Training Session on Energy
Equipment
Furnaces and
Refractories
Presentation from the
“Energy Efficiency Guide for Industry in Asia”
www.energyefficiencyasia.org
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© UNEP 2006
Training Agenda: Steam
Introduction
Type of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
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© UNEP 2006
Introduction
What is a Furnace?
• Equipment to melt metals
• Casting
• Change shape
• Change properties
• Type of fuel important
• Mostly liquid/gaseous fuel or electricity
• Low efficiencies due to
• High operating temperature
• Emission of hot exhaust gases
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© UNEP 2006
Introduction
Chimney:
remove
combustion
gases
Burners: raise or
maintain chamber
temperature
Furnace Components
Furnace chamber:
constructed of
insulating materials
Hearth: support or
carry the steel.
Consists of
refractory materials
Charging & discharging doors for
loading & unloading stock
(The Carbon Trust)
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© UNEP 2006
Introduction
What are Refractories:
Materials that
• Withstand high temperatures and sudden
changes
• Withstand action of molten slag, glass, hot
gases etc
• Withstand load at service conditions
• Withstand abrasive forces
• Conserve heat
• Have low coefficient of thermal expansion
• Will not contaminate the load
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Introduction
Refractories
Refractory lining of a
furnace arc
Refractory walls of a
furnace interior with
burner blocks
(BEE India, 2005)
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© UNEP 2006
Introduction
Properties of Refractories
• Melting point
• Temperature at which a ‘test pyramid’ (cone)
fails to support its own weight
• Size
• Affects stability of furnace structure
• Bulk density
• Amount of refractory material within a
volume (kg/m3)
• High bulk density = high volume stability,
heat capacity and resistance
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© UNEP 2006
Introduction
Properties of Refractories
• Porosity
• Volume of open pores as % of total refractory
volume
• Low porosity = less penetration of molten
material
• Cold crushing strength
• Resistance of refractory to crushing
• Creep at high temperature
• Deformation of refractory material under
stress at given time and temperature
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© UNEP 2006
Introduction
Properties of Refractories
• Pyrometric cones
• Used in ceramic industries
to test ‘refractoriness’ of
refractory bricks
• Each cone is mix of oxides
that melt at specific
temperatures
(BEE India, 2004)
• Pyrometric Cone Equivalent (PCE)
• Temperature at which the refractory brick and
the cone bend
• Refractory cannot be used above this temp
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Introduction
Properties of Refractories
• Volume stability, expansion &
shrinkage
• Permanent changes during refractory service
life
• Occurs at high temperatures
• Reversible thermal expansion
• Phase transformations during heating and
cooling
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Introduction
Properties of Refractories
• Thermal conductivity
• Depends on composition and silica content
• Increases with rising temperature
• High thermal conductivity:
• Heat transfer through brickwork required
• E.g. recuperators, regenerators
• Low thermal conductivity:
• Heat conservation required (insulating
refractories)
• E.g. heat treatment furnaces
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© UNEP 2006
Training Agenda: Steam
Introduction
Type of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
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© UNEP 2006
Type of Furnaces and Refractories
• Type of Furnaces
• Forging furnaces
• Re-rolling mill furnaces
• Continuous reheating furnaces
• Type of Refractories
• Type of Insulating Materials
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Type of Furnaces and Refractories
Classification Combustion Furnaces
Classification method
Types and examples
1. Type of fuel used
Oil-fired
Gas-fired
Coal-fired
2. Mode of charging materials
Intermittent / Batch
Periodical
 Forging
 Re-rolling (batch/pusher)
 Pot
Continuous
 Pusher
 Walking beam
 Walking hearth
 Continuous recirculating bogie furnaces
 Rotary hearth furnaces
3. Mode of heat transfer
Radiation (open fire place)
Convection (heated through medium)
4. Mode of waste heat
recovery
Recuperative
Regenerative
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© UNEP 2006
Type of Furnaces and Refractories
Forging Furnace
• Used to preheat billets/ingots
• Use open fireplace system with
radiation heat transmission
• Temp 1200-1250 oC
• Operating cycle
• Heat-up time
• Soaking time
• Forging time
• Fuel use: depends on material and
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number of reheats
© UNEP 2006
Type of Furnaces and Refractories
Re-rolling Mill Furnace – Batch type
• Box type furnace
• Used for heating up scrap/ingots/billets
• Manual charge / discharge of batches
• Temp 1200 oC
• Operating cycle: heat-up, re-rolling
• Output 10 - 15 tons/day
• Fuel use: 180-280 kg coal/ton material
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© UNEP 2006
Type of Furnaces and Refractories
Re-rolling Mill Furnace –
Continuous pusher type
• Not batch, but continuous charge and
discharge
• Temp 1250 oC
• Operating cycle: heat-up, re-rolling
• Output 20-25 tons/day
• Heat absorption by material is slow,
steady, uniform
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
• Continuous material flow
• Material temp 900 – 1250 oC
• Door size minimal to avoid air
infiltration
• Stock kept together and pushed
• Pusher type furnaces
• Stock on moving hearth or structure
• Walking beam, walking hearth, continuous
recirculating bogie, rotary hearth furnaces
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
1. Pusher Furnace
• Pushers on ‘skids’ (rails) with water-cooled
support push the stock
• Hearth sloping towards discharge end
• Burners at discharge
end or top and/or
bottom
• Chimney with
recuperator for
waste heat recovery
(The Carbon Trust, 1993)
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
2. Walking Beam Furnace
• Stock placed on stationary ridges
• Walking beams raise the stock and move forwards
• Walking beams lower stock onto stationary ridges
at exit
• Stock is removed
• Walking beams
return to furnace
entrance
(The Carbon Trust, 1993)
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
3. Walking Hearth Furnace
• Refractory blocks extend through hearth
openings
• Stock rests on fixed refractory blocks
• Stock transported
in small steps
‘walking the hearth’
• Stock removed
at discharge end
(The Carbon Trust, 1993)
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
4. Continuous Recirculating Bogie
Furnace
• Shape of long and narrow tunnel
• Stock placed on bogie (cart with wheels) with
refractory hearth
• Several bogies
move like train
• Stock removed
at discharge end
• Bogie returned
to entrance
(The Carbon Trust, 1993)
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© UNEP 2006
Type of Furnaces and Refractories
Continuous Reheating Furnaces
5. Rotary Hearth Furnace
•
•
•
•
Walls and roof remain stationary
Hearth moves in circle on rollers
Stock placed on hearth
Heat moves in
opposite direction
of hearth
• Temp 1300oC
(The Carbon Trust, 1993)
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Type of Furnaces and Refractories
Classification of Refractories
Classification method
Examples
Chemical composition
ACID, which readily combines with bases
Silica, Semisilica, Aluminosilicate
BASIC, which consists mainly of metallic
oxides that resist the action of bases
Magnesite, Chrome-magnesite, Magnesitechromite, Dolomite
NEUTRAL, which does not combine with
acids nor bases
Fireclay bricks, Chrome, Pure Alumina
Special
Carbon, Silicon Carbide, Zirconia
End use
Blast furnace casting pit
Method of manufacture
Dry press process, fused cast, hand
moulded, formed normal, fired or chemically
bonded, unformed (monolithics, plastics,
ramming mass, gunning castable, spraying)
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© UNEP 2006
Type of Furnaces and Refractories
Fireclay Refractories
• Common in industry: materials available and
inexpensive
• Consist of aluminium silicates
• Decreasing melting point (PCE) with increasing
impurity and decreasing AL2O3
High Alumina Refractories
• 45 - 100% alumina
• High alumina % = high refractoriness
• Applications: hearth and shaft of blast furnaces,
ceramic kilns, cement kilns, glass tanks
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© UNEP 2006
Type of Furnaces and Refractories
Silica Brick
• >93% SiO2 made from quality rocks
• Iron & steel, glass industry
• Advantages: no softening until fusion point is
reached; high refractoriness; high resistance to
spalling, flux and slag, volume stability
Magnesite
• Chemically basic: >85% magnesium oxide
• Properties depend on silicate bond concentration
• High slag resistance, especially lime and iron
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© UNEP 2006
Type of Furnaces and Refractories
Chromite Refractories
• Chrome-magnesite
•
•
•
•
15-35% Cr2O3 and 42-50% MgO
Used for critical parts of high temp furnaces
Withstand corrosive slags
High refractories
• Magnesite-chromite
•
•
•
•
>60% MgO and 8-18% Cr2O3
High temp resistance
Basic slags in steel melting
Better spalling resistance
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© UNEP 2006
Type of Furnaces and Refractories
Zirconia Refractories
• Zirconium dioxide ZrO2
• Stabilized with calcium, magnesium, etc.
• High strength, low thermal conductivity, not
reactive, low thermal loss
• Used in glass furnaces, insulating refractory
Oxide Refractories (Alumina)
• Aluminium oxide + alumina impurities
• Chemically stable, strong, insoluble, high
resistance in oxidizing and reducing atmosphere
• Used in heat processing industry, crucible shaping28
© UNEP 2006
Type of Furnaces and Refractories
Monolithics
• Single piece casts in equipment shape
• Replacing conventional refractories
• Advantages
•
•
•
•
•
•
•
Elimination of joints
Faster application
Heat savings
Better spalling resistance
Volume stability
Easy to transport, handle, install
Reduced downtime for repairs
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© UNEP 2006
Type of Furnaces and Refractories
Insulating Materials Classification
• Material with low heat conductivity:
keeps furnace surface temperature
low
• Classification into five groups
•
•
•
•
•
Insulating bricks
Insulating castables and concrete
Ceramic fiber
Calcium silicate
Ceramic coatings (high emissivity coatings)
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Type of Furnaces and Refractories
Castables and Concretes
• Consist of
• Insulation materials used for making piece
refractories
• Concretes contain Portland or high-alumina
cement
• Application
• Monolithic linings of furnace sections
• Bases of tunnel kiln cars in ceramics
industry
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© UNEP 2006
Type of Furnaces and Refractories
Ceramic Fibers
• Thermal mass insulation materials
• Manufactured by blending alumina
and silica
• Bulk wool to make insulation
products
• Blankets, strips, paper, ropes, wet felt etc
• Produced in two temperature grades
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Type of Furnaces and Refractories
Ceramic Fibers
Remarkable properties and benefits
•
•
•
•
•
•
•
•
•
•
Low thermal conductivity
Light weight
Lower heat storage
Thermal shock resistant
Chemical resistance
Mechanical resilience
Low installation costs
Ease of maintenance
Ease of handling
Thermal efficiency
• Lightweight furnace
• Simple steel fabrication
work
• Low down time
• Increased productivity
• Additional capacity
• Low maintenance costs
• Longer service life
• High thermal efficiency
• Faster response
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© UNEP 2006
Type of Furnaces and Refractories
High Emissivity Coatings
• Emissivity: ability to absorb and
radiate heat
• Coatings applied to interior furnace
surface:
•
•
•
•
emissivity stays constant
Increase emissivity from 0.3 to 0.8
Uniform heating and extended refractory life
Fuel reduction by up to 25-45%
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© UNEP 2006
Type of Furnaces and Refractories
High Emissivity Coatings
(BEE India, 2005)
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© UNEP 2006
Training Agenda: Steam
Introduction
Type of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
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© UNEP 2006
Assessment of Furnaces
Heat Losses Affecting Furnace
Performance
Heat input
FURNACE
Heat in stock
Other losses
Furnace surface/skin
Openings in furnace
Hydrogen in fuel
Moisture in fuel
Flue gas
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© UNEP 2006
Assessment of Furnaces
Instruments to Assess Furnace
Performance
Parameters
to be measured
Location of
measurement
Instrument
required
Required
Value
Furnace soaking zone
temperature (reheating
furnaces)
Soaking zone and side
wall
Pt/Pt-Rh thermocouple
with indicator and
recorder
1200-1300oC
Flue gas temperature
In duct near the discharge
end, and entry to
recuperator
Chromel Alummel
Thermocouple with
indicator
700oC max.
Flue gas temperature
After recuperator
Hg in steel thermometer
300oC (max)
Furnace hearth pressure
in the heating zone
Near charging end and
side wall over the hearth
Low pressure ring gauge
+0.1 mm of Wc
Oxygen in flue gas
In duct near the discharge
end
Fuel efficiency monitor for
oxygen and temperature
5% O2
Billet temperature
Portable
Infrared pyrometer or
optical pyrometer
-
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© UNEP 2006
Assessment of Furnaces
Calculating Furnace Performance
Direct Method
• Thermal efficiency of furnace
= Heat in the stock / Heat in fuel
consumed for heating the stock
• Heat in the stock Q:
Q = m x Cp (t1 – t2)
Q = Quantity of heat of stock in kCal
m = Weight of the stock in kg
Cp= Mean specific heat of stock in kCal/kg oC
t1 = Final temperature of stock in oC
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t2 = Initial temperature of the stock before it enters the furnace in©oC
UNEP 2006
Assessment of Furnaces
Calculating Furnace Performance
Direct Method - example
m = Weight of
the stock = 6000
kg
Cp= Mean
specific heat of
m x Cp (t1 – t2)
stock = 0.12
kCal/kg oC
6000 kg X 0.12 X (1340 – 40)
t1 = Final
temperature of
936000 kCal
stock = 1340 oC
t2 = Initial
temperature of
the stock = 40 oC
(heat input / heat output) x 100
Calorific value of
oil = 10000
[936000 / (368 x 10000) x 100 = 25.43%
kCal/kg
Fuel consumption
= 368 kg/hr
• Heat in the stock Q =
•
•
•
• Efficiency =
•
•
• Heat loss = 100% - 25% = 75%
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© UNEP 2006
Assessment of Furnaces
Calculating Furnace Performance
Indirect Method
Heat losses
a) Flue gas loss
= 57.29 %
b) Loss due to moisture in fuel
= 1.36 %
c) Loss due to H2 in fuel
= 9.13 %
d) Loss due to openings in furnace
= 5.56 %
e) Loss through furnace skin
= 2.64 %
Total losses
= 75.98 %
Furnace efficiency =
• Heat supply minus total heat loss
•
100% – 76% = 24%
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© UNEP 2006
Assessment of Furnaces
Calculating Furnace Performance
Typical efficiencies for industrial furnaces
Furnace type
Thermal efficiencies (%)
1) Low Temperature furnaces
a. 540 – 980 oC (Batch type)
20-30
b. 540 – 980 oC (Continous type)
15-25
c. Coil Anneal (Bell) radiant type
5-7
d. Strip Anneal Muffle
7-12
2) High temperature furnaces
a. Pusher, Rotary
7-15
b. Batch forge
5-10
3) Continuous Kiln
a. Hoffman
25-90
b. Tunnel
20-80
4) Ovens
a. Indirect fired ovens (20 oC –370 oC)
35-40
b. Direct fired ovens (20 oC –370 oC)
35-40
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© UNEP 2006
Training Agenda: Steam
Introduction
Type of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
43
© UNEP 2006
Energy Efficiency Opportunities
1. Complete combustion with minimum excess air
2. Proper heat distribution
3. Operation at the optimum furnace temperature
4. Reducing heat losses from furnace openings
5. Maintaining correct amount of furnace draft
6. Optimum capacity utilization
7. Waste heat recovery from the flue gases
8. Minimize furnace skin losses
9. Use of ceramic coatings
10.Selecting the right refractories
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Energy Efficiency Opportunities
1. Complete Combustion with
Minimum Excess Air
• Importance of excess air
• Too much: reduced flame temp, furnace
temp, heating rate
• Too little: unburnt in flue gases, scale losses
• Indication of excess air: actual air /
theoretical combustion air
• Optimizing excess air
•
•
•
•
Control air infiltration
Maintain pressure of combustion air
Ensure high fuel quality
Monitor excess air
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© UNEP 2006
Energy Efficiency Opportunities
2. Proper Heat Distribution
When using burners
• Flame should not touch or be obstructed
• No intersecting flames from different burners
• Burner in small furnace should face upwards
but not hit roof
• More burners with less capacity (not one big
burner) in large furnaces
• Burner with long flame to improve uniform
heating in small furnace
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© UNEP 2006
Energy Efficiency Opportunities
3. Operate at Optimum Furnace
Temperature
• Operating at too high temperature:
heat
loss, oxidation, decarbonization, refractory stress
• Automatic controls eliminate human
error
Slab Reheating furnaces
1200oC
Rolling Mill furnaces
1200oC
Bar furnace for Sheet Mill
800oC
Bogie type annealing furnaces
650oC –750oC
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© UNEP 2006
Energy Efficiency Opportunities
4. Reduce Heat Loss from Furnace
Openings
• Heat loss through openings
• Direct radiation through openings
• Combustion gases leaking through the openings
• Biggest loss: air infiltration into the furnace
• Energy saving measures
• Keep opening small
• Seal openings
• Open furnace doors less frequent and shorter
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© UNEP 2006
Energy Efficiency Opportunities
5. Correct Amount of Furnace Draft
• Negative pressure in furnace: air
infiltration
• Maintain slight positive pressure
• Not too high pressure difference: air
ex-filtration
Heat loss only about 1% if furnace
pressure is controlled properly!
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© UNEP 2006
Energy Efficiency Opportunities
6. Optimum Capacity Utilization
• Optimum load
• Underloading: lower efficiency
• Overloading: load not heated to right temp
• Optimum load arrangement
• Load receives maximum radiation
• Hot gases are efficiently circulated
• Stock not placed in burner path, blocking flue
system, close to openings
• Optimum residence time
• Coordination between personnel
• Planning at design and installation
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stage © UNEP 2006
Energy Efficiency Opportunities
7. Waste Heat Recovery from Flue Gases
• Charge/Load pre-heating
• Reduced fuel needed to heat them in furnace
• Pre-heating of combustion air
• Applied to compact industrial furnaces
• Equipment used: recuperator, selfrecuperative burner
• Up to 30% energy savings
• Heat source for other processes
• Install waste heat boiler to produce steam
• Heating in other equipment (with care!)
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© UNEP 2006
Energy Efficiency Opportunities
8. Minimum Furnace Skin Loss
• Choosing appropriate refractories
• Increasing wall thickness
• Installing insulation bricks (= lower
conductivity)
• Planning furnace operating times
• 24 hrs in 3 days: 100% heat in refractories
lost
• 8 hrs/day for 3 days: 55% heat lost
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© UNEP 2006
Energy Efficiency Opportunities
9. Use of Ceramic Coatings
• High emissivity coatings
• Long life at temp up to 1350 oC
• Most important benefits
• Rapid efficient heat transfer
• Uniform heating and extended refractory life
• Emissivity stays constant
• Energy savings: 8 – 20%
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© UNEP 2006
Energy Efficiency Opportunities
10. Selecting the Right Refractory
Selection criteria
• Type of furnace
• Type of metal charge
• Structural load of
furnace
• Presence of slag
• Stress due to temp
gradient & fluctuations
• Area of application
• Chemical compatibility
• Working temperatures
• Heat transfer & fuel
conservation
• Extent of abrasion
and impact
• Costs
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© UNEP 2006
Training Session on Energy
Equipment

Furnaces and
Refractories
THANK YOU
FOR YOUR ATTENTION
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© UNEP 2006
Disclaimers 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