draft and efficiency evaluation of furnace and boilers.ppt

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Session III

Draft, Performance indicators, Draft, Performance indicators, efficiency evaluation of furnaces efficiency evaluation of furnaces

and boilersand boilers

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DraftDraft

• 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

• If the hot gas is confined in a column of suitable height, the buoyancy of the gas as contained in column is capable of creating draft or less than atmospheric pressure required to cause atmospheric air to be drawn into furnace.

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Minimum DraftMinimum 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.

• If the draft at arc is -0.01 inch WC at arc the draft at below 10 feet would be -0.13 inch WC

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Convection tube Shock tubes

Radiant tubes

Natural draft furnace

Arc draft -0.03 inch WC

Convection bank Pr drop -0.5 inch WC

Draft at convectional outlet -0.53 inch WC

Prime rule of furnace operation is such that there should never be positive pressure at any point within furnace structure

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DraftDraft

• 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

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Performance indicators

The performance indicators of furnaces and boilers are:

• Excess Air• Temperature of flue gas to stack

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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

Typical Excess Air Level for Oil Fuel is 15 –25 %

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Oxygen content in flue gas vs Excess air

% O2 Excess Air %

2 10

3 16

4 22

5 29

6 37

7 46

8 57

9 69

10 83

10 % reduction in excess air can increase efficiency by 1 %

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Relation Between Residual O2 and Excess air

Relation between residual oxygen and excess air

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Oxygen (%)

Exc

ess

air

(%

)

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How to find Excess Air?

• O2 in flue gas from analyzer or portable instrument

Excess air = (O2 %)* 100 / (21- O2 %)

Excess air in flue gas having 3 % O2 in flue gas = (3*100)/(21-3)=16.6 say 17%

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How to find Excess Air?

• ORSAT analysis of flue gas and read excess air from BETZ chart using data on O2 and CO2

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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.

• For effectiveness on air tightness track O2 content in flue gas at convection outlet and at APH outlet

It is true that none of the furnaces nor boilers are air tight !

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How to control Excess Air?

• Combustion Control

• Minimize Air ingress

• Operate and maintain APH to avoid air leak

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How to control Excess Air?

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

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How to control Excess Air?

Air ingress• Ensure tight closing of peep holes / igniter

points• Seal all openings in convection banks i.e.

return bend covers• Ensure “no leak” in APH• Apply castables at joints of return bend

covers

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Flue Gas Sampling points

• At ARCH for combustion control (continuous)• 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)

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• Draft

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““Efficiency evaluation of Efficiency evaluation of FURNACE & BOILER”FURNACE & BOILER”

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Boiler Economiser

APH

Induced Draft Fan

Damper

Boiler System

Oil FD fan

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USEFUL ENERGY OUTPUT EFFICIENCY = --------------------------------------- ENERGY INPUT

DISADVANTAGES:• 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 MethodBoiler and Furnace efficiency - Direct Method

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USEFUL ENERGY OUTPUT - LOSSES EFFICIENCY = -----------------------------------------------------

ENERGY INPUT

ADVANTAGES:

•LOSSES ARE ESTIMATED AND DEDUCTED FROM 100

•IMPACT OF 1 % VARIATION ON EFFICIENCY IS

100 – (10 +/- 0.1) = 90 +/- 0.1 = 89.9 TO 90.1 %

•IT INDICATES CLUES TO OPERATING PERSONNEL

Boiler & Furnace efficiency - Indirect MethodBoiler & Furnace efficiency - Indirect Method

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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

Define : Losses during Furnace OperationDefine : Losses during Furnace Operation

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Calculation of Furnace efficiency – Indirect Method

Fuel fired : Only IFO

Fuel Composition: C=87 %, H2= 12 %, S = 0.7 %, Water = 0.2 %, Ash = 0.1 %

GCV = 10400 Kcal /kg

CO2 content in flue gas: 10.8 %

O2 content in flue gas: 5 %

Stack Temp : 170 C

Ambient Temp : 30 C

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Calculation of Furnace efficiency – Indirect Method

Theoretical Air required per kg of fuel:

{11.6 C + 34.8 (H2-O2/8)+ 4.35 S}

100

= (11*87)+(34.8 * 12) + (4.35*0.4)

100

= 14.30 kg of air per kg of fuel

Excess Air = 5 * 100 = 31.25 %

(21-5)

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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

= 18.68 kg of flue gas per kg of fuel

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Calculation of Furnace efficiency – Indirect Method

L1 : Heat loss to dry flue gas

M * cp * (Tf - Ta ) * 100

GCV

= 18.68 * 0.23 * (170 – 30) * 100 = 5.78 %

10400

L2 : Loss due to hydrogen in fuel

9H2 {584 + 0.45 (Tf – Ta)} * 100

10400

= 6.72 %

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Calculation of Furnace efficiency – Indirect Method

L3 : Loss due to moisture in fuel

M { 584 + 0.45 (Tf – Ta)} * 100

GCV

= 0.002 { 584 + 0.45 (170 – 30)}* 100 = 0.01 %

10400

L4 : Loss due to moisture content in air

AAS * (Kg moisture per kg air) * 0.45 * (Tf – Ta)* 100

GCV

= 18.77 * 0.025 * 0.45 * (170 – 30 ) * 100

10400

= 0.28 %

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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%

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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%•10 degree C drop in surface temperature increases efficiency from 86.5 % to 86.59%

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Success Factors of Boilers Efficiency

Stack temperature and oxygen content in flue gas leaving the stack are the results of:•Combustion Efficiency•Design of system•Operational Efficiency•Maintenance Effectiveness•Monitoring & Control

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Performance indicators of Boilers

• Flue gas temperature to stack

• 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

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Estimation of Stack Losses

• Stack losses can be estimated with the help of flue gas temperature to stack and flue gas analysis

• Monographs like BETZ energy chart can be used for assessing stack losses and furnace efficiency

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Furnace efficiency AU-4

Basis

Throughput : 7600 MT per day, % SRF 1.14%

Stack temperature 265 C = 509 F

Ambient temperature = 30 C = 86 F

Flue gas analysis : CO2 – 9.5%, O2 – 6%

Excess air = about 40%

APH under maintenance

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Furnace efficiency AU-4

• 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 %

• Efficiency = (100 – 18.62 – 2) x 100 / (100 – 8) = 86.29%

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Furnace efficiency AU-4

• Current efficiency = 86.29%

• Achievable efficiency = 90%

• Deviation in efficiency = - 3.71%

• Additional Fuel consumption = 3.59 MT per day

= 3.59 x 18,000

= Rs. 64,557/- per day

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Boiler efficiency B-4

Basis

Stack temperature 180 C = 360 F

Ambient temperature = 30 C = 86 F

O2 analyzer : 4%

CO2 about 12%

Excess air = about 22%

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Boiler efficiency B-4

• 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 % • Efficiency = (100 – 12.79 – 2) x 100 / (100 – 6.5) =

91.1% (erroneous as temperature and O2 are not at same point)

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Boiler efficiency B-4

• 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 % • Efficiency = (100 – 16.14 – 2) x 100 / (100 – 9) =

89.9%

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Case study on Furnace

Type of furnace: Forced Draft, Vertical cylindrical

• Impact of variation in stack temperature at constant oxygen level of 6 %

• Impact of variation in O2 content in flue gas at constant stack temperature of 325 o C

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Case study - Impact of stack temperature at constant oxygen level of 6 %

STACK TEMP (o C ) vs EFFICIENCY

84.27

84.0383.7983.5583.31

82

82.5

83

83.5

84

84.5

85

325 320 315 310 305

STACK TEMP (o C)

EFFI

CIE

NC

Y (%

)

20 o C drop in stack temp reduces fuel consumption by 1 %

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Case study - Impact of variation in O2 content in flue gas at constant stack temp of 325 o C

1% drop in O2 content in flue gas reduces fuel consumption by 0.5 to 1 %

OXYGEN (%) vs EFFICIENCY

86.29

85.7384.88

83.8483.31

82838485868788

6 5 4 3 2

OXYGEN (%)

EF

FIC

IEN

CY

(%

)

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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

• This also occurs where area in convection section is less

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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

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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

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Case studies on combined effect of variation in O2 content in flue gas &

stack temp

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EFFICIENCY OF FURNACE IN DCUEFFICIENCY OF FURNACE IN DCU

  Target Actual

O2 Content in flue gas 4.0% 7.9%

Excess Air 23% 55.3%

Stack Temp o C 165 215

Furnace Efficiency 90% 86.53%

Fuel saving Potential (MT/Day)   1.69

Saving potential , Rs per Day   23660

Hot air temp to furnace is 279 o C against normal of 305 o C

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ACTION FOR IMPROVING EFFICIENCY OF ACTION FOR IMPROVING EFFICIENCY OF FURNACE IN DCUFURNACE IN DCU

•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

•Provision of draft gauge at APH inlet / outlet for assessing pressure drop

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EFFICIENCY OF FURNACE IN CDUEFFICIENCY OF FURNACE IN CDU

  Target Actual

O2 Content in flue gas 4.0% 7.6%

Excess Air 23% 53.3%

Flue gas Temp at ID suction o C

Stack Temp o C

150

150

193

172

Furnace Efficiency 92% 88.38%

Fuel saving Potential (MT/Day)   1.17

Saving potential , Rs per Day   16405

Hot air temp to furnace is 140 o C against normal of 175 o C

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ACTION FOR IMPROVING EFFICIENCY OF ACTION FOR IMPROVING EFFICIENCY OF FURNACE IN CDUFURNACE IN CDU

• Hot water washing of APH / Revamp of APH

•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

•Provision of draft gauge at APH inlet / outlet for assessing pressure drop

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Factors of stack losses 1. Incomplete combustion

• Incomplete combustion leads to formation of CO & soot.

• 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

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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

Increase in firing rate increase stack temperature

4. Position of main stack damper

• Design of main stack damper affects stack loss

• Position of main stack damper also has direct impact on stack loss

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Factors of stack losses 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

Oxygen analyzers are installed with good intention. But these instruments fail very often which affects control of excess air.

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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

• Use of portable oxygen analyzers for monitoring of oxygen profile in flue gas.

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REVAMP OF FURNACE IN CRUDE DISTILLATION UNIT

Furnace

convection

APH

ID

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Issue of Monitoring and control of stack gas temperature and oxygen in flue gas

Questions are:

a. Where should you measure the stack gas temperature and why do you recommend this location?

b. Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?

c. Where should you place the oxygen sensor and why do you select this location?

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Issue of Monitoring and control of stack gas temperature and oxygen in flue gas

Question:

a. 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.

• For balanced draft furnace measure flue gas temperature at final stack

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Issue of Monitoring and control of stack gas temperature and oxygen in flue gas

Question:

a. Which is the best solution to measure either O2 or CO2 in stack gas and why is it the best solution?

Answer:

Best solution is to measure oxygen in flue gas.

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Issue of Monitoring and control of stack gas temperature and oxygen in flue gas

Question:

a.Where should you place the oxygen sensor and why do you select this location?

Answer:

The oxygen sensor should be placed at the end of fire box / radiation section.

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