Fine solids recycle in a circulating fluidized bed

Details Fine solids recycle in a circulating fluidized bed Fine solids recycle in a circulating fluidized bed

1999-12-17

2001-08-07

Wilson, Gregory (Department: 3749)

Liquid heaters and vaporizers

Miscellaneous

C110S245000, C422S145000, C422S147000

Reexamination Certificate

active

06269778

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of circulating fluidized bed (CFB) reactors, combustors or boilers and, in particular, to a simple system which permits recycling of the finest particles that would otherwise be carried over with the gas flow exiting a separator used in combination with such CFB reactors, combustors or boilers. The invention thus permits enhanced utilization of reagents in such CFB equipment.
2. Description of the Related Art
A necessary condition for efficient utilization of reagents in a CFB reactor, combustor or boiler, such as combustion of fuel and/or sulfation of sorbent in a CFB boiler, is reagent particle circulation in the unit providing sufficient residence time for reactions to complete. This is achieved by solids separation from gases leaving the reactor and recycling these solids back to the unit.
The solids recycle systems may be single-stage or multi-stage. For a CFB boiler, as shown in
FIG. 1
, a single-stage system typically includes a cyclone separator
10
located downstream of the furnace
20
and a solids recirculation loop comprised of a standpipe
30
connected to a lower portion
35
of the cyclone
10
and a non-mechanical device
40
to seal against gas by-passing the separator. For example, the non-mechanical device
40
can be a syphon valve.
A double-stage system may include two cyclones connected in series (not shown in the Figures), each having its own recirculation loop, or an impact-type particle separator
11
, as shown in FIG.
2
. The impact-type particle separator
11
is typically an array of U-shaped beams or similarly shaped elements arranged at the furnace exit
21
. A secondary particle collection device
31
is positioned after the impact-type particle separator
11
(downstream with respect to a flow of gases and entrained particles through the CFB reactor). A common secondary particle collection device
31
is a mechanical dust collector, such as a multicyclone or multiclone dust collector (MDC). In this type of system, the bulk of the solids leaving the furnace
20
are collected and recycled by the primary stage particle separator
11
, while the secondary stage collects and returns most of the fine particles passing through the primary particle separator
11
back to the furnace
20
.
The CFB process could benefit if the above-identified particle separation/collection devices were more effective in collecting fine particles from the flue gases. The effect is that fewer fine particles are recycled prior to leaving the CFB unit, and thus less time is available for reaction of the particles. Although fine particles require less reaction time, the majority of unreacted material exiting the system, such as unburned carbon and unsulfated sorbent in CFB boilers, is concentrated in the finest particles. These fine particles usually have diameters below 50-70 microns.
Fine particles of this size are commonly collected in a baghouse or electrostatic precipitator. U.S. Pat. No. 5,343,830 to Alexander et al. discloses one recycling method which recycles the fine particles collected in the baghouse or electrostatic precipitator back to the reactor. flowever, this method requires installation of a complex solids recycle system.
Any notable improvement of fine particle collection in the cyclone or any other inertial-type separation device presently known using increasing swirling and outlet gas velocity, if possible, will result in a prohibitively high pressure drop and increased parts wear.
Alternatively, a mechanical dust collector can be used to increase fine particle collection, as taught by a Russian publication,
Aerodynamic Calculation of Boiler Units
(
Standard Method
), Edited by S. I. Mochan, 3
rd
Ed., Leningrad, “Energia”, 1977. As shown on page 87 thereof, gas is pulled out from the mechanical dust collector ash hopper and recycled back to the mechanical dust collector inlet using a dedicated fan. The recycle gas stream is cleaned of ash using high efficiency cyclones placed in the recycle loop.
Gas flow pulled from the separator in the same direction as collected solids entrains some the finest particles that otherwise would be carried over with the gas flow leaving the separator, thus, improving collection efficiency for those particles. This method does not cause a gas velocity increase in the collecting elements outlet pipes that normally contributes to a large share of the elements' pressure drop and erosion potential.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve utilization of reagents in a CFB reactor unit with a simple, low-cost method and apparatus. The present invention utilizes a similar concept as that described immediately above not only in the context of a CFB reactor unit but also with a simpler arrangement allowing lower capital and operating costs.
Accordingly, a CFB furnace or reactor unit is provided having enhanced circulation of the finest reagents particles. The improved circulation is achieved by recycling gas having entrained fine particles from a solids collection hopper connected to the solids separator back into the reactor. A system of one or more conduits connects the upper portion of the solids collection hopper with the reactor. The conduit system is equipped with means for recycling gas from the hopper to the reactor.
The invention is an inexpensive system which permits recycling of the finest particles that would otherwise be carried over with the gas flow exiting the separator.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.


REFERENCES:
patent: 4747852 (1988-05-01), Engstrom
patent: 4931260 (1990-06-01), Molerus et al.
patent: 4969930 (1990-11-01), Arpalahti
patent: 5343830 (1994-09-01), Alexander et al.
patent: 5345883 (1994-09-01), Panos
patent: 5463968 (1995-11-01), Abdulally
patent: 5682828 (1997-11-01), Phalen et al.
patent: 6067943 (2000-05-01), Morin et al.
Aerodynamic Calculation of Boiler Units (Standard Method), Third Edition—Edited By S.I. Mochan—copyright page and p. 79 and 87.

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