3150360 study of combustion process[1]
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STUDY OF COMBUSTION PROCESS BOILERS
A Mini-Project ReportSubmitted In Partial Fulfillment Of
The Requirements For The Award Of The Degree Of
BACHELOR OF TECHNOLOGY
IN
MECHANICAL ENGINEERING
Submitted
By
Mr. D. Venkata Suresh
Mr. Srinivas Balakrishnan Mr.CH. PranayajoshiMr. Sudhakar Mr.T. Sivaji
Under Estimated Guidance ofSri.Ch. Venkata Prasad (M.Tech)
Department of Mechanical Engineering
St.Anns College of Engineering and Technology(Affiliated to Jawaharlal Nehru Technological University)
Chirala 523155
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ACKNOWLWDGEMENT
It is my bounded duty to acknowledge my gratefulness to the people whohave helped me directly or indirectly to prepare this project.
I am extremely thankful to Sri. K. Satyanarayana C.A general manager, for
his kind cooperation by giving valuable suggestions and continuous support
during the course of the project.
At the outset I wish to record my gratitude to Sri. P. Uma Maheswar Rao,
the production development manager of CAPOL, for his encouragement andvaluable support.
I express my sincere thanks to Sri. CH. Venkata Prasad, our project guide at
SACET, vetapalem, for giving continuous support in the period of project.
Also I am very much thankful to the in charges of the plant and the staff who
have cooperated with me.
(D. VENKATA SURESH)
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DECLARATION
I, D. VENKATA SURESH, here by declare that the project work titledSTUDY OF COMBUSTION PROCESS BOILERS is on my own effort.
This project report is submitted in partial fulfillment of requirements of the
award of the degree of Bachelor of Technology in Mechanical Engineering from
JNTU.
I also declare that this project report is a result of my own effort and I havenot submitted to any other university for the award of any other degree or
diploma.
Date:
Place:
(D. VENKATA SURESH)
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ABSTRACT
The combustion calculations play an important role in STUDY OFCOMBUSTION PROCESS. Combustion calculations by the designer are one
of the first step in proportioning the steam generator. The scope of these
calculations would include the determination of thermal efficiencies of the boiler,economizer the calculation of air and gas weights.
Combustion calculations were done as per the existing conditions in
CAPOL( CORAMANDAL AGRO PRODUCTS & OILS LTD.), Jandrapeta,
Chirala. Heat balance sheet is prepared for the obtain results.
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CONTENTS
COAL
Introduction
Fuel for combustion
Analysis of coal
COAL BURNING METHODS
Classification of coal burning methods
Pulverized fuel firing
Pulverized fuel and handling system
Unit system
Central or Bin system
COMBUSTION
Combustion equipment for steam boilers
The basic requirements of combustion equipment
Combustion for solid fuels selection considerationTypes of combustion
Stages of combustion
Temperature profile of pulverized coal combustion
Combustion reaction
Combustion of fuel
Combustion losses
Practical combustion
Fuels and corrosion
COMBUSTION CALCULATIONS
Data collection
Solution
Heat balance sheet
CONCLUSION
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INTRODUCTION
Combustion consists of process causing chemical reaction between the
combustible part of a fuel and oxygen, with a view to generate heat the products
formed as a result of process is CO2, CO and water vapor. CO2 is the result of
complete combustion of carbon whereCO is a result of incomplete combustion,
as compared to CO2, contains only half the amount of oxygen for unit weight of
carbon the oxygen for combustion is supplied from air which from combustion
considerations contains 21% oxygen and 79% of nitrogen by volume. Industrially
good combustion aims a producing maximum amount of heat by resulting from
combustion. The products of combustion, called flue gases, contain CO2, water
vapor, nitrogen and compounds of sulphur like SO2 and SO3.
The phenomenon of combustion may be considered in its chemical and
technical aspects. Besides the fuelsize, rate of firing, supply of primary air,secondary air and moisture content in coal also greatly affect the combustion
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FUEL FOR COMBUSTION (COAL)
Coal is the oldest fuel and still used on large scale throughout the world for
power generation. It has sustained most of our combustion activity until oil was
struck and even now continues to be a major source of energy.
In India, coal is the only source of power generation, and will play very
important role in coming years.
ANALISYS OF COAL
The classification of coal is based upon the physical and chemicalcomposition of the coal and it is there fore necessary to study about the chemical
composition of the coal.
The common tests, which are used to find the constituent value of the coal, are
proximate analysis and ultimate analysis of the coal.
Proximate analysis
The proximate analysis of the coal gives the composition of coal in respect of
moisture, volatile matter, ash and carbon. The proximate analysis of most coals
indicates the following ranges of various constituencies:
CONSTITUENTS PERCENTAGE
Moisture 3 to 30%
Volatile Matter 3 to 40%
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Ash 2 to 30%
Fixed Carbon 16 to 92%
The constituents given by the proximate analysis mostly decide the adoption of
coal and design of power plant. The role played by each constituent in adapting
coal for power plant is discussed below
Moisture
All coal contains some percentage of moisture and generally varies
from 1 to 40%.The moisture in the coal exists in two forms as inherent and
free moisture. The inherent moisture is the combined moisture and is held
in the ores of the coal. The percentage of inherent moisture is determined
by heating the coal to 1100 C in the current nitrogen. The inherent moisture
is never removed from the coal used for power plants, it is costly
procedure. The free moisture is defined as moisture present in the coalwhich can be removed just by exposing the coal to the natural airflow or by
drying with the help of air of 500.
Volatile Matter
The volatile matter present in the coal maybe as high as 50%. The
volatile matter present in coal may be combustible gases such as methane,
hydrogen, carbon monoxide and other hydrocarbons or incombustible gaseslike CO2 and N2. The presence of incombustible gases is always undesirable
as they do not add in heat value but increase the volume of furnace
required. Overall, the volatile undesirable as they do not add in heat value
but increase the volume of furnace required. Overall, the volatile matters
affect the furnace volume, and arrangement of heating sufaces.
Ash
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Ash is another most undesirable constituent of coal. The ash present in
the coal is of two forms as fixed ash and free ash. The fixed ash present in
the coal comes from the original vegetable matter and it cannot beremoved
from coal before burning the coal
COAL BURNING METHODS
The efficient combustion of fuel in the combustion chamber and efficient
transfer of heat energy to the water for steam generation are essential for
economical working of the boiler. The two commonly methods for burning coalare stoker firing and pulverized firing. The pulverized firing method is used for
pulverized coal. The selection of firing method adopted for a particular boiler
depends upon the following factors:
The characteristics of the available coal.
Capacity of the plant.
Load actor of the boiler
Nature of load fluctuation and Reliability and efficiency of various combustion equipments available.
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CLASSIFICATION OF COAL BURNING METHEDS
The classifications of the combustion system used for coal burning is given
below:
PULVERIZED FUEL FIRING
In pulverized fuel firing system the coal is reduced to fine particles with the
type of grinding mill and then projected into the combustion chamber with the
help oh air current. The amount of air required to complete the combustion is
supplied separately to the combustion chamber. The resulting turbulence in the
combustion chamber helps for uniform mixing of fuel and air and through
combustion. The amount of air which is used to carry the coal and to dry it beforeentering into the combustion chamber is known as primary air and the amount
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of air which is supplied separately for complete the combustion is known as
secondary air.
The efficiency of the pulverized fuel firing system mostly depends upon
the size of the powder. The fineness of the coal should be such as 70% of it
would pass through a 200 mesh sieve and 90% through 50 mesh sieve.
Advantages
Any grade of coal can be used since coal is powder before used.
The rate of feed of the fuel can be regulated properly resulting in the fuel
economy
Since there is almost complete combustion of the fuel there is increased the
rate of evaporation and higher boiler efficiency.
Greater capacity to meet peak loads.
The system is practically free from sagging and clinkering troubles.
No stand by losses due to banked fires.
Practically no ash handling problems.
No moving part in the furnace is subjected to high temperatures.
This system works success fully with or in combustion with gas and oil.
Much smaller quantity of air is required as compared to that of stroke
firing.
Disadvantages
High capital cost
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Lot of fly ash in exhaust, which makes the removing of fine dust
uneconomically
The possibility of explosion is more as coal burns like gas.
The maintenance of furnace is brick work is costly.
Special equipment is needed to start the system.
PULVERIZED FUEL HANDLING SYSTEMS
Two methods are in general use to feed the pulverized fuel to the combustion
chamber of the boiler. First is unit system and second is central or bin
system.
In unit system, each burner of the plant is fired by one or more unit pulverizes
connected to the burners, while in the central system, the fuel is pulverized in the
central plant and then distributed to each furnace with the help of high pressure
air current. Each type of fuel handling system consists of crushers, magnetic
separators, driers , pulverizing mills, storage bins, conveyors and feeders.
The arrangement of deferent equipments required in both systems is shown in
the figure:
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The coal received by the plant from the mine may vary widely in sizes. It is
necessary to make the coal of uniform size before passing the pulverizer for
efficient grinding. The coal received from the mine is passed through a
preliminary crusher to reducer the size to allowable unit (30mm). the crushed
coal is further passed over magnetic separator, which removes pyrites and tramp
iron.
Unit system
In a unit system, each burner or a group of burners and a pulverizer constitute a
unit. Crushed coal is fed to the pulvorizer through a feeder at a variable rate
governed by the combustion requirements of furnace and steam generating rate
required in the boiler. The arrangement of unit systems is shown the figure. Hot
air or fuel gases are passed through a feeder to dry the coal before feeding to thepulverizer. The pulverized coal is carried from the mill with the help of primary
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air fan as shown in the figure. This further carries the coal through short delivery
pipe to the burner. The secondary air is supplied to the burner before entering the
fuel in the combustion chamber as shown in the figure. A plant feeding 1 tone of
pulverized fuel per hour consumes approximately 10 to 15 kwhr.energy.
The advantages and disadvantages of unit system over central are listed
below:
Advantages
It is simple in lay out and cheaper than central system.
The coal transportation system is simple.
It allows direct control of combustion from the pulverizer.
The maintenance of charges are less, as spares required are less.
Coal which requires drying for satisfactory function of the central
system is generally supplied without drying in the unit system.
Disadvantages
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The advantages and disadvantages of unit system our central are listed
below:
Advantages
The central system is flexible and changes can be made to accommodate
quick changes in demand. There is always a supply of fuel available in
reserve in the boiler bunkers. Since any mill can be used to supply any
boiler, the outage of parts of the mills or even a short outage of entire
pulverizing plant will not cause a boiler plant outage.
There is greater degree of flexibility as the quantity of fuel and air can be
separately controlled.
The pulverizer always runs at its rated load irrespective of the load on the
plant, therefore its power consumption per ton of coal crushed per hour isless.
Burners can be operated independently of the operation of the coal
preparation.
Disadvantages
Central system is higher in first cost and occupies a large space.
There is possibility of fire hazard due to the coal pulverized coal.
Dryers are essential
COMBUSTION
The object of combustion with reference to the furnaces is to provide
orderly and uniform production of it to be transmitted to a heat absorbing
medium. One of the most important items is that the correct amount of oxygen
must be supplied per unit weight of the fuel burned to provide complete
combustion. In addition to the correct air-fuel mixer time must be allowed for
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complete mixing and burning and the furnace temperature must be such as to
support combustion.
The fuels used are hydrocarbons. The fuels appear in the forms solid, liquid
and gas. Chemically the hydrogen of the fuel and the oxygen of the air form
water, and the carbon and the oxygen form carbon dioxide and sometimes carbonmonoxide. These oxidation processes, when accompanied by rapid liberation of
heat are known as combustion. To start combustion the hydrocarbons and the
oxygen must be brought to certain minimum ignition temperature.
COMBUSTION EQUIPMENT FOR STEAM BOILERS.
The combustion equipment is a component of the steam generator. Since thesource of heat is the combustion of a fuel, a working unit must have, whatever,
equipment is necessary to receive the fuel and air, proportioned to each other and
to the boiler steam demand, mix, ignite and perform any other special
combustion duties, such as distillation of volatile from coal prior to ignition.
The fuels are mainly bituminous coal, fuel oil and natural gas mentioned
in order of importance. All composed of hydrocarbons and coal has as well
much fixed carbon and little sulphur to burn these fuels to the desired endproducts CO2 and H2O requires:
Air in sufficient proportions.
A good mixing of fuel and air.
A turbulence or relative motion between fuel and air.
The combustion equipment must fulfill these requirements and in addition,
be capable of close regulation of rate of firing the fuel for boilers which
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ordinarily operate an variable load coal firing equipment must also take a
means for holding and discharging the ash residue.
THE BASIC REQUIREMENTS OF COMBUSTION EQUIPMENT
Through mixing of fuel and air.
Optimum fuel air ratios leading to most complete combustion
possible maintained over full load range.
Ready and accurate response of rate of fuel feed to load demand.
Continues and reliable ignition of fuel.
Practical distillation of volatile components of coal.
Adequate control over point of formation and accumulation of ash,
when the coal is the fuel.
COMBUSTION EQUIPMENT FOR SOLID FUELS SELECTION AND
CONSIDERATION:
While selecting the combustion equipment for solid fuels the following
considerations should be taken into account.
Initial cost of the equipment. Sufficient combustion space and its ability to with stand high flame
temperature.
Operating cost.
Minimum smoke nuisance.
Arrangements for through mixing of air with fuel for efficient
combustion
TYPES OF COMBUSTION
Based upon the type of fuel combustion can be divided into two types.
1. Homogenous combustion
2. Heterogeneous combustion
HOMOGENOUS COMBUSTION
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Combustion of gaseous fuel atomized liquid fuel is called homogenous
combustion.
The rate of homogenous combustion at constant temperature at any particular
moment is the product of the concentration of the reacting species.
r = K x Cxfuel x Cyo2
where k is the rate constant which depends on the temperature and chemical
nature of the reagents.
C fuel = concentration of fuel
C02 = concentration of oxygen
x , y are the mole of fuel and oxygen involved in the stoichiometric chemicalequation.
HETROGENOUS COMBUTION
The combustion of solid fuel is called heterogeneous combustion. In
heterogeneous combustion the concentration of combustible substance is
constant and hence the rate of this relation is dependent only on the concentration
of oxygen on the surface of the solid fuel.
r = K. Cso2
where Cso2 concentration of oxygen on the fuel surface.
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COMBUSTION CALCULATIONS
Data collection:
Coal consumption : 0.375 tons/hr
Steam produces : 2 tons/hr
Boiler operating conditions:-
Steam pressure = 10 barSteam temperature = 179.90 C
Economizer :
Water temperature = 380 C
Flue gas temperature =250 C
Boiler house temperature = 370 C
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COAL ANALYSYS (BY WEIGHT)
CONSTITUENT PERCENTAGE (%)
C 62.5H 4.25
O 5.11
N 1.2
S 0.82
ASH 9.85
MOISTURE 16.24
TOTAL 100
FLUE GAS ANALYIS (BY VOLUME)
CONSTITUENT PERCENTAGE (%)
CO2 13.2
O2 4.85
N2 81.95
TOTAL 100
Gross caloric value = 30550 kj/kg
Boiler house temperature = 370 C
= 310k
SUBTANCES SPESIFIC HEATS [Kj/Kg]
Dry flue gas......... 1.005
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Water vapor in flue gas 2.095
Water 4.187
Solution :
Theatrical air requirement:
Basis : 100 kg coal
Constituent
Element
Percentage
By weight
Molecular
Weight
K Mol K mole of O2required for
combustion
C 62.5 12 5.208 5.208H 4.25 2 2.125 1.625
O 5.11 32 0.159 -0.159
N 1.2 28 0.042 .
S 0.85 32 0.027 0.027
ASH 9.85 .
MOISTURE 16.24 .
TOTAL = 6.1385
There, theatrical air requirement
= 6.1385(100/21)/ kg of coal
= 29.23 kmol/100 kg coal
= 29.23 (28.9) kg/kg coal
( 28.9 is the average molecular weight of air)
= 844.744 kg/kg coal
= 8.447 kg/kg coal
Actual air supplied:
100 kg coal contains 5.208 kmol of C.100 kmol of flue gas contains 13.2 kmol of C.
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Therefore the amount of flue gas produced,
= 5.208 (100/32) kmol/ 100 kg of coal
= 39.45 kmol/ 100 kg of coal
Let x mol of air be supplied per 100 kg of coal burnt.Therefore by nitrogen balance we get,
79x/100 + 0.042 = 81.95/100 (39.45)
x = 40.87 kmol/100kg coal
therefore the weight of air supplied
= 40.87 (28.9) kg/100kg of coal
= 1181.14 kg/100kg coal= 11.81 kg/kg coal
Excess air :
= actual air supplied theatrical air supplied x 100
Theatrical air supplied
= (11.81 8.447)(100/8.447)
= 39.81%.
WEIGHT OF FLUE GAS:
FLUE GAS
CONSTITUENT
KMOL MOLECULAR
WEIGHT
WEIGHT WEIGHT IN 39.45KMOLE OF FLUE
GAS
CO2 13.2 44 13.2 (44) 13.2(44)x39.45/100= 229.125
O2 4.85 32 4.85 (32) 4.8(32) x 39.45/100= 61.226
N2 81.95 28 81.95 (28) 81.95(28)x39.45/100= 905.219
TOTAL 1195.57
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Water produced due to combustion of hydrogen content of coal
= 2.125 kmol= 2.125 (18) kg
= 38.25 kg
Free moisture = 16.24 kg
Therefore the total weight of flue gas
= 119.57 + 38.25 + 16.24
= 1250.06 kg/100kg coal
= 12.50 kg/kg coal
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Thermal Efficiencies
1) Boiler :
Total heat content of steam at 10 bar
= 2776.2 kj/kg
Total content of feed water charged to boiler
= 4.187 x 38
= 159.106 kj/kg
The net heat transferred to the steam
= 2776.2 159.106
= 2617.094 kj/kg
The rate of steam generation/t of coal =
2 ton/hr
0.375 ton/hr= 5.33 t/t of coal
The heat transferred to steam / kg of coal burned
= 5.33 x 2617.094
= 13949.11 kj/kg
Gross calorific value of coal as fired
= 30550 (100-16.24)/100= 25588.68 kj/kg of coal
Thermal efficiency of the boiler
= 13949.1 / 25588.68
= 0.5451
= 54.51%
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ECONOMIZER
Net heat transferred to the boiler feed water
= 5.33 x 4.187 x 38
= 848.03 kj/kg
Thermal efficiency of the economizer
= (848.03 / 25588.68) 100
= 3.31%
HEAT LOST TO THE DRY FLUE GAS
Weight of the dry flue gas
= 11.95 kj/kg of coal
Enthalpy of the dry flue gas= 11.95 x 1.005 (523.15 310.15)
= 2558.076 kj/ kg
The percentage of heat lost to the dry flue gas
= (2558.076 / 25588.68) 100
= 10%.
HEAT LOST TO THE WET FLUE GAS
Water content in the flue gases when 100 kg of coal burnt
=38.25+16.24
=54.49kg/100kg coal
The weight of water vapor per kg of coal burnt
= 0.5449 kg/ kg coal
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Fluegas
Constituent
Kmol % composition
CO2 (13.2/100)39.45
= 5.2
12.24
O2 (4.8/100)39.45
= 1.9
4.47
N2 (81.95/100)39.45
= 32.33
76.13
H2O (54.49/18)
= 3.02
7.11
total 42.46 99.95
The vapor pressure of water vapor= (7.1/99.95) 101.32
= 7.2 kN/ m2
Which corresponds to the due point temperature of water vapor is 3110K and
latent heat of evaporation is 2411.2 kj/kg.
The enthalpy of water in flue gas
= 0.5449[2.095(523.15 311)+ 2411.2+4.187(523.15-310.15)]
= 2041.83 kj/kg
The percentage of heat lost to the wet flue gas
= 2041.83/25588.68
= 7.98%
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HEAT BALANCE SHEET
UNIT HEAT RECOVER
(KJ/KG)
% HEAT RECOVERY
BOILER 13949.11 54.51
ECONOMIZER 848.03 3.31
HEAT TO DRY FLUE
GAS
2558.076 10
HEAT TO WET FLUEGAS
2041.83 7.98
HEAT UNACCOUNTED 6191.63 24.2
TOTAL 25588.68 100
RESULTS
Theoretical air supplied = 8.4478 kg/ kg of coal
Actual air supplied = 11.81 kg/kg of coal
% Excess air = 39.81
THERMAL EFFICIENCIES
1) Boiler = 54.51%2) Economizer = 3.31%
PARCENTAGE HEAT LOSSES
Heat to dry flue gas = 10%
Heat to wet flue gas= 7.98%
Heat unaccounted = 24.2%
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Study of combustion process - Boilers
Conclusion
From combustion calculations, it is found that the excess air should
be supplied in between 30 to 40% of theoretical air requirement for
the complete combustion of coal.
As shown in graph the relation between percentage of excess air
and percentage heat lost is that as the increase in excess air flow
there is a decrease in heat loss.
Thermal efficiencies of boiler, economizer are calculated and also
heat balance sheet is prepared for the obtained result.
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