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Thermal plant boiler optimisation
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Welcome To
Presentation on
Boiler Performance Optimization
Kanchan Nath Date: 18.02.08DGM(OS-NCR) Venue: PMI
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Emerging Market Requirements High Reliability & Availability
Economic Generation
Suitable for Differing Modes Of Operation
Suitable for Different Quality Of Fuel
Ability to Operate Under Adverse Grid
Conditions / Fluctuations
Minimum Emission Of Pollutants
Lowest Life Cycle Cost
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Threats
Increasing no. of Power Majors in the country Tariff-Market Driven Possibilities of losing out to competitors in
the changing business scenario causing hurdles for further capacity addition
Strategic changes in Government Policy for Power Sector
Stringent environmental norms
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DPNL SH
Platen SHT
R
RHTR
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCWpump
Bottom ash
stack
screentubes
Thermal Structure of A SG
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Boiler Performance
Boiler Performance consists of: Combustion Performance Air Heater Performance Milling System Performance Burner Performance Draft System Performance ESP Performance
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Boiler Performance
Contd… Boiler Losses Boiler Efficiency and Heat Rate Slagging / Clinkering Tube Leakage Performance Optimization
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Types of Combustion
Complete Combustion: In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen.
Incomplete Combustion: Incomplete combustion occurs when there isn't enough oxygen to allow the fuel to react completely with the oxygen to produce carbon dioxide and water
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Combustion Efficiency
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Combustion Efficiency
Combustion efficiency is a calculated measurement (in percent) of how well the heating equipment is converting a specific fuel into useable heat energy at a specific period of time in the operation of a heating system.
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Combustion Efficiency
Complete combustion efficiency (100%) would extract all the energy available in the fuel. However, 100% combustion efficiency is not realistically achievable due to stack loss and boiler shell losses.
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Combustion Efficiency Improvement
Combustion Efficiency relates to the part of the reactants that combine chemically. Combustion efficiency increases
with increasing temperature of the reactants increasing time that the reactants are in
contact increasing surface areas and increasing stored chemical energy
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Combustion Efficiency
Three Ts of Combustion:
Time: All combustion requires certain time which depends on types of reaction
Temperature: Temperature must be more than ignition temperature
Turbulence: Proper turbulence helps in bringing the fuel and air in intimate contact and gives them enough time to complete reaction
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Combustion Factors
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Factors Affecting PF Combustion
Coal Fineness Mill Outlet Temperature Excess Oxygen and CO Air Flow SA and PA Temperature Burner Tilt Residence time in the Furnace Air Distribution
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Coal Fineness It is a major factor which determines the
combustion efficiency of the Boiler Thumb Rule for coal fineness: 70% thro’ -
200 mesh, less than 1% for +50 Mesh Lower the coal fineness-losses in terms of
unburnt carbon in BA, slagging and subsequent deposits on the heating surface
Higher the coal fineness-losses in terms of energy consumption in the Milling system. High wear rate of Milling components
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Mill Outlet Temperature
Mill outlet temperature is an important factor for combustion and efficiency of milling system
Lower the mill outlet temperature-delay in combustion, improper air-fuel mixture, choking of mill/mill discharge pipes resulting in high rejects, reduction in mill throughput etc.
Very high mill outlet temperature-possibility of fire in the mill
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Air Flow
Air for combustion is divided into three types depending upon its role which are primary, secondary and excess air.
Primary air provides a percentage of the
combustion air, but more importantly, controls the amount of fuel that can be burned.
Secondary air improves combustion efficiency by promoting the fuel to burn completely.
Excess air is supplied to the combustion process to ensure each fuel molecule is completely surrounded by sufficient combustion air. As a burner tune-up improves the rate at which mixing occurs, the amount of excess air required can be reduced.
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Air Flow Too much, or too little fuel with the
available combustion air may potentially result in unburnt fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and additional (excess) air is required for good combustion. Too much additional air can contribute to CO generation, lower efficiencies and perhaps unsafe conditions with heating equipment not out living its full service life.
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Air Flow
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Excess Air Typically 20 % excess air is recommended for boiler
operation; Actual optimal value would vary from boiler to boiler depending on coal quality, fineness and other operating practices.
O2 instruments are installed at the economizer exit, where they can be influenced by air infiltration. The O2 reading in control room may not be necessarily representative of the actual O2 in furnace.
The most important variable in boiler operation is operating O2 / excess air and its availability / reliability is critical to efficient operation
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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 High DFG losses
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Effect of Excess O2 Level on Boiler Efficiency
High excess air reduces efficiency and increases ash erosion; Lower excess air can result in incomplete combustion
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Air Distribution Role of Excess air is important to achieve
efficient combustion. Equally important is achieve improvement in air
distribution to reduce NOx emissions. SADC help mitigate minor deficiencies in equal
distribution of combustion air. Typical upgrade in this area cover use of baffles,
perforated plate or compartmentalization of wind box.
Computer assisted CFD modeling is used at design stage to improve air distribution .
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Oxygen and CO
CO in the flue gas is an indicator of incomplete combustion
Excess oxygen is a measure for confirmation of complete combustion
Lower the excess oxygen-incomplete combustion resulting in losses in terms of high unburnt
Higher the excess oxygen-improper air-fuel mixture resulting in deterioration of combustion efficiency, high energy consumption in draft system, higher dry flue gas loss etc.
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SA and PA Temperature
SA and PA Temperatures are important parameters to be monitored for Boiler combustion
Air temperatures mainly depend upon APH performance, mass flow of Gas/Air, Gas/Air inlet temperatures etc.
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Burner Tilt
Burner Tilt is operated to maintain RH steam temperature at no/minimum RH spray
Lower the Burner Tilt associated with poor coal fineness-chances of high unburnt in BA
Higher or upward the tilt-possibility of coal particles from upper Mills escaping to 2nd pass without being burnt completely
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Residence Time
Coal particles require certain time inside the furnace to burn completely
Less the residence time-possibility of escaping of coal particles going to the 2nd pass without being burnt completely
Higher the residence time-possibility of High Furnace temperature and heat loss
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13 Essentials of Optimum Combustion
Fuel Preparation 1. Fuel feed quality and size shall be consistent. 2. Fuel feed shall be measured and controlled as accurately as
possible. Gravimetric feeders are preferred. 3. Fuel line fineness >75% passing a 200-mesh screen, and 50 mesh
particles <0.1%. Distribution to Burners 4. Primary airflow shall be accurately measured and controlled to
±3% accuracy. 5. Primary air to fuel ratio shall be accurately controlled when
above minimum. 6. Fuel line minimum velocities shall be 16.5 m/s. 7. Fuel lines shall be balanced by “Clean Air” test to within 2% of
average.
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8. Fuel lines shall be balanced by “Dirty Air” test to within 5% of average.
9. Fuel lines shall be balanced in fuel flow to within 10% of average.
10. Over-fire air shall be accurately measured and controlled to ±3% accuracy.
11. Furnace exit shall be oxidizing; 3% oxygen is preferable.
12. Mechanical tolerances of burners and dampers shall be ± 6.5mm.
13. Secondary air distribution to burners shall be within 5-10% of average.
Combustion
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AH Baskets Condition of AH Seals Amount of air flow, Primary and
Secondary Air Condition of Sector Plates, Axial and
Radial Air Ingress in the Boiler
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AH Leakage % Gas Side Efficiency X-Ratio Flue Gas/Air dp across AH Gas/Air Temperature
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The leakage of the high pressure air to the low pressure
flue gas is due to the Differential Pressure between fluids, increased seal clearances in hot condition, seal erosion / improper seal settings.
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 at times restricting unit loading
Performance-Factors
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Air Heater Leakage Air heater leakage levels affect exit
temperature and APC of fans. Accurate measurement of AH leakage
important to assess degradation of performance level
High leakage can even adversely affect unit capability.
Five mill operation results in higher level of leakage in 210/200 mw units.
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Gas Side Efficiency
Ratio of the temperature drop to the temperature Head expressed as a percentage.
Temperature drop is Gas O/L temp. minus Gas I/L temp.
Temp Head is Gas I/L temp minus Air I/L temp
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X-Ratio
Ratio of Heat capacity of air passing through the AH to the Heat capacity of Flue Gas passing through the AH
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Flue Gas Exit Temp At A.H. Outlet
Flue Gas Temp. at AH Outlet is an indication of amount of heat lost from the Boiler
Lower the flue gas exit temperature, lower will be losses
DP across AH is an important indication of AH performance
Very low temperature at AH outlet can also cause Cold End Corrosion
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Oxygen in Flue Gas at AH A Inlet / Outlet
0
2
4
6
8
10
A B C D E F
Probe
Inle
t O2
%
0
2
4
6
8
10
12
Out
let O
2 %
Inlet O2 Outlet O2
Oxygen in Flue Gas at AH B Inlet / Outlet
0
2
4
6
8
10
A B C D E F
ProbeIn
let O
2
0
2
4
6
8
10
Out
let O
2
Inlet O2 Outlet O2
Typical Oxygen Levels at AH Inlet / Outlet
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MILL PERFORMANCE
0
0.4
0.8
1.2
1.6
60 65 70 75 80 85 90 95 100
FINENESS - % THRU 200 MESH
CA
PA
CIT
Y
FA
CT
OR
0.85
0.9
0.95
1
1.05
0 4 8 12 16 20
% MOISTURE
CA
PA
CIT
Y
FA
CT
OR
0
0.5
1
1.5
2
40 50 60 70 80 90 100
HARDGROOVE INDEX (HGI)
MIL
L
OU
TP
UT
X 1
00%
• GRINDABILITY (HGI)
• FINENESS
• MOISTURE
• SIZE OF RAW COAL
• MILL WEAR (YGP)
• MTC PRACTICES
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PF fineness
Typical recommended value of pulverised fuel fineness through 200 mesh Sieve is more than 70% and less than 1% retention on 50 mesh sieve.
Fineness is expressed as the percentage pass through a 200-mesh screen (74µm).
Coarseness is expressed as the percentage retained on a 50-mesh screen (297µm).
Screen mesh indicates the number of openings per linear inch.
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Excess PF fineness 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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Effect Of Fineness On Boiler Operation
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48Control Room
Boiler
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2 3
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A B C D E F
Burner Imbalance
• Mill discharge pipes offer different resistance to the flows due to unequal lengths and different 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.
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Burner Imbalance
• 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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Boiler Losses
Dry Flue Gas Loss Wet Flue Gas Loss Unburnt Carbon Losses Radiation Losses
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Boiler Losses
Stack Losses (Dry Flue Gas & Wet Flue Gas Loss)
Stack losses represent the heat in the flue gas that is lost to the atmosphere upon entering the stack. Stack losses depend on fuel composition‚ firing conditions and flue gas temperature.
Dry Flue Gas Losses – the (sensible) heat energy in the flue gas due to the flue gas temperature
Flue Gas Loss Due to Moisture – the (latent) energy in the steam in the flue gas stream due to the water produced by the combustion reaction being vaporized from the high flue gas temperature.
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Reasons For DFG Loss Air heater baskets corroded/eroded Air heater baskets fouled Air Ingress in the Furnace Low Furnace Heat Absorption Running of more no. of Mills/Top
Mills Bypass dampers miss-positioned High Seal leakage
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Unburnt in Bottom Ash Unburnt in Fly Ash
Unburnt Carbon Loss
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• 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
• Burner balance / worn orifices
• Primary Air Flow / Pressure
Un-burnt Carbon Loss
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Higher Primary air Flow PA flow optimization / calibration Optimum Mill Outlet Temperature Mill Fineness problem Increased Mill Loading (more than
rated mill capacity)
Reason for High Unburnt Carbon
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Radiation and convection losses are independent of the fuel being fired in a boiler and represent heat lost to the surroundings from the warm surfaces of a boiler or high-temperature water generator. These losses depend mainly on the size of the equipment and thermal insulation
Radiation and Convection Losses
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Thermal Insulation Difference in temp between ambient and surface in deg
C
Heat LossKcal/m2hr
25 340
40 600
100 1910
150 3225
200 5330
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Loss due to Spray
HR deviation on account of variation in spray rate should be investigated.
Zero target for spray rate - deviation from this is dependent on type of coal, excess air level , mill combination & availability of burner tilt control
Furnace cleanliness levels also affect spray rates.
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Unaccountable Loss
It is the loss which are generally not measured with the available system or equipments
“Unaccountable” heat rate loss is defined as the deference between the actual heat rate based on test and the sum of the expected heat rate and all “accountable” heat rate deviations
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Air Ingress
• Cold air leaks into the boiler from openings in the furnace and convective pass
• 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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Apart from degradation of AH baskets’ performance, another reason for lower heat recovery across air heaters is boiler operation at lesser SA flows.
This is on account of air ingress from furnace bottom, peep holes, penthouse roof and expansion joints.
The actual oxygen in the furnace is much less than what is being read at economiser outlet by online zirconia.
Difference between oxygen at furnace outlet and AH inlet / economizer outlet has been observed to be in the range of 1.0 to 2.5 % in many boilers.
Air Ingress
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Boiler operation under adverse conditions continues as in majority of units ‘On line’ CO feedback is not available.
Air ingress across AH outlet to ID suction observed to be generally in the range of 5 to 9%.
Flue gas ducts & expansion joints at Eco outlet and APH inlet / outlet to be inspected thoroughly during O/H
Air Ingress
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Furnace Outlet
Air-in-leakage
Zirconia O2 Probe
AH Seal Lkg
ESP
Expansion Joints
Air Ingress Points – Furnace Roof , Expansion joints, Air heaters, Ducts, ESP Hoppers, Peep Holes, Manholes, Furnace Bottom
ID fan amperages should be trended and tracked from OH to OH
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The HR of a conventional Coal fired power plant is a measure of how efficiently it converts the chemical energy contained in the fuel into electrical energy.
If a power plant converted 100% of the chemical energy in the fuel into electricity, the plant would have a heat rate of 860 kcal/kWh.
A modern conventional power plant might have at best a design full load heat rate of 2200 kcal/kWh, which is about 39% efficient.
Heat Rate-Concepts
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Heat Rate-Definition Gross Unit Heat Rate :Includes all heat
input to the boiler and the “gross” electrical generation.
Net Unit Heat Rate :Based on “net” electrical generation
Design Unit Heat Rate : It is the heat rate the designer anticipates will occur at the design condenser pressure and load, make up
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Heat Rate = Sp. Coal consumption X GCV of Coal
Definition
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Modus- OperandiModus-
Operandi
Accurate Gap Assessment
PIPs
Gap analysis
Action Plans
Resource Mobilization
Implementation
Retrofits
Evaluation & Dissemination
Focus on Equipment degradations
Systematic Approach
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Typical Performance Gaps
Identification of the performance gaps is the key to improvement
Major Heat Rate Losses
18-20%
30-32%
15-17%
19-21%
6-8%
6-8%
Condenser & CTs
Turbine (HP/IP)
Dry Flue Gas Loss
Unaccountables
RH Spray
Others
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DFG Loss Unburnt Carbon Loss Radiation Loss Unaccounted Losses
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Causes
Soot blowing not effective IDT of ash is low Coal fineness not proper Poor Coal Quality-High ash content Improper air distribution-SADC
problem Low excess air
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Effects
Less heat transfer in furnace High DFG loss High spray flow-losses Uncontrolled BAH Load restriction Tripping of Boiler and Unit High metal temperature
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ESP Performance-Boiler Efficiency
ESP’s handle very high flue gas volumes. Particulate properties and gas stream
conditions dictate ESP Performance. Particle Size distribution, resistivity, flue gas
flow, fuel quality and process temperature affect the ability of ash to be collected and removed from ESP’s.
Increase in flue gas velocities and temperature in Electric fields can all be related to degradation of boiler and Air heater Performance.
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Major Causes
Steam/Flue Gas Erosion Corrosion Weld Joint Failure Overheating Fatigue Material Failure
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Overheating of tube due to internal oxide/deposition
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Increase in metal temperature due to internal scale
•
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Boiler improvements
Air heater modifications to reduce flue gas exit temperature and minimize air leakage. Efficiency Improvement potential
Reduction in flue gas percentage oxygen Improvement in PF Fineness for improved
efficiency. Balancing of coal & air flows Modification of heat transfer surfaces Control system upgrade for improved fuel
air ratio
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Improvements in Critical Measurements
In order to operate power plants efficiently, the operators must have reliable and accurate information on the unit. Small errors in sensors can result in inaccurate estimation of performance gaps and “unaccountable” heat rate deviations increases.
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Profit to utility Benefit to consumer Less pollution Conserve natural resources Minimize equipment life cycle cost Minimize Cost of Generation Customer satisfaction Be in competition
Why Optimization?
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• 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
• Boiler loading, insulation etc.
Boiler Efficiency-Factors
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Major auxiliaries Consuming Power in a Boiler are FD fans, PA fans, ID fans and mills. Reasons for higher 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
Auxiliary Power Consumption
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Excess oxygen percentage CO percentage Unburnt percentage in BA and FA PF fineness in 50/100/200 mesh FEGT Gas/Air inlet/outlet temperature at
APH
Critical Parameters for Optimization
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RH/SH Spray flow Metal temperature Loading in the draft system Amount of slagging Boiler Efficiency Heat Rate of the Unit
Critical Parameters for Optimization
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OFF – Design/Optimum Conditions
Parameter Deviation Effect on Heat
Rate Excess Air (O2) per % 7.4 Kcal/kWh Exit Gas Temp per oC 1.2 Kcal/kWh Unburnt Carbon per % 10-15 Kcal/kWh Coal moisture per % 2-3 Kcal/kWh
Boiler Efficiency per % 25 Kcal/kWh
Effect of Boiler side Parameters
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Savings Potential
Fine tuning a boiler’s combustion air and fuel input has a direct impact on the amount of fuel consumed by a boiler.
For each 1% decrease in excess air levels introduced into the combustion process, the boiler’s efficiency increases by 1/4 to 1 of a percent. While some excess air is necessary to ensure complete combustion, flue gas analysis will verify that excess air is within the manufacturer’s specifications and optimize efficient operation.
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Air Leakage Weight of air passing from air side to gas side; This leakage is assumed to occur entirely between air inlet and gas outlet
Hot End / Cold End / Entrained Leakage
Calculation 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
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 C 100
Gas Side Efficiency = (333.5-150.5) / (333.5-36.1) = 61.5 %
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X – Ratio
Ratio of heat capacity of air passing through the air heater to the heat capacity of flue gas passing through the air heater.
= Wair out * CpaWgas in * Cpg
= Tgas in - Tgas out (no leakage)Tair out - Tair in
Say AH leakage – 17.1%, Gas In Temp – 333.5 C, Gas Out Temp – 133.8 C , Air In Temp – 36.1 C, Air Out Temp – 288 C
X ratio = (333.5 – 150.5) / (288 –36.1) = 0.73
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Boiler Efficiency - the % of heat input to the boiler absorbed by the working fluid
a) Direct method or Input / Output method b) Indirect method or Loss method
For coal fired boilers, it’s difficult to measure coal flow and heating value accurately on real time basis. Also, there’s no clue to operator as to the extent and nature of the losses.
For utility boilers efficiency is generally calculated by heat loss method wherein the losses are calculated and subtracted from 100. Commonly used standards are
ASME PTC 4 BS – 2885 (1974) IS: 8753: 1977DIN standards
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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) * 100 where L – lossesHf – heat
input
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.
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Effect of Operating Parameters on Boiler Losses
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Loss method
• Boiler efficiency is calculated by loss method
• Individual losses are calculated as % of Heat input lost
• Dry Flue gas loss
• Loss due to moisture and Hydrogen in coal
• Loss due to moisture in air
• Unburnt carbon in ash loss
• Radiation loss
• Unaccounted loss
•Boiler efficiency (%) = 100 – Loss%
