Training Session on Energy
Equipment
Boilers & Thermic
Fluid Heaters
Presentation from the
“Energy Efficiency Guide for Industry in Asia”
www.energyefficiencyasia.org
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© UNEP 2006
Training Agenda: Boiler
Introduction
Type of boilers
Assessment of a boiler
Energy efficiency opportunities
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© UNEP 2006
Introduction
What is a Boiler?
• Vessel that heats water to become
hot water or steam
• At atmospheric pressure water
volume increases 1,600 times
• Hot water or steam used to transfer
heat to a process
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© UNEP 2006
Introduction
STEAM TO
PROCESS
EXHAUST GAS
STACK
VENT
DEAERATOR
PUMPS
ECONOMIZER
VENT
BOILER
BURNER
BLOW DOWN
SEPARATOR
WATER
SOURCE
FUEL
BRINE
CHEMICAL FEED
SOFTENERS
Figure: Schematic overview of a boiler room
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© UNEP 2006
Training Agenda: Boiler
Introduction
Type of boilers
Assessment of a boiler
Energy efficiency opportunities
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© UNEP 2006
Types of Boilers
What Type of Boilers Are There?
1. Fire Tube Boiler
2. Water Tube Boiler
3. Packaged Boiler
4. Fluidized Bed (FBC) Boiler
5. Stoker Fired Boiler
6. Pulverized Fuel Boiler
7. Waste Heat Boiler
8. Thermic Fluid Heater (not a boiler!)
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© UNEP 2006
Type of Boilers
1. Fire Tube Boiler
(Light Rail Transit Association)
•
Relatively small steam
capacities (12,000
kg/hour)
•
Low to medium steam
pressures (18 kg/cm2)
•
Operates with oil, gas
or solid fuels
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© UNEP 2006
Type of Boilers
2. Water Tube Boiler
(Your Dictionary.com)
•
Used for high steam
demand and pressure
requirements
•
Capacity range of 4,500
– 120,000 kg/hour
•
Combustion efficiency
enhanced by induced
draft provisions
•
Lower tolerance for
water quality and needs
water treatment plant 8
© UNEP 2006
Type of Boilers
3. Packaged Boiler
To
Chimney
Oil
Burner
(BIB Cochran, 2003)
• Comes in complete
package
• Features
• High heat transfer
• Faster evaporation
• Good convective
heat transfer
• Good combustion
efficiency
• High thermal
efficiency
• Classified based on
number of passes
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© UNEP 2006
Type of Boilers
4. Fluidized Bed Combustion
(FBC) Boiler
•
Particles (e.g. sand) are suspended in high
velocity air stream: bubbling fluidized bed
•
Combustion at 840° – 950° C
•
Capacity range 0,5 T/hr to 100 T/hr
•
Fuels: coal, washery rejects, rice husk, bagasse
and agricultural wastes
•
Benefits: compactness, fuel flexibility, higher
combustion efficiency, reduced SOx & NOx
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© UNEP 2006
Type of Boilers
4a. Atmospheric Fluidized Bed
Combustion (AFBC) Boiler
• Most common FBC boiler that uses preheated
atmospheric air as fluidization and combustion air
4b. Pressurized Fluidized Bed
Combustion (PFBC) Boiler
• Compressor supplies the forced draft and
combustor is a pressure vessel
• Used for cogeneration or combined cycle power
generation
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© UNEP 2006
Type of Boilers
4c. Atmospheric Circulating Fluidized
Bed Combustion (CFBC) Boiler
•
Solids lifted from bed,
rise, return to bed
•
Steam generation in
convection section
•
Benefits: more
economical, better space
utilization and efficient
combustion
(Thermax Babcock & Wilcox Ltd, 2001)
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© UNEP 2006
Type of Boilers
5. Stoke Fired Boilers
a) Spreader stokers
• Coal is first burnt in suspension then in
coal bed
• Flexibility to meet load fluctuations
• Favored in many industrial applications
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© UNEP 2006
Type of Boilers
5. Stoke Fired Boilers
b) Chain-grate or traveling-grate stoker
• Coal is burnt on moving
steel grate
• Coal gate controls coal
feeding rate
• Uniform coal size for
complete combustion
(University of Missouri, 2004)
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© UNEP 2006
Type of Boilers
6. Pulverized Fuel Boiler
•
Pulverized coal powder blown with combustion
air into boiler through burner nozzles
•
Combustion
temperature at 1300 1700 °C
•
Benefits: varying coal
quality coal, quick
response to load
changes and high preheat air temperatures
Tangential firing
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© UNEP 2006
Type of Boilers
7. Waste Heat Boiler
• Used when waste heat
available at medium/high
temp
• Auxiliary fuel burners
used if steam demand is
more than the waste heat
can generate
Agriculture and Agri-Food
Canada, 2001
• Used in heat recovery
from exhaust gases from
gas turbines and diesel
engines
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© UNEP 2006
Type of Boilers
8. Thermic Fluid Heater
•
Wide application for indirect process heating
•
Thermic fluid (petroleum-based) is heat transfer
medium
•
Benefits:
• Closed cycle = minimal losses
• Non-pressurized system operation at 250 °C
• Automatic controls = operational flexibility
• Good thermal efficiencies
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© UNEP 2006
Type of Boilers
8. Thermic Fluid Heater
3. Heat transfer
through heat
exchanged
User equipment
2. Circulated
to user
equipment
4. Fluid
returned to
heater
Control
panel
Insulated
outer wall
1. Thermic
fluid heated
in the heater
Blower
motor
unit
Fuel oil
filter
Exhaust
(Energy
Machine India)
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© UNEP 2006
Training Agenda: Boiler
Introduction
Type of boilers
Assessment of a boiler
Energy efficiency opportunities
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© UNEP 2006
Assessment of a boiler
1. Boiler
2. Boiler blow down
3. Boiler feed water treatment
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Assessment of a Boiler
1. Boiler performance
• Causes of poor boiler performance
-Poor combustion
-Heat transfer surface fouling
-Poor operation and maintenance
-Deteriorating fuel and water quality
• Heat balance: identify heat losses
• Boiler efficiency: determine
deviation from best efficiency
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© UNEP 2006
Assessment of a Boiler
Heat Balance
An energy flow diagram describes geographically
how energy is transformed from fuel into useful
energy, heat and losses
Stochiometric
Excess Air
Un burnt
Stack Gas
FUEL INPUT
STEAM
OUTPUT
Convection &
Radiation
Blow
Down
Ash and Un-burnt parts
of Fuel in Ash
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© UNEP 2006
Assessment of a Boiler
Heat Balance
Balancing total energy entering a boiler against
the energy that leaves the boiler in different forms
12.7 %
8.1 %
1.7 %
100.0 %
BOILER
Fuel
0.3 %
2.4 %
1.0 %
73.8 %
Heat loss due to dry flue gas
Heat loss due to steam in fuel gas
Heat loss due to moisture in fuel
Heat loss due to moisture in air
Heat loss due to unburnts in residue
Heat loss due to radiation & other
unaccounted loss
Heat in Steam
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© UNEP 2006
Assessment of a Boiler
Heat Balance
Goal: improve energy efficiency by reducing
avoidable losses
Avoidable losses include:
- Stack gas losses (excess air, stack gas
temperature)
- Losses by unburnt fuel
- Blow down losses
- Condensate losses
- Convection and radiation
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© UNEP 2006
Assessment of a Boiler
Boiler Efficiency
Thermal efficiency: % of (heat) energy input that is
effectively useful in the generated steam
BOILER EFFICENCY
CALCULATION
DIRECT METHOD:
2) INDIRECT METHOD:
The energy gain of the
working fluid (water and steam)
is compared with the energy
content of the boiler fuel.
The efficiency is the
different between losses
and energy input
1)
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© UNEP 2006
Assessment of a Boiler
Boiler Efficiency: Direct Method
Boiler efficiency () =
Heat Input x 100
Q x (hg – hf) x 100
=
Heat Output
Q x GCV
hg -the enthalpy of saturated steam in kcal/kg of steam
hf -the enthalpy of feed water in kcal/kg of water
Parameters to be monitored:
- Quantity of steam generated per hour (Q) in kg/hr
- Quantity of fuel used per hour (q) in kg/hr
- The working pressure (in kg/cm2(g)) and superheat
temperature (oC), if any
- The temperature of feed water (oC)
- Type of fuel and gross calorific value of the fuel (GCV) in
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kcal/kg of fuel
© UNEP 2006
Assessment of a Boiler
Boiler Efficiency: Direct Method
Advantages
•
•
•
•
Quick evaluation
Few parameters for computation
Few monitoring instruments
Easy to compare evaporation ratios with
benchmark figures
Disadvantages
•
•
No explanation of low efficiency
Various losses not calculated
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© UNEP 2006
Assessment of a Boiler
Boiler Efficiency: Indirect Method
Efficiency of boiler () = 100 – (i+ii+iii+iv+v+vi+vii)
Principle losses:
i) Dry flue gas
ii) Evaporation of water formed due to H2 in fuel
iii) Evaporation of moisture in fuel
iv) Moisture present in combustion air
v) Unburnt fuel in fly ash
vi) Unburnt fuel in bottom ash
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vii) Radiation and other unaccounted losses © UNEP 2006
Assessment of a Boiler
Boiler Efficiency: Indirect Method
Required calculation data
• Ultimate analysis of fuel (H2, O2, S, C, moisture
content, ash content)
• % oxygen or CO2 in the flue gas
• Fuel gas temperature in ◦C (Tf)
• Ambient temperature in ◦C (Ta) and humidity of air in
kg/kg of dry air
• GCV of fuel in kcal/kg
• % combustible in ash (in case of solid fuels)
• GCV of ash in kcal/kg (in case of solid fuels)
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© UNEP 2006
Assessment of a Boiler
Boiler Efficiency: Indirect Method
Advantages
•
•
Complete mass and energy balance for each
individual stream
Makes it easier to identify options to improve
boiler efficiency
Disadvantages
•
•
Time consuming
Requires lab facilities for analysis
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© UNEP 2006
Assessment of a Boiler
2. Boiler Blow Down
•
Controls ‘total dissolved solids’ (TDS) in the
water that is boiled
•
Blows off water and replaces it with feed water
•
Conductivity measured as indication of TDS
levels
•
Calculation of quantity blow down required:
Blow down (%) =
Feed water TDS x % Make up water
Maximum Permissible TDS in Boiler water
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© UNEP 2006
Assessment of a Boiler
Boiler Blow Down
Two types of blow down
• Intermittent
• Manually operated valve reduces TDS
• Large short-term increases in feed water
• Substantial heat loss
• Continuous
• Ensures constant TDS and steam purity
• Heat lost can be recovered
• Common in high-pressure boilers
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© UNEP 2006
Assessment of a Boiler
Boiler Blow Down
Benefits
• Lower pretreatment costs
• Less make-up water consumption
• Reduced maintenance downtime
• Increased boiler life
• Lower consumption of treatment
chemicals
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© UNEP 2006
Assessment of a Boiler
3. Boiler Feed Water Treatment
• Quality of steam depend on water
treatment to control
• Steam purity
• Deposits
• Corrosion
• Efficient heat transfer only if boiler
water is free from deposit-forming
solids
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© UNEP 2006
Assessment of a Boiler
Boiler Feed Water Treatment
Deposit control
• To avoid efficiency losses and
reduced heat transfer
• Hardness salts of calcium and
magnesium
• Alkaline hardness: removed by boiling
• Non-alkaline: difficult to remove
• Silica forms hard silica scales
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© UNEP 2006
Assessment of a Boiler
Boiler Feed Water Treatment
Internal water treatment
•
Chemicals added to boiler to prevent scale
•
Different chemicals for different water types
•
Conditions:
• Feed water is low in hardness salts
• Low pressure, high TDS content is tolerated
• Small water quantities treated
•
Internal treatment alone not recommended
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© UNEP 2006
Assessment of a Boiler
Boiler Feed Water Treatment
External water treatment:
•
Removal of suspended/dissolved solids and
dissolved gases
•
Pre-treatment: sedimentation and settling
•
First treatment stage: removal of salts
•
Processes
a) Ion exchange
b) Demineralization
c) De-aeration
d) Reverse osmoses
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© UNEP 2006
Assessment of a Boiler
External Water Treatment
a) Ion-exchange process (softener plant)
• Water passes through bed of natural zeolite of
synthetic resin to remove hardness
• Base exchange: calcium (Ca) and magnesium (Mg)
replaced with sodium (Na) ions
• Does not reduce TDS, blow down quantity and
alkalinity
b) Demineralization
• Complete removal of salts
• Cations in raw water replaced with hydrogen ions
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© UNEP 2006
Assessment of a Boiler
External Water Treatment
c) De-aeration
• Dissolved corrosive gases (O2, CO2)
expelled by preheating the feed water
• Two types:
• Mechanical de-aeration: used prior to addition
of chemical oxygen scavangers
• Chemical de-aeration: removes trace oxygen
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© UNEP 2006
Assessment of a Boiler
External Water Treatment
Mechanical
de-aeration
Vent
Spray
Nozzles
Boiler Feed
Water
Stea
m
Scrubber
Section
(Trays)
• O2 and CO2 removed by
heating feed water
• Economical treatment
process
• Vacuum type can reduce
O2 to 0.02 mg/l
Storage
Section
De-aerated
Boiler Feed
Water
( National Productivity Council)
• Pressure type can
reduce O2 to 0.005 mg/l
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© UNEP 2006
Assessment of a Boiler
External Water Treatment
Chemical de-aeration
• Removal of trace oxygen with scavenger
• Sodium sulphite:
• Reacts with oxygen: sodium sulphate
• Increases TDS: increased blow down
• Hydrazine
• Reacts with oxygen: nitrogen + water
• Does not increase TDS: used in high pressure
boilers
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© UNEP 2006
Assessment of a Boiler
External Water Treatment
d) Reverse osmosis
• Osmosis
• Solutions of differing concentrations
• Separated by a semi-permeable membrane
• Water moves to the higher concentration
• Reversed osmosis
• Higher concentrated liquid pressurized
• Water moves in reversed direction
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© UNEP 2006
Assessment of a Boiler
External water treatment
d) Reverse osmosis
Pressure
Fresh Water
Feed
Water
More
Concentrated
Solution
Concentrate
Flow
Water Flow
Semi Permeable
Membrane
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© UNEP 2006
Training Agenda: Boiler
Introduction
Type of boilers
Assessment of a boiler
Energy efficiency opportunities
44
© UNEP 2006
Energy Efficiency Opportunities
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Stack temperature control
Feed water preheating using
economizers
Combustion air pre-heating
Incomplete combustion
minimization
Excess air control
Avoid radiation and convection
heat loss
Automatic blow down control
Reduction of scaling and soot
losses
Reduction of boiler steam
pressure
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Variable speed control
© UNEP 2006
Energy Efficiency Opportunities
1. Stack Temperature Control
• Keep as low as possible
• If >200°C then recover waste heat
2. Feed Water Preheating
Economizers
• Potential to recover heat from 200 – 300 oC flue
gases leaving a modern 3-pass shell boiler
3. Combustion Air Preheating
• If combustion air raised by 20°C = 1% improve
thermal efficiency
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© UNEP 2006
Energy Efficiency Opportunities
4. Minimize Incomplete Combustion
• Symptoms:
• Smoke, high CO levels in exit flue gas
• Causes:
• Air shortage, fuel surplus, poor fuel distribution
• Poor mixing of fuel and air
• Oil-fired boiler:
• Improper viscosity, worn tops, cabonization on
dips, deterioration of diffusers or spinner plates
• Coal-fired boiler: non-uniform coal size
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© UNEP 2006
Energy Efficiency Opportunities
5. Excess Air Control
• Excess air required for complete combustion
• Optimum excess air levels varies
• 1% excess air reduction = 0.6% efficiency rise
• Portable or continuous oxygen analyzers
Fuel
Kg air req./kg fuel
%CO2 in flue gas in practice
Solid Fuels
Bagasse
Coal (bituminous)
Lignite
Paddy Husk
Wood
3.3
10.7
8.5
4.5
5.7
10-12
10-13
9 -13
14-15
11.13
Liquid Fuels
Furnace Oil
LSHS
13.8
14.1
9-14
9-14
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© UNEP 2006
Energy Efficiency Opportunities
6. Radiation and Convection Heat
Loss Minimization
•
Fixed heat loss from boiler shell, regardless of
boiler output
•
Repairing insulation can reduce loss
7. Automatic Blow Down Control
•
Sense and respond to boiler water conductivity
and pH
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© UNEP 2006
Energy Efficiency Opportunities
8. Scaling and Soot Loss Reduction
•
Every 22oC increase in stack temperature = 1%
efficiency loss
•
3 mm of soot = 2.5% fuel increase
9. Reduced Boiler Steam Pressure
•
Lower steam pressure
= lower saturated steam temperature
= lower flue gas temperature
•
Steam generation pressure dictated by process
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© UNEP 2006
Energy Efficiency Opportunities
10. Variable Speed Control for Fans,
Blowers and Pumps
•
Suited for fans, blowers, pumps
•
Should be considered if boiler loads are
variable
11. Control Boiler Loading
•
Maximum boiler efficiency: 65-85% of rated load
•
Significant efficiency loss: < 25% of rated load
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© UNEP 2006
Energy Efficiency Opportunities
12. Proper Boiler Scheduling
•
Optimum efficiency: 65-85% of full load
•
Few boilers at high loads is more efficient than
large number at low loads
13. Boiler Replacement
Financially attractive if existing boiler is
•
Old and inefficient
•
Not capable of firing cheaper substitution fuel
•
Over or under-sized for present requirements
•
Not designed for ideal loading conditions
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© UNEP 2006
Training Session on Energy
Equipment

Boilers & Thermic
Fluid Heaters
THANK YOU
FOR YOUR ATTENTION
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© UNEP GERIAP
Disclaimer and References
• This PowerPoint training session was prepared as part of
the project “Greenhouse Gas Emission Reduction from
Industry in Asia and the Pacific” (GERIAP). While
reasonable efforts have been made to ensure that the
contents of this publication are factually correct and
properly referenced, UNEP does not accept responsibility for
the accuracy or completeness of the contents, and shall not
be liable for any loss or damage that may be occasioned
directly or indirectly through the use of, or reliance on, the
contents of this publication. © UNEP, 2006.
• The GERIAP project was funded by the Swedish
International Development Cooperation Agency (Sida)
• Full references are included in the textbook chapter that is
available on www.energyefficiencyasia.org
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© UNEP 2006