boiler efficiency by miss sonali priyadarshini mohanta

7/31/2019 Boiler Efficiency by Miss Sonali Priyadarshini Mohanta

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Thermal efficiency of boiler by Heat loss method

or indirect method.


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A. Photograph

B. Personal Details:1. Name :

2. Company name:3. Designation:4. Qualifications:

Sonali priyadarshini Mohanta

Assistant Manager (CPP)B.E.(Mechanical) + MBA ( Pursuing)

C. Total experience in years

Energy Management relatedexperience details

2 years.

Monitoring and improving the thermal efficiency ofboiler.Energy conservation by reducing the steam losses

in the plant.

D. Area of Specialization MBA in power management, power generation

E. Major Achievements Won 1st prize in technical paper presentationduring technical fest in the college.

F. Details of Energy ManagementProjects undertaken, if any


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Thermal efficiency of boiler by Heat loss method

or indirect method.

Learning objective:

1) Various methods to calculate the thermal

efficiency of the boiler.

2) Various heat losses occurred in the boiler while

under operation.

3) How to calculate each loss occurred.

4) Methods and practice to do the coal analysis, ash

analysis and flue gas analysis.

5) Controllable losses and uncontrollable losses.

6) Various practices to reduce the controllable losses.

Abstract:

Boiler is a system that converts the chemical energy of the coal

in to the thermal energy of the steam. The performance of the boiler is judge

by calculating the thermal efficiency of boiler. It is the ratio of energy

absorbed to total energy input. We have various methods to calculate the

thermal efficiency of boiler. The heat loss method or indirect method is best

used in practice. In this we come across the various losses occurred in the

boiler out of them some are controllable and some uncontrollable and we

can put our endeavor to reduce the controllable losses.


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1) Boiler:

Boiler is system which converts the chemical energy of the coal in to the thermal

energy of steam. It act as a heat exchanger which exchange the heat between flue gassesthat generated by the combustion of fuel and feed water which later on converted in to

steam.

In the boiler there are certain inputs and output as shown in the figure 1.1

Figure 1.1

To check the performance of the boiler we calculate the thermal efficiency of the boiler.

FLUE GASES

BOILER

Coal

Water

AirAsh

SuperheaSteam


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2) Thermal efficiency of the boiler:

2.1) Definition: Thermal efficiency of boiler is the ratio of the output energy available

to the total energy supplied to the boiler. It is denoted by

i.e. = Heat absorbed / input energy.

We have two method to calculate the thermal efficiency.

1. Direct Method or Input Output Method.

2. Indirect Method or Heat Loss Method.

2.2) Input output method:

Efficiency = (Heat absorbed by working fluid / (Heat in fuel +Heat credits)*100)

2.3) Heat loss method:

Efficiency = 100 - (heat losses/(Heat in fuel+ Heat credits) *100)

Heat loss method is used for efficiency calculation.

Heat loss is better to use in practice. The main reasons for that are following.

It is easier to calculate the losses as compared to measure the flow rate of coal.

We can check the controllable & uncontrollable losses & can do endeavor to reduce

the controllable losses.


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We can come with more ideas for efficient boiler operation.

Efficiency can be improved by combined effect of these entire things.

It is more accurate as compared to direct method.

2.4) Heat losses in boiler:

Heat loss due to unburned carbon in refuse.

Heat loss due to heat in dry flue gas.

Heat loss due to moisture in the as fired coal.

Heat loss due to moisture from burning of hydrogen.

Heat loss due to moisture in air.

Heat loss due to formation of carbon monoxide.

Sensible Heat loss in the ash.

Heat loss due to surface radiation and convection.

Some other losses are:

a) Heat loss due to soot blowing.

b) Heat in pulverized rejects.

2.5) Heat credits:

Heat content in the entering air.

Sensible heat of the fuel.

Pulverizer or crusher power.

Boiler circulating pumps.


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Recirculating gas fan power ( ID fan, FD fan, PA fan)

3) Methodology followed to calculate the various losses:

3.1) Data & Sample Collection

Flue gas temperature at air heater outlet.

Dry & wet bulb temperature of ambient air.

Flue gas analysis for O2, CO2, CO & Excess air.

Relative humidity from psychometric chart.

Front ash temperature at furnace outlet.

Coal sampling.

Front & fly ash sampling.

3.2) Analysis of Sample Collected.

Coal sample collected is analyzed for proximate analysis.

Ash sample collected is analyzed for unburned carbon.

The ultimate analysis of coal is done by standard formulas.

3.3) Proximate Analysis of coal:

In the proximate analysis we find out the various constituent of coal are as follow.

(The date has been taken for B grade coal.)


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Constituent Percentage

Fixed Carbon 40.08

Volatile Material 32.87

Total Moisture 14.5

Inherent Moisture 7.54

Ash 18.7

Table 1

Steps involved in proximate analysis of coal:

The Sample is taken at the entrance of coal banker.

It is weighted and kept in the oven at 40C in the absence of humidity.

The sample is weighted again and keeps it in the oven at temperature 1005C for

4 hr to calculate the sample moisture.

The sample is weighted again keep at 1005C for 15 hr to calculate the total

moisture.

A sample of coal is burned at temperature of 825C for 4 hr to calculate the ash %

in the coal.

A sample of coal is heated at temperature of 920 C for exact 7 minute to calculate

the volatile matter.

A sample of coal is burned in the bomb calorimeter to calculate the calorific value

of coal.


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3.4) Ultimate analysis of coal:

Constituent Percentage

Carbon 57.87

hydrogen 4.147

Oxygen 9.947

Nitrogen 1.44

Sulphur 0.35

Table 2.

Formulation used for the ultimate analysis:

C = 0.97*FC + 0.7(VM + 0.1*ash) - TM (0.6-0.01*TM).

N = 2.1 0.02*VM.

H = 0.036*FC + 0.086( VM- 0.1*ash) 0.0035*TM*TM*(1-0.02*TM)

O = 100 ( C+H+N)

C- Carbon content.

N- Nitrogen

H- Hydrogen.

O- Oxygen.

VM-Volatile Matter.

TM- Total moisture

FC- Fixed Carbon.


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3.5) Flue gas analysis:

In the flue gas analysis we find out the following constituents:

Carbon dioxide (Co2).

Carbon monoxide (Co).

Oxygen (O2)

Excess Air.

Flue Gas Analyzer

Steps involved in flue gas analysis:

Flue gas analyzer (K M 900) is used.

Fresh air is sucked (purging) to reduce the effect of earlier gases present in it.

The flue gas is sucked (location: inlet of Economizer). FGA gives reading directly.


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L2. Heat losses due to flue gasses: (kcal).(4.2)

Heat loss = Wg* Cpg* (Tg-Ta)

Wg = weight of flue gases/kg of as fired coal.

Cpg = specific heat of flue gases.

Tg = temperature of flue gases at APH outlet.

Ta = ambient air temperature.

Total flue gasses produced per kg of as fired coal :.(4.2.1)

WA (air required) =((28.02 N2 * ( Cb+ 12.01 S/32.02))/ (12.01 ( CO2 + CO))

N)/0.7685

ATh (Theoretical Air) = 11.51 C + 34.30(H O/7.937) + 4.335 S

Excess Air = (WA ATh)/ATh * 100.

Cb -Amount of carbon burned / kg of as fired coal

Cb = C Wash*Hash/8040 (coal).

8040 kcal/kg carbon (heat value of 1 kg of carbon)

Total flue gas = WA + (Wcoal Wash) = ATh + Excess air + (Wcoal Wash).

L3. Heat loss due to moisture in as fired coal: (kcal).(4.3)


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Heat loss = Wm *(LH +2*Tg-4.2*Ta)

Wm = Weight of moisture / kg of as fired coal..

LH = Latent heat of water / kg of water.

Tg = Temperature of flue gas outlet.

Ta = Dry bulb temperature.

L4. Heat losses due to moisture from burning of hydrogen: (kcal).(4.4)

Heat Loss = 8.936* H (LH + 2*Tg-4.2*Ta)

L5. Heat loss due to moisture in air: (kcal) .(4.5)

Heat Loss = WA * WmA * Cps*(Tg-Ta).

WA = weight of air used per kg of coal.

WmA = weight of moisture per kg of coal.

Cps = specific heat of superheated steam

L6. Heat loss due to formation of carbon monoxide: (kcal) ..(4.6)

Heat Loss = CO / (CO2 + CO) * 5632 * Cb

Cb = Amount of carbon burned / kg of as fired coal.

L7. Sensible heat loss in the ash: (kcal).(4.7)


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Heat Loss = Wash *Cpash (Tash Ta).

Wash = weight of ash per kg of coal.

Tash = temperature of ash at furnace outlet.

Ta = atmospheric temperature.

L8. Heat loss due to radiation: .........(4.8)

It is taken about 3 % of total heat losses.

5) Calculation of heat credits:

B1. Heat in the entering air: .(5.1)

Hea = WA Cpa (Ta2 Ta).

Hea = heat in the entering air.

Cpa = specific heat of air.

Ta2 = temperature of air at the air pre-heater outlet.

Ta = ambient air temperature.

B2. Pulveriser or crusher power, boiler circulating pump power and fans (primary

air, FD fan, ID fan) power..(5.2)

The power input is determined by the following formula using the reading of the reliable

ammeter and voltmeter and power factor. Heat equivalent for the usual 3 phase power is:


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= (3 * volts*ampere* per cent power factor*857)/ 100000/Wc kcal/hr/kg of fuel

Wc = Amount of coal fired per hr.

Or

Heat equivalent = kilowatt input * 857 kcal per hr.

Total losses = L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8.

Total heat credits = B1 + B2

= 100 - (heat losses/ (Heat in fuel+ Heat credits) *100)

6) Controllable losses:

Heat Loss in Flue gas.

Heat Loss due to Moisture in as fired coal.

Heat Loss due to formation of carbon monoxide.

Heat Loss due to unburned carbon in ash.

7) Uncontrollable losses:

Heat loss due to moisture from burning of Hydrogen.

Heat Loss due moisture in the air.

Heat Loss due to sensible heat in the ash


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8) How to control

8.1) Heat losses due to flue gasses:

Excess air should remains in the range of 60-70%.

Figure 8.1

Temperature of flue gas should be controlled should not be more than 140C.

There should be corresponding variation in the air flow rate as the load on the

boiler varies.

8.2) Heat losses due to unburned carbon:

Size of the coal, fired in boiler should be controlled. For example for sticker fired

coal, it should not be more than 20 mm.

Proper amount of excess air should be supplied to the boiler.

Boiler should not be overloaded.

Homogenous mixing of air and fuel should be maintained.


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8.3) Heat losses due to formation of carbon monoxide:

Excess air should not be less than 60%. Air should be entered with higher turbulence so that each carbon atom comes in

contact with oxygen atoms.

Ratio of Actual air to theoretical air should be more than 1.3.

Z

Figure 8.3

8.4) Heat losses due to moisture in the as fired coal:

Proper storage of coal should be done. (Specially in rainy season).

Coal preheating.

boiler efficiency by miss sonali priyadarshini mohanta

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