BACKGROUND OF THE INVENTION While great strides have and continue to be made in developing different and better methods of producing power for society's needs, steam generation using nitrogen- bearing fuels, mainly in coal burning boilers remains a keystone of many present day power production systems. Air pollution from nitrogen oxides ("NOx") is among the lingering problems of combustion driven steam generators. If not converted or destroyed by costly to build and operate catalytic reduction systems, NOx generated as byproducts of the combustion process escape to ambient air. Nitrogen oxides are well known to contribute to creating adverse health and environmental effects. Hence, formation of significant quantities of NOx is a serious drawback to operation of conventional steam generation facilities.

Advances in nitrogen-bearing fuel burning steam boilers, including coal-fired boilers, have been made throughout the years. Some of the changes have enabled the construction of newer power plants that are capable of operating with lower NOx byproduct than older plants. However, a large number of early technology steam generation units are still in operation around the world and are responsible for emitting substantial amounts of NOx into the environment. The economic hurdles necessary to modify or replace older power plants with modern technology are formidable.

Moreover, improvements are still wanting in the reduction of NOx emissions from more recently developed power production facilities.

With respect to modern boiler systems, further improved NOx reduction methods are desired which can be successfully implemented without compromising efficiencies provided by more recently developed technologies. It is further seen that new NOx- reducing systems also should be adaptable for incorporation into still functioning, older

power plants. Thus there is a large need to provide an improved method of reducing the generation of NOx in steam boiler power plants.

Today combustion of coal, heavy oils and the like is a very popular method of providing heat to generate steam in industrial and utility power plants. A major area of improvement of the operation of coal-fired boilers involves so-called"staged combustion". See for example a seminal work titled"Interpretation of Small and Intermediate Scale Test Results from a Low NOx Combustion System for Pulverized Coal", Johnson, S. A. , Yang, R. J. , and Sommer, T. M. , Technical Paper: International Flame Research Foundation Advanced Combustion Technology Meeting, Noordwijkerhout, Netherlands, May 12-14,1980.

Broadly described, staged combustion relates to dividing the combustion chamber into zones including an upstream Zone I and a downstream Zone II. Zone I, sometimes referred to as the ignition zone is where the fuel (s) and some air enter the combustion chamber and the fuel is ignited. The combustion air can enter the combustion chamber together with the fuel and/or through different inlets. Part of the air, sometimes called the"primary air", enters the combustion chamber through the same nozzle (s) as the fuel. For liquid fuels such as fuel oil, the primary air can be injected with the fuel so as to atomize the liquid for more efficient combustion. For solid fuels such as coal, the primary air can serve as a conveying medium to transport coal into the combustion chamber. Additional combustion air, sometimes referred to as"secondary air", enters Zone I through inlets different from the primary air inlets. The secondary air inlets are close to the primary air and fuel inlets.

Zone II is intended to complete the combustion of fuels left unreacted by combustion in Zone I. A fundamental characteristic of staged combustion is that the combustible mixture of fuel plus the total of primary and secondary air in Zone I is maintained rich in fuel. Thus an excess of fuel exists as a product of Zone I combustion.

Air is introduced to Zone II to burn the remaining fuel. The air charged to Zone II is sometimes called"over fire air". Modern staged combustion systems usually provide air flows such that a significant portion (e. g., about 20%) of the total oxygen enters in Zone II. Certain staged combustion systems utilize air fed into Zone II through a multitude of streams, for better mixing.

The oxidant for combustion of fuel in furnaces can include enriched air. By "enriched air"is meant a gas mixture composition comprising components of air in which one or more components is outside the respective typical concentration range in ambient air. For example, nitrogen enriched air can have about 80-100 vol. % nitrogen and oxygen enriched air can contain about 21-100 vol. % oxygen.

The prior art discloses that NOx emissions can be reduced from staged combustion coal fired steam generation when the air in the ignition zone is maintained in the nitrogen enriched concentration range. Other disclosures suggest that similar results can occur when both of the Zone I and Zone II combustion air concentrations are maintained in the nitrogen enriched air range. Still other disclosures propose to reduce NOx emissions by using oxygen enriched air concentrations in both of Zone I and Zone II.

US 6,568, 185 discloses the use of a high velocity, oxygen-enriched gas introduced into Zone II to address the problem of mixing excess air with main flue gas air within Zone II. The patent also presents processes and apparatus to reduce NOx in boiler operation, particularly in integrated air separation/power generation plants in which an air separation unit provides enriched air for combustion.

US 6,418, 865 discloses a staged combustion process for a boiler in which oxygen enriched air is fed to each of a radiation zone and a convection zone. Oxygen depleted flue gas air is recirculated to either or both of these zones while the amount of oxygen enrichment and total gas flow is controlled to maintain the heat transfer patterns within the boiler at the originally-designed specification for operation by air combustion.

US 2003/0108833 Al discloses a low NOx staged combustion process using coal in which the gaseous oxidant contains more than 21 vol. % oxygen and the stoichiometric ratio of fuel and oxidant is maintained in the first combustion stage below that which would produce the same amount of NOx if the stage were operated with air as the only oxidant.

There remains a need to burn nitrogen-bearing fuel to generate thermal energy in a way that cost effectively reduces the generation of NOx. It is also desirable to have a process for upgrading existing furnace operations to reduce their presently high NOx-

containing emissions to lower NOx concentrations in a way that is minimally disruptive of the existing operations and causes only slight changes to combustion process equipment already in place.

SUMMARY OF THE INVENTION Accordingly, the present invention provides a combustion process comprising the steps of (A) providing a furnace adapted to operate a two-zone, staged combustion reaction of a nitrogen-bearing fuel which furnace comprises a first zone and a second zone downstream of the first zone, (B) charging into the first zone a nitrogen-bearing fuel at a fuel flow rate and an ignition gas mixture comprising an oxidant at an ignition gas mixture flow rate such that a first zone oxidant: fuel ratio is less than the stoichiometric oxidant: fuel ratio operative to completely combust the fuel, (C) combusting the ignition air mixture and a portion of the fuel at a first stage flame temperature to generate thermal energy and thereby forming in the first zone a first stage flue gas comprising residual combustible fuel, (D) admitting the first stage flue gas to the second zone, (E) introducing into the second zone a secondary oxidant feed stream at a rate such that a second oxidant: fuel ratio in the second zone is greater than the stoichiometric oxidant: fuel ratio operative to completely combust the residual combustible fuel, and (F) combusting the residual combustible fuel and oxidant in the second zone at a second stage flame temperature to generate thermal energy, in which the first stage flame temperature is greater than a first reference temperature defined as the actual temperature that would be attained by combustion in the first zone of the nitrogen-bearing fuel charged at the fuel flow rate and an ambient air composition fed at a rate to provide the first zone oxidant: fuel ratio and to thereby produce a reference first stage flue gas, and

in which the second stage flame temperature is at most a second reference temperature defined as the actual temperature that would be attained by combustion in the second zone of the reference first stage flue gas and an ambient air composition fed at a rate to provide the second oxidant: fuel ratio.

This invention also provides a combustion process for burning a fossil fuel comprising nitrogen, the process comprising the steps of (A) providing a furnace adapted to operate a staged combustion reaction of the fuel, the staged combustion reaction consisting of a first stage carried out in a first zone and a second stage carried out in a second zone downstream of the first zone, (B) charging into the first zone the fuel and an ignition gas mixture comprising an oxidant such that oxidant: fuel ratio in the first zone is less than the stoichiometric oxidant: fuel ratio operative to completely combust the fuel, (C) causing combustion in the first zone of ignition air mixture and the fuel to generate thermal energy and thereby forming in the first zone a first stage flue gas comprising residual combustible fuel, (D) admitting the first stage flue gas to the second zone, (E) introducing into the second zone a secondary oxidant feed stream comprising an oxidant such that oxidant: fuel ratio in the second zone is greater than the stoichiometric oxidant: fuel ratio operative to completely combust the residual combustible fuel, and (F) combusting the residual combustible fuel and oxidant in the second zone to generate thermal energy, in which the ignition gas mixture comprises oxygen and nitrogen and has a concentration of oxygen greater than 21 vol. %, and in which the secondary oxidant feed stream comprises oxygen and nitrogen and has a concentration of nitrogen greater than 79 vol. % nitrogen.

DETAILED DESCRIPTION OF THE INVENTION In this disclosure the following terms have the corresponding definitions.

"Ignition gas mixture"means the total gas fed to the first zone of a staged combustion furnace and includes oxidant for combustion of fuel.

"First stage flue gas"means the gas mixture in the furnace immediately downstream of the first zone combustion and which flows from the first combustion zone to the second combustion zone. The first stage flue gas consists of residual combustible fuel, product of combustion and residual ignition gas mixture.

"Residual combustible fuel"means the components of the fuel that were incompletely combusted in the first stage and which are available in the first stage flue gas to be combusted with an oxidant to form carbon dioxide and water.

"Secondary oxidant feed stream"means the oxidant-containing gas mixture fed from external sources to the second combustion zone of a staged combustion furnace.

Typically the fuels burned to produce thermal energy, especially in steam generating power plants, are fossil fuel material such as coal. These materials usually contain various compounds having nitrogen atoms in their structure. When the fuel is burned in presence of oxygen, nitrogen liberates from decomposition of the compounds in a form which reacts with the oxygen to generate nitrogen oxides (NOx). Molecular nitrogen is typically fed to fossil fuel furnaces in the air used for combustion, conveying, atomization of liquid fuel, etc. However, molecular nitrogen is very stable and usually does not react with oxygen to form NOx unless the furnace is improperly operated.

The amount of NOx produced is influenced by such factors as the combustion temperature, amounts of nitrogen and volatile matter in the fuel, residence time in the combustion zone, and the like. If the concentration of nitrogen borne by the fuel is small, lowering the peak combustion temperature tends to reduce the amount of NOx produced. In many practical circumstances, nitrogen in the fuel is substantial. The equilibrium concentration of NOx produced by combustion rises with increasing temperature but can be maintained at low values by operating with a fuel rich condition.

Hence, tactics for reducing NOx emissions for more abundantly nitrogen-loaded fuels involve adjusting the oxidant: fuel ratio in relation to combustion temperature.

This invention is primarily directed to reducing the production of NOx in emissions from combustion of high nitrogen content fuels carried out in staged combustion furnace operations. Thus it relies upon burning the fuel initially at a high temperature that promotes formation of molecular nitrogen. Of course, the temperature should be below that at which a substantial amount of molecular nitrogen decomposes such that increased NOx would likely result.

More specifically this invention is directed to furnaces using a two-staged combustion process in which the first zone is fuel rich. That is, the oxidant supplied to the first stage is less than is theoretically necessary, i. e., the stoichiometric amount, to completely reduce the hydrocarbon components of the fuel to carbon dioxide and water.

The ratio of oxidant to fuel, that is the oxidant: fuel ratio is thus said to be less than the stoichiometric oxidant: fuel ratio. This disclosure adopts the convention that the stoichiometric ratio of oxidant to fuel is designated numerically as 1: 1 or"1.0".

Accordingly, oxidant: fuel ratios in the fuel rich range have numerical values less than 1.0 and oxidant: fuel ratios in the oxidant rich range have numerical values greater than 1.0.

Combustion of the remainder of the fuel is carried out in the second zone of the two-staged combustion process. Maximum fuel efficiency is obtained when all of the fuel is burned for its heat value. To assure that the fuel is completely combusted, additional oxidant is combined in a second combustion zone with residual combustible fuel present in the first stage flue gas from the first zone. The additional oxidant is fed at a rate effective to provide an oxidant rich mixture such that sufficient oxidant is present to consume all of the residual combustible fuel.

It has now been discovered that burning high nitrogen content fuels in a two- stage combustion process having a fuel rich first zone and an oxidant rich second zone very effectively provides low NOx emissions generally when the first zone is operated at high temperature and the second zone is operated at low temperature. As mentioned the temperature in the first zone should be high enough to favor decomposition of nitrogen in the fuel to a form of nitrogen which then recombines significantly to benign molecular nitrogen. However, this temperature should not be so high as to react

molecular nitrogen. The temperature in the second zone should be kept below temperatures which accelerate reaction of nitrogen compounds to molecular nitrogen which can then react with excess oxygen to form NOx.

In one aspect the novel process can be operated by utilizing a temperature in the first zone above a first reference temperature while utilizing a temperature in the second zone that is below a second reference temperature. The reference temperatures are those at which the same combustion process operates with the condition that all of the oxidant in the ignition air mixture and the secondary oxidant feed stream is supplied by oxygen in an ambient air composition. More specifically, the reference temperatures can be determined as follows.

A particular two-staged combustion furnace geometry is selected. The furnace can be an existing installation or a new design can be proposed. The term"geometry"is intended to embrace not only the physical dimensions of the furnace but also the configuration, that is the number and position of the inlet streams, the type of burners, lances etc. position of flue gas outlet (s) and heat transfer loads. Next the intended fuel composition and flow is selected. For the first and second zones, fuel rich and oxidant rich oxidant: fuel ratios are chosen, respectively. From these ratios the corresponding first and second zone oxidant flows are calculated. The system is operated under actual load using the pre-selected geometry, fuel flow, oxidant: fuel ratio with all of the oxidant provided in the form of oxygen in an ambient composition air (i. e. , nominally 21 vol. % oxygen/79 vol. % nitrogen). The term"actual load"is means the actual heat produced and transferred from the furnace at the above-described combustion conditions. Thus the procedure provides the actual temperatures achieved by the furnace in the first and second zones. The actual temperatures are distinguished from the adiabatic temperatures which would be achieved if there were no heat loss, i. e. , zero heat load, on the furnace during operation. Basically, the reference temperatures are the first and second zone combustion temperatures that actually result when the selected furnace is actually operated under load at design fuel rate and oxidant: fuel ratio but with all of the oxidant being oxygen in ambient air composition.

The foregoing method of determining the reference temperatures is largely empirical. Thus it is seen that the novel method is perhaps more readily adapted to use

for retrofitting an existing power generating facility. The existing facility can be operated with ambient air composition oxidant feed streams to determine the reference temperatures. Once the reference temperature for each zone has been identified, the amount of NOx in flue gas emissions can be reduced by operating with combustion temperatures above and below the respective first and second zone reference temperatures.

In an embodiment, the novel two-stage combustion process can be operated with a first zone combustion temperature of 1500°C or higher and a second zone combustion temperature of 1600°C or lower. Having identified the broad range of"high"first zone and"low"second zone combustion temperatures, the operator can further adjust combustion temperatures within the ranges to optimize reduction of NOx in the emissions.

Combustion temperature can be controlled within the desired ranges by any appropriate method. For example, a higher or lower heat load can be used in the furnace zones. In a preferred embodiment of this invention, the combustion temperatures in the first and second zones are controlled primarily by using enriched air compositions for the oxidant supply streams. More specifically, the ignition air mixture for the first zone comprises an oxygen-enriched air composition and the secondary oxidant feed mixture for the second zone comprises a nitrogen-enriched air composition. The concentration of oxygen in the gas mixture fed to the first zone should be higher than 21 vol. %, preferably higher than 22 vol. %, and more preferably higher than 24 vol. %. The concentration of nitrogen in the secondary oxidant feed stream to the second zone should be higher than about 80 vol. %, preferably higher than about 82 vol. %, and more preferably higher than about 88 vol. %.

This combination of enriched air oxidant compositions can effectively control the temperatures within desired ranges to suppress NOx formation because the nitrogen entrained in the enriched air serves as a thermal drain. For example, in the first zone the ignition gas mixture has less nitrogen than ambient air. It therefore absorbs a proportionately smaller amount of heat from combustion per unit of oxidant than would the larger amount of nitrogen in an ambient air composition at the same oxidant: fuel ratio. Because less heat is taken up by the nitrogen, the combustion temperature rises in

the first zone. Similarly, nitrogen-enriched air in the secondary oxidant feed stream has more nitrogen than ambient air at the same oxidant: fuel ratio. Thus the second zone nitrogen removes more sensible heat from combustion than would ambient air composition and the combustion temperature is lower.

The nitrogen enriched air and oxygen enriched air can be supplied from any suitable source, for example by adding pure component to a gas mixture. Preferably according to this invention the oxygen enriched air and nitrogen enriched air for the ignition gas mixture and the secondary oxidant feed stream, respectively, are generated by an air separation operation. This usually involves separating ambient air into a nitrogen enriched fraction and an oxygen enriched fraction. Advantageously, because oxygen enriched air is used in the first zone and nitrogen enriched air is used in the second zone, both product fractions of air separation can be put to use. Neither is entirely wasted.

In one contemplated embodiment, an air separation unit is provided to generate a supply of oxygen-enriched air fraction and nitrogen enriched air fraction. These supplies are incorporated into the ignition gas mixture and secondary oxidant feed stream respectively. That is, the enriched air fractions can be supplemented with amounts of ambient air composition gas such that the desired total oxidant: fuel ratios are obtained.

The overall ignition gas mixture supplied to the first zone should be oxygen enriched.

That is, the average concentration of oxygen in the aggregate of all of the gas streams feeding the first zone, should be in the oxygen enriched range. Thus a plurality of feed gas streams can be utilized in which one or more stream is not oxygen enriched provided that oxygen supplied in other streams renders the ignition gas mixture of the first zone to be oxygen enriched.

Preferably, the air separation unit is designed such that either all of the ignition gas mixture required to operate the first zone is provided entirely by the oxygen enriched air fraction or all of the secondary oxidant feed stream required to operate the second zone is provided entirely by the nitrogen enriched air fraction. In a rare situation, it is contemplated that all of ignition gas mixture and secondary oxidant feed stream consumed by the first and second stages, respectively, will be exactly matched by the

oxygen enriched air fraction and the nitrogen enriched air fraction from the air separation unit.

Air separation operations for making enriched air fractions for use according to this invention may be any of the well known types in the art, including cryogenic, membrane, pressure swing adsorption, and the like. Preferably the air separation unit for this embodiment is a selectively gas permeable membrane.

Membranes which provide good flux and at least moderate selectivity between oxygen and nitrogen are useful. Preferred membranes include those having selectively permeable portions comprised of rubbery polymers or high free volume, glassy polymers. Rubbery polymers are characterized as having glass transition temperatures below the temperature at which gas separation occurs. Silicone rubber is an example.

Glassy polymers have glass transition temperatures above the temperature of gas separation. Representative high free volume, glassy polymers membranes are polytrimethylsilylpropyne, PDD copolymers (copolymers of 2, 2-dimethyl-1, 3- pefluorodioxole), and HyflonOO copolyrners of tetrafluoroethylene and 2,2, 4-trifluoro-5 trifluoromethoxy-1,3 dioxole (Ausimont, Thorofare, New Jersey). Great preference is given to using PDD copolymer membranes which have been found to provide a uniquely favorable combination of high flux and selectivity between oxygen and nitrogen.

Therefore, they are ideal for producing the enriched air fractions utilized in the present invention.

More preferably, these selectively gas permeable membranes are in the form of hollow fibers. Hollow fiber membranes useful for separation of air to provide the enriched gas mixtures for this invention are typically assembled in so-called"membrane modules". These modules have an elongated, usually cylindrical gas-tight case which surrounds a bundle of multiple fibers that are aligned largely but not absolutely parallel to the longitudinal axis of the elongated case. The ends of the fiber bundles are potted, usually within a polymeric resin which forms tube sheets for the bundles. The outer peripheries of the tube sheets are sealed against the inner wall of the case. Such membrane modules are well known in the gas separation art. Gas permeable membranes of PDD copolymers and hollow fiber modules with PDD copolymer membranes are available from Compact Membrane Systems, Inc., Wilmington, Delaware.

It has been disclosed that the present invention is expected to find great use primarily in the field of fuel-fired boilers for steam generation in power plants. It is contemplated that this invention may also find use in a variety of industrial applications, for example, those that employ directly fired, nitrogen-bearing fuel to produce heat in glass, steel, aluminum or cement manufacturing.

Although specific forms of the invention have been selected in the preceding disclosure for illustration in specific terms for the purpose of describing these forms of the invention fully and amply for one of average skill in the pertinent art, it should be understood that various substitutions and modifications which bring about substantially equivalent or superior results and/or performance are deemed to be within the scope and spirit of the following claims.