US2868857A - Combustion chambers - Google Patents

Description

Jan. 1 3, 1959 A. H. HOWLAND ET AL COMBUSTION CHAMBERS 2 Sheets-Sheet 1 Filed Sept. 1, 1955 A. H. HOWLAND. E. M. HUGHES. H. FORD & N. P. W. MOORE 7 BY 1 Attorfzeys ,1959 A. H. HOWLAND ETAL 6 COMBUSTION CHAMBERS 2 Sheets-Sheer. 2

Filed Sept. 1, 1955 A F/GZ.

W V 5% E 23 5 V -g2 Inventors A. H. HOWLAND. E. M. HUGHES. H.FORD a. N.P. w. MOORE BY W M Attorneys United States Patent COMBUTION CHAMBERS Application September 1, 1955, Serial No. 531,976

Claims priority, application Great Britain September 3, 1954 11 Claims. (Cl. 260-679) The present invention relates to combustion apparatus and has for an object the provision of an apparatus in which there may be burnt, under stable conditions, mix

tures which are so weak or so rich or contain such large proportions of. diluents or have such a high velocity that combustion under normal conditions is unstable or impossible.

In the process for the production of unsaturated hydrocarbons by partial combustion and cooling as usually practised, the hydrocarbon and the oxidant are preheated, premixed and passed into a combustion chamber through a grid consisting of a large number of passages each of small diameter. Theflames stabilized at theoutlet rims of these passages are necessarily short in length and it has in consequence been necessary to employ a combustion chamber whose diameter is predominantly greater than its length. If an attempt were made to feed the combustion products to a gas turbine, the large area of the grid would necessitate the provision of a combustion products manifold adapted to collect the combustion products and such a manifold would give rise to structural difiiculties and furthermore tend to lengthen the time during which the combustion products are at a high temperature.

According to the present invention there is provided an apparatus for the combustion of a mixture of a gaseous or vaporized or atomized fuel with an oxidizing gas which comprises a combustion chamber long in relation to its transverse dimension and having a mixture inlet at one end and a combustion products outlet at the other end and a flame stabilizing device disposed Within said combustion chamber in the path of the combustion mixture flowing therethrough; the flame stabilizing device may be a bafile so disposed as to promote turbulent flow. A pilot burner or electrically heated Wire may be used within the combustion chamber as an active source of heat.-

It is known that a high rate of heat release per unit volume can be obtained by cyclic combustionof a premixed gaseous fuel and an oxidant in a plant which includes a combustion chamber having a length predominantly greater than its diameter when the gaseous fuel is used in a proportion, relative to the oxidant, which is equal to or weaker than the stoichiometric proportion and when the combustion chamber has its combustion products exit open to the surrounding atmosphere so that the combustion process occurs nominally at atmospheric pressure. Such a combustion chamber may consist of a refactory-lined tube into which the premixed reactants are introduced through an inlet section in the form of one or more small diameter passages in which the gas velocity is greater than the velocity of flame propagation in the mixture. Combustion in the chamber can, by adjustment of the operating conditions, be made to occur in cycles wherein unburnt reactants passing down the tube during one cycle are ignited on admixture with the hot products of combustion of the previous cycle, the

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flame then travelling rapidly up the tube until it is extinguished at the inlet section allowing fresh unburnt reactants to pass down the tube to be ignited as before.

The cyclic frequency depends upon the composition of the mixture of reactants introduced, their temperature and pressure, the dimensions of the combustion chamber and the design of the inlet section.

We have found that, although cyclic combustion in hitherto known forms of apparatus is diflicult to achieve when the ratio of the reactants is substantially different from the stoichiometric, cyclic combustion under such conditions is readily initiated and maintained in the apparatus of the present invention.

A feature of the invention consists in that said flame stabilizing device comprises a fuel nozzle adapted to inject a jet of fuel into the chamber parallel with the longitudinal axis thereof, an obstructing element arranged in the path of the jet thereby to spread the fuel into a thin sheet, and distributor means for supplying air or oxygen to the vicinity of said obstructing element, i. e. the thin sheet of fuel for combustion thereof to provide a region of high temperature in the combustion chamber for stabilizing the combustion of the premixed combustion mixture; the nozzle may be directed substantially along the axis either in the upstream or downstream direction. With combustion chambers of this type it is a diflicult matter to arrange a pilot burner of the conventional type in such a manner that it remains stable in operation when the gas velocity is high. The apparatus provided by the present invention is, however, capable of stable operation under such circumstances.

In a convenient arrangement, the distributor means comprises a tube arranged coaxially with the fuel nozzle to provide an annular port for the air or oxygen although other arrangements may be adopted if desired; for example, the obstructing element may be porous and the oxidizing gas fed therethrough to produce a sweat-cooling elfect. This effect may be produced in other parts of the apparatus, e. g. in a support for the obstructing element, by forming these of porous material and passing fuel or oxidant therethrough. Where the gas velocity through the combustion chamber is likely to be very high, it is convenient to provide for stabilizing the flame against distortion by arranging the distributor means in such a manner as to supply the air or oxygen to both sides of the thin sheet of fuel; a similar effect is obtainable in the case where the nozzle and distributor means are directed upstream by careful adjustment of the velocity of the air or oxygen. Alternatively, the obstructing element may be made sufliciently large to carry the whole of the thin sheet of fuel upon its surface thereby providing the flame produced therefrom with mechanical support against distortion. In the latter case, the obstructing element behaves as a strongly and efficiently heated baflle for the combustible mixture.

It is to be realized that the fuel introduced through the fuel nozzle of the stabilizing device may, but need not necessarily, be identical with the fuel supplied through the mixture inlet of the chamber. For instance, it is advantageous to use hydrogen as the fuel for the stabilizing device on account of the very high temperature it gives on combustion with oxygen and the fact that the steam produced is an inert diluent which can be easily condensed from the combustion products.

It is within the scope of the invention to provide an apparatus of the type aforesaid in a form adapted to the production of acetylene or other unsaturated hydrocarbons (among which may be mentioned ethylene) from a hydrocarbon fuel by using the heat of combustion of the fuel with air and/or oxygen to promote pyrolysis and/orcracking of an excess of the fueland subjecting the resulting gaseous product to sufficiently rapid cooling to a temperature at which the unsaturated hydrocarbons are stable (usually below 200 C.) to produce a cooled mixture containing a significant proportion thereof. In such a process, the inherent instability of combustion of the fuel-rich mixture has in the past given rise to serious difficulties. A combustion chamber of the type provided by the present invention is eminently suitable for use in such processes. Thus the combustion products outlet may be in communication with cooling means adapted to subject the combustion products to the required rapid cooling. In a preferred arrangement, the said cooling means is provided in the form of gas turbine in which the combustion products may be allowed to expand adiab-atically and produce useful work. The use of a gas turbine to effect the rapid cooling of hot gases has been described in British Patent Specification No. 711,208. Flash-back into the inlet section from the combustion chamber can be prevented not only by ensuring that the velocity of flow of the mixture of gases within the inlet section is greater than the velocity of flame propagation in the mixture but also by placing a suitable flame trap at the mouth of the inlet section to prevent the passage of flame further upstream in which case the velocity of flow in the inlet section need not be greater than the velocity of flame propagation.

The process can be initiated by any known means such as spark ignition, glow plug or by the deliberate striking back of a flame initially stabilized at some outlet from the system, which outlet may be the normal outlet duct or some additional outlet used only for the lighting up operation and intended to be closed subsequently.

The yield of acetylene obtained by the pyrolysis and/ or cracking reaction is dependent upon the time that the reacting mixture resides in the combustion chamber at the high temperature. The residence time for different experiments can be expressed on a relative basis as the time taken by the mixture to traverse the combustion chamber when at atmospheric temperature and pressure. In an experiment using a cylindrical combustion chamber having a ratio of diameter to length of 2:9, a residence time of about 30 milliseconds was found to give optimum yield from a mixture of 1:6 parts by volume of methane with 1 part by volume of oxygen. A considerable reduction in yield resulted with this combustion tube for a residence time of under 15 milliseconds or over 40 milliseconds. This combustion tube has given a similar performance when operating under a pressure of 10 lbs. per square inch above atmospheric pressure and stable operation with a maintained cyclic combustion process and an appreciable yield of acetylene or more) is obtainable at higher pressures-up to 5 atmospheres at leastbut with a reduced residence time. Preheating the infiowing mixture has-also a markedly beneficial influence upon the yield, particularly when operating under pressure.

The invention will be further understood from the following description of various embodiments of the apparatus, in which reference is made to the accompanying drawings.

In these drawings:

Figure 1 shows in longitudinal section an apparatus intended for use at high gas velocity,

Figure 2 shows in longitudinal section an apparatus suitable for use at relatively low gas velocity, and

Figure 3 shows diagrammatically a modified form of combustion apparatus.

In the apparatus shown in Figure 1, a combustion chamber in the form of a cylindrical ceramic duct 1 adapted to carry a combustible mixture at high velocity passing between end A, where the duct is reduced in diameter to provide an inlet, and outlet end B is provided with a refractory cross-piece 2 provided, on the longitudinal axis of the duct, with a cylindrical projec- Cir tion 3 which acts as an obstructing element against which may be projected a jet of hydrocarbon fuel from axial nozzle 4 supplied from outside the duct via a pipe 5'. Tubes 6 and 7 adapted to be supplied with air or oxygen via pipes 8, 9 and 10 cooperate respectively with the nozzle 4 and the cylindrical projection 3 to provide annular ports 11 and 12 for supplying air or oxygen to both sides of the thin disc of fuel formed by the jet of fuel impinging on the projection 3.

In the apparatus shown in Figure 2, a combustion chamber in the form of a refractory tunnel 21 closed at end A and open at exit end B is provided with a crosspiece 22 having formed thereon an axially-directed cylindrical projection 23 which acts as an obstructing element for spreading fuel supplied via a pipe 24 and terminating in a nozzle 25 into a thin disc. Pipe 24 is axially located in a wide passageway 26 constricted at its inner end to provide an annular air or oxygen port 27 for distributin'g air or oxygen over the said disc. A plurality of inlets leading into the chamber at positions 28 which lie on a circle concentric with the nozzle 25 and the port 27 enable the premixed mixture for combustion to be fed into the chamber. If desired, mixtures of different compositions may be fed through different members of the plurality of inlets for final admixture in the chamber.

It will be understood that, although for convenience herein the nozzle directed at the obstructing element is referred to as a fuel nozzle and although the distributor means is stated to have the function of supplying air or oxygen to the thin sheet, the functions of these two structural elements may be reversed. Thus the apparatus may successfully be used with air or oxygen supplied to the nozzle and spread out into a thin sheet to which the fuel is supplied by the said distributor means.

The manner in which the embodiments of the apparatus shown in Figures 1 and 2 may be arranged for producing unsaturated hydrocarbons is shown diagramrnatically in Figure 3.

In Figure 3 a combustion chamber having disposed therein the flame stabilizing devices, has an inlet pipe I leading from a preheater H adapted to be supplied by a mixture obtained by mixing, in an adjustable mixing valve V, fuel and molecular oxygen-containing gas, e. g. air, supplied respectively via pipes Q and R. A turbine shown at T is provided for quenching the combustion products to give a mixture containing unsaturated hydrocarbons which are obtained at exhaust outlet E. If demanded by the form of flame stabilizing devices, there are provided the oxygen and fuel lines A and B shown in broken lines in Figure 3.

Experiments with an apparatus of the type shown in Figure 2 have given the following results.

The main part of the combustion chamber B was circular in shape and had a diameter of 2 /2 inches and a length of 15 inches. There were 7 inlets (28) forthe pre-mixed mixture of hydrocarbon fuel and oxygen. The hydrocarbon fuel used was a gas-having the following composition: methane; 7 /z% carbon dioxide; 7 /2% nitrogen. With a flow of 3250 cubic feet per hour of hydrocarbon fuel and 1660 cubic feet per hour of oxygen at substantially atmospheric pressure and temperature to the combustion chamber and a flow of 300 cubic feet per hour of hydrocarbon fuel. through the nozzle (25') and 200 cubic feet per hour of oxygen through the port 27, stable combustion of the main mixture was maintained with this fuel/oxygen ratio of 1.96:1 by volume.

A yield of acetylene in excess of 6% by volume of the dry combustion products was obtained. This does not necessarily present the optimum mixture ratio that can be reacted under stable, conditions nor the optimum yield of acetylene. The product gases also contained substantial quantities of hydrogen and carbon monoxide in the ratio of 2:1 which could therefore'be used as a synthesis gas in subsequent reactions.

The invention includes a process for the conversion of saturated hydrocarbons to unsaturated hydrocarbons by pyrolysis and/or cracking of the saturated hydrocarbons in the apparatus above described.

We claim:

1. Apparatus for the conversion of substantially saturated hydrocarbons to unsaturated hydrocarbons comprising an elongated combustion chamber, means for introducing a premixed combustion mixture at a high rate of flow into one end of the combustion chamber for passage through the combustion chamber, and a flame stabilizing device disposed within said combustion chamber in the path of the combustion mixture flowing therethrough, said flame stabilizing device comprising a nozzle for introduc tion of fuel into said combustion chamber parallel to the longitudinal axis thereof, an obstructing element disposed in the fuel path from said nozzle and distributor means for supplying molecular oxygen to the vicinity of said obstructing element.

2. Apparatus according to claim 1 wherein a preheater is associated with the inlet to the combustion chamber.

3. Apparatus for the conversion of substantially saturated hydrocarbons to unsaturated hydrocarbons comprising an elongated combustion chamber, means for introducing a premixed combustion mixture at a high rate of flow into one end of the combustion chamber for passage through the combustion chamber, a flame stabilizing de vice disposed within said combustion chamber in the path of the combustion mixture flowing therethrough, said flame stabilizing device comprising a nozzle for introduction of fuel into said combustion chamber parallel to the longitudinal axis thereof, an obstructing element disposed in the fuel path from said nozzle and distributor means for supplying molecular oxygen to the vicinity of said obstructing element, and combustion product quenching means directly connected with the other end of said combustion chamber.

4. Apparatus according to claim 3 wherein said quenching means is a gas turbine.

5. A process for the conversion of substantially saturated hydrocarbons to unsaturated hydrocarbons which comprises feeding a premixed mixture of saturated hydrocarbon and molecular oxygen at a high rate of flow into a combustion zone long in relation to its transverse dimension, said mixture containing said hydrocarbon and molecular oxygen in ratios substantially difierent from the stoichiometric ratio, igniting said mixture to efiect a pyrolysis of the saturated hydrocarbon, stabilizing combustion in said zone by means of an independent disc-like stabilizing fiame extending transversely across the whole of the combustion chamber and thus maintained in the path of flow of said mixture, withdrawing the hot combustion products from the combustion chamber and cooling said combustion products.

6. A process according to claim 5 wherein said premixed mixture is preheated prior to introduction into said combustion zone. 1

7. A process according to claim 5 wherein said combustion chamber is operated under superatmospheric pressure.

8. A process according to claim 5 wherein the cooling of said hot combustion products is effected by quenching to a temperature not exceeding 200 C.

9. A process according to claim 5 wherein said hot combustion products are cooled by adiabatic expansion in a gas turbine.

10. A process according to claim 5 wherein said independent disc-like stabilizing flame is produced by introducing a jet of one of the complementary combustion, reactants contained in said premixed mixture separately into said combustion zone, obstructing said jet to form a thin sheet of said one of said complementary combustion reactants, supplying to said sheet the other of said complementary combustion reactants and burning said reactants to form the disc-like flame.

11. A process according to claim 10 wherein the rate of flow of said premixed mixture through said combustion zone is adjusted to a value such that cyclic combustion thereof occurs.

References Cited in the file of this patent UNITED STATES PATENTS 1,823,503 Mittasch et al. Sept. 15, 1931 1,965,770 Burgin July 10, 1934 1,965,771 Groll et al. July 10, 1934 2,377,245 Krejci May 29, 1945 2,498,444 Orr Feb. 21, 1950 2,529,598 Deanesly Nov. 14, 1950 2,556,196 Krejci June 12, 1951 2,630,461 Sachsse et al Mar. 3, 1953 2,721,227 Mungen Oct. 18, 1955 2,727,932 Evans et al Dec. 20, 1955 2,750,434 Krejci June 12, 1956

Claims (1)

1. APPARATUS FOR THE CONVERSION OF SUBSTANTIALLY SATURATED HYDROCARBONS TO UNSATURATED HYDROCARBONS COMPRISING AN ELONGATED COMBUSTION CHAMBER, MEANS FOR INTRODUCING A PREMIXED COMBUSTION MIXTURE AT A HIGH RATE OF FLOW INTO ONE END OF THE COMBUSTION CHAMER FOR PASSAGE THROUGH THE COMBUSTION CHAMBER, AND A FLAME STABILIZING DEVICE DISPOSED WITHIN SAID COMBUSTION CHAMBER IN THE PATH OF THE COMBUSTION MIXTURE FLOWING THERETHROUGH, SAID FLAME STABILIZING DEVICE COMPRISING A NOZZLE FOR INTRODUCTION OF FUEL INTO SAID COMBUSTION CHAMBER PARALLEL TO THE LONGITUDINAL AXIS THEREOF, AN OBSTRUCTING ELEMENT DISPOSED IN THE FUEL PATH FROM SAID NOZZLE AND DISTRIBUTOR MEANS FOR SUPPLYING MOLECULAR OXYGEN TO THE VICINITY OF SAID OBSTRUCTING ELEMENT.

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