US3237677A

US3237677A - Heater and process suitable for lean gaseous fuels

Info

Publication number: US3237677A
Application number: US353948A
Authority: US
Inventors: Wiesenthal Peter Von; Robert G Buchholz; Carmelo J Jacono
Original assignee: Alcorn Combustion Co
Current assignee: Linde GmbH; Alcorn Combustion Co
Priority date: 1964-03-23
Filing date: 1964-03-23
Publication date: 1966-03-01
Anticipated expiration: 1983-03-01

Description

Mrch 1, 1966 P. voN wlEsr-:NTHAL ETAL 3,237,677

HEATER AND PROCESS SUITABLE FOR LEAN GASEOUS FUELS Filed March 25, 1964 3 Sheets-Sheet 1 l ATTORNEY WIN March l, 1966 P. voN wu-:sENTHAL ETAL 3,237,677

HEATER AND PROCESS SUITABLE FOR LEAN GASE'OUS FUELS Filed March 25, 1964 3 Sheets-Sheet 2 G 8 8 H 5 f j 7 E ith 4 f fxi/2,? x/,e 4

March 1, 1966 P. voN wxEsENTHAL ETAI. 3,237,577

HEATER AND PROCESS SUITABLE FOR LEAN GASEOUS FUELS Filed March 25, 1964 5 Sheets-Sheet 5 l f f Filed Mar. 23, 1964, ser. No. 353,948 12 Claims. (ci. 15s-11) This disclosure relates to tiring lean fuels in substantially inert media. The invention is embodied in an apparatus and related process especially suited to this task.

To date commercial application of this invention has been most successful in the rin-g of carbon monoxide exhausted in off gas from hydrocarbon catalytic cracking units. For illustrative purposes this teaching will be set forth in that application; however, it should be understood that the apparatus taught herein is -also applicable to other lean gas ring opportunities.

Ideally, carbon monoxide will burn at about l128 F. But in practice, even where combustors are provided with high heat capacity settings, satisfactory CO conversion usually requires tire box temperatures above 1500 F. The firing of an alternate fuel in addition to CO assures adequate temperature levels.

The present advance offers CO conversion at lower plant cost and at lower alternate fuel requirements than any heretofore achievedl in commercial installations. Fundamental analysis indicated a need for the following features in CO tiring:

More intimate mixing of CO and O2.

Initiating conversion as soon as the off gas reaches the combustion chamber.

Avoidance of chilling the reacting CO.

Adequate and reliable detention time for complete CO conversion in the combustion chamber. Stated somewhat diiferently, potential short circuit paths through the combustion chamber had to be corrected.

Design for these features is complicated by the fact that off gas heaters must also be suitable for tiring an alternate fuel alone.

Intimate mixing of reactants is essential to efcient CO conversion. Jets of air (or some other oxygen-rich gas) are directed against streams of the olf gas to achieve this mixing. The air jets and olf gas streams rapidly lose their individual identities to form a comingled mass of reacting gases.

The setting is arranged to dene a plurality of parallel elongated combustion chambers. Each of these chambers has an upstream portion wherein a high degree of mixing can be achieved. Thereafter the mass of reacting gases is coursed longitudinally through the chamber for exit via a downstream portion.

Off gas is usually available at from 900 to 1100 F. and it usually contains from 4 to 10% CO in an otherwise inert gas medium. Even with ily wheel heat storage provided by a high heat capacity setting, a temperature boost in the order of from 500 F. to 750 F. is generally necessary to stabilize firing of the CO. Boosting the CO temperature also involves boosting the temperature of the 90 to 96% of inerts in the off gas. Further, combustion air for CO iirin'g is not generally preheated so this air adds to the temperature boost requirement. Attempting to achieve the required temperature boost solely through alternate fuel firing has given rise to inordinate fuel demands. Itis desired to have reaction heat from the CO conversion contribute as much as possible to this temperature boost. Here rapid initiation of conversion and avoidance of chilling come into play. To trigger United States Patent O 3,237,677 Patented Mar. 1, 1966 ICC the reaction, some combustion air is injected into the olf gas upstream of the combustion chamber. Heat liberation from CO to CO2 conversion acts as soon as the kindling temperature of CO is reached. To mitigate the chilling etfect which combustion air and off gas have on the mass of reacting gases, the preponderant portions of the air and olf gas are introduced in increments along the length of the upstream portion of the combustion chamber. As the reacting mass of gases progresses through the combustion chamber, the potential chilling effect of combustion air and Voff gas additions are far more than compensated for by the heat release of the conversion.

At the core of the basic CO tiring application here contemplated is the principle of completely converting CO to CO2 before attempting to recover heat from the combustion products. This principle must be reconciled with refinery space limitations. Compelling geometry considerations associated with hydrocarbon catalytic cracking units make it very convenient to arrange each of the elongated combustion chambers horizontally at or close to ground level. This arrangement simplifies duct work and allows auxiliary burners to be mounted along a single end wall where they can be serviced from grade level. With horizontal combustion chambers, to prevent the ground larea requirements for such units from becoming excessive, the heat recovery portion generally has to be situated above the downstream portions of the combustion chambers. This positioning of the heat recovery Section presents two problems. First, the possibility of reactants finding short circuit paths through the combustion chambers is increased. Second, the presen-ce of cold heat exchange tubes directly above the downstream portions of the combustion chambers poses the danger of thermal inversion which would defeat the basic scheme Iof these elongated combustion chambers.

Fundamental inquiry into thermal effects on flow characteristics was necessary in preventing short circuit paths. In this design the setting has a roof arch which includes a downstream arch embracing the downstream portion of ea-ch of the combustion chambers. The setting also denes a heat recovery section above the downstream portions of the combustion chambers. The downstre-am arch describes outlets which communicate the downstream portions of the combustion chambers in flow series with the heat recovery section. A comingled mass of reacting gases progresses horizontally through the lcombustion chambers and exits upward through the outlets. This ow calls for a upward turn so an upstream inward wall and a downstream outward wall of the heat recovery section are involved. Cold model studies indicated that with the geometries, pressure, quantities, and` velocities of this design, in making this turn there would be little or no ow adjacent the inward wall. But it was discovered that, under the peculiar' inuence of heat liberation from a reacting uid and with the presence of heat extraction coils abovevthe outlet, flow adjacent the inside wall was surprisingly high. It was found that gases tended toward premature exit up the inward wall of the heat recovery section. To cope with this problem a transverse lip was connected across the downstream arch upstream of and in the vicinity of the outlet. The lip projects downward into the downstream portion of the combustion chamber so that lgases are deflected downward as they approach the outlet. By this expedient retention of reactants 1n the combustion chamber has been prolonged, and the reliability of retention has been assured.

To avoid thermal inversions a row of deck baffles span-s the Outlets. These baffles are spaced from each other and from the roof arch so as to dene a plurality of openings which communicate the downstream portions of the combustion chambers with the heat recovery section. As the gases exit the combustion chambers via these openings, velocity is increased by virtue of reduced flow section. This increased upward velocity develops an upward ow gradient which overcomes potential downward convective ow. Thus thermal inversions back into the downstream portions of the combustion chambers are obviated. Spacing of these deck baffles also offer-s an opporunity for control of gas distribution in the heat recovery section.

A lternae fuel firing The key difference between ring CO and firing an alternate fuel in this heater is the difference in temperature levels generated. It is desirable that the unit be capable of accommodating various alternate fuels. But here it should be kept in mind that alternate fuels are employed for only a small portion of the time, so this firing condition should not dominate the overall design. The cost of the unit increases substantially with protection from high temperatures. When firing alternate fuel, addition of excess air permits the reduction of combustion chamber temperatures. For example, when firing oil Without excess airvcombustion chamber temperatures in the order of 3500 F. are reached. These high temperatures would require expensive construction. Additions of from 60 t 75% excess air reduces these temperature levels to the vicinity of 2200 F. which lis more compatible with the class of structure called for by usual CO firing.

During alternate fuel firing, protection of gas-air plena along the upstream arch becomes critical. Even with high excess air for firing an alternate fuel, if combustion gases were to enter the air and ga-s plena, these plena would have to be built to withstand substantially the same temperatures as the combustion chambers. Clearly in view of the limited extent of alternate fuel iiring, the .design of these plena for such severe conditions is not generally justifiable.

The present design recognizes that excess air, associated with alternate fuel firings, offers an opportunity for cooling and protecting the gas and air plena. When firing an alternate fuel in this unit, a major portion of the excess air is introduced via the air plenum into the upper portions of the combustion chambers as air jets. Flowing air has a cooling effect. Further, the air forms a barrier below and along the upstream arch prohibiting access of combustion gases from the combustion chambers into either the air plenum or the lgas plenum.

These and other features will appear more fully from the accompanying drawings wherein:

FIGURES I and IA show an elevation view in section of a heater according to this invention and taken along line I-I of FIGURE II.

FIGURE IA joins FIGURE I along connecting line FIGURE II is an elevation view in section looking toward the burners.

In the drawings setting 1 is supported by steel frame 2 which in turn transmits loads to pier 3. Setting 1 includes oor 4, roof arch 6, upstream wall 7, longitudinal walls 8 and outward wall 9 all of which cooperate to define a plurality of elongated, substantially horizontal combustion chambers 11. The combustion chamber 11 seen in FIGURE I is representative of the other combustion chambers 11. It should be understood that details of the combustion chamber 11 in FIGURE I also pertain to the other combustion chambers arranged in parallel therewith. As best seen in FIGURE I, roof arch 6 includes upstream arch 12 embracing upstream portion 13 of combustion chamber 11 as well as downstream arch 14 embracing downstream portion 16. Upstream wall 7 encloses upstream portion 13 which is spaced longitudinally relative combustion chamber 11 from downstream portion 16.

This heater is generally intended for firing a burnable waste gas in a mixture of inert gases. Such a mixture of gases is usually produced by means of carbon burn off in catalyst regenerators or by some similar regenerative process. These sources usually exhaust waste products out their upper ends so the CO off gas or other lean gaseous fuel becomes available for heat recovery purposes in a downward flowing stream. Gas inlet means introduce the gas mixture into upstream portion 13 of combustion chamber 11 to form a plurality of gas streams 17 Gas shroud 19 forrns gas plenum 21 above upstream arch 12. The gas reaches plenum 21r by way of conduit 18.

Combustion of the burnable gas is supported by air or some other oxygen-rich gas. Air inlet means are shown to introduce air into upstream portion 13 of combustion chamber 11 to form a plurality of air jets 22. Forced draft fans (not shown) serve as a source of supply for air and also serve as a means for pressurizing the air delivered to the heater. In some installations fans are unnecessary. Air shroud 23 cooperates with upstream arch 12 to define air plenum 24 between gas plenum 21 and upstream arch 12. Air duct 26 communicates air plenum 24 for supply from the forced draft fans.

To deliver the gas mixture t-o upstream portion 13 of combustion chamber 11, gas shroud 19 defines first gas ports 27 communicating in flow series with gas plenum 21. Upstream arch 12 defines gas inlet ports 28 communicating in iiow series with upstream portion 13 of combustion chamber 11. Gas conduits 29 are each Connected to gas shroud 19 about one of the rst gas ports 27. These gas conduits 29 traverse air plenum 24 and are each connected to upstream arch 12 above one of the gas inlet ports 28 to communicate gas plenum 21 in flow series with upstream portion 13 of the combustion chamber for introducing the gas mixture thereto as gas streams 17.

For premixing air with the gas mixture before the gas mixture enters upstream portion 13, premix ports 31 communicate air plenum 24 in flow series with the interiors of gas conduits 29 to inject air into the-se conduits. The major portion of the air enters upstream portion 13 of the combustion chamber as air jets 22. The air enters through air inlet ports 32 dened by upstream arch 12 to communicate air plenum 24 in flow series with upstream portion 13.

Preheating of combustion chambers 11 is accomplished by use of burners 33 (of known design) which penetrate setting 1 through upstream wall 7. Burners 33 communicate by way of alternate fuel lines 34 with a source of alternate fuel (not shown). The alternate fuel may be practically any gaseous or liq-uifable hydrocarbon. Cornbustion air is delivered to burners 33, through air duct 26 and burner air passages 36.

Intimate mixing of off gas and air is accomplished by directing air jets 22 diagonally into the downward directed gas streams 17. Gas conduits 29 and air inlet ports 32 are organized so that each of the gas streams 17 has at least one of the air jets 22 directed thereagainst for intimate mixing of fuel and oxygen whereby a comingled mass of reacting gases is formed.

Admission of off gas and air is organized along the length of each upstream portion 13 of combustion chambers 11 so that chilling of the comingled mass is avoided. Gas inlet ports 28 and air inlet ports 32 are spaced along the length .of upstream arch 12 so that the gas mixture and the air are admitted into the comingled mass in increments along the length of each upstream portion 13 of the combustion chambers.

Considering all of the parallel combustion chambers in gross, the comingled masses are arranged to course through combustion chamber 11 for exit via downstream portions 16. Downstream arch 14 defines outlets 38 communicating in fiow series with each downstream portion 16 of the combustion chambers to exhaust gases therefrom so that a flow gradient is developed to cause the comingled mass to ow longitudinally through each of the combustion chambers 11. To prevent premature exit of gases up inward wall 39, lip 41 `is disposed transverse the coml bustion chambers and is connected across downstream arch 14 upstream of and in the vicinity of outlets 38. Lip 41 has end 42 projectingA downward into downstream portions 16 of the combustion chambers so that gases are deflected downward as they approach outlets 38 whereby detention of the comingled mass is made more reliable.

In order to avoid loss of ignition or pulsating detonation, during load changes or varying concentrations of CO in the off gas, high heat capacity setting 1 is fabricated from refractory blocks 43 (or a castable refractory) to give a fly wheel heat storage effect. The reacting comingled mass passes in sweeping relationship over surfaces 44 of refractory blocks 43 to store heat in the refractory. Even in the event of failure of burners 33, heat from the refractory blocks 43 would support stable combustion for a considerable period of time. The absence of any heat sink in combustion chamber 11 approaches the ideal adiabatic oxidation condition for CO. Thus ignition stability is assured.

This design calls for complete CO conversion to CO2 before any attempt is made to recover heat from the combustion products. Heat recovery section 46 is formed by setting 1 above downstream portions 16 of combustion chambers 11. Outlets 38 communicate in flow series with heat recovery section 46 to exhaust gases thereto. Tube coils 47 are mounted in heat recovery section 46. Means are provided for circulating one or more fluids through coils 47 for non-contact heat exchange with the combustion products passing through heat recovery section 46. Flue means shown as stack 48 and hood 49 communicate in flow series with heat recovery section 46 to exhaust gases therefrom.

In hydrocarbon catalytic cracking oi gas applications, heat recovery is used for either hydrocarbon heating or for steam generation or for both. The relatively low equilibrium temperatures of this design suggest that the preponderance of heat transfer to tube coils 47 is by means of convection. In steam generation, forced circulation is employed rather than natural circulation boiling because in natural circulation units these low temperature levels have on occasion resulted in serious circulation problems. Forced circulation overcomes these problems by assuring positive and uniform velocities in all tubes of a coil. By this expedient local mineral deposition and scaling are reduced.

lWhere steam is to be generated, screen tubes 51, located in the hottest portion of heat recovery section 46, are filled with water under positive circulation at all times thereby keeping tube metal temperatures close to those of the water. This is opposite to the condition found in natural circulation units wherein screen tubes 51 generally steam at the highest rates and are, therefore, most subject to fouling. In the case of overfiring, the screen tubes of a conventional natural circulation boiler may even go to complete dryness with serious consequences. 'This situation is obviated in the present forced circulation unit. Even at maximum overload conditions, screen tubes 51 are always filled with water.

Space availability from mounting heat recovery section 41 above downstream portions 16 of combustion chambers 11 makes possible the use of relatively large tube sizes in the order of 4.5" O.D. These large tubes permit several worthwhile advantages. First, large tubes minimize any problems resulting from high concentrations of solids. Second, the substantial wall thickness of large tubes offer protection should external erosion conditions be severe. Third, large tube sizes eliminate potential vibration problems. In this regard the tendency in CO red units has been to attempt increasing heat transfer coeicients. Toward this objective velocities are stepped up. At critical velocities however, the Von Karman effect induced along the tube lengths results in vibrations which at resonance frequencies can have most distressing consequences. Small diameter bent tubes rolled into drums have been the worst performers on this score. In the present design, the generous sizes of tube coils 47 in conjunction with the straight tube lengths and positive supports makes destructive vibration most unlikely.

Flow restriction in outlets 38 is used to prevent ther mal inversions between heat recovery section 46 and downstream portions 16 of combustion chambers 11. Rows of deck bailles 52 have sides 56 spaced from each other and from downstream arch 14 to define a plurality of openings 57 which communicate downstream portions 16 of combustion chambers 11 with heat recovery section 46. By reduction of sectional flow area, velocity of flow through these openings 57 is increased producing an upward flow gradient so that potential downward inversion flow potentials are overcome.

Soot blowers 58 are fitted for complete cleaning of tube coils 47.

When firing alternate fuel alone, air duct 26, air plenum 24, premix ports 31, and air inlet ports 32 serve to communicate with the forced draft fans for introducing air to cool air plenum 24 and to form an air barrier below and along upstream arch 12 prohibiting entry of combustion gases into gas plenum 21.

A series of full scale performance tests were conducted on heaters embodying the apparatus claims and operated according to the process claim of this specification. The lean fuel was off gas from a hydrocarbon catalytic cracking unit. The following table sets forth typical results from these tests.

Off gas composition (percent by weight):

N2 73.1 O2 0.2 CO 9.1

CO2 13.5 H2O 4.1

Flow lbs/hour-.. 462,000 Temperature F" 1040 Total heat input B.t.u./hour 297,000,000 Alternate fuel required B.t.u./hour 23,800,000 Air required lbs/hour-- 142,000 Normal temperature of chamber Fu 1720 Velocities over the cross sectional flow area were found to be relatively uniform throughout the combustion chamber. No thermal inversions from the heat recovery section to the downstream portion of the combustion chamber were experienced. Adequate CO conversion was achieved at combustion chamber temperatures as low as l4l0 F. On shutting oi the auxiliary burners stable CO conversion was sustained. When alternate fuel alone was red, the temperatures in the gas and air plena were kept within limits where special protection for the gas and air shrouds is unnecessary.

It will be apparent to those skilled in iired heater design that wide changes can be made in the shown embodiment without departing from the main theme of invention set forth in the claims.

What is claimed is: j

1. A combustor suitable for a lean fuel occurring in a predominantly inert gas mixture, the combustor comprising a sketting which defines an elongated combustion chamthe combustion chamber having an upstream portionV and a downstream portion spaced longitudinally from each other,

gas inlet means for introducing the gas mixture into the upstream portion of the combustion chamber to form a plurality of gas streams,

a source of a combustion-supporting gas,

combustion-supporting gas inlet means for introducing a combustion-supporting gas into the upstream portion to form a plurality of combustion-supporting jets,

the combustion-supporting gas inlet means organized relative the gas inlet means so that each of the gas streams has at least one of the combustion-supporting jets directed thereagainst for intimate mixing p of the gas mixture and combulstionsupporting gaswhereby a comingled mass of reacting gases is formed.

at least one burner means penetrating the setting into the upstream portion of the combustion chamber and firing an alternate fuel to heat the lean fuel above its kindling temperature,

the gas inlet means and the combustion-supporting gas inlet means arranged so that the gas mixture and the combustion-supporting gas are introduced into the comingled mass in increments along the length of the upstream portion whereby chilling of the comingled mass is avoided,

uncooled refractory lining the interior of the chamber so that the comingled mass passes in sweeping relationship thereover to provide heat storage in the refractory,

the setting vdeiining an outlet communicating in fiow series with the downstream portion of the combustion chamber to exhaust gases therefrom so that a flow gradient is developed causing the comingled mass to iiow longitudinally through the combustion chamber from the upstream portion for exit via the downstream portion.

2. The combustor of claim 1 with premix means for injecting combustion-supporting gas into the gas mixture before the gas mixture reaches the combustion chamber.

3. A fired heater suitable for a lean fuel occurring in a predominantly inert gas mixture, the heater including a setting with walls and a floor as well as a roof arch all cooperating to define an elongated substantially horizontal combustion chamber,

the combustion chamber having an upstream portion and a downstream portion spaced longitudinally from each other,

the roof arch including an upstream arch embracing the upstream portion of the combustion chamber and a downstream arch embracing the downstream portion,

at least one burner means penetrating the setting into the upstream portion of the ,combustion chamber and iiring an alternate fuel for heating the lean fuel above its kindling temperature,

a gas shroud defining a gas plenum above the upstream arch,

means communicating the gas plenum in ow series with a source of the gas mixture,

an air shroud cooperating with the upstream arch to define an air plenum between the gas plenum and the upstream arch,

a source of air,

means communicating the air chamber in iiow series with the source of air, g

the gas shroud defining a plurality of first gas ports communicating in iiow series with the gas plenum,

the upstream arch defining a plurality of gas inlet ports communicating in iiow series with the upstream portion of the combustion chamber,

a plurality of gas conduits each connected to the gas shroud about one of the first gas ports and traversing the air plenum and connected to the upstream arch about one of the gas inlet ports to communicate the gas plenum in flow series with the upstream portion of the combustion chamber for introducing the gas mixture thereto in a plurality of gas streams,

means for pressurizing the air in the air plenum,

at least one of the gas conduits defining a premix port communicating the air plenum in flow series with that conduit to inject air into the gas mixture before the gas mixture reaches the upstream portion of the combustion chamber,

the upstream arch defining a plurality of air ports communicating the air plenum in flow series with theupstream portion of the combustion chamber so that the air issues into the combustion chamber in a plurality of air jets,

the gas conduits and the air ports organized so that each of the gas streams has at least one of the air jets directed thereagainst for intimate mixing of the lean fuel and oxygen whereby a comingled mass of reacting gases is formed,

the gas inlet ports and the air inlet ports spaced along the length of the upstream arch so that the gas mixture and the air are admitted into the comingled mass in increments along the length of the upstream portion of the combustion chamber whereby chilling ofthe comingled mass is avoided,

the downstream arch defining an outlet communicating in flow series with the downstream portion of the combustion chamber to exhaust gases therefrom so that a flow gradient is developed to cause the comingled mass to fiow longitudinally through the combustion chamber from the upstream portion for exit via the downstream portion.

4. A fired heater suitable for a lean fuel occurring in a predominantly inert gas mixture, the heater comprising a setting with walls and a floor as well as a roof arch all cooperating to define an elongated substantially horizontal combustion chamber,

a source of air,

a source of alternate fuel,

at least one burner communicating with the source of alternate fuel and the source of air and penetrating the setting into the upstream portion of the combustion chamber for firing the alternate fuel to heat the lean fuel above its kindling temperature,

a gas shroud defining a gas plenum above the upstream arch,

a source of the gas mixture,

means communicating the gas plenum in flow series with the source of the gas mixture,

a plurality of gas conduits each connected to the gas shroud about one of the first gas ports and traversing the air plenum and connected to the upstream arch about one of the gas inlet ports to communicate the gas plenum in iiow series with the upstream portion of the combustion chamber for introducing the Ygas mixture thereto in a plurality of gasstreams,

means communicating the air chamber in ow series with the source of air,

means for pressurizing the air in the air plenum,

the upstream arch defining a plurality of air ports communicating the air plenum in flow series with the upstream portion of the combustion Achamber so that the air issues therein to form a plurality of air jets,

the gas conduits and the air ports organized so that each of the gas streams has at least one of the -air jets directed thereagainst for intimate mixing of the lean fuel and oxygen whereby a comingled mass of reacting gases is formed,

the gas inlet ports and the air inlet ports spaced along the length ofthe upstream arch sothat the gas mixture and the air are admitted into the comingled 9 mass in increments along the length of the upstream portion of the combustion chamber whereby chilling of the comingled mass is avoided,

the downstream arch defining an outlet communicating in flow series with the downstream portion of the combustion chamber to exhaust gases therefrom so that a fiow gradient is developed to cause the comingled mass to flow Vlongitudinally through the combustion chamber from the upstream portion to exit via the downstream portion,

a lip disposed transverse relative the combustion chamber and connected across the downstream arch upstream of and in the vicinity of the outlet,

the lip projecting downward into the downstream portion of the combustion chamber so that gases are defiected downward as they approach the outlet.

5. The heater of claim 4 with the setting defining a heat recovery section above the downstream portion of the combustion chamber,

the heat recovery section communicating in iiow series with the outlet to receive gases exhausted therefrom,

at least one tube coil mounted in the heat recovery section,

means circulating a fluid through the tube coil for non-contact heat exchange with gases in the heat recovery section,

flue means communicating in ow series with the heat recovery section to exhaust gases therefrom,

a row of deck baiiies supported to span the outlet,

the deck bafiies having sides spaced from each other and from the downstream arch to define a plurality of openings which communicate the downstream portion of the combustion chamber' with the heat recovery section.

6. The heater of claim 5 with the walls including an upstream .end wall which encloses the upstream portion of the combustion cham- Y ber,

the burner .penetrating the setting through the upstream end wall.

7. A fired heater suitaible for a lean fuel occurring in a predominantly inert gas mixture, the heater comprising a setting with a oor and a roof arch and an upstream end wall and an outside wall as well as at least three longitudinal walls all cooperating to dene a plu- `rality of elongated parallel substantially horizontal combustion chambers, i

each of the combustion chambers having an upstream portion and a downstream portion spaced longitudinally from `each other,

each of the upstream ends disposed at the same end of the setting,

the roof arch including an upstream archembracing the upstream portions of each of the combustion chambers and a downstream arch embracing each of the downstream portions,

a source of air, f

a source of alternate fuel,

at least one burner communicating With the source of alternate fuel and the source of airand penetrating the upstream end wall into the upstream portion of each of the combustion chambers for firing the alternate fuel to heat the lean fuel above its kindling temperature,

a gas shroud defining a gas plenum above the-upstream arch,

a source of the gas mixture,

means communicating the gas plenum in f'low series with the source of the gas mixture,

the gas shroud defining a plurality of first gas ports communicating in fiow series with the gas plenum,

the upstream arch defining a plurality of gas inlet ports communicating in flow series with each of the upstream portions of the combustion chamber,

a plurality of gas conduits each connected to the gas shroud about one of the first gas ports and traversing the air plenum and connected to the upstream arch about one of the gas inlet ports to communicate the gas plenum in flow series with the upstream portions of each of the combustion chambers for introducing the gas mixture into each of the chambers in a plurality of gas streams,

means communicating the air chamber in flow series with the source of air,

means for pressurizing the air in the air plenum,

the upstream arch defining a plurality of air ports communicating the air plenum in ow series with the upstream portions of the combustion chambers so that the air issues into each of the chambers to form a plurality of air jets,

the gas conduits and the air ports in each of the combustion chambers organized so that each of the gasv streams has at least one of the air jets directed thereagainst for intimate mixing of the lean fuel and oxygen whereby a comingled mass of reacting gases is formed in each of the combustion chambers,

the gas inlet ports and the air inlet ports communieating with each of the combustion chambers spaced along the length of the upstream arch so that the gas mixture and the air are admitted into the comingled mass in increments along the length of each of the upstream portions of the combustion chambers whereby chilling of the comingled mass is avoided,

the downstream arch defining a plurality of outlets each communicating in flow series with one of the downstream portions of the combustion chambers to exhaust gases therefromv so that fiow gradients are developed in each of the combustion chambers to cause the comingled masses to fiow longitudinally v through the combustion chamber from the upstream portions to exit via the downstream portions,

a lip disposed transverse relative the combustion charnbers and connected across the downstream arch upstream of and in the vicinity of the outlets,

the lip projecting downward into the downstream portions of the combustion chambers so that gases are defiected downward as they approach the outlets,

the settling defining a heat recovery section above the downstream portions of the combustion chambers,

the heat recovery section communicating in flow series with the outlets to receive gases exhausted therefrom,

at least one tube coil mounted in the heat recovery section,

means for circulating a fluid through the tube coil for non-contact heat exchange with gases in the heat recovery section, l

flue means communicating in fiow series with the heat recovery section to exhaust gases therefrom,

a row of deck bafiies having ends supported on the longitudinal Walls and arranged to span each of the outlets,

the deck baffles having sides spaced from each other and from the downstream arch to define a plurality of openings which communicate the downstream Aportions of the combustion chambers with the heat recovery section. i

8. A fired heater suitable for alternately firing either a lean gaseous fuel occurring in an inert gas mixture with auxiliary firing of an alternate fuel or firing the alternate fuel alone, the heater comprising a setting with walls and a oor as well as a roof arch all cooperating to define an elongated substantially horizontal combustion chamber,

the combustion 4chamber having an upstream portion and a downstream portion spaced longitudinally from each other,

the walls including ian upstream end wall enclosing the upstream portion of the combustion chamber,

a source of the alternate fuel,

a sounce of air,

at least one burner communicating with the source of alternate fuel and with the source of air and penetrating the upstream end wall into the upstream portion of the combustion chamber for firing the alternate fuel to heat the lean fuel above its kindling temperature,

ra gas shroud defining a gas plenum above the upstream arch,

a source of the gas mixture,

an air shroud cooperating with the upstream arch to define an air plenum lbetween the gas plenum and the upstream arch,

the gas shroud defining -a plurality of first gas ports Communicating in liow series with the gas plenum,

the upstream arch defining a plurality of gas inlet ports communicating in flow series with the upstream portion of the combustion chamber,

a plurality of gas conduits each connected to the gas shroud about one of the lirst gas ports and traversing the air plenum and connected to the upstream arch about one of t'ne gas inlet ports to communicate the gas plenum in flow series with the upstream portion of the combustion chamber for introducing the gas mixture into the upstream portion in a plurality of gas streams,

means communicating the air plenum in flow series with the source of air,

means for pressurizing the lair in the air plenum,

the upstream arch defining a plurality of air ports communicating the air plenum in ow series with the upstream portion of the combustion chamber so that the air issues into the combustion chamber in a plurality of air jets,

the gas conduits and the air ports organized so that each of the gas streams has at least one of the air jets directed thereagainst for intimate mixing of the lean fuel and oxygen whereby a commingled mass of reacting gases is formed,

the gas inlet ports and the air inlet ports spaced along the length of the upstream arch so that the gas mixture and the air are admitted into the commingled mass in increments along the length of the upstream portion of the combustion chamber whereby chilling of the com-mingled mass is avoided,

the setting defining an outlet communicating in ow series with the downstream portion of the combustion chamber to exhaust gases therefrom so that a flow gradient is developed to cause the commingled mass to flowlongitudinally through the combustion chamber from the upstream portion for exit via the downstream portion,

cooling means communicating with the source of air and operable when the alternate fuel alone is being fired for introducing air into the upstream portion of the combustion chamber by way of the air plenum to cool the air plenum and form an air barrier below and along the upstream arch to prohibit entry of combustion gases into the gas plenum.

9. A process -for recovering Aheat from CO occurring in a predominantly inert gas mixture containing less than 10% CO, the process comprising the steps of providing an elongated combustion zone with an upstream portion and a downstream portion, preheating the upstream portion to a temperature in excess of 1128 F., introducing the mixture into the upstream portion in increments along its length wit-h the mixture entering as a plurality of gas streams directed substantially transverse relative the combustion zone, introducing a combustion-supporting gas into the upstream portion in increments along its length with the combustion-supporting gas entering as a plurality of 'combustion supporting jets,

arranging for each of the gas streams to be intersected by at least one of the combustion-supporting jets so that intimate mixing lof the CO and combustion-supporting gas will ensue whereby a commingled mass of reacting gases 4is formed,

providing a high heat capacity refractory interior lining for the combustion zone so that the zone is substantially adiabatic whereby the mass washes over the refractory for fly-wheel heat storage in the refractory to stabilize potential ignition pulsations.

10. The process of claim 9 and providing a heat-recovery zone in flow communication with and laterally disposed relative the downstream portion,

exhausting the mass via the downstream portion into the heat-recovery zone,

deecting flow of the mass transversely away from the heat-recovery zone before its entry therein, extracting heat from the mass in the heat-recovery zone.

11. The process of claim 10 wit-h the combustion-supporting gas selected from la group consisting of atmospheric air, air enrichedwith oxygen, gas turbine exhaust having an appreciable oxygen content and off gases from catalyst regenerators having an appreciable oxygen content.

12. The process of claim 11 and positioning the heat-recovery zone above the downstream portion,

restricting the section area of flow as the mass passes from the downstream portion into the heat-recovery zone so that potential thermal inversions are overcome.

References Cited by the Examiner UNITED STATES PATENTS 1,441,721 1/ 1923 `Caracristi 122-7 3,119,440 1/1964 Cruise et al. 158-117.5 3,207,201 9/ 1965 Zink et al 158-1 FOREIGN PATENTS 101,135 9/1925 Austria. 441,106 2/ 1927 Germany. 38,001 12/ 1914 Sweden.

FREDERICK L. MATTESON, IR., Primary Examiner.

MEYER PERLIN, JAMES W. WESTHAVER,

Examiners. E. G. FAVORS, Assistant Exwmner,

Claims

1. A COMBUSTOR SUITABLE FOR A LEAN FUEL OCCURRING IN A PREDOMINANTLY INERT GAS MIXTURE, THE COMBUSTOR COMPRISING A SETTING WHICH DEFINES AN ELONGATED COMBUSION CHAMBER, THE COMBUSTION CHAMBER HAVING AN UPSTREAM PORTION AND A DOWNSTREAM PORTION SPACED LONGITUDINALLY FROM EACH OTHER, GASE INLET MEANS FOR INTRODUCING THE GAS MIXTURE INTO THE UPSTREAM PORTION OF THE COMBUSTION CHAMBER TO FORM A PLURALITY OF GAS STREAMS, A SOURCE OF A COMBUSTION-SUPPORTING GAS, COMBUSTION-SUPPORTING GAS INLET MEANS FOR INTRODUCING A COMBUSTION-SUPPORTING GAS INTO THE UPSTREAM PORTION TO FORM A PLURALITY OF COMBUSTION-SUPPORTING JETS, THE COMBUSTION-SUPPORTING GAS INLET MEANS ORGANIZED RELATIVE THE GAS INLET MEANS SO THAT EACH OF THE GAS STREAMS HAS AT LEAST ONE OF THE COMBUSTION-SUPPORTIN JETS DIRECTED THEREAGAINST FOR INTIMATE MIXING OF THE GAS MIXTURE AND COMBUSTION-SUPPORTING GASWHEREBY A COMINGLED MASS OF REACTING GASES IS FORMED. AT LEAST ONE BURNER MEANS PENETRATING THE SETTING INTO THE UPSTREAM PORTION OF THE COMBUSTION CHAMBER AND FIRING AN ALTERNATE FUEL TO HEAT THE LEAN FUEL ABOVE ITS KINDLING TEMPERATURE, THE GAS INLET MEANS AND THE COMBUSTION-SUPPORTING GAS INLET MEANS ARRANGED SO THAT THE GAS MIXTURE AND THE COMBUSTION-SUPPORTING GAS ARE INTRODUCED INTO THE COMINGLED MASS IN INCREMENTS ALONG THE LENGTH OF THE UPSTREAM PORTION WHEREBY CHILLING OF THE COMINGLED MASS IS AVOIDED, UNCOOLED REFRACTORY LINING THE INTERIOR OF THE CHAMBER SO THAT THE COMINGLED MASS PASSES IN SWEEPING RELATIONSHIP THEREOVER TO PROVIDE HEAT STORAGE IN THE REFRACTORY, THE SETTING DEFINING AN OUTLET COMMUNICATING IN FLOW SERIES WITH THE DOWNSTREAM PORTION OF THE COMBUSTION CHAMBER TO EXHAUST GASES THEREFROM SO THAT A FLOW GRADIENT IS DEVELOPED CAUSING THE COMINGLED MASS TO FLOW LONGITUDINALLY THROUGH THE COMBUSTION CHAMBER FROM THE USPTREAM PORTION FOR EXIT VIA THE DOWNSTREAM PORTION.