Method of increasing the separating efficiency of a cyclone separator and a cyclone for carrying out the method

The separating efficiency of a cyclone separator used for removing solid particles from a gas stream (for example ash particles from the combustion gas which is passed to a gas turbine) is increased by retarding the particles before they arrive at the cyclone and thereafter accelerating them over a short distance before they enter the cyclone. In this way large particles will have a lower speed than small particles when entering the cyclone. Despite a high velocity of the transport gas and a high inlet velocity for small particles, it is possible to obtain an inlet velocity for larger particles which is desirably low from the point of view of reducing erosion of the cyclone separator. The separation of fine particles is improved. The retardation of the particles may take place in a T-shaped branch pipe, which has one branch connected to the cyclone, a second branch connected to a conveying pipe and a third branch which is formed as a blind space.

BACKGROUND OF THE INVENTION 
The invention relates to a method of increasing the separating efficiency 
in a cyclone separator and to a cyclone separator for separating particles 
having varying size. 
The separating efficiency of a cyclone is highly dependent on the inlet 
velocity of the particles entering the cyclone and on the particle size of 
the particles. An increased inlet velocity gives a higher separating 
efficiency. Small particles are more difficult to separate than large 
particles. This is due to the fact that small particles have a low falling 
velocity and are drawn more easily with the gas stream into the vortex in 
the central part of the cyclone separator. 
To increase the separating efficiency, the most obvious thing to do would 
be to increase the inlet velocity of the particles entering the cyclone. 
However in a plant of conventional design this results in: 
1. the pressure drop increasing, and 
2. the erosion rate of the envelope surface of the cyclone increasing. The 
erosion is caused mainly by the larger particles. 
A pressure drop increase can often be accepted, but an increased erosion 
rate with increasing entry speed leads to a drastic reduction of the life 
of the cyclone which is unacceptable for commercial reasons. To keep the 
erosion rate to an acceptable level, a maximum inlet velocity of about 
20-30 m/s is normally used. 
OBJECT OF THE INVENTION 
The object of the invention is to increase the separating efficiency, in a 
plant with a cyclone separator, without the above-mentioned negative 
effects associated with increased gas transport velocity at the inlet of 
the cyclone. 
SUMMARY OF THE INVENTION 
According to the invention, the improvement is brought about by the 
particles in the transport gas flow being slowed down, suitably to a 
standstill, at some distance upstream from the cyclone inlet. After the 
retardation, the particles are accelerated by the transport gas flow. The 
large heavy particles are accelerated more slowly than the small, light 
particles. By locating the retardation region at an appropriate distance 
from the cyclone inlet, a desired "velocity profile" for the particles at 
the inlet may be obtained. What constitutes an appropriate distance 
depends on a number of different factors but is chosen so that the 
particles exceeding a certain size and having the greatest erosion effect 
will have a velocity which does not exceed about 20 m/s. The smallest 
particles are accelerated rapidly, and preferably this will be to almost 
the same velocity as that of the transport gas. The high entry velocity of 
the smaller particles results in an improved separating efficiency for 
those particles while substantially the same separating efficiency is 
obtained for the large particles as would be obtained in a conventional 
cleaning plant. The total separating efficiency is thus improved by the 
method of the invention without any increase in erosion with the resultant 
reduced life of the cyclone that that produces. 
The retardation of the particles may take place in a T-shaped branch pipe, 
where the stem of the T is connected to the cyclone inlet, a second branch 
is connected to a conveying pipe and the third branch is blanked off and 
forms a blind space. In this blind space a "cushion" of particles 
accumulates which forms a pad and prevents direct contact of the particles 
with the wall of the branch pipe in the blanked-off part and thus prevents 
erosion of the T-shaped pipe. 
The invention may, for example, be applied to a pressurized fluidized bed 
combustion plant (a PFBC plant) and gas turbines which are driven with the 
combustion gases from such a plant. In such plants it is most important to 
remove the particles accompanying the combustion gases to prevent erosion 
damage in the gas turbines. When applying the method of the invention to 
known situations either the number of cleaning stages disposed in series 
may be maintained and a higher separating efficiency achieved, or the 
number of cleaning stages disposed in series may be reduced while 
maintaining at least the same degree of gas purification. In the latter 
case not only will a smaller amount of cyclones be required but a smaller 
space for such cyclones will be needed, thus reducing the size of the 
plant. The pressure vessel of the plant may also be able to be made 
smaller. The installation cost will also be considerably reduced. The 
pressure drop caused by the deflection of the gas flow in the T-branches 
can, in practice, be compensated for by the smaller number of cyclones 
required in series.

DESCRIPTION OF PRIOR ART 
In the drawing, the numeral 1 designates a cyclone separator which is 
supplied with gas, mixed with particles, through a conduit 2. The 
particles, for example dust accompanying the combustion gases leaving a 
pressurized fluidized bed in a power plant, have approximately the same 
velocity in the conveying pipe 2 as the transport gas. In the prior art 
arrangement shown in FIG. 1, the conveying pipe 2 opens out tangentially 
directly into the cyclone separator 1, gas and particles will then have 
the same velocity when entering the separator 1. 
In case of a high inlet velocity, particularly coarse particles will cause 
strong erosion within the portion of the cyclone wall marked 3. For 
practical reasons, to give an acceptable working life, the upper limit for 
the inlet velocity of the particles entering the separator normally lies 
between 15 and 20 m/s. At this inlet velocity, the separation is 
unsatisfactory for the smallest particles. 
DESCRIPTION OF PREFERRED EMBODIMENT 
In the embodiment of a cleaning plant according to the invention shown in 
FIG. 2, a T-shaped branch pipe 4 has its stem part 5 connected to the 
inlet of the cyclone separator 1 and the conveying pipe 2 is connected to 
the part 6 of the branch pipe. The part 7 of the branch pipe is sealed off 
by a plate 8 and forms a blind space 9 which will be filled with particles 
which form a "brake cushion" or pad against which the particles in the 
conveying pipe are slowed down. After having slowed down, the particles 
are accelerated as they travel along the branch 5 of the branch pipe. 
Small particles are accelerated rapidly, large particles more slowly. By 
selecting a suitable length x of the branch pipe part 5 in relation to the 
particle load, the particle size distribution, the particle density, the 
pressure, temperature, viscosity etc. of the transport gas, a suitable 
"velocity profile" of the particle mass in the gas flow can be achieved. 
It will be possible to use gas speeds of 50 m/s or thereabove and still 
obtain a velocity of the larger particles which is lower than 15-20 m/s, 
and this is desirable from the point of view of reducing wall erosion of 
the separator. 
The effect of the invention is clearly illustrated in FIG. 3. The velocity 
of the transport gas in the conveying pipe 2 and in the branch pipe is 
indicated by the line 10. The particle velocity at the cyclone inlet is 
indicated by the curve 11 which shows a "velocity profile" of the 
particles. The curve shows that the particle velocity is reduced with 
increased particle size. The shape and position of the curve 11 are 
dependent on the length x of the part 5 of the branch pipe as well as on 
the particle density and shape and gas properties (pressure, temperature, 
viscosity, etc.). At increased length x, the curve is displaced upwards 
and to the right, as shown by the arrow 12. The dotted curve 11a and chain 
line curve 11b, respectively, show the velocity profile for increased and 
decreased lengths x, respectively, of the branch pipe part 5. The dashed 
line 13 represents the normal inlet velocity of gas and particles in a 
conventional cyclone design. As will be clear from the curve 11, the inlet 
velocity of the larger particles lies below the line 13, which is 
desirable from the point of view of erosion and working life of the 
separator. 
A cyclone separator according to the invention is most valuable for the 
separation of bed material or ashes from transport gas in a PFBC plant 
with a bed equipment and ash discharge equipment shown schematically in 
FIG. 4. A PFBC plant of this type is shown and described in U.S. patent 
application Ser. No. 563,427 filed on the 20th Dec. 1983 in the name of 
Roine Brannstrom and reference should be made thereto for further details. 
The cyclone separator 21 in FIG. 4 is positioned at the outlet end of a 
gas-retarding/accelerating device 20 which in turn is connected to a 
source 22 of particle-contaminated gas. Particles separated from the 
separator 21 can be collected in a container 24 and the purified gas led 
on to a gas-utilising device 23 such as a gas turbine. 
When using the method of the invention it is possible to work with high 
transportation speeds for gas entering the device 21, for example 50-60 
m/s. A direct supply of the gas-particle mixture into the cyclone 
separator 21 at this high speed would result in an intolerable erosion and 
a short life of the separator. The invention makes it possible to obtain 
both a tolerable wear of the separator and a high degree of separation of 
fine particles from its outlet stream. 
The cyclone separator 20, 21 according to the invention can also, with a 
good result, be used for cleaning the gas leaving a PFBC fluidized bed 
(e.g. 22) before entering the gas turbine (e.g. 23).