US2742276A - Calcining apparatus for producing lime or the like - Google Patents

Description

April 1K7, 1956 v. J. AZBE 2,742,276

CALCINING APPARATUS FOR PRODUCING LIME OR THE LIKE Filed April 8, 1955 2 Sheets-Sheet 1 FIGB.

Apri 17, 1956 v. J. AZBE 2,742,276

CALCINING APPARATUS EOE PRODUCING LIME @E THE LIKE 2 Sheets-Sheet 2 Filed April 8, 1955 ffl/f aga@ CALCINING APPARATUS Foa PnoDUcnsG Lnvruonrrlimcs y j Victor I. Azbe, Webster Groves, Mo., assigner to Albe Corporation, Clayton, Mo., a corporation of Missouri Application April S, 1953, Serial No. 347,466

is claims. (ci. 263-29) j Vnotedthe provision of a thermally efficient, high-capacity,

United safes Patent o compact and integrated kiln and gas producer system adapted to operate satisfactorily with the lower grade fuels and to produce relatively better lime from lower grade of stone, and having the following improved properties:

(l)V integrated gas-producer and kiln temperature and operating controls; l

(2) Recirculation of gas of both high heat and high CO2 content; Y

(3) Introduction of all fresh air at -one point at the base of the kiln with distribution so arranged and controlled with regard to the gas producer and the kiln that no additional air need be supplied to the gas producer;

' (4) Control in the kiln of air, and of recirculating gas so as to obtain a controlled calcining iinishing zone operating at low temperature with a radiant-heating (i. e., longllame) high-temperature calcim'ng Zone and final positive termination of combustion in a precalcining zone;

V(5) Low-,temperature stone preheating before high- "-rateprehe'ating to avoid thermal shock to fragile stone sensitive to heat spalling;

(6) A system applicable `to coalor wood-tired ga producers .and one which can employ in combination with the gas producer `a gas preheater or auxiliary fuelsuch as oil or natural gas;

(7) .A system permitting recirculation from .the terminus of the calcining zone through the medium of a Y single fan;

(8) A system effect-ing recirculation of kilnr gases yof .high CO2 contentblendcd with air and injected into the gas producer as a blast at high temperature,` so as beneiiciallyY to cendition'the gas producer reactions; and, (9) The provision vof high thermal efliciency (barring v.unavoidable radiation loss) without inducing excessive and damaging loperating temperatures. t

Other objects and features will be in part apparent and in part pointed out hereinafter. .Y

The invention accordingly comprises the constructions and methods hereinafter described, the scope of the nventon being indicated in the following claims.

In the accompanying drawings, in which severalyof various possible embodiments of the .invention vare illusgas producer for fuels lean .in volatiles, .such as wood, bagasse, lignite, anthracite, coke and oil shale; p

2,742,276 Patented Apr. 17, 1956 Fice Fig. 4 is a fragmentary cross section taken on line 4-4 of Fig. 3 but with vfuel removed; and,

Fig. 5 is an enlarged detail of an auxiliary inlet.

Corresponding reference characters indicate correspending parts throughout the several views of the `drawings.. Y Y

Referringk now more particularly to the drawings, numeral 1 in general-indicates the kiln component of my invention and 3 the combined gas-producer component. The kiln 1 is located on a supporting structure 5 under which isa four-section draw hopper 7 for finished lime and a take-off `conveyor 9. The kiln is of the vertical shaft type, having in descending order an upper storage zone 1l,- a low-rate preheating zone 13, a high-rate preheating zone 15, a precalcining zone 17, a radiant calcining zone 19, a calcination finishing zone 21, a precooling zone 23 and a ual cooling Zone 25.`

`Mounted on the top of the kiln shaft is a structure 27 forbringing into position a charging car 29 which introduces limestone (for example) into the upper openings 31 ofthe shaft. The side. walls of the kiln are indicated at 33.

v The shaft has a single passage 35 extending downward through the zones 11, 13 and into zor1e'15.V From this point down, beginning with zone 15 and extending through zones 17-23 (inclusive) and into zone 25, the shaft carries cross walls 37 and 39, which dividey it into four vertical quadrate passages 41. These terminate at the bottom in four. tapered outlet Vboots 43 respectively having bottom draw Vfeeders 45. These feeders, assho'wn, may be of the automatic shakertype, `4and as is known are constructed to admit air while discharging iinished lime. Further description of items such as cross walls 37, 39, the four passages 41, outlet boots 43 and feeders 45 will be unnecessary herein because they are known (see, for example, my'United States Patent 2,199,384, 2,300,860 and 2,532,077).

'In' the cross Wall 37 in ascending order are located ducts 47, 49, 51, 53 and 55. The lowerrnost duct 47 is a cooler-air exit duct, having air inlets 57 from the vertical passages 41. The duct 49 is a lower gasrport, having gas outlets 59 leading into said passages 41. The duct 51 is an upper gas port, having gas outlets 61 into said passages 41. Ducts 49 and 51 are connected by ports 60, the arcas of which are under control of slide blocks 62. vThe duct 53 is a combustion-terminating air duct, having outlets 63 into said passages 41. Duct`53 is fed 4by a high-pressure air fan 54. The duct 55 is a recircu- Y lation duct, having inlets 65 leading from the passages 471. In addition `to the above-mentioned ducts in cross Wall 37, there is a lower exhaust duct 56, having oi'ltake passages 67 communicating with the chamber 35; also an upper exhaust duct 5S, having otftake passages 69 ccmmunicating with the chamber 35. The ducts 56 and 58 are, as `indicated by the dotted lines, located preferablyv in the side walls 33. It will be understood that any or all of the other ducts 47, 49, 51, 53 and 55 may, in addition to thepassages shown in the wall 37, have additional limbs thereof in the walls 33. 0r ducts 56 and 58V may have limbs bridging the space 35. These variations are known (see, for example, Patents 2,199,384 and 2,532,077).

The duct 47 communicates with an outside air-dust 4collector 71 from which a .r'ecirculatingtrunk pipe 73 leads by one vbranch 74 to the fan 54. This is for direct reintroduction of air into an upper part of the kiln. A second branch 75 of duct 73 leads to a recirculating duct (77,79). In the duct 75 is a damper 81; also an orifice 'control83f The portion 77 of duct (77, 79) connects with the recirculation duct 55. The exhaust ducts 56 and 58 have connections and 87, respectively, with an exhaust grates 109 which may be of the chopping, shaking andv dumping type. Below the grates is an ash hopper 111 feeding to an ash removal conveyor 113. On top of the gas producer are bell-controlled coal inlets 115, supplied with coal from a bin 117 through coal feeders 119. The counterweighted bell controls are numbered 116. At 121 is indicated a nozzle connecting the duct portion 79 with the inside of the gas producer. At this point is located a damper 123 (see also Fig. 5).

Operation is as follows, assuming that the kiln shaft 1 has been charged with limestone and that the gas producer 3 is being fed with coal and is in operation. There will then be in the gas producer an ash zone 127, an oxidizing zone 129, a reducing zone 131 and a volatilizing zone 133. It is the function of the gas producer to gasify solid fuels for delivery to the kiln through neck 107.

All (100% by weight) of the air employed in the system is drawn in by induction at the draw feeders 45, through the boots 43 and into the vertical passages 41. This air absorbs heat from the finished lime which is passing down lcalled the final cooling zone 25. Operations of the fans 54 and 99 draw off approximately one half (50% by weight) of the preheated air into the trunk duct 73 after dust collection in 71. One fifth of the 50%, or approximately of the original 100% of air, is handled by the fan 54 and reintroduced directly into the kiln through duct 53. The purpose of this will appear below. Four fths of the 50%, or approximately 40% of the original 100% of air, is drawn by fan 99 into the recirculating line 77, 79. This fan also abstracts gas from the kiln shaft through the duct 55. The gases handled in duct (77, 79) are also recirculatory, since they reenter the kiln 1 indirectly through neck 107 after having passed through the gasification chamber 105 (note in this connection that duct 79 is connected to the chamber below the grates 109, as indicated at 135).

The 50% of air that is not withdrawn over duct` 47 passes upward through the kiln above duct 47 to produce the precooling zone 23. At the top of this zone this air meets gas entering the kiln through neck 107 from the gas producer 3. The gas from the neck 107 (which includes fuel and CO2) enters duct 49, the major portion rising through openings 60 into duct 51. Thus the minor portion of the fuel enters the kiln through the lower passages 59 and the major portion through the upper passages 61. As a result the stated 50% of air-is caused to meet with a deficiency of gas from duct 49, which is to say that the air is in excess. The resulting combustion results in a mild heating, producing the calcination finishing zone 19. Then the remaining part of the air which has not been used in'combustion of gas from lower ports 59 is employed for combustion of gas from upper ports 61. At the latter elevation there is a large amount of gas and a deficiency of air, which produces a state of delayed combustion of volatile matter resulting in cracking of hydrocarbons and long-ame instead of rapid complete combustion to CO2, thus creating controllably the much to be desired radiant state calcining zone 19. Long-fiame radiant combustion is superior to short-flame combustion because the resulting radiation throughout the entire zone 19 is better able to distribute heat throughout the interstitial spaces among the stone particles. Consequently, there is an even application of heat at high but controlled ln the section 79 is an auxiliary exhaust temperature throughout the zone 19. The recirculating stream aids in producing a homogeneous temperature state throughout the kiln cross section. With short-ame combustion, the heat distribution in the upper parts of this zone 19 would otherwise be primarily by convection and conduction and would not be as evenly distributed nor as eiciently introduced into the stone, even though the recirculating stream were introduced.

At the upper end of the zone 19, at which the longiiame combustion should terminate, the above-mentioned 10% of air is reintroduced by fan 54 via duct 53 through openings 63. This provides oxygen for converting the remaining CO to CO2 in a positive manner, which gives a final short-flame termination of combustion. Therefore, calcination above duct 53 (in zone 17) is in a lower temperature than in zone 19. Zone 17 is therefore referred to as the precalcining zone. Calcination starts at its upper end at which is located the duct 55. All the'gas reaching duct 55 has a relatively high COz content and is relatively hot.. It may therefore be used for the production of a hot-blast high-COz-content gas stream into the bottom of the gas producer via recirculating line (77, 79) which is under action of the fan 99.

Most of the gas rising in the kiln is eventually drawn over duct 55, but some is allowed to rise up to the lower exhaust duct 56. This gas is also fairly hot and produces the high-rate preheating zone 15. After withdrawal of some of this gas over duct 56, the remainder is left to rise to duct 53. This remainder is fairly cool and produces a low-rate preheating zone 13 wherein the more fragile rock may receive its initial heat without thermal shock. Substantially all of the remaining gas is then drawn off through duct 58. The part of the kiln above duct 58 is the storage zone 11, for rock from the charging cars 29.

The exhaust fan 89 is responsible for suction through the ducts 56 and 58. Such a fan, in order to operate at a constant capacity, requires gas at a constant temperature, impossible of attainment when ystone charging is intermittent except through the arrangement shown, which permits the blending of hot gases from duct 56 with those of duct 58. It will be noted in this connection that fan 54 also handles only relatively cool gas (only preheated air). The fan 99 on the other hand must be of the high-temperature variety.

The purpose of the duct connecting duct 55 with fan 89 is Vto permit the kiln to operate with small stone. Duct 95 is therefore called a small-stone conversion bypass. In the event it is used (damper 97 opened), the dampers 91 and 93 are closed. The reason for such operation in the case of small stone is to reduce gas friction by eliminating gas flow through the upper portions of the kiln. It may be mentioned that in this event, due to the larger heat-absorbing area of the small stone, the temperatures are lower for a given active height, so that the gases flowing from duct 55 are of sufficiently low temperature to be handled by the fan 89.

We return now to the iow of gas over the recirculation line (77, 79) from duct 55. This gas is handled by the fan 99 over line (77, '79) and is directed into the bottom of the gasification chamber 105. Here it passes through the ash zone 127 and enters the oxidizing zone 129, oxidizing the carbon left from volatilization of the coal to CO. Substantially all of the oxygen is used up, thus producing the next-higher reducing zone 131. Because of the high CO2 content and high temperature of the blast stream, elinkering of the ash is effectively prevented, thus preventing undue interference with injection and distribution. Simultaneously, and for the same reasons, a substantial portion of the kiln CO2 is reconverted to CO.

No steam is required for controlling either producer or kiln temperatures, for although the reactions in oxidizthe zone 129 are exothermic (C+O2=CO2 and 2C+02=2C0) the reaction (C +CO2=2C0) is endothermic. Thus, due

Vattendant variabilities.

arf-raars to' 'theI systematic provision of the 4high-C022 high-'teinperature blast in the gas producer, the endothermic reaction is possible.` This results in` temperature modulation possibilities far beyond what is possible in ordinary systems where only steam or `small qantities of endothermic CO2v are introduced at' relatively much lower temperatures. This tends to solve completelythe perplexing gas producer clinkering problem and 'the many Moreover, the gas obtained, while of a lower heat value per cubic foot,is of a much greater volume and higher total heat Acontent and the 'system as a Whole is more controllable. The lime is thus calcined with a greater volume'of gas at lower temperature, rather than a smaller `voiu'in'e at `excessive temperatures. This produces better lime. By means `of the` present construction the kiln temperature, starting with a suiciently high but not too high temperature from neck 107, is controlled by adjustment of the air-fuel ratios brought about by the 'air passing up through the kiln past ports 59 and 61, as already made clear; also by the long-flame combustion above duct 5I and the short-flame combustion termination at duct 5-3, supplemented by the readily regulatable quantity of kiln gas recirculated through the systems, modifying beneficially both producer and kiln functions.

In addition to the advantages already set forth, the following will be clear. The gases flowing through the recirculation line (77, 79) are sufhciently hot to pro; vide a high-temperature gas blast for the gas producer. The high CO2 content results in the temperature control at the gas producer bed and subsequently of the kiln. The air introduced into line (77, 79) from branch -75 is to provide sufficient oxidation in the gas producer to maintain a high rate of gasification. While the air flowing from 75 into 77 has some cooling effect, it will be .understood that the heat absorbed from the gas stream from duct 55 is not lost, butreenters the system, except for some small loss from insulated line (77, 79), without over-burning or dead burning of lime. Nevertheless, above duct 51 higher calcination temperatures are applied and the radiant calcining zone created, which `is an effective heat transfer zone not excessive in temperature and notconducive to damage of lime structure. Y An important feature is the two-level burning from 49 and 51 to produce the calcination finishing zone 21, wherein a mild finishing action occurs without overburning tof lime. Nevertheless, above duct 51 higher calcination temperatures are rapidly applied to the stone in order to produce the radiant calcining zone. In this zone there is no danger of spoiling finished lime by rapid application of heat.

Besides the termination of combustion 'effected by the air' flowing from duct 53, there is also the advantage at this point of high turbulence due to pressure air injection by the fan 54.

It is an advantage that all of the air for primary combustion at d uct 49, secondary combustion at duct 51 and producer combustion passes through the lower portion of the kiln through the cooling zone 25. This means that all gases for all combustion operations are preheated, the lime well cooled and heat recovered to enhance kiln thermal efficiency.

VThe double-level upper exhaust effected bygducts 56v and 58 (operative when the kiln is used for large stone) acts not only as protection for the fan 89, as already stated,

'but effects the low-rate preheating zone 13, which will ,not damage fragile types of stone physically or chemically sensitive to thermal shock.

Regarding the high CO2 content of the gaseous mixture entering the producer and the endothermic action in the reducing zone 131, for example, about 16% of the gas entering the gas producer is CO2 instead of the usual 2% to 7%, and the temperature thereof is substantially above 500 and may be 1,000 F. The end result is a steady generation of a greater volume of gas of milder temperature for the ki1n,whichV can be done without the remployment of extra-high-grade fuels.

Additional purposes of the auxiliary exhaust an control damper shown at 103 are to relieve the kiln preheating zone of excess gas if desired, or to provide a hot blast for fuel drying purposes or other such supplementary uses conducive to kiln unit performance etilciency. Damper 106 is to control the volume of air-recirculating gas mixture to the producer.

The nozzle 121 is for the purpose of providing over-fire preheating or tempering. If connected` to another source of fuel, instead of to line 79, it may be used to enrich the producer gas with natural gas or fuel oil or to use the gas producer chamber as a combustion chamberfor pulverized coal, fuel oil, natural or blast furnace gas.

While my gas producer has been described as being of the typefor gasification of coal, it will be understood that it may also be the type for gasi'cation of other fuels, such as wood, bag'asse, et cetera. The preferable producer for the latter purpose is called the constant feed type and is illustrated in Figs. 3 and 4. Referring to these ig'ures, indicates4 the support, 137 the ash hopper and 139 the interior, with a triple gas neck 141. Bulk wood may be introduced from a tilting basket such as shown at 1743,'v or ba'gasse, coke, etc. from a chute upon opening of a cover such as shown at 145. The wood is lled to the top* of chamber 139 and it gradually sinks into the active section below arches 140 as it burns away. The wood dropsbelow the arches 14@ through inlets 142 leading from the top of the producer to the arches, respectively.- This accounts for reference to such producers as being of the constant feed type. The advantages' are a constant depth of active fuel bed, constant gas temperature and a low-temperature feed compartment. Wood .fuel is shown only in Fig. 3. Fig. 4 shows the chamber empty and without auxiliaries.v The roughly pyramidal lay vof the fuel, as indicated in Fig. 3, is sometimes referred to as being hogged.

With regard to the kiln, it should be noted that it is of a' triple rfiring-level type, the primary gas being introduced 'at 59,secondary gas in excess at 51, with surplus gas being burned with secondary air at S3. Thus is created a greatly lengthened calcining zone. Y

As indicated by the cross-hatch symbol for heat insulation, .the following portions of the device will b'e 'understood toV be well heat insulated or lagged: -kiln 1 from n the bottom to the ports 67; gas producer 3; collector 7.1; and passages 73, 74, 75, 77 and 79.

In view of the above, it will be seen that the several objects of the invention are achieved and otheradvantageous results attained.

As various changes could be made in the Iabove constructions and methods without departing from the scope ofthe invention, 'it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense, and applicable whether or not the unit entails the `usefof thegas producer.

I claim: i

1. A vkiln comprising a vertical shaft for descending materialand having a lower air inlet, fuel inlet ports above said air inlet, said ports being located at two levels withl interconnections adapted for issuance of more fuel into the shaft from the upper level than from the lower level, a fuel connection joined with the fuel inlet ports, means for abstracting air from-the kiln at a level below said fuel inlets andabove the lower air inlet, the remainder of the unabstracted air rising past said gas inlets in an amount providing excess air relative to fuel issuing from the lower fuel inlet level to provide initial short-name combustion to `CO2 and in an amount deficient with respect to fuel issuing from the upper fuel inlet level to provide long-flame combustion to CO, and connecting means adapted to circulate some of said abstracted air to a shaftamanteV "7 inlet point above the upper fuel inlet levels to provide nal short-dame combustion to CO2.

2. Calcining apparatus wherein an upright kiln having a gas inlet, and a gas producer having a gas outlet, are connected between said outlet and'inlet; comprising a lower air inlet means and air exit means in the kiln, the latter being located between the airinlet means and said gas inlet, a trst branch from said air exit means adapted to reintroduce some of the exit air to the kiln at a rcdelivery point beyond said gas inlet, a CO2 gas recirculation exit fromthe kiln beyond said redelivery point, means connecting said recirculation exit with the gas producer below its fuel bed, and a second'branch from said air exit means adapted to deliver air from said ai exit means to said recirculation exit.

3. Apparatus made according to claim 2, including means connecting said gas recirculation exit with the gas producer at a point above its fuel bed.

4. Apparatus made according to claim 2, including a first fan in said iirst branch adapted to produce a turbulent blast of reintroduced air which is preheated, and a second fan connected with said recirculation exit and adapted yto vproduce an independent blast into the producer of hot recirculating CO2 gas and air which has been preheated and further raised in temperature by said CO2 gas while cooling the latter.

. 5. Apparatus made according to claim 4, including spaced exhaust exits from the kiln above said recirculation exit, an exhaust fan, and connections between said exhaust fan and said exhaust exits.

6. Apparatus made according to claim 5, including also a connection between said exhaust fan and said recirculation exit, and means for controlling all of said connections with the exhaust fan.

7. Calcining apparatus wherein an upright kiln having a gas inlet, and a gas producer having a gas outlet, are connected between said outlet and` inlet; a lowermost air inlet yfor the kiln, the kiln having an air outlet thereabove dening .a nal cooling Vzone, multiple gas inlet ports at two'levels above the air inlet and fed by the connection between the producer and the kiln, said ports being connected between said levels, whereby an initial cooling zone is defined between the air outlet and the lowermost gas ports and a calcination finishing Zone is detined between the two levels of the gas ports, and means for directing the reintroduction of preheated air from said air outlet into the kiln at a level above the gas ports to define between said point of reintroduction and the upper gas ports a radiant calcining zone.

8. Apparatus made according to claim 7, wherein substantially all of the air required Yfor kiln and gas producer operations enters said lowermost air inlet and passes through said final cooling Zone, including gas exit means for abstracting gas from the kiln above the level of air reintroduction to define a precalcining zone, and means for directing some air from said air outlet and abstracted gas to and through the gas producer for redelivery to the kiln at said gas inlet, whereby a volume f of CO2 is supplied to the producer and also hotl air for combustion, and whereby CO2 and hot combustible gas are supplied at said gas inlet.

9. Apparatus made according to claim 8, including low-level gas exhaust means in the kiln above said gas exit means defining a high-rate preheating zone, and a high-level gas exhaust means defining a low-rate preheating zone, the kiln having a storage zone above the highlevel gas exhaust means.

l0. lApparatus made according to claim V9, wherein a fan is located in said air inlet, a second fan is located in said gas exit, and including an exhaust fan provided with controlled connections to said gas exit means and said low-level and high-level exhaust means.

1l. Calcining apparatus wherein an upright kiln having a gas inlet, and a gas producer having a gas outlet, are connected between said outlet and inlet; comprising means for substantially heat insulating the kiln and producer, a lower air inlet in said kiln for substantially all of the air employed by the kiln and the gas producer, and gas ports in the kiln above the lower air inlet, means for abstracting a substantial portion of the air from the kiln below the gas ports and reintroducing a relatively small fraction of the abstracted airinto the kiln above said gas ports, means for abstracting gas including a substantial amount of CO2 from the kiln above said point of air reintroduction, said last-named abstracting means being connected to the gas producer, said abstracting means being substantially heat insulated, whereby substantially all of the air required for calcination and production of gas is preheated by cooling of finished material, and whereby both air and gas recirculation are accomplished without substantial loss of heat.

l2. In calcining apparatus wherein an upright kiln having a fuel gas inlet, and a gas producer having a fuel gas outlet, have a fuel connection between said outlet and inlet, said kiln having a rst lower air inlet; fuel ports in the kiln connected with its fuel gas inlet and located above its air inlet, said fuel ports being located at two levels, means effecting issuance of more fuel into the kiln from the upper level ports than from the lower level ports, means for abstracting air from the kiln at a level below said ports and above the lower air inlet to provide a final cooling zone, unabstracted air rising to form a precooling zone and passing said ports in an amount providing excess air relative to fuel issuing from the lower ports so as to provide initial short-name combustion to CO2 in a calcination finishing zone and in an amount deficient with respect to fuel issuing from the upper ports to provide long-flame combustion to CO in a radiant calcining zone, connecting means adapted to circulate some of said abstracted air to a second kiln air inlet point above the fuel ports to provide final short flame combustion to CO2 in a precalcining zone, gas exit means for abstracting CO2 gas from the kiln above the level of said second air inlet, means for directing some air from said. air-abstracting means into mixture with said abstracted CO2 gas, and means for delivering the mixture to said gas producer.

13, Apparatus made according to claim l2, including spaced exhaust exits from the kiln above said means for abstracting gas therefrom to establish a lower high-rate preheating zone and a higher low-rate preheating zone.

References Cited in the le of this patent UNITED STATES PATENTS 829,956 Eldred Sept. 4, 1906 2,199,384 AZbe May 7, 1940 2,370,281 Azbe Feb. 27, 1945 2,532,077 Azbe Nov. 28, 1950

Claims (1)

1. 2. CALCINING APPARATUS WHEREIN AN UPRIGHT KILN HAVING A GAS INLET, AND A GAS PRODUCER HAVING A GAS OUTLET, ARE CONNECTED BETWEEN SAID OUTLET AND INLET; COMPRISING A LOWER AIR INLET MEANS AND AIR EXIT MEANS IN THE KILN, THE LATTER BEING LOCATED BETWEEN THE AIR INLET MEANS AND SAID GAS INLET, A FIRST BRANCH FROM SAID AIR EXIT MEANS ADAPTED TO REINTRODUCE SOME OF THE EXIT AIR TO THE KILN AT A REDELIVERY POINT BEYOND SAID GAS INLET, A CO2 GAS RECIRCUTATION EXIT FROM THE KILN BEYOND SAID REDELIVERY POINT, MEANS CONNECTING SAID RECIRCULTAION EXIT WITH THE GAS PRODUCER BELOW ITS FUEL BED, AND A SECOND BRANCH FROM SAID AIR EXIT MEANS ADAPTED TO DELIVER AIR FROM SAID AIR EXIT MEANS TO SAID RECIRCULATION EXIT.

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