Furnaces

Furnaces and Refractories
FURNACES AND REFRACTORIES
1. INTRODUCTION............................................................................................................... 1
2. TYPES OF FURNACES, REFRACTORIES AND INSULATION................. 5
3. ASSESSMENT OF FURNACES................................................................................. 18
4. ENERGY EFFICIENCY OPPORTUNITES.......................................................... 27
5. OPTIONS CHECKLIST................................................................................................ 35
6. WORKSHEETS.............................................................................................................. 35
7. REFERENCES .................................................................................................................. 36
1. INTRODUCTION
This section introduces furnaces and refractories and explains the various design and operation
aspects.
1.1 What is a furnace?
A furnace is an equipment used to melt metals for casting or to heat materials to change their
shape (e.g. rolling, forging) or properties (heat treatment).
Since flue gases from the fuel come in direct contact with the materials, the type of fuel chosen is
important. For example, some materials will not tolerate sulphur in the fuel. Solid fuels generate
particulate matter, which will interfere the materials placed inside the furnace. For this reason:
Most furnaces use liquid fuel, gaseous fuel or electricity as energy input.
Induction and arc furnaces use electricity to melt steel and cast iron.
Melting furnaces for nonferrous materials use fuel oil.
Oil-fired furnaces mostly use furnace oil, especially for reheating and heat treatment of
materials.
Light diesel oil (LDO) is used in furnaces where sulphur is undesirable.
Furnace ideally should heat as much of material as possible to a uniform temperature with the
least possible fuel and labor. The key to efficient furnace operation lies in complete combustion
of fuel with minimum excess air. Furnaces operate with relatively low efficiencies (as low as 7
percent) compared to other combustion equipment such as the boiler (with efficiencies higher
than 90 percent. This is caused by the high operating temperatures in the furnace. For example, a
furnace heating materials to 1200 o
C will emit exhaust gases at 1200 o
C or more, which results in
significant heat losses through the chimney.
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Chapter - 1

All furnaces have the following components as shown in Figure 1 (Carbon Trust, 1993):
Refractory chamber constructed of insulating materials to retain heat at high operating
temperatures.
Hearth to support or carry the steel, which consists of refractory materials supported by a
steel structure, part of which is water-cooled.
Burners that use liquid or gaseous fuels to raise and maintain the temperature in the chamber.
Coal or electricity can be used in reheating furnaces.
Chimney to remove combustion exhaust gases from the chamber
Charging and discharging doors through which the chamber is loaded and unloaded. Loading
and unloading equipment include roller tables, conveyors, charging machines and furnace
pushers.
Figure 1: Typical Furnace Components (The Carbon Trust, 1993)
1.2 What are refractories?
Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or
corrosive solids, liquids or gases at high temperatures. The various combinations of operating
conditions in which refractories are used, make it necessary to manufacture a range of refractory
materials with different properties. Refractory materials are made in varying combinations and
shapes depending on their applications. General requirements of a refractory material are:
Withstand high temperatures
Withstand sudden changes of temperatures
Withstand action of molten metal slag, glass, hot gases, etc
Withstand load at service conditions
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Withstand load and abrasive forces
Conserve heat
Have low coefficient of thermal expansion
Should not contaminate the material with which it comes into contact
Table 1 compares the thermal properties of typical high density and low density refractory
materials.
Table 1. Typical Refractory Properties (The Carbon Trust, 1993)
Property High Thermal Mass
(High density refractories)
Low Thermal Mass
(Ceramic fiber)
Thermal conductivity (W/m K) 1.2 0.3
Specific heat (J/kg K) 1000 1000
Density (kg/m3) 2300 130
Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and
atmospheres encountered different types of refractories are used. Typical installations of
refractories are shown in Figure 2.
Figure 2a. Refractory lining of a furnace
arch (BEE, 2005)
Figure 2b. Refractory walls of a furnace
interior with burner blocks (BEE, 2005)
Some of the important properties of refractories are:
Melting point: Pure substances melt instantly at a specific temperature. Most refractory materials
consist of particles bonded together that have high melting temperatures. At high temperatures,
these particles melt and form slag. The melting point of the refractory is the temperature at which
a test pyramid (cone) fails to support its own weight.
Size: The size and shape of the refractories is a part of the design of the furnace, since it affects
the stability of the furnace structure. Accurate size is extremely important to properly fit the
refractory shape inside the furnace and to minimize space between construction joints.
Bulk density: The bulk density is useful property of refractories, which is the amount of
refractory material within a volume (kg/m3). An increase in bulk density of a given refractory
increases its volume stability, heat capacity and resistance to slag penetration.
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Porosity: The apparent porosity is the volume of the open pores, into which a liquid can
penetrate, as a percentage of the total volume of the refractory. This property is important when
the refractory is in contact with molten charge and slag. A low apparent porosity prevents molten
material from penetrating into the refractory. A large number of small pores is generally
preferred to a small number of large pores.
Cold crushing strength: The cold crushing strength is the resistance of the refractory to
crushing, which mostly happens during transport. It only has an indirect relevance to refractory
performance, and is used as one of the indicators of abrasion resistance. Other indicators used are
bulk density and porosity.
Pyrometric cones and Pyrometric cones equivalent (PCE): The ‘refractoriness’ of (refractory)
bricks is the temperature at which the refractory bends because it can no longer support its own
weight. Pyrometric cones are used in ceramic industries to test the refractoriness of the
(refractory) bricks. They consist of a mixture of oxides that are known to melt at a specific
narrow temperature range. Cones with different oxide composition are placed in sequence of
their melting temperature alongside a row of refractory bricks in a furnace. The furnace is fired
and the temperature rises. One cone will bends together with the refractory brick. This is the
temperature range in oC above which the refractory cannot be used. This is known as Pyrometric
Cone Equivalent temperatures. (Figure 3)
Figure 3: Pyrometric Cones
(Bureau of Energy Efficiency, 2004)
Creep at high temperature: Creep is a time dependent property, which determines the
deformation in a given time and at a given temperature by a refractory material under stress.
Volume stability, expansion, and shrinkage at high temperatures: The contraction or expansion
of the refractories can take place during service life. Such permanent changes in dimensions may
be due to:
The changes in the allotropic forms, which cause a change in specific gravity
A chemical reaction, which produces a new material of altered specific gravity
The formation of liquid phase
Sintering reactions
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Fusion dust and slag or by the action of alkalies on fireclay refractories, to form alkali-
alumina silicates. This is generally observed in blast furnaces.
Reversible thermal expansion: Any material expands when heated, and contracts when cooled.
The reversible thermal expansion is a reflection on the phase transformations that occur during
heating and cooling.
Thermal conductivity: Thermal conductivity depends on the chemical and mineralogical
composition and silica content of the refractory and on the application temperature. The
conductivity usually changes with rising temperature. High thermal conductivity of a refractory
is desirable when heat transfer though brickwork is required, for example in recuperators,
regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat, as the
refractory acts as an insulator. Additional insulation conserves heat but at the same time
increases the hot face temperature and hence a better quality refractory is required. Because of
this, the outside roofs of open-hearth furnaces are normally not insulated, as this could cause the
roof to collapse. Lightweight refractories of low thermal conductivity find wider applications in
low temperature heat treatment furnaces, for example in batch type furnaces where the low heat
capacity of the refractory structure minimizes the heat stored during the intermittent heating and
cooling cycles. Insulating refractories have very low thermal conductivity. This is usually
achieved by trapping a higher proportion of air into the structure. Some examples are:
Naturally occurring materials like asbestos are good insulators but are not particularly good
refractories
Mineral wools are available which combine good insulating properties with good resistance
to heat but these are not rigid
Porous bricks are rigid at high temperatures and have a reasonably low thermal conductivity.
2. TYPES OF FURNACES, REFRACTORIES AND INSULATION
This section describes the types of furnaces, refractories and insulation materials used in
industry. It also gives criteria for selecting refractory types for optimum results.
2.1 Types of furnaces
Furnaces are broadly classified into two types based on the heat generation method: combustion
furnaces that use fuels, and electric furnaces that use electricity. Combustion furnaces can be
classified in several based as shown in Table 2: type of fuel used, mode of charging the
materials, mode of heat transfer and mode of waste heat recovery. However, it is not possible to
use this classification in practice, because a furnace can be using different types of fuel, different
ways to charge materials into the furnace etc. The most commonly used furnaces are described in
the next sections
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Table 2. Classification of furnaces
Classification method Types and examples
Type of fuel used Oil-fired
Gas-fired
Coal-fired
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
Mode of heat transfer Radiation (open fire place)
Convection (heated through medium)
Mode of waste heat recovery Recuperative
Regenerative
2.1.1 Forging furnace
The forging furnace is used for preheating billets and ingots to attain a ‘forge’ temperature. The
furnace temperature is maintained at around 1200 to 1250 o
C. Forging furnaces use an open
fireplace system and most of the heat is transmitted by radiation. The typical load is 5 to 6 ton
with the furnace operating for 16 to 18 hours daily. The total operating cycle can be divided into
(i) heat-up time (ii) soaking time and (iii) forging time. Specific fuel consumption depends upon
the type of material and number of ‘reheats’ required.
2.1.2 Re-rolling mill furnace
a) Batch type
A box type furnace is used as a batch type re-rolling mill. This furnace is mainly used for heating
up scrap, small ingots and billets weighing 2 to 20 kg for re-rolling. Materials are manually
charged and discharged and the final products are rods, strips etc. The operating temperature is
about 1200 o
C. The total cycle time can be further categorized into heat-up time and re-rolling
time. During heat-up time the material gets heated up-to the required temperature and is removed
manually for re-rolling. The average output from these furnaces varies from 10 to 15 tons / day
and the specific fuel consumption varies from 180 to 280 kg. of coal / ton of heated material.
b) Continuous pusher type
The process flow and operating cycles of a continuous pusher type is the same as that of the
batch furnace. The operating temperature is about 1250 o C. Generally, these furnaces operate 8
to 10 hours with an output of 20 to 25 ton per day. The material or stock recovers a part of the
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heat in flue gases as it moves down the length of the furnace. Heat absorption by the material in
the furnace is slow, steady and uniform throughout the cross-section compared with batch type.
eat in flue gases as it moves down the length of the furnace. Heat absorption by the material in
the furnace is slow, steady and uniform throughout the cross-section compared with batch type.
2.1.3 Continuous reheating furnace2.1.3 Continuous reheating furnace
In continuous reheating, the steel stock forms a continuous flow of material and is heated to the
desired temperature as it travels through the furnace. The temperature of a piece of steel is
typically raised to between 900°C and 1250o
C, until it is soft enough to be pressed or rolled into
the desired size or shape. The furnace must also meet specific stock heating rates for
metallurgical and productivity reasons.
In continuous reheating, the steel stock forms a continuous flow of material and is heated to the
desired temperature as it travels through the furnace. The temperature of a piece of steel is
typically raised to between 900°C and 1250
To ensure that the energy loss is kept to a minimum, the inlet and outlet doors should be minimal
in size and designed to avoid air infiltration. Continuous reheating furnaces can be categorized
by the two methods of transporting stock through the furnace:
To ensure that the energy loss is kept to a minimum, the inlet and outlet doors should be minimal
in size and designed to avoid air infiltration. Continuous reheating furnaces can be categorized
by the two methods of transporting stock through the furnace:
Stock is kept together to form a stream of material that is pushed through the furnace. Such
furnaces are called pusher type furnaces.
Stock is kept together to form a stream of material that is pushed through the furnace. Such
furnaces are called pusher type furnaces.
Stock is placed on a moving hearth or supporting structure which transports the steel through
the furnace. The furnaces include walking beam, walking hearth, continuous recirculating
bogie furnaces, and rotary hearth furnaces.
Stock is placed on a moving hearth or supporting structure which transports the steel through
the furnace. The furnaces include walking beam, walking hearth, continuous recirculating
bogie furnaces, and rotary hearth furnaces.
Table 3 compares the main types of continuous reheating furnaces used in industry.Table 3 compares the main types of continuous reheating furnaces used in industry.
o
C, until it is soft enough to be pressed or rolled into
the desired size or shape. The furnace must also meet specific stock heating rates for
metallurgical and productivity reasons.
Figure 4. Pusher Furnace (The Carbon Trust, 1993)

Table 3. Comparison of Different Continuous Reheating Furnaces (Adapted from The Carbon Trust, 1993 and BEE, 2005)
Type Description Advantages Disadvantages
Pusher
furnace
(Figure 4)
The main features are:
Furnaces may have solid hearth, but in most
cases pushers are used to charge and
discharge stock, that move on “skids” (rails)
with water-cooled supports.
These furnaces typically have a hearth sloping
towards the discharge end of up to 35 meters
divided into five zones in top-fired furnaces.
Firing of furnace by burners located at the
discharge end of the furnace, or at top and/or
bottom to heat stock from both top and/or
bottom
The discharge ends of these furnaces have a
chimney with a recuperator for waste heat
recovery.
Low installation and
maintenance costs (compared
with moving hearth furnaces)
Advantages of top and bottom
firing:
Faster heating of stock
Lower temperature
differences within stock
Reduced stock residence
time
Shorter furnace lengths
(compared to solid hearth
furnaces)
Water cooling energy losses from the
skids and stock supporting structure
in top and bottom fired furnaces
Discharge must be accompanied by
charge
Stock sizes/weights and furnace
length are limited by friction and
possibility of stock pile-ups
Furnace needs facilities to be
completely emptied
Quality reduction by (a) physical
marking by skids or ‘skid marks’ (b)
temperature differences along the
stock length caused by the water
cooled supports in top and bottom
fired furnaces
Walking
beam
furnace
(Figure 5)
These furnaces operate as follows:
Stock is placed on stationary ridges
Walking beams are raised from the bottom to
raise the stock
Walking beams with the stock move forwards
Walking beams are lowered at end of the
furnace to place stock on stationary ridges
Stock is removed from furnace and walking
beams return to furnace entrance
Initially temperatures were limited 1000 0
C but
new models are able to reach 1100 0
C
Overcomes many of the
problems of pusher furnaces
(skid marks, stock pile-ups,
charge/discharge)
Possible to heat bottom face
of the stock resulting in
shorter stock heating times
and furnace lengths and thus
better control of heating
rates, uniform stock
discharge temperatures and
operational flexibility
High energy loss through water
cooling (compared with walking
hearth furnaces)
Much of the furnace is below the
level of the mill; this may be a
constraint in some applications
Sometimes when operating
mechanism of beam make it
necessary to fire from the sides, this
results in non-uniform heating of the
stock
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Type Description Advantages Disadvantages
Walking
hearth
furnace
(Figure 6)
These furnaces are designed so that the stock rests
on fixed refractory blocks, which are extended
through openings in the hearth. The stock is
transported towards the discharge end in discrete
steps by “walking the hearth”, similar to walking
beam furnaces
Simplicity of design
Ease of construction
Ability to cater for different
stock sizes (within limits)
Negligible water cooling
energy losses
Can be emptied
Minimal physical marking of
the stock
Temperatures across the stock are
not uniform because the bottom of
stock cannot be heated and small
spaces between the stock limits
heating of the sides. Large spaces
between stocks can partially alleviate
this. But this increases stock
residence time to up to several hours,
which affects furnace flexibility and
yield
Continuous
recirculating
bogie
furnace
(Figure 7)
The furnace has the shape of a long and narrow
tunnel with rails inside and works as follows:
Stock is placed on a bogie (cart with wheels)
with a refractory hearth
Several bogies move like a train over the
entire furnace length through the furnace
Stock is removed at the discharge end and the
bogie returns to the charge end of the furnace
Suitable for compact stock of
variable size and geometry
The stock in the bogie has to undergo
a cycle of heating and cooling then
again heating
Heat storage loss through heating
and cooling of the bogies
Inadequate sealing of the gap
between the bogies and furnace shell,
difficulties in removing scale, and
difficulties in firing across a narrow
hearth width caused by the narrow
and long furnace shape
Rotary
hearth
furnace
(Figure 8)
More recent developed furnace type that is
overtaking the bogie furnace. The walls and the
roof of the furnace remains stationery while the
hearth moves in a circle on rollers, carrying the
stock. Heated gas moves in opposite direction of
the hearth and flue gases are discharged near the
charging door. The temperature can reach 1300 o
C
Suitable for stock of variable
size and geometry
Reduced heat storage loss
compared to bogie furnace
More complex design with an
annular shape and revolving hearth
Possible logistical problems in layout
of some rolling mills and forges
because of close location of charge
and discharge positions

Figure 5. Walking Beam Furnace (The Carbon Trust 1993)
Figure 6. Walking Hearth Furnace (The Carbon Trust, 1993)
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Figure 7. Continuous Re-circulating Bogie Furnace (The Carbon Trust, 1993)
Figure 8. Rotary Hearth Furnace (The Carbon Trust, 1993)
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2.2 Types of refractories
Refractories can be classified on the basis of chemical composition, end use and methods of
manufacture as shown below.
Table 4. Classification of refractories based on chemical composition (Adapted from
Gilchrist)
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, Magnesite-chromite,
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)
2.2.1 Fireclay refractories
Firebrick is the most common form of refractory material. It is used extensively in the iron and
steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many
others.
Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories
consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3
content of up to 44 percent. Table 5 shows that the melting point (PCE) of fireclay brick
decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces,
kilns and stoves because the materials are widely available and relatively inexpensive.
Table 5. Properties of typical fireclay bricks (BEE, 2005)
Brick type Percentage
SiO2
Percentage
Al2O3
Percentage other
constituents
PCE o
C
Super Duty 49-53 40-44 5-7 1745-1760
High Duty 50-80 35-40 5-9 1690-1745
Intermediate 60-70 26-36 5-9 1640-1680
High Duty (Siliceous) 65-80 18-30 3-8 1620-1680
Low Duty 60-70 23-33 6-10 1520-1595
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2.2.2 High alumina refractories
Alumina silicate refractories containing more than 45 percent alumina are generally termed as
high alumina materials. The alumina concentration ranges from 45 to 100 percent. The
refractoriness of high alumina refractories increases with increase in alumina percentage. The
applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic
kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
2.2.3 Silica brick
Silica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw material is
quality rocks. Various grades of silica brick have found extensive use in the iron and steel
melting furnaces and the glass industry. In addition to high fusion point multi-type refractories,
other important properties are their high resistance to thermal shock (spalling) and their high
refractoriness. The outstanding property of silica brick is that it does not begin to soften under
high loads until its fusion point is approached. This behavior contrasts with that of many other
refractories, for example alumina silicate materials, which begin to fuse and creep at
temperatures considerably lower than their fusion points. Other advantages are flux and stag
resistance, volume stability and high spalling resistance.
2.2.4 Magnesite
Magnesite refractories are chemically basic materials, containing at least 85 percent magnesium
oxide. They are made from naturally occurring magnesite (MgCO3). The properties of magnesite
refractories depend on the concentration of silicate bond at the operating temperatures. Good
quality magnesite usually results from a CaO-SiO2 ratio of less than two with a minimum ferrite
concentration, particularly if the furnaces lined with the refractory operate in oxidizing and
reducing conditions. The slag resistance is very high particularly to lime and iron rich slags.
2.2.5 Chromite refractories
Two types of chromite refractories are distinguished:
Chrome-magnesite refractories, which usually contain 15-35 percent Cr2O3 and 42-50
percent MgO. They are made in a wide range of qualities and are used for building the
critical parts of high temperature furnaces. These materials can withstand corrosive slags and
gases and have high refractoriness.
Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent
Cr2O3. They are suitable for service at the highest temperatures and for contact with the most
basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance
than chrome-magnesite.
2.2.6 Zirconia refractories
Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before
application as a refractory, which is achieved by incorporating small quantities of calcium,
magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization,
quantity of stabilizer and quality of the original raw material. Zirconia refractories have a very
high strength at room temperature, which is maintained up to temperatures as high as 15000
C.
They are therefore useful as high temperature construction materials in furnaces and kilns. The
thermal conductivity of zirconium dioxide is much lower than that of most other refractories and
the material is therefore used as a high temperature insulating refractory. Zirconia exhibits very
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low thermal losses and does not react readily with liquid metals, and is particularly useful for
making refractory crucibles and other vessels for metallurgical purposes. Glass furnaces use
zirconia because it is not easily wetted by molten glasses and does not react easily with glass.
2.2.7 Oxide refractories (Alumina)
Alumina refractory materials that consist of aluminium oxide with little traces of impurities are
known as pure alumina. Alumina is one of the most chemically stable oxides known. It is
mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and
alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate,
sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing
atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is
used for lining furnaces operating up to 1850o
C.
2.2.8 Monolithics
Monolithic refractories are single piece casts in the shape of equipment, such as a ladle as shown
in Figure 9. They are rapidly replacing the conventional type fired refractories in many
applications including industrial furnaces. The main advantages of monolithics are:
Elimination of joints which is an inherent weakness
Faster application method
Special skill for installation not required
Ease of transportation and handling
Better scope to reduce downtime for repairs
Considerable scope to reduce inventory and eliminate special shapes
Heat savings
Better spalling resistance
Greater volume stability
Monolithics are put into place using various methods, such as ramming, casting, gunniting,
spraying, and sand slinging. Ramming requires proper tools and is mostly used in cold
applications where proper consolidation of the material is important. Ramming is also used for
air setting and heat setting materials. Because calcium aluminate cement is the binder, it will
have to be stored properly to prevent moisture absorption. Its strength starts deteriorating after 6
to 12 months.
Figure 9. A Monolithic Lining for Ladel
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2.3 Insulating materials
Insulating materials greatly reduce the heat losses through walls. Insulation is achieved by
providing a layer of material with low heat conductivity between the internal hot surface of a
furnace and the external surface, thus keeping the temperature of the external surface low.
Insulating materials may be classified into the following groups:
Insulating bricks
Insulating castables
Ceramic fiber
Calcium silicate
Ceramic coating
Insulating materials owe their low conductivity to their pores while their heat capacity depends
on the bulk density and specific heat. Air insulating materials consist of minute pores filled with
air, which have a very low thermal conductivity. Excessive heat affects all insulation material
adversely, but at what temperatures this takes place varies widely. Therefore the choice of an
insulating material must be based on its ability to resist heat conductivity and on the highest
temperature it will withstand. One of the most widely used insulating materials is diatomite, also
known as kiesel guhr, which consists of a mass of skeletons of minute aquatic plants deposited
thousands of years ago on the beds of seas and lakes. Its chemical composition is silica
contaminated with clay and organic matter. A wide range of insulating refractories with wide
combinations of properties is now available. Table 6 shows important physical properties of
some insulating refractories.
Table 6. Physical properties of insulating materials (BEE, 2005)
Type Thermal
conductivity at
400o
C
Max. safe
temperature (o
C)
Cold crushing
strength
(kg/cm2
)
Porosity
percent
Bulk
density
(kg/m3
)
Diatomite Solid
Grade
0.025 1000 270 52 1090
Diatomite
Porous Grade
0.014 800 110 77 540
Clay 0.030 1500 260 68 560
High Alumina 0.028 1500-1600 300 66 910
Silica 0.040 1400 400 65 830
2.3.1 Castables and concretes
Monolithic linings of furnace sections can be constructed by casting refractory insulating
concretes, and stamping lightweight aggregates into place that are suitably bonded. Other
applications include the bases of tunnel kiln cars used in the ceramic industry. The ingredients
are similar to those insulation materials used for making piece refractories, except that concretes
contain either Portland or high-alumina cement.
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2.3.2 Ceramic fiber
Ceramic fiber is a low thermal mass insulation material, which has revolutionized furnace design
lining systems. Ceramic fiber is manufactured by blending and melting alumina and silica at a
temperature of 1800 – 2000o
C, and breaking the molten stream by blowing compressed air or
dropping the molten stream on a spinning disc to form loose or bulk ceramic fiber. The bulk
fiber is used to produce various insulation products including blankets, strips, veneering and
anchored modules, paper, vacuum formed boards and shapes, ropes, wet felt, mastic cement etc.
Fibers are usually produced in two temperature grades based on Al2O3 content. A new product is
ZrO2 added alumino-silicate fiber, which helps to reduce shrinkage levels and thereby making
the fiber suitable for higher temperatures. Continuous recommended operating temperature for
fibers are given in the Table 7.
Table 7. Continuous recommended operating temperature for fibers (BEE, 2005)
Al2O3 SiO2 ZrO2
1150o
C 43 – 47 percent 53 – 57 percent -
1250o
C 52 – 56 percent 44 – 48 percent -
1325o
C 33 – 35 percent 47 – 50 percent 17 – 20 percent
Ceramic fibers are generally produced in bulk wool form and needled into a blanket mass of
various densities ranging from 64 to 190 kg/m3
. Converted products and over 40 different forms
are made from blankets to suit various requirements.
The characteristics of ceramic fibers are a remarkable combination of the properties of
refractories and traditional insulation material.
a) Lower thermal conductivity
Because of the low thermal conductivity (0.1 kCal/m per hour per o
C at 600 o
C for a blanket with
128 kg/m3
density) it is possible to construct thinner linings with the same thermal efficiency as
conventional refractories. As a result of thinner lining, the furnace volume is higher. It is 40
percent more effective than good quality insulation brick and 2.5 times better than asbestos.
Ceramic fiber is a better insulator than calcium silicate.
b) Light weight
The average density of ceramic fiber is 96 kg/m3
. It is one tenth of the weight of insulating brick
and one third of the weight of asbestos / calcium silicate boards. For new furnaces structural
supports can be reduced by 40 percent.
c) Lower heat storage
Ceramic fiber linings absorb less heat because of their lower density. Furnaces can therefore be
heated and cooled at faster rates. Typically the heat stored in a ceramic fiber lining system is in
the range of 2700 - 4050 kCal/m2
(1000 – 1500 Btu/Ft2
) as compared to 54200-493900 kCal/m2
(20000 – 250000 Btu/Ft2
) for conventionally lined systems.
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d) Thermal shock resistant
Ceramic fiber linings resist thermal shock due to their resilient matrix. This also allows for
faster heat up and cool down cycles, thereby improving furnace availability and productivity.
e) Chemical resistance
Ceramic fiber resist most of the chemical attack and is unaffected by hydrocarbons, water and
steam present in flue gases.
f) Mechanical resilience
The high mechanical resilience of ceramic fiber makes it possible to manufacture fiber-lined
furnaces off-site, transport them to the site in assembled form without the risk of damage.
g) Low installation cost
As the application of ceramic fibers is a standardized process, no special skills are required.
Fiber linings require no dry out or curing times and there is no risk of cracking or spalling when
they are heated after installation.
h) Ease of maintenance
In case of physical damage, the section of damaged ceramic fiber can be quickly removed and
replaced with a new piece. Entire panel sections can be prefabricated for fast installation with
minimal down time.
i) Ease of handling
All product forms are easily handled and most can be quickly cut with a knife or scissors.
Vacuum formed products may require cutting with a band saw.
j) Thermal efficiency
Thermal efficiency of a furnace lined with ceramic fiber is improved in two ways. First, the low
thermal conductivity of ceramic fiber allows the lining to be thinner and therefore the furnace
can be smaller. Second, the fast response of ceramic fiber to temperature changes also allows for
more accurate control and uniform temperature distribution within the furnace.
Other advantages offered by ceramic fiber are:
Lightweight furnace
Simple steel fabrication work
Low down time
Increased productivity
Additional capacity
Low maintenance cost
Longer service life
Higher thermal efficiency
Faster response
2.3.3 High emissivity coatings
Emissivity (i.e. the measure of a material’s ability to both absorb and radiate heat) is often
considered as an inherent physical property that does not normally change (other examples are
density, specific heat and thermal conductivity). However, the development of high emissivity
17

coatings allows the surface emissivity of materials to be increased. High emissivity coatings are
applied on the interior surface of furnaces. Figure 10 shows that the emissivity of various
insulating materials reduces with increasing process temperatures. The advantage of high
emissivity coatings is that the emissivity remains more or less constant.
The emissivity of furnaces that operate at high temperatures is 0.3. By using high emissivity
coatings this can go up to 0.8, resulting in an increase of heat transfer through radiation.
Other benefits of high emissivity coatings in furnace chambers are uniform heating and extended
life of refractories and metallic components such as radiant tubes and heating elements. For
intermittent furnaces or where rapid heating is required, use of such coatings was found to reduce
fuel or power by 25 - 45 percent.
Figure 10. Emissivity of Refractory Materials at Different
Temperatures (BEE, 2005)
3. ASSESSMENT OF FURNACES
This section describes the various methods and techniques used to quantify the losses from the
furnace and the methods to carry out performance assessment of typical furnaces.
3.1 Heat losses affecting furnace performance
Ideally, all heat added to the furnaces should be used to heat the load or stock. In practice,
however, a lot of heat is lost in several ways as shown in Figure 11.
18

FURNACE
Fluegas
Moistureinfuel
Openingsinfurnace
Furnacesurface/skin
Otherlosses
Heat input
Heat in stock
Hydrogeninfuel
FURNACE
Fluegas
Moistureinfuel
Openingsinfurnace
Furnacesurface/skin
Otherlosses
Heat input
Heat in stock
Hydrogeninfuel
Figure 11. Heat Losses in a Furnace
These furnace heat losses include (BEE, 2005 and US DOE, 2004):
Flue gas losses: part of the heat remains in the combustion gases inside the furnace. This
loss is also called waste-gas loss or stack loss.
Loss from moisture in fuel: fuel usually contains some moisture and some of the heat is used
to evaporate the moisture inside the furnace
Loss due to hydrogen in fuel which results in the formation of water
Loss through openings in the furnace: radiation loss occurs when there are openings in the
furnace enclosure and these losses can be significant, especially for furnaces operating at
temperatures above 540°C. A second loss is through air infiltration because the draft of
furnace stacks/chimneys cause a negative pressure inside the furnace, drawing in air through
leaks or cracks or when ever the furnace doors are opened.
Furnace skin / surface losses, also called wall losses: while temperatures inside the furnace
are high, heat is conducted through the roof, floor and walls and emitted to the ambient air
once it reaches the furnace skin or surface.
Other losses: there are several other ways in which heat is lost from a furnace, although
quantifying these is often difficult. Some of these include
− Stored heat losses: when the furnace is started the furnace structure and insulation is also
heated, and this heat only leaves the structure again when the furnace shuts down.
Therefore this type of heat loss increases with the number of times the furnace is turned
on and off
− Material handling losses: the equipment used to move the stock through the furnace, such
as conveyor belts, walking beams, bogies etc, also absorb heat. Every time equipment
leave the furnace they loose their heat, therefore heat loss increases with the amount of
equipment and the frequency by which they enter and leave the furnace
19

− Cooling media losses: water and air are used to cool down equipment, rolls, bearing and
rolls, but heat is lost because these media absorb heat
− Incomplete combustion losses: heat is lost if combustion is incomplete because unburnt
fuel or particles have absorbed heat but this heat has not been put to use
− Loss due to formation of scales
3.2 Instruments to assess furnace performance
Furnace efficiency is calculated after subtracting the various heat losses. In order to find out
furnace efficiency using the indirect method, various parameters must be measured, such as
hourly furnace oil consumption, material output, excess air quantity, temperature of flue gas,
temperature of furnace at various zones, and others. Date for some of these parameters can be
obtained from production records while others must be measured with special monitoring
instruments. Table 8 lists the instruments that are needed to measure these parameters.
Table 8. Instruments for Measuring Furnace Performance Parameter (BEE, 2005)
Parameters
to be measured
Location of
measurement
Instrument
required
Required
Value
1200-1300o
CFurnace soaking zone
temperature (reheating
furnaces)
Soaking zone and side
wall
Pt/Pt-Rh thermocouple
with indicator and
recorder
700o
C max.Flue gas temperature In duct near the discharge
end, and entry to
recuperator
Chromel Alummel
Thermocouple with
indicator
300o
C (max)Flue gas temperature After recuperator Hg in steel thermometer
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
-
3.3 Calculating furnace performance
A furnace’s efficiency increases when the percentage of heat that is transferred to the stock or
load inside the furnace increases. The efficiency of the furnace can be calculated in two ways,
similar to that of the boiler: direct method and indirect method. Both methods are explained
below.
20

3.3.1 Direct method
The efficiency of a furnace can be determined by measuring the amount heat absorbed by the
stock and dividing this by the total amount of fuel consumed.
Heat in the stock
Thermal efficiency of the furnace =
Heat in the fuel consumed for heating the stock
The quantity of heat (Q) that will be transferred to stock can be calculated with this equation:
Q = m x Cp (t1 – t2)
Where, Q = Quantity of heat of stock in kCal
m = Weight of the stock in kg
Cp= Mean specific heat of stock in kCal/kg o
C
t1 = Final temperature of stock in o
C
t2 = Initial temperature of the stock before it enters the furnace in o
C
An example calculation is given in section 3.3.3.
3.3.2 Indirect method
The furnace efficiency can also be determined through the indirect method, similar to the
evaluation of boiler efficiency. The principle is simple: the heat losses are substracted from the
heat supplied to the furnace. Different types of heat losses are illustrated in Figure 11. Typical
thermal efficiencies for common industrial furnaces are given in the Table 9.
Table 9. Thermal Efficiencies for Common Industrial Furnaces (BEE 2005)
Furnace type Typical thermal efficiencies (percent)
1) Low Temperature furnaces
a. 540 – 980 o
C (Batch type) 20-30
b. 540 – 980 o
C (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 o
C –370 o
C) 35-40
b. Direct fired ovens (20 o
C –370 o
C) 35-40
21

An example calculation using the indirect method is given in the next section.
3.3.3 Example calculation of furnace efficiency
Calculate the efficiency of an oil-fired reheating furnace with the direct and indirect method
using the data below.
Operating temperature: 1340o
C
Exit flue gas temperature after preheater: 750o
C
Ambient temperature: 40o
C
Preheated air temperature: 190o
C
Specific gravity of fuel oil: 0.92
Average fuel oil consumption: 400 liters / hr = 400 x 0.92 =368 kg/hr
Calorific value of oil 10000 kCal/kg
Average O2 percentage in flue gas: 12 percent
Moisture in 1 kg of fuel oil: 0.15 kg
H2 in 1 kg of fuel oil: 0.1123 kg
Theoretical air required to burn 1 kg of oil: 14 kg
Weight of stock: 6000 kg/hr
Specific heat of billet: 0.12 kCal/kg/0
C
Furnace wall thickness (D): 460 mm
Billet extraction outlet (X): 1 m x 1 m
Average surface temperature
of heating + soaking zone: 122 o
C
Average surface temperature of area
other than heating and soaking zone: 80 o
C
Area of heating + soaking zone: 70.18 m2
Area other than heating and soaking zone: 12.6 m2
Direct method calculation
The heat input is 400 liters per hour. The specific gravity of fuel is used to convert this into kg.
Therefore: 400 l/hr x 0.92 kg/l = 368 kg/hr
The heat output is calculated as follows:
= m x Cp x ΔT
= 6000 kg x 0.12 x (1340 – 40)
= 936000 kCal
The efficiency is:
= (heat input / heat output) x 100
= [(936000 / (368 x 10000)] x 100 = 25.43 percent
The approximate heat loss is 100% – 25% = 75%
22

The different heat losses are calculated below.
a) Heat loss in flue gas
Excess air (EA)
= O2 percent / (21 – O2 percent)
= 12 / (21 – 12)
= 133 %
Mass of air supplied
= (1 + EA/100) x Theoretical air
= (1+ 1.13) x 14
= 32.62 kg/kg fuel oil
m x Cp x ΔT x 100
% Heat loss in flue gas =
GCV of fuel
Where,
m = weight of flue gas (air + fuel) = 32.62 + 1.0 = 33.62 kg/kg oil
Cp = specific heat
ΔT = temperature difference
% Heat loss = {33.62 x 0.24 x (750 – 40)} x 100 = 57.29%
10000
b) Heat loss from moisture in fuel
M x {584 + Cp (Tf – Tamb)} x 100
% Heat loss from moisture in fuel =
GCV of fuel
Where,
M = kg of moisture in 1 kg of fuel oil
Tfg = Flue gas temperature, 0
C
Tamb = Ambient temperature, 0
C
GCV = Gross Calorific Value of fuel, kCal/kg
% Heat loss = 0.15 x {584 + 0.45 (750 – 40)} x 100 = 1.36%
10000
23
Indirect method

c) Loss due to hydrogen in fuel
9 x H2 x {584 + Cp (Tf – Tamb)} x 100
% Heat loss due to hydrogen in fuel =
GCV of fuel
Where,
H2 = kg of H2 in 1 kg of fuel oil (= 0.1123 kg/kg of fuel oil)
% Heat loss = 9 x 0.1123 x {584 + 0.45 (750 – 40)} x 100 = 9.13%
10000
d) Heat loss due to openings in furnace
(Black body radiation factor x emissivity x
factor of radiation x area of opening) x 100
% Heat loss from openings in
furnace =
Quantity of oil x GCV of oil
The factor of radiation through openings and the black body radiation factor can be obtained
from standard graphs as shown in Figure 12 and Figure 13.
Factor of radiation (refer Figure 12) = 0.71
Black body radiation at 1340 0
C (refer Figure 13) = 36 kCal/kg/cm2/hr
The area of the opening is 100 cm x 100 cm = 10000 cm2
Emissivity = 0.8
% Heat loss from furnace openings = 36 x 0.8 x 0.71 x 10000 x 100 = 5.56%
368 x 10000
24

Figure 12. Radiation Factor for Heat Release through Openings relative to
the Quality of Heat Release from Perfect Black Body (BEE, 2005)
Figure 13. Black Body Radiation at Different Temperatures (BEE, 2005)
25

e) Heat loss through furnace skin
To determine the heat loss through the furnace skin, first the heat loss through the roof and
sidewalls and through other areas must be calculated separately.
i). Heat loss through roof/ceiling and sidewalls (= heating and soaking zone):
Total average surface temperature = 122o
C
Heat loss at 122o
C (Refer Figure 14) = 1252 kCal /m2
hr
Total area of heating + soaking zone = 70.18 m2
Heat loss from roof and walls
Heat loss through furnace roof =
Area of roof and walls
Total heat loss = 1252 kCal / m2
hr x 70.18 m2
= 87865 kCal/hr
ii) Heat lost from area other than heating and soaking zone
Total average surface temperature = 80 oC
Heat loss at 80o
C (Refer Figure 14) = 740 kCal / m2
hr
Total area = 12.6 m2
Heat loss from roof other areas
Heat loss through other areas =
Area of other areas
Total heat loss = 740 kCal / m2
hr x 12.6 m2
= 9324 kCal/hr
(Heat loss i + heat loss ii) x 100
% Heat loss through furnace skin =
GCV of oil x Quantity of oil per hour
% Heat loss through furnace skin = (87865 kCal/hr + 9324 kCal/hr) x 100 = 2.64%
10000 kCal/kg x 368 kg/hr
f) Unaccounted losses
The unaccounted losses cannot be calculated unless the other types of losses are known.
Furnace efficiency
Adding the losses a to f up gives the total losses:
a) Flue gas loss = 57.29 %
26

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 %
The furnace efficiency calculated through the indirect method = 100 – 75.98 = 24.02%
Figure 14. Heat Loss from the Ceiling, Sidewall and Hearth of Furnace (BEE, 2005)
4. ENERGY EFFICIENCY OPPORTUNITES
This section explains the various energy saving opportunities in furnaces.6
Typical energy
efficiency measures for an industry with furnace are:
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. Minimum refractory losses
9. Use of ceramic coatings
10. Selecting the right refractories
27

4.1 Complete combustion with minimum excess air
The amount of heat lost in the flue gases (stack losses) depends on the amount of excess air. To
obtain complete combustion of fuel with the minimum amount of air, it is necessary to control
air infiltration, maintain pressure of combustion air, fuel quality and monitor the amount excess
air. Too much excess air will reduce flame temperature, furnace temperature and heating rate.
Too little excess air will result in an increase in unburnt components in flue gases that are carried
away through the stack and it also causes more scale losses.
Optimizing combustion air is the most attractive and economical measure for energy
conservation. Potential savings are higher when the temperature of furnace is high. The air ratio
(= actual air amount / theoretical combustion air amount) gives an indication of excess air air. If
a reheating furnace is not equipped with an automatic air/fuel ratio controller, it is necessary to
periodically take a sample of gas in the furnace and measure its oxygen contents with a gas
analyzer.
4.2 Proper heat distribution
A furnace should be designed to ensure that within a given time the stock is heated uniformly to
a desired temperature with the minimum amount of fuel.
Where burners are used to fire the furnace, the following should be ensured for proper heat
distribution:
The flame should not touch or be obstructed by any solid object. Obstruction causes the fuel
particles to de-atomize, which affects combustion and causes black smoke. If the flame
impinges on the stock scale losses will increase. If the flame impinges on refractories,
products from incomplete combustion can settle and react with the refractory constituents at
high temperatures.
The flames of different burners should stay clear of each other, as intersecting flames cause
incomplete combustion. It is also desirable to stagger burners on opposite sides.
The burner flame has a tendency to travel freely in the combustion space just above the
material. For this reason, the axis of the burner in small furnaces is never placed parallel to
the hearth but always at an upward angle, but the flame should not hit the roof.
Large burners produce longer flames, which may be difficult to contain within the furnace
walls. More burners of less capacity ensure a better heat distribution inside the furnace and
also increase the furnace life.
In small furnaces that use furnace oil, a burner with a long flame with a golden yellow color
improves uniform heating. But the flame should not be too long, because heat is lost of the
flame reaches the chimney or the furnace doors.
4.3. Operation at the optimum furnace temperature
It is important to operate the furnace at its optimum temperature. Operating temperatures of
various furnaces are given in Table 10. Operating at too high temperatures causes heat loss,
excessive oxidation, de-carbonization and stress on refractories. Automatic control of the furnace
temperature is preferred to avoid human error.
28

Table 10. Operating Temperatures of Various Furnaces
Slab Reheating furnaces 1200o
C
1200o
CRolling Mill furnaces
800o
CBar furnace for Sheet Mill
650o
C –750o
CBogie type annealing furnaces
4.4. Prevent heat loss through openings
Heat can be lost by direct radiation through openings in the furnace, such as the charging inless,
extracting outlet and the peephole in the wall or ceiling. Heat is also lost due to pressure
differences between the inside of the furnace and the ambient environment causing combustion
gases to leak through the openings. But most heat is lost if outside air infiltrates into the furnace,
because in addition to heat loss this also causes uneven temperatures inside the furnace and stock
and can even lead to oxidization of billets.
It is therefore important to keep the openings as small as possible and to seal them. Another
effective way to reduce the heat loss through furnace openings is by opening the furnace doors
less frequent and for the shortest time period as possible (another option is described under item
4.5). This heat loss is about 1 percent of the total quantity of heat generated in the furnace, if
furnace pressure is controlled properly.
Section 3.3.3 already explained one way of calculating heat loss through openings. But an
alternative way is calculating heat loss with the following equation:
Where,
Q = heat loss
T = absolute temperature (K)
a = factor for total radiation
A = area of opening, m2
H = time (hours)
For example, a reheating furnace with a temperature of 1340 oC, the wall thickness is 460 mm
(X) and the door is 1 m high (D) by 1 m wide. D/X = 1/0.460 = 0.71, and in Figure 12 this
corresponds with a factor for total radiation of 0.71. The heat loss from openings in therefore:
29

4.5. Control of furnace draft
If negative pressures exist inside the furnace, air can infiltrate through cracks and openings and
affect the air-fuel ratio control. This in turn can cause metal to not reach the desired temperature
or non-uniform temperatures, which affects the next processes like forging and rolling. Fuel
consumption and product rejection rates increase. Tests conducted on seemingly airtight furnaces
have shown air infiltration up to 40 percent. To avoid this, slight positive pressure should be
maintained inside the furnace (in addition to the measures mentioned under 4.4).
But the pressure difference should not be too high because this will cause ex-filtration. While
this is less of a problem than infiltration, it can still result in flames reaching out of the furnace,
overheating of refractories leading to reduced brick life, increased furnace maintenance, and
burnout of ducts and equipment.
Proper management of the pressure difference between the inside and outside of the furnace is
therefore important to minimize heat loss and adverse impacts on products.
4.6. Optimum capacity utilization
One of the most vital factors affecting the furnace efficiency is the load. This includes the
amount of material placed in the furnace, the arrangement inside the furnace and the residence
time inside the furnace.
a) Optimum load
If the furnace is under loaded the proportion of total heat available that will be taken up by the
load is smaller, resulting in a lower efficiency. Overloading can lead to the load not heated to the
right temperature within a given period of time.
There is a particular load at which the furnace will operate at maximum thermal efficiency, i.e.
where the amount of fuel per kg of material is lowest. This load is generally obtained by
recording the weight of material of each charge, the time it takes to reach the right temperature,
and the amount of fuel used. The furnace should be loaded to the optimum load at all times,
although in practice this may not always be possible.
b) Optimum arrangement of the load
The loading of materials on the furnace hearth should be arranged so that
It receives the maximum amount of radiation from the hot surfaces of the heating chambers
and flames
Hot gases are efficiently circulated around the heat receiving surfaces of the materials
Stock is not placed in the following position:
− In the direct path of the burners or where flame impingement is likely to occur
− In an area that is likely to cause a blockage or restriction of the flue system of the furnace
− Close to any door openings where cold spots are likely to develop
30

c) Optimum residence time of the load
Fuel consumption is kept at a minimum and product quality is best if the load only remains
inside the furnace until it has the required physical and metallurgical properties.
Sometimes the charge and production schedule does not correspond with the capacity of the
furnace. If this is the case, either the
Load is higher or lower than the optimum load
Residence time is longer or shorter than the ideal residence time. Excessive residence time
will increase oxidation of the material surface, which can result in rejection of products. The
rate of oxidation is dependent upon time, temperature, as well as free oxygen content
Temperature is increased to make up for shorter residence time. The higher the working
temperature, the higher is the loss per unit of time.
All three result in fuel wastage and sometimes in reduced product quality. Coordination between
the furnace operator, production and planning personnel is therefore essential.
Optimum utilization of furnace can be planned at design stage, by selecting the size and type
(batch, continuous) that best matches the production schedule.
The overall efficiency of a continuous type furnace will increase with heat recuperation from the
waste gas stream. If only batch type furnace is used, careful planning of the loads is important.
Furnace should be recharged as soon as possible to enable use of residual furnace heat.
4.7. Waste heat recovery from furnace flue gases
In any industrial furnace the combustion products leave the furnace at a temperature higher than
the stock temperature. Flue gases carry 35 to 55 percent of the heat input to the furnace with
them through the chimney. The higher the amount of excess air and flue gas temperature, the
higher the amount of waste heat that is available. However, the primary objective should be to
minimize the amount of waste heat generated through energy conservation measures. Waste heat
recovery should only be considered when further energy conservation is not possible or practical.
Waste heat in flue gases can be recovered for preheating of the charge (stock, load), preheating
of combustion air or for other processes as described below.
a) Charge pre-heating
When raw materials are preheated by exhaust gases before being placed in a heating furnace, the
amount of fuel necessary to heat them in the furnace is reduced. Since raw materials are usually
at room temperature, they can be heated sufficiently using high-temperature flue gases to
noticeably reduce the fuel consumption rate.
b) Preheating of combustion air
For a long time, fuel gases were only use for preheating of combustion air for large boilers,
metal-heating furnaces and high-temperature kilns. But preheating using heat from flue gases is
now also applied to compact boilers and compact industrial furnaces.
31

A variety of equipment is available to recover waste heat. External recuperators are most
common, but other techniques are also used, such as self-recuperative burners. For example, a
modern recuperator use furnace exhaust gas of 1000°C can preheat the combustion air to over
500 o
C, which results in energy savings of up to 30 percent compared with using cold
combustion air entering the furnace.
Since the volume of combustion air increases when it is preheated, it is necessary to consider this
when modifying air-duct diameters and blowers. It should be noted that preheating of
combustion gases from high-density oils with a high sulphur content, could cause clogging with
dust or sulphides, corrosion or increases in nitrogen oxides.
c) Utilizing waste heat as a heat source for other processes
Other process (to generate steam or hot water by a waste heat boiler)
The temperature of furnace exhaust gas can be as high as 400- 600 °C, even after heat has been
recovered from it for preheating the charge or combustion air. One possibility is to install a waste
heat boiler to produce steam or hot water from this heat, especially when large quantities steam
or hot water are needed in a plant. Sometimes exhaust gas heat can be used for heating purposes
in other equipment, but only if the heat quantity, temperature range, operation time etc are
suitable for this. Fuel consumption can be greatly reduced. One existing example is the use of
exhaust gas from a quenching furnace as a heat source in a tempering furnace.
4.8. Minimizing furnace skin losses
About 30 to 40 percent of the fuel used in intermittent or continuous furnaces is used to make up
for heat lost through the furnace skin/surface or walls. The extent of wall losses depend on:
Emissivity of wall
Thermal conductivity of refractories
Wall thickness
Whether the furnace is operated continuously or intermittently
There are several ways to minimize heat loss through the furnace skin:
Choosing the appropriate refractory materials
Increasing the wall thickness
Installing insulating bricks. Outside wall temperatures and heat losses of a composite wall
are much lower for a wall of firebrick and insulation brick compared to a wall of the same
thickness that consists only of refractory bricks. The reason is that insulating bricks have a
much lower conductivity.
Planning operating times of furnaces. For most small furnaces, the operating periods
alternate with the idle periods. When the furnaces are turn off, heat that was absorbed by the
refractories during operation gradually dissipates through radiation and convection from the
cold face and through air flowing through the furnace. When the furnace is turned on again,
additional fuel is needed to heat up the refractories again. If a furnace is operated
continuously for 24 hours in three days, practically all the heat stored in the refractories is
lost. But if the furnace is operated 8 hours per day all the heat stored in the refractories is not
dissipated. For a furnace with a firebrick wall of 350 mm thickness, it is estimated that during
32

the 16 hours that the furnace is turned off, only 55 percent of the heat stored in the
refractories is dissipated from the cold surface. Careful planning of the furnace operation
schedule can therefore reduce heat loss and save fuel.
The quantity (Q) of heat loss from the furnace skin is the sum of natural convection and thermal
radiation. In addition to the method explain in section 3.3.3, the following equation can also be
used:
Where,
Q = Quantity of heat released (kCal/hr)
a = factor regarding direction of the surface of natural convection ceiling = 2.8,
side walls = 2.2, hearth = 1.5
tl = temperature of external wall surface of the furnace (°C), based on the average of as
many measurements as possible to reduce the error margin
t2 = temperature of air around the furnace (°C)
E = emissivity of external wall surface of the furnace
The first part of the equation gives the heat loss though natural convection, and the second part
the heat loss through radiation. Figure 14 shows the relation between the temperature of external
wall surface and the quantity of heat release calculated with this formula.
An example calculation of the heat loss from a furnace’s surface is as follows:
A reheating furnace has a ceiling, sidewalls and hearth with a 20 m2
, 50 m2
and 20 m2
surface
area respectively. Their average measured surface temperatures 80°C, 90°C and 100°C
respectively. Based on Figure 14, the quantities of heat release from ceiling, sidewalls and hearth
per unit area are respectively 650 kCal/m2
h, 720 kCal/m2
h and 730 kCal/m2
h.
Therefore, the total quantity of heat release Q
= loss through ceiling + loss through sidewalls + loss through hearth
= (650 x 20) + (720 x 50) + (730 x 20)
= 13000 + 36000 +14600= 63,600 kCal/hr
33

Figure 15. Relationship between Surface Temperature and
Quantity of Heat Loss (BEE, 2005)
4.9 Use of ceramic coatings (high emissivity coatings)
Ceramic coatings in the furnace chamber promote rapid and efficient transfer of heat, uniform
heating and extended life of refractories. The emissivity of conventional refractories decreases
with increase in temperature whereas for ceramic coatings it increases slightly. This outstanding
property has been exploited by using ceramic coatings in hot face insulation. Ceramic coatings
are high emissivity coatings and a have a long life at temperatures up to 1350o
C. There are two
types of ceramic coatings: those used for coating metal substrates, and those used for coating
refractory substrates. The coatings are non-toxic, non-flammable and water based. Applied at
room temperatures, they are sprayed and air-dried in less than five minutes. The coatings allow
the substrate to maintain its designed metallurgical properties and mechanical strength.
Installation is quick and can be completed during shut down. Energy savings of the order of 8-20
percent have been reported depending on the type of furnace and operating conditions. High
emissivity coatings are further described in section 2.3.3.
4.10 Selection of refractories
The selection of refractories aims to maximize the performance of the furnace, kiln or boiler.
Furnace manufacturers or users should consider the following points in the selection of a
refractory:
Type of furnace
34

Type of metal charge
Presence of slag
Area of application
Working temperatures
Extent of abrasion and impact
Structural load of the furnace
Stress due to temperature gradient in the structures and temperature fluctuations
Chemical compatibility to the furnace environment
Heat transfer and fuel conservation
Cost considerations
5. OPTIONS CHECKLIST
It is difficult to make a checklist of general options for furnaces, because options to improve
energy efficiency vary between furnaces. But the main options that are applicable to most
furnaces are:
Check against infiltration of air: use doors or air curtains
Monitor O2 /CO2/CO and control excess air to the optimum level
Improve burner design, combustion control and instrumentation
Ensure that the furnace combustion chamber is under slight positive pressure
Use ceramic fibers in the case of batch operations
Match the load to the furnace capacity
Retrofit with heat recovery device
Investigate cycle times and reduce
Provide temperature controllers
Ensure that flame does not touch the stock
35

1
FUELS & COMBUSTION
1. INTRODUCTION..........................................................................................................1
2 TYPE OF FUELS............................................................................................................1
3. PERFORMANCE EVALUATION OF FUELS ...............................................11
4. ENERGY EFFICIENCY OPPORTUNITIES...................................................17
5. OPTION CHECKLIST..............................................................................................20
6. WORKSHEETS...........................................................................................................23
7. REFERENCES..............................................................................................................24
1. INTRODUCTION
This section briefly describes the main features of fuels.
Energy from the Sun is converted into chemical energy by photosynthesis. But, as we know,
when we burn dried plants or wood, producing energy in the form of heat and light, we are
releasing the Sun’s energy originally stored in that plant or in that wood through
photosynthesis. We know that, in most of the world today, wood is not the main source of
fuel. We generally use natural gas or oil in our homes, and we use mainly oil and coal to heat
the water to produce the steam to drive the turbines for our huge power generation systems.
These fuels - coal, oil, and natural gas - are often referred to as fossil fuels.
The various types of fuels (like liquid, solid and gaseous fuels) that are available depend on
various factors such as costs, availability, storage, handling, pollution and landed boilers,
furnaces and other combustion equipments.
The knowledge of the fuel properties helps in selecting the right fuel for the right purpose and
for the efficient use of the fuel. Laboratory tests are generally used for assessing the nature
and quality of fuels.
2 TYPE OF FUELS
This section describes types of fuels: solid, liquid, and gaseous.
2.1 Liquid Fuels
Liquid fuels like furnace oil and LSHS (low sulphur heavy stock) are predominantly used in
industrial applications. The various properties of liquid fuels are given below.
Chapter - 2

2
Density is defined as the ratio of the mass of the fuel to the volume of the fuel at a reference
temperature of 15°C. Density is measured by an instrument called a hydrometer. The
knowledge of density is useful for quantitative calculations and assessing ignition qualities.
The unit of density is kg/m3.
2.1.2 Specific gravity
This is defined as the ratio of the weight of a given volume of oil to the weight of the same
volume of water at a given temperature. The density of fuel, relative to water, is called
specific gravity. The specific gravity of water is defined as 1. Since specific gravity is a ratio,
it has no units. The measurement of specific gravity is generally made by a hydrometer.
Specific gravity is used in calculations involving weights and volumes. The specific gravity
of various fuel oils are given in Table below:
Table 1. Specific gravity of various fuel oils (adapted from Thermax India Ltd.)
Fuel Oil L.D.O
(Light Diesel Oil)
Furnace oil L.S.H.S
(Low Sulphur
Heavy Stock)
Specific Gravity 0.85 - 0.87 0.89 - 0.95 0.88 - 0.98
2.1.3 Viscosity
The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on
the temperature and decreases as the temperature increases. Any numerical value for
viscosity has no meaning unless the temperature is also specified. Viscosity is measured in
Stokes / Centistokes. Sometimes viscosity is also quoted in Engler, Saybolt or Redwood.
Each type of oil has its own temperature - viscosity relationship. The measurement of
viscosity is made with an instrument called a Viscometer.
Viscosity is the most important characteristic in the storage and use of fuel oil. It influences
the degree of pre-heating required for handling, storage and satisfactory atomization. If the
oil is too viscous, it may become difficult to pump, hard to light the burner, and difficult to
handle. Poor atomization may result in the formation of carbon deposits on the burner tips or
on the walls. Therefore pre-heating is necessary for proper atomization.
2.1.4 Flash Point
The flash point of a fuel is the lowest temperature at which the fuel can be heated so that the
vapour gives off flashes momentarily when an open flame is passed over it. The flash point
for furnace oil is 66 0
C.
2.1.5 Pour Point
The pour point of a fuel is the lowest temperature at which it will pour or flow when cooled
under prescribed conditions. It is a very rough indication of the lowest temperature at which
fuel oil is ready to be pumped.
2.1.1 Density

– 3
Specific heat is the amount of kCals needed to raise the temperature of 1 kg of oil by 10
C.
The unit of specific heat is kcal/kg0
C. It varies from 0.22 to 0.28 depending on the oil specific
gravity. The specific heat determines how much steam or electrical energy it takes to heat oil
to a desired temperature. Light oils have a low specific heat, whereas heavier oils have a
higher specific heat.
2.1.7 Calorific Value
The calorific value is the measurement of heat or energy produced, and is measured either as
gross calorific value or net calorific value. The difference is determined by the latent heat of
condensation of the water vapour produced during the combustion process. Gross calorific
value (GCV) assumes all vapour produced during the combustion process is fully condensed.
Net calorific value (NCV) assumes the water leaves with the combustion products without
fully being condensed. Fuels should be compared based on the net calorific value.
The calorific value of coal varies considerably depending on the ash, moisture content and
the type of coal while calorific value of fuel oils are much more consistent. The typical GCVs
of some of the commonly used liquid fuels are given below:
Table 2. Gross calorific values for different fuel oils (adapted from Thermax India Ltd.)
Fuel Oil Gross Calorific Value (kCal/kg)
Kerosene - 11,100
Diesel Oil - 10,800
L.D.O - 10,700
Furnace Oil - 10,500
LSHS - 10,600
2.1.8 Sulphur
The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a
lesser extent on the refining process. The normal sulfur content for the residual fuel oil
(furnace oil) is in the order of 2 - 4 %. Typical figures for different fuel oils are shown in
Table 3.
Table 3. Percentages of sulphur for different fuel oils (adapted from Thermax India Ltd.)
Fuel oil Percentage of Sulphur
Kerosene 0.05 - 0.2
Diesel Oil 0.05 - 0.25
L.D.O 0.5 - 1.8
Furnace Oil 2.0 - 4.0
LSHS < 0.5
The main disadvantage of sulphur is the risk of corrosion by sulphuric acid formed during
and after combustion, and condensation in cool parts of the chimney or stack, air pre-heater
and economizer.
2.1.6 Specific Heat

4
The ash value is related to the inorganic material or salts in the fuel oil. The ash levels in
distillate fuels are negligible. Residual fuels have higher ash levels. These salts may be
compounds of sodium, vanadium, calcium, magnesium, silicon, iron, aluminum, nickel, etc.
Typically, the ash value is in the range 0.03 - 0.07 %. Excessive ash in liquid fuels can cause
fouling deposits in the combustion equipment. Ash has an erosive effect on the burner tips,
causes damage to the refractories at high temperatures and gives rise to high temperature
corrosion and fouling of equipments.
2.1.10 Carbon Residue
Carbon residue indicates the tendency of oil to deposit a carbonaceous solid residue on a hot
surface, such as a burner or injection nozzle, when its vaporizable constituents evaporate.
Residual oil contains carbon residue of 1 percent or more.
2.1.11 Water Content
The water content of furnace oil when it is supplied is normally very low because the product
at refinery site is handled hot. An upper limit of 1% is specified as a standard.
Water may be present in free or emulsified form and can cause damage to the inside surfaces
of the furnace during combustion especially if it contains dissolved salts. It can also cause
spluttering of the flame at the burner tip, possibly extinguishing the flame, reducing the flame
temperature or lengthening the flame.
Typical specifications of fuel oils are summarized in the Table below.
Table 4. Typical specifications of fuel oils (adapted from Thermax India Ltd.)
Fuel OilsProperties
Furnace Oil L.S.H.S L.D.O
Density (Approx.
g/cc at 150C)
0.89 - 0.95 0.88 - 0.98 0.85 - 0.87
Flash Point (0C) 66 93 66
Pour Point (0C) 20 72 18
G.C.V. (kCal/kg) 10500 10600 10700
Sediment, % Wt.
Max.
0.25 0.25 0.1
Sulphur Total, %
Wt. Max.
Up to 4.0 Up to 0.5 Up to 1.8
Water Content, %
Vol. Max.
1.0 1.0 0.25
Ash % Wt. Max. 0.1 0.1 0.02
2.1.12 Storage of Fuel oil
It can be potentially hazardous to store furnace oil in barrels. A better practice is to store it in
cylindrical tanks, either above or below the ground. Furnace oil that is delivered may contain
dust, water and other contaminants.
2.1.9 Ash Content

5
The sizing of the storage tank facility is very important. A recommended storage size
estimate is to provide for at least 10 days of normal consumption. Industrial heating fuel
storage tanks are generally vertical mild steel tanks mounted above the ground. It is prudent
for safety and environmental reasons to build bund walls around tanks to contain accidental
spillages.
As a certain amount of settlement of solids and sludge will occur in tanks over time, tanks
should be cleaned at regular intervals: annually for heavy fuels and every two years for light
fuels. Care should be taken when oil is decanted from the tanker to the storage tank. All leaks
from joints, flanges and pipelines must be attended to at the earliest. Fuel oil should be free
from possible contaminants such as dirt, sludge and water before it is fed to the combustion
system.
2.2 Solid Fuel (Coal)
Subject not detailed

9
2.3 Gaseous Fuel
Gas fuels are the most convenient because they require the least amount of handling and are
used in the simplest and most maintenance-free burner systems. Gas is delivered "on tap" via
a distribution network and so is suited for areas with a high population or industrial density.
However, large individual consumers do have gasholders and some produce their own gas.
2.3.1 Types of gaseous fuel
The following is a list of the types of gaseous fuel:
§ Fuels naturally found in nature:
− Natural gas
− Methane from coal mines
§ Fuel gases made from solid fuel
− Gases derived from coal
− Gases derived from waste and biomass
− From other industrial processes (blast furnace gas)
§ Gases made from petroleum
− Liquefied Petroleum gas (LPG)
− Refinery gases
− Gases from oil gasification
§ Gases from some fermentation process
Gaseous fuels in common use are liquefied petroleum gases (LPG), Natural gas, producer
gas, blast furnace gas, coke oven gas etc. The calorific value of gaseous fuel is expressed in
Kilocalories per normal cubic meter (kCal/Nm3
) i.e. at normal temperature (20 0
C) and
pressure (760 mm Hg).
2.3.2 Properties of gaseous fuels
Since most gas combustion appliances cannot utilize the heat content of the water vapour,
gross calorific value is of little interest. Fuel should be compared based on the net calorific
value. This is especially true for natural gas, since increased hydrogen content results in high
water formation during combustion.
Typical physical and chemical properties of various gaseous fuels are given in Table 9.

10
Table 9. Typical physical and chemical properties of various gaseous fuels
Fuel Gas Relative
Density
Higher Heating
Value kcal/Nm3
Air/Fuel
ratio-
m3
of air to m3
of Fuel
Flame
Temp.
o
C
Flame
Speed m/s
Natural
Gas
0.6 9350 10 1954 0.290
Propane 1.52 22200 25 1967 0.460
Butane 1.96 28500 32 1973 0.870
2.3.3 LPG
LPG is a predominant mixture of propane and Butane with a small percentage of unsaturates
(Propylene and Butylene) and some lighter C
2
as well as heavier C
5
fractions. Included in the
LPG range are propane (C
3
H
8
), Propylene(C
3
H
6
), normal and iso-butane (C
4
H
10
)and
Butylene(C
4
H
8
). LPG may be defined as those hydrocarbons, which are gaseous at normal
atmospheric pressure, but may be condensed to the liquid state at normal temperature, by the
application of moderate pressures. Although they are normally used as gases, they are stored
and transported as liquids under pressure for convenience and ease of handling. Liquid LPG
evaporates to produce about 250 times volume of gas.
LPG vapour is denser than air: butane is about twice as heavy as air and propane about one
and a half times as heavy as air. Consequently, the vapour may flow along the ground and
into drains sinking to the lowest level of the surroundings and be ignited at a considerable
distance from the source of leakage. In still air vapour will disperse slowly. Escape of even
small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and
thus cause considerable hazard. To aid in the detection of atmospheric leaks, all LPG’s are
required to be odorized. There should be adequate ground level ventilation where LPG is
stored. For this very reason LPG cylinders should not be stored in cellars or basements,
which have no ventilation at ground level.
2.3.4 Natural gas
Methane is the main constituent of natural gas and accounting for about 95% of the total
volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon
Dioxide, and traces of other gases. Very small amounts of sulphur compounds are also
present. Since methane is the largest component of natural gas, generally properties of
methane are used when comparing the properties of natural gas to other fuels.
Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air
readily and does not produce smoke or soot. It contains no sulphur. It is lighter than air and
disperses into air easily in case of leak. A typical comparison of carbon contents in oil, coal
and gas is given in the table below.
Fuels and Combustion

11
Table 10. Comparison of chemical composition of various fuels
Fuel Oil Coal Natural Gas
Carbon 84 41.11 74
Hydrogen 12 2.76 25
Sulphur 3 0.41 -
Oxygen 1 9.89 Trace
Nitrogen Trace 1.22 0.75
Ash Trace 38.63 -
Water Trace 5.98 -
3. PERFORMANCE EVALUATION OF FUELS
This section explains the principles of combustion, how fuel performance can be evaluated
using the stochiometric calculation of air requirement, the concept of excess air, and the draft
system of exhaust gases.
3.1 Principles of Combustion
3.1.1 Combustion process
Combustion refers to the rapid oxidation of fuel accompanied by the production of heat, or
heat and light. Complete combustion of a fuel is possible only in the presence of an adequate
supply of oxygen.
Oxygen (O2) is one of the most common elements on earth making up 20.9% of our air.
Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to
a gas before they will burn. Usually heat is required to change liquids or solids into gases.
Fuel gases will burn in their normal state if enough air is present.
Most of the 79% of air (that is not oxygen) is nitrogen, with traces of other elements.
Nitrogen is considered to be a temperature reducing diluter that must be present to obtain the
oxygen required for combustion.
Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and
diluting the flue gases. This reduces the heat available for transfer through the heat exchange
surfaces. It also increases the volume of combustion by-products, which then have to travel
through the heat exchanger and up the stack faster to allow the introduction of additional
fuel-air mixture.
This nitrogen also can combine with oxygen (particularly at high flame temperatures) to
produce oxides of nitrogen (NOx), which are toxic pollutants. Carbon, hydrogen and sulphur
in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and sulphur
dioxide, releasing 8,084 kcals, 28,922 kcals and 2,224 kcals of heat respectively. Under
certain conditions, carbon may also combine with oxygen to form carbon monoxide, which
results in the release of a smaller quantity of heat (2,430 kcals/kg of carbon). Carbon burned
to CO2 will produce more heat per unit of fuel than when CO or smoke are produced.

12
C + O2 → CO 2 + 8,084 kcals/kg of Carbon
2C + O2 → 2 CO + 2,430 kcals/kg of Carbon
2H 2 + O2 → 2H2O + 28,922 kcals/kg of Hydrogen
S + O2 → SO2 + 2,224 kcals/kg of Sulphur
Each kilogram of CO formed means a loss of 5654 kCal of heat (8084 –2430).
3.1.2 Three T’s of combustion
The objective of good combustion is to release all of the heat in the fuel. This is
accomplished by controlling the "three T's" of combustion which are (1) Temperature high
enough to ignite and maintain ignition of the fuel, (2) Turbulence or intimate mixing of the
fuel and oxygen, and (3) Time, sufficient for complete combustion.
Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen.
Water vapor is a by-product of burning hydrogen. This removes heat from the flue gases,
which would otherwise be available for more heat transfer.
Natural gas contains more hydrogen and less carbon per kg than fuel oils and as such
produces more water vapor. Consequently, more heat will be carried away by exhaust while
firing natural gas.
Too much, or too little fuel with the available combustion air may potentially result in
unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for
perfect combustion and some additional (excess) air is required for ensuring complete
combustion. However, too much excess air will result in heat and efficiency losses.
Not all of the fuel is converted to heat and absorbed by the steam generation equipment.
Usually all of the hydrogen in the fuel is burned and most boiler fuels, allowable with today's
air pollution standards, contain little or no sulfur. So the main challenge in combustion
efficiency is directed toward unburned carbon (in the ash or incompletely burned gas), which
forms CO instead of CO2.
Figure 1. Perfect, good and incomplete combustion

13
3.2 Stochiometric Calculation of Air Requirement
3.2.1 Calculation of stochiometric air needed for combustion of furnace oil
For combustion air is needed. The amount of air needed can be calculated using the method
given below.
The first step is to determine the composition of the furnace oil. Typical specifications of
furnace oil from lab analysis is given below:
Constituents % By weight
Carbon 85.9
Hydrogen 12
Oxygen 0.7
Nitrogen 0.5
Sulphur 0.5
H2O 0.35
Ash 0.05
GCV of fuel 10880 kcal/kg
If we take these analysis data, and considering a sample of 100 kg of furnace oil, then the
chemical reactions are as follows:
Element Molecular Weight (kg / kg mole)
C 12
O2 32
H2 2
S 32
N2 28
CO2 44
SO2 64
H2O 18
C + O2 → CO2
H2 + 1/2O2 → H2O
S + O2 → SO2
Constituents of fuel
C + O2 → CO2
12 + 32 → 44
12 kg of carbon requires 32 kg of oxygen to form 44 kg of carbon dioxide therefore 1 kg of
carbon requires 32/12 kg i.e 2.67 kg of oxygen
(85.9) C + (85.9 x 2.67) O2 → 315.25 CO2
2H2 + O2 → 2H2O
4 + 32 → 36
4 kg of hydrogen requires 32 kg of oxygen to form 36 kg of water, therefore 1 kg of hydrogen
requires 32/4 kg i.e. 8 kg of oxygen.

14
(12) H2 + (12 x 8) O2 → (12 x 9 ) H2O
S + O2 → SO2
32 + 32 → 64
32 kg of sulphur requires 32 kg of oxygen to form 64 kg of sulphur dioxide, therefore 1 kg of
sulphur requires 32/32 kg i.e. 1 kg of oxygen
(0.5) S + (0.5 x 1) O2 → 1.0 SO2
Total oxygen required = 325.57 kg
(229.07+96+0.5)
Oxygen already present in
100 kg fuel (given) = 0.7 kg
Additional oxygen required = 325.57 –0.7
= 324.87 kg
Therefore quantity of dry air needed = (324.87) / 0.23
(air contains 23% oxygen by weight)
= 1412.45 kg of air
Theoretical air required = (1412.45) / 100
= 14.12 kg of air / kg of fuel
Therefore, in this example, for each kg of furnace oil burnt, 14.12 kg of air is required.
3.2.2 Calculation of theoretical CO2 content in the flue gases
It is necessary to also calculate the CO2 content in the flue gases, which then can be used to
calculate the excess air in the flue gases. A certain amount of excess air is needed for
complete combustion of furnace oils. However, too much excess air points to heat losses and
too little excess air points to incomplete combustion. The CO2 in flue gases can be
calculated as follows:
Nitrogen in flue gas = 1412.45 –324.87
= 1087.58 kg
Theoretical CO2% in dry flue gas by volume is calculated as below:
Moles of CO2 in flue gas = (314.97) / 44 = 7.16
Moles of N2 in flue gas = (1087.58) / 28 = 38.84
Moles of SO2 in flue gas = 1/64 = 0.016
Theoretical CO2 % by Volume = (Moles of CO2 x 100) / Total Moles (Dry)
= (7.16 x 100) / (7.16 + 38.84 + 0.016)
= 15.5%

15
3.2.3 Calculation of constituents of flue gas with excess air
Now we know the theoretical air requirements and the theoretical CO2 content of flue gases.
The next step is to measure the actual CO2 percentage in the flue gases. In the calculation
below it is assumed that the measured %CO2 in the flue gas is 10%.
% Excess air = [(Theoretical CO2%/Actual CO2) –1] x 100
= [(15.5/10 –1)] x 100
= 55%
Theoretical air required for 100kg of fuel burnt = 1412.45 kg
Total quantity of air supply required with 55% excess air = 1412.45 x 1.55
= 2189.30 kg
Excess air quantity (actual –theoretical excess air) = 2189.30 –1412.45
= 776.85
O2 (23%) = 776.85 x 0.23
= 178.68 kg
N2 (77%) = 776.85 –178.68
= 598.17 kg
The final constituents of flue gas with 55% excess air for every 100 kg fuel is as follows:
CO2 = 314.97 kg
H2O = 108.00 kg
SO2 = 1 kg
O2 = 178.68 kg
N2 = 1685.75 kg (= 1087.58 in air + 598.17 in excess air)
3.2.4 Calculation of theoretical CO2% in dry flue gas by volume
Now that we have the constituents by weight, we can calculate the constituents on a volume
basis as follows:
Moles of CO2 in flue gas = 314.97 / 44 = 7.16
Moles of SO2 in flue gas = 1/64 = 0.016
Moles of O2 in flue gas = 178.68 / 32 = 5.58
Moles of N2 in flue gas = 1685.75 / 28 = 60.20
Theoretical CO2% by volume = (Moles of CO2 x 100) / Total moles (dry)
= (7.16 x 100) / (7.16 + 0.016 + 5.58 + 60.20)
= 10%
Theoretical O2% by volume = (5.58 x 100) / 72.956
= 7.5%

16
3.3 Concept of Excess Air
For optimum combustion, the real amount of combustion air must be greater than that
required theoretically. Part of the stack gas consists of pure air, i.e. air that is simply heated to
stack gas temperature and leaves the boiler through the stack. Chemical analysis of the gases
is an objective method that helps to achieve finer air control. By measuring CO2 or O2 in flue
gases (by continuous recording instruments or Orsat apparatus or some cheaper portable
instruments) the excess air level and stack losses can be estimated. The excess air to be
supplied depends on the type of fuel and the firing system.
A faster way to calculate the excess air is by using the figures 2 and 3, provided the
percentage of CO2 or O2 in the flue gases have been measured.
Figure 2. Relation between CO2 & Excess Air
Figure 3. Relationship between residual oxygen and excess air

17
For optimum combustion of fuel oil the CO2 or O2 in flue gases should be maintained as
follows:
CO2 = 14.5–15 %
O2 = 2–3 %
3.4 Draft System
The function of draft in a combustion system is to exhaust the products of combustion, i.e.
flue gases, into the atmosphere. The draft can be classified into two types namely natural
draft and mechanical draft.
3.4.1 Natural draft
Natural draft is the draft produced by a chimney alone. It is caused by the difference in
weight between the column of hot gas inside the chimney and column of outside air of the
same height and cross section. Being much lighter than outside air, chimney flue gas tends to
rise, and the heavier outside air flows in through the ash pit to take its place. Draft is usually
controlled by hand-operated dampers in the chimney and breeching connecting the boiler to
the chimney. Here no fans or blowers are used. The products of combustion are discharged at
such a height that it will not be a nuisance to the surrounding community.
3.4.2 Mechanical draft
It is draft artificially produced by fans. Three basic types of drafts that are applied are:
§ Balanced draft: Forced-draft (F-D) fan (blower) pushes air into the furnace and an
induced draft (I-D) fan draws gases into the chimney thereby providing draft to remove
the gases from the boiler. Here the pressure is maintained between 0.05 to 0.10 in. of
water gauge below atmospheric pressure in the case of boilers and slightly positive for
reheating and heat treatment furnaces.
§ Induced draft: An induced-draft fan draws enough draft for flow into the furnace,
causing the products of combustion to discharge to atmosphere. Here the furnace is kept
at a slight negative pressure below the atmospheric pressure so that combustion air flows
through the system.
§ Forced draft: The Forced draft system uses a fan to deliver the air to the furnace, forcing
combustion products to flow through the unit and up the stack.
4. ENERGY EFFICIENCY OPPORTUNITIES
This section includes energy efficiency opportunities in Fuel Combustion
4.1 Pre-heating of the Combustion Oil
The viscosity of furnace oil and LSHS (Low Sulphur Heavy Stock) increases with decreasing
temperature, which makes it difficult to pump the oil. At low ambient temperatures (below 25
0
C), furnace oil cannot be pumped easily. To circumvent this, preheating of oil is
accomplished in two ways:
§ The entire tank may be preheated. In this form of bulk heating, steam coils are placed at
the bottom of the tank, which is fully insulated;

18
§ The oil can be heated as it flows out with an outflow heater. To reduce steam
requirements, it is advisable to insulate tanks where bulk heating is used.
Bulk heating may be necessary if flow rates are high enough to make outflow heaters of
adequate capacity impractical, or when a fuel such as LSHS is used. In the case of outflow
heating, only the oil, which leaves the tank, is heated to the pumping temperature. The
outflow heater is essentially a heat exchanger with steam or electricity as the heating
medium.
4.2 Temperature control of Combustion Oil
Thermostatic temperature control of the oil is necessary to prevent overheating, especially
when oil flow is reduced or stopped. This is particularly important for electric heaters, since
oil may get carbonized when there is no flow and the heater is on. Thermostats should be
provided at a region where the oil flows freely into the suction pipe. The temperature at
which oil can readily be pumped depends on the grade of oil being handled. Oil should never
be stored at a temperature above that necessary for pumping as this leads to higher energy
consumption.
4.3 Combustion Controls
Combustion controls assist the burner in regulation of fuel supply, air supply, (fuel to air
ratio), and removal of gases of combustion to achieve optimum boiler efficiency. The amount
of fuel supplied to the burner must be in proportion to the steam pressure and the quantity of
steam required. The combustion controls are also necessary as safety device to ensure that
the boiler operates safely.
Various types of combustion controls in use are:
§ On/Off control: The simplest control, ON/OFF control means that either the burner is
firing at full rate or it is OFF. This type of control is limited to small boilers.
§ High/low/off control: Slightly more complex is HIGH/LOW/OFF system where the
burner has two firing rates. The burner operates at slower firing rate and then switches to
full firing as needed. Burners can also revert to the low firing position at reduced load.
This control is fitted to medium sized boilers.
§ Modulating control: The modulating control operates on the principle of matching the
steam pressure demand by altering the firing rate over the entire operating range of the
boiler. Modulating motors use conventional mechanical linkage or electric valves to
regulate the primary air, secondary air, and fuel supplied to the burner. Full modulation
means that boiler keeps firing, and fuel and air are carefully matched over the whole
firing range to maximize thermal efficiency.

20
5. OPTION CHECKLIST
This section includes most important options to improve energy efficiency of fuel use and in
combustion processes.
Fuel Checklist
§ Daily check: Oil temperature at the burner and oil/steam leakages
§ Weekly task: Cleaning of all filters and draining of water from all tanks
§ Yearly task: Cleaning of all tanks
Troubleshooting for fuels
1. Oil not pumpable
• Viscosity too high
• Blocked lines and filters
• Sludge in oil
• Leak in oil suction
• Vent pipe choked
2. Blocking of strainers
• Sludge or wax in oil
• Heavy precipitated compounds in oil
• Rust or scale in tank
• Carbonization of oil due to excessive heating
3. Excess water in oil
• Water delivered along with oil
• Leaking manhole
• Seepage from underground tank
• Ingress of moisture from vent pipe
• Leaking heater steam coils
4. Pipeline plugged
• Sludge in oil
• High viscosity oil
• Foreign materials such as rags, scale and wood splinters in line
• Carbonization of oil

21
Combustion Checklist
1. Start up
• Check for correct sized burner/nozzle.
• Establish air supply first (start blower). Ensure no vapour/gases are present before
light up.
• Ensure a flame from a torch or other source is placed in front of the nozzle.
• Turn ON the (preheated) oil supply (before start-up, drain off cold oil).
2. Operations
• Check for correct temperature of oil at the burner tip (consult viscosity vs.
temperature chart).
• Check air pressure for LAP burners (63.5 cm to 76.2 cm w.c. air pressure is
commonly adopted).
• Check for oil drips near burner.
• Check for flame fading/flame pulsation.
• Check positioning of burner (ensure no flame impingement on refractory walls or
charge).
• Adjust flame length to suit the conditions (ensure flame does not extend beyond the
furnace).
3. Load changes
• Operate both air and oil valves simultaneously (For self-proportioned burner, operate
the self-proportioning lever. Do not adjust valve only in oil line).
• Adjust burners and damper for a light brown (hazy) smoke from chimney and at least
12 percent CO2.
4. Shut down
• Close oil line first.
• Shut the blower after a few seconds (ensure gases are purged from combustion
chamber).
• Do not expose the burner nozzle to the radiant heat of the furnace. (When oil is shut
off, remove burner/nozzle or interpose a thin refractory between nozzle and furnace).
Troubleshooting for combustion
The checklist in the Table below can help find the causes and solutions for typical problems
found with fuel combustion.

22
TROUBLESHOOTING CHART FOR COMBUSTION
No Problems Causes & solutions
1. Starting difficult 1. No oil in the tank.
2. Excess sludge and water in storage tanks.
3. Oil not flowing due to high viscosity/low temperature.
4. Choked burner tip.
5. No air.
6. Strainers choked.
2. Flame goes out or
splutters
1. Sludge or water in oil.
2. Unsteady oil and air pressures.
3. Too high a pressure for atomizing medium which tends to blow
out flame.
4. Presence of air in oil line. Look for leakages in suction line of
pump.
5. Broken burner block, or burner without block.
3. Flame flashes back 1. Oil supply left in ‘ON’position after air supply cut off during
earlier shut off.
2. Too high a positive pressure in combustion chamber.
3. Furnace too cold during starting to complete combustion (when
temperature rises, unburned oil particles burn).
4. Oil pressure too low.
4 Smoke and soot 1. Insufficient draft or blower of inadequate
2. Oil flow excessive.
3. Oil too heavy and not preheated to
4. Suction air holes in blower plugged.
5. Chimney clogged with soot/damper
6. Blower operating speed too low.
5. Clinker on refractory 1. Flame hits refractory because combustion chamber is too small
or
2. is not correctly aligned.
3. Oil dripping from nozzle.
4. Oil supply not ’cut off’before the air supply during shut-offs.
6. Cooking of fuel in
burner
1. Nozzle exposed to furnace radiation after shut-
2. Burner fed with atomizing air over 300 °C.
3. Burner block too short or too wide.
4. Oil not drained from nozzle after shut off.
7. Excessive fuel oil
consumption
1. Improper ratio of oil and air.
2. Burner nozzle oversized.
3. Excessive draft.
4. Improper oil/air mixing by burner.
5. Air and oil pressure not correct
6. Oil not preheated properly.
7. Oil viscosity too low for the type of burner used.
8. Oil leaks in oil pipelines/preheater.
9. Bad maintenance (too high or rising stack gas temperature).

23
6. WORKSHEETS
Worksheet 1: Excess Air Calculation
No Parameters Formula Units Value
1 Carbon (C) % by Weight
2 Hydrogen (H) % by Weight
3 Oxygen (O4) % by Weight
4 Nitrogen % by Weight
5 Sulphur % by Weight
6 H2O % by Weight
7 Ash % by Weight
8 GCV of Fuel kCal/kg
9 Oxygen Required for
burning of Carbon (O1)
C x (32/12) kg/100 kg of Fuel
burning of Hydrogen (O2)
H x (32/4) kg/100 kg of Fuel
burning of Sulphur (O3)
S x (32/32) kg/100 kg of Fuel
12 Total Oxygen Required
(O)
O1 + O2 + O3 –O4 kg/100 kg of Fuel
13 Stochiometric Amount of
Air Required (S.A)
O / 0.23 kg/100 kg of Fuel
14 Excess Air (EA) %
15 Actual Amount of Air
Required
S.A x (1+ EA/100) kg/100 kg of Fuel

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