Arrangement for fine dust separation in a fluidized bed reactor

An arrangement for fine dust separation in a fluidized bed reactor with a clone and dust return conduit. A venturi tube extends from the solids discharge of the cyclone intermediate the cyclone and the dust return conduit, and a gas injector is located at the inlet of the venturi tube. The fine dust which leaves the fluidized bed is separated from the exhaust gas in the cyclone, and the separated dust is conducted from the cyclone, through the venturi tube and the dust return conduit, and into the lower region of the fluidized bed. Gas is conducted through the gas injector to help force the separated dust through the return conduit and into the fluidized bed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a fluidized bed reactor including a 
cyclone for fine dust separation, and a dust conveying conduit to return 
the dust to the fluidized bed. 
During operation of a fluidized bed having changing particle sizes, and 
especially during fluidized bed combustion, for instance, of graphite, 
fine material is carried out of the fluidized bed together with the 
exhaust gas, and the fine material must be separated from the exhaust gas 
and returned into the fluidized bed or further conducted elsewhere (for 
example, bunkered). It is difficult to separate the fine material or dust 
from the exhaust gas when the fine dust possesses poor flow properties. 
2. Discussion of the Prior Art 
As has been known for a considerable period of time, the separation of the 
fine dust material from the exhaust gas and the return of the fine dust 
into the fluidized bed during a graphite combustion could not be achieved 
by means of a system located within the fluidized bed reactor and 
consisting of a cyclone with an attached dust return conduit extending to 
the lower region of the fluidized bed layer. In contrast therewith, for a 
long period external separator systems have been employed in which the 
dust is recycled into the fluidized bed either with the aid of a gate 
valve or an injection system positioned downstream thereof. 
Disadvantages of these systems reside in the large demand for mechanically 
articulated components, the cooling of the dust material prior to re-entry 
into the fluidized bed and, above all, the flow and blockage problems 
encountered in the bunker and the material return system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to improve upon the 
state of the technology with the aid of a system for separating dust from 
exhaust gas from a fluidized bed which, without necessitating any 
extensive technical demands, facilitates producing a surprisingly 
troublefree recycling of the separated dust into the fluidized bed, and 
which can be incorporated without difficulty into the fluidized bed 
reactor. 
The system of the above-mentioned type distinguishes itself from other 
systems for separating and recycling dust from a fluidized bed through a 
venturi tube, which forms an extension of the solids discharge of the 
cyclone, between the cyclone and the dust return conduit, and a gas 
injector at the inlet of the venturi tube. 
In this arrangement, the fine dust which leaves the fluidized bed is 
separated from the exhaust gas by means of a cyclone whose dust outlet, in 
a deviation from the usual arrangement, is not closed, but instead is 
extended by a venturi tube. There is additionally provided a gas injector 
means at the inlet to the venturi tube, which is used to blow the 
material, separated from the exhaust gas in the cyclone, into the lower 
region of the fluidized bed. 
In this way, the gas injector means serves as a gate valve between the 
pressure level in the cyclone and the pressure level in the material 
return conduit, and also propels the driving jet of the poorly flowable 
fine dust through the venturi tube and the material return conduit. 
Accordingly, the operation of the cyclone can be either actuated or 
terminated by switching on and off the driving jet through the venturi 
tube. The cyclone may have a discharge cone leading to the dust outlet of 
the cyclone; and preferably an annular gap is provided at an inlet of the 
discharge cone, and a supplemental gas is conducted through that inlet, 
and along the wall of the cone, to prevent any caking of dust on that 
wall. This arrangement is especially helpful to conduct or discharge from 
the cyclone dusts that have extremely poor flow characteristics.

DETAILED DESCRIPTION 
As illustrated in FIG. 1, fluidized bed reactor 1 includes a fluidized bed 
layer 2 having upper and lower halves 2a and 2b, and which is maintained 
in a suspended state by means of a fluidizing medium 3. At the upper end 
of the fluidized bed reactor, exhaust gas from bed 2, together with 
entrained fine granular material, is tangentially conveyed into the 
cyclone 4, which separates the granular material from the exhaust gas. The 
gas which has been cleansed of the granular material exits the 
installation through 5; and the separated fine granular material is 
propelled by the injector 7 and is conducted through the venturi tube 6 
and material return conduit 8, and thus reconveyed into the fluidized bed 
layer, specifically, into lower half 2b thereof. Material return conduit 8 
extends into, and has a bottom outlet 8a located in, the lower half 2b of 
fluidized bed 2. 
In this way, the gas injection means 7 effectively develops and maintains 
at the inlet of the venturi constriction 6, a fluid pressure greater than 
the fluid pressure at the outlet end of the material return conduit 8. 
This, first, assists conducting the separated granular material from 
cyclone 4, through venturi constriction 6 and the material return conduit 
8, and into lower half 2b of fluidized bed layer 2, and second, prevents 
gases from passing upward through material return conduit 8 and into 
cyclone 4 from lower half 2b of the fluidized bed layer 2. 
The cyclone, the attached venturi tube 6 and the conveying conduit 8 are 
illustrated in more detail in FIG. 2 of the drawings. As can be 
ascertained, the lower end of the cyclone forms an uninterrupted 
transition into the venturi tube 6, and the nozzle 9 of the injector 7 is 
arranged at the inlet of the venturi tube. Through the aid of injector 7, 
the separated fine granular material is conducted into the conveying 
conduit 8. The exhaust gas enters the cyclone through the inlet opening 
10, and the cleansed gas is discharged from the installation through 
outlet 5. Finally, operation of the above-arrangement may be monitored by 
measuring a pressure difference by means of tube 11. 
An embodiment of the invention which has been investigated on a laboratory 
pilot scale, and which is of the type schematically illustrated in FIGS. 1 
and 2, has the following measurements: The cyclone has an internal 
diameter of 65 mm, and is extended by a venturi tube having a diameter of 
8 mm at its narrowest width, to which there is connected a conveying tube 
with a diameter of 20 mm. The nozzle opening has a diameter of 1.5 mm and 
is arranged at a distance of 25 mm from the narrowest cross section of the 
venturi tube. 
Through this pilot arrangement, the fine dust quantity which was discharged 
from the furnace was reduced, with the aid of the invention, from about 5 
kg/hr to less than 200 g/hr, even though the gas velocity in the fluidized 
bed was increased by means of the blown-in drive gas. In essence, the 
exhaust gas quantity consisted of a total of about 20 Nm.sup.3 /h, and the 
gas quantity for the drive jet consisted of about 1.2 Nm.sup.3 /h, and the 
measured pressure difference between tube 11 was 20 mbar. 
A still further improvement could be obtained by employing a multi-stage 
cyclone system, as shown in FIG. 5. 
The gas feed to the injector 7 was conducted through the furnace itself for 
a suitable tempering. As shown in FIG. 4, a heat loop 20 may be arranged 
in the inlet to the gas injector 7 to provide for temperature control. 
FIG. 3 illustrates an annular gap 12 located at the end of the cyclone and 
which extends around the outside of a conical surface of a discharge cone 
of the cyclone. To assist the discharge of dusts having poor flow 
characteristics from the cyclone, gas can be introduced through the 
annular gap 12 so that the conical surface at the cyclone discharge is 
constantly traversed by a gas flow which prevents the adherence of dust 
particles to the conical surface. 
The components shown in FIG. 3 which correspond to those of the arrangement 
of FIG. 2 are identified by the same reference numerals. However, at the 
inlet to the discharge cone of the cyclone there is additionally provided 
the annular gap 12, which is supplied with supplemental gas through an 
inlet connection 13. An annular lip 21 is located adjacent the outlet of 
annular gap 12 to direct gas therefrom against the inside surface of the 
discharge cone. A guide cone 14 is located at the end of the nozzle 9 to 
smooth the flow of the dust grains into the discharge cone, and the guide 
cone is shaped to conform with the flow behavior of the dust through the 
cyclone in this discharge region. Centering pins 15 provide for a precise 
orientation of the guide cone 14 and of the nozzle 9. 
In the illustrated arrangement, the cyclone has an internal diameter of 80 
mm. The diameter at the narrowest location 6 of the venturi tube has a 
dimension of 8 mm, and the nozzle 9 has a diameter of 2.2 mm. The diameter 
of the immersion or material return tube 8 is 32 mm, and that of the gas 
exhaust conduit 5 is 45 mm. The gas is exhausted at a rate of 40 Nm.sup.3 
/hr and gas is introduced through the drive gas conduit 7 at 2.2 Nm.sup.3 
/hr. The annular gap 12 has an inner width of 0.15 mm and was supplied 
with supplemental gas at a rate of 1 Nm.sup.3 /hr. 
By means of the inventive system, which includes the venturi tube at the 
cyclone discharge and an injector nozzle 9 connected ahead or upstream 
thereof, there is achieved a reentrainment of even poorly flowing dust 
into a fluidized bed under differential pressures of up to 1500 to 2000 
mbar. A drive gas flow through injector 7 of from about 5% of the gas 
throughput of the cyclone, is adequate. 
In addition, the quantity of gas introduced into the cyclone via the 
annular gap is from about 2.5% of the gas throughput of the cyclone. 
Advantages of the systems are elimination of mechanically movable 
components, as well as any cooling of the fine dust, and any danger of 
blockage in the material return conduit. Furthermore, the amount of dust 
that would have to be handled outside the fluidized bed reactor can be 
reduced considerably. 
Quite understandably, the inventive dust separating and return arrangement 
does not have to be located inside the fluidized bed reactor, and 
depending upon need, can also be arranged outside the fluidized bed 
reactor. Moreover, it may also be advantageous to provide for additional 
processing of the separated granular material, or means to conduct that 
material into a bunker.