DRY FLUE GASES LOSSES
Rising fuel costs which in most of
power generating companies are
passed through to customer ,caught
the attention to regulating agencies.
Regulating agencies are now paying
more attention to efficiency of power
BENEFIT -identification of losses
Green house gases emission decrease
Particulate emission reduction
Reduction in O&M cost
Less capacity addition
Dry Gas Losses ( excess air)
Dry gas losses are those heat losses resulting
from the heating of combustion air and fuel
from ambient conditions to the temperature of
the flue gas leaving the unit .
The magnitude of dry gas losses is directly
proportional to the excess airflow and to the
difference between the inlet air temperature
and the flue gas outlet temperature.
Reducing these losses presents the greatest
potential for improving boiler efficiency .
Dry gas losses
High excess air and lower heat
absorption in the boiler system can
cause exit gas temperature higher
than expected resulting dry gas loss.
A 22 deg C rise in exit temperature
can rise the heat rate one percent.
LOSSES IN BOILER –Design (IN
210 MW THERMAL POWER PLANT( AS PER BS 2886)
at 100% TMCR 210 MW
DRY GASH2O AND H2 IN FUEL
H2O IN AIR
LOSSES IN BOILER –Design (IN
DRY GASH2O IN FUEL
H2O FROM COMP OF H2
H2O IN AIR
500 MW THERMAL POWER PLANT( AS PER BS 2886)
at 100% TMCR 500 MW
Cause of deviation
Excess air, high O2 at boiler outlet .
In leakage from bottom hopper air preheater/ESP
inlet door leakage /open
In leakage from duct, expansion joints.
Excess mill tempering air due to low mill temperature
Incorrect operation of pre warming airpreheater inlet
Air preheater bypass dampers open
Poor mill performance
Poor preheater efficiency- plugage/fouled;
Oxygen measurement calibration
Correct on of mills in service for given load
Excess furnace draft (higher draft is worst for in
Reduce excess air
Ensure hopper ,slag ports etc closed tightly
Adjust mill primary air
Achieve design mill outlet temperature or
Proper soot blowing for air reheater
Proper operation of steam coil to prevent
fouling and corrosion
Proper soot blowing for boiler
Proper number of mills in service
Repair expansion joints, door gaskets
on hoppers and preheater seals.
Repair/replace preheater baskets.
Plugging or fouling of APH
Plugging or fouling of preheaters can occur on the
hot side but is more common on the cold side where
moisture has formed due to reaching the dewpoint. In
addition to raising the exit gas temperatures, plugging
can lead to load reductions.
Ensure proper soot blowing for preheater. Dry
superheated steam is normally used as a cleaning
Periodic high pressure washing may be necessary if
the pressure drop across the preheater starts to limit
Air inleakage in boiler,preheater or
Run O2 rise test on boiler to locate air inleakage and
Air inleakage in the furnace, boiler ducts, expansion
joints, or preheater can adversely affect the heat
transfer, give false indications of percent O2, and
increase fan auxiliaries.
Incoming oxygen rich air also tends to increase the
rate of acid deposition increasing the corrosion
O2 readings should be taken at several locations
simultaneously to isolate cause of air in leakage.
Corrosion or erosion in Air
During the process of combustion, sulfur in the fuel is
converted to SO2 and depending on the excess air
available, part of the SO2 is converted to SO3. The
SO3 reacts with any water vapor present in the
preheater to form sulfuric acid (H2SO4).
Ensure proper operation of steam coils or preheater
bypass damper to keep the preheater above dew
If steam coils are used, perform periodic inspections
for leaks which would increase water vapor to the
Incoming oxygen-rich air, as a result of air in
leakage, tends to increase the rate of acid deposition,
increasing the corrosion potential.
Corrosion in ductwork and ID
The ductwork and ID fan housings have
suffered damage due to dewpoint corrosion.
Taking into account the money spent on repairs
to ductwork and pattern of corrosion ,high
nickel alloy should used since it will afford a
good degree against corrosion and also
provide adequate resistance against
mechanical abrasion .
If Carpenter 20(CN7M) is selected
,performance will increase the life of ductwork
by a factor of 8 as compared to carbon steel.
Air and flue gas system -500 MW
Air Leakage in AirPreheater
Air Leakage in APH %
90 * (CO2 in – CO2 out )
= --------------------------CO2 out
NOTE1.CO2 in – percentage CO2 at air preheater inlet
2.CO2 out- percentage CO2 at air preheater