Thermal efficiency of boiler by Heatloss method

or indirect method.

A. Photograph

B. Personal Details:1. Name :2. Company name:3. Designation:4. Qualifications:5. Email Address: 6. Complete postal

address 7. Fax No:

Ashwani KumarNational power training instituteStudent MBA Power ManagementB.E.(Mechanical) + MBA ( Pursuing)ashwani.kumar.goel@gmail.com Room no 302, trainee Hostel nationalpower training institute faridabad.

C. Total experience in years

Energy Management related experience details

2 years.

Monitoring and improving the thermalefficiency of boiler.Energy conservation by reducing thesteam losses in the plant.

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

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

F. Details of Energy ManagementProjects undertaken, if any

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.

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 gasses that

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.

2) Thermal efficiency of the boiler:

FLUE GASES

BOILER

Coal

Water

Air Ash

Superheated

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 η

i.e. η = 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.

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.

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.)

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 40°C in the

absence of humidity.

The sample is weighted again and keeps it in the oven

at temperature 100±5°C for 4 hr to calculate the sample

moisture.

The sample is weighted again keep at 100±5°C for 15 hr

to calculate the total moisture.

A sample of coal is burned at temperature of 825°C

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.

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.

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.

Sample location

4) Calculation of various heat losses:

L1. Heat loss due to unburned carbon in ash: (kcal)

………………………….(4.1)

• Heat Loss = Wash * Hash

Wash = weight of ash per kg of coal.

Hash = heating value of ash

Ash sampling:

• Front ash sampling for calculation of unburned in front

ash.

• Fly ash sampling for calculating the unburned in fly

ash.

Heating value of ash = ash*GCV (0.2*unburned in Front

ash+ 0.8*unburned in fly ash)

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)

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)

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:

= (√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

η = 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

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 140°C.

• 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.

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.