Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for the fluid-
bed gasification of ash-containing carbonaceous solids
with steam to produce synthesis gas.
The world's supply of recoverable gas and petroleum,
at present levels of consumption, cannot last much beyond
the year 2000. Coal reserves, on the other hand, are
relatively abundant. Coal and lignite, for example, contain
55.9 x 10l5 thermal kilowatt-hours of energy and represent
88.8 percent of the energy obtainable from the world's
initial supply of recoverable ~ossil fuels. Even if
present coal production of 3 billion metric tons per
year should double three times, coal supplies would lask
about 300 years. In a modern industrial society such
as the U.S., 95.9% of the energy consumed (19693 comes
from the burning of fossil fuels; 20.0% from coal and
75.8% from gas and oil. Non-fossil fuel sources include
3.8% hydropower and 0.3% nuclear. Although the use of
atomic power is increasing, it is anticipated that
fossil fuels will continue to be the principal source
20 of energy for the remainder of this century. ~ -
In order to relieve the enormous drain on petroleum
resources~ more use of coal is being strongly advocated.
It is the most plentiful of the fossil fuels, and con-
stitutes a prodigious reserve of energy. However~ the
burning of coal results in severe atmospheric pollution ;~
due to the release of sulfur dioxide and particulate -
matter. If it is to contribute significantly as a ~-
world energy source, coal must be substantially freed of ; -
its pollution causing elements.
One method of upgrading coal into a clean energy
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fuel regards coal as a raw material for the manufacture
of synthetic gas and oil. In this approach, finely
divided coal is heated at successively higher tempera-
tures in a series of fluid-bed reactors to give a medium
btu gas (about 500 btu), tar and devolatilized coal
particles or char. On hydrotreatment in a catalytic
reactor, the tar gives a synthetic crude oil which can `
be refined in the usual manner. A detailed description
of the process is disclosed in U. S. Patent 3,375,175
to Eddinger et al. This char constitutes an excellent
grade of carbonaceous solid for fluid-bed steam gasifica-
tion to form synthesis gas containing hydrogen and carbon
monoxide which can be catalytically methanated to give
methane commonly referred to as substitute natural gas
(SNG). Appropriate measures must be taken to remove
atmospheric pollutants such as sulfur dioxide, ammonia
and ash residues. Such upgrading of coal, although
embodying certain known technQlogy, has yet to be
realized on a scale capable of providing oil and gas
in quantities sufficient to ease the acute shortage of
natural petroleum products.
One of the major difficulties in coal conversion
processes, as above described, occurs in the gasification
stage wherein a bed of fluidized char is contacted with
steam to form synthesis gas. The heart of the problem .
is providing enough heat to sustain the gasification
reaction which is highly endothermic.
Desirably, the heat is supplied by burning a portion
of the char and various means of achieving this have been
proposed and tried. For instance, oxygen has been mixed
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with the steam in order to combust some of the char in the
gasifier. ~owever, the use of oxygen is economically
ob~ectionable. Air can be substituted for pure oxygen but
the synthesis gas is then contaminated with nitrogen. In ;
another approach, a recycle stream of char is withdrawn
from the gaisfication bed and partially combusted to raise
its temperature to such a point that on reintroduction to `~
the gasifier along with fresh char, a substantial portion
of the heat of reaction is provided. Such a scheme is
disclosed in U. S. Patent 3,440,117 to Patton et al.
One of the drawbaoks to using recycle char streams for
heating fluid-beds is the attritional generation of fines
which tend to be blown out of the gasifier with the synthesis -
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gas along with the fines formed by breakdown of char -
particles due to reaction with steam. The fines either -
are lost or must be recovered and returned to the system
thereby contributing to increased operation costs. More- ~- -
over, the build-up of fines necessitates reduction in
gas velocity in the reactor resulting in an over-all
lower throughput.
Another proposal for heating a gasification zone is
set forth in U. S. Patent 3,171,369 to Stephens et al.
In this scheme a portion of the char is burned in a com- -
bustion zone at just below the incipient fusion temperature
of the ash formed during combustion. The hot ash particles,
being tacky, adhere to one another and form agglomerates
which are withdrawn from the combustor and introduced
into the gasifier to supply heat to the highly endothermic ~;
steam-carbon reaction. After emerging from the gasifier,
the ash agglomerates can be recycled to the combustion
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zone for reheating. The advantages of the Stephens et al
method over the prior methods of supplying heat to a steam
gasification zone are: (1) it does not contaminate the
synthesis gas with nitrogen as occurs when air is admitted
to the gasifier to burn a portion of the char; (2) it
is less costly than processes that use oxygen in place
of air to avoid nitrogen dilution; (3) it is not sub~ect
to the high carbon losses which occur when a recycle
char stream is heated by contact with hot combustion gases
as disclosed in the previously cited Patton et al patent;
(4) it utilizes a by-product particulate heat exchanger,
namely fused ash, with concomitant savings in raw material
costs; and (5) the circulating fused ash is mechanically
superior to recycle char which undergoes excessive ~ -
attrition thereby engendering the formation of carbon
fines which must be recovered or confined to avoid further `
carbon losses and atmospheric pollution.
Although it constitutes an improved concept in heating
fluid-bed carbon gasifiers, the Stephens et al process
is not commercially feasible. Its chief drawback resides
in failing to provide a practicable means of balancing ^
the heat requirements of the gasifier, on the one hand,
and the combustor, on the other. The need and reasons
for maintaining such heat balance is discussed below.
In the Stephens et al process of fluidiæed carbon
gasification, there are two separable heat requirements
for gasification; one for supplying heat to the endothermic
carbon-steam reaction and the other for agglomerating char
ash to form a recycle heat carrier between the gasifier
and the combustor. In order to maintain the gasifier
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temperature at a given level, either the recycle rate
of heat carrier must be varied or the temperature of
the primary combustor must be varied. Varying the
circulation rate of heat carrier is not practical as
this would require large complex valves and associated
control systems. Varying the temperature of the primary
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combustor is practical, provided the combustor is `
operated below the fusion temperature of the char-ash heat
carrier. However, to control the agglomeration of char-
ash and to produce a desired particle size for use as
heat carrier requires a precise control of temperature ;
and other operating variables. Either the gasifier
temperature will swing undesirably, causing inefficiencies,
or the combustor temperature will vary leading to defluidiza-
tion or to inadequate agglomeration. Stephens et al propose
control of combustor temperature by injection o~ water
but such an expedient is both wasteful and inefficient.
So far as we are aware, the utilization of agglomerated
ash for supplying heat to a steam-carbon gasi~ier has
not been achieved on a practical scale.
In accordance with the present invention there is
provided an improvement in the fluid-bed gasification of ;
ash-containing carbonaceous solids with steam to produce
synthesis gas wherein heat is supplied to the gasification
zone by passing therethrough a stream of agglomerated ash
particles formed by heating ash particles derived from
the carbonaceous solids in a combustion zone at a
temperature sufficient to cause agglomeration of tacky ash
particles; a substantial improvement is reallzed wherein -~
the agglomerated ash particles are formed in a separate
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agglomerating combustion zone and the resulting
agglomerated ash particles introduced into a main
combustion zone where they are maintained below
agglomerating temperatures but at sufficient temperatures
to sustain the gasification zone.
The present process is an improvement in the known
process of producing synthesis gas from solid carbonaceous
materia].s in a fluid-bed. In such present process,
the carbonaceous raw material, of the size such that lt
can be suspended in a gas stream to form a suspended bed
of solids surrounded with gas which acts like a fluid,
is reacted with steam in accordance with the following
equation: C ~ H20 ~ CO + H2. An excellent source of
carbonaceous material is the char produced by the pyrolysis
of finely divided coal in a series of fluid-bed reactors
at successively higher temperatures until all the tar
forming components have been removed from the coal as
described in the previously cited U. S. Patent 3,375,175.
The temperature at which the steam-carbon reaction takes
20 place is generally from about 1600F (871C) to about
1800F (982C), pre~erably about 1600F (871C) to 1700F
(971C) using the char material of the patent.
The steam-carbon reaction is exceedingly endothermic,
requiring about 2700 calories per gram of carbon. Part of
this heat can be supplied from the superheat put into the
steam used in the process, as reactant and as fluidizing gas
for the bed of carbonaceous material, but large amounts
of additional heat are needed.
In accordance with this invention, such heat is
supplied by introducing into the gasification zone, a
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circulating stream of agglomerated ash particles which -~
have been heated to about 1900~ (1038C) by contact with ;;
the hot combustion gases produced by burning a separate
source of char, preferably the char fines which are -
expelled from the gasifier. The agglomerated ash particles,
in passing through the gasification zone, transfer their
heat to the reacting system, from whence they are conveyed `
back to the combustion zone of burnlng fines to be reheated
for another passage through the gasification zone and
so on in a continuous stream between the two stations.
The agglomerated ash particles are produced by burning -
a portion of the char in a separate agglomerating combustion
zone at temperatures sufficient to form partially fused
or tacky ash particles which undergo agglomeration.
~ eferring to the single figure drawing, there is pro- -
vided a gasifier 10 in which a bed 12 of char is maintained
on a grid 14 by a fluidizing stream 16 of superheated
steam. The char is fed into bed 12 via entry port 18. `~
In gasifier 10, the char reacts with steam to form
20 a gaseous mixture consisting mostly of synthesis gas (CO -
and Hz) but also containing some C02 and H2O. These
gases, carrying entrained solids, pass through internal
cyclone 11, and then are exhausted through line 20 to
external cyclone system 22. In the cyclones, the entrained
solids are separated from the synthesis gas stream which
exits from the process through line 24. The larger
solids are returned to the reactor bed by the cyclone - ;
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system, while the finer solids are withdrawn through
line 26 to combustion chamber 28.
The solids fed to combustion chamber 28 comprise
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the finest solids coming from the fluid-bed in gasifier
10, since the synthesis gas stream picks up this fines
fraction selectively. By burning them, fines are prevented -
from building up in the system, thus reducing the load
on the cyclone system, permitting a much smaller capital
investment and less maintenance in this area. At the
same time, selective removal of fines stabilizes the size
consist of the bed solids to a larger size consist, per-
mitting a high throughput of gas without excessive losses
of solids from the bed.
Combustion of the char fines in combustor 28 is effected
with air, preferably preheated to about 400F (204C) and
supplied from line 30. Steam conveys agglomerated ash pel-
lets from the lower portion of gasifier 10 via line 17 to
combustor 28 where the particles are m~aintained at a
temperature of about 1900~ (1038C) to 2100~ (1149C).
The heated particles emerge from the top of combustor 28 and
are propelled upward with combustion gases through lift tube
19 into separator 23 and then drop down through separator
exit tube 27 and into gasifier 10. The heated agglomerated
ash pellets cascade down through the bed of fluidized
char particles imparting heat thereto and exiting from ~:
the bottom of the gasifier and so back to the combustor
28 for a complete cycle.
Combustion gases and attrited ash particles formed ;~
in combustor 28 move up lift tube 19 into separator 23
where they are disengaged from circulating agglomerated
ash pellets and exit from the top of separator 23 via
line 32 into primary cyclone 34. Refractory fines from
cyclone 34 are recycled through line 25 to combustor 28
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while combustion gases and attrited refractory not removed ~ :
by primary cyclone 34 enter secondary cyclone 38 through .
line 36. Fly ash and combustion gases are discharged from
secondary cyclone 38 and the refractory fines are conveyed ~ :
via line 40 to agglomerator combustor 43 which is heated ~ ~ .
to a sufficiently high temperature to effect agglomeration :
of incoming refractory fines to produce agglomerated ash ::
pellets. Heat for agglomerator combustor 43 is provided . :
by burning char fines in cyclone furnace 45. Char fines
10 enter the furnace through conduit 49 which picks up the ~ -
fines at ~uncture 52 of char fines supply line 26. I.ine ..
50 is a source of preheated air ( 400F, 204C) which
supplies combustion air via take-off line 56 to cyclone
furnace 45. Combustion gases from cyclone furnace 45
are conveyed through connecting pipe 59 to neck-portion
61 of agglomerator combustor 43. The heated agglomerated
ash pellets pass out of agglomerator combustor 43 through
the bottom of neck-portlon 61 and are carried to primary `
combustor 28 by make-up refractory line 64. The hot
20 ~flue gases exiting from cyclone 65, located within : .
agglomerator combustor 43, are led via line 67 into line ~ .
30 which supplies preheated air for primary combustor : :
.: .
28. Line 50 divides at junction 70 to form tempering air .
line 54 and preheated air line 30 to combustor 28. Waste
slag from cyclone furnace 45 exits through line 73.
In the description aforesaid cyclone furnace 45 can
be dispensed with and the char fines burned directly in
fluidized bed agglomerator 43. The agglomerated ash
pellets drop to the bottom of the vessel and exit from
30 boot section 61. Although the detailed description herein
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is based on forming and heating the agglomerated ash
pellets in a fluid-bed, a transport heater can be
substituted for fluid heating.
Example
17 ~ 900 lb/min ( 8119 ~ ~4 Kg/min) of make-up char
produced in accordance with U. S. Patent 3 ~ 375 ~ 175 at
1000F (538C)~ containing 15 percent ash or 2710 lb/min
(1229~25 Kg/min), (feed stream 18 of the drawing) is fed
to the fluidized bed 12 of the gasifier 10. The tempera-
ture of the bed of char is 1600F (871s)~ and the
pressure is 6 atmospheric (atm.) absolute (normal range is ~ r
4 to 10 atm. absolute). In the gasifier, the char mass
i3 fluidized by 22~800 lb/min (10342~08 Kg/min) of steam
entering at 1000F (538C)~ The exit gases (stream 24)
contain 18~290 lb/min (8296~ 34 Kg/min) of C0 plus
hydrogen (mole ratio 0. 627 moles C0:1 mole hydrogen) along
with unconverted steam (5986 lb/min, 2715 ~ 25 Kg/min) and
an equilibrium amount of C02 (16~420 lb/min, 7448~11 Kg/min)
as defined by the chemical reaction H20 + C0 = C02 + H2.
The fluidized-bed gasifier provides an efficient `~
means of mixing the steam and the particulate char to effect
the water-gas reaction, and simultaneously to elute the
fine portion of the char from the bed. These fines are
separated from the product gases in a series of cyclones,
the last of which will collect the finest portion of the
char which is preferentially used as fuel for the process.
These fines contain about the same concentration of ash as
the char in the gasifier; about 32% by weight. Thus, when
the char fines are burned completely, all of the ash
introduced with the feed (2710 lb/min, -1229~26 Kg/min) is
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removed from the system either as fly ash with combustion
gases from the primary combustor 28 or as molten slag from ~
the cyclone furnace 45. ~ ;
To supply the necessary heat for the water-gas reaction
in the gasifler, a refractory stream made up of char-ash
agglomerates is recycled from the gasifier 10 to the
primary combustor 28 by line 17 at a rate of 666,667 lb/min
(302,400.15 Kg/min). The recycle refractory stream is
heated to 1900F (1038C) by the combustion of 4950 lb/min
10 (2245.32 Kg/min) of carbon in 7800 lb/min (3538.08 Kg/min)
of char fines fed to the combustor 28 from line 26.
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50,400 lb/min (22861.44 Kg/min) of preheated air from
line 30 and 6667 lb/min (3024.15 Kg/min) of combustion
products from the agglomerator 43 are combined in line
30 at 400F (204C) and used to fluidize the recycle
refractory and to combust the char fines in the primary
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combustor. The heated recycle refractory is carried over-
head by combustion gases in line 19 to the separator 23,
where the refractory is separated and returned to the gasifier ;
by line 27.
During the recycling of the refractory, attrition
causes a breakdown of the particles to fines. In addition,
combustion of the char fines is incomplete in the primary
combustor. A ma~or portion of the refractory fines and
char is removed from the combustion gases by cyclone 34
and returned to the primary combustor. Additional
refractory fines and char-ash fines are separated in
cyclone 389 with a balance of the fine solids exiting
as fly ash at a rate of 2542 lb/min (1153.05 Kg/min) with
the combustion gases. The amount of fines separated by
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cyclone 38 is controlled to equal the amount of makeup
refractory required to of~set the attrition losses, 6667
lb/min (3024.15 Kg/min). These fines are carried by
line 40 into the agglomerator 43, which operates at a '~
temperature of about 2000F (1093C) where the char is
derived from a typical high volatile, bituminous B coal
such as that obtained from the Illinois No. 6 seam. The
agglomerator is heated by the combustion of char fines,
taken as a slipstream from line 26 at ~unction 52,
carried to the cyclone furnace 45 by line 49. Products
of combustion from the cyclone furnace are tempered with
air to control the temperature of the agglomerator.
Burning about 400 lb/min (181.44 Kg/min) of char fines
with 4030 lb/min (1828 Kg/min) or air in the cyclone furnace,
followed by tempering with 2300 lb/min (1043.28 Kg/min) of
air will provide a temperature of 2000F (1093C) in the
agglomerator. The agglomerator is operated to produce a ;~
makeup refractory stream of 1/4- to 3/8-inch (.63 cm to
.93 cm) diameter ash agglomerates. These are conveyed
through line 64 to primary combustor 28 for reheating.
About 174 lb/min (78.93 Kg/min) of ash is removed from
the cyclone furnace as molten slag.
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