Transcript Energy Efficiency in Boiler Operation

BOILERS
FUNDAMENTALS/COMBUSTION
AJAY SHUKLA
DGM NTPC PMI
23rd 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
Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four
processes:
•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)
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
Basic boiler :
Steam
Water
Steam / water system
Blow down
Mixing of fuel
and air
Furnace
Heat transfer
Surface
Flue gas
AIR
FUEL
Ash
Phenomenological Model
Hot Flue
Gas
Thermal Structure
SH
Convection &
Radiation HT
Steam
Convection HT
Rise in Enthalpy of
Steam
Drop in Enthalpy
of Flue Gas
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
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
Boiler fundamentals
-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 COMBUSTIONTIME,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)
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)


Density difference &
height of water
column
Assisted by external
circulating pump (CC/
BCW pump)
Forced/ assisted
circulation (185-200
ksc)
Once thru boiler
 Below 221.5 bar
1. Sub critical
 240-360 bar
2. Supercritical
Circulation ratio
It may be defined as the ratio of feed
water flow thru down comers to the
steam generated in water wall.





CR
CR
CR
CR
CR
=
=
=
=
=
30-35 Industrial boilers
6-8 Natrual cir. Boilers
2-3 Forced cir. Boilers
1 Once thru boilers (Sub critical)
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
Sodium carry over
Chloride carry over
Copper carry over
Iron carry over
-
<10 ppb
<3 ppb
<2 ppb
<1 ppb
not detectable
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.
Extract from the steam table
Enthalpy in Kcal/kg
Pressure
Temperature
o
C
2
Water
(kg/cm )
(hf )
Evaporation
(hfg)
Steam
(hg)
Specific Volume
(m3/kg)
1
100
100.09
539.06
639.15
1.673
2
120
119.92
526.26
646.18
0.901
3
133
133.42
517.15
650.57
0.616
4
143
143.70
509.96
653.66
0.470
5
151
152.13
503.90
656.03
0.381
6
158
159.33
498.59
657.92
0.321
7
164
165.67
493.82
659.49
0.277
8
170
171.35
489.46
660.81
0.244
Steam Properties : a re-look
Pressure,
Bar A
Density,
3
kg/m
Water
Enthalpy,
kcal/kg
Dry steam
Enthalpy,
kcal/kg
Latent
Heat,
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.
w/w
HPH+Eco
SH RH
HPT
IPT
BFP
LPT
LPH
CEP
condenser
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
0
0
 200 MW: 157 bar MS Pressure, 540 C/540 C
0
0
 500 MW: 179 bar MS Pressure, 540 C/540 C
0
0
 660 MW: 246 bar MS Pressure, 545 C/563 C
Advanced Supercritical Parameter
0
0
 310 bar MS Pressure, 610 C/610 C
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 Critical
Operates 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.
Universal Pressure
Operate at constant pressure
Drum type
Provides a thermodynamic fixed point at
drum, which remains at constant temp.
Sliding Pressure
Operate at sub-critical pressure at reduced
loads.
Natural Circulation
Boilers use the difference in water and
steam density to drive the water/steam
mixture through the water tubes.
Assisted Circulation
Boilers have Circulating Water Pump
which assists the natural convective flow
through the water tubes.
EFFECT OF SUPERCRITICAL PARAMETERS
Temperature (C)
538
240 kg/cm2
Expansion Line
170 kg/cm2
Critical Point 225 kg/cm2
Condensation
Enthalpy
4
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 Type
All 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 Fired
Burners 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 Fired
Burners 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 Fired
Burners 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 Bottom
Bottom Ash collected in slag
form. Mostly used for low ash
coals with low fusion
temperatures.
Dry Bottom
Bottom 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
Lignite
Peat
Bagasse
Gas
Husk
Kerosene
Petrol
HSD
LDO
Natural gas
Methane
LPG
Producer
FO
LSHS
MAIN CONSTITUENTS OF
FUEL
Carbon
Hydrogen
Sulphur
Nitrogen
Oxygen
Water Vapour
Ash
PROXIMATE ANALYSIS OF TYPICAL INDIAN COAL
DESIGN WORST BEST
TOTAL MOISTURE
ASH
VOLATILE MATTER
FIXED CARBON
%
%
%
%
15
42
21
22
16.5
44
19.5
20
14
38
23
25
TOTAL
%
100
100
100
PROPERTIES OF FUEL
(Typical Analysis of F.O.)
Carbon
Hydrogen
Sulphur
Calorific value
Sp. Gravity at 30oC
Flash point
Viscosity at 40oC
Water Percentage
Sediment Percentage
83.52%
11.68%
3.27%
10,000 Kcal/kg
0.95
65oC
1500 RW Sec No 1
0.15
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
+
COMBUSTION PROCESS
LIQUID FUEL
PRESSURISED + PREHEATED
ATOMISED
HEATED BY FURNACE HEAT
VAPORISED
IGNITED BY FLAME
COMBUSTION
COMBUSTION
REACTIONS
C O
C O
C
O O
2C + O2
2CO + LESS HEAT
C
COMBUSTION INCOMPLETE
COMBUSTION
REACTIONS
C
O
O C+O
2
C
O
CO2 + HEAT
H O
H
H H
O O
O
2H2 + O2
H H
H O
H
2H20 + HEAT
COMBUSTION COMPLETE
COMBUSTION
FLAME & FLAME FRONT
*

*


FLAME :
IT IS AN ENVELOPE OR ZONE WITHIN WHICH
COMBUSTION REACTION IS OCCURRING AT SUCH A RATE
AS TO PRODUCE VISIBLE RADIATION.
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 COMBUSTION
REQUIRES .......

3 T’s - 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)
Pulverized Fuel Boiler (Contd..)
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
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