Description:DUAL FURNACE FLUIDIZED BED BOILER

FIELD OF THE INVENTION

The present invention in general, relates to a fluidized bed boiler to cater to lower capacities of steaming and for efficient multi-fuel firing with very high carbon burn up and lower emissions.

More particularly, the present invention relates to a fluidized bed boiler with a dual furnace, the unit being modular, low height and compact. Thus, it requires very short lead time for erection at site.

It is capable of catering to lower capacities of steaming and is capable of firing coal as well as other alternative fuel like biomass and the like. The combustion is done very efficiently.

TECHNICAL BACKGROUND OF THE INVENTION

It is known that in a typical circulating fluidized bed combustion (hereinafter referred to as CFB Boiler at places), the fuel and the cyclone ash fly ash recirculated to the fluidized bed is well mixed with the hot bed material resulting in a uniform temperature distribution in the bed. This is achieved by a higher turbulence caused by introducing fluidizing air in smaller bed dimensions.
For a Dense Bed Cold Cyclone CFB Boiler the flue gas / ash temperature is cooled to 420-440°C in the first pas itself by positioning the majority of pressure parts (viz. Evaporators, Superheaters, final economizer etc.) in the first pass with lower gas/ fluidizing velocities and also lower dust load due to colder ash recirculation through plate construction cold cyclones (no water/steam cooling).
In Bubbling Bed- Atmospheric Classic Fluidized Bed Combustion System (hereinafter referred to as AFBC at places) boilers, the Final Superheater heating surface or some part of evaporator is also embedded in the Fluidized bed with lower bed velocity to avoid excessive erosion and simultaneously have very effective heat transfer, due to direct conductive heat transfer with ash hence lower superheater heating area requirements.
There is no ash recirculation in AFBC Boiler. In case of CFB Boiler, the recirculated fly ash takes over the cooling of the fluidized bed. The same is more effective in a Cold cyclone CFB compared to hot cyclone units due to excellent temperature control due to lower ash temperature. It is in the convection zone that the heat taken is transferred to the convectional heating surfaces.
The existing design CFB boilers in the market have a tower-type arrangement with enormous support structure arrangement and huge erection complexity and lead time at site.
The first boiler pass is formed by water-cooled, gas-tight membrane tube walls in case of Cold cyclone CFB Boilers. Its membrane tube walls in cyclone, cross over duct and second pass in case of typical Hot Cyclone Units available in the market. The membrane walls are part of the evaporator system and are designed for natural circulation.
Currently, all the CFB Boilers lack capabilities to cater to lower capacity requirements below 50-60TPH in a cost effective and lower time frame project execution. Also these boilers lack higher percentage of Biomass firing capabilities, especially at higher steam temperature cyclones due to higher Alkali and Chlorine content in the Biomasses and other alternate fuels, available in market. That apart, the erosion rates and hence the tube replacement frequency and downtimes of many units of such boilers are common and hence a concern.
Furthermore, as stated before, the CFB Boilers in vogue have tower-type arrangement with top supporting arrangement and huge erection complexity and lead time at site.
Accordingly, there exists a need for a fluidized bed boiler which can cater to lower capacities of steaming and for efficient multi-fuel firing, with very high carbon burn up and lower emissions.
There is also a need for a fluidized bed boiler which avoids known erection complexities and is easy to erect and requires very short lead time for erection at site.
The present invention meets the above mentioned needs and other associated needs by providing a fluidized bed boiler with dual furnace having a modular construction, is of lower height and have overall small cross section as compared to known boilers in market.
The boiler is adapted to cater to lower capacity of steaming (<50-60TPH) and ensures efficient multi-fuel firing inclusive of biomass and other alternative fuel. The boiler is also protected from high temperature chloride corrosion and alkali slagging, as the high temperature final superheater is protected from corrosive flue gases.
OBJECTIVES OF THE INVENTION

It is the prime objective of the present invention to provide a fluidized bed boiler with dual furnace which can cater to lower capacity of steaming (<50-60TPH) and ensures efficient multi-fuel firing inclusive of biomass and other alternative fuel.
It is another important objective of the present invention to provide a fluidized bed boiler with dual furnace which is easy to erect and requires very short lead time for erection at site.
It is a further objective of the present invention, to provide a fluidized bed boiler with dual furnace which is protected from high temperature Chloride corrosion, as the final high temperature superheater is protected from the corrosive flue gas.
It is yet another objective of the present invention to provide a fluidized bed boiler with dual furnace which meets all the above objectives and simultaneously incorporates all advantages of existing Cold Cyclone and Hot cyclone CFBC boilers and AFB boilers and Grate type boilers.
How the foregoing objectives are achieved should be clear from the contents of the specification hereinafter.
All through the specification including the claims, the words such as “boiler”, “primary furnace/secondary furnace”, “modular”, “free board zone”, “cyclone”, “siphon”, “primary/secondary bed”, “primary/secondary, windbox/nozzles”, “pressure parts” are to be interpreted in the broadest sense of the respective terms and includes all similar items in the field known by other terms, as may be clear to persons skilled in the art. Restriction/limitation, if any, referred to in the specification, is solely by way of example and understanding the present invention.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a fluidized bed boiler having a primary furnace and a secondary furnace, with a common middle wall therebetween. The primary furnace has at least a primary bed above which exists at least a first free board zone. The secondary furnace has at least a secondary bed above which exists at least a second free board zone. In accordance with the invention, the boiler is designed as a modular unit and the primary furnace has a primary wind box with air nozzles atop it and pressure parts enclosed within a water wall panel. The secondary furnace has a secondary wind box with air nozzles atop it and pressure parts enclosed within another water panel. Both the primary furnace and the secondary furnace have secondary air nozzles and a first set of tertiary air nozzles as well as a second set of specially designed tertiary air nozzles. Adjacent to the primary furnace there exists a cyclone the clean gas outlet of which is connected to at least two passes for safe routing of flue gas to the chimney/stack after emission control.
In accordance with preferred embodiments of the present invention:
-the water wall panel encloses an evaporative screen, a secondary super heater, a third stage Economizer and a Subcooler, said Subcooler being a steaming portion of the economizer that is connected to the steam drum of the boiler and said Subcooler has inclined coils whereby higher levels of steaming is allowed in the coils and bubble traps inside the tubes are prevented due to its inclined geometry;
-the another water wall panel encloses an evaporative screen, a primary superheater and a second stage Economizer;
-the secondary bed has ash-embedded in-bed coils, mostly in-bed superheater coils, capable of unique four stage combustion process for efficient multi-fuel firing inclusive of biomass and other alternative fuel;
-the cyclone is connected to first of the two passes through a cross over duct and the ash, unreacted calcium oxide, limestone and unburnt material are collected in the high efficiency cyclone for recirculation back to the primary furnace alongwith other boiler fuel feed via a siphon located beneath the cyclone and said siphon being operatively connected to said primary furnace;
-the first of the two passes encases a horizontal recuperativel tubular air preheater, an economizer located benath it and a finned vertical recuperative tubular air pre heater beneath the economizer, the second of the two passes is located atop the vertical preheater and opens into Electrostatic Precipitator or bag filter for final expelling of flue gas through the chimney;
-said horizontal preheaters have air inside and gas outside while said vertical preheaters have gas inside and air outside, the preheaters at the cold end section being provided with special corrosion resistant material;
-the evaporative screens are tapped off from the middle water wall and extends further up to act as support tubes for all pressure parts of both boilers and serve as heat absorption surfaces;
-the middle water wall can cause heat transfer on both of its sides and has reducing tube diameter to manage the circulation circuit healthiness;
-the primary furnace is fed with coal from a coal bunker through coal feeders while the secondary furnace is fed with biomass to the secondary bed from a biomass bunker through a biomass feeder;
-both the primary furnace and the secondary furnace are fed with coal from said coal bunker;
-the primary furnace is designed for burning majority of the fuel, rest in the secondary furnace;
-said boiler is adapted to cater to lower capacity of steaming (<50-60TPH) and for efficient multi-fuel firing inclusive of biomass and other alternative fuel;
-the entire unit is modular, has a lower height and compact design and is bottom supported.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Having described the main features of the invention above, a more detailed and non-limiting description of a preferred embodiment will be given in the following with reference to the drawings.

Figure 1 is a front view of a preferred embodiment of the fluidized bed boiler according to the present invention.
Figure 2 is a side view of the embodiment, front view of which is shown in figure 1.
Figure 3 is a rear view of the embodiment, front view of which is shown in figure 1.
Figure 4 is another side view.

DETAILED DESCRIPTION OF THE INVENTION

The following describes a preferred embodiment of the present invention which is for the sake of understanding the invention and non-limiting.

It should be understood that the figures are non-limiting and the present invention embodies variations in the number of components shown and orientation thereof. Furthermore, fuel application has been described as coal and biomass by way of example. Other than biomass it can be other alternative fuel source as well and it is within the scope of the invention to maneuver the process and configuration accordingly. In this context, it should be understood that the fuel can be 100% coal as well.
In all the figures, like reference numerals represent like features.
Now, the constructional features of the boiler and functions of such features are described in more detail to explain how the technical deficiencies as discussed under “Technical background of the invention” are sorted out and how the objectives of the instant invention, as stated under the heading “Objectives of the invention” are met.
Referring to figure 1 the fluidized bed boiler has a primary furnace (100) which consists of a primary bed (124), above which there is a free board zone. It has a primary windbox (142) with air nozzles (130) mounted on the same. The pressure parts are enclosed inside a water wall panel (104). The pressure parts include an evaporator referred to as a Screen (104a) which has an evaporative surface, a Secondary Superheater (110), a third stage Economizer (114) and a Subcooler (116). The Subcooler is the Steaming portion of the Economizer which finally connected to the Steam Drum (138).
It will be clear from figure 1 that the fluidized bed boiler of the present invention also has a secondary furnace (102). It consists of a secondary bed (126), above which there is a free board zone.
From figure 1, it will be also clear that the free board zone of both the primary furnace (100) and the secondary furnace (102) has secondary air nozzles (132) and a first row of tertiary air nozzles (134) and a second row of specially designed tertiary air nozzles (168). This additional row of tertiary air nozzles in the free board zone ensures required injection velocity through all sets of nozzles. This facilitates more turbulence inside the furnace, rendering greater residence time and blanketing effect in the boiler. Thus more carbon burn up efficiency is achieved and complete combustion is ensured in the event of biomass burning, which takes place in the secondary furnace (102) as elaborated hereinafter. It’s to be noted that in the absence of Biomass/ AFR the secondary furnace also combusts Coal just like the primary furnace.
Hence, apart from traditional three stage combustion as known in respect of CFB boilers, caused by primary nozzles (130) in the bed and secondary and tertiary air nozzles (132, 134) in the free board zone, this additional row of air nozzles (168) ensures four stage combustion. This is explained again, a little later in this specification while explaining the functioning of the boiler.
The secondary furnace also has a secondary windbox (144) with air nozzles (130 a) mounted on the same. The pressure parts are enclosed within a water wall panel (106). The pressure parts include an evaporator (106a), Primary Superheater (108) and Second Stage Economizer (112). The secondary bed (126) has ash embedded In-Bed Coils (170) [mostly In-Bed Final Superheater coils].
The common Middle Water Wall (166) in-between Primary & Secondary Furnaces also services a major heat duty contributor in the Evaporation circuit.
There are two downcomer pipes (171, 172) located at the middle of the boiler and in connectivity with the steam drum (138). These supply water from the steam drum (138) to the bottom header frame. Through water walls (104, 106), the steam water mixture rises and enters the steam drum (138) through the top header frame as can be seen from figure 1.

The Screen Coils (104 a & 106 a) in the Primary (100) and Secondary Furnaces (102) are tapped off from the Middle Water Wall Panel (166). These are extended further up and serves as support tubes which supports other pressure parts in both the furnaces. These support tubes are heat transfer absorption surfaces as well.

Also it’s to be noted that the middle wall (166) has heat transfer sides with respective furnaces and it being cautiously designed to ensure healthy circulation. Finally it extends to form the roof wall & is terminal at a header which is finally connected via risers to the steam drum (138).

Adjacent to the primary furnace (100) there exists a cyclone (118)
in the flue gas exit path. The clean gas outlet of the Cyclone (118) is connected to a pass (160) through a steel construction cross over duct (140) as clearly shown in figure 1. The ash and unburnt material are collected in the high efficiency cyclone (118). There is a siphon (128) located beneath the cyclone (118).
The ash, other inventory and unburnt materials are recirculated back to the primary furnace (100) via the siphon (128), through the feed chutes mounted over the furnace along with coal, limestone.
From figure 1 and figure 3 it would be clear that the pass (160) encases a horizontal recuperative tubular air preheater (120), beneath which lies an economizer (146), followed by a finned vertical tubular air heater (122).
Referring to figures 3 and 4 in particular there is a further pass (162), beneath the horizontal preheater (120) and above the vertical preheater (122). Through this pass (162) the flue gas is routed to the Emission control equipment like Electrostatic Precipitator or Bag filter and finally to Induced draft (ID) fan and then the Chimney/ Stack. This would be particularly clear from figure 4.
Referring to figure 2, the boiler is fed with coal from the coal Bunker (150) though the coal feeders (152). The biomass is fed to the boiler from the biomass bunker (148) through the biomass feeder (154). In case of 100% Coal firing then the coal from a single bunker is fed to both the Furnaces through duel feeders.
The cyclone (118) can be steel construction (non-water cooled wall) refractory one. The passes (160, 162) are of steel casing. The economizer (146) is a horizontal bare tube economizer.
Having described the constructional features of the boiler, now the functioning of the boiler is explained in detail and for that purpose, the same figures are once again referred to.
The primary furnace (100) is designed for firing around 63-65% of fuel and hence meet 63-65% of overall heat duty of the boiler. It is fed with coal from the coal bunker (150) through the coal feeders (152). The secondary furnace (102) is fed with biomass to the secondary bed (126) from the biomass Bunker (148) through the biomass Feeder (154) which contributes to around 35-38% of overall heat duty of the Boiler. In this context figure 2 should be referred to.
It is also within the scope of the present invention to feed both the primary and secondary furnace with coal using the coal bunker (150) and coal feeders (152).
As explained in the foregoing with reference to figure 1, primary furnace (100) has a primary Windbox (142) with air nozzles (130) mounted on the same. The primary Windbox (142) is supplied with primary air from the Forced Draft (FD) Fan. In the boiler there is only a single Draft fan which is the Force Draft (FD) Fan and no separate primary and secondary air fans are incorporated.

The combustion of majority of fixed carbon happens in the bed under sub-stoichiometric combustion, rest of the fuel fines and volatiles are combusted in the free board zone utilizing specially designed secondary air nozzles (132) and the two rows of tertiary air nozzles (134, 168) in both the Primary (100) and Secondary Furnaces (102). Majority of desulphurization reaction also happens in the bed. Cyclone helps reutilize the unreacted calcium oxide and Limestone to be recirculated back into the furnace bed for effective desulphurization and optimal Limestone consumption. It should be clear from figure 1 that these nozzles (132, 134, 168) are also present in the free board zone of the primary furnace (100), just like the air nozzles identified with reference numeral 130.

The flue gas velocities are in the range of 4.5m/s or below which ensures higher furnace residence time of the fuel particles & also ensures very low levels of erosion. The primary furnace (100) acts more like a Circulating fluidized bed boiler with cyclone ash recirculation through the furnace (100) and cyclone (118).
The secondary furnace (102) acts more like typical AFBC Boiler, but still the flue gas with ash/ unburnt particles travel across the CFB Boiler top with some amount of ash getting collected to the primary furnace (100) as well due to loss of inertia while traversing the pass an rest gets collected and gets recirculated back to the CFB Boiler bed. This ensures maintaining a uniform, optimal bed temperature which is vital for the smooth combustion process. This also ensures the Limestone particles, un-reacted calcium oxide and other inventory to be reutilized in the combustion process as those are recirculated back to the furnace through the cyclone, which is common for the flue gas from both the furnaces.
The bed temperature in the secondary furnace (102) is maintained by the in-bed coils (170), mostly superheater coils. This reduces the effective superheater surface area. The superheater coils have unique four stage combustion process for multi-fuel firing inclusive of biomass and other alternative fuel. This ensures lower emissions and excellent fuel combustion flexibility with finer temperature control. More precisely, with reference to figure 1, in addition to the traditional three stage combustion through normal nozzles with tube bending done on the furnace wall in Cold Cyclone CFB units, there is introduced suitably sized fourth stage tertiary air nozzles (168) injection, without effecting the furnace membrane wall. The nozzles are inserted through the flats of the membrane wall without disturbing the membrane wall tubes. It is shown in a circular fashion as depicted in figure 1 for better understanding. This ensures more turbulence inside the furnace, rendering greater residence time and blanketing effect in the boiler for ensuring more carbon burn up efficiency and also for ensuring complete combustion in case of biomass firing.

The pressure parts inside the primary furnace (100) which includes firstly the evaporative surface, namely the screen (104a), and absorb the heat from the flue gases. Above this, there is the secondary superheater (110) and the third stage economizer (114) and subcooler coils (116). All of these absorb heat and the flue gas is expelled from the primary furnace pass at around 600 to 635°C.
Secondary furnace (102) is having the evaporative surface (106a), above which is the Economizer (112) which absorbs heat and expels the flue gas at around 400 to 415°C from the secondary furnace. Both the flue gases along with the dust load gets mixed and enters the cyclone (118) at around 590 to 610°C.
Then the flue gas enters the pass (160) through the cross over duct (140). The ash and unburnt material collected in the cyclone (118) is recirculated back into the primary bed (124) through the siphon (128) which is supplied with fluidizing / sealing air from the FD fan itself. This ensures that there is positive sealing and there is no flue gas bypass through the cyclone.
The coal and limestone also gets along with this recirculation cyclone ash to the primary bed (124) which gives an excellent uniformly mixed inventory in the fluidized bed and ensures optimal chemical reactions in the combustion process and desulphurization reaction.
The flue gas after entering the pass (160) faces the first set of Air Preheater bundles (120) which is having air inside the tubes and flue gas over and outside the tube surface. The final hot air of primary and secondary air preheater bundles (120) as depicted in figure 1 is routed to the respective primary wind box (142) and secondary wind box (144) for bed fluidization and combustion in primary zone of the boiler. Rest of the air is routed to the secondary and tertiary nozzles (132, 134, 168) of the respective furnaces for completion of the combustion in the free board zone.
The flue gas then enters the economizer (146) after which it enters the gas inside air outside vertical finned Air Preheater Tubes (122) which is located in both end section of the pass (160) and majority in lower heighted pass (162). Both the primary and secondary bundles (120) of Air preheater at the cold end section is provided with special corrosion resistant material, as the flue gas exit temperatures are maintained as low as 120-145°C in the unit.
Feed water is fed to the boiler starting from economizer (146) then to the economizer identified with reference numeral 112 and then to the economizer identified with reference numeral 114. Then it goes to Subcooler (116) and finally to steam drum (138). As the Subcooler (116) is having inclined coils higher levels of steaming is allowed in the coils and there won’t be any bubble trap inside the tubes due to its inclined geometry.
The saturated steam from the steam drum (138) is routed though the saturated steam piping to the primary superheater (108) of the secondary furnace (102) then to the second stage superheater (110) of the primary furnace (100), then finally to In-bed final superheater coils (170) from where it goes to the final superheated steam outlet from the boiler, for end use of superheated steam. The fluidizing velocity is maintained really very low, for example around 1.5 to 1.8 m/s to avoid any accelerated tube erosion of the in-bed coils.
Middle wall Panel (166) absorbs heat from both the primary furnace (100) and secondary furnace (102) thus acting as a partition between both.
The boiler being lower in height as compared to known boilers is easy to erect. Further, it should be clear from the foregoing description that the boiler is of modular nature and comprises of modular arrangement of components. The low heighted boiler which is for example, around 10 to 12 metres lower than traditional CFB units, is bottom supported (164) as well, simplifying the structural frame work of this Boiler.
The boiler is protected from high temperature chloride corrosion and alkali slagging, as the final supeheater is protected from corrosive flue gases.
The boiler also incorporates all advantages of existing Cold Cyclone and Hot cyclone CFBC boilers and AFB boilers and Grate type boilers.
The boiler has in bed superheater coils (170). This reduces the effective superheater surface area. The superheater coils have unique four stage combustion process for multi-fuel firing inclusive of biomass and other alternative fuel. This ensures lower emissions and excelllent fuel combustion flexibility with finer temepratre control.
As explained in the foregoing the unit is specially designed for lower capacity (<50-60TPH) Boiler units which ensures lower height of the Boiler, lower foot print, controlled emissions, very low rate of erosion, lower execution period at site. Comparatively lower gas velocity profile and strategic pressure part positioning ensure, lower rate of erosion as well, in comparison to typical CFB units.
The fluidized bed boiler of the present invention as described in the foregoing is capable of being scaled to higher capacities as well and this is within the scope of the present invention.

It should be clear from the description hereinbefore and the various figures that all the objectives of the present invention have been met. This would be even more clear from the appended claims.

The present invention has been described with reference to a preferred embodiment and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate modifications within the ambit of what has been described hereinbefore and claimed in the appended claims.
, Claims:We claim:

1. A fluidized bed boiler having a primary furnace (100) and a secondary furnace (102), with a common middle wall (166) therebetween, said primary furnace (100) having at least a primary bed (124) above which exists at least a first free board zone, said secondary furnace (102) having at least a secondary bed (126) above which exists at least a second free board zone characterized in that the boiler is designed as a modular unit and the primary furnace (100) has a primary wind box (142) with air nozzles (130) atop it and pressure parts enclosed within a water wall panel (104) while the secondary furnace (102) has a secondary wind box (144) with air nozzles (130a) atop it and pressure parts enclosed within another water panel (106) and in that both the primary furnace (100) and the secondary furnace (102) have secondary air nozzles (132) and a first set of tertiary air nozzles (134) as well as a second set of specially designed tertiary air nozzles (168), and adjacent to the primary furnace (100) there exists a cyclone (118) the clean gas outlet of which is connected to at least two passes (160, 162) for safe routing of flue gas to the chimney/stack after emission control.

2. The fluidized bed boiler as claimed in claim 1 wherein the water wall panel (104) encloses an evaporative screen (104a), a secondary super heater (110), a third stage economizer (114) and a subcooler (116), said subcooler (116) being a steaming portion of the economizer (114) that is connected to the steam drum (138) of the boiler and said subcooler (116) has inclined coils whereby higher levels of steaming is allowed in the coils and bubble traps inside the tubes are prevented due to its inclined geometry.

3. The fluidized bed boiler as claimed in claim 1 wherein the another water wall panel (106) encloses an evaporative screen (106a), a primary superheater (108) and a second stage economizer (112).

4. The fluidized bed boiler as claimed in claim 1 wherein the secondary bed (126) has ash-embedded in-bed coils (170), mostly in-bed superheater coils, capable of unique four stage combustion process for efficient multi-fuel firing inclusive of biomass and other alternative fuel.

5. The fluidized bed boiler as claimed in claim 1 wherein the cyclone (118) is connected to first (160) of the two passes (162) through a cross over duct (140) and the ash, unreacted calcium oxide, limestone and unburnt material are collected in the high efficiency cyclone (118) for recirculation back to the primary furnace (100) alongwith other boiler fuel feed via a siphon (128) located beneath the cyclone (118) and said siphon (128) being operatively connected to said primary furnace (100).

6. The fluidized bed boiler as claimed in claim 5 wherein the first of (160) the two passes (162) encases a horizontal recuperativel tubular air preheater, an economizer (146) located benath it and a finned vertical recuperative tubular air pre heater (122) beneath the economizer (146), the second (162) of the two passes (160, 162) is located atop the vertical preheater (122) and opens into Electrostatic Precipitator or bag filter for final expelling of flue gas through the chimney.

7. The fluidized bed boiler as claimed in claim 6 wherein said horizontal preheaters (120) have air inside and gas outside while said vertical preheaters (122) have gas inside and air outside, the preheaters at the cold end section being provided with special corrosion resistant material.

8. The fluidized bed boiler as claimed in claims 1, 2 and 3 wherein the evaporative screens (104a, 106a) are tapped off from the middle water wall (166) and extends further up to act as support tubes for all pressure parts of both boilers (100, 102) and serve as heat absorption surfaces.

9. The fluidized bed boiler as claimed in claim 8 wherein the middle water wall (166) can cause heat transfer on both of its sides and has reducing tube diameter to manage the circulation circuit healthiness.

10. The fluidized bed boiler as claimed in claim 1 wherein the primary furnace (100) is fed with coal from a coal bunker (150) through coal feeders (152) while the secondary furnace (102) is fed with biomass to the secondary bed (126) from a biomass bunker (148) through a biomass feeder (154).

11. The fluidized bed boiler as claimed in claim 10 wherein both the primary furnace (100) and the secondary furnace (102) are fed with coal from said coal bunker.
12. The fluidized bed boiler as claimed in any preceding claim wherein the primary furnace is designed for burning majority of fuel and rest in the secondary furnace.

13. The fluidized bed boiler as claimed in claim 12 wherein said boiler is adapted to cater to lower capacity of steaming (<50-60TPH) and for efficient multi-fuel firing inclusive of biomass and other alternative fuel.

14. The fluidized bed boiler as claimed in claiam 13 wherein the entire unit is modular, has a lower height and compact design and is bottom supported (164).

15. The fluidized bed boiler as claimed in any preceding claim which is capable of being scaled to higher capacities.

Dated this 14th day of July, 2023