embodiments of the invention describe a novel burner - mixer apparatus which burns an ultra - lean premixed fuel - air mixture with a stable flame that operates without a mechanical fan or blower to flow air through the burner apparatus . one embodiment of the invention utilizes fuel pressure to induce air flow though the burner assembly and achieve good mixing of the air and fuel prior to burning ( hereafter referred to as a natural draft burner ). an embodiment of the invention also establishes a weak swirl , or low swirl , on the fuel - air flow . the exit flow has a swirl number between about 0 . 01 and 3 . 0 . the fuel pressure supplies the energy to create a well - mixed flow of air , fuel , and optionally , a diluent , to exit the system with adequate velocity and the exit flow has rotation in a plane normal to the axial flow . the flame burning in the exit region performs in the manner of a low swirl flame described in u . s . pat . nos . 5 , 735 , 681 and 5 , 879 , 148 incorporated herein by reference as if fully set forth in their entirety . the low swirl burner ( lsb ) is adaptable for a number of applications , including industrial heating , boilers , and gas turbines . in one embodiment , the low swirl concept has been adapted to natural draft operation , in which the fuel pressure , instead of an electrically powered fan or blower , induces air flow through the burner . natural draft burners are used at petroleum refineries and other sites where flammable materials are processed to avoid the hazard of electric spark generation . refineries currently use commercially available natural draft burners in their process heaters , but are under pressure by regulatory agencies to reduce the emissions from their facilities , particularly for nox . the natural draft burners currently available cannot satisfy upcoming emissions limits , so refineries may be faced with installation of post - combustion emission control strategies such as selective catalytic reduction ( scr ). while scr systems are capable of lowering nox emissions , they have substantial installation and operating costs . the systems require careful monitoring to avoid release of ammonia , which is used in the nox control chemistry . the low swirl burner design offers ultra - low emissions as well as good fuel flexibility and turndown . the low swirl design has been adapted to operate in a natural draft configuration . in embodiments of the invention , the burner and mixer components have been sized and oriented appropriately to provide good mixing , suitable air - fuel ratio , adequate lsb exit velocity , and a low swirl flow pattern . suitable lsb geometries were developed and testing was conducted . emissions measurements demonstrated that the natural draft lsb had emissions that were equivalent to the standard lsb design that has been commercialized . the petroleum industry anticipates that they will need a burner design that can provide less than 10 ppm nox with 3 % residual oxygen in the exhaust . in premixed burner systems , nox production increases with flame temperature and the fuel - air ratio . at fuel - air ratios that produce 3 % oxygen in the exhaust , the flame temperature is sufficiently high such that more than 10 ppm nox is produced . to avoid excessive nox production , a diluent can be added to the fuel - air mix to reduce nox production while maintaining low excess oxygen in the exhaust . flue gas recirculation ( fgr ) is an example of an effective nox control strategy using diluent addition . flue gas recirculation was incorporated into various embodiments of the natural draft burner design by adding a flow path for the burner flue gas to the lsb mixer inlet . by adjusting the effective areas for air and flue gas into the lsb inlet , up to 30 % flue gas recirculation may be achieved . the addition of fgr to the lsb design provided significant nox reduction , and a range of conditions were identified that can satisfy the requirement of less than 10 ppm nox at 3 % oxygen in the exhaust . in one embodiment , a natural draft low swirl burner has been developed that is capable of achieving the nox emissions requirements for refineries . additional embodiments address scale - up , performance optimization , fuel switching , and insuring stable operation at the full range of environmental conditions . one application of an embodiment of the invention is for process heating in petroleum refineries . for safety reasons , refineries use natural draft burners in their petroleum refining operations . the existing burners are capable of switching between operation with natural gas and with hydrogen - containing refinery gas , but their nox emissions exceed the levels necessary to satisfy upcoming limits that are being implemented by air pollution control districts , particularly in california . the only currently - available method for refineries to satisfy the emissions limits promulgated by air quality management districts in california is to install post - combustion control systems such as selective catalytic reduction ( scr ). scr and similar technologies have substantial installation , maintenance , and operating costs . the scr system must be monitored carefully to avoid inadvertent release of ammonia . refineries would greatly prefer to install burners in their process heaters that would allow them to satisfy the new emissions requirements . if the natural draft adaptation of the low swirl burner is commercialized and available for installation at refineries , it will allow them to process petroleum at a lower cost , and provide lower cost products to consumers than the alternative of post - combustion emission control . embodiments of the invention can be scaled up to a much larger heat output , and the components will be optimized for the operating conditions found at refineries . embodiments of the invention have been tested to insure that the design has adequate durability , reliability , and sufficient margin of safety over the entire range of potential operating conditions . besides refinery process heating , there are a number of other applications for the natural draft low swirl burner design . one embodiment of the invention has demonstrated that the natural draft lsb operates well with vitiated ( reduced oxygen content ) air . consequently , embodiments of the invention have significant potential for use in duct burner applications . duct burners are used to heat air and exhaust streams for heat recovery steam generators , combined heat and power systems , scr reheating , and similar applications . the natural draft low swirl burner design can also be applied to devices that currently use natural draft burners , either alone or with induced draft exhaust . potential applications include boilers , water heaters , and residential and commercial furnaces . burners for some of these appliances are relatively close in heat output to embodiments of the invention and therefore should not be difficult to adapt the design to these systems . an embodiment of the low swirl burner ( lsb ) design is suitable for natural draft operation with 30 psig gaseous fuel . the emissions , turndown , and flame stability of the lsb have been assessed with natural gas and hydrogen - methane blends . the emissions have been compared with predicted emissions levels for premixed flames . flue gas recirculation and fuel staging , techniques for emissions reduction at low excess air , have been incorporated into the natural draft low swirl burner to demonstrate the capability of achieving significant emissions reductions at low excess air levels . demonstration of natural draft operation with a small scale lsb with at least 3 : 1 turndown using the following fuels as discussed above , the low swirl burner is an innovative burner design for premixed flames that utilizes a unique flame stabilization mechanism ( littlejohn and cheng , 2007 ). the design has been commercialized for process heating by maxon / honeywell and is under development for gas turbines , as well as adaptation for residential and commercial appliances . referring to fig1 , burner apparatus 100 includes a venturi system 102 attached to the inlet of a 2 inch ( 5 cm ) diameter low swirl burner 104 . the venturi 102 uses a 0 . 043 inch ( 0 . 11 cm ) diameter fuel jet 106 to entrain air into the burner 100 . larger fuel jets were found to have insufficient air entrainment for good flame stability with 30 psig methane . the venturi 102 has air inlets 108 on the side and bottom . the air inlets 108 can be partially blocked to decrease the air / fuel ratio . the burner system 100 is also shown schematically in fig2 a . methane and hydrogen are supplied by standard gas cylinders at 30 psig , and fuel flows are measured with rotameters and / or mass flow meters . emissions measurements were performed by enclosing the flame with a quartz cylinder ( not shown ) to prevent dilution of the exhaust gas with outside air . a continuous gas sample was collected at the top of the quartz cylinder , cooled and dried and flowed through a horiba pg250 multi - gas analyzer . the analyzer measures nox , co , co 2 and o 2 . the analyzer was calibrated daily with calibration gases . for measurements of turndown capability , methane flow to the venturi orifice was gradually increased until a stable flame could be maintained . emission measurements were recorded while the methane flow was ramped up to the maximum flow possible with a 30 psig source . the data are compiled in table 1 below . the fuel flow and the measured oxygen concentration in the exhaust were used to calculate the bulk flow velocity through the low swirl burner . the values of nox in lb / mmbtu were calculated using the following equation : this calculation is based on molar volumes at 0 ° c . ( 32 ° f . ), and gives slightly higher numbers than some of the equations used in regulatory documents that use 60 ° f . ( 15 . 6 ° c .) as the reference temperature . fig4 illustrates the burner 100 heat output ( diamonds ) and nox output ( circles ) as a function of exit velocity . even in a non - optimized system , the lsb demonstrates over 4 : 1 turndown capability . the nox emissions readings were multiplied by 10 to make the values comparable to the heat output . similar behavior is observed when methane / hydrogen blends are used instead of methane alone . some measurements were obtained with the air inlet partially blocked so the system operated at lower excess air levels . as can be seen in fig4 , nox emissions increase more rapidly than the fuel flow to the burner . this is a result of less air entrainment at higher fuel flows due to the increasing backpressure of the burner system 100 . as the air / fuel ratio of the burner decreases , the flame temperature , and nox production , increase . at high flow rates , the flow through the orifice transitions to choked flow . four times the volume of hydrogen is needed to consume the oxygen that is consumed by burning a given volume of methane , and to obtain a fixed level of excess air , the volume of fuel increases as the fraction of hydrogen in the fuel increases . the diameter of the fuel injection 106 orifice was fixed for these initial studies , and it was difficult to obtain low excess air levels with 42 % hydrogen - 58 % methane fuel blends since the higher volumetric flow created choked flow conditions at the orifice . as part of the burner optimization , a larger orifice may be used to assess lsb performance at low excess air and high hydrogen fuel content . the horiba analyzer uses a ndir ( non - dispersive infra - red ) detection system for carbon monoxide . this type of detector is subject to interference from other gases in the exhaust system , which can result in negative signals when the co concentration is low . a bendix model 8501 ndir co analyzer was used to confirm this observation . a report by jernigan et al ( 2002 ) from thermo environmental instruments discusses potential interference in ndir co systems in more detail . the flame stabilization mechanism of the low swirl burner 100 limits the minimum velocity at which the burner can operate . the flame stabilization is a result of the gas velocity downramp that occurs as the gases flow out of the burner 100 . the flame front settles to the location where the air / fuel velocity matches the flame speed . if the velocity out of the burner is too low , the flame will propagate back into the burner and attach to the swirler body 104 . the swirler 104 will then act as a mechanical flame holder , similar to conventional burner designs . this is shown graphically in fig5 which illustrates flame position versus bulk velocity out of burner . u o represents the bulk velocity out of burner , and the flame position is the height above the burner exit . s l represents laminar flame speed . flame speed is dependent on the composition of the fuel . for example , hydrogen has a higher flame speed than methane . as shown in the fig5 , a flame with a higher flame speed will propagate into the burner at a higher exit velocity than a flame with a lower flame speed . this information may be utilized for designing a low swirl burner system that will have stable operation with all fuel types and fuel flows that the system will experience . burner manufacturers prefer to build in a safety margin so the burners can tolerate off - normal conditions . the performance of the lsb system used for these measurements has not been fully optimized , and significant performance gains are likely by reducing the burner backpressure and matching the fuel mixing venturi to the lsb swirler . a computational fluid dynamics ( cfd ) analysis of the venturi - swirler assembly will assist in defining the important system parameters . turndown studies were also done with approximately 20 % hydrogen and 40 % hydrogen in the fuel . the fuel measurement and control systems were operating near their lower limits at times , and it was difficult to obtain precise values of hydrogen content . the results are also tabulated in table 1 . the performance with fuel blends was found to be similar to methane - only flames shown in fig4 . obtaining baseline nox emissions from the same burner in forced draft operation emissions measurements from the low swirl burner under forced draft conditions provide a good reference for understanding the performance of the burner under natural draft operation . the venturi 100 was removed from the lsb 100 ( see for example fig2 b and fig3 a ), and the burner was attached to a flow system that supplies well - mixed air and fuel . well - defined flows were established by feeding house compressed air through a turbine meter and methane through a mass flow controller . when hydrogen was included in the fuel blend , it was flowed through a mass flowmeter and flow was controlled with a needle valve . the combined flow was fed into the lsb , and the same arrangement was used to measure the burner emissions . the results are shown in fig6 , nox data correlated well with a well - regarded study on nox production in premixed combustion systems from leonard and stegmaier ( 1994 ). the data is also compiled in table 2 below . as can be seen from the fig6 , the nox emissions data measured here agree well with the data from the leonard - stegmaier study . for fuels without significant fuel - bound nitrogen , nox production in flames is closely correlated with flame temperature , and flame temperature increases as excess air decreases . thermal nox production ( zeldovich mechanism ) is the primary production process in flames with relatively low excess air . the most straightforward method to reduce nox in combustion exhaust gases is to operate with sufficient excess air to bring nox within the necessary limits . when operating with excess air is not practical ( for example a higher temperature or higher efficiency is needed ), techniques such as flue gas recirculation ( fgr ) ( see fig3 a and 3 b ) or fuel staging can be used to reduce emissions . these techniques are explored for use with the natural draft low swirl burner . the emissions observed with the forced draft and natural draft lsb testing can be compared to assess the effectiveness of the venturi in mixing air and fuel . if the air and fuel are not well mixed , there will be fuel - rich regions in the flame zone that will burn hotter than the bulk mixture and generate excess nox . similarly , fuel - poor regions will burn cooler and can incompletely burn the fuel , potentially leading to high levels of co , formaldehyde , and unburned hydrocarbons . the nox emissions of the low swirl burner under forced draft operation and under natural draft operation are shown in fig7 . within the accuracy of the measurements , the nox emissions of the two operating modes are the same . this indicates that the venturi is producing a fairly uniform mixture of fuel and air , in spite of relatively simple fabrication from standard pipe fittings . the mixedness of the flow out of the venturi can be assessed by conducting a series of measurements of the fuel concentration to determine the variation in concentration . however , since the mixing is satisfactory , this is not necessary . another embodiment seeks to improve the performance of the natural draft burner by incorporating a commercially available venturi and testing swirlers with lower backpressures . commercial venturis have been optimized for operation with natural draft burners . swirler designs that have been adapted to reduce backpressure should allow operation at higher bulk velocities , which should expand the range of stable operation . this embodiment expands the operating range of the natural draft low swirl burner . also , the improved lsb system is utilized for the studies on flue gas recirculation and fuel staging described below . optimization of natural draft lsb to improve performance . vary injection and premixing configuration to optimize natural draft burner to achieve the same emissions as forced draft in the preceding , the low swirl burner under natural draft operation demonstrated emissions that were comparable to those obtained under forced draft operation . the burner was close to the flashback limit at the lowest range of exit velocities studied . to allow for at least 3 : 1 turndown while maintaining a good safety margin to avoid flashback , it is desirable to operate at higher velocities than those described above . there is a limited amount of energy in the 30 psig fuel to entrain air and push it through the burner assembly , so an efficient venturi premixer and a low backpressure lsb are needed to optimize system performance . a lower backpressure swirler for the lsb has been designed and fabricated . the new swirler has fewer vanes with a more aerodynamic profile to provide less backpressure . an illustration of the lower backpressure swirler is shown in fig8 . the original swirler described above has 8 overlapping vanes and significantly more backpressure . with the existing venturi used earlier , the new swirler allows operation at & gt ; 6 m / s exit velocity , whereas the swirler used previously was limited to & lt ; 5 m / s . the new design offers at least 20 % increase in flow rate , and there is potential for additional improvement through a cfd ( computational fluid dynamics ) optimization of the flow system components . fig9 illustrates that for a given fuel flow ( heat output ), the new swirler provides higher air entrainment and higher bulk velocity using the same venturi and fuel injection orifice . a screen or other flow restriction is used in the center section of the swirler to obtain the proper flow split between the non - swirling core flow and the swirling annular flow . an equation has been developed to define a swirl number based on the flows and geometry of the lsb ( johnson et al , 2005 ). a swirl number of ˜ 0 . 5 has found to work well for most applications . several center screens were tested with the new swirler . screens with little blockage reduce the lsb backpressure but can result in unstable flames . a center screen with 32 % open area was found to provide good flame stability and relatively low backpressure . the screen can be replaced with a tapered central body to reduce the potential for flame attachment to the swirler . in fig1 , the dry nox emissions of the lsb with the new swirler are compared with the emissions of the lsb in the original configuration . the new design shows the same emissions behavior as the original lsb system operated in natural draft mode . these emissions are consistent with the values expected from burning well - mixed fuel and air . data from the tests of the new swirler with methane and methane - hydrogen blends are compiled in table 3 below . air entrainment in venturis has been studied for many years ( von elbe and grumer , 1948 ) and commercial venturi designs incorporate the guidelines developed from these studies . a commercial burner venturi was acquired and its performance with the low swirl burner is reported below . the optimum fuel injection orifice for the venturi will provide good air entrainment , good air / fuel mixing , and accommodate a range of fuel compositions . a parametric study of fuel injection orifice sizes with the existing venturi was performed . fuel injection orifices with holes equivalent to # 57 , # 53 , and # 52 drill sizes were tested . for a given fuel flow rate , the smallest orifice generated the highest burner exit velocity . however , the larger orifices allow operation at higher heat outputs at a given fuel supply pressure . an excessively small orifice will limit the fuel flow to unacceptably low values , while an excessively large orifice may not create adequate fuel / air mixing . fuel injectors with orifices in the range of # 57 to # 55 drill size provide the best performance in the current venturi - lsb configuration . in fuel blends , the fuel heat content per unit volume varies . for example , natural gas has over three times the energy content of hydrogen on a volumetric basis . therefore , to maintain steady heat input into a burner , it is necessary to modulate the fuel flow in conjunction with the hydrogen content of the fuel . the fuel injection orifice must be capable of delivering a range of fuel flow rates in response to fuel composition and heating needs while supplying well - mixed air and fuel to the burner . following simulated flue gas recirculation studies , the system was adapted to utilize actual flue gas in the recirculation system . the goal is to reduce nox emissions at low excess air conditions . in the previous example , the emissions from the natural draft low swirl burner were assessed as a function of excess air . the nox emissions increased as excess air was reduced , as would be expected from the increasing flame temperatures associated with the richer stoichiometries . flue gas recirculation ( fgr ) is often used in industrial combustion systems to improve emissions ( baltasar et al , 1997 ). fgr does not alter the oxygen concentration in the exhaust gas since it does not alter the air - fuel ratio in the flame zone . the effect of simulated flue gas recirculation on lsb emissions was studied . to simulate fgr , a measured flow of nitrogen was fed into the venturi inlet feeding the low swirl burner . see fig3 a and 3 b . the nitrogen , along with some of the air flowing into the inlet , creates a blend with an oxygen concentration that matches the oxygen concentration in the exhaust gas . this is a simplification of a combustion system using fgr , since flue gas also contains carbon dioxide and water vapor . however , it can provide an indication of how fgr affects burner emissions . measurements were made on the burner with simulated fgr flows ranging from zero to & gt ; 40 % fgr . only methane was used as a fuel for this series of measurements . the simulated dry nox emissions , corrected to 3 % oxygen , are plotted against excess air in fig1 . the data are also tabulated in table 4 below . some of the data in the fig1 for the “ no fgr ” case are from measurements reported above . the addition of fgr shifts the nox vs . excess air curve to the left , so that nox at a given excess air value decreases as fgr increases . this demonstrates that with sufficient fgr , single digit nox levels can be obtained at low excess air . while the data obtained with simulated flue gas recirculation indicate that it is possible to operate a natural draft lsb with single digit nox emissions at low excess air , the use of actual flue gas in the experiments provides a more compelling case for the benefits of fgr . when fgr is used , it is desirable to avoid cooling the flue gas to maintain high system efficiency . feeding hot flue gas into the air - fuel blend will raise the flame temperature , which , in turn , can raise nox levels . demonstration of nox reduction with actual flue gas recirculation makes a strong case for use of fgr to control nox . to explore the impact of actual fgr on lsb emissions , four 0 . 5 inch diameter tubes were placed around the burner between the flame enclosure and the venturi inlet ( see fig3 a ). the negative pressure created by the venturi was sufficient to provide up to approximately 20 % fgr . nitrogen was also injected into the venturi inlet to access higher apparent levels of fgr . for this series of measurements , an all - metal swirler assembly was used that could tolerate the higher inlet temperatures associated with fgr . the flow rate through the tubes to recirculate flue gas was estimated from the fuel flow rates , the residual oxygen concentration , and the estimated total air entrainment rate by the venturi . both methane and methane - hydrogen blends were used as fuels for these measurements . the actual dry nox emissions ( corrected to 3 % o 2 ) as a function of excess air are shown in fig1 , and the data are compiled in table 5 below . due in part to the uncertainties with the flows in the system , there is more scatter in the data . however , there is a clear trend that shows fgr shifts the nox vs . excess air curve to the left . again , single digit nox levels can be achieved at low excess air with sufficient flue gas recirculation . a commercial venturi / inspirator sized for ˜ 100 kbtu / h burners was obtained from fives north american . it was coupled with the low swirl burner assemblies used in the earlier testing and operated over a range of conditions . the lsbs displayed good performance when fed by the commercial venturi . dry nox emissions were comparable to those obtained in the earlier experiments , as shown in fig1 . the red and blue symbols represent earlier measurements obtained with the simple venturi , and the green symbols represent new data obtained with the commercial venturi . this also confirms that the simple venturi is achieving adequate air / fuel mixing . the data obtained with the commercial venturi are compiled in table 6 . the commercial venturi has been optimized to entrain combustion air and achieve good mixing . this was demonstrated by the firing rates and burner exit velocities obtained in testing the low swirl burners . it was possible to operate at 10 % higher flow rates than those obtained with the simple venturi system . this will translate into better turndown capability and better flame stability . the commercial venturi also allows for finer adjustment of the air - fuel ratios by use of a threaded flange to control the air inlet gap width . assess potential of fgr and fuel staging with natural draft lsb . the use of flue gas recirculation ( fgr ) with the low swirl burner ( lsb ) operating in natural draft mode demonstrated that fgr has the potential to maintain low nox emissions at low excess air conditions . another strategy to control nox emissions from combustion is fuel staging . a system with fuel staging uses a lean primary flame to limit initial nox production . the remainder of the fuel is injected into the primary flame exhaust to consume some of the residual oxygen , and the exhaust from the system can have 3 % oxygen or less . since the secondary flame is burning vitiated ( oxygen - depleted ) air , it can have lower nox emissions than if it burned in non - vitiated air . the fuel staging tests were conducted by injecting fuel into the base of the enclosure for the lsb flame . the same low swirl burner and venturi that were used in the earlier measurements were incorporated into the test system . several configurations were tried for injection of the secondary fuel . generally , a circular tube with a number of small holes was placed around the exit cone of the low swirl burner . a cross - section of the system is shown schematically in fig1 . it was found that the secondary fuel injection holes tended to produce discrete yellow - tipped flames . the flame appearance indicated that the secondary fuel was not mixing well with the exhaust gas from the primary flame zone . to improve mixing , nitrogen was added to the secondary fuel flow to produce a ˜ 1 : 1 mixture of methane and nitrogen . the larger volume of gas should increase the jet velocity , and the nitrogen should slightly impede the onset of combustion . the position of the secondary fuel injection ring was also moved to various heights above the base of the enclosure . none of these variations had a significant effect on the appearance of the flames in the secondary combustion zone . similarly , the measured nox emissions were no better than that obtained by injecting all of the fuel into the lsb . with higher secondary fuel flows , the nox emissions may be slightly higher than equivalent flames without fuel staging . to improve the dispersion of the secondary fuel injection , a very fine stainless steel mesh was placed over the secondary fuel injection tube . the mesh was clamped down at the edges . the mesh dispersed the secondary fuel flow into the exhaust gas over an extended area , and prevented the creation of fuel jets in the exhaust gas . the flame generated by the combustion of the secondary fuel was now more evenly dispersed . however , the nox emissions were no better than those from equivalent flames in which all fuel was fed to the low swirl burner . nitrogen was also added to the secondary fuel flow in some tests to improve dilution and mixing . again , there was no reduction in nox levels . the fuel staging test results are compiled in table 7 below . the lack of reduction in nox levels with the addition of secondary fuel suggests that there is inadequate mixing in the secondary fuel injection zone . from our studies of the low swirl burner , we have found that turbulence levels are not very high beyond the primary flame zone . without rapid mixing of the secondary fuel with the exhaust gas , the secondary combustion zone will not be well mixed , and nox emissions will be high . fuel staging may be more effective at higher burner velocities . however , it will be difficult to achieve sufficiently high velocities with a natural draft low swirl burner for staged combustion . alternatively , a very highly dispersed secondary fuel injection system may demonstrate better emissions performance . another approach for staged combustion is to create a rich ( oxygen - deficient ) primary combustion zone , and then blend in air to achieve the desired overall stoichiometry . such burner systems are sometimes called rql burners , for rich - quench - lean combustion , or rich - quick mix - lean combustion . the rich primary zone has a relatively low flame temperature and consequently does not produce high nox levels . the rich primary zone can produce high levels of co and unburned hydrocarbons since there is not sufficient oxygen for complete combustion . the addition of secondary air lowers the overall stoichiometry to the desired level of excess air . one of the problems with rql burners is the combustion gas passes through stoichiometric conditions as the secondary air is added , and significant nox production can occur at that point . also , there can be issues with adequate burnout of co and unburned hydrocarbons after the secondary air addition . this version of fuel staging is not likely to provide sufficient emissions reduction with the low swirl burner for refinery conditions due to the low excess air levels in the refinery system exhaust . also , a power source would be needed to supply the secondary air to the burner assembly since the fuel is only inducing the flow of the primary air . while it may be possible to develop a suitable fuel staging arrangement for a natural draft low swirl burner , it is likely to require significant development to create a satisfactory fuel injection and mixing system for the secondary fuel . methane - hydrogen fuel blends have higher flame speeds than methane alone and it will be more difficult to establish a fuel staging system for these blends that provides significant emissions reduction . embodiments of the invention demonstrate that fgr appears to be significantly more promising than fuel staging for controlling nox emissions . the flue gas recirculation system can be built into the natural draft low swirl burner assembly to minimize heat loss and to utilize the induced draft from the fuel injection . with the low back pressure associated with the low swirl burner , a 30 psig fuel stream has sufficient energy to entrain both combustion air and flue gas , and the system will be capable of achieving low emissions and good turndown . due to the limitations of the current test system used to recirculate flue gas , there was limited firing rate turndown capability when real fgr was used . the relatively small flow path for flue gas recirculation resulted in low fgr at low firing rates where there was small pressure differential between the burner inlet and the combustion chamber . measurements were conducted over a 2 : 1 firing rate range . over this range , the firing rate did not appear to have any significant influence on emissions . results from this study indicated that the two factors that significantly influenced emissions were excess air level and the fraction of fgr . the natural draft lsb that did not incorporate fgr demonstrated ˜ 10 : 1 turndown , so should be feasible to expand the turndown range with fgr . adding fgr to a natural draft lsb is a matter of incorporating a suitable flow path for flue gas to be entrained into the burner inlet with the combustion air . dampers with low actuation force can be incorporated into the inlet air and flue gas flows to control the fgr rate . the low swirl burner , integrated with a suitable venturi , works well in natural draft configuration when fuel is available at ˜ 30 psig . for best performance , a low pressure drop swirler should be matched with the designed flow output of the venturi . the emissions of the natural draft low swirl burner agree well with the emissions predicted by leonard and stegmaier ( 1994 ). the natural draft lsb works well with both natural gas and natural gas - hydrogen blends . nox emissions increase as the system excess air is reduced and flame temperature increases . it is desirable to operate refinery process heaters at low excess air conditions to improve efficiency . techniques such as flue gas recirculation ( fgr ) or fuel staging can lower nox emissions from burners operating at low excess air conditions . the natural draft low swirl burner was operated with simulated and actual fgr and with fuel staging . the lsb with fuel staging did not show any improvement in nox emissions over the unstaged natural draft lsb . however , tests with simulated and actual flue gas recirculation indicated that the low swirl burner can achieve single digit nox levels at low excess air when 20 - 30 % fgr is utilized . embodiments of the invention demonstrate that the natural draft low swirl burner has the capability to achieve low emissions at low excess air conditions , and are adaptable to a commercial product for process heating . the foregoing detailed description of the present invention is provided for purposes of illustration and is not intended to be exhaustive or to limit the invention to the embodiments disclosed .