array 160 and 162 of
The controller 90 in each embodiment is configured to provide and shift between differing modes of primary and staged reactant injection. Each controller 90 can thus shift back and forth between primary injection modes by extinguishing a flame at a premix burner, inducing diffuse combustion in the associated combustion chamber, and reestablishing stable flame combustion by reigniting a flame at the burner, as described above. Each controller 90 can modulate the fuel to oxidant ratio of premix in a primary reactant stream by modulating the flow rates of fuel and/or combustion air. While continuing the same heating process without interruption, and preferably while maintaining fuel-lean combustion in each of the primary and staged combustion zones, each controller 90 can also shift back and forth between staged injection modes by initiating, modulating, and/or terminating the injection of secondary and tertiary fuel separately and independently from each other.
For example, the controller 90 in the embodiment of
In a more specific example, the controller 90 is configured to provide and shift between differing modes, including two or more modes selected from: a mode in which no fuel is delivered to adjacent secondary and tertiary fuel injectors while fuel and combustion air are being delivered to the adjacent premix burner, a mode in which fuel is delivered to a secondary fuel injector but not to an adjacent tertiary fuel injector while fuel and combustion air are being delivered to the adjacent premix burner, a mode in which fuel is delivered to a tertiary fuel injector but not to an adjacent secondary fuel injector while fuel and combustion air are being delivered to the adjacent premix burner, and a mode in which fuel is delivered to both the secondary and tertiary fuel injectors while fuel and combustion air are being delivered to the adjacent premix burner. The controller 90 may shift between modes in response to information indicating combustion conditions including, for example, the presence or absence of a flame, the length of a flame, and/or a heat release profile.
In another specific example, the controller 90 is configured to (a) deliver fuel and combustion air to a premix burner at a lean fuel to oxidant ratio and to simultaneously (b) deliver fuel to the burner, an adjacent secondary fuel injector structure, and an adjacent tertiary fuel injector structure at an overall rate that is stoichiometric relative to the combustion air delivered to the burner while (c) varying the heat release profile of a combusting primary reactant stream by initiating, regulating, and terminating delivery of fuel to the secondary and tertiary fuel injectors separately from each other in response to a temperature profile sensor system, and/or varying the length of a flame by initiating, regulating, and terminating delivery of fuel to the secondary and tertiary fuel injectors separately from each other in response to a flame detector system.
In yet another specific example, the controller 90 is configured to respond to a flame detector by (a) initiating, terminating, and regulating the delivery of fuel and combustion air to a premix burner in a flame mode in the presence of a flame, and (b) initiating, regulating, and terminating the delivery of fuel and combustion air to the burner in a diffuse combustion mode in the absence of a flame, and (c) initiating, regulating, and terminating the delivery of fuel to adjacent staged fuel injectors separately from each other while delivering fuel and combustion air to the burner in the flame mode and in the diffuse combustion mode.
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.
