Bidesh SenguptaDesign Engineer, Engineering - Rotary Air

Preheaters, Alstom India Ltd., India

Chittatosh Bhattacharya Deputy Director (Tech.)

National Power Training Institute, E. Region, India

Proceedings of ICAER 2013: IIT BombayDay – 2, Session – D (FN) , Paper Ref:011December 10-12, 2013; Mumbai; INDIA

Effect of Ambient Air Temperature on the

Performance of Regenerative Air

Preheater of Pulverised Coal Fired Boilers

World Energy Outlook 2010

The shift from coal power is not as fast as the growth of RE over the years !!

Starting with Electric Power Energy Realities

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.

To Meet Sustainable Power Demand

Energy & Environment Challenges & Needs

For Sustainable Coal Power Generation at Affordable PRICE with

Environment protection must have two straight forward approaches

Objective - Reduce coal & auxiliary power consumption :lesser amount fuel to produce same amount of energy with lesser emission.

1

Benefit – Acceptable environmental impact from the pollutantsproduced by coal power generation process

2

Air

Water

Coal

InputElectricity

Output

OBJECTIVE - Reduce Coal, Water, Air Consumption per unit power produced

OBJECTIVEReduce

Auxiliary Power& LossesBENEFIT

Reduction in emission of pollutants

Energy & Environment Modeling for Sustainable Coal Power

Few Identified Primary Actions

1 Increase “As Fired” Coal Quality by adequate Drying

2

3

R EDUCTION OF COAL & AUXIL IARY P OWER CONSUMPTION

Reduce Induced & Forced Draught Fans Power Consumption

Complete burning of Coal & maximizing heat trapping in Boiler

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

APH Hot END

APH Cold END

HOT Primary Air OUT

Cold Primary Air IN

HOT Secondary Air OUT

Hot Flue Gas IN

Regenerative Air Preheater Basics.......

COLD Flue Gas OUT

COLD Secondary Air IN

Arrangement of Regenerative APH

340℃Mill

FD Fan

PA Fan

ESP

ID Fan130℃

STACK

Boiler

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.

Performance Impact of Air Preheater on Overall Equipment Efficiency

FD & PA Fans Pulverizers Burners

Boiler Input Devices

Boiler Operation Combustion Control ID Fans

Environmental Emission Control DevicesBoiler Output Devices

Electrostatic Precipitators Bag houses SCR FGD

FD FAN+ P

PA FAN

ID Fan- P

1

2

3

4WB/ MillEconomizer

Evaluation of APH Leakage

1

2

4

Hot Flue Gas APH Bypassing

Ambient Cold Air APH Gas Side

Hot Air APH Gas Side

Ambient Cold Air APH Bypassing

APH helps to dry up pulverized coalwith hot primary air, rapidattaining of ignition temperatureand creating turbulence with morevolumetric flow of Secondary air.

3

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.

The leakage of air to flue gas side & increases

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.

Understanding APH Leakage…….

2 3

Regenerative APH Heat Exchange MechanismHot side Cold side

On Rotation Heating SurfaceHeating SurfaceHeating Surface

Flue Gas H/E Storage H/E Transfer H/E in Air

APH Heat Exchange Performance Evaluation

Flue Gas & Air Side Heat Balance

Heat Transferred by Flue Gas qg：

1.

Heat Transferred to Air qa：

2.

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 = q

a

mgeｘ Cpg x （Tge- Tgl) = mal ｘ Cpa x （Tal-

Tae)

qg

= mge ｘ Cpg x （Tge-

Tgl)

qa = mal ｘ Cpa x （Tal- Tae)

Gas EntryAir Leaving

Tal

mal

Tae

Tgl

Tge

mge

Gas LeavingAir Entry

qae

qal

+Qa

qgl

- Qg

qge

+Qa - Qg

ηThermal

: Ratio of ΔT between Gas inlet and Air inlet

and between Gas/Air inlet and Gas/Air outlet .

1. ηgas - Gas side Efficiency

2. ηair

-Air side Efficiency

⊿Tmax Tge- Tae

⊿Tg Tge- Tglηg = =

⊿Ta Tal - Tae

⊿Tmax Tge - Tae

ηa = =

ΔTmax

ΔTa

ΔTg

1000 20000

50

100

200

150

250

300

400

350

Length of Heating Elements

Te

mp

er

at

ur

e(℃

)

Tae

Tal

Tge

Tgl

Cold Hot

APH Thermal efficiency Evaluation Matrix

ηa x XR = ηg

Xratio

( XR) : heat capacity ratio of air Vs flue gas

Alternately may be expressed as below from the equation of qg=q

a

Relation for thermal efficiency -

(Tal – Tae )

Tge – Tglmal x Cpa

mge x Cpg

XR =⊿Tg

⊿Ta

Cmin mal x Cpa

Cmax mge x Cpg

XR = =

Xratio

: APH Thermal efficiency Indicator

==

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……………..

・ Quantity of Air flow（ma, ）、 when (Tae) AAT

・ (Tae) AAT Humidity Ratio

・ Humidity Ratio Heat loss to dry air

・ (Tae) AAT ηAPH

・ As (Tae) AAT ΔPh（Δpa :O/L – ΔPg I/L）

thus Air leakage increases

The result of Plant Findings as follows…..

Boiler (Air/Flue gas)

FD/PA Fan ESP

Tae

ma

mg

TgeDPh

Tgl

ΔPa ΔPg

mL

Hot Air Flue Gas

Effect of AAT in PerformanceAmbient air temperature (AAT) affects - mass flow rate (m

a), coefficient of heat capacity (Cpa);

density, humidity ratio (moisture content capacity of air on RH)

FIELD STUDY ON THE EFFECT OF AAT IN ηAPH

A set of operating parameters governing APHperformance was collected having a considerabledifference in AAT . All data are taken at nearly equalpower generating condition (having a variation in PLFwithin + 0.5%) and collected within 24 hours span toavoid variation in RH factor on the same day, (~ 40%).

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

Effect of AAT (Tae

) on mass flow (ma) variation

As ma decreases ;

ma.Cpa decreases

too, reducing the

capability of air to extract

the heat from APH

element matrix absorbed

from flue gas..!!!

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……

Effect of AAT (Tae

) on Relative Humidity (RH)

The APH efficiency :

Effect of AAT (Tae

) on APH efficiency (ηAPH

)

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.

The modified efficiency is given by:

The modified efficiency is calculated considering the

moisture of air . Decrease of 22% of η

APHmfor 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!!

Effect of AAT (Tae

) on APH modified efficiency (ηAPHm

)

=

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

output power.

Effect of APH Performance Deterioration..!!!

The use of regenerative APH in tropical countries with higher AAT is notan energy efficient option where RH of air is substantially high andconsiderable fuel energy is wasted to dry up air-moisture beside thehigh moisture low-rank coal used for pulverised coal fired powergeneration system. As such, high moisture coal drying can be moreeconomically achieved through atmospheric fluidized bed drier usingwaste heat of flue gas upstream of ID fan and before exhaust throughstack. The partial flue gas recirculation through pulverizer (PFGR)system can reduce the APH heat transfer loading, since coal dryingcapacity sometimes get restricted due to insufficient hot primary airtemperature and thereby causes power generation capacity restriction.Besides, a reduction in coal drying need in turn increases moresecondary air temperature at APH ensuring better combustion withlower NOx generation potential with less excess air.

CONCLUSION

THANK YOU

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