Boiler Plant Operations and Maitainance
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Transcript of Boiler Plant Operations and Maitainance
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2
Fuel Combustion and SteamGeneration Process
This chapter deals exclusively with the fundamental aspects related to fuel combustion
and steam generation in boilers. The characteristics of various boiler fuels are explainedin detail along with their influence on boiler design. The discussion also includes adetailed account of the fundamental principles governing combustion such as air supply,combustion temperature, air-fuel mixing & combustion rate. Boiler firing forms a vitalpart of overall boiler operation & therefore necessitates a detailed study of the different
firing methods & related appliances. Boiler efficiency is a critical parameter indicatingthe ability of a boiler to transfer heat. In order to enable a better understanding of theconcepts related to efficiency, the discussion focuses on the conditions determiningefficiency & also the methods employed to measure it.
Learning objectives
Types of emissions. Methods to control boiler emissions.
Impact of various pollutants.
Effectiveness of cleaning equipment.
Standard emission levels.
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2.1 Overview of the Boiler Heating and Steam GenerationProcess
Boiler heating takes place through the process of combustion where heat is produced bythe rapid chemical combination of oxygen with the combustible elements of a fuel. This is
accomplished by mixing air and fuel at elevated temperatures. The combustion process isguided by certain fundamental principles which are discussed below.
Air supply control
It is always important to ensure that the right proportion of air and fuel be maintained, in
order to obtain the highest possible efficiency. Air quantity depends on the fuel type,
operating conditions and the equipment used for combustion. This is often determined on
the basis of manufacturers recommendations, which are in turn based on past operational
experience and actual performance. While excess air may result in large scale release of
hot gases from the stack with a correspondingly high heat loss and reduced efficiency,
deficiency in the same allows some of the unburned or partially burned fuel to pass
through the furnace, again resulting in a loss in efficiency.
Combustion temperature
It is important in the combustion process to maintain the fuel/air mixture at a sufficiently
high temperature, in order to promote combustion. When operated at low capacities,
temperatures tend to be lower and this can result in incomplete combustion and excessive
Smoke formation, especially if combustion controls are not set properly.
Mixing of Air and fuel
Proper mixing of the air-fuel mixture is again essential for the combustion process to
initiate and it is important that each combustible particle comes into intimate contact withthe oxygen present in the air. Poor mixing and air distribution will result in an excess of
air in some portions of the combustion chamber and a deficiency in others. Combustion
equipments are therefore designed keeping this principle in mind, so that the best possible
mixing is achieved.
Time required
The combustion rate is determined by a host of factors such as air supply, temperature and
mixing and normally an appreciable amount of time is needed to complete the process.
When operating at excess capacities, there may be insufficient time left to complete the
combustion process, as a consequence of which considerable amount of unburned fuel is
discharged from the furnace and resulting in appreciable losses.
The fundamental principles governing the combustion process along with the three Ts of
combustion namely time, temperature and turbulence are illustrated in the block diagram
below.
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Figure 2.1Combustion Process
2.2 Steam Generation Fundamentals
Boiling
This is the process by which water is boiled to make steam. After attaining boiling
temperature (100C or 212F at14.7 psia), the heat energy from the fuel effects a change
in phase from liquid to gaseous, i.e. from water to steam. A continuous process for this is
provided by a steam generating system known as boiler. The figure below illustrates a
kettle type boiler in which a fixed quantity of water is heated.
Figure 2.2
Kettle Type Boiler
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In this case, there is an increase in water temperature and the boiling or saturation
temperature is reached with formation of bubbles, for a specific pressure. With
continuous application of heat, the temperature tends to remain constant, with the
steam flowing from the water surface. If a provision is made to remove the steam
continuously, the temperature would remain the same and the water content tends to
evaporate, unless there is additional water added. In a continuous process, water isregulated into the vessel at the same flow rate as the steam being generated and
leaving the vessel.
Circulation
Most boilers have water and steam flowing through tubes where they absorb heat
resulting from the combustion process. For continuous generation of steam, water
circulation through the tubes is a must. This is usually accomplished either by the
process of
Natural or thermal circulation
Or
Forced or pumped circulation
Let us discuss these two types in detail.
Natural circulation
Figure 2.3
Natural Circulation
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Referring the above figure, in the case of natural circulation, there is no presence of steam
in the unheated tube segment AB. With heat addition, a steam-water mixture is generated
in segment BC. The density of the steam-water mixture in BC is less when compared with
the water segment AB, resulting in the water flowing down in AB on account of gravity
and the consequent flowing up of the steam-water mixture in BC, into the steam drum.
The circulation rate is dependent on the difference in average density between the
unheated water and the steam-water mixture. The total circulation rate is a function of
a. Operating pressure Higher pressures give rise to higher density steam andsteam-water mixtures. This tends to reduce the flow rate by reducing the total
weight difference between the unheated and heated segments.
b. Heat input An increase in heat input will result in an increase in the amountof steam in the heated segment and a decrease in the average density of the
steam-water mixture, resulting in a higher total flow rate.
c. Boiler height With taller designs, there is a larger total pressure differencebetween the heated and unheated legs, resulting in higher total flow rates.
d. Free-flow area Larger free-flow or cross-sectional areas for water or steam-water mixtures will result in increased circulation rates.
Forced circulation
Figure 2.4
Forced Circulation
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As seen in the figure, in this type, a pump is added to the flow loop and the pressure
difference generated by this pump controls the flow rate. This is normally employed
when the boilers are designed to operate near or above the critical pressure of 3206
psia, with there being little difference in density between water and steam. This type
of circulation is also useful with certain designs in the sub-critical pressure range. The
pumps provide sufficient head for circulation and for the required velocities and thetubes used in forced circulation boilers are normally smaller in diameter.
Separation of steam and water
This takes place in the steam drum. This is easily accomplished in small, low-pressure
boilers by the use of a large drum that is roughly half full of water and having
natural gravity steam-water separation. On the other hand, high-capacity, high-
pressure units need mechanical separators for economically providing moisture-free
steam from the steam drum.
2.3 Influence of the Type of Fuel on Boiler Design
The fuel type used determines the overall design of the boiler to a great extent.
Whatever the type and nature of fuel whether fossil fuels such as coal, oil or natural
gas or by-product fuels, plant design requires different provisions to be incorporated
with regard to fuel preparation, fuel handling and combustion, heat recovery, material
corrosion, environmental considerations, pollution control etc.
To enable a better understanding, let us compare a boiler which is pulverized-coal-
fired and a boiler which is natural-gas-fired. In case of the former using a solid fuel
such as coal, the design involves several complexities. Solid fuels tend to have a high
ash percentage which is not combustible and this is a factor in plant design. These
boilers also require extensive fuel handling, storage and preparation, a comparatively
larger furnace for combustion and wider spacing of the heat transfer surfaces. Other
additional equipments that are needed include air-heaters to enhance combustion,
special cleaning equipment in the form of sootblowers in order to reduce the impact of
fouling and erosion, environmental control equipment such as electrostatic
precipitators and SO2 scrubbers and ash handling and disposal systems.
A natural gas-fired boiler on the other hand requires only minimal storage and
handling facilities, since the gas is supplied directly to the boiler via the pipeline.
Additionally, it requires a relatively smaller furnace for combustion. As there is no
formation of ash, there is complete absence of fouling in the boiler and therefore this
design permits close spacing of the heat-transfer surfaces. The smaller furnace
requirement and closer spacing of the heat-transfer spacing results in a compact boilerdesign. The allowance made for corrosion is also relatively small and emission control
is related chiefly to nitrogen oxide (NOx) that is formed during combustion.
Considering all these factors, the overall picture is that of a small and economical
design.
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2.4 Firing Appliances
Boiler firing consists of feeding the coal for combustion, into the boiler furnace. The
commonly applied methods for boiler firing include hand shoveling, use of stokers,
pulverizers and fluidized bed combustors. Among these, hand firing is seldom used
these days, but is still included in our discussion, to enable a better understanding of
the combustion fundamentals.
2.4.1 Hand firing
They are used on small capacity boilers in view of their low heat release rates. Here,
the grates serve the twin purposes of supporting the fuel bed as well as admitting
primary air. Although the original designs comprised stationary beds, shaking or
moving grates were introduced later. The agitation of the fuel bed by the grates helps
keep it even and prevents holes from forming in the fuel bed. Presently, the method of
hand firing has almost become obsolete and replaced by mechanical devices such as
stokers and pulverizers.
2.4.2 Stokers
Stokers are located in the furnace and are designed to feed solid fuel onto a grate
where the fuel burns as primary air is introduced, with over-fire air also being
introduced for enhancing the process of combustion. Stokers are also designed to
remove ash residues that remain after combustion. They are used on large boilers,
giving high heat release rates and employed for handling a variety of solid fuels such
as coal, wood, bark, bagasse, rice hulls and municipal waste.
Stokers essentially consist of:
a. Fuel feed system.b. A moving or stationary grate assembly for supporting the burning fuel and
admitting the majority of combustion air.
c. An over-fire air system for completing the combustion process and to reduceemissions such as NOx.
d. An ash-discharge system.
Generally, two types of stokers systems are available
1. Underfeed stokers where both the fuel and air supply are from under the grate.2. Overfeed stokers where the fuel is supplied from above the grate and air
supply is done from below.
Overfeed stokers are further classified into two types
a. Mass fed stokers where the fuel which is continuously fed to one end of thegrate travels horizontally or inclined across the grate as it burns, with the ash
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being removed from the opposite end. Here, the combustion air is introduced
from below the grate and moves up through the burning fuel bed.
b. Spreader stokers where the fuel is spread uniformly over the grate as it isthrown into the furnace. The combustion air in this case, enters from below.
The fuel fines burn in suspension as they fall against the upward moving airflow. The heavier fuel gets burned on the grate and the ash is removed from
the discharge end. Spreader stokers are the most common among the ones inuse presently and have the capacity to handle a wide variety of solid fuels.
The various stoker types are dealt with in detail, in the discussion to follow.
2.4.3 Underfeed stokers
As the name suggests and as described earlier, the fuel and air supply in underfeed
stokers are made from under the grate. The figure below illustrates an underfeed
stoker system used on a fire-tube boiler.
Figure 2.5
Underfeed Stoker
Boilers may be provided either with single or twin-retort stokers as in the case of
small boilers or equipped with multiple-retort stokers as with larger boilers, for
obtaining higher combustion rates.
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To enable a simple understanding of how the underfeed stoker works, consider the
single-retort stoker shown in the figure below.
Figure 2.6Single-retort Underfeed Stoker
The ram pushes the raw coal into the furnace along a feed trough. As the fresh coal is
pushed in, the coal in the furnace starts to rise, causing more coal to be exposed to the
air from the tuyeres or openings in the grate. The furnace heat along with the
incoming air heats up the raw coal which ignites and burns as it moves up toward the
fuel bed outline. The burning coal tends to be pushed to the ash discharge end, either
due to the pressure exerted by the incoming fuel or grate motion.
Multiple-retort stokers operate on the same principle as single or twin-retort stokers.
The figure below illustrates a multiple-retort stoker with steam-operated ash dumping
plates and coal and air distribution mechanisms.
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Figure 2.7
Multiple-retort Underfeed Stoker
As seen from the figure, an adequate number of retort and tuyere sections are arranged
side by side for making the required stoker width. Coal supply to each retort is made
by means of a ram. These stokers incline from the rams towards the end where ash-
discharge takes place. Secondary rams are also provided and this together with the
effect of gravity that the stoker inclination provides causes the fuel to move toward
the ash discharge end. The rate of fuel movement and consequently the fuel bed shape
can be regulated by an adjustment of the stroke length of the secondary rams.
Underfeed stokers are well suited for continuous operation at their rated capacity,
especially highly volatile fuels, with some limitations with regard to the type of coal
used. Underfeed stoker types are defined by the mechanism used for moving the coalsuch as single and multiple retorts, screw feed and ram feed.
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2.4.4 Overfeed stokers
Mass-fed Overfeed Stoker
In this type, fuel which is continuously fed to one end of the grate travels horizontally
or inclined across the grate as it burns and ash is removed from the opposite end.
Combustion air is introduced from below the grate and moves up through the burning
fuel bed. Mass-fed overfeed stokers are further categorized into two types:
a. Moving-grate stokers chain or traveling grate
b. Water-cooled vibrating-grate stokers
Moving-grate stoker
Chain-grate stokers use an endless chain that supports the fuel bed and passes over the
drive & return bend sprockets. Traveling-grate stokers also use an endless chain, but
carry small grate bars to support the fuel bed. This provides better control of fine ash
sifting through the grate. In both the types, the chain travels over two sprockets, one
at the front & one at the rear of the furnace. They are equal in length to the furnacewidth. The front sprocket is connected to a variable-speed mechanism. The air
openings in the grates depend on the fuel burned. A traveling-grate stoker is shown.
Figure 2.8
Traveling-grate Stoker
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The coal here is fed from a hopper at the front, by gravity. A hand-adjusted gate
regulates the fuel depth on the grate. The burning progresses with the travel of the
grate through the furnace and the ash is carried over the rear end and deposited in the
ash pit.
Vibrating-grate stoker
The operation here is similar to the moving-grate stoker, but the fuel-fed & bedmovement is achieved by vibration.
Figure 2.9
Vibrating-grate Stoker
The grates consist of iron blocks attached to water-cooled tubes. The tubes are equallyspaced between headers connected to the boiler. The connecting tubes between the
headers & the boiler circulation system have long bends to permit vibration of the
grates. Flexible plates are used to divide the space beneath the stoker into
compartments. Air distribution through the bed is regulated by individual supply ducts
with dampers.
A vibration generator driven by a constant-speed motor actuates the grates. This isessentially made of two unbalanced weights rotating in opposite directions, in order to
impart the required vibrations. The depth of the feed is regulated by an adjustment of
the hopper gate. The feed rate is automatically controlled by variations in the vibrating
cycle. The vibration along with the inclination of the grate makes the bed move
toward the ash pit.
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Spreader stoker
In this type, the fuel is spread uniformly over the grate as it is thrown into the furnace
and combustion air enters from below. The fuel fines burn in suspension as they fall
against the upward moving air flow. The heavier fuel gets burned on the grate and ash
is removed from the discharge end. Spreader stokers are the most common among the
stokers in use presently and have the capacity to handle a wide variety of solid fuels.
Spreader stokers consist of a variable feeding device, a mechanism for throwing the
fuel into the furnace and grates with suitable openings to admit air. A traveling-grate
spreader stoker is shown in the figure.
Figure 2.10
Traveling-grate Spreader Stoker
Coal falls on the grate & combustion is completed as it slowly moves through the
furnace. The ash falls into the pit when the grates pass over the sprocket. The rate of
grate movement is varied, to produce the required depth of ash at the discharge end.
2.4.5 Pulverized firing
In pulverized firing, fine and ground coal is flown through ducts or pipes into the
furnace, by means of air and coal in suspension. Pulverizing exposes the fuel elements
in coal to rapid oxidation even as the ignition temperature is reached. This results in a
more complete burning process. Upon entry into the furnace, the fine particles are
exposed to radiant heat with increase in temperature. The volatile coal matter is
distilled off in the form of gas. Sufficient primary air mixes intimately with the coal
particle stream, supporting combustion. The volatile matter burns first and then heats
up the remaining carbon to incandescence. The secondary air introduced around the
burner supplies oxygen for completing the combustion process.
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Features of Pulverized Firing
Coal feeder regulating coal flow from bunker to pulverizer.
Heat source for pre-heating primary air.
Primary air fan.
Piping to direct the coal & primary air from the pulverizer to the burners.
Burners to mix coal and air.
Suitable controls.
Pulverizers are generally classified as
- Contact mills
- Impact mills and
- Ball mills
Contact Mills
They contain stationary & power-driven elements arranged to have rolling action with
respect to each other. Coal is passed between them again & again, until the desired
pulverization is obtained. The grinding elements may consist of balls rolling in a raceor rollers running over a surface.
An air stream is circulated through the grinding compartment of the mill. A rotatingclassifier permits fine particles to pass in the air stream and rejects the oversized
particles which are returned for re-grinding.
Ball and Race Mill
This design uses steel balls & races as grinding elements. The lower race is power
driven while the upper is stationary. Springs are provided to exert pressure on the
upper race and coal is pulverized between the balls & the lower race.
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Figure 2.11
Ball and Race Mill
Ball Mill
It consists of a large drum partly filled with steel balls of different sizes to about 30%
volume. The drum is rotated as the coal is fed. The coal mixes with the balls & getscrushed.
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Figure 2.12
Ball Mill
Hot air entering the drum dries & carries the crushed coal through the classifiers, to
the burners and the oversized particles are returned to the drum for further grinding.
Impact Mills
In this design, the impact principle is employed and coal remains in suspension duringthe pulverizing process. Pulverization occurs due to the impact of coal on coal as wellas the stationary & moving parts. This design provides faster response rates & short
startup & shutdown times.
2.4.6 Fluidized Beds
Fluidized beds are capable of burning low grade fuels in an economically friendly
manner. The fuels that are fired range from wet biomass sludges to high ash low CV
coals. The bed is comprised of inert materials such as sand and the particle size
depends on the stable bed depth required as well as air flows.
Turbulent mixing of fuel and air occurs in fluidized beds. This results in good mixing
& heat transfer rates and lower combustion temperatures in the range of 815 875C.
The emissions are reduced considerably on account of the lower combustion
temperature. By the addition of limestone (CaCO3)) to the bed, a significant reduction
in sulphur-dioxide emission levels is achieved.
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There are basically two types of fluidized bed designs that are available
a. Bubbling fluidized bed (BFB)
b. Circulating Fluid Bed (CFB)
Bubbling fluidized bed (BFB)
In this process, a mixture of particles is suspended in an upwardly flowing stream
mixture of air and combustion gases, resulting in fluid like properties. Optimum
combustion is produced by an intimate mixing of the fuel-air mixture. The transition
between the bed and the space above is known as freeboard area.
In BFB boilers, the combustion system is two-staged. While solid fuel particles burn
within the bed, volatiles and very fine fuel particles are burned in the freeboard area.
The freeboard area is also injected with secondary air, in order to optimize
combustion in the second stage of the combustion process. A basic BFB design and a
bottom supported towerpak BFB boiler design are shown.
Figure 2.13 (a)Bubbling Fluidized Boiler
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Figure 2.13 (b)
Bubbling Fluidized Boiler (Courtesy Babcock and Wilcox))
Circulating Fluid Bed (CFB)
Here, tube bundles are not present in the dense bed. The required heat-transfer surface
comprises the furnace enclosure (waterwalls) and internal division walls located
across the boiler width. It is possible to eliminate the in-bed tube bundle on account of
the large quantity of solids that are recycled internally and externally around the
furnace. CFB designs vary primarily with regard to the method of collecting and
recycling of solids. Two different CFB designs are depicted.
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Figure 2.14 (a)
Circulating Fluid Bed Design
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liquid and heated, this vapor will form gas. There is no exact point at which a
substance changes from vapor to gas or gas to vapor. While steam formed as a result
of boiling water at atmospheric pressure can be considered as vapor since it is just
above the liquid state, air may be considered as gas as it is far removed from the liquid
state under normal conditions. Gases tend to follow certain definitive laws of behavior
when subjected to pressure, volume and temperature changes and the more nearly a
vapor approaches a gas, the more closely it follows the gas laws.
During combustion, the temperature of gas varies widely and because the gases are
maintained near atmospheric pressure, the volume also varies. This is a very important
consideration as fans, boiler passes and flue ducts have to be designed accordingly.
Along with the physical changes, a lot of chemical reactions occur during the
combustion process. All substances are made of one or more chemical elements, with
the atom being the smallest particle an element can be divided into. These atoms
combine in various combinations to form molecules which in turn are the smallest
particles of a substance or compound, whose characteristics are determined by the
atoms that make up its molecules. Combustion is a chemical process involving the
reaction of carbon, hydrogen and sulphur with oxygen and we shall discuss this
further in detail, in the discussion to follow.
2.5.1 Stoichiometric air and excess air requirements
Stoichiometric air is the air that is needed for complete combustion of one unit of fuel
under ideal conditions. Excess air on the other hand is the extra air used in a furnace
beyond the air required for Stoichiometric or complete combustion. The process of
combustion requires a proper proportioning of fuel and air with the fuel elements. The
burning of coal, oil or gas is a chemical reaction involving the fuel and oxygen present
in the air. Air contains 23% oxygen by weight and 21% by volume. The remainder
mostly consists of nitrogen which has no role in the combustion but does have an
effect on the volume of air required. Although it does not burn, nitrogen absorbs thereleased heat. Thus nitrogen has an effect on the combustion process in that it
influences the temperature and time needed for completing the burning of the fuel.
With pure oxygen, the combustion is more rapid and spontaneous. It is required to
supply 4.78 ft3 (0.135 m3) of air for combustion for every 1 ft3 (0.028 m3) of oxygen.
In the event that not enough air or oxygen is supplied during combustion; the mixture
is rich in fuel and the fire is reduced, resulting in a flame that is longer as well as
smoky. In this case, the combustion process is incomplete and the flue gases will
contain residues of unburned fuel. This also results in less heat production. On the
other hand, in case too much oxygen or air is supplied during combustion, the mixture
as well as the burning becomes leaner, resulting in a shorter flame and cleaner fire.
Some of the released heat is taken away by the excess air and carried up the stack.
The burning process is always carried out with excess air to ensure proper andcomplete burning of all fuel and more efficient heat release. This also results in
reduced smoke formation and soot deposits. An accurate analysis of the ideal air/fuel
ratio is made with the help of an Orsat apparatus. This helps determine the percentage
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of inadequate or excess air. The usual percentage of excess air for coal is around 50%,
whereas for oil, gas or pulverized coal, the value is between 10 and 30%. The table
below shows the values of recommended excess air for various fuels and furnace
types.
Fuel Furnace typePercentage of
Carbon-dioxide
measured
Percentage of excess air(rated)
Coal Stoker fired, naturaldraft
7.0-8.3 50-65
Coal Stoker fired, forceddraft underfed
3.5-7.0 20-50
Pulverized coal Partially watercooled furnace fordry ash removal
2.7-6.0 15-40
Crushed coal Cyclone furnace -suction / pressure
1.9-2.7 10-15
Bagasse All furnace 4.2-5.4 25-35
Wood Dutch oven & hofttype
3.5-4.2 20-25
Furnace oil Multi fuel burners &flat flame
3.0-4.0 16-22
Black liquor Recovery furnaces 1.0-1.4 5-7
Natural gas Multi-fuel burners 1.4-2.3 7-12
Natural gas Register type burners 1.0-1.9 5-10
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2.5.2 Combustion chemistry and products of combustion
Any combustion process involving a fossil fuel will have only three elements
combining with the oxygen in the air and releasing heat. These are carbon, hydrogen
and sulphur. The combustion process comprises of the following basic chemicalreactions.
1. C + O2 = CO2
In the above equation, carbon (C) in complete combustion combines with the oxygen
(O2) in the air to form carbon-dioxide (CO2).
2. 2C + O2 = 2CO
In the equation shown above, we have carbon combining with oxygen in the air toform carbon-monoxide (CO). This happens in the case of incomplete combustion
where the carbon does not burn completely.
3. 2H2 + O2 = 2H2O
In this reaction, the hydrogen (H2) in the fuel combines with oxygen (O2) in the air to
form di-hydrogen oxide (H2O) or water.
4. S + O2 = SO2
Sulphur(S) is the last flammable constituent in the fuel and it combines with oxygen
in the air to form sulphur-dioxide (SO2).
The other reactions involved in the combustion process include
Carbon-monoxide burned to carbon-dioxide
2CO + O2 = 2CO2
Sulphur combining with oxygen to form sulphur-trioxide
2S + 3O2 = 2SO3
Methane (CH4) burned to carbon-dioxide and water
CH4 + 2O2 = CO2 + 2H2O
Acetylene combining with oxygen to form carbon-dioxide and water
2C2H2 + 5O2 = 4CO2 + 2H2O
Ethylene combining with oxygen to form carbon-dioxide and water
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C2H4 + 3O2 = 2CO2 + 2H2O
Ethane burned to carbon-dioxide and water
2C2H6 + 7O2 = 4CO2 + 6H2O
2.6 Boiler Efficiency
Boiler efficiency is defined as the measure of its ability to transfer the heat given to it
by the furnace, to the water and steam. Here, the furnace performance is always taken
into account and also sometimes that of the pre-heater, super-heater, re-heater and
economizer. Boiler efficiency is considered to be a combination of the efficiencies of
its elements.
Determining the boiler efficiency
Efficiency may be expressed as a percentage figure or in terms of evaporation which
is the steam rate per unit mass of fuel fired. The evaporation may again be actual or
equivalent. When determining the steaming rate per unit of the heating surface, the
effect of the economizer and pre-heater surfaces is always excluded from calculations
for a separate comparative efficiency. The overall efficiency is therefore higher than
the comparative efficiency, by the percentage of heat absorbed by the heat recovery
equipment.
Efficiency as applied in boiler performance guarantees is normally construed for
different fuels as follows:
Solid fuels Efficiency of the boiler alone is the ratio of the heat absorbed by the
water and steam in the boiler per unit mass of combustible burned on the grate, to the
calorific value of unit mass of combustible as fired. The combined boiler, furnace and
grate efficiency is the ratio of the heat absorbed by the water and steam in the boiler
per unit mass of fuel fired, to the calorific value of unit mass of fuel as fired.
Liquid and Gaseous fuels The combined boiler, furnace and burner efficiency is the
ratio of the heat absorbed by the water and steam in the boiler per unit mass or volume
of fuel, to the calorific value of unit mass or volume of fuel.
The efficiencies of solid fuel boilers are the same whether it is on a dry fuel or a fuel-
as-fired basis. Sometimes, the lower heating value of the fuel is used, as the latent heat
of the moisture formed by the burning of hydrogen in the fuel is not available to
generate steam in the boiler. While the European practice is to generally use the lower
heating value, the practice in the United States is to use the as-fired heat value.
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Fixed and Variable conditions determining boiler efficiency
The maximum attainable efficiency is dependent on the following fixed conditions:
Boiler design and construction This includes arrangement of the heatingsurface, shape and volume of the furnace, water and steam circulation within
the boiler and the flow of the combustion product through the boiler passes.
Type of fuel used and its characteristics. Heat-recovery equipments such as super-heaters, economizers, pre-heaters
and feed-water heaters.
In-built heat losses such as those occurring through boiler walls and settingand the heat losses in the flue gases and ash that is not recoverable.
Firing rate in relation to the furnace volume and heating surface.
Ability to exercise control over the variable conditions.
Variable conditions affecting boiler efficiency
Type of operation whether continuous or intermittent, on-off, high-low ormodulating.
Fuel condition during firing and firing rate. Percentage of excess air.
Draft as affected by barometric pressure.
Cleanliness of the heat-absorbing surfaces.
Burner adjustment.
Incomplete combustion and unburned carbon.
Temperature and humidity of the combustion air.
Determining the efficiency of a boiler is in reality a performance test conducted on it.
While these tests can be carried out during actual operation in the case of large
installations, smaller boilers can either be tested in the laboratory or in the field under
semi-controlled conditions.
Direct method of determining efficiency
This involves the measurement of energy input to the useful energy output. While the
energy input is based on the gross calorific value of the fuel and further corrected for
site reference temperature, the useful energy output is a measure of the sum of the
steam heat output plus blowdown heat.
The efficiency in this case is given by
Efficiency = Steam weight (steam heat feedwater heat) X 100
Fuel weight x fuel heating value
Indirect method of determining efficiency
Here, the various heat losses such as dry gas losses, moisture losses, unburned fuel
losses, convection and radiation losses and other unaccounted lossesare determined.
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Thus, efficiency determination by this method is a direct function of heat losses from
the boiler and the combustion process. The efficiency is given by
Efficiency = Fuel heating value - losses X 100
Fuel heating value
The measuring, testing and calculation procedures for boiler efficiency are specified
in detail in the international boiler standards such as BS, DIN and ASME. The table below contains some of the typical maximum economically achievable efficiency
values.
Rated capacity range in MW
Type of fuel
3-5 5-30 30-70
Coal
a. Stoker
b. Pulverized
81.0
83.3
83.9
86.8
85.5
88.8
Oil 84.1 86.7 88.3
Gas 80.1 81.7 84.0
Table 2.1
Efficiency Values for Different Fuels
2.7 Fireside Deposits and Corrosion
The accumulation of slag and soot on the fireside influences the heat transfer rate
greatly. The deposition of foreign particles results in efficiency loss. While soot and
other fireside deposits may not cause direct damage, the acids that are formed by the
reaction of moisture with the sulfur products may lead to corrosion and tube failure.
Again, fly-ash and other small hard particles cause fireside corrosion.
Solid refuse which are part of the products of combustion cause severe operational
and maintenance problems. They clog the gas passages by sticking to the heat transfersurfaces and depositing in areas of low gas velocity. The refuse may be in the form of
flue dust, slag or soot and smoke.
Flue dust includes fly ash containing fine particles of ash and cinder which are
particles of partially burned fuel carried from the furnace and from which volatile
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gases have been driven off and sticky ash which is ash at a temperature between initial
deformation and softening.
Slag which may be molten or fused refuse consists of vitreous slag, semi-fused slag
consisting of particles partly fused together, plastic slag which is viscous in nature and
liquid slag.
Sooth and smoke consist of unburned products formed out of hydrocarbon vaporsdeprived of oxygen or adequate temperature for ignition. The table below shows how
boiler efficiency is affected by combustion deposits.
Soot thickness in mm Heat conductivity loss in %
0.8
1.6
3.2
4.8
9.5
26.2
45.3
69.0
Table 2.2
Lowering of Boiler Efficiency due to Combustion deposits
2.7.1 Slagging and clogging phenomena
Molten fly ash has a tendency to stick to the furnace walls of a boiler as softened slag,
leading to an increase in surface temperature and reduced heat transmission. As themolten slag runs down the walls, a chemical reaction takes place, resulting in erosion
or slag penetration. If the furnace temperature is not high enough, there may be a
solidification of fly ash that may in turn deposit on the walls and cause the surface
temperature to equal the ash fusion temperature. Furnace temperature variations will
cause the fly ash to either melt or buildup until equilibrium is reached. When burning
fuel particles become embedded in this mass, there will be a further rise tin
temperature. The slag may harden and develop into large masses around cool
openings in the hot zone. This characteristic of the ash to melt, fuse and coalesce intoa homogeneous mass is dependent on the ash-softening properties of the fuel as well
as the temperature.
Clogging results when coal or oil deposits resulting from burning, choke the gaspassages and reduce the rate of heat transfer. The accumulations may cause spongeash agglomeration, fouling, bridging, segregation and bird-nesting. Sponge ash
agglomeration results in the transformation of dry ash particles into soft, spongy
structures. Fouling is the agglomeration of refuse in gas passages or heat-absorbing
surfaces leading to restrictions in gas and heat flow. Bridging is again the
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agglomeration of slag and refuse, leading to a partial or complete blockage of the
spaces or apertures between heat-absorbing tubes. Segregation involves selective
deposition of refuse of varying compositions, while bird-nesting is the agglomeration
of porous masses of loosely adhering particles of refuse and slag in the first tube bank
of a watertube boiler.
Ash assumes a less agglomerate nature beyond the hot zones when it begins to cool. It
possesses the soft and flaky properties of soot in the rear passes and is thus easilyblow away.
2.7.2 Fireside Corrosion
This tends to occur when the flue gases cool below the dew point temperature and
water vapor condenses onto the surfaces. The process of corrosion is further
accelerated in the presence of sulphur products which result from the combustion of
fuels. The dew point of sulphuric acid is about 93C or 200F higher than water, with
the value varying with the proportion of acid and water vapor. High sulphur fuels havehigher dew points and the corrosion rate is found to increase with an increase in the
dew point.
Moisture collection on tubes forms a bond for deposition of ash with hygroscopic dust
and ferric sulfate, causing moisture films to form at temperatures as much as 10 to 24
C (50 - 75F) above the dew point of the flue gases. The phenomenon of air heater
fouling and corrosion normally tends to increase when the metal temperature falls
below 150C (300F). The temperature limit must be raised in the case of fuels with
higher sulfur content. The corrosion and clogging caused by both, the sulphur content
as well as the dust burden of the flue gases, affect equipment design.
While the gases in contact with tubes and plates tend to attain dew point that muchfaster, the ones in the main stream are relatively slower in doing this. Normally,
corrosion is most noticeable at the cold end of the air heater or economizer. Some
amount of sulphuric acid corrosion does tend to occur at elevated temperatures in the
range of 325C (620F). But major corrosion related difficulties occur at temperatures
below the dew point of the acid which under normal conditions varies between 138C
(280F) and 160C (320F).
In coal fired boilers, sulphuric acid may react with fly ash at feedwater temperatures
in excess of 260C (500F), to form a glassy insoluble deposit.
Deposits may accumulate in cold-end equipments such as air heaters, economizers
and dust collectors in which the gas temperatures drop below or close to dew point.Soot deposits in particular have an affinity for moisture. Also, coal soots have traces
of sulphur-dioxide and sulphur-trioxide, while oil soots have potassium and sodiumsulphates in addition. These in reaction with moisture form a dilute, but corrosive
sulphuric and sulphurous acid. Fuel oil slags may contain vanadium pentoxide that
attacks and corrodes even high chromium steels. Apparently, cold-end corrosion can
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be reduced by injecting ammonia into the flue gases, thereby neutralizing the acids
that form as a result of the presence of sulphur-bearing ash deposits
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