EP0083116A2

EP0083116A2 - Improved metal burner

Info

Publication number: EP0083116A2
Application number: EP82112114A
Authority: EP
Inventors: Martin Luther Weirick
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1981-12-30
Filing date: 1982-12-30
Publication date: 1983-07-06
Also published as: CA1187395A; JPS58150707A; EP0083116A3

Abstract

here is described an improved metal burner-combustion chamber assembly for generating a continuous stream of high-temperature combustion gas of at least 1,000°C comprising (i) a metal burner having nozzles for feeding a fuel and an oxidizing gas, and a means for conducting heat away from the metal burner by a fluid and (ii) a combustion chamber made of a refractory material one end of which is connected to the metal burner, wherein the inner surface of said metal burner facing the interior of the combustion chamber is coated with a layer of a reflective metal having a reflectivity of at least 0.50, more preferably at least 0.80 and most preferably at least 0.90. Said coating reduces heat loss through the metal burner, contributes to a more efficient use of the fuel value and protects the burner material by lowering the temperature of the burner.

Description

Technical Field

This invention relates to an improved metal burner-combustion chamber assembly for generating a continuous stream of high temperature combustion gas of at least 1,000°C. The invention is particularly useful for an efficient utilization of fuel value and for the protection of the metal material under circumstances where a high-temperature combustion gas stream is continuously generated, for instance, in the so-called Advanced Cracking Reactor where a hydrocarbon feed is cracked by virtue of a heat carrier generated in a high temperature combustion zone.

Background Art

In operating high temperature burner-combustion chamber assemblies, the suppression of the burner temperature is a cumbersome problem. The term burner as used herein refers only to a metal portion supplying fuel and oxygen to a combustion chamber, and does not mean the entire assembly. Since the burner is made of a metal rather than of a refractory material, the burner temperature must be kept below a certain temperature level in order to maintain the integrity and the operability of the burner. Means conventionally adopted to control the burner temperature in such high temperature burner-combustion chamber assemblies include (i) controlling the flame temperature by adjusting the combustion condition, (ii) conducting and convecting heat away from the burner or from the refractory material in contact with the burner, and/or (iii) adjusting the geometry of the assembly so as to r'educe the amount of radiation energy impinging upon the burner. However, there are always some limitations and disadvantages inherent to these conventional means. Thus, for instance, one would often prefer to achieve as high a flame temperature as possible. In fact, this invention relates to temperature reduction of the metal burner, not of the flame. Heat removal from the burner or from the refractory material in contact therewith by conduction and convection may not be adequate because a large temperature gradient is still created within the bulk of the burner material, and in any event such heat removal often means energy loss and inefficient use of the fuel values. There are usually other considerations which put a practical limitation to the extent of the geometrical adjustment one can make on the burner-combustion chamber assembly.
Thus, alternative means for controlling the burner temperature are very much needed. This is true, for instance, in the case of an Advanced Cracking Reactor (ACR) where a hydrocarbon feed is cracked by a heat carrier generated in a combustion chamber upstream of the hydrocarbon feed inlet, and the temperature of the heat carrier often reaches 2,000°C or higher. In fact, the adiabatic flame temperature is often about 3,000°C or even higher, depending upon the combustion condition.
In contrast to the aforementioned conventional means of controlling the metal burner temperature in a high temperature burner-combustion chamber assembly, the present invention affords an alternative means for reducing the burner temperature and preserving the integrity and operability of the burner by virtue of a reflective coating on the inner surface of the burner facing the interior.of the constuction chamber. The chamber side of the burner is coated with a reflective metal having a reflectivity of at least 0.50, more preferably at least 0.80 and most preferably at least 0.90.
This invention has a general applicability to metal burner-combustion chamber assemblies where the temperature therein attains a high value, namely, at least 1,000°C. It becomes more useful as the temperature of the combustion gas becomes higher than 1,000°C.

Disclosure of the Invention

There is described an improved metal burner-combustion chamber assembly for generating a continuous stream of high-temperature combustion gas of at least 1,000°C comprising (i) a metal burner having nozzles for feeding a fuel and an oxidizing gas, and a means for conducting heat away from the metal burner by a fluid, and (ii) a combustion chamber, made of a refractory material one end of which is connected to the metal burner, wherein the inner surface of said metal burner facing the interior of the combustion chamber is coated with a layer of a reflective metal having a reflectivity of at least 0.50, more preferably at least 0.80, and most preferably at least 0.90. Said coating reduces heat loss through the metal burner because it increases the percentage of reflection of radiation impinging upon the burner and reduces the percentage of absorption of same by the burner. Thus, it contributes to a more efficient use of the-fuel value and protects the burner material by preventing the burner from reaching undesirably high temperatures. The reflective coating reduces the burner temperature of the metal by reducing the amount of absorption of the radiation energy coming from both the flame and the combustion chamber surfaces and impinging upon the metal burner. The lower metal temperature results in lower metal stresses, higher metal strength, and lower overall heat loss from the combustion chamber.
The surface of the reflective coating should be highly polished to maximize radiation reflection and to minimize radiation absorption. The coating should be stable and should bond well to the base metal.
The term "reflectivity" as used herein is defined by Reflectivity - 1 - total hemispherical emissivity. The term total hemispherical emissivity is well defined in the art and will be abbreviated as emissivity. See, for instance "Chemical Engineer's Handbook" edited by R.H. Perry et al, McGraw-Hill, New York, 4-th edition (1963), page 34 et seq of Chapter 10. Reflectivity can be determined by a standard procedure using suitable radiation source and a photometer. The emissivity of the metal surface varies depending upon inter alia, the surface condition of the metal, the temperature of the metal, and the wavelength of the radiation impinging upon the metal surface. The values of the emissivity in this specification and the appended claims refer to the values at 600°C. The extent of dependence of the emissivity upon the spectral distribution of the incident radiation is small.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 pictures a typical metal burner as viewed from the combustion chamber side. It shows metal, double concentric nozzles for fuel and oxygen, and cooling water tubes running through the burner metal.
Fig. 2 depicts a cut-away side view of the metal burner of Fig. 1 showing the position of the reflective coating and the flow directions of fuel and oxygen.

DETAILED DESCRIPTION

Figures 1 and 2 are presented only for the purpose of illustrating this invention. The numeral 1 is the metal portion of the burner. The numeral 2 is a fuel tube and 3 is an oxygen annulus. The numeral 4 depicts a cooling water tube running through the burner. The numeral 5 depicts a reflective coating on the inner surface of the metal burner facing the interior of the combustion chamber (not shown). The number 6 depicts the flow direction of fuel and oxygen.
The burner in this invention is made of a metal. It is preferred to use a heat- and corrosion- resistant metal particularly when the burner temperature becomes high. Various types of stainless steel can be used for this purpose, an example being Hastelloy^(R) X manufactured by Cabot Corporation, Stellite Division, 2010 West Park Avenue, Kokomo, Indiana 46910, which is a heat- and corrosion- resistant alloy comprising nickel, chromium, iron and molybdenum.
The metal burner has nozzles for feeding a fuel and an oxidizing gas. In Advanced Cracking Reactions it is preferred to use a fuel material which is a gas at ordinary temperature and pressure. If the fuel material is not a gas at ordinary temperature, it is preferred to preheat the material and vaporize the fuel before it is fed to the burner. Even where the fuel material is a gas at ordinary temperature, it may be preferable, from the standpont of process economics, to preheat the fuel and the oxidizing gas. Examples of a preferred fuel are lighter hydrocarbons such as methane and ethane. Methane is particularly preferred.
The term "oxidizing gas" as used in this specification and claims means oxygen or oxygen-containing gas such as air. When one seeks to obtain a higher temperature combustion gas, it is preferred to use oxygen as the oxidizing gas.
A preferred mode of arrangement of the nozzles is a concentric annulær arrangement where each narrower tube/nozzle is surrounded by a wider tube/nozzle in a concentric configuration. One can feed fuel through the inner tube/nozzle and oxidizing gas through the outer tube/nozzle, or vice versa. An example of such arrangement is illustrated in Figures 1 and 2. Where there is such a concentric arrangement of the nozzles, it is preferred to place the individual sets of concentric nozzles more or less equally spaced from each other in the tubesheet. Such concentric arrangement is very helpful for an efficient mixing between fuel vapor and oxygen, and hence for an efficient utilization of the fuel value. It should be understood, however, that the fuel is not always brought to a complete oxidation or combustion, although the term "combustion chamber" is used in the specification and the claims. For some economical and/or engineering considerations, the fuel may be only partially oxidized in the combustion chamber.
The metal burner, because of the high temperature of the combustion chamber, has a means for conducting heat away from it by use of a fluid. A typical way to accomplish heat removal is to provide one or more cooling channels provided inside the bulk material of the burner. Water is the usual cooling medium going through the cooling channels. An example of the arrangement of the cooling channels is shown in Figures 1 and 2.
In order to reflect the radiation energy flux impinging upon the inner surface of the burner facing the interior of the combustion chamber, said inner surface of the burner is coated with a reflective metal having a reflectivity of at least. 0.50, more preferably at least 0.80, and most preferably at least 0.90. Such reflective metal may be a single component metal or an alloy. Examples of such highly-reflective metal suitable for this invention include precious metals such as gold, platinum and rhodium. Not only do these metals have high reflectivities but also they are chemically stable and corrosion-resistant. Gold is a particularly preferred species for this invention. The surface of the reflective coating should be highly polished in order to maximize the reflection of the radiation flux coming from the combustion gas and the inner wall surface of the combustion chamber. It has been observed that the reflective coating substantially reduces the temperature of the burner metal by virtue of reduced absorption of radiation by the burner. This results in lower metal stress, higher metal strength, lower overall heat loss from the combustion system. Thus, the instant invention enhances the reliability and operability of the high-temperature burner and the overall economy of the combustion process.
This invention is generally useful where the temperature of the combustion gas is at least 1,000°C. It becomes increasingly more useful as the temperature goes higher. An example where this invention is particularly useful is in the Advanced Cracking Reactor (ACR), although this invention is not limited in scope to its utilization in conjunction with ACR. The subject of ACR is well known in the art. Basically it involves (i) continuous creation of a high temperature -combustion gas within a combustion zone, (ii) cracking of a hydrocarbon feed by use of the combustion gas as a heat carrier, and (iii) subsequent quenching of the product. An example of review articles on the subject is, Hosoi and Keister, "Ethylene from Crude Oil", Chemical Engineering Progress, Volume 71, No. 11, pages 63-67 (1975). Typically the temperature of the combustion gas in ACR reaches about 2,000°C. In such a situation, the reflective coating of the burner according to this invention becomes .particularly useful.
It has been observed that the reflective coating according to this invention stays shiny and highly reflective during continuous operation of the burner-combustion chamber assembly provided one takes due precautions to maintain the combustion condition favorable to the preservation of the reflective coating. For example, methane is a preferred fuel because the tendency for the deposition of foreign substances such as soot on the reflective coating is minimal. Contamination of a corrosive substance in the fuel should be avoided. The fact that the reflective coating is on the back side of the burner and that there is a continuous high speed flow of gas stream down the combustion chamber is undoubtedly helpful for the preservation of the reflective coating.
The thickness of the reflective coating is typically of the order of 1/5000 5 mm inch. The reflective layer should be strongly bonded to the s-ubstrate metal. This invention is not limited to any particular method of coating a reflective metal on the burner. Various methods are known in the art. An example is electro-plating. One may provide another.layer sandwiched between the reflective layer and the burner metal. Where the reflective layer is gold, it is useful to provide a thin layer of palladium as a sandwiched layer, because it considerably suppresses the diffusion of gold into the burner metal substrate.
The afore-mentioned combustion chamber is made of a refractory material. Since the inside wall of the chamber often reaches very high temperatures, it is preferred to use a special high temperature resistant refractory material such as zirconia. This is particularly true where the temperature of the combustion gas reaches about 2,000'C. The combustion chamber may be constructed with several layers of different refractory materials. There is no particular limitation as to the size and shape of the combustion chamber, but usually it has an axis of symmetry.
The following example is given only to illustrate this invention, and it should not be construed as limiting the scope of this invention.

Example 1

A combustion was conducted using a metal burner-combustion chamber assembly and the effect of gold plating the inner surface of the metal burner facing the interior of the combustion chamber upon the temperature of the metal burner was studied. The metal burner made of a stainless steel Hasteloy^(R) X had three sets of double-concentric nozzles. The cross-sectional area of each internal nozzle was 32.2 mm² and that of each outer nozzle was 159.4 mm at the tip. An internal cooling channel was provided within the bulk of the metal burner. It curled around the three sets of double concentric nozzles. Cooling water was allowed to run through the internal cooling channel. Fuel was fed through the inner nozzles and oxygen was fed through the outer nozzles. The combustion chamber was a cylindrical shape of 1.8m long and 12 cm inner diameter, a tapering being provided at both ends of the chamber. The fuel composition was (on the weight basis) 10.6 % H₂, 80.6 % CH₄, 4.7 X C₂H₄ and C₂H₆, 1.5 % CO, 0.7 % CO₂ and 1.9 % N₂. The feed rate of the fuel was 112 kg/h 249 lb/hr and that of oxygen was 453 kg/h 1,006 lb/hr. Dilution steam of 350°C temperature was fed to the combustion chamber at the rate of 764 kg/h 1,698 lb/hr through a set of steam injection tubes provided at the upper part of the combustion chamber wall 229 mm downstream from the metal burner. Steam curtain was provided around the upper part of the interior wall of the combustion chamber by feeding 200°C steam into the combustion chamber at the rate of 79 kg/h 176 lb/hr through an annulus provided adjacent the perimeter of the metal burner and vertically downward along the interior wall. The flow rate of the cooling water of the metal burner was 79 kg/h 110 lb/hr. Under this set of conditions, the fuel/oxygen stoichiometric ratio was 1.1, the adiabatic flame temperature after the mixing of fuel, oxygen, steam curtain and dilution steam was complete was 1950°C. The temperature of a location within the metal burner 6mm away from the bottom surface thereof facing the combustion chamber was measured with a thermocouple for two situations, one where the burner bottom surface was coated with gold and the other where there was no gold plating. In the former situation, the emissivity was 0.35 because of a matte finish rather than shiny finish. By improving the. gold plating technique the surface reflectivity can be made better than this. In the no-gold plating situation, the emissivity of the burner bottom surface was 0.50. The emissivity values were determined by use of a portable emissometer. The results for the two situations are summarized below:
In the above data the bottom surface temperature is an extrapolated-value, whereas the data at 6mm away from the bottom surface is a' measured value. The difference of bottom surface temperature between the two situations is substanital and clearly demonstrates the advantage of the reflective coating of this invention. With a better (shinier) surface coating technique, the effect of reflective coating can be made more apparent. It should also be kept in mind that the above described experiment is a small scale experiment. Needless to say the temperature of the metal burner depends on numerous factors including combustion condition (type of fuel used, feed rate of fuel and feed rate of exygen, mixing condition between fuel and oxygen, amount of dilution steam, preheating of the feed, etc.), size and configuration of the combustion chamber, emissivities of the interior surfaces of the combustion chamber wall and of the metal burner, extent of heat removal from the metal burner by the cooling fluid, and presence of optional steam curtain which cools at least a portion of the inner wall of the combustion chamber. Therefore, the extent of temperature reduction cannot be stated in any simple quantitative manner as a function of the reflectivity of the coated surface. It is generally true, however, that as the size of the combustion chamber becomes larger, the effect of the reflective coating becomes more significant. This is due to the fact, inter alia, that the radiation heat transfer from the flame to the metal burner becomes more important relative to the other heat transfer mechanisms as the combustion chamber becomes larger. Thus, the reflective coating of this invention becomes more useful in large scale operations such as commercial scale ACR. It is also true that in large scale operations of ACR, various feeds such as dilution steam are preheated to higher temperatures than the temperature used in the above-described experiment.
It is predicted that in a large scale ACR the emissivity difference of 0.03 versus 0.5, for instance, can make a difference of several hundred degrees in the metal burner temperature. This difference is very critical from the standpoint of safe and economical operation of the process.

Claims

1. A high temperature metal burner - combustion chamber assembly which comprises,

(i) a metal burner having nozzles for feeding a fuel and an oxidizing gas, and a means for conducting heat away from the metal burner by a fluid; and

(ii) combustion chamber made of a refractory material one end of which is connected to the metal burner,

wherein the inner surface of said metal burner facing the interior of the combustion chamber is coated with a reflective metal having a reflectivity of at least 0.50.

2. The high temperature metal burner-combustion chamber assembly according to claim 1, wherein the reflective metal has a reflectivity of at least 0.80.

3. The high temperature metal burner-combustion chamber assembly according to claim 1, wherein the reflective metal has a reflectivity of at least 0.90.

4. The high temperature metal burner-combustion chamber assembly according to any of claims 1-3, wherein the reflective metal is gold.

5. The high temperature metal burner-combustion chamber assembly according to any of claims 1-3 wherein the nozzles are arranged in a double concentric configuration.