PowerPoint Presentation

1TN - 28 Nov 2013 V0

COMBUSTION IN A CFBC BOILERTAPASH NAGCFBC CHARACTERISTICS

2TN - 28 Nov 2013 V0Operates under special fluid dynamic conditionSolid particles are mixed through furnace at a velocity exceeding the average terminal velocity of the particlesMajor fractions of solids are captured by cycloneHigh recycle rate intensifies solid mixing and evens out combustion temperatureHigh turbulence, solid mixing and absence of a defined bed levelSolids are distributed through out the furnace with steadily decreasing density from bottom to top of furnace

COMBUSTION FACTORS

3TN - 28 Nov 2013 V0COMBUSTIONPASACOAL QUALITYCOAL SIZEBED HEIGHTBED TEMPASH RECFURNACE VOLUME

Conveys coalUniform distribution of coal in furnaceProvides air for combustionFluidizes the bed ashEnsures proper mixingProvides sealing air to feeders

4TN - 28 Nov 2013 V0PRIMARY AIR

Staged CombustionMaintains bed temperatureBurner airProvides excess air requirementControls overboard temperatureEliminates NOx formation

5TN - 28 Nov 2013 V0SECONDARY AIR

RU - 30 July 2013 V06

The primary air shall be adjusted in such a manner that it provides minimum fluidization and better combustion

The secondary air shall be adjusted in such a manner that it has maximum penetration so that there is adequate reaction of oxygen with fuel.

High pressure for bottom SA is required, to guarantee high penetration and better mixing of air with bed material

If PA quantity is high and SA quantity is not adequate, the fuel combustion in dense phase of furnace, i.e., the lower portion will decrease but increase in the lean phase or the upper portion resulting in more water wall erosion.

Secondary combustion shall occur in boiler cyclone and seal pot due to less SA quantity.

The temperature of flue gas at cyclone outlet will rise and there will be wide variation between cyclone inlet and outlet temperatures The heat transfer in back pass will increase due to high flue gas temperature. Attemperation will increase and heating surface will be damaged

AIR DISTRIBUTION CRITERIA

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AIR ADJUSTMENT TABLESRLOAD(MW)SA DAMPER POSITION (%)SA HEADER PRESSURE (kPa)PA QUANTITYBED HEIGHT (kPa)BED TEMP()OXYGEN(%)TOPBOTTOM15045256.0Critical fluidizing air10.0

Around 7004.0-5.0250-8040-5030-406.0-7.5Critical fluidizing air11.0

700-8503.5-4.2380-10060-7040-507.5-8.0Adjust bed temperature 11.5

800-8503.5-4.04100-12070-8050-608.0-9.0Adjust bed temperature12.0800-900~ 3.55120-13580-9060-709.5Adjust bed temperature12.5800-9103.0-3.5

7RU - 30 July 2013 V08

SECONDARY AIRPOOR

OXYGEN

COREOXYGEN PROFILE IN CFBC FURNACEPoor oxygen core formed due to the combustion in reducing atmosphere due to insufficient mixing of airResults in bad combustion efficiency when burning coals with low volatile contentStaged air supply to be provided for better combustionProper PA to SA ratio should be maintainedProper air velocities should be maintainedAngle and size of SA duct should be such that the penetration length of air jet is deep in to the furnace

RU - 30 July 2013 V09OPERATION PRACTICES

FUELVOLATILE CONTENT(%)RATIO OF PA TO TOTAL AIRANTHRACITE40~50RATIO OF PRIMARY AIRVELOCITY OF AIR (m/s)PA(Based on throat area of Nozzle)SA~4055~85

RU - 30 July 2013 V010

Coefficient of Excess air is a function of the fuelReasonable air supply ensures better temperature distribution in the furnace and better combustion efficiencyEnsures lower NOX generation

EXCESS AIR REQUIREMENTSEXCESS AIR AT FURNACE OUTLETFUELVOLATILE CONTENT(%)COEFFICIENT OF EXCESS AIRANTHRACITE401.20~1.22

GCVMoistureAsh ContentVolatile materialCarbon content

11TN - 28 Nov 2013 V0COAL QUALITY

FinesOversizeDistributionVolatile materialCarbon content

12TN - 28 Nov 2013 V0COAL SIZE

RU - 30 July 2013 V013

Hard Coal with less volatile dont burn effectively in CFBC boiler

Reactivity of carbon decreases while in circulation for a long time

Coals having low fusion temperature can be burnt as temperature in dense bed and free board is around 850-900oC

Fuel grain size distribution depends upon the material balance, the rate of combustion in the dense phase and free board and combustion efficiency

The biggest grain size will be fluidized in dense phase and will start burning

The finer grains shall be entrained in the flue gas and burn in free board zone

The finest grains up to 90 microns shall be returned by the separator

The coal of size higher than 6 mm may be permitted, if the coal has fragmentation characteristics during heating

FEW FACTS

13RU - 30 July 2013 V014

COAL INDEXI (MJ/kg)%C IN ASH LEAVING FURNACECOAL INDEX

I = Vdaf QLHV

Vdaf = Volatile content in coal on dry basis

QLHV = Lower heating value of coal

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SIZE DISTRIBUTIONPARTICLE SIZE (m)SIZE FRACTION (%m)

15Should be sufficient enough to burn the large size particles in dense phaseBed height should be such that it provides enough resistance to the PA supplied to the furnaceLower bed height will increase the PA velocity resulting in erosion of water walls and entrainment of high size particles in lean phaseDepends on the coal quality and sizeBed height higher than normal will reduce the bed temperature and will reduce combustion efficiencyBed height is a function of load

16TN - 28 Nov 2013 V0BED HEIGHT

Should be around 850-900oCToo low the temperature will result in inefficient combustion, will increase the cyclone and seal pot temperatures. Back pass temperatures will increase. Will increase attemperation.Too high the temperature will result in clinker formation and agglomeration Depends on coal quality, bed height, air velocity and quantityMaintaining bed temperatures within the specified range is very important for ensuring that the heat transfer takes place in the boiler as per the design specifications

17TN - 28 Nov 2013 V0BED TEMPERATURE

Cross section determines the velocity(5 to 6m/s) of air in the furnaceIf cyclone separators are arranged in one direction, the ratio of furnace width to depth is equal to the no of cyclonesIf cyclone separators are arranged in side, the ratio of furnace width to depth is equal to th of no of cyclonesRatio of width and depth determines the SA penetration in the furnaceMinimum 20m height is required for a CFBC boiler for ensuring better combustion efficiencyFurnace height is too high for the water wall length required

18TN - 28 Nov 2013 V0FURNACE VOLUME

RU - 30 July 2013 V019RECOMMENDED FURNACE HEIGHT

FUELBOILER CAPACITY(TPH)35751302204106801000ANTHRACITE20212632364354BITUMINOUS20202529343950LIGNITE20202428303747IT IS OF PRIME IMPORTANCE THAT THE FURNACE HEIGHT SHOULD BALANCE THE COMBUSTION EFFICIENCY AND THE HEATING SURFACE. BELOW IS THE RECOMMENDATIONS FOR DIFFERENT CAPACITY CFBC BOILERSIf Combustion is the heart, loop seal is the valve of the heart

20TN - 28 Nov 2013 V0ASH RECIRCULATION

RU - 30 July 2013 V021

Returns solids captured by the cyclone to the furnace

Prevents backflow of flue gas from furnace to cyclone

Failure of loop seal to transfer the solids at required rate will result in an entrained bed conditions

Large temperature gradient along the furnace, low heat absorption & higher back pass temperature

Loss in steam output, excessive steam temperatures & high stack temperature

Height of dip leg is very important. It shall be always more than riser leg

Pressure difference between dip leg and riser shall be around 2kPa

The air velocity in dip leg shall be around 0.3m/s and in riser leg shall be around 1.3 m/s

The velocity of circulating material in loop seal shall be around 0.2m/s

High air velocity may result in combustion in loop seal causing agglomeration

LOOP SEAL FUNCTION & ADVANTAGES21RU - 30 July 2013 V022

QUESTIONS PLEASE

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