CA1185794A - Fluidized bed gasification ash separation and removal - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Method and apparatus for production of a com-bustible product gas from particulate coal in a fluidized bed system. A vertically positioned vessel includes an enlarged upper section and a smaller diameter cylindrical lower separator section, of diameter d, bounded at the bottom by a perforated or porous conical distribution plate of shallow angle with respect to horizontal. Solids are injected upwardly into or at the top of the separator section through tubular inlets terminating a distance ?
above the distributor plate. The ratio ?/d is less than 2.5. A cool fluidizing gas is injected into the separator section through the distributor plate, maintaining fluidization velocity in the separator section of about 1.2 Umf. Char and ash prticles are separated in the separator section, agqlomerated ash being withdrawn from the bottom of the lower separator section and char being recycled to the combustion jet above the top of the tub-ular inlets.

Description

i7~

~ 49,212 FLUIDIZED BED GASIFICATION ASH

GOVERNMEMT CONTRACT CLAUSE
The invention disclosed herein was made or conceived in the course of, or under, a contract with the United States Government identified as No.
EF-77~C-01-1514.
BACKGROUND OF THE INVENTION
E'ield of the Invention:
This invention relates to gasification of car-bonaceouq materials, and more particularly to method and apparatus ~or separation and removal of ash from fluidized bed gasiication reactors.
Description of_the Prior Art:
In reactor~ for the gasification o~ carbonaceous materials, such as coal, a combustible product gas is produced, as well as solid waste products such as agglo-merated ash. In the Process Development Unit (PDU~ fluid-ized bed gasification reactor being operated for the United States Government, particulate coal is injected through the central one of a number of concentric tubes extending upwardly into the center of a vertical bed-containing pressure vessel. The vessel typically includes one or more upper sections of enlarged diameter relative to a lower section of smaller diameter, joined by a sloped transitional region. Fluidization occurs in the upper section~s at a velocity greater than the commonly described minimum fluidization velocity (Um~).
., ~.

S ~

2 . ~9,212 Fluidization and combustion support gases have been injected into the PDU in various manners, including vertically through the concentric tubes, radially from the concentric tubes, and through sparger rings disposed at selected elevations within the vessel. Other gasification reactors discharge a fluidizing gas into vertical vessels through perforated plates positioned near the bottom of - the vessel.
~n the PDU fluidized bed gasification reactor, feed particulated coal, in addition to producing a com-bustible product gas, intermediately forms char, and ultimately forms waste ash. The process takes place at temperatures in the range of 1400F to 1900F, and above.
The ash must be removed from the vessel, preferably con-tinuously or by an on-line batch process, in order to maintain the process efficiently operational. It is desirable to remove only the ash as opposed to the incom-pletely reacted char, in order to maintain a high effici-ency. It is also desirable to remove the ash at a low temperature, less than about 500F, to minimize the impact of heat transfer on downstream components and to decrease heat loss. This can necessitate a long vessel with an elongated lower section through which downward movement o~
the dense ash takes place over an extended period of time, ~5 thus allowing sufficient cooling of the ash prior to removal from the vessel. It is, conversely, desirable to maintain gasification system components, including the containing vessel, at reasonable sizes for fabrication, structural integrity, cost and other purposes. An en-larged vessel also tends to require increased amounts offluidization and combustion gases, thus detracting from system efficiency.
The fluidized bed gasification Process Develop-ment Unit has been successfully operated at a coal throughput of approximately 15 tons per hour for air blown operation and 35 tons per hour for oxygen blown operation.
The unit has a single set of vertically positioned concen-

3 49,212 tric injection tubes through which, in addition to parti-culate coal, various process mediums, such as recycled product gas, steam and oxygen, are injected. Additional fluidizing gas is injected through a sparger ring of circular cross section, concentrically disposed within the lower region of the gasifier vessel. In order to provide similar gasification systems of enlarged throughput, larger vessels will be required. It is also contemplated that larger concentric tubes, or multiple sets of tubes will be rèquired. With such enlarged e~lipment, and particularly with multiple sets of concentric injection tubes, it has now been recognized that a single sparger will likely be insufficient to provide a relatively ba-lanced distribution of fluidizing gases across the vessel cross section. This has not been previously recognized in the art. An imbalanced distribution can lead to channell-ing of the fluidizing gas flowing upwardly through the particulate matter. This can cause local slugging, exces-sive mixing, and local stagnation, as opposed to separa-tion, of the char and ash particles.
It is thus desirable to provide gasifiers cap-able of large throughput. Preferably such gasifiers should provide essentially complete gasification and combustion of coal or other carbonaceous feed material, 2S and should provide the capability to discharge product ash on-line and at a temperature below approximately 500F.
Such gasification reactors should be comprised of reason-ably sized components, and not require excessive quanti-ties of feed gases.
SUMMARY OF THE INVENTION
This invention provides gasification processes and apparatus capable of large throughput of feed parti-culate carbonaceous material, such as coal.
In preferred form, a fluidized bed reactor operating at combustion temperatures in the range of 1800E to 2000F, includes a vertically disposed vessel having an upper section of enlarged inside diameter and a

4 - 49,212 lower section of smaller inside diameter (d). The lower section operates at a fluidization v~locity of about 1.2 Umf. The lower or separation section is bounded at the lower end by a conical distribution plate, preferably ~n inverted conical distribution plate. The angle of the co~ical surfaces of the plate is, with respect to hori-zontal, greater than 7~ and less than 30, and preferably between 7 and 15~ The plate is perforated, and a fluid-ization gas is passed upwardly through the perforations and into the separation section. The combination of a flowing gas and the slope of the plate provide a small equivalent angle of repos~ such that ash particles can readily be removed from the plate through an enlarged opening at the center of the plate or through discharge outlets at the side of the plate.
Extending upwardly from the plate are multiple sets of solid and gaseous injection tubes. Each set includes several concentric tubes, as well known, for injection of particulate coal, gasification, combustion 2~ and fluidization gases. The solid injection tubes, extend upwardly into the separation section length (Q), and can extend to an elevation at the top of the separation sec-tion. By maintaining the ratio Q/d in this structure at less than about 2.5, sufficient and evenly distributed fluidization gas flows through the perforations and up-wardly through the separation section in a manner which facilitates separation o the char and ash and which avoids imbalanced pressure drop and associated channelling of the fluidizing gas within the separator section. The system avoids slugging operation in the separator section and associated intermixing of the char and ash which defeats the separation function, and can successfully operate with slugging occurring above the separator sec-tion. The system also provides sufficient residence time of ash particles within the separation section and suffi-cient ~heat transfer between the particles and the fluidiz-ing gas to allow cooling of the ash particles to tempera-tures below about 500F prior to removal from the reactor.
': ,;

i7~
49,212 BRIEE DESCRIPTION OE THE ~RAWINGS
The advantages, nature and additional features o the invention will become more apparent from the fol-lowing description, taken in connection with the accom-panying drawing, in which:
Figure 1 is an elevation view, in section, of a vessel for containment of a fluidized bed gasification process;
Figure 2 is a cut-away perspective view of the lower section of the containing vessel;
Figures 3 and 4 are simplified elevation views, in section~ of the vessel and internal components for alternate embodiments;
Figure 5 is a plan view of a conical distribu-tion plate; and Figure 6 is a graphical representation o test data, plotting solid separation rate (kg/min-m2, X-axis) versus feed rate (kg/min-m2, Y-axis).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
, Referring now to Figure 1 there is shown a vertically oriented pressure vessel 10 for containing a fluidized bed gasification process. Process mediums, such as air or oxygen, recycle gas, steam, and particulate carbonaceous material, such as coal, are fed through concentric tubular inlets 12 into the vessel 10. The vessel includes several sections including one or more upper sections 14 of large inside diameter, and a lower separator section 16 of small inside diameter, d, relative to the upper sections. The vessel lO can also include one or more transitional regions along ths inside and outside diameters of the vessel.
The process mediums, upon injection, form a gasifying and combusting pressuriæed fluidized bed within the vessel 10, generally above the top of the tubular inlets 12, where coal particles are intermediately devola-tilized to char and a product gas, and ultimately the char is gasified and combusted to ash. The combustible product ~ t7~ ~
6 - 49,212 gas from the reaction is discharged through an outlet 18, and agglomerated ash is discharged near the bottom of the vessel. The process operates at a temperature in the range of 1400F to 2000F, and above.
One, or preferably a plurality of sets of con-centric tubes, extends upwardly from a distributor plate 20 to an elevation at or below the top of the lower sep-arator section 16, as shown in Figures 2 through 4. The distributor plate 20 forms the bo~tom of the separator 16.
The distributor plate 20 is perforated, including perfor-ations 22 for passage of a fluidizing gas. The distri-butor plate also includes openings 24 through which pass one or more of the concentric tubular inlets 12. Each of the tubes of the inlets 12 can penetrate the distributor plate 20, as shown in Figure 2 or, as shown in Figure 3, some o the tubular inlets can enter the lower separator section 16 laterally.
The distributor plate 22 is generally conical, and preferably is oriented as an inverted cone as shown in Figure 3. It can also be oriented as an upright cone, as shown in Figure 4. With the inverted conical arrangement, product ash particles are withdrawn from the lower separ-ator section 16 through one or more enlarged openings 26 at the lower elevation of the plate and an outlet conduit 28. The conduit 28 can include well known valve appara-tus. With an upright conical arrangement ash particl~s are withdrawn from the lower elevation of the plate 20, at the outer periphery, preferably through several outlets 30 about the periphery.
The perforations 22 are sized and configured to allow upward passage therethrough of a fluidizing gas, such as recycled product gas, which enters a plenum 32 below the plate 20 through an inlet 34, and to restrict downward passage of the ash particles through the perfor-3S ations. The fluidizing gas is injected at a fluidization veloci~ty of about 1.2 Umf~ such that the lower separator section 16 operates under the mechanism commonly referred ' `;

7~
7 - 49,212 to as the minimum fluidization mechanism. Preferably the perforations 22, as shown in Figure 5, are circular of a 0.125 inch to l.0 inch diameter. A metallic mesh can also be placed over the perforations. Alternatively, a sinter-ed porous distribution plate of metal or ceramic materialscan.be utilized, for example, a plate comprised of sinter-ed porous stainless steel or brass.
The ash withdrawal opening 26, if located at a position where no tubular inlets 12 penetrate the plate, is circular of a diameter of approximately six to eight inches. If the opening 26 is disposed about a concentric tube 12, the width of the annulus formed between the outer tube 12 and the ash withdrawal opening 26 is approximately six to eight inches.
The angle ~, the slope of the top of the dis-tribution plate relative to horizontal, is between 7~ and 30~, and preferably between 7 and 15. It is desirable to minimize the angle in order to alleviate an uneven pressure differential across the interior volume of the lower separator section 16. A small angle minimizes the static head pressure drop differential among the uppermost and lowermost perforations. A differential causes a preferent1al flow path, or channelling through the lower pressure volume of the separator and a fluidizing maldis-tribution, and poor separation of char and ash results.It is also desirable to increase the angle to assist in removal of ash particles on the distributor plate. Below a slope of 7, even with fluid flowing through the perfor-ations 22, ash particles tend to accumulate in position on the distributor plate and not migrate toward the outlet opening.
The lower separator section 16 is the region in which char and ash are separated, the char being upwardly recycled for further combustion, and the ash migrating downward for ultimate removal. A basic mechanism contrib-uting to the separation is the ability of a rising bubble to carry a relatively low-density char particle upwardly 75~
8 49,212 in its wake, as opposed to a higher density ash particle.
A mechanism which can defeat separation is slugging within the separator section. Slugging, the formation of a large gaseous bubble across the entire cross section of the separator 16, tends to push both ash and char particles upwardly. A slugging regime can, however, be avoided by maintaining the ratio ~/d at less than about 2.5, where is the distance between the distributor plate and the point within or near the top of the separator section at which raw carbonaceous material, such as coal, is dis-charged from a feed tube 12 into the free interior of the vessel 10, and where d is the internal diameter of the lower separator section.
The diameter d is preferably the minimum dia-meter which will prevent slugging in ~he separator sec-tion, while maintaining an appropriate Q/d ratio, and will provide enough cross-sectional area for the char-ash sepàration. Additionally, in order to avoid a net accumu-lation of ash particles, the internal diameter of the ~eparator section 16 is large enough to assure that the rate of char-ash separation is larger than the rate of ash agglomeration. Figure 6 graphically presents the results of experimentation on a system utilizing dolomite, simu-lating ash particles, and char fed into a fluidizing zone similar to the lower separator section 16. As shown, the maximum separation rate for the system, normalized on a cross-sectional area basis, is approximately 750 Kg/min-m2, regardless of the feed rate. The separation rate is heavily dependent upon the relative concentration of the two feed materials and their relative density and size ratios. The test unit operated at a pressure of 15.5 kPa and ambient temperature. The injection rates were Vl=0.24 m/sec, V2-25.3 m/sec, and V3=0.27 m/sec, where Vl, V2 and V3 are injected as shown in the sketch of the test apparatus in the upper left corner of Figure 6, and corre-spond ~respectively to (Vl) the fluidizing gas injection through the distributor plate 20, (V2) the injection of 9 49,212 solids in a transport gas through the tubular inlets 12, and (V3) a gaseous injection through a truncated conical grid above the lower separator section 16.
In addition to limiting slugging, the separator section 16 preferably is of sufficient height to provide cooling of the ash particles, prior to removal from the vessel 10, to a desirable temperature, preferably less than 500F.
The top of the concentric tubular inlets 12 is disposed at the top of or within the lower separator section 16. It is desirable to minimize the length Q to shorten the overall height of the vessel 10 and to improve solids recirculation for gasification and combustion, since particle velocity is greater in the smaller, lower section 16 cross-sectional area. Where the top of the tubular inlets 12 is within the separator section, suffi-cient penetration of the combustion jet into the section of the vessel 10 ahove the separator section 16 exists to ensure that either the combustion jet penetrates into the gaslfication region above the separator section or the bubble size generated above the combustion jet within the separator section is smaller than the char~ash separator section 16 inside diameter, so as to avoid slugging in the separator section. Increased bubble size above the separ-ator section is acceptable.
An exemplary vessel 10 including a char-ash separator section 16 approximately three feet in diameter and five feet high, with concentric tubes extending be-tween three and five feet upwardly into the separator section meets the described relationships. Table I iden-tifies other system parameters:
Ash removal rate 5000 lbs/hr Superficial gas velocity in separator section 1.5 - 2.5 ft/sec ~ Mean ash particle SizP 1650 microns 7~4 49,212 Ash particle density 100 lbs/ft3 Gasification bed temperature 1800~-1950F
Ash discharge temperature 500F
Air jet velocity from tubular inlets 60 - 120 ft/sec Diameter of outer tubular inlet 16"
Separator bed voidage 0.48 Inverted distributor cone angle 15 Perforation diameter 0.125"
Number of perforations 194 For the exemplary system the combustion jet penetration is up to five feet and the maximum bubble size i~ 3.3 feet at an elevation within the gasifier bed.
However, because of the elongated jet penetration the separator section diameter of three feet is sufficient to alleviate slugging interference ~ith khe separation func-tion since large bubbles appear only in the gasifiersection.
A fluidized bed gasification system operating With a lower separation section of the minimum fluidiza-tion type, as disclosed, will provide efficient separation of char and ash, alleviate slugging in the separator section, and provide separation of char and ash at a rate compatible with the ash agglomeration rate. Additionally, ash particles will have sufficient residence time in khe separator section in contact with a cool gas to assure discharge from the containing vessel at acceptable temper-atures.

Claims (5)

We claim:

1. A method of operating a gasification reactor of the fluidized-bed type which operates within a vert-ically disposed vessel having an enlarged upper section of large inside diameter and a lower section of a smaller inside diameter, said method intermediately forming char and ultimately forming ash and a combustible product gas, said method comprising:
injecting particulate carbonaceous material upwardly into said vessel at an elevation below the top of said lower section;
injecting gasification and combustion support gas into said lower section such that a combustion jet extends upwardly above said lower section and into said enlarged upper section;
injecting fluidizing gas upwardly into said lower section at a rate so as to maintain a fluidization velocity in said lower section of about 1.2 Umf; and removing ash from the bottom of said lower section.

2. A coal gasification reactor of the fluid-ized-bed type comprising:
a vertically disposed vessel, said vessel having an upper section of large inside diameter and a lower separator section of a smaller inside diameter d;
a conical perforated distribution plate disposed at the bottom of said separator section, the angle of said plate relative to horizontal being greater than 7° and less than 15°;

means for injecting a fluidizing gas upwardly through said perforated distribution plate and into said separator section so as to maintain a minimum fluidization velocity of about 1.2 Umf in said lower separator section;
a plurality of solids injection tubes extending vertically upward from said distributor plate a length ?, the ratio ?/d being less than about 2.5; and means for removing solids from atop said dis-tributor plate.

3. The reactor of claim 2 wherein said dis-tributor plate is comprised of sintered porous stainless steel.

4. The reactor of claim 2 wherein said distri-butor plate is comprised of sintered porous brass.

5. The reactor of claim 2 wherein said solids injection tubes extend upwardly from said distributor plate and terminate within said separator section.