US3656878A

US3656878A - High luminosity burner

Info

Publication number: US3656878A
Application number: US22812A
Authority: US
Inventors: Franklin J Wright
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1970-03-26
Filing date: 1970-03-26
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

A method and apparatus for burning, at increased luminosity intensities. A fuel such as natural gas whose combustion is normally characterized by a low luminosity flame. A first hydrocarbon fuel which may include a fixed amount of a free radical promoter is burned in a diffusion flame. The products of this combustion, which include soot particles, are burned along with a second fuel such as natural gas. The flame produced in this secondary combustion has a luminosity greater than that produced by the combustion of said second fuel in a single-stage burner.

Description

United States Patent Wright [4 Ar. 1, 1972 [54] HIGH LUMINOSITY BURNER 1,747,676 2/1930 Kerr ..431/4 x [72] Inventor: Franklin J. Wright, Watchung, NJ. FOREIGN PATENTS OR APPLICATIONS [731 Assignefil Research and Engineering Company 317,042 7/1929 Great Britain ..431/4 [22] Filed: Mar. 26, 1970 Primary Examiner-John J. Camby PP NOJ 22,812 Assistant Examiner-Robert A. Dua

- Attorney-Manahan & Wright and Donald F. Wohlers [52] U.S.Cl ..431/10 431/5,43l/353 51 Int. Cl ..F23m3/04 [571 RA [58] Fleld 0f Search .Lu43l/2, 5, 4, 10, 126, 353, A method and apparatus f i g at increased luminosity 431/158 115 intensities. A fuel such as natural gas whose combustion is normally characterized by a low luminosity flame. A first [56] References cued hydrocarbon fuel which may include a fixed amount of a free UNITED STATES PATENTS radical promoter is burned in a diffusion flame. The products of this combustion; which include soot particles, are burned 2,506,972 5/1950 Schellentrager et al. ..431/5 X alo g with a second fuel such as natural gas, The flame 2,829,731 4/1958 Clayton X produced in this secondary combustion has a luminosity 2.879.362 1959 Burden, X greater than that produced by the combustion of said second Pl'yOI' fuel in a 5ing]e-stage burner 2,780,538 2/1957 Chilton ....43l/4 X 1,972,259 9/1934 Brassert ..431/4 5 Claims, 3 Drawing Figures i\ f T PATENTEDAPR 18 I972 SHEET 2 BF 2 T IIO .518 59. z zommg 6 850m mov PRE-MIXED WITH THE FUEL R I w W F M EW 1" 0 WM n I A M a ram E W R R W -fiE N 00.. w j m s M T O A M R E f ON 1. E 10L A E 1 0 N M R A an w an R RT P FE m F H 0 N as LL 1 00 MM 5 O 2 O 0 53m 6 wJOE Ema POOw m0 924mm mun LUMINOSITY BURNER BACKGROUND OF THE DISCLOSURE The instant invention is directed to a method and apparatus for increasing the luminosity of flames produced by low luminosity fuels. Specifically, the instant invention is directed to a method and apparatus in which a first fuel is burned in a diffusion flame to produce a primary flame and thereafter the products of combustion are burned with a second low luminosity flame-producing fuel to produce a high luminosity secondary flame. More specifically, the instant invention is directed to a method and apparatus for increasing the luminosity of a hydrocarbon gas fuel flame in which a hydrocarbon fuel is combined with a fixed quantity of a free radical promoter and burned in a diffusion primary flame, the products of which are again burned in a secondary high luminosity flame in which the gaseous hydrocarbon mixture provides the main fuel.

By the term difi'usion flame is meant one which results when the fuel and the oxidizer are not premixed. Quoting from Flame Structure, R. M. Fristrom and A. A. Westenburg, Mc- Graw-Hill, 1965 at page 10, If they the reactants are not premixed, the resulting flame is called a diffusion flame since the mixing of fuel and oxidizer must be accomplished by a diffusion process.

Large-scale furnace design has been limited, in the prior art, primarily to the use of heavy hydrocarbon fuel oils. The use of these heavy liquid fuels has several disadvantages based primarily on the wide range of molecular weight constituents of the fuel mixture. A typical fuel oil contains, in addition to light-end hydrocarbons, long-chained heavy molecular weight constituents. This is often manifested in the high viscosity of the mixture.

Among the disadvantages inherent in burning a fuel such as fuel oil is incomplete combustion, which results in pollution of the atmosphere. The high molecular weight constituents take much of the heat of combustion in their vaporization. Thus, there is insufficient heat to completely combust all the components in the liquid mixture. Hence, the formation of soot, carbon monoxide and other incomplete combustion products which all contribute to air pollution.

A second disadvantage inherent in a mixture of heavy hydrocarbon liquids is the inefficiency that results therefrom. As stated above, much of the heat of combustion of such a mixture is expended in latent heat losses resulting from the vaporization of the heavier molecular constituents. In rare cases, further heat is expended in melting such constituents in addition to vaporizing them. Thus, the flame temperatures of such mixtures are much lower than a fuel which enters the combustion reaction in the gaseous state.

In the prior art, these disadvantages have been overlooked in favor of the conspicuous advantage of such fuel oils. That is, the high luminosity flame produced by such a fuel. The 'soot and incompletely burned solid and liquid constituents in the flame are heated and thus emit electromagnetic waves which contribute to the radiative heat transfer constituent of the heat transfer in the furnace. It should be appreciated that in fur- I nace design an endeavor is made to provide a maximum amount of radiant heat transfer to the materials being heated.

Gaseous hydrocarbon fuels, such as natural gas, which comprise a relatively narrow range of low-end hydrocarbons, do not share many of the disadvantages of heavy fuel oil. For instance, gaseous fuels do not require the expending of any of the heat of combustion in vaporizing the fuel. This results in a higher temperature flame. In turn, this efiectuates a higher percentage of complete combustion of gaseous hydrocarbon fuels as compared to liquid hydrocarbon fuels such as fuel oil. Hence, the combustion of gaseous fuel as compared to liquid fuel occurs at a higher temperature, results in more complete combustion and causes less pollution of the atmosphere.

In the prior art, these advantages were outweighed because of the low luminosity of the flame produced by gaseous hydrocarbon fuels compared to flames resulting from liquid hydrocarbon fuel combustion. Although the natural gas combustion resulted in high convective heat transfer rates and low pollution of the atmosphere, the radiation heat transfer rate which must be appreciable in furnace design was too low. It is obvious to those skilled in the art that a burner design which can produce high luminosity flames from the burning of gaseous hydrocarbon fuels will be well received by those skilled in the furnace art.

SUMMARY OF THE INVENTION The instant invention is directed to a method and apparatus in which gaseoushydrocarbon fuels are burned to produce high luminosity flames. This is accomplished by the incomplete combustion of a first-fuel in a diffusion flame to produce a controlled quantity of solid soot particles. The particles, along with the other products of the diffusion flame combustion move downstream where they are again combusted with a gaseous fuel in the presence of excess air to produce a secondary flame whose luminosity is much greater than the luminosity normally associated with the combustion of gaseous fuels.

The instant invention is also directed to the unique combination of conventional hydrocarbon fuels with fixed quantities of free radical promoters which increase the quantity of soot produced in the diffusion flame primary combustion.

In accordance with the instant invention, a first hydrocarbon fuel is burned in a first burning zone of the burner of the instant invention by means of a diffusion flame. The products of the diffusion flame combustion are passed into a second burning zone of the burner of the instant invention. There, the products of the first burning zone combustion are combined with a second hydrocarbon fuel in the presence of air. These combustion reactants are then burned to produce a secondary flame of high luminosity. The first hydrocarbon fuel may be augmented with a fixed quantity of a free radical promoter which increases the amount of solid combustion products produced in the first burning zone resulting in a high luminosity flame.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood by reference to the accompanying drawings of which:

FIG. 1 is a schematic representation of the burner of the instant invention;

FIG. 2 is a plot relating the mass of soot produced by the combustion, in a diffusion flame, of various hydrocarbon fuels as a function of percentage stoichiometric oxygen premixed with the fuels;

' FIG. 3 is a plot relating mass of soot produced by the combustion of ethylene, in a diffusion flame, as a function of the percentage of two free radical promoters premixed with the ethylene.

DETAILED DESCRIPTION FIG. 1 is a schematic representation of a preferred embodiment of the high luminosity gas burner of the instant invention. The burner generally indicated at 1 comprises, in a preferred embodiment, a hollow cylindrical body member or conduit 2 whose material of construction is any suitable high temperature resistant material. It should be appreciated that the body member 2 is not limited to cylindrically shaped tubes. Thus, any other convenient shape such as a rectangular duct may be substituted. A second hollow member 6 is disposed inside the outer member 2. Hollow member 6 begins at the inlet end 4 of the furnace 1 and extends a variable distance into the burner 1. The member 6 comprises the inlet means for the introduction of a first hydrocarbon fuel into the burner 1. Thus, hollow member 6, which may be of any convenient shape, i.e. cylindrical or square-shaped, is in communication (not shown) with a hydrocarbon fuel source. The fuel may be premixed with a free radical promoter as will be discussed in greater detail hereinafter.

The inlet end 4 of the burner l is either open to the introduction of air or alternatively in communication with a source of air if the inlet is closed. Therefore, air is introduced at the inlet end 4 inwardly into the burner 1. This is represented in FIG. 1 by arrows 3. The fuel enters the inlet means represented in FIG. 1 by the hollow member 6 and is burned at the outlet thereof. The flow of the fluid into the member 6 is represented by the arrow 5. The fuel exiting the inlet member 6 is immediately burned. In order to initiate burning, an ignition initiation means is provided. In a preferred embodiment illustrated in FIG. 1, a spark plug 11 is provided to perform this function. The resultant primary flame 8 is a diffusion flame. That is, the fuel is drawn toward the flame by vaporization and diffusion through member 6. At the outermost edge of the flame 8 the fuel is combined with the air surrounding the flame 8 to sustain combustion. The diffusion flame combustion occurs in a first heating zone designated generally as 10.

The diffusion flame combustion products which include soot as well as other combustion products move in a downstream direction toward the outlet of the burner 1, as will be described in greater detail below, to a section of burner 1 designated for convenience as the second burning zone 14. Burning zone 14 is not physically distinguished from burning zone 10. Their distinguishing feature lies in the combustion that occurs in these zones. Zone 14 is characterized by a plurality of gaseous fuel inlets 12 which may be disposed in any convenient manner so as to provide an efficient dispersal of the fuel into the second burning zone 14 of the burner 1. A back-up ignition means, which in the preferred embodiment may be a spark plug such as that illustrated at 11, is disposed also in zone 14. Usually the flame 8 is swept by means of the air stream 3 so as to ignite the fuel in burning zone 14. However, in order to ensure that there will be no bum-out if the primary ignition means fails, for any reason, spark plug 11a is provided.

The gas fuel is combined with the air which enters the burner 1 through inlet 4 as illustrated by arrows 3 to provide a secondary combustion in which a secondary flame 16 results. In addition to the fuel and air which combine to support combustion, the combustion products of the primary flame 8 are also burned. That is, those fractions of the combustion products which were not oxidized to carbon dioxide and water in the flame 8 are further combusted. As will be described hereinafter, the burning of these products, primarily soot, increases the luminosity of the flame 16.

In operation, a fuel with or without a free radical promoter is fed into the inlet conduit member 6 as illustrated by the arrow 5. The fuel used in the first combustion zone is a fluidized hydrocarbon. Preferably the hydrocarbon will be unsaturated. More preferably, the unsaturated hydrocarbon will be either an oleflnic or aromatic hydrocarbon. As will be discussed in greater detail in the examples, ethylene and propylene among the olefins, and benzene, an aromatic, have been successfully tested for their application to the instant invention. It should be appreciated that hydrocarbons of the saturated variety can also be used in the first burning zone 10. Saturated hydrocarbons do not provide as great a quantity of combustion products as the above-mentioned groups. However, they do improve the luminosity of the secondary flame 16 when combined with free radical promoters as will be described below. Thus, their use is predicated on their availability as compared to olefins or aromatics. Since gaseous fuels, such as those contemplated for use in the second burning zone 14, are primarily saturated hydrocarbons, it is obvious that the use of such fuels in the first-stage burn is convenient and economical. The use of the same fuel eliminates the requirement for two separate storage facilities as well as lowers the unit cost of the fuel because it is purchased in larger quantities.

It should be appreciated that saturated hydrocarbons may be blended with unsaturated hydrocarbons to provide a suitable soot producing fuel. Furthermore, a blend of saturated and unsaturated hydrocarbons combined with fixed amounts of free radical promoters, as will be discussed in greater detail hereinafter, may also be used as the fuel in the first burning zones. These blends, if used, may be selected on the basis of availability, economic considerations, or operating conditions without fundamentally changing the increased luminosity effect of the burner of the instant invention.

The first-stage burn in the first bunting zone 10 is initiated by activation of the spark plug 16. The spark plug 16 is not used further unless there is a burn out. The first stage burn is characterized by a diffusion flame. Diffusion flames usually result in incomplete combustion due to the inability of the oxidizer to completely mix with the fuel. Thus, diffusion flame combustion occurs at the peripheral of the flame. Oxidizer, in this case air, can only mix with the fuel at the point of combustion. Therefore, no burning occurs in the hollow cone shaped center of the flame 8. This results in incomplete combustion with the resultant formation of soot.

Soot is essentially a multiplicity of carbon atoms. The formation of soot has been explained by several theories. However, it is generally believed that polymerization of the fuel, which occurs under the elevated temperatures of combustion contributes to the ultimate formation of soot. Unsaturated compounds are the usual monomeric raw material for the formation of polymers. Hence, it follows that an unsaturated hydrocarbon is more apt to produce soot. Saturated hydrocarbons can also be used, although not as satisfactorily, since the heat of combustion causes portion of the saturated hydrocarbons to be thermally cracked and thus become unsaturated also.

It has been found that premixing of the first-stage burning fuel with certain free radical promoters results in increased soot formation. This is in accordance with the polymerization theory of soot formation. Polymerization is promoted by the presence of free radicals. The amount of promoter added increases the soot formed up to a fixed percentage, depending upon the fuel used. A maximum is reached after which increased percentage promoter in the fuel results in marked decreases in soot formation and ultimately in complete elimination of soot.

Among the free radical promoters tested, oxygen has been found to be an excellent promoter of increased soot formation. That oxygen increases soot formation is an indicia of the significant effect of free radical promoters on the formation of soot in hydrocarbon combustion. One would expect oxygen, being a strong oxidizer, to promote complete combustion. Surprisingly, the free radical promoting effect of oxygen predominates at low concentrations, resulting in increased soot formation.

Turning to FIG. 2, the effect of oxygen on various hydrocarbon fuels is illustrated. In FIG. 2, the mass of soot formed per unit mass of carbon in the fuel being burnt, in a diffusion flame, is plotted as a function of the amount of oxygen premixed with the fuel. The amount of oxygen is expressed as the percentage of stoichiometric oxygen required to fully convert the fuel burnt to carbon dioxide and water. Since the percentage of stoichiometric oxygen is the independent variable, it is the abscissa on the curve, while the amount of soot formed, expressed as mass of soot per unit mass of carbon, is the ordinate.

A typical saturated hydrocarbon, propane, is included to illustrate the effect of burning saturated hydrocarbons on soot formation. The curve clearly indicates that saturated hydrocarbons provide the least favorable ratio of mass of soot formed per unit mass of carbon in the fuel burnt. The propane line, based on experimental data denoted by the circular points, demonstrates that although saturated hydrocarbons are not effective to produce soot, without the addition of a free radical promoter, they can be used when premixed with such a promoter. Thus, when no oxygen is added to propane, no soot is formed at all. However, as the percentage of stoichiometric oxygen premixed with the fuel increases, so does the formation of soot. It is seen that generally 5 to I25 percent of the stoichiometric oxygen required to convert all the propane to carbon dioxide and water is a preferred mix to be used in the first stage of the burner l of the instant invention. More particularly, a range of 8 to 12 percent of stoichiometric oxygen premixed with the fuel will yield optimum formation of soot per unit weight of carbon in the fuel burnt. Most particularly, the addition of oxygen, premixed with a saturated hydrocarbon in the amount of 11 percent of the stoichiometric oxygen required to fully oxidize the hydrocarbon, yields the optimum formation of soot per unit weight of carbon in the saturated hydrocarbon.

FIG. 2 also illustrates the effect of the combustion, in a dif fusion flame, of two olefins, ethylene and propylene, with or without premixing with oxygen, on the formation of soot. For both olefins, no oxygen need be premixed with these olefin fuels in order to produce significant amounts of soot. Therefore, it is clear that olefins provide an excellent source for use in the first heating zone of the burner of the instant invention. The curves, based on experimental data, also clearly indicate that the addition of a free radical promoter, which in this case is oxygen, markedly increased the yield of soot produced per unit weight of carbon in the fuel burned. Generally, for olefins, FlG. 2 indicates that the addition of oxygen in the amount of l to 12.5 percent of the stoichiometric oxygen required to fully oxidize the olefin appreciably increases the amount of soot produced per unit weight of carbon burnt. More particularly, 7 to 12 percent of the stoichiometric oxygen required to fully oxidize the olefin results in markedly improved soot formation per unit weight of carbon in the fuel burnt. Most particularly, the addition of oxygen premixed with the fuel in the amount of to l I percent of the stoichiometric oxygen required to fully oxidize the fuel will result in optimum formation of soot per unit weight of carbon in the fuel burnt. It should be noted that the range of percentage addition of oxygen to the fuel burnt does not vary among olefins and saturated hydrocarbons.

FIG. 2 also includes a curve for a typical aromatic hydrocarbon, benzene. Benzene has been used to illustrate the effect of an aromatic in the formation of soot with or without a free radical promoter because it is probably the most basic of all aromatic compounds. Like olefins, but unlike saturated hydrocarbons, benzene does not require the addition of any free radical promoters in order to produce a significant quantity of soot when burned in a diffusion flame. However, FIG. 2 clearly indicates that the addition of a fixed percentage, based on the amount of fuel, of a free radical promoter increases the amount of soot produced per unit weight of carbon originally available in the fuel. Thus, the addition of oxygen in the amount of 5 to 21 percent of the stoichiometric oxygen required to convert all the benzene to water and carbon dioxide markedly increases the milligrams of soot produced per gram of carbon in the benzene burnt. More specifically, the addition of oxygen in the amount of 10 to percent of the stoichiometric oxygen required to fully oxidize the benzene produces a more optimum yield of milligrams of soot per gram of carbon in the benzene burnt. Most specifically, the addition of oxygen, premixed with benzene in the amount of 16 to 18 percent of the stoichiometric oxygen required to fully oxidize the benzene, results in the most optimum yield of soot per unit weight of carbon in the benzene. Thus, the optimum percentages of the free radical promoter, oxygen, are slightly different for a typical aromatic than for typical olefins and saturated hydrocarbons.

FIG. 3 is a curve relating the grams of soot produced per mole of fuel when the fuel is premixed with small amounts of free radical promoters. In FIG. 3 only one fuel, ethylene, is employed. Again, the fuel burns as an air diffusion flame. The grams of soot per mole of fuel reacted, being the dependent variable is the ordinate, while the independent variable, moles of the free radical promoter added per mole of ethylene is the abscissa.

The curve designated nitropropane indicates the curve tracing the eflect of increasing amounts of nitropropane added per mole of ethylene. The second curve, designated amyl nitrite, indicates a similar curve in which the free radical promoter is amyl nitrite instead of nitropropane. Although the absolute values are different for these curves, the optimum yield of soot formed occurs at approximately the same molar percentage of free radical promoter per mole of fuel. Moreover, the shape of the curves are almost parallel. FIG. 3 indicates that for free radical promoters, other than oxygen, the addition of 0.005 to 0.075 moles of free radical promoter per mole of fuel, with which it is premixed, markedly improves the yield of soot per mole of fuel burned. More particularly, the addition of 0.015 to 0.05 moles of free radical promoter per mole of fuel increases the mass of soot produced per unit mole of fuel burned. Most particularly, the addition of 0.02 to 0.03 moles of free radical promoter per mole of fuel burnt provides the best increase in the production of soot per mole of fuel burnt.

Returning now to FIG. 1, the soot formed in the diffusion flame 8 is entrained by the momentum of the downstream movement of the stream of air 3. The soot thus moves downstream into the second burning zone 14. In zone 14 the gaseous fuel, which in a preferred embodiment is natural gas or other gaseous hydrocarbons, is fed into the burner through a plurality of inlets 12. The primary diffusion flame 8 is similarly entrained to some extent and thereby ignites the gas in the presence of the air stream 3. If, perchance the flame goes out, the spark plug 11a automatically is activated to rekindle the flame. The combustion of the gaseous fuel, which is indicated in FIG. 1 by arrows 15, would normally result in a flame of low luminosity. However, the products of combustion from the primary diffusion flame 8 are also ignited by the gas fuels combustion reaction. The soot particles are heated and emit electromagnetic waves, which is manifested visually as glowing particles. The plurality of glowing particles imparts to the flame a higher luminosity than would normally be associated with a gaseous fuel-air type combustion flame. The emitted electromagnetic waves contribute to increased radiative heat transfer rates to the furnace. It should be appreciated that the soot particles, as well as all the incomplete combustion products of the primary flame 8, are totally reacted to water and carbon dioxide. In this way the advantage of a fuel oil combustion, that is high luminosity, is combined with the advantage of a gaseous fuel, that is relatively small amounts of incompletely combusted products.

The high luminosity flame 16 extends outward from the external body member 2 of the burner 1 into the furnace. This is illustrated in FIG. 1 by a furnace wall 18 across which the flame 16 extends. It should be appreciated that the primary flame 8 is totally enclosed by the external member 2. The secondary high luminosity flame l6 sweeps across the furnace providing high convection and radiation heat transfer to the material being heated.

While the above described preferred embodiment illustrates the invention in detail, it should be understood that the present invention in its broadest aspects is not necessarily limited to its preferred embodiment. Other embodiments which do not depart from the scope and spirit of this invention should be understood to be covered by the invention as defined by the claims.

Iclaim:

1. A twostage burner for increasing the luminosity of a hydrocarbon flame comprising in combination:

a first buming means to burn a first hydrocarbon fuel in a diffusion flame confined in a conduit through which air is passed; and

a second burning means, disposed downstream in said conduit of said first burning means, to burn a second hydrocarbon fuel and the combustion products of said first hydrocarbon fuel in said air whereby a secondary flame of high luminosity is produced.

2. A method for increasing the luminosity of a hydrocarbon flame comprising the following steps in combination:

a. burning a first hydrocarbon fuel consisting essentially of unsaturated hydrocarbon in a diffusion flame in a first burning zone thereby producing soot by incomplete combustion of said fuel, said unsaturated hydrocarbon being selected from the group consisting of olefins and aromatics;

b. passing the combustion products associated with the burning of said first hydrocarbon fuel downstream into a second burning zone; and

c. burning said combustion products in said second burning zone with a second hydrocarbon fuel in the presence of air whereby a secondary flame of high luminosity is produced.

3. A method for increasing the luminosity flame comprising the steps in combination:

a. burning a first hydrocarbon fuel including a free radical promoting compound in a diffusion flame in a first buming zone, thereby producing soot by incomplete comof a hydrocarbon 8 bustion of said fuel;

4. The method of claim 3 wherein the free radical promoting compound is selected from the group consisting of nitropropane and amyl nitrite.

5. The method of claim 3 wherein the free radical promoting compound is a predetermined amount of oxygen less than required for complete combustion of said first hydrocarbon fuel.

Claims

2. A method for increasing the luminosity of a hydrocarbon flame comprising the following steps in combination: a. burning a first hydrocarbon fuel consisting essentially of unsaturated hydrocarbon in a diffusion flame in a first burning zone thereby producing soot by incomplete combustion of said fuel, said unsaturated hydrocarbon being selected from the group consisting of olefins and aromatics; b. passing the combustion products associated with the burning of said first hydrocarbon fuel downstream into a second burning zone; and c. burning said combustion products in said second burning zone with a second hydrocarbon fuel in the presence of air whereby a secondary flame of high luminosity is produced.
3. A method for increasing the luminosity of a hydrocarbon flame comprising the steps in combination: a. burning a first hydrocarbon fuel including a free radical promoting compound in a diffusion flame in a first burning zone, thereby producing soot by incomplete combustion of said fuel; b. passing the combustion products associated with the burning of said first hydrocarbon fuel downstream into a second burning zone; and c. burning said combustion products in said second burning zone with a second hydrocarbon fuel in the presence of air whereby a secondary flame of high luminosity is produced.
4. The method of claim 3 wherein the free radical promoting compound is selected from the group consisting of nitropropane and amyl nitrite.
5. The method of claim 3 wherein the free radical promoting compound is a predetermined amount of oxygen less than required for complete combustion of said first hydrocarbon fuel.