CA2093572A1

CA2093572A1 - Fluidized bed reactor

Info

Publication number: CA2093572A1
Application number: CA002093572A
Authority: CA
Inventors: Rafal Berezowski; Lesley W. Weir; Jerry Doell
Original assignee: DRW INCINERATORS Inc
Current assignee: DRW INCINERATORS Inc
Priority date: 1993-04-07
Filing date: 1993-04-07
Publication date: 1994-10-08

Abstract

ABSTRACT OF THE DISCLOSURE
Common problems in fluidized bed reactors include uneven distribution of particulate material in the bed, poor contact between fuel and fluidized bed particles, and difficulties in separating solids from flue gases. These problems are solved in a fluidized bed reactor including a rectangular cross section, elongated housing containing a large area, lower reactor chamber and a smaller area, freeboard, fluidizing air being fed into the bottom of the reactor chamber via a plurality of nozzles in parallel inlet pipes, the nozzles being inclined slightly outwardly towards opposite sides of the housing; a bottom central solids discharge pipe for recycling a portion of the solids to a fuel/sorbent inlet in one side of the housing above the fluidized bed; a flow divider near the top of the fluidized bed when activated for diverting particles outwardly in a spiral motion towards the sides of the bed; shelves in the freeboard for causing the flue gas to follow a circuitous route through the freeboard, causing the settling of entrained solids; water pipes lining the housing and extending between bottom and top boilers for creating steam;
separators for receiving flue gas to remove solids therefrom for recycle to the fluidized bed; and a heat exchanger for preheating fluidizing/combustion air before introducing such air into the fluidized bed.

Description

~93572 This invention relates to a fluidized bed reactor apparatus.
The present invention was designed specifically to burn fuels which are difficult to burn, and consequently require special combustion techniques, such as long residence times ~in the order of minutes rather than seconds)/ low combustion temperatures and in furnace control of emissions. Typical difficult fuels include municipal ~ ~ -solid waste, low grade coals, peat, lignite and high water -content ~ludges. Such fuels are burned for waste disposal, to produce heat, especially in the form of high temperature steam, or for both reasons. The term fuel as used hereinafter is intended to mean various carbonaceous materials for use in the fluidized bed combustion apparatus.
While the apparatus was designed specifically for burning or combustion, it will be appreciated that a fluidized bed reactor of the type described herein can be used for other purposes such as drying, absorption and adsorption. For the sake of simplicity, the following description is directed to a combustion apparatus only.
The use of bubbling and f luidized bed reactors using carbonaceous fuels to generate steam from heat exchangers in the reactors is well documented in the literature. In general, air is passed through a bed of particulate material including a fuel and possibly a sorbent for absorbing polluting gases such as sulfur and nitrogen oxides, chlorine and/or compounds containing chlorine. The ~93~72 air is used to fluidize the bed and to promote combustion in suspension at a relatively low temperature. Fuel fluidization followed by combustion occurs in a combustion chamber and above the chamber in a so-called freeboard.
Water passes through pipes in heat exchange relationship to the fluidized bed and the freeboard to generate steam.
The reactor is connected to a separator for removing particulate materials entrained in the flue gas as a result of fluidization. The solids are recycled to the fluidized bed or conveyed to disposal. The result is a useful combination of high combustion efficiency, high ~
sulfur and chlorine adsorption, low nitrogen oxides emission ~-and fuel flexibility.
Fluidized bed combustion involves the fluidizing of a bed of particulate material by injecting air into the bed from a windbox located beneath a reactor chamber via a -~
distributor plate and injection devices. Two basic types of fluidized bed reactors have been developed, namely:
(1) The bubbling bed reactor in which fluidizing air upon being introduced through small orifices creates small bubbles, which coalesce to form large gas bubbles, which rise through the bed because of buoyancy forces. The bubbles explode at the surface and splash the bed particles. The fuel is introduced into the bed where it is enveloped by hot bed material, which prompts it to

2~93~72 ' : .
gasify and then burn on contact with the -oxygen in the fluidizing air. The bed has a relatively high bulk or aerated density and a well defined, discrete upper surface.
(2~ The circulating bed reactor in which air velocity conveys some of the basic bed material (sand) and the fuel upwardly through the combustion zone and the freeboard out of the reactor to a cyclone where solids are separate from hot flue gas. The fuel is mixed thoroughly with the particulate bed material and burned in the entire interior of the reactor, including the lower combustion chamber and in the freeboard. The bulk density of the bed is lower than that of the bubbling bed and there is no defined border between the bed surface and the freeboard section of the reactor.
In a bubbling bed reactor, most of the burning occurs in the bed or in its immediate vicinity. Fuel is usually introduced into the combustion chamber from one feeder located at one side of the reactor housing, the fuel migrating to the entire bed. Consequently fuel distribution in the bed is not uniform-fuel concentration being higher in the area close to the fuel inlet and lower at the opposite wall. There is a combustion air deficiency in the area close to the fuel inlet and a significant excess of air at . ' .,.~

2~93~72 locations far from the inlet. Upon becoming entrained in rising gases, unburned combustible particles tend to escape from the system without burning. Some of the disadvantages ~ -of the bubbling bed system include reduced combustion efficiency, relatively high carbon monoxide emissions, and the creation of nitrogen oxides because of the existence of hot spots in the reactor.
The circulating fluidized bed system provides better mixing of the fuel with the particulate bed material, and consequently higher combustion efficiency. However, -~
significantly larger volumes of fluidizing air at high velocities are required, resulting in a variety of problems, including difficult process control, a need for high air output and flue gas handling equipment such as blowers and cyclone separators, a requirement for high temperature flue gas handling equipment, and the erosion of heat exchange tubes and other elements of the system.
A variety of fluidized bed systems having features in common with the present invention are disclosed by Canadian Patent Nos. 809,948, issued to M. Tada on April 8, 1969 and 1,100,817, issued to J.J. Wieminen et al on May 12, 1981, and U.S. Patents Nos. 3,696,793, issued to A. 3ell on October 10, 1972; 4,075,953, issued to N.K. Sowards on February 28, 1978; 4,111,158, issued to L. Reh et al on September 5, 1978, 4,271,126, issued to M. Marschollek et al on June 2, 1981; 4,330,502, issued to F. Engstrom on May 18, ;
1982; 4,363,292, issued to F. Engstrom on December 14, 1982;

2~93572 4,457,289, issued to J. Korenberg on July 3, 1984, 4,770,128, issued to D. K. McDonald on September 13, 1988;
4,898,533, issued to T. Okamoto et al on February 6, 1990, 4,938,170, issued to T. Ohshita et al on July 3, 1990;
5,005,528, issued to M.J. Virr on April 9, 1991; 5,037,617, issued to D. S. Soni on August 6, 1991; 5,039,301, issued to J.V. Allen et al on August 13, 1991; 5,138,982, issued to T.
Ohshita et al on August 18, 1992 and 5,141,708, issued to W.R. Campbell, Jr. et al on August 25, 1992.
The object of the present invention is to improve upon existing systems, particularly in the areas of (i) distribution of particulate material in the fluidized bed, (ii) the contact of fuel and fluidized bed particles, (iii) the separation of solids from flue gases, and (iv) the recycling of solids removed from the flue gas and from the combustion chamber of the apparatus.
Accordingly, in the broadest embodiment, the present invention relates to a fluidized bed reactor comprising housing means defining a lower reaction zone and an upper freeboard zone, the reaction zone containing particulate material defining a bed, said housing means including bottom wall means, and side wall means, one said side wall means having an inclined portion extending between two opposed side wall means at the top of the reaction zone said inclined portion sloping upwardly to the bottom, inlet end of the freeboard zone; a plurality of nozzle means in the bottom of said reaction zone for introducing a gas under ~ ~-~357~
, :
pressure into said bed for reaction and fluidizing of the ~ ;~
particulate material; reactant inlet means for introducing ~:
reactant into the reaction zone above the fluidized bed;
flow divider means extending across the center of said reaction zone; secondary air inlet means in said flow divider means for introducing gas into the middle of the reaction zone; upper boiler drum means at the top of said ,~
housing means; lower boiler drum means at the bottom of said housing means; heat exchange tube means extending substantially the entire length of the interior of said housing means between said upper and lower boiler means;
particulate material outlet means in the bottom center of said housing means for discharging particulate material from ~ .
the housing means for recycle to said reactant inlet means;
and flue gas outlet means at the upper end of said housing means for discharging flue gas from the freeboard zone.
More specifically, the invention relates to a fluidized bed reactor comprising housing means defining a lower combustion zone and an upper freeboard zone, the combustion zone containing particulate material defining a bed, said housing means including bottom wall means, and side wall means, one said side wall means having an inclined portion extending between two opposed side wall means at the top of the combustion zone said inclined portion sloping upwardly to the bottom, inlet end of the freeboard zone; a plurality of nozzle means in the bottom of said combustion~;
zone for introducing air under pressure into said bed for~ ~

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combustion and fluidizing of the particulate material; fuel inlet means for introducing fuel into the combustion zone above the fluidized bed; flow divider means extending across the center of said combustion zone; secondary air inlet means in said flow divider means for introducing air into the middle of the combustion zone; upper boiler drum means at the top of said housing means; lower boiler drum means at the bottom of said housing means; heat exchange tube means extending substantially the entire length of the interior of said housing means between said upper and lower boiler means; particulate material outlet means in the bottom center of said housing means for discharging particulate material from the housing means for recycle to said fuel inlet means; and flue gas outlet means at the upper end of said housing means for discharging flue gas from the freeboard zone.
The invention will be described in greater detail with reference to the accompanying drawings, which illustrate a preferred embodiment of the invention, and wherein~
Figure 1 is a schematic, longitudinal sectional view of a fluidized bed reactor in the form of a combustion apparatus in accordance with the present invention;
Figure 2 is a schematic, side view of the apparatus of Fig. 1 as seen from the left of Fig. 1;
Figure 3 is a cross sectional view of a bottom corner of the apparatus of Figs. 1 and 2;

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~ ` 2093~72 Figure 4 is a schematic side view of a section of air inlet pipe used in the apparatus of Figs. 1 to 3;
Figure 5 is a perspective view of a flow divider used in the apparatus of Figs. 1 to 3;
Figure 6 is an end view of the flow divider of Fig. 5; and Figure 7 is a schematic, block diagram of the controls used to operate the apparatus of Figs. 1 to 3.
Referring to Fig. 1, a fluidized bed combustion apparatus in accordance with the present invention includes ~ -~
an elongated, rectangular cross section housing generally indicated at 1, the lower portion of which defines a combustion chamber or zone 2 containing particular bed material, in this case sand, and the upper portion of which defines a freeboard 3. The housing 1 is rectangular in cross section, the lower portion being substantially larger than the upper portion thereof. Fuel is introduced into the combustion chamber 2 from a fuel bin 4 via a pipe 5, a valve 6, a hopper ~, and a screw conveyor 8. Of course, other feed mechanisms, which prevent flue gas/air exchange between the combustion chamber and the outside can be used. When required, lime or another ground sorbent such as dolomite is introduced into the chamber 2 from a bin 9 via a line 10, a valve 11 and the screw conveyor 8. Combustion of the fuel in the chamber 3 heats water which circulates from lower boiler drums 13 through boiler tubes 14, which line the interior of the housing 1, to upper boiler drum 15. The

3~72 elevated temperature of the hot bed material and of the screw feeder (heated by the heat from the combustion chamber) effects preheating of the fuel and sorbent to start gasification of the fuel and calcination of the sorbent.
The fuel, grit and sorbent mixture falls into the bed of hot particulate material to which air for fluidizing and combustion is supplied via nozzles 17. The air is blown into the nozzles 17 by a pump 18, a pipe 19, valves 20, and inlet pipes 21, which extend across the bottom 22 (Fig. 3) of the housing 1. The spacing between the nozzles 17 is such that the cones 23 of air emanating therefrom overlap a short distance from the pipes 21. The nozzles 17 in the pipe 21 nearest the center of the housing 1 are inclined outwardly by an angle of 5+2 to the vertical, the nozzles 17 in the next rows moving away from the center are inclined at an angle of 10~2 to the vertical and the outermost nozzles are inclined at an angle of 15l2 to the vertical.
As shown schematically in Fig. 2, a motor 24 can be provided at one end of each pipe 21 for rotating the latter, i.e. for changing the inclination of the nozzles 17. Of course, -other modifications (not shown) would be required in the air inlet system. The air flow through the nozzles 17 is such that it lifts the grit, fuel and sorbent particles a short distance above the expanded bed level. ;
A flow divider 25 located at the center of the housing, diverts the gaseous medium and entrained particles so that the particles fall back into the sides of the bed ;

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2~93572 above the inclined lower side walls 26 thereof. At its lowest point, the flow divider is at a distance from the -bottom of the housing approximately equal to four-fifths of the bed height in its dynamic (operating) mode. As best shown in Figs. 5 and 6, the flow divider 25 is generally diamond-shaped in cross section. The rectangular sides 27 of the flow divider slope outwardly in both directions, i.e.
vertically and horizontally from a central peak 28 and from ridges 30 and 31 extending vertically and horizontally, respectively from the central peak 28. Moreover, the surfaces in each of the top and bottom quadrants 32 and 33 of each side 26 thus defined are concave in both directions (vertically and horizontally). Thus, upwardly moving air and particles are diverted outwardly and towards the ends of the flow divider 25, i.e. they are given a spiral or helical movement. The top half of the flow divider is inclined to the vertical along the entire length of the flow divider. A
central passage 35 extends the length of the flow divider for receiving combustion air. Smaller inclined passages 36 extend downwardly and outwardly from the central passage 35 to the sides in the lower quadrants 33. The passages 36 are intended to supply secondary combustion air to the combustion chamber 2 during normal operation, and to supply a combustible gas for heating the bed during start up operations. As illustrated schematically in Fig. 2, a motor 37 can be provided on one side of the housing 1 for rotating 3~72 the flow divider 25 to change the inclination thereof for changing flow patterns in the combustion zone 2.
During normal operations a portion of the solid particles are discharged through an outlet pipe 38 in the bottom center of the housing 1. A valve 39 in the pipe 38 controls the flow of particulate material through a pipe 41 and screens 42 to an elevator 43. The elevator 43 moves the particulate material into the hopper 7 of a screw conveyor 46, which feeds the material into the hopper 47 of the screw conveyor 8. The recirculation system is thermally insulated to conserve the heat content of the sand, and to provide a heat source for preheating fuel and sorbent. The purpose of the solids circulating system is to remove heavier, non~
combustible particles such as stones and tramp metal from the sand and combustible material. ~-~
As mentioned above, the combustion chamber 2 is ~;
rectangular in cross section with two opposed inclined lower side walls 26 so that the chamber 2 tapers downwardly at its bottom end. The nozzles 17 are inclined to direct bed ~
material upwardly and outwardly in the direction of arrows -47. Any air and particulate material above the center of the bed is deflected by the flow divider 25. Because the middle portion of the bed is not fluidized, particulate -material can freely fall into the outlet pipe 38. By regulating the flow through the nozzles in the rows adjacent to the outlet pipe 38, the amount of bed material being recirculated can be controlled. One wall of the casing :.. : - , ' -' . `. ' : ' ' ~ : . : ,: ~ . . :. , - ~0~3~72 includes an inclined portion 48, which deflects particles out of the gas stream for return to the bed.
The flue gas with entrained small particles is funnelled into the freeboard 3. Shelves 50 formed of refractory material such as firebrick extend outwardly from opposite sides of the freeboard 3. The shelves 50 on one side are staygered with respect to the shelves on the other side of the freeboard 3 for diverting the flow of gas, creating a longer flow path and consequently a longer residence time of gases and particulate material in the freeboard area. Moreover, rapid changes in flow direction causes particulate material to settle out of the flue gas.
Flue gases leaving the freeboard 3 pass through a tapering throat 52 in an outlet pipe 53 to first and second stage particulate material gravity separators 54 and 55, respectively. Because of the sudden reduction in gas velocity, particulate material (flyash) falls out of the gas stream. Gas leaving the separator 55 passes through a heat exchanger 56 in which combustion air is preheated. The thus heated air passes through pipe 57 to the pump 18. The cooled gases are cleaned in cyclone separators 58 (one shown) and discharged through an outlet 59 to a bag house installation (not shown). Particulate material is discharged from the separators 54, 55 and 58 via outlet pipes 60 and valves 61, and returned to the hopper 7 of the screw feeder 8 via pipe 63 or discharged for disposal.

- ~093~72 Referring to Fig. 7, the apparatus described above is controlled by a control 65 in the form of central processing unit. The temperature of the water tubes 14 and in the boiler drum 15 is monitored by thermocouples 66, 67 and 69 which are connected to the control 65 by a lines 71, 72 and 73, respectively. The temperature of the air entering the pump 17 from the air preheater 56 via the pipe ~
57 is also monitored by a thermocouple 76. The temperature ~-information is transmitted to the control 65 via line 77.
Gas analyzers 79 are connected to the pipe 53 for determining the quantities of carbon monoxide, hydrochloric acid, combustibles other than Co, sulfur dioxide, nitrogen ~ -oxides and oxygen in the flue gas. The gas analyzers 79 are -connected to the control 65 by lines 80. A level detector 82 is provided in the combustion chamber for determining the -~
level of the top of the fluidized bed. The detector 82 is connected to an indicator 83 which feeds information to the control 65 via line 84.
The information concerning temperatures, fluidized bed level, and flue gas components is fed to the computer control 65. The control is connected to the fuel feed valve 6 and the sorbent feed valve 11 by lines 86 and 87 for varying the rate of feed of the fuel and sorbent. The valves 20 which determine the quantity of air entering the fluidized bed are controlled by separate controls 88, which are connected to the control 65 by a line 89. It will be noted that in the apparatus of Fig. 1 separate valves are ~093572 provided for all pipes 20, while in the apparatus of Fig. 7 one valve controls the flow of air to a pair of pipes 20-one on each side of center. Separate controls 90 and 91 are also provided for controlling the flow of air to the flow divider 25 via pipe 92, and the flow of water to the boiler -tubes 14, respectively. The controls 90 and 91 are connected to the control 65 by lines 94 and 95.
The use of the above described controls permits the operation of the apparatus at varying loads, and at the optimum temperatures for efficient combustion, heat transfer and emission control. . ~:- ;
- . :

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A fluidized bed reactor comprising housing means defining a lower reaction zone and an upper freeboard zone, the reaction zone containing particulate material defining a bed, said housing means including bottom wall means, and side wall means, one said side wall means having an inclined portion extending between two opposed side wall means at the top of the reaction zone said inclined portion sloping upwardly to the bottom, inlet end of the freeboard zone; a plurality of nozzle means in the bottom of said reaction zone for introducing a gas under pressure into said bed for reaction and fluidizing of the particulate material;
reactant inlet means for introducing reactant into the reaction zone above the fluidized bed; flow divider means extending across the center of said reaction zone;
secondary air inlet means in said flow divider means for introducing gas into the middle of the reaction zone; upper boiler drum means at the top of said housing means; lower boiler drum means at the bottom of said housing means; heat exchange tube means extending substantially the entire length of the interior of said housing means between said upper and lower boiler means; particulate material outlet means in the bottom center of said housing means for discharging particulate material from the housing means for recycle to said reactant inlet means; and flue gas outlet means at the upper end of said housing means for discharging flue gas from the freeboard zone.

2. A fluidized bed reactor comprising housing means defining a lower combustion zone and an upper freeboard zone, the combustion zone containing particulate material defining a bed, said housing means including bottom wall means, and side wall means, one said side wall means having an inclined portion extending between two opposed side wall means at the top of the combustion zone said inclined portion sloping upwardly to the bottom, inlet end of the freeboard zone; a plurality of nozzle means in the bottom of said combustion zone for introducing air under pressure into said bed for combustion and fluidizing of the particulate material; fuel inlet means for introducing fuel into the combustion zone above the fluidized bed; flow divider means extending across the center of said combustion zone; secondary air inlet means in said flow divider means for introducing air into the middle of the combustion zone; upper boiler drum means at the top of said housing means; lower boiler drum means at the bottom of said housing means; heat exchange tube means extending substantially the entire length of the interior of said housing means between said upper and lower boiler means;
particulate material outlet means in the bottom center of said housing means for discharging particulate material from the housing means for recycle to said fuel inlet means; and flue gas outlet means at the upper end of said housing means for discharging flue gas from the freeboard zone.

3. A fluidized bed reactor according to claim 2, wherein said nozzle means are arranged in parallel rows on each side of the center of the bottom of said housing means, the nozzle means on one side of center being inclined in one direction away from the center, and the nozzle means on the other side of center being inclined in the opposite direction away from the center.

4. A fluidized bed reactor according to claim 3, including a plurality of parallel inlet pipe means extending across the bottom of said housing means, said nozzle means extending out of said inlet pipe means; and valve means for separately controlling the flow of air under pressure into each said inlet tube means, whereby the flow pattern of the particles in the fluidized bed can be varied.

5. A fluidized bed reactor according to claim 4, including motor means for rotating said inlet pipe means, whereby the inclination of said nozzle means can be changed.

6. A fluidized bed reactor according to claim 4, wherein said flow divider means includes elongated body means extending centrally across the combustion zone above and parallel to said inlet pipe means, said body means having a generally diamond-shaped cross section, the top half thereof being inclined slightly to the vertical, a first longitudinal passage in said body means extending centrally along substantially the entire length of said body means for receiving air under pressure; and a plurality of second passages inclined outwardly and downwardly from said first passage to lower sides of said body means for introducing air into the fluidized bed.

7. A fluidized bed reactor according to claim 6, including burner means in selected of said second passage means for initiating or maintaining combustion in said combustion chamber.

8. A fluidized bed reactor apparatus according to claim 6, wherein the sides of said flow divider body means are rectangular, each side including a central peak with concave ridges extending vertically and horizontally therefrom dividing the side into quadrants, each said quadrant being concave in the vertical and horizontal directions for imparting a spiral flow to gas and particles diverted thereby.

9. A fluidized bed reactor according to claim 6, 7 or 8, including drive means for rotating said body means, whereby the flow pattern of the gas and particles in the combustion zone can be changed.

10. A fluidized bed reactor according to claim 2, including first screw conveyor means for introducing fuel into said combustion zone through said fuel inlet means and for receiving recycled particulate material from said particulate material outlet means; and second screw conveyor means for feeding recycled particulate material from said particulate material outlet means into said first screw conveyor means.

11. A fluidized bed reactor according to claim 2, including separator means for receiving flue gas from said flue gas outlet means and for separating solids therefrom;
and return pipe means for returning the solids to the fuel inlet means.

12. A fluidized bed reactor according to claim 11, wherein said separator means includes first and second gravity separators aligned in the direction of flue gas flow; and at least one cyclone separator downstream of said gravity separators in the direction of flue gas flow.

13. A fluidized bed reactor according to claim 12, including heat exchange means between said gravity and cyclone separators for utilizing flue gas heat to preheat air prior to introduction into said nozzle means.

14. A fluidized bed reactor according to claim 2, including shelf means in said freeboard zone for lengthening the path of travel of gas exiting said combustion zone to promoting the separation of solids therefrom.

15. A fluidized bed reactor according to claim 14, wherein said shelf means includes a plurality of shelves extending outwardly from opposite sides of said freeboard zone, the shelves on one side being staggered with respect to the shelves on the other side thereof.