Fluidized bed boiler and high temperature separators used therein

A fluidized bed boiler and a high temperature separator used therein. Each separator consists of a horizontal S-shaped passage formed in a casted block and having an inlet for receiving flue gases from the boiler. A divider wall divides a portion of the passage into a dense-phase gas bypass and a vertical-flow ash collecting chamber. As a result flyash of a diameter (50 .mu.m) can be separated, and the combustion cycle efficiency can reach over 98%.

BACKGROUND OF THE INVENTION 
This invention relates to a fluidized bed boiler and a high temperature 
separator used in said boiler and, more particularly, to such a boiler and 
separator in which the separator has a low resistance and achieves a high 
separation efficiency. 
Current state-of-the-art fluidized bed boilers are not equipped with 
high-temperature separators for separating the fine particulate material 
from the gases exiting the boiler. Consequently, a large amount of fine, 
unreacted materials is blown out of the boiler which results in poor 
utilization of fuel and sulfur removal agent. For boilers burning fuels 
with high ash content, this can also cause severe abrasion of the 
downstream heat-transfer surfaces. 
High-speed, or circulating, fluidized bed boilers are equipped with 
high-temperature separators which capture the flyash and reuse it. The 
separators are generally of cyclone type, and the flow resistance through 
the separators at high temperatures is as high as 100-150 mm of water 
column. As a result, the increase of electricity consumption by the draft 
fan is equivalent to 4% of fuel cost. This type of separator is difficult 
to scale up and its efficiency decreases as its capacity increases. 
Additionally, its structure is complicated and its ash and gas discharge 
locations are rather localized, which have adverse effects on ash recycle, 
heat transfer, and the arrangement of convective heat-transfer surfaces. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore an object of the present invention to provide fluidized bed 
boiler equipped with S-shaped separators having low resistance and high 
separation efficiency. 
It is a further object of the present invention to provide a fluidized bed 
boiler of the above type with features that include high combustion 
efficiency, easy-to-build construction, and steady performance when 
scaling up. 
It is a still further object of the present invention to provide a high 
temperature separator for use in a fluidized bed boiler and having the 
aforementioned advantages. 
Towards the fulfillment of these and other objects, the fluidized bed 
boiler of the present invention includes one or more separators having 
S-shaped passages installed at the flue gas exit in the upper section of 
the fluidized bed furnace. Each separator consists of a horizontal-flow 
S-shaped passage formed in a casted block. A bypass for the densephase 
gas, and a vertical-flow ash collecting chamber are formed in the S-shaped 
passage by a flow dividing wall. The separator's modules and the 
supporting wall underneath them constitute one or several sidewalls of the 
furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, the S-shaped separator of the present invention is 
formed by two casted blocks 1 and 2 and a dividing wall 3. A horizontal 
passage is formed through the separator and consists of five sections: an 
entrance section I, a concentrical annular section II, an eccentrical 
annular section III, a turning section IV, and an exit section V. The 
entrance section I is a long vertical slot along the height of the 
furnace, and the flat surface of the block 1 at the entrance section makes 
an angle of 15.degree.-30.degree. with its frontal flat surface. The block 
2 has a cylindrical surface and to assure proper flow direction at the 
entrance, the front edge of the block 2 is located 50-100 mm ahead of the 
front edge of the block 1. The concentrical annular section II joins 
smoothly with the entrance section I. The vertical cross-section area of 
the S-shaped passage is sized to assure a horizontal flow velocity of 
20-25 m/s. The flow dividing wall 3 divides the horizontal passage into 
two portions. The portion that leads the dense gas stream to the ash 
collecting chamber 4 occupies 10%-20% of the total vertical cross-section 
area of the S-shaped passage, and preferably 15%. This flow bypass leads 
to the ash collecting chamber 4 via a smooth transition section. At the 
exit section of the S-shaped passage, there are two types of exit openings 
i.e. a straight-flow exit 5 and a tangential-flow exit 6. When multiple 
separators are assembled together, the exit openings 5 and 6 are 
alternately arranged on different levels along the height of the furnace 
in order to reduce the exit gas velocity as quickly as possible. To 
achieve better flow distribution, the exit openings of two adjacent 
separators are arranged laterally in a symmetric manner. This is shown in 
the arrangement of FIG. 2 in which a symmetric layout of four 
straight-flow exits 5 form a middle row, and a series of four 
tangential-flow exits 6 form the top and bottom rows. 
For a larger capacity boiler, separators can be added either in a vertical 
direction, a lateral direction, or both. A flat partition wall 7 is 
installed between any two levels of separators and flow openings are 
provided through partition walls at the locations corresponding to the 
respective ash collecting chambers to discharge the dense-phase gas 
stream. 
FIG. 3 depicts a pluarlity of the separators of the present invention shown 
mounted in a sidewall 10 of the furnace section of a boiler, with the 
sidewall extending coextensive with, and supported by, a support wall 9. A 
passage 8 is formed through the upper portion of the wall 9 and is 
connected to the chamber 4 for providing for the discharge of flyash from 
the latter chamber. 
In operation, after entering the separator of the present invention the 
flue gas with flyash from the furnace is accelerated to 20-25 m/s and 
rotated 180.degree.-240.degree. in the S-shaped, horizontal passage. A 
large majority of flyash particles in the gas stream are separated and 
forced toward the wall by centrifugal action. The flow dividing wall (3) 
divides the flue gas into two streams. The outer stream, which carries 
most of flyash, is guided into an ash collecting chamber (4) and from this 
chamber, exits through the opening 8 (FIG. 3). The inner gas stream, which 
contains a very small amount of flyash, follows the remaining S-shaped 
passage and leaves the separator through an exit opening 5 or 6 before 
passing to and through the heat recovery sections of the boiler shown in 
the right portion of FIG. 3. 
The S-shaped separator of the present invention can be casted from 
refractory materials. Its structure is simple and is easy to fabricate. 
Also flyash of a median diameter (approximately 50 microns) can be 
separated, and the combustion cycle efficiency (bituminous coal with a 
heating value higher than 4000 Kcal/kg) can reach over 98%. This invention 
has the further advantages of reducing electric power consumption of the 
draft fan and maintaining steady performance in scaling up or down. When 
the separator of the present invention is applied to a industrial boiler 
of 4-35 ton/hr capacity, the boiler efficiency can be improved by 8-15%, 
and the coal consumption can be reduced by 10-20%. For boilers having even 
greater capacity, the utilization rate of the sulfur removal agent can be 
doubled by use of the separator of the present invention. For fluidized 
bed boilers burning high ash content coal, the separator of the present 
invention can reduce the abrasion of the downstream heat-transfer 
surfaces. When the boiler is connected with a staged precipitator, the 
first stage precipitator can be eliminated with the addition of separators 
of the present invention.