CA1069306A - System for withdrawing solids from a high temperature fluidized bed - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

Particulate solids are continuously discharged from a high temperature fluidized bed through a descending solids withdrawal line into a fluidized bed cooling vessel while controlling the solids discharge rate indirectly by regulating the cooling vessel pressure, preferably by means of a control valve which is used to throttle gag taken off overhead from the vessel.
Fluidized solids may be discharged from the cooling vessel into a second vessel in which they are slurried in water and removed from the system through one or more pressure letdown valves. Alternatively, the particles may be passed from the fluidized bed colling vessel into a lock hopper from which they are withdrawn periodically.

Description

106~306

2 1. Field of the Invention: This invention

3 relates to fluidized solids operations and i8 particularly

4 concerned with the removal of hot solid particles from high temperature fluidized beds during ooal gasification and 6 similar operations 7 2. Desc~tion of the Prior Art: Fluidized bed 8 system~ for the gasification of coal and similar carbona- -9 ceous solids in which coal par~icles are devolatilized to produce hydrocarbon gases and char and the char is reacted 11 with steam to fo~m synthesis gas have been developed in 12 recent years. The reactions involved, which may be carried 13 out in a single vessel or in two or more reactors, are high- --14 ly endothermic and require that large amounts of heat be ~upplied This is generally done by burning a portion of 16 the char, either by injecting oxygen into the flu~dized bed 17 with the steam or by withdrawing char from the bed, passing 18 it to a sepArate combustion zone, and then returning hot 19 char particles to the fluldized bed reaction vessel. The gasificatio:n and c~mbustion reactions which thus take place 21 result in ~he production of significant quantities of ash 22 derived from mineral matter in the feed coal. The ash not 23 oarried overhead with the product gases tends to accumulate 24 in the sy~tem and must be removed if the process is to oper-ate continuously.
26 There have been several different methods proposed 27 for coping with the ash removal problem. Much of the early 28 coal gasifica~ion work was carried out with slagging type 29 gasi~ier~ which were operated at temperatures above the ash fusion point and therefore re~ulted in the formation of a ,, , ,, -~069306 1 molten ash which could be quenched and withdrawn ~9 slag 2 from the lower part of the ga~ifier. Such a system i5 3 useful for the removal o~ a~h from ga~ifiers designed for 4 the production of synthesis ga~es of low methane content but poses problem~ where higher B.t.u. product gases are 6 desired. The high tempera~ures required to melt the ash 7 tend to crack any methane present in the system and hence 8 the B.t.u. content of the product gas will normally be low.
9 A more effect~ve system where gases containing signiicant quantities of methane are required is to operate at tem-11 peratures below the a~h fusion point and wi~hdraw a portion 12 of the char solids from the gasifier in order to keep ~he 13 ash content of the system at satisf~ctory levels~
14 The intermittent withdrawal of hot char solids by means of lock hoppers and similar equipment had been pro-16 posed as a means for removing ash from gasifiers and simi~
17 lar reaction ve~sel~ but pose~ problems because it may 18 result in operating upsets in the gasifica~ion process and 19 because lt requires ~he use of flow s~utoff valves wkich must oper~te at high temperatures. Sultable valves are 21 difficult to design and maintainO An alternative approach ~22~ to form a slurry of solids and water in the lower portion 23 of the gasifier beneath the high temperature fluidized bed 24 and then depressure the slurry across a series of valves 2~ to permit the eontinuou~ withdrawal of ~o~ids from the 26 system. It is difficult to fonm such a slurry and control 27 the solids content because of the proximity of the high 28 temperature fluldized bed.
29 More recently, it has been suggested that a por-~0 tion of the ~team or other reactant gas to be used in the ~069306 fluidized bed be injected into the lower portion of the gasifier at a relatively low rate which is sufficient to suspend the lighter char particles of low ash content but insufficient to suspend heavier particles containing more ash. The additional gas required to maintain the bed in the fluidized state is introduced at a somewhat higher rate above the lower gas inlet. Coal particles fed into such a system near the top of the gasifier become fluidized and circulate within the bed. Lighter particles of low ash content which find their way into the zone below the level at which the main fluidizing gas is injected are entrained and carried back into the bed. Heavier particles which fall into the zone below the main gas injection level and cannot be entrained tend to accumulate in the lower portion of the vessel and can be discharged into lock hoppers or similar equipment. A system of this type has advantages over earlier methods proposed for the removal of ash but requires very careful control of the gas velocities if effective separation of the particles is to be obtained and necessitates that the particles be quenched in the lower part of the gasifier or that vaLves designed to operate at high temper-atures be provided. Moreover, the internal equipment which must be provided in the lower part of the gasifier may inter-fere with the circulation of hot char between the gasifier and an external combustion vessel. Efforts to avoid these and related problems associated with the removal of ash particles from gasifiers and other fluidized bed vessels have in the past been largely unsuccessful.
SUMMARY OF THE INVENTION
Here described is an improved system for the with-drawal of hot solid particles from a high temperature fluid-ized bed which at least in part avoids the difficulties out-lined above. It has now been found that particulate solids can be continuously discharged from a high temperature fluid-ized bed through a descending solids withdrawal line into a fluidized bed cooling vessel while controlling the solids discharge rate indirectly by regulating the cooling vessel pressure, preferably by means of a control valve which is used to throttle gas taken overhead from the vessel. The heat contained in the withdrawn particles can in part be recovered as sensible heat in the overhead gas. In addi-tional, water can be injected to cool the particles and generate additional steam. The gas taken overhead can be returned to the high temperature fluidized bed, vented to the atmosphere after treatment to recover heat and remove contaminants, or employed in other process applications.
The withdrawn solids are passed from the fluidized bed cooling vessel into a depressuring system which may include a slurrying drum and one or more pressure letdown valves or lock hoppers which operate at relatively low temperatures and thus do not require the high temperature valves needed in other systems. This permits close control of the rate at which hot solids are withdrawn from the gasifier with-: . ' .~ , . . . , !
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. : ' ' . ' out the difficulties associated with ash withdrawal systems _ employed in the past.
In a preferred embodiment of the invention, thehot solids withdrawn from the gasifier or similar high tem-perature fluidized bed vessel pass downwardly through the descending solids withdrawal line in countercurrent flow to a stream of steam or gas which is injected at a rate suffi-cient to maintain the solids in the withdrawal line in a dense phase fluidized state. The velocity of the upflowing gas is controlled so that heavier particles of high ash content accumulate in the withdrawal line and move downward-ly past the point at which gas is injected and enter the fluidized bed cooling vessel. By a phenomenon which may be called "dense phase particle segregation", lighter particles of low ash content and high carbon content will tend to remain in the fluidized bed, rather than flow down the with-drawal line with the denser, high ash particles. This permits the preferential withdrawal from the gasifier of relatively large or dense particles which are high in ash content and reduces the loss of carbon from the gasification process. The process thus results in a significant improve-ment in carbon utilization in the process, alleviates difficulties due to the buildup and accumulation of ash de-posits in the gasifier, and provides an effective method for the elimination of ash without the difficulties that have characterized methods used in the past.
~RIEF DESCRIPTION OF THE DRAWING
-Fig. 1 in the drawing is a schematic flow diagram ,.. ,... , ', . '; " ., ."` " . ,, ., ., '.' . , ' ' " ........ ' ' -:
' ~ ` . . . . ' . ' ' ,' ` " " ',"' ' ' . ' ' ' " ' '.'. ' ' ' ` ~ ` . ' " ' ' ' ,~'` ` `'`' ' ' " ~ .' of a coal gasification process in which ash is withdrawn from a high temperature fluidized bed gasifier in accordance with the invention;
Fig. 2 is an enlarged sectional view of the lower end of the gasifier of Fig. 1 which illustrates the manner - in which solids are withdrawn from the fluidized bed; and Fig. 3 is a schematic flow diagram of a portion of a process similar to that of Fig. 1 which illust~ates an alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process depicted in Fig. 1 of the drawing is a gasification process for the production of a product gas stream of relatively high methane content by the treatment of bituminous coal, subbituminous coal, lignite or similar carbonaceous material with steam at high temperatures. It will be ur.derstood that the invention is not restricted to the particular process shown and can be used in conjunction with fluidized bed processes for the carbonization of coal and similar feed solids, for the gasification of petroleum coke and similar carbonaceous solids, for the retorting of oil shale and the li~e, for the partial combustion of carbonaceous solids, and in other high temperature fluidized bed processes in which high temperature particles must be continuously removed from the fluidized bed for further treatment or discharge from the system.
In the process shown in Fig. 1, a solid carbona-_ , ~ - 7 -, -. ~ . . - ~ ~ .

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ceous feed material such as bituminous coal, subbituminous coal, lignite or the like which has been crushed to a par-ticle size of about 8 mesh or smaller on the Tyler Screen Scale is fed into the system through line 10 from a feed . . .
preparation plant or storage facility which does not appear in t~e drawing. If desired, this coal or other carbonaceous feed material may be impregnated or mixed with an alkali metal ~onstituent to catalyze the gasification reaction.
The feed solids introduced through line 10 are fed into a ! -closed hopper or similar vessel 11 from which they are dis-charged through star wheel feeder or an equivalent device 12 in line 13 at an elevated pressure sufficient to permit their introduction into the gasifier at the system operating pressure or a somewhat higher pressure. In lieu of or in addition to this particular type of arrangement, parallel lock hoppers, pressurized hoppers, aerated standpipes operated in series, or other apparatus may be employed to raise the input feed solids stream to the required pressure level. The use of such equipment for handling coal and other finely divided solids at elevated pressure has been described in the literature and will therefore be familiar to those skilled in the art. Equipment which may be employed for this purpose is generally available from commercial sources.
A carrier gas stream is introduced into the system of Fig. 1 through line 14 to permit the entrainment of coal particles or other solid feed materials from line 13 and ~, : . . :., :,-,. ,,, - : . ., : ... ,. .-facilitate introduction of the solids into ~asifier 15.
The carrier gas employed may be high pressure steam, recycle product gas, inert gas, or the like. The use of recycle product gas avoids reduction of the hydrogen concentration in the gasifier and is therefore generally preferred. The carrier gas stream is introduced into the system at a pres-sure between about 50 and about 2000 psig, depending upon the pressure at which gasifier 15 is operated and the solid feed material employed, and is preferably fed into the system at a pressure between about 100 and about 1000 p5ig.
The gas may be preheated to a temperature in excess of about 300F. but below the initial softening point of the coal or other carbonaceous feed material if desired. For the gas-ification of bituminous coals, the use of carrier gas at . - , . , \ - 8A -. ~

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1 temperatures within the range between about 400 and about 2 550F. is often advantageous. The c081 or other feed par-3 ticles, preferably less than about 8 mesh in size on the 4 Tyler Screen Scale, are suspended in the carrier gas stream S in a ratio between about 0.2 and about 5.0 pounds of solid 6 feed materials per pound of carrier gas. The optimum ratio 7 for a particular system will depend in part on the feed par-8 ticle size and density, the molecular weight of the gas 9 employed, the temperature of the solid feed material and - r input gas stream, and other factors. In general, ratios 11 between abou~ 0.5 and about 4.0 pounds of coal or other ~
12 solid feed mater~al per pound of carrier gas are preferred.
13 The feed stream prepared by the entrainment of 14 coal or other solid particles from line 13 in the gas intro-duced through line 14 is normally fed into the gasifier 16 through one or more fluid-cooled nozzles not shcwn in the 17 drawing. The cooling fluid will normally be low pressure 18 steam but may al~o be water or the like. This fluid may be 19 circulated in the nozzle for cooling purposes or injected into the gasifier around the ~tream of feed ga~ and en-21 trained solids ~o control entry of the so1ids into a fluid- `
22 ized bed in the gasifier. In the system shown in Fig. 1, 23 the gas and entrained solids flow into injection manifold 24 16 and then pass ~nto the gasifier through four injection line~ 17 ~paced abou~ the gasifier periphery. The number 26 of injection lines and nozzles employed will depend in part 27 upon the ~asifier diameter and the feed rates used and may 28 be ~aried as necessary.
29 The gasifier employed in the system shown in Fig.
1 comprises a refractory lined vessel containing a fluidized -~069306 bed of char particles introduced into the lower portion of the system through bottom inlet line 18. The inlet line extends upwardly through the bottom of the gasifier to a point above an internal grid or similar distribution device not shown in Fig. 1. Steam for maintaining the char par-ticles in a fluidized state and reacting with the char to produce a synthesis gas containing hydrogen and carbon monoxide is introduced into the lower portion of the gasifier - below the grid or other distribution device through manifold -19 and steam injection lines 20. The installation shown employs four steam injection lines spaced at 90 intervals about the gasifier periphery but a greater or lesser number may be employed if desired. Steam thus introduced will normally be fed into the system at a rate within the range of about 0.5 and about 2.0 pounds of steam per pound of coal or other solids feed. The upflowing steam and sus-; pended char particles form a fluidized bed which extends up-wardly in the gasifier to a level above that at which the coal or other solid feed particles are introduced with the feed gas from line 14. The upper surface of this fluidized bed will normally be located a substantial distance above the feed injection level but sufficiently below the upper end of the gasifier to permit disengagement of the heavier char particles that may otherwise tend to be entrained with the gas leaving the bed.

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106g306 In the particular gasifier shown in the drawings, the lower portion of the fluidized bed between the grid or similar distribution device and the level at which the coal or other solids feed material is introduced into the system serves as a steam gasification zone. Here the steam in-jected through the manifold and steam injection lines reacts with carbon in the hot char particles to form synthesis gas ~ containing hydrogen and carbon monoxide. The hydrogen con-centration in the gaseous phase of the fluidized bed in-creases from essentially zero at the bottom of the bed to a value of about 30 to about 50 volume percent or more near the upper surface of the bed. The temperature in the steam gasification zone will generally range bétween about 1450 and about 1950~. Depending upon the particular feed ma-terial and particle size employed, the gas velocities in the fluidized bed will normally range between about 0.2 and about 2.0 feet per second or more.
The upper portion of the fluidized bed in gasifier 15 serves as a hydrogasification zone where the feed coal is devolatilized and at least part of the volatile matter which is liberated reacts with hydrogen generated in the steam gasification zone below to produce methane. Other reactions, including the reaction of hydrogen with carbon to form methane, also take place. The level at which the solids feed stream is introduced and hence the location of the steam gasification and hydrogasification zones depends in part on the proper~ties of the particular coal or carbonaceous feed e 106g306 ;`
material which is employed in the process. It is generally preferred to select the injection level so that the methane yield from the gasifier will be maximized and the tar yield minimized. The amount of methane produced generally in-creases as the coal feed injection point is moved upwardly towards the top of the fluidized bed. The tar produced from the input coal or other feed solids normally increases as the feed injection point is moved upwardly and decreases as it is moved toward the bottom of the fluidized bed, other operating conditions being the same. The solids feed stream should be generally introduced into the gasifier at a point where the hydrogen concentration in the gas phase is in excess of about 20 percent by volume, preferably between about 30 and about 50 volume percent.
In general, it is preferred that the upper level of t~he fluidized bed in gasifier 15 be maintained sufficently above the feed injection level to provide at least about four seconds of residence time for the gas phase in contact with the fluidized solids in the hydrogasification zone. A
residence time between 1~ and a~out 20 seconds is normally advantageous. The optimum hydrogen concentration at the feed injection point and the gas residence time above that point will vary with different types and grades of coal or other feed solids and will also change with variations in the gasification temperature, pressure, steam rate and other process variables. Higher rank bituminous coals normally require somewha~ more severe reaction conditions and longer ~0~i9306 residence times to obtain high methane yields and low taryields than coals of lower rank. Similarly, higher reaction temperatures generally tend to increase the hydrogen concen-tration of the gas phase and reduce the gas residence times - needed to secure acceptable methane and tar yields from a particular feed material.
The raw product gas from the fluidized bed in gas-ifier 15 moves upwardly from the upper surface of the bed, carrying entrained solids with it. This gas is withdrawn from the gasifier through overhead line 21 and passes to a cyclone separator or equivalent device 22 where the larger entrained solids are separated from the gas. In lieu of an external separator as shown in the drawing, the gasifier t~ may contain one or more internal cyclones or similar devices 1 for the removal of entrained solids from the upflowing gas ?,:
stream. The solids removed from the gas in separator 22 are conveyed downwardly through diplegs 23 and 24 for rein-jection into the system as described hereafter. The overhead gas from the separation unit 22 is passed through line 25 to a second separation unit 26 where additional entrained fine solids are removed. .These particles are withdrawn by means - of dipleg 27 and may be passed with the solids from the first separation unit through dipleg 24 for injection into a trans-fer line burner as shown in the drawing or for reinjection into the gasifier. The raw product gas taken overhead from separation unit 26 through line 28 may be treated for the recovery of heat and the removal of acid gases and other . , , .. .. ., . . ~ , ,~. .

undesirable constituents and then employed as a low heating value fuel gas or upgraded into a product gas of higher B.T.U. content. Conventional gas clean-up and methanation procedures may be employed.
The heat required for the gasification process shown in Fig. 1 is generated by continuously withdrawing char particles from the fluidized bed in the lower portion - of the gasifier by means of line 30, passing these particles and fines from dipleg 24 into an upflowing stream of carrier gas introduced into the system through line 31, and injecting this stream into the lower end of transfer line burner 32.
The carrier gas employed may be recycled flue gas, inert gas or the like. An oxygen-containing gas, normally air, is introduced into the system through line 33 and injected into the lower end of the burner through manifold 34 and peripherally spaced injection lines 35. It is sometimes preferred to dilute the oxygen-containing gas introduced at the bottom of the burner with recycied flue gas or inert gas introduced through line 36 so that the oxygen content of the gas entering the burner at this point is about 15 percent or less, preferably less than about 6 percent.
Additional oxgyen-containing gas, normally air, is intro-duced into the upper portion of the burner through line 37, manifold 38 and peripherally spaced injection lines 39. The combustion of carbon as the solids move upwardly through the burner in the presence of the oxygen-containing gas results in heating of the solid particles to a temperature ~ - 14 -,. . .

~ ; 3 ; 10~;9306 in excess of that within the gasifier.
It is generally preferred to control the oper-ation of the burner so that the solid particles leaving the ; - upper end of the unit have a temperature of from about 50 to about 300~. above the fluidized bed temperature in the gasifier. The solids leaving the burner enter cyclone sepa-rator or similar device 40 where the larger particles are removed from the gas stream and conveyed downwardly through line 41 for reintroduction into the gasifier with the car-rier gas introduced through line 18. The overhead gases from separation unit 40 are passed through line 42 to a second separation unit 43 where entrai~ed fine solids are removed and conveyed downwardly through dipleg 44. These fine particles may be introduced into a stream of carrier gas, normally recycle flue gas or inert gas, introduced through line 45 and.reinjected into the burner with the car-rier gas and solid particles introduced through line 31.

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1 The raw flue gas taken overhead from separation unit 43 2 through line 46 may be processed for the recovery of heat 3 and the removal of pollutants and other undesirable con-4 stituents and then d~scharged into the a~mosphere or em-ployed in various process applications.
6 The system employed in accordance with the 7 invention for the withdrawal of ash from the gasifier of 8 Fig. 1 is shown in greater detail in Fig. 2 of the drawings.
9 Fig. 2 is a fragmentary longitudinal cross-section showing the lower end of gasifier 15 and the lines connected there-11 to. Hot char particles from transfer line burner 32 are 12 returned through line 41 and carried upwardly into the gas-13 ifier through line 18 as pointed out above. SteEm inJected ` 3 14 through several injection lines such as line 20 maintains the particles in a fluidized bed above internal grid 47. A
16 stream of solid char particles is continuously withdrawn 17 from the fluidized bed through a descending solids with-18 drawal line 48 which passes through and extends below the 19 grid. A stream of steam9 recycle product gas, or inert gas is injected into line 48 through line 49 a short distance 21 below the gasifier to maintain the solids in the withdrawal 22 line in a fluidized state. Lighter solids of relatively 23 low ash content which may enter the withdrawal line tend to 24 be carried back into the gasifier by the upwardly flowing gas. The heavier particles of relatively high ash content 26 tend not to be carried by the upflowing gas and instead 27 settle downwardly in line 48 to form a stream of solids 28 which eventually leaves line 48 through line 50O The veloc-29 ity and volume of gas injected through line 49 will depend in part upon the average particle size and ash content of .

1 the solids in the fluidized bed, ~he desired rate of solids 2 withdrawalJ and other factors. In general, superficial gas 3 velocities between about 0.05 and about S.0 feet per second 4 will be employed within line 48. Velocities between about 0.2 and about 2.0 feet per second are generally adequate 6 and usually preferred. The velocity required in a partic-7 ular operation can be readily detenmined by monitoring the 8 ash content of the particles withdrawn from the system and g increasing or reducing the velocity with which the gas is injected until the desired ash content is obtained. Because 11 the volume of gas injected through line 49 is smallJ the 12 iniected gas has little effect upon the fluidized bed main-13 tained within the gasifier above grid 47. Changes in the 14 gas velocity within line 48 can be made as desired without disrupting the fluidized bed or seriously affecting the 16 amount of solids carried overhead from the bed~ By properly 17 adjusting the gas velocity, the selective withdrawal of the lB particles having a significantly higher ash content than the 19 average ash content of the particles making up the fluidized bed is possible. This permits more effective removal of 21 ash from the system and results in better carbon utilization 22 than might otherwise be obtained.
23 The dense phase stream of solids moving down-24 wardly through line 50 passes into fluidized bed cooling vessel 51. Here the solid particles are maintained in the 26 fluidized state by means of steamJ recycled product gas or 27 other fluidizing gas introduced into the lower end of the 28 vessel through line 52 containing con~rol valve 53. Vessel 29 51 may include an internal grid or similar distribution device which does not appear in the drawing to aid in - 16 ~

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la6s306 ; 1 fluidization of the solids. In lieu of a single in3ection2 line as shown, a manifold and multiple lines may be employed 3 for introduction of the fluidizing gas. The rate at which 4 fluidizing gas in introduced into the vessel ~hrough line 52 is adjusted by means of control valve 53. The velocity 6 and volume of fluidizing gas injected will depend in part 7 upon the average particle size and ash content of the solids 8 in the cooling bed. In general, superficial gas velocities 9 between about 0.05 and about 3.0 feet per second will be employed within cooling vessel 51. Velocities between about 11 0.2 and about 1~0 feet per second are usually preferred.
12 The rate at which solid particles pass downwardly into cool- -13 ing vPssel 51 is controlled indirectly by controlling the 14 pressure in that vessel using control valve 55 to throttle the fluidizing gas as ~t is taken overhead through llne 54.
16 This eliminates the necessity for passing the high tempera-17 ture solids in line 50 ~hrough a slide valve or similar 18 device and thus does away with one of the principal diffi-19 culties encountered in withdrawing solids fr~m gasifiers and slmilar high temperature fluidized bed systems. The 21 gases taken overhead through line 54 will normally be at a 22 substantially lower temperature than the incoming solids 23 and in addition will be relatively free of entrained solid 24 particles. If desired, a cyclone separator or similar dev~e can be provided in the upper portion of vessel 51 or in line 26 54 above the vessel to reduce the amount of particles that 27 may be entrained in the overhead gas. Valve 55 is thus not ; 28 sub;ected to the severe operating conditions that a slide 29 valve or similar device intendèd to handle the incoming solids would be subjected to and hence a commercially , ,, , , ~ . . .. .

1~)69306 1 a~ailable valve designed or intermediate temperature oper-2 ations may be used. The &ases discharged through valve 55 3 may be vented through line 56 and valve 57 or recycled to 4 the gasifier through line 58. In either case, the cooling

5 vessel 51 will normally be operated at a controlled pressure

6 close to that in gasifier 15. The pressure will normally

7 be adjusted using control valve 55 so as to maintain the

8 fluidized bed level in cooling vessel 51 at a preferred ~;

9 position above solids discharge line 61 and sufficiently

10 below the upper end of the vessel to permit disengagement ll of most entrained particles. If desired, water or other 12 vaporizable cooling fluid may be introduced into the upper 13 end of the cooling vessel through line 59 to further aid in 14 reducing the temperature of the incoming solids. In lieu 15 of or in addition to introducing cooling fluid near the top 16 of the cooling vessel, water or other vaporizable cooling 17 fluid may also be introduced directly into the fluidized bed 18 in a lower part of ~he cooling vessel through line 62. If 19 the gases taken overhead from the cooling vessel are not 20 returned to the gasifier~ means may be provided for the -21 recovery of heat from the gas stream prior to-discharge or 22 use of the gas in other applica~ions.
23 Solids are continuously discharged from cooling 24 vessel 51 through line 61 containing control valve 63. This 25 stream of solids will normally be at a considerably lower 26 temperature than the solid particles in line 50 and thus a `
`27 slide valve, rotary valve, or other conventional device can 28 be employed to control the soli~s withdrawal rate without 29 the difficulties encountered with valves used for the with-30 drawal of solids fr~m a gasifier or similar high temperature - 18 - .

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1 fluidized bed system. The particles discharged through 2 valve 63 may be passed into a solids slurry vessel 64 into 3 which water or other cooling fluid is introduced through 4 line 65 to fonm a slurry of the solid particles. This S slurry is withdrawn from the bottom of vessel 64 through 6 line 66 containing pressure reduction valve 67. Gases are 7 vented from the slurry preparation vessel through line 68.
8 This system is particularly advantageous in processes using 9 an alkali metal or similar gasification catalyst which can be leached from the solid particles and recovered for reuse.

11 Alternatively, however3 vessel 64 may be a lock hopper or

12 the like into wh~ch the solid particles are passed from

13 line 61 downstream of valve 63 and accumulated and from

14 which they are withdrawn periodically. In either case, solids discharge line 61 may be located in a lower part or 16 at the bo~om of the cooling vessel 51 in lieu of at the 17 location shown in Figo lo 18 Fig. 3 in the d~awings illustrates an alternative 19 embodiment of the invention which includes an angle of repose valve ~hrough which the solid particles pass into the cooling 21 vessel. Figo 3 is a fragmentary longitudinal cross-section 22 of the lower end of the gasifier similar to Fig~ 2. A
23 stream of solids entrained in a carrier gas is passed up~
2~ wardly through line 70 into the gasifier 71 above grid or distribution de~ice 720 Steam is introduced through several 26 injection lines such as line 73 and passes upwardly through 27 the grid or similar device to maintain the solid particles 28 i~ the fluidized state. A stream of solid char particles 29 is continuously'withdrawn from the ~uidized bed through side exit nozzle 74 and descending solids withdrawal line ~ 19 -/

1 75. A stream of steam or gas may be introduced upwardly : 2 through line 76 to maintain the solids in the withdrawal 3 line in a fluidized state. Lighter solids of low ash con~nt~
4 which may enter the withdrawal line tend to be carried back - 5 into the gasifier bed by the upwardly flowing gas. The : 6 heavier solids of higher ash content move downwardly in 7 the withdrawal line and form a dense phase stream o~ solids 8 in line 77. The solids in line 77 pass through angle of 9 repose valve 78 before enter~ng cooling vessel 79. The solids flow rate through the horizontal angle of repose 11 valve is controlled by adjusting the flow of fluidization 12 steam or gas through line 80. This use of an angle of rep .~ 13 valve aids in providing for uniform discharge of the solids 14 from the fluidized bed on a continuous basis and thus in some cases pr~motes smoother operation of the overall sys-16 tem. It should be apparent that the ~wo principal modifi-17 cations æhown in Fig. 3, the side exit nozzle 74 and the .~ 18 angle of repose valve 78~ are independent of each other and 19 may be separate~y incorporated in the system of Fig. .2.
` 20 As pointed out above, the rate at which solid 21 particles are withdrawn from the fluidizéd~bed will depend 22 in part on the rate at w.hich ash must be discharged from 23 the system, the properties of the particles making up the 24 fluidized bed, and other factors~ In a typical coal gasifi-cation process using coal having an ash content of about 9 26 weight percent and a coal feed rate of about 400,000 pounds 27 per hour, for example, a solids withdrawal rate of about 28 15~000 pounds per hour will normally be satisfactory~ This ~ 29 will result in the discharge of solids having an average ;~ 30 ash content of aboùt 80 weight percent. Additional ash will ~ .

1~69306 1 be taken overhead in the form of fines in the product gas 2 and flue gas. This permits substantially complete with-3 drawal of the ash fed into the system with the feed coal 4 and thus precludes ash buildup in the gasifier or transfer S line burner. It also aids in alleviating difficulties 6 which are normally encountered due to the formation of ash 7 deposits in the gasifier and burner and results in better 8 carbon utilization and greater heat efficiency than might 9 otherwise be o~tained.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the gasification of carbonaceous solids wherein hot carbonaceous solid particles are withdrawn from a fluidized bed gasifier to prevent the buildup of ash in the system, the improvement which comprises continuously discharging a stream of hot carbonaceous solid particles from said fluidized bed gasifier through a descending solids withdrawal line into a fluidized bed cooling vessel at a rate sufficient to prevent the buildup of substantial quantities of ash in said gasifier and without passing said hot particles through a control valve, injecting a fluidizing gas upwardly into said cooling vessel at a velocity sufficient to maintain said solid particles discharged into said cooling vessel in the fluidized state, continuously withdrawing gas substantially free of solids overhead from said cooling vessel, controlling the pressure within said cooling vessel to regulate the rate at which said hot carbonaceous solids are discharged from said gasifier into said cooling vessel by throttling said gas with-drawn overhead from said cooling vessel, and continuously with-drawing said solid particles from said cooling vessel for discharge from the system through a line containing a control valve at a temperature substantially below that at which said solid particles are discharged into said cooling vessel.

2. A process as defined by claim 1 wherein at least part of said gas withdrawn from said cooling vessel is injected into said gasifier.

3. A process as defined by claim 1 wherein said hot carbonaceous solid particles comprise coal char particles.

4. A process as defined by claim 1 wherein said solid particles withdrawn from said cooling vessel are passed to a slurry preparation vessel in which said particles are slurried in water.

5. A process as defined by claim 1 wherein a gas is introduced upwardly into said descending solids withdrawal line at a rate sufficient to effect dense phase particle segregation.

6. A process as defined by claim 1 wherein a vaporizable liquid coolant is introduced into said cooling vessel to further cool said solid particles.

7. A process as defined by claim 1 wherein said hot carbonaceous solid particles are discharged from said fluidized bed gasifier into said descending solids withdrawal line through a side exit nozzle in said gasifier and passed from said descending solids withdrawal line into said fluidized bed cooling vessel through an angle of repose valve.

8. A process as defined by claim 1 wherein said hot carbonaceous solid particles comprise petroleum coke.

9. A process as defined by claim 1 wherein said stream of hot carbonaceous solid particles includes particles of a gasification catalyst.

10. A process as defined by claim 1 wherein said solid particles are continuously withdrawn from said cooling vessel into a pressurized lock hopper.