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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011

Boiler Efficiency Analysis Using Direct Method

Sunit Shah and D.M.Adhyaru,Member, IEEE

Abstract- Real-time data of boiler thermal efficiency can reallyreflect the boiler operation condition, heat generation and heatloss. Performance of the boiler, like efficiency and evaporationratio reduces with time, due to poor combustion, heat transferfouling and poor operation and maintenance. Boiler efficiencycan be also useful in analysis of boiler and can also be used inpredictive maintenance of the boiler. How to reduce problems of

the boiler efficiency real-time computations and be successful incalculating the boiler thermal efficiency on line are the mainconcern of operation departments of power systems. Using HeatInput-Output method plant people can evaluate quickly theefficiency of boilers using few parameters

Index TermsReal-Time, Boiler Efficiency, Input-Output

Method .

I. INTRODUCTION

A boiler is an enclosed vessel that provides a means for

combustion heat to be transferred into water until it becomes

heated water or steam. The hot water or steam under pressure

is then usable for transferring the heat to a process. Water is auseful and cheap medium for transferring heat to a process.

When water is boiled into steam its volume increases about

1,600 times, producing a force that is almost as explosive as

gunpowder. This causes the boiler to be extremely dangerousequipment that must be treated with utmost care.

Fig. 1. Boiler and its various components

The process of heating a liquid until it reaches its gaseous state

is called evaporation. Heat is transferred from one body toanother by means of (1) radiation, which is the transfer of heat

from a hot body to a cold body without a conveying medium,

(2) convection, the transfer of heat by a conveying medium,such as air or water and (3) conduction, transfer of heat by

actual physical contact, molecule to molecule. Boiler

Specification: The heating surface is any part of the boiler

metal that has hot gases of combustion on one side and water

on the other. Any part of the boiler metal that actually

contributes to making steam is heating surface. The amount of

heating surface of a boiler is expressed in square meters. The

larger the heating surface a boiler has, the more efficient itbecomes. The quantity of the steam produced is indicated in

tons of water evaporated to steam per hour. Maximum

continuous rating is the hourly evaporation that can bemaintained for 24 hours. F & A means the amount of steam

generated from water at 100 0C to saturated steam at 100 0C.[5]

A. Boiler OperationThe basic purpose of a boiler is to turn water into steam, in

this case saturated steam. This operation sounds relativelysimple but is actually more complicated. Other components

and processes such as the deaerator and economizer are

necessary to help the overall operation run more efficiently.

The boilers utilized on campus are of the stack drum type,

which means there are drums within the boilers stacked one

above the other. In these particular boilers there are two

drums. The upper drum is called a steam drum and is wheresaturated steam leaves the boiler. While the lower drum is

called the mud drum and is where liquid feed water enters. It

is also where sediment carried into the boiler settles. Tubescalled risers and down comers are used to connect the two

drums. All of the energy required within the boiler is produced

by the combustion of a fuel. The burner acts very similar tothe gas stove at home, just more complicated. It is comprised

of a wind box, igniter, fuel manifold and/or atomizing gun,observation port and flame safety scanner. Currently the

boilers can burn either No. 2 fuel oil or natural gas.

Fluctuating prices of fuel can raise or lower the cost to

produce steam. Having the choice between two different fuels

gives the option of burning the lower cost fuel.

Operation of the boiler begins with feed water entering the

mud drum where it is heated. The combustion of fuel within

978-1-4577-2168-7/11/$26.00 2011 IEEE

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Fig. 2. Combustion in Boiler

the furnace provides the required energy which is imparted by

a combination of convection and radiation. A two-phase water

mixture forms within the riser and begins to ascend to the

steam drum due to its decreasing density. Boiling to 100%

quality in the tubes is undesirable because water vapour has

different heat transfer characteristics than liquid water. This

can lead to high wall temperatures and eventual tube burnout.

Once it reaches the steam drum the majority of saturated

vapour will be removed from the two-phase mixture; thereby

increasing the remaining mixtures density. The increase indensity will initiate its descent in the down comers back to themud drum. This natural circulation continuously allows for a

constant flow of saturated steam exiting the boiler.[3]

B. Combustion in BoilerCombustion occurs when fossil fuels, such as natural gas, fuel

oil, coal or gasoline, react with oxygen in the air to produce

heat. The heat from burning fossil

fuels is used for industrial processes, environmental heating or

to expand gases in a cylinder and push a piston. Boilers,

furnaces and engines are important users of fossil fuels. Fossil

fuels are hydrocarbons, meaning they are composed primarilyof carbon and hydrogen. When fossil fuels are burned, carbon

dioxide (CO2) and water (H2O) are the principal chemical

products, formed from the reactants carbon and hydrogen in

the fuel and oxygen (O2) in the air. The simplest example of

hydrocarbon fuel combustion is the reaction of methane

(CH4), the largest component of natural gas, with O2 in the air.When this reaction is balanced, or stoichiometric, each

molecule of methane reacts with two molecules of O2

producing one molecule of CO2 and two molecules of H2O.

When this occurs, energy is released as heat. The combining

of oxygen (in the air) and carbon in the fuel to form carbon

dioxide and generate heat is a complex process, requiring theright mixing turbulence, sufficient activation temperature and

enough time for the reactants to come into contact and

combine. Unless combustion is properly controlled, high

concentrations of undesirable products can form. Carbonmonoxide (CO) and soot, for example, result from poor fuel

and air mixing or too little air. Other undesirable products,

such as nitrogen oxides (NO, NO2), form in excessive amountswhen the burner flame temperature is too high. If a fuel

contains sulphur, sulphur dioxide (SO2) gas is formed. For

solid fuels such as coal and wood, ash forms from

incombustible materials in the fuel.[3]

II. BOILEREFFICIENCY

Efficiency in general describes the extent to which time or

effort is well used for the intended task or purpose. It is often

used with the specific purpose of relaying the capability of aspecific application of effort to produce a specific outcome

effectively with a minimum amount or quantity of waste,

expense, or unnecessary effort.

A. What is boiler efficiency?

A typical heat balance for a boiler is shown in Fig.3 . Asshown in the figure, only part of the heat content of the fuel isconverted into useful heat, while the rest is lost through

exhaust gases, blowdown, and radiation losses. The efficiency

of boilers is usually rated based on combustion efficiency,

thermal efficiency, and overall efficiency.

Combustion efficiencyThe typical combustion process in boilers involve burning of

fuels that contain carbon (oil,gas, and coal) with oxygen to

generate heat. Oxygen required for combustion is normally

taken from air supplied to the burner of the boiler. The amount

of air needed for combustion depends on the type of fuel used.

To ensure complete combustion of fuel, more air than required(excess air) for combustion is provided to ensure that the fuel

is completely burnt. Since excess air leads to lower boiler

efficiency (due to removal of heat by the excess air as it passesthrough the boiler), the objective is to ensure that the optimumamount of excess air is provided .Combustion efficiency is anindication of the burners ability to burn fuel and the ability of

the boiler to absorb the heat generated. The amount of unburnt

fuel and excess air in the exhaust are used to assess a burners

combustion efficiency. Burners performing with extremely

low levels of unburned fuel while operating at low excess air

levels are considered efficient. Burners firing gaseous and

liquid fuels operate at excess air levels of 15% or less and

negligible amounts of unburned fuel. By operating at only

15% excess air, less heat from the combustion process is beingused to heat excess air which increases the available heat forthe boiler load. Combustion efficiency is not the same for all

fuels; generally, gaseous and liquid fuels burn more efficiently

than solid fuels do. As combustion efficiency does not account

for several other factors needed to determine a boilers fuel

usage, it should not be the sole factor used in economic

evaluations. Combustion Efficiency is also referred to as

Flue Loss or Stack Loss Efficiency.[6]

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Thermal efficiencyThermal efficiency is a measure of the efficiency of the heat

exchange in the boiler. It provides an indication of how wellthe heat exchanger can transfer heat from the combustion

process to water or steam in the boiler. It does not take into

consideration the conduction and convection losses from the

boiler.

Overall efficiencyAnother measure of boiler efficiency is the overall boiler

efficiency, which is a measure of how well the boiler can

convert the heat input from the combustion process to the

steam or hot water. It is also called fuel-to-steam efficiency.[8]

InputHeat

OutputHeatEfficiencyBoiler (1)

Heat output is define as the heat absorbed by the working fluid

Heat Input is defined as the chemical heat in fuel plus heat

credits. [1]

The heat input depends on the amount of fuel burnt and its

calorific value (heating value). The calorific value, normally

expressed in kJ/kg,multiplied by the amount of fuel burnt in

kg/s gives the heat input in kJ/s (kW). The heat output is the

difference in the heat content of the feedwater and steam (or

hot water) produced multiplied by the flow rate of water or

steam. The heat content of water and steam is expressed in

kJ/kg and the flow rate of water or steam is expressed in kg/s,

which yields the heat output in kW.

The overall efficiency of a boiler is lower than the combustionefficiency as it takes into account radiative and convective

losses from the boiler and other losses, such as cycle losses,due to passing of air through the boilerduring the off cycle.

The efficiency of a boiler can also be estimated by subtracting

stack losses, radiative losses, and convective losses from the

combustion efficiency. While combustion efficiency can be

measured directly by using a combustion analyzer, the stack,

radiative, and convective losses can be estimated using boiler

manufacturers data.

B. Significance of boiler efficiency

Electricity, would be needed in ample amount in coming years

in India. Primarily coal, oil and gas generate about 80 % of

electricity of nation. Boiler is heart in generating electricityusing coal, oil and gas. Efficient boiler operation has always

been critical in power plants. Online efficiency monitoring ofboiler can help to know the operating condition of boiler

.Knowing performance characteristics of boiler would be

optimizing boiler operation , and improving overall efficiency

of boiler and power plant Beside this it can also encourage use

of coal technology and reduce emission.

Performance of the boiler, like efficiency and evaporationratio reduces with time, due to poor combustion, heat transfer

fouling and poor operation and maintenance. Deterioration of

fuel quality and water quality also leads to poor performanceof boiler. Efficiency testing helps us to find out how far the

boiler efficiency drifts away from the best efficiency. Any

detected errors could therefore be investigated to pinpoint the

problem area for necessary corrective action. Hence it is

necessary to find out the current level of efficiency for

performance evaluation, which is a pre requisite for energy

conservation action in industry. This limitation concerning

monitoring of the boilers, which has a significant effect on

their optimization, is caused by the fact that the majority of

these power plants were designed at a time when the plants

production (and not its emissions or efficiency) was the

critical operating parameter. The inadequate monitoringdescribed means that, in general, the operation of the boilers is

based on the use of certain combinations of global or indirectvariables, derived either from the recommendations of the

boiler supplier or from the accumulated experience of the

operators of each particular facility. These combinations

frequently have more to do with operational stability andhistorical inertia, i.e. following customary practices, than with

true optimum operating conditions.[4]

III. DIRECT METHOD

This is also known as input-output method due to the fact

that it needs only the useful output (steam) and the heat input

(i.e. fuel) for evaluating the efficiency. Efficiency calculated

by the InputOutput method is based upon measuring the fuel

flow and steam generator fluid side conditions necessary tocalculate output. The uncertainty of efficiency calculated by

the InputOutput method is directly proportional to the

uncertainty of determining the fuel flow, a representative fuelanalysis, and steam generator output. Therefore, to obtain

reliable results, extreme care must be taken to determine these

items accurately. [2]

Efficiency determination by the InputOutput method requires

direct and accurate measurement of all output as well as all

input. The primary measurements required are the following:

(a) feedwater flow rate entering the steam generator

(b) flow rates of all secondary output streams such as boiler

blowdown, auxiliary steam, etc.

(c) pressure and temperature of all working fluid

streams such as entering feedwater, superheater outlet,

reheater inlet and outlets, auxiliary steam, etc.(d) additional measurements in the turbine cycle as required to

determine reheater flows by energy balance methods

(e) fuel flow rate

(f) higher heating value of the fuel

This efficiency can be evaluated using the formula:

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100)(

)(GCVq

hHQEfficiencyBoiler (2)

where

Q is Quantity of steam (dry) generated in T/hrq is Quantity of coal consumed in T/hr

H is Enthalpy of steam kJ/kgh is Enthalpy of feed water in kJ/kg

GCV is Gross Calorific Value of coal in kJ/kg

Heat inputBoth heat input and heat output must be measured. The

measurement of heat input requires knowledge of the

calorific value of the fuel and its flow rate in terms of

mass or volume, according to the nature of the fuel.

For gaseous fuel: A gas meter of the approved type canbe used and the measured volume should be corrected for

temperature and pressure. A sample of gas can be

collected for calorific value determination, but it is

usually acceptable to use the calorific value declared bythe gas suppliers.

For liquid fuelHeavy fuel oil is very viscous, and this property varies

sharply with temperature. The meter, which is usually

installed on the combustion appliance, should be regarded

as a rough indicator only.For test purposes, a meter

calibrated for the particular oil is to be used and over arealistic range of temperature should be installed. Even

better is the use of an accurately calibrated day tank.

For solid fuel

The accurate measurement of the flow of coal or othersolid fuel is very difficult. The measurement must be

based on mass, which means that bulky apparatus must be

set up on the boiler-house floor. Samples must be taken

and bagged throughout the test, the bags sealed and sentto a laboratory for analysis and calorific value

determination. In some more recent boiler houses, theproblem has been alleviated by mounting the hoppers

over the boilers on calibrated load cells, but these are yet

uncommon.

Heat output

There are several methods, which can be used formeasuring heat output. With steam boilers, an installed

steam meter can be used to measure flow rate, but this

must be corrected for temperature and pressure. In earlier

years, this approach was not favoured due to the change

in accuracy of orifice or venturi meters with flow rate. It

is now more feasible with recent flow meters of the

variable-orifice or vortex-shedding types. The alternative

with small boilers is to measure feed water, and this can

be done by previously calibrating the feed tank and noting

down the levels of water during the beginning and end of

the trial. Care should be taken not to pump water during

this period. Heat addition for conversion of feed water atinlet temperature to steam, is considered for heat output.

In case of boilers with intermittent blowdown, blowdownshould be avoided during the trial period. In case of

boilers with continuous blowdown, the heat loss due to

blowdown should be calculated and added to the heat in

steam. The greatest advantage of the direct method is thatit is easy. In addition, it can cover any period of time. It

takes into account any losses occurring during the period

under consideration and reflects the actual steam

generation and fuel consumption for that period. The

mass of steam is usually taken at the crown valve whichmeans that auxiliary steam such as deaerator steam, steam

used for boiler fans and feed water pumps or any other

steam related to the boiler operation is included. The

boiler feed water may or may not include boiler blow

down.[5]

A. Case Study-Ukai Thermal Power Station

Technical Data Of Power Plant [11]

Design Data of 200 MW Boiler

Boiler Type: - Direct tangentially coal fired, balanced

draught, natural circulation,

Design Fuel: - Bituminous coal

Mass of coal burnt = 85 T/hr (q)

Mass of steam generated = 600 T/hr (Q)

Coal

1) Fixed carbon = 39%

2) Volatile matter = 25%

3) Moisture = 8%

4) Ash = 28%

Gross Calorific Value = 4900 kcal/kg = 20501.6 kJ/kg

Mean feed water temperature =35 0C

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Pressure of steam = 131.9 kg/cm2= 129.5 bar

At 129.35 bar - 130 bar enthalpy of steam is as follow

hf = 1531.5 kJ/kg ; hf g = 1130.7 kJ/kg

H = hf + hf g

= 1531.5+ (0.85) (1130.7)

= 2492.595 kJ/kg

Heat of feed water

= 1x4.18x(35-0)

=146.3 kJ/kg

Total net heat given to produce 1 Kg of steam

= H- h

=2492.595-146.3

=2346.295 kJ/kg

where,

H is enthalpy of steam

h is enthalpy of water

100)(

)(GCVq

hHQEfficiencyBoiler (3)

1006.2050185

)3.146595.2492(600)( xEf ficiencyBoiler (4)

%77.80)(EfficiencyBoiler (5)

IV. CONCLUSIONIn this paper direct method (i.e. input -output method) is

explored for boiler calculation of boiler efficiency. The directmethod (i.e., the input-output method) is the simplest method

to determine boiler efficiency. In this method, the heat

supplied to the boiler and the heat absorbed by the water in the

boiler in a given time period are directly measured. Realizing

the direct boiler efficiency method online would help to to

know the operating condition of boiler .Knowing performance

characteristics of boiler would be optimizing boiler operation ,

and improving overall efficiency of boiler and power plant

REFERENCES

[1]John M Driscoll, John V Clearly, W.G.Mclean, J.W.Murdock, SteamGenerating Units,Power Test Codes The American Society of Mechanical

Engineers,December 1964.

[2] M.P.McHale,J.R.Friedmen, J. H. Karian , Steam Generating Units,Power

Test Codes The American Society of Mechanical Engineers, January 2009.[3] Wayne C. Turner, Steve Doty Energy Management Handbook The

Fairmont Press Inc.,6 Editon,2007[4] Dr. Lal Jayamaha , Energy - Efficient Builiding System:green strategiesfor operation and maintenance, The McGraw-Hill Companies Inc.,2006,pg

77-103[5] Boilers,Bureau of Energy Efficiency

[6] Combustion Analysis Basics:An Overview of Measurements, Methodsand Calculations Used in Combustion Analysis,TSI Incorporated

[7] Waste Heat Reduction and Recovery for Improving Furnace Efficiency,

Productivity and Emissions Performance,U.S. Department of Energy Efficiency and Renewable Energy[8] Determining & Testing Boiler Efficiency for Commercial/Institutional

Packaged Boilers ,American Boiler Manufacturing Association[9] Energy Management Series-Boiler Plant Systems

[10]British Power Plant Journal,,Plant Performance and Performance

Monitoring, Vol G,pg 480-488

[11] Analysis Of Boiler Efficiency Case Study Of Thermal Power Stations[12] A. Copado, F. Rodriguez Boiler Efficiency and NOx Optimisation

through Advanced Monitoring and Control of Local Combustion Conditions

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