Method and apparatus for operating a circulating fluidized bed system

In association with a circulating fluidized bed reactor, a bed of particles is established in the return duce for particles from the particle separator to the combustion chamber. Inlet openings extend from the bed of particles into the combustion chamber, and the height of the particle bed is maintained above the inlet openings a sufficient distance to form a gas seal. The particles in the bed are fludized, and also transporting nozzles are arranged at different levels for transporting the particles from the bed through the inlet openings into the combustion chamber. Heat exhangers (e.g. superheaters) may be provided in the bed to recover heat from the particles.

BACKGROUND OF THE INVENTION 
This invention relates to a novel method and an apparatus for operating a 
circulating fluidized bed system. 
Circulating fluidized bed (CFB) systems such as CFB combustors include a 
combustion chamber having a fast fluidized bed of solid particles therein. 
A particle separator is connected to a discharge opening in the upper part 
of the combustion cheer, for separating solid particles from the 
suspension of flue gases and entrained solid material being discharged 
from the combustion chamber. A return duct is connected between the 
particle separator and the lower part of the combustion chamber for 
recirculating separated solid particles from the particle separator into 
the combustion chamber. A gas outlet is arranged in the particle separator 
for discharging flue gases. 
Cyclone separators are commonly used as particle separators. A dip leg 
recirculates the separated particles from the cyclone to the lower part of 
the combustion chamber. A loop seal has to be arranged in the dip leg in 
order to prevent gases from flowing from the combustion chamber backward 
into the cyclone therethrough. The loop seal constructions are very large 
and complicated. It has also been suggested to use L-valves as loop seals. 
The L-valve is, however, also space consuming, as a rather long connection 
channel filled with bed material is needed between the return duct and the 
combustion chamber in order establish a loop seal. 
The circulating fluidized bed reactors are used in a variety of different 
combustion, heat transfer, chemical or metallurgical processes. Depending 
on the process, different bed materials are fluidized or circulated in the 
system. In combustion processes particulate fuel such as coal, coke, 
lignite, wood, waste or peat, as well as other particulate matter such as 
sand, ash, sulfur absorbent, catalyst or metal oxides can be the 
constituents of the fluidized bed. The velocity in the combustion chamber 
usually is in the range of 3,5 to 10 m/s, but can be substantially higher. 
Typically heat is recovered from fluidized bed combustion processes by heat 
transfer surfaces in the combustion chamber and in the convection section 
arranged in the gas pass after the particle separator. In circulating 
fluidized bed (CFB) combustors or boilers the peripheral walls of the 
combustion chamber are usually made as membrane walls in which vertical 
tubes are combined by flat plate material or fins to form evaporating 
surfaces. Additional heat transfer surfaces such as superheaters may be 
arranged within the upper part of the combustion chamber for superheating 
the steam. 
Additional superheaters as well as reheaters, preheaters and air preheaters 
are arranged in the convection section. It has also been suggested to 
forth the return duct of heat transfer surfaces. 
The heat transfer surfaces are normally designed to give optimal 
superheated steam also at a low load range. At higher loads steam 
production is then controlled by spraying water in the convection section. 
Superheating at low load often constitutes a problem. The combustion 
chamber exit gas temperature decreases with decreasing load and the 
superheaters in the convection section do not have enough capacity to 
provide the desired results. Additional superheaters arranged in the 
combustion chamber would increase costs and control problems in the boiler 
improperly. Additional heat transfer surfaces within the combustion 
chamber would further decrease the temperature of the flue gases, which 
still contain unburned fuel, to e.g. 700.degree. to 750.degree. C., which 
would have an negative effect on NOX and N.sub.2 O reduction. 
Additional separate heat transfer surfaces within a fluidized bed would on 
the other hand also be exposed to the high velocity (3-10 m/s or even 
higher) flow of gas and particles therein. Corrosion and erosion would 
cause sever problems. Any heat transfer surface arranged within the 
combustion chamber would have to be made of heat resistant material, most 
probably also protected by some erosion resistant material. Such heat 
transfer surfaces would thus become very heavy and expensive. The 
corrosion constitutes a severe problem in the gas atmosphere in the 
combustion chamber, when burning fuels containing gaseous chlorine and 
alkali components. 
Especially in pressurized applications it is even less desirable to have to 
add heat transfer surfaces and increase the size of the combustor, which 
leads to a need to increase the size of the pressure vessel, as well. In 
pressurized applications, having smaller combustion chambers, heat 
transfer surfaces are already very close to each other. It would therefor 
be very difficult to add any additional heat transfer surface into the 
combustion chamber. A very compact arrangement of heat transfer surfaces 
also prevents horizontal mixing of bed material within the combustion 
chamber and results in decreased combustion efficiency. Besides space 
problems, clogging may also become a problem if heat transfer surfaces are 
arranged very close to each other. 
It has been suggested to use external heat exchangers (EHE) for increasing 
the superheating capacity. In such superheaters in a separate fluidized 
bed of hot circulating solid material, the solid material is introduced 
into the EHE from the particle separator. The suggested external heat 
exchangers would be large and expensive as well as difficult to control 
independently from the main combustion process. Erosion would also 
constitute a problem when exposing heat transfer surfaces to a fluidized 
bed of large hot particles. Further at very low loads the amount of solid 
material being discharged with the flue gases from the combustion chamber 
and introduced in the EHE would decrease to such a level that superheating 
could be achieved. A simpler and less expensive solution is needed. 
It is an object of the present invention to provide a method and an 
apparatus for operating circulating fluidized bed systems in which the 
above mentioned drawbacks are minimized. 
It is especially an object of the present invention to provide an improved 
loop seal arrangement for circulating fluidized bed systems. 
It is further an object of the present invention to provide an improved 
method for heat recovery in circulating fluidized bed systems. 
It is still further an object of the present invention to provide an 
improved method for controlling the heat recovery in a circulating 
fluidized bed system. 
It is thereby also an object of the present invention to provide an 
improved method for superheating of steam in a circulating fluidized bed 
boiler system, at different load conditions. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method of operating 
a CFB system comprising the steps of 
establishing a fast fluidized bed of solid particles in the combustion 
chamber so that a suspension comprising flue gases and solid particles 
entrained therein is caused to flow upwardly in the combustion chamber, 
collecting solid particles separated from said suspension, 
directing the collected solid particles into a recycling duct, having an 
inlet for solid particles in its upper part and being connected via inlet 
means with the lower part of the combustion chamber, 
establishing a bed of solid particles in the recycling duct, for preventing 
gases from the combustion chamber from flowing through the inlet means 
into the recycling duct, and 
introducing transporting gas into the recycling duct, for recycling 
particles from the bed through the inlet means into the combustion 
chamber, thereby recycling particles from the recycling duct through two 
or more superimposed openings, constituting a gas seal. 
The particles are preferably directly recycled into the combustion chamber, 
but can, if needed, be recycled into an intermediate chamber which has 
further connection with the combustion chamber. 
According to one preferred embodiment of the invention particles are 
collected in a particle separator and recycled into the combustion chamber 
through a return duct, forming the recycling duct of the invention. A 
slowly bubbling bed of particles is established in the lower end of the 
return duct, from which particles are continuously introduced through 
inlet conduits into the combustion chamber. The bed in the return duct 
constitutes a loop seal for preventing combustion gases from flowing 
backwards from the combustion chamber through the inlet conduits into the 
return duct. 
The bed in the return duct is formed of particles circulating in the CFB 
system. Particles circulating in the system have a smaller particle size 
distribution than the mean size distribution of the total mass of 
particles in the system. The bed moves slowly downwards as solid material 
therefrom is reintroduced into the combustion chamber and new solid 
material is continuously added on top of the bed. The height of the bed 
may be controlled by controlling the reintroduction of solid material 
therefrom into the combustion chamber. 
Solid material is according to the invention reintroduced into the 
combustion chamber with the help of transporting gas through two or 
several inlet openings or inlet conduits connecting the lower parts of the 
return duct preferably directly with the combustion chamber. Thereby two 
or preferably several horizontal or inclined slot like openings or 
conduits on top of each other constitute a connection between the return 
duct and the combustion chamber. The slot like openings also constitute a 
gas seal. 
The transporting gas is introduced into the bed in the return duct at 
locations from which it can mainly flow towards the inlet conduits and not 
to the upper part of the return duct. The gas flow thereby transports 
particles from the bed through the inlet conduits into the combustion 
chamber. The inlet conduits are preferably located in the return duct at a 
level substantially lower than the upper surface of the bed, so that the 
bed portion above the inlet conduits is sufficient to prevent gases from 
flowing upwards into the return duct. The higher the bed the higher is the 
pressure difference forming the loop seal preventing gases from flowing 
backwards through the return duct into the particle separator. 
Transporting gas may be introduced through nozzles in the bottom of the 
return duct or through nozzles at different levels in the side walls of 
the return duct. It is possible to control the recirculation of particles 
into the combustion chamber by controlling the amount of gas introduced at 
different locations. Transporting gas introduced through nozzles in the 
bottom of the return duct will mainly transport particles through the 
lowermost inlet conduits, whereas transporting gas introduced through 
nozzles higher up will transport particles through inlet conduits higher 
up in the return conduit. It is also possible to transport particles 
horizontally or in other desired directions. 
Air from the windbox or air from a separate blower, at a slightly higher 
pressure, or some other cheap gas, e.g. recycled flue gas, may be used as 
transporting gas. Inert gases could also be used especially if inert, 
non-oxidizing conditions are needed. 
According to a preferred embodiment of the invention, the inlet conduits 
comprise several slot like conduits or openings formed on top of each 
other in a free like construction arranged in a common wall between the 
return duct and the combustion chamber. The conduits according to the 
invention, being divided into two or more narrow slot like conduits on top 
of each other, i.e. having a gill-like structure, can be made with very 
short length between return duct and combustion chamber, compared to 
conduits needed in known L-valve loop seals and they can therefor easily 
be included in a conventional membrane wall construction. Known L-valve 
loop seals with only one single conventional conduit, having a large cross 
section especially with a large vertical extension, have to be very long 
in order to build up enough particles in the conduit to constitute a loop 
seal and prevent gases from flowing from the combustion chamber into the 
return duct. 
The loop seal effect of an L-valve type inlet conduit depends on the ratio 
(h/l) between the vertical extension (h) of the conduit and the length (l) 
of the conduit. This ratio should be h/l&lt;0.5 in order to prevent solids 
from flowing uncontrollably through the inlet conduit, for keeping a high 
enough solid surface level in the conduit to prevent gas from flowing 
backwards through the conduit. The larger the cross section of the conduit 
the bigger the vertical extension (h) of the opening, i.e. the longer 
conduit is needed. In inclined conduits, having outlet ends on a higher 
level than inlet ends, the length (l) of the conduit can be further 
decreased. The special design of the inlet conduit makes it possible to 
control the level of bed surface in the return duct and thereby the loop 
seal effect achieved by the bed. 
The inlet conduit or conduits are, according to a preferred embodiment of 
the present invention, divided into several slot like openings or 
conduits, having a small vertical extension, arranged on top of each 
other. Thereby the total vertical extension h.sub.tot needed can be 
divided into h.sub.1, h.sub.2, h.sub.3 . . . , each divided vertical 
extension being just a fraction of the total h.sub.tot needed. The length 
(l) of each conduit can be decreased in the same proportion as the 
vertical extension (h), without the loop seal effect of the inlet conduit 
being decreased. Thereby short inlet conduits, only long enough to extend 
through a common membrane wall, can be used. This definitely simplifies 
the construction of the return duct and its connection to the combustion 
chamber as well as the whole circulating fluidized bed system. 
The return duct may according to the present invention be constructed as a 
very simple channel having a common wall with the combustion chamber. The 
connection between the return duct and the combustion chamber, which in 
earlier constructions has been very large and complicated, may now be a 
simple frame like construction with a set of gill like inlet openings or 
conduits, in the conventional tube panel wall used in boilers, for 
reintroducing material into the combustion chamber. 
Heat transfer surfaces may, according to a further important embodiment of 
the invention, be arranged in a heat transfer zone in the recycling or 
return duct in order to recover heat from the circulating mass of 
particles in the CFB system, thereby constituting an Integral Heat 
EXchanger (IHEX) in the return duct. 
The heat transfer surfaces are preferably arranged in the bed but may 
extend upwards beyond the bed. Heat may also be recovered by heat transfer 
surfaces arranged in the walls of the return duct. The mean size 
distribution of the particles, flowing from the combustion chamber into 
the particle separator and therefrom into the return duct, is smaller than 
the mean size distribution of particles in the combustion chamber, as a 
larger portion of small particles are entrained with the flue gases than 
coarse particles. Fine particles, having medium sizes ranging below 
500.mu., typically 150-250.mu. in the return duct provide for a dense bed 
in the return duct with a very high heat transfer coefficient for particle 
convection, k=100-500 W/m.sup.2 k. 
If heat transfer surfaces are used then the return duct is preferably 
extended in its lower part, having in the extension part a larger 
horizontal cross section than in its upper part, thereby providing more 
space for heat transfer surfaces and the bed of solid particles. 
Superheating of steam may advantageously take place in the return duct. In 
CFB systems heat is readily available for superheating in the circulating 
mass of hot particles. As a further advantage, achieved by arranging 
superheaters in the return duct, there is no need to unnecessarily cool 
the flue gases in the combustion chamber and unfavorably decrease 
temperatures before unburned fuel and ash have been separated from them. 
The invention thereby provides for good NOX and N.sub.2 O reduction in the 
combustion chamber. 
The gas atmosphere in the heat transfer zone in the bed, being very limited 
and containing mainly clean gas without alkaline, chlorine or other 
corrosive gaseous components, provides very advantageous conditions for 
superheating. The superheaters may be heated to much higher temperatures 
than what normally is the case in corrosive conditions prevailing in the 
combustion chamber. Steam of up to 500.degree.-550.degree. C. may be 
produced also when burning corrosive gaseous components containing fuels. 
It has especially been a problem in waste/RDF burning boilers to utilize 
the heat for superheating, due to the dirty gases, containing different 
kinds of corrosion causing components. The present invention provides a 
system in which superheater surfaces contacts hot circulating material in 
a safe gas atmosphere. 
Also erosion is minimized in the slowly bubbling bed having gas velocities 
of &lt;0.5 m/s, e.g. 10 cm/s, whereby particles colliding with the heat 
transfer surfaces have a very low impact velocity. In combustion chambers 
in conventional or circulating fluidized beds the velocities are in the 
range of 0.5 to 50 m/s the particle flow thereby causing severe erosion on 
additional surfaces therein. Additionally erosion in the present bed is 
relatively small due to the small particle size of bed material. 
The heat transfer from particles to superheater surfaces in the heat 
transfer zone in the bubbling bed may be advantageously controlled by 
introducing a fluidizing flow of gas into at least a part of the heat 
transfer zone, providing movement of particles close to the superheater 
surfaces. Increased gas flow around the surfaces will increase heat 
transfer to the surfaces. Gas, such as air or inert gas, may be introduced 
for heat transfer control through several separate nozzles. The 
transporting gas may also be used for controlling heat transfer. 
The heat transfer may according to the invention be controlled by the 
location and/or flow rate of gas introduced into different parts of the 
heat transfer zones. 
Very small gas flows are needed for providing a suitable heat transfer. The 
gas needed may mainly be discharged from the return duct through the inlet 
conduits together with the transport gas, but may, as the gas flows needed 
for heat transfer is very small, also be allowed to flow up into the 
return duct. Especially if heat transfer surfaces are disposed high up in 
the bed it may be more preferable to let at least some of the gas flow 
upwardly in the return duct. Normally the height of the bed will prevent 
gas introduced for heat transfer reasons from flowing upwardly into the 
particle separator. 
It may to some extent be possible to control the heat transfer also by 
controlling the total height of the bed, especially if a part of the heat 
transfer surfaces extends above the bed. 
Heat transfer zones and inlet conduits may be arranged in the same parts of 
the return duct or they may be arranged in adjacent parts of the return 
duct. 
According to one embodiment of the present invention heat transfer zones 
and inlet conduit zones are arranged side by side. Gas flows are 
introduced into the heat transfer zones for controlling heat recovery and 
for transporting the recirculated particles through the inlet conduit into 
the combustion chamber. Transporting gas is preferably also introduced 
into the heat transfer zones for transporting particles therefrom 
horizontally towards the adjacent inlet conduit zones and further into the 
inlet conduits. 
The level of the bubbling bed may be controlled by measuring the pressure 
difference in the return duct between a first preset level below the upper 
surface of the bed and another second preset level above the upper surface 
of the bed, the two preset levels being chosen to include therebetween 
both the actual and optimal levels of the upper surface. The flow of 
transporting gas discharging particles through the inlet conduits may be 
controlled according to the pressure difference measured in order to keep 
the upper surface of the bed at an optimal level. 
The present invention provides a very simple CFB boiler construction. The 
return duct is preferably constructed as a narrow vertical channel having 
one wall common with the combustion chamber, the wall being e.g. a typical 
membrane wall used in CFB boilers. The opposite wall may be a similar 
membrane wall. The inlet conduit connecting the return duct with the 
combustion chamber may be prefabricated as a frame like construction 
having several inlet conduits on top of each other. Such a frame structure 
may easily be connected to a membrane wall on site. There is no need for 
complicated large conventional loop seal constructions. 
The present invention provides a boiler system with a very wide load range. 
Also in the present new system, at extremely low loads, with a velocity of 
about 2 m/s in the combustion chamber, too few particles may be entrained 
with the flue gases to provide the heat transfer capacity needed in the 
return duct. In these cases and if additional superheating is needed, it 
may be possible to utilize the particles flowing downwardly along the 
peripheral walls of the combustion chamber. 
The downward flow of particles may be guided into a vertical narrow 
recycling duct or pocket arranged on a side wall inside the combustion 
chamber. The pocket is open in its upper end for capturing particles 
flowing along the wall. The pocket may be formed by arranging a partition 
wall close to the side wall, the partition wall separating the recycling 
duct or pocket from the main part of the combustion chamber. The pocket 
may be formed as the narrow vertical slot like return duct within the 
combustion chamber. Particles captured by the pocket are allowed to form a 
bed in the lower part of the pocket, similar to the bed in the return 
duct. The bed is controlled to flow slowly downwardly in the pocket and to 
recirculate particles into the main part of the combustion chamber 
through, e.g., gill type inlet conduits, disposed in a frame like 
construction on the partition wall. Heat transfer surfaces, preferably 
superheater surfaces may be disposed in the pocket. Gas nozzles may be 
arranged in the bottom of the pocket as well as on the sides of the 
pocket, for controlling recirculation of particles and heat transfer. 
Such a pocket construction should be disposed in the combustion chamber at 
not too high a level, so that enough solid particles still may be 
collected from the particle flow along the walls above the pocket, at the 
loads in question, for heat transfer purposes. Superheater surfaces are 
well protected in the bed in the pocket as the gas atmosphere there is 
similar to that in the return duct and contains very little corrosive 
gaseous components. 
It is also possible to control the heat transfer by controlled gas flows 
introduced into the pocket. An inlet conduit construction as described for 
the return duct provides a loop seal for the pocket, preventing gases from 
flowing backwards through inlet conduits into the pocket. 
Heat transfer may very easily be controlled in this new system. The heat 
transfer surfaces in the combustion chamber itself may be designed for 
smaller loads than what has been previously the case. The additional heat, 
e.g. for superheating, may be obtained by heat transfer surfaces in the 
pocket and the return duct. This additional heat needed may be controlled 
by the gas flows in the corresponding heat transfer zones. 
At high loads an increased circulating mass of particles increases the heat 
recovery in the return duct whereas at low loads, as the circulating mass 
of particles is decreased leading to decreased heat transfer in the return 
duct, the heat recovery can be increased in the pocket. 
In earlier known systems heat transfer surfaces in the combustion chamber 
have to be designed to secure a satisfactory superheating of steam at low 
loads also. In order to prevent overheating at high loads in such systems 
and for controlling the temperature spray nozzles have had to be installed 
in the convection section. In the present new system spray nozzles are 
needed only for control of steam temperature during operation not for 
controlling steam production at different loads.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a circulating fluidized bed combustor 10, having a combustion 
chamber 12 with an expanded fluidized bed of particles therein. A particle 
separator 14 is connected to the upper part of the combustion chamber 12, 
for separating particles entrained with the mixture of flue gases and 
solid material being discharged from the combustion chamber. A return duct 
16 is provided for recirculating separated solid material from the 
separator into the lower part of the combustion chamber. A convection 
section 18 is connected to a gas outlet 20 arranged in the upper part of 
the separator 14. 
The walls of the combustion chamber 12, of which only walls 22 and 24 are 
shown in FIG. 1, as well as, the walls 26, 28 of the particle separator 14 
and the walls 30, 32 of the vertical channel-like return duct 16, are 
preferably constructed of water wall panels or membrane panels. Water is 
evaporated in the membrane walls. The panels in the bottom part of the 
combustion chamber 12 are protected by a refractory lining 33. Also the 
panels in the return duct 16 may be partly or completely protected by 
refractory lining (not shown). In the embodiment shown in FIG.1 the wall 
28 in the separator and wall 32 in the return duct form the wall 24 in the 
combustion chamber. 
The lower part 34 of the return duct 16 has a larger horizontal cross 
section than the upper part 35 of the return duct. A bubbling bed 36 of 
recycling particles is provided in the lower part 34. 
A superheater 38, preferably the last superheater surface in the steam 
system, is arranged in the bubbling bed 36 for superheating steam produced 
in the panel walls 22, 24, 26 and 30 in the combustion chamber 12 and the 
return duct 16. Steam may also be superheated in a superheater 40 in the 
convection section 18 after the separator 14. Further heat transfer 
surfaces 42, 44, 46, 48 for reheating, preheating and air preheating are 
also arranged in the convection section 12. 
Gill like inlet conduits or openings 50 in a frame like structure 60, shown 
in FIG.1 and FIG.2 connect the lower part of the return duct 16 with the 
lower part of the combustion chamber 12. The height of the bed 36 (over 
openings 50) and the gill like inlet conduit construction 50, 60 
constitute a loop seal preventing combustion chamber gases, at a high 
pressure p1 in the combustion chamber 12, from flowing through the inlet 
conduits 50 into the return duct 16 and further upwardly to the gas space 
above the bubbling bed 36 having a lower pressure p2. The height of the 
bed 36 above inlet conduit 50 should preferably be sufficient to provide a 
pressure (head) greater than the pressure difference p1-p2. 
Transport gas (e.g. air, inert gas, recycled flue gas, or the like) is 
introduced into the return duct 16 through bottom nozzles 52, which can be 
conventional nozzles used in fluidized beds. Additional transport gas is 
introduced through inlets 54, 56, 58 and 59. Transport gas introduced 
through bottom nozzles 52 transports particles from the lowermost part of 
the bed towards the inlet openings 50. Transport gas introduced through 
inlets 54 and 56 mainly transports particles from the middle parts of the 
bed towards the inlet conduits 50. Transport gas introduced through 
nozzles 58 and 59 transports particles from the upper part of the bed 
towards the upper openings of the inlet conduits 50. 
By controlling the flow of transport gas through the different nozzles 52, 
54, 56, 58, 59 at bottom and side wall locations it is possible to control 
the amount of particles being reintroduced from the bed 36 into the 
combustion chamber 12 and thereby the loop seal effect. By increasing the 
gas flow through the bottom nozzles 52 and correspondingly decreasing the 
gas flow through side wall nozzles 54, 56, 58, 59 an increased flow of 
particles is achieved and a decrease in the height of the bed 36. By 
increasing the gas flow through the uppermost nozzles 58, 59 and 
decreasing the flow through the bottom nozzles 52, 54 a decreased 
recirculation of particles is achieved and an increase in the height of 
the bed 36. 
The inlet openings 50 can be grouped in a frame like structure 60 located 
in an inlet conduit zone 62 in the return duct. Superheaters 38 are 
located in adjacent heat transfer zones 64. In other embodiments both 
zones 62 and 64 may overlap. 
The frame structure 60 can easily be inserted in a conventional panel wall 
32 and the slot like inlet conduits 50 can be prefabricated into the wall 
when covering the wall with refractory lining. Tubes in the panel wall 32 
are normally bent (not shown in Figures) during construction to provide 
any opening needed for the inlet conduit frame construction 60. A mold, 
for the slot like openings 50, made of e.g. Styrox or other combustable 
material is inserted in the opening between the tubes, before covering the 
panel wall 32 with refractory lining. The mold is burnt away during 
heating of the refractory lining, leaving only slot like inlet conduits or 
openings 50 in the wall. 
The inlet conduits 50 constitute horizontal or upwardly inclined gill type 
channels that is slot-like openings disposed one above the other as 
clearly illustrated in FIGS. 1 and 2. The transporting gas flow nozzles 52 
are preferably arranged at a distance from the inlet conduits in order to 
prevent the gas from flowing directly into the conduits without entraining 
particles thereby. The distance is preferably at least twice the distance 
between two inlet openings. 
Gas nozzles 66 (see FIG. 2) are also arranged in the bottom of heat 
transfer zones 64 for providing a gas flow around the heat transfer 
surfaces 38 and for transporting the particles in the heat transfer zone 
64 towards the inlet conduit zone. Additional gas nozzles 68, 70 may be 
arranged in the heat transfer zone, as shown in FIG.1, at different levels 
in the wall of the return duct for controlling the heat transfer at 
different locations in the heat transfer zones 64. 
Heat transfer can be controlled by changing the proportion of gas 
introduced through nozzles 68 and 70, the total gas flow thereby being 
constant. Heat transfer is increased by increasing the gas flow through 
nozzles 68 situated below the heat transfer surfaces 38, and decreased by 
increasing the gas flow through nozzles 70 situated at a higher level 
above the lowermost heat transfer surfaces 38. 
It may in some embodiments be necessary to arrange heat transfer zones and 
conduit zones in the same part of the return duct 16, the heat transfer 
surfaces 38 then being arranged directly in front of the inlet conduits 
50. Transport gas flows through nozzles 52, 54, 56, 68 and 70 would then 
effect both heat transfer and transport of particles. 
Transport of particles may however be controlled separately from heat 
transfer by arranging a primarily vertical partition wall 31 in front of 
the inlet conduits 50, for separating the bed 36 in a heat transfer 
section 61 and a transport section 71 as seen in FIG. 3. The partition 
wall 31 reaches from the return duct wall 32 downwardly below the inlet 
conduits 50 and is arranged between the heat transfer surfaces 38 and 
inlet conduits 50. Transport gas is then preferably introduced through 
nozzles 53 directly below the transport zone or through nozzles (not 
shown) arranged in the partition wall 31. Gas introduced through nozzles 
54,56,68 and 70 will effect the heat transfer but have little or no affect 
on the transport of particles. Gas introduced through nozzles 56, 58, 68 
and 70 will flow upwardly in the return duct if no gas seal is arranged in 
the duct. 
FIG. 4 illustrates an embodiment of the present invention, according to 
which heat recovery from particles flowing downwardly along the side walls 
22 of the combustion chamber 12 is obtained. In FIG.4 the same reference 
numerals as in FIG.1 and FIG.3 have been used where applicable. 
In FIG. 4, the return duct 16, may or may not have heat transfer surfaces 
arranged therein or heat may be recovered only through membrane walls 30 
and 32 of the return duct 16. A partition wall 124 is arranged inside the 
combustion chamber 12 close to the wall 22, thereby forming a pocket 135 
(or return duct) construction close to the wall 22, in which case the 
return duct 35 is a second return duct. Particles flowing downwardly along 
the upper part of wall 22 fall down into the pocket 135 and form a bed 136 
of particles therein. Particles are recycled from the pocket 135 into the 
combustion chamber 12 through inlet conduits 150, similar to inlet 
conduits 50. Transport gas is introduced through nozzles 152, 156 and 158. 
A heat transfer surface 138, e.g- a superheater, is arranged in the pocket 
135. The heat transfer surfaces 138 may be arranged in heat transfer zones 
adjacent to transport zones similar to the FIG. 2 construction. The side 
wall 22 may be as shown in FIG.4, bent outwardly to increase the cross 
section of the lower portion of the pocket 135. Nozzles 152, 168 and 170 
for controlling the heat transfer may be arranged in the bottom of the 
pocket 135 or in the side wall 22. 
FIG. 5 shows an embodiment of the present invention in which particles 
collected in a return duct 16 and additionally in a recycling (second 
return) duct 135' are gathered in a common bed 34'. At high loads the 
recycling of particles through duct 135' into the bed 34' may be prevented 
and mainly particles recycled through the return duct are used to form the 
bed 34'. The flow of particles through the internal recycling duct 135' 
may be controlled by a valve preventing particles from flowing from the 
duct into the bed, or by controlling the fluidization in the combustion 
chamber 12 in the vicinity of the partition wall 124. At low load 
particles may mainly flow through recycling duct 135' providing enough 
particles into heat transfer zones in the bed 34'. 
While the invention has been described in connection with what is presently 
considered to be most practical and preferred embodiments of the 
invention, it is to be understood that the invention is not to be limited 
to the disclosed embodiments, but on the contrary, is intended to cover 
various modifications and equivalent arrangements included within the 
spirit and scope of the appended claims.