BOILERS FUNDAMENTALS/COMBUSTION AJAY SHUKLA DGM NTPC PMI 23rd March ,10
Dec 22, 2015
BOILERS FUNDAMENTALS/COMBUSTION
AJAY SHUKLADGM NTPC PMI23rd March ,10
In early 19th Century boiler were low pressure Invention of water tube removed the pr barrier
and boiler pr rise to super critical Between 70- 90 utility operated conservatively
and used low steam pr in boiler . Now renewed interest in high efficiency
supercritical boiler .The interest arose from the environmental need to attain higher efficiency and dividend of higher eff is reduce CO2
Rankine Cycle
•1 to 2: Isentropic expansion (Steam turbine) •2 to 3: Isobaric heat rejection (Condenser) •3 to 4: Isentropic compression (Pump) •4 to 1: Isobaric heat supply (Boiler)
Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four processes:
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.
500 MW Boiler – Typical Arrangement Drum type
OUTLINE
• Boiler fundamentals
• Boiler components (water side)
• Boiler combustion (air side)
• Boiler classification
Basic Knowledge of Boiler
FUEL
Flue gas
Blow down
Steam
AIR
Water
Ash
Basic boiler :
Steam / water system
Mixing of fuel and air
Furnace Heat transferSurface
Phenomenological Model
Hot Flue Gas
Thermal Structure SH
Steam
Convection &Radiation HT
Convection HT
Drop in Enthalpyof Flue Gas
Rise in Enthalpy ofSteam
Mechanism of Heat Transfer
Source/Supply Thermal Structure Sink /Demand
STEAM GENERATOR COMPONENTS
FURNACE DRUM BOILER CIRCULATING PUMPS CONVECTION PASS
SUPERHEATER REHEATER ECONOMISER
• AIR HEATER• STEAM COILED AIR PREHEATER• SOOT BLOWERS• COAL FEEDERS• PULVERIZERS• COAL PIPING• BURNERS• IGNITOR AND WARM UP BURNERS• DUCTWORK AND• INSULATION AND LAGGING
BOILER LAYOUT AND PA FAN
DPNLSHTR
Platen S
HT
R
SCREEn
LTSH
ESPAPH
ID fan
Chimney
Economiser
Bottom Ash
Downcomer
Drum
waterwallFireball
Gooseneck
Reheater
-BOILER=CONTROLLED COMB.+HEAT TRANSFER
-CHEMICAL =THERMAL
-COMBUSTION-FUEL,TEMP,O2
-FUEL - BITUMINOUS COAL
Boiler fundamentals
Combustion in furnace :-• Pulverized fuel by coal burners• Ignition temp. By oil firing• O2 by means of fans.
Reactions:-• C+O2 = CO2,• 2H2+O2 = 2H2O• S+O2 = SO2• Theoretical air = O2/.233
Boiler fundamentals
FACTORS AFFECTING COMBUSTION- TIME,TEMP., INTER MIXING OF AIR WITH
FUEL(TTT), COAL FINENESS,I. Excess Air:- - (20%)-bituminous coal -(15%)-lignite
A. Lower excess air:- -High unburnt loss
B. Higher excess air:- -Higher heat loss (ma*cpa*dt)
Boiler fundamentals
Water and Steam Circulation System
Economiser Boiler drum Down Comers Water walls Primary super heater Platen super heater Final super heater Reheater
Drum The boiler drum forms a part of the
circulation system of the boiler. The drum serves two functions, the first and primary one being that of separating steam from the mixture of water and steam discharged into it. Secondly, the drum houses all equipments used for purification of steam after being separated from water. This purification equipment is commonly referred to as the Drum Internals.
Type of Circulation Natural circulation
(upto 165 ksc)
Forced/ assisted circulation (185-200 ksc)
Once thru boiler1. Sub critical2. 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 the ratio of
feed water flow thru down comers to the steam generated in water wall.
CR = 30-35 Industrial boilers CR = 6-8 Natrual cir. Boilers CR = 2-3 Forced cir. Boilers CR = 1 Once thru boilers (Sub critical) CR = 1 Supercritical boilers
Waterwall construction Made of carbon steel (Grade-C) hollow
circular tubes and DM water flows inside 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.
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 20-25% of the operating pressure.
Carbon steel, alloy steel & SS used for tubing of SH & RH.
Superheaters
Convection Superheaters Radiant Superheaters
Important Components of Boiler
• Economizer• Boiler drum• Water wall • Superheater• Reheater
Boiler Pressure Part Design Code – IBR/ASME. Selection of Material based on:
Creep and Fatigue strength at design temperature.
Fire side oxidation resistance. Design Temperature and
thickness: as per IBR. Allowable stress for chosen
material – as per ASME.TWO PASS BOILER ARRANGEMENT
More Details of Pulverized Fuel fired SG
Additional allowance on tube design thickness to take care of erosion.
Selection of Material Upto 4000C: Carbon Steel for boiler tubes and plates. Upto 5500C: Low Alloy Steels like T11/P11, T22/P22, T23 etc. Upto 5900C: Medium Alloy Steel like T91/P91. Above 5900C: Austenitic Stainless Steel like TP347H, Super
304H.
Drum internals designed for removal of maximum moisture and provide required purity.
TDS in Feed Water restricted to 15 to 20 ppm Dissolved solids carryover not to exceed
Silica carry over - <10 ppb Sodium carry over - <3 ppb Chloride carry over - <2 ppb Copper carry over - <1 ppb Iron carry over - not detectable
Erosion shield/Cassette baffles on erosion prone areas.
Boiler Auxiliaries
Steam Theory
Within the boiler, fuel and air are forced into the furnace by the burner.
There, it burns to produce heat. From there, the heat (flue gases)
travel throughout the boiler. The water absorbs the heat, and
eventually absorb enough to change into a gaseous state - steam.
To the left is the basic theoretical design of a modern boiler.
Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.
Why Steam is so popular as heat conveying media in industry?
Highest specific heat and latent heat
Highest heat transfer coefficient
Easy to control and distribute
Cheap and inert
Properties of Steam Liquid Enthalpy
Liquid enthalpy is the "Enthalpy" (heat energy) in the water when it has been raised to its boiling point is measured in kcal/kg, its symbol is hf
Also known as "Sensible Heat” Enthalpy of Evaporation
It is the heat energy to be added to the water in order to change it into steam.
There is no change in temperature, the steam produced is at the same temperature as the water from which it is produced.
Also known as latent heat and its symbol is hfg
The temperature at which water boils, also called as boiling point or saturation temperature (It increases as the pressure increases. )
As the steam pressure increases, the usable heat energy in the steam (enthalpy of evaporation), which is given up when the steam condenses, actually decreases.
The total heat of dry saturated steam or enthalpy of saturated steam is given by sum of the two enthalpies
hf +hfg When the steam contains moisture the total
heat of steam will be hg = hf +q hfg where q is the dryness fraction.
Superheated Steam Superheat is the addition of heat to
dry saturated steam without increase in pressure.
Degree of Superheat The temperature of superheated
steam, expressed as degrees above saturation corresponding to that pressure.
Steam Properties : a re-look
Pressure,Bar A
Density,kg/m3
WaterEnthalpy,kcal/kg
Dry steamEnthalpy,kcal/kg
LatentHeat,kcal/kg
2 1.109 119.87 645.8 525.9
6 3.112 159.3 657.8 498.5
10 5.049 181.2 663 481.8
30 17.7 239.5 669.7 430.2
50 24.85 274.2 667.3 396.5
70 35.78 300.9 662.1 361.2
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.
Heat transfer in furnace and enclosed superheater takes place thru radiation.
condenserCEP
LPH
BFP
HPH+Ecow/w
SHHPTIPT
RH
LPT
Basic Knowledge of Boiler Purpose
To produce steam (Main Steam and Reheat Steam) at rated pressure and temperature
To Convert the heat of combustion of coal/oil/gas to thermal energy of steam
Steam Parameters are decided by Turbine Cycle Requirements
Steam Parameters adopted by NTPC 200 MW: 157 bar MS Pressure, 5400C/5400C 500 MW: 179 bar MS Pressure, 5400C/5400C 660 MW: 246 bar MS Pressure, 5450C/5630C
Advanced Supercritical Parameter 310 bar MS Pressure, 6100C/6100C
Engineering Function Selection of Unit Size
Based on load demand, coal and water availability. Input from Feasibility Report
Selection of Steam Parameters Choice of steam parameters is governed by overall cost of the plant. Sub-critical boilers are more suited in places where fuel cost is low.
Both drum type and once through boilers are acceptable based on manufacturer’s experience.
Super-critical boilers are costly because of greater use of high temperature material in boiler pressure parts.
Selection of Firing System Firing systems are generally left to manufacturer’s discretion as each
manufacturer prefers his standard design.
CLASSIFICATION OF BOILER
Based on Steam Parameters
Sub Critical Operates below the critical pressure of
water (221.2 bar)
Super CriticalOperates above the critical pressure of
water (221.2 bar).
Once Through No Thermodynamic fixed point i.e.
evaporation point keeps shifting in the water tubes depending on firing rate.
Drum typeProvides a thermodynamic fixed point at drum, which remains at constant temp.
Natural CirculationBoilers use the difference in water and steam density to drive the water/steam
mixture through the water tubes.
Assisted CirculationBoilers have Circulating Water Pump
which assists the natural convective flow through the water tubes.
Universal Pressure Operate at constant pressure
Sliding PressureOperate at sub-critical pressure at reduced
loads.
4
Tem
p er a
t ur e
(C
)
Enthalpy
538
Expansion Line
170 kg/cm2
240 kg/cm2
Critical Point 225 kg/cm2
Condensation
EFFECT OF SUPERCRITICAL PARAMETERSEFFECT OF SUPERCRITICAL PARAMETERS
CLASSIFICATION OF BOILER
Based on Flue Gas Arrangement
Two Pass Most of the SH, RH and Eco heat
transfer surfaces are placed in the horizontal and second passes. Some pendant SH and RH surfaces placed above the furnace. Pendant section
tubes cannot be drained.
Tower TypeAll heat exchangers are arranged
horizontally above the furnace. Provides easy draining of the SH and RH tubes and
headers.
OT Boiler Tower type
Typical Layout
CLASSIFICATION OF BOILER
Based on Firing Arrangement
Tangential FiredBurners are arranged over
many elevation to fire around an imaginary circle. One mill
normally feeds one coal elevation. individual Sec. Air
control is not provided.
Wall FiredBurners are arranged in rows over many elevation on front and rear walls. Mill to burner
distribution optimized for stable combustion at low loads. Each burner flame independent with
individual Sec. Air control.
Downshot FiredBurners are arranged to fire
downwards in rows over many elevation on front and rear walls. Better suited to low
volatile coals as it gives a high furnace residence time.
CLASSIFICATION OF BOILER
Based on Bottom Ash
Wet BottomBottom Ash collected in slag form. Mostly used
for low ash coals with low fusion temperatures.
Dry BottomBottom ash is cooled in
water in the hopper before removal in the
clinker form. Suited for Indian coals with high
ash content.
Boiler
Combustion
Combustion
•Burning of fuel (chemical reaction)
•Rapid combination of o2 with fuel, resulting in the release of heat
•For fuel to burn ,the following conditions must be present
• The fuel must be gasified
•The oxygen and fuel mixture should be proper.
•Temp should be above ignition
FUELS
Combustible substances which, when combined with oxygen in air & ignited, burn giving heat.
CLASSIFICATION OF FUELS
Solids Liquids Gaseous
Coal Kerosene Natural gasLignite Petrol MethanePeat HSD LPGBagasse LDO Producer GasHusk FO
LSHS
MAIN CONSTITUENTS OF FUEL
CarbonHydrogenSulphurNitrogenOxygenWater VapourAsh
DESIGN WORST BEST
TOTAL MOISTURE % 15 16.5 14ASH % 42 44 38VOLATILE MATTER % 21 19.5 23FIXED CARBON % 22 20 25
TOTAL % 100 100 100
PROXIMATE ANALYSIS OF TYPICAL INDIAN COAL
PROPERTIES OF FUEL (Typical Analysis of F.O.)
Carbon 83.52%Hydrogen 11.68%Sulphur 3.27%Calorific value 10,000 Kcal/kgSp. Gravity at 30oC 0.95Flash point 65oCViscosity at 40oC 1500 RW Sec No 1Water Percentage 0.15Sediment Percentage 0.3 (Variable)
COMBUSTION• Combustion is rapid oxidation of fuel resulting
in constituents getting converted into respective oxides, liberating heat.
Fuel +Air Oxides + Heat (Prs of combustion)
C +O2 : CO2 + Heat 43,968 Kcal
2H2 +O2 : 2H2O + Heat 61,979 Kcal
S +O2 : SO2 + Heat 3175 Kcal
Incomplete Combustion
2C + O2 : 2CO + Heat 26,429 Kcal
1 Kg of liquid fuel + 15 Kg of Air Oxides + Heat
(5.26M3)
HEATED BY FURNACE HEAT
PRESSURISED + PREHEATED
LIQUID FUEL
ATOMISED
VAPORISED
IGNITED BY FLAME
COMBUSTION
COMBUSTION PROCESS
COMBUSTIONREACTIONS
2C + O2 2CO + LESS HEAT
C
OOC
C O C O
COMBUSTION INCOMPLETE
COMBUSTIONREACTIONS
C + O2 CO2 + HEAT
C
OO
CO
O
2H2 + O2 2H20 + HEAT
H H
OOH H
HH
O HH
O
COMBUSTION COMPLETE
COMBUSTIONFLAME & FLAME FRONT* FLAMEFLAME : IT IS AN ENVELOPE OR ZONE WITHIN WHICH
COMBUSTION REACTION IS OCCURRING AT SUCH A RATE AS TO PRODUCE VISIBLE RADIATION.
* FLAME FRONT FLAME FRONT : IT IS THE 3 D CONTOUR ALONG WHICH COMBUSTION
STARTS IT IS THE DIVIDING LINE BETWEEN FUEL-AIR MIXTURE
AND COMBUSTION PRODUCTS.
REF. : NORTH AMERICAN COMBUSTION HANDBOOK
EXCESS AIR
Fuel + Theoretical air required + 15% to 40% T.A. Combustion
FOR COMPLETE COMBUSTION...
Fuel has to be atomised.
Raise the temperature to ignition temperature.
Electrical spark of ignition.
Proper mixing of fuel and air.
Distribution of Primary and Secondary air.
GOOD COMBUSTIONREQUIRES .......
3 T’s - TIME, TIME, TEMPERATURE & TURBULENCE
PROPER PROPORTIONING OF FUEL & AIR
CORRECT CONTROL OF FUEL & AIR
THOROUGH MIXING OF FUEL & AIR
INITIAL & SUSTAINED IGNITION
MEASUREMENT OF COMBUSTION
CO2 : 12 - 13%
SMOKE INDEX : 2 - 3
STACK TEMPERATURE : As per design.
O2 : 3%
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)
Advantages Its ability to burn all ranks of coal from
anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces.
Disadvantages High power demand for pulverizing Requires more maintenance, flyash
erosion and pollution complicate unit operation
Pulverized Fuel Boiler (Contd..)
SAFETIES
ý Unauthorised flame presence during pre-purge and after controlled shut down.
ý Pilot flame safety
ý Main flame safety
ý High gas pressure safety
ý Low gas pressure safety (optional)
ý Double Block & Bleed valves in main gas line
ý Combustion air failure safety
ý Interlock with boiler safeties
Any question please ?
THANK YOU