US3738819A - Method of using combustion adjuvant - Google Patents

Abstract

A METHOD OF EMPLOYING AN ADJUVANT FOR HYDROCARBON FUELS IS PROVIDED BY ADDING AN ADJUVANT COMPRISING A CALCIUM BASED MONTMORILLONITE CLAY, A PHOSPHATE, AND A SOURCE OF BORON OXIDE TO A HYDROCARBON FUEL IN A COMBUSTION ZONE. A PREFERRED FORMULATION COMPRISES 85 WEIGHT PERCENT CALCIUM BENTIONITE, 10 WEIGHT PERCENT ANHYDROUS TRISODIUM PHOSPHATE, AND 5 WEIGHT PERCENT SODIUM BORATE. THE ADJUVANT IS COMBINED WITH THE HYDROCARBON FUEL OR WITH COMBUSTIONN AIR IN AN AMOUNT OF FROM ABOUT 0.00001 UP TO LESS THAN ABOUT 0.1 WEIGHT PERCENT, BASED ON THE WEIGHT OF THE HYDROCARBON FUEL. COMBUSTION EFFICIENCY IS SUBSWTANTIALLY IMPROVED AND OXIDATION IS SUBSTANTIALLY MORE COMPLETE, SO THAT COMBUSTION PRODUCTS ARE PRODUCED IN LESS NOXIOUS FORMS. IN ADDITION, THE NAATURE OF SLAG OR OTHER DEPOSITS UPON SURFACES IN A FURNACE OR COMBUSTION CHAMBER ARE SUBSTANTIALLY ALTERED, SO THAT CORROSIVE CONDITIONS DO NOT OCCUR AND THE DEPOSITION OF SLAG IS PREVENTED OR MATERIALLY REDUCED, AND THE ASH IS PRODUCED IN A SOFT, FRIABLE FORM.

Description

United States Patent Int. Cl. C101 9/00, 1/32 U.S. Cl. 44-4 3 Claims ABSTRACT OF THE DISCLOSURE A method of employing an adjuvant for hydrocarbon fuels is provided by adding an adjuvant comprising a calcium based montmorillonite clay, a phosphate, and a source of boron oxide to a hydrocarbon fuel in a combustion zone. A preferred formulation comprises 85 weight percent calcium bentonite, 10 weight percent anhydrous trisodium phosphate, and 5 Weight percent sodium borate. The adjuvant is combined with the hydrocarbon fuel or with combustion air in an amount of from about 0.00001 up to less than about 0.1 weight percent, based on the weight of the hydrocarbon fuel. Combustion efficiency is substantially improved and oxidation is substantially more complete, so that combustion products are produced in less noxious forms. In addition, the nature of slag or other deposits upon surfaces in a furnace or combustion chamber are substantially altered, so that corrosive conditions do not occur and the deposition of slag is pre- 'vented or materially reduced, and the ash is produced in a soft, friable form.

' This application is a continuation-in-part of applicants copending application, Ser. No. 852,867, filed Aug. 25, 1969 now abandoned, and Ser. No. 11,827, filed Feb. 16, 1970 (US. Pat. No. 3,628,925).

This invention relates to a method for employing an adjuvant for combustion processes, and to a method for increasing the efliciency thereof. Additionally, the utilization of the method of this invention substantially reduces the relative amounts of undesirable, harmful and toxic components in the end products of hydrocarbon fuel combustion.

As is well known, the combustion of fuel oil, coal and natural gas produces a large number of by-products, including dust, fly-ash, sulfur dioxide, etc. Incomplete combustion results in the discharge of smoke, soot and carbon monoxide into the atmosphere. Anticipated industrial expansion threatens to increase the hazards of such air pollution appallingly in the relatively near future. Such pollution adversely affects vegetative plants and the health of animals and humans. Such efiects range from petty annoyance to chronic illness and death.

Polluted air has been linked to irritation of nose, throat, and eyes, aggravation of the respiratory tract, including bronchitis, emphysema, and cardiovascular ailments.

, Whether coal, gas, oil or other organic material comprises the fuel, with even the most efficient furnace design and operating conditions, complete combustion is seldom if ever attained. Build-up of tar, coke, soot and mineral slag on boiler surfacesconstitutes a serious problem, promoting chemical corrosion of metallic parts and greatly reducing efficiency of heat transfer. Burning of additional fuel to offset this reduced heat transfer merely increases 3,738,819 Patented June 12, 1973 the production of pollutants and adversely affects economy of operation. Furthermore, the procedures now commonly employed for removal of boiler depots are costly, generally unsatisfactory, sometimes requiring shut-downs, and such practices as blow-off of soot and fly-ash are increasingly prohibited by law.

Under conditions imposed by practical furnace construction, the formation of undesirable residues is a normal result of the combustion of fossil fuels. Except for the burning of elemental carbon, the combustion comprises rapid chain reactions in the gas phase. Furnace operation can be described, in fact, as a controlled explosion. The type and predominance of the various chain reaction steps depends partly upon the type of fuel; but inasmuch as the principal ingredients are carbon and hydrogen, the process of burning is controlled more by external factors such as concentrations, initial gas temperature and manner of mixing of fuel with the combustion air.

Majority of combustion occurs in the flame front, which measures fractions of a millimeter in thickness. Whatever combustion occurs must be practically complete within this boundary between burned and unburned gases, wherein all locally available oxygen is consumed. Under conditions of rapid furnace firing, ignition and combustion occur almost simultaneously. Propagation of the flame front generally is a thermal process, in that the flame must transfer heat to the unburned gas to cause it to ignite.

In oil burners the fuel is either vaporized or atomized before ignition. On heating and vaporization, a certain amount of decomposition of such oils occurs and some non-volatile carbonaceous material forms. Droplets of heavy oils are partly carbonized within the flame. The tendency to deposit carbon on and around the burner is a function of both molecular weight and molecular structure of the fuel. Tendency of fuel oils to smoke" increases with their carbon-hyrogen ratio.

Pulverized coal is 50% combusted in 0.05 second after the particles leave the burner port. At 0.1 and 0.3 second, approximately 5% remains unburned. Further reduction of unburned fixed carbon proceeds very slowly; elementary carbon does not vaporize at ordinary flame temperatures.

The combustion flame front impinges on furnace Walls and other heat absorbing surfaces, particularly under the conditions of hard firing. Although such surfaces may initiate some combustion steps through production of free radical chain carriers, other combustion intermediates are destroyed by such contact. Additionally, in the presence of insufficient air for complete combustion, lighter fractions evaporate, but the more complex compounds decompose and form carbonaceous deposits. Other factors contributing to carbon deposition include insuflicient secondary air, insuflicient mixing of air with volatile matter, temperature of air and fuel falling below the critical temperature, insufiicient time of contact between air and fuel, or impingement upon a cool surface. Incomplete secondary combustion results in formation of tarry vapors, solid carbon, gaseous hydrocarbons, carbon monoxide and hydrogen. Finely divided carbon is swept away in suspension in the flue gases to cooler zones of the furnace or is discharged from the stack as smoke or soot.

; Furnaces currently are constructed to provided for removal of deposits by blowers'and scrapers (or lances). Out-ofservice steam and water washing frequently is employed, although disposal of the wash water often becomes a problem. In-service boiler water washing can result in damage and should never be used with high-alloy super heater tubes because of thermal shock damage. tAlSO, chloride in the water can initiate cracking of austenitic tubing.

Contributing to inefiiciency of furnace operations is deposition on the tubes of inorganic fuel ash. This slag not only creates resistance to transfer of heat energy, but is also generally acid in reaction, causing sulfuric acid corrosion of affected metal surfaces. Whereas coal ash tends to neutralize some of the acid formed in the boiler, the vanadium contained in most oils increases the formation of sulfuric acid from sulfur dioxide. Consequently, the rapid removal of deposits on metal surfaces is extremely important.

In view of the problems arising from furnace operation, there has been an increasingly urgent need for means to enhance completeness of combustion, minimize formation of tarry and carbonaceous residues on boiler tubes and to prevent deposition of molten mineral slag on the metal surfaces.

It is accordingly an object of the present invention to provide a method of employing an adjuvant for hydrocarbon fuels which increases the efficiency of combustion and alters the nature of the combustion products.

These and still other objects are realized by the method of using the composition of the combustion adjuvant in accordance with the present invention, comprising a calcium based montmorillonite, a phosphate, and a source of boron oxide. The calcium based montmorillonite constitutes at least about 75% weight percent of the adjuvant, while the phosphate makes up about to 15 weight percent and the boron oxide source constitutes about 1 to weight percent. In addition to the foregoing, the adjuvant can further include, if desired, an essentially inert diluent in amounts ranging from 0 up to several hundred, or even several thousand weight percent, based on the weight of the adjuvant. As an inert diluent, any material can be used which does not detrimentally hinder combustion or the functioning of the adjuvant. For example, the diluent can be a hydrocarbon fuel oil, a substantially inert solid, such as a diatomaceous earth, or even an excess of the calcium based montmorillonite. The adjuvant composition is disclosed and claimed in the aforesaid application, Ser. No. 11,827.

The calcium based montmorillonite is preferably one of the naturally occurring montmorillonite based clays, such as bentonite. The material known as Southern Bentonite is preferred, since it is readily available at low cost in a form which is directly usable in the combustion adjuvant of the present invention, i.e. it is a calcium based montmorillonite. Other montmorillonite based clays can be used, but since such materials are not ordinarily calcium based, it is necessary that they be treated to replace at least a part of another metal with calcium.

The term calcium based is used to indicate that a substantial proportion of the metallic ions replacing aluminum in the montmorillonite crystalline lattice are calcium. The montmorillonite clays are crystalline aluminosilicates of a specific, known composition, having a planar structure of alternating sheets" of silica and alumina layer bonded to two silica layers. In other clays, such as kaolinite and illite, the structure differs by bonding of each silica layer to two layers of alumina, while the montmorillonite has each silica layer bonded to one alumina layer and one silica layer. Thus, designating silica as Si and alumina by Al, the C-dimension of the montmorillonite crystal lattice can be represented by the formula:

The adjacent i layers of the montmorillonite lattice gives the clay its distinctive properties.

Within the crystalline lattice of naturally occurring clays, a portion of the aluminum atoms are replaced by other metals in minor amounts, including iron, zinc, nickel, lithium, magnesium, calcium, potassium and sodium. In most montmorillonite clays of the bentonite variety, about 50 to 75 milliequivalents of exchangeable metallic bases occur per 100 grams of clay, and of this amount, sodium, calcium, magnesium and iron constitute the bulk. The relative amounts of sodium and calcium are of significance in the present invention, it being necessary to utilize a material having a predominant proportion of calcium and a relatively minor proportion of sodium. The material commonly known as Southern Bentonite is suitable, having about 1.3 to 3.5 milliequivalents calcium and only about 0.3 to 0.45 milliequivalents sodium per 100 grams of clay. Other clays of the montmorillonite type ordinarily predominate in sodium, which is detrimental in the combustion adjuvant of the present invention, and such clays, if used, must be ion exchanged to remove sodium and add calcium. The sodium content of the clay should not exceed 1.0 weight percent. Since such manipulations add considerably to the cost of the product, it is preferred to use a calcium based montmorillonite of a naturally occurring variety, e.g. Southern Bentonite.

The phosphate component of the combustion adjuvant can be, insofar as is presently known, any phosphate functional material, although some will, of course, be preferred for reasons of availability, cost or efliciency. For example, while in some contexts it will be desirable to utilize organo phosphates, e.g. tricresyl phosphate and the like, to reduce dusting of the composition, in most circumstances, such materials will be prohibitively expensive in comparison with inorganic phosphates. Common phosphate rock might be effective but for the highly corrosive nature of the hydrofluoric acid produced upon combustion. Preferred for economic considerations and effectiveness are the alkali metal phosphates, particularly anhydrous trisodium phosphate, which is inexpensive, readily available, and in a form conducive to ease of handling and formulation.

Similarly, the boron oxide can be supplied by any convenient source, so long as it does not further contain any constituent which is corrosive or detrimental to combustion. Boron oxide per se can be used, but a cheaper, more readily available source is sodium borate or common borax, which is accordingly preferred. Other alkali and alkaline earth metal borates and boric acid are further examples of suitable sources of the boron oxide.

The adjuvant composition is formed of the foregoing essential components as an intimate admixture in finely divided particulate form. The materials should be ground or pulverized to pass a 200 mesh, preferably a 325 mesh screen (to provide a maximum particle size of not more than about 44) The finer particle sizes enhance dispersion in the hydrocarbon fuel and minimize atomizer wear. The materials in such finely divided form are often subject to dusting, which can be effectively prevented by including a minor amount of light oil or other suitable oiling agent.

The composition is effective in rather broad relative proportions of the essential components, with at least about 75 weight percent of the calcium based montmorillonite being used, preferably about to 94 weight percent, while the phosphate is preferably about 5 to 15 percent. When circumstances are appropriate, the essential components can be combined with varying amounts of an inert diluent. For example, when large scale furnaces are utilized, their operation is often automated, and the introduction of the adjuvant of the present invention is also desirably automated. Measurement, handling and distribution are often facilitated by increasing the bulk of the adjuvant. The inert diluent can be utilized in amounts ranging from 0 to several hundred or even several thousand, percent, e.g. 5,000%, based on the weight of the adjuvant. The nature of such an inert diluent is limited only to materials which do not detrimentally afiect the operation of the furnace or of the adjuvant. Many such materials will be readily apparent to those of ordinary skill in the art, and can include, for example, both solid and liquid materials. For example, as illustrations of solid diluents which can be used, there can be mentioned coal, coke, carbon blacks, diatomaceous earth, siliceous materials, and the like. A particularly advantageous inert diluent is an excess of the calcium based montmorillonite. Liquid diluents can include such materials as kerosene, fuel oil, cycle oil, residual oils or the like.

The amount of the combustion additive to be added to a furnace will vary with the size and type of furnace and with the nature of the fuel. The considerations vary greatly and no general rule can be given, although it has now been found that substantial degrees of effectiveness are attained when the adjuvant is employed at levels from as little as 0.00001 up to less than about 0.1 weight percent, based on the Weight of the hydrocarbon fuel.

When the product of this invention, in finely ground form, is injected into the firing chamber, either independently, in intimate admixture with the fuel, or in the combustion air, completeness of combustion in the firebox is greatly enhanced, indicated by composition of stack gases and lowering of stack temperature. Formation of smoke, soot and tars is greatly reduced, and the deposition of slag and other materials on tubes and refractive surfaces is almost nullified. In fact, under proper firing conditions and without resort to mechanical cleaning methods, metal surfaces are maintained clean and bright. Accordingly, heat transfer is appreciably improved. Deposition of ash and slag is prevented almost entirely. Further, the slag removed from the ash pit is in a readily friable, powdery condition.

While the exact mode of operation of the composition is not clearly understood, the following explanation is offered, but it should be understood that applicants do not wish to be bound thereby. The minutely ground material, mixed with the fuel as it is sprayed or injected into the firing chamber, is broken into multitudinous finer particles at the flame front temperature. Heat energy absorbed by the crystalline material is surrendered and exchanged to combustion products as flame front temperature decreases with flow through the furnace, thereby promoting more complete combustion. Whether added continuously or intermittently, the material of this invention, broken into particles by the extreme temperatures, provides a very thin but frequently renewed highly refractory surface, upon which unburned compounds impinge and thereby undergo further additional oxidative reaction.

In addition, it has been observed that the effect of the method of use of the combustion adjuvant is probably at least in part catalytic in nature, although the precise nature of the catalytic effect has not been ascertained. The evidence supporting the probable catalytic nature of the effects of the present method are apparent from the following example, where it is clearly shown that the magnitude of the changes produced by the method is far greater than could result from stoichiometric effects alone.

EXAMPLE I A full scale test operation in an electric utility power plant, in service to a small city, is conducted. The full scale test is conducted in a modern boiler which burns on the average, about forty thousand pounds per hour No. 6 fuel oil. The boiler is operated under normal service conditions and monitored for the period of the test. The test program is conducted as follows:

The selected burner is removed from service, shut down and cleaning and routine maintenance are performed in the usual fashion. The furnace is then started up and placed in service to base load generating equipment under normal operating conditions with a magnesium oxide anticorrosion additive for about one month. When stable operating conditions are assured, base measurements, in

addition to the continuous monitoring by the operators, are taken. Three days thereafter, injection of the adjuvant of Table I is started at a rate of 0.023 weight percent based on the weight of the fuel, by injection with service air. On the twenty-eighth day of injection data is again taken.

TABLE I 1 Wt. percent Calcium bentonite Trisodium phosphate (anhydrous) 10 Sodium borate 5 The following results are obtained from the test period:

TABLE II Day Change percent (Base) 28 of base Adjuvant rate, wt. percent 0 0.023 Sulfur content, fuel wt. percent (1 2. 25 2. 25 Fuel rate, percent base (1.) 96 -4.0 Air rate, percent base. 100 92 8. 0 Reheater temp., F 990 1,010 +2.0 s03, p.p.m 22.1 8.2 64 502, p. .m 928.9 829.0 1l.0 00, v0 percent. i N.A. 0 a 100 002, vol. percent 12.2 13. 0 +6.0

1 Fuel rates and air rates are averages over the period of operation for the boiler in service to base line generating capacity.

2 Base measurements of carbon monoxide were lost because of damage to the samples taken.

a Carbon monoxide measurements were made by the Orsat system.

While Table II indicates substantial benefits to be derived from the use of the adjuvant, still other benefits accrue. For example, during the period of operation prior to injection of the adjuvant, relatively substantial amounts of slag and ash were produced, and deposits formed upon the surfaces of the furnace components. After a few hours of combustion with the adjuvant, slag and ash production was substantially reduced, and, additionally, the accumulated deposits were gradually eliminated.

EXAMPLE H In a test furnace, the adjuvant of Example I, Table I, is employed at a rate of about 0.00001 weight percent, based on the weight of fuel. Reductions in S0 SO ,CO, and fuel rate are noted.

As stated above, when bentonite is used as the silicate, it must be of low sodium content, not more than 5% and preferably less than 1% as N320. Likewise, in order to preclude slagging, the bentonite should possess not more than 10% by weight of iron, calculated as Fe O Should either sodium or iron exceed the indicated maxima, these can be reduced in amount by partial pyroxying of the exchangeable bases with hydrogen ions, utilizing acid treatment, or the like. Such procedure is familiar to those skilled in the art.

What is claimed is:

1. The method of promoting combustion efliciency of hydrocarbon fuels comprising burning said fuels in a combustion zone and adding to said combustion zone about 0.0001 to less than about 0.1 weight percent, based on the weight of said fuel, of a combustion adjuvant comprising about 80 to 93 weight percent calcium montmorillonite, about 5 to 15 weight percent of an alkali metal phosphate, and about 1 to 10 weight percent of a source of boron oxide selected from the group consisting of boron oxide, boric acid, alkali metal borates, and alkaline earth borates.

2. The method of claim 1 wherein said adjuvant is added to said combustion zone dispersed in combustion air.

3. The method of claim 1 wherein said adjuvant is added to said combustion zone dispersed in said fuel.

(References on following page) 7 8 References Cited 3,316,070 4/ 1967 Scott 4451 3,628,925 12/ 1971 Milner 44-4 UNITED STATES PATENTS 3,630,696 12/1971 Milner et a1 444 1/19'16 Barba 444 10/1940 Rick 61 5 CARL F. DEES, Primary Examiner 10/ 1961 Thompson 444 10/ 1967 Kukin 444 US. Cl. X.R. 11/1968 Booth 444