furnace and boilers
DESCRIPTION
Furnace and BoilersTRANSCRIPT
Slide 1*
*
Draft
Draft at any point inside furnace is the difference in energy between column of hot gas and the atmosphere outside furnace
Volume of gases increases with temperature and the weight per unit volume becomes less
*
Minimum Draft
Draft to overcome the pressure drop of combustion gases in their flow from furnace proper to the point of entry to stack (plus minus 0.03 inch WC)
Increase in temperature increase draft
Draft varies (-) 0.1 inch per 10 feet height in furnace at about 650 C.
*
Convection bank Pr drop -0.5 inch WC
Draft at convectional outlet -0.53 inch WC
*
Where to control?
How to assess the profile of draft in case of pressurization of boiler / furnaces?
Case on draft survey in Russian Boilers
Case on VBU furnace
Excess Air
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Optimising Excess Air
In practice Excess Air over Stoichiometric air is needed for complete combustion
Less Air Incomplete combustion & Smoke
More Air Heat loss through stack
CO2 or O2 values will indicate excess air level
*
As already mentioned, for complete combustion 14.1 Kg of air for every kg of fuel oil is needed. In practice, mixing of fuel and air is never perfect, a certain amount of excess air is needed to complete combustion and ensure that release of the entire heat contained in fuel oil.
If too much air than what is required for completing combustion were allowed to enter, additional heat would be lost in heating the surplus air to the chimney temperature. This would result in increased stack losses.
Too little air is supplied would lead to the incomplete combustion and smoke (CO).
*
10 % reduction in excess air can increase efficiency by 1 %
% O2
*
Alternatively, excess air can be calculated by measuring oxygen (O2) percentage in flue gas.
Question
Ask the participants if O2 is 6.5%, what is the excess air?
Answer
O2 in flue gas from analyzer or portable instrument
Excess air = (O2 %)* 100 / (21- O2 %)
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
How to find Excess Air?
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Where to measure Excess Air?
For combustion control measure O2 at ARCH for furnaces and at the inlet of super heater for boilers i.e. at end of fire box.
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Combustion Control
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Combustion Control
Better atomization of oil (right temp of oil, dry superheated atomizing steam)
Delta ‘P’ (Steam – Oil)
Supply of quality fuel oil as per design condition of burners
Preparation of fuel oil tank (BS&W and foreign materials in oil)
Right quantity of combustion air
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Air ingress
Seal all openings in convection banks i.e. return bend covers
Ensure “no leak” in APH
Apply castables at joints of return bend covers
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
At APH inlet for detecting air ingress in convection bank
At APH out let for assessing leaks if any
Ensure proper sample points (like that in DHDT)
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
*
*
Excess air - O2/(21 - O2)
-200%
Dry flue gas loss - (10.37 x 3*1 x .23 (125 - 30)
4200
- 16.7%
% Excess air - 9/(21 – 9)
- 75%
Dry flue gas loss [(10.37 x 1.75) +1] x .23 x (95) 4200
- 9.96%
ACCURACY DEPENDS UPON MEASUREMENT OF ENERGY UTILIZED & FUEL CONSUMPTION
FOR 90 % OPERATING EFFICIENCY FURNACE 1% ERROR IN ESTIMATION RESULTS IN SIGNIFICANT VARIATION
90 +/- 0.9 = 89.1 TO 90.9 %
IT DOES NOT INDICATES CLUES TO OPERATING PERSONNEL
Boiler and Furnace efficiency - Direct Method
*
*
100 – (10 +/- 0.1) = 90 +/- 0.1 = 89.9 TO 90.1 %
IT INDICATES CLUES TO OPERATING PERSONNEL
Boiler & Furnace efficiency - Indirect Method
*
*
FURNACE EFFICIENCY IS ALWAYS LESS THAN ONE DUE TO FOLLOWING LOSSES:
SENSIBLE HEAT LOSS ALONG WITH DRY FLUE GAS
LATENT HEAT LOSS DUE TO PRESENCE OF H2 IN FUEL
LOSS DUE TO MOISTURE IN FUEL
LOSS DUE TO MOISTURE IN AIR
LOSS DUE TO INCOMPLETE COMBUSTION
RADIATION & CONVECTION HEAT LOSS
*
*
Fuel fired : Only IFO
Fuel Composition: C=87 %, H2= 12 %, S = 0.7 %, Water = 0.2 %, Ash = 0.1 %
GCV = 10400 Kcal /kg
Stack Temp : 170 C
Ambient Temp : 30 C
Theoretical Air required per kg of fuel:
{11.6 C + 34.8 (H2-O2/8)+ 4.35 S}
100
100
Excess Air = 5 * 100 = 31.25 %
(21-5)
Calculation of Furnace efficiency – Indirect Method
Actual Air supplied = 1.3125 * 14.3 = 18.77 kg of air per kg of fuel
Mass of flue gas : Mass of CO2 + Mass of N2+ Mass of SO2+Mass of oxygen in flue gas
= 0.87 * (44/12) + 18.77 * 0.77 + 0.007 * 2 + (18.77 – 14.30) * 0.23
= 3.19 + 14.45 + 0.014 + 1.028
*
*
L1 : Heat loss to dry flue gas
M * cp * (Tf - Ta ) * 100
GCV
10400
9H2 {584 + 0.45 (Tf – Ta)} * 100
10400
= 6.72 %
L3 : Loss due to moisture in fuel
M { 584 + 0.45 (Tf – Ta)} * 100
GCV
10400
AAS * (Kg moisture per kg air) * 0.45 * (Tf – Ta)* 100
GCV
10400
= 0.28 %
Calculation of Furnace efficiency – Indirect Method
L5 : Heat loss due to partial combustion (formation of CO) = 0.02%
L6: Radiation and Convection Loss = 2.09%
Total Loss = (L1+L2+L3+L4+L5+l6)
= ( 5.78+6.72+0.01+0.28+0.02+2.09) = 14.91%
Efficiency = 100 – 14.91 = 85.09%
Calculation of Furnace efficiency – Indirect Method
20 C drop in stack temperature increases efficiency from 85.09% to 86.05%
Drop in excess air from 31% to 20% increases efficiency from 86.05% to 86.5%
*
*
Success Factors of Boilers Efficiency
Stack temperature and oxygen content in flue gas leaving the stack are the results of:
Combustion Efficiency
Level of excess air in flue gas
Reality byte:
The temperature of flu gas leaving stack remains higher than desired
Excess air level remains more than requirement
*
*
Estimation of Stack Losses
Stack losses can be estimated with the help of flue gas temperature to stack and flue gas analysis
*
*
Stack temperature 265 C = 509 F
Ambient temperature = 30 C = 86 F
Flue gas analysis : CO2 – 9.5%, O2 – 6%
Excess air = about 40%
Connect 9.5 % on CO2 scale with 6 % on O2 scale
Read gas loss = 2.51% per 100 F of difference between stack and ambient
Sensible heat loss = 2.51 * (509 – 86) / 100 = 10.62%
Read latent heat loss from scale 2 = 8%
Stack loss = 10.62 + 8 = 18.62 %
Setting loss say 2 %
*
*
= 3.59 x 18,000
O2 analyzer : 4%
CO2 about 12%
Connect 12 % on CO2 scale with 4 % on O2 scale
Read gas loss = 2.3% per 100 F of difference between stack and ambient
Sensible heat loss = 2.3 * (360 – 86) / 100 = 6.29%
Read latent heat loss from scale 2 = 6.5%
Stack loss = 6.29 + 6.5 = 12.79 %
Setting loss say 2 %
*
*
Connect 8 % on CO2 scale with 7 % on O2 scale
Read gas loss = 2.61% per 100 F of difference between stack and ambient
Sensible heat loss = 2.61 * (360 – 86) / 100 = 7.14%
Read latent heat loss from scale 2 = 9%
Stack loss = 7.14+9 = 16.14 %
Setting loss say 2 %
*
*
Type of furnace: Forced Draft, Vertical cylindrical
Impact of variation in stack temperature at constant oxygen level of 6 %
*
*
*
*
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Increase in stack temp with decrease in oxygen level
With decrease in supply rate of air to furnace at high firing rate, the oxygen level in flue gas comes down but stack temperature increases marginally.
This effect is mainly due to un-cleaned surface in convection bank.
Observation based on furnace where fuel fired is 80 % oil and 20 % gas
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Decrease in stack temp with decrease in oxygen level
This phenomena observed in the furnace of Delayed Coking Unit
Whenever air supply is controlled at high firing rate, stack temperature drops and skin temperature of coils in radiation section increases
The fact – More heat is available for absorption
90 % fuel fired in this furnace is refinery fuel gas
Cleaner heating surface due to type of fuel used
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Decrease in stack temp with increase in supply of air (increase of oxygen level in flue gas)
Stack temperature drops due to heat absorbed by additional quantity of air.
Less heat is available for process
Skin temperature drops with high level of excess air
*
*
*
*
EFFICIENCY OF FURNACE IN DCU
4.0%
7.9%
Closing of stack damper
Control of excess air
Sealing of openings in return bend covers of convection bank
Provision of sample points at convection outlet, APH inlet and APH outlet for monitoring of O2 profile in flue gas along the path of flue gas
The sample point as above shall be used for leak test of APH
*
*
EFFICIENCY OF FURNACE IN CDU
4.0%
7.6%
23%
53.3%
Flue gas Temp at ID suction o C Stack Temp o C
150 150
193 172
Furnace Efficiency
Control of excess air
Stack temperature is indicating lower due to air ingress from flange just below the thermocouple point in stack
Sealing of openings in return bend covers of convection bank
Provision of sample points at convection outlet, APH inlet for monitoring of O2 profile in flue gas along the path of flue gas
The sample point as above shall be used for leak test of APH
*
*
Insufficient Turbulence : Non-uniformity in distribution of air for furnace with forced draft fan.
Insufficient air supply may be due to malfunctioning of air dampers
Lower Temperature of fuel oil and lower viscosity at burner tip leads to incomplete combustion
Presence of water in fuel / atomizing steam results in poor atomization of fuel, formation of small fire balls which leads to formation of soot and increased stack temperature
Formation of coke / clinker at burner tip affects combustion
*
*
Factors of stack losses
2. Soot blowing system
Improper soot blowing system results in increased stack temperature and loss of fuel
Positioning of soot blowers and supply of steam quality affects soot blowing
3. Firing rate
4. Position of main stack damper
Design of main stack damper affects stack loss
*
*
5. Excess Air
Supply of excess air leads to increased stack loss and additional fuel consumption.
6. Mechanical effectiveness / Air Ingress
Fire box and flue gas side remains under negative pressure
Any loose fittings results in air ingress and loss of heat through stack
7. Peep holes
Improper closing of peep holes leads to air ingress and stack loss.
8. Malfunctioning of monitoring instruments
*
*
Opportunities for minimizing stack losses
Uniform air distribution to burners
The oil must be heated to desired temperature at burner tip
Water free oil to burner
Regular cleaning of burners to minimize clinker formation
Use of dry superheated steam for atomization
Use of castables for sealing openings in convection bank of furnaces
Installation of glass window peep holes in place of door type peep holes
Survey of APH for leak
*
*
Furnace
convection
APH
ID
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Questions are:
Where should you measure the stack gas temperature and why do you recommend this location?
Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Where should you measure the stack gas temperature and why do you recommend this location?
Answer:
Measure stack temperature at final exit point of boiler i.e. at APH outlet of ID suction.
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?
Answer:
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Where should you place the oxygen sensor and why do you select this location?
Answer:
*
*
*
0
50
100
150
200
250
123456789101112131415
Oxygen (%)
*
Draft
Draft at any point inside furnace is the difference in energy between column of hot gas and the atmosphere outside furnace
Volume of gases increases with temperature and the weight per unit volume becomes less
*
Minimum Draft
Draft to overcome the pressure drop of combustion gases in their flow from furnace proper to the point of entry to stack (plus minus 0.03 inch WC)
Increase in temperature increase draft
Draft varies (-) 0.1 inch per 10 feet height in furnace at about 650 C.
*
Convection bank Pr drop -0.5 inch WC
Draft at convectional outlet -0.53 inch WC
*
Where to control?
How to assess the profile of draft in case of pressurization of boiler / furnaces?
Case on draft survey in Russian Boilers
Case on VBU furnace
Excess Air
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Optimising Excess Air
In practice Excess Air over Stoichiometric air is needed for complete combustion
Less Air Incomplete combustion & Smoke
More Air Heat loss through stack
CO2 or O2 values will indicate excess air level
*
As already mentioned, for complete combustion 14.1 Kg of air for every kg of fuel oil is needed. In practice, mixing of fuel and air is never perfect, a certain amount of excess air is needed to complete combustion and ensure that release of the entire heat contained in fuel oil.
If too much air than what is required for completing combustion were allowed to enter, additional heat would be lost in heating the surplus air to the chimney temperature. This would result in increased stack losses.
Too little air is supplied would lead to the incomplete combustion and smoke (CO).
*
10 % reduction in excess air can increase efficiency by 1 %
% O2
*
Alternatively, excess air can be calculated by measuring oxygen (O2) percentage in flue gas.
Question
Ask the participants if O2 is 6.5%, what is the excess air?
Answer
O2 in flue gas from analyzer or portable instrument
Excess air = (O2 %)* 100 / (21- O2 %)
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
How to find Excess Air?
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Where to measure Excess Air?
For combustion control measure O2 at ARCH for furnaces and at the inlet of super heater for boilers i.e. at end of fire box.
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Combustion Control
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Combustion Control
Better atomization of oil (right temp of oil, dry superheated atomizing steam)
Delta ‘P’ (Steam – Oil)
Supply of quality fuel oil as per design condition of burners
Preparation of fuel oil tank (BS&W and foreign materials in oil)
Right quantity of combustion air
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
Air ingress
Seal all openings in convection banks i.e. return bend covers
Ensure “no leak” in APH
Apply castables at joints of return bend covers
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
At APH inlet for detecting air ingress in convection bank
At APH out let for assessing leaks if any
Ensure proper sample points (like that in DHDT)
*
Oils are petroleum derived fuels. While kerosene, Diesel, Gas oil etc are direct products of distillation of crude oil, there other fuels which are mixed to make the product usable. This varies from country to country. L.D.O (Light Diesel oil) for example is a mixture of furnace oil and diesel. Furnace oil, LSHS (Low Sulphur Heavy Stock) and H.P.S (Heavy Petroleum Stock) are the bottom residues in a distillation column.
Of the properties viscosity, Calorific value and sulphur are key.
Viscosity determines the flowability of a fuel and is a significant factor in atomisation in the burner.
Calorific value gives the heating value of the fuel and is important again in burner selection.
*
*
*
Excess air - O2/(21 - O2)
-200%
Dry flue gas loss - (10.37 x 3*1 x .23 (125 - 30)
4200
- 16.7%
% Excess air - 9/(21 – 9)
- 75%
Dry flue gas loss [(10.37 x 1.75) +1] x .23 x (95) 4200
- 9.96%
ACCURACY DEPENDS UPON MEASUREMENT OF ENERGY UTILIZED & FUEL CONSUMPTION
FOR 90 % OPERATING EFFICIENCY FURNACE 1% ERROR IN ESTIMATION RESULTS IN SIGNIFICANT VARIATION
90 +/- 0.9 = 89.1 TO 90.9 %
IT DOES NOT INDICATES CLUES TO OPERATING PERSONNEL
Boiler and Furnace efficiency - Direct Method
*
*
100 – (10 +/- 0.1) = 90 +/- 0.1 = 89.9 TO 90.1 %
IT INDICATES CLUES TO OPERATING PERSONNEL
Boiler & Furnace efficiency - Indirect Method
*
*
FURNACE EFFICIENCY IS ALWAYS LESS THAN ONE DUE TO FOLLOWING LOSSES:
SENSIBLE HEAT LOSS ALONG WITH DRY FLUE GAS
LATENT HEAT LOSS DUE TO PRESENCE OF H2 IN FUEL
LOSS DUE TO MOISTURE IN FUEL
LOSS DUE TO MOISTURE IN AIR
LOSS DUE TO INCOMPLETE COMBUSTION
RADIATION & CONVECTION HEAT LOSS
*
*
Fuel fired : Only IFO
Fuel Composition: C=87 %, H2= 12 %, S = 0.7 %, Water = 0.2 %, Ash = 0.1 %
GCV = 10400 Kcal /kg
Stack Temp : 170 C
Ambient Temp : 30 C
Theoretical Air required per kg of fuel:
{11.6 C + 34.8 (H2-O2/8)+ 4.35 S}
100
100
Excess Air = 5 * 100 = 31.25 %
(21-5)
Calculation of Furnace efficiency – Indirect Method
Actual Air supplied = 1.3125 * 14.3 = 18.77 kg of air per kg of fuel
Mass of flue gas : Mass of CO2 + Mass of N2+ Mass of SO2+Mass of oxygen in flue gas
= 0.87 * (44/12) + 18.77 * 0.77 + 0.007 * 2 + (18.77 – 14.30) * 0.23
= 3.19 + 14.45 + 0.014 + 1.028
*
*
L1 : Heat loss to dry flue gas
M * cp * (Tf - Ta ) * 100
GCV
10400
9H2 {584 + 0.45 (Tf – Ta)} * 100
10400
= 6.72 %
L3 : Loss due to moisture in fuel
M { 584 + 0.45 (Tf – Ta)} * 100
GCV
10400
AAS * (Kg moisture per kg air) * 0.45 * (Tf – Ta)* 100
GCV
10400
= 0.28 %
Calculation of Furnace efficiency – Indirect Method
L5 : Heat loss due to partial combustion (formation of CO) = 0.02%
L6: Radiation and Convection Loss = 2.09%
Total Loss = (L1+L2+L3+L4+L5+l6)
= ( 5.78+6.72+0.01+0.28+0.02+2.09) = 14.91%
Efficiency = 100 – 14.91 = 85.09%
Calculation of Furnace efficiency – Indirect Method
20 C drop in stack temperature increases efficiency from 85.09% to 86.05%
Drop in excess air from 31% to 20% increases efficiency from 86.05% to 86.5%
*
*
Success Factors of Boilers Efficiency
Stack temperature and oxygen content in flue gas leaving the stack are the results of:
Combustion Efficiency
Level of excess air in flue gas
Reality byte:
The temperature of flu gas leaving stack remains higher than desired
Excess air level remains more than requirement
*
*
Estimation of Stack Losses
Stack losses can be estimated with the help of flue gas temperature to stack and flue gas analysis
*
*
Stack temperature 265 C = 509 F
Ambient temperature = 30 C = 86 F
Flue gas analysis : CO2 – 9.5%, O2 – 6%
Excess air = about 40%
Connect 9.5 % on CO2 scale with 6 % on O2 scale
Read gas loss = 2.51% per 100 F of difference between stack and ambient
Sensible heat loss = 2.51 * (509 – 86) / 100 = 10.62%
Read latent heat loss from scale 2 = 8%
Stack loss = 10.62 + 8 = 18.62 %
Setting loss say 2 %
*
*
= 3.59 x 18,000
O2 analyzer : 4%
CO2 about 12%
Connect 12 % on CO2 scale with 4 % on O2 scale
Read gas loss = 2.3% per 100 F of difference between stack and ambient
Sensible heat loss = 2.3 * (360 – 86) / 100 = 6.29%
Read latent heat loss from scale 2 = 6.5%
Stack loss = 6.29 + 6.5 = 12.79 %
Setting loss say 2 %
*
*
Connect 8 % on CO2 scale with 7 % on O2 scale
Read gas loss = 2.61% per 100 F of difference between stack and ambient
Sensible heat loss = 2.61 * (360 – 86) / 100 = 7.14%
Read latent heat loss from scale 2 = 9%
Stack loss = 7.14+9 = 16.14 %
Setting loss say 2 %
*
*
Type of furnace: Forced Draft, Vertical cylindrical
Impact of variation in stack temperature at constant oxygen level of 6 %
*
*
*
*
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Increase in stack temp with decrease in oxygen level
With decrease in supply rate of air to furnace at high firing rate, the oxygen level in flue gas comes down but stack temperature increases marginally.
This effect is mainly due to un-cleaned surface in convection bank.
Observation based on furnace where fuel fired is 80 % oil and 20 % gas
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Decrease in stack temp with decrease in oxygen level
This phenomena observed in the furnace of Delayed Coking Unit
Whenever air supply is controlled at high firing rate, stack temperature drops and skin temperature of coils in radiation section increases
The fact – More heat is available for absorption
90 % fuel fired in this furnace is refinery fuel gas
Cleaner heating surface due to type of fuel used
*
*
Case study - Combined effect of variation in O2 content in flue gas & stack temp
Decrease in stack temp with increase in supply of air (increase of oxygen level in flue gas)
Stack temperature drops due to heat absorbed by additional quantity of air.
Less heat is available for process
Skin temperature drops with high level of excess air
*
*
*
*
EFFICIENCY OF FURNACE IN DCU
4.0%
7.9%
Closing of stack damper
Control of excess air
Sealing of openings in return bend covers of convection bank
Provision of sample points at convection outlet, APH inlet and APH outlet for monitoring of O2 profile in flue gas along the path of flue gas
The sample point as above shall be used for leak test of APH
*
*
EFFICIENCY OF FURNACE IN CDU
4.0%
7.6%
23%
53.3%
Flue gas Temp at ID suction o C Stack Temp o C
150 150
193 172
Furnace Efficiency
Control of excess air
Stack temperature is indicating lower due to air ingress from flange just below the thermocouple point in stack
Sealing of openings in return bend covers of convection bank
Provision of sample points at convection outlet, APH inlet for monitoring of O2 profile in flue gas along the path of flue gas
The sample point as above shall be used for leak test of APH
*
*
Insufficient Turbulence : Non-uniformity in distribution of air for furnace with forced draft fan.
Insufficient air supply may be due to malfunctioning of air dampers
Lower Temperature of fuel oil and lower viscosity at burner tip leads to incomplete combustion
Presence of water in fuel / atomizing steam results in poor atomization of fuel, formation of small fire balls which leads to formation of soot and increased stack temperature
Formation of coke / clinker at burner tip affects combustion
*
*
Factors of stack losses
2. Soot blowing system
Improper soot blowing system results in increased stack temperature and loss of fuel
Positioning of soot blowers and supply of steam quality affects soot blowing
3. Firing rate
4. Position of main stack damper
Design of main stack damper affects stack loss
*
*
5. Excess Air
Supply of excess air leads to increased stack loss and additional fuel consumption.
6. Mechanical effectiveness / Air Ingress
Fire box and flue gas side remains under negative pressure
Any loose fittings results in air ingress and loss of heat through stack
7. Peep holes
Improper closing of peep holes leads to air ingress and stack loss.
8. Malfunctioning of monitoring instruments
*
*
Opportunities for minimizing stack losses
Uniform air distribution to burners
The oil must be heated to desired temperature at burner tip
Water free oil to burner
Regular cleaning of burners to minimize clinker formation
Use of dry superheated steam for atomization
Use of castables for sealing openings in convection bank of furnaces
Installation of glass window peep holes in place of door type peep holes
Survey of APH for leak
*
*
Furnace
convection
APH
ID
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Questions are:
Where should you measure the stack gas temperature and why do you recommend this location?
Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Where should you measure the stack gas temperature and why do you recommend this location?
Answer:
Measure stack temperature at final exit point of boiler i.e. at APH outlet of ID suction.
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?
Answer:
*
*
Issue of Monitoring and control of stack gas temperature and oxygen in flue gas
Question:
Where should you place the oxygen sensor and why do you select this location?
Answer:
*
*
*
0
50
100
150
200
250
123456789101112131415
Oxygen (%)