US2218281A - Method for cooling flue gas - Google Patents

Description

' Oct. l5, 1940- 5A F. DE RIDDER ET AL 2,218,281

METHQD FOR COOLING FLUE GAS Filed NOV. 1'7, 1936 Waff/@ray Patented Oct. 15, 1940 STATES PAT ortica METHOD FOR COOLING FLUE GAS Application November 17, 1936, serial No. 111,246

4 claims. (c1. 252-372) This invention relates to the production of ue gases under superatmospheric pressures, and more particularly is concerned with a method for preventing undue deterioration of the furnace by the action of extremely high temperatures obtained in the vicinity of the combustion zone.

Production of Iiuc gases by burning suitable fuels such as hydrocarbon gases, or liquids, or powdered coal under considerable superatmospheric pressure may be required in a number of industrial processes. For instance, it may be desired to use large quantities of a gas substantially free from oxygen at high temperatures and pressures. To this end a fuel may be burned at normal pressures in a furnace, and after cooling the products of combustion suciently to enable their being handled by compressors, they are compressed and then reheated in suitable coils. On the other hand, the same result may be had by the much more simple method of combining compressed air and fuel and burning the resulting mixture under a superatmospheric pressure.

In this manner, diiculties due to the condensation of water upon compressing relatively cool flue gases, are avoided, liquid water not only tending to interfere with the proper operation of the compressors, but also frequently causing serious corrosion therein.

Again, it may be desired to crack hydrocarbons under considerable superatmospheric pressures by directly contacting them'with hot combustion gases. In this case the indirect heating method of the flue gases is impractical because the temperatures to which they must be heated are so high that they would tend to react with the materials of good heating conductivities used in the construction of heating coils, and .moreover these materials would lose a good deal of their tensile strength, thus being unable to Withstand high pressures.

When, however, the combustion is carried out under pressure, the pressure-resisting material need not be exposed to high temperatures since it may be lined on the inside with heat-resisting, insulating material such as fire brick, magnesite brick, silica brick, etc.

It is well .recognized that for structural reasons as well as to minimize the cost, it is desirable to build pressure Vessels as small as possible. This means that usually the combustion space in a pressure furnace is limited to a minimum necessary for complete combustion. Moreover, as the result of the combustion under pressure, unusually high local temperatures are reached which, within the small area available, give rise to the evolution of a very large amount of radiant heat. Under these conditions, the burner and the lreproof brick lining in the vicinity of the place of actual combustion are bound to be damaged due to exposure to excessive temperatures, with the result that the maintenance cost of such furnaces is very high.

It is the purpose of our invention to lower the flame temperature in the furnaces which are damaged due to excessively high flame temperatures by injecting into the flame a non-combustible cooling liquid.

Heretofore, when it was desired to lower the temperature of the combustion gases in a furnace the means usually resorted to consisted of cooling a portion of the combustiongases and recirculating the same through the furnace, or by introducing steam in some suitable manner. Such cooling Agases were used vin the form of blankets to protect certain parts of the furnace or theyv were simply commingled with the fuelair mixture undergoing combustion to produce a flue gas mixture having a more moderate average temperature.

We have found that the above means are insufcient adequatelyto protect the burner and furnace lining in many instances, particularly if combustion iscarried out under substantially superatmospheric pressures, and that even by injecting large quantities of steam or cooled flue gases, the maintenance cost of pressure furnaces remains excessive due to a premature -de struction of parts thereof. However, we have discovered that we can successfully overcome these diiculties by injecting, instead of steam or flue gases, a nely divided spray of water from a point within or near the mouth of the burner into the fuel-air mixture, into the space back of the flame.

Our invention will be fully understood from the following description` of the attached drawing: Figure 1 represents a partial sectional elevation of a pressure furnace with burner. Figure 2 shows the burner of Figure 1 on an enlarged scale.

Referring to Figure 1: A pressure vessel l is lined on the inside with a layer 2 ofv a suitable insulating material such as kieselguhr, rock wool, asbestos, expanded vermiculite, reproof cement,

etc., which layer is faced with a lining 3 of fire consists of the following essential parts: Tube 6, for introducing a combustible mixture into the furnace proper, protrudes into pressure vessel I through insulating layer 2 as far as brick lining 3, being welded to the vessel to hold the desired superatmospheric pressures. An opening in the re brick lining of approximately the same or slightly larger cross sectional area as that of tube 6 allows the combustible mixture to enter the combustion space of the furnace. Screwed onto tube 6 at the end outside the furnace is T 'I connecting .with valved air line 8. Wedge nipple 9 is attached to T 1 to form a straight line continuation of' tube 6. The wedge nipple 9 is blanked olf by plate Il), through which pass two pipes, one pipe I I for the fuel such as natural gas, cracked gas, illuminating gas, acetylene, methane, ethane, propane, butane or liquid fuels such as naphtha, kerosene, distillate oil, residual fuel oil, or even powdered coal whichmay be moved by high pressure air or by mechanical devices such as a conveyor, not shown, within pipe I I, and the other pipe I2 for water or an equivalent cooling liquid. Fuel pipe Il extends beyond the T 'I 'into tube Ii to within a short distance of the mouth of this tube. At the end of pipe II in tube 6 is a suitable mechanism I3 of conventional design to eifect rapid mixing of the fuel with the air, the latter entering through line 8. Water pipe I2 extends farther than gas pipe Il to a point within tube l6 close to its mouth. Pipe I2 carries a spray device I4 capable of spraying a controlled amount of water in a nely divided state in the direction of travel of the fuel-air mixture.

When the furnace is in operation the fuel-.air mixture produced in tube 6 burns in the combustion space of the furnace, the flame back being at some short distance away from the mouth of the burner as indicated in Figure 2. After the inside of the furnace is well heated so as to prevent a possible extinction of the flame, water is sprayed through nozzle I4 into the space back of the flame. While the water droplets travel from the nozzle towards the burning mixture, they are partly vaporized by the radiant heat before reaching the flame itself, thereby effecting cooling of the burner and nearby parts of the brick lining. The remainder of the Water is vaporized in the combustion space proper, considerably lowering the temperature in this area, and greatly reducing the radiant heat emanating therefrom. In this manner the brick lining near this area can be adequately protected.

For the successful operation of our invention two things are essential: first, the amount of Water must be so controlled that at no time the temperature in the combustion space is lowered below the ignition temperature of the fuel-air mixture, and secondly the water must be finely sprayed to allow its rapid vaporization. If the water droplets-are so large as to enable accumulation of water oki'the floor of the furnace, serious damage may be done to it and possible explosion may result, due to a sudden evaporation of this water.

Due to the necessity of fine division of the spray water and to avoid operating difflculties, we usually prefer to use a separate water line which is independent of the gas and air lines, as shown in the drawing. By controlling the w-ater pressure in line I2 and the setting of nozzle I4, the neness of the spray can be regulated more easily than if the water were introduced together with air or gas. If desired, however, the burner may be designed so that the water is atomized by forcing it through a suitable nozzle together with gas or air or both. f

'I'he pressures in the furnace may vary within wide limits depending upon the purpose for which the flue gases are to be used. Pressures from slightly above atmospheric to above lbs. may be required, pressures up to about 20 lbs. being most common.

The advantages of our method of cooling furnace linings over those employing steam or recirculating ilue gases comprise among others greater efficiency which results in a considerable saving of repair costs and cheaper operation. To obtain an effective reduction of temperature in the combustiorrspace with flue gases or steam, a relatively large amount of cooling medium is required. Thus the cost of recirculation or steam generation may form a considerable portion of the cost of producing the flue gases. On the other hand, when using water the required amount of cooling medium is very much smaller and the cost of introducing same is but a small fraction of that of introducing gases for the same purpose.

By our method of cooling, the flue gases remain clean and free from soot and other impurities y in such cracking processes, the oil is not injected into the fuel-air mixture undergoing combustion, but into the flue gases after the combustion is substantially complete, because otherwise coke formation is excessive and the yield of desided cracked hydrocarbons accordingly low.

Thus, while liquids of the type of hydrocarbon oils are unsuitable in our process for the purpose of cooling the flame, certain non-combustible liquids other than water or mixtures of a predominant amount of such non-combustible liquids with combustible substances may be used, such as liquid carbondioxide, liquid sulfur dioxide, dilute aqueous solutions of ammonia, alcohols, ketones, aldehydes, carboxylic acids, amines, etc. When using cooling liquids containing combustible substances care should be taken to introduce with the fuel-air mixture an amount of air in excess of that necessary to burn the fuel, to

insure substantially complete combustion of the combustible material in the solution to avoid formation of soot or other impurities.

We claim as our invention:

1. A pressure furnace for the production of flue gases under superatmospheric pressure comprising a pressure-resistant shell lined with siliceous material forming a combustion chamber susceptible to excessive damage, such as spalling, under the temperature and pressure conditions developed in the furnace in the absence of injected water, a burner consisting essentially of a mixing device for producing under pressure a fuel-air mixture at one end of the furnace, a conduit forming the mouth of the burner for continuously conveying this mixture into the combustion chamber and maintaining a ame therein,fan independent water conduit in said burner extending into said first conduit, the end of the water conduit being neai the mouth of the burner and having a spray nozzle for spraying liquid water in a nely divided state into the `,pace back of the flame and throughout the ame in a quantity suflicient to cool the same to prevent spalling While maintaining the ame, and an outlet for combustion gases at another end of the furnace.

2. A pressure furnace for the production of ue gases under superatmospheric pressure comprisinga pressure-resistant shell lined with siliceous material forming a combustion chamber susceptible to excessive damage, such as spalling, under the temperature and pressure conditions developed inthe furnace in the absence of injected water, a burner consisting essentially of a mixing device for producing under pressure a fuel-air mixture at one end of the furnace, a conduit forming the mouth of the burner for continuously conveying this mixture into the combustion chamber and maintaining a name therein, an independent water conduit communieating with said burner, the end of the Water conduit terminating in the first-named conduit which forms the mouth of the burner and being adapted to introduce Water in the form of a spray into the space back of the name and throughout the name in a quantity suicient to cool the same to prevent spalling While maintaining the flame, and an outlet for combustion gases at another end of the furnace.

3. A pressure furnace for the production of flue gases under superatmospheric pressure comprising a pressure-resistant shell lined with siliceous material `forming a combustion chamber susceptible to excessive damage, such as spalling,

under the temperature and pressure conditions developed in the furnace in the absence of injected Water, a burner comprising a mixing device for producing under pressure a fuel-air mixture at one end of the furnace in communi- 5 cation with the combustion chamber, and disposed to maintain a flame therein, independent Water conduit means terminating in communication with the combustion chamber in the near vicinity of said burner for introducing Water in the form of a spray into the space back of the fiame and throughout the ame in a quantity sucient to cool the same to prevent spalling while maintaining the ame, and an outlet for combustion gases at another end of the furnace.

4. In a method for producing combustion gas under superatmospheric pressure in a combustion chamber lined With siliceous material susceptible to excessive damage, such as spalling, under temperature and pressure conditions developed in the chamber in the absence of injected Water, wherein a fuel and an oxygencontaining gas is burned in the combustion chamber to produce a flame and the resultant combustion gases are removed from the combustion chamber, the improvement of injecting Water in the form of a spray into the space back of the ame and throughout the ame from the near vicinity of the point at which the fuel and oxygen-containing gas are introduced into the combustion chamber in a quantity sulcient to cool the same to prevent spalling Whilev main- 'taining the ame.

GYSBERT F. DE RIDDER. RALPH HOAGLAND CROSBY.

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