Patent Publication Number: US-2021164652-A9

Title: Control system and method for a burner with a distal flame holder

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation-in-Part application which claims priority benefit under 35 U.S.C. § 120 (pre-AIA) of co-pending International Patent Application No. PCT/US2018/042935, entitled “CONTROL SYSTEM FOR A BURNER WITH PERFORATED FLAME HOLDER,” filed Jul. 19, 2018 (docket number 2651-303-04). International Patent Application No. PCT/US2018/042935 claims priority benefit from U.S. Provisional Patent Application No. 62/534,193, entitled “CONTROL SYSTEM FOR A BURNER WITH PERFORATED FLAME HOLDER,” filed Jul. 18, 2017 (docket number 2651-303-02), now expired. Each of the foregoing applications, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     SUMMARY 
     Embodiments include a combustion system including a distal flame holder. The combustion system is configured to operate in a preheating state and in a standard operating state. In the preheating state, the combustion system supports a pilot flame with a pilot fuel and an oxidant. The pilot flame is positioned to heat the distal flame holder to an operating temperature. In the standard operating state, the distal flame holder holds a combustion reaction of a main fuel and an oxidant. Optionally, the main fuel and the pilot fuel may be the same fuel. According to some embodiments, the main fuel and/or the pilot fuel may include fuel mixtures. In an embodiment, the main fuel and the pilot fuel are natural gas. 
     According to an embodiment, a combustion system includes a pilot flame sensor configured to sense a condition of the pilot flame and to output sensor signals indicative of the condition of the pilot flame. The combustion system includes a distal flame holder sensor configured to sense a condition of the distal flame holder or a combustion reaction held by the distal flame holder and to output sensor signals indicative of the condition of the distal flame holder or the combustion reaction. The combustion system includes a controller configured to receive the sensor signals from the pilot flame sensor and the distal flame holder sensor. The controller is configured to execute software instructions stored on a non-transitory computer readable medium to automatically adjust parameters of the combustion system and to automatically transition the combustion system between the preheating state and the standard operating state responsive to the sensor signals from the distal flame holder sensor and the pilot flame sensor and/or to engage alternate methods or devices to maintain stable and safe combustion or stable and safe states other than the preheating state and the standard operating state. The controller adjusts the combustion system and transitions between states by controlling one or more actuators configured to adjust components of the combustion system. 
     According to an embodiment, a flame stability sensor is positioned to sense a flame condition (e.g., the presence or absence of a flame) in a region between (e.g., main) fuel nozzles and a distal (e.g., perforated) flame holder, said region being found by the inventors to characterize a main combustion reaction instability. For example, the flame stability sensor may be positioned halfway between the fuel nozzles and a distal flame holder, for example a perforated flame holder. 
     In an embodiment, a variable-output pilot burner may be positioned at least 0.62 of the distance from main fuel nozzles to a distal flame holder (the larger portion of the distance being between the main fuel nozzles and variable-output pilot burner). The variable output pilot burner may be driven to output a load corresponding to preheating of the distal flame holder or, alternatively, to output a continuous pilot. The inventors have found that by maintaining a continuous pilot flame adjacent to and below (subjacent, or upstream from) the distal flame holder, a transition step wherein a flame location is shifted between two discrete, different positions may be eliminated. In addition to, or advantageously instead of, the transition step, the continuous pilot is configured to hold a pilot flame according to a plurality of output loads. In an example system, the output loads principally used were two—either stable pilot flame or high output preheat flame where the temperature of the distal flame holder is raised to a main fuel operating temperature over a specified duration. The inventors contemplate that pluralities greater than two output levels may be used to maintain a very low, flame stability limited operation, a throttled system heat output mode (which in an embodiment may result in elimination of a second cold climate “HVAC” subsystem), a routine and minimum fuel pressure drop pre-heat mode, a demand pre-heat mode, and an emergent demand pre-heat mode. System damage recovery modes may one day prove advantageous. The inventors contemplate that a relatively high turndown ratio of the continuous pilot may be obtained by disposing a perforated or porous tile (pilot tile) superjacent to a plurality of 1 atm fuel nozzles, a low output pilot flame may be stabilized to minimize variable pilot stable heat output. In an embodiment, the system, at moderate to high output, supports low output stable pilot operation to cause greater than 98% of CO 2  generation is provided by main fuel nozzle during a normal operating mode. This mode may help reduce NOx production during normal operation compared to a higher ratio of pilot burner output to main fuel output. 
     The flame holder may include plural porous and/or solid bodies (tiles) with spaces therebetween. 
     A controller may, upon receipt of an instability signal from the flame stability sensor corresponding to at least transient presence of a flame in the positioned region, responsively execute a logical decision that the combustion reaction instability exists, at least transiently. The controller may responsively write an incident of the combustion reaction instability to a log file and/or cause an electronic display state corresponding to the incident to be provided to an operating engineer or the like. Optionally, the controller may cause one or more actuators to modify an operating condition to increase main combustion reaction stability. For example, the controller may cause actuation of a flame blow off apparatus to increase fluid flow velocity or fluid cooling between the fuel nozzles and the distal flame holder, cause a damper to open to increase air volume delivery, cause a blower to increase power to increase air volume delivery, cause a valve to momentarily pause fuel delivery, and/or cause increased pilot fuel output to increase combustion heat of the pilot flame. 
     One embodiment is a combustion system including a distal flame holder. The combustion system is configured to operate in a preheating state and in a standard operating state. In the preheating state, the combustion system supports a pilot flame by outputting a pilot fuel into a furnace volume. The pilot flame is positioned to heat the distal flame holder to an operating temperature. In the standard operating state, the distal flame holder holds a combustion reaction of a main fuel and an oxidant. The combustion system includes a pilot flame sensor configured to sense a condition of the pilot flame and to output sensor signals indicative of the condition of the pilot flame. The combustion system includes a distal flame holder sensor configured to sense a condition of the distal flame holder and to output sensor signals indicative of the condition of the distal flame holder. The combustion system includes a controller configured to receive the sensor signals from the pilot flame sensor and the distal flame holder sensor. The controller is configured to execute software instructions stored on a non-transitory computer readable medium to output messages on a display prompting an operator of the combustion system to adjust parameters of the combustion system and to transition the combustion system between the preheating state and the standard operating state responsive to the sensor signals from the distal flame holder sensor and the pilot flame sensor. 
     One embodiment is a combustion system including a distal flame holder. The combustion system is configured to operate in a preheating state and in a standard operating state. In the preheating state, the combustion system supports a pilot flame with a pilot fuel and an oxidant. The pilot flame is positioned to heat the distal flame holder to an operating temperature. In the standard operating state, the distal flame holder holds a combustion reaction of a main fuel and an oxidant. The combustion system includes a pilot flame sensor configured to sense a condition of the pilot flame and to output sensor signals indicative of the condition of the pilot flame. The combustion system includes a distal flame holder sensor configured to sense a condition of the distal flame holder and to output sensor signals indicative of the condition of the distal flame holder. The combustion system includes a controller configured to receive the sensor signals from the pilot flame sensor and the distal flame holder sensor. The controller is configured to execute software instructions stored on a non-transitory computer readable medium to adjust parameters of the combustion system and to transition the combustion system between the preheating state and the standard operating state responsive to the sensor signals from the distal flame holder sensor and the pilot flame holder sensor. The controller is configured to output messages on a display prompting an operator of the combustion system to approve adjusting parameters of the combustion system or transitioning between the preheating state and the standard operating state responsive to the sensor signals. The controller adjusts the combustion system and transitions between states by controlling one or more actuators configured to adjust components of the combustion system if the operator indicates approval of the adjustment or the transition. The controller can also maintain desired combustion within the distal flame holder via control of actuators in accordance with sensor signals output by the various sensors of the combustion system. Additionally, or alternatively, the controller may be configured to operate in an automatic mode wherein the controller automatically controls the one or more actuators. In the automatic mode, the controller preferably creates a log file to indicate sensed parameters and/or actuations performed under automatic control. 
     According to an embodiment, a computer method for operating a burner having a distal flame holder includes receiving a heat demand datum via a hardware digital interface operatively coupled to a network, and comparing, using a logic device and computer-readable non-transitory memory, the heat demand datum with previously received heat demand data. The computer method includes determining, with the logic device and the computer-readable non-transitory memory as a function of the heat demand datum, a heating state of a burner system including at least one distal flame holder and at least one continuous pilot apparatus. The computer method includes, responsive to an increase in the heat demand datum compared to previously received heat demand data, driving the burner system to place the continuous pilot apparatus into a high heat output state for a duration sufficient to raise the distal flame holder to a normal, main fuel, operating temperature. The computer method includes, after a main fuel operating state has been reached, ramping down the continuous pilot apparatus heat output while ramping up a main fuel flow through main fuel nozzles aligned to output fuel for entrainment in combustion air, then entering an input face of at least one tile of the distal flame holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a combustion system including a distal flame holder, according to an embodiment. 
         FIG. 2  is a simplified diagram of a combustion system including a distal flame holder configured to hold a combustion reaction, according to an embodiment. 
         FIG. 3  is a side sectional diagram of a portion of the distal flame holder of  FIGS. 1 and 2 , according to an embodiment. 
         FIG. 4  is a flow chart showing a method for operating a burner system including the distal flame holder of  FIGS. 1-3 , according to an embodiment. 
         FIG. 5A  is a simplified diagram of a combustion system including a reticulated ceramic distal flame holder configured to hold a combustion reaction, according to an embodiment. 
         FIG. 5B  is a side sectional diagram of a portion of the reticulated ceramic distal flame holder of  FIG. 5A , according to an embodiment. 
         FIG. 6  is a block diagram of components of a combustion system, according to an embodiment. 
         FIG. 7  is a flow diagram of a process for operating a combustion system, according to an embodiment. 
         FIG. 8  is a flow diagram of a process for operating a combustion system, according to an embodiment. 
         FIG. 9  is a flow diagram of a process for operating a combustion system, according to an embodiment. 
         FIG. 10A  is a diagram of a combustion system, according to an embodiment. 
         FIG. 10B  is a diagram of the combustion system of  FIG. 10A  in a preheating state, according to an embodiment. 
         FIG. 10C  is a diagram of the combustion system of  FIG. 10A  in a standard operating state, according to an embodiment. 
         FIG. 11  is a diagram of a combustion system, according to an embodiment. 
         FIG. 12  is a flow chart showing a computer method for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus, according to an embodiment. 
         FIG. 13  is a flow chart showing a computer method for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
       FIG. 1  is a diagram of a combustion system  100  including a distal flame holder  102 , according to an embodiment. The combustion system  100  is configured to preheat the distal flame holder  102  to an operating temperature at which the distal flame holder  102  can sustain a combustion reaction of a main fuel and an oxidant at least partially within the distal flame holder  102 . Thus, the combustion system  100  is configured to operate in two general operating conditions: a preheating state and a standard operating state. In the preheating state, the combustion system  100  preheats the distal flame holder  102  to the operating temperature. When the distal flame holder  102  has reached the operating temperature, the combustion system  100  transitions to the standard operating state in which the distal flame holder  102  holds a combustion reaction of the main fuel and the oxidant at least adjacent to the distal flame holder  102 . In one embodiment, the distal flame holder  102  includes a perforated flame holder, and in the standard operating state the distal flame holder  102  holds a majority of the combustion reaction of the main fuel and the oxidant within the perforated flame holder. 
     In one embodiment, the combustion system  100  utilizes an oxidant source  104 , a pilot fuel distributor  106  and a pilot fuel source  108  in at least the preheating state. In the preheating state, the oxidant source  104  outputs an oxidant into the furnace volume in which the distal flame holder  102  is positioned. The pilot fuel source  108  supplies a pilot fuel to the pilot fuel distributor  106 . The pilot fuel distributor  106  outputs the pilot fuel into the furnace volume. The pilot fuel and the oxidant mix together in the furnace volume. In an embodiment, the combustion system  100  may utilize an igniter to ignite the mixture of the pilot fuel and the oxidant, thereby generating a pilot flame. The pilot flame is positioned between the pilot fuel distributor  106  and the distal flame holder  102 . In an embodiment, the pilot flame may be positioned just upstream of the distal flame holder  102 . In the preheating state, the pilot flame preheats the distal flame holder  102  until the distal flame holder  102  reaches the operating temperature. The terms pilot flame, preheat flame, and preheating flame may be used interchangeably throughout this disclosure, except where specifically noted. Specifically, when the pilot flame is engaged in a manner to preheat the distal flame holder  102 , it is a preheating flame. When the distal flame holder  102  reaches the operating temperature, the combustion system  100  transitions to the standard operating state. 
     In one embodiment, in the standard operating state the combustion system  100  utilizes the main fuel distributor  110 , the main fuel source  112 , and the oxidant source  104  to support a combustion reaction of the main fuel and the oxidant at least adjacent to the distal flame holder  102  during the standard operating state. In the standard operating state, the main fuel source  112  supplies a main fuel to the main fuel distributor  110 . The main fuel distributor  110  outputs the main fuel with a trajectory to be received by the distal flame holder  102 . The main fuel and the oxidant mix as the main fuel travels toward the distal flame holder  102 . The distal flame holder  102  receives the mixture of the main fuel and the oxidant at the distal flame holder  102 . Because the distal flame holder  102  has been heated to the operating temperature, the distal flame holder  102  supports a combustion reaction of the main fuel and the oxidant supported by the distal flame holder  102 . 
     According to an embodiment, the distal flame holder includes a perforated flame holder configured to support the combustion reaction of the main fuel and the oxidant at least partially within the perforated flame holder during the standard operating state. In another embodiment, the distal flame holder includes at least one refractory tile. 
     According to an embodiment, the combustion system  100  further includes an oxidant source configured to provide the oxidant to the furnace volume, and one or more actuators communicatively coupled to the controller configured to adjust a flow of the oxidant from the oxidant source. The controller may be configured to control the one or more actuators to adjust the flow of the oxidant responsive to the received sensor signals. 
     In an embodiment, the oxidant source may be a natural draft combustion air source. In another embodiment, the oxidant source may be a forced convection combustion air source. 
     In one embodiment, the oxidant source  104  includes multiple sources of oxidant. In the preheating state, the oxidant source  104  may supply oxidant from all sources of oxidant, e.g., through slots in a barrel register and from a common upstream supply. In the standard operating state, the barrel register can be closed so that all oxidant comes from upstream the slots of the barrel register. 
     In one embodiment, the oxidant source  104  includes dampers whose positions can be adjusted to direct all of the flow of the oxidant closer in proximity to a location of the main fuel distributors during the standard operating state and during transition to the standard operating state. During the preheating state, the position of the dampers can be adjusted to enable flow of the oxidant proximate to the pilot fuel distributors  106 . 
     In one embodiment, various conditions can arise during the preheating state, the standard operating state, and the transition between the preheating state and the standard operating state. The conditions in the combustion system  100  can indicate that the preheating state is progressing normally, that the time to transition to the standard operating state has arrived, or that the combustion system  100  is operating as expected in the standard operating state. However, in some cases the conditions can indicate a problem with one or more components, processes, or operations of the combustion system  100 . The conditions within the combustion system  100  can indicate that one or more parameters of the combustion system  100  should be adjusted in order to bring operations to a desired state, that the combustion system  100  should revert from a standard operating state to the preheating state, or that the combustion system  100  should shut down. 
     In one embodiment, the combustion system  100  utilizes a sensor array  114 , a controller  116 , actuators  118 , and a display  120  in order to monitor and address the conditions within the combustion system  100 . In particular, the sensor array  114  includes multiple sensors configured to sense various parameters of the combustion system  100 . The sensors of the sensor array  114  can provide sensor signals to the controller  116 . The controller  116  receives the sensor signals, identifies conditions within the combustion system  100 , and controls the actuators  118  to adjust the conditions within the combustion system  100 . The sensor signals can indicate that the preheating state is progressing normally, that the time to transition to the standard operating state has arrived, or that the combustion system  100  is operating as expected in the standard operating state. The sensor signals can also indicate a problem with the conditions or components of the combustion system  100 . The controller  116  can adjust the components or the conditions of the combustion system  100  in response to the sensor signals by controlling the actuators  118  to physically adjust components or parameters of the combustion system  100 . The display  120  can indicate the present conditions within the combustion system  100  in accordance with the sensor signals, can indicate that the controller  116  is taking one or more corrective actions, or can indicate that an operator of the combustion system  100  should operate one or more manual controls  123  in order to adjust conditions within the combustion system  100 . 
     In one embodiment, the sensor array  114  includes a pilot flame sensor  124 . The pilot flame sensor  124  senses parameters relating to the pilot flame during the preheating state of the combustion system  100 . The pilot flame sensor  124  provides sensor signals to the controller  116  indicating the conditions of the pilot flame. Based on the sensor signals provided by the pilot flame sensor  124 , the controller  116  can adjust parameters of the combustion system  100 . 
     In one embodiment, the pilot flame sensor  124  detects whether the pilot flame is present during the preheating state. When the combustion system  100  enters the preheating state, the controller  116  controls one or more of the actuators  118  to cause the oxidant source  104  to output the oxidant into the furnace volume. The controller  116  can also control the actuators  118  to operate a valve or other mechanism enabling the pilot fuel source  108  to supply the pilot fuel to the pilot fuel distributor  106 . The controller  116  can then cause an ignition mechanism (i.e., igniter), such as a sparker, to ignite the pilot fuel and the oxidant, thereby initiating the pilot flame. The pilot flame sensor  124  senses whether the pilot flame is present during the preheating state. The pilot flame sensor  124  provides sensor signals to the controller  116  indicating whether or not the pilot flame is present. If the sensor signals indicate that the pilot flame is not present, the controller  116  can take action such as causing the igniter to generate additional electric arcs in order to ignite the pilot fuel and the oxidant. If the pilot flame sensor  124  indicates that the pilot flame is still not present, then the controller  116  can control the actuators  118  to attempt to cause the oxidant source  104  to supply the oxidant or to attempt to cause the pilot fuel source  108  to supply the pilot fuel to the pilot fuel distributor  106 . This can be followed by causing the igniter to generate additional electric arcs. If the sensor signals continue to indicate that the pilot flame is not present, the controller  116  can indicate that a system fault has occurred that requires that the combustion system  100  be shut down until an operator can inspect the oxidant source  104 , the pilot fuel source  108 , the pilot fuel distributor  106 , the valves connecting the pilot fuel distributor  106  and the pilot fuel source  108 , and the actuators  118  in order to identify and correct any faulty conditions with these components. The operator can then inspect the various components and correct any issues. 
     In one embodiment, the pilot flame sensor  124  may sense the position of the pilot flame in at least the preheating state. For example, the pilot flame may be present and may not be in a desired position. The sensor signal can indicate that the pilot flame is too close to the distal flame holder  102  or too far from the distal flame holder  102 , i.e., too close to the pilot fuel distributor  106 . In response to these conditions, the controller  116  can adjust the flow of the oxidant into the furnace volume by increasing or decreasing the flow of the oxidant into the furnace volume. In response to these conditions, the controller  116  can adjust the flow of the pilot fuel into the furnace volume by increasing or decreasing the flow rate of the pilot fuel, or by increasing or decreasing the velocity of the pilot fuel. By adjusting the flow of the oxidant and the pilot fuel, the controller  116  can adjust the position of the pilot flame relative to the distal flame holder  102 . 
     In one embodiment, the pilot flame sensor  124  can indicate a temperature of the pilot flame. The pilot flame may be generating more or less heat than desired for the preheating of the distal flame holder  102 . The sensor signals can inform the controller  116  of the temperature of the pilot flame. In response, the controller  116  can adjust the parameters of the flow of the oxidant and the pilot fuel in order to adjust the temperature of the pilot flame during the preheating state. 
     In one embodiment, the pilot flame sensor  124  can include multiple sensors. The pilot flame sensor  124  can include one or more of a flame scanner, a flame rod, a temperature sensor, an image capture device or other kinds of sensors for detecting the presence and parameters of the pilot flame. The pilot flame sensor  124  can include an electro-capacitive flame sensor. Structures and methods of using electro capacitive flame sensors are described in International Patent Application No. PCT/US2019/039467, entitled “VARIABLE COMPOSITION GAS MIXTURE SENSOR,” filed Jun. 27, 2019 (docket number 2651-333-04), and International Patent Application No. PCT/US2019/039475, entitled “COMBUSTION SYSTEM INCLUDING A COMBUSTION SENSOR AND A PLASMA GENERATOR,” filed Jun. 27, 2019 (docket number 2651-342-04), incorporated herein by reference thereto. 
     In one embodiment, the sensor array  114  can include a distal flame holder sensor  122 . The distal flame holder sensor  122  can monitor parameters of the distal flame holder  102 . The distal flame holder sensor  122  senses the parameters of the distal flame holder  102  during the preheating state and at least while entering the standard operating state. The distal flame holder sensor  122  generates sensor signals and provides them to the controller  116 . The controller  116  receives the sensor signals from the distal flame holder sensor  122  and can take action to adjust the parameters of the combustion system  100  based on the conditions of the distal flame holder  102 . 
     In one embodiment, the distal flame holder sensor  122  includes a temperature sensor configured to sense the temperature of the distal flame holder  102  during the preheating state. During the preheating state, the combustion system  100  supports a pilot flame positioned to heat the distal flame holder  102  to the operating temperature. Throughout the preheating state, the distal flame holder sensor  122  monitors the temperature of the distal flame holder  102 . If the sensor signal indicates that the distal flame holder  102  has not yet reached the operating temperature, then the controller  116  keeps the combustion system  100  in the preheating state, thereby causing the pilot flame to continue to heat and increase the temperature of the distal flame holder  102 . If the sensor signals indicate that the distal flame holder  102  has reached the operating temperature, then the controller  116  can cause the combustion system  100  to transition to the standard operating state. 
     In one embodiment, the controller  116  causes the combustion system  100  to transition to the standard operating state by removing the pilot flame. The controller  116  can remove the pilot flame by causing the actuators  118  to stop the pilot fuel source  108  from supplying the pilot fuel to the pilot fuel distributor  106 . The controller  116  can cause the pilot fuel source  108  to stop providing the pilot fuel to the pilot fuel distributor  106  by closing one or more valves that connect the pilot fuel source  108  to the pilot fuel distributor  106 . When the pilot fuel distributor  106  no longer outputs the pilot fuel, the pilot flame will be extinguished. 
     The controller  116  continues the transition from the preheating state to the standard operating state by causing the main fuel source  112  to supply the main fuel to the main fuel distributor  110  by controlling the actuators  118  to open one or more valves that enable the flow of the main fuel from the main fuel source  112  to the main fuel distributor  110 . The main fuel distributor  110  outputs the main fuel into the furnace volume. The oxidant source  104  continues to output oxidant into the furnace volume during the transition to the standard operating state. The main fuel and the oxidant mix as they travel toward the distal flame holder  102 . The distal flame holder  102  receives the mixture of the main fuel and the oxidant. Because the distal flame holder  102  has reached the operating temperature, the distal flame holder  102  outputs heat sufficient to ignite the mixture of the main fuel and the oxidant at the distal flame holder  102 . In the standard operating state, the distal flame holder  102  supports a stable combustion reaction of the main fuel and the oxidant adjacent to or at least partially within the distal flame holder  102 . In this way, the controller  116  can cause the transition of the combustion system  100  from the preheating state to the standard operating state responsive to the sensor signals from the distal flame holder sensor  122 . 
     In one embodiment, the distal flame holder sensor  122  continues to monitor the distal flame holder  102  in the standard operating state and to output sensor signals to the controller  116 . The distal flame holder sensor  122  can detect the presence or absence of the combustion reaction within and adjacent to the distal flame holder  102 . If the distal flame holder sensor  122  indicates that the combustion reaction of the main fuel and the oxidant is not present at the distal flame holder  102 , then the controller  116  can take corrective action. The controller  116  can cause the actuators  118  to adjust or reopen the valves that enable the flow of the main fuel from the main fuel source  112  to the main fuel distributor  110 . The controller  116  can output a message on the display  120  indicating to the operator to check whether the main fuel source  112  is supplying the main fuel to the main fuel distributor  110  and to take corrective action, if necessary, by operating the manual controls  123 . If, after the controller  116  has taken corrective actions, the distal flame holder sensor  122  indicates the absence of a combustion reaction of the main fuel and the oxidant, the controller  116  can cause the combustion system  100  to enter a fault state in which all fuel sources shut down so that neither the main fuel nor the pilot fuel is output into the furnace volume. 
     In one embodiment, the distal flame holder sensor  122  can indicate that the combustion reaction of the fuel and the oxidant is localized below the distal flame holder  102  or in an otherwise undesirable location. The controller  116  can take actions such as adjusting the flow of the main fuel, adjusting the output of the oxidant, or adjusting of the parameters of the components of the combustion system  100  in order to adjust the position of the combustion reaction of the main fuel and the oxidant. Alternatively, the controller  116  can output messages on the display  120  indicating to the operator of the combustion system  100  that the combustion reaction is not properly held by the distal flame holder  102  and that the operator should take corrective action. 
     In one embodiment, the distal flame holder sensor  122  can indicate that the temperature of the distal flame holder  102  has fallen below the operating temperature. In this case, the controller  116  can cause the combustion system  100  to reenter the preheating state. The controller  116  can cause the main fuel distributor  110  to again output the main fuel in order to reenter the standard operating state in which the distal flame holder  102  sustains a combustion reaction of the main fuel and the oxidant. 
     In one embodiment, the distal flame holder sensor  122  includes one or more of a flame scanner, a flame rod, a temperature sensor, a visible light sensor, an infrared light sensor, an ultraviolet light sensor, an image capture device that captures images in one or more of the visible light spectrum, the infrared light spectrum, or the ultraviolet light spectrum, or any other type of sensor that can detect parameters of a combustion reaction. The distal flame holder sensor  122  can include multiple sensors of the same type. The distal flame holder sensor  122  can include multiple sensors of different types, such as those set forth above. Thus, while  FIG. 1  indicates a single distal flame holder sensor  122 , the distal flame holder sensor  122  can include multiple individual sensors of different kinds or of the same kind. 
     In one embodiment, if neither the pilot flame sensor  124  nor the distal flame holder sensor  122  indicate the presence of a pilot flame or a main combustion reaction, the controller  116  can stop the flow of all fuel into the furnace volume. 
     In one embodiment, the pilot flame sensor  124  includes one or more of a flame scanner, a flame rod, a temperature sensor, a visible light sensor, an infrared light sensor, an ultraviolet light sensor, an image capture device that captures images in one or more of the visible light spectrum, the infrared light spectrum, or the ultraviolet light spectrum, or any other type of sensor that can detect parameters of a combustion reaction. The pilot flame sensor  124  can include multiple sensors of the same type. The pilot flame sensor  124  can include multiple sensors of different types, such as those set forth above. Thus, while  FIG. 1  indicates a single pilot flame sensor  124 , the pilot flame sensor  124  can include multiple individual sensors of different kinds or of the same kind. 
     In one embodiment, the sensor array  114  includes sensors other than the distal flame holder sensor  122  and the pilot flame sensor  124 . For example, the sensor array  114  can include one or more of a bridge wall temperature sensor, a CO monitor, an NOx monitor, an O 2  monitor, a process monitor, a draft pressure sensor, a dynamic pressure sensor, a pressure differential sensor, or other kinds of sensors. Some of the sensors can be included in the distal flame holder sensor  122  or the pilot flame sensor  124 . All the sensors of the sensor array  114  provide control signals to the controller  116 . The controller  116  can take actions to adjust conditions in the combustion system  100  responsive to the sensor signals from the various sensors of the sensor array  114 . 
     In one embodiment, the controller  116  includes a non-transitory computer readable medium and one or more processors. The non-transitory computer readable medium can include one or more memories and store instructions encoded in software for controlling the combustion system  100 . The one or more processors are configured to execute the instructions. The instructions can include data related to the various operating conditions of the combustion system  100 . The instructions can include data related to both faulty or undesirable operating conditions and proper or desirable operating conditions. The instructions can include actions to be taken by the controller  116  responsive to the sensor signals received by the controller  116 . The actions can include adjusting conditions of the combustion system  100  by causing the actuators  118  to adjust, activate, or deactivate various components of the combustion system  100 . The actions taken by the controller  116  can also include outputting messages to the display  120 . The messages can include data indicating the current conditions of the combustion system  100 . The messages can also include data prompting the operator of the combustion system  100  to take various actions in order to maintain or adjust the conditions of the combustion system  100 . The messages can include prompts to approve an action proposed by the controller  116  to adjust or maintain conditions in the combustion system  100 . The controller  116  can also output data via wired or wireless connections to one or more other computing systems. The data can include the data related to current conditions of the combustion system  100 , the data related to actions taken by the controller  116 , the data related to actions proposed by the controller  116 , or prompts to the operator of the combustion system  100  to take actions or to approve proposed actions. 
     In one embodiment, the software instructions include one or more algorithms, state diagrams, decision trees, or other instructions by which the controller  116  makes decisions to adjust the parameters of the combustion system  100 . The controller  116  can also include a state machine that determines actions to be taken by the controller  116  responsive to the sensor signals. 
     In one embodiment, the actuators  118  include mechanisms that can control, adjust, or otherwise affect physical components of the combustion system  100 . The actuators  118  can include motors, motivators, electrical switches, electrical connectors, electrical transmitters, or other types of mechanisms that can physically affect or manipulate components of the combustion system  100 . For example, the actuators  118  can include motors or switches for physically opening, closing, or otherwise adjusting valves that control the flow of fuel or oxidant into the furnace volume. The actuators  118  can include mechanisms that control the movements of a stack damper. The actuators  118  can include mechanisms that activate an igniter to ignite the pilot flame or the main combustion reaction. The actuators  118  can include mechanisms that adjust the mixture of fuels included in the pilot fuel or the main fuel by increasing or decreasing the concentration of various components of the pilot fuel or the main fuel. The actuators  118  can include mechanisms for adjusting or activating the oxidant source  104 . The actuators  118  can include other kinds of mechanisms for physically manipulating components of the combustion system  100  other than those set forth above. These other kinds of mechanisms can also include mechanisms for controlling components of the combustion system  100  not shown in  FIG. 1  or expressly described herein. 
     In one embodiment, the manual controls  123  enable the operator of the combustion system  100  to physically manipulate components of the combustion system  100  in order to adjust conditions of the combustion system  100 . The manual controls  123  can include switches, buttons, dials, levers, keypads, touchscreens, keyboards, or other types of mechanisms that can enable the operator to manipulate the components of the combustion system  100 . The manual controls  123  can include manual devices for opening and closing valves. The manual controls  123  can include the valves themselves. The manual controls  123  can enable the operator to activate, deactivate, or adjust the oxidant source  104 , the main fuel source  112 , the pilot fuel source  108 , the main fuel distributor  110 , the pilot fuel distributor  106 , the igniter, the stack damper, or any other components of the combustion system  100 . 
     In one embodiment, the manual controls  123  can control the actuators  118 . The manual controls  123  can control some or all of the same actuators  118  that can be controlled by the controller  116 . The manual controls  123  can also control actuators  118  that cannot be controlled by the controller  116 . In some cases, the manual controls  123  include some or all of the actuators  118 . In one embodiment, the manual controls  123  enable the operator to shut down the combustion system  100  entirely or to override actions taken by the controller  116 . 
     According to embodiments, the distal flame holder  102  may be formed from perforated or porous tiles or bodies, from solid tiles or bodies, or from a combination of perforated and solid tiles or bodies. The inventors have found that a distal flame holder  102  using a combination of perforated and solid bodies has performance properties similar to and operates in a manner similar to a distal flame holder made exclusively of structural elements plus perforated tiles. The following description of  FIGS. 2-4 , while referring specifically to a distal flame holder  102  or a distal flame holder  102  including a perforated flame holder body  208 , will be understood to also be applicable to distal flame holders that use perforated tiles, solid bodies spaced apart, or a combination of perforated tiles and solid bodies. 
       FIG. 2  is a simplified diagram of a burner system  200  including a distal flame holder  102  configured to hold a combustion reaction, according to an embodiment. As used herein, the terms distal flame holder and distal reaction holder shall be considered synonymous unless further definition is provided. Likewise, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided. According to embodiments, the distal flame holder  102  may include a perforated flame holder. 
     Experiments performed by the inventors have shown that distal flame holders  102  described herein can support very clean combustion. Specifically, in experimental use of burner systems  200  ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O 2 ) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion. 
     According to embodiments, the burner system  200  includes a fuel and oxidant source  202  disposed to output fuel and oxidant into a combustion volume  204  to form a fuel and oxidant mixture  206 . As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The distal flame holder  102  is disposed in the combustion volume  204  and positioned to receive the fuel and oxidant mixture  206 . 
       FIG. 3  is a side sectional diagram  300  of a portion of the distal flame holder  102  of  FIGS. 1 and 2 , according to an embodiment. Referring to  FIGS. 2 and 3 , a distal flame holder  102  may include a perforated flame holder body  208  defining a plurality of perforations  210  aligned to receive the fuel and oxidant mixture  206  from the fuel and oxidant source  202 . As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of a perforated flame holder, shall be considered synonymous unless further definition is provided. The perforations  210  are configured to collectively hold a combustion reaction  302  supported by the fuel and oxidant mixture  206 . 
     The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H 2 ), and methane (CH 4 ). In another application, the fuel can include natural gas (mostly CH 4 ) or propane (C 3 H 8 ). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein. 
     According to an embodiment, the perforated flame holder body  208  can be bounded by an input face  212  disposed to receive the fuel and oxidant mixture  206 , an output face  214  facing away from the fuel and oxidant source  202 , and a peripheral surface  216  defining a lateral extent of the distal flame holder  102  incorporating a perforated flame holder body  208 . The plurality of perforations  210  which are defined by the perforated flame holder body  208  extend from the input face  212  to the output face  214 . The plurality of perforations  210  can receive the fuel and oxidant mixture  206  at the input face  212 . The fuel and oxidant mixture  206  can then combust in or near the plurality of perforations  210  and combustion products can exit the plurality of perforations  210  at or near the output face  214 . 
     According to an embodiment, the distal flame holder  102  incorporating a perforated flame holder body  208  is configured to hold a majority of the combustion reaction  302  within the perforations  210 . For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume  204  by the fuel and oxidant source  202  may be converted to combustion products between the input face  212  and the output face  214  of the perforated flame holder. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction  302  may be output between the input face  212  and the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction  302 . As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations  210  can be configured to collectively hold at least 80% of the combustion reaction  302  between the input face  212  and the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . In some experiments, the inventors produced a combustion reaction  302  that was apparently wholly contained in the perforations  210  between the input face  212  and the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . According to an alternative interpretation, the distal flame holder  102  incorporating a perforated flame holder body  208  can support combustion between the input face  212  and the output face  214  when combustion is “time-averaged.” For example, during transients, such as before the distal flame holder  102  is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face  212  of the perforated flame holder. 
     While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations  210 , but the “glow” of combustion heat is dominated by a visible glow of the distal flame holder  102  itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face  212  of the distal flame holder  102  incorporating a perforated flame holder  208 , and a fuel nozzle  218 , within the dilution region D D . Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations  210  of the perforated flame holder, between the input face  212  and the output face  214 . In still other instances, the inventors have noted apparent combustion occurring downstream from the output face  214  of the distal flame holder  102 , but still a majority of combustion occurred within the perforated flame holder of the distal flame holder  102  as evidenced by continued visible glow from the distal flame holder  102  that was observed. 
     The distal flame holder  102  incorporating a perforated flame holder  208  can be configured to receive heat from the combustion reaction  302  and output a portion of the received heat as thermal radiation  304  to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume  204 . As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body  208 . 
     Referring especially to  FIG. 3 , the perforated flame holder of the distal flame holder  102  outputs another portion of the received heat to the fuel and oxidant mixture  206  received at the input face  212  of the perforated flame holder. The perforated flame holder body  208  may receive heat from the combustion reaction  302  at least in heat receiving regions  306  of perforation walls  308 . Experimental evidence has suggested to the inventors that the position of the heat receiving regions  306 , or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls  308 . In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face  212  to the output face  214  (i.e., somewhat nearer to the input face  212  than to the output face  214 ). The inventors contemplate that the heat receiving regions  306  may lie nearer to the output face  214  of the perforated flame holder of the distal flame holder  102  under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions  306  (or for that matter, of heat output regions  310 , described below). For ease of understanding, the heat receiving regions  306  and the heat output regions  310  will be described as particular regions  306 ,  310 . 
     The perforated flame holder body  208  can be characterized by a heat capacity. The perforated flame holder body  208  may hold thermal energy from the combustion reaction  302  in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions  306  to the heat output regions  310  of the perforation walls  308 . Generally, the heat output regions  310  are nearer to the input face  212  than are the heat receiving regions  306 . According to one interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via thermal radiation, depicted graphically as  304 . According to another interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via heat conduction along heat conduction paths  312 . The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions  306  to the heat output regions  310 . In this way, the distal flame holder  102  may act as a heat source to maintain the combustion reaction  302 , even under conditions where a combustion reaction  302  would not be stable when supported from a conventional flame holder. 
     The inventors believe that a distal flame holder  102  incorporating a perforated flame holder body  208  causes the combustion reaction  302  to begin within thermal boundary layers  314  formed adjacent to the walls  308  of the perforations  210 . Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the distal flame holder  102  incorporating a perforated flame holder body  208 , it is apparent that at least a majority of the individual reactions occur within the distal flame holder  102  incorporating a perforated flame holder body  208 . As the relatively cool fuel and oxidant mixture  206  approaches the input face  212 , the flow is split into portions that respectively travel through individual perforations  210 . The hot perforated flame holder body  208  transfers heat to the fluid, notably within the thermal boundary layers  314  that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture  206 . After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction  302  occurs. Accordingly, the combustion reaction  302  is shown as occurring within the thermal boundary layers  314 . As flow progresses, the thermal boundary layers  314  merge at a merger point  316 . Ideally, the merger point  316  lies between the input face  212  and the output face  214  that define the ends of the perforations  210 . At some position along the length of a perforation  210 , the combustion reaction  302  outputs more heat to the perforated flame holder body  208  than it receives from the perforated flame holder body  208 . The heat is received at the heat receiving region  306 , is held by the perforated flame holder body  208 , and is transported to the heat output region  310  nearer to the input face  212 , where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature. 
     In an embodiment, each of the perforations  210  is characterized by a length L defined as a reaction fluid propagation path length between the input face  212  and the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . As used herein, the term reaction fluid refers to matter that travels through a perforation  210 . Near the input face  212 , the reaction fluid includes the fuel and oxidant mixture  206  (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction  302 , molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face  214 , the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant. 
     The plurality of perforations  210  can be each characterized by a transverse dimension D between opposing perforation walls  308 . The inventors have found that stable combustion can be maintained in the distal flame holder  102  incorporating a perforated flame holder  208  if the length L of each perforation  210  is at least four times the transverse dimension D of the perforation  210 . In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for the thermal boundary layers  314  to form adjacent to the perforation walls  308  in a reaction fluid flowing through the perforations  210  to converge at the merger points  316  within the perforations  210  between the input face  212  and the output face  214  of the distal flame holder  102  incorporating a perforated flame holder body  208 . In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion). 
     The perforated flame holder body  208  can be configured to convey heat between adjacent perforations  210 . The heat conveyed between adjacent perforations  210  can be selected to cause heat output from the combustion reaction portion  302  in a first perforation  210  to supply heat to stabilize a combustion reaction portion  302  in an adjacent perforation  210 . 
     Referring especially to  FIG. 2 , the fuel and oxidant source  202  can further include the fuel nozzle  218 , configured to output fuel, and an oxidant source  220  configured to output a fluid including the oxidant. For example, the fuel nozzle  218  can be configured to output pure fuel. The oxidant source  220  can be configured to output combustion air carrying oxygen, and optionally, flue gas. 
     The distal flame holder  102  can be held by a distal flame holder support structure  222  configured to hold the distal flame holder  102  at a dilution distance D D  away from the fuel nozzle  218 . The fuel nozzle  218  can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture  206  as the fuel jet and the oxidant travel along a path to the distal flame holder  102  through the dilution distance D D  between the fuel nozzle  218  and the distal flame holder  102 . Additionally or alternatively (particularly when a blower is used to deliver the oxidant contained in combustion air), the oxidant or combustion air source  220  can be configured to entrain the fuel and the fuel and the oxidant travel through the dilution distance D D . In some embodiments, a flue gas recirculation path  224  can be provided. Additionally, or alternatively, the fuel nozzle  218  can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D D  between the fuel nozzle  218  and the input face  212  of the distal flame holder  102 . 
     The fuel nozzle  218  can be configured to emit the fuel through one or more fuel orifices  226  having an inside diameter dimension that is referred to as “nozzle diameter.” The distal flame holder support structure  222  can support the distal flame holder  102  to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  greater than 20 times the nozzle diameter. In another embodiment, the distal flame holder  102  is disposed to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  between 100 times and 1100 times the nozzle diameter. Preferably, the distal flame holder support structure  222  is configured to hold the distal flame holder  102  at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle  218 . When the fuel and oxidant mixture  206  travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction  302  to produce minimal NOx. 
     The fuel and oxidant source  202  can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the distal flame holder  102  and be configured to prevent flame flashback into the premix fuel and oxidant source. 
     The oxidant source  220 , whether configured for entrainment in the combustion volume  204  or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source  202 . 
     The distal flame holder support structure  222  can be configured to support the distal flame holder  102  from a floor or wall (not shown) of the combustion volume  204 , for example. In another embodiment, the distal flame holder support structure  222  supports the distal flame holder  102  from the fuel and oxidant source  202 . Alternatively, the distal flame holder support structure  222  can suspend the distal flame holder  102  from an overhead structure (such as a flue, in the case of an up-fired system). The distal flame holder support structure  222  can support the distal flame holder  102  in various orientations and directions. 
     The distal flame holder  102  can include a single perforated flame holder body  208 . In another embodiment, the distal flame holder  102  can include a plurality of adjacent distal flame holder sections that collectively provide a tiled distal flame holder  102 . In an embodiment, one or more of the plurality of adjacent distal flame holder sections may include a perforated flame holder body  208 . In other embodiments, the distal flame holder  102  may include a plurality of distal flame holder sections disposed apart from each other at positions about a central flow axis of the fuel and the oxidant. 
     The distal flame holder support structure  222  can be configured to support the plurality of distal flame holder sections. The distal flame holder support structure  222  can include a metal superalloy, a cementatious, and/or a ceramic refractory material. In an embodiment, the plurality of adjacent distal flame holder sections can be joined with a fiber reinforced refractory cement. The distal flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least twice a thickness dimension T between the input face  212  and the output face  214 . In another embodiment, the distal flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least three times, at least six times, or at least nine times the thickness dimension T between the input face  212  and the output face  214  of the distal flame holder  102 . 
     In an embodiment, the distal flame holder  102  can have a width dimension W less than a width of the combustion volume  204 . This can allow the flue gas recirculation path  224  from above to below the distal flame holder  102  to lie between the peripheral surface  216  of the distal flame holder  102  and the combustion volume wall (not shown). 
     Referring again to both  FIGS. 2 and 3 , the perforations  210  can be of various shapes. In an embodiment, the perforations  210  can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations  210  can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations  210  can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations  210  can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face  212  to the output face  214 . In some embodiments, the perforations  210  can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations  210  may have lateral dimension D less than a standard reference quenching distance. 
     In one range of embodiments, each of the plurality of perforations  210  has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations  210  has a lateral dimension D between 0.1 inch and 0.5 inch. For example, the plurality of perforations  210  can each have a lateral dimension D of about 0.2 to 0.4 inch. 
     The void fraction of a distal flame holder  102  that incorporates a perforated flame holder body  208  is defined as the total volume of all perforations  210  in a section of the perforated flame holder body  208  divided by a total volume of the perforated flame holder  102  including the perforated flame holder body  208  and the perforations  210 . In such embodiments the distal flame holder  102  should have a void fraction between 0.10 and 0.90. In an embodiment, the distal flame holder  102  can have a void fraction between 0.30 and 0.80. In another embodiment, the distal flame holder  102  can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx. 
     A distal flame holder  102  incorporating the perforated flame holder body  208  can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder body  208  can be formed to include mullite or cordierite. Additionally, or alternatively, the perforated flame holder body  208  can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body  208  can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known. 
     The inventors have found that the perforated flame holder body  208  can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C. 
     The perforations  210  can be parallel to one another and normal to the input and the output faces  212 ,  214 . In another embodiment, the perforations  210  can be parallel to one another and formed at an angle relative to the input and the output faces  212 ,  214 . In another embodiment, the perforations  210  can be non-parallel to one another. In another embodiment, the perforations  210  can be non-parallel to one another and non-intersecting. In another embodiment, the perforations  210  can be intersecting. The perforated flame holder body  208  can be one piece or can be formed from a plurality of sections. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder body  208  may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder body  208  may be formed from a ceramic material that has been punched, bored or cast to create channels. 
     In another embodiment, the perforated flame holder body  208  can include a plurality of tubes or pipes bundled together. The plurality of perforations  210  can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band. 
     The perforated flame holder body  208  can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body  208  can include discontinuous packing bodies such that the perforations  210  are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g., Super Raschig Rings) that may be held together by a metal cage. 
     The inventors contemplate various explanations for why burner systems including the distal flame holder  102  provide such clean combustion. 
     According to an embodiment, a perforated flame holder body  208  may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream  206  contacts the input face  212  of the perforated flame holder body  208 , an average fuel-to-oxidant ratio of the fuel stream  206  is below a (conventional) lower combustion limit of the fuel component of the fuel stream  206 —lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture  206  will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.). 
     The perforated flame holder body  208  and systems including the perforated flame holder body  208  described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O 2 , i.e., an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O 2 . Moreover, the inventors believe the perforation walls  308  may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx. 
     According to another interpretation, production of NOx can be reduced if the combustion reaction  302  occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder body  208  is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder body  208 . 
       FIG. 4  is a flow chart showing a method  400  for operating a burner system including the distal flame holder  102  shown and described herein. To operate a burner system including a distal flame holder, the distal flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture. 
     According to a simplified description, the method  400  begins with step  402 , wherein the distal flame holder (e.g.,  102 ) is preheated to a start-up temperature, T. After the distal flame holder is raised to the start-up temperature, the method proceeds to step  404 , wherein the fuel and oxidant are provided to the distal flame holder and combustion is held by the distal flame holder. 
     According to a more detailed description, step  402  begins with step  406 , wherein start-up energy is provided at the distal flame holder. Simultaneously or following providing start-up energy, a decision step  408  determines whether the temperature T of the distal flame holder is at or above the start-up temperature, T S . As long as the temperature of the distal flame holder is below its start-up temperature, the method loops between steps  406  and  408  within the preheat step  402 . In decision step  408 , if the temperature T of at least a predetermined portion of the distal flame holder is greater than or equal to the start-up temperature, the method  400  proceeds to overall step  404 , wherein fuel and oxidant is supplied to and combustion is held by the distal flame holder. 
     Step  404  may be broken down into several discrete steps, at least some of which may occur simultaneously. 
     Proceeding from decision step  408 , a fuel and oxidant mixture is provided to the distal flame holder, as shown in step  410 . The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the distal flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the distal flame holder incorporating a perforated flame holder body  208  at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder. 
     Proceeding to step  412 , the combustion reaction is held by the distal flame holder. 
     In step  414 , heat may be output from the distal flame holder. The heat output from the distal flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example. 
     In optional step  416 , the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the distal flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step  416 , a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the distal flame holder. 
     Proceeding to decision step  418 , if combustion is sensed not to be stable, the method  400  may exit to step  424 , wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step  402 , outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in decision step  418 , combustion at the distal flame holder is determined to be stable, the method  400  proceeds to decision step  420 , wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step  404 ) back to step  410 , and the combustion process continues. If a change in combustion parameters is indicated, the method  400  proceeds to step  422 , wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step  404 ) back to step  410 , and combustion continues. 
     Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step  422 . Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally, or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the distal flame holder over one or more iterations of the loop within step  404 . 
     As described in conjunction with  FIGS. 3 and 4 , the distal flame holder  102  operates by outputting heat to the incoming fuel and oxidant mixture  206 . After combustion is established, this heat is provided by the combustion reaction  302 ; but before combustion is established, the heat (or “startup energy,” in  FIG. 4 ) is provided by combustion of a mixture of pilot fuel from a pilot fuel distributor  106  and an oxidant. 
     In some embodiments, the pilot fuel distributor  106  may itself support a pilot flame the intensity of which is controlled to heat the distal flame holder  102 . In other embodiments, the pilot fuel distributor  106  may include a flame holder configured to support a pilot flame disposed to heat the distal flame holder  102 . The fuel and oxidant source  202  can include a fuel nozzle  218  configured to emit a fuel stream  206  and an oxidant source  220  configured to output oxidant (e.g., combustion air) adjacent to the fuel stream  206 . The fuel nozzle  218  and oxidant source  220  can be configured to output the fuel stream  206  to be progressively diluted by the oxidant (e.g., combustion air). The distal flame holder  102  can be disposed to receive a diluted fuel and oxidant mixture  206  that supports a combustion reaction  302  that is stabilized by the distal flame holder  102  when the distal flame holder  102  is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated distal flame holder  102 . 
     The burner system  200  can further include a controller  230  operatively coupled to the pilot fuel distributor  106  and to a data interface  232 . For example, the controller  230  can be configured to control ignition, and change (e.g., turn up) a flow rate, of a pilot fuel provided by the pilot fuel distributor  106  in order to provide a start-up flame and effect a preheating state of the combustion system  100  when the distal flame holder  102  needs to be pre-heated and to change (e.g., turn down) the flow rate of the pilot fuel provided by the pilot fuel distributor  106  when the distal flame holder  102  is at an operating temperature (e.g., when T≥T S ). In some embodiments, the pilot fuel distributor is controlled to provide pilot fuel at the same time that a main fuel distributor  110  provides a main fuel to the distal flame holder  102 , thus supplementing the combustion capacity of the combustion system  100 . 
     Various approaches for actuating a start-up flame are contemplated. In one embodiment, the pilot fuel distributor includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture  206  to cause heat-recycling and/or stabilizing vortices and thereby hold the start-up flame; or to be actuated to not intercept the fuel and oxidant mixture  206  to cause the fuel and oxidant mixture  206  to proceed to the distal flame holder  102 . In another embodiment, a fuel control valve, blower, and/or damper may be used to select a pilot fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a distal flame holder operating temperature, the mixture flow rate may be decreased to just maintain a pilot flame, or increased to supplement main combustion. 
     An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture  206 . The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder. Other forms of start-up apparatuses are contemplated. For example, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture  206  that would otherwise enter the distal flame holder  102 . The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller  230 , which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture  206  in or upstream from the distal flame holder  102  before the distal flame holder  102  is heated sufficiently to maintain combustion. 
     The burner system  200  can further include a sensor  234  (corresponding in some embodiments with at least one of the pilot flame sensor  124  and the distal flame holder sensor  122  in  FIG. 1 ) operatively coupled to the control circuit  230 . The sensor  234  can include a heat sensor configured to detect infrared radiation or a temperature of the distal flame holder  102 . The control circuit  230  can be configured to control the heating apparatus  228  responsive to input from the sensor  234 . Optionally, a fuel control valve  236  can be operatively coupled to the controller  230  and configured to control a flow of the fuel to the fuel and oxidant source  202 . Additionally or alternatively, an oxidant blower or damper  238  can be operatively coupled to the controller  230  and configured to control flow of the oxidant (or combustion air). 
     The sensor  234  can further include a combustion sensor operatively coupled to the control circuit  230 , the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction  302  held by the distal flame holder  102 . The fuel control valve  236  can be configured to control a flow of the fuel from a fuel source to the fuel and oxidant source  202 . The controller  230  can be configured to control the fuel control valve  236  responsive to input from the combustion sensor  234 . The controller  230  can be configured to control the fuel control valve  236  and/or the oxidant blower or damper  238  to control a preheat flame to heat the distal flame holder  102  to an operating temperature. The controller  230  can similarly control the fuel control valve  236  and/or the oxidant blower or damper  238  to change the fuel and oxidant mixture  206  flow responsive to a heat demand change received as data via the data interface  232 . 
       FIG. 5A  is a simplified perspective view of a combustion system  500 , including another alternative perforated flame holder body  208 , according to an embodiment. The perforated flame holder body  208  is a reticulated ceramic perforated flame holder, according to an embodiment.  FIG. 5B  is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder body  208  of  FIG. 5A , according to an embodiment. The distal flame holder  102  of  FIGS. 5A, 5B  can be implemented in the various combustion systems described herein, according to an embodiment. A distal flame holder  102  incorporating a perforated flame holder body  208  is configured to support a combustion reaction  302  of the fuel and oxidant mixture  206  at least partially within the perforated flame holder body  208 . According to an embodiment, the distal flame holder  102  incorporating a perforated flame holder body  208  can be configured to support a combustion reaction  302  of the fuel and oxidant mixture  206  upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder body  208 . 
     According to an embodiment, the perforated flame holder body  208  can include reticulated fibers  539 . The reticulated fibers  539  can define branching perforations  210  that weave around and through the reticulated fibers  539 . According to an embodiment, the perforations  210  are formed as passages through the reticulated ceramic fibers  539 . 
     According to an embodiment, the reticulated fibers  539  are formed as a reticulated ceramic foam. According to an embodiment, the reticulated fibers  539  are formed using a reticulated polymer foam as a template. According to an embodiment, the reticulated fibers  539  can include alumina silicate. According to an embodiment, the reticulated fibers  539  can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers  539  can include Zirconia. According to an embodiment, the reticulated fibers  539  can include silicon carbide. 
     The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers  539  are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant mixture  206 , the combustion reaction  302 , and heat transfer to and from the perforated flame holder body  208  can function similarly to the embodiment shown and described above with respect to  FIGS. 2-4 . One difference in activity is a mixing between perforations  210 , because the reticulated fibers  539  form a discontinuous perforated flame holder body  208  that allows flow back and forth between neighboring perforations  210 . 
     According to an embodiment, the reticulated fiber network is sufficiently open for downstream reticulated fibers  539  to emit radiation for receipt by upstream reticulated fibers  539  for the purpose of heating the upstream reticulated fibers  539  sufficiently to maintain combustion of a fuel and oxidant mixture  206 . Compared to a continuous perforated flame holder body  208 , heat conduction paths  312  between reticulated fibers  539  are reduced due to separation of the reticulated fibers  539 . This may cause relatively more heat to be transferred from the heat-receiving region  306  (heat receiving area) to the heat output region  310  (heat output area) of the reticulated fibers  539  via thermal radiation  304 . 
     According to an embodiment, individual perforations  210  may extend from an input face  212  to an output face  214  of the perforated flame holder body  208 . Perforations  210  may have varying lengths L. According to an embodiment, because the perforations  210  branch into and out of each other, individual perforations  210  are not clearly defined by a length L. 
     According to an embodiment, the perforated flame holder body  208  is configured to support or hold a combustion reaction  302  or a flame at least partially between the input face  212  and the output face  214 . According to an embodiment, the input face  212  corresponds to a surface of the distal flame holder  102  proximate to the fuel nozzle  218  or to a surface that first receives fuel. According to an embodiment, the input face  212  corresponds to an extent of the reticulated fibers  539  proximate to the fuel nozzle  218 . According to an embodiment, the output face  214  corresponds to a surface distal to the fuel nozzle  218  or opposite the input face  212 . According to an embodiment, the input face  212  corresponds to an extent of the reticulated fibers  539  distal to the fuel nozzle  218  or opposite to the input face  212 . 
     According to an embodiment, the formation of boundary layers  314 , transfer of heat between the perforated flame holder body  208  and the gases flowing through the perforations  210 , a characteristic perforation width dimension D, and the length L can be regarded as related to an average or overall path through the perforated flame holder body  208 . In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight-line distance T RH  from the input face  212  to the output face  214  through the perforated flame holder body  208 . According to an embodiment, the void fraction (expressed as (total distal flame holder  102  volume—reticulated fiber  539  volume)/total volume)) is about 70%. 
     According to an embodiment, the reticulated ceramic perforated flame holder body  208  is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder body  208  includes about 100 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder body  208  in accordance with principles of the present disclosure. 
     According to an embodiment, the reticulated ceramic distal flame holder  102  can include shapes and dimensions other than those described herein. For example, the distal flame holder  102  can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic distal flame holder  102  can include shapes other than generally cuboid shapes. 
     According to an embodiment, the reticulated ceramic distal flame holder  102  can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single distal flame holder  102 . Alternatively, each reticulated ceramic tile can be considered a distinct distal flame holder  102 . 
     According to an embodiment, referring back to  FIG. 1 , the pilot fuel distributor  106  includes a fuel nozzle disposed proximate to the main fuel distributor  110 . The controller  116  may be configured to cause the flow of the pilot fuel to stop and the flow of the main fuel to start when the distal flame holder  102  is determined to be at a predetermined operating temperature. 
     According to another embodiment, referring again to  FIG. 1 , the pilot fuel distributor  106  includes a pilot flame support assembly disposed distal from a primary fuel distributor, at a distance intermediate between the primary fuel distributor and the distal flame holder  106 . The controller  116  may be configured to cause the flow of the pilot fuel to decrease so as to maintain a pilot flame supported by the pilot flame support assembly and the flow of the main fuel to start when the distal flame holder  102  is determined to be at a predetermined operating temperature. 
       FIG. 6  is a block diagram of components of a combustion control system  600 , according to an embodiment. The combustion control system  600  includes a controller  116 , a set of sensors  114 , a set of actuators  118 , a display  120 , and a control input  670 . The set of sensors  114 , the set of actuators  118 , the control input  670 , and the display  120  are communicatively coupled to the controller  116  such that the controller  116  can send, or receive signals, instructions, or data from the components. These components are utilized to monitor, control, and adjust operation of the combustion system  600  with respect to holding a combustion reaction  302  in a distal flame holder  102  (see  FIGS. 1-3, 5A, and 5B ). 
     In one embodiment, the set of sensors  114  includes a bridgewall temperature sensor  640 , a pilot flame scanner  642 , a CO monitor  644 , a NO X  monitor  646 , an O 2  monitor  648 , a dynamic pressure sensor  649 , a distal flame holder flame scanner  650 , a pressure differential sensor  651 , a process monitor  652 , a camera  653 , a pressure sensor  654 , and a distal flame holder temperature sensor  655 . These sensors monitor various parameters of the combustion system  600  and output sensor signals to the controller  116 . The sensor signals indicate various parameters of the combustion system  600 . The set of sensors  114  can include fewer sensors, more sensors, or different kinds of sensors than those shown in  FIG. 6 . 
     In one embodiment, the set of actuators  118  include a stack damper actuator  656 , a main fuel actuator  658 , a pilot fuel actuator  660 , an oxidant source actuator  662 , a process actuator  664 , and an igniter actuator  666 . The set of actuators  118  receive electrical commands and instructions from the controller  116 . The set of actuators  118  activate, control, or adjust components of the combustion system  600  responsive to the commands from the controller  116 . Additionally, or alternatively, the set of actuators  118  can be operated manually by an operator of the combustion system  600 . 
     In one embodiment, the display  120  displays messages, data, or other indications from the controller  116 . The operator or technician of the combustion system  600  can receive information via the display  120 . The controller  116  can output messages via the display  120  indicating various parameters of the combustion system  600  as measured by the set of sensors  114 . The controller  116  can output messages via the display  120  indicating operations that the controller  116  will undertake, such as transitioning from a preheating state to a standard operating state or controlling one or more of the set of actuators  118  responsive to the sensor signals. The display  120  can also display prompts requesting input from the operator of the combustion system  600  requesting that the operator provide approval or permission to execute one or more proposed actions. Upon receiving input from the operator, the controller  116  can undertake actions or refrain from action in accordance with the instructions received from the operator. 
     In one embodiment, the control input  670  enables the operator of the combustion system  600  to enter commands to the controller  116 . The control input  670  can include one or more of a keypad, a keyboard, a touchscreen, buttons, switches, a mouse, a trackpad, or any other suitable way for the operator of the combustion system  600  to input data or commands to the controller  116 . The control input  670  can communicate with the controller  116  via any suitable data transfer interface. In one embodiment, when the controller  116  outputs a message on the display  120  requesting input from the operator to proceed with a proposed adjustment to the combustion system  600 , the operator can include a command to the controller  116  via the control input  670  responsive to the message on the display  120 . The operator can also utilize the control input  670  to override actions taken by the controller  116  in controlling the combustion system  600 . 
     In one embodiment, the bridgewall temperature sensor  640  senses a temperature of a furnace bridgewall. The temperature of the furnace bridgewall provides an indication of whether a process of the furnace is ready for operation. As the distal flame holder  102  sustains a combustion reaction  302 , the temperature of the furnace bridgewall will increase. When the bridgewall of the furnace has reached a selected threshold temperature, the combustion system  600  can initiate a process. 
     In an embodiment, the controller  116  receives the temperature of the furnace bridgewall from the bridgewall temperature sensor  640  and takes one or more actions based on the temperature of the furnace bridgewall and on one or more algorithms, state machines, or other software instructions implemented by the controller  116 . In one example, if the sensor signal from the bridgewall temperature sensor  640  indicates that the temperature of the furnace bridgewall is above the threshold temperature, then the controller  116  can send the sensor signal to the process actuator  664 . The process actuator  664  can activate a process, such as initiating the flow of a working fluid to be heated by the furnace. In one embodiment, if the bridgewall temperature sensor  640  indicates that the temperature of the furnace bridgewall is below the threshold temperature, then the controller  116  refrains from activating the process actuator  664 . In one embodiment, if the process is already active, and the sensor signal from the bridgewall temperature sensor  640  indicates that the temperature of the furnace bridgewall has fallen below the threshold temperature, then the controller  116  can take measures to increase the heat output by the distal flame holder  102  by operating the main fuel actuator  658  to adjust flow of the main fuel, the oxidant source actuator  662  to adjust the flow of the oxidant, the stack damper actuator  656  to adjust the movement or position of the stack damper, or any other actuators that can adjust a parameter of the combustion system  600  to increase output from the distal flame holder  102 . The controller  116  can also cause the process actuator  664  to adjust or stop the process until the temperature of the furnace bridgewall increases beyond a threshold temperature. 
     In one embodiment, the pilot flame scanner  642  monitors parameters of the pilot flame while the combustion system  600  is in the preheating state. The pilot flame scanner  642  can detect whether the pilot flame is present. The pilot flame scanner  642  can detect the position of the pilot flame. The pilot flame scanner  642  outputs a sensor signal to the controller  116  indicating the presence, the absence, the position, or other parameters of the pilot flame. 
     In one embodiment, when the controller  116  receives the sensor signal from the pilot flame scanner  642 , the controller  116  takes one or more actions based on the parameters of the pilot flame. If the sensor signal from the pilot flame scanner  642  indicates that the pilot flame is not present, then the controller  116  can send command signals to the oxidant source actuator  662  to adjust the flow of the oxidant, to the pilot fuel actuator  660  to adjust the flow of the pilot fuel, and to the igniter actuator  666  to ignite the pilot flame by generating an electric arc or in any other suitable way. In one embodiment, if the sensor signal from the pilot flame scanner  642  indicates that the position of the pilot flame is too far from the distal flame holder  102  or too close to the distal flame holder  102 , then the controller  116  can output control signals that cause the pilot fuel actuator  660  to adjust a flow rate of the pilot fuel, or to adjust a fuel mixture of the pilot fuel. The controller  116  can also issue commands to the oxidant source actuator  662  causing the oxidant source actuator  662  to increase or decrease the flow of the oxidant. 
     In one embodiment, the CO monitor  644  monitors the concentration of CO and flue gases generated by the combustion reaction  302  of the main fuel and the oxidant in the standard operating state. The CO monitor  644  outputs sensor signals to the controller  116  indicating concentration of CO in the flue gases generated by the combustion reaction  302  held by the distal flame holder  102 . The controller  116  receives the sensor signals and takes one or more actions based on the CO concentration as indicated by the sensor signals from the CO monitor  644 . 
     In one embodiment, if the concentration of CO in the flue gas is below an acceptable value, then the controller  116  may not adjust any parameters of the combustion system  600  in order to maintain the current state of the combustion reaction  302 . If the concentration of CO in the flue gas is higher than an acceptable value, then the controller  116  can send signals to the main fuel actuator  658 , the oxidant source actuator  662 , or the stack damper actuator  656  in order to adjust the combustion reaction  302  of the main fuel and the oxidant. The controller  116  can cause the main fuel actuator  658  to adjust the flow of the main fuel or the mixture of fuels that make up the main fuel in order to cause the combustion reaction  302  of the main fuel and the oxidant to generate less CO. The controller  116  can also cause the oxidant source actuator  662  to adjust the flow of the oxidant into the furnace in order to reduce the concentration of CO in the flue gas. 
     In one embodiment, the NO X  monitor  646  senses the concentration of NO X  in the flue gas generated by the combustion reaction  302  of the main fuel and the oxidant held by the distal flame holder  102  in the standard operating state. The NO X  monitor  646  outputs a sensor signal to the controller  116  indicating the concentration of NO X  in the flue gas. The controller  116  can take one or more actions based on the concentration of the NO X  in the flue gas as indicated by the sensor signal. 
     In one embodiment, if the sensor signal from the NO X  monitor  646  indicates that the NO X  concentration is higher than a threshold value, for example higher than 10 ppm, then the controller  116  can take actions to reduce the concentration of NO X  in the flue gas. In one embodiment, the controller  116  can control the oxidant source actuator  662  to increase the flow of the oxidant from the oxidant source. Additionally, or alternatively, the controller  116  controls the main fuel actuator  658  to decrease the flow of the main fuel or to otherwise adjust parameters of the flow of the main fuel in order to decrease the concentration of NO X  in the flue gas. 
     In one embodiment, the O 2  monitor  648  monitors the presence of O 2  in the flue gas. The O 2  monitor  648  outputs a sensor signal to the controller  116  indicating the concentration of O 2  in the flue gas. The controller  116  receives the sensor signal from the O 2  monitor  648  and undertakes one or more actions based on the concentration of O 2  in the flue gas. 
     In one embodiment, it is desirable that the concentration of O 2  in the flue gas fall within a selected range, e.g., greater than or equal to 2% and less than or equal to 5%. If the sensor signal from the O 2  monitor  648  indicates that the concentration of O 2  is below the selected range, then the controller  116  can control the oxidant source actuator  662  to increase the flow of the oxidant into the furnace. Additionally, or alternatively, the controller  116  can increase the concentration of O 2  in the flue gas by decreasing the flow of the main fuel into the furnace. If the sensor signal from the O 2  monitor  648  indicates that the concentration of O 2  is greater than the selected range, then the controller  116  can cause the oxidant source actuator  662  to decrease the flow of the oxidant into the furnace. Additionally, or alternatively, the controller  116  can cause the main fuel actuator  658  to increase the flow of the main fuel into the furnace in order to decrease the concentration of O 2  in the flue gas. In some cases, a higher than desired concentration of O 2  can be the result of incomplete fuel burn. Thus, the controller  116  can control the main fuel actuator  658  to reduce the velocity (or flow rate) of the main fuel in order to more completely combust the main fuel. 
     In one embodiment, the dynamic pressure sensor  649  detects changes in pressure with time at one or more locations in the combustion environment. The dynamic pressure sensor  649  generates sensor signals indicative of the change in pressure in the furnace over time or of the draft of the oxidant. The sensor signals from the dynamic pressure sensor  649  can indicate a slope or derivative of the pressure with respect to time and/or may be converted to frequency domain to detect audible or inaudible noise caused by pressure waves. The inventors note that the dynamic pressure sensor  649  produces a signal indicative of stability of a combustion reaction in the distal flame holder  102 . When the combustion reaction is stable, there is relatively constant pressure at the dynamic pressure sensor  649 . When the combustion reaction is unstable, the dynamic pressure sensor  649  produces a signal corresponding to rapid fluctuations in pressure, a condition that has been noted by the inventors to correspond to relatively high audible noise produced by the flowing gas in the furnace. The controller  116  can undertake one or more actions to adjust the pressure or other combustion parameters responsive to the sensor signal from the dynamic pressure sensor  649 . 
     In one embodiment, the controller  116  can increase or decrease the pressure by controlling the oxidant source actuator  662  to adjust the flow of the oxidant responsive to the sensor signals provided by the dynamic pressure sensor  649 . The controller  116  can adjust the pressure by causing the stack damper actuator  656  to adjust the stack damper. The controller  116  can adjust the pressure by causing the main fuel actuator  658  to adjust the flow of the main fuel. The controller  116  can also undertake other actions to adjust the pressure responsive to sensor signals provided by the dynamic pressure sensor  649 . 
     In one embodiment, the pressure differential sensor  651  detects pressure differentials or differences across two or more locations in the furnace, such as across the distal flame holder  102 . The controller  116  can undertake one or more actions to adjust the pressure or other combustion parameters responsive to the sensor signal from the pressure differential sensor  651 . 
     In one embodiment, the controller  116  can increase or decrease the pressure by controlling the oxidant source actuator  662  to adjust the flow of the oxidant. The controller  116  can adjust the pressure by causing the stack damper actuator  656  to adjust the stack damper. The controller  116  can adjust the pressure by causing the main fuel actuator  658  to adjust the flow of the main fuel. The controller  116  can also undertake other actions to adjust the pressure responsive to sensor signals provided by the dynamic pressure sensor  649 . 
     In one embodiment, the distal flame holder flame scanner  650  monitors parameters of the combustion reaction  302  of the main fuel and the oxidant held by the distal flame holder  102 . The distal flame holder flame scanner  650  outputs a sensor signal to the controller  116  indicating the parameters of the combustion reaction  302  held by the distal flame holder  102  in the standard operating state. 
     In one embodiment, the distal flame holder flame scanner  650  can detect whether the combustion reaction  302  of the main fuel and the oxidant is present at the distal flame holder  102 . If the sensor signals output by the distal flame holder flame scanner  650  indicate that the combustion reaction  302  of the main fuel and the oxidant is not present, then the controller  116  can undertake one or more actions. For example, the controller  116  can cause a flow of the pilot fuel to the pilot flame to increase to provide additional heat to the distal flame holder  102  so that the distal flame holder  102  is at a sufficient temperature to initiate a combustion reaction  302  of the main fuel and the oxidant. The controller  116  can thus cause the combustion system  600  to revert back to the preheating state by controlling the main fuel actuator  658 , the pilot fuel actuator  660 , the oxidant source actuator  662 , and the igniter actuator  666  to cease the flow of the main fuel, to adjust the flow of the oxidant, to initiate a flow of the pilot fuel, and to ignite the pilot flame until the distal flame holder  102  has reached the threshold temperature. 
     In one embodiment, if the sensor signals output by the distal flame holder flame scanner  650  indicate that the combustion reaction  302  of the main fuel and the oxidant is concentrated too far upstream from the distal flame holder  102  or too far downstream from the distal flame holder  102 , then the controller  116  can control the main fuel actuator  658  to adjust the flow rate, the velocity, the mixture, or other parameters of the main fuel. The controller  116  can also cause the oxidant source actuator  662  to adjust the flow of the oxidant in order to cause the combustion reaction  302  of the main fuel and the oxidant to be held by the distal flame holder  102 . 
     In one embodiment, the distal flame holder flame scanner  650  can indicate how much heat is generated by the combustion of the main fuel and the oxidant. If the sensor signals from the distal flame holder flame scanner  650  indicate that the combustion reaction  302  of the main fuel and the oxidant is generating too much heat or too little heat, then the controller  116  can take one or more actions. For example, the controller  116  can adjust the flow or mixture of the main fuel by controlling the main fuel actuator  658 . The controller  116  can also cause the oxidant source actuator  662  to adjust the flow of the oxidant to increase or decrease the temperature of the combustion reaction  302  of the main fuel and the oxidant. 
     In one embodiment, the process monitor  652  measures parameters of the process, such as the transfer of heat from the combustion reaction  302  of the main fuel and the oxidant to a working fluid. The process monitor  652  outputs sensor signals to the controller  116  indicating the parameters of the process. The controller  116  can take one or more actions to adjust the parameters of the process responsive to the sensor signals. 
     In one embodiment, the controller  116  can control the process actuator  664  in order to adjust one or more aspects of the process responsive to the sensor signal from the process monitor  652 . Additionally, or alternatively, the controller  116  can control one or more other actuators to adjust parameters of the combustion reaction  302  of the main fuel and the oxidant in order to adjust the parameters of the process. 
     In one embodiment, a camera  653  monitors one or more conditions within the furnace and outputs sensor signals indicative of the monitored condition. The camera  653  can include a charge coupled device (CCD) camera, a CMOS camera, or other types of cameras. The camera  653  can be part of one or more other sensors in the sensor array  114 . The camera  653  can monitor visual parameters of the distal flame holder  102 , the combustion reaction  302  within the distal flame holder  102 , the pilot flame, flashback of the combustion reaction  302 , the physical condition of components, actuators, sensors, or other conditions within the furnace. The controller  116  can take one or more actions in response to the sensor signals from the camera  653 . The camera  653  can detect UV wavelengths, IR wavelengths, and/or visible light wavelengths. The camera  653  can include a video camera or other kinds of cameras. 
     In one embodiment, the camera  653  can convert the field of view with a phase mask and detect a signal with a planar CCD or a CMOS array, not as an image of the field of view, but as matrix data that can be decoded to focus at a range of focal planes. 
     In one embodiment, the sensor array  114  can include a flashback sensor configured to detect flashback of the combustion reaction  302  from the distal flame holder  102  towards the main fuel distributor(s)  110 . The flashback sensor can be part of one or more other sensors in the sensor array  114 . The flashback sensor can include one or more of a camera, an infrared sensor, a flame rod, a UV sensor, a CCD camera, thermocouples, photo cells, electrodes, or other kinds of devices capable of sensing flashback. 
     In one embodiment, the controller  116  can control the turndown ratio in the furnace response to sensor signals from one or more of the sensors in the sensor array  114  or from sensors not shown or described herein. The controller  116  can control or adjust the turndown ratio by operating one or more actuators  118  to adjust parameters of the combustion environment such as fuel flow parameters, oxidant parameters, operating state parameters, or other parameters. 
     In one embodiment, the combustion system  600  can include multiple distal flame holders  102 . The combustion system  600  can include multiple main fuel distributors  110 , multiple oxidant sources  104 , multiple pilot fuel distributors  106 , and multiple other components to operate the multiple distal flame holders  102 . The combustion system  600  can include multiple of the various sensors  114  to sense the parameters related to the multiple distal flame holders  102 . The controller  116  can adjust the parameters related to the multiple distal flame holders  102  in response to the sensor signals from the various sensors  114 . The sensors can control the operations related to the multiple distal flame holders  102  based on huffing, instability, and turndown as indicated by the sensors of the sensor array  114 . The controller  116  can also cease operation of one or more of the distal flame holders  102  or can select which and how many of the multiple distal flame holders  102  should be in operation. The controller  116  can control the set of actuators  118  to control, operate, select, or stop operations related to the multiple distal flame holders  102 . 
     The combustion system  600  can also be a multi-fuel system that utilizes multiple fuels or kinds of fuel in holding a combustion reaction  302  in one or more distal flame holders  102 . The controller  116  can control the flow of the multiple fuels, select which fuels to use, or select mixtures or blends of fuel based on the sensor signals from the various sensors of the sensor array  114 . 
     In one embodiment, the pressure sensor  654  monitors pressure in the furnace or the draft pressure of the oxidant. The pressure sensor  654  sensor signals and outputs into the controller  116  a signal indicative of the pressure in the furnace or of the draft of the oxidant. The controller  116  can undertake one or more actions to adjust the pressure responsive to the sensor signal from the pressure sensor  654 . 
     In one embodiment, the controller  116  can increase or decrease the pressure by controlling the oxidant source actuator  662  to adjust the flow of oxidant. The controller  116  can adjust the pressure by causing the stack damper actuator  656  to adjust the stack damper. The controller  116  can adjust the pressure by causing the main fuel actuator  658  to adjust the flow of the main fuel. The controller  116  can also undertake other actions to adjust the pressure responsive to sensor signals provided by the pressure sensor  654 . 
     In one embodiment, the distal flame holder temperature sensor  655  monitors the temperature of the distal flame holder  102 . The distal flame holder temperature sensor  655  generates sensor signals indicating the temperature of the distal flame holder  102  and transmits them to the controller  116 . The controller  116  can undertake one or more actions to adjust the temperature of the distal flame holder  102  based on the sensor signals from the distal flame holder temperature sensor  655 . 
     In one embodiment, the distal flame holder temperature sensor  655  monitors the temperature of the distal flame holder  102  during the preheating state of the combustion system  600 . Thus, as the pilot flame of the pilot fuel and the oxidant heats the distal flame holder  102 , the distal flame holder temperature sensor  655  monitors the temperature of the distal flame holder  102 . If the sensor signal indicates that the temperature of the distal flame holder  102  is below the threshold temperature or an operating temperature, then the controller  116  causes the combustion system  600  to remain in the preheating state in which the pilot flame remains present and continues to heat the distal flame holder  102 . If the sensor signal from the distal flame holder temperature sensor  655  indicates that the temperature of the distal flame holder  102  has reached the threshold temperature or the operating temperature, then the controller  116  can control the pilot fuel actuator  660  and the main fuel actuator  658  to transition from the preheating state to the standard operating state by ceasing the flow of the pilot fuel and initiating the flow of the main fuel. 
     In one embodiment, the distal flame holder temperature sensor  655  continues to monitor the temperature of the distal flame holder  102  during the standard operating state. If the sensor signal from the distal flame holder temperature sensor  655  indicates that the temperature of the distal flame holder  102  has dropped below the threshold temperature or the operating temperature, then the controller  116  can take one or more actions. For example, the controller  116  can cause the pilot flame to begin heating the distal flame holder  102 . For example, the controller  116  can cause the combustion system  600  to revert to the preheating state by stopping the flow of the main fuel and increasing the flow of the pilot fuel. 
     In one embodiment, the controller  116  automatically controls the various actuators  118  responsive to the sensor signals from the set of sensors  114  in accordance with one or more sets of software instructions, algorithms, state machines, or other protocols that indicate what actions the controller  116  will take based on the values of the sensor signals generated by the set of sensors  114 . In one embodiment, the controller  116  does not automatically control one or more of the actuators  118  responsive to the sensor signals. Instead, the controller  116  outputs prompts or instructions via the display  120  to an operator indicating that the operator should manually adjust components of the combustion system  600  based on the sensor signals. The controller  116  can also prompt the operator to approve actions to be undertaken by the controller  116  so that the controller  116  can control the various actuators  118 . The controller  116  can use a mixture of automatic controlling actuators  118 , prompting an operator to control the actuators  118 , and prompting an operator to approve proposed actions of the controller  116 . 
       FIG. 7  is a flow diagram of a process  700  for operating a combustion system in a preheating state, according to an embodiment. The process  700  can be controlled by a controller  116  executing process steps in accordance with one or more algorithms, sets of software instructions, or state machines. The controller  116  can implement the process  700  by utilizing one or more processors to execute instructions stored on a non-transitory computer readable medium. 
     At step  702 , the process  700  begins by pre-purging a furnace of the combustion system. The pre-purging process includes purging gases, particulates, or debris from the furnace. The pre-purging process can include controlling an oxidant source to flow an oxidant through the furnace in order to clear unwanted gases, particulates, and debris from the furnace. Additionally, or alternatively, the pre-purging process can include passing an inert gas into the furnace in order to remove unwanted gases, particulates, and debris from the furnace. Once the furnace has been purged, the process  700  can proceed to step  704 . 
     In one embodiment, at step  704 , the process  700  opens a pilot fuel valve in order to initiate a flow of pilot fuel into the furnace. If the process  700  has not yet begun flowing oxidant into the furnace, then the process  700  can control an oxidant source to begin flowing oxidant into the furnace. From step  704 , the process  700  proceeds to step  706 . 
     At step  706 , the process  700  ignites the pilot fuel and the oxidant to produce a pilot flame. In one embodiment, the process  700  may ignite the pilot fuel and the oxidant by generating an electric arc. In another embodiment, the process  700  may ignite the pilot fuel and the oxidant by generating a gliding arc. In another embodiment, the process  700  may ignite the pilot fuel and the oxidant by dissipating current through a hot surface igniter. In particular, the controller  116  can control the igniter in order to ignite the pilot flame. From step  706 , the process  700  proceeds to decision step  708 . 
     In one embodiment, at decision step  708 , the process  700  determines whether or not the pilot flame is present. If the pilot flame is not present, then the process  700  can revert to step  706  and can attempt again to initiate the pilot flame. If the pilot flame is present at decision step  708 , then the process  700  can proceed from decision step  708  to step  710 . 
     In one embodiment, at step  710 , the process  700  preheats the distal flame holder positioned in the furnace. In particular, the distal flame holder is positioned to receive heat from the pilot flame. The pilot flame heats the distal flame holder, causing the temperature of the distal flame holder to increase. From step  710 , the process  700  proceeds to step  712 . 
     In one embodiment, at step  712  the process  700  measures the temperature of the distal flame holder. From step  712 , the process  700  proceeds to decision step  714 . 
     In one embodiment, at decision step  714 , if the temperature of the distal flame holder T PFH  is less than a threshold or operating temperature T TH , then the process  700  returns to step  710  and continues to preheat the distal flame holder. At decision step  714 , if the temperature of the distal flame holder is greater than the threshold or operating temperature, then the process  700  proceeds to step  716 . Typically, the threshold or operating temperature T TH  is at or above the auto-ignition temperature of the pilot fuel at the conditions of the system (temperature, humidity, atmospheric pressure). The inventors have noted a very slight transient reduction in distal flame holder temperature T PFH  when cold fuel is first introduced to the distal flame holder. The inventors have found it advantageous, therefore, to set the threshold or operating temperature T TH  slightly above the pilot fuel auto-ignition temperature. 
     In one embodiment, at step  716 , the process  700  transitions from the preheating state to the standard operating state. In the standard operating state, the pilot flame may be reduced and a combustion reaction of the main fuel and the oxidant is held by the distal flame holder. In one embodiment, the pilot flame used for preheating may be maintained or increased to supplement the combustion reaction of the main fuel and the oxidant at the distal flame holder. 
       FIG. 8  is a flow diagram of a process  800  for operating a combustion system in a standard operating state, according to an embodiment. The process  800  can be controlled by a controller  116  executing process steps in accordance with one or more algorithms, sets of software instructions, or state machines. The controller  116  can implement the process  800  by utilizing one or more processors to execute instructions stored on a non-transitory computer readable medium. 
     In one embodiment, at step  802 , the process  800  transitions from a preheating state to the standard operating state by opening a main fuel valve. With the main fuel valve open, main fuel is output into a furnace. If an oxidant source is not already supplying oxidant to the furnace, then at step  802  the process  800  can also cause the oxidant source to supply the oxidant into the furnace. The main fuel and the oxidant travel towards the distal flame holder and mix together as they travel toward a distal flame holder. In some embodiments, the distal flame holder includes a perforated flame holder that receives the mixture of the main fuel and the oxidant into perforations or channels of the perforated flame holder. Because the distal flame holder has been heated to the operating temperature or threshold temperature, the distal flame holder ignites a combustion reaction of the main fuel and the oxidant. The distal flame holder holds at least a portion of the combustion reaction adjacent to the distal flame holder. Portions of the distal flame holder can also occur downstream and/or upstream from the distal flame holder. From step  802 , the process  800  proceeds to step  804 . 
     In one embodiment, at step  804 , the process  800  reduces a flow of pilot fuel via a pilot fuel valve, thereby reducing a pilot flame. In some embodiments, the pilot fuel valve can be used to reduce the flow of the pilot fuel prior to opening the main fuel valve. From step  804 , the process  800  proceeds to step  806 . 
     In one embodiment, at step  806 , the process  800  checks measurables or parameters of the combustion system. These measurables can include whether the combustion reaction of the main fuel and the oxidant is present, the location of the combustion reaction of the main fuel and the oxidant, a concentration of various gases in a flue gas, pressure in the furnace, a temperature of a bridgewall of the furnace, parameters of a process receiving heat from the combustion reaction, or other parameters of the combustion system. From step  806 , the process  800  proceeds to decision step  808 . At decision step  808 , the process  800  determines whether the measured conditions of the combustion system are acceptable. If the measured conditions of the combustion system are not acceptable, the process  800  proceeds to step  810 . If the measured conditions of the combustion system are acceptable, the process  800  proceeds to step  812 . 
     In one embodiment, at step  810 , the process  800  takes corrective action to adjust the parameters of the combustion system. The corrective actions can include adjusting the flow of the main fuel, adjusting the flow of the oxidant, adjusting a stack damper, adjusting a mixture of the main fuel and an oxidant, shutting down the combustion system, reversing to the preheating state, or other kinds of corrective actions. From step  810 , the process  800  proceeds to step  806 . 
     In one embodiment, at step  812  the process  800  maintains the present conditions of the distal flame holder and of the combustion system in general. From step  812 , the process  800  can proceed back to step  806  for the measurables to be checked again. Alternatively, if the combustion system has accomplished the desired work, the process  800  can proceed to step  814 . 
     In one embodiment, at step  814  the process  800  shuts down the combustion system. 
       FIG. 9  is a flow diagram of a process  900  for operating a combustion system in a standard operating state, according to an embodiment. The process  900  can be controlled by a controller  116  executing process steps in accordance with one or more algorithms, sets of software instructions, or state machines. The controller  116  can implement the process  900  by utilizing one or more processors to execute instructions stored on a non-transitory computer readable medium. 
     In one embodiment, at step  902 , the process  900  checks measurables of the combustion system. These measurables can include whether a combustion reaction of main fuel and oxidant is present, the location of the combustion reaction of the main fuel and the oxidant, a concentration of various gases in a flue gas, pressure in the furnace, a temperature of a bridgewall of a furnace, parameters of the process  900  receiving heat from the combustion reaction, or other parameters of the combustion system. From step  902 , the process  900  proceeds to decision step  904 . At decision step  904 , the process  900  determines whether the measured conditions of the combustion system are acceptable. If the measured conditions of the combustion system are not acceptable, the process  900  proceeds to one or more of steps  906 ,  908 ,  910 , or  912 . If the measured conditions of the combustion system are acceptable, the process  900  proceeds to step  914 . 
     In one embodiment, at step  906  the process  900  adjusts position of a stack damper responsive to the measured parameters of the combustion system. In one embodiment, at step  908  the process  900  adjusts an oxidant flow responsive to the measured parameters of the combustion system. In one embodiment, at step  910  the process  900  adjusts a main fuel flow responsive to the measured parameters of the combustion system. At step  912 , the process  900  re-transitions to a preheating state, responsive to the measured parameters of the combustion system. 
     In one embodiment, at step  914  the process  900  maintains the present conditions of the distal flame holder and of the combustion system in general. From step  912 , the process  900  can proceed back to step  902  for the measurables to be checked again. Alternatively, if the combustion system has accomplished the desired work, the process  900  can proceed to step  916 . 
     In one embodiment, at step  916  the process  900  shuts down the combustion system. 
     Structures for and methods of using a continuous pilot are described in U.S. Provisional Patent Application No. 62/844,669, entitled “PILOT STABILIZED BURNER,” filed May 7, 2019 (docket number 2651-348-02), incorporated by reference herein. As used herein, the terms continuous pilot and distal pilot may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. 
     In an embodiment, a variable-output pilot burner may be positioned at least 0.62 of the distance from main fuel nozzles to a distal flame holder (the larger portion of the distance being between the main fuel nozzles and variable-output pilot burner). The variable output pilot burner may be driven to output a load corresponding to preheating of the distal flame holder or, alternatively, to output a continuous pilot. The inventors have found that by maintaining a continuous pilot flame adjacent to and below (upstream or subjacent) the distal flame holder, a transition step wherein a flame location is shifted between two discrete, different positions may be eliminated. In addition to, or advantageously instead of, the transition step, the continuous pilot is configured to hold a pilot flame according to a plurality of output loads. In an example system, the output loads principally used were two—either stable pilot flame or high output preheat flame where the temperature of the distal flame holder is raised to a main fuel operating temperature over a specified duration. The inventors contemplate that pluralities greater than two output levels may be used to maintain, for example, a very low, flame stability limited operation, a throttled system heat output mode (which in an embodiment may result in elimination of a second cold climate “HVAC” subsystem), a routine and minimum fuel pressure drop pre-heat mode, a demand pre-heat mode, and/or an emergent demand pre-heat mode. System damage recovery modes may one day prove advantageous. The inventors contemplate that a relatively high turndown ratio of the continuous pilot may be obtained by disposing a perforated or porous tile (pilot tile) superjacent to (i.e., downstream from) a plurality of 1 atm fuel nozzles, a low output pilot flame may be stabilized to minimize variable pilot stable heat output. In an embodiment, the system, at moderate to high output, supports low output stable pilot operation to cause greater than 98% of CO 2  generation is provided by main fuel nozzle during a normal operating mode. This mode may help reduce NOx production during normal operation compared to a higher ratio of pilot burner output to main fuel output. 
     The distal flame holder may include plural porous and/or solid bodies (tiles) with spaces therebetween. 
       FIGS. 10A-10C  are diagrams of a combustion system  1000  in different states. Descriptions of elements described above having the same reference numbers as in the description below may be incorporated wholly or in various combinations by reference thereto.  FIG. 10A  is a diagram of the combustion system  1000  in a non-operating state, according to an embodiment. The combustion system  1000  includes a furnace  1071  defining a furnace volume  1073 . The combustion system  1000  includes a distal flame holder  102  positioned within the furnace volume  1073 . The combustion system  1000  includes one or more main fuel distributors  110 , a pilot fuel distributor  106 , an igniter  1077 , a pilot flame sensor  124 , and a distal flame holder sensor  122  positioned within the furnace volume  1073 . The combustion system  1000  includes an oxidant source  104 , a controller  116 , actuators  118 , a display  120 , a control input  670 , manual controls  123 , a main fuel source  112 , and a pilot fuel source  108 . The combustion system  1000  includes one or more main fuel valves  1074  controlling a flow of main fuel from the main fuel source  112  to the main fuel distributors  110 . The combustion system  1000  includes one or more pilot fuel valves  1076  controlling a flow of pilot fuel from the pilot fuel source  108  to the pilot fuel distributor  106 . The combustion system  1000  includes a stack damper  1084  positioned in a flue of the furnace  1071 . The combustion system  1000  further includes a bridgewall temperature sensor  640  and a gas composition sensor  1072 . 
     In one embodiment, the controller  116  may receive sensor signals from the pilot flame sensor  124 , the distal flame holder sensor  122 , the bridgewall temperature sensor  640 , the gas composition sensor  1072 , and/or the pressure sensor  654 . The controller  116  is coupled to the actuators  118 . The various actuators  118  are capable of physically adjusting the main fuel valves  1074 , the pilot fuel valves  1076 , the oxidant source  104 , the main fuel distributors  110 , the pilot fuel distributor  106 , and the stack damper  1084 . In one embodiment, the controller  116  is configured to control the actuators  118  to adjust various parameters of the combustion system  1000 . 
     In one embodiment, the controller  116  is configured to output messages, sensor readings, prompts, warnings, alerts, or other types of data on the display  120 . An operator of the combustion system  1000  can view the data output on the display  120  and can operate the combustion system  1000  responsive to the data output on the display  120 . 
     In one embodiment, the operator of the combustion system  1000  can utilize the manual controls  123  to operate the components of the combustion system  1000 . The manual controls  123  can control the actuators  118  to adjust the parameters of the combustion system  1000 . Alternatively, or additionally, the manual controls  123  can enable the operator to physically adjust the components of the combustion system  1000  separate from the actuators  118 . 
     In one embodiment, the control input  670  may enable an operator of the combustion system  1000  to input commands or data to the controller  116 . In one embodiment, the controller  116  can output requests for the operator to approve one or more actions proposed by the controller  116  responsive to sensor signals provided by the various sensors. The operator of the combustion system  1000  can input selections or commands approving or disapproving the proposed actions of the controller  116  via the other control inputs  670 . 
       FIG. 10B  is a diagram of the combustion system  1000  of  FIG. 10A  in the preheating state, according to an embodiment. In the preheating state, the combustion system  1000  generates a pilot flame  1075  to preheat the distal flame holder  102  to an operating temperature. When the distal flame holder  102  has been heated to the operating temperature, the combustion system  1000  can transition to the standard operating state. 
     In one embodiment, in the preheating state the controller  116  controls one or more of the actuators  118  to open the pilot fuel valves  1076 . With the pilot fuel valves  1076  open, the pilot fuel source  108  supplies the pilot fuel to the pilot fuel distributor  106 . The pilot fuel distributor  106  outputs the pilot fuel into the furnace volume  1073 . In one embodiment, the pilot fuel distributor  106  includes one or more pilot fuel nozzles each coupled onto the end of a pilot fuel riser. The pilot fuel is output from orifices in the fuel nozzles. 
     In one embodiment, in the preheating state the controller  116  controls one or more of the actuators  118  to cause the oxidant source  104  to supply oxidant into the furnace volume  1073 . The oxidant source  104  supplies the oxidant into the furnace volume  1073 . The oxidant mixes with the pilot fuel in the furnace volume  1073 . 
     In one embodiment, the oxidant source  104  includes a barrel register. The barrel register includes apertures that can be opened to a selected degree in order to draft the oxidant into the furnace volume  1073 . The actuators  118  can control the degree to which the apertures are open, and thus the degree to which the oxidant is drafted into the furnace volume  1073 . 
     In one embodiment, the controller  116  controls one or more of the actuators  118  to cause the igniter  1077  to ignite the pilot fuel and the oxidant to produce a pilot flame  1075 . The controller  116  can cause the igniter  1077  to generate an electric arc capable of igniting the pilot flame  1075  in the presence of the mixed pilot fuel and the oxidant. The electric arc can cause ignition of the pilot fuel and the oxidant, thereby initiating the pilot flame  1075 . 
     In one embodiment, the pilot flame sensor  124  (see  FIG. 10A ) monitors the parameters of the pilot flame  1075  and provides sensor signals to the controller  116  indicating the sensed parameters of the pilot flame  1075 . The pilot flame sensor  124  can sense whether the pilot flame  1075  is present. The pilot flame sensor  124  can also sense the position of the pilot flame  1075 . The pilot flame sensor  124  can also sense the temperature of the pilot flame  1075 . The pilot flame sensor  124  outputs the sensor signals to the controller  116  indicative of the parameters of the pilot flame  1075 . 
     In one embodiment, the controller  116  can adjust the parameters of the pilot flame  1075  responsive to the sensor signals provided by the pilot flame sensor  124 . For example, if the pilot flame sensor  124  signals indicate that the pilot flame  1075  is not present, then the controller  116  can control one or more of the actuators  118  to generate additional electric arcs from the igniter  1077 , to adjust the distribution of the pilot fuel into the furnace volume  1073 , or to adjust the flow of the oxidant into the furnace volume  1073 . The controller  116  can also control the flow of the pilot fuel and the oxidant in order to adjust the position of the pilot flame  1075  responsive to the sensor signals from the pilot flame sensor  124 . 
     In one embodiment, the distal flame holder sensor  122  measures the temperature of the distal flame holder  102  during the preheating state and provides the sensor signals to the controller  116  indicating the temperature of the distal flame holder  102 . If the sensor signals from the distal flame holder sensor  122  indicates that the temperature of the distal flame holder  102  is below an operating or threshold temperature, then the controller  116  allows the pilot flame  1075  to continue to heat the distal flame holder  102 . If the sensor signals from the distal flame holder sensor  122  indicate that the temperature of the distal flame holder  102  is equal to or greater than the operating or threshold temperature, then the controller  116  can cause the combustion system  1000  to transition to the standard operating state. 
     In one embodiment, an operator of the combustion system  1000  can activate, operate, or adjust the various components of the combustion system  1000  during the preheating state by operating the manual controls  123 . The operator can adjust the parameters of the combustion system  1000  responsive to messages provided by the controller  116  via the display  120 . 
       FIG. 10C  is a diagram of the combustion system  1000  in the standard operating state, according to an embodiment. In the standard operating state, the combustion system  1000  sustains a combustion reaction  1086  of at least the main fuel and the oxidant at the distal flame holder  102 . 
     In one embodiment, the combustion system  1000  transitions to the standard operating state by first reducing a flow of the pilot fuel supplying the pilot flame  1075 . The controller  116  reduces the pilot flame  1075  by causing one or more of the actuators  118  to reduce flow of the pilot fuel to the pilot flame  1075  via the pilot fuel valves  1076 , thereby ceasing the flow of the pilot fuel to the pilot fuel distributor(s)  106 . When the pilot fuel distributor(s)  106  cease to output the pilot fuel, the pilot flame  1075  is reduced from a preheating size to a maintenance size. 
     In an embodiment, the controller  116  causes the combustion system  1000  to enter the standard operating state by causing one or more of the actuators  118  to open the main fuel valves  1074 , thereby enabling the main fuel to flow from the main fuel source  112  to the main fuel distributors  110 . The main fuel distributors  110  output the main fuel toward the distal flame holder  102 . The controller  116  can also cause the oxidant source  104  to output the oxidant into the furnace volume  1073 , if the oxidant source  104  is not already outputting the oxidant into the furnace volume  1073 . The main fuel entrains and mixes with the oxidant as it travels toward the distal flame holder  102 . Because the distal flame holder  102  is at the operating temperature, the distal flame holder  102  ignites and sustains the combustion reaction  1086  of the mixture  206  of the main fuel and the oxidant. In one embodiment, the distal flame holder  102  holds a portion of the combustion reaction  1086  adjacent to the distal flame holder  102 . In an embodiment in which the distal flame holder  102  includes a perforated flame holder body (e.g.,  208 ), the distal flame holder  102  can sustain at least a portion of the combustion reaction  1086  within the perforated flame holder body. The distal flame holder  102  may also sustain a portion of the combustion reaction  1086  upstream and/or downstream from the distal flame holder  102 . 
     In one embodiment, in the standard operating state, the distal flame holder sensor  122 , the pressure sensor  654 , the bridgewall temperature sensor  640 , and the gas composition sensor  1072  output sensor signals to the controller  116 . The distal flame holder sensor  122  monitors parameters of the combustion reaction  1086 , including the position, distribution, and temperature of the combustion reaction  1086 . The bridgewall temperature sensor  640  senses the temperature of the bridgewall of the furnace  1071  and the pressure sensor  654  senses the pressure within the furnace volume  1073 . The gas composition sensor  1072  senses the concentration of various gases, such as NO X , CO, and O 2 , in the flue gases  1082  and exit through the flue of the furnace  1071 . 
     In one embodiment, the controller  116  can cause the actuators  118  to adjust the flow of the main fuel, the flow of the oxidant, the orientation of the stack damper  1084 , and other components of the combustion system  1000  in order to adjust the parameters of the combustion system  1000 . The controller  116  can control the flow of the oxidant and the main fuel, as well as a position of the stack damper  1084  to adjust the concentration of gases in the flue gas  1082 , to adjust the location and distribution of the combustion reaction  1086 , to adjust the pressure within the furnace volume  1073 , or to adjust other parameters of the combustion system  1000 . 
       FIG. 11  is a diagram of a combustion system  1100 , according to an embodiment. The combustion system  1100  is substantially similar to the combustion system  100  of  FIG. 1 , except that the sensor array  114  of the combustion system  1100  may include a flashback sensor  1123 . The inventors have found that positioning a variable pilot (e.g., pilot fuel distributor  106 ) between the main fuel nozzles  110  and the distal flame holder  102  may reduce the incidence of flashback. The inventors have successfully run such systems without flashback sensors. Accordingly, use of a variable pilot may obviate the need for a flashback sensor  1123 . Nevertheless, some embodiments may employ the flashback sensor  1123  as follows. 
     In one embodiment, the flashback sensor  1123  is configured to sense flashback of the combustion reaction held by the distal flame holder  102  toward the main fuel distributor  110  during the standard operating state. Flashback is a potentially dangerous condition in which the combustion reaction travels upstream, igniting the fuel stream closer than desired to the main fuel distributor  110 . The flashback sensor  1123  senses the flashback and transmits sensor signals to the controller  116  indicating the presence of the flashback. The controller  116  can then take one or more actions to stop the flashback condition. 
     In one embodiment, the controller  116  stops the flashback condition by increasing a velocity of the flow of the main fuel from the main fuel distributor  110 . The increased velocity of the flow of the main fuel inhibits the combustion reaction from traveling upstream because the fuel travels faster than the combustion reaction can travel upstream. The controller  116  can operate one or more of the actuators  118  to adjust the flow of the main fuel from the main fuel distributor  110  responsive to the sensor signals from the flashback sensor  1123 . Alternatively, the controller  116  can output an indication on the display  120  prompting the operator to manually adjust the flow of the main fuel to inhibit the flashback. 
     In one embodiment, the controller  116  stops the flashback condition by stopping the flow of the main fuel, thereby bringing the combustion system  1100  out of the standard operating state. The controller  116  can operate one or more of the actuators  118  to stop the flow of the main fuel from the main fuel distributor  110  responsive to the sensor signals from the flashback sensor  1123 . Alternatively, the controller  116  can output an indication on the display  120  prompting the operator to manually stop the flow of the main fuel to inhibit the flashback. The controller  116  can shut down the combustion system  1100  entirely when the flashback occurs. 
     In one embodiment, the controller  116  can take other actions than those described above in order to deal with the flashback condition. 
     In one embodiment, the flashback sensor  1123  senses the flashback during the preheating state of the combustion system  1100 . In particular, in the preheating state the flashback sensor  1123  detects a flashback of the pilot flame  1075  toward the pilot fuel distributor  106 . The controller  116  can respond to the flashback condition in the preheating state by increasing the flow of the pilot fuel, by stopping the flow of the pilot fuel, or in any other suitable manner. 
     In one embodiment, the flashback sensor  1123  is positioned to sense flashback between the input face  212  of the distal flame holder  102  and the main fuel distributor  110 . Thus, in a vertically fired combustion system  1100 , the flashback sensor  1123  can have a vertical position between the distal flame holder  102  and the main fuel distributor  110 . In a laterally fired combustion system  1100 , the flashback sensor  1123  can have a lateral position between the distal flame holder  102  and the main fuel distributor  110 . 
     In one embodiment, the flashback sensor  1123  can include one or more of a camera, an infrared sensor, a flame rod, a UV sensor, a CCD camera, thermocouples, photo cells, electrodes, or other kinds of devices capable of sensing flashback. 
       FIG. 12  is a flow chart showing a computer method  1200  for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus, according to an embodiment. Computer method  1200  corresponds to a preheating procedure that prepares a distal flame holder (e.g.,  102 ) to carry sufficient heat to ignite the main fuel and oxidant flowing thereto. 
     According to an embodiment, a computer method  1200  for operating a burner having at least one distal flame holder and at least one continuous pilot apparatus includes, in step  1202 , receiving a heat demand datum via a hardware digital interface operatively coupled to a network. Step  1204  includes comparing, using a logic device, the heat demand datum with previously received heat demand data stored in a computer-readable non-transitory memory. Step  1206  includes determining, with the logic device and the computer-readable non-transitory memory, as a function of the heat demand datum, a heating setting from among a plurality of heating settings of the burner system. Step  1208  includes, responsive to an increase in the heat demand datum compared to previously received heat demand data, driving the burner system to place the continuous pilot apparatus into a high heat output setting, of the plurality of heating settings, for a preheat duration sufficient to raise the distal flame holder to a temperature corresponding to a normal main fuel operating state. 
     According to an embodiment, in step  1206 , the plurality of heating settings of the burner system includes one or more positions corresponding to each of a plurality of fuel flow control valves, a first fuel flow control valve of the plurality of fuel flow control valves being operatively coupled to the continuous pilot apparatus, and a second fuel flow control valve of the plurality of fuel flow control valves being operatively coupled to the one or more main fuel nozzles. 
     According to an embodiment, in step  1206 , the plurality of heating settings of the burner system includes a plurality of positions corresponding to each of the plurality of fuel flow control valves, a first subset of the plurality of fuel flow control valves being operatively coupled to the continuous pilot apparatus, and a second subset of the plurality of fuel flow control valves each being operatively coupled to a respective main fuel nozzle of the one or more main fuel nozzles. 
     According to an embodiment, the computer method  1200  for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus further includes (not illustrated) receiving sensor data substantially determinate that the distal flame holder has reached the temperature corresponding to the normal main fuel operating state. The determination that the distal flame holder has reached the temperature corresponding to the normal main fuel operating state is performed by the logic device and the non-transitory computer memory as a function of the received sensor data. In an alternative, or additional embodiment, the computer method  1200  for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus further includes (not illustrated) receiving a preheat time clock datum corresponding to expiration of the preheat duration. 
       FIG. 13  is a flow chart showing a computer method  1300  for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus, according to an embodiment. Step  1302  includes changing the heating setting to a normal main fuel operating setting by ramping down the at least one continuous pilot apparatus heat output while ramping up a main fuel flow through one or more main fuel nozzles aligned to output a main fuel for entrainment in combustion air, and for entrance to an input face of at least one tile of the distal flame holder. 
     Step  1304  includes determining, with the logic device and the computer-readable non-transitory memory, as a function of the preheat time clock datum, that the distal flame holder has reached the temperature corresponding to the normal main fuel operating state. 
     In an embodiment, the heat demand datum corresponds to a capacity requirement proportional to completely burning a fuel at a given flow rate of the fuel. In another embodiment, the fuel is the main fuel output through the main fuel nozzles. 
     According to an embodiment, the computer method  1300  for operating a burner system having at least one distal flame holder and at least one continuous pilot apparatus further includes, in step  1306 , responsive to a second received heat demand datum compared to previously received heat demand data, driving the burner system to place one or more main nozzles into a reduced heat output setting, of the plurality of heating settings, by driving a plurality of fuel control valves to ramp down the main fuel flow while ramping up a pilot fuel flow to the continuous pilot apparatus. The comparison of the second received heat demand datum to the previously received heat demand data may be performed with the logic device and the non-transitory computer memory. 
     In an embodiment, the normal main fuel operating setting includes a ratio of pilot fuel flow to main fuel flow corresponding to a particular heat demand datum. The ratio of pilot fuel flow to main fuel flow corresponding to the particular heat demand datum may be a function of previous heat demand data. 
     Those of skill in the art will recognize, in light of the present disclosure, that the combustion system in accordance with principles of the present disclosure can include sensors and actuators other than those disclosed herein, other combinations of sensors and actuators, as well as other kinds of actions to be taken by the controller  116  responsive to sensor signals. All such other sensors, actuators, combinations, and actions fall within the scope of the present disclosure. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.