Air-preheater for Conservation of Flue Gas Energy

P M V Subbarao

Professor

Mechanical Engineering Department

Minimize Final Exhaust Gas Temperature….Properly Utilize Enthalpy of Flue Gas….

Gas Temperatures

• Platen Super Heater:• Inlet Temperature: 1236.4 0C• Outlet Temperature: 1077 0C• Final Super Heater:• Inlet Temperature: 1077 0C• Outlet Temperature: 962.4 0C• Reheater:• Inlet Temperature: 962.4 0C• Outlet Temperature: 724.3 0C• Low Temperature Super Heater:• Inlet Temperature: 724.30C• Outlet Temperature: 481.3 0C• Economizer:• Inlet Temperature: 481.3 0C• Outlet Temperature: 328.5 0C

Steam Temperatures

• Platen Super Heater:• Inlet Temperature: 404 0C• Outlet Temperature: 475 0C• Final Super Heater:• Inlet Temperature: 475 0C• Outlet Temperature: 540 0C• Reheater:• Inlet Temperature: 345 0C• Outlet Temperature: 5400C• Low Temperature Super Heater:• Inlet Temperature: 3590C• Outlet Temperature: 404 0C• Economizer:• Inlet Temperature: 254 0C• Outlet Temperature: 302 0C

Furnace absorption

Platen SH

Pendent SHCSH

Reheater

Economizer

Combustion Losses C & R losses Hot Exhaust Gaslosses

~3280C

Design 1: 500 MW

Design 1: 500 MW

LMTD for various Devices: Model 1

Design 1: 500 MW

DPNL SH

Platen SHT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A Modern Fuel Fired SG

Thermal Balance in Air Pre-Heater.

• The energy rate gained by air )( ,, inairoutairairair hhmQ

LMTDaphgasair TUAQQ

adust

dust

me

me

as

as

g hh

U11

1

• The energy rate lost by flue gas

• Overall Coefficient of Heat Transfer, U

)( ,, outgasingasgasgas hhmQ

• Overall Convective rate of Heat Exchange

The Creative Design……

• Heat exchanger is complete only if there are Donor, Receiver and Mediator in Equilibrium.

• Presence of all the three necessary?• Time sharing & Space Sharing• Time Sharing : Donor And Mediator for sometime and

Mediator and Receiver for sometime : Repeat!• Space sharing: All present always.• Central Limit Theorem : It is impossible to have time and

space sharing in one system.• Time Sharing : Regenerators• Space Sharing : Recuperators

The concept of Time Sharing

• At any time:

• The overall heat transfer coefficient, U

me

me

as

as

g

gas

h

U

1

1

adust

dust

me

meair

h

U1

1

OR

• At stead operation:

condg

gas

Rh

U

1

1

acond

air

hR

U1

1

OR

Stockholm 1920The Ljungström Air Preheater

Historical Significance of Landmark

• Throughout the history of boilers there have been many advancements in order to obtain a better performance and lower fuel consumption.

• Ljungström Air Preheater invented by Fredrik Ljungström, then Technical Director at Aktiebolaget Ljungström Ångturbin (ALÅ) is the most successful invention.

• The first installation in a commercial boiler saved as much as 25% of the fuel consumption.

• In a modern Steam generator the Ljungström Air Preheater provides up to 20% of the total heat transfer in the boiler process, but the Ljungström Air Preheater only represents 2% of the investment.

Economic Impact of the Landmark

• The use of a Ljungström Air Preheater in a modern power plant saves a considerable quantity of fuel.

• The cost of the preheater is generally recovered after only a few months.

• The total world-wide fuel savings resulting from all Ljungström Air Preheaters which have been in service is equivalent to 4,500,000,000 tons of oil (1994).

• An estimate shows that the Ljungström Air Preheaters in operation annually saves about $30 Billion US.

• The distribution of thermal power capacity in which Ljungström Air Preheaters are installed over the world is shown in the table below.

Landmark Contribution to Developmentof New World Wide Industry

• The use of the Ljungström Air Preheater started in the 1920s throughout the whole world.

• In the beginning the marketing of the Ljungström Air Preheater was made in close connection with ALÅ.

• but most of the deliveries have actually been made through a network of licensees throughout the world.

Rotary or Regenerative Air Pre-Heater

Stationary-Plate Type Air Pre-Heater

Regenerative Pre-Heaters

• Rotates with a low speed

• Rotor is divided into 12 or 24 sections and 12 or 24 radial divisions.

• Each sector is divided into several trapezoidal sections with transverse division plates.

• Heat storage pales are placed in these sections.

• Weight : 500 tons.

• This consists of : rotor, sealing apparatus, shell etc.

Schematic diagram of the rotary air preheater in thermal power plant.

Thermodynamic process and flow diagram

Leakage in APH

Multiple Channel RAPH

Fluid flows in rotary regenerator