FLUIDIZED BED COMBUSTION SYSTEM (FBC)

INTRODUCTION:-

• With Escalating Price of oil & gas during the last decade the world power industry shifted from oil to coal as coal is abundant compared with oil & gas.

• Note – now days Indian power industry struggling with coal also , but to know how the FBC technology comes in to the picture, it is necessary to go through this.

• With growing realization that the oil & gas are not going to be available for electric power generation in the near future. The world power industry is slowly moving for the old faithful fuel i.e. coal.

PULVERISED FUEL FIRED BOILER:-

• Pulverized fuel firing was developed earlier this century and universally used throughout the world till today for power generation.

Draw back of P.F. Boilers:-

• A pulverized fuel fired furnace designed for a particular type of coal cannot be used for any type of coal with same efficiency and safety.

• The size of the coal used is limited i.e. 70-100 micron therefore large investment is needed for coal preparing equipment & its maintenance

• The ignition of the coal particles becomes easy & combustion becomes steady when the temp in the furnace is 1650 deg centigrade.

• The amount of NOx formed is large compare with any type of combustion system as the temperature maintained in the furnace is high. The removal of SO2 demands high capital cost equipment.

• New rules & regulation imposed by government for the air pollution the cost of power generation went high as extra equipment are needed to control the air pollution to the required level.

• At present the boiler are designed to suit the fuel characteristics. The configuration and size of the boiler furnaces and the burner differ considerably depending upon whether the coal is anthracite,bituminous,lignite ,oil or gas.

• On other hand FBC can accept any fuel including low grade coals, oil, gas or municipal waste.& also control of Sox, Nox, emissions.

PRINCIPAL OF FBC SYSTEM:-

• When a air is passed through a packed bed of finely divided solid particles it experiences the pressure drop across the bed. The mixture of solid particles and air is like a fluid. Burning of a fuel in such a state is known as fluidized bed combustion.

• The fuel & inert material dolomite/limestone are fed on a distributor plate and air is supplied from the bottom of the distributor plate. High velocity of air keeps the solid feed material in suspended condition during burining.the generated heat is rapidly transferred to the water passing through the tube immersed in the bed & generated steam taken out.

• During the burning SO2 formed is absorbed by the dolomite /lime stone and prevent it escape with the exhaust gases. The molten slag is tapped from the surface of the bed in form of bed drain.

• The combustion efficiency remain very high 99.5 % as very high heat transfer rate are maintained over the surface of the tube.

• Even the poorest grade coal could burnt without sacrificing combustion efficiency.

• The heat transfer rate to the surface is high as the system behave like a violently boiling liquid & nearly 50% of heat transfer released in the bed is absorbed by the tubes.

• The bed operating temperature of 800-900 is ideal for sulphur retention by addition of limestone or dolomite to the bed bring down SO2 emission level to 15% of that in conventional firing method. Low Nox emission is automatically achieved in FBC due to low bed temperature & low excess air compared to pulverized fuel furnace.

• The cost economics shown that a saving of about 10% in operating cost & 15% in capital cost. Could achieved for unit rating 120mw.

• Size the of the coal used to preferred is 6 to 13mm.

FUNDAMENTAL OF FLUIDIZATION :-• Fluidization is the operation by which the solid are

transformed into fluid like state through air. When air is passed vertically upward through a bed of solid particles supported by grid.

• Fixed bed:- The air at low velocity will tends to follow the path of least

resistance & pass upward through the bed with pressure drop. This is called fixed bed.

Minimum Fluidization:-• With increase in velocity of air a point is reached when the

solid particles just suspended in the upward flowing air. The bed is considered to be just fluidized and referred to as a bed at minimum fluidization.( the weight of the air in any section of bed is equal to weight of air & solid particles in that section)

BUBBLING BED:-• Any excess air above the minimum fluidization will cause

bubble formation and the excess air will escape the bed as bubble. This state is called as bubbling bed.

• TURBULENT BED :-As the velocity of air through bubbling bed is increased the bed expands & the bubbles constantly collapse and reform the bed surface is highly diffused & particles are thrown off into freeboard above. Such a bed is called as turbulent bed.

Circulating fluidized bed:-

• With further increase in air velocity the bed particles are entrained, separated from air and returned to the base of furnace. This is called circulating bed.

• Umf Ut

Fix bedMinimum fluidization bed

Bubbling bed

Tabulating bed

Circulating bed

FIXED BED MINIMUM FLUDISATION

BUBBLING BED

TURBULENT BED

CIRCULETING FLUDIZED BED

Minimum fluidization velocity (Umf):-

• The air velocity at which the pressure drop across the bed becomes equals to the weight of the particles per unit cross sectional area of the weight.

• Terminal velocity (Ut):-

The air velocity at which particles entrainmentOccurs is called the terminal velocity. When the drag forced of upward moving air is greater than the weight of the particle.

The classification of FBC boilers

• 1) Bubbling bed.• 2) Circulating bed.

FBC

Bubbling Circulating

AFBC PFBC ACFBC PCFBCV=3-5 m/sec 1-2m/sec up to 10m/sec ----

AFBC Boiler:-

• In AFBC boiler coal is cursed to a size of 1-6mm and fed in to the combustion chamber. the air enter through distributors in to bed of sand at a velocity of 3-5 m/sec turbulence is created.

• The gaseous product of combustion pass over the superheater,economiser,APH,ESP being exhausted to atmosphere.

PFBC TYPE:-

• THE PFBC is similar to AFBC but operated at high pressure .the FD fan is replaced with a compressor and the combustion is in pressure vessel. The heat release rate in the bed be proportional to the bed pressure and hence deep bed is used to extract large amount of heat. This is not yet commercialized.

CFBC TYPE :-

• In circulating system the bed parameter are so maintained as to promote solids elutriation from bed. they are lifted in relatively dilute phase for solid riser and a down-comer with a cyclone provide a return path for solid. There are no steam generation tubes immersed in the bed. Generation of superheating steam takes place in the convection section.

ADVANTAGES OF FBC:-• It can use solid liquid and gaseous fuel or mix as well as domestic &

industrial waste.• High combustion efficiency.• The SO2 formed due to combustion of sulphur can be absorbed in

the fluidized bed.• The combustion in conventional system become unstable when the

ash exceed 48% but even 70% ash containing coal can efficiently burnt.

• Flue gases from conventional oil-fired boiler plant-300-500ppm Nox• Coal fired plant -400-800ppm• FBC with oil -100-120ppm• FBC with coal – 60-100ppm.• Pulverization of solid fuel is not necessary with fluidized bed

combustor. the maximum size of coal particle is limited to 6mm.

BED MATERIAL SIZE & SPECIFICATIONS:-• Bed Material Size:• 0.85mm to 2.36mm (not below 0.85mm)• Distribution:• 0.85 mm to 1.0 mm: 10%• 1.0 mm to 1.5 mm: 50%• 1.5 to 2.36 mm: 40%• Bed Material Specifications:• Crushed Fire Bricks Castable IS8 grade Bricks or River Silica Sand• Fusion Temperature : 1300 Deg. C.• Shape : Spherical Angular• Bulk Density (Kg/m3) : 1050• Silica (SiO2) : 68 %• Al203 : 28 %• Fe203 : 1.05%• PiO2 : 1.67%• MnO : Trace• CaO : 0.54%• MgO : 0.23%• P2O5 : 0.08%• A2O : 0.22%• K2O : 0.45%

Differential Pressure Test :-• Before going to bed material filling .we must know the cleanliness of

bubble caps. the fluidization depends on this aspect. Hence for checking bubble cap cleanliness we have conduct DP test.

• For conducting DP test following procedure to be follow.• Initially close the wind box & burner damper• ID fan PA fan suction discharge damper closed.• First start ID fan open discharge damper then suction damper maintain the

draft inside the furnace -5mmwc.• Start PA fan-1 open discharge damper then suction damper.• Start PA fan -2 open discharge damper then suction damper.• Open all burner damper & wind box damper• Increase the air flow by increasing the VFD of both PA fan and the maintain

the draft -5mmwc with the help of ID fan.• Boiler having total air flow requirement of 60-65 TPH for carring out DP

test.• Step by step increasing the air flow with help of PA fan increase upto 60

TPH by maintaining the draft -5mmwc and note down the WB Pr.

• Note down the wind box pressure reading all reading are to be taken in table given

S.NO. Air Flow in TPH Wind Box Pr mmwc

1

2

3

4

5

• BED MATERIAL FILLING:-

• After completion of DP test with reading obtained we can ensure the cleanliness of bubble caps .then we can proceed for bed material filling.

• Note down the wind box pressure reading at 60 TPH keeping wind box & burner damper in open condition.

• Maintain draft -5mmwc then start filling of bed material.

• While filling the bed material intermittently observe the wind box readings by observing the wind box pressure reading also we can judge fluidization inside the furnace.

• For judging the bed height we have to subtract the initial wind box pressure reading from the present wind box pressure reading.

• Bed height=(Present WB Pr – Initial WB Pr)

Case study:-• Effect of high bed height:-a) Bed temperature will be low b) Main steam temperature will be lowc) Pressure will be lowd) Air flow is also reduce

How bed height will be increase:-

• All bed drain point is chocked .like emergency drain ,normal drain, bed ash cooler problems.

• If coal contain, percentage of ash more compare to design.

• If using dolachar with coal.• If coal size is more • Unburnt present in bed • Shells present in coal.

• When your bed height is increase your temp will drop simultaneously your pressure will be drop.

• To maintain the pressure & temperature to desired level in simple way we will increase the coal feeding. So your sp.coal consumption will increase. Because your bed height is increase condition.

• If your bed height continuously maintain or increase above from your desired level even your bed ash cooler is in running condition.

• That means your bed height not maintaining with the help of 1 BAC. In that case you have to start another bed ash cooler.

• The cost of running another bed ash cooler is less when we compare it with increase coal feeding.

• I have consider the data of 1 hrs for calculation of cost difference.

• On the bases of data we have.• Our coal feeding has increase 2T/hrs more or less. due to bed eight

increase from 820mmwc to 868mmwc• If cost of coal we have is -3500/- per tonne• So in 1hr we consume 2T more coal.• The cost of increase coal is = 2x3500 = 7000/-So in 1hr we consume

7000 Rs coal more .• This is because of increase bed height.• On other hand if we run another bed ash cooler to maintain the

desired bed height we save 10 times more with compare to coal cost.

• So to maintain the bed height 820mmwc we have to run another bed ash cooler.

• The cost of running 1 BAC .• The rating of BAC motor = 7.5kw.• The energy consumed by BAC in 1 hr =7.5kw.• As 1unit = 1kw• The cost of 1unit = 4.50/- • So cost to run the BAC = 4.50X7.5 =33.75/- in 1hrs.• The cost of running BAC is also less than 33.75 Rs because

we cannot run the BAC in full load. We always run it on 70% VFD.

• Another thing is that .if to convey the bed material from vessel to silo with the help of service air.

• If service air pressure will be drop due to running of another bed ash cooler form 6kg/sq.cm to 4.5kg/sq.cm.

• To maintain the pressure we start another compressor.

• The cost of running another comp for 1hrs is• Rating of compressor = 160kw• 1 unit = 1kw.• Unit consume =160kw in 1 hr• Running cost of 1 hr =160 x 4.5 = 720/-

• Cost of run the comp for 1hr =720 Rs.• The cost of increased coal feeding = 7000 Rs.• The cost to run the BAC & comp • =720 +33.75=755.75 Rs.• So from this we have seen that 10 times more

operating cost increases in 1hr because of increase coal feeding.

• From this to run another BAC to maintain bed height is beneficial to instead of increase coal feeding.

EMISSION CONTROL :-• Sulpher dioxide• Sulphur in coal react with oxygen present in

combustion air forms sulphur dioxide (SO2)• Why sulphur dioxide control.• When gaseous SO2 combine with water it forms

dilute solution of sulphurous acid (H2SO3)• SO2 +H2O ------- H2SO3

• Sulphurous acid can easily oxidize in the atmosphere to form sulphuric acid.

• Dilute sulphuric acid is major constituent in acud rain.

• SO2 control in the combustor:-• Inexpensive material like lime stone ,dolamite use

as a sorbent to SO2 from fuel gas when lime stone or dolamite added in bed reaction occurs.

• CACO3 + 766btu/lb ------- CAO +CO2• The Cao lime react with gaseous SO2 and O2 at the

temp of around 815-830 decent exothermically to form CASO4.

• CAO +SO2+1/2 O2 ----------- CASO4.• CASO4 is chemically stable at fluidized bed

operating temp and is removed from the system as a solid along with ash for disposal.

Control of Nox emission:-• In bubbling bed ,Nox emission are controlled to an

acceptable level due .• 1) low operating temp around 815 descent.

Compared to pf fired boiler• This temp is too low for the N2 in the air to react

with oxygen• Nox also form by the nitrogen present in the fuel.

this reaction is highly dependent on the amount of oxygen avaible for the reaction.

TROUBLE SHOOTING CHART:-• 1) Clinker formation:-• Probable cause :- • a) more fuel in the combustor.• b) less air flow to the combustor.• c) low ash fusion temp.

Solution:- a) Control fuel/air flow to the combustor.

b) bed temp should not be above ash fusion temp.

• Good start up avoid clinker formation.• 2 High bed temp:-• Probable cause-a) Less fines in the fuelb) Low air to the combustor.c) Bigger fuel size.d) Low bed height.e) More fuel in combustor.

• Solution:-a) Correct the fuel size & adjust the air flow.b) Cut the fuel feed.c) Increase the bed height by feeding the bed

material.LOW BED TEMP:-Probable cause

d) high fines in fuel.e) excess air to the combustor.f) bed packing due to stone & shells.g) clincker in the region.

• SOLUTION :-a) correct the fuel size & adjust the air flow.b) drain the bed & clear the heavy material

from the bed.BED TEMP VARITION:-Probable cause :-a)uneven coal concentration in bed.SOLUTION :-a)adjust the fuel feeding setting.

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