US2872906A

US2872906A - Method for the operation of a boiler and apparatus therefor

Info

Publication number: US2872906A
Application number: US409334A
Authority: US
Inventors: Eugene J Houdry
Original assignee: Oxy Catalyst Inc
Current assignee: Oxy Catalyst Inc
Priority date: 1954-02-10
Filing date: 1954-02-10
Publication date: 1959-02-10
Anticipated expiration: 1976-02-10

Description

Feb. 10, 1959 E. J. HOUDRY 2,872,906

I METHOD FOR THE OPERATION OF A BOILER AND APPARATUS THEREFOR Filed Feb. 10, 1954 4 Sheets-Sheet 1 FIG.

INVENTOR.

EUGENE J. HOUDRY \IATTQRNEY Feb. 10, 1959 E. J. H DRY METHOD FOR THE 0P TION AND APPAR s THEIR 2,872,906 OF OILER EF 4 Sheets-Sheet 2 Filed Feb. 10, 1954 m E N u U u;

H u u u u H mm u u xws \ IN V EN TOR.

EUGENE J'. HOUDRY ATTORN EY 1959 E J. HQUDRY 2,872,906

METHOD FOR TRE OPERATION OF A BOILER AND APPARATUS THEREFOR Filed Feb. 10. 1954 4 Sheets$heet 3 v WZl I I I I I 1 I I l I pk l "Zl 25 4- 4 FIG. 3. 25

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BY f I T MT 4%.M% %e- ATTORNEY 4 Sheets-Sheet 4 $5 H 5E2 A E. J. HOUDRY E OPERATION OF A BOILER AND APPARATUS THEREFOR mohbmikma O.

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v EUGENE J. HOUDRY m h ATTORNEYS Feb. 10, 1959 Filed Feb. 10, 1954 METHOD FOR THE GPERATKON (19F A EQILER AND APPARATUS THEREFOR Eugene J. floudrm Ardmore, Pm, assignor to Oxy- Catalyst, Inc, a corporation of Pennsylvania Application February 10, 1954, Serial No. 409,334

11 Claims. (Cl. 122-4) This invention relates to an improved method for the operation of boilers to provide increased efliciency and capacity, and is also concerned with apparatus for carrying out the improved method.

In conventional boiler systems of any appreciable size, the boiler heat exchange system is usually arranged in a number of sections (or passesas they are generally called) which the hot combustion gases from the furnace traverse in series in a generally counter-current direction with respect to the flow of Water, or other fluid to be heated, through the boiler tubes. In the operation of such a plural pass system, usually the major portion of the total heat transfer occurs in the early passes, while only a minor portion of the heat is transferred in the succeeding passes although these comprise the major portion of the total heat exchange surface in the boiler. This is due to the relatively higher temperature differentials between the gas and heat exchange surface existing in the earlier passes as compared to that in the succeeding passes, which of course produces relatively higher rates of heat transfer in the earlier passes. In a typical conventional three-pass boiler, for example, producing 250 pound saturated steam, the average temperature differential in the first pass might be of the order of 1100 F.; drop to about 340 F. in the second pass; and to about 225 F. in the third pass. In such a system it is apparent that the major portion of the capacity of the boiler, as expressed, for example, in B. t. u. actually transferred per hour, is contributed by the first pass, and that the succeeding passes do not contribute capacity in proportion to the heat exchange area which they contain. To increase the capacity of such a system it is necessary to increase the size of the entire boiler including the boiler furnace and the heat exchange area provided in the first and in the remaining passes.

Inone aspect the invention has for its object to provide a method for the operation of a plural-pass boiler which makes possible a substantial increase in its overall capacity without a corresponding increase in its overall size.

In another aspect, the invention is concerned with increasing boiler efficiency. As is well known one of the factors affecting boiler efficiency at a given temperature is the amount of sensible heat loss in the stack gases, which is indicated by the ratio of the volume of stack gases exhausted per unit of heat produced or transferred in the boiler. The larger the value of this ratio, the lower the boiler elficiency. This factor in the boiler efficiency (which is often measured in terms of CO concentration in the stack gas) is determined largely by the amount of excess air employed in the boiler furnace. As is well known, there is a certain percentage of excess air required to produce efficient combustion and smooth operation in the boiler furnace, the percentage required depending upon the type of boiler and type of fuel which is burned. With reference to this second aspect, it is the object of the invention to provide an increase inoperation.

boiler efficiency by reducing the value of this ratio without necessarily reducing the amount of excess air employed in the boiler furnace, thus reducing sensible heat losses and increasing the overall boiler efficiency. As will be apparent from the description which follows, the method provided by the invention combines in the same process both of the above advantages, namely, increased capacity without a corresponding increase in overall size plus increased efiiciency.

A further object of the invention is to provide apparatus adapted to carry out such a method.

Other and more specific objects of the invention will be apparent from the detailed description which follows and the accompanying drawings.

The method of the invention providing the above advantages may be described in general terms as follows. Any of the ordinary fuels is burned in the usual manner in a boiler furnace preferably using sufficient excess air over the stoichiometric amount theoretically required for combustion to provide for efficient combustion and smooth The hot stream of combustion gases thus generated, containing free oxygen are then conducted through at least one pass of the boiler heat exchange system where the gases give up a major portion of their heat and undergo a substantial lowering of their temperature. An auxiliary fuel such as a fuel gas or a volatilized hydrocarbon vapor is distributed in the cooled stream of boiler gases in such proportion as to produce a mixture which is outside the limits of flammability thus obviating any danger of explosion which otherwise would accompany such an operation. This non-flammable mixtur of combustion gases and added fuel is then conducted into contact with a bed of oxidation catalyst arranged across the path of flow of the boiler gases, causing oxidation of the non-flammable mixture entirely by catalytic oxidation at the surface of the catalyst and thereby sub stantially increasing the temperature of the stream of boiler gases. The thus reheated gas stream is then conducted through the next succeeding pass of the heat exchange system with the result thatin this pass the temperature differential between the gas and heat exchange surface, and consequently the rate of heat exchange, is substantially increased in comparison to the conventional methods of operation. This substantially increased rate of heat exchange beyond the first pass in turn effects a substantial increase in the overall capacity of the boiler without a corresponding increase in the overall size of the furnace or, generally speaking, of the overall size of the boiler.

In addition to this increase in the overall boiler capacity, catalytic combustion of the added fuel in the manner described above makes possible a decrease in the volume of stack gases per unit of heat produced as a result of the fact that the catalytic oxidation of the added fuel consumes at least a portion of the excess oxygen present in the combustion gases from the furnace. Since catalytic oxidation in most instances can be effectively carried out in the presence of as little as 2% or even less of excess oxygen over the stoichiometric amount required, the percentage of oxygen in the stack gases can be reduced to 2% or even less.

For a better understanding of the invention reference is now made to the accompanying drawings in which:

Fig. 1 is a semi-diagrammatic view showing a typical horizontal tube, three-pass steam boiler, constructed in accordance with the invention;

Fig. 2 is a plan view taken on line 2-2 of Fig. 1 showing a portion of the boiler tubes and showing means for distributing auxiliary fuel in the boiler gases, and a bed of oxidation catalyst supported on the boiler tubes;

Fig. 3 is an enlarged plan View of a section of the bed of oxidation catalyst;

Fig. 4 is a view partly in section taken on the line 4-4 of Fig. 3;

Fig. is a cross-sectional view taken on the line 5--5 of Fig. 4;

' Fig. 6 is a perspective view showing one typical catalytic unit suitable for employment in the invention; and

Fig. 7 is a diagrammatical view of a system for controlling the distribution of auxiliary fuel.

Referring now particularly to Fig. 1, this figure shows a side elevation of a boiler having burners designated generally by the letter B which fire into the furnace space designated by the letter F. The boiler tubes T are supported upon the usual masonry pillars P.

Referring now to the circulation system in the boiler, the reference numeral 1 refers to the steam drum having a safety valve 1a and a water level indicator 1b. Headers 2 and 3 respectively are disposed at either end of the tubes, and communicate with the steam drum by lines 5 and 4 respectively. The interior of the boiler is divided into three sections or passes designated generally by the Roman numerals I, II and III by means of bafiies 6 and 7 which cause the gases to traverse the boiler tubes in series in three separate passes.

The top of the furnace is provided with the usual masom'y roof 8 and insulating material 9 as an outer covering.

Primary fuel to be burned in the furnace is supplied by means of a plurality of burners B through line 10 which is controlled by a valve 10A. Reference numeral 11 indicates an inlet for primary air which is inspirated and mixed with the fuel through the action of the venturi inspirator 12. Openings 13 provide for the ingress of secondary air, all in a manner well known in the art. The combustion gases which take a path as indicated by the arrows 26, leave the tube section of the boiler through outlet 14 and may thereafter be passed through the usual air heater or economizer or directly to stack as desired.

Auxiliary fuel is supplied to the combustion gases on the downstream side of the first pass by line 15 which supplies fuel to a distributing spider having a distributing conduit 16 running parallel to the boiler tubes T which feeds transverse conduits 17 having nozzles or holes 18. Flow of auxiliary fuel from the nozzles or holes 18 into the stream of combustion gases is indicated by the arrows 27.

On the upstream side of the second pass, a bed of oxidation catalyst designated by the letter C occupies the area between baffles 6 and 7, being disposed across the entire cross-section of the path of flow of the boiler gases. The bed of oxidation catalyst C is made up of a plurality of units 19 (one of which is shown in enlarged perspective in Fig. 6) which are laid upon the generally horizontal boiler tubes in a manner to be hereafter described in more detail, and held in place principally by gravity. Since the catalytic units are rectangular in shape, a ceramic filler piece 20 is provided to occupy the irregular space produced by the slanting bafiie 6 to avoid by-passing of the boiler gases.

Referring now to Fig. 6 showing one of the units 19 in perspective, it may be seen that the unit consists of a pair of end plates 21 preferably formed of high quality porcelain and maintained in fixed relation to one another by means of a center post 23 cemented in sockets provided in each of the end plates. A plurality of rod like elements 22, also of high quality porcelain, are supported between the end plates and carried in apertures provided therein. As may be best seen in Fig. 5 the rod like elements 22 are arranged in a number of rows, the elements in each row being staggered with respect to those of adjacent rows. The elements themselves are tear drop in cross-section to provide an air foil surface to minimize turbulence and pressure drop in the gases flowing through the unit. These units are more fully described in copending application Serial No. 159,191 filed May 1, 1950 4 of Eugene J. Houdry for Process and Apparatus for Contacting Operations now Patent Number 2,730,434.

The porcelain rods 22 are each provided with a catalytic film, preferably a thin film of catalytic alumina, having a thickness of from .001 to .006", and impregnated with a small amount for example from 1% to 2% by weight based on the weight of the alumina film, of finely divided metal, preferably a metal such as platinum or palladium which, deposited on the alumina base, provides an oxidation catalyst of excellent activity. Catalysts of this general type are more fully described in C0- pending application Serial No. 312,152 filed September 29, 1952 of Eugene J. Houdry for Catalytic Structure and Composition now Patent Number 2,742,437. Such a catalyst existing as a thin film deposited on porcelain is both active and durable, and capable of remaining active after prolonged exposure to temperatures as high as 1800 F. It has a relativelylow activation temperature, that is, the minimum temperature at the catalyst surface at which it will initiate an oxidation reaction. Minimum activation temperature varies somewhat depending on the reactants to be oxidized, but for many gaseous fuels, such as carbon monoxide, propane, hydrogen, etc., will be as low as 500 F. in the case of a platinum-on-alumina catalyst such as that described above.

As may be seen in Figs. 2, 3, 4, and 5, the units 19 are stacked side-by-side and end-to-end in a plurality of rows, and in two superimposed layers. As may be best seen in Figs. 4 and 5 the bottom layer of units is carried on H-shaped ceramic beams 24 which are laid directly across the boiler tubes T. As may be seen in Fig. 4 the channel provided by the H-shaped beam 24 is of a width sufiicient to accommodate two back-to-back end plates 21 thus retaining the units against end-wise separation and thus tying the entire assembly together.

As may be seen in Figs. 4 and 5 the second layer of units is supported on the first layer, the same type of ceramic H-shaped beams 24 being interposed between the two layers, pairs of back-to-back end plates in the upper and lower layers being carried in the upper and lower channels respectively of the H-shaped beams. To prevent by-passing of the gases between units in the upper layer, T-shaped ceramic inserts 25 are disposed between each pair of side-by-side units. The insert 25 is merely held in place by gravity and rests upon a pair of rod like elements 22 as may be best seen in Fig. 5. Bypassing between the units in the lower layer is obviated by arranging the units in the two layers in staggered relation in the manner shown in Fig. 5.

The advantages of the catalytic assembly described above (which is more fully described in copending application Serial Number 308,838, filed September 10, 1952 by William M. Bowen III for Catalytic Assembly now Patent Number 2,718,460) are many. in the case of a generally horizontal tube boiler as in the described embodiment, the entire assembly may be laid in place and retained in position entirely by gravity. Since the entire assembly is constructed of ceramic material, preferably good quantity porcelain, it is capable of withstanding high temperatures. By properly dimensioning the H- shaped beams 24 permitting the proper tolerances, expansion and contraction of each of the units can take place substantially independently of the others in the assembly, thus permitting localized variations in temperature which are apt to occur in a large assembly. Pressure drop through such a two layer assembly, using air-foil rods 22 approximately .15 in thickness and with a center-tocenter spacing between rods in each row of about .28", is of the order of .05" of water at a gas velocity of about 55 ft./second and a gas temperature of 1500 F. through the assembly.

It is to be understood that although the use of catalytic units like those shown, assembled in the manner described, is a preferred embodiment of the invention, the invention is not limited to any particular type of catalyst, or catalytic unit, or manner of assembly of such catalysts or units, and that any suitable type of oxidation catalyst which is capable of withstanding high temperatures and at the same time of imposing a minimum of pressure drop in the system may be employed.

In carrying out the method of the invention, which will now be described in more detail with particular reference to the embodiment shown in the drawings, any of the usual types of fuel may be burned in the boiler furnace such as fuel gas, fuel oil, coal or other types of fuel. In burning fuels such as coal, where a considerable amount of fly ash is carried in the stream of boiler gases, suitable provision should be made to protect the catalyst from undue erosive effects occasioned by the impingement of the fly ash on the surface of the catalyst. Likewise, disposition of the catalyst bed in a zone where the fly ash is in a molten condition should be avoided to prevent deposition of the fly ash upon the surface of the catalyst.

As in ordinary practice, burning of the fuel in the furnace should preferably take place in the presence of the usually required percentage of excess air over the theoretical stoichiometric amount required for complete combustion. The amount of excess air employed should be chosen in the usual manner so as to provide for eflicient combustion and smooth operation and to provide a stream of combustion gases containing negligible amounts of combustibles. As is Well known, the amount of excess air required to afford' smooth operation and eflicient combustion depends upon a number of factors, including for example the type of burner, type of furnace, and type of fuel employed. As will be explained in more detail subsequently, it is preferable to restrict the amount of excess air so that the amount of oxygen in the resulting combustion gases is such that it is impossible to form a flammable mixture with the auxiliary fuel subsequently added irrespective of the proportion of the added fuel.

The stream of hot combustion gases from the boiler furnace are led into the first pass of the boiler heat exchange system, entering at temperatures of for example from 1800 F. to 2200 F. and leaving the first pass at for example from 800 F. to 1000 F.

The thus cooled gases leaving the first pass are then mixed with an auxiliary fuel which is distributed in the gas stream by a distributor such as the one illustrated in the drawings. Desirably, the distribution means is so arranged as to provide thorough mixing and uniform distribution of the added fuel in the stream of boiler gases. In the distributing means shown in the drawings for example, provision should be made for equalizing the flow in the various pipes making up the spider, for example, by proper sizing of the pipes as is indicated in Fig. 2. In a distribution device such as that shown in the drawings, the distributing orifices may be disposed on the upstream side of the distributing pipe, instead of on the downstream side as shown, so that the auxiliary fuel is delivered into the stream of boiler gases counter-current thereto, thus affording increased turbulence and better mixing. Other types of distributors of course than that shown may be employed.

Fuel gas, or vaporizable hydrocarbons such as volatile fuel oils are preferred for use as auxiliary fuels since these may be most readily distributed uniformly and intimately in the gas stream, and thus most readily undergo catalytic oxidation when brought into contact with a bed of oxidation catalyst. However, in some cases solid fuels may be employed for this purpose, particularly ash free solid fuels, provided these are reduced to an extremely finely divided condition.

The amount of auxiliary fuel to be added depends upon the desired outlet temperature from the bed of oxidation catalyst. Sufiicient fuel should be added to raise the temperature of the gas stream leaving the oxidation catalyst, through the heat released by the catalytic oxidation .of the added fuel, to such desired outlet temperature.

I Thus, if the temperature of the boiler gases leaving the first pass is 800 F. and it is desired to reheatthis stream to 1600" Frbefore flowing through the second pass, sufficient auxiliary fuel must be added at the proper rate to produce this 800 F. rise in gas temperature. It is clear that the required amount of auxiliary fuel can be readily calculate-d knowing the B. t. u. content of the fuel and the total flow of boiler gases. The most desirable range of temperatures to which the relatively cool gases leaving the first pass or first series of passes of the boiler heat exchange system should be reheated depends upon economic considerations and the characteristics of the particular boiler involved. Generally speaking, however, it is desirable to reheat to relatively high temperature of the order of 1300 F. to 1800 F. to obtain the advan tages of a high rate of heat exchange. Upper limit of reheat temperature will usually be set by the practical operating limit of the oxidation catalyst. Most known oxidation catalysts lose their activity when operated over 1800 F.

Another factor to be considered in selecting the rate of addition of auxiliary fuel is the necessity from a safety standpoint of avoiding the production of flammable or explosive mixtures in the boiler. The mixture of auxiliary fuel with the boiler gases must always be kept outside the limits of flammability to avoid the danger of explosion.

There are two factors which affect the flammability of any particular mixture of a given fuel with an oxygen containing gas: first, the fuel concentration, and secondly, the oxygen concentration. For any given fuel in a gas of given oxygen content, there are certain upper and lower limits of fuel concentration outside of which the mixture is non-fiamma le. It is usually diflicult to predict these limits, particularly in complex mixtures, although they can be readily determined empirically. For example, the upper and lower limits of flammability of many gases and vapors in admixture with air have been determined. (See U. S. Bureau of Mines Bulletin 279 Limits of lnflammability of Gases and Vapors 1939.)

Similarly, for any given fuel admixed with an oxygen containing gas, there is a fairly definite lower limit in the oxygen concentration below which it is impossible to form an explosive mixture irrespective of the concentration of the fuel gas or vapor in the mixture. Thus, for example, if the oxygen c ntent of air is gradually re placed by nitrogen or some other inert gas, when the oxygen concentration is reduced to about 12%, it is no longer possible to form a flammable mixture containing methane and the oxygen-deficient gas irrespective of the methane concentration. This lower limit of oxygen concentration varies depending upon the particular fuel or mixture of fuels involved, but for most fuels consisting primarily of hydrocarbons it is of the order of about 10% by volume.

There are accordingly two ways to insure that the mixture of boiler gases and added fuel remains outside the limits of flammability, namely, by maintaining a relatively low oxygen concentration in the mixture, and secondly by maintaining a relatively low fuel concentration therein. The concentration of oxygen in the boiler gases depends upon the amount of excess air employed in the boiler furnace. When burning natural gas for example, the use of 20% excess air will result in a concentration of 3.7% oxygen by volume in the stack gases; using 80% excess air there will be 9.7% by volume of oxygen in the stack gases. Preferably, the use of excess air should be controlled so that the concentration of oxygen in the resulting combustion gases is suchthat when the auxiliary fuel is added to the combustion gases it will be impossible to form a flammable mixture irrespective of the concentration of added fuel therein. As indicated above, the limit of oxygen concentration to satisfy this condition will depend upon the particular auxiliary fuel employed.

The concentration of added fuel in the boiler gas 7 stream cariof course be controlled directly by properly metefin'g the added fuel relative to the output of combustion gases. Preferably, the concentration should be adjusted so that the concentration of fuel in the boiler gases always remains less than that concentration correspondihgto the lower limit of flammability of the particular fuel used in air. The lower limit of flammability in air of various fuels varies depending on the articular fuel. For rriethane, for example, this value is 5.3% by volume; for ethane 3.2%; fof pfoiian'e 2.4%; for natuial gas about 413% ar v v I p If both of these c'o'riditions are satisfiedsifiiultarieoiisly, namely, a low oxygen concentration making it impossible to form a flammable mixture irrespective of fuel concentration and at the same time a fuel concentration Which is below the lower limit of flammability of the particular fuel in air, the possibility of forming an explosive mixture becomes doubly impossible. Preferably, the process is carried out in this manner. With this double safeguard should the concentration ofadded fuel in the boiler gas stream bec'onie' excessively high through some iiialfu'rietioning of the system, an explosive mixture would never: thclcss not be formed because of the low oxygen concentration. if, on the other hand, the oxygen concentration in the stream of combustion gases rose to higher levels for some reason, an explosive mixture would nevertheless not be formed because of the low concentration of fuel in the stream.

If desired, additional oxygen besides that introduced in the boiler furnace and not consumed by combustion of primary fuel, may be introduced into the stream of boiler gases at other points in the system. For example, the auxiliary fuel may be mixed with air, or with air and combustion gases, and such mixture distributed in the boiler gases in the manner already described. Alternatively, or in addition, air, a mixture of air and combus tion gases, may be independently added by an additional distributor. Ordinarily, sutiicient oxygen is initially present in the boiler gas stream from the furnace, but in some cases, particularly where the procedure of adding fuel and catalytic oxidation thereof is repeated a number of times, supply of additional oxygen in this manner may prove desirable.

The non-flammable mixture of oxygen containing combustion gases and added fuel passes through the bed of oxidation catalyst and the added fuel is oxidized at the surface of the catalyst, releasing heat and raising the temperature of the stream of boiler gases. Since most oxidation catalysts capable of withstanding the severe conditions inherent in the process are not active below about 1 500 F., the temperature of the gas stream entering the catalyst should be about 500 F. and preferably of the order of about 700 F. The temperature of the gas leaving the catalyst bed will of course depend upon the initial gas temperature and the amount of added fuel. Using an efficient oxidation catalyst such as the type described above, substantially complete oxidation of the combustible material can be obtained in the presence of an oxygen concentration of 2% by volume or even less. The thus heated gases in the embodiment shown immediately encounter the boiler tubcs in the second pass of the heat exchange system at a much higher temperature than in the conventional system. Instead of entering the second pass at a temperature for example of 800 F. to 1000 F. as in many conventional boilers for producing saturated steam, according to the invention the gas would enter at a temperature of the order of from 1300 F. to 1800 F. Average temperature differentials, accordingly, between the gas stream and the heat exchange surface in the second pass may be raised from. the order of 300 F. to 400 F. for example to from 500 F. to 900 F. The corresponding increase in the heat exchange rate results in a substantial increase in the overall capacity of the boiler without any increase in the size of the boiler furnace or of the heat exchange surface in the first or second 8 pass. Operating according to the invention, in the embodiment described and shown in the drawings, the gases leaving the second pass may be at a somewhat higher temperature because of the increased second pass inlet temperature, for example of the order of from 50 F. to 100 F. higher, and it may be in some cases advantageous to increase the heat exchange surface in the third pass somewhat in order to reduce the stack temperature to the same levels as in conventional operation. Alternatively, of course, the size of the air heater or economizer could be somewhat increased.

If desired, the procedure of adding auxiliary fuel to the stream of combustion gases and of oxidizing this fuel in the presence of an oxidation catalyst in the manner' described can be repeated in succeeding passes a number of times. Thus, if the boiler was provided with a relatively large number of passes such as six, two beds of oxidation catalysts could be employed, one arranged for example on the upstream side of the second pass, and another on the upstream side of the third pass, auxiliary fuel being distributed in the stream of boiler gases on the downstream side of the first and second passes.

As previously mentioned, the catalytic oxidation of the auxiliary fuel results in consumption of the oxygen present in the combustion gases by virtue of the use of excess air in the boiler furnace. This of course results in lower oxygen content in the stack gases than in conventional operations, indicating a favorable ratio for the volume of stack gases per B. t. u. delivered. Preferably, of course, conditions should be arranged so that the concentration of oxygen in the stack gases is reduced to as low a value as possible. In a preferred embodiment of the invention, the concentration of oxygen in the stack gases is reduced, through consumption in the process of catalytically oxidizing added fuel, to a value of 2% or less. As previously stated, using an efficient oxidation catalyst, 2% excess oxygen is all that is required to give elficient operation. Reduction of the oxygen concentration may be accomplished using one or more stages of catalytic oxidation of added fuel as proves convenient or desirable.

Suitable precautions should be taken to insure that auxiliary fuel will be supplied only when the boiler is operating normally. When the boiler furnace is not operating for example, provision should be made to automatically shut off the supply of auxiliary fuel to avoid filling the boiler with an explosive mixture. Likewise, if for some reason the temperature of the gases into which the auxiliary fuel is distributed drops below the minimum activation temperature of the oxidation catalyst, provision should likewise be made for shutting off the supply of auxiliary fuel, since the added fuel would otherwise pass through the oxidation catalyst without undergoing oxidation. Reference is now made to Fig. 7 which shows a control system capable of controlling the supply of auxiliary fuel in the boiler system illustrated in Fig. 1.

In Fig. 7 line 10 supplying the main burners B of the boiler furnace with fuel is controlled by a valve 28 operated by a solenoid 29 which biases the valve 28 in a normally closed position. This solenoid operated valve is connected by leads 30 and 31 to the main boiler control system. The branch line 15 which supplies fuel to the fuel distributor on the downstream side of the first pass is controlled by

valves

32, 33, 34 and 35. Valve 32 is a manually operated cut-off valve. Valve 33 is a conventional motor operated pressure regulating valve.

Valve 34 is a cut-off valve operated by a solenoid 36 which biases valve 34 in a normally closed position, and which is connected by

leads

37 and 38 and leads 30 and 31 respectively to the main boiler control system. Solenoids 29 and 36, controlling

valves

28 and 34 respectively, are thus connected in parallel in the same circuit and operate simultaneously so that when the fuel for the main burners is cut off, the supply of auxiliary fuel will also be cut off.

To insure that auxiliary fuel will be supplied into the 9 stream of boiler gases only when the boiler gas temperature at the point of distribution is above the minimum catalyst activation temperature, a thermoswitch 39 is provided in series with the solenoid 36 operating valve 34. Switch 39 is controlled by thermocouples located in the boiler gas stream after the first pass in the vicinity of the auxiliary gas distributor, switch 39 being connected with these thermocouples by line 40. In the system illustrated, switch 39 is set so as to remain open so long as the boiler gas temperature in the vicinity of the auxiliary gas distributor is below the minimum catalyst activation temperature, thus preventing a supply of auxiliary fuel under conditions such that oxidation of the added fuel at the surface of the catalyst might not take place.

As a further safeguard, to provide for the eventuality that the oxidation catalyst might partially or entirely lose its activity, a second switch 42 is placed in series with the solenoid 36, the second switch being controlled by a continuous combustible gas analyzer located downstream from the oxidation catalyst. Any of the well known analyzers which gives a continuous indication of the concentration of combustibles in a gas stream may be employed. A sample of the boiler gases at any convenient point downstream from the oxidation catalyst is continuously withdrawn and passed through the analyzer which gives a continuous indication of the concentration of combustibles and this indication in turn is converted into a signal by well known means to operate switch 42. The concentration of combustibles will, of course, be an indication of the efficiency and activity of catalyst and the system may be adjusted to open the switch 42 when the concentration of combustibles exceeds a predetermined maximum, thus automatically shutting off the supply of auxiliary fuel when the activity of the catalyst falls below a predetermined level.

Motor operated valve 35 located in line 15 is a flow controller for controlling the flow of auxiliary fuel in ac cordance with the temperature of the gases leaving the second pass over the boiler tubes. Within the limits indicated by the necessity of maintaining a non-flammable mixture the rate of flow of auxiliary fuel to the system can be regulated to maintain a relatively constant temperature in the gas stream after the first pass. Thermocouples disposed after the second pass and connected to the control for valve 35 by line 41 sense the gas temperature at this point. It is apparent that the rate at which auxiliary gas is applied to a system determines the amount of heat liberated in the bed of oxidation catalyst, and this in turn controls under a given set of conditions, the temperature at the outlet of the second pass.

EXAMPLE This example illustrates the advantages obtainable through the use of the invention on a typical steam boiler by comparing the results obtainable without the addition of auxiliary fuel (Example A) to those obtainable with the addition of auxiliary fuel in the manner described (Example B). In both cases a gas-fired steam boiler of the horizontal tube type, similar in construction to that shown in the drawings and designed to deliver about 45,000 pounds per hour of 250 pound steam is employed; In Example A the boiler is operated in the conventional manner, while in Example B auxiliary fuel is added after the first pass in the manner indicated in the drawings and oxidized in the presence of a bed of oxidation catalyst made up of units similar to those shown in the drawings and previously described, having. a film of catalytic alumina about .003 in thickness impregnated with about 1% of platinum by weight (based on the weight of the alumina film) deposited on the surface of the porcelain rods 22. Total catalytic surface in the bed provided by rods 22 amounts to 835 square feet provided by 1190 units approximately six inches in length and having 37 rods 22 per unit, stacked in two layers across the boiler tubes of the second pass as shown in 1'0 the drawings. In both cases arefinery gas having a heat content of 1684 B. t. u./cu. ft. is employed as the fuel supplied to the main burners and to the auxiliary fuel distributor. The table below outlines the results of the two different modes of operation:

Table 1 Example A Example B Fuel gas to main burners (S. C. F. M.) 556 556 Heat content of gas to main burners (B.

r. 56, 250, 000 56, 250, 000 Fuel gas to auxiliary gas distributor (S. C. F. M.) 97.2 Heat control of gas to auxiliary gas distributor (B. t. u./hr.) 9, 800, 000 Excess air used in primary fuel combustionpercent over theoretical 28 28 Gas flow through Pass I (S. O. F. M.) 12, 400 12,400 Gas flow through Pass II (S. C. F. M.) 12, 400 14, 583 Gas temperature-entry to Pass I (Degrees F. 21-00 2200 Gas temperature-exit from Pass I (Degrees F.) 800 800 Average AT (approx.)-Pnss I (Degrees F.) 1100 1100 Gas temperature-entry to Pass II (Degrees I 800 1500 Gas temperature it from Pass 11 (Degrees F. 675 735 Average AT (approx.) ass II (Degrees Ii)... 340 720. Oxygen concentration in gas at: exit; from Pass I (percent by vol.) 5 5. Auxiliary Fuel concentration in gas at distributor (percent by vol.) 0. 78 Oxygen concentration in gas at exit from Pass II (percent by vol.) 5 2. 1' Combustiblcs concentration at exit from Pass II. Negligible Negligible a S. C. F. M.=Standard cubic feet per minute. A'l=lcmperature differential between gas temperature and temperature otsaturatcd steam in tubes.

It is apparent that with a substantially increased gasboiler tube temperature differential, a correspondingly increased rate of heat exchange is obtained in the second pass using the method of the invention. The fuel capacity increase is of the order of 9,800,000 B. t. u./hr. 56,250,000 B. t. u./h r.

or 17.4%. With the same overall efiiciency in both cases the actual capacity (B. t. u. output) is increased in the same proportion.

It will be noted also that in Example B the oxygen concentration in the stack gas is lower. At the same stack temperature in each case this factor gives rise to an increase in boiler efliciency due to the smaller volume of stack gas per unit of heat output.

Complete safety of operation in Example B is guaranteed by two factors (1) the low oxygen concentration in the mixture of boiler gas and added fuel at the distributor, and (2) the low fuel concentration. With an oxygen concentration of 5% formation of an explosive mixture with the fuel employed is impossible irrespective of fuel concentration. The fuel concentration (0.78%) is below the lower limit of flammability of this fuel in air, and accordingly, for all practical purposes it would be impossible to form an explosive mixture regardless of oxygen concentration since the concentration of oxygen in the boiler gas cannot exceed that of atmospheric air.

In the foregoing description and in the claims which follow it is understood that the term boiler is used in a broad sense and includes any heat exchange system in which a hot gas stream is generated by combustion of a fuel and thereafter passed over heat exchange surfaces to exchange heat from the hot gas to some other medium. It is to be understood further that modifications of the invention within the spirit thereof other than those specifically described are included within its scope which is to be determined by reference to the appended claims.

I claim:

1. In a boiler having a furnace for burning a fuel in the presence of air, and having a heat exchange system pro viding heat exchanges surfaces over which the boiler gases flow, and in which the heat exchange surfaces of 11 said system are divided into a plurality of sections arranged in series with one another with respect to the how of boiler gases such that said boiler gases traverse the heat exchange surfaces in said sections successively in a plurality of passes, the improvement comprising means for distributing an auxiliary fuel into the stream of boiler gases, said distributing means being disposed within said boiler between two of said sections of the boiler heat exchange system thereby to distribute said fuel into the stream of said boiler gases assaid gases flow from one of the said sections to the next section in series therewith, and a bed of oxidation catalyst arranged across the path of fiow of the boiler gases such that said gases flow through said bed in direct contact with said catalyst, said bed of catalyst being operative to catalytically oxidize said auxiliary fuel and being situated downstream from said distributing means and upstream from at least one section of the boiler heat exchange system, means for controlling the flow of auxiliary fuel to said distributing means, said controlling means comprising temperature sensing means disposed in the vicinity of said distributing means for sensing the temperature of said boiler gases, and means operatively coupled with said temperature sensing means for preventing the flow of auxiliary fuel' when the temperature of boiler gas at said distributing means is below a predetermined minimum temperature.

2. In a boiler having a furnace for burning a fuel in the presence of air, and having a heat exchange system providing heat exchange surfaces over which the boiler gases flow, and in which the heat exchange surfaces of said system are divided into a plurality of sections arranged in series with one another with respect to the flow of boiler gases such that said boiler gases traverse the heat exchange surfaces in said sections successively in a plurality of passes, the improvement comprising means for distributing an auxiliary fuel into the stream of boiler gases, said distributing means being disposed within said boiler between two of said sections of the boiler heat exchange system thereby to distribute said fuel into the stream of said boiler gases as said gases flow from one of the said sections to the next section in series therewith, and a bed of oxidation catalyst arranged across the path of flow of the boiler gases such that said gases flow through said bed in direct contact with said catalyst, said bed of catalyst being operative to catalytically oxidize said auxiliary fuel and being situated downstream from said distributing means and upstream from at least one section of the boiler heat exchange system, means for controlling the flow of auxiliary fuel to said distributing means, said controlling means comprising detecting means for continuously detecting the concentration of combustible material in the stream of boiler gases at a point downstream from said bed of oxidation catalyst and means operatively coupled with said detecting means for preventing the flow of auxiliary fuel to said distributing means when the concentration of combustible material in the boiler gas downstream from said bed of oxidation catalyst exceeds a predetermined maximum value.

3. In a boiler having a furnace for-burning a fuel in the presence of air, and having a heat exchange system providing heat exchange surfaces over which the boiler gases flow, and in which the heat exchange surfaces of said system are divided into a plurality of sections arranged in series with one another with respect to the flow of boiler gases such that said boiler gases traverse the heat exchange surfaces in said sections successively in a plurality of passes, the improvement comprising means for distributing an auxiliary fuel into the stream of boiler gases, said distributing means being disposed within said boiler between two of said sections of the boiler heat exchange system thereby to distribute said fuel into the stream of said boiler gases as said gases flow from one of the said sections to the next section in series therewith, and a bed of oxidation catalyst arranged across the path of flow of the boiler gases such that said gases flow through said bed in direct contact with said catalyst, said bed of catalyst being operative to catalytically oxidize said auxiliary fuel and being situated downstream from said distributing means and upstream from at least one section of the-boiler heat exchange system, and means for regulating the flow of auxiliary fuel to said distributing means, said regulating means comprising temperature sensing means disposed downstream from said bed of oxidation catalyst for sensing the temperature of said boiler gases, and means operatively coupled with said temperature sensing means for regulating the flow of auxiliary fuel to said distributing means in inverse proportion to the temperature sensed.

4. In a boiler having a furnace for burning a fuel in the presence of air, and having a heat exchange system providing heat exchange surfaces over which the boiler gases flow, and in which the heat exchange surfaces of said system are divided into a plurality of sections arranged in series with one another with respect to the fiow of boiler gases such that said boiler gases traverse the heat exchange surfaces in said sections successively in a plurality of passes, the improvement comprising means for distributing an auxiliary fuel into the stream of boiler gases, said distributing means being disposed within said boiler between two of said sections of the boiler heat exchange system thereby to distribute said fuel into the stream of said boiler gases as said gases flow from one of the said sections to the next section in series therewith, and a bed of oxidation catalyst arranged across the path of fiow of the boiler gases'such that said gases flow through said bed in direct contact with said catalyst, said bed of catalyst being operative to catalytically oxidize said auxiliary fuel and being situated downstream from said distributing means and upstream from at least one section of the boiler heat exchange system, means for controlling the flow of auxiliary fuel to said distributing means, said controlling means comprising temperature sensing means disposed in the vicinity of said distributing means for sensing the temperature of said boiler gases, and means operatively coupled with said temperature sensing means for preventing the flow of auxiliary fuel when the temperature of the boiler gas at said distributing means is below a predetermined minimum temperature, the flow of auxiliary fuel to said distributing means being further controlled by a second control means, said second control means comprising detecting means for continuously detecting the concentration of combustible material in said stream of boiler gas at a point downstream from said bed of oxidation catalyst, and means operatively coupled with said detecting means for preventing the fiow of auxiliary fuel to said distributing means when the concentration of combustibles in said boiler gas stream exceeds a predetermined maximum value.

5. In a boiler having a furnace for burning a fuel in the presence of air, and having a heat exchange system providing heat exchange surfaces over which the boiler gases flow, and in which the heat exchange surfaces of said system are divided into a plurality of sections arranged Y in series with one another with respect to the flow of boiler gases such that said boiler gases traverse the heat exchange surfaces in said sections successively in a plurality of passes, the improvement comprising means for distributing an auxiliary fuel into the stream of boiler gases, said distributing means being disposed within said boiler between two of said sections of the boiler heat exchange system thereby to distribute said fuel into the stream of said boiler gases as said gases flow from one of the said sections to the next section in series therewith, and a bed of oxidation catalyst arranged across the path of flow of the boiler gases such that said gases flow through said bed in direct contact with said catalyst, said bed of catalyst being operative to catalytically oxidize said auxiliary fuel and being situated downstream from said distributing means and upstream from at least one section'of the boiler heat exchange system, means for controlling theflow of auxiliary fuel to said distributing means, said control meanscomprising' temperature sensing means disposed in the vicinity of said distributing means for sensing thetemperature-of said boiler gases, and means operative'ly coupled with said temperature sensing means for preventing the flow of auxiliary fuel to said distributing means when the temperature of boiler gasat said distributing means is below a predetermined minimum temperature, the'flow of auxiliary fuel to said distributing means being further controlled by a second control means, said second control means comprising detecting means for continuously detecting the concentration of combustible material in said stream of boiler gas at a point downstream from said bed of oxidation catalyst, and means operatively coupled with said detecting means for preventing the flowaof auxiliary fuel to said distributing means when the concentration of combustibles in said boiler gasstream vexceeds'a predetermined maximum value, the flow of auxiliary fuel to said distributing means being still further controlled by regulating means, said regulating means comprising temperature sensing means disposed downstream from said bed of oxidation catalyst for sensing the temperature of said boiler gases, and means operatively coupled with said temperature sensing means for regulating the flow of auxiliary fuel to said distributing means in inverse proportion to the temperature sensed.

6. A method for the operation of a boiler which comprises the steps of charging fuel and air to a boiler furnace, controlling the proportion of air to fuel such that sufiicient air is added to provide an excess of air over the stoichiometric amount required for complete cornbustion, burning said fuel in the presence of said air thereby generating a stream of hot combustion gases containing free oxygen, passing said hot stream through at least one section of the boiler heat exchange system over heat exchange surfaces having a substantially lower temperature than the temperature of said hot stream, thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to produce a mixture therewith, controlling the proportion of the auxiliary fuel in said mixture to produce a mixture which is outside the limits of flammability, passing said mixture into direct contact with an oxidation catalyst to effect catalytic oxidation of said auxiliary fuel through the agency of said catalyst, and thereby substantially raise the temperature of said stream of gases, and thereafter passing the thus heated gas stream through succeeding portions of the boiler heat exchange system.

7. A method for the operation of a boiler which comprises the steps of charging fuel and air to a boiler furnace, controlling the proportion of air to fuel such that suflicient air is added to provide an excess of air over the stoichiometric amount required for complete combustion, burning said fuel in the presence of said air thereby generating a stream of hot combustion gases containing free oxygen, passing said hot stream through at least one section of the boiler heat exchange system over heat exchange surfaces having a substantially lower temperature than the temperature of said hot stream thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to produce a mixture therewith, controlling the proportion of auxiliary 7 fuel in said mixture to produce a mixture which is outside the limits of flammability, passing said mixture into direct contact with an oxidation catalyst to effect catalytic oxidation of said auxiliary fuel through the agency of said catalyst and thereby substantially raise the temperature ofsaid stream of gases, and thereafter passing the thus heated gas stream through succeeding sections of the boiler heat exchange system, and controlling the proportion of auxiliary fuel added to said combustion gases such that the concentration of oxygen in the stream of combustion gases leaving the boiler after catalytic oxidation of said auxiliary fuel does not exceed about 2% by volume.

8. A method for the operation of a boiler which com- 14 prises the steps of, charging fuel and air to a boiler furinace, controlling the proportion of air to fuel such that suflicient excess air is added to insure complete combustion, burning said fuel in the presence of said air thereby generating a stream of hot combustion gases containing free oxygen and negligible amounts of combustibles, passing said hot stream of gases through at least one section of the boiler heat exchange system over heat exchange surface having a substantially lower temperature than the temperature of said hot stream, thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to produce a mixture therewith, controlling the proportion of auxiliary fuel in saidmixture so as to produce a mixture outside the limits of flammability, passing said mixture into direct contact with oxidation catalyst arranged across the path of flow of said gases to effect catalytic oxidation of said auxiliary fuel through the agency of said catalyst and thereby substantially raise the temperature of said stream of gases, and thereafter passing the thus heated gas stream through succeeding sections of the boiler heat exchange system.

9. A method for the operation of a boiler which comprises the steps of charging fuel and air to a boiler furnace, controlling the proportion of air to fuel such that sutficient excess air is added to insure efficient combustion but limiting the excess air such that the resulting combustion gases contain less than the required concentration of free oxygen necessary to provide a flammable mixture with subsequently added fuel irregardless of the proportion of said added fuel, burning said fuel in the presence of said air in the boiler furnace thereby generating a stream of combustion gases containing free oxygen and negligible amounts of combustibles, passing said hot stream of gases through at least one section of the boiler heat exchange system over heat exchange surfaces having a substantially lower temperature than the temperature of said hot stream thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to produce a nonflammable mixture therewith, passing said mixture into direct contact with oxidation catalyst arranged across the path of flow of said gases to eifect catalytic oxidation of said auxiliary fuel through the agency of said catalyst and thereby substantially raise the temperature of said stream of gases, and thereafter passing the thus heated gas stream through succeeding portions of the boiler heat exchange system.

10. A method for the operation of a boiler which comprises the steps of charging fuel and air to a boiler furnace, controlling the proportion of air to fuel such that sutficient excess air is added to insure eflicient combustion but limiting the excess air such that the resulting combustion gases contain less than the required concentration of free oxygen necessary to provide a flammable mixture with subsequently added fuel irregardless of the proportion of said added fuel, burning said fuel in the presence of said air in said furnace thereby generating a stream of hot combustion gases containing free oxygen and negligible amounts of combustibles, passing said hot stream through at least one section of the boiler heat exchange system over heat exchange surfaces having a substantially lower temperature than the temperature of said hot stream thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to form a mixture therewith and regulating the proportion of added fuel to the total stream such that the concentration of fuel in said stream is less than that required to form a flammable mixture with atmospheric air, passing said mixture into direct contact with oxidation catalyst arranged across the path of flow of said stream to effect catalytic oxidation of said auxiliary fuel through the agency of said catalyst and thereby substantially raising the temperature of said stream of gases, and thereafter passing the thus heated gas stream 15' a through succeeding sections of the boiler heat exchange system. l 1 a 11. A method for the operation of a boiler which comprises the steps of charging fuel and air to a boiler furnace, controlling the proportion of air to fuel such that the mixture contains sufficient excess air to insure efficient combustion of said fuel, burning said fuel in the presence of said air in said boiler furnace thereby generating a stream of combustion gases containing free oxygen and negligible amounts of combustibles, passing said hot stream through at least one section of the boiler heat exchange system over heat exchange surfaces having a substantially lower temperature than the temperature of said hot stream thereby substantially lowering the temperature of said stream, distributing auxiliary fuel in the cooled stream to form a mixture therewith, and regulating the proportion of added fuel to the total stream such that the concentration of fuel in the stream is less than that required to form a flammable mixture with atmospheric air, passing said mixture into direct contact with oxidationcatalyst arranged across the path of flow of said stream to effect catalytic oxidation of said auxiliary fuel through theagency of said catalyst and thereby substantially raising the temperature of said stream of gases, and thereafter passing the thus heated gas stream through succeeding sections of the boiler heat exchange system.

References Cited in the file of this patent UNITED STATES PATENTS