Sure Sh 1

Study of combustion process - Boilers

1. COAL

1.1INTRODUCTION

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 where CO 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 fuel size, rate of firing, supply of primary air, secondary air and moisture content in coal also greatly affect the combustion 1.2 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.

1

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

1.3 ANALYSIS OF COAL

The classification of coal is based upon the physical and chemical composition 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%

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

2

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers from the coal used for power plants, it is costly procedure. The free moisture is defined as moisture present in the coal which 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 gases like 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

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 be removed from coal before burning the coal

3

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

4

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

5

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

6

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

2. 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 coal are 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. 2.1 CLASSIFICATION OF COAL BURNING METHEDS

The classifications of the combustion system used for coal burning is given below:

7

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers 2.2 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 before entering into the combustion chamber is known as “primary air” and the amount 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.

8

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

 Disadvantages

 High capital cost  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. 2.3 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.

9

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers The arrangement of deferent equipments required in both systems is shown in the

figure: 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

10

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers to allowable unit (30mm). the crushed coal is further passed over magnetic separator, which removes pyrites and tramp iron. 2.4 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 the pulverizer. The pulverized coal is carried from the mill with the help of primary 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.

11

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

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

 The mill operates at variable load as per the load on the power plant which results on the poor performance of the pulverizing mill (more power consumption per ton of coal at part load).  The total capacity of all mills must be higher than for the central system with the load factors common in practice.  The degree of flexibility is less than of central system.  The fault in the preparation unit may put the entire steam generator out of use.  There is excessive wear and tear of the fan blades as it handles air and coal particles. 2.5 CENTRAL OR BIN SYSTEM

In the system the crushed coal is fed to the drier from the raw coal bunker by gravity. The drying of coal is effected either by using hot gases, preheated air or bled steam. The dried coal is fed to pulverizer with the help of feeder. The pulverized coal is carried from the pulverizer mill with the help of air as shown in figure and it is separated in the cyclone separator. The separated pulverized coal is transferred to the central bunker with the help of conveyor. The central system uses practically all the

12

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers equipments as used in unit system with higher capacity of each part. The storage bins are also used in addition to other equipments. This bin may contain from 12 to 24 hours supply of pulverized coal. The energy consumption of this system lies between 15 to 25 Kwhr per ton of pulverized coal. 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 is less.

 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

13

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

3.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 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 carbon monoxide. 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.

3.1 COMBUSTION EQUIPMENT FOR STEAM BOILERS.

14

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

The combustion equipment is a component of the steam generator. Since the source 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 end products 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 ordinarily operate an variable load coal – firing equipment must also take a means for holding and discharging the ash residue. 3.2 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. 3.3

COMBUSTION

EQUIPMENT

FOR

SOLID

FUELS

SELECTION

AND

CONSIDERATION:

15

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

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 Homogeneous combustion 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

16

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers C02 = concentration of oxygen x , y are the mole of fuel and oxygen involved in the stoichiometric chemical equation. Heterogeneous combustion 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.

17

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

18

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

19

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

20

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

21

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

22

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

23

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

24

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

25

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

26

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

27

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

28

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

29

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

30

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

31

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

32

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

33

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

34

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

35

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

4.COMBUSTION CALCULATIONS TYPE OF BOILER: COAL BOILER Data collection:

Coal consumption Steam produces

: 0.375 tons/hr : 2 tons/hr

Boiler operating conditions:Steam pressure

= 10 bar

Steam temperature = 179.90 C Economizer : Water temperature

= 380 C

Flue gas temperature

=2500 C

Boiler house temperature = 370 C COAL ANALYSYS (BY WEIGHT)

CONSTITUENT C H O N S ASH MOISTURE TOTAL

PERCENTAGE (%) 62.5 4.25 5.11 1.2 0.82 9.85 16.24 100

FLUE GAS ANALYIS (BY VOLUME)

36

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers 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 Water vapor in flue gas…………… 2.095 Water ……………………………… 4.187 Solution

Theoretical air requirement: Basis : 100 – kg coal Constituent

Percentage

Molecular

Element

By weight

Weight

required

12 2 32 28 32 … …

combustion 5.208 1.625 -0.159 …. 0.027 …. …. 6.1385

C H O N S ASH MOISTURE TOTAL

62.5 4.25 5.11 1.2 0.85 9.85 16.24

K Mol

5.208 2.125 0.159 0.042 0.027 … …

K mole of O2 for

There, theoretical air requirement = 6.1385(100/21)/ kg of coal = 29.23 kmol/100 kg coal

37

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers = 29.23 (28.9) kg/kj ( 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. 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

KMOL

CONSTITUENT

MOLECULAR

WEIGHT

WEIGHT

WEIGHT IN

39.45

KMOLE OF FLUE GAS

CO2

13.2

44

13.2 (44)

13.2(44)x39.45/100

38

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers O2

4.85

N2

32

81.95

28

4.85 (32)

= 229.125 4.8(32) x 39.45/100

81.95 (28)

= 61.226 81.95(28)x39.45/100 = 905.219 1195.57

TOTAL 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 = 1195.57 + 38.25 + 16.24 = 1250.06 kg/100kg coal = 12.50 kg/kg coal 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

39

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers 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% 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

40

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers =38.25+16.24 =54.49kg/100kg coal The weight of water vapor per kg of coal burnt = 0.5449 kg/ kg coal Fluegas

Kmol

% composition

Constituent CO2

(13.2/100)39.45

12.24

O2

= 5.2 (4.8/100)39.45

4.47

N2

= 1.9 (81.95/100)39.45

76.13

H2O

= 32.33 (54.49/18)

7.11

total

= 3.02 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%

41

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers

HEAT BALANCE SHEET

UNIT

HEAT

RECOVER % HEAT RECOVERY

BOILER ECONOMIZER HEAT TO DRY FLUE

(KJ/KG) 13949.11 848.03 2558.076

54.51 3.31 10

GAS HEAT TO WET FLUE

2041.83

7.98

GAS HEAT UNACCOUNTED TOTAL

6191.63 25588.68

24.2 100

42

St.Ann’s college of Engineering & Technology

Study of combustion process - Boilers 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%

5.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.

43

St.Ann’s college of Engineering & Technology