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