Fluidized bed steam temperature enhancement system

A reactor in which a furnace and a heat recovery area are provided. A bed of solid particulate material including fuel is supported in the furnace and air is introduced into the bed at a velocity sufficient to fluidize same and support the combustion or gasification of the fuel. The products of combustion (or flue gases) pass upwardly through the furnace and transfer heat energy to the walls thereof to produce steam. Flue gases leaving the upper region of the furnace section are transported to a heat recovery area, which functions to remove additional heat energy from the flue gases for producing the steam. A flue gas by-pass system is provided which transports relatively hot flue gases from a lower region of the furnace section to the heat recovery area for improving isothermal operating conditions and optimizing reactor performance. One or more conduits pass flue gases directly from selected extraction points within the lower region of the furnace to an upper portion of the heat recovery area. A dust collector may be connected to the gas extraction conduits for separating particulate material from the flue gases, if needed.

BACKGROUND OF THE INVENTION 
This invention relates to a fluidized bed reactor and a method of operating 
same and, more particularly, to such a reactor and method in which a flue 
gas by-pass system is provided for channeling a portion of flue gases to a 
heat recovery area. 
Fluidized bed reactors, such as qualifiers, steam generators, combustors, 
and the like are well known. In these arrangements, air is passed through 
a bed of particulate material, including a fossil fuel such as coal and an 
absorbent for the sulfur generated as a result of combustion of the coal, 
to fluidize the bed and promote the combustion of the fuel at a relatively 
low temperature. The entrained particulate solids are separated externally 
of the bed and recycled back into the bed. The heat produced by the 
fluidized bed is utilized in various applications such as the generation 
of steam, which results in an attractive combination of high heat release, 
high sulfur absorbtion, low nitrogen oxides, emissions and fuel 
flexibility. 
The most typical fluidized bed reactor is commonly referred to as a 
"bubbling" fluidized bed in which the bed of particulate material has a 
relatively high density and a well defined, or discrete, upper surface. 
Other types of fluidized bed reactors utilize a "circulating" fluidized bed 
in which the fluidized bed density is well below that of a typical 
bubbling fluidized bed, the air velocity is greater than that of a 
bubbling bed and the flue gases passing through the bed entrain a 
substantial amount of particulate solids and are substantially saturated 
therewith. 
Also, circulating fluidized beds are characterized by relatively high 
solids recycling which makes them insensitive to fuel heat release 
patterns, thus minimizing temperature variations, and therefore 
stabilizing the emissions at a low level. The high solids recycling 
improves the efficiency of the mechanical device used to separate the gas 
from the solids for solids recycle, and the resulting increase in sulfur 
absorbent and fuel residence times reduces the absorbent fuel consumption. 
However, several problems do exist in connection with these types of 
fluidized bed reactors, and more particularly, those of the circulating 
type. For example, a circulating fluidized bed reactor typically must be 
designed to function at near isothermal conditions within a fairly precise 
and narrow range of temperatures for maximum sulfur capture and solids 
stabilization. When operating at a relatively low load, it is very 
difficult to maintain these temperature conditions since the flue gas 
temperature leaving the furnace section and entering the heat recovery 
area tends to drop significantly. The furnace exit flue gases become 
cooled to the point where the efficiency of the downstream convection heat 
exchange surfaces suffer and thus more elaborate or extra surfaces are 
required. A superheater so modified, in addition to requiring larger and 
more expensive superheat and/or reheat surfacing, also produces 
undesirably large attemperation requirements at full load. In order to 
maintain acceptable temperatures during operation of these modified 
superheaters, recycle solid stream temperature and flow control 
techniques, variable external heat exchangers and other expensive means of 
temperature control have also been employed. However, the addition of 
these components also adds to the cost and complexity of the system. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a fluidized 
bed reactor and method for controlling same which overcomes the 
aforementioned disadvantages of previous techniques. 
It is a further object of the present invention to provide a reactor and 
method of the above type which provides higher flue gas temperatures to 
the heat recovery area, especially at low loads. 
It is a still further object of the present invention to provide a reactor 
and method of the above type in which unusually large superheater 
surfacing and/or otherwise expensive means of temperature control normally 
required at low loads is eliminated. 
It is a still further object of the present invention to provide a reactor 
and method of the above type in which the efficiency of the heat exchange 
surfaces is increased. 
It is a still further object of the present invention to provide a reactor 
and method of the above type in which optimum system temperatures are 
achieved. 
Toward the fulfillment of these and other objects, the fluidized bed 
reactor of the present invention includes a flue gas by-pass system 
operative between a furnace section and a heat recovery area of the 
reactor. One or more conduits channel a portion of the flue gases from a 
lower region of the furnace section above a dense bed directly to the heat 
recovery area of the reactor. The comparatively hot flue gases passing 
through the one or more conduits and received within the heat recovery 
area enhance the steam/reheat temperatures, especially at low loads.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring specifically to the drawing, the reference numeral 2 refers, in 
general, to a fluidized bed reactor which includes a furnace section 4, a 
separating section 6, a heat recovery area 8 and a flue by-pass assembly 
10. The furnace section 4 includes an upright enclosure 12 and an air 
plenum 12a disposed at the lower end portion of the enclosure for 
receiving air from an external source. An air distributor 14 is provided 
at the interface between the lower end of the enclosure 12 and the air 
plenum 12a for allowing the pressurized air from the plenum to pass 
upwardly through the enclosure 12. A dense bed 15 of particulate material 
is supported on the air distributor 14, one or more inlets 16 are provided 
through a front wall of the enclosure 12 for introducing a particulate 
material onto the bed, and a drain pipe 17 resisters with an opening in 
the air distributor 14 for discharging spent particulate material from the 
bed 15. The particulate material can include coal and relatively fine 
particles of an adsorbent material, such as limestone, for adsorbing the 
sulfur generated during the combustion of the coal, in a known manner. The 
air from the plenum 12a fluidizes the particulate material in the bed 15. 
It is understood that the walls of the enclosure 12 include a plurality of 
water tubes (not shown) disposed in a vertically extending relationship 
and that flow circuitry (also not shown) is provided to pass water through 
the tubes to convert the water to steam. Since these types of walls and 
flow circuitry are conventional, they will not be described in any further 
detail. 
The separating section 6 includes one or more cyclone separator 18 provided 
adjacent the enclosure 10 and connected thereto by ducts 20 which extend 
from openings formed in the upper portion of the rear wall of the 
enclosure 12 to inlet openings formed in the upper portion of the 
separators 18. The separators 18 receive the flue gases and entrained 
particulate material from the fluidized bed 15 in the enclosure 12 and 
operate in a conventional manner to disengage the particulate material 
from the flue gases due to the centrifugal forces created in the 
separator. The separated flue gases pass, via one or more ducts 22, into 
and through the heat recovery area 8 under the control of a sliding gate 
valve, or damper 23, associated with the duct 22. 
The heat recovery area 8 includes an enclosure 24 housing a superheater 26, 
a reheater 28 and an economizer 30, all of which are formed by a plurality 
of heat exchange tubes (not shown) extending in the path of the gases that 
pass through the enclosure 24. The superheater 26, the reheater 28 and the 
economizer 30 all are connected to the aforementioned fluid flow circuitry 
(also not shown) extending from the tubes forming the walls of the furnace 
section 12 to receive heated water or vapor for further heating. After 
passing through the superheater 26, the reheater 28 and the economizer 30, 
the gases exit the enclosure 24 through an outlet 32 formed in the rear 
wall thereof. 
The separated solids from the separator 18 pass into a hopper 18a connected 
to the lower end of the separator and then into a dipleg 33 connected to 
the outlet of the hopper. The dipleg 33 extends into a relatively small 
fluidized seal pot 34 having a discharge conduit 36 extending into the 
lower portion of the furnace section 4 for reasons to be described later. 
The flue by-pass assembly 10 of the present invention includes two gas 
extraction conduits 38a, 38b, a dust collector 40 and a gas introduction 
conduit 42. The gas extraction conduits 38a, 38b extend through the rear 
wall of the enclosure 12 and communicate with the lower region of the 
furnace section 4. It is understood that the conduits 38a and 38b may 
optionally extend further into the furnace section 4 to an area generally 
above the dense bed 15. 
The dust collector 40 is located in the lower portion of the enclosure 24 
and the conduits 38a and 38b extend through a wall of the enclosure 24 in 
registery with the dust collector 40. Thus a portion of the furnace gases 
enter the conduits 38a and 38b, pass through the conduits and are 
discharged into the dust collector 40. It is understood that each of the 
conduits 38a and 38b may include grillwork or other means (not shown) for 
filtering or otherwise controlling the passage of material through the 
assembly 10. Suitable dampers 46a, 46b are also included within gas 
extraction conduits 38a, 38b, respectively, to control and/or prevent the 
passage of furnace flue gases through the flue by-pass assembly 10. 
The dust collector 40 may include one or more separators (not shown) which 
receive the flue gases and entrained particulate material from the furnace 
section 4 through the conduits 38a, 38b and operates in a conventional 
manner to disengage the particulate material from the flue gases. The 
separated particulate material passes into a hopper 40a connected to the 
lower end of the dust collector 40 and then into a dipleg 48 connected to 
the outlet of the hopper. The dipleg 48 is connected to an injector 
conduit 50 having a branch conduit 50a. The conduits 50 and 50a extend 
through the rear wall of the enclosure 12 and to the conduit 36, 
respectively, for introducing the material into the bed 15 and the 
discharge conduit 36, respectively. The separated flue gases pass upwardly 
through the dust collector 40 and into the gas introduction conduit 42. 
The gas introduction conduit 42 registers with the upper portion of the 
dust collector 40 and extends for the length of the enclosure 24 to an 
upper portion of the heat recovery area 8. An outlet opening is provided 
in the upper end portion of the conduit 42 so that gases from the furnace 
pass through the assembly 10 and are discharged into the upper portion of 
the heat recovery area 8 through the upper end of the conduit 42. 
In operation, particulate fuel material from the inlet 16 is introduced 
into a lower region of the enclosure 12 and adsorbent material can also be 
introduced in a similar manner, as needed. Pressurized air from an 
external source passes into and through the air plenum 12a, through the 
air distributor 14 and into the bed 15 of particulate material in the 
enclosure 12 to fluidize the material. 
A lightoff burner (not shown) or the like is disposed in the enclosure 12 
and is fired to ignite the particulate fuel material. When the temperature 
of the material reaches a relatively high level, additional fuel from the 
inlet 16 is discharged into the enclosure 12. 
The material in the enclosure 12 is self combusted by the heat in the 
furnace section 4 and the mixture of air and gaseous products of 
combustion (also referred to as "flue gases") passes upwardly through the 
enclosure 12 by natural convection and entrains, or elutriates, the 
relatively fine particulate material in the enclosure. The velocity of the 
air introduced, via the air plenum 12a, through the air distributor 14 and 
into the interior of the enclosure 12 is established in accordance with 
the size of the particulate material in the enclosure 12 so that a 
circulating fluidized bed is formed, i.e. the particulate material is 
fluidized to an extent that substantial entrainment or elutriation of the 
particulate material in the bed is achieved. Thus, the flue gases passing 
into an upper region of the enclosure 12 are substantially saturated with 
the particulate material. The saturated flue gases passing into the upper 
region of the enclosure 12 exit through the ducts 20 and pass into the 
cyclone separators 18. 
As the relatively hot flue gases pass upwardly from the lower region of the 
furnace 4 to the upper region thereof, heat energy is radiated or 
conducted to the water tubes of the enclosure 12. The flue gases in the 
upper region of the furnace section 4 which pass to the separating section 
6 and the heat recovery area 8 will therefore experience a reduction in 
temperature. This temperature reduction may be especially significant when 
the reactor 2 is operating at low fuel loads. 
Once the flue gases have passed from the upper region of the furnace 
section 8 and into the separators 18, the solid particulate material is 
separated from the flue gases and the former passes through the hoppers 
18a and is injected, via the dipleg 33, into the seal pot 34. The cleaned 
flue gases from the separators 18 exit, via duct 22, to the heat recovery 
area 8 with the valve 23 regulating the flow thereof. The flue gases pass 
through the enclosure 24 and across the superheater 26, the reheater 28 
and the economizer 30, before exiting through the outlet 32 to external 
equipment. 
A portion of the flue gases passing upwardly through the enclosure 12 are 
intercepted at one or more selected extraction points within the lower 
region of the enclosure 12 just above the dense bed 15 by the conduits 38a 
and 38b of the flue by-pass assembly 10 for direct introduction to dust 
collector 40. Within the dust collector 40, solid particulate material is 
separated from the flue gases and the former passes through the hopper 40a 
and is injected, via the dipleg 48, into injector line 50. The particulate 
material is then reintroduced to the dense bed 15 for additional 
combustion via the line 50, or via the branch line 50a and the conduit 36. 
The cleaned flue gases from the dust collector 40 pass upwardly through gas 
introduction conduit 42 and exit into the upper portion of the heat 
recovery area 8. This flow of the relatively hot flue gases into the heat 
recovery area 24 through the flue by-pass assembly 10 may be regulated by 
adjustment of the dampers 46a, 46b. The relatively hot flue gases 
discharging into the upper portion of the heat recovery area 8 combine 
with the flue gases from the ducts 22 and pass across the superheater 26, 
the reheater 28 and the economizer 30, as described above. 
Water is passed through the economizer 30, to a steam drum (not shown), 
then through the walls of the furnace section 4 to exchange heat with the 
fluidized bed 15 and generate steam. The steam then passes through the 
above-mentioned fluid flow circuitry (not shown) and through the 
superheater 26, the reheater 28 and the economizer 30 in the heat recovery 
area 8. The steam thus picks up additional heat from the hot gases passing 
through the heat recovery area 8 before the steam is discharged to 
external equipment such as a steam turbine. 
It is apparent that several advantages result from the foregoing. The 
by-pass of relatively hot flue gases through the flue gas assembly to the 
heat recovery area provides for generally higher gas temperatures in the 
heat recovery area, and hence enhanced steam temperatures, especially at 
low loads. Isothermal reactor conditions which are especially difficult to 
maintain at low operating loads of the reactor can be economically and 
efficiently maintained and regulated by the flue by-pass assembly. 
Further, the need for larger and more expensive superheater and/or 
reheater surfacing is eliminated and the efficiency of the downstream heat 
exchange surfaces is increased. 
Several variations may be made in the foregoing without departing from the 
scope of the invention. For example, it is contemplated that one or any 
number of gas extraction conduits may be provided according to the 
requirements of the system, there being described herein the two conduits 
38a, 38b for purposes of illustration. It is also understood that the 
selection and number of the extraction points and thus the positioning and 
number of the gas extraction conduits may vary according to the particular 
design requirements of the reactor. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure and in some instances, some features of the invention 
will be employed without a corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the scope of the invention.