Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Back~round of Invention
The present invention is in a circulating fluidized bed
system for carrying out exothermic processes. The system
consists of a fluidized bed reactor, a solids separator and
a recycling line wherein oxygen-containing primary gas is
supplie~'~ through lines through the bottom of the fluidized
bed reactor and oxygen-containing secondary gas is supplied
to the reactor at an elevation o at least 1 meter above the
reactor bottom but not in excess of 30% of the height of the
reactor. Fuel is introduced into the fluidized bed reactor
between the primary and secondary gas inlets. The fluidized
bed reactor has a bottom surface area, 40 to 75~ of which is
covered by one or more displacers having a height not in
excess of one-half of that of the reactor.
Processes and equipment for handling circulating
fluidized beds, particularly for combusting carbonaceous
materials, have proved most advantageous and for numerous
reasons are superior to processes and equipment in which
so-called orthodox or conventional fluidized beds are
employed.
; The basic prooess for combustion in a circulating
fluidized bed is describ:ed in U.S. Patent 4,165,717. In
that process the; combustion is in two stages and the heat
released by the combustion is extracted by means of cooling
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surfaces disposed in the fluidized bed reactor.
significant advantage of that process is that the combustion
process can be adapted to the required output in a
technic~lly simple manner by controlling the suspension
density, and hence the hea~ transfer to the cooling
surfaces, especially in the upper portion of the reactor
space.
In the combustion process using a circulating fluidized
bed in accordance with U.S. Patent 4,111,158 at least a
portion of the heat released by combustion is extracted in a
fluidized bed cooler, which succeeds the flui~ized bed
reactor. Cooled solids from the cooler are recycled to the
fluidized bed reactor to maintain or control the reactor
temperature. In that case an adapt~tion to the required
output can be achieved by an increase or decrease of the
rate at which solids flow through the fluidized bed cooler
and then back into the fluidized bed reactor.
While the absve referred to processes have pro~en
highly satisfactory, the existing trend towards plant units
for progressively larger heat outputs results in certain
difficulties in the control of the process because the
higher thermal demands require larger reactor dimensions,
particularly larger reactor cross-sections. In the larger
cross-section uni~s, satisfactory transverse mixing of fuel
and the like, and of oxygen-containing secondary gas,
+) (thermal), i.e. ~ 3 ~ 0808
throughout the cross-section of the fluidized bed reactor is
no longer ensured in the inlet region although such mixing
is required for the reaction~ As a result, a considerahle
part of the reaction can occur in the upper portion of the
reactor space. Also afterburning may occur when the solids
and gas have been separated in the solids separator
following the reactor. This sequence of events is
encountered in plants having combustion heat outputs above
about 300 MW ln fluidized bed reactors having reactor
areas in excess of about 50 m2.
In a prior proposal for solving the above pro~lem in
reactors having a large cross-sectional area 40 to 75~ of
the bottom surface area of the fluidized bed reactor was
covered by one or more displacers, each displacer having a
height not in excess of one-half of the height of the
fluidized bed reactor. The geome~ric configuration of the
displacer was selected subs~antially as desired. For
instance, in a fluidized bed reactor having a circular
cross-section the displacer may have the shape of a cylinder
or of a-frustum of a cone and the center of the bottom
displacer surface may lie ~pproximately on the center of the
bottom surface. In a reactor having a rectangular
cross-section the displacer may have the shape of a dam,
which at its ends may optionaIly adjoin the parallel Wall5
of the reac~or so that the lower portion of the reactor
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space was effectively divided into two separate chambers.
Two dams may be provided, which may be virtually at right
angles to each other and which - if they adjoin the walls of
the reactor - would divide the lower part of the reactor
space into ~our separate chambers.
It has been found that difficulties in the operation of
a fluidized bed plant designed with the above displacer may
arise if the displacer divides the lower part of the reactor
space into separate chambers because in that case the
primary air or fluidized gas stream may entrain bed material
from one chamber so that the internal circulation of solids
always taking place in a fluidized bed reactor wiL-l cause
such entrained bed material to enter another chamber or
other chambers. Unless an expensive control is provided
there will be no reverse or compensating flow of material
because ~he flow of the 1uidizing air through the
"depleted" chamber will be restricted owing to the different
hydrostatic pres~ures over the bottoms of the respective
chambers.
S~m~arv of the Inven~ion
It is an object of the invention to provide a fluidized
bed system which consists of a fluidized bed reactor, a
solids separator and a recycling path and serves to carry
out exothermic processes in a circulating fluid~2ed bed
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which system ensures a satisfactory and reliable operation
without the need for an expensive or elaborate control even
when the plant is opera-ted at a high combustion power.
According to the presen-t invention there is
provided a fluidized bed system for conducting an exothermic
process comprising:
- a fluidized bed reactor, a solids separator and
a recycling means;
- means for supplying oxygen-containing primary
gases through the bottom oE the fluidized bed reactor;
- means for supplying oxygen-containing secondary
gases in an eleva-tion of at leas-t about 1 meter above the
reactor bottom but not in excess of 30% of the height of the
reactor;
- means for introducing fuel into the fluidized
bed reactor between the primary and secondary gas inlets;
- one or more displacer means covering 40 to 75%
of the bottom surface area of the fluidized bed reactor, the
remaining bot-tom surface of the fluidized bed reac-tor
constituting a single coherent surface, said displacer means
having a height not in excess of one-half of the height of
the fluidized bed reac-tor.
In order to provide a single coherent surface the
one or more displacers is so designed that individual
segments oE the reactor bottom communicate with each other.
This can be achieved, e.g., in that a gas-permeable bottom
surface is left between the displacer at its largest
dimension and the reac-tor wall or - if the displacer extends
from wall to wall - the displacer is formed with at least
one through passage, which may be open--topped or tunnal-
like, so that individual bottom segments are joined to each
other. The displacers must not be so designed or positioned
such that separate bottom segments and separate fluidizing
chambers are formed. The fluidizing gas may optionally flow
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at a lower velocity through -the connecting surfaces which
are left by the displacer or displacers or through the base
surfaces whlch define the passage compared to -the velocity
of the fluidizing gas flowing through the main surfaces of
the grate.
The configuration of -the displacer may
substantially be selec-ted as desired. For instance, in a
fluidi2ed bed reactor which is circular in cross-section,
the displacer
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may consis~ of a cylinder or of a frustum of a cone and the
center of the circular bottom surface of the displacer may
lie approximately on the center of the bottom surface of the
reactor. In a reactor having a rec~angular cross-section
the displacer may consist of a dam. Two dams may be
provided which extend substantially at right angles to each
other.
It is particularly desirable to provide displacers
which are square or rectangular in cross-section. Certain
departures from the exact geometric figure, e.g., the
provision of rounded corners, are permissibleO
The displacer may be made from a refractory material
which is conventional in furnace construction.
Alternatively it may be made of membrane walls or finned
walls protected by a covering consisting of ram~ed compound
on the surface or surfaces exposed to the reactor space. A
coolant may pass through the protected walls. The one or
more displacers is firmly affix~d to the reactor and
together with the reactor constitute a structural unit.
The material which is capable of an exothermic reaction
lS charged throu~h a plurality of charging devices so that
individual segments formed in the lower part of the reactor
space can be separately supplied.
In accordance with a preferred feature of the invention
the one or more displacers is provided with a device for
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charging fuel. Fuel is charged on a plurality of levels so
that an effective distribution o the fuel will be ensured.
In accordance with a further preferred feature of the
invention, the displacer is provided with means for
supplying oxygen~containing secondary gas. Such means may
optionally be arranged on a plurality of levels. In
accordance with that feature of the invention the -wY~Y~L~
chamber portions may be supplied with secondary gas through
inlets which are provided in the wall of the fluidized bed
reactor and in the interior of the reactor so that an
optimal admixing of the secondary gas will be ensured.
If secondary gas is supplied through ports provided in
the wall of ~he fluidized bed reactor, the uppermost point
or surface of the displacer should be disposed above such
ports. If secondary gas is supplied on a plurality of
superimposed levels the displacer must extend ahove ths
level of at least the lower st inlet.
In accordance with another desirable embodiment of the
invention, the one or more displacers has a cross-sectional
area which decreases from the bottom of the displacer to its
top. In that case and in conjunction with the last
described embodiment it will be possible to maintain the
velocity of flow in the reactor part providea with the
displacer within certain limits in spite of the supply of
secondary gas.
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The principle of the circulating fluidized bed which is
used in the fluidized bed plant is distinguished in that
states of distrihution having no defined boundary layer are
provided, contrary to the "orthodox" fluidized bed, in which
a dense phase is separated by a distinct density step from
the overlying gas space. In the circulating fluidized bed
there is no density step between a dense phase and overlying
gas space but the solids concentration in the reactor
decreases over substantially the height of the reactor.
By means of the Froude and Archimedes numbers the
following ranges may be defined for the operating
conditions:
0.,1 c 3/~ x ~r2 x ~ ~ lO
a~d
1~ Ar ~ 100
0 - 0 _ --
wherel n 3
Ar ,~
,~ x ~,~ 2
r2 U2
~; x dk
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a~d
u ~ ralative gas velocit;~ m./~
Ar Archimede~ numb~r
Fr 8 Froud~ number
~ ~ . deII~ity of ga~ i~ kg/n~3
3 k 8 den~it;y of ~olid particle il~ kg/m3
d ~3 ~ diam~ter of 3pherical particle irl m
~J ~ kinema~i¢ ~iscosity i~ m2/~
e 8 co~s~ant of grav~tatio~l in mJQ2
The exothermic reaction is carried out at least in two stages
with oxygen-containing gases supplied on different levels.
This techniques provides the advantage that the reaction is
"soft", local overheating is avoided and the formation of
NX i5 substantially suppressed. The uppermost inlet for
oxygen-containinq gas should be sufficiently above the
lowermost one to ensure that the oxygen content of the gas
supplied at the lower inlet is substantially consumed at the
height of the reactor corresponding to the upper inlet.
Under an operating condition when the fluidizing and
secondary ga5eg are supp}ied at predetermined volume rates,
a given average suspension density is obtained and hence, a
certain heat transfer is achieved. The heat ~ransfer to the
cooLing surfaces can be increased by increasing the
suspension density by an increase of the rate of fluidizing
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+) This e~bodiment has been explained in more detail
in U.S. Patent 4 165 717.
gas and optionally o the rate of secondary gas. At a
virtually constant combustion temperature the higher heat
transfer will permit an extraction of the quantities of heat
which are generated at a higher combustion heat output.
Whereas the higher combustion heat output involves a higher
oxygen demand, this wil] virtually automatically be met
hecause a higher fluidizing gas rate and optionally a higher
secondary gas rate is required in order to increase the
suspension density. ~)
In accordance with another suitable feature of the
invention, the fluidized bed system is provided with at
least one fluidized bed cooler, which is connected to the
reactor via a solids supply line and a solids recycle line.
Hot solids are withdrawn from the circulating fluidized bed
and are coolea by direct and indirect heat exchange in a
fluidized state, and at least a partial stream of cooled
solids is recycled to the circulatin~ fluidized bed.
That embodiment has been explained in more detail in
U.S. Patent 4,111,158. In that system the temperature can
be kept constant virtually without a change of the operating
conditions in the fluldized bed reactor, e.g., without a
change of the suspension density and other parameters,
merely by a control of the withdrawal of hot solids and a
controlled recycling of the cooled solids. In dependence on
the output and on the selected reaction temperature the
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recycling will be effected at a higher or lower rate. Any
desired temperature can be adjusted from those very close to
the ignition point to higher temperatures, which may be
limited, e.g., by a softening of the reaction residues. The
temperature may lie between about 450 and 950C.
Heat is extracted in the fluidized bed cooler under
conditions which effect an extxemely high heat transfer,
e.g., in a range from 300 to 500 watts~m2 C.
To control the temperature in the fluidized bed
reactor, at least a partial stream of cooled solids ls
recycled from the fluidized bed cooler. For instance, the
required partial stream of cooled solids may be fed directly
into the fluidized bed reactor. In addition the exhaust gas
may also be cooled by a supply of cooled solids, which may
be fed, e.g., to a pneumatic conveyor or to a suspension
heat exchanger stage, and such solids may subsequently be
separated from the exhaust gas and be recycled to the
fluidized bed cooler~ As a result, the heat of the exhaust
gas will enter the fluidized bed cooler, too.
It is particularly desirable to supply one partial
current of cooled solids directly into the fluidized bed
reactor and to supply another partial stream of cooled
solids indirectly to the fluidized bed reactor after a
cooling of the exhaust gases.
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A recooling of the hot solids from the fluidized
bed reactor should be effected by a countercurrent flow of
the solids to the cooling fluid in a fluidized bed cooler
which has a plurality of cooling chambers which are flown
through in succession and contain interconnected cooling
registers. In -tha-t case the combustion heat can be absorbed
by a relatively small quantity of coolant.
In accordance with another feature of the
fluidized bed system that is provided with a fluidized bed
cooler, the latter is combined with the fluidized bed
reactor in a unit of construction. In that case the
fluidized bed reactor and the f]uidized bed cooler comprise
a common wall, which is suitably cooled and which has an
opening -through which cooled solids can flow in-to the
fluidized bed reactor. As has been mentioned hereinbefore
the fluidized bed cooler may comprise a plurality of cooling
chambers but it may a:Lternatively consis-t of a plurality of
units which are pxovided with cooling surfaces and each of
which has in common with the fluidized bed reactor a wall
having an opening for the Elow of solids and a separate
; solids supply line. Such an arrangement is described in
EP-A-206 066 published on december 30, 1986.
The system utilizing the fluidized bed cooler is
of wide application particularly because almost any desired
heat carrier medium can be heated in the fluidized bed
cooler. From a technological aspect the generation of steam
in various forms and the heating of heat carrier salts are
of special significance.
Within the scope of the invention, air or oxygen-
enriched air or commercially pure oxygen may be used asoxygen-containing gases. The output can be increased by
carrying ou-t the reaction under pressure, e.g., up to 20
bars.
Basically all materials which are capable of a
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self-sus-taining combustion may be used in -the fluidized bed
plant in accordance wi-th -the inven-tion. Examples of such
materials are coals of all kinds, par-ticularly of inferior
quality, such as coal washery refuse, sludge coal, high-sal-t
coal, but also brown coal and oil shale. Additional fields
of applica-tion include the roasting of various sulfide ores
or ore concentrates.
The various fea-tures of novel-ty which characterize
the inven-tion are pointed out with particularity in the
claims annexed -to and forming a part of this specification.
For a be-tter understanding of the inven-tion, its operating
advantages and specific objects obtained by its use,
reference should be had -to the accompanying drawings and
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descriptive matter in which there .is illustrated and
described a preferred embodiment of the invention.
Brief Description of the Drawings
Figure 1 illustrates in a top plan view various
examples of illustrative confiyurations of displacers which
are circular or rectangular in cross-section and may be used
in a fluidized bed reactor;
Figure 2 is a perspective view showing the lower
portion of the fluidized bed reactor provided with a
displacer; and
Figure 3 is a longitudinal sectional view showing the
:~ fluidized bed reactor.
;~
. escription of_Preferred Embodiment
A fluidized bed reactor 1 is diagrammatically indicated
: in Figure 2 and its bottom surface is partly covered by at
least one prismatic displacer 7 so that there are a
: :piuraIity of fluidizing segments 6. :The displa~ers 7 are
provided with secondary gas openings:ll in their ~n*~ upper
portion.
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The fluidized bed reactor 1 shown in Figure 3 has a
cooling surface 2, which is indicated to consist of a
membrane wall~ The lower reactor chamber 8 is divided by a
damlike displacer 7 into four segments, namely, two segments
which are parallel to the dam, onP segment in front of the
dam and one segment behind the dam. An oxygen-containing
fluidizing gas is supplied to the lower reactor chamber 8
through a line 5 and a fluidi2ing grate 6, with fuel through
lines 3 and with oxygen-containing secondary gas through
lines 9. Additional secondary gas is fed through line 10
and the secondary gas openings 11. Additional fuel is fed
through lines 3. The gas-solids suspension exits through
line 4 passing into a separator, such as a cyclone. The
entrained solids in line 4 are separated in the cyclone and
recycled to a lower por~ion of the reactor.
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- Example
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Coal was com~usted with air to produce saturated steam.
; The fluidized bed reactor 1 of the fluidized bed plant
had a base surface area of 12.5 m x 10.1 m and a height of
30.5 m. Its bottom surface was partly covered by a
displacer 7 having a bottom surface area of 8.5 m x 8~1 m in
such a manner that four segments were obtained, which were
provided with fluidizing grates 6. Two segments having a
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width of 2 m each extended parallel to the longer wall of
the reactor. Two segments having a width of 1 m each were
disposed between respective ends of the displacPr 7 and the
reactor wall. The displacer 7 had the shape of a prism
which had a height of 6.8 m. Each segment was in
communication with at least one other segment.
The wall surface of the 1uidized bed reactor 1 was
entirely lined with water-cooled membrane walls. The walls
of the displacer 7 consisted also of water-cooled membrane
walls, which on the side facing the reactor were protected
with refractory material.
Coal at the rate of 110,400 kg/h was supplied to the
fluidiæed bed reactor 1. The coal had a lower heating value
of 15.9 MJ/kg and an average particle diameter of 0.2 mm.
Limestone having approximately the same particle size was
introduced into the reactor at a rate of 10,400 kg/h. The
feeding of the coal and lîmestone was effected through a
total of six lines with the aid of entraining air at 100C
and at a rate of 11,040 sm3/h. The fluidizing gas consisted
of 230,00 sm /h of air at a temperature of 260C which was
introduced into the reactor through the fluidizing grates 6.
The secondary gas lines 9 and 11 were used to supply
additional air at 260C at a total rate of 206,000 sm3/h on
three levels respectively disposed 2 m (51,500 sm3~h), 4.6 m
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(51,500 sm3/h) and 7.3 m ~103,000 sm3/h) above the
fluidizing grate 6.
Under the selected operating conditions a temperature
of 850C was maintained in the fluidized bed reactor 1.
Saturated steam at 140 bars and at a rate o~ corresponding
to a heat output of 102 MW was produced at the cooling
surfaces 2 and on the membrane walls of the displacer 7.
It will be understood that the specification and
; examples are illustrative but not limitative of the p:resent
invention and that other embodiments within the spirit and
scope of the invention will suggest themselves to those
skilled in the art.
~: +) (thermal)
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