This document summarizes research on the effect of ambient air temperature on the performance of regenerative air preheaters used in pulverized coal-fired boilers. The summary is: 1) Ambient air temperature affects the mass flow rate and heat capacity of air entering the preheater, reducing its ability to extract heat from flue gases. 2) Higher ambient temperatures also increase the humidity ratio of air, resulting in heat losses from moisture that do not contribute to heat transfer. 3) Field studies showed regenerative air preheater efficiency decreases by around 14% for an 8°C rise in ambient temperature, and modified efficiency that accounts for moisture decreases by 22% for the same temperature rise

1. Effect of Ambient Air Temperature on the
Performance of Regenerative Air
Preheater of Pulverised Coal Fired Boilers
Chittatosh Bhattacharya
Deputy Director (Tech.)
National Power Training Institute, E. Region, India
Bidesh Sengupta
Design Engineer, Engineering - Rotary Air
Preheaters, Alstom India Ltd., India
Proceedings of ICAER 2013: IIT Bombay
Day – 2, Session – D (FN) , Paper Ref:011
December 10-12, 2013; Mumbai; INDIA

2. World Energy Outlook 2010
Starting with Electric Power Energy Realities
The shift from coal power is not as fast as the growth of RE over the years !!

3. To Meet Sustainable Power Demand
Coal Power has
A Proven Past
A Progressive Present
& A Promising Future
With the support of low cost, low to high
quality of coal resources to run at least for
another Century (if not more) to provide
affordable quality power for all with
negotiable emission.

4. Energy & Environment Challenges & Needs
For Sustainable Coal Power Generation at Affordable PRICE with
Environment protection must have two straight forward approaches
1
Objective - Reduce coal & auxiliary power consumption :lesser
amount fuel to produce same amount of energy with lesser emission.
2
Benefit – Acceptable environmental impact from the pollutants
produced by coal power generation process
Input
Air
Water
Coal
OBJECTIVE -
Electricity
Output
OBJECTIVE
Reduce
Auxiliary Power
& Losses
BENEFIT
Reduction in emission of pollutants
Reduce Coal, Water, Air Consumption per unit power produced

5. Energy & Environment Modeling for Sustainable Coal Power
R E D U C T I O N O F C OA L & A U X I L I A RY P OW E R C O N S U M P T I O N
Few Identified Primary Actions
1
Increase “As Fired” Coal Quality by adequate Drying
2
Complete burning of Coal & maximizing heat trapping in Boiler
3
Reduce Induced & Forced Draught Fans Power Consumption
4
Reduce Heat lost through stack & improve Furnace heat transfer
All the above actions are
linked to the Performance
of one Boiler Component
better known as
Air Preheater

6. Regenerative Air Preheater Basics.......
HOT Secondary Air OUT
HOT Primary Air OUT
APH Hot END
Hot Flue Gas IN
Boiler
130℃
ESP
APH
Mill
ID Fan
340℃
STACK
FD Fan
PA Fan
APH Cold END
Cold Primary Air IN
COLD Flue Gas OUT
COLD Secondary Air IN
Arrangement of Regenerative APH
The APH accounts for 10-12 % of a unit’s thermal efficiency and is a critical component of plant system.
Ambient Air Temperature has a major impact on overall APH performance and therefore the intent is to
focus on the factors affecting APH performance and how to overcome or eliminate the issues.

7. Performance Impact of Air Preheater on Overall Equipment Efficiency
Boiler Input Devices
FD & PA Fans
Pulverizers
Burners
Economizer
Boiler Output Devices Environmental Emission Control Devices
Boiler Operation
Electrostatic Precipitators
Combustion Control Bag houses SCR FGD
ID Fans
WB/ Mill
1
Evaluation of APH Leakage
4
1
Hot Flue Gas
2
Hot Air
APH Bypassing
APH Gas Side
3
3
ID Fan
P
FD FAN
+ P
PA
FAN
APH Gas Side
4
2
Ambient Cold Air
Ambient Cold Air
APH Bypassing
APH helps to dry up pulverized coal
with hot primary air, rapid
attaining of ignition temperature
and creating turbulence with more
volumetric flow of Secondary air.

8. Understanding APH Leakage…….
The leakage of Flue Gas/Air across the circumference bypassing
APH is not creating any fan power loss but causing a heat rate
penalty of ƞboiler loss of 1% for every ~ 220 C rise in Texit gas . With
increase in ΔTCold Hot side (average 185-2000C ), the leakage
increases more in large dia. Regenerative APH.
An air leakage of 5-7% is already acknowledged in the design and
supply condition of the regenerative APH and a loss of 10 -13%
reduction in ƞoverall APH is already observed for every 10% increase
in leakage.
2
3 increases
The leakage of air to flue gas side
&
loading of ID/FD fan power without contributing to improve ƞboiler
rather causes LOI. Though these two leakages are measurable with
O2 concentration in flue gas before & after APH, the Flue Gas /Air
bypassing APH are not accurately measurable as per ASME PTC
4.3 neither it is a part of PG test, and only approximated.

9. APH Heat Exchange Performance Evaluation
Regenerative APH Heat Exchange Mechanism
Hot side
Heating Surface
On Rotation Heating Surface
Flue Gas H/E
Storage H/E
Flue Gas & Air Side Heat Balance
Heat Transferred by Flue Gas qg：
1.
qg = mge ｘ Cpg x （TgeTgl)
Cold side
Heating Surface
Transfer H/E in Air
Air Leaving
Tal
mal
qal
Gas Entry
qge
+Qa
Tge
mge
Heat Transferred to Air qa：
2.
qa = mal ｘ Cpa x （Tal- Tae)
+Qa
- Qg
qae
- Qg
qgl
m ： Mass of Fluid [ Flue Gas / Air ] （ｋｇ/ｈ）
Cｐ： Mean Specific Heat between Tae and Tgl （kｃａｌ/ｋｇ
/℃）
T ： Temperature of Hot/Cold Fluid [ Flue Gas / Air ] （℃）
Heat Balance： qg = qa
mgeｘ Cpg x （Tge- Tgl) = mal ｘ Cpa x （Tal-
Tae
Air Entry
Tgl
Gas Leaving

10. APH Thermal efficiency Evaluation Matrix
ηThermal: Ratio of ΔT between Gas inlet and Air inlet
and between Gas/Air inlet and Gas/Air outlet .
1. ηgas - Gas side Efficiency
⊿Tmax
=
Tge- Tgl
350
Tge- Tae
300
2. ηair -Air side Efficiency
ηa =
⊿Ta
⊿Tmax
=
Tal - Tae
Tge - Tae
Temperature (℃)
ηg =
⊿Tg
Tge
400
250
Tal
ΔTg
200
150
ΔTa
Tgl
100
50
Tae
0
1000
Cold
Length of Heating Elements
2000
Hot
ΔTmax

11. Xratio: APH Thermal efficiency Indicator
Xratio ( XR) : heat capacity ratio of air Vs flue gas
XR =
Cmin
Cmax
=
mal x Cpa
mge x Cpg
Alternately may be expressed as below from the equation of qg=qa
XR =
mal x Cpa
mge x Cpg
=
Tge – Tgl
(Tal – Tae )
Relation for thermal efficiency -
=
⊿Tg
⊿Ta
ηa x X R = ηg
Thus we find ambient air temperature (AAT) is a determining factor
of regenerative air pre heater efficiency and its performance – the
effect of which is evaluated in next few slides……………..

12. Effect of AAT in Performance
Ambient air temperature (AAT) affects - mass flow rate (ma), coefficient of heat capacity (Cpa);
density, humidity ratio (moisture content capacity of air on RH)
・ Quantity of Air flow（ma, ）、 when (Tae) AAT
・ (Tae) AAT
・
Humidity Ratio
・ (Tae) AAT
・
As (Tae) AAT
Humidity Ratio
Boiler (Air/Flue gas)
Hot Air
ma
Flue Gas
DP
h
mg
Tge
Heat loss to dry air
ηAPH
ΔPh（Δpa :O/L – ΔPg I/L）
thus Air leakage increases
The result of Plant Findings as
ΔPa
Tae
ΔPg
m
L
FD/PA Fan
Tgl
ESP

13. FIELD STUDY ON THE EFFECT OF AAT IN
ηAPH
A set of operating parameters governing APH
performance was collected having a considerable
difference in AAT . All data are taken at nearly equal
power generating condition (having a variation in PLF
within + 0.5%) and collected within 24 hours span to
avoid variation in RH factor on the same day, (~ 40%).

14. Effect of AAT (Tae ) on mass flow (ma) variation
Va = Constant: Space available for the fluid flow through the device is constant.
Pa = Constant: atmospheric pressure.
Mass Flow rate through the device is; ma = Pa Va / R Tae
Tae(AAT) ma
Almost 15% drop in mass flow
rate of air for 8 0C rise in AAT
ma
ma.Cpa
As
decreases ;
decreases
too,
reducing
the
capability of air to extract
the heat from APH
element matrix absorbed
from flue gas..!!!

15. Effect of AAT (Tae ) on Relative Humidity (RH)
Tae(AAT)
RH constant : humidity ratio
Humidity ratio
ha
The moisture in air does not help in any
heat transfer from the element matrix
through flue gas and therefore is
considered as loss
HL= heat loss due to moisture/kg of air
Cm = specific heat content of moisture
For 8 0C rise in AAT there is
̰12 KJ/kg loss of energy……

16. Effect of AAT (Tae ) on APH efficiency (η )
APH
The APH efficiency :
The APH efficiency is
calculated
without
considering the effect
of moisture in air. For
8 0C rise AAT , ηAPH
decreases by almost
14% of the initial η.

17. Effect of AAT (Tae ) on APH modified efficiency (η )
APHm
The modified efficiency is given by:
=
The modified efficiency is
calculated considering the
moisture of air . Decrease
of 22% of ηAPHm for 8 0C
rise in AAT or for 1 0C rise in
AAT the efficiency decreases
by 2.74%. Means more input
is required to obtain rated
output: ultimately causing
Commercial
LOSS!!

18. Effect of APH Performance Deterioration..!!!
 Inadequate drying of coal in coal mills.
 Increase of AAT increases leakage; increases ID / FD fan
power consumption.
 Due to leakage of secondary air in flue gas reduces O2 in the
furnace causing incomplete combustion .
 Due to incomplete combustion; more fuel is required to obtain
the required output, also incomplete combustion badly affects
the environment ultimately leading to commercial losses.
 Auxiliary power is supplied from plant. Increase in
consumption of auxiliary power leads to decrease the net

19. CONCLUSION
The use of regenerative APH in tropical countries with higher AAT is not
an energy efficient option where RH of air is substantially high and
considerable fuel energy is wasted to dry up air-moisture beside the
high moisture low-rank coal used for pulverised coal fired power
generation system. As such, high moisture coal drying can be more
economically achieved through atmospheric fluidized bed drier using
waste heat of flue gas upstream of ID fan and before exhaust through
stack. The partial flue gas recirculation through pulverizer (PFGR)
system can reduce the APH heat transfer loading, since coal drying
capacity sometimes get restricted due to insufficient hot primary air
temperature and thereby causes power generation capacity restriction.
Besides, a reduction in coal drying need in turn increases more
secondary air temperature at APH ensuring better combustion with
lower NOx generation potential with less excess air.

20. THANK YOU
???????