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
6RU - 30 July 2013 V07
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
14RU - 30 July 2013 V015
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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