Thermal Power Plants

A presentation on Steam Generator

Coal to Electricity ….. Basics Pollutants

Super Heated Steam

Coal Heat

Heat Loss In Condense r

ASH

Loss

Chemical Energy

Thermal Energy

Kinetic Energy

Turbine Torque Mech. Energy Loss

Electrical Energy

Alternating current in Stator Elet. Energy Loss

Major Energy Sources of India

Why Coal? RES 5%

Nuclear 3%

Ad va ntage s of Coa l Fu el Shortcomi ngs of Coa l Hydel 26% Coal 55% Diesel 1% Gas 10%

Share of Coal in Power Generation

•Abundantly available in India •Low cost •Technology for Power Generation well developed. •Easy to handle, transport, store and use

•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 Coa l Proper ti es •Calorific Value •Grade of Coal (UHV) •Proximate Analysis •Ultimate Analysis •Ash and Minerals •Grindability •Rank Coa l pr oduction

•Physical Characteristics

•Surface Mining

Coal Be nefic ia tion

•Underground Mining

•Why? •Processes •Effectiveness

Coa l Tra nspor ta ti on •Rail •Truck •Conveyor •Ship Coa l pr oduction •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 2.

Raw materials for design of boilers o Generating heat energy Coal from mines

4.

Ambient air

o

Air for combustion

6.

Water from natural resources (river, ponds)

o

Working fluid for steam generation, possessing heat energy



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

Coal analysis 

o o

Typical composition (Proximate analysis) 1. Fixed carbon 2. Fuel ash 3. Volatile material 4. Total Moisture 5. Sulfur High calorific value/ Lower calorific value (Kcal/kg) 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 reactions C

2CO + O2 = 2CO2 + 4347BTU/lb + 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 Sulfur reaction S + (undesirable)

O2

= 2H2O +61095 BTU/lb

= SO2 + 3980 BTU/lb

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

Q Furnace =  

MWElect

ηCycle

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 120-145 T/hr

For 500 MW units For 200 MW units

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

C L A S S IF IC A T IO N O F M IL L S

XRP (B H E L )

V E R T IC A L S P IN D L E

TUBE

BO W L/ BALL & RACE

P R E S S U R IZ E D

E M IL L S (B A B C O C K )

M PS

BOWL MILL Model no. 623XRP 703XRP 763XRP 803XRP 883XRP 903XRP 1003XRP 1043XRP

Base capacity(T/Hr) 18.4 26.4 33.8 36.5 51.1 54.1 68.1 72.0

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

BALL& RACE MILL(E MILL)

Model no. 7E9 8.5E10 8.5E9 10E10 10.9E11 10.9E10 10.9E8

Base capacity(T/Hr) 25 35 40 55 61 70 80

TUBE MILL

Model no. BBD4760 BBD4772

Base capacity(T/Hr) 83 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

Typical Boiler Problems •

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 contd.. •

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

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  Improvements

(1)

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 BFP Higher feed water quality required. More complex supporting and framing in Boiler due to Spiral Wall tubes. Slight higher capital cost.

COMPARISION OF 660 MW Vs 500 MW BOILER Description

unit

S/H STEAM FLOW SH

STEAM PR

SH STEAM TEMP

RH STEAM TEMP

INLET

RH STEAM TEMP OUTLET RH STEAM PRESS INLET FEED WATER TEMP

500

T/HR

2225

1625

KG/CM2

256

179

C

540

540

T/HR

1742

1397.4

C

303.7

338.5

C

568

540

KG/CM2

51.17

46.1

C

291.4

255.2

0

RH STEAM FLOW

660

0 0

0

COST COMPARISON FOR 660 MW vs. 500 MW

1.

DESCRIPTION

660 MW

Cost of Boiler alone

1970.73 Cr 1020.54 Cr

500 MW

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

5 Cost of TG for entire stage

1204.72 Cr 634.31 Cr

6 Cost of TG Per MW

0.6Cr

1.02 Cr

0.63 Cr