Boiler Fundamentals

Presentation on Power Plant Boiler

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 boilers 1. 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

Coal analysis  Typical composition (Proximate analysis) 1. Fixed carbon 2. Fuel ash 3. Volatile material 4. Total Moisture 5. Sulfur o High calorific value/ Lower calorific value (Kcal/kg) o Hardgrove Index (HGI)

Coal analysis  Typical composition (Ultimate analysis) 1. Carbon 2. Hydrogen 3. Sulfur 4. Oxygen 5. Nitrogen 6. Fuel Ash o Initial Deformation temperature (IDT) o High calorific value/ Lower calorific value (Kcal/kg)

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 of reactants  Reaction rate [k] is expressed by Arrhenius law,

Where

k = k0e

− E / RT

k0 = pre-exponential factor E =Activation energy (i.e. sufficient energy to destroy the molecular bonds) R= Gas constant T= Absolute temperature of the process

Combustion Reactions (Carbon)  Main reactions 2C

+ O2

= 2CO + 3950 BTU/lb (Deficit air)

C

+ O2

= CO2 +14093 BTU/lb

Secondary reactions 2CO + O2 = 2CO2 + 4347BTU/lb + CO2 = 2CO -7.25MJ/kg

C

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

Combustion Reaction (N2)  Nitrogen reaction [NO] = K1 e(-K2/T) [N2][O2]1/2 t Where K1, K2 [] t T

= constants = mole fraction = time = Temperature

Combustion air  Theoretical air for complete combustion is known as stoichiometric air.  Excess air for completion of combustion (20% at NCR i.e. 3.6% O2)

O2 % ExcessAir = 21 − O2 %

 Total combustion is divided as   

Primary air For drying & pulverised coal transportation Secondary air Additional air for combustion Ratio of SA/ PA = 2:1 (Approx)

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 =

MW Elect

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

Arrangement of fuel input in furnace  Coal is pulverized in mills at a fineness of 70% thru 200 mesh. Dried powdered coal is conveyed to furnace (at a temperature < 95-100oC)  Total coal flow is distributed among running mills and fed thru coal burners at 20-25 m/sec.  Coal flow is arranged in tiers. Maximum heat release rate must not exceed plain area heat loading. It generates excessive NOx and making ash fused.

Combustion air arrangement in furnace  Fuel air is supplied around coal nozzles (at velocity of 30-35 m/sec).  Secondary air is supplied in adjacent tiers of sec. air dampers from wind box (Hot air from Secondary APH)  Overfire/ Tempering air is supplied at the top of the burnaer zone for NOx control.  Gas recirculation is adopted for steam temperature control in oil/ gas fired units.  Furnace draft is maintained at -5 mmwcl with Forced and Induced draft fans (balanced draft)

Method of firing  Down shot firing   Front fired  Single wall Opposed wall  Corner fired   Tangential tilted  -30o to +30o  Fluidized bed combustion 

For Anthracite (Low VM) Bituminous (Medium VM)

Bituminous (Medium VM)

Lignite, peat, garbage (For low CV, high ash, high Sulfur) Corner and Front fired furnaces are suitable for power plants In India for Bituminous coals

Tangential corner fired boilers  Suitable for sub bituminous and bituminous coal  More uniform heat absorption in furnace wallls  Inherent feature of low NOx formation due air deficit region at centre  Air plenum near walls reduce slagging and prevent fireside corrosion.  Arrangement of steam temperature control by tilting the coal burners (-30o to +30o)

Furnace Design  Heat generated in furnace is managed in such a way that it fulfills 1. No dry out of waterwall tubes 2. Enough residence time for combustion 3. Gas temperature before convection super heater < Ash fusion temperature 4. Selection of metallurgy (Carbon steel Grade-C)

Furnace Design  For criteria of keeping flue gas temperature below Ash fusion temperature, following parameter are designed. 1. Furnace Height, Width, Depth 2. Furnace Volume 3. Furnace residence time (2.5 to 3.0 seconds) 4. Rate of steam generation in furnace, going to drum 5. Additional heat transfer surface as Platen SH, Wall Reheater, Divisinal Panel SH 6. Furnace exit gas temperature (FEGT) is the criteria for design of furnace.

Waterwall construction  Made of carbon steel (Grade-C) hollow circular tubes and DM water flows inside  Above burner zone, tubes are internally spiraled for better drying thru spinning action  Furnace walls are made as tangent wall whereas 2nd pass walls are made as membrane walls.  Waterwalls are stiffened by the vertical stays and buck stays to safeguard from furnace pressure pulsation & explosion/ implosion  The boiler as a whole is hanging type, supported at the top in large structural columns.  Vertical expansion is allowed downwards and provision is made at bottom trough seal near ring header.

Dry/ Wet bottom furnaces  Ash is characterized by 1. Initial deformation temperature 2. Ash fusion temperature (IDT-50oC Approx) 3. Hemispherical temperature 4. Flow temperature Indian coals are suitable for dry bottom furnace design only, because of a. High fuel ash (35-50%) b. High silica in ash (55-65% & vitrification temp. is 2000oK) c. High flow temperature (1600oC approx)

Configuration of Dead chamber Steam drum

ARCH TUBES

HORIZONTAL PASS w/w hanger Screen tubes tubes

GOOSENECK

DOGHOUSE BACKPASS

Downcomer

BCW p/p

FURNACE

To APH

DPNL SHTR

Platen SHTR

Drum

Reheater S C R E E n Gooseneck

LTSH

Chimney

Downcomer waterwall

Fireball

Economiser

ID fan

APH Bottom Ash

ESP

Steam generation principle  Steam power plants operate on Rankine Cycle, DM water as working fluid.  Sensible heat is added in economiser +furnace  Steam generation takes place in waterwall.  Typical furnace efficiency is 45% approx.  Heat transfer in furnace and enclosed superheater takes place thru radiation.

w/w HPH+Eco

SH RH HPT IPT

BFP LPT LPH CEP

condenser

Superheater & Reheater  Heat associated with the flue gas is used in superheaters & Reheater, LTSH, economiser.  Maximum steam temperature is decided by the operating drum pressure and metallurgical constraints of the turbine blade material.  Reheating is recommened at pressure above 100 ksc operating pressure. Reheating is done at 2025% of the operating pressure.  Carbon steel, alloy steel & SS used for tubing of SH & RH.

w/w HPH+Eco

SH RH HPT IPT

BFP LPT LPH CEP

condenser

Principle of circulation  Density water and steam changes with pressure as shown.  At higher pressure, density difference reduces.  Flow establishment in down comer, waterwall and drum is due to density difference and height of water column (i.e. waterwall) at lower pressure.

185 ksc Sp. gravity

225 ksc 165 ksc Pressure (KSC)

Type of Circulation  Natural circulation (upto 165 ksc)  Forced/ assisted circulation (185-190 ksc)  Once thru boiler 1. Sub critical 2. Supercritical

 Density difference & height of water column  Assisted by external circulating pump (CC/ BCW pump)

 Below 221.5 bar  240-360 bar

Circulation ratio  It may be defined as  ratio of feed water flow thru down comers to the steam generated in water wall.  Ratio of the weight of 2phase mixture to the weight of dry steam in waterwall.  Ratio of the total fluid contained to the weight of the dry steam in waterwall.

 CR = 30-35 Industrial boilers  CR = 6-8 Natrual cir. Boilers  CR = 2-3 Forced cri. Boilers  CR = 1 Once thru boilers  (Sub critical)  CR = 1 Supercritical boilers

Representation of steam/ water parameters on T-S diagram 3

Temperature 374.16oC

Entropy

2

1

1. Sub critical parameter 2. Critical parameter, (225.65 ksc/ 374.16oC) 3. Supercritical parameter

Design concepts of once thru steam generators Controlled point in circuitry

Mix locations

Sulzer design









Valved control of furnace ckt

External downcomers

{0 from 60% to Full Load}

External downcomers

C/R flow

C/R pump Combined C/R design

Feed Benson Design

Comparision of measures to improve efficiency % 46

0.03 bar double 300bar 600/600

44 250bar 540/560

42 120 40

1.15 1.25

130

39 Excess exit gas Air temp.

0.065bar single

250bar 540/560

170bar 535/535 steam conditions

Reheat cond. Pressure

Effect of supercritical parameters on efficiency (%) Pressure/ Temp 540oC/ 540oC

175 bar Ref.

245 bar 1.92

295 bar 2.44

540oC/ 566oC

0.72

2.74

3.33

565oC/ 565oC

1.35

3.45

4.12

600oC/ 600oC

3.10

5.30

6.09

600oC/620oC Advantage

3.62

5.91

6.61

1. Saving on coal consumption on higher efficiency 2. Reduction in fuel handling system & pollution control.