Refractory Materials for Gas Combustion Equipment EiMIL BLAHA U
. I
Selas Corp. of America, Philadelphia, Pa.
The main objective of industrial gas-burning equipment is to provide flexible, controlled heat-controlled heat that follows a laboratory-established pattern. On an industrial scale, ceramic refractory materials have proved to be capable of aiding the combustion process and functioning of such equipment. Investigations on an unprecedented scale by private and government-sponsored agencies promise to provide basic knowledge that will enable the ceramic engineer to improve these products further and thereby utilize more completely the inherent properties of gaseous fuel.
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HE recent evolution in industrial heat processing is based on the necesaity for complete control of temperature-time cycle, and gaseous fuel is available to aid materially in accomplishing these results. The complex system of large scale product heating involves many components that control individual phases before the end product-a product that has been heated to an exact temperature a t a predetermined rate for a set time-has been obtained. One of the important components of a gas heat-treating system is the burner, Many distinctly different types are in industrial use, but this discussion is limited to burners that incorporate unconventional features and materials of construction. These materials are refractories or other materials that are capable of withstanding the heat and the stresses due to high temperature gradients in such a manner as t o retain their functional shape. A distinct function of a t least one burner component is to act as a combustion accelerator or catalyst; if combustion is to be fully controlled, it must take place in a specified space and time.
affect the time of reactioh to complete the combustion. As COIYbustion is to be continuous in a flowing stream, the d i ~ t a , m c cthak ~ the gases must travel before being completely burned is a functiom of flame propagation of the gas and stream velocity. Any change in the velocity of the gas stream affects the distance proportiaor ately.
CONTROL OF GAS-AIR RATIO
For the purpose of this discussion it is assumed that the gasair mixture is being supplied to the gas burner rn a complete and controlled mixture; this is being practiced in industry, even on very large installations. Carbureting machines in single units of up to 7,000,000 B.t.u. per hour are available and used in the metal, glass, and ceramic industries. An outstanding example is the production of glass fiber. These carburetors are compressors of the centrifugal or positive displacement type, operating in conjunction with a proportioning intake valve that controls and maintains any required proportion of gas-air automatically regardless of atmospheric conditions, and delivers the gas-air mixture through a suitable distributing system to the burners a t pressures up to 15 pounds per square inch. Other means of proportioning the fuel gas and air may be employed, usually, however, a t a sacrifice of accuracy of proportioning and quantity of fuel burned for a given space. BURNER AND COMPONENTS
The function of the individual burner is simple. Regardless of whether the combustion process is considered to be mainly molecular diffusion or thermal conduction ( I ) , it is a fact that molecular changes and energy release occur. It is of importance to know the rate a t which combustion takes place. Flame propagation rate characteristics of the different fuel gases vary and January 1954
u Figure 1. Radiant Burner AssembIy
Practical considerations dictate that the position of the burner be fixed. Therefore, in order to eliminate the impinging effect of the longest flame when the burner is performing a t the highest gas velocity, the burner position in relation to the product being heated must correspond to the longest flame a t maximum combustion rate. Tangential firing and over- and underfiring of the product to be heated are attempts t o circumvent this condition. A direct approach in the form of acceleration of combustion rate has proved to be more effective in providing the required flexibility of heat cycle control from the standpoint of temperature, uniformity of product heating, and heating rate. In addition, realignment of heat energy forms delivered from the burner permits greater geometric control of heat patterns through arrange-
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ment of burner groups. The radiant gas burner is a typical example of these rapid combustion burners which have been relatively recently developed. The effective functioning of this burner is obtained by combustion acceleration provided by a refractory catalyst part of the burner (Figure 1). I n its present form there are two main functional components: the catalyst (Figure 2) and the diffuser
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A
Figure 2. Catalytic Shapes of Radiant Burner
(Figures 3 and 4). They require the attention of the ceramist and physical chemist, because of the effect that the high rate of combustion, in a confined space, has on these two parts. The catalytic component and diffuser are subjected to excessive thermally induced stresses. The face of the diffuser is heated by radiation and convection currents, in many instances to an incandescent heat (1400' C.), while cooling gases are being fed through the ports approximately only inch from the face of this component. A less severe but still excessive stress is developed in the combustion accelerator, the surface of which may reach a temperature of 1650" C. The materials of construction utilized a t present are nonductile and have low thermal conductivity and low tensile strength. Therefore, under the conditions of use, the structure must undergo a physical change. These destructive forces are being minimized by proper design.
Aluminum oxide and stabilized zirconium oxide are suitable for working in the higher temperature regions: alumina up to 1850" C. and zirconia up to 2100" C. Zirconia in a form known &B stabilized zirconia is now available. Stabilization is obtained by high temperature treatment with an admixture of calcium or magnesium oxide. DIFFUSER.The second important part of a gas burner is the diffuser, which functions to distribute the gas-air mixture over the surface of the catalyst. To accomplish this, its position in relation to the catalytic surface is determined by the design and intended functioning of the burner. In the case of a radiant type of burner, the diffuser is placed in the vortex of a cup-shaped cavity. The diffuser incorporates openings a t the periphery that connect with the burner t,ube and is intended, in this case, to discharge the gas-air mixture radially, in thin streams over the cup cavity. Intense heat is developed within this cavity owing t o rapid combustion and therefore the face of the diffuser is subjected to back-radiation and heating by involuted combustion gases.
Figure 3.
Standard Diffuser
REFRACTORY COMPONENTS OF GAS BURNER
The shape of the refractory part that provides the combustion acceleration has been developed experimentally with due consideration to gas flow dynamics. Gases that burn slowly present difficulties that have been overcome by a design of the catalyst shown in Figure 2. A series of circumferential depressions tends to increase turbulence and provide surface contact. Considerations involving the selection of the materials for this refractory are obvious. Temperature stability must be beyond the highest temperature that may be developed when the burner functions. Effective combustion acceleration is dependent not only on the material but also on the physical character. In general, a porous structure tends to increase the reaction rate of combustion. A long list of materials that effectively influence the combustion speed of fuel gas-air mixtures comprises virtually all the hightemperature-resisting oxides and silicates. Practical considerations exclude some of the best, from the standpoint of high cost, thermal expansion, thermal instability, etc. Examples of those excluded are:
Because of the cooling effect of the gas-air mixture flowing through the ports, an extreme thermal gradient is established, resulting in stresses that exceed the low elastic limit of any known refractory material. From the standpoint of reliable performance of a radiant burner that is to be heated or cooled rapidly, only a high-temperature noble metal could fulfill the requirements
Magnesium oxide-high thermal expansion Beryllium oxide-cost and toxicity Rare earth oxides-cost Of importance are materials that have low thermal expansion, a property that is met by the silicates of aluminum and zirconium, and also by calcium aluminate. The last named is a very recent development and promises to be increasingly important because of its low thermal expansion. Hydraulic setting high temperature mortars and concrete have been developed that incorporate varying amounts of calcium aluminate, the aluminate being the hydraulic setting agent. Zirconium silicate or zircon also finds extensive use as a combustion catalyst, the principal advantage being relatively low thermal expansion and ease of fabricating of even intricate shapes to close tolerance by ordinary ceramic forming practices. 184
Figure 4. Diffuser Modified to Withstand Rapid Temperature Changes
for the material of construction for the diffuser. As this is impossible, from the standpoint of both cost and availability of sufficient quantities of the metal, other means had to be found to make this burner practical for high temperature and rapid cycle operations. The above considerations apply only to burner equipment that is being cycled-Le., heated and cooled rapidly-as many thousands of burners have been in use for years of uninterrupted, continuous duty or low number of shutdowns
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-Ceramics The problem of diffuser failure when subjected t o rapid cycling has been solved principally by design changes of the diffuser, best illustrated by comparing Figures 3 and 4. Without a change in the material used, the diffuser tip shown in Figure 4 is virtually indestructible. The reason for such a striking improvement can also be recognized by comparing the two figures. The part of the diffuser subject to failure has simply been removed. Of interest should be the possible use of recently developed high temperature metal-ceramice as materials of construction for the diffuser and other parts of the burner. Oxidation rate of most high temperature metals and carbides is rapid, but can be reduced materially by incorporating varying amounts of refractory oxides that form an impervious protective coating. Fabricating parts by ceramic methods from metal and oxide powder and subsequent sintering has advanced to the point where shapes of intricate design can be produced in quantity to close tolerances. The principal advantages of these materials are high heat conductivity, high strength a t elevated temperatures, and relatively low oxidation rate. Very recent newcomers in this field are molyb-
and Glass-
denum and silicon, together with chromium and titanium carbide. Some of these recent developments are really not new. More than 50 years ago many investigators and inventors proposed metal-ceramics incorporating substantially identical components as those of today. However, there were few, if any, needs for materials of construction for severe temperature fluctuation a t that time. Chromium and silicon ( 5 ) are being recognized as valuable metals for high temperature service, especially from the standpoint of low oxidation rate a t elevated temperature. A ductile form of silicon (8) has also been reported. LITERATURE CITED
(1) E v a n s , M. W., Chem.Revs.,51, No. 3,363 (1952). (2) Gen. Elect. Rev.,56, No. 1, 14 (1953). (3) McAdam, D. J., Jr., J. Research Natl. Bur. Standards, 28, 693 (1942). RECEIVED for review April 14, 1953.
ACCEPTED August 28, 1963.
for Nuclear L. R. MCCREIGHT, Knolls Atomic Power Laboratory, .
I
I
General Electric Co., Schenectady, N . Y .
The nonmetallic inorganic materials that comprise the field of ceramics have been associated directly and indirectly with atomic energy work since the beginning of the Manhattan Project. Today, ceramic materials are being investigated to an even greater extent than during the war for possible applications in a reactor that will produce useful power. Such applications are many and varied. They include the use of conventional electrical and thermal insulations, refractories, and ceramic coatings. Other less conventional ceramics are being studied for use in reactors as fuels, moderators, poisons, shields and bearings, and in other special applications.
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HE five major interior components of a power-producing nuclear reactor are listed in Table I, with the general
properties and types of material required for each application. These are illustrated schematically in Figure 1 (6).
TABLE I. COMPONENTS OF POWER-PRODUCING NUCLEAR REACTOR Fuel Moderator Shielding Coolant structural materials
Under oes fission U236 PuzZ9 or U2as Slows $own neutions b Be ‘etc. Prevents esca e df ‘radioactivity. Hydrogenous material to agsorb neutrons. Heavy metals t o absorb beta and gamma radiation Heat transfer air, water, and liquid metals TO support other components
Several reactors (3) have been built using a ceramic fueluranium dioxide-as part or all of their fuel. The very first reactor, which wm built in the west stands of Stagg Field at the January 1954
University of Chicago in 1942, had about 10 tons of uranium metal and 40 tons of uranium dioxide for fuel. It also had a ceramic moderator-graphite. The second British reactor, nicknamed “Gleep” (for graphite, low energy, experimental pile), also uses uranium metal and uranium oxide (12 and 21 tons, respectively) as fuel and graphite as a moderator. The French reactor, “Zoe” (zero, oxyde d’urane, eau lourde), uses about 30 tons of purified sintered uranium dioxide rods for fuel. Another reactor similar to Zoe is reported to be under construction by Norway and Belgium. The construction of nuclear reactors for the generation of useful power will probably result in higher and higher operating tempere tures. When the operating temperature of the fuel is to be more than 1000’ C., ceramic fuel elements will be almost a necessity. Even below this temperature there is considerable interest in ceramic materials for fuels. Among the significant research work being done is the preparation of phase diagrams of uranium dioxide with other refractory oxides such as magnesium oxide, beryllium oxide, calcium oxide, aluminum oxide,
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