Note: Descriptions are shown in the official language in which they were submitted.
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FLUIDIZED BED GASIFICATION ASH
SEPARATION AND REMOVA1
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~).
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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-
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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
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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.
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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
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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
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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
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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
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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
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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.
