ASH HANDLING ARRANGEMENT (ASHA)

JISHNU VEMP NO:-6106188BHEL-ISG

Arrangement of Boiler Auxiliaries

Coal bunker: Stores crushed coal from CHP Made up of welded steel plates Kept at the top of the mills, so as to assist

induced gravity flow Normally 6 in number Coal feeder: Drag link mechanism used for the

transport of coal from bunker

Boiler Auxiliaries

Mills: For every 200 MW unit, there are 6 mills (25%

capacity) which powder the coal to the required fineness

Located at the same elevation as that of furnace Pulverization is done in the mill and crushing in

the bunker Primary Air fans (PA fans): 2 fans per 200 MW unit operating at 50%

capacity Heating air up to 50C

Located at datum level

Boiler Auxiliaries

Air pre-heaters: It is used to preheat primary air and

secondary air using regenerative heat exchangers utilizing the flu gases

2 such heaters for each 200MW unit 16m elevation from datum• Burners: - to burn the pulverised coal

with oil

A short note on various heat exchangers

1) Shell and tube heat exchanger: used in suction heaters of oil fired TPPs

2) Plate heat exchangers 3) Adiabatic wheel heat exchangers 4) Plate fin heat exchangers 5) Regenerative heat exchangers

Ljungstrom type Bisector

Boiler Auxiliaries

FD fans: Forced draft fans , 2 per 200MW unit Used to handle the secondary air 0 metres elevation, at the same level as of

the PA fan Wind box: To supply the excess air, placed on either

sides of the furnace facing chimney Distributing medium Scanner fan: To cool the scanner

Boiler Auxiliaries

Soot blowers: There are 3 types of soot blowers, viz

Long retractable soot blowers, air heater blowers, Wall blowers

• Pressure reducing valves

BOILER AIR AND DRAFT SYSTEM

Why we need a fan

The fan is required for 2 purposes: -A) To provide sufficient oxygen to assist the

combustionB) To remove the products of combustionC) Evacuation of flue gases

Selection criterion for fan

Air or gas flow-mass flow rate (kg/hr) Density of gas (function of state variables

like temperature and pressure) System resistance

DRAFT SYSTEM

What is draft?

The term draft refers to the pressure difference between the atmosphere and that existing in the furnace

It is required to ensure that required amount of oxygen is available for combustion, removal of products of combustion, losses due to flow is to be minimized

Types of draft

Various types of drafts are A) Natural draft: Vacuum is created in

furnace by making very tall chimneys, this pressure difference draws air to furnace

B)Induced draft: The air is sucked into the furnace by the height of chimney and flue gases are pushed out by the ID fans. Negative pressure is created

C) Forced draft: It has positive pressure and air is pushed out by the pressurization by FD fan

Requirement of TPP

Mechanical drafts (ID and FD) are more advantageous than natural draft because proper combustion is ensured, easy flow of flue gases and more efficiency

Air is required to carry coal from mill to furnace

To remove the flue gases To ensure proper combustion

FURNACE

PRIMERY AIR HEATER

SECONDRY AIR HEATERMILLS

FD FAN A

PA FAN BPA FAN A

FD FAN B

ESP

ID FAN A

ID FAN B

CHIMNEY

Atmospheric air

Atmospheric air

Primary air system

The primary air is driven by axial flow reaction fan

Air discharging from each fan is divided into two parts, one passes first through a air pre-heater then through a gate into the P.A bus duct. The second goes to the cold air duct. The mix of both is used to carry the pulverized coal to the boiler.

Ignitor fan

These fans, again two per boiler, are used to supply air for cooling ignitors and combustion of ignitor air fuel mixture. Fan makes the suction from atmosphere directly and supplies air to the wind boxes of individual igniters at a fixed constant uncontrolled rate at ambient temperature.

Burner Box Dampers

Coal/Air Dampers

Secondary Air Dampers

Oil/Secondary Air Dampers

Bottom Tier Secondary Air Damper

Over Fire Damper

Secondary air Damper control

Fuel Air Dampers: which regulate air surrounding a fuel compartment. Fuel air dampers are modulated in proportion to the rate of fuel fired in that elevation.

Auxiliary Air Dampers: which regulate air

adjacent to a fuel compartment are termed. Auxiliary air dampers are modulated to maintain a fixed differential pressure between the wind box and the furnace.

Primary and secondary air system

Tangential Firing System

Variation of Pressure in Boiler

Furnace Pressure At Various Points in Boiler

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Points in Boiler

Furn

ace

Pre

ss

ure

1FD Fan Inlet

2FD Fan Outlet

3Airheater Inlet

4Airheater Outlet

5Windbox Pressure

6 Furnace

7

Superheater Platen Inlet

8Reheater Inlet

9 LTSH Inlet

10 Economiser Inlet

11Airheater Inlet

12E.P. Inlet

13I.D. Fan Inlet

14I.D. Fan Outlet

Hoisting machines used in ASHA

Chains: 2 types. Viz coil chains and stud link chains

Coil chains are used in hoists, cranes and dredges

Made of open hearth steel for smaller ones and high grade wrought iron for larger ones

Load capacity equals 1.5 times proof load or load at the elastic limit whichever may be the least

FOS=2 for normal applications, 3 for

3 for intermittent applications, 3.5 for continuous machine operations

Stud link chains are used for anchors and moorings

Can carry loads which are 20-25% more than open link chains

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