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PRINCIPLE OF FLUIDIZATION
When a bed of fine particles, for example sand, is subjected to an upward stream of air, the particles become suspended as the airflow reaches a certain velocity. This condition is referred to as the minimum fluidizing velocity and it varies according to the particle size and the depth of the bed. When the bed is fluidized it resembles a boiling liquid. Such a turbulent mass of solid particles is named a fluidized bed. Coal can be fed into the bed and as it burns it resembles molten lava. If the gas velocity becomes too high, then the particles are entrained in the gas stream and are lost. A fluidized bed behaves like a liquid, so both the bed level and temperature can be easily controlled.
Coal fuel enters the furnace slightly above the fluid bed splash zone where the rising stream of air and combustion gases burst from the sand bed. The vigorous action of the bed causes the fuel to rapidly mix and be quickly raised on temperature to its igniting point. Solid fuel particles mixing with the rising air stream are jostled by the hot bed material (sand, ash & fuel) causing rapid release of surface moisture and volatile matter from within the fuel.
The relatively large mass of moving bed material continually exposes new surfaces of the fuel particles for combustion, sustaining the rapid combustion within the bed. The rapid combustion enables very good load response to be maintained. (Closely approaches that of oil fuel)
WHEN the fluidized bed design is compared with conventional grate systems the following specific capabilities of the former enable a practical unattended control procedure (automatic controls) to be devised and implemented;
Combustion temperatures are significantly lower and thus much safer conditions are continually maintained (well under 1000 oC)
The nature of a fluid bed ensures that mal-distribution is not likely to occur. Fuel migration through the bed is fast and even. Air distribution is insured by the designed configuration and location of the air nozzles.
The bed combustion can be stopped instantly by turning off the air supply to the bed. This action results in the bed slumping, thereby smothering combustion. The bed is inherently safe because it contains only 3-5% combustible material as a maximum. This is evenly distributed throughout the inert bed material. When slumped, the bed is safe and permits an easy restart even after several hours simply by re-introducing the airflow into the bed. This is possible because of the substantial quantity of entrained heat in the bed material.
The furnace is located atop the air distribution plenum. It has an overall freeboard height of about 5 meter. At its base, and assembly of In-bed tubes are positioned so that at maximum steam output they are fully immersed in the expanded fluidized bed. As the fluidizing air supply is decreased, the expanded bed depth is reduced and the in-bed tubes are progressively uncovered such that at all outputs approximately 50% of the heat release by combustion is transferred to the in-bed tubes when burning coal fuel. This allows a turndown of at least 3:1 at near constant excess air with the entire bed in operation. Co-efficiency of heat transfer of the in-bed tubes immersed in the sand bed is 5-6 times
igher than that of the conventional convection tube bank area. If design of boiler requires 30 m2 for convection area to generate 1000 Kg of steam per hour, we can safely take 1/6 of it or 5 m2 to generate 1000 Kg of steam from the immersed in-bed tube area.
The fluidizing air enters through air distributors mounted on a flat base plate. The bed material is silica sand having a mean size of 0.9 mm (#3). Bed depth is about 400 mm.
Start-up of the bed is achieved by the firing of an above bed distillate burner, which is lit by Oil. Start-up is normally achieved in 45 minutes from cold status. Hot starts normally are achieved in less than 10 minutes, 6 minutes of which is needed for purging to ensure there are no pockets of ignitable gas in the boiler passes. If the bed temperature is above 600 oC no oil fuel is needed to re-start the bed. Usually bed can maintain enough heat or above 600 oC for 1-2 hours.
The average bed temperature varies between 950 oC at full load to 750 oC at minimum load. In fluidised bed operation bed temperature is monitored and is in fact a better indication of combustion condition than a flame scanner used for oil burner.
FREE BOARD ZONE/ OVERFIRE 2ND AIR
The major proportion of the fuel is burnt within the bed, with the remainder burning in the free board zone or disengaging space above the bed. With the injection of overfire 2nd air into the free board zone, disengaging space temperature will rise higher than the temperature of the bed. The final gas temperature leaving the furnace will be similar to the bed temperature as the elevation in temperature due to free board combustion is partially offset by the heat transfer to the uncovered portion of in-bed tubes and cooling effect of the fresh air injection.
COAL FEEDING SYSTEM
Fine coal is stored on the ground and normally flat conveyor system transport coal from the in-ground hopper to the silo mounted at the front of the boiler. The coal runs from the storage silo by gravity to the screw conveyor with variable speed gear motor, then the coal enters the furnace through an air swept spout.
Soot blowers are not necessary to be installed as the combustion temperature of the fluidized bed is controlled at well below the ash fusion temperature and fly-ash entrained in the gases entering the convection tube banks are dry and non-adhering and possess a self-cleaning action.
COASE ASH REMOVAL / CIRCULATING SAND BED SYSTEM
Coarse ash which is not elutriated from the bed must be removed. This material is removed continuously by means of circulating sand bed. We call this system as CIRCULATING BED. Air nozzles are screwed in to the multiple air distribution pipes instead of base plate which was used before. Because of this air distribution pipes, coarse ash can fall below the pipe level travelling downward in between the pipes and those materials will be discharged from the rotary valve below together with the silica sand onto the vibrating screen which will segregate those coarse ash and other foreign materials from the pure silica sand. Then the pure silica sand will be returned to the furnace through the sand returning pipe by blower.
a) Fluidized Bed Temperature Control
The bed temperature is controlled between approximately 750 - 950 oC by varying the speed of the coal screw feeder in response to a signal from the steam pressure and the bed temperature (2 factors)
b) Free Board Draught Control
The draught in the free board is controlled at approximately -50 Pa by adjusting the induced draught fan damper located after the multi-cyclone dust collector.
c) Fluidized Bed Load Control
Under normal operating conditions, a manual set point forms the load signal. This signal is used to adjust the fluidizing fan damper. As the amount of air is varied to the furnace, the coal feed rate will be regulated to maintain the bed temperature. The fluidizing fan damper control is overridden if the flow measurement indicates a fluidizing velocity of less than 1 m/s.
d) Bed Level Control
Bed pressure is used to control the bed depth between an effective static height of 120 mm to 180 mm. As the bed pressure reaches the lower limit, the bed material is introduced to the bed by starting the bed material feed screw.
ACID GAS EMISSION ( FBC, most environmentally friendly system)
One of the main advantages of the fluidized bed boiler is the fact that
the temperature spread in the furnace is small due to solid mobility in the fluidized state. This feature is commonly exploited for sulphur capture using limestone;
CaCO3 CaO + CO2
2CaO + 2SO2 + O2 2CaSO4
The thermodynamic stability of the sulphur capture product declines sharply as temperature is increased above 900 oC and for this reason, bed temperature is maintained between 800 - 900 oC. Ca/S molar feed ratio of around 2 - 2.5 are typically used and sulphur capture efficiency for this type of system is usually in the range 70-90 percent, depending on limestone reactivity. Soft, dolomitic limestones are generally the most effective for this service.
In an overall sense, FBC acid gas emissions are dramatically lower than those from other combustion system such as chain grate stoker, underfeed stoker, pulverized coal spreader etc.,( without external flue gas treatment). Emission limits of (typically) 200-300 ppm for Sox and 100-150 ppm for Nox can usually be met without resorting to post treatment of flue gas.
ASH / CARBON / SAND / LIMESTONE
The carbon content of the bed is typically around 2-3 percent (as coal being 5% of the total bed material) and the balance of the bed meterial can be ash, sulphated limestone or sand. The preferred mode of operation is one in which ash and limestone make up the bed. In some instances limestone is not needed and ash content is too low and friable enough to break down to fine particle and be carried out of the bed. This case is specially with the "ADARO COAL" of Kalimantan, Indonesia, which contains only 1 % ash and 0.1 % sulphur. In such cases it is necessary to add a material such as sand to the bed to maintain a suitable inventory in the system. The performance of a particular coal in relation to bed ash formation can be highly significant from a fuel selection point of view - a boiler operator may not wish to add sand/limestone purely for maintenance of a suitable bed. In such cases the supplier of a coal which does not have good bed-forming properties may need to "spike" the fuel with bed-forming agents.
Ash softening temperature is an important parameter. The fluidized bed must operate in a "Dry" condition since any stickiness has the potential to cause uncontrolled agglomeration and ultimately de-fluidization. Relatively few coals can produce sufficient stickiness below about 900 oC to upset fluidization, though coals which contain appreciable amounts of chlorine and/or alkali metals such as Na and K are known to give problems.
ONLY FLUIDIZED BED SYSTEM CAN OPERATE JUST LIKE OIL/GAS BURNER
(Turn-down ratio 1/4)
Generation of heat can be adjusted freely up to the turn-down ratio of about 1/4 by controlling coal fuel feeding and forced fan damper openig to the minimum fluidizing velocity. At the minimum fluidizing velocity, sand bed is still fluidizing and continue to generate steam at 1/4 capacity. Between this range of evaporation, Fluidized Bed can react most likely to the Oil/Gas burner among any conventional coal firing system such as chain grate and/or underfeed stoker.
NO LOSS OF REMAINING FUEL WHEN SHUT DOWN YOUR BOILER
Not like with the other systems, you do not waste any remaining coal fuel when you want to stop the boiler. Remaining coal still inside the sand bed when you turn off the air supply to the bed shall have no more Oxygen supply and trapped inside the hot sand without wasting carbon for no use.
For any other coal fire system like chain grate, all coal fuel already stay on top of the moving chain grate shall have to finish buring even the boiler stop operating and you do not want steam any more. Those loss will count greatly in a long time when comparing Fluidized Bed and Chain Grate.
NO CLINKER, NO ASH FOULING/SLUGGING
All ash will go out of Fluidizing Sand bed in a very dry condition under low temperature of about 900 oC. This condition of ash will never make problems of clinkering and never stick on the surface of boiler tubes. Therefore, soot blower is not needed for Fluidized Bed as mentioned previously. Only fine and dry ash are collected by the multi-cyclone system aftr the boiler and to be discharged through the rotary valve. Since the quality of ash coming out from Fluidized Bed is very fine and equal in its size, ash from the Fluidized Bed Boiler will find commercial value for fertilizer, mixing agent for cement moulded product, construction material, filler for road making etc. Ash from other coal firing system such as chain grate are very rough/coarse due to its high temperature above ash melting point, and can not find its commercial value.
FBC IN-BED TUBES
Maintaining temperature alltime low of 850-900 oC is because of the remarkably high co-efficiency of heat transfer in the FBC in-bed tube submerged inside the fluidizing sand. Compared to the normal co-efficiency of heat transfer of radiation tube or boiler convection tube area, co-efficiency of FBC in-bed tubes can generate steam 5 - 6 times more from the same heating area. This produces great amount of evaporation in this FBC inbeded tube area although in relatively small area and the flue gas entering into the convection boiler tube zone will be below 600 oC which is almost 200 oC lower than that of other conventional boilers.