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BOILERS FUNDAMENTALS/COMBUSTION

AJAY SHUKLA

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

EFFECT OF VARIOUS PARAMETERS IN RANKINE CYCLE

SUPERHEATING – improves efficiencyINCREASING PRESSURE – marginal improvement in efficiencyLOWERING EXHAUST PRESSURE- improves efficiencyREHEATING – improves efficiencyREGENERATIVE FEEDWATER HEATING – improves efficiency

Boiler/ steam generatorSteam 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.

Basic Knowledge of BoilerPurpose

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 NTPC200 MW: 157 bar MS Pressure, 5400C/5400C500 MW: 179 bar MS Pressure, 5400C/5400C660 MW: 246 bar MS Pressure, 5450C/5630C

Advanced Supercritical Parameter310 bar MS Pressure, 6100C/6100C

Engineering FunctionSelection of Unit Size

Based on load demand, coal and water availability.Input from Feasibility Report

Selection of Steam ParametersChoice 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 SystemFiring 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.

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.

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 Classification (contd…)

Once Through BoilerContinuous Path of water through economizer to water walls and to superheaters.These are necessarily forced circulation boilers.They are suited for fast start-up and load cycling, as the thick drum is eliminated.

Once Through Boiler

Boiler Classification (contd…)

Drum Type Boiler (Natural Circulation)

Drum Type Boiler (Assisted Circulation)

Boiler Classification (contd…)

Supercritical Boiler (Universal Pressure)

Supercritical Boiler (Sliding Pressure)

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 :

Heat transferSurface

Furnace Mixing of fuel and air

Steam / water system

Phenomenological Model

Hot Flue Gas

Thermal StructureSH

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

FURNACEDRUMBOILER CIRCULATING PUMPSCONVECTION PASS

SUPERHEATERREHEATERECONOMISER

• 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 SHTR

SCREEn

LTSH

ESPAPH

ID fan

Chimney

Economiser

Bottom Ash

Downcomer

Drum

waterwallFireball

Gooseneck

Reheater

Water and Steam Circulation System

EconomiserBoiler drumDown ComersWater wallsPrimary super heaterPlaten super heaterFinal super heaterReheater

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 columnAssisted by external circulating pump (CC/ BCW pump)

Below 221.5 bar240-360 bar

Circulation ratioIt may be defined as the ratio of feed water flow thru down comers to the steam generated in water wall.

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

Waterwall constructionMade of carbon steel (Grade-C) hollow circular tubes and DM water flows insideWaterwalls are stiffened by the vertical stays and buck stays to safeguard from furnace pressure pulsation & explosion/ implosionThe 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 & ReheaterHeat 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 SuperheatersRadiant Superheaters

Important Components of Boiler• Economizer• Boiler drum• Water wall • Superheater• Reheater

Boiler Pressure Part DesignCode – 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

Boiler Auxiliaries

TO

Session on Boiler

Combustion

-BOILER=CONTROLLED COMB.+HEAT TRANSFER

-CHEMICAL =THERMAL

-COMBUSTION-FUEL,TEMP,O2

-FUEL - BITUMINOUS COAL

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

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 Kcal2H2 +O2 : 2H2O + Heat 61,979 KcalS +O2 : SO2 + Heat 3175 KcalIncomplete Combustion2C + O2 : 2CO + Heat 26,429 Kcal1 Kg of liquid fuel + 15 Kg of Air Oxides +

HEATED BY FURNACE HEAT

PRESSURISED + PREHEATEDLIQUID 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 + O2CO2 + 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)

AdvantagesIts 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.

DisadvantagesHigh 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

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