17634_Steam Power Plant and Coal

THERMAL POWER PLANT

Coal to Electricity

ECONOMISER

LOW

TEMP

SH

REHEATE

R SUPER

HEATER

COMBUSTION

ZONE

AIR PRE HEATER

BOILE

R

DRUM

WATER

WALLS

WIND BOX FROM

BOILER

FEED

PUMP

TO HP

TURBINE

FROM HP TURBINE

TO IP TURBINE

TO CHIMNEY

BOILER

TYPICAL SCHEME OF COAL / OIL FIRED POWER PLANT

SUPERHEATER TUBES

REHEATER PANEL

VIEW OF ESP

CONDENSER

BOILER

Condenser

LP

TURBINE

IP

TURBINE

HP

TURBINE

MS

MS

CRH

NRV

HRH

HPSV

HPSV

IPSV

IPSV GENERATOR

HPCV

HPCV

IPCV

IPCV

CRH

NRV

TG SET

VIEW OF CONTROL ROOM

EFFICIENCY OF THERMAL POWER PLANT

TYPE EFFICIENCY

BOILER (1) 88-90 %

TURBINE (2) 45-50 %

GENERATOR (3) 96-98 %

OVERALL CYCLE (4)

= 1x2x3

34-42 %

EFFICIENCY OF THERMAL POWER PLANT

How much coal was consumed to produce one unit of

electricity

Specific Coal = Heat Rate / Calorific Value of Coal

Consumption (Unit :- Kg/KWHr)

For Example, If Heat rate is 2000 Kcal / KWhr & Coal

Calorific Value is 4000 Kcal / Kg

Then, Specific Coal Consumption will be = 2000/4000 =

0.5 Kg/KWHr

Specific coal consumption

Specific Steam Consumption :- 3 Ton of Steam flow

required for 1 MW of power

Specific Coal Consumption :- 0.5 Kg coal is required for

generation of 1 unit of electricity

Auxiliary Power :- Around 8-10% power is consumed by

power plant auxiliaries

Some thumb rules w.r.t thermal plant

Efficiency of Thermal power ranges from 33 to 40 %

One 500 MW thermal power plant generates 12 million

units of electricity daily

Around 22000 TO 69000 m3/hr of water is required for

condensing steam in Condenser for a 210 TO 660 MW

Each power station has to maintain stock of minimum 7

days (Critical Stock)

Some thumb rules w.r.t thermal plant

MAJOR COMPONENT OF BOILER SIDE

COMPONENT PURPOSE

COAL BUNKER TO STORE PULVERISED COAL

COAL MILL FOR PULVERISING COAL

PA FAN FOR PRIMARY AIR

FD FAN FOR SECONDARY AIR

ID FAN TO MAINTAIN VACUUM INSIDE FURNANCE SO

THAT FLUE GASES CAN BE EVACUATED

WATER WALLS,

SUPERHEATER,

REHEATER, LOW

TEMPERATURE

SUPERHEATER,

ECONOMISER

VARIOUS WATER / STEAM CARRYING TUBES

INSIDE THE BOILER. IN ALL THESE TUBES

WATER / STEAM TAKES HEAT FROM

FURNACE WHERE COMBUSTION OF COAL /

OIL TAKES PLACE.

BOILER DRUM TO SEPARATE WATER FROM STEAM

COAL AND

COMBUSTION

Why Coal?

Share of Coal in Power

Generation

Advantages of Coal Fuel

•Abundantly available in India

•Low cost

•Technology for Power Generation well developed.

•Easy to handle, transport, store and use

Shortcomings of Coal

•Low Calorific Value

•Large quantity to be Handled

•Produces pollutants, ash

•Disposal of ash is Problematic

•Reserves depleting fast

•India’s Coal Reserves are estimated to be 206 billion tonnes. Present consumption is about 450 million tonnes.

•Cost of coal for producing 1 unit of electricity (Cost of coal Rs 1000/MT)is Rs 0.75.

•Cost of Gas for producing 1 unit of electricity (Cost of Gas Rs 6/SMC)is Rs 1.20.

Coal

55%

Gas

10%

Diesel

1%

Hydel

26%

RES

5%

Nuclear

3%

Coal – what is it?

65-95% C

2-7% H

<25% O

<10% S

1-2% N

20-70% Char

5-15% Ash

2-20% H2O

20-45% VM

• Inhomogeneous organic fuel

formed mainly from

decomposed plant matter.

• Over 1200 coals have been

classified.

Time, Temperature

Coal Rank

• Coalification forms different

coal types:

(Peat)

Lignite

Bituminous coal

Anthracite

(Graphite)

Pro

xim

ate

An

aly

sis

E

lem

en

tal C

om

positio

n

Knowing more about Coal

Coal Transportation

•Rail

•Truck

•Conveyor

•Ship

Coal production

•Surface Mining

•Underground Mining

Coal Properties

•Calorific Value

•Grade of Coal (UHV)

•Proximate Analysis

•Ultimate Analysis

•Ash and Minerals

•Grindability

•Rank

•Physical Characteristics

Coal Beneficiation

•Why?

•Processes

•Effectiveness

Coal production

•Surface Mining

•Underground Mining

Useful Heat Value (UHV) UHV= 8900-138(A+M)

Coal Applications

• Homes – heat and cooking

• Transportation – steam

engines

• Industry – metal works

• Electricity – power plants

Coal classification

• Anthracite: hard and geologically the oldest

• Bituminous

• Lignite: soft coal and the youngest

• Further classification: semi- anthracite,

semi-bituminous, and sub-bituminous

24

Physical properties

• Heating or calorific value (GCV)

• Moisture content

• Volatile matter

• Ash

Chemical properties

• Chemical constituents: carbon, hydrogen, oxygen, sulphur

Coal Properties

25

Heating or calorific value

• The typical GVCs for various coals are:

Parameter Lignite

(Dry

Basis)

Indian

Coal

Indonesian

Coal

South

African

Coal

GCV

(kCal/kg)

4,500 4,000 5,500 6,000

Physical properties

26

Moisture content

• % of moisture in fuel (0.5 – 10%)

• Reduces heating value of fuel

• Weight loss from heated and then cooled powdered

raw coal

Volatile matter

• Methane, hydrocarbons, hydrogen, CO, other

• Typically 25-35%

• Easy ignition with high volatile matter

• Weight loss from heated then cooled crushed coal

Physical properties

27

Ash

• Impurity that will not burn (5-40%)

• Important for design of furnace

• Ash = residue after combustion

Fixed carbon

• Fixed carbon = 100 – (moisture + volatile matter + ash)

• Carbon + hydrogen, oxygen, sulphur, nitrogen

residues

• Heat generator during combustion

Physical properties

28

Proximate analysis of coal

• Determines only fixed carbon, volatile matter,

moisture and ash

• Useful to find out heating value (GCV)

• Simple analysis equipment

Ultimate analysis of coal

• Determines all coal component elements: carbon,

hydrogen, oxygen, sulphr, other

• Useful for furnace design (e.g. flame temperature,

flue duct design)

• Laboratory analysis

Analysis of Coal

29

Analysis of Coal

Proximate analysis(Physical properties)

Typical proximate analysis of various coals (%)

Indian

Coal

Indonesian

Coal

South African

Coal

Moisture 5.98 9.43 8.5

Ash 38.63 13.99 17

Volatile

matter

20.70 29.79 23.28

Fixed Carbon 34.69 46.79 51.22

30

Analysis of Coal

Ultimate analysis (Chemical Properties)

Typical ultimate analysis of coal (%)

Parameter Indian Coal, % Indonesian Coal, %

Moisture 5.98 9.43

Mineral Matter (1.1 x Ash) 38.63 13.99

Carbon 41.11 58.96

Hydrogen 2.76 4.16

Nitrogen 1.22 1.02

Sulphur 0.41 0.56

Oxygen 9.89 11.88

GCV (kCal/kg) 4000 5500

31

Performance Evaluation

• Combustion: rapid oxidation of a fuel

• Complete combustion: total oxidation of fuel (adequate supply of oxygen needed)

• Air: 23.2% oxygen, 76% nitrogen and other

• Nitrogen: (a) reduces the combustion efficiency (b) forms

NOx at high temperatures

• Carbon forms (a) CO2 (b) CO resulting in less heat

production

Principles of Combustion

32

Performance Evaluation

• Control the 3 Ts to optimize combustion:

Principles of Combustion

1T) Temperature

2T) Turbulence

3T) Time

33

Performance Evaluation

Oxygen is the key to combustion

Principle of Combustion

Bureau of Energy Efficiency, India, 2004

34

Performance Evaluation

Stoichiometric calculation of air

required

Stoichiometric air needed for combustion of furnace oil

Theoretical CO2 content in the flue gases

Actual CO2 content and % excess air

Constituents of flue gas with excess air

Theoretical CO2 and O2 in dry flue gas by volume

35

Performance Evaluation

• Measure CO2 in flue gases to estimate excess air level and

stack losses

Concept of Excess Air

Carbon dioxide (%)

Excess a

ir (

%)

Source: Bureau of Energy Efficiency, India, 2004

36

Performance Evaluation

Concept of Excess Air

Residual oxygen (%)

Excess a

ir (%

)

Bureau of Energy Efficiency, India, 2004

• Measure O2 in flue gases to estimate excess air level and

stack losses

Flame Inside Furnace

Fuel Burning furnaces

• Furnace provide controlled, efficient conversion of chemical energy of

fuel into heat energy which is transferred to the heat absorbing surfaces

of the steam generator.

• The firing system introduce the fuel and air for combustion, mix these

reactants, ignite the mixture and distribute the flame and combustion.

• Furnace can be divided into two types:

a. Grate-fired furnaces

b. Chamber-type or flame furnace

Combustion Equipment's

Coal may be fed into furnace for combustion in lump pieces or in

powder form.

a. Fuel bed furnaces (for coarse particles)

b. Pulverized coal furnaces (fine particles)

c. Cyclone furnaces (crushed particles)

d. Fluidized bed furnaces (crushed small particles)

Fuel bed combustion

A grate is used at furnace bottom to hold a bed of fuel. There are two ways of

feeding coal on to the grate:

a. Overfeeding

b. Underfeeding

In overfeeding, the coal is fed into the grate above the point of air admission

and in case of underfeeding, the coal is admitted into the furnace below the

point of air admission.

Primitive Method of solid Combustion

Primary Air

Secondary Air Flame

Green Coal

Incandescent coke

Grate

CO+CO2+N2+H2

VM+CO+CO2+N2+H2

O2+CO2+N2+H2O

ASH

Top-feed updraft combustor

Physics of Fixed Bed Combustion

• A bed of stoker-sized coal particles is supported by a grate.

• Air flows upwards through the grate and the fuel bed.

• Upon heating, coal particles first undergo a stage of Thermal

preparation.

– Evaporation of Moisture drying.

– Distillation of VM (Pyrolosis)

– Production of enough VM to start ignition.

– Combustion of char.

• The coal flows slowly downward at a velocity Vs, as the coal

burns out in the lower layer of the bed.

Overfeed and underfeed fuel bed

section

Difference

Automation in Fuel Bed Combustion

Also called Mechanical grate or Stokers.

• Uniform operating condition

• Higher burning rate

• Greater efficiency

TYPES:

• Travelling Grate Stoker

• Chain Grate Stoker

• Spreader Stoker

• Vibrating Stoker

• Underfeed Stoker

C o n v e y o r t y p e

Conveyor Type

Stokers are used for burning non-coking free burning high volatile high ash coals. Although initial cost of this stoker is high but operation and maintenance cost is low.

Spreader Stoker

Spreader Stoker

Advantages • Its operation cost is low.

• A wide variety of coal can be burnt easily by this stoker.

• A thin fuel bed on the grate is helpful in meeting the fluctuating loads.

• Ash under the fire is cooled by the incoming air and this minimizes

clinkering.

• The fuel burns rapidly and there is little coking with coking fuels.

Disadvantages • The spreader does not work satisfactorily with varying size of coal.

• In this stoker the coal burns in suspension and due to this fly ash is

discharged with flue gases which requires an efficient dust collecting

equipment.

Underfeed stoker

Closing Remarks on Mechanical

stokers

• In all MS forced draft fans are used both for PA and over fire SA.

• A practical engineering limit seems to be reached when the

length and width of the grate are about 9 m with grate area 80

m2.

• In practice stokers have rarely exceeded a capacity of 135

tons/hour.

• Heat release rate-1340 kW/m2

• Stoker firing is limited to relatively low capacity up to 50 kg/s of

steam due to low efficiency. However, in their size range (19 to

32 mm) stokers remains important in steam generator system.

• Beyond a certain size of boiler, stokers become impracticable

and uneconomical (increase floor area).

Pulverized Fuel Combustion

• Invented in 1920.

• Fine particles of coal ~ 75 microns.

• Huge heat release per unit area : 2 – 5 MW/m2.

• Steam generation : 2000 tons/hour.

Schematic of typical coal pulverized

system

A Inlet Duct;

B Bowl Orifice;

C Grinding Mill;

D Transfer Duct to Exhauster;

E Fan Exit Duct.

Ash disposal

• A large quantity of ash is, produced in steam power plants using coal.

• Ash produced in about 10 to 20% of the total coal burnt in the furnace.

• Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some poisonous gases.

• It is desirable to quench the ash before handling due to following reasons:

1. Quenching reduces the temperature of ash.

2. It reduces the corrosive action of ash.

3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will

disintegrate.

4. Quenching reduces the dust accompanying the ash.

Types of Ash

• Burning of Coal leads to generation of Ash

• Can be the non- combustible part of fuel, or

generated after the burning of the coal

• Two Types are :-

• Bottom Ash

• Fly Ash

Bottom Ash

• Non- Combustible Part of Combustion

• Comprises traces of combustibles embedded in

forming clinkers and sticking to hot side walls of a

coal-burning furnace

• The clinkers fall by themselves into the water or

sometimes by poking manually, and get cooled

• May be used as an aggregate in road construction

and concrete

Fly Ash

• Residual of Coal Combustion

• Comprises of fine particles that rises with Flue

Gases

• All types of fly ash includes substantial amounts of

silicon dioxide and Calcium Oxide

• Size range – from

0.5 µm to 100 µm

Challenges of Ash handling

• Indian coal presents high ash content generally

which tends to be inconsistent. Design of the

system has to adequately cover anticipated

variations and be capable of handling the worst

scenario.

• System has to be environmentally friendly.

• System has to be reliable with least maintenance

problems.

• System has to be energy efficient

Ash and dust collecting

Equipment's

• Hydraulic system

• Pneumatic system

• Mechanical system.

Dust collection system

The size of dust particles is designated in microns (1 μ = 0.001 mm). Dust particles are mainly

ash particles called fly ash intermixed with some quantity of carbon ash material called cinders.

Gas borne particles larger than 1μ in diameter are called dust and when such particles become

greater in size than 100µ they are called cinders. Smoke is produced due to the incomplete

combustion of fuels, smoke particles are less than 10µ in size.

The disposal smoke to the atmosphere is not desirable due to the following reasons :

1. A smoky atmosphere is less healthful than smoke free air.

2. Smoke is produced due to incomplete combustion of coal. This will create a big economic

loss due to loss of heating value of coal.

3. In a smoky atmosphere lower standards of cleanliness are prevalent. Buildings, clothing's,

furniture etc. becomes dirty due to smoke. Smoke corrodes the metals and darkens the paints.

To avoid smoke nuisance the coal should be completely burnt in the furnace.

Types of dust collectors

The various types of dust collectors are as follows :

1. Mechanical dust collectors.

2. Electrical dust collectors.

Mechanical dust collectors. Mechanical dust collectors are sub-divided into wet and dry types.

In wet type collectors also known as scrubbers water sprays are used to wash dust from the air.

The basic principles of mechanical dust collectors are shown in Fig. As shown in Fig. by

increasing the cross-sectional area of duct through which dust laden gases are passing, the

velocity of gases is reduced and causes heavier dust particles to fall down. Changing the direction

of flow of flue gases causes the heavier particles of settle out. Sometime baffles are provided as

to separate the heavier particles. Mechanical dust collectors may be wet type or dry type. Wet

type dust collectors called scrub scrubbers make use of water sprays to wash the dust from flue gases.

Dry type dust collectors include gravitational, cyclone, and baffle dust collectors.

Electrostatic Precipitators. It has two sets of electrodes, insulated from each other that maintain

an electrostatic field between them at high voltage. The flue gases are made to pass between these

two sets of electrodes. The electric field ionises the dust particle; that pass through it attracting

them to the electrode of opposite charge. The other electrode is maintained at a negative potential

of 30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by

rapping the electrode periodically. The electrostatic precipitator is costly but has low maintenance

cost and is frequently employed with pulverised coal fired power stations for its effectiveness on

very fine ash particles and is superior to that of any other type.

4.13.1 FLY ASH SCRUBBER

It is similar to a mechanical ash collector but has a flowing water film on its inner walls. Due to

this film, the collected ash is removed more rapidly from the apparatus to the bin. The degree of

ash collection in scrubbers varies from 0.82 to 0.90. The dust laden gas enters through the inlet

pipe.

Cinder Catcher. Cinder catcher is used to remove dust and cinders from the gas. In this catcher

the dust laden gas is made to strike a series of vertical baffles that change its direction and

reduce its velocity.

The separated dust and cinders fall to the

Hopper for removal. Cinder catchers

are ordinarily used with stoker firing.