ENERGY PERFORMANCE ASSESSMENT OF BOILERS

Introduction

Performance of the boiler, like efficiency and evaporation ratio 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 performance of boiler. Efficiency testing helps us to
find out how far the boiler efficiency drifts away from the best efficiency. Any observed abnormal
deviations 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.

Purpose of the Performance Test

The purpose of the performance test is to determine actual performance and efficiency of the
boiler and compare it with design values or norms. It is an indicator for tracking day-to-day and
season-to-season variations in boiler efficiency and energy efficiency improvements.

Performance Terms and Definitions:

Scope:

The procedure describes routine test for both oil fired and solid fuel fired boilers using coal, agro
residues etc. Only those observations and measurements need to be made which can be readily
applied and is necessary to attain the purpose of the test.

Reference Standards

British standards, BS845: 1987

The British Standard BS845: 1987 describes the methods and conditions under which a boiler
should be tested to determine its efficiency. For the testing to be done, the boiler should be
operated under steady load conditions (generally full load) for a period of one hour after which
readings would be taken during the next hour of steady operation to enable the efficiency to be
calculated.

The efficiency of a boiler is quoted as the % of useful heat available, expressed as a percentage
of the total energy potentially available by burning the fuel. This is expressed on the basis of
gross calorific value (GCV).

This deals with the complete heat balance and it has two parts:

Part one deals with standard boilers, where the indirect method is specified

Part Two deals with complex plant where there are many channels of heat flow. In this case, both
the direct and indirect methods are applicable, in whole or in part.

ASME Standard: PTC-4-1 Power Test Code for Steam Generating Units

This consists of

Part One: Direct method (also called as Input -output method)

Part Two: Indirect method (also called as Heat loss method)

IS 8753: Indian Standard for Boiler Efficiency Testing

Most standards for computation of boiler efficiency, including IS 8753 and BS845 are designed
for spot measurement of boiler efficiency. Invariably, all these standards do not include blow
down as a loss in the efficiency determination process.

Basically Boiler efficiency can be tested by the following methods:

1) The Direct Method: Where the energy gain of the working fluid (water and steam) is
compared with the energy content of the boiler fuel.
2) The Indirect Method: Where the efficiency is the difference between the losses and the
energy input.

The Direct Method Testing

Description: 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. This efficiency
can be evaluated using the formula:
Measurement Required For Direct Method Testing:

Heat input

Both 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 can be 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 by the gas
suppliers.

For liquid fuel: Heavy 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 and, for test purposes, a meter calibrated for the particular oil
is to be used and over a realistic 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 other solid 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 sent to a laboratory for analysis and calorific value determination. In some more
recent boiler houses, the problem 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 for measuring 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 viable with
modern flow meters of the variable-orifice or vortex-shedding types.

The alternative with small boilers is to measure feedwater, 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 feedwater at inlet temperature to steam, is considered for heat output.

In case of boilers with intermittent blowdown, blowdown should 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.

Boiler Efficiency by Direct Method: Calculation and Example

Test Data and Calculation: Water consumption and coal consumption were measured in a
coal-fired boiler at hourly intervals. Weighed quantities of coal were fed to the boiler during the
trial period. Simultaneously water level difference was noted to calculate steam generation
during the trial period. Blow down was avoided during the test. The measured data is given
below.
Merits and Demerits of Direct Method:

Merits

1. Plant people can evaluate quickly the efficiency of boilers

2. Requires few parameters for computation
3. Needs few instruments for monitoring

Demerits

1. Does not give clues to the operator as to why efficiency of system is lower
2. Does not calculate various losses accountable for various efficiency levels
3. Evaporation ratio and efficiency may mislead, if the steam is highly wet due to
water carryover.

The Indirect Method Testing

Description: The efficiency can be measured easily by measuring all the losses occurring in the
boilers using the principles to be described. The disadvantages of the direct method can be
overcome by this method, which calculates the various heat losses associated with boiler. The
efficiency can be arrived at, by subtracting the heat loss fractions from 100.An important
advantage of this method is that the errors in measurement do not make significant change in
efficiency.

Thus if boiler efficiency is 90%, an error of 1% in direct method will result in significant
change in efficiency. i.e. 90 – 0.9 = 89.1 to 90.9. In indirect method, 1% error in measurement
of losses will result in
Efficiency = 100 - (10 – 0.1) = 90 – 0.1 = 89.9 to 90.1

The various heat losses occurring in the boiler are:

The following losses are applicable to liquid, gas and solid fired boiler
L1= Loss due to dry flue gas (sensible heat)
L2 =Loss due to hydrogen in fuel (H2)
L3 =Loss due to moisture in fuel (H2O)
L4 =Loss due to moisture in air (H2O)
L5 =Loss due to carbon monoxide (CO)
L6 =Loss due to surface radiation, convection and other unaccounted*.
*Losses which are insignificant and are difficult to measure.
The following losses are applicable to solid fuel fired boiler in addition to above
L7 =Unburnt losses in fly ash (Carbon)
L8 Unburnt losses in bottom ash (Carbon)

Boiler Efficiency by indirect method = 100 - (L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8)

Measurements Required for Performance Assessment Testing
The following parameters need to be measured, as applicable for the computation of boiler
efficiency and performance.
a) Flue gas analysis
1. Percentage of CO2 or O2 in flue gas
2. Percentage of CO in flue gas 3. Temperature of flue gas
b) Flow meter measurements for
1. Fuel
2. Steam
3. Feedwater
4. Condensate water 5. Combustion air
c) Temperature measurements for
1. Flue gas
2. Steam
3. Makeup water
4. Condensate return
5. Combustion air
6. Fuel
7. Boiler feedwater
d) Pressure measurements for
1. Steam
2. Fuel
3. Combustion air, both primary and secondary
4. Draft
e) Water condition
1. Total dissolved solids (TDS)
2. pH
3. Blow down rate and quantity
The various parameters that were discussed above can be measured with the instruments that are
given in Table 1.1

TABLE 1.1 TYPICAL INSTRUMENTS USED FOR BOILER PERFORMANCE

ASSESSMENT.
Instrument Type Measurements
Flue gas analyzer Portable or fixed % CO2 , O2 and CO
Temperature indicator Thermocouple, liquid in Fuel temperature, flue gas
glass temperature, combustion air
temperature, boiler surface
temperature, steam temperature
Draft gauge Manometer, differential Amount of draft used
pressure or available
TDS meter Conductivity Boiler water TDS, feedwater TDS,
make-up water TDS.
Flow meter As applicable Steam flow, water flow, fuel flow,
air flow
Test Conditions and Precautions for Indirect Method Testing:

A) The efficiency test does not account for:

1. Standby losses. Efficiency test is to be carried out, when the boiler is operating under a
steady load. Therefore, the combustion efficiency test does not reveal standby losses,
which occur between firing intervals
2. Blow down loss. The amount of energy wasted by blow down varies over a wide range.
3. Soot blower steam. The amount of steam used by soot blowers is variable that depends
on the type of fuel.
4. Auxiliary equipment energy consumption. The combustion efficiency test does not
account for the energy usage by auxiliary equipment’s, such as burners, fans, and
pumps.

B) Preparations and pre conditions for testing

1. Burn the specified fuel(s) at the required rate.

2. Do the tests while the boiler is under steady load. Avoid testing during warming up of
boilers from a cold condition
3. Obtain the charts /tables for the additional data.
4. Determination of general method of operation
5. Sampling and analysis of fuel and ash.
6. Ensure the accuracy of fuel and ash analysis in the laboratory.
7. Check the type of blow down and method of measurement
 8. Ensure proper operation of all instruments.
9. Check for any air infiltration in the combustion zone.

C) Flue gas sampling location

It is suggested that the exit duct of the boiler be probed and traversed to find the location of the
zone of maximum temperature. This is likely to coincide with the zone of maximum gas flow
and is therefore a good sampling point for both temperature and gas analysis.

D) Options of flue gas analysis

Check the Oxygen Test with the Carbon Dioxide Test

If continuous-reading oxygen test equipment is installed in boiler plant, use oxygen reading.

Occasionally use portable test equipment that checks for both oxygen and carbon dioxide. If the
carbon dioxide test does not give the same results as the oxygen test, something is wrong. One
(or both) of the tests could be erroneous, perhaps because of stale chemicals or drifting
instrument calibration.

Another possibility is that outside air is being picked up along with the flue gas. This occurs if
the combustion gas area operates under negative pressure and there are leaks in the boiler
casing.

Carbon Monoxide Test

The carbon monoxide content of flue gas is a good indicator of incomplete combustion with all
types of fuels, as long as they contain carbon. Carbon monoxide in the flue gas is minimal with
ordinary amounts of excess air, but it rises abruptly as soon as fuel combustion starts to be
incomplete.

E) Planning for the testing

1. The testing is to be conducted for a duration of 4 to 8 hours in a normal production day.

2. Advanced planning is essential for the resource arrangement of manpower, fuel, water
and instrument check etc. and the same to be communicated to the Operation and
Maintenance Departments.
3. Sufficient quantity of fuel stock and water storage required for the test duration should
be arranged so that a test is not disrupted due to non-availability of fuel and water.
4. Necessary sampling point and instruments are to be made available with working
condition.
5. Lab Analysis should be carried out for fuel, flue gas and water in coordination with lab
personnel.
6. The steam table, psychometric chart, calculator computer etc. are to be arranged for
computation of boiler efficiency.
Calculation Procedure and Formulae:

In order to calculate the boiler efficiency by indirect method, all the losses that occur in the
boiler must be established. These losses are conveniently related to the amount of fuel burnt. In
this way it is easy to compare the performance of various boilers with different ratings.

Step-I Conversion of Proximate Analysis to Ultimate Analysis:

However it is suggested to get an ultimate analysis of the fuel fired periodically from a reputed
laboratory. The Air Required: Theoretical (stoichiometric) air fuel ratio and excess air supplied
are to be determined first for computing the boiler losses.
Boiler Losses Calculations:

1. Heat loss due to dry flue gas

This is the greatest boiler loss and can be calculated with the following formula:

2. Heat loss due to evaporation of water formed due to H2 in fuel (%)

The combustion of hydrogen causes a heat loss because the product of combustion is water.
This water is converted to steam and this carries away heat in the form of its latent heat.

3. Heat loss due to moisture present in fuel: Moisture entering the boiler with the fuel leaves
as a superheated vapour. This moisture loss is made up of the sensible heat to bring the moisture
to boiling point, the latent heat of evaporation of the moisture, and the superheat required to
bring this steam to the temperature of the exhaust gas. This loss can be calculated with the
following formula:
4. Heat loss due to moisture present in air:

Vapour in the form of humidity in the incoming air, is superheated as it passes through the
boiler. Since this heat passes up the stack, it must be included as a boiler loss. To relate this loss
to the mass of coal burned, the moisture content of the combustion air and the amount of air
supplied per unit mass of coal burned must be known.

The mass of vapour that air contains can be obtained from psychometric charts.

5. Heat loss due to incomplete combustion:

Products formed by incomplete combustion could be mixed with oxygen and burned again with
a further release of energy. Such products include CO, H2, and various hydrocarbons and are
generally found in the flue gas of the boilers. Carbon monoxide is the only gas whose
concentration can be determined conveniently in a boiler plant test.
6. Heat loss due to radiation and convection:

The other heat losses from a boiler consist of the loss of heat by radiation and convection from
the boiler casting into the surrounding boiler house.

Normally surface loss and other unaccounted losses is assumed based on the type and size
of the boiler as given below

For industrial fire tube / packaged boiler = 1.5 to 2.5%

For industrial water tube boiler = 2 to 3%

For power station boiler = 0.4 to 1%

However it can be calculated if the surface area of boiler and its surface temperature are known.
 SUMMARY OF HEAT BALANCE FOR COAL FIRED BOILER

Input/output Parameter kCal / kg of % loss

coal
Heat Input = 3501 100
Losses in boiler
1. Dry flue gas, L1 = 275.88 7.88
2. Loss due to hydrogen in fuel, L2 = 120.43 3.44
3. Loss due to moisture in fuel, L3 = 206.91 5.91
4. Loss due to moisture in air, L4 = 10.15 0.29
5. Partial combustion of C to CO, L5 = 90.32 2.58
6. Surface heat losses, L6 = 8.75 0.25
7. Loss due to Unburnt in fly ash, L7 = 3.85 0.11
8. Loss due to Unburnt in bottom ash, L8 = 61.97 1.77
Boiler Efficiency = 100 (L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8) = 77.77 %

Factors Affecting Boiler Performance:

1. Periodical cleaning of boilers

2. Periodical soot blowing
3. Proper water treatment programme and blow down control
4. Draft control
5. Excess air control
6. Percentage loading of boiler
7. Steam generation pressure and temperature
8. Boiler insulation
9. Quality of fuel

Suggestion to improve Boiler Efficiency:

Boiler Efficiency is of prime importance to the process plants. Improving the boiler efficiency
even slightly can cut down the fuel bills significantly. Improvement in boiler efficiency can be
achieved if certain steps are followed.

1. Monitor Feedwater Quality

The quality of boiler feedwater affects the overall boiler performance and hence the boiler
efficiency. Poor feedwater quality increases the TDS levels in the boiler drum. Increased TDS
calls for more blowdown which results in increased blowdown losses and lower boiler
efficiency. If the TDS level is not brought down in the approved limits, it leads to water
carryover which results in poor operational efficiency of the steam system.
2. Boiler Sizing
The first step to ensure high boiler efficiency is selecting a correctly sized boiler. If the boiler is
over sized, it will result in frequent on-off. Every time a boiler shuts down and re-starts, boiler
efficiency comes down drastically.

3. Blowdown Control and Heat Recovery from Blowdown

In order to maintain the TDS level within specified limits, we need to drain out water from
boiler drum with higher salt concentration and replace it with fresh water. Draining out the
water from boiler drum results in loss of energy as the water inside will be hot and pressurized.
Manual blowdown is rarely accurate and results in either under or over blowdown. Over
blowdown brings down the boiler efficiency. Instead of depending on manual blowdown,
implementing a blowdown control system will certainly help.
At the same time, heat from blowdown water should be recovered by using a blowdown vessel.

4. Furnace Draft Tuning

Pressure inside the boiler furnace should be tuned properly (-2 to -5 mm of water column for
small boilers) to get a good boiler efficiency. If the pressure falls below -5, it will result in
increased unburnt losses and will bring down the boiler efficiency. This makes furnace pressure
draft tuning an essential adjustment to improve the boiler efficiency.

5. Excess Air
Excess air increases the enthalpy losses and as a result, the boiler efficiency goes down. For
proper combustion of fuel inside the furnace, certain amount of air (oxygen) is required. If
sufficient quantity of air is not supplied, the carbon present in the fuel is incompletely oxidized
to carbon monoxide and less amount of heat is released which brings down the overall
efficiency of the fuel. On the other hand, if the excess air is more than required, this air absorbs
the energy by absorbing the heat from combustion and this energy is lost along with the flue
gases which again brings down the boiler efficiency. Monitoring stack oxygen levels and
controlling them within the required band is essential for high boiler efficiency.

6. Fuel Quality
Quality of fuel is of prime importance as far as boiler efficiency is concerned. Just as an
example, if fuel has high moisture content, the enthalpy losses taking place will be much higher
and as a result, the boiler efficiency will come down. In case of solid fuel fired boilers, drying
the fuel before combustion can avoid enthalpy losses and hence improve the boiler efficiency.

7. Tube Cleaning
Over the period, soot deposition takes place on the fire side of the boiler tubes and scaling on
the water side. The layer of soot/scales acts as insulator and brings down the heat transfer rate.
As a result, the hot gases pass away without actually transferring the heat to the water. Cleaning
boiler tubes periodically removes all the soot and scales and improves the efficiency of the heat
transfer and results in improved boiler efficiency.
8. Online Monitoring of Boiler
Boilers do not operate at the rated efficiencies all the time. The operating practices play an
important role in determining the real time boiler efficiency. Using an online efficiency
monitoring system for boilers can give insights about actual real-time efficiency and can
generate suggestions to improve the boiler efficiency based on that.

9. Boiler Automation
Boiler automation results in efficient and safe boiler operation. Many times, manual operation
leads to following operating practices that bring down the boiler efficiency. Boiler automation
ensures that boiler operates only in the safest and most efficient way and hence significantly
improve the boiler efficiency.

10. Heat Recovery from Exhaust

Flue gases coming out from the boiler contain considerable amount of heat energy due to their
hot temperature. This energy from the hot flue gases can be recovered and used to heat
feedwater. This system improves the boiler efficiency. If the sulphur contents of the fuel are
high, there might a risk of dew point corrosion and hence, the heat should be recovered to a
certain extent only.