Abstract:
A heating system according to the present invention is divided into two interconnected compartments, the first having at least one burner feeder and the second having at least one receiver burner. The thermal output of these units may be equal or unequal to match the heat recovery requirements and requirements of the receiver burner for feeder flue gas. Fuel is fed to both the feeder and receiver burners. Heat from both burners heats the fluid to be heated within the heating system. The flue gas exiting from the feeder compartment is introduced and mixed into the receiver burners air supply, similar to conventional flue gas recirculation (FGR) type low NOx burners. The feeder may use a lean premix or rich burn quench technique to achieve low-NOx. Only one combustion air fan is required. The Ductwork is not exposed or the external exposure is very short as compared to much greater lengths of external duct work in conventional flue gas recirculation systems. The compartments and their interconnection are arranged to make maximum contact with the container or fluid flow path of the fluid to be heated. Thus, there is minimal heat loss, and no condensation to cause possible corrosion. The methods have steps consistent with the system described. There is also a system employing multiple heaters with only receiver burners and the flue gas from yet another separate feeder heater which is fed to the other heaters and combined with air at the burner(s).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to residential and commercial heat release processes, which requires the burning of fuel with air. Specifically, the invention relates to processes regulated for achieving low nitrogen oxide (NOx), carbon monoxide (CO) and volatile organic chemicals (VOC) emissions. 
     This invention provides more economical means for achieving low emission levels, particularly oxides of nitrogen, while maintaining high efficiency. High efficiency also reduces greenhouse gases. 
     Residential units may include, among others, forced hot air heating and water heaters. Commercial units may include boilers and water heaters. Industrial units may include indirect heating such as radiant tubes. 
     The fuels combusted might be natural gas, refinery or process gas, and liquid distillate fuels like diesel fuel or #6 (residual) fuel oil. 
     NOx is an abbreviation for the collective species of nitrogen oxide and nitrogen dioxide, which are responsible for health problems, and the creation of smog. NOx is created in the intense heat of a combustion flame where nitrogen and oxygen are present. It has been long known that the suppression of flame temperature can reduce NOx significantly. NOx can also be created from fuel bound nitrogen and ammonia or other nitrogen bearing compounds that find its way into the fuel or combustion air. In this case, NOx is not reduced by a suppression of flame temperature, but NOx can be “chemically reduced” to N 2  and carbon dioxide (CO 2 ) in a hot fuel-rich flame that is carbon monoxide (CO) rich. 
     There are a number of methods for reducing NOx, as listed below. However, all come with some economic penalty or degradation in process performance. It is an object of this invention to provide a novel method and structure that will minimize the economic penalties associated with these NOx reducing methods. 
     FIELD OF THE INVENTION 
     There are a number of techniques for reducing NOx that are well known in the prior art. Some definition of terms employed in the art and some explanation of the nature of processes known in the prior art are described below. 
     Ballast is a mass of gas added to a region of combustion that reduces flame temperature. 
     A nozzle mix burner is one in which air and gas enter the combustion zone unmixed. Such a burner has the advantage that there is essentially no chance of flashback. However, it has the disadvantage that combustion is slower than with premixing so that more CO and VOC are produced, and usually the NOx is higher. 
     A premix burner brings air and gas together before entering the combustion zone. This results in lower NOx and cleaner combustion. However, it allows the possibility that flashback can happen. 
     Diluents are gases added to a region of combustion which reduce speed of reaction and heat liberation by reducing molecular collisions between reactive molecules. 
     Flue gases are gas mixtures which are the results of combustion, gases which normally flow out the stack. 
     Rich mix has more fuel than air in the burn mixture and results in insufficient air to complete combustion. 
     Lean mix has more air than required for stoichiometric combustion of the fuel. Normal natural gas combustion produces about 2-3% oxygen in the flue gases. The flame extinguishes at about 12-15% oxygen in the flue gases.. 
     Fuel bound nitrogen is nitrogen that is chemically bound in the fuel molecules. 
     Oxygen trim means reducing the excess oxygen too close to stoichiometric levels while maintaining a watch on the carbon monoxide regulated limits. This produces a small decrease in NOx from oil and gas nozzle-mix burners but an increase in the NOx from premix burners. Operating closer to stoichiometric improves thermal efficiency but increases pollutants such as CO and VOC. 
     Lean premix is a lean mix mixed before it is introduced into the combustion space. One can achieve very low-NOx by diluting the stoichiometric gas mixture with large amounts of air ballast. This moderates the flame temperature. However, the air ballast carries large amounts of heat out the stack, so that the process is thermally inefficient. This method is acceptable for air dryers where the moderated temperatures are required. The air ballast increases NOx for a nozzle-mix burner. 
     Quick Mix is another burner design technology that uses a nozzle mix type burner and rapidly mixes the air and the fuel in the combustion space before it can ignite. This simulates premix and is reported to obtain low-NOx. 
     Rich burn quench introduces air over the fire of a burner. It is mainly used to reduce oxides of nitrogen emission for nozzle-mix burners using fuel oils or gases with high levels of fuel bound nitrogen. Most of the fuel bound nitrogen is converted to NOx in the flame. This is in excess of the thermal NOx created in the hot flame. However the CO in the very rich flame reduces some of the nitrogen oxide back to nitrogen and carbon dioxide. Therefore, the remainder of the fuel requires burning before venting to atmosphere, usually with natural gas and overfire air. The proplem is matching the heat release to the heat absorbing capabilities (e.g. steam/water tubes in a boiler) and using natural gas in place of less expensive fuel oil and the additional piping and costs of natural gas. 
     Water or steam injection employed as ballast (as used in lean premix) and diluent injected to cool the flame temperature and reduce the combustion reaction rate thus reducing thermal NOx. The ballast carries heat out the stack. Thus it is inefficient compared to stoichiometric combustion. Water and steam may be expensive in arid areas like California. 
     Staged air combustion is similar to “rich burn quench” except that there is no quench so that the introduction of additional air combusts the remainder of the fuel. Usually the final combustion is close to stiochiometric; thus the process is thermally efficient. Often staged air combustion is used in combination with other techniques for lowering NOx. 
     Flue gas recirculation (FGR) is a technique using an extra fan to suck flue gases from the stack and usually to force them into the air stream. This is a very popular method of reducing NOx on existing and new boilers and works well with both liquid and gaseous fuels. Usually 10-30% flue gas is recirculated. There is a loss of efficiency due to recirculation fan power and cooling and condensing of the recirculated flue gas in the piping. With high sulfur fuels, condensing must be avoided to prevent corrosion; thus recirculation usually requires clean fuels. Flue gas recirculation does not reduce NOx from fuel bound nitrogen. 
     In situ FGR are methods of FGR whereby flue gas from the furnace is inspirated into the burner body and mixed with the air or fuel without an external loop or very short external loop. This technique is only moderately successful in reducing NOx since the inspiratation of hot flue gases requires large amounts of energy. 
     Zone combustion or bias combustion for long or tall furnaces allows the combustion to be split up into zones. One patent has a lean premix burner firing at one end of the furnace and fuel injectors before the end of the furnace (zone  2 ) where combustion and heat transfer can be completed before exiting. Thus, there are two types of burners. However, the flue products of one burner does not enter into the combustion of the other burner as in the present invention. 
     Co-generation is normally thought of as method for improving heat recovery from turbine-generator set or diesel-generator set. However, it does reveal a method of reducing NOx. The waste gas stream from the generator set contains a lot of oxygen and heat. To reclaim some of the waste-heat the waste gas can be sent (15% in the case of a gas-fired turbine at about 900 F.)directly to a waste heat recovery unit (boiler). Still, thermal efficiency is low due to the large amounts of excess air (ballast). Some waste recovery unit&#39;s (boiler) have burners, which uses the oxygen in the exhaust gas to complete combustion close to stoichiometric. Thus, the overall heat balance of the plant is improved and the thermal efficiency is high. The problem, of course, is that in the summer when steam is not required in large amounts, the waste heat recover unit&#39;s (boiler) are shut off and the plant is less efficient. The testing of these waste heat recovery units (boilers) burners taught that low NOx emissions could be obtained with the turbine exhaust. The turbine exhaust is similar to a mix of preheated air and flue gas recirculation. 
     In summary, when applying the above techniques to residential heating, flue gas recirculation is expensive to operate and maintain. Lean premix and water injection are inefficient. Staged air combustion and rich quenched lean mixtures may not fit the process requirements. Also rich quench lean creates the presence of high carbon monoxide (from rich combustion), which makes any leak in the heat exchanger or malfunction of the stack deadly. Such risks are unacceptable for residential heating units. 
     SUMMARY OF THE INVENTION 
     This invention provides a number of methods by which the above mentioned techniques for achieving low-NOx can be applied in a new system, using an improved more efficient method. Thus the invention avoids the losses in efficiency, operating complexity and high cost of current industrial low-NOx systems. 
     A heating system according to the present invention is divided into two interconnected compartments, the first having at least one feeder burner and the second having at least one receiver burner. The thermal output of these units may be equal or unequal to match the heat recovery requirements and requirements of the receiver burner for feeder flue gas. Fuel is fed to both the feeder and receiver burners. Heat from both burners heats the fluid to be heated within the heating system. The flue gas exiting from the feeder compartment is introduced and mixed into the receiver burners air supply, similar to conventional flue gas recirculation (FGR) type low NOx burners. The feeder may use a lean premix or rich burn quench technique to achieve low-NOx. Only one combustion air fan is required. The ductwork is not exposed or the external exposure is very short as compared to much greater lengths of external ductwork in conventional flue gas recirculation systems. The compartments and their interconnection are arranged to make maximum contact with the container or fluid flow path of the fluid to be heated. Thus there is minimal heat loss, and no condensation to cause possible corrosion. 
     A related system of the invention system employs multiple heaters with only receiver burners and the flue gas is obtained from yet another separate feeder heater which is fed to the other heaters and combined with air at the burner(s). 
     More specifically, the present invention relates to a two stage heater containing a body of fluid to be heated. A first combustion compartment within the heater has at least one feeder burner connectable to a supply of fuel and having an air supply enabling the fuel to be burned to produce a first combustion product. A second combustion compartment has at least one receiver burner connectable to a supply of fuel and having an air supply. Connection between the first and second compartments whereby the first combustion product can flow into the second combustion compartment in position to combine with air and fuel and be burned to make a second combustion product. The walls of the container for the fluid to be heated are in good thermal contact with at least the first and second combustion compartments. 
     The present invention also relates to a method of reducing NOx in a heater having a fluid body to be heated. The method steps are as follows: Burning fuel in a first compartment of the heater to produce a first combustion product. Applying heat from the first combustion product to the fluid body to be heated. Burning fuel in a second compartment of the heater into which the first combustion product as well as new air is introduced to produce a second combustion product. Applying heat from the second combustion product to the fluid body to be heated. Then exhausting the second combustion product. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a horizontal cross sectional view of a hot-air gas-fired furnace taken beneath the air heating compartment above the burners. 
     FIG. 2 is a sectional view of the furnace or water heater of FIG. 1 taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a sectional view of the furnace or water heater of FIG. 1 taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a sectional view of the furnace or water heater of FIG. 1 taken along line  4 — 4  of FIG.  1 . 
     FIG. 5 is a schematic view representing a plurality of heaters having receiver burners which receive and employ the exhaust from separate heater having a supply burner which together provide the advantages of the present invention. 
     FIG. 6 is a vertical sectional view of a water heater employing the present invention. 
     FIG. 7 is a horizontal sectional view taken through the water heater of FIG. 6 taken along line  7 — 7  of FIG. 6 to show the burner from above with only the suggestion of the relative location of other heater parts. 
     FIG. 8 is a sectional view through a fire-tube boiler employing the present invention. 
     FIG. 9 is a partial view of an insulated furnace employing the present invention showing a duct in section at the location of a radiant tube heating system, shown in elevation. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1-4 show a hot-air heater unit  1  in which the present invention has been incorporated. Forced hot air for heating is drawn into the heating compartment  8  of the heater by fan  2  shown in FIG.  2 . Fan  2  forces air through supply duct  3  and forces the cold air into the air heating compartment  8  defined by one of the internal heat transmitting walls  14  providing a floor to the compartment  8 . The compartment encloses feeder combustion tubes  27  and.  35  which provide radiant heating together with heat transfer through the floor combustion tubes heat the passing air. In some embodiments the tubes may be equipped with heat transfer fins  7  to improve heat transfer to the air. Hot air exits the air heating compartment  8  of the furnace through the supply duct  4  directing the heated air to the space to be heated. Natural gas or other suitable gases is supplied as fuel  9  to the fuel pressure regulator  10  and the fuel distribution header  11 , which supplies feeder burner nozzles  17  and receiver burner nozzles  32 . Chamber walls and ceiling  14  create three separate compartments: the feeder burner compartment  41 , the receiver burner compartment  42  and the unit flue gas compartment  43 . 
     The feeder burner air enters by feeder combustion air register  15  drawn by suction created by exhaust fan  39  in flue  40  as shown in FIG.  1 . Fan  39  maintains the combustion side under suction to prevent combustion gas leaks into the hot air compartment  8  or elsewhere outside the combustion exhaust chambers. Air is distributed to each feeder burner  17  via the feeder burner primary air register  18  and fuel is supplied by a metered feeder burner nozzle  17  and premixed in feeder premix chamber  22  as seen in FIG.  2 . The mixture passes through the feeder burner screen  19  and is ignited and monitored by the feeder burner igniter/flame safety monitor  20 . Secondary feeder burner air, if required, enters through feeder burner secondary air registers  21 . The flame enters the feeder combustion tube  27  which provides a duct through which heat is transferred to the heating air in compartment  8 . The cooled combusted flue gas exits the feeder combustion tube  27  and enters the receiver burner compartment  42  as seen in FIG.  3 . 
     Additional combustion air, if required, is drawn in to the receiver burner compartment  42  by the receiver combustion air register  31  and mixes with the feeder flue gas. This supplies internally of the furnace or heating unit an air and flue gas mix, which is similar to a mix generated by recirculating flue gases (FGR technology) external to the heating unit. Some of the flue gas mix is drawn into the receiver burner intake  37  by the inspirating action of the gas leaving the receiver burner nozzle  32 . The receiver burner assembly  34  is equipped with a flame holder/ glow plug  33 . The mix combusts and enters the receiver combustion tube  35  through which heat is transferred to the heating air flow through compartment  8 . The cooled combusted flue gas exits the receiver combustion tubes  35  through a metering orifice  36  to control flow distribution, then into the unit flue gas compartment  43 . The flue gas exits through the flue gas duct  38  through the combustion air fan  39  to the atmosphere through a chimney (not shown). 
     Industrial experience has shown that the lowest NOx can be obtained with either lean-premix (or quick mix technology) or external flue gas recirculation (FGR). In a first embodiment of the invention, the concept of both of these methods are used to obtain low NOx without (1) the extra capital and operating cost of an extra fan for FGR, (2) the cost of external duct work associated with FGR, (3) the loss of efficiency due heat losses from the external duct work for FGR, and (4) the loss of efficiency of ballast (excess air) carrying heat out the stack with lean-premix. Most or all of the air for the total and complete combustion of the fuel fired in the hot-air heater unit  1  enters in through the feeder combustion air register  15  into the feeder premix chamber  22 . The percent of total fuel to the feeder burner assembly  23  is generally between 50% to 60%. If there is too much air in the premix to the feeder burner assembly  23 , combustion could be quenched, therefore there is means to bypass some air around the feeder premix chamber  22  by way of the feeder burner secondary air registers  21  or receiver combustion air register  31 . The merits of very lean premix for generating low NOx are well known in the industry. The cooled combusted flue gas exits the feeder combustion tube  27  and enters the receiver burner compartment  42 . Additional combustion air (if required) is drawn in the receiver combustion air register  31 . The mixture of diluted air is the same as if the unit were equipped with an external flue gas recirculation (FGR). This mix is then used by receiver burner assembly  34  to burn with low-NOx essentially in the manner known from the practice of external flue gas recirculation (FGR). 
     In a second embodiment of the invention the feeder burner  19  operates in a normal high NOx mode. It supplies about one-third to one-fifth the heat input of the heater unit  1 . Only slightly more than stoichiometric air for the feeder burner assembly  23  fuel consumption enters in through the feeder combustion air register  15  into the feeder premix chamber  22  or feeder burner secondary air registers  21 . The feeder burner employed may thus be a conventional burner. The cooled combusted flue gas exits the feeder combustion tube  27  and enters the receiver burner compartment  42 . Air for receiver gas combustion is drawn in the receiver combustion air register  31 . The mixture of diluted air which is equivalent to having 20-30% flue gas recirculation is then used by the receiver burner to burn with low-NOx as is well known from the practice of external flue gas recirculation (FGR). The NOx is not as low as the first embodiment of the invention, since it is the average of one conventional burner and four to six low-NOx burners of the same size. 
     FIG. 5 schematically represents a novel configuration of conventional prior art heater units or boilers. Additional parallel units  60  normally make for greater efficiency. A plurality of typical receiver units  60  are equipped with low-NOx flue gas recirculation type burners as are commercially available. All are connected in parallel to a common flue gas feeder duct  55  provided with a back pressure regulator  56  and the receiver flue gas orifice  65  controls the flow of the feeder flue gas to each receiver unit  60 . Each receiver unit  60  receives air from a receiver air supply duct  62  pressured by a receiver combustion air fan  66 . Each receives fuel from a supply line  61  from a fuel supply which is mixed with air and flue gas from a flue gas injector  63 . Each may receive flue gas from a feeder unit  50 , similar to each of the receiver units  60 . The feeder unit supplied fuel through line  52  and air from fan  52  through air supply  51 . Feeder ducts  58  receive flue gas from feeder unit  50  through a common duct  55 . Combustion and heat transfer occurs in the receiver units  60  and the resulting flue gas is exhausted to atmosphere through the exhausted duct  64  to the receiver stack  64 . The feeder unit combustion air fan operates at a higher pressure than the receiver unit&#39;s air fan. 
     An advantage of this parallel configuration over flue gas recirculation systems is the elimination of separate recirculation fans for each of the units with a capital and operating savings. The passing of flue gases from the exhaust of a feeder unit  50  through a boiler which is not operating will provide a hot stand-by unit without expending extra fuel. Thus the overall efficiency of the boiler system is increased. A disadvantage is that when only one boiler is operating there is no decrease in NOx. However, regulatory agencies exempt certain age and size boilers from NOx regulations. Thus depending on the boiler house arrangement, this invention may satisfy regulatory requirements at all times. 
     Those skilled in the art will understand the system permits use of an alternate backup feeder unit, that is normally a receiver unit  60  by the use of shut off valves  59  in the feedback ducting  58 , for use when the feeder unit  50  requires repairs. 
     FIG. 5 shows that improvements in heating systems can be separated by an internal wall or can be two or more completely independent heating systems. Thus, FIG. 5 shows the flue gases of one boiler fed into a multiple of other boilers. A number of scenarios may be used here; for example the feeder system supplies flue gas for four or five other systems. 
     Another scenario would be the feeder system or systems supplies rich flue gas recirculation from rich burn quench low-NOx technology, and the receiver Systems supply more air and fuel to meet their heat input requirements and the requirements of good combustion. 
     FIG. 6 and 7 show a water heater, which typically might be used in homes, modified to employ features of the present invention. Many of the structured parts will be recognized as conventional. Typically the water to be heated  80  is contained in a torroidal tank  76  provided by the metal walls designed to sustain some steam as well as water pressure. The outer walls of the tank  76  is shown in an embodiment of the invention for a small capacity hot-water heater that is found in homes and commercial facilities. A hot-water heater pressure shell  76  is surrounded by insulation  77 . A cold water feed pipe  75  enters through the top of the tank and extends to near the tank&#39;s bottom. Hot water exits through a hot water supply pipe  78 . The pressure shell  76  is equipped with a pressure relief valve  79  extending supported on a pipe fixed through the top wall of the tank, as is well known in the practice. 
     The main burner, here receiver burner  104 , is centered below the tank. Fuel supply pipe  82  enters to gas regulator  83  which supplies gas to the gas distribution header  84 . Header  84  then distributes gas both to the receiver gas supply pipe  103  and the receiver burner tip  104 , and to feeder gas supply pipe  86  and then the feeder gas orifice  90 . The feeder burner air is drawn in by the combined effects or the receiver air tube venturi  101  and the inspirating action of the fuel gas in the feeder venturi  89 . Combustion air is supplied by a combustion air fan  81  (FIG.  7 ), which is equipped with a receiver air register  108  at its entrance for controlling combustion air flow to the receiver burner tip  104  through the receiver air tube  100  and receiver air tube venturi  101 . 
     The feeder combustion air is metered by and enters by the feeder air register  88  and the feeder fuel is metered by the feeder gas orifice  90 . The feeder fuel and feeder air is mixed in the feeder venturi  89  and combusted on the downstream side of the feeder burner screen  93 . It is ignited and monitored by the feeder igniter/flame monitor  92 . The flame enters the generally annular feeder combustion tube  94 , which is firmly attached to the outside of the bottom of the hot water heater pressure shell  76 . Heat is transferred to the water  80  in the tank and the cooled flue gas is drawn from feeder combustion tube  94  into the receiver air tube venturi  101  to mix with the receiver air  99  to create a mixture equivalent to 15-25% external flue gas recirculation. The mixture mixes with receiver gas discharged from the receiver burner tip  104  and is ignited by the receiver glow wire  105  and combustion takes place in the main heat transfer tube  106  where heat is transferred to the water  80  in tank  76  and then exits to the atmosphere through the exhaust duct  107 . Thus the feeder burner, generates conventional Nox, or slightly lower NOx than the conventional type burner, if it is operated slightly lean, while the receiver burner generators low NOx as would be expected from an FGR type burner, and the units NOx is the proportional average of the two streams. 
     FIG. 8 shows an embodiment of the invention employed with in a commercial large pressure shell  125  of a water heater or steam boiler. A cold water inlet pipe  126  supplies the water  127  to be heated, The hot water/steam outlet pipe  128  at the top of the boiler supplies hot water or steam to users. A pressure relief valve  130 , and gauges as desired, are also provided at the top of the boiler. Two compartments are formed outside the pressure shell  125  by the front door  135  and the back door  136  of the boiler. A blow-down and water preparation system (not shown) are normally employed in accordance with the practice well known in the industry. The feeder pilot  143  is supplied pilot gas and air mix  146  through inlet pipes, which gas is ignited by feeder pilot igniter  144 . Flame safety is provided by a UV sensor acting as feeder pilot flame sensor  145  as is commercially available and well known in the industry. 
     Air enters the feeder burner assembly  147  through the feeder forced air supply duct  132 . Oil fuel is introduced by the feeder burner fuel line  133  into the feeder oil gun  134  along with atomizing steam through line  137  if required. The air and the fuel mix and, ignited by the pilot burner, in the feeder burner block  140  and enter the feeder heat transfer tube  141 . The receiver pilot  174  is provided with air and pilot gas premix  180  and is ignited by receiver pilot igniter  176 . Flame safety is provided by UV sensor receiver pilot flame sensor  178 . Gas from the receiver gas supply  160  enters the receiver premix chamber  164 . Receiver forced air  161  is also introduced through the receiver premix chamber  164 , and mixes with fuel and burns on the receiver burner screen  166 . The ratio is sub-stoichiometric so that NOx created by fuel bound nitrogen and combustion is reduced in the rich atmosphere. This is the well known and proven technique for reducing NOx referred to as a rich burn quench with natural gas re-burn. 
     Thus the exiting feeder flue gas  142  is fuel-rich and must be re-burned by introducing more air and increasing the temperature of the air with a portion of natural gas injection so that combustion is sustained and completed before exiting the unit. The receiver pilot  143  is provided with air and pilot gas premix  146  and is ignited by receiver pilot igniter  144 . Flame safety is provided by UV sensor receiver pilot flame sensor  178 . [Gas from the receiver gas supply  160  enters the receiver premix chamber  164 .] Receiver forced air  172  is also introduced through the receiver premix chamber  134 , and mixes with fuel and burns (on the receiver burner screen  166  out of the burner block  140 ). Alternately one might use quick mix type burner. The receiver burner premix has enough air to complete combustion for the fuelrich feeder flue gas  142  and the re-burn natural gas injection. The mix completes combustion in receiver heat transfer tube  168  and exits through exhaust chamber  170  and unit stack  172 . Some boilers use more unfired passes to increase efficiency, as is well known in the art. Such variations are not shown for simplicity but would occur to the man skilled in the art. The added capital expense of an extra burner can be offset by the ability to burn less expensive, and possibly more available, fuel, for example, during natural gas curtailment. 
     Another embodiment of the invention for burning all natural gas involves some modifications. The feeder burner assembly  147  uses lean-premix or quick mix technology to obtain low-NOx combustion and the receiver burner assembly  159  uses flue gas recirculation type burner technology. The flue gas exiting feeder flue gas  142  is equivalent to 20-30% flue gas recirculation mixed with the required amount of combustion air. Additional air, if required, may be provided through receiver forced air  161 . The advantage of the invention over flue gas recirculation (FGR) is that there is no extra operating expense for the recirculation fan electricity and no heat-loss (loss of efficiency) due to external ducts. 
     Still, another embodiment of the invention modifies the feeder burner assembly  147  to provide about one-third to one-fifth the total heat-input as the receiver burner assembly ( 159 ). Thus the feeder burner assembly  147  would supply only the equivalent of 20-30% flue gas recirculation, which would be mixed with the receiver forced air  161  and then mixed with the fuel in a burner designed for FGR (flue gas recirculation burners commercially available). 
     Two examples of use of the type of system of FIG. 8 as follows illustrates the versatility of the system: 
     Case 1: A scotch tube boiler (commercial type boiler) is equipped with the invention. The feeder unit burner is supplied with all the air and about half or more of the fuel, which is combusted in the lower combustion tube. The burner uses Lean Premix low-NOx technology. The products of combustion are then feed into the “air” inlet of the receiver burner and the remainder of the fuel is added. The exhaust of the feeder burner is equivalent to 20-30% flue gas recirculation which is well mixed with receiver unit combustion air. The exhaust from the receiver is close in temperature and composition to a normal fired unit with same efficiency. 
     Case 2: Since the above has the additional cost of another burner, it may not be economically feasible for gas firing. However when used with fuels with large amounts of fuel bound nitrogen it may be the only method to significantly reduce NOx. The feeder burner will combust most of the fuel at a very rich stoichiometry. The combustion is then completed in the receiver tube with additional (“overfire”) air and natural gas reburn as is well known in the art. The exhaust from the receiver is close in temperature and composition to a normal fired unit with same efficiency. 
     FIG. 9 illustrates an alternative furnace arrangement employing the invention. The furnace casing  200  is lined with refractory lining  201  which encloses the furnace work space  202 . The feeder radiant tube  210  and receiver radiant tube  233  pass through the furnace casing  200  and refractory lining  201  and provide radiant heating to the furnace work space  202 , usually with an air-tight seal formed by radiant tube mounting flanges  205 . The feeder radiant tube  210  and receiver radiant tube  233  are provided with radiant tube end caps  218 . Normally the feeder radiant tube  210  and receiver radiant tube  233  would not be connected but exhaust directly to a collection duct (not shown) and the stack (not shown). This invention pairs and connects the tubes so that there is feeder radiant tube  210  and receiver radiant tube ( 233 ). Most furnaces of this type are equipped with many radiant tubes so that multiple pairing of radiant tubes is convenient and relatively easily accomplished. 
     The unit forced combustion air is delivered through duct  203  and feeder fuel is delivered through line  213 . Both or either of these supplies may or may not be preheated as they enter the feeder burner  215  which uses either lean pre-mix or quick mix technology to obtain low-NOx. The feeder radiant tube is equipped with a feeder pilot  211 , a feeder pilot air and gas mix  214  and an igniter/flame sensor  212 . Combustion is completed in the feeder radiant tube  210  and the flow exits at the feeder radiant tube exhaust  217  into feeder exhaust connector pipe  225 , through expansion connector  226  into the receiver burner mix supply region  229 . 
     The receiver burner mix supply region  229  contains enough air for complete combustion of the receiver fuel supplied through line  227  and they mix and combust in the receiver radiant tube  233  and exit through the unit exhaust  204 . A receiver pilot  230  is supplied with air/fuel mixture receiver burner pilot air and gas mix  228 , which is ignited by the receiver pilot igniter/flame sensor  231 . The heat release to the furnace work space  202  from the feeder radiant tube  210  and receiver radiant tube  233  are normally equal. However the feeder radiant tube  210  will always be fired with more fuel, the proportional amount depending on the temperature of preheat of the unit forced combustion air  203 . 
     Some low-NOx radiant tubes use staged combustion which in the long term causes metal dusting and holes in the radiant tube. This is due to the change of the atmosphere in the radiant tube from reducing (fuel-rich) to oxidizing (excess air) caused by the staged flame pattern. This invention maintains an oxidizing atmosphere throughout the radiant tube, thus reducing dusting problems. 
     Some commercial low NOx radiant tubes use the forced combustion air acting through a venturi to create suction which through a connection leg to the radiant tube exhaust, inspirates flue gases. This generates a FGR type low NOx combustion. However, inspirating of hot flue gas and the inefficiency of the venturi/inspirator result in a costly electrical energy penalty for the combustion air fan. 
     The advantages of the invention embodiment of FIG.  9  and its variations are that there is less chance of metal dusting in the radiant tubes which results in longer life, lower electrical energy consumption, and also a reduction in ducting since the pair requires only one combustion air inlet and one exhaust outlet rather than the two required for the conventional arrangement. 
     The radiant tube is a method of indirect heat exchange used in industry. Heating is by radiation, therefore the external surface temperature of the tubes is from 1200-2000 F. (depending on process requirements and temperature limits of the tube materials). There are often a large number of radiant tubes in an industrial furnace, thus enough to make many feeder/receiver pairs as shown in FIG.  9 . 
     The above description of specific embodiments of the invention not only gives various specific forms which the invention may take but also suggests how variations in those form may be made. There are also various ways in which heater systems in accordance with the present invention can be used and some of the alternate ways of using them are suggested. It will, therefore, be clear to those skilled in the art that there are many possible variations to the present invention, some of which have not been specifically covered and some which have not even been suggested. Such variations within the scope of the claim are intended to be within the scope and spirit of the present invention.