blr ah perf indices
TRANSCRIPT
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Factors affecting Boiler Performance
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Boiler Performance Characterisation
Combustion / Thermal Efficiency- Conversionof chemical heat in fuel to production of steam
adequate Time / Temperature / Turbulence
Auxiliary Power Consumption The total powerbeing consumed by ID, FD, PA fans and the mills.
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OFF Design/Optimum Conditions
Parameter Deviation Effect on Heat
RateExcess Air (O2) per % 7.4 Kcal/kWhExit Gas Temp per
oC 1.2 Kcal/kWh
Unburnt Carbon per % 10-15 Kcal/kWhCoal moisture per % 2-3 Kcal/kWh
Boiler Efficiency per % 25 Kcal/kWh
Effect of Boiler side Parameters (Approx.)
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Boiler Control Volume
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Factors affecting Boiler efficiency include
Design Coal Quality
Mill Performance - PF Fineness
Burner-to-burner PF balance
Excess Air Level
Boiler Air Ingress
AH Performance Furnace / Convective section Cleanliness
Quality of Overhauls
Water Chemistry, boiler loading, insulation etc.
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Efficiency Vs HHV of
Coal
AssumptionsExit Gas Temp - Constt.
Fuel Moisture - Constt
Fuel Hydrogen - ConsttExcess Air - 20 %
GCV - 3700 kal/kg
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Proximate Analysis, Ultimate Analysis, CalorificValue, Ash Constituents, Ash Fusion Temperatures,FC/VM ratio, Hard Grove Index, YGP (Yeer Geer Price)Index
Typical Proximate Coal Analysis - Fixed Carbon - 32.4%, Volatile matter - 21.6 %, Moisture 16.0 %, Ash30.0 %, GCV 4050 kcal/kg
+ve aspects - Low Sulfur, Low chlorine, Low iron
content and High Ash fusion temp-ve aspects - High ash, moisture, high silica /alumina ratio, low calorific value, high electricalresistivity of ash,
Problem
The Coal
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CAsh H O N S Mi M
As fired basis
Air dry basis
Dry basis
Dry & Ash free basis
A FC VM MCoke Volatile
Ultimate
Proximate
Coal Composition -
Different bases of representation
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Coal characteristics decide the heat release rates,furnace wall conditions and consequently the
furnace heat transfer
Deterioration in Coal quality affects boiler capabilityto operate at rated parameters.
Change in coal quality affects capacity, efficiencyand combustion stability.
Increase in moisture affects mill drying, temperingair requirement, gas velocities, ESP & Boilerefficiency.
Ash quality / quantity affects boiler erosion, millwear, slagging and fouling propensity, ashhandling system, sprays, sootblowingrequirements etc.
Change in coal characteristics affects mill wearparts life & throughput of Pulverizers.
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PF fineness
Typical recommended value of pulverised fuel
fineness through 200 mesh Sieve is 70% and 1%
retention on 50 mesh sieve.
Fineness is expressed as the percentage pass
through a 200-mesh screen (74m).
Coarseness is expressed as the percentage
retained on a 50-mesh screen (297m).
Screen mesh indicates the number of openings per
linear inch.
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Excessive PF fineness would cause
Reduction in mill capacity
Increased mill component wear
Increased mill and fan power combustion
Excessive PF fineness may not necessarily result
in improved combustion
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Control Room
Boiler
1
2 3
4
Mills
Mill discharge pipes offer different resistance to the flows dueto unequal lengths and different geometry layouts.
Fixed orifices are put in shorter pipes to balance velocities /
dirty air flow / coal flows. The sizes of the orifices are specified
by equipment supplier.
A B C D E F
Burner Imbalance
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Burner Imbalance
Primary Air Flow
Coal Flow
Dirty air flow distribution should be with in +/-5.0% of the average of fuel pipes
Coal distribution should be with in +/-10% of
the average of fuel pipes
Balanced Clean air flows do not necessarily
result in balanced Dirty air flows.
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Burner Balance
Balanced PF flows are an essential pre-requisite to an
optimized combustion. Usually the imbalance gets
camouflaged by additional excess air, thereby losing out on
boiler efficiency and operating flexibility.
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Excess Air
Low excess air operation can lead to unstable combustion (furnace puffs)
increased slagging of waterwalls and SH sections
Loss in boiler efficiency due to increased CO / unburnt
combustibles
High excess air operation can lead to
Increased boiler losses High SH / RH temperatures
Higher component erosion
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Boiler Air Ingress
Cold air leaks into the boiler from openings in the furnace andconvective pass and through open observation doors.
Some of the boiler leakage air aids the combustion process;
some air that leaks into the boiler in the low temperature zones
causes only a dilution of the flue gas.
This portion of air appears as a difference in O2 level between
the furnace exit and oxygen analysers at economizer exit.
Actual oxygen in the furnace could be much less.
Also, boiler casing and ducting air ingress affects ID fans power
consumption and margins in a major way.
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The difference between oxygen at furnace outlet(HVT) and economizer outlet (zirconia) was in therange of 1.0 to 2.5 % in many boilers.
Apart from degradation of AH baskets performance,another reason for lower heat recovery across airheaters is boiler operation at lesser SA flows due tohigh air-in-leakage.
Air Ingress
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Boiler operation under adverse conditions continues as inmajority of units On line CO feedback is not available.
All boilers need to be equipped with On line CO monitors
at Eco Outlet / ID fan discharge.
Air ingress across AH outlet to ID suction observed to begenerally in the range of 5 to 9%.
Flue gas ducts & expansion joints at Eco outlet and APH
inlet / outlet inspected thoroughly during O/H
Replacement of Metallic / Fabric Expansion joints in 10
years / 5 years cycle
Air Ingress
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Air Heaters
Factors affecting performance include
Operating excess air levels
PA/SA ratio
Inlet air / gas temperature
Coal moisture
Air ingress levels
Sootblowing
No. of mills in service
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Air Heaters
Factors affecting performance include
PA Header Pressure
High pressure results in increased AH leakage, higher
ID fan loading, higher PA fan power consumption,deteriorates PF fineness & can increase mechanical
erosion
Upstream ash evacuation
Maintenance practices
Condition of heating elements, seals / seal setting,
sector plates / axial seal plates, diaphragm plates,
casing / enclosure, insulation
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Boiler Exit Gas Temperature
Ideal flue gas temperature at stack outlet should be just above thedew point to avoid corrosion; Higher gas temperatures reduce
efficiency; Possible causes of temperature deviations are
Dirty heat transfer surfaces High Excess air
Excessive casing air ingress
Fouled/corroded/eroded Air heaterbaskets
Non - representative measurement
Contd..
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Air Heaters - Exit Gas Temperatures
Factors affecting EGT include
Entering air temperature - Any changes would
change exit gas temperature in same direction
Entering Gas Temperature - Any changes wouldchange exit gas temperature in same direction
X-ratio - An increase in X-ratio would decrease exit
gas temperatures & vice versa
Gas Weight - Increase in gas weight would result inhigher exit gas temperatures
AH leakage - An increase in AH leakage causes
dilution of flue gas & a drop in As read exit gas
temperatures
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AUXILIARY POWER CONSUMPTION
Major auxiliaries Consuming Power in a Boiler areFD fans, PA fans, ID fans and mills. Reasons forhigher APC include
* Boiler air ingress
* Air heater air-in-leakage
* High PA fan outlet pressure
* Degree of Pulverisation
* Operation at higher than optimum excess air
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Main Steam/ Reheated Steam Temperature
While an increase in steam temperatures is beneficial
to Turbine Cycle Heat Rate, theres no benefit to
boiler efficiency, infact it affects reliability adversely.
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Testing Techniques & Performance Optimisation
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Boiler & Air Heater Tests
Tests to be conducted underdefined operating regime (O2
level / PA Header Pressure / no. of mills) at nominal load
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Pre Test Stabilisation Period
Prior to the test run, equipment must be operated at steadystate conditions to ensure that there is no net change in
energy stored in steam generator envelope.
Minimum Stabilisation Time - 1 hour
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Test Duration
Should be sufficient to take care of deviations in parametersdue to controls, fuel variations & other operating conditions.
When point by point traverse of Flue gas ducts is done, test
should be long enough o complete atleast two traverses.
In case of continuous Data Acquisition System & use of
composite sampling grids, shall be based on collection of
representative coal & ash samples.
Could be 1/2 to 2 hours in case of parametric optimisation
tests or 4 hours for Acceptance Tests.
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Frequency of Observations
Parameter readings to be taken at a maximum interval of 15minutes & a preferred interval of 2 minutes or less
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Measurements during a Boiler Test Coal Sample for Proximate analysis & GCV
Bottom Ash and Flyash Samples
Flue Gas Composition at AH Outlet
Flue Gas Temperature at AH Inlet / Outlet Primary / Secondary air temp at AH inlet / outlet
Dry / Wet bulb temperatures
Control Room Parameters
(All measurements / sampling to be done simultaneously)
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Coal Sampling
Coal Samples are drawn from all individual running
feeders from sampling ports in feeder inlet chutes
Composite sample is collected from all running feeders
One sample is sealed in an air tight container for total
moisture determination
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Flyash Sampling
Flyash is collected in several hoppers as Flue Gas goes
to stack; Heavier particles fall out first due to turns in gas
stream
Relative distribution of ash to various hoppers is notaccurately known
Preferred way to collect a) a representative sample b)
sample of the test period is to use High Volume Sampler
probes on both sides of boiler
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Need for Off line Grid Measurement
On Line Instruments are adequate to monitor airheater performance but not good for assessing
degradation. PG tests also necessitates installation of
grid in air and flue gas ducts.
a) Flue gas O2 measurement at AH outlet is not
available
b) Single point Orsat can be misleading due to
stratification in flue gas
c) The grid also validates & cross checks
representative ness of online feedback
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FG
Economizer
FG
APH
Sampling
Locations
APH
Expansion
Bellow
Test Locations - AH Inlet & Outlet
Inlet Sampling plane to be as close to AH as possible; Outletgrid to be a little away to reduce stratification
AH hopper / Manhole air ingress can influence test data
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Sampling Ports in Flue Gas Ducts (Typical )
Sampling Point for Flue Gas Temperature & Composition
100mm
Flue Gas Duct is divided into equal cross-sectional areas and
gas samples are drawn from each center
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Typical problems
High Economiser / AH exit gas temperature Air ingress from furnace bottom, penthouse and
second pass
Boiler operation at high excess air
Metal temperature excursions
High Unburnt carbon in ashes
Uneven Flyash Erosion
Flame failures
Shortfall in steam temperatures
Imbalance in Left - Right steam temperatures
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Deterioration of Boiler efficiency and increase in auxiliary poweris generally on account ofAir Heater performancedegradation from O/H to O/H.
Major symptoms of this degradation include the following
Increased flue gas volume - Affects ESP performance
Lower flue gas exit temperatures due to high air heater leakage- An
erroneous boiler efficiency feedback generates complacency
Lower fan margins - Limit the unit output at times
Boileroperation at less than optimum excess air- Specially in units
where in ID fans are running at maximum loading
Air Heaters
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Air Heater - PerformanceIndicators
Air-in-Leakage
Gas Side Efficiency
X - ratio
Flue gas temperature drop Air side temperature rise
Gas & Air side pressure drops
(The indices are affected by changes in entering
air or gas temperatures, their flow quantities and
coal moisture)
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AH Performance Monitoring
O2 & CO2 in FG at AH Inlet
O2 & CO2 in FG at AH Outlet
Temperature of gas entering / leaving air heater Temperature of air entering / leaving air heater
Diff. Pressure across AH on air & gas side
(Above data is tracked to monitor AH performance)
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Air Heater Leakage (%)
The leakage of the high pressure air to the lowpressure flue gas is due to the Differential Pressure between
fluids, increased seal clearances in hot condition, seal erosion /
improper seal settings. Typically air heater starts with a
baseline leakage of 6 to 10% after an overhaul.
Increased AH leakage leads to
Reduced AH efficiency
Increased fan power consumption Higher gas velocities that affect ESP performance
Loss of fan margins leading to inefficient operation and attimes restricting unit loading
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Air Heater Leakage (%)
Direct - Hot End / Cold End(60% through radial seals + 30% through Circumferential
bypass)
Air leakage occurring at the hot end of the airheater affects its thermal and hydraulic
performance while cold end leakage increases
fans loading.
Entrained Leakage due to entrapped air between the
heating elements (depends on speed of rotation &
volume of rotor air space)
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Air Heater Leakage - Calculation
This leakage is assumed to occur entirely between air inlet
and gas outlet; Empirical relationship using the change in
concentration of O2 or CO2 in the flue gas
= CO2in - CO2out * 0.9 * 100 CO2out
= O2out - O2in * 0.9 * 100 = 5.7 2.8 * 90
(21- O2out) (21-5.7)
= 17.1 %CO2 measurement is preferred due to high absolute values; In
case of any measurement errors, the resultant influence on
leakage calculation is small.
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Gas Side Efficiency
Ratio of Gas Temperature drop across the air heater,
corrected for no leakage, to the temperature head.
= (Temp drop / Temperature head) * 100
where Temp drop = Tgas in -Tgas out (no leakage)
Temp head = Tgasin - T air in
Gas Side Efficiency = (333.5-150.5) / (333.5-36.1) = 61.5 %
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Tgas out (no leakage) = The temperature at which the gas
would have left the air heater if there were no AH leakage
= AL * Cpa * (Tgas out - Tair in) + Tgas out
Cpg * 100
Say AH leakage 17.1%, Gas In Temp 333.5 C, Gas Out Temp
133.8 C, Air In Temp 36.1 C
Tgasnl = 17.1 * (133.8 36.1) + 133.8 = 150.5 C100
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Air Heaters Good Practices
AH sootblowing immediately after boiler light up.
Monitoring of Lub oil of Guide & Support bearings
through Quarterly wear-debris analysis.
Hot water washing of air heaters after boilershutdown - flue gas temperature ~ 180 to 150 C
with draft fans in stopped condition. (Ideally pH
value can verify effective cleaning)
Basket drying to be ensured by running draft fans
for atleast four hours after basket washing.
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Air Heaters
Baskets cleaning with HP water jet cleaning during
Overhauls after removal from position
Heating elements to be covered with templates during
maintenance of air heaters.
Gaps between diaphragms & baskets to be closed for
better heat recovery & lower erosion rate at edges.
Ensuring healthiness offlushing apparatus of Eco &
AH ash hoppers
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Boiler Performance
Boiler Efficiency
The % of heat input to the boiler absorbed by the
working fluid (Typically 85-88%)
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Boiler Efficiency
Boiler Efficiency can be determined by
a) Direct method or Input / Output method
b) Indirect method or Loss method
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Boiler Efficiency
Direct method or Input / Output method measures theheat absorbed by water & steam & compares it with the
total energy input based on HHV of fuel.
Direct method is based on fuel flow, GCV, steam flow
pressure & temperature measurements. For coalfired boilers, its difficult to accurately measure coal
flow and heating value on real time basis.
Another problem with direct method is that the extentand nature of the individual components losses is not
quantified.
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Boiler Efficiency
Indirect method or Loss method
For utility boilers efficiency is generally calculated by heat loss
method wherein the component losses are calculated andsubtracted from 100.
Boiler Efficiency = 100 - Losses in %
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Indirect or Loss method
In Heat Loss method the unit of heat input is the higher
heating value per kg of fuel. Heat losses from various
sources are summed & expressed per kg of fuel fired.
Efficiency = 100 (L/Hf) * 100L losses
Hf heat input
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Indirect or Loss method
This method also requires accurate determination of
heating value, but since the total losses make a relatively
small portion of the total heat input (~ 13 %), an error in
measurement does not appreciably affect the efficiency
calculations.
In addition to being more accurate for field testing, the
heat loss method identifies exactly where the heat
losses are occurring.
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Boiler Efficiency
Commonly used standards for boiler performance testing are
ASME PTC 4 (1998)
BS 2885 (1974)IS: 8753: 1977
DIN standards
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rameters required for computing Boiler Efficiency
AH flue gas outlet O2 / CO2 / CO
AH flue gas inlet and outlet temp C
Primary / Secondary air temp at AH inlet / outlet C
Total Airflow / Secondary Air Flow t/hr
Dry/Wet bulb temperatures C
Ambient pressure bar a
Proximate Analysis & GCV of Coal kcal / kgCombustibles in Bottom Ash and Flyash
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Boiler Losses Typical values
Dry Gas Loss 5.21Unburnt Loss 0.63
Hydrogen Loss 4.22
Moisture in Fuel Loss 2.00
Moisture in Air Loss 0.19
Carbon Monoxide Loss 0.11
Radiation/Unaccounted Loss 1.00
Boiler Efficiency 86.63
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Dry Gas Loss (Controllable)
This is the heat carried away by flue gas at AH outlet
Its a function of flue gas quantity and the temperature
difference between air heater exit gas temperature andFD fan inlet air temperature
Typically 20 C increase in exit gas temperature ~ 1%
reduction in boiler efficiency.
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Dry Gas Loss
Sensible Heat of flue gas (Sh)
Sh = Mass of dry flue gas X Sp. Heat X (Tfg Tair)
Dry Flue Gas Loss % = (Sh / GCV of Fuel) * 100
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Unburnt Carbon Loss (Controllable)
Loss due to Unburnt Carbon
= U * CVc * 100 / GCV of Coal
CVc CV of Carbon 8077.8 kcal/kg
U = Carbon in ash / kg of coal
= Ash * C (Carbon in coal)
100 100 - C
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Influencing Factors - Unburnt Carbon Loss
Type of mills and firing system
Furnace size
Coal FC/VM ratio, coal reactivity
Burners design / condition
PF fineness (Pulveriser problems)
Insufficient excess air in combustion zone
Air damper/ register settings
Burnerbalance / worn orifices
Primar Air Flow / Pressure
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Computation - Moisture Loss
Total Moisture Loss
= (9H+M) * Sw / GCV of Coal
Sw Sensible Heat of water vapour
= 1.88 (Tgo 25) + 2442 + 4.2 (25 - Trai)
The moisture in flue gases (along with Sulphur in fuel) limits
the temperature to which the flue gases may be cooled due tocorrosion considerations in the cold end of air heater, gas
ducts etc.
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