US3358731A

US3358731A - Liquid fuel surface combustion process and apparatus

Info

Publication number: US3358731A
Application number: US539522A
Authority: US
Inventors: James J Donnelly
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1966-04-01
Filing date: 1966-04-01
Publication date: 1967-12-19
Anticipated expiration: 1984-12-19

Description

Dec. 19, 1967 3,358,731

LIQUID FUEL SURFACE COMBUSTION PRocEss AND APPARATUS .J. J. DONNELLY 2 Sheets-Sheet 1 Filed April 1966 7 z ,xx z

F/gure I'INVENTOR. y L/Umm JDonne//y f cuz,

Afforney 2 Sheets-Sheet 2 Dec. 19, 1967 J. J. DONNELLY LIQUID FUEL SURFACE COMBUSTION PROCESS AND APPARATUS Filed April 1966 United States Patent O 3,358,731 LIQUID FUEL SURFACE CGMBUSTION PRGCESS AND APPARATUS lames J. Donnelly, Cinnaminson, NJ., assignor to Mobi Oil Corporation, a Corporation of New York Filed Apr. 1, 1966, Ser. No. 539522 7 Clairns. (CE. 158-470 ABSTRACT GF THE DCLOSURE A liquid fuel is atomized in a manner to form electro- This invention relates to a process and apparatus for burning fuels, especially hydrocarbon fuels. More particularly, this invention relates to a process and apparatus whereby fuels in the form of electrostat ically charged droplets are subjected to surface combustion.

The term "surface combustion as employed herein is meant to describe the phenomenon whereby fuels are burned on a surface either in the absence of a flame or in the form of a microfiame, but in any event, in the absence of a normally visible fiame. At the present time, there is a diversity of opinion as to whether surface combustion takes place in the absence of a flame or in the presence of a microflame. It is my intention not to limit the process of this invention to either theory, but to merely distinguish the phenomenon from combustion having a normally visible fiame.

Surface combustion is a desirable means for burning certain fuels since this technique results in higher thermal efliciencies than can be attained with flame combustion techniques. Present surface combustion techniques employ relatively volatile fuels since previously no effective way has been found to efilciently direct liquid fuels to the combustion surface. Gaseous hydrocarbons have been conducted to the surface of porous refractory materials by means of internal channels. However, it is not practical to feed normally liquid hydrocarbons to the surface in this manner, because of the tendency of such materials to thermally decompose within the passages and cause pluggng.

On the other hand, the use of liquid fuels, especially higher boiling fuels, such as middle distillates and mixtures of middle distillates with residual oils in previous surface combustion techniques has been of limited success and restricted to special applications. Surface combustion is used to some extent in industrial furnaces and boilers using liquid hydrocarbon fuels, but in general, normal fiame combustion accounts for the bulk of the heat producing process. Furthermore, in such installations, special air handlng equipment which consumes large quantities of power is required to obtain eiiicient burning.

There is an economic incentive to employ the lower cost, higher boiling hydrocarbons, such as middle distillates and mixtures of middle distillates with residual oil, as fuels; especially in the smaller furnaces and boilers employed for space heating purposes. There is a further economic incentive to reduce the cost of burning such fuels in commercial units by reducing Operating power costs, promoting more complete combustion and reducing significantly or elim-inating the emission of undesirable smoke and soot.

In the usual process of burning liquid fuels, the fuels are first atomized. The resultant droplets are fully or partially vaporized by the inflow of heat by convection or radiation. The fuel vapors d-ifiuse into and are mixed with an oxygen containing gas, usually air, and the mixture is ignited and burned. Ordinarily, in burning liquid fuels, fuel droplets and an encompassing air stream are injected into a heterogeneous burning mixture composed of vaporizing and burning droplets, burning fuel vapors and hot combustion products. When conditions within the mixture are appropriate, the fuel is burned completely with minimum excess air.

In burning liquid hydrocarbon fuels, the difi'iculty of effecting complete combustion increases as the fuels become heavier, that is, as their viscosities increase and their volatilities decrease. As their viscosities increase, fuels become more difficult to atomize so that high injection pressures, up to 1000 p.s.i.g., must be used or, alternatively, two-fluid atomizers with steam or air as an atomizing agent must be employed. As their volatilities decrease, the resultant droplets become more difficult to vaporize, so that greater effort must be made to increase heat transfer. This may be accomplished by increasing turbulence by injecting the combustion air at higher pressures. Increasing fuel and air -injection pressures required the use of more costly fuel and air handling equipment and significant increases in the power required to operate such equipment.

it is possible to overcome the need for the production of fine droplets in the precombustion process of atomization, if there is a compensating increase in the rate of heat transfer to the droplets. Surface combustion can provide such increased heat transfer, provided the droplets can be made to approach the incandescent surface on which combustion is occurrng. Up to the time of the present invention, it had been difficult to obtain complete combustion of middle distillates and mixtures of middle distillates and residual oil for extended periods of time while employing surface combustion techniques.

In surface combustion, it is desirable to concentrate the fuel to be burned on or Very close to the combustion surface. When this fuel concentration is attained, high combustion eficiencies are obtained. One means proposed for concentrating fuel in a relatively small area is through the use of electrostatic charges. In such processes, atomized fuel droplets are electrostatically charged and are directed from a charging electrode to an attracting electrode maintained at a substantially different voltage. Unfortunately, in prior arrangements, when the thus charged fuels are ignited, a fiame is formed which contains ions. The ionized atmos-phere thus produced promotes arcing between the two electrodes with the result that the fuel no longer becomes electrostatically charged, and furthermore, the electrostatic field which propels the droplets is destroyed. Unless this arcing is prevented it renders electrostatic charging as a means for atomizing and concentrating fuels relatively useless in processes where it is desirable to maintain fuel combustion over extended periods of time.

As an alternative to fuel electrostatic charging, it has been proposed to employ a stream of air to move suspended fuel droplets to the surface combustion surface and as a means for supplying sufiicient free oxygen to support combustion. Unfortunately, with liquid hydrocarbon fuels volumetrically a relatively large amount of air is needed to attain complete combustion. Furthermore, air vclocities through the combustion space are necessarily high. When large amounts of air are employed as a fuel motive force in surface combustion, the atomized fuel tends to follow the air flow lines and by-pass the combustion surface. This results in lower combustion eficiency. It is therefore desirable to minirnze the influence of m-oving air which might divert fuel from moving toward the cornbustion surface.

In the combustion of hydrocarbon fuels, it is desirabie to atomize the fuel to obtain small droplets. This facilitates control of droplet trajectories and improves fnel dispersion on the combustion surface. For the reasons given above, it is undesirable to suppiy the free oxygen combustion requirements in a manner which seriously disrupts fuel flow to the combuston surface. The present invention provides a means for supplying sufcient free oxygen to the combustion surface without seriously disrupting fuel flow. Furthermore, in the present invention, the free oxygen is supplied in such a way that it prevents or controls the counterflow of flame ions to the attracting electrode so that arcing between electrodes is avoided. Up to the time of the present invention, complete combustion of hydrocarbon fuels, by a surface combustion technique, has not been attained.

It is an object of the present invention to provide a process and apparatus for effecting complete conbustion of liquid fuels for eXtended periods of time.

It is a further object of the present invention to provide apparatus for dispersing liquid fuels and subsequently effecting complete combustion thereof by a surface combustion technique.

By the process of this invention, the fuel to be burned is atomized to form droplets in a manner so that they are electrostatically charged. The electrostatically charged droplets are then directed to a heated surface or a surface on which surface combustion is occurring. The droplets are charged by application of high voltage with the atomizer being employed as one electrode with a second droplet attracting electrode being positioned downstream in the direction of droplet flow. A surface upon which fuel combustion is to take place is interposed between the two electrodes so that the droplets will impinge thereon. The use of the two electrodes create an electrostatic field which helps atomize the fuel and Supplies a motive force for the fuel droplets to direct them to the combustion surface. The combustion surface is initially supplied with heat to initiate combustion and with sufhcient free oxygen to initiate and maintain combustion. The free oxygen necessary to effect complete fuel combustion is supplied to the combustion surface in a manner which does not seriously disrupt the flow of fuel. At least some of the free oxygen is conducted to the combustion surface through Channels in the surface combustion device at velocities suflicient to prevent or control the counterfiow through such Channels to the attracting electrode of electrical charges initially carried by the fuel droplets to the flame zone. The flow of charged particles is regulated to prevent arcing between electrodes and build-up of a space-charge. In another aspect of this invention, the possibility of arcing can be minimized by coating the downstream electrode with an insulating material such as porcelain.

Atomization of the fuel is carried out in a manner so as to form droplets of a sufi'iciently small size to avoid subsequent localized concentration of fuel on the comhustion surface. In addition, atomization is carried out in a manner to permit electrostatic charging of the droplets formed. It has been found that coincident atomization and electrostatic charging can be accomplished by Contacting the fuel with a surface maintained at a voltage substantially different from an attracting electrode, while at the same time subjecting the fuel to disruptive forces which cause droplet formation. The resultant droplets are electrostatically charged and are of a sufliciently small size so that upon contact of the droplets or resultant vapor with the combustion surface, complete combustion thereof is effected. The droplets may be partially or completely vaporized prior to contacting the combustion surface by heat transfer from the radiant combustion surface or the hot combustion products owing from the surface.

In the process of this invention, fuel atomization is carried out so as to produce electrostatically charged fuel droplets of a sufiiciently small size so that localized quenching of the combustion surface is avoided. In the process of this invention, desirable fuel atomization can be attained by various device including an ultrasonic horn, a two-fluid nozzle and a standard high pressure nozzle or similar devices. But in each instance the atomizer is maintained at a voltage substantially different from an attracting electrode so that the droplets formed are electrically charged. With some atomizers, modifications may be made to facilitate the droplet charging process. In some cases clectrostatic forces alone may be used to effect atomization.

It is sometimes preferable to employ an ultrasonic born as an atornizer. The horn can be caused to oscillate by a driving means such as a piczo-electric Crystal which is connected with a suitable electrical power supply. The horn is constructed to amplify the crystal oscillations. The fuel to be atomized is directed to the oscillating flat surface of the horn. The fuel can be directed to the oseillating fiat surface either from a source separate from the transducer or from a source which forms part of the transducer structure. In either case, the fuei is contacted with the horn surface for a sufcient period of time to cause the resultant fuel droplets to be electrostatically charged. The fuel droplets are directed to the combustion surface because of the voltage ditferential between the atomizer and the electrode positioned downstream which is associated with the combustion surface. The accumulation of electrical charges at the combustion surface is avoided by permitting controlled migration of the charged particles through the channels of the surface combustion element. Counte'flowing air velocities are regulated to obtain the desired rate of charge migration. The charges may also be drained from the combustion zone by a collecting electrode connected to the attracting electrode through an electrical resistance.

In the process of this invention, it is possible to employ a variety of fuels including hydrocarbon fuels such as middle distillates or mixtures of residual oi and middle distillate. In order to supply sufiicient air to support complete surface combustion, while not disruptng fuel flow to the combustion surface, it is supplied thereto through the porous combustion surface and through ports on the combustion surface in a direction opposite to fuel flow and in a manner to prevent or control the flow of electrical charges to the attracting electrode. The -ports are uniformly distributed to provide substantially uniform air concentration on the combustion surface. It is within the scope of this invention to provide additional air to the combustion surface which ows in a direction arallel to fuel flow but in amounts less than that which disrupts fuel flow. In addition, apparatus can be employed which assists in attaining the proper balance of air flow.

For a more complete understanding of the present invention, reference is made to the accompanying drawings.

FIGURE 1 shows one form of the present invention wherein an oscillating transducer driven atornizer is ernployed.

FIGURE 2 shows an alternative form of this invention wherein a ring shaped electrode atomizer and a cone shaped combustion surface are employed.

FIGURE 3 is a detailed view of the surface combustion device shown in FIGURE l.

Referring now to FIGURE l, the fuel to be burned is directed to the interier of the oscillating atomizer 2 through conduit i. The atomizer comprises a driving crystal 6, a horn 8 and a horn extension 10 which is placed in a horn housing 12. The Crystal transducer 6 is caused to oscillate by means of an electrical energy source not shown. The fuel flow is regulated by valve 14. The fuel is directed to the surface 16 of the oscillating horn extension 1@ through conduit 18. The horn 3 is subjected to an electrical potential in order to electrostatically charge the fuel drcplets emitted from the surface 16 of horn extension 10. The electrostatically charged droplets are attracted to a metal electrode 20', maintained at a voltage significantly different from the horn 8, located at the interior of surface combustion device 22. Coincident with fuel droplet flow, primary air is directed to surface combustion device 22 through conduit 24. Within surface combustion device 22 the air is uniformly distributed and then directed to the surface 26 of combustion device 22 by means of uniformly spaced ports located on the surface 26 as well as through pores in the combustion device 22. This air flow is regulated to control counterflow of the charged particles from the combustion zone to the attracting electrode so that neither arcing between electrodes nor space charge build-up occurs. Prior to contact of fuel and air, the combustion surface 26 is preheated to the fuel ignition temperature by a heating device not shown. On the combustion surface 26, the fuel is burned in the absence of a visible flame. The heat resulting from the fuel combsution is maintained within the combustion zone 28 by an insulated shell 39. The heat is captured by means of a heat exchanging arrangement wherein a heat exchange fluid, as for example, air or water is directed to the combustion zone 28 through

conduits

32 and 34. The fluid is directed through the combustion Zone 28 through

conduts

36 and 38 wherein it is heated by radiation and convection. The heated fluid is then withdrawn from the combustion Zone 28 through conduit 40. The combustion products are withdrawn from the combustion zone 28 through exhau'st 42.

Referring now to FIGURE 2, the fuel to be burned is directed to the interior of hollow atomizing ring 1 through conduit 3. The fuel in the atomizing ring 1 is subjected to a high electrical potential and is forced through the small perforations located on the interior of atomizing ring 1. The fuel is atomized by electrostatic forces resulting from the electrical potential applied to the ring 1. The resultant electrostatically charged fuel droplets are attracted by the low electrical potential metal screen 5, located within the heated conical combustion surface 7. The combustion surface is heated to the fuel ignition temperature by heating element 9. Coincident with the fuel flow, primary air is directed to the interior of cone 7 through

conduits

11 and 13 and thence to the combustion surface through ports 15 as well as through pores in the combustion device. This air flow is regulated to control counterflow of charged particles from the combustion zone to the attracting electrode so that neither arcing between electrodes nor space charge build-up occurs. Secondary air to be employed in the combustion process is allowed through the opening 17 into combustion zone casing 19 by normal convection. On the combustion surface 7, fuel is burned in the absence of a visible flame. The usable heat generated thereby is captured by a suitable heat exchange apparatus not shown.

Referring now to FIGURE 3, the surface combustion device shown comprises two mating symmetrical portions 44 which have been joined in a complementary manner with a metal screen electrode 20 interposed therebetween. The surface combustion device is made of a porous ceramic material which will promote surface combustion. Air employed in the combustion process enters the interior of the combustion device 22 through conduit 24. Within the combustion device, a plurality of interior walls 46, which extend through a portion of its interier length, function to promote uniform air distribution. The exterior combustion surfaces 26 are each provided with a plurality of uniformly spaced ports which permit air flow from the interior of the surface combustion device 22 to the surfaces 26. Atomized fuel impinges upon the exte'rior surface 26 and is contacted with air flowing from ports 48 as well as from pores within the surfaces 26. When the surface is in a heated condition, surface combustion ensues.

Within the scope of this invention, the surface combustion device can be of any configuration as for example, conical, flat, spherical and the like so long as the atomized fuel will impinge thereupon substantially uniformly. The surface combustion device is made from a material which will promote surface combustion and will not deteriorate under the influence of heat. Suitable materials include porous ceramics, firebrick, asbestos and the like which may or may not be impregnated with an addition al catalytic agent as for example, Group VIII metals -or their oXides, iron oxide, nickel oxide or platinum.

The voltages employed should be sufliciently high to efect satisfactory control of droplet trajectories without causing an undesirable electrical discharge between the electrodes. The Voltages necessary to provide suitable droplet trajectory control or electrostatic atomization and subsequent droplet trajectory control vary with the electrical charge on the droplets, the shape and spacing of the electrodes, the nature of the intervening matter, and the velocities of the gas streams through which the droplets move. In general, the voltages used should be such that the electrical field gradient is kept below 30 kilovolts per centimeter. It is preferable to keep the voltage difference low enough so that the electrical field gradient is below 5 kilovolts per centimeter. Depending upon burner configuration and Operating conditions, it may be desirable to use a constant voltage, an alternating voltage or a combination of the two types.

Depending upon the arrangement of the surface combustion element and the nature of the beat removing techniques employed such as radiation, convection, expansion through a turbine or nozzle, varying amounts of combustion air can be introduced through the porous surface combustion device. In some instances all the combustion air may be introduced through the porous combustion surface. In this case the quantity of combustion gas can range from that corresponding to the stoichiometric equivalent amount for complete combustion of the fuel to an amount corresponding to 200 percent excess gas. Usually in introducing all the c-ombustion gas through the surface combustion element, it is desirable to maintain the combustion gas quantity between the stoichiometric equivalent and that corresponding to 50 percent excess gas.

In some instances -only a portion of the combustion gas is introduced through the porous surface combustion element. In this case the quantity of combustion gas so introduced might range upward from a fraction of the amount necessary to combine with the fuel to produce all gaseous products to an amount greater than the stoichiometric equivalent for complete combustion. Usually in introducing part of the combustion gas through the porous element it is desirable to maintain the gas quantity so that the ratio of oxygen atoms in this incoming primary gastto carbon atoms in the incoming fuel is greater than one. Secondary gas is introduced into the combustion space priman'ly to provide sufficent additional oxygen to complete combustion of the fuel, although in some applications (eg. gas turbines) it may be used for tempering purposes as well. The quantity of combustion gas in the combined primary and secondary combustion gas streams can range from the stoichiometric equivalent for complete combustion to 1500 per cent excess combustion gas. However, it is usually desirable to a maintain the total combustion gas supply between that corresponding to the stoichiometric equivalent for complete combustion and that corresponding to 500 percent excess combustion gas.

Having fully described my invention I claim:

1. The process for burning a fuel which comprises Coincidentally atomizing and electrostatically charging a liquid fuel, directing the resultant atomized charged fuel to a heated surface which promotes surface combustion, and supplying free oxygen substantially uniformly to said combustion surface in a direction opposite to the flow of 7 said atomized fuel to effect surface comb'ustion of said fuel on said surface.

2. Apparatus for efecting fuel surface combustion comprising in combination:

(A) Means for coincidentally -atomizing and electrostatically charging a liquid fuel to prod'uce a flow of atomized fuel to a surface combustion means,

(B) Surface combuston means located downstream in the direction of atomized fuel flow comprising;

(1) A surface capable of promoting surface com- 10 b'uston having a plurality of unifornly paced openings to permit contact of free oxygen and said atomized fuel thereon,

(2) A free oxygen inlet means adapted to direct free oxygen to said surface and,

(3) An electrode located downstream from said surface in the direction of fuel flow having an electrical potential suficiently different from (A) to electrostatically attract said atomized fuel to said surface.

3. The apparatus of claim 2 whercin said surface (Bl) is fiat.

4. The apparatus of claim 2 wherein said surface (Bl) is conical.

5. The process of claim I wherein the fuel is a liqud petroleum hydrocarbon.

6. The process of claim 1 Wherein the fuel is a middle distillate petroleum hydrocarbon.

7. The process of claim 1 Wherein the fuel is a mixture of a middle distillate petroleum hydrocarbon and a residual oil.

References Cited UNITED STATES PATENTS 1,S4S,155 7/1925 Mehner 263-40 2,604,936 7/1952 Kaehni et al. 158-113 3,224,485 12/1965 Blomgren et al. ISS-1175 X 3,269,446 8/ 1966 Luther 158--4 FREDERICK L. MATTESON, JR., Prma'y Examiner.

E. G. FAVORS, Assistant Examner.

Claims

2. APPARATUS FOR EFFECTING FUEL SURFACE COMBUSTION COMPRISING IN COMBINATION: (A) MEANS FOR COINCIDENTALLY ATOMIZING AND ELECTROSTATICALLY CHARGING A LIQUID FUEL TO PRODUCE A FLOW OF ATOMIZED FUEL TO A SURFACE COMBUSTION MEANS, (B) SURFACE COMBUSTION MEANS LOCATED DOWNSTREAM IN THE DIRECTION OF ATOMIZED FUEL FLOW COMPRISING; (1) A SURFACE CAPABLE OF PROMOTING SURFACE COMBUSTION HAVING A PLURALITY OF UNIFORMLY SPACED OPENINGS TO PERMIT CONTACT OF FREE OXYGEN AND SAID ATOMIZED FUEL THEREON, (2) A FREE OXYGEN INLET MEANS ADAPTED TO DIRECT FREE OXYGEN TO SAID SURFACE AND, (3) AN ELECTRODE LOCATED DOWNSTREAM FROM SAID SURFACE IN THE DIRECTION OF FUEL FLOW HAVING AN ELECTRICAL POTENTIAL SUFFICIENTLY DIFFERENT FROM (A) TO ELECTROSTATICALLY ATTRACT SAID ATOMIZED FUEL TO SAID SURFACE.