A presentation on

Steam Generator

Coal to Electricity ….. Basics

Coal

Chemical Energy

Super Heated Steam

Pollutants

Thermal Energy

Turbine Torque

Heat Loss In Condenser

Kinetic Energy

Electrical Energy

Alternating current in Stator

Mech. Energy Loss

ASHHeat

Loss

Elet. Energy Loss

Major EnergySources of India

Why Coal?

Coal55%

Gas10%

Diesel1%

Hydel26%

RES5%

Nuclear3%

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.

Knowing more about Coal

Coal production

•Surface Mining

•Underground Mining

Coal Transportation

•Rail

•Truck

•Conveyor

•Ship

Coal Properties

•Calorific Value

•Grade of Coal (UHV)

•Proximate Analysis

•Ultimate Analysis

•Ash and Minerals

•Grindability

•Rank

•Physical CharacteristicsCoal Beneficiation

•Why?

•Processes

•Effectiveness

Coal production

•Surface Mining

•Underground Mining

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

Boiler/ steam generator Steam generating device for a specific purpose.

Capable to meet variation in load demand

Capable of generating steam in a range of operating pressure and temperature

For utility purpose, it should generate steam uninterruptedly at operating pressure and temperature for running steam turbines.

Boiler/ steam generator Raw materials for design

of boilers1. Coal from mines

2. Ambient air

3. Water from natural resources (river, ponds)

o Generating heat energy

o Air for combustion

o Working fluid for steam generation, possessing heat energy

A 500MW steam generator consumes about 8000 tonnes of coal every dayIt will be considered if it requires about 200 cubic meter of DM water in a dayIt will produce about 9500 tonnes of Carbon di Oxide every day

Coal analysis

Typical composition (Proximate analysis)1. Fixed carbon2. Fuel ash3. Volatile material4. Total Moisture5. Sulfur

o High calorific value/ Lower calorific value (Kcal/kg)

o Hardgrove Index (HGI)

Combustion of coal Carbon, hydrogen, sulfur are sources of heat

on combustion

Surface moisture removed on heating during pulverization.

Inherent moisture and volatiles are released at higher temperature, making coal porous and leading to char/ coke formation. (Thermal preparation stage)

Fuel Oil Three liquid fuels used in power plants

• 1. Heavy Fuel Oil (HFO)• 2. LSHS (Low Sulfur Heavy stock)• 3. High speed Diesel (HSD)

Oil firing is preceded by Lowering viscosity and increasing flowability on

heating for better combustion in given turn down ratio.(125oC)

Droplet formation on atomization (by steam/ compressed air/ mechanical pressurization)

Combustion initiation by High energy spark ignition

Combustion of reactants Reaction rate depends on concentration of one of

the reactants

Concentration varies on partial pressure of the reactants.

Partial pressure is a function of gas temperature.

Therefore, reaction rate depends on temperature and substance that enter the reaction.

Combustion Reactions (Carbon)

Main reactions 2C + O2 = 2CO + 3950 BTU/lb (Deficit air)

C + O2 = CO2 +14093 BTU/lb

Secondary reactions2CO + O2 = 2CO2 + 4347BTU/lb

C + CO2 = 2CO -7.25MJ/kg

Combustion Reactions (Carbon)

Carbon reaction 2C + O2 =2CO [Eco =60kJ/mol]

C + O2 =CO2 [Eco2 =140kJ/mol]

reaction at 1200oC

4C + 3O2 =2CO + 2CO2 (Ratio 1:1)

Reaction at 1700oC

3C + 2O2 = 2CO +CO2 (Ratio 2:1)

It is desirable to supply combustion air at lower temperature regime in furnace

Combustion Reaction (H2, S)

Hydrogen reaction

2H2 + O2 = 2H2O +61095 BTU/lb

Sulfur reaction

S + O2 = SO2 + 3980 BTU/lb (undesirable)

Coal for combustion

Anthracite Semi-anthracite Bituminous Semi-Bituminous Lignite Peat

High CV, low VM High CV, low VM Medium CV, medium VM Medium CV, medium VM Low CV, high VM, high TM Very low CV, high VM & TM

Heat Generation in furnace Heat input in the furnace

Efficiency of thermal power plants is 37%-45% for different types of cycle

For typical conventional P.F. boilers, coal flow rate is

290-350 T/hr For 500 MW units120-145 T/hr For 200 MW units

Cycle

ElectFurnace

MWQ

Tangential Firing System

MAIN EQUIPMENTS OF FUEL & FIRING SYSTEM

• MILLS OR PULVERISERS

• FEDDERS

• BURNERS

TYPES OF FEEDERS

• VOLUMETRIC FEEDRES

• GRAVIMETRIC FEEDERS

PULVERIZERS

OBJECTIVES

• TO CRUSHED THE COAL

• REDCED TO A FINENESS SUCH THAT 70-80% PASSES THROUGH A 200MESH SIEVE

ADVANTAGES OF PULVERISED COAL FIRING

• EFFICIENT UTILISATION OF CHEAPER GRADE OF COALS

• FLEXIBILITY IN FIRING WITH ABILITY TO MEET FLUCTUATING LOADS

• BETTER COAL COMBUSTION INCREASING THE BOILER EFFICIENCY

• HIGH AVAILIBILITY

XR P(B H EL)

E M ILLS(B AB C O C K )

M PS

B O W L/B ALL & R ACE

VER TIC AL SPIN D LE

PR ESSU R IZED

TU B E

C LASSIFIC ATIO N O F M ILLS

BOWL MILL

Model no. Base capacity(T/Hr)

623XRP 18.4703XRP 26.4763XRP 33.8803XRP 36.5883XRP 51.1903XRP 54.11003XRP 68.11043XRP 72.0

BASE CAPACITY(T/HR)AT HGI -55Total Moisture-10%Fineness-70% THRU 200 MESH

BALL& RACE MILL(E MILL)

Model no. Base capacity(T/Hr)

7E9 258.5E10 358.5E9 4010E10 5510.9E11 6110.9E10 7010.9E8 80

TUBE MILL

Model no. Base capacity(T/Hr)

BBD4760 83BBD4772 90

AIR AND DRAFT SYSTEM

OBJECTIVES

• THE AIR WE NEED FOR COMBUSTION IN THE FURNACE AND FLUE GAS THAT WE MUST EVACUATE• TRANSPORT AND DRY THE PULVERISED COAL• SEALING OF BEARINGS FROM COAL/DUST

DRAFT SYSTEM

DRAFT MEANS THE DIFFRENCE BETWEEN THE ATMOSPHERIC PRESSRE AND PRESSURE EXISTING IN THE FURNACE

•NATURAL DRAFT- OBTAINED BY TALL CHIMNEY

• INDUCED DRAFT- BY ID FANS

• FORCED DRAFT- BY FD FANS

• BALANCE DRAFT - BY ID AND FD FANS•GENERALLY IN POWER PLANT BALANCE DRAFT SYSTEM IS USED.

FANS IN POWER PLANT

• FORCED DRAFT FAN

• INDUCED DRAFT FAN

• PRIMARY AIR FAN

• SEAL AIR FAN

• SCANNER AIR FAN

THE BASIC INFORMATION NEEDED TO SELECT A FAN ARE

• AIR OR GAS FLOW-KG/HR

• DENSITY(FUNCTION OF TEMPERATURE AND PRESSURE)

• SYSTEM RESISTANCE(LOSSES)

AIR PRE HEATERS

OBJECTIVES

• TO RAISE THE TEMPERATURES OF PRIMARY AND SECONDARY AIR BY UTILISING HEAT FROM FLUE GAS AT LOW TEMPERATURE

ADVANTAGES OF AIR PREHEATERS

• INCREASE THE BOILER EFFICIENCY

• STABILITY OF COMBUSTION IMPROVED BY USE OF HOT AIR

• PERMITTING TO BURN POOR QUALITY COAL

Ljungstrom type Bisector

TWO PASS BOILER ARRANGEMENT

Electro Static Precipitator

To remove fly ash from the flue gases electrostatic precipitators are used.

They have collection efficiency over 99.5%

The efficiency depends on various parameters such as velocity of flow, quantity of gas, resistivity of ash, voltage of fields, temperature etc

Principle of Operation

The fluegas laden with flyash is sent through ducts having negatively charged plates which give the particles a negative charge. The particles are then routed past positively charged plates, or grounded plates, which attract the now negatively-charged ash particles. The particles stick to the positive plates until they are collected by periodically rapping.

SELECTION OF BOILER

TYPE OF BOILER

Based on steam parameter- Subcritical/ Supercritacal

Based on steam/ water circuit-Once throuh/ drum type

Based on air/ flue gas path- Tower/Two path/ T-type

Type of fuel- Coal fired/ oil fired

Type of draft system-

Type of burner arrangement- Tangential/Front/ opposed

Selection of Firing system- Type of mills

Single reheat/ double reheat

Type of water wall tube- Plain, rifled

Type of tubing arrangement- Spiral/ straight

• Tube leakages from boiler pressure parts.

• Erosion of tubes due to high ash content and velocities

• Over heating of tubes

• Passing from valves causing difficulty in maintaining the parameters

• Failure or incorrectness of measured parameters

• Overloading of boiler due to very poor quality of coal

• Deposition of ash (clinkers) on furnace walls.

• Difficulties in removal of ash from the boiler

• Reduced effectiveness of heat transfer leading to loss of efficiency.

• Improper combustion of coal in the boiler.

Typical Boiler Problems

• Air ingress from the nose arch, penthouse and boiler second pass and quantification thereof

• Difference between on line reading and the actual oxygen in the flue gas duct

• Difference between actual and 'on line' temperature

• measurement of air heater air / gas outlet temperatures

• Fouling and Slagging

• High unburnt Carbon in flyash or bottomash

• High air heater leakage

• Boiler operation at high excess air

Typical Boiler Problems contd..

A Few words on Super Critical Boiler

Definition “CRITICAL” is a thermodynamic expression

describing the state of a substance beyond which there is no clear distinction between the liquid and gaseous phase.

The critical pressure & temperature for water are

Pressure = 225.56 Kg / cm2 Temperature = 374.15 C

SUPERCRITICALTHERMAL CYCLE

ADVANTAGES (1)

Improvements in plant efficiency by more than 2 %

Decrease in Coal Consumption Reduction in Green House gases. Overall reduction in Auxiliary Power

consumption. Reduction in requirement of Ash dyke

Land & Consumptive water.

SUPERCRITICAL – ADVANTAGES (2)

Sliding pressure operation because of Once

through system .

Even distribution of heat due to spiral wall

arrangement leading to less Boiler tube failure,

thereby improving system continuity and

availability of the station.

Low thermal stress in Turbine .

The startup time is less for boiler.

SUPERCRITICAL – DISADVANTAGES

Higher power consumption of BFPHigher feed water quality required.

More complex supporting and framing in Boiler due to Spiral Wall tubes.

Slight higher capital cost.

Description unit 660 500

S/H STEAM FLOW T/HR 2225 1625

SH STEAM PR KG/CM2 256 179

SH STEAM TEMP 0C 540 540

RH STEAM FLOW T/HR 1742 1397.4

RH STEAM TEMP INLET 0C 303.7 338.5

RH STEAM TEMP OUTLET 0C 568 540

RH STEAM PRESS INLET KG/CM2 51.17 46.1

FEED WATER TEMP 0C 291.4 255.2

COMPARISION OF 660 MW Vs 500 MW BOILER

COST COMPARISON FOR 660 MW vs. 500 MW

DESCRIPTION 660 MW 500 MW

1. 1Cost of Boiler alone 1970.73 Cr 1020.54 Cr

2 Cost of ESP 153.00 Cr Included above

3 Total cost of Boiler + ESP 2124.00 Cr 1020.54 Cr

4 Boiler cost Per MW 1.07 Cr 1.02 Cr

5 Cost of TG for entire stage 1204.72 Cr 634.31 Cr

6 Cost of TG Per MW 0.6Cr 0.63 Cr