boiler plant operations and maitainance

8/2/2019 Boiler Plant Operations and Maitainance

1/32


2/32


3/32

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.


4/32

28 < Practical Boiler Plant Operation and Management >28

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.


5/32

29

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


6/32


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


7/32

31

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


8/32


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.


9/32

33

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


10/32


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.


11/32

35

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.


12/32


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.


13/32

37

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


14/32


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.


15/32

39

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.


16/32


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.


17/32

41

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.


18/32


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.


19/32

43

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


20/32


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.


21/32

45

Figure 2.14 (a)

Circulating Fluid Bed Design


22/32


23/32

47

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


24/32


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


25/32

49

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


26/32


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.


27/32

51

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.


28/32


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


29/32

53

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


30/32


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


31/32

55

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


32/32


boiler plant operations and maitainance

Documents