DME-II_Thermal Engineering-II_Boiler Performance
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 test- ing 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
•
To find
out the efficiency of the boiler
•
To find
out the Evaporation ratio
The purpose
of the performance test is to determine actual
performance and
efficien- cy
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
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 eval- uated using the formula:
Measurements
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, accord- ing 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 tem- perature.
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 partic- ular 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 determi- nation. 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 tem- perature 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.
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 inter-vals. 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 Demerit of Direct Method
Merits
• Plant
people can evaluate quickly the efficiency of
boilers
• Requires
few parameters for computation
• Needs
few instruments for monitoring
Demerits
• Does not give clues to the operator as to why efficiency of system is lower
• Does
not calculate various losses accountable for various efficiency levels
• 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 effi- ciency 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.
Feed
water
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 feed water
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 glass
|
Fuel temperature, flue gas
temperature, combustion air temperature, boiler surface temperature, steam
temperature
|
Draft gauge
|
Manometer, differential pressure
|
Amount of draft used or available
|
TDS meter
|
Conductivity
|
Boiler water TDS, feed water 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:
•
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
•
Blow
down loss. The amount of energy wasted by blow down varies over a wide range.
•
Soot blower steam. The amount of steam used by soot blowers is variable that
depends on the type of fuel.
•
Auxiliary equipment energy consumption. The combustion
efficiency test does not account for
the energy usage by auxiliary equipments, such as burners, fans, and pumps.
B) Preparations and pre conditions for testing
•
Burn the specified fuel(s) at the
required rate.
•
Do the tests while the boiler
is under steady
load. Avoid testing during
warming up of boil-
ers from a cold condition
•
Obtain the charts /tables for the
additional data.
•
Determination of general method of operation
•
Sampling and analysis of fuel and ash.
•
Ensure the accuracy of fuel and ash
analysis in the laboratory.
•
Check the type of blow down and method
of measurement
•
Ensure proper operation of all instruments.
•
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 car- bon 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 incom-
plete.
Boiler Efficiency by Indirect Method: Calculation Procedure and Formula
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.
Conversion formula for proximate analysis to ultimate analysis
%C = 0.97C
+ 0.7 (VM + 0.1A) – M(0.6 – 0.01M)
|
||
%H2
|
=
|
0.036C + 0.086 (VM – 0.1xA) –
0.0035M2 (1 – 0.02M)
|
%N2
|
=
|
2.10 – 0.020 VM
|
where C
|
=
|
% of fixed
carbon
|
A
|
=
|
% of ash
|
VM
|
=
|
% of volatile matter
|
M
|
=
|
% of moisture
|
However it is suggested to get a
ultimate analysis of the fuel fired periodically from a reputed laboratory.
Theoretical (stoichiometric) air
fuel ratio and excess air supplied are to be determined first for computing the
boiler losses. The formula is given below for the same.
The various losses associated with
the operation of a boiler are discussed below with required formula.
1. Heat loss due to dry flue gas
This is the greatest boiler loss and can be calculated
with the following formula:
m x Cp
x (Tf - Ta )
L1 = x 100
GCV of fuel
Where,
|
||
L1
|
=
|
% Heat loss due to dry flue gas
|
m
|
=
|
Mass of dry flue gas in kg/kg of
fuel
|
=
|
Combustion products from fuel: CO2
+ SO2 + Nitrogen in fuel +
Nitrogen in the actual mass of air supplied + O2
in flue gas. (H2O/Water vapour
in the flue gas should not be considered)
|
|
Cp
|
=
|
Specific
heat of flue gas in kCal/kg°C
|
Tf
|
=
|
Flue gas temperature in °C
|
Ta
|
=
|
Ambient temperature in °C
|
Note–1:
For
Quick and simple calculation of boiler efficiency use the following.
A:
Simple method can be used for determining the dry flue gas loss as given below.
m x Cp x (Tf – Ta) x 100
a) Percentage heat loss due to dry flue gas =
GCV of fuel
Total mass of flue gas (m)/kg of fuel = mass
of actual air supplied/kg of fuel + 1 kg of fuel
Note-2: Water vapour
is produced from Hydrogen in fuel, moisture present in fuel and air dur- ing
the combustion. The losses due to these components have not been included in
the dry flue gas loss since they are separately calculated as a wet flue gas
loss.
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
evapora- tion 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
where
|
||
M
|
=
|
kg moisture in fuel on 1 kg basis
|
Cp
|
=
|
Specific heat of superheated steam
in kCal/kg°C
|
Tf
|
=
|
Flue gas temperature in °C
|
Ta
|
=
|
Ambient temperature in °C
|
584
|
=
|
Latent heat corresponding to partial
pressure of water vapour
|
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 boil- er. 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 psychrometric charts and typical values are
included below:
Dry-Bulb
|
Wet
Bulb
|
Relative
Humidity
|
Kilogram
water
per Kilogram dry
air (Humidity Factor)
|
Temp ºC
|
Temp ºC
|
(%)
|
|
20
|
20
|
100
|
0.016
|
20
|
14
|
50
|
0.008
|
30
|
22
|
50
|
0.014
|
40
|
30
|
50
|
0.024
|
L4
|
=
|
AAS x
humidity factor x Cp x (Tf – Ta ) x 100
|
|
GCV of fuel
|
|||
where
|
|||
AAS
|
=
|
Actual mass of air supplied per kg
of fuel
|
|
Humidity factor
|
=
|
kg of water/kg of dry air
|
|
Cp
Tf Ta
|
=
=
=
|
Specific heat of superheated steam
in kCal/kg°C
Flue gas
temperature in °C
Ambient
temperature in °C (dry bulb)
|
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
concen- tration 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 watertube 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 as given below
:
L6
|
=
|
0.548 x [ (Ts / 55.55)4 – (Ta / 55.55)4] + 1.957 x (Ts – Ta)1.25 x sq.rt of [(196.85 Vm + 68.9) / 68.9]
|
where
L6
|
=
|
Radiation loss in W/m2
|
Vm
|
=
|
Wind velocity in m/s
|
Ts
|
=
|
Surface temperature (K)
|
Ta
|
=
|
Ambient temperature (K)
|
Heat loss due to unburned carbon in fly ash and bottom ash:
Small amounts of carbon
will be left in the ash and this constitutes a loss of potential heat in the
fuel. To assess these heat losses, samples of ash must be analyzed for carbon
content. The quantity of ash produced per unit of fuel must also be known
Heat Balance:
Having established the magnitude
of all the losses mentioned above, a simple heat balance would give the
efficiency of the boiler. The efficiency is the difference between the energy
input to the boiler and the heat losses calculated.
Boiler Heat
Balance:
Input/Output Parameter
|
kCal / kg of fuel
|
%
|
|
Heat Input in fuel
|
=
|
100
|
|
Various Heat losses in boiler
|
|||
1. Dry flue
gas loss
|
=
|
||
2. Loss due
to hydrogen in fuel
|
|||
3. Loss due
to moisture in fuel
|
=
|
||
4. Loss due
to moisture in air
|
=
|
||
5. Partial
combustion of C to CO
|
=
|
||
6. Surface
heat losses
|
=
|
||
7. Loss due
to Unburnt in fly ash
|
=
|
||
8. Loss due
to Unburnt in bottom ash
|
=
|
||
Total Losses
|
=
|
||
Boiler efficiency = 100 –
(1+2+3+4+5+6+7+8)
|
|||
Example:
Boiler Efficiency Calculation
For Coal fired Boiler
The following are the data collected for a
boiler using coal as the fuel. Find out the boiler effi- ciency by indirect
method.
Fuel firing
rate
|
=
|
5599.17
kg/hr
|
Steam
generation rate
|
=
|
21937.5
kg/hr
|
Steam
pressure
|
=
|
43 kg/cm2(g)
|
Steam
temperature
|
=
|
377 °C
|
Feed water
temperature
|
=
|
96 °C
|
%CO2
in Flue gas
|
=
|
14
|
%CO in flue gas
|
=
|
0.55
|
Average flue
gas temperature
|
=
|
190 °C
|
Ambient
temperature
|
=
|
31 °C
|
Humidity in
ambient air
|
=
|
0.0204 kg /
kg dry air
|
Surface
temperature of boiler
|
=
|
70 °C
|
Wind
velocity around the boiler
|
=
|
3.5 m/s
|
Total
surface area of boiler
|
=
|
90 m2
|
GCV of
Bottom ash
|
=
|
800 kCal/kg
|
GCV of fly
ash
|
=
|
452.5
kCal/kg
|
Ratio of
bottom ash to fly ash
|
=
|
90:10
|
Fuel Analysis (in %)
|
||
Ash content
in fuel
|
=
|
8.63
|
Moisture in
coal
|
=
|
31.6
|
Carbon
content
|
=
|
41.65
|
Hydrogen
content
|
=
|
2.0413
|
Nitrogen
content
|
=
|
1.6
|
Oxygen
content
|
=
|
14.48
|
GCV of Coal
|
=
|
3501 kCal/kg
|
Boiler Efficiency By Indirect Method
Step-1: Find the theoretical air requirement
Theoretical air required for
complete
combustion
|
=
|
[(11.6 x C) + {34.8 x (H2
– O2/8)} + (4.35 x S)] /100
kg/kg of
coal
|
=
|
[(11.6 x
41.65) + {34.8 x (2.0413 – 14.48/8)} +
(4.35 x 0)]
/ 100
|
|
=
|
4.91 kg / kg
of coal
|
SUMMARY OF HEAT
BALANCE FOR COAL FIRED BOILER
|
|||
Input/Output Parameter
|
kCal / kg of coal
|
% loss
|
|
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 %
|
|||
Comments
Post a Comment