US2976853A - Steam generation - Google Patents

Description

March 28, 1961 A. T. HUNTER Erm. 2,976,853

STEAM GENERATION Filed March 51, 1959 2 Sheets-Sheet 1 ARTHUR T. HUNTER ROBERT C. PATTERSON EVERETT C. LEW|$ ATTORNEY March 28, 1961 A. T. HUNTER Erm. 2,976,853

STEAM GENERATION Filed March C51, 1959 2 Sheets-Sheet 2 INVENTORS: ARTHUR T. HUNTER ROBERT C. PATTERSON EVERETT C. LEWIS Unit States STEAM GENERATION Filed Mar. '31, 1959, Ser. No. 803,305 4 Claims. (Cl. 122-4) This invention relates to an improved method for generating and/ or heating steam or for heating other desired fluids and has particular relation to an improved method for this purpose employing a uidized bed within which fuel and air are introduced with the fuel being oxidized within the bed and with the `fluid to be heated passed in indirect heat exchange relation with the bed.

A bed of discrete material may be fluidized by passing a stream of gas upwardly therethrough, with uidization for a particular particle size occurring at or above a predetermined velocity and temperature. When in this fluidized state the particles of the material move rather rapidly throughout the body of material with the entire mass being in an agitated state resembling a boiling liquid and with the violence of the agitation depending upon the velocity of the gas passing upwardly through the material. When in this iluidized state the material is not carried along with the iluidizing gas and although the body of material will be expanded from its packed or nonfluidized condition, the uidizing gas passes through the material and leaves the same at what is termed a disengaging zone. With a given particle size and density the velocity of the gases passing through the material must be maintained lwithin specific minimum and maxi.

mum limits in order to have proper fluidization. The maximum limit is that where substantial quantities of the material are carried away from the bed in the gas stream, or in other words, the material is entrained within the gas stream, while the minimum limit is that below which fluidization does not occur, the mass of material remaining in the so-called packed condition and the particles not moving from their at rest position.

It is well recognized that within fluidized beds an extremely high rate of heat transfer prevails among the lluidizing gas, the lluidized particles and a heat exchange coil or the like immersed within the bed. In addition to this high rate of heat transfer, which is many times that obtained with conventional systems, i-t has been found that by injecting fuel and air directly into a liuidized bed and oxidizing the fuel therewithin very high heat release rates are obtainable which are many times those obtainable by conventional firing methods. These char acteristics render the use of liuidized beds within which atent the generating and/ or heating of steam or the heating of other desired luids because of the great reduction in size of equipment over that heretofore required for accomplishing the same result.

It has been found desirable, for severalA reasons, to operate these beds in the neighborhood of l800 F. or within the range of 1600 to 2000 F. At this temperature complete oxidation of commercially feasible fuels, which have relatively high ignition temperatures is had with a bed of material that is not as costly, extremely active oxidation catalyst but which can be obtained very economically and will not catalyze the oxidation process at a temperature substantially below these temperatures but does catalyze the process at these rather high temperatures. Materials such as activated alumina, tubular alumina and the like have proved satisfactory for such beds. The working fuels referred to have an ignition temperature of from 1000 to 1400" F. and it is found that with a bed temperature somewhat above the ignition temperature such as the 16002000 F. range mentioned satisfactory oxidation is obtained. Another result of operating at these high temperatures is the attainment of very high heat transfer rates, much higher than at lower temperatures, with 50-55 B.t.u./sq. ft./ F. differential being obtained with a particle size of about 1A; inch and apparent density of 60 lbs. per cubic ft., with there being no serious metallurgical problems experienced with regard to the tube coil immersed within the bed since this coil is adequately cooled by the iluid flowing through it so as to keep its temperature below an excessive value.

While With these economic materials satisfactory oxidation of a mixture of such a working fuel and air is had when the bed of material is in the temperature range of 1600-2000 F. there exists the very diicult problem of heating the bed of material to this high temperature in a simple, economical and feasible manner. The present invention is directed to the solution of this problem.

In accordance with the invention there is provided a bed of material, such as activated alumina which is heated to a temperature sufficiently high to oxidize an ignition fuel which as a much lower ignition temperature than the commercially feasible working fuel and at which temperature the bed of material is incapable of oxidizing the working fuel. This heating of the material may be accomplished by passing hot .air upwardly therethrough with the air being heated by means of a special air heater which may take the form of a duct burner. It is preferable, although not necessary to maintain the bed in a packed condition during this heat-up period so that the rate of heat transfer to the coil is at a low value. After thus heating Ithe bed of material the ignition fuel is introduced thereinto and air is passed upwardly therethrough with this ignition fuel being oxidized within 'the bed of material. Sucient of this ignition fuel is introduced into the bed so that the heat evolved by its oxidation will raise the temperature of the bed to where the working fuel will be effectively oxidized or in other words, to the 1600-2000" F. range. Thereafter the supply of ignition fuel to the bed is stopped and the Working fuel is introduced thereinto along with air being passed upwardly through the bed with the gas flow through the bed being regulated Ito maintain the bed -uidized The working fuel is completely oxidized within the bed (complete oxidation meaning only minute traces of unburned fuel being evidenced in the efluent from lthe bed) and heat is impaited to a luid, such as converting water to steam by passing it in indirect heat exchange relation with the bed with a tube bundle or the like being immersed with the bed for this purpose.

In a modification of this method a still lower preheating is had by utilizing a bed of material that is a very active oxidation catalyst. In this modification a separate bed is provided and into which the ignition fuel is introduced with this separate bed being comprised at least in part of a very active oxidation catalyst. This bed is heated, as by hot air, to a temperature sulciently high to elect ,oxidation of the low ignition temperature ignition fuel and the hot eluent from this catalytic bed is passed through the main bed to heat it to its required temperature for oxidizing the working fuel with the working fuel then being supplied to this main bed, the ignition fuel turned off and the main bed maintained fluidized as in the previously described embodiment.-

It is an object of this invention to provide an improved method for the generating and/or heating of steam or other desired'uids employing alluidized bed..

of material within which fuel is introduced and burned and from which heat is absorbed to heat the desired lluid.

Fig. l is -a diagrammatic representation in the form of a vertical section through an apparatus suitable for carrying out one form of the method of the invention,

Fig. 2 is a sectional view taken along line 2--2 of Fig. l.

Fig. 3 is a view similar to that of Fig. 1 but showing a modified apparatus suitable for carying out another form of the method of theinvention.

Referring now to the drawing, wherein like reference characters are used throughout to designate like elements, the stnuctural organization diagrammatically represented in Figs. 1 and 2, comprises ahousing 10 into the lower end of which is introduced air or other suitable combustion supporting gas, with fan or pump 12 forcing this air into and upwardly through the housing with the air prior to its entrance into the 4housing passing through preheater 14, of any desired construction, where it is preheated as required. g

Within housing there is disposed an upper mass or bed of discrete material 16-and a lower mass or bed of discrete material 18 which is separated from the upper bed being 'supported on a perforated plate or closely spaced tubes 20 while the lower bed is supported on a similar perforate plate or closely spaced tubes 22 with each of these plates permitting a gas to pass upwardly throughthem and accordingly through housing 10 while preventing the passage of the material which they support downwardly through them. The upper plate or tubes 20 must be cooled in order to withstand the high temperature to which they are subjected and accordingly if a plate is'employed cooling tubes are attached to it while if tubes are employed a Vcooling medium is passed -through them.

In lieu of separating material'lslfrom material 16 by plate or tubes 20 the vparticles of` material 18 may be of suiciently greater size or density than those of the mate rial 16 so that they do not uidize within the range of fluidization lvelocity of the material 16 whereby material 18 will remain in the lower endof housing 10 with maferial 16 disposed 'immediately thereabove and restingY thereon.

The'bed of material 18 is comprised at least in party of a very active oxidation catalyst such as activated alumina coated or impregnated with platinum, palladium, rhodium,v ruthenium, silver, copper, chromium, manga-7 nese, nickel,l cobaltor combinations of these metals while the bed 'ofnlaterial '16 is comprised of a material which is "much Vless VALactive as `an oxidation catalyst and is accordingly much more' economical such as activated alu-` mina, per se, or tubular alumina. Fuel may be intro,1 duced into each ofthese beds with the fuel being introduced into the lower portion of bed y18 through the distribution conduits 24 which are connected to lieaderk26` supply header v38`a1idreturn header 40. Fluid to be heated is conveyed through the tube bundle and this bundle may advantageously take the form of a steam generator wherein water is'evaporated. The tube bundle may bee'modied to yform or include part or all of the walls of the enclosure or housing 10.

The so-called working fuel lfor vthe organization of Figs. 1 and-2 is suppliedto header 3i) and introduced into the lower portion-of bed 16 .through the distribution foonduits 28. Thisffu'el is Yexrployedlfor sustained operation f' of the device and as previously mentioned fuels that are economically feasible have a characteristically high ignition temperature. Therefore an ignition fuel which is employed only during the starting-up periods of the unit is introduced into the bed 18 through distribution conduits '24 with this ignition fuel having a substantially lower ignition temperature than that of the working fuel. When operation of the unit is initiated the air that is forced up through housing 10 and accordingly through the beds 18 and 16 by means of the fan 12 is preheated by air preheater 14 sufliciently to heat the bed of very active catalyst 18 to a temperature somewhat about its activation temperature for the ignition of the fuel. Thereafter valve 32 is opened and this ignition fuel is introduced into the bed 18 where it is oxidized. The eiiluent from this bed -18 passes upwardly through the bed 16 and the supply of ignition fuel to the bed 18 is regulated so that this effluent will heat the material in bed 16 to a suiciently high temperature that it is capable of oxidizing the working fuel when introduced thereinto from 'distribution conduits 28 and with the gas ow through bed 18 preferably being maintained below that required for uidization during this heating period. When the material in bed 16 is so heated valve 32 is closed and valve 34 is opened so that the supply of ignition fuel to bed 18 is shut olf and the Working -fuel is supplied to the lower portion of bed 16. Since bed 16 has been heated suiciently to oxidize this working fuel the fuel lwill be oxidized within this bed of material. The passage of air upwardly through the bed of material is then `regulated so that the bed is maintained iu a liuidized state and the heat evolved or a portion ofthe heat evolved from the oxidation of the working fuel within the bed is imparted to the iluid that is conveyed through the heat exchange bundle 36.

By the use of an ignition fuel which has an ignition 'temperature substantially lower than that of the working fuel and by the use of a bed of very active oxidation catalyst which accordingly has a low activation temperature or minimum temperature at which it is capable of oxidizing `a fuel and into which the ignition fuel is introduced the capacity that is required for the 'preheater 14 is Ysufficiently low so that from an economic standpoint it is within reason and is feasible.

In accordance with this method the following opera tion has been carried out. When methane was employed as Lthe working fuel, which has an ignition temperature of approximately 1200" F., an ignition fuel was 'ernployed which had an ignition temperature of approximately 500 F. and was introduced into a bed of catalyst which was capable of oxidizing this ignition fuel at a temperature of 600-700 F. The bed of catalyst was heated to a temperature within this range and the ignition fuel then supplied to the catalyst. The supply of this fuel was regulated so that the effluent from this bed of catalyst heated the `bed of material 16 to a temperature generally within the range of l600-2000 F. at

which temperature this material was capable of completely oxidizing the working fuel. When this temperature was obtained the supply of ignition fuel to the bed of catalyst 18 was terminated and the working fuel was supplied to the bed of material 16 with Ithis bed being maintained in a fluidized condition and with complete oxidation of `the working fuel being had within this bed.

In the modiiication represented in Fig. 3, in lieu of employing abed of catalyst a single bed (16a) of material is employed which is comprised of the previously mentioned economical material that is capable of oxidizing the working fuel to produce complete lcombustion only at the rather high temperatures mentioned with the ignition fuel being introduced into the lower portion of this bed through distribution conduits 28a. The operation of this organization is lsimilar to that previously described except that the bed of material'16a "must be heated to a'temperaturesuihciently' high to oxidizethe ignition fuel with this temperature of course being somewhat higher than that required when a very active oxidation catalyst is employed `as in the previously described embodiment although it is substantially lower than that required to oxidize the working fuel and has been found p to be low enough to render the operation commercially feasible. When the bed of material has been heated to this required temperature by means of air heater 14 the ignition fuel is introduced into the lower portion of the bed through distribution conduits 24a with this fuel being oxidized within the bed and with the heat evolved heatying the bed to a suiciently high temperature so that it is capable of oxidizing the working fuel. Thereafter the supply of ignition fuel is terminated and the working fuel is introduced into the -bed and completely oxidized therewithin with the gas ow upwardly through the bed being regulated to maintain the bed fluidized.

With this modified form of method, operating with methane (ignition temperature 12.00 F.) and with an ignition fuel having an ignition temperature 500 F., the bed of material 16a was heated by means of hot air to a temperature of between 80G-900 F. after which the ignition fuel was introduced thereinto and completely oxidized or substantially `oxidized therewithin. The supply of ignition fuel Was regulated so that the bed was heated to between 1600-2000 F. after which the supply of ignition fuel was terminated and the working fuel was supplied to the bed, the bed iluidized and with oxidation of the working fuel being effected in the bed and with the working fuel being regulated to maintain the temperature of the bed within this range.

Thus in accordance with the present invention the heating of a bed `of material to the high temperature required for oxidation of a commercially feasible fuel which has a high ignition temperature is greatly simplified and brought within the realm of reality with the method being highly practicable and highly satisfactory and overcoming an extremely difficult problem in starting units to which the invention is directed.

The term air as used through the application includes all combustion supporting gases and gas mixtures.

This description is intended for the purpose of explanation and illustration only and is not to limit the invention since many modifications which are within the scope and spirit of the invention will occur to those skilled in the art. Y

What is claimed is:

l. The method of steam generation through oxidation of a working fuel and transfer of heat to water which comprises heating a stream of air to a temperature below the ignition temperature of the working fuel but above the ignition temperature of an ignition fuel which has an ignition tem-perature well below that of the Working fuel, passing said air upwardly through a ybody of discrete material at a suflicient-ly low velocity so that the material does not uidize and thereby heating the same to a temperature above the ignition temperature of the ignitionfuel, thereafter introducing said ignition fuel into said body of material and oxidizing the same therewithin, utilizing a portion of the heat thus liberated to heat a body of discrete material to a temperature well above the ignition tempera-ture of the working fuel, and Where the material will effect substantially complete oxidation of a mixture of the working fuel and air that is introduced thereinto, after this body of material attains this desired temperature introducing such a mixture into and passing it upwardly through this body of material and terminating he flow of said ignition fuel, regulating the gas flow through the 'body of material so as to lluidize the same and completely oxidize the working fuel within said iluidized lbody of material, and converting' Water to steam by passing it in indirect heat exchange relation with this iluidized body of material.

2. The method of uid heating through oxidation of a Y when heated to a given temperature much lower than said predetermined high, heating the air prior to its passage through the bed sufficiently to raise the temperature of the bed incident to passage of the `air therethrough to a point where it is capable of effecting oxidization of the ignition fuel but incapable of effecting oxidation of the working fuel, thereafter introducing said ignition fuel into the lower portion of the bed and oxidizing the same therewithin, regulating the supply of fuel and air to maintain the bed packed and impart sufficient heat to the bed to raise its temperature to atleast said predetermined high, thereafter introducing said Working fuel into the bed, increasing the air flow therethrough to uidize the bed and terminating the introduction of the ignition fuel thereinto, substantially completely oxidizing said working fuel within said bed, imparting heat to a uid by passing it in indirect heat exchange relation with the fluidized bed, and regulating the supply of said Working fuel to maintain said bed of material at a temperature at least as high -as said predetermined high temperature.

3. The method of fluid heating through oxidation of a working fuel and transfer of heat to -a lluid comprising passing air upwardly through a bed of material which is capable of effecting substantially complete oxidation of the working fuel when the material is heated to a predetermined high temperature and of an ignition fuel when heated to a given temperature much lower than said predetermined high with this given temperature being generally in the range of G-900 F., heating the air prior to its passage through the bed sufliciently to raise ignition fuel into the lower portion of the bed and oxi` dizing the same therewithin, regulating the supply of fuel and air to maintain the bed packed and impart sufficient heat to the bed to raise its temperature to at least said predetermined high, thereafter introducing said working fuel into the bed, increasing the rair flow therethrough to iluidize the bed :and terminating the introduction of the ignition fuel thereinto, substantially completely oxidizing said working fuel within said bed, imparting heat to -a fluid by passing it in indirect heat exchange relation with the iluidized bed, and regulating the supply of said working fuel to maintain the temperature of said bed of material at least as high as said predetermined high temperature.

4. The method of uid heating through oxidation of a working fuel and transfer of heat to =a fluid comprising passing air upwardly through a bed of material which is capable of effecting substantially complete oxidation of a working fuel having an ignition temperature of approximately 1200 F. when the material is heated to a temperature generally in the range of 1600-2000" F. and an ignition fuel having an ignition temperature of approximately 500 F. when the material is heated to a temperature generally in the range of 800 to 900 F., heating the air prior to its passage through the bed sufficiently to raise the temperature of the bed incident to passage of the air therethrough to a temperature generally in the range of SOO-900 F., thereafter introducing said ignition fuel into the lower portion of the bed and oxidizing the same therewithin, regulating the supply of fuel and air to maintain the bed packed and impart suliicient heat to the bed so that its temperature is generally in the range of 1600-2000" F., thereafter introducing said working fuel into the bed, increasing the air ilow therethrough t u'idize the bed vand terminating the introduction of the References.A Cited inthe leof this patent ignition 'fuel' thereinto, substantially completely oxidiz- UNITED- STATESV-PATENTS f fidby'pass'ing it in indirect heat exchange relation with 420056 Hackney "j Ian' 28 'f1890 Y Y Y. 2,664,346 Mayhew Dec.29, 1953 the uidized bed, `and regulating the supply of said work- 5 l ing fuel to maintain said bed of material at a tempera- FOREIGN PATENTS 4 ture generally in the range of x1600-2000 F. 7922682 ",Great Britain Api-Q2; `14958

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