US3421824A - Method of burning industrial fuels - Google Patents

Description

Jan. 14, 1969 w. A. HERBST 3,421,324

METHOD OF BURNING INDUSTRIAL FUELS Filed June 1, 1967 AIR-I FUEL AIR 1 I 4 COOLING JACKET COMBUSTION CHAMBER I w. A. I-IERBST NT/mm e rfm Siia PATENT. ATTORNEY United States Patent 3,421,824 METHOD OF BURNING INDUSTRIAL FUELS Walter A. Herbst, Union, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed June 1, 1967, Ser. No. 642,928 US. Cl. 431 7 Claims Int. Cl. F23m 3/04; F23c 1/00 ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method of burning liquid industrial fuels in which the corrosivity and air polluting tendencies of the combustion products of such burning are minimized, while utilizing to the highest possible degree the heat content of the fuel. Specifically, the invention relates to a two-stage burning process in which atomized fuel is burned to produce impurity-containing intermediate solid products which are removed and thereafter burning any remaining gases so as to accomplish substantially complete combustion of the fuel.

Description of the prior art Major problems of corrosion and air pollution often occur in the operation of industrial combustion equipment. Generally, when fuel such as residual petroleum, coal, and other byproduct residue of industrial processes is burned, large amounts of corrosion promoting compounds, usually oxides of sulfur, vanadium, other metals and more complex interreaction products are deposited on heat utilization equipment, causing poor heat transfer, equipment breakdown and general overall lack of efiicency. These corrosion promoting compounds may chemically damage combustion equipment or may physically cause damage by interfering with moving parts.

Steam boilers and gas turbines are particularly vulnerable to loss of efficiency from combustion product deposits. Hot rotor and stator blades of turbines can become coated and pitted by combustion deposits. Boiler tubes, when subjected to large combustion deposits, cannot satisfactorily perform their heat transfer function. In addition to these aforesaid problems, air pollution occurs when sulfur compounds, e.g., S0 and S0 and other combustion products, are emitted from the heat utilization equipment.

In vol. 51, No. 494, Journal of the Institute of Petroleum, pages 7172 (February 1965), it is shown that atomized droplets of fuel oil burn in two stages. In the first stage of combustion, depending on the temperature of this initial burning, the fuel oil droplets form intermediate solid particles which may be in the form of hollow carbon skeletons called cenospheres, or in the form of small, dense, coke-like particles. If the original fuel contains sulfur, vanadium and other elements which form corrosion promoting and air polluting oxides, such undesirable substances concentrate in these intermediate solid particles. In the second stage of combustion the intermediate 3,421,824 Patented Jan. 14, 1969 solid particles are completely consumed, releasing the sulfur, vanadium and other impurities in the form of corrosion promoting and air polluting oxides and other complex reaction products.

SUMMARY OF THE INVENTION It has now been found, in accordance with this invention, that the corrosivityjand air polluting tendency of the combustion products of liquid, high boiling, industrial fuels can be reduced by introducing air and atomized fuel in substantial stoichiometric equivalents into a combustion zone, burning the fuel to form intermediate solid and gaseous combustion products, separating the intermediate solid products from the intermediate gaseous products, then adding additional air to the intermediate gaseous products in an amount sufficient to give substantially complete combustion of the gases, and finally, burning the intermediate gaseous products.

In this invention the initial step in the combustion process is to introduce air and fuel into a combustion zone under conditions which, upon combustion of the fuel, favor the production of the intermediate solid combustion products described above. The fuel should be mixed with the air in the form of atomized particles, these particles generally having a droplet diameter in the range of approximately 3 to 700 microns.

The amount of air to be mixed with the fuel in the initial combustion step which favors the formation of the solid intermediate combustion products depends upon the amount of time which elapses between the combustion step and the separation step. An air to fuel ratio of up to 1.2 times the theoretical stoichiometric equivalent of air to fuel is satisfactory for the production of the solid intermediate combustion products if the initial combustion and separation steps occur almost simultaneously. If such be the case the separation occurs so rapidly that the amount of stoichiometrically excessive air does not have the opportunity to completely burn the solid intermediate combustion products. If, however, the initial combustion and separation steps are separated by a relatively long period of time as, for example, in the case where these steps are carried out in separate pieces of equipment, air to fuel ratios of 0.8 to 1.03 times the stoichiometric equivalent must be used. Intermediate solid combustion products will be formed using this ratio, and even though a substantial length of time separates the two steps, because of the limited amounts of air available, the danger of undesired burning of the solid intermediate combustion products is minimal.

The next step in the process after the proper mixture of air and fuel has been introduced into the combustion zone is to burn the fuel and form the intermediate combustion products. The temperature in the combustion zone in this burning step is usually in the range of 2500 Fl to 4000 F. In addition to the formation of the solid intermediate combustion products, intermediate gaseous products such as H CO, CH, and C H are formed, which are subsequently consumed in the final burning step. The amount of intermediate solids produced by the initial burning step will be in the range of approximately 0.1 to 20 wt. percent of the fuel.

After the initial burning stage has been completed, the solid intermediate combustion products are then removed from the gaseous combustion products. The manner in which the separation is effected bears directly upon the air to fuel ratio employed in the initial combustion step. If the low air to fuel ratios (0.8 to 1.03 times stoichiometric) have been used and solid intermediate combustion products formed, the process of separating the solids from any unburned gases may be delayed for a time, because, as most of the air has been used for the formation of the intermediate solid products, the danger of completely burning these products and thus liberating the corrosive compounds is minimal. The separation of the solids from the gases may then be carried out in a step separate from the initial combustion step by known methods in equipment such as cyclones, settling chambers, filtering beds, etc. If, however, higher air to fuel ratios (1.03 to 1.2 times stoichiometric) are employed in the initial combustion step, the intermediate solid products must be separated almost immediately from any unburned gases, i.e., the solids must be separated from the gases within about 0.001 to 0.05 second from the time of initial combustion. This procedure minimizes the danger of combusting the .solid intermediate combustion products to its corrosive oxides. This separation may be accomplished by carrying out the combustion under cyclonic conditions so that the solids are thrown from the periphery of the flame by centrifugal force. Other methods such as the use of baflles or impact plates will suggest themselves to those skilled in the art.

After the separation of the solid intermediate condensation products from the intermediate gaseous products, the next step of the process is to combust these gases so as to utilize as much of the heat content of the original fuel as possible. To these gases, e.g., H CO, CH C H etc., is added an amount of air necessary to give substantial smokeless combustion. The amount of additionl air added to the stream containing the particle free intermediate gaseous products is in the range of approximately 0.2 to 0.7 times the stoichiometric amount of air to the original amount of fuel. This would bring the overall air to fuel stoichiometric range for the entire combustion process of this invention to 1.0 to approximately 1.73 if air to fuel ratios of 0.8 to 1.03 were employed in the initial combustion stage and approximately 1.03 to 1.9 if the air to fuel ratios of 1.03 to 1.2 were initially employed. The temperature of the particle-free gas stream which is to be fully burned will be such that upon the addition of air, combustion will be spontaneous, i.e., approximately 2500 F. to 4000 F. The temperature of the fully-combusted gas stream may be reduced by introducing an amount of air over and above that needed for full combustion. This may be done when the fully combusted gas stream is too hot to be employed in heat utilization processes. In such a case the additional air may amount to approximately 1 to 10 times the stoichiometric amount of air to the original fuel. The final combustion products will consist mainly of N H 0 and C0 The heat in this stream is utilized by industrial equipment such as boilers and gas turbines.

The type of fuel to which the process of the invention is applicable is carbonaceous fuels in general. It will be particularly applicable to liquid fuels high in sulfur content (e.g., up to 6 wt. percent) and/or containing ap.- preciable amounts of corrosion promoting materials (e.g., up to 0.2 wt. percent). Liquid residual fuels rich in vanadium (up to 1,000 p.p.m.) or other materials that form corrosive products would find this process advantageous. Such fuels will usually be relatively viscous (up to 200 seconds at 210 F. by the Saybolt Universal viscosimeter) The process may, however, be applied also to distillate fuels to reduce the corrosive effect of sulfur compounds even though such distillates will not normally produce harmful amounts of other corrosion promoting com pounds. The process also may be applied to conditions where corrosion promoting constituents are brought in With the combustion air as in marine applications where the air may contain relatively large amounts of salt from sea water spray.

BRIEF DESCRIPTION OF THE DRAWING The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawing in which is presented a flow diagram showing the preferred embodiment of the invention.

4. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the attached drawing, residual petroleum fuel containing undesirable trace elements of sulfur, vanadium, sodium, and the like is sprayed by means of at atomizer 1, e.g., pressure or pneumatic types, into combustion chamber 2. The fuel droplets thus formed are burned with an amount of air introduced in conjunction with the droplets through inlet 11 which is in the range of 0.8 to 1.03 times the theoretical stoichiometric equivalent of air to fuel. After the initial combustion, the fuel gas stream comprised of solid intermediate combustion products and intermediate combustion gases such as H CO, CH.,, and C H formed by the initial combustion is sent to particle separator 4 via conduit 3. Different types of particle separators, depending on the temperature and velocity of the incoming fuel gas streams may be employed to separate the solid and gas components, e.g., a gravity settling chamber, cyclone separator, etc. It is preferred, however, to use a cyclone separator for this process. As the particle-containing intermediate combustion stream enters separator 4, it is swirled in a circular manner to separate the light and heavy components. The particular design of this apparatus depends on gas velocities, particle sizes and the relative density of the particles, i.e., the density of the particles compared to the density of the gases, and need not be explained here as such design technique is well within the purview of one skilled in the art. Relatively heavy particles need not be centrifuged as rapidly as lighter particles. The particular operating conditions depend on the physical properties of the combustion products of the particular fuel being used. The swirling action centrifuges out the solid intermediate combustion products, leaving the gaseous component to pass upward through outlets 5 to the final combustion step. The temperature in the separator is approximately the same as that in combustion chamber 2.

Incases where the air to fuel ratio is greater than 1.03 times stoichiometric, the initial combustion will occur in the separation device so that the cyclone is used directly as a combustion zone and the particles are centrifuged directly from the flame. In this case conduit 3 would be eliminated and separator 4 would be the combustion chamber into which the fuel would be directly sprayed. Separation times would be in the range of approximately 0.001 to 0.05 second. In this manner residence times are reduced and equipment size made more compact.

The separated gases are thus carried to boiler 9, or other device for the utilization of hot gases, e.g., a gas turbine, and additional air is added through inlet 8 to complete the combustion of the fuel gas stream. The temperature at the point of introduction of the additional air is approximately that of the cyclone, i.e., approximately 2500 F. to 4000 F. The final combustion products consisting mainly of N H 0 and CO pass from the heat utilization equipment through exhaust port 10 and then to exhaust stacks.

In this description the additional air has been shown as being added directly to the heat utilization equipment. This, however, is only the preferred mode of operation as full heat utilization is accomplished. The additional air may be added at some point prior to the introduction of gases into the heat utilization equipment with full combustion occurring at that prior point. The hot, fully combusted gases would then be sent to the heat utilization equipment. Heat loss would, however, occur between the point at which complete combustion takes place and the point at which the heat from that combustion is used.

In the preferred embodiment of this invention, solid intermediate combustion products rich in sulfur and other corrosion promoting compounds are removed from dip leg 12 of the cyclone through fluidized solids trap 13. The fluidized solids may consist of the finely divided solid, intermediate combustion products themselves, or finely divided silica or alumina or other high melting finely divided solid added as a suspension in the initial combustion air or added to the cyclone. Alternately the solids may be collected in a storage zone and removed periodically. Other means of removing the solids will occur to those skilled in the art. A cooling jacket 14 may be employed on the cyclone separator to reduce the rate of deterioration of the cyclone caused by excessively high temperatures. The cooling fluid may be air or steam. In the latter case, the cyclone jacket may be used as a steam generator or a steam preheater and incorporated into the steam system of the boiler.

It can be seen then that by this process sulfur and other solid corrosion promoting constituents which contribute to air pollution and degradation of hot metal parts of industrial heat utilization equipment are eliminated, thereby enhancing the operability and life of such equipment. The amount of smoke produced by this process will have also been abated as the amount of air ultimately available is more than enough for the complete combustion of the fuel.

What is claimed is:

1. A method of reducing the corrosivity and air polluting tendencies of the combustion products of carbonaceous fuels containing corrosion promoting materials which comprises: (a) introducing air and atomized fuel in substantial stoichiometric ratios into a first combustion zone, said ratio being sufficient to form separable intermediate solid and gaseous combustion products upon combustion; (b) burning the fuel to form intermediate solid and gaseous combustion products, said intermediate solid combustion products having concentrated therein the corrosion promoting materials; (c) separating the intermediate solid products from the intermediate gaseous products; (d) adding additional air to the intermediate gaseous products in amounts sufficient to give substantially complete combustion; and (e) burning the intermediate gaseous products in a second combustion zone to give substantially complete combustion.

2. The method of claim 1 wherein the additional air added to complete combustion after the intermediate combustion solids have been removed is in an amount within the range of approximately 0.2 to 0.7 times the amount of air used in the initial combustion step.

3. The method of claim 1 wherein the additional air added to complete combustion after the intermediate combustion solids have been removed is in an amount within the range of approximately 1.0 to 10 times the amount of air used in the initial combustion step.

4. The method of claim 1 wherein the stoichiometric ratio of air to the fuel in the first combustion zone is in the range of 0.8 to 1.03.

5. The method of claim 4 wherein the fuel is residual fuel oil having a sulfur content up to 6 wt. percent.

6. The method of claim 5 wherein the temperature of the gaseous combustion products at the region of introduction of additional air to the gaseous products is at a level sufficient to induce the spontaneous combustion of any incompletely burned gases.

7. The method of claim 6 wherein the temperature of the gaseous combustion products at the point of introduction of additional air to the gaseous products is in the range of from about 2500 F. to about 4500 F.

References Cited UNITED STATES PATENTS 2,625,791 1/1953 Yellot -28 2,927,632 3/1960 Fraser 1584 3,080,855 3/1963 Lewis 110-1 3,228,451 1/1966 Fraser et al. 158l17.5

JAMES W. WESTHAVER, Primary Examiner.

US. Cl. X.R.

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